Common Origin of Enthalpic and Entropic Substituent Effects in Reactions of Benzhydryl Cations with Nucleophiles

Abstract

Dedicated to Professor Christoph Riichardt on the occasion of his 65th birthdayThe investigation of substituent effects is one of the most important tools for the determination of reaction mechanisms. While effects of substituents on equilibria and rates of chemical reactions are usually attributable to differences in enthalpy, changes in the entropy of activation, particularly for reactions of reactive intermediates (fast reactions), have been recognized to be the origin of observed substituent effects." -31 For the reactions of para-substituted benzhydryl cations with n-nucleophiles, we now demonstrate that the change from enthalpic to entropic substituent effects occurs within these reaction series on going from slow to fast reactions. Consequently, the reactivity of the nucleophile determines whether changing substituents in the electrophile affects AH * or AS * .Detailed mechanistic investigations of the reactions of benzhydryl cations with a l k e n e~[~] and other n-systemsr5] have shown that generally the C-C bond formation is the rate-determining step of the reaction sequence depicted in Scheme 1, that I ir Scheme 1. The reaction of benzhydryl cations with compounds containing n-elec-is, the measured rate constant represents the attack of the carbenium ion at the n-system. Reduction of the electron-donating ability of the substituents X and Y in the carbenium ion 1 gives rise to an acceleration of the reactions, which is caused entirely by the decrease of the enthalpy of activation while the entropy of activation remains constant within the accuracy of the measurements (Fig. l). An analogous pattern has been observed forFor the reaction of the bis(p-toly1)carbenium ion (1 ; X = Y = CH,) with 2-methyl-2-butene (2) in CH,Cl,, rate constants k of (6.9 k 0.3) x lo4 Lmol-'s-', almost independent of temperature between -70 and -4O"C, have been observed, from which an activation energy E, of (0.4 0.6) kJmol-' and an enthalpy of activation AH * of (-1.4 k 0.6) kJ mol- (Fig. 1 top) can be calculated. Though no enthalpic barrier is present, the observed rate constants are five orders of magnitude below the diffusion limit as a result of the large entropy term -TAS * .How do carbenium ions, which are less stabilized by substituents than 1 (X = Y = CH,), behave? To investigate the reactions of more electrophilic benzhydryl cations (1) with the same nucleophiles (2-5), we used the laser pulse method, employing acetonitrile as a solvent because photolysis of benzhydryl chlorides in dichloromethane almost entirely causes homolytic cleavage of the C-Cl bond.[*] As variation of the solvent has only a very small influence on the rates of the reactions of carbenium ions with neutral nucleophiles,~'~ 91 the rate constant measured for the reaction of the bis(p-chloropheny1)carbenium ion (1, X = Y = Cl) with 2 ( k =7.4 x lo7 Lrnol-'s-', CH,CN, 20 "C) indicates that the replacement of X = Y = CH, by C1 in 1 causes an increase of reactivity by almost three orders of magnitude.['0] Since there was no enthalpic barrier even for...

Substituent Effects on the Reactivity of the Silicon−Carbon Double Bond. Mechanistic Studies of the Ene-Addition of Acetone to Reactive Arylsilenes

“…In such situations, the negative activation energy results from the free energy of activation for the second step being dominated by the entropic term, with the enthalpy of the transition state lower than that of the free reactants. This is a common feature of fast reactions which proceed by mechanisms of this general type, − but it does not in itself rule out the possibility that the reaction proceeds by a concerted mechanism. ,,,− The Arrhenius plots for addition of acetone to 4c , 4d , and 4g depend on the substituent in a manner which is strikingly similar to alcohol additions, however, for which excellent evidence for the involvement of complexes exists. ,− Thus, we favor the analogous mechanism for ketone additions, even though there is presently no direct evidence for the involvement of complexes in the reaction. Within the confines of this mechanism, the trend in the temperature dependences can be rationalized in terms of the variations of the individual rate constants for formation of ( k 1 ), reversion of ( k - 1 ), and hydrogen transfer within ( k 2 ) the association complex as a function of substituent.…”

Cationic Copolymerization and Multicomponent Polymerization of Isobutylene with C4 Olefins

“…It is obvious, therefore, that the propagation rate constant of any monomer that is less reactive than styrene will also be diffusion limited (e.g., p -chlorostyrene). The linear free energy relationship (LFER) − is usually used to predict rates of reactions of carbocations with alkenes and other types of nucleophiles: where E is an electrophilicity parameter, N is a nucleophilicity parameter, and s is a nucleophile-specific slope parameter, which is usually close to 1 and can be neglected in semiquantitative treatments of reactivity. Therefore, monomers with N values less than styrene (0.78) should show diffusion-limited propagation.…”

Nucleophilic substitution of the nitro group, fluorine and chlorine in aromatic compounds

“…105 It is known that the energies of bimolecular S N Ar reactions depend on the nature of the leaving group and the nucleophile, 1 whereas the reactivities in these processes are under enthalpy control. 11 Entropy control itself is effective in fast bimolecular reactions occurring in solution involving radicals (k 2 = 10 6 ± 10 8 litre mol 71 s 71 from 785 to 710 8C), 151 carbenes (k 2 = 10 4 ± 10 9 litre mol 71 s 71 at 25 8C) 152 or carbocations (k 2 = 10 71 ± 10 8 litre mol 71 s 71 from 770 to 730 8C) 153 where the contribution of enthalpy is either very small or constant as a result of which the transition state is shifted to earlier position on the reaction coordinate. 152,153 Recently, it has been found that the selectivity of substitution of the nitro group and fluorine in S N Ar reactions is connected with entropy control, 154,155 although bimolecular S N Ar reactions occur at a relatively slow rate (k 2 = 10 74 ± 10 71 litre mol 71 s 71 at 25 8C in DMSO or DMF).…”

“…11 Entropy control itself is effective in fast bimolecular reactions occurring in solution involving radicals (k 2 = 10 6 ± 10 8 litre mol 71 s 71 from 785 to 710 8C), 151 carbenes (k 2 = 10 4 ± 10 9 litre mol 71 s 71 at 25 8C) 152 or carbocations (k 2 = 10 71 ± 10 8 litre mol 71 s 71 from 770 to 730 8C) 153 where the contribution of enthalpy is either very small or constant as a result of which the transition state is shifted to earlier position on the reaction coordinate. 152,153 Recently, it has been found that the selectivity of substitution of the nitro group and fluorine in S N Ar reactions is connected with entropy control, 154,155 although bimolecular S N Ar reactions occur at a relatively slow rate (k 2 = 10 74 ± 10 71 litre mol 71 s 71 at 25 8C in DMSO or DMF). 156 However, the relative activities of the nitro group and fluorine in the competitive reactions between nitrobenzotrifluorides 69 and 70, phenols 71 and arenethiols 72 in the presence of K 2 CO 3 in DMF (40 ± 95 8C) are controlled by the entropy of the activation, since TDS = > DDH = in all cases (Table 8).…”