Asymmetric Synthesis of Axially Chiral Biaryls by Nickel-Catalyzed Grignard Cross-Coupling of Dibenzothiophenes
journal contributionposted on 28.05.2004 by Yong-Hwan Cho, Asato Kina, Toyoshi Shimada, Tamio Hayashi
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Catalytic asymmetric Grignard cross-coupling of 1,9-disubstituted dibenzothiophenes (6a−c) and dinaphthothiophene (6d) with aryl- and alkyl-Grignard reagents (7) proceeded with high enantioselectivity (up to 95% ee) in the presence of a nickel catalyst (3−6 mol %) coordinated with 2-diphenylphosphino-1,1‘-binaphthyl (H-MOP) or oxazoline-phosphine ligand (i-Pr-phox) in THF to give 2-mercapto-2‘-substituted-1,1‘-biphenyls (8a−c) and 2-mercapto-2‘-substituted-1,1‘-binaphthyls (8d) in high yields. The mercapto group in the axially chiral cross-coupling products was converted into several functional groups by way of the methylsulfinyl group. The rate of flipping in dinaphthothiophene was measured by variable-temperature 31P NMR analysis of methylphenylphosphinyldinaphthothiophene derivative (21).