Anti-Markovnikov functionalizations of unsaturated compounds, Part 5 - Thefirst rhodium-catalyzed anti-Markovnikov hydroamination: Studies on hydroamination and oxidative amination of aromatic olefins

The first transition-metal-catalyzed regiospecific anti-Markovnikov hydroam ination of aromatic olefins is reported. Styrene and substituted styrenes r eact with secondary aliphatic amines, especially morpholine and N-arylpiper azines, in the presence of cationic rhodium complexes to give 2-aminoetheny lbenzene and 2-aminoethylbenzene derivatives. Cationic [Rh(cod)(2)]+BF4- an d various phosphines (1:2-mixture) were employed as in situ catalysts. Acco rding to labeling experiments, there is no evidence that the hydroamination is a consecutive hydrogenation of a previously formed enamine. Hydroaminat ion with simple secondary amines, for example piperidine, can also be achie ved by the use of a higher olefin concentration and higher reaction tempera tures than those given in previously published reaction procedures. Kinetic investigations of the major reaction pathway reveal that the reaction rate of the oxidative amination and the hydroamination is dependent on the styr ene and on the catalyst concentration, and independent of the amine concent ration. Experiments that employed deuterium-labeled amines (N-D) provided e vidence that the mechanism involves an amine-activating pathway, The substi tuents on the styrene, the phosphine ligand, and the solvent influence the yield of the aminations and the enamine:alkylamine ratio.