| Abstract
| - We show here that palladium(0) (dibenzylideneacetone) complexes bearing 1,10-phenanthrolineconstitute efficient catalysts for the cyclotrimerization of aromatic isocyanates. For the first time, themechanism of this reaction has been investigated experimentally and theoretically with group 10 catalysts.This investigation provides a very consistent picture of the catalytic cycle. Notably, we establish that thereaction does not proceed by stepwise cycloadditions or ring insertions involving metallacyclic intermediates,as might have been anticipated. Rather, in our proposal, the initial steps of the mechanism resemble thechain-growth process operative during the anionic polymerization of isocyanates and feature charge-separated intermediates. These steps are then followed by ring closure on the metal center of the lastintermediate formed to yield a seven-membered metallacycle that reductively eliminates the cyclotrimerand re-forms the active species. In addition, we conclusively show that the (known) palladacycles thatcould be isolated during the experimental investigations are not catalytic intermediates but result from catalystdeactivation. Thus, with Pd(0) diimine catalysts, the actual trimerization mechanism appears to be a blendbetween the two types of mechanisms proposed thus far for the oligomerization of heterocumulenes withvery different catalysts. In conclusion, this work contributes to a better understanding of the reactivity ofarylisocyanates in the vicinity of late group 10 metal centers in low oxidation state and sheds some lighton the detrimental self-poisoning processes observed during the reductive carbonylation of nitroaromaticsubstrates catalyzed by related catalysts in non-nucleophilic media.
|
