Upper rim guanidinocalix[4]arenes as artificial phosphodiesterases

Article abstract of DOI:10.1021/jo300193y

Calix[4]arene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosp

Full text of DOI:10.1021/jo300193y

Article
pubs.acs.org/joc
Upper Rim Guanidinocalix[4]arenes as Artificial Phosphodiesterases
Laura Baldini,† Roberta Cacciapaglia,‡ Alessandro Casnati,† Luigi Mandolini,‡ Riccardo Salvio,*,‡
Francesco Sansone,† and Rocco Ungaro†
†Dipartimento di Chimica Organica e Industriale, Universita
̀
degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy
‡Dipartimento di Chimica and IMC - CNR Sezione Meccanismi di Reazione, Universita
̀
La Sapienza, 00185 Roma, Italy
S
* Supporting Information
ABSTRACT: Calix[4]arene derivatives, blocked in the cone conformation and functionalized with
two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the
cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared
with the behavior of a monofunctional model compound, the catalytic superiority of the calix[4]arene
derivatives points to a high level of cooperation between catalytic groups. Combination of acidity
measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite
for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral
guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator. The
additional guanidinium (guanidine) group in the diprotonated (monoprotonated) trifunctional
calix[4]arene acts as a more or less innocent spectator. This is not the case with the tetrasubstituted
calix[4]arene, whose mono-, di-, and triprotonated forms are slightly less effective than the corre-
sponding di- and triguanidinocalix[4]arene derivatives, most likely on account of a steric interference
with HPNP caused by overcrowding.
on a guanidinium unit in conjunction with another active unit
INTRODUCTION
■
such as a metal center,36−39a hydroxyl group,40 or another unit
In recent years many research groups have achieved significant
results in the synthesis and study of simple, nonpeptidic mole-
cules that mimic structural and functional aspects of enzymes.1−3
In this context artificial catalysts that efficiently cleave DNA and
RNA fragments have received considerable attention4−19 because
phosphodiester bonds are extremely reluctant to undergo
hydrolysis20 and because cleavage agents of these substrates have
a potential application in health-related goals, such as the antisense
therapy.21,22
In artificial enzymes a primary role is played by the molecular
scaffold. An efficient scaffold should keep the active functions at
the proper distance as a result of a good compromise between
preorganization and flexibility. Previous studies in the field have
shown that the calix[4]arene scaffold, blocked in the cone
conformation by proper alkylation of the lower rim hydroxyl
groups, is suitable for the design of catalysts active in the
cleavage of carboxylic and phosphoric esters.4,9,23−29
acting as a general base.41,42
In the past years the guanidinium unit has been successfully
used in the design of molecular receptors.43−46 Guanidinocalixar-
enes were synthesized for the purpose of binding to negatively
charged substrates through electrostatic interactions, and they
were initially used to build up supramolecular assemblies on
surfaces.47,48 Thanks to their remarkable affinity for nucleic acid
strands,49 more recently they were successfully used to vehicle
DNA through cell membranes, thus inducing transfection.45,46
In this paper we report on the synthesis and kinetic
investigation of the catalytic activity of guanidinocalix[4]arenes
1−4 in the transesterification of the RNA model compound
2-hydroxypropyl p-nitrophenyl phosphate (HPNP), (eq 1).
Bi- and trimetallic derivatives of calix[4]arenes decorated
with suitable ligating units proved to be highly efficient catalysts
in phosphodiester hydrolysis, thanks to a high degree of
cooperation between the metal centers4,9,27 which can act as
binding centers as well as activating units.
The guanidinium unit also has a great importance as an
activating and/or anchoring group in hydrolytic reactions.
Guanidinium plays an important role in nature as it is present
in many proteins. The active site of staphylococcal nuclease
contains two arginine units, which, in conjunction with a calcium
ion, lead to an electrophilic activation of phosphodiesters for
hydrolysis.30 Studies are also available of the catalytic activity of
artificial systems based on one or more guanidinium units.15,31−35 or
These catalysts are based on the calix[4]arene scaffold blocked
in the cone conformation by alkylation of the phenolic OH
groups with ethoxyethyl chains for the purpose of increasing
the solubility in polar solvents and getting rid of the self-
aggregating properties typical of amphiphilic p-guanidinocalix-
arenes endowed with lipophilic chains at the lower rim.45 The
Received: January 25, 2012
Published: February 24, 2012
© 2012 American Chemical Society
3381
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
upper rim is functionalized with two to four guanidinium units.
In view of the well established mechanism of general acid−base
catalysis in the cleavage of HPNP, involving the synergic action
of neutral and protonated forms of guanidine,35 compounds 1−4
offer wide opportunities for achieving guanidine−guanidinium
combinations in varying proportions, depending on pH. As a
control, the monofunctional compound 5 was also synthesized
and investigated under the same conditions.
obtained by crystallization of the crude reaction mixture from
MeOH, the 1,2-vicinal dinitro- and the trinitro derivatives 7 and
9, respectively, were isolated after a careful chromatographic
separation. The structures of the isomeric derivatives 7 and 8
were assigned on the basis of the highly diagnostic patterns of
1
the H NMR signals of ArCH2Ar, well precedented for other
upper rim substituted calix[4]arenes.52 The 1,3-distal dinitro
derivative 8 originates a pair of doublets at 4.50 and 3.40 ppm,
while the proximal 1,2-vicinal dinitro derivative 7 gives rise to
three pairs of doublets (1:2:1 ratio) at 4.66 and 3.28, 4.56 and
3.23, 4.45 and 3.16 ppm. The tetranitro derivative 10 was
obtained by ipso-nitration of the p-tert-butylated ether 6b in the
cone structure.50 The four nitro compounds 7−10 were then
reduced with hydrazine hydrate and Pd/C to the corresponding
amino derivatives 11−14 in 70−79% yields. Particular care was
paid in the workup and storage of these multivalent
aminocalixarenes 11−14 because they may absorb atmospheric
CO2, giving rise to carbamates.53 The guanidinylation reaction,
carried out by using bis-Boc-thiourea and HgCl2, gave the
expected di- (15 and 16) and triguanidinylated (17) products
in 45−54% isolated yields after column chromatography. As
previously reported for other upper rim functionalized
calixarenes,45,49 the low yields of isolated products were
ascribed to partial Boc removal and consequent loss of material
during the chromatographic purification process. The tetragua-
nidinylated compound 18 was instead isolated by crystallization
in moderate yield (59%). The deprotection reactions of 15−18
with hydrochloric acid proceeded smoothly and afforded
compounds 1−4 as hydrochlorides in quantitative yields.
Potentiometric Titrations. Knowledge of the acidity
constants of the hydrochlorides of 1−4 and of the monofunctional
model compound 5 is a prerequisite for a meaningful investiga-
tion of their catalytic properties. A mixture of DMSO/H2O,
80:20 (v/v), hereafter referred to as 80% DMSO, was used
as reaction medium for titration experiments, as well as for
kinetic analysis. This mixture is well-known to be suitable for
pKa determination54,55 and for investigation of the kinetics of
hydrolytic reactions of phosphodiesters.35,39,56−58 The binding
RESULTS AND DISCUSSION
■
Catalyst Synthesis. The synthesis of guanidinylated
tetrakis(2-ethoxyethoxy)calix[4]arenes 1−4 was carried out
according to Scheme 1. It is known that the 1,3-distal dinitro
derivative 8 can be efficiently obtained by a selective ipso-
nitration of a 5,17-bis-tert-butylcalix[4]arene derivative,50 but
since the 1,2-vicinal dinitro derivative and the trinitro
derivatives 7 and 9 were also needed, we preferred to carry
out the nitration51 of tetrakis(2-ethoxyethoxy)calix[4]arene 6a,
followed by separation of the various substitution products.
Indeed the nitration reaction showed a rather poor selectivity,
yielding a mixture of mononitro-, dinitro-, and trinitrocalixar-
enes. While the 1,3-distal dinitrocalix[4]arene 850 could be
Scheme 1. Synthesis of Calix[4]arene Derivatives 1−4
3382
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
of guanidinium to phosphate in 80% DMSO is stronger than in
pure water59−61 and, consequently, in this solvent mixture a
catalyst based on guanidinium as binding/activating group is
potentially favored. In 80% DMSO the pKw for water
autoprotolysis rises to 18.4,62 but the pKa values of amines
and other nitrogen bases are approximately the same or slightly
lower than those in water.63
Acidity constants of protonated bases were determined by
standard potentiometric titrations of their chloride salts with
Me4NOH in 80% DMSO. All titrations were carried out on
2 mM solutions of guanidinocalix[4]arenes. A typical titration
plot is reported in Figure 1. Figure 2 shows the distribution
diagrams of the species for 2−4.
Figure 1. Titration of 2·2HCl (2 mM) with Me4NOH in 80% DMSO,
25 °C, in the presence of 10 mM Et4NBr.
The acidity constants of guanidinocalix[4]arenes (Table 1)
range over almost 3 orders of magnitude. A number of factors
contribute to determine the variations in the acidity of the
various guanidinium units on the calixarene scaffold relative to
that of the monofunctional compound 5H+. First of all
statistical factors must be taken into account. As shown by
the statistically corrected pKi values given in parentheses in
Table 1, statistical corrections are sizable in a number of cases,
but do not alter to a significant extent the general picture. More
importantly, as one would expect from electrostatic consid-
erations, repulsion between two or more guanidinium groups
facilitates the departure of a proton from one of the groups.
The operation of this strong acidity-enhancing effect makes the
pKi values of each one of the polysubstituted guanidinocalix[4]-
arenes increase markedly on increasing i. It might seem odd,
however, that the statistically corrected pK1 values of the
diprotic, triprotic, and tetraprotic acids are remarkably similar
to one another, showing an apparent insensitivity to the varying
number of acidity-enhancing positive groups. Similar con-
clusions are drawn when the pK2 and pK3 of the triprotic acid
3(H+)3 are compared with the corresponding values of the
tetraprotic acid 4(H+)4. The apparent insensitivity to the action
of multiple positive poles finds an at least partial explanation in
the operation of medium effects on the stability of multiply
charged ions (see Supporting Information, Appendix 1).
Figure 2. Distribution diagrams of the species for 2 (top), 3 (middle),
and 4 (bottom) as a function of pH under the conditions of the
titration experiments in Table 1.
kinetics of HPNP transesterification in 80% DMSO at 37 °C in
guanidine buffers, Yatsimirsky et al. established the rate law of
eq 2 (B = guanidine),
+
k
= k [B] + k [B][BH ]
(2)
obs
1
2
Kinetic Measurements. The catalytic activity of calix[4]-
arene derivatives 1−4 in the transesterification of the RNA
model compound HPNP (eq 1) was investigated in the same
solvent mixture used for the potentiometric measurements,
namely, 80% DMSO at 25.0 °C. In a recent study of the
showing contributions from simple general base catalysis (k1 =
0.025 M−1 s−1), and a reaction involving concerted action of
neutral and protonated forms of the buffer (k2 = 1.68 M−2 s−1).35
On the basis of a proton inventory study, these authors con-
vincingly argued that the protonated buffer component acts via
3383
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
a,b
Table 1. Acidity Constants of Guanidinocalix[4]arenes 1−4 and of the Model Compound 5 in 80% DMSO, 25 °C
entry
species
pK1
11.5
pK2
pK3
pK4
1
2
3
4
5
5·HCl
1·2HCl
2·2HCl
3·3HCl
4·4HCl
9.3 (9.6)
9.3 (9.6)
8.8 (9.3)
9.0 (9.6)
11.6 (11.3)
11.5 (11.2)
10.4 (10.4)
10.3 (10.5)
11.5 (11.0)
11.5 (11.3)
12.3 (11.7)
a
pKi data from potentiometric titrations carried out on 2 mM substrate solutions in the presence of 10 mM Et4NBr. Experimental uncertainty = 0.1
b
pK units or less. Statistically corrected values are in parentheses. Under the same conditions, the pK of guanidine·HCl is 13.7.
electrostatic transition state stabilization, rather than proton
transfer.
(Table 1, entry 1 and footnote b), and of the different
temperatures of the two sets of experiments, (25 °C vs 37 °C).
These findings clearly indicate that the mechanisms of HPNP
transesterification are essentially the same in both cases, i.e.
general base catalysis in combination with general-base/general-
acid catalysis in which the protonated form of the catalyst
stabilizes the anionic transition state via electrostatic interaction,
most likely reinforced by a bidentate hydrogen-bonding
interaction, as schematically depicted in Figure 4.
Preliminary to the kinetic investigation of the catalytic
efficiency of di-, tri-, and tetrafunctional calix[4]arene
derivatives 1−4, the transesterification of HPNP was
investigated in 80% DMSO, 25 °C, in N-(4-methoxyphenyl)-
guanidine (5) buffer (50% free base). The results are reported
in Figure 3. The plot does not approach zero at low buffer
Figure 4. General-acid/general-base catalytic mechanism of HPNP
transesterification in N-(4-methoxyphenyl)guanidine (5) buffer.
A different kinetic behavior was found when free and
protonated guanidine units are bound to the same molecular
framework. Half-neutralization of the bifunctional precatalyst
2·2HCl with one molar equivalent of Me4NOH gave buffer
solutions of 2·H+ (pH = 10.4) which were used for the
transesterification of HPNP. A strictly linear dependence of kobs
vs buffer concentration was observed (Figure 5, eq 4), showing,
Figure 3. Catalysis of transesterification of HPNP by N-(4-
methoxyphenyl)guanidine (5) buffer (50% free base, pH = 11.5) in
80% DMSO at 25 °C. kobs = vo/[HPNP], where vo is the spectro-
photometrically determined initial rate of p-nitrophenol liberation.
The ionic strength was adjusted to 0.1 M by the addition of Et4NBr.
The line is a plot of eq 3, with the best fit parameters given in
the text.
concentrations, indicating that the rate of background reaction
at pH 11.5, albeit very small, is not negligible. A nonlinear least-
squares procedure was used to fit data points to the parabolic
eq 3, with the following values of best fit parameters: ko =
(0.11 0.05) × 10−5 s−1; k1 = (2.5 0.7) × 10−4 M−1 s−1, k2 =
(1.0 0.2) × 10−2 M−2 s−1.
+
k
= k + k [B] + k [B][BH ]
(3)
obs
o
1
2
Comparison of the best fit specific rate of background
reaction ko with the value 0.17 × 10−5 s−1 measured for the
rate of HPNP transesterification in phosphate buffer at pH
11.5 (see Supporting Information, Table 3S in Appendix 2)
shows that a major component of the background reaction is
due to hydroxide catalysis. The numerical values of k1 and k2
are 2 orders of magnitude lower than the corresponding
values of the reaction catalyzed by guanidine buffers,35 which
is easily understandable on the basis of the lower basicity of
N-(4-methoxyphenyl)guanidine (5) relative to guanidine
Figure 5. Plot of kobs for the liberation of p-nitrophenol from 0.1 mM
HPNP vs the concentration of 2H+. Buffer solutions obtained by
mixing equimolar amounts of 2·2HCl and Me4NOH, pH = 10.4 in
DMSO 80% at 25 °C. From the slope of the straight line: kcat = (2.3
0.05) × 10−2 M−1 s−1. (The value k = 1.6 × 10−7 s−1 calculated for
background reaction of HPNP at pH 10.4 (kbg = 10 (pH−17.2)
;
Supporting Information, Appendix 2) was used as the intercept
value with the y-axis).
3384
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
inter alia, that in the investigated concentration range the
catalyst works
Table 2. Transesterification of HPNP Catalyzed by
Guanidinocalix[4]arene Derivatives in 80% DMSO
at 25.0 °C
a
+
k
= k [2H ]
(4)
obs
cat
entry
precatalyst
Me4NOH (mol equiv)
pH
105 × kobs (s−1
)
under subsaturating conditions, with a second-order catalytic
1
1·2HCl
0.5
1.0
1.5
0.5
1.0
1.5
1.0
1.5
2.0
2.5
1.0
2.0
3.0
9.3
10.4
11.5
9.3
1.9
3.3
rate constant kcat = (2.30 0.05) × 10−2 M−1 s−1.
2
Compared with the reaction carried out in the presence of
the monofunctional control 5, the efficiency of the bifunctional
catalyst is so high as to obscure contributions from background
b
3
2.0
4
2·2HCl
3·3HCl
4.6
6.9
5
10.4
11.5
9.7
reaction (kbg = 1.6 × 10−7 s−1 at pH 10.4; from kbg = 10 (pH−17.2)
,
b
6
5.0
Supporting Information, Appendix 2). As to the simple general-
base catalyzed mechanism, the second-order rate constant k1 for
the reaction of 5 (eq 3) is 2 orders of magnitude lower than kcat, in
spite of the fact that 5 is 2 orders of magnitude more basic than
2H+ (Table 1). We conclude therefore that the catalysis in buffer
solutions of 2H+ is entirely due to a mechanism involving
concerted actions of the neutral and protonated forms of
the guanidine moieties in 2, as shown in Figure 6. In other
7
4.9
9.7
11.4
9.6
1.0
3.2
8
10.4
11.1
11.5
9.8
9
10
11
12
13
4·4HCl
10.9
11.8
b
2.5
a
Pseudo-first-order specific rates kobs calculated as vo/[HPNP], where
vo is the spectrophotometrically determined initial rate of p-nitrophenol
liberation in 0.1 mM HPNP solutions in the presence of 3 mM
catalyst. The rate constant (kbg, s−1) for the hydroxide-catalyzed
reaction as a function of pH is given by the following expression:
b
kbg = 10(pH−17.2) (Supporting Information, Appendix 2). Corrected for
the background reaction. kbg = 0.2 × 10−5 s−1 at pH 11.5; kbg = 0.4 ×
10−5 s−1 at pH 11.8.
Consistent with the distribution diagram in Figure 2, the
maximum catalytic activity in the reaction catalyzed by the
1,3-distal bifunctional catalyst 2 is achieved at pH 10.4 (compare
entries 4 and 6 with entry 5), which provides a definite con-
firmation that the catalytically active species is the monop-
rotonated form 2H+. Similar considerations apply to the 1,2-
vicinal regioisomer 1 (entries 1−3), whose acid−base behavior
is very similar to that of 2 (Table 1, entries 2 and 3). In
previous studies, homobimetallic complexes of calix[4]arenes
decorated at the upper rim with suitable ligating units were
investigated as catalysts for the transesterification of HPNP. It
was found that the degrees of synergism critically depend on
the nature of the catalytic units and on whether the substitution
pattern is 1,2-vicinal or 1,3-distal,25,27 and that two effective
catalytic groups at the upper rim of a calix[4]arene backbone do
not necessarily make a good catalyst.27 It is remarkable,
therefore, to find that both 1,2-vicinal 1H+ and 1,3-distal 2H+
exhibit a high degree of synergism between catalytic groups, the
1,3-distal bifunctional catalyst being slightly more than twice as
effective as its 1,2-vicinal regioisomer.
As to the trifunctional catalyst 3, comparison of the distribution
diagram in Figure 2 with kinetic data in Table 2 (entries 7−10)
shows that the species 3H+, featuring one guanidinium and two
guanidine groups, is catalytically more effective than the doubly
protonated species 3(H+)2.64 The former is also slightly more
effective than 2H+, which is easily ascribed to statistical factors,28
thus showing that the inherent catalytic effectiveness of 1,2-vicinal
or 1,3-distal guanidine−guanidinium pairs is not affected to a
significant extent by the presence of an additional guanidine
moiety, which, as a matter of fact, behaves as an innocent spectator.
This is clearly not the case with the tetrafunctional catalyst 4,
for which all the species obtained by partial neutralization with 1, 2,
and 3 mol equiv of Me4NOH, i.e., 4(H+)3, 4(H+)2, and 4H+
are somewhat less effective than 3H+ (Table 2, compare entries
11−13 with entry 9). It appears, therefore, that the extra guanidine
moiety in 4 no longer behaves as an innocent spectator but
destabilizes the transition state, presumably through a moderate
Figure 6. Bifunctional general acid−base catalysis of 2H+ in the
transesterification of HPNP.
words, this mechanism is the intramolecular counterpart of the
general-base/general-acid mechanism depicted in Figure 4. The
ratio kcat/k2 = 2.3 M is the effective molarity (EM) of the reaction
undergoing intramolecular acid−base catalysis. This means that
2.3 M concentrations of 5 and 5H+ display the same catalytic rate
as that of a 2.3 M solution of 2H+ or, more realistically, that at
millimolar catalyst concentrations the reaction catalyzed by 2H+ is
3 orders of magnitude faster than that catalyzed by the 1:1 mixture
of 5 and 5H+. Thus, an EM value as high as 2.3 M provides a
measure of the high degree of synergism displayed by neutral and
protonated guanidinium units of 2H+ in the stabilization of the
transition state of the HPNP transesterification.
In order to carry out a comparison of the catalytic efficiencies
of the various calix[4]arene derivatives in the transesterification
of HPNP, a set of rate measurements was carried out in buffer
solutions obtained by partial neutralization of 3 mM solutions
of guanidinocalix[4]arene precatalysts with varying amounts of
Me4NOH. The results are collected in Table 2. HPNP
transesterifications in the presence of guanidinocalix[4]arene
catalysts were in all cases faster than background transesterification,
with rate enhancements as high as 3 orders of magnitude in a
number of cases (entries 1, 4, 7, and 8). In three cases only a minor
correction for background was required (entries 3, 6, and 13), as the
background contributions were larger than the 5% estimated
uncertainties in kobs.
3385
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
The first column in DCM/AcOEt (96:4) gave enriched fractions of
compound 7 which was further purified by using a second column in
hexane/acetone (8:2) to obtain 300 mg of a yellow oil (13% yield). 1H
NMR (400 MHz, CDCl3): δ 7.50 (s, 2H), 7.47 (s, 2H), 6.68−6.50 (m,
6H), 4.66 (d, J = 14.0 Hz, 1H), 4.56 (d, J = 14.0 Hz, 2H), 4.45 (d, J =
14.0 Hz, 1H), 4.31−4.20 (m, 2H), 4.21−4.00 (m, 6H), 3.88−3.68 (m,
8H), 3.58−3.42 (m, 8H), 3.28 (d, J = 14.0 Hz, 1H), 3.23 (d, J = 14.0
Hz, 2H), 3.16 (d, J = 14.0 Hz, 1H), 1.18 and 1.15 (2t, J = 7.0 Hz, 6H
each). 13C NMR (100 MHz, CDCl3): δ 162.3, 156.2, 142.6, 137.0,
135.5, 135.3, 133.7, 129.0, 128.1, 124.3, 123.3, 122.6, 74.0, 73.4, 69.8,
69.5, 66.4, 31.0, 30.8, 15.3, 15.2. MS (ESI): m/z 825.3 [M + Na]+.
Anal. Calcd for C44H54N2O12: C, 65.82; H, 6.78; N, 3.49. Found: C,
65.75; H, 6.89; N, 3.58.
steric interference with the altered substrate in the transition
state. In other words, the steric overcrowding caused by the
presence of two extra guanidine or guanidinium moieties, in
addition to the guanidine−guanidinium pair directly involved in
the acid−base catalysis, causes a modest steric repulsion with
the substrate in the activation process. Such an interference is
apparently avoided in the reaction catalyzed by 3H+.
To sum up, the data reported in this work, when combined
with analogous data from previous studies, strongly reinforce the
notion that cone-calix[4]arenes are quite useful for the design of
efficient catalysts. The catalytic efficiency results from a proper
organization of catalytic groups brought about by the calix[4]arene
platform, coupled with a high level of adaptability resulting from
the conformational flexibility of the platform itself.
A still open question is whether cone-calix[4]arenes are suitable
for the construction of multifunctional catalysts whose catalytic
performance results from the cooperative action of three or more
catalytic groups. Some evidence was obtained of the operation of
trimetallic catalysis of zinc(II) and copper(II) complexes in the
cleavage of carboxylate and phosphate esters.4,25,26,28 On the
contrary, there seems to be no sign of the operation of multi-
functional catalysis in the transesterification of HPNP catalyzed by
calix[4]arene derivatives 3 and 4, which behave as bifunctional
catalysts, with no evidence of additional stabilization of the transi-
tion state arising from functional groups other than the guanidine−
guanidinium pair involved in the general acid−base catalysis.
5,17-Dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]-
arene (8). This compound (270 mg; 12% yield) showed the same
physicochemical and spectroscopic properties as those reported in the
literature.51
5,11,17-Trinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix-
[4]arene (9). A sample was obtained as a slightly pink solid (280 mg;
1
10% yield): mp 147−150 °C. H NMR (300 MHz, CDCl3) δ 7.83−
7.80 (m, 4H), 7.20 (s, 2H), 6.33 (s, 3H), 4.66 and 4.55 (2d, J = 13.8
Hz, 2H each), 4.40−4.20 (m, 4H), 4.14 and 3.99 (2t, J = 5.6 Hz, 2H
each), 3.81−3.72 (m, 8H), 3.56−3.42 (m, 8H), 3.32 and 3.27 (2d, J =
13.8 Hz, 2H each), 1.18 and 1.12 (2t, J = 7.0 Hz, 6H each). 13C NMR
(100 MHz, CDCl3): δ 163.0, 161.0, 155.3, 143.0, 142.6, 137.5, 136.2,
135.0, 132.8, 128.4, 124.6, 123.7, 123.6, 123.0, 74.4, 74.0, 73.9, 69.8,
69.4, 66.6, 66.5, 66.3, 31.0, 30.9, 29.7, 15.2. ES-MS: m/z 870.2 (M + Na)+.
Anal. Calcd (%) for C44H53N3O14: C, 62.33; H, 6.30; N, 4.96. Found:
C, 62.37; H, 6.42; N, 5.02.
5,11-Diamino-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]-
arene (11). A sample of 1,2-vicinal dinitro compound 7 (0.15 g, 0.187
mmol) was dissolved in absolute EtOH (10 mL), and hydrated
hydrazine (98%, 0.1 mL, 2.12 mmol) and a catalytic amount of Pd/C
were added. The reaction mixture was refluxed overnight under
nitrogen. After cooling, the catalyst was removed by filtration and the
solvent evaporated to dryness to obtain compound 11 as a colorless oil
EXPERIMENTAL SECTION
■
Instruments. NMR spectra were recorded on either a 400- or 300
MHz spectrometer. Partially deuterated solvents were used as internal
standards to calculate the chemical shifts (δ values in ppm). Mass
spectra were obtained on an electrospray ionization (ESI) time-of-
flight spectrometer.
Spectrophotometric measurements were carried out on either a
double beam or on a diode array spectrophotometer.
Materials. All reactions were carried out under nitrogen
atmosphere. Dry DMF was prepared according to standard procedures
and stored over molecular sieves. Anhydrous CH2Cl2 was obtained by
distillation over CaCl2. DMSO, purged 30 min with argon, and mQ
water were used in the preparation of 80% DMSO used in kinetic and
potentiometric experiments. All the other solvents and reagents were
used as commercially available without any further purification. Flash
chromatography purifications were carried out on 230−240 mesh
silica gel. HPNP65 and tetrakis(2-ethoxyethoxy)calix[4]arene 6a51
were prepared according to literature procedures. The 5,11,17,23-tetra-
amino-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]arene 14 was pre-
pared according to a literature procedure.66 Aminocalixarenes 11−14
were stored under nitrogen atmosphere to avoid the formation of their
carbamate derivatives.53
1
(0.11 g; 79% yield). H NMR (400 MHz, CDCl3): δ 6.72−6.50 (m,
6H), 6.01 (d, J = 2.5 Hz, 2H), 5.97 (d, J = 2.5 Hz, 2H), 4.50 (d, J =
13.4 Hz, 1H), 4.42 (d, J = 13.4 Hz, 2H), 4.33 (d, J = 13.4 Hz, 1H),
4.09 (t, J = 5.8 Hz, 4H), 4.00 (t, J = 6.0 Hz, 4H), 3.89−3.75 (m, 8H),
3.54 (q, J = 6.8 Hz, 8H), 3.15 (d, J = 13.4 Hz, 1H), 3.05 (bs, 4H), 3.03
(d, J = 13.4 Hz, 2H), 2.90 (d, J = 13.4 Hz, 1H), 1.24−1.14 (m, 12H).
13C NMR (100 MHz, CDCl3); δ 156.6, 149.5, 140.6, 135.5, 135.1,
128.2, 128.1, 121.8, 115.4, 73.1, 69.7, 69.6, 66.4, 30.9, 15.3. ES-MS: m/z
765.34 (M + Na)+, m/z 743.38 (M + H)+. Anal. Calcd for
C44H58N2O8: C, 71.13; H, 7.87; N, 3.77. Found: C, 71.03; H, 7.95:
N, 3.91.
5,17-Diamino-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]-
arene (12). The reaction was carried out as for the preparation of
compound 11, starting from the 1,3-distal dinitro derivative 8 (0.17 g,
0.212 mmol). Compound 12 was obtained as a reddish oil (0.116 g;
1
74% yield). H NMR (400 MHz, CDCl3) δ 6.73 (d, J = 7.2 Hz, 4H),
6.63 (t, J = 7.2 Hz, 2H), 5.98 (s, 4H), 4.44 (d, J = 13.2 Hz, 4H), 4.12
(t, J = 5.5 Hz, 4H), 4.03 (t, J = 5.3 Hz, 4H), 3.92 (m, 8H), 3.57 and
3.55 (2q, J = 7.3 Hz, 8H), 3.19 (s, 4H), 3.06 (d, J = 13.2 Hz, 4H), 1.23
and 1.21 (2t, J = 7.3 Hz, 6H each). 13C NMR (100 MHz, CDCl3): δ
156.6, 149.5, 140.7, 135.5, 135.2, 128.2, 122.0, 115.7, 73.2, 73.0, 69.7,
69.6, 66.4, 66.3, 30.9, 15.3. ES-MS: m/z 743.38 (M + H)+. Anal. Calcd
for C44H58N2O8: C, 71.13; H, 7.87; N 3.77. Found: C, 71.06; H, 7.92;
N, 3.88.
5,11,17-Triamino-25,26,27,28-tetrakis(2-ethoxyethoxy)calix-
[4]arene (13). The reaction was carried out as for the preparation of
compound 11, starting from trinitro compound 9 (0.235 g, 0.278
mmol). Compound 13 was obtained as a dark-red oil (0.147 g; 70%
yield). 1H NMR (300 MHz, CDCl3): δ 6.75 (d, J = 6.9 Hz, 2H), 6.65
(t, J = 6.9 Hz, 1H), 6.09 (s, 2H), 6.00 and 5.95 (2d, 2H each, J = 2.1
Hz), 4.42 and 4.32 (2d, J = 13.2 Hz, 2H each), 4.10 (t, J = 5.7 Hz,
2H), 4.05−3.90 (m, 6H), 3.90−3.70 (m, 8H), 3.50 (q, J = 6.8 Hz,
8H), 3.27 (s, 6H), 3.03 and 2.90 (2d, J = 13.2 Hz, 2H each), 1.20 and
1.18 (2t, J = 6.8 Hz, 6H each). 13C NMR (100 MHz, CDCl3): δ 157.1,
Nitration of 25,26,27,28-Tetrakis(2-ethoxyethoxy)calix[4]-
arene (6a). To a stirring solution of 25,26,27,28-tetrakis(2-
ethoxyethoxy)calix[4]arene 6a (2.0 g, 2.8 mmol) in DCM (95 mL)
and glacial acetic acid (5.6 mL) was added HNO3 (65%, 93.3 mL).
The reaction turned purple and then dark brown and was quenched
after 135 min by adding H2O (60 mL). The organic phase was
separated and washed with a 5% K2CO3 aqueous solution until basic
pH. The solvent was removed under reduced pressure to afford an
orange residue, from which most of the 1,3-distal dinitro derivative (8)
was obtained through crystallization from MeOH. The solvent was
removed from the mother liquor and the residue chromatographed
(SiO2, petroleum ether/AcOEt, 7:3; petroleum ether/AcOEt, 2:3; and
AcOEt/MeOH, 98:2) to afford, in the given order, the mononitro
derivative, a mixture of 1,2-vicinal dinitro (7) and 1,3-distal dinitro (8)
compounds, and finally the trinitro derivative (9).
5,11-Dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]-
arene (7). A pure sample of compound 7 was obtained by two
consecutive flash column chromatography separations on SiO2 gel.
3386
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
150.2, 140.4, 138.7, 138.5, 138.4, 136.0, 135.5, 135.4, 128.4, 122.0,
116.5, 115.7, 73.3, 72.9, 69.7, 69.6, 66.4, 66.3, 30.9, 29.7, 15.3. ES-MS:
m/z 780.3 (M + Na)+, 758.3 (M + H)+. Anal. Calcd for C44H59N3O8:
C, 69.72; H, 7.85; N, 5.54. Found: C, 69.64; H, 7.97; N, 5.65.
5,11-Bis[(N,N′-di-Boc)guanidine]-25,26,27,28-tetrakis(2-
ethoxyethoxy)calix[4]arene (15). A stirring solution of the 1,2-
vicinal diaminocalix[4]arene 11 (0.1 g, 0.135 mmol) and bis-Boc
thiourea (0.088 g, 0.314 mmol) in dry DMF (6 mL) was cooled to
0 °C with an ice-bath and then Et3N (0.131 mL, 0.94 mmol) and
HgCl2 (0.085 g, 0.314 mmol) were added. After 2 h, the reaction was
quenched by adding AcOEt (15 mL) and filtering the HgS precipitate.
The solvents were removed from the collected filtrate at the rotavapor
and the resulting reddish foam chromatographed (SiO2; hexane/
AcOEt, 3:1). Compound 15 was obtained as a white solid (0.089 g;
with H2O (15 mL). The organic phase was separated and dried over
Na2SO4 and the filtrate evaporated under reduced pressure. The
residue was recrystallized from Et2O/hexane to obtain pure compound
18 (0.44 g; 59% yield): mp > 300 °C. 1H NMR (400 MHz, CDCl3): δ
11.59 (s, 4H), 9.83 (s, 4H), 6.94 (s, 8H), 4.46 (d, J = 13 Hz, 4H), 4.09
(t, J = 5.6 Hz, 8H), 3.83 (t, J = 5.6 Hz, 8H), 3.53 (q, J = 7 Hz, 8H),
3.15 (d, J = 13 Hz, 4H), 1.47 and 1.44 (2s, 36H each), 1.20 (t, J = 7
Hz, 12H). 13C NMR (100 MHz, CDCl3): δ 163.6, 153.6, 153.0, 134.7,
131.0, 123.2, 83.0, 79.1, 73.3, 69.4, 66.4, 31.2, 28.2, 28.1, 15.3. ES-MS:
m/z 893.46 (M + 2Na)2+/2, 1763.85 (M + Na)+.
5,11-Diguanidine-25,26,27,28-tetrakis(2-ethoxyethoxy)-
calix[4]arene Bis-hydrochloride (1·2HCl). To a stirring solution of
the 1,2-vicinal derivative 15 (0.083 g, 0.068 mmol) in 1,4-dioxane (6.8
mL), 37% HCl (0.23 mL, 2.76 mmol) was added. After 5 days at room
temperature, the solvent was removed at the rotavapor; pure
compound 1·2HCl was obtained in quantitative yield as a white solid
(0.061 g; 99% yield): mp 136−139 °C. 1H NMR (300 MHz, CD3OD) δ
6.78−6.60 (m, 10H), 4.62 (d, J = 13.5 Hz, 1H), 4.60 (d, J = 13.5 Hz,
2H), 4.57 (d, J = 13.5 Hz, 1H), 4.28−4.09 (m, 8H), 4.00−3.82 (m,
8H), 3.62 and 3.57 (2q, J = 7.0 Hz, 4H each), 3.25 (d, J = 13.5 Hz,
1H), 3.23 (d, J = 13.5 Hz, 2H), 3.20 (d, J = 13.5 Hz, 1H), 1.22 and
1.21 (2t, J = 7.0 Hz, 6H each). 13C NMR (100 MHz, CDCl3): δ 156.5,
156.3, 156.0, 137.3, 136.4, 135.3, 134.4, 128.3, 127.9, 125.5, 125.0,
122.3, 73.8, 73.3, 70.0, 69.7, 66.2, 66.0, 30.4, 30.3, 14.3, 14.2. ES-MS:
m/z 849.32 (1 + Na)+, 827.35 (1 + H)+: HR ES-MS: m/z Calcd for
1
54% yield): mp 104−106 °C. H NMR (300 MHz, CDCl3) δ 11.58
(s, 2H), 9.79 (s, 2H), 6.74 (s, 2H), 6.73 (s, 2H), 6.62 (s, 6H), 4.49 (d,
J = 12.8 Hz, 1H), 4.45 (d, J = 12.8 Hz, 2H), 4.41 (d, J = 12.8 Hz, 1H),
4.08 and 4.04 (2t, J = 6.1 Hz, 4H each), 3.81 and 3.79 (2t, J = 6.1 Hz,
4H each), 3.53 (q, J = 7.2 Hz, 8H), 3.14 (d, J = 12.8 Hz, 1H), 3.13 (d,
J = 12.8 Hz, 3H), 1.51 and 1.47 (2s, 18H each), 1.25−1.12 (m, 12H).
13C NMR (100 MHz, CDCl3): δ 163.6, 156.5, 153.9, 153.4, 153.2,
135.4, 135.1, 134.8, 130.6, 128.3, 122.7, 122.6, 122.1, 83.3, 79.2, 73.1,
69.7, 69.4, 66.4, 30.9, 28.2, 28.1, 15.3. ES-MS: m/z 1249.5 (M + Na)+,
1227.7 (M + H)+. Anal. Calcd for C66H94N6O16: C, 64.58; H, 7.72; N,
6.85. Found: C, 64.63; H, 7.81; N, 6.95.
5,17-Bis[(N,N′-di-Boc)guanidine]-25,26,27,28-tetrakis(2-
ethoxyethoxy)calix[4]arene (16). The reaction was carried out as
for the preparation of compound 15, starting from 1,3-distal
diaminocalix[4]arene 12 (0.12 g, 0.16 mmol). A pure sample of 16
was obtained by flash column chromatography (SiO2, cyclohexane/
AcOEt, 15:4) as a white solid (0.086 g; 45% yield): mp 157−160 °C.
1H NMR (300 MHz, CDCl3): δ 11.65 (s, 2H), 10.24 (s, 2H), 7.29 (s,
4H), 6.32−6.19 (m, 6H), 4.45 (d, J = 13.2 Hz, 4H), 4.24 (t, J = 6.6 Hz,
4H), 3.95−3.81 (m, 8H), 3.76 (t, J = 5.0 Hz, 4H), 3.56 and 3.51 (2q,
J = 7.2 Hz, 4H each), 3.12 (d, J = 13.2 Hz, 4H), 1.54 and 1.49 (2s,
18H each), 1.23 and 1.18 (2t, J = 7.2 Hz, 6H each). 13C NMR (100
MHz, CDCl3): δ 163.7, 155.0, 154.7, 153.4, 153.3, 136.9, 133.0, 130.7,
127.8, 122.6, 122.4, 83.5, 79.4, 73.9, 72.5, 69.6, 69.5, 66.5, 66.2, 30.8,
28.2, 28.1, 15.4, 15.3. ES-MS: m/z 1227.6 (M + H)+. Anal. Calcd (%)
for C66H94N6O16: C, 64.58; H, 7.72; N, 6.85. Found: C, 64.51; H,
7.83; N, 6.93.
+
C46H63N6O8 827.4707 (1 + H)+, found 827.4693.
5,17-Diguanidine-25,26,27,28-tetrakis(2-ethoxyethoxy)-
calix[4]arene Bis-hydrochloride (2·2HCl). The reaction was carried
out according to the preparation of compound 1, stirring for 4 days a
mixture of compound 16 (0.086 g, 0.07 mmol), 0.23 mL of 37% HCl
(2.76 mmol), and 5 mL of 1,4-dioxane. Compound 2·2HCl was
obtained in quantitative yield as a pale-yellow oil (0.077 g; 99% yield).
1H NMR (400 MHz, CD3OD) δ 7.00 (d, J = 7.2 Hz, 4H), 6.83 (t, J =
7.2 Hz, 2H), 6.42 (s, 4H), 4.62 (d, J = 13.0 Hz, 4H), 4.33 (t, J = 6.0
Hz, 4H), 4.09 (t, J = 5.0 Hz, 4H), 3.97 (t, J = 6.0 Hz, 4H), 3.91 (t, J =
5.0 Hz, 4H), 3.64 and 3.57 (2q, J = 7.0 Hz, 4H each), 3.25 (d, J = 13.0
Hz, 4H), 1.27 and 1.20 (2t, J = 7.0 Hz, 6H each).13C NMR (75 MHz,
CD3OD): δ 158.2, 157.8, 156.5, 137.7, 136.7, 130.0, 129.5, 126.1,
124.0, 75.4, 74.2, 71.2, 67.6, 67.3, 31.8, 15.72, 15.68. ES-MS: m/z 414.1
[(2 + 2H)2+]/2, 827.4 (2 + H)+; HR ES-MS m/z Calcd for C46H63N6O8+
827.4707 (2 + H)+, found 827.4738.
5,11,17-Triguanidine-25,26,27,28-tetrakis(2-ethoxyethoxy)-
calix[4]arene Tris-hydrochloride (3·3HCl). The reaction was
carried out according to the preparation of compound 1, stirring for
4 days a mixture of the trifunctional derivative 17 (0.1 g, 0.068 mmol),
37% HCl (0.335 mL, 4.0 mmol), and 6.7 mL of 1,4-dioxane. The
reaction was quenched, removing the solvent at the rotavapor, and
pure compound 3·3HCl was obtained in quantitative yield as a pale-
yellow oil (0.077 g; 99% yield). 1H NMR (400 MHz, CD3OD): δ 6.83
(d, J = 7.2 Hz, 2H), 6.79−6.71 (m, 3H), 6.55 (s, 4H), 4.61 and 4.58
(2d, J = 13.3 Hz, 2H each), 4.25 (t, J = 3.9 Hz, 2H), 4.19 (t, J = 5.1 Hz,
2H), 4.20−4.05 (m, 4H), 3.99−3.81 (m, 8H), 3.72−3.50 (m, 8H),
3.26 and 3.23 (2d, J = 13.3 Hz, 2H each), 1.26−1.11 (m, 12H). 13C
NMR (75 MHz, CD3OD) δ: 157.87, 157.82, 157.76, 157.58, 156.9,
138.1, 138.0, 137.3, 136.1, 129.7, 127.0, 126.6, 126.2, 124.0, 75.3, 75.1,
74.5, 71.4, 71.3, 71.0, 67.6, 67.4, 31.8, 31.7, 15.72, 15.68. ES-MS: m/z
442.8 [(3 + 2H)2+]/2, 295.6 [(3 + 3H)3+]/3. HR ES-MS m/z Calcd
5,11,17-Tris[(N,N′-di-Boc)guanidine]-25,26,27,28-tetrakis(2-
ethoxyethoxy)calix[4]arene (17). The reaction was carried out
according to the preparation of compound 15, starting from triamino
compound 13 (0.147 g, 0.194 mmol), 3 equiv of bis-Boc-thiourea and
HgCl2 and 9 equiv of Et3N. A pure sample of 17 was obtained by flash
column chromatography (SiO2; hexane/AcOEt, 3:1) as a slightly pink
1
oil (0.145 g; 50% yield). H NMR (300 MHz, CDCl3): δ 11.67 (bs,
2H), 11.55 (bs, 1H), 10.27 (bs, 2H), 9.31 (bs, 1H), 7.33 and 7.28 (2d,
2H each, J = 2.3 Hz), 6.37 (t, 1H, J = 7.6 Hz), 6.22 (d, 2H, J = 7.6 Hz),
6.17 (s, 2H), 4.45 and 4.41 (2d, J = 13.5 Hz, 2H each), 4.20 (t, J = 5.9
Hz, 4H), 3.60−3.47 (m, 12H), 3.60−3.46 (m, 8H), 3.13 and 3.11 (2d,
J = 13.5 Hz, 2H each), 1.53, 1.49, 1.47, and 1.42 (4s, 54H), 1.22 and
1.16 (2t, J = 6.9 Hz, 6H each). 13C NMR (100 MHz, CDCl3): δ 163.6,
155.1, 154.9, 153.8, 153.4, 153.2, 153.0, 152.7, 136.9, 136.7, 133.6,
133.2, 130.8, 130.4, 127.8, 123.3, 122.5, 122.4, 122.1, 83.4, 83.1, 79.4,
79.0, 73.8, 72.5, 69.6, 69.5, 69.4, 66.6, 66.5, 66.2, 31.0, 28.2, 28.1, 15.4,
15.2. ES-MS: m/z 1484.6 (M + H)+, 1506.6 (M + Na)+. Anal. Calcd
for C77H113N9O20: C, 62.29; H, 7.67; N, 8.49. Found: C, 62.18; H,
7.73; N, 8.56.
5,11,17,23-Tetrakis[(N,N′-di-Boc)guanidine]-25,26,27,28-
tetrakis(2-ethoxyethoxy)calix[4]arene (18). The reaction was
carried out according to the preparation of compound 15, starting
from tetraamino compound 14 (0.650 g, 0.84 mmol), 4.5 equiv of bis-
Boc-thiourea and HgCl2 and 12 equiv of Et3N in dry DMF (30 mL).
After 18 h, DCM was added and HgS filtered off. After removal
of DCM at the rotavapor, the DMF solution was left standing,
and a white solid precipitated, was filtered off, and washed with
hexane. The solvents were completely removed from the filtrate at the
rotavapor, and the residue was uptaken in AcOEt (15 mL) and washed
2+
for C47H67N9O8 442.7556 [(3 + 2H)2+]/2, found 442.7570.
5,11,17,23-Tetraguanidine-25,26,27,28-tetrakis(2-
ethoxyethoxy)calix[4]arene Tetrakis-hydrochloride (4·4HCl).
To a stirring solution of compound 18 (0.425 g, 0.24 mmol) in 1,4-
dioxane (30 mL) at rt were added a 37% HCl aqueous solution (3.5
mL) and triethylsilane (TES) (0.39 mL, 2.44 mmol). After 24 h, the
solvent was evaporated and the residue dissolved in water and washed
with Et2O (2 × 10 mL). The aqueous layer was lyophilized, thus
leaving a sample of pure compound 4·4HCl (0.250 g; 96% yield):
mp > 300 °C. 1H NMR (300 MHz, D2O): δ 6.78 (s, 8H), 4.53 (d, J =
13.5, 4H), 4.25 (t, J = 4.5 Hz, 8H), 3.99 (t, J = 4.5 Hz, 8H), 3.66 (q,
J = 7 Hz, 8H), 3.37 (d, J = 13.5, 4H), 1.20 (t, J = 7 Hz, 12H). 13C
NMR (100 MHz, D2O/MeOD, 7/3): δ 156.8, 156.5, 137.2, 129.1,
3387
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
126.1, 74.15, 70.8, 67.2, 31.0, 14.9. ES-MS: m/z 941.9 (4 + H)+, 471.5
AUTHOR INFORMATION
■
+
[(4 + 2H)2+]/2. HR ES-MS m/z Calcd for C48H69N12O8 941.5361
(4 + H)+, found 941.5352.
Corresponding Author
*E-mail: riccardo.salvio@uniroma1.it.
N-(4-Methoxyphenyl)-N,N′-bis-(tert-butoxycarbonyl)-
guanidine (19). N,N′-Bis-(tert-butoxycarbonyl)-N″-triflyl-guanidine
(304 mg, 0.77 mmol), and p-methoxyaniline (333 mg, 2.7 mmol)
were dissolved in anhydrous DCM (100 mL). The reaction mixture
was kept under stirring at room temperature for 40 h and then was
washed with KHSO4 (1 M, 50 mL) and a saturated solution of
NaHCO3 (50 mL). The organic phase was dried over MgSO4 and the
solvent was evaporated under reduced pressure. The residue was
purified by flash column chromatography (SiO2; hexane/AcOEt (9:1)
eluent), and active carbon was added to the eluted fractions containing
the product. After filtration over Celite and evaporation of the solvent,
compound 19 was obtained as a pale-yellow oil (229 mg, 0.63 mmol;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial contribution from MIUR/PRIN 2008 is acknowledged.
We also thank Ms. Claudia Savelli for technical assistance in the
synthesis of the compounds and in the potentiometric
experiments.
1
82% yield). H NMR (300 MHz; CDCl3): δ 1.53 (s, 9H), 1.49 (s,
REFERENCES
■
9H), 3.79 (s, 3H), 6.86 (d, 2H, J = 9 Hz), 7.47 (d, 2H, J = 9 Hz). 13
C
(1) Brinker, U. H., Mieusset, J.-L., Eds. Molecular Encapsulation -
Organic Reactions in Constrained Systems; John Wiley & Sons, Ltd.:
New York, 2010.
NMR (75 MHz; CDCl3) 28.1, 28.2, 55.5, 114.0, 129.9, 127.6, 129.7,
153.6, 156.8. Anal. Calcd for C18H27N3O5: C, 59.16; H, 7.45; N, 11.50.
Found: C, 58.90; H, 7.69; N, 11.42.
(2) van Leeuwen, P. W. N. M., Ed. Supramolecular Catalysis; Wiley-
VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2008.
(3) Breslow, R. A. E. I. Artificial Enzymes; Breslow, R., Ed.; Wiley-
VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2005.
(4) Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. Chem. Soc.
Rev. 2000, 29, 75−86.
N-(4-Methoxyphenyl)guanidine Hydrochloride (5·HCl). In
20 mL of a 1:1 mixture of dioxane and HCl 0.1 M in water were
dissolved 229 mg (0.627 mmol) of compound 19. The solution was
kept under stirring at room temperature for 2 days. The solvent
mixture was removed by evaporation, obtaining compound 5·HCl as a
1
white solid (115 mg, 0.571 mmol; 91% yield): mp 108−110 °C. H
(5) Morrow, J. Curr. Opin. Chem. Biol. 2004, 8, 192−200.
(6) Mancin, F.; Scrimin, P.; Tecilla, P.; Tonellato, U. Chem. Commun.
2005, 2540−2548.
NMR (300 MHz; CD3OD): δ 3.75 (s, 3H), 6.96 (d, 2H, J = 9 Hz);
7.17 (d, 2H, J = 9 Hz). 13C NMR (75 MHz, CD3OD): δ 56.2, 115.8,
124.8, 127.5, 128.9, 159.3. Anal. Calcd for C8H12ClN3O: C, 47.65; H,
6.00; N, 20.84. Found: C, 47.27; H, 6.09; N, 20.50.
(7) Niittymaki, T.; Lonnberg, H. Org. Biomol. Chem. 2006, 4, 15−25.
̈
̈
(8) Scarso, A.; Zaupa, G.; Houillon, F. B.; Prins, L. J.; Scrimin, P.
J. Org. Chem. 2007, 72, 376−385.
Potentiometric Titrations. Potentiometric titrations were
performed by an automatic titrator equipped with a combined
microglass pH electrode. The electrode was calibrated using standard
HClO4 and Me4NOH solutions at different concentrations, I = 10 mM
(Et4NBr). The time required to obtain a stable pH reading increased
from 1 min in acid medium, up to 6 min at pH above 9. The
(9) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Peracchi, A.;
Reinhoudt, D. N.; Salvio, R.; Sartori, A.; Ungaro, R. J. Am. Chem.
Soc. 2007, 129, 12512−12520.
(10) Tseng, T.-S. A.; Burstyn, J. N. Chem. Commun. 2008, 6209−
6211.
+
(11) Bonomi, R.; Selvestrel, F.; Lombardo, V.; Sissi, C.; Polizzi, S.;
Mancin, F.; Tonellato, U.; Scrimin, P. J. Am. Chem. Soc. 2008, 130,
15744−15745.
(12) Bazzicalupi, C.; Bencini, A.; Bonaccini, C.; Giorgi, C.; Gratteri,
P.; Moro, S.; Palumbo, M.; Simionato, A.; Sgrignani, J.; Sissi, C.;
Valtancoli, B. Inorg. Chem. 2008, 47, 5473−5484.
(13) Nwe, K.; Andolina, C. M.; Morrow, J. R. J. Am. Chem. Soc. 2008,
130, 14861−14871.
(14) Katada, H.; Komiyama, M. ChemBioChem 2009, 10, 1279−
1288.
(15) Lindgren, N. J. V.; Lars Geiger, J. R.; Schmuck, C.; Baltzer, L.
Angew. Chem., Int. Ed. 2009, 48, 6722−6725.
(16) Thomas, J. M.; Yoon, J.-K.; Perrin, D. M. J. Am. Chem. Soc.
2009, 131, 5648−5658.
(17) Hollenstein, M.; Hipolito, C. J.; Lam, C. H.; Perrin, D. M.
ChemBioChem 2009, 10, 1988−1992.
(18) Mohamed, M. F.; Brown, R. S. J. Org. Chem. 2010, 75, 8471−
8477.
calibration plot of calculated −log cH values vs experimental pH
readings was linear in the range 2−17, with −log cH+ = a + b × pHread
,
and best fit values a = −0.7852 1.5%, and b = 0.965 5%. The pKw
values determined in several titrations coincided, within experimental
errors, with the value reported in the literature.62 Potentiometric
titrations were carried out under a nitrogen atmosphere, on 6 mL of
1−3 mM solutions of the compound, in the presence of 0.1 M Et4NBr,
(80% DMSO, 25 °C). A 0.05−0.2 M Me4NOH solution in 80%
DMSO was added to the titration vessel in small increments. Analysis
of titration plots was carried out by the program HYPERQUAD
2000.67,68
Kinetic Measurements. Spectrophotometric measurements were
carried out on either a double beam or on a diode array spectro-
photometer. Kinetic measurements of HPNP transesterification
were carried out on 0.1 mM substrate solutions in the presence of
3 mM catalyst by UV−vis monitoring of p-nitrophenol liberation at
400 nm. Hydrochlorides were neutralized by the addition to the
reaction mixture of the calculated amount of Me4NOH before kinetic
runs. Rate constants were obtained by an initial rate method,
error limit on the order of 10% for rate constants of 10−8−10−7 s−1,
and 5% for faster reactions.
(19) Lonnberg, H. Org. Biomol Chem. 2011, 9, 1687−1703.
̈
(20) Wolfenden, R. Chem. Rev. 2006, 106, 3379−3396.
(21) Demesmaeker, A.; Haner, R.; Martin, P.; Moser, H. E. Acc.
Chem. Res. 1995, 28, 366−374.
(22) Trawick, B. N.; Daniher, A. T.; Bashkin, J. K. Chem. Rev. 1998,
98, 939−960.
ASSOCIATED CONTENT
■
(23) Dospil, G.; Schatz, J. Tetrahedron Lett. 2001, 42, 7837−7840.
(24) Cacciapaglia, R.; Casnati, A.; Di Stefano, S.; Mandolini, L.;
Paolemili, D.; Reinhoudt, D. N.; Sartori, A.; Ungaro, R. Chem.Eur. J.
2004, 10, 4436−4442.
S
* Supporting Information
1H and 13C NMR spectra, plots of potentiometric acid−base
titrations, and medium effects on acid−base equilibrium and
transesterification rates. Background rates of HPNP trans-
esterification in 80% DMSO, 25 °C. This material is available
free of charge via the Internet at http://pubs.acs.org.
(25) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.;
Salvio, R.; Sartori, A.; Ungaro, R. J. Org. Chem. 2005, 70, 624−630.
(26) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.;
Salvio, R.; Sartori, A.; Ungaro, R. J. Org. Chem. 2005, 70, 5398−5402.
3388
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389

The Journal of Organic Chemistry
Article
(27) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.;
Salvio, R.; Sartori, A.; Ungaro, R. J. Am. Chem. Soc. 2006, 128, 12322−
12330.
(28) Cacciapaglia, R.; Casnati, A.; Mandolini, L.; Reinhoudt, D. N.;
Salvio, R.; Sartori, A.; Ungaro, R. Inorg. Chim. Acta 2007, 360, 981−
986.
(60) Kneeland, D. M.; Ariga, K.; Lynch, V. M.; Huang, C. Y.; Anslyn,
E. V. J. Am. Chem. Soc. 1993, 115, 10042−10055.
(61) Chu, F. Y.; Flatt, L. S.; Anslyn, E. V. J. Am. Chem. Soc. 1994, 116,
4194−4204.
(62) Kreevoy, M. M.; Baughman, E. H. J. Phys. Chem. 1974, 78, 421−
423.
(63) Pawlak, Z.; Bates, R. G. J. Solution Chem. 1975, 4, 817−829.
(64) As shown in the Supporting Information, Appendix 1, a
significant fraction of the difference in reactivity between 3H+ and
3(H+)2 is accounted for on the basis of medium effects.
(65) Brown, D. M.; Usher, D. A. J. Chem. Soc. 1965, 6558−6564.
(66) Yang, X.; McBranch, D.; Swanson, B.; Li, D. Angew. Chem., Int.
Ed. 1996, 35, 538−540.
(29) Cacciapaglia, R.; Di Stefano, S.; Mandolini, L. Molecular
Encapsulation, Organic Reaction in Constrained Systems; John Wiley &
Sons: New York, 2010; Chapter 8.
(30) Cotton, F. A.; Hazen, E. E. Jr.; Legg, M. J. Proc. Natl. Acad. Sci.
U.S.A. 1979, 76, 2551−2555.
(31) Kurz, K.; Gobel, M. W. Helv. Chim. Acta 1996, 79, 1967−1979.
̈
(32) Cuevas, F.; Di Stefano, S.; Magrans, J. O.; Prados, P.; Mandolini,
(67) Gans, P.; Sabatini, A.; Vacca, A. Talanta 1996, 43, 1739−1753.
(68) Alderighi, L.; Gans, P.; Ienco, A.; Peters, D.; Sabatini, A.; Vacca,
A. Coord. Chem. Rev. 1999, 184, 311−318.
L.; de Mendoza, J. Chem.Eur. J. 2000, 6, 3228−3234.
(33) Scheffer, U.; Strick, A.; Ludwig, V.; Peter, S.; Kalden, E.; Gobel,
̈
M. W. J. Am. Chem. Soc. 2005, 127, 2211−2217.
(34) Gnaccarini, C.; Peter, S.; Scheffer, U.; Vonhoff, S.; Klussmann,
NOTE ADDED AFTER ASAP PUBLICATION
S.; Gobel, M. W. J. Am. Chem. Soc. 2006, 128, 8063−8067.
■
̈
(35) Corona-Martinez, D. O.; Taran, O.; Yatsimirsky, A. K. Org.
Biomol. Chem. 2010, 8, 873−880.
This paper was published ASAP on March 15, 2012. Two
changes were made to the text in the Experimental Section. The
revised paper was reposted on March 23, 2012.
(36) Sheng, X.; Lu, X.; Y., C.; Lu, G.; Zhang, J.; Shao, Y.; Liu, F.; Xu,
Q. Chem.Eur. J. 2007, 13, 9703−9712.
(37) Ait-Haddou, H.; Sumaoka, J.; Wiskur, S. L.; Folmer-Andersen, J. F.;
Anslyn, E. V. Angew. Chem., Int. Ed. 2002, 41, 4014−4016.
(38) Suh, J.; Moon, S.-J. Inorg. Chem. 2001, 40, 4890−4895.
(39) Salvio, R.; Cacciapaglia, R.; Mandolini, L. J. Org. Chem. 2011, 76,
5438−5443.
(40) Muche, M. S.; Gobel, M. W. Angew. Chem., Int. Ed. 1996, 35,
̈
2126−2129.
(41) Smith, J.; Ariga, K.; Anslyn, E. V. J. Am. Chem. Soc. 1993, 115,
362−364.
(42) Piatek, A. M.; Gray, M.; Anslyn, E. V. J. Am. Chem. Soc. 2004,
126, 9878−9879.
(43) Best, M. D.; Tobey, S. L.; Anslyn, E. V. Coord. Chem. Rev. 2003,
240, 3−15 and references therein.
(44) Tobey, S. L.; Anslyn, E. V. J. Am. Chem. Soc. 2003, 125, 14807−
14815.
(45) Sansone, F.; Dudic, M.; Donofrio, G.; Rivetti, C.; Baldini, L.;
Casnati, A.; Cellai, S.; Ungaro, R. J. Am. Chem. Soc. 2006, 128, 14528−
14536.
(46) Bagnacani, V.; Sansone, F.; Donofrio, G.; Baldini, L.; Casnati,
A.; Ungaro, R. Org. Lett. 2008, 10, 3953−3956.
(47) Corbellini, F.; Mulder, A.; Sartori, A.; Ludden, M. J. W.; Casnati,
A.; Ungaro, R.; Huskens, J.; Crego-Calama, M.; Reinhoudt, D. N.
J. Am. Chem. Soc. 2004, 126, 17050−17058.
(48) Kopaczynska, M.; Wang, T. Y.; Schulz, A.; Dudic, M.; Casnati,
A.; Sansone, F.; Ungaro, R.; Fuhrhop, J. H. Langmuir 2005, 21, 8460−
8465.
(49) Dudic, M.; Colombo, A.; Sansone, F.; Casnati, A.; Donofrio, G.;
Ungaro, R. Tetrahedron 2004, 60, 11613−11618.
(50) Verboom, W.; Durie, A.; Egberink, R. J. M.; Asfari, Z.;
Reinhoudt, D. N. J. Org. Chem. 1992, 57, 1313−1316.
(51) van Loon, J. D.; Heida, J. F.; Verboom, W.; Reinhoudt, D. N.
Recl. Trav. Chim. Pays-Bas 1992, 111, 353−359.
(52) Sartori, A.; Casnati, A.; Mandolini, L.; Sansone, F.; Reinhoudt,
D. N.; Ungaro, R. Tetrahedron 2003, 59, 5539−5544.
(53) Baldini, L.; Melegari, M.; Bagnacani, V.; Casnati, A.; Dalcanale,
E.; Sansone, F.; Ungaro, R. J. Org. Chem. 2011, 76, 3720−3732.
(54) Georgieva, M. Anal. Chim. Acta 1977, 90, 83−89.
(55) Wrob
308.
́ ́
el, R.; Chmurzynski, L. Anal. Chim. Acta 2000, 405, 303−
(56) Taran, O.; Yatsimirsky, A. K. Chem. Commun. 2004, 1228−
1229.
(57) Gomez-Tagle, P.; Vargas-Zuniga, I.; Taran, O.; Yatsimirsky, A. K.
J. Org. Chem. 2006, 71, 9713−9722.
(58) Sanchez-Lombardo, I.; Yatsimirsky, A. K. Inorg. Chem. 2008, 47,
2514−2525.
(59) Ariga, K.; Anslyn, E. V. J. Org. Chem. 1992, 57, 417−420.
3389
dx.doi.org/10.1021/jo300193y | J. Org. Chem. 2012, 77, 3381−3389