Efficient and highly chemoselective direct reductive animation of aldehydes using the system silane/oxorhenium complexes

Article abstract of DOI:10.1002/adsc.201000246

This work reports a novel method for direct reductive amination of aldehydes with silanes catalyzed by several high-valent oxorhenium(V) and oxorhenium (VII) complexes. The catalytic system PhSiH₃/ReIO ₂(PPh₃)₂ (2.5mol%) was very efficient for the synthesis of secondary amines and highly chemoselective, tolerating a wide range of functional groups such as -NO₂, -CF₃, -SO ₂R, -CO₂R, -F, -Cl, -Br, -I, -CN, -OH, -OCH₃, -SCH₃, NCOR, and double bonds. This novel method was also employed in the synthesis of tertiary amines with moderate yields.

Full text of DOI:10.1002/adsc.201000246

FULL PAPERS
DOI: 10.1002/adsc.201000246
Efficient and Highly Chemoselective Direct Reductive Amination
of Aldehydes using the System Silane/Oxorhenium Complexes
Sara C. A. Sousaa and Ana C. Fernandesa,*
a
Centro de Quꢀmica Estrutural, Complexo I, Instituto Superior Tꢁcnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Fax : (+351)-21-846-4457; phone: (+351)-21-841-9264; e-mail: anacristinafernandes@ist.utl.pt
Received: March 29, 2010; Published online: September 8, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000246.
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Abstract: This work reports a novel method for
direct reductive amination of aldehydes with silanes NCOR, and double bonds. This novel method was
catalyzed by several high-valent oxorhenium(V) and also employed in the synthesis of tertiary amines
F, Cl, Br, I, CN, OH, OCH3, SCH3,
oxorhenium
ACHTUNGTRENNUNG
PhSiH3/ReIO CHTUNGTRENNUNG
À
for the synthesis of secondary amines and highly che- Keywords: amines; C N bond formation; homogene-
moselective, tolerating a wide range of functional ous catalysis; oxorhenium complexes; reductive ami-
groups such as
À
NO2,
À
CF3,
À
SO2R,
À
CO2R, nation; silanes
Introduction
is unstable to heat and must be handled with extreme
care.
In contrast, organosilanes are mild and environ-
Amines and their derivatives exist in many biological-
ly important molecules, and are relevant intermedi- mentally benign reducing agents, and in recent years,
ates in the synthesis of pharmaceutically active sub- several reagent systems containing silanes such as
stances, dyes, and fine chemicals. Consequently, the Et3SiH/InCl3,[6]
Et3SiH/
G
E
PhSiH3/
synthesis of amines is an active field in medicinal Bu2SnCl2,[8] PhSiH3/MoO2Cl2,[9] polymethylhydrosilox-
chemistry and modern organic synthesis.
ane (PMHS)/Ti
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Direct reductive amination is a powerful tool to
oxane (PMHS)/trifluoroacetic acid (TFA)[11] have also
synthesize a higher order amine from a carbonyl com- been employed in reductive amination.
pound and an amine, which proceeds through the for-
Due to the biological and chemical importance of
mation of an imine intermediate, followed by an in amines, the search for novel catalyst systems, leading
situ reduction. The importance of reductive amination to efficient and highly chemoselective methods for
procedures is exemplified by the enormous number of their synthesis, remains an important target in organic
its synthetic uses.
synthesis.
A variety of methods have been developed to ac-
In recent years, Tosteꢂs group[12] and our group[13]
complish direct reductive amination including catalyt- have demonstrated that oxomolybdenum and -rhe-
ic hydrogenation,[1] or using a reducing agent such as nium complexes activate the Si H and B H bonds.
NaBH4,[2] NaBH3CN,[3] NaBH(OAc)3,[4] pyridine- Toste[13] has also reported that the catalytic system
BH3.[5] However, in terms of functional group toler- PhMe2SiH/ReIO
2A(PPh3)2 is very efficient for the hy-
ance, and side reactions, most of these reagents may drosilylation of aldehydes and ketones, toleranting
À
À
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
À
À
have one or more drawbacks. For example, catalytic several functional groups such as NO2, Br, ester
hydrogenation is incompatible with compounds con- and double bonds. We have also described the effi-
taining other reducible functional groups such as cient hydrosilylation of aldehydes and ketones,[13] and
double and triple bonds, nitro, and cyano groups. the reduction of several functional groups such as aro-
NaBH4 sometimes requires harsh reaction conditions, matic nitro compounds,[14] imines,[15] amides,[16]
cyanoborohydride is highly toxic and generates toxic esters,[17] sulfoxides,[18,13c] pyridine N-oxides,[18a] and al-
by-products such as HCN or NaCN, and pyridine-BH3 kenes[19] to the corresponding amines, alcohols, sul-
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Adv. Synth. Catal. 2010, 352, 2218 – 2226

Efficient and Highly Chemoselective Direct Reductive Amination of Aldehydes
fides, pyridines, and alkanes using the catalytic sys- oxorhenium complexes were very efficient, affording
tems silane/oxo-complexes or borane/oxo-complexes.
high conversions of the amine. However, the reaction
2A(PPh3)2 was very fast
Very recently, we investigated the catalytic activity catalyzed by 2.5 mol% of ReIO CTHNUGTRENNUNG
À
À
À
of oxo-complexes in C C, C S and C P bond forma- (10 min), producing the amine with 96% conversion
tion. We demonstrated that MoO2Cl2 catalyzes Frie- (Table 1, entry 1). A similar reaction using only
del–Crafts acylation and sulfonylation, affording aro- 1 mol% of ReIO2ACTHNUTRGEN(UNG PPh3)2 required 5 h 10 min
matic ketones and sulfones in moderate to good (Table 1, entry 2). At room temperature, the reductive
yields.[20] We also reported the use of MoO2Cl2 as an amination was much slower, taking 15 h (Table 1,
excellent catalyst for the addition of HP(O)ACHTNUGTRNEUNG(OEt)2 to entry 3). No reductive amination was observed with
aldehydes or imines, producing a-hydroxyphospho- phenylsilane in the absence of ReIO2ACHTNUGTRENUNG(PPh3)2 (Table 1,
nates or a-aminophosphonates in excellent yields.[21,22] entry 8), demonstrating the catalytic role of the oxo-
These results show that MoO2Cl2 is an excellent cata- rhenium complex.
À
À
À
lyst for C C, C S and C P bond formation.
Direct reductive amination of aldehydes was also
In continuation of our interest in developing organ- studied with several silanes, namely, phenylsilane, di-
À
ic reductions and C X bond forming reactions cata- methylphenylsilane, triethylsilane, triphenylsilane and
lyzed by high-valent oxo-complexes, herein, we report polymethylhydrosiloxane (PMHS). Of these silanes,
a novel method for C N bond formation through phenylsilane and dimethylphenylsilane were the most
direct reductive amination of aldehydes.
À
efficient reagents, affording the desired amine in ex-
cellent conversions (Table 2, entries 1 and 2). How-
AHCTUNTGRENeNGNU ver, the reaction with PhSiH3 was much faster, re-
quired only 10 min (Table 2, entry 1). Moderate con-
versions were obtained with Et3SiH and PMHS
Results and Discussion
The condensation of 4-nitrobenzaldehyde with aniline (Table 2, entries 3 and 4). No reaction was observed
was chosen as the model reaction for studying the in- using triphenylsilane (Table 2, entry 5) or in absence
fluence of catalysts, silanes and critical reaction pa- of silane (Table 2, entry 6).
rameters such as solvent and temperature (Table 1,
The influence of the solvent on the direct reductive
Table 2 and Table 3). The progress of the reactions amination of aldehydes was also explored (Table 3).
was monitored by thin layer chromatography and by THF was the best solvent at reflux temperature, af-
1H NMR.
fording the amine in 96% conversion after only
In order to compare the catalytic activity of oxorhe- 10 min (Table 3, entry 1). At room temperature, this
nium complexes, the reaction of 4-nitrobenzaldehyde reaction required 15 h (Table 3, entry 2). Chloroform,
with aniline was carried out with PhSiH3 (120 mol%) toluene, and benzene also gave excellent conversions
in the presence of the catalysts ReIO CHTUNGTREN(NNUG PPh3)2, of the amine (Table 3, entries 3–5), however, the reac-
2A
(PPh3)2, Re2O7, MTO (CH3ReO3), and tion in chloroform was much faster, requiring 1 h
3A(dppm) in refluxing THF under air atmos- 30 min. In contrast, dichloromethane and acetonitrile
ReOCl
ReOCl CHTUNGTRENNUNG
3ACHTUNGTRENNUNG
phere, as summarized in Table 1. In general, all the
Table 1. Direct reductive amination with phenylsilane catalyzed by oxorhenium complexes.[a]
Entry
Catalyst
Catalyst [mol%]
Temperature [8C]
Time
Conversion [%][b]
1
2
3
4
5
6
7
8
ReIO
ReIO
ReIO
ReOCl
ReOCl
Re2O7
MTO
G
2.5
1.0
2.5
2.5
2.5
2.5
2.5
–
reflux
reflux
r.t
reflux
reflux
reflux
reflux
reflux
10 min
5 h 10 min
15 h
96
91
95
100
96
94
92
1 h
1 h 10 min
1 h 30 min
2 h 20 min
6 h 30 min
without catalyst
no reaction
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of aniline, and 120 mol% of PhSiH3.
Conversion was determined by H NMR.
[b]
1
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Sara C. A. Sousa and Ana C. Fernandes
Table 2. Direct reductive amination with different silanes catalyzed by ReIO2AHCTUNGTERNNUNG
(PPh3)2.[a]
FULL PAPERS
Entry
Silane
Silane [mol%]
Time
Conversion [%][b]
1
2
3
4
5
6
PhSiH3
DMPSH
Et3SiH
PMHS
Ph3SiH
120
250
300
200
250
–
10 min
30 min
24 h
24 h
18 h
96
100
65
51
no reaction
no reaction
without silane
6 h 30 min
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of aniline, and 2.5 mol% of ReIO
Conversion was determined by H NMR.
(PPh3)2.
[b]
1
Table 3. Direct reductive amination with phenylsilane and ReIO2ACTHNUTRGNEUNG
(PPh3)2 in different solvents.[a]
Entry
Solvent
Temperature [8C]
Time
Conversion [%][b]
1
2
3
4
5
6
7
THF
THF
reflux
r.t.
10 min
15 h
1 h 30 min
9 h
9 h
1 h
96
95
92
87
85
60
47
CHCl3
toluene
benzene
CH2Cl2
CH3CN
reflux
reflux
reflux
reflux
reflux
24 h
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of aniline, 120 mol% of PhSiH3, and 2.5 mol% of
ReIO2A(PPh3)2.
Conversion was determined by H NMR.
CHTUNGTRENNUNG
[b]
1
only afforded the amine in moderate conversions successfully accomplished with the catalytic system
(Table 3, entries 6 and 7). PhSiH3/ReIO2A(PPh3)2 (2.5 mol%) (Table 4, entries 1–6
To evaluate the general applicability of the catalytic and 8–10), showing the high chemoselectivity of this
system PhSiH3/ReIO2A(PPh3)2 (2.5 mol%), the direct novel methodology.
reductive amination was explored with a variety of al- The substrate versatility of this novel method was
CTHUNGTRENNUNG
CTHUNGTRENNUNG
dehydes and aniline (Table 4). Generally, good to ex- further demonstrated by the chemoselective reductive
cellent yields of secondary amines were obtained amination of cinnamaldehyde with aniline in 79%
under the optimized reaction conditions, including al- yield (Table 4, entry 11). Another interesting result
dehydes bearing electron-withdrawing or electron-do- was the reductive amination of ferrocenecarboxalde-
nating groups. The reactions were very fast (10– hyde with aniline in 72% yield (Table 4, entry 12).
40 min), except the reductive amination of 4-cyano-
benzaldehyde that required 7 h (Table 4, entry 6).
Reductive amination of 4-methoxybenzaldehyde
was also explored using different anilines with the
Reductive amination of aldehydes containing sever- system PhSiH3/ReIO CHTUNTGREN(NUNG PPh3)2 (2.5 mol%) (Table 5).
2A
À
À
À
al functional groups such as NO2, CF3, SO2R, All these reactions gave good to excellent yields of
CO2R, Br, CN, OCH3, SCH3, NCOR were the corresponding amine. The reactions carried out
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Efficient and Highly Chemoselective Direct Reductive Amination of Aldehydes
Table 4. Direct reductive amination of aldehydes with the catalytic system PhSiH3/ReIO2ACHTUNGTRNEUNG
(PPh3)2.[a]
Entry
1
Aldehyde
Product
Time
Yield [%][b]
10 min
90
2
3
20 min
5 min
89
83
4
15 min
84
5
6
7
8
40 min
82
90
87
7 h
10 min
5 min
87
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Sara C. A. Sousa and Ana C. Fernandes
FULL PAPERS
Table 4. (Continued)
Entry
Aldehyde
Product
Time
Yield [%][b]
9
10 min
82
10
11
10 min
30 min
73
79
12
30 min
72
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of aniline, 120 mol% of PhSiH3, and 2.5 mol% of
ReIO2A(PPh3)2.
Isolated yield.
CHTUNGTRENNUNG
[b]
with anilines containing electron-withdrawing groups
were much faster (Table 5, entries 2–5) than the reac-
tions with anilines containing electron-donating
groups (Table 5, entries 6–8).
Finally, we investigated the reaction of N-methyl-
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
the synthesis of tertiary amines by direct reductive
amination (Table 6). The reactions were carried out
with different aromatic aldehydes bearing electron-
withdrawing or electron-donating groups, affording
the corresponding tertiary amines in moderate yields
in few minutes (5–20 min). A small amount of N- this reductive amination. No reaction was observed
methylaniline and alcohol, resulting of the reduction when 4-nitrobenzaldehyde and aniline was treated
of aldehyde with the system PhSiH3/ReIO
was also obtained as secondary products [Eq. (1)]. In or when 4-nitrobenzaldehyde and aniline was treated
order to minimize the amount of alcohol formed, the with 2.5 mol% of ReIO2A(PPh3)2 without silane
reductive amination of 4-methoxybenzaldehyde and (Table 2, entry 6), after 6 h 30 min. These results show
2ACHTUNGTRENN(NUG PPh3)2 with phenylsilane without catalyst (Table 1, entry 8)
CTHUNGTRENNUNG
N-methylaniline was carried out by slowly addition of that silane and ReIO2ACTHNUGTRENNUG(PPh3)2 play an active role in
the silane. However, similar yields of alcohol were ob- the reduction.
tained.
The previous works of Toste[12] have demonstrated
À
Several experiments were carried out in order to that ReIO
ascertain some fundamental mechanism aspects of silanes,
2A
producing
the
hydride
species
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Efficient and Highly Chemoselective Direct Reductive Amination of Aldehydes
Table 5. Direct reductive amination of 4-methoxybenzaldehyde with different anilines using the catalytic system PhSiH3/
ReIO2ACHTUNGTRENNUNG
(PPh3)2.[a]
Entry
1
Aniline
Product
Time
5 min
Yield [%][b]
87
2
3
4
5
6
5 min
93
93
94
94
91
75
5 min
5 min
35 min
1 h 15 min
2 h 30 min
7
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of aniline, 120 mol% of PhSiH3, and 2.5 mol% of
ReIO2A(PPh3)2.
Isolated yield.
CHTUNGTRENNUNG
[b]
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Sara C. A. Sousa and Ana C. Fernandes
(PPh3)2.[a]
FULL PAPERS
Table 6. Direct reductive amination of aldehydes with N-methylaniline using the catalytic system PhSiH3/ReIO2ACHTNUGTRENNUNG
Entry
Aldehyde
Product
Time
5 min
Yield [%][b]
1
62
2
10 min
64
3
4
5
10 min
20 min
10 min
65
64
59
6
20 min
65
[a]
All reactions were carried out with 1.0 mmol of aldehyde, 1.0 mmol of N-methylaniline, 120 mol% of PhSiH3, and 2.5
mol% of ReIO
Isolated yield.
2ACHTUNGTRENNUNG(PPh3)2.
[b]
(PPh3)2(O)(I)Re(H)OSiMe2Ph [Eq. (2)]. Based on
this result, and to investigate the reduction of the
imine intermediate by a hydride species, we carried
out the reductive amination of 4-nitrobenzaldehyde
with aniline using dimethylphenylsilane-d and
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Efficient and Highly Chemoselective Direct Reductive Amination of Aldehydes
2.5 mol% of ReIO2ACHTNUTRGNE(UGN PPh3)2 in refluxing THF [Eq.
(3)]. This reaction resulted in the incorporation of the
deuterium only in the carbon of the double bond, as
1
confirmed by H NMR. This result is consistent with
the hydrosilylation of the imine to give an intermedi-
ate N-silylamine, followed by a rapid hydrolysis, prob-
ably, due to the presence of a trace of water in the re-
action mixture.
A plausible mechanism (Scheme 1) for the direct
reductive amination of aldehydes with the catalytic
system PhSiH3/ReIO2ACHTNUTRGNE(UNG PPh3)2 should initiate by the
formation of the imine, and coordination of this mole-
cule to the catalyst by substitution of the two
phosphines, affording the complex ReIO
In the second step, the hydride species
(imine)2(O)IRe(H)OSiR3 is formed as result of the
2ACHTUNGTERN(NUNG imine)2.
ACHTUNGTRENNUNG
À
addition of the Si H bond of the silane to one of the
oxorhenium bonds. This reaction is similar to the for-
mation of the hydride (PPh3)2(O)IRe(H)OSiMe2Ph
reported by Toste,[12] reacting ReIO
PhMe2SiH [Eq. (2)].
2ACHTUNGERTN(NUNG PPh3)2 with
In the next step occurs the hydrosilylation of the
imine, followed by hydrolysis to the corresponding
amine. Finally, the ReIO2ACTHNUTRGENUGN(imine) species formed will
be stabilized by the entry of another molecule of
imine, regenerating the dioxorhenium complex
ReIO2ACHTUNGTRENNUNG(imine)2.
Scheme 1. Proposed catalytic cycle for the direct reductive
amination of aldehydes with the system silane/
ReIO2ACHTNUGTRNEUNG(PPh3)2.
Conclusions
Other advantages of this procedure include high
In conclusion, we have developed a new and efficient isolated yields, low catalyst loading (2.5 mol%), fast
method for direct reductive amination of aldehydes to reactions, simple experimental operation, stability of
the corresponding secondary amines using the catalyt- the catalysts towards air and moisture, allowing the
ic system silane/oxorhenium(V) and oxorhenium
complexes. This mild and practical method is highly
chemoselective, tolerating a large range of functional the literature, this method is more chemoselective
ACHTUNGTRENNUNG
À
À
À
À
À À
groups such as NO2, CF3, SO2R, CO2R, F, Cl,
than some hydrogenation systems in the reductive
À
À
À
À
À
À
À
Br, I, CN, OH, OCH3, SCH3, NCOR, and amination of substrates containing double bonds or
double bond. This novel methodology was also ap- nitro, cyano, and halo groups. Furthermore, the reac-
plied to the synthesis of tertiary amines in moderate tions carried out with our catalytic system were much
yields.
faster, using less amount of catalyst and silane, than
the method reported in the literature employing the
Adv. Synth. Catal. 2010, 352, 2218 – 2226
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Sara C. A. Sousa and Ana C. Fernandes
FULL PAPERS
system PhSiH3/MoO2Cl2.[9] All these features make
this method an attractive and useful alternative in or-
ganic synthesis.
[4] a) C. D. Gutierrez, V. Bavetsias, E. McDonald, Tetrahe-
dron Lett. 2005, 46, 3595–3597; b) A. F. Abdel-Magid,
K. G. Carson, B. D. Harris, C. A. Maryanoff, R. D.
Shah, J. Org. Chem. 1996, 61, 3849–3862.
The results obtained extend the scope of the use of
high-valent oxorhenium complexes as effective cata-
[5] M. D. Bomann. I. C. Guch, M. DiMare, J. Org. Chem.
1995, 60, 5995–5996.
À
lysts for C N bond formation. Further applications of
[6] a) O.-Y. Lee, K.-L. Law, C.-Y. Ho, D. Yang, J. Org.
Chem. 2008, 73, 8829–8837; b) O.-Y. Lee, K.-L. Law,
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[7] T. Mizuta, S. Sakaguchi, Y. Ishii, J. Org. Chem. 2005,
70, 2195–2199.
the catalytic system silane/oxo-rhenium complexes to
À
other organic reductions and C X bond forming reac-
tions are now in progress in our group.
[8] a) R. Apodaca, W. Ziao, Org. Lett. 2001, 3, 1745–1748;
b) J. J. Kangasmetsa, T. Johnson, Org. Lett. 2005, 7,
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Experimental Section
[9] C. A. Smith, L. E. Cross, K. Hughes, R. E. Davis, D. B.
Judd, A. T. Merritt, Tetrahedron Lett. 2009, 50, 4906–
4911.
General Procedure for Direct Reductive Amination
of Aldehydes with PhSiH3/ReIO2ACHTNUGRTNEUNG(PPh3)2
[10] S. Chandrasekhar, C. R. Reddy, M. Ahmed, Synlett
2000, 1655–1657.
To a mixture of aldehyde (1.0 mmol), aniline (1.0 mmol)
and ReIO2A(PPh3)2 (2.5 mol%) in THF (3 mL) at reflux tem-
CHTUNGTRENNUNG
[11] J. P. Patel, A.-H. Li, H. Dong, V. L. Korlipara, M. J.
Mulvihill, Tetrahedron Lett. 2009, 50, 5975–5977.
[12] a) J. J. Kennedy-Smith, K. A. Nolin, H. P. Gunterman,
F. D. Toste, J. Am. Chem. Soc. 2003, 125, 4056–4057;
b) K. A. Nolin, J. R. Krumper, M. D. Puth, R. G. Berg-
man, F. D. Toste, J. Am. Chem. Soc. 2007, 129, 14684–
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[13] a) A. C. Fernandes, R. Fernandes, C. C. Rom¼o, B.
Royo, Chem. Commun. 2005, 213–214; b) P. J. Costa,
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M. J. Calhorda, Chem. Eur. J. 2007, 13, 3934–3941;
c) A. C. Fernandes, J. A. Fernandes, F. A. Almeida Paz,
C. C. Rom¼o, Dalton Trans. 2008, 6686–6688.
[14] R. G. Noronha, C. C. Rom¼o, A. C. Fernandes, J. Org.
Chem. 2009, 74, 6960–6964.
perature was added PhSiH3 (120 mol%). The reaction mix-
ture was stirred under air atmosphere (the reaction times
are indicated in Table 4, Table 5, and Table 6) and the prog-
1
ress of the reaction was monitored by TLC and H NMR.
Upon completion, the reaction mixture was evaporated and
purified by silica gel column chromatography with the ap-
propriate mixture of n-hexane and ethyl acetate to afford
the amines.
Acknowledgements
This research was supported by FCT through project PTDC/
QUI/71741/2006. S. C. Sousa thanks FCT for grant (SFRH/
BD/63471/2009). The authors thank the Portuguese NMR
Network (IST-UTL Center) for providing access to the NMR
facilities and the Portuguese MS Network (IST Node) for the
ESI measurements.
[15] A. C. Fernandes, C. C. Rom¼o, Tetrahedron Lett. 2005,
46, 8881–8883.
[16] A. C. Fernandes, C. C. Rom¼o, J. Mol. Catal A: Chem.
2007, 272, 60–63.
[17] A. C. Fernandes, C. C. Rom¼o, J. Mol. Catal A: Chem.
2006, 253, 96–98.
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Adv. Synth. Catal. 2010, 352, 2218 – 2226