Dual Iminium- and Lewis Base Catalyzed Morita-Baylis-Hillman Reaction on Cyclopent-2-enone

Article abstract of DOI:10.1055/s-0036-1591521

The application of iminium catalysis to the challenging Morita-Baylis-Hillman reaction on cyclopenten-2-one leads to the corresponding allylic alcohols in excellent yields. Experimental evidence shows that secondary amines act as co-catalysts activating t

Full text of DOI:10.1055/s-0036-1591521

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
R. Innocenti et al.
Letter
Synlett
Dual Iminium- and Lewis Base Catalyzed Morita–Baylis–Hillman
Reaction on Cyclopent-2-enone
N
Riccardo Innocenti
Gloria Menchi
N
N
H
Andrea Trabocchi*
0
0
0
0
0-
0
0
3
1-
7
7
4
9-
3
0
1
OH
O
O
iminium ion
catalysis
Lewis base
catalysis
O
R
+
Department of Chemistry ‘Ugo Schiff’, University
of Florence, Via della Lastruccia 13, 50019 Sesto
Fiorentino, Florence, Italy
R
H
THF–1 M NaHCO3 (1:4)
25 °C, 40 h
37–99%
andrea.trabocchi@unifi.it
16 examples
R = alkyl,aryl
Received: 02.11.2017
Accepted after revision: 16.11.2017
Published online: 13.12.2017
hyde. A final H-shift and elimination of the base furnishes
the corresponding adduct (Scheme 1). Recent insight on the
mechanism of alcohol-mediated MBH reaction have been
reported by Plata and Singleton.10
DOI: 10.1055/s-0036-1591521; Art ID: st-2017-d0809-l
Abstract The application of iminium catalysis to the challenging Mori-
ta–Baylis–Hillman reaction on cyclopenten-2-one leads to the corre-
sponding allylic alcohols in excellent yields. Experimental evidence
shows that secondary amines act as co-catalysts activating the enone
moiety towards the nucleophilic attack at the β-position by DABCO as
the Lewis base catalyst, resulting in an augmented nucleophilic charac-
ter towards the reaction with aldehydes.
:B
O
O
O
O
O
O
H
B
Ph
H
:B
B
B
Key words catalysis, enones, synthetic methods, C–C coupling, basicity
H–B
O
O
O
HO
It is well recognized that the development of multibo-
nd-forming protocols, such as multicomponent reactions,1
domino reactions,2 or one-pot multistep processes3 are im-
portant synthetic approaches in organic chemistry. The
Morita–Baylis–Hillman (MBH) reaction4 is an aldol-type
chemical transformation used for the generation of C–C
bonds. The first paper on this reaction was reported by
Morita in 1968,5 in which acrylonitrile was reacted with a
range of aldehydes under phosphine catalysis. In 1972, Bay-
lis and Hillman subsequently patented a similar reaction
catalyzed by amines.6 Typical substrates for these reactions
are electron-poor alkenes, which react by their α-carbon
with carbonyl compounds or imines as the electrophiles. In
the latter case the reaction is called the ‘aza-MBH’.7 Interest
in this reaction grew in the 1990s,8 due to the high level of
atom economy, the versatility in the use of aldehydes and
α,β-unsaturated carbonyls, and the polyfunctional charac-
ter of the reaction product. Accordingly, this reaction met
our interest as an efficient synthetic tool addressing cou-
pling reactions to achieve polyfunctional molecules for di-
versity-oriented synthesis strategies.9
Scheme 1 The MBH reaction mechanism between cyclopent-2-enone
and benzaldehyde
The reactivity of α,β-unsaturated compounds is modu-
lated by the potency of the electron-withdrawing group at-
tached to the double bond; as the higher such character the
higher the reactivity of the β-position for the base attack to
generate the nucleophile. Cyclopent-2-enones are poorly
reactive under MBH reaction conditions, although this moi-
ety is represented in many natural product compounds,11
such as prostaglandins and didemnenones, as illustrated in
Figure 1. Furthermore, many synthetic processes allow for
formation and functionalization of the cyclopentenone
ring, such as the Pauson–Khand reaction, the Nazarov cy-
clization, the asymmetric allylic alkylation, and cross-cou-
pling reactions, demonstrating the value of such cyclic
structures in organic synthesis.11 The MBH reaction plays a
key role in the derivatization of cyclopentenones, because it
allows for the generation of a stereocenter adiacent to the
α-carbon of the α,β-unsaturated system, and several syn-
thetic approaches have been reported in the literature.12
The mechanism of the MBH reaction consists of the ac-
tivation of the α,β-unsaturated compound at the β-position
to form a zwitterionic compound, which subsequently at-
tacks the electrophilic carbonyl carbon atom of the alde-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
R. Innocenti et al.
Letter
Synlett
ter,15 as the application of a 1:4 ratio of THF and 1 M NaHCO3
improved the yield to 43%. The stoichiometry of the reac-
tion was optimal when the aldehyde was used in a three-
fold excess, giving the product in 65% yield as compared to a
stoichiometric amount (43%) or to a threefold excess in fa-
vor of the cyclopentenone (42%).21
O
O
H
O
O
O
OR2
O
HO
OR1
HO
pentenomycins
parthenin
guaiane
O
O
O
O
H
OH
Table 1 Study of the Base Catalyst in the MBH Reaction of Cyclopent-
OH
O
2-enone with Benzaldehydea
H
HO
H
OH
O
O
H
base (0.2 equiv)
25 °C
OH
CHO
OH
+
HO
OH
THF–water (1:1)
didemnenones A/B
przewalskin B
4α-4-deoxyphorbol
1
2
3
Figure 1 Representative natural products containing the cyclopent-2-
enone ring
Entry
Base
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
DABCO
N-Me-Py
imidazole
Ph3P
168
168
168
168
40
0
0
The application of iminium catalysis on the MBH reac-
tion is somewhat recent. The first report, by Shi in 2002,13
described the combined use of imidazole and L-proline for
the reaction of methyl vinyl ketone and aromatic alde-
hydes; whereas no product was achieved without it. Tom-
kinson14 subsequently reported the importance of water in
the solvent system to enable efficient iminium catalysis. In
2003, the application of aqueous NaHCO3 instead of water
was reported by Cheng et al.,15 and Gruttadauria16 con-
firmed the role of NaHCO3 in improving the MBH reactivity,
due to a direct role in the mechanism. Iminium catalysis
was also reported for the intramolecular MBH reaction.17
Nevertheless, none of these approaches has been applied
for the MBH on cyclopentenone thus far, which has been re-
ported to occur only under Lewis base catalysis.12 In this
work we report our studies of the MBH on cyclopent-2-
enone using iminium catalysis, demonstrating that a sec-
ondary amine is important in activating the poorly reactive
cyclopentenone.
Diverse catalysts suitable for the MBH reaction were
screened using different solvent systems (Table 1). Initially,
DABCO failed to react in all solvents but aqueous THF.18
Then, different MBH catalysts were screened using such a
solvent system. Ph3P and the tertiary amines Et3N and N-
Me-pyrrolidine did not work. DMAP and pyrrolidine proved
to promote the reaction only in the presence of water, con-
firming its important role in the catalytic cycle. The use of
DMAP in the reaction of α,β-unsaturated ketones and its
limitations have been already reported in the litera-
ture.12d,19 The finding of pyrrolidine as a secondary amine
catalyst for the MBH reaction was interesting, as it suggest-
ed the possible dual role as a nitrogen nucleophile to acti-
vate the β-position of cyclopentenone and as a carbonyl ac-
tivator through iminium catalysis to promote the reac-
tion.20 Interestingly, no Mannich-type adducts were
observed in all cases, and starting material was recovered
when the MBH did not occur. A noticeable improvement
was observed when 1 M NaHCO3 was used in place of wa-
8
0
DMAP
35
37
0
DMAP
168
168
40
Et3N
pyrrolidine
pyrrolidine
23
22
168
a The reaction was carried out using cyclopent-2-enone (1 equiv), benzal-
dehyde (1 equiv), base (0.2 equiv), with 0.25 M concn for the cyclopente-
none.
b Yields were calculated following product isolation and purification.
The concomitant application of pyrrolidine and DABCO
as the base resulted in improved yields as, using catalytic
DABCO (0.2 equiv), the yield was raised to 75%. Although
the reaction worked in the presence of pyrrolidine as the
sole catalyst, these results showed that a second nitrogen
nucleophilic catalyst has a beneficial effect on the yield. In
fact, DABCO alone is not able to attack the β-carbon of cy-
clopent-2-enone, but the formation of the iminium species
by pyrrolidine results in the activation of the β-carbon as an
electrophilic center allowing DABCO to react. Then, this in-
termediate attacks the aldehyde, and after oxygen protona-
tion and DABCO elimination, the final unsaturated product
is formed, releasing the amine on hydrolysis (Scheme 2).
An attempt to carry out the reaction using L-proline in
place of pyrrolidine reduced the yield from 75% to 32%. We
considered a possible correlation of the yield with the pKa of
the secondary amine, as pyrrolidine possesses a higher val-
ue than proline (11.4 vs 10.6). Accordingly, we tested a
range of secondary amines in the MBH reaction, and the re-
sults are shown in Table 2.
Indeed, the weaker the basic character of the amine, the
lower the yield of the MBH reaction, possibly due to a role
of the basicity of the nitrogen atom in the attack of inter-
mediate II onto the aldehyde (Scheme 2, step II). Morpho-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
R. Innocenti et al.
Letter
Synlett
line, possessing a pKa around 8, failed to react, suggesting
the inability of the iminium ion to proceed to the nucleo-
philic species as in step II in Scheme 2 (Table 2, entry 4). All
proline derivatives showed reduced yields with respect to
pyrrolidine, and a correlation between the pKa and the yield
could be established (Table 2, entries 5–8). The use of an
imidazolidinone organocatalyst,22 though well established
in the formation of the iminium ion, did not give the ex-
pected reaction (Table 2, entry 9), suggesting that its low
pKa (5.8) could not facilitate the attack of the activated spe-
cies to the aldehyde (step II of Scheme 2). Chiral (R)-N-ben-
zyl-1-phenylethanamine and (S)-2-benzhydryl-pyrrolidine,
although possessing a calculated pKa value of 10 and 11, re-
spectively, did not react under our MBH conditions, possi-
bly due to steric hindrance impairing the formation of the
iminium species with the cyclopentenone (Table 2, entries
10 and 12). Indeed, (R)-α-methyl-pyrrolidine reacted simi-
larly to pyrrolidine, confirming that a high pKa value is im-
portant for the iminium-catalyzed MBH reaction (Table 2,
entry 11). When chiral amines were used, poor degrees of
enantiomeric excess were obtained (Table 2, entries 5–8,
11). The optimized conditions consisted of the reaction of
cyclopentenone with 3 equiv of aldehyde, using catalytic
DABCO and pyrrolidine, in 1:4 THF–1 M NaHCO3 at 25 °C for
40 h.23 Insights on the mechanism were obtained with ki-
netics experiments, which allowed us to calculate pseudo-
first-order rate constants and to establish linearity with
catalyst concentration, along with ESI-MS identification of
iminium intermediates, suggesting that only one secondary
amine molecule is involved before the rate-limiting step
(see Supporting Information).
The scope of the iminium-catalyzed MBH reaction on
cyclopentenone using pyrrolidine and DABCO as catalysts
was studied for a range of aliphatic and aromatic aldehydes
(Table 3).
The MBH reaction on nitrobenzaldehydes proceeded in
quantitative yields (products 4–6), as the nitro group im-
proved the electrophilic character of the aldehyde. A similar
tendency was observed for halogen-substituted compounds
8 and 9, which were obtained in 84% and 80% yield, respec-
tively. Conversely, methoxy-substituted aldehydes showed
reaction yields ranging from 37% to 70%, with the lowest
Table 2 Screening of Amines as Iminium Ion Activators for the MBH
Reaction
O
O
OH
CHO
amine, base
+
THF–1 M NaHCO3 (1:4)
40 h, 0 °C
a
Entry
Amine
pKa
Yield, (%)b,c
1
2
3
4
5
6
7
8
pyrrolidine
piperidine
11.4
10.8
10.8
8.3
75
62
OH
O
diethylamine
morpholine
proline
64
R
O
0
10.6
10.7
8.3
32 (9)
52 (25)
20 (10)
24 (0)
prolinol
N
OH
H
N
HCO3
N
proline methyl ester
R
9.4
HO
N
CO2Me
V
N
H
N
9
5.8
0
0
O
I
N
N
H
R
HN
N
N
O
H
O
10
10.0
N
O
O
N
H
N
N
IV
N
11
12
11.5
11.0
81 (23)
0
O
H
N
II
N
H
R
HCO3
N
O
N
III
Ph
R
H
N
N
H
Ph
a Calculated as the mean value between those obtained with ACD/pKa DB
v6.0, and Marvin v6.2.2. (see Supporting Information and ref.24).
b Yields were calculated following product isolation and purification.
c % ee in parentheses, calculated using GC with chiral BetaDex120 column.
Scheme 2 Proposed mechanism of the MBH reaction mediated by a
dual catalysis via iminium formation with pyrrolidine and DABCO as the
Lewis base
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
R. Innocenti et al.
Letter
Synlett
Table 3 (continued)
Product
outcome experienced for the preparation of 13, possessing
two methoxy groups on the phenyl ring. This was also
found for the methyl-substituted compound 7, which was
obtained in 54% yield. Heterocyclic aldehydes showed reac-
tion yields around 70%, as observed for compounds 14 and
15, and the aliphatic isobutyraldehyde produced compound
16 in good yield as well.
Yield (%)
37
OH
O
O
O
O
13
73
OH
Table 3 Scope of the Iminium-Catalyzed MBH on Cyclopent-2-enone
OH
14
DABCO (0.2 equiv)
pyrrolidine (0.2 equiv)
25 °C, 40 h
O
O
O
R
69
65
+
OH
O
R
H
THF–1 M NaHCO3 (1:4)
S
15
Product
Yield (%)
OH
O
OH
75
O
16
3
>99
OH
O
O
In conclusion, report the study of the Morita–Baylis–
Hillman reaction on cyclopent-2-enone using dual iminium
and Lewis base catalysis, demonstrating that a secondary
amine is important in activating the poorly reactive enone.
We have succeeded in demonstrating that the iminium-cat-
alyzed MBH will occur on cyclopentenone with 1:4 THF–1 M
NaHCO3 solvent mixture, and using DABCO as the Lewis
base catalyst and pyrrolidine as the iminium ion generator.
The screening of a range of secondary amines showed a cor-
relation of the reactivity to the basicity, quantified by the
pKa value of the conjugated acid and also to steric hin-
drance.
4
O2N
O2N
95
OH
5
>99
54
NO2 OH
O
6
OH
O
O
7
Funding Information
84
OH
Financial support from the University of Florence and MIUR
(PRIN2015, cod. 20157WW5EH) is acknowledged.
)(
8
F
80
70
OH
O
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1591521.
Br
9
S
u
p
p
o
nrtogI
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
OH
O
O
References and Notes
O
O
10
OH
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thesis;
2
V
o.
l
Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford,
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11
O
OH
O
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Organic Molecules; University Science Books: Sausalito, 1999.
(b) Tsuji, J. Transition Metal Reagents and Catalysts; John Wiley
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Letter
Synlett
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(18) This base catalyst did not work using other solvents such as ace-
tonitrile, dichloromethane, methanol, and toluene, as well as
aqueous mixtures like DMF–water (9:1) and MeOH–water (1:1).
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(20) The concomitant iminium activation of the aldehyde was
excluded in our MBH reaction conditions as a consequence of
model experiments with ESI-MS (see Supporting Information)
and of the fact that no Mannich-type reaction was observed.
(21) A standard approach in this way was not found in the literature:
Cheng reported the use of an excess of aldehyde;14 whereas in
the paper by Gruttadauria15 an excess of cyclopent-2-enone
was used.
(4) (a) Basavaiah, D.; Rao, A. J.; Satyanarayana, T. Chem. Rev. 2003,
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(23) General Procedure for the Morita–Baylis–Hillman Reaction
In a flask containing a solution of DABCO (0.12 mmol, 14 mg) in
THF (0.5 mL) and 1 M aqueous solution of NaHCO3 (2 mL), pyr-
rolidine (0.12 mmol, 10 μL), cyclopent-2-enone (0.6 mmol), and
the aldehyde (1.8 mmol) were successively added. The reaction
was stirred at room temperature for 40 h. Then, ethyl acetate
(20 mL) was added, and the organic phase was washed with 1 M
HCl, sat. aqueous NaHCO3, and brine. The organic phase was
dried over Na2SO4, filtered, concentrated under reduced pres-
sure, and the crude product was purified by flash chromatogra-
phy column (FCC).
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2-[Hydroxy(3,4-dimethoxyphenyl)methyl]cyclopent-2-
enone (13)
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2004, 69, 8413. (c) Ito, H.; Takenaka, Y.; Fukunishi, S.; Iguchi, K.
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Obtained in 37% yield. 1H NMR (400 MHz, CDCl3): δ = 6.95–6.81
(m, 4 H), 5.50 (s, 1 H), 3.87 (s, 3 H), 3.86 (s, 3 H) 2.58 (m, 2 H),
2.45 (m, 2 H). 13C NMR (50 MHz, CDCl3): δ = 209.7, 159.3, 149.1,
148.6, 147.8, 133.9, 118.6, 110.9, 109.5, 69.7, 55.9 (2 C), 35.3,
26.6. ESI-MS: m/z (%) = 271.17 (100) [M + Na]+. Anal. Calcd for
C
14H16O4 (248.27): C, 67.73; H, 6.50. Found: C, 67.80; H, 6.58.
2-[Hydroxy(2-thienyl)methyl]cyclopent-2-enone (15)
Obtained in 69% yield. 1H NMR (400 MHz, CDCl3): δ = 7.45 (m, 1
H), 7.27–7.23 (m, 1 H), 7.00–6.95 (m, 2 H), 5.81 (s, 1 H), 3.68 (br,
1 H), 2.65–2.62 (m, 2 H), 2.49–2.47 (m, 2 H). 13C NMR (100 MHz,
CDCl3): δ = 209.3, 159.6, 146.9, 145.1, 126.7, 125.2 124.7, 66.2,
35.3, 26.7. ESI-MS: m/z (%) = 217.08 (100) [M + Na]+. Anal. Calcd
for C10H10O2S (442.52): C, 61.83; H, 5.19. Found: C, 62.00; H,
5.28.
(13) Shi, M.; Jiang, J. K.; Li, C. Q. Tetrahedron Lett. 2002, 43, 127.
(14) Davies, H. J.; Ruda, A. M.; Tomkinson, N. C. O. Tetrahedron Lett.
2007, 48, 1461.
(24) Liao, C.; Nicklaus, M. C. J. Chem. Inf. Model. 2009, 49, 2801.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E