Anti-Markovnikov Reactions, Part 8 - Rhodium-catalyzed amination of aromatic olefins

The oxidative amination of styrene with secondary amines in the presence of cationic rhodium catalysts yields regiospecifically the corresponding anti -Markovnikov enamines. Styrene as the hydrogen acceptor gave concomitantly ethylbenzene. In the presence of 1,5-cyclooctadiene (cod) preferential redu ction to cyclooctene takes place. The addition of cod reduces the rate of t he reaction, but also the amount of ethylbenzene produced. Here, for the fi rst time the ratio of enamine: ethylbenzene is > 1, which is favourable in case of more expensive styrene derivatives. A screening of various ligands for oxidative amination reveals that hemiIabile 2-(omega -phosphino-n-alkyl )-pyridines are superior ligands for this reaction compared to simple alkyl and aryl phosphines.