Palladium(II) Acetate [Pd(OAc)₂]: A Versatile Catalyst

Biographical Sketches

Rakesh Kumar Vats was born in Rindhana Village (Gohana), ­Haryana, India in 1976. He received his Bachelor of Pharmacy (B.Pharm.) degree from M. D. University, Rohtak (Haryana), India. He obtained M.Tech. (Pharmacy) from National Institute of ­Pharmaceutical Education and Research (NIPER), Mohali, Punjab. Currently he is working as Senior Research Fellow (CSIR-SRF) towards his Ph.D. under the supervision of Prof. Uma Ramachandran at NIPER. His area of research includes synthesis of novel bioactive heterocyclic compounds especially PPAR agonists using organometallics in environmentally benign conditions.

Introduction

Palladium(II) acetate [Pd(OAc)₂] [CAS: 3375-31-3] is a commercially available reagent, which is stable and soluble in organic solvents. It has a melting point of 195 °C (dec.). It can be prepared from metallic palladium by dissolving in acetic acid containing nitric acid. It may contain nitrate anion as impurity. Pd(OAc)₂ is purified by dissolving it in hot benzene and concentrating the benzene solution after removing the insoluble part. Pure Pd(OAc)₂ can be obtained as needle-like crystals by recrystallization.

Palladium(II) acetate is used for oxidative addition, insertion, transmetalation and reductive elimination reactions. It is used for allylic oxidation (acetoxylation), e.g. oxidation of cyclohexene to 2-acetoxycyclohexene. Unsaturated aldehydes can be elongated by one carbon atom. Silyl enols undergo transmetalation followed by intramolecular alkene insertion and β-elimination. Acetoxybenzene is prepared by reaction of benzene with Pd(OAc)₂. This is a useful method for phenol production from benzene. Pd(OAc)₂ is widely used in the presence of phosphine ligand and as a base in Heck reaction, for coupling aryl or vinyl halides with alkenes. [1] In the presence of TBAB, it catalyses direct homocoupling of aryl halides. [2] It is also used to improve Wacker oxidation of terminal alkenes to 2-alkanones with p-benzoquinone, which improves the reaction rate 50-fold. [3] It is efficient in ligandless Suzuki cross-coupling of aryl boronic acids with aryl iodides. [4] A stoichiometric quantity is required in Buchwald-Hartwig reaction of C-N bond formation. Selective reduction of alkynes is catalyzed by Pd(OAc)₂ with NaOMe [5] and reduction of aryl/enol triflates by this catalyst is reported. [6] Pd(OAc)₂ was microencapsulated in polyurea for making it reusable and recoverable catalyst for hydrogenation. [7]

Abstracts

(A) Oxygenation of unactivated sp3 C-H bonds can be achieved with Pd(OAc)2 and PhI(OAc)2 as oxidant. The unactivated sp3 C-H bonds of both oxime and pyridine substrates undergo highly regio- and chemoselective oxygenation. [8]
(B) Preparation of benzolactams by Pd(OAc)2 catalyzes direct ­aromatic carbonylation in an atmosphere of CO gas with air. [9]
(C) Distannylation of strained C-C triple bonds is catalyzed by Pd(OAc)2. Oxidative addition of a distannane to a palladium complex occurs during the distannylation of in situ generated arynes with distannanes in the presence of catalytic amount of Pd/tert-octyl isocyanide (t-OcNC) complex to give 1,2-distannylarenes in moderate to high yields. [10] The synthesis of highly substituted 1,3-butadienes by arylation of internal alkynes can be achieved with the help of Pd(OAc)2. [11]
(D) Pd(OAc)2-catalyzed functionalization of indoles with 2-acetoxy methyl substituted electron deficient alkenes was performed under neutral conditions. This protocol is very efficient and may open new area for functionalization of indoles. [12] Pd(OAc)2-catalyzed tandem Heck and aldol reaction between 2-bromobenzaldehyde and functionalized alkenes leads to naph­thalene. [13]
(E) Pd(OAc)2 catalyzes multiple arylation via successive C-C and C-H bond cleavages. These unprecendented reactions seems to provide useful information for designing new catalytic cycles that occur via C-C and C-H cleavages. [14]
(F) The asymmetric desymmetrization of prochiral meso compounds represents a powerful strategy for the expedient synthesis of two or more contiguous stereogenic centres in one operation. Pd(OAc)2 provides the desired keto acid in good yield and enantio­meric excess. [15]
(G) Pd(OAc)2 in pyridine is developed as an effective catalyst for highly regioselective hydroselenation of alkynes. [16]
(H) The α-arylation of methanesulfonamides using phosphine ligands and NaOt-Bu as a base was also achieved with Pd(OAc)2. [17]

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