Dehydrogenative synthesis of imines from alcohols and amines catalyzed by a ruthenium N-heterocyclic carbene complex

Article abstract of DOI:10.1021/om201095m

A new method for the direct synthesis of imines from alcohols and amines is described where hydrogen gas is liberated. The reaction is catalyzed by the ruthenium N-heterocyclic carbene complex [RuCl₂(IiPr)(p-cymene)] in the presence of the ligand DABCO and molecular sieves. The imination can be applied to a variety of primary alcohols and amines and can be combined with a subsequent addition reaction. A deuterium labeling experiment indicates that the catalytically active species is a ruthenium dihydride. The reaction is believed to proceed by initial dehydrogenation of the alcohol to the aldehyde, which stays coordinated to ruthenium. Nucleophilic attack of the amine affords the hemiaminal, which is released from ruthenium and converted into the imine.

Full text of DOI:10.1021/om201095m

Article
pubs.acs.org/Organometallics
Dehydrogenative Synthesis of Imines from Alcohols and Amines
Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex
Agnese Maggi and Robert Madsen*
Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark
S
* Supporting Information
ABSTRACT: A new method for the direct synthesis of imines
from alcohols and amines is described where hydrogen gas is
liberated. The reaction is catalyzed by the ruthenium N-heterocyclic
carbene complex [RuCl2(IiPr)(p-cymene)] in the presence of the
ligand DABCO and molecular sieves. The imination can be applied
to a variety of primary alcohols and amines and can be combined
with a subsequent addition reaction. A deuterium labeling
experiment indicates that the catalytically active species is a ruthenium dihydride. The reaction is believed to proceed by
initial dehydrogenation of the alcohol to the aldehyde, which stays coordinated to ruthenium. Nucleophilic attack of the amine
affords the hemiaminal, which is released from ruthenium and converted into the imine.
INTRODUCTION
■
The imine is an important functional group in organic
chemistry and is often used for the synthesis of amines by
various addition reactions.1 Imines are usually prepared by
condensation of an aldehyde or a ketone with a primary amine
but can also be formed by oxidation of secondary amines,2
Figure 1. Structure of ruthenium N-heterocyclic carbene complex 1.
oxidative condensation of primary amines,3 and the aza-Wittig
reaction.4 In addition, imines can be prepared by coupling of
primary alcohols and amines where hydrogen gas is liberated
alcohols and amines in the presence of an oxidant.5
(Scheme 1).
Recently, new dehydrogenative reactions have been
developed for the coupling of alcohols and amines where
hydrogen gas is liberated and no stoichiometric additives are
Scheme 1. Dehydrogenative Imine Synthesis
necessary. These procedures constitute more environmentally
benign methods for oxidative couplings and produce a
minimum of waste. Ruthenium pincer complexes have been
shown to mediate the coupling to form both amides6 and
RESULTS AND DISCUSSION
For the initial studies equimolar amounts of benzyl alcohol and
■
imines,7 depending on the structure of the ligand. An osmium
pincer complex has been shown to catalyze the formation of
tert-octylamine were selected as test substrates (Table 1). It was
quickly discovered that imine formation occurred in the
absence of potassium tert-butoxide. The reaction was
performed with 5% of complex 1 in refluxing toluene under a
flow of argon. Molecular sieves were added to secure
continuous removal of water during the reaction. Under these
conditions a 40% yield of the imine was obtained after 24 h
with 55% conversion of the alcohol (Table 1, entry 1). Only
about 3% of the ester from self-condensation of the alcohol13
was observed as a byproduct, and no secondary amine or amide
could be detected.
9
imines,8 while the heterogeneous catalysts Ag/Al2O3 and Pt/
10
TiO2 mediate the formation of amides and imines,
respectively. We have shown that ruthenium N-heterocyclic
carbene complexes can catalyze the synthesis of amides from
primary alcohols and amines with the extrusion of hydrogen
gas.11 Following our initial findings, several ruthenium N-
heterocyclic carbene and related complexes have been shown to
mediate the amidation.12 Among these, the reaction is most
efficiently performed with ruthenium complex 1 (Figure 1) in
the presence of tricyclohexylphosphine (PCy3) and potassium
tert-butoxide.12c,d
During the study of the mechanism of this reaction, we
observed that imines in some cases were formed to a significant
degree12d and we speculated whether the conditions could be
altered into a dehydrogenative imine synthesis. Herein, we
describe a new ruthenium-catalyzed synthesis of imines from
Imines are usually easy to reduce, and it is noteworthy that
the CN bond is not saturated under the reaction conditions.
To improve the conversion of the alcohol, different ligands
Received: November 8, 2011
Published: December 7, 2011
© 2011 American Chemical Society
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Article
Table 1. Optimizing Imine Formation
a
a
entry
ligand
none
amt of ligand (%)
BnOH conversn (%)
imine yield (%)
1
2
3
4
5
6
7
8
9
10
55
67
80
42
36
52
17
48
84
83
83
89
48
40
56
40
60
65
41
32
44
17
44
71
PCy3
5
5
DABCO
dppe
5
xantphos
phenanthroline
PPh3
5
5
10
10
10
10
10
10
10
10
pyridine
PCy3
DABCO
DABCO
DABCO
DABCO
DABCO
none
81
b
c
11
74
d
12
82
34
e
13
f
14
31
g
h
15
35
a
b
c
d
Determined by GC with nonane as internal standard. Without molecular sieves. 9% of secondary amine was also formed. With [RuCl2(IMe)(p-
e
f
cymene)] (5%). With [RuCl2(p-cymene)]2 (2.5%), IiPr·HCl (5%), and KOtBu (5%). With [RuCl2(p-cymene)]2 (2.5%), ItBu·HCl (5%), and
KOtBu (5%). With [RuCl2(p-cymene)]2 (2.5%). 15% of the secondary amine was also formed.
g
h
were investigated as additives. With 5% of PCy3 or 1,4-
diazabicyclo[2.2.2]octane (DABCO) the alcohol conversion
increased (Table 1, entries 2 and 3), while bidentate ligands as
well as PPh3 and pyridine gave lower conversions (entries 4−
8). A further improvement could be achieved by increasing the
amount of ligand to 10% (entries 9 and 10), and since DABCO
gave the best result, this ligand was selected for general use.
With DABCO only a trace amount (∼2%) of the secondary
amine from reduction of the imine was observed as a
byproduct. When the experiment in entry 10 was repeated in
the absence of molecular sieves, the same conversion of benzyl
alcohol was observed, but the amount of secondary amine had
increased to 9% (entry 11). Hence, the molecular sieves do not
influence the rate of the imination, but instead the selectivity is
affected by the continuous removal of water.
The importance of the ruthenium complex was also
investigated. First, the isopropyl wingtips in 1 were replaced
by methyl groups and the reaction performed with the complex
[RuCl2(IMe)(p-cymene)] (Table 1, entry 12). This gave a
slightly higher conversion than with 1, but the product imine
was obtained together with 6% of the corresponding secondary
amine. Due to the lower selectivity, the methyl-substituted
complex [RuCl2(IMe)(p-cymene)] does not constitute a better
precatalyst than the isopropyl-substituted 1. In our amidation
reaction it was possible to generate the ruthenium N-
heterocyclic carbene complex in situ from [RuCl2(p-cymene)]2,
1,3-diisopropylimidazolium chloride (IiPr·HCl), and potassium
tert-butoxide.11 This was also attempted for the imination
reaction, but a significantly lower conversion was observed as
compared to the experiment with complex 1 (entries 10 and
13). The use of the more sterically hindered 1,3-di-tert-
butylimidazol-2-ylidene as the carbene ligand gave even lower
conversion (entry 14). The importance of the carbene ligand
was confirmed by performing the reaction in the absence of this
ligand, where a significant amount of the corresponding
secondary amine was formed as a byproduct (entry 15).
Thus, for general use complex 1 in the presence of DABCO
and molecular sieves presents the optimum catalyst system for
the imination. The formation of hydrogen during the
transformation was confirmed by collecting the argon−
hydrogen gas mixture from the reaction and using it for
hydrogenating an alkyne in a separate flask.
With the optimized catalyst system in place, our attention
then turned to other alcohols and amines in order to investigate
the scope of the imination. First, different alcohols were studied
in the reaction with tert-octylamine (Table 2). Para-substituted
benzyl alcohols with methyl, methoxy, and fluoro substituents
participated well in the imine formation, and only trace
amounts of the corresponding secondary amines were detected
in these reactions (entries 1−4). The methoxy group could also
be tolerated in the ortho position without affecting the yield of
the imine (entry 5). Notably, a small amount of anisole was
observed in entries 3 and 5, which presumably arises from
decarbonylation of the intermediate aldehyde.14 A methyl ester
in the para position gave a slightly lower yield, and with this
substrate the reaction was accompanied by significant
formation of both the secondary amine and the secondary
amide (entry 6). p-Chloro- and p-bromobenzyl alcohol were
poor substrates due to considerable dehalogenation as a side
reaction (results not shown).
o-Hydroxybenzyl alcohol was also an inferior substrate since
the product was obtained as a 1:1 mixture of the desired imine
and the corresponding secondary amine (Table 2, entry 7). p-
Nitrobenzyl alcohol gave the imine in moderate yield due to
competing reduction of the nitro group (entry 8). Hex-5-enyl
alcohol was converted into the imine with complete reduction
of the olefin (entry 9). In this reaction the imine was the only
product detected and the moderate yield is presumably due to
the poor stability of alkylimines toward purification by flash
column chromatography.
In addition to tert-octylamine other primary amines were also
investigated as substrates, and in this case the reaction was
performed with benzyl alcohol (Table 3). Cyclohexylamine
gave the imine in 60% isolated yield and with only a trace
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Table 2. Imination of Alcohols with tert-Octylamine
Table 3. Imination of Amines with Benzyl Alcohol
a
b
Determined by GC with nonane as internal standard. Isolated yield.
c
d
10% of secondary amine was also formed. 7% of secondary amine
e
f
was also formed. Reacted for 48 h. 5% of secondary amine was also
formed. Reacted for 52 h.
g
a
b
Determined by GC with nonane as internal standard. Isolated yield.
c
d
The imination reaction provides access to a variety of imines
which may be used directly in a subsequent addition reaction.
This was illustrated with the enantiomerically pure imine from
Table 3, entry 3, which has previously been reacted with a
variety of nucleophiles.1b After the imine was formed from
benzyl alcohol and (R)-1-phenylethylamine, the solvent was
replaced with THF or Et2O followed by addition of an
allylating agent (Scheme 2). With allylzinc bromide the
3% of anisole was also formed. Performed in mesitylene at 163 °C
e
with PCy3 instead of DABCO. 14% of secondary amine, 17% of
f
amide, and 3% of methyl benzoate were also formed. 37% of
secondary amine was also formed. 15% of N-(p-aminobenzylidene)-
g
tert-octylamine was also formed.
amount of the secondary amine (entry 1). 1-Adamantylamine
afforded the product in 70% yield together with 10% of the
secondary amine (entry 2). Optically pure (R)-1-phenylethyl-
amine and (R)-1-(1-naphthyl)ethylamine gave the correspond-
ing imines without any sign of racemization (entries 3 and 4).
The more hindered benzhydrylamine reacted very slowly and
only gave 40% yield after 2 days (entry 5). Further steric
hindrance inhibited the imination almost completely, as seen
with tritylamine, where only a trace amount of the imine was
observed together with benzyl benzoate from self-condensation
of the alcohol (entry 6). These experiments indicate that the
amine has to attack the ruthenium complex in order for the
imination to proceed.15 Aniline reacted very sluggishly with
benzyl alcohol and only gave the imine in low yield together
with several byproducts (result not shown). Reacting benzyl
alcohol with complex 1 in refluxing toluene in the absence of an
amine gave about 10% of benzaldehyde after 2 h, as judged by
GC-MS analysis, and this did not change upon prolonged
treatment, where small amounts of benzyl benzoate were also
observed.
Scheme 2. Sequential Imination and Allylation
addition product was obtained in 61% overall yield from
benzyl alcohol, but with almost no diastereoselectivity. With the
more hindered B-allyl-9-BBN the product was isolated in 53%
yield and with a diastereomeric ratio of 9:1.16
To obtain more information about the mechanism of the
imination, two experiments with deuterium-labeled benzyl
alcohol were performed. First, benzyl alcohol-α,α-d2 was
reacted with tert-octylamine under the standard conditions
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(Scheme 3). Interestingly, the product imine was obtained as a
1.4:1 mixture of the deuterium-labeled imine and the
Scheme 4. Proposed Mechanism for Imination
Scheme 3. Imination with Benzyl Alcohol-α,α-d2
nonlabeled product, as shown by GC-MS analysis. This result
was observed in both toluene and toluene-d8 and is not a result
of a deuterium−hydrogen exchange with the solvent. A control
experiment showed that the scrambling occurred during the
imination reaction and not by a competing transformation from
the imine. When the product H-imine from Table 2, entry 1,
was treated with PhCD2OH under the same conditions as in
Scheme 3, no deuterium incorporation in the imine was
observed.
When the imination in Scheme 3 was monitored by 1H NMR
in toluene-d8, deuterium−hydrogen scrambling in the starting
alcohol was observed already after 5 h. Reisolating the alcohol
at this time showed that it consisted of PhCD2OH (56%),
PhCDHOH (34%), and PhCH2OH (10%). This indicates that
the initial β-hydride elimination to form benzaldehyde is a
reversible reaction and, more importantly, the catalytically
active ruthenium species is a dihydride. The same observation
was made in our very recent dehydrogenative ester synthesis
from primary alcohols with complex 1.13
6. Recently, Crabtree and Eisenstein performed a computa-
tional study on a similar hemiaminal bonded to a ruthenium(II)
hydride in order to determine whether the amide or the imine
would be formed.20 They showed that the amide is formed after
hydrogen transfer to hydride, while imine formation requires
hydrogen transfer to oxygen.20 Under the more acidic
conditions of the imination, hydrogen transfer to oxygen may
be more facile, e.g. through an outer-sphere proton transfer,
which would afford complex 7 and then the imine after
decomplexation of the hemiaminal. In this way, the fate of the
intermediate hemiaminal determines whether the amide or the
imine is formed in the coupling. The scrambling observed in
Scheme 3 can be explained by the observation that ruthenium
dihydride complexes are able to scramble hydrogen and
deuterium when exposed to hydrogen/deuterium gas.21 In
combination with a reversible β-hydride elimination, this
provides a route by which O−H or N−H hydrogens can be
scrambled into the α positions of the alcohol.
The influence of the β-hydride elimination was further
probed by measuring the primary kinetic isotope effect. The
initial rate was determined both with PhCH2OH/tert-octyl-
NH2 and with PhCD2OD/tert-octyl-ND2, which gave a kinetic
isotope effect (kH/kD) of 1.1
0.3. This negligible value
indicates that β-hydride elimination from the alcohol is not the
rate-determining step in the imination mechanism.
On the basis of these experiments and our previous studies
on the amidation,12d we propose the imination mechanism in
Scheme 4. It has previously been shown by NMR that the p-
cymene ligand in complexes such as 1 is lost upon reflux in
toluene.17 This is followed by replacement of the two chloride
ligands with hydride through substitution with the alcohol,
release of hydrogen chloride, and β-hydride elimination. The
formation of ruthenium hydrides in this way has been
established for other ruthenium(II) chloride complexes.18 By
this catalyst initiation small amounts of the aldehyde will be
formed and converted into the imine after reaction with the
amine. The catalytically active species is believed to be
dihydride 2, which coordinates the alcohol to afford complex
3. Hydrogen gas is then liberated by hydrogen transfer to
hydride, as demonstrated earlier.19 This gives rise to alkoxide
complex 4, which is then converted into aldehyde complex 5 by
β-hydride elimination. It is possible that the aldehyde is
released from 5 and imine formation then occurs with the
amine in solution. However, since the imination is sensitive to
the steric demands of the amine, it seems more reasonable that
the amine attacks the coordinated aldehyde to afford
hemiaminal 6 (as the zwitterion protonated at nitrogen).
This is, so far, fairly similar to the mechanism proposed for
the amidation with complex 1.12d The major difference,
however, is the lack of a strong base in the imination, and
the more acidic environment may affect the stability of complex
In summary, we have presented a new method for the direct
synthesis of imines from primary alcohols and amines in which
water and hydrogen gas are formed as the only byproducts. The
reaction is catalyzed by the ruthenium N-heterocyclic carbene
complex 1, which is easy to handle and straightforward to
prepare. A mechanism is proposed with a ruthenium dihydride
species as the catalytically active component.
EXPERIMENTAL SECTION
■
General Information. Toluene was distilled from sodium and
benzophenone under an argon atmosphere. Column chromatography
was performed on silica gel 60 (0.035−0.070 mm) saturated with
Et3N. NMR chemical shifts were measured with TMS or the residual
solvent signal in CDCl3 (δH 7.26 ppm, δC 77.0 ppm) as internal
reference.
General Procedure for Imination. Ruthenium complex 112d
(22.9 mg, 0.05 mmol), DABCO (11.2 mg, 0.1 mmol), and 4 Å
molecular sieves (150 mg) were placed in an oven-dried Schlenk flask
equipped with a cold finger. Vacuum was applied, and the flask was
then filled with argon (repeated twice). Toluene (1 mL), alcohol (1
mmol), amine (1 mmol), and nonane (0.2 mmol as internal standard)
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were added by syringe, and the mixture was refluxed with stirring
under a flow of argon for 24 h. The reaction mixture was cooled to
room temperature and the solvent removed in vacuo. The residue was
purified by silica gel column chromatography (hexane/Et2O 10/0 →
9/1 with 2% Et3N) to afford the imine.
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N-(4-Methylbenzylidene)-tert-octylamine (Table 2, Entry 2): 1H
NMR (300 MHz, CDCl3) δ 8.21 (s, 1H), 7.64 (d, 2H, J = 8.1 Hz),
7.21 (d, 2H, J = 8.1 Hz), 2.38 (s, 3H), 1.69 (s, 2H), 1.32 (s, 6H), 0.96
(s, 9H); 13C NMR (75 MHz, CDCl3) δ 154.2, 140.0, 134.8, 129.1,
127.8, 60.8, 56.6, 32.0, 31.7, 29.6, 21.4; HRMS m/z calcd for C16H26N
232.2021 [M + H]+, found 232.2059.
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N-(4-Methoxybenzylidene)-tert-octylamine (Table 2, Entry 3): 1H
NMR (300 MHz, CDCl3) δ 8.18 (s, 1H), 7.70 (d, 2H, J = 8.7 Hz),
6.93 (d, 2H, J = 8.7 Hz), 3.84 (s, 3H), 1.68 (s, 2H), 1.32 (s, 6H), 0.96
(s, 9H); 13C NMR (75 MHz, CDCl3) δ 161.0, 153.6, 130.4, 129.3,
113.8, 60.6, 56.6, 55.3, 32.0, 31.8, 29.7; HRMS m/z calcd for
C16H26NO 248.1970 [M + H]+ found 248.2010.
N-(4-Fluorobenzylidene)-tert-octylamine (Table 2, Entry 4): 1H
NMR (300 MHz, CDCl3) δ 8.21 (s, 1H), 7.76−7.70 (m, 2H), 7.10
(bt, 2H), 1.69 (s, 2H), 1.32 (s, 6H), 0.96 (s, 9H); 13C NMR (75 MHz,
CDCl3) δ 163.8 (d, JC−F = 248.0 Hz), 152.9, 133.6 (d, JC−F = 2.85 Hz),
129.6 (d, JC−F = 8.4 Hz), 115.5 (d, JC−F = 21.6 Hz), 60.9, 56.5, 32.0,
31.7, 29.6; HRMS m/z calcd for C15H23FN 236.1770 [M + H]+, found
236.1809.
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N-(2-Methoxybenzylidene)-tert-octylamine (Table 2, Entry 5): 1H
NMR (300 MHz, CDCl3) δ 8.67 (s, 1H), 7.96 (bd, 1H), 7.35 (bt,
1H), 6.98 (bt, 1H), 6.91 (bd, 1H), 3.87 (s, 3H), 1.70 (s, 2H), 1.33 (s,
6H), 0.96 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 158.6, 150.4, 131.0,
127.0, 125.8, 120.8, 110.8, 61.3, 56.6, 55.4, 32.0, 31.8, 29.8; HRMS m/
z calcd for C16H26NO 248.1970 [M + H]+, found 248.2008.
N-(4-Carbomethoxybenzylidene)-tert-octylamine (Table 2, Entry
6): 1H NMR (300 MHz, CDCl3) δ 8.27 (s, 1H), 8.07 (d, 2H, J = 8.1
Hz), 7.80 (d, 2H, J = 8.1 Hz), 3.93 (s, 3H), 1.70 (s, 2H), 1.33 (s, 6H),
0.95 (s, 9H); 13C NMR (75 MHz, CDCl3) δ 166.8, 153.5, 141.2,
131.1, 129.7, 127.7, 61.5, 56.5, 52.2, 32.0, 31.7, 29.5; HRMS m/z calcd
for C17H26NO2 276.1919 [M + H]+, found 276.1960.
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Umani-Ronchi, A. J. Chem. Soc., Perkin Trans. 1 1996, 875−882.
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N-(4-Nitrobenzylidene)-tert-octylamine (Table 2, Entry 8): 1H
NMR (300 MHz, CDCl3) δ 8.30 (s, 1H), 8.26 (d, 2H, J = 8.7 Hz),
7.91 (d, 2H, J = 8.7 Hz), 1.71 (s, 2H), 1.34 (s, 6H), 0.94 (s, 9H); 13C
NMR (75 MHz, CDCl3) δ 152.3, 142.7, 128.5, 123.8, 61.9, 56.5, 32.0,
31.7, 29.5; HRMS m/z calcd for C15H23N2O 263.1715 [M + H]+,
found 263.1753.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text and figures giving experimental procedures, character-
1
ization data, and H and 13C NMR spectra. This material is
(18) (a) Solari, E.; Gauthier, S.; Scopelliti, R.; Severin, K.
Organometallics 2009, 28, 4519−4526. (b) Aranyos, A.; Csjernyik,
available free of charge via the Internet at http://pubs.acs.org.
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G.; Szabo, K. J.; Backvall, J.-E. Chem. Commun. 1999, 351−352.
̈
AUTHOR INFORMATION
(c) Mizushima, E.; Yamaguchi, M.; Yamagishi, T. Chem. Lett. 1997,
237−238.
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Corresponding Author
* E-mail: rm@kemi.dtu.dk.
(19) Chatwin, S. L.; Davidson, M. G.; Doherty, C.; Donald, S. M.;
Jazzar, R. F. R.; Macgregor, S. A.; McIntyre, G. J.; Mahon, M. F.;
Whittlesey, M. K. Organometallics 2006, 25, 99−110.
(20) Nova, A.; Balcells, D.; Schley, N. D.; Dobereiner, G. E.;
Crabtree, R. H.; Eisenstein, O. Organometallics 2010, 29, 6548−6558.
ACKNOWLEDGMENTS
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We thank the Danish Council for Independent Research−
Technology and Production Sciences for financial support.
(21) Burling, S.; Kociok-Kohn, G.; Mahon, M. F.; Whittlesey, M. K.;
̈
Williams, J. M. J. Organometallics 2005, 24, 5868−5878.
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