Recent Mechanistic Studies

Prokof ev [18] studied the effects of oligomeric polysulphides on the rate of sulphidation of copper foil in m-xylene. These oligomers contained linear polysulphide chains. It was reported that the polysulphides themselves did not sulphidise the copper at an appreciable rate. However, when used together with elemental sulphur, sulphidation occurred and the sulphide was more homogeneous and finer than by sulphur alone. These results were used to explain the more stable bonding to rubber containing polysulphides. Apparently, linear sulphides are not reactive but they insert sulphide and then become active for sulphidation. 

Chandra [19] evaluated the effect of an adhesion promoter by investigating the failed interfaces between NR skim stocks and tyre cords to determine the effect of a cobalt adhesion promoter on the mode of failure. The main conclusions were that the model by Van Ooij was confirmed. The cobalt strengthened and stabilised the interface, but had a negative effect on the compound properties in the interface region, as expected. As a result, the failure mode became more cohesive. 

Stage I Formation of active intermediate products from the accelerator  

The rate-determining step in this sequence is reaction la, the slow scission of CBS. It was found that in squalene, the induction period was as follows  

The data indicate that the CBS decomposition is catalysed by the double bonds of squalene thus confirming the observations in a). 

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Recent mechanistic studies have shown that the many combinations of alcohols, carboxylic acids and solvents cannot be correctly described by a uniform mechanism. In certain cases the reaction appears to involve a pentavalent dialkoxyphos-phorane 10 as an intermediate, which is in equilibrium with oxyphosphonium salt 8 4 ... 

As a rule, short nucleation times are the prerequisite for monodisperse particle formation. A recent mechanistic study showed that when Pt(acac)2 is reduced by alkylalu-minium, virtually all the Pt cluster nuclei appear at the same time and have the same size [86]. The nucleation process quickly consumes enough of the metal atoms formed initially to decrease their concentration below the critical threshold. No new metal cluster nuclei are created in the subsequent diffusion-controlled growth stage. 

As in the alkylation reaction, the reactive intermediate in Friedel-Crafts acylation can be a dissociated acylium ion or a complex of the acid chloride and Lewis acyl.49 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.50... 

Recent mechanistic studies were conducted by Ragaini et al. using tetracarbonyl rhodate complexes. Several potential intermediates such as (152) have been isolated.584 585 The catalytic system has been optimized and the influence of solvent was examined.586 2-Hydroxypyridine has a large activating effect on the [PPN][Rh(CO)4]-based catalytic system for the reductive carbonylation of nitrobenzene to Me phenylcarbamate 587... 

Widenhoefer has also disclosed an interesting extension consisting of hydrosilylative cyclization of a diene catalyzed by palladium. High enantioselectivity (up to 95% ee) was achieved by using palladium catalysts with Ci-symmetric pyridine-oxazoline ligands351,364 and recent mechanistic studies have confirmed the involvement of an intramolecular carbometallation step.365... 

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

These recent mechanistic studies have provided the foundation for the most recent work that has expanded the scope of iridium-catalyzed allylic substitution. The synthesis and characterization of the ethylene-bound complex lb resulted directly... 

Recent mechanistic studies using HP infrared equipment, as well as HP-NMR measurements involving the use of CO and CH3I, have allowed the iridium intermediates which are present in solution as methyl acetate and water, and are consumed to produce acetic acid [.12, 34, 41-43], to be followed. All of these observations can be rationalized by a single catalytic cycle (see Figure 8.5), in which equilibria exist between the neutral and anionic complexes for all species. The main species involved in the carbonylation, which are detected in batch mode under carbonylation conditions [34], and correspond to the slower steps of catalysis, are the methyl—iridium and acetyl-iridium complexes [Ir(CH3)l3(CO)2] and [Ir(COCH3)l3(CO)2] respectively. 

Interest in the field has existed since the late 19th century and to attempt a comprehensive review would be a gargantuan task. This chapter attempts to draw together some of the principles involved and to deal only in some detail with the more recent mechanistic studies. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Oxidation of primary N-aminobenzimidazole 32 with PhI(OAc)2 4 in the presence of olefins gives aziridines 34 [54]. Similar oxidations are effected by lead tetraacetate. The reaction was initially proposed to involve the intermediacy of AT-nitrene as a reactive species, thought to be produced through reductive a-elimination of amino-A3-iodane 33. Recent mechanistic studies on lead tetraacetate oxidation, however, suggests that the acetoxyamine 35 instead of AT-nitrene is the aziridinating species, and the reaction proceeds through a transition state 36 similar to that of epoxidation using peracids [55]. 

In a recent mechanistic study it was shown that ferf-butyl hydroperoxide acts most likely as a precursor for the formation of a cobalt peroxide intermediate (entry 3)... 

Recently, mechanistic studies have been extended to the substitution reactions of octahedral complexes of Pt(IV). It has been found, for example, that the organic base pyridine replaces chloride in the complex Pt(NH3)3ClJ at a rate proportional to the concentration of pyridine ... 

Cathodic conversions of the carboxyl group and its derivatives have been long known, and tabulations and discussions of these reactions can be found in several places [1-5]. Most of the earlier work on these processes involved constant-current reduction with careful selection and control of solvent, temperature, pH, and electrode material. More recently, mechanistic studies of these processes have been made using modern electroana-lytical techniques, but our knowledge in this area remains far from complete. 

The predominance of nuclear methoxylation over competing alkyl side-chain oxidation processes for these systems was largely attributed to an intramolecular attack by the appended side-chain alkoxy radical on the anodically generated aromatic radical cation in the usual EEQCp mechanism [99]. A more recent mechanistic study of anodic oxidation of chiral alkoxynaphthalene derivative (LXXXIX) sheds some additional light on this process [Eq. (46)] [100]. The isolation of a 2 1 mixture of enantiomeric methoxylated products indicated that intramolecular attack by the appended alkoxy radical on the aromatic radical cation (path B) was disfavored at —7S°C (a nearly 1 1 mixture of enantiomers was obtained at 25 C), since the intermediate cyclic ketal resulting from such a process would be a meso form that would lead to a racemic mixture of methoxylated products. Note that the chirality of the appended hydroxyether side-chain disappears upon cycliza-tion to the acetal. 

The mechanism of the deprotonation of dipole-stabilized anions has been studied in detail. It has been shown by IR spectroscopy that a preequilibrium exists between the butyllithium base and the amide or amidine, forming a coordination complex prior to deprotonadon. A recent mechanistic study has shown that, in cyclohexane solvent, this prior coordination is between the amide (or added TMEDA) and aggregated s-butyllithium, and that the effect of the coordination is to increase the reactivity of the complex. The diastereoselectivity of proton removal in chiral benzylic systems has also been examined,but since the anions invert, this selectivity is of little consequence in the alkylation step. 

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