New half-sandwich alkyl, aryl, aryloxide, and propargyloxide titanium(iv) complexes containing a cyclopentadienyl ligand with a pendant ether substituent: behavior and influence in the hydroamination of alkynes of the ether group

Abstract

The complex CpOTiCl3 (1; CpO = C5H4CH2CH2OCH3) reacts with 1.0, 2.0, and 3.0 equiv of MeMgCl to give CpOTiMeCl2 (2), CpOTiMe2Cl (3), and CpOTiMe3 (4). In the solid state, the ether group of the pendant substituent of the cyclopentadienyl ligand is coordinated to the metal center (d(Ti−O) = 2.3373(18) Å in 2 and 2.2818(10) Å in 3) disposed transoid to a methyl ligand. In solution the O-donor substituent is involved in a coordination−dissociation equilibrium (ΔH° = 3.0 ± 0.7 kcal mol-1 and ΔS° = 12 ± 4 cal mol-1 K-1 for 2, ΔH° = 4.3 ± 0.1 kcal mol-1 and ΔS° = 17.2 ± 0.1 cal mol-1 K-1 for 3, and ΔH° = 2.3 ± 0.1 kcal mol-1 and ΔS° = 11.9 ± 0.3 cal mol-1 K-1 for 4). The reactions of 1 with 3.0 equiv of PhCH2MgCl, PhMgCl, and (p-tolyl)MgBr lead to CpOTiR3 (R = PhCH2 (5), Ph (6), p-tolyl (7)) containing a free pendant ether group. The X-ray structure of 6 shows a β-agostic Ti−H interaction between the metal center and a phenyl group (Ti−Cα−Cβ = 139.3(3) and 105.3(2)°). Complex 1 reacts with 1.0 equiv of Li(O-2,6-tBu2-4-MeC6H2) to give the six-coordinate aryloxide derivative CpOTi(O-2,6-tBu2-4-MeC6H2)Cl2 (8), which has been also characterized by X-ray diffraction analysis. The structure suggests a significant multiple bond character for the Ti−aryloxide bond (Ti−O−C = 151.31(14)°). The addition of 1.0 and 2.0 equiv of MeMgCl to 8 affords CpOTiMe(O-2,6-tBu2-4-MeC6H2)Cl (9) and CpOTiMe2(O-2,6-tBu2-4-MeC6H2) (10). Treatment of 1 with 1.0, 2.0, and 3.0 equiv of Li(O-2,6-iPr2C6H3) gives rise to CpOTi(O-2,6-iPr2C6H3)Cl2 (11), CpOTi(O-2,6-iPr2C6H3)2Cl (12), and CpOTi(O-2,6-iPr2C6H3)3 (13). Complex 11 reacts with 1.0 and 2.0 equiv of MeMgCl to afford CpOTiMe(O-2,6-iPr2C6H3)Cl (14) and CpOTiMe2(O-2,6-iPr2C6H3) (15), whereas the reaction of 12 with 1.0 equiv of MeMgCl leads to CpOTiMe(O-2,6-iPr2C6H3)2 (16). Complexes 15 and 16 can be also obtained by addition of 1.0 and 2.0 equiv of 2,6-diisopropylphenol to 4. Treatment of 4 with 1.0 and 2.0 equiv of 1,1-diphenyl-2-propyn-1-ol gives the propargyloxide derivatives CpOTiMe2(OCPh2CCH) (17) and CpOTiMe(OCPh2CCH)2 (18). Like 5−7, complexes 8−18 contain a free pendant ether group. Complex 4 is a more efficient catalyst precursor than the related CpTiMe3 (19) for the regioselective anti-Markovnikov hydroamination of phenylacetylene with cyclohexylamine, 2,6-dimethylaniline, tert-butylamine, and 2,6-diisopropylaniline and for the hydroamination of 1-phenylpropyne and diphenylacetylene with cyclohexylamine and 2,6-dimethylaniline.