Benzhydrylium ions

Picosecond absorption spectroscopy was employed to study the dynamics of contact ion pairs produced upon the photolysis of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol.66 A review appeared of the equation developed by Mayr and co-workers log k = s(N + E), where k is the rate constant at 20 °C, s and N are nucleophile-dependent parameters, and is an electrophilicity parameter 67 This equation, originally developed for benzhydrylium ions and n-nucleophiles, has now been employed for a large number of different types of electrophiles and nucleophiles. The E, N, and s parameters now available can be used to predict the rates of a large number of polar organic reactions. Rate constants for the reactions of benzhydrylium ions with halide ions were obtained... 

Reactions of carbocations with free CN- occur preferentially at carbon, and not nitrogen as predicted by the principle of hard and soft acids and bases.69 Isocyano compounds (N-attack) are only formed with highly reactive carbocations where the reaction with cyanide occurs without an activation barrier because the diffusion limit has been reached. A study with the nitrite nucleophile led to a similar observation.70 This led to a conclusion that the ambident reactivity of nitrite in terms of charge control versus orbital control needs revision. In particular, it is proposed that SNl-type reactions of carbocations with nitrite only give kinetically controlled products when these reactions proceed without activation energy (i.e. are diffusion controlled). Activation controlled combinations are reversible and result in the thermodynamically more stable product. The kinetics of the reactions of benzhydrylium ions with alkoxides dissolved in the corresponding alcohols were determined.71 The order of nucleophilicities (OH- MeO- < EtO- < n-PrCT < / -PrO ) shows that alkoxides differ in reactivity only moderately, but are considerably more nucleophilic than hydroxide. 

The scale was developed using a series of benzhydrylium ions having the general structure 54, where the E value was taken to be 0.00 for system with both X and Y = OCH3. This system allowed the study of a wide range of reactivity, because the E value Ifor 54 having X and Y = Cl is 6.0, while that for 54 with X and Y = N(CH3)2 is —7.4. The nucleophiles in the initial study were all 7i-nucleophiles, and the nucleophile-specific slope parameter was chosen to be 1.00 for the case of 2-methyl-l-pentene, which was foimd to have an N value of 0.96. Each nucleophile is identified with both an N and an Sn value, so the listing for 2-methyl-l-pentene is 0.96 (1.00). 

A DFT study addressed the relationship between electrophilicity and electro-fugality. A number of descriptors obtained by semi-empirical and DFT methods were evaluated for estimations of the Mayr electrophilicity parameters. For a set of 64 electrophiles, good correlations were found using reactivity descriptors calculated by the PM6 semi-empirical and DFT methods. Rate constants for reactions of amines and benzhydrylium ions were calculated using various theoretical methods. A combined ONIOM/DFT approach gave good agreement with experiment. 

Electrophilicity parameters E were reported for highly reactive benzhydrylium ions. This study shows that correlations of log 2 versus E remain linear even when = 0, showing that a change from activation control to entropy control does not cause a bend in the linear free energy relationship. The correlation lines do flatten when the diffusion limit is approached. Nucleofugality parameters and for 5 1 ionization based on the Mayr equation log kjon = + N ) were reported for fluoride in protic solvents, var-... 

The second-order rate constants for the reaction of several substituted hydrazines, hydrazides, hydroxylamines, methylamines, and ammonia with benzhydrylium ions and quinone methides in water and MeCN have been measured and the N and % values [in the log k = %(At + E) equation] of the various nucleophiles determined. The effect of adding methyl or larger group(s) to the a- or -position of the nucleophiles is discussed. Although the amine and hydrazine nucleophiles react much slower in water... 

The second-order rate constants for the reaction of the anions formed from imidazoles, pyrimidines, and purines with quinone methides and benzhydrylium ions in DMSO and in water have been measured and their N and % values in the log k = Sf N+E) equation determined. As expected, the nucleophiles are more reactive in DMSO than in water and the anions are 10" -10 times more reactive than their neutral counterparts. However, the solvent effects vary markedly with each nucleophile. The % values range from 0.50 to 0.77, whereas the N values for the anionic nucleophiles vary from approximately 15 to 21 in DMSO and from 10 to 12 in water. The Brpnsted basicity is correlated with the nucleophilicity in DMSO but not in water. 

In order to further validate the theoretical scale of electrophilicity based on the global electrophilicity index,105 we first compared the theoretical scale with the experimental electrophilicity determined from kinetic data by Mayr et al. for a series of 28 carbenium ions.106,107 They are displayed in Chart 6. Included in these series are the tritylium, benzhydrylium and benzylium ions. These charged electrophiles are expected to show enhanced electrophilicity patterns as compared with the neutral electron donors we will discuss later. The electrophilicity values are in the range [9.0-14.0] eV (see Table 13, second column). The local electrophilicity pattern at the carbocation site is displayed in Table 13, fourth column. They are obtained by projecting the global electrophilicity w with the electrophilic Fukui function ff, using (5). These values have been used to... 

A series of benzhydrylium cations was also used to estimate the nulceophilicities of hexafluoroisopropanol (HFIP)/water mixtures. Using laser flash photolysis, benzhydryhum cations were generated from the corresponding phosphonium ion... 

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