Methyl species

Successive hydrogenation produces a metal-methyl species accompanied hy the release of water ... 

The biological cycle of arsenic in the surface ocean involves the uptake of arsenate by plankton, the conversion of arsenate to a number of as yet unidentified organic compounds, and the release of arsenite and methylated species into the seawater. Biological demethylation of the methyl-arsenicals and the oxidation of arsenite by as yet... 

The EPR spectrum shows, in accordance with the XPS results, no feature that can be attributed to Ti centers. What is the nature of the radical observed in the EPR spectrum It might be thought that methyl radicals are the most natural products in the reduction of a mixed titaniiun-chlorine-methyl species according to the following simple reaction scheme ... 

The results obtained indicate that in the reaction between phenol and methanol, formaldehyde is the trae methylating agent when basic catalysts are used. This indicates that the type of transformation occurring with methanol is the factor that mainly differentiates performances in phenol methylation when catalyzed by either basic or acid catalysts. The catalyst plays its role in the generation of the methylating species the nature of the latter then determines the type of phenolic products obtained. 

In order to gather more information about this problem, it was deemed worthwhile to follow the energetics of the alkylation reaction of water by methyl-, ethyl-, and fluo-roethyldiazonium ions. The main goal of these calculations was to establish whether transition-state calculations can provide information about hard versus soft electrophilic character of these species.12 Computations at Hartree-Fock and MP2 level were performed using the 6-31G basis set. It was found that both at the Hartree-Fock level and when correlation energy affects were included, the ethyl and fluoroethyl species do not show the presence of a transition state, while the methyl species show a small transition state. It was concluded that transition state computations cannot shed light on the characters of these species. 

Fig. 12 Calculated structure of an ethylene-coordinated cationic methyl species derived from FI catalyst 1...

To summarize briefly, our approach involves initial attack by a relatively nucleophilic metal hydride on coordinated CO. Such reactivity has been demonstrated repeatedly for main-group metal hydrides perhaps the most elegantly worked-out system involves CpRe(C0)2(N0)+ (Cp = Tl-C H ) which, under varying conditions, can be converted to an entire range of products containing CO at different stages of reduction, including formyl, carbene, hydroxymethyl and methyl species (Scheme l). Reactions lead-... 

The rate-determining step in this process is the oxidative addition of methyl iodide to 1. Within the operating window of the process the reaction rate is independent of the carbon monoxide pressure and independent of the concentration of methanol. The methyl species 2 formed in reaction (2) cannot be observed under the reaction conditions. The methyl iodide intermediate enables the formation of a methyl rhodium complex methanol is not sufficiently electrophilic to carry out this reaction. As for other nucleophiles, the reaction is much slower with methyl bromide or methyl chloride as the catalyst component. 

In spite of the difficulties mentioned above, Brookhart and co-workers succeeded in measuring the barrier for ethene insertion into (dppp)PdC(0)CH3+ at 160 K, starting from the ethene adduct, generated at still lower temperatures, in the absence of CO. The barrier measured (AG ) amounted to only 51.4 kJ/mol, i.e. the reaction is faster than the insertion of CO in an ionic alkylpalladium complex. The barrier of insertion of ethene into a palladium methyl species or palladium ethyl species was higher, at 67 kJ/mol at 233 K. As for the CO insertion described above, these values concern the barriers in preformed ethene adducts at higher temperatures the overall barrier will be higher, because alkene coordination will be disfavoured by entropy and competition with CO and solvent. Formation of CO adducts will also be less favourable at higher temperatures. 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

Another experiment in which sequential adsorption of phenol and pyridine then followed by methanol shows formation of pyridinium ion and phenolate anion whereas no traces of methanol or electrophilic methyl species or formation of methylated products were identified on the catalysts surface. This result was supposedly confirmed from another experiment in which anisole and methanol were co-adsorbed on the catalyst. The spectra were referred to the molecular species of methanol and anisole without any significant interaction among them and above 200°C they simply desorbed from the catalyst. 

A summary of aniline N-methylation mechanistic features on Cui xZnxFe204 ferrospinel catalysts is given in Figure 27. It was possible, due to in-situ IR studies, to observe a dissociative adsorption and possible orientation of reactants on the catalyst surface, their conversion to product at low temperatures, and desorption-limited kinetics, all under conditions that are close to the reaction conditions. Although Cu is the active center for the aniline A-methylation reaction, and IR studies reveal that Zn acts as the main methyl species source. 

Needle like nanoparticles of Mg-Al mixed spinel catalysts synthesized under hydrothermal conditions were used for the synthesis of 1-methylimidazole by the gas phase imidazole methylation with methanol performed at atmospheric pressure [113]. High yield and selectivity to desired product were obtained at a temperature range between 320 and 350°C. It was proposed that one of the nitrogen atoms participates in the bonding of the imidazole with the basic site of the catalyst, and the second nitrogen atom is accessible for the reaction with electrophilic methyl species formed from methanol on acid site of the catalysts. 

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

S.2 Identification of the Cataiytically Active Species The Chemistry of Croup 4 Metal Methyl Species 313, CI AICIR2... 

Identification of the Catalfticaily Active Species The Chemistry of Group 4 Metal Methyl Species 315... 

Thermochemical data on ahphatic hydroxylamines is sparse. Indeed, it is almost totally limited to methylated species and so we include both N- and O-methyl substituted species... 

MI33>. Covalent hydration of pyrimido[4,5- pyrimidines was reported in CHEC-II(1996) <1996CHEC-II(7)737> covalent hydration across the 7-8 bond of 6- and 8-methylated species 68 and 69 has been investigated in detail <2003EJM719>. 

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