Ketene-Ketene Rearrangement: Substituent Effects on the 1,3-Migration in .alpha.-Oxo Ketenes

The mechanism of the 1,3 migration in a series of a-ore ketenes, RCOCH=C=O (R = H, CH3, NH2, OH, SH, SiH3, OCH3, SCH3, N(CH3)(2), and C6H5),was studied by high-level ab initio molecular orbital calculations up to the QCISD(T)/6-311+G(2d,p) + ZPVE level of theory, The ore ketene-oxo ketene rearrangement is predicted to proceed via a four-membered cyclic transition structure. The calculated activation barrier depends strongly on the nature of the migrating group (R), ranging from 20 to 200 kJ mol(-1). n-Electron donor substituents (e.g., NH2 and SH) are found to stabilize the four-centered transition structures. For the dimethylamino substituent, the 1,3-shift is predicted to be a facile process (barrier = 20 kJ mol(-1)), and a stable four-membered cyclic interemediate is found to exist on the reaction profile. The predicted 1,3-migratory aptitude is in the order N(CH3)(2) > SCH3 > SH > NH2 > OCH3 > OH > SiH3 > H > C6H5 > CH3. This calculated trend is consistent with experimental observations. The stabilization of the transition structure can be rationalized by the donor-acceptor interaction between an appropriate n- or pi-electron donor substituent and the vacant carbon p orbital of the ketene LUMO. The E/Z conformational preferences (gas phase and solution), rotational barriers, and IR spectra of alpha-ore ketenes are also reported.