Pathways to Metal–Ligand Cooperation in Quinoline-Based Titanium(IV) Pincers: Nonelectrophilic N-methylation, Deprotonation, and Dihydropyridine Formation

Abstract

A series of titanium­(IV) complexes stabilized by quinoline-based pincer ligands have been synthesized and characterized. The reaction of [TiCp*Me3] with 8-hydroxy-2-quinolinecarboxaldehyde results in [TiCp*Me­{κ3-N,O,O-(OCH)­(8-O-N-Me-quin)}] (1), which shows an uncommon N-methylation/dearomatization of the pyridinic ring; in contrast, when 8-hydroxy-2-quinolinemethanol reacts with the same Ti­(IV) trimethyl derivative, the expected monoalkyl complex [TiCp*Me­{κ3-N,O,O-(OCH2)­(8-O-quin)}] (2) is formed. The pincer ligand in 2 can be dearomatized by deprotonation of the methylene fragment, yielding [TiCp*Me­{κ3-N,O,O-(OCH)­(8-O-quin)}]­[Li­(Et2O)] (3·Et2O) and [TiCp*Me­{k3-N,O,O-(OCH)­(8-O-quin)}]­[Li­(Py)2] (3·2Py), or by incorporation of a hydride group into the para position of the pyridinic ring, giving [TiCp*Me­{κ3-N,O,O-(OCH2)­(4-H-8-O-quin)}]­[Li·THF] (5). Compounds 1, [TiCp*­{κ3 -N,O,O-(OCH2)­(4-H-8-O-quin)}]2 (6), and [TiCp*­{κ3-N,O,O-(OCH2)­(4-H-8-O-quin)}]2[Li·THF]2(μ-O)] (7) have been studied by X-ray diffraction. Additionally, DFT quantum mechanical calculations were performed on complexes 1 and 2.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

“…Selected examples involving a variety of transition-metal and main-group centres are included here (Scheme 5). 84,85 With this approach, there are additional selectivity challenges including transmetallation pathways 86,87 or HT to the metal rather than the pyridyl ring. Overreduction needs to be monitored and can usually be circumvented through the use of pure reagents, careful stoichiometric additions, and controlled lowtemperature reactions.…”