CHEMOSELECTIVITY IN THE REACTIONS OF ACETYLKETENE AND ACETIMIDOYLKETENE - CONFIRMATION OF THEORETICAL PREDICTIONS

Acetylketene (1) was generated by flash pyrolysis of 2,2,6-trimethyl-4 H-1,3-dioxin-4-one (6). The selectivities of 1 toward a number of repr esentative functional groups were measured for the first time in a ser ies of competitive trapping reactions. The trend in reactivities towar d 1 follows the general order amines > alcohols >> aldehydes approxima te to ketones and can be rationalized by considering both the nucleoph ilicity and the electrophilicity of the reacting species. Alcohols sho w significant selectivity based on steric hindrance, with MeOH approxi mate to 1 degrees > 2 degrees > 3 degrees. These selectivities are con sistent with the activation energies and the pseudopericyclic transiti on structure preciously calculated for the addition of water to formyl ketene. The results, presented here, of ab initio transition structure calculations for the addition of ammonia to formylketene are qualitat ively consistent with the experimental trends as well. N-Propylacetace timidoylketene (a) was generated by the solution pyrolysis of tert-but yl N-propyl-3-amino-2-butenaate (gal and showed similar selectivity to ward alcohols as opposed to ketones and similar steric discrimination toward alcohols. This is again in agreement with previous ab initio ca lculations. Taken together, these experimental trends in the reactivit ies of both 1 and 2 toward a variety of reagents provide strong, altho ugh indirect support for the planar, pseudopericyclic transition struc tures for these reactions which are predicted by ab initio calculation s.