Efficient preparation of trans-α,β-unsaturated aldehydes from saturated aldehydes by oxidative enamine catalysis

Article abstract of DOI:10.1007/s11426-011-4432-6

A mild and highly efficient amine-catalyzed, IBX-mediated oxidation of aldehydes to (E) selective α,β-unsaturated aldehydes has been achieved in good yields. The process features a new oxidation of enamines to iminium ions in a catalytic fashion.

Full text of DOI:10.1007/s11426-011-4432-6

SCIENCE CHINA
Chemistry
December 2011 Vol.54 No.12: 1932–1936
doi: 10.1007/s11426-011-4432-6
• ARTICLES •
Efficient preparation of trans-,-unsaturated aldehydes from
saturated aldehydes by oxidative enamine catalysis
ZHANG ShiLei1, XIE HeXin1, SONG AiGuo1, WU DeYan2, ZHU Jin2, ZHAO SiHan1,
LI Jian2*, YU XinHong2* & WANG Wei1,2*
1 Department of Chemistry & Chemical Biology, University of New Mexico, MSC03 2060, NM 87131, USA
2 School of Pharmacy, East China University of Science & Technology, Shanghai 200237, China
Received September 6, 2011; accepted September 19, 2011; published online November 14, 2011
A mild and highly efficient amine-catalyzed, IBX-mediated oxidation of aldehydes to (E) selective , -unsaturated aldehydes
has been achieved in good yields. The process features a new oxidation of enamines to iminium ions in a catalytic fashion.
aldehydes, enals, enamines, IBX, oxidative enamine catalysis
1 Introduction
Recently we reported a novel organocatalytic oxidative
enamine catalysis for direct transformation of an enamine
derived from a saturated aldehyde into an ,-iminium ion
(Scheme 1, Eq. 1) [1, 2]. The iminium ion is efficiently
produced from its corresponding enamine by IBX mediated
oxidation via 2e transfer. Notably, the power of this pro-
cess has been realized in the context of direct asymmetric
-functionalization of simple aldehydes in high efficiency.
The chemistry is complementary to the widely studied
MacMillan’s iminium catalysis [3], which relies on the use
of ,-unsaturated aldehydes as reactants. In our continuing
effort on exploring the potential of this new chemistry, we
contemplate that it can be extended for the synthesis of C=C
double bonds through direct dehydrogenation of saturated
C–C bonds since the reaction pathway involves the for-
mation of an iminium species 3. Such an intermediate can
undergo hydrolysis in the absence of a nucleophile to pro-
duce ,-unsaturated aldehydes 5 as products (Scheme 1,
Eq. 2). Successful implementation of the process will pro-
Scheme 1 Oxidative enamine catalysis.
vide a new direct approach to highly valuable enals from
readily available aldehydes since this class of compounds
have arguably become the most widely used substrates in
iminium catalysis [3, 4].
2 Results and discussion
Using IBX as oxidant [5], we initially explored several
*Corresponding author (email: jianli@ecust.edu.cn, xhyu@ecust.edu.cn, wwang@unm.edu)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

Zhang SL, et al. Sci China Chem December (2011) Vol.54 No.12
1933
secondary amines I–V [6] and Et2NH as catalysts for the
reaction. Most of the catalysts are capable of promoting the
dehydrogenation process at room temperature even though
the yields vary broadly (Table 1, entries 1–6). Tertiary
amine is inert for this transformation (Table 1, entry 7) be-
cause it cannot promote formation of an enamine or imini-
um intermediate with 1a, indicating that the resulting
enamine is essential for the process. Furthermore, when the
reaction is conducted in CH2Cl2 without any catalyst, only a
trace amount dehydrogenation product 5a (<1%) is ob-
served after 4 h (Table 1, entry 8). The influence of solvent
is also investigated. DMSO affords a higher yield than other
solvents (Table 1, entries 1 and 9–12) maybe due to the
highest solubility of IBX in it. However, considering the
workup simplicity with CH2Cl2, DMSO is not selected for
further optimizing reaction conditions in our studies. Finally,
decreasing the amount of IBX from 1.5 to 1.2 equiv gives a
slightly higher yield and therefore this was adopted as the
best reaction condition (Table 1, entry 13). It should be
noted that Nicolaou et al. reported an IBX mediated oxida-
tion of ketones to enones [7]. Nevertheless, in their proce-
dures, a large excess of IBX (up to 8.0 equiv) was used and
high temperature (60–85 °C) was necessary with long reac-
tion time (up to 60 h). An improved protocol was also re-
ported using IBX-NMO (4-methylmorpholine N-oxide) com-
plex as co-oxidant at ambient temperature, but still requiring
long reaction time (15–48 h) [8]. Moreover, DMSO was the
only effective reaction medium for these processes.
Switching to other solvents led to very low reaction yields.
Furthermore, the system was much less effective for 3-aryl-
propionaldehydes. Only one example of 3-(2-pyridinyl)-
propionaldehyde was disclosed with low efficiency (3.0
equiv of IBX-NMO, 45 h and 43% yield). The observation
was demonstrated in our comparison studies. We found that
under the same reaction conditions they reported, no reac-
tion occurred for 3-phenyl-propionaldehyde 1a in a mixture
of DMSO and toluene at 65 °C (Table 1, entry 14). Fur-
thermore, a moderate yield (58%) was obtained using an
excess of IBX (2.4 equiv) and NMO (2.4 equiv). Again the
use of DMSO as reaction solvent was essential for an effec-
tive transformation and much longer reaction time was re-
quired (Table 1, entry 15). In contrast, the protocols we
have developed are superior to Nicolaou’s. Remarkably, the
reaction occurs efficiently under very mild reaction condi-
tions (room temperature) using only 1.2 equiv of IBX in a
short period of time in high yields in various reaction sol-
vents, thus rendering it more practical in organic synthesis.
Having established the optimized reaction conditions, we
examined the scope and limitation of the dehydrogenation
process. As seen from Table 2, the substrates bearing
electron-neutral (Table 2, entry 1), -withdrawing (Table 2,
entries 2, 3 and 7–11) -donating (Table 2, entries 5–6) and a
Table 1 Optimization of reaction conditionsa)
Conversion
(%)
Yield
(%) b)
80
77
68
44
Entry
Cat.
Solvent
Time (h)
1b)
2
I
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
2.5
4
100
100
100
54
II
3
III
IV
4
Table 2 Scope of the organocatalytic dehydrogenation reactions of
4
4
aldehydes a)
5
6
7
V
CH2Cl2
CH2Cl2
CH2Cl2
4
4
4
54
34
<1
41
15
<1
Et2NH
TEA
8
CH2Cl2
Et2O
4
4
<1
85
<1
20
14
78
84
82
<5
58

I
9
Entry
R1, R2, 5
Ph, H, 5a
Time (h)
Yield (%) b)
10
I
hexane
4
39
1
2
2.5
4
82
81
80
85
84
83
77
80
82
79
75
0
11
I
toluene
4
86
4-NO2C6H4, H, 5b
2-NO2C6H4, H, 5c
4-NO2-3-MeOC6H3, H, 5d
4-MeOC6H4, H, 5e
2-MeOC6H4, H, 5f
3-FC6H4, H, 5g
3-NO2C6H4, H, 5h
4-BrC6H4, H, 5i
4-CNC6H4, H, 5j
2-py, H, 5k
12
I
DMSO
2
100
100
60
13 c)
14 d)
15 e)
I
CH2Cl2
2.5
10
28
3
4
4
5
DMSO/toluene
DMSO


5
6
95
6
6
a) Unless stated otherwise, the reaction was carried out with aldehyde
1a (0.2 mmol), oxidant (0.30 mmol) and catalyst (0.02 mmol) in solvent
(1.0 mL) at rt for shown time. b) Conversion and yield were determined by
1H NMR of the crude product with 1,1,2,2-tetrachloroethane as internal
standard. c) 0.24 mmol (1.2 equiv.) of IBX was used. d) Reaction condition:
aldehyde 1a (0.2 mmol), IBX (0.26 mmol) in DMSO (0.33 mL) and tolu-
ene (0.66 mL) was stirred at 65 °C for 1 h or 10 h. e) Reaction condition:
aldehyde 1a (0.2 mmol), IBX (0.48 mmol) and NMO (0.48 mmol) in
DMSO (0.33 mL) were stirred at rt for 28 h. NMO: 4-methylmorpholine
N-oxide.
7
4
8
4
9
4
10
11 c)
12
4
24
10
Ph, Me, 5l
a) Unless specified, see Experimental section for detail. b) Isolated
yields. c) 2.5 equiv of Et3N was used.

1934
Zhang SL, et al. Sci China Chem December (2011) Vol.54 No.12
combination of –withdrawing and -donating (Table 2, entry
4) groups can effectively engage at the process to give good
yields of ,-unsaturated aldehydes 5. It seems that the ste-
ric hindrance of substitutes on benzene ring also have very
limited influence on the dehydrogenation reactions (Table 2,
entries 2 vs. 3, 5 vs. 6). It is noted that as for 2-pyridinyl
aldehyde (Table 2, entry 11), 2.5 equiv of Et3N is required.
It is found that without Et3N as an additive, almost no de-
sired unsaturated aldehyde is observed after 24 h, which
might be due to the interaction between the pyridine moiety
of the substrate and the acid moiety of the IBX. The limita-
tion of the reaction is also realized. No dehydrogenation
product can be observed for the ,-disubstituted aldehyde
5l (Table 2, entry 12). It should be pointed out that for all of
the reactions showed above, only E-isomers of ,-
unsaturated aldehydes formed exclusively, no Z-isomers are
detected.
were used as received, unless otherwise stated. Merck 60
silica gel was used for chromatography, and Sorbent Tech-
nologies silica gel plates with fluorescence F254 were used
1
for thin-layer chromatography (TLC) analysis. H and 13C
NMR spectra were recorded on Bruker Avance 500, and
tetramethylsilane (TMS) was used as a reference. Data for
1H are reported as follows: chemical shift (ppm), and multi-
plicity (s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet). Data for 13C NMR are reported as ppm.
4.2 Experimental procedures and characterization of
products
Typical Procedure: Aldehyde 1 (0.2 mmol) was dissolved
in CH2Cl2 (1.0 mL), followed by addition of IBX (67 mg,
0.24 mmol) and catalyst I (6.5 mg, 0.02 mmol). The reac-
tion mixture was stirred at rt for shown time as listed in
table 2. The product was purified directly by column
chromatography to yield the desired (E) ,-unsaturated
aldehyde.
The -alkyl substituted aldehyde 6 is also performed
under the similar reaction conditions at 0 °C (Scheme 2).
Nevertheless only 20% of desired enal 7a is produced along
with 14% of dienal 7b and 10% of starting material. The
low yield might result from the dienamine formed in the
process, which complicates the process.
trans-Cinnamaldehyde (5a)
The title compound was prepared according to the general
1
procedure, as described above in 82% yield. H NMR (500
MHz, CDCl3):  9.71 (d, J = 8.0 Hz, 1H), 7.57 (m, 2H),
7.48 (d, J = 16.0 Hz, 1H), 7.44 (m, 3H), 6.72 (dd, J = 16.0,
8.0 Hz, 1H); 13C NMR (125 MHz, CDCl3):  193.6, 152.7,
134.0, 131.2, 129.1, 128.6, 128.4.
(E)-3-(4-Nitrophenyl)acrylaldehyde (5b)
The title compound was prepared according to the general
1
procedure, as described above in 81% yield. H NMR (500
MHz, CDCl3):  9.78 (d, J = 7.5 Hz, 1H), 8.29 (d, J = 8.5
Hz, 2H), 7.74 (d, J = 8.5 Hz, 2H), 7.54 (d, J = 16.0 Hz, 1H),
6.81 (dd, J = 16.0, 7.5 Hz, 1H); 13C NMR (125 MHz,
CDCl3):  192.7, 149.0, 148.8, 139.9, 131.7, 129.0, 124.3.
Scheme 2 Dehydrogenation of -alkyl substituted aldehyde.
3 Conclusions
(E)-3-(2-Nitrophenyl)acrylaldehyde (5c)
In conclusion, we have developed an efficient approach to
highly valuable (E) selective ,-unsaturated aldehydes
from saturated aldehydes. The process involves an unprec-
edented catalytic IBX-mediated oxidation of enamines, re-
cently developed from our laboratory [1, 2]. The mild reac-
tion condition renders the process particularly attractive in
the synthesis of biologically interesting complex natural
products which bear sensitive functionalities. The applica-
tion of this powerful method in the synthesis of natural
products and therapeutic agents is being pursued in our la-
boratory.
The title compound was prepared according to the general
procedure, as described above in 80% yield. H NMR (500
1
MHz, CDCl3):  9.79 (d, J = 7.5 Hz, 1H), 8.11 (d, J = 8.0
Hz, 1H), 8.05 (d, J = 16.0 Hz, 1H), 7.74–7.69 (m, 2H),
7.64–7.61 (m, 1H) 6.64 (dd, J = 16.0, 7.5 Hz, 1H); 13C
NMR (125 MHz, CDCl3):  193.0, 148.0, 147.2, 133.8,
132.6, 131.1, 130.0, 129.0, 125.1.
(E)-2-Methoxy-4-(3-oxoprop-1-enyl)phenyl acetate (5d)
The title compound was prepared according to the general
procedure, as described above in 85% yield. H NMR (500
1
MHz, CDCl3):  9.70 (d, J = 7.5 Hz, 1H), 7.44 (d, J = 16.0
Hz, 1H), 7.18–7.09 (m, 3H), 6.67 (dd, J = 16.0, 7.5 Hz, 1H);
3.88 (s, 3H), 2.33 (s, 3H); 13C NMR (125 MHz, CDCl3):
 193.4, 168.6, 151.8, 151.6, 142.3, 132.9, 128.7, 123.5,
121.9, 111.4, 56.0, 20.6.
4 Experimental
4.1 General experimental section
Commercial reagents and solvents from VWR or Aldrich

Zhang SL, et al. Sci China Chem December (2011) Vol.54 No.12
1935
(E)-3-(4-Methoxyphenyl)acrylaldehyde (5e)
(E)-3-(Pyridin-2-yl)acrylaldehyde (5k)
The title compound was prepared according to the general
procedure, as described above in 84% yield. H NMR (500
The title compound was prepared according to the general
procedure, as described above in 75% yield. H NMR (500
1
1
MHz, CDCl3):  9.69 (d, J = 8.0 Hz, 1H), 7.84 (d, J = 16.0
Hz, 1H), 7.55 (d, J = 8.0 Hz, 1H), 7.43–7.39 (m, 1H), 7.00
(t, J = 7.0 Hz, 1H), 6.95 (d, J = 8.0 Hz, 1H), 6.64 (dd, J =
16.0, 8.0 Hz, 1H), 3.91 (s, 3H); 13C NMR (125 MHz,
CDCl3):  194.5, 158.3, 148.2, 132.7, 129.1, 128.9, 123.0,
120.9, 111.3, 55.6.
MHz, CDCl3):  9.80 (d, J = 8.0 Hz, 1H), 8.70 (d, J = 4.5
Hz, 1H), 7.77 (d, J = 7.5 Hz, 1H), 7.567.51 (m, 2H),
7.34–7.31 (m, 1H), 7.09 (dd, J = 15.5, 7.5 Hz, 1H); 13C
NMR (125 MHz, CDCl3):  193.5, 152.6, 151.0, 150.3,
136.8, 131.5, 124.7, 124.1.
(E)-Undeca-2,10-dienal (7a)
(E)-3-(2-Methoxyphenyl)acrylaldehyde (5f)
The title compound was prepared according to the general
procedure at 0 °C as described above in 20% yield. H
1
The title compound was prepared according to the general
procedure, as described above in 83% yield. H NMR (500
1
NMR (500 MHz, CDCl3):  9.51 (d, J = 8.0 Hz, 1H),
6.88–6.82 (m, 1H), 6.12 (dd, J = 15.5, 8.0 Hz, 1H),
5.85–5.76 (m, 1H), 5.02–4.93 (m, 2H), 2.36–2.32 (m, 2H),
2.07–2.03 (m, 2H), 1.55–1.35 (m, 8H); 13C NMR (125 MHz,
CDCl3):  194.1, 158.8, 138.9, 133.0, 114.3, 33.7, 32.7,
29.0, 28.8, 28.7, 27.8.
MHz, CDCl3):  9.65 (d, J = 8.0 Hz, 1H), 7.52 (d, J = 9.0
Hz, 2H), 7.42 (d, J = 16 Hz, 1H), 6.95 (d, J = 9.0 Hz, 2H),
6.61 (dd, J = 16.0,7.5 Hz, 1H), 3.86 (s, 3H); 13C NMR (125
MHz, CDCl3):  193.6, 162.2, 152.6, 130.3, 126.8, 126.5,
114.6, 55.4.
(E)-3-(3-Fluorophenyl)acrylaldehyde (5g)
(2E, 4E)-Undeca-2,4,10-trienal (b)
The title compound was prepared according to the general
procedure, as described above in 77% yield. H NMR (500
The title compound was prepared according to the general
procedure at 0 °C as described above in 14% yield. H
1
1
MHz, CDCl3):  9.72 (d, J = 7.0 Hz, 1H), 7.45–7.39 (m,
2H), 7.34 (d, J = 7.5 Hz, 1H), 7.26 (d, J = 8.0 Hz, 1H), 7.14
(t, J = 8.5 Hz, 1H), 6.69 (dd, J = 16.0, 8.0 Hz, 1H); 13C
NMR (125 MHz, CDCl3):  193.2, 164.0, 162.1, 150.9,
136.2, 136.2, 130.7, 130.6, 129.6, 124.4, 118.1, 118.0,
114.8, 114.6.
NMR (500 MHz, CDCl3):  9.54 (d, J = 8.0 Hz, 1H), 7.08
(dd, J1 = 15.5 Hz, J2 = 10.0 Hz, 1H), 6.35–6.24 (m, 2H),
6.08 (dd, J1 = 15.0 Hz, J2 = 7.5 Hz, 1H), 5.84–5.76 (m, 1H),
5.03–4.97 (m, 2H), 2.25–2.21 (m, 2H), 2.10–2.05 (m, 2H),
1.55–1.40 (m, 4H); 13C NMR (125 MHz, CDCl3):  193.9,
152.7, 147.0, 138.5, 130.1, 128.8, 114.6, 33.5, 33.0, 28.4,
28.0.
(E)-3-(3-Nitrophenyl)acrylaldehyde (5h)
The title compound was prepared according to the general
procedure, as described above in 80% yield. H NMR (500
We are grateful for the financial support from the National Science Foun-
dation (CHE-1057569, W. W.) and the China 111 Project (B07023, J. L.
and W. W.), the National Natural Science Foundation of China (20972051,
X. Y.-H.), and the Science and Technology Commission of Shanghai Mu-
nicipality (08430703900 & 08431901800, X. Y.-H.).
1
MHz, CDCl3):  9.77 (d, J = 7.5 Hz, 1H), 8.41 (s, 1H), 8.28
(d, J = 8.0 Hz, 1H), 7.89 (d, J = 7.5 Hz, 1H), 7.64 (t, J = 7.5
Hz, 1H), 7.53 (d, J = 16.0 Hz, 1H), 6.81 (dd, J = 16.0, 8.0
Hz, 1H); 13C NMR (125 MHz, CDCl3):  192.7, 148.9,
148.8, 135.7, 133.6, 130.9, 130.2, 125.3, 123.0.
1
2
3
4
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(E)-3-(4-Bromophenyl)acrylaldehyde (5i)
The title compound was prepared according to the general
1
procedure, as described above in 82% yield. H NMR (500
MHz, CDCl3):  9.70 (d, J = 7.5 Hz, 1H), 7.56 (d, J = 8.5
Hz, 2H), 7.43–7.39 (m, 3H), 6.69 (dd, J = 16.0, 7.5 Hz, 1H);
13C NMR (125 MHz, CDCl3):  193.2, 151.0, 132.9, 132.4,
129.7, 129.0, 125.6.
(E)-4-(3-Oxoprop-1-enyl)benzonitrile (5j)
The title compound was prepared according to the general
1
procedure, as described above in 79% yield. H NMR (500
MHz, CDCl3):  9.75 (d, J = 7.5 Hz, 1H), 7.72 (d, J = 8.0
Hz, 2H), 7.66 (d, J = 8.0 Hz, 2H), 7.47 (d, J = 16.0 Hz, 1H),
6.76 (dd, J = 16.0, 7.5 Hz, 1H); 13C NMR (125 MHz,
CDCl3):  192.8, 149.4, 138.1, 132.8, 131.2, 128.7, 118.1,
114.2.
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6
7
8
Nicolaou KC, Montagnon T, Baran PS, Zhong YL. Iodine (V) rea-
gents in organic synthesis. Part 4. o-Iodoxybenzoic acid as a chemo-
specific tool for single electron transfer-based oxidation processes. J
Am Chem Soc, 2002, 124(10): 2245–2258
Nicolaou KC, Montagnon T, Baran PS. Modulation of the reactivity
profile of IBX by ligand complexation: Ambient temperature dehy-
dration of aldehydes and ketones to -unsaturated carbonyls.
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5
For selected reviews of IBX oxidations, see: (a) Duschek A, Kirsch
SF. 2-Iodoxybenzoic acid: A simple oxidant with a dazzling array of
potential applications. Angew Chem Int Ed, 2011, 50(7): 1524–1552;
(b) Satam V, Harad A, Rajule R, Pati H. 2-Iodoxybenzoic acid (IBX):
An efficient hypervalent iodine reagent. Tetrahedron, 2010, 66(39):
LI Jian received his BS and MS degree in medicinal chemistry from Shenyang
Pharmaceutical University in 2000 and 2003, respectively, and PhD from Shang-
hai Institute of Materia Medica, Chinese Academy of Sciences in 2006. He
worked as an associate professor in School of Pharmacy, East China University of
Science & Technology, then professor and director of the Department of Phar-
maceutical Sciences in March 29, 2011. He also worked in Prof. Wei Wang’s
laboratory in University of New Mexico as a visiting professor from June to Au-
gust 2009. He has received the Servier Youth Medicinal Chemistry Award (2007)
and Shanghai Rising-Star Award (2007). His research interests are currently fo-
cused on three areas-medicinal chemistry, chemical biology, and small molecular
organocatalytic asymmetric synthesis.
YU XinHong received his BS degree in 1982 and MS degree under the super-
vision of Professors Maoqin Liu and Cuili Zhu in 1986 in medicinal chemistry
from School of Pharmacy, Fudan University, China. He obtained his PhD from
East China University of Science & Technology (ECUST) in 2006 under the di-
rection of Professor Yongjia Shen. Currently he holds a professor position at the
School of Pharmacy, ECUST. His research interests include organic synthesis,
particularly practical synthetic methods and medicinal chemistry.
WANG Wei received his BS degree in chemistry in 1988 from Nanjing Nor-
mal University, China and MS degree in organic chemistry from Shanghai Insti-
tute of Meteria Medica, the Chinese Academy of Sciences in 1993. He obtained
his PhD from North Carolina State University in 2000 before carrying out his
postdoctoral studies at the University of Arizona. He is an associate professor in
the Department of Chemistry & Chemical Biology at the University of New
Mexico and a professor of East China University of Science & Technology. His
current research interests span from organic synthesis, molecular imaging, and
drug discovery to chemical biology.