Dimerisation catalysis

G. Lefebvre and Y. Chauvin Dimerisation and codimerisation of olefinic compounds by coordination catalysis, pp. 108-203. 

When dicarbene complexes of the form 21 were tested for 1-bntene or propene dimerisation, npon activation with AfEt Cl or MAO in tolnene, rapid deactivation took place yielding Ni(0) [25]. It was shown that this decomposition did indeed involve carbene-hydride and carbene-aUcyl rednctive elimination. Some dimerisation was evident at -15°C (TON = 50), however decomposition of the intermediate Ni species seemed too rapid for effective catalysis. In contrast, when the complexes were... 

A bottle of analytical grade material exploded in laboratory storage at 25°C (undoubtedly from internal pressure of carbon dioxide). Pure material, protected from light and air, is only stable on a long term basis if kept refrigerated. Otherwise slow decomposition and decarboxylation occurs [1], possibly accelerated by enzymic catalysis from ingress of airborne yeasts. At ambient temperature, the acid dimerises and dehydrates to 2-oxo-4-carboxyvalerolactone [2],... 

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. 

The first class includes non-redox reactions like isomerisation, dimerisation or oligomerisation of unsaturated compounds, in which the role of the catalyst lies in governing the kinetic and the selectivity of thermodynamically feasible processes. Electrochemistry associated to transition metal catalysis has been first used for that purpose, as a convenient alternative to the usual methods to generate in situ low-valent species which are not easily prepared and/or handled [3]. These reactions are not, however, typical electrochemical syntheses since they are not faradaic they will not be discussed in this review. 

The reductive dimerisation of organic halides is, to our knowledge, the first reported reaction involving both a faradaic electroreduction and a catalysis by... 

One of the most attractive features of the IL/CO2 approach to homogeneous catalysis is the development of continuous processes [7]. Consequently it needs to be demonstrated that the combination of a suitable IL and compressed CO2 can offer more potential for process optimisation than just a simple protocol for batch-wise catalyst recycling. As an example we were able to activate, tune and immobilise Ni catalyst 13 in a continuous-flow system for the hydroviny-lation of styrene (Scheme 3). Styrene is co-dimerised with ethene yielding 3-substituted 1-butenes [26,27]. We could show that this powerful carbon-carbon bond-forming reaction can be achieved with high enantioselectivity in batch-wise operation and in continuous-flow systems. 

The cycloaddition of ketenes to carbonyl compounds also shows the expected regioselectivity. Both HOMO,keI() c/LlJMO(kctcnc) and LUMO(ketone)/ HOMO(ketcnc) interactions may be important, but they lead to the same conclusions about regioselectivity. Lewis acid catalysis is commonly employed in this reaction presumably the Lewis acid lowers the energy of the LUMO of the ketene (or that of the ketone) in the same way that it does with dienophiles. Ketenes also dimerise with ease, since they are carbonyl compounds. The regiochemistry, whether it is forming a /3-lactone 6.256, 6.257 or a 1,3-cyclobutanedione 6.258, is that expected from the frontier orbitals of Fig. 6.39. 

The first example for biphasic oligomerisation of olefins in ionic liquids was published in 1990, reporting on the dimerisation of propene by nickel(II) catalysts in chloroaluminate ionic liquids of the general formula [cation]Clx-(AlCl3)y with either [C4Ciim]+, [C4py]+ or [(C4)4P]+ as cation.[10] It was found that in basic ionic liquids, y < 0.5, no catalysis took place. Excess chloride, which is present in such basic chloroaluminates, poisons the catalyst and it was shown that nickel compounds of the type NiCkCPRok... 

The description of the steric properties of phosphanes using the Tolman cone angle [113] proved to be an excellent concept capable of explaining many phenomena in the coordination chemistry of phosphanes and their applications, especially in homogenous catalysis. That there is a steric influence connected with NHC was noticed very early, in fact it was thought that the steric hindrance introduced by the N-mesityl substituents was a contributing factor in the isolation of the first stable carbene in 1991 as opposed to dimerisation to the known tetraaminoethylenes [1],... 

In a related context, 2-(p-toluenesulfonyl)ethylamine was used as an ammonia equivalent in an aza-ene reaction by which aldehyde 92.1 was converted to the bicycle 92.2 [Scheme 8.92]. After N-acetylation and Pd(0)-catalysed hydro-stannylation, the alkenylstannane 92.4 dimerised under copper(II) nitrate catalysis. The 2-(p-toluenesulfonyl)ethyl group was then discharged by -elimination with potassium rerf-butoxide. 

Some data are available about catalysis in 1,2-cycloadditions. Tributyl phosphine catalyses dimerisation of phenyl isocyanate to uretidinedione in toluene . The reaction is kinetically of first order with respect to catalyst and overall third order the reverse process is first order with respect to catalyst and overall second order. The mechanism is complex, as revealed by the value of the apparent activation energy of the forward reaction (E= l.l 0.7 kcal.mole" ), which presumably results from the combined temperature dependence of two or more steps, including formation of an isocyanate-phosphine complex (see eqn. (13), p. 113). 

Butadiene dimerisation is very sensitive to catalysis by zero-valent nickel complexes, which can direct the reaction towards 1,2-cycloaddition or (4-1-4) cycloaddition, through a bis-7r-allyl intermediate, with small amounts of Diels-Alder product . Larger quantities of the latter were obtained in other cases . This and other examples of activation of olefins by transition metal complexes have been associated with excitation of the coordinated 7r-system . 

In conclusion, there is no satisfactory experimental data concerning the actual barrier to dimerisation of a diaminocarbene. In their study of equilibration of benzoimidazoI-2-yIidenes with dimers, Hahn et al. [67] proposed that this was due to unimolecular dissociation, but Liu and Lemal [66] were more cautious, suggesting electrophilic catalysis might be occurring. 

Fritsch, D., Randjelovic, 1., Keil, F. (2004). Application of a forced-flow catalytic membrane reactor for the dimerisation of isobutene. Catalysis Today, 98, 295—308. 

In 2000, Hashmi reported one of the first applications of gold(m) chloride to homogeneous catalysis. In this report, several examples of intramolecular and intermolecular cyclisations involving, carbon-carbon and carbon-oxygen bond formation, were efficiently promoted by gold at room temperature and at low catalyst loading. The authors reported a cyclo-isomerisation/dimerisation of allenyl ketones and a,(3-unsaturated ketones (Scheme 16.50). This one-pot transformation was catalysed by 1 mol % of gold(m) chloride and proceeded at room temperature. 

Scheme 7.16 Dimerisation of a,p-unsaturated aldehydes catalysed by chiral JV-heterocyclic carbene catalysis and titanium catalysis.

Several multicomponent metal oxide catalysts, developed for this process, have achieved excellent product selectivity with a high conversion of propene Mo-Bi-Fe-Co-M-K-O (M = V or W) used for the first step can attain >90% acrolein yields [6,7] while for the second step Mo-V-Cu-based oxides can lead to >97% acrylic acid yields [8,9], giving, in theory, an overall acrylic acid yield from propene of 87%. In addition to the compositional differences in fhe catalysis for the two-step process, there is also a difference in the optimal reaction temperatures 320-330°C for the first step and 210-255°C for the second step. One has to keep in mind that propene and oxygen can form an explosive mixture and therefore, certain limitations in the feed composition (propene oxygen (air) steam) exist. In addition, the acrylic acid easily dimerises at temperatures above 90 C, meaning that the reactor effluent should be quickly quenched after the second catalyst bed to temperatures below this critical value. 

Baltzer s group has recently described a fully-synthetic protein that is also capable of hydrolysing p-nitrophenyl esters the polypeptide, which contains 42 amino acids, was designed to fold into a hairpin helix-loop-helix motif that dimerises into a four-helix bundle. The dimer is predicted to present on its surface a shallow reactive site containing several histidine residues. The spectroscopic properties of the peptide are consistent with the predicted folded structure, and the molecule does indeed catalyse ester hydrolysis (and transesterification) more effectively than 4-methylimidazole does. However, there is little substrate selectivity, and not much turnover. The histidine array does not seem to act via general acid-base catalysis, but rather to bind and stabilise ester oxygens in the transition state. We return to this molecule below. 

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