One-pot approach for C-C bond formation through ruthenium-amido complex catalyzed tandem aldol reaction/hydrogenation

Article abstract of DOI:10.1055/s-0029-1218815

A one-pot novel and efficient approach was developed for the -alkylation of various nitriles with carbonyl compounds using ruthenium-amido complex catalyst 1. The C-C bond was formed through aldol reaction followed by hydrogenation with triethylamine-formic acid (TEAF) and 1. Moderate to high yields were obtained, and a variety of functional groups were tolerated, including nitro and chloro groups, and a furan ring. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0029-1218815

PAPER
2577
One-Pot Approach for C–C Bond Formation through Ruthenium-Amido
Complex Catalyzed Tandem Aldol Reaction/Hydrogenation
C
–C BondFormat
a
ionvia
T
an
i
demA
f
ldol Reac
e
tion/Hydro
n
genation g Sun, Deju Ye, Hualiang Jiang, Kaixian Chen, Hong Liu*
The Center for Drug Discovery and Design, State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica,
Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, 201203, P. R. of China
Fax +86(21)50807088; E-mail: hliu@mail.shcnc.ac.cn
Received 7 January 2010
Abstract: A one-pot novel and efficient approach was developed
for the a-alkylation of various nitriles with carbonyl compounds us-
ing ruthenium-amido complex catalyst 1. The C–C bond was
formed through aldol reaction followed by hydrogenation with tri-
ethylamine–formic acid (TEAF) and 1. Moderate to high yields
were obtained, and a variety of functional groups were tolerated, in-
cluding nitro and chloro groups, and a furan ring.
O
S
HN
N
Ru
O
Key words: ruthenium-amido complex, tandem reaction, aldol re-
action, hydrogenation, C–C bond
1
Figure 1 Ruthenium-amido complex 1
Two-carbon homologation is a very important transfor-
mation in organic chemistry. The numerous methods1
available mostly involve redox functional group transfor-
mations rather than carbon–carbon bond formation. In this
regard, the development of two-carbon homologation
through carbon–carbon bond formation using organocata-
lytic tandem methodology can provide an expedient ac-
cess to homologated products from simple starting
materials.
Initially we found that the reaction of various aldehydes
with ethyl cyanoacetate could efficiently proceed using
ruthenium amido complex, Ru[(R,R)-Tsdpen](h6-p-
cymene) (1) as a catalyst via tandem aldol reaction/hydro-
genation reaction. p-Nitrobenzaldehyde (2a) and ethyl cy-
anoacetate (3a) were used as model substrates to optimize
the reaction conditions, including various solvent, reac-
tion time, reaction temperatures, and different amounts of
catalyst (Table 1). As illustrated in Table 1, the prelimi-
nary survey was carried out in toluene at 40 °C for 24
hours. The one-pot reaction provides good result using
0.05 equivalent catalyst (90%, Table 1, entry 1). The na-
ture of solvent was found to have a pronounced impact on
the process. Moderate yields were observed when we se-
lected tetrahydrofuran, acetonitrile, and dichloromethane
as solvent (Table 1, entries 2, 3, and 4). tert-Butyl alcohol
was proved to be better than toluene as a solvent and the
reaction time was shortened to 12 hours (98%, Table 1,
entry 5). There is no significant change in yield when the
reaction temperature was down to room temperature
while the reaction time had to be prolonged to 24 hours
(Table 1, entry 6). The yields of reaction were decreased
and the reaction times were longer when the concentration
of the catalyst 1 was lower (0.02, 0.01 equiv) (Table 1, en-
tries 7 and 8). This indicated that the optimization of al-
dol/hydrogenation conditions by increasing the amount of
1 and reaction temperature may be more necessary. To our
knowledge, this is the first report of tandem aldol/hydro-
genation catalyzed by a ruthenium-amido complex. The
reaction proceeded as follows: treatment of p-nitrobenzal-
dehyde and ethyl cyanoacetate with 0.05 equivalent of 1
gave an olefin product, which was successively reduced
by 1 and TEAF that was directly added to the reaction so-
lution.
Of the various types of tandem reactions, an aldol reaction
combined with a simultaneous catalytic hydrogenation for
the formation of C–C bond is of importance, and several
studies with different catalysts have been reported.2 Re-
cently, ruthenium/Hydrotalcites3 (HTs) catalyzed direct
a-alkylation of nitriles with primary alcohols was demon-
strated for the effective synthesis of a-alkylated nitriles,
which are important building blocks of various biological-
ly active compounds.4 However, this catalyst system usu-
ally required high reaction temperature and was only
limited to arylacetonitriles as substrates. Ruthenium-ami-
do complexes,5 which have sufficient Brønsted basicities
to deprotonate hydrogen donors, were found to efficiently
catalyze asymmetric Michael addition of 1,3-dicarbonyl
compounds to cyclic enones, nitroalkenes, and
azodicarboxylates6, and have been applied to the reduc-
tion of various activated olefins.7 Herein we report a ru-
thenium-amido complex catalyst, {Ru[(R,R)-Tsdpen](h6-
p-cymene), Figure 1}, for the one-pot formation of C–C
bond from various nitriles and aldehydes through tandem
aldol reaction/hydrogenation.
SYNTHESIS 2010, No. 15, pp 2577–2582
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Advanced online publication: 15.06.2010
DOI: 10.1055/s-0029-1218815; Art ID: Z00210SS
© Georg Thieme Verlag Stuttgart · New York

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H. Sun et al.
PAPER
Table 1 Optimization of Reaction Conditions Using p-Nitrobenzaldehyde and Ethyl Cyanoacetatea
O
CHO
OEt
Ru complex
TEAF
OEt
NC
+
CN
O2N
O
O2N
2a
3a
5a
Entry
Solvent
toluene
THF
Ru complex (equiv)
Temp (°C)
Time (h)
24
Yield (%)b
1
2
3
4
5
6
7
8
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.01
40
40
40
40
40
r. t.
40
40
90
54
65
45
98
95
89
56
48
MeCN
CH2Cl2
t-BuOH
t-BuOH
t-BuOH
t-BuOH
48
48
12
24
48
48
a Reaction conditions: aldehyde 2a (0.3 mmol), nitrile 3a (0.3 mmol), solvent (6 mL), Et3N (0.2 mL)–formic acid (0.2 mL).
b Isolated yields.
After determining the optimized conditions, we examined iments for establishing the scope of this method, we
the generality of the process as summarized in Table 2. selected heteroaryl aldehydes such as furan-2-carbalde-
It was found that the method was applicable to a broad hyde and thiophene-2-carbaldehyde as reactants and the
range of various substituted aldehydes, including aromat- yields were also found to be good (Table 2, entries 8 and
ic, aliphatic, and heterocyclic aldehydes. The results indi- 9). Furthermore, alkyl aldehyde 3-phenylpropanal and the
cated that the electronic effects on the reaction were not ketone cyclohexanone were relatively less reactive under
significant. Aldehydes containing various electron-with- the same conditions (Table 2, entries 11 and 12). At the
drawing and electron-donating substituents were used un- same time, the scope of the process with respect to the
der the optimal reaction conditions (Table 2, entries 1–6), variation of steric influence of cyanoacetates was investi-
although those containing electron-donating group such gated (Table 2). As shown, here also the products were
as p-SMe, o-OMe and m-Cl gave moderate products obtained in good yields (Table 2, entries 13–16). Unfortu-
yields (Table 2, entries 4–6). No obvious steric effects nately we could not get chiral products even though we
were observed and the reactions with para-, ortho-, and adopted a chiral ruthenium-amino complex as catalyst and
meta-substituted benzaldehydes proceeded in good yields also attempted to use some sterically hindred cyanoace-
(Table 2, entries 1–3), while 3,4,5-trimethoxybenzalde- tates as reactants.
hyde gave lower yield (Table 2, entry 7). In further exper-
Table 2 a-Alkylation of Various Nitriles with Carbonyl Compounds Using the Ruthenium Complex
CN
1. Ru complex
R1
OR2
R1 CHO
+
NC
R2
2. TEAF
3
2
O
5a–q
Entry
1
Donor
Acceptor
Time (h)
12
Product
Yield (%)a
98
CHO
OEt
5a
NC
O
O2N
CHO
OEt
2
12
5b
91
NC
O
NO2
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PAPER
C–C Bond Formation via Tandem Aldol Reaction/Hydrogenation
2579
Table 2 a-Alkylation of Various Nitriles with Carbonyl Compounds Using the Ruthenium Complex (continued)
CN
1. Ru complex
R1
OR2
R1 CHO
NC
R2
+
2. TEAF
3
2
O
5a–q
Entry
3
Donor
Acceptor
Time (h)
12
Product
Yield (%)a
O2N
CHO
CHO
OEt
5c
94
80
NC
O
OEt
OEt
4
5
24
24
5d
5e
NC
NC
O
O
MeS
CHO
83
84
OMe
CHO
OEt
OEt
6
24
24
5f
NC
NC
O
O
Cl
MeO
CHO
7b
5g
60
MeO
O
OMe
CHO
OEt
OEt
8
9
12
12
5h
5i
92
83
NC
NC
O
O
S
CHO
CHO
OEt
OEt
OEt
10
12
48
48
5j
5k
5l
87
NC
NC
NC
O
O
O
O
11b
12b
trace
49
CHO
CHO
O
O
13
14
12
12
5m
5n
92
90
NC
NC
O
O
O2N
O2N
CHO
O
CHO
CHO
15
16
12
12
5o
5p
91
88
O
O
NC
NC
O2N
O2N
O
O
S
a Isolated yields.
b Reaction temperature = 60 °C.
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2580
H. Sun et al.
PAPER
CHO
Ts
N
CO2Et
Ph
Ph
O2N
Ru
N
CN
O2N
N
COOEt
4
H
H
6
O2N
O
NC
TEAF
OEt
Ts
H
H
Ru
N
1
2a
EtOOC
COOEt
H
N
Ph
CN
Ph
7
Ts
NC
Ph
N
CO2Et
Ru
CN
H
Ph
N
H
O2N
O2N
5a
8
1
Scheme 1 Proposed mechanism for the tandem aldol reaction/hydrogenation
Ruthenium-Amido Complex-Catalyzed Tandem Aldol Reac-
tion/Hydrogenation; General Procedure
We propose a reaction mechanism for the transformation
in Scheme 1. The studies recently reported6a,b imply that
the reaction of the amido complex 1 with aldol donors
may proceed to give the C-bound Ru cyanoacetate inter-
mediate 6, which further reacts with aldehydes to give an
olefin product 4. Successively, the transfer hydrogenation
of C=C bond was completed by ruthenium-amido com-
plex 7 with a hydrogen source to give the final product 5a
(Scheme 1). Considering the characteristics of activated
olefins and the hydridic amido-ruthenium complex
(RuH), which is a coordinately saturated complex, the re-
duction of the polarized a,b-unsaturated compounds is
proposed to proceed in a stepwise conjugate reduction
procedure. After a RuH conjugate addition to the a-carbon
of the C=C bond (asymmetry generated step), the interme-
diate 8 might eliminate from the metal complex and sub-
sequently catches another proton rapidly from the excess
Et3NH+ in the reaction mixture.6,7 Thus, no asymmetric in-
duction could be generated in the a-carbon center of 5a.
Aldehyde 2 (0.3 mmol), nitrile 3 (0.3 mmol), and the catalyst 1
(0.05 equiv) were dissolved in t-BuOH (6 mL) and the mixture was
stirred at 40 °C for 10 h. Then Et3N (0.2 mL) and formic acid (0.2
mL) were directly added to the reaction mixture and stirred for an-
other 2 h. After completion of the reaction, the solvent was evapo-
rated and the crude products 5a–p were purified by column
chromatography on silica gel using PE–EtOAc (6:1) as eluent
(Table 2).
Ethyl 2-Cyano-3-(4-nitrophenyl)propanoate (5a)
1H NMR (300 MHz, CDCl3): d = 1.299 (t, J = 7.2 Hz, 3 H), 3.36 (m,
2 H), 3.67–3.82 (m, 1 H), 4.26 (q, J = 7.2 Hz, 2 H), 7.47 (d, J = 8.7
Hz, 2 H), 8.22 (d, J = 8.7 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 13.7, 34.7, 38.6, 63.2, 115.5, 123.8,
130.1, 142.6, 147.3, 164.7.
HRMS (ESI): m/z calcd for C12H11N2O4 [M – H]–: 247.0719; found:
247.0706.
Ethyl 2-Cyano-3-(2-nitrophenyl)propanoate (5b)
1H NMR (300 MHz, CDCl3): d = 1.31 (m, 3 H), 3.26–3.31 (m, 1 H),
3.67–3.73 (m, 1 H), 4.10–4.14 (m, 1 H), 4.25–4.30 (m, 2 H), 7.50–
7.59 (m, 2 H), 7.62–8.7.66 (m, 1 H), 8.09–8.10 (m, 1 H).
In conclusion, we have demonstrated a catalyst of ruthe-
nium-amido complex for the one-pot synthesis of a-alky-
lated nitriles from the reaction of various nitriles with
aldehydes through aldol/hydrogenation reaction. This
work presents the first successful application of rutheni-
um-amido complex 1 with an M/NH bifunctional unit to
catalytic aldol/hydrogenation reaction, and we are now
working on the expansion of the scope of the reaction.
13C NMR (75 MHz, CDCl3): d = 13.9, 33.3, 38.1, 63.2, 115.9, 125.5,
129.3, 130.7, 133.6, 133.9, 165.1.
HRMS (ESI): m/z calcd for C12H11N2O4 [M – H]– : 247.0719; found:
247.0705.
Ethyl 2-Cyano-3-(3-nitrophenyl)propanoate (5c)
1H NMR (300 MHz, CDCl3): d = 1.31 (t, J = 7.2 Hz, 3 H), 3.27–3.34
(m, 1 H), 3.68–3.75 (m, 1 H), 4.10–4.16 (m, 1 H), 4.27 (q, J = 7.2
Hz, 2 H), 7.50–8.11 (m, 4 H).
13C NMR (75 MHz, CDCl3): d = 13.9, 34.8, 39.0, 63.3, 115.5, 122.9,
124.0, 129.9, 135.4, 137.1, 148.3, 164.8.
The reagents (chemicals) were purchased from commercial sources,
and used without further purification. Analytical TLC was done us-
ing plates coated with HSGF 254 silica gel (0.15–0.20 mm thick-
ness). All products were characterized by their NMR and MS
spectra. 1H and 13C NMR spectra were recorded in CDCl3 on a 300
MHz instrument. Chemical shifts were reported in d (ppm) down-
field from TMS. Low- and high-resolution mass spectra (LRMS
and HRMS) were recorded on a Finnigan MAT-95 LCQ-DECA
spectrometer.
HRMS (ESI): m/z calcd for C12H11N2O4 [M – H]–: 247.0719; found:
247.0718.
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PAPER
C–C Bond Formation via Tandem Aldol Reaction/Hydrogenation
2581
Ethyl 2-Cyano-3-[4-(methylthio)phenyl]propanoate (5d)
1H NMR (300 MHz, CDCl3): d = 1.29 (t, J = 7.1 Hz, 3 H), 3.12–3.28
(m, 2 H), 3.67–3.72 (m, 1 H), 4.27 (q, J = 7.1 Hz, 2 H), 7.18–7.27
(m, 4 H).
13C NMR (75 MHz, CDCl3): d = 13.8, 15.5, 35.0, 39.5, 62.9, 116.0,
126.6, 129.4, 131.8, 138.0, 165.3.
HRMS (ESI): m/z calcd for C12H12NO2 [M – H]–: 202.0868; found:
202.0849.
Ethyl 2-Cyano-5-phenylpentanoate (5l)
1H NMR (300 MHz, CDCl3): d = 1.27–1.34 (m, 3 H), 1.81–1.98 (m,
3 H), 2.66–2.71 (m, 2 H), 3.45–3.50 (m, 2 H), 3.66–3.69 (m, 1 H),
4.22–4.30 (m, 2 H), 7.31–7.32 (m, 5 H).
HRMS (ESI): m/z calcd for C13H14NO2S [M – H]–: 248.0745;
found: 247.0765.
13C NMR (75 MHz, CDCl3): d = 14.0, 28.3, 29.241, 34.91, 37.4,
62.8, 116.4, 128.5, 128.5, 140.7, 166.0.
Ethyl 2-Cyano-3-(2-methoxyphenyl)propanoate (5e)
1H NMR (300 MHz, CDCl3): d = 1.28 (t, J = 7.2 Hz, 3 H), 3.08–3.15
(m, 1 H), 3.86 (s, 3 H), 3.33–3.40 (m, 1 H), 4.20–4.27 (q, J = 7.2 Hz,
2 H), 7.53–8.20 (m, 4 H).
13C NMR (75 MHz, CDCl3): d = 13.9, 31.5, 37.3, 55.2, 62.6, 110.3,
116.5, 120.6, 123.5, 129.7, 131.1, 157.3, 165.9.
HRMS (ESI): m/z calcd for C14H17NO2 [M – H]–: 230.1249; found:
230.1243.
tert-Butyl 2-Cyano-3-(4-nitrophenyl)propanoate (5m)
1H NMR (300 MHz, CDCl3): d = 1.47 (s, 9 H), 3.68–3.73 (m, 1 H),
3.25–3.38 (m, 2 H), 7.45 (d, J = 8.7 Hz, 2 H), 8.21 (d, J = 8.7 Hz, 2
H).
13C NMR (75 MHz, CDCl3): d = 27.6, 34.9, 39.6, 84.9, 115.9, 123.9,
130.1, 142.8, 147.4, 163.6.
HRMS (ESI): m/z calcd for C13H15NO3 [M – H]–: 232.1038; found:
232.1042.
Ethyl 3-(3-Chlorophenyl)-2-cyanopropanoate (5f)
1H NMR (300 MHz, CDCl3): d = 1.29 (t, J = 7.2 Hz, 3 H), 3.14–3.30
(m, 2 H), 3.70–3.75 (m, 1 H), 4.27 (q, J = 7.2 Hz, 2 H), 7.19–7.20
(m, 1 H), 7.29–7.30 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 13.9, 35.1, 39.2, 63.0, 115.7, 127.2,
127.9, 129.1, 130.1, 134.5, 137.1, 165.1.
HRMS (ESI): m/z calcd for C14H15N2O4 [M – H]–; 275.1032; found:
275.1032.
Benzyl 2-Cyano-3-(4-nitrophenyl)propanoate (5n)
1H NMR (300 MHz, CDCl3): d = 3.82–3.84 (m, 1 H), 3.32–3.35 (m,
2 H), 5.22 (s, 2 H), 7.30–7.39 (m, 7 H), 8.10–8.14 (d, J = 8.7 Hz, 2
H).
13C NMR (75 MHz, CDCl3): d = 35.0, 38.8, 68.8, 115.3, 124.0,
128.7, 128.7, 129.0, 130.1, 134.1, 142.1, 147.5, 164.6.
HRMS (ESI): m/z calcd for C12H11ClNO2 [M – H]–: 236.0478;
found: 236.0457.
Ethyl 2-Cyano-3-(3,4,5-trimethoxyphenyl)propanoate (5g)
1H NMR (300 MHz, CDCl3): d = 1.28 (t, J = 7.2 Hz, 3 H), 3.09–3.20
(m, 2 H), 3.68–3.73 (m, 1 H), 3.83 (s, 3 H, CH3), 3.86 (s, 6 H, CH3),
4.26 (q, J = 7.2 Hz, 2 H), 6.49 (s, 2 H).
13C NMR (75 MHz, CDCl3): d = 13.9, 36.0, 39.7, 56.0, 60.7, 62.9,
105.8, 116.2, 130.8, 137.3, 153.2, 165.4.
HRMS (ESI): m/z calcd for C17H13N2O4 [M – H]–: 309.0875; found:
309.0871.
Furan-2-ylmethyl 2-Cyano-3-(4-nitrophenyl)propanoate (5o)
1H NMR (300 MHz, CDCl3): d = 3.28 (m, 2 H), 3.82–3.88 (m, 1 H),
5.18 (m, 2 H), 6.38–6.39 (m, 1 H), 6.46–6.49 (m, 1 H), 7.38–7.48
(m, 3 H), 8.14–8.17 (d, J = 6.4 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 34.8, 38.7, 59.9, 110.7, 112.0,
115.2, 123.8, 130.0, 142.2, 143.7, 147.3, 164.5.
HRMS (ESI): m/z calcd for C15H18NO5 [M – H]–: 292.1185; found:
292.1180.
Ethyl 2-Cyano-3-(furan-2-yl)propanoate (5h)
HRMS (ESI): m/z calcd for C15H11N2O5 [M – H]–: 299.0668; found:
299.0665.
1H NMR (300 MHz, CDCl3): d = 1.31 (t, J = 7.2 Hz, 3 H, CH3),
3.23–3.36 (m, 2 H), 3.81–3.86 (m, 1 H), 4.29 (q, J = 7.2 Hz, 2 H),
6.25–6.26 (m, 1 H), 6.32–6.34 (m, 1 H), 7.37 (m, 1 H).
Thiophen-2-ylmethyl 2-Cyano-3-(4-nitrophenyl)propanoate
(5p)
13C NMR (75 MHz, CDCl3): d = 13.8, 28.4, 37.0, 63.0, 108.3, 110.5,
115.8, 142.5, 148.8, 165.1.
HRMS (ESI): m/z calcd for C10H11NO3 [M – H]–: 192.0661; found:
192.0645.
1H NMR (300 MHz, CDCl3): d = 3.17–3.34 (m, 2 H), 3.81–3.85 (m,
1 H), 5.36 (m, 2 H), 6.97–7.01 (m, 1 H), 7.10–7.11 (m, 1 H), 7.34–
7.37 (m, 3 H), 8.10–8.12 (d, J = 6.3 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 35.0, 38.7, 62.5, 115.205, 124.0,
127.0, 127.9, 129.7, 130.1, 135.7, 142.0, 147.5, 164.6.
HRMS (ESI): m/z calcd for C15H11N2O4S [M – H]–: 315.0440;
found: 315.0437.
Ethyl 2-Cyano-3-(thiophen-2-yl)propanoate (5i)
1H NMR (300 MHz, CDCl3): d = 1.3 (t, J = 6.9 Hz, 3 H), 3.4–3.5
(m, 2 H), 3.75–3.77 (m, 1 H), 4.24–4.31 (q, J = 6.9 Hz, 2 H), 6.97–
7.00, 7.22–7.24 (m, 1 H).
13C NMR (75 MHz, CDCl3): d = 13.9, 29.8, 39.8, 63.1, 115.9, 125.3,
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
127.2, 127.3, 136.6, 165.0.
HRMS (ESI): m/z calcd for C10H10NO2S [M – H]–: 208.0432;
found: 208.0447.
Acknowledgment
Ethyl 2-Cyano-3-phenylpropanoate (5j)
1H NMR (300 MHz, CDCl3): d = 1.27 (t, J = 7.2 Hz, 3 H), 3.16–3.32
(m, 2 H), 3.90–3.74 (m, 1 H), 4.20–4.27 (q, J = 7.2 Hz, 2 H), 7.26–
7.38 (m, 5 H).
We gratefully acknowledge the financial support from the National
Natural Science Foundation of China (Grants 20721003 and
20872153), International Collaboration Projects (Grants
20720102040), the 863 Hi-Tech Program of China (Grant
2006AA020602), and National
(2009ZX09501-001 and 2009ZX09501-010).
13C NMR (75 MHz, CDCl3): d = 13.9, 35.7, 39.6, 62.9, 116.1, 127.7,
128.8, 129.0, 135.2, 165.5.
S
&
T
Major Projects
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PAPER
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