Igor Tkatchenko

Ionic Liquids: Media for Better Molecular Catalysis

Ionic liquids (ILs) are more and more suggested as substitutes for traditional solvents in organic synthesis and catalysis. They are suitable candidates for the dissolution of ionic complexes. They can activate and retain them in a polar state: in fact, they act as sequestrands, opening the route to two-phase processes and easier catalyst recovery. This paper reviews authors results with the development of new syntheses of ILs and applications in carbon–carbon bond formation (dimerisation of methyl acrylate) and redistribution (ring closing metathesis) reactions using ionic precatalysts.

Small particles in zeolites as selective catalysts for the hydrocondensation of carbon monoxide

Abstract Small metal particles in Y zeolite supercages can be prepared by the controlled thermal decomposition of binary metal carbonyls and their derivatives. The presence of such aggregates is clearly demonstrated in the case of ruthenium. Indirect evidence for iron and cobalt aggregates is gained from the study of the product distribution observed for the hydrocondensation of carbon monoxide. These metal-zeolite materials are shown to catalyse the selective formation of hydrocarbons in the C1-C9 range.

1,3-Dimethylimidazolium-2-carboxylate: the unexpected synthesis of an ionic liquid precursor and carbene-CO2 adduct

1,3-Dimethylimidazolium-2-carboxylate is formed in good yield, rather than the anticipated organic salt, 1,3-dimethylimidazolium methyl carbonate, as the reaction product resulting from both N-alkylation and C-carboxylation of 1-methylimidazole with dimethyl carbonate; the crystal structure of the zwitterion exhibits pi-stacked rings and two-dimensional sheets constructed by hydrogen-bonds from imidazolium-ring hydrogens to the carboxylate group.

Cationic cyclopalladated complexes: new catalyst precursors for the telomerization of butadiene with alcohols

Abstract The cationic cyclopalladated complexes derived from N,N-dimethylbenzylamine 4, N-benzylidene-(S)-(−)-α-methylbenzylamine 5 and 8-methyl quinoline 6 have been prepared and their properties as catalyst precursors for the telomerization of butadiene with methanol were investigated. The butadiene conversion was 60–70% for the three complexes and a mixture of butadiene dimers and telomers containing 2, 4 and 6 butadiene units were formed. The product selectivity is strongly influenced by the nature of the cyclopalladated complex. Thus, with complex 6 the C16 and C24 telomers were formed preferentially (more than 85%). On the other hand, with complex 4 the amount of butadiene dimers is greater than 40%. Complex 6 is also active for the telomerization of other aliphatic alcohols, but in these cases the 1- and 3-alkoxy octadiene telomers were produced preferentially.

Selective palladium-catalysed dimerisation of methyl acrylate in ionic liquids: towards a continuous processThis work was presented at the Green Solvents for Catalysis Meeting, held in Bruchsal, Germany, 13–16th October 2002.

The activity and stability of cationic palladium complexes [Pd(PBu3)2S2]2+ used for the selective tail-to-tail dimerisation of methyl acrylate are significantly improved with the utilisation of ionic liquids like [BMIM][BF4] or the protonated N-butyl-imidazole, [HBIM][BF4]. Problems related to product inhibition and catalyst recycling are overcome by running the reaction in a two-phase mode, toluene being used as extractant. Catalyst stabilisation is further improved by trapping the ancillary ligand into the ionic liquid with an ionic tail: with the use of 1-dibutylphosphino-2-dimethylaminoethane, the catalyst is stable for more than 100 h, therefore demonstrating the feasibility of a continuous process for methyl acrylate dimerisation.

Cationic benzyl nickel complexes as homogeneous catalysts for styrene oligomerization. X-ray crystal structure of [Ni(η3-CH2C6H5)(PPh3)2]PF6 · CH2CI2 *

Abstract New cationic complexes [Ni(η3-CH2C6H5)(PPh3)2]PF6 (3) and [Ni(η3-CH2C6H5)((+)-DIOP)]PF6 (4) were synthesized from the corresponding neutral compounds [Ni(η3-CH2C6H5)X(PR3)2] (X = Cl, Br), by metathetical halide abstraction with TIPF6. Complex 3 was characterized by X-ray crystallography and shown to have a distorted square-planar geometry with the benzylic group coordinated in a quasi-allylic fashion. Crystals are monoclinic, space group P21/n, a = 11.798(5), b = 19.251(2), c = 18.561(3) A, β = 93.03°, V = 4208.6 A3. Solution NMR studies showed benzyl fluxionality and phosphine lability in compound 3, but not in 4. The latter one is catalytically inactive towards styrene oligomerization, whereas 3 produces styrene oligomers ( M ¯ n ≈ 1000 ) with 67% isotactic content. Although [Ni(η3-CH2C6H5)CI(PCy3)] and trans[Ni(Cl)(H)(PCy3)2] are inactive, addition of TlPF6 induced catalytic activity with formation of highly isotactic (90%) styrene oligomers with similar M ¯ n and a different terminal group.

Carbonylation reactions 7. Regioselective synthesis of 2-arylpropionic acids by catalytic carbonylation of styrene derivatives in the presence of palladium compounds : the critical role of the counter anion

The regioselectivity of alkoxycarbonylation of 1-arylethanols catalysed by Pd/acid systems is very sensitive to the nature of the counter anion of the acid: chloride anion favours branched products while tetrafluoroborate and sulfonates anions favour linear products, the latter ones enhancing also product yields. Alkoxycarbonylation of styrenes is also sensitive to the nature of the counter anion.

Synthesis of (η3-allyl)(η4-cyclo-octa-1,5-diene) nickel cations: valuable catalysts for the oligomerization of alkenes

η n 3 n -Allylnickel cations stabilised by ‘labile’ ligands, obtained by metathetical exchange of η3-allylnickel halides with non-complexing anions in the presence of dienes or donating solvents, are efficient catalysts for the oligomerization of unsaturated hydrocarbons and lead to unusual selectivities and to new η3-allylnickel complexes which may be intermediates in these reactions.

Nickel complexes with heterofunctionalized phosphine ligands. Catalytic oligomerization of ethylene with [Ni(C5Ph5){Ph2PCHC(O)Ph}]

Reaction of [Ni(C5Ph5)Br(CO)] with Ph2PCH2C(O)Ph in the presence of 1,8-bis(dimethylamino)naphthalene (proton sponge) gives the chelate complex [Ni(C5Ph5){Ph2PCHC(O)Ph}]1. The complexes [Ni(C5Ph5){Ph2PCHC(O)(C5H4)Fe(C5H5)}]2, [Ni(C5Ph5){Ph2PCHC(O)(1,3-C6H4)C(O)HCPPh2}Ni(C5Ph5)]3, [Ni(C5H5){Ph2PCHC(O)Ph}]4 and [Ni(C5H5){Ph2PCHC(O)(C5H4)Fe(C5H5)}]5 have been synthesised using the appropriate phosphine ligand and applying a procedure similar to that for 1. Complex 1, activated with NaBH4, exhibits catalytic activity in the low-pressure oligomerization of ethylene. At 130 °C, under 38 bar of ethylene, selectivities of up to 98% towards linear α-olefins were achieved. Protonation of the bimetallic complex 2 with HBF4 gave the cationic complex [Ni(C5Ph5){Ph2PCH2C(O)(C5H4)Fe(C5H5)}]BF46. Crystais of 6 belong to the orthorhombic space group Pbca with a= 19.427(6), b= 38.595(11), c= 13.001(4)A and Z= 8. The structure was refined to R= 0.049 (R′= 0.067). The bonding of C5Ph5 contrasts with the bonding modes usually found for cyclopentadienyl ligands. The ring contains two short [1.384(8) and 1.389(8)], two medium [1.444(8) and 1.443(8)], and one long [1.474(8)A] C–C bonds so that the C5Ph5 may be viewed as an (alkyl, diene) ligand. This unusual bonding is likely to result from the ‘chemical’ dissymmetry of the P,O ligand and the low trans influence of the oxygen atom. The extremely low ν, (CO) frequency of the co-ordinated ketone (1525 cm–1) reflects the strong electron-withdrawing effect of the Ni(C5Ph5)+ moiety.

Stoicheiometric dimerisation of methyl acrylate mediated by Pd(acac)2·HBF4 systems and its relevance to the mechanism of catalytic dimerisation of acrylates (Hacac = MeCOCH2COMe)

Protonation of Pd(acac)2(Hacac = MeCOCH2COMe) by HBF4·OEt2 leads to ionic [Pd(acac)S2][BF4] complexes, isolated for S = MeCN, which react with methyl acrylate to give selectively linear dimers via a palladium(IV) metallocycle.