Synthesis of N-arylpyrroles, hetero-Diels-Alder adducts, and allylic amines by reaction of unfunctionalized dienes with nitroarenes and carbon monoxide, catalyzed by Ru(CO)(3)(Ar-BIAN)

The reaction between an un-functionalized conjugated diene and a nitroarene under CO pressure, catalyzed by Ru-3(CO)(12)/Ar-BIAN (Ar-BUN = bis(arylimino)acenaphthene), affords the corresponding allylic amine (1), the hetero-Diels-Alder adduct (oxazine) (2), and the N-arylpyrrole (3) in different ratios depending on the experimental conditions. The synthesis of the allylic amine involves an intermolecular catalytic C-H functionalization by a transition metal complex. Compounds I and 2 are primary products of the reaction, whereas 3 derives from a following reaction of 2. By running the reaction at 120 degreesC, the decomposition of 2 to 3 is completely suppressed, allowing for the isolation of 2 in good yields. On the contrary, at 200 degreesC 2 is completely transformed into 3 during the reaction. The selectivity in allylic amine is somewhat sensitive to the experimental conditions and always ranges between 15 and 25%. Electron-withdrawing substituents on the nitroarene give better results, but electron-donating ones slow the reaction and give lower selectivities. Steric hindrance on the nitroarene strongly retards the reaction, but use of 2-methylnitrobenzene allowed for the isolation and X-ray structural characterization of a resting state of the catalytic system, Ru[N(o-CH3C6H4)C(O)N(o-CH3C6H4)C(O)](CO)(2)(Ph-BIAN) (9).