Stereospecificity of the Hofmann elimination in the gas phase

摘要

>The Hofmann elimination of ammonium ions having a single positive charge is demonstrated to exhibit stereospecificity with regard to expulsion of neutral alkene. For the 3-hexyl series ofthreoanderythro4-monodeuterated ions (3-hexylammonium ion;N,N,N-trimethyl-d₃-3-hexylammonium ion;N-ethyl-3-hexylammonium ion;N-methyl,N-ethyl-3-hexylammonium ion;N,N-dimethyl,N-ethyl-3-hexylammonium ion), the upper limit of theE:Zratio of the expelled alkene (r) approaches 2 (the stereospecificity) with a deuterium isotope effect close to 2.0, although the effects of isotopic substitution diminish theE:Zratio somewhat. Two fragmentations compete with that reaction: hydride shift (which gives the same products but with hydrogen scrambling) and loss of a neutral amine to give an alkyl cation. These competing reactions render our calculation approximate, but the results suggest a value not too far from the upper limit.]]>