An Overlooked Pathway in 1,3-Dipolar Cycloadditions of Diazoalkanes with Enamines

Methyl diazoacetate reacts with 1-(N-pyrrolidino)cycloalkenes to give products of 1,3-dipolar cycloadditions and azo couplings. The kinetics and mechanisms of these reactions were investigated by NMR spectroscopy and DFT calculations. Orthogonal pi-systems in the 1,3-dipoles of the propargyl-allenyl type allow for two separate reaction pathways for the (3+2)-cycloadditions. The commonly considered concerted pathway is rationalized by the interaction of the enamine HOMO with LUMO+1, the lowest unoccupied orbital of the heteropropargyl anion fragment of methyl diazoacetate. We show that HOMO/LUMO(pi*(N=N)) interactions between enamines and methyl diazoacetate open a previously unrecognized reaction path for stepwise cycloadditions through zwitterionic intermediates with barriers approximately 40 kJ mol(-1) lower in energy in CHCl3 (DFT calculations) than for the concerted path.