Aluminium-alkyl compounds

Most catalysts are based on chromium that has been studied for this purpose since the mid-seventies, probably started by Union Carbide Corporation. Chromium is the metal of the Phillips ethene polymensation catalysts and presumably it was discovered accidentally that under certain conditions 1-hexene was obtained as a substantial by-product. Neither the precise catalytic cycle nor the intermediate complexes or precursors are known. It is generally accepted that an alkyl aluminium compound first reduces the chromium source and that coordination of two molecules of ethene is followed by cyclometallation, giving a chromocyclopentane. During the cyclometallation the valence of chromium goes up by two and thus a starting valence of either one or zero seems reasonable. This cyclic mechanism explains why such high selectivity is obtained [5],... 

The Phillips process uses a threefold excess of 2,5-dimethylpyrrole, a chromium salt, and an excess of an alkyl aluminium compound [6], In Figure 9.9 we have drawn only one ligand per chromium, but we do not know the... 

More recently there have been reports of a number of catalyst systems involving alkyl aluminium compounds (15, 16) (such as AlEts, AlEt2Cl, AlEtCl2) used together with cocat alsyts (usually water or epichlorohydrin). 

Note that only ca 1% of the titanium atoms introduced with TiCl3 into the system give rise to the formation of surface active sites (since most of the titanium atoms remain inside the solid TiCl3 particles) [40], However, such active sites exhibit rather low stereospecificity in propylene polymerisation. The activity and stereospecificity of catalysts based on the / -TiCl3 modification also depend on the type of alkyl aluminium compound used as the activator. The application of triethylaluminium leads to a catalyst of much higher activity but of much lower stereospecificity, and on account of this diethylaluminium chloride is used for the polymerisation of propylene and other a-olefins, while triethylaluminium (and also triisobutylaluminium) is used for ethylene polymerisation [28],... 

The five membered cydic 1,3-dioxolane (CHjOCHjCHjO) can be polymerised by a variety of catalysts including sulphuric acid (P7), perchloric acid (98), phosphorus pentachloride (PP) and alkyl aluminium compounds with water as a co-catalyst (100). The effect of the catalyst boron trifluoride diethyl etherate on the polymerisation of 1,3-dioxolane has also been studied and it has been found that equilibrium between monomeric 1,3-dioxolane and poly(l, 3-dioxolane) is set up in both the undiluted polymer and in solution (101-104). Controverf has arisen as to whether the equilibrium is between cyclic monomer and cyclic polymer (98) or between cyclic monomer and chain polymer (104). 

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. 

The toluene solution in the trap contains alkyl aluminium compounds and this solution should be carefully... 

It is very likely that one of the roles played by the solid support in polymerizations of this type is to stabilize the bound titanium alkyl, either from further reduction by an alkyl aluminium compound or from a bimolecular reduction reaction such as formulated in equation (2). 

Successful homogeneous catalyst systems were soon developed by Natta and his research team at Milan. These were anionic coordinated catalysts obtained by reaction of an alkyl aluminium compound (e.g. A1(C2Hs)2C1) with a hydrocarbon-soluble vanadium compound (e.g. VOCI3)—at least one of the components requiring a halogen in the... 

Average molecular weight increases with reaction time, with decrease in reaction temperature, with decrease in catalyst concentration, with decrease in monomer concentration and with increase in the ethylene/propylene ratio. It also depends on the nature of the alkyl aluminium compound and on the Al V ratio. 

A fourth mechanism for chain transfer involves the transfer of the polymer chain to the large excess of aluminium alkyls present. This has also been observed. Since the reaction rate for this process depends on the concentration of the aluminium compound, the molecular weight distribution now also depends on the latter concentration. For oligomer production this is only relevant when a very large excess of aluminium alkyls is present. 

The influence of the metal atom on mechanism is particularly well illustrated by the pronounced tendency of the alkyls of boron and aluminium to react via coordination complexes in which the metal atom is four-co-ordinate. It is no coincidence that the prime examples of substitution by mechanism SE2(co-ord) occur in reactions of alkyl-boron compounds. 

Organometallic compounds (except for alkyl aluminium and alkyl lithium compounds)... 

A further interesting category of aluminium compounds are the aluminium tetrahydroborates for which mixed species of the type A1(BH4) R3 (n = 1,2 R = alkyl) have been reported. (161) While in the parent compound A1(BH4)3 the Al resonance is found at 97 ppm (162) the mixed forms lead to resonances between this and those of the alanes. Like the parent compound the mixed species show a tendency towards adduct formation. The ether and amine adduct exhibits Al resonances in the region 100-150 ppm with excessively broad lines ( 2kHz). (161)... 

Unipositive manganese appears in the ion [Mn(CN)g] , obtained when yellow solutions of [Mn (CN)g] ions are reduced with aluminium powder and alkali. When Mnig is treated with alkyl isocyanides, compounds are formed containing the [Mn(RNC)g]+ ion (Sacco and Naldini, 1956). 

Conventionally, HAS are blended with PO during processing. 2-(Diethy-lamino)-4,6-bis[butyl(l,2,2,6,6-pentamethyl-4-piperidyl) amino]-l,3,5-triazine may be fed with an olefin directly into the low pressure polymerization process catalyzed with a modified MgCl2 supported Ziegler-Natta catalyst [142]. The catalytic activity was not impaired [143], Tetramethylpiperidine was reported to be a useful component in MgC -supported Ziegler-Natta catalysts as well. Very high stereospecificity of the synthesised PO was achieved. A complex of HAS with the alkyl aluminium activator was envisaged without interaction with the catalytically active alkyl titanium compound [144],... 

Alkyl aluminium hydrides are obtained by reaction 18.29. These compounds, although unstable to both air and water, are important catalysts for the polymerization of alkenes and other unsaturated organic compounds. Ziegler-Natta catalysts containing trialkyl aluminium compounds are introduced in Box 18.3. 

The stereochemistry of alkylation was next investigated. 2-Methylcyclohexanone (43) was treated with MeLi to afford predominantly the trans methylated product 45. However, when the aluminium compounds MAD or MAT were present, they coordinate to the carbonyl group and cause the configuration to be reversed (14). Very complex cases using other sophisticated alkylating reagents are not reported. When 2-methyl-l-propenyl magnesium bromide was reacted with the ketone 27, the isomer of natural product 46 was the... 

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