Ionic Thiourea Organocatalysis of the Morita-Baylis-Hillman Reaction

Article abstract of DOI:10.1071/CH15596

An ionic thiourea based organocatalyst has been shown to promote a 1,4-diazabicyclo[2.2.2]octane, (DABCO) catalysed Morita-Baylis-Hillman reaction between benzaldehyde and cyclohex-2-en-1-one. The ionic thiourea catalyst was easily prepared from a pyrroli

Full text of DOI:10.1071/CH15596

CSIRO PUBLISHING
Aust. J. Chem. 2016, 69, 759–762
Full Paper
http://dx.doi.org/10.1071/CH15596
Ionic Thiourea Organocatalysis of the
Morita–Baylis–Hillman Reaction
A
A
A
Trevor McGrath, Katherine N. Robertson, Jason D. Masuda,
A
A B
,
Jason A. C. Clyburne, and Robert D. Singer
A
Atlantic Centre for Green Chemistry, Department of Chemistry,
Saint Mary’s University, Halifax, Nova Scotia, Canada.
Corresponding author. Email: robert.singer@smu.ca
B
An ionic thiourea based organocatalyst has been shown to promote a 1,4-diazabicyclo[2.2.2]octane, (DABCO) catalysed
Morita-Baylis-Hillman reaction between benzaldehyde and cyclohex-2-en-1-one. The ionic thiourea catalyst was easily
prepared from a pyrrolidinium salt containing an arylamine moiety and 3,5-di(trifluoromethyl)phenylisothiocyanate.
X-ray crystallographic analysis of the ionic thiourea catalyst shows an acetone molecule doubly hydrogen bonded to the
Lewis acidic thiourea N-H protons. Entrainment of the ionic thiourea co-catalyst in the ionic liquid N-butyl-N-
methylpyrrolidinium bistriflimide, [BMPyr][N(Tf)2], facilitates catalyst recycling and affords very good yields with
reaction times reduced through use of microwave heating.
Manuscript received: 23 September 2015.
Manuscript accepted: 23 November 2015.
Published online: 28 January 2016.
Introduction
ionic liquid moiety based on the pyrrolidinium cation into a
thiourea organocatalyst allows dissolution of the ionic organoca-
talyst in the ionic liquid solvent, thereby facilitating homogenous
reaction conditions in the ionic liquid phase. Furthermore, a
pyrrolidinium cationic tag would avoid any potential side reac-
tions that have been shown to occur when other ions, such as the
imidazolium cation, are present in MBH reaction mixtures.[4]
Separation of the ionic catalyst and ionic liquid phase from the
reactants and products using an organic solvent such as diethyl
ether, Et2O, is then possible. This strategy provides a very simple
method to recycle the catalyst for use in subsequent reactions and
mimics the advantages of both homogeneous and heterogeneous
catalytic systems.
Microwave heating using ionic liquids as a reaction medium
has been shown to be very efficient due to the microwave-
absorbing properties of ionic liquids.[9] Microwave irradiation
has also been shown to accelerate the Morita–Baylis–Hillman
reaction.[10] Inspired by previous studies using ionic liquid-
tagged molecules for catalytic applications,[11] an ionic liquid-
tagged thiourea was designed for use as an organocatalyst.
Herein, we report the synthesis and application of an ionic
thiourea organocatalyst, wherein the ionic nature of the thiourea
catalyst renders the catalyst ionophilic and thus very suitable for
use and recycling in an ionic liquid. This system is hence well
suited to the use of microwave heating due to the ionic nature of
the organocatalyst and the use of an ionic liquid reaction medium.
We report herein the use of an ionic thiourea oragnocatalyst
for the MBH reaction using an ionic liquid reaction medium
with microwave irradiation. The ionic thiourea derivative 3
reported herein is a salt and solid at room temperature and hence
is not an ‘ionic liquid’. Rather, as a salt, it is ‘ionophilic’ and
readily dissolves in an ionic liquid reaction medium. Hence,
The Morita–Baylis–Hillman (MBH) reaction is a valuable
carbon–carbon bond-forming reaction, affording allylic alcohol
products with high functional group density and is typically
catalyzed by a tertiary amine or tertiary phosphine.[1] The salient
drawback of the MBH reaction is the slow rate of reaction,
resulting in long reaction times. Several methods are known to
accelerate the MBH reaction such as the use of protic addi-
tives,[2] Lewis acids,[3] and ionic liquid solvents.[4] Schreiner
and coworkers have shown that electron-deficient thiourea
derivatives can behave as Lewis acids and accelerate Diels–
Alder reactions.[5] As Lewis acids can also promote the
MBH reaction, electron-deficient thiourea derivatives are
therefore expected to also accelerate the MBH reaction. This
was demonstrated by Nagasawa and coworkers – the authors
showed that the MBH reaction could be accelerated using a
thiourea organocatalyst.[6]
Organocatalysis, which refers to the use of a sub-stoichio-
metric quantity of an organic molecule to accelerate a chemical
reaction, is an area that has experienced rapid growth since the
turn of the twenty-first century.[7] Despite the emergence of
organocatalysis over the past number of years, organocatalysis
using ionic liquid systems remains relatively unexplored,
namely with respect to thiourea-based organocatalysts. Several
examples of ionic liquid-functionalized proline derivatives have
been reported for use as organocatalysts.[8] To our knowledge,
there are no reports on the use of ionic liquid-based thiourea
deriviatives as organocatalysts.
The ionic liquid N-butyl-N-methylpyrrolidinium bistriflimide,
[BMPyr][N(Tf)2], is an excellent solvent for a variety of organic,
organometallic, and inorganic compounds; however, it is immis-
cible in selected organic solvents. Hence, the incorporation of an
Journal compilation ꢀ CSIRO 2016
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760
T. McGrath et al.
CF3
NH2
NO2
NO2
CF3
N
1)
Fe0/H2O
NCS
F3C
S
N
ϩ
r.t./24 h
73 %
MeCN/MW
50ЊC/2 h
81 %
MeCN/MW
80ЊC/15 min
Ϫ
PF6
Ϫ
N
H
PF6
F3C
N
H
Ϫ
81 %
PF6
N
N
Br 2) HPF6(aq)/H2O
ϩ
ϩ
3
r.t./3 h
85 %
1
2
Scheme 1. Synthesis of ionic thiourea catalyst 3. MW ¼ microwave heating; r.t. ¼ room temperature.
this reaction system and protocol offers the advantages of
(1) catalytic recyclability, (2) solvent recyclability, and
(3) reduced reaction times while permitting comparable yields
of products relative to those obtained from more conventional
systems reported to date.
2.621
2.751
Results and Discussion
The criteria necessary for organocatalytic activity in thiourea
derivatives include the presence of sufficiently electron-
deficient secondary amine protons, thereby enabling them to act
as effective Lewis acids.[12] Thus, the aryl substituents on the
thiourea moiety need to bear electron-withdrawing groups.
Substitution of the aryl groups at the ortho position has been
shown to be unfavourable and ought to be avoided. Rigidifying
interactions between the ortho protons of the aryl rings and
the sulfur lone pair electrons are also required. A previously
prepared ionic thiourea derivative, specifically and intentionally
designed for metal extractions, lacked these structural and
electronic features and hence would be unlikely to be organo-
catalytic.[13] To incorporate these required features in our ionic
thiourea derivative, p-nitrobenzylbromide was alkylated using
N-methylpyrrolidine to afford a pyrrolidinium bromide salt that
was then metathesized to the hexafluorophosphate salt 1, thus
enabling facile purification (Scheme 1). The nitro group was
then easily reduced using Fe0 in water to afford the amine 2,
which then underwent reaction with 3,5-di(trifluoromethyl)
phenyl isothiocyanate under microwave irradiation to give the
ionic thiourea derivative 3.
2.117(17)
2.107(17)
Fig. 1. X-ray crystal structure of ionic thiourea co-catalyst 3. Ellipsoids
drawn at 50% probability level. (Disorder of the cation and anion, and a
complete disordered acetone solvent molecule have been omitted for clarity).
obtained using Schreiner’s neutral thiourea derivative 4 when
the reaction was performed over 2 days in ionic liquid [BMPyr]
[N(Tf)2] while using conventional heating (Table 1, Entries 4
and 5a).
An advantage of our ionic thiourea co-catalyst 3 employing
the ionic liquid [BMPyr][N(Tf)2] as a reaction medium was
revealed in a series of recycling experiments (Table 1, Entries
5a–5c); no apparent loss of catalytic activity was observed over
three cycles using the ionic thiourea catalyst/ionic liquid system.
Furthermore, we conducted leaching experiments that revealed
that no ionic thiourea co-catalyst 3 was leached out of the ionic
liquid phase/reaction medium, as observed by 1H NMR analysis,
during Et2O extractions of reaction products. Hence, the ionic
thiourea co-catalyst 3 was successfully entrained in the ionic
liquid phase. Conversely, neutral thiourea catalyst 4 readily
leached into Et2O from the ionic liquid phase during extraction
of products, thereby making the recycling of the neutral catalyst
4 problematic. Unsurprisingly, no allylic alcohol product was
observed after 2 days of conventional heating at 258C in the
ionic liquid-mediated reaction in the absence of catalyst 3
(Table 1, Entry 6).
Another advantage of our ionic thiourea co-catalyst 3 used
in an ionic liquid reaction medium becomes evident when the
reaction is heated using microwave irradiation (Table 1, Entry 7).
Owing to the enhanced microwave-absorbing properties of
the ionic liquid used, [BMPyr][N(Tf)2], reaction times were
significantly decreased to 6 h (versus 48 h) when microwave
heating at 508C was employed affording useful percentage
conversion to allylic alcohol product. It is possible that the ionic
nature of 3 also was a factor on the microwave-irradiated
The product was recrystallized from acetone, and the struc-
ture was determined unequivocally as an acetone solvate of 3
using single-crystal X-ray diffraction (Fig. 1).
The X-ray structure displays an acetone molecule doubly
hydrogen-bonded to the Lewis acidic thiourea N–H protons of 3.
Such an interaction has been proposed as a key interaction for
the acceleration of the MBH reaction by thiourea organocata-
lysts.[12] Additionally, the rigidifying interactions determined to
be necessary for catalytic activity by Schreiner and coworkers
between the ortho protons of the phenyl rings and the sulfur
atom of the thiourea backbone are also observed.[12]
Using Schreiner’s thiourea, N,N0-bis[3,5-bis(trifluoromethyl)
phenyl]thiourea (4), as a benchmark, the ionic thiourea derivative
3 was tested for catalytic activity using neat conditions in an ionic
liquid and in an ionic liquid under microwave irradiation (Table 1).
Using the ionic thiourea organocatalyst 3 afforded a slightly
lower conversion to the allylic alcohol product under neat
reaction conditions at 258C under conventional heating using
a hotplate (Table 1, Entries 1 and 2); no conversion to product
was observed in the absence of 3 (Table 1, Entry 3). The use of
our ionic thiourea co-catalyst 3 gave comparable yields to those

Ionic Thiourea Organocatalysis of the MBH Reaction
761
Table 1. Morita–Baylis–Hillman reaction using ionic thiourea co-catalyst 3
O
O
OH
O
10 % co-catalyst 3
H
10 % DABCO
1 equiv.
5 equiv.
Entry
Co-Catalyst
Time [h]
Temperature [8C]
Solvent
Conversion [%]A
1
3
24
24
24
48
48
25
25
25
25
25
Neat
80
96
0
2
4
Neat
3
None
Neat
4
4
[BMPyr][NTf2]
[BMPyr][NTf2]
98
92
90
91
0
5a
5b
5c
6
3
3
3
None
3
48
6
25
[BMPyr][NTf2]
[BMPyr][NTf2]
[BMPyr][NTf2]
[BMPyr][NTf2]
7
50B
50B
50B
78
89
5
8
4
6
9
None
6
APercentage conversions determined by 1H NMR.
BDielectric heating using microwave irradiation.
reaction and hence provided a synergistic effect. However, this
is yet to be confirmed in future experiments using a broader
array of substrates. Although the neutral thiourea derivative 4
also gave comparable useful conversion to the allylic alcohol
product under similar conditions (Table 1, Entry 8), it is not
recyclable due to the leaching problems described earlier.
Heating the ionic liquid-mediated reaction under microwave
irradiation in the absence of ionic thiourea co-catalyst 3 afforded
only a very small conversion to allylic alcohol product (Table 1,
Entry 9), thus demonstrating the generally slow nature of the
MBH reaction in the absence of a thiourea catalyst even at
higher temperatures in an ionic liquid reaction medium.
solvent (see Table 1) with stirring at room temperature.
Cyclohex-2-en-1-one (0.49 mL, 5.00 mmol) and benzaldehyde
(0.10 mL, 1.00 mmol) were then added sequentially to the
reaction mixture. 1,4-diazabicyclo[2.2.2]octane (DABCO;
0.056 g, 10 mol-%) was then added to the reaction mixture. The
reaction was stirred at room temperature (258C) for a specified
reaction time. Alternatively, reactions were heated using a CEM
Discover microwave reactor operating at 20 W. Upon comple-
tion of the reaction, the organic materials were extracted using
diethyl ether (5 ꢀ 10 mL). An aliquot of the ether extract was
dissolved in CDCl3, and the sample was analyzed via 1H NMR
spectroscopy to determine the percentage conversion to the
allylic alcohol product, 2-(hydroxyphenylmethyl)cyclohex-2-
en-1-one. To recycle the ionic liquid phase containing catalyst 3,
the ionic liquid phase was placed under vacuum to remove
any residual ether. Benzaldehyde, cyclohex-2-en-1-one, and
DABCO were added in the same quantities as those used in the
first run. The mixture was stirred for 48 h and extracted using the
same procedure with diethyl ether (5 ꢀ 10 mL). The third con-
secutive reaction in [BMPyr][N(Tf)2] was performed using
the same procedure as that used for the second reaction, and the
results are summarized in Table 1 (Entries 5a–5c).
Conclusion
Herein, we demonstrate that ionic thiourea co-catalyst 3 can be
easily synthesized and successfully catalyzes the MBH reaction in
a recyclable fashion and in shortened reaction times under
microwave irradiation in an ionic liquid reaction medium. X-ray
crystallographic analysis confirms the ionic nature of our catalyst
(3). Very good percentage conversion to allylic alcohol product
comparable with that obtained using a neutral thiourea catalyst 4)
can be obtained. The ionophillic nature of co-catalyst 3 facilitates
preferential partitioning/entrainment into the ionic liquid [BMPyr]
[N(Tf)2], consequently enabling recycling of the catalyst and
solvent system under efficient microwave heating, leading to
reduced reaction times. The MBH reaction involves charged
intermediates making ionic solvents ideal reaction media. Our
catalytic system presents the possibility for the scale-up of the
MBH reaction to produce large quantities of allylic alcohol pro-
ducts with a high functional group density using a continuous flow
approach. The scope and limitations of this catalytic strategy using
a wider variety of aldehydes and enones are currently under
investigation. The effect of varying the ionic liquid medium as
well as the ionic thiourea derivative through simple alterations via
our modular synthetic approach is also being further investigated.
1-Nitrobenzyl-1-methylpyrrolidinium
Hexafluorophosphate (1)
p-Nitrobenzyl bromide (1.00 g, 4.63 mmol) was dissolved
in acetonitrile (MeCN; 10 mL), and N-methylpyrrolidine
(0.48 mL, 4.63 mmol) was added dropwise at ambient tempera-
ture. The reaction vessel was heated using microwave irradiation
to 808C and held for 15 min. The product was precipitated from
MeCN using an equal volume of ethyl acetate, filtered, and dried
under vacuum to afford the intermediate product (1.13 g, 81 %),
mp 168–1698C. nmax (KBr)/cmꢁ1 2963, 1605, 1528, 1424, 1346.
dH ([D6]DMSO, 300 MHz) 8.35 (2H, d, J 8.7), 7.94 (2H, d, J 8.7),
4.85 (2H, s), 3.65–3.69 (2H, m), 3.46–4.51 (2H, m), 2.97 (3H, s),
2.14–2.16 (2H, m). dC ([D6]DMSO, 75 MHz) 149.0, 136.9, 134.6,
124.3, 64.1, 63.5 47.7, 21.2. m/z (electrospray ionization; ESIþ)
221.9 (100 %, [C12H17N2O2]þ); calcd 221.1.
Experimental
General Procedure
Ionic thiourea catalyst 3 (0.304 g, 10 mol-%) was placed in a
50-mL round-bottom flask and dissolved in the appropriate
N-Methyl-N-(p-nitrobenzyl)pyrrolidinium bromide (1.23 g,
4.08 mmol) was dissolved in a minimum amount of water, and

762
T. McGrath et al.
an aqueous solution of HPF6 (1.25 equiv., 0.43 mL) was added
dropwise. The product which precipitated was then filtered off
after 3 h, and dried under vacuum. The product obtained was a
white solid (1.27 g, 85 %), mp 155–1568C. nmax (KBr)/cmꢁ1
3081, 1610, 1530, 1431, 1359, 834, 557. dH ([D6]DMSO,
300 MHz) 8.34 (2H, d, J 8.8), 7.86 (2H, d, J 8.8), 4.72 (2H, s),
3.58–3.62 (2H, m), 3.41–3.47 (2H, m), 2.93 (3H, s), 2.14–2.16
(2H, m). dC ([D6]DMSO, 75 MHz) 149.0, 136.4, 134.5, 124.3,
64.5, 63.6, 47.8, 21.2. dP ([D6]DMSO, 121 MHz) ꢁ144.2
(septet, J 711.4). m/z (ESIþ) 221.1 (100 %, [C12H17N2O2]þ);
calcd 221.1. m/z (ESIꢁ) 144.6 (100 %, [PF6]ꢁ); calcd 144.9.
reaction products are available on the Journal’s website. The
crystallographic data for 3 have been deposited (CCDC
1010910). The data can be obtained from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; Fax þ44 1223 336033; Email: deposit @ccdc.cam.
ac.uk or from www.ccdc.cam.ac.uk.
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of Canada
(Natural Sciences and Engineering Research Council of Canada (NSERC)
Discovery to RDS and Undergraduate Research Summer Award (USRA) to
TM), Canadian Foundation for Innovation (for the X-ray diffractometer and
NMR experiments), Nova Scotia Research and Innovation Trust, and Saint
Mary’s University (Faculty of Graduate Studies and Research) for funding
for this research.
1-Aminobenzyl-1-methylpyrrolidinium
Hexafluorophosphate (2)
FeSO4ꢂ7H2O (5.75 g, 20.69 mmol) and sodium citrate (0.44 g,
1.72 mmol) were added to water (100 mL). NaBH4 (1.30 g,
34.49 mmol) was added slowly, and the iron was reduced to
black Fe0. The water was decanted, and the nanoparticles were
washed and decanted twice more with water (50 mL). N-Methyl-
N-(p-nitrobenzyl)pyrrolidinium hexafluorophosphate (1.26 g,
3.45 mmol) was added, and the reaction was stirred at ambient
temperature for 24 h. The reaction mixture was passed through a
vacuum frit to remove water and other aqueous impurities.
The residue in the frit was washed with MeCN (3 ꢀ 20 mL).
MeCN from the washings was removed under vacuum, and the
product was dried under vacuum. The product was obtained as a
yellow solid (0.85 g, 73 %), mp 143–1448C. nmax (KBr)/cmꢁ1
3489, 3401, 2981, 1632, 1426, 835, 558. dH ([D6]DMSO,
300 MHz) 7.16 (2H, d, J 8.4), 6.60 (2H, d, J 8.4), 5.53 (s, 2NH),
4.30 (2H, s), 3.44–3.48 (2H, m), 3.26–3.30 (2H, m), 2.83 (3H, s),
2.09 (2H, s). dC ([D6]DMSO, 75 MHz) 150.8, 133.8, 115.4,
114.0, 66.1, 62.3, 47.5, 21.3. dP ([D6]DMSO, 121 MHz) ꢁ144.2
(septet, J 711.3). m/z (ESIþ) 191.2 (99.5 %, [C12H19N2]þ); calcd
191.1. m/z (ESIꢁ) 144.6 (100 %, [PF6]ꢁ); calcd 144.9. ESI
HRMS (positive mode) m/z 191.1535 (99.5 %, [C12H19N2]þ);
calcd for [C12H19N2]þ 191.1548.
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Supplementary Material
Spectroscopic data, including 1H and 13C NMR spectra, of
all intermediate products, ionic thiourea derivative 3, and