Ylid and Related Molecular Rearrangements

Ylid and Related Molecular Rearrangements A fundamental problem associated with mechanisms of rearrangement reactions is concerned with whether the transformation occurs in a concerted or stepwise manner. The alternatives are illustrated in a general way in equation (62). 

A radical pair pathway has been identified for the conversion of 29 into 30 as indicated by the E/A multiplet due to the A2B2 system in the amine side chain at S 2-5-3 0 ppm (Baldwin and Brown, 1969 for CIDNP in the formation of a methylenecyclohexadiene, see Bethell et al., 1972c). The lowenhancementfactor (5-20) at maximum polarization suggests that a concerted [1,3] sigmatropic rearrangement may occur concurrently. 

The occurrence of characteristic by-products can be taken as indicative of a radical pair mechanism in rearrangements. Thus the observation 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier 

N—C PhCHaNMeaCHCOPh MeaN. CH(COPh)CHaPh E Schollkopf et al., 1969b 

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