Allyl system solvolysis

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

Allylic systems have also provided fertile ground for investigation of ion-pair phenomena. Young, Winstein, and Goering established the importance of ion pairs in solvolysis of these compounds. They showed that ion pairs are responsible for the rearrangement of a,a-dimethylallyl chloride to y,y-dimethyl-allyl chloride (Equation 5.8).24 Goering s labeling methods have subsequently supplied a number of details about allylic ion-pair structure.25... 

For (58) and (60) keq was found to equal krac80,82. Thus it appears that internal return in allylic systems results in racemization and complete oxygen equilibration. However, this result is also consistent with an ion pair (63) which retains coordination of the ether oxygen to the carbon atom from which it departed. (63) would produce an equimolar mixture of (62a) and (62b). Determination of the 180 distributions of both recovered enantiomers from the solvolysis of (60) revealed identical scrambling, i.e. rapid interconversion of ion pairs (63) and (64)S2 With (55), however, predominant product formation from (63) was indicated by excess carbonyl-180 in the inverted ester and excess ether-180 in the ester of retained configuration80. ... 

Reminiscent of the effects encountered in the corresponding allylic systems (Section 14-3), benzylic resonance can affect strongly the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, the 4-methylbenzenesulfonate (tosylate) of 4-methoxyphenylmethanol (4-methoxybenzyl alcohol) reacts with solvent ethanol rapidly via an SnI mechanism. This reaction is an example of solvolysis, specifically ethanolysis, which we described in Chapter 7. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

The principal exception to this statement involves tertiary carbon, allylic carbon, and benzyl carbon atoms atl ached to X. In such cases it is often found that the second-order reaction with solvate ion such as RO (in ROH solvent) is much slower than apparent first-order reaction with solvent. These systems may be recognized as those giving quite stable carbonium ions, and the solvolysis has b( en ascribed to an Sifl mechanism. 

Initial interest in the solvolyses of cyclopropyl derivatives stemmed from the observation that they underwent solvolysis with concerted ring-opening , and that the reaction was strongly dependent on the nature and stereochemistry of substituents on the ring. This was explained by Woodward and Hoffmann who predicted from orbital symmetry considerations that the electrocyclic transformation in which a cyclopropyl carbocation is converted to an allyl cation should occur in a disrotatory fashion. Also, the particular disrotatory path a given system will take should be dependent on the stereochemistry of the leaving group. This is illustrated as follows. 

Brown and coworkers have published several papers concerning solvolyses of 1-aryl-1-cyclopropyl 3,5-dinitrobenzoates in 80 % aqueous acetone. A plot of log k versus <7 revealed, by its non-linearity, a change in mechanism as electron demand at the cationic center increases. The more electron-releasing substituents give more unrearranged cyclopropyl product. Thus the cyclopropanol to allyl alcohol ratios were p-MeO, 87 13 p-MeS, 70 30 and p-Me, 5 95. Extrapolation of the tertiary data to the parent cyclopropyl system indicates that solvolysis of the latter must be enhanced by concerted ring-opening by a factor of 10. ... 

Marshall and Huffman have employed a neat combination of the aforementioned principles to access the hydroazulene system (109) by solvolysis of (107) in the presence of aqueous dioxane the reaction (107) —> (109 equation 42) proceeds via the incipient allylic carbonium ion intermediate (lOS). Indeed, subsequent to this woric, a wide range of cyclodecenols (many naturally occurring) have been used to... 

Goering and his associates studied the solvolysis of several allylic p-nitrobenzoates which can give symmetrical allyl cations, (55)—(67)80 82 For all four systems, the rate of racemization, ka, was found to be greater than k. External return was not involved. In the cyclic systems (59) and (60) geometrical purity was preserved during... 

The in situ generated peroxocomplexes were tested for the catalytic epoxidation of various olefins, such as allyhc alcohols, homoallylic alcohols and non-functionalized olefins. The results of these H2O2 oxidations in an alcohol-water system are summarized in Table 2 for the hydrophilic catalyst A, and in Table 3 for the lipophihc material C. Especially for the more reactive alkenes, the turnover number comes close to the maximum of 300. The epoxide selectivity generally exceeds 90%, with minimal solvolysis. With catalyst A, some substrates gave a lower selectivity. For instance, the product distribution for cyclohexene is 65% epoxide, 27% of allylic oxidation products and only 4% of the diol. The epoxycyclohexane selectivity increases to 91% with the hydrophobic material C. 

Solvolysis rates of tertiary systems where a methyl group is present, have been used to estimate the rate of solvolysis of the corresponding secondary compounds good agreement was obtained between predicted and observed solvolysis rates for several systems which involved nucleophilic solvent assistance. Solvolysis in 60% aqueous acetone of the 3,5-dinitrobenzoate esters of cycloalk-2-en-l-ols labelled with deuterium at the 1-position, showed that little scrambling of the label or racemization occurred in the cyclo-octenol case, ca. 33 % scrambling occurred in the cycloheptenol case, and 56% scrambling occurred in the cyclohexenol case. Therefore, allylic participation, especially in the cyclo-octenol case, was rather weak. ... 

Two papers deal with doubly-unsaturated [3,2,0]-systems. The rates of solvolysis of the epimers (179) are almost equal and the products are unrearranged. It is concluded from this and from work on (161) that the solvolysis of (179) proceeds via the allyl cation (180) but that rearrangement via (181) to (182) is possible under certain conditions, e.g. treatment of the alcohols related to (179) with FSOjH. Just such a rearrangement is observed by Hart who finds that the labelled hexamethyl-ketone (183) with FSO3H in methylene chloride at — 78°C warmed to 10 C and worked up with NaHCOj gives 65% of (184). The ion (185) was detectable by H n.m.r. at —78 °C and (187) was present before the quenching. (186) is presumed to be an intermediate. 

Heteroatoms are not the only groups that will facilitate an SnI reaction when in proximity to the cationic center. Resonance effects in allyl or benzyl carbenium ions stabilize the cationic center and hence facilitate the substitution reaction. Yet, n systems further away from the cationic center can also get involved if their geometry is such that they are oriented toward the carbenium ion s empty p orbital. Consider the solvolysis of cholesterol tosylate in Figure 11.7. Two products are formed, and the solvolysis rate is approximately 100 times... 

Formation of allylic products is characteristic of solvolysis reactions of cyclopropyl halides and arenesulfonates. Similarly, diazotization of cyclopropylamine in aqueous media gives only allyl alcohol. The ring opening of the cyclopropyl cation is an electrocyclic reaction of the 4n H- 2 type, where n is equal to zero and should be disrotatory. Another facet of stereochemistry arises in substituted cyclopropyl systems. Note that for a d5-2,3-dimethylcyclopropyl cation, two nonequivalent disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The solvolysis rate constant of 3-indenyl 3,5-dinitrobenzoate 44a was 10 less than that of cyclo-pentenyl dinitrobenzoate 44d and 5 x 10 less than that of indanyl dinitrobenzoate 44b. Methoxy and methyl substituents at the 6-position on the ring in 44a and a methyl substituent at the 1-position on the double bond have accelerating effects expected for benzylic and allylic stabilization, respectively. The cyclopropanated system 44c showed no enhancement of reactivity compared to 44b, and this was taken as evidence for a diminuation in reactivity for 44c by a factor of 10 from that expected for a fully conjugating cyclopropyl group. The rate retardation was ascribed to due to homoantiaromaticity, of the type depicted for 41a. 

The concern for the possibility of generation of an ion pair that might be induced by the aqueous solvent system led to an examination of the solvolysis of 1,1-dideuterioallyl mesylate in an aqueous methanol solvent system. Here, however, no allylic rearrangement was observed in the formation of the substitution products thus, there was no rearrangement in the starting mesylate as well [47] ... 

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