Isotope Effects on Hydride Transfer Reactions from Transition Metal Hydrides to Trityl Cation. An Inverse Isotope Effect for a Hydride Transfer

Citations Over TimeTop 11% of 1999 papers

Abstract

Hydride transfer from transition metal hydrides (MH) to Ph3C+BF4- gives M−FBF3 and Ph3CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH2Cl2 solution, 25 °C). For hydride transfer from Cp*(CO)3MoH (Cp* = η5-C5Me5) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Phn(p-MeOC6H4)3-nC+BF4-; n = 3, 2, 1, 0], the isotope effect remains essentially constant at kMoH/kMoD = 1.7−1.9 as the rate constant decreases from kH− = 6.5 × 103 to 1.4 M-1 s-1. For hydride transfer to Ph3C+BF4- from five metal hydrides [Cp(CO)3MoH, Cp*(CO)3WH, (indenyl)(CO)3WH, Cp*(CO)3MoH, and trans-Cp(CO)2(PCy3)MoH; Cp = η5-C5H5] with second-order rate constants kH− ≥ 3.8 × 102 M-1 s-1, the kinetic isotope effects are also kMH/kMD = 1.7−1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from kWH/kWD = 1.8 to 0.47 as the rate constant decreases (from kH− = 2.0 × 103 to 0.72 M-1 s-1). The steadily decreasing values of kMH/kMD with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows down in progressing from more electron-donating Cp ligands to less electron-rich Cp ligands. The inverse isotope effect (kWH/kWD = 0.47) found for the slowest tungsten hydride, (C5H4CO2Me)(CO)3WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.

Related Papers

• → SOME DEUTERIUM KINETIC ISOTOPE EFFECTS: IV. β-DEUTERIUM EFFECTS IN WATER SOLVOLYSIS OF ETHYL, ISOPROPYL, AND tert-BUTYL COMPOUNDS(1960)14 cited
• → Nitrogen and deuterium kinetic isotope effects on the Menshutkin reaction(1996)17 cited
• → Secondary Deuterium Isotope Effects on Reactions Proceeding Through Carbocations(1983)24 cited
• → Calculations of intrinsic isotope effects and the detection of tunneling in enzyme-catalyzed reactions(1990)10 cited
• Kinetic Deuterium Isotope Effects on the Reactions of 2-(4-Methoxyphenyl)Oxirane in Water Solutions(2006)