Organometallic compounds in organic synthesis. Part 13. Stereoselectivity of complexation of cyclohexadiene esters

Abstract

Complexation of cyclohexadienes with Fe(CO)₃ as the entering group are subject to classical steric hindrance by alkyl groups, but CO₂R and related groups introduce a competing factor because they favour sterically the entering group, probably through an intermediate complex. Increase of π-electron availability in the initial 1,4-diene by the attachment of OMe reduces this influence, probably because it favours direct olefin complexation. Non-polar conditions increase the directing effect of CO₂R, but complete stereospecificity cannot be achieved except through additional alkyl substitution. Nevertheless, many stereoisomeric pairs can be separated and the route leads to precursors of a number of sterically defined products which are otherwise not readily accessible.