Studies of Grignard reagent rearrangements by means of nuclear magnetic resonance spectroscopy. I. The structure of allylic Grignard reagents. II. The interconversion of cyclopropylcarbinyl and allylcarbinyl Grignard reagents

Abstract

I. The structure of allylic Grignard reagents: The nuclear magnetic resonance (n.m.r.) spectra of several simple allylic Grignard reagents in diethyl ether solution, of benzylmagnesium chloride in ether, and of diallylmagnesium and dibutenylmagnesium in dioxane have been investigated. The results for the simple allylic systems are best interpreted in terms of an extremely mobile tautomeric exchange of magnesium between allylic positions, whereas the conventional structure is indicated for the benzyl Grignard reagent. The results are related to the rearrangements known to occur in the Grignard reactions of allylic halides. II. The interconversion of cyclopropylcarbinyl and allylcarbinyl Grignard reagents: The Grignard reagent, in ether solution, from either cyclo-propylcarbinyl chloride or allylcarbinyl chloride has been shown by its n.m.r. spectrum to possess the allylcarbinyl structure. Nortricyclcylchloride and exo- or endo-5-dehydronorbornyl chloride also give a common Grignard reagent, whose n.m.r. spectrum seems best fitted to the nortricyclyl structure. The results explain the products formed in the Grignard reactions of these sets of isomeric halides. They are also discussed with respect to the recently discovered interchange between the 1- and 2-positions in allylcarbinylmagnesium halides.