Titanium hydroamination catalysts bearing a 2-aminopyrrolinato spectator ligand: monitoring the individual reaction steps

Abstract

A series of new titanium half sandwich complexes, containing a 2-aminopyrrolinato ligand {N(Xyl)N}(-) as the ancillary ligand, have been prepared and are shown to be pre-catalysts for the hydroamination of alkynes. The coordination of {N(Xyl)N}(-) to titanium was achieved by reaction of [Cp*TiMe(3)] with the protioligand N(Xyl)NH giving [Cp*Ti(N(Xyl)N)(Me)(2)] (). Upon reaction of complex with an excess of tert-butylamine, the imido complex [Cp*Ti(N(Xyl)N)(N(t)Bu)(NH(2)(t)Bu)] () was formed. The latter provided the preparative entry to the synthesis of a range of N-aryl substituted imido complexes. Imido ligand exchange with 2,6-dimethylaniline, 2,4,6-trimethylaniline as well as 2,6-diisopropylaniline gave the corresponding arylimido complexes in clean reactions. Reaction of the titanium imido complex [Cp*Ti(N(Xyl)N)(N(t)Bu)(NH(2)(t)Bu)] with terminal arylacetylenes, such as phenylacetylene and tolylacetylene, led to C-H activation and the formation of alkynyl/amido complexes, whereas the arylimido complexes and cleanly underwent {2 + 2} cycloaddition, giving the azatitanacyclobutene derivatives. A single-crystal X-ray structure analysis of the azatitanacyclobutene [Cp*Ti(N(Xyl)N){kappa(2)N(2,6-C(6)H(3)Me(2))CTol[double bond, length as m-dash]CH}] () provided the first crystallographically characterized Markovnikov cycloaddition product of an imidotitanium complex with a terminal alkyne. The mechanistic aspects of the hydromanination of alkynes with the new Ti half sandwich complexes were studied and established a reversible {2 + 2} cycloaddition step and the cleavage of the metallacyclic intermediate as the rate determining step in the catalytic cycle. The titanium half sandwich imido complexes were found to be active catalysts for the inter- and intramolecular hydroamination of a broad range of alkynes and omega-aminoalkynes.

Hydroamination as a route to nitrogen-containing oligomers

“…Literature precedent suggests that the mechanism of hydroamination for this group 4 catalyst follows a [2 + 2] cycloaddition pathway (Scheme 1). [2][3][4]6,7,44 This mechanism accounts for chain growth because the hydroamination product after each turnover contains a primary amine functionality which can form a new titanium-imide species. However, this mechanism does not account for the observation of the major set of peaks in the MALDI-TOF mass spectrum of (2a) (Fig.…”

Transition‐Metal‐Catalyzed Hydroamination Reactions

“…Benchmark reactivity for new catalyst systems is often established using diphenylacetylene, where regioselectivity is not a challenge (Table 15.2). Doye has carried out extensive investigations with a variety of amines and a range of catalysts (3a-c, 4a-c, 6a-c), including various preprepared imido complexes, 15 in the work completed in collaboration with Gade [31,32]. Notably, arylamines are the most reactive substrates, while less sterically demanding alkylamines, such as benzylamine and linear alkylamines, are some of the most challenging substrates (Table 15.2, entries [9][10][11][12][17][18][19].…”

“…Indeed, as shown in Table 15.3, it is the amine substrate that most significantly impacts both reactivity (as measured by yield) and regioselectivity. Although Doye and coworkers [31,37] have prepared and screened a number of distinct metal complexes, it is known that ligand dissociation can be observed in sterically congested Ti systems [38]. Thus, even though a variety of different precatalysts have been explored, they may yield very similar catalytically active species in situ.…”

“…Ogata and Fukuzawa [201] also developed dinuclear Ir and Rh systems with NHCs for the related intramolecular hydroamination reaction to give substituted indole products. Intermolecular alkyne hydroamination was recently investigated by Otero and coworkers [202], where they have used a cationic Rh complex ligated with easily varied iminopyridine ligands (31). This catalyst system is limited to arylamine substrates and the highest conversions of about 70% with a modest TOF of approximately 3 h −1 are achieved with sterically bulky aniline derivatives (Scheme 15.30), suggesting that steric bulk helps with product decomplexation during the catalytic cycle.…”

From Five to Seven: Ring Expansion of Monoazadiene Titanium Complexes by Insertion of Aldehydes, Ketones and Nitriles

“…The three‐membered azatitanacycles IIIa are defined as η 2 ‐imine complexes . The four‐membered rings exist as azatitanacyclobutanes IVa or ‐butenes IVb, which can be generated via [2+2] cycloaddition reactions of olefins or alkynes– and titanium imido species, or by employing titanium carbenes and carbodiimides . The family of the five‐membered rings is represented by insertion products of olefins and acetylenes into titanaaziridines, leading to azatitanacylopentanes Va and −3‐pentenes Vb ,.…”