New Titanium Complexes Containing a Cyclopentadienyl Ligand with a Pendant Aminoalkyl Substituent: Preparation, Behavior of the Amino Group, and Catalytic Hydroamination of Alkynes

Complex CpNTiCl3 (1) (CpN = C5H4CH2CH2NMe2) reacts with 1.0, 2.0, and 3.0 equiv of
MeMgCl to give CpNTiMeCl2 (2), CpNTiMe2Cl (3), and CpNTiMe3 (4), respectively. In the
solid state, the amino group of the pendant substituent of the cyclopentadienyl ligand is
weakly coordinated to the metal center (d(Ti−N) = 2.445(2) Å in 2 and 2.433(4) Å in 3),
transoid disposed to a methyl ligand. In solution, the N-donor substituent is involved in a
coordination−dissociation equilibrium (ΔH° = 2.4 ± 0.3 kcal·mol-1 and ΔS° = 6.5 ± 0.9
cal·mol-1·K-1 for 2, ΔH° = 3.8 ± 0.4 kcal·mol-1 and ΔS° = 12.3 ± 1.0 cal·mol-1·K-1 for 3,
and ΔH° = 4.4 ± 0.1 kcal·mol-1 and ΔS° = 17.9 ± 0.4 cal·mol-1·K-1 for 4). In moist benzene,
complex 3 affords the dinuclear species CpNTiCl(μ-O)2ClTiCpN (5), containing a planar Ti2(μ-O)2 core. In the solid state, the pendant amino groups of 5 are also coordinated to the
metal centers (d(Ti−N) = 2.421(2) Å). Like in 2, 3, and 4, the amino groups of 5 are involved
in a coordination−dissociation equilibrium (ΔH° = 4.9 ± 0.2 kcal·(mol of Ti)-1 and ΔS° =
20.7 ± 0.6 cal·K-1·(mol of Ti)-1). Complex 1 also reacts with LiNH(2,6-iPr2C6H3). The reaction
leads to the six-coordinate amido-imido derivative {(2,6-iPr2C6H3)NH}CpNTi{N(2,6-iPr2C6H3)}
(6), which shows a strong coordination of the pendant amino group to the titanium atom
(d(Ti−Namino) = 2.227(2) Å). In solution the amino group remains coordinated. Complex 4
has been found to be an efficient catalyst precursor for the intermolecular regioselective
anti-Markovnikov hydroamination of asymmetric alkynes. The reactions give enamine−imine mixtures, which are transformed into the corresponding secondary amines.