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Enhanced Reactivities toward Amines by Introducing an Imine Arm to the Pincer Ligand: Direct Coupling of Two Amines To Form an Imine Without Oxidant Li-Peng He,†,‡,∥ Tao Chen,†,‡,∥ Dirong Gong,†,‡ Zhiping Lai,†,§ and Kuo-Wei Huang*,†,‡ †
Division of Chemicals and Life Sciences and Engineering, ‡KAUST Catalysis Center, and §Advanced Membranes and Porous Material Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: Dehydrogenative homocoupling of primary alcohols to form esters and coupling of amines to form imines was accomplished using a class of novel pincer ruthenium complexes. The reactivities of the ruthenium pincer complexes for the direct coupling of amines to form imines were enhanced by introducing an imine arm to the pincer ligand. Selective oxidation of benzylamines to imines was achieved using aniline derivatives as the substrate and solvent.
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occurs with the deprotonation/protonation of the methylene arm during the O−H and H−H bond activation events.5 In comparison to C−H bonds, N−H bonds are in general stronger yet more acidic,6 and therefore, replacement of the CH2 group with an NH spacer may favor the deproronation/ dearomatization of the pincer ligand and offer different reactivities. We have recently developed a new class of pincer complexes based on the phosphinoaminopyridine pincer ligands (complexes 1−3, Scheme 1) and demonstrated that
mines are an important class of versatile synthetic intermediates for the preparation of nitrogen-containing compounds, many of which are useful building blocks and reagents for biological and pharmaceutical applications.1 While imines can be prepared by the condensation of ketones or aldehydes and amines in the presence of an acid catalyst, the oxidation of amines to imines offers a direct and alternative strategy and has received increasing attention.2 Extensive studies have been carried out in the development of mild, selective, and green oxidation procedures for the synthesis of imines from primary and secondary amines.2 In these reactions, however, the use of stoichiometric oxidants is necessary. In principle, an imine can be achieved by dehydrogenating an amine molecule, which could eliminate the formation of waste from oxidants and allow the further collection and utilization of the valuable hydrogen byproduct, but in contrast to alcohol dehydrogenation, analogous work for amines is scarce.3 Herein, we demonstrate the enhanced reactivities of the ruthenium pincer complexes for the direct coupling of two amines to form an imine by introducing an imine arm to the pincer ligand.
Scheme 1. Ruthenium Catalysts 1−4
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RESULTS AND DISCUSSION Pyridine-based pincer transition-metal complexes are of particular interest, due to their reactivitiy and flexibility for modification. Studies on the applications of complexes of PNP (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine) and PNN (PNN = 2-(di-tert-butylphosphinomethyl)-6(diethylaminomethyl)pyridine) ligands have revealed their diverse activities in catalyzing a variety of organic transformations, such as dehydrogenative homocoupling of primary alcohols to form esters, dehydrogenative coupling of alcohols and amines to form amides, and hydrogenation of esters, amides, carbonates, etc.3b,4 These reactions are believed to proceed via the metal−ligand cooperation mechanism, where the dearomatization/aromatization of the pyridinyl group © 2012 American Chemical Society
these dearomatized ruthenium complexes display efficient catalytic activities in the transfer hydrogenation of ketones in isopropyl alcohol.7 When one of the strong and bulky phosphine donors in 1 was replaced by a weaker and smaller oxazoline donor in 2, the catalytic activity in transfer hydrogenation was significantly increased, indicating that the Received: May 16, 2012 Published: July 9, 2012 5208
dx.doi.org/10.1021/om300422v | Organometallics 2012, 31, 5208−5211
Organometallics
Note
high conversion and yields (entries 16 and 17). This methodology offers a straight strategy for the selective oxidation of benzylamines to the corresponding imines under the base- and oxidant-free conditions. Since the Milstein catalyst is capable of catalyzing the dehydrogenative coupling of amines and alcohols to amides,4a complexes 1−3 were also studied using 1-hexanol and benzylamine (eq 1). While it has been demonstrated that a
reactivity of the Ru center is sensitive to the ligand environment.8 With these novel complexes, we first studied their catalytic activities for the well-known transformation of alcohols to esters via a dehydrogenative homocoupling reaction. An initial test was carried using 1-hexanol as the model substrate with 0.1 mol % catalyst loading, as shown in Table 1. It was found that Table 1. Esterification of Primary Alcohol with Ruthenium Complexesa
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number of ruthenium/ligand combinations show great activity in this type of dehydrogenative amide generation,9 it was rather surprising that our ruthenium complexes only afforded low yields for the amide products (
