高配位化手法を機軸とした有機スズエノラートを用いる新しい合成反応

抄録

Highly coordinated organotin (IV) enolates were generated by the coordination of appropriate ligands to four-coordinated tin (IV) enolates. NMR studies revealed formation of five-coordinate O-stannyl enolates in the presence of hexamethylphosphoric triamide (HMPA) or tetrabutylammonium bromide (Bu₄NBr). The highly coordinated enolates, which attained a marked change in chemoselectivity, have higher nucleophilicity to organic halides and lower reactivity to carbonyl compounds than four-coordinated reagents. The effective control of chemoselectivity in the intermolecularly competitive reaction between organic halides and carbonyl compounds was demonstrated using two types of tin enolates, a four-coordinated enolate and a highly coordinated one. An ab initio computational study uncovered the mechanism of the selective reaction of tin enolates in both cases with or without a ligand. It suggests that the coordination by a bromide anion causes an increase of nucleophilicity and a decrease of Lewis acidity of tin enolates. Michael addition of organotin (IV) ketone enolates to α, β-unsaturated esters took place with a catalytic amount of tetrabutylammonium bromide (Bu₄NBr) to give δ-keto esters.