Masked aldehyde makes mild radical reaction possible

Few functional groups are as popular in synthesis as the carbonyl group. Composed of an oxygen molecule doubly bonded to carbon, the carbonyl welcomes reaction with all types of nucleophiles. Carbonyls will even accept electrophiles, but it takes some convincing. Chemists access this reversed reactivity by using single-electron strategies, which typically require strong, stoichiometric reductants to form a ketyl radical. Now, researchers led by David A. Nagib of Ohio State University have found a way to form ketyl radicals under much milder conditions (Science 2018, DOI: 10.1126/science.aau1777). By reacting an aldehyde with acyl iodide, they formed an intermediate that’s easier to reduce than a carbonyl. Then the team used a cheap, photoactivated manganese catalyst to pluck off iodide and form a ketyl radical. The radical reacts with an alkyne, and the redox cycle is capped off when iodine adds back to the final product. Forming complex Z-selective vinyl iodides, the new approach can stitch alkyl and benzyl aldehydes with a wide range of alkynes. To demonstrate the synthetic utility of their approach, the researchers subjected reaction products to additional coupling reactions to rapidly connect five molecular components.