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
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Abstract
Intermolecular equilibrium isotope effects (EIEs) were measured (26.5 °C) and calculated for (silox)2(tBu3SiND)TiRD (1-ND-RD) + RHH ⇄ (silox)2(tBu3SiNH)TiRH (1-RH) + RDD: RHH/RDD = CH4/CD4, 2.00(6), calcd 1.88; C2H6/C2D6, 2.22(8), 1.93; cC3H6/cC3D6, 1.71(4), 1.48; C2H4/C2D4, 1.41(11), 1.34; C6H6/C6D6, 1.22(7), 1.26; C7H8/C7D8 (50.0 °C), 1.59(6), 1.55. Related intramolecular EIEs for (silox)2(tBu3SiND)TiR‘ (1-ND-R‘) ⇄ (silox)2(tBu3SiNH)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-cC3H4D2, 2.58(6), 2.53; trans-HDCCHD, 1.00(2), 1.00; 1,3,5-C6H3D3, 1.273(4), 1.25; PhCH2D (50.0 °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 × MMI × EXC × EXP[−(ΔΔZPE/kBT)]. The large MMI term in the intermolecular casesa consequence of using perprotio vs perdeuterio small molecule substratesis attenuated by EXC and EXP[−(ΔΔZPE/kBT)] contributions derived from low-energy core vibrations. The EIEs are differentiated on the basis of the EXP [−(ΔΔZPE/kBT)] 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)2TiNSitBu3 (2) were inferred via EIE = KIEaddn/KIEelim. Extraordinary intermolecular KIEaddn values ranging from ∼30 (CH4/CD4, C2H6/C2D6) to ∼9 (C6H6/C6D6) are consistent with previous mechanistic accounts and may be conventionally rationalized.
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