Allyl palladium II Complexes

Submitted by YOSHITAKA TATSUNO, TOSHIKATSU YOSHIDA, and SEIOTSUKA  

A 200-mL, two-necked, round-bottomed flask equipped with a magnetic stirring bar, a gas inlet tube, and a condenser topped with a bubbler is charged with an aqueous solution of palladium(ll) chloride (4.4 4 g, 25 mmole) and sodium chloride (2.95 g, 50 mmole) in 10 mL of H20, followed by methanol (60 mL) and allyl chloride (3-chloro-l-propene) (6.0 g, 67 mmole). Carbon monoxide is passed slowly (2-2.5 L/hr) under stirring through the reddish-brown solution by way of a gas-inlet tube for 1 hour. The bright yellow suspension thus obtained is poured into water (300 mL) and extracted with chloroform (2 X 

100 mL). The extract is washed with water (2 X 150-mL portions) and dried over calcium chloride. Evaporation under reduced pressure (20 torr) gives yellow crystals. Yield 4.3 g (93%). The crude product can be used without further purification. The analytically pure sample can be obtained by recrystallization from a mixture of dichlorome thane/hexane. The compound decomposes at 155-156° And. Calcd. for C6H10Cl2Pd2 C, 19.49 H, 2.73. Found C, 19.60 H, 2.75. 

The air-stable, yellow, crystalline compound is soluble in benzene, chloroform, acetone, and methanol. The H NMR spectrum (CDC13) shows two doublets at 5 3.03 (anti CH2, J = 12.0 Hz) and 4.10 ppm (syn CH2, J = 7.1 Hz), and a triplet at 5 5.48 ppm (CH) in a relative ratio of 2 2 1. A variety of reactions of this compound are summarized in several reviews.3 

A tetrahydrofuran (THF) solution of sodium cyclopentadienide4 is prepared by adding freshly distilled cyclopentadiene5 to a sodium suspension in THF. The concentration of the resulting pale-pink solution can be determined by titration with acid. 

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Allenyl alcohols 10 react with lithium bromide in the presence of a palladium(II) catalyst to afford tetrahydrofurans and tetrahydropyrans 11 in good yield (Scheme 17.6) [7]. The mechanism of the reaction is similar to that discussed in Sect 17.2.1. i.e. it proceeds via a 2-bromo(jt-allyl)palladium(II) complex. In this case, however, the second nucleophile is not bromide ion but the alcohol moiety. As stoichiometric oxidant p-benzoquinonc (BQ) or copper(II) together with oxygen can be used. 

Allyl esters, carbonates, and carbamates readily undergo C-O bond cleavage upon reaction with palladium(O) to yield allyl palladium(II) complexes. These complexes are electrophilic and can react with nucleophiles to form products of allylic nucleophilic substitution. Linkers based on this reaction have been designed, which are cleavable by treatment with catalytic amounts of palladium complexes [165,166], For the immobilization of carboxylic acids, support-bound allyl alcohols have proven suitable (Figure 3.12, Table 3.7). 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

Palladium-catalyzed reductive silylation has also been reported. The cathodic reduction of allylic acetates in the presence of silylating agents and a catalytic amount of (Ph3P)4Pd [53] gives the corresponding allylsilanes. The initially formed rr-allyl-palladium(II) complex seems to be reduced at the cathode to generate the allyl anion intermediate, which reacts with chlorosilane to give the final product. 

Ammonium formate hydrogenolysis of 7t-allyl-palladium(II) complexes formed via oxidative addition of Pd(0) to propargylic carbonate esters is a useful method for preparing disubstituted alkynes, as depicted below. This method of deoxygenation avoids the use of expensive and toxic reagents often associated with the free-radical Barton-McCombie deoxygenation. °... 

The generally accepted mechanism of palladium-catalyzed allylic substitutions is shown in Scheme 1. An allylic substrate 1, typically an acetate or a carbonate, reacts with the catalyst, which enters the catalytic cycle at the Pd(0) oxidation level. Both Pd(0) and Pd(II) complexes can be used as precatalysts, because Pd(II) is easily reduced in situ to the active Pd(0) form. Presumably, the reaction is initiated by formation of a Ji-complex which eliminates X to produce an (ri -allyl)palladium(II) complex. The product of this oxidative addition can... 

In contrast to the processes based on the external attack of a nucleophile on the coordinated CO or olefin ligands on Pd(II) species, where re-oxidation of the Pd(0) produced to reactive Pd(II) presents a considerable problem, no such problem is involved in reaction of a Pd(0) complex with allylic substrates. As we have already discussed in Schemes 1.9 and 1.10, allylic compounds such as allylic acetates or carbonates readily oxidatively add to Pd(0) species to form 7 -allyl palladium(II) complexes that are susceptible to nucleophilic attack. The catalytic process converting allylic substrates to produce allylation products of nucleophiles has found extensive uses in organic synthesis, notably in the work of Tsuji and Trost. Employment of a chiral ligand in the catalytic allylation of nucleophiles allows catalytic asymmetric synthesis of allylation... 

Once oxidative addition of allyhc carboxylate occurs to form r/ -allyl-palladium(II) complex, both a- and y-carbons of the allyhc moiety are susceptible to attack of various nucleophiles to give products, where allylation usually takes place at the less-hindered side of the -allylic ligand (Scheme 3.25). 

When phosphine-free di-valent complexes such as PdCl2 [98] or [PdCl] OTf [99] are employed for the reaction with allyl alcohol, dimeric (rj -allyl)palladium(II) complex is formed in good yield (Eq. 3.28). 

An attempt has been made to predict the sites of nucleophilic attack on [M(CO)3(fl--hydrocarbon)] complexes using the perturbation theory of reactivity. For the model allyl substrate [Co(CO)3( j -C3H5)] the site preference CO > M > C3H5 was predicted for reaction with hard nucleophiles in polar solvents. On the other hand, with soft nucleophiles initial attack at the ir-allyl ligand was favored. Mechanistic studies have suggested only a small energy difference between attack by alkoxide ions on the allyl ligand and the metal in related ( Tr-allyl) palladium(II) complexes. ... 

A variety of 3-vinyl-substituted imidazo[l,5-a]indole derivatives were synthesized by intramolecular Pd catalyzed cyclization of the indole-2-carboxylic acid al-lenamides through either a domino carbopalladation/exo-cyclization process or a novel hydroamination reaction that proceeds smoothly under microwave irradiation. Both the observed pathways involve a Tu-allyl-palladium (II) complex arising from insertion of the allene group into a palladium (II) species, the latter being formed in situ by the intervention of an aryl iodide or of the N-H group. In both these cases, the role of nucleophile is covered by the indole nitrogen (Beccalli et al., 2010). 

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