C‐Nitroso compounds. Part XXXIII. Reaction of α‐chloronitrosoadamantane with Grignard reagents

Abstract

AbstractThe reaction of α‐chloronitrosoalkanes with aliphatic and aromatic Grignard reagents RMgX has been studied using the model compound α‐chloronitrosoadamantane (AdClNO)1. In addition to adamantanone oxime 7 (4‐60%) and the adamantanone oxime ether 8 (trace −28%), the expected nitrone 2 is formed. The nitrone can be isolated for synthetic purposes using a non‐ chromatographic procedure in widely varying yields ( 6 87 %). The structure of all nitrones has been determined using spectroscopic techniques.Formation of oxime and oxime ether has been explained with the mechanism outlined in Scheme 3. This also accounts for the drastic changes in product distribution with the nature of the Grignard reagent. The mechanism involves nitrone formation via polar 1,2‐addition of the Grignard reagent to the nitroso group, and single electron transfer (SET) from the Grignard reagent to the nitroso compound which leads to an iminoxy radical together with radical R*. The latter combines with the multident iminoxy radical on nitrogen and oxygen (not on carbon) producing the nitrone and the oxime ether. Disproportionation can result in the formation of oxime. The oxime can also be formed when iminoxy radicals accept an electron from the Grignard reagent.As the electron donating power is strongly influenced by the nature of the Grignard reagent (i.e. SET vs. polar addition), widely varying product distributions are obtained. The phenyl Grignard reagent is amongst the poorest of electron donors, and therefore gives almost exclusively nitrone by polar 1,2‐addition.Since nitroso compounds are normally excellent free radical scavengers, it is difficult to explain why trapping of transient radicals R* by the nitroso compound, followed by loss of halogen, appears to be relatively unimportant as a route to nitrone (Scheme 3, reaction iii). The formation of dibenzyl (42%) from the reaction of AdClNO 1 with benzylmagnesium bromide is equally remarkable.