Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)
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.
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.
References (34)
Tetrahedron
(2003)- et al.
J. Organomet. Chem.
(2007) - et al.
Inorg. Chim. Acta
(2006) - et al.
J. Organomet. Chem.
(2006) - et al.
Tetrahedron Lett.
(1997) - et al.
Tetrahedron Lett.
(2007) - et al.
Tetrahedron Lett.
(2006) - et al.
J. Organomet. Chem.
(2009) - et al.
Tetrahedron
(2007) - et al.
Tetrahedron
(2005)