Butadiene, catalytic selectivity

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

Platinum complexes Pt(PPh3)4 and PtCl2(PPh3)2, either alone or in combination with CP3CO2H, catalyze the selective monohydroamination of 1,3-butadiene with BnNH2 or PhNH2 (Eq. 4.49) in moderate yields [185]. Pd(0Ac)2/CF3C02H is also an efficient but less active catalytic system for this reaction [185]. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

Fig. 2. Selected geometric parameters (A) of the optimized structures of the key species for oxidative coupling for the catalytically active generic [Ni0(r(2-butadiene)2PH3] species la and the [Ni°(ri2-butadiene)3] species Fb of the C8- and Ci2-product channel, respectively, via the most feasible pathway for p2-/rans/r 2-ds-butadiene coupling (of opposite enantiofaces) along la -> 2a and Fb -> 2b. Free energies (AG, AG 5 in kcalmol-1) are given relative to the favorable stereoisomer of the respective bis(r 2-/rans-butadiene) and tris(r 2-/r

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

Reactions of 3-methylthio-4-trimethylsilyl-l,2-butadiene with electron-poor monosub-stituted and disubstituted alkenes were promoted by a catalytic amount of ethylaluminum dichloride, affording the corresponding methylenecyclobutanes with high selectivities and with yields ranging from 37% for methyl crotonate to 97% for methacrylonitrile15. 

The resulting complex remained dissolved in the biphasic catalytic system. The 4-vinyl-l-cyclohexene product, obtained with 100% selectivity in [BMIM]PF6, was continuously separated from the reaction mixture by decantation, allowing the reuse of the remaining catalyst solution. The 1,3-butadiene conversion in the biphasic system was higher than that observed in homogeneous systems. Because the unconjugated product has a lower solubility in the ionic liquids than the conjugated butadiene feed, continuous separation of product contributes to the increased reaction rate in the ionic liquid. 

On the other hand, the highest reactivity is achieved in the presence of the complex prepared in sim from Pd(acac)2 and N-heterocyclic carbene IMes.HCl (TON = 4.299). With this catalytic system, the dimerization of butadiene is negligible (1%). However, in that case, due to very high activity of the catalytic system, there is no selectivity towards the monotelomer, and a large amount of the ditelo-mer was formed (51 and 41% respectively). 

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