Article pubs.acs.org/Organometallics
A Bis(phosphaethenyl)pyridine Complex of Iridium(I): Synthesis and Catalytic Application to N‑Alkylation of Amines with Alcohols Yung-Hung Chang,† Yumiko Nakajima,†,‡ and Fumiyuki Ozawa*,† †
International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ‡ JST-PRESTO, Uji, Kyoto 611-0011, Japan S Supporting Information *
ABSTRACT: The iridium(I) complex [IrCl(BPEP-H)] (1), coordinated with 2,6-bis[2-(2,4,6-tri-tert-butylphenyl)-2phosphaethenyl]pyridine (BPEP-H) as a PNP-pincer-type phosphaalkene ligand, has been synthesized and fully characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction analysis. Complex 1 (1 mol %) catalyzes N-alkylation of primary and secondary amines with alcohols, leading to the selective formation of secondary and tertiary amines, respectively. Primary amines are smoothly alkylated with a variety of benzylic and aliphatic alcohols (1 or 3 equiv) at 100 °C under basic conditions (CsOH, 10 mol %) to give the corresponding secondary amines in good to high yields. On the other hand, Nalkylation of secondary amines with benzyl alcohol (3 equiv) proceeds in the presence of KH2PO4 (5 mol %) at 140 °C to afford tertiary amines in high yields.
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INTRODUCTION There has been considerable interest in the coordination chemistry of low-coordinate phosphorus compounds, owing to their unique ligand properties.1,2 Phosphaalkenes having a P C bond are among the central subjects in such chemistry.2,3 They possess an extremely low lying π* orbital around the phosphorus atom and thus exhibit a marked tendency to engage in effective π back-bonding with transition metals. We have demonstrated with bidentate diphosphinidenecyclobutene ligands (DPCB-Y) that this property is useful for catalysis, leading to highly efficient organic transformations in conjunction with group 8−10 metals.2c,d Representative examples include hydroamination of 1,3-dienes (Pd),4 dehydrative allylation with allylic alcohols (Pd),5 and Z-selective hydrosilylation of alkynes (Ru).6 Our recent efforts have been devoted to the coordination chemistry of PNP-pincer type phosphaalkene ligands (BPEPY),7,8 in which two phosphaethenyl groups are connected to the 2,6-positions of a pyridine ring (Chart 1).9 The PNP-pincer ligands of this type have been extensively studied for phosphine
analogues and proven to be a valuable structural scaffold for developing functional transition-metal complexes.10 We have considered that the meridional coordination of the pincer ligands can be utilized as a useful structural motif also in phosphaalkene chemistry. Thus, in this geometry, the two P C bonds in BPEP-Y adopt a parallel orientation to the coordination plane, suitable to undergo a dπ−pπ interaction with central metals. We have confirmed the occurrence of such orbital interactions in Fe(I) and Cu(I) complexes.7a−c This paper reports the synthesis of [IrCl(BPEP-H)] (1) and its catalytic application to N-alkylation of amines with alcohols. This catalysis has attracted a great deal of attention in recent years as an environmentally benign process of synthesizing amines, which produces water as the sole byproduct.11 While both heterogeneous and homogeneous catalysts have been known to promote the reaction,11b ruthenium12−14 and iridium15−19 complexes have constituted a vast majority of the homogeneous catalysts because of their relatively high catalytic performance with high product selectivity.11c We have found that complex 1 exhibits catalytic activity comparable to that of those complexes.
Chart 1. BPEP-Y Ligands
RESULTS AND DISCUSSION Synthesis and Characterization of [IrCl(BPEP-H)] (1). The reaction of BPEP-H with 1/2 mol of [Ir(μ-Cl)(coe)2]2 (coe = cyclooctene) in CH2Cl2 afforded [IrCl(BPEP-H)] (1), which
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Received: January 28, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/om4000743 | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
Table 1. Effects of Bases on N-Alkylation of Aniline with Benzyl Alcohol Catalyzed by 1a
was isolated as a green crystalline solid in 61% yield and characterized by elemental analysis, NMR spectroscopy, and Xray diffraction analysis (eq 1). Complex 1 is stable under a nitrogen atmosphere and can be stored at ambient temperature without decomposition.
The NMR data observed in CD2Cl2 at room temperature were consistent with a PNP-pincer-type square-planar structure with C2v symmetry. The two phosphorus atoms were observed equivalently at δ 249.1 in the 31P NMR spectrum; the chemical shift is in a typical range for phosphaalkene ligands. The 1H NMR spectrum exhibited two singlet signals assignable to ortho and para t-Bu groups of BPEP-H at δ 1.71 (36 H) and 1.39 (18 H), respectively. Figure 1 depicts the X-ray structure of 1, which adopts a slightly distorted square planar geometry; the sum of the four
entry
base (amt, mol %)
conversn, %b
yield, %c
1 2 3 4 5 6 7
none K2CO3 (30) KOAc (30) K3PO4 (30) KO(t-Bu) (30) KOH (30) CsOH (10)
6 15 23 52 67 81 100
