Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 23: Organocobalt and Organomercury Compounds

Chapter 23 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 23 button below. The drawing underneath the button pictures radical formation from two organocobalt epimers. Beside the drawing is a description of the reaction. Below the drawing and its description is a summary of Chapter 23.

Chapter 23: Organocobalt and Organomercury Compounds

This drawing shows the interconversion of two organocobalt epimers.

Drawing Description

Radicals such as ·Co(dmgH)2py are responsible for a type of reactivity known as the persistent-radical effect. This effect causes a reaction that generates a persistent radical [e.g., ·Co(dmgH)2py] and a transient radical (typically a carbon-centered one) in equal amounts to give a higher yield of the cross-coupling product than would be expected from random radical coupling. This effect is responsible for only cross-coupling products (5 and 6) being detected in the reaction shown in the drawing on the left (Scheme 1 in Chapter 23).

Summary of Chapter 23

Organocobalt complexes are sources of free radicals because heating, photolysis, or enzy­matic reaction cleaves a carbon–cobalt bond homolytically to produce carbon-centered and cobalt-centered radicals. Cleaving the carbon–cobalt bond in this way changes the oxidation state of cobalt from Co(III) to Co(II). Complexes with cobalt in the Co(II) oxidation state exhibit radical reactivity. Cobalt containing carbohydrates easily undergo epimer­ization reactions because the radicals formed by bond fragmentation readily recombine. Carbon-centered radicals produced from organocobalt complexes also undergo the characteristic radical reactions of addition and cyclization.

Organocobalt and organomercury compounds have a similarity in reactivity because each contains a carbon-metal bond that is easily cleaved by heating or photolysis. Carbon-centered radicals produced from organomercury compounds undergo hydrogen-atom abstraction and radical addition reactions. Concern about the toxicity of organomercury compounds reduces their usefulness as radical precursors.