Reaction of organozinc using external chiral ligands

6 Reaction of organozinc using external chiral ligands 

Since the bisphosphine as well as a monophosphine greatly accelerate the copper-catalyzed reaction [59], a survey of the known diphosphine was carried out to find that 0.5% of copper(II) triflate and 0.5% of phosphine are sufficient, though enantioselectivity was at most 44% [60]. Chiral phosphite ligand bearing tartrate moiety 28 accelerated the reaction [61], but the ee was not so satisfactory at 40% (Eq. (12.30)) [62]. Chiral thiazolidinone 29 was developed as a chiral ligand to afford the product in 63% ee [63]. 

Binaphthol-based phosphorus amidite 30 was developed by Feringa to afford 3-ethylcyclohexanone in over 98% ee [64, 65]. However, high enantioselectivity is limited to cyclohexenone, and rather poor selectivity was observed in the reaction of cyclopentenone (10% ee) and cycloheptenone (53% ee). Symmetric aminophosphine ligand 31 was synthesized and the reaction with cyclohexenone was examined in the presence of 5 mol% of copper triflate to afford the product in 55% ee [66]. Amide-phosphine 32 was examined in the reaction to afford the product in 35% ee. Higher selectivity (64%) was observed in the reaction of 4,4-dimethylcyclohexenone [67, 68]. 

Based on the reaction of diorganozinc with cycloalkenone catalyzed by N-monosubstituted sulfonamide and copper(I) [69], the effect of chiral sulfonamide 33 was examined. It was found that catalytic amounts of both sulfonamide and copper(I) are necessary to catalyze the reaction, but ee was at most 32% [70]. 

The asymmetric conjugate addition of diethylzinc with chalcone was also catalyzed by nickel and cobalt complex (Eq. (12.31)) [71]. A catalytic process was achieved by using a combination of 17 mol% of an aminoalcohol 34 and nickel acetylacetonate in the reaction of diethylzinc and chalcone to provide the product in 90% ee [72, 73]. Proline-derived chiral diamine 35 was also effective, giving 82% ee [74]. Camphor-derived tridentate aminoalcohol 36 also catalyzes the conjugate addition reaction of diethylzinc in the presence of nickel acetylacetonate to afford the product in 83% ee [75]. Similarly, the ligand 37-cobalt acetylacetonate complex catalyzes the reaction to afford the product in 83% ee [76]. 

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