1,3-sigmatropic shifts in carbonylketenes, carbonyl isocyanates and analogous compounds

Antarafacial 1,3-sigmatropic shifts in carbonyl derivatives of ketenes, iso cyanates, thioketenes and thioisocyanates have been studied by means of ab initio MO calculations. Energy barriers in 20 different systems have been u niformly determined at the MP4SDTQ/6-31G(d,p) level, based on MP2/6-31G(d,p ) geometries and corrected for zero-point energies. For formylketene, highe r-level calculations using the QCISD(T) method and larger basis sets [up to 6-311++G(2df,2p)] have also been carried out. In carbonylketenes, the migr atory aptitude of the substituents R is established as follows: Cl> SH > NF 2 > NH2 > BH2 > PH2 > F > OCH3 > OH > SiH3 > H > C6H5 > CH3. The barrier he ights range from 10 kcal/mol for CI migration to 35 kcal/mol for phenyl mig ration. Several factors influencing the energy barriers including the exist ence of an n-electron pair at the migrating atom. its size, electronegativi ty and ability to adapt to a hypervalent state, as well as the strengths of the breaking and forming bonds have been examined in detail. Generally spe aking, 1,3-sigmatropic rearrangements are feasible thermal unimolecular rea ctions even under mild experimental conditions.