Yukawa-Tsuno analysis

Bromodeboronation has acquired a particular significance in recent theories of electrophilic substitution and briefly this has arisen since it was supposed to have a very high r factor relative to its p factor in a Yukawa-Tsuno analysis. (For a fuller discussion see ref. 729). It had been suggested729 that some of the rate coefficients determined for the reaction (Table 254) may be in error due to concurrent bromodeprotonation, and a reinvestigation730 of this possibility has revealed a number of points ... 

Table 15 Substituent constants used for the Yukawa-Tsuno analysis of gas-phase substituent effects.

A kinetic study of the nucleophilic substitution of Y-phenyl diphenylphosphinoth-ioates (125 X = S) by alkali metal ethoxides (MOEt M = Li, Na, K) in anhydrous ethanol at 298 K was reported (Scheme 39). Plots of pseudo-first-order rate constants ( obsd) versus [MOEt] showed distinct upwards (KOEt) and downwards (LiOEt) curvatures, respectively, pointing to the importance of ion-pairing phenomena and a differential reactivity of dissociated EtO and ion-paired MOEt. The reactivity of MOEt towards the 4-nitro compound (125 X = S, Y = 4-N02> increases in the order LiOEt < EtO < NaOEt < KOEt, which differs from the reactivity order LiOEt > NaOEt > KOEt > EtO reported previously for the reaction of 4-nitrophenyl diphenylphosphinate (125 X = O, Y = 4-NO2). Yukawa-Tsuno analysis revealed that the reactions of (125 X = S) and its P=0 analogue (125 X=O) with MOEt proceeded through the same concerted mechanism, which involved M+ ions increasing the electrophilicity of the reaction centre via TS (126). The P=0 compounds (125 X = O) were approximately 80-fold more reactive towards the dissociated EtO than the P=S compounds (125 X = S) (regardless of the electronic nature of substituent Y) but were up to 3100-fold more reactive towards ion-paired LiOEt. ... 

There are extensive data for the acid-catalyzed protiodesilylation of XCgELrSiMes in methanol-aqueous perchloric acid or acetic acid-aqueous sulphuric acid at 50°C225. Correlation analysis of the partial rate factors (relative rate constants) by means of the Yukawa-Tsuno equation (Section n.B) finds p = —5.3 and r+ = 0.65. These values are consistent with a relatively low demand for stabilization of the transition state by electron delocalization, i.e. the transition state is early along the reaction coordinate, p-NO2 is highly deactivating with / = 14 x 10 but 0-NO2 is even more deactivating, with / = 6.8 x 10-5. This contrasts with the deactivation order discussed above for nitration and chlorination (Table 6), and may be explained in terms of the early transition state, well removed from the Wheland intermediate. 

Data from Dzvinchuk and Lozinskii (88ZOR2167). Determined by H-NMR at 25°C (in CHCI3 at 20°C). The values (Xt)o were directly measured, rather than obtained from regression analysis. The standard errors of the slope are in the range 0.01-0.03. n = 8 (X = H, 3-NO2, 4-NO2, 4-Br, 4-Ph, 4-Me, 4-MeCONH, 4-MeO). Molar ratio. This is the coefficient of the Yukawa-Tsuno equation, not the correlation coefficient. The correlation coefficients lie in the range 0.997-0.999. 

First-order dependence is observed with respect to both the oxidant and reductant in the oxidation of substituted anilines with peroxomonosulfate anion. Addition of acid causes retardation of the reaction. Yukawa-Tsuno correlation of the rates gave a negative reaction constant (p -1.7) and analysis of the effect of solvent in terms of Grunwald-Winstein equation (m 0.4) indicated an S -type reaction. 

It should be stressed that there is nothing exceptional about the benzo[f>] compounds in not providing constant literature data for practically every aromatic shows the ct+ values to be variable. For some molecules that are not very polarizable, the variation in ct+ is quite small, leading to the erroneous conclusion of their constancy. The presently described molecules are particularly polarizable and show that satisfactory treatment of reactivity data can generally be achieved only through the use of a multiparameter equation, notably the Yukawa-Tsuno equation (59BCJ971). 

Although the Hammond shift of the transition state is clearly important, the change in p value caused thereby may not generally be significant in the solvolysis process. Consequently, we still wish to emphasize that in spite of the wide spectrum of observed r values, the intrinsic resonance demand is an inherent property of the carbocation to be formed and remains essentially the same in the intermediate and in the transition states of the related reactions of the intermediate carbocation. This is the basis of mechanistic analysis based on the Yukawa-Tsuno relationship. 

The Yukawa-Tsuno equation continues to find considerable application. 1-Arylethyl bromides react with pyridine in acetonitrile by unimolecular and bimolecular processes.These processes are distinct there is no intermediate mechanism. The SnI rate constants, k, for meta or j ara-substituted 1-arylethyl bromides conform well to the Yukawa-Tsuno equation, with p = — 5.0 and r = 1.15, but the correlation analysis of the 5 n2 rate constants k2 is more complicated. This is attributed to a change in the balance between bond formation and cleavage in the 5 n2 transition state as the substituent is varied. The rate constants of solvolysis in 1 1 (v/v) aqueous ethanol of a-t-butyl-a-neopentylbenzyl and a-t-butyl-a-isopropylbenzyl p-nitrobenzoates at 75 °C follow the Yukawa-Tsuno equation well, with p = —3.37, r = 0.78 and p = —3.09, r — 0.68, respectively. The considerable reduction in r from the value of 1.00 in the defining system for the scale is ascribed to steric inhibition of coplanarity in the transition state. Rates of solvolysis (80% aqueous ethanol, 25 °C) have been measured for 1-(substituted phenyl)-l-phenyl-2,2,2-trifluoroethyl and l,l-bis(substi-tuted phenyl)-2,2,2-trifluoroethyl tosylates. The former substrate shows a bilinear Yukawa-Tsuno plot the latter shows excellent conformity to the Yukawa-Tsuno equation over the whole range of substituents, with p =—8.3/2 and r— 1.19. Substituent effects on solvolysis of 2-aryl-2-(trifluoromethyl)ethyl m-nitrobenzene-sulfonates in acetic acid or in 80% aqueous TFE have been analyzed by the Yukawa-Tsuno equation to give p =—3.12, r = 0.77 (130 °C) and p = —4.22, r — 0.63 (100 °C), respectively. The r values are considered to indicate an enhanced resonance effect, compared with the standard aryl-assisted solvolysis, and this is attributed to the destabilization of the transition state by the electron-withdrawing CF3 group. 

Fujio and co-workers studied the reaction of pyridine with a wide range of 1-arylethyl bromides in acetonitrile. By careful analysis of the kinetic data, they were able to dissect each reaction into a first-order and a second-order component, as shown in the table below. The first-order components were correlated by a Yukawa-Tsuno equation logk/k = 5.0(a° + 1.15ct+). The second-order component gave a curved plot, as shown in Figure 4.P18. Analyze the responses of the reaction to the aryl substituents in terms of transition state structures. 

A series on substituent effects on the solvolysis of l,l-diphenyl-2,2,2-trifluoroethyl systems has continued in a study of the effects of electron-donating substituents in the fixed aryl moiety. " The fixed substituent in one benzene ring was p-MeO, p-PhO, or p-Me, and the substituent in the other ring was varied. The substrates were chlorides and bromides, and the reactions were carried out in 80% aqueous ethanol at 25.0 °C. The substituent effects were complex and required detailed analysis through Hammett-Brown and Yukawa-Tsuno treatments. The complexities arise largely from the twisting of the aryl groups out of coplanarity. 

Numerous solvolytic studies on diarylmethyl derivatives have been carried out under a variety of conditions. The analysis of substituent effects in the solvolysis of the monosubstituted chlorides [22] was reported earlier (Yukawa and Tsuno, 1959 Yukawa etal., 1966). The purpose of this analysis is to clarify the effect of a fixed substituent Y in one ring on the substituent effect of the variable substituents X on the second ring. Three extensive sets of kinetic data for the solvolysis of X, Y-disubstituted benzhydryl systems under fixed conditions have been reported one for the ethanolysis (Nishida, 1967), one for the chloride hydrolysis in 85% aqueous acetone at 0°C (Fox and Kohnstam, 1964) and one for the bromide hydrolysis (Mindl et al., 1972 Mindl and... 

Yukawa and Tsuno have still another approach, also involving dual parameters Yukawa, Y Tsuno, Y. Bull. Chem. Soc. Jpn. 1959, 32, 971. For a review and critique of this method, see Shorter, J., in Chapman, N.B. Shorter, J. Correlation Analysis in Chemistry Recent Advances, Plenum, NY, 1978, pp. 119-173, 126-144. This article also discusses the Swain-Lupton and Taft a/, Or approaches. For yet other approaches, see Afanas ev, LB. ]. Org. Chem. USSR 1981, 17, 373 J. Chem. Soc. Perkin Trans. 2 1984, 1589 Ponec, R. Coll. Czech. Chem. Common. 1983, 48, 1564. 

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