Reductions of C=O and C=N groups with the systems composed of (η5-C5H5)2MoH2 and acids

Article abstract of DOI:10.1016/S0022-328X(98)00782-7

Selective reductions of organic compounds, such as carbonyl compounds and imines, using a system composed of Cp₂MoH₂ and acids are examined. This system can reduce the substrates under mild conditions. Extremely high diastereoselectivity was achieved in the reduction of 4-t-butylcyclohexanone. The reactivity of imines depends on their structure. Aromatic imines are found to be more reactive than aliphatic imines.

Full text of DOI:10.1016/S0022-328X(98)00782-7

Journal of Organometallic Chemistry 569 (1998) 139–145
Reductions of CꢀO and CꢀN groups with the systems composed of
(p5-C5H5)2MoH2 and acids1
Makoto Minato, Yutaka Fujiwara, Miho Koga, Naoya Matsumoto, Susumu Kurishima,
Makoto Natori, Norihiro Sekizuka, Ken-ichiro Yoshioka, Takashi Ito *
Department of Materials Chemistry, Faculty of Engineering, Yokohama National Uni6ersity, 79-5 Tokiwadai, Hodogaya-ku,
Yokohama 240-8501, Japan
Received 31 March 1998
Abstract
Selective reductions of organic compounds, such as carbonyl compounds and imines, using a system composed of Cp2MoH2
and acids are examined. This system can reduce the substrates under mild conditions. Extremely high diastereoselectivity was
achieved in the reduction of 4-t-butylcyclohexanone. The reactivity of imines depends on their structure. Aromatic imines are
found to be more reactive than aliphatic imines. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Aromatic imines; Aliphatic imines; Diastereoselectivity
1. Introduction
We have been investigating molybdenum and tung-
sten polyhydride compounds, which have unique fea-
tures and have proved to be a useful starting material
for the syntheses of a number of molybdenum and
tungsten complexes [3]. During the course of our de-
tailed studies on the chemistry of the molybdenum
dihydride Cp2MoH2 (Cp=p5-C5H5) (1), we found
that the system consisting of 1 and protonic acids
(HA) such as carboxylic acids, HCl, and TsOH (Ts=
p-MeC6H4SO2) reduces aldehydes and ketones to yield
the corresponding alcohols under mild conditions. An
important feature of this reaction is that the system
can reduce carbonyl compounds chemoselectively. In
this paper, we report full details on this reduction
system. We also report that the use of the system for
imines results in facile reductions to the corresponding
amines. Although much is known about the reduction
of ketones and aldehydes, the analogous reduction of
imines has received less attention. Some of these re-
sults have appeared in preliminary communications
[4,5].
Reduction of a carbonyl group using main group
elements of hydride such as LiAlH4 or NaBH4 has
been carried out routinely for many years in modern
synthetic chemistry and there is a huge number of
results in this area. However, there has been a growing
demand among organic chemists for new systems
which are capable of reducing a carbonyl group of
broad spectrum under especially mild reaction condi-
tions and occasionally with remarkable selectivities [1].
In order to find excellent reducing agents, considerable
efforts have been made. In this respect, recently the
reduction of a carbonyl group with transition metal
hydrido complexes has received increasing attention
since the properties of these complexes are easily ‘fine-
tuned’ by the selection of a wide variety of supporting
ligands [2].
* Corresponding author. Fax: +81 45 3393971.
1 Dedicated to Professor Nakamura on the occasion of his retire-
ment from Osaka University.
0022-328X/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00782-7

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M. Minato et al. / Journal of Organometallic Chemistry 569 (1998) 139–145
Table 1
Reduction of organic carbonyl compounds with Cp2MoH2 (1) and protonic acid
Substrate/mmol
Cp2MoH2/mmol Acid/mmol
Time/h
Product(s)
Yield/%
CH3CHO/excess
(CH3)2CO/27.0
(CH3)2CO/20.3
(CH3)2CO/26.9
(CH3)2CO/13.4
0.470
0.504
0.370
0.598
0.560
0.543
0.430
0.501
0.331
0.501
0.369
0.397
AcOH/33
AcOH/26
CH3CH2COOH/19
(CH3)2CHCOOH/30
(CH3)3CCOOH/29
HCl/4.0
TsOH/1.0
AcOH/35
AcOH/20
AcOH/26
0.5
7
20
29
108
2
CH3CH2OH
(CH3)2CHOH
(CH3)2CHOH
(CH3)2CHOH
(CH3)2CHOH
(CH3)2CHOH
(CH3)2CHOH
Cyclohexanol
167
189
176
173
104
207
176
157
152
122
(CH3)2CO/3.91
(CH3)2CO/27.1
6
Cyclohexanone/1.45
CH3COCH2CH3/0.662
CH3COC(CH3)3/29.1
CH3COOCH2CH3/0.738
CH2ꢀCHCOCH3/1.23
24
46
82
62
20
CH3CH(OH)CH2CH3
CH3CH(OH)C(CH3)3
No reaction
AcOH/23
AcOH/35
CH3COCH2CH3, CH3CH(OH)CH2CH3
56, 40
Cp2MoH2+2RCOR%2“HACp2MoA2+2RCHOHR%;
2. Results and discussion
1
HA=protonic acid
2.1. Reduction of carbonyl compounds with 1 and
protonic acids
Acetaldehyde, acetone methylethylketone, and cyclo-
hexanone were reduced by this system easily at room
temperature, while ethyl acetate was not reduced at all.
When one equivalent of acetic acid (AcOH) was used,
reduction without solvent was too slow at room tem-
perature to give reasonable yields of the alcohols. How-
ever, the use of methanol as solvent resulted in a
surprising enhancement of the rate so that high yields
of the product resulted from reactions at 50°C. It is
noteworthy that 1 reacts with two equivalents of sub-
strate, indicating two hydrido ligands were consumed
for the reaction. Bulky acids such as pivalic acid require
prolonged stirring. On the other hand, in the presence
of strong acid, such as HCl or TsOH, the reaction was
completed within 6 h in good yield and two equivalents
of acid sufficed for the reduction. The sterically hin-
dered pinacolone was hydrogenated smoothly to give
the corresponding alcohol in good yield. h,i-Unsatu-
rated ketones were found to be reduced to yield satu-
rated ketones and alcohols indicating that 1,4-reduction
takes place. We confirmed that simple unactivated alke-
nes, such as ethylene, 1-heptene, and cyclohexene could
not be reduced by this system, although 1 reacted with
allylic alcohols to give cationic cyclic k-hydroxypropyl-
molybdenum derivatives and y-allyl complexes [11].
The stereochemical behavior of this reduction system
was examined by using 4-t-butylcyclohexanone.
The molybdenum dihydride Cp2MoH2 (1) was first
synthesized by Green and co-workers [6]. It has long
been known that this complex has basic character and
is easily protonated to give cationic trihydride
[Cp2MoH3]+ [7]. However, its property has scarcely
been studied, partly because of the difficulty in its
isolation in analytically pure form. Recently we have
found that the trihydride cation can be successfully
isolated as tosylate when the hydride is protonated with
TsOH in non-aqueous solvent [8]. The trihydride com-
plex [Cp2MoH3]+OTs− was labile and was easily con-
verted to monohydridotosylato complex Cp2MoH(OTs)
(2), which behaves as a highly reactive molybdenocene
precursor, with accompanying evolution of 1 mol of H2
when warmed in ethanol. We also found that 2 can be
successfully obtained in the presence of hydrogen ac-
ceptor such as acetone, that was hydrogenated to afford
2-propanol quantitatively. This result indicates a con-
trast with that of Nakamura and Otsuka [9]. Reactivity
of 1 toward various olefins and acetylenes was investi-
gated by them in detail [10]. They showed catalytic
hydrogenation of 1,3-and 1,4-dienes, methyl acrylate,
methyl crotonate, crotonaldehyde, or mesityl oxide us-
ing 1 as catalyst at 140–150°C under 160 atm of
hydrogen without solvent. In this case, the carbonyl
groups were found to survive the reduction and 1 was
active only for the carbon–carbon double bond. Ac-
cordingly, the addition of acid alters the chemoselectiv-
ities of this reduction system. We were much interested
in reducing carbonyl group using 1 and protonic acids,
and tried the hydrogenation of several carbonyl
compounds.
Its reduction afforded cis-4-t-butylcyclohexanol (ax-
OH) as the main product when more than two equiva-
lents of acid, e.g. RCO2H (R=CF3, Me, Et, or But),
HCl, or TsOH, were used. The diastereoselectivity was
found to decrease with increasing bulk of the alkyl
group in carboxylic acids and by reducing the amount
At first we carried out the reduction in the presence
of large excess of substrates and acids at room temper-
ature. Yields were determined by GLC and were based
on the complex used. The results are shown in Table 1.

M. Minato et al. / Journal of Organometallic Chemistry 569 (1998) 139–145
141
Table 2
Reduction of 4-t-butylcyclohexanone with Cp2MoH2 (1) and protonic acid
Entry
Cp2MoH2/mmol
Acid/mmol
Time/h
Products/%
ax-OH
eq-OH
1
2
3
4
5
6
7
8
9
0.341
0.437
0.363
0.412
0.483
0.366
0.370
0.392
AcOH/17
25
51
96
30
30
17
22
72
72
24
122
111
20
99
61
138
49
128
47
29
33
18
37
43
17
17
18
79
0
CH3CH2COOH/24
(CH3)3CCOOH/20
CF3COOH/0.82
CF3COOH/0.48
HCl/0.84
HCl/0.38
TsOH/1.0
TsOH/0.40
TsOH/1.5
0.403
Cp2MoH(OTs)/1.46
10
68
the existence of an unidentified intermediate, which is
gradually increased with decreasing concentration of 2
and is decreased as 3 was formed, in step (ii) (Scheme
2). Assuming that the reaction obeys a successive
pseudo first order kinetics, we obtained the experimen-
of acid. In particular, use of one equivalent of TsOH
resulted in inversion of the diastereoselectivity from
excess of the cis-isomer (ax-OH) to excess of the trans-
isomer (eq-OH) as shown in Table 2. Similar results
were obtained when 4-t-butylcyclohexanone was re-
duced with 1 and acetic acid in methanol at 50°C.
These stereochemical studies suggest that the reaction
proceeds via two successive pathways, in which mono-
hydrido complex Cp2MoHA (2) is a key intermediate.
Thus, in step (i) in Scheme 1, the starting dihydride 1
reacts with the acid and the substrate to give 1 mol each
of alcohol and the monohydrido complex 2. In the next
step (ii), another mole of the substrate interacts with 2
to give a second mole of product together with the
disubstituted molybdenum complex 3.
The existence of these two successive pathways was
further substantiated by following 1H-NMR spec-
troscopy of the reduction of 2,2,2-trifluoroacetophe-
none with 1 and acetic acid in benzene-d6. Cp signals of
each compound (1, 2, and 3) were found to be a reliable
indication in estimating the reaction path. The signal
assignable to the Cp protons of the monohydrido 2 and
the diacetato 3 were observed at l 4.77 and 5.07 ppm,
respectively. The signal of 1 disappeared immediately
indicating step (i) is very fast and step (ii) is a rate-de-
termining stage in this reaction sequence. The intensity
of the signal of 2 increased during the first stage and
decreased as 3 was formed. The detailed study showed
tal rate constants k1(0.042 min−1) and k2(0.101 min−1
)
for step (ii) by means of the Runge–Kutta procedure.
The diastereoselectivity of this reducing system may
be rationalized by assuming that there is no stereoselec-
tivity in step (i), whereas step (ii) proceeds with remark-
able selectivity. In accordance with this assumption,
100% selectivity (cis-isomer) was achieved when 4-t-
butylcyclohexanone was reduced using the indepen-
dently prepared monohydrido complex Cp2MoH(OTs).
The isolated yield of pure cis-t-butylcyclohexanol was
68% based on the ketone. Furthermore, we observed
that the chiral monohydridocarboxylato complex, ob-
tained by the reaction of 1 with tartaric acid, reduced
prochiral ketones to give the corresponding enan-
tiomerically enriched alcohols [12].
Two pathways are possible for both steps (i) and (ii)
as shown in Scheme 3. One involves nucleophilic attack
of the metal on the carbonyl carbon, where the elec-
trophilicity is enhanced by protonation at the neighbor-
ing carbonyl oxygen atom, then the hydride migrates
from the metal to the carbon (Path A). An alternative
path involves initial protonation of the basic complex 1,
then the carbonyl group inserts into the Mo–H bond
giving an alkoxo intermediate, which decomposes with
the acid (Path B). The former pathway, which is similar
Scheme 1.
Scheme 2.

142
M. Minato et al. / Journal of Organometallic Chemistry 569 (1998) 139–145
Scheme 3.
to those proposed for ketone and aldehyde reduction
with [Mo(CO)5]−, seems to be more plausible than the
latter [13]. We observed that the rate of reduction of
2,2,2-trifluoroacetophenone was much faster than that
of acetophenone suggesting the initial nucleophilic at-
tack of the molybdenum on the positive carbonyl car-
reduced in methanol without protonic acid. In toluene
or in THF, no product was obtained in the absence of
protonic acid. Since the imines are known to be more
susceptible to the nucleophilic attack than ketones or
aldehydes, it is conceivable to consider that methanol
played a role of protonic acid in this case. As N-cyclo-
hexylaniline seems to be more reactive than ketones, we
examined the reduction of this imine in the presence of
acetophenone in our system and confirmed that the
imine was reduced selectively and acetophenone was
recovered unreacted.
bon because the trifluoromethyl is
a
powerful
electron-withdrawing group, and in 2,2,2-trifluoroace-
tophenone this attack seems to be more favorable.
2.2. Reduction of imines with 1 and protonic acids
As shown above, significant success has been
achieved in the reduction of ketones and aldehydes
through use of 1 and protonic acid system, and then we
tried the reduction of imines using this system.
At first we carried out the reduction of N-cycldo-
hexylideneaniline to N-cyclohexylaniline at room tem-
perature under various conditions in order to obtain
the optimum conditions. Three different types of sol-
vents, namely methanol, THF, and toluene were tested.
The results are shown in Table 3.
Subsequently, we carried out the reduction of several
imines and the results are shown in Table 4.
The reactions proceeded in satisfactory yields and all
solvents tested were suited to this process. The reduc-
tions were completed within 70 h at room temperature
in all cases. It was noted that, in contrast to the
reduction of ketones and aldehydes, the imine was
Table 4
Table 3
Reduction of imines with Cp2MoH2 (1)
Reduction of N-cyclohexylideneaniline with 1
Entry
Cp2MoH2/mmol Imine/mmol
Time/h
Yield/%a
Entry
Cp2MoH2/
Imine/mmol Solvent
Yield/%a
mmol
1
2
3
4
5
6
7
1.10
1.05
1.47
0.228
1.06
I/3.10
18
72
24
24
92
92
92
78
48
II/2.69
III/1.48
IV/0.48
V/2.26
VI/1.93
VII/2.03
1
2
3
4
5
0.73
0.77
0.81
0.81
0.72
1.47
0.77
1.69
1.69
1.44
Toluene
Toluene
THF
THF
Methanol
89b
0
53b
0b
81b
0
125b
107b
103b
0.972
0.993
79
a
a
Isolated yield based on the 1.
Based on the 1 used.
b AcOH (1.71 mmol) was added.
b The reduction was carried out in the presence of TsOH.

M. Minato et al. / Journal of Organometallic Chemistry 569 (1998) 139–145
143
Table 5
Reduction of N-cyclohexylidenecyclohexylamine with 1 and Lewis acid
Entry
Cp2MoH2/mmol
Imine/mmol
Lewis acid/mmol
Solvent
Temperature/°C
Time/h
Yield/%a
1
2
3
4
5
6
7
1.74
1.17
2.01
0.838
0.712
0.893
0.738
3.44
1.19
2.00
0.847
0.713
0.893
4.30
TiCl4/1.32
TiCl4/1.18
TiCl4/1.97
BF3/0.853
Ti(C3H7O)4/0.704
Ti(C2H5O)4/0.880
Yb(OTf)3/0.871
Toluene
r.t.
r.t.
40
24
24
24
24
92
92
24
0
20
22
19
7
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
40
r.t.
r.t.
50
14
90
a Isolated yield based on the 1 used.
specific coordination number and is a stable Lewis acid
in aqueous media [17]. These unique properties may be
responsible for the best result in the present system. In
our system, methanol was used as a solvent, and the
Lewis acidity of Yb(OTf)3 seems to be not influenced
by this protic media In contrast with the protonic acid
system, 1 reacts with one equivalent of the substrate
indicating only one hydrido ligand was consumed for
the reduction.
The imines, other than N-cyclohexylidenecyclohexy-
lamine (IV), were hydrogenated to the corresponding
amines. It is worth pointing out that the reduction of
the imines bearing phenyl group in their structure pro-
ceeds smoothly, although the aliphatic imine was not
reduced at all. This result indicates a contrast with that
of Ba¨ckvall and Wang [14]. Recently they reported that
ruthenium-catalyzed transfer hydrogenation of imines
and they found aliphatic imines are more reactive than
aromatic ones. As shown in Table 4, in the case of a
reducible imine, the phenyl group may or may not be
attached directly to the CꢀN moiety, suggesting that the
phenyl group would not activate the imino group with
the aid of an electronic effect. Although the mechanism
of this reduction is not known with certainty, it seems
likely that the complex 1 or the trihydrido complex
[Cp2MoH3]+ interacts with the phenyl group of the
imine at the beginning, then it attacks the CꢀN moiety.
Intuitively, if the complex and the aliphatic imine are
initially held together, the reduction will proceed. It is
well known that 1 reacts with a Lewis acid to form a
1:1 adduct which can be expected to bind further to an
imine [15]. Therefore we decided to study the reduction
in association with Lewis acids.
2.4. Reduction of aliphatic imines with 1 and Yb(OTf )3
Then we carried out the reduction of several aliphatic
imines using Cp2MoH2–Yb(OTf)3 system and the re-
sults are shown in Table 6.
The reduction proceeded smoothly in all cases. The
success in the reduction of the aliphatic imine using
Lewis acid may be the result of a favorable five-mem-
bered cyclic transition state, where the complex 1 is
effectively linked by a Lewis acid to an imine, for
hydrogen transfer as shown in Scheme 4. A similar
‘bidentate effect’ can not be expected for protonic
acids.
2.3. Reduction of N-cyclohexylidenecyclohexylamine
with 1 and Lewis acids
Several Lewis acids were tested and the results are
shown in Table 5. Expectedly, the Lewis acids were
found to be efficient in the reduction of N-cyclohexyli-
denecyclohexylamine to dicyclohexylamine. Especially,
the reaction using Yb(OTf)3 gave a high product yield
(90%). This compound has been utilized as a promising
Lewis acid in organic synthesis since Kobayashi’s pio-
neering work demonstrating Yb(OTf)3 to be a good
catalyst in the Diels–Alder reaction [16]. It has the
Scheme 4.

144
M. Minato et al. / Journal of Organometallic Chemistry 569 (1998) 139–145
Table 6
Reduction of aliphatic imines with Cp2MoH2 (1) and Yb(OTf)3
Entry
Cp2MoH2/mmol
Imine/mmol
Yb(OTf)3/mmol
Time/h
Yield/%a
1
2
3
0.791
1.11
0.678
VIII/1.71
IX/2.25
X/1.39
0.846
1.48
0.808
92
48
48
40
35
49
a
Based on the 1 used.
3. Experimental details
Cp2MoH2 (0.110 g, 0.483 mmol) and 4-t-butylcyclohex-
anone (0.156 g, 1.01 mmol) in THF (5 ml) was added
equimolar amount of CF3COOH (37 ml, 0.483 mmol).
The reaction mixture was stirred at ambient tempera-
ture for 30 h under argon. The resultant solution was
concentrated to dryness under reduced pressure and the
residue was extracted with hexane. The resulting solu-
tion was analyzed by GLC.
3.1. General
All manipulations were conducted under purified ar-
gon or nitrogen. Air-sensitive reagents and products
were handled by standard Schlenk techniques. Solvents
were dried and purified in the usual manner, and stored
under an atmosphere of argon. Commercially available
chemicals were used as such without any further purifi-
cation. Unhydrated TsOH was prepared by refluxing
the hydrate in benzene for 5 h followed by crystalliza-
tion by allowing the resulting solution to stand at
ambient temperature. Infrared spectra were determined
on a Perkin–Elmer 1600 series spectrometer. NMR
spectra were recorded on a JEOL JNMEX-270 spec-
3.1.3. Reduction of N-cyclohexylideneaniline with 1 and
protonic acid
Typical procedure is as follows. To a solution of
Cp2MoH2 (0.165 g, 0.725 mmol) and the imine (0.255 g,
1.47 mmol) in toluene (10 ml) was added equimolar
amount of acetic acid (0.103 g, 1.71 mmol). The reac-
tion mixture was stirred at ambient temperature for 34
h under argon. The resultant solution was concentrated
to dryness under reduced pressure. To this residue was
added a proper quantity of 5% sodium hydrogen car-
bonate solution and the mixture was extracted with
ether in three portions. The extracts were combined and
dried over MgSO4. The product was purified by
medium pressure column chromatography using hex-
ane–ethyl acetate (9:1).
1
trometer. H-NMR chemical shifts were referenced to
tetramethylsilane (TMS). Cp2MoH2 was synthesized by
the published method [6]. Imines were prepared from
the appropriate amine and ketone by the usual water
removal procedure (Dean Stark, or using a molecular
sieve 4A). The structure assignment of the products was
carried out by comparison of the IR and 1H-NMR
spectra with those of the authentic samples. Gas chro-
matography (GLC) for qualitative and quantitative
analyses were carried out on a Shimadzu Model GC-7A
chromatograph and GC-14A chromatograph. Yields
were determined using an internal standard.
3.1.4. Reduction of imines with 1 and protonic acid
The reductions of imines were carried out by the
same procedures as above. The detailed reaction condi-
tions and the results are compiled in Table 4.
3.1.1. Reduction of carbonyl compounds with 1 and
protonic acids
Typical procedure is as follows. To the flask contain-
ing Cp2MoH2 (1: 0.115 g, 0.504 mmol) was added
AcOH (1.5 ml, 26 mmol) and acetone (2 ml, 27 mmol)
by a trap-to-trap method. The mixture was stirred at
room temperature in vacuo for 7 h. From the system,
volatile liquids were removed under reduced pressure
and the resulting solution was analyzed by GLC. 2-
Propanol so obtained was 0.95 mmol (189%). Experi-
ments listed in Table 1 were carried out under
essentially the same conditions, and the detailed reac-
tion conditions and the results are compiled in Table 1.
3.1.5. Reduction of N-cyclohexylidenecyclohexylamine
with 1 and Lewis acids
Typical procedure is as follows. To a solution of
Cp2MoH2 (0.168 g, 0.738 mmol) and the imine (0.770 g,
4.30 mmol) in methanol (50 ml) was added equimolar
amount of Yb(OTf)3 (0.540 g, 0.871 mmol). The reac-
tion mixture was stirred at 50°C for 24 h under argon.
The resultant solution was concentrated to dryness
under reduced pressure. To this residue was added a
proper quantity of 5% sodium hydrogen carbonate
solution and the mixture was extracted with ether in
three portions. The extracts were combined and washed
with brine. The resulting solution was analyzed by
GLC.
3.1.2. Reduction of 4-t-butylcyclohexanone with 1 and
protonic acids
Typical procedure is as follows. To a solution of

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3.1.6. Reduction of aliphatic imines with 1 and
Yb(OTf )3
The reductions of imines were carried out by the
same procedures as above. The detailed reaction condi-
tions and the results are compiled in Table 6.
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Acknowledgements
This work was supported by a Grant-in-Aid for
Scientific Research (B) (No. 07455354) and by a Grant-
in-Aid for Scientific Research on Priority Area (No.
09239216) from the Ministry of Education, Science,
Sports and Culture of Japan.
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