727425-45-8Relevant articles and documents
The cycloaddition reaction between α-bromo vinylketenes and imines: A combined experimental and theoretical study
Benfatti, Fides,Bottoni, Andrea,Cardillo, Giuliana,Fabbroni, Serena,Gentilucci, Luca,Stenta, Marco,Tolomelli, Alessandra
experimental part, p. 2261 - 2273 (2009/11/30)
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 b-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.
