A general study of [(n5-Cp')2Ti(n2-Me3SiC2SiMe3)]-catalyzed hydroamination of terminal alkynes:: Regioselective formation of Markovnikov and anti-Markovnikov products and mechanistic explanation (Cp' = C5H5, C5H4Et, C5Me5)

A general study of the regioselective hydroamination of terminal alkynes in the presence of [(eta(5)-CP)(2)Ti(eta(2) -Me3SiC2SiMe3)] (1), [(eta(5)- CpEt)(2)Ti(eta(2)-Me3SiC2SiMe3)] (CpEt = ethylcyclopentadienyl) (2), and [(eta(5)- CP*)(2)Ti(eta(2)-Me3SiC2SiMe3)] (Cp* = pentamethylcyclopentadienyl) (3) is presented. While aliphatic amines give mainly the anti-Markovnikov products, anilines and aryl hydrazines yield the Markovnikov isomer as main products. Interestingly using aliphatic amines such as n-butylamine and benzylamine the different catalysts lead to a significant change in the observed regioselectivity. Here, for the first time a highly selective switch from the Markovnikov to the anti-Markovnikov product is observed simply by changing the catalyst. Detailed theoretical calculations for the reaction of propyne with different c substituted anilines and tert-butylamine in the presence of [(eta(5)-C5H5)Ti(= NR)(NHR)] (R = 4-C6H4X; X = H, F, Cl, CH3, 2,6-dimethylphenyl) reveal that the experimentally observed regioselectivity is determined by the relative stability of the corresponding pi-complexes 10. While electrostatic stabilization favors the Markovnikov performance for aniline, the steric repulsive destabilization disfavors the Markovnikov performance for tert-butylamine.