Isotope effects on hydride transfer reactions from transition metal hydrides to trityl cation. An inverse isotope effect for a hydride transfer
- Brookhaven National Lab., Upton, NY (United States). Chemistry Dept.
Kinetic isotope effects are useful in mechanistic studies, since they can provide insight into the transition state of the reaction being examined. Hydride (H{sup {minus}}) transfer reactions between carbons are pertinent to the chemistry of nicotinamide adenine dinucleotide (NAD{sup +}) analogues. Kinetic and mechanistic studies have established details of hydride transfers from 1,4-dihydropyridines and related hydride donors to carbon-based hydride acceptors such as pyridinium or acridinium cations. Hydride transfer from transition metal hydrides (MH) to Ph{sub 3}C{sup +}BF{sub 4}{sup {minus}} gives M-FBF{sub 3} and Ph{sub 3}CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH{sub 2}Cl solution, 25 C). For hydride transfer from Cp*(CO){sub 3}MoH (Cp{sup *} = {eta}{sup 5}-C{sub 5}Me{sub 5}) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Ph{sub n}(p-MeOC{sub 6}H{sub 4}){sub 3{minus}n}C{sup +}BF{sub 4}{sup {minus}}; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k{sub MoH}/k{sub MoD} = 1.7--1.9 as the rate constant decreases from k{sub H{sup {minus}}} = 6.5 {times} 10{sup 3} to 1.4 M{sup {minus}1} s{sup {minus}1}. For hydride transfer to Ph{sub 3}C{sup +}BF{sub 4}{sup {minus}} from five metal hydrides [Cp(CO){sub 3}MoH, Cp{sup *}(CO){sub 3}WH, (indenyl)(CO){sub 3} WH, Cp{sup *}(CO){sub 3}MoH, and trans-Cp(CO){sub 2}(PCy{sub 3})MoH; Cp = {eta}{sup 5}-C{sub 5}H{sub 5}] with second-order rate constants k{sub H{sup {minus}}} {ge} 3.8 {times} 10{sup 2} M{sup {minus}1} s{sup {minus}1}, the kinetic isotope effects are also k{sub MH}/k{sub MD} = 1.7--1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k{sub WH}/k{sub WD} = 1.8 to 0.47 as the rate constant decreases (from k{sub H{sup {minus}}} = 2.0 {times} 10{sup 3} to 0.72 M{sup {minus}1} s{sup {minus}1}). The steadily decreasing values of k{sub MH}/k{sub MD} 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 (k{sub WH}/k{sub WD} = 0.47) found for the slowest tungsten hydride, (C{sub 5}H{sub 4}CO{sub 2}Me)(CO){sub 3}WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.
- Research Organization:
- Brookhaven National Laboratory (BNL), Upton, NY
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC02-98CH10886
- OSTI ID:
- 682617
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 13 Vol. 121; ISSN JACSAT; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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