"Chirality transfer" in iron-mediated dienylic substitutions via highly enantiomerically enriched planar chiral 1-phenylsulfonyl-substituted tricarbonyl(η5-pentadienyl)iron(1 +) complexes

Article abstract of DOI:10.1016/S0022-328X(96)06850-7

The preparation of highly diastereo-and enantiomerically enriched planar chiral 1-phenylsulfonyl-substituted tricarbonyl(η⁵-pentadienyl)iron(1 +) complexes 7 [syn,syn-(1R,5R)-7 and syn,syn-(1S,5S)-7 (cisoid-or U-forms); de > 99% ≡ 5-syn-CH₃/5-anti-CH₃ ? 100:1, ee > 99%; 87% quant. from resolved 6] is described. Starting from the enantiopure diene (1E,3E,S)-5 both enantiomers of the cationic complexes syn,syn-7 become readily accessible via chromatographic resolution of the diastereomeric mixture of the corresponding neutral tricarbonyl(η⁴-diene)iron(0) complexes 6 [(1R,5S)-6 ≡ ψ-endo-6 and (1S,5S)-6 ≡ ψ-exo-6; de > 99%, ee > 99%; 85% quant. prior to resolution]. The nucleophilic addition of hetero and carbon atom nucleophiles (morpholine, silyl enol ether 8 and silyl ketene acetal 9) to the racemic complex syn,syn-(1R/S,5R/S)-7 afforded the neutral ε-substituted tricarbonyl(η⁴-diene)iron(0) complexes rac-ψ-exo-10a-c in moderate yields [43-68% from syn,syn-(1R/S,5R/S)-7] as single geometrical isomers [(E,Z) or (E,E); kinetic (U-form/strong nucleophile) or thermodynamic (S-form/less reactive nucleophile) control]. Likewise, nucleophilic addition to the stereochemically homogeneous complexes syn,syn-(1R,5R)-7 or syn,syn-(1S,5S)-7 followed by oxidative cleavage of the carbonyliron fragment offers an access to ε-substituted dienes 11a-c in moderate to fair yields [45-65%, (E,Z)/(E,E) = > 85:1-1:3] with enantiomeric excesses ranging from > 99%/98.9% [(1E,3Z,R)-11a/(1E,3Z,S)-11a] to 93% [(6E,8E,S)-11b]. The stereochemistry of the formation and the stereochemical pathways of the nucleophilic addition reactions of the nonracemic complexes syn,syn-7 leading to the dienes 11a-c as well as spectroscopic and structural details are discussed. Furthermore, the reaction proceeds with virtually complete "chirality transfer" from C-O via C-Fe to C-N or C-C, respectively, with either retention or inversion of stereochemistry of the stereogenic centre with respect to the starting material (S)-1 depending strongly on the reaction conditions. The observed ε-regioselectivity of the nucleophilic addition reaction displays the synthetic equivalence of the cationic complexes of type syn,syn-7 with a planar chiral a⁶-synthon allowing an umpolung of the classical d⁶-chemistry.