Premium
N–Alkylation of Amines Catalyzed by a Ruthenium–Pincer Complex in the Presence of in situ Generated Sodium Alkoxide
Author(s) -
Das Kanu,
Nandi Pran Gobinda,
Islam Khadimul,
Srivastava Hemant Kumar,
Kumar Akshai
Publication year - 2019
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201901310
Subject(s) - chemistry , medicinal chemistry , alkoxide , alkylation , catalysis , pincer movement , ruthenium , alcohol , methanol , organic chemistry
We report the use of ruthenium–NNN‐pincer complexes of the type ( R2 NNN)RuCl 2 (PPh 3 ) (R = t Bu, i Pr, Cy and Ph) for the catalytic N ‐alkylation of primary amines under solvent‐free conditions. For the first time, the base that is required to promote these reactions is generated in situ from the alcohol by the use of sodium. The resulting sodium alkoxide regenerates the alcohol substrate while acting as the water scavenger thus mitigating the need of an additional base. Among the catalysts screened, ( t Bu2 NNN)RuCl 2 (PPh 3 ) (0.02 mol‐%) gives very high turnovers and good yields at 140 °C. The ( t Bu2 NNN)RuCl 2 (PPh 3 ) catalyzed N ‐alkylation tolerates a variety of amine and alcohol substrates. While excellent turnover (29000) was obtained for the ( t Bu2 NNN)RuCl 2 (PPh 3 ) (0.002 mol‐%) catalyzed alkylation of aniline with cyclohexyl methanol, the turnovers obtained in the corresponding catalytic methylation of p ‐anisidine was also very high (12000). The ( t Bu2 NNN)RuCl 2 (PPh 3 ) catalyzed reactions have also been accomplished under open‐vessel conditions resulting in a net dehydrogenative coupling reaction. This protocol has been used to transform benzene‐1,2‐diamines to benzimidazoles with high productivity (12000 turnovers). DFT studies indicate that while β‐hydride elimination is rate‐determining (RDTS: 24.31 kcal/mol) for the alcohol dehydrogenation segment which is endothermic, insertion of the imine is rate‐determining (RDTS: 11.26 kcal/mol) for its hydrogenation that is exothermic.