The effects of sulfur substitution in chiral amino thiols on the enantioselective addition of organozinc reagents to aldehydes: a novel method for estimation of free energies of dimerization in monomer–dimer equilibria

Differences between the thiol ligand 1 and the corresponding alcohol ligand 2 were observed in the catalytic asymmetric alkylation of benzaldehyde with diethylzinc. The thiol ligand 1 was superior for reaction rate, enantioselectivity and asymmetric amplification. The effects of chiral amino thiols are discussed and compared with the results of chiral amino alcohol counterparts. The quantitative and thermodynamic aspects of the monomer–dimer equilibria involved in thiazazincolidine or oxazazincolidine catalysts have also been studied on the basis of colligative properties.