Mayr equation

The Mayr Equation. Mayr and his co-workers have developed Equation (31) which correlates the reactivity of carbocations and related electrophiles with k nucleophiles. 

Nucleophilicity parameters, N and % for electrophile-nucleophile combination based on the Mayr equation ogk = Sj (N+E), were reported for highly nucleophilic pyridines bearing amino groups at carbons 3, 4, and 5, isothioureas," and thiocarboxylates, dithiocarbamate, and dithiocarbonate." Electrophilicity parameters E were reported for triarylmethyl cations with para and meta fluorine substituents," 1,3-diarylallyl cations," and aldehydes, imines, and enones." Nucleofugality parameters Nf and Sf for ionization based upon the Mayr equation logki = Sf(Ef + Nf) were reported for acetate in aqueous methanol" and chloride in aprotic solvents." The latter study also shows that common solvent parameters do not reliably predict ionization rates in aprotic solvents. Electrofugality parameters Ef were reported for triarylmethyl cations with para and meta fluorine substituents." ... 

Nucleophilicity parameters N and % for electrophile-nucleophile combination based on the Mayr equation log k=+E) were reported for fluorides in protic solvents, enamines derived from imidazolidinones, trimethylsilyl enol ethers with perfluori-nated substituents at the a-carbon, O-methylated Breslow intermediates, anions of nucleobases and their subunits, enamides, symmetrical and unsymmetrical hydrazines, and heteroarylboron compounds. Of note, replacement of CH3 with CF3 and C6H5 with CgF5 in the trimethylsilyl enol ethers reduces the nucleophilicity by 8 and 4.5 orders of magnitude, respectively. Hydrazines have very similar nucle-ophilicities to alkyl amines in other words, there is no evidence of an a-effect. With NH2NMC2, there is a fast reversible reaction corresponding to addition of the tertiary amine, followed by a slow irreversible reaction for addition at the primary amine, with a 3000-fold difference in nucleophilicity between the two sites. ... 

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 N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

Mayr has reviewed the nucleophilicity parameters of allylstannanes and related compounds.276 The allyl-tin bond is easily cleaved, and this has been exploited in the preparation of pentafluorophenyltin chlorides and bromides (Equation (98)).277... 

In a review on the addition of carbenium ions to alkenes (equation 19) as a general procedure for carbon-carbon bond formation50, Mayr reported on investigations which also include the reactions of a variety of 1,3-dienes toward electrophilic carbon species generated by Lewis acid-promoted heterolysis of alkyl chlorides. 

In a series of reports published over the last 10-15 years, Mayr and co-workers obtained second-order rate constants for reactions of carbocations and other electrophiles such as metal-7i complexes with a series of nucleophiles, especially 7t-nucleophiles where a C C bond is formed. An impressive body of reactivity data has been accumulated, and, including data from other groups, correlated by the following equation. 

As pointed out by Mayr,28 Ritchie,15 and Hine33,34 KR also measures the relative affinities of R+ and H30+ for the hydroxide ion. It can be regarded as providing a general affinity scale applicable to electrophiles other than carbocations.33,35 It can also be factored into independent affinities of R+ and H30+ as shown in Equations (2) and (3). Such equilibrium constants have been denoted If by Hine.33 AR corresponds to the ratio of constants for reactions (2) and (3) and, in so far as Kc for H30+ is the inverse of Kw the autoprotolysis constant for water, KR = KCKW... 

Rappoport and TaShma s work removed a major difficulty for Ritchie s analysis and helped pave the way for Mayr to exploit fully the wide applicability and simplicity of Equation (29) for predicting rates of reactions of electrophiles with nucleophiles. Mayr pointed out that Equation (29) could be rewritten as Equation (30), in which log ka corresponds to the rate constant for reaction of the electrophile under study with a reference nucleophile266 (chosen as water by Ritchie) which, in so far as it is characteristic of the... 

Eiowever, the equation that Mayr adopts is not Equation (32) but Equation... 

In recognition of the excellent correlation that exists between his own and Swain and Scott s (or Kevill s) parameters, Mayr suggests modifying Equation... 

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... 

Olah and coworkers, ° and Mayr and Striepe discussed the scope and limitations of aliphatic Friedel-Crafts alkylations. In particular, they considered factors that would favor reactions of the type shown in equation (121), where an alkene is alkylated by an alkyl halide. ° They reasoned that formation of the 1 1 addition products (42) can be expected, if (41) reacts faster with the alkene than (42), otherwise higher addition products will be formed. Mayr ° suggested that the relative dissociation rates of (41) and (42) induced by the Lewis acid should reflect their relative rates of addition to a common alkene. Furthermore, it was assumed that the solvolysis rates in 80% ethanol were proportional to the Lewis acid induced dissociation constants. A few examples where good yields of alkylated (addition) products were obtained are shown in equations (122) and (123). 

---