Reductions palladium® bromide

Soderberg and coworkers have developed a palladium-phosphine-catalyzed reductive iV-het-eroannuladon of 2-nitrostyrenes forming indoles in good yields For example, reaction of 6-bromo-2-nitrostyrene with carbon monoxide in the presence of a catalytic amount of palladium diacetate (6 mol% and triphenylphosphine 124 mol% in acetonitrile at 30 gives 4-bromoindole in 86% yield fEq 10 62 Several functional groups, such as esters, ethers, bromides, tnflates, and additional nitro groups, have been shown to be compatible with the reaction conditions... 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

If, instead of a palladium catalyst, a nickel catalyst, such as the bipyridylnickel(II) bromide, is used for the arylation of amines (Fig. 7), the reduction of the aryl halide into the corresponding aromatic hydrocarbon is still present for the primary or secondary benzylamines but, the arylation into substituted anilines is the main reaction even most often the only one, for the other types of amines. 

Palladium NHC systems for the hydrodehalogenation of aryl chlorides and bromides and polyhalogenated aromatic substrates originate from about the same time as the first reports on Ni chemistry, and show many similarities. Initial efforts showed that the combination of PdCdba) (2 mol%), one equivalent of imidazolium chloride and KOMe produced an effective system for the reduction of 4-chlorotolu-ene, especially upon use of SIMes HCl 2 (96% yield of toluene after 1 h at 100°C) [7]. Interestingly, higher ligand to metal ratios severely inhibited the catalysis with only 7% yield of toluene achieved in the same time in the presence of two equivalents of SIMes HCl 2. Variation of the metal source (Pd(OAc)2, Pd(CjHjCN)jClj), alkoxide (NaOMe, KO Bu, NaOH/ ec-BuOH) or imidazolium salt (IMes HCl 1, IPr HCl 3, lAd HCl, ICy HCl) all failed to give a more active catalyst. 

There are few reports of oxidative addition to zerovalent transition metals under mild conditions three reports involving group 10 elements have appeared. Fischer and Burger reported the preparation of aTT -allylpalladium complex by the reaction of palladium sponge with allyl bromide(63). The Grignard-type addition of allyl halides to aldehydes has been carried out by reacting allylic halides with cobalt or nickel metal prepared by reduction of cobalt or nickel halides with manganese/iron alloy-thiourea(64). 

Alkyl iodides, benzyl chlorides, benzyl bromides, and adamantyl bromides and iodides undergo reduction with triethylsilane/palladium chloride.195 The reduction of a /3-chloro ether occurs in excellent yield with this system (Eq. 56).195... 

Vinyl and Aryl Halides and Triflates. The organosilane reduction of aryl halides is possible in high yields with triethylsilane and palladium chloride.195 The reaction is equally successful with aryl chlorides, bromides, and iodides. Aryl bromides and iodides, but not chlorides, are reduced with PMHS/Pd(PPh3)4 in moderate to excellent yields.199 This system also reduces vinyl bromides.199 p-Chlorobenzophenone is reduced to benzophenone with yym-tetramethyldisilo-xane and Ni/C in excellent yield (Eq. 59).200 There is a report of the organosilane reduction of aryl and vinyl triflates in very high yields with the combination of Et3SiH/Pd(OAc)2/dppp (l,3-bis(diphenylphosphino)propane) (Eq. 60).201... 

The key intermediate 124 was prepared starting with tryptophyl bromide alkylation of 3-acetylpyridine, to give 128 in 95% yield (Fig. 37) [87]. Reduction of 128 with sodium dithionite under buffered (sodium bicarbonate) conditions lead to dihydropyridine 129, which could be cyclized to 130 upon treatment with methanolic HC1. Alternatively, 128 could be converted directly to 130 by sodium dithionite if the sodium bicarbonate was omitted. Oxidation with palladium on carbon produced pyridinium salt 131, which could then be reduced to 124 (as a mixture of isomers) upon reaction with sodium boro-hydride. Alternatively, direct reduction of 128 with sodium borohydride gave a mixture of compounds, from which cyclized derivative 132 could be isolated in 30% yield after column chromatography [88]. Reduction of 132 with lithium tri-f-butoxyaluminum hydride then gave 124 (once again as a mixture of isomers) in 90% yield. 

The Pd-catalyzed amination of / -rm-butylphenyl bromide with pyrrole in the presence of Pd(OAc)2, dppf and one equivalent of NaOr-Bu led to the Af-arylation product 88. A simplified version of the mechanism commences with the oxidative addition of p-te/t-butylphenyl bromide to Pd(0), giving rise to the palladium complex 89. Ligand exchange with pyrrole followed by deprotonation by the base (NaOr-Bu) results in amido complex 90. Reductive elimination of 90 then gives the amination product 88 with concomitant regeneration of Pd(0) catalyst. If the amine had a (3-hydride in amido complex 90, a (3-hydride elimination would be a competing pathway, although reductive elimination is faster than P-hydride elimination in most cases. 

With Pd(0) generated in situ, the oxidative addition of aryl bromide 102 to Pd(0) proceeds to form Pd(II) intermediate 104. Migratory insertion of 104 then occurs to furnish the cyclized indoline intermediate 105. Subsequent reductive elimination of 105 takes place in a cis fashion, giving rise to exo-cyclic olefin 107, which then tautomerizes spontaneously to the thermodynamically more stable indole 103. The reductive elimination by-product as a palladium hydride species 106 reacts with base, regenerating Pd(0) to close the catalytic cycle. 

In the early 1980s, one of the first preparations of substituted allenes was reported, which employed a palladium-catalyzed cross-coupling reaction of allenyl halides [9]. In this study, allenyl bromides 13 and various Grignard reagents 14 were coupled in the presence of catalytic amounts of a Pd(0) species, generated in situ by reduction of a Pd(II) salt. Trisubstituted allenes 15 were obtained with high regioselectivity (allene 15 alkyne 16 = 90 10 to 99 1) (Scheme 14.5). 

It has been proposed that in this reaction CO2 reacts as an electrophile with [ArPd (PPh3)2] formed by reduction of the aryl-palladium(II) [102]. Aryl chlorides react too slowly with Pd° to enable an efficient carboxylation reaction. On the other hand aryl triflate and aryl bromide have similar reactivity. The synthesis of aryl carboxylic acids can then be obtained from phenols via the formation of the corresponding aryltriflate (Eq. 15) [29, 30] ... 

Palladium metallic clusters have been prepared at room temperature by sonochemical reduction of Pd(OAc)2 and a surfactant, myristyltrimethylammonium bromide, in THE or MeOH [160[. It is noteworthy that nanosized amorphous Pd is obtained in THE, but in a crystalline form in MeOH. In this solvent, and in higher homologous alcohols, sonolysis of tetrachloropalladate(II) leads to Pd nanoclusters in which carbon atoms, formed by complete decomposition of the solvent, can diffuse. This results in an interstitial solid having the formula PdQ (0 < x < 0.15) [161]. Noble metal nanoparticles of Au, Pd, and Ag are obtained by sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilise the naked col-... 

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

Palladium(o) triphenylphosphine complexes catalyse the reduction of aryl bromides and iodides in a divided cell to give the diaryl [230]. The catalytic species... 

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