Dioxygen activation and two consecutive oxidative decarboxylations of phenylpyruvate by nonheme iron(ii) complexes: Functional models of hydroxymandelate synthase (HMS) and CloR

Article abstract of DOI:10.1039/c5cc01652e

Two mononuclear iron(ii)-phenylpyruvate complexes of monoanionic facial N₃ ligands are reported to react with dioxygen to undergo two consecutive oxidative decarboxylation steps via an iron-mandelate complex mimicking the function of HMS and CloR.

Full text of DOI:10.1039/c5cc01652e

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ARTICLE TYPE
Dioxygen Activation and Two Consecutive Oxidative Decarboxylations
of Phenylpyruvate by Nonheme Iron(II) Complexes: Functional Models
of Hydroxymandelate Synthase (HMS) and CloR**
Debobrata Sheet,a Shrabanti Bhattacharya a and Tapan Kanti Paine* a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
phenylpyruvate (3DMAꢀ4HPP) is catalyzed by CloR via the
formation of 3ꢀdimethylallylꢀ4ꢀhydroxymandelic acid (3DMAꢀ
50 4HMA) (Scheme 1b).18 CloR does not have sequence similarity
to any known oxygenases. However, HMS and CloR are similar
in terms of reactivity and are involved in benzylic hydroxylation.
The enzyme CloR uses 3DMAꢀ4HPP (in the first step of the
reaction) whereas HMS uses 4HPP as substrate/αꢀketo acid
55 cofactor.
Two mononuclear iron(II)-phenylpyruvate complexes of
monoanionic facial N3 ligands are reported to react with
dioxygen
to
undergo
two
consecutive
oxidative
10 decarboxylation steps via an iron-mandelate complex
mimicking the function of HMS and CloR.
Alphaꢀketo acidꢀdependent oxygenases constitute a large
superfamily of nonheme iron enzymes that couple the oxidative
decarboxylation of the αꢀketo acid cofactors concomitant with the
15 oxidation of substrates.1ꢀ4 Hydroxymandelate synthase (HMS)
and 4ꢀhydroxyphenylpyruvate dioxygenase (HPPD) belong to the
same superfamily of αꢀketo acid dependent enzymes.5ꢀ9 HMS is
involved in the first step of the biosynthesis of pꢀ
hydroxyphenylglycine, a nonproteiogenic amino acid component
20 of antibiotics such as vancomycin.6, 10 HMS, expressed from a
calcium dependent antibiotic (CDA) Amycolatopsis orientalis,
catalyzes the chiral oxidation of 4ꢀhydroxyphenylpyruvate
10ꢀ12
(4HPP) to (S)ꢀ4ꢀhydroxymandelate (4HMA) (Scheme 1a).7,
The structure of cobalt(II) substituted HMS was solved with the
25 product hydroxymandelate coordinated to the metal center in a
bidentate fashion.7 HMS exhibits high structural similarity to that
of HPPD, an evolutionary related enzyme that catalyzes the
second step of the tyrosine catabolism pathway.7, 13 HPPD and
HMS are the exceptions in the αꢀketo acidꢀdependent enzymes as
Scheme 1 Reactions catalyzed by (a) HMS and HPPD, and by (b) CloR.
In biomimetic chemistry, a large number of functional models
for αꢀketo acidꢀdependent oxygenases have been reported.4, 19ꢀ21
60 We have recently reported a functional model for the second step
of CloR.22 While some progress has been made in understanding
the decarboxylation/oxidative CꢀC bond cleavage mechanism of
phenylpyruvic acid by iron complexes of polydentate ligands,23, 24
there is no nonheme iron(II)ꢀphenylpyruvate complex mimicking
65 the function of HMS or both the steps catalyzed by CloR. Since
monoanionic facial tris(pyrazolyl)borate ligands (Tp) have widely
been used to develop models for nonheme iron oxygenases,4, 19, 21,
30 both the enzymes use
a
common substrate 4ꢀ
hydroxyphenylpyruvate (4HPP).5 The common goal is to
hydroxylate 4HPP with the incorporation of both the atoms of O2
into the substrate (Scheme 1a). These enzymes follow the
mechanism typical of αꢀketo acidꢀdependent dioxygenases. An
35 iron(IV)ꢀoxo intermediate,14 generated in the reaction with O2,
hydroxylates the substrate 4HPP. In the case of HMS, the
iron(IV)ꢀoxo abstracts an αꢀH atom from 4ꢀhydroxyphenylacetate
intermediate and hydroxylates the benzylic carbon via OH
rebound mechanism affording hydroxymandelate (4HMA).6, 7, 15,
40 16 In HPPD, on the other hand, spatial orientation of the substrate
(4HPP) and steric factors controls the regiospecific delivery of
the oxo atom from the iron(IV)ꢀoxo oxidant.5, 17
25ꢀ28
we have investigated the iron(II)ꢀphenylpyruvate chemistry
of different Tp ligands. As an outcome of our investigation, we
70 report herein the synthesis and characterization of two nonheme
mononuclear iron(II) complexes, [(TpPh,Me)FeII(PPH)] (1) and
[(TpMe2)FeII(PPH)] (1a), where TpPh,Me = hydrotris(3ꢀphenylꢀ5ꢀ
methylpyrazolyl)borate,
dimethylpyrazolyl)borate,
TpMe2
PPH
=
hydrotris(3,5ꢀ
monoanionic
Another functionally related bifunctional nonheme iron
oxygenase CloR is involved in the biosynthesis of
45 aminocoumarin antibiotics such as chlorobiocin.18 These
antibiotics contain a 3ꢀdimethylallylꢀ4ꢀhydroxybenzoate (3DMAꢀ
4HB) moiety whose formation from 3ꢀdimethylallylꢀ4ꢀhydroxyꢀ
and
is
75 phenylpyruvate (Fig. 1). The phenylpyruvate complexes react
with O2 to undergo oxidative decarboxylation and concomitant
hydroxylation of the benzylic carbon of phenylacetic acid
resulting in the formation of iron(II)ꢀmandelate complexes. The
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latter further activates O2 to undergo oxidative decarboxylation.
The reactivity of the iron(II)ꢀPPH complexes toward O2 and the 40 trigonal plane is formed by the pyrazole nitrogens N5 and N3,
bipyramidal coordination geometry (τ = 0.57),29 where the
mechanism of two consecutive oxidative decarboxylations of
phenylpyruvate is reported in this communication.
and the carboxylate O1. The axial positions are occupied by the
pyrazole nitrogen (N2) and the keto oxygen (O2) with the N2–
Fe1–O2 angle of 174.3(2)° (Table S1). The Fe1–O2 bond
distances of 2.258(6) Å and C31–C32 of 1.555(12) Å clearly
45 indicates that PPH does not enolize upon binding with iron(II)
center.23, 24 Although single crystals of complex 1a could not be
isolated, the spectroscopic data indicate a very similar binding
mode of PPH as in 1.
5
Fig. 1 Ligands and model substrates.
The iron(II)ꢀαꢀketo acid complexes (1 and 1a) were
synthesized by mixing equimolar amounts of the respective Tp
ligand, FeCl2 and NaPPH in dichloromethaneꢀmethanol solvent
10 mixture (Experimental Section, ESI). The ESIꢀmass spectra of 1
and 1a exhibit molecular ion peaks at m/z = 703.03 and 516.06
with the isotope distribution patterns calculated for [1+H+] and
[1a]+, respectively (Figs. S1 and S2, ESI). The complexes display
1
paramagnetically shifted H NMR spectra in CD3CN indicating
15 highꢀspin nature of the iron(II) centers (Figs. S3 and S4, ESI).
The optical spectrum of complex 1 in acetonitrile shows broad
bands at around 422 and 552 nm, in which the latter peak arises
due to the iron(II)ꢀtoꢀketo metal to ligand charge transfer
transition (MLCT). The MLCT bands suggest a bidentate
20 chelation of the αꢀketo acid (PPH) to the iron(II) centers via one
keto oxygen and one carboxylate oxygen in solution. For
complex 1a, the MLCT bands are shifted to 413 and 560 nm.
Similar MLCT bands with low extinction coefficients (~1000 Mꢀ
1cmꢀ1) have been observed in previously reported iron(II)ꢀαꢀketo
50 Fig. 3 Optical spectral changes during the reaction of 1 (1 mM) with
dioxygen in acetonitrile at 295 K. Inset: ESIꢀmass spectra (positive ion
mode, acetonitrile) of the oxidized solution after the reaction of 1 with (a)
16O2 and with (b) 18O2 in acetonitrile for 10 min.
The red solution of complex 1 in acetonitrile rapidly changes
55 to green upon reaction with O2 over a period of 1h during which
the CT bands at 422 and 552 nm gradually decays (Fig. 3). The
ESIꢀmass spectrum of the oxidized solution displays an ion peak
at m/z = 675.2 with the isotope distribution pattern for
[{(TpPh,Me)FeII(PAA)}+H+] (PAA = phenylacetate) which is
60 shifted two mass units higher to m/z = 677.2 after reaction with
18O2, indicating incorporation (>95%) of one oxygen atom from
dioxygen into the decarboxylated product (PAA) (Fig. 3, Inset).
Complex 1a takes about 1 h for complete decay of the CT bands
and the ESIꢀmass spectrum of the oxidized solution suggests the
65 formation of [(TpMe2)FeII(PAA)] (Figs. S5 and S6, ESI).
25 acid complexes of
hydrotris(3,5ꢀdiphenylpyrazolyl)borate)
(TpPh2) ligand.20, 23
1
Timeꢀdependent H NMR studies (Fig. S7, ESI) in CD3CN
1
reveal the formation of benzaldehyde and the H NMR spectrum
of the final oxidized solution bears resemblance to that of an
independently isolated complex [(TpPh,Me)FeII(PAA)] (2) (Fig. S7
70 and Experimental Section, ESI). Further, the GCꢀmass spectra
reveal the formation of phenylacetic acid, benzoic acid and
benzaldehyde with the ion peaks at m/z = 136, 122 and 106,
respectively (Figs. S8 and S9, ESI). With18O2, the phenylacetic
acid peak is shifted to m/z = 138 while that of benzaldehyde is
75 shifted to m/z =108 with (50% 18O). The ion peak of benzoic acid,
on the other hand, is shifted two and four mass unit to m/z = 124
(25% 18O) and 126 (70% 18O) (Figs. S8 and S9, ESI). GCꢀMS of
the methyl ester derivatives of phenylacetic acid and benzoic acid
also confirm the incorporation of labeled oxygen (Fig. S10, ESI).
80 The labeling experiment data unambiguously establish that while
one oxygen atom from O2 is incorporated each into phenylacetic
acid and benzaldehyde, two oxygen atoms from O2 are
Fig. 2 ORTEP plot of [(TpPh,Me)FeII(PPH)] (1). All the hydrogen atoms
except for B1 have been omitted for clarity.
30
The single crystal Xꢀray structure of 1 reveals a fiveꢀ
coordinate mononuclear complex where the iron center is
coordinated by three pyrazole nitrogens from the TpPh,Me ligand,
and one carbonyl oxygen (O2) and one carboxylate oxygen (O1)
atom of a monoanionic phenylpyruvate (Fig. 2). Similar binding
35 mode of keto acid is observed in the reported fiveꢀcoordinate
iron(II)ꢀαꢀketo acid complexes of TpPh2 ligand.20, 23 The average
iron(II)ꢀN(pyrazole) bond length is typical of highꢀspin iron(II)
23
complexes.20,
The iron center resides in a distorted trigonal
2
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incorporated into benzoic acid. In addition to phenylacetic acid, 50 benzoic acid and benzyl alcohol in the reaction with O2.30
A
the formation of benzaldehyde and benzoic acid from
phenylpyruvic acid is intriguing. Of note, the reported iron(II)ꢀ
PPH complex [(TpPh2)FeII(PPH)] undergoes oxidative
nucleophilic ironꢀoxygen oxidant generated after decarboxylation
of mandelate has been implicated to carry out the oxidation of
benzaldehyde to benzoic acid. Benzyl alcohol is formed through
Cannizzaro reaction of benzaldehyde.30 These experimental
5
decarboxylation to form phenylacetic acid as the only product.23
1
Analysis of organic products from 1 through timeꢀdependent H 55 results clearly establish that benzaldehyde (and benzoic acid) is
NMR spectroscopy reveals that in addition to phenylacetic acid,
the amounts of benzaldehyde and benzoic acid steadily increase
with time. Between 10ꢀ20 min, a new peak at 5.24 ppm,
derived from mandelate and leaves no doubt about the
intermediacy of an iron(II)ꢀmandelate complex in the
decarboxylation of complex 1. As observed in complex 3, ligand
hydroxylation is also observed upon longer exposure of the final
10 attributable to mandelic acid, is observed which subsequently
decays (Fig. S11, ESI). Mandelic acid therefore forms as an 60 oxidized solution of 1 (Fig. S19, ESI). Thus, on the basis of total
intermediate product which does not accumulate in a large
amount. Finally, at the end of the reaction a product distribution
of phenylacetic acid (58%), benzaldehyde (33%), and benzoic
15 acid (8%) is observed. Complex 1a, after reaction with O2,
yield of benzaldehyde and benzoic acid, formation of about 41%
mandelic acid is estimated for 1 (Scheme S1). The formation of
mandelic acid through oxidative decarboxylation of
phenylpyruvate with the incorporation of both the oxygen atoms
affords phenylacetic acid (77%), benzaldehyde (5%), benzoic 65 of O2 functionally mimics the reaction catalyzed by
acid (4%), and mandelic acid (3%). In the reaction, about 8ꢀ10%
phenylpyruvic acid remains unreacted (Fig. S12, ESI). While the
ironꢀmandelate complex from 1 could not be observed in the ESIꢀ
20 mass spectrum, the final oxidized solution of 1a exhibits a small
hydroxymandelate synthase (HMS).
As no ironꢀoxygen intermediate could be observed for
spectroscopic characterization, external substrates were used to
intercept the active ironꢀoxygen species involved in the reaction
ion peak at m/z = 504.2 with the isotope distribution pattern 70 pathway (Scheme S2).31 In the reaction of 1 with 10 eqv
calculated for [(TpMe2)Fe(mandelate)]+ (Fig. S13, ESI). The GCꢀ
MS of the methyl ester of mandelic acid shows a molecular ion
peak at m/z = 166. When the reaction is carried out with 18O2, the
25 ion peaks are shifted to 168 (50% 18O atom) and 170 (45% 18O
thioanisole, 25% thioanisole oxide is obtained as the only product
(Fig. S20, ESI). However, with reduced amount of thioanisole (2
eqv), a mixture of sulfoxide and sulfone is formed. With dimethyl
sulfoxide as a substrate, 35% dimethyl sulfone is obtained (Fig.
atom) (Fig. 4). Labeling experiments clearly indicate that both the 75 S21, ESI). The reaction of 1 with O2 affords 30% fluorenone and
oxygen atoms in mandelic acid are derived from dioxygen. A
mixed labeling experiment with O2 and H2O18 reveals partial
incorporation of single labeled oxygen into mandelic acid (Fig.
30 S14, ESI).
32% anthracene from fluorene and 9,10ꢀdihydroanthracene,
respectively (Fig. S22ꢀS23, ESI). While cyclohexene yields cisꢀ
cyclohexaneꢀ1,2ꢀdiol (5%), a mixture cisꢀcyclooctaneꢀ1,2ꢀdiol
(5%) and cyclooctene oxide (5%) are obtained from cyclooctene
80 (Figs. S24ꢀS27, ESI). With 1ꢀoctene, 33% octaneꢀ1,2ꢀdiol is
observed (Fig. S28, ESI). Reaction of 1 with 18O2 indicates the
incorporation of both the labeled oxygen into octaneꢀ1,2ꢀdiol and
a single labeled oxygen into thioanisole oxide (Figs. S29ꢀS30,
ESI). Additionally, a mixed labeling experiment with 16O2 and
85 H2O18 shows no incorporation of labeled oxygen from 18O
enriched water into the products. In the interception experiments
with thioanisole or 1ꢀoctene, the yield of benzoic acid is
decreased but the total amount of benzaldehyde and benzoic acid
remains constant. Also, the yield of phenylacetic acid remains
90 unaltered in the presence of these substrates. Moreover, the yields
of substrateꢀderived products are found to be less than 40%. All
these results clearly illustrate that a metalꢀbased nucleophilic
oxidant derived from in situ generated iron(II)ꢀmandelate is
mainly responsible for substrate oxidation.
Fig. 4 GCꢀmass spectra of the methyl ester of mandelic acid. Mandelic
acid is formed in the reaction of 1 with (a) 16O2 and (b) with 18O2.
1
In the timeꢀdependent H NMR spectra of the reaction of 1
35 with O2, a transient complex appears after 5 min, which
1
completely decays after 25 min (Fig. S7, ESI). The H NMR
spectrum of the transient species is similar to that of an
95
Taking into account all the results, a mechanistic proposal is
put forward (Scheme 2). In the first step, iron(II)ꢀphenylpyruvate
complex undergoes oxidative decarboxylation to generate an
iron(IV)ꢀoxo species as proposed for αꢀketo acidꢀdependent
enzymes. Since nonheme iron(IV)ꢀoxo complexes have been
indepedently
prepared
iron(II)ꢀmandelate
complex
[(TpPh,Me)FeII(mandelate)(MeOH)](3) (Experimental Section and
40 Fig. S15, ESI). The colorless solution of 3 in acetonitrile turns
light green over a period of 45 min and the ESIꢀMS of the
oxidized solution exhibits ion peaks with the isotope distribution
patterns calculated for [(TpPh,Me*)Fe]+, and {[(TpPh,Me*)Fe(OBz)]
+ H}+ (Figs. S16 and S17, ESI). At end of the reaction about 19%
45 benzoic acid, 59% benzaldehyde and 5% benzyl alcohol are
formed (Fig. S18, ESI). Importantly, the iron(II)ꢀphenylacetate
complex 2 does not afford mandelic acid or benzaldehyde or
benzoic acid upon exposure to O2. An iron(II)ꢀmandelate
complex of TpPh2 ligand has been reported to form benzaldehyde,
100 reported to hydroxylate aliphatic CꢀH bonds,32 the iron(IV)ꢀoxo
species in the present system hydroxylates the benzylic carbon of
the coordinated PAA to form an iron(II)ꢀmandelate species. The
iron(II)ꢀmandelate rapidly reacts with another molecule of O2 in
the same manner as reported for iron(II)ꢀαꢀhydroxy acid
105 complexes of TpPh2 ligand.27, 30 Initially an iron(III)–superoxide
complex is formed that abstracts the hydrogen atom from the ꢀOH
group of mandelate to form an iron(III)–hydroperoxide species
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thus formed carries out the oxidation of various substrates
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60
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Scheme
2 Mechanistic proposal of two consecutive oxidative
decarboxylations of phenylpyruvic acid on iron(II) complexes.
19. T. K. Paine and L. Que, Jr., Struct. Bond., 2014, 160, 39ꢀ56.
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10
In conclusion, we have isolated and characterized two
biomimetic iron(II)ꢀphenylpyruvate complexes supported by
tris(pyrazolyl)borate ligands. The complexes react with O2 to
undergo oxidative decarboxylation to form phenylacetic acid,
benzaldehyde and benzoic acid. Mechanistic studies reveal the
22. T. K. Paine, S. Paria and L. Que, Jr., Chem. Commun., 2010, 46,
80
1830ꢀ1832.
15 involvement of a putative iron(IV)ꢀoxo oxidant in the oxidation
of phenylacetate to mandelate. The iron(II)ꢀmandelate complex,
generated in situ, further reacts with O2 to form benzaldehyde and
benzoic acid. The formation of mandelic acid from the iron(II)ꢀ
phenylpyruvate complex represents the first functional model of
20 HMS and the first step of CloR through hydroxylation at the
benzylic position of PAA. The conversion of in situ formed
mandelate to benzaldehyde and benzoic acid represents the
functional model mimicking the second step of CloR. The results
described herein demonstrate the role of steric and electronic
25 factors of the ligand in directing the specific oxygenꢀdependent
transformation reaction.
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2821.
TKP acknowledges the DST, India (Project: SR/S1/ICꢀ
51/2010) for financial support. DS thanks CSIR, India, and SB
thanks DST(INSPIRE) for research fellowships.
31. A. Mukherjee, M. Martinho, E. L. Bominaar, E. Münck and L. Que,
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30 Notes and references
aDepartment of Inorganic Chemistry, Indian Association for the
Cultivation of Science, 2A&2B Raja S. C. Mullick Road, Jadavpur,
Kolkata-700032, India. Fax: +91-33-2473-2805; Tel: +91-33-2473-
4971; E-mail: ictkp@iacs.res.in
35 † Electronic Supplementary Information (ESI) available: Experimental
procedure, spectral and crystalographic data. CCDCꢀ1013639. See
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4
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