The Mechanism of 1,4-Addition of Dibromomethylene

Article abstract of DOI:10.1021/jo00298a017

Dibromomethylene produced from the Seyferth reagent (PhHgCBr3) may react as a free species or while complexed with Lewis bases.Recent reports of the long-sought 1,4-addition to dienes of dibromomethylene, produced from the Seyferth reagent, to form five-membered rings therefore could be based on reactions of either free or complexed carbenes.These modes of reaction may be distinguished by product ratio dependence on the diene concentration, on the solvent basicity, and on the basicity of additives.We have found that the addition of Seyferth-generated dibromomethylene to the exo dienes 1 and 2 gives a ratio of 1,2- and 1,4-addition that is independent of all these considerations of concentration, solvent, and additive.These observations lend strong support to the assertion that 1,4-addition occurs by a concerted, symmetry-allowed pathway.When the carbane source is 11,11-dibromo-1,6-methano<10>annulene, addition to 1,2-dimethylenecyclohexane (1) gives the same proportion of 1,4-addition as for the Seyferth reagent, indicating a free carbene in both cases.Addition of dibromomethylene from the annulene to the sterically congested 2, however, gives a much augmented proportion of 1,4-addition, in fact the major pathway of the first time.This observation may indicate a complexed carbene in this case.