Anchimeric effect, transition state

The foregoing results indicate that a powerful anchimeric rate-accelerating effect (38) must operate in the transition state of the electrophilic substitutions of II which exceeds the electron-with drawing effect of the immonium form D in the mono-positive cation II ... 

Closely analogous transition states in the solvolysis of yG-chloroethylamines indicate that the anchimeric effect in these systems may accelerate the reaction rate by a factor of 103 to 104 as compared to the aliphatic halide in which the nitrogen is replaced by a carbon atom (2,4). 

The study of the addition of trifluoroacetic acid to several 5-substituted 1-pentyne derivatives 22 led Peterson and Duddey (1963, 1966) and Peterson and Bopp (1967) to the conclusion that for some of the compounds investigated the intermediate resulting from protonation of the triple bond may be a five-membered ring cation of type 23, rather than a linear vinyl cation. This is indicated by the observation of rearranged products deriving from 1-5 shift when Y = Cl, F, OMe, and OAc (see scheme 3). Kinetio data indicate that Y-participation is important in the transition state of the reaction. Anchimeric assistance effects, expressed in terms of the ratio of assisted to unassisted mechanisms, kjjkg (Heck and Winstein, 1957), have been estimated to be 3 4 for Y = Cl and 6 5 for Y = OMe. The entire kinetic picture almost matches... 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

The u-methoxy-group on PMegCo-MeOCeHi) obviously has little effect on the rates whereas an acceleration of 100 times is observed in oxidative additions involving methyl iodide. This effect is attributed to anchimeric assistance due to interaction of the methoxy oxygen with iridium. However, in the present case the lack of effect is probably a reflection of the much lower polarity of the transition state (2) compared with that for methyl iodide addition (3). 

In a related study of triarylphosphine, P(XC6H4)3, addition to the dienyl ring of 15, rates varied with X in the order 2-MeO > 4-MeO > 4-Me > H > 2Me (relative rates 90/9/4/1/10 ). The unexpectedly rapid reaction with P(2-MeOC6H4)3 was rationalized in terms of stabilization of the transition state 16 via an anchimeric effect involving overlap between... 

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