Palladium-catalyzed dimerization

It is apparent from mechanistic considerations that an active species in the palladium-catalyzed dimerization of butadiene is a zero-valent palladium complex, which forms bis-ir-allylic complex 20. 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Palladium-catalyzed dimerization of arylselenocarboxamides 274 gave 3,5-diaryl-l,2,4-selenadiazoles 79 under mild reaction conditions (Scheme 37) <2002JOM274>. Treatment of arylselenocarboxamides 274 with aqueous... 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

Scheme 5.7.17 Palladium-catalyzed dimerization-carbostannylation ofalkynes...

Scheme 5.7.18 Proposed mechanism of the palladium-catalyzed dimerization-carbostannylation of alkynes...

Palladium-catalyzed dimerization of 2-arylpyridines by employing oxone as the terminal oxidant has been reported [75], The reaction was shown to proceed via a Pd(II)-Pd(TV) pathway. A method for cross-coupling of simple arenes with 2-arylpyridine derivatives by using catalytic palladium acetate in the presence of two equivalents of silver carbonate requires use of a large excess of the less reactive arene [76],... 

The palladium-catalyzed dimerization of methyl acrylate was found to proceed smoothly in a [BMIM][BF4]/scC02 biphasic system [42] (BMIM = l-butyl-3-methyl-imidazolium). The catalytic system comprises phosphonium salts and HBF4 as cocatalysts, making the IL environment particularly attractive. The partitioning of substrates and products between the IL and scC02 was investigated and the data were used for planning of the reaction conditions. Turnover numbers up to 560 and turnover frequencies up to 195 h-1 were obtained under optimized conditions. 

Precursors for the synthesis of ( + )- and (—)-citronellol (2) are enantioselectively prepared via palladium-catalyzed dimerization of isoprene in the presence of methanol which gives ( )-8-methoxy-3,7-dimelhyl-l,6-octadiene (1) in up to 93 % yield with optical purities of up to 35% when 71-allylpalladium chloride dimer, with the addition of optically active phosphane ligands derived from menthol, is used as catalyst31,32. 

Later, Ballivet-Tkatchanko and coworkers compared the palladium catalyzed dimerization of methyl acrylate in ionic liquids (monophasic) and IL/SCCO2 (biphasic) [20]. They found that both reactions exhibited similar selectivities for tail-to-tail... 

In the palladium catalyzed dimerization of mono substituted allenes in DMF a mixture of the linear dimers 33 and 34 is obtained in 39-83 % yields. ... 

Scheme 18 Palladium-catalyzed homo-dimerization of 2(llf)-pyrazinones...

In another reductive coupling, substituted alkenes (CH2=CH Y Y = R, COOMe, OAc, CN, etc.) can be dimerized to substituted alkanes (CH3CHYCHYCH3) by photolysis in an H2 atmosphere, using Hg as a photosensitizer. Still another procedure involves palladium-catalyzed addition of vinylic halides to triple bonds to give 1,3-dienes. ... 

The palladium-catalyzed formation of sulfides can generate polyphenylene sulfide from a dithiol and a dibromoarene, or from 4-bromobenzenethiol (Equation (38)).17 In 1984 Asahi Glass obtained patents for the formation of this polymer in the presence of palladium and nickel catalysts.125,126 In addition, Gingras reported palladium-catalyzed couplings of aryl halides and thiols to form discrete phenylene sulfide oligomers.127,128 A number of polyphenylene sulfide wires, ranging from dimeric to pentameric structures, were prepared by the palladium coupling, albeit in modest yields ... 

An unexpected dimerization product was obtained in 30% yield during the palladium-catalyzed hydrogenation of 2-(2-pyridylmethylene)-3(2H)-benzo[b]furan-3-one. A couple of similar types of substrates were investigated, indicating the scope of the substrates was limited... 

The main path of the palladium-catalyzed reaction of butadiene is the dimerization. However, the trimerization to form /j-1, 3,6,10-dodeca-tetraene takes place with certain palladium complexes in the absence of a phosphine ligand. Medema and van Helden observed, while studying the insertion reaction of butadiene to 7r-allylpalladium chloride and acetate (32, 37), that the reaction of butadiene in benzene solution at 50°C using 7r-allylpalladium acetate as a catalyst yielded w-1,3,6,10-dodecatetraene (27) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 hours. 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

The same type of bis-functionalization has been reported for the palladium-catalyzed borylstannylative carbocy-cyclization of 1,6-, 1,5-, 1,7-diynes, bis-propargylamine, and ether.377 It should be noted that even 1,2-dialkylidene cyclobutane can be obtained in reasonable yield. Ito has proposed the related silaborative reaction involving nickel(O) catalysis.378 This reaction has been performed in an intra- and intermolecular fashion. The intramolecular reaction allows the formation of cyclic dienes and the intermolecular process proceeds through a dimerization of alkynes to give acyclic dienes. 

Palladium-catalyzed cyclic carboxylation of dienes can be utilized for the synthesis of lactones.2 Polymer-supported Pd catalyst could also be used for this reaction (Scheme 42).61 The reaction is initiated by dimerization of two molecules of diene to give a bis-7r-allylpalladium intermediate such as 123. The incorporation of C02 takes place at the internal position of an allyl unit to afford the 7r-allylpalladium carboxylate 124 which, after reductive elimination/ cyclization, yields the (5-lactone 121 (Scheme 43). 

The transition metal-catalyzed domino reactions will undoubtedly have a splendid future which is underlined by the increasing number of publications in this area Steglich et al. presented an approach to arcyriacyanin A via a domino Heck reaction between a bromo(indolyl)maleiimide and 4-bromoindole.1811 The synthesis of 33 -bifurans by Ling et al. was achieved using a novel palladium-catalyzed domino dimerization and subsequent cyclization of acetylenic ketones.1821 Other applications of a combination of Heck reactions for domino processes are the syntheses of aza-heterocydes developed by our group.1831... 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

In addition to the aforementioned total syntheses. Shannon et al. observed the formation of an N-C3-linked dimer during the transformation of a 3-bromocarbazole to a 3-cyanocarbazole by reaction with copper(I) cyanide in DMF under reflux (668). Harrity et al. reported the synthesis of non-natural (+ )-N,N -dimethylbismurrayafoline A via a chromium-mediated benzannulation, followed by a palladium-catalyzed oxidative coupling reaction (669). 

Yamamoto and coworkers utilized the resultant product from the palladium catalyzed reaction of l-fluoro-2-iodobenzene with the trifluorovinylzinc reagent to prepare a tri-fluorovinylimide, which on thermal dimerization at 140-160 °C gave predominantly the -cyclobutane, whose structure was established by X-ray crystallography (Scheme 4)60. 

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