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The Cycloaddition Reaction Between α‐Bromo Vinylketenes and Imines: A Combined Experimental and Theoretical Study
Author(s) -
Benfatti Fides,
Bottoni Andrea,
Cardillo Giuliana,
Fabbroni Serena,
Gentilucci Luca,
Stenta Marco,
Tolomelli Alessandra
Publication year - 2008
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.200800348
Subject(s) - chemistry , cycloaddition , imine , reactivity (psychology) , stereochemistry , population , bromine , computational chemistry , diene , reaction mechanism , substrate (aquarium) , catalysis , organic chemistry , medicine , natural rubber , alternative medicine , demography , oceanography , pathology , sociology , geology
Abstract 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.