Mechanism of the von Braun amide degradations with carbonyl bromide or phosphorus pentabromide
1973; Elsevier BV; Volume: 29; Issue: 21 Linguagem: Inglês
10.1016/s0040-4020(01)93483-0
ISSN1464-5416
AutoresBarry A. Phillips, Gábor Fodor, J. Gál, F. Letourneau, Justin J. Ryan,
Tópico(s)Synthesis and Biological Evaluation
ResumoThe isolation of the α-bromobenziminium hexafluoroantimonate 6, the first intermediate in the amide degradation with carbonyl bromide, was previously announced. Present studies reveal that using either COBr2 or PBr5, the very first intermediate is the iminium tribromide (7). Reaction of the crystalline tribromide with cyclohexane gave the monobromide 8 nearly quantitatively and in crystalline form. Heating of the cyclic iminium monobromide 8 to 120° generates in high yield the α,ω-dihalogenoalkane (type 13) and benzonitrile, while heating to 100° in bromobenzene allows isolation of the ω-bromoalkyl benzimidoyl bromide (9 or 10). An independent synthesis of this and other imidoyl bromides is elaborated. the equilibrium of imidoyl bromides with N-alkyl nitrilium (11) can be shifted toward the nitrilium salt by dissolving in liquid sulfur dioxide or reacting with methyl fluorosulfate. All these steps have been monitored by NMR. The ω-bromoalkyl imidoyl bromide at 120° undergoes fragmentation via the nitrilium ion to an α,ω-dibromoalkane and benzonitrile. In addition to gaining mechanistic information (1) we achieved isolation of crystalline n-alkylene-α-bromoiminium bromides, and their smooth thermal decomposition, which makes the von Pechmann-von Braun type of degradation competitive to the Hofmann methylation; (2) new procedure was found for preparing ammonium tribromides and iododibromides using carbonyl bromide; (3) a number of the heterofore unknown imidoyl bromides have been prepared and characterized, and their thermolysis and mass spectra studied.
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