A simple and practical catalytic electron transfer system composed of TiCl4 and metallic Yb: application in carbonyl olefination and insight into the mechanism

Abstract

The McMurry-type olefinations have become a powerful tool in organic synthesis. However, the use of stoichiometric amounts of unmanageable Ti species and the generation of a large amount of salts limit their applications on the industrial scale. This paper outlines a simple and practical catalytic electron transfer (ET) system composed of TiCl₄ and Yb, which turned out to be effective for homo- and cross-deoxygenative coupling of aldehydes and ketones. Specifically, the method is based upon the strategy that the low valent Ti species abstracts oxygen from carbonyl and delivers them to highly oxophilic Yb in an unprecedented manner. Remarkably, the present process is operationally simple, minimizes the generation of chemical waste, and makes the only by-product Yb₂O₃ as a solid easily removed and utilized; furthermore, the yield improved significantly of that obtained in the stoichiometric version in our system. Our mechanistic data not only provide the direct evidence that Ti(IV) can facilitate the part deoxygenation of pinacol at room temperature, but also demonstrate, for the first time, that for bulky ketones the turnover-limiting step is the Ti-mediated carbonyl coupling rather than the subsequent deoxygenation of the resulting pinacolate intermediates, which are different from the previous observations.