Demonstration of prominent Cu(ll)-promoted leaving group stabilization of the cleavage of a homologous set of phosphate Mono-, Di-, and triesters in methanol

Article abstract of DOI:10.1021/ja910111q

A series of phosphate mono-, di-, and triesters with a common leaving group (LG) (2′-(2-phenoxy)1, 10-phenanthroline) was prepared, and the kinetics of decomposition of their Cu(II) complexes was studied in methanol at 25°C under _s^spH-controlled conditions. The Cu(II) complexes of 2-[2′-phenanthrolyl]phenyl phosphate (Cu(ll):6), 2-[2′- phenanthrolyl]phenyl methyl phosphate (Cu(II):7), and 2-[2′-phenanthrolyl] phenyl dimethyl phosphate (Cu(II):8) are tightly bound, having dissociation constants K_d < 3 × 10^-7 M, with the Cu(II) being in contact with the departing phenoxide. The _s^spH/rate profile for cleavage of Cu(II): 6 has a low _s^spH plateau (k₀ = 6.3 × 10^-3 s^-1), followed by a bell-shaped maximum (k_cat^max= 14.7 ± 0.4 s ^-1) dependent on two ionizations with _s^spK _a³ and _s^spK_a 21 = 7.8 ± 0.1 and 11.8 ± 0.2. The _s^spH/rate profile for cleavage of Cu(ll):7 has a broad plateau from _s^spH 3 to _s^spH 10 followed by a descending wing at higher _s^spH with a gradient of -2. The _s ^spH/rate profile for cleavage of Cu(ll):8 is sigmoidal with two plateaus (Zc1 = (2.0 ± 0.2) × 10^-5S^-1, k ₂ = (1.2 ±0.2) × 10^-6S^-1), connected by an ionization with a _s^spK_a of 6.03. Activation parameters are given for the reactions in the plateau regions: all three species show similar ΔH^# terms of 21.4-21.6 kcal/mol, with major differences in the ΔS^- terms, which vary from 18 to 2.3 to -7.4 cal/(mol-K) passing from the mono- to di- to triester. Detailed analyses of the kinetics indicate that the reactions involve spontaneous solvent-mediated cleavage of the Cu(ll)-coordinated phosphate dianion [Cu(ll):6b]° and phosphate diester monoanion [Cu(ll):7b]⁺ and, for the triester, complexes containing Cu(II) and Cu(II): ^-OCH ₃ designated as [Cu(ll):8a]²⁺ and [Cu(ll):8b]⁺. Reactions where methoxide is the active nucleophile are not observed. Comparisons of the rates of the decomposition of these species at their _s^spH maxima in the neutral _s^spH region with the estimated rates of the background reactions indicate that leaving group assistance provided by the coordinated Cu(II) accelerates the cleavage of the phosphate mono-, di-, and triesters by 10¹⁴ to 10¹⁵, 10¹⁴, and 10⁵. Detailed Hyperquad 2000 analysis of titration data indicates that phenoxide 9^- is bound 23 kcal/mol stronger than the phosphate triester 8. It is the realization of part of this energy in the emerging products resulting from P-O(LG) cleavage that provides the driving force for the catalyzed reactions.

Full text of DOI:10.1021/ja910111q

Published on Web 02/17/2010
Demonstration of Prominent Cu(II)-Promoted Leaving Group
Stabilization of the Cleavage of a Homologous Set of
Phosphate Mono-, Di-, and Triesters in Methanol
C. Tony Liu, Alexei A. Neverov, Christopher I. Maxwell, and R. Stan Brown*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6
Received November 30, 2009; E-mail: rsbrown@chem.queensu.ca
Abstract: A series of phosphate mono-, di-, and triesters with a common leaving group (LG) (2′-(2-phenoxy)-
1,10-phenanthroline) was prepared, and the kinetics of decomposition of their Cu(II) complexes was studied
s
in methanol at 25 °C under pH-controlled conditions. The Cu(II) complexes of 2-[2′-phenanthrolyl]phenyl
s
phosphate (Cu(II):6), 2-[2′-phenanthrolyl]phenyl methyl phosphate (Cu(II):7), and 2-[2′-phenanthrolyl]phenyl
dimethyl phosphate (Cu(II):8) are tightly bound, having dissociation constants Kd e 3 × 10-7 M, with the
Cu(II) being in contact with the departing phenoxide. The sspH/rate profile for cleavage of Cu(II):6 has a low
sspH plateau (ko ) 6.3 × 10-3 s-1), followed by a bell-shaped maximum (kcatmax ) 14.7 ( 0.4 s-1) dependent
on two ionizations with pK2a and pKa3 ) 7.8 ( 0.1 and 11.8 ( 0.2. The pH/rate profile for cleavage of
s
s
s
s
s
s
s
s
s
Cu(II):7 has a broad plateau from pH 3 to pH 10 followed by a descending wing at higher pH with a
s
s
s
gradient of -2. The sspH/rate profile for cleavage of Cu(II):8 is sigmoidal with two plateaus (k1 ) (2.0 ( 0.2)
× 10-5 s-1, k2 ) (1.2 ( 0.2) × 10-6 s-1), connected by an ionization with a sspKa of 6.03. Activation parameters
are given for the reactions in the plateau regions: all three species show similar ∆Hq terms of 21.4-21.6
kcal/mol, with major differences in the ∆Sq terms, which vary from 18 to 2.3 to -7.4 cal/(mol ·K) passing
from the mono- to di- to triester. Detailed analyses of the kinetics indicate that the reactions involve
spontaneous solvent-mediated cleavage of the Cu(II)-coordinated phosphate dianion [Cu(II):6b]0 and
phosphate diester monoanion [Cu(II):7b]+ and, for the triester, complexes containing Cu(II) and Cu(II):
-OCH3 designated as [Cu(II):8a]2+ and [Cu(II):8b]+. Reactions where methoxide is the active nucleophile
are not observed. Comparisons of the rates of the decomposition of these species at their sspH maxima in
s
the neutral pH region with the estimated rates of the background reactions indicate that leaving group
s
assistance provided by the coordinated Cu(II) accelerates the cleavage of the phosphate mono-, di-, and
triesters by 1014 to 1015, 1014, and 105. Detailed Hyperquad 2000 analysis of titration data indicates that
phenoxide 9- is bound 23 kcal/mol stronger than the phosphate triester 8. It is the realization of part of this
energy in the emerging products resulting from P-O(LG) cleavage that provides the driving force for the
catalyzed reactions.
1. Introduction
Due to the great accelerations required to bring the various
phosphate cleavage reactions into a useful time scale for living
systems, considerable effort has been expended to understand
the mechanistic diversity of phosphoryl transfer reactions
mediated by enzymes. In the absence of catalysts, the solvolytic
reaction rates of phosphate mono- and diesters with unactivated
leaving groups (LGs) are such that the half-time for hydrolysis
of a monoester is ∼1012 years, and those for hydrolysis of RNA
and DNA under physiological conditions are ∼110 and ∼108-10
years, respectively.6 However, the rate of phosphoryl transfer
reactions in the presence of the most efficient enzymes can be
Phosphate mono-, di-, and triesters have important roles in
living systems. Phosphorylation and hydrolysis reactions of
phosphate monoesters play vital roles in protein function, energy
regulation, metabolism, signal transduction, and many other
processes.1-3 Phosphate diesters are extremely resistant to
solvolytic cleavage, making them suitable functionalities for the
backbones for DNA and RNA, which are responsible for storing
genetic information.3 Phosphate triesters are not naturally
occurring and have no known natural biological function, but
they are commercially important as pesticides4 owing to their
toxicity as acetylcholinesterase inhibitors.5
(4) (a) Toy, A.; Walsh, E. N. Phosphorus Chemistry in EVeryday LiVing,
2nd ed.; American Chemical Society: Washington, DC, 1987; Chapters
18-20. (b) Quin, L. D. A Guide to Organophosphorus Chemistry;
Wiley: New York, 2000. (c) Gallo, M. A.; Lawryk, N. J. Organic
Phosphorus Pesticides. The Handbook of Pesticide Toxicology;
Academic Press: San Diego, CA, 1991.
(5) (a) Main, R. A.; Iverson, F. Biochem. J. 1966, 100, 525. (b) Emsley,
J.; Hall, D. The Chemistry of Phosphorus; Wiley: New York, 1976; p
494.
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9
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A R T I C L E S
Liu et al.
accelerated by 1011- to 1012-fold for phosphate triesters,7 1015-
to 1021-fold for diesters,6 and >1017-fold for phosphate mo-
noesters.8
Many enzymes that cleave phosphate esters contain two or
more transition-metal ions (Zn2+, Ca2+, Mg2+, Fe3+, and Mn2+
)
in close proximity in their active sites,3 and their catalytic roles
have been discussed at length.2,3,9 Four main catalytic modes
that metalloenzymes are proposed to employ are (1) Lewis acid
activation of the substrate via Mx+ · · ·OdP binding, (2) delivery
of a metal-bound hydroxide or alkoxide that serves as a
nucleophile or a base, (3) electrostatic stabilization of the anionic
substrate and nucleophile/base through binding to its +-charged
active site and subsequent lowering of the transition-state energy
of the reaction,10 and (4) stabilization of the leaving group
through metal ion coordination. Model catalysts have been
designed that employ several, but rarely all, of these modes of
catalysis in the hopes of achieving the efficiency of enzymatic
catalysis in man-made systems.11,12 Our recent work demon-
strated that a simple dinuclear catalyst (1) promotes the cleavage
of phosphate diesters 2 and 3 in methanol13 and ethanol14 with
up to 1011-13- and 1014-17-fold rate accelerations over the
uncatalyzed processes. That no such acceleration is seen for
the cleavage of a phosphate diester promoted by 1 in water15
indicates that medium effects16 play a key role in producing
large rate enhancements with metal ions, perhaps amplifying
weak effects and inducing other modes of catalysis not easily
seen in water.
There are surprisingly few, albeit informative, reports docu-
menting LGA promoted by general acids or metal ions even
though this should be a vital component of phosphoryl transfer
reactions catalyzed by enzymes where the natural substrates
contain nonactivated leaving groups. This type of catalysis is
expected to be most readily observed for a reaction considered
to proceed via a dissociative transition state (TS)2 with extensive
leaving group departure and little nucleophile participation.
Consistent with this, significant metal ion-promoted LGA is seen
for the alkaline phosphatase-catalyzed cleavage of dianionic
phosphate monoesters.17 Accelerations up to 108-fold have been
documented for intramolecular general-acid catalysis of the
hydrolyses of 8-(dimethylammonium)naphth-1-yl phosphate
(4a)18 and salicyl phosphate19 monoesters relative to the
uncatalyzed reactions of these dianions. There are also a handful
of examples of LGA for cleavage of phosphate monoesters
promoted by a metal ion, notable examples being the Cu(II)-
catalyzed cleavages of 2-(4(5)-imidazoyl)phenyl phosphate,20
8-quinolyl phosphate,21 salicyl phosphate,22 and 2-(1,10-phenan-
throlyl) phosphate (5)23 where the metal ion-promoted LGA
increases the reactivities of these by 104- to 108-fold over those
of the background reactions.
(6) (a) Wolfenden, R. Chem. ReV. 2006, 106, 3379. (b) Wolfenden, R.;
Snider, M. J. Acc. Chem. Res. 2001, 34, 938. (c) Schroeder, G. K.;
Lad, C.; Wyman, P.; Williams, N. H.; Wolfenden, R. Proc. Natl. Acad.
Sci. U.S.A. 2006, 103, 4052. (d) Wolfenden, R.; Ridgeway, C.; Young,
G. J. Am. Chem. Soc. 1998, 120, 833. (e) Warshel, A.; Sharma, P. K.;
Kato, M.; Xiang, Y.; Liu, H.; Olsson, M. H. Chem. ReV. 2006, 106,
3210.
The hydrolysis of the phosphate diester methyl 8-(dimethy-
lammonium)naphth-1-yl phosphate (4b)24 is accelerated by
intramolecular general-acid catalysis as is the cleavage of bis(8-
hydroxyquinoline) phosphate.25 The cleavage of adenosine 3′-
alkyl phosphate diesters in the presence of some transition metals
and lanthanides is suggested26 to involve coordination of the
departing group to the metal ions, particularly La3+. Our recent
work demonstrated an LGA of 1012-fold for the cleavage of
(7) Donarski, W. J.; Dumas, D. P.; Heitmeyer, D. P.; Lewis, V. E.;
Raushel, F. M. Biochemistry 1989, 28, 4650.
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Neverov, A. A. J. Phys. Org. Chem. 2009, 22, 1. (b) Williams, N. H.;
Takasaki, B.; Wall, M.; Chin, J. Acc. Chem. Res. 1999, 32, 485. (c)
Morrow, J. Comments Inorg. Chem. 2008, 29, 169.
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M.; Manci, F.; Saielli, G.; Chin, J.; Tonellato, U. Chem.sEur. J. 2007,
13, 2246.
diester 2c promoted by Yb3+ 27 as well as an apparent LGA for
,
the dinuclear Zn(II) complex 1 in promoting cleavage of diesters
having an o-NO2 or -CO2Me substituent on the aryloxide leaving
groups (2b,c).13c
For triesters such as diethyl 8-(dimethylammonium)naphth-
1-yl phosphate (4c),28 intramolecular general-acid catalysis
enhances the attack of water and oxyanion nucleophiles.
However, unambiguous examples of metal ion-promoted LGA
(12) (a) Feng, G.; Natale, D.; Prabaharan, R.; Mareque-Rivas, J. C.;
Williams, N. H. Angew. Chem., Int. Ed. 2006, 45, 7056. (b) Iranzo,
O.; Kovalevsky, A. Y.; Morrow, J. R.; Richard, J. P. J. Am. Chem.
Soc. 2003, 125, 1988. (c) O’Donoghue, A.; Pyun, S. Y.; Yang, M.-
Y.; Morrow, J. R.; Richard, J. P. J. Am. Chem. Soc. 2006, 128, 1615.
(d) Williams, N. H.; Cheung, W.; Chin, J. J. Am. Chem. Soc. 1998,
120, 8079.
(13) (a) Neverov, A. A.; Lu, Z.-L.; Maxwell, C. I.; Mohamed, M. F.; White,
C. J.; Tsang, J. S. W.; Brown, R. S. J. Am. Chem. Soc. 2006, 128,
16398. (b) Bunn, S. E.; Liu, C. T.; Lu, Z.-L.; Neverov, A. A.; Brown,
R. S. J. Am. Chem. Soc. 2007, 129, 16238. (c) Neverov, A. A.; Liu,
C. T.; Bunn, S. E.; Edwards, D. R.; White, C. J.; Melnychuk, S. A.;
Brown, R. S. J. Am. Chem. Soc. 2008, 130, 16711.
(21) Hay, R. W.; Basak, A. K.; Pujari, M. P. J. Coord. Chem. 1991, 23,
43.
(22) Hay, R. W.; Basak, A. K.; Pujari, M. P.; Perotti, A. J. Chem. Soc.,
Dalton Trans. 1986, 2029.
(14) Liu, C. T.; Neverov, A. A.; Brown, R. S. J. Am. Chem. Soc. 2008,
130, 16711.
(23) Fife, T. H.; Pujari, M. P. J. Am. Chem. Soc. 1988, 110, 7790.
(24) Kirby, A. J.; Lima, M. F.; da Silva, D.; Roussev, C. D.; Nome, F.
J. Am. Chem. Soc. 2006, 128, 16944.
(15) Kim, J.; Lim, H. Bull. Korean Chem. Soc. 1999, 20, 491.
(16) Sa´nchez-Lombardo, I.; K Yatsimirsky, A. Inorg. Chem. 2008, 47, 2514.
(17) Zalatan, J. G.; Catrina, I.; Mitchell, R.; Grzyska, P. K.; O’Brien, P. J.;
Herschlag, D.; Hengge, A. C. J. Am. Chem. Soc. 2007, 129, 9789.
(18) Kirby, A. J.; Dutta-Roy, N.; da Silva, D.; Goodman, J. M.; Lima,
M. F.; Roussev, C. D.; Nome, F. J. Am. Chem. Soc. 2005, 127, 7033.
(19) Bromilow, R. H.; Kirby, A. J. J. Chem. Soc. B 1972, 149.
(20) Benkovic, S. J.; Dunikoski, L. K., Jr. J. Am. Chem. Soc. 1971, 93,
1526.
(25) Browne, K. A.; Bruice, T. C. J. Am. Chem. Soc. 1992, 114, 4951.
(26) Bruice, T. C.; Tsubouchi, A.; Dempcy, R. O.; Olson, L. P. J. Am.
Chem. Soc. 1996, 118, 9867.
(27) Edwards, D. R.; Neverov, A. A.; Brown, R. S. J. Am. Chem. Soc.
2009, 121, 368.
(28) (a) Kirby, A. J.; Tondo, D. W.; Medeiros, M.; Souza, B. S.; Priebe,
J. P.; Lima, M. F.; Nome, F. J. Am. Chem. Soc. 2009, 131, 2023. (b)
Asaad, N.; Kirby, A. J. J. Chem. Soc., Perkin Trans. 2 2002, 1708.
9
3562 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
Scheme 1. Cu(II)-Assisted Cleavages of Phosphate Esters 6-8 in Methanola
a R′ and R′′ ) H or CH3.
for triesters are exceedingly rare,29 and the only unambiguous
example of which we are aware where metal-assisted LG
departure was quantified is that of La3+-promoted methanolysis
of triester 3b, which is cleaved about 60 times faster than
triesters without LGA.30
ing group, which is known to bind strongly to Mx+ ions (Scheme
1). The results show that Cu(II)-promoted LGA gives a 1014-
fold or more acceleration for the methanolysis of monoester 6
and diester 7 in the neutral sspH region. By contrast, the
methanolysis of triester 8 in the presence of Cu(II) shows a
more moderate, but still appreciable, 105-fold rate acceleration
at neutrality due to metal ion-promoted LGA.
2. Experimental Section
2.1. Materials. Methanol (99.8%, anhydrous), sodium methoxide
(0.50 M in methanol, titrated against N/2 certified standard aqueous
HCl solution and found to be 0.50 M), Cu(CF3SO3)2 (98%),
Zn(CF3SO3)2, 2-picoline (98%), 2,6-lutidine (99+%), 2,4,6-collidine
(99%), triethylamine (99%), 2,2,6,6-tetramethylpiperidine (99+%),
Amberlite IR-120H ion-exchange resin (functionalized as sulfonic
acid), sodium carbonate, imidazole (99%), dimethyl chlorophos-
phate (96%), p-nitrophenol (98%), 1,10-phenanthroline (99+%),
2-bromoanisole (97%), lithium (high sodium granule, 99%), KMnO4
(99+%), pyridine hydrochloride (98%), methanol-d (99.5 atom %
D), NaOH (reagent grade, 97%), and phosphorus oxychloride (99%)
were purchased from Aldrich and used as supplied. HClO4 (70%
aqueous solution, titrated to be 12.09 M) was obtained from Acros
Organics. Lithium chloride and MnSO4 ·H2O were purchased from
Anachemia Chemical Ltd., while 1-methylpiperidine (99%) was
obtained from Alfa Aesar and used as supplied. MgSO4 anhydrous
and 1-ethylpiperidine (99%) were obtained from Fisher Scientific
and TCI America Laboratory Chemicals, respectively. 2-(2-Hy-
droxyphenyl)-1,10-phenanthroline was prepared following a pub-
lished procedure.31 The disodium salt of phosphate monoester 6
was synthesized following a literature method32 using 2-(2-
hydroxyphenyl)-1,10-phenanthroline and POCl3. Both the phosphate
diester 7 and the triester 8 were prepared according to a general
method33 with small modifications.13c 1H NMR, 31P NMR, and
exact MS spectra of phosphates 6-8 are consistent with the
structures (see the Supporting Information).
While phosphoryl mono-, di-, and triester systems with the
8-(dimethylamino)-1-naphthyl leaving group18,24,28 allow one
to compare the effectiveness of general-acid assistance of LG
departure on the three ester types, there is no analogous set of
esters from which one can draw comparisons of the effectiveness
of metal ion LGA. In this study we describe the kinetics of
Cu(II)-promoted methanolyses of a set of phosphoesters 6-8
containing the 2-(2-hydroxyphenyl)-1,10-phenanthroline31 leav-
1
2.2. General Methods. H NMR and 31P NMR spectra were
determined at 400 and 162.04 MHz. CH3OH2+ concentrations were
determined potentiometrically using a combination glass Fisher
Scientific Accumet electrode model no. 13-620-183A calibrated
with certified standard aqueous buffers (pH 4.00 and 10.00) as
described in previous papers.34 The pH values in methanol were
s
s
determined by subtracting a correction constant of -2.2434 from
the electrode readings, and the autoprotolysis constant for methanol
was taken to be 10-16.77 M2. The sspH values for the kinetic
experiments were measured at the end of the reactions to avoid
the effect of KCl leaching from the electrode. For methanolysis of
(29) (a) Trogler, W. C.; Morrow, J. R. Inorg. Chem. 1989, 28, 2330. (b)
Ku¨hn, U.; Warzeska, S.; Pritzkow, H.; Kra¨mer, R. J. Am. Chem. Soc.
2001, 123, 8125.
(30) (a) Edwards, D. R.; Liu, C. T.; Garrett, G.; Neverov, A. A.; Brown,
R. S. J. Am. Chem. Soc. 2009, 131, 13738. (b) Liu, T.; Neverov, A. A.;
Tsang, J. S. W.; Brown, R. S. Org. Biomol. Chem. 2005, 3, 1525.
(31) Holligan, B. M.; Jeffery, J. C.; Ward, M. D. J. Chem. Soc., Dalton
Trans. 1992, 3337.
(32) Williams, A.; Naylor, R. A. J. Chem. Soc. B 1971, 10, 1973.
(33) Padovani, M.; Williams, N. H.; Wyman, P. J. Phys. Org. Chem. 2004,
17, 472.
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A R T I C L E S
Liu et al.
Cu(II):7 the sspH values measured at the beginning of the reactions
were the same as those determined at the end.
syringe was loaded with 0.02 mM Cu(OTf)2 in methanol. When
mixed, the final concentrations of Cu(OTf)2, 6, and buffer were
2.3. General UV-Vis Kinetics. The Cu(II)-catalyzed metha-
nolyses of 7 and 8 were followed at 414 nm for the appearance of
the Cu(II)-bound phenoxide 9 using a UV-vis spectrophotometer
with the cell compartment thermostated at 25.0 ( 0.1 °C. The
reactions were conducted in the presence of buffers composed of
various ratios of HClO4 and amines (2-picoline (sspH 4.8-6.9), 2,6-
lutidine (sspH 7.0-7.5), 2,4,6-collidine (sspH 7.8-8.5), N-isopropy-
lmorpholine (sspH 8.5-9.0), 1-methylpiperidine (sspH 9.4-9.7),
1-ethylpiperidine (sspH 9.8-10.5), triethylamine (sspH 10.9), and
2,2,6,6-tetramethylpiperidine (sspH 11.0-12.1)) to maintain the sspH
in methanol. For reactions at sspH 3 or lower, an appropriate amount
of HClO4 was added to achieve the targeted initial [CH3OH2+] in
solution. A typical kinetic experiment involved preparing a methanol
solution of the phosphate substrate (between 0.01 and 0.05 mM)
containing excess buffer (0.2-1.0 mM) in a 1 cm path length UV
cuvette. The rate constants for the methanolyses of 6 and 7 in the
presence of Cu(II) did not exhibit buffer concentration effects (from
0.1 to 20 mM 2-picoline or 2,2,6,6-tetramethylpiperidine buffer),
nor was the rate sensitive to the presence of 0.01-20 mM
tetrabutylammonium chloride, tetrabutylammonium bromide, or free
pyridine. Initiation of the reaction involved addition of an aliquot
of Cu(II) stock solution in methanol to the substrate solution in
the cuvette to achieve the desired concentrations of the reaction
components in a final volume of 2.5 mL. Duplicate kinetics were
done, and in cases where the reactions are reasonably fast, the
absorbance vs time traces for product appearance were fit to a
standard first-order exponential equation to obtain the observed first-
order rate constants (kobsd). For very slow reactions, such as those
of the Cu(II)-promoted methanolysis of 8 at higher sspH, initial rates
of the reactions were obtained by fitting the first 5-10% of the
absorbance vs time traces to a linear regression, followed by
dividing the initial rates by the expected absorbance change (∆Abs)
if the reaction were to reach 100% completion to get the kobsd
0.01, 0.01, and 0.2 mM. At least five kinetic runs were recorded at
s
each pH, and the average values gave the kobsd constants. A
s
concentration-dependent experiment was also conducted by fol-
lowing the reactions of 4 × 10-6 M e [Cu(II):6] e 5 × 10-4 M in
the presence of a 40-fold excess of 1-methylpiperidine buffer (sspH
10.4 ( 0.2). The effect of increasing [Cu(II)] was investigated by
increasing [Cu(OTf)2] from 1× 10-5 to 7 × 10-5 M while keeping
[6] constant at 1 × 10-5 M in the presence of 0.2 mM 1-meth-
ylpiperidine buffer (sspH 10.2 ( 0.2). A solvent kinetic isotope
experiment was done in a fashion similar to that described for
substrate 7, where stock solutions Cu(OTf)2, 6, and 1-methylpip-
eridine buffer were 0.5, 5, and 50 mM in CH3OD. Aliquots of the
solutions were diluted into solutions of CH3OH or CH3OD to attain
the desired concentrations. In this case, one of the syringes of the
stopped-flow analyzer was loaded with 0.02 mM 6 and 0.4 mM
buffer, and the other syringe contained 0.02 mM Cu(OTf)2. Two
separate experiments were conducted with 1-methylpiperidine
s
buffer, setting the measured but uncorrected pD values at 9.6 (
s
0.2 and 10.4 ( 0.2 in methanol. (These uncorrected values were
measured at the end of the reactions simply as the electrode readings
+ 2.24 and have not been corrected for the effect of the deuterated
s
solvent on the electrode reading or on the pKa of the buffer. As
s
s
will be discussed, the actual pD is less important since this is in
s
the plateau region of the sspH/rate profile.) The average of the first-
order rate constants from seven kinetic runs was used for analysis.
2.5. Methanolysis of 4-Nitrophenyl Phosphate (10) at 50
°C. The progress for the methanolysis of 10 mM 4-nitrophenyl
phosphate at 50 °C in 20 vol % CD3OD in CH3OH was followed
using 31P NMR. To six separate standard NMR tubes, each
containing 10 mM disodium 4-nitrophenyl phosphate, were added
100 mM NaOMe, 10 mM NaOMe, 10 mM HClO4, 20 mM HClO4,
30 mM HClO4, and 50 mM HClO4. In another sample, no additional
base or acid was added to a 10 mM concentration of the phosphate.
The seven samples (all with a final volume of 1 mL of solution)
were capped and sealed with Parafilm before being incubated in a
water bath set at 50.0 ( 0.5 °C. At different times samples were
removed from the water bath and 31P NMR spectra were acquired
with at least 600 scans on each. 31P NMR spectra of the starting
phosphate and monomethyl phosphate in methanol were used as
references. All kinetic reactions were followed to a maximum of
15% completion, and for each sample (each concentration of added
base or acid) at least triplicate experiments were conducted. The
product conversion (%) vs time plots were fit to a linear regression
to obtain the rates of the reactions. The rates were converted to
first-order rate constants by dividing the conversion (%)/time data
constants.
s
Separate pH/rate profiles were constructed for the 0.05 mM
s
Cu(OTf)2-catalyzed cleavages of 0.05 mM 7 and 0.05 mM 8 in
the presence of 1 mM buffer in methanol. A concentration-
dependent study was also conducted with increasing [Cu(II)] and
[7] in a 1:1 ratio from 0.05 to 0.3 mM in the presence of excess
HClO4, such that the sspH was kept at 3.5 ( 0.2. The solvent kinetic
isotope experiment for the Cu(II)-assisted breakdown of 7 was
carried out as follows. Stock solutions of 5 mM Cu(OTf)2, 7.0 mM
substrate 7, and 50 mM picoline (with 1/2 equiv of added HClO4)
buffer in CH3OD were prepared. Aliquots of these solutions were
diluted in UV cells containing either CH3OH or CH3OD so that
the final concentrations of Cu(OTf)2, 7, and picoline were 0.05,
s
0.05, and 1 mM. Duplicate kinetic runs were done, and the pH
s
by 100%.
values measured after the reactions were found to be 6.2 ( 0.2. A
similar experiment was also done for substrate 8 with final
concentrations of Cu(OTf)2, 8, and picoline buffer (sspH 3.5 ( 0.2)
of 0.05, 0.05, and 1 mM.
2.4. Stopped-Flow Kinetics. The Cu(II)-promoted cleavage of
6 was monitored at 414 nm for the appearance of product using a
stopped-flow apparatus thermostated at 25.0 ( 0.1 °C. One syringe
of the stopped-flow analyzer was loaded with 0.02 mM 6 and a
0.4 mM concentration of the desired buffer in methanol. The second
s
2.6. Spectrophotometric Titrations. pKa of 9. A 1 cm UV
s
cell was charged with 0.1 mM phenol 9 in 2.5 mL of anhydrous
methanol. Small aliquots of a 1.0 M tetrabutylammonium hydroxide
in methanol were added to vary its concentration from 0 to 1.0 M
in small increments, and the UV-vis spectrum (250-500 nm) of
the mixture was collected at 25 °C after each addition of base. The
titration was done in duplicate. The absorbance values at 400 nm
(appearance of the phenoxide) were corrected for volume change,
and the log of the corrected absorbance at 400 nm was plotted
against the -(log [H+]) in solution. The data were fit to the
following expression to yield the acid dissociation constant of the
phenolic OH group of compound 9:
(34) (a) Gibson, G.; Neverov, A. A.; Brown, R. S. Can. J. Chem. 2003,
81, 495. (b) 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, and 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 since the autoprotolysis constant of methanol is 10-16.77, the neutral
sspH is 8.4.
K
log(Abs) ) log A
+ A0
(1)
+
( (
)
)
K + [H ]
In eq 1, A is the overall absorbance change expected from
converting one species completely into another, A0 is the initial
absorbance when the initial species (phenol 9) exists at 100%, and
9
3564 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
s
K is the dissociation constant of interest. The average pKa value
3. Results
s
of phenol 9 was determined to be 16.16 from duplicate titrations.35
2.7. Binding Constant of Cu(II) and Phenol 9. To a 2.5 mL
methanol solution containing 0.1 mM phenol 9 and 0.1 mM
Cu(OTf)2 were added small aliquots of 0.6 M triflic acid in
methanol, and the UV-vis spectrum (220-500 nm) was recorded
after each addition. The absorbance values at 450 nm corresponding
to the decrease in the complex concentration were corrected for
volume changes, and the titration data were analyzed using
Hyperquad 200036 with a model defined by species 9, 9-H+, Cu(II):
9, and Cu(II):9- (see the Supporting Information). The acid
dissociation constants of 9 and 9-H+ were fixed to be 10-16.16 and
10-4.73 M as determined independently (see the Supporting Infor-
mation), while the autoprotolysis constant of methanol was taken
to be 10-16.77 M2.34 With these inputs, the dissociation constants
of Cu(II):9- a Cu(II) + 9- and Cu(II):9 a Cu(II) + 9- + H+
were computed as -[log(Kd)] values of 23.64 ( 0.03 and 24.13 (
0.01, respectively, from duplicate titrations (see Table 8S, Sup-
porting Information).
3.1. Cu(II) Complexation with 8 and 9 in Methanol. The
X-ray diffraction structure of [(Cu(II)2:9-2:(µ-MeCO2)][PF6]
shows the Cu(II) bound to 9 in a planar fashion with Cu(II)-N
bond distances of ∼2 Å and a Cu(II)-phenoxide-O bond
distance of ∼1.9 Å.31 Similar structures are reported with
Ni(II),31 Pd(II),39a and Fe(III)39b which indicate that, once bound,
the metal ion directly interacts with the phenoxide oxygen. The
binding constant for Cu2+ and 9 was determined by spectro-
photometric titration under acidic conditions (see the Supporting
Information, Figure 9S), and the data were analyzed using
Hyperquad 2000 to give a Kd of 2.3 × 10-24 M. Ancillary
information from the Hyperquad fitting points to a significant
drop of the sspKa of 9 from 16.16 to 0.49 when bound to Cu(II),
indicative of a large electrostatic interaction of the phenoxide
oxygen of 9- with the Cu ion.
Triester 8 reacts slowly in the presence of Cu(II) and allows
spectrophotometric titration studies. At sspH 7.4 (1 mM collidine
containing 7 × 10-5 M 8 and varying [Cu2+]), the dissociation
constant for Cu(II):8 was found to be (5.1 ( 2.4) × 10-7 M. A
separate study monitoring [Cu(II):8] under acidic conditions
2.8. Binding Constant of Cu(II) and 8. To a UV cell containing
7 × 10-5 M 8 and 1 mM collidine buffer (sspH 7.4) in 2.5 mL of
methanol were added aliquots of 5.6 mM Cu(OTf)2 stock solution
in 5 µL increments. The absorbance at 490 nm after each addition
was plotted against [Cu(OTf)2], and the data were fit to a binding
equation37,38 applicable to strong and weak situations.
yields a similar dissociation constant of (2.8 ( 0.4) × 10-7
M
after analysis of the data with Hyperquad 2000 (see the
Supporting Information). The reactivities of the Cu(II) com-
plexes of 6 and 7 are too great to allow experimental determi-
nation of their dissociation constants, although we assume that
these are similar to that of Cu(II):8 but may have somewhat
stronger binding due to electrostatic attraction between the metal
ion and anionic phosphates.
A more accurate value for the dissociation constant of Cu(II):8
was acquired as follows. To a UV cell containing a 0.1 mM
concentration of the complex in 2.5 mL of anhydrous methanol
were added small aliquots of triflic acid in methanol with subsequent
determination of the UV-vis spectrum from 220 to 500 nm after
each addition. The absorbance values at 263 nm for the reduction
in the complex concentration were corrected for volume change
and plotted against the -(log [H+]) of the solution. Data from
duplicate titrations were analyzed using Hyperquad 2000 to compute
an average dissociation constant (Kd) of 2.8 × 10-7 M.
3.2. Background Reaction for Methoxide with 8 and 7. The
second-order rate constant of (2.94 ( 0.09) × 10-3 M-1 s-1
for the CH3O--promoted cleavage of 8 was determined under
pseudo-first-order conditions at three [-OCH3] values (see the
Supporting Information).
2.9. Activation Parameters. The kinetic runs at different
temperatures for substrates 6 and 7 were done using a thermostated
stopped-flow analyzer, and the rate constants determined at each
temperature were used to construct Eyring plots. Kinetic runs for
substrate 8 at different temperatures were done by UV-vis
spectrophotometry and the solution temperatures determineded with
a thermometer inserted into the cell at the end of the reaction. First-
order rate constants were measured in quadruplicate at six different
temperatures ranging from 10 to 35 °C for the Cu(II)-promoted
The CH3O--promoted methanolysis of 7 is too slow to follow
experimentally, so the sspKa value for the leaving group phenol
9 (16.16), as determined above, was used to estimate the second-
order rate constant for the methoxide background reaction of
7. The Brønsted plot for the methoxide-promoted cleavages of
a series of methyl aryl phosphate diesters in methanol13c fits
the expression log(k2-OMe) ) (-0.57 ( 0.06)sspKa + (0.14 (
methanolysis of 6 (both at 0.01 mM) in the presence of 0.2 mM
-OMe
s
0.68). From this, k2
for methoxide attack on 7 is 8.5 ×
1-methylpiperidine buffer, pH 10.4 ( 0.2. Fitting the first-order
s
10-10 M-1 s-1 assuming the reactivity of 7 adheres to the same
relationship as do those of dimethyl aryl phosphates.
rate constant vs 1/T plot to the Eyring equation gives the ∆Hq and
∆Sq values. For substrate 7, the activation parameters were
determined with 0.05 mM Cu(OTf)2, 0.05 mM 7, and 0.25 mM
HClO4 (sspH 3.6 ( 0.2) and using seven temperatures ranging from
12.0 to 39.8 °C. For substrate 8, duplicate experiments were
conducted with 0.05 mM Cu(OTf)2, 0.05 mM 8, and 1.0 mM
2-picoline buffer (sspH 3.8 ( 0.2) at seven temperatures ranging
from 16.0 to 42.5 °C.
3.3. Methanolysis of 4-Nitrophenyl Phosphate (10). An
estimate for the background reaction of 6, which reacts too
slowly to observe, was made from the rate constant for
methanolysis of 4-nitrophenyl phosphate (10), which exhibits
a plateau region between sspH ≈ 10 and sspH 14. The cleavage
of 10 at 50.0 ( 0.5 °C in methanol was monitored using
s
initial rate techniques at three pH values greater than 10.6
s
s
(the second pKa value of 10 determined by half-neutraliza-
(35) The total DA for conversion of a 0.1 M solution of phenol 9 to
phenoxide 9- was determined to be 0.9 unit at 400 mm from fits of
the absorbance vs [+NBu4-OCH3] to a standard single ionization model
(see the Supporting Information).
s
tion) at various times using 31P NMR (see the Supporting
Information), and the average kobsd for the methanolysis of
dianionic 10 between sspH 11.6 and sspH 14.0 was determined
to be 1.4 × 10-7 s-1. This value can be compared with the
reported value for cleavage of the dianion of 10 in water of
1.55 × 10-8 s-1 at 39 °C,40 from which a kobsd of 8 × 10-8
s-1 is calculated at 50 °C on the basis of the activation
parameters in water listed by Guthrie and Jencks.41 No
(36) Hyperquad 2000 is a computer program that fits titration data into
nonlinear expressions containing a series of formation constants (log
ꢀ) for species input to a model: Gans, P.; Sabatini, A.; Vacca, A.
Talanta 1996, 43, 1739.
(37) kobsd ) kcat(1 + KB[S] + [cat]KB- X)/(2KB)/[S], where X ) (1 +
2KB[S] + 2[cat]KB + KB2[S]2-2KB2[cat][S] + [cat]2KB
) .
2
0.5
(38) A detailed description and examples where the equation has been
applied can be found in ref 13.
(39) (a) Bardwell, D.; Black, D.; Jeffery, J. C.; Ward, M. D. Inorg. Chem.
1993, 12, 1577. (b) Jeffery, J. C.; Moore, C. S. G.; Psillakis, E.; Ward,
M. D.; Thornton, P. Polyhedron 1995, 14, 599.
(40) Kirby, A. J.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 3209.
(41) Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3565

A R T I C L E S
Liu et al.
s
Scheme 2. Kinetic Scheme for the pH Dependence of the
Cleavage of Cu(II):6 in Methanol Hsaving a Reactive Form with
s
s
Maximum Activity in the pH Region between the pKa Values of
s
the Two Ionizable Groupss
kcatmax ssK2a
[H+]
[H+]
kobsd
)
+ k0
s
2
+
s
3
+
s
2
+
(
)
(
)
(
)
sKa + [H ] sKa + [H ]
sKa + [H ]
(2)
Figure 1. log(kobsd) vs sspH for the Cu(II)-promoted cleavage of 6 (b, 0.01
mM Cu(II) and 6) in methanol in 0.2 mM buffered solutions and 7 (9,
0.05 mM Cu(II) and 7) in methanol with 1 mM amine buffer as described
in the Experimental Section or excess added HClO4 in the acidic region in
methanol at 25 °C. The line through the b data is computed by an NLLSQ
The deuterium kinetic isotope effect was determined at two
measured estimated values of “sspD”, 10.4 ( 0.2 and 9.6 ( 0.2,
in the plateau region where the rate is independent of sspH (0.2
s
mM 1-methylpiperidine buffer). The pD values were simply
s
s
s
fit to eq 2, giving two macroscopic pK2a and pKa3 values of 7.8 ( 0.1 and
s
s
11.8 ( 0.2, a maximum rate constant (kcatmax) of 14.7 ( 0.4 s-1, and k0 )
(6.3 ( 0.4) × 10-3 s-1 (r2 ) 0.9883). The line through the 9 data is
computed by an NLLSQ fit of the data to eq 5 derived for the process in
Scheme 4, giving log(ssKa2/Kdim) ) -20.5 ( 0.2 and kcat ) 0.0024 ( 0.0001
s-1 (r2 ) 0.9798).
those from the reading of the electrode made in the deuterated
solvent plus the added correction factor of 2.24 for passing from
water to methanol. The actual sspD values measured in the plateau
region are only approximate but are less important than
demonstrating that the rate constant obtained at two different
sspD values is invariant. This proves to be the case since the
kMeOH/kMeOD value determined at pD 9.6 is (14.7 ( 0.3 s-1)/
s
methanolysis product (methyl phosphate) was observed for
the cleavage of 3-nitrophenyl phosphate (sspKa ) 12.41 for
3-nitrophenol in methanol)13c after incubation at 50 °C in
methanol for 300 h. Assuming the limit of detectability of
the product would be 5%, this corresponds to an upper limit
for the decomposition rate constant of 1 × 10-8 s-1. An upper
limit for the Brønsted ꢀ value of ∼-1.0 can be computed
from these two data, and while there is a large error in the
number, it is similar to the values of -1.23 and -1.11
obtained for the cleavage of the dianions of monoesters in
water42 and in tert-amyl alcohol,43 indicating that sensitivity
to the substituent is not greatly affected by these sorts of
hydroxylic solvents.
s
(14.3 ( 0.4 s-1) ) 1.03 ( 0.04 and at sspD 10.4 is (15.9 ( 0.6
s-1/14.7 ( 0.4 s-1) ) 0.95 ( 0.05.
In Scheme 3 is an expanded version of Scheme 2 where the
s
species [Cu(II):6a]+ formed at lower pH through binding of
s
6- to Cu2+ undergoes two possible microscopic ionizations44
to yield two formally neutral complexes, [Cu(II):6b]0 and
[Cu(II):6c]0. Chemical intuition suggests that [Cu(II):6b]0, drawn
as having a P-O- · · ·Cu(II) coordination, should be the more
active of the two since both nonbridging oxyanions on P can
facilitate the departure of the leaving group with assistance of
an electropositive Cu(II). A second set of microscopic ioniza-
tions convert the neutral complex into a less active or inactive
anionic complex, [Cu(II):6d]-. It is known that the simple
phenanthroline:Cu(II):-OCH3 complex dimerizes to produce
(phenanthroline:Cu(II):-OMe)2 at low concentrations in metha-
nol,45 so rate vs complex concentration experiments were
conducted in the plateau region at sspH 10.5 where the dominant
species are the two [Cu(II):6]0 forms given in Scheme 3. The
data in Figure 2 follow a square root dependence on [[Cu(II):
6]0] which stems from the equilibrium formation of an inactive
dimer as in eq 2. Such monomer/dimer equilibrium behavior is
treated by NLLSQ fitting of the rate vs concentration data to
eq 445 derived for the process in eq 3 where the complex is
completely formed at all concentrations used, and the active
monomer [Cu(II):6]0 which decomposes with a rate constant
kcat is in equilibrium (with dissociation constant Kdis) with an
Because the Brønsted slopes for the cleavage of the
dianions are not very different in water, methanol, and tert-
amyl alcohol, one may estimate a rate constant for the
spontaneous cleavage of 6 in methanol on the basis of
s
the above kobsd for 10 and the difference in pKa values for
s
the two leaving groups (11.1813 and 16.16). We assume the
ꢀlg value in methanol has limits of -1.23 and -1.0.
An approximate kobsd for the spontaneous meth-
anolysis of the dianion of 6 at 50 °C is computed
s
as kobsd ) 10-(ꢀ( pKa4-nitrophenol-sspKa9)-log(kobsd4-nitrophenol)) )
6
s
10-(ꢀ(11.18-16.16)-(-6.85)). The rate constant spans a 13-fold range
of 1.1 × 10-13 to 1.5 × 10-15 s-1 if ꢀ ) -1.23 or -1.0,
respectively, but despite the ambiguity, this is an exceedingly
slow reaction, and such an error will not greatly affect the
analysis.
inactive dimer, [Cu(II):6]0 . Because it is the rate, not the rate
2
constant, used for the y axis in Figure 2, kcat has the units (Abs/
s)/M. A kcat value of ∼1.3 × 105 (Abs/s)/M is estimated from
the rate constants and the changes in absorbance obtained at
low [Cu(II):6] where the majority species in solution is the
monomer. Fixing this kcat, the Figure 2 data were fit to eq 3 to
give Kdis ) (1.12 ( 0.03) × 10-5 M.
3.4. Cu(II)-Promoted Methanolysis of 6. The sspH/rate profile
given in Figure 1 for the Cu(II)-catalyzed cleavage of 6 in
methanol (b symbols) can be fit to eq 2 derived for a model
given in Scheme 2 with two ionizable groups having sspKa values
of 7.8 and 11.8, a maximum rate constant (kcatmax) of 14.7 (
s
0.4 s-1 in the plateau region, and a low pH plateau of (6.3 (
s
0.4) × 10-3 s-1. The kcat
value in the neutral pH region
max
s
P 7 2[Cu(II):6]0 y z [Cu(II):6]0
(3)
s
2
corresponds to a t1/2 ) 50 ms for catalyzed cleavage of 6.
kcat
Kdis
9
3566 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
Scheme 3. Possible Microscopic Ionizations for Cu(II):6 Speciesa
a Counterions omitted for simplicity.
[H+]2
log(kobsd) ) log(kcat) + log
(5)
s
2
+ 2
(
)
sKa/Kdim + [H ]
Finally, a concentration dependence experiment was con-
ducted by monitoring the rate constant for the solvolytic
cleavage of 7 as a function of increasing [Cu(II):7] in the plateau
region of the sspH/rate profile in Figure 1 by adding excess HClO4
s
to keep the pH at 3.5 ( 0.2. Unlike the case for Cu(II):6,
s
increasing [Cu(II):7] from 0.05 to 0.3 mM does not affect the
rate constant of the reaction (see the Supporting Information).
This is in agreement with the monomeric [Cu(II):7b]+ being
the only significant species in solution from sspH 3.3 to sspH 10.
3.6. Cu(II)-Catalyzed Methanolysis of 8. Inspection of the
sspH/rate profile for the Cu(II)-catalyzed methanolysis of 8 given
in Figure 3 reveals two reactive species connected by a single
ionization. The data in Figure 3 are fit to eq 6 derived for the
process in Scheme 5 with two different complexes, each
cleaving with a different rate constant (k1 or k2), interconnected
Figure 2. Rate of the reaction (Abs/s) vs [[Cu(II):6]0] for the cleavage of
6 monitored at 414 nm in the presence of 8 mM 1-methylpiperidine buffer
(sspH 10.4 ( 0.2) at T ) 25 °C. The dotted line through the data is computed
from a fit of the data to eq 3 having a fixed kcat of 1.3 × 105 (Abs/s)/M as
described in the text. The computed Kdis is (1.12 ( 0.03) × 10-5 M with
r2 ) 0.9784.
rate ) {k K ( (1 + 8[[Cu(II):6]0])/K - 1)/4} (4)
by an acid dissociation sspKa of 6.03. These reactive species are
√
cat dis
dis
s
designated as [Cu(II):8a]2+ at pH < 5.5 with k1 ) (2.0 ( 0.2)
s
3.5. Cu(II)-Catalyzed Methanolysis of 7. Also shown in
Figure 1 is the sspH/rate profile for the Cu(II)-promoted cleavage
of 7 under buffered conditions in methanol which exhibits a
broad sspH-insensitive region from sspH 3.3 to sspH 10. At higher
sspH the plot has a gradient of -2, suggesting that two
methoxides are responsible for forming an inactive dimeric
entity. In the proposed mechanism the most reactive species is
[Cu(II):7b]+ shown in Scheme 4 with the monoanionic phos-
phate bound to the metal ion or its kinetic equivalent where the
phosphate is not bound (not shown). A fit of the kinetic data to
the expression in eq 5 gives the line through the 9 data in Figure
1, log(ssKa2/Kdim) ) -20.5 ( 0.2, and a kcat of 0.0024 ( 0.0001
s-1. A solvent kinetic isotope study at a measured sspD of 6.2 (
0.2 at the midpoint of the plateau in the sspH/rate profile (Figure
1) gives kMeOH/kMeOD ) 1.01 ( 0.04 (kMeOH ) (2.37 ( 0.03) ×
10-3 s-1; kMeOD ) (2.35 ( 0.05) × 10-3 s-1).
× 10-5 s-1 and [Cu(II):8b]+ at sspH > 7.5 with k2 ) (1.2 ( 0.2)
× 10-6 s-1. A solvent kinetic isotope study at sspH 3.5 ( 0.2 in
the plateau region of Figure 3, where the majority of the species
in solution are [Cu(II):8a]2+, gives k1
) (2.03 ( 0.04) ×
MeOH
10-5 s-1, k1
) (9.3 ( 0.2) × 10-6 s-1, and k1MeOH/k1
MeOD
MeOD
) 2.2 ( 0.1 for the phosphoryl transfer reaction to solvent.
ssKa
[H+]
log(kobsd) ) log k1
+ k2s
(6)
ssKa + [H+]
sKa + [H ]
+
(
)
3.7. Activation Parameters. To probe further the nature of
the Cu(II)-catalyzed reactions of these phosphate esters, the
(44) For the ionization process of the dibasic acid shown in Scheme 3, the
macroscopic ionization constants K1a and Ka2 are given in terms of the
microscopic ionization constants as K1a ) k1a + ka2, 1/Ka2 ) 1/k3a
+
1/ka4, and K1aKa2 ) ka1k3a ) k2ak4a: Cookson, R. F. Chem. ReV. 1974, 74,
(42) Kirby, A. J.; Varvoglis, A. G. J. Am. Chem. Soc. 1967, 89, 415.
(43) Hoff, R. H.; Hengge, A. C. J. Org. Chem. 1998, 63, 6680.
5.
(45) Neverov, A. A.; Brown, R. S. Org. Biomol. Chem. 2004, 2, 2245.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3567

A R T I C L E S
Liu et al.
a
s
Scheme 4. Proposed Scheme for the Reaction of Different pH-Dependent Cu(II):7 Species
s
a Counterions omitted for simplicity.
Table 1. Activation Parameters (∆Hq, ∆Sq, and ∆Gq at 25 °C) and
the kMeOH/kMeOD Values for the Cu(II)-Assisted Cleavages of
s
Phosphates 6-8 in the Plateau Region of the Respective pH/
s
Rate Profiles
phosphate
complexa
∆Hq
(kcal/mol)
∆Sq
[cal/(mol·K)]
∆Gq(25 °C)
(kcal/mol)
kMeOH/kMeOD
b
[Cu(II):6]0
[Cu(II):7]+
21.4 ( 0.7
21.6 ( 0.4
21.6 ( 0.5
18 ( 2
2.3 ( 1.1
-7.4 ( 1.7
16.0
20.9
23.8
0.95 ( 0.05
1.01 ( 0.04
2.2 ( 0.1
c
d
[Cu(II):8]2+
a The activation parameters for substrate
standard-state conditions with M nucleophile. This allows for
8
are computed at
1
comparison with the values obtained for substrates 6 and 7. b Activation
parameters determined at sspH 10.4
(
0.2. c Activation parameters
d
s
s
determined at pH 3.6 ( 0.2. Activation parameters determined at pH
s
s
s
Figure 3. log(kobsd) vs pH for the Cu(II)-promoted cleavage of 8 (0.05
s
3.8 ( 0.2.
mM Cu(II) and 8) with 1 mM amine buffer or excess added HClO4 in the
acidic region, T ) 25 °C. The dotted line is obtained from the fit of the
data to eq 6 derived for the process of Scheme 5, giving computed k1 )
(2.0 ( 0.2) × 10-5 s-1, k2 ) (1.2 ( 0.2) × 10-6 s-1, and Ka ) (9.4 ( 2.0)
× 10-7 (sspKa ) 6.03) (r2 ) 0.9643).
Scheme 5. Proposed Mechanism for the Formation of Different
Cu(II):8 Speciesa
a Counterions omitted for simplicity.
Figure 4. Eyring plots of ln(k/T) vs 1/T for the methanolysis of phosphate
esters: (9) 0.01 mM Cu(II):6, 0.2 mM 1-methylpiperidine buffer, sspH 10.4
s
( 0.2; (b) 0.05 mM Cu(II):7, 0.25 mM HClO4, pH 3.6 ( 0.2; (O) 0.05
activation parameters were determined in the plateau regions
of the plots given in Figures 1 and 3. The activation data are
given in Table 1, and the Eyring plots are presented in Figure
4 (for raw experimental data see the Supporting Information).
The lines in Figure 4 are essentially parallel, indicative of very
similar ∆Hq values for cleavage of each complex, while the
∆Sq values differ by about 26 cal/(mol ·K), passing from
negative for the triester to positive for the monoester. The values
from the solvent kinetic isotope experiments are also included
in Table 1 for comparison.
s
mM Cu(II):8, 1.0 mM 2-picoline buffer, sspH 3.8 ( 0.2. Fits of the data to
the Eyring equation give the activation parameters presented in Table 1.
4. Discussion
The concept of, and requirement for, LGA of the cleavage
of phosphate esters mediated by enzymes is widely ack-
nowledged,9b,46 particularly where the leaving group is poor.
However, LGA is difficult to demonstrate in small-molecule
chemistry13,20-23,26,27,29,30,47 due to the limitations imposed
by size and the synthetic difficulties in precisely positioning
the metal ions for assisting LG departure. Demonstration of
metal ion-promoted LGA in small-molecule turnover catalysts
is a challenge yet to be met in any substantial way unless
(46) (a) Morrow, J. Comments Inorg. Chem. 2008, 29, 169. (b) Catrina, I.;
O’Brien, P. J.; Purcell, J.; Nikolic-Hughes, I.; Zalatan, J. G.; Hengge,
A. C.; Herschlag, D. J. Am. Chem. Soc. 2007, 129, 5760. (c) Admiraal,
S. J.; Herschlag, D. Chem. Biol. 1995, 2, 729. (cc) Cleland, W. W.;
Hengge, A. C. Chem. ReV. 2006, 106, 3252.
9
3568 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
Table 2. Rate Constants for the Cleavage of Phosphates 6-8 and Their Cu(II) Complexes
a
b
)
reactant
kobsd (s-1
)
k2-OMe (M-1 s-1
ksc (s-1
)
acceleration (kobsd/kbackground)
d,g
[Cu(II):6b]0
14.7 ( 0.4b (200)c
(6.3 ( 0.4) × 10-3
(2.5 ( 0.1) × 10-3
(2.0 ( 0.2) × 10-5
(1.2 ( 0.2) × 10-6
(1-14) × 1013
7.1 × 1014
d
b
b
b
[Cu(II):6a]+
b,h
[Cu(II):7b]+
b,i
[Cu(II):8a]2+
4.0 × 109
1.0 × 105
b,j
[Cu(II):8b]+
6
7
8
(1.1-15) × 10-13
e
8.5 × 10-10
f
(2.9 ( 0.1) × 10-3
a
kobsd indicates the spontaneous rate constant for decomposition of the Cu(II) complexes at the indicated pH values in footnotes g-i. b T ) 25 °C.
s
s
c T ) 50 °C. sspH < 2.5. e Determined from application of the sspKa of 16.16 for the 2-(2′-phenanthrolyl)phenol leaving group (9) to log(k2-OMe) )
(-0.57 ( 0.06)sspKa + (0.14 ( 0.68).13c f Experimentally determined as described in the Supporting Information. g Determined by comparing the
spontaneous rate of decomposition of the complex at sspH 10.4 with the projected rate of cleavage of the dianion of 6 as computed in section 3.3.
d
h Determined by comparing the kobsd for cleavage of [Cu(II):7b]+ at neutral pH 8.38 with that calculated for the methoxide-promoted reaction of 7 from
s
s
-OMe
s
k2
.
i Determined by comparing the kobsd for cleavage of [Cu(II):8a]2+ at neutral pH 5.0 with that calculated for the methoxide-promoted reaction of
s
8 from the observed rate constant k2
) (2.94 ( 0.09) × 10-3 M-1 s-1
.
j Determined by comparing the kobsd for cleavage of [Cu(II):8b]+ at neutral
-OMe
sspH 8.38 with that calculated for the methoxide-promoted reaction of 8 from the observed rate constant k2
) (2.94 ( 0.09) × 10-3 M-1 s-1
.
-OMe
the substrates contain specially positioned auxiliary groups
that transiently coordinate to the metal center during cleav-
age.48
In this proof-of-concept study we have employed a strategy
where a departing phenoxy group is connected to an o-
phenanthroline ligand that firmly positions a Cu(II) ion within
∼1.9 Å of the departing oxygen in a series of mono-, di-, and
triesters, Cu(II):6, Cu(II):7, and Cu(II):8. We note that the sspKa
of 16.16 for phenol 9 suggests this is not a particularly activated
leaving group, lying between CF3CH2OH (sspKa ) 15.8) and
49
s
CFH2CH2OH (sspKa ) 17.2). The unusually high pKa value
s
(relative to those of normal phenols (the sspKa of phenol is 14.3))
possibly results from some steric hindrance to solvation of the
phenoxy O- imposed by the bulky 2-phenanthrolyl group, but
this same feature ideally sets up the tight coordination environ-
ment for Cu(II). Each of 6-8 binds a Cu(II) ion sufficiently
strongly that the complexes are fully formed at all concentrations
and sspH values by in situ addition of 1 equiv each of the metal
ion and ligand. Once bound, these are not turnover catalysts,
but rather molecular complexes that cleave quickly by a
unimolecular reaction, but the principle of LGA is amply
demonstrated and can be quantified because the sspH/rate profiles
in Figures 1 and 3 show plateau regions that correspond to a
solvent-mediated decomposition of fairly well-defined species.
The ground-state forms of [Cu(II):6b]0 and [Cu(II):7b]- in
Schemes 3 and 4 are depicted having a nonbridging oxyanion
of the mono- and diester coordinated to the Cu(II). This is based
on a marked difference in the UV/vis spectrum of Cu(II):7 where
the P-O- can coordinate relative to that of Cu(II):8 where such
coordination is unlikely (see the Supporting Information) along
with computational data that show the PO- groups of the
monoester and diester are capable of coordinating to the Cu(II)
as in [Cu(II):6b]0 and [Cu(II):7b]- (see the Supporting Informa-
tion). In the case of the monoester, the phosphoryl-bound form
is computed to be more stable than the uncoordinated structure
[Cu(II):6b′]0 by 5.3 kcal/mol (see the Supporting Information).
In any event, we have no information on which of these two
kinetically equivalent forms of the mono- and diester is the
preferred reactive species. Such a distinction, while of funda-
mental interest, does not change the conclusions of the study.
Of relevance to the question of LGA is how much accelera-
tion one can expect from positioning the Cu(II) metal ion close
to the departing phenoxy oxygen of the leaving group in the
three complexes investigated here. In principle, the catalysis is
evaluated in terms of the energy of binding the Cu(II) of the
TS for each of the cleavage reactions relative to the TS for the
uncatalyzed reaction. Hyperquad 2000 analysis of the titration
data (section 3.1) indicates that the dissociation constants of
Cu(II):9 and Cu(II):9- are 1.1 × 10-8 M and 2.3 × 10-24
M
which translate into free energies of binding of 10.8 and 32.2
kcal/mol at 25 °C (see the calculation based on the Hyperquad-
determined ꢀ3 and ꢀ4 equilibrium values in the Supporting
Information). The 21.3 kcal/mol difference in these values can
be taken as the additional energy of binding attributable to a
fully bound phenoxide O-, a portion of which is realized in the
TS for cleavage of the P-O(LG) bond. Not all of this needs be
realized in a given case, but the magnitude of the stabilization
should be directly proportional to the extent of ArO · · ·P bond
cleavage as measured by the Leffler index R, which is given as
the ratio of the Brønsted ꢀlg for the P · · ·OAr cleavage reaction
relative to the ꢀeq for equilibrium transfer of phosphoryl groups
between oxyanion nucleophiles.50
(47) (a) Bone, R.; Frank, L.; Springer, J. P.; Atack, J. R. Biochemistry
1994, 33, 9468. (b) Tsubouchi, A.; Bruice, T. C. J. Am. Chem. Soc.
1994, 116, 11614. (c) Tsubouchi, A.; Bruice, T. C. J. Am. Chem. Soc.
1995, 117, 7399. (d) Tibbits, T. T.; Xu, X.; Kantrowitz, E. R. Protein
Sci. 1994, 3, 2005. (e) Rishavy, M. A.; Hengge, A. C.; Cleland, W. W.
Bioorg. Chem. 2000, 28, 283. (f) Dempcy, R. O.; Bruice, T. C. J. Am.
Chem. Soc. 1994, 116, 4511. (g) Bruice, T. C.; Tsubouchi, A.; Dempcy,
R. O.; Olson, L. P. J. Am. Chem. Soc. 1996, 118, 9867. (h) Souza,
B. S.; Branda˜o, T. A. S.; Orth, E. S.; Roma, A. C.; Longo, R. L.;
Bunton, C. A.; Nome, F. J. Org. Chem. 2009, 74, 1042.
(48) While the Yb3+- and 1-promoted cleavage of methyl o-carbomethoxyar-
yl phosphate diesters13c,27 2b,c and La3+-promoted cleavage of
triesters30 3b can exhibit turnover, binding of the lanthanide-metal
ions to the departing phenoxy group is very strong so that product
inhibition occurs.
(49) Tsang, W.-Y.; Edwards, D. R.; Melnychuk, S. A.; Liu, C. T.; Neverov,
A. A.; Williams, N. H.; Brown, R. S. J. Am. Chem. Soc. 2009, 131,
4158.
(50) (a) Thea, S.; Williams, A. Chem. Soc. ReV. 1986, 15, 125. (b) Williams,
A. Acc. Chem. Res. 1984, 17, 425. (c) Williams, A. Concerted Organic
and Bio-organic Mechanisms; CRC Press: Boca Raton, FL, 2000; pp
161-181.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3569

A R T I C L E S
Liu et al.
A compendium of the various kinetic rate constants for
cleavage of the three complexes is presented in Table 2 to allow
easy comparison. Below we deal with Cu(II)-promoted cleavage
of esters 6-8 in the context of extant relevant data for the
background solvolytic cleavages of mono-, di-, and triesters,
particularly the solvent-mediated cleavages, when such data are
available.
of 16.16 for the 2′-(2-phenoxy)-1,10-phenanthroline leaving
group we estimate (as in the Results) that the spontaneous
decomposition of the dianion of 6 should have a rate constant
between 1.1 × 10-13 and 1.5 × 10-12 s-1 at 50 °C. Comparison
of this rate constant with the 200 s-1 rate constant for the
decomposition of Cu(II):6 computed at 50 °C indicates that the
Cu(II)-induced LGA is worth about 1.3 × 1014 to 1.8 × 1015
times or ∼19.3-20.8 kcal/mol in free energy terms.53 Although
we do not have reliable activation parameters for the cleavage
of 6 in methanol, this LGA should be somewhat higher if the
comparison is made at 25 °C. All these numbers must be
regarded as estimates given the uncertainties in the rates of the
background reactions due to the long extrapolations required,
but this will not change the following analysis substantially.
Using the above free energy data as limits for the rate
acceleration for the Cu(II)-catalyzed vs uncatalyzed reactions,
and the 21.3 kcal/mol for the differential binding of the
phenolate leaving group to Cu(II) relative to its binding to the
2′-(2-phenoxy)-1,10-phenanthroline unit in the starting material,
one can compute a Leffler index of greater than 90% P · · ·OAr
cleavage in the TS. Interestingly, the Leffler index for uncata-
lyzed cleavage of phosphate monoesters in water is reported to
be42 -1.23/-1.37 ) 90%, suggesting that the strong leaving
group catalysis we see here does not alter the amount of cleavage
of the departing group to any great extent.
4.1. Monoester. Kirby and Varvoglis42 showed that phosphate
monoesters with good LGs (e.g., 2,4-dinitrophenol, pKa ) 4.07)
react faster through the dianionic form than the monoanion via
a “dissociative-like” transition state with the P-O(LG) bond
being almost completely broken (ꢀlg ) -1.23).42 However, as
the LG becomes poorer, the monoanion reacts faster due to a
rate-enhancing protonation of the departing oxygen, either
through the formation of a zwitterionic (-O)2P-O(H+)-LG
species prior to the rate-limiting chemical step or through an
intramolecular proton transfer process with an intervening water
molecule. Because of the reduced charge on the now-protonated
phenoxy oxygen in the TS, the latter reaction is less sensitive
to the nature of the leaving group (ꢀlg ) -0.27).42 Subsequently,
Hengge and Hoff determined that the solvolytic cleavage of
4-nitrophenyl phosphate dianion in tert-butyl alcohol is about
2000 times faster than that of the monoanion in tert-butyl
alcohol.43 They attributed the increased difficulty in the proton
transfer process for the monoanion to a higher pKa value of the
phosphate group in tert-butyl alcohol coupled with a less
favorable four-membered transition state for the intramolecular
proton transfer process in alcohol, contrasting the more favorable
six-membered arrangement in water.
4.2. Diester. Kirby and Younas54 showed that the aqueous
cleavage of phosphate diesters at 100 °C is an exceedingly slow
process that is highly dependent on the LG basicity. The reaction
with a reasonably good leaving group, 4-nitrophenoxy, involves
hydroxide attack on the monoanion down to pH ≈ 5 with a
short, but discernible, plateau region indicative of a water
reaction, flanked by a hydronium ion-catalyzed process. The
water reaction was observable with leaving groups as good as,
or better than, 4-nitrophenoxy but not with poorer ones such as
phenoxy or 4-methoxyphenoxy. The methanolytic cleavage of
aryl methyl phosphate diesters at 25 °C has been reported,13c
but the only observed reaction was methoxide attack on the
monoanion under the reported conditions. On the basis of the
In the present work in methanol we find that 6, when
coordinated to Cu(II) as [Cu(II):6]0, reacts rapidly (∼15 s-1) in
the plateau region of the sspH/rate profile given in Figure 1. The
plateau stems from spontaneous, solvent-mediated decomposi-
tion of dianionic phosphate complexed to Cu2+ as [Cu(II):6]0
where the nonbridging phosphoryl oxygens (one perhaps
coordinated to the Cu(II)) partake in pushing the Cu(II)-
complexed 2′-(2-phenoxy)-1,10-phenanthroline LG away from
an emerging metaphosphate-like unit with little nucleophilic
assistance from solvent methanol. This is consistent with the
∆Sq of 18 ( 2 cal/(mol ·K) and a solvent kinetic isotope effect
of unity, indicating little change in the participating solvent
CH3O-L force constant during the solvent capture of the
sspKa of 16.16 for 2-(2′-phenanthrolyl)phenol (9), the predicted
-OMe
k2
for methoxide attack on diester 7 is 8.5 × 10-10 M-1
s-1 assuming that this leaving group falls on the Brønsted
relationship described for methoxide attack on methyl aryl
phosphate diesters.13c,55
emerging PO3- unit. On the other hand, the Figure 1 data show
s
The long plateau between sspH 3 and sspH 10 given in Figure
1 is attributable to the spontaneous cleavage of [Cu(II):7b]+
(see Scheme 4) having a single nonbridging phosphate oxyanion.
The data given in Table 1 for this process indicate a lower ∆Sq
(2.3 ( 1.1) than is the case for decomposition of [Cu(II):6b]0
as well as a solvent kinetic isotope effect of 1.0, indicating little
nucleophilic participation of the solvent in the rate-limiting step.
a plateau at pH < 2.5 where the monoprotonated complex of
s
the monoester [Cu(II):6a]+ reacts with a pH-independent rate
s
s
constant similar to that of the diester [Cu(II):7b]+. The data
show that the extra phosphoryl oxyanion created by deproto-
nation enhances the spontaneous decomposition of [Cu(II):6]0
by ∼2500-fold.
In methanol, which is structurally more similar to water than
tert-butyl alcohol and has an intermediate dielectric constant
(εr ) 80.2, 32.7, and 12.5 for water, methanol, and tert-butyl
alcohol, respectively),51 we find that the decompositions of
4-nitrophenyl phosphate mono- and dianions at 50 °C have
approximately equal rate constants of 1.4 × 10-7 s-1 (see the
Supporting Information).52 From this rate constant and the sspKa
This process has a rate constant of (2.5 ( 0.1) × 10-3 s-1 (t1/2
s
) 4.6 min) and at the neutral pH of 8.38 is about 7 × 1014
s
(53) Computed as 130 > Gqcat ) -RT ln(kcat/kbackground) from the Eyring
equation at 298 K (kcat ) 15 s-1 and kbackground ) 1.1 × 10-13 to 1.5
× 10-12 s-1).
(54) Kirby, A. J.; Younas, M. J. Chem. Soc. B 1970, 510.
(55) Given that the reactions of phosphate diesters are exceedingly slow,
the only estimate that we have for the reaction of 7 at neutral pH is
that obtained by the presumed fit to the Brønsted relationship, but
this is probably too low an estimate on the basis of the data we have
(51) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R.
Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman
Scientific & Technical with John Wiley & Sons, Inc.: Harlow, U.K.,
New York, 1989; pp 1442-1443.
(52) While 4-nitrophenyl phosphate monoanion and dianion react at 39
°C with rate constants of 1.07 ×10-6 and 1.55 × 10-8 s-1, respectively,
the reported rate constants for 2-chloro-4-nitrophenyl phosphate at 39
°C are 2.43 × 10-7 and 2.26 × 10-7 s-1, respectively.42
with methoxide attack on dimethyl aryl triesters at 25 °C where the
-OMe
Brønsted relationship is log k2
) (-0.51 ( 0.04)sspKaLG + (4.68
( 0.56).30a The predicted rate constant for methoxide attack of 8 is
2.7 × 10-4 M-1 s-1, which is 10 times less than the experimental rate
constant of 2.94 × 10-3 M-1 s-1 determined here.
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3570 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
faster than the estimated methoxide reaction for cleavage of 7.
Evaluation of the energetics of the Cu(II)-induced LGA is
complicated by the fact that the catalyzed reaction appears to
proceed by a solvent-mediated mechanism while such a process
is probably not present for cleavage of uncomplexed 7 given
s
the high pKa for the leaving group.
s
4.3. Triester. Kirby and Khan56 have reported that the
cleavage of dialkyl aryl phosphate triesters (actually 2-(aryloxy)-
2-oxo-1,3,2-dioxaphosphorinanes) having good leaving groups
in water at 39 °C is subject to HO--, H3O+-, and solvent-
promoted reactions. For the least reactive esters (such as the
4-nitrophenyl derivative) the base and acid wings account for
the hydrolysis over almost all the pH range except for a small
deviation at ∼pH 4 that may result from a water reaction. We
reported that the methoxide-promoted reactions of a series of
dimethyl aryl phosphates at 25 °C exhibit a Brønsted ꢀlg value
of -0.51 ( 0.0430a and have no evidence for spontaneous
-OMe
reactions of triesters in methanol. The k2
for the cleavage
of 8 determined here ((2.94 ( 0.09) × 10-3 M-1 s-1) gives a
computed pseudo-first-order rate constant for the background
reaction of 1.2 × 10-11 s-1 at neutral pH 8.38 and 5 × 10-15
s
s
s-1 at sspH 5 assuming the methoxide reaction dominates at both
sspH values. Comparing these with the rate constants obtained
from the Figure 3 data for the decomposition of [Cu(II):8a]2+
and [Cu(II):8a]+ in the two plateau regions indicates that Cu(II)
complexation gives accelerations of 1 × 105 at sspH 8.38 and 4
× 109 at sspH 5. The calculated acceleration at lower sspH is larger
than at neutrality mainly because the background base-promoted
reaction decreases linearly with [-OCH3]. The data rule out an
active role for the Cu(II)-coordinated methoxide as a nucleophile
or general base since this would imply that the methoxide form
should react faster than the one without, contrary to what is
observed. The favored reaction at sspH < 5 involves spontaneous
decomposition of [Cu(II):8a]2+, which reacts about 20 times
faster than [Cu(II):8b]+ due to the greater Lewis acidity of the
Cu(II) in the former. The decomposition of [Cu(II):8b]+ must
involve a larger participation of methanol as a nucleophile than
with the diester and monoester, possibly with the assistance of
a second methanol to aid in displacing the Cu(II)-coordinated
LG as judged by the solvent kinetic isotope effect of 2.2 and
the negative ∆Sq of -7.4 ( 1.7, consistent with some restriction
of the degrees of freedom of the reacting species.
4.4. General Considerations Based on Activation Parameters
and Solvent Kinetic Isotope Effects. The data in Table 1 suggest
that the major differences in the activation free energies of the
decomposition of [Cu(II):6]0, [Cu(II):7]+, and [Cu(II):8]2+ stem
not from enthalpy differences, but from entropy differences
which can be analyzed in terms of the tightness or looseness of
the transition states for the cleavage processes. In Figure 5 is a
More O’Ferrall-Jencks diagram for the solvolytic cleavage of
the mono-, di-, and triesters. For simple, noncatalyzed solvolysis
reactions, the reaction coordinate profiles for the three sets of
esters follow the lower curve (monoester), central diagonal
(diester), and upper curve (triester), with the TSs being marked
with “q”.2 Binding of Cu(II) (designated in red) to the O(LG)
component of the lower left and upper left corners should
stabilize these corners by similar amounts, assumed for all esters
to be on the order of 8.9 kcal/mol from the dissociation constant
of Cu(II):8 of 3 × 10-7 M. However, Cu(II) binding to the
-O(LG) (9-) is determined by potentiometric titration to be very
much stronger as indicated by Kd ) 10-23.64 M. Thus, the entire
Figure 5. Simplified More O’Ferrall-Jencks diagram illustrating the energy
surface for three general phosphoryl transfer reactions where the decomposi-
tion of a phosphate monoester is believed to have a loose transition state
which becomes an increasingly “tighter” TS for phosphate diesters and
triesters.2 The three TSs for the uncatalyzed reactions of tri-, di-, and
monoesters are shown as black q symbols. Binding of the (red) Cu(II) to
substrates 6-8 moves each TS toward the starting material in the Hammond
sense and toward the metaphosphate-like corner in the anti-Hammond sense,
leading to a new TS (red q symbols) for each of the catalyzed reactions
with a similar amount of P-O(LG) cleavage as in the uncatalyzed reaction,
but significantly less CH3O-P bond development.
right-hand side of the diagram is pulled down by ∼32.2 kcal/
mol. The net effect of Cu(II) binding to all the corner species
is to shift the TS positions (red arrows in Figure 5) toward the
starting material along the reaction coordinate in the Hammond
sense, and in the anti-Hammond sense toward the dissociative
metaphosphate-like corner. Thus, the predicted three new
transition states (indicated with “q” in Figure 5) have little
perturbation of the extent of P-O(LG) cleavage, but have
significantly less nucleophilic CH3(H)O· · ·P bond development.
4.4.1. Monoester. Activation entropies of 3.5 cal/(mol·K)40
and 24.5 cal/(mol ·K)43 are reported for the solvolysis of
4-nitrophenyl phosphate dianion in water and tert-butyl alcohol.
Considerable evidence2 indicates that PO3 transfer from
-
phosphate monoesters involves concerted processes in water
with a small nucleophilic participation of solvent through a loose
TS situated in the lower right corner of the diagram in Figure
5. In the weaker nucleophilic solvent, tert-butyl alcohol, the
∆Sq and observed production of racemic tert-butyl phosphate
from chiral [16O,17O,18O]-p-nitrophenyl phosphate supports a
two-step mechanism with a rate-limiting dissociation to form a
freely diffusing metaphosphate intermediate which is subse-
quently captured by solvent. The large positive ∆Sq of 18 cal/
(mol·K) for [Cu(II):6]0 obtained at sspH 3.8 is consistent with a
considerably looser TS than is generally the case for decomposi-
tion of phosphate monoester dianions in water and can be
compared with the 14.1 cal/(mol·K) that is seen for cleavage
of 8-(dimethylammonium)naphth-1-yl phosphate which involves
strong intramolecular H-bonding in the starting material fol-
lowed by general-acid assistance to the leaving group depar-
ture.18 The Cu(II)-mediated LGA moves the TS for decompo-
(56) Khan, S. A.; Kirby, A. J. J. Chem. Soc. B 1970, 1172.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3571

A R T I C L E S
Liu et al.
sition of the coordinated dianion further toward the bottom side
of the diagram with less participation of the nucleophilic solvent
and perhaps closer to the point where the process becomes a
stepwise one.
and a significant solvent deuterium kinetic isotope effect kH/kD
) 2.0, both data being consistent with a process where an
attacking water on P is assisted by a second water acting as a
general base. Depending on the leaving group and the nature
of the attacking oxynucleophile, there appears to be a continuum
of mechanisms from concerted displacement of the leaving
group to a stepwise reaction for cyclic six-membered phosphate
diesters.56,57 However, the bulk of the linear free energy58 and
heavy atom kinetic isotope59 evidence concerning phosphoryl
transfer from acyclic diesters to oxygen acceptors indicates that
the reactions are concerted.
4.4.2. Diester. The neutral hydrolysis of bis(2,4-dinitrophenyl)
phosphate monoanion in water has a ∆Hq ) 19 kcal/mol and
∆Sq of -25.5 cal/(mol ·K),54 which is 30 entropy units lower
than that for decomposition of the corresponding monoester
dianion. This, coupled with the fact that there is a significant
solvent deuterium isotope effect kH/kD ) 1.55 at 39 °C and 1.45
40
o
at 100 C, whereas kH/kD is unity for the monoester dianion,
is consistent with the notion that the diester reacts by a
bimolecular mechanism with some involvement of a molecule
of water (assisted by a second) in the TS.54 The decomposition
of a phosphate diester subject to intramolecular H-bonding LGA
still has a negative ∆Sq of -12.0 cal/(mol ·K) in the neutral pH
As far as we are aware, there are no solvolytic data pertaining
to acyclic phosphate triesters (or even cyclic ones with poorer
leaving groups than 4-nitrophenoxy) that support a water- or
solvent-mediated reaction, and it is likely that the high sspKa of
9 precludes triester 8 from reacting with the poorly nucleophilic
methanol solvent. Hence, a comparison between the ∆Sq and
solvent kinetic isotope data for those reactions with good leaving
groups in water and what we observe here for the decomposition
of [Cu(II):8]2+ in methanol cannot be made unless we are
allowed to use Khan and Kirby’s data56 for the 2-(aryloxy)-2-
oxo-1,3,2-dioxaphosphorinanes. The reaction of [Cu(II):8]2+ at
sspH 3.8 gives a ∆Sq of -7.4 entropy units and a solvent kinetic
isotope effect kH/kD ) 2.2. The latter effect supports a direct
nucleophilic attack by methanol possibly with general-base
assistance by a second alcohol, and the entropy term, being at
least 20 units higher than the value for the 2,4-dinitrophenyl
derivative above, suggests that there is a considerably looser
TS for the Cu(II)-assisted process probably due to decreased
formation of the developing P-OCH3 bond.
region.24 The solvent-promoted methanolysis of the Yb3+
:
phosphate diester complex 11 exhibits a 1012-fold acceleration
relative to the uncatalyzed reaction as well as a kH/kD of 1.10
( 0.15, a ∆Hq of 16.1 kcal/mol, and a ∆Sq of -15.0
cal/(mol·K). These data support a reaction (eq 7) proceeding
via a looser TS (12) having less involvement of a nucleophilic
CH3O(H) relative to the solvent reaction alone.27 The spontane-
ous decomposition of [Cu(II):7]+ has a ∆Sq of 2.3 cal/(mol ·K),
which is more positive than what has been observed to date for
the purely solvolytic and LG-assisted cleavages of phosphate
diesters and closer to what was observed for the hydrolysis of
phosphate monoester monoanions40,42 (typically -0.6 to -6.0
entropy units for the monoanions of methyl, phenyl, 4-nitro-
phenyl, and 2,4-dinitrophenyl phosphate), which are considered
to proceed through loose dissociative-like transition states
subject to LGA by protonation of the departing group. Thus,
whereas the solvolytic reactions of phosphate diesters are
generally considered2 to be concerted ones having a reaction
coordinate close to the diagonal of the diagram shown in Figure
5, the LGA brought about by the Cu(II) coordination to 7 shifts
the new, looser TS closer toward the bottom side of the figure
with little change in the extent of cleavage of the P-O(LG)
bond and less development of the CH3O· · ·P bond.
5. Conclusions
As part of a continuing effort13,27,30 to investigate the
magnitudes and mechanisms of catalysis resulting from metal
ion-promoted LGA, the reactions of the Cu(II) complexes of
substrates 6-8 were investigated. In this system, the coordinated
Cu(II) is bound tightly by the phenanthroline moiety60 and
positioned to directly interact with the bridging phenoxy oxygen
as in Scheme 1. Each complex decomposes by one or more
solvent-mediated pathways through relatively well-defined spe-
cies. The most active form of the monoester is [Cu(II):6b]0,
where both of the nonbridging phosphoryl oxygens are depro-
tonated, and is 2500 times more reactive than [Cu(II):6a]+,
where one of the nonbridging oxygens is protonated. The active
forms for the diester and triester in the neutral sspH plateau region
are [Cu(II):7b]+, [Cu(II):8a]2+, and [Cu(II):8b]+, where the
diester has a negatively charged phosphoryl oxygen and the
triester has, respectively, Cu(II):methanol coordination (sspH <
5.5) or a Cu(II)-bound methoxide (sspH > 7). The process of
transforming Cu(II):6,7,8 into Cu(II):9- releases ∼23.3 kcal/
mol (8.9-32.2 kcal/mol on the basis of the experimental binding
constants for the triester and phenoxide), and the endothermicity
of progressively greater P · · ·O(LG) bond cleavage will be offset
4.4.3. Triester. Khan and Kirby56 studied the hydrolysis of
dialkyl aryl phosphate triesters (2-(aryloxy)-2-oxo-1,3,2-diox-
aphosphorinanes) in water at 39 °C and found that those with
leaving groups better than 4-nitrophenoxy have pH/rate profiles
comprising hydroxide and hydronium ion wings with a plateau
region below pH 7, the breadth of which increases with
decreasing pKa of the leaving phenoxy group. In the case of
the 2,4-dinitrophenoxy leaving group, the spontaneous hydroly-
sis is characterized by a very low entropy of -35.6 cal/(mol ·K)
(57) Rowell, R.; Gorenstein, D. G. J. Am. Chem. Soc. 1981, 103, 5894.
(58) (a) Bourne, N.; Williams, A. J. Am. Chem. Soc. 1984, 106, 7591. (b)
Bourne, N.; Chrystiuk, E.; Davis, A. M.; Williams, A. J. Am. Chem.
Soc. 1988, 110, 1890. (c) Ba-Saif, S. A.; Davis, A. M.; Williams, A.
J. Org. Chem. 1989, 54, 5483.
(59) (a) Cassano, A. G.; Anderson, V. E.; Harris, M. E. J. Am. Chem. Soc.
2002, 124, 10964. (b) Hengge, A. C.; Tobin, A. E.; Cleland, W. W.
J. Am. Chem. Soc. 1996, 117, 5919.
(60) The stability constant reported for Cu(II) + phenanthroline a Cu(II):
phenanthroline is log K ) 9.20 in water: Sille´n, L. G., Martell, A. E.
Spec. Publ.sChem. Soc. 1964, 664-665; 1971, Suppl. No. 1, 676.
9
3572 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Prominent Cu(II)-Promoted Leaving Group Stabilization
A R T I C L E S
by a larger transition-state binding attributable to the emerging
Cu(II):-O(LG) interaction. It is interesting that the ∆Hq values
for the reactions of all three coordinated esters are essentially
the same and the differences in the reaction free energies are
dictated solely by the ∆Sq values, which span 25 entropy units,
passing from +18 to +2.4 to -7.4 cal/(mol ·K) for mono-, di-,
and triester. The data support the interpretation that Cu(II)-
promoted LGA loosens the transition states for the cleavage
reactions by reducing both the extent of solvent involvement
in forming the CH3O-P bond and restriction of the degrees of
methanols of solvation, with little change in the extent of
cleavage of the P-O(LG) bond.
There has been much interest in whether enzyme and man-
made catalysts of phosphoryl transfer employ the same or altered
transition states as the solution-based reactions,61 and evidence
consistent with the same transition states61a,b or different
transition states61,61c-f in the catalyzed as uncatalyzed reactions
exists. In the present study, the available data indicate that, for
the triester 8 and diester 7, the Cu(II)-promoted LGA brings
on a solvent-mediated reaction that replaces the lyoxide reaction:
now a weakly nucleophilic solvent (methanol) is sufficient to
displace the coordinated leaving group. This is to be expected
from extant data provided by Kirby et al. demonstrating that
the ꢀnuc for attack of oxynucleophiles on phosphate triesters
drops as the leaving group becomes better,56 and the rate
constants for water attack on phosphate diesters54 and the
dianions of monoesters42 increase as the phenoxide leaving
groups get better. In the present case the pKa of 9 drops from
16.16 to 0.49 when fully coordinated to Cu(II), which now
provides a very good leaving group capable of being displaced
by the weakly nucleophilic solvent. This is reminiscent of the
discussion and predictions put forth by Jencks and Kirby about
the nucleophilic cleavage of 4-nitrophenyl phosphate dianion
by nitrogen-containing nucleophiles where the ꢀnuc was 0.13.40
They suggested that increasing the reactivity of nucleophilic
groups in the active site of an enzyme that cleaves phosphate
monoester dianions “will not, in itself, make a large contribution
to the rate enhancement brought about by enzymatic catalysis
of reactions of phosphate dianions. More effective catalysis
might be brought about by the enzyme by enhancing the leaving
ability of the leaving group, by reduction of the activation energy
through distortion or compression of the substrate(s), or by
bringing about a change in the mechanism of the reaction,
perhaps by metal ion catalysis.”
Chin and co-workers have presented an insightful account62
detailing the magnitudes of the accelerating effects brought on
by metal ions for each mode of catalysis to reach the rates of
the enzymatic cleavages of phosphate esters and have concluded
that LG assistance giving a 106-fold acceleration would be
required to cleave natural substrates with poor leaving groups.
Previously we have demonstrated that simple systems compris-
ing a metal-containing catalyst and a medium effect brought
on by the light alcohols can provide rate accelerations of up to
1017-fold for the cleavage of phosphate triesters30b,9a and
diesters9a by capitalizing on the first three of the four main
effects by which metal ions can accelerate the cleavage of
phosphate esters. What we have shown here is that a simple
man-made system taking advantage of a close positioning of a
metal ion and the bridging phenoxy oxygen departing from
phosphate mono-, di-, and triesters as well as a medium effect
provided by methanol solvent is easily capable of providing
1012- to 1015-fold acceleration of the cleavage reactions for the
mono- and diesters and 105 to 109-fold for the triester (depending
on the comparison pH) relative to a background reaction at the
s
same pH.
s
Acknowledgment. We gratefully acknowledge the financial
assistance of the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the Canada Foundation for
Innovation (CFI). We also acknowledge stimulating discussion with
Dr. David R. Edwards of this department. C.T.L. thanks the NSERC
for a PGS-2 postgraduate scholarship, and R.S.B. thanks the Canada
Council for the Arts for a Killam Research Fellowship, 2007-2009.
This project also received support (Grant No. HDTRA1-08-1-0046)
from the Defense Threat Reduction AgencysJoint Science and
Technology Office, Basic and Supporting Sciences Division.
Supporting Information Available: Figures giving kinetic data
s
as a function of pH, text describing compound synthesis and
s
spectral data, details of Cu(II) binding studies and titration
studies with 9 and 8, kinetic studies of 4-nitrophenyl phosphate,
computational details for phosphoryl- and methanol-bound forms
of [Cu(II):6]0 and [Cu(II):7]+, and optimized computed struc-
tures. This material is available free of charge via the Internet
at http://pubs.acs.org.
(61) (a) O’Brien, P. J.; Herschlag, D. Biochemistry 2002, 41, 3207. (b)
Zalatan, J. G.; Herschlag, D. J. Am. Chem. Soc. 2006, 128, 1293. (c)
McWhirter, C.; Lund, E. A.; Tanifum, E.; Feng, G.; Sheikh, Q. I.;
Hengge, A. C.; Williams, N. H. J. Am. Chem. Soc. 2008, 130, 13673.
(d) Humphry, T.; Forconi, M.; Williams, N. H.; Hengge, A. C. J. Am.
Chem. Soc. 2004, 126, 11864. (e) Humphry, T.; Iyer, S.; Iranzo, O.;
Morrow, J. R.; Richard, J. P.; Paneth, P.; Hengge, A. C. J. Am. Chem.
Soc. 2008, 130, 17858. (f) Feng, G.; Tanifum, E. A.; Adams, H.;
Hengge, A. C.; Williams, N. H. J. Am. Chem. Soc. 2009, 131, 12771.
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