Squaramide equilibrium acidities in DMSO

Article abstract of DOI:10.1021/ol5005017

A number of popular squaramide organocatalysts' acidities were determined by an overlapping indicator method in DMSO via UV spectrophotometric titrations. The pK_a values are in the range of 8.37-16.46. The results may be helpful for the rational design and development of squaramide type new catalysts.

Full text of DOI:10.1021/ol5005017

Letter
pubs.acs.org/OrgLett
Squaramide Equilibrium Acidities in DMSO
Xiang Ni, Xin Li,* Zhen Wang, and Jin-Pei Cheng*
State Key Laboratory of Elemento-Organic Chemistry, Department of Chemistry, Nankai University, and Collaborative Innovation
Center of Chemical Science and Engineering, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: A number of popular squaramide organo-
catalysts’ acidities were determined by an overlapping indicator
method in DMSO via UV spectrophotometric titrations. The
pKa values are in the range of 8.37−16.46. The results may be
helpful for the rational design and development of squaramide
type new catalysts.
ompounds possessing a hydrogen-bond donor motif
of the N−H bonds between squaramides and thioureas was
Cassociated with a chiral spacer framework, which are ∼8°. These two natural differences were considered to be
attributed to the “squara structure” of cyclobutendione.5,16 In
universally existent in nature, have become a prominent type of
addition to the structure feature investigated above, the
distinctly different pKa values of N−H bonds between
squaramide and (thio)urea compounds should be one of the
most crucial parameters rationalizing the extraordinary
performance of squaramides (Figure 1). However, to the best
organocatalyst that enables the realization of a range of
transformations with high efficiency and stereoselectivity.1
Especially, chiral bifunctional (thio)urea-tertiary amines have
received special attention owing to their double hydrogen-
bonding interaction of the N−H bond, in which several
functionalities, such as carbonyl, nitro, imine groups and vinyl
sulfones, have been successfully activated.2
Squaramides, due to their structure feature of possessing
both a hydrogen-bond acceptor at the carbonyl group and
hydrogen-bond donor donated by the N−H group, have been
designed as efficient receptors capable of not only recognizing
anions and cations3 but also forming complexes with a variety
of neutral molecules.4 In 2008, Rawal and co-workers applied a
chiral squaramide-based organocatalyst derived from a cinchona
alkaloid in the Michael addition of 1,3-dicarbonyl compounds
to nitro olefins, which performed excellently in both chemical
efficiency and stereocontrol compared to the results using the
counterpart thiourea-based catalyst.5 From then on, squar-
amides with different skeletons have been widely developed and
proven efficient in a broad range of asymmetric trans-
formations, including aldol,6 Mannich,7 Friedel−Crafts,8
Michael addition,9 Henry,10 Morita−Baylis−Hillman,11
Diels−Alder,12 cycloaddition,13 cascade reaction,14 asymmetric
cycloaddition of homophthalic anhydrides,15 and so on.16 It is
worth mentioning that the squaramide catalyst exhibited
superior activity evidenced by the fact that a very low catalyst
loading was needed to complete the full conversion of the
substrate. In fact, the crystallographic structure and computa-
tional data obtained by Rawal demonstrated that squaramide’s
two N−H bonds are coplanar with the rigid four-element-ring
framework of the cyclobutendione unit.5 Compared to the
counterpart thioureas, the distances between the two N−H
groups of squaramides are about 0.6 Å broader than in the case
of thioureas. And the difference in value of the dihedral angles
Figure 1. Situation of the pKa values of bifunctional hydrogen-bonding
catalysts.
of our knowledge, the fundamental physical organic parameters
of squaramides, i.e. the acidities, which are helpful for
understanding the mechanism of asymmetric catalysis, are still
unknown. It is worth noting that the absence of the
corresponding pKa values seriously limited the rational design
and development of this emerging organocatalyst.
In 2010, Luo and Cheng first determined the pKa values of a
series of chiral bifunctional thiourea catalysts in DMSO by an
overlapping indicator method, in which a linear free energy
relationship between the pKa and both the reactivity and
enantioselectivity of studied Michael reactions was found.17
Subsequently, Schreiner et al. measured the acidities of some
widely used thiourea and urea organocatalysts with the same
method.18 Furthermore, pKa values of some widely used chiral
Brønsted acid catalysts, such as phosphoric acids and super
acids, were also reported by experimental methods.19 In the
Received: February 17, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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Letter
present study, we reported the determination of the pKa values
of squaramide type catalysts.20
Eighteen squaramide catalysts, including achiral and chiral
ones, were selected as the target compounds (Figure 2). The
Figure 2. Studied squaramides.
Figure 3. (a) Absorption spectra of the anion derived from 2-Br-PhS-
FH for various added amounts of 2-Br-PhS-FH during the titration.
(b) Absorption spectra of the anion derived from 2-Br-PhS-FH for
various added amounts of squaramide 10 during the titration.
pKa values were measured in DMSO by the spectrophotometric
method of an overlapping indicator.21 A solution of an indicator
containing a known concentration of its colored anion was
titrated with a solution of squaramide whose anion absorbs at a
different wavelength. The concentration of the indicator’s anion
is monitored by a UV/vis spectrophotometer after each
addition of the squaramide solution (Figure 3). The
equilibrium acidity of the squaramide could then be calculated
according to eqs 1 and 2.
Figure 4. Indicators’ structures and their pKa values in DMSO.22
The determinations of the pKa values were carried out under
an atmosphere of argon. The treatment of DMSO and base (K-
dimsyl basic solution) strictly followed that in literature.21 The
carbon acid indicators used in this study are shown in Figure 4.
The results of the pKa values of squaramides are shown in
Table 1.
As shown in Table 1, the pKa values of the studied
squaramides cover the range 8.37−16.46. The electronic effect
on acidity was then investigated. General information shows
that the di-3,5-CF3 substituted phenyl type squaramides have
pKa values lower than 11.9, indicating that they are more acidic
than AcOH (pKa = 12.330). For the three achiral squaramides,
in which both N-atoms were substituted with an aromatic
group, fitting trifluoromethyl groups onto the aromatic ring
sharply increases their acidities. Particularly, the pKa values’
decrements correspond to the increment of trifluoromethyl
groups, in which each trifluoromethyl group strengthens the
acidity of achiral squaramides by more than 1 pKa unit. Owning
to the lack of a second aromatic ring, most of the pKa values of
chiral squaramides are higher than 2. With regard to the
electron-withdrawing ability diminished by each type of
electronic tuning group, the pKa values increase in the order
type II < type I < type III.
We next examined the structural effect on the acidities of
squaramides. It is not difficult to find that the squaramides with
a cyclohexane-1,2-diamine motif (11 and 14) exhibit higher pKa
values than the squaramides with the other chiral skeletons (5,
8, 17, and 18). With regard to 10−15, the acidities of
squaramides with cyclic tertiary-amine unit are higher than the
corresponding acyclic tertiary-amine ones. Further inspection
of the data shows that the pKa gap between 11 and 14 (0.04
pKa units) is smaller than the values in the other two pairs (0.2
pKa units for 10 and 13, 0.18 pKa units for 12 and 15),
indicating that the structure effect on the acidity is weaker
accompanied by the stronger influence of the electronic effect.
Although it shares the same chiral skeleton with 10−15, catalyst
16 has a remarkably lower pKa value, which can be explained by
the addition of the second squaramide moiety. And for the
squaramides 4−9, which possess cinchona alkaloid skeletons,
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Organic Letters
Letter
chiral squaramides and their thiourea analogues, respectively
(Figure 6). This interesting correlation implies that the pKa of
squaramide may be estimated on the basis of the knowledge of
the pKa value of the corresponding thiourea.
Table 1. pKa Values of Squaramides in DMSO
entry
squaramide
pKa value
indicator
1
1
2
12.48 0.05
10.55 0.03
8.37 0.04
12.17 0.03
10.54 0.04
14.99 0.05
12.18 0.04
11.03 0.03
15.13 0.05
13.10 0.04
11.83 0.05
16.28 0.05
13.30 0.04
11.87 0.03
16.46 0.05
10.78 0.03
11.42 0.03
11.57 0.04
2-Br-PhS-FH
2
2,7-diBr-PhS-FH
CN-FH
3
3 (Schmidt, 2013)23
4 (Du, 2010)24
5 (Du, 2010)24
6 (Jørgensen, 2010)25
7 (Du, 2010)24
8 (Du, 2010)24
9 (Rawal, 2010)5
10 (Du, 2011)26
11 (Rawal, 2010)27
12 (Rodriguez, 2012)28
13 (Rawal, 2010)27
14 (Rawal, 2010)27
15 (Rawal, 2010)29
16
4
2,7-diBr-PhS-FH
2,7-diBr-PhS-FH
(4-Br-PhS)-FH
2,7-diBr-PhS-FH
2,7-diBr-PhS-FH
(4-Br-PhS)-FH
2-Br-PhS-FH
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2,7-diBr-PhS-FH
(3-Cl-Ph)-FH
2-Br-PhS-FH
2,7-diBr-PhS-FH
(3-Cl-Ph)-FH
2,7-diBr-PhS-FH
2,7-diBr-PhS-FH
2,7-diBr-PhS-FH
17 (Rawal, 2010)29
18
the pKa values decrease when the quinoline rings were
substituted by 6′-methoxy groups.
The rapid development of chiral squaramide catalysis can be
attributed to their excellent catalytic performance rather than
the corresponding thiourea, which is generally considered due
to its higher acidity. However, more quantitative information
regarding the pKa gap between squaramide and thiourea is still
needed. Comparison of the pKa value between squaramides and
corresponding thiourea analogues was then conducted. It was
found that the pKa values of squaramides are lower than their
thiourea analogues (Figure 5), in which 0.13−1.97 pKa gap
Figure 6. Correlation of pKa values of squaramides with their thiourea
analogues.
In summary, we have determined the pKa values of 18
squaramide organocatalysts in DMSO by the classical over-
lapping indicator method. The pKa values lie in the range 8.37
to 16.46. The acidifying effect of the catalysts’ structure and
substituted trifluoromethyl groups were investigated. Moreover,
comparisons of the acidity values between squaramides and
their thiourea analogues were also conducted. We believe that
the data reported here will provide a basis for developing new
hydrogen-bonding catalysts and will be helpful in obtaining
deeper insight into the corresponding mechanisms.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, analytic data for all the new compounds,
and Figures S1−S17. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xin_li@nankai.edu.cn.
*E-mail: chengjp@nankai.edu.cn.
Notes
Figure 5. pKa values of thiourea analogues in DMSO.
The authors declare no competing financial interest.
units were obtained. Although the magnitude of the pKa gap
between squaramide and thiourea is sharply dependent on the
parent structure, the fact that squaramide is more acidic
indicated that squaramide engages in stronger hydrogen bonds
than the corresponding thiourea. This result may be one of the
most important reasons explaining why lower loadings of
squaramide catalysts can perform with even higher activity in
completing a broad range of asymmetric transformations. To
our delight, two good straight lines with R values of 0.995 and
0.932 were obtained in the regression analysis between the pKa
values of achiral squaramides and their thiourea analogues, and
ACKNOWLEDGMENTS
■
We thank the Most (2012CB821600 and 2010CB833300),
NSFC (21390400, 21172112, and 21172118), the State Key
Laboratory on Elemento-organic Chemistry, and the Program
for New Century Excellent Talents in University (NCET-12-
0279) for financial support.
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