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 (<b>1</b>, <b>2</b>) react with 1.0, 2.0, and 3.0 equiv of MeMgCl to give Ind^XTiMeCl₂ (<b>3</b>, <b>4</b>), Ind^XTiMe₂Cl (<b>5</b>, <b>6</b>), and Ind^XTiMe₃ (<b>7</b>, <b>8</b>), respectively (X = CH₂CH₂OMe (<b>1</b>, <b>3</b>, <b>5</b>, <b>7</b>), CH₂CH₂NMe₂ (<b>2</b>, <b>4</b>, <b>6</b>, <b>8</b>)). Complexes <b>3</b>, <b>6</b>, and <b>8</b> 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 (<i>d</i>(Ti−O) = 2.296(3) Å (<b>3</b>); <i>d</i>(Ti−N) = 2.4006(19) (<b>6</b>), 2.4214(17) Å (<b>8</b>)) disposed transoid to a methyl ligand. In solution the pendant donor groups are involved in coordination−dissociation equilibria (Δ<i>H</i>° = 4.2 ± 0.6 kcal mol^-1 and Δ<i>S</i>° = 15.5 ± 3 eu for <b>3</b>; Δ<i>H</i>° = 3.4 ± 0.2 kcal mol^-1 and Δ<i>S</i>° = 11.6 ± 0.3 eu for <b>4</b>; Δ<i>H</i>° = 3.5 ± 1.1 kcal mol^-1 and Δ<i>S</i>° = 15.2 ± 4.4 eu for <b>5</b>; Δ<i>H</i>° = 4.3 ± 1.3 kcal mol^-1 and Δ<i>S</i>° = 16.0 ± 2.8 eu for <b>6</b>; Δ<i>H</i>° = 2.2 ± 0.5 kcal mol^-1 and Δ<i>S</i>° = 11.0 ± 2.2 eu for <b>7</b>; Δ<i>H</i>° = 4.9 ± 0.5 kcal mol^-1 and Δ<i>S</i>° = 20.8 ± 2.2 eu for <b>8</b>). Complexes <b>7</b>, <b>8</b>, IndTiMe₃ (<b>9</b>), and H₄IndTiMe₃ (<b>10</b>; 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 (<i>tert</i>-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 <i>tert</i>-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 <b>7</b>−<b>10</b> and those of their cyclopentadienyl counterparts is also included (Table ).