Dimerization of ethylene

Dimerization of ethylene to butene-1 has been developed recently by using a selective titanium-based catalyst. Butene-1 is finding new markets as a comonomer with ethylene in the manufacture of linear low-density polyethylene (LLDPE). 

Feed and product quality from dimerization of ethylene to 1-butene ... 

Scheme 25a,b The symmetry-forbidden (a) and -free (b) frontier orbital interactions for the dimerization of ethylenes... 

The frontier orbital interaction is forbidden by the symmetry for the dimerization of ethylenes throngh the rectangular transition state. The HOMO is symmetric and the LUMO is antisymmetric (Scheme 25a). The overlap integrals have the opposite signs at the reaction sites. The overlap between the frontier orbitals is zero even if each overlap between the atomic p-orbitals increases. It follows that the dimerization cannot occur throngh the fonr-membered ring transition states in a concerted and stereospecfic manner. 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

Dimethyltitanium complex 25, bearing an ethylene and methyl ligands, catalyzed the dimerization of ethylene via a metallacyclopentane intermediate 26 (Eq. 1) [30]. During the dimerization, no insertion of ethylene into the Ti-Me bond was observed due to the perpendicular orientation between methyl and ethylene ligands. This inertness could be attributed to the low oxidation state of 25, i.e. Ti(II). 

The rate equation for the dimerization of ethylene (5) can be used to describe the codimerization in the presence of large excesses of butadiene. The rate of the addition reaction as measured by the disappearance of ethylene is represented in Eq. (5). It is first order in ethylene, proton, chloride, and rhodium. 

Formation of a titanacyclopentane via cyclic dimerization of ethylene was reported as early as 197644 (Scheme 17). In marked contrast with the corresponding Zr-promoted reactions discussed later in this chapter, the intramolecular version of the Ti-promoted alkene-alkene coupling reactions does not appear to have been well developed. Consequently, detailed aspects of pair -selectivity and regioselectivity still remain largely unknown. 

The concept of the conservation of orbital symmetry can be extended to intermolecular cycloaddition reactions which occur in a concerted manner. The simplest case is the dimerization of ethylene molecules to give cyclobutane, the 2n + 2je cycloaddition. The proper geometry for the concerted action would be for the two ethylene molecules to orient one over the other. Two planes of symmetry are thereby set up -perpendicular to the molecular plane bisecting the bond axes oy-parallel to the molecular plane lying in between the two molecules (Figure 8.10). 

Ta1 adducts with ethylene have been obtained as highly air sensitive solids by reduction of the corresponding Ta111 compounds under argon (equation 85),292 or by reductive elimination of H2 from [TaH2ClL4] (Scheme 9). A similar procedure, but under dinitrogen, gave Tav nitrenes (Section 34.2.3.6). The same Tam precursor (60) provided o alkyl derivatives (equation 86). Complex (63) catalyzes the selective dimerization of ethylene to 1-butene. 

Butadiene d4 complexes were obtained (i) from [NbCU(dmpe)2] and magnesium butadiene (equation 87) j706 (ii) by dimerization of ethylene using alkylidenes (equation 88) 707 or (iii) by metal vapor techniques (equation 89), which yielded sublimable methylallyl derivatives.70 Compound (62) could not be prepared by Na/Hg reduction of (22) in the presence of butadiene. Compound (63) is also accessible from [TaH2ClL4] (Scheme 9). 

The simplest possible alkene oligomerization reaction, the dimerization of ethylene to butenes, is a well-studied reaction, and an industrial process was also developed for the selective formation of 1-butene42 (IFP Alphabutol process). 

Institute for Industrial Research of Oslo, Norway. A unique metallo-organic catalyst system has been discovered which enables the selective trimerization of ethylene to 3-methyl-2-pentene. High temperature de-methanation of this compound results in the formation of isoprene in good yields. Similarly, since 1-butene and 2-butene are dimers of ethylene, they react with ethylene selectively in the liquid phase to produce 3-methyl-2-pentene. 

ReCl5 has been found to act as a Friedel-Crafts catalyst for the alkylation of benzene with ethylene. Ethylbenzene, x-butylbenzene and hexaethylbenzene were formed.612 When propylene was used in place of ethylene, cumene and di-, tri- and tetra-isopropylbenzenes were obtained.613 Ethylbenzene and anisole were also alkylated with ethylene. A carbonium ion mechanism was proposed, in some cases with dimerization of ethylene preceding alkylation. 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

The catalytic cycle of the Ni-catalysed dimerization of ethylene to give 1-butene (65) is explained by the insertion of ethylene to the nickel hydride 62 twice to form the ethyl complex 63 and the butyl complex 64. The elimination of /1-hydrogen gives 1-butene (65), and regenerates the Ni—H species 62. The reaction is chemoselective. Curiously, no further insertion of ethylene to 64 occurs. 

Consider the dimerization of ethylene. The frontier orbitals having different symmetries (1), their overlap is zero and the reaction is forbidden. In the Diels-Alder reaction, the FOs have the same symmetry, their overlap is positive (2 and 3) and the reaction becomes allowed. 

FIGURE 1 Manufacture of butene-1 (n-butene) by dimerization of ethylene. 

Fig. 14. Orbital and state symmetry correlations for the dimerization of ethylene. The orbital symmetry designations in the upper diagram are with respect to the planes of symmetry tjyt and < ...

The dimerization of ethylene is the first of another group of dimerizations leading to closed structures, e.g. in (56) and (57). 

At first sight the orbital changes in (92) appear to be similar to that in the dimerization of ethylene, or forbidden in the ground state. 

But (92) is different. Since there are four a orbitals to consider, there is an additional symmetry plane here. If the atomic orbitals are suitably combined pictorially, according to the general plan of Fig. 13, or analytically, one would find >p2 and 4>s to be degenerate. That is, the energy gap one has in the second and third MO s for the dimerization of ethylene (Fig. 14) vanishes the process becomes symmetry-allowed. 

As a test of the method, we examined the dimerization of ethylene. The first and second excited states are Az and Alt respectively (Fig. 14). For a four-center square species, the normal vibrations which could lead to dimerization are (Alg) and vz (Blg) (Herzberg, 1945). Accordingly, dimerization would be possible but difficult, since the symmetry of vx matches that of the second excited state. This conclusion agrees with the results of orbital-symmetry arguments. 

Recently, Angelescu et a/.[92] have studied the activity and selectivity for dimerization of ethylene of various catalysts based on Ni(4,4-bipyridine)Cl2 complex coactivated with A1C1(C2H5)2 and supported on different molecular sieves such as zeolites (Y, L, Mordenite), mesoporous MCM-41 and on amorphous silica alumina. They found that this type of catalyst is active and selective for ethylene dimerization to n-butenes under mild reaction conditions (298 K and 12 atm). The complex supported on zeolites and MCM-41 favours the formation of higher amounts of n-butenes than the complex supported on silica alumina, which is more favourable for the formation of oligomers. It was also found that the concentration in 1-butene and cw-2-butene in the n-butene fraction obtained with the complex supported on zeolites and MCM-41, is higher compared with the corresponding values at thermodynamic equilibrium. 

Cai, F. X., Lepetit, C., Kermarec, M. and Olivier, D. Dimerization of ethylene into 1-butene over supported tailor-made nickel-catalysts. J. Mol. Catal., 1987, 43, 93-116. 

Carlu, J. C. and Caze, C. Dimerization of ethylene catalyzed by a nickel-catalyst supported on porous polymers. React. Polym., 1990, 13, 153-160. 

Ceder, R., Muller, G., Saleh, J. and Vidal, J. Catalytic dimerization of ethylene to 1-butene by square-planar nickel-complexes. J. Mol. Catal., 1991, 68, 23-31. 

On the other hand, the participation of Pd2+ and/or Pd+ species as active sites in the reaction of dimerization of ethylene was suggested some years ago (222). Recently, two independent groups established that Pd + species in Pd/X and Pd/Y zeolites are sites of highest activity (124-126, 223). Starting with a Pd2+/Y or Pd2+/X catalyst, they monitored its activity in the course of reaction. Parallel study with XPS, ESR, and solid-state NMR confirmed that Pd+ is an active species in the reaction. Its concentration grows with time on-stream. 

A number of zeolite-based catalysts are active for the dimerization of ethylene. The major products are n-butenes (1-butene, tram-2-butene, m-2-butene), i.e.,... 

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