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)2Ti=NStBu3

Intermolecular equilibrium isotope effects (EIEs) were measured (26.5 degrees C) and calculated for (silox)(2)-((Bu3SiND)-Bu-t)TiRD (1-ND-R-D) + RHH reversible arrow (silox)(2)((Bu3SiNH)-Bu-t)TiRH (1-R-H) + RDD: RHH/RDD = CH4/CD4, 2.00(6), calcd 1.88; C2H6/C2D6, 2.22(8), 1.93; (C3H6)-C-C/(C3D6)-C-C, 1.7(4), 1.48; C2H4/C2D4, 1.41(11), 1.34; C6H6/C6D6, 1.22(7), 1.26; C7H8/C7D8 (50.0 degrees C), 1.59(6), 1.55. Related intramolecular EIEs for (silox)(2)((Bu3SiND)-Bu-t)TiR' (1-ND-R') reversible arrow (silox)2((Bu3SiNH)-Bu-t)TiR (1-R) are provided: RH = R'D = CH3D, 3.16(25), calcd 2.60; CH2D2, 1.13(8), 0.911; CHD3, 0.389(20), 0.303; CH3CD3, 1.53(3), 1.44; 1,1-(C3H4D2)-C-C, 2.58(6), 2.53; trans-HDC=CHD, 1.00(2), 1.00; 1,3,5-C6H3D3, 1.273(4), 1.25; PhCH2D (50.0 degrees C), 2.06(2), 1.98. Calculations of pertinent model complexes (e.g, (HO)(2)(H2N)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 x MMI x EXC x EXP[-(Delta Delta ZPE/k(B)T)]. The large MMT term in the intermolecular cases-a consequence of using perprotio vs perdeuterio small molecule substrates-is attenuated by EXC and EXP[-(Delta Delta ZPE/k(B)T)] contributions derived from low-energy cole vibrations. The EIEs are differentiated on the basis of the EXP [-(Delta Delta ZPE/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 (KIEelim) from 1-R, KIEaddn values for 1,2-RH-addition to putative intermediate (silox)(2)Ti=(NSiBu3)-Bu-t (2) were inferred via EIE = KIEaddn/KZE(elim). Extraordinary intermolecular KIEaddn values ranging from similar to 30 (CH4/CD4, C2H6/C2D6) to similar to 9 (C6H6/C6D6) are consistent with previous mechanistic accounts and may be conventionally rationalized.