Enantioselectivity asymmetric cross-coupling

Carbon-carbon bond-forming reactions are one of the most basic, but important, transformations in organic chemistry. In addition to conventional organic reactions, the use of transition metal-catalyzed reactions to construct new carbon-carbon bonds has also been a topic of great interest. Such transformations to create chiral molecules enantioselectively is therefore very valuable. While various carbon-carbon bond-forming asymmetric catalyses have been described in the literature, this chapter focuses mainly on the asymmetric 1,4-addition reactions under copper or rhodium catalysis and on the asymmetric cross-coupling reactions catalyzed by nickel or palladium complexes. 

Among chiral phosphine ligands used for the nickel- or palladium-catalyzed asymmetric cross-coupling, a series of the ferrocenylphosphines has shown a great success in terms of enantioselectivity and a variety of modified chiral ferrocenylphosphines has been applied to the reaction (10-16). The first example of asymmetric cross-coupling catalyzed by... 

An example of practical application of the asymmetric cross-coupling, enantioselective synthesis of a-curcumene (19), is shown in Scheme 8F.6 [21]. The compound was synthesized with 66% ee through Ni/(S)-(/ )-PPFA (lOa)-catalyzed cross-coupling of 1 -(p-tolyl)ethylmag-nesium chloride (17) and vinyl bromide (4b). 

They next studied the asymmetric oxidative polymerization of achiral 2,3-dihydroxynaphthalene (Scheme 42). The polymerization of this monomer with CuCl2-(-)-sparteine complex resulted in a low yield and gave a low molecular weight oligomer, whereas the polymerization with CuCl-(S)-Phbox quantitatively gave a polymer with Mn of 10 600-15 300. The enantioselectiv-ity attained in this polymerization, however, was estimated to be low, with 43% ee from the model reaction [169]. When vanadyl sulfate (VOSO -Phbox complex was used instead of the copper catalyst system, the enantioselectivity was improved up to 80% ee [170]. Asymmetric cross-coupling polymerization of two kinds of naphthol derivatives was also reported [171,172]. 

In 2005, the asymmetric cross-coupling reactions of aldehydes 33 with arylsul-fonylamides 34 were reported by Miller and co-workers [37]. In the presence of 15 mol% of peptide-derived NHC 32, the corresponding aza-benzoin adduct 35 was obtained in good yields with good enantioselectivities (Scheme 7.19). 

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