Inter- and Intramolecular Experimental and Calculated Equilibrium Isotope Effects for (silox)₂(tBu₃SiND)TiR + RH (silox = tBu₃SiO):  Inferred Kinetic Isotope Effects for RH/D Addition to Transient (silox)₂TiNSitBu₃

Intermolecular equilibrium isotope effects (EIEs) were measured (26.5 °C) and calculated for (silox) 2 ( t Bu 3 SiND)TiR D ( 1 -ND-R D ) + R H H ⇄ (silox) 2 ( t Bu 3 SiNH)TiR H ( 1 -R H ) + R D D: R H H/R D D = CH 4 /CD 4, 2.00(6), calcd 1.88; C 2 H 6 /C 2 D 6, 2.22(8), 1.93; c C 3 H 6 / c C 3 D 6, 1.71(4), 1.48; C 2 H 4 /C 2 D 4, 1.41(11), 1.34; C 6 H 6 /C 6 D 6, 1.22(7), 1.26; C 7 H 8 /C 7 D 8 (50.0 °C), 1.59(6), 1.55. Related intramolecular EIEs for (silox) 2 ( t Bu 3 SiND)TiR‘ ( 1 -ND-R‘) ⇄ (silox) 2 ( t Bu 3 SiNH)TiR ( 1 -R) are provided: RH = R‘D = CH 3 D, 3.16(25), calcd 2.60; CH 2 D 2, 1.13(8), 0.911; CHD 3, 0.389(20), 0.303; CH 3 CD 3, 1.53(3), 1.44; 1,1- c C 3 H 4 D 2, 2.58(6), 2.53; trans -HDC CHD, 1.00(2), 1.00; 1,3,5-C 6 H 3 D 3, 1.273(4), 1.25; PhCH 2 D (50.0 °C), 2.06(2), 1.98. Calculations of pertinent model complexes (e.g., (HO) 2 (H 2 N)TiR ( 1 ‘-R)) generated the vibrational frequencies necessary to interpret the EIEs in terms of a statistical mechanics description utilizing gas-phase partition functions; EIE = SYM × MMI × EXC × EXP[−(ΔΔZPE/ k B T )]. The large MMI term in the intermolecular cases a consequence of using perprotio vs perdeuterio small molecule substrates is attenuated by EXC and EXP[−(ΔΔZPE/ k B T )] contributions derived from low-energy core vibrations. The EIEs are differentiated on the basis of the EXP [−(ΔΔZPE/ k B T )] term, with CH-based bending vibrations playing the major role. Substrate bending vibrations that are absent in intramolecular cases are primarily responsible for the greater intermolecular values. Using measured (or calculated) EIEs and kinetic isotope effects for 1,2-RH-elimination (KIE elim ) from 1 -R, KIE addn values for 1,2-RH-addition to putative intermediate (silox) 2 Ti NSi t Bu 3 ( 2 ) were inferred via EIE = KIE addn /KIE elim . Extraordinary intermolecular KIE addn values ranging from ∼30 (CH 4 /CD 4, C 2 H 6 /C 2 D 6 ) to ∼9 (C 6 H 6 /C 6 D 6 ) are consistent with previous mechanistic accounts and may be conventionally rationalized.