Palladium-catalyzed trimerization

Hexa(oligophenyl)benzenes (e. g. 31 or 33) present one possible approach to the realization of this aim. Two efficient synthetic routes have been elaborated for the preparation of hexa(terphenyl)- and hexa(quaterphenyl)benzene. The first, involving palladium-catalyzed trimerization of diarylacetylenes [54] as the key step, was demonstrated by the synthesis of a hexakis-alkylated hexa(terphenyl)benzene derivative 31 from the corresponding bis(terphenyl) acetylene (32). The peripheral tert-alkyl substituents serve to solubilize the molecule. 

Palladium-catalyzed trimerization of alkynes has been developed, " but simple terminal alkynes undergo dimerization to form enynes. A mechanism for the formation of head-o-tail enynes has been proposed that proceeds through palladium(iv) complexes 202 or 203. Probably, however, the acidic terminal alkyne will cleave the palladium-alkenyl bond to give the enyne product and an alkynylpalladium(ii) species that can enter a new catalytic cycle instead." ... 

Scheme 2.86 Palladium-catalyzed trimerization of diynes 2.254a-c.

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. 

The palladium-catalyzed cyclotrimerization of arynes can also be used to construct flat hydrocarbons with extended conjugation, such as hexabenzotri-naphthylene (supertriphenylene, 87). In this case the required aryne 86 was generated in the presence of palladium by treatment of triflate 85 with tetra-butylammonium fluoride, and afforded trimer 87 in 20% isolated yield from 85 (Scheme 18) [51]. As in other cases (see below), the use of a soluble fluoride source was necessary because the extended planar reaction product is extremely... 

V. Gevorgyan, U. Radhakrishnan, A. Takeda, M. Rubina, M. Rubin, Y. Yamamoto, J. Org. Chem. 2001, 66, 2835-2841. Palladium-catalyzed highly chemo- and regioselective formal [2+2+2] sequential cycloaddition of alkynes a renaissance of the well known trimerization reaction ... 

In palladium-catalyzed [2 - - 2 - - 2] cycloaddition, arynes can be used as alkyne components. In 1994, Pena, Romero, and co-workers reported palladium-catalyzed homo-[2- -2- -2] cycloaddition of arynes (Scheme 6.9) [12]. The reactions of 2-silylaryl trifluoromethanesulfonates 26 with CsF generated the corresponding arynes 27, which were trimerized in the presence of a catalytic amount of Pd(PPh3)4 or Pd2(dba)3 to afford substituted triphenylenes 28. In the reactions of unsymmetric arynes, moderate to high regioselectivities were observed. A mechanism via pallada-cycle intermediates, generated through the oxidative cyclization of two molecules of arynes, was proposed. 

Interestingly, under the same reaction conditions the benzannulation reaction did not take place in the case of electron-deficient substrates such as 36, providing the corresponding alkyne trimerization product 37 instead (Scheme 14.16). These results highlight the difference between cobalt- and palladium-catalyzed benzannulation reactions, where electron-poor enynes were the most reactive substrates for the [4 - - 2] benzannulation reaction. 

The palladium-catalyzed [4- -2]-benzannulation of diynes by enynes, such as the [2- -2- -2]-trimerization sequence, is accelerated in the presence of a combination of a Lewis acid/phosphine or by a Bronsted base [11]. This allows the reaction conditions to be modified and to increase the number of substrates used and the reaction yield, especially for the synthesis of problematic penta-substituted benzenes. One of the features of the reaction of [4-1-2]-benzannulation is that it requires the presence of an activating group (AG, alkenyl or alkynyl) in the enynophile (2.216, Scheme 2.74). Development of this trend has led to the discovery of three different series of alkynyl trimerization in the presence of Pd(0) catalyst. A formal [2- -2- -2]-trimerization passes through reductive coupling of two identical (R = R, = H) or different R and R groups... 

Palladium-catalyzed coupling of the 2-biphenylmagnesium bromide to 9,9-bis(2-bromopropenyl)fluorene which was prepared from fluorene and 2,3-dibromopropene followed by an acid catalyzed (intramolecular) Friedel-Crafts alkylation led to a trimer. However, this method has a limitation that it can only produce a short chain, not polymer but up to tetramer. 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

In high polarity solvents, such as acetonitrile, dimethyl sulfoxide, methyl acetate, and methanol, the branched dimer, 4-methyl azelate precursor, is formed in high yield. In methanol, a methoxy dimer (CH302CCgH] 4(0013)0020113) is also formed in moderate yield. Heavies contain both an acyclic methyl, 4-pentadienoate trimeric product and high molecular weight methyl, 4-pentadienoate homopolymer. Polymerization in the absence of air(48) appears to be catalyzed by traces of the tertiary phosphine which is used to prepare the palladium dimerization catalyst. 

The trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

The vinyl ester exchange has also been studied in the chloride-free palladium(II) acetate system, using vinyl propionate as substrate. Of the three Pd(II) species present in this system (Section II, A, 2), the dimer is most reactive, with the trimer Pd3(OAc)g next, and the monomer unreac-tive. The rate expression for exchange catalyzed by the dimer is (214). 

Alkyne cyclotrimerization occurs at various homogeneous and heterogeneous transition metal and Ziegler-type catalysts [7], Substituted benzenes have been prepared in the presence of iron, cobalt, and nickel carbonyls [8] as well as trialkyl- and triarylchromium compounds [9]. Bis(acrylonitrile)nickel [10] and bis(benzonitrile)palladium chloride [11] catalyze the cyclotrimerization of tolane to hexaphenylbenzene. NiCl2 reduced by NaBH4 has been utilized for the trimer-ization of 3-hexyne to hexaethylbenzene [12]. Ta2Cl6(tetrahydrothiophene)3 and Nb2Cl6(tetrahydrothiophene)3 as well as 7 -Ind-, and 77 -Ru-rhodium... 

Unlike nickel catalysts which form cyclic dimers and trimers (1,5-cyclooctadiene and 1,5,9-cyclododecatriene), palladium compounds catalyze linear dimerization of conjugated dienes. 1,3-Butadiene itself is converted to 1,3,7-octatriene. The reaction most characteristic of palladium is the formation of various telomers. 1,3-Buta-diene dimerizes with incorporation of various nucleophiles to form telomers of the following type ... 

A single example is known for a palladium(0)-catalyzed cotrimerization reaction involving two molecules of an alkene and a methylenecyclopropane molecule, leading to the formation of a seven-membered-ring product.The reaction is limited to unsubstituted allene and, along with the cyclotrimer, l,3,5-tris(methylene)cycloheptane (3), 1,3-bis(methylene)cyclopen-tane (2), the product of a [3 + 2] cycloaddition, is also obtained. The product ratio 2/3 is markedly dependent on the catalyst composition. Additionally, the allene trimer, 1,2,4-tris(methyl-ene)cyclohexane (4), and the methylenecyclopropane homodimer, 5-methylenespiro[2.4]hep-tane (5), are formed. 

Fig. 23. Catalytic formation of benzene molecules on different palladiiun cluster sizes by the temperature-programmed-reaction (TPR) experiment. The benzene was produced through the cyclo trimerization of acetylene catalyzed by size-selected palladium clusters, Pdni supported on a MgO film. (Adapted from Ref. 35.)...

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

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