Ti(iv)-amino triphenolate complexes as effective catalysts for sulfoxidation

Article abstract of DOI:10.1039/c0dt00228c

C ₃-symmetric Ti (iv) amino triphenolate complexes efficiently catalyze, without previous activation and in excellent yields, the oxidation of sulfides at room temperature, using both CHP and the more environment friendly aqueous hydrogen peroxide as terminal oxidants, with catalyst loadings down to 0.01%. The Ti(iv) catalysts and the intermediate Ti(iv)-peroxo complexes have been characterized in solution by ¹H NMR and ESI-MS techniques and via density functional studies.

Full text of DOI:10.1039/c0dt00228c

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Ti(IV)-amino triphenolate complexes as effective catalysts for sulfoxidation†
Miriam Mba,a Leonard J. Prins,a Cristiano Zonta,a Massimo Cametti,b Arto Valkonen,b Kari Rissanenb and
Giulia Licini*a
Received 30th March 2010, Accepted 26th May 2010
First published as an Advance Article on the web 8th July 2010
DOI: 10.1039/c0dt00228c
C3-symmetric Ti (IV) amino triphenolate complexes efficiently catalyze, without previous activation and
in excellent yields, the oxidation of sulfides at room temperature, using both CHP and the more
environment friendly aqueous hydrogen peroxide as terminal oxidants, with catalyst loadings down to
0.01%. The Ti(IV) catalysts and the intermediate Ti(IV)-peroxo complexes have been characterized in
solution by 1H NMR and ESI-MS techniques and via density functional studies.
Introduction
One of the current trends in catalyst design is the use of multiden-
tate ligands for complexation of metal ions.1 Two main advantages
of this strategy are the high thermodynamic stability of the metal
complexes, which allows low catalyst concentrations without loss
of catalyst integrity, and the inhibited formation of multimeric
metal species under turnover conditions. This important feature
is obtained by the nearly complete filling of all vacant sites of
the metal by a single ligand. In addition, the presence of single
mononuclear species greatly facilitates mechanistic studies and
Fig. 1 Ti(IV) mononuclear complexes 1 and 2.
catalyst optimization, especially in stereoselective processes. While
C2 symmetric bidentate ligands have been extensively used, the
C3 symmetric ligands2 have attracted less attention in spite of
their advantages when octahedral complexes are involved in the
catalytic process.2a In recent years, their successful application has
been demonstrated in a significant number of reactions.3
These are very similar to 1 with respect to the trigonal-
bipyramidal coordination geometry of Ti(IV), and to the number
and type of donor atoms. On the other hand, because of the
higher acidity of phenol compared to alcohol, we can expect a
beneficial effect on the stability which allows for lower catalyst
concentrations and loadings.
Our interest in this topic comes from the use of chiral C3-
symmetric Ti(IV)-trialkanolamine complexes 1 (Fig. 1) as oxida-
tion catalysts.3b,c,e,4 In the complexes the tetradentate coordination
occurs through three alkoxides and a tertiary amine. This arrange-
ment results in a high thermodynamic stability of the complex. In
the presence of alkyl hydroperoxides these complexes are able to
catalyze the stereoselective oxidation of alkyl aryl sulfides with
enantiomeric purity up to 93% and turnover numbers (TON) up
to 1000.3b,j We also showed that secondary amines are effectively
oxidized to nitrones with high chemoselectivities, quantitative
yields and TONs reaching 700.3e In addition, we have shown that
the high thermodynamic stability of 1 allows their incorporation
in polyvinylidene fluoride membranes without affecting their
performance, even after five recycles of the catalytic membrane.5
More recently, we have been interested in the structural analogous
Ti(IV) amino triphenolate complexes 2 (Fig. 1).
A substantial number of publications have appeared regarding
the synthesis and complexation behaviour of amino triphenolate
ligands 3 with transition metals.6 Taking advantage of the high
stability of these complexes, their applications are increasing
rapidly. Among these metals, Ti(IV) amino triphenolate complexes
2 have been used successfully as polymerization catalysts.7,8 In
other organic transformations the variants of 2 have been reported
to be poor Lewis acid catalysts and that activity is only observed
after exchange of the apical isopropoxy ligand for a more labile
triflate anion.9 Recently, we found that Ti(IV) complexes 2 are
efficient sulfoxidation10 and N-oxidation11 catalysts without the
need of prior activation using aqueous hydrogen peroxide as
oxidant.12 Bull and co-workers have achieved ee¢s up to 47% in
the benzyl phenyl sulfide oxidation by cumyl hydroperoxide using
an enantiopure pseudo-C3 Ti(IV) complex.13 Herein we report a
more detailed and complete study on the use of these catalytic
systems in sulfoxidations using peroxides as primary oxidants.
The speciation of the catalysts in solution has been done by NMR
and ESI-MS techniques and the X-ray structure of complex 2c,
the most effective catalyst in these processes, has been determined.
We have also explored the nature of the Ti(IV) peroxo interme-
diates in these reactions using NMR and ESI-MS techniques and,
with the aid of computational methods, structures for the metal
peroxo intermediates are proposed.
aDipartimento di Scienze Chimiche, Universita` di Padova, via Marzolo 1,
35131, Padova, Italy. E-mail: giulia.licini@unipd; Fax: (+) 39 0498275239
bNanoscience Center Department of Chemistry, University of Jyva¨skyla¨,
Jyva¨skyla¨, Finland FIN-403512
† Electronic supplementary information (ESI) available: ESI-MS and
1H NMR spectra. CCDC reference number 756168. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00228c
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Table 1 ESI-MS-TOF data for complexes 2a–ca
Results and discussion
Ion (m/z most abundant isotopic
peak, rel. % intensity)
Synthesis and characterization of complexes 2a–c
Entry
Complex
The nature of the substituent R in ortho position has been found
to be very important to the stability of aminotriphenolate Ti(IV)
complexes.10,14 In the case of small substituents, for instance R =
CH3, the formation of aggregates, in particular m-oxo dinuclear
complexes, have been observed after exposure of the solid
crystalline compounds to wet air.14,15 This phenomenon was not
observed when bulky substituents such as tert-butyl were present.
In order to test whether the peripheral substituents R might have
an effect on the catalytic activity of 2, we prepared complexes 2a–
c carrying substituents of increasing size (R = H, CH3 and t-Bu,
respectively, Scheme 1). The complexes were obtained by mixing
the appropriate ligand 3a–c with an equimolar amount of Ti(Oi-
Pr)4 in CDCl3 under nitrogen atmosphere. Ligands 3a–c were
synthesized using a procedure developed in our group, based on
a threefold reductive amination of the corresponding substituted
1
2
3
2a
2b
2c
Ia (412, 10); IIa (434, 22); IIIa
(791, 34); IVa (845, 100)
Ib (454, 100); IIb (476, 40); IIIb
(875, 37); IVb (929, 32)
Ic (580, 100)
a Methanol as mobile phase, [2a–c] = 2 ¥ 10-5.
highly symmetric species. In contrast, complex 2c remains stable
in solution showing no variations in the 1H NMR spectrum. No
changes occur even after solvent removal under vacuum and
re-dissolution in CDCl3 (except for the disappearance of free
isopropanol).
In methanol, the solvent used in the oxidation with H2O2,10
1
the H NMR of complex 2c shows a single, highly symmetrical
1
salicylic aldehydes.16 The H NMR spectra of the in situ formed
species in which the apical isopropoxy ligand has been displaced
(one equivalent of isopropanol is released) by a methoxy group.
Complexes 2a and 2b show a similar behaviour.
complexes in CDCl3 show a single set of signals for each aromatic
proton, for the methine proton of the coordinated isopropoxy
apical ligand and for the Me and t-Bu ortho substituents in
complexes 2b and 2c, respectively.
The speciation in solution of complexes 2 was also investigated
using ESI-MS spectrometry. Solutions of complexes 2a–c using
methanol as mobile phase (15 mM) were studied. The high stability
of complexes 2a–c was confirmed by the fact that, even working
in a coordinating solvent like methanol and under electrospray
conditions, the main signals in the spectra are related to the
complexes, while signals corresponding to the free ligands are
present only in trace amounts (Table 1, and Fig. 2 and 3).
Scheme 1 In situ synthesis of complexes 2a–c.
The behaviour of complexes 2a–c in solution was investigated by
the combined use of 1H NMR and ESI-MS techniques. Complexes
2 are obtained as a racemic mixture, resulting from the helical
wrapping of the ligand around the metal ion. The chirality of
the complexes is reflected on the diastereotopicity of the benzylic
methylene protons. For these protons, an interesting difference
is observed between complexes 2a and 2b on one hand and
complex 2c on the other. For complexes 2a and 2b a single broad
◦
signal is observed at 28 C for the methylene protons, indicating
that the exchange between the two enantiomers is occurring on
1
the H NMR time scale. In contrast, complex 2c showed two
doublets at 3.94 and 2.89 ppm, as a consequence of a much slower
racemization rate.17
Accordingly to the previous results,6,14 the presence of peri-
pheral bulky t-Bu groups increases the activation barrier for
racemization.
The stability of the complexes is also affected by the size of
the substituents. Upon standing in solution (18 days), complexes
2a and 2b slowly convert into other species in which the apical
isopropoxy ligand is released in solution as free isopropanol. In
the case of complex 2b a single signal is observed for the methyl
group in ortho position, consistent with the formation of another
Fig. 2 Ions Ia–c, IIa–c, IIIa–c, IVa–c detected in ESI-MS experiments
on complexes 2a–c in methanol.
In all the cases ions corresponding to the protonated mononu-
clear complexes bearing a methoxy group as apical ligand (I
or II) were detected (Fig. 3). For complexes 2a and 2b signals
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Fig. 4 Molecular structure of 2c¢ in the solid state (ORTEP plot, 50%
probability ellipsoids). The disorder of the methoxy carbon is removed for
clarity.
2a–c was tested by mixing each complex, prepared in situ, with
10 equivalents of cumyl hydroperoxide (CHP) and 10 equivalents
of thioanisole 4a in CDCl3 (69 mM) (Scheme 2).
Fig. 3 ESI-MS spectra of the in situ formed complexes 2a (a), 2b (b) and
2c (c) in methanol as mobile phase: [2a–c] = (2.0 ¥ 10-5M).
corresponding to dinuclear complexes IIIa, IIIb, IVa and IVb were
also present (Table 1, entries 1 and 2 and Fig. 3, spectra a and b).
Significantly, in the case of 2a, ions IIIa and IVa are the most
abundant peaks in the spectrum (Table 1, entry 1 and Fig. 3).
These species have isotopic clusters and m/z ratio consistent
with dinuclear complexes containing one (III) or two methoxy
residues (IV), likely bridging between the two Ti(IV) nuclei.
Importantly, these dinuclear species were not detected for complex
2c (Table 1, entry 3 and Fig. 3, spectrum c), confirming the
extreme stability of the mononuclear complex and its resistance
to aggregation.18 Therefore, we can conclude that two analytical
techniques, 1H NMR and ESI-MS, provide useful information for
the speciation in solution of titanatrane complexes 2a–c, clarifying
the nature of the species present in solution and their relative
stability.
Scheme 2 Oxidation of methyl phenyl sulfide (4a) with CHP catalyzed
by complexes 2a–c.
Reactions were carried out at 28 ◦C and monitored by 1H NMR.
The kinetic profiles (Fig. 5) clearly evidenced the formation of
the oxidation products phenyl methyl sulfoxide 5a and phenyl
methyl sulfone 6a (Table 2). While in the case of transesterification
and Diels–Alder reactions these complexes have been reported
to need the substitution of the apical ligand,9 in oxygen-transfer
reactions we found that all complexes are very effective Lewis acid
catalysts without the need of prior activation. This is illustrated
by the time for the half consumption of the reagent (t1/2) in the
order of 30–60 min and by the complete consumption of the
oxidant. The high reactivity of these catalysts is evident from
Suitable crystals (methanol–dichloromethane)for X-raydiffrac-
tion have been obtained for complex 2c¢, bearing the methoxy
apical ligand (Fig. 4).
The X-ray determined structure of complex 2c¢ presents a
propeller-like conformation around the Ti(IV) atom, a classical
arrangement in such types of metal complexes. At the molecular
level, no significant structural differences with respect to the X-ray
structure of analogous Ti(IV) complex with additional para tert-
butyl substituents18 were observed, whereas the packing is clearly
affected by the lack of such voluminous groups. In addition, a
slight disorder for the apical methoxy carbon was found out.
Catalytic sulfoxidations
Fig. 5 Oxidation of thioanisole (4a) to methyl phenyl sulfoxide (5a) and
methyl phenyl sulfone (6a) with CHP in CDCl3 at 28 ◦C catalyzed by
complexes 2a–c (10%). [4a]0 = [CHP]0 = 69.0 mM. Reaction catalyzed by
2a: 5a (᭹), 6a (᭺). Reaction catalyzed by 2b: 5a (ꢀ), 6a (ꢀ). Reaction
catalyzed by 2c: 5a (ꢀ), 6a (ꢁ). Concentrations were determined by
1H NMR, internal standard 1,2-dichloroethane.
Ti(IV)-trialkanolamine complexes 1 are known to efficiently
catalyse the oxidation of sulfides using alkyl hydroperoxides as
primary oxidants.3b,j,4 Because of the structural similarities, we
tested the catalytic activity of complexes 2a–c in the same reaction
under similar conditions. The catalytic activity of complexes
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Table 2 Oxidation of thioanisole (4a) by cumene hydroperoxide (CHP)
to methyl phenyl sulfoxide (5a) and methyl phenyl sulfone (6a) catalyzed
by complexes 2a–ca
after two weeks (Table 3, entry 3). The fact that at even at very
low concentrations (0.07 mM) and in the presence of a large
excess of competing ligands (cumyl alcohol and CHP) catalyst
2c retains its activity indicating a very high stability of the catalyst
under turnover conditions. In the second series of experiments,
performed at much higher substrate concentrations (0.5 M), the
reactions occurred much faster and, when performed with 0.1% of
catalyst, afforded a complete conversion into products after 42 h.
An interesting concomitant effect is that lowering catalyst loading
yields to a significant increase of chemoselectivity in favour of
sulfoxide (Table 3, entries 3 and 5). Moreover, in all reactions, at
the end of the oxidation signals corresponding to the catalyst 2c
were still detected in the reaction mixture.
Entry
Catalyst
t1/2b/h
5ac (%)
5a : 6ac
1
2
3
2a
2b
2c
1.1
0.9
0.5
80
78
65
89 : 11
88 : 12
84 : 16
a Reactions were carried out in CDCl3 at 28 ◦C using a molar ratio
substrate/CHP 1 : 1 and 10% of catalyst: [4a]0 = [CHP]0 = 69.0 mM.
b Time required for a 50% decrease of the initial concentration of oxidant.
c Determined by 1H NMR (CDCl3, 300 MHz) using DCE as internal
standard and quantitative GC analysis of the crude reaction mixture after
total oxidant consumption (iodometric test).
Reactions carried out using 1% of Ti(OiPr)4 afforded the sul-
foxide in lower yields (72%) and after much longer reaction times
(Table 3, entry 8) and with lower chemoselectivities (5a : 6a =
83 : 17).
Stimulated by the elevated catalytic activity of the 2c/CHP
system and the high stability of complex 2c under turnover
conditions, the activity of complex 2c using hydrogen peroxide
as terminal oxidant was explored as well.10 The development of
catalytic systems that use hydrogen peroxide as terminal oxidant
has been the focus of intense research since this is a cheap, easy
to handle, and much more environment benign oxidant.20 It is
important to note that the Ti(IV) trialkanolamine complexes 1
are absolutely not active under these conditions and they do not
afford significant amounts of oxidized products. Only few Ti(IV)
complexes have been reported to be able to activate H2O2 or its
urea complex in the oxidation of sulfides or olefins, mainly based
in salen, salan and salalen ligands.21,22
As we have previously communicated, complex 2c resulted an
excellent catalysts in sulfide oxidations using hydrogen peroxide as
terminaloxidant.10 Thereactions were carried out inhomogeneous
conditions (methanol as solvent) and sulfoxides were obtained in
high yields and selectivities.
The system 2c/H2O2 was found to be more active than the
system 2c/CHP. TOFs up to 1700 h-1 were obtained and catalyst
loadings could be decreased down to 0.01% (8000 TON).10 The
2c/H2O2 system shows better chemoselectivities favouring the
sulfoxide formation with comparable catalyst loadings. Indeed,
under these conditions, sulfone 6a forms only in very small
amounts and at the end of the reaction, in contrast to the 2c/CHP
system where it is formed from the very beginning and in much
larger amounts (Fig. 5 and Table 2). Catalyst 2c was revealed to be
stable under turnover conditions also in this case and the 1H NMR
signals relative to 2c were detected unchanged at the end of the
reactions.
a comparison with the Ti(IV)-trialkanolamine catalysts 1 which
exhibit similar reactivities.3b,4b Noteworthy, the increase of the
steric bulk on the periphery of the catalyst leads to an increased
catalytic activity. This might be related to the tendency of the
catalysts with smaller ortho substituents to form aggregates, which
may have no or lower catalytic activity. Ti(IV)-amino triphenolate
catalysts 2a–c give sulfoxide : sulfone ratios from 89 : 11 to 84 : 16
and clearly outperform the Ti(IV)-trialkanolamine catalysts 1,
which generally yield sulfoxides and sulfones in a ratio of about
60 : 40 when the same oxidant : substrate ratio is used.3b,4b,19 Blank
reactions (no catalyst) did not afford significant conversion into
the products under comparable reaction times, as well as the
reactions performed in the presence of triphenolamines 3a–c.
On the basis of these preliminary results, the catalytic activity of
complex 2c, which results in the best results as far as the reactivity
is concerned, was explored in detail (Table 3).
The system remains active after decreasing the catalyst loading
down to 0.1% (Table 3, entries 2 and 3). In this case, even if the
reaction becomes extremely slow, a yield of 94% was obtained
Table 3 Oxidation of thioanisole (4a) by CHP to methyl phenyl sulfoxide
(5a) and methyl phenyl sulfone (6a) catalyzed by 2c.a Effect of concentra-
tion and catalyst loading
Entry
2c (%)
[4a]0/M
t/h
5ac (%)
5a : 6ab
1
2
3
4
5
6
7
8
10
1
0.1
1
0.1
—
—
0.069
0.069
0.069
0.50
0.50
0.50
2
30
380
3
42
18
18
18
65
90
94
81
> 99
5
84 : 16
95 : 5
99 : 1
90 : 10
99 : 1
> 99 : 1
> 99 : 1
83 : 17
The scope of the reaction has been explored working at
millimolar scale. A series of sulfides was oxidized with CHP and
H2O2 (Table 4)10 in the presence of 2c at 10 and 1% loading,
respectively.
Included in the series are aryl/alkyl and dialkyl sulfides.
In all cases, fast reactions and complete consumption of the
oxidant were observed. In analogy with thioanisole 4a, also with
the other substrates the oxidation with H2O2 shows a higher
reactivity and selectivity for the formation of the corresponding
sulfoxides in comparison to CHP. In the aryl-methyl series, the
effect of electron-donating and withdrawing groups present in
the aromatic group was examined (4a, 4e, 4f). A significant
c
0.50
0.50
5
72
d
—
a Reactions were carried out in CDCl3 at 28 ◦C using a molar ratio
4a/CHP = 1 : 1. b Determined by 1H NMR (CDCl3, 300 MHz) using DCE
as internal standard and quantitative GC analysis of the crude reaction
mixture after total oxidant consumption (iodometric test). c Reaction
performed in the presence of 1% of ligand 3c. d Reaction performed in
the presence of 1% of Ti(Oi-Pr)4.
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Table 4 Oxidation of sulfides 4a–f by CHP aand H2O2 bcatalyzed by 2c.
of the peroxo intermediates in these catalytic reactions is an
important task. In the case of the Katsuki–Sharpless system for
the asymmetric epoxidation of allylic alcohols, studies on the
formation of the titanium peroxo species have been performed
via IR measurements,24 while with the Ti(IV) trialkanolamine
complexes 1, Ti(IV)-peroxo species have been characterized via
single-crystal diffractometric analysis,25 1H NMR, ESI-MS ex-
periments and computational methods.3j,26 Recently Katsuki and
co-workers reported the X-ray crystal structure of a peroxo Ti(IV)
salan complex which is thought to be part of the catalytic cycle
in the epoxidation reaction.21i,27,28 Peroxo species of Ti(IV)-amino
triphenolate complexes 2 have not been isolated and characterized
so far.
Scope of the reaction
Entry Substrate R1
R2
Oxidant 5cd (%) 5 : 6c
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
Ph
Me
n-Bu
CHP
H2O2
CHP
H2O2
72 (70) 84 : 16
97 (93) 98 : 2
80 (75) 89 : 11
95 (89) 95 : 5
69 (65) 82 : 18
91 (84) 93 : 7
75 (70) 86 : 14
p-Me
PhC6H4
n-Bu
CH2Ph CHP
H2O2
n-Bu
CHP
H2O2
CHP
H2O2
CHP
H2O2
In order to obtain more information on the nature of the active
species in the oxygen transfer process, we decided to investigate
more in detail the behaviour of catalyst 2c in the presence of CHP
and H2O2.
92 (83)
85 (81)
94 (86)
93 : 7
92 : 8
94 : 6
9
p-MeOC6H4 Me
p-NO2C6H4 Me
10
11
12
49 (46) 66 : 34
77 (61) 85 : 15
2c and CHP. The addition of increasing amounts of cumyl
hydroperoxide (1–100 equiv) to a chloroform-d3 solution of
complex 2c (both formed in situ or after removal of the excess
of isopropanol) did not allow the detection of any new species via
1H NMR the formation, even though a change in the colour of the
solution was observed. A similar behaviour was observed for the
addition of increasing amounts of cumyl alcohol. This observation
is in agreement with the kinetic experiments monitored via
1H NMR, where the signals of the catalyst are detected during the
course of the reaction. The peroxo-complex could not be detected
of any new species by ESI-MS (CH3CN, 2 ¥ 10-5 M), even in
the presence of a large excess of CHP (100–1000 equivalents).
The inertness of the isopropoxy apical ligand has been already
evidenced by other authors showing that it can be exchanged only
with more acidic ligands such as trifluoroacetic or triflic acid,9,18
bidentate derivatives.29 or in alcoholic solution (methanol). In
the reaction with CHP it seems that the equilibrium toward the
reactive peroxo complex allows its formation only in very low
a Reactions were carried out at 0.5 mmol scale in CHCl3 at 28 ◦C using
[4a]0 = [CHP]0 = 0.5 M, [2c]0 = 0.005 M. b Reactions were carried out at
0.5 mmol scale in MeOH at 28 ◦C using [4a–f]0 = [CHP]0 = 0.5 M, [2c]0 =
0.005 M. c Determined by 1H NMR (CD3OD, 300 MHz) and quantitative
GC analysis of the crude reaction mixture after total oxidant consumption
(iodometric test). d Isolated yields are given in parentheses.
Table 5 1H NMR chemical shifts (d, ppm) from PhCH2 and t-Bu protons
of complex 2c: [2c] = 0.008 M, after H2O2 addition (10 equivalents) in
different deuterated solvents
d (PhCH2)
d (t-Bu)
Solvent
2c
+H2O2
2c
+H2O2
CD3ODb
CD2Cl2
CDCl3
CD3COCD3
CD3CN
3.49
3.64
nda
1.43
1.47
1.45
1.47
1.45
1.35
1.41
1.41
1.40
1.38
3.91, 2.93
2.89, 3.95
3.50
nda
3.60
3.54
3.43
a Not detected due to the overlapping of H2O signals. b 1000 equivalents of
H2O2 were added.
1
concentrations (not detectable via H NMR) and once formed it
reacts immediately in the presence of a nucleophile (sulfide).
2a–c and H2O2. The 2a–c–hydrogen peroxide systems were
investigated via 1H NMR in methanol-d4. Initially, after dissolving
in CD3OD a concentrated chloroform solution of the in situ
formed complex 2c, the complete displacement of the apical
isopropoxy ligand by CD3OD was observed. The characteristic
1H NMR signals for the apical isopropoxy ligand disappeared
and only signals deriving from free isopropanol were observed
at 3.91 (1H, q, J = 6.3 Hz) and 1.12 (6H, d, J = 6.3 Hz) ppm
(Scheme 3 and Fig. 6, spectrum a).
effect of the para substituents (H, OMe and NO2) was found,
especially in the oxidation with CHP (Table 5, entries 1, 9–11).
Although with both substrates a quantitative consumption of the
oxidant was observed, the sulfoxide : sulfone ratio is much different
(92 : 8 for the electron-rich p-methoxyphenyl methyl sulfide 4e,
84 : 14 for phenyl methyl sulfide 4a and 66 : 34 for the electron
poor p-nitrophenyl methyl sulfide 4f). A similar behaviour was
previously observed with the Ti(IV)-trialkanolamine 1 catalyzed
sulfoxidations in which the formation of high amounts of sulfone
was found to originate from the biphilic nature of the Ti(IV) peroxo
complex.19 The fact that a comparable behaviour is observed for
the Ti(IV)-amino triphenolate complexes 2c is indicative that a
similar mechanistic pathway could be occurring. Currently, we
are examining this aspect in more detail.
Scheme 3 Peroxo-Ti(IV) formation by reaction of 2c with H2O2 in
CD3OD.
Characterization of the peroxo-titanium complexes
Ti(IV) catalyzed sulfoxidations and allylic alcohol oxidations
usually occur by in situ activation of peroxides via formation
of Ti(IV) peroxo complexes which are the active species in the
oxygen transfer process.23 The identification and characterization
Addition of hydrogen peroxide to 2c¢ affords the formation of
a new species in amounts which depend on the excess of H2O2
employed. Up to 100 equivalents no significant changes in the
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appearance of the free isopropanol and a shift of the methylene
and t-Bu signals (Table 5). This phenomenon is consistent with
the formation of a new species via apical isopropoxy/peroxy
exchange, which leads to the peroxo complex 7c. In all these
systems, in contrast with what was observed in methanol-d4, the
complete formation of complex 7c was evidenced after addition
of only a ten-fold excess of hydrogen peroxide. On the other hand,
extensive decomposition of the peroxo species happened in 24 h.
This observation can be correlated to the low conversions obtained
in the reactions carried out in acetonitrile (52%) or acetone (44%)
associated with H2O2 decomposition.10,30
Further evidence on the nature of the peroxo Ti(IV) complex
could be achieved via ESI-MS experiments. Spectra obtained in
negative mode, with [2c] = 1 ¥ 10-5 M in methanol in the presence
of a large excess of hydrogen peroxide (1000 equiv.) showed the
presence of a single peak at m/z = 580 (Vc) (Fig. 7 and 8), whose
isotopic cluster corresponds to the monomeric anionic peroxo
specie originating from 7c (Scheme 3).31 The same species was
obtained when perdeuterated methanol or acetonitrile were used
as solvent.
Fig. 7 Ti(IV) peroxo anions Va–c, from ESI-MS experiments on com-
plexes 2a–c in the presence of H2O2 and using methanol as mobile phase.
Fig. 6 1H NMR spectra (CD3OD, 300 MHz) in the region from 0 to
4.3 ppm of (a) complex 2c (0.8 ¥ 10-3 M). (b) after the addition of hydrogen
peroxide (35%, 0.8 M) and (c) after addition of thioanisole 4a (0.8 M).
Signals correspond to complex 2c (∑), newly formed species (*) and methyl
groups from 4a, 5a and 6a.
1H NMR spectrum were observed. Only in the presence of a larger
excess of hydrogen peroxide (1000 equivalents) the yellow solution
became brighter and a new species was detected in the 1H NMR
spectrum, characterized by an upfield shift of the tert-butyl signal
(from 1.43 to 1.35 ppm) and a downfield shift of the methylene
signal (from 3.49 to 3.64 pm) (Fig. 6, spectrum b). Upon standing
for one week no evident signs of decomposition were observed.
On the other hand, addition of stoichiometric amounts of sulfide
4a to the solution, led to the formation of sulfoxide 5a and sulfone
6a. Contemporaneously, the disappearance of the newly formed
species was observed with the rebuilding of the initial catalyst 2c¢
(Fig. 6, spectrum c). This experimental evidence is consistent with
the occurrence of a ligand exchange process in which the solvent
(CD3OD) and the bidentate peroxide compete for coordination to
the metal center (Scheme 3).
The formation of the peroxo-complex 7c was also monitored by
1H NMR in other, less coordinating, solvents like CD2Cl2, CDCl3,
acetone-d6 and CD3CN (Table 5).
The addition of hydrogen peroxide in chlorinated solvents led to
a change of colour from yellow to dark orange and in acetonitrile
or acetone from yellow to dark green. These variations are
indicative of a change in the coordination sphere of the metal ion
(Table 5). In all cases the 1H NMR spectra showed the decreasing
of the signals corresponding to the apical isopropoxy ligand
just after addition of one equivalent of H2O2 with the parallel
Fig. 8 Experimental (A) and calculated (B) isotopic distribution of anion
Vc (R = t-Bu), m/z = 580.
The formation of the peroxo complexes deriving from 2a and
2b was also explored (Fig. 7). In both cases addition of aqueous
hydrogen peroxide to a solution of 2a and 2b in methanol-d4 led to
the formation of systems with complex 1H NMR spectra, referable
to the formation of aggregates. Nevertheless, ESI-MS analysis of
the resulting mixtures allowed the detection of ion Vb (m/z = 454)
(Fig. 9), even if in small amounts, while for complex 2a no peroxo
species could be detected at all.
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hydrogen peroxide (or alkyl hydroperoxide) activation by early
transition d0 metals, such as Ti(IV), the active species originates
from exchange of one of the alkoxo ligands by the oxidant. The new
resulting hydroperoxo (or alkylperoxo) metal complex becomes
active for the oxygen transfer process when the peroxo moiety is
2
h -coordinated.32
The peroxo complexes detected experimentally are the hydroper-
oxo 7c (1H NMR) and the corresponding anionic peroxo species
Vc (ESI-MS). In both cases the optimization afforded structures
with distorted bipyramidal geometries, in which the peroxo moi-
2
eties are h -coordinated and occupy the upper part of the complex,
trans to the nitrogen atom. In all cases the aminotriphenoxide
ligand maintains a propeller like conformation, in analogy with
what we observe in the X-ray structure of catalyst 2c (Fig. 4)
The neutral adduct with hydrogen peroxide 7c displays peroxidic
˚
Ti–O, Ti–O(H) and O–O bonds of 1.904, 2.517 and 1.515 A,
2
Fig. 9 Experimental (A) and calculated (B) isotopic distribution of ion
Vb (R = Me), m/z = 454.
respectively, while the anionic Vc has a more symmetric h -
˚
˚
peroxo group (Ti–O = 1.872 A and O–O = 1.520 A). In both
cases attempts to force the peroxo structure in an octahedral
conformation failed.
Theoretical studies
We also considered the possible geometries of the cumylperoxo
complex 8c, species that we could not observe experimentally. It
is interesting to note that in the most stable structure the peroxo
moiety presents an end-on coordination mode (Ti–O, Ti–OCum
In order to obtain more insight on the nature of the peroxo
species under study, the structures of the peroxo species obtained
by reaction with CHP and hydrogen peroxide were optimized
by means of density functional calculations (B3LYP) using the
LANL2DZ basis set (Fig. 10). It is largely accepted that in the
˚
and O–O bonds of 1.864, 2.927 and 1.487 A, respectively) Very
likely the steric hindrance between the tert-butyl substituents and
2
the cumyl group disfavours the peroxo h -coordination mode.
Conclusion
In summary, we have synthesized and characterized a series of
mononuclear titanium(IV) complexes able to catalyze the oxidation
of sulfides to sulfoxides in high yields and chemoselectivities and,
thanks to their intrinsic stability to hydrolysis, high turnovers and
turnover frequencies. The best catalyst 2c is able to activate both
alkyl and hydrogen peroxides as terminal oxidants with catalyst
loadings down to 0.01% and TONs up to 8000. Spectroscopic
evidence (1H NMR and ESI-MS), supported by DFT calculation,
indicate that the active species in the process are monoperoxo-
Ti(IV) complexes.
Experimental
General procedures
All synthetic operations for catalyst preparation were carried out
under nitrogen atmosphere in a MBraun MB 200MOD glove-
box, equipped with a MB 150 G-I gas recycling system (nitrogen
working pressure: 6 bar). Unless otherwise noted, Fluka dry-
quality solvents were used (water < 0.005–0.010%, stored over
molecular sieves). When necessary, solvents were further purified
˚
or dried by standard techniques, stored over 4 A molecular
sieves and degassed prior to use. Deuterated solvents were
˚
purchased from Aldrich and stored over 4 A molecular sieves.
All chemicals were purchased from Aldrich and Fluka as high-
purity products (>95%) and used without further purification.
Cumyl hydroperoxide (Fluka, 80% solution in cumene) was
◦
˚
stored at 4 C over 4 A molecular sieves. Ti(IV) tetraisopropox-
Fig. 10 Optimized geometries of peroxo-complexes 7c, Vc and 8c (B3LYP
LANL2DZ). Hydrogen atoms have been omitted for clarity.
ide, thioanisole, dibutyl sulfide, benzyl phenyl sulfide, n-butyl
7390 | Dalton Trans., 2010, 39, 7384–7392
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p-tolyl sulfide, p-methoxy thioanisole, p-nitro thioanisole and 35%
aqueous hydrogen peroxide from Aldrich. Ligands 3a–c were
synthesized as previously reported.16 Ti(IV) complexes, prepared
as previously reported,10 were always handled and stored in a
glovebox, with the exception of complex 2c which could be handled
in open air.
consumed (iodometric test). CHCl3 was added and the mixture
was washed with 5% sodium metabisulfite aqueous solution, the
layers were separated and the aqueous one extracted twice with
chloroform. The organic layers were washed with brine, dried
over MgSO4 and the solvent was removed under reduce pressure.
Sulfoxide : sulfone ratios were determined by quantitative GC
analysis and by 1H NMR (CDCl3, 300 MHz) of the crude reaction
mixtures. Yields were determined on the isolated products after
flash chromatography on silica gel, eluent petroleum ether–ethyl
acetate. All analytical data of sulfoxides 5a–f and sulfones 6a–f
match those already reported in the literature.33
Analysis
Melting points are uncorrected and were determined by a Reichert
Austria apparatus, 1 ◦C precision. 1H NMR spectra were recorded
on a Bruker AC 250 (250.18 MHz) or Bruker Avance DRX 300
(300.13 MHz) spectrometer, using the partially deuterated solvent
or TMS as internal references (TMS d = 0 ppm, CHCl3 d =
X-Ray crystal analysis of 2c¢
1
7.26 ppm, CH3OH d = 4.78, 3.35 ppm). H NMR kinetics (at
Colourless crystals of 2c¢ for single-crystal X-ray diffraction anal-
ysis were obtained from methanol. The obtained single crystals
were mounted on a Nylon loop sample holder with perfluoro
polyether and the data collected at 123.0(2) K using Bruker-
Nonius KappaCCD diffractometer with APEX-II detector and
28 ◦C) were recorded on a Bruker Avance DRX 300 (300.13 MHz)
spectrometers equipped with probe temperature control units. 13
C
NMR spectra were recorded on Bruker AC 250 (62.9 MHz)
or Bruker Avance DRX 300 (75.5 MHz) spectrometers in H-
decoupled mode, using the solvent carbon resonance as the
internal standard: CHCl3 d = 77.0 ppm (t). ESI-MS experiments
of complexes 2a–c were performed in a ESI-TOF MarinerTM
BiospectrometryTM Workstation, Applied Biosystems by flow
injection analysis using methanol as mobile phase. ESI-MS
experiments of peroxo complexes Va–c were performed in a
Agilent LC/MSD Trap serie SL by flow injection analysis using
methanol as mobile phase. GC analysis were performed using a
Shimadzu GC-2010 gas chromatograph with a FID detector and
a capillary column EQUITYTM-5 using dodecane as external
standard.
˚
graphite-monochromatized Mo-Ka (l = 0.71073 A) radiation.
COLLECT34 software was used for the data collection (q and w
scans) and DENZO-SMN35 for the processing. The structure was
solved by direct methods with SIR200436 and refined by full-matrix
least-squares methods with WinGX-software,37 which utilizes the
SHELXL-97 module38 Lorentzian polarization correction and
multi-scan absorption correction (SADABS39) were applied on all
data. All C–H hydrogen positions were calculated and refined as
riding atom model with 1.2 and 1.5 times of the thermal parameter
of the C-atoms, respectively.
Crystal data for 2c¢. C34H45NO4Ti, Mr = 579.61, crystal size
0.30 ¥ 0.23 ¥ 0.20 mm, orthorhombic, space group Pna21 (no.
General procedure for monitoring the sulfoxidation reactions
catalyzed by 2a–c with CHP as oxidant (Table 2). A screw-cap
NMR tube was charged with a solution of the corresponding
in situ formed Ti(IV)-catalyst 2 in CDCl3 (0.00414 mmol), the
internal standard (1,2-dichloroethane, DCE), 35% aqueous H2O2
(0.0414 mmol) and thioanisole (0.0414 mmol) were added up to a
final volume of 0.6 mL. Concentrations of sulfide, sulfoxide and
sulfone were monitored by integration of the methyl group signals
by 1H NMR: PhSMe (2.4 ppm), PhSOMe (2.8 ppm) and PhSO2Me
(3.1 ppm) with respect to the internal standard, DCE (3.78 ppm).
˚
33), a = 11.8776(2), b = 23.2811(3), c = 11.4274(2) A, V =
3
-3
-1
˚
3159.95(9) A , Z = 4, Dc = 1.218 Mg m , m = 0.307 mm ,
F(000) = 1240, 30406 collected reflections (2qmax = 25.00◦) of
which 5547 independent (Rint = 0.0649), Tmax = 0.9411, Tmin
=
0.9134, full-matrix least-squares on F2 with 8 restraints and 372
parameters, GOF = 1.021, R1 = 0.0384 [I > 2s(I)], wR2 (all
-
-3
40
˚
data) = 0.0823, largest peak/hole = 0.180/-0.275 e A , Flack
parameter = 0.10(2).
Acknowledgements
General procedure for monitoring the sulfoxidation reactions
catalyzed by 2c with CHP as oxidant (Table 3). A screw-cap
NMR tube was charged with the correct amount of a solution
of the in situ formed complex 2c in CDCl3. The internal standard
(1,2-dichloroethane), 35% aqueous H2O2 and thioanisole (4a) were
added with final concentrations as reported in Table 3 to a final
volume of 0.6 mL. Concentrations of sulfide, sulfoxide and sulfone
were monitored by integration of the methyl group signals in
1H NMR: PhSMe (2.4 ppm), PhSOMe (2.8 ppm) and PhSO2Me
(3.1 ppm) with respect to the internal standard, DCE (3.78 ppm).
Final yields were determined by quantitative GC analysis after
complete CHP consumption (iodometric test).
We acknowledge support of the University of Padova - Assegni
di Ricerca di Ateneo 2006 (M. M.) and COST, Action D40
‘Innovative Catalysis - New Processes and Selectivities’
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7392 | Dalton Trans., 2010, 39, 7384–7392
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©