Cyclodimerization, butadiene

An active catalyst can sometimes be made simply in situ for the target reaction, avoiding a pre-synthesis step. [Fe(NO)2Cl]2 in [BMIM]BF4 or [BMIM]PF6 was in situ treated with a reducing agent, Zn(0), (Et)2AlCl, or -butyllithium for the 1,3-butadiene cyclodimerization reaction (Scheme 22), 206). 

Maxwell et al. 177, 178) studied the deactivation of reduced Cu2+Y catalysts for butadiene cyclodimerization in some detail. This work showed that the catalyst stability could be markedly improved by using NH3 as a reducing agent and choosing the activation conditions such that excess NH3 remains selectively chemisorbed on the zeolite acidic sites. Further, the Cu2+Y-derived catalyst was thermally stable to 850°C and was therefore able to withstand a regeneration procedure which involved a polymer burn-off at 550°C. By contrast, the catalysts prepared by direct exchange with monovalent copper, i.e., Cu+Y, formed CuO irreversibly when heated above 330°C. 

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont-morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH (>95%). Preexchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange. 

A number of transition metal ion-exchange zeolites are active for acetylene trimerization (159, 160), and the criterion for activity appears to be an even, partially filled d-orbital, i.e., d8 (Ni2 +, Co+), d( (Fe2+), d4 (Cr2 + ). This has led to the suggestion that the mechanism must involve a complex in which there is simultaneous coordination of two acetylene molecules to the transition metal ion. The active oxidation state for CuNaY butadiene cyclodimerization catalysts has been unambiguously defined as monovalent copper (172-180). The d10 electronic configuration of Cu+ is consistent with the fact that isoelectronic complexes of Ni° and Pd° are active homogeneous catalysts for this reaction. The almost quantitative cyclodimerization selec-... 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

A similar Ni(0) species is derived from (cyclooctadiene) nickel and a phosphinated polystyrene.The catalyst has little intrinsic activity in butadiene cyclodimerization to cyclooctadiene and vinylcyclohexene, but this was enhanced to a level of about 60-100 g-product/g-Ni/hr by the addition of AlEt2(OEt). Cyclododecatriene was not produced, indicating coordination of a phosphine to the nickel throughout the process. 

Figure 3. Ni(0) catalysts for butadiene cyclodimerization and cyclotrimerization. Filled circles, triangles and squares represent results for 1,5 cyclodiene, vinyl-cyclohexene and cyclododecatriene formation, respectively with a low molecular weight catalyst (-C H40P(0C(SH4)2) Open circles, triangles and squares are for PEoiig-bound Ni(0) catalysts using PEoiig-C H40P(OC H4)2 as a ligand.

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Production of ethylbenzene from butadiene has been iavestigated by many researchers. It consists of two steps cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene and dehydrogenation of the vinylcyclohexene to ethylbenzene. 

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). 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

The cyclodimers are liberated from the respective elimination products 8a and 10a via successive substitution processes with incoming butadiene, that regenerates the active catalyst la in an overall exergonic process. For the rate determining reductive elimination step of the C8-channel free-energy activation barriers of 20.1-24.1 kcalmol-1 are predicted for catalysts TTV, that are in excellent agreement with experimental estimates.43 Thus, moderate reaction conditions are required for the catalytic cyclodimerization of 1,3-butadiene.6... 

Unlike nickel catalysts, palladium catalysts undergo neither cyclodimerization nor cyclotrimerization to form COD or CDT. Only one paper by Chepaikin and Khidekel reported that a mixture of divinylcyclobu-tanes was obtained from butadiene using palladium salts with noncom-plexing anions such as perchlorate and boron tetrafluoride (15). This is a big difference between the catalyses of palladium and nickel. 

C4 hydrocarbons in presence of water selfdiffusion coefficients, 39 391-393 cyclodimerization, butadiene, 31 35 ethane, self-diffusion coefficients, 39 371-373... 

The cyclodimerization of 1,3-butadiene and isoprene by zinc reduction of an iron nitrosyl complex dispersed in [BMIM]BF4 (and alternatively [BMIMJPFg) showed... 

In the system nickel/L/butadiene, secondary amines can shift the cyclodimerization of butadiene to the acyclic products (7a) and (75) Its cocatalyst functfon can be visualized by the corresponding [L]-control map (Scheme 3.3-2). In the three-component system nickel/morpholine/butadiene the open-chain products are formed for log ([morpholine]o/[Ni]o) > -1. Both octatrienes (7a) and (75) are formed at the constant ratio of 1.8 over the entire range of the examined amine/nickel scale. However, the efficiency of the catalytic system is low. After a turnover of 30% butadiene, the catalytic activity ends because of the formation of stop complexes of the nickel amide type. 

Dupont and co-workers studied the Pd-catalyzed dimerization [108] and cyclodimerization [109] of butadiene in non-chloroaluminate ionic liquids. The biphasic dimerization of butadiene is an attractive research goal since the products formed, 1,3,5-octatriene and 1,3,6-octatriene, are sensitive towards undesired polymerization, so that separation by distillation is usually not possible. These octa-trienes are of some commercial relevance as intermediates for the synthesis of fragrances, plasticizers, and adhesives. Through the use of PdCl2 with two equivalents of the ligand PPhj dissolved in [BMIM][Pp6], [BMIM][Bp4], or [BMIM][CF3S03], it was possible to obtain the octatrienes with 100 % selectivity (after 13 % conversion) (Scheme 5.2-23) [108]. The turnover frequency (TOP) was in the range of 50 mol butadiene converted per mol catalyst per hour, which represents a substantial increase in catalyst activity in comparison to the same reaction under otherwise identical conditions (70 °C, 3 h, butadiene/Pd = 1250) in THF (TOP = 6 h ). 

For example, the dinitrosyliron(O) complex can be formed cathodically. This complex is able to catalyze the cyclodimerization of conjugated dienes (Scheme 8) 259,260) in case of 1,3-butadiene, 20000 turnovers are obtained per hour with... 

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