Addition of a carbonyl functionality to titanium carbenes. A study of the mechanism and intermediates in the Tebbe reaction

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Abstract
The Tebbe transformation of carbonylic substrates such as aldehydes, ketones, esters and amides into alkenes has been analysed theoretically. The active complex in the Tebbe reaction was modelled by various X2TiCH2(X =η5-C5H5, H or Cl) complexes. The extended-Hückel approach was used to obtain a qualitative molecular orbital interaction picture of the reaction giving the expected titanaoxetane intermediate. A concerted path is proposed. A formally 2π+ 2π reaction path is made allowed due to the existence of a very polarized titanium–carbon π-type orbital (highest occupied molecular orbital) providing two electrons more for the cycloaddition. The geometrical structures of the titanaoxetanes were analysed by extended-Hückel, semiempirical as well as ab initio methods. These indicate that the four-membered ring will take a planar geometry, as puckering of the metallacycle results in weakning of the titanium–oxygen bond. A comparison of the 4- and 3-exo-methylenetitanaoxetane isomers revealed that the former is the most stable.