Vitamin B12 (cobalamin, 1) is one of a few naturally occurring organometallic molecules. As a cofactor for adenosylcobalamin-dependent and methylcobalamin-dependent enzymes, it plays a crucial role in biological processes, including DNA synthesis and regulation, nervous system function, red blood cell formation, etc. Enzymatic reactions, such as isomerisation, dehalogenation, and methyl transfer, rely on the formation and cleavage of the Co-C bond. Because it is a natural, nontoxic, environmentally benign cobalt complex, cobalamin (1) has been successfully utilised in organic synthesis as a catalyst for Co-mediated reactions. This tutorial review concisely describes cobalamin-catalysed organic reactions that hold promise for environmentally friendly cobalt catalysis, leaving the reader with basic knowledge and the ability to harness the catalytic potential of this fascinating molecule.

Cobalester, a natural nontoxic vitamin B12 derivative, was found to catalyse unusual olefinic sp(2) C-H alkylation with diazo reagents as a carbene source instead of the expected cyclopropanation.