Solvolytic studies

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

Vinyl trifluoromethanesulfonates (triflates) are a new class of compounds, unknown before 1969, that have been used most extensively in solvolytic studies to generate vinyl cations.2,3,812 Three methods have been used to prepare these sulfonic esters. The first, involving the preparation and decomposition of acyltriazines,4 requires several steps to prepare the acyltriazines and is limited to the preparation of fully substituted vinyl triflates. The second method involves the electrophilic addition of trifluoromethanesulfonic acid to acetylenes5,8,15 and, consequently, is not applicable to the preparation of trisubstituted vinyl triflates and certain cyclic vinyl triflates. However, this second procedure is relatively simple and often gives purer products in higher yield than the subsequently discussed reaction with ketones. Table I lists vinyl triflates that have been prepared by this procedure. ... 

Early solvolytic studies by Winstein and coworkers clearly established the anchimeric assistance provided by the neighboring cyclobutyl ring in the... 

Presumably, in these systems the energy cost of ring puckering is greater than the stabilization due to bishomoaromaticity (Bartlett and Rice, 1963). However, if the ring is already forced into a puckered conformation (e.g. by bridging between the 3 and 5 positions) then homoaromatic stabilization becomes effective. Solvolytic studies on the bridged systems [30] and [31]... 

A parallel was drawn between stable ion and AMI studies of methylphenanthrenes and solvolytic studies of K-region and non-K-region phenanthrene oxides. The carbocation formed by opening of the 1,2-epoxide closely resembled the 2-methylphenanthrene cation (and 7H ), and the regiochemistry of phenol formation (1-phenanthrol) could be understood. Similarly, phenanthrenium cations derived from the 3-methyl and dimethylated compounds served as models for carbo-cations formed by solvolysis of phenanthrene-3,4-epoxide (formation of 4-phenanthrol following hydride shift). 

Materials. Air dried wood (Populus deltoides) was ground to pass through a screen of 0.5 mm mesh size. These fines were used in the subsequent solvolytic studies. The chemicals used were ethylene glycol (practical grade) sodium hydroxide (practical grade) ethanol (95%) diethyl ether. Calibration-phenols were purchased from Aldrich Chemicals Limited and Alfa Chemicals Limited. 

Historically, physical organic chemists first explored this field and numerous studies devoted to the bicyclobutylium ion problem document this early period. From a preparative point of view, these studies were rather discouraging as the inherent capability of bicyclobutylium ions for multi-side reactions gave little hope that clear-cut ring enlargements could be achieved. Consequently, preparative applications, and even product analyses, within the countless solvolytic studies performed to learn more about the cyclopropylmethyl-cyclobutyl-ho-moallyl family, remained rare. 

Solvolytic studies provided the first structural indication for almost every carbocation-ic intermediate and the C4H,+ ion is no exception. Roberts observed that the solvolysis of cyclopropylcarbinyl or cyclobutyl systems and the diazotative deamination reactions of cyclopropylcarbinyl amine or cyclobutyl amine gave similar product mixtures consisting of cyclopropylcarbinyl, cyclobutyl and allylcarbinyl derivatives in essentially the same ratio1,2. A common cationic intermediate of C3v structure, the tricyclobutonium ion 1, was... 

From solvolytic studies of iso topically labeled substrates it was shown that cyclopropyl-carbinyl-cyclobutyl interconversion is stereospecific51 52. The stereospecific interconversion of cyclobutyl cations to the corresponding cyclopropylcarbinyl cation was also cleanly observed in superacid medium, and was used to prepare otherwise unstable cis-(a-methylcyclopropyl)carbinyl cation 1753. Thus ionization of d.s-2-chloro- or cw-3-chloro-l-methylcyclobutane in SbF5-S02ClF at -135 °C yielded the ris-isomer which rapidly rearranged irreversibly into the trans-isomer 18 at about -100 °C. The trows-isomer 18 is the only cation obtained when the preparation was carried out at -80 °C, or when prepared from the cyclopropylmethyl carbinol20b 38 50ac (equation 24). 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

Since it was difficult to make an exclusive choice between mechanisms for the degenerate rearrangement shown in Eqs. (3.141) and (3.142) and the average structure of 572 on the basis of H NMR data only, further arguments were taken from a solvolytic study, and the mechanism involving 574 [Eq. (3.141)] was preferred as an explanation for the behavior of 572 in superacid. 

A summary of the empirical scales of solvent polarity for the pure solvents frequently used for solvolytic studies is shown in Table 5. 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). 

Numerous solvolytic studies on diarylmethyl derivatives have been carried out under a variety of conditions. The analysis of substituent effects in the solvolysis of the monosubstituted chlorides [22] was reported earlier (Yukawa and Tsuno, 1959 Yukawa etal., 1966). The purpose of this analysis is to clarify the effect of a fixed substituent Y in one ring on the substituent effect of the variable substituents X on the second ring. Three extensive sets of kinetic data for the solvolysis of X, Y-disubstituted benzhydryl systems under fixed conditions have been reported one for the ethanolysis (Nishida, 1967), one for the chloride hydrolysis in 85% aqueous acetone at 0°C (Fox and Kohnstam, 1964) and one for the bromide hydrolysis (Mindl et al., 1972 Mindl and... 

Data from solvolytic studies on 4-methylated derivatives of cholesteryi tosylate [28,46] and mesylate [2g] have been interpreted as evidence for an intermechate cation close to the... 

Solvolytic studies of the 3-methyl and 3-phenyl substituted exo-tricyclo[3.2.1.0 ]oct-exo-3-yl derivatives 19 and 20 shown below have revealed" steric rate enhancements of... 

An interesting recent case of a solvolytic study of a fused-ring cyclopropylcarbinyl system is that of Hudrlik and coworkers on 38 as part of a synthesis of a-methylene-a-butyrolactones. 

Paquette and coworkers have carried out a solvolytic study of a tricyclic system in which a 3,4-ethano group is attached to the 2-bicyclo[3.1.0]hexyl system. This was done to assess the importance of cyclopropyl versus cyclobutyl neighboring group involvement. Some relative rate data are given in Scheme 10 for 3,5-dinitrobenzoate hydrolyses in 80 % aqueous acetone at 115 °C. Product results are also shown. Deuterium labeling revealed the lack of any cyclopropylcarbinyl-cyclopropylcarbinyl cation rearrangements. Also, the results showed that cyclopropyl participation predominates over initial involvement of the cyclobutyl group. 

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