Titanium Hydrazinediido Half-Sandwich Complexes: Highly Active Catalysts for the Hydrohydrazination of Terminal Alkynes at Ambient Temperature

Abstract

Reaction of [Cp*Ti(N Xyl N)(N t Bu)(NH 2 t Bu)] with 1 molar equiv of diphenylhydrazine yielded the hydrazinediido complex [Cp*Ti(N Xyl N)(NNPh 2 )(NH 2 t Bu)] (1a), whereas the orange pyridine adduct [Cp*Ti(N Xyl N)(NNPh 2 )(py)] (1b) was obtained by reacting the imide [Cp*Ti(N Xyl N)(N t Bu)(NH 2 t Bu)] with diphenylhydrazine in the presence of pyridine. The tert-butylamine coordinated to the metal center in 1a could be removed by heating the solid at 60 °C and 10 -6 mbar for 72 h, yielding [Cp*Ti(N Xyl N)(NNPh 2 )] (1c). In the presence of pyridine or 4-dimethylaminopyridine (dmap) as neutral co-ligands the hydrazinediido complexes [Cp*Ti(N Xyl N)(NNMePh)(py)] (2a) and [Cp*Ti(N Xyl N)-(NNMePh)(dmap)] (2b) as well as [Cp*Ti(N Xyl N)(NNMe 2 )(dmap)] (3) were prepared. Upon replacement of dmap by the weaker donor ligand pyridine in the synthesis of the pyridine adduct analogous to 3, a mixture of [Cp*Ti(N Xyl N)(NNMe 2 )(py)] (4a) and the dinuclear complex [Cp* 2 Ti 2 (N Xyl N) 2 (µ-η 1 ,η 1 -NNMe 2 )(µ-η 1 ,η 2 -NNMe 2 )] (4b) was obtained. Reaction of the dimethylhydrazinediido complex 3 with phenylacetylene gave the Markovnikov cycloadduct, which had sufficient lifetime to allow its 1 H, 13 C, and 15 N NMR spectroscopic characterization in solution. All three hydrazinediido compounds 1a, 2a, and 3 were found to display remarkable activities in catalytic hydrohydrazinations at ambient temperatures. For catalyst loadings of 5 mol % complete conversions of the terminal alkynes and diynes with Markovnikov regioselectivities of over 99% selectivity were observed within 1 h.

Efficient Anti‐Markovnikov‐Selective Catalysts for Intermolecular Alkyne Hydroamination: Recent Advances and Synthetic Applications

“…[34] The most efficient hydrohydrazination catalysts reported to date are titaniumimido complexes, with which hydrohydrazination of terminal alkynes is carried out at room temperature. [35] In our case, treatment of phenylacetylene and diaryl-, mixed arylalkyl-and dialkylhydrazines with 5 mol-% of 10 gave hydrohydrazination products at elevated temperatures in good yields ( Table 6). Exclusive anti-Markovnikov selectivity is observed with 1,1-diphenylhydrazine; however, in the reactions with 1-methyl-1-phenylhydrazine and 1,1-dimethylhydrazine, regioselectivity is diminished.…”

“…This often requires catalyst modification, due to the different reactivity imparted by the two adjacent nitrogen atoms in hydrazine 34. The most efficient hydrohydrazination catalysts reported to date are titanium‐imido complexes, with which hydrohydrazination of terminal alkynes is carried out at room temperature 35. In our case, treatment of phenylacetylene and diaryl‐, mixed arylalkyl‐ and dialkylhydrazines with 5 mol‐% of 10 gave hydrohydrazination products at elevated temperatures in good yields (Table 6).…”

A Remarkable Switch from a Diamination to a Hydrohydrazination Catalyst and Observation of an Unprecedented Catalyst Resting State

“…The best conversions were observed for 6 a , 6 b , and 6 e : 91–95 % conversion after 47–68 h at room temperature with a catalyst loading of 10 mol %. However, with the exception of a system reported recently by Gade,3d these performances are not untypical of titanium hydrohydrazination catalysts at room temperature 2c,d. 3c All systems showed first‐order consumption of the hydrazine (see Figure 2 for the reaction with HCC4‐C 6 H 4 CF 3 and Figure S3 in the Supporting Information for HCCTol).…”

Zirconium and Hafnium Hydrazinediido Half-Sandwich Complexes: Synthesis and Reactivity

“…6−15 Stoichiometric reactions with alkynes, 4,5,7,8,12 allenes, 14 and heteroallenes, such as isocyanates, 3,10,11,14 isothiocyanates, 10,11,14 carbon dioxide, 3,9,11 carbodiimides, 16 and carbon disulfide, 11 resulted in formal [2 + 2] cycloadditions. In some cases this initial reaction step was followed by Ti−X insertion, 10,17 or insertion into the N−N bond of the hydrazinediido unit, depending on the ancillary ligand.…”

“…In some cases this initial reaction step was followed by Ti−X insertion, 10,17 or insertion into the N−N bond of the hydrazinediido unit, depending on the ancillary ligand. Complexes of this type have been employed as catalysts in the hydrohydrazination of alkynes 7,12,18 and carbodiimides 16 and related three-component coupling reactions. 19 However, zirconium 20−25 and hafnium 21 hydrazinediido complexes remain relatively scarce, despite the interesting and diverse reactive behavior found for some of them.…”