Kinetic substituent effects

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their 

The additivity of substituent effects, demonstrated by (16), was the first kinetic evidence for a symmetrical charge distribution in the rate-limiting transition states of alkene bromination. 

As expected, deviations from (16) are found for alkenes bearing one or several branched groups. In order to take into account the steric effects, the authors applied the extended Taft relationship, log(/c//c0) = p a + S Es. The resulting correlation is (17) where p, now —5.4, is considerably more 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for 

Because of the absence of any obvious reference value, the p -value of — 3.1 is not readily discussed in terms of charge magnitude or brominebridging at the rate-limiting transition states. For alkene hydration, it is now accepted that the intermediates are carbocations (20). The corresponding structure-reactivity relationship (21) is obtained by using o and x+ 

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It is still not clear whether steric constraints can modify the magnitude of bromine bridging. There is at least one example that suggests that this is possible. Non-additive kinetic substituent effects and cis-dibromoadducts imply that the intermediate of cis-cyclooctene bromination in methanol is a /J-bromocarbocation (Dubois and Fresnet, 1974). 

The question of bridged and/or open intermediates has been considered in Section 4, where the data on kinetic substituent effects were discussed with the help of the multipathway scheme (Scheme 7) to determine the relative importance of bromonium and carbocation paths. It is not straightforward to obtain significant p-values for each of them from the complex pa relationship (34) and (35) corresponding to this scheme for an a,/f-Ri,R2... 

An extended form of (39) has been used to analyse kinetic substituent effects on bromination of alkenes GRaC=CR()R) , where G is a conjugatively electron-donating group and R is alkyl (Bienvenue-Goetz and Dubois, 1981). 

Steric control has been invoked to explain the kinetic substituent effects as well as the syn stereoselectivity observed in these additions, for example to fraws-cyclooctene and traws-cyclononene. In these cyclic compounds, one side of the TT-bond is more shielded by the rest of the molecule and hence anti attack by a nucleophile is difficult. 

The Y-T equation has been used to analyse the substituent effects on carbocation formation equilibria in the gas phase. These correlations are compared with those for the kinetic substituent effects in the corresponding solution phase solvolyses in Table 17 and substituent effects on thermodynamic basicities of carbonyl groups in both phases are compared in Tables 18 and 19. 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

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