Ruthenium-catalyzed transfer hydrogenation of imines by propan-2-ol in benzene

Article abstract of DOI:10.1002/1521-3765(20020703)8:13<2955::AID-CHEM2955>3.0.CO;2-Q

Transfer hydrogenation of a variety of different imines to the corresponding amines by propan-2-ol in benzene catalyzed by [Ru₂(CO)₄(μ H)(C₄Ph₄COHOCC₄Ph₄)] (1) has been studied. The reaction is highly efficient with turnover frequencies of over 800 per hour, and the product amines were obtained in excellent yields. A remarkable concentration dependence of propan-2-ol was observed when the reaction was run in benzene as cosolvent. An optimum was obtained at 24 equivalents of propan-2-ol to imine, and further increase of the propan-2-ol led to a dramatic decrease in rate. Also the use of polar cosolvents with 24 equivalents of propan-2-ol gave a low rate. It was found that ketimines react faster than aldimines and that electron-donating substituents on the imine increase the rate of the catalytic transfer hydrogenation. Electron-withdrawing substituents decreased the rate. An isomerization was observed with imines having an α-hydrogen at the N-alkyl substituent, which is in accordance with a mechanism involving a ruthenium-amine intermediate. It was demonstrated that the ruthenium-amine complex from α-methylbenzylamine, corresponding to the postulated intermediate, can replace 1 as catalyst in the transfer hydrogenation of imines. A primary deuterium isotope effect of k_CH/CD = 2.7 ± 0.25 was observed when 2-deuterio-propan-2-ol vas used in place of propan-2-ol in the ransfer hydrogenation of N-phenyl-(1-phenylethylidene)amine.

Full text of DOI:10.1002/1521-3765(20020703)8:13<2955::AID-CHEM2955>3.0.CO;2-Q

FULL PAPER
Ruthenium-Catalyzed Transfer Hydrogenation of Imines by Propan-2-ol
in Benzene
Joseph S. M. Samec and Jan-E. B‰ckvall*[a]
Abstract: Transfer hydrogenation of a
variety of different imines to the corre-
sponding amines by propan-2-ol in ben-
zene catalyzed by [Ru2(CO)4(m-
H)(C4Ph4COHOCC4Ph4)] (1) has been
studied. The reaction is highly efficient
with turnover frequencies of over 800
per hour, and the product amines were
obtained in excellent yields. A remark-
able concentration dependence of prop-
an-2-ol was observed when the reaction
was run in benzene as cosolvent. An
optimum was obtained at 24 equivalents
of propan-2-ol to imine, and further
increase of the propan-2-ol led to a
dramatic decrease in rate. Also the use
of polar cosolvents with 24 equivalents
of propan-2-ol gave a low rate. It was
found that ketimines react faster than
aldimines and that electron-donating
substituents on the imine increase the
rate of the catalytic transfer hydrogena-
tion. Electron-withdrawing substituents
decreased the rate. An isomerization
was observed with imines having an a-
hydrogen at the N-alkyl substituent,
which is in accordance with a mecha-
nism involving a ruthenium amine in-
termediate. It was demonstrated that the
ruthenium amine complex from a-
methylbenzylamine, corresponding to
the postulated intermediate, can replace
1 as catalyst in the transfer hydrogena-
tion of imines. A primary deuterium
isotope effect of kCH/CD ˆ 2.7 Æ 0.25 was
observed when 2-deuterio-propan-2-ol
was used in place of propan-2-ol in the
transfer hydrogenation of N-phenyl-(1-
phenylethylidene)amine.
Keywords: homogeneous catalysis ¥
hydrogen transfer¥ imines ¥ reaction
mechanisms
tion of imines using [RuCl2(PPh3)3], propan-2-ol, and K2CO3
as base nearly a decade ago[4c] Noyori et al. reported the first
asymmetric transfer hydrogenation of imines by formic acid
using a chiral ruthenium catalyst with excellent yields and
enantioselectivities.[5a] More recently, Baker and Mao report-
ed transfer hydrogenation of imines by formic acid using
chiral Noyori ligands on a rhodium catalyst.[5b] No asymmetric
transfer hydrogenation of imines by propan-2-ol has so far
been reported.
Transfer hydrogenation with propan-2-ol is operationally
very simple since the latter can be used as solvent or
cosolvent. Furthermore, propan-2-ol is a solvent that does
not affect the pHand therefore it is preferred over formic acid
as hydrogen donor. The lack of efficient procedures for
transfer hydrogenation of imines by propan-2-ol motivated
further studies of this reaction.
Introduction
Transfer hydrogenation of polar functional groups has sig-
nificantly contributed to the recent growth in organic syn-
thesis.[1] The reaction is recommended for its simplicity since
no hydrogen gas is required. When propan-2-ol is used as
hydrogen donor, the only side product formed is acetone,
which is easily removed by distillation during workup. While
the transfer hydrogenation reaction of ketones[1, 2, 3] has been
widely explored during the last two decades the correspond-
ing reaction of imines[1, 4, 5] has been less studied (Scheme 1).
The latter transformation is of importance for the synthesis
of pharmaceuticals and agrochemicals and therefore further
studies on transfer hydrogenation of imines seemed highly
desirable. After our group reported the transfer hydrogena-
We have recently been involved in the use of catalyst 1 in
hydrogen transfer reactions.[6] Catalyst 1 promoted efficient
Oppenauer-type oxidation of alcohols[6a,b] and it was also used
as an efficient racemization catalyst in enzymatic dynamic
kinetic resolution of alcohols.[6c,d] Dimeric catalyst 1 is also
known to efficiently disproportionate aldehydes to esters.[7] In
solution, catalyst 1 dissociates into species 2 and 3,[7] where the
former can hydrogenate and the latter can dehydrogenate
(Scheme 2). Casey et. al[4a] recently studied the stoichiometric
hydrogenation of carbonyls and imines by the 18 eÀ species 2,
Scheme 1. The transfer hydrogenation of imines.
[a] Prof. Dr. J.-E. B‰ckvall, J. S. M. Samec
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 10691 Stockholm (Sweden)
Fax : (‡46)8-154908
E-mail: jeb@organ.su.se
Chem. Eur. J. 2002, 8, No. 13
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J.-E. B‰ckvall and J. S. M. Samec
FULL PAPER
THF)[4a] it was of interest to study the effect of an added
cosolvent in the transfer hydrogenation of imines by propan-
2-ol. A preliminary screening indicated that benzene was an
efficient cosolvent. To find the optimal ratio between propan-
2-ol and cosolvent we studied the rate of transfer hydro-
genation of 4, with 1 as catalyst, as a function of the
concentration of propan-2-ol in benzene [Eq. (2), Figure 1].
Scheme 2. The dissociation of the dimeric catalyst precursor.
formed from dimeric 1. From results on kinetic isotope effects
for the reaction of benzaldehyde, a concerted mechanism for
À
À
the addition of the two hydrogens (C OHand Ru H; see
Scheme 2) of 2 to carbonyls was proposed.
In the present work we have studied the catalytic transfer
hydrogenation of imines by employing ruthenium catalyst 1.
Species 2 hydrogenates the imine to the desired amine, and
species 3 dehydrogenates the propan-2-ol to acetone. In this
process complexes 2 and 3 are interconverted (Scheme 3).[7]
A
remarkable concentration dependence of propan-2-ol was
observed, and a highly efficient transfer hydrogenation was
obtained by using 24 equivalents of propan-2-ol (to imine) in
benzene. A mechanism involving a ruthenium amine inter-
mediate is proposed.
Figure 1. Transfer hydrogenation of 4 according to Equation (2).
Surprisingly, the results show that there is a maximum of the
rate, measured as in terms of the initial turnover frequency
(TOF), at about 24 equivalents of propan-2-ol (Figure 1),
which corresponds to a benzene:propan-2-ol ratio of 1.7:1.
The concentration dependence is most likely due to two
effects: at low concentrations of propan-2-ol there is a large
increase in rate with an increased concentration. This effect
levels out at higher concentrations of propan-2-ol and the
negative solvent effect by propan-2-ol compared to benzene
will dominate, leading to a lower rate.[9]
The effect of propan-2-ol encouraged us to screen different
solvents using the optimal concentration of propan-2-ol, that
is, 24 equivalents. The initial TOF as well as the conversion to
amine after 90 min was measured. Toluene gave almost the
same rate as benzene and both of these solvents gave full
conversion to amine within 90 min (Table 1, entries 5 and 6).
Using more polar systems, such as THF,[4a, 7] tert-butanol, or
propan-2-ol decreased the TOF and did not give full con-
versions after 90 min. For tert-butyl alcohol and THF this may
be explained by a coordination of the solvent to 3, which has
been previously suggested by Shvo and Menashe in hydrogen
transfer to aldehydes.[7] However, for propan-2-ol this should
not lead to a rate deceleration since propan-2-ol is expected to
coordinate to the ruthenium center prior to dehydrogenation.
Casey et al. have previously reported the effect of water
using 2 in the transfer hydrogenation of benzaldehyde.[4a]
Therefore, we screened the influence of water on the catalytic
activity. We observed a rate acceleration upon addition of
Scheme 3.
Results and Discussion
Synthesis of starting materials: The imines were prepared
from the corresponding ketone and the appropriate amine.
The reaction was carried out in the presence of molecular
sieves with NaHCO3 as base [Eq. (1)]. The reactions proceed
smoothly and in most cases the imines were isolated in good
yields.
Transfer hydrogenation of imines
Effect of solvent: Transfer hydrogenations are usually carried
out in a mixture of triethylamine and formic acid or in propan-
2-ol without any additional solvent.[1a, 8] Since Casey et al.
observed a dramatic solvent effect in the stoichiometric
reaction of 2 with benzaldehyde (toluene 50 times faster than
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Ru-Catalyzed Transfer Hydrogenation
2955 2961
Table 1. The screening of different solvents.[a]
Table 2. The transfer hydrogenation of different imines to the corresponding
amine.
Entry
Solvent
TOF[b]
% Conversion[c]
Entry Imine
Amine
C:S ratio Time
[min][a]
Yield[b]
1
2
3
4
5
6
THF
140
190
250
440
650
700
72
97
97
100
100
100
tert-butyl alcohol
propan-2-ol
cyclohexane
toluene
1
1:330
1:330
90
97
benzene
[a] These reactions were performed by using the catalyst 1 (1 mmol, 0.33%)
stirred for 8 min in the solvents above (0.95 mL) at 708C. The imine 4
(0.6 mmol) dissolved in propan-2-ol (0.55 mL, 7.2 mmol, 24 equiv) pre-
heated at 738C was then added. The reactions were monitored by 1HNMR
spectroscopy. [b] The TOF was measured during the first 10 min (mol
substrate  conversion after 10 min  mol catalystÀ1  timeÀ1 (h)). The
TOF based on Ru will be half of the values given. [c] Conversion after
90 min.
2
3
60
45
95
97
1:330
4
5
1:330
1:100
45
96
95
small amounts of water (20% rate acceleration with 0.8% of
water). Above 1% of water content the results are incon-
sistent and difficult to interpret.
480
Catalytic procedure and substituent effects: The optimized
conditions from above with 24 equivalents of wet propan-2-ol
in benzene (i.e. propan-2-ol:benzene:H2O ˆ 1:1.7:0.02) were
employed for the transfer hydrogenation of various imines. In
most cases the reaction proceeds within a few hours at 708C
by using 0.3 1 mol% of catalyst, to give excellent yields (93
98%). For example, imines 4 7 (Table 2, entries 1 4)
undergo transfer hydrogenation by propan-2-ol within 0.75
1.5 h by employing 0.3 mol% of 1 to give 95 97% of the
corresponding amine.
6
7
1:330
1:330
240
300
94
98
8
9
1:200
1:200
360
240
97
98
Some variations of the substrates were made to elucidate
the mechanism of the reaction. Our model substrate was N-
phenyl-(1-phenylethylidene)amine 4, which is a ketimine. In
our report from 1992[4c] one of the conclusions was that
aldimines react faster than ketimines, which has been
confirmed by Yamagishi et al.[10] This is also the normal order
of reactivity for reduction of imines with hydride reagents.[11]
However, with catalyst 1 the results indicate the reversed
reactivity (Table 2, entries 1 and 7), that is ketimines react
faster than aldimines. This order of reactivity is unusual and in
contrast to that for hydrogenation of imines.[9a] Placing an
ethyl group instead of a methyl group on the imine-carbon
atom further increased the rate (Table 2, entry 2). To examine
if this was an electronic effect the phenyl ring was substituted
with electron-donating and electron-withdrawing groups in
the para position. Both p-methoxy and p-methyl groups
increased the rate (Table 2, entries 3 and 4, respectively)
whereas a p-fluoro substituent decreased the rate (Table 2,
entry 5). A more quantitative comparison of the electronic
properties of the substrates is given in Table 3 in which the
rate is measured as the initial TOF after 10 min.
10
11
12
1:200
1:200
1:100
360
360
240
94
94
93
13
1:100
240
94[c]
[a] The reactions were carried out using imine (1.0 mmol), benzene (3.15 mL),
propan-2-ol (24 mmol, 1.85 mL) and catalyst 1 (0.003 0.01 mmol) at 708C. Full
conversion was measured by 1HNMR spectroscopy. [b] Yield of isolated
product. [c] In this case yield was determined by GC.
relative rate for electron-rich imines is also consistent with the
concerted mechanism,[4a] in which proton transfer from 2 to
the imine is crucial for promoting hydride transfer to
ruthenium. Substituting the phenyl group by a naphthyl group
(Table 2, entry 9) decreased the reaction rate. This is also
consistent with the discussion above in which the naphthyl
group is more electron-withdrawing than the phenyl group
thus creating an electron-poor imine. Substituting the aniline
component with a benzyl group (Table 2 entries 11 and 12)
It is expected that an electron-rich imine would coordinate
better to ruthenium. The increased reaction rate observed
with electron-donating groups on the imine may therefore
reflect that coordination of imine to ruthenium comes into the
rate expression. This would also account for the increased
reactivity of ketimines versus aldimines in which the methyl
and even more the ethyl group on the ketimine are better
electron donors than the aldimine-hydrogen atom. The faster
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J.-E. B‰ckvall and J. S. M. Samec
FULL PAPER
Table 3. Influence of electronic properties of the substrate.[a]
be to prepare a ruthenium amine complex and test it as
catalyst.[13] Attempts to synthesize the ruthenium complex
with amines 17 or 27 by employing the procedure described
for primary amines[12] were unsuccessful. When raising the
temperature to 1008C using amine 27 we observed the
formation of imines 14 and 15. This suggests that a ruthe-
nium amine complex is formed but that it easily undergoes
b-elimination.
Amine complexes with primary amines, analogous to 29
have been previously reported.[12] To get more information
concerning possible intermediate amine complexes we there-
fore synthesized the primary amine complex 30 from 1-phe-
nylethylamime [Eq. (3)]. It was found that the amine complex
Imine
X
Y
TOF[b]
6
7
5
4
10
8
MeO
Me
HEt
HMe
H3H00
F
Me
Me
730
700
840
800
Me
120[c]
[a] The reactions were run using imine (0.3 mmol), benzene (0.95 mL), and
propan-2-ol (7.2 mmol, 0.55 mL) with 0.3 mol% of 1 at 708C. [b] TOF was
1
measured after 10 min by HNMR spectroscopy and based on catalyst 1.
The TOF based on Ru will be half of the values given. [c] 1 mol% of 1 was
used.
decreased the rate. This effect may be due to the observed
isomerization between 14 and 15 (vide infra). The catalyst 1 is
even effective in catalyzing the transfer hydrogenation of
imine 16. Substituting the aromatic group with an aliphatic
group decreased the rate (Table 2, entries 6, 8, and 10). The
lower rate for these imines may be due to the fact that the
aliphatic groups are less efficient in stabilizing a carbonium
ion at the imine-carbon atom.
30 worked as a catalyst for the transfer hydrogenation of 4
with propan-2-ol to give amine 17 (Scheme 5). The reaction
rate was lower (40% conversion after 90 min using 1 mol% of
29 compared to 100% conversion after 90 min using
0.3 mol% of 1), which is likely due to the slower b-elimination
form the primary amine.[14]
Isomerization: In the transfer hydrogenation of 14 (and 15)
with 1 as catalyst, we observed an isomerization of 14 to 15
(Table 2, entries 11 and 12). The imine 15 appears to be more
stable than 14. When starting from 14 there is a fast
conversion to 27 with concomitant isomerization to 15. The
latter imine reacts slower in transfer hydrogenation. When
starting from 15 the conversion to 27 is slower and the
isomerization to 14 occurs at a much lower rate than that of
isomerization of 14 to15 (Scheme 4). This isomerization is
likely to proceed through a ruthenium amine intermedi-
ate.[12]
Scheme 5.
Deuterium isotope effect: The deuterium isotope effect for the
catalytic hydrogen transfer reaction of imine 4 was measured
by the use of 2-deuteriopropan-2-ol. The initial rate was
measured (between 200 and 600 s) for both the deuterated
and the nondeuterated propan-2-ol in the transfer hydro-
genation of imine 4 to amine 17. The initial rate using
nondeuterated
propan-2-ol
was
kobs ˆ (6.6 Æ 0.4) Â
10À8 molsÀ1 and for the 2-deuteriopropan-2-ol was kobs
ˆ
(2.4 Æ 0.3) Â 10À8 molsÀ1. The deuterium isotope effect of the
reaction was therefore kH/D ˆ (2.7 Æ 0.25).
Mechanistic considerations: Two different mechanisms have
been proposed for the transfer hydrogenation of aldehydes
and ketones with 1 as catalyst.[4a, 7] In the first mechanism
proposed by Shvo and Menashe[7] the carbonyl compound is
coordinated to ruthenium in 2 followed by migratory
insertion. This mechanism was supported by the isolation of
the corresponding amine complex.[12] The second mechanism,
proposed recently by Casey et al.,[4a] involves a concerted
addition of the hydride and the acidic proton from catalyst 2
to the carbonyl compound without prior coordination of the
Scheme 4. The isomerization of imine 14 and 15.
Ruthenium amine complexes as catalysts: Attempts to isolate
a ruthenium amine intermediate such as 29 from the
reaction mixture or to observe it by NMR spectroscopy
during the reaction have so far been unsuccessful. Another
approach to investigate if the transfer hydrogenation of
imines proceeds via a ruthenium amine intermediate would
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Ru-Catalyzed Transfer Hydrogenation
2955 2961
used for flash chromatography. Unless otherwise noted, all materials were
obtained from commercial suppliers and used without further purification.
All reactions were performed under a dry argon atmosphere in oven-dried
(1408C) glassware. Distillations were conducted in a B¸chi Glass oven
B-580.
latter. This mechanism was supported by the fact that
selective deuteration of the hydroxy and the hydride positions
of 2 gave individual isotope effects in agreement with the
combined isotope effect observed by deuteration of 2 in both
positions.
It is expected that an electron-rich imine would coordinate
better to ruthenium. The increased reaction rate observed
with electron-rich imines is consistent with a coordination of
the imine to ruthenium. In the concerted mechanism pro-
posed by Casey et al.[4a] the protonation would be expected to
be faster with an electron-rich imine. However, the hydride
addition should be slower with an electron-rich imine and
therefore the effect is expected to cancel to some extent in the
concerted mechanism.
In the transfer hydrogenation of 14 (and 15) with 1 as
catalyst, we observed an isomerization between 14 and 15
(Table 2, entries 11 and 12). This shows that the transfer
hydrogenation is reversible, that is the catalyst 1 can
b-eliminate a ruthenium amine intermediate leading to the
formation of the imine. This resembles the mechanism
proposed for transfer hydrogenation of carbonyls[1, 15] involv-
ing coordination of the substrate to the catalyst and migratory
insertion. Isomerization between 14 and 15 has also been
observed by Brune et al.[4e] with the Wilkinson catalyst and
more recently by Yamagishi et al.[4b] with the active catalyst
[RuH2(PPh3)4].[16] The transfer hydrogenation of imine 4 using
ruthenium amine complex 30 instead of 1 as catalyst
(Scheme 5) also supports that the reaction proceeds via a
ruthenium amine intermediate.
[Ru2(CO)4(m-H)(C4Ph4COHOCC4Ph4)] (1): The title compound was
synthesized according to the literature.[17]
General procedure for the synthesis of imines: N-phenyl-(1-phenylethyl-
idene)amine (4): In a typical experiment NaHCO3 (4.2 g, 50 mmol), aniline
(0.91 mL, 10 mmol), acetophenone (1.17 mL, 10 mmol), activated molec-
ular sieves (7 g; 4 ä), and benzene (4 mL) were added to a Schlenk-tube
and the mixture was exposed to an argon atmosphere and heated to reflux
overnight. The reaction mixture was filtered through Celite, the Celite
washed with CH2Cl2, the filtrate was collected, and the solvents were
evaporated in vacuo. The starting material was removed by distillation
(1208C, 1 mbar) and the product was subsequently distilled (1608C,
1 mbar). Yield (1.52 g, 78%) of yellow crystals. 1HNMR (300 MHz,
CDCl3, 258C, TMS): d ˆ 7.95 8.00 (m, 2H), 7.41 7.49 (m, 3H), 7.32 7.39
(m, 2H), 7.06 7.12 (m, 1H), 6.77 6.83 (m, 2H), 2.24 (s, 3H); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d ˆ 165.6, 151.9, 139.7 130.8, 129.2, 128.6,
127.4, 123.4, 119.6, 17.6; IR (CDCl3): nÄ ˆ 3058, 3027,1638, 1593, 1482, 1447,
1288, 1214 cmÀ1; elemental analysis calcd (%) for C14H13N (195.3): C 86.12,
H6.71, N 7.17; found: C 85.96, H6.63, N 7.30.
N-Phenyl-(1-phenylpropylidene)amine (5): Prepared according to the
general procedure. Yield (1.59 g, 76%) of yellow crystals as a 10:1 mixture
of geometric isomers. 1HNMR (300 MHz, CDCl 3, 258C, TMS): major
isomer: d ˆ 7.91 7.95 (m, 2H), 7.43 7.48 (m, 3H), 7.31 7.37 (m, 2H),
7.04 7.11 (m, 1H), 6.77 6.81 (m, 2H), 2.66 (q, J ˆ 7.7 Hz, 2H), 1.08 (t, J ˆ
7.7 Hz, 3H); minor isomer: d ˆ 2.80 (q, J ˆ 7.4 Hz, 2H), 1.23 (t, J ˆ 7.4 Hz,
3H), aromatic resonances are obscured by the major isomer; 13C NMR
(75 MHz, CDCl3, 258C, TMS): d ˆ 170.9, 151.8, 138.2, 130.5, 129.1, 128.7,
127.8, 123.2, 119.3, 23.7, 13.1. According to the 1HNMR spectrum there was
4% of the corresponding enamine in the product mixture. 1HNMR
(300 MHz, CDCl3, 258C, TMS): d ˆ 8.34 (s, 1H), 5.59 (q, J ˆ 7.0 Hz, 1H),
1.79 (d, J ˆ 7.0 Hz, 3H), aromatic resonances are obscured by the major
isomer; IR (CDCl3): nÄ ˆ 3058, 3027, 2977, 1633, 1593, 1485, 1210 cmÀ1
elemental analysis calcd (%) for C15H15N (209.3): C 85.67, H7.19, N 7.14;
found: C 85.83, H7.26, N 6.56.
;
Conclusion
N-Phenyl-[1-(4-methoxyphenyl)ethylidene]amine (6): Prepared according
to the general procedure. Yield (1.8 g, 79%) of white crystals. 1HNMR
(300 MHz, CDCl3, 258C, TMS): d ˆ 7.95 (d, J ˆ 9.0 Hz, 2H ), 7.31 7.37 (m,
2H), 7.04 7.09 (m, 1H), 6.94 (d, J ˆ 9.0 Hz, 2H), 6.77 6.80 (m, 2H); 3.87
(s, 3H), 2.20 (s, 3H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d ˆ 164.7,
161.8, 152.1, 132.5, 129.1, 129.1, 123.2, 119.8, 113.8, 55.6, 17.4; IR (CDCl3):
nÄ ˆ 3058, 2971, 1636, 1604 1364 cmÀ1; elemental analysis calcd (%) for
C15H15NO (225.1): C 79.97, H6.71, N 6.22; found: C 79.81. H6.95, N 6.40.
The transfer hydrogenation of imines using catalyst 1 and
propan-2-ol shows an interesting solvent effect, where polar
solvents decrease the rate. Less polar solvents with 24
equivalents of propan-2-ol to imine showed a higher rate
and the best solvent system (of those studied) was benzene
(benzene/propan-2-ol ˆ 1.7:1). The substrate itself had a
significant influence on the process. Ketimines react faster
than aldimines. Electron-donating groups increase the rate
while electron-withdrawing groups decrease the rate. An
isomerization was observed between 14 and 15, suggesting
that the process is reversible proceeding through a rutheni-
um amine intermediate. Involvement of a ruthenium
amine intermediate is supported by the fact that the isolated
ruthenium amine complex 30 acts as a catalyst in transfer
hydrogenation of imines.
N-Phenyl-[1-(4-methylphenyl)ethylidene]amine (7): Prepared according
to the general procedure. Yield (1.1 g, 35%) of yellow crystals. Spectral
data were in accordance with those previously reported.[18]
N-Phenyl-[1-(4-fluorophenyl)ethylidene]amine (8): Prepared according to
the general procedure. Yield (1.61 g, 76%) of white-yellow crystals.
1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ 7.96 8.00 (m, 2H), 7.33
7.37 (m, 2H), 7.07 7.14 (m, 3H), 6.78 6.80 (m, 2H), 2.22 (s, 3H); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d ˆ 164.5 (d, JC,F ˆ 251 Hz), 164.4, 151.7
135.9, 129.5 (d, JC,F ˆ 8.6 Hz), 129.2, 123.6, 119.6, 115.5 (d, JC,F ˆ 21.5 Hz),
17.5; IR (CDCl3): nÄ ˆ 3065, 3049, 1636, 1591, 1507, 1486, 1221, 1164 cmÀ1
.
N-Phenyl-(1,2-dimethylpropylidene)amine (9): Prepared according to the
general procedure. Yield (0.63 g, 39%) of a brown oil as a 7:1: mixture of
geometric isomers. 1HNMR (400 MHz, CDCl 3, 258C, TMS): major
isomer: d ˆ 7.25 7.30 (m, 2H), 7.00 7.04 (m, 1H), 6.66 6.68 (m, 2H),
2.62 (heptet, J ˆ 7.0, 1H), 1.73 (s, 3H), 1.20 (d, J ˆ 7.0 Hz, 6H); minor
isomer: d ˆ 2.69 2.76 (m, 1H), 2.07 (s, 3H), 1.03 (d, J ˆ 7.0 Hz, 6H),
aromatic resonances are obscured by those of the major isomer. 13C NMR
(100 MHz, CDCl3, 258C, TMS): major isomer: d ˆ 176.3, 152.0, 129.0,
123.0, 119.5, 39.4, 20.1, 17.2; minor isomer: d ˆ 129.0, 122.9, 119.4, 31.9, 20.3;
Experimental Section
General methods: H(400 or 300 MHz) and 13C (100 or 75 MHz) spectra
1
were recorded on a Varian Mercury spectrometer. Chemical shifts (d) are
reported in ppm, using residual solvent as internal standard, and coupling
constants (J) are given in Hz. IR spectra were obtained by using a Perkin-
Elmer 1600 FT-IR instrument, and the samples were examined as CDCl3
solutions on NaBr plates. Only the strongest/structurally most important
peaks (cmÀ1) are listed. Elemental analyses were performed by Analytische
Laboratorien, Lindlar, Germany. Merck silica gel 60 (240 400 mesh) was
IR (CDCl3): nÄ ˆ 3060, 2966, 1661, 1594, 1484 cmÀ1
.
N-Phenyl-(1-phenylmethylidene)amine (10): Prepared according to the
general procedure. Yield (3.6 g, 99%) of white crystals. Spectral data were
in accordance with those previously reported.[19]
Chem. Eur. J. 2002, 8, No. 13
¹ WILEY-VCHVerlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0813-2959 $ 20.00+.50/0
2959

J.-E. B‰ckvall and J. S. M. Samec
FULL PAPER
N-Phenyl-[1-(cyclohexyl)ethylidene]amine (11): Prepared according to the
general procedure. Yield (3.1 g, 77%) of a yellow oil as an 11:1 mixture of
geometric isomers. 1HNMR (400 MHz, CDCl 3, 258C, TMS): major
isomer: d ˆ 7.25 7.30 (m, 2H), 6.99 7.04 (m, 1H), 6.65 6.69 (m, 2H),
1.86 1.98 (m, 4H), 1.73 (s, 3H), 1.33 1.72 (m, 7H); minor isomer: d ˆ
2.24 2.29 (m, 4H), 2.09 (s, 3H), All other resonances are obscured by
those of the major isomer. 13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ
175.7, 152.0, 129.0, 122.9, 119.5, 49.6, 30.4, 26.3, 26.3, 17.8; IR (CDCl3): nÄ ˆ
N-phenyl-1-phenylpropylamine (18): The general procedure was followed
by using imine 5 (0.209 g, 1.0 mmol) and catalyst 1 (3.26 mg, 3.0 mmol). The
product was distilled (2208 C, 1 mbar) to afford 0.2 g (97%) of amine 18.
1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ 7.30 7.37 (m, 4H), 7.21 7.25
(m, 1H), 7.07 7.11 (m, 2H), 6.62 6.66 (m, 1H), 6.51 6.54 (m, 2H), 4.24 (t,
J ˆ 6.8 Hz, 1H), 4.07 (bs, 1H), 1.85 (m, 2H), 0.97 (t, J ˆ 6.8 Hz, 3H);
13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ 147.7, 144.1, 129.2, 128.7,
127.0, 126.7, 117.3, 113.4, 59.9, 31.8, 11.0; IR (CDCl3): nÄ ˆ 3412, 3052, 2965,
1603, 1505, 1317 cmÀ1; elemental analysis calcd (%) for C15H17N (211.3): C
85.26, H8.11, N 6.63; found C 85.12, H7.99, N 6.48.
3019, 2929, 2952, 1659, 1595, 1484, 1448, 1166 cmÀ1
.
N-Phenyl-[1-(2-naphthyl)ethylidene]amine (12): Prepared according to
the general procedure. Yield (2.0 g, 80%) of yellow crystals. 1HNMR
(400 MHz, CDCl3, 258C, TMS): d ˆ 8.35 (s, 1H), 8.23 (m, 1H), 7.86 7.96
(m, 3H), 7.50 7.58 (m, 2H), 7.35 7.41 (m, 2H), 7.11 (m, 1H), 6.85 (m, 2H),
2.36 (s, 3H); 13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ 165.5, 152.0,
137.1, 134.6, 133.1, 129.2, 129.1, 128.2, 127.9, 127.8, 127.4, 126.6, 124.4, 123.5,
119.6, 17.6; IR (CDCl3): nÄ ˆ 3430, 1626, 1483, 1446, 1369 cmÀ1; elemental
analysis calcd (%) for C18H15N (245.3): C 88.13, H6.61, N 5.71; found: C
87.96, H6.43 N 5.90.
N-phenyl-1-(4-methoxyphenyl)ethylamine (19): The general procedure
was followed by using imine 6 (0.222 g, 1.0 mmol) and catalyst 1 (3.26 mg,
3.0 mmol). The product was purified by distillation (2508 C, 1 mbar) to
1
afford amine 19 (0.22 g; 98%). HNMR (400 MHz, CDCl 3, 258C, TMS):
d ˆ 77.27 7.29 (m, 2H), 7.07 7.11 (m, 2H), 6.84 6.87 (m, 2H), 6.62 6.66
(m, 1H), 6.50 6.53 (m, 2H), 4.45 (q, J ˆ 6 Hz, 1H), 3.98 (bs, 1H), 3.78 (s,
13
3H), 1.49 (d, J ˆ 6 H z, 3H ); C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ
158.7, 147.5, 137.4, 129.3, 127.1, 117.3, 114.2, 113.5, 55.4, 53.0, 25.2; IR
(CDCl3): nÄ ˆ 3407, 3051, 2963, 1603, 1509, 1244 cmÀ1
.
N-Phenyl-(1-methylhexylidene)amine (13): Prepared according to the
general procedure. Yield (1.55 g, 80%) of a yellow oil as a 3:1: mixture
of geometric isomers. 1HNMR (400 MHz, CDCl 3, 258C, TMS): major
isomer: d ˆ 7.25 7.30 (m, 2H), 7.00 7.04 (m, 1H), 6.67 6.70 (m, 2H),
2.38 2.42 (m, 2H), 1.77 (s, 3H), 1.66 1.69 (m, 2H), 1.35 1.40 (m, 4H)
0.91 0.94 (m, 3H); minor isomer: 2.15 (s, 3H), 2.09 2.13 (m, 2H), 1.44
1.51 (m, 2H), 1.14 1.24 (m, 4H), 0.83 (t, J ˆ 7.0, 3H), aromatic resonances
are obscured by those of the major isomer. 13C NMR (100 MHz, CDCl3,
258C, TMS): major isomer: d ˆ 172.4, 151.8, 129.5, 123.1, 119.7, 41.9, 31.8,
26.3, 22.7, 19.6, 14.2; minor isomer: 129.0, 123.0, 115.3, 34.2, 31.7, 26.8, 26.1,
22.4, 14.0; IR (CDCl3): nÄ ˆ 3310, 2955, 2929, 1662, 1595, 1484, 1365,
N-phenyl-1-(4-methylphenyl)ethylamine (20): The general procedure was
followed by using imine 7 (0.209 g, 1.0 mmol) and catalyst 1 (3.26 mg,
3.0 mmol). The product was purified by distillation (2208C, 1 mbar) to
afford amine 20 (0.20 g (96%)). 1HNMR (400 MHz, CDCl 3, 258C, TMS):
d ˆ 7.24 7.27 (m, 2H), 7.07 7.13 (m, 4H), 6.61 6.66 (m, 1H), 6.50 6.53
(m, 2H), 4.46 (q, J ˆ 6.6 Hz, 1H), 3.99 (bs, 1H), 2.32 (s, 3H), 1.50 (d, J ˆ
6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ 147.6, 142.4,
136.6, 129.5, 129.3, 125.9, 117.3, 113.5, 53.3, 25.2, 21.2; IR (CDCl3): nÄ ˆ 3410,
3052, 2967, 1603, 1504, 1318 cmÀ1; elemental analysis calcd (%) for C15H17N
(211.3): C 85.26, H8.11, N 6.63; found: C 85.11, H8.09, N 6.67.
1242 cmÀ1
.
N-phenyl-1-(4-fluorophenyl)ethylamine (21): The general procedure was
followed by using imine 8 (0.213 g, 1.0 mmol) and catalyst 1 (10.85 mg,
10.0 mmol). The product was purified by distillation (2208C, 1 mbar) to
afford amine 20 (0.202 g; 95%). 1HNMR (400 MHz, CDCl 3, 258C, TMS):
d ˆ 7.32 7.37 (m, 2H), 7.08 7.14 (m, 2H), 6.98 7.09 (m, 2H), 6.65 6.70
(m, 1H), 6.48 6.53 (m, 2H), 4.48 (q, J ˆ 6.6 Hz, 1H), 4.01 (bs, 1H), 1.51 (d,
J ˆ 6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ 162.0 (d,
N-benzyl-(1-phenylethylidene)amine (14): Prepared according to the
general procedure. Yield (2.5 g, 60%) of white crystals as a 14:1: mixture
of geometric isomers. 1HNMR (400 MHz, CDCl 3, 258C, TMS): major
isomer: d ˆ 7.88 7.90 (m, 2H), 7.27 7.46 (m, 9H), 4.46 (s, 2H), 2.35 (s,
3H); minor isomer: d ˆ 4.44 (s, 2H), 2.40 (s, 3H), aromatic resonances are
obscured by those of the major isomer. 13C NMR (100 MHz, CDCl3, 258C,
TMS): d ˆ 166.2, 141.3, 140.8, 129.8, 128.6, 128.5, 127.9, 127.0, 126.8, 55.9,
JC,F ˆ 244), 147.3, 141.1, 129.4, 127.5 (d, JC,F ˆ 7.6 Hz), 117.7, 115.65 (d, JC,F
21.4 Hz), 113.6, 53.1, 25.41; IR (CDCl3): nÄ ˆ 3410, 3051, 2968, 1603, 1507,
1318, 1220 cmÀ1
ˆ
16.1; IR (CDCl3): nÄ ˆ 3084, 3026, 1633, 1494, 1446, 1280 cmÀ1
.
.
N-(1-phenylethyl)benzylidenamine (15): Prepared according to the general
procedure. Yield (3.1 g, 98.7%) of colorless oil. Spectral data were in
accordance with those previously reported.[20]
N-phenyl-(3-methyl-2-butyl)amine (22): The general procedure was fol-
lowed by using imine 9 (0.156 g, 0.97 mmol) and catalyst 1 (3.26 mg,
3.0 mmol). The product was purified by distillation (2008C, 1 mbar) to
afford amine 22 (0.147 g; 90%). 1HNMR (400 MHz, CDCl 3, 258C, TMS):
d ˆ 7.15 7.19 (m, 2H), 6.58 6.68 (m, 3H) 3.48 (bs, 1H), 3.36 (m, 1H), 1.85
(m, 1H), 1.11 (d, J ˆ 7.3 Hz, 3H), 0.98 (d, J ˆ 7.1 Hz, 3H), 0.92 (d, J ˆ
7.3 Hz, 3 H) ; 13C NMR (100 MHz, CDCl3, 258C, TMS): d ˆ 148.0, 129.4,
116.8, 113.2, 53.6, 32.4, 19.4, 17.7, 16.8; IR (CDCl3): nÄ ˆ 3407, 3052, 2961,
1602, 1505, 1320 cmÀ1; elemental analysis calcd (%) for C11H17N (163.2): C
80.93, H10.50. N 8.58; found: C 80.96, H10.42, N 8.75.
N-phenyl-(diphenylmethylidene)amine (16): Prepared according to the
general procedure. Yield (3.0 g, 58%) of yellow crystals. 1HNMR
(300 MHz, CDCl3, 258C, TMS): d ˆ 7.73 7.77 (m, 2H), 7.37 7.50 (m,
3H), 7.24 7.27 (m, 3H), 7.10 7.17 (m, 4H), 6.89 6.94 (m, 1H), 6.70 6.74
(m, 2H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d ˆ 130.9, 129.7, 129.5,
128.7, 128.6, 128.3, 128.0, 123.3, 121.1; IR (CDCl3): nÄ ˆ 3058, 3027, 1638,
1593, 1482, 1447, 1289, 1214 cmÀ1; elemental analysis calcd (%) for C19H15N
(257.3): C 88.68, H5.88, N 5.44; found: C 88.80, H5.89, N 5.34.
N-(phenyl)benzylamine (23): The general procedure was followed by using
imine 10 (0.181 g, 1.0 mmol) and catalyst 1 (3.26 mg, 3 mmol). The product
was purified by distillation (2208C, 1 mbar) to afford amine 23 (0.198 g;
94%). 1HNMR (300 MHz, CDCl 3, 258C, TMS): d ˆ 7.28 7.42 (m, 5H),
7.15 7.23 (m, 2H), 6.70 6.76 (m, 1H), 6.63 6.68 (m, 2H), 4.35 (s, 2H),
4.03 (bs, 1H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d ˆ 148.3, 139.6,
129.4, 128.8, 127.7, 127.4, 117.8, 113.0, 48.1; IR (CDCl3): nÄ ˆ 3419, 3052, 3035,
1602, 1506, 1324 cmÀ1; elemental analysis calcd (%) for C13H13N (183.3): C
85.21, H7.15, N 7.64; found: C 85.09, H6.96, N 7.63.
Screening the influence of propan-2-ol: In these reactions imine 4 (58.6 mg,
0.3 mmol), catalyst (0.98 mg, 0.9 mmol, 0.33%), and benzene were added to
a reaction flask. The reaction flask was heated at 708C for 5 min. The
equivalents of propan-2-ol were varied (1 64 equivalents), preheated, and
added to the reaction flask. The total volume was held constant at 1.5 mL.
1
The reactions were analyzed by HNMR spectroscopy after 10 min. TOF
was measured after 10 min (mol substrate*conversion after 10 min*mol
catalystÀ1*timeÀ1 (h)).
Screening solvents: The catalyst (1 mg, 0.9 mmol, 0.33%) was dissolved in
the solvents (0.95 mL) reported in Table 1 and stirred for 8 min at 708C. A
solution of imine 4 (58.6 mg, 0.3 mmol) dissolved in propan-2-ol (0.55 mL,
7.2 mmol) heated at 738C for 5 min was added to the catalyst dissolved in
N-phenyl-1-cyclohexylethylamine (24): The general procedure was fol-
lowed by using imine 11 (0.201 g, 0.96 mmol) and catalyst 1 (5.5 mg,
5 mmol). The product was purified by distillation (2208C, 1 mbar) to afford
amine 24 (0.190 g; 93%). 1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ
7.13 7.17 (m, 2H), 6.62 6.66 (m, 1H), 6.56 6.58 (m, 2H), 3.48 (bs, 1H),
4.03 (quintet, J ˆ 6.3 Hz, 1H), 1.66 1.82 (m, 5H), 1.44 1.47 (m, 1H),
1.02 1.27 (m, 5H), 1.11 (d, J ˆ 6.3 Hz, 3H), 13C NMR (75 MHz, CDCl3,
258C, TMS): d ˆ 148.1, 129.4, 116.7, 113.1, 53.1, 43.2, 30.0, 28.6, 26.8, 26.7,
1
the solvent. The reaction was monitored by HNMR spectroscopy.
Transfer hydrogenation of imines: N-phenyl-1-phenylethylamine (17): In a
typical experiment the imine 4 (0.195 g, 1.0 mmol) catalyst 1 (3.26 mg,
3.0 mmol), benzene (3.15 mL), and propan-2-ol (1.84 mL, 24.0 mmol) were
added to a 10 mL round-bottomed flask. After 90 min the solvents were
evaporated in vacuo. The product was distilled (2208C, 1 mbar) to afford
amine 17 (191 mg; 97%). Spectral data were in accordance with those
previously reported.[21]
26.5, 17.6; IR (CDCl3): nÄ ˆ 3407, 3051, 2924, 2851, 1601, 1506, 1319 cmÀ1
.
N-phenyl-1-(2-naphtyl)ethylamine (25): The general procedure was fol-
lowed by using imine 12 (0.24 g, 0.98 mmol) and catalyst 1 (5.5 mg, 5 mmol).
The product was purified by distillation (2508C, 1 mbar) to afford amine 25
2960
¹ WILEY-VCHVerlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0813-2960 $ 20.00+.50/0
Chem. Eur. J. 2002, 8, No. 13

Ru-Catalyzed Transfer Hydrogenation
2955 2961
(0.231 g; 98%). 1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ 7.82 7.86
(m, 4H), 7.46 7.55 (m, 3H), 7.10 7.14 (m, 2H), 6.58 6.70 (m, 3H), 4.68 (q,
J ˆ 6.6 Hz, 1H), 4.16 (bs, 1H), 1.62 (d, J ˆ 6.6 Hz, 3H); 13C NMR
(100 MHz, CDCl3, 258C, TMS): d ˆ 147.5, 142.9, 133.8, 132.9, 129.3,
128.6, 128.0, 127.8, 126.2, 125.7, 124.6, 124.4, 117.5, 113.6, 53.9, 25.2; IR
P. M. Maitlis, J. Organomet. Chem. 1984, 269; j) H. T. Nishiguchi, K.
Fukuzumi, J. Org. Chem. 1976, 41, 665; k) Y. Sassons, J. Blum, J. Org.
Chem. 1975, 40, 887; l) Y. Sassons, G. L. Rempel, Tetrahedron lett.
1974, 4133.
[3] a) D. A. Alonso, P. Brandt, S. J. M. Nordin, P. G. Andersson, J. Am.
Chem. Soc. 1999, 121, 9580; b) T. Sammakia, E. L. Stangeland, J. Org.
Chem. 1997, 62, 6104; c) T. Langer, G. Helmchen, Tetrahedron Lett.
1996, 37, 1381; d) S. Hashiguchi, A. Fujii, J. Takehera, T. Ikariya, S.
Hashiguchi, A. Fujii, J. Takehera, T. Ikariya, R. Noyori, J. Am. Chem.
(CDCl3): nÄ ˆ 3412, 3052, 2967, 1602, 1505, 1319 cmÀ1
.
N-phenyl-1-methylhexylamine (26): The general procedure was followed
by using imine 13 (0.189 g, 1.0 mmol) and catalyst 1 (5.5 mg, 5.0 mmol). The
product was purified by distillation (1828C, 1 mbar) to afford amine 26
(0.180 g; 94%). 1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ 7.13 7.18
(m, 2H), 6.64 6.68 (m, 1H), 6.56 6.59 (m, 2H), 3.45 (m, 2H), 1.26 1.58
(m, 9H), 1.17 (d, J ˆ 6.2 Hz, 3H), ), 0.89 (t, J ˆ 7.1 Hz, 3H), 13C NMR
(100 MHz, CDCl3, 258C, TMS): d ˆ 147.9, 129.4, 116.9, 113.2, 48.6, 37.4,
32.1, 26.0, 22.8, 21.0, 14.2; IR (CDCl3): nÄ ˆ 3403, 3052, 2958, 2929, 1602,
1505, 1319 cmÀ1; elemental analysis calcd (%) for C13H21N (191.3): C 81.61,
H11.06, N 7.32; found: C 81.49, H11.15, N 7.36.
√
Soc. 1995, 117, 7562; d) J. P. Genet, V.Ratovelomanana-Vidal, C.
Pinel, Synlett, 1993, 478.
[4] a) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana, J.
Am. Chem. Soc. 2001, 123, 1090; b) E. Mizushima, M. Yamaguchi, T.
Yamagishi, Chem. Lett. 1997, 237; c) G.-Z. Wang, J.-E. B‰ckvall, J.
Chem. Soc. Chem. Commun. 1992, 980; d) S. Bhaduri, N. Sapre, K.
Sharma, P. G. Jones, G. Carpenter, J. Chem. Soc. Dalton Trans. 1990,
1305; e) H. A. Brune, J. Unsin, R. Hemmer, M. Reichhardt, J.
Organomet. Chem. 1989, 369, 335; f) Y. Watanabe, Y.Tsuji, H. Ige, Y.
Ohsugi, T. Ohta, J. Org. Chem. 1984, 49, 3359; g) F. Martinelli, G.
Mestroni, A. Camus, G. Zassinovich, J. Organomet. Chem. 1981, 220,
383; h) R. Grigg, T. R. B. Mitchell, N. Tongpenyai, Synthesis 1981, 442.
[5] a) N. Uematsu, A. Fujii, S. Hashiguchi, T. Ikariya, R. Noyori, J. Am.
Chem. Soc. 1996, 118, 4916; b) J. Mao, D. C. Baker, Org. Lett. 1999, 6,
841.
[6] a) G.-Z. Wang, U. Andreasson, J.-E. B‰ckvall, Chem. Commun. 1994.
1037; b) M. L. S. Almeida, M. Beller, G.-Z. Wang, J.-E. B‰ckvall,
Chem. Eur. J. 1996, 2, 1533; c) A. L. E. Larsson, B. A. Persson, J.-E.
B‰ckvall, Angew. Chem. 1997, 109, 1256; Angew. Chem. Int. Ed. Engl.
1997, 36, 1211; d) B. A. Persson, A. L. E. Larsson, M. L. Ray, J.-E.
B‰ckvall, J. Am. Chem. Soc. 1999, 121, 1645.
N-benzyl-1-phenylethylamine (27): The general procedure was followed by
using imine 14 (0.209 g, 1.0 mmol) and catalyst 1 (5.5 mg, 5 mmol). The
product was purified by distillation (2508C, 1 mbar) to afford amine 27
(0.198 g; 94%). 1HNMR (400 MHz, CDCl 3, 258C, TMS): d ˆ 7.24 7.38
(m, 10H), 3.82 (q, J ˆ 6.6 Hz, 1H), 3.67, 3.60 (AB, J ˆ 13.2 Hz, 2H), 1.57
(bs, 1H), 1.37 (d, J ˆ 6.6 Hz, 3H); 13C NMR (100 MHz, CDCl3, 258C,
TMS): d ˆ 145.8, 140.9, 128.7, 128.6, 128.4, 127.2, 127.1, 126.9, 57.7, 51.9, 24.7;
IR (CDCl3): nÄ ˆ 3325, 3025, 2962, 1602, 1584, 1505, 1493, 1452, 1304 cmÀ1
elemental analysis calcd (%) for C15H17N (211.3): C 85.26, H8.11, N 6.63;
found: C 85.09, H8.20, N 6.63.
;
N-benzyl-1-phenylethylamine (27): The general procedure was followed by
using imine 15 (0.209 g, 1.0 mmol) and catalyst 1 (10.85 mg, 10 mmol). The
product was purified by distillation (2508C, 1 mbar) to afford amine 27
(0.198 g; 94%). See above for spectral data.
[7] N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885.
N-phenyl-1-benzhydrylamine (28): The imine 16 (77.2 mg, 0.3 mmol),
catalyst 1 (3.26 mg, 3 mmol), propan-2-ol (0.55 mL, 7.2 mmol), and benzene
(0.95 mL) were stirred at 708C. The reaction was monitored by GC, and
spectral data for NMR spectroscopy were in accordance with those
previously reported.[22]
[8] G. Zassinovich, G. Mestroni, S. Gladiali, Chem. Rev. 1992, 92, 1051.
¡
[9] a) B. R. James, Catal. Today 1997, 37, 209; b) D. Beaupere, L. Nadjo,
R. Uzan, P. Bauer, J. Mol. Catal. 1983, 18, 73; c) D. Beaupere, L.
Nadjo, R. Uzan, P. Bauer, J. Mol. Catal. 1982, 14, 129.
[10] E. Miizushima, M. Yamaguchi, T. J. Yamagishi, Mol. Catal. A. 1999,
148, 69.
[11] K. Harada, in Patai The Chemistry of the Carbon-Nitrogen Double
Bond, pp. 276 293.
Dicarbonyl(tetraphenylcyclopentadienone)(phenylethylamine)ruthenium
(30): The complex was synthesized according to a literature procedure.[13]
[12] M. Abed, I. Goldberg, Z. Stein, Y. Shvo, Organometallics 1988, 7, 2054.
[13] Ruthenium amine complexes have been isolated by Shvo, see refs
[12] and [7].
Acknowledgement
[14] Secondary amines were found to isomerize faster than primary
amines: N. Hermanns, unpublished results from these laboratories.
[15] a) O. Blum, D. Milstein, J. Am. Chem. Soc. 1995, 117, 4582; b) S.
Murahashi, T. Naota, Y. Oda, N. Hirai, Synlett, 1995, 733; c) G.-Z.
Wang, U. Andreasson, J.-E. B‰ckvall, J. Chem. Soc. Chem. Commun.
1994, 1037; d) U. Karlsson, G.-Z. Wang, J.-E. B‰ckvall, J. Org. Chem.
1994, 59, 5850; e) S. Murahashi, T. Naota, Synthesis, 1993, 4, 433; f) D.
Morton, D. J. Cole-Hamilton, J. Chem. Soc. Chem. Commun. 1988,
1154; g) S. Murahashi, T. Naota, K. Ito, Y. Maeda, H. Taki, J. Org.
Chem. 1987, 52, 4319; h) S.-I. Murahashi, T. Naota, N. Nakajima,
Tetrahedron Lett. 1985, 7, 925.
[16] J. E. B‰ckvall, J. Organometal. Chem., in press.
[17] F. F. Huerta, Y. R. Santosh Laxmi, J.-E. B‰ckvall, Org. Lett. 2000, 3,
1037.
[18] P. Milart, J. Cioslowski, Synthesis 1984, 4, 328.
[19] J. R. Hwu, W. N. Tseng, H. V. Patel, F. F. Wong, D.-N. Horng, B. R.
Liaw, L. C. Lin, J. Org. Chem. 1999, 64, 2211.
This work was supported by grants from the Swedish Natural Science
Research Council and the Swedish Foundation for Strategic Research.
Discussions with Prof. Brian R. James is gratefully acknowledged. We
¡
thank Drs Claudia Brasse, Fernando F. Huerta, and Oscar Pamies for
helpful suggestions during this work.
[1] a) S. Gladiali, G. Mestroni in Transition Metal Organic Synthesis, Vol.
2 (Eds.: M. Beller, C. Bolm,) Wiley-VCHWeinheim, 1998, p. 97; b) J.-
E. B‰ckvall, R. L. Chowdhury, U. Karlsson, G.-Z. Wang in Perspec-
tives in Coordination Chemistry (Eds.: A. F. Williams, C. Floriani,
A. E. Merbach), Verlag Helvetica Chimica Acta, Basel, 1992, p. 463;
c) M. J. Palmer, M. Wills, Tetrahedron: Asymmetry 1999, 10, 2045.
[2] a) Y. R. Santosh Laxmi, J.-E. B‰ckvall, Chem. Commun. 2000, 611;
b) A. Aranyos, G. Csjernyik, K. Szabo, J.-E. B‰ckvall, Chem.
Commun. 1999, 4, 351; c) G.-Z. Wang, J.-E. B‰ckvall, J. Chem. Soc.
Chem. Commun. 1992, 337; d) R. L. Chowdhury, J.-E. B‰ckvall, J.
Chem. Soc. Chem. Commun. 1991, 1063; e) D. E. Linn, J. Halpern, J.
Organomet. Chem. 1987, 330, 155; f) D. E. Linn, J. Halpern, J. Am.
Chem. Soc. 1987, 109, 2969; g) W. A. Fordyce, R. Wilczynski, J.
Halpern, J. Organomet. Chem. 1985, 296, 115; h) T. A. Smith, P. M.
Maitlis, J. Imai, J. Organomet. Chem. 1985, 289, 385; i) T. A. Smith,
[20] F. Texier-Boullet, Synthesis 1985, 6/7, 679 681.
[21] T. Kawakami, T. Sugimoto, I. Shibata, A. Baba, H. Matsuda, N.
Sonoda, J. Org. Chem. 1995, 60, 2677.
[22] A. R. Katritzky, M. Qi, J. Org. Chem. 1997 62, 4116.
Received: January 30, 2002 [F3838]
Chem. Eur. J. 2002, 8, No. 13
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