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 π‐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(π*(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 CHCl(3) (DFT calculations) than for the concerted path.