Acetic deactivation mechanism

The first step of the reaction is likely to be the protonation of ethylene to produce a carbocation that undergoes the direct addition of acetic acid to produce ethyl acetate. The successive addition of ethylene to the carbocation leading to the production of alkene oligomers is a likely side reaction Formation and accumulation of these oligomers could eventually deactivate the catalyst. Detailed studies for a better understanding of the complex reaction mechanism are in progress. 

Extensive investigations in our laboratories on the deactivation of rhodium and iridium catalysts has shown there to be a number of different mechanisms involved. Both, rhodium and iridium catalysts are generally less stable at higher temperatures, and have more labile ligands than their ruthenium counterparts. All of the catalysts are affected by pH, but the ruthenium catalysts seem to be more readily deactivated by acid. Indeed, these reactions are often quenched with acetic acid, whilst stronger acids are used to quench the rhodium reactions. Each of the catalysts can be deactivated by product inhibition, the ruthenium catalyst with aromatic substrates such as phenylethanol, and the rhodium and iridium ones by bidentate chelating products. 

The preferred position for electrophilic substitution in the pyridine ring is the 3 position. Because of the sluggishness of the reactions of pyridine, these are often carried out at elevated temperatures, where a free radical mechanism may be operative. If these reactions are eliminated from consideration, substitution at the 3 position is found to be general for electrophilic reactions of coordinated pyridine, except for the nitration of pyridine-N-oxide (30, 51). The mercuration of pyridine with mercuric acetate proceeds via the coordination complex and gives the anticipated product with substitution in the 3 position (72). The bromina-tion of pyridine-N-oxide in fuming sulfuric acid goes via a complex with sulfur trioxide and gives 3-bromopyridine-N-oxide as the chief product (80). In this case the coordination presumably deactivates the pyridine nucleus in the 2 and... 

Pyridine /V-oxide is unreactive toward iron-catalyzed bromination at 110°C (55JA2902), but silver sulfate-catalyzed bromination in sulfuric acid at 200°C gives a 10% yield of 2- and 4-bromination in the ratio 1 2 (6ITL32). With bromine in oleum the main product is 3-bromopyridine /V-oxide (60%) together with the 2,5-dibromo (—35%), and 2,3- and 3,4-dibromo compounds (—5%) (62T227). Presumably the N-oxide function is here complexed with sulfur trioxide, which causes deactivation and 3-orientation. Bromination in acetic anhydride also gives 3-substitution (35%) an addition-elimination mechanism has been proposed (65JPJ62). 

Deactivating chain transfer to monomer is quite common in polymerization of allyl monomers [40-42], Allyl radicals such as that of allyl acetate are resonance-stabilized, with the result that polymerization rates and molecular weights remain low. Moreover, with chain transfer as the dominant termination mechanism, the termination rate is first order in free radicals. This lets the free-radical population become proportional to the initiator concentration and leads to a polymerization rate that is first order rather half order in initiator and zero order in monomer. 

In the light of this mechanism, several well-known facts concerning diazonium coupling reactions are readily explicable. Sodium acetate or sodium carbonate is usually necessary for coupling to lower the acidity of the reaction mixture and to increase correspondingly the concentration of free base or phenoxide ion. There is little probability of reaction between diazonium and substituted anilinium ions since they are of like charge and the aromatic nucleus of the anilinium ion is deactivated ... 

The ester/acetal groups are disarmed by different mechanisms while esters disarm electronically, benzylidene and isopropylidene acetals were considered to be disarmed by torsional strain. This deactivation was extensively studied by Bols,180 who used the... 

Formation of cyclic acetals by sugar hydroxyls generally retards nucleophilic displacements at the anomeric centre of the same sugar residue. In the case of 4,6-benzylidene derivatives, the mechanism of deactivation appears to be that the dipole of the C6-06 bond is constrained with its positive end directed towards the anomeric centre.In the case of the 1,2-diketals, the deactivation arises from the increased difficulty of forming half-chair or boat conformations in a six-membered ring, which is part of a traw -fused decalin structure. 

The acylation of benzofuran by acetic anhydride was carried out in the presence of Y zeolites in the liquid phase (60°C, atmospheric pressure). It is shown that the reaction procedure has a significant influence on the activity of the catalyst. Deactivation takes place but the zeolite can be completely regenerated by reactivation in air. A reaction mechanism is proposed in which the acylium ion adsorbed on the zeolite reacts with non activated benzofuran. 

A plausible mechanism would be electrophilic attack of Pd(OAc)2 followed by alkene insertion and subsequent 3-H elimination to afford the vinylated product and HPdOAc. It is believed that lithium acetate prevents deactivation of palladium(O), which is formed when HPdOAc releases acetic acid. ... 

Transition metal halides can also act as transfer agents. For example, copper(ii) chloride or iron(iii) chloride may be applied. Transfer coefficients for these two halides have been determined in DMF at 60 °C. For copper(ii) chloride, the transfer coefficients are C= 10", C= 10 and C= 10 for styrene, MMA and acrylonitrile, respectively. Iron(iii) chloride is less efficient and gives values of C=626, C=306, C=86, C=4 and C=2 for vinyl acetate, styrene, vinyl chloride, MMA and acrylonitrile, respectively. " The transfer process forms alkyl halides and metal species in lower oxidation states, the latter occasionally being capable of activating the former. Such a mechanism of reversible activation and deactivation is utilized in atom transfer radical polymerization (ATRP). 

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