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Weakening C−O Bonds: Ti(III), a New Reagent for Alcohol Deoxygenation and Carbonyl Coupling Olefination

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journal contribution
posted on 13.01.2010, 00:00 by Horacio R. Diéguez, Armando López, Victoriano Domingo, Jesús F. Arteaga, José A. Dobado, M. Mar Herrador, José F. Quílez del Moral, Alejandro F. Barrero
Investigations detailed herein, including density functional theory (DFT) calculations, demonstrate that the formation of either alkoxy− or hydroxy−Ti(III) complexes considerably decreases the energy of activation for C−O bond homolysis. As a consequence of this observation, we described two new synthetic applications of Nugent’s reagent in organic chemistry. The first of these applications is an one-step methodology for deoxygenation−reduction of alcohols, including benzylic and allylic alcohols and 1,2-dihydroxy compounds. Additionally, we have also proved that Ti(III) is capable of mediating carbonyl coupling−olefination. In this sense, and despite the fact that for over 35 years it has been widely accepted that either Ti(II) or Ti(0) was the active species in the reductive process of the McMurry reaction, the mechanistic evidence presented proves the involvement of Ti(III) pinacolates in the deoxygenation step of the herein described Nugent’s reagent-mediated McMurry olefination. This observation sheds some light on probably one of the mechanistically more complex transformations in organic chemistry. Finally, we have also proved that both of these processes can be performed catalytically in Cp2TiCl2 by using trimethylsilyl chloride (TMSCl) as the final oxygen trap.