trans-dichloro-bis(benzonitrile)palladium(II): A versatile reagent in organic synthesis

Abstract

(A) Pd(PhCN)2Cl2 in combination with P(t-Bu) 3, indeed a highly active catalyst for Sonogashira couplings, provides a mild, efficient and general method for the reaction of aryl bromides at room temperature. Pd(PhCN)2Cl2/P(t-Bu)3 can even effect Sonogashira coupling of hindered aryl bromides. (B) Reaction of Pd(PhCN)2Cl2 with the ligand LL [3-C6H 4(CONMe-4-C5H4N)2] gave the neutral dipalladium(II) macrocycle trans,trans-[Pd2Cl4(μ-LL) 2], with a short separation between the palladium atoms of the lantern complex [Pd2(μ-LL)4]4+, which acts as a host for chloride ions and forms the complex [Pd2(μ-Cl)(μ- LL)4]3+, in which the distance between the palladium atoms is significantly larger [Pd-Pd = 6.56 Å]. (C) Miura's group reacted various aroyl chlorides (1) with styrene (2a) in the presence of the catalyst system Pd(PhCN)2Cl2/(PhCH2)Bu3NCl to give the corresponding stilbene derivatives 3a in satisfactory yields. The aroyl chlorides 1 also reacted with butyl acrylate (2b) to afford butyl (E)-cinnamate and its derivatives (3b) in 79-95% yields. (D) Pd(PhCN) 2Cl2 in the presence of polyvinylpyrrolidone (PVP) as a polymer support is an efficient catalyst for the direct synthesis of diphenyl carbonate (DPC) via oxidative carbonylation of phenol using carbon monoxide and air. This is the first example of the successful usage of polymer support in the oxidative carbonylation yielding DPC. (E) Recently Ray et al. used Pd(PhCN)2Cl2 to stabilize azirines containing an aldehyde functionality via the formation of 2:1 and 1:1 complexes. (G) The group of McNulty and Capretta described the synthesis of a small library of phosphorinanes and demonstrated their utility in cross-coupling chemistry using Pd(PhCN)2Cl2. The phosphorinanes allowed for modification of one of the alkyl moieties permitting steric and electronic fine-tuning of the ligands. In addition, optimization revealed that Pd(PhCN)2Cl 2 was the best palladium source for these couplings. © Georg Thieme Verlag Stuttgart.