Cu(II)-ion-catalyzed solvolysis of N,N- bis(2-picolyl)ureas in alcohol solvents: Evidence for cleavage involving nucleophilic addition and strong assistance of bis(2-picolyl)amine leaving group departure

Article abstract of DOI:10.1021/ic500620k

The kinetics and products for solvolysis of N-p-nitrophenyl-N′, N′-bis(pyridin-2-ylmethyl) urea (7a), N-methyl-N-p-nitrophenyl-N′, N′-bis(pyridin-2-yl methyl) urea (7b), and N-phenyl-N′,N′- bis(pyridin-2-yl-methyl) urea (DPPU) (7c) promoted by Cu(II) ion in methanol and ethanol were studied under s^spH-controlled conditions at 25 °C. Methanolysis and ethanolysis of these substrates proceeds rapidly at a 1:1 ratio of substrate:metal ion, the half-times for decomposition of the Cu(II):7a complexes being ～150 min in methanol and 15 min in ethanol. In all cases, the reaction products are the Cu(II) complex of bis(2-picolyl)amine and the O-methyl or O-ethyl carbamate of the parent aniline, signifying that the point of cleavage is the bis(2-picolyl) - N - C - O bond. Reactions of the Cu(II):7b complexes in each solvent proceed about 3-5 times slower than their respective Cu(II):7a complexes, excluding an elimination mechanism that proceeds through an isocyanate which subsequently adds alcohol to give the observed products. The reactions also proceed in other solvents, with the order of reactivity ethanol > methanol >1-propanol >2-propanol > acetonitrile (with 0.2% methanol) > water spanning a range of 150-fold. The mechanism of the reactions is discussed, and the reactivity and mode of cleavage are compared with that of the M(II)-promoted ethanolytic cleavage of a mono-2-picolyl derivative, N-p-nitrophenyl-N′-(pyridin-2-yl-methyl) urea (4a), which had previously been shown to cleave at the aniline N-C - O bond. The large estimated acceleration of the rate of attack of ethoxide on 7b of at least 2 × 10¹⁶ provided by associating Cu(II) with the departing group in this urea is discussed in terms of a trifunctional role for the metal ion involving Lewis acid activation of the substrate, intramolecular delivery of a Cu(II)-coordinated ethoxide, and metal-ion-assisted leaving group departure.

Full text of DOI:10.1021/ic500620k

Article
pubs.acs.org/IC
Cu(II)-Ion-Catalyzed Solvolysis of N,N-Bis(2-picolyl)ureas in Alcohol
Solvents: Evidence for Cleavage Involving Nucleophilic Addition and
Strong Assistance of Bis(2-picolyl)amine Leaving Group Departure
Mei-Ni Belzile, Alexei. A. Neverov, and R. Stan Brown*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
S
* Supporting Information
ABSTRACT: The kinetics and products for solvolysis of N-p-
nitrophenyl-N′,N′-bis(pyridin-2-ylmethyl) urea (7a), N-meth-
yl-N-p-nitrophenyl-N′,N′-bis(pyridin-2-yl methyl) urea (7b),
and N-phenyl-N′,N′-bis(pyridin-2-yl-methyl) urea (DPPU)
(7c) promoted by Cu(II) ion in methanol and ethanol were
s
s
studied under pH-controlled conditions at 25 °C. Meth-
anolysis and ethanolysis of these substrates proceeds rapidly at
a 1:1 ratio of substrate:metal ion, the half-times for decomposition of the Cu(II):7a complexes being ∼150 min in methanol and
15 min in ethanol. In all cases, the reaction products are the Cu(II) complex of bis(2-picolyl)amine and the O-methyl or O-ethyl
carbamate of the parent aniline, signifying that the point of cleavage is the bis(2-picolyl)NCO bond. Reactions of the
Cu(II):7b complexes in each solvent proceed about 3−5 times slower than their respective Cu(II):7a complexes, excluding an
elimination mechanism that proceeds through an isocyanate which subsequently adds alcohol to give the observed products. The
reactions also proceed in other solvents, with the order of reactivity ethanol > methanol >1-propanol >2-propanol > acetonitrile
(with 0.2% methanol) > water spanning a range of 150-fold. The mechanism of the reactions is discussed, and the reactivity and
mode of cleavage are compared with that of the M(II)-promoted ethanolytic cleavage of a mono-2-picolyl derivative, N-p-
nitrophenyl-N′-(pyridin-2-yl-methyl) urea (4a), which had previously been shown to cleave at the aniline N−CO bond. The
large estimated acceleration of the rate of attack of ethoxide on 7b of at least 2 × 1016 provided by associating Cu(II) with the
departing group in this urea is discussed in terms of a trifunctional role for the metal ion involving Lewis acid activation of the
substrate, intramolecular delivery of a Cu(II)-coordinated ethoxide, and metal-ion-assisted leaving group departure.
(or hydroxide) to all of its substrates including acetamide,
formamide, semicarbazide, and N-methyl urea,9 although recent
computational10 and experimental evidence with small
molecule dinuclear complexes11 indicates that elimination
might be a competitive pathway. Whatever its mechanism,
the enzyme produces very large rate accelerations since its kcat is
estimated to be 1014−1015 times greater than the computed
spontaneous rate constant for cleavage of urea at pH 7.12,8
More recent computational studies indicate that the enzymatic
acceleration may be as much as 1025 times that for the
computed rate constant for hydroxide attack on H2NC(
O)NH2.10,13
I. INTRODUCTION
Ureases (EC 3.5.1.5) are Ni(II)-dependent metalloenzymes
that functionally belong to the class of amidohydrolases that
catalyze the hydrolysis of urea into carbon dioxide and
ammonia.1,2 The widely studied jack-bean urease has a
remarkable substrate fidelity since the catalytic efficiency for
its second-best urea-type substrate semicarbazide (NH2C(
O)NHNH2) is 1/1000th that of urea itself.3 The active sites of
all known ureases are highly conserved and contain a bis-μ-
hydroxo-bridged dimeric nickel center with a metal−metal ion
separation of ∼3.5 Å. The nickel ions are also bridged by an N-
carbamylated lysine through its O atoms and by a hydroxide
ion. One nickel ion is coordinated to N atoms of histidines
residues and one water molecule, while the other is coordinated
to two histidines, an aspartic acid, and two water molecules.4
Urease activity is specific for its metal ion, namely, Ni(II), with
the Zn(II)- and Cu(II)-substituted enzymes being inactive.5
Urea is resistant toward hydrolysis because of its resonance
stabilization estimated at 30−40 kcal/mol.3 Its pH-independent
decomposition in water between 2 and 12 proceeds not by
hydrolysis involving nucleophilic addition to the CO unit but
by elimination of NH3 to yield cyanic acid (HNCO)6 with
a rate constant from 8.3 × 10−10 to 1.2 × 10−11 s−1 at 25 °C.7,8
The catalytic reaction mediated by urease itself is generally
believed to operate via hydrolytic nucleophilic addition of water
Recent work14 in our lab has centered on mechanisms by
which metal-containing complexes can facilitate cleavage of
esters, phosphates, amides, and ureas in alcohol media. The
pathways by which metal ions promote acyl and phosphoryl
transfer reactions can involve one or more of several roles such
as (a) Lewis acid activation, (b) intracomplex delivery of a
metal-coordinated lyoxide nucleophile, (c) stabilization of the
substrate−nucleophile activated complex, and (d) assistance of
the departure of the leaving group (leaving group assistance,
LGA).15,16 Kinetic studies of the methanolysis of N-acyl
Received: March 18, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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derivatives of N,N-bis(2-picolyl)amine and N,N-bis-
(benzimidazol-2-yl-methyl)amine (1−3)17−19 catalyzed by
various transition metal ions under pH-controlled conditions20
revealed trifunctional roles for the metal ions. These involved a
pre-equilibrium coordination of the metal ion to the bis(2-
picolyl)amino or bis(benzimidazol-2-yl)amino portion of the
ligand system, intramolecular attack of the bound metal ion’s
coordinated methoxide on the CO, and metal-ion-assisted
C−N departure. The resulting catalytic effect of the metal ions
is striking for Cu(II)-promoted cleavage of 2, being at least 1017
times faster than the reaction involving free methoxide.19
aqueous acid at 55 °C led to elimination of the anilines and
produced the (en)2Co(III) complex of (2-picolyl)amine.
In view of the above works and our interest in LGA for
cleavage of bis(2-picolyl)-substituted amides, we have under-
taken a more detailed study of the Cu(OTf)2-catalyzed
alcoholysis of bis(2-picolyl)-substituted ureas 7a−c under
sspH-controlled conditions. As will be shown, in contrast to
the reported cleavages of 4a−c mediated by Ni(II) (and
Cu(II)), which gives the products shown in eq 1, the Cu(II)-
promoted methanolyses and ethanolyses of 7a−c proceeds
rapidly at 25 °C to give bis(2-picolyl)amine and the O-methyl
or O-ethyl carbamate of the corresponding aniline.
II. EXPERIMENTAL SECTION
Our attention was drawn recently to two studies12,21 dealing
with transition-metal-ion-promoted ethanolytic cleavage of
some substituted ureas bearing an N-(2-picolyl)amine portion
(4a,b). The first12 reported that 4a, in aqueous ethanol in the
presence of excess NiCl2 at elevated temperature (50−80 °C),
produced O-ethyl-N-(2-picolyl) carbamate (5) and by compet-
ing hydrolysis gave smaller amounts of (2-picolyl)amine (6) as
in eq 1. The mechanism was proposed to involve metal binding
II.a. Materials. Nickel perchlorate hexahydrate (reagent grade) was
purchased from GFS Chemicals. Copper(II) trifluoromethanesulfo-
nate (98%), trifluoromethanesulfonic acid (≥99%), phenyl isocyanate
(98%), and 4-nitrophenyl isocyanate (98%) were obtained from
Aldrich. 2,2′-Dipicolyl amine (97%) was obtained from Pure
Chemistry Scientific Inc., while 2,6-lutidine (98%) was obtained
from Sigma-Aldrich. 2,4,6-Collidine (98%) was obtained from BDH
Laboratory Reagents, and methanol (99.8%, anhydrous) was
purchased from EMD Chemicals.
II.b. General Methods. 1H NMR spectra were determined at 400
+
+
MHz. [CH3OH2 ] and [C2H5OH2 ] were determined potentiometri-
cally using a Fisher Scientific Accumet combination glass electrode
(model no. 13-620-292) calibrated with certified standard aqueous
buffers (pH 4.00 and 10.00) as described previously.24 The sspH values
in methanol were determined by subtracting a correction constant of
−2.2420 from the electrode readings, and the autoprotolysis constant
s
s
for methanol was taken to be 10−16.77 M2. The pH values in ethanol
were determined by subtracting a correction constant of −2.5425 from
the electrode readings, and the autoprotolysis constant for methanol
was taken to be 10−19.1 M2. The sspH values for the kinetic experiments
were measured postreaction to avoid the possibility of inhibitory
effects of KCl leaching from the electrode.
to the CO unit with subsequent delivery of a Ni(II)-bound
lyoxide leading to products. A subsequent report21 describing
the X-ray diffraction structure of the Cu(II) complex of 4a
showed that the metal was actually bidentate bound by both the
pyridyl and the urea nitrogens in a five-membered metallocyclic
ring. Further infrared spectral studies showed that the Ni(II)
complexes of more heavily substituted ureas 4a−c were subject
to a similar mode of coordination as the Cu(II) complexes and
that these differed in structure from that of the Zn(II) complex
of 4a which was bidentate bound through the pyridyl N and
urea CO in a seven-membered ring. Sczepanski’s22 more
detailed results for the Ni(II)-catalyzed ethanolysis reactions
indicated that the phenyl- (4b) and 4-methoxyphenyl- (4c)
substituted ureas reacted faster than the 4-nitrophenyl ureas
(4a) and that the Cu(II) complexes reacted more quickly (by
about 7 times) than the Ni(II) complexes. Product studies
showed that the primary products of all ethanolysis reactions
were the corresponding anilines plus the ethyl carbamate of (2-
picolyl)amine (5) as in eq 1; no products corresponding to the
ethyl carbamate of the anilines could be identified. Finally,
Sargeson and co-workers23 investigated the structure and
reactivity of a ligand exchange inert bis(ethylenediamine)Co-
(III) adduct of 4, showing that decomposition of these in
II.c. Synthesis. II.c.i. Synthesis of N-Phenyl-N′-(pyridin-2-yl-
methyl) Urea (PPU), (4b). This was prepared following a previously
reported method with some modifications.22 2-Picolylamine (0.477
mL, 4.62 mmol) was dissolved in 10 mL of dry CH2Cl2 and cooled to
0 °C under Ar. Phenyl isocyanate (0.503 mL, 4.62 mmol) was then
added dropwise, giving a clear colorless solution which, after 10 min,
turned slightly yellow with formation of white precipitate. The reaction
mixture was stirred overnight at room temperature, after which it was
filtered under vacuum and the white powder was collected. Mp 127−
1
128 °C. The product was analyzed by H NMR in CDCl3, and the
resulting spectrum was consistent with the structure and previously
reported data22 and is presented in Figure 8S (Supporting
Information).
II.c.ii. Synthesis of N-4-Nitrophenyl-N′-(pyridin-2-yl-methyl) Urea,
(p-nitro-PPU), (4a). This was prepared following the above procedure
from 2-picolylamine (0.477 mL, 4.62 mmol) dissolved in 10 mL of dry
CH2Cl2 cooled to 0 °C under Ar. After addition of 4-nitrophenyl
isocyanate (0.76 g, 4.62 mmol) the reaction mixture appeared as a pale
yellow cloudy mixture which, with further stirring, gave a pale yellow
solid precipitate. The reaction mixture was stirred overnight at room
temperature and then filtered under vacuum to give a pale yellow
powder. Mp 179−181 °C. The 1H NMR spectrum was consistent with
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Table 1. Kinetic pKa and kmax Constants for Cleavage of Various 7a,b,c:Cu(II):(−OR) Complexes Reacting in Methanol or
s
s
a
Ethanol at 25 °C
complex
7a:Cu(II):(−OR)
7b:Cu(II):(−OR)
7c:Cu(II):(−OR)
methanol
s
kinetic pKa
7.16 0.10
(7.5 1.3) × 10−5
<6.7
6.24 0.06
(1.9 0.14) × 10−5
s
kmax (s−1
)
(2.9 0.3) × 10−5
ethanol
s
s
kinetic pKa
kmax (s−1
6.25 0.09
(7.0 0.7) × 10−4
6.47 0.06
(1.3 0.1) × 10−4
6.39 0.08
(6.2 0.5)x10−4 s−1
)
a
Complexes are generated in situ and assumed to be fully formed at 1:1 concentrations of ligand and metal ion.
the structure and previously reported data22 and is presented in Figure
7S, Supporting Information.
yellow-orange reaction mixture was stirred for 72 h at room
temperature. The product mixture was purified by MPLC (silica gel
stationary phase). The first elution using a 50:50 ratio of hexanes and
ethyl acetate removed unreacted phenyl isocyanate. Subsequent
elution using a 50:50 ratio of ethyl acetate/methanol yielded the
II.c.iii. Synthesis of N-p-Nitrophenyl-N′,N′-bis(pyridin-2-ylmethyl)
Urea, (4-nitro-DPPU), 7a. 2,2′-Dipicolylamine (0.287 g, 1.51 mmol)
was dissolved in 7 mL of dry CH2Cl2 and cooled to 0 °C under Ar. 4-
Nitrophenyl isocyanate (0.237 g, 1.44 mmol) was dissolved in 5 mL of
dry CH2Cl2, which was then added dropwise to the reaction mixture.
The bright yellow solution was stirred overnight at room temperature,
after which a portion of the reaction mixture was purified by MPLC
(silica stationary phase, ethyl acetate/methanol, 50:50 v/v ratio) to
1
desired product as an oil. The purified product was analyzed by H
NMR and 13C NMR in CDCl3. 1H NMR (400 MHz, CDCl3, 25 °C):
δ 10.11 (bs, 1H), 8.58 (d, 2H, J = 4.74 Hz), 7.60 (td, 2H, J = 7.64 Hz,
J = 1.77 Hz), 7.51 (m, 2H), 7.30 (m, 2H), 7.24−7.18 (m, 4H), 7.00
(tt, 1H, J = 7.45 Hz, J = 1.1 Hz), 4.64 (s, 4H). NMR (100.58 MHz,
CDCl3, 25 °C): δ 157.55, 156.84, 148.56, 140.14, 136.92, 128.52,
122.81, 122.51, 121.97, 119.15, 52.82. 1H NMR and 13C NMR spectra
can be found in the Supporting Information (Figure 1S and 2S). The
ATR-IR spectrum showed a CO absorbance at 1663 cm−1. UV−vis:
ε240 = 2.72 × 104 M−1 cm−1 at 240 nm. EI-MS: C19H18N4O; m/z calcd
318.1481 (+); found 318.1469 (+).
1
give yellow solid which was analyzed by H and 13C NMR in d4-
1
methanol. H NMR (400 MHz, CD3OD, 25 °C): δ 8.57 (d, 2H, J =
4.4 Hz), 8.21 (m, 2H; AA′), 7.77 (td, 2H, J = 7.78 Hz, J = 1.77), 7.69
(m, 2H; BB′), 7.36−7.30 (m, 4H), 4.75 (s, 4H). 13C NMR (100.58
MHz, CD3OD, 25 °C): δ 158.55, 158.16, 150.05, 147.92, 143.56,
138.98, 125.80, 124.28, 123.92, 120.00, 53.81. These spectra can be
found in Figures 3S and 4S, Supporting Information. The FTIR-IR
spectrum of 7a showed a CO absorbance at 1680 cm−1 as well as
NO2 symmetrical and asymmetrical stretches at 1329 and 1507 cm−1.
UV−vis: ε326 = 3.53 × 104 M−1 cm−1. EI-MS: C19H17N5O3; m/z calcd
363.1331 (+); found 363.1319 (+). Mp 142−144 °C
II.d. General UV−Vis Kinetics. Cu(CF3SO3)2-catalyzed meth-
anolyses and ethanolyses of ureas 7a−c were followed by monitoring
the rate of disappearance of the starting materials at 360 nm using a
UV−vis spectrophotometer with the cell compartment thermostated
at 25.0 0.1 °C. Reaction mixtures were buffered using 2,6-lutidine or
2,4,6-collidine, both partially neutralized with HOTf in various ratios,
II.c.iv. Synthesis of N-Methyl-N-p-nitrophenyl-N′,N′-bis(pyridin-2-
yl-methyl) Urea, 7b. N-p-Nitrophenyl-N′,N′-bis(pyridin-2-yl methyl)
urea (0.72 g, 1.98 mmol) was mixed with 6 mL of dry acetone with
stirring at room temperature to give a pale yellow slurry, after which
potassium hydroxide (0.12 g, 2.18 mmol) was added in one portion,
causing the slurry to turn bright orange. The mixture was stirred for 10
min under argon, and dimethyl sulfate (0.28 mL, 2.97 mmol) was
added in one portion by a syringe to the reaction mixture. The
reaction mixture was then heated to reflux under Ar for 2 h, after
which it was hot filtered, and the solvent was removed by rotatory
evaporation. Part of the mixture was purified by MPLC (silica gel
stationary phase, methanol/ethyl acetate, gradient elution from 0:100
and by ytterbium triflate partially neutralized by tetrabutylammonium
s
s
methoxide to maintain the constant pH.
A typical kinetic experiment for determining the sspH dependence of
the Cu(II)-catalyzed cleavage of the NR(CO) bond in methanol
was performed as follows. Three cells with concentrations of
Cu(OTf)2 at 0.2, 0.4, and 0.6 mM, using aliquots of a 50 mM stock
solution in methanol, were prepared in methanol containing 4 mM of
buffer. A 50 μL amount of a 10 mM acetonitrile stock solution of
substrates 7a−c (0.2 mM) was added to each cell, and the reactions
were allowed to run until completion. The total cell volume was 2.5
mL.
1
to 50:50 v/v ratio). H NMR analysis of the eluted fraction showed
Kinetic experiments in ethanol were performed as follows. Two cells
with concentrations of Cu(OTf)2 at 0.1 and 0.2 mM were prepared in
ethanol containing 4 mM of buffer. A 50 μL amount of 10 mM stock
solution of substrates 7a−c in acetonitrile was added to each cell to
give a substrate concentration of 0.1 mM (total cell volume was 2.5
the presence of both the nonmethylated and the N-methylated N-p-
nitrophenyl-N′,N′-bis(pyridin-2-yl methyl) urea. The product was
further purified by dissolving the mixture in methanol followed by
addition of water until the solution became turbid, after which it was
placed in a freezer (−11 °C) overnight. The liquid was separated from
the crystals by filtration, and the solvent was removed by rotary
evaporation followed by further evaporation under reduced pressure
mL). Duplicates of each cell were run to obtain two averaged data
s
points at each pH value and for each [Cu2+].
s
1
The first-order rate constants for the disappearance of the starting
material were calculated from the Abs vs time kinetic curves collected
at 360 nm or using the initial rate method at 360 nm when necessary.
The Abs vs time data were fit to a standard first-order exponential
provided by a vacuum pump. The product was analyzed by H NMR
and 13C NMR in CDCl3. 1H NMR (400 MHz, CDCl3, 25 °C): δ 8.59
(d, 2H, J = 4.48 Hz), 8.12 (m, 2H; Aphenyl), 7.65 (td, 2H, J = 7.75 Hz, J
= 1.77 Hz), 7.25−7.10 (m, 6H), 4.59 (s, 4H) 3.25(s, 3H). 13C NMR
(100.58 MHz, CDCl3, 25 °C): δ 160.92, 156.62, 151.20, 149.61,
141.85, 136.64, 125.24, 122.51, 122.21, 118.61, 53.74, 37.52. These
spectra can be found in the Supporting Information (Figure 5S and
6S). The FTIR-IR spectrum of 6 showed a CO absorbance at 1667
cm−1 as well as NO2 symmetrical and asymmetrical stretches at 1311
and 1504 cm−1. UV−vis: ε337 = 1.25 × 104 M−1 cm−1 at 337 nm. EI-
MS: C20H19N5O3; m/z calcd 377.1488 (+); found 377.1475 (+). Mp
184−186 °C.
equation to obtain the kobs values. Data collected at various sspH values
s
s
generated a full pH profile for cleavage of ureas 7a−c catalyzed by
Cu2+ at 25 °C. The so-obtained rate constants are presented in Table
1, located in the Results and Discussion section.
II.d.I. Kinetic Experiments for Cleavage of 7a in Various Solvents.
The effect of various solvents on the Cu2+-catalyzed cleavage of N-p-
nitrophenyl-N′,N′-bis(pyridin-2-yl methyl) urea (7a) was studied at 25
°C. Since the maximum activity was shown to occur at a 1:1:1 ratio of
Cu(II):substrate:alkoxide (vide infra), cells were charged with 1-
propanol, 2-propanol, or acetonitrile followed by 0.1 mM each of
Cu(OTf)2 (5 μL aliquot of a 50 mM stock solution in acetonitrile), 0.1
mM NaOCH3 in methanol (5 μL of a 50 mM stock solution of
II.c.v. Synthesis of N-Phenyl-N′,N′-bis(pyridin-2-yl-methyl) Urea
(DPPU), 7c. 2,2′-Dipicolyl amine (0.172 g, 0.861 mmol) was dissolved
in 10 mL of dry CH2Cl2 and cooled to 0 °C under Ar. Phenyl
isocyanate (93.6 μL, 0.861 mmol) was then added slowly, and the
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NaOCH3 in methanol), and 7a (25 μL of a 10 mM stock solution in
acetonitrile) to give a total cell volume of 2.5 mL.
The kinetics for decomposition of 7a were monitored as described
above.
II.e. Product Analysis. Following completion of the kinetic runs
for the alcoholysis reactions of the ureas 7a−c run in each type of
buffer, solutions were combined and all solvent was removed in vacuo.
1
The residue was redissolved in d4-methanol and then analyzed by H
NMR. The Cu(II)-bound N′,N′-bis(2-picolyl)amine leaving group was
not detected in the sample due to the paramagnetic effect of the
Cu(II) ion, but the presence of a carbamate product was detected, and
its concentration was determined from integration of characteristic 1H
NMR peaks for each product.
max
s
Figure 1. Plot of log(k ) vs pH for cleavage of 7a:Cu(II) (0.2 mM
obs
s
each of Cu(II) triflate and p-nitro-DPPU (7a)) in anhydrous methanol
under buffered conditions at T = 25 °C. Data were NLLSQ fit to eq 2
The preparative product analysis in ethanol was conducted in a vial
with a total volume of 15 mL containing 0.1 mM of ureas, 0.1 mM
Cu(OTf)2, and 4 mM of buffer. The reaction mixture was left at room
temperature for 2 days, after which the solvent was removed in vacuo
s
s
to give a kinetic pKa of 7.16 0.10 and a maximum rate constant
(kmax) for decomposition of 7a:Cu(II):(−OCH3) of (7.5 1.3) × 10−5
s−1.
1
and the residue was redissolved in d4-methanol and analyzed by H
NMR. As was the case with the reaction in methanol the Cu(II)-
bound N′,N′-bis(2-picolyl)amine leaving group was not detected due
to the paramagnetic effect of the Cu(II) ion, but the presence of
carbamate products was.
equiv of Cu(OTf)2 were NLSSQ fit to eq 2 to give a kinetic
sspKa value of 6.25 0.09 and a maximum rate constant (kmax
)
of (7.0 0.7) × 10−4 s−1, (log (k ) vs pH profile shown in
max
s
s
obs
Figure 16S, Supporting Information).
A product analysis for cleavage of 4b was performed as follows. A
vial with a total volume of 15 mL of ethanol, containing 0.1 mM of 4b,
0.5 mM Ni(ClO4)2, and 0.1 mM (20% molar equivalence per
Ni(ClO4)2) of NaOEt base was prepared. It was heated at 80 °C for 2
days, after which the solvent was removed and the residue redissolved
To better understand the stoichiometric requirements for the
metal ion we determined the initial rates of the decomposition
of 0.1 mM 7a as a function of increasing [Cu(OTf)2] in ethanol
at pH 7.6, where 7a:Cu(II):(−OEt) is essentially completely
s
s
1
in d4-methanol and analyzed by H NMR and high-resolution EI-MS.
formed. The plot of the initial rate vs [Cu2+] shown in Figure 2
III. RESULTS AND DISCUSSION
III.a. Kinetics of Cu(II)-Promoted Alcoholysis of N-4-
Nitrophenyl-N′,N′-bis(pyridin-2-yl-methyl) Urea (7a).
Our initial studies involved Ni(II)-catalyzed methanolysis and
ethanolysis of ureas 7; however, these were far slower than the
reactions with Cu(II), exhibiting very small spectral changes
with 7a and none with 7c, so further studies with Ni(II) were
discontinued.
Cu(II)-promoted methanolyses of 0.2 mM solutions of 7a in
s
s
methanol were studied over a pH range of 5.6−8.2 under
buffered conditions in the presence of varying [Cu(OTf)2]
between 0.2 and 0.6 mM. In all cases the maximal rate of
reaction occurred at [Cu2+] = [7a] with a slight decrease in rate
being observed with increasing [Cu(OTf)2]. These observa-
tions are consistent with strong 1:1 binding of the metal ion to
7a as has been previously observed in the cases of Cu2+-
promoted cleavage of N′,N′-bis(pyridin-2-yl-methyl)
amides.17,18 (The diminution of rate with added Cu(OTf)2
will be shown later to result from an inhibitory effect of a
second metal ion.) NLLSQ (nonlinear least-squares) fits of the
maximum experimentally observed first-order rate constants
(kmobasx) for decomposition of 7a in the presence of equimolar
Figure 2. Plot of the initial rate of decomposition of 0.1 mM 7a in
s
ethanol as a function of [Cu(OTf)2] at pH 7.6, T = 25 °C.
s
maximizes at a 1:1 ratio and then asymptotically diminishes
with increasing metal-ion concentration. These observations are
consistent with binding of a second Cu(II) (with or without an
associated ethoxide) to a reactive 7a:Cu(II):(−OEt) complex,
resulting in formation of a less reactive or nonreactive complex
of proposed stoichiometry 7a:Cu(II)2:(−OEt)1,2 as is depicted
in Scheme 1. This seems to be a common feature for such
complexes as we observed a similar behavior for the Cu(II)-
catalyzed cleavage of N,N-bis(2-picolyl) carbamates.26
Cu2+ vs pH to eq 2, derived for a one-proton dissociation
s
s
s
s
model, gave a kinetic pKa of 7.16 0.10 and a maximum rate
constant (kmax) of (7.5 1.3) × 10−5 s−1. These constants are
presented in Table 1 along with the values for cleavage of other
complexes in methanol and ethanol (vide infra).
Scheme 1. Hypothetical Process Rationalizing the
Concentration Effects of Cu2+ for Decomposition of
7a:Cu(II) Shown in Figure 2
kmax ssKa
⎛
⎜
⎝
⎞
⎟
⎠
log(kmax) = log
obs
sK + [H+]
(2)
s
a
The appearance of the sspH rate profile in Figure 1 suggests that
the active form of the complex is 7a:Cu(II):(−OCH3).
Cu(II)-promoted ethanolysis of 7a was studied under similar
s
conditions (a pH range of 5.6−7.7 with small variations
s
max
obs
outlined in the Experimental Section. The k
values for
decomposition of 7a:Cu(II) in ethanol in the presence of 1
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On the basis of Scheme 1 an equation was derived27 defining
the relationship between [Cu2+] and the observed rate of the
reaction. NLLSQ fitting of the rate vs [Cu2+] data to eq 327
constants increased slightly with increasing [Cu2+]. In these
cases the experimental data were fitted using a universal binding
equation30 at each sspH to provide a rate constant at saturation,
taken to be the kmobasx for decomposition of Cu(II):7c complex at
provided the line through the data in Figure 2; the derived kmax
s
s
s
max
s
and pKa are given in Table 1.
a given pH. The log(k ) vs pH data for decomposition of
s obs s
Cu(II):7c were subsequently fit to eq 2 to yield the kinetic sspKa
and kcat values in Table 1.
III.a.i. Kinetics of the Cu(II)-Promoted Cleavage of N-
Methyl-N-p-nitrophenyl-N′,N′-bis(pyridin-2-yl methyl) Urea
(7b) in Methanol and Ethanol. The kmax for decomposition of
7a:Cu(II):(−OCH3) in methanol is 7.5 × 10−5 s−1, 50 times less
than that for cleavage of 2:Cu(II):(−OCH3) (3.9 × 10−3
s−1).18,19 Ureas are notoriously inert to solvolytic attack on
their CO unit as it is flanked by two nitrogen atoms.
However, an alternative explanation for the rate reduction
relative to that of the corresponding amide might be the
presence of the relatively labile N−H proton in 7a which, when
abstracted, reduced the electrophilicity of the N−CO unit
similar to what is observed for Cu(II)-promoted hydrolysis of
secondary amides.28,29
Cu(II)-promoted ethanolysis of 7c was studied similarly to
s
max
the case of 7a, and at all pH values the k
values were
s
obs
obtained at a 1:1 ratio of [Cu2+] and [7c]. Subsequent data
fitting to eq 2 gave a kinetic sspKa of 6.25 0.09 and kmax = (6.2
0.5) × 10−4 s−1; 7c:Cu(II) reacts about 30 times faster in
ethanol than in methanol, while 7a:Cu(II) reacts ∼10 times
faster in ethanol.
III.b. Product Analysis of the Cleavage Reaction of
7a−c in Methanol and Ethanol. Depicted in Scheme 2 are
two sets of products that may result from Cu(II)-assisted
1
cleavage of 7. Cleavage pathways were probed by H NMR
The rate of the Cu(II)-promoted methanolysis of the N-
analysis of the products after completion of the alcoholysis
reactions of 7 in the various buffers, after removal of the
solvents and redissolution of the residues in d4-methanol. The
O-methyl and O-ethyl carbamate products were identified by
characteristic chemical shifts of the CH3O singlet at δ = 3.75
ppm and CH3CH2O signals at δ = 4.25 d (J = 7.07 Hz) and
1.32 t (J = 7.07 Hz) (Figures 9S and 10S, Supporting
Information) showing that these reactions proceeded by path a
in Scheme 2. No traces of aniline, the product formed by path
b, were found by 1H NMR analysis or using UV−vis
spectoscopy.
s
methyl analogue, 7b, was studied as a function of pH under
s
conditions similar to those used for 7a:Cu(II). The first-order
rate constants were obtained in the presence of 1, 2, and 4
equiv of Cu(OTf)2 relative to that of the substrate (0.1 mM).
Contrary to what was observed for Cu(II):7a, the observed rate
constants increased slightly with the increase in [Cu(OTf)2]
(Figure 19S, Supporting Information). NLLSQ fits of these
experimental first-order rate constants to a strong binding
equation30 yielded k
values (Supporting Information) for
max
obs
s
decomposition of 7b:Cu(II) indicative of a plateau in the pH
s
range of 6.8−8.0, so the anticipated kinetic sspKa would be <6.7.
The average kmax value of (2.9 0.3) × 10−5 s−1 for cleavage of
Scheme 2. Cleavage Pathways for the Cu(II)-Promoted
Alcoholysis of Ureas 7a,c, X = H, NO2
s
s
7b:Cu(II):(−OCH3) in this pH range is about 3 times slower
than that for 7a:Cu(II):(−OCH3), the small rate reduction
perhaps resulting from a steric encumbrance of intramolecular
nucleophilic addition of the coordinated Cu(II):(−OCH3) to
the CO unit. Nevertheless, that the rates of decomposition of
these two complexes are so similar suggests that N−H
deprotonation of the complex of 7a, followed by nucleophilic
attack of the Cu(II):(−OCH3), is not a reasonable mechanism
for methanolysis of the latter, further suggesting that the N−H
deprotonation and elimination of the Cu(II):bis(2-picolyl)
amine is not the preferred pathway for production of the O-
methyl carbamate of p-nitroaniline.
s
s
The pH dependence of the rate of cleavage of 0.1 mM
7b:Cu(II):(−OCH2CH3) was also studied at 25 °C at the two
1H NMR analysis of the combined product mixtures from
the kinetic experiments showed that cleavage of 7b and 7c also
produced only the corresponding O-methyl or O-ethyl
carbamates following path a in Scheme 2. No traces of the
corresponding anilines were observed; UV−vis scanning kinetic
studies with 7a also confirm that reaction proceeds only by path
a since the presence of p-nitroaniline would result in a
significant increase of absorbance at 380 nm which was not
observed.
III.c. Medium Effects on the Rate of Solvolysis of 7a
Catalyzed by Cu(OTf)2. From Table 1 the respective
solvolyses of 7a,b,c:Cu(II):(−OR) are, respectively about 10,
5, and 30 times faster in ethanol than methanol. Solvolyses of
7a:Cu(II) were also investigated in water, 1-propanol, 2-
propanol, and acetonitrile containing 0.2% methanol or ethanol
by volume, these amounts being contributed by the stock
solutions containing base. In these solvents, by analogy with the
methanol and ethanol cases, the active forms are considered to
concentrations of 0.1 and 0.2 mM Cu(OTf)2 in ethanol
containing 4 mM of buffer. Duplicate kinetics were run to
s
obtain averaged data points at each pH value and for each
s
max
obs
[Cu2+]. The maximal rate constants (k ) were observed at a
1:1 ratio of 7b:Cu(II). A full sspH profile for its ethanolysis at 25
°C was constructed (Figure 17S, Supporting Information) by
fitting of the rate constant vs sspH data to eq 2, yielding a kinetic
sspKa of 6.47 0.06 and kmax of (1.3 0.1) × 10−4 s−1. Cleavage
of 7b:Cu(II):(−OR) is about 5 times faster in ethanol than
methanol.
III.a.ii. Kinetics of the Cu(II)-Promoted Cleavage of N-
Phenyl-N′,N′-bis(pyridin-2-yl methyl) Urea (7c) in Methanol
s
s
and Ethanol. The pH dependence of the Cu(II)-catalyzed
cleavage of 0.2 mM 7c in methanol was determined at T = 25
°C using 0.2, 0.4, and 0.6 mM concentrations of Cu2+ in
s
s
max
obs
buffered solutions. Above pH 6 the k was obtained at a 1:1
ratio of 7c:Cu(II) with further increases in [Cu(OTf)2] leading
s
s
to a slight inhibitory effect. Below pH 6 the observed rate
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involve Cu(II):(−OR) bound to 7a, solvolyzing by a
mechanism analogous to what occurs in the two former
alcohols.
Table 2. Comparison of First-Order Rate Constants for the
Solvolysis of 7a at 25 °C in Various Solvents Containing a
1:1:1 Mixture of 7a, Cu(OTf)2, and NaOCH3 or TBAE, each
at 0.1 mM
III.c.I. Hydrolysis of 7a Catalyzed by Cu(OTf)2. Reaction of
7a in water was studied at concentrations of 0.01 mM for each
of Cu(OTf)2, urea 7a, and NaOH base.31 Kinetic traces were
obtained at 390 nm, the wavelength of maximum change under
these conditions, corresponding to the appearance of p-
nitroaniline (see below). The fit of the Abs vs time traces to
a standard exponential equation gave a kcat of 4.8 × 10−6 s−1 at
pH of 7.0, T = 25 °C, which is about 15- and 150-times,
respectively, slower than the reactions in methanol and ethanol.
Increasing [Cu2+] to 0.2 mM led to only a slight increase in the
rate constant, and further increases in [Cu2+] led to formation
of a yellow-brown solid, surmised to be oligomeric Cu(II)
hydroxide.
solvents
H2O
103 kmax, s−1
0.0046
0.075
0.70
MeOH
EtOH
a
a
1-PrOH
2-PrOH
0.057
0.021
0.011
a
acetonitrile
a
In the presence of 0.2% v/v MeOH.
amides 1−3,17−19 which have limited solubility and stability in
water with no noticeable cleavage reaction being observed.
General retardation of the metal-ion catalysis of acyl and
phosphoryl transfer reactions one often sees in water stems
from many sources such as limited complex solubility, weaker
binding of the metal ion and substrate, and oligomerization/
precipitation of the metal-ion:hydroxo species. Notably, water
heavily solvates the metal ion which, when coupled with a
weaker electrostatic association of metal ion and substrate in a
high dielectric constant solvent, reduces the binding energy of
the ensuing complex. In addition, relative to less polar alcohols,
an aqueous solvent tends to raise the activation energy for
metal-ion-promoted cleavages where charge is being dispersed
in the transition states relative to ground state.33 The fact that
7:Cu(II) complexes undergo hydrolysis indicates that the effect
of the Cu(II) ion in promoting solvolytic cleavage is not limited
to light alcohols but is effective in water for this system.
Presumably this is aided by the very tight binding of the metal
ion provided by the bis(2-picolyl) portion of the substrate and
the solubility characteristics of the urea complex. The unusual
ability of Cu(II) to provide powerful leaving group assistance
for hydrolysis of a specially designed set of phosphate mono-,
di-, and triesters has been noted,16b which may signify a far
more general process for accelerating hydrolysis of such
processes where the leaving groups are poor.
The product mixture from the hydrolysis reaction of 7a was
1
analyzed by H NMR and UV−vis spectrometry with the only
product observed32 being p-nitroaniline. This is consistent with
the cleavage reaction proceeding in the same fashion as the
methanolysis or ethanolysis via path a (Scheme 3), where the
Scheme 3. Proposed Pathways for Hydrolytic
Decomposition of 7a:Cu(II):(−OH) in Water
initial product is the carbamic acid of p-nitroaniline which
spontaneously decarboxylates to give aniline. It is also
consistent with reaction proceeding via path b, where aniline
is directly formed along with the carbamic acid of the
Cu(II):bis(2-picolyl)amine complex which spontaneously
loses CO2.
III.c.ii. Solvolysis of 7a Catalyzed by Cu(OTf)2 in Various
Organic Solvents. Solvolyses of 7a:Cu(II) in 1- or 2-propanol
should also have the metal-bound alkoxides acting as the
intramolecular nucleophiles. Methanol cannot be avoided given
that a small amount (0.2% by volume) arises from addition of 1
equiv of NaOCH3 base solution in methanol. Solvolysis of
Cu(II):7a in neat acetonitrile does not occur unless there is
added alcohol and base.
III.d. Ni(II)-Promoted Cleavage of N-(2-Picolyl)-N′-aryl
Ureas 4a,b in Ethanol. We confirmed and expanded upon the
previous results for the metal-ion-catalyzed ethanolysis of the
N-(2-picolyl)-N′-aryl ureas.22 A series of experiments with 4a,b
(respective aniline substituents, p-NO2 and p-H) was
s
s
performed in ethanol with pH control in the presence of a
noninhibitory perchlorate counterion. We could not use Cu2+
with 4a,b since their complexes were sparingly soluble in
ethanol, which precluded accurate kinetic determination of the
rate constants, although the mode of cleavage of the Cu(II)
complexes could be determined.
The reactions were conducted in the above solvents along
with 0.1 mM each of 7a, Cu2+, and NaOCH3 or TBAE base.
These were monitored by an initial rate method at 360 nm for
the disappearance of 7a:Cu(II). Controls confirming that a
reaction actually occurred with 7a:Cu(II) were run under
identical conditions observing no changes in the spectrum of
0.1 mM of dipicolylamine or Cu2+ alone in the different
solvents. The observed rate constants given in Table 2 span a
range of ∼150-fold, following the order ethanol > methanol >1-
propanol >2-propanol > acetonitrile > water.
Since the available evidence indicates that metal-ion-
promoted cleavage of bis(2-picolyl) ureas and amides17−19
involves an active form containing a ligand-complexed M(II):
(−OR), our experiments were conducted in the presence of
20% molar equivalents of tetrabutylammonium ethoxide
s
s
relative to the concentration of Ni(ClO4)2 to hold the pH
constant. These conditions were chosen to avoid precipitation
of M(II) alkoxides that can occur at high [alkoxide]. The plot
of the kobs for cleavage of 4a as a function of [Ni(ClO4)2] was
linear over a range of 0.0−1.0 mM, and from the gradient we
obtained a k2obs of (0.87 0.05) × 10−2 M−1 s−1 at 58.4 °C and
(1.45 0.03) × 10−2 M−1 s−1 at 68.1 °C. In the same fashion,
Observation of a hydrolysis reaction in the aqueous solution
containing 7a:Cu(II) is notable relative to the attempted
hydrolyses of the Cu(II) complexes of structurally related
obs
k2 values were obtained for the Ni(II)-promoted decom-
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position of 4b of 5.2 × 10−2 and 14 × 10−2 M−1 s−1 at 58.4 and
68.1 °C.
Scheme 5. Proposed Pathway for the Cu(II)-Mediated
Methanolysis of 7
These k2obs values for 4a and 4b obtained at 68 °C are larger
by ∼18 and 3 times, respectively, than was previously22
observed for these substrates at 80 °C. Given that only 20%
molar equivalents of base were used here for the reasons stated
above, our experimental numbers are suboptimal by at least a
factor of 5, meaning that the rate constants for the fully active
Ni(II):4a,b:(−OEt) complexes are ∼100 and 15 times larger
than what was reported previously.22 This is likely due to our
use of Ni(ClO4)2 with its non or poorly inhibitory counterion
rather than NiCl2 where the chloride is known to form
complexes with transition-metal ions, particularly in solvents
with lower polarity such as alcohols.34 Contrary to what we
observe for the metal-ion-catalyzed cleavage of the bis(2-
picolyl)amine containing ureas 7 (where the presence of the
NO2 group accelerates the rate of the reaction), the presence of
an electron-withdrawing −NO2 group in 4a slows the rate of
the reaction down by approximately 1 order of magnitude
relative to that observed with the phenyl derivative, 4b.
Our product results also confirm that both Ni(II)- and
Cu(II)-catalyzed ethanolysis of 4a,b in the presence of a
noninhibiting counterion gives the corresponding aniline and
O-ethyl-N-(2-picolyl) carbamate, following path b in Scheme 4.
decomposes to products. The latter cleavage step involves a
simultaneous, or nearly so, fracturing of the Cu−OR and N−
CO bond (identified by the corrugated line in the INT in
Scheme 5) to yield the O-methyl N-anilino carbamate (CARB)
and is highly dependent on the metal-ion-facilitated departure
of the bis(2-picolyl)amide portion through increasingly strong
Cu(II) coordination of the emerging N. Once free, or nearly so,
from INT the Cu(II)-coordinated bis(2-picolyl) amide anion is
protonated by the medium or buffer components therein.35
The Cu(II)- and Ni(II)-promoted ethanolyses of mono(2-
picolyl)ureas 4a−c have been shown by prior studies21,22 and
the current work to exclusively produce aniline and O-ethyl-N-
(2-picolyl) carbamate, suggesting that, if there is a tetrahedral
intermediate comparable to INT in Scheme 5, the Cu(II)-
coordinated mono(2-picolyl)amide(amine) is either a poorer
leaving group than aniline or there is a different mechanism for
cleavage of the metal-ion complexes of 4a−c. While more work
would be required to ascertain this, an obvious possibility is that
the Cu(II)- and Ni(II)-promoted reactions of 4 involve an
elimination of aniline through deprotonation of the NH of the
picolyl amine coordinated to the metal ion. Roecker and
Sargeson23 studied the decomposition of a Co(III) complex of
ureas 4 for which the structure (8) and mechanism for
hydrolysis in acidic media are shown in Scheme 6. An
a
Scheme 4
Scheme 6. Proposed Pathway for Decomposition of Co(III)
a
a
Complexes of N-(2-Picolyl) N′-Aryl Ureas (8)23
X = H, NO2.
This mode of cleavage of Ni(II):4b in ethanol is also supported
by MS experiments, where peaks attributable to both O-ethyl-
N-(2-picolyl)carbamate (C9H12N2O2; m/z calcd 180.0899;
found 180.0908) and aniline (C6H7N; m/z calcd 93.0578;
found 93.0574) were detected.
III.e. Mechanistic Aspects and Differences between
the Cleavage Pathways for the Bis(2-picolyl) and
Mono(2-picolyl) Ureas. Each of the ligands in 7a:Cu(II):
(−OR) and 7c:Cu(II):(−OR) binds metal ion sufficiently
strongly that saturation is observed in the kobs vs [Cu2+]
plots, and decomposition of these complexes exclusively
produces bis(2-picolyl)amine:Cu(II) and an O-methyl or O-
ethyl carbamate (CARB) of the parent aniline (path a in
Scheme 2). Although we have not performed detailed
calculations on the Cu(II)-mediated cleavage of 7a−c, the
available experimental evidence, by analogy with decomposition
of amide complexes such as 2:Cu(II):(−OCH3),17−19 points to
the mechanism presented in Scheme 5 involving a 3-fold role
for the metal ion. Following initial binding to 7, CuII:(HOR) is
deprotonated to form a kinetically active 7:CuII:(−OR)(HOR)
complex. The metal ion activates the urea via complexation (or
partial complexation) of the bis(2-picolyl)amino nitrogen,
which allows a subsequent intramolecular delivery of the
metal-bound alkoxide to the CO to give a tetrahedral
intermediate (INT, suggested to be fleeting), which then
a
Ethylene diamine ligands represented as two N’s connected by
curved line for clarity.
important feature of the mechanism is the proposed formation
of a chelated isocyanate ligand (10) that rapidly decomposes to
the final products. The pathway is consistent with other
literature examples36 and with the fact that free phenyl
isocyanate, the initial product if cleavage had occurred at the
2-picolyl N−C(O) linkage, is not observed. The mechanism
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is driven by the fact that the Co(III)-coordinated N−H is quite
acidic and therefore easily deprotonated to form 8− with a
potential equilibrium formation of a zwitterionic form (9)37
which decomposes to 10.
be estimated as follows. The kobs for decomposition of
7a:Cu(II):(−OCH3) in methanol is 7.5 × 10−5 s−1 (t1/2
=
150 min), which is 50 times less than that for cleavage of
2:Cu(II):(−OCH3).18,19 The rate of decomposition of 7a:Cu-
(II):(−OCH2CH3) complex in ethanol is about 10 times faster,
giving a t1/2 of ∼15 min at 25 °C. The acceleration of the
alkoxide reactions of amide 2 produced by complexation to the
Cu(II) is at least 1017 in methanol and 1020 in ethanol.19
Due to the fact that solvolysis of ureas is very slow there are
only a limited number of kinetic data available for the rate of
hydrolysis6,8,13b,43 and to the best of our knowledge no kinetic
data for the rate of ethanolysis. Therefore, several assumptions
are needed in order to calculate the acceleration achieved by
the presence of Cu(II) ions for decomposition of 7 in ethanol.
The reactivity of hydroxide and ethoxide in the simple reaction
of cleavage of p-nitrophenyl acetate is measured by their relative
second-order rate constants of 9.544a and 107 M−1 s−1.44b
Extrapolation of the experimental data13b for the alkaline
hydrolysis of urea to 25 °C gives a second-order rate constant
of 10−8 M−1 s−1, and considering that alkaline hydrolysis of
tetramethyl urea is ∼4 times slower than that of urea43 one
might estimate a rate constant for its hydrolysis at 25 °C of 2.5
× 10−9 M−1 s−1. Coupling the latter with an assumed 10-fold
IV. CONCLUSIONS
Despite our systems being mononuclear in Cu(II) or Ni(II)
and as such cannot be considered as models of the dinuclear
Ni(II) site of urease, the results of the above study may have
both direct and indirect implications for enzymatic cleavage of
urea. The roles of the metal ions in urease (designated as
Ni1(II) and Ni2(II)) have been postulated to involve urea
coordination by Ni1(II) followed by nucleophilic delivery of a
metal-bound hydroxide to the Ni1(II)-coordinated CO unit
by the second Ni2(II). There is experimental11d,23 and
computational10 support for a possible enzymatic elimination
mechanism, but more favored are hydrolytic mechanisms for
which there are three current variants.38 These differ in whether
the urea is coordinated to one or two metal ions (the latter via
an Ni1(II)--ON-NH2--Ni2(II)) bridge and whether the
nucleophilic hydroxide is bound terminally to one metal ion
or bridged between the two Ni ions. All involve formation of a
tetrahedral intermediate from which the NH3 departs aided by
general acid catalysis from an active site His-H+ or from the di-
Ni-bridged hydroxide. These hydrolytic variants have been
discussed38 alongside the enzymatic alternative of general-
base−general-acid-promoted elimination of ammonia to yield
cyanic acid, which hydrates and then eliminates CO2. There are
instances where Ni(II) complexes are shown to accelerate
ethanolysis of ureas,12,21,22,39 but as far as we are aware, none of
these has ruled out the possibility of an elimination reaction
giving a transient isocyanate followed by addition of ethanol to
form a carbamate. In fact, there have been calls to re-evaluate
the hydrolytic mechanism postulated for the enzyme to
consider elimination.11d,23,10,40
increase in activity gives an estimated 2.5 × 10−8 M−1 s−1 for
s
s
ethoxide attack on tetramethylurea. The pKa and log kmax data
from Figure 17S, Supporting Information, for the Cu(II)-
promoted ethanolysis of 7b allows one to calculate45 a second-
order rate constant for ethoxide attack on 7b:Cu(II) of 5 × 108
M−1 s−1. By this analysis, the acceleration provided by the
Cu(II) for ethoxide attack on 7b corresponds to about 2 × 1016
times.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectral data for 7a−c, plots of rate constant vs metal-ion
s
s
concentration or pH, NMR data for products arising from
An important contribution bearing upon the possible role of
metal coordination of the urea N in a solvolytic process has
been provided by Kostic and co-workers, who have shown that
́
cleavage of 7a−c in methanol and ethanol. This material is
available free of charge via the Internet at http://pubs.acs.org.
Pd(II) complexes like (en)Pd(OH2)2 can promote hydrolysis
and alcoholysis of urea through a mechanism that involves
initial CO coordination, subsequent rearrangement to a Pd-
NH2-bound form, and apparent nucleophilic attack by a metal-
AUTHOR INFORMATION
Corresponding Author
*Phone: 613-533-2400. Fax: 613-533-6669. E-mail: rsbrown@
chem.queensu.ca.
Notes
■
bound hydroxide or alkoxide.41 In none of the Kostic
́
studies
nor any of the currently favored enzymatic42 or model
processes is one, or both, of the metal ions (Pd(II) or Ni(II))
postulated or demonstrated to assist the departure of the NH2
group either from a decomposing tetrahedral intermediate or in
an elimination mechanism, although under special circum-
stances this phenomenon can impart very large accelerations to
remove leaving groups that are difficult to displace in solvolytic
processes17−19 including hydrolysis.16a,b
The present results indicate that the Cu(II)-promoted and in
all likelihood, the Ni(II)-promoted cleavages of 7a,b in
methanol proceed with very similar rates, giving the respective
O-methyl carbamates of p-nitroaniline and N-methyl p-
nitroaniline. This excludes an elimination mechanism, at least
for these mononuclear complexes. That the reaction is also
prominent in ethanol and water suggests that these solvolyses
also proceed by nucleophilic addition, followed by metal-ion-
promoted departure of the leaving amine. In terms of reaction
rate, there is a very large acceleration of the cleavage of 7a−c
relative to solvent-induced reactions, and although the exact
amount of acceleration is difficult to quantify, a lower limit can
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the generous support of the Natural
Sciences and Engineering Research Council of Canada
(NSERC) and Queen’s University. M.-N.B. also thanks the
NSERC for an Undergraduate Summer Research Assistant
award (2013).
REFERENCES
■
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(3) Karplus, P. A.; Pearson, M. A.; Hausinger, R. P. Acc. Chem. Res.
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(4) Benini, S.; Rypniewski, W. R.; Wilson, K. S.; Miletti, S.; Ciurli, S.;
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(5) (a) Park, I.-S.; Hausinger, R. P. Biochemistry 1996, 35, 5345.
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Inorganic Chemistry
Article
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δAbs/δt = Δεkobs((1 + K[7a] + [Cu(OTf)2]K − (1 + 2K[7a]
+ 2[Cu(OTf)2]K + (K)2[7a]2 − 2(K)2[Cu(OTf)2][7a]
+ [Cu(OTf)2]2(K)2)0.5)/(2K))Km/(Km + [Cu(OTf)2]
− (1 + K[7a] + [Cu(OTf)2]K − (1 + 2K[7a] + 2[Cu(OTf)2]K
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2013, 11, 7595. It should be pointed out that the apparent
acceleration of 1025 quoted in this work is based on a computed free
energy of activation for addition of hydroxide to urea of 40.9 kcal
mol−1, while the only available experimental number13b derived from
the Arrhenius activation parameter and entropy is 27 kcal mol −1 at 25
°C. (b) Lynn, K. R. J. Chem. Phys. 1965, 69, 687.
+ (K)2[7a]2 − 2(K)2[Cu(OTf)2][7a] + [Cu(OTf)2]2(K)2)0.5
/(2K))
)
(3)
where δAbs/δt is the initial reaction rate and Δε is the differential
extinction coefficient at 360 nm; Km is the dissociation constant for 7:
(Cu(II))2 ↔ Cu(II) + 7, Cu(II); K is an effective association constant
for formation of 7:Cu(II):(−OR), which incorporates acid dissociation
of 7:Cu(II):(HOR) so is pH dependent; kobs is the observed first-order
rate constant for decomposition of the 7:Cu(II) complex at a given
pH. (b) Equation 3 was obtained from the equations for equilibrium
binding and conservation of mass using the commercially available
MAPLE software: Maple 9.00, June 13, 2003, Built 136194; Waterloo
Maple Inc.: Waterloo, Ontario, Canada.
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(31) At higher concentrations of Cu(II), the Cu(II) precipitated out
as a brown-yellow solid. As a result, the UV−vis spectrum monitoring
the reactions could not be analyzed.
(c) Brown, R. S.; Neverov, A. A. Adv. Phys. Org. Chem. 2008, 42, 271.
(d) Brown, R. S.; Lu, Z.-L.; Liu, C. T.; Tsang, W. Y.; Edwards, D. R.;
Neverov, A. A. J. Phys. Org. Chem. 2009, 23, 1 and references therein.
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(32) No products containing the dipicolylamine moiety were
1
observed by H NMR due to the paramagnetic nature of the Cu(II)
ions that were complexed to them.
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s
s
(35) While we have no direct experimental data for the pKa of the
Cu(II)-coordinated bis(2-picolyl)amine in methanol, Cu(II) coordi-
s
s
nation in a related system reduces the pKaof the OH group of 2-(2′-
hydroxyphenyl)phenanthroline from 16.16 to 0.49 in methanol.16
A
(18) Raycroft, M. A. R.; Maxwell, C. I.; Oldham, R. A. A.; Andrea, A.
S.; Neverov, A. A.; Brown, R. S. Inorg. Chem. 2012, 51, 10325.
(19) Raycroft, M. A. R.; Cimpean, L.; Neverov, A. A.; Brown, R. S.
Inorg. Chem. 2014, 53, 2211.
similar large reduction in amine sspKawould result from Cu(II)
coordination, resulting in the significant stabilization of the departing
anion during Cu(II)-promoted methanolysis of 7. The timing of the
subsequent protonation of the departing Cu(II)-coordinated bis(2-
picolyl)amide cannot be early in the cleavage pathway since the two
picolyl nitrogens and departing N are coordinated to Cu(II), leaving
no available lone pair on the emerging N to which a H bond can
develop until significant or complete C−N cleavage has occurred.
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Inorg. Chem. 1989, 28, 1983. Farlie, D. P.; Jackson, G. W.; McLaughlin,
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(20) For the designation of pH in nonaqueous solvents we use the
nomenclature recommended by IUPAC: Compendium of Analytical
Nomenclature. Definitive Rules 1997, 3rd ed.; Blackwell: Oxford, U.K.,
1998. The pH meter reading for an aqueous solution determined with
an electrode calibrated with aqueous buffers is designated as wwpH; if
the electrode is calibrated in water and the “pH” of the neat buffered
methanol solution then measured, the term swpH is used; if the
electrode is calibrated in the same solvent in which the “pH” reading is
made, then the term sspH is used. In methanol ws pH-(−2.24) = sspH, and
(37) A kinetically equivalent process could involve decomposition of
8− with trapping of the emerging anilide by H3O+ acting as a general
acid.
since the autoprotolysis constants of methanol and ethanol are 10−16.77
s
and 10−19.1, respectively, the neutral pH is 8.4 and 9.55.
s
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Model Studies for Jack Bean Urease and Photochemistry of Phosphoryl
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Suzuki, S. J. Am. Chem. Soc. 1997, 119, 5752.
7924
dx.doi.org/10.1021/ic500620k | Inorg. Chem. 2014, 53, 7916−7925

Inorganic Chemistry
Article
(40) In ref 11d, it has been suggested that the all substrates handled
by urease9 could react by elimination, although it would be difficult to
understand how elimination from formamide, the second best
substrate, could lead to formic acid.
(41) (a) Kaminskaia, N. V.; Kostic,
5917. (b) Kaminskaia, N. V.; Kostic,
4302. (c) Kaminskaia, N. V.; Guzei, I. A.; Kostic,
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Dalton Trans. 1998, 3879.
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No. 364891, DOI: 10.1155/2010/364891.
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(45) The second-order rate constant is given as log kmax
/
s
exp(−2.3(pKauto − pKa)) where pKauto (the pKauto of ethanol) is
s
19.125 and pKa refers to the pKa.
s
s
7925
dx.doi.org/10.1021/ic500620k | Inorg. Chem. 2014, 53, 7916−7925