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(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 = 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) A (3); d(Ti-N) = 2.4006(19) (6), 2.4214(17) A (8) disposed transoid to a methyl ligand. In solution the pendant donor groups are involved in coordination-dissociation equilibria (Delta H degrees = 4.2 +/- 0.6 kcal mol(-1) and Delta S degrees = 15.5 +/- 3 eu for 3 ; Delta H degrees = 3.4 +/- 0.2 kcal mol(-1) and Delta S degrees = 11.6 +/- 0.3 eu for 4; Delta H degrees = 3.5 +/- 1.1 kcal mol(-1) and Delta S degrees = 15.2 +/- 4.4 eu for 5; Delta H degrees = 4.3 +/- 1.3 kcal mol(-1) and Delta S degrees = 16.0 +/- 2.8 eu for 6; Delta H degrees = 2.2 +/- 0.5 kcal mol(-1) and Delta S degrees = 11.0 +/- 2.2 eu for 7; Delta H degrees = 4.9 +/- 0.5 kcal mol(-1) and Delta S degrees = 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).