Solvolytic studies intermediates

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... 

The 7-Norbomyl Cation In solvolytic studies, 7-norbomyl derivatives exhibited extremely low reactivity and gave products with predominant retention of conligura tion. " - Winstein et al. consequently concluded that the cationic intermediate is a nonclassical ion. However, isolation of the... 

Four papers have appeared in which a Russian group report details of solvolytic studies on tetrafluorobenzobicyclo[2,2,2]-octenyl and -octadienyF " tosylates as well as on the addition of trifluoroacetic acid to the octadiene discussion centres on the role of non-classical carbonium ion intermediates. 

While only an ancillary topic in this discussion, the nonclassical ion controversy, extensively reviewed elsewhere,has a foundation in the historical context of the Wagner-Meerwein rearrangement. After completing a solvolytic study of structurally similar exo- and e cto-2-norbomyl brosylates, Winstein and Trifan suggested that the reaction s cationic intermediate was instead a o-delocalized, symmetrically bridged norbomyl ion 9. This concept deviated from the accepted classical cation structure proposed by Meerwein as the equilibrium between 7 and 8, where the positive charge was considered to be localized on a single atom. 

Labelling studies with (525) clearly establish that either ion (526) is stable at -115 °C or equilibration between less symmetric structures must be rapid at that temperature. Now, in accordance with earlier predictions, evidence for a square-pyramidal ion is obtained from the n.m.r. spectrum of (527) in superacid media. Although solvolytic studies with the benzoate of (527) suggested that (528) was not an intermediate, the speetrum from (527) is best interpreted on the basis of strueture (528) or a set of rapidly equilibrating ions. 

In the case of 60, the ion has been prepared and has been shown to be a diradical in the ground state, as predicted by the discussion on page 58. Evidence that 60 is not only nonaromatic but also antiaromatic comes from studies on 64 and 66. When 64 is treated with silver perchlorate in propionic acid, the molecule is rapidly solvolyzed (a reaction in which the intermediate 65 is formed see Chapter 5). Under the same conditions, 66 undergoes no solvolysis at all that is, 60 does not form. If 60 were merely nonaromatic, it should be about as stable as 65 (which of course has no resonance stabilization at all). The fact that it is so much more reluctant to form indicates that 60 is much less stable than 65. It is noted that under certain conditions, 65 can be generated solvolytically. ... 

Finally, in a recent study by Walling and El-Taliawi (216) it was shown that solvolytically generated vinyl cations may be reduced by Fe ions in solution to the corresponding vinyl radical. When 2-buten-2-yl triflate was solvolyzed in concentrated ferrous perchlorate solution in the presence of acrylonitrile monomer, polymerization of the acrylonitrile was observed. No such polymerization occurred under identical conditions in the absence of Fe ions. It seems that the polymerization of acrylonitrile was initiated by the vinyl radicals formed by reduction of the intermediate vinyl cation by Fe as follows (216) ... 

In summary our study of a-substituted nitrosamines has given us a very interesting view of the chemical reactivity of this class of compounds. We have been able to show that the chemical behavior is governed by the intermediate existence of N-alkyl N-nitrosoimmonium ions in the thermal and solvolytic reactions. The significance of these N-alkyl N-nitrosoimmonium ions in the biological activity of dialkyInitrosamines is not clear at the moment. 

This chapter deals with silyl-substituted carbocations. In Section II results of quantum chemical ab initio calculations of energies and structures of silyl-substituted carbocations are summarized1. Throughout the whole chapter results of ab initio calculations which relate directly to the experimental observation of silyl-substituted carbocations and their reactions are reviewed. Section m reports on gas phase studies and Section IV on solvolytic investigations of reactions which involve silyl-substituted carbocation intermediates and transition states. Section V summarizes the structure elucidation studies on stable silyl-substituted carbocations. It includes ultra-fast optical spectroscopic methods for the detection of transient intermediates in solution, NMR spectroscopic investigations of silyl-substituted carbocations in superacids and non-nucleophilic solvents, concomitant computational studies of model cation and X-ray crystallography of some silyl-substituted carbocations which can be prepared as crystals of salts. 

In these later sections, interpretations of quantitative data for product mixtures are emphasised, and the relationship between kinetics and product analysis will be developed. Mechanistic applications of kinetic data are limited to steps of reactions prior to and including the rate-determining step. As separate later steps often determine the reaction products, detailed product studies and investigations of reactive intermediates are important supplements to kinetic studies. Examples of solvolytic and related (SN) reactions have been chosen first because they provide a consistent theme, and second because SN reactions provide an opportunity to assess critically many of the mechanistic concepts of organic chemistry. Product composition in solvolytic reactions will be discussed next followed by product selectivities (Section 2.7.2) and rate-product correlations (Section 2.7.3). 

If rearrangement proceeded only by 1,3-hydrogen shifts there is no way in which deuterium could appear in the 1-position. Even an unlikely series of 1,2-shifts can be ruled out by the greater amount of C2H5CD2OH formed than of (C2H4D)CHDOH. However, the results are easily accommodated by the intermediacy of partially equilibrating protonated cyclopropanes. A number of other solvolytic and deamination studies also support the idea that protonated cyclopropanes are reactive intermediates 28,29)... 

Full details of a study of leaving group-promoted solvolytic elimination reactions of 1-(1-methyl-l-arylethyl)pyridinium cations in 25 vol.% acetonitrile (aqueous) have been reported. Reactions of (34) and (35) are found to proceed via a common carbocation intermediate of ion-molecule pair type to give the suhstitution product (36) and elimination product (37) (Scheme 4). The total rate of reaction of (35) exceeds that for (34) by 1100-fold, corresponding to a Bronsted parameter of )S g = —0.93, and the fraction of (37) obtained is governed by = 0.12 for the dehydronation (kg) of the ion-molecule pair by the leaving group the product ratio is hardly affected hy the presence of substituted pyridines. For (34) and (35), = 1.85 0.10 (60 °C) and... 

Alkyl-bridged ions may be generated as potential intermediates in 1,2-alkyl shifts ( a route ), by double bond participation ( tt route ), and by protonation of cyclopropanes. In the acylic series, kinetic methods are not very useful for detecting alkyl participation. The solvolyses of alkyl sulfonates proceed with 1,2-alkyl shifts only in the case of neopentyl-type substrates, (612) and (613). Isobutyl (610) and 3-methyl-2-butyl sulfonates (611) prefer solvolytic displacement and 1,2-H shifts. Alkyl shifts in (610) and (611) are induced only by deamination (X=Nj) and related reactions which are not amenable to kinetic studies. 

It should be noted that tracer studies, such as the determination of the position of bond fission in hydrolysis and oxygen transfer in various oxidation reactions, are not considered to be within the scope of this review. The use of O18 for studying ion-pair intermediates in solvolytic... 

Silyl-substituted carbenium ions have attracted considerable experimental interest because they are believed to be intermediates in electrophilic additions to vinyl, ethynyl and aryl silanes, in solvolytic reactions and in cationic cyclization reactions1 -4,322. Despite this wide interest, knowledge concerning the effect of silyl substitution on the stabilities of carbenium ions was rather qualitative and only recently more quantitative data became available. Theoretical studies centered mainly around a- and / -silyl-substituted carbenium ions, but y-silyl effects have been also studied. 

---