Titanium Hydrazido and Imido Complexes:  Synthesis, Structure, Reactivity, and Relevance to Alkyne Hydroamination

Abstract

Treatment of Ti(NMe(2))(2)(dpma) (1) with aniline results in the protonation of the dimethylamido ligands, which are retained as dimethylamines, and generation of a titanium imido complex Ti(NPh)(NHMe(2))(2)(dpma) (2) in 94% yield. The monomeric imido 2 is converted to the reactive dimeric micro-imido [Ti(NPh)(dpma)](2) (3) on removal of the labile dimethylamine donors. The dimer 3 is converted to monomeric terminal imido complexes in the presence of added donors, e.g., 4,4'-di-tert-butyl-2,2'-bipyridine (Bu(t)-bpy) and DME. Compounds 1-3 exhibit the same rate constant for 1-phenylpropyne hydroamination by aniline and are all kinetically competent to be involved in the catalytic cycle. Attempts to use 1 as a catalyst for hydroaminations involving 1,1-dimethylhydrazine resulted in only a few turnovers under the best conditions. Consequently, the chemistry of 1 with hydrazines to generate hydrazido complexes was scrutinized for comparison with the imido species. Through these studies, titanium hydrazido complexes including Ti(eta(2)-NHNC(5)H(10))(2)(dpma) (5), Ti(eta(2)-NHNMe(2))(2)(dpma) (6), and [Ti(micro:eta(1),eta(2)-NNMe(2))(dpma)](2) (7) were characterized. In addition, a terminal hydrazido(2-) complex was available by addition of Bu(t)-bpy to 1 prior to 1,1-dimethylhydrazine addition, which provided Ti(eta(1)-NNMe(2))(Bu(t)-bpy)(dpma) (8). Compound 8 was structurally characterized and compared to Ti(NPh)(Bu(t)-bpy)(dpma) (4b), an imido derivative with the same ancillary ligand set. Compound 8 has a nucleophilic beta-nitrogen consistent with a hydrazido(2-) formulation, as determined by reaction with MeI to form the ammonium imido complex [Ti(NNMe(3))(Bu(t)-bpy)(dpma)]I (9). Analogous pyridinium imido complexes [Ti(N-1-pyridinium)(Bu(t)-bpy)(dpma)](+) (10) are available by addition of 1-aminopyridinium iodide to 1. From the investigations, some conclusions regarding the activity of titanium pyrrolyl complexes in hydroamination were drawn. The lack of conversion of the bis[micro-hydrazido(2-)] 7 to monomeric species in the presence of donor ligands is put forth as one explanation for the poor hydrazine hydroamination activity of 1. This problem was combated in the synthesis of Ti(NMe(2))(2)(dap)(2), which is an active catalyst for hydrazine hydroamination of alkynes.

Pyrazole Synthesis Using a Titanium‐Catalyzed Multicomponent Coupling Reaction and Synthesis of Withasomnine

“…Titanium was added as the dimethylamidocontaining precatalysts, which are easily synthesized from commercially available TiA C H T U N G T R E N N U N G (NMe 2 ) 4 . [8] The dimethylamido ligands are protolytically removed by the primary amine substrate [9] to generate titanium imido complexes that undergo [2 + 2] cycloaddition with alkynes. [10] The resulting azatitanacyclobutenes undergo 1,1-insertion of isonitriles to generate 5-membered metallacycles.…”

Modular N,O‐Chelating Ligands: Group‐4 Amidate Complexes for Catalytic Hydroamination

“…[65][66][67][68] Following an initial report of titanium catalyzed hydroamination by Rothwell and coworkers, [69] research into hydroamination catalysis has also followed the trend of focusing on Cp-based ligand systems and then moving onto non-Cp-based catalysts. A variety of systems have been developed for hydroamination reactions including work from the groups of Bergman, [15,64,[70][71][72][73] Livinghouse, [74,75] Odom, [67,[76][77][78][79][80][81] Beller, [65,[82][83][84][85][86] Doye, [87][88][89][90][91][92][93][94] ourselves, [59,61,62,95,96] and others. [11,[97][98][99][100][101][102][103][104]…”

Titanium-catalyzed iminohydrazination of alkynes

“…The iminohydrazination reaction is a modification of hydrohydrazination of alkynes [5][6][7][8] catalyzed by some titanium complexes where a 1,1-disubstituted hydrazine is added to an alkyne to generate a hydrazone (Scheme 2). In a previous study, we developed two catalysts for alkyne hydrohydrazination, a titanium pyrrolyl complex Ti(NMe 2 ) 2 (dap) 2 (1), where dap is 2-(N,N-dimethylaminomethyl)pyrrolyl, and thiolate-containing Ti(NMe 2 ) 2 -(SC 6 F 5 ) 2 (NHMe 2 ) (2) (Chart 1).…”

“…Using titanium catalysts, alkynes can be hydrohydrazinated [4] (Scheme 2) with 1,1-disubstituted hydrazines, a process that generates hydrazones and indoles (after Fischer cyclization) [5,6]. This procedure recently has been elegantly expanded by Ackermann [7] and Beller [8] using alternative titanium catalysts to generate a variety of indole derivatives.…”