Titanium-salan-catalyzed asymmetric sulfoxidations with H2O 2: Design of more versatile catalysts

Article abstract of DOI:10.1002/aoc.2968

Titanium-salan complexes with 3,3'-diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H₂O₂ with good to high enantioselectivities. In this paper, substituent effects on titanium-salan-catalyzed enantioselective oxidation of sulfides to sulfoxides have been systematically investigated. Titanium-salan catalysts with halogen substituents at the 5,5'-positions (3,3'-H₂dihydrogen substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species; a consistent reaction mechanism is proposed. Copyright 2013 John Wiley & Sons, Ltd. Titanium-salan complexes with halogen substituents at the 5,5'-positions are versatile catalysts for the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides, demonstrating good to high chemoselectivity (up to 92 %) and enantioselectivity (up to 93 % ee). Copyright

Full text of DOI:10.1002/aoc.2968

Full Paper
Received: 6 December 2012
Revised: 23 December 2012
Accepted: 28 December 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2968
Titanium–salan-catalyzed asymmetric
sulfoxidations with H2O2: design of more
versatile catalysts
Evgenii P. Talsi and Konstantin P. Bryliakov*
Titanium–salan complexes with 3,3’-diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation
catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H2O2 with good to high enantio-
selectivities. In this paper, substituent effects on titanium-salan-catalyzed enantioselective oxidation of sulfides to sulfoxides have
been systematically investigated. Titanium–salan catalysts with halogen substituents at the 5,5’-positions (3,3’-H2dihydrogen
substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with
good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species;
a consistent reaction mechanism is proposed. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: asymmetric catalysis; hydrogen peroxide; mechanism; oxidation; sulfides; titanium
and by introduction of mild electron acceptors at the 5,50-positions.
In addition, the mechanism has been probed by kinetic methods;
Introduction
The urgency of novel, more efficient, sustainable and economically
effective catalytic asymmetric sulfoxidations for the sustainable
progress of the fine chemicals industry has been noted extensively
in recent review papers.[1–9] To date, titanium occupies the most
important position in asymmetric sulfoxidations: the majority of
practical sulfoxidations have so far been developed on the basis
of the Kagan–Modena protocol, which uses chiral titanium–
dialkyltartrate catalysts and alkyl hydroperoxides as oxidants.[1–9]
On the other hand, several titanium-based catalyst systems
appeared in the last few years, some of them surpassing the
Kagan–Modena systems in terms of chemo- and stereoselectivity,
as well as in ease of handling and sustainability, especially those
relying on H2O2 as the oxidant.[10–18]
Previously, we reported that the enantioselective oxidation of
various sulfides with ’green’ 30% aqueous hydrogen peroxide could
be performed in the presence of catalytic amounts of titanium–
salan complexes (0.2–1.0 mol%); the asymmetric induction was
generated in a tandem stereoconvergent asymmetric oxidation/
kinetic resolution process.[16] Pessoa and Correia with co-workers
synthesized a series of related titanium catalysts with 1,2-cyclohex-
anediamine and 1,2-diphenylethylenediamine as chiral moieties
and reported the catalytic oxidation of thioanisole, with moderate
enantiomeric excess (ee) values.[17] Further studies revealed that
titanium–salan catalyst 1 had an optimal structure only for the
highly enantioselective oxidation of bulky aryl benzyl sulfides,
while the oxidation of smaller sulfides (such as thioanisole
derivatives and alkyl phenyl sulfides) proceeded with lower
chemo- and enantioselectivity.[18]
the nature of catalytically active species is discussed.
Experimental
General
H2O2 was used as a commercial analytical-grade 30% aqueous
solution. Silica gel 60 (0.063–0.200mm) for column chromatography
was purchased from Panreac. Hexane and methylene chloride for
catalysts preparation were dried over 4 Å molecular sieves. All other
chemicals were Aldrich, AlfaAesar, or Acros commercial reagents.
Chiral salan ligands and titanium–salan catalysts 1, 5–7 were
prepared as reported.[16,18] The synthesis of new catalysts 4, 8, 9 is
1
described in the supporting information. H and 13C NMR spectra
were measured on a Bruker Avance 400 instrument at 400.13 and
100.613 MHz, respectively, in 5 mm cylindrical tubes. Chemical shifts
were referenced to internal standard tetramethylsilane, with positive
values in the low-field direction.
Catalytic Oxidation Procedure and Product Analysis
Sulfide (0.1 mmol) was added to the Ti–salan catalyst (1 mmol)
dissolved in CH2Cl2 (10 ml). The mixture was thermostated at
ꢀ10 ꢁC, and an appropriate amount of 30% aqueous hydrogen
peroxide (0.10–0.13 mmol H2O2) was then introduced into one
portion. Stirring (500 rpm) was continued at that temperature
*
Correspondence to: Konstantin P. Bryliakov, Boreskov Institute of Catalysis,
Siberian Branch of the Russian Academy of Sciences, Pr. Lavrentieva 5,
630090, Novosibirsk, Russian Federation. E-mail: bryliako@catalysis.ru
In the present work, we aimed at overcoming the steric limitation
of titanium–salan catalyst systems and designing catalysts capable
of oxidizing both bulky and small sulfides with good enantio-
selectivities. This has been achieved by reducing the steric bulk at
the 3,30-positions of the salicylidene rings (by replacing Ph with H),
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of
Sciences, 630090, Novosibirsk, Russian Federation
Appl. Organometal. Chem. 2013, 27, 239–244
Copyright © 2013 John Wiley & Sons, Ltd.

E. P. Talsi and K. P. Bryliakov
for 16 h (for catalysts 1, 4–6) or 40 h (for less active catalysts 7–9),
to ensure completion of the reaction. The reaction progress was
periodically monitored by thin-layer chromatography (eluent:
hexane–acetone). The reaction mixture was dried with CaSO4,
and a 1 ml aliquot of the mixture was taken and carefully evapo-
rated in a stream of air. The residue was dissolved in 0.6 ml CCl4
or CDCl3–CCl4 and the composition of products was analyzed
by 1H NMR as previously described.[16,18]
Competitive oxidation experiments were performed using an
excess of sulfide substrates ([S]o/[H2O2] = 20:7), to avoid a contribu-
tion of over-oxidation of sulfoxides. The latter might introduce
unnecessary complications into the kinetic analysis, since sulfoxides
with different absolute configurations are oxidized with different
rates (for details see Bryliakov and Talsi[16]).
For entries 3–8 (Table 1), the enantiomeric excess values were
measured by H NMR with Eu(hfc)3 chiral shift reagent in CCl4 or
Results and Discussion
1
CDCl3/CCl4 as previously reported.[16,18–20] For entries 1 and 2
(Table 1), crude reaction mixtures were separated on a short column
(SiO2, petrol ether–acetone), and the ee values of the sulfoxides
were measured on a Shimadzu LC-20 high-performance liquid
chromatograph equipped with chiral columns as previously
reported.[18] The absolute configurations of sulfoxides were estab-
lished (for entries 3–8) by comparison of the Eu(hfc)3-shifted
NMR patterns of sulfoxides with those of the sulfoxides with
known absolute configuration, as previously described,[14,19,20]
or (for entries 1 and 2) based on the comparison of retention
times of major and minor enantiomers: benzyl phenyl sulfoxide
(Chiralcel OD-H, hexane–iPrOH = 90:10, 1.0 ml minꢀ1, 25 ꢁC),
tR = 13.7 min (R), tR = 16.7 min (S); benzyl 2-naphthyl sulfoxide
(Chiralpak AD-H, hexane–iPrOH = 90:10, 1.0 ml minꢀ1, 25 ꢁC):
tR = 17.5 min (R), tR = 19.6 min (S).[18]
Recently, we examined the relationship between the electronic
properties of the ligand and the performance of the titanium
catalysts 1–3 (X = Ph, Y = H, Br, CH3; Scheme 1).[18] In particular,
while catalyst 1 containing 3,30-diphenyl substituents in the
salicylidene rings demonstrated high enantioselectivities for the
oxidation of bulky aryl benzyl sulfides, the introduction of electron-
donating substituent at the 5,50-positions of the aromatic rings
(complex 3: Y= CH3) reduced the selectivity and turnover numbers
sustained, and the electron-withdrawing one (2: Y = Br) reduced
the catalyst activity; the enantioselectivity decreased in both cases.
Apparently, an appropriate interplay of electronic and steric
properties of the ligand structure is crucial for achieving high
stereoselectivity. In this work, we first varied steric properties of sub-
stituents at the 3,30-positions of the aromatic rings (the nearest
positions to the reactive metal center; see catalysts 1, 4–6
in Scheme 1), and then electronic effects caused by electron-
withdrawing groups at the 5,50-positions (the farthest from the
reactive metal center; see catalysts 7–9, Scheme 1). To probe the
steric effects, the enantioselective oxidation of various sulfides with
H2O2 in the presence of catalysts 1, 4, 5 and 6 under optimized
conditions formulated in Bryliakov and Talsi[18] was studied. To
benefit from the stereoconvergent kinetic resolution (which
becomes most significant at high substrate conversions[16,18]),
oxidant excess of 10–20 % was applied in most cases. For all
catalysts studied, (nearly) quantitative conversions were achieved
within 10–16 h. The results are summarized in Table 1.
For the experiment with tBuOOH as the oxidant, the entire
procedure was the same (catalyst: 1; sulfide: PhSCH2Ph), with
the only exception that 0.11 mmol tBuOOH (as 8.4 M solution in
decane) instead of H2O2 was added.
Parallel Oxidations
Three different sulfides (PhSCH2Ph, p-CH3PhSCH2Ph, p-NO2PhSCH2Ph;
0.1 mmol each sulfide) were added to three flasks containing
Ti–salan catalyst 1 (1 mmol) in CH2Cl2 (10 ml). The mixtures were
thermostated at ꢀ10 ꢁC, and 30% aqueous hydrogen peroxide
(0.2 mmol) was then added in one portion to each mixture upon
stirring (500 rpm). From each flask, 0.5 ml aliquots were taken
every 30–40 min and evaporated in a stream of air. Collection
of probes was stopped after 4 h, to limit the experiment
with low sulfide conversion (<25%) conditions. The probes
were dissolved in 0.6 ml CCl4 or CDCl3/CCl4 and the com-
As expected, catalyst 1 showed the highest ee values in the
oxidation of bulky benzyl phenyl sulfide and benzyl 2-naphthyl
sulfide (Table 1, entries 1 and 2; cf. Bryliakov and Talsi[16]). With less
sterically demanding substrates, 1 demonstrated lower effective-
ness, which resulted in lower sulfoxide yields (and higher sulfone
formation) and in lower ee values of the sulfoxides (entries 3–8).
The replacement of phenyls at the 3,30-position with smaller
susbstituents (OMe, Me, H) dramatically deteriorated the catalytic
performance of the resulting catalysts 4, 5 and 6 in the oxidation
of bulky substrates (entries 1 and 2). Rather surprisingly, catalysts
4–6 demonstrated higher enantioselectivities with less bulky
substrates (entries 3–8). In particular, the least sterically demanding
catalyst (6) showed the highest ee values with five out of eight
substrates considered (Table 1, entries 3, 4, 6–8), in those cases
when catalyst 1 was particularly poorly stereoselective. Catalysts 4
and 5 occupy an intermediate position between 1 and 6 in terms
of bulkiness of the 3,30-substituents (OCH3 and CH3). In effect,
catalyst 4 demonstrated high enantioselectivities only with
PhSCH2Ph and 2-naphthyl-SCH2Ph (albeit with rather low sulfoxide
selectivity), while for most of smaller substrates it was only slightly
more enantioselective than 1 (entries 3, 6–8). Catalyst 5 exhibited
equally moderate enantioselectivities with both bulky and less
bulky sulfides (33–59% ee).
1
position of products was analyzed by H NMR. The ln(S/So) vs.
time dependences can be found in the supporting information.
The first point was excluded in all three cases because the
reactions were found to require ~30 min to approach steady-
state conditions.
Competitive Oxidations
A mixture of four different sulfides (PhSCH3, p-CH3PhSCH3,
p-BrPhSCH3, p-NO2PhSCH3; 0.05 mmol each) was added to
the Ti–salan catalyst (1mmol) dissolved in CH2Cl2 (10 ml). The
mixture was thermostated at ꢀ10 ꢁC, and 30% aqueous
hydrogen peroxide (0.07 mmol) was then added in one portion.
Stirring (500 rpm) was continued at that temperature for 48 h.
After that, the reaction mixture was dried with CaSO4, and a
1 ml aliquot of the mixture was taken and carefully evaporated
in a stream of air. The residue was dissolved in 0.6 ml CDCl3/CCl4
and the composition of products was analyzed by 1H NMR.
kX/kH values were calculated as {ln([PhSCH3]/[PhSCH3]o)}/{Ln
([p-XPhSCH3]/[p-XPhSCH3]o)}.
Thus the steric effects of 3,30-substituents are exhibited very
clearly: the bulkiest 3,30-diphenyl substituents are most advanta-
geous for the enantioselective oxidation of bulkiest aryl benzyl
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Appl. Organometal. Chem. 2013, 27, 239–244

Versatile catalysts for Ti-salan-catalyzed asymmetric sulfoxidations
Table 1. Asymmetric oxidation of various sulfides on catalysts 1, 4–6
N
Substratea
Catalyst
Conversion (%)
Sulfoxide/sulfone yield (%)
ee (configuration)
1
2-NaphSCH2Ph
1
4
5
6
1
4
5
6
1
4
5
6
1
4
5
6
1
4
5
6
1
4
5
6
1
4
5
6
1
4
5
6
100
93.0
95.5
96.0
99.0
90.0
98.0
99.0
100
79.0 / 21.0
63.0 / 30.0
82.5 / 13.0
80.5 / 15.5
79.5 / 19.5
56.0 / 34.0
76.0 / 22.0
74.0 / 25.0
83.5 / 16.5
80.0 / 18.0
83.0 / 4.0
97.0 (R)
87.5 (R)
57.0 (S)
82.0 (R)
94.5 (R)
86.5 (R)
55.0 (S)
83.5 (R)
15.0 (R)
35.0 (S)
52.0 (S)
56.5 (R)
53.0 (R)
46.0 (R)
46.5 (S)
63.0 (R)
73.0 (R)
68.0 (R)
33.0 (S)
65.5 (R)
9.0 (R)
2
3
4
5
6
7
8
PhSCH2Ph
2-NaphSMe
PhS(cyclo-C6H11
PhSiPr
98.0
87.0
98.5
91.0
64.0
85.0
97.0
83.0
74.5
83.0
89.0
96.0
96.5
98.5
99.0
91.5
73.0
83.0
99.5
100
91.0 / 7.5
)
72.5 / 18.5
45.0 / 19.0
65.0 / 20.0
62.5 / 33.5
53.0 / 30.0
45.5 / 29.0
57.5 / 25.5
60.0 / 29.0
83.5 / 12.5
67.0 / 29.5
90.0 / 9.5
p-BrPhSMe
p-CH3PhSMe
PhSMe
44.0 (R)
46.5 (S)
65.5 (R)
3.0 (R)
75.5 / 23.5
66.0 / 15.5
58.0 / 15.0
60.0 / 23.0
80.5 / 19.0
70.5 / 29.5
76.5 / 18.0
77.0 / 22.5
77.0 / 22.0
38.5 (R)
53.0 (S)
64.5 (R)
20.5 (R)
43.0 (R)
59.0 (S)
69.5 (R)
94.5
99.5
99.0
aReactions were performed at ꢀ10 ꢁC, sulfide initial concentration 10 mM, [H2O2]/[S] = 1.2, catalyst loading 1.0 mol%, reaction time 16 h.
sulfides, while the smallest 3,30-dihydrogen substituents also favor
the enantioselective oxidation of small aryl methyl sulfides.
Interestingly, when a relatively mild electron-withdrawing
substituent (5,50-Br2) was introduced into the structure of the
salan ligand, the resulting catalyst 7 surpassed 6 in terms of both
enantioselectivity (cf. Tables 1 and 2, entries 3, 4, 5 and 8) and
sulfoxide selectivity (cf. Tables 1 and 2, entries 4–7) with most
of the less bulky substrates. Moreover, in the oxidation of bulky
aryl benzyl sulfides, 7 displayed enantioselectivities only a few
per cent lower than those obtained with catalyst 1, while the
sulfoxide selectivity was even higher with 7 (entries 1 and 2).
To study the effect of electron-withdrawing substituents system-
atically, complexes 8 and 9 (Scheme 1) were prepared and
tested in the oxidation of the same set of sulfides (Table 2).
Catalyst 8 (X = H, Y = I) in most cases demonstrated either higher
enantioselectivities or higher sulfoxide selectivities, as compared
to 7. In contrast, catalyst 9 with the strongest electron acceptor
(X= H, Y = Cl) was both less enantioselective and in most cases
less efficient (which is evident from lower conversions under the
same conditions).
What is the nature of the active species responsible for the
enantioselective oxygen transfer to the sulfide? In 2005–2009,
Katsuki and co-workers contributed a series of papers dedicated
Scheme 1. Titanium–salan catalysts discussed in Bryliakov and Talsi[18]
(1–3) and in this work (1, 4–9).
Appl. Organometal. Chem. 2013, 27, 239–244
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E. P. Talsi and K. P. Bryliakov
Table 2. Asymmetric oxidation of various sulfides on catalysts 7–9
N
Substratea
Catalyst
Conversion (%)
Sulfoxide/ sulfone yield (%)
ee (configuration)
1
2-NaphSCH2Ph
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
7
8
9
100
100
97.0
99.0
99.0
78.0
100
100
96.5
93.5
100
88.0
94.5
100
100
99.5
100
100
100
100
100
99.5
100
100
81.5 / 18.5
75.5 / 24.5
87.0 / 10.0
70.5 / 28.5
81.0 / 18.0
73.0 / 5.0
91.5 (R)
93.0 (R)
87.0 (R)
92.5 (R)
92.5 (R)
83.0 (R)
72.0 (R)
71.5 (R)
60.0 (R)
82.0 (R)
81.0 (R)
76.0 (R)
73.5(R)
2
3
4
5
6
7
8
PhSCH2Ph
2-NaphSMe
PhS(cyclo-C6H11
PhSiPr
77.5 / 22.5
74.0 / 26.0
92.5 / 4.0
)
75.0 / 18.5
72.0 / 28.0
73.5 / 14.5
68.0 / 26.5
72.0 / 28.0
63.0 / 37.0
84.0 / 15.5
88.5 / 11.5
87.5 / 13.0
85.0 / 15.0
80.5 / 19.5
91.0 / 9.0
79.5 (R)
76.5 (R)
58.5 (R)
60.5 (R)
62.0 (R)
65.0 (R)
67.5 (R)
63.0 (R)
72.0 (R)
77.5 (R)
72.5 (R)
p-BrPhSMe
p-CH3PhSMe
PhSMe
72.0 / 27.5
78.0 / 22.0
84.0 / 16.0
aReactions were performed at ꢀ10 ꢁC, sulfide initial concentration 10 mM, [H2O2]/[S] = 1.3, catalyst loading 1.0 mol%, reaction time 40 h.
to enantioselective epoxidations with H2O2 in the presence
of chiral titanium(IV) salan and salalen complexes.[21–28] The
authors managed to isolate and X-ray characterize a binuclear
m-oxo-m-peroxo titanium–salan complex, formed in the reaction
of the starting (also binuclear) catalyst with H2O2. The authors
believed the m-oxo-m-peroxo complex to be a precursor of the
elusive active epoxidizing species.[26] Using high-resolution electro-
spray mass spectrometric technique, Berkessel with co-workers
identified a titanium–salalen complex (formed in situ from the
salalen ligand and Ti(OiPr)4) as a mononuclear species.[29] On the
basis of kinetic and mass spectrometric studies, they predicted
that the active epoxidizing agent could be mononuclear peroxoti-
tanium(IV) salalen species.[30]
Although the real active species has never been detected by
any physical method, kinetic data could also provide insight into
the nature and reactivity of the oxygen-transferring species and
into the entire reaction mechanism. Such data are of particular
value when they are obtained under practical catalytic condi-
tions. To evaluate the reaction mechanism, we first ran parallel
oxidations of three different substrates under the same condi-
tions (for details see Experimental) over catalyst 1 at the initial
stages of the reaction, and measured the decay of sulfide concen-
tration with time. Although one might expect that PhSCH2Ph,
p-MePhSCH2Ph and p-NO2PhSCH2Ph would be oxidized at
different rates, the observed decay of all three substrates was
found to obey a pseudo-first-order kinetics with nearly equal
(within experimental error) rate constants: for PhSCH2Ph,
k = 1.6 ꢂ 10ꢀ2 minꢀ1; for p-MePhSCH2Ph, k = 1.5 ꢂ 10ꢀ2 minꢀ1
;
for p-NO2PhSCH2Ph, k = 1.7 ꢂ 10ꢀ2 minꢀ1, see also supporting
information. The kinetic plots are shown in Fig. S1 of the sup-
porting information. In all experiments, the reactions required
at least 30 min to approach steady-state conditions. This is
indicative of a stepwise mechanism: at first, the rate-determining,
step, H2O2 interacts with the catalyst to form the active intermediate,
which further rapidly reacts with the sulfide.
To evaluate the electronic effect of para-substituent in substi-
tuted thioanisoles, a series of competitive oxidation experiments
were performed (for details see Experimental). The observed log
(kX/kH) values displayed linear correlation (R2 > 0.99) with Hammett
parameters sp and slope r-values in the range ꢀ1.35 to ꢀ1.40
for four different catalysts (see Figs 1 and S2 and Table 3; cf.
other studies[31–36]). Good linearity suggests that the oxidation
proceeds via a single mechanism; the negative r-values indi-
cate a positive charge build-up on sulfur in the sulfoxidation
reaction.[37,38] The use of Hammett parameter sp+ gave a relatively
poorer correlation (R2 ≤ 0.98). These data are consistent with a
concerted mechanism in which an electrophilic active oxygen
species is directly attacked by a nucleophilic sulfide,[39,40] ruling
out an alternative electron transfer mechanism (with formation
of a transient sulfenium radical cation intermediate).[41,42]
Substituent Y in catalysts 5–9 did not significantly affect the
apparent r-values (Table 3).
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Appl. Organometal. Chem. 2013, 27, 239–244

Versatile catalysts for Ti-salan-catalyzed asymmetric sulfoxidations
Conclusions
Titanium–salan complexes bearing bulky phenyl substituents at
the 3,30-positions of aromatic rings are the most stereoselective
catalysts for the oxidation of sterically demanding aryl benzyl
sulfides with H2O2. At the same time, catalysts with smaller substi-
tuents at the 3,30-positions (CH3, OCH3, and particularly H) demon-
strate higher enantioselectivities in the oxidations of less bulky
substrates, such as aryl methyl sulfides. The introduction of mild
electron acceptors (particularly, 5,50-I2 and 5,50-Br2) yields catalysts
7–9 that demonstrate good to high enantioselectivities with both
bulky and less bulky sulfides, along with the highest sulfoxide
selectivities (up to 92%). Kinetics suggests that the rate-limiting step
of the catalytic reaction is the interaction between the titanium–
salan catalyst and H2O2 to form the active species, presumably
titanium(IV) peroxo complex, which further rapidly reacts with the
sulfide. Hammett analysis indicates a moderately electrophilic
nature of the active oxygenating species (r-values of ꢀ1.35 to
ꢀ1.40), which is directly attacked by a nucleophilic sulfide, ruling
out an alternative electron transfer mechanism. A consistent
reaction mechanism is proposed.
Figure 1. Hammett plot (log(kX/kH) vs. s) for the competitive oxidation
of substituted p-XPhSCH3 by the system 5/H2O2. For details see
Experimental.
Table 3. Hammett r parameters for catalysts 5 and 7–9
Catalyst
5
7
8
9
r
ꢀ1.36
ꢀ1.39
ꢀ1.35
ꢀ1.40
Acknowledgements
This work was supported by the Russian Foundation for Basic
Research, grant 12-03-33180.
Altogether, these data can be explained as follows. At the first
(rate-determining) step, an adduct is formed between the titanium
complex and hydrogen peroxide (cf. other studies[22,26,27,29,30]).
Coordination of H2O2 to the titanium center in the chiral ligand
framework is (1) a precondition to accomplish the oxygen transfer
in a stereoselective fashion and (2) believed to increase the peroxide
electrophilicity and activate it for the sulfide oxidation: in the
absence of Ti-salan catalysts, no oxidation takes place. A plausible
reaction mechanism is presented in Scheme 2. The mode of H2O2
coordination to titanium (Z1 or Z2) is not reliably established as
yet. However, we favor the hypothesis of Z2 coordination since a
test reaction with tBuOOH (for which case a Z2 coordination is less
likely, especially with most sterically demanding titanium–salan
catalysts, where X ¼ H) as the terminal oxidant demonstrated
only a 3% sulfide conversion (see Experimental). Recently, a
similar Z2-peroxotitanium-reactive intermediate was predicted
by density functional theory calculations in non-stereoselective
titanium–salan-catalyzed sulfoxidation, and its formation was
experimentally corroborated by atmospheric pressure chemical
ionization (APCI) mass spectrometric studies.[43]
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