Preparation of Half-Sandwich Alkyl−Titanium(IV) Complexes Stabilized by a Cyclopentadienyl Ligand with a Pendant Phosphine Tether and Their Use in the Catalytic Hydroamination of Aliphatic and Aromatic Alkynes

Abstract

Complex CpPTiCl3 (1; CpP = C5H4CH2CH2PPh2) reacts with 3.0, 2.0, and 1.0 equiv of MeMgCl to give CpPTiMe3 (2), CpPTiMe2Cl (3), and CpPTiMeCl2 (4). In solution the P-donor substituent of the cyclopentadienyl ligand is involved in a coordination−dissociation equilibrium (ΔH° = 7.7 ± 0.1 kcal·mol-1 and ΔS° = 36.9 ± 0.4 cal·mol-1·K-1 for 2, ΔH° = 6.0 ± 0.2 kcal·mol-1 and ΔS° = 22.3 ± 0.6 cal·mol-1·K-1 for 3, and ΔH° = 6.1 ± 0.2 kcal·mol-1 and ΔS° = 24.4 ± 1 cal·mol-1·K-1 for 4). The reaction of 1 with 3.0 equiv of PhCH2MgCl affords CpPTi(CH2Ph)3 (5), containing a free phosphine pendant group between −90 and 20 °C. In contrast to 5, the PPh2 group of {(2,6-iPr2C6H3)NH}CpPTi{N(2,6-iPr2C6H3)} (7), which is formed from the reaction of 2 with 2,6-diisopropylaniline, remains coordinated to the titanium atom between 60 and −60 °C. Complex 2 is an efficient catalyst precursor for the regioselective hydroamination of aliphatic (1-octyne and cyclohexylacetylene) and aromatic (phenylacetylene and 1-phenylpropyne) alkynes with aromatic (2,6-dimethylaniline and 2,6-diisopropylaniline) and aliphatic (tert-butylamine, dodecylamine, and cyclohexylamine) amines. The reactions give imines 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 aliphatic alkynes and aromatic amines, while anti-Markovnikov derivatives with regioselectivies of 100% are obtained from the reactions of aromatic alkynes with all the studied amines and from the reactions of the aliphatic alkynes with tert-butylamine and dodecylamine. The reaction of 1-octyne and cyclohexylacetylene with cyclohexylamine gives mixtures of both types of products. Some considerations about the mechanism of the catalysis are also presented.