Copper-Catalysed Asymmetric 1,4-Addition of Organozinc Compounds to Linear Aliphatic Enones Using 2,2′-Dihydroxy 3,3′-Dithioether Derivatives of 1,1′-Binaphthalene

Abstract

Directed ortho dilithiation of bis(diethylcarbamate) or bis(MOM)-protected (Sa)-1,1′-bi(2-naphthol) followed by treatment with R2S2 [R = Me, Ph (X-ray structure)] or Me2Se2 cleanly affords the 3,3′ derivatives; the free naphthols are produced on deprotection. In the case of the bis(MOM) series, but not that of the bis(carbamates), some racemisation occurs. The ligand 2,2′-dihydroxy-3,3′-dimethylthio-1,1′-binaphthalene shows optimal performance in the addition of ZnEt2 to linear aliphatic enones (E)-R1C(O)CH=CHR2. Variation of the steric demands of R1 and R2 generates catalytic results consistent with binding of a zinc-based Lewis acid anti to the ene function and with the reactive conformation being s-cis. With enones containing the functions R2 = (CH2)nCH(OAlkyl)2 (n = 0−2), the ZnEt2 addition products undergo base-promoted cyclisation.