Technical articles
Experimental Selection Logic for C-C Bond Construction: Understanding Fragment Coupling, Carbonyl Chain Extension, Olefination, and Late-Stage Bond-Forming Pathways by Bond-Forming Task
Experimental Selection Logic for C-C Bond Construction: Understanding Fragment Coupling, Carbonyl Chain Extension, Olefination, and Late-Stage Bond-Forming Pathways by Bond-Forming Task
1. First, define the bond-forming task clearly
In C-C bond construction, three questions should be considered first: what the starting substrate is, what type of carbon fragment needs to be introduced, and whether the system can tolerate organometallic reagents, transition-metal catalysis, or relatively strong basic conditions. Based on these three questions, common tasks can be divided into six categories: (1) coupling two preassembled fragments, (2) directly extending a carbon chain from a carbonyl group, (3) converting a carbonyl group into an alkene, (4) installing special fluorinated carbon fragments, (5) adjusting the skeleton through alkene reorganization, and (6) directly activating a C-H bond on an existing skeleton to form a new bond.
1.1 | Preliminary routing of C-C bond construction pathways by bond-forming task
Bond-forming task | Common starting substrates | Reaction type to examine first | What to assess first |
Coupling two preassembled fragments | Aryl or alkenyl halides, triflates, organoboron reagents, terminal alkynes, organozinc reagents, etc. | Cross-coupling | Whether the coupling partner is accessible and whether the functional groups are tolerated |
Direct carbon-chain extension from a carbonyl group | Aldehydes, ketones, esters, epoxides | Organometallic nucleophilic addition or epoxide ring opening | Whether strong nucleophiles, strong bases, and strictly anhydrous, oxygen-free conditions are acceptable |
Converting a carbonyl group into an alkene | Aldehydes, ketones | Wittig reaction; Horner-Wadsworth-Emmons reaction (abbreviated as the HWE reaction) | Whether the target requires a double bond or a saturated carbon chain |
Installing special carbon fragments | Arenes, alkenes, nucleophilic substrates, radical acceptors | Trifluoromethylation; difluoromethylation | Whether the fluorinated fragment should be introduced at an early or late stage |
Adjusting the skeleton through double bonds | Dienes, alkene fragments | Olefin metathesis | Whether the goal is ring closure or exchange of alkene fragments |
When installation of a leaving group is undesirable | C-H sites on an already assembled skeleton | C-H functionalization | Whether site selectivity is controllable and whether a directing group is required |
2. When two fragments are already prepared, examine cross-coupling first
Cross-coupling is well suited to routes in which “both fragments are already prepared and only need to be joined.” These reactions are catalyzed by transition metals and are widely used to form C-C bonds. Cross-coupling has become one of the core methods for connecting aryl, alkenyl, and alkynyl fragments.
2.1 | Typical applications of common cross-coupling reactions
Reaction type | Common coupling partners | Problems it is suited to solve | Main points of attention |
Suzuki-Miyaura coupling | Aryl or alkenyl halides, triflates + organoboron reagents | Aryl-aryl, aryl-alkenyl, and alkenyl-alkenyl bond formation | Matching among base, catalyst, and substrate |
Sonogashira coupling | Aryl or alkenyl halides + terminal alkynes | Aryl-alkynyl and alkenyl-alkynyl bond formation | Terminal alkyne homocoupling and copper-salt-related side reactions |
Heck reaction | Aryl or alkenyl halides, triflates + alkenes | Introduction of aryl or alkenyl fragments onto alkenes | Regioselectivity and alkene geometry |
Negishi coupling | Aryl or alkenyl halides + organozinc reagents | Fragment coupling involving substrates of insufficient reactivity | Preparation and handling of organozinc reagents |
Stille coupling | Aryl or alkenyl halides + organotin reagents | Coupling of certain difficult substrates | Organotin residues and purification burden |
The strength of cross-coupling lies in its clear fragment-based design, which makes it convenient to rapidly vary substituents on the same molecular framework. A typical prerequisite is that the substrate already contains a couplable site, or that prefunctionalization must first be carried out.
3. When starting from a carbonyl compound, first distinguish whether a single bond or a double bond is needed
Carbonyl substrates often correspond to two different routes. If the goal is to extend the carbon chain and obtain an alcohol intermediate, organometallic nucleophilic additions such as Grignard addition should be considered first. If the goal is to convert the carbonyl group directly into an alkene, the Wittig reaction and the HWE reaction should be examined first.
3.1 | Two common C-C bond construction pathways from carbonyl substrates
Pathway | Representative reaction | Bond type formed | Suitable target | Main limitations |
Nucleophilic addition | Grignard addition | C-C single bond | Organometallic nucleophilic addition or chain extension by epoxide ring opening; direct carbon-chain extension to afford alcohol intermediates | Sensitive to water, oxygen, and acidic protons; limited functional group tolerance |
Carbonyl olefination | Wittig reaction | C=C double bond | Direct conversion of aldehydes or ketones into alkenes | Often accompanied by phosphine oxide byproducts, so workup must be considered |
Carbonyl olefination | HWE reaction | C=C double bond | Commonly used for constructing alkenes bearing electron-withdrawing groups, and frequently applied to α,β-unsaturated esters, ketones, nitriles, and related systems | Base strength and substrate sensitivity must be considered |
The key question at this stage is whether the target structure ultimately requires a single bond or a double bond. If subsequent steps will continue to elaborate an unsaturated fragment, carbonyl olefination is usually more direct. If later steps require alcohols, diols, or further functional-group interconversions, organometallic addition is often the more suitable route.
4. When the skeleton needs to be adjusted through double bonds, examine olefin metathesis
The essence of olefin metathesis is the cleavage and recombination of carbon-carbon double bonds to redistribute alkene fragments. This class of methods has been widely used in modern organic synthesis for ring closure, fragment exchange, and skeletal reorganization.
4.1 | Typical uses of olefin metathesis
Use | Common form | Suitable target | Main points of attention |
Ring closure | Ring-closing metathesis | Construction of medium rings, macrocycles, and unsaturated rings | Dilution conditions, dimerization side reactions, and substrate conformation |
Fragment exchange | Cross metathesis | Exchange and reorganization of two alkene fragments | Substrate matching and selectivity |
Skeletal reorganization | Redistribution of double bonds | Reorganization of alkene fragments through metathesis | Catalyst stability and functional-group compatibility |
5. When special carbon fragments need to be installed, examine trifluoromethylation and difluoromethylation
Trifluoromethylation and difluoromethylation are not simply about “introducing fluorinated substituents.” Rather, they involve installing fluorinated carbon fragments as intact units into the target molecule.
5.1 | Questions to consider first when introducing fluorinated carbon fragments
Question | Selection focus |
Is the required fragment trifluoromethyl or difluoromethyl? | These are not the same fragment and cannot be interchanged casually |
Is the target substrate a nucleophilic substrate, arene, alkene, or radical acceptor? | The reaction mode changes with substrate type |
Should the fluorinated fragment be introduced early or late in the route? | This depends on compatibility with the preceding and subsequent steps |
Does the reaction rely on an electrophilic, nucleophilic, or radical reagent? | This directly affects the reaction conditions and substrate scope |
6. When prefunctionalization is undesirable, C-H functionalization should be carefully evaluated
C-H functionalization refers to the selective and controllable conversion of a C-H bond into a new C-C, C-N, C-O, or C-X bond. When used to construct C-C bonds, the key issues are site control and directing strategies.
6.1 | Situations in which C-H functionalization should be prioritized
Situation | Why it is suitable |
The skeleton has already been assembled and only one more fragment needs to be attached at a late stage | Enables direct site-selective modification on an existing framework |
A single coupling would otherwise require multiple additional prefunctionalization steps | Direct C-H activation may shorten the route |
A usable directing group is present in the molecule | Helps improve site selectivity |
The goal is to rapidly expand a set of closely related analogues | Facilitates late-stage structural diversification |
If C-H functionalization suffers from insufficient site selectivity, an unsuitable directing group, or incompatibility between the substrate and the catalytic system, it may not be more efficient than the conventional strategy of “installing a reactive site first, then coupling.”
7. Selection comparison of different C-C bond construction pathways
Goal | Bond-construction pathway to examine first | Suitable starting substrates | Main limitations |
Coupling two preassembled fragments | Cross-coupling | Halides, triflates, organoboron reagents, organozinc reagents, terminal alkynes, etc. | A couplable site usually needs to be available in advance |
Direct carbon-chain extension from a carbonyl group | Organometallic additions such as Grignard addition | Aldehydes, ketones, esters, epoxides | Condition-sensitive, with limited functional group tolerance |
Direct conversion of a carbonyl group into an alkene | Wittig reaction, HWE reaction | Aldehydes, ketones | Workup and control of alkene geometry need to be considered in advance |
Ring closure or skeletal adjustment through double bonds | Olefin metathesis | Dienes, alkene fragments | Selectivity and substrate matching are critical |
Installation of trifluoromethyl or difluoromethyl fragments | Introduction of fluorinated carbon fragments | Arenes, alkenes, nucleophilic substrates, radical acceptors | The timing of introduction affects the entire synthetic route |
Direct attachment of a new fragment onto an existing skeleton | C-H functionalization | Preassembled skeletons | Site control and directing strategies are the core issues |
8. Classification, Features, and Applications of Representative Chemicals Related to Experimental Selection for C-C Bond Construction (Tables 1-5)
Table 1 | Cross-Coupling Catalysts, Cocatalysts, and Typical Substrates
Category | CAS No. | Aladdin Cat. No. | Name | Grade or Purity | Product Features and Applications |
Copper cocatalyst for Sonogashira coupling | 7681-65-4 | Copper(I) iodide | Anhydrous grade, ≥99.995% metals basis | Commonly used in Cu-cocatalyzed Sonogashira coupling systems to promote the participation of terminal alkynes in C-C bond formation; also applicable to certain Cu-mediated coupling and alkynylation reactions | |
Palladium catalyst for cross-coupling | 13965-03-2 | Bis(triphenylphosphine)palladium dichloride | Pd 15.2% | One of the commonly used palladium sources for constructing catalytic systems for cross-coupling reactions such as Suzuki, Sonogashira, and Heck reactions | |
Palladium catalyst for cross-coupling | 14221-01-3 | Tetrakis(triphenylphosphine)palladium(0) | Pd ≥8.9% | A zerovalent palladium precatalyst commonly used in cross-coupling reactions involving aryl halides or triflates; suitable for reaction development and condition screening | |
Aryl halide coupling substrate | 108-86-1 | Bromobenzene | Standard for GC, ≥99.5% (GC) | A representative aryl halide substrate for establishing conditions and comparing methods in Suzuki, Sonogashira, Heck, Negishi, and Stille coupling reactions | |
Alkene coupling substrate | 100-42-5 | Styrene | Standard for GC, ≥99.5% (GC), contains 10-15 ppm TBC stabilizer | A common terminal alkene substrate for evaluating arylation performance in Heck reactions; also a representative alkene substrate for cross metathesis | |
Terminal alkyne coupling substrate | 536-74-3 | Phenylacetylene | ≥97% | A common terminal alkyne substrate used in Sonogashira coupling to construct aryl-alkynyl bonds; also used in alkynyl chain extension and method screening | |
Aryl triflate substrate | 17763-67-6 | Phenyl Trifluoromethanesulfonate | ≥98% (GC) | A representative aryl triflate substrate that can replace certain aryl halides in palladium-catalyzed coupling systems; suitable for evaluating leaving-group effects | |
Triflation reagent | 358-23-6 | Trifluoromethanesulfonic anhydride | ≥99% | Commonly used to activate phenolic hydroxyl or enolic substrates into triflates, thereby providing highly reactive leaving-group substrates for subsequent cross-coupling |
Table 2 | Organoboron, Organozinc, and Organotin Coupling Partners and Related Reagents
Category | CAS No. | Aladdin Cat. No. | Name | Grade or Purity | Product Features and Applications |
Organoboron substrate for Suzuki coupling | 98-80-6 | Phenylboronic acid (PBA) (contains varying amounts of Anhydride) | ≥99.5% | A representative organoboron coupling partner that can undergo Suzuki coupling with aryl halides or triflates; commonly used in the construction of biaryl structures | |
Borylation reagent | 73183-34-3 | Bis(pinacolato)diboron | ≥99% | Commonly used for borylation of aryl or alkenyl substrates to prepare organoboron intermediates required for subsequent Suzuki coupling | |
Borylation reagent | 25015-63-8 | 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane | ≥97% | Commonly used for the borylation of alkenes, alkynes, and certain halide substrates, providing boronate ester intermediates for subsequent cross-coupling | |
Organozinc reagent for Negishi coupling | 557-20-0 | D684313 | Diethylzinc solution | 2 M in toluene | A typical organozinc reagent that can be used in Negishi coupling or in addition and transmetalation processes involving organozinc reagents; suitable for evaluating highly reactive coupling systems |
Organotin reagent for Stille coupling | 7486-35-3 | Tributyl(vinyl)stannane | ≥98% | A common vinyl organotin coupling partner used in Stille coupling to construct aryl-alkenyl or alkenyl-alkenyl structures |
Table 3 | Reagents Related to Grignard Addition and Carbonyl Olefination
Category | CAS No. | Aladdin Cat. No. | Name | Grade or Purity | Product Features and Applications |
Metal feedstock for Grignard reagent preparation | 7439-95-4 | M109153 | Magnesium | AR, ≥99.5% | Used for the in situ preparation of Grignard reagents; a basic raw material for converting aryl and alkyl halides into organomagnesium reagents |
Grignard reagent | 75-16-1 | M130050 | Methylmagnesium Bromide | 3.0 M solution in diethyl ether | A commonly used methylating Grignard reagent that can add to electrophilic substrates such as aldehydes, ketones, and esters to achieve methyl introduction and carbon-chain extension |
Grignard reagent | 100-58-3 | P103163 | Phenylmagnesium Bromide | 1.0 M in THF | A commonly used aryl Grignard reagent that can be used in carbonyl addition to construct aryl-substituted alcohols; also applicable to certain transmetalation processes and the preparation of coupling precursors |
Electrophile for Grignard reaction | 75-21-8 | Ethylene oxide solution | 2.5-3.3 M in THF | Commonly used in ring-opening reactions with Grignard reagents to achieve two-carbon chain extension and afford terminal hydroxyl intermediates | |
Strong base for Wittig/HWE reactions | 7646-69-7 | S110860 | Sodium hydride | 60% dispersion in mineral oil | Commonly used for deprotonation of phosphonium salts or activation of phosphonates to generate ylides or phosphonate carbanions, thereby driving carbonyl olefination reactions |
Strong base for Wittig/HWE reactions | 109-72-8 | n-Butyllithium solution | 2.7 M in hexane (25% solution) | Commonly used under strongly basic conditions to generate ylides or phosphonate carbanions; also applicable to halogen-lithium exchange and intermediate construction | |
Phosphonium salt precursor for Wittig reaction | 1779-49-3 | Methyltriphenylphosphonium bromide | ≥98% | A classical Wittig reaction precursor that forms the methylene ylide upon deprotonation and is used to convert aldehydes and ketones into terminal alkenes | |
HWE reagent | 867-13-0 | Triethyl phosphonoacetate | ≥98% | A commonly used stabilized phosphonate reagent that undergoes the HWE reaction with aldehydes or ketones to construct α,β-unsaturated ester structures | |
HWE reagent | 2537-48-6 | Diethyl cyanomethylphosphonate | ≥98% (GC) | A commonly used cyano-substituted phosphonate reagent that can construct cyano-substituted alkene intermediates through the HWE reaction and is suitable for further functional-group transformations |
Table 4 | Reagents Related to the Introduction of Special Fluorinated Carbon Fragments
Category | CAS No. | Aladdin Cat. No. | Name | Grade or Purity | Product Features and Applications |
Nucleophilic trifluoromethylation reagent | 81290-20-2 | (Trifluoromethyl)trimethylsilane (TFMTMS) | ≥98% | A commonly used nucleophilic trifluoromethylation reagent that can introduce a trifluoromethyl fragment into electrophilic substrates such as carbonyl compounds | |
Electrophilic trifluoromethylation reagent | 1895006-01-5 | Dimesityl(trifluoromethyl)sulfonium Trifluoromethanesulfonate | ≥98% (HPLC) | An electrophilic trifluoromethylation reagent that can be used to study trifluoromethyl introduction into heteroatom-containing substrates or electron-rich substrates | |
Electrophilic trifluoromethylation reagent | 887144-94-7 | 1-Trifluoromethyl-1,2-benziodoxol-3-(1H)-one | ≥97% | A common hypervalent iodine trifluoromethylation reagent that can be used for radical-type or electrophilic trifluoromethyl introduction | |
Difluoromethylation reagent | 58310-28-4 | (Difluoromethyl)triphenylphosphonium bromide | ≥97% | Can be used for introducing difluoromethyl fragments and can also serve as a precursor for related difluoromethylation or difluoroolefination transformations | |
Difluoromethylation reagent | 1355729-38-2 | Zinc difluoromethanesulfinate | ≥95% | One of the commonly used difluoromethylation reagents; can introduce difluoromethyl groups into aryl, heteroaryl, and related substrates under transition-metal or radical conditions |
Table 5 | Reagents Related to Olefin Metathesis and C-H Functionalization
Category | CAS No. | Aladdin Cat. No. | Name | Grade or Purity | Product Features and Applications |
First-generation metathesis catalyst | 172222-30-9 | Grubbs Catalyst | Ru 12.3% | A classic first-generation Grubbs catalyst that can be used for ring-closing metathesis and for developing certain cross-metathesis reactions | |
Second-generation metathesis catalyst | 246047-72-3 | Grubbs Catalyst, 2nd Generation | ≥99.95% metals basis | A highly active ruthenium metathesis catalyst commonly used for ring-closing metathesis, cross metathesis, and alkene skeletal adjustment involving more challenging substrates | |
Second-generation Hoveyda-Grubbs metathesis catalyst | 301224-40-8 | Hoveyda-Grubbs Catalyst 2nd Generation | ≥97% | Commonly used in cross metathesis and ring-closing metathesis; suitable for evaluating stability and substrate compatibility | |
Palladium catalyst for C-H functionalization | 3375-31-3 | Palladium acetate | AR, Pd 46.0-48.0% | A commonly used palladium source for C-C bond-forming reactions such as directed C-H arylation, alkenylation, and oxidative coupling | |
Silver salt oxidant for C-H functionalization | 563-63-3 | Silver acetate | AR, ≥99.5% | Commonly used in oxidative regeneration systems for palladium-catalyzed C-H functionalization; can also participate in halide scavenging and reaction activation | |
Copper salt oxidant for C-H functionalization | 6046-93-1 | Copper(II) acetate monohydrate | Suitable for analysis, ACS, premium grade | Commonly used in oxidative C-H functionalization and coupling systems; can serve as an oxidant or as a copper-containing catalytic component | |
Additive for C-H functionalization | 75-98-9 | Pivalic acid (PA) | ≥99% | Commonly used as a carboxylic acid additive to promote the C-H activation step, especially in palladium-catalyzed directed C-H functionalization systems | |
Substrate for directed C-H functionalization | 1008-89-5 | 2-Phenylpyridine | ≥98% | A classical model substrate for directed C-H activation, commonly used to evaluate C-C bond-forming reactions such as arylation, alkenylation, and oxidative coupling | |
Common bidentate directing group | 578-66-5 | 8-Aminoquinoline | ≥98% | Commonly used to construct bidentate amide-directed systems and suitable for studies on directed C-H arylation, alkylation, and related C-C bond-forming reactions |
Note: The above are representative Aladdin products. For more product specifications, please search the Aladdin website using the “product name/CAS/catalog number”.
References
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[2] Maryanoff B E, Reitz A B. The Wittig Olefination Reaction and Modifications Involving Phosphoryl-Stabilized Carbanions. Chemical Reviews, 1989, 89(4): 863-927.
[3] Roman D, Sauer M, Beemelmanns C. Applications of the Horner-Wadsworth-Emmons Olefination in Modern Natural Product Synthesis. Synthesis, 2021, 53: 2713-2739.
[4] Liu X, Xu C, Wang M, Liu Q. Trifluoromethyltrimethylsilane: Nucleophilic Trifluoromethylation and Beyond. Chemical Reviews, 2015, 115(2): 683-730.
[5] Ogba O M, Warner N C, O’Leary D J, Grubbs R H. Recent advances in ruthenium-based olefin metathesis. Chemical Society Reviews, 2018, 47: 4510-4544.
[6] Rej S, Ano Y, Chatani N. Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds. Chemical Reviews, 2020, 120(3): 1788-1887.
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