Solvolysis anchimeric assistance

The constant kt is determined experimentally, and ks can be calculated by use of a Hammett plot as described below. Then FkA can be calculated by simply subtracting ks from kt. If FkA is the rate of formation of anchimerically assisted solvolysis product, it should lead to threo-acetate. The product arising from ks should lead to erythro-acetate. Therefore the fraction of /Ara>-acetate expected can be calculated by FkA/(FkA + ks) or FkA/kt. Column five of Table 6.1 shows the calculated fraction of threo-acetate. 

In 1960, Winstein reported a record-breaking anchimerically assisted solvolysis rate acceleration 7-norbomadienyl precursors reacted lO times faster than their saturated 7-norbomyl analogs [47]. The cation favors a C, rather than a Qv structure, [47, 48] since interaction between the cationic center and... 

The hydrolysis of 2,2 -bischloroethyl sulfide (mustard gas) was one of the first neighboring-group-accelerated reactions to get widespread attention. Initially the hydrolysis of this compound is purely first order in the sulfide, and the rate is unaffected by added alkali or other nucleophiles the increasing amount of chloride ion, however, slows the rate with time. Since formation of the primary carbocation is not expected, these data suggest an anchimerically assisted solvolysis [Eq. (1)], leading to the formation of the intermediate sulfonium ion (1), which may react with water, chloride ion, or any other nucleophile present. A second anchimerically assisted process that leads to displacement of the second chloro group can occur. 

Winstein, one of the most brilliant chemists of his time, concluded that it is attractive to account for these results by way of the bridged (non-classical) formulation for the norbornyl cation involving accelerated rate of formation from the exo precursor [by anchimeric assistance His formulation of the norbornyl cation as a cr-bridged species stimulated other workers in the solvolysis field to interpret results in a variety of systems in similar terms of rr-delocalized, bridged carbonium... 

Anchimeric assistance by oxirane oxygen may accelerate the solvolysis of (44 Scheme 33) (by a factor of ca. 10 -l6 ) compared to the exo isomer (45 Scheme 34) (81JOC3512). 

Based on kinetic investigations the solvolysis of ochloroalkyl sulphoxides 506 in 80% ethanol was found to proceed via a cyclic intermediate formed via anchimeric assistance of the sulphinyl oxygen atom601,602. For a solvolysis of 4-halogenothian-l-oxides see Reference 603 (equation 309). 

Anchimeric assistance in the solvolysis of /3-arylthiovinyl sulfonates was demonstrated by means of kinetic studies on model compounds (182). In a variety of solvents ranging from nitromethane and methanol to acetic acid, the /3-arylthiovinyl sulfonate 216 was shown to react 20 to 33 times faster than the triphenylvinyl sulfonate 217. Different accelerating factors were... 

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... 

Rearrangement across the double bond and possible anchimeric assistance involving carbon also has been investigated. Rappoport and Gal (134) have reported that the solvolysis of l-anisyl-2,2-diphenyl bromide, 224, in aqueous ethanol as well as in formic acid gave a,a-diphenyl-p-methoxyacetophenone, 225, as the only product i.e., no rearrangement product 226 was observed. 

More mechanistic work undoubtedly will be done on the stereochemistry of solvolysis, particularly of simple alkyl-substituted acyclic vinyl systems. The questions of concerted anchimerically assisted or nonconcerted rearrangements both to the double bond and across the double bond need further development. 

Explain the rate ratio for compounds 3 and 4 estimated for limiting SN1 solvolysis by correcting the cyclopropyl system for anchimeric assistance which occurs when the ring opens. 

The excellent correlation between the percentage of anchimeric assistance calculated from the rate (column 5) and product (column 6) data is compelling evidence that the above analyses are accurate and thus that separate, discrete pathways for solvolysis exist—one solvent-assisted and one aryl-assisted.30... 

In discussing the cyclopropylcarbinyl cation before the norbornyl cation we have, chronologically, put the cart before the horse. The first experimentally documented example of anchimeric assistance by a C—C a bond was announced by Winstein and Trifan in 1949.96 These workers studied the solvolysis of exo- and... 

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 solvolysis product distribution and the kinetic data (solvent effect33) m = 0.583 — 0.505, substituent effect34) q = —2.98) were clearly consistent with a SNj ionization process involving the anchimeric assistance of the triple bond (kA) 35) leading to the resonance stabilized cation 63 with its positive charge delocalized through the adjacent triple bond into the aryl (or cyclopropyl) group 13). 

The stereochemical course of solvolysis of a diastereomeric /3-bromo-silane, which was found to be trans elimination, as well as the observation of 1,2-silyl group migration in a process of substitution at the 0 carbon 143) are evidence of anchimeric assistance of the silyl group in ionization of the bond to the /3 carbon. It was further noticed 144) that after quenching the solvolysis reaction of /3-bromo-/3-dideuterioethylsilane the substrate recovered is a mixture of a and /3 deuterated isomers [Eq. (33)]. These... 

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. 

An alternative model system, potentially superior to t-butyl chloride in the evaluation of solvent ionizing power, is based on the specific rate of anchimeric, y9-aryl assisted solvolysis or decomposition of 4-methoxyneophyl tosylate at 75 °C according to Eq. (7-20) [37]. Use of this compound avoids some practical disadvantages of t-butyl... 

The solvolysis of 4,4-dimethyl-e -cholesteryi tosylate (5) is interesting in that a simple elimination reaction leading to a conjugated 3,5-diene is impossible. Ionisation of the 3 a-tosylate, which derives no anchimeric assistance from the 5,6-double bond, is followed instead by migration of the 4/9 methyl group to C(3). The data available are consistent with Shoppee s postulate [y8] that the heterolysis occurs as the slow step, and is followed by rapid rearrangement of the classical ion... 

---