STEREOSELECTIVITY IN THE WITTIG REACTION OF AROMATIC KETONES - ORIGIN OF PREFERENCE FOR THE OLEFIN GEOMETRY

Investigation of the stereoselectivity observed in the Wittig reaction of aromatic ketones with ''nonstabilized'' phosphonium ylides revealed that the nature of the substituent on the phenyl ring of phenyl S-pyridyl ketone determined the stereoselectivity. Generally the Wittig reaction of such ketones with carboxy phosphonium ylides proceeded preferentially to yield (Z)-olefin, albeit with modest selectivity. However, the reaction with aryl sulfonamido-substituted aromatic ketones resulted in high (E)-stereoselectivity. In order to understand the origin of this high (E)-selectivity, a semiempirical conformational analysis of the four uncharged diastereomeric oxaphosphetane intermediates was performed with a cumulatively modified sampling procedure to generate initial conformations, followed by full energy optimization. Computational studies of the unsubstituted and 4-nitrophenyl-substituted oxaphosphetane intermediates were consistent with the low (Z)stereoselectivity observed. The results in the calculations of the aryl sulfonamido-substituted intermediate likewise were consistent with the high (E)-stereoselectivity observed. Calculations of the potassium-coordinated acid anion of the latter species were also performed. All calculations supported interaction of the sulfonamido and carboxylate groups by either hydrogen bonding or salt bridge formation which appears to effect the final stereochemical outcome. Furthermore, we investigated the stereoselectivity of Wittig reactions in which the sulfonamido NH or the carboxylate were removed. In both cases, the (Z)-olefin was formed preferentially, thereby supporting the existence of intramolecular hydrogen bonding or salt bridge formation.