Carbene radical
Carbene radicals are a special class of transition metal [carbene complex|organometallic carbenes]. The carbene radical can be formed by one-electron reduction of Fischer-type carbenes using an external reducing agent, or directly upon carbene formation at an open-shell transition metal complex using diazo compounds and related carbene precursors.
Cobalt-carbene radicals have found catalytic applications in cyclopropanation reactions, as well as in a variety of other catalytic radical-type ring-closing reactions.
Theoretical calculations and EPR studies confirmed their radical-type behaviour and explained the bonding interactions underlying the stability of the carbene radical.
Stable carbene radicals of other metals are known, but the catalytically relevant cobalt-carbene radicals have thus far only been synthesized as long-lived reactive intermediates.
Bonding interactions and radical reactivity
The chemical bond present in carbene radicals is surprising in that it possesses aspects of both Fischer and Schrock type carbenes.As a result, the cobalt carbene radical complexes have discrete radical-character at their carbon atom, thus giving rise to interesting catalytic radical-type reaction pathways.The mechanism of formation of a carbene radical at cobalt typically involves carbene generation at the metal with simultaneous intramolecular electron transfer from the metal into the metal-carbene π* antibonding [molecular orbital|anti-bonding] molecular orbital constructed from the metal d-orbital and the carbene p-orbital. As such, carbene radicals are perhaps best described as 'one-electron reduced Fischer-type carbenes'.
Discrete electron transfer from a sigma-type metal d-orbital occurs, leads the typical radical character of the carbene carbon. This behaviour not only explains the carbon-centered radical-type reactivity of these complexes, but also their reduced electrophilicity as well as their enhanced reactivity to electron-deficient substrates. Furthermore, second coordination sphere hydrogen-bonding interactions give rise to faster reactions because H-bonds are stronger to the reduced carbene as compared to the precursor.
Such H-bonding interactions can also facilitate chirality transfer in enantioselective carbene-transfer reactions.
In order for the σ bond to be stabilized, a back-bonding action from the π molecular orbital to the anti-bonding π* molecular orbital is necessary and the porphyrin ring serves as an electron π-symmetry "buffer" to ensure this interaction is obtained.
The back-donation to the π* orbital would result in unfavorable excess electron density on the carbene carbon but the presence of adjacent functional groups relieve this electron build-up and yield the final radical electron, which occupies a single p atomic orbital state on the carbon.