Solvolysis anchimeric effect

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... 

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 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 experimental dependency of the /J-silyl effect on 0 in solvolysis reactions is sketched in Figure 764. Obviously, it differs from that anticipated for a k mechanism with rate-determining formation of siliconium ion or from the cosine-squared function expected for the pure hyperconjugative stabilization model. Apparently, the /J-silyl effect is operative in the solvolysis of both the syn- and rmt/ -periplanar conformations. The rate acceleration in the latter might be ascribed to a more favourable geometry for the (T-anchimeric assistance. 

The large amounts of five- and six-membered cyclic tosylates, products of internal return (49% yield in one case 12a of Table 3) points also to a highly selective intermediate, although the reason for such a remarkable selectivity towards external nucleophiles is not clear. On the other hand, acetolysis of 6-phenyl-5-hexynyl brosylate, which is apparently anchimerically assisted by the triple bond (4>bs./ caic. = 1 6) yields only the five-membered ring product in addition to open-chain solvolysis products (13 of Table 3). The effect of the phenyl group in orienting the oyclization reaction would indicate that intermediate species like 43 may become important when R =Ph. 

There is much additional evidence to support the postulate that the effect of neighboring sulfur is due to anchimeric assistance. Cyclohexyl chloride undergoes solvolysis in ethanol-water to yield a mixture of alcohol and ether. As usual for secondary alkyl substrates, reaction is SnI with nucleophilic assistance from the solvent (see Sec. 14.17). A C5H5S— group on the adjacent carbon can speed... 

The development of quantitative scales of solvent nucleophilicity based on solvolysis reactions is reviewed. Effects of solvent nucleophilicity are illustrated by product studies, by correlations of kinetic data, and by quantitative estimates of competing nucleophilic pathways, including competing solvent-assisted and anchimerically assisted pathways. The problem of separating quantitatively the nucleophilic and electrophilic solvent contributions to reactivity is discussed. Recent results on the nucleophilicities of aqueous sulfuric acid mixtures are presented. 

Introduction to of electron-releasing substituents sharply decreases the exo endo rate ratio there is no such effect if two geminal methyl groups are introduced to C. The value 3 for the exo endo rate ratio resembles greatly the usual epimeric rate ratio for secondary substrates without anchimeric assistance (see above). Introduction of an electron-releasing substituent to levels the o-participation in the solvolysis of the secondary 2-exo-epimer, but it does not affect the rate ratio of the tertiary analogues for which the solvolysis rate is predominantly determined by steric factors, for example ... 

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. 

Brown has shown that on introduction of a double bond to isomeric p-nitro-benzoates 52 and 53 the rate of solvolysis in either case decreases equally (about fivefold) which is due to the inductive double-bond effect and clearly points to the lacking anchimeric assistance in the solvolysis of206 this is also evidenced by the absence of tricyclic compounds among the reaction prcKiucts. Tlte isomers 206 and 207 are characterized by a high exotendo rate ratio and pr iominant formation of exo alcohol. 

Tanida et al. have studied 6- and 7-substituted 2-exo-benzonorbomenyl brosylates with highly deactivating substituents As already remarked, the homo-para-OCHj group accelerates the solvolysis of exo isomer 264 178-fold (77.6 °C) this effect is the highest for the anchimeric participation of the p-anisyl group in all studied systems. Thus, the benzene ring participation in the C —X bond ionization is more effective than in that of the C —X bond. 

Subsequently the same authors established that the solvolysis of optically active 6,7-dimethoxy-l,2-dimethyl-2-exo-norbornyl chloride or -p-nitrobenzoate is accompanied by anchimeric acceleration as distinct from the unsul tituted analogues 275 and 276 the resulting products are optically active Thus, anchimeric assistance leads to the formation of an unsymmetrical cation which may be the homobenzylic ion 278 while the partial loss of activity is due to the interconversion of enantiomeric ions. The authors have also shown that the accelerating effect of the homopara-methoxyl group is practically the same for the solvolysis in methanol, 50% acetone... 

Another widely used comparison, especially in bicyclic systems, is the exojendo rate ratio. This comparison is also fraught with pitfalls. Problems arise because the endo and exo isomers of a polycyclic system have unique configurations and hence different steric requirements. This can affect anchimeric assistance by neighboring groups. In addition there are inherent differences in field effects and nucleophilic solvent assistance. The 350-fold difference in rate constant for acetolysis of exo- and endo-2-norbornyl p-bromobenzenesulfonates [(11) and (12), respectively] was taken as evidence for a participation in the solvolysis of (11) by Winstein... 

The kinetic data in Table 1 demonstrates that S-4 anchimeric assistance is not important in the hydrolysis of acyclic alkyl chlorides [e.g., n-hexyl chloride solvolyzes 1.8 times faster than PhS(CH2)3Cl in 20 mole % aqueous dioxane at 100°C ]. Some data on cyclic systems will be presented here to demonstrate that proximity effects can drastically affect S-4 participation. The formation of (35) and (36) from treatment of 3j -methanesul-fonyloxy-5-a-methylthiocholestane (34) with tetrapropylammonium acetate was rationalized in terms of S-4 participation as shown. Ireland and Smith found that the rates of solvolysis of (37) and (38) were slower [/Crei(77°C) = 0.46 and 0.21, respectively] than the rate for trans-4-t-huty ... 

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