Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

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Abstract

The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile manner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the precursors (3a–3e) with Pd(OAc)2 in refluxing HOAc, followed by treatment with LiCl at room temperature provided convenient access to the corresponding pincer palladium(II) complexes (4a–4e). The molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlorides with phenylboronic acid.

Graphical abstract

A series of unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors (3a–3e) have been easily synthesized in four steps starting from commercially available isophthalaldehyde. The corresponding pincer Pd complexes (4a–4e) were prepared via direct C2 palladation of the precursors and used as efficient catalysts for Suzuki reactions.

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Introduction

Palladium complexes with NCN pincer ligands have been extensively studied in recent years due to their high stability, feasible structural modifications, and remarkable catalytic activities in organometallic catalysis [1], [2], [3], [4]. Much of the research has focused on symmetrical NCN pincer palladium complexes, which are symmetrical with two identical N donors such as amines [5], [6], [7], imines [8], [9], pyridines [10], [11], [12], oxazolines [13], [14], [15], [16], [17], [18], [19], [20] or other N-containing heterocycles [21], [22] and 2 equiv. five-membered palladacycles (Scheme 1). Some of the complexes have been successfully applied as catalysts for stannylation of allyl [5] or propargylic substrates [6], Diels–Alder reaction [15], Heck [8], [10], [21], [22], Suzuki and Sonogashira [22] coupling reactions. And the chiral complexes proved to be effective in asymmetric Michael reaction between α-cyanocarboxylates and methyl vinyl ketone (up to 83% ee) [8], [17]. Following our interest in the metal pincer complexes and their applications, we recently reported the symmetrical chiral NCN pincer Pt(II) and Pd(II) complexes with 1,3-bis(2′-imidazolinyl)benzenes [23], [24] and particularly unsymmetrical PCN pincer Pd(II) complexes containing phosphinito group by one-pot phosphorylation/palladation reaction [25], [26]. We reasoned that different donors such as “hard” N and “soft” P in PCN pincers might provide a better tuning of the catalytic properties or give unique reactivity of the corresponding metal pincer complexes. In fact, some unsymmetrical pincer Pd complexes have been found to be much more active than the related symmetrical ones under certain circumstances [26], [27]. Herein, we would like to report a simple protocol for the synthesis of the unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors (3a–3e) and their corresponding Pd (II) derivatives (4a–4e) (Scheme 2). To the best of our knowledge, there are no reports concerning the preparation and use of unsymmetrical NCN pincer Pd(II) complexes. The Pd complexes (4a–4d) were unusual in that they not only had different N-heterocyclic donors, but also contained both five- and six-membered metallacycles in the molecules. The obtained Pd complexes were applied to the Suzuki reactions of aryl halides with phenylboronic acid.

Section snippets

Synthesis and characterization

The unsymmetrical NCN pincer ligand precursors 3a–3e were prepared from commercially available isophthalaldehyde in four steps as shown in Scheme 2. First, selective reduction of one aldehyde group in isophthalaldehyde by NaBH4 at 0 °C in MeOH readily afforded 3-(hydroxymethyl) benzaldehyde, which after bromination with PBr3 led to 3-(bromomethyl)benzaldehyde (1). Then nucleophilic substitution of 1 with 3,5-dimethylpyrazole, pyrazole or diethylamine in the presence of K2CO3 in DMF or dioxane

General

Compounds 1 [30], [31], 3,5-dimethylpyrazole [32], 2 [25], [33] and (S)-valinol [34] were prepared according to the literature methods. All the other reagents were used as commercial sources. Melting points were measured using a WC-1 microscopic apparatus and were uncorrected. IR spectra were collected on a Bruker VECTOR22 spectrophotometer in KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker DPX-400 spectrometer. Mass spectra were performed on the Agilent LC/MSD Trap XCT

Supplementary material

CCDC 748266 contains the supplementary crystallographic data for 4e. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgements

We are grateful to the National Science Foundation of China (Nos. 20872133, 20572102) and the Innovation Fund for Outstanding Scholar of Henan Province (074200510005) for financial support of this work.

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