The new palladium complexes with NHC and phosphine ligands, cis-PdCl 2 (L 2 )(PPh 3 ) (2), cis-PdBr 2 (L 2 )(PPh 3 ) (3), and cis-PdCl 2 (L 3 )(PPh 3 ) (4) were prepared following a general protocol of a one pot reaction between PdCl 2 (COD), PPh 3 , and the ligand precursors LH 3 Y (Y=Cl, Br, BF 4 ) (L 2 = 1,3-dibenzylimidazolin-2-ylidene; L 3 =1,3-dibenzylimidazol-2-ylidene). The cis-PdCl 2 (L 3 )(PCy 3 ) complex (5) was prepared by the ligand substitution reaction between 4 and PCy 3 . The palladium complexes with NHC and pyridine complexes, trans-PdCl 2 (L)(py) (6: L = L 2 ; 7: L = L 3 ) were obtained by heating a mixture of PdCl 2 (COD) and LH 3 BF 4 in pyridine. A similar reaction condition using CH 3 CN as solvent with KO t Bu as base afforded cis-PdCl 2 (L 3 ) 2 (8). Complexes 2-8 were successfully characterized by X-ray crystallographic studies, among which, intriguingly, two polymorphs of 8 were obtained. Thermogravimetric analysis showed that the cis-PdX 2 (NHC)(PR 3 ) complexes are more thermally stable than the trans-PdCl 2 (NHC)(py) complexes. Together with the known cis-PdCl 2 (L 1 )(PCy 3 ) (1) (L 1 =1-benzyl-3-(N-phenylacetamido)imidazol-2-ylidene), they were screened for the direct arylation reaction between aryl halides and alkynes. The result indicate that the carbene/phosphine complexes 1-5 are superior precatalysts than 6-8 with higher activities than the commonly-used system of Pd(OAc) 2 /2PPh 3 .

An electrochemical dehydrogenative C−H/N−H cross coupling of imidazopyridines with diarylamines has been developed. A variety of triarylamine derivatives could be obtained in high regioselectivities and moderate‐to‐good yields. In this transformation, the reaction is conducted in a simple undivided cell without using a metal catalyst or a stoichiometric amount of external chemical oxidants. Mechanistic studies indicate that the C−N bond is likely to be formed through the cross coupling between an arene cation radical and a nitrogen radical.

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Electrochemical oxidative [4 + 2] annulation for the synthesis of pyrazines.