Vitamin B12, Catalyst of Radical Reactions in Organic Synthesis

Abstract

Vitamin B12 and related metal complexes act as mediators in the electron transfer from the electron source (chemical reducing agent or cathode in electrochemical reductions) to an electrophilic substrate R-X (R-X = alkyl-, vinyl-or acyl-derivative) [IJ. On B 12 -catalyzed reduction R-X is transformed to the radical R' or the carbanion R-(depending on the structure of R and the reaction conditions) either directly or via an organometallic intermediate. The vitamin B 12 -catalyzed electrolysis of R-X at ca. -0.9 V vs.SCE under simultaneous irradiation of visible light [4J is the mildest way to form radicals R' by umpoZung of R-X. The thus created radicals may undergo follow-up reactions 'as e.g.: intra-or intermolecular addition to olefins [5J, elimination of a-leaving groups [6J, reduction followed by protonat,ion [7], disproportionation to olefin and hydrocarbon [BJ or rearrangements [9J.To the synthetically most useful reactions belongs the the C,C-bond formation by inter-or intramolecular addition to the (activated) double or triple bond of alkenes or alkines as well as the (concerted) formation of more than one C,C-bond by subsequent inter-or intramolecular additions (tandem-reaction).The application of the Organic Photo Electro Catalysis (OPEC-reaction) with B12 as catalyst is illustrated by the synthesis of some natural products. B12-catalyzed intermolecular addition [1, 3, 4JAs an example the synthesis of all stereoisomers of brevicomin (an insect pheromon) starting from cheap enantiomerically pure compounds like tartaric acid, mannitol, vitamin C and ribonolactone is described [10J. The strategy for the construction of bicyclic ketales is outlined in the scheme.