Transfer with metal-alkyl

For most catalysts it is usual to employ an excess of metal alkyl above that which is required to alkylate the transition metal, and it is well established that this excess reduces the molecular weights of the polymers formed. The transfer reaction is formally alkyl interchange, XV, viz. 

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Average lifetimes were calculated from molecular weights and polymerization rates (Table 9, p. 188) [244] lifetimes were independent of VCI3 concentration but fell with increase in Al/V ratio above 2/1, because of transfer with metal alkyl. At a fixed ratio of Al/V = 2/1 the molecular weight was dependent on the nature of the aluminium alkyl employed, being much lower with AlMe3 than with the higher alkyls. 

Alkyl group transfer. When metal alkyls react with compounds of a different metal, the products frequently are those in which one product forms a stable crystal lattice. For example, in the reaction... 

Polymerization occurs at active sites formed by interaction of the metal alkyl with metal chloride on the surface of the metal chloride crystals. Monomer is chemisorbed at the site, thus accounting for its orientation when added to the chain, and propagation occurs by insertion of the chemisorbed monomer into the metal—chain bond at the active site. The chain thus grows out from the surface (31). Hydrogen is used as a chain-transfer agent. Chain transfer with the metal alkyl also occurs. 

A MOF constructed from rhodium paddlewheel clusters linked to porphyrinic ligands already discussed in Section 4.3.1.1 shows an interesting synergetic behavior when the porphyrinic rings are loaded with metals like Cu , Ni , or Pd . In the hydrogenation of olefins, the hydride species at the rhodium center is transferred to the coordinated olefin adsorbed on a metal ion in the center of the porphyrin ring to form an alkyl species, and next this alkyl species reacts with a hydride species activated at the rhodium center to form the alkane [81]. 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

Insertion of coordinated NO into one of the metal alkyls would yield a nitrosoalkane, which could dissociate and in turn react with the starting material via 0-atom transfer. The independent observation that PhNO reacted with Cp W(NO)(CH2SiMe3)2 to form Cp W(0)2(CH2SiMe3) in low yield supports the role of the nitrosoalkane as an alternative oxidant. 

Reaction with an alkyl halide takes place at the metal surface. In the first step, an electron is transferred from lithium to the alkyl halide. 

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