Skeleton rearrangement

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. 

Wilkes H, R Rabus, T Fischer, A Armstroff, A Behrends, F Widdel (2002) Anaerobic degradation of -hexane in a denitrifying bacterium further degradation of the initial intermediate (l-methylpentyl)succinate via a C-skeleton rearrangement. Arch Microbiol 111 235-243. 

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... 

Anion-radicals are the first products of reductive electron transfer. Mntnal repulsion of the primary and snbseqnent excess electrons forms the basis of the impediment of polyelectron reduction at the initial step. However, an orbital, which already has one electron, can be populated by another electron if a proper redncer is chosen. This reducer must be able to overcome Coulomb repulsion. As a resnlt, more or less stable dianions are formed. Like anion-radicals, dianions can reversibly give excessive electrons back. If skeleton rearrangement is absent, the initial uncharged molecules are regenerated. 

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. 

The acyloxymercuration-demercuration of alkenes provides an alternative route to esters which is probably less prone to caibon skeleton rearrangements than the direct addition of carboxylic acids to alkenes (equation 282). This reaction has recently been reviewed.477 The reaction is most commonly run using mercury(II) acetate in acetic acid, though other mercury salts may be used and aprotic solvents can also be employed. Equilibria have been measured for the reaction of mercury(II) trifluoroacetate and alkenes in tetrahydrofuran, and were found to be solvent dependent.478... 

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