The vinylketene-acylallene rearrangement: Theory and experiment

Alkoxyvinylketenes 4 are generated by flash vacuum thermolysis (FVT) or photolysis of 3-alkoxycyclobutenones 3. The thermal interconversion of 4 and allene carboxylic acid esters 5 under FVT conditions is demonstrated by Ar matrix FTIR spectroscopy. In addition, ethoxyvinylketene 4 b undergoes thermal elimination of ethene with formation of s-cis- and s-trans-acetylketene (8). An analogous aminovinylketene-to-allenecarboxamide conversion is observed on FVT of 3-dimethylaminocyclobutenone 3 c. A facile 1,3-chlorine migration in 2,3-buta-dienoyl chloride (5 d) is also reported. Consistent with the experimental observations, 1,3-methoxy, 1,3-chloro, and 1,3-dimethylamino migrations in vinylketene are calculated (G2(MP2,SVP) level) to have moderate barriers of 169, 157, and 129 kJ mol(-1), respectively, significantly less than the corresponding 1,3-H shift barrier (273 kJ mol(-1)). The stabilization of the four-center transition structures is rationalized in terms of the donor-acceptor interaction between the lone pair electrons of the migrating donor substituent and the vacant central carbon p orbital of the ketene LUMO. The predicted migratory aptitude in the series of substituted vinylketenes, R-C(=CH2)-CH=C=O, is in the order N(CH3)(2)>SCH3>SH>Cl>NH2>OCH3>OH>F>H>CH3, and correlates well with the electron-donating ability of the R group.