Stereoselective conjugate addition reactions of lithium amides to α,β-unsaturated chiral iron acyl complexes [(η 5 -C 5 H 5 )Fe(CO)(PPh 3 )(COCH=CHR)]
Davies SG., Dupont J., Easton RJC., Ichihara O., McKenna JM., Smith AD., De Sousa JAA.
The conjugate additions of a range of achiral and homochiral lithium amides derived from primary and secondary amines to α,β-unsaturated chiral iron acyl complexes [(η 5 -C 5 H 5 )Fe(CO) (PPh 3 )(COCHCHR)] have been studied to determine the extent of enantiorecognition in these reactions, and for the asymmetric synthesis of β-amino acids and β-lactams. Conjugate addition of achiral lithium dimethylamide to the chiral iron cinnamoyl complexes (S,E)- and (S,Z)-[(η 5 -C 5 H 5 )Fe(CO)(PPh 3 )(COCHCHPh)] proceeds with high diastereoselectivity, with this protocol being used to establish unambiguously the absolute configuration of Winterstein's acid (3-N,N-dimethylamino-3-phenylpropanoic acid) as (R). The highly diastereoselective conjugate addition of lithium N-benzyl-N- trimethylsilylamide to a range of α,β-unsaturated iron acyl complexes, followed by in-situ elaboration of the derived enolate by either alkylation or aldol reactions is also demonstrated, facilitating the stereoselective synthesis of both cis- and trans-β-lactams. This methodology has been used to effect the formal asymmetric syntheses of (±)-olivanic acid and (±)-thienamycin. Addition of chiral lithium amides derived from primary and secondary amines to the iron crotonyl complex [(η 5 -C 5 H 5 )Fe(CO)(PPh 3 )(COCHCHMe) ] indicates that lithium N-α-methylbenzylamide shows low levels of enantiorecognition, while lithium N-3,4-dimethoxybenzyl-N-α- methylbenzylamide and lithium N-benzyl-N-α-methylbenzylamide show high levels of enantiodiscrimination. The high level of observed enantiorecognition was used to facilitate a kinetic resolution of (RS)-[(η 5 -C 5 H 5 )Fe(CO)(PPh 3 )(COCHCHMe)] with homochiral lithium (R)-N-3,4-dimethoxybenzyl-N-α-methylbenzylamide. Further mechanistic studies show that conjugate additions of (RS)-lithium N-benzyl-N-α-methylbenzylamide to either the (RS)- or homochiral iron crotonyl complex show 2:1 stoicheiometry, while homochiral lithium N-benzyl-N-α-methylbenzylamide shows 1:1 stoicheiometry. © 2004 Elsevier B.V. All rights reserved.