Cleavage, and formation reaction

The kinetic parameters for the bridge cleavage and formation reactions show a number of trends which in some cases parallel those... 

A large body of thermodynamic as well as kinetic data for intramolecular bridge-cleavage and -formation reactions has been accumulated during the last two decades. These reactions are usually very fast and reversible, and in this sense are related to the preequilibria observed during the hydrolysis of bidentate ligands. 

Carbon-Carbon Bond Cleavage and Formation Reactions, a Comparison with CO Oxidation... 

The metathesis of linear alkynes has also been reported, e.g. the metathesis of propyne, 1-pentyne, 2-pentyne, and 2-hexyne (31-33). This reaction can be visualized as the cleavage and formation of carbon-carbon triple bonds ... 

The NMR spectra have shown the formation of Schiff base as an intermediate product in the synthesis of the fully N-deacetylated oligomers from chitosan.32 The mechanism of the Schiff base reaction leading to chain cleavage and formation of 5-hydroxymethyl-2-furfural has been proposed. 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The collision theory is silent on the cleavage and formation of bonds involved in the reaction. 

Electrochemical fluorination in anhydrous hydrogen fluoride (Simons process) involves electrolysis of organic compounds (ahphatic hydrocarbons, haloalkanes, acid halides, esters, ethers, amines) at nickel electrodes. It leads mostly to perfluori-nated compounds, but is accompanied to a high extent by cleavage and rearrangement reactions. The mechanism of the formation of carbocations according to Eq. (1) and Scheme 1 is assumed... 

Jenner [275] has presented a thorough description of several possible contributions to both the intrinsic and the environmental parts of the activation volumes, based on accurate experimental observation of pressure effect on reactions in solutions. The intrinsic contribution to the activation volume essentially derives from the differences in structure between the transition state and the reacting species, so it is directly related to the partial cleavage and formation of chemical bonds in the transition state. In cases where the environmental contribution is negligible, the activation volume variation gives a direct insight in the molecular mechanism [275, 280]. In this case in fact, considering... 

Treatment of 2-, 3- or 4-MePhCu with DPPM affords the trimeric aggregate Cu3[CH(PPh2)2]3 in quantitative yield [56]. This is in fact a simple acid-base reaction (Cu/H exchange). It should be noted that the latter compound is also accessible by means of a transmetalation reaction between Li[CH(PPh2)2] and CuCl. In contrast, treatment of [Cu4(QH4CH2NMe2-2)4] with DPPE results in selective C-P bond cleavage and formation of Cu2(PPh2)2(DPPE)2 (see Scheme 1.11) [57]. 

Such a cleavage and formation of bonds occur in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. 

In addition to simple electron transfers in which no chemical bond is either broken or formed, numerous organic reactions, previously formulated by movements of electron pairs, are now understood as processes in which an initial electron transfer from a nucleophile (reductant) to an electrophile (oxidant) produces a radical ion pair, which leads to the final products via the follow-up steps involving cleavage and formation of chemical bonds [11-23], The follow-up steps are usually sufficiendy rapid to render the initial electron transfer the rate-determining step in an overall irreversible transformation [24], In such a case, the overall reactivity is determined by the initial electron-transfer step, which can also be well designed based on the redox potentials and the reorganization energies of a nucleophile (reductant) and an electrophile (oxidant). 

Griengl, H., Hickel, A., Johnson, D.V., Kratky, C., Schntidt, M. and Schwab, H. (1997) Enzymatic cleavage and formation of cyanohydrins a reaction of biological and synthetic relevance. Chem. Commun., 1997,1933-1940. 

The description of the borderline between stepwise and concerted nucleophilic substitution remains murky in cases where there is no significant stabilization of the transition state for the concerted reaction through the coupling of bond cleavage and formation. The reason is that there are no simple experimental protocols to detect the point at which the energy well for the carbocation intermediate of the stepwise reaction in the upper right hand corner of Figure 2.3 is transformed into... 

From mechanistic considerations and assuming that cleavage and formation of (R)-benzoin are in equilibrium, BAL should also catalyze carboligation. Consequently, BAL-catalyzed acyloin condensation of benzaldehyde in an aqueous buffer/DMSO mixture resulted in almost quantitative formation of enantiomeri-cally pure (R)-benzoin [Scheme 2.2.7.21, Eq. (1)]. The reaction was carried out on a preparative scale with different aromatic and heteroaromatic aldehydes [62]. From the viewpoint of the organic-preparative chemist, it is important to mention that crude cell extracts of the recombinant E. coli strain overexpressing the BAL gene are sufficient for catalysis, hence, purification of the enzyme is not necessary. 

The reactions of dinuclear thioaldehyde and -ketone complexes with PR3 take a different course. When the dimeric complexes 112 are treated with PMe3 a Ti-S bond in 112 is cleaved and the monomeric complexes 113 are reversibly formed (see Scheme 26), 69 The reaction of 112 with benzene-thiol led to a Ti-C cleavage and formation of 1,3-propanedithiolato-(phenylthiolato) complex.69 The reaction of binuclear p-191 rf-thioketone molybdenum complexes related to 115 (M = Mo, E = S, R = Aryl) with P(OEt)3 also gave mononuclear complexes [Mo(CO)2(Et) i72-S = C(Aryl)2 (i75-C5H5)] by a rather complicated reaction sequence. When other phosphites were employed additionally binuclear complexes derived from addition of the phosphite to one molybdenum atom were isolated.225... 

The data in Table XXXV show that common features for these ammonia and amine complexes are very fast isomerization between the cis and trans isomers of the diaqua species and the fact that the trans diaqua isomers are generally more stable than the cis isomers. In the ammine system the activation parameters for k2 and k 2 are consistent with an isomerization process at cobalt(III), but it is at present not clear how this occurs. It need not be a simple cis-trans isomerization occurring at one of the Co(III) centers, but might involve the participation of both metal centers. The isomerization reaction may proceed via intramolecular proton transfer between a water ligand and one of the two hydroxo bridges with simultaneous bridge cleavage and formation... 

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