Cycloadducts from Diazocumulenes and 1,2,3(λ3)-Diazaphospholes: Thermolysis Generates Products Derived from 3-Alkenylidene-1,2,3(λ5)-diazaphospholes

Abstract

(1-Diazo-2-oxoalkyl)silanes 1 react with 2-acyl-1,2,3(λ3)-diazaphospholes 4 to form [3+2] cycloaddition products 5, which indicate that 1-diazo-2-silyloxy-1-alkenes 2, coexisting with 1 as the minor equilibrium component, have been trapped. Thermal impact on cycloadducts 5 generates the tricyclic phosphorus heterocycles 9; their formation can be rationalized by two competing processes, namely thermal [3+2] cycloreversion into 2 and 4, and extrusion of molecular nitrogen. The latter process is likely to generate 3-alkenylidene-1,2,3(λ5)-diazaphospholes 8, which are trapped intermolecularly by diazaphospholes 4. Intermediates 8 could not be intercepted with dimethyl acetylenedicarboxylate (DMAD); rather, 5i reacts with DMAD to form the spiro-λ5-phosphorane 11 in low yield.