Cycloadducts from diazocumulenes and 1,2,3(lambda(3))-diazaphospholes: Thermolysis generates products derived from 3-alkenylidene-1,2,3(lambda(5))-diazaphospholes

(1-Diazo-2-oxoalkyl)silanes 1 react with 2-acyl-1,2,3(lambda(3))diazaphosph oles 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 componen t, have been trapped. Thermal impact on cycloadducts 5 generates the tricyc lic phosphorus heterocycles 9; their formation can be rationalized by two c ompeting processes, namely thermal [3+2] cycloreversion into 2 and 4, and e xtrusion of molecular nitrogen. The latter process is Likely to generate 3- alkenylidene-1,2,3(lambda(5)) -diazaphospholes 8, which are trapped intermo lecularly by diazaphospholes 4. Intermediates 8 could not be intercepted wi th dimethyl acetylenedicarboxylate (DMAD); rather, 5i reacts with DMAD to f orm the spiro-lambda(5)-phosphorane 11 in low yield.