Chiral dienolates: formation and stereoselective α-alkylation of the lithium dienolate derived from (Z)-[(η5-C5H5)Fe(CO)(PPh3)COCHCHMe]. X-Ray crystal structure of (RS)-(Z)-[(η5-C5H5)Fe(CO)(PPh3)COCHCHMe]

Abstract

An X-ray crystal structure analysis of (RS)-(Z)-[(η⁵-C₅H₅)Fe(CO)(PPh₃)COCHCHMe](2) shows that the crotonoyl group adopts a cisoid conformation in the solid state. In solution it is the cisoid conformation that is deprotonated by butyl-lithium to give the corresponding dienolate (3). Alkylation (Mel, Etl, or PhCH₂Br) of the lithium dienolate (3) occurs regiospecifically in the α-position to give stereoselectively the single diastereoisomers (RS,SR)-[(η⁵-C₅H₅)Fe(CO)(PPh₃)COCHRCHCH₂](R = Me, Et, or PhCH₂). Protonation of the dienolate (3) gives the β,γ-unsaturated acyl complex [(η⁵-C₅H₅)Fe(CO)(PPh₃)COCH₂CHCH₂] exclusively.