Diene active species

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

The pyridine-containing ruthenium-based complex XXVI developed by Nolan [59] from the indenylidene complex DC, promoted the RCM of various dienes (Equation 8.6). Kinetic studies were carried out and showed that, despite a rapid initiation, the presence of pyridine in the reaction mixture has a negative effect on the stability of the active species and only moderate catalytic conversions were obtained [59] (Scheme 8.21). 

Recently several studies have focused on the nature of the active species in the polymerization of ethylene and conjugated dienes initiated by the chelate of butyllithium and N,N,N, N -tetramethylethylenediamine (TMEDA). 

The r/5-cyclopentadienylcobalt unit, [Co(t/5-C5H5)], is believed to be a key active species in catalytic cyclooligomerization of acetylenes or cooligomerization of acetylenes with other unsaturated reactants. It can be generated from several precursors, [Co( /5-CjH5)(CO)2],1 [Co(t/5-C5H5)(t/4-diene)],2... 

To generate the catalytically active species, the diene ligand of 18 was removed by hydrogenation with 1 atm H2 to afford the bis-aquo species (PMc3)2Rh(H2O)2 (23). In the absence of 1, addition of small allyl alcohols quickly yielded the isomerized product. However, the reaction did not tolerate terminal substitution, and isomerization of crotyl alcohol was not observed (Table 7.4). Significantly, when both crotyl alcohol and allyl alcohol were added to 23, neither substrate underwent isomerization, suggesting that crotyl alcohol inhibits the reaction. 

In developing the mechanism, it was assumed that the adsorbed state of buta-1 2-diene active in hydrogenation was the di-7r-adsorbed species C in Fig. 32. The close similarity in the ZV-profiles for each butene formed in a given reaction suggests that each was formed as a primary product and the general mechanism shown in Fig. 33 was proposed. The difference in behaviour of the type A and B catalysts was explained by proposing that, at the type A surface, a-bonded and 7r-olefinic species are of importance as... 

In discussions about the nature of the active species in the polymerization of dienes by Ziegler/Natta catalyst systems allyl species have already been suggested in the 1960s [273-278]. This discussion has continued through the past decades [139,279-283]. Today, it is widely accepted that Nd-allyl-groups are the key element in the insertion of dienes into the Nd carbon bond. 

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

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