ELECTROPHILIC REACTIVITY OF BISKETENES - AN EXPERIMENTAL AND THEORETICAL-STUDY, AND PHOTOINDUCED HYDRATION

The kinetics of the acid-catalysed hydration of the bisketene [C(SiMe( 3))=C=O](2) (1) are indicative of rate limiting protonation at C-beta followed by hydration to form the (monoketenyl)acetic acid 5, previous ly proposed as an unobserved intermediate in the neutral hydrolysis of 1, On further reaction 5 undergoes a slower uncatalysed conversion to form the stereoisomeric 2,3-bis(trimethylsilyl)succinic anhydrides 4. Confirmation of the intermediacy of 5 is obtained by its formation an d observation on photohydration of 1. Reaction of 1 with one equivalen t CF3CO2H yields the (monoketenyl)acetic trifluoroacetic anhydride 8. Ab initio MO calculations at the MP2/6-31G//MP2/6-31G* level for prot on addition and at the HF/6-31G//HF/6-31G* level for chloronium ion a ddition to the parent bisketene buta-1,3-diene-1,4-dione (9) indicate that the most stable products result from attacks at C-alpha, the carb onyl carbon, while the ions from attack at C-beta are at least 4.7 and 18.4 kcal mol(-1) less stable, respectively. Further calculations at the MP2/3-21G//HF/3-21G level indicate that silylation of the bisketen e causes a relative favouring of electrophilic attack at C-beta, in ag reement with the experimental results.