The mechanisms of stereoselective Michael addition and stereoselective metallation yielding Z- and E-lithio-3,3-diphenylpropionylmesitylene

Abstract

Equilibration of the Z- and E-lithium enolate isomers of 3,3-diphenylpropionylmesitylene, obtained via reactions (i) and (ii), respectively, has shown a gradual change in the E:Z ratio with the cation-solvating PhCH:CH·CO·Mes + PhLi → Ph₂CH·CH:C(OLi)Mes (Z)(i), Ph₂CH·CH₂·CO·Mes + PhLi → Ph₂CH·CH:C(OLi)Mes (E)(ii) ability of the medium from 95:5 in ether to 18:32 in 1 : 1 ether–HMPT (hexamethylphosphoric triamide). The kinetic E:Z ratio of metallation (ii) decreases, though less sensitively, in the same direction. Michael addition (i) yields the Z isomer under kinetic control regardless of the cation-solvating ability of the medium. According to i.r. and n.m.r. data, cinnamoylmesitylene prefers the s-trans conformation, which would lead to the E isomer in a fast Michael reaction, fixing the conformational equilibrium in the initial state. The formation of the Z isomer via reaction (i) is accounted for by the steric demands of the attack of phenyl-lithium in the transition state. The role of the cation association to the oxygen atom is discussed with respect to the equilibrium and kinetic results.