Asymmetric aminohydroxylation of substituted styrenes: applications in the synthesis of enantiomerically enriched arylglycinols and a diamine

The catalytic asymmetric aminohydroxylation of a variety of styrene derivatives and vinyl aromatics using osmium tetroxide in conjunction with alkaloid-derived ligands [e.g. (DHQ)(2)PHAL or (DHQD)(2)-PHAL] and haloamine salts of alkyl carbamates (e.g. ethyl carbamate or tert-butyl carbamate) has been investigated. By observing the effect of different aromatic substituents and alkyl carbamates on the regioselectivity, yield and enantioselectivity of the aminohydroxylation reactions, a number of conclusions have been reached: (i) the 1-aryl-2-hydroxyethylamine regioisomers were obtained as the major products in reasonable yield and high (greater than or equal to 87%) enantiomeric excess; (ii) tert-butyl carbamate was superior to ethyl carbamate in terms of yield, enantioselectivity and ease of removal of the N-protecting group; (iii) high (greater than or equal to 96%) enantioselectivity was observed with a 4-methoxy-substituted styrene whereas ortho-substituted styrenes gave lower enantioselectivities; (iv) chiral ligands (DHQ)(2)PHAL and (DHQD)(2)PHAL gave essentially equal and opposite senses and degrees of asymmetric induction; (v) regioselectivity was ligand dependent with better regioselectivity (and therefore higher isolated yields) obtained with (DHQ)(2)PHAL than with (DHQD)(2)PHAL. The products of the aminohydroxylation reactions were used to prepare enantiomerically enriched arylglycinols and a chiral diamine.