Casey and Johnson
1345
to somewhat stronger hydrogen bonding of water to 4 than
of THF to 4 that must be replaced in the transition state.
Despite the retardation of the reaction rate observed upon
addition of ethanol to toluene in the catalytic reactions de-
scribed in the previous section, no retardation was observed
during the course of stoichiometric reduction. This may re-
sult from the relatively low ultimate concentration of alco-
hol, or from competition for hydrogen bonding with the
carbonyl species.
lowing Schlenk techniques or in a nitrogen atmosphere
glovebox. Toluene-d8 and THF-d8 were distilled from a so-
dium benzophenone solution immediately prior to use.
CD2Cl2 was dried over CaH2 and distilled prior to use. Other
solvents were dried and deoxygenated with activated alu-
mina and Q5 purification columns (11). Benzaldehyde and
acetophenone were distilled prior to use. NMR spectra were
recorded on a 360 MHz spectrometer, and are referenced to
residual protons in the deuterated solvent. The temperature
of the NMR probe was determined before and after each run
with a calibrated internal thermocouple. Ruthenium hydride
4 and isotopomers were prepared according to literature pro-
cedure (5).
Addition of water to THF alters isotope effects
The presence of water also results in significant alteration
of the magnitude of the individual isotope effects for the re-
duction of benzaldehyde in THF. As the concentration of
water increases, both individual isotope effects change: the
OH/OD isotope effect increases with water concentration,
while the RuH/RuD isotope effect decreases. The doubly la-
beled isotope effect also increases with higher concentra-
tions of water (Table 3).
General kinetic procedure
The general kinetic procedure will be illustrated with a
specific example. A standard THF solution of 5 (0.30 mL,
0.007 09 mol/L, prepared in a nitrogen glovebox from
20.2 mg of 5 and 2.5 mL of THF) was prepared in a
resealable NMR tube. The sample was degassed by three
freeze–pump–thaw cycles. Upon cooling the sample to
–196 °C, 1 atm of H2 was added (1 atm = 101.325 kPa). The
sample was then heated at 80 °C to ensure complete conver-
sion to 4-RuHOH. The solvent was evaporated under low
pressure, and the remaining solid was dissolved in 0.25 mL
of toluene-d8 in a nitrogen atmosphere glove box. The sam-
ple was cooled to –78 °C and benzaldehyde (100 µL,
0.0492 mmol, 20 equiv.) was added from a 100 µL gas-tight
syringe. The NMR tube was resealed, inserted into an NMR
spin collar, shaken for ~3 s, and inserted into an NMR spec-
trometer precooled to –37 °C. The disappearance of 4 and
the appearance of benzyl alcohol and ruthenium dimer 5
Deuterium labeling of the hydroxyl proton displayed an
isotope effect of 1.30 in THF in the absence of water. Addi-
tion of 0.079 mol/L of water (H2O and D2O, respectively)
led to an increase in the isotope effect to 2.22, and addition
of 0.120 mol/L of water led to a further increase to 3.10.
Deuterium substitution of ruthenium hydride results in an
isotope effect of 2.60 in the absence of water. Addition of
water (0.079 mol/L) led to the decrease of this isotope effect
to 1.57, and addition of more water (0.120 mol/L) resulted
in a further decrease to 1.36.
DFT calculations of the transition state for reduction of
H2C=O by (η5-C5H4OH)Ru(CO)2H indicate an early transi-
tion state with only moderate proton transfer from the CpOH
to the carbonyl oxygen and moderate hydride transfer from
Ru to the carbonyl carbon (5b). The increase in OH/OD iso-
tope effect upon addition of water suggests more extensive
proton transfer to the carbonyl oxygen at the transition state.
Similarly, the decrease in the RuH/RuD isotope effect upon
addition of water suggests less extensive transfer of hydride
to the carbonyl carbon at the transition state. This is consis-
tent with water stabilizing proton transfer in the transition
state.
1
were followed by H NMR spectroscopy for over three half-
lives.
[2,5-Ph2-3,4-Tol2(5-C4COH)]Ru(CO)2CH3 (6)
A THF solution of [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru-
(CO)2Cl (106 mg, 0.175 mmol) and excess CH3MgBr was
refluxed under nitrogen for 4 h and then neutralized with
1.5 mL of 5% HCl (aq). This mixture was extracted with to-
luene. Evaporation of the toluene extract under vacuum gave
6 as a deep maroon solid (88 mg, 0.150 mmol, 86% yield).
IR (CH2Cl2, cm–1): 1946, 2008. 1H NMR (toluene-d8,
360 MHz) δ: 0.82 (s, RuCH3), 1.89 (s, 6H, CpTolCH3), 4.29
Conclusion
3
These studies provide detailed mechanistic evidence that
the reduction of ketones and aldehydes occurs through a
concerted transfer of the acidic and hydridic hydrogens from
the tolyl analogue of Shvo’s hydroxycyclopentadienyl ruthe-
nium hydride 4 in toluene, methylene chloride, and THF. In
each case, hydrogen transfer is the rate-limiting step of the
reaction. The rate of carbonyl compound reduction is much
faster in non-hydrogen bonding solvents. Use of the hydro-
gen bonding solvent THF results in slower reduction rates,
in addition to a larger isotope effect on labeling of the acidic
proton. These changes are presumably due to stabilization of
4 via hydrogen bonding to the acidic hydrogen.
(s, OH), 6.63 (d, J = 8.2 Hz, 4H, aromatics), 6.9–7.2 (m,
10H, aromatics), 7.43 (d, 3J = 8.2 Hz, 4H, aromatics).
13C{1H} NMR (toluene-d8, 90 MHz) δ: –15.6 (RuCH3), 21.3
(CpTolCH3), 88.6 (C3,4 of Cp), 104.2 (C2,5 of Cp), 137.8
(C1 of Cp), 125–137 (eight resonances, aromatics), 203.8
(CO).
Acknowledgements
Financial support from the United States Department of
Energy, Office of Basic Energy Sciences, is gratefully ac-
knowledged. Jeffrey Johnson thanks the ACS Organic Divi-
sion Emmanuil Troyansky Graduate Fellowship for support.
Grants from the National Science Foundation (NSF) (CHE-
962-9688) and the National Institutes of Health (NIH) (I S10
RR04981-01) for the purchase of NMR spectrometers are
acknowledged.
Experimental
General
All syntheses and sample preparations were prepared fol-
© 2005 NRC Canada
Casey, Charles P.
