Six-Membered Cyclopalladated Complexes of 2-Benzylpyridine

Cyclopalladated complexes provide a potential use for regiospecifically controlled organic syntheses. Various substituents can be introduced onto the palladated carbon position by reactions of cyclopalladated complexes with various reagents. Cyclopalladation of aryl-substituted nitrogen or phosphorus bases has been performed mainly by use of CPdCL] or PdCl2(NCPh)2 in the presence or absence of an acetate salt, but this method appears to have limit in applicability. Here, we describe a preparation of a rare six-membered cyclopalladated complex of 2-benzylpyridine by use of palladium(II) acetate, which seems to be a better starting material for syntheses of cyclopalladated species. Only five reports of six-membered cyclopalladated complexes have appeared.  

Palladium(II) acetate was purchased from Nippon Engelhard, and used without further purification. 2-Benzylpyridine (0.46 g, 2.72 mmol) is added to a solution of palladium(II) acetate (0. g, 2.67 mmol) in acetic acid (50 mL). This solution is stirred at 25 °C for 24 h affording a pale yellow precipitate. This precipitate is collected on a medium-porosity fritted glass filter and washed with water, methanol, and diethyl ether. Yield 0.80 g (89%), mp 268 °C (dec under air in a capillary tube). 

In a 50-mL round-bottomed flask containing a magnetic bar are placed bis(/i-acetato)bis[2-(2-pyridinylmethyl)phenyl-C Af]dipalladium(II) (see 

Section A) (0.30 g, 0.45 mmol) and 20 mL of acetone. To this suspension, an aqueous solution (10 mL) of lithium chloride (0.076 g, 1.8 mmol) is added. The reaction mixture is stirred for 2 days at room temperature. The resulting pale yellow solid is filtered through a medium-porosity fritted glass and washed with aqueous methanol (50%, 20 mL). Yield 0.25 g (90%). 

This complex is stable and almost insoluble in common organic solvents. It has a mp 248 °C (dec under air in a capillary tube). The complex reacts with styrene in refluxing m-xylene in the presence of triethylamine to form trans-2-[[2-(2-phenylethenyl)phenyl]methyl]pyridine in 44% yield.  

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