Asymmetric Transfer Hydrogenation
FULL PAPER
(CH), 77.8 (CH), 127.6–139.2 (several m, Ar), 176.9 (CH=N), 178.1 ppm
(CH=N); 31P NMR (CD2Cl2, 121 MHz): 51.6 (d, JP, P =31 Hz), 49.2 ppm
(d, JP, P =31 Hz); IR (KBr): n˜ =2004 cmꢀ1 (nCꢁO); MS (ESI+): m/z (%):
Conclusions
In summary, we have described highly active and well-de-
fined iron precatalysts 1 and 3 for the transfer hydrogena-
tion of aromatic and non-aromatic ketones. The iron com-
plexes were fully characterized, including by their crystal
structures. They have excellent catalytic activities at room
temperature, competitive with those of the best-established
ruthenium catalysts.[18] With bulky substrates, excellent en-
antiomeric selectivities were observed when 3 was used as
the precatalyst. This is the only iron catalyst so far reported
to give such high ee values at high levels of conversion. In
addition, we have further optimized our recently reported
catalytic iron system involving 2, now achieving high turn-
over frequencies (up to 2600 hꢀ1), high turnover numbers
and high enantiomeric excesses. This is an important step in
the journey to replace precious and toxic platinum metal
catalysts with cheap and environmentally friendly iron com-
pounds. Further mechanistic work is required to explain the
unique selectivities of the ketone reductions described in
this work.
406.1 [MꢀMeCNꢀCO]2+
;
elemental analysis (%) calcd for
C55H45B2F8N3P2Fe1: C 62.59, H 4.30, N 3.98; found: C 61.93, H 4.96, N
3.67.
The complex (S,S)-3 was also prepared with similar yield and properties.
Crystals of this complex for the X-ray diffraction study were obtained by
diffusion of ether into an acetone solution.
Transfer hydrogenation: A solution of substrate (1, 3 or 20 mmol) in
propan-2-ol (6 mL) was added to a mixture of solid precatalyst [1, (R,R)-
2 or (R,R)-3, 0.005 mmol] and KOtBu (0.04 mmol). The immediately
formed dark solution was stirred vigorously. Samples were taken from
the mixture, quenched by exposure to air and analysed by gas chroma-
tography. When the samples are exposed to air the solutions turn yellow
and the reaction stops immediately. The alcohol/ketone concentrations
do not change in these solutions, even after several days. The GC reten-
tion times were obtained by comparison of the GC retention times for
product mixtures with known literature values. All of the catalytic results
were reproduced.
Structural determination of 1 and (S,S)-3: X-ray crystallographic data for
1 and (S,S)-3 were collected on a Bruker–Nonius Kappa-CCD diffrac-
tometer with use of monochromated MoKa radiation (l = 0.71073 ꢂ)
and were measured with a combination of f scans and w scans with k off-
sets, to fill the Ewald sphere. Data collection parameters and crystallo-
graphic information are given in Table 2. The data were processed by use
Experimental Section
Table 2. Summary of crystal data and details of intensity collection and
least-squares refinement parameters for 1 and (S,S)-3.
General procedures: All reactions were carried out under N2 by standard
Schlenk or drybox techniques. Dry, oxygen-free solvents were prepared
by distillation from appropriate drying reagents and employed through-
out. Substrates 4–11 and 14–16 were distilled from P2O5. All other com-
mercially available reagents were used without further purification. The
1
ACHTUNGTRNE(NUNG S,S)-3
empirical formula
Fw
C43H37B2F8FeOP2
903.17
150(2)
C58H51B2F8FeN3O2P2
1113.43
150(2)
T [K]
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
¯
complexes trans-
G
E
R
[BF4]2 (2),[6a] trans-[Fe-
ACHTUNGTRENNUNG
P1
P21
[6b]
9.9487(4)
14.2587(4)
14.4888(4)
92.1580(2)
94.0940(2)
100.3860(2)
2013.8(1)
2
1.489
22594
9142 [Rint =0.0656]
9142; 542
0.0622; 0.1737
11.4484(3)
23.0724(6)
22.4850(6)
90
91.941(1)
90
5935.8(3)
4
1.246
A
C
ACHTUNGTRENNUNG
[6b]
ACHTUNGTRENNUNG[BF4]2
300 MHz spectrometers were employed for recording 1H (400 MHz and
300 MHz), 13C{1H} (100 MHz and 75 MHz) and 31P{1H} (121 MHz) NMR
spectra at ambient temperature. The 1H and 13C{1H} NMR spectra were
referenced to solvent resonances. The 31P{1H} NMR spectra were refer-
enced to 85% H3PO4 (0 ppm). Gas chromatography was carried out on a
Perkin–Elmer Autosystem XL instrument. All infrared spectra were re-
corded on a Nicolet 550 Magna-IR spectrometer. The elemental analysis
were performed on a Perkin–Elmer 2400 CHN elemental analyser. Sam-
ples were handled under argon when appropriate.
b [8]
g [8[
V [ꢂ3]
Z
1 [mgcmꢀ3
]
reflections collected
unique reflections
data; parameters
R1;[a] wR2[b]
45379
20334 [Rint =0.075]
20334; 1369
0.0661; 0.18116
2
General procedure for the synthesis of trans-[Fe
[BF4]2 (1) and trans-(R,R)-[Fe(NCMe)2(diph-ethP2N2)]
A solution either of trans-[Fe(NCMe)2A(ethP2N2)][BF4]2 (1.31 g, 1.5 mmol)
or trans-(R,R)-[Fe(NCMe)2(diph-ethP2N2)][BF4]2 (0.51 g, 0.5 mmol) in
A
ACHTUNGTRENNUNG
A
E
E
N
ACHTUNGTRENNUNG
2
[a] R1=ꢀjjFo jꢀjFc jj/ꢀjFo j. [b] wR2={ꢀ[w
(Fo ꢀFc2)2]/ꢀ[w
(Fo )2]}1/2
ACHTUNGTERNNUNG .
A
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
acetone was stirred under CO for 2 h. The reaction mixture was concen-
trated to dryness to give an orange solid. The whole procedure was re-
peated three times. The solution was concentrated to dryness and the re-
maining orange residue was washed with toluene and ether. Crystalliza-
tion from CH2Cl2/Et2O or acetone/Et2O gave the analytically pure com-
pounds.
of the Denzo-SMN package. Absorption corrections were carried out
with SORTAV. The structures were solved and refined with
SHELXTLV6.1 for full-matrix, least-squares refinement based on F2. All
H atoms were included in calculated positions and allowed to refine in
riding-motion approximation with Uiso tied to the carrier atom.
CCDC-710194 (1) and CCDC-710195 [(S,S)-3] contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
1
Compound 1: Yield: 1.14 g (1.3 mmol, 87%); H NMR (CD2Cl2): d=1.86
(s, 3H; MeCN), 4.09 (brs, 2H; CH2), 4.25 (brs; CH2), 6.46–8.06 (several
br s, 18H; Ar), 9.30 ppm (s, 2H; CH=N); 13C NMR (CD2Cl2): d=65.6
(CH2), 129.5–138.6 (several m, Ar), 175.3 ppm (CH=N); 31P NMR
(CD2Cl2): d=50.8 (s); IR (KBr): n˜ =2002 cmꢀ1 (nCꢁO); MS (ESI+): m/z
(%): 330.1 [MꢀMeCNꢀCO]2+
; elemental analysis (%) calcd for
C43H37B2F8N3P2Fe1: C 57.18, H 4.13, N 4.65; found: C 56.12, H 4.15, N,
4.83. Crystals of 1 for the X-ray diffraction study were obtained by diffu-
sion of ether into a CH2Cl2 solution.
Acknowledgements
Compound (R,R)-3: Yield: 0.47 g (0.4 mmol, 80%); 1H NMR (CD2Cl2):
d=1.20 (brs, 3H; MeCN), 5.28 (brs, 2H; CH), 5.81–8.07 (several brs,
28H; Ar), 9.33 ppm (brs, 2H; CH=N); 13C NMR (CD2Cl2): d=76.3
RHM thanks NSERC Canada for a Discovery Grant and the Petroleum
Research Fund, as administered by the American Chemical Society, for a
type AC grant.
Chem. Eur. J. 2009, 15, 5605 – 5610
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5609