Imines in the Titanium Coordination Sphere: Highly Reactive Titanaaziridines and Larger Titanacycles Formed by Subsequent C-C Coupling Reactions

The reductive complexation of aldimines is one of the promising synthetic tools to obtain titanaaziridines by reaction of the corresponding titanium halides and magnesium as a reducing agent. The effects of the substitution pattern of the imine ligand and the nature of the titanium fragment on the degree of activation of the (2)-coordinated N-C double bond were investigated. With (CpTiCl3)-Ti-# (Cp-# = cyclopentadienyl or pentamethylcyclopentadienyl) as the precursor for the reductive complexation of the imines N-(4-methylbenzylidene)aniline (2e) and 2-methyl-1-pyrroline (7), the formation of diazatitanacyclopentanes by McMurry-like C-C coupling is found. A six-membered titanacycle 8 is obtained by a Michael-like coupling reaction of 7. By these insights, it is possible to derive a general reaction pattern for low-valent titanium centers with imines. Three-membered titanaaziridines are formed for sterically demanding imines. If such a formation is not possible, coupling reactions occur. Imines that have the opportunity to release a proton at the -position to the nitrogen atom undergo Michael-like reactions, whereas McMurry-like coupling reactions are found at less-shielded titanium centers. Although the titanoceneaziridines show the expected reactivity with aldehydes, ketones, and nitriles, the insertion of alkynes into the titanium-carbon bond proceeds with a remarkably inverse regioselectivity compared with that for the methyleneaniline complex [(Cp2Ti)-Ti-Ad((2)-RN=CH2)] (6; Ad = adamantanyl, R = p-ClC6H4).