Iron-mediated cleavage of C-C bonds in vicinal tricarbonyl compounds in water

Full text of DOI:10.1002/anie.200806296

Communications
DOI: 10.1002/anie.200806296
Iron Catalysis
À
Iron-Mediated Cleavage of C C Bonds in Vicinal Tricarbonyl
Compounds in Water**
´
Jasmin Mecinovic, Refaat B. Hamed, and Christopher J. Schofield*
Iron is the second most abundant metal in the Earthꢀs crust
and plays an essential role in biology. Iron-containing proteins
are involved in biological processes including the modifica-
tion of nucleic acids, proteins, carbohydrates, and lipids;
oxygen transport and sensing; small-molecule metabolism;
and electron transfer.[1] Iron and its complexes are also widely
used in organic synthesis, for example as catalysts for
oxidation, reduction, and coupling reactions.[2–5] Although
(Scheme 1; Figure S1 in the Supporting Information).
1H NMR spectroscopy (700 MHz) and LC-MS analysis indi-
cated that, in the presence of PHD2 and FeII (PHD2/FeII/
1 1:1:5) 1 reacted to form N-oxalylglycine (NOG 3) and
benzoic acid (4). Unexpectedly, we found that 3 and 4
(Figure S2 in the Supporting Information) also formed in the
absence of PHD2 in an iron-dependent manner (Table 1).
When FeIII was substituted for FeII, it also catalyzed cleavage
of 1 (both at unbuffered pH 4 and buffered pH 7; Figure 1)
under aerobic conditions. However, when 1 was treated with
FeII at pH 3, little fragmentation was observed (< 5%
conversion, 14 h), which is consistent with the slower oxida-
tion of FeII to FeIII at low pH (Figure S3 in the Supporting
Information). Similarly, when FeII was used in the presence of
a 50-fold excess of reducing agent (sodium ascorbate or
potassium dithionate), no reaction (< 5% conversion) of 1
À
there are reports of the use of iron salts and complexes for C
C/O/N/S bond formation, these reactions typically require
organic solvents and relatively harsh reaction conditions.
[6–9]
[10]
À
À
There are few examples of iron-catalyzed C C
and C O
bond-cleavage reactions and, to our knowledge, there are no
À
reported efficient C C bond-cleavage reactions catalyzed by
iron that occur in aqueous solutions in the absence of added
ligands/catalysts. Herein, we report the unexpected finding
À
À
that ferric ions catalyze the cleavage of C C bonds of vicinal
tricarbonyl compounds in water at room temperature in the
absence of added ligands.
took place. Under anaerobic conditions, cleavage of the C C
bond was observed in the presence of FeIII, but not in the
presence of FeII. Taken together, these observations led us to
conclude that the active species is FeIII rather than FeII.
During our studies on the evaluation of tricarbonyl
compounds as inhibitors of a human oxygen-sensing enzyme
(prolyl hydroxylase domain 2 (PHD2))[11,12] we found that
Table 1: Screening metal ions for their potential to mediate oxidative
cleavage of tricarbonyl compound 1 in water at room temperature.[a]
tricarbonyl compounds
1 and 2 underwent reaction
Entry
Reagent (equiv)
t [h]
Conversion [%][b]
1
2
3
4
FeSO4 (2)
2
5
2
9
2
100
n.d.
100
70
K4[Fe(CN)6] (2)
Fe2(SO4)3 (1)
Fe2(SO4)3 (0.1)
FeCl3 (2)
Scheme 1. Synthesis of the tricarbonyls 1 and 2. EDCI=1-(3-dimethyla-
minopropyl)-3-ethylcarbodiimide hydrochloride, HOBT=1-hydroxy-1H-
benzotriazole, TFA=trifluoroacetic acid.
5
100
100
traces
n.d.
traces
n.d.
n.d.
n.d.
n.d.
traces
n.d.
n.d.
n.d.
n.d.
100
100
88
6
7
8
9
Fe(NO3)3 (2)
K3[Fe(CN)6] (2)
CoCl2 (2)
MnSO4 (2)
NiSO4 (2)
CuSO4 (2)
ZnSO4 (2)
RuCl3 (2)
PdCl2 (2)
AgNO3 (2)
PtCl2 (2)
Hg(OAc)2 (2)
Ce(SO4)2 (2)
KMnO4 (1)
KRuO4 (1)
NaIO4 (1)
2
24
5
5
5
5
5
5
5
5
5
5
5
´
10
11
12
13
14
15
16
17
18
19
20
21
[*] J. Mecinovic, R. B. Hamed, Prof. Dr. C. J. Schofield
Chemistry Research Laboratory
Department of Chemistry, University of Oxford
Mansfield Road, Oxford, OX1 3TA (United Kingdom)
Fax: (+44)1865-275-674
E-mail: christopher.schofield@chem.ox.ac.uk
Homepage: http://www.chem.ox.ac.uk/researchguide/cjscho-
field.html
[**] We thank the Newton–Abraham Fund (J.M.), the Ministry of Higher
Education, Egypt (R.B.H.), the Biotechnology and Biological
Research Council, the European Union, and the Wellcome Trust for
funding. We also thank Dr. John M. Brown for helpful discussions
on mechanisms.
1.5
5 min
24
[a] Reaction conditions: 1 (500 mm), reagent (metal ion 1 mm) in H2O,
258C. [b] Conversion was determined by 1H NMR spectroscopy and LC-
MS analysis. n.d.=not detected.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806296.
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Chemie
À
C C bonds, to give 4 and benzoylformic acid in a ratio of
approximately 6:4 (Table 2, entry 5; Figure S8 in the Support-
ing Information).
À
Diketones were then screened for potential C C frag-
mentation. The analogue of 1 that lacks a carbonyl group at
C2 (11), the 1,3-dione 12 (which can potentially chelate iron
through two oxygen atoms to form a six-membered ring), and
diethyl oxomalonate 13 were not cleaved (Table 2, entries 6–
8). The need for a tricarbonyl-containing substrate was
supported by the lack of conversion observed for the vicinal
dicarbonyl compounds benzil (14) and 2,2’-pyridyl (15;
Table 2, entries 9 and 10). Importantly, the dicarbonyl com-
pounds phenylglyoxal (16) and the trifluoromethyl derivative
17, both existing predominantly in their hydrated forms in
water, were not fragmented (Table 2, entries 11 and 12). Also,
the reaction of 2-oxoglutaric acid (18) in the presence of
2 equivalents of FeIII did not result in cleavage (Table 2,
entry 13). Overall, these results imply that the reaction
requires a tricarbonyl motif with a vicinal dicarbonyl arrange-
ment; the third carbonyl group can come from an ester,
amide, or ketone functionality.
Figure 1. 1H NMR spectra of the iron(III)-mediated cleavage of 1 to
give benzoic acid (4). Reaction of 1 in the presence of 2 equivalents of
FeIII in water at room temperature; a) before the reaction and after
reaction times of b) 15 min, c) 45 min, d) 90 min, and e) 2 h.
Next we studied ninhydrin and dehydro-l-ascorbate (21),
which are tricarbonyl compounds of biological interest.
Ninhydrin, which is widely used for the detection of amino
acids,[15] was cleaved to give a-ketoacid 19 and phthalic acid
(20); substituting FeIII with KMnO4 or KRuO4 also gave 19
and 20 (Scheme 2; Figures S9 and S10 in the Supporting
Information). Consistent with the observed iron(III)-cata-
lyzed fragmentation of ninhydrin, DꢀAniello et al. have
reported that the standard ninhydrin test for amino acids
fails in the presence of FeIII.[16]
Dehydro-l-ascorbate (21), the oxidized form of l-ascorbic
acid (22; vitamin C) is probably the most important naturally
occurring tricarbonyl-containing compound (Scheme 3). A
biologically and commercially important reducing agent, 22 is
degraded by both non-oxidative and oxidative pathways—
potentially via more than 100 different intermediates.[17,18]
Oxidative degradation of 22 proceeds via 21,[19,20] which can
undergo further oxidation and hydrolytic lactone ring-open-
ing to l-threonic acid (23) and oxalic acid (24).[21]
Compound 21 was cleaved to give 23 and 24 under
standard reaction conditions after 18 h. However, the cleav-
age did not proceed in the presence of other noncomplexed
biologically relevant transition-metal ions (ZnII, CuII, CoII,
NiII, MnII), even at higher temperatures (378C, 508C;
Figure S11 in the Supporting Information). As compounds
21–24 had similar LC retention times, 5,6-isopropylidene-l-
ascorbic acid was used in further studies. 5,6-Isopropylidene-
l-ascorbic acid was oxidized to 5,6-isopropylidene-l-dehy-
droascorbate when 4 equivalents of FeIII was used, and
cleavage occurred to give 3,4-isopropylidene-l-threonic acid
and 24 (ca. 50% conversion after 6 h; Figure S12 in the
Supporting Information).
Despite the biological importance of ascorbate, pathways
for its degradation are not fully elucidated. In some plants l-
ascorbic acid (22) is degraded to l-tartrate via l-idonate.
However, in most plants, degradation occurs via 21 to yield 23
and 24. Recent work has shown that the pathway occurs in
plant extracts and involves both enzymatic and nonenzymatic
We investigated the cleavage of 1 to give 3 and 4 in the
presence of other first-row transition-metal ions (Table 1).
Only iron was found to catalyze the cleavage of 1 in water at
room temperature under aerobic conditions. In contrast to
Fe2(SO4)3, FeCl3, and Fe(NO3)3 (Table 1, entries 1 and 3–6),
K3[Fe(CN)6] (Table 1, entry 7) did not catalyze the cleavage
of 1. Under the standard reaction conditions, RuIII, AgI, HgII,
PdII, PtII, and CeIV reagents did not mediate the cleavage of 1
(< 5% conversion; 5 h, room temperature; Table 1,
entries 13–18). Cleavage of 1 was mediated by the strong
oxidizing agents KMnO4 (Figure S4 in the Supporting Infor-
mation), KRuO4, and NaIO4 (Table 1, entries 19–21).
Cleavage of 1 by FeIII also occurred in methanol. 1H NMR
spectroscopy and LC-MS analysis (2 equiv of FeIII, CD3OD)
revealed the products to be the N-oxalylglycine C1-methyl
ester (5) and methyl benzoate ( ꢀ 80% in 3 h; Figure S5 in the
Supporting Information).
Other vicinal tricarbonyl compounds[13,14] were then
tested to see if they underwent cleavage under the standard
conditions. (Unless stated otherwise, the reaction conditions
were 2 equivalents of FeIII in H2O at room temperature;
Table 2.) Analogues of 1 (6 and 7; Table 2, entries 1 and 2;
Figure S6 in the Supporting Information) that lacked a
carboxylic acid side chain were cleaved similarly to 1, thereby
excluding the possibility that chelation of iron by a carboxylic
acid side chain plays essential role in the cleavage process.
Ethyl 2,3-dioxo-3-phenylpropanoate 8 reacted to form
benzoic acid (4) and ethanol (Table 2, entries 3; as monitored
by 1H NMR spectroscopy, Figure S7 in the Supporting
Information). LC-MS analysis revealed oxalate to be the
other product (an a-ketoacid intermediate was not observed).
In contrast to the cleavage of 1, which was complete in 2 h,
reaction of 8 required 9 h, possibly because of iron chelation
by the oxalate product. Cleavage of diphenylpropanetrione 9
resulted in the formation of 4 and benzoylformic acid
(Table 2, entry 4). 1H NMR spectroscopy and LC-MS analysis
revealed that 10 is cleaved at both of the two potentially labile
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Communications
steps.[21,22] Interestingly, the in vivo
labeling studies showed that C1 of
22 is incorporated into 24.[21] Our
work has implications in this regard
as the demonstration that FeIII can
catalyze the conversion of 21 into 23
and 24 reveals the potential for non-
enzymatic iron(III)-catalyzed deg-
radation of 22 (there is also evi-
dence for oxygenase involvement
in vivo[23]), and may in part account
for loss of vitamin C in food prep-
aration.
To investigate the mechanism of
cleavage of the tricarbonyl com-
pound, we then performed labeling
experiments. Consistent with the
observed cleavage under anaerobic
conditions, when 1 was reacted in
the presence of FeIII in H2O under
18O2, no (< 5%) 18O was incorpo-
rated into either benzoic acid (4) or
NOG 3 according to LC-MS analy-
sis. In contrast, when the reaction
was performed in H218O, two 18O
atoms were incorporated into both
4 and 3 (Figure 2). When 1 was
reacted without FeIII in H218O, only
one 18O atom was incorporated,
likely into its central carbonyl
group, which is consistent with the
observation that the central keto
group of tricarbonyl compounds
exists predominantly in a hydrated
form in water (Figure S13 in the
Supporting Information);[13] 18O
incorporation into either 4 or 3
was not observed in H218O.
Table 2: Scope of the iron-catalyzed cleavage of carbonyl compounds in water at room temperature.
Entry
Starting material
Products
t
[h]
Conversion
[%][a]
1
2
3
4
6
7
8
9
2
2
9
2
100
100
100
100
5
10
2
100
6
7
8
11
12
13
–
–
–
5
5
5
n.d.
n.d.
n.d.
9
14
–
–
–
–
–
5
24
24
5
n.d.
n.d.
n.d.
n.d.
n.d.
10
11
12
13
15
When ninhydrin was treated
with FeIII in H218O, five 18O atoms
were incorporated into a-keto acid
19 and four 18O atoms into phthalic
acid (20; Figure S14 in the Support-
ing Information). With dehydro-l-
ascorbic acid (21), the 18O labeling
experiments led to the incorpora-
tion of two 18O atoms from H218O
into threonic acid (23; Figure S15 in
16[b]
17[b]
18
24
[a] Conversion was determined by 1H NMR spectroscopy and LC-MS analysis. [b] Shown to be
predominantly hydrated in water.
III
À
À
Scheme 2. C C cleavage of ninhydrin (2 equiv of Fe ) to give a-keto
acid 19 and phthalic acid (20). The major products of the reaction in
H218O are shown.
Scheme 3. Iron(III)-mediated cleavage of the C C bond in dehydro-l-
ascorbate (21) to give threonic acid (23) and oxalate 24. The result of
a
13C labeling experiment is shown.
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reaction involves a complex stoichiometry of two FeIII ions
À
and two tricarbonyl compounds with a single C C cleavage
occurring per cycle).
Overall, we have shown that aqueous ferric ions in the
absence of added ligands mediate the cleavage of vicinal
tricarbonyl compounds. The reaction has relevance for the
detection of amino acids by ninhydrin and for the metabolism
of vitamin C. We note that if the reverse reaction; that is,
ligation of a 2-oxocarboxylate and a carboxylate to give a
carbonyl compound, could be achieved under mild aqueous
conditions it would be of interest—perhaps even from a
prebiotic perspective. Such a process may have precedent in
the acyloin reaction, which requires much harsher non-
aqueous conditions.[24]
Figure 2. Mass spectra from the oxygen incorporation studies for the
cleavage reaction of 1 (2 equiv of FeIII for 90 min). The two oxygen
atoms, incorporated into NOG 3 (left) and benzoic acid (4; right)
come from water; a) non-labeled; b) only H218O was labeled; c) only
18O2 was labeled; d) non-labeled, e) only H218O was labeled, f) only
18O2 was labeled.
Experimental Section
Solutions of tricarbonyls/dicarbonyls (10 mm, deionized water) and
ferric sulfate (10 mm, deionized water) were mixed in a 1:1 ratio (final
concentration of tricarbonyl/dicarbonyl 500 mm, FeIII 1 mm, pH 4) and
then reacted at room temperature prior to LC-MS analysis; samples
were analyzed by using a Waters LCT Classic MS Machine). For
1H NMR analyses samples were prepared in the same manner as for
LC-MS, except that 2H2O was used as the solvent; samples were
analyzed by using a Bruker AVIII 700 MHz spectrometer with an 1H
inverse TCI cryoprobe. For 18O labeling experiments, reactions were
carried out under the standard reaction conditions, except that 18O2 or
H218O were used. Anaerobic experiments were carried out in a Belle
Technology glove box (< 0.1 ppm O2).
the Supporting Information). The reaction of [13C-1]-labeled
l-ascorbic acid with FeIII revealed 13C incorporation into 24,
but not into 23 (Scheme 3; Figure S16 in the Supporting
Information).
The overall process for the fragmentation of tricarbonyl
compounds involves a two-electron oxidation; that is, it
requires reduction of two FeIII to two FeII ions. Any proposed
mechanisms must account for the labeling results, the require-
ment of a tricarbonyl moiety, the range of substrates, and the
likely involvement of a dihydrated intermediate. Tricarbonyl
compounds can potentially form a six-membered chelate ring
involving their C1 and C3 carbonyl groups, form a five-
membered ring, or chelate through all three carbonyl groups.
We propose a “minimal” mechanism involving the initial
formation of a dihydrated tricarbonyl compound, which
complexes with two FeIII ions (Scheme 4), one of which is
complexed by the “third” carbonyl oxygen. Concomitant with
Received: December 23, 2008
Published online: March 11, 2009
À
Keywords: C C cleavage · homogeneous catalysis · iron ·
redox chemistry · tricarbonyl compounds
.
[1] R. Crichton, Inorganic Biochemistry of Iron Metabolism, 2nd
ed., Wiley, 2001.
[2] C. Bolm, J. Legros, J. Le Paih, L. Zani, Chem. Rev. 2004, 104,
6217 – 6254.
[3] A. Correa, O. G. Mancheno, C. Bolm, Chem. Soc. Rev. 2008, 37,
1108 – 1117.
[4] L. Que, Jr., W. B. Tolman, Nature 2008, 455, 333 – 340.
[5] B. D. Sherry, A. Fꢁrstner, Acc. Chem. Res. 2008, 41, 1500 – 1511.
[6] L. Bꢂnisvy, J.-C. Chottard, J. M. Y. Li, Eur. J. Inorg. Chem. 2005,
999 – 1002.
À
C C cleavage, each of the ferric ions can then accept an
electron to give a complex formed from two ferrous ions,
benzoic acid, and an a-keto acid (it is also possible that the
[7] A. Dhakshinamoorthy, K. Pitchumani, Tetrahedron 2006, 62,
9911 – 9918.
[8] H. Nakazawa, M. Itazaki, K. Kamata, K. Ueda, Chem. Asian J.
2007, 2, 882 – 888.
[9] T. K. Paine, J. England, L. Que, Jr., Chem. Eur. J. 2007, 13, 6073 –
6081.
[10] C. C. Tzschucke, N. Pradidphol, A. Diꢂguez-Vꢃzquez, B. Kong-
kathip, N. Kongkathip, S. V. Ley, Synlett 2008, 1293 – 1296.
´
[11] J. Mecinovic, R. Chowdhury, B. M. R. Liꢂnard, E. Flashman,
M. R. G. Buck, N. J. Oldham, C. J. Schofield, ChemMedChem
2008, 3, 569 – 572.
[12] C. J. Schofield, P. J. Ratcliffe, Nat. Rev. Mol. Cell Biol. 2004, 5,
343 – 354.
[13] M. B. Rubin, R. Gleiter, Chem. Rev. 2000, 100, 1121 – 1164.
[14] H. H. Wasserman, J. Parr, Acc. Chem. Res. 2004, 37, 687 – 701.
[15] M. M. Joulliꢂ, T. R. Thompson, N. H. Nemeroff, Tetrahedron
1991, 47, 8791 – 8830.
Scheme 4. A proposed mechanism for the iron(III)-catalyzed cleavage
of tricarbonyl compounds.
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Communications
[16] A. DꢀAniello, G. DꢀOnofrio, M. Pischetola, L. Strazzullo, Anal.
[20] W. Jabs, W. Gaube, Z. Anorg. Allg. Chem. 1984, 514, 179 – 184.
[21] M. A. Green, S. C. Fry, Nature 2005, 433, 83 – 87.
[22] M. M. Taqui Khan, A. E. Martell, J. Am. Chem. Soc. 1967, 89,
4176 – 4185.
[23] K. Saito, J. Ohmoto, N. Kuriha, Phytochemistry 1997, 44, 805 –
809.
Biochem. 1985, 144, 610 – 611.
[17] R. D. Hancock, R. Viola, Crit. Rev. Plant Sci. 2005, 24, 167 – 188.
[18] M. W. Davey, M. V. Montagu, D. Inzꢂ, M. Sanmartin, A.
Kanellis, N. Smirnoff, I. J. Benzie, J. J. Strain, D. Favell, J.
Fletcher, J. Sci. Food Agric. 2000, 80, 825 – 860.
[19] A. Schulz, C. Trage, H. Schwarz, L. W. Kroh, Int. J. Mass
Spectrom. 2007, 262, 169 – 173.
[24] K. T. Finley, Chem. Rev. 1964, 64, 573 – 589.
2800 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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