cis-Dihydroxylation of alkenes by a non-heme iron enzyme mimic

Article abstract of DOI:10.1055/s-2008-1078248

Using the non-heme iron oxidase active site as a template, a peptidomimetic ligand has been designed, synthesized, and used to effect the dihydroxylation of alkene substrates. Fenton-type radical pathways are also observed. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1078248

2172
LETTER
cis-Dihydroxylation of Alkenes by a Non-Heme Iron Enzyme Mimic
c
is-Dihydroxy
a
lation of
A
r
School of Chemistry F11, University of Sydney, Sydney, NSW 2006, Australia
Fax +61(2)93513329; E-mail: p.rutledge@chem.usyd.edu.au
Received 2 June 2008
Dedicated to Professor Sir Jack Baldwin, on the occasion of his 70th birthday
O
X
X
Abstract: Using the non-heme iron oxidase active site as a tem-
plate, a peptidomimetic ligand has been designed, synthesized, and
used to effect the dihydroxylation of alkene substrates. Fenton-type
radical pathways are also observed.
N
O
Et
O
O
N
X
Asp
Fe
N
Fe
N
His
Ph
OH2
O
H
X
Ph
N
Key words: alkenes, bioorganic chemistry, dihydroxylations, iron,
N
oxygenations
His
1
2
Figure 1 The generalized active-site environment of non-heme iron
oxidase enzymes 1, incorporating the ‘2-His-1-carboxylate’ facial
triad of protein-derived ligands, and two vacant sites for substrate
and/or oxygen binding; and the small-molecule iron(II) complex 2 de-
Nature uses a diverse range of iron-based enzyme systems
to effect the oxidation of hydrocarbon substrates with re-
markable efficiency and exquisite control.1–3 These in-
clude the porphyrin-based cytochrome P450s,1 the soluble signed to mimic the NHIO active site in structure and function
di-iron mono-oxygenases3 and non-heme iron oxidases2
such as naphthalene dioxygenase,4 and isopenicillin N
synthase.5 These biological systems have served as the
starting point for wide-ranging inquiry in search of biomi-
metic iron-based oxidizing systems for application in or-
ganic synthesis.6–9 Amongst the most noteworthy are the
Gif systems pioneered by Sir Derek Barton,6 terpyridyl-
amine-based dihydroxylating agents and other systems
developed by Que et al.,7,9,10 and the bis(pyridinyl/pyrro-
lidinyl) systems applied by the White group to execute an
O
O
t-Bu
O
OH
Ph
Et
HO
N
NH
O
Et
Ph
4
Ph
+
+
6
N
N
HO
OH
Ph
3
5
Scheme 1 The tetradentate ligand 3 required to generate complex 2,
and its retrosynthesis to N-ethylaniline 4, pyridine-2,6-dimethanol 5,
impressive range of selective C–H activation chemistry.11 and the Seebach dioxolonone 6
As part of our endeavor to develop simple amino acid
based systems as the basis of iron-centered catalysts for
The complex 2 is designed to incorporate two vacant cis
hydrocarbon oxidation,12 we report here the synthesis and
application of a tetradentate ligand based on pyridine-2,6-
dimethanol and (S)-mandelic acid that combines with
iron(II) acetate to effect cis-dihydroxylation of alkene
substrates.
sites at the iron center for dioxygen/peroxide binding: Que
has proposed that the presence of two vacant sites cis to
each other is an absolute requirement for small-molecule
iron complexes to effect cis-dihydroxylation using H2O2
as the oxidant.7 Pyridine and tertiary amine nitrogen at-
oms model the histidine ligands, while mandelic acid
mimics both the carboxylate and a water ligand. This de-
sign incorporates two aromatic groups to increase steric
density around the metal and counter the propensity of
iron complexes to give bridged products via competing in-
termolecular oxidation reactions [e.g., Fe(III)–O–Fe(III)
species].
The active site of non-heme iron oxidase enzymes
(NHIOs) is highly conserved and contains an iron(II) cen-
ter coordinated by two histidine residues and an aspartate
or glutamate residue (referred to as the ‘2-His-1-carboxy-
lic acid’ facial triad)13 and one or more water ligands
(Figure 1).
We envisaged a biomimetic complex 2, formed between
iron(II) and the peptidomimetic ligand 3 to mimic the
structure and function of the NHIO active site. The ligand
3 is tetradentate and incorporates two oxygen and two ni-
trogen donors to replicate the iron-binding environment
created by the 2-His-1-carboxylate triad and one water
molecule.
Our retrosynthesis of 3 (Scheme 1) leads to N-ethylaniline
4, pyridine-2,6-dimethanol 5, and the dioxolonone 6 [de-
rived from (S)-mandelic acid and pivaldehyde as reported
by Seebach].14 Ligand 3 was prepared from these starting
materials in seven steps and an overall yield of 27%
(Scheme 2).12,15
Thus diol 5 was first monoprotected using sodium hydride
and tert-butyldimethylsilyl chloride in moderate yield
(57%). The unprotected alcohol was brominated in excel-
lent yield (94%) using carbon tetrabromide and triphe-
SYNLETT 2008, No. 14, pp 2172–2174
Advanced online publication: 05.08.2008
DOI: 10.1055/s-2008-1078248; Art ID: D19208ST
© Georg Thieme Verlag Stuttgart · New York
2
2
.0
8
.2
0
0
8

LETTER
cis-Dihydroxylation of Alkenes
2173
O
O
O
O
O
O
O
t-Bu
t-Bu
t-Bu
OH
Ph
OH
N
OTBS
X
HO
N
O
O
O
Ph
Ph
Ph
N
N
N
Et
Et
iv
vi
vii
i–iii
OH
N
N
Ph
Ph
5
7
8
9
X= OH
X = Br
10
3
v
Scheme 2 Synthesis of ligand 3. Reaction conditions: i. NaH, TBSCl, CH2Cl2, r.t., 5 h, 57%; ii. CBr4, PPh3, CH2Cl2, r.t., 2 h, 94%; iii. LDA-
6 (premixed), THF, –78 °C, 6 h, 91%; iv. TBAF, THF, r.t., 90 min, 88%; v. CBr4, PPh3, CH2Cl2, r.t., 2 h, 77%; vi. EtNHPh, n-BuLi, DMPU
(premixed), THF, –40 °C, 5 h, 85%; vii 1 M LiOH, THF, reflux, 17 h then 1 M HCl, 95%.15
OH
O
OH
OH
OH
OH
H2O
path B
path A
+
+
O
Fe(II), H2O2
2, H2O2
12
11
13
14
15
16
Scheme 3 Potential products from the reaction of cyclohexene 11 under the turnover conditions used: the desired cis-dihydroxylation reaction
mediated by the iron complex 2 (path A), and competing Fenton-type reactivity via the allylic radical, to the alcohol 13, ketone 14, and epoxide
15 products (path B); the trans-diol product 16 could arise from subsequent hydrolysis of 15
nylphosphine, then the newly established bromide was two-oxygen’ peptidomimetic ligand to mimic the NHIO
displaced using the lithium enolate of 6.14 Removal of the active site we have effected alkene cis-dihydroxylation.
silyl ether and bromination of the unmasked alcohol both
proceeded smoothly (88% and 77%) to give 9. Nucleophi-
lic attack on 9 by the lithium amide derived from N-ethyl-
aniline 4 introduced the second nitrogen, and alkaline
hydrolysis to open the dioxolonone revealed the target
ligand 3 in excellent conversion (95%). The complex 2
was prepared in situ by mixing equimolar amounts of
product profile evinces competing reactions via Fenton-
However, the yields and turnover numbers (TON) of diol
products are generally low, and competing pathways are
evidently in operation. Further investigation into the reac-
tion of cyclohexene 11 revealed that as well as the desired
cis-diol 12, significant amounts of the allylic oxidation
products 13 and 14 are also formed (Scheme 3). This
ligand 3 (as its sodium salt) and iron(II) acetate in metha-
nol, then used directly in oxidation reactions with five al-
iphatic alkenes using hydrogen peroxide as the oxidant
(Table 1).16 Diol products were observed for all five of
these substrates and epoxides for four of them. In all cases
yields of diol are greater than of epoxide. In control exper-
iments carried out in the absence of ligand 3 no diol prod-
ucts were observed. Thus ligand 3 and iron(II) acetate
combine to mediate an alkene dihydroxylation reaction
not observed without the ligand: using a ‘two-nitrogen-
type pathways,17 by which iron(II) and hydrogen peroxide
combine to unleash hydroxyl radicals that effect allylic
C–H abstraction and give rise to products 13 and 14.
These radical processes may also give rise to the epoxide
product 15 and thence the trans-diol 16 via epoxide open-
ing.
Thus while we have observed iron-mediated dihydroxyl-
ation of alkene substrates using a biomimetic ligand,
yields are generally low and turnover numbers show that
Table 1 Turnover Products from Reaction of Alkene Substrates with 3, Fe(OAc)2, and H2O2 in Methanol
Substrate
Epoxidea
cis-Diola
mmol
trans-Diola
mmol
Diol/epoxide
Ratioc
mmol
n.d.d,e
0.10
0.18
0.01
0.08
TONb
TONb
0.08
0.01
0.02
0.15
0.10
TONb
0.01
Cyclopentene
Cyclohexene
Cyclooctene
1-Heptene
0.78
0.13
0.21
1.48
0.92
n.d.d
0.13
n.d.d
n.a.
n.a.
0.010
0.019
0.001
0.008
1.3:1
1.2:1
148:1
11.5:1
1-Octene
n.a.
a Results are the average of at least two runs. Yields were determined by gas chromatography, using the single point internal standard method.
b Turnover number (TON) represents the amount of product (mmol) per amount Fe complex (mmol). Actual yields may be higher as complex
was synthesized in situ and used immediately.
c Ratio cis-diol/epoxide.
d n.d. = none detected.
e The epoxide product was not detected for the reaction of cyclopentene, however, it is possible that this product was formed in the reaction but
lost during workup (cyclopentene oxide, bp 102 °C).
Synlett 2008, No. 14, 2172–2174 © Thieme Stuttgart · New York

2174
S. M. Barry, P. J. Rutledge
LETTER
–40 °C. The reaction was stirred at –40 °C for 5 h, then left
to warm to r.t. overnight, The mixture was poured onto half-
saturated NH4Cl solution (4 mL), and the aqueous phase was
extracted with Et2O (3 × 8 mL). The organic extracts were
combined, dried, and concentrated in vacuo. The crude
product was purified by column chromatography (SiO2, 10:1
cyclohexane–EtOAc) to give 10 as a yellow oil (470 mg,
85%).
Compound 10 (150 mg, 0.34 mmol) was dissolved in THF
(0.30 mL); aq LiOH (1 M, 0.44 mL) was added. The mixture
was heated at reflux for 17 h, then cooled to r.t., adjusted to
pH 7 with 1 M HCl and extracted with CH2Cl2 (3 × 1 mL).
The organic phases were combined, dried, and evaporated to
a yellow oil which solidified under vacuum. Trituration with
Et2O (3 × 2 mL) gave 3 as an off-white solid (120 mg, 95%).
Characterization Data for (S)-3-{6-[(Ethyl-phenyl-
amino)-methyl]-pyridin-2-yl}-2-hydroxy-2-phenyl-
propionic Acid 3
the reaction is far from catalytic. This is due primarily to
competing oxidative chemistry mediated by hydroxyl rad-
icals, which diverts a significant amount of the oxidizing
power and gives rise to C–H abstraction and allylic oxida-
tion. Hydroxyl radical intermediates are generated from
the hydrogen peroxide oxidant either by reaction with un-
complexed iron(II) in solution (the traditional Fenton re-
action) or by reaction with an iron-ligand species on a
‘Fenton-type’ path. Either way, the ligand 3 does not af-
ford sufficient control over the chemistry of iron(II) and
hydrogen peroxide. Work is under way to prepare im-
proved ligand architectures which combine more effec-
tively with iron(II) to suppress these competing Fenton
pathways.
Rf = 0.2 (CH2Cl2–MeOH, 10:1); [a]D20 –92.6 (CHCl3, c
0.38); mp 110–112 °C. IR (KBr): nmax = 3355 (w, OH str),
1612 (s, C=O str), 1505 (m, C=C str), 1575 (m, C=C str),
807, 745 cm–1. 1H NMR (300 MHz, CD3OD): d = 1.24 (3 H,
t, J = 7.0 Hz, NCH2CH3), 3.56 (2 H, q, J = 7.0 Hz,
NCH2CH3), 3.64 (1 H, d, J = 14.5 Hz, 1 of CH2CC6H5), 3.53
(1 H, d, J = 14.5 Hz, 1 of CH2CC6H5), 4.57 (2 H, s, CH2N),
6.65 (3 H, m, 3 NC6H5), 7.05 (1 H, d, J = 7.5 Hz, py-Hd),
7.12–7.29 (6 H, m, 3 of CC6H5, 2 of NC6H5, py-Hb), 7.54 (1
H, t, J = 7.0 Hz, py-Hg), 7.76 (2 H, m, 2 of CC6H5). 13C NMR
(75 MHz, CDCl3): d = 11.26 (NCH2CH3), 44.82
Acknowledgment
We wish to thank the Irish Research Council for Science, Enginee-
ring, and Technology for an Embark Award to SMB. This work was
further supported in Ireland by the Centre for Synthesis & Chemical
Biology at University College Dublin under the Programme for Re-
search in Third Level Institutions (PRTLI) administered by the
HEA, and in Australia by the University of Sydney.
References and Notes
(1) Ortiz de Montellano, P. R. Cytochrome P-450 Structure,
Mechanism and Biochemistry; Plenum: New York, 1995.
(2) (a) Que, L.; Ho, R. Y. N. Chem. Rev. 1996, 96, 2607.
(b) Kappock, T. J.; Caradonna, J. P. Chem. Rev. 1996, 96,
2659.
(3) Wallar, B. J.; Lipscomb, J. D. Chem. Rev. 1996, 96, 2625.
(4) Karlsson, A.; Parales, J. V.; Parales, R. E.; Gibson, D. T.;
Eklund, H.; Ramaswamy, S. Science 2003, 299, 1039.
(5) Burzlaff, N. I.; Rutledge, P. J.; Clifton, I. J.; Hensgens, C. M.
H.; Pickford, M.; Adlington, R. M.; Roach, P. L.; Baldwin,
J. E. Nature (London) 1999, 401, 721.
(6) Barton, D. H. R. Tetrahedron 1998, 54, 5805.
(7) Chen, K.; Que, L. Angew. Chem. Int. Ed. 1999, 38, 2227.
(8) (a) Bugg, T. D. H. Tetrahedron 2003, 59, 7075. (b) Costas,
M.; Mehn, M. P.; Jensen, M. P.; Que, L. Chem. Rev. 2004,
104, 939. (c) Shan, X. P.; Que, L. J. Inorg. Biochem. 2006,
100, 421.
(9) Oldenburg, P. D.; Que, L. Catal. Today 2006, 117, 15.
(10) (a) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.;
Bukowski, M. R.; Stubna, A.; Munck, E.; Nam, W.; Que, L.
Science 2003, 299, 1037. (b) Oldenburg, P. D.; Shteinman,
A. A.; Que, L. J. Am. Chem. Soc. 2005, 127, 15672.
(c) Oldenburg, P. D.; Ke, C. Y.; Tipton, A. A.; Shteinman,
A. A.; Que, L. Angew. Chem. Int. Ed. 2006, 45, 7975.
(11) Chen, M. S.; White, M. C. Science 2007, 318, 783.
(12) Krall, J. A.; Rutledge, P. J.; Baldwin, J. E. Tetrahedron
2005, 61, 137.
(13) Hegg, E. L.; Que, L. Eur. J. Biochem. 1997, 250, 625.
(14) (a) Seebach, D.; Naef, R. Helv. Chim. Acta 1981, 64, 2704.
(b) Seebach, D.; Naef, R.; Calderari, G. Tetrahedron 1984,
40, 1313.
(15) Conversion of 5 into 9 was achieved as detailed in ref. 12.
Then a solution of ethylaniline 4 (0.19 mL, 1.49 mmol) in
THF (10 mL) was cooled to 0 °C and n-BuLi (1.5 M, 1.19
mL) added via syringe. The yellow solution was stirred at
0 °C for 20 min, then DMPU (0.22 mL, 1.78 mmol) was
added and the solution stirred for a further 20 min. The
mixture was cooled to –40 °C and added via cannula to a
solution of 9 (500 mg, 1.24 mmol) in THF (10 mL) also at
(CH2CC6H5), 48.85 (NCH2CH3), 55.11 (CH2N), 79.64
(CC6H5), 111.09 (CH of C6H5), 115.25 (py-Cb), 117.43 (py-
Cd), 121.94, 125.17, 125.35, 126.35, 128.15 (CH of C6H5),
136.61 (py-Cg), 141.97 (Cipso of CC6H5), 146.55 (Cipso of
NC6H5), 157.02 (py-Ca), 158.21 (py-Ce), 176.53 (C=O). MS
(ES+): m/z (%) = 377 (100) [MH]+, 359 (45) [MH –H2O]+.
HRMS (ES+): m/z calcd for C23H24N2O3: 377.1865; found:
377.1860 (100%) [MH]+.
(16) Turnover Procedure
Methanol was distilled over CaH2 and degassed in three
freeze–thaw cycles before use. All substrates were distilled
over CaH2 and passed through activated alumina before use
to remove any peroxides. All reactions were carried out
under an atmosphere of argon. Ligand 3 (50 mg, 0.13 mmol)
was dissolved in anhyd CH2Cl2 (0.75 mL) and treated with
NaH (12 mg, 0.53 mmol). After stirring for 45 min at r.t. the
solvent was removed in vacuo to give the sodium salt of 3 as
a white powder. To a suspension of this salt (4.0 mg, 10
mmol) in MeOH (0.20 mL) was added Fe(OAc)2 (1.7 mg, 10
mmol) in MeOH giving a yellow solution which was stirred
at r.t. for 45 min. The solution was diluted with MeOH (15
mL), and the substrate alkene (10 mmol) was added.
Hydrogen peroxide (100 mmol, 30% aq) diluted in MeOH
(1 mL) was added to the stirring solution over 4 h, and the
solution was stirred at r.t. for a further 12 h. The reaction was
reduced in vacuo, diluted with EtOAc and filtered through
SiO2. n-Decane was added as an internal standard. Products
12–16 were analyzed by gas chromatography and GC-MS
and identified unambiguously by comparison with authentic
samples.
Gas chromatography was carried out on a Hewlett-Packard
5890 Series II gas chromatograph fitted with an HP-1ms
column (30 m × 0.25 mm ID, 0.25 mm;S/N US2469051H),
and (to distinguish cis- and trans-diols) a Hewlett-Packard
5890A gas chromatograph fitted with a BP-20 column
(25 m × 0.22 mm ID, 0.25 mm) and ChemStation software.
Both chromatographs were equipped with split/splitless
capillary inlets and flame-ionisation detectors (FID).
(17) Walling, C. Acc. Chem. Res. 1975, 8, 125.
Synlett 2008, No. 14, 2172–2174 © Thieme Stuttgart · New York

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