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

( 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 Fe 2 (CO) 9 ( 13 ) afforded diastereomeric mixtures of their (η 2 -alkene)tetracarbonyliron(0) complexes 14a ‘/ a ‘ ‘− f ‘/ f ‘ ‘ (acceptor group, Acc = SO 2 Ph) and 15 ‘/‘ ‘ ( Acc = CO 2 Me) (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- exo - 14, 15 ) were expected to yield syn -Me, syn - Acc -configured and “S”-shaped complexes 14a ‘ ‘, 15 ‘ (Ψ- endo - 14, 15 ) accordingly anti -Me, syn - Acc -configured cationic complexes 18 and 19 upon treatment with HBF 4 . Complex 14a ‘ (de = ee > 99%) reacted quantitatively to the syn -Me-substituted (η 3 -allyl)tetracarbonyliron(1+) complex 18 ‘ ( syn -Me, syn -SO 2 Ph- 18 ) ( syn -Me/ anti -Me > 99:1, ee > 99%). Diastereomeric mixtures of complexes 14a ‘/ a ‘ ‘ gave mixtures of complexes 18 ‘, 18 ‘ ‘ ( anti -Me, syn -SO 2 Ph- 18 ) and ent - 18 ‘ ‘ ( ent - syn -Me, syn -SO 2 Ph- 18 ). Conversion of complex 14a ‘ ‘ to 18 ‘ ‘ or complex 18 ‘ ‘ itself was subjected to an anti -Me → syn -Me isomerization process, yielding eventually a diastereomeric mixture of complexes 18 ‘ ‘ and ent - 18 ‘ ‘, thus lowering the overall enantiomeric purity of syn -Me, syn -SO 2 Ph-substituted complexes 18 . Conversion of a mixture of 15 ‘/‘ ‘ (de = 10%) to cationic complexes 19 ‘/‘ ‘ did not exhibit significant anti -Me → syn -Me isomerization ( syn -Me: anti -Me = 1:1.19, ee > 96% for both diastereomers). Nucleophilic anti -addition of silyl enol ether 20 to complex 18 ‘ or silyl ketene acetal 21 to a complex mixture 19 ‘/‘ ‘ afforded enantiopure alkenyl sulfone ( R )- 23 or ester ( S )- 24 (82% − quant., ee >96 to >99%). Addition to a complex mixture containing 18 ‘, 18 ‘ ‘, and ent - 18 ‘ yielded 23, albeit with lower enantiomeric purity (ee = 59−66%). The chirality transfer process of the iron-mediated allylic substitution proceeds with overall retention (double inversion) of stereochemistry with respect to the stereogenic center of the starting materials, conservation of ( E )-double bond geometry, and complete γ-regioselectivity for the nucleophilic addition reactions. Differences of configurative stability of the anti -configured Me groups in the cationic π-allyl complexes 18 ‘ ‘ and 19 ‘ were found requiring appropriate consideration if used in stereocontrolled organic synthesis.