Epimerization of Cyclic Vinylphosphirane Complexes: The Intermediacy of Biradicals

The phosphinidene complex Ph−P−W(CO)5 reacts with
cyclopentadiene, 1,3-cyclohexadiene, 1,3-cycloheptadiene, and 1,3-cyclooctadiene to give in each case isomeric
mixtures of syn- and
anti-vinylphosphiranes.
With longer reaction times and/or higher reaction temperatures,
the anti adducts isomerize to the syn adducts. These
epimerizations are likely to proceed via biradical intermediates.
Only the syn-vinylphosphirane of
1,3-cyclohexadiene
undergoes a [1,3]-sigmatropic shift to yield a
syn-phopholene. The dichotomy of biradical and
concerted mechanisms
is discussed in relationship with the analogous mechanism for the
vinylcyclopropane → cyclopentene rearrangement.