Abstract
The mechanism of the ruthenium–N-heterocyclic-carbene-catalyzed formation of amides from alcohols and
amines was investigated by experimental techniques (Hammett studies, kinetic isotope effects) and by a computational study by using dispersion-corrected density functional theory (DFT/
M06). The Hammett study indicated that a small positive charge builds-up at the benzylic position in the transition
state of the turnover-limiting step. The kinetic isotope effect was determined to be 2.29ACHTUNGTRENUNG(!0.15), which suggests that the breakage of the C"H bond is not the rate-limiting step, but that it is one of several slow steps in the catalytic cycle. Rapid scrambling of hydrogen and deuterium at the a position of the alcohol was observed with
deuterium-labeled substrates, which implies that the catalytically active species is a ruthenium dihydride. The experimental results were supported by the characterization of a plausible catalytic cycle by using DFT/M06. Both cisdihydride and trans-dihydride intermediates were considered, but when the theoretical turnover frequencies
(TOFs) were derived directly from the calculated DFT/M06 energies, we found that only the trans-dihydride
pathway was in agreement with the experimentally determined TOFs.
amines was investigated by experimental techniques (Hammett studies, kinetic isotope effects) and by a computational study by using dispersion-corrected density functional theory (DFT/
M06). The Hammett study indicated that a small positive charge builds-up at the benzylic position in the transition
state of the turnover-limiting step. The kinetic isotope effect was determined to be 2.29ACHTUNGTRENUNG(!0.15), which suggests that the breakage of the C"H bond is not the rate-limiting step, but that it is one of several slow steps in the catalytic cycle. Rapid scrambling of hydrogen and deuterium at the a position of the alcohol was observed with
deuterium-labeled substrates, which implies that the catalytically active species is a ruthenium dihydride. The experimental results were supported by the characterization of a plausible catalytic cycle by using DFT/M06. Both cisdihydride and trans-dihydride intermediates were considered, but when the theoretical turnover frequencies
(TOFs) were derived directly from the calculated DFT/M06 energies, we found that only the trans-dihydride
pathway was in agreement with the experimentally determined TOFs.
Original language | English |
---|---|
Journal | Chemistry - A European Journal |
Volume | 18 |
Issue number | 49 |
Pages (from-to) | 15683–15692 |
Number of pages | 11 |
ISSN | 0947-6539 |
DOIs | |
Publication status | Published - 2012 |
Keywords
- Amides
- Density functional calculations
- Isotope effect
- Reaction mechanisms
- Ruthenium