Build the Three-Membered Ring First, or Form the Carbon–Nitrogen Bond First?—Retrosynthetic Strategies, Route Selection, and Stereochemical Control in Cyclopropylamine Synthesis

1 Introduction

Cyclopropylamines combine a three-membered cyclopropane carbon ring and a nitrogen-containing functional group within the same molecule. Cyclopropane possesses high ring strain and distinctive carbon–carbon bond orbital characteristics, while the nitrogen atom can participate in protonation, coordination, nucleophilic reactions, and redox processes. The combination of these two structural units allows cyclopropylamines to serve both as structural motifs in drug molecules and as synthetic intermediates in ring-opening, cycloaddition, and functional-group transformation reactions.[1]

The synthesis of cyclopropylamines requires coordination of three processes:

① construction of the three-membered ring;

② formation of the carbon–nitrogen bond;

③ control of the substitution pattern and stereochemistry on the ring.

The corresponding retrosynthetic routes can be broadly divided into two categories:

① constructing the cyclopropane framework first, followed by formation of the carbon–nitrogen bond through rearrangement, coupling, reductive amination, or difunctionalization;

② establishing the carbon–nitrogen bond in a one-carbon or two-carbon synthon first, followed by construction of the three-membered ring through cyclopropanation or intramolecular ring closure.

These two classes of routes are suitable for different nitrogen-connectivity patterns, degrees of ring substitution, and stereochemical requirements. The availability of starting materials, whether the target requires a single stereoisomer, and whether the reaction conditions are suitable for scale-up also influence route selection.

Keywords: cyclopropylamines; aminocyclopropanes; N-cyclopropylamines; retrosynthetic analysis; carbon–nitrogen bond formation; cyclopropanation; ring strain; regioselectivity; diastereoselectivity; enantioselectivity; Kulinkovich reaction

2 Two Types of Synthetic Tasks Involving Cyclopropylamines

For the purpose of retrosynthetic analysis, cyclopropylamine-related targets are discussed here according to two types of synthetic tasks. Both types contain a cyclopropyl–nitrogen linkage and may overlap structurally, but the principal considerations in route design are different.

Type of synthetic task	General structure	Main synthetic task	Representative structures
Construction of an aminocyclopropane scaffold	A cyclopropane carbon directly bonded to —NH₂, —NHR, or —NR₂	Forming a carbon–nitrogen bond on a ring carbon, or constructing the three-membered ring from a nitrogen-containing synthon	1-Aminocyclopropane, 2-substituted cyclopropylamines, polysubstituted chiral cyclopropylamines
N-Cyclopropylation of amines	A nitrogen atom bonded to a cyclopropyl group, represented as R₂N—cyclopropyl	Installing a cyclopropyl group on the nitrogen atom of an existing amine	N-Cyclopropylanilines, N-cyclopropyl aliphatic amines, N-cyclopropyl nitrogen heterocycles

The synthesis of aminocyclopropanes usually requires control over the ring carbon bearing the amino group, the positions of other substituents, and the relative and absolute configurations on the ring. By contrast, the synthesis of N-cyclopropylamines primarily addresses cyclopropylation at the nitrogen atom.

For example, copper-promoted oxidative coupling of cyclopropylboronic acid with amines is suitable for preparing N-cyclopropylamines, but it does not directly address the cis/trans configuration or enantioselectivity of polysubstituted aminocyclopropanes.[3]

3 Three Structural Questions to Resolve Before Selecting a Route

3.1 In What Form Does Nitrogen Enter the Reaction?

Nitrogen may enter a synthetic route in the form of a free amine, amide, imine, hydroxylamine derivative, nitro group, azide, or nitrile.

Different nitrogen-containing forms serve different reaction functions:

① free amines generally act as nucleophiles;

② O-acylhydroxylamines can serve as electrophilic nitrogen sources;

③ amides can provide both the carbonyl carbon and the nitrogen substituent;

④ the carbon and nitrogen atoms of a nitrile can be converted together into the C—NH₂ unit of a primary cyclopropylamine;

⑤ nitro and azido groups can be converted into amino groups after ring formation;

⑥ imines, enamines, and other nitrogen-containing alkenes can participate directly in cyclopropanation.

The form in which nitrogen is introduced affects not only protection and deprotection, but also the electronic properties of the substrate, its ability to coordinate to metals, the rate of ring formation, and the types of side reactions that may occur.[1]

3.2 Is the Three-Membered Ring Already Present Before the Reaction?

When the cyclopropanecarboxylic acid, cyclopropanol, cyclopropylboronic acid, cyclopropene, or methylenecyclopropane required for the target structure is readily available, the three-membered ring can be prepared first, followed by carbon–nitrogen bond formation.

When the target contains multiple ring substituents, contiguous stereocenters, or a quaternary carbon stereocenter, nitrogen can be preinstalled in a nitrogen-containing alkene, amide, nitrile, or another synthon, allowing the carbon skeleton and stereochemistry to be established simultaneously during ring formation.

3.3 At Which Step Should the Stereochemistry Be Established?

The conformation of cyclopropane is constrained by the three-membered ring, and the cis/trans relationship between substituents cannot be interconverted through ordinary single-bond rotation. Consequently, the relative configuration on the ring is generally determined during the ring-forming, addition, or ring-closing step.

Route design should therefore clarify:

① whether the amino group and the adjacent substituent should be cis or trans;

② whether the target requires a single enantiomer;

③ whether the ring contains multiple contiguous stereocenters;

④ whether a quaternary carbon stereocenter must be established;

⑤ whether subsequent deprotection or functional-group transformation could affect the established configuration.

4 Route I: Constructing the Three-Membered Ring Before Forming the Carbon–Nitrogen Bond

This class of routes starts from carboxylic acids, carboxamides, ketone equivalents, alcohols, boronic acids, cyclopropenes, or methylenecyclopropanes that already contain a three-membered ring.

The basic carbon framework of the three-membered ring is already defined in the starting material. Subsequent steps are mainly responsible for forming the carbon–nitrogen bond, adjusting the substituents on nitrogen, or introducing additional substituents onto the existing ring.

4.1 Rearrangements of Cyclopropanecarboxylic Acids and Carboxamides

Cyclopropanecarboxylic acids can be converted into primary cyclopropylamines through the Curtius rearrangement, while cyclopropanecarboxamides can give primary cyclopropylamines through the Hofmann rearrangement.

4.1.1 Curtius Rearrangement

The reaction process can be summarized as follows:

Cyclopropyl—C(O)OH

→ Cyclopropyl—C(O)N₃

→ Cyclopropyl—N=C=O

→ Cyclopropyl—NH₂

The carboxylic acid is first converted into an acyl azide, which subsequently rearranges to form an isocyanate. Hydrolysis of the isocyanate affords the primary amine. Alternatively, the isocyanate can react with an alcohol to form a carbamate, thereby directly providing a protected cyclopropylamine after rearrangement.

4.1.2 Hofmann Rearrangement

The reaction process can be summarized as follows:

Cyclopropyl—C(O)NH₂

→ Cyclopropyl—N=C=O

→ Cyclopropyl—NH₂

The Hofmann rearrangement begins with an unsubstituted primary carboxamide. Under oxidative conditions, an isocyanate is formed and is subsequently hydrolyzed to give a primary amine.

Both reactions remove the carbonyl carbon of the original carboxylic acid or carboxamide. The ring substitution pattern is mainly determined by the starting material, and the rearrangement step generally does not establish new stereocenters on the ring.

These routes are suitable for primary cyclopropylamines when the corresponding cyclopropanecarboxylic acids or carboxamides are readily available. In practical applications, the compatibility of acyl azides, isocyanates, oxidants, and basic conditions with other functional groups must be evaluated.[1]

4.2 Formal Reductive Amination of Cyclopropanone Equivalents

Cyclopropanone can react with an amine to form an imine or iminium intermediate, which is then reduced to give an N-substituted cyclopropylamine. Because free cyclopropanone is highly reactive, practical preparations generally employ stable equivalents that can generate cyclopropanone-like reactivity under the reaction conditions.

A 1995 study used [(1-ethoxycyclopropyl)oxy]trimethylsilane in reactions with primary or secondary amines, followed by reduction with sodium cyanoborohydride, to prepare mono-, di-, and tricyclopropylamines.[2]

The reaction can be summarized as follows:

Cyclopropanone equivalent + R¹R²NH + reducing agent → R¹R²N—cyclopropyl

This route is mainly used to install a cyclopropyl group on nitrogen and is suitable for preparing N-cyclopropylamines. The steric hindrance and nucleophilicity of the amine, together with the reaction rate of the cyclopropanone equivalent, influence the reaction outcome.

4.3 Copper-Promoted N-Cyclopropylation with Cyclopropylboronic Acid

Cyclopropylboronic acid can undergo oxidative coupling with anilines, primary aliphatic amines, and secondary aliphatic amines to generate the corresponding N-cyclopropyl derivatives.

A representative study used copper(II) acetate, 2,2′-bipyridine, and either sodium carbonate or sodium bicarbonate to achieve N-cyclopropylation under an air atmosphere. In the original conditions, both copper acetate and 2,2′-bipyridine were used in stoichiometric amounts.[3]

The reaction can be summarized as follows:

R¹R²NH + cyclopropyl—B(OH)₂ → R¹R²N—cyclopropyl

This route has the following characteristics:

① cyclopropylboronic acid is relatively stable and convenient to measure and use;

② it avoids the use of cyclopropyl halides that are prone to side reactions;

③ it can directly modify the N-substitution pattern of an amine or nitrogen heterocycle;

④ it is primarily used for N-cyclopropylation and does not construct complex ring substitution patterns.

4.4 Conversion of Cyclopropanols into trans-Cyclopropylamines through Zinc Homoenolates

Under the action of zinc reagents, cyclopropanols can undergo selective carbon–carbon bond cleavage to form zinc homoenolates. A 2017 study demonstrated that these zinc homoenolates can display carbonyl-like electrophilic reactivity, react with nucleophilic amines, and form trans-cyclopropylamines.[4]

The main reaction process can be summarized as follows:

Substituted cyclopropanol → ring-opened zinc homoenolate → amine trapping → ring closure of a nitrogen-containing intermediate → trans-substituted cyclopropylamine

This route has two important characteristics:

① the three-membered ring undergoes controlled opening during the reaction and is then re-formed through an intramolecular process;

② the conformation of the open-chain intermediate and the subsequent ring-closing process favor formation of the trans product.

This method is suitable for certain trans-2-substituted cyclopropylamines. The yield and diastereoselectivity depend on the substitution pattern of the cyclopropanol, the structure of the amine, and the zinc-reagent conditions.

4.5 Copper-Catalyzed Aminoboration of Methylenecyclopropanes

Methylenecyclopropanes contain an exocyclic carbon–carbon double bond that can undergo difunctionalization while retaining the three-membered ring.

A 2014 study used bis(pinacolato)diboron, B₂pin₂, and O-benzoyl-N,N-dialkylhydroxylamines to achieve copper-catalyzed aminoboration of methylenecyclopropanes, producing borylmethyl-substituted cyclopropylamines.[5]

The reaction can be summarized as follows:

Methylenecyclopropane + B₂pin₂ + electrophilic nitrogen source → borylmethyl-substituted cyclopropylamine

The amino group and the boron-containing group are added to the two carbons of the original exocyclic double bond. The resulting carbon–boron bond can undergo further oxidation, coupling, or other functional-group transformations. The products therefore contain both a cyclopropylamine structure and a site for subsequent derivatization.

The reaction must simultaneously control:

① the regioselectivity determining which carbon of the original exocyclic double bond receives the amino group and which receives the boryl group;

② the diastereomeric relationship between the newly formed amino-substituted center and other substituents on the ring;

③ the chemoselectivity between retention of the three-membered ring and competing ring opening.

4.6 Asymmetric Carbometalation–Electrophilic Amination of Cyclopropenes

Cyclopropenes already contain a three-membered ring while retaining a carbon–carbon double bond that can undergo selective addition. Chiral catalytic carbometalation can establish a carbon–carbon bond and a carbon–metal bond at the two ends of the double bond, after which an electrophilic nitrogen source is used to trap the cyclopropylmetal intermediate.

Simaan and Marek reported catalytic asymmetric carbometalation of cyclopropenes followed by electrophilic amination, affording polysubstituted cyclopropylamines with high diastereomeric purity and enantiomeric enrichment.[6]

The reaction can be summarized as follows:

Substituted cyclopropene → asymmetric carbometalation → cyclopropylmetal intermediate → electrophilic amination → polysubstituted chiral cyclopropylamine

This method enables the sequential introduction of a carbon substituent and a nitrogen-containing functional group onto an existing three-membered ring. Its selectivity depends on the matching among the cyclopropene precursor, organometallic reagent, chiral catalytic system, and electrophilic nitrogen source.

5 Route II: Forming the Carbon–Nitrogen Bond Before Constructing the Three-Membered Ring

This class of routes uses nitrogen-containing synthons that already contain a carbon–nitrogen bond or can be converted into an amino group. The linkage between nitrogen and a specific carbon atom is established before ring formation, while the ring-forming step constructs the carbon–carbon bonds of the three-membered ring.

Representative nitrogen-containing synthons include:

① enamines and enamides: the carbon–nitrogen bond is already present, and the carbon–carbon double bond can undergo carbene or carbenoid cyclopropanation;

② 2-azadienes: these can serve as nitrogen-containing two-carbon synthons and react with donor–acceptor carbenes to construct polysubstituted chiral cyclopropylamines;

③ nitroalkenes: the nitro group activates the double bond and serves as an amino-group precursor, giving cyclopropylamines through addition–ring closure followed by reduction;

④ carboxylic acid dialkylamides: in the Kulinkovich–de Meijere reaction, these provide both the carbonyl carbon and the nitrogen substituent;

⑤ aliphatic nitriles: in the Kulinkovich–Szymoniak reaction, these provide the ring carbon bonded to the amino group and the final primary amino group;

⑥ nitrogen-containing carbanion equivalents derived from α-amino esters, aminonitriles, and related compounds: these can react with bifunctional electrophiles to construct three-membered rings through sequential alkylation or intramolecular substitution;

⑦ chain-like nitrogen-containing precursors capable of intramolecular nucleophilic substitution: these molecules contain both a nucleophilic carbon center and a leaving group and can generate cyclopropylamine derivatives through three-membered-ring closure.

The common feature of these categories is that the carbon–nitrogen linkage already exists before ring formation, or that nitrogen is present in a form such as a nitro or nitrile group that can later be converted into an amino group.[1]

5.1 Asymmetric Cyclopropanation of 2-Azadienes

2-Azadienes can serve as nitrogen-containing two-carbon synthons and react with metal carbenes to form aminocyclopropanes.

A 2019 study employed terminal or (Z)-internal 2-azadienes in chiral dirhodium-catalyzed cyclopropanation with donor–acceptor carbenes generated from α-diazo esters.[7]

The reaction can be summarized as follows:

2-Azadiene + α-diazo ester → cyclopropanation involving a chiral dirhodium carbene → polysubstituted aminocyclopropane

In a single ring-forming process, this method can establish:

① two new carbon–carbon bonds;

② a quaternary carbon stereocenter adjacent to the amino group;

③ the relative configuration of the ring substituents;

④ the absolute configuration of the product.

When internal 2-azadienes are used, the products can contain three contiguous stereocenters. The reaction selectivity is jointly influenced by the geometry of the 2-azadiene, the substituents on the carbene, and the steric environment of the chiral catalyst.[7]

5.2 Kulinkovich–de Meijere Reaction: Construction of Cyclopropylamines from Carboxylic Acid Dialkylamides

The Kulinkovich reaction family uses titanium alkoxides and Grignard reagents containing β-hydrogen atoms to form organotitanium intermediates.

The classical Kulinkovich reaction uses carboxylic esters as substrates and mainly produces cyclopropanols. The Kulinkovich–de Meijere reaction instead uses carboxylic acid dialkylamides, reorganizing the amide carbonyl and the organotitanium carbon fragment into N,N-disubstituted cyclopropylamines.[8]

The reaction can be summarized as follows:

R—C(O)NR¹R² + Grignard reagent + titanium alkoxide → N,N-disubstituted cyclopropylamine

The carbon-skeleton relationships are as follows:

① the amide carbonyl carbon becomes the ring carbon bonded to nitrogen;

② the amide nitrogen and its substituents are retained in the product;

③ the organotitanium species provides the carbon fragment required to construct the other two ring carbons;

④ the formation and exchange processes of the organotitanium intermediate influence the substitution pattern of the product.

This method can be used to prepare N,N-disubstituted cyclopropylamines. Intramolecular variants developed subsequently can also be used to construct certain spirocyclic and bicyclic structures.[1,8] The reaction requires Grignard and titanium reagents and is sensitive to water, oxygen, and functional groups containing active hydrogen atoms.

5.3 Kulinkovich–Szymoniak Reaction: Construction of Primary Cyclopropylamines from Nitriles

The Kulinkovich–Szymoniak reaction uses aliphatic nitriles as nitrogen-containing precursors and generates primary cyclopropylamines through the combined action of divalent titanium, a Lewis acid, and a Grignard reagent.

The reaction can be summarized as follows:

R—C≡N + Grignard reagent + titanium reagent + Lewis acid → 1-substituted primary cyclopropylamine

In this reaction:

① the nitrile carbon becomes the ring carbon bonded to the amino group;

② the nitrile nitrogen is converted into the primary amino group;

③ the organotitanium intermediate provides the carbon fragment required to construct the remaining ring carbons;

④ workup releases the primary cyclopropylamine.

Compared with the Kulinkovich–de Meijere reaction, the nitrile route directly provides a primary amine and does not require removal of alkyl substituents from nitrogen.[9]

Subsequent stereochemical studies have shown that intramolecular and intermolecular variants of the Kulinkovich cyclopropanation of nitriles may display different stereochemical outcomes. Product configuration therefore cannot be predicted solely from the configuration of the starting alkene.[10]

5.4 Addition–Ring-Closure Routes Using Nitroalkenes

Nitroalkenes can serve as nitrogen-containing two-carbon synthons. The strongly electron-withdrawing nitro group activates the carbon–carbon double bond toward nucleophilic addition. The resulting addition intermediate then forms a second carbon–carbon bond through intramolecular substitution to give a nitrocyclopropane, after which reduction of the nitro group affords the amino group.

The general reaction sequence is:

Nitroalkene → nucleophilic addition → intramolecular ring closure → nitrocyclopropane → nitro-group reduction → cyclopropylamine

This route divides construction of the three-membered ring and adjustment of the nitrogen oxidation state into two stages. During ring formation, the nitro group activates the double bond and identifies the ring carbon that will ultimately bear nitrogen. After ring formation, it is converted into the target amino group.

When selecting this route, the following factors must be evaluated:

① regioselectivity of nucleophilic addition;

② the ratio of cis and trans isomers formed during ring closure;

③ compatibility of the nitro-group reduction conditions with other functional groups;

④ whether the three-membered ring remains intact during reduction.[1]

5.5 Other Ring-Closing Modes Using Nitrogen-Containing Synthons

Enamines, enamides, α-amino esters, aminonitriles, and chain-like nitrogen-containing precursors bearing leaving groups can also be used to construct cyclopropylamines.

These routes mainly involve three ring-forming strategies:

① cyclopropanation of a nitrogen-containing alkene with a carbene or carbenoid;

② sequential formation of two carbon–carbon bonds through reactions of nitrogen-containing carbanion equivalents with dihalides or other bifunctional electrophiles;

③ closure of a three-membered ring through intramolecular nucleophilic substitution of a chain-like intermediate.

These methods can be used to prepare cyclopropane amino acids, aminonitriles, and other cyclopropylamines bearing electron-withdrawing substituents. Their applicability generally depends on the mode of carbanion stabilization, the properties of the leaving group, and the conformational requirements of the ring-closing step.[1]

6 How the Target Structure Determines the Synthetic Route

6.1 Simple Primary Cyclopropylamines

The following routes can be considered:

① Curtius rearrangement of a cyclopropanecarboxylic acid;

② Hofmann rearrangement of a cyclopropanecarboxamide;

③ Kulinkovich–Szymoniak reaction of an aliphatic nitrile.

When the corresponding cyclopropanecarboxylic acid or carboxamide is readily available, the structural relationship in the rearrangement route is relatively straightforward. When a 1-substituted cyclopropylamine must be constructed directly from a simple aliphatic nitrile, the Kulinkovich–Szymoniak reaction can be considered.

6.2 N-Cyclopropylamines

The following routes can be considered:

① formal reductive amination of a cyclopropanone equivalent;

② copper-promoted oxidative coupling of cyclopropylboronic acid.

These two types of methods primarily modify the cyclopropyl substitution on nitrogen and do not establish complex arrays of contiguous stereocenters on the ring.

6.3 trans-2-Substituted Cyclopropylamines

Certain trans-2-substituted cyclopropylamines can be obtained by forming a zinc homoenolate from a cyclopropanol, followed by amine trapping and ring closure. The diastereoselectivity of this route arises from the conformation of the open-chain intermediate and the subsequent ring-closing process.[4]

6.4 Cyclopropylamines Containing Sites for Subsequent Derivatization

Aminoboration of methylenecyclopropanes can introduce both an amino group and a boron-containing group. The resulting carbon–boron bond can subsequently participate in oxidation, coupling, and other functional-group transformations.[5]

6.5 Polysubstituted Chiral Cyclopropylamines

The following routes can be considered:

① asymmetric carbometalation–electrophilic amination of cyclopropenes;

② asymmetric carbene cyclopropanation of 2-azadienes.

The former begins with an existing three-membered ring and sequentially establishes substituents across the cyclopropene double bond. The latter constructs the carbon skeleton and stereocenters simultaneously during formation of the three-membered ring.[6,7]

6.6 N,N-Disubstituted, Spirocyclic, or Bicyclic Cyclopropylamines

The Kulinkovich–de Meijere reaction of carboxylic acid dialkylamides can directly construct N,N-disubstituted cyclopropylamines. Intramolecular variants developed subsequently can also be used to construct certain spirocyclic and bicyclic aminocyclopropanes.[1,8]

6.7 Comparison of Routes for Different Target Structures

Target structure	Representative precursor	Route to consider	Main bond-forming information	Key issues
Simple primary cyclopropylamine	Cyclopropanecarboxylic acid or carboxamide	Curtius or Hofmann rearrangement	The carbonyl carbon is removed to generate a primary amine	Precursor preparation and azide-forming or oxidative conditions
1-Substituted primary cyclopropylamine	Aliphatic nitrile	Kulinkovich–Szymoniak reaction	The nitrile carbon and nitrogen are converted into a C—NH₂ unit	Grignard reagent, titanium reagent, and Lewis acid
N-Cyclopropylamine	Amine and cyclopropylboronic acid	Copper-promoted N-cyclopropylation	Installation of a cyclopropyl group on nitrogen	Copper loading and oxidative conditions
N-Alkylcyclopropylamine	Cyclopropanone equivalent and amine	Formal reductive amination	The nitrogen substituent is adjusted through the amine component	Amine steric hindrance and reduction conditions
trans-2-Substituted cyclopropylamine	Substituted cyclopropanol and amine	Zinc-homoenolate ring opening–ring closure	Control of certain trans relative configurations	Dependence on substrate and metal conditions
Borylmethyl-substituted cyclopropylamine	Methylenecyclopropane	Copper-catalyzed aminoboration	Simultaneous formation of C—N and C—B bonds	Nitrogen source, regioselectivity, and ring retention
Polysubstituted chiral cyclopropylamine	Cyclopropene	Asymmetric carbometalation–electrophilic amination	Formation of substituents and stereochemistry on an existing three-membered ring	Chiral catalytic system and electrophilic nitrogen source
Cyclopropylamine containing a quaternary carbon stereocenter	2-Azadiene	Asymmetric carbene cyclopropanation	Establishment of multiple stereocenters during ring formation	Matching of the diazo precursor and catalyst
N,N-Disubstituted cyclopropylamine	Carboxylic acid dialkylamide	Kulinkovich–de Meijere reaction	The amide carbonyl carbon and nitrogen both enter the product	Organotitanium system and Grignard reagent

7 Selectivity Control in Cyclopropylamine Synthesis

Cyclopropylamine synthesis requires not only construction of the three-membered ring and formation of the carbon–nitrogen bond, but also control over the positions at which substituents are attached, the relative configuration on the ring, and the absolute configuration. Regioselectivity, diastereoselectivity, and enantioselectivity respectively determine “which carbon receives a particular group,” “whether substituents are located on the same or opposite faces of the three-membered ring,” and “which enantiomer is formed preferentially.”

7.1 Regioselectivity

Regioselectivity determines which carbon atom of an unsymmetrical double bond or reaction intermediate receives the amino group, carbon substituent, boryl group, or metal-containing group.

The principal influencing factors include:

① the electronic difference between the two ends of the double bond;

② steric hindrance of the substituents;

③ coordination of the metal with nitrogen, oxygen, or other heteroatoms;

④ the steric environment created by the catalyst and ligand;

⑤ the geometric requirements for subsequent ring closure of the intermediate.

In aminoboration of methylenecyclopropanes, regioselectivity determines which end of the original exocyclic double bond receives the amino group and which receives the boron-containing group. In asymmetric carbometalation–electrophilic amination of cyclopropenes, regioselectivity determines which ring carbon receives the carbon substituent and which receives the nitrogen-containing functional group.[5,6] Catalytic asymmetric carbometalation of cyclopropenes followed by electrophilic amination can generate polysubstituted cyclopropylamines with high diastereomeric purity and enantiomeric enrichment.

7.2 Relative and Absolute Configuration

Diastereoselectivity primarily determines the cis or trans relationship between substituents on the ring. Its origins include the geometry of the starting alkene, the preferred conformation of an open-chain intermediate, the direction of intramolecular ring closure, steric repulsion between substituents, and conformationally restricted transition states formed through metal coordination.

When a cyclopropanol undergoes ring opening to a zinc homoenolate, followed by amine trapping and ring closure, a trans-cyclopropylamine can be formed. This selectivity is related to the conformation of the open-chain intermediate and the geometry of ring closure.[4] The original study showed that zinc homoenolates can react with amines as carbonyl electrophiles and produce trans-cyclopropylamines with high diastereoselectivity.

Enantioselectivity depends on whether the chiral catalyst can distinguish between the two enantiotopic faces of the substrate or intermediate. In the cyclopropene route, stereocenters are established during asymmetric carbometalation, after which electrophilic amination fixes the carbon–nitrogen connectivity. In the 2-azadiene route, two new carbon–carbon bonds within the ring and the stereocenters are formed simultaneously during chiral dirhodium-catalyzed carbene cyclopropanation.[6,7] Reactions of 2-azadienes with donor–acceptor carbenes can achieve both enantioselective and diastereoselective cyclopropanation.

7.3 Polysubstituted Structures and Quaternary Carbon Stereocenters

As the number of substituents on the ring increases, regioselectivity, cis/trans selectivity, and enantioselectivity increasingly influence one another. Highly substituted cyclopropyl carbons are sterically congested, making late-stage formation of new carbon–carbon or carbon–nitrogen bonds through substitution reactions relatively difficult.

Direct asymmetric cyclopropanation of 2-azadienes with substituted carbenes can establish a quaternary carbon stereocenter adjacent to the amino group while forming the three-membered ring. When internal 2-azadienes are used, multiple contiguous stereocenters can also be generated.[7] Therefore, for highly substituted targets or targets containing contiguous stereocenters, establishing the substitution pattern and stereochemistry simultaneously during the ring-forming step generally provides a clearer route to structural control than preparing a sterically congested three-membered ring first and then performing late-stage functionalization.

8 Evaluation of Cyclopropylamine Synthetic Routes

The suitability of a route for a specific target cannot be judged solely by the yield of an individual step. Starting-material availability, stereoselectivity, functional-group compatibility, separation difficulty, and scale-up safety collectively determine the practical feasibility of a route.

8.1 Starting Materials and Route Length

The first consideration is whether the key precursors are easy to prepare, purify, and store.

Cyclopropanecarboxylic acids, carboxamides, and certain aliphatic nitriles can generally be used for structurally simple primary cyclopropylamines. Polysubstituted cyclopropenes, 2-azadienes, and specific cyclopropanols may require multistep preparation. The stability, storage conditions, and practical working concentrations of cyclopropylboronic acids, cyclopropanone equivalents, and organometallic reagents must also be considered.

Route length should therefore be evaluated together with the difficulty of precursor preparation. Even when the principal reaction involves only a few steps, the overall preparative efficiency may remain limited if the key substrate is difficult to synthesize or has low stability.

8.2 Yield, Selectivity, and Separation

Cyclopropylamine reactions should be evaluated comprehensively in terms of:

① chemical yield;

② ratio of regioisomers;

③ ratio of cis/trans isomers;

④ enantiomeric composition;

⑤ ring-opened and other by-products;

⑥ overall yield after protection and deprotection;

⑦ difficulty of separating the product from its isomers.

A reaction with a high single-step yield may still have low practical efficiency if it produces multiple regioisomers or stereoisomers that are difficult to separate. Conversely, a route with a moderate yield but a simple isomeric composition and readily purified products may be operationally advantageous in target-compound preparation.

8.3 Functional-Group Compatibility

Different reaction systems show substantial differences in functional-group tolerance.

Grignard reagents and low-valent titanium systems are generally unsuitable for direct use with substrates containing free hydroxy groups, carboxylic acids, or other active-hydrogen functionalities. Copper-promoted oxidative coupling requires evaluation of the stability of readily oxidized groups. Metal-carbene conditions require assessment of other alkenes, alkynes, and bonds capable of undergoing insertion reactions within the molecule. Curtius and Hofmann rearrangements require consideration of azide-forming, oxidative, and basic conditions, respectively.

When nitro, azido, or other nitrogen-containing precursors are used, it is also necessary to confirm that subsequent reduction or deprotection conditions will not affect the three-membered ring or other sensitive functional groups in the molecule.

8.4 Scale-Up and Operational Safety

Routes involving acyl azides, diazo compounds, Grignard reagents, low-valent titanium intermediates, cyanoborohydrides, and electrophilic hydroxylamine derivatives require evaluation of intermediate stability, reaction exotherms, and quenching procedures during scale-up.

Reaction concentration, addition rate, order of addition, heat-transfer efficiency, gas evolution, and accumulation of reactive intermediates can all alter the reaction outcome. Yields and selectivities observed in small-scale experiments can demonstrate the feasibility of a method, but they cannot replace evaluations of safety, reproducibility, and downstream processing under scale-up conditions.

9 Major Research Directions in Cyclopropylamine Synthesis

9.1 Polysubstituted Structures and Stereodivergent Synthesis

A variety of methods are available for preparing monosubstituted and certain 1,2-disubstituted cyclopropylamines. However, general routes applicable to broad classes of substrates remain limited for cis-2,3-disubstituted, trisubstituted, and other cyclopropylamines containing multiple sterically congested stereocenters.[1]

Future research needs to expand the substrate scope for polysubstituted systems and enable the selective preparation of different cis/trans isomers and enantiomers from the same or closely related starting materials by changing the catalyst, ligand, or reaction conditions. A comprehensive review published in 2025 identified scope, stereochemistry, and future development trends as important criteria for evaluating cyclopropylamine synthetic methods.

9.2 Reducing Nitrogen-Protecting-Group Interconversions

Some ring-forming methods require nitrogen to be introduced as an amide, carbamate, nitro group, azide, or another protected form. These nitrogen sources help regulate substrate reactivity but also introduce additional reduction, deprotection, or reprotection steps.

Nitrogen sources that are compatible with free amines, or that can be converted into primary amines under mild conditions, can reduce functional-group interconversions and minimize the effects of subsequent reaction conditions on the three-membered ring and other sensitive groups.

9.3 Improving Coordination Between Direct Bond Formation and Three-Membered-Ring Retention

Direct amination, addition-based amination, and difunctionalization can reduce the need to preinstall carboxylic acids, amides, halides, or other leaving groups. However, these methods must still control regioselectivity, nitrogen-source reactivity, and competing ring opening.

The future development of these methods should focus not only on reducing the number of reaction steps, but also on lowering catalyst or promoter loading, expanding substrate scope, improving stereoselectivity, and making the reaction conditions suitable for reproducible preparation and scale-up. The development of cyclopropylamine synthesis has gradually shifted from simply obtaining the target framework toward integrated control over substitution pattern, stereochemistry, and preparative feasibility.[1]

10 Classification and Research Applications of Representative Chemicals Related to Cyclopropylamine Scaffold Construction, Carbon–Nitrogen Bond Formation, and Stereoselectivity Studies

Note: The following products cover rearrangements of cyclopropanecarboxylic acids and amides, cyclopropanol ring opening–reclosure, coupling of cyclopropyl organoboron reagents, electrophilic amination, ring formation from nitrogen-containing synthons, titanium-mediated cyclopropanation of amides or nitriles, carbene cyclopropanation, and subsequent reduction and amino-group protection. Different products serve as substrates, reagents, catalysts, promoters, or analytical references in the corresponding routes and are not necessarily interchangeable under the same reaction conditions.

Table 1. Core Cyclopropylamine Compounds, Cyclopropane Precursors, and Cyclopropyl-Transfer Reagents

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Rearrangement precursor for primary cyclopropylamines	1759-53-1	C106979	Cyclopropanecarboxylic acid	≥98%	Used in studies of the Curtius rearrangement, isocyanate intermediate formation, and preparation of primary cyclopropylamines.
Amide-rearrangement precursor for primary cyclopropylamines	6228-73-5	C134392	Cyclopropanecarboxamide	≥98% (GC)	Used in studies of the Hofmann rearrangement, amide degradation, and construction of primary cyclopropylamine scaffolds.
Core target compound and analytical reference	765-30-0	C108049	Cyclopropylamine	≥98%	Used for structural confirmation, derivatization, reactivity comparison, and analytical method development for cyclopropylamines.
Structural reference for cyclopropane amino acids	22059-21-8	A101238	1-Aminocyclopropanecarboxylic acid	≥98%	Used in studies of cyclopropane amino-acid structures, carboxyl-group transformations, amino-group protection, and derivative synthesis.
Cyclopropyl nitrile structural starting material	5500-21-0	C153826	Cyclopropanecarbonitrile	≥97% (GC)	Used in studies of cyclopropyl nitrile structures, nitrile-group transformations, carbon–nitrogen bond derivatization, and ring retention.
Zinc homoenolate precursor	16545-68-9	C174869	Cyclopropanol	≥97%	Used in studies of cyclopropanol ring opening, zinc homoenolate formation, amine trapping, and ring closure to cyclopropylamines.
Model substrate for substituted cyclopropanols	29526-96-3	P1063594	1-Phenylcyclopropan-1-ol	≥95%	Used in studies of the ring-opening reactivity of 1-aryl-substituted cyclopropanols, the effects of zinc reagents, and substrate-structure effects.
Cyclopropyl halide substrate	4333-56-6	B107476	Cyclopropyl bromide	≥99%	Used in studies of cyclopropyl halide metalation, cross-coupling, radical transformations, and competition between ring retention and ring opening.
Organoboron reagent for N-cyclopropylation	411235-57-9	C120273	Cyclopropylboronic acid (contains variable amounts of anhydride)	≥90%	Used in copper-promoted oxidative coupling with amines, N-cyclopropylation, and substrate-scope studies.
Stable cyclopropyl boronic ester	126689-01-8	C120277	Cyclopropylboronic acid pinacol ester	≥96%	Used in cyclopropyl boronic ester coupling, boryl-group transformations, and cyclopropyl functional-group transfer studies.
Stable cyclopropyl trifluoroborate salt	1065010-87-8	P160814	Potassium cyclopropyltrifluoroborate	≥97% (W)	Used in coupling of stable cyclopropyl borate salts, comparison of amination conditions, and cyclopropyl-transfer studies.
Cyclopropyl organometallic reagent	23719-80-4	C115964	Cyclopropylmagnesium bromide	1 mol/L in THF	Used in studies of cyclopropyl nucleophilic addition, electrophilic amination, carbon–nitrogen bond construction, and ring retention.

Table 2. Nitrogen-Containing Synthons, Carbene Precursors, and Addition–Ring-Closure Reagents

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Lower aliphatic nitrile substrate	75-05-8	A119012	Anhydrous acetonitrile (ACN)	Anhydrous grade, ≥99.8%, H₂O ≤0.003%	Used in studies of aliphatic nitrile ring formation, incorporation of nitrile carbon into primary cyclopropylamine scaffolds, and the effects of water content.
Nitro one-carbon synthon	75-52-5	N119666	Nitromethane (regulated explosive precursor)	Anhydrous grade, ≥98.5% (GC)	Used in studies of nitro carbanion formation, nucleophilic addition, nitrocyclopropane construction, nitro-group reduction, and amino-group conversion.
Aliphatic dialkylamide substrate	685-91-6	I136012	N,N-Diethylacetamide	≥99% (GC)	Used in titanium-mediated amide cyclopropanation, construction of N,N-disubstituted cyclopropylamines, and reaction-condition screening.
Aromatic dialkylamide substrate	611-74-5	I134083	N,N-Dimethylbenzamide	≥99% (GC)	Used in aromatic amide ring formation, preparation of aryl-substituted cyclopropylamines, and studies of substrate effects.
α-Amino ester synthon	623-33-6	G105477	Glycine ethyl ester hydrochloride	≥99%	Used as an α-amino ester precursor. Following amino-group protection, imine formation, or other activation treatments, it is used in α-carbanion formation, sequential alkylation, and construction of cyclopropane amino acids.
Protected nitrogen source and nitrogen nucleophile	85-41-6	P104067	Phthalimide	≥99%	Used in studies of protected nitrogen-source introduction, cyclopropyl amination, intramolecular substitution, and subsequent deprotection.
Model nitroalkene substrate	5153-67-3	N122699	trans-β-Nitrostyrene	≥98%	Used in Michael addition–ring-closure reactions, construction of aryl nitrocyclopropanes, and stereoselectivity studies.
Diazo ester carbene precursor	623-73-4	E433136	Ethyl diazoacetate	15% in toluene	Used for general metal-carbene generation, cyclopropanation of nitrogen-containing alkenes, and comparison of non-asymmetric reaction conditions.
Bifunctional alkylating reagent	106-93-4	D104774	1,2-Dibromoethane	≥99%	Used in sequential alkylation of nitrogen-containing carbanions, intramolecular ring closure, and three-membered-ring construction studies.
Halomalonate ring-closing reagent	685-87-0	D139177	Diethyl bromomalonate	≥90%	Used in nitroalkene addition–ring-closure reactions, construction of ester-substituted cyclopropanes, and stereochemical studies.

Table 3. Copper and Rhodium Catalytic Systems and Borylation Reagents

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Base for copper-promoted coupling	584-08-7	P485463	Potassium carbonate	Anhydrous grade, reagent grade, high-purity grade, ≥99%	Used in carbonate-base screening and reaction-condition comparison for N-cyclopropylation with cyclopropyl organoboron reagents.
Organocopper catalyst precursor	7681-65-4	C433811	Copper(I) iodide	Anhydrous grade, ≥99.995% metals basis	Used in organocopper-species formation, cyclopropene carbometalation, electrophilic amination, and ligand screening.
Copper catalyst precursor for aminoboration	7758-89-6	C112392	Copper(I) chloride	PrimorTrace™, ≥99.999% metals basis	Used in aminoboration of methylenecyclopropanes, formation of copper–boron species, and regioselectivity studies.
Ligand for copper-coordination control	366-18-7	D108977	2,2′-Bipyridine	AR, ≥99%	Used in copper-promoted N-cyclopropylation, regulation of the copper-center coordination environment, and reaction-condition screening.
Copper promoter for N-cyclopropylation	142-71-2	C1520978	Anhydrous copper(II) acetate	≥99.9% metals basis	Used in oxidative coupling of cyclopropylboronic acid with amines, N-cyclopropylation, and comparison of copper salts.
Boron source for aminoboration	73183-34-3	B396365	Bis(pinacolato)diboron	≥99%	Used in aminoboration of methylenecyclopropanes, carbon–boron bond formation, and subsequent derivatization studies.
Reagent for organocopper-species and cyclopropanol-transformation studies	544-92-3	C305340	Copper(I) cyanide	≥99%	Used in organocopper-species formation, ring opening and rearrangement of specific cyclopropanol derivatives, and comparison of metal-mediated cyclopropyl-transformation conditions.
Catalyst for carbene cyclopropanation	15956-28-2	R102671	Rhodium(II) acetate dimer	Rh: 43.0%–46.6%	Used in diazo-compound decomposition, non-asymmetric metal-carbene cyclopropanation, and comparison of dirhodium catalytic systems.

Table 4. Titanium, Zinc, and Grignard Reagents and Strong-Base Systems

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Lewis acid for nitrile ring formation	109-63-7	B431394	Boron trifluoride diethyl etherate	Distillation grade, ≥46.5% BF₃ basis	Used in titanium-mediated cyclopropanation of nitriles, nitrile activation, and studies of conditions for primary cyclopropylamine formation.
Strong base for deprotonation of nitrogen-containing synthons	7646-69-7	S110860	Sodium hydride	60% dispersion in mineral oil	Used in deprotonation of amino esters and nitro compounds and in intramolecular cyclopropane ring-closure studies.
Grignard reagent for titanium-mediated ring formation	925-90-6	E434581	Ethylmagnesium bromide solution	3.4 M in 2-methyltetrahydrofuran	Used in the formation of low-valent organotitanium species, cyclopropanation of amides or nitriles, and carbon-skeleton construction.
Reagent for zinc homoenolate formation	557-20-0	D684313	Diethylzinc solution	2 M in toluene	Used in substituted cyclopropanol ring opening, zinc homoenolate formation, amine trapping, and ring reclosure studies.
Grignard reagent for titanium-mediated ring formation	1068-55-9	I107831	Isopropylmagnesium chloride	2.0 M in THF	Used in organotitanium-intermediate formation, studies of Grignard-reagent structural effects, and screening of cyclopropanation conditions.
Strong base for carbanion formation	865-47-4	P111075	Potassium tert-butoxide	≥98%	Used in the deprotonation and ring formation of nitro compounds, amino esters, and other substrates containing activated methylene groups.
Titanium reagent for titanium-mediated cyclopropanation	546-68-9	T105736	Titanium(IV) isopropoxide	≥99.9% metals basis	Used in the formation of low-valent titanium species and in cyclopropanation studies involving dialkylamides or aliphatic nitriles.

Table 5. Reagents for Rearrangement, Electrophilic Amination, Reductive Amination, and Amino-Group Protection

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Organic base and acid scavenger	121-44-8	T140677	Triethylamine	Anhydrous grade, ≥99.5%, water ≤50 ppm	Used in rearrangements, electrophilic amination, amino-group protection, acid scavenging, and adjustment of reaction conditions.
General borohydride reducing agent	16940-66-2	S432207	Sodium borohydride (regulated explosive precursor)	purum p.a., ≥96%	Used in transformations of carbonyl compounds, imines, and reducible nitrogen-containing intermediates and in comparison of reduction conditions.
tert-Butoxycarbonyl protecting reagent	24424-99-5	D106159	Di-tert-butyl dicarbonate	≥99%	Used in amino-group protection of cyclopropylamines, protecting-group interconversion before and after ring formation, and derivative preparation.
Selective reductive-amination reagent	25895-60-7	S105661	Sodium cyanoborohydride	≥95%	Used in reductive amination of cyclopropanone equivalents with amines and in the construction of N-cyclopropylamines.
Benzyloxycarbonyl protecting reagent	501-53-1	B105737	Benzyl chloroformate	≥96%, contains 0.1% sodium carbonate as stabilizer	Used in benzyloxycarbonyl protection of cyclopropylamines, regulation of nitrogen nucleophilicity, and subsequent deprotection studies.
Mild reductive-amination reagent	56553-60-7	S106573	Sodium triacetoxyborohydride (STAB)	≥90%	Used in selective reduction of imine or iminium intermediates and preparation of N-substituted cyclopropylamines.
Curtius rearrangement reagent	26386-88-9	D106412	Diphenyl phosphoryl azide (DPPA)	≥97%	Used in acyl azide formation from cyclopropanecarboxylic acids, isocyanate formation, and preparation of primary cyclopropylamines.
Electrophilic amination reagent	2950-43-8	H100568	Hydroxylamine-O-sulfonic acid (HOSA)	≥97%	Used in electrophilic amination of cyclopropylmetal reagents, carbon–nitrogen bond formation, and construction of primary amines.

Note: The products listed above are representative Aladdin research products. Additional product specifications, grades, and certificate of analysis information can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Bertus P.; Caillé J. Advances in the Synthesis of Cyclopropylamines. Chemical Reviews, 2025, 125(6), 3242–3377. DOI: 10.1021/acs.chemrev.4c00674.

[2] Gillaspy M. L.; Lefker B. A.; Hada W. A.; Hoover D. J. A Simple Method for the Formation of Cyclopropylamines: The First Synthesis of Tricyclopropylamine. Tetrahedron Letters, 1995, 36(41), 7399–7402. DOI: 10.1016/0040-4039(95)01560-4.

[3] Bénard S.; Neuville L.; Zhu J. Copper-Promoted N-Cyclopropylation of Anilines and Amines by Cyclopropylboronic Acid. Chemical Communications, 2010, 46(19), 3393–3395. DOI: 10.1039/B925499D.

[4] Mills L. R.; Barrera Arbelaez L. M.; Rousseaux S. A. L. Electrophilic Zinc Homoenolates: Synthesis of Cyclopropylamines from Cyclopropanols and Amines. Journal of the American Chemical Society, 2017, 139(33), 11357–11360. DOI: 10.1021/jacs.7b07104.

[5] Sakae R.; Matsuda N.; Hirano K.; Satoh T.; Miura M. Highly Stereoselective Synthesis of (Borylmethyl)cyclopropylamines by Copper-Catalyzed Aminoboration of Methylenecyclopropanes. Organic Letters, 2014, 16(4), 1228–1231. DOI: 10.1021/ol5001507.

[6] Simaan M.; Marek I. Asymmetric Catalytic Preparation of Polysubstituted Cyclopropanol and Cyclopropylamine Derivatives. Angewandte Chemie International Edition, 2018, 57(6), 1543–1546. DOI: 10.1002/anie.201710707.

[7] Shao X.; Malcolmson S. J. Catalytic Enantio- and Diastereoselective Cyclopropanation of 2-Azadienes for the Synthesis of Aminocyclopropanes Bearing Quaternary Carbon Stereogenic Centers. Organic Letters, 2019, 21(18), 7380–7385. DOI: 10.1021/acs.orglett.9b02692.

[8] Chaplinski V.; de Meijere A. A Versatile New Preparation of Cyclopropylamines from Acid Dialkylamides. Angewandte Chemie International Edition in English, 1996, 35(4), 413–414. DOI: 10.1002/anie.199604131.

[9] Bertus P.; Szymoniak J. New and Easy Route to Primary Cyclopropylamines from Nitriles. Chemical Communications, 2001, 37(18), 1792–1793. DOI: 10.1039/B105293B.

[10] Astashko D.; Lee H. G.; Bobrov D. N.; Cha J. K. On the Stereochemistry of the Kulinkovich Cyclopropanation of Nitriles. The Journal of Organic Chemistry, 2009, 74(15), 5528–5532. DOI: 10.1021/jo900823h.

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