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 grou p at C(3) (allylic position) and an electron acceptor substituent at C(l), were synthesized from enantiopure (S)-ethyl lactate [(S)-1]. Complexation w ith Fe-2(CO)(9) (13) afforded diastereomeric mixtures of their (eta (2)-alk ene)tetracarbonyliron(0) complexes 14a'/a " -f'/f " (acceptor group, Ace = SO2-Ph) and 15'/" (Acc = CO2Me) (48% - quant.; de < 3-70%), each diastereom er in enantiopure form (Note: descriptors ' and " denote major and minor di astereomer). 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 molec ular structures were determined by X-ray diffraction, allowing for an assig nment of the absolute configurations of complexes 14a'/a " -f'/f " and 15'/ ". "W"-shaped complexes 14a', 15 " (Psi -exo-14,15) were expected to yield syn-Me,syn-Acc-configured and "S"-shaped complexes 14a ", 15' (Psi -endo-14 ,15) accordingly anti-Me,syn-Acc-configured cationic complexes 18 and 19 up on treatment with HBF4. Complex 14a' (de = ee > 99%) reacted quantitatively to the syn-Me-substituted (eta (3)-allyl)tetracarbonyliron(1+) complex 18' (syn-Me,syn-SO2Ph-18) (syn-Melanti-Me > 99:1, ee > 99%). Diastereomeric mi xtures of complexes 14a'/a " gave mixtures of complexes 18', 18 " (anti-Me, syn-SO2Ph-18) and ent-18 " (ent-syn-Me,syn-SO2Ph-18). Conversion of complex 14a " to 18 " or complex 18 " itself was subjected to an anti-Me --> syn-M e isomerization process, yielding eventually a diastereomeric mixture of co mplexes 18" and ent-18 ", thus lowering the overall enantiomeric purity of syn-Me,syn-SO2Ph-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 diaster eomers). Nucleophilic anti-addition of silyl enol ether 20 to complex 18' o r silyl ketene acetal 21 to a complex mixture 15'/" afforded enantiopure al kenyl sulfone (R)-23 or ester (S)-24 (82% - quant., ee > 96 to > 99%). Addi tion to a complex mixture containing 18', 18 ", and ent-18' yielded 23, alb eit with lower enantiomeric purity (ee = 59-66%). The chirality transfer pr ocess of the iron-mediated allylic substitution proceeds with overall reten tion (double inversion) of stereochemistry with respect to the stereogenic center of the starting materials, conservation of (E)-double bond geometry, and complete gamma -regioselectivity for the nucleophilic addition reactio ns. Differences of configurative stability of the anticonfigured Me groups in the cationic pi -allyl complexes 18 " and 19' were found requiring appro priate consideration if used in stereocontrolled organic synthesis.