Solvent-Free N-Alkylation and Dehydrogenative Coupling Catalyzed by a Highly Active Pincer-Nickel Complex

Abstract

The synthesis and characterization of a pincer-nickel complex of the type ( iPr2NNN)­NiCl2(CH3CN) is reported here. We have demonstrated the utility of this pincer-nickel complex (0.02 and 0.002 mol %) for the catalytic N-alkylation of amines using various alcohols. Under solvent-free conditions, while the highest yield (ca. 90%) was obtained for the alkylation of 2-aminopyridine with naphthyl-1-methanol, excellent turnovers (34000 TONs) were observed for the alkylation of 2-aminopyridine with 4-methoxybenzyl alcohol. To demonstrate the synthetic utility of these systems, high-yield reactions (up to 98%) have been probed for representative substrates with a higher loading of the pincer-nickel catalyst (4 mol %). DFT studies indicate that while β-hydride elimination is the RDS for alcohol dehydrogenation, the N-alkylated product can be formed either via hydrogenation with a rate-determining σ-bond metathesis or by alcoholysis that has imine insertion as the RDS. All of the corresponding resting states have been observed by HRMS (ESI) analysis. The labeling experiments are also complementary to DFT studies and show evidence for the involvement of the benzylic C–H bond in the RDS with a k CHH/k CHD value of about 2.5. This method has been applied to accomplish efficient (2000 TONs) dehydrogenative coupling leading to various benzimidazoles.

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

“…The mass loss could be correlated to loss of fragments either from 2 or from 3 (Figure and Figure S56). The TGA profile of 2a has been previously discussed …”

“…Encouraged by our recent success in obtaining unprecedented turnovers in the N -alkylation of alcohols using the NNN pincer-nickel catalyst ( i Pr2 NNN)­NiCl 2 (CH 3 CN) ( 2a ) under solvent-free conditions, we wished to probe the activity of these pincer-nickel systems toward the synthetically valuable β-alkylation of alcohols. In this study, we report the synthesis of a series of pincer-nickel complexes of the type ( R2 NNN)­NiCl 2 (CH 3 CN) ( 2a , R = i Pr; 2b , R = t Bu; 2c , R = Cy; 2d , R = Ph; and 2e , R = p -F-C 6 H 4 ) and their utility in catalyzing the β-alkylation of alcohols (Figure ).…”

Rational design of pincer‐nickel complexes for catalytic cyanomethylation of benzaldehyde: A systematic DFT study

“…The first combination BS1, consist of B3LYP functional 25 in conjunction with 6‐31G(d,p) BS, 26 the second combination BS2, consist of B3LYP functional 25 in conjunction with 6‐311G(d) BS 26 for the nonmetal elements, while for Ni we used an effective core Stuttgart/Dresden (SDD) pseudopotential and the associated BS with a polarization function 22 . In our previous reports 27 the Perdew‐Burke‐Ernzerhof (PBEPBE) functionals and LANL2DZ BS for similar complexes gave better results. Therefore, we used the third combination BS3, that consists of PBEPBE 28 functional in conjunction with the double‐ζ quality LANL2DZ 29 BS with effective core potential for Ni atom and triple‐ζ quality 6‐311G(d,p) BS with polarization function for all other atoms.…”

Cyclometalated Iridium Complex-Catalyzed N-Alkylation of Amines with Alcohols via Borrowing Hydrogen in Aqueous Media

“… 3 Since the introduction of this kind of synthetic method by Grigg and Watanabe in 1981, 4 it has aroused great interest in chemists as well as become one of the research hotspots. 5 For example, the nickel-, 6 iron-, 7 cobalt-, 8 manganese-, 9 silver-, 10 and copper- 11 atalyzed N -alkylation of amines with alcohols via borrowing hydrogen was extensively reported. Besides, the ruthenium 12 and iridium complexes 13 are also employed in this transformation of N -alkylation.…”