Tosylates solvolysis

Diels-Alder reaction of 2-bromoacrolein and 5-[(ben2yloxy)meth5i]cyclopentadiene in the presence of 5 mol % of the catalyst (35) afforded the adduct (36) in 83—85% yield, 95 5 exo/endo ratio, and greater than 96 4 enantioselectivity. Treatment of the aldehyde (36) with aqueous hydroxylamine, led to oxime formation and bromide solvolysis. Tosylation and elimination to the cyanohydrin followed by basic hydrolysis gave (24). 

Solvolysis of tosylate (303) yields, in addition to aziridinyl alcohol (304), ring expanded products (305) and (306) (68TL6179). These products and the observed rates of solvolysis were explained in terms of bicyclic intermediate (301). 

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geomeby. In tosylate 1, the cyclopropane ring is locked into an orientation which affords a perpendicular arrangement. It reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geomeby, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate 4. ... 

Table 5.12. Tosylate/Bromide Rate Ratios for Solvolysis of RX in 80% Ethanol ...

When enantiomerically pure fni/is-2-acetoxycyclohexyl tosylate is solvolyzed, tiie product is racemic /mn -diacetate. This is consistent with the proposed mechanism, since the acetoxonium intennediate is achiral and can only give rise to racemic material. Additional evidence for diis interpretation comes from the isolation of a cyclic ortho ester when the solvolysis is carried out in ethanol. In this solvent the acetoxonium ion is captured by the solvent. 

Table 5.21. Extent of Aryl Rearrangement in 2-PhenyIetiiy1 Tosylate Solvolysis"...

Table 5.22. Extent of Solvolysis with Aryl Participation as a Function of Substituent and Solvent for l-Aryl-2-propyl Tosylates...

The occurrence and extent of rearrangement of the 2-butyl cation have also been investigated by solvolysis studies using isotopic labeling. When 2-butyl tosylate is solvolyzed in acetic acid, C-2/C-3 rearrangement occurs only to the extent of 9% in the 2-butyl acetate which is isolated.Thus, under these conditions, most of the reaction proceeds by direct participation of the solvent. 

Shifts of Itydride between carbon atoms sqiarated by several atoms are possible if the molecular geometry is favorable. Particularly clear-cut examples have been found in medium-sized rings. For example, solvolysis of cyclononyl-l- C tosylate can be shown by degradation of the product cyclononene to occur with about 20% of the becoming located at the 5-, 6-, and 7-positions. 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

The solvolysis of the tosylate of 3-cyclohexenol has been studied in several solvents. The rate of solvolysis is not very solvent-sensitive, being within a factor of 5 for all solvents. The product distribution is solvent-sensitive, however, as shown below. 

The solvolysis of isomeric 5-fluoro and 5-trimethyIstaimyl 2-adamantyl tosylates has been examined. The relative rates depend on the substituents and the stereochemistry of the reactants as shown. 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

Our next concern is the solvent nucleophilicity. Schadt et al. " chose the solvolysis of methyl tosylate, which should be an Sn2 process, as the defining process. For this standard reaction the parameter / in Eq. (8-69) was set at 1.00. An empirical estimate of m, describing the sensitivity of methyl tosylate to solvent ionizing power, was obtained as the slope of the two-point line for methyl tosylate solvolysis in... 

Table 8-14. Regression Parameters for Tosylate Solvolysis Reactions"...

Another deviation from the normal displacement reaction of primary tosylates occurs in nucleoside derivatives (39, 81) where cyclonucleosides and anhydronucleosides are formed by participation of a nitrogen atom (as in purine nucleosides) and oxygen atom (as in pyrimidine nucleosides ), respectively. Iodonucleosides can result from these reactions only if these cyclic compounds are prone to attack by iodide ion. Several new examples of unexpected reactions during the solvolysis of sulfonate esters in sugar derivatives have been recorded in the past few years (2, 4,5,7,15,44,62,63,94). 

When the reaction was followed in an nmr probe, the appearance and disappearance of two intermediates 66 and 67 was observed along with the buildup of product 65. The rate of reaction of 64 at 75° in formic acid, k = 3xl0" sec, is six times faster than the rate of the corresponding saturated system, 2,2-dimethyl-1-pentyl tosylate, k = 5xl0" sec, under identical conditions. If the inductive rate retarding effect of the triple bond is taken into account, then the calculated rate enhancement resulting from triple-bond participation in the solvolysis of 64 is about 3000(81). The... 

Participation of more remote triple bonds has also been investigated. Peterson and Kamat (82) have carried out a careful study of the solvolysis in a variety of solvents of 6-heptyn-2-yl tosylate, 69, and 6-octyn-2-yl tosylate, 70. The observed products of solvolysis are given in Schemes IV and V. 

Transannular participation of a triple bond has been examined by Hanack and Heumann (86). Acetolysis or solvolysis in aqueous acetone of cyclodecyn-5-yl-l-tosylate 104a gave, besides some unidentified unsaturated hydrocarbons, a mixture of the rearranged ketones 105 and 106. None of the unrearranged... 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

Essentially similar results and conclusions were obtained by Peterson and Indelicato (158) in the solvolysis of the corresponding tosylates and brosylates, 171 b (R = p-CHj Cfi H4 or p-BrCg H4 ) and 171 c (R = p-CHj 5 H4, p-BrCg H4 ), in 50% aqueous methanol at 130°. In this case, the trans isomer was found to react at a rate 10 times that of the cis isomer. Furthermore, the trans isomer gave 95% 2-butyne and 5% 2-butanone, whereas the cis isomer gave 72% 2-butyne and 28% 2-butanone as products. Also, as expected (vide supra) for a unimolecular solvolysis reaction, the cis brosylate reacts at a rate four times that of the corresponding tosylate. 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

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