Rearrangements carbon skeleton

Carboxylic acids with an electron donating substituent in the a-position decarboxylate in a two-electron oxidation to carbocations (see chap. 7). These can react with the solvent (alcohol, acetic acid, water) or the unreacted carboxylate to ethers, esters, or alcohols (Eq, 14). In some cases the carbon skeleton rearranges, which is a clear indication of the cationic pathway. 

Therefore, Markovnikov s mle is followed, and carbon skeleton rearrangements and double-bond isomerizations (prior to attack by CO) are frequent. 

When carbocations are involved as intermediates, carbon skeleton rearrangement can occur during electrophilic addition reactions. Reaction of f-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged chloride.5... 

The electroreductive dehalogenation of a-haloacetic acids has been achieved with cobalamin [387]. The hydrophobic vitamin B12 Co complex immobilized on a glassy carbon electrode (252) may catalyze the electrochemical carbon-skeleton rearrangements of... 

D. M. Smith, S. D. Wetmore, and L. Radom, Theoretical Studies of Coenzyme-Bi2-Dependent Carbon-Skeleton Rearrangements, in Theoretical Biochemistry—Processes and Properties of Biological Systems, L. A. Ericksson, Ed., Elsevier, Amsterdam, The Netherlands, 2001, pp. 183-214. Electronic structure calculations are applied to the understanding and prediction of how enzymes can lower the barriers to the 1,2-shifts in radicals that occur in reactions catalyzed by B12. 

An analogous reaction occurs with diphenylethanedione, which results in carbon-skeleton rearrangement. This is one of the few carbon-skeleton rearrangements brought about by basic reagents, and is known as the benzilic acid rearrangement. ... 

Write structural formulas for all of the possible isomers of C8H10 that contain one benzene ring. Show how many different mononitration products each could give if no carbon skeleton rearrangements occur but nitration is possible either in the ring or side chain. Name all of the mononitration products by an accepted system. 

Since these addition reactions appear to proceed through carbocation intermediates, carbon skeleton rearrangements can be expected (equations 18 and 19).36-39... 

Carbon skeleton rearrangements during either hydroxymercuration or demercuration are also more common with dienes. 

One major advantage of the alkoxymercuration-demercuration approach to ethers over the acid-catalyzed process is the fact that carbon skeleton rearrangements are seldom observed. Only unsaturated cyclopropanes,42S>426 or aryl-substituted alkenes427 428 in the presence of highly electrophilic mercury salts afford rearranged products. 

C is a primary alcohol and is quite prone to undergo Carbon skeleton rearrangement when treated with hoi acid. To obviate the carbon skeleton rearrangement and effect the dehydration so that B is the major product, it is suggested that the acetate ester of C be formed and pyrolyzed as shown below. The acetate ester pyrolysis is known to yield olefins in which there is no carbon skeleton rearrangement. 

This reaction is useful because carbon skeleton rearrangement... 

Bothe, H., Darley, D. J., Albracht, S. P., Gerfen, G. J., Golding, B. T., and Buckel, W., 1998, Identification of the 4-glutamyl radical as an intermediate in the carbon skeleton rearrangement catalyzed by coenzyme Bi2-dependent glutamate mutase from Clostridium cochlearium. Biochemistry 37 4105n4113. 

Dowd, P., Wilk, B., and Wilk, B. K., 1992, 1st Hydrogen abstraction rearrangement model for the coenzyme B12-dependent methylmalonyl-CoA to succinyl-CoA carbon skeleton rearrangement reaction. J. Am. Chem. Soc. 114 794997951. 

Thermal decomposition of quaternary ammonium salts and bases is most valuable in structural investigations of amines, particularly heterocyclic secondary amines (Hofmann exhaustive methylation). The course of the elimination (A or B) is determined by the nature of the four alkyl groups on the nitrogen atom. The reaction has found little use in the synthesis of pure olefins. The yields are low even when three of the alkyl groups are methyl radicals. Carbon-skeleton rearrangement does not occur. Thus, the only olefin obtained by pyrolysis of pinacolyltri-methylammonium hydroxide, (CHj),CCH(CHj)N(CHj)j OH, is /-butylethylene (50%). ... 

Migrations of the double bond and carbon-skeleton rearrangements are important in the preparation of several olefinic compounds. A number of alkyl cyclopentenes are available in quantities suitable for synthetic work by the isomerization of cyclohexene and its homologs over alumina at 470- 480°. o-Allylphenol is isomerized by methanolic potassium hydroxide at 110° to o-propenylphenol (75%). Several /5, y-olefinic acids are conveniently obtained from the corresponding a /3-isomers by equilibration in basic media. The two isomeric acids are readily separated by partial esterification of the resulting mixtures since the /3,y-isomers are more easily esterified. "... 

The alkene metathesis reaction represents a unique carbon skeleton rearrangement in organic chemistry. This reaction, which grew out of the studies of catalysts related to alkene polymerization, results in the cleavage of the carbon-carbon double bond of an alkene and the redistribution of the resulting alkyl-idene fragments (equation 1). The same reaction carried out with cyclic alkenes generally results in the formation of polymers (equation 2). 

Oliagnier S, Kervio E, Retey J. The role and source of 5 -de-oxyadenosylradical in a carbon skeleton rearrangement catalyzed by a plant enzyme. FEES Lett. 1998 437 309-312. 

Wetmore SD, Smith DM, Golding BT, Radom L. Interconversion of (S)-glntamate and (2S,3S)-3-methylaspartate a distinctive B-12-dependent carbon-skeleton rearrangement. J. Am. Chem. Soc. 2001 123 7963-7972. 

Theoretical Studies of Coenzyme Bi2-Dependent Carbon-Skeleton Rearrangements... 

The main focus of the work to be discussed in this chapter is the mechanism by which the B 12-dependent carbon-skeleton rearrangements occur. However, before discussing these reactions specifically, we present more information about how coenzyme-B 12 mediates rearrangements in general. 

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