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 IndXTiCl3 (1, 2) react with 1.0, 2.0, and 3.0 equiv of MeMgCl to give IndXTiMeCl2 (3, 4),
IndXTiMe2Cl (5, 6), and IndXTiMe3 (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
).