Mode III cleavages of carbon-cobalt bonds

5 -Deoxyadenosylcobalamin has long been known to be acid-labile, decomposing in 

This is, in fact, a particular example of a general class of reactions in which organocobalt complexes with )8-oxy substituents (including 2-hydroxy-, 2-alkoxy-and 2-alkyloxyalkyl-cobalts) [52,91-93] are decomposed in acid via elimination of aquocobalt complexes (Eqn. 55). 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

Evidence that the intermediate formed is a w complex rather than an alkylco-baloxime carbonium ion was obtained by direct observation of the intermediate (in D2SO4/D2O) by H-nmr spectroscopy. 

5 -Deoxyadenosylcobalamin also undergoes a mode III cleavage when treated with alkaline cyanide (Eqn. 57) [95]. 

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