Palladium dibenzylideneacetone

For the palladium dibenzylideneacetone complex (1), NMR data to support the proposition that the bis-phosphine acts as a bidentate ligand has been reported. A triflate salt of the TT-allyl palladium complex has been isolated and is stable in the solid state. However, no crystals suitable for X-ray analysis were obtained. An X-ray crystal structure of the ligand and a bis-palladium complex has been reported. The palladium complexes are generated just before use under an inert atmosphere exposure to air affords a catalytically inactive tetra-coordinated palladium(II) species. ... 

In some cases, quite stable palladium(O) complexes such as palladium dibenzylideneacetone (Pd(dba)2, Pd2(dba)3, or Pd2(dba)3 CHCI3) can be utilized advantageously, especially when the substrate is sensitive to oxidation. 

Phase-transfer hydrocarboxylation of terminal acetylenes 254 (R = Bu, pentyl, Ph or PhCH2CH2) with carbon monoxide, catalysed by nickel(II) cyanide, affords the alkenoic acids 255 Similarly, phenylacetylene reacts with carbon monoxide and alcohols in the presence of palladium(dibenzylideneacetone)2 to give esters of unsaturated acids, e.g. 256192. 

A reported diastereoselective synthesis of precursor A of vitamin D3 involved the use of 2-methylcyclopent-2-enone as starting material. The Mukaiyama-Michael conjugate addition of ketene acetal 269 in the presence of trityl hexachloroaniimonate afforded the adduct 270. The lateral chain was introduced, according the procedure of Tsuji, by the treatment of crude 270 with allyl carbonate and palladium dibenzylideneacetone " (Scheme 63). The expected product 271 was obtained in 63% yield from 269. Reduction of 271 with LAH afforded a mixture of diols that was selectively tosylated at the primary hydroxy group. The secondary hydroxy group was protected with the methoxymethyl group and further functional modifications afforded the lactone 272. The reaction of lithium dimethyl methylphosphonate with the lactone 272 completed the synthesis of the AB-des-cholestane derivative 273. 

Pd(dba)2 was prepared by the literature procedure 3 palladium chloride (1.05 g, 5.92 mmol) was added to hot (ca. 50°C) methanol (150 mL) containing dibenzylideneacetone (DBA) (4.60 g, 19.6 mmol) and sodium acetate (3.90 g, 47.5 mmol). The mixture was stirred for 4 hr at 40°C to give a reddish-purple precipitate and allowed to cool to complete the precipitation. The precipitate was isolated by filtration, washed successively with water and acetone, and dried in vacuo. 

B. (E)-4-(2-Phenylethenyl)benzonitrile (2). An oven-dried, 250-mL, three-necked, round-bottomed flask equipped with an argon inlet adapter, rubber septum, glass stopper, and a teflon-coated magnetic stir bar is cooled to room temperature under a flow of argon. The flask is charged successively with bis (tri-tert-butylphosphinc)palladium [(Pd(P(t-Bu)3)2] (0.238 g, 0.466 mmol, 1.5 mol% Pd) (Notes 1, 2), tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (0.213 g, 0.233 mmol, 1.5 mol% Pd) (Note 3), and... 

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

Raboisson et al. <2002JOC8063> elaborated a new general approach to pyrazolo[l,5- ][l,3,5]triazine-based C-nucleosides 140 by application of palladium-mediated cross-coupling reaction. The reaction of 139 was carried out by using bis(dibenzylideneacetone)Pd(0), triphenylarsine, and triethylamine to give the product 140 in high yield (75%). 

The complex is obtained in high yield by reaction of dimethyl acetylenedicarbox-ylate with bis(dibenzylideneacetone)palladium. The complex is only slightly soluble in the usual solvents. 

Scheme 18.7 Synthesis ofthe insect pheromone 12 by palladium-catalyzed SN2 -substitution (dba = dibenzylideneacetone).

The palladium nanoparticle is prepared from the reaction of the stabilizer, 4,4 -bis(perfluorooctyl)dibenzylideneacetone with palladium(II) chloride. The average size of the nanoparticle varied according the ratio of PdCF to the stabilizer, but was typically around 4 or 5 nm. The initial yield observed in the Suzuki coupling reaction was 90%, but decreased to 78% after five consecutive runs. Fluorous boronates (alternative precursors in Suzuki reactions), have also been developed for use in fluorous biphasic processes [12], A generic structure of a fluorous boronate is shown in Figure 10.2. 

Buchwald has shown that, in combination with palladium(II) acetate or Pd2(dba)3 [tris(dibenzylideneacetone)dipalladium], the Merrifield resin-bound electron-rich dialkylphosphinobiphenyl ligand (45) (Scheme 4.29) forms the active polymer-supported catalysts for amination and Suzuki reactions [121]. Inactivated aryl iodides, bromides, or even chlorides can be employed as substrates in these reactions. The catalyst derived from ligand (45) and a palladium source can be recycled for both amination and Suzuki reactions without addition of palladium. 

Cyclocarbonylation of o-iodophenols 503 with isocyanates or carbodiimides and carbon monoxide in the presence of a catalytic amount of a palladium catalyst (tris(dibenzylideneacetone)dipalladium(O) Pd2(DBA)3) and l,4-bis(di-phenylphosphino)butane (dppb) resulted in formation of l,3-benzoxazine-2,4-diones 504 or 2-imino-l,3-benzoxazin-4-ones 505 (Scheme 94). The product yields were dependent on the nature of the substrate, the catalyst, the solvent, the base, and the phosphine ligand. The reactions of o-iodophenols with unsymmetrical carbodiimides bearing an alkyl and an aryl substituent afforded 2-alkylimino-3-aryl-l,3-benzoxazin-4-ones 505 in a completely regioselective manner <1999JOC9194>. On the palladium-catalyzed cyclocarbonylation of o-iodoanilines with acyl chlorides and carbon monoxide, 2-substituted-4f/-3,l-benzoxazin-4-ones were obtained <19990L1619>. 

The use of soluble metal catalysts makes it possible to react 8 and its substituted derivatives with aryl bromides and triflates at 100 °C. The catalyst systems that have been used are Pd2(dba>3 and ( )BINAP with calcium carbonate as base (dba = dibenz[ ,. ]anthracene, BINAP = 2,2-bis(diphenyl-phosphanyl)-l,l-binaphthyl)<2000JA2178>, and 2-(di-r-butylphosphino)biphenyltris(dibenzylideneacetone)palladium with sodium yt-butoxide as base <2003T3109>. 

The choice of catalyst is important, for instance the use of tetrakis(triphenylphos-phane)palladium(O) complex results in the quantitative cyclotrimerization of 3,3-dimethylcy-elopropene.17 In similar fashion 3,3-dimethoxycyclopropene cyciodimerizes to 3 (R = OMe, 74%) using bis(dibenzylideneacetone)paUadium(0) [Pd(dba)2] complex.18 The trisubstituted cyclopropene 4 is transformed to the head-to-head dimer 5 in the presence of copper(I) iodide.19... 

Other reactions of dienes with metal atoms are only of a limited synthetic use. Dibenzylideneacetone (PhCH=CH—CO—CH=CHPh DBA) reacts with palladium vapor to afford Pd2(DBA)3, a complex in which the coordination is through the two C=C units and does not involve the C=0 (5, 92). Cobalt vapor undergoes an extremely complicated reaction with 1,4-pentadiene, producing pentenes, C5H6, and various polymers as well as the organometallic product, HCo( 1,3-pen tadiene)2, which involves isomerization from a nonconjugated to a conjugated diene (104, 110). 

In certain cases, when the palladium or nickel catalyzed coupling is not efficient or fails completely, an alternate solution is provided by the use of copper based catalyst systems. The 5-iodouracil derivative shown in 7.77. was unreactive towards imidazole using either the Buchwald-Hartwig conditions or the copper(I) triflate promoted the carbon-nitrogen bond formation reported by Buchwald98 These latter conditions, however, were effective in coupling the iodouracil with a series of other amines (7.77.), The optimal catalyst system consisted of copper(I) triflate, phenantroline and dibenzylideneacetone (dba).99... 

Highly selective transformation of terminal acetylenes to either linear or branched carboxylic acids or esters may be achieved by appropriately selected catalyst systems. Branched esters are formed with high selectivity when the acetylenes are reacted with 1-butanol by the catalyst system Pd(dba)2/PPh3/TsOH (dba = dibenzylideneacetone) or palladium complexes containing PPh3. Pd(acac)2 in combination with various N- and O-containing phosphines and methanesulfonic acid is also an efficient catalyst for the alkoxycarbonylation of 1-alkynes to yield the branched product with almost complete selectivity.307,308... 

Palladium(II) chloride (Johnson Matthey) and 10% Pd/C (Aldrich) were used as received. Bis[(methallyl)chloropalladium(II)] (ref. 8), bis(dibenzylideneacetone)palladium(0) (ref. 9), metho-xyoctadienes (ref. 10), l-methoxy-3-octene (ref. 11), methoxyallyle and cis + traru-methoxycrotyle (ref. 12) were prepared as described in the literature. 

The phase-transfer method has also been employed for the carbonylation of benzylic halides to carboxylic acids.477 The palladium(O) complexes [Pd(PPh3)4] (103), [Pd(diphos)2] (104) and [Pd(DBA)2] (105 DBA = dibenzylideneacetone) were used as catalysts. With (103) and (104) the carboxylic acid was the major product. Complex (105) gave little or none of the acid, the toluene and bibenzyl derivatives corresponding to the benzyl halide used being formed. Benzyl esters of the carboxylic acid were sometimes present as minor products. The reaction has been adapted to provide a new synthesis of anthranilic acid derivatives (equation 106).478 Tri-n-butylamine was used to neutralize the HBr formed. 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

---