Anchimeric effect

The 1-mesyloxy intermediate (6) was similarly prepared via the equatorial monobenzoate, and it reacted with azide ion by a single SN2 displacement, since no anchimeric effect was possible here. The scyllo... 

Finally the dimesyl derivative (7) was prepared in a similar manner. When it reacted with azide ion, configurational inversions were observed at positions 1, 2, and 3 (formula 7), owing in part to the anchimeric effect of the position 3 acetoxy group. The vibo or dl (145/23) configuration (same as in the starting material) was retained in the final product 10 (Y = NHAc), a derivative of 3,5-diamino-l,2,4-cyclo-hexanetriol (42). 

Recently, anchimeric assistance was also reported in the solvolysis of dialkyl-j3-thiovinyl sulfonates (184). In particular, vinyl ester 221 reacts 3.8 X 10 times faster and vinyl ester 222, 4.2 x 10 times faster, respectively, than model compound 223 in 9 1 CH3N03 CH3 0H at 25°. The large anchimeric effects, 10 to 10, in the solvolysis of dialkyl- 3-thiovinyl sulfonates... 

One obvious area in which anchimeric effects might materialize is in chelation reactions this is the underlying reason that chelation is dominated by the first step (Sec. 4.4.1). We have seen it operate in the formation of Pt(II) chelates (Sec. 4.7.6). The accompanying values of AS are less negative than is usual for the associative substitution in Pt(II), consistent with the intramolecular mechanism. The rate constant for the ring-closure reaction (6.1)... 

Since the bridged complex reacts without formation of the diaqua intermediate (this can be separately prepared and shown to be much less reactive than would be required), the anchimeric effect of the adjacent coordinated water is established. See also Ref. 8. 

The discovery of the anchimeric effect of the N-nitroso group led to several exciting possibilities. Kruger (12) postulated that if the side chain of nitrosamines is larger than ethyl, the compounds are degraded to methylating agents by B-hydroxyl-ation. He postulated that the function of this side chain... 

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). 

Unfortunately, preparative experiments of Iwaoka and Kondo (35) are of no direct relation to mechanistic investigations the use of a low pressure mercury lamp provides no selectivity as far as excitation of substrate or products is concerned. However, the fact that photolysis in strong acid solution decreased the bleaching rate would indicate the absence of an anchimeric effect and the results of their investigations by flash photolysis are in agreement with the electron (Equation 2) and energy transfer (Equation 4) reactions upon direct excitation. 

One of the anchimeric effects that hampered the stmcture elucidation of picrotoxinin (1) considerably was the high stability of its oxirane (Scheme 1). When combined with the fact that this functionality seemed highly unusual for a naturally occurring compound at the time, tests to confirm the presence of this funcionality failed. For example, even gaseous hydrochloric acid in acetic acid did not cleave this epoxide, but led to anhydropicrotoxinin (103) by intramolecular ether formation (Scheme 2). Under strongly basic condition the many functionalities of the... 

The acceleration caused by the participation of the cyclobutane ring (10 ) upon solvolysis of compound 650 amounts to about a third of the anchimeric effect of the cyclopropane ring in the compound 565. The calculated extent of bond bending for cyclobutane is also about a third of bond bending for cyclopropane. Similar data have also been obtained in the acetolysis of ojmpound 651. 

The 3,5-dinitrobenzoate (113), however, hydrolyzes at a rate 48,500 times that of cyclooctyl p-bromobenzenesulfonate, after appropriate leaving group conversions. These results dramatically demonstrate transannular anchimeric R2O-5 assistance in this medium ring system. Paquette and Scott discussed inductive, steric, dipolar field, and anchimeric effects that are operative in the medium ring systems. 

The copper-catalysed thermolysis of toluene-/)-sulphonyl azide in the presence of various sulphides also affords the corresponding sulphimides. A kinetic study on the reactions of Chloramine-T with orrAo-substituted sulphides indicates a marked steric hindrance by or//ro-substituents, while the anchimeric effect of ortho-groups that have a carbonyl function decreases in the order CHjCOjH CH COjMe < CH COj- < COjMe COJti < COr-... 

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... 

This effect can be explained in the following way the d orbitals of the sulfur atom in a modified polybutadiene unit overlaps the n orbital of an adjacent double bond and activates it towards electrophilic attack of another sulfenyl chloride molecule (Scheme 36). This mechanism is supported by the fact that the substitution of the more strongly bridging alkylthio group for an arylthio group enhances the anchimeric effect and increases the blocky character of the resulting copolymer, as shown in experiments carried out with methanesulfenyl and n-butanesulfenyl chlorides. 

A third recent example of such cis labilization is provided by the (formation) reactions of edta with [Cr(OH ) ] and with carboxylato-complexes [Cr(0Hj)6(02CR)] +. In the latter cases, the observed faster rates of replacement of water have been ascribed to an analogous cis anchimeric effect to that operating in the ammine aquations mentioned (M. V. Olson, Inorg. Chem., 1973, 12, 1416). This type of anchimeric effect has been observed previously, as for example in the rapid aquation of 0-nitrito-chromium(m) complexes [T. C. Matts and P. Moore, J. Chem. Soc. (A), 1971, 1632]. 

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