Self-supported N-heterocyclic carbenes and their use as organocatalysts

Article abstract of DOI:10.3390/molecules21081100

The study of N-heterocyclic carbenes (NHCs) as organocatalysts has proliferated in recent years, and they have been found to be useful in a variety of reactions. In an attempt to further expand their utility and to study their recyclability, we designed and synthesized a series of self-supported NHCs in which the catalytic carbene groups form part of a densely functionalized polymer backbone, and studied them as organocatalysts. Of the self-Supported NHCs examined, a benzimidazole derived polymer with flexible linkers connecting the catalytic groups was found to be the most efficient organocatalyst in a model benzoin condensation reaction, and thus it was used in a variety of such reactions, including some involving catalyst recycling. Furthermore, it was also used to catalyze a set of redox esterification reactions involving conjugated unsaturated aldehydes. In all of these reactions the catalyst afforded good yield of the desired product and its polymeric nature facilitated product purification.

Full text of DOI:10.3390/molecules21081100

molecules
Article
Self-Supported N-Heterocyclic Carbenes and Their
Use as Organocatalysts
Shuang Ma and Patrick H. Toy *
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong, China;
u3002732@connect.hku.hk
* Correspondence: phtoy@hku.hk; Tel.: +852-28592167; Fax: +852-28571586
Academic Editor: Maurizio Benaglia
Received: 19 July 2016; Accepted: 15 August 2016; Published: 20 August 2016
Abstract: The study of N-heterocyclic carbenes (NHCs) as organocatalysts has proliferated in recent
years, and they have been found to be useful in a variety of reactions. In an attempt to further expand
their utility and to study their recyclability, we designed and synthesized a series of self-supported
NHCs in which the catalytic carbene groups form part of a densely functionalized polymer backbone,
and studied them as organocatalysts. Of the self-Supported NHCs examined, a benzimidazole
derived polymer with flexible linkers connecting the catalytic groups was found to be the most
efficient organocatalyst in a model benzoin condensation reaction, and thus it was used in a variety of
such reactions, including some involving catalyst recycling. Furthermore, it was also used to catalyze
a set of redox esterification reactions involving conjugated unsaturated aldehydes. In all of these
reactions the catalyst afforded good yield of the desired product and its polymeric nature facilitated
product purification.
Keywords: self-supported catalyst; organocatalyst; N-heterocyclic carbene; benzoin condensation;
redox esterification
1. Introduction
As research regarding N-hereocyclic carbenes (NHCs) has increased in recent years, it has become
clear that they are highly useful organocatalysts for a wide range of molecular transformations
due to their ability to react with aldehydes and generate the corresponding homoenolate reactive
intermediates [1–12]. In light of their great utility and specialized reactivity, efforts have been made to
simplify their use and facilitate their recycling, and one strategy to achieve these aims has been to attach
NHCs to a polymer support that allows for them to be easily removed at the end of a reaction and
potentially reused. For example, imidazolium and related ionic liquids have been attached to various
inert polymer carriers [13–17], and these can serve as protonated precursors to the corresponding
NHC organocatalysts [18 24]. However, a drawback of this strategy is that the use of an inert polymer
–
support lowers the loading level of the catalyst, even more so if a linker group is used for attachment,
and increases the amount of solvent required for reactions (Figure 1a). An emerging strategy to work
around this issue that potentially reduces the amount of dead weight, especially for imidazolium NHC
precursors that require substituents on both nitrogen atoms (vide infra), is to self-support the catalyst
by linking the catalytic groups in such a manner so that they are located in the main polymer chain,
rather than attached to it (Figure 1b).
(a)
(b)
NHC group
linker
NHC group
n
dead weight
Figure 1. (a) Polymer-supported NHC organocatalysts. (b) Self-supported NHC organocatalysts.
Molecules 2016, 21, 1100; doi:10.3390/molecules21081100
www.mdpi.com/journal/molecules

Molecules 2016, 21, 1100
2 of 12
For example, Cowley et al. described the use of
1
as a recyclable self-supported organocatalyst
precursor for benzoin condensation reactions (Figure 2) [25]. However, relatively large solubilizing
groups were used to link the imidazolium groups, which significantly lowered the catalytic group
loading level of the polymer. Additionally, the linking groups were not attached to the nitrogen
atoms, reducing the loading efficiency even further. In related research, Ying and co-workers used
multifunctional linker groups to join imidazolium groups that did not bear solubilizing groups,
and were attached at the nitrogen atoms to generate heterogeneous polymers, such as
These polymers were used as NHC precursors in catalytic applications, often in the presence of a metal
such as Pd or Cu. Also, Liu et al. synthesized polymers, such as , using fluorinated linker groups
2 [26–29].
3
to connect imidazolium groups [30]. However, these polymers were not used as NHC precursors,
but were directly applied as salts to catalyze CO2 cycloaddition reactions with epoxides to form
cyclic carbonates.
Figure 2. Examples of self-supported imidazolium salts used as N-hereocyclic carbene (NHC)
precursors and catalysts.
Our group has had a longstanding interest in studying polymer-supported catalysts and reagents
in general [31–34], and we have recently reported a self-supported oxime ligand for use in Pd-catalyzed
Suzuki-Miyaura reactions [35]. This research has motivated us to examine self-supported imidazolium
and benzimidazolium salts with flexible linker groups as NHC organocatalyt precursors. The premise
underlying this research was that while the self-supported polymeric salts themselves would be
insoluble in most organic solvents, the use of flexible linker groups would allow for the polymers
to become soluble after deprotonation to generate the corresponding neutral self-supported NHC
groups, which could then serve as homogeneous catalytic groups. Furthermore, it was expected
that reprotonation of the polymers at the end of reactions would precipitate them from the reaction
mixtures so that they could be separated from the desired products simply by filtration, and be ready
for reuse. Herein we report the results of our research.
2. Results and Discussion
2.1. Synthesis of Self-Supported NHC Precursors
Since one of the major objectives of this research project was to examine what effect, if any,
the use of flexible linker groups to connect the NHC groups has on their solubility, and thus
catalytic utility, we targeted polymers
utilized by Ying and co-workers were used for their preparation [26]. For the synthesis of
, two equivalents of imidazole ( ) were deprotonated using NaH and then alkylated with a single
equivalent of 1,4-bis(bromomethyl)benzene ( ) to afford in quantitative yield. Alkylation of using
4
–
6
for study (Scheme 1), and methods similar to those
4
and
5
7
8
9
9

Molecules 2016, 21, 1100
3 of 12
8
again in DMF at elevated temperature afforded polymeric imidazolium salt
1,3-dibromopropane (10) to alkylate afforded polymer
of benzimidazole (11) were alkylated as before with one equivalent of 10 to afford 12 in quantitative
yield, and 12 was subsequently alkylated with 10 a second time to afford polymer in 90% yield.
Thus, polymers (with relatively rigid linkers), (with one relatively rigid linker and one flexible
linker), and (with two flexible linkers) were prepared in short order and high overall yield from
simple starting materials.
4
in 75% yield. Using
9
5 in 86% yield. Similarly, two equivalents
6
4
5
6
Scheme 1. Synthesis of polymers 4–6.
2.2. Self-Supported NHCs as Catalysts in Benzoin Condensation Reactions
With our target polymeric salts in hand, we examined their utility as NHC precursors in the
benzoin condensation of benzaldehyde (13a) to produce the alpha-hydroxy ketone product 14a
.
The base DBU was used in DMSO for deprotonation of the polymers to generate the NHC catalytic
groups, and in side-by-side reactions when no polymer or polymer
14a was detected after 18 h (Table 1, entries 1 and 2). DBU was chosen as the base for these studies
since it is typically used in such reactions, and we believe that the lack of reactivity of was due
to its complete insolublity. When polymers and were used, 14a was formed in 74% and 82%
4 was used, no desired product
4
5
6
isolated yield, respectively (entries 3 and 4). Thus, it does appear that incorporating flexible linker
groups into the polymers does increase the catalytic utility of the self-supported NHC organocatalysts
by increasing polymer solubility. While polymers
5 and 6 performed similarly, we chose to use
6
for our further studies. Changing the reaction solvent to THF resulted in significantly lower
yield (entry 5), but using DMF allowed for high yield (84%) after only 5 h (entry 6). Increasing
the reaction concentration further enhanced the isolated yield to 92% (entry 7). When no DBU
was added, no reaction occurred, confirming the role of NHC catalysis (entry 8). For the sake of
comparison, we prepared N,N’-diethylbenzi midazolium bromide (15, see Experimental Section) as
a small molecule analogue of polymer
6, and found that the corresponding NHC was much less
effective as an organocatalyst (entry 9). Perhaps locating the NHC groups closely in a polymer allows

Molecules 2016, 21, 1100
4 of 12
for increased local concentration of the catalytic groups compared to the case of a molecular catalyst
what would be more evenly distributed throughout the solution, and this high local concentration
allows for synergism between the functional groups that results in improved catalytic efficiency [36,37].
Table 1. Benzoin condensation reaction of 13a using polymers 4–6 and 15 a.
Entry
NHC-HBr
Solvent
Time (h)
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
–
4
5
6
6
6
6
6
15
DMSO (2 mL)
DMSO (2 mL)
DMSO (2 mL)
DMSO (2 mL)
THF (2 mL)
DMF (2 mL)
DMF (1 mL)
DMF (1 mL) b
DMF (1 mL)
18
18
18
18
18
5
5
5
5
0
0
74
82
41
84
92
0
7
a
Unless otherwise specified, the reactions were performed on a 0.5 mmol scale of benzaldehyde with 10 mol %
NHC precatalysts and 15 mol % DBU at room temperature; b Without DBU.
Using the best identified reaction conditions, a variety of aromatic aldehydes were used as
substrates in benzoin condensation reactions using polymer as the precatalyst. As can be seen
6
in Table 2, simple electron-donating and electron–withdrawing group substituted benzldehydes
13b–d worked well in these reactions to produce the expected products 14b–d in high isolated yields
(entries 1–3). Highly electron-rich 13e and highly electron-poor 13f also worked well in these reactions,
producing 14e and 14f, respectively (entries 5 and 6). Even napthaldehyde 13g could be converted
into product 14g in excellent yield.
Table 2. Substrate scope of benzoin condensation reactions using polymer 6 a.
Entry
Substrate
Time (h)
Product
Isolated Yield (%)
1
2
3
4
5
6
a
Ar = 4-Cl-C6H4- (13b)
Ar = 4-Br-C6H4- (13c)
Ar = 4-MeO-C6H4- (13d)
Ar = 3,4,5-(MeO)3 -C6H2- (13e)
Ar = 4-CF3-C6H4- (13f)
Ar = 2-Napth- (13g)
12
12
12
5
12
5
Ar = 4-Cl-C6H4- (14b)
Ar = 4-Br-C6H4- (14c)
Ar = 4-MeO-C6H4- (14d)
Ar = 3,4,5-(MeO)3 -C6H2- (14e)
Ar = 4-CF3-C6H4- (14f)
Ar = 2-Napth- (14g)
94
88
82
82
78
88
The reactions were performed on a 0.5 mmol scale of aldehyde with 10 mol %
5
and 15 mol % DBU at room
temperature in 1 mL dry DMF.
The possibility of recovering and reusing
6 was also explored using 13a as the substrate (Table 3).
After the reaction to form 13a was complete, a solution of HCl in 1,4-dioxane (4.0 M) was added slowly
dropwise into the mixture. The reaction mixture was stirred for an additional 1 h at room temperature,
followed by the addition of ethyl acetate (20 mL). The liquid containing the crude 13a was decanted
and the light yellow precipitate
6 was washed with additional ethyl acetate (10 mL), and dried under
vacuum (quantitative recovery). This was then directly used in the next reaction cycle. As can be seen
in Table 3, polymer 6 was used five times with no obvious decrease in catalytic efficiency.

Molecules 2016, 21, 1100
5 of 12
Table 3. Recycling experiments using polymer 6 a.
Cycle
1
2
3
4
5
Isolated Yield (%) 92
92
91
90
92
a
The reactions were performed on a 0.5 mmol scale of 12a with 10 mol %
6
precatalysts and 15 mol % DBU at
room temperature in 1 mL dry DMF.
2.3. Self-Supported NHCs as Catalysts in Redox Esterification Reactions of α,β-Unsaturated Aldehydes
We next applied in the redox esterification reactions of conjugated unsaturated aldehydes
6
described by Chan and Scheidt [38]. Since it was previously reported that these reactions
require a balance between the ability to generate the NHC catalyst through deprotonation of
the benzimidazolium salt and the acidity needed to protonate the homoenolate equivalent [39],
we examined the use of weakly acidic phenol additives in these reactions. Initial reactions were
performed using cinnamaldehyde (16A) and benzyl alcohol (17a) as substrates (Table 4). When no
◦
additive was used, only 44% yield of ester 18Aa was obtained in toluene at 100 C after 4 h using
10 mol % 6 (entry 1). However, when PhOH (1 equiv.) was added and the reaction time was increased
to 48 h, 86% of 18Aa was obtained (entry 2). Switching to 4-nitrophenol (1 equiv.) further increased the
yield of 18Aa to 91% under otherwise identical reaction conditions (entry 3). Doubling the amount
of
only served to lower the yield and increase the reaction time necessary for completion (entry 5).
Interestingly, using 15 in place of lead to lowering of the isolated yield of 18Aa to 51% even after 8 h.
6 to 20 mol % allowed the reaction time to be shortened to 3 h (entry 4), but diluting the reaction
6
Table 4. NHC-catalyzed redox esterification reactions using polymer 6 a.
Entry
6 (mol %) Additive (1 equiv.)
Time (h)
Isolated Yield (%)
1
2
3
10
10
10
20
20
20
–
4
48
48
3
8
8
44
86
91
90
72
51
phenol
4-nitrophenol
4-nitrophenol
4-nitrophenol
4-nitrophenol
4
5 b
6 c
a
Unless otherwise specified, the reactions were performed on a 1.5 mmol of 16A and 3.0 mmol of 17a with
a 1:1.25 molar ratio of
6
to DBU in 1 mL toluene; b 2 mL toluene was used; c 15 was used as the precatalyst
instead of 6.
Using the best identified reaction conditions we next examined the substrate scope of such
redox esterification reaction using as summarized in Table 5. Primary benzyl alcohols 17b were
reacted with 16A to produce the corresponding esters 18Ab in excellent yields (entries 1–3). When
6
–d
–d
secondary benzyl alcohol 17e was used, only low yield of 18Ae was obtained (entry 4), as was the
case with menthol (17f) to form 18Af (entry 5). Perhaps not surprisingly, when no substrate alcohol
was used, 4-nitrophenyl ester 18Ag was obtained in very low yield (entry 6). Electron-withdrawing
group substituted cinnamaldehydes 16B and 16C reacted with 17a to form 18Ba and 18Ca in excellent
yields, respectively (entries 7 and 8), as did electron-donating group substituted cinnamaldehydes

Molecules 2016, 21, 1100
6 of 12
16D and 16E to form 18Da and 18Ea, respectively (entries 9 and 10). Even heterocyclic analogue of
cinnamaldehyde 16F produced the corresponding ester 18Fa in very high yield (entry 11).
Table 5. Substrate scope of redox esterification reactions using 6 a.
Entry
Ar
R
Time (h)
Product
Isolated Yield (%)
1
2
3
4
5
Ph- (16A)
Ph- (16A)
Ph- (16A)
Ph- (16A)
Ph- (16A)
4-Br-C6H4CH2- (17b
2-Br-C6H4CH2- (17c)
)
7
12
12
20
48
48
4
3
3
5
7
18Ab
18Ac
18Ad
18Ae
18Af
18Ag
18Ba
18Ca
18Da
18Ea
18Fa
94
92
94
30
30
7
89
89
90
79
89
2-Cl-C6H4CH2- (17d
PhCH(CH3)- (17e)
menthyl- (17f)
–
)
6 b
7
8
9
Ph- (16A)
4-Br-C6H4- (16B)
4-CN-C6H4- (16C)
4-tBu-C6H4- (16D)
4-MeO-C6H4- (16E)
2-furanyl- (16F)
Bn- (17a)
Bn- (17a)
Bn- (17a)
Bn- (17a)
10
11
a
Bn- (17a)
Unless otherwise specified, the reactions were performed on a 1.5 mmol scale of aldehyde 16 with 2.0 equiv of
alcohol 17, 20 mol %
of cinnamaldehyde and 4-nitrophenol.
6
and 25 mol % DBU at 100 ◦C in 1 mL toluene; b Product (18Ag) formed from the reaction
3. Experimental Section
3.1. General Information
Unless otherwise noted, all reagents were obtained from the Acros (Bridgewater, NJ, USA),
Aldrich (St. Louis, MO, USA), or Alfa Aesar (Ward Hill, MA, USA) companies, and were used directly
without purification. Toluene was dried using a Solv-Tek purification system employing activated
Al2O3 (Innovative Technology China Ltd., Hong Kong, China). Tetrahedrofuran (THF) was distilled
from Na metal under a N2 atmosphere before use. All reactions were carried out in dry glassware
under a N2 atmosphere, and were monitored by TLC analysis using GF254 silica gel coated plates.
The corresponding Rf values and solvents used as eluents are listed. Column chromatography was
performed with silica gel (230–400 mesh). NMR spectra were recorded on a Bruker (Billerica, MA,
USA) DRX-300 or DRX-400 spectrometer operating at 300/400 MHz for 1H and 75/100 MHz for 13
C
analysis. Chemical shift data is expressed in ppm with reference to TMS. The following abbreviations
were used for the assignment of the signals and their multiplicities: s (singlet), bs (broad singlet),
d (doublet), t (triplet), q (quartet), p (pentet), m (multiplet), dt (double of triplet). Mass spectra data
were obtained on a Finnigan MAT 96 mass spectrometer (Thermo Electron Corp., Madison, WI, USA).
3.2. Synthesis of NHC Precursors
Synthesis of
imidazole (
, 3.5 g, 51.4 mmol) or benzimidazole (11, 6.0 g, 50.8 mmol) in dry THF (150 mL) at 0 ◦C,
and the resulting suspension was stirred at rt for 2 h (until the hydrogen gas evolution ceased). Then
(for , 13.2 g, 50.0 mmol) or 10 (for 12, 10.1 g, 50 mmol) was added, and the resulting mixture was
9 and 12. Sodium hydride (60% in oil, 2.2 g, 55.0 mmol) was slowly added to
7
8
9
refluxed for 6 h more. The solvent was then removed under vacuum and the thus obtained crude
product was dissolved in dichloromethane (150 mL) and washed with water (100 mL). The aqueous
phase was extracted with dichloromethane (2
×
150 mL). The combined organic layers were washed
with a saturated NaCl solution (200 mL), dried over anhydrous magnesium sulfate and filtered.
After removal of the solvent under reduced pressure, the product was redissolved in dichloromethane
(15 mL) and slowly added dropwise into diethyl ether (350 mL) to precipitate the pure product.

Molecules 2016, 21, 1100
7 of 12
The product
See Supplementary Materials for NMR spectra.
9
or 12 was dried at 70 ◦C under vacuum overnight and obtained in quantitative yield.
1
1,10-(p-Phenylenedimethylene)bisimidazole (
9
). White solid. H-NMR (400 MHz, CDCl3, ppm):
δ
5.11
(s, 4H), 6.88 (s, 2H), 7.09 (s, 2H), 7.13 (s, 4H), 7.53 (s, 2H). 13C-NMR (100 MHz, CDCl3):
127.9, 130.1, 136.5, 137.5. LRMS for C14H14N4: calcd 238.1, found 238.0.
δ 50.4, 119.3,
1
1,10-(1,3-Propanediyl)bis-1H-benzimidazole (12). Light yellow solid. H-NMR (400 MHz, CDCl3, ppm):
δ
2.56 (p, 2H, J = 4.0 Hz), 4.21 (t, 4H, J = 4.0 Hz), 7.277.34 (m, 6H), 7.84–7.86 (m, 2H), 7.87 (s, 2H).
13C-NMR (100 MHz, CDCl3):
δ
29.4, 41.7, 109.3, 120.6, 122.5, 123.3, 133.3, 142.6, 143.9. LRMS for
C17H16N4: calcd 276.1, found 276.0.
Synthesis of the polymeric salts
4–6. The polymeric salts were made via direct alkylation of the 9
or 12 using
8
or 10 in a 1:1 molar ratio on a 10 mmol scale in dry DMF (150 mL). The reaction mixture
◦
was heated for two days at 85 C. The solid product that precipitated from the solution and was filtered,
washed with additional DMF (50 mL) and Et2O (2
24 h. See Supplementary Materials for NMR spectra.
Polymer was obtained in 75% yield as a white powder. It was completely insoluble in all
solvents examined and thus was not characterized by NMR analysis.
×
50 mL), and dried under vacuum at 70 ◦C for
4
1
Polymer
ppm): 2.42 (bs, 2H), 4.24–4.30 (m, 4H), 5.39–5.49 (m, 4H), 7.31–7.53 (m, 4H), 7.78–7.88 (m, 4H),
9.33–9.57 (m, 2H).
Polymer was obtained as white powder with 90% yield. H-NMR (400 MHz, d6-DMSO, ppm):
5 was obtained as light yellow powder with 86% yield. H-NMR (400 MHz, d6-DMSO,
δ
1
6
δ 2.67 (bs, 2H), 4.46–4.76 (m, 4H), 7.63–7.71 (m, 4H), 9.88–10.27 (m, 1H).
Synthesis of 15. Sodium hydride (60% in oil, 2.2 g, 55.0 mmol) was added to a solution of 11 (6.0 g,
50.8 mmol) in THF (150 mL) at 0 ◦C and the resulting suspension was stirred at room temperature
for 2 h (until hydrogen gas evolution ceased). Bromoethane (5.4 g, 50.0 mmol) was added to the
◦
reaction mixture and the resulting solution was heated at 70 C for 6 h more. The solvent was removed
under vacuum and the crude product was dissolved in dichloromethane (150 mL) and washed with
water. The aqueous phase was extracted with dichloromethane (2
×
150 mL). The combined organic
layers were washed with a saturated NaCl solution (200 mL), dried over anhydrous magnesium
sulfate and filtered. The crude product was purified by silica gel chromatography to afford the desired
product N-ethyl-1H-benzimidazole (93% yield) as yellow/brown oil. N-Ethyl-1H-benzimidazole (5.8 g,
40.0 mmol) and bromoethane (4.4 g, 40.0 mmol) were dissolved in 100 mL of THF. The reaction vial was
sealed and the resulting solution was heated at 70 ◦C for 12 h. The slightly yellow solid product that
precipitated from the solution was filtered, washed with diethyl ether (2
×
25 mL), and dried under
vacuum at 70 ◦C for 24 h to afford 15 (95% yield). See Supplementary Materials for NMR spectra.
1,3-Diethyl-1H-benzimidazolium bromide (15). Pale yellow solid. 1H-NMR (400 MHz, CDCl3, ppm):
δ
1.76 (t, 6H, J = 8.0 Hz), 4.71 (q, 4H, J = 8.0 Hz), 7.68–7.70 (m, 2H), 7.77–7.80 (m, 2H), 11.42 (s, 1H).
13C-NMR (100 MHz, CDCl3):
δ
15.0, 43.0, 113.2, 127.3, 131.3, 142.2. LRMS for [C11H15N2]+: calcd 175.1,
found 175.0.
3.3. General Procedure for the Benzoin Condensation Reactions
A Schlenk flask was charged with 13 (2.94 mmol) and 6 (10 mol %). Dry, degassed DMF (2 mL)
was added via syringe, followed by DBU (15 mol %). The reaction mixture was stirred at room
temperature under an atmosphere of nitrogen for the indicated time. Then a solution of HCl in dioxane
(4.0 M, 0.2 mL) was added via syringe, and the reaction mixture was stirred for an additional 1 h
at room temperature under nitrogen. Ethyl acetate (20 mL) was then added dropwise to precipitate
the polymer. The liquid phase was decanted and the light yellow precipitate
additional ethyl acetate (10 mL), and dried under vacuum. The combined organic phase was then
extracted into ethyl acetate (100 mL), washed with brine (3 100 mL) and the aqueous layer was
6 was washed with
×

Molecules 2016, 21, 1100
8 of 12
back-extracted with ethyl acetate (100 mL). The organic layers were combined, dried with anhydrous
magnesium sulfate, filtered, concentrated and purified by column chromatography (SiO2; hexanes:
ethyl acetate). Characterization details for each compound are shown below. See Supplementary
Materials for NMR spectra.
2-Hydroxy-1,2-diphenylethanone (14a). TLC (3:1 v/v hexanes:ethyl acetate): Rƒ = 0.45. 1H-NMR
(400 MHz, CDCl3, ppm):
7.39–7.44 (m, 2H), 7.52–7.58 (m, 1H), 7.89–7.93 (m, 2H). 13C-NMR (100 MHz, CDCl3):
128.7, 128.8, 129.2, 133.6, 134.0, 139.1, 199.1. LRMS for C14H12O2: calcd 212.1, found 212.1.
δ 4.56 (d, 1H, J = 8.0 Hz), 5.95 (d, 1H, J = 8.0 Hz), 7.24–7.36 (m, 4H),
δ
76.4, 127.9,
1
2-Hydroxy-1,2-di(4-chloro-phenyl)ethanone (14b). TLC (3:1 v/v hexanes:ethyl acetate): Rƒ = 0.45. H-NMR
(400 MHz, CDCl3, ppm):
(m, 2H), 7.37–7.40 (m, 2H), 7.81–7.84 (m, 2H). 13C-NMR (75 MHz, CDCl3):
130.6, 131.7, 134.9, 137.3, 140.9, 197.6. LRMS for C14H10Cl2O2: calcd 280.0, found 280.0.
δ
4.52 (d, 1H, J = 8.0 Hz), 5.88 (d, 1H, J = 8.0 Hz), 7.23–7.26 (m, 2H), 7.30–7.32
δ
75.6, 129.2, 129.3, 129.6,
1
2-Hydroxy-1,2-di(4-bromo-phenyl)ethanone (14c). TLC (3:1 v/v hexanes:ethyl acetate): Rƒ = 0.45. H-NMR
(400 MHz, CDCl3, ppm): 4.49 (d, 1H, J = 8.0 Hz), 5.86 (d, 1H, J = 8.0 Hz), 7.17–7.20 (m, 2H), 7.44–7.47
(m, 2H), 7.54–7.57 (m, 2H), 7.72–7.76 (m, 2H). 13C-NMR (100 MHz, CDCl3):
75.7, 129.5, 129.7, 130.6,
δ
δ
132.1, 132.3, 132.6, 137.8, 138.3, 197.8. LRMS for C14H10Br2O2: calcd 367.9, found 367.9.
2-Hydroxy-1,2-di(4-methoxy-phenyl)ethanone (14d). TLC (1:1 v/v hexanes:ethyl acetate): Rƒ = 0.60.
1H-NMR (400 MHz, CDCl3, ppm):
δ
3.73 (s, 3H), 3.80 (s, 3H), 4.55 (d, 1H, J = 8.0 Hz), 5.85 (d, 1H,
J = 8.0 Hz), 6.87–6.83 (m, 4H), 7.27–7.23 (m, 2H), 7.92–7.88 (m, 2H). 13C-NMR (100 MHz, CDCl3):
55.7, 56.0, 75.7, 115.0, 126.9, 129.3, 131.7, 132.1, 160.1, 164.1, 197.6. LRMS for C16H16O4: calcd 272.1,
δ
found 272.1.
2-Hydroxy-1,2-di(3,4,5-trimethoxy-phenyl)ethanone (14e). TLC (1:5 v/v hexanes:ethyl acetate): Rƒ = 0.56
.
1H-NMR (300 MHz, CDCl3, ppm):
δ
3.81 (s, 3H), 3.82–3.83 (m, 12H), 3.90 (m, 3H), 4.58 (d, 1H,
J = 6.0 Hz), 5.77 (d, 1H, J = 6.0 Hz), 6.56 (s, 2H), 7.20 (s, 2H). 13C-NMR (75 MHz, CDCl3):
δ
56.29, 56.31,
60.93, 61.09, 76.54, 104.83, 106.95, 128.31, 135.12, 153.07, 153.98, 197,43. LRMS for C20H24O8: calcd
392.1, found 392.2.
2-Hydroxy-1,2-di(4-trifluoromethyl-phenyl)ethanone (14f). TLC (3:1 v/v hexanes:ethyl acetate): Rƒ = 0.22.
1H-NMR (400 MHz, CDCl3, ppm):
δ
4.45 (bs, 1H), 6.03 (s, 1H), 7.46 (d, 2H, J = 8.0 Hz), 7.60 (d, 2H,
J = 8.0 Hz), 7.69 (d, 2H, J = 8.0 Hz), 8.00 (d, 2H, J = 8.0 Hz). 13C-NMR (100 MHz, CDCl3):
δ
76.1,
122.0, 124.7, 126.1, 126.4, 128.2, 129.5, 136.1, 138.3, 142.1, 197.8. LRMS for C16H10F6O8: calcd 348.1,
found 348.1.
1
2-Hydroxy-1,2-di(2-napthyl)ethanone (14g). TLC (3:1 v/v hexanes:ethyl acetate): Rƒ = 0.52. H-NMR
(300 MHz, CDCl3, ppm):
δ
4.83 (s, 1H), 6.31 (s, 1H), 7.42–7.57 (m, 5H), 7.75–8.03 (m, 8H), 8.52 (s, 1H),
76.5, 124.3, 125.0, 126.5, 126.6, 127.0, 127.6, 127.78,
7.72–7.76 (m, 2H). 13C-NMR (75 MHz, CDCl3):
δ
127.81 128.1, 128.7, 129.1, 129.2, 129.8, 130.9, 131.5, 132.3, 133.3, 133.5, 135.9, 136.7, 199.0. LRMS for
C22H16O2: calcd 312.1, found 312.1.
3.4. Recycling of Polymer 5 from Benzoin Condensation Reactions
After the reaction was complete, the precipitated polymer was separated as above, and the
light yellow polymer was washed with additional ethyl acetate (10 mL) and dried under vacuum
(quantitative recovery of 6), and used directly for the next reaction cycle.
3.5. General Procedure for the Redox Esterification of α,β-Unsaturated Aldehydes
Aldehyde 16 (1.5 mmol), alcohol 17 (3.0 mmol), and 6 (20 mol %) were mixed in toluene (1 mL)
in a reaction vial. The vial was flushed with N2. DBU (25 mol %) was then added slowly dropwise
into the reaction vial and the reaction mixture, followed by the addition of 4-nitrophenol (100 mol %),
the reaction mixture was then stirred at 100 ◦C. The reaction was monitored by TLC and stirred for

Molecules 2016, 21, 1100
9 of 12
time indicated in Table 5. The mixture was then cooled to room temperature and the reaction was
quenched by a solution of HCl in 1,4-dioxane (4.0 M, 0.2 mL). Ethyl acetate (20 mL) was dropwise into
the vial to precipitate the polymeric catalyst. The precipitate was filtered and the filtrate was washed
with additional ethyl acetate (10 mL). The combined organic phase was concentrated under vaccum
and purified by column chromatography (SiO2; hexanes: ethyl acetate). Characterization details for
each compound are shown below. See Supplementary Materials for NMR spectra.
1
Benzyl 3-phenylpropanoate (18Aa). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.52. H-NMR (300 MHz,
CDCl3, ppm):
CDCl3): 31.0, 36.0, 66.3, 118.0, 128.6, 128.7, 129.0, 130.4, 136.0, 140.5, 145.3, 172.8. LRMS for C14H12O2:
calcd 240.1, found 240.1.
δ
2.67 (t, 2H, J = 7.5 Hz), 2.96 (t, 2H, J = 7.5 Hz), 7.15–7.39 (m, 10H). 13C-NMR (75 MHz,
δ
1
4-Bromobenzyl 3-phenylpropanoate (18Ab). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.45. H-NMR
(300 MHz, CDCl3, ppm): 2.68 (t, 2H, J = 7.5 Hz), 2.94 (t, 2H, J = 7.5 Hz), 5.04 (s, 2H), 7.14–7.50 (m, 9H).
13C-NMR (75 MHz, CDCl3)
31.0, 35.9, 65.6, 122.4, 126.5, 128.4, 128.7, 130.0, 131.8, 135.1, 140.4, 172.7.
HRMS for C16H15BrO2: calcd 318.0255, found 318.0245.
δ
δ
1
2-Bromobenzyl 3-phenylpropanoate (18Ac). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.50. H-NMR
(300 MHz, CDCl3, ppm): 2.76 (t, 2H, J = 7.5 Hz), 3.04 (t, 2H, J = 7.5 Hz), 5.23 (s, 2H), 7.17–7.34 (m, 8H),
7.60 (d, 1H, J = 9.0 Hz). 13C-NMR (75 MHz, CDCl3)
31.0, 35.8, 65.9, 123.5, 126.4, 127.5, 128.4, 128.6,
δ
δ
129.7, 129.9, 132.9, 135.3, 140.4, 172.6. HRMS for C16H15BrO2: calcd 318.0255, found 318.0245.
1
2-Chlorobenzyl 3-phenylpropanoate (18Ad). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.50. H-NMR
(300 MHz, CDCl3, ppm):
13C-NMR (75 MHz, CDCl3)
140.4, 172.6. HRMS for C16H15BrO2: calcd 274.0761, found 274.0759.
δ
2.72 (t, 2H, J = 7.5 Hz), 2.99 (t, 2H, J = 7.5 Hz), 5.22 (s, 2H), 7.19–7.40 (m, 9H).
δ
31.0, 35.9, 63.7, 126.4, 126.9, 128.2, 128.4, 128.6, 129.0, 129.5, 129.9, 133.7,
1
1-Phenylethyl 3-phenylpropanoate (18Ae). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.50. H-NMR
(300 MHz, CDCl3, ppm): 1.49 (d, 3H, J = 6.0 Hz), 2.64 (t, 2H, J = 7.5 Hz), 2.94 (t, 2H, J = 7.5 Hz), 5.88
(q, 1H, J = 6.0 Hz), 7.15–7.35 (m, 10H). 13C-NMR (75 MHz, CDCl3)
22.2, 31.0, 36.2, 72.4, 126.2, 126.3,
δ
δ
127.9, 128.4, 128.6, 140.5, 141.7, 172.2. LRMS for C17H13O2: calcd 254.1, found 254.1.
2-Isopropyl-4-methyl-cyclohexyl 3-phenylpropanoate (18Af). TLC (1:20 v/v hexanes:ethyl acetate):
1
Rƒ = 0.56. H-NMR (400 MHz, CDCl3, ppm):
δ 0.69 (d, 3H, J = 4.0 Hz), 0.80–0.93 (m, 8H), 1.25–1.36
(m, 1H), 1.41–1.52 (m, 1H), 1.61–1.76 (m, 4H), 1.91–1.95 (m, 1H), 2.58–2.62 (m, 2H), 2.92–2.96 (m, 2H),
4.67 (dt, 1H, J = 12.0 Hz), 7.18–7.28 (m, 5H). 13C-NMR (100 MHz, CDCl3):
δ
16.4, 20.9, 22.1, 23.5, 26.3,
31.2, 31.5, 34.4, 36.3, 41.0, 47.1, 74.3, 126.3, 128.4, 128.5, 140.7, 172.6. HRMS for C19H28O2: calcd 288.2089,
found 288.2082.
1
4-Nitrophenyl-3-phenylpropanoate (18Ag). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.50. H-NMR
(400 MHz, CDCl3):
δ 2.94 (t, 2H, J = 8.0 Hz), 3.09 (t, 2H, J = 8.0 Hz), 7.17–7.20 (m, 2H), 7.26–7.27 (m, 2H),
7.32–7.36 (m, 2H), 8.24–8.26 (m, 2H, J = 7.5 Hz). 13C-NMR (CDCl3, 100 MHz): 30.1, 35.6, 120.5, 122.3,
125.2, 130.1, 131.7, 139.0, 145.3, 155.5, 170.3. LRMS for C15H13NO4: calcd 271.1, found 271.1.
1
Benzyl 3-(4-bromophenyl)propanoate (18Ba). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.38. H-NMR
(300 MHz, CDCl3, ppm):
7.29–7.35 (m, 6H). 13C-NMR (75 MHz, CDCl3):
130.17, 131.59, 135.85, 139.38, 172.40. LRMS for C16H15BrO2: calcd 318.0, found 318.0.
δ
2.68 (t, 2H, J = 9.0 Hz), 2.95 (t, 2H, J = 7.5 Hz), 5.11 (s, 2H), 7.11–7.14 (m, 2H),
δ
30.36, 35.65, 66.40, 120.14, 128.30, 128.32, 128.61,
1
Benzyl 3-(4-cyanophenyl)propanoate (18Ca). TLC (1:15 v/v hexanes:ethyl acetate): Rƒ = 0.40. H-NMR
(400 MHz, CDCl3, ppm): 2.69 (t, 2H, J = 8.0 Hz), 3.02 (t, 2H, J = 6.0 Hz), 5.10 (s, 2H), 7.26–7.29 (m, 4H),
7.33–7.36 (m, 2H), 7.63–7.55 (m, 2H). 13C-NMR (100 MHz, CDCl3):
30.8, 35.0, 66.4, 110.1, 118.9, 128.2,
δ
δ
128.3, 128.5, 129.1, 132.2, 135.6, 145.9, 171.9. HRMS for C17H15O2N: calcd 265.1103, found 265.1101.
1
Benzyl 3-(4-tert-butylphenyl)propanoate (18Da). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.42. H-NMR
(300 MHz, CDCl3, ppm):
δ 2.68 (t, 2H, J = 9.0 Hz), 2.95 (t, 2H, J = 7.5 Hz), 5.11 (s, 2H), 7.11–7.14 (m, 2H),

Molecules 2016, 21, 1100
10 of 12
7.29–7.35 (m, 6H). 13C-NMR (75 MHz, CDCl3):
δ
30.5, 31.3, 31.5, 36.0, 66.4, 125.5, 128.1, 128.3, 128.6,
136.1, 137.4, 149.2, 173.0. HRMS for C20H24O2: calcd 296.1776, found 296.1174.
Benzyl 3-(4-methoxyphenyl)propanoate (18Ea). TLC (1:15 v/v hexanes:ethyl acetate): Rƒ = 0.55. H-NMR
(300 MHz, CDCl3, ppm): 2.65 (t, 2H, J = 7.5 Hz), 2.91 (t, 2H, J = 7.5 Hz), 3.78 (s, 3H), 5.10 (s, 2H),
6.82 (d, 2H, J = 9.0 Hz), 7.09 (d, 2H, J = 9.0 Hz), 7.29–7.36 (m, 5H). 13C-NMR (75 MHz, CDCl3):
30.2,
1
δ
δ
36.2, 55.3, 66.3, 113.9, 128.3, 128.6, 129.3, 132.5, 135.8, 158.1, 172.8. LRMS for C17H16O3: calcd 270.1,
found 270.0.
Benzyl 3-(furan-2-yl)propanoate (18Fa). TLC (1:20 v/v hexanes:ethyl acetate): Rƒ = 0.36. 1H-NMR
(400 MHz, CDCl3, ppm):
δ
2.68 (t, 2H, J = 8.0 Hz), 2.97 (t, 2H, J = 8.0 Hz), 5.11 (s, 2H), 5.98–5.99 (m, 1H),
23.5, 32.7, 66.4, 105.4, 110.0, 110.2,
6.23–6.24 (m, 1H), 7.26–7.35 (m, 6H). 13C-NMR (100 MHz, CDCl3):
δ
128.3, 128.6, 135.9, 141.3, 154.1, 172.3. LRMS for C14H14O3: calcd 230.1, found 210.1.
4. Conclusions
In conclusion, we have prepared a series of self-supported imidazolium and benzimidazolium
polymeric salts (4–6) that can function as NHC precursors by attaching the linker groups as nitrogen
substituents. These were heterogeneous at the outset of reactions, but when deprotonated in situ using
DBU to generate the corresponding NHC organocatalytic groups, they became soluble, and were
examined in benzoin condensation reactions. Polymer
was used in a range of benzoin condensation and conjugated unsaturated aldehyde redox esterification
reactions. Importantly polymer could be recovered and reused from the former reactions by
6 performed best in the model reaction and it
6
protonation to regenerate and precipitate it with no apparent decrease in utility through five reaction
cycles. Furthermore, it appeared that the self-supported catalytic groups exhibited synergistic effects
that were not observed when analogous molecular catalyst precursor 14 was used. Further study of
this observation is currently ongoing, as is identification of additional applications for
this research will be reported shortly.
6. Results of
Supplementary Materials: Copies of the NMR spectra can be accessed at: http://www.mdpi.com/1420-3049/21/
8/1100/s1.
Acknowledgments: This research was supported financially by the University of Hong Kong and the Research
Grants Council of the Hong Kong S. A. R., P. R. of China (Project No. 17305915).
Author Contributions: Experiments were performed by Ms. Shuang Ma and supervised by Prof. Patrick H. Toy.
Conflicts of Interest: The authors declare no conflicts of interest.
References
1.
2.
Breslow, R. Rapid deuterium exchange in thiazolium salts. J. Am. Chem. Soc. 1957, 79, 1762–1763. [CrossRef]
Breslow, R. On the mechanism of thiamine action. IV. Evidence from studies on model systems. J. Am.
Chem. Soc. 1958, 80, 3719–3726. [CrossRef]
3.
4.
5.
Bugaut, X.; Glorius, F. Organocatalytic umpolung: N-heterocyclic carbenes and beyond. Chem. Soc. Rev.
2012, 41, 3511–3522. [CrossRef] [PubMed]
Grossmann, A.; Enders, D. N-heterocyclic carbene catalyzed domino reactions. Angew. Chem. Int. Ed. 2012
51, 314–325. [CrossRef] [PubMed]
,
Izquierdo, J.; Hutson, G.E.; Cohen, D.T.; Scheidt, K.A. A continuum of progress: Applications of
N-heterocyclic carbene catalysis in total synthesis. Angew. Chem. Int. Ed. 2012, 51, 11686–11698. [CrossRef]
[PubMed]
6.
7.
8.
Douglas, J.; Churchill, G.; Smith, A.D. NHCs in asymmetric organocatalysis: Recent advances in azolium
enolate generation and reactivity. Synthesis 2012, 44, 2295–2309.
Ryan, S.J.; Candish, L.; Lupton, D.W. Acyl anion free N-heterocyclic carbene organocatalysis. Chem. Soc. Rev.
2013, 42, 4906–4917. [CrossRef] [PubMed]
Hopkinson, M.N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014
510, 485–496. [CrossRef] [PubMed]
,

Molecules 2016, 21, 1100
11 of 12
9.
Flanigan, D.M.; Romanov-Michailidis, F.; White, N.A.; Rovis, T. Organocatalytic reactions enabled by
N-heterocyclic carbenes. Chem. Rev. 2015, 115, 9307–9387. [CrossRef] [PubMed]
10. Mahatthananchai, J.; Bode, J.W. On the mechanism of N-heterocyclic carbene-catalyzed reactions involving
acyl azoliums. Acc. Chem. Res. 2014, 47, 696–707. [CrossRef] [PubMed]
11. Martin, D.; Canac, Y.; Lavallo, V.; Bertrand, G. Comparative reactivity of different types of stable cyclic and
acyclic mono- and diamino carbenes with simple organic substrates. J. Am. Chem. Soc. 2014, 136, 5023–5030.
[CrossRef] [PubMed]
12. Paul, M.; Breugst, M.; Neudörfl, J.-M.; Sunoj, R.B.; Berkessel, A. Keto-Enol thermodynamics of Breslow
intermediates. J. Am. Chem. Soc. 2016, 138, 5044–5051. [CrossRef] [PubMed]
13. Kim, D.W.; Chi, D.Y. Polymer-supported ionic liquids: Imidazolium salts as catalysts for nucleophilic
substitution reactions including fluorinations. Angew. Chem. Int. Ed. 2004, 43, 483–485. [CrossRef] [PubMed]
14. Yuan, J.; Antonietti, M. Poly(ionic liquid)s: Polymers expanding classical property profiles. Polymer 2011, 52,
1469–1482. [CrossRef]
15. Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An update. Prog. Polym. Sci. 2013, 38, 1009–1036.
[CrossRef]
16. Nishimura, N.; Ohno, H. 15th anniversary of polymerized ionic liquids. Polymer 2014, 55, 3289–3297.
[CrossRef]
17. Prakash, C.; Shinde, S.S.; Biradar, A.V. Tailor made ionic liquids: catalyst and media for organic
transformations. Curr. Org. Chem. 2015, 19, 728–742.
18. Zhou, H.; Zhang, W.-Z.; Wang, Y.-M.; Qu, J.-P.; Lu, X.-B. N-Heterocyclic carbene functionalized polymer for
reversible fixation-release of CO2. Macromolecules 2009, 42, 5419–5421. [CrossRef]
19. Pawar, G.M.; Buchmeiser, M.R. Polymer-supported carbon dioxide-protected N-heterocyclic carbenes:
Synthesis and application in organo- and organometallic catalysis. Adv. Synth. Catal. 2010, 352, 917–928.
[CrossRef]
20. Pinaud, J.; Vignolle, J.; Gnanou, Y.; Taton, D. Poly(N-heterocyclic carbene)s and their CO2 adducts as
recyclable polymer-supported organocatalysts for benzoin condensation and transesterification reactions.
Macromolecules 2011, 44, 1900–1908. [CrossRef]
21. Gondo, C.A.; Bode, J.W. Catalytic redox amidations of aldehydes with a polymer-supported
peptide-N-heterocyclic carbene multifunctional catalyst. Synlett 2013, 24, 1205–1210. [CrossRef]
22. Coupillaud, P.; Pinaud, J.; Guidolin, N.; Vignolle, J.; Fèvre, M.; Veaudecrenne, E.; Mecerreyes, D.;
Taton, D. Poly(ionic liquid)s based on imidazolium hydrogen carbonate monomer units as recyclable
polymer-supported N-heterocyclic carbenes: Use in organocatalysis. J. Polym. Sci. Part A Polym. Chem. 2013
,
51, 4530–4540.
23. Bortolini, O.; Cavazzini, A.; Dambruoso, P.; Giovannini, P.P.; Caciolli, L.; Massi, A; Pacifico, S.; Ragnoa, D.
Thiazolium-functionalized polystyrene monolithic microreactors for continuous-flow umpolung catalysis.
Green Chem. 2013, 15, 2981–2992. [CrossRef]
24. Coupillaud, P.; Vignolle, J.; Mecerreyes, D.; Taton, D. Post-polymerization modification and organocatalysis
using reactive statistical poly(ionic liquid)-based copolymers. Polymer 2014, 55, 3404–3414. [CrossRef]
25. Powell, A.B.; Suzuki, Y.; Ueda, M.; Bielawski, C.W.; Cowley, A.H. A recyclable, self-supported organocatalyst
based on a poly(N-heterocyclic carbene). J. Am. Chem. Soc. 2011, 133, 5218–5220. [CrossRef] [PubMed]
26. Zhang, Y.; Zhao, L.; Patra, P.K.; Hu, D.; Ying, J.Y. Colloidal poly-imidazolium salts and derivatives. Nano Today
2009, 4, 13–20. [CrossRef]
27. Tan, M.; Zhang, Y.; Ying, J.Y. Hydrosilylation of ketone and imine over poly-N-heterocyclic carbene particles.
Adv. Synth. Catal. 2009, 351, 1390–1394. [CrossRef]
28. Yu, D.; Zhang, Y. Copper- and copper-N-heterocyclic carbene-catalyzed C-H activating carboxylation of
terminal alkynes with CO2 at ambient conditions. Proc. Natl. Acad. Sci. USA 2010, 107, 20184–20189.
[CrossRef] [PubMed]
29. Zhang, Y.; Ying, J.Y. Main-chain organic frameworks with advanced catalytic functionalities. ACS Catal. 2015
,
5, 2681–2691. [CrossRef]
30. Yang, Z.-Z.; Zhao, Y.; Ji, G.; Zhang, H.; Yu, B.; Gao, X.; Liu, Z. Fluoro-functionalized polymeric ionic liquids:
Highly efficient catalysts for CO2 cycloaddition to cyclic carbonates under mild conditions. Green Chem.
2014, 16, 3724–3728. [CrossRef]

Molecules 2016, 21, 1100
12 of 12
31. Xia, X.; Toy, P.H. Rasta resin-triphenylphosphine oxides and their use as recyclable heterogeneous reagent
precursors in halogenation reactions. Beilstein J. Org. Chem. 2014, 10, 1397–1405. [CrossRef] [PubMed]
32. Ma, S.; Yang, Y.-C.; Toy, P.H. Rasta resin-TBD-catalyzed γ-selective Morita–Baylis–Hillman reactions of
α,γ-disubstituted allenones. Synlett 2015, 26, 1732–1736. [CrossRef]
33. Xia, X.; Toy, P.H. Polyethyleneimine-supported triphenylphosphine and its use as a highly loaded
bifunctional polymeric reagent in chromatography-free one-pot Wittig reactions. Synlett 2015, 26, 1737–1743.
[CrossRef]
34. Ma, S.; Toy, P.H. Chromatography-free esterification reactions using a bifunctional polymer. Synlett 2016, 27,
1207–1210.
35. Yang, Y.-C.; Toy, P.H. Self-supported ligands as a platform for catalysis: Use of a polymeric oxime in
a recyclable palladacycle precatalyst for Suzuki–Miyaura reactions. Synlett 2014, 25, 1319–1324.
36. Marti, J.; López-Calahorra, F.; Bofill, J.M. A theoretical study of benzoin condensation. J. Mol. Struct. THEOCHEM
1995, 339, 179–194. [CrossRef]
37. Moore, J.L.; Silvestri, A.P.; de Alaniz, J.R.; DiRocco, D.A.; Rovis, T. Mechanistic investigation of the
enantioselective intramolecular Stetter reaction: Proton transfer is the first irreversible step. Org. Lett.
2011, 13, 1742–1745. [CrossRef] [PubMed]
38. Chan, A.; Scheidt, K.A. Conversion of α,β-unsaturated aldehydes into saturated esters: An umpolung
reaction catalyzed by nucleophilic carbenes. Org. Lett. 2005, 7, 905–908. [CrossRef] [PubMed]
39. Maki, B.E.; Chan, A.; Scheidt, K.A. Protonation of homoenolate equivalents generated by N-heterocyclic
carbenes. Synthesis 2008, 40, 1306–1315.
Sample Availability: Samples of the compounds are available from the authors.
©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).