Aliphatic C?C Bond Cleavage in α-Hydroxy Ketones by a Dioxygen-Derived Nucleophilic Iron–Oxygen Oxidant

Article abstract of DOI:10.1002/chem.201605672

A nucleophilic iron–oxygen oxidant, formed in situ in the reaction between an iron(II)–benzilate complex and O₂, oxidatively cleaves the aliphatic C?C bonds of α-hydroxy ketones. In the cleavage reaction, α-hydroxy ketones without any α-C?H bond afford a 1:1 mixture of carboxylic acid and ketone. Isotope labeling studies established that one of the oxygen atoms from dioxygen is incorporated into the carboxylic acid product. Furthermore, the iron(II) complex cleaves an aliphatic C?C bond of 17-α-hydroxyprogesterone affording androstenedione and acetic acid. The O₂-dependent aliphatic C?C bond cleavage of α-hydroxy ketones containing no α-C?H bond bears similarity to the lyase activity of the heme enzyme, cytochrome P450 17A1 (CYP17A1).

Full text of DOI:10.1002/chem.201605672

A Journal of
Accepted Article
Title: Aliphatic C-C Bond Cleavage of α-Hydroxy Ketones by a
Dioxygen-Derived Nucleophilic Iron-Oxygen Oxidant
Authors: Tapan Kanti Paine, Shrabanti Bhattacharya, Rubina
Rahaman, and Sayanti Chatterjee
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To be cited as: Chem. Eur. J. 10.1002/chem.201605672
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COMMUNICATION
Aliphatic C-C Bond Cleavage of α-Hydroxy Ketones by a
Dioxygen-Derived Nucleophilic Iron-Oxygen Oxidant
[
a]
[a]
[a]
[a],
Shrabanti Bhattacharya, Rubina Rahaman, Sayanti Chatterjee and Tapan Kanti Paine *
[
16]
[3, 17-22]
Abstract: A nucleophilic iron-oxygen oxidant, formed in situ in the
reaction between an iron(II)-benzilate complex and O , oxidatively
reaction by DAD.
For CYP17A1,
an iron-peroxide
2
species is implicated to initiate the lyase reaction. Only very
recently, the proposed intermediate in the lyase step has been
trapped and spectroscopically characterized as ‘iron(III)-peroxo-
hemiacetal’ species. Although both the enzymes cleave the C-
C bond of α-hydroxy ketones, the substrates are different. DAD
acts on a α-hydroxy ketone with α-C-H bond, whereas the α-
hydroxy ketone unit in 17α-hydroxypregnenolone does not
contain any α-C-H bond. Thus the cleavage pathways are
expected to be different in the two cases.
cleaves the aliphatic C-C bonds of α-hydroxy ketones. In the
cleavage reaction, α-hydroxy ketones without any α-C-H bond afford
a 1:1 mixture of carboxylic acid and ketone. Isotope labeling studies
establish that one of the oxygen atoms from dioxygen is
incorporated into the carboxylic acid product. Furthermore, the
iron(II) complex cleaves an aliphatic C-C bond of, 17-α-
hydroxyprogesterone, affording androstenedione and acetic acid.
[23]
The
2
O -dependent aliphatic C-C bond cleavage of α-hydroxy
ketones containing no α-C-H bond bears similarity to lyase activity of
the heme enzyme, cytochrome P450 17A1 (CYP17A1).
The selective oxidative cleavage of C-C bonds are important
reactions in the biodegradation of toxic compounds and
[
1-4]
biosynthesis of natural products.
These reactions are
catalyzed by dioxygen-activating metalloenzymes with the
incorporation of oxygen atoms from dioxygen. Based on the
nature of substrates, these reactions are classified into two
categories: aromatic ring cleavage and aliphatic C-C bond
cleavage. The aromatic C-C bond cleavage reactions by ring-
cleaving dioxygenases have been extensively studied over the
Scheme 1. Aliphatic C-C bond cleavage reactions catalyzed by (a) 2,4’-
dihydroxyacetophenone dioxygenase (DAD) and (b) cytochrome P450 17A1
(CYP17A1).
[
1, 5-7]
last several decades.
These studies provided useful
mechanistic insights into the role of metal ion and ligand
environment in controlling the selectivity of ring fission reactions.
On the contrary, the aliphatic C-C bond cleaving oxygenases
Synthetic model complexes provide useful insights into the
mechanistic aspects of aliphatic C-C bond cleavage reactions.
[2,
[
2, 8]
have received less attention.
The aliphatic C-C bond cleaving
24]
We recently reported the C-C bond cleavage of α-hydroxy
oxygenases are structurally diverse and act on a wide range of
ketones containing α-H atom by iron(II) complexes of nitrogen
[
9]
substrates.
Moreover, structurally and functionally distinct
[25, 26]
donor polydentate ligands.
. But the iron(II) complexes were
enzymes cleave the aliphatic C-C bonds of similar type of
substrates. One such example is the oxidative aliphatic C-C
unable to cleave the C-C bond of the α-hydroxy ketones without
α-C-H bond. Since a nucleophilic iron(III)-hydroperoxide species
has been invoked as a reactive intermediate in the C-C bond
cleavage reaction by CYP17A1, metal based nucleophilic iron-
oxygen species is expected to cleave the C-C bond of α-hydroxy
ketones. In this direction, we have investigated the reactivity of
bond
cleavage
of
α-hydroxy
ketones.
2,4’-
[
10, 11]
Dihydroxyacetophenone dioxygenase (DAD),
a bacterial
nonheme iron enzyme involved in the catabolism of 4-
hydroxyacetophenone, catalyzes the oxygenative cleavage of
2,4’-dihydroxyacetophenone into 4-hydroxy benzoate
and
Ph2
II
[27]
an iron(II)-benzilate complex, [(Tp )Fe (benzilate)] (1)
supported by a facial N ligand (Scheme 2), toward various α-
formate (Scheme 1a). On the other hand, the heme enzyme
3
[
12-14]
cytochrome P450 17A1 (CYP17A1),
cleaves the C17–C20
hydroxy ketones as model substrates. As an outcome of our
investigation, we present herein the C-C bond cleavage of α-
hydroxy ketones with dioxygen by complex 1 and a mechanism
of the oxidative transformation reaction.
bond (lyase) of 17α-hydroxypregnenolone in the biosynthesis of
steroid hormone, dehydroepiandrosterone from pregnenolone
[
15]
(
Scheme 1b).
Initial binding of the α-hydroxy ketone prior to
dioxygen activation has been proposed in the oxidative cleavage
Ph2
II
The iron(II)-benzilate complex [(Tp )Fe (benzilate)] (1)
reductively activates dioxygen and performs number of
putative iron(II)-hydroperoxide
a
[
27-30]
[
a]
Ms. S. Bhattacharya, Ms. R. Rahaman, Dr. S. Chatterjee, Prof. T. K.
Paine
bioinspired oxidations.
A
species, intercepted by external substrates, has been proposed
as the active oxidant. Since the iron-oxygen oxidant from 1 has
been reported to exhibit nucleophilic reactivity, the oxidant is
likely to react with electrophilic carbonyl compounds. Although
ketones are not oxidized by complex 1, aldehydes are converted
to mixtures of carboxylic acids and alcohols (Experimental
Section and Table S1 in the Supporting Information, SI). In the
reaction with substituted benzaldehydes, the yields of the
Department 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
E-mail: ictkp@iacs.res.in
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
corresponding carboxylic acids are higher than the alcohols
suggest that two different pathways are operational. The
nucleophilic oxidant directly oxidizes benzaldehydes to benzoic
acid, and in another pathway, benzaldehyde is converted to
equimolar amounts of benzyl alcohol and benzoic acid via
The reactivity of 1 was then tested toward different α-
hydroxy ketones. Two different types of α-hydroxy ketones were
investigated: one without α-C-H bond, and the other with α-C-H
bond (Table 1). The complex oxidatively cleaves the C-C bond
of α-hydroxy ketones and the metal-coordinated benzilate
undergoes two-electron oxidation to form benzophenone
quantitatively. The reaction of 1 with 1-hydroxycyclohexyl phenyl
ketone (1 equiv) affords benzoic acid and cyclohexanone with
50% yield (Figures S2 and S3). It is known that complex 1 reacts
[
28]
Cannizzaro mechanism.
The yield of the major product,
benzoic acid was used to obtain the relative rates for the
competitive oxidations of pairs of aldehydes. Hammett analysis
on 1:1 mixtures of benzaldehyde and para-substituted
benzaldehydes (p-X-C
OMe) gives a ρ value of +0.82 (Figure S1). Thus the iron-oxygen
species from is nucleophilic oxidant, which oxidizes
aldehyde substrates.
6
H
4
CHO, where X = -NO
2
, -Br, -Me, -
with O
2
to undergo intramolecular ligand hydroxylation (90%) in
[
28]
the absence of any substrate.
In the reaction with 1-
1
a
hydroxycyclohexyl phenyl ketone, 35% ligand hydroxylation is
estimated (Figure S4). Therefore, 50% active oxidant is involved
in the C-C bond cleavage reaction, and the rest of the oxidant
hydroxylates the ortho position of one of the phenyl rings of
Table 1. Aliphatic C-C bond cleavage products of different α-hydroxy ketones
in the reaction with complex 1 and dioxygen.
Ph2
Tp ligand. Thus intra-ligand hydroxylation causes low yields of
α-Hydroxy ketone(s)
Product(s)
% Yield/% Intra-
ligand hydroxylation
the
C-C
cleavage
products.
When
2-hydroxy-2-
methylpropiophenone is used as a substrate, benzoic acid and
acetone are obtained in 58(±2)% and 60(±2)% yield, respectively,
along with 27% ligand hydroxylation (Figures S5-S7). No
appreciable change in product yield is observed upon increasing
the concentration of substrate.
5
0(±3)/35
0(±3)
5
The reaction between 1 and 1-hydroxycyclohexyl phenyl
1
8
ketone when carried out with
from the substrate displays an ion peak at m/z = 138, which is
two mass unit higher than that obtained in the reaction with O
This supports the incorporation of one oxygen atom of O into
the product (Figure S8). The other product cyclohexanone,
2
O , (methyl) benzoate derived
5
6
9
8(±2)/27
2
.
0(±2)
2
1
8
1(±3)/43
however, does not contain any labeled oxygen from
O
2
. In a
1
6
18
mixed labeling experiment with
O and H O, no incorporation
2 2
of labeled oxygen is observed into the product.
8
0(±3)
(±3)
8
6(±3)
7
1
1
5(±2)/46
0(±2)
Scheme 2. Oxidative C-C bond cleavage of α-hydroxy ketones by complex 1.
2(±3)
To check the importance of the O-H group in the C-C bond
cleavage pathway, reaction of
methylpropiophenone was carried out under oxygen atmosphere
Scheme 2, Experimental Section in SI). In the reaction, the
methoxy derivative of 2-hydroxy-2-methylpropiophenone
remains unreacted without any detectable C-C bond cleavage
product. When natural substrate of CYP17A1, 17-α-
1
with 2-methoxy-2-
6
4(±3)/53
(
8
1
6
(±2)
a
hydroxyprogesterone is used, the C-C bond cleavage of the α-
hydroxy ketone moiety takes place to an extent of 10% forming
androstenedione and acetic acid (Figure 1 and Scheme 3). A
4(±3)
1
8
labeling experiment with
2
O reveals that acetic acid contains
one labeled oxygen atom while androstenedione does not
contain any labeled oxygen product (Figure 1).
8(±3)/30
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To understand the difference in C-C bond cleavage reactivity
1
of two different types of α-hydroxy ketones, time-dependent H
NMR experiments were carried out for the reaction between 1
and benzoin (Figure S14). In the reaction, the yield of benzoic
acid increases with time, whereas that of benzaldehyde remains
constant after 3 min. Thereafter benzoin is linearly converted to
benzoic acid. Thus benzoic acid is not derived from
benzaldehyde via oxidation by a metal-oxygen species. For
unsymmetrical substrate 4-chlorobenzoin, the yield of 4-
chlorobenzoic acid is slightly (8-11%) higher than that of benzoic
acid. While benzaldehyde is detected as a minor product, 4-
chlorobenzaldehyde is not observed in the reaction. Therefore
benzaldehyde and 4-chlorobenzoic acid are formed (with about
8% yield for each product) via a pathway similar to that for the α-
hydroxy ketones without α-C-H bond. The labeling experiment
1
8
with
2
O for the reaction of 1 with 4-chlorobenzoin reveals
partial incorporation of labeled oxygen into 4-chlorobenzoic acid
and benzoic acid, whereas benzaldehyde does not contain any
labeled oxygen (Figure S15). The partial incorporation of labeled
oxygen atom into carboxylic acid products suggests that a
metal−oxygen intermediate species exchanges its oxygen atom
Figure 1. GC-mass spectra of (a) androstenedione, and of acetic acid formed
[26]
with water in the reaction pathway.
A mixed labeling
16
in the reaction between 1 and 17α-hydroxyprogesterone with (b)
2
O and (c)
18
16
18
18
16
2
experiment with O and H2 O for the reaction between 1 and
2
O . Inset c: molecular ion peak for acetic acid at m/z = 60 ( O) and 62 ( O).
4-chlorobenzoin further supports that labeled oxygen atom from
water is incorporated both into benzoic acid and 4-chlorobenzoic
acid (Figure S16).
All these observations indicate that the C-C bond cleavage
reaction of benzoin (or substituted benzoin) by complex 1 takes
place via two different pathways. In one pathway, it forms a 1:1
mixture of benzoic acid and benzaldehyde and in other pathway
benzoin forms two equivalents of carboxylic acids. With benzoin-
type substrates, only 8-10% reaction takes place in the former
pathway. For cyclic substrate, 2-hydroxycyclohexanone, only the
second pathway is followed to afford the dicarboxylic acid.
Scheme 3. Layase of the C17-C20 bond of 17-α-hydroxyprogesterone by
complex 1.
On the contrary, the α-hydroxy ketones with α-C-H bond
afford the corresponding acids as the major cleavage products,
whereas aldehydes are obtained in trace amounts (Scheme 4).
The reaction of 1 with benzoin (1 equiv) yields 75(±2)% benzoic
acid, 10(±2)% benzaldehyde and 12(±3)% benzil (Table 1,
Figure S9). 1-(4-Chlorophenyl)-2-hydroxy-2-phenylethanone
forms a mixture of 4-chlorobenzoic acid 91(±3)%, benzaldehyde
8(±3)%, benzoic acid 80(±3)%, and 4-chlorobenzil 6(±3)%
(
Figure S10). 4,4'-Dimethoxy benzoin is converted to 4-
methoxybenzoic acid 64(±3)%, 4-methoxybenzaldehyde 8(±2)%
and 4,4'-dimethoxybenzil 14(±3)% (Figure S11). In all these
reactions, intramolecular ligand hydroxylation is also observed
(
Figure S12). When 2-hydroxycyclohexanone is made to react
Scheme 5. Proposed mechanism for the oxidative C-C bond cleavage of α-
hydroxy ketones with dioxygen by a biomimetic iron(II)-benzilate complex.
with complex 1 and dioxygen, adipic acid is obtained as the sole
product (Figure S13).
With the results discussed above, a mechanistic proposal is
put forward in Scheme 5. The reaction of complex 1 with
dioxygen proceeds via the formation of an iron(II)-hydroperoxide
[
28]
(
I) species. In the enzymatic system, it has been shown that
Scheme 4. C-C bond cleavage of α-hydroxy ketones containing α-C-H bond.
the hydroxy group of substrate and the oxygen atoms of
peroxide could be responsible in initiating the aliphatic C-C
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COMMUNICATION
[
23]
bond cleavage. The experimental result from the reaction of 1
with O and the methoxy derivative of α-hydroxy ketone strongly
supports the role of hydroxy group. In analogy to the "peroxo-
TKP acknowledges the Indian National Science Academy for the
financial support (Young Scientist Project). SB thanks DST
(INSPIRE) and RR thanks CSIR for research fellowships.
2
[
23]
hemiacetal" intermediate observed in CYP17A1,
an iron(II)-
alkylperoxo species (II) is thus proposed to form upon
nucleophilic iron(II)-hydroperoxide to the electrophilic carbonyl
carbon of α-hydroxy ketone. Intermediate II further undergoes C-
C bond cleavage to yield an iron(II)-benzoate-hydroxide species
Keywords: iron • dioxygen • nucleophilic oxidant • C-C bond
cleavage • α-hydroxy ketone
[
[
1]
2]
S. Fetzner, Appl. Environ. Microbiol. 2012, 78, 2505.
C. J. Allpress, L. M. Berreau, Coord. Chem. Rev. 2013, 257, 3005.
M. Akhtar, J. N. Wright, P. Lee-Robichaud, J. Steroid. Biochem. Mol.
Biol. 2011, 125, 2.
(
III) and ketone. For benzoin-type substrates, the minor pathway
involves the above mechanism. The major pathway, which forms
two equivalents of carboxylic acid with incorporation of oxygen
atom from water, involves the formation of iron(II)-α-hydroxy
ketone complex after replacing the hydroperoxide group of
intermediate I (Scheme S1). Benzoin is likely to undergo
enolization and subsequent formation of a planar chelate ring at
the iron(II) center is a driving force to replace hydroperoxide.
[3]
[4]
T. D. H. Bugg, C. J. Winfield, Nat. Prod. Rep. 1998, 5, 513.
M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev. 2004,
[
5]
104, 939.
[
[
[
[
6]
7]
8]
9]
T. D. H. Bugg, S. Ramaswamy, Curr. Opin. Chem. Biol. 2008, 12, 134.
E. G. Kovaleva, J. D. Lipscomb, Nat. Chem. Biol. 2008, 4, 186.
D. Buongiorno, G. D. Straganz, Coord. Chem. Rev. 2013, 257, 541.
G. D. Straganz, B. Nidetzky, ChemBioChem 2006, 7, 1536.
2
Detection of hydrogen peroxide in the reaction of 1 and O with
benzoin supports this hypothesis (Experimental section, SI). The
substrate coordinated species (II') then reacts with dioxygen to
afford the cleavage products following a mechanism (Scheme
[10] D. J. Hopper, Biochem. J. 1986, 239, 469.
[11] J. Guo, P. Erskine, A. R. Coker, J. Gor, S. J. Perkins, S. P. Wood, J. B.
Cooper, Acta Cryst. 2015, F71, 1258.
[
26]
[
[
12] S. Nakajin, P. F. Hall, J. Biol. Chem. 1981, 256, 3871.
13] H. J. Barnes, M. P. Arlotto, M. R. Waterman, Proc. Natl. Acad. Sci. U. S.
A. 1991, 88, 5597.
S1) similar to that reported recently by us.
The resulting
iron(II)-benzoate product rapidly gets oxidized to form iron(III)-
phenolate-benzoate complex. The above mechanism explains
the formation of two equivalents of carboxylic acid as well as the
formation of diketone.
[
14] M. K. Akhtar, S. L. Kelly, M. A. Kaderbhai, J. Endocrinol. 2005, 187,
267.
[
[
15] P. F. Hall, J. Steroid. Mol. Biol. 1991, 40, 527.
16] R. Keegan, A. Lebedev, P. Erskine, J. Guo, S. P. Wood, D. J. Hopper,
S. E. J. Rigby, J. B. Cooper, Acta Cryst. 2014, D70, 2444.
17] D. L. Corina, S. L. Miller, J. N. Wright, M. Akhtar, J. Chem. Soc., Chem.
Commun. 1991, 782.
In conclusion, the reactivity of a nucleophilic iron-oxygen
oxidant from an iron(II)-benzilate complex toward different -
hydroxy ketones has been investigated. Hammett analysis and
interception studies with external substrates suggest formation
of a putative iron(II)-hydroperoxide species. The oxidant cleaves
the aliphatic C-C bonds of α-hydroxy ketones to afford carboxylic
acids and ketones. Isotope labeling studies establish that that
one of the oxygen atoms from dioxygen is incorporated into
carboxylic acid. Furthermore, the iron(II) complex cleaves the
C17-C20 bond of 17-α-hydroxyprogesterone affording
androstenedione and acetic acid. Although the coordination
environment and spin state of iron-oxygen oxidant are different
than those in heme systems, the iron complex discussed here
shows reactive similarities to the lyase activity of cytochrome
P450 17A1 (CYP17A1). Additionally, this work demonstrates
[
[
18] P. Robichaud, J. N. Wright, M. Akhtar, J. Chem. Soc., Chem. Commun.
1994, 1501.
[19] M. Akhtar, D. Corina, S. Miller, A. Z. Shyadehi, J. N. Wright,
Biochemistry 1994, 33, 4410.
[
20] P. S. Pallan, L. D. Nagy, L. Lei, E. Gonzalez, V. M. Kramlinger, C. M.
Azumaya, Z. Wawrzak, M. R. Waterman, F. P. Guengerich, M. Egli, J.
Biol. Chem. 2015, 290, 3248.
[
[
21] M. C. Gregory, I. G. Denisov, Y. V. Grinkova, Y. Khatri, S. G. Sligar, J.
Am. Chem. Soc. 2013, 135, 16245.
22] F. K. Yoshimoto, E. Gonzalez, R. J. Auchus, F. P. Guengerich, J. Biol.
Chem. 2016, 291, 17143.
[23] P. J. Mak, M. C. Gregory, I. G. Denisov, S. G. Sligar, J. R. Kincaid,
Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 15856.
[
[
[
[
24] M. Sallmann, C. Limberg, Acc. Chem. Res. 2015, 48, 2734.
2
that the O -dependent aliphatic C-C bond cleavage pathway
depends on the nature of substrate.
25] S. Paria, P. Halder, T. K. Paine, Angew. Chem. Int. Ed. 2012, 51, 6195.
26] R. Rahaman, S. Paria, T. K. Paine, Inorg. Chem. 2015, 54, 10576.
27] S. Paria, L. Que, Jr., T. K. Paine, Angew. Chem. Int. Ed. 2011, 50,
11129.
Acknowledgements
[28] S. Paria, S. Chatterjee, T. K. Paine, Inorg. Chem. 2014, 53, 2810.
[
[
29] S. Chatterjee, T. K. Paine, Angew. Chem. Int. Ed. 2015, 54, 9338.
30] S. Chatterjee, T. K. Paine, Angew. Chem. Int. Ed. 2016, 55, 7717.
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Chemistry - A European Journal
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Cleavage of Aliphatic C-C
Bonds. A nucleophilic iron-
oxygen oxidant from an
Shrabanti Bhattacharya,
Rubina Rahaman, Sayanti
Chatterjee and Tapan Kanti
Paine*
iron(II)-benzilate
complex
cleaves the aliphatic C-C
bonds of α-hydroxy ketones
including the C17-C20 lyase
of 17-α-hydroxyprogesterone.
Page No. – Page No.
Aliphatic
C-C
Bond
Cleavage of α-Hydroxy
Ketones by a Dioxygen-
Derived
Nucleophilic
Iron-Oxygen Oxidant
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