Diorganozinc reagents chiral

In mosl allylation reactions, only a catalytic amount of CuCN-2LiCl is required [41]. Use of die chiral ferrocenylamine 104 as a catalyst makes enables asymmetric allylation of diorganozinc reagents to be effected witli allylic chlorides iScbeme 2.3G) [78]. Related electropb des such as propargylic bromides [79] and unsaturated epoxides [80] also undergo Su2 -substitution reactions iScbeme 2.37). 

Figure 85 Examples of chiral ligands used for the alkylation and arylation of imines by diorganozinc reagents.

The addition of diorganozinc reagents to a-alkoxyaldehydes furnishes selectively protected 1,2-diols.19 Applications toward the synthesis of pheromones like (-)-exo- and (-)-endo-brevicomin 2 and 3 exploits the catalytic nature of the stereochemical induction, e.g. the newly formed chiral centre depends only on the configuration of the chiral catalyst 1. 

The enantioselective addition of organometallics to aldehydes is a useful approach to optically active secondary alcohols. Diorganozinc reagents add with excellent enantioselectivity to aldehydes in the presence of a chiral catalyst such as 1,2- or 1,3-amino alcohols (see equation 14 and Table 2). In most cases, diethylzinc has been used, but the reaction could be extended to some other dialkylzinc reagents and to divinylzinc. Alkylzinc halides afford secondary alcohols with a substantially lower enantiomeric excess. Many aldehydes are good substrates, "- but the best results are usually obtained with aromatic aldehydes. ... 

Based on Noyori s finding that f -monosubstituted sulfonamide and cop-per(I) catalyze the addition of diorganozinc reagents to cycloalkenone [63], the effect of the chiral sulfonamide 27 (Scheme 14) was examined by Sewald it was found that catalytic amounts of both sulfonamide and copper(I) are necessarily to effect the reaction,but the ee was at the most 32% [64]. 

Conjugate addition. The parent chiral BINAMINE is an excellent ligand for CUCI2 to promote the conjugate addition of diorganozinc reagents. When complexed to carbene 3 palladium dicarboxylates exhibit catal3dic activity in the aryl transfer from ArB(OH)2 to... 

Immobilization of chiral ligands to effect asymmetric induction in alkylation of aromatic aldehydes by diorganozinc reagents promoted by PEG-im-mobilized ligands 54-57 can also be promoted by soluble polystyrene-bound species. A recent example of this is work where a polystyrene-bound BINOL was prepared [ 105]. This polymer 69 was used to form titanium-BINOLate and AlLibis(binaphthoxide) catalysts for Et2Zn reaction with benzaldehyde and for asymmetric Michael additions of stabilized carbanions to cyclohexenone. While good stereoselectivities were obtained with these catalysts, the synthetic yields were modest. 

Similarly, diorganozinc reagents undergo carbometallation of cyclopropene-3-carboxamides of type 122 with a chiral oxazolidinone moiety, and the best combination for the addition of diethylzinc to 122 was CuBr-SMe2 and MgBr2-OEt2 (Scheme 10.39). 

A copper-catalyzed asymmetric addition of diorganozinc reagents to N-phosphi-noylimines has been developed for the synthesis of chiral a,a,a-trifluoromethyl-amides (249). Ketimines (250), generated in situ from the corresponding hemiaminals, led to the chiral amides in high yields and excellent enantiocontrol (Scheme 101). ... 

Further applications to the preparation of chiral polyoxygenated molecules ]302] and to the synthesis of the natural product (-)-mucocin [303] have been reported. The enantioselective addition of the functionalized diorganozinc reagent... 

Addition of Organozinc Reagents to Aldehydes, Ketones, and a-Ketoesters 2.06.16.2.1 Chiral ligands used in addition reactions of diorganozincs with aldehydes... 

The hydroboration of dienic silyl enol ethers, such as 124, with LItBH leads to organob-oranes which can be converted to new diorganozincs, such as 125 (Scheme 42) . More importantly, this method allows the preparation of chiral secondary alkylzinc reagents. 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

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