Reactions carbon monoxide dissociation

M. Torrent, M. Duran, and M. Sola, Density Functional Study on the Preactivation Scenario of the Dotz Reaction Carbon Monoxide Dissociation versus Alkyne Addition as the First Reaction Step, Organometallics 17, 1492-1501 (1998). 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Alcohols and jlkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.188 The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reducing agent. A variety of heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide dissociates on the catalytic surface to give carbides and that these are in turn hydrogenated to give surface carbenes 1 " n ... 

This variance in intimate mechanisms is likely seen in the reactions of HCr(CO)5 and HW(CO)j with C02 to provide HC02M(C0)j derivatives (45). For example, carbon monoxide dissociation in the chromium anion, as evinced by l3CO exchange studies, occurs at a rate indistinguishable by conventional techniques from that of C02 insertion. Consistent with this observation, the rate of decarboxylation of HC02Cr(CO)jis retarded in an atmosphere of carbon monoxide. Similar behavior was noted in decarboxy-... 

While the hydrogenation of the active surface carbon that forms from CO dissociation appears to be the predominant mechanism of CH4 formation, it is not the only mechanism that produces methane. Poutsma et al. [85] have detected the formation of CH4 over paliadium surfaces that do not readily dissociate carbon monoxide. They also observed methane formation over nickel surfaces at 300 K under conditions in which only molecular carbon m.onoxide appears to be present on the catalyst surfaces [81]. Vannice [86] also reported the formation of methane over platinurh, palladium, and iridium surfaces, and independent experiments indicate the absence of carbon monoxide dissociation over these transition-metal catalysts in most cases. It appears that the direct hydrogenation of molecular carbon monoxide can also occur but that this reaction has a much lower rate than methane formation via the hydrogenation of the active carbon that is produced from the dissociation of carbon monoxide in the appropriate temperature range. 

The mechanism of this reaction was investigated to find out if carbon monoxide dissociation is the rate-determining step (16). The rate of the reaction of acetylcobalt tetracarbonyl with iodine is too fast to measure under conditions which allow dissociation rate to be measured easily. Thus, dissociation is not rate-determining, and the acylcobalt tetracarbonyl and the iodine must be reacting directly. Further studies with the less reactive acyl(triphenylphosphine)cobalt tricarbonyls showed that the first step in the reaction with iodine is a rapid cleavage of the cobalt-carbon bond to form acyl iodide and iodo(triphenylphosphine)cobalt tricarbonyl. 

Coordinatively Unsaturated Cobalt Carbonyls Relevant to Hydro-formylation. The negative effect of carbon monoxide partial pressure on the rate of hydroformylation was the first indication of the participation of coordinatively unsaturated cobalt carbonyls in the catalysis of aldehyde formation and of the accompanying olefin isomerization. The retarding effect of carbon monoxide has also been observed in cobalt-catalyzed olefin and aldehyde hydrogenation and in various other reactions of cobalt carbonyls as well. It was assumed that in these reactions in fast reversible carbon monoxide dissociation highly reactive coordinatively unsaturated complexes are formed in very low concentrations, undetectable by conventional analytical methods. By using sophisticated new methods, in some cases the detection and characterization of these elusive species has become possible. 

Only recently has a mechanism been proposed for the copper-cataly2ed reaction that is completely satisfactory (58). It had been known for many years that a small amount of carbon dioxide in the feed to the reactor is necessary for optimum yield, but most workers in the field beHeved that the main reaction in the formation of methanol was the hydrogenation of carbon monoxide. Now, convincing evidence has been assembled to indicate that methanol is actually formed with >99% selectivity by the reaction of dissociated, adsorbed hydrogen and carbon dioxide on the metallic copper surface in two steps ... 

These values suggest that the two hydroxycarbene isomers convert into one another very easily. The barrier to molecular dissociation of the cis form is significant, however, and so this structure probably does not dissociate directly, but rather first converts to the trans isomer, which is subsequently transformed into formaldehyde, which dissociates to carbon monoxide and hydrogen gas. The article from which this study was drawn computes the activation energy for the trans to cis reaction as 28.6 kcal- moT at RMP4(SDQ)/6-31G(d,p) (it does not consider the other reactions). 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

The reaction of CDI with primary phosphines was expected to lead first to an azolide ImCOPHR, analogous to imidazole-N-carboxamide as the reaction product of primary amines and CDI. In fact, reaction of phenylphosphine with CDI leads directly to imidazole, carbonmonoxide, and tetraphenylcyclotetraphosphine (THF, reflux, 5h). In analogy to the dissociation of imidazole-AT-carboxamide into isocyanates and imidazole, this can be explained by the assumption that the first-formed ImCOPHC6H5 dissociates into an isocyanate analogue, C6H5P=C=0, which is unstable and decomposes into carbon monoxide and phenylphosphene (C6H5P) which tetramerizes. However, the intermediate formation of phenylphosphene has not yet been definitely proved. 

At the present, it is difficult to predict a distinct rhodium catalyst showing the appropriate properties. Furthermore, the reaction conditions applied will influence the outcome of the reaction also. Low carbon monoxide pressure favours p-hydride elimination by enhanced CO dissociation which allows for the formation of vacant sites at the metal... 

The bidentate formate ligand of OsH(K2-02CH)(CO)(P,Pr3)2 is converted into a monodentate group by carbonylation. Thus, the reaction of this compound with carbon monoxide gives 0sH K1-0C(0)H (C0)2(P Pr3)2. Similarly, the addition of a stoichiometric amount of trimethylphosphite yields 0sH k -0C(0)H (C0) P(OMe)3 (P Pr3)2, and the addition of a stoichiometric amount of ethyne di-carboxylic methyl ester leads to 0sH K1-0C(0)H (C0)(r 2-Me02CC=CC02Me) (P Pr3)2, which in solution partially dissociates the alkyne. As is shown in... 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

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