NHC-Cu-Catalyzed Silyl Conjugate Additions to Acyclic and Cyclic Dienones and Dienoates. Efficient Site-, Diastereo- and Enantioselective Synthesis of Carbonyl-Containing Allylsilanes.

Efficient and highly diastereo- and enantioselective conjugate additions of phenyldimethylsilyl units to acyclic and cyclic dienones and dienoates are disclosed. The C-Si bond forming reactions are catalyzed by 2.0-2.5 mol % of a copper complex of a chiral monodentate N-heterocyclic carbene; the requisite reagent, PhMe(2)Si-B(pin), is commercially available or can be easily prepared. Transformations generate allylsilanes in up to 98% yield and >99:1 enantiomeric ratio, and proceed with complete 1,4-selectivity, unless the dienone or dienoate carries a trisubstituted alkene conjugated to the carbonyl group; in the latter cases, 1,6-addition products are obtained exclusively and in up to >98% Z selectivity.

Enantioselective synthesis of boron-substituted quaternary carbons by NHC-Cu-catalyzed boronate conjugate additions to unsaturated carboxylic esters, ketones, or thioesters.

A Cu-catalyzed method for enantioselective boronate conjugate additions to trisubstituted alkenes of acyclic alpha,beta-unsaturated carboxylic esters, ketones, and thioesters is disclosed. All transformations are promoted by 5 mol % of a chiral monodentate NHC-Cu complex, derived from a readily available C(1)-symmetric imidazolinium salt, and in the presence of commercially available bis(pinacolato)diboron. Reactions are efficient (typically, 60% to >98% yield after purification) and deliver the desired beta-boryl carbonyls in up to >98:2 enantiomer ratio (er). Processes involving unsaturated thioesters proceed with higher enantioselectivity (vs carboxylic esters or ketones), and the resulting products can be functionalized by Ag-mediated or Pd-catalyzed reactions that furnish the derived carboxylic ester or various ketones. Routine oxidation affords beta-hydroxy ketones or carboxylic esters, ketone aldol products that cannot be otherwise prepared efficiently by an alternative catalytic enantioselective protocol.

Enantioselective conjugate silyl additions to cyclic and acyclic unsaturated carbonyls catalyzed by Cu complexes of chiral N-heterocyclic carbenes.

An efficient Cu-catalyzed protocol for enantioselective addition of a dimethylphenylsilanyl group to a wide range of cyclic and acyclic unsaturated ketones, esters, acrylonitriles, and alpha,beta,gamma,delta-dienones is disclosed. Reactions are performed in the presence of 1-2 mol % of commercially available and inexpensive CuCl, a readily accessible monodentate imidazolinium salt, and commercially available (dimethylphenylsilyl)pinacolatoboron. Cu-catalyzed enantioselective conjugate additions proceed to completion within only 2 h to afford the desired silanes in 87-97% yield and 90:10-99:1 enantiomeric ratio (er). Use of a proton source (e.g., MeOH) is not required; accordingly, synthetically versatile alpha-silyl boron enolates can be obtained. The special utility of the present protocol, in comparison with the related catalytic enantioselective aldol and boronate conjugate additions, is discussed and illustrated through various functionalizations of the enantiomerically enriched beta-silylcarbonyls.

Efficient C-B bond formation promoted by N-heterocyclic carbenes: synthesis of tertiary and quaternary B-substituted carbons through metal-free catalytic boron conjugate additions to cyclic and acyclic alpha,beta-unsaturated carbonyls.

Metal-free nucleophilic activation of a B-B bond has been exploited in the development of a highly efficient method for conjugate additions of commercially available bis(pinacolato)diboron to cyclic or acyclic alpha,beta-unsaturated carbonyls. The reactions are readily catalyzed by a simple N-heterocyclic carbene (NHC) present at 2.5-10 mol %. A variety of cyclic and acyclic unsaturated ketones and esters can serve as substrates. The transformations deliver beta-boryl carbonyls bearing tertiary or quaternary B-substituted carbons in up to >98% yield. Preliminary studies indicate that although related Cu-NHC-catalyzed reactions are equally efficient, the metal-free variant is more functional-group-tolerant; in contrast to the Cu-catalyzed reactions, the metal-free processes proceed readily in the presence of a terminal alkyne and do not promote concomitant diboration of an aldehyde. Representative functionalization of the resulting boron enolates demonstrates the strong influence of the Lewis acidic B atom of the beta-boronate.

Monodentate non-C(2)-symmetric chiral N-heterocyclic carbene complexes for enantioselective synthesis. Cu-catalyzed conjugate additions of aryl- and alkenylsilylfluorides to cyclic enones.

A new class of enantioselective conjugate addition (ECA) reactions that involve aryl- or alkenylsilyl fluoride reagents and are catalyzed by chiral non-C(2)-symmetric Cu-based N-heterocyclic carbene (NHC) complexes are disclosed. Transformations have been designed based on the principle that a catalytically active chiral NHC-Cu-aryl or NHC-Cu-alkenyl complex can be accessed from reaction of a Cu-halide precursor with in situ-generated aryl- or alkenyltetrafluorosilicate. Reactions proceed in the presence of 1.5 equiv of the aryl- or alkenylsilane reagents and 1.5 equiv of tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF). Desired products are isolated in 63-97% yield and 73.5:26.5-98.5:1.5 enantiomeric ratio (47%-97% ee). A major focus of the present studies is the design, evaluation, and development of new chiral imidazolinium salts and their derived NHC-Cu complexes as catalysts that promote reactions of various carbosilanes to a range of electrophilic substrates. Toward this end, nearly 20 new chiral monodentate imidazolinium salts, most of which are non-C(2)-symmetric, have been prepared and fully characterized and their ability to serve as catalysts in the ECA reactions has been investigated.

A practical method for enantioselective synthesis of all-carbon quaternary stereogenic centers through NHC-Cu-catalyzed conjugate additions of alkyl- and arylzinc reagents to beta-substituted cyclic enones.

We present the first examples of Cu-catalyzed enantioselective conjugate additions of alkyl- and arylzinc reagents to unactivated cyclic beta-substituted enones. Transformations are promoted in the presence of 2.5-15 mol % of a readily available chiral NHC-based Cu complex, affording the desired products bearing all-carbon quaternary stereogenic centers in 67-->98% yield and in up to 97% ee. Catalytic enantioselective reactions can be carried out on a benchtop, with undistilled solvent and commercially available (not further purified) Cu salts. Mechanistic models, accounting for the observed levels and trends in enantioselectivity are provided.

Novel route to azobenzenes via Pd-catalyzed coupling reactions of aryl hydrazides with aryl halides, followed by direct oxidations.

[reaction: see text] N-Boc aryl hydrazines undergo Pd-catalyzed coupling reactions with aryl halides to provide N-Boc diaryl hydrazines in excellent yields. The resulting N-Boc diaryl hydrazines were directly oxidized with NBS/pyridine in CH(2)Cl(2) at room temperature to the azobenzenes.