N–Alkylation of Amines Catalyzed by a Ruthenium–Pincer Complex in the Presence of in situ Generated Sodium Alkoxide

Das, Kanu; Nandi, P.; Islam, Khadimul; Srivastava, H. K.; Kumar, Akshai

doi:10.1002/ejoc.201901310

Cited by 42 publications

(39 citation statements)

References 171 publications

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“…The yields of 7 dropped steadily upon lowering the NaOH loading (entries 6 and 7, Table ). Employing Na (5 mol %) to generate the base in situ (prior to addition of 2d ) resulted in yields that are comparable to that obtained with the use of NaOH (5 mol %) (entry 5 vs entry 8, Table ). The yield of 7 improved (75%) with the use of NaO t Bu (5 mol %) in the 2d (0.01 mol %) catalyzed β-alkylation of 5 with 4 (entry 9, Table ).…”

Section: Resultsmentioning

confidence: 80%

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

2021

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We report here the synthesis and characterization of a series of NNN pincer-nickel complexes of the type ( R2 NNN)-NiCl 2 (CH 3 CN) (R = i Pr, t Bu, Cy, Ph, and p-F-C 6 H 4 ) based on bis(imino)pyridine ligands. In solution, these complexes are found to be equilibrium mixtures containing one and two pincer ligands, respectively. While the crystal structure of the former was reported by us recently for R = i Pr, we report the crystal structure of the latter in this study for R = p-F-C 6 H 4 . The considered NNN pincer-Ni complexes have been successfully employed to accomplish the catalytic β-alkylation of several secondary alcohols with a variety of benzyl alcohols at 140 °C with high yields and unprecedented turnovers. A maximum of 92% yield of the β-alkylated product at 18 400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyl)phenyl)ethane-1-ol in the presence of 0.005 mol % of ( Ph2 NNN)NiCl 2 (CH 3 CN) and 5 mol % of NaO t Bu at 140 °C after 24 h. The reaction exhibits zero-order dependence of rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed β-alkylation of alcohols and, hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.

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Section: Resultsmentioning

confidence: 80%

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

2021

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“…In comparison, its precursor NiCl 2 (DME) is relatively less stable and readily loses its Cl – ligands. This offers promise for utilizing 2 in highly endothermic reactions such as the dehydrogenation of hydrocarbons and their derivatives. − ,, …”

Section: Resultsmentioning

confidence: 99%

“…While the 2 -catalyzed N-alkylation reaction of 3 with 4 also did not proceed in the absence of a base (entry 1, Table ) or in the absence of catalyst (entry 10, Table ), very poor results were obtained upon use of 0.75 equiv of NaOH and KOH (entries 2 and 3, Table ). Though the turnovers improved upon use of Na and KO t Bu (entries 4 and 5, Table ), the best turnovers were obtained with the use of 0.75 equiv of NaO t Bu in the presence of 0.02 mol % of 2 (entry 6, Table ). While lowering the amount of NaO t Bu resulted in poor turnovers (entries 7 and 8, Table ), use of 1 equiv NaO t Bu gave comparable yields (entry 9, Table ).…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2020

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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.

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“…Five-coordinated ruthenium hydride complexes were proposed to be the active catalysts for dehydrogenation, and previous DFT studies generally omitted the kinetical details of the generation of such active catalysts from their precursors. − Herein, we performed the detailed calculations for this process to gain more information (Figure ). In the presence of a catalytic amount of strong base KO t Bu, the N–H deprotonation of 1 can undergo smoothly via TS1 to afford t BuOH and anionic intermediate 4 .…”

Section: Results and Discussionmentioning

confidence: 99%

“…The development of transition-metal-catalyzed acceptorless dehydrogenative or borrowing-hydrogen coupling of alcohols has also attracted the participation of computational mechanistic studies . For the Ru-catalyzed systems, great efforts were devoted to the dehydrogenation of alcohols, the dehydrogenative homo-coupling of alcohols to esters, and the dehydrogenative cross-coupling of alcohols with N-containing compounds to imines, α,β-unsaturated nitriles, alkylated amines, and amides . Herein, a DFT study on the Ru-catalyzed dehydrogenative cross-coupling of 1-arylethanol and secondary cyclic alcohols to β-disubstituted ketones was performed to elucidate the detailed mechanism and, more importantly, the origin of the selective cross-coupling manner.…”

Section: Introductionmentioning

confidence: 99%

Mechanism and Origin of Chemoselectivity of Ru-Catalyzed Cross-Coupling of Secondary Alcohols to β-Disubstituted Ketones

Liu

Tang

et al. 2020

J. Org. Chem.

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Ru-catalyzed cross-coupling of secondary alcohols with only byproducts H 2 and H 2 O provides a green synthetic strategy to prepare β-disubstituted ketones. Density functional theory (DFT) calculations were performed with the coupling of 1-phenylethanol and cyclohexanol as a model reaction to gain deeper mechanistic insights herein. The mechanistic details of the main reaction and the key steps of possible side reactions were clarified, and the obtained results are consistent with reported selectivity. Hydrogenation of α,β-unsaturated ketones and dehydrogenation of ruthenium hydride intermediate are direct chemoselectivity-determining stages. The hydrogenation via 1,4-addition generates more stable intermediates, being favored over that via 1,2-addition, and thus avoids the formation of alkene products. The conjugation and π−π stacking effects of phenyl and the weak electronic effect of alkyls explain the dominance of specific ketone products in the hydrogenation stage. Hydrogenation of ketone products is kinetically operative but not exergonic enough to stop the irreversible dihydrogen release in an open reaction system, and thus alcohol products are absent. Furthermore, water evaporation in aldol condensation is found to be a double-edged sword, as it can accelerate the hydrogenation stage to prevent α,β-unsaturated ketones from being the main products but decrease the selectivity therein from thermodynamics overall.

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N–Alkylation of Amines Catalyzed by a Ruthenium–Pincer Complex in the Presence of in situ Generated Sodium Alkoxide

Cited by 42 publications

References 171 publications

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

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

Mechanism and Origin of Chemoselectivity of Ru-Catalyzed Cross-Coupling of Secondary Alcohols to β-Disubstituted Ketones

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