Allenylketenes: Versatile substrates in nucleophilic, electrophilic, and cycloaddition reactions

4-Methylene-2-trimethylsilyl-3-phenylcylobutenones 3 are obtained from the corresponding cyclobutenedione 7 by Tebbe and Peterson methylenation reacti ons, and upon photolysis give 3-phenyl substituted allenylketenes (RCH)-C-1 =C=C(Ph)C(SiMe3)=C=O 4. These are more reactive in ring closure back to 3 t han are the 3-trimethylsilyl analogs 1, as predicted on the basis of molecu lar orbital calculations. Reactions of allenylketenes 1 and 4 include addit ion of H2O followed by cyclization to form butenolides 13, addition of anil ine to give allenyl carboxamide 14 and lactam 15, addition of CF3CO2H to fo rm allenyl carboxylic acid 17, addition of Br, in a 1,4-fashion to give pen ta-2,4-dienoic acid derivatives 18, and reaction with m-chloroperbenzoic ac id to give the unique allenyl acylsilane 23. [4+1]-Cycloaddition reactions of 1 and 4 with Me3SiCHN2 give methylenecyclopentenones 24 and 25 while ace taldehyde undergoes BF3. OEt2 catalyzed [2+2] cycloadditions forming p-lact ones. [4+2] Cycloadditions occur with benzaldehyde to form the highly crowd ed exomethylene pentenolactone 29, and with TCNE forming methylenecyclohexe nones 31.