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Cob(I)alamin als Katalysator. 6. Mitteilung . Bildung und Fragmentierung von Alkylcobalaminen, ein Gleichgewichtsprozess zwischen nukleophiler Addition und reduktiver Fragmentierung
Fischli, Albert,Mueller, Peter Michael
, p. 529 - 554 (1980)
Isolated olefines can be saturated using catalytic amounts of cob(I)alamin in aqueous acetic acid; as electron source an excess of zinc dust is added to the solution containing the homogeneous catalyst.During this overall hydrogenation of isolated double bonds intermediate alkylcobalamins are formed (compare e.g.Schemes 2, 4, 5, 7 and 12).Clear evidence is presented that the nucleophilic attack on the isolated double bond is carried out by cob(I)alamin and not by cob(II)alamin also present in the system (see Scheme 3b and 3c).As this catalytic saturation of olefins depends on the pH of the solution, characterized by a slow reaction at pH=7.0 compared to the same reduction in aqueous acetic acid (see Scheme 2, 2 -> 4, and Scheme 3a), it is reasonable to accept the participation of an electrophilic attack by a proton during the generation of alkylcobalamins. - We use the term nucleophilic addition to describe the formation of alkylcobalamins from a proton, an olefin and cob(I)alamin (compare Schemes 4-7 and 12).A special sequence of experiments showed the nucleophilic addition to beregioselective.Preferentially the higher substituted alkylcobalamin revealed to be produced.Therefore, the nucleophilic addition of cob(I)alamin follows the Markownikoff rule (compare chap. 4: formation and fragmentation of β-hydroxyalkylcobalamins).Under the reaction conditions applied the intermediate alkylcobalamins can be present in base-on and base-off forms.They are known to exist as octahedral complexes and might also be stable to some extent as tetragonal-pyramidal species.In addition the base-off forms can partially be protonated at the dimethylbenzimidazole moiety in aqueous acetic acid (compare Scheme 12).From this equilibrium of intermediate alkylcobalamins three modes of decay disclosed to be possible: (i) The reductive fragmentation leading to an olefin, a proton, and cob(I)alamin is the formal retro-reaction of the nucleophilic addition (see Schemes 2, 4 and 6-12).This equilibrium of an associated alkylcobalamin and the corresponding dissociation products revealed to be a fast process compared to the reductive cleavage of the Co, C-bond cited below (s. (iii)). (ii) As the second reaction pattern an oxidative fragmentation producing an olefin, a hydroxy anion (or water, respectively) and cob(III)alamin has been observed (see Schemes 7, 8, 10 and 12). (iii) The slow reductive cleavage of the Co, C-bond, initiated by addition of electrons (see ), was the third reaction path observed (see Schemes 2, 4-8 and 10-12. - The stereochemistry of the three transformations originating from the intermediate alkylcobalamins is unknown up to now.The antiperiplanar pattern of the fragmentation reactions presented in the Schemes has been chosen arbitrarily (see e.g.Scheme 12).
