Structure and thermal isomerization of the adducts formed in the reaction of cyclohexyl isocyanide with dimethyl acetylenedicarboxylate

Reaction of cyclohexyl isocyanide (Id) with dimethyl acetylenedicarboxylate (DMAD) gave a mixture of products such as the cyclopenta[b]pyridine derivatives 14 (28%) and 19 (1%), the azaspirononatriene derivative 13 (2%), and the azabicyclononatriene 16 (3%). Interestingly, 14 on refluxing in xylene for 3 h gave exclusively 13 (92%), whereas when the reaction was continued for 6 h, a mixture of 13 (85%) and 19 (9%) was formed. In a separate reaction, when 13 was heated in a sealed tube at 200 degrees C for 1 h, 19 was isolated in 63% yield. Activation energies for the thermal isomerization of 14 to 13 and 13 to 19 have been found to be 19.4 and 16.7 kcal/mol, respectively. Hydrogenation of 13 and 14 gave a mixture of the tetrahydro derivatives, 22 and 23, in each case. The structures of 13, 14, 16, 19, 22, and 23 were established unambiguously through X-ray crystallographic analysis.