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

Addition of 2.0 equiv of Na(silox) to TiCl4(THF)(2) afforded (silox)(2)TiCl2 (1), which yielded (silox)(2)((Bu3SiNH)-Bu-t)TiCl (2-Cl) upon treatment with (Bu3SiNLi)-Bu-t. Grignard or alkyllithium additions to 2-Cl or 1,2-RH-addition to transient (silox)(2)Ti=(NSiBu3)-Bu-t (3) produced (silox)(2)((Bu3SiNH)-Bu-t)TiR (2-R; R = Me, Et, CH2Ph = Bz, CH=CH2 = Vy, Bu-c, Bu-n, Ph, H, Pr-c, (c)Pe, CH2-3,5-Me2C6H3 = Mes, (neo)Hex, (c)Hex, eta(3)-H2CHCH2, eta(3)-H2CCHCHMe). Insertions of C2H4, butadiene, HC2H, and (HC2Bu)-Bu-t into the titanium-hydride bond of 2-H generated (silox)(2)((Bu3SiNH)-Bu-t)TiR (2-R; R = Et, eta(3)-H2CCHCHMe, Vy, E-CH=(CHBu)-Bu-t). Trapping of 3 by donors L afforded (silox)(2)LTi=(NSiBu3)-Bu-t (3-L; L = OEt2, THF (X-ray, two independent molecules: d(Ti=N) = 1.772(3), 1.783(3) Angstrom), py, PMe3, NMe3, NEt3) and metallacycles (silox)(2)((Bu3SiN)-Bu-t)TiCR=CR' (3-RC2R'; RC2R' = HC2H, MeC2Me, EtC2Et, (HC2Bu)-Bu-t) and (silox)(2)((Bu3SiN)-Bu-t)TiCH2CH2 (3-C2H4). Kinetics of 1,2-RH-elimination from 2-R revealed a first-order process (24.8 degrees C): R = Bz < Mes < H < Me (1.54(10) x 10(-5) s(-1)) < (neo)Hex < Et < Bu-n < Bu-c < (c)Pe < (c)Hex < Pr-c < Vy < Ph. Kinetics data, large 1,2-RH/D-elimination KIE's (e.g., MeH/D, 13.7(9), 24.8 degrees C), and Eyring parameters (e.g., 2-Me, Delta H double dagger = 20.2(12) kcal/mol, Delta S double dagger = -12(4) eu) portray a four-center, concerted transition state when 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-Pr-n similar to 2-Bu-n > 2-(neo)Hex > 2-Et, 2-Bu-c > 2-CH2SiMe3 > 2-Ph > 2-Me > 2-Bz > 2-Pr-c similar to 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 Delta G degrees(3). Using thermochemical cycles, relative activation energies for 1,2-RH-addition were assessed: (c)HexH > (c)PeH > (BuH)-Bu-n > (neo)HexH > EtH > BzH > (BuH)-Bu-c > MesH > MeH > PhH > (PrH)-Pr-c > VyH > 3-C2H4 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.