Bonding, proton transfer, and diradical stabilization in phosphonium ylides

The phosphorus-carbon bond of phosphonium ylides is studied with semiempirical molecular orbital calculations. The primary effect of the inclusion of 3d orbitals on phosphorus is the anticipated electron transfer from the ylide carbon to the 3d orbitals, with an accompanying increase in P-C bond order. A novel effect of some significance is the hyperconjugative a-type interaction of CHZ orbitals in a model H3PCHz with a P 3d orbital of the appropriate symmetry. Intermolecular and intramolecular mechanisms for proton transfer in (CH3)3PCH2 are studied. A concerted hydrogen switch involving two molecules is calculated to possess a high activation energy. The potential surface for a concerted intramolecular hydrogen migration leads to a transition state best described as a pentavalent phosphorus derivative. Carbon diradicals should be stabilized by intervening phosphorus or other second-row atoms. This type of stabilization is studied for a hypothetical CH2PR3CH2. Such species are potential singlet states and should possess barriers to rotation of CHZ groups but not of the PRa moiety. hosphonium ylides or alkylidene phosphoranes, P whose electronic structure is commonly described as a resonance hybrid 1, derive much of their current \ + -/ \ / -P-c /P=c\ / \ 1 importance from the synthetic utility of the Wittig reaction. -P=C \ ’ \R2 R4 /R1 + o=c\ /R3 + \ R, \c=c /R3 -P=O + / &’ \R, The ylides are fascinating compounds in their own right, exhibiting a remarkable variety of structural types exemplified by the species 2-5. 3-6 The resonance hybrid formulation 1 implies the verifiable existence of double bond character and the less verifiable concomitant participation of phosphorus 3d orbitals in bonding. The presence of partial double bond character is indicated by P-C interatomic distance~’-’~ of 1.66-1.74 A, intermediate between an (1) G. Wittig and G. Geissler, Jusfus Liebigs Ann. Chem., 580, 44 (1953), and subsequent papers. (2) An excellent source of information on phosphonium ylides is A. W. Johnson, “Ylid Chemistry,” Academic Press, New York, N. Y., 1966. (3) F. Rarnirez, N. B. Desai, B. Hansen, and N. McKelvie, J . Amer. Chem. Soc., 83,3539 (1961). (4) F. Ramirez and S. Levy, ibid., 79,67 (1957). ( 5 ) C. N. Matthews, J. S. Driscoll, and G. H. Birum, Chem. Com(6) C. N. Matthews and G. H. Biruni, Tefruhedron Lef t . , 5707 mun., 736 (1966). (1966); Accounts Chem. Res . , 2 , 373 (1969). \ / 7p=c=p\ 2