Cupric superoxo-mediated intermolecular C-H activation chemistry

Article abstract of DOI:10.1021/ja110466q

The new cupric superoxo complex [LCu^II(O₂ ^-)]⁺, which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH).

Full text of DOI:10.1021/ja110466q

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pubs.acs.org/JACS
Cupric Superoxo-Mediated Intermolecular C-H Activation Chemistry
†
,‡
†
§
§
||
†
Ryan L. Peterson, Richard A. Himes, Hiroaki Kotani, Tomoyoshi Suenobu, Li Tian, Maxime A. Siegler,
,
||
,‡,§
,†,‡
Edward I. Solomon,* Shunichi Fukuzumi,* and Kenneth D. Karlin*
†
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
‡
§
Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S Supporting Information
b
II
ABSTRACT: The new cupric superoxo complex [LCu -
•
-
þ
(
O2 )] , which possesses particularly strong O-O and
Cu-O bonding, is capable of intermolecular C-H activa-
tion of the NADH analogue 1-benzyl-1,4-dihydronicotina-
mide (BNAH). Kinetic studies indicated a first-order
dependence on both the Cu complex and BNAH with a
deuterium kinetic isotope effect (KIE) of 12.1, similar to
that observed for certain copper monooxygenases.
Figure 1. (left) Representation of the cationic portion of the X-ray
I þ
-
structure of [LCu ] (B(C
6 5 4 4
F ) ), revealing the N O(amide) coordina-
tion (Cu-O = 2.190 Å). (right) Calculated structure (see the Support-
II
•-
þ
opper(I) reactions with molecular oxygen play fundamental
roles in many chemical and biological processes. Copper-
ing Information) of [LCu (O )] (1), indicating the H-bonding
2
1,2
interaction between the ligand and the superoxo β-oxygen atom. See the
C
text for further discussion.
dependent proteins perform a diverse array of oxidative and
oxygenative reactions. This has inspired considerable efforts in
the design of novel ligands and copper-coordinated complexes as
well as the study of ligand-copper(I) dioxygen adducts to eluci-
date their structures, electronic characteristics, and substrate
superoxo complexes have been shown to exhibit phenol O-H
1
0a,12
bond cleavage reactions,
and in one case, Itoh and co-
13
II
•-
workers provided evidence for a Cu (O2 )-mediated intra-
molecular benzylic C-H oxygenation. Here, for the first time, an
intermolecular C-H substrate oxidation reaction has been
achieved, with kinetic data clearly implicating the involvement
2
-4
reactivities.
derived species, mononuclear analogues have been synthetically
In comparison with binuclear copper dioxygen-
3,5
II
•-
challenging and hence are less well understood. However, they
are fundamentally important and directly relevant to copper pro-
teins, including dopamine-β-monooxygenase (DβM) and pepti-
dylglycine-R-hydroxylating monooxygenase (PHM). These en-
zymes possess a so-called noncoupled binuclear active site,
of the Cu (O2 ) complex in rate-limiting substrate C-H bond
cleavage.
I
I þ
-
14
The Cu complex [LCu ] [as the B(C F ) salt ] of a
ligand L previously employed by Masuda and co-workers,
namely, [bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]meth-
6
5 4
6
14a
7
which comprises two Cu centers separated by ∼11 Å. Dioxygen
yl}amine], was exposed to O (by bubbling via a syringe needle)
2
binding and substrate hydroxylation occur at one of the copper
in 2-methyltetrahydrofuran (MeTHF) solvent at -125 °C to
II
•-
þ
sites (designated Cu ). In an important PHM X-ray structure, a
form the adduct [LCu (O )] (1) (Figure 1), which exhibited
2
1
M
-
-1
dioxygen-derived species presumed to be an end-on bound
UV-vis absorptions [λmax = 410 nm (3700 M cm ), 585 nm
-1 -1 -1 -1 15
II
•-
cupric superoxide species (i.e., Cu -O-O ) resides adjacent
(900 M cm ), 741 nm (1150 M cm )] characteristic of
6
c
6a,6b,8
II
to an inhibitory substrate analogue. Along with biochemical,
a mononuclear end-bound Cu superoxo complex. This tem-
5,9
chemical, and computational studies, the cupric superoxo
species is thought by many to be the reactive intermediate
responsible for initiating oxidation via hydrogen-atom abstrac-
tion. However, other species have been considered as important
intermediates in enzymatic turnover, either prior to or following
substrate attack, including cupric hydroperoxo (Cu - OOH)
and high-valent cupryl (Cu -O T Cu dO) entities.
perature well below -80 °C was necessary in order to observe
this 1:1 Cu/O adduct. Under these conditions, this green, EPR-
2
silent species was quite stable, decaying only very slowly (half-
II
2- 2þ
2
life > 4 h) with conversion to [{(LCu )} (O )] (2), a μ-1,2-
2
-
1
-1
peroxodicopper(II) complex [λ = 541 nm (9900 M cm )]
that is observed when [LCu ] is oxygenated at -80 °C.
A resonance Raman (rR) spectrum (77 K, excitation at
max
I þ
14b
II
-
10
II
•
III
4b,9c,11
4
13 nm) of 1 in frozen MeTHF solution is shown in Figure 2.
In our own research program, we seek to elucidate the
chemical nature of all of these mononuclear species. In this
report, we describe the generation and characterization of a new
-
1
The O-O stretch was observed at 1130 cm as a single peak
II
•-
Cu (O ) species and an example of substrate C-H activation
Received: November 21, 2010
Published: January 25, 2011
2
(
i.e., oxidative C-H bond cleavage). To this point, cupric
r 2011 American Chemical Society
1702
dx.doi.org/10.1021/ja110466q
|J. Am. Chem. Soc. 2011, 133, 1702–1705

Journal of the American Chemical Society
COMMUNICATION
a ν(O-O) of 1120 cm- and a ν(Cu-O) of 472 cm
1
-1 12
; thus,
-
1
ν(O-O) and ν(Cu-O) are both ∼10 cm higher in 1.
II
•-
þ
II
Optimized geometries of [Cu TMPA(O )] and [Cu -
2
•-
þ
(
NMe -TMPA)(O2 )] were also obtained through DFT
2
calculations and compared to that of 1. These calculations gave
an increase in both ν(Cu-O) and ν(O-O) when H-bonding,
particularly to the β oxygen of the superoxo ligand, was included
15,19
in 1.
The calculated structure showed a decrease of 0.005 Å in
the Cu-O bond length, indicating that a slightly stronger Cu-O
bond is associated with the higher ν(Cu-O). From the DFT
calculation of the structure with Cu-N bond lengths constrained
at the values of the optimized structure of 1 but with no
pivalamido group, this appears to reflect a distortion of the
Nequatorial ligand system that decreases its donor interaction with
the Cu. The donor interaction of the superoxo with the Cu thus
increases, leading to a stronger Cu-O bond. Alternatively, the
calculations showed that the O-O bond length actually in-
creases by 0.005 Å in 1, indicating that the increase in ν(O-O)
Figure 2. Solvent-subtracted rR spectra of MeTHF solutions of 1 (λex
=
16
4
13 nm): (A) ν(Cu-O) region; (B) ν(O-O) region. Red,
2
O
2
; blue,
1
8
O .
1
6
when the complex was formed with O (Figure 2B, red), but
2
1
8
II
•-
þ
upon O isotopic substitution, two features were observed
relative to [Cu TMPA(O )] does not reflect a stronger
2
2
(
Figure 2B, blue). This behavior is consistent with a Fermi
O-O bond but derives from structural coupling of vibrations
within the ligand system due to the H-bond. This H-bonding
stabilizes the superoxo species (relative to the binuclear peroxo
species) and allows its reactivity to be studied.
18
resonance between the O vibration and a nonenhanced mode
at similar energy. From the energy and intensity of the two
observed mixed-modal features, the preinteraction energy of
the resonantly enhanced O- O stretch was calculated to be
1
by 63 cm upon O substitution, consistent with a bound
superoxo species. For the lower-energy region, both the O2
and O data showed two peaks with an intensity distribution
that changed with isotope. Therefore, these are both mixed as a
2
18
18
II
•-
þ
[LCu (O2 )] (1) is unreactive toward a number of com-
monly employed C-H substrates, such as dihydroanthracene,
xanthene, and 10-methyl-9,10-dihydroacridine, which are sub-
strates possessing C-H bonds that are significantly weaker than
those found for the DβM and PHM substrates (dopamine,
-
1
067 cm . Thus, the O-O stretch was shifted to lower energy
-1 18
2
1
6
16
1
8
2
6b
85 kcal/mol; hippuric acid, 87 kcal/mol). However, the addi-
15
result of Fermi resonance. Analysis of the lower-energy region
tion of an excess of 1-benzyl-1,4-dihydronicotinamide (BNAH),
1
6
-1
•
(
Figure 2A) showed a Cu- O stretch at 482 cm that was
an NADH analogue that is both a strong H atom (H ) and
-
1
18 17
-
19
II
•-
þ
shifted to 462 cm with O.
hydride (H ) donor, to solutions of [LCu (O2 )] leads to
the decay of the latter, as observed by UV-vis spectroscopy
(Figure 3a). Kinetic interrogation of this reaction (-125 °C)
showed pseudo-first-order decay behavior with respect to 1
(for 10-40 equiv of BNAH). The decay was also first-order in
[BNAH]. Thus, the cupric superoxo complex is responsible for
Thus, the rR spectroscopic data confirmed the formulation of
II
•-
þ
[
LCu (O )] as an end-on superoxo-containing complex
2
1
-
with O-O and Cu-O stretches of 1130 and 482 cm
,
1
5
respectively. Notably, these values are higher than those
found for all cupric superoxo complexes previously described:
ν(O-O) for the one structurally defined side-on bound cupric
promoting the substrate oxidation (see below). A second-order
-
1
-1 -1
superoxo complex is 1043 cm , while those for the end-on
bound species range up to 1122 cm ; ν(Cu-O) varies from
4
Cu-O and O-O vibrational frequencies of 1, a related structure
having the pivalamido group at the para position instead of the
ortho position, and a structure with Cu-N bonds constrained
but no pivalamido group. The comparison between para and
ortho substitution eliminated an inductive effect from the
pivalamido group as the origin of the higher frequencies. Con-
rate constant of 0.19 M
s
was obtained (Figure 3b); when
-
1
0
the substrate was deuterated in the 4 and 4 positions, i.e., when
22 to 474 cm-
1 15
.
Using DFT calculations, we evaluated the
1-benzyl-1,4-dihydro[4,4 - H ]nicotinamide (BNAD) was used,
0
2
2
-
1 -1
a significant slowing of the reaction occurred (k = 0.016 M
s ;
Figure 3b). This gives a kinetic isotope effect (KIE) of 12.1. This
KIE value is comparable to the KIE of 10 reported for C-H bond
15
20
cleavage of BNAH by a trans-dioxomanganese(V) porphyrin.
II
•-
þ
Product analysis of the [LCu (O2 )] /BNAH reaction (fol-
15
lowing quenching with HCl at -130 °C) confirmed that
-
1
0
sistent with the ν(O-O) of 1130 cm , 1 was calculated to be an
end-on bound Cu(II) superoxo species with a triplet ground state
(
BNAH underwent oxidation by 1. The substrate’s 4 C-H bond
was oxidatively cleaved to form 1-benzylnicotinamidium ion
-
1 15
þ
1
the singlet/triplet splitting was calculated to be 1581 cm
,
(BNA ) in 42% yield ( H NMR) based on the initial copper
concentration (Scheme 1). Additionally, upon acidification,
liberated hydrogen peroxide was also detected in ∼30% yield,
approximately corresponding to the amount of the peroxo-
after correction for spin contamination). For L and its close ana-
14a
logues, Masuda and co-workers found that the pivalamidopyr-
idyl substituent forms intramolecular H-bonds to the R oxygen
atoms of a peroxide and/or hydroperoxide moiety ligated to the
II
2- 2þ
2
dicopper(II) complex [{(LCu )} (O )] (2), which was
2
1
4a,18
copper(II) ion
or to the R nitrogen atom of an azide
also a reaction product as identified by its characteristic UV-vis
absorption bands (see above).
14a
15
coordinated to copper(II). However, our calculations sug-
II
•-
þ
gested that the superoxo moiety in [LCu (O2 )] (1) forms
an intramolecular H-bond with either the R or β oxygen. This
would contribute to the relative stability of 1 and can account
for the higher O-O and Cu-O frequencies. rR data on
the analogues [Cu (TMPA)(O )] [TMPA = tris(2-
pyridylmethyl)amine] and [Cu (NMe -TMPA)(O )] show
Cupric superoxo-promoted cleavage of the C-H bond likely
follows one of two possible pathways (Scheme 1). One is initial
H-atom transfer (HAT), in which the C-H bond is cleaved
homolytically (with the thus-formed BNA radical rapidly losing a
second electron); the other involves hydride transfer resulting
from heterolytic cleavage. To provide further mechanistic insight
II
•-
þ
2
II
•-
þ
2
2
1
703
dx.doi.org/10.1021/ja110466q |J. Am. Chem. Soc. 2011, 133, 1702–1705

Journal of the American Chemical Society
COMMUNICATION
Scheme 1
15
steps as there are researchers in the field. The steps to products
following the transition state are also difficult to surmise in our
25
chemical systems. Despite these uncertainties, the importance
of the present studies lies in the observation of intermolecular
C-H activation by a cupric superoxo complex.
To summarize, a new ligand-supported cupric superoxo com-
plex with reactivity behavior of relevance to the DβM and PHM
enzymes has been described. Significant advances include the
following: (a) [LCu (O2 )] (1) is the first cupric superoxo
complex that incorporates a hydrogen-bonding ligand feature.
Figure 3. (a) Spectral changes of 0.4 mM [LCu (O
2
•-)]þ in the
II
II
•-
þ
presence of 8 mM BNAH at -125 °C in MeTHF. The first spectrum
recorded (green) is that immediately following bubbling of O through a
solution containing [LCu ] and BNAH. Inset: pseudo-first-order
kinetics fit of the 741 nm data. (b) Plots of kobs as a function of BNAH,
BNAD, or BzIMH concentration, used to determine the second-order
rate constants.
2
I þ
(
b) This results in a compound with stronger Cu-O and O-O
bonds than in previously known examples. (c) Not necessarily
related to the latter properties, the reactions of 1 with BNAH and
BzImH provide the first examples of intermolecular C-H bond
activation by a cupric superoxo complex. Notably, it is not
unexpected that relatively weak C-H bonds are cleaved for an
intermolecular situation; one does not have the advantage of a
proximate substrate, as found for the single known example of
into the mode of C-H activation by 1, a second substrate, 1,3-
21
dimethyl-2,3-dihydrobenzimidazole (BzImH), was studied for
reactivity. Like BNAH, BzImH possesses a weak C-H bond but
has markedly different bond strengths than BNAH (homolytic,
bond dissociation energy = 73.4 kcal/mol vs 70.7 kcal/mol for
BNAH; heterolytic, hydride affinity = 49.5 kcal/mol vs 64.2 kcal/
II
•-
13
intramolecular Cu (O2 )-mediated C-H oxidation. (d) Fi-
nally, the relative rates for C-H oxidation of these two substrates
support rate-limiting homolytic C-H bond activation. Further
corroborating studies with other C-H mechanistic probes will
be investigated. The clean kinetic behavior, striking deuterium
KIE, and determination of rate-limiting HAT demonstrate that
2
1
mol for BNAH). Kinetic studies revealed that the oxidation of
BzImH occurs ∼2.4 times slower than that of BNAH, with a
-
1 -1
second-order rate constant of 0.078 M
s
(Figure 3b). The
-
lower rate of C-H oxidation of BzImH (stronger H donor)
than of BNAH (stronger H donor) thus suggests that at least for
these substrates, the preferred mode of C-H activation by
LCu (O )] is via rate-limiting homolytic C-H bond
II
•-
þ
•
the reaction of [LCu (O
2
)] (1) with C-H substrates may
possess biological significance in direct comparison to the reac-
tion mechanism of the DβM and PHM enzymes.
II
•-
þ
[
2
cleavage, i.e, HAT.
’
ASSOCIATED CONTENT
On the basis of extensive studies of the enzymes PHM and
DβM, the most accepted mechanistic proposal is that these
S
Supporting Information. Experimental procedures, spec-
6b,22
b
enzymes initially proceed by (nonclassical)
promoted by a cupric superoxo complex generated at the Cu
HAT chemistry
tra, DFT calculations, explanations and supporting diagrams, and
crystallographicdata (CIF). Thismaterialisavailablefreeof charge
via the Internet at http://pubs.acs.org.
M
8
a,9a
site.
Interestingly, enzyme kinetic studies carried out on
6,23
DβM and PHM revealed KIEs of 10-14.
(depending on
the conditions), which are very similar to that found here for our
system. The studies reported here with a “model” cupric super-
oxide complex suggest that this mechanism is followed, at least
for the relatively weak C-H substrates investigated. In one case,
theoretical calculations inspired by results on a model system led
to the suggestion that an initial hydride abstraction may occur for
’
AUTHOR INFORMATION
Corresponding Author
edward.solomon@stanford.edu; fukuzumi@chem.eng.osaka-u.ac.jp;
karlin@jhu.edu
2
4
PHM/DβM. The present findings suggest that this is not likely
to be a favorable pathway.
’
ACKNOWLEDGMENT
Following C-H activation, the overall mechanism of sub-
strate hydroxylation by the enzymes DβM and PHM is poorly
understood, with nearly as many proposals for the subsequent
This research was supported by the National Institutes of
Health (GM28962 to K.D.K. and DK31450 to E.I.S.), WCU
Program R31-2008-000-10010-0 (to K.D.K. and S.F.), by a
1
704
dx.doi.org/10.1021/ja110466q |J. Am. Chem. Soc. 2011, 133, 1702–1705

Journal of the American Chemical Society
COMMUNICATION
Grant-in-Aid 20108010 (S.F.), and the Global COE Program
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (S.F.)
(20) Lee, J. Y.; Lee, Y.-M.; Kotani, H.; Nam, W.; Fukuzumi, S. Chem.
Commun. 2009, 704–706.
(
21) Zhu, X.-Q.; Zhang, M.-T.; Yu, A.; Wang, C.-H.; Cheng, J.-P. J.
Am. Chem. Soc. 2008, 130, 2501–2516.
22) Francisco, W. A.; Blackburn, N. J.; Klinman, J. P. Biochemistry
003, 42, 1813–1819.
23) Bollinger, J. M., Jr.; Krebs, C. Curr. Opin. Chem. Biol. 2007, 11,
51–158.
24) de la Lande, A.; Parisel, O.; G ꢀe rard, H.; Moliner, V.; Reinaud, O.
Chem.;Eur. J. 2008, 14, 6465–6473.
25) For BNAH, no organic products besides BNA were detected
(
’
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2
(
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in the reaction mixture. The sole inorganic product identified was the
trans-peroxo complex 2, though it accounted for only ∼50% of the total
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(
(
3) Itoh, S. Curr. Opin. Chem. Biol. 2006, 10, 115–122.
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-
(
(
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2
and HO
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. This would lead to consumption of 2 equiv of 1 per
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þ
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quantitative) yield of BNA . The trans-peroxo species 2 was not reactive
toward BNAH at -125 °C and decomposed only very slowly in the
(
7) Chen, P.; Solomon, E. I. Proc. Natl. Acad. Sci. U.S.A. 2004, 101,
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presence of large excesses of BzImH. Further incubation of the final
þ
1
reaction mixture did not lead to higher detected yields of BNA , i.e.,
(
other copper species present following the conclusion of the reaction did
not react with the substrate at the temperature examined.
4
A. L.; Blackburn, N. J. J. Biol. Chem. 2006, 281, 4190–4198.
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(
(
II
2- 2þ
2
4
386.(b) [{(LCu )} (O )]
(2) possesses distinctive UV-vis
2
-
1
absorptions and a ν(O-O) of 837 cm
.
(
15) See the Supporting Information.
18
16
(16) The complex formed with
O
2
had a small O contamination
2
-
1
estimated to be 18% from the weak O-O feature at 1130 cm observed
in the O sample spectrum. The signal of the O sample was scaled and
subtracted from the O spectrum in the analysis of the Cu- O
vibrational region.
18
16
1
8
18
-
1
(
17) A resonance-enhanced peak at 485 cm was observed for the
16
complex formed with O , together with a less intense peak at
4
at 468 and 448 cm , with their relative intensities reversed in com-
parison with the spectrum of the O sample. This pattern also indicates
a Fermi resonance with a nonenhanced mode at similar energy, in this
2
-
1
18
52 cm . For the complex formed with
O
2
, two peaks were observed
-
1
16
16
18
case for both the Cu- O and Cu- O spectra. From the analysis (see
the Supporting Information), the preinteraction Cu-O stretching
16
-1
18
frequency for the O complex is at 482 cm and that for the
O
-
1
complex is at 462 cm
18) Wada, A.; Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.;
Mukai, M.; Kitagawa, T.; Einaga, H. Angew. Chem., Int. Ed. 1998, 37, 798–799.
19) (a) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am.
.
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