Carbon atom insertion into N-heterocyclic carbenes to yield 3,4-dihydroquinoxalin-2(1H)-ones

Carbon atom insertion into a cyclic framework is an attractive form of molecular editing since it can modify the core skeleton of a molecule, allowing for controlled increases in molecular size and complexity. We have discovered that when benzimidazoliums and 2-(methylsulfonyl)chromones were mixed under basic conditions, followed by the addition of a basic aqueous solution/mixture, the carbon atom located at position 2 of the chromone could be inserted into the in situ-generated N-heterocyclic carbenes (NHCs) to afford (Z)-3-(2-phenyl-2-oxoethylidene)-3,4-dihydroquinoxalin-2(1H)-ones, a scaffold found in numbers of bioactive compounds. Under the same conditions, 1-methyl-3-phenylbenzimidazolium iodide provided an unexpected pentacyclic product with a [1]benzopyrano[2,3-b]phenazine framework in an excellent yield presumably via a pathway involving a single-carbon transfer from the NHC.