The Cycloaddition Reaction Between α‐Bromo Vinylketenes and Imines: A Combined Experimental and Theoretical Study

The unusual behaviour of α‐bromo vinylketenes in the cycloaddition reactions with imines is described. This class of vinylketenes behaves as dienophiles in [2+2] reactions, but also displays an unusual diene reactivity in [4+2] reactions. Interestingly, the reactivity of α‐bromo vinylketenes can be modulated via a fine tuning of the substituents. For instance, a methyl group in the β‐position completely inhibits the [2+2] reaction and the [4+2] pathway is almost exclusively followed. A more hindered ketimine (instead of a simple imine) is enough to activate again the [2+2] mechanism. We have carried out a DFT theoretical investigation to rationalise these experimental observations. We have considered two pathways for the [2+2] reaction involving imine, that is, the endo and exo pathways. The former is favoured in the case of α‐bromo vinylketenes, while the latter is preferred for non‐substituted vinylketenes. Since the [4+2] cycloaddition becomes possible only when the s ‐ Z ‐conformation of vinylketene is significantly populated, the presence of bromine substituents in this substrate is crucial in determining the [2+2] or [4+2] mechanisms. For unsubstituted vinylketenes, the barrier connecting the s‐E ‐ to the s‐Z ‐conformation is too high to be easily overcome. Thus, the s ‐ Z ‐structure has a low population and the [2+2] mechanism is favoured. In the case of α‐bromo vinylketenes (especially the β‐methyl‐substituted ones), this barrier can be surmounted and the [4+2] mechanism becomes available.