Propene dimerization

Table 5.2-2 Selected results from Ni-catalyzed propene dimerization in chloroaluminate ionic...

The dimerization of propene has been extensively studied because the propene dimers are of considerable interest as fuel additives and as starting materials for the preparation of monomers (4, 48, 49, 101). The reaction course can be controlled to give methylpentenes, 2,3-dimethyl-butenes (2, 4, 7, 47, 51), or hexenes (44-46) as the main products. 

The influence of the phosphine on the composition of the propene dimers for a series of phosphines is given in Table V. From Table V it can be seen that the yield of hexenes and of methylpentenes decreases from 21.6 to 0.1% and from 73.9 to 19.0%, respectively, while at the same time the yield of 2,3-dimethylbutenes increases from 4.5 to 80.9%. 

An important contribution to an understanding of the mechanism of propene dimerization has been obtained by studying the reaction of nickel-hydride model complexes (55). The formation of the propyl- and isopropyl-nickel complexes 48 and 49 has been observed in the reactions of HNi(PR3)Cl complexes (50) with propene at -78°C [Eq. (15)] ... 

A convincing example of the operation of steric effects is to be seen in the difference in the composition of propene dimers obtained using epimers of menthyl(f-C4H9)PCH3 (Table VIII) with opposite configuration at the phosphorous atom (103). 

From the results discussed above, it appears that the main influence of phosphines in controlling the course of propene dimerization consists in hindering, through steric interference, the second insertion step from proceeding according to Ni — C2 type addition. An explanation for the predominant Ni — C2 type addition in the absence of steric hindrance in terms of the polarity of metal-carbon, metal-H. and C=C bonds has been recently presented (104). The extremely bulky phosphine (t-C4H9)2P-i-C3H7 is even able to reverse the direction of Ni—H addition to propene, a step which is obviously less sensitive toward steric hindrance. 

Figure 13.33 Yield (Y) of propene dimers, trimers, tetramers and total true oligomers (di- to hexamers, without products with intermediate carbon numbers obtained by cracking and recombination of fragments)...

Scheme 3.3-3. Propene dimerization using homogeneous nickel catalysts (product distribution in mol%) l. Reaction conditions [Ni]o (A1 (Et) Cl2lo (propene]o = 1 4 200, (NiJo ...

The behaviour of these catalysts in the oligomerization of higher alkenes, propene and butene has not previously been reported. This paper addresses this aspect, and in addition focuses on the nature of the structure of the oligomers produced by propene dimerization. 

It is however clear that the use of carefully ion-exchanged amorphous supports can lead to catalysts that have superior regioselectivities in propene dimerization compared with many homogeneous systems. The approach may well be more generally applicable to, for example, other areas of transition metal mediated catalysis and this aspect is now attracting our attention. 

Several (bis)carbene nickel complexes were tested as catalysts for the dimerisation of propene or 1-butene (Scheme 8.3). IS While the activity of these complexes was very poor in the dimerisation of 1-butene when toluene was used as solvent, turnover frequencies as high as 7,000 mol mol h 1 were observed at ambient temperature with C4Ciim]Cl-AlCL-iV-methylpyrrole (0.45 0.55 0.10) as solvent. With propene as substrate, TOFs of 75,000 mol-moL -lf1 were achieved. Compared to NiCl2(PCy3)2, the activity of the carbene complexes is considerably higher, but selectivity towards the desired, highly branched propene dimer is low. 

The dimerization mechanism can be visualized as a polyaddition reaction to Ni-H and Ni-C bonds. The Ni-H species is formed by yff-hydrogen abstraction. The reaction pathway for the formation of various propene dimer structures is given in Scheme 1. 

Scheme 1. Propene dimerization by cationic nickel complexes. Reaction pathways of dimer formation.

Thus propene dimers, e. g., at 50 °C, have a composition of 22 % n-hexenes, 72 % 2-methylpentenes, and 6% 2,3-dimethylbutenes. Under the same conditions, isomer structures of n-butene dimers are 6 % n-octenes, 59 % 3-methylheptenes, and 34 % 3,4-dimethylhexenes propene-butene codimerization yields the following isomer distribution 12% n-heptenes, 12% 2-methylhexenes, 40% 3-methyl-hexenes and 35 % 2,3-dimethylpentenes. The relative rate constants for codimerization are... 

More than 25 plants are in operation, mainly for propene dimerization their total output is 3.4 Mt per year. Plant investments are comparatively low. 

The hydrovinylation reaction has its origin in the observations made in 1963 that propene dimerizes at a quite remarkable rate in the presence of certain organo-nickel catalysts and that the product distribution can be influenced by introducing auxiliary P-donor ligands [1]. In 1967 it was discovered that in the presence of the chiral ligand P( ranx-myrtanyl)3, 2-butene can be co-dimerized with propene to give 4-methyl-2-hexene in an enantioselective manner and the extension of this co-dimerization reaction to ethylene has become known as hydrovinylation. 

These results led to the postulate that the bismuth component of bismuth molybdate catalysts is responsible for the abstraction of the first hydrogen from propene, but that this oxide is not capable of inserting oxygen into the reaction intermediate which forms. Instead any allyl species which form over this oxide from allyl iodide and less easily from propene, dimerize to hexadiene in the absence of a suitable source of oxygen. By contrast, allyl species do not form from propene over molybdenum oxide. However, when this oxide is exposed to a facile source of allyl species, namely allyl iodide, acrolein forms readily, consistent with the oxygen insertion and second hydrogen abstraction steps occurring over this component of the bismuth molybdate catalysts. 

Using the ionic liquid catalyst, the Dimersol reaction can be performed as a two-phase liquid-liquid process at atmospheric pressure at between — 15 and 5 °C. Under these conditions, alkenes are immersed with activities well in excess of that found in both solvent-free and conventional solvent systems. The products of the reaction are not soluble in the ionic liquid, and form a second less-dense phase that can be separated easily. The nickel catalyst remains selectively dissolved in the ionic liquid phase, which permits both simple extraction of pure products and efficient recycling of the liquid catalyst phase. In addition to the ease of product/catalyst separation, the key benefits obtained using the ionic liquid solvent are the increased activity of the catalyst (1250 kg of propene dimerized per 1 g of Ni catalyst), better selectivity to desirable dimers (rather than higher oligomers) and the efficient use of valuable catalysts through simple recycling of the ionic liquid. 

Propene dimerization using a nickel catalyst modified by r< r/-butyldiisopropylphosphane to give 2,3-dimethyl-l-butene has been intensively investigated to elucidate the mechanism. Nickel hydride and alkylnickel intermediates have been trapped at low temperatures4-5. The role of the phosphane ligand has also been intensively studied and changes in its steric bulk have been found to be extremely effective for asymmetric induction. 

Figure4.8.4 Example of homogeneous two-phase catalysis propene dimerization with a homo Ni-catalyst and an ionic liquid as solvent. Adapted from Eichmann (1999).

Shmidt, E K., Mironova, L. V, Thach, V S., and Kalabina, A. V, Propene dimerization in the presence of a Ni(PPh3)4 complex with Lewis and Bronsted acids, Kinet Katal, 16, 270, 1975. 

The products of propene dimerization using alkali metals dispersed on a variety of supports are given in Table 4.18. The reactions were performed over a support containing 0.1 mole of dispersed alkali metal at 423 K and about 10 MPa pressure. The main product is 4-methyl — 1-pentene, as expected from the allylic carbanion addition to double bond. 

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