Solvolysis reactions tosylates

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

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. 

To test the reactivity of these tosylates, the kinetics of the solvolysis reaction in glacial acetic acid were measured. Tosylate II reacted so rapidly that the kinetics could not be measured titrimetrically (ti/2 30 sec. at 15°C). At 40° compound III reacted very slowly, out compound I reacted at a convenient rate (ti/2 = 7 min. at 40°) and accurate kinetic data were determined for it. The kinetic data for III were obtained at higher temperatures. The results for both compounds are shown in Table 1. 

Although under solvolytic conditions carbocations are formed only as short-lived (transient) species, it is of interest to discuss here some recent work involving substituted C4H7+ cations which are relevant to the general discussions in the review. Wiberg and coworkers have carried out extensive studies on solvolysis reactions involving 3-substituted cyclobutyl tosylates 12 (equation 18), and rationalized the observed products by ab initio molecular orbital calculations at the MP2/6-31G level46,47. 

Possibly this is due to a greater charge development in the transition states of the chromyl acetate oxidations. Solvolysis reactions at the 10-position are known to be highly unfavorable 9S). The solvolytic reactivity of 10-tricyclo [5.2.1.0 4>10] decyl tosylate is retarded relative to 1-adamantyl tosylate by a factor of approximately 106 at 70°C in acetic acid. 

In contrast to typical mono- or acyclic substrates (e. g.,isopropyl), 2-adaman-tyl derivatives are also found to be insensitive to changes in solvent nucleophilicity. A variety of criteria, summarized in Table 13, establish this point. In all cases, the behavior of 2-adamantyl tosylate is comparable to that observed for its tertiary isomer but quite unlike that observed for the isopropyl derivative. Significant nucleophilic solvent participation is indicated in the solvolysis reactions of the isopropyl system. The 2-adamantyl system, on the other hand, appears to be a unique case of limiting solvolysis in a secondary substrate 296). The 2-adamantyl/ isopropyl ratios in various solvents therefore provide a measure of the minimum rate enhancement due to nucleophilic solvent assistance in the isopropyl system 297). 

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

The solvolysis reactions of cis and tram f-butylcyclohexyl tosylates also have been studied in NMA238. In both cases the primary product was 4-f-butylcyclohexene (no other cyclohexene products were detected) and much smaller quantities of cyclohexyl acetates and cyclohexanols were also recovered. The reaction rate was first order with respect to the tosylates. Similar to results of studies of the reaction in other solvents, the c/s-tosylate was solvolyzed more readily than was the tram compound. The stereochemical distribution of the minor products was significantly altered by small amounts of water (<1%) added to the NMA solvent. 

Although the reactions of cyclopropylcarbinyl cations have been investigated in considerable detail, corresponding reactions of the cyclopropenyl system have received much less attention, but they appear to be similar . Solvolysis of tosylate 291 proceeds only three times more rapidly than its saturated analogue a,j5-unsaturated carbonyl compounds ensue, presumably from ring expansion of cation 292 (equation 90). Analogous processes account for the first steps of the dehydration of 293 to naphthalene 294 and azulene 295 under various conditions as shown opposite. 

Other reactions involving heterocycloalkane to heterocycloalkene transformations have met with quite limited success. Dehydrogenation reactions of saturated silacyclopentane derivatives with Pt/C or with an Al-Cr-K oxide catalyst yield complex mixtures of unsaturated cyclic products with one and two double bonds 140, 272), Dehydration or solvolysis reactions lead to ring cleavage [356) for instance, solvolysis of 4-tosyl-1,1-dimethyl-1-silacyclohexane affords low yields of 1,1-dimethyl-1-silacyclo hex-3-ene and several ring-opening products (356). 

Cycloheptadienes, and derivatives thereof, have been made by solvolysis of tosyl and 4-ni-trobenzyl esters of bicyclo[4.1.0]hept-2-en-5-ol and related compounds. These reactions, which were designed to give mechanistic information, produced complex product mixtures which were not synthetically useful. 4,4-Dimethyl-6-methoxybicyclo[4.1.0]heptan-entfo-2-ol (11) was cleaved by dilute hydrochloric acid to yield 3,3-dimethylcyclohept-5-enone (12) quantitatively. ... 

Solvent Dependence of Reactivity. Solvolysis reactions were investigated to obtain structure-reactivity relationships, but these studies were complicated by the solvent dependence of relative rates (Table I). These results show a 1010 variation in relative rates of solvolyses of methyl and 2-adamantyl tosylates (2-AdOTs) in trifluoroacetic acid (TFA) compared with those of ethanolysis. Even for two secondary systems, relative rates for 2-AdOTs-(CH3)2CHOTs vary from 36 in trifluoroacetic acid to 0.0011 in ethanol (4). Hence, separate intrinsic structural effects must be separated from solvent-induced effects. 

The TT electrons of carbon-carbon double bonds can also become involved in nucleophilic substitution reactions. This participation can facilitate the ionization step if it leads to a carbocation having special stability. Solvolysis reactions of the syn and anti isomers of 7-norbornenyl tosylates provide some dramatic examples of the influence of participation by double bonds on reaction rates and stereochemistry. The anfi-tosylate is more reactive by a factor of about 10 than the saturated analog toward acetolysis. The reaction product, anft -7-acetoxynorbornene, is the product of retention of configuration. These results can be explained by participation of the tt electrons of the double bond to give the ion 3, which is stabilized by delocalization of the positive charge. ... 

Participation of carbon-carbon double bonds in solvolysis reactions is revealed in some cases by isolation of products with new carbon-carbon a bonds. A particularly significant case is the formation of the bicyclo[2.2.1]heptane ring during solvolysis of 2-cyclopent-3-enylethyl tosylate. ... 

If in the solvolysis of tosylates 163 and 164 the reaction is interrupted after 50 % of the initial compound has been spent the same mixture of products results ... 

The reaction of tosylate 382 was studied with sodium trifluoroacetate in trifluoro-acetic acid and in dimethylsulphoxide. In the former case the resulting mixture is practically identical with that obtained by the solvolysis of tosylate 372, i.e. it contains only a slight admixture of equatorial epimer 381a quite stable under the reaction conditions. Dimethylsulphoxide (which properly solvates carbocations) yields a mixture of trifluoroacetates 380a and 381a in a 1 1.8 ratio, i.e. the yield of the equatorial isomer is nearly twice as large as that of the axial one (fairly stable under the reaction conditions) in this reaction mixture the products resulting from... 

The reduction of ketone 559 by LiAlH results mainly in the endo alcohol 556. The thermodynamic equilibrium of exo and endo alcohols leads mainly to exo alcohol 560. Hence, the formation of endo alcohol 556 upon solvolysis of tosylate 558 cannot be due to steric factors — the endo-side attack is sterically less favourable than the exo-side one the resulting alcohol 556 is not a thermodynamically controllable product. Consequently, the data obtained cannot be used to assume a classical structure of the intermediate cation 561. At the same time the participation of the C —C bond and the intermediate non-classical ion are quite compatible with these facts. The acetolysis of the optically active tosylate 558 is accompanied by complete racemization, the rate of the latter being 3 times as high as that of acid elimination (internal return). The completeness of racemization shows the reaction to proceed via a symmetrical trishomocyclopropenyl ion or rapid equilibration of unsymmetrical cations 561 or to be accompanied by a 1,3-hydride shift from to C . 

An olefinic n system can also participate in a solvolysis reaction. Acetolysis of cholesteryl tosylate (41, Figure 8.21) gave the -acetoxy product (43), plus some rearrangement product (44) with a rate constant around 100 times greater than the rate constant for acetolysis of cyclohexyl... 

Due to the strain imposed by the three-membered ring, the cyclopropyl cation is not a stable intermediate and electrocyclic ring opening occurs readily. Therefore, in the solvolysis of cyclopropyl tosylate in acetic acid, allyl acetate is obtained rather than cyclopropyl acetate. Solvolysis reactions of other cyclopropyl halides, sulphonates, and diazotization of cyclopropylamine in aqueous solution also give the allylic products. 

Why is there a difference in the isotope effects in the solvolysis reactions of the following alkenyl tosylates Explain why the second example has the larger isotope effect. 

An Example of a Change in Mechanism in a Solvolysis Reaction Studied Using cr" 452 A Swain-Lupton Correlation for Tungsten-Bipyridine-Catalyzed Allylic Alkylation 453 Using Taft Parameters to Understand the Structures of Cobaloximes Vitamin 612 Mimics 455 The Use of the Schleyer Method to Determine the Extent of Nucleophilic Assistance in the Solvolysis of Aryl vinyl Tosylates 459... 

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