Carbocations, from alcohols

With these conditions, Oiah was able to make carbocations from alcohols. He treated t-butanol with SbFsand HF in liquid SO2. This is the reaction. 

FIGURE 4.16 Production of relatively stable carbocations from alcohols. 

Aliphatic ketones can be reduced to hydrocarbons by triethylsilane and gaseous BF3.178 The BF3 is a sufficiently strong Lewis acid to promote formation of a carbocation from the intermediate alcohol. 

Figure 7.7 Free-energy diagrams for the formation of carbocations from protonated tertiary, secondary, and primary alcohols. The relative free energies of activation are tertiary < secondary primary.

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

Subsequently it was found that the same cations could also be generated from alcohols in super acid-S02 at - 60°C 1 and from alkenes by the addition of a proton from super acid or HF-SbF5 in S02 or SO20IF at low temperatures.12 Even alkanes give carbocations in super acid by loss of H. For example,13 isobutane gives the r-butyl cation... 

FIGURE 4.17 Energies of activation for formation of carbocations from alkyloxo-nium ions of methyl, primary, secondary, and tertiary alcohols. 

Alcohols react with compound 22 at low temperature in ca. 30 min, yielding an alkoxydimethylsulfonium salt 24 and one equivalent of trifluoroacetic acid. This mixture is normally stable at room temperature for several days. Nonetheless, alkoxydimethylsulfonium salts, derived from alcohols whose radicals are able to stabilize carbocations—particularly allylic and benzylic alcohols—suffer solvolyses by the action of trifluoroacetic acid from 0°C to room temperature, already in the absence of an amine, yielding the corresponding trifluoroacetates. This differential stability of alkoxydimethylsulfonium salts, derived from diverse alcohols, dictate different protocols in the Omura-Sharma Swern oxidation depending on the alcohol (vide infra). 

Dehydrations produce olehns from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rearrangement, as in file case of tertiary alcohols, mixtures of regioisomers are often produced during file loss of a proton from file carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method. 

Extensive measurements of heats of formation of carbocations in the gas phase exist and there have been more limited measurements in solution for nonhydroxylic solvents.39 For comparison with equilibrium measurements in water, however, the most appropriate measurement would appear to be free energies of formation in aqueous solution. It is fortunate therefore that a convenient compilation of values of AGf(aq) at 25°C has been provided by Guthrie.38 This allows us for example to derive a value of AGf(aq) for a carbocation from a measurement of its pXR value, provided that the free energy of formation of the corresponding alcohol [R OH in Equation (1)] is known. 

Surprisingly, the kinetic measurements now available for the nucleophilic trapping of carbocations with water are not always matched by measurements of rate constants for formation of the carbocation from the corresponding alcohol required to evaluate the equilibrium constant AR. Although carbocations are reactive intermediates in the acid-catalyzed dehydration of alcohols to form alkenes,85,86 the equilibrium in this reaction usually favors the alcohol and the carbocation forming step is not rate-determining. Rate constants may... 

The Lucas test involves adding the Lucas reagent to an unknown alcohol and watching for the second phase to separate (see Table 11-2). Tertiary alcohols react and show a second phase almost instantly because they form relatively stable tertiary carbocations. Secondary alcohols react in about 1 to 5 minutes because their secondary carbocations are less stable than tertiary ones. Primary alcohols react very slowly. Since the activated primary alcohol cannot form a carbocation, it simply remains in solution until it is attacked by the chloride ion. With a primary alcohol, the reaction may take from 10 minutes to several days. 

Polar solvents also favour El reactions because they stabilize the intermediate carbocation. El eliminations from alcohols in aqueous or alcohol solution are particularly common, and very useful. 

In this process, a carbocation is formed, either formally or in reality. This cation can be formed, for example the elimination of water from alcohols, of alcohols from acetals or by oxidation of a positive particle such as a proton to an alkene or an epoxide. The carbocation then reacts with a nucleophile to form a new carbocation, that undergoes one or more comparable further transformations in a cationic-cationic process, finally being trapped by a nucleophile or stabilized by elimination of a proton. 

Elimination from halides almost always gives a mixture of elimination and solvolysis products. The acid-catalysed elimination of water from alcohols provides a preparative alternative (reaction 5.29). The proto-nated alcohol 43 loses a water molecule to give the carbocation, which can v eliminate a proton to form the alkene. Even if some of the carbocation is trapped as the sulfate ester 44, this reaction is reversible, and the alkene can be distilled out of the reaction mixture to bring the reaction to completion. 

Chapter 5 continues the chemistry of alcohols and alkyl halides by showing how they can be nsed to prepare alkenes by elimination reactions. Here, the students see a second example of the formation of carbocation intermediates from alcohols, but in this case, the carbocation travels a different pathway to a different destination. 

Carbenes photogenerated from diazo precursors undergo proton transfer from alcohols to generate carbocations that are eventually trapped by nucleophiles [Eq. (11)] [106],... 

Schmidt reaction. Nascent carbocations generated from alcohols (using TfOH or SnCl ) are trapped by the added alkyl azides. ... 

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