Diruthenium Diacetate Catalysed Aerobic Oxidation of Hydroxylamines and Improved Chemoselectivity by Immobilisation to Lysozyme

Article abstract of DOI:10.1002/cctc.201701083

A new green method for the preparation of nitrones through the aerobic oxidation of the corresponding N,N-disubstituted hydroxylamines has been developed upon exploring the catalytic activity of a diruthenium catalyst, that is, [Ru₂(OAc)₄Cl]), in aqueous or alcoholic solution under mild reaction conditions (0.1 to 1 mol % catalyst, air, 50 °C) and reasonable reaction times. Notably, the catalytic activity of the dimetallic centre is retained after its binding to the small protein lysozyme. Interestingly, this new artificial metalloenzyme conferred complete chemoselectivity to the oxidation of cyclic hydroxylamines, in contrast to the diruthenium catalyst.

Full text of DOI:10.1002/cctc.201701083

Accepted Article
Title: Diruthenium Diacetate-Catalyzed Aerobic Oxidation
of Hydroxylamines and Improved Chemoselectivity by
Immobilization to Lysozyme
Authors: Flavia Lupi, Tiziano Marzo, Giampiero D'Adamio, Sara
Cretella, Francesca Cardona, Luigi Messori, and Andrea Goti
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10.1002/cctc.201701083
ChemCatChem
FULL PAPER
Diruthenium Diacetate-Catalyzed Aerobic Oxidation of
Hydroxylamines and Improved Chemoselectivity by
Immobilization to Lysozyme
Flavia Lupi,[a,b] Tiziano Marzo,#[c,a] Giampiero D’Adamio,#[a] Sara Cretella,[a] Francesca Cardona,*[a,d]
Luigi Messori,*[a] and Andrea Goti*[a,d]
Dedication ((optional))
Abstract: A new green method for the preparation of nitrones
through the aerobic oxidation of the corresponding N,N-disubstituted
hydroxylamines has been developed while exploring the catalytic
activity of a diruthenium catalyst, i.e. [Ru2(OAc)4Cl]), in aqueous or
alcoholic solution under mild reaction conditions (0.1 to 1 mol%
catalyst, air, 50 °C) and reasonable reaction times. Notably, the
catalytic activity of the dimetallic center is retained after its binding to
the small protein lysozyme. Interestingly, this new artificial
metalloenzyme conferred complete chemoselectivity to the oxidation
of cyclic hydroxylamines, in contrast to the diruthenium catalyst.
We were intrigued by the possible use of [HEWL/Ru2(OAc)2]
in catalysis and speculated that the persistence of the intact
diruthenium motif would guarantee preservation of the
interesting and peculiar catalytic properties exhibited by
Ru2(OAc)4Cl. This latter complex showed indeed significant
catalytic properties, particularly in redox processes, either as a
catalyst for reduction[2] or, more commonly, for oxidation
reactions.[3] Likely, this dual behaviour is to be ascribed to the
mixed valence nature of the dimetallic center, containing two
ruthenium atoms in +2 and +3 oxidation state. We were
particularly attracted by the reported oxidation reactions of
amines to imines,[3b] that utilize oxygen as the stoichiometric
reagent, thus offering the chance of developing a green
oxidation methodology. At the same time, in view of the scientific
Some of us recently reported the synthesis and structural
characterisation of a mixed valence diruthenium adduct of
lysozyme, ([HEWL/Ru2(OAc)2], hereafter) originating from its
direct reaction with the lantern-like complex diruthenium
tetraacetate chloride, under physiological-like conditions.[1]
Interestingly, combined ESI-MS and RX studies, revealed that
the binding of the [Ru2(µ-O2CCH3)2-(H2O)2]3+ fragment occurs
selectively in two different sites, at the level of two exposed
aspartate groups, acting as bidentate ligands for the ruthenium
atoms (Figure 1).
interest in the use of nitrones (as spin traps or agents for drug-
[5],[6]
related diseases),[4] their synthetic applications
and their
obtainment by oxidation methods,[5a],[7] we also aimed at
investigating the catalytic use of Ru2(OAc)4Cl in the oxidation of
N,N-hydroxylamines to the corresponding nitrones. This
transformation occurs readily with a number of reagents;[5a],[8]
however, only very few catalytic methods are usually successful,
when simply using air or oxygen as the stoichiometric oxidant.[9]
Moreover, in the presence of the above diruthenium catalyst, the
use of hydroxylamines as substrates opens a new question.
Indeed, hydroxylamines can be also reduced to the
corresponding amines;[10] consequently, the use of a catalyst
which is also able to promote reduction reactions, as
Ru2(OAc)4Cl is, poses
a
problem
concerning
the
chemoselectivity of the reaction itself to produce either nitrones
or amines.
Figure 1. Structure of lysozyme-Ru2(OAc)2 complex determined by
crystallography.
In this paper we report the results emerging from preliminary
studies, which demonstrate: i. the feasibility of green aerobic
catalytic oxidation of hydroxylamines to nitrones under
Ru2(OAc)4Cl catalysis; ii. the proof-of-concept for the catalytic
use of [HEWL/Ru2(OAc)2] in this same reaction; iii. an increase
in chemoselectivity for the oxidation reaction when the
Ru2(OAc)4Cl complex is replaced by its protein derivative.
[a]
F. Lupi, Dr. T. Marzo, G. D'Adamio, Prof. F. Cardona, Prof. L.
Messori, Prof. A. Goti
Dipartimento di Chimica "Ugo Schiff"
Università di Firenze
Via della Lastruccia 3-13, 50019 Sesto Fiorentino (FI), Italy
E-mail:francesca.cardona@unifi.it; luigi.messori@unifi.it;
andrea.goti@unifi.it
Table 1. Aerobic oxidation of N,N-dibenzylhydroxylamine catalyzed by
Ru2(OAc)4Cl.[a]
[b]
[c]
LENS, University of Florence, Via Nello Carrara 1, 50019 Sesto
Fiorentino (FI), Italy.
Dr. T. Marzo
Entry
1
Cat
(mol%)
Ox
air
Solvent
H2O
Conc
(M)
Time
(h)
Conv.
(%)[b]
Dipartimento di Chimica e Chimica Industriale (DCCI)
Università di Pisa, Via Moruzzi, 13, 56124 Pisa, Italy
Associated with CNR-INO, Via Nello Carrara 1, Sesto Fiorentino
(FI), Italy
[d]
0
1x10-3
24
0
#
T. M. and G. D. equally contributed to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cctc.201701083
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catalyst concentration to 0.1 mol% and use of oxygen as the
oxidant afforded the same 98% conversion and 82% yield of
pure 2a recovered after FCC, albeit after longer reaction time
(168 h, Table 1, Entry 4). A good 85% conversion was also
achieved when a solution of 1a in a 1:1 H2O/AcOEt mixture (10-3
M) was stirred at 50 °C for 24 h (Table 1, Entry 6). Even in this
2
3
4
5
1.0
1.0
0.1
0
air
air
O2
air
H2O
H2O
H2O
1x10-3
1x10-4
1x10-4
1x10-3
24
93
50
98[c]
98[d]
0
168
24
H2O/AcOEt
1:1
case,
a blank experiment without catalyst excluded the
spontaneous oxidation of the model substrate under the same
reaction conditions. A final oxidation test was performed with a
solution of 1a in a 3:1 H2O/toluene mixture (10-4 M) using oxygen
as the oxidant and 0.1 mol% of Ru2(OAc)4Cl catalyst. However,
after heating at 50 °C for 72 h, only 50% conversion was
reached (Table 1, Entry 13), comparable to that obtained with
toluene alone employed as the solvent.
6
1.0
air
H2O/AcOEt
1:1
1x10-3
24
85
7
1.0
1.0
1.0
0.1
1.0
1.0
0.1
air
air
air
O2
air
O2
O2
MeOH
1x10-3
1x10-4
1x10-3
0.5x10-3
1x10-3
1x10-3
1x10-4
40
67
72
67
29
50
65
50
8
EtOH
72
9[e]
10[e]
11
12[e]
13
(CF3)2CHOH
(CF3)2CHOH
Toluene
24
Having identified the optimal reaction conditions for the
complete conversion of substrate 1a (Table 1, Entry 3), the
substrate scope of the reaction using 1 mol% of Ru2(OAc)4Cl
catalyst, water as solvent and air as the oxidant was addressed.
In isolated cases MeOH or EtOH were used instead of water in
order to avoid hydrolysis of sensitive nitrones. A series of both
alkyl and benzyl hydroxylamines 1a-1i, including cyclic and
enantiopure ones, were subjected to the above mentioned
conditions (Table 2; for detailed references and characterization
of new products, see the Supporting Information). Carbohydrate-
derived polyhydroxylated hydroxylamines 1d-1i were included in
the selected substrates, since the corresponding nitrones are
useful tools for the total synthesis of natural products and other
biologically active polyhydroxylated alkaloids.[11] Good to
excellent conversions were obtained in all cases, apart from
hydroxylamine 1i, which did not react even after prolonged
reaction times, at 50 °C (Table 2, Entry 12). In this single case,
we may postulate that the steric hindrance of the isopropylidene
protecting groups hampers the coordination of the substrate to
the catalyst, and therefore inhibits the reaction. Concerning
hydroxylamine 1b, the moderate conversion observed is
ascribed to the milder reactions conditions used. Indeed, it was
stirred in H2O at room temperature for 72 h, and 50% of
192
24
Toluene
102
72
H2O/Toluene
3:1
[a] Reaction conditions: substrate (0.21-2.4 mmol), oxidant, catalyst, 50 °C;
[b] Determined by 1H NMR; [c] 71% of nitrone was obtained by FCC; [d] 82%
of nitrone was obtained by FCC; [e] Reaction performed at room temperature.
N,N-Dibenzylhydroxylamine (1a) was selected as a model
substrate (0.21-2.3 mmol) to study the best reaction conditions
for its oxidation to nitrone 2a, in the presence of 1 mol% or 0.1
mol% of Ru2(OAc)4Cl catalyst and a baloon of air or pure oxygen,
respectively, as the stoichiometric oxidants. An initial screening
of different solvents was undertaken. First, due to the good
solubility of hydroxylamine 1a in alcohols, we investigated
MeOH and EtOH, using air as the oxidant, but the conversion of
the starting material was not complete even after 40-72 h at
50 °C (respectively 67% and 72%, Table 1, Entry 7 and 8). The
use of hexafluoro-2-propanol (HFIP) as fluorinated alcoholic
solvent gave a good conversion (67%) after 24 h at room
temperature, but only 29% conversion was observed employing
0.1 mol% of Ru2(OAc)4Cl catalyst and oxygen as the oxidant
after 192 h (Table 1, Entry 10 vs 9). By heating hydroxylamine
1a at 50 °C in a 10-3 M solution in toluene for 24 h with 1.0 mol%
catalyst, an unsatisfactory conversion (50%) was obtained
(Table 1, Entry 11). A better conversion (65%) was observed
using oxygen as the oxidant and the same catalytic amount of
Ru2(OAc)4Cl after 72 h without heating (Table 1, Entry 12). In
view of using [HEWL/Ru2(OAc)2] in catalysis, water was the ideal
solvent in consideration of the solubility and stability of lysozyme
in water. The low solubility of hydroxylamine 1a in H2O prompted
us to use higher dilution ([substrate] = 10-4 M) and heating at
50 °C, that turned to be a good compromise between solubility
and reactivity. Under these conditions, a nearly complete
conversion (98%) of the starting material was afforded in 50 h
(Table 1, Entry 3 vs 2), and 71% of pure 2a was recovered after
purification by FCC. A control experiment was carried out in the
absence of catalyst and no conversion was detected under the
same reaction conditions (Table 1, Entry 1). Reduction of
1
substrate conversion was observed by analysis of the H NMR
spectrum of the crude mixture. The crude mixture was not
purified by FCC due to high volatility of both 1b and 2b. When
unsymmetrically disubstituted hydroxylamines were tested, two
isomeric nitrones were always formed (Table 2, Entries 3-5 and
7–10). The oxidation of N-benzyl-N-octylhydroxylamine (1c)
gave an almost equimolar mixture of 2Ac and 2Bc isomers
independently on the solvents used (H2O, MeOH or EtOH). As
previously observed,[9b] heating favoured the hydrolysis of the
oxidation products, especially for the less stable nitrone 2Bc.
For this reason, the oxidation was performed at room
temperature for a prolonged time (Table 2, Entries 3-5). The
best solvent for this substrate was EtOH (100% conversion, 74%
yield), since MeOH gave only 45% conversion and H2O led to
62% conversion after 24 h with formation of hydrolysis products
when the reaction times exceeded 24 h The oxidation of
unsymmetrically disubstituted hydroxylamine 1h, performed in
H2O for 5 h, proceeded with 46% of conversion and formed the
two isomeric nitrones 2Ah and 2Bh in 2:1 ratio (Table 2, Entry
10). The reaction went to completion upon 46 h heating, and a
better 56% yield (56%) was obtained (Table 2, Entry 11).
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Table 2. Aerobic oxidation of N,N-disubstituted hydroxylamines with 1.0 mol% Ru2(OAc)4Cl.[a]
Entry
Substrate
Temp
(°C)
Solvent
Time
(h)
Product(s)
Conv
(%)[b]
2A:2B:2C
Ratio[b]
Yield
(%)[c]
1
2
50
r.t.
H2O
H2O
50
72
98
50
-
-
71
-
3
4
5
r.t.
r.t.
r.t.
MeOH
H2O
EtOH
264
24
168
45
62
100
43:57:0
50:50:0
50:50:0
-
-
74
6
50
H2O
60
100
-
40
7
8
9
50
50
50
H2O
H2O
H2O
4
2
2
100
100
100
27:19:54[d]
24:44:32
7:56:37
62 (29)
47
68 (43)
10
11
50
50
H2O
H2O
5
46
46
100
67:33:0
35
56
71:29:0[e]
50
H2O
45
-
0
-
-
12
[a] Reaction conditions: substrate (0.13-0.23 mmol), Ru2(OAc)4Cl (1.0 mol%), solvent, air; [b] Determined by 1H NMR; [c] Combined recovered yield of products
after FCC (the combined yield of nitrones are reported in parenthesis); [e] The formation of amines was reduced using NMO or aqueous hydroxylamine as
sacrificial agents; [f] Traces (<5% yield) of a decomposition product were isolated in this case.
The oxidation of cyclic carbohydrate-derived hydroxylamines
1e-1g afforded both isomeric nitrones 2A and 2B after heating at
50 °C in H2O for only 2-4 h. In all cases, considerable amounts
of the reduced amines 3Ce-3Cg were observed (Table 2,
Entries 7-9). The mixed valence property of the Ru2(OAc)4Cl
catalyst might well account for the reduction of hydroxylamine to
amine. In order to limit the formation of the reduced products, N-
methylmorpholine N-oxide (NMO) or aqueous NH2OH were
employed as additional sacrificial agents. Their use, albeit giving
some success, led to lower conversion of the starting materials
(see the Supporting Information for further details). Interestingly,
at least with hydroxylamine 1e, lowering the amount of catalyst
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10.1002/cctc.201701083
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to 0.1 mol% but with oxygen instead of air as the oxidant (Table
3, Entry 3 vs Table 2 Entry 7) led to unchanged regioselectivity
in the formation of the two nitrones 2Ae/2Be, but improved
chemoselectivity. Indeed, the amount of undesired amine was
decresed by half, though the reaction was slightly slower (7 h vs
4 h). A slight reduction in the amount of formed amine was also
observed in the reaction of 1g when oxygen was used as the
stoichiometric oxidant (Table 3, Entry 5 vs Table 2, Entry 9).
were run with 0.1 mol% catalyst under an oxygen atmosphere,
due to the best results in terms of chemoselectivity obtained
under these conditions with the chosen substrates. The results,
shown in Table 3, clearly demonstrate that the catalytic activity
is retained in [HEWL/Ru2(OAc)2]. Even more significant, the use
of [HEWL/Ru2(OAc)2] completely suppressed the formation of
the undesired amine for all hydroxylamines (Table 3, Entries 2, 4
and 6), leading to improved yields of nitrones with negligible
effects on regioselectivity.
Oxidation of the hydroxylamine 1d occurred smoothly at
50 °C with 100% conversion after 60 h at 50 °C, and pure
nitrone 2d was recovered in 40% yield after FCC (Table 2, Entry
6). This relatively low yield can be ascribed to formation of the
reduction product (the amine) which is volatile and, for this
reason, was not detected in the crude mixture. In summary,
Table 2 discloses how Ru2(OAc)4Cl (1 mol %) catalyst is able to
In order to exclude that oxidation was effected by the
diruthenium moiety released from the adduct, the following
control experiments were undertaken on substrate 1e. When the
oxidation of 1e was carried out with 0.1 mol% Ru2(OAc)4Cl in the
presence of an equimolar amount or of a ten-fold excess of
HEWL (0.1 mol% or 1 mol%, respectively), very similar results to
the use of Ru2(OAc)4Cl alone were obtained (see Supporting
afford the aerobic oxidation of
a wide range of N,N-
hydroxylamines to nitrones in water, the best sustainable solvent. Information). Particularly, the nitrones 2Ae and 2Be were
This result is remarkable, also considering that the few existing
aerobic procedures for this transformation[9] are scarcely
reproducible,[9a] use non green solvents[9a] or employ higher
amounts (5-10 mol%) of gold-based heterogenous catalysts in
MeOH as solvent.[9b,c]
obtained in 1.5-1.7 ratio, and, more importantly, the reduction
product, e.g., the corresponding amine 3e, was obtained in
relative amounts comparable to entry 3, Table 3 (32% and 38%
of the crude mixture, respectively, in the presence of HEWL, vs.
26% in its absence). This is a clear evidence that oxidation with
lysozyme-Ru2(OAc)2 complex is catalyzed by the adduct, or a
species derived thereof, which is responsible for the
achievement of complete chemoselectivity.
With the purpose to assess whether the interesting catalytic
properties exhibited by the Ru2(OAc)4Cl catalyst are retained by
the protein adduct [HEWL/Ru2(OAc)2], for its prospective
application as an artificial metalloenzyme, the hydroxylamines
1d, 1e and 1g were selected as model substrates The reactions
Table 3. Aerobic oxidation of N,N-disubstituted hydroxylamines with 0.1 mol% Ru2(OAc)4Cl and 0.1% mol% (lysozyme)Ru2(OAc)2 adduct in H2O.[a]
Entry
Substrate
Catalyst
(0.1 mol%)
Time
(h)
Product(s)
Conv
(%)[b]
2A:2B:2C
Ratio[b]
Yield
(%)[c]
1
2
Ru2(OAc)4Cl
192
264
87
-
-
<40
(lysozyme)Ru2(OAc)2
100
81
3
4
Ru2(OAc)4Cl
7
100
100
45:29:26
66:34:0
68 (39)
95
(lysozyme)Ru2(OAc)2
192
5
6
Ru2(OAc)4Cl
4
100
100
9:62:29
14:86:0
66 (46)
59
(lysozyme)Ru2(OAc)2
168
[a] Reaction conditions: substrate (0.14-0.65 mmol), Oxygen (balloon), H2O, 50 °C, 0.1 mol% catalyst; [b] Determined by 1H NMR; [c] Combined recovered yield
of products after FCC (the combined yield of nitrones are reported in parenthesis).
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(Fig. 2). [1]
In conclusion, we have demonstrated here the feasibility of
green aerobic catalytic oxidation of hydroxylamines to nitrones
under Ru2(OAc)4Cl catalysis and provided the proof-of-concept
for the catalytic use of [HEWL/Ru2(OAc)2] in this same reaction,
thus implying a role of the latter species as a novel artificial
metalloenzyme.[12] We also demonstrated that the reaction
becomes completely chemoselective when Ru2(OAc)4Cl was
replaced by its protein derivative. Additional work is underway to
elucidate the mechanism of the oxidation when the artificial
metalloenzyme is employed and to further extend the scope of
the oxidations with this catalyst.
Figure 2. ESI-MS spectrum recorded after synthesis and purification of
lysozyme-Ru2(OAc)2 complex.
The determination of adduct percentage in the lyophilized powder was
performed in triplicate by a Varian 720-ES Inductively Coupled Plasma
Atomic Emission Spectrometer (ICP-AES). Before the analysis, the
sample was weighed in PE vials and digested in a thermo-reactor at
80 °C for 24 h with 2 mL of aqua regia (HCl suprapure grade and HNO3
suprapure grade in 3:1 ratio). After digestion, the sample was diluted to 6
mL with ultrapure water (≤18 MΩ); 5.0 mL of sample were spiked with 1
ppm of Ge used as an internal standard and analysed. Calibration
standards were prepared by gravimetric serial dilution from a commercial
standard solution of Ru at 1000 mg L . The wavelength used for Ru
determination was 245.657 nm whereas for Ge the line at 209.426 nm
was used. The operating conditions were optimized to obtain maximum
signal intensity, before sample analysis, a rinse solution of HCl suprapure
grade and HNO3 suprapure grade in 3:1 ratio was used in order to avoid
any “memory effect”. A degree of protein metalation of 95.6% was
obtained.
Experimental Section
−
1
General Methods: Commercial reagents were used as received. All
reactions were carried out with magnetic stirring, and were monitored by
TLC on 0.25 mm silica gel plates (Merck F254). Column
chromatographies were carried out on silica gel 60 (32–63 μm) or on
silica gel (230–400 mesh, Merck). Yields refer to spectroscopically and
analytically pure compounds unless otherwise stated. 1H NMR spectra
were recorded with a Varian Mercury-400 or a Varian INOVA 400
instrument at 25 °C. 13C NMR spectra were recorded with a Varian
Gemini-200 instrument. Chemical shifts were calibrated using
tetramethylsilane (1H: δ = 0.00 ppm) and CDCl3 (13C: δ = 77.0 ppm).
Integrals are consistent with assignments, and coupling constants are
given in Hz. For detailed peak assignments, 2D spectra were measured
(COSY, HSQC, NOESY, and NOE when necessary). IR spectra were
recorded with a BX FTIR Perkin–Elmer System spectrophotometer. ESI-
MS data were recorded with a Thermo Scientific™ LCQ fleet ion-trap
mass spectrometer. Elemental analyses were carried out with a Perkin–
Elmer 2400 analyzer. Optical rotation measurements were carried out
with a JASCO DIP-370 polarimeter.
General Procedure for the oxidation of N,N-disubstituted
hydroxylamines to nitrones catalyzed by Ru2(OAc)4Cl complex: The
Ru2(OAc)4Cl catalyst (1.0 mol%) was added to
a solution of
hydroxylamines 1a-i (0.13-0.23 mmol) in H2O, methanol or ethanol, and
the mixture was stirred at the appropriate temperature (see Table 2)
under a flow of air for 2–264 h. The reaction was monitored by TLC or 1H
NMR spectroscopic analysis. Then water was added, the mixture was
extracted with CH2Cl2 or AcOEt and the organic layers were dried over
Na2SO4. Filtration on cotton and evaporation under reduced pressure
afforded nitrones 2a-i. Where possible, flash column chromatography
was used to afford pure nitrones, including separation of constitutional
isomers.
Synthesis of [Ru2(µ-O2CCH3)4Cl]: The complex was synthesized as
reported in literature, using commercially available starting materials and
solvents without further purifications.[1],[13]
General Procedure for the oxidation oxidation of N,N-disubstituted
hydroxylamines to nitrones by (Lysozyme)Ru2(OAc)2 adduct: The
(Lysozyme)Ru2(OAc)2 adduct (0.1 mol%) was added to a solution of
hydroxylamine 1d, 1e or 1g (0.14-0.65 mmol) in H2O and the mixture was
stirred at 50 °C (see Table 3) under a flow of oxygen for 168-264 h. The
reaction was monitored by TLC or 1H NMR spectroscopic analysis. The
catalyst was recovered by filtration through a Millipore Membrane Filter.
The recovered solution was extracted with CH2Cl2 or AcOEt and the
organic layers were dried over Na2SO4. Filtration on cotton and
evaporation under reduced pressure afforded crude mixtures, that were
purified by flash column chromatography to obtain pure nitrones 2d, 2e
and 2g (59-95% yields, see Table 3).
Synthesis and characterisation of [Ru2(µ-O2CCH3)2]-lysozyme
adduct: [Ru2(µ-O2CCH3)4Cl] (1.92 mg, 4 x 10-3 mmol) was slowly added
(8 h), under stirring, to a solution of lysozyme (57.35 mg, 4 x 10-3 mmol)
previously prepared in 20 mM ammonium acetate buffer. The resulting
solution was then dialyzed using Sigma-Aldrich Pur-A-Lyzer™ devices
with nominal cut-off 3.5 KDa. The solution was the lyophilized and the
resulting solid mixture of metalated and non metalated protein, stored at -
20° C.
To assess adduct formation and purity, FIA-HR ESIMS (positive mode,
see ESI for instrumental details and setup) measurements were
performed solubilizing lyophilized powder in 20 mM ammonium acetate
buffer (pH=6.8). Along the signal of native protein at 14303.8 Da, another
signal falling at 14656.7 Da was observed and assigned to the adduct
formed by HEWL with a fragment of the type [Ru2(µ-O2CCH3)2-(H2O)2]3+
Acknowledgements
F.C. and A.G. thank the Ministero della Salute, Regione
Toscana (Ricerca Finalizzata, RF-2011-02347694) and Ente
Cassa di Risparmio di Firenze (grant No 2014/0303) for a
fellowship to G.D. L.M. and T.M. thank Beneficentia Stiftung,
Ente Cassa di Risparmio di Firenze and COST Action CM1105.
T.M. thanks AIRC-FIRC (Fondazione Italiana per la Ricerca sul
Cancro, 3-years Fellowship for Italy - Project Code: 18044) for
financial support. CISM (University of Florence) is acknowledged
for recording ESI-MS spectra and Dr. M. Severi and Prof. R.
Udisti (University of Florence) for ICP-AES measurements.
CIRCMSB is also acknowledged.
Keywords: nitrones • aerobic oxidation • diruthenium catalyst •
lysozyme • chemoselectivity
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[1]
L. Messori, T. Marzo, R. N. Fernandes Sanches, H.-U. Rehman, D. de
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10.1002/cctc.201701083
ChemCatChem
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Flavia Lupi, Tiziano Marzo, Giampiero
D’Adamio, Sara Cretella, Francesca
Cardona,* Luigi Messori,* and Andrea
Goti*
Page No. – Page No.
The novel aerobic oxidation of hydroxylamines to nitrones catalyzed by
Ru2(OAc)4Cl or its adduct with lysozyme [HEWL/Ru2(OAc)2] is reported.
Remarkably, coordination of the diruthenium moiety to the protein imparts complete
chemoselectivity with respect to the metallic complex alone.
Diruthenium Diacetate-Catalyzed
Aerobic Oxidation of Hydroxylamines
and Improved Chemoselectivity by
Immobilization to Lysozyme
This article is protected by copyright. All rights reserved.