Oligomerization selectivity

These selectivities obtained with Amberlyst-15 seem to be rather optimistic in view of recent work, where only with overstoichiometric ratios high di-+tri-ether selectivity is obtained. It is also not settled why addition of tert-butanol to the reaction mixture suppresses isobutene oligomerization selectivity [34, 35], At this stage it should also be stressed that the tert-butylglycerol ethers (TBGE) mentioned are excellent substitutes for MTBE (ETBE) as gasoline octane boosting components [36]. 

C02 has been found to have a promoting effect (191-193) on the overall catalyst activity, but decreases oligomerization selectivity. 

Phosphinated polystyrene resins have been functionalized with both bisphosphine nickel carbonyl and tristriphenylphosphinehydridocarbonylrhodium. Using this system, butadiene can be cyclodimerized to vinylcyclohexane and selectively hydroformylated at the terminal double bond in sequence. Similarly, the use of a NiBr2 chelate, reduced by NaBHj, together with the rhodium polymer permitted the linear oligomerization-selective hydroformylation of butadiene. These were both one-pot processes. 

Because of its significance, some basic principles of the Ni-catalyzed dimerization of propene in chloroaluminate ionic liquids should be presented here. Table 5.2-2 displays some reported examples, selected to explain the most important aspects of oligomerization chemistry in chloroaluminate ionic liquids [97]. 

In addition to the neutral nickel/phosphine complexes used in the Shell Higher Olefins Process (SHOP), cationic Ni-complexes such as [(mall)Ni(dppmo)][SbF6] (see Figure 5.2-7) have attracted some attention as highly selective and highly active catalysts for ethylene oligomerization to HAOs [106]. 

It was recently demonstrated in the author s group that the use of hexafiuo-rophosphate ionic liquids allows, for the first time, selective, biphasic oligomerization of ethylene to 1-olefins with the aid of the cationic Ni-complex [(mall)Ni(dppmo)][SbF6] (Scheme 5.2-22) [25, 107]. 

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

Epoxides are often encountered in nature, both as intermediates in key biosynthetic pathways and as secondary metabolites. The selective epoxidation of squa-lene, resulting in 2,3-squalene oxide, for example, is the prelude to the remarkable olefin oligomerization cascade that creates the steroid nucleus [7]. Tetrahydrodiols, the ultimate products of metabolism of polycyclic aromatic hydrocarbons, bind to the nucleic acids of mammalian cells and are implicated in carcinogenesis [8], In organic synthesis, epoxides are invaluable building blocks for introduction of diverse functionality into the hydrocarbon backbone in a 1,2-fashion. It is therefore not surprising that chemistry of epoxides has received much attention [9]. 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

An oligomeric protein that spans a cell membrane forming a regulated pore through which Ca2+ can pass. Ca2+ channels differ considerably in their selectivity for Ca2+ over other cations For example DP3R are poorly selective, voltage-dependent Ca2+ channels are vety selective. 

Ca2+ Channel Blockers. Figure 1 Most voltage-gated Ca2+ channels exist as a hetero-oligomeric complex of several subunits, a 1 subunits form the Ca2+-selective ion pore and contain the voltage-sensors of the channel. 

HVA calcium channels are biochemically hetero-oligomeric complexes of five proteins encoded by four gene families (Fig. 1) The ax subunits of 190-250 kDa contain the voltage-sensor, the selectivity filter, the ion-conducting pore, the binding sites for most calcium... 

A consequence of the selectivity for electrophilic radicals is that not all products are trapped with equal efficiency. With electron-rich monomers (e.g. S) oligomerization may complicate analysis. Other possible complications in the utilization of this method have been discussed by Russell.491... 

The competition at 200°C between an aliphatic and an aromatic amine toward the formation of an imide is a very selective reaction (Fig. 5.1) for the formation of an aliphatic imide.141 This reaction suggests that the reactive processing in extruder, for example, could be used to transform a melt-processable polyimide with an oligomer end capped with an aliphatic amine. In order to get a perfecdy alternate block polyimide-block siloxane, Rogers et al. used low-temperature transimidization.142 An oligomeric aromatic imide was end capped... 

Higher selectivity concerning the formation of a-olefins during oligomerization Easier separation of the catalyst from the oligomer... 

ZSM-5 Cracking of n-octane Higher activity Higher propylene selectivity Reduced oligomerization [71]... 

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