Reaction rates codimerizations

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... 

Increased stereoselectivity is found at low reaction temperatures but is then accompanied by a decrease in reaction rate. Below — 20"C no codimerization at all occurs with the nickel(O) catalyst, thus further improvement in the stereoselectivity could not be achieved. 

A selective dimerization of propylene to 2,3-dimethylbutene catalyzed by R4P[(i-Pr,P)NiCl3] with Et3Al2Cl3 in a toluene medium has been reported 1617]. The increasing temperature (—20 to +20°C) leads to the formation of C, olefins at the expense of 4-methyl-1-pentene. This suggests a secondary codimerization of the product olefin with propylene. Most of the olefins are the thermodynamically less favored a-olefins, indicating the absence of double-bond isomerization under these conditions. The Al/Ni ratio, although having a predominant effect on reaction rate and yield at low values, has no influence on the catalyst selectivity. 

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. 

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-... 

A tetracoordinated complex (20)4 was actually isolated. Complex 20 in the presence of ethylene forms the coordinated complex 21, as can be seen from H NMR. Complex 21 is a model of the intermediate for the additional reaction to form C6 dienes. The model catalyst had been shown to be a codimerization catalyst under more severe conditions (high temperature), although the rate of reaction was very slow compared to the practical systems. These studies are extremely useful in demonstrating the basic steps of the codimerization reactions taking place on the Ni atom. The catalytic cycle based on these model complexes as visualized by Tolman is summarized in Scheme 7. A more complete scheme taking into consideration by-product formation can be found in Tolman (40). 

Both the Co and the Fe systems have very similar chemistry for the 1 1 codimerization reaction. Although they are almost identical in catalytic selectivity, they do differ in other catalytic properties, especially the rate of reaction (66). In practice, the Co system is superior to the Fe system our discussion will therefore focus mainly on the former system. 

Polymer ligands Content of P, wt.% Swelling in heptane, mg/g Composition of reaction products, wt.% Composition of pentene fraction, wt.% Relative rate of codimerization, g of product... 

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