Silicon‐Based Cross‐Coupling Reactions in the Total Synthesis of Natural Products

Unlike other variants of transition‐metal‐catalyzed cross‐coupling reactions, those based on organosilicon donors have not been used extensively in natural product synthesis. However, recent advances such as: 1) the development of mild reaction conditions, 2) the expansion of substrate scope, 3) the development of methods to stereoselectively and efficiently introduce the silicon‐containing moiety, 4) the development of a large number of sequential processes, and 5) the advent of bifunctional bis(silyl) linchpin reagents, signify the coming of age of silicon‐based cross‐coupling reactions. The following case studies illustrate how silicon‐based cross‐coupling reactions play a strategic role in constructing carbon–carbon bonds in selected target molecules.     

Photosensitized Intermolecular Carboimination of Alkenes through the Persistent Radical Effect

An intermolecular, two‐component vicinal carboimination of alkenes has been accomplished by energy transfer catalysis. Oxime esters of alkyl carboxylic acids were used as bifunctional reagents to generate both alkyl and iminyl radicals. Subsequently, addition of the alkyl radical to an alkene generates a transient radical for selective radical–radical cross‐coupling with the persistent iminyl radical. Furthermore, this process provides direct access to aliphatic primary amines and α‐amino acids by simple hydrolysis.     

Intramolecular aminolysis of trichloroethyl esters: a mild macrocyclization protocol for the preparation of cryptophycin derivatives

The bifunctional catalyst 2-hydroxypyridine (2-pyridone) is shown to promote the intramolecular aminolysis of the polyfunctionalized long-chain amino trichloroethyl ester 8 to afford cryptophycin-51 (4). This process for the construction of the macrocyclic core of cryptophycin derivatives is noteworthy for its convenience, avoidance of expensive coupling reagents, and use of mild reaction conditions.     

Recent advance in heterocyclic organozinc and organomanganese compounds; direct synthetic routes and application in organic synthesis

A practical synthetic route for the preparation of 2-pyridyl and 3-pyridyl derivatives has been accomplished by utilizing a simple coupling reaction of stable 2-pyridylzinc bromides and 3-pyridylzinc bromides. The organozincs used in this study were easily prepared via the direct insertion of active zinc into the corresponding bromopyridines. The subsequent coupling reactions with a variety of different electrophiles have afforded the corresponding coupling products. Using highly active manganese, a variety of Grignard-type organomanganese reagents have been obtained. The subsequent coupling reactions of the resulting organomanganese reagents with several electrophiles have also been accomplished under mild conditions.     

Synthesis and reactions of thioamides of 5-amino-2-aryl-2H-1,2,3-triazole-4-carboxylic acid

A method has been developed for obtaining thioamides of 5-amino-2-aryl-2H-1,2,3-triazole-4-carboxylic acid. Their heterocyclization reactions with bifunctional reagents has been studied. New heterocyclic polycyclic ensembles have been synthesized containing a 1,2,3-triazole fragment, a thiazole, and a residue of the natural alkaloid cytisine.     

The synthesis of polyamine networks using cationic living poly(1-tert-butylaziridine)

Polyamine networks have been synthesized by end-linking bifunctional living poly-TBA with polyfunctional nucleophilic reagents. The bifunctional living poly-TBA was obtained by polymerization of TBA initiated by low molecular weight bifunctional living poly(tetrahydrofuran). The swelling degrees of the networks in several solvents are reported.     

Novel annulation reactions: A simple preparation of spirocycles and an allylsilane based bifunctional acceptor-donor annulating reagent

A simple, one pot, stereoconvergent synthesis of spirocycles has been achieved in excellent yield. Further studies on one step bifunctional annulating reagents have resulted in the development of a new allylsilane based cyolopentannulation reaction.     

Microwave-Assisted Carbonyl–Carbonyl Coupling Route for the Preparation of a Useful Intermediate in the Synthesis of Carbapenems

Microwave-assisted organic synthesis has been used for the cyclization of a monocyclic oxoamide to the corresponding bicyclic derivative via a Wittig-type coupling by means of phosphite reagents. The parameters influencing the reaction yields under microwave (dielectric heating) and classical conditions (convective heating) have been evaluated.     

Direct Arylation/Alkylation/Magnesiation of Benzyl Alcohols in the Presence of Grignard Reagents via Ni-, Fe-, or Co-Catalyzed sp(3) C-O Bond Activation

Direct application of benzyl alcohols (or their magnesium salts) as electrophiles in various reactions with Grignard reagents has been developed via transition metal-catalyzed sp(3) C-O bond activation. Ni complex was found to be an efficient catalyst for the first direct cross coupling of benzyl alcohols with aryl/alkyl Grignard reagents, while Fe, Co, or Ni catalysts could promote the unprecedented conversion of benzyl alcohols to benzyl Grignard reagents in the presence of (n)hexylMgCl. These methods offer straightforward pathways to transform benzyl alcohols into a variety of functionalities.     

Synthesis of Functionalized o‐, m‐, and p‐Terphenyl Derivatives by Consecutive Cross‐Coupling Reactions of Triazene‐Substituted Arylboronic Esters

Triazene‐substituted arylboronic esters were prepared readily from the corresponding aryl magnesium derivatives and shown to function as a new class of donor–acceptor‐substituted coupling reagents. The selective functionalization of these aromatic derivatives led to a wide variety of terphenyl derivatives in which the original bifunctional unit (often further substituted with another functional group) formed the central aromatic ring. The functionalized terphenyl derivatives were formed in two efficient cross‐coupling steps from the triazene‐substituted boronic esters: Suzuki cross‐coupling with an aryl halide was followed by BF₃?OEt₂‐induced palladium‐catalyzed coupling of the diazonium salt generated in situ from the triazene with an arylboronic acid.     

Preparation of aryl ketones via Ni-catalyzed Negishi-coupling reactions with acid chlorides

A Ni-catalyst-catalyzed cross-coupling reaction of organozinc reagents with acid chlorides has been successfully developed. Mild reaction conditions were required to complete the coupling reactions affording the corresponding aryl ketones in good to excellent yields.     

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl-Oxime-Ethers as Bifunctional Reagents

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl-oxime-ethers as bifunctional reagents under metal-free photochemical conditions. Via concomitant C−S and C−C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom-economic fashion, providing rapid access to multi-functionalized β-sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N−O bond cleavage is suggested for the process, based on our results on mechanistic investigations.     

钯催化剂催化交叉偶联反应形成P-C键和复杂含磷化合物的研究进展

综述了利用金属钯催化交叉偶联反应形成P-C键以合成各种复杂的含磷化合物特别是手性膦配体的研究进展;介绍了参加偶联反应的不同价态的P化合物亲核试剂,以及卤代烯烃、卤代芳烃、三氟甲磺酸烯基脂、三氟甲磺酸芳基脂、乙烯基硼酸脂等亲电试剂;探讨了相应的偶联反应的反应条件、反应机理及其在材料、医药、农药等领域的应用.     

Nickel(0)-catalyzed diastereoselective three-component coupling of 1,3-dienes, aldehydes, and organometallic reagents: influence of organometallic reagents on diastereoselectivity

A nickel-catalyzed diastereoselective alkylative three-component coupling of 1,3-diene and aldehyde with organoboron or organosilicon reagents has been realized. The diastereoselectivity was dramatically changed depending on the class of organometallic reagents. The reaction using ArB(OH)₂ in the presence of PPh₃ afforded 1,3-syn-substituted 4-penten-1-ol derivative as a single diastereomer. On the other hand, the coupling reaction with tetraorganosilicon reagent using NHC as a ligand under similar conditions exclusively produced the corresponding 1,3-anti isomer.     

Kinetically Controlled,Highly Chemoselective Acylation of Functionalized Grignard Reagents with Amides by N−C Cleavage

The direct transition-metal-free acylation of amides with functionalized Grignard reagents by highly chemoselective N−C cleavage under kinetic control has been accomplished. The method offers rapid and convergent access to functionalized biaryl ketones through transient tetrahedral intermediates. The direct access to functionalized Grignard reagents by in situ halogen–magnesium exchange promoted by the versatile turbo-Grignard reagent (iPrMgCl ⋅ LiCl) permits excellent substrate scope with respect to both the amide and Grignard coupling partners. These reactions enable facile, operationally simple and chemoselective access to tetrahedral intermediates from amides under significantly milder conditions than chelation-controlled intermediates. This novel direct two-component coupling sets the stage for using amides as acylating reagents in an alternative paradigm to the metal-chelated approach, acyl metals and Weinreb amides.     

Palladium-catalyzed coupling of thiol esters with aryl and primary and secondary alkyl organoindium reagents

Thiol esters and organoindium reagents undergo palladium-catalyzed cross-coupling under mild conditions to give ketones in moderate to excellent yields. Aryl and primary/secondary alkyl organoindium reagents can be used as coupling partners. This method has two advantages over the cross-coupling of thiol esters with boron and tin reagents: (1) no added copper reagent is required to mediate the reaction and (2) for the case of alkyl transfer, no added base is required to activate organoindium reagents for cross-coupling as is required for the coupling of alkyl boron reagents with thiol esters.     

Enantiomeric analysis of chiral compounds using phosphorus acid chlorides

New achiral separating bifunctional reagents, dichlorides of methylphosphonic and O-ethyl-thiophophoric acids, have been used for the quantitative determination of the enantiomeric composition of α-amino acids (alanine, valine, proline), secondary alcohols (2-octanol, 2-pentanol, 1-methoxy-2-propanol) and α-phenylethylamine. The determination of the enantiomeric composition of optically active α-amino acids, secondary alcohols, and amines is based on the transformation of compounds into symmetric diastereometers using organophosphorous achiral bifunctional reagents followed by the determination of the derivatives by gas chromatography with a mass spectral detector.     

Single‐Electron Transmetalation: Synthesis of 1,1‐Diaryl‐2,2,2‐trifluoroethanes by Photoredox/Nickel Dual Catalytic Cross‐Coupling

Novel methods for the incorporation of fluorinated subunits into organic frameworks are important in pharmaceutical, agrochemical, and materials science applications. Herein, the first method for the cross‐coupling of benzylic α‐trifluoromethylated alkylboron reagents with (hetero)aryl bromides is achieved through application of a photoredox/nickel dual catalytic system. The harsh conditions and high temperatures required by conventional Suzuki‐coupling protocols are avoided by exploitation of an odd‐electron pathway that permits room temperature transmetalation of these recalcitrant reagents. This method represents the first direct and general route for the synthesis of unsymmetrical 1,1‐diaryl‐2,2,2‐trifluoroethanes, thereby providing efficient access to a previously unexplored chemical space.     

Enantioconvergent Fukuyama Cross‐Coupling of Racemic Benzylic Organozinc Reagents

The first enantioconvergent palladium‐catalyzed Fukuyama cross‐coupling of racemic benzylic organozinc reagents with thioesters has been developed. The reaction furnishes enantioenriched acyclic α‐disubstituted ketone products in good yields and high enantioselectivities. A broad substrate scope is achieved under mild reaction conditions to prevent racemization of the potentially labile tertiary stereocenters.     

Nested Arg-specific bifunctional crosslinkers for MS-based structural analysis of proteins and protein assemblies

The combination of chemical probing and high-resolution mass spectrometry constitutes a powerful alternative for the structural elucidation of biomolecules possessing unfavorable size, solubility, and flexibility. We have developed nested Arg-specific bifunctional crosslinkers to obtain complementary information to typical Cys- and Lys-specific reagents available on the market. The structures of 1,4-phenyl-diglyoxal (PDG) and 4,4′-biphenyl-diglyoxal (BDG) include two identical 1,2-dicarbonyl functions capable of reacting with the guanido group of Arg residues in proteins, as well as the base-pairing face of guanine in nucleic acids. The reactive functions are separated by modular spacers consisting of one or two benzene rings, which confer greater rigidity to the crosslinker structure than it is afforded by typical aliphatic spacers. Analysis by electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has shown that the probes provide both mono- and bifunctional products with model protein substrates, which are stabilized by the formation of diester derivatives in the presence of borate buffer. The identification of crosslinked sites was accomplished by employing complementary proteolytic procedures and peptide mapping by ESI-FTICR. The results showed excellent correlation with the solvent accessibility and structural context of susceptible residues, and highlighted the significance of possible dynamic effects in determining the outcome of crosslinking reactions. The application of nested reagents with different spacing has provided a new tool for experimentally recognizing flexible regions that may be involved in prominent dynamics in solution. The development of new bifunctional crosslinkers with diverse target specificity and different bridging spans is expected to facilitate the structure elucidation of progressively larger biomolecular assemblies by increasing the number and diversity of spatial constraints available for triangulating the position of crosslinked structures in the three dimensions.