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 reactions, 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 than are the 3-trimethylsilyl analogs 1, as predicted on the basis of molecular orbital calculations. Reactions of allenylketenes 1 and 4 include addition of H2O followed by cyclization to form butenolides 13, addition of aniline to give allenyl carboxamide 14 and lactam 15, addition of CF3CO2H to form allenyl carboxylic acid 17, addition of Br, in a 1,4-fashion to give penta-2,4-dienoic acid derivatives 18, and reaction with m-chloroperbenzoic acid to give the unique allenyl acylsilane 23. [4+1]-Cycloaddition reactions of 1 and 4 with Me3SiCHN2 give methylenecyclopentenones 24 and 25 while acetaldehyde undergoes BF3. OEt2 catalyzed [2+2] cycloadditions forming p-lactones. [4+2] Cycloadditions occur with benzaldehyde to form the highly crowded exomethylene pentenolactone 29, and with TCNE forming methylenecyclohexenones 31.