Carbonium ions sulfur-stabilized

The initiating step in these reactions is the attachment of a group to the sulfoxide oxygen to produce an activated intermediate (5). Suitable groups are proton, acyl, alkyl, or almost any of the groups that also initiate the oxidations of alcohols with DMSO (40,48). In a reaction, eg, the one between DMSO and acetic anhydride, the second step is removal of a proton from an a-carbon to give an yUde (6). Release of an acetate ion generates the sulfur-stabilized carbonium ion (7), and the addition of acetate ion to the carbonium ion (7) results in the product (eq. 15) ... 

As has been suggested in the previous section, explanations of solvent effects on the basis of the macroscopic physical properties of the solvent are not very successful. The alternative approach is to make use of the microscopic or chemical properties of the solvent and to consider the detailed interaction of solvent molecules with their own kind and with solute molecules. If a configuration in which one or more solvent molecules interacts with a solute molecule has a particularly low free energy, it is feasible to describe at least that part of the solute-solvent interaction as the formation of a molecular complex and to speak of an equilibrium between solvated and non-solvated molecules. Such a stabilization of a particular solute by solvation will shift any equilibrium involving that solute. For example, in the case of formation of carbonium ions from triphenylcarbinol, the equilibrium is shifted in favor of the carbonium ion by an acidic solvent that reacts with hydroxide ion and with water. The carbonium ion concentration in sulfuric acid is greater than it is in methanol-... 

These orders are consistent with the order of stability of the alkyl car-bonium ions produced from the corresponding alkyl groups. A linear relationship was obtained between the logarithms of apparent rate constants for cracking and the heats of formation of the corresponding carbonium ions. Sugioka and Aomura therefore conclude that catalytic cracking of sulfur compounds over silica-alumina takes place by a carbonium ion mechanism. 

Rearrangements of tricyclic systems in concentrated sulfuric acid are often unlike those observed in SbFs-S02 solutions. Not only do intermolecular hydride shifts occur readily with ordinary substrate concentrations, but also the stabilities of the product alcohols control product distributions in sulfuric acid, whereas the stabilities of the carbonium ions are the controlling factors in SbFs-S02 solution. 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

As these phenyl-anologs of isobutane all have a single tertiary hydrogen atom, they may be considered as possible precursors of tertiary carbonium ions. Phenyl derivatives were selected because phenyl groups are known to stabilize carbonium ions and independent existence of these ions have been amply demonstrated by cryoscopic measurements of sulfuric acid solutions of the corresponding carbinols (87, 88) and by conductivity measurements in liquid sulfur dioxide solutions of the corresponding halides and perchlorates (89, 90, 91). Also, the electronic spectra of these ions have been well characterized (92-96). Isobutane is not included in this list since only indirect evidence is available for the independent existence of the tertiary-butyl ion and its electronic spectrum is still in doubt. Studies involving this compound and other aliphatic hydrocarbons will be discussed separately in Section VI, C, 2, c. 

It is not possible to examine alkyl cations such as er -butyl cation under similar conditions because of the intervention of a myriad of condensation, cyclization, and rearrangement reactions. In 96% sulfuric acid, fer -butanol is converted within minutes to a mixture containing 50% alkanes and 50% cyclopentenyl cations. " One of the major developments in organic chemistry during the decade of the 1960 s was the application of NMR spectroscopy in so-called superacid media to probe the structure of carbonium ions. The most obvious use of this technique is in examining alkyl cations and other less stable ions, the p s of which are not readily measured. In fact, the method is so versatile and the information gained so much more valuable than simple stability measurements that it is now the method of first choice in probing carbonium ion structure. 

As increasing numbers of alkyl and/or phenyl groups are added to the parent benzyl system, cations of increasing stability result. Deno et al. [59] studied the alcohol-carbonium ion equilibria as a function of sulfuric acid concentration and derived an empirical acidicity function, Co, for the water-sulfuric acid system. The acidity function allowed the pKji values for the equilibrium expressed in eq. 7.7 to be determined (Table 7.4). Furthermore, Deno and Schriesheim demonstrated quantitatively the relationship between the rate of an 8 1... 

Cyclization with the participation of hydroigrl groups or double bonds equivalent to them with aromatic rings is used in the syntheses of estrogens (aromatic ring A Schemes 49, 84, 88) or intermediates for the production of nonaromatic steroids (aromatic ring C Scheme 51). On the basis of the cyclization of a model tricyclic carbinol (81) under the action of 90% sulfuric acid, it was shown that when R = Me the formation of a cyclo-pentanophenanthrene derivative (83) takes place predominantly, while when R = H the spirane derivative (85) is the main product. This was explained by the comparative stabilities of the intermediate carbonium ions (82) and (84). When R = H, the tertiaiy carbonium ion (84) leading to the spirane (84) is more stable, and when R = Me the formation of the isomeric tertiary ion (82) is possible, and this cyclizes to the product (83) [55] (Scheme 88). 

While any substituent on the carbon atom a to the oxime is expected to stabilize the carbonium ion at the site of fragmentation [R. K. Hill, J. Org. Chem., 27, 29 (1962)], the fragmentation governed by sulfur has been rationalized as proceeding more likely by the relatively strain-free 1,2-thiazetine intermediate (i). Loss of a proton p to the sulfur leads to the fragmented product. 

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