Corrins nucleophilic reactions

Coproporphyrin I synthesis, 816 Coronands classification, 919 metal ion complexes, 928,938 Corphins, 855 Coninoids, 983 Corrins, 871-888 demetallation, 882 deuteration, 879 electrophilic reactions, 879 metallation, 882 NMR, 878 nucleophilicity, 886 nucleophilic reactions, 879 oxidation, 879 oxidative lactamization, 880 oxidative lactonization, 880 photochemistry, 887 reactions, 879 at metal, 885 rearrangements, 879 redox chemistry, 888 spectra, 877 synthesis, 878 Corroles, 871-888 demetallation, 874 deuteration, 872 hydrogenation, 872 metallation, 874 reactions, 872 at metal, 875 redox chemistry, 876 synthesis, 871 Corticotropin zinc complexes medical use, 966 Cotton effect anils, 717... 

Two major mechanisms have to be taken into consideration for the alkylation of Co -corrins. The classical mechanism of a bimolecular nucleophilic substitution reaction at carbon (the Co -corrin acts as a nucleophile) leads to /3-aUcylated Co -corrins with high diastereoselectivity. Secondly, an electron transfer-induced radical process (where the Co -corrin acts as a one-electron reducing agent) may also lead to cobalt alkylation. The observed formation of incomplete a-aUcylated Co -corrins under kinetically controlled conditions has been proposed to occur via this path. The high nucleophilic reactivity of Co -corrins and their diastereoselective nucleophilic reaction on the ( upper ) /3-face are not increased by the nucleotide function on the ( lower ) a-face rather they appear to be an inherent reactivity of the corrin-bound tetracoordinate Co -center. Among the organometallic B12 derivatives prepared to date, neopentylcobalamin, benzylcobalamin, and... 

The N-oxides of special position, although their reactions with nucleophilic reagents are similar to the reactions of other imines. These compounds have been studied particularly in connection with attempts to synthesize the cyclic skeleton corrin of vitamin B12. 

Soft nucleophiles (X = CN , SCN , 1 ) attack Ni(5-C1CH2- or 5-HOCH2-ODC) at C-10 to give Ni(5-Me-10-X-ODC), white 5 2 reactions occur with hard nucleophiles (HO , CF3C02 ). Oxidation of natural cobalt(III) corrins by KMuO -pyridine gives 5- or 15-carboxy and 5,I5-di-carboxy derivatives. Acetoxylation of the meso methyl groups has also been reported. ... 

The second important type of organometallic reactivity of B 12-derivatives concerns the highly nucleophilic/nucleofugal Co(I)-corrins [75,91,132[. These provide the basis of the heterolytic mode of formation/cleavage of the Co - C bond, important in methyl-corrinoids in enzyme-catalyzed methyl-transfer reactions [125-127]. This mode is represented by the reaction of Co(I)-corrins with alkylating agents in the formation of the Co - CH3 bond and the nucleophile-induced demethylation of methyl Co(III)-corrins for the... 

The reverse process, the nucleophile-induced dealkylations of methyl-Co(III)-corrins, has been less studied due to the impediment of the intramolecular coordination of the nucleotide base [134,137]. Indeed, thiolates demethylate methylcobinamide (45) to cob(l)inamide (43) approximately 1000 times faster than MeCbl (3) to B s (40") [137], reflecting the strong stabilizing effect of the coordinated nucleotide in 3 [86,134]. This is of relevance for enzymatic methyl group transfer reactions involving protein bound Co(l)- and methyl-Co(lll)-corrins, where considerable axial base effects are expected [125,138]. 

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