Palladium-Catalyzed Cross-Coupling Reaction of Bis(cyclopentadienyl)diaryltitaniums with Terminal Alkynes

Organotitanium compounds find application in diverse reactions, including carbon-carbon bond formation and oxidation. While titanium (IV) compounds have been used in various applications, the potential of bis(cyclopentadienyl)diaryltitanium in cross-coupling reactions remains unexplored. This study focuses on Sonogashira-type cross-coupling reactions involving terminal alkynes and organotitanium compounds. Diaryltitanocenes were synthesized using titanocene dichloride with lithium intermediates derived from aryl iodide. Under open-flask conditions, reactions of diphenyltitanocenes with ethynylbenzene in the presence of 20 mol% Pd(OAc)2 in DMF produced coupling products in a remarkable 99% yield. Various diaryltitanocenes and alkynes under standard conditions yielded corresponding cross-coupling products with moderate to good yields. Notably, the Sonogashira-type alkynylation proceeds under mild conditions, including open-flask conditions, and without the need for a base. Furthermore, this cross-coupling is atom-economical and involves the active participation of both aryl groups of the diaryltitanocene. Remarkably, this study presents the first example of a Sonogashira-type cross-coupling using titanium compounds as pseudo-halides.