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About the structure of phosphorus ylids: Electron distribution and geometry

✍ Scribed by Manfred Schlosser; Titus Jenny; Bruno Schaub

Publisher
John Wiley and Sons
Year
1990
Tongue
English
Weight
469 KB
Volume
1
Category
Article
ISSN
1042-7163

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✦ Synopsis

Abstract

Using ^13^C‐chemical shifts as a probe for the electronic environment of carbon centers, triphenylphosphoniomethanide, the model case of a “reactive” phosphorus ylid, was found to have much more zwitterion than PC double bond character (resonance structure 1a being dominant). Triphenylphosphonio‐propenide (“triphenylphosphonium‐allylid”), a “moderated” ylid, accumulates electron excess mainly at the α‐carbon atom (resonance structure 6a being dominant) whereas triphenylphosphonio‐ethenoate (formyl‐methylene‐triphenylphosphorane) and other “stabilized” ylids carry roughly equal fractions of negative charge at the α‐carbon and the oxygen atom (resonance formulas a and b being of comparable importance). The one‐bond C,H coupling constant of triphenylphosphonio‐methanide (1) argues against a perfectly planar ylid center. The three‐bond P,C coupling constants permit the assignment of endo‐ or exo‐configurations to ylids having an allyl‐type side‐chain.

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