A R T I C L E S
Samec et al.
resonances were too weak or obscured by the other isomer or 2. The
spectrometer was then heated to -25 °C where the free amine appeared
with characteristic resonances at δ 1.44 (d, J ) 8.3 Hz, 3H), 3.75
(s, 3H).
with ketones (aldehydes). The kinetic isotope effects observed
in the transfer dehydrogenation of an amine clearly show that
the hydride transfer alone is rate-limiting, which requires a
stepwise mechanism. Exchange studies performed with both an
external amine trap (in the presence of H2) and an internal amine
trap are consistent with an inner-sphere mechanism where the
substrate is coordinated to the ruthenium prior to hydrogen
transfer. The employment of an internal amine trap gave only
the ruthenium complex with the amine originating from the
imine at low temperatures. An outer-sphere mechanism can
explain this result only by assuming that the hydrogen bonded
intermediate X′′ (between A and diamine 21) coordinates
nitrogen to ruthenium before the hydrogen bond is broken and
the diamine can reorient in the solvent cage. Since we view
this assumption as highly improbable, we favor an inner-sphere
mechanism where the imine coordinates to ruthenium via an
η5fη3 ring slippage. We believe this mechanism is consistent
with all experimental data concerning the reactions of 2 with
imines. Computations at a high level are also consistent with
the proposed mechanism.
[2,3,4,5-Ph4(η4-C4CO)]Ru(CO)2NH(CH3)(CHCH3Ph) (15). Imine
14 (0.1 mL, 0.03 mmol, 0.3 M in CD2Cl2) was added to a solution of
2 (0.5 mL, 0.04 mmol, 0.08 M in CD2Cl2) at -196 °C. The NMR tube
was inserted to a precooled spectrometer (-90 °C), and the complex
appeared as diastereomers in total conversion. The solvent was then
evaporated, C6D6 (0.6 mL) was added, and the spectrometer was grad-
ually heated to 47 °C at which temperature the complex decomposed.
The two diastereomers were also synthesized according to a literature
procedure13 and separated by column chromatography using CH2Cl2/
pentane. 1H NMR (400 Hz, CDCl3, 25°C) major isomer: δ 1.56 (d, J
) 7.3 Hz, 3H), 2.07 (d, J ) 5.5 Hz, 3H), 3.88 (m, J ) 5.5, 7.3 Hz,
1H), 6.86 (m, 4H), 7.11 (m, 18H), 7.55 (m, 2H), 7.75 (m, 1H). 13C
NMR (100 Hz, CDCl3, 25°C) δ 14.9, 38.0, 65.5, 83.3, 83.8, 102.9,
104.5, 126.4-139.3 (20 resonances), 163.6, 200.8, 201.5. IR (CDCl3):
ν ) 2004, 1946, 1600, 1578, 1567, 1499, 1446 cm-1. HRMS (ES+)
(M + H)+ calcd for C40H33NO3Ru, 677.1504; found, 677.1496. 1H
NMR (400 Hz, CDCl3, 25°C) minor isomer: δ 0.90 (d, J ) 6.8 Hz,
3H) 2.15 (d, J ) 5.9 Hz, 3H), 3.70 (m, 1H), 7.10 (m, 23H), 7.62 (m,
2H). 13C NMR (100 Hz, CDCl3, 25°C) δ 23.4, 46.6, 68.5, 83.7,
84.0, 103.4, 105.1, 126.2-141.4 (20 resonances), 162.4, 200.7, 201.4.
The results presented provide strong support for an inner-
sphere process via coordination of the substrate in the trans-
fer hydrogenation of imines and transfer dehydrogenation of
amines with the monomers 2 and A of Shvo’s catalyst 1,
respectively.
IR (CDCl3): ν ) 2005, 1948, 1600, 1577, 1562, 1499, 1446 cm-1
.
HRMS (ES+) (M + H)+ calcd for C40H33NO3Ru, 677.1504; found,
677.1492.
Kinetic Isotope Study: Stoichiometric Hydrogenation. Complex
2 and 2-d2 were prepared from 3 and H2 or D2, respectively, as described
in the Supporting Information. The THF was evaporated, and CD2Cl2
saturated with H2O or D2O (depending on whether 2 or 2-d2 was used)
was added. The solution of 2 (0.50 mL of a 2 M solution, 0.10 mmol)
was syringed into an NMR tube under an argon atmosphere and cooled
to -196 °C. Freshly distilled imine 12 (0.100 mL of a 0.04 M solution
in CD2Cl2, 4 µmol) was added by syringe to the NMR tube, and the
mixture was warmed to -78 °C and carefully shaken. The NMR tube
was recooled to -196 °C and put into the spectrometer precooled to
-65 °C. At this temperature, no reaction occurred and the sample was
initially locked and shimmed, and an acquisition was run to double-
check the concentrations (that pseudo-first-order kinetics was followed)
and that no reaction had taken place. The temperature was set to -54
°C, the sample was shimmed, and t0 was set when the temperature had
reached -54 °C. The time between when the temperature was initially
set to -54 °C and the first acquired spectrum was ∼140 s (reactions
that had proceeded more than 25% were discarded). The reaction was
followed until at least 2 half lives (24 min), integrating the methoxy
peaks of 12 and free imine.
Experimental Section
[2,3,4,5-Ph4(η4-C4CO)]Ru(CO)2NH(Ph)(CHCH3Ph) (9). Imine 8
(0.1 mL, 0.03 mmol, 0.3 M in CD2Cl2) was added to a solution of 2
(0.5 mL, 0.06 mmol, 0.12 M in CD2Cl2) at -196 °C. The NMR tube
was inserted to a precooled spectrometer (-50 °C). The temperature
was increased to -40 °C, and at this temperature complex 9 started to
form as a mixture of diastereoisomers. After 3 h the conversion to 9
was determined to be 75% by integrating the doublets at δ 0.86 and
1.49 for complex 9 and the singlet at δ 2.2 for imine 8. 1H NMR
(CD2Cl2, 400 MHz) major isomer: δ 0.86 (d, J ) 6.4 Hz, 3H), 3.46
(d, J ) 11.0 Hz, 1H), 4.47 (dq, J ) 6.4, 11 Hz, 1H), 5.62 (m, 1H),
5.98 (m, 1H); the other aromatic resonances were obscured by 2. Minor
isomer: 1.49 (d, J ) 8.3 Hz, 3H), 4.32 (brd, J ) 7.1 Hz, 1H), 4.49
(m, 1H); the other resonances were obscured by the other isomer and
2. 13C NMR (100 Hz, CD2Cl2, -35°C) major isomer: δ 24.3, 25.4,
83.7, 85.9, 102.2, 104.7, 113.1, 119.5, 123-133 (21 resonances), 141.0,
162.0, 199.3, 201.6. Minor isomer: δ 27.1, 53.4, 82.4, 87.8, 102.3,
103.6, 117.1; all other resonances were too weak or obscured by the
other isomer or 2. The spectrometer was heated to -30 °C where the
free amine appeared with a characteristic resonance at δ 1.47 (d, J )
6.4 Hz).
Kinetic Isotope Study: Catalytic Transfer Dehydrogenation. An
NMR tube was charged with 2,6-dimethoxy-1,4-benzoquinone (11.2
mg, 0.07 mmol) under argon. The amine 4 was added by syringe (0.3
mL, 0.026 mmol, 0.086 M in toluene-d8). The NMR tube was inserted
into a prewarmed spectrometer (100 °C) for 5 min to dissolve the qui-
none. The NMR tube was ejected, and complex 1 was added by syringe
(0.45 mL, 2.16 µmol, 4.85 mM in toluene-d8) and the NMR tube was
reinserted into the spectrometer. The reactions were followed at 100
°C until at least 2 half lives, integrating the signals for the amine (δ
1.15-1.23) and the imine (δ 1.84-1.92) using ferrocene as internal
standard. The first 10% conversion was not taken into consideration
in obtaining the rate as catalyst 1 took a few minutes to completely
dissociate.
[2,3,4,5-Ph4(η4-C4CO)]Ru(CO)2NH(Ph)(CHCH3Ph-p-MeO) (13).
Imine 12 (0.1 mL, 0.03 mmol, 0.3 M in CD2Cl2) was added to a solution
of 2 (0.5 mL, 0.04 mmol, 0.08 M in CD2Cl2) at -196 °C. The NMR
tube was inserted into a precooled spectrometer (-60 °C), and the
temperature increased to -58 °C where the complex appeared as a 3:1
mixture of diastereomers. The conversion to complex 13 was 56% after
3 h of integrating the singlets at δ 3.58 and 3.62 for complex 13 and
1
the singlet at δ 3.85 for imine 12. H NMR (CD2Cl2, 400 MHz, -35
°C) of the major isomer: δ 0.82 (d, J ) 6.4 Hz, 3H), 3.35 (d, J ) 11.0
Hz, 1H), 3.58 (s, 3H), 4.49 (dq, J ) 11.0, 6.4 Hz, 1H), 5.57 (m, 1H),
5.98 (m, 1H). All other resonances were obscured by 2. Minor isomer:
δ 1.46 (d, J ) 8.3 Hz, 3H), 3.62 (s, 3H), 4.06 (m, CH). All other
resonances were obscured by the major isomer and 2. 13C NMR (100
Hz, CD2Cl2, -35 °C) major isomer: δ 24.3, 55.2, 67.5, 83.7, 85.8,
102.2, 104.7, 113.4, 124-133 (22 resonances), 150.6, 158.4, 199.4,
201.6. Minor isomer: δ 25.4, 67.2, 83.1, 85.6, 102.3, 103.6; the other
Exchange Study with External Amine Trap. Complex 2 (55 mg,
0.1 mmol) dissolved in CD2Cl2 (0.4 mL), and the 0.25 M solution of
2 was added by syringe into an NMR tube under Ar and cooled to
-196 °C. Freshly distilled 16 (0.5, 1, or 2 mmol; 0.5, 1, or 2 M; 0.1
mL of CD2Cl2) and 14 (0.1 mmol, 1 M, 0.1 mL) were added, and the
sample was warmed to -78 °C and carefully shaken. The sample was
9
14304 J. AM. CHEM. SOC. VOL. 128, NO. 44, 2006