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

Abstract

Complexes Ind X TiCl 3 ( 1, 2 ) react with 1.0, 2.0, and 3.0 equiv of MeMgCl to give Ind X TiMeCl 2 ( 3, 4 ), Ind X TiMe 2 Cl ( 5, 6 ), and Ind X TiMe 3 ( 7, 8 ), respectively (X = CH 2 CH 2 OMe ( 1, 3, 5, 7 ), CH 2 CH 2 NMe 2 ( 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 (Δ H ° = 4.2 ± 0.6 kcal mol -1 and Δ S ° = 15.5 ± 3 eu for 3; Δ H ° = 3.4 ± 0.2 kcal mol -1 and Δ S ° = 11.6 ± 0.3 eu for 4; Δ H ° = 3.5 ± 1.1 kcal mol -1 and Δ S ° = 15.2 ± 4.4 eu for 5; Δ H ° = 4.3 ± 1.3 kcal mol -1 and Δ S ° = 16.0 ± 2.8 eu for 6; Δ H ° = 2.2 ± 0.5 kcal mol -1 and Δ S ° = 11.0 ± 2.2 eu for 7; Δ H ° = 4.9 ± 0.5 kcal mol -1 and Δ S ° = 20.8 ± 2.2 eu for 8 ). Complexes 7, 8, IndTiMe 3 ( 9 ), and H 4 IndTiMe 3 ( 10; H 4 Ind = 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 4).