Preparation and X-ray Structures of Alkyl−Titanium(IV) Complexes Stabilized by Indenyl Ligands with a Pendant Ether or Amine Substituent and Their Use in the Catalytic Hydroamination of Alkynes

Complexes Ind^XTiCl₃ (1, 2) react with 1.0, 2.0, and 3.0 equiv of MeMgCl to give Ind^XTiMeCl₂ (3, 4), Ind^XTiMe₂Cl (5, 6), and Ind^XTiMe₃ (7, 8), respectively (X = CH₂CH₂OMe (1, 3, 5, 7), CH₂CH₂NMe₂ (2, 4, 6, 8)). Complexes 3, 6, and 8 have been characterized by X-ray diffraction analysis. The structures prove that in the solid state the pendant substituents of the indenyl ligands are coordinated to the metal center (d(Ti−O) = 2.296(3) Å (3); d(Ti−N) = 2.4006(19) (6), 2.4214(17) Å (8)) disposed transoid to a methyl ligand. In solution the pendant donor groups are involved in coordination−dissociation equilibria (ΔH° = 4.2 ± 0.6 kcal mol^-1 and ΔS° = 15.5 ± 3 eu for 3; ΔH° = 3.4 ± 0.2 kcal mol^-1 and ΔS° = 11.6 ± 0.3 eu for 4; ΔH° = 3.5 ± 1.1 kcal mol^-1 and ΔS° = 15.2 ± 4.4 eu for 5; ΔH° = 4.3 ± 1.3 kcal mol^-1 and ΔS° = 16.0 ± 2.8 eu for 6; ΔH° = 2.2 ± 0.5 kcal mol^-1 and ΔS° = 11.0 ± 2.2 eu for 7; ΔH° = 4.9 ± 0.5 kcal mol^-1 and ΔS° = 20.8 ± 2.2 eu for 8). Complexes 7, 8, IndTiMe₃ (9), and H₄IndTiMe₃ (10; H₄Ind = 4,5,6,7-tetrahydroindenyl) are efficient catalyst precursors for the regioselective hydroamination of 1-octyne, phenylacetylene, and 1-phenylpropyne with aromatic (2,6-dimethylaniline and 2,6-diisopropylaniline) and aliphatic (tert-butylamine, dodecylamine, and cyclohexylamine) amines. The reactions give imine or imine−enamine mixtures, which are reduced to the corresponding secondary amines. The Markovnikov or anti-Markovnikov nature of the obtained products depends on the aliphatic or aromatic character of both the alkyne and the amine. Markovnikov products with regioselectivities of 100% are formed from the reactions between 1-octyne and aromatic amines, while anti-Markovnikov derivatives with regioselectivities of 100% are obtained from the reactions of aromatic alkynes with all the studied amines and from the reactions of 1-octyne with tert-butylamine and dodecylamine. The reactions of 1-octyne with cyclohexylamine give mixtures of both types of products. A comparative study between the catalytic efficiencies of 7−10 and those of their cyclopentadienyl counterparts is also included (Table ).