N-allylhydroxylamines from 1,2-addition of allyl Grignard reagents to nitro compounds: generality and drawbacks of the reaction

Abstract

Allyl Grignard reagents react with both aromatic and aliphatic nitro derivatives via 1,2-addition to the nitro group. Conversely, nitroalkenes give either intractable mixtures or exclusive 1,4-conjugate addition. Nitroarenes with high steric hindrance at the ortho position and low aromatic stabilisation give competitive or exclusive conjugate addition at the reactive para position. The ‘in situ’ treatment of the unstable 1,2-adducts with aluminium hydrides in the presence of catalytic amounts of palladium on charcoal provides a general method of synthesis of N-allylhydroxylamines. LiAlH₄ is a very efficient reducing agent, but, in some cases, it does not allow the reduction to be stopped at the hydroxylamino stage. Red-Al^® and DIBAL-H are less efficient but they ensure greater selectivity. Red-Al avoids the complete reduction to amines except when a strongly electron-donating substituent such as a methoxy group is present in the para position of the aromatic ring. Hydroxylamines can be obtained by reaction of nitrosoarenes followed by aqueous quenching. However, this alternative reaction does not offer any improvement, since relevant amounts of azo and azoxy derivatives are recovered as by-products.