Selectivities in hydrocarbon activation: Kinetic and thermodynamic investigations of reversible 1,2-RH-elimination from (silox)2((t)Bu3SiNH)TiR (silox = (t)Bu3SiO)

Article abstract of DOI:10.1021/ja9707419

Addition of 2.0 equiv of Na(silox) to TiCl₄(THF)₂ afforded (silox)₂TiCl₂ (1), which yielded (silox)₂-((t)Bu₃SiNH)TiCl (2-Cl) upon treatment with (t)Bu₃SiNLi. Grignard or alkyllithium additions to 2-Cl or 1,2-RH-addition to transient (silox)₂Ti=NSi(t)Bu₃ (3) produced (silox)₂((t)Bu₃SiNH)TiR (2-R; R = Me, Et, CH₂Ph = Bz, CH=CH₂ = Vy, (c)Bu, (n)Bu, Ph, H, (c)Pr, (c)Pe, CH₂-3,5-Me₂C₆H₃ = Mes, (neo)Hex, (c)Hex, η³-H₂CHCH₂, η³-H₂CCHCHMe). Insertions of C₂H₄, butadiene, HC₂H, and HC₂(t)Bu into the titanium-hydride bond of 2-H generated (silox)₂((t)Bu₃SiNH)TiR (2-R; R = Et, η³-H₂CCHCHMe, Vy, E-CH=CH(t)Bu). Trapping of 3 by donors L afforded (silox)₂LTi=NSi(t)Bu₃ (3-L; L = OEt₂, THF (X-ray, two independent molecules: d(Ti=N) = 1.772(3), 1.783(3) A?) py, PMe₃, NMe₃, NEt₃) and metallacycles (silox)₂((t)Bu₃SiN)TiCR=CR' (3-RC₂R'; RC₂R' = HC₂H MeC₂Me, EtC₂Et, HC₂(t)Bu) and (silox)₂((t)Bu₃SiN)TiCH₂CH₂ (3-C₂H₄) Kinetics of 1,2-RH-elimination from 2-R revealed a fist-order process (24.8 °C): R = Bz < Mes < H < Me (1.54(10) x 10^-5 s^-1) < (neo)Hex < Et < (n)Bu < (c)Bu < (c)Pe < (c)Hex < (c)Pr < Vy < Ph. Kinetic's data, large 1,2-RH/D-elimination KIE's (e.g., MeH/D, 13.7(9), 24.8°C), and Eyring parameters (e.g., 2-Me, ΔH≠ = 20.2(12) kcal/mol, ΔS≠ = -12(4) eu) portray a four-center, concerted transition state where the N···H···R linkage is nearly linear. Equilibrium measurements led to the following relative standard free energy scale: 2-(c)Hex > 2-(c)Pe > 2-(n)Pr ~ 2-(n)Bu > 2-(neo)Hex > 2-Et > 2-(c)Bu > 2-CH₂SiMe₃ > 2-Ph > 2-Me > 2-Bz > 2-(c)Pr ~ 2-Mes > 2-Vy > 3-C₂H₄ > 3-NEt₃ > 2-H > 3-OEt₂ > 3-EtC₂Et > 3-MeC₂Me > 3-THF > 3-NMe₃ > 3-PMe₃ > 3-py. A correlation of D(TiR)(rel) to D(RH) revealed greater differences in titanium-carbon bond energies. THF loss from 3-THF allowed a rough estimate of ΔG°(3). Using thermochemical cycles, relative activation energies for 1,2-RH-addition were assessed: (c)HexH > (c)PeH > (n)BuH > (neo)HexH > EtH > BzH > (c)BuH > MesH > MeH > PhH > (c)PrH > VyH > 3-C₂H₄ formation > H₂. On the basis of a parabolic model, C-H bond activation selectivities are influenced by the relative ground state energies of 2-R and a:parameter representing the reaction coordinate, A more compressed reaction coordinate for sp²- vs sp³-substrates eases their activation.