A novel selective aza-Morita-Baylis-Hillman (aza-MBH) domino reaction and aza-MBH reaction of N-sulfonated imines with acrolein catalyzed by a bifunctional phosphine organocatalyst

Article abstract of DOI:10.1002/chem.200800753

An efficient, bifunctional phosphine organocatalyst catalyzed aza-MBH reaction and aza-MBH domino reaction was developed between N-sulfonated imines and acrolein under mild conditions in moderate to excellent yields. Acrolein was added to a solution of N-(2-chlorobenzylidene)-4-methylhenzenesulfonamide and catalyst LBBA in CHCl₃. The stirring was maintained at room temperature until completion of the reaction. The residue was purified by a flash column chromatography to yield 3a as a colorless solid. The OH group is utilized to stabilize the intermediate A through hydrogen bonding. The Michael addition of A with N-sulfonated imine gives intermediate B, when THF is used as the solvent. The aza-MBH/Michael/aldol/dehydrate domino reaction also provides an efficient method to synthesize tetrahydropyridine derivatives, which can be used as building blocks in organic synthesis.

Full text of DOI:10.1002/chem.200800753

DOI: 10.1002/chem.200800753
A Novel Selective Aza-Morita–Baylis–Hillman (aza-MBH) Domino
Reaction and Aza-MBH Reaction of N-Sulfonated Imines with Acrolein
Catalyzed by a Bifunctional Phosphine Organocatalyst
Xiangtai Meng, You Huang,* and Ruyu Chen[a]
A practical and efficient construction of highly functional-
ized and diversified molecules from simple starting materials
is highly desirable and remains a great challenge. The
domino process is a very appealing strategy as it favors the
formation of complex molecules starting from readily avail-
able substrates in two or more steps without isolation of in-
termediates.[1] These reactions avoid time-consuming and
costly processes, including the purification of intermediates
and steps involving the protection and deprotection of func-
tional groups, and they are environmental friendly and often
proceed with excellent stereoselectivities.[2] Thus, considera-
ble efforts have been made to develop catalytic domino
transformations catalyzed by an organocatalyst,[3] one of the
most successful class of organocatalysts used for this pur-
pose are secondary amines and usually initiated reactions
are Michael additions.[3] Although, significant advances have
been made in the development of two-step domino reac-
tions,[4] the development of a new catalyst for a new domino
reaction and a multicomponent three-step domino reaction
proved to be a challenging task, especially since such pro-
cesses are rare.[5,6] To the best of our knowledge, a domino
sequence starting with aza-Morita–Baylis–Hillman (aza-
MBH) reaction, followed by an intermolecular Michael ad-
dition and aldol/dehydration reaction has not been reported
so far.[7] Herein, we report for the first time a novel selective
aza-MBH domino reaction and aza-MBH reaction of N-sul-
fonated imines with acrolein catalyzed by a bifunctional
phosphine organocatalyst.
ucts.[8] However, this reaction has traditionally suffered from
low reaction rates and limited substrate scopes. Recently,
Chong and Shi reported that octanol and p-nitrophenol can
accelerate Baylis–Hillman reaction, respectively.[9] Sasai and
Shi also reported several bifunctional organocatalysts, which
can efficiently catalyze the aza-MBH reaction.[10] This pro-
moted us to search for an easy synthesis and high efficient
bifunctional phosphine organo-
catalyst for the aza-MBH reac-
tions. (2’-Hydroxy-biphenyl-2-
yl)-diphenylphosphane
(see
below) was first reported in
1994 by Takaya and co-work-
ers.[11] It can also be easily pre-
pared in high yields from 2-phe-
nylphenol and usually applied
to Rh-catalyzed hydrogenation and hydroformylation reac-
tions.[12] Although, it incorporates a basic and an acidic
moiety in one molecule and can be used as a Lewis base–
Brønsted acid (LBBA) bifunctional organocatalyst, there
have been no reports on its utility as a bifunctional organo-
catalyst to catalyze the aza-MBH reaction thus far. On the
other hand, acrolein, different from methyl vinyl ketones
and methyl acrylate, is an interesting and more activated
Michael acceptor, which is rarely used as the conjugated car-
bonyl substrate in Baylis–Hillman reaction, clearly because
of its propensity to form oligomers or polymers under the
basic catalysis employed.[13] Recently, more and more re-
ports are available on the aza-MBH reaction using acrolein
as Michael acceptor.[14] So, it is very important to study the
aza-MBH reaction using acrolein as Michael acceptor cata-
lyzed by bifunctional organocatalysts (LBBA).
The Baylis–Hillman reaction, first reported in 1972by
À
Baylis and Hillman, is an important carbon carbon bond-
forming process that affords densely functionalized prod-
We first studied the reaction of N-(2-chlorobenzylidene)-
4-methylbenzenesulfonamide (1a) with acrolein (1.5 equiv)
in the presence of 20 mol% LBBA in THF at room temper-
ature. As indicated by TLC, the reaction proceeded smooth-
ly. One hour later, 1a disappeared and a new compound
was obtained in high yield (90%) (Scheme 1i). The product
[a] Dr. X. Meng, Prof. Dr. Y. Huang, Prof. Dr. R. Chen
State Key Laboratory and
Institute of Elemento-organic Chemistry Nankai University
Tianjin 300071 (China)
Fax : (+86)22-23503627
E-mail: hyou@nankai.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800753.
1
was characterized by H NMR and 13C NMR spectroscopy
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Chem. Eur. J. 2008, 14, 6852– 6856

COMMUNICATION
Table 1. LBBA-catalyzed domino reaction and aza-MBH reaction of 1a
as a normal aza-MBH product 2a. To our delight, when the
reaction time was extended, according to TLC, 2a was
slowly reduced and another new product was formed
(Scheme 1ii). After 24 h, unfortunately only 37% yield of
and acrolein.[a]
Entry
Solvent
t [min]
Product[b]
Yield [%] 2a
Yield [%] 3a
1[c]
2[d]
3[e]
4[e]
5[e]
6[e]
7[e]
THF
THF
THF
DMF
CH3CN
CH2Cl2
CHCl3
60
90
0
0
70
0
0
0
24 h
24 h
10
10
10
37
38
0
31
42
48
10
0
[a] All reactions were performed on a 0.5 mmol scale with LBBA (20
mol%) as the catalyst with the use of 2mL of solvent. [b] Product isolat-
ed by flash chromatography. [c] 1.2equiv of acrolein were used.
[d] 1.5 equiv of acrolein were used. [e] 3.0 equiv of acrolein were used.
tries 2,3) whether the ratio of the substrates was 1:1.5 or 1:3.
When the reaction was carried out in a more polar solvent
such as DMF, the aza-MBH product 2a was obtained in
70% yield within 10 min (entry 4). In less polar solvents
such as CH3CN, CH2Cl2, CHCl3 (entries 5, 6, 7), all reactions
exclusively gave the aza-MBH domino reaction product 3a
in a short reaction time (10 min); also no aza-MBH product
2a was found. The highest yield of 3a was obtained in
CHCl3. Therefore, CHCl3 was selected as the best solvent
for the aza-MBH domino reaction product 3a (Scheme 1iii)
and THF was used as the best solvent for the aza-MBH re-
action product 2a (Scheme 1i). Although the exact reason
for the solvent effect is not known (THF or DMF), it can be
assumed that the oxygen atom of the solvent could stabilize
the reaction intermediate anion by a hydrogen bond, and
then the intermediate can easily be converted to the aza-
MBH product 2a. However, when the reaction was carried
out in solvents such as CH3CN, CH2Cl2, CHCl3, the reaction
intermediate can not be stabilized by a hydrogen bond, so
that the intermediate has a high reactivity and yields the
aza-MBH domino reaction product 3a.
Scheme 1. i) LBBA (20% mol), THF, 1 h; ii) LBBA (20% mol), THF,
24 h; iii) LBBA (20% mol), CHCl3, 10 min; iv) LBBA (20% mol), THF,
18 h; v) LBBA (20% mol), CHCl3, 12 h.
the new product was isolated. The new product was also
1
characterized by H NMR and 13C NMR spectra. There are
two doublets (J=19 Hz) at d 3.33 and 4.43 ppm in the
1H NMR and one peak at d 38.20 ppm in the 13C NMR spec-
trum (see supporting information). These NMR spectral
data indicate that the new product is not the regular aza-
MBH reaction product. Further confirmation of the struc-
ture of this new product was obtained by X-ray diffraction
measurements. The ORTEP diagram is shown in Figure 1.[15]
A tetrahydropyridine derivative 3a was formed.
Under identical conditions for 2a and 3a, we then exam-
ined a range of N-sulfonated imines 1 to explore the gener-
ality of this novel catalyst (LBBA) for the aza-MBH reac-
tion and the domino reaction. The results are summarized in
Tables 2and 3. As can be seen in Table 2, it was observed
that all reactions proceeded smoothly in THF under the
mild conditions to afford the corresponding aza-MBH reac-
tion products 2 in high yields. The results exhibited the
scope with respect to a range of N-sulfonated imines substi-
tuted with both electron-donating (Table 2, entry 8) and
electron-withdrawing (Table 2, except entry 8) groups to
afford the desired aza-MBH products in excellent yields in a
short time. From Table 3, the one-pot three component
domino reaction of N-sulfonated imines with acrolein can
also be catalyzed by the bifunctional organocatalyst
(LBBA) efficiently in CHCl3. In these reactions, a range of
N-sulfonated imines react with acrolein by way of a cata-
Figure 1. X-ray structure of 3a.
With these results at hand, we next examined how to con-
trol this reaction to selectively form different products.
Thus, we have studied the solvent effect, the results are
shown in the Table 1. When THF was used as the solvent,
the reaction is finished in one hour to give aza-MBH prod-
uct 2a in high yields using 1.2equiv acrolein (entry 1), if the
reaction time was prolonged. The aza-MBH domino reac-
tion product was obtained in the almost same yields (en-
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6853

Y. Huang et al.
Table 2. LBBA-catalyzed aza-MBH reaction.
lyzed aza-MBH/Michael/aldol/dehydration domino sequence
affording the tetrahydropyridine derivatives with good
À
À
yields (41–50%). Moreover, one new C C bond, one C N
bond and one new C=C bond were formed under mild and
rapid reaction conditions. This methodology provides a new
protocol for the construction of a variety of tetrahydropyri-
dine derivatives with different substituents.
Entry
Ar
Compound
t [h]
Yield 2 [%][a,b]
1
2-ClC6H4
6H4
1a
1b
1c
1d
1e
1 f
1g
1h
1i
1
1
1
1
0.5
0.5
1
1.5
1
90 (2a)
85 (2b)
88 (2c)
80 (2d)
95 (2e)
92( 2 f)
91 (2g)
86 (2h)
80 (2i)
85 (2j)
23-ClC
3
4
5
6
7
8
9
To extend the scope of this reaction, cross aza-MBH
domino reactions of two different Michael acceptors were
investigated (Scheme 3). The aza-MBH reaction of 1a with
methyl vinyl ketone (MVK) (1.2mmol) catalyzed by LBBA
in CHCl3 was performed at room temperature for one hour;
after 1a had disappeared and a new product had formed
acrolein (1.5 equiv) was added to the reaction mixture.
After 12h, the reaction was finished and the new aza-MBH
domino reaction product 3k was obtained in 52% yield.
This reaction demonstrates the potential utility of this novel
domino reaction for the efficient synthesis of other tetrahy-
dropyridine derivatives.
4-ClC6H4
2,4-Cl2C6H3
4-NO2C6H4
3-NO2C6H4
4-BrC6H4
4-CH3C6H4
4-FC6H4
10
C6H5-CH=CH
1j
1
[a] Isolated yields. [b] 1.2equiv of acrolein were used.
Table 3. LBBA-catalyzed domino reaction.
Entry
Ar
Compound
t [min]
Yield 3 [%][a,b]
1
2-ClC6H4
6H4
1a
1b
1c
1d
1e
1 f
1g
1h
1i
10
10
10
10
5
48 (3a)
45 (3b)
50 (3c)
50 (3d)
49 (3e)
46 (3 f)
48 (3g)
50 (3h)
46 (3i)
41 (3j)
23-ClC
3
4
5
6
7
8
9
4-ClC6H4
2,4-Cl2C6H3
4-NO2C6H4
3-NO2C6H4
4-BrC6H4
4-CH3C6H4
4-FC6H4
5
10
15
10
10
Scheme 3. The LBBA catalyzed cross aza-MBH domino reaction.
10
C6H5-CH=CH
1j
[a] Isolated yields [b] 3.0 equiv of acrolein were used.
In order to obtain more information for the study of the
aza-MBH domino reaction, we first carried out the
31P NMR spectroscopic measurements of the bifunctional
catalyst LBBA in the absence and presence of acrolein in
CDCl3 and [D8]THF, respectively, The 31P NMR spectra of
LBBA showed
a
signal at
d
À12.98 (CDCl3) and
À47.32ppm ([D 8]THF), but the 31P NMR spectra of LBBA
and acrolein (molar ratio=1:8) gave a new signal at d
+32.03 (CDCl3) and À5.52ppm ([D 8]THF), which appears
to correspond to the phosphonium enolate A (Scheme 2).
The difference in the 31P NMR spectrum for CHCl3 and
THF indicated that intermediate A may have a different re-
activity, and yields product 2 and 3. Secondly, the reaction
of 2b with acrolein (1.5 equiv) in the presence of 20 mol%
LBBA in THF and CHCl3 at room temperature was carried
out (Scheme 1iv, v). As indicated by TLC, after 18 h, the tet-
rahydropyridine derivative 3b was obtained in 48% yield in
THF(Scheme 1iv). A similar result was obtained with 49%
yield in CHCl3 after 12h. On the base of these results,
though in contrast to the literature,[10b] a proposed mecha-
nism for the domino reaction and the aza-MBH reaction is
shown in Scheme 2, in which LBBA acts as a bifunctional
catalyst in this reaction. The phosphine acts as a Lewis base
Scheme 2. Proposed catalytic cycle of the domino reactions and aza-
MBH reaction.
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Morita–Baylis–Hillman Reactions
COMMUNICATION
and the phenolic OH group acts as a Brønsted acid.[10b] In
the first step, LBBA reacts with acrolein to produce inter-
mediate A; the OH group is utilized to stabilize the inter-
mediate A through hydrogen bonding. The following Mi-
chael addition of A with N-sulfonated imine gives inter-
mediate B, when THF is used as the solvent. Elimination of
the catalyst LBBA from B gave aza-MBH product 2. How-
ever, in the case of CHCl3 as the solvent, intermediate B did
not stop at this step to give the aza-MBH product 2, but
continued to react rapidly with another acrolein, which un-
dergoes an intermolecular Michael addition to give another
zwitterionic intermediate C. In the subsequent third step,
the intermediate C undergoes an intramolecular aldol con-
densation and dehydration to regenerate catalyst LBBA to
give the tetrahydropyridine derivative 3.
In conclusion, we have developed an efficient, bifunction-
al phosphine organocatalyst catalyzed aza-MBH reaction
and aza-MBH domino reaction between N-sulfonated
imines and acrolein under mild conditions in moderate to
excellent yields. The aza-MBH/Michael/aldol/dehydrate
domino reaction also provides an efficient method to syn-
thesise tetrahydropyridine derivatives which can be used as
building blocks in organic synthesis. Further investigations
are underway to expand the scope and application of the bi-
functional phosphine organocatalyst LBBA and this new ef-
ficient domino process.
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Experimental Section
General procedure of aza-MBH domino reaction for synthesis the tetra-
hydropyridine derivatives: Acrolein (1.5 mmol) was added to a solution
of N-(2-chlorobenzylidene)-4-methylbenzenesulfonamide (0.5 mmol) and
catalyst LBBA (0.1 mmol) in CHCl3 (2mL). The stirring was maintained
at room temperature until completion of the reaction (the reaction was
monitored by TLC plate). The residue was purified by a flash column
chromatography to yield 3a as a colorless solid.
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General procedure for the aza-MBH reaction: Acrolein (0.6 mmol) was
added to a solution of N-(2-chlorobenzylidene)-4-methylbenzenesulfona-
mide (0.5 mmol) and catalyst LBBA (0.1 mmol) in THF (2mL) at room
temperature. The reaction mixture was monitored by TLC plate. After
1 h, the residue was purified by a flash column chromatography to yield
2a as a colorless solid.
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Acknowledgements
We thank the National Natural Science Foundation of China (20772061)
and the Natural Science Foundation of Tianjin (05YFJMJC00600) for fi-
nancial support.
Keywords: aldol reaction · domino reactions · Michael
reaction · Morita–Baylis–Hillman reaction · organocatalysis
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Received: April 21, 2008
Published online: June 25, 2008
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Chem. Eur. J. 2008, 14, 6852– 6856