Electrophilic reactivity of bisketenes: an experimental and theoretical study, and photoinduced hydration

Abstract

The kinetics of the acid-catalysed hydration of the bisketene [C(SiMe₃)CO]₂(1) are indicative of rate limiting protonation at C_β followed by hydration to form the (monoketenyl)acetic acid 5, previously 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 intermediary of 5 is obtained by its formation and observation on photohydration of 1. Reaction of 1 with one equivalent CF₃CO₂H yields the (monoketenyl)acetic trifluoroacetic anhydride 8. Ab initio MO calculations at the MP2/6–31G*//MP2/6–31G* level for proton addition and at the HF/6–31G*//HF/6–31G* level for chloronium ion addition to the parent bisketene buta-l,3-diene-l,4-dione (9) indicate that the most stable products result from attacks at C_β, the carbonyl carbon, while the ions from attack at C_β 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 bisketene causes a relative favouring of electrophilic attack at C_β, in agreement with the experimental results.