A General Study of [(η5-Cp′)2Ti(η2-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 [(η5‐Cp)2Ti(η2‐Me3SiC2SiMe3)] (1), [(η5‐CpEt)2Ti(η2‐Me3SiC2SiMe3)] (CpEt=ethylcyclopentadienyl) (2), and [(η5‐Cp*)2Ti(η2‐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 substituted anilines and tert‐butylamine in the presence of [(η5‐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 π‐complexes 10. While electrostatic stabilization favors the Markovnikov performance for aniline, the steric repulsive destabilization disfavors the Markovnikov performance for tert‐butylamine. A systematic study of the regioselective hydroamination of terminal alkynes in the presence of titanocene complexes is reported. Aliphatic amines give mainly the anti‐Markovnikov imine, anilines and aryl hydrazines yield the Markovnikov isomer as main products (see scheme).