Cross-interaction constant , nucleophilic

Gas-phase nucleophilic substitution reactions of Y-benzyl chlorides and X-phenoxide or X-thiophenoxide nucleophiles have been investigated by using the PM3 semiempirical MO method. The structure of the transition state was examined. The values of the gas-phase Hammett constants px and py are much greater than for the solution reactions, but a theoretical cross-interaction constant pxy (ca —0.60 for both phenoxides and thiophenoxides) agrees well with an experimental value of —0.62 for the thiophenoxide reactions in MeOH at 20 °C. Other work by the same group has involved theoretical studies of competitive gas-phase 5 n2 and E2 reactions of NCCH2CH2CI with HO and An ab initio method at the 6-31-l-G level was... 

As a mechanistic criterion we introduce the cross-interaction constant (CIC) p,7, defined in equations la and lb based on the adoption of a convention denoting substituents on the nucleophile, substrate and leaving groups, respectively, as X, Y and Z15. 

A number of other examples of useful applications of KIEs involving deuteriated aniline nucleophiles are reported15e g. Elucidation of the TS structures is facilitated by means of nucleophile KIEs in conjunction with the sign and magnitude of the cross-interaction constants. 

A stepwise mechanism with a rate-limiting leaving group expulsion from the intermediate was proposed for the reactions of Z-aryl dimethyl- (43a), methylphenyl-(43b), and diphenyl-phosphinates (43c) with substituted anilines in DMSO at 333 K, based on the positive cross-interaction constants, The steric effects of the two ligands (R, R ) play a role in determining the reactivity of the phosphinates, but are relatively small compared to other phosphinate systems. A dominant frontside nucleophilic attack involving a hydrogen-bonded four-centre-type transition state (44) was proposed... 

The kinetics and mechanism of the aminolysis of phenyl-substituted phenyl phosphonyl chlorides (136) with anilines (135) were investigated in MeCN at 55.0 °C. Very sensitive variation of /Oy(5/Oy2>0) with the change of substituent on the nucleophile Sax) led to a large negative cross-interaction constant, pxY = Spy)/ Sox) = —1.31. The secondary kinetic isotope effects observed with deuteriated aniline nucleophiles were of the inverse type (/th/ d = 0.61-0.87), and small A// (1.6-9.7kcalmor ) and large negative A5 (—43 to —65 e.u.) values were... 

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