About the structure of phosphorus ylides: electron distribution and geometry

Using 13C-chem. shifts as a probe for the electronic environment of carbon centers, triphenylphosphoniomethanide, the model case of a reactive phosphorus ylide, have much more zwitterion than PC double bond character. Triphenylphosphoniopropenide (triphenylphosphonium-allyide), a moderated ylide, accumulates electron excess mainly at the a-carbon atom, whereas triphenylphosphonioethenoate (formyl-methylenetriphenylphosphorane) and other stabilized ylides carry roughly equal fractions of neg. charge at the a-carbon and the oxygen atom. The one-bond C,H coupling const. of triphenylphosphoniomethanide argues against a perfectly planar ylide center. The three-bond P,C coupling consts. permit the assignment of endo- or exo-configurations to ylides having an allyl-type side-chain. [on SciFinder (R)]