Paul Frédéric, Moulin Solenne, Piechaczyk Olivier, Le Floch Pascal, Osborn John A
Laboratoire Sciences Chimiques de Rennes, Université de Rennes 1, Campus de Beaulieu, CNRS, 35042 Rennes Cedex, France.
J Am Chem Soc. 2007 Jun 13;129(23):7294-304. doi: 10.1021/ja068291k. Epub 2007 May 18.
We show here that palladium(0) (dibenzylideneacetone) complexes bearing 1,10-phenanthroline constitute efficient catalysts for the cyclotrimerization of aromatic isocyanates. For the first time, the mechanism 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 the reaction 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 the chain-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 last intermediate formed to yield a seven-membered metallacycle that reductively eliminates the cyclotrimer and re-forms the active species. In addition, we conclusively show that the (known) palladacycles that could be isolated during the experimental investigations are not catalytic intermediates but result from catalyst deactivation. Thus, with Pd(0) diimine catalysts, the actual trimerization mechanism appears to be a blend between the two types of mechanisms proposed thus far for the oligomerization of heterocumulenes with very different catalysts. In conclusion, this work contributes to a better understanding of the reactivity of arylisocyanates in the vicinity of late group 10 metal centers in low oxidation state and sheds some light on the detrimental self-poisoning processes observed during the reductive carbonylation of nitroaromatic substrates catalyzed by related catalysts in non-nucleophilic media.
我们在此表明，带有1,10 - 菲咯啉的钯(0)（二亚苄基丙酮）配合物是芳族异氰酸酯环三聚反应的有效催化剂。首次使用第10族催化剂对该反应的机理进行了实验和理论研究。该研究提供了一个非常一致的催化循环图景。值得注意的是，我们确定该反应并非如预期的那样通过涉及金属环中间体的逐步环加成或环插入进行。相反，在我们的提议中，该机理的初始步骤类似于异氰酸酯阴离子聚合过程中起作用的链增长过程，并以电荷分离中间体为特征。随后这些步骤是在最后形成的中间体的金属中心上进行环化，生成一个七元金属环，该金属环通过还原消除环三聚体并重新形成活性物种。此外，我们确凿地表明，在实验研究过程中可以分离出的（已知）钯环不是催化中间体，而是催化剂失活的产物。因此，对于Pd(0)二亚胺催化剂，实际的三聚机理似乎是迄今为止针对使用非常不同的催化剂进行杂累积二烯低聚所提出的两种机理的混合。总之，这项工作有助于更好地理解低氧化态的第10族晚期金属中心附近芳基异氰酸酯的反应性，并为在非亲核介质中由相关催化剂催化硝基芳族底物的还原羰基化过程中观察到的有害自毒化过程提供了一些线索。
