Free‐Radical Carbonylations: Then and Now

Abstract Although known since the 1950s, free‐radical carbonylation has not received much attention until only recently. In the last few years the application of modern free‐radical techniques has revealed the high synthetic potential of this reaction as a tool for introducing CO into organic molecules. Clearly now is the time for a renaissance of this chemistry. Under standard conditions (tributyltin hydride/CO) primary, secondary, as well as tertiary alkyl bromides and iodides can be efficiently converted into the corresponding aldehydes. Aromatic and α,β‐unsaturated aldehydes can also be prepared from the parent aromatic and vinylic iodides. If the reaction is carried out in the presence of alkenes containing an electron‐withdrawing substituent, the initially formed acyl radical subsequently adds to the alkene, leading to a general method for the synthesis of unsymmetrical ketones. This three‐component coupling reaction can be extended successfully to allyltin‐mediated reactions. Thus, β,γ‐enones can be prepared from organic halides, CO, and allyltributylstannanes. In a remarkable one‐pot procedure alkyl halides can be treated with a mixture of alkene, allyltributylstannane, and carbon monoxide in a four‐component coupling reaction that provides β‐functionalized δ,ϵ‐unsaturated ketones by the formation of three new CC bonds. The reaction of 4‐pentenyl radicals with CO leads to acyl radical cyclization, which provides a useful method for the synthesis of cyclopentanones. Certain useful one‐electron oxidations can be combined efficiently with free‐radical carbonylations. These findings and others discussed in this article clearly demonstrate that free‐radical carbonylation can now be considered a practical alternative to transition metal mediated carbonylation.