From Dialkyltitanium Species to Titanacyclopropanes: An Ab Initio Study
Résumé
Although the dialkoxytitanacyclopropanes are presented as important reagents in organic synthesis, the mechanism of their generation from dialkyltitanium species is not firmly established, as shown by the various propositions cited in the literature. An ab initio study of these transformations was investigated, namely, β-H elimination, β-H abstraction, epimetalation, as well as the related α-H abstraction. The results tend to show that the formation of a titanium hydride intermediate is unlikely, but, most probably, a direct hydrogen transfer (β-H abstraction) would occur. Compared to titanocene analogues, the α-H and β-H abstraction reactions were less favorable with dialkoxytitanium complexes. The overall reaction was found to be more favorable in the case of dialkoxydialkyltitanium complexes by the prior formation of bimetallic intermediates, underlining the importance of titanium ate complexes in this chemistry. A prior agostic interaction between the ethyl moiety and the metal was found to facilitate the hydrogen transfer and the formation of the metallacyclopropane. This phenomenon was illustrated by an easier β-H abstraction from a complex bearing an agostic interaction in its ground state. The intrinsic bond orbital method was used throughout this work to better visualize the electronic processes involved in these transformations.