ORIGIN OF STEREOCHEMISTRY IN THE ALPHA-AMINO-ACID ESTERS AND AMIDES GENERATED FROM OPTICALLY-ACTIVE ZIRCONAAZIRIDINE COMPLEXES

Methane elimination from ethylenebis(tetrahydroindenyl)(methyl)zirconi um amides (R = alkyl or aryl, R' = aryl) (S,S)-26 gives a mixture of e pimeric zirconaaziridines 25 and 30. When zirconaaziridines with R = a lkyl are trapped with ethylene carbonate as they are formed, methyl al pha-amino acid esters 16 are obtained in poor ee (+14% to -56%); the e e's of 16 reflect the kinetic ratio of 25 to 30. When epimeric zircona aziridines with R = alyl are allowed to equilibrate before ethylene ca rbonate is added, the esters 16 are obtained in > 96% ee. When epimeri c zirconaaziridines with R = alkyl are allowed to equilibrate before e thylene carbonate is added, 16 is obtained in 21-97% ee. When isocyana tes are added to the zirconaaziridine epimers 25 and 30, phenylglycina mides are obtained in 80-99% ee, The ee's of the esters and amides are better when R' is o-anisyl than when R' is Ph. A Curtin-Hammett- Wins tein-Holness analysis explains the stereochemistry in the esters and a mides. When an equilibrated mixture of epimers (R = CH(2)Ph, R' = o-an isyl) 25g and 30g is treated with increasing ethylene carbonate concen trations, the ee of the ester 16g increases from 53% to 89%. The ee of 16g at saturation with ethylene carbonate implies that K-eq (k(25g)/k (30g)) is 17.2 for the 30g --><-- 25g equilibrium. A similar result (1 9.0) is obtained from the de of the insertion product rac-38 when 30g --><-- 25g is treated with an excess of t-BuNCO.