trans-Dichloro-bis(benzonitrile)palladium(II): A Versatile Reagent in Organic Synthesis

Biographical Sketches

Sulagna Brahma was born in Midnapur, India. She received her M.Sc. (Chemistry) from Vidyasagar University (India) in 2000. She is currently working towards her Ph.D. under the supervision of Prof. J. K. Ray at the Indian Institute of Technology Kharagpur ­(India). She received the "Young Scientist" award of the Indian Chemical Society in 2003. Her research interests include molecular recognition, host-guest interactions and synthesis of new receptors.

Introduction

The use of Pd(II) complexes as catalysts in organic reactions has been well-established since the beginnings of ­organic synthesis. Due to its versatility, trans-dichloro-bis(benzonitrile)palladium(II) receives special attention as activating agent and also as stabilizing agent. Recently, Khinast and his coworkers reacted ligands like (N-[3-(trimethoxysilyl)propyl]ethylenediamine, 2-(2¢-pyridyl)ethyltrimethoxysilane, etc. with Pd(PhCN)₂Cl₂ to prepare immobilized catalysts with improved activity in Suzuki coupling reactions. [1] The presence of polar groups in the ligand can increase the interaction of the metal complexes with a substrate which is able to improve enantioselectivity in asymmetric catalysis. [2] With this aim Condom et al. used Pd(PhCN)₂Cl₂ for the synthesis of the new asymmetric, water-soluble phosphine (S)-(-)-(3-diphenylphosphino-2-hydroxy-propyl)trimethylammonium chloride from the accessible (S)-(-)-(3-chloro-2-hydroxypropyl)tri­methylammonium chloride. [3]

Various fluorinated arene ligands based on diimine and diacetylpyridine backbones, synthesized using Pd(PhCN)₂Cl₂, serve as precursor compounds for the investigations of π -stacking interaction. Pd(PhCN)₂Cl₂ is suitable for single-crystal X-ray diffraction studies, obtained by the growth from solution in benzonitrile, as it readily loses benzonitrile to form the cubic cluster Pd₆Cl₁₂, which co-crystallizes with a variety of planar aromatic hydro­carbons. [4]

Pd(PhCN)₂Cl₂ appears to be a versatile activating agent for the alcoholic and epoxide functionality under un­usually mild conditions. [5] Another important aspect of this reagent is that it can be used as stabilizing agent for unstable azirine compounds via formation of 1:2 and 1:1 complexes. [6]

This reagent is generally prepared by condensation of PdCl₂ in benzonitrile and it is also commercially ­available.

Abstracts

(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. [7] 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-C6H4(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]. [8]
(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. [9]
(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. [10]
(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. [11]
(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)2Cl2 was the best palladium source for these couplings. [12]

References

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