Rearrangement of Allylic Organomagnesium Compounds

On the basis of considerable spectroscopic and chemical evidence, allylic organomagnesium compounds appear to be best represented as a mixture of rapidly equilibrating allylic isomers [Eq. (44)]. As such, by 

It was recently reported that reaction of this Grignard reagent with trimethylchlorosilane gave about 50% of reaction at each end of the allylic system, but there is no indication as to which allylic Grignard isomer was the reacting species (61). 

The fairly recent literature has included a review of the preparation and reactions of allyl and butenyl Grignard reagents (5), a review of the reactions of allylic organometallic compounds (II), and a review of organomagnesium rearrangements, in which allylic derivatives were discussed in some detail (29). Since more recent publications do not appear to include new information that would significantly modify earlier conclusions, the previous discussions should suffice. 

One relatively recent development might be treated briefly. It was reported by Benkeser in 1969 (7) and by Miginiac in 1970 (51) that the products of some allylic Grignard addictions to hindered ketones are isomerized after longer reaction times to products with inversion of the allylic group [e.g., Eq. (46)]. 

In the case of allylic zinc derivatives, which isomerize much more readily in a similar fashion, the rearrangement takes place by reversal of the addition, and readdition of the allylic organometallic to give the thermodynamically favored, less hindered product (51). More recently, Benkeser has demonstrated the reversibility of the addition of the allyl Grignard itself (6). 

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