Palladium-catalysed reactions reductive

Now the alkene must be moved yet one more time around the ring to prepare the way for another intramolecular Heck reaction. Hydroboration (chapter 17) of 153 is regioselective because of the large N-Boc group and Swem oxidation completes the insertion of the ketone 155. Reduction and elimination use another palladium-catalysed reaction. Conversion to the triflate 157 is followed by Pd-catalysed transfer hydrogenation, the H atom coming from formic acid HC02H. 

These palladium-catalysed reactions involve insertion of the palladium into the carbon-bromine or carbon-triflate bond, reaction with the organometallic species (alkenyl stannane in a Stille reaction, arylboronic acid in a Suzuki reaction, or alkynyl copper species in a Sonogashira reaction) and reductive elimination to give the products shown below. 

The use of transition-metal catalysts in the substitution of aryl halides by amines has been reviewed. There has also been a summary of the use of palladium catalysts in the reaction of aryl and heteroaryl halides with primary and secondary amines. It has been shown that the amination of aryl sulfamates by aliphatic amines may be achieved using a nickel catalyst with an A-heterocyclic carbene ligand. Suitable ligands for nickel and palladium in the catalysed amination reactions of aryl sulfamates and imidazolylsulfonates have been identified. " The palladium-catalysed reaction of aryl nonafluorobutanesulfonates with primary sulfonamides may yield substituted products such as (22). Kinetic data suggest that reductive elimination from the palladium intermediate is likely to be rate limiting. A-Arylmethanesulfonamides may also be formed from aryl bromides and chlorides using a palladium catalyst... 

The acylation of arenes with alcohols has been shown to be possible using a palladium chloride catalyst in the presence of f-butylhydroperoxide. In 2-arylpyridines, substitution is directed to the ortho-position and, after initial paUadation, the formation of intermediate (59) is likely before reductive elimination yields the acylated product. The regioselective acetoxylation of indoles, at the 3-position, has been achieved using the palladium-catalysed reaction with phenyliodonium acetate. 3-Acyl indoles may also be prepared using acetyl chlorides with zirconium tetrachloride as a Lewis acid catalyst. 

The coupling of anilines with arylboronic acids has been described in a process catalysed by palladium acetate without the need for ligands, bases, or salts. The mechanism shown in Scheme 17, where S represents solvent, involves in situ formation of the dia-zonium salt from the aniline, and formation of an arylpalladium alkoxo complex which allows the transmetallation step with arylboronic acids. It has been shown that a free amine may be used as a DG in a palladium-catalysed reaction promoted by soluble silver salts. The latter aid the formation of intermediates, (136), which may undergo cyclopalladation followed by transmetallation with an arylboronic acid and reductive elimination. Related work has shown thatbiaryl-2-amines may react with aryl iodides in the presence of palladium acetate and silver acetate to give mono- or di-arylated species such as (137). 

Several carbon-hydrogen bond substitutions are involved in the palladium-catalysed reaction of arylsulfonic acids with arenes to yield aromatic sulfones. A plausible mechanism, shown in Scheme 9, involves the initial formation of the palladacycle (99) which, after eoupling with the arene, yields the biphenyl derivative (100). Further coordination and carbon-hydrogen activation gives the seven-membered palladacycle (101) that affords the sulfone, (102), after reductive elimination. ... 

Coordination of palladium with the adjacent pyridine ring is likely to facilitate the acylation of 2-arylpyridines with ct-diketones. In the presence of t-butyl hydroperoxide (TBHP), cleavage of the diketones yields radicals that may allow the formation of intermediates such as (115), which yield the acylated products after reductive elimination. Interestingly the ortfto-aroylation of 2-aryl pyridines may also be achieved using the palladium-catalysed reaction with toluene in the presence of TBHP. Here the mechanism is likely to involve initial benzylation followed by oxidation at the benzylic position to give the acylated product. Acetanilides may also be acylated at the ort/jo-position using toluene and TBHP to yield products such as (116). Here a possible mechanism involves initial formation of a cyclopalladated intermediate followed by reaction with an acyl radical formed by oxidation of the toluene. ... 

The palladium-catalysed reaction of the pyrazolo-pyrimidine derivative (141) with 3-bromotoluene may result in arylation at the 3-position in the pyrazole ring or at an sp hybridized site in the 7-methyl side-chain depending on the base and ligands used. After initial insertion of the palladium catalyst into the aryl halide bond, palladation of (141) occurs by a concerted metalation-deprotonation pathway and is followed by reductive elimination. Concerted metalation-deprotonation is also likely in the palladium-acetate-catalysed reaction of imidazo[l,2-a]pyridines with aryl bromides to give 3-substituted derivatives such as (142). A careful mechanistic study of the arylation of pyridine A-oxide by bromotoluene, catalysed by palladium acetate and t-butylphosphine, has shown that direct reactions of an aryl palladium complex with... 

It was found that sorbed palladium might catalyse reaction of Mn(III) reduction by Cf not only after it s removing from coal, but AC with palladium, Pd/AC, has also his own catalytic effect. On the base of dependence between characteristics of AC, chemical state of palladium on AC surface and catalytic action of Pd/AC in indicator reaction it might establish, that catalytic action concerns only to non-reduced or partly reduced palladium ions connected with chloride ions on coal surface. The presence or absence of catalytic action of Pd/AC in above-mentioned reaction may be proposed for determination of chemical state of palladium on AC surface. Catalytic effect was also used for palladium micro-amounts determination by soi ption-catalytic method. 

Kuroda and Tarui [498] developed a spectrophotometric method for molybdenum based on the fact that MoVI catalyses the reduction of ferric iron by divalent tin ions. The plot of initial reaction rate constant versus molybdenum concentration is rectilinear in the range 0.01-0.3 mg/1 molybdenum. Several elements interfere, namely, titanium, rhenium, palladium, platinum, gold, arsenic, selenium, and tellurium. 

Other Addition Reactions.—At variance with previous reports, the palladium-catalysed reduction of 9a-fluoro-ll/3-hydroxy-A -and-A -corticosteroids proceeded stereospecifically to give the 5/8-isomers. The 9a-fluorine atom appears to be responsible for an increased folding of ring A towards the a-face, thus exposing the -face to the catalyst. 

Indole derivatives might also be prepared by the palladium catalysed reductive heteroannulation of o-nitrostyrene derivatives. The bicyclic olefin, shown in 3.75. was converted to the indole derivative in good yield.94 The reaction, which was run under forcing conditions utilises carbon monoxide as the reducing agent. 

A palladium-catalysed reduction of ethyl thiolesters to aldehydes with triethylsilane has been proposed by Fukuyama [2611. The reaction, coupled with a mild conversion of carboxylic acids to thiolesters (see [261], footnote 2), affords a versatile method for the overall transformation of... 

Thus the alkyl ester end groups of convergently grown poly(benzyl ether) dendrimers can be subjected to post-synthetic modification by hydrolysis [28], reduction [29], transesterification/amidation [28] in a variety of ways. Subsequent modification of poly(benzyl ether) dendrons bearing p-bromobenzyl end groups by palladium-catalysed coupling reactions permitted preparation of dendrons with phenyl, pyridinyl, or thiophenyl end groups [30]. 

An electrophilic palladation by a phenyl palladium intermediate at C(3) and a C(3) to C(2) migration of a palladium species, followed by reductive elimination, is indicated. 2-Phenylpyridine has been formed by the reaction of pyridine and iodobenzene at 150 °C in the presence of phosphido-bridged ruthenium dimer complexes.49 A catalytic cycle involving one of the complexes in the system was proposed. Optimum conditions for the efficient and regioselective palladium-catalysed C(2) arylation of ethyl 4-oxazolecarboxylate (47) with iodobenzene have been presented.50... 

Reductive coupling of iodonium salts catalysed by a palladium-zinc system also produced biaryls in good yield [38]. Also very effective was the palladium-catalysed cross-coupling of iodonium salts with sodium tetraphenylborate in water [39]. The reaction of 3-indolyl phenyliodonium trifluoroacetate with several alkyl and aryl lithium reagents gave 3-substituted indoles [40] ... 

The additional adamantyl substituent on the phenol moiety was introduced by an acid catalysed reaction with 1-adamantol prior to the reduction step. The significance of this ligand is that it stabilises a Pd-alkyl group cis to the NHC ligand in a palladium(II) carbene complex. These transition metal carbene complexes with a cis alkyl ligand are still rare... 

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