Solvolysis and elimination

The interpretation of product data for competitive solvolysis and elimination reactions requires that the mechanism for these reactions be known. Two experiments are sufficient to show that the formation of solvolysis and elimination products occurs by partitioning of a common carbocation intermediate (Scheme 3 a) rather than by competing bimolecular reactions of the substrate (Scheme 3b).3... 

The demonstration that formation of the nucleophile adduct R-Nu results in the same proportional decrease in the yields of the alkene and solvent adducts, so that the ratio of the yields of these reaction products is independent of [Nu-]. If the solvolysis and elimination reactions proceed by competing stepwise and concerted pathways, respectively, then the yield of R-OSolv will decrease with increasing trapping of the carbocation intermediate by added nucleophile, while the yield of alkene from elimination will remain constant, so that the ratio [R-OSolv]/[Alkene] will decrease as [Nu ] is increased. 

Only low yields of the azide ion adduct are obtained from the reaction of simple tertiary derivatives in the presence of azide ion 2145 46 and it is not possible to rigorously determine the kinetic order of the reaction of azide ion, owing to uncertainties in the magnitude of specific salt effects on the rate constants for the solvolysis and elimination reactions. Therefore, these experiments do not distinguish between stepwise and concerted mechanisms for substitution reactions at tertiary carbon. 

There are at least two other studies of competitive reactions to form the products of solvolysis and elimination reactions that may provide insight into the relationships between carbocation structure and reactivity toward nucleophile addition and deprotonation. 

The intramolecular nitrogen-trapping protocol used by Banwell has also been successfully exploited in the assembly of spirocyclic frameworks relating to the aromatic erythrina alkaloids.24 However, when it was applied to the synthesis of nonaromatic spirocycles, as found in histrionicotoxin, the flexible alkyl tether proved to be problematic.25 In this study, gem-dichlorocyclopropane substrate 37 was initially subjected to silver(I) salts under a variety of conditions only to provide solvolysis and elimination products without any indication of trapping by the pendent... 

Besides the aforementioned Kolbe dimers, alcohols, esters or ethers can become the major products in the electrolysis of carboxylic acids. These results have suggested that in anodic decarboxylation the intermediate radicals were further oxidized to carbocations that yielded solvolysis and elimination prod-ucts. °2 This part of the anodic decarboxylation, which leads to carbenium ions, is frequently called nonKolbe electrolysis. Applications of the nonKolbe electrolysis to synthesis and to mechanistic investigation of carbocations are summarized in refs. 8,19,20 and 23. ... 

The ready availability of the pinenes makes them attractive potential feedstocks for /-menthol production. A number of routes have been published, but the only commercially successfiil one is that described above, which uses myrcene as an intermediate. One alternative approach is as shown in Fig. 8.41 using citronellene, prepared from (—)-P-pinene, as a key intermediate. The difficult step is to convert the citronellene to citronellol and two ways of achieving this are shown. The first uses an aluminium alkyl as described above under citronellene and citronellol. The second uses hydrochlorination of the trisubstituted double bond followed by anti-Markownikoff addition of hydrogen bromide to the other, then selective solvolysis and elimination. A process along these lines was developed by GUdden in the 1960s [206, 230], but was never commercialized. 

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). 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

Scheme 2.17 Formation of substitution and elimination product in the solvolysis of f-butyl substrates.

As can be deduced from discussions presented above and in Chapter 5, it is very important to recognize that when designing reactions involving carbocations, both migration reactions and elimination reactions can complicate the outcome of intended SnI transformations. An example illustrating the potential formation of multiple side products is shown in Scheme 6.3 with the solvolysis of 2-bromo-2,3-dimethylpentane in methanol. 

Like Problems 3(b) and 3(c), this is a solvolysis reaction. However, due to the increased complexity of the starting compound, the potential product mixture is more complex. Specifically, if we consider the initial solvolysis step and elimination of water, we notice that an allyl carbocation is formed that is adjacent to a migratable hydrogen atom. While this carbocation can undergo an El elimination reaction, a... 

When an o-nitroaniline is acylatcd by ethyl chloroformate and then catalyti-cally reduced, thermolysis of the reduction product (33) gives a 1-substituted 2-benzimidazolone (34) (Scheme 2.1.15) f99J. Presumably the carbamates (33) eliminate ethanol as they cyclize. and so the reactions bear similarities to those which proceed through isocyanates (see Scheme 2.1.18). In the presence of magnesium chloride, which appears to activate the urea carbonyl group to solvolysis and condensation, some benzimidazolones are converted into 2-alkyl- and 2-arylbenzimidazoles [100],... 

The intermediate radical can be further oxidized to form a carbenium ion, that undergoes solvolysis, rearrangement and elimination. This conversion is strongly influenced by the structure of the carboxylic acid, the electrolyte, the presence of foreign anions and the anode material (see Section 2.8.2). Besides... 

Solvolysis, Substitution, Elimination, and Reduction.—Studies on the solvolysis of 3p-tosyloxy-5,10-secocholest-1 (10)-en-5-ones have been reported5 and are complementary to those reported earlier6 on similar compounds. Considerable double bond participation was observed for the Z-3a-compound (1), the i>3a-compound (2),... 

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