Palladium dimerisation

Equations 12, 14 and 17 require the presence of H20. Thus H20 plays an important role in promoting the catalytic activity, but can also cause deactivation. Catalysis will be more efficient when all the reactions directly involved in the catalytic cycle are faster than the side reactions subtracting active species. Deactivation is related to the requirement of the palladium centre to have a vacant coordination site to ensure high catalytic activity. However, palladium tends to achieve the usual coordination number four, for example through dimerisation. Dimerisation/deactivation can be prevented by coordination of labile ligands, like H20, which acts also as an efficient hydride source. Also deprotonation leads to dimerisation/deactivation an acid can prevent it. 

These nickel- or palladium-catalyzed homocoupling reactions can also be conducted in the presence of zinc as reducing agent, leading usually to comparable results as shown recently in the palladium-catalyzed dimerisation of aryltrif-lates [30]. 

Thus, there are several different mechanisms involved in the nickel- or palladium-catalyzed dimerisation of aryl halides. Especially in the case of the Ni-bpy system, as indicated in Scheme 2, one of the two routes can become the more favorable by changing the experimental conditions, notably the cathode potential. 

Summaries on dimerisation and oligomerisation reactions in ionic liquids have been published previously.[3,4] Predominantly complexes of either palladium or nickel have been employed in ionic liquids for dimerisation/oligomerisation reactions, but there are also examples where iron,[5] tungsten1 and rhodium17 have been used. Apart from metal catalysed dimerisation/oligomerisation reactions in ionic liquids, examples of electrochemical dimerisation have been reported, which include arylhalidcs17 and 3-(4-fluorophenyl) tluophcnc1 as substrates. 

The dimerisation of methyl acrylate is an important reaction since the product, dimethyl dihydromuconate, represents a useful precursor for speciality chemicals such as cyclopentenones, as well as for the production of nylon-6,6. The synthetic challenge is to avoid formation of head-to-tail dimerisation products. The tail-to-tail dimerisation of methyl acrylate with palladium catalysts has been evaluated in protic and common imidazolium ionic liquids.17 21"231... 

The linear dimerisation of butadiene with palladium(II) catalyst precursors has been investigated in [C4Ciim]+ with a variety of different anions.[24] Observed turnover frequencies, which range from 37-49 mol mol h, are affected only slightly by the nature of the ionic liquid or catalyst precursor. Best activities were obtained with four equivalents of triphenylphosphine per palladium at a reaction temperature of 70°C. Contrary to the reaction in THF, no formation of metallic palladium was observed and reuse of the catalyst solution was possible. Pressurising the reaction mixture with 5-10 bar of carbon dioxide led to a decrease in reaction rates, which was explained by decreased substrate solubility in the C02-expanded ionic liquid. 

Tail-to-tail dimerisation of methyl acrylate has been investigated with a rhodium catalyst in protic and neutral imidazolium ionic liquids.171 While acidic ionic liquids showed some advantage with palladium catalysts (vide supra), reaction rates with Rh(Cp )(C2H4)2 as catalyst were much lower in [HC4im][BF4] and [HCiim][BF4] relative to those in [C4Ciim][BF4], It was suggested that this was due to coordination of imidazole to the rhodium centre. 

Head-to-tail dimerisation of methyl acrylate to the dimethyl ester of 2-methylenepentane-dioic acid (126) occurred in 82-85% yield in the presence of catalytic amounts of P(RNCH2CH2)3N with R = PP, Bu or Bz but the less sterically hindered proazaphosphatrane with R = Me, gave oligomer or pol-ymer. The proazaphosphatrane, P(RNCH2CH2)3N with R = Bu also acts as an effective ligand for the palladium-catalyzed direct arylation of ethyl cyano-acetate (127) with aryl bromides (e.g. 128) to form (129) in high yield. ... 

A further evidence on the acceleration enjoyed by a typical Pd-catalysed reaction, the Heck reaction, in an ionic phase ( V-mcthyl-Y.Y. V.-trioctylammonium chloride or Aliquat 336) is found in a triphasic protocol developed by Tundo and coworkers. 7b.The arylation of electron poor olefins is catalysed by palladium supported on charcoal (Pd/C) and is carried out in the heterogeneous isooctane/Aliquat 336/water system (Figure 27). Under this multiphasic condition, Aliquat 336 forms a third liquid phase between the organic and the aqueous phase that traps the catalyst. The use of phosphines is not necessary. As a matter of fact, Aliquat 336 incorporates the solid-supported catalyst and ensures an efficient mass transfer between the bulk phases resulting in an increase of the reaction rate of an order of magnitude compared to the reaction in the absence of the ionic liquid. A determing role is played by the base while I LN drives the reaction towards the formation of ethyl cinnamate, reaction carried out in the presence of KOH lead to formation of Ullmann dimerisation products. 

These dimerisation reactions of terminal alkynes have been further extended to the catalytic cyclisation of a,co-diynes. For example, treatment of 1,15-hexa-decadiyne with 10 mol% of 7a affords the endo-msLCTOcydic product, (Z)-l-cyclohexadecen-3-yne with complete stereoselectivity (Equation 5). This novel cyclisation is of particular utility, because synthetic routes to endo-cyc ic (Z)-l-en-3-ynes are extremely limited. A related palladium-catalysed cyclisation of a,co-diynes to give the corresponding exo-cyc c l-en-3-ynes has been reported by Trost and co-workers. 

MIBK) change resin with palladium/ nickel uses acetone and hydrogen as feed acetone is dimerised and dehydrated in the same column to give mesityl oxide. ... 

Palladium compounds are also useful catalysts for oligomerisation and co-oligomerisation reactions. Dimerisation or co-dimerisation of olefins catalysed by palladium(n) chloride or, better, PdCl2(C6H5CN)2 involves a palladium(ii)-hydride intermediate as the catalytically active species. A similar reaction to dimerisation is coupling, e.g. of benzene to give biphenyl, which is also catalysed by palladium(ii) chloride and proceeds via palladium-phenyl u-bonded species. The nature of intermediates in... 

Boronic acid derivatives are easily available and useful substrates especially in the palladium-catalysed Suzuki-Miyaura coupling reaction. In copper-catalysed reactions, they are also versatile starting materials. Dimerisation of arylboronic acids was first reported with copper acetate as a catalyst under ojgrgen at 100 Later, it was found that CuCl(OH)(phen) complex was much more efficient as a catalyst to afford dimer in high yields at 28 °C (Scheme 15.17). ... 

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