Carbonates relative cleavage rates

These results indicate that proton transfer occurs in the ratedetermining step and there is little carbon—carbon bond cleavage in the transition state. Clear-cut evidence for a two-step mechanism is supplied by Lynn and Bourns results of variable carbon-13 isotope effects in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers [249] (Table 22). Slow proton transfer occurs in the first step and C—C bond cleavage takes place in the second step. At high concentrations of the buffer base, the rate of reversal of the first step becomes comparable to the (relatively fast) rate of the second step and, consequently, the second step becomes partially rate-determining which causes a weak carbon isotope effect. The most reasonable structure of the intermediate is that of the sigma complex. 

The increase of the rate coefficient with increasing acidity of the solution is probably caused by an additional reaction between ArCOOH and H30+, according to rate eqn. (68) (involving [H30+]) as observed also in the decarboxylation of aromatic amino-acids. This assumption is confirmed by comparisons of isotope effect data. According to Bourns results [247] of carbon isotope effects (Table 22), carbon—carbon bond cleavage is still relatively unimportant in the transition state of the reaction in 1 M HC104. On the other hand, the solvent isotope effect is decreased from ca. 6 in 0.02 M HC1 to 2.0 in 1.0 M HC1 (Table 24) [259]. If the reaction sequence... 

The thermal reactions of the pyridinium borate salts are likely to follow the same electron-transfer path. Experimental evidence for this conclusion is the fact that the 5cc-butyl transfer is substantially faster than methyl transfer although a nucleophilic substitution mechanism would predict the less hindered group to be transferred preferentially. The fast rates of 5cc-butyl transfer can be readily explained on the basis of the electron-transfer mechanism (Eqs. 69-71) by considering the different boron-carbon bond strength [189, 190] for the various alkylborates. The boron-carbon bond cleavage (Eq. 70) is apparently the critical step, and its relative rate [191] as compared to that of the back electron transfer determines the overall rate for thermal alkyl transfers in pyridinium tetraalkylborate salts. 

The propensity for C-N vs. N-H activation correlates well with substituent Hammet parameters groups that increase the basicity of aniline increase the relative rate of N-H activation, suggesting that nucleophilic attack by the amine at an empty d /dy orbital of Ta(silox)3 preceeds oxidative addition. On the other hand, electron-withdrawing substituents decrease the rate of N-H activation and increase the rate of C-N activation, similarly to the effects observed on electrophilic aromatic substitution. Nucleophilic attack by the filled d a orbital of Ta(silox)3 is expected to occur at the arylamine ipso carbon preceding C-N oxidative addition. The carbon-heteroatom cleavages can be accomodated by mechanisms using both electrophilic and nucleophilic sites on the metal center. 

Grignard carbon relative reactivities are 1° < 2° > 3°. In ring cleavage, alkyl substitution at Rj appears to have little effect on rate, while alkyl at R4 decreases the rate. Halogen at Rj or Rj or phenyl at R3 decreases the rate of cyclization. Phenyl at Ri, R2 produces a sizable decrease in cyclization rate for three-membered ring formation, but a slight increase for the five-membered ring. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

The catalysis of the cleavage of carbon-halogen bonds by complexation with metal ions such as silver or mercuric ion is a well-known phenomenon. The compounds susceptible to this action are alkyl halides capable of forming car-bonium ions. The complexed anions such as in mercuric nitrate, mercuric perchlorate, or hydrated mercuric ion do not exhibit a simple relationship between their effect on the total rate and on the relative distribution of products stemming from water or the anion. This evidence is indicative of the following catalytic mechanism ... 

The first step of the activation of butane and cyclohexane has been assumed to be the cleavage of a secondary C—H bond, with minor contributions from primary C — H bonds in the case of butane. This picture is supported only by indirect evidence. When the relative rates of reaction of various alkanes were compared on a V-Mg oxide and Mg2V207 catalyst (Table VIII), it was found that alkanes with only primary carbons (ethane) reacted most slowly. Those with secondary carbons (propane, butane, and cyclohexane) reacted faster, with the rate being faster for those with more secondary carbon atoms. Finally, the alkane with one tertiary carbon (2-methylpropane) reacted faster than the ones with either a single or no secondary carbon (26). From these data, it was estimated that the relative rates of reaction of a primary, secondary, and tertiary C—H bond in alkanes on the V-Mg oxide catalyst were 1, 6, and 32, respectively (26). 

Stereochemical retention of configuration around the saturated a-carbon can only result from the closed transition state, whereas the open transition state can lead to inversion (10, 193). Configurational inversions occur in many biological substrates, and much future work on biomethylation will likely involve the use of substituted methyl groups and the study of any optical changes that occur. Determination of the relative rates of cleavage for a series of normal and branched alkyl derivatives enables a distinction between retention or inversion in the SK2 (open) pathway (194). 

RELATIVE RATES OF ELECTROPHILIC CLEAVAGE OF THE CARBON-METAL BOND IN SOME /7-SUBSTITUTED BENZYL-METAL AND... 

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