Pincer Type Rh Complex Catalyzed Asymmetric Diboration of Terminal Alkenes and Their Conversion to the Optically Active 1,2-Diols

Nishiyama et al. have reported that optically active 1,2-diols are given by the pincer type Rh complexes [Rh(Phebox), 1] catalyzed asymmetric 1,2-diboration of terminal alkenes and subsequent oxidation. According to their results, the asymmetric diboration of 4-chlorostyrene with a diboron ester proceeds with high enantioselectivity by using a rhodium complex coordinated with a pincer type ligand (1a), and the desired 1,2-diborylated product is formed. Subsequent oxidation of it with sodium peroxoborate in THF-water gives the corresponding 1,2-diol. In this reaction, allyl ethers and allyl anilines can be transformed into the related optically active 1,2-diols. In addition, when using trans 1,3-dienes as reactants, terminal alkene moieties are selectively reacted and the optically active allylic alcohols are given. Thus, this asymmetric diboration using 1 is an efficient synthetic method to produce the optically active 1,2-diols.