FROM CARBENE AND PHOSPHINE TO YLIDE - SYMMETRY-BREAKING ALLOWED PATHWAY

Nonempirical pseudopotential SCF calculations have been performed on the PH3 + CH2 (1A1) → PH3CH2 reaction, using a double ζ (+ d orbitals on the phosphorus) basis set. The calculated properties of the methylenephosphorane molecule are in perfect agreement with previous all-electron ab initio calculations. The heat of reaction is 44 kcal/mol. A very large barrier (>35 kcal/mol) prohibits the direct addition if the P and CH2 atoms are maintained in the same plane as suggested by the ylide equilibrium geometry. If the CH2 plane is perpendicular to the phosphorus lone pair, the barrier disappears; then a sudden rocking of the methylene group, occurring at short P-C distances, leads monotonically to the ylide potential well. The generality of such a symmetry-breaking mechanism is discussed. © 1979, American Chemical Society. All rights reserved.