Carbocations sulfur containing

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. 

Disubstituted Alkenes. Simple 1,2-disubstituted alkenes such as 2-octene or cyclohexene, which produce only secondary aliphatic carbocation reaction intermediates, do not undergo reduction upon treatment with a Brpnsted acid and an organosilicon hydride. Even when extreme conditions are employed, only traces of reduction products are detected.192 203 207-210,214 An exception is the report that 4-methyl-2-pentene forms 2-methylpentane in 70% yield when heated to 50° for 20 hours with a mixture of Et3SiH/TFA containing a catalytic amount of sulfuric acid. It is believed that 4-methyl-2-pentene is isomerized to 2-methyl-2-pentene prior to reduction.203... 

Compounds containing a pyramidally arranged (hence, chiral) sulfur to which are linked three alkyl or aryl groups, resulting in a net positive charge on the sulfur. A biologically important example is S-adenosyl-L-methi-onine chloride. Sulfonium salts can also be utilized as analogs or mimics of carbocation intermediates in enzyme-catalyzed reactions. For example, methyl-(4-meth-ylpent-3-en-l-yl)vinylsulfonium perchlorate proved to be an excellent inhibitor (Ki = 2.5 tM) of the enzyme that catalyzes the formation of the bicyclic (+)- -pinene ... 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

Benzo-1,3-ditellurolylium tetrafluoroborate 76 turned out to be the first preparatively isolated salt of a heterocyclic carbocation containing two tellurium atoms in a ring (91UP3). The synthesis of 76 was carried out, similar to its sulfur analog benzo- 1,3-dithiolylium boron tetrafluoride (76BCJ3567), by treatment of a solution of 2-butoxybenzo-l,3-ditellurole in acetic anhydride with 2 equivalents of HBF4. 

Samples of lignin, wood, or cellulose were suspended in phosphate buffer (pH 7.0, y = 0.5) containing varying concentrations of sodium cholate, sodium deoxycholate or sodium taurocholate. The concentration remaining in the supernatant after equilibration for 20-24 h was determined by measuring the absorption spectra of polyenylic carbocations formed from bile acids in 72% sulfuric acid ( ). ... 

STRATEGY AND ANSWER As we have learned, in a strongly acidic medium such as methanol containing catalytic sulfuric acid, an alkene can accept a proton to become a carbocation. In the reaction above, the 2° carbocation formed initially can undergo an alkanide shift and a hydride shift as shown below to become a 3° carbocation, which can then react with the solvent (methanol) to form an ether. 

In the context of this chapter and reactions of alkenes, an alternative method for the preparation of polymers is possible. In the presence of strong acids, many alkenes form carbocation intermediates, as in Section 10.2, but under the proper reaction conditions, the carbocation intermediate may react with another molecule of the alkene and cationic polymerization is possible. Alkenes that polymerize under cationic conditions usually contain electronreleasing groups attached to the C=C unit. Examples are substituted alkenes such as isobutylene (166 2-methyl-l-propene) or methyl vinyl ether (168). The reaction of isobutylene (166) and sulfuric acid leads to poly(isobutylene), 167, which is also known as butyl rubber. 

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