Selectivities in Hydrocarbon Activation:  Kinetic and Thermodynamic Investigations of Reversible 1,2-RH-Elimination from (silox)₂(tBu₃SiNH)TiR (silox = tBu₃SiO)

Addition of 2.0 equiv of Na(silox) to TiCl 4 (THF) 2 afforded (silox) 2 TiCl 2 ( 1 ), which yielded (silox) 2 ( t Bu 3 SiNH)TiCl ( 2 -Cl) upon treatment with t Bu 3 SiNLi. Grignard or alkyllithium additions to 2 -Cl or 1,2-RH-addition to transient (silox) 2 Ti NSi t Bu 3 ( 3 ) produced (silox) 2 ( t Bu 3 SiNH)TiR ( 2 -R; R = Me, Et, CH 2 Ph = Bz, CH CH 2 = Vy, c Bu, n Bu, Ph, H, c Pr, c Pe, CH 2 -3,5-Me 2 C 6 H 3 = Mes, neo Hex, c Hex, η 3 -H 2 CHCH 2, η 3 -H 2 CCHCHMe). Insertions of C 2 H 4, butadiene, HC 2 H, and HC 2 t Bu into the titanium−hydride bond of 2 -H generated (silox) 2 ( t Bu 3 SiNH)TiR ( 2 -R; R = Et, η 3 -H 2 CCHCHMe, Vy, E -CH CH t Bu). Trapping of 3 by donors L afforded (silox) 2 LTi NSi t Bu 3 ( 3 -L; L = OEt 2, THF (X-ray, two independent molecules: d (Ti N) = 1.772(3), 1.783(3) Å), py, PMe 3, NMe 3, NEt 3 ) and RC 2 R‘ = HC 2 H, MeC 2 Me, EtC 2 Et, HC 2 t Bu) and Kinetics of 1,2-RH-elimination from 2 -R revealed a first-order process (24.8 °C): R = Bz < Mes < H < Me (1.54(10) × 10 - 5 s - 1 ) < neo Hex < Et < n Bu < c Bu < c Pe < c Hex < c Pr < Vy < Ph. Kinetics 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, Δ H ⧧ = 20.2(12) kcal/mol, Δ S ⧧ = −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 -CH 2 SiMe 3 > 2 -Ph > 2 -Me > 2 -Bz > 2 - c Pr ∼ 2 -Mes > 2 -Vy > 3 -C 2 H 4 > 3 -NEt 3 > 2 -H > 3 -OEt 2 > 3 -EtC 2 Et > 3 -MeC 2 Me > 3 -THF > 3 -NMe 3 > 3 -PMe 3 > 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 Δ G °( 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 -C 2 H 4 formation > H 2 . 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 sp 2 - vs sp 3 -substrates eases their activation.