Asymmetric transfer hydrogenation of ketones catalyzed by rhenium complexes with chiral ferrocenylphosphane ligands

Article abstract of DOI:10.1002/ejic.201200693

We have prepared a series of new rhenium complexes containing chiral ferrocenyldiphosphane ligands of the Josiphos family, starting from commercially available rhenium sources. These new Re^V oxido and nitrido complexes, several of which have been characterized by X-ray crystallography, are air- and moisture-stable and are active catalysts in the asymmetric transfer hydrogenation of ketones using 2-propanol as the hydrogen source in the presence of substoichiometric amounts of triethylamine (TEA). The reaction proceeds cleanly with good to excellent yields (50-99 %) but with moderate enantioselectivity (up to 58 % ee). A mechanism not involving hydridic species is proposed.

Full text of DOI:10.1002/ejic.201200693

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FULL PAPER
DOI: 10.1002/ejic.201200693
Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Rhenium
Complexes with Chiral Ferrocenylphosphane Ligands
Esteban Mejía,[a] Raphael Aardoom,[a] and Antonio Togni*[a]
Keywords: Asymmetric synthesis / Homogeneous catalysis / Hydrogen transfer / Rhenium / Phosphane ligands
We have prepared a series of new rhenium complexes con-
taining chiral ferrocenyldiphosphane ligands of the Josiphos
family, starting from commercially available rhenium
sources. These new ReV oxido and nitrido complexes, several
of which have been characterized by X-ray crystallography,
are air- and moisture-stable and are active catalysts in the
asymmetric transfer hydrogenation of ketones using 2-propa-
nol as the hydrogen source in the presence of substoichio-
metric amounts of triethylamine (TEA). The reaction pro-
ceeds cleanly with good to excellent yields (50–99%) but
with moderate enantioselectivity (up to 58% ee). A mecha-
nism not involving hydridic species is proposed.
ketones and imines[14] and that of Kwong on the asymmet-
ric cyclopropanation of alkenes[15] using chiral ReV and ReI
catalysts stand alone. Other important achievements in the
Re-catalyzed reduction of carbonyl groups have been re-
ported by Abu-Omar[16] and Royo.[17]
An important class of ligands used in those kinds of
transformations are bi-, tri- or tetradentate ferrocenylphos-
phanes,[6b,18] which were moderately successful until Baratta
and coworkers reported outstanding results [(enantio-
selectivity and turnover numbers (TONs)][9,19] by using
Josiphos ligands (Figure 1), some of the most successful in
asymmetric catalysis and hence one of the so called “privi-
leged ligands”.[20] They have been widely used in several
applications both in academia and in industry.[20b]
Introduction
In the search for new, more versatile, more environmen-
tally friendly or greener chemical processes, transfer hydro-
genation reactions are a possible attractive alternative to
classical reductions with molecular hydrogen because of
their high selectivity, mild reaction conditions and opera-
tional simplicity.[1] Depending on the nature of the metal
and the ligands used, the mechanism of these processes can
be classified in two main classes: direct H transfer or a
metal-templated, hydride-mediated process.[1b] The most
popular hydrogen sources are 2-propanol,[2] formic acid[3]
and the azeotropic formic acid/triethylamine mixture.[4] The
metal complexes used to catalyze these transformations are
limited to Rh,[2–3,5] Ru,[4,6] Ir,[5,7] Ni,[8] and more recently
Os[9] and Fe.[10] The ligands used in asymmetric transfer
hydrogenation feature different architectures and donor
atoms including oxygen, sulfur, nitrogen and phosphorus
in different combinations and in bi-, tri- and tetradentate
fashions.[1b]
Probably due to its scarcity,[11] the coordination chemis-
try of rhenium remains relatively underdeveloped as com-
pared to that of other transition metals. Nevertheless, in the
last decade this element has gained growing popularity as
a catalyst in different processes.[12] As Re is less expensive
than Rh or Ir and due to its unique chemistry,[13] Re-cata-
lyzed reactions are an interesting field of exploration. In the
Figure 1. General structure of Josiphos ligands. R and RЈ substitu-
ents presented here account for the commercially available ligands
(Solvias AG) though some other new combinations are presented
in this work (vide infra). A new class of Josiphos ligands
stereogenic at the phosphorus atoms has been recently developed
field of Re-catalyzed asymmetric synthesis, the pioneering in our group.[21]
work of Toste on the enantioselective hydrosilylation of
[a] Department of Chemistry and Applied Biosciences, Swiss
Federal Institute of Technology, ETH Zürich,
Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland
Fax: +41-44-632-1310
Here, we report the synthesis and characterization of a
series of new ReV oxido and nitrido complexes with chiral
bidentate ferrocenylphosphane ligands of the Josiphos fam-
ily, and their successful use as catalysts for the asymmetric
transfer hydrogenation of ketones. A plausible reaction
mechanism is also proposed.
E-mail: atogni@ethz.ch
Homepage: http://www.tognigroup.ethz.ch/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200693.
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E. Mejía, R. Aardoom, A. Togni
Table 1. Complexes of the type [ReOX3(Josiphos)] (3).
FULL PAPER
Results and Discussion
Ligand Synthesis
3
R
RЈ
X
Yield [%]
a
b
c
d
e
f
g
h
i
Ph
Cy
Ph
Ph
Cy
Ph
Cy
Cy
Cy
tBu
Ph
Cy
Ph
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
94
78
85
71
73
97
85
16
Ͼ99
72
Ferrocenyldiphosphane ligands of the Josiphos family
were synthesized by using a slight modification of the re-
ported procedure.[22] The common precursor in all cases
was the commercially available (R)-Ugi’s amine,[23] which in
a first step was selectively ortho-lithiated and the resulting
ferrocenyllithium derivative was allowed to react with the
corresponding secondary chlorophosphane to give the
(R,S)-pfa (phosphanylferrocenylamine) intermediate 1. The
dimethylamino moiety was then substituted by the corre-
sponding secondary phosphane with retention of configura-
tion[24] to give the desired Josiphos ligand 2 (Scheme 1). For
this study only the “classic” Josiphos (R = Ph, RЈ = Cy, 2a)
and the noncommercially available analogues were synthe-
sized. All other ferrocenyldiphosphane ligands were ob-
tained from Solvias AG.
adamantyl
3,5-(Me)2Ph
Cy
tBu
tBu
3,5-(CF3)2Ph
3,5-(Me)2Ph
Ph
j
k
Cy
97
Scheme 1. General procedure for the synthesis of Josiphos ligands.
Figure 2. Time-dependent 31P NMR (101.2 MHz, r.t., CDCl3)
spectra for complex 3d.
All complexes were obtained in good yields except 3h,
which contains trifluoromethylated substituents at phos-
Synthesis and Characterization of Complexes
The parent oxidotrihalobis(triphenylarsane)rhenium(V) phorus. This compromises not only the donor and coordi-
complexes, mer-[ReOX3(AsPh3)2] (X = Cl, Br), were synthe- nating abilities of the ligand due to its electron-withdrawing
sized by following the reported procedures[25] but using properties, but also confers additional steric demand. The
KReO4 instead of HReO4 as the starting material. They complexes are solids stable to air and moisture that can
reacted readily with Josiphos ligands in CH2Cl2 at room be stored for several months at room temperature without
temperature to give the corresponding fac-[ReOX3(Josi- apparent decomposition. They decompose slowly in solu-
phos)] complexes (3, Table 1) in a procedure analogous to tion (in non-degassed solvents) to give oxidation products
the one reported by Parr.[26] As depicted in Scheme 2, the (see below).
coordination of the chiral diphosphane ligands to the rhe-
X-ray quality single crystals of exo-3a, exo-3e, exo-3i,
nium centre gives a mixture of two isomeric products, endo- endo-3f and 3g (both isomers) were obtained by slow dif-
3 and exo-3. This was confirmed by X-ray diffraction (see fusion of n-hexane, ethyl ether or benzene into a solution
below). It was observed that upon ligand substitution on of the corresponding compounds in dichloromethane or
mer-[ReOX3(AsPh3)2], the endo isomer is produced first (ki- chloroform (for the X-ray structures of exo-3e and exo-3i
netic product). However, this species slowly isomerizes in see the Supporting Information). The crystal structure of
solution to the more stable exo isomer (thermodynamic exo-3a is presented in Figure 3. The geometry around the
product) (Figure 2). The lability of the P–Re bonds in the Re centre is a distorted octahedron with a fac–cis configura-
endo complex is evidenced in the 31P NMR spectra by its tion. One of the main features of the structure is the small
broad peaks, as a consequence of this dynamic behaviour.
O–Re–Cltrans angle of 170.46(12) Å with the oxygen atom
Scheme 2. General procedure for the synthesis of Re oxido complexes.
2
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Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
bent towards the bidentate ligand as reported for other ReV
diphosphane monooxido complexes.[26–27] This is due to
electronic repulsion between the Re–O moiety and the neg-
atively charged chloride ligands.[27b] The short Re–O bond
length [1.685(4) Å] is in good agreement with those for
other complexes of this kind[28] and is better described as a
triple bond (one σ bond and two π bonds), as it is clearly
shorter than a double bond, which has a theoretical length
of 1.86 Å.[27a,29] The reason for the increased bond order is
the overlap of both px and py oxygen orbitals with the rhe-
nium dxz and dyz orbitals (z axis through the Re–O bond).
The two d electrons would then occupy the nonbonding dxy
orbital,[28] which explains the observed diamagnetism of the
complexes.[29]
Figure 4. ORTEPIII drawing of complex 3g. Ellipsoids at 50%
probability. The asymmetric unit consists of both endo (down) and
exo (up) isomers. Relevant distances [Å] and angles [°]: exo isomer
Re(1)–P(1) 2.493(3), Re(1)–P(2) 2.578(3), Re(1)–O(1) 1.707(8), Re–
Cl(1) 2.393(3), Re–Cl(2) 2.395(3), Re–Cl(3) 2.385(3), P(1)–Re(1)–
P(2) 95.63(10), O(1)–Re(1)–Cl(3) 171.1(3); endo isomer Re(2)–P(3)
2.458(3), Re(2)–P(4) 2.579(3), Re(2)–O(2) 1.751(8), Re(2)–Cl(4)
2.363(3), Re(2)–Cl(5) 2.414(3), P(3)–Re(2)–P(4) 95.13(9), O(2)–
Re(2)–Cl(6) 167.0(3).
Figure 3. ORTEPIII drawing of complex exo-3a·(CD2Cl2)·(C6H6).
Solvent molecules are omitted for clarity. Ellipsoids at 50% prob-
ability. Relevant distances [Å] and angles [°]: Re–P(1) 2.4604(13),
Re–P(2) 2.4875(12), Re–O 1.685(4), Re–Cl(1) 2.4024(12), Re–Cl(2)
2.3972(13), Re–Cl(3) 2.4143(13), P(1)–Re–P(2) 89.05(4), O–Re–
Cl(3) 170.46(12).
In the case of complex 3g, both endo and exo isomers
crystallize together in a one to one ratio in the asymmetric
unit (Figure 4). The superposition of both structures shows
that they are indeed very similar, and both have the same
conformation in the six-membered ring formed by the metal
and the chelating ligand. The only important differences are
in the conformation of the cyclohexyl rings and in the O–
Re–Cltrans angle; the endo isomer has a more pronounced
deviation from linearity and, hence, a more congested steric
situation (see Figure 5). Taking this observation into ac-
count, the decrease of steric congestion could be the driving
force for the observed isomerization process.
An attempt to crystallize the tribromo complex 3k under
noninert conditions gave as a result the oxidation product
[ReOBr4(2a(κO-OPPh2)) (4, Figure 6), in which the neutral
rhenium(VI) ReOBr4 moiety is coordinated with the mono-
oxidized form of the Josiphos ligand. Rhenium(VI) com-
plexes are rare and often very reactive.[30] In this case, the
Figure 5. MacMoMo drawing of the superimposed crystal struc-
tures of both endo-3g and exo-3g.
ReVI centre is stabilized by the strongly bonded oxido group core is a redox-active unit and could serve as an oxido-
[Re–O 1.612(7) Å] and the weakly bonded phosphanyloxide transfer agent,[31] it is likely that the oxidation of the phos-
ligand [Re–O 2.127(7) Å]. The mechanism of formation of phane ligand occurs by intramolecular oxygen transfer
4 is still unclear but considering the fact that the [ReVO]+3 rather than by O2 in solution.
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E. Mejía, R. Aardoom, A. Togni
FULL PAPER
Scheme 4. Procedure for the synthesis of 6.
Figure 6. ORTEPIII drawing of 4·(CD2Cl2). Solvent molecules are
omitted for clarity. Ellipsoids at 50% probability. Relevant dis-
tances [Å] and angles [°]: Re–O(1) 2.127(7), Re–O(2) 1.612(7), P(1)–
O(1) 1.522(7), O(1)–Re–O(2) 178.0(3).
The dioxido derivative [ReO2I(2a)] (5) was synthesized in
an analogous procedure by simple ligand substitution of the
parent complex [ReO2I(PPh3)2] with the bidentate ligand
at room temperature in dichloromethane (Scheme 3). The
elemental analysis of the resulting paramagnetic compound
confirms the proposed stoichiometry with the two oxido
ligands in a trans configuration due to the required cis che-
lation of the biphosphane ligand.
Figure 7. ORTEPIII drawing of endo-6·(CD2Cl2). Solvent mole-
cules are omitted for clarity. Ellipsoids at 50% probability. Relevant
distances [Å] and angles [°]: Re–P(1) 2.3985(9), Re–P(2) 2.4152(9),
Re–N 1.679(3), Re–Cl(1) 2.3879(9), Re–Cl(2) 2.4107(8), P(1)–Re–
P(2) 91.45(3).
trido ligand as previously reported for other rhenium ni-
trido complexes with chelating diphosphanes.[32]
Scheme 3. Procedure for the synthesis of 5.
The nitrido complex [ReNCl2(2a)] (6) was synthesized
from [NBu4][ReNCl4] and the biphosphane ligand by
adapting the procedure reported by Duatti[32] (Scheme 4).
Rhenium-Catalyzed Transfer Hydrogenation of Ketones
With the Re complexes in hand, their catalytic activity
It was also obtained as a mixture of the endo and exo iso- in the transfer hydrogenation of ketones was tested using
mers, and upon crystallization single crystals of endo-6 were acetophenone as the model substrate. In a typical pro-
obtained, the structure of which was confirmed by single cedure, a certain amount of catalyst (ca. 1 mol-%) was dis-
crystal X-ray analysis (Figure 7).
solved in dry 2-propanol under an argon atmosphere along
Complex endo-6 exhibits a distorted square pyramidal with one equivalent of acetophenone. Then a base was
geometry around the Re centre with the nitrido ligand in added (ca. 10 mol-%) and the mixture was heated over-
the apical position. The rhenium atom is displaced 0.447 Å night. For optimization of the conditions, complex 3a and
from the mean plane P(1)–P(2)–Cl(1)–Cl(2) towards the ni- other ReV complexes were used as catalysts (Table 2).
4
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Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
Table 2. Result for the transfer hydrogenation of acetophenone un- satisfactory; no conversion was obtained with the former
der different reaction conditions.
(Table 2, Entry 10) and low conversion and no selectivity
Entry[a] Catalyst
Base
Time Temp. Yield
ee[c] [%]
with the latter (Table 2, Entry 9). When the catalyst was
synthesized in situ, by mixing equimolar amounts of mer-
[ReOX3(AsPh3)2] and the Josiphos ligand in 2-propanol
prior to the addition of the ketone, similar results were ob-
[h]
[°C]
[%][b]
1
2
3
4
5
3a
3a
3a
3a
3a
12
12
48
21
20
80
80
80
80
80
0
–
[d]
[d]
K2CO3
K2CO3
iPrONa
(iPr)
2NEt
NEt3
NEt3
NEt3
50
99
67
76
11 (S)
rac
rac
17 (S)
Table 4. Results for the transfer hydrogenation of ketones using
complex 3h as catalyst.
6
7
8
3a
3a
3a
24
48
48
20
20
20
80
25
40
80
80
80
83 (75)
17 (S)
–
19 (S)
rac
0
(46)
15
0
9[e]
10[f]
11
[ReOCl2(Binap)] NEt3
[ReOCl2(PPh3)2] NEt3
3a (in situ)
–
NEt3
92
14 (S)
[a] Reaction conditions: conc. of substrate 0.4 m; ca. 1 mol-% of
catalyst; 8 mol-% of base. [b] Calculated by integration of the peaks
1
in the H NMR spectra. Numbers in brackets correspond to iso-
lated yields. [c] Determined by using chiral HPLC (see Exp. Sec-
tion). [d] Excess (ca. 20-fold). [e] S-Binap was used. Complex syn-
thesized as reported previously.[26] [f] Synthesized as reported pre-
viously.[33]
The addition of a base proved crucial because a lack of it
(Table 2, Entry 1) resulted in no product formation. When
potassium carbonate was used (Table 2, Entries 2 and 3),
good conversion was obtained but with a loss of enantio-
selectivity. The same effect was observed with the addition
of sodium isopropoxide (Table 2, Entry 4). This could be
due to decomposition of the complex to generate different
active species that do not contain the chiral ligand. When
triethylamine (TEA) was used instead, moderate enantio-
selectivities were obtained. The same selectivity was ob-
served when bulkier diisopropylethylamine was added,
however with lower yield (Table 2, Entry 5). The reaction
temperature also had a dramatic effect on the reaction
yield. When the reaction was run at 25 °C (Table 2, Entry
7) no conversion was observed. At 40 °C, a yield of only
46% was obtained after two days, whereas at 80 °C a yield
of 80% was obtained (Table 2, Entry 6). When other ReV
complexes were used as catalysts, namely mer-[ReOCl3-
(PPh3)2] and fac-[ReOCl3(S-Binap)], the results were not
Table 3. Results for the transfer hydrogenation of acetophenone
catalyzed by rhenium(V) complexes with Josiphos ligands.
Entry[a] Cat.
Yield
[%][b]
ee[c] [%] Entry[a] Cat. Yield
ee[c] [%]
[%][b]
1
2
3
4
5
6
7
8
9
3b
96
96
79 (35)
50 (31)
97 (85)
92
89
70
97
37 (R)
37 (R)
8 (S)
12 (R)
10 (R)
19 (R)
30 (R)
55 (R)
43 (R)
10
11
12
13
14
15
16
17
18
3i[e]
3j
67
4
95
52 (R)
23 (S)
15 (S)
20 (S)
11 (S)
20 (S)
11 (S)
15 (R)
15 (R)
3b[d]
3c
3k
3d
3e
3f
3g
3k[f] 49
5
5[f]
5[d]
6
97
25
93
88
6
3h
3h[d]
6[f]
[a] Reaction conditions: conc. of substrate 0.4 m; ca. 1 mol-% of
catalyst; 8 mol-% of TEA; 80 °C, 20 h. [b] Calculated by integration
of the peaks in the 1H NMR spectra. Numbers in parentheses
correspond to isolated yields. [c] Determined by using chiral HPLC
(see Exp. Section). [d] Made in situ. [e] 14 h. [f] 40 °C.
[a] Reaction conditions: concentration of substrate 0.4 m; catalyst
prepared in situ, 1 mol-% of ligand and 1 mol-% of [Re-
OCl3(AsPh3)2]; ca. 20 mol-% of TEA; 80 °C, 20 h. [b] Calculated
by integration of the peaks in the 1H NMR spectra. [c] Determined
by using chiral HPLC (see Exp. Section).
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E. Mejía, R. Aardoom, A. Togni
FULL PAPER
tained as when the isolated complex was used (Table 2, En- 3a but still shows significant triple bond character. The
try 11). Having selected the best conditions for the asym- Re–O(alkoxido) distance [1.875(6) Å] suggests strong π-do-
metric transfer hydrogenation of acetophenone with com- nation so it can be regarded as a double bond. Another
plex 3a (Table 2, entry 6), we assessed the catalytic perform- remarkable structural feature of 7 is the inverted configura-
ance of the ReV complexes prepared previously. The results tion relative to the parent complex exo-3a, with the ter-
are summarized in Table 3.
minal oxido ligand in the endo position.
In general, good to excellent yields were obtained for the
Re-catalyzed transfer hydrogenation of acetophenone with
the different complexes. The only exception was for com-
plex 3j (Table 3, Entry 11) probably because of the steric
hindrance imparted by the bis-tert-butylphosphane moiety
on the ferrocene. It is important to note that in spite of the
fact that all complexes have the same chirality (R,SP), the
stereochemical outcome of the reaction changes depending
on the nature and relative position of the organic residues
on the P atoms. Complex 3a (R = Ph, RЈ = Cy) gives prefer-
entially the S isomer (17% ee, Table 2, Entry 6), whereas its
“inverted” analogue 3b (R = Cy, RЈ = Ph) gives an excess
of the R isomer (37% ee, Table 3, Entry 1). The same effect
was observed for 3f and 3j (Table 3, Entries 6 and 11). In
terms of selectivity, the best results were obtained for those
complexes with R = Cy (Table 3, Entries 1, 7, 8 and 10),
which preferentially produce the R isomer. The same sense
of induction was obtained with complex 3e (R, RЈ = Cy),
although with lower ee (Table 3, Entry 5). The best results
were obtained with complex 3h (Table 3, Entries 8 and 9),
which was then selected for screening with different ketones
(Table 4). For these reactions, the catalyst was generated in
situ as it showed similar selectivity and higher yield than
the isolated complex.
Figure 8. ORTEPIII drawing of 7. Ellipsoids at 50% probability.
Relevant distances [Å] and angles [°]: Re–P(1) 2.458(3), Re–P(2)
2.457(3), Re–O(1) 1.727(7), Re–O(2) 1.875(6), Re–Cl(1) 2.456(2),
ReCl(2) 2.477(3), P(1)–Re–P(2) 92.29(9), O(1)–Re–O(2) 178.4(3).
For all of the substrates tested, the reaction proceeded
Metal alkoxide complexes are common intermediates in
cleanly and in general with good to excellent yields. The transfer hydrogenation reactions, namely those that proceed
only exception was for 4-cyanophenyl methyl ketone either through an inner-sphere monohydridic route or
(Table 4, Entry 4), probably because coordination of the cy- through a Meerwein–Ponndorf–Verley type hydrogen trans-
ano moiety to the metal centre hampers the reactivity of fer.[1c] These complexes are rather stable and have been pro-
the catalyst. Conversely, the same substrate gave the highest posed to serve simply as inactive catalyst reservoirs in Ru-
enantioselectivity, most likely because of steric factors.
catalyzed TH[35] or as the resting state of the catalyst in Rh-
catalyzed TH.[36] The fact that the isolated alkoxy interme-
diate 7 still contains two equatorial chloride ligands suggest
that the mechanism does not proceed through a metal hy-
dride pathway. As observed by Bäckvall and coworkers for
Mechanistic Aspects
Transfer hydrogenation (TH) of ketones has been shown the RuCl2(PPh3)3-catalyzed TH,[37] successive alkoxide dis-
to proceed by different pathways, which depend heavily on placement/β-elimination reactions lead to the formation of
the metal, ligands and reaction conditions.[1a,1c] As this is metal-dihydride species in basic media; in our case, this has
the first report using a rhenium catalyst in this kind of reac- not been observed. Taking these observations into account,
tion, no prediction can be made concerning their preferred the reaction mechanism depicted in Figure 9 is proposed.
modus operandi. Nevertheless, having assessed the geome-
The first step of the mechanism is the dissociation of a
try, reactivity and coordination behaviour of our ReV com- phosphane ligand in 3, to allow the coordination of the
plexes, a plausible reaction pathway may be proposed (vide incoming alcohol. The assumption that the Josiphos ligand
infra).
could alternate between monodentate and bidentate is sup-
After completion of the TH of acetophenone in 2-prop- ported by the observed thermal isomerization of the com-
anol using complex exo-3a, a very stable rhenium-isoprop- plexes (vide supra) and by the fact that the alkoxido com-
oxide complex, [ReOCl2(iPrO)(2a)] (7), was isolated and plex 7 exhibits an inverted configuration compared with the
characterized. The X-ray structure of 7 is presented in Fig- parent complex exo-3a. Of the two phosphorus donors of
ure 8. Complex 7 exhibits a slightly distorted octahedral the bidentate Josiphos ligand, the one which is more likely
geometry containing a trans-oxido-alkoxido unit typical for to dissociate first is the less electron-rich and hence the
this kind of complex.[27a,34] The Re–oxido distance weaker σ donor. For complex 3a this will be the one directly
[1.727(7) Å] is longer than that in the parent complex exo- attached to the ferrocene core, whereas for 3h the one bear-
6
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Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
with chiral bidentate ferrocenylphosphane ligands. The re-
action uses 2-propanol as a solvent and as the sole hydrogen
source, and requires the addition of substoichiometric
amounts of a base. The reaction is clean and gives generally
good to excellent yields (up to Ͼ99%), although the
enantioselectivity is still moderate (up to 58%). The new
rhenium(V) complexes are bench stable and easy to handle.
An alkoxido complex was isolated after completion of the
reaction with catalyst exo-3a. Based on this, and consider-
ing the previously proposed reaction pathways, a plausible
reaction mechanism was proposed, in which the reduction
occurs by direct hydrogen transfer between the simulta-
neously coordinated alkoxy and ketone substrate. To the
best of our knowledge, this is the first example of a transfer
hydrogenation reaction catalyzed by rhenium complexes.
Experimental Section
General Methods: All the reactions were carried out under an inert
argon atmosphere using standard Schlenk techniques. Solvents
were distilled and dried prior to utilization by using reported pro-
cedures.[39] All commercially available reagents were used as re-
ceived without further purification. (R)-[1-(dimethylamino)ethyl]-
ferrocene (Ugi’s amine) was kindly provided by SOLVIAS AG as
the tartrate salt. The ligand precursors dimethyl{(R)-1-[(S)-2-(di-
phenylphosphanyl)ferrocenyl]ethyl}amine (1a) and dimethyl{(R)-1-
[(S)-2-(dicyclohexylphosphanyl)ferrocenyl]ethyl}amine (1b) were
synthesized as described previously.[40] The Josiphos ligand (R)-1-
[(SP)-2-(diphenylphosphanyl)ferrocenyl]ethyldicyclohexylphos-
phane (2a) was prepared as previously reported.[22] The ligands (R)-
1-[(SP)-2-(dicyclohexyphosphanyl)ferrocenylethyl]diphenylphos-
phane (2b), (R)-1-[(SP)-2-(diphenylphosphanyl)ferrocenyl]ethyldi-
(3,5-xylyl)phosphane (2d), (R)-1-[(SP)-2-(dicyclohexylphosphanyl)-
ferrocenyl]ethyldicyclohexylphosphane (2e), (R)-1-[(SP)-2-(di-
phenylphosphanyl)ferrocenyl]ethyldi-tert-butylphosphane (2f), (R)-
1-[(SP)-2-(dicyclohexylphosphanyl)ferrocenyl]ethyldi-tert-butyl-
phosphane (2g) and (R)-1-[(SP)-2-(di-tert-butylphosphanyl)ferro-
cenyl]ethyldiphenylphosphane (2j) were purchased from Aldrich
(Solvias Chiral Ligands Kit) and used as received. The parent rhe-
nium complexes [ReOCl3(AsPh3)2] (X = Cl, Br)[25] and [NBu4]-
[ReNCl4][41] were synthesized by following literature procedures.
NMR Spectra were recorded with Bruker Avance DPX300 and
DPX250 instruments. The chemical shifts are reported in parts per
million (ppm). 1H and 13C NMR shifts are referenced to TMS and
calibrated with the residual solvent peak; 31P shifts are relative to
H3PO4 (85%) as an external reference. Coupling constants J are
given in Hz. HR ESI-MS and MALDI-MS measurements were
performed by the MS service of the Laboratorium für Organische
Chemie der ETH Zürich. Values are given as m/z. Elemental analy-
ses were performed by the microelemental analysis service of the
Laboratorium für Organische Chemie der ETH Zürich. FTIR mea-
surements were performed with a Perkin–Elmer Spectrum BX
Series spectrometer with solid state samples by using an ATR
Golden Gate sampling accessory. HPLC was performed with Ag-
ilent Series 1100 or HP 1050 instruments equipped with a UV di-
ode array detector (DAD); flow in mL/min, eluent (hexane/IPrOH-
ratio) are given in each case; columns: Chiralcel OD-H, OB, AD-
H (4.6ϫ250 mm, particle 5 μm) or Chiralcel OJ (4.6ϫ250 mm,
particle 10 μm). X-ray structural measurements were performed
Figure 9. Proposed mechanism for the transfer hydrogenation of
acetophenone catalyzed by 3.
ing the trifluoromethyl substituents shall be more prone to
dissociate. The resulting alcohol complex 8 eliminates HX
with the aid of the added base to give the alkoxido complex
9. Upon coordination of the ketone (complex 10), a direct
hydrogen transfer from the coordinated alkoxide to the
ketone takes place. This bears a strong resemblance to the
mechanism found for TH with Ir-COD (COD = 1,5-cyclo-
octadiene) derivatives.[38] The resulting complex 11 releases
an acetone molecule upon coordination of another alcohol
ligand and this complex (12) undergoes intramolecular pro-
ton exchange and dissociation of the reduced ketone to re-
generate metal alkoxide 9. The central role of the latter is
not only supported by its isolation in the form of the spe-
cific compound 7, but more importantly by the fact that the
use of either 3a or 7 as catalysts precursors give essentially
identical results, both in terms of activity and selectivity.
The stereochemical outcome of the reaction is believed
to depend heavily on steric factors (as the proposed mecha-
nism depicts). However, considering that several factors
(solvation, dispersion, steric and electrostatic interactions)
have been reported to determine the degree of enantio-
selectivity,[1c] a definite conclusion cannot be drawn.
Conclusions
We have developed an asymmetric transfer hydrogena-
tion reaction of ketones using new rhenium(V) complexes with a Siemens CCD diffractometer (Siemens SMART PLAT-
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7

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Date: 12-09-12 17:18:48
Pages: 13
E. Mejía, R. Aardoom, A. Togni
FULL PAPER
FORM, with CCD detector, graphite monochromator, Mo-Kα ra-
diation). Data were collected using the program SMART. Integra-
tion was performed with SAINT. Structure solution (direct meth-
ods) was accomplished with SHELXTL 97. CCDC-869344 (for
was dissolved in dichloromethane and washed successively with a
saturated Na2CO3 solution, brine and water, dried with MgSO4
and filtered. It was concentrated to dryness with a rotary evapora-
tor to give the crude product. The crude product was purified either
3a), -869345 (for 3d), -869346 (for 3e), -869347 (for 3f), -869348 by column chromatography or by recrystallization to give pure 2
(for 3g), -869349 (for 3i), -869353 (for 4), -869350 (for 6) and -
869351 (for 7) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
as an orange solid.
(R)-1-[(SP)-2-(Diphenylphosphanyl)ferrocenyl]ethyldicyclohexyl-
phosphane (2a):[22] From 1a (1.394 g, 3.1614 mmol, 1 equiv.) and
dicyclohexylphosphane (0.7 mL, 3.4597 mmol, 1.1 equiv.) accord-
ing to the general procedure. Recrystallized from a saturated
dichloromethane solution at –20 °C to give orange crystals; yield:
General Method for the Synthesis of (R,S)-pfa (1): In an oven-dried
Schlenk tube, (R)-[1-(dimethylamino)ethyl]ferrocene (Ugi’s amine)
(1 equiv.) was dissolved in dry diethyl ether (to a concentration of
1–2 m), and this solution cooled to –78 °C. To this solution tert-
butyllithium (1.6 m) in pentane (1.2 equiv.) was added dropwise
with a syringe. The reaction mixture was stirred at this temperature
for 30 min. Then the cooling bath was removed to allow the mix-
ture to warm to room temperature. It was stirred for a further
60 min leading to the formation of a red-orange precipitate. The
same amount of ether used before was added and the mixture was
stirred for another 60 min giving a clear solution. The solution was
cooled again to –78 °C and the corresponding secondary phos-
phane chloride (1.1 equiv.) was added dropwise with a syringe. The
reaction was warmed to room temperature overnight. To the re-
sulting orange suspension a saturated solution of Na2CO3 was
added slowly to quench the reaction. The biphasic mixture was
separated in a separatory funnel and the organic phase was washed
with water and brine and dried with MgSO4, then filtered and con-
centrated to dryness in vacuo to give a sticky orange solid. The
crude product was purified by either column chromatography or
by recrystallization to give pure 1 as an orange solid.
1
1.813 g (96%). H NMR (300 MHz, CD2Cl2): δ = 7.73–7.58 (m, 2
H, Ph), 7.46–7.33 (m, 3 H, Ph), 7.16 (d, J = 3.2 Hz, 5 H, Ph), 4.36
(s, 1 H, HCp), 4.32 (t, J = 2.2 Hz, 1 H, HCp), 4.07 (s, 1 H, HCp),
3.79 (s, 5 H, Cp), 3.30 (dd, J = 7.0, 2.0 Hz, 1 H, HCMe), 1.77 (br.
s, Cy), 1.70 (br. s, Cy), 1.63 (br. s, Cy), 1.59 (dd, J = 7.2, 4.7 Hz,
3 H, Me), 1.55–1.43 (m, Cy), 1.35–0.91 (m, Cy) ppm. 31P NMR
(121 MHz, CD2Cl2): δ = 15.46 (d, J = 37.6 Hz), –25.76 (d, J =
37.6 Hz) ppm.
(R)-1-[(SP)-2-(Dicyclohexylphosphanyl)ferrocenyl]ethyldiadamantyl-
phosphane (2c): According to the general method, from 1a
(500.6 mg, 1.13 mmol, 1 equiv.), bisadamantylphosphane[42]
(412.8 mg, 1.36 mmol, 1.2 equiv.) and with the addition of tri-
fluoroacetic acid (0.08 mL). Purified by flash chromatography in
silica eluted with a mixture of c-hexane/ethanol, 10:1, to give an
orange powder; yield 268.3 mg (34 %). 1H NMR (250 MHz,
CD2Cl2): δ = 7.67–7.57 (m, 2 H, Ph), 7.40–7.31 (m, 3 H, Ph), 7.26–
7.12 (m, 5 H, Ph), 4.43 (d, J = 1.2 Hz, 1 H, HCp), 4.23 (t, J =
2.3 Hz, 1 H, HCp), 4.01–3.95 (m, 1 H, HCp), 3.81 (s, 5 H, Cp),
3.43 (dt, J = 7.0, 5.5 Hz, 1 H, HCMe), 2.00 (br. s, adamantyl), 1.93
(dd, J = 7.2, 3.0 Hz, 3 H, Me), 1.79 (br. s, adamantyl), 1.73 (br.
s, adamantyl), 1.64 (br. s, adamantyl) ppm. 13C NMR (63 MHz,
CD2Cl2): δ = 143.22 (dd, J = 8.0, 1.7 Hz), 140.96 (dd, J = 12.2,
5.3 Hz), 136.45 (d, J = 23.4 Hz), 133.50 (dd, J = 16.7, 3.0 Hz),
129.21 (s), 128.27 (d, J = 8.0 Hz), 127.58 (d, J = 6.0 Hz), 127.11
(s), 103.45 (dd, J = 25.1, 24.4 Hz), 74.93 (dd, J = 12.3, 4.3 Hz),
72.17 (dd, J = 5.0, 0.9 Hz), 70.92 (dd, J = 4.4, 2.5 Hz), 69.97 (s),
68.67 (s), 42.96 (dd, J = 11.0, 3.0 Hz), 42.52 (d, J = 12.0 Hz), 40.19
(d, J = 2.8 Hz), 39.84 (s), 39.62 (d, J = 2.7 Hz), 39.35 (s), 37.63 (s),
29.88–29.29 (m), 19.10 (s) ppm. 31P NMR (101 MHz, CD2Cl2): δ
= 49.79 (d, J = 54.1 Hz), –26.35 (d, J = 54.1 Hz) ppm. HRMS
(MALDI): calcd. for [M + H]+ 699.2967; found 699.2973.
Dimethyl{(R)-1-[(S)-2-(diphenylphosphanyl)ferrocenyl]ethyl}amine
(1a):[40] From Ugi’s amine (4.024 g, 15.652 mmol, 1 equiv.), tert-
butyllithium (12 mL, 1.6 m in pentane, 19.2 mmol, 1.2 equiv.) and
chlorodiphenylphosphane (3 mL, 16.652 mmol, 1.1 equiv.), accord-
ing to the general method. Recrystallized at –20 °C from a satu-
rated dichloromethane/hexane solution to give orange crystals;
1
yield 5.133 g (74%). H NMR (250 MHz, CDCl3): δ = 7.66 (m, 2
H, Ph), 7.48–7.39 (m, 3 H), 7.27 (m, 5 H), 4.45 (s, 1 H, HCp), 4.32
(t, J = 2.3 Hz, 1 H, HCp), 4.23 (ddd, J = 13.2, 6.4, 2.3 Hz, 1 H,
HCMe), 4.02 (s, 5 H, Cp), 3.93 (s, 1 H, HCp), 1.84 (s, 6 H, NMe),
1.34 (d, J = 6.7 Hz, 3 H, HCMe) ppm. 31P NMR (101 MHz,
CDCl3): δ = –22.82 (s) ppm.
(R)-1-[(SP)-2-(Dicyclohexylphosphanyl)ferrocenyl]ethyldi[3,5-bis-
(trifluoromethyl)phenyl]phosphane (2h): From 1b (106.6 mg,
0.235 mmol, 1 equiv.) and bis[3,5-bis(trifluoromethyl)phenyl]phos-
phane[43] (111 mg, 0.242 mmol, 1 equiv.) according to the general
procedure. Purified by recrystallization from methanol at –20 °C, to
Dimethyl{(R)-1-[(S)-2-(dicyclohexylphosphanyl)ferrocenyl]ethyl}-
amine (1b):[40] From Ugi’s amine (1.821 g, 7.08 mmol, 1 equiv.),
tert-butyllithium (5.4 mL, 1.6 m in pentane, 8.5 mmol, 1.2 equiv.)
and chlorodicyclohexylphosphane (1.8 mL, 8.15 mmol, 1.1 equiv.)
according to the general method. Flash chromatography through
silica, eluted with a mixture of n-hexane/ethyl acetate, 5:1. Recrys-
tallized at –20 °C from a saturated ethanol solution to give an
orange powder; yield 1.896 g (59%). 1H NMR (300 MHz, CD2Cl2):
δ = 4.28 (s, 1 H, HCp), 4.24 (t, J = 2.1 Hz, 1 H, HCp), 4.10 (s, 1
H, HCp), 4.05 (s, 5 H, Cp), 3.98 (m, 1 H, HCMe), 2.34 (br. s, Cy),
2.09 (br. s, Cy), 2.00 (br. s, Cy), 1.84 (br. s, Cy), 1.68 (m, Cy), 1.48–
1.33 (m, Cy), 1.28 (d, J = 6.7 Hz, 3 H, Me), 1.23–1.01 (m, Cy)
ppm. 31P NMR (121 MHz, CD2Cl2): δ = –11.84 (s) ppm.
1
give an orange powder; yield 141 mg (69%). H NMR (300 MHz,
CD2Cl2): δ = 7.97 (s, Ph), 7.93 (s, Ph), 7.85 (s, Ph), 7.80 (s, Ph),
7.71 (s, Ph), 7.69 (s, Ph), 4.30 (s, Cp, minor conformer), 4.22 (s, 5
H, Cp, major conformer), 4.12 (s, HCp), 4.07 (s, HCp), 3.93 (br. s,
1 H, HCMe), 3.64 (s, HCp), 2.66–2.52 (m, Cy), 2.44 (dd, J = 16.5,
8.8 Hz, Cy), 2.22 (d, J = 8.5 Hz, Cy), 1.90 (br. s, Cy), 1.84–1.68
(m, Cy), 1.53 (dd, J = 37.7, 22.3 Hz, Cy), 1.43–1.32 (m, 3 H, Me),
1.28 (d, J = 7.6 Hz, Cy), 1.19 (d, J = 16.4 Hz, Cy), 1.01–0.70 (m,
Cy) ppm. 13C NMR (63 MHz, CD2Cl2): δ = 135.94 (dd, J = 21.8,
2.8 Hz), 132.16 (dd, J = 19.0, 3.6 Hz), 131.78 (d, J = 3.1 Hz),
131.45 (d, J = 7.0 Hz), 125.99 (d, J = 5.7 Hz), 124.17 (s), 122.46
(s), 121.65 (d, J = 5.6 Hz), 95.93 (dd, J = 23.1, 18.5 Hz), 72.97 (s),
70.70 (s), 70.62 (s), 70.05 (s), 69.20 (s), 67.94 (s), 38.26 (s), 35.55
(s), 33.90 (s), 32.03 (s), 30.63 (d, J = 10.2 Hz), 30.40 (s), 30.24 (s),
29.92 (s), 28.88 (s), 28.61 (s), 28.03 (s), 27.87 (s), 27.67 (d, J =
General Method for the Synthesis of Josiphos Ligands 2: In an oven-
dried Schlenk tube, 1 (1 equiv.) was dissolved in degassed glacial
acetic acid (to a concentration of 0.5–1 m). To this solution, the
corresponding secondary phosphane (1.1 equiv.) was added, and
the mixture was stirred at 80 °C for three hours. The solvent and
volatiles were removed in vacuo to give an orange sticky solid. It
8
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Eur. J. Inorg. Chem. 0000, 0–0

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Date: 12-09-12 17:18:48
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Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
11.3 Hz), 26.88 (d, J = 8.0 Hz), 17.50 (s) ppm. 31P NMR
(121 MHz, CD2Cl2): δ = 11.96 (s, minor conformer), 8.31 (br. s,
major conformer), –11.77 (s, minor conformer), –16.16 (br. s, major
conformer) ppm. 19F NMR (282 MHz, CD2Cl2): δ = –63.24 (d, J
= 28.9 Hz, minor conformer), –63.34 (d, J = 26.5 Hz, major con-
former) ppm. HRMS (MALDI): calcd. for [M + H]+ 867.1836;
found 867.1826.
(903.10): calcd. C 47.88, H 4.91, P 6.86; found C 48.15, H 4.93, P
6.82.
[ReOCl3(2b)] (3b): From [ReOCl3(AsPh3)2] (37.7 mg, 0.041 mmol,
1 equiv.) and 2b (25.9 mg, 0.043 mmol, 1 equiv.) according to the
general method to give a brown solid; yield 29 mg (78%). 1H NMR
(250 MHz, CD2Cl2): δ = 7.84 (s, Ph), 7.74 (s, Ph), 7.73–7.60 (m,
Ph), 7.53 (br. s, Ph), 7.41 (d, J = 6.9 Hz, Ph), 7.37–7.18 (m, Ph),
4.90 (s, 1 H, HCp), 4.74 (s, 1 H, HCp), 4.69 (s, 1 H, HCp), 4.56 (s,
(R)-1-[(SP)-2-(Dicyclohexylphosphanyl)ferrocenyl]ethyldi(3,5-
xylyl)phosphane (2i): From 1b (200.7 mg, 0.443 mmol, 1 equiv.) and 1 H, endo), HCp, 4.38 (s, 5 H, Cp, exo), 4.34 (s, 5 H, endo), 4.04–
bis(3,5-xylyl)phosphane (120 μL, 0.529 mmol, 1.2 equiv.) according
to the general procedure. Purified by flash chromatography in silica
eluted with a mixture of n-hexane/ethyl acetate, 20:1 and 5% of
TEA, to give an orange powder; yield 155 mg, 95% purity (53%).
1H NMR (250 MHz, CD2Cl2): δ = 7.00 (s, 1 H, Xyl), 6.98 (s, 2 H,
3.81 (m, 1 H, HCMe), 3.40 (s, Cy), 3.17 (q, J = 12.2 Hz, Cy), 2.76
(dd, J = 24.5, 11.9 Hz, Cy), 2.48 (d, J = 28.1 Hz, Cy), 2.20–1.99
(m, Cy), 1.98–1.77 (m, Cy), 1.69 (dd, J = 12.7, 6.9 Hz, 3 H, Me),
1.62–1.33 (m, Cy), 1.33–1.07 (m, Cy), 0.94–0.76 (m, Cy) ppm. 13C
NMR (63 MHz, CD2Cl2): δ = 135.21 (d, J = 6.9 Hz), 134.27 (d, J
Xyl), 6.95 (s, 2 H, Xyl), 6.89 (s, 1 H, Xyl), 4.19 (s, 2 H, HCp), 4.16 = 7.2 Hz), 132.02 (d, J = 2.1 Hz), 131.59 (d, J = 3.1 Hz), 129.09
(s, 5 H, Cp), 3.89 (s, 1 H, HCp), 3.52 (ddd, J = 13.8, 6.9, 3.5 Hz,
1 H, HCMe), 2.27, [s, 6 H, Me(Xyl)], 2.26, [s, 6 H, Me(Xyl)], 1.87
(br. s, Cy), 1.70 (dd, J = 24.2, 15.9 Hz, 3 H, HCMe), 1.40 (m, Cy),
(d, J = 9.2 Hz), 127.85 (d, J = 10.3 Hz), 91.32–90.61 (m), 75.61 (d,
J = 4.4 Hz), 71.64 (s), 70.68 (s), 70.38 (d, J = 6.5 Hz), 40.91 (d, J
= 23.2 Hz), 38.28 (d, J = 22.2 Hz), 30.22 (s), 29.23 (s), 29.07 (s),
1.28 (br. s, Cy), 1.14 (br. s, Cy), 0.89 (br. s, Cy) ppm. 13C NMR 28.74 (s), 28.34 (s), 27.85 (s), 27.73–27.09 (m), 26.77 (s), 26.09 (s),
(63 MHz, CD2Cl2): δ = 138.97 (d, J = 18.1 Hz), 138.06 (d, J =
5.0 Hz), 137.67 (d, J = 7.8 Hz), 136.27 (d, J = 19.9 Hz), 133.70 (d,
J = 21.1 Hz), 131.20 (s), 130.02 (d, J = 16.0 Hz), 129.69 (s), 99.25
16.72 (d, J = 6.4 Hz) ppm. 31P NMR (101 MHz, CD2Cl2): δ = 1.43
(br. s, endo), –2.86 (d, J = 13.4 Hz, exo), –12.19 (br. s, exo), –19.89
(br. s, endo) ppm. HRMS (MALDI): calcd. for [M – Cl]+ 867.1134,
(dd, J = 22.4, 19.4 Hz), 78.94 (dd, J = 22.5, 3.3 Hz), 71.70 (s), 69.71 found 867.1128.
(s), 69.17 (dd, J = 9.7, 3.9 Hz), 68.07 (s), 37.34 (d, J = 13.0 Hz),
[ReOCl3(2c)] (3c): From [ReOCl3(AsPh3)2] (122 mg, 0.132 mmol,
35.91 (dd, J = 12.3, 3.5 Hz), 33.82 (d, J = 22.6 Hz), 32.15 (dd, J =
13.5, 3.5 Hz), 31.60 (d, J = 13.9 Hz), 30.49 (d, J = 4.4 Hz), 28.77
(d, J = 14.9 Hz), 27.86 (d, J = 9.9 Hz), 27.09 (d, J = 4.9 Hz), 21.61
(d, J = 6.9 Hz), 19.84 (t, J = 3.9 Hz) ppm. 31P NMR (101 MHz,
CD2Cl2): δ = 3.66 (d, J = 8.4 Hz), –14.61 (d, J = 7.6 Hz) ppm.
HRMS (MALDI): calcd. for [M + H]+ 651.2967; found 651.2968.
1 equiv.) and 2c (99 mg, 0.142 mmol, 1.1 equiv.) according to the
general method to give a brown solid; yield 113 mg (85%). 1H
NMR (250 MHz, CD2Cl2): δ = 8.43–8.28 (m, Ph), 8.28–8.13 (m,
Ph), 8.02 (dd, J = 18.7, 10.2 Hz, Ph), 7.71–7.39 (m, Ph), 7.34 (t, J
= 9.0 Hz, Ph), 5.21 (dd, J = 10.9, 7.6 Hz, HCMe), 4.86 (s, HCp),
4.78 (s, HCp), 4.75–4.67 (m, HCp), 4.57 (s, HCp), 4.47 (s, HCp),
4.40 (s, HCp), 4.36 (s, Cp), 4.33 (s, HCp), 3.78 (s, Cp), 3.49 (s,
adamantyl), 2.97 (br. s, adamantyl), 2.70 (br. s, adamantyl), 2.49
(br. s, adamantyl), 2.38 (dd, J = 9.1, 7.2 Hz, Me), 2.33 (br. s, ada-
mantyl), 2.29 (br. s, adamantyl), 2.19 (br. s, adamantyl), 2.07 (d, J
= 8.1 Hz, adamantyl), 1.96 (br. s, adamantyl), 1.88 (br. s, ada-
mantyl), 1.80 (br. s, adamantyl), 1.75 (br. s, adamantyl), 1.69 (br.
s, adamantyl), 1.65 (br. s, adamantyl), 1.52 (br. s, adamantyl), 1.46
(br. s, adamantyl), 1.39 (br. s, adamantyl), 1.34 (br. s, adamantyl),
1.26 (br. s, adamantyl), 0.73 (br. s, adamantyl) ppm. 31P NMR
(101 MHz, CD2Cl2): δ = 26.88 (d, J = 11.3 Hz, exo), 8.50 (d, J =
14.4 Hz, endo), –28.08 (d, J = 11.5 Hz, exo), –32.99 (d, J = 14.5 Hz,
endo) ppm. HRMS (MALDI): calcd. for [M – Cl]+ 971.1761; found
971.1739.
General Method for the Synthesis of Complexes 3: In an oven-dried
Schlenk tube, [ReOCl3(AsPh3)2] (X = Cl, Br)[25] (1 equiv.) and the
corresponding Josiphos ligand 2 were dissolved in dry dichloro-
methane (to a concentration of ca. 0.1 m). The mixture was then
stirred at room temperature for 3–12 h. It was then concentrated
in vacuo to a fourth of its original volume, and to the resulting
brown solution n-hexane was added (two or three times the initial
volume) to precipitate a brown solid. This was decanted and
washed thoroughly with hexane and finally with ethyl ether. The
product was then dried in vacuo and purified (when necessary) by
recrystallization.
[ReOCl3(2a)] (3a): From [ReOCl3(AsPh3)2] (298.5 mg, 0.324 mmol,
1 equiv.) and 2a (194.3 mg, 0.327 mmol, 1 equiv.) according to the
general method to give a brown-green solid; yield 274 mg (94%).
X-ray quality crystals were obtained by layering a saturated dichlo-
romethane solution with benzene. 1H NMR (250 MHz, CD2Cl2): δ
= 8.28–8.10 (m, 2 H, Ph), 7.57 (m, 3 H, Ph), 7.50–7.37 (m, 2 H,
Ph), 7.27 (m, 3 H, Ph), 4.90 (s, 1 H, HCp), 4.60 (d, J = 7.6 Hz, 1
H, HCp), 3.73 (s, 5 H, Cp), 3.41 (s, 1 H, HCp), 2.52 (br. s, 1 H,
HCMe), 2.07 (dd, J = 12.0, 7.4 Hz, 3 H, Me), 2.00–1.74 (m, Cy),
1.74–1.31 (m, Cy), 1.22 (s, Cy), 1.05–0.80 (m, Cy), 0.74–0.44 (m,
[ReOCl3(2d)] (3d): From [ReOCl3(AsPh3)2] (61 mg, 0.066 mmol,
1 equiv.) and 2d (43 mg, 0.067 mmol, 1 equiv.) according to the ge-
neral method to give a brown solid; yield 54 mg (71%). The ob-
tained endo isomer isomerizes slowly in solution at room tempera-
ture to give the exo isomer. X-ray quality crystals of exo-3d were
obtained by layering a saturated chloroform solution with n-hex-
ane. endo isomer: 1H NMR (250 MHz, CD2Cl2): δ = 8.18–8.02 (m,
Ar), 7.66 (dd, J = 11.0, 7.2 Hz, Ar), 7.58–7.38 (m, Ar), 7.32 (m,
Cy) ppm. 13C NMR (63 MHz, CD2Cl2): δ = 141.28 (d, J = Ar), 7.06 (s, 1 H, Xyl), 6.97 (s, 1 H, Xyl), 4.78 (s, 1 H, HCp), 4.60
53.5 Hz), 135.98 (d, J = 8.4 Hz), 133.67 (d, J = 61.3 Hz), 132.63 (t, J = 2.4 Hz, 1 H, HCp), 4.22 (s, 1 H, HCp), 4.19 (m, HCMe),
(d, J = 8.9 Hz), 131.97 (d, J = 2.6 Hz), 130.88 (d, J = 2.4 Hz),
128.37 (d, J = 10.6 Hz), 128.02 (d, J = 10.1 Hz), 92.78 (d, J = 1.49 (dd, J = 13.1, 7.2 Hz, 3 H, HCMe) ppm. 31P NMR (101 MHz,
15.5 Hz), 76.47 (d, J = 2.8 Hz), 72.44 (d, J = 7.7 Hz), 71.97 (s), CDCl3): δ = –9.27 (br. s), –22.00 (d, J = 14.8 Hz) ppm. HRMS
71.33 (d, J = 6.5 Hz), 39.86 (d, J = 19.2 Hz), 37.67 (d, J = 19.1 Hz), (MALDI): calcd. for [M – Cl]+ 911.0821; found 911.0820. exo iso-
3.79 (s, 5 H, Cp), 2.30 [s, 6 H, Me(Xyl)], 2.22 [s, 6 H, Me(Xyl)],
31.04 (d, J = 21.7 Hz), 29.91 (s), 29.80 (s), 28.68 (d, J = 8.7 Hz),
28.17 (s), 27.84 (s), 27.42 (s), 27.23 (s), 26.80 (s), 26.28 (s), 17.54
(d, J = 6.7 Hz) ppm. 31P NMR (101 MHz, CD2Cl2): δ = 3.06 (d, J
= 15.5 Hz), –34.81 (d, J = 15.6 Hz) ppm. HRMS (MALDI): calcd.
for [M – Cl]+) 867.1133, found 867.1138. C36H44Cl3FeOP2Re
mer: 13C NMR (63 MHz, CDCl3): δ = 140.46 (s), 139.59 (s), 137.51
(d, J = 8.2 Hz), 137.34 (d, J = 7.4 Hz), 135.56 (d, J = 8.4 Hz),
134.81 (s), 133.37 (s), 132.80 (s), 132.28 (d, J = 8.2 Hz), 131.94 (d,
J = 9.5 Hz), 131.62 (d, J = 7.4 Hz), 130.01 (s), 129.42 (s), 128.63
(s), 128.03 (d, J = 11.1 Hz), 127.27 (d, J = 11.1 Hz), 92.54 (dd, J
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9

Job/Unit: I20693
/KAP1
Date: 12-09-12 17:18:48
Pages: 13
E. Mejía, R. Aardoom, A. Togni
FULL PAPER
= 17.6, 3.5 Hz), 77.30 (s), 75.99–75.72 (m), 73.70 (s), 72.77 (s),
71.54 (s), 70.99 (s), 70.23 (d, J = 6.9 Hz), 32.70 (d, J = 29.9 Hz),
21.70 (s), 21.29 (s), 15.28 (dd, J = 4.9, 1.7 Hz) ppm. 31P NMR
(101 MHz, CDCl3): δ = 1.33 (d, J = 15.7 Hz), –30.73 (d, J =
15.9 Hz) ppm.
(br. s, Cy), 2.25 (dd, J = 10.0, 7.2 Hz, HCMe), 2.16 (dd, J = 9.9,
7.2 Hz, HCMe), 1.94 (br. s, Cy), 1.79 (br. s, Cy), 1.75 (br. s, Cy),
1.53 (d, J = 12.2 Hz, tBu), 1.41 (d, J = 13.3 Hz, tBu), 1.33–1.16
(m, tBu), 0.94–0.81 (m, Cy) ppm. 31P NMR (101 MHz, CD2Cl2):
δ = 24.89 (s), 23.61 (d, J = 10.4 Hz), –23.29 (s), –37.56 (s) ppm.
HRMS (MALDI): calcd. for [M – 2Cl]+ 792.2077; found 792.2085;
calcd. for [M – Cl]+ 827.1759; found 827.1758.
[ReOCl3(2e)] (3e): From [ReOCl3(AsPh3)2] (67 mg, 0.073 mmol,
1 equiv.) and 2e (45 mg, 0.074 mmol, 1 equiv.) according to the ge-
neral method to give a brown powder; yield 48 mg (73%). X-ray
quality crystals were obtained by layering a saturated dichloro-
[ReOCl3(2h)] (3h): From [ReOCl3(AsPh3)2] (25 mg, 0.027 mmol,
1 equiv.) and 2h (25 mg, 0.029 mmol, 1.1 equiv.) according to the
general method to give a brown powder; yield 5.3 mg (16%). 1H
NMR (300 MHz, CD2Cl2): δ = 8.22 (d, J = 8.4 Hz, Ar), 8.13 (d, J
1
methane solution with ethyl ether. H NMR (250 MHz, CD2Cl2):
δ = 4.74 (s, 1 H, HCp), 4.55 (s, 1 H, HCp), 4.46 (s, 1 H, HCp),
4.30 (s, 5 H, Cp), 3.50 (s, 1 H, Cy), 3.01 (s, 1 H, Cy), 2.80 [q, J = = 7.4 Hz, Ar), 7.98 (br. s, Ar), 7.61 (br. s, Ar), 4.99 (s, 1 H, HCp),
13.3 Hz, 1 H, HC(Me)], 2.48 (s, 2 H, Cy), 2.30 (d, J = 9.4 Hz, 1
H, Cy), 2.10 (d, J = 7.4 Hz, 3 H), 2.05 (d, J = 7.4 Hz, 3 H), 1.85
4.78 (d, J = 8.5 Hz, 1 H, HCp), 4.64 (s, 1 H, HCp), 4.58 (s, 1 H,
HCp), 4.41 (s, 5 H, Cp), 4.40 (s, 5 H, Cp), 4.30 (d, J = 8.8 Hz, 1
(m, Cy), 1.53 (m, Cy), 1.39 (m, Cy), 1.25–0.97 (m, Cy), 0.85 (m, H, HCp), 4.25 (s, 1 H, HCp), 3.95–3.83 (m, 1 H, HCMe), 3.48 (m,
Cy) ppm. 13C NMR (63 MHz, CD2Cl2): δ = 91.75 (br. s), 71.27 (s), 1 H, HCMe), 2.90 (d, J = 12.1 Hz, Cy), 2.82 (br. s, Cy), 2.57 (br.
70.50 (s), 70.39 (s), 70.09 (s), 70.00 (s), 40.53 (d, J = 16.8 Hz), 36.94
(d, J = 23.3 Hz), 32.17 (d, J = 34.6 Hz), 30.02 (d, J = 5.5 Hz), 29.71 J = 13.2, 6.7 Hz, 3 H, Me), 1.64 (dd, J = 13.8, 7.0 Hz, 3 H, Me),
(d, J = 7.1 Hz), 28.96 (s), 28.89–28.38 (m), 27.98 (d, J = 9.6 Hz), 1.53 (br. s, Cy), 1.47–1.20 (m, Cy), 0.92–0.83 (m, 1 H, Cy) ppm.
s, Cy), 2.34 (br. s, Cy), 2.12 (br. s, Cy), 2.07–1.78 (m, Cy), 1.70 (dd,
27.69 (d, J = 2.9 Hz), 27.50 (d, J = 1.8 Hz), 26.74 (d, J = 4.1 Hz), 31P NMR (121 MHz, CD2Cl2): δ = 2.71 (s), –4.26 (s), –10.27 (s),
26.55 (d, J = 4.4 Hz) ppm. 31P NMR (101 MHz, CD2Cl2): δ = 6.19
(d, J = 13.4 Hz), –27.57 (d, J = 13.7 Hz) ppm. HRMS (MALDI):
calcd. for [M – Cl]+ 879.20849; found 879.2065.
–18.27 (s) ppm. 19F NMR (188 MHz, CD2Cl2): δ = –63.30 (s),
–63.43 (s), –63.46 (s), –63.47 (s) ppm. HRMS (MALDI): calcd. for
[M – Cl]+ 1139.0629; found 1139.0646.
[ReOCl3(2f)] (3f): From [ReOCl3(AsPh3)2] (49.1 mg, 0.053 mmol, [ReOCl3(2i)] (3i): From [ReOCl3(AsPh3)2] (61.8 mg, 0.067 mmol,
1 equiv.) and 2f (31.5 mg, 0.058 mmol, 1.1 equiv.) according to the
general method to give a brown powder; yield 44 mg (97%). X-ray
quality crystals of endo-3f were obtained by layering a saturated
chloroform solution of the isomer mixture with n-hexane. 1H NMR
1 equiv.) and 2i (44.9 mg, 0.069 mmol, 1 equiv.) according to the
general method to give a brown solid; yield 65 mg (Ͼ99%). X-ray
quality crystals were obtained by layering a saturated dichloro-
1
methane solution with n-hexane. H NMR (250 MHz, CD2Cl2): δ
(300 MHz, CDCl3): δ = 8.46–8.32 (m, Ph), 8.25–8.13 (m, Ph), 8.13– = 7.39 (s, 1 H, Xyl), 7.35 (s, 1 H, Xyl), 7.17 (s, 1 H, Xyl), 6.98 (s,
7.96 (m, Ph), 7.85 (d, J = 7.4 Hz, Ph), 7.80–7.71 (m, Ph), 7.64 (t,
J = 7.3 Hz, Ph), 7.45 (br. s, Ph), 7.33 (br. s, Ph), 5.38 (dt, J = 12.9,
6.9 Hz, 1 H, HCMe), 4.79 (s, 1 H, HCp), 4.70 (s, 1 H, HCp), 4.66
1 H, Xyl), 6.94 (s, 1 H, Xyl), 4.61 (br. s, 1 H, HCMe), 4.54 (br. s,
2 H, HCp), 4.35 (s, 1 H, HCp), 4.33 (s, 5 H, Cp), 3.12 (dd, J =
22.8, 11.6 Hz, 1 H, Cy), 2.88 (br. s, 1 H, Cy), 2.72 (d, J = 11.4 Hz,
(s, 1 H, HCp), 4.44 (s, 1 H, HCp), 4.33 [s, 5 H, Cp (exo)], 4.09 (s, 1 H, Cy), 2.35 [s, 6 H, Me(Xyl)], 2.13 [s, 6 H, Me(Xyl)], 2.08 (d, J
1 H, HCp), 3.87 (dt, J = 17.0, 7.2 Hz, 1 H, HCMe), 3.76 [s, 5 H, = 8.9 Hz, Cy), 1.97–1.67 (m, Cy), 1.57 (dd, J = 13.3, 7.1 Hz, 3 H,
Cp (endo)], 3.56 (s, 1 H, HCp), 2.29 [dd, J = 9.2, 7.5 Hz, 3 H, HCMe), 1.49–1.13 (m, Cy), 1.13–0.71 (m, Cy), 0.38 (br. s, Cy) ppm.
HCMe (exo)], 2.19 [dd, J = 9.9, 7.6 Hz, 3 H, HCMe (endo)], 1.56 13C NMR (63 MHz, CD2Cl2): δ = 137.95 (s), 137.79 (s), 137.61 (s),
[s, 9 H, Me(tBu) (exo)], 1.51 [s, 9 H, Me(tBu) (exo)], 1.15 [s, 9 H,
133.39 (d, J = 2.0 Hz), 133.35 (d, J = 1.4 Hz), 133.09 (d, J =
Me(tBu) (endo)], 1.10 [s, 9 H, Me(tBu) (endo)] ppm. 13C NMR 7.5 Hz), 132.48 (d, J = 6.6 Hz), 129.96 (s), 129.13 (s), 91.34 (s),
(63 MHz, CDCl3): δ = 138.05 (d, J = 50.0 Hz), 136.08 (d, J =
8.4 Hz), 135.20 (d, J = 8.1 Hz), 135.05 (d, J = 47.5 Hz), 133.80 (s),
133.39 (d, J = 8.9 Hz), 132.98 (d, J = 10.4 Hz), 132.36 (s), 131.46
(d, J = 2.6 Hz), 131.20 (s), 130.69 (d, J = 2.3 Hz), 130.51 (s), 130.17
(s), 128.74 (s), 128.55 (s), 128.11 (d, J = 4.5 Hz), 127.97 (s), 127.89
76.44 (s), 71.62 (s), 70.27 (d, J = 5.8 Hz), 42.62 (d, J = 27.5 Hz),
34.33 (d, J = 28.2 Hz), 30.05 (d, J = 3.3 Hz), 28.99 (s), 28.81 (s),
28.50 (s), 28.06 (s), 27.89 (s), 27.65 (s), 27.46 (s), 27.01 (d, J =
2.0 Hz), 26.90 (d, J = 1.3 Hz), 26.33 (s), 21.87 (s), 21.37 (s), 15.66
(d, J = 5.3 Hz) ppm. 31P NMR (101 MHz, CD2Cl2): δ = 1.45 (br.
(d, J = 2.4 Hz), 127.69 (d, J = 1.5 Hz), 127.51 (d, J = 1.0 Hz), 93.62 s), –20.41 (br. s) ppm. HRMS (MALDI): calcd. for [M – Cl]+
(s), 82.11 (s), 77.36 (s), 75.91 (s), 75.70 (d, J = 6.6 Hz), 72.22 (s),
71.67 (s), 71.06 (d, J = 8.1 Hz), 70.63 (s), 69.92 (d, J = 7.8 Hz),
45.33 (d, J = 8.9 Hz), 44.02 (d, J = 6.5 Hz), 43.24 (d, J = 9.7 Hz),
41.55 (d, J = 10.4 Hz), 39.04 (d, J = 10.0 Hz), 33.19 (s), 32.11 (s),
30.52 (s), 19.71 (s), 18.67 (s) ppm. 31P NMR (101 MHz,
CDCl3): δ = 27.61 (d, J = 12.0 Hz, exo), 23.14 (d, J = 14.6 Hz,
endo), –27.55 (d, J = 11.9 Hz, exo), –33.81 (d, J = 14.7 Hz, endo)
ppm. HRMS (MALDI): calcd. for [M – Cl]+ 815.0820; found
815.0826.
923.1777; found 923.1776; calcd. for [M + Na]+ calcd: 981.1341;
found 981.1340.
[ReOCl3(2j)] (3j): From [ReOCl3(AsPh3)2] (47 mg, 0.051 mmol,
1 equiv.) and 2j (30.6 mg, 0.056 mmol, 1.1 equiv.) according to the
general method to give a brown solid; yield 31.4 mg (72%). 1H
NMR (250 MHz, CD2Cl2): δ = 7.91–7.21 (m, Ph), 5.08 (s, 1 H,
HCp), 5.00 (dd, J = 15.4, 7.4 Hz, 1 H, HCMe), 4.84 (s, 1 H, HCp),
4.76 (s, 1 H, HCp), 4.28 (s, 5 H, Cp), 2.07 (s, 9 H, tBu), 2.02 (s, 9
H, tBu), 1.64 (dd, J = 13.2, 7.3 Hz, 3 H, HCMe) ppm. 13C NMR
(63 MHz, CD2Cl2): δ = 134.95 (d, J = 7.3 Hz), 134.60 (d, J =
7.1 Hz), 131.98 (d, J = 2.6 Hz), 131.63 (d, J = 2.9 Hz), 128.63 (d,
J = 3.8 Hz), 128.47 (d, J = 4.6 Hz), 90.82 (dd, J = 12.7, 5.1 Hz),
78.61 (s), 76.32 (d, J = 29.0 Hz), 72.75 (s), 71.57 (s), 71.18 (d, J =
5.3 Hz), 45.21 (d, J = 16.8 Hz), 43.71 (d, J = 16.6 Hz), 34.50 (d, J
[ReOCl3(2g)] (3g): From [ReOCl3(AsPh3)2] (49 mg, 0.053 mmol,
1 equiv.) and 2g (32 mg, 0.058 mmol, 1.1 equiv.) according to the
general method to give a brown solid; yield 39 mg (85%). X-ray
quality crystals were obtained by layering a saturated dichloro-
1
methane solution with n-hexane. H NMR (250 MHz, CD2Cl2): δ
= 4.71 (s, 1 H, HCp), 4.65 (s, 1 H, HCp), 4.63 (s, 1 H, HCp), 4.59 = 28.7 Hz), 32.76 (s), 31.05 (s), 16.66 (d, J = 6.1 Hz) ppm. 31P
(s, 1 H, HCp), 4.56 (s, 1 H, HCp), 4.50 (s, 1 H, HCp), 4.37 (s, 5 NMR (101 MHz, CD2Cl2): δ = 18.12 (d, J = 12.8 Hz), 2.97 (d, J
H, Cp), 4.32 (s, 5 H, Cp), 3.82 (dt, J = 13.2, 5.5 Hz, 1 H, HCMe),
3.64–3.41 (m, 1 H, HCMe), 3.09 (br. s, Cy), 2.99–2.50 (m, Cy), 2.44
= 13.2 Hz), –8.18 (d, J = 13.3 Hz), –8.69 (d, J = 13.0 Hz) ppm.
HRMS (MALDI): calcd. for [M – Cl]+ 815.0820; found 815.0816.
10
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Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20693
/KAP1
Date: 12-09-12 17:18:48
Pages: 13
Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
[ReOBr3(2k)] (3k): From [ReOBr3(AsPh3)2] (138.7 mg, 0.131 mmol,
1 equiv.) and 2a (82.6 mg, 0.139 mmol, 1 equiv.) according to the
general method to give a greenish-brown solid; yield 132 mg (97%).
1H NMR (250 MHz, CD2Cl2): δ = 8.31 (dd, J = 16.7, 8.7 Hz, Ph),
8.15–7.97 (m, Ph), 7.64–7.52 (m, Ph), 7.48 (s, Ph), 7.35 (br. s, Ph),
CD2Cl2): δ = 135.07 (d, J = 11.3 Hz), 134.29 (d, J = 9.2 Hz), 134.18
(d, J = 61.2 Hz), 131.32 (d, J = 2.8 Hz), 131.18 (d, J = 2.0 Hz),
129.95 (d, J = 51.6 Hz), 128.63 (d, J = 7.3 Hz), 128.46 (d, J =
9.0 Hz), 92.36 (d, J = 17.0 Hz), 75.26 (s), 72.33 (s), 70.60 (d, J =
15.3 Hz), 68.30 (s), 43.47 (d, J = 28.2 Hz), 36.27 (d, J = 21.0 Hz),
7.34 (s, Ph), 7.26 (m, Ph), 5.29–5.13 (m, 1 H, HCMe), 4.94 (s, 1 H, 33.97 (d, J = 23.9 Hz), 30.94 (s), 29.15 (s), 28.84 (s), 28.13 (s),
HCp), 4.74 (s, 1 H, HCp), 4.65–4.59 (m, 1 H, HCp), 4.56 (d, J = 27.47–25.80 (m), 25.18 (s), 24.84 (s), 23.35 (s), 16.03 (s) ppm. 31P
1.9 Hz, 1 H, HCp), 4.47 (s, 1 H, HCp), 4.27 (s, 5 H, Cp), 4.25 (s,
1 H, HCp), 3.78 (s, 5 H, Cp), 3.72–3.49 (m, 1 H, HCp), 2.82–2.58
(m, Cy), 2.44–2.22 (m, Cy), 2.13 (dd, J = 12.2, 7.5 Hz, 3 H, Me),
2.05 (dd, J = 12.0, 8.4 Hz, 3 H, Me), 1.98–1.46 (m, Cy), 1.42 (d, J
= 8.6 Hz, Cy), 1.34–0.83 (m, Cy), 0.82–0.60 (m, Cy), 0.58–0.33 (m,
Cy) ppm. 13C NMR (63 MHz, CD2Cl2): δ = 142.13 (d, J =
52.8 Hz), 136.28 (d, J = 8.0 Hz), 135.93 (d, J = 8.2 Hz), 135.70 (d,
J = 9.7 Hz), 133.24 (d, J = 61.4 Hz), 133.09 (d, J = 9.9 Hz), 132.31
(d, J = 8.6 Hz), 131.94 (d, J = 2.6 Hz), 131.78 (s), 131.29 (s), 130.75
(d, J = 2.1 Hz), 128.23 (s), 128.05 (s), 127.94 (s), 127.83 (d, J =
5.7 Hz), 92.10 (dd, J = 17.8, 2.3 Hz), 91.13 (d, J = 14.4 Hz), 76.83
(d, J = 7.2 Hz), 76.36 (d, J = 2.4 Hz), 73.03 (d, J = 5.7 Hz), 72.67
(d, J = 9.0 Hz), 71.95 (s), 71.85 (s), 71.63 (s), 71.47–71.23 (m), 71.03
(s), 70.15 (d, J = 8.9 Hz), 65.92 (s), 41.54 (d, J = 19.7 Hz), 38.74
(d, J = 18.5 Hz), 36.69 (d, J = 16.3 Hz), 32.69 (d, J = 22.8 Hz),
31.00 (d, J = 5.4 Hz), 30.67 (d, J = 4.0 Hz), 30.42 (d, J = 4.1 Hz),
30.25 (s), 28.61 (s), 28.41 (d, J = 6.7 Hz), 28.15 (s), 27.99 (s), 27.86
(s), 27.64 (s), 27.50 (s), 27.22 (s), 27.04 (s), 26.92 (s), 26.72 (s), 26.60
(s), 26.50 (s), 26.18 (s), 26.08 (s), 18.21 (d, J = 6.8 Hz), 17.31 (s)
ppm. 31P NMR (101 MHz, CD2Cl2): δ = –0.56 (s, endo), –3.73 (d,
J = 14.8 Hz, exo), –29.68 (s, endo), –43.80 (d, J = 14.7 Hz, exo)
ppm. HRMS (MALDI): calcd. for [M – Br]+ 957.0110; found
957.0088.
NMR (101 MHz, CD2Cl2): δ = 31.37 (d, J = 11.9 Hz), 15.82 (d, J
= 12.0 Hz) ppm.
General Procedure for the Catalytic Transfer Hydrogenation of
Ketones: In an oven-dried Young–Schlenk tube, the corresponding
amount of catalyst (ca. 1 mol-%) was dissolved in dry 2-propanol
at room temperature. When the catalyst was synthesized in situ, the
ferrocenyl ligand and the corresponding rhenium precursor were
mixed in a 1:1 ratio (ca. 1 mol-% each). To this solution 1 equiv. of
the ketone substrate was added (final concentration ca. 0.4 m).
Then an excess of TEA (8–20%) was added and the system was
heated for 20–24 h at 80 °C. The mixture was then allowed to cool
to room temperature, and the solvent and volatiles are removed in
vacuo. n-Hexane was then added, and the solution was filtered
through a plug of silica, eluted with n-hexane and concentrated
in a rotary evaporator to give the crude product. The yields were
determined by integration of 1H NMR signals using a 90° pulse
width of 3.33 μs and a relaxation delay between scans of 3 s. The
ee was measured by using chiral HPLC. In every case the absolute
configuration of the product was determined by comparison of the
retention time with that of an authentic sample of a pure enanti-
omer (see Supporting Information).
Isolation of the Rhenium-Alkoxy Intermediate [ReOCl2(iPrO)(2a)]
(7): After the extraction of the transfer-hydrogenation product with
n-hexane, the remaining insoluble orange solid was dried in vacuo,
dissolved in dichloromethane, and this solution was filtered
through a plug of silica and eluted with the same solvent. The
solution was concentrated to dryness to give the product as a light
orange powder. X-ray quality crystals were obtained by layering a
saturated dichloromethane solution with n-hexane. 1H NMR
(300 MHz, CD2Cl2): δ = 8.16 (s, 2 H, Ph), 7.87–7.69 (m, 2 H, Ph),
7.52 (s, 3 H, Ph), 7.41–7.22 (m, 3 H, Ph), 4.76 (s, 1 H, HCp), 4.71
(s, 1 H, HCp), 4.64 (s, 1 H, HCp), 3.83–3.66 [m, 1 H, HC(Me)2],
3.56 (s, 5 H, Cp), 3.27 (dd, J = 12.1, 5.8 Hz, 1 H, HCMe), 3.18
(dd, J = 24.4, 11.9 Hz, 1 H, Cy), 2.85 (s, 1 H, Cy), 2.63 (dd, J =
22.9, 12.3 Hz, 1 H, Cy), 2.11–1.94 (m, Cy), 1.89 (dd, J = 10.2,
7.6 Hz, 3 H, HCMe), 1.75 (br. s, Cy), 1.55 (br. s, Cy), 1.49–1.32
(m, Cy), 1.19–0.99 (m, Cy), 0.85 (dd, J = 24.3, 11.8 Hz, Cy), 0.66
(s, Cy), 0.61 [d, J = 6.0 Hz, 3 H, HC(Me)2], 0.05 [d, J = 6.0 Hz, 3
H, HC(Me)2] ppm. 13C NMR (63 MHz, CD2Cl2): δ = 137.94 (d, J
= 58.8 Hz), 135.84 (d, J = 9.3 Hz), 135.33 (d, J = 8.8 Hz), 133.80
(d, J = 55.9 Hz), 131.49 (d, J = 2.5 Hz), 130.98 (d, J = 2.4 Hz),
128.31 (d, J = 10.5 Hz), 127.91 (d, J = 10.3 Hz), 93.63 (d, J =
19.0 Hz), 76.21 (d, J = 2.0 Hz), 75.49 (s), 71.38 (s), 70.86 (s), 70.71
(s), 37.66 (d, J = 20.5 Hz), 36.92 (d, J = 21.4 Hz), 36.02 (d, J =
19.5 Hz), 29.99 (s), 29.06 (d, J = 6.0 Hz), 28.24 (s), 28.09 (d, J =
3.9 Hz), 27.81 (s), 27.64 (s), 27.46 (s), 27.14 (s), 26.44 (d, J =
6.6 Hz), 25.92 (s), 23.24 (s), 22.52 (s), 17.32 (d, J = 5.4 Hz) ppm.
31P NMR (101 MHz, CD2Cl2): δ = 15.03 (br. s), –11.44 (d, J =
12.7 Hz) ppm. HRMS (MALDI): calcd. for [M – Cl]+ 891.1949;
found 891.193.
[ReO2I(2a)] (5): In a nitrogen-filled glove-box, 2a (116.2 mg,
0.195 mmol, 1.1 equiv.) and [ReO2I(PPh3)2] (154 mg, 0.177 mmol,
1 equiv.) were dissolved in dichloromethane (3 mL). The resulting
deep dark red solution was stirred at room temperature overnight.
To this solution n-hexane was added until the precipitation of an
orange solid occurred. The crude product was decanted, washed
thoroughly with n-hexane and ethyl ether and dried in vacuo to
afford an orange-brown paramagnetic powder; yield 140 mg (84%).
HRMS (MALDI): calcd. for [M – I]+ 813.1720; found 813.1750.
FTIR (neat): ν = 3052.48, 2923.36, 2848.79, 1433.99, 1559.78,
˜
1096.27, 908.60, 790.73, 744.09, 692.46 cm–1. C36H44FeIO2P2Re
(939.64): calcd. C 46.02, H 4.72, P 6.59; found C 45.85, H 4.92, P
6.44.
[ReNCl2(2a)] (6): In an oven-dried Schlenk tube, 2a (59.3 mg,
0.099 mmol, 1 equiv.) and [NBu4][ReNCl4][41] (52.8 mg) were dis-
solved in dry toluene (5 mL). The orange solution was stirred at
90 °C for two hours to give a dark brown solution. It was then
allowed to cool to room temperature and n-hexane (10 mL) was
added with a syringe leading to the precipitation of a brown solid.
The solid was decanted and washed with n-hexane (3ϫ10 mL) and
dried in vacuo to give a green-brown solid. The crude product was
dissolved in dichloromethane, filtered through celite and concen-
trated to dryness to give the product as a greenish-black solid; yield
40.2 mg (47%). X-ray quality crystals (endo isomer) were obtained
by recrystallization from a saturated dichloromethane/n-hexane
1
solution at –20 °C. H NMR (250 MHz, CD2Cl2): δ = 8.16 (dd, J
= 12.1, 7.7 Hz, 2 H, Ph), 7.65 (dd, J = 15.0, 6.9 Hz, 5 H, Ph), 7.35
(dt, J = 13.2, 6.5 Hz, 3 H, Ph), 4.82 (s, 1 H, HCp), 4.60 (s, 1 H,
HCp), 4.45 (s, 1 H, HCp), 4.11–3.95 (m, 1 H, HCMe), 3.68 (s, 5
H, Cp), 3.05 (dd, J = 24.2, 11.9 Hz, 1 H, Cy), 2.78 (dd, J = 24.9,
12.2 Hz, 1 H, Cy), 2.28 (m, Cy), 1.94 (m, Cy), 1.73 (dd, J = 11.8,
7.5 Hz, 3 H, Me), 1.36–1.20 (m, Cy), 1.14 (m, Cy), 1.00–0.77 (m,
Supporting Information (see footnote on the first page of this arti-
cle): Spectroscopic and analytical characterization data (NMR,
MS, elemental analyses) for ligands and complexes, summary of
crystallographic data, and characterization data for products of
Cy), 0.24 (m, Cy), 0.08 (br. s, Cy) ppm. 13C NMR (63 MHz, transfer hydrogenation reactions.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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/KAP1
Date: 12-09-12 17:18:48
Pages: 13
E. Mejía, R. Aardoom, A. Togni
FULL PAPER
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Received: June 24, 2012
Published Online: ᭿
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Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20693
/KAP1
Date: 12-09-12 17:18:48
Pages: 13
Re-Catalyzed Asymmetric Transfer Hydrogenation of Ketones
Rhenium Catalysis
A series of new rhenium complexes con-
taining chiral ferrocenyldiphosphane li-
gands of the Josiphos family was prepared.
The complexes are active catalysts in the
asymmetric transfer hydrogenation of ke-
tones using 2-propanol as the hydrogen
source and in the presence of triethylamine,
thus representing the first example of Re
catalysts for this kind of transformation.
E. Mejía, R. Aardoom, A. Togni* ... 1–13
Asymmetric Transfer Hydrogenation of
Ketones Catalyzed by Rhenium Complexes
with Chiral Ferrocenylphosphane Ligands
Keywords: Asymmetric synthesis / Homo-
geneous catalysis / Hydrogen transfer /
Rhenium / Phosphane ligands
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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