[52522-40-4] · C52H43Cl3O3Pd2 · Tris(dibenzylideneacetone)dipalladium · (MW 1035.14)
(air-stable Pd0 complex used directly as homogeneous Pd0 catalyst with or without ligand)
Solubility: insol H2O; sol chloroform, benzene.
Form Supplied in: deep purple needle-like crystals; commercially available.
Preparative Methods: the reaction of Na2[Pd2Cl6] (from Palladium(II) Chloride and NaCl) and dibenzylideneacetone (dba) gives Bis(dibenzylideneacetone)palladium(0), [Pd(dba)2], which is formulated more correctly as [Pd2(dba)3]dba.1,3 Recrystallization from chloroform displaces the uncoordinating dba with chloroform to give the title reagent as deep purple needles.1
Many reactions are catalyzed by Pd0 catalysts.4 The Pd0 catalysts are prepared in two ways. One common method for generation of Pd0 species involves in situ reduction of PdII salts, most conveniently Palladium(II) Acetate. An alternative involves the use of air-stable, commercially available Pd2(dba)3 as a Pd0 catalyst with or without added phosphine. The dba ligand is displaced by the addition of phosphine ligands (eq 1). Some reactions catalyzed with and without added phosphine ligands are described below.
An intramolecular palladium-ene reaction that proceeds smoothly using Pd2(dba)3 in the presence of Triphenylphosphine is shown in eq 2.5
Silyl enol ethers react with allylic carbonates using Pd2(dba)3 and dppe (1,2-Bis(diphenylphosphino)ethane), yielding the allylated ketones under neutral conditions (eq 3).6
Sometimes good or different results are obtained when the title reagent is used without added phosphine ligands. Dibenzylideneacetone itself is a good ligand which effects the catalytic activity of the Pd0 differently than phosphines. The following Pd0-catalyzed intramolecular cyclization catalyzed by Pd2(dba)3 without added phosphine ligand is more selective than in the presence of the ligand (eq 4). The cyclization under the ligandless conditions affords (1) and (2) in ratios as high as 9:1. In addition, the cyclization conducted without phosphine ligands in the presence of Ag salts occurs with virtually complete selectivity to give (2).7
Coupling of an enol triflate with (Z)-1-propenyltri-n-butyltin proceeds smoothly using Pd2(dba)3 as a catalyst in the absence of a phosphine ligand (eq 5).8
Pd2(dba)3 is a good catalyst, but it is not always as good as other Pd0 catalysts. When Pd0-catalyzed reactions of congested molecules are carried out, use of Pd2(dba)3 is not recommended. For example, in the elimination reaction of a steroidal a-allylic carbonate, the ligand-free catalyst prepared from Pd(OAc)2/Bu3P (1:1) gives the homoannular conjugated diene regioselectively.9 No regioselectivity is observed with the Pd2(dba)3/Bu3P catalyst (eq 6). Also, Pd2(dba)3/Bu3P is less active than Pd(OAc)2/Bu3P in a provitamin D synthesis proceeding by the same elimination reaction of the cyclic allylic carbonates.10
Okayama University of Science, Japan