Substituted carbocations

To be able to prepare and study these elusive species in stable form, acids billions of times stronger than concentrated sulfuric acid were needed (so called superacids). Some substituted carbocations, however, are remarkably stable and are even present in nature. You may be surprised to learn that the fine red wine we drank tonight contained carbocations which are responsible for the red color of this natural 12% or so alcoholic solution. I hope you enjoyed it as much as I did. 

This suggests that as water attacks the bromonium ion positive charge develops on the carbon from which the bromine departs The transition state has some of the character of a carbocation We know that more substituted carbocations are more stable than less substituted ones therefore when the bromonium ion ring opens it does so by breaking the bond between bromine and the more substituted carbon... 

Ruorme-Substituted Carbocations Allen, A D, Tidwell, T T Adv Carbocation Chem /, 173 3 S-... 

The benzo-fused 1,3-dithietane 6, from which the formation of two dithiiranes (7 and 8) is possible depending on the course of the reaction, only gave the alkylaryldithiirane 7 in 59% yield by treatment with NaOCl-NaC104 (97BCJ509). Tliis is suggestive of the intermediate formation of the more stable, diaryl-substituted carbocation 9 and not the other counterpart 10. 

Markovnikov s rule In the addition of HX to an alkene, the more highly substituted carbocation is (restated) formed as the intermediate rather than the less highly substituted one. 

U Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation intermediate. A more highly substituted carbocation forms faster than a less highly substituted one and, once formed, rapidly goes on to give the final product. 

U A more highly substituted carbocation is more stable than a less highly substituted one. That is, the stability order of carbocafions is tertiary > secondary > primary > methyl. 

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Addition of a proton occurs to give the more-substituted carbocation, so addition is regioselective and in accord with Markovnikov s rule. A more detailed discussion of the reaction mechanism is given in Section 6.2 of Part A. Owing to the strongly acidic and rather vigorous conditions required to effect hydration of most alkenes, these conditions are applicable only to molecules that have no acid-sensitive functional groups. The reaction is occasionally applied to the synthesis of tertiary alcohols. 

To what extent is the partitioning of simple aliphatic and benzylic a-CH-substituted carbocations in nucleophilic solvents controlled by the relative thermodynamic driving force for proton transfer and nucleophile addition reactions It is known that the partitioning of simple aliphatic carbocations favors the formation of nucleophile adducts (ksjkp > 1, Scheme 2) and there is good evidence that this reflects, at least in part, the larger thermodynamic driving force for the nucleophilic addition compared with the proton transfer reaction of solvent (A dd U Scheme 6).12 21,22,24... 

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

In this chapter, die term carbene includes the species called carbenoid which is often considered as an a-metal-substituted carbocation or its equivalent. In some cases the term carbenoid also includes encumbered carbenes complexed with metal halides. 

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

Chapter 1 by H.-U. Siehl discusses parallel stable ion NMR spectroscopic and computational studies on various classes of silyl-substituted carbocations. Chapter 2 by K. Komatsu focuses on unusually stable n-conjugated carbocations that are formed as a result of annelation to bicyclic frameworks. 

Experimental and Computational NMR Spectroscopic Investigation of Siiyl-Substituted Carbocations... 

Organosilicon compounds are widely used in organic synthesis. The understanding of the structure and properties of the intermediates involved in their reactions is a prerequisite for further development and optimization of useful synthetic transformations involving silicon substituted compounds. Trialky lsilyl-substituted carbocations are particularly... 

Early attempts to generate ot-aryl-(3-silyl substituted carbocations by ionization of 1,1-diphenyl-2-(trimethylsilyl)ethanol 10 usipg FS03H in S02C1F even at very low temperature of -140 °C were unsuccessful (77). Only 1,1-diphenylethyl cation 11 and trimethylsilyl fluorosulfate, fhe products of P-silyl cleavage were observed. 

The ferrocenyl group is a very good electron donor. The a-ferrocenyl P-silyl substituted carbocation 20 is accessible by protonation of ( )-1 -ferrocenyl-2-(triisopropylsilyl)alkene 21 with trifluoroacetic acid in SO2CIF at - 95 °C (13, 22). 

A similar pattern had been observed earlier with p-phenyl-substituted carbocations. Relative migration ability of phenyl and substituted phenyl groups were determined for the first time, not by focusing on carbocations formed via aliphatic series as structural models, but rather by studying 9-ary 1-9,10-dimethylphenanthrenium ions (18, 19). 

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