Steric control and the mechanism of benzaldehyde oxidation by polypyridyl oxoiron(IV) complexes: Aromatic: versus benzylic hydroxylation of aromatic aldehydes

Article abstract of DOI:10.1039/c7dt03727a

The present study describes the first example of the hydroxylation of benzaldehydes by synthetic nonheme oxoiron(iv) complexes, where the reactivity, chemoselectivity, and mechanism were strongly influenced by the ligand environment of the iron center.

Full text of DOI:10.1039/c7dt03727a

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DOI: 10.1039/C7DT03727A
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Steric Control and Mechanism of Benzaldehyde Oxidation by
Polypyridyl Oxoiron(IV) Complexes: Aromatic versus Benzylic
Hydroxylation of Aromatic Aldehydes
Received 00th January 20xx,
Accepted 00th January 20xx
Ramona Turcas,a Dóra Lakk-Bogáth,a Gábor Speier,a and József Kaizer*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: The present study describes the first example of the (FeII(BPMEN)/BA/H2O2),8 and m-chloroperbenzoic acid
hydroxylation of benzaldehydes by synthetic nonheme oxoiron(IV) (FeII(TPA)/m-CPBA;9 FeII(BPMEN)/m-CPBA8, where BPMEN =
complexes, where the reactivity, chemoselectivity, and N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)ethane-1,2-diamine,
mechanism was strongly influenced by the ligand enviroment of and TPA = tris(2-pyridylmethyl)amine) by nonheme iron
the iron center.
complexes with two cis-labile sites have been described in
detail. Based on their observations oxoiron(IV) and/or
oxoiron(V) via the formation of a (2-acylperoxo)iron(III)
species have been proposed as key intermediates for the
aromatic hydroxylation.7-9 Benzaldehydes may serve as good
candidates to probe biologically relevant C-H hydroxylation,
and to study the mechanism of the hydrogen atom abstraction
and the formation of the concomitant product from the caged
radical pair. Only few examples can be found in the literature,
where high-valent oxometal complexes are directly involved in
the benzaldehyde oxidation.10 The kinetics and mechanisms of
the oxidation of aromatic aldehydes to carboxylic acids, by cis-
[RuIV(bpy)2(py)(O)]2+ and [RuIV(tpy)(bpy)(O)]2+ (bpy = 2,2’-
bipyridine, py = pyridine, and tpy = 2,2’:6’,2”-terpyridine) in
water and acetonitrile have been extensively studied.10b
Oxoiron(IV) complex supported by pentadentate N4Py (N4Py =
N,N’-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) ligand
has been reported to oxidize various substrates by a high
variety of mechanisms, including electron transfer (ET),11a
electron transfer-proton transfer (ET-PT),11b hydrogen atom
abstraction (HAT),11c,d and oxygen atom transfer (OAT).11e,f
Herein, we present the comparison of the stoichiometric
oxidation of benzaldehyde to benzoic acid and/or salicylic acid
by previously characterized (UV-vis, ESI-MS, Mössbauer…)
Pterin-dependent hydroxylases, including phenylalanine
(PheH), tyrosine (TyrH), and tryptophan (TrpH) hydroxylase are
non-heme mononuclear iron enzymes with 2-His-1-carboxylate
platform,1 which are responsible for essential biological
functions such as biosynthesis of tyrosine (PheH), and various
neurotransmitters like dopamine, norepinephrine, epinephrine
(TyrH), and serotonin (TrpH).2 For such enzymes, catalytic
oxoiron(IV) intermediates have been postulated to introduce a
hydroxyl group on the aromatic ring via their electrophilic
attack on the aromatic ring supported by inverse KIE and a NIH
shift.3 High-valent oxoiron complexes are also present as the
reactive intermediates for the oxidative catalysis by
ketoglutarate dependent non-heme iron-containing
-
oxygenases, which can perform hydroxylation of aliphatic C-H
bonds on various substrates.4 In this case the key step is the
hydrogen atom abstraction from the substrate by oxoiron(IV)
species resulting in the caged radical pair [Fe(III)-OH-.R]
followed by the formation of R-OH via oxygen rebound
pathway based on the large KIE values of ~50 for taurin: -KG
dioxygenase4b,d and ~60 for prolyl-4-hydroxylase4e. Synthetic
model systems5 including mononuclear nonheme oxoiron(IV)
intermediates may play a key role to get more insight into the
mechanism of action in these enzymes.
[FeIV(N4Py)(O)]2+,12a ) and [FeIV(asN4Py)(O)]2+,12b
(2 (4) (asN4Py =
As a functional models of pterin-dependent hydroxylases a
stoichiometric hydroxylation of aromatic compounds such as
pendant aromatic ring on the ligand [FeII(6-PhTPA)/PhIO;6
N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine)(Fig. 1).
FeII(6-PhTPA)/tBuOOH) (6-PhTPA
phenyl-2-pyridylmethylamine),7
=
bis(2-pyridylmethyl)-6-
benzoic acid
Department of Chemistry, University of Pannonia, H-8201 Veszprém,
Hungary
E-mail: kaizer@almos.vein.hu.†Electronic Supplementary Information (ESI)
available: Experimental details of oxydations, and kinetic data. See
DOI: 10.1039/x0xx00000x
Fig. 1. Iron(II) and iron(IV) complexes used in this study.
J. Name., 2013, 00, 1-3 | 1
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Interestingly, the aromatic hydroxylation does not occur in the those obtained for the oxidation of PhCHO by cis-
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DOI: 10.1039/C7DT03727A
IV
2+
reaction of
replacement of one of the pyridyl moiety by a more labile 2- and 3.67 M-1 s-1 at 25°C, respectively).10b Finally, activation
pyridylmethyl arm on the N4Py ligand. parameters of
H≠ = 20.5 kJ mol-1 andS≠ = -276 J mol-1 K-1
The oxidation potential of has been studied at 25°C in a were calculated from plots of ln(k/T) versus 1/T in CH3CN over
2 but does with 4
, which may be explained by the [RuIV(bpy)2(py)(O)]2+ and [Ru (tpy)(bpy)(O)] complexes (1.05

2
HAT processes with p-substituted benzaldehydes. The the temperature range 278 to 303 K for the oxidation of
oxidation reactions were carried out anaerobically at room PhCHO (Fig. S1-S2, ESI†)). Activation parameters obtained for
temperature in dry acetonitrile. Organic products were cis-[RuIV(bpy)2(py)(O)]2+ and [RuIV(tpy)(bpy)(O)]2+ were


H≠
S≠
=
=
characterized by GC/MS and quantitated using naphthalene as 45 kJ mol-1,
an internal standard. Complex
of with 2 equiv. of PhIO (<40 min), and the rate of its rapid entropies are typical of associative processes, and indicate that
decomposition, which coincided with the regeneration of
max = 380 and 449 nm), was measured as a function of the

S≠ = -96 J mol-1 K-1, and

H≠ = 20 kJ mol-1,
2
was generated by the reaction -168 J mol-1 K-1, respectively.10b The large negative activation
1
1
the transition state is better organized than the prior step.
(
Competitive reactions were done with para-substituted
concentration of added PhCHO. The yield of benzoic acid was benzaldehyde derivatives in order to evaluate the influence of
almost quantitative (~90%), indicating that the UV-vis spectral electronic factors on the reaction (Fig. 2c, Table S2, ESI†). The
change corresponds to the HAT process. An isobestic point was relative reactivity shows linear correlation (r = 0.99, n = 6) with
observed at 560 nm, indicating that there were no long-lived Hammett’s
intermediates in the conversion of the green species into the 1.21), demonstrating that the rate constant for oxidation of
FeII product. Furthermore, no shifts have been observed in the the benzaldehydes by
is sensitive to changes in the electronic
max value of after the addition of PhCHO, excluding any properties of the aldehyde; electron-donating groups on the
 constant. The reaction constant,  is negative (
=

2
2
complexation with the oxidant (Fig. 2a). The spectral changes substrate increases and electron-withdrawing groups
observed for the reactions between the various benzaldehyde decreases the reaction rate. This result suggests that the
derivatives and
presence of a large excess of PhCHO (130-330 equiv.) obeyed benzaldehydes mediated by complex
pseudo-first order kinetics, and the pseudo-first order rate transition states with charge transfer from the substrate to the
constants increased proportionally with the substrate iron. The relatively small value suggests that there is a small
concentration (Fig. 2b, Table S1, ESI†). From this linear development of positive charge on the substrate in the
2
are virtually identical. The rates in the metal-based oxidant is electrophilic, where the oxidation of
2
proceeds through polar

plotting, the second order rate constant (k2) was determined transition state. Comparison with literature
 values for
to be 8.17 × 10-2 M-1 s-1 at 25°C, which is comparable to that related ET and direct HAT processes suggests that the benzylic
measured in oxidation of benzyl alcohol (9.9 × 10-2 M-1 s-1)11d (aldehydic) oxidation by the oxoiron(IV) intermediate occurs
by
2. Similar values were observed under air and in the by the latter mechanism rather than reaction of the oxo
presence of 5 equivalent of benzyl alcohol as inhibitor (8.36 × species with the aromatic ring, where a more negative
 value
10-2 and 8.29 × 10-2 M-1 s-1, respevtively), excluding the would be expected. This value is also considerably less than
involvement of the dioxygen (autoxidation of benzaldehyde) in -3.0 for the oxidative N-dealkylation reaction of anilines by

=
2
in
the rate-determining step. This value is much smaller than a mechanism that has been described as electron-proton
transfer (ET-PT).11b The Hammett
value is twofold larger than
that observed in the closely related cis-
[RuIV(bpy)2(py)(O)]2+/PhCHO system.10b
-0.65)
Furthermore, threefold larger than those obtained for the
oxoiron(IV)(porphyrin radical cation)/PhCH2OH (
= -0.4),13 and
17-fold larger than those obtained for bona fide nonheme
oxoiron(IV)/PhCH2OH systems (
= -0.07),13 so the oxidation of

(
=


benzaldehydes is much more sensitive to electronic effects
than the benzyl alcohol oxidation was.
Substrate PhCHO/PhCDO kinetic isotope effect (KIE) was
also investigated (Fig. 2b, Table S3, ESI†). The involvement of
the aldehydic C-H bond is indicated by the magnitude of the
kCH/kCD kinetic isotope effects of 26.5. This value is larger than
“classical” KIE values (KIE ~7) and those measured in the
oxidation of PhCHO by cis-[RuIV(bpy)2(py)(O)]2+ and
[RuIV(tpy)(bpy)(O)]2+
complexes
(KIE
~8
and
~6,
Fig. 2. Reactions of [FeIV(N4Py)(O)]2+
CH3CN at 298 K. (a) UV-vis spectral change of
addition of 267 equiv. PhCHO. Inset shows time course of the
decay of monitored at 695 nm. (b) Plot of k’ versus
[substrate] for reactions of (1.5 mM) with PhCHO, and
(2
) with benzaldehydes in
(1.5 mM) upon
respectively),10b lower than that observed for the oxidation of
2
benzyl alcohol (KIE of ~50),11d but comparable to that
measured in the oxidation of series of alkanes (KIE ~20)11c by
2.
2
Based on literature data above, such a large KIE value indicates
that nonheme oxoiron(IV) intermediate activates
benzaldehyde by an H-atom abstraction from the aldehydic C-
2
PhCDO. (c) Hammett plot of log krel against p of para-
substituted benzaldehydes. (d) The logk2 versus BDEC-H plots.
2 | J. Name., 2012, 00, 1-3
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H of benzaldehyde via a tunneling-like HAT mechanism, and
that this C-H bond cleavage is the rate-determining step.15
Very large KIEs were also found in the HAT for different
nonheme iron enzymes, which shows that the C-H bond
cleavage is the rate-determining step, and hydrogen tunneling
plays an important role in these reactions.4b,d Homolytic bond
dissociation energies (BDEs) are known for a variety of
substrates including C-H bond. A good linear correlation exists
between logk2 and C-H BDE for several of the aldehydes
studied here (Fig. 2d). It has a slope of -0.33, which is twofold
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larger than the slope for the oxidation of alkanes by complex
2
(-0.18).11c Similar correlations have been observed in C-H bond
oxidations by mononuclear nonheme oxoiron(IV) species with
a mechanism that involves rate-determining HAT.16
Fig. 3. Reactions of [FeIV(asN4Py)(O)]2+
in CH3CN at 298 K. (a) UV-vis spectral change of
upon addition of 267 equiv. PhCHO. Inset shows time course
of the decay of , and the formation of monitored at 705 and
568 nm. (b) Plot of k’ versus *substrate+ for reactions of (1.5
mM) with PhCHO, and PhCDO. (c) Hammett plot of log krel
against p of para-substituted benzaldehydes. (d) The logk2
versus BDEC-H plots.
(
4
) with benzaldehydes
(1.5 mM)
Similarly to the
2
4
/PhCHO system, the reactivity of
) has been also studied at 25°C in the
FeIV(O)(asN4Py)](ClO4)2 (
4
oxidation reaction of PhCHO and several of its derivatives. The
oxidation reactions were carried out anaerobically at room
temperature in dry acetonitrile. Organic products, derived
from the hydrolyzed complexes, were characterized by GC/MS
4
5
2
(ESI†). Complex
4 was generated by the reaction of 3 with 2
equiv. of PhIO (<40 min), and the rate of its rapid
decomposition at 705 nm, and the formation of a new purple
species (5) with a max at 568 nm were measured as a function
benzaldehyde) in the rate-determining step. Finally, activation
parameters of
 
H≠ = 32 kJ mol-1 and S≠ = -284 J mol-1 K-1
were calculated from plots of ln(k/T) versus 1/T in CH3CN over
the temperature range 278 to 303 K for the oxidation of
PhCHO (Fig. S5-S6, ESI†). The large negative activation
entropies are typical of associative processes, and indicate that
the transition state is better organized than the prior step. To
of the concentration of added PhCHO (Fig. 3a, Table S4, ESI†).
An isobestic point was observed at ~675 nm, indicating that
there were no long-lived intermediates in the conversion of
the green species into the new purple species. UV/VIS spectra
of the stable endpoint chromophores was identical to that of
the independently synthesized iron(III) salicylate complex,
[FeIII(asN4Py)(O2C(C6H4)O)](ClO4), with a characteristic ligand-
get insight into the mechanism of the 4-mediated oxidation,
we have investigated the effect of the substituent in the para-
position of benzaldehyde, and the observed kinetic data were
treated in terms of the Hammett equation (Table S5, ESI†). The
Hammett plot in Fig. 3c shows that a correlation exists
to-metal charge-transfer (LMCT) band at 568 nm (
~2000 M-1
cm-1) (Fig. S3, ESI†). No reaction has been observed by the use
of benzoic acid as a possible candidate, but similar species has
between logkrel and
demonstrates that the rate constant for oxidation of the
benzaldehydes by is also sensitive to changes in the
 with a slope of  = -1.97. The correlation
been formed in the reaction of
3 with m-CPBA (max at 560
nm), and almost identical spectral features were observed for
FeIII(TPA) and FeIII(BPMEN) salicylate complexes.8,9 The
4
electronic properties of the aldehyde, with electron-donating
substituents increasing reactivity and electron-attracting
maximum formation of 3 and 5 was determined to be ~50-50%
in the presence of a large excess of PhCHO by comparing the
intensities of absorption bands at 409 and 568 nm with the
groups decreasing it. The magnitude of the
 value is in a good
known
complexes (Fig. S3, ESI†).
 , and the independently prepared 5
values of 3 12b
agreement with a hydrogen-atom-transfer (HAT) model. Its
value is significantly larger than those observed for the
2
/PhCHO and RuIVO/PhCHO systems above, but considerably
Competitive reactions were also done with para-
substituted benzaldehyde derivatives, and the observed
characteristic LMCT bands (max-1) show linear correlation with
smaller than what is expected for the ET-PT reaction (

~3.0),11b and for the direct oxidation of the aromatic ring (
 ~-
3.9).11a
Hammet’s
 constants (Fig. S4, ESI†). The rates in the presence
Substrate C6H5CHO/C6H5CDO and C6H5CHO/C6D5CHO
kinetic isotope effects (KIEs) were also investigated (Table S5-
S6, ESI†). As shown in Fig. 3b benzaldehyde oxidation can be
associated with two KIEs: ~37 for deuteration of the aldehydic
group and 3.7 for deuteration of the aromatic ring.
Furthermore, there is a clear correlation between the rate
constant for H-atom abstraction and the C-H bond dissociation
energy (BDE) of the aldehydes studied here (Fig. 3d). It has a
slope of -0.50, which is significantly larger than the slope for
of a large excess of PhCHO (133-367 equiv.) showed pseudo-
first order kinetics, and the pseudo-first order rate constants
increased proportionally with the substrate concentration (Fig.
3b). From this linear plotting, the second order rate constant
(k2) was determined to be 2.19 × 10-2 M-1 s-1 at 25°C, which is
comparable to that measured in oxidation of benzyl alcohol
(1.25 × 10-2 M-1 s-1) under same conditions, but 4-fold smaller
than that observed in oxidation of benzaldehyde (8.17 × 10-2
M-1 s-1) by
2. Similar values were observed under air and
dioxygen (2.27 × 10-2 and 2.34 × 10-2 M-1 s-1, respevtively),
the oxidation of alkanes by complex
2 (-0.18). This correlation
together with the large deuterium isotope effect for the
excluding the involvement of the dioxygen (autoxidation of
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oxidation of C6H5CHO/C6H5CDO indicates that the aldehydic C-
H bond breaking is important part of the rate-limiting step, and
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DOI: 10.1039/C7DT03727A
the reaction of
like mechanism. In contrast to the PhCHO oxidation, a large
Hammett value of -3.9, and inverse KIE = kH/kD value of ~0.9
were calculated for the aromatic ring oxidation by complex
involving an initial electrophilic attack on the
aromatic ring resulting in a tetrahedral radical or cationic
4 proceeds by H-atom transfer via tunneling-

2
,

-system of the
-
complex, instead of HAT mechanism. Fractional 1,2 hydrogen
shifts (“NIH shifts”) have also been suggested for the
intramolecular iron(IV)-mediated aromatic substitution
reaction including oxene and nitrene transfer reactions.6 Based
on these results above, the observed C6H5CHO/C6D5CHO KIE of
3.7 may be explained by intramolecular hydrogen shift
between the aromatic and benzylic positions after the HAT
process but still in the rate-determining step before the OH
rebound.
Sceme 1. Proposed mechanisms for benzylic (Route a) and
aromatic (Route b) hydroxylation by oxoiron(IV) complexes.
4
(a) J. C. Price, E. W. Barr, B. Tirupati, J. M. Bollinger, C. Krebs,
Biochemistry, 2003, 42, 7497-7508; (b) Sinnecker, N.
Svensen, E. W. Barr, S. Ye, J. M. Bollinger, F. Neese, C. Krebs,
J. Am. Chem. Soc., 2007, 129, 6168-6179; (c) D. A.
Proshlyakov, T. F. Henshaw, G. R. Monterosso, M. J. Ryle, R.
P. Hausinger, J. Am. Chem. Soc., 2004, 126, 1022-1023; (d) J.
C. Price, E. W. Barr, T. E. Glass, C. Krebs, J. M. Bollinger, J.
Am. Chem. Soc., 2003, 125, 13008-13009; (e) L. M. Hoffart, E.
W. Barr, R. B. Guyer, J. M. Bollinger, C. Krebs, Proc. Natl.
Acad. Sci., 2006, 103, 14738-14743.
In summary, both the benzylic and arene oxidation can be
drawn with the two different tautomers of the intermediate
benzoyl radical as shown in Scheme 1. Since the redox
properties of
1 and 3 complexes are almost identical (E1/2 =
1.01 V and 0.95 V vs. SCE, respectively), the difference in the
mechanisms can be explained by different geometries around
the iron centers. Similarly to the recently published results, the
5
(a) A. R. McDonald, L. Que, Jr., Coord, Chem. Rev., 2013, 257
414-428; (b) L. Que, Jr., Acc. Chem. Res., 2007, 40, 493-500;
(c) W. Nam, Acc. Chem. Res., 2007, 40, 465-465.
M. P. Jensen, M. P. Mehn, L. Que, Jr., Angew. Chem. Int. Ed.,
2003, 42, 4357-4360.
,
aromatic ring oxidation occurs only in the presence of
has two cis-labile sites for the formation of a six-membered-
ring transition state. The benzylic oxidation by and can be
4, which
6
7
8
9
2
4
M. P. Jensen, S. J. Lange, M. P. Mehn, E. L. Que, L. Que, Jr., J.
Am. Chem. Soc., 2003, 125, 322.
S. Taktak, M. Flook, B. M. Foxman, L. Que, Jr., E. V. Rybak-
Akimova, Chem. Commun., 2005, 5301-5303.
N. Y. Oh, M. S. Seo, M. H. Lim, M. B. Consugar, M. J. Park, J.-
U. Rohde, J. Han, K. M. Kim, J. Kim, L. Que, Jr., W. Nam,
Chem. Commun., 2005, 5644-5646.
drawn by an intermolecular tunneling-like HAT processes in
both cases. In summary, we can conclude that the
replacement of a pyridyl arm with a 2-pyridylethyl arm can
significantly affect the chemoselectivity of the oxoiron(IV)
species in hydroxylation reactions of benzaldehydes.
10 (a) G. Green, W. Griffith, D. H. Hollinshead, S. V. Ley, M.
Schroder, J. Chem. Soc., Perkin Trans. 1, 1984, 681-686; (b)
W. K. Seok, T. J. Meyer, Inorg. Chem., 2005, 44, 3931-3941.
11 (a) S. P. de Visser, K. Oh, A.-R. Han, W. Nam, Inorg. Chem.,
2007, 46, 4632-4641; (b) K. Nehru, M. S. Seo, J. Kim, W. Nam,
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Y. Oh, J. U. Rohde, W. J. Song, A. Stubna, J. Kim, E. Munck, W.
Nam and L. Que Jr., J. Am. Chem. Soc., 2004, 126, 472; (d) N.
Y. Oh, Y. Suh, M. J. Park, M. S. Seo, J. Kim, W. Nam, Angew.
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Seo, M. J. Park, K. M. Kim, W. Nam, Chem. Commun., 2005,
1405-1407; (f) D. Lakk-Bogáth, G. Speier, J. Kaizer, New J.
Chem., 2015, 39, 8245.
Notes and references
§ This work was supported by a grant from The Hungarian
Research Fund (OTKA) K108489, GINOP-2.3.2-15-2016-00049,
and the New National Excellence Program of the Ministry of
Human Capacities-2016-3-IV. (D. Lakk-Bogáth).
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Dalton Transactions
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DOI: 10.1039/C7DT03727A
Steric Control and Mechanism of Benzaldehyde Oxidation by Polypyridyl Oxoiron(IV)
Complexes: Aromatic versus Benzylic Hydroxylation of Aromatic Aldehydes
Ramona Turcas, Dóra Lakk-Bogáth, Gábor Speier, and József Kaizer*
Stoichiometric oxidation of benzaldehyde to benzoic acid or salicylic acid by [FeIV(N4Py)(O)]2+
and [FeIV(asN4Py)(O)]2+ complexes have been carried out.