STABILIZED AND PERSISTENT ALLENYLKETENES

Photolyses of 2,3-bis(trimethylsilyl)-substituted methylenecyclobuteno nes 22-26 give essentially quantitative conversion to the alkenylketen es 28-32 which have been isolated as long-lived species at room temper ature. As predicted by molecular orbital calculations, the methylenecy clobutenones 22 and 26, with carbethoxy and hydrogen substitution on t he methylene group, respectively, undergo thermal equilibration with t he corresponding alkenylketenes 28 and 32 with equilibrium constants [ 22]/[28] = 1.0 and [26]/[32] = 0.029, at 100 degrees C. The X-ray stru cture of the phenyl-substituted alkenylketene 29 confirms the anti-pla nar conformation as predicted by calculation. The bis(allenylketene) 3 5 has been made by an analogous procedure. Hydration rates of the alle nylketenes show acceleration by the CO(2)Et substituent, but in all ca ses these are modestly less reactive than the 1,2-bisketene (Me(3)SiC= C=O)(2).