NITRONES FROM ADDITION OF BENZYL AND ALLYL GRIGNARD-REAGENTS TO ALKYL NITRO-COMPOUNDS - CHEMOSELECTIVITY, REGIOSELECTIVITY, AND STEREOSELECTIVITY OF THE REACTION

Reaction of allyl and benzyl Grignard reagents with functionalized nitroalkanes affords nitrones in good yield. This process shows considerable chemoselectivity; carbonyl groups and other highly reactive electrophilic functions are unaffected by the reaction conditions (THF, -70-degrees-C). A mixture of regioisomers 4 and 5 is usually obtained, and the product distribution depends on the nature of the alkyl framework. An intermediate 3 is postulated, and the isomeric pair of nitrones arises, very likely through a selective syn elimination. Alpha-substituted alkyl chains give mostly conjugated products 4 while unbranched chains afford predominantly the nonconjugated nitrones 5. The 4/5 ratio can be strongly modified by using a proton source of suitable strength; trichloroacetic acid produces 4 exclusively in the reaction of nitroethane with benzyl Grignard, while 2,6-dimethylphenol affords completely the nonconjugated nitrone 5. The stereochemistry of the double bond is affected by the nature of the reagent used. Benzyl Grignard gives only Z nitrones 4 and 5; 2-butenylmagnesium chloride gives nonconjugated Z nitrones and a predominance of E isomer in the conjugated nitrone 5.