Iron-Mediated Allylic Substitution Reactions with Chirality Transfer. Stereochemistry of the Formation of Diastereo- and Enantiomerically Enriched Olefinic and Allylic Tetracarbonyl Iron Complexes

Abstract

(E)-Configurated allylic ligands (S)-6a−f and (S)-8, bearing a leaving group at C(3) (allylic position) and an electron acceptor substituent at C(1), were synthesized from enantiopure (S)-ethyl lactate [(S)-1]. Complexation with Fe2(CO)9 (13) afforded diastereomeric mixtures of their (η2-alkene)tetracarbonyliron(0) complexes 14a‘/a‘ ‘−f‘/f‘ ‘ (acceptor group, Acc = SO2Ph) and 15‘/‘ ‘ (Acc = CO2Me) (48% − quant.; de < 3−70%), each diastereomer in enantiopure form (Note: descriptors ‘ and ‘ ‘ denote major and minor diastereomer). Synthetically useful results were obtained for allylic ligands bearing a benzylic protecting group [(S)-6a and (S)-8] and using hexane or diethyl ether as solvent (14a‘/a‘ ‘: quant., de = 70%; 15‘/‘ ‘: 75−88%, de = 10−16%). Complexes 14a‘/a‘ ‘ were fractionally crystallized, and their molecular structures were determined by X-ray diffraction, allowing for an assignment of the absolute configurations of complexes 14a‘/a‘ ‘−f‘/f‘ ‘ and 15‘/‘ ‘. “W”-shaped complexes 14a‘, 15‘ ‘ (Ψ-e...