Glyoxalase I-type hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex

Article abstract of DOI:10.1021/ja0601336

The synthesis, characterization, and hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex, [(bppppa ^-)Ni]ClO₄·CH₃OH (1, bppppa^- = monoanion of N,N-bis-[(6-phenyl-2-pyridyl)methyl]-N-[(6-pivaloylamido-2- pyridyl)methyl]amine), are reported. Complex 1 was characterized by X-ray crystallography, ¹H NMR, UV-vis, FTIR, and elemental analysis. Treatment of 1 with an equimolar amount of the hemithioacetal PhC(O)CH(OH)SCD₃ in dry acetonitrile results in the production of the thioester PhCH(OH)C(O)SCD₃ in ～60% yield. This reaction is conveniently monitored via ²H NMR spectroscopy. A protonated analogue of 1, [(bppppa)Ni](ClO₄)₂ (2), is unreactive with the hemithioacetal, thus indicating the requirement of the anionic chelate ligand in 1 for hemithioacetal isomerization reactivity. Complex 1 is unreactive with the thioester product, PhCH(OH)C(OH)SCD₃, which indicates that the pK_a value for the PhCH(OH)C(O)SCD₃ proton of the thioester must be significantly higher than the pK_a value of the C-H proton of the hemithioacetal (PhC(O)CH(OH)SCD₃). Complex 1 is the first well-characterized Ni(II) coordination complex to exhibit reactivity relevant to Ni(II)-containing E. coli glyoxalase I. Treatment of NiBr₂-2H ₂O with PhC(O)CH(OH)SCD₃ in the presence of 1-methylpyrrolidine also yields thioester product, albeit the reaction is slower and involves the formation of multiple -SCD₃ labeled species, as detected by ₂H NMR spectroscopy. The results of this study provide the first insight into hemithioacetal isomerization promoted by a synthetic Ni(II) coordination complex versus a simple Ni(II) ion.

Full text of DOI:10.1021/ja0601336

Published on Web 12/07/2006
Glyoxalase I-type Hemithioacetal Isomerization Reactivity of a
Mononuclear Ni(II) Deprotonated Amide Complex
Katarzyna Rudzka,† Atta M. Arif,‡ and Lisa M. Berreau*,†
Contribution from the Department of Chemistry and Biochemistry, Utah State UniVersity,
Logan, Utah 84322-0300, and Department of Chemistry, UniVersity of Utah,
Salt Lake City, Utah 84112
Received January 8, 2006; E-mail: berreau@cc.usu.edu
Abstract: The synthesis, characterization, and hemithioacetal isomerization reactivity of a mononuclear
Ni(II) deprotonated amide complex, [(bppppa-)Ni]ClO4‚CH3OH (1, bppppa- ) monoanion of N,N-bis-[(6-
phenyl-2-pyridyl)methyl]-N-[(6-pivaloylamido-2-pyridyl)methyl]amine), are reported. Complex 1 was char-
1
acterized by X-ray crystallography, H NMR, UV-vis, FTIR, and elemental analysis. Treatment of 1 with
an equimolar amount of the hemithioacetal PhC(O)CH(OH)SCD3 in dry acetonitrile results in the production
2
of the thioester PhCH(OH)C(O)SCD3 in ∼60% yield. This reaction is conveniently monitored via H NMR
spectroscopy. A protonated analogue of 1, [(bppppa)Ni](ClO4)2 (2), is unreactive with the hemithioacetal,
thus indicating the requirement of the anionic chelate ligand in 1 for hemithioacetal isomerization reactivity.
Complex 1 is unreactive with the thioester product, PhCH(OH)C(O)SCD3, which indicates that the pKa
value for the PhCH(OH)C(O)SCD3 proton of the thioester must be significantly higher than the pKa value
of the C-H proton of the hemithioacetal (PhC(O)CH(OH)SCD3). Complex 1 is the first well-characterized
Ni(II) coordination complex to exhibit reactivity relevant to Ni(II)-containing E. coli glyoxalase I. Treatment
of NiBr2‚2H2O with PhC(O)CH(OH)SCD3 in the presence of 1-methylpyrrolidine also yields thioester product,
albeit the reaction is slower and involves the formation of multiple -SCD3 labeled species, as detected by
2H NMR spectroscopy. The results of this study provide the first insight into hemithioacetal isomerization
promoted by a synthetic Ni(II) coordination complex versus a simple Ni(II) ion.
Scheme 1
Introduction
The glyoxalase system catalyzes the detoxification of cyto-
toxic 2-oxoaldehydes (e.g., methyl glyoxal, CH3C(O)C(O)H)
by a two-step reaction pathway.1 In the first step, glyoxalase I
(GlxI) catalyzes the isomerization of a hemithioacetal to produce
a thioester (Scheme 1). Glyoxalase II (GlxII) then catalyzes the
hydrolysis of the thioester to produce free glutathione and a
2-hydroxy acid. The GlxI enzyme from Escherichia coli contains
a mononuclear Ni(II) center within the enzyme active site.2,3
Bacterial GlxI enzymes from Y. pestis, P. aeruginosa, and N.
meningitidis also exhibit maximal activity in the presence of
Ni(II).4 Recently, the GlxI from the human parasite Leishmania
major was also found to be a Ni(II)-dependent enzyme.5 An
X-ray crystal structure of the E. coli GlxI enzyme revealed a
distorted octahedral Ni(II) center having a mixture of nitrogen
and oxygen donor ligands, [(NHis)2(OGlu)2Ni(OH2)2] (Scheme
1).6 The role of the Ni(II) center in the E. coli GlxI-mediated
hemithioacetal isomerization reaction is not yet defined.7,8 On
the basis of X-ray absorption spectroscopic studies of product-
and inhibitor-bound E. coli GlxI, a mechanistic pathway is
favored in which the role of Ni(II) ion is to generate a nickel
hydroxide moiety that can serve as a general base for the
isomerization reaction.7,8 Specifically, the Ni(II)-OH is pro-
posed to abstract a proton from the hemithioacetal (Scheme 2).
In this reaction, an anion is produced that is subsequently
protonated at the former carbonyl carbon to yield the thioester
product.9 It is unclear whether the intermediate anion in this
reaction transiently interacts with the Ni(II) center. X-ray
† Utah State University.
‡ University of Utah.
(1) Thornalley, P. J. Biochem. J. 1990, 269, 1-11.
(2) Clugston, S. L.; Barnard, J. F. J.; Kinach, R.; Miedema, D.; Ruman, R.;
Daub, E.; Honek, J. F. Biochemistry 1998, 37, 8754-8763.
(3) Clugston, S. L.; Yajima, R.; Honek, J. F. Biochem. J. 2004, 377, 309-
316.
(6) He, M. M.; Clugston, S. L.; Honek, J. F.; Matthews, B. W. Biochemistry
2000, 39, 8719-8727.
(4) Sukdeo, N.; Clugston, S. L.; Daub, E.; Honek, J. F. Biochem. J. 2004,
384, 111-117.
(5) Vickers, T. J.; Greig, N.; Fairlamb, A. H. Proc. Natl. Acad. Sci. U.S.A.
2004, 101, 13186-13191.
(7) Davidson, G.; Clugston, S. L.; Honek, J. F.; Maroney, M. J. Inorg. Chem.
2000, 39, 2962-2963.
(8) Davidson, G.; Clugston, S. L.; Honek, J. F.; Maroney, M. J. Biochemistry
2001, 40, 4569-4582.
9
17018
J. AM. CHEM. SOC. 2006, 128, 17018-17023
10.1021/ja0601336 CCC: $33.50 © 2006 American Chemical Society

Hemithioacetal Reactivity of Ni(II) Complex
A R T I C L E S
Scheme 2
Scheme 3
the results of this study provide the first insight into hemith-
ioacetal isomerization promoted by a well-characterized syn-
thetic Ni(II) complex versus simple Ni(II) ion.
Results and Discussion
Synthesis and Characterization of [(bppppa-)Ni]ClO4 (1).
Chelate ligands containing secondary amide appendages have
been shown to stabilize a variety of mononuclear metal
hydroxide complexes.12-16 With this in mind, we attempted to
prepare a new mononuclear Ni(II)-OH complex starting from
the X-ray crystallographically characterized mononuclear Ni-
(II) complex [(bppppa)Ni](ClO4)2 (2, Scheme 3).17 Treatment
of this complex with 1 equiv of Me4NOH‚5H2O in CH3CN
resulted in the formation of an orange/brown solution. Following
workup and recrystallization from CH3CN/CH3OH/Et2O,
[(bppppa-)Ni]ClO4‚CH3OH (1) was isolated in 68% yield as
orange-brown crystals. Complex 1 has been characterized by
X-ray crystallography, 1H NMR, UV-vis, FTIR, and elemental
analysis. These combined characterization methods indicate that,
instead of formation of the desired mononuclear Ni(II)-OH
complex, treatment of 2 with 1 equiv of base resulted in the
formation of a deprotonated amide complex.
absorption studies of the enzyme-product complex of E. coli
GlxI show no evidence of Ni(II)-product interactions, leading
previous researchers to favor the proton-transfer mechanism
shown in Scheme 2.
To date, only three structurally characterized mononuclear
Ni(II)-OH complexes have been reported in the literature.10
Neither these complexes nor any other mononuclear Ni(II)
complex has been shown to promote the isomerization of a
hemithioacetal in a fashion akin to that proposed for E. coli
GlxI. In 1970, a single brief report appeared in the literature in
which the use of NiBr2‚2H2O and the base 1-methylpyrrolidine
was indicated to promote the isomerization of a hemithioacetal
to produce a thioester product in DMF.11 No experimental details
or yield of the thioester product were reported for this reaction.
In the work described herein, we have explored the hemi-
thioacetal isomerization reactivity of mononuclear Ni(II) com-
plex supported by a chelate ligand containing an oxygen-
coordinated deprotonated amide ligand. This complex, which
was produced during attempts to generate a mononuclear Ni-
(II)-OH complex, promotes the isomerization of a hemithio-
acetal to produce a thioester product. Importantly, this is the
first synthetic Ni(II) complex to be reported that exhibits
glyoxalase I-type reactivity. Control studies indicate that the
presence of the deprotonated amide in the supporting chelate
ligand is required for hemithioacetal isomerization reactivity.
Additionally, we have reexamined the previously reported
hemithioacetal isomerization reaction promoted by NiBr2‚2H2O/
1-methylpyrrolidine in DMF.11 Using a deuterium-labeled
hemithioacetal and 2H NMR spectroscopy, we have found that,
while thioester formation does occur, this reaction is significantly
slower than the reaction involving the deprotonated amide
complex. These hemithioacetal isomerization reactions also
An ORTEP drawing of the cationic portion of 1 is shown in
Figure 1. Details of the X-ray data collection and refinement
are given in Table 1. Selected bond distances in 1 and its parent
17
complex [(bppppa)Ni](ClO4)2 (2) are provided in Figure 1.
Additional bond distances and angles for 1 are given in Table
2. As expected, the presence of the deprotonated amide in 1
results in a slight contraction of the C(5)-N(1) bond and
elongation of the amide C(5)-O(1) bond (Figure 1 (bottom))
relative to the distances found in the structurally similar 2
wherein a protonated amide is present. The shorter C(6)-N(1)
distance in 1 (1.373(4) Å) may be attributed to delocalization
1
of the anionic charge into the pyridyl ring. H NMR spectro-
scopic evidence for such delocalization has been previously
reported for a zinc analogue complex.18 This delocalization,
along with the presence of the Ni(II) ion, stabilizes the
deprotonated amide moiety in 1. Notably, the average Ni(1)-
NPhPy distance increases slightly in 1 (2.11 Å) relative to that
found in 2 (2.09 Å). This is an indication of a less Lewis acidic
Ni(II) center in 1. The Ni(II) center in both complexes exhibits
2
differ in the nature of species that are detectable by H NMR
spectroscopy. For the reaction involving the deprotonated amide
coordination complex, no evidence was found for Ni(II)-
coordinated species, whereas in the NiBr2‚2H2O-promoted
reaction several spectroscopically identifiable new species are
present in the reaction mixture, some of which may involve
coordination between the hemithioacetal and Ni(II). Overall,
(12) Berreau, L. M.; Mahapatra, S.; Halfen, J. A.; Young, V. G., Jr.; Tolman,
W. B. Inorg. Chem. 1996, 35, 6339-6342.
(13) Cheruzel, L. E.; Cecil, M. R.; Edison, S. E.; Mashuta, M. S.; Baldwin, M.
J.; Buchanan, R. M. Inorg. Chem. 2006, 45, 3191-3202.
(14) Borovik, A. S. Acc. Chem. Res. 2005, 38, 54-61.
(15) MacBeth, C. E.; Hammes, B. S.; Young, V. G., Jr.; Borovik, A. S. Inorg.
Chem. 2001, 40, 4733-4741.
(9) Proton-transfer reactions involving the oxygen atoms of the hemithioacetal
are also required for production of the thioester product.
(16) MacBeth, C. E.; Gupta, R.; Mitchell-Koch, K. R.; Young, V. G., Jr.;
Lushington, G. H.; Thompson, W. H.; Hendrich, M. P.; Borovik, A. S. J.
Am. Chem. Soc. 2004, 126, 2556-2567.
(10) (a) Kieber-Emmons, M. T.; Schenker, R.; Yap, G. P. A.; Brunold, T. C.;
Riordan, C. G. Angew. Chem., Int. Ed. 2004, 43, 6716-6718. (b) Ca´mpora,
J.; Palma, P.; del R´ıo, D.; Alvarez, E. Organometallics 2004, 23, 1652-
1655. (c) Ca´mpora, J.; Matas, I.; Palma, P.; Graiff, C.; Tiripicchio, A.
Organometallics 2005, 24, 2827-2830.
(17) Rudzka, K.; Arif, A. M.; Berreau, L. M. Inorg. Chem. 2005, 44, 7234-
7242.
(18) Szajna, E.; Makowska-Grzyska, M. M.; Wasden, C. C.; Arif, A. M.;
Berreau, L. M. Inorg. Chem. 2005, 44, 7595-7605.
(11) Hall, S. S.; Poet, A. Tetrahedron Lett. 1970, 2867-2868.
9
J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006 17019

A R T I C L E S
Rudzka et al.
Table 2. Selected Bond Distances (Å) and Angles (deg)a
1
Ni-O(1)
1.916(2)
1.989(3)
2.056(3)
2.148(3)
2.076(3)
Ni-N(2)
Ni-N(3)
Ni-N(4)
Ni-N(5)
O(1)-Ni-N(2)
O(1)-Ni-N(3)
N(2)-Ni-N(3)
O(1)-Ni-N(5)
N(2)-Ni-N(5)
N(3)-Ni-N(5)
O(1)-Ni-N(4)
N(2)-Ni-N(4)
N(3)-Ni-N(4)
N(5)-Ni-N(4)
89.94(10)
163.44(11)
83.72(11)
115.51(10)
110.17(11)
81.05(11)
95.63(10)
136.87(10)
78.97(10)
105.72(10)
a Estimated standard deviations indicated in parentheses.
Figure 1. Top: ORTEP drawing of the cationic portion of 1. Ellipsoids
are drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity. Bottom: Comparison of bond distances within the cationic portions
of 1 and 2.
Table 1. Summary of X-ray Data Collection and Refinementa
1
empirical formula
formula weight
crystal system
space group
a (Å)
C36H38ClN5NiO6
730.87
orthorhombic
Pb21a
13.0931(3)
13.8192(2)
18.4276(4)
90
b (Å)
c (Å)
Figure 2. A region of the 1H NMR spectra of 1 (a) and 2 (b). Both spectra
were acquired in CD3CN solution at 302 K.
R (deg)
â (deg)
90
90
γ (deg)
(ꢀ ) 320 M-1 cm-1). This feature is not present in the UV-vis
spectrum of 2.17
V (Å3)
3334.22(12)
4
1.456
Z
density (calcd), Mg m-3
temp (K)
Hemithioacetal Isomerization Reactivity of 1. Our initial
goal was to generate a new mononuclear Ni(II)-OH complex
and examine its reactivity with a hemithioacetal. Although 1
does not contain a Ni(II)-OH moiety, it does contain a Ni(II)-
bound anionic ligand in the form of a deprotonated amide. While
this functional group is not directly relevant to the chemistry
proposed for E. coli GlxI, we decided to initially continue our
studies with this complex, as to date no synthetic Ni(II)
coordination complex has been shown to promote the isomer-
ization of a hemithioacetal. Treatment of 1 with a stoichiometric
150(1)
color
brown
plate
0.33 × 0.20 × 0.08
Nonius KappaCCD
0.717
54.96
99.8%
7306
7306 [R(int) ) 0.0000]
476
0.0437/0.0926
1.102
0.460/-0.581
crystal habit
crystal size (mm)
diffractometer
abs. coeff. (mm-1
)
2θ max (deg)
completeness to θ ) 27.48°
reflections collected
indep. reflections
variable parameters
R1/wR2b
21
GOF (F2)
amount of the hemithioacetal PhC(O)CH(OH)SCD3 in dry
largest diff. (e Å-3
)
CH3CN at 302 K results in hemithioacetal isomerization over
the course of ∼1.5 h to produce the thioester PhCH(OH)C(O)-
SCD3 in ∼60% yield (Scheme 4). As shown in Figure 3, this
GlxI-type reaction can be conveniently monitored via 2H NMR
spectroscopy, where PhC(O)CH(OH)SCD3 exhibits a 2H NMR
signal at 1.88 ppm, and the product PhCH(OH)C(O)SCD3 has
a signal at 2.13 ppm when referenced to an internal C6D6
standard (7.37 ppm). A third minor resonance at 2.38 ppm has
been tentatively identified as indicating the presence of a non-
coordinated anionic hemithioacetal-derived species in solution
(vida infra). After 24 h, some unreacted hemithioacetal remains,
and the combined integrated intensity of the -SCD3 species
a Radiation used: Mo KR (λ ) 0.71073 Å). b R1 ) ∑||Fo| - |Fc||/∑|Fo|;
wR2 ) [∑[w(Fo - Fc )2]/[∑(Fo )2]]1/2, where w ) 1/[σ2(Fo ) + (aP)2 +
bP].
2
2
2
2
a geometry that is intermediate between trigonal bipyramidal
and square pyramidal (1, τ ) 0.44; 2, τ ) 0.54).19
A region of the 1H NMR spectra of 1 and 2 is shown in Figure
2. Three sharp resonances are present in the spectrum of 1 in
the range of 30-50 ppm that clearly differentiate the spectrum
of this complex from that of its protonated parent complex.
Based on studies of a series of mononuclear Ni(II) complexes
of the 6-Ph2TPA ligand, these resonances are assigned to the
â/â′-H’s of the pyridyl rings.20 Complex 1 is orange when
dissolved in acetonitrile, with an absorption feature at 440 nm
(20) Szajna, E.; Dobrowolski, P.; Fuller, A. L.; Arif, A. M.; Berreau, L. M.
Inorg. Chem. 2004, 43, 3988-3997.
(21) Russell, G. A.; Mikol, G. J. J. Am. Chem. Soc. 1966, 88, 5498-5504.
(22) In the 2H NMR spectra acquired in CH3CN, the natural abundance 2H NMR
signal for the solvent methyl group is positioned under the signal associated
with the -SCD3 group of the hemithioacetal.
(19) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J.
Chem. Soc., Dalton Trans. 1984, 1349-1356.
9
17020 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Hemithioacetal Reactivity of Ni(II) Complex
A R T I C L E S
Scheme 4
has decreased by ∼30%. This decrease appears to correlate with
the formation of a dark orange-brown precipitate in the NMR
tube. The chemical composition of this solid remains under
investigation. It is insoluble in common organic solvents, thus
precluding its characterization by solution spectroscopic meth-
ods.
Figure 3. 2H NMR spectra of a reaction mixture of 1 and PhC(O)CH-
(OH)SCD3 in dry CH3CN at various times after mixing. The first spectrum
(a), which was obtained within 5 min of mixing of the reagents, is taken as
t ) 0 min; (b) 19 min; (c) 43 min; (d) 83 min; and (e) 23 h 18 min. The
integrated intensity of the combined -SCD3 signals remains constant for
(a)-(d) relative to the integrated intensity of a C6D6 internal standard.
However, for (e) the integrated intensity of the combined -SCD3 species
is reduced by ∼30% relative to that found for (a)-(d). This appears to
correlate with the appearance of a dark orange-brown precipitate in the
reaction mixture. All spectra were recorded at 302 K. Opening of the spectral
window to a chemical shift range of ∼180 to -20 ppm did not reveal any
additional signals.
The anionic species (-SCD3, 2.38 ppm) can also be produced
in dry acetonitrile by treatment of PhC(O)CH(OH)SCD3 with
an equimolar amount of Me4NOH‚5H2O. Notably, no change
2
in the H NMR spectral features of this mixture is identifiable
after ∼18 h at room temperature, thus indicating that hemi-
thioacetal isomerization to produce thioester does not occur in
the presence of only Me4NOH‚5H2O in acetonitrile in this time
period.
2
The H NMR studies of the reaction of 1 with PhC(O)CH-
(OH)SCD3 suggest that the hemithioacetal, thioester, and the
spectroscopically observable anionic species do not directly
coordinate to the Ni(II) center in 1. Specifically, the chemical
shifts of the -SCD3 resonances of these molecules are identical
to those found for the individual molecules in the absence of
the paramagnetic Ni(II) complex. However, this is not conclu-
sive evidence and does not rule out the possibility of transient
1
interactions. H NMR spectroscopy was also used to monitor
the reaction of 1 with PhC(O)CH(OH)SCH3 in CD3CN (Figure
4). During the time period required for the isomerization reaction
at 302 K (∼1.5 h as determined by the 2H NMR studies
described above), the 1H NMR spectrum looks generally similar
to that of analytically pure 1, with only subtle broadening of
selected resonances of 1. One new broad signal is present at
∼32 ppm. This signal is not indicative of the presence of
protonated amide complex 2, as it does not have a distinct
resonance at this chemical shift (Figure 2). Opening of the
chemical shift window to examine a range of 180 to -20 ppm
did not reveal any additional new resonances. Overall, the results
of these NMR experiments suggest that the majority of the Ni-
(II) complex present in the hemithioacetal isomerization reaction
mixture is unaltered 1.
Control Reactions. Treatment of the protonated amide
complex [(bppppa)Ni](ClO4)2 (2) with PhC(O)CH(OH)SCD3
(Scheme 4) resulted in no reaction after 18 h as determined via
2H NMR spectroscopy, thus indicating the requirement of the
anionic chelate ligand in 1 for hemithioacetal isomerization
reactivity. A deprotonated form of the ligand, Na(bppppa), does
not promote hemithioacetal isomerization or deprotonation over
Figure 4. (a) A region of the 1H NMR spectrum of 1. (b) The same region
for compound 1 in the presence of an equimolar amount of PhC(O)CH-
(OH)SCD3. Both spectra were acquired in CD3CN at 302 K.
a time period of ∼40 h at 298 K, thus indicating the requirement
of the Ni(II) center for hemithioacetal isomerization reactivity.
Treatment of 1 with an equimolar amount of the thioester PhCH-
(OH)C(O)SCD3 in dry acetonitrile at 302 K resulted in no
reaction after 16 h. This suggests that the pKa value for the
PhCH(OH)C(O)SCD3 proton of the thioester product must be
significantly higher than the pKa value of the PhC(O)CH(OH)-
SCD3 proton of the hemithioacetal. Overall, the results of these
combined studies suggest that 1 may serve as a general base
for the hemithioacetal isomerization reaction. The basic site of
1 that accepts and subsequently distributes the proton may be
either the amide oxygen or the nitrogen. Because the bound
amide oxygen atom is protected by the hydrophobic phenyl
groups of the chelate ligand, as well as by the bulky tert-butyl
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A R T I C L E S
Rudzka et al.
versus that found in 1, allows for the formation of Ni(II)
complexes with hemithioacetal-derived species. Over the course
of 1.5 h, which is the time required for ∼60% yield of thioester
in the hemithioacetal isomerization reaction involving 1, only
a trace amount of thioester is produced in the NiBr2‚2H2O/1-
methylpyrrolidine reaction (Figure 5b). Similar to the reaction
involving 1, after ∼1.5 h a heavy red-brown precipitate began
to deposit in the NMR tube. At this point, the overall integrated
intensity of the combined -SCD3 species had decreased by 15%.
After 24 h at 25 °C (Figure 5c), the amounts of thioester and
hemithioacetal present in solution are similar, but the overall
integrated intensity for all -SCD3 species has declined ∼50%.
Conclusions
The glyoxalase pathway (Scheme 1) is ubiquitous in biologi-
cal systems, and the role of the metal center in the hemithioacetal
isomerization catalyzed by GlxI remains to be fully defined. In
1970, Hall and Poet demonstrated that the conversion of model
hemithioacetals to the corresponding R-hydroxy thioesters was
accelerated in the presence of a divalent metal ion (e.g., Mg-
(II)).11 Specifically, the rate of thioester formation from a
hemithioacetal was found to increase by 30-fold in a DMF
solution containing Mg(NO3)2 and sodium acetate (or tertiary
amines), versus a DMF solution containing only the base. The
role of the Mg(II) center was proposed to be stabilization of an
enediol(ate) intermediate. Similar reactivity was indicated to
occur using other divalent metal ions (including Ni(II)) and
bases, albeit no experimental details or yields were provided.11
Notably, in a later study involving an aqueous environment and
using imidazole as the base, the rate of hemithioacetal isomer-
ization was accelerated only by a factor of 1.8 in the presence
of Mg(II).24 Under these conditions, water and imidazole likely
compete as ligands for the Mg(II) center, thus limiting interac-
tions between the metal and any anionic enediol(ate) intermedi-
ate. Overall, these studies represent the entirety of what has
been previously reported in terms of metal-containing model
studies relevant to the chemistry of GlxI enzymes. The studies
reported herein, although not of direct structural relevance to
E. coli GlxI, provide interesting new insight into the chemistry
of metal ion-promoted hemithioacetal isomerization.
During attempts to prepare a mononuclear Ni(II)-OH
complex of the amide-appended bppppa ligand, we found that
a deprotonated amide complex is produced. This type of
reactivity has been recently identified in a structurally similar
amide-appended Zn(II) complex.18 Stabilization of the depro-
tonated amide moiety occurs both through amide oxygen
coordination to the divalent metal center and via delocalization
of the anionic charge into the pyridyl ring. While exploration
of the acid/base properties of 1 and related compounds will be
the subject of a future study, we hypothesize that the stabilization
factors noted above produce an amide linkage having enhanced
acidity.
Figure 5. 2H NMR spectra of a reaction mixture of NiBr2‚2H2O,
1-methylpyrrolidine, and PhC(O)CH(OH)SCD3 in dry DMF at various times
after mixing. The first spectrum (a), which was obtained within 10 min of
mixing of the reagents, is taken as t ) 0 min; (b) 1.5 h; and (c) 23 h. The
signals marked DMF are the natural abundance 2H NMR signals for protio
DMF.22
substituent, we favor a reaction pathway wherein the deproto-
nated amide nitrogen atom is involved in proton-transfer
reactivity.
If 1 is acting as a general base for the hemithioacetal
isomerization reaction, the reaction should be catalytic under
appropriate conditions. To evaluate this possibility, an NMR
tube was prepared containing 1 and PhC(O)CH(OH)SCD3 in a
1:5 molar ratio and C6D6 (internal standard). Spectroscopic
monitoring of this reaction mixture using 2H NMR spectroscopy
over the course of 66 h revealed the formation of ∼1.5 equiv
of the thioester PhCH(OH)C(O)SCD3. Overall, the spectra for
this mixture were very similar to those shown in Figure 3, albeit
a larger amount of the anionic 2.38 ppm species was present.
Similar to the stoichiometric reaction, the combined integrated
intensity of the -SCD3 species decreased by ∼37% after 66 h
and a deep orange-brown precipitate deposited in the NMR tube.
Evaluation of the Hemithioacetal Isomerization Reactivity
of NiBr2‚2H2O/1-Methylpyrrolidine in DMF Using PhC(O)-
CH(OH)SCD3 and 2H NMR Spectroscopy. There is one
previous report in the literature of hemithioacetal isomerization
promoted by NiBr2‚2H2O and 1-methylpyrrolidine in DMF.11
No experimental details or yield were reported for this reaction.
To gain insight into this reaction, we again used the 2H-labeled
hemithioacetal and 2H NMR spectroscopy. Solutions were
prepared containing an equimolar mixture of NiBr2‚2H2O, PhC-
(O)CH(OH)SCD3, and C6D6 (internal standard) in dry DMF.
Addition of 1 equiv of 1-methylpyrrolidine to this mixture
2
produced a color change from yellow-green to red-brown. H
NMR spectra collected at various times following addition of
the base are shown in Figure 5. Unlike the relatively simple
spectra found for hemithioacetal isomerization promoted by 1
(Figure 3), several broad resonances in the 2H NMR spectra of
the NiBr2‚2H2O/1-methylpyrrolidine indicate the presence of
multiple -SCD3 containing species. For example, at least three
new resonances are present in the region of 1.0-1.6 ppm.23
Another new resonance is also present at ∼2.35 ppm. We
hypothesize that the more exposed Ni(II) center in NiBr2‚2H2O,
The reactivity of 1 with a hemithioacetal provides the first
example of a well-characterized Ni(II) complex, which promotes
a hemithioacetal isomerization reaction. Because the Ni(II)
(23) Control experiments confirm that the -SCD3 derived resonances in the
region of 1.0-1.6 ppm were only produced when NiBr2‚2H2O, 1-meth-
ylpyrrolidine, and PhC(O)CH(OH)SCD3 were all present in the reaction
mixture.
(24) Hall, S. S.; Doweyko, A. M.; Jordan, F. J. Am. Chem. Soc. 1978, 100,
5934-5939.
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17022 J. AM. CHEM. SOC. VOL. 128, NO. 51, 2006

Hemithioacetal Reactivity of Ni(II) Complex
A R T I C L E S
on a larger scale, starting with ∼60 mg of [(bppppa)Ni](ClO4)2. Anal.
Calcd for C35H34N5O5ClNiCH3OH: C, 59.24; H, 5.25; N, 9.60.
Found: C, 58.79; H, 5.09; N, 9.64. UV-vis (CH3CN), nm (λmax, M-1
cm-1): 440 (320). FTIR (KBr, cm-1): 1088 (νClO ), 628 (νClO ).
Monitoring Stoichiometric Hemithioacetal Isomerization via 2H
NMR Spectroscopy. A NMR tube containing equimolar amounts of
1 and PhC(O)CH(OH)SCD3 in dry protio acetonitrile (700 µL) was
prepared under a nitrogen atmosphere. An internal standard (C6D6, 1
µL) was added, and the tube was sealed using a stopcock. The tube
was then quickly transferred to the NMR system that had been
previously optimized for data collection. Each 2H NMR spectrum was
referenced to the chemical shift of the C6D6 standard. The C6D6 signal
was also used as an internal integration standard.
complex is paramagnetic, monitoring of this reaction was
performed using a deuterium-labeled hemithioacetal and H
2
NMR spectroscopy. This novel approach is also applicable to
monitoring hemithioacetal isomerization reactions involving
simple Ni(II) salts, as was outlined for the reaction involving
NiBr2‚2H2O and 1-methylpyrrolidine. Importantly, comparison
of the reactions promoted by 1 and NiBr2‚2H2O/1-methylpyr-
rolidine provides clear evidence that the reaction involving the
coordination complex is faster and involves fewer spectroscopi-
cally identifiable species. This is likely a consequence of the
fact that, while five coordination positions in 1 are occupied
by the chelate ligand, the Ni(II) ion derived from NiBr2‚2H2O
in DMF has multiple available coordination positions for
interaction with hemithioacetal-derived species. In terms of E.
coli GlxI, the presence of four amino acid ligands to the Ni(II)
center likely limits interaction with the hemithioacetal or anionic
species derived from deprotonation of this substrate.
A problem encountered in using either 1 or NiBr2‚2H2O/1-
methylpyrrolidine is the precipitation of a red-brown solid in
the reaction mixture after several hours. Based on the integrated
intensity of the combined -SCD3 species in each reaction, this
solid contains hemithioacetal-derived species. Ongoing efforts
are focused on determining the chemical composition of this
solid and its relevance to hemithioacetal isomerization reactivity.
4
4
Na(bppppa). The bppppa ligand (27 mg, 4.9 × 10-5 mol) was
treated with NaH (1.2 mg, 5.1 × 10-5 mol) in dry THF. After the
mixture was stirred for 1.5 h at ambient temperature, the solvent was
removed under reduced pressure, and a 1H NMR spectrum of the
remaining pasty solid was obtained. 1H NMR (CD3CN, 400 MHz): δ
7.83-7.77 (br m, 7H), 7.64 (br d, J ) 7.8 Hz, 3H), 7.47-7.45 (m,
6H), 7.37-7.31 (br m, 2H), ∼6.6 (br, 1H), 3.79 (s, 4H), 3.65 (s, 2H),
1.03 (s, 9H). These resonances are shifted relative to those found for
neutral bppppa.17 Addition of water to Na(bppppa) in acetonitrile yields
1
the neutral bppppa ligand, as determined by H NMR spectroscopy.
Treatment of Na(bppppa) with PhC(O)CH(OH)SCD3 in Protio
2
Acetonitrile. This reaction was set up and monitored via H NMR
spectroscopy in a fashion identical to that employed for 1. No evidence
for hemithioacetal isomerization or deprotonation was found over a
time period of ∼40 h at 298 K.
Experimental Section
General Methods. All reagents were obtained from commercial
sources and were used as received without further purification. Solvents
were dried according to published procedures and were distilled under
N2 prior to use.25 Water-sensitive reactions were performed in an
MBraun Unilab glovebox under an atmosphere of purified N2. The
organic compounds bppppa (N,N-bis[(6-phenyl-2-pyridyl)methyl]-N-
[(6-pivaloylamido-2-pyridyl)methyl]amine),17 PhC(O)CH(OH)SCD3
(hemithioacetal),21 and PhCH(OH)C(O)SCD3 (thioester)11 were prepared
according to literature procedures.
Physical Methods. 1H and 2H NMR spectra were collected as
previously described.20 UV-vis spectra were recorded at ambient
temperature using a Hewlett-Packard 8453 diode array spectrophotom-
eter. Solid-state IR spectra were recorded using a Shimadzu FTIR-
8400 spectrometer as KBr pellets. Elemental analyses were performed
by Atlantic Microlabs of Norcross, GA.
Hemithioacetal Reactions Involving NiBr2‚2H2O and 1-Meth-
ylpyrrolidine. A solution of NiBr2‚2H2O (10 mg, 3.9 × 10-5 mol) in
DMF (700 µL) was prepared. Stirring of this solution for ∼30 min at
room temperature was required for the NiBr2‚2H2O to fully dissolve.
To this solution were added the internal standard C6D6 (1 µL), the
hemithioacetal PhC(O)CH(OH)SCD3 (7.3 mg, 3.9 × 10-5 mol), and
1-methylpyrrolidine (4.5 µL, 3.9 × 10-5 mol). The resulting red-brown
mixture was transferred to an NMR tube that was sealed using a
stopcock. This tube was transferred within 10 min to the NMR
2
instrument that had been previously optimized for data collection. H
NMR spectra were collected at timed intervals and were referenced to
the chemical shift of the C6D6 internal standard (7.37 ppm). Reactions
of this type were monitored for a minimum of 24 h at 298 K. After
approximately 40 min, the color of the solution had intensified and a
red-brown precipitate began to appear. Subtle loss of color and an
increasing amount of precipitate are apparent over the course of the
reaction. The integrated intensity of all -SCD3 labeled species in the
reaction mixture decreases by ∼50% after 24 h. As a prelude to
evaluating the results of these experiments, individual 2H NMR spectra
of the hemithioacetal PhC(O)CH(OH)SCD3 (1.97 ppm) and thioester
PhC(OH)CH(O)SCD3 (2.11 ppm) were collected in DMF.
CAUTION! Perchlorate salts of metal complexes with organic
ligands are potentially explosiVe. Only small amounts of material should
be prepared, and these should be handled with great care.26
[(bppppa-)Ni]ClO4‚CH3OH (1). [(bppppa)Ni](ClO4)2 (26 mg,
17
0.032 mmol) dissolved in CH3CN (∼2 mL) was added to a slurry of
Me4NOH‚5H2O (5.8 mg, 0.032 mmol) in CH3CN (∼2 mL). The
resulting mixture was stirred under a dry nitrogen atmosphere for 4 h.
The solvent was then removed under reduced pressure. The orange
residue was dissolved in CH2Cl2, and the solution was filtered through
a glass wool/Celite plug. The CH2Cl2 was removed under reduced
pressure. The remaining orange solid was recrystallized by Et2O
diffusion into a CH3CN/CH3OH (2:1) solution to give deep orange-
brown crystals (15 mg, 68%). This reaction may be safely performed
Acknowledgment. We thank the National Science Foundation
(CAREER Award CHE-0094066) for financial support of this
work.
Supporting Information Available: X-ray crystallographic
file for 1 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(25) Armarego, W. L. F; Perrin, D. D. Purification of Laboratory Chemicals,
4th ed.; Butterworth-Heinemann: Boston, MA, 1996.
(26) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.
JA0601336
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