Stabilized and Persistent Allenylketenes

Photolyses of 2,3-bis(trimethylsilyl)-substituted
methylenecyclobutenones 22−26 give essentially
quantitative
conversion to the allenylketenes 28−32 which
have been isolated as long-lived species at room temperature.
As
predicted by molecular orbital calculations, the
methylenecyclobutenones 22 and 26, with
carbethoxy and hydrogen
substitution on the methylene group, respectively, undergo thermal
equilibration with the corresponding allenylketenes
28 and 32 with equilibrium constants
[22]/[28] = 1.0 and
[26]/[32] = 0.029, at 100 °C. The
X-ray structure of the
phenyl-substituted allenylketene 29 confirms the
anti-planar conformation as predicted by calculation.
The bis(allenylketene) 35 has been made by an analogous procedure.
Hydration rates of the allenylketenes show
acceleration
by the CO2Et substituent, but in all cases these are
modestly less reactive than the 1,2-bisketene
(Me3SiCCO)2.