Codimerizations

Phenyl-1,4-hcxadicnc (122) is obtained as a major product by the codimerization of butadiene and styrene in the presence of a Lewis acid[110]. Pd(0)-catalyzed addition reaction of butadiene and aiiene (1 2) proceeds at 120 C to give a 3 1 mixture of trans- and c -2-methyl-3-methylene-l,5.7-octatriene (123)[lll]. 

Other Dimer Olefins. Olefins for plasticizer alcohols are also produced by the dimerization of isobutene [115-11-7] 4 8 codimerization of isobutene and / -butene [25167-67-3]. These highly branched octenes lead to a highly branched isononyl alcohol [68526-84-1] product. BASE, Ruhrchemie, ICl, Nippon Oxocol, and others have used this source. 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

Propjiene (qv) [115-07-1] is the predominant 0x0 process olefin feedstock. Ethylene (qv) [74-85-1J, as well as a wide variety of terminal, internal, and mixed olefin streams, are also hydroformylated commercially. Branched-chain olefins include octenes, nonenes, and dodecenes from fractionation of oligomers of C —C olefins as well as octenes from dimerization and codimerization of isobutylene and 1- and 2-butenes (see Butylenes). 

Lastly, in perfluorobutadiene s codimerization reaction with butadiene, a significant amount of Diels-Alder adduct is obtained, with the perfluorodiene acting as the diene component [125] (equation 105)... 

Materials. 5-Methyl-1,4-hexadiene was obtained by the codimerization of isoprene and ethylene with a catalyst (18) consisting of iron octanoate, triethylaluminum and 2,2 -bi-pyridyl. The product mixture which contained principally 5-methyl-1,4-hexadiene and 4-methy1-1,4-hexadiene was fractionated through a Podbielniack column to yield high purity (>99%) 5-methylxhexadiene, b.p. 92.80C,njj 1.4250 (Lit. (19) b.p. 88-89°C, np 1.4249). 1-Hexene (99.9% purity), 1-decene (99.6% purity), 4-methyl-1-hexene (99.5% purity) and 5-methyl-l-hexene (99.7% purity) were obtained from Chemical Samples Co. 6-TiCl3 AA (Stauffer Chemical Co.j contains 0.33 mole AICI3 per mole of TiClj). Diethylaluminum Chloride was obtained from Texas Alkyls (1.5 M in hexane). 

MCP (1) has been codimerized with several different types of compounds containing C = C double bonds (Tables 38,39, and 40). 

As in the case of dimerizations, MCP derivatives are known to undergo metal-catalysed [2 + 2] codimerizations with other alkenes in a few cases [2]. The examples are limited to strained olefins, such as norbornadiene (572) (Scheme 79) [152] and cyclobutene (574) (Scheme 80) [153], and to alkyl acrylates (Table 46) [154] and always compete with the alternative [3 + 2] addition of TMM species. 

Stereospecific codimerization of 120 with diethyl fumarate and maleate is accomplished by the Ni(COD)2 catalyst as shown in Scheme 43 [64], (Scheme 42 and 43)... 

It should be noted that codimerization was achieved from diphenyl cyclopropenone and unsubstituted cyclopropenone (2JS)197. Phenyl hydroxy cyclopropenone, which appears to be an associated dimer in (dioxane) solution52, formed a dimeric pulviniv acid lactone 260 on treatment with thionyl chloride51, probably via oxidative rearrangement of a dimer 259 ... 

Included are the dimerization, codimerization, oligomerization, double-bond isomerization, and cyclization of olefins. 

Further examples which show that the direction of Ni—C addition to propene can be changed from Ni — C2 to Ni — Cj by introducing P(i-C3H7)3 are to be found in the codimerizations of ethylene with propene (4, 12, 29) and of 1,3-cyclooctadiene with propene (4, 79). The last example is particularly informative, for in this case no ambiguity as to the assignment of the products to Ni - C4 or Ni - C2 type addition exists. 

In one example of an asymmetric codimerization, a pronounced effect on the optical yield has been observed by increasing the phosphine/nickel molar ratio (94, 95). This effect may be ascribed to suppression of a dissociation process or to complexation of a second molecule of the inducing phosphine to the nickel atom. 

Asymmetric Codimerization of Styrene with Ethylene Influence of the Complex Anion of the Catalyst on Optical Yield... 

Finally, in the codimerization of styrene with ethylene to optically active 3-phenyl-1-butene (52) (Table XI), it has been shown that the complex anion (Y) can exert a strong influence on the optical yield 103, 105). 

Table I). Codimerization of this dimer with norbomadiene also gives preferentially one stereoisomer, the exo,trans,exo,trans,exo isomer (30). 

Codimerization of butadiene with dicyclopentadiene (example 8, Table II) was shown to proceed via a crotyl-nickel complex (62). Ring contraction of cyclooctadiene (example 10, Table II) appears to be a hydride promoted reaction. The hydride-promoted dimerization of norbomadiene to -toly 1 norbornene (example 9, Table II) appears to be quite different from dimerization via a metallacycle (see Table I, example 16). 

The codimerization reaction we will discuss in this chapter belongs to the last type. The reaction and the reactants involved will be the simplest of them all, i.e., a 1 1 codimerization of ethylene and butadiene to form C dienes. 

Catalysts for this codimerization reaction can be derived from prac-tially all the Group VIII transition metal compounds. Their catalytic properties, such as rate, efficiency, yield, selectivity, and stereoselectivity, vary from poor to amazingly good. Some better-known catalyst systems and their product distributions are listed in Table I. As one can see, the major codimerization product under the given condition is the linear 1 1 addition product, 1,4-hexadiene. The formation of this diene and its related C6 products will become the center of our discussions. The catalyst systems that have been investigated rather extensively are derived from Rh, Ni, Co, and Fe. We shall cover these systems in some detail. A lesser-known catalyst system based on Pd will also be briefly discussed. 

Before we continue I would like to mention briefly the practical reasons leading to the study of this codimerization reaction. 

Typical Isomer Distributions in 1 1 Codimerization of Ethylene and Butadiene by Various Transition Metal Catalysts... 

The most general and comprehensive reaction mechanism of the 1 1 codimerization has been reported by Cramer (4). The results were based on reaction properties measured in an alcoholic medium under relatively mild conditions. 

Scheme 2. Catalytic cycle for Rh catalyst for 1 1 codimerization of ethylene and butadiene to form 1,4-hexadiene.

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. 

In the codimerization reaction, both reactants are present in large excess compared to the catalyst concentration. The selectivity toward a 1 1 codimerization to form 1,4-hexadiene, instead of a random oligomerization, represents a rather unique reaction, especially in view of the fact that the same catalyst also dimerizes ethylene to butene (3) at about the same rate as the codimerization. The explanation forwarded by Cramer (4, 7) is based on the overwhelmingly favored stability of the tt-... 

In this codimerization reaction, the predominant complex is 3, which should lead to the ethylene-butadiene codimerization product. If the ethylene in complex 3 is displaced by butadiene to form 7 before the insertion reaction takes place, then a C8 or higher olefin could be formed... 

Hexadiene is the immediate product found in the codimerization reaction described above in a mixture of ethylene and butadiene. However, the reaction will not stop at this stage unless there is an overwhelming excess of butadiene and an adequate amount of ethylene present. As the conversion of butadiene increases, some catalyst begins to isomerize... 

Hexadiene which is formed by 1,4-addition of hydrogen and a vinyl group to butadiene, is the predominant product in the codimerization reaction. However, there is always a small amount (1-3%) of 3-methyl-... 

It was found that, in a nonpolar medium, the crotyl rhodium complex 1 is relatively inactive as a codimerization catalyst. However, it becomes very active in the presence of a small amount of donors such as alcohol. The activity generally increases linearly with the amount of the added donors and then depends on the strength of the donors, either leveling off or decreasing with further increases in the donor concentration. Strong donors improve the activity at lower concentration but inhibit the reaction at higher concentration. Some representative donors and their rate enhancement efficiency are shown in Table VI. The relationships between the concentrations of various donors and the reaction rates are summarized in Figure 5. The rate enhancement efficiency (expressed as relative reactivity) of a donor was measured based on the maximum rate attainable by addition of a suitable quantity of the donor to the reaction mixture, i.e., the maximum in the activity curve of Fig. 5. The results in Table VI show that those donors with p Ka values (25) between -5 and... 

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