Titanium tetra-tert-butoxide-tert-butyl hydroperoxide oxidizing system: Physicochemical and chemical aspects

Article abstract of DOI:10.1007/s11176-008-2015-9

The reaction of titanium tetra-tert-butoxide with tert-butyl hydroperoxide (1: 2) (C₆H₆, 20 C) involves the steps of formation of the titanium-containing peroxide (t-BuO)₃TiOOBu-t and peroxytrioxide (t-BuO)₃TiOOOBu-t. The latter decomposes with the release of oxygen, often in the singlet form, and also homolytically with cleavage of both peroxy bonds. The corresponding alkoxy and peroxy radicals were identified by ESR using spin traps. The title system oxidizes organic substrates under mild conditions. Depending on the substrate structure, the active oxidant species can be titanium-containing peroxide, peroxytrioxide, and oxygen generated by the system.

Full text of DOI:10.1007/s11176-008-2015-9

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 2, pp. 266 276.
Original Russian Text L.P. Stepovik, M.V. Gulenova, I.M. Martynova, B.A. Mar’yasin, V.K. Cherkasov, 2008, published in Zhurnal Obshchei Khimii,
008, Vol. 78, No. 2, pp. 288 298.
Pleiades Publishing, Ltd., 2008.
2
Titanium Tetra-tert-butoxide tert-Butyl Hydroperoxide
Oxidizing System: Physicochemical and Chemical Aspects
a
a
a
L. P. Stepovik , M. V. Gulenova , I. M. Martynova ,
a
b
B. A. Mar’yasin , and V. K. Cherkasov
a
Lobachevskii Nizhni Novgorod State University,
pr. Gagarina 23, Nizhni Novgorod, 603950 Russia
e-mail: gulmv@rambler.ru
b
Razuvaev Institute of Organometallic Chemistry,
Russian Academy of Sciences, Nizhni Novgorod, Russia
Received June 13, 2007
Abstract The reaction of titanium tetra-tert-butoxide with tert-butyl hydroperoxide (1 : 2) (C H , 20 C)
6
6
involves the steps of formation of the titanium-containing peroxide (t-BuO) TiOOBu-t and peroxytrioxide
3
(
t-BuO) TiOOOBu-t. The latter decomposes with the release of oxygen, often in the singlet form, and also
3
homolytically with cleavage of both peroxy bonds. The corresponding alkoxy and peroxy radicals were
identified by ESR using spin traps. The title system oxidizes organic substrates under mild conditions.
Depending on the substrate structure, the active oxidant species can be titanium-containing peroxide, peroxy-
trioxide, and oxygen generated by the system.
DOI: 10.1134/S1070363208020151
Titanium alcoholates are widely used as catalysts
products suggests the radical mechanism of the reac-
tion [6].
of hydroperoxide oxidation of various classes of or-
ganic substrates. They catalyze the synthesis of epox-
idized alcohols from alkenes [1] and stereoselective
epoxidation of allyl alcohols [2, 3]. When treated with
tert-butyl hydroperoxide in the presence of Ti(OPr-i)4,
primary and secondary benzyl alcohols selectively
transform into the corresponding aldehydes and ke-
tones [4], and substituted phenols and naphthols are
oxidized to o-quinones [5]. The above oxygen-con-
taining derivatives are oxidized along intramolecular
pathway, without participation of free radicals.
We suggested that the reaction of the components
of the alcoholate I hydroperoxide II system in ben-
zene is similar to that observed in the aluminum tri-
Table 1. Reaction products formed in the (t-BuO) Ti
4
t-BuOOH system in benzene at 20 C (moles per mole of
titanium alcoholate)
(
t-BuO) Ti : t-BuOOH ratio
4
Reaction productsa
The reaction of titanium tetra-tert-butoxide I with
tert-butyl hydroperoxide II in 1 : 2 molar ratio in
benzene at room temperature is accompanied by the
release of oxygen (0.85 0.90 mol per mole of I) and
regeneration of the starting titanium alcoholate [6]
1: 2
1: 10
Volatile reaction products
b
O2
0.85
3.59
0.08
0.02
4.42
9.63
0.29
t-BuOH
(t-BuO)2
(CH ) C=O
(
Table 1). As the II : I molar ratio is increased to
0 : 1, the amount of the released oxygen increased to
.2 mol. No products of solvent oxidation were de-
1
4
Traces
3
2
c
Products of hydrolysis of nonvolatile residue
tected. The above system oxidizes under mild condi-
tions (20 C) the C H bonds of the methyl (toluene),
methylene (hexane, ethylbenzene), and methine (1,1-
diphenylmethane, triphenylmethane) groups [6], and
t-BuOH
2.15
0.08
0.07
3.16
0.11
0.24
H O
2
2
CH COOH
3
a
b
also the E H bonds of organometallic hydrides R EH
Averaged results.
Determined by the reaction with benz-
1 mol of titanium in all the experiments.
3
c
(E = Si, Ge; R = Et, Ph) [7]. The composition of the
aldehyde.
266

TITANIUM TETRA-tert-BUTOXIDE tert-BUTYL HYDROPEROXIDE OXIDIZING SYSTEM:
267
tert-butoxide tert-butyl hydroperoxide system [8]. The
initial step of the process is formation of the titanium-
containing peroxide. The reaction of the latter with
tert-butyl hydroperoxide leads to peroxytrioxide III
which decomposes along two pathways: (1) release of
oxygen and regeneration of titanium alcoholate and
(2) homolytic pathway with the formation of alkoxy
and peroxy radicals [scheme (1)] [6, 7, 9].
C H , 20 C
6
6
(
t-BuO) Ti + t-BuOOH
t-BuOH + (t-BiO) TiOOBu-t,
4
3
I
II
(
t-BuO) TiOOBu-t + t-BuOOH
t-BuOH + (t-BiO) TiOOBu-t,
3
3
III
(
1)
1
2
3
1
O
(
t-BuO) Ti + O ,
4
2
(
t-BuO) Ti
O
(t-BuO) TiOO + t-BuO ,
3
3
t-BuO
IIIa
(t-BuO)3TiO + t-BuOO .
The goal of this study was to examine and prove
the processes indicated in scheme (1) by physico-
chemical and chemical methods.
radicals are detected only at low temperatures [12].
With time, this signal becomes strongly broadened,
suggesting generation of oxygen in the reaction mix-
ture [13]. In 30 min the signal fully disappears. In the
vitrified solvent matrix (ethylbenzene, 90 C), an
The formation of radicals was monitored by ESR
in the presence of spin traps and without them. As
spin traps we used C-phenyl-N-tert-butylnitrone IV
and 2-methyl-2-nitrosopropane V. The former scav-
enges alkylperoxy radicals, and the latter, alkyl and
alkoxy radicals [10, 11]. It is known that nitroso com-
pounds react with hydroperoxides to form adducts of
alkoxy radicals. The possibility of using spin trap V
as acceptor of free radicals in the system metal alco-
holate hydroperoxide was demonstrated in [8].
anisotropic spectrum of (t-BuO) TiOO is observed.
3
Its parameters are g = 2.0249, g = 2.0092, and g =
1
2
3
2.0025. The quantity gi = 2.0122 calculated from
these parameters is well consistent with the g value
i
determined from the isotropic ESR spectrum (Fig. 2).
This fact confirms that, at room temperature, only the
titanium-containing peroxy radical is detected.
A mixture formed by the reaction of an authentic
sample of (t-BuO) TiOOBu-t with t-BuOOH (20 C)
3
After mixing I and II (1 : 2) in benzene at room
also gives a broad singlet with the same g-factor in
the absence of spin traps. Because tri-tert-butoxy-tert-
butylperoxytitanium gives ESR signals neither at 90
temperature without spin traps, a singlet (g 2.0125)
i
appears in the ESR spectrum (Fig. 1). This signal can
be assigned to (t-BuO) TiOO (VI), as alkylperoxy
(ethylbenzene) nor at 25 C (benzene), (t-BuO) TiOO
3
3
H
0
.5 mT
H
0
.5 mT
Fig. 1. ESR spectrum recorded in the (t-BuO) Ti
4
t-BuOOH (1 : 2) system immediately after mixing the
reactants in the absence of spin traps (benzene, 20 C,
Fig. 2. ESR spectrum of the reaction mixture
c[Ti(OBu-t) ] = 0.3 M).
(t-BuO) Ti t-BuOOH (1 : 2) in ethylbenzene ( 90 C).
4
4
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

268
STEPOVIK et al.
(
a)
VII
(b)
IX
VIII
IX
VIII IX
VII IX
IX
VIII IX
VII
VII
VIII
VII
VIII VII
VIII
H
0.25 mT
Fig. 3. ESR spectrum recorded in the (t-BuO) Ti t-BuOOH system (1 : 2) in the presence of spin trap IV: (a) immediately
4
after mixing the reactants, c[Ti(OBu-t) ] 0.025 M, and (b) 10 min after mixing the reactants, c[Ti(OBu-t) ] 0.1 M, the
4
4
sample was degassed. Figures at peaks are compound nos.
is apparently generated in the step of the reaction of
the titanium-containing peroxide with II.
sopropane contains a symmetrical triplet (1 : 1 : 1)
1
5
[g = 2.0056, a ( N) = 2.715 mT] corresponding to
i i
the adduct of spin trap V with t-BuO radicals, t-Bu
N(O ) OBu-t (X) [8].
Addition of C-phenyl-N-tert-butylnitrone IV into
the reaction system in benzene gives rise to an ESR
spectrum shown in Fig. 3. After mixing the reactants
We determined the half-life of the (t-BuO) TiOO
3
radical in ethylbenzene, 500 s. The kinetics of the
decay of this radical follows a first-order equation,
suggesting a monomolecular mechanism of its forma-
tion.
{
c[Ti(OBu-t) ] = 0.025 M}, a triplet with isotropic
4
1
5
parameters [g = 2.0063, a ( N) = 0.795 mT] typical
i
i
of benzoyl-tert-butylnitroxyl radical VII [10] appears
in the ESR spectrum (Fig. 3a). Radical VII is formed
by oxidation of the spin trap with either singlet oxy-
gen or peroxy radicals [11]. A triplet of doublets with
asymmetric lines was assigned to adducts of C-phen-
yl-N-tert-butylnitrone. With an increase in the concen-
tration of titanium tert-butoxide to 0.1 M, a signal of
radical VII practically disappears from the ESR spec-
trum in 10 min, and two triplets (1 : 1 : 1) of doublets
An indirect evidence for the formation of peroxy
radicals t-BuOO and (t-BuO) TiOO is the presence
3
in reaction products of tert-butyl peroxide ( 0.08 mol)
and bound hydrogen peroxide ( 0.10 mol) (per mole
of starting alcoholate) (Table 1) [Eq. (2)]:
2ROO
ROOR + O2,
(2)
(
1 : 1) are recorded (Fig. 3b). Their constants were
R = t-Bu, (t-BuO) Ti.
3
determined after degassing the samples. One of the
signals with the parameters g = 2.0053, a ( N) =
1
5
i
i
To prove the formation of peroxy radicals and
quantitatively estimate their concentration, we per-
1
.450 mT should be assigned to the adduct of trap IV
with alkoxy radical VIII, t-BuO , or (t-BuO) TiO [8].
3
formed the reactions of (t-BuO) Ti t-BuOOH with
4
At low concentrations of the starting alcoholate I
(
c[Ti(OBu-t) ] = 0.1 M its relative concentration is
low, and within 10 15 min it fully disappears. The
major signal in the spectrum (Fig. 3b) at g = 2.0056
with the hyperfine coupling constants a ( N) = 1.323,
ai(H) = 0.139 mT belongs to an adduct of spin trap
IV with one of peroxy radicals [(t-BuO) TiOO or
sterically hindered phenols [2,6-di-tert-butyl-4-methyl-
phenol (Ionol), 2,4,6-tri-tert-butylphenol], because it
is known that alkylperoxy radicals are scavenged by
,4,6-substituted phenols with the formation of 4-al-
kylperoxy-2,6-di-tert-butyl-4-alkylhexa-3,5-dienones
0.025 M), this signal is not detected (Fig. 3a), and at
4
2
i
1
5
i
[15 17].
In the absence of metal alcoholates (20 C, C H ),
6
6
3
hydroperoxide II does not react with any of the above-
mentioned phenols. From the reactions of system I II
with these phenols, we isolated in both cases 2,6-di-
tert-butyl-4-tert-butylperoxy-4-alkylcyclohexa-2,5-di-
enone ( 30%) and 2,5-di-tert-butyl-p-benzoquinone
(0.04 0.17 mol) (Table 2).
t-BuOO ] (IX) [14]. The first alternative is more prob-
able, because (t-BuO) TiOO is more stable under
3
these conditions.
The isotropic ESR spectrum (benzene, 20 C) of the
reaction mixture in the presence of 2-methyl-2-nitro-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

TITANIUM TETRA-tert-BUTOXIDE tert-BUTYL HYDROPEROXIDE OXIDIZING SYSTEM:
269
OH
O
R
O
XO
t-BuOO
XOH
R
OOBu-t
(
3)
R
O
O
O
R = Me, t-Bu
X = t-Bu, (t-BuO) Ti
(
t-BuO) TiOO
3
3
+
Ti(OBu-t)4
R = t-Bu
R
OOTi(OBu-t)3
XI
The formation of the above products can be ac-
counted for by scheme (3).
Table 2. Products of oxidation of 2,6-di-tert-butyl-4-R-
phenols (R = Me, t-Bu) with the system Ti(OBu-t)4
t-BuOOH (1 : 1 : 2), C H , 20 C (moles per mole of metal
6
6
As seen from the scheme, the phenoxyl radical
alcoholate)a
reacts either with t-BuOO or with (t-BuO) TiOO
3
radical. In the former case, the reaction yields a per-
oxy ketone, and in the latter case, 2,6-di-tert-butyl-
p-benzoquinone via titanium-containing peroxide XI.
The presence of peroxy ketones in the reaction mix-
tures is also confirmed by IR spectroscopy. The IR
spectra of the reaction products contain strong absorp-
Ti(OBu-t)4
Reaction productsa
R = Me
R = t-Bu
Volatile reaction products
1
t-BuOH
Acetone
3.31
0.04
3.17
tion bands belonging to quinoid structures, cm :
1
1
647 (C=C), 1667 1669 (C=O), 796 800 (O O) [17,
8]. Thus, the formation of alkylperoxy ketones con-
b,c
Products of hydrolysis of nonvolatile residue
t-BuOH
t-BuOOH
,6-Di-tert-butyl-p-
benzoquinone
,6-Di-tert-butyl-4-tert-
butylperoxy-4-R-cyclohexa-
,5-dienone
Bis(1,3,5-tri-tert-butylcyclo-
hexa-2,5-dien-4-on-1-yl)
peroxide
2.64
0.02
0.04
2.75
Traces
0.17
firms the generation of peroxy radicals in the system
I II and allows quantitative estimation of their con-
centration.
2
2
0.30
0.29
0.30
0.07
0.24
Along with the peroxy ketone, with 2,4,6-tri-tert-
butylphenol we expected the formation of the corre-
sponding alkoxy ketone, 2,4,6-tri-tert-butyl-4-tert-
butoxycyclohexa-2,5-dienone. However, we have not
detected this product. It is known that the related
compounds are not formed from Ionol [19]. In the
case of 2,4,6-tri-tert-butylphenol, the phenoxyl radical
interacts not only with peroxy radicals, but also with
2
Starting phenol
a
b
Averaged results.
c
1 mol of titanium in all the experiments.
An unidentified epoxy compound (yield 0.10 mol) was also
detected in the case of R = Me.
oxygen generated by the system (t-BuO) Ti t-BuOOH
4
to form bis(1,3,5-tri-tert-butylcyclohexa-2,5-dien-4-
on-1-yl) peroxide.
Tri(tert-butoxy)titanium tert-butyl trioxide III can-
not be subjected to an experimental structural study
because of its instability and is postulated as the nec-
essary unit in the chain of the general mechanism
03W program package [20]. The results of geometry
optimization and calculation of the total energy of
peroxytrioxide III suggest that it should be fairly
1
stable ( E = E1
E = 19.3 kcal mol ) (Fig. 4).
2
[scheme (1)]. To substantiate this approach theoreti-
To determine whether this point on the potential
energy surface is a minimum, we calculated the vibra-
tion frequencies of a molecule of this kind. In so
doing, to decrease the number of heavy atoms and
thus to simplify the calculation, we replaced tert-butyl
groups by methyl groups. We assumed that this re-
placement would not cardinally alter the problem,
because the tert-butyl groups are relatively remote
from each other. Therefore, their repulsion in the open
cally, we performed a quantum-chemical calculation
of the trioxide molecule (an isolated molecule in the
gas phase was considered).
All the calculations were performed by the DFT
method (density functional theory) in the B3LYP ver-
sion using the 6-31G basis set and the 6-31G(d,p)
basis set supplemented by polarized d,p functions.
We used the standard version of the GAUSSIAN
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

270
STEPOVIK et al.
form of the trioxide molecule should not be signifi-
cant, despite the fact that the strain strain caused by
close location of tert-butyl groups is certainly higher
compared to methyl groups. The calculation showed
that the open form of titanium peroxytrioxide III cor-
responds to a minimum on the potential energy sur-
face (Fig. 5), i.e., this molecule is an intermediate.
Selected bond angles and bond lengths in this mole-
cule are given in Table 3.
peroxide II and its homolytic decomposition across
two peroxy bonds.
Table 1 shows that the major transformation path-
way of III is the decomposition with the release of
oxygen. In the process, the tert-butoxy group migrates
to the metal atom with an electron pair, which, in
accordance with the rule of spin preservation, should
lead to the formation of singlet oxygen [scheme (1),
pathway 1]. However, quantum-chemical calculations
showed that cyclic form IIIa of the peroxytrioxide
cannot be realized because of strong steric strain pro-
duced by the bulky tert-butyl groups. Presumably,
simultaneous formation of oxygen and titanium alco-
holate I is realized in a dimer of the open form of III
[scheme (4)]:
Thus, the results of a quantum-chemical calculation
and evidences for the presence of the radicals
(
t-BuO) TiOO , t-BuO , (t-BuO) TiO , and t-BuOO
3 3
confirm the formation of titanium-containing peroxy-
trioxide III in the reaction of alcoholate I with hydro-
Bu-t
O
^
O
O
O
^
21O2 + 2(t-BuO)4Ti.
|
(4)
Ti(OBu-t)3
|
(
t-BuO) Ti
3
O
|
O
t-Bu
Table 3. Selected interatomic distances d in (MeO) Ti
OOOMe
With the aim to identify and quantitatively deter-
3
a
1
mine O , we studied the reaction of the (t-BuO) Ti
2
4
t-BuOOH system with typical acceptors of singlet
oxygen: anthracene and 9,10-dimethylanthracene. The
primary oxidation products are 9,10-epidioxyanthra-
cenes. 9,10-Dihydro-9,10-epidioxyanthracene under
the reaction conditions transforms into anthraquinone
[21]; its yield reaches 0.30 mol. The reaction with
9,10-dimethylanthracene yields up to 0.45 mol of
Distance
d,
Distance
d,
1
6
7
8
2
2
3
4
4
Ti O
1.792
1.784
1.782
1.903
O O
1.447
1.446
2.346
1
3
Ti O
O O
1
2
Ti O
O O
1
Ti O
9
,10-dimethylepidioxyanthracene, which was isolated
a
2
3
4
1
Bond angle O O O
= 108.4 .
pure and characterized by H NMR spectroscopy [22].
The formation of anthraquinone and 9,10-dimethyl-
9
,10-epidioxyanthracene unambiguously proves the
occurrence of the reaction with singlet oxygen.
It is well known that -carotene in concentrations
of down to 10 4 10 M efficiently quenches free
singlet oxygen irrespective of its generation pathway.
5
One -carotene molecule induces conversion of more
1
than 250 O molecules. The quenching is physical,
2
9
1
1
occurs at a rate of (2.5 5.0) 10 mol l s , and
consists in the energy transfer to the triplet level of
-carotene [23, 24]:
1
O ( ) + 3( -carotene). (5)
3
O ( ) + -carotene
2
2
Fig. 4. Results of calculation of the (t-BuO) TiOOOBu-t
molecule in the open form [FOPT (geometry optimiza-
tion to reach the minimum); DFT-B3LYP; 6-31G; E =
Addition of -carotene in concentrations as high as
10 4 10 M to the system under consideration de-
creased the anthraquinone yield by a factor of no more
than 2.
3
3
1
931.997810 au].
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TITANIUM TETRA-tert-BUTOXIDE tert-BUTYL HYDROPEROXIDE OXIDIZING SYSTEM:
271
We studied the reaction of the system alcoholate I
hydroperoxide II (1 : 2) with anthracene and 9,10-di-
methylanthracene simultaneously (molar ratio 1 : 1),
C5
taking into account that the rate constants of the reac-
tions of free singlet oxygen with anthracene and 9,10-
dimethylanthracene differ by two orders of magnitude
O4
4
6
1
1
O3
(
k1 = 7 10 , k = 4 10 l mol s , respectively)
2
[
9
25]. However, in our case the yield of 9,10-dimethyl-
,10-epidioxyanthracene (0.35 mol) exceeds the yield
O2
of anthraquinone (0.19 mol) by a factor of less than 2.
These results, in combination with the lack of signif-
icant effect of -carotene on the anthracene conver-
sion, show that there is no free singlet oxygen in the
system.
O6
Ti1
C9
C10
O7
The (t-BuO) Ti t-BuOOH system oxidizes aryl-
4
2
alkenes containing at the sp -carbon atom only phenyl
substituents and hydrogen atoms (1,1-diphenylethyl-
ene, triphenylethylene). It is known that these com-
pounds are relatively inert to free singlet oxygen. The
O8
[
2+2]-cycloaddition reactions are performed by pass-
1
C11
ing O through an arylalkene solution for 24 h and
2
more [26].
By the example of the reaction of system I II with
Fig. 5. Results of calculation of trimethoxytitanium
methyl trioxide molecule [FOPT (geometry optimization
to reach the minimum); DFT-B3LYP; 6-31G(d, p); E =
1460.44767616 au; no imaginary frequencies].
Ph C=CH , we demonstrated that the reaction was
2
2
complete within 1 1.5 h, as judged from the disap-
pearance of both the substrate and hydroperoxides.
Prolongation of the reaction time to 24 h did not lead
to higher conversion of the substrate and higher yields
of oxidation products (carbonyl compounds, Table 4).
Table 4. Products of reactions of phenylalkenes with the
Ti(OBu-t) t-BuOOH system (1 : 2) (benzene, 20 C,
4
a
moles per mole of titanium alcoholate)
We assume that the oxidation of phenylethylenes
with the (t-BuO) Ti t-BuOOH system involves the
4
Reaction products
Ph C=CH2
Ph C=CHPh
2
2
steps of the formation and subsequent decomposition
of 1,2-dioxetanes [27]. With the aim to elucidate the
pathways of formation of dioxetanes, we performed
the reactions with 1,1-diphenylethylene in the pres-
ence of -carotene. Addition of -carotene exerted no
appreciable effect on the quantitative composition of
the Ph C=CH oxidation products and on the degree
of its conversion (Table 4). Hence, in this case the
active oxidizing agent is other than free singlet oxy-
gen. The most probable agent is either singlet oxygen
coordinated on the metal as in vanadium compounds
Volatile reaction products
2.86b
t-BuOH
PhCHO
2.78
2.85
0.04
Products of hydrolysis of nonvolatile residuec
2.86b
0.65b
2.80
0.19
0.12
2
2
t-BuOH
2.58
0.59
Ph C=O
2
PhCOOHd
HCOOH
0.33
0.35
0.36b
0.30b
Starting alkylarene
0.73
[
22, 28] or the metal-containing peroxytrioxide. How-
ever, in contrast to the systems based on vanadium
compounds [22, 28] when the yield of benzaldehyde
in oxidation of trans-stilbene reaches 50%, in our
system we have not detected the aldehyde or benzoic
acid among products of similar reaction.
a
b
Averaged results. In the presence of -carotene, concentra-
4
c
tion 7.7 10 M; v(O theor.) : v( -carotene) = 160.
1 mol
2
d
of metal in all the experiments. Formaldehyde was identified
qualitatively.
ably generated in the step of the reaction of the sub-
strates with peroxytrioxide III [Eq. (6)].
Thus, the products of the reactions of system I II
with anthracenes and phenylalkenes suggest that the
substrates are oxidized with singlet oxygen. The data
we obtained suggest that singlet oxygen is most prob-
The possibility of the direct oxidation with organo-
metallic polyoxides was demonstrated by the example
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

272
STEPOVIK et al.
Ph
Ph
R
Ph
Ph
R
H
[
(t-BuO) TiOOOBu-t] +
C=C
C C
Ph C=O + RCHO,
(6)
3
2
(
t-BuO) Ti
4
H
O O
of trialkylsilyl hydrotrioxides [29]. These compounds
react with typical acceptors of singlet oxygen to form
the corresponding products in 25 60% yield. At the
same time, the amount of singlet oxygen determined
by the chemiluminescence method does not exceed
oxidation yielded acetophenone, 1-phenyl-1-ethanol,
and 1-hydroperoxy-1-phenylethane.
To prove the homolytic character of the process,
we examined the oxidation of ethylbenzene by ESR.
On addition of 2-methyl-2-nitrosopropane (20 C),
an ESR spectrum containing signals of adducts of
the -phenylethyl radical PhCH(Me)N(O )Bu-t (XII)
0
.1%. Oxygen released in the free form is triplet,
because under the influence of oxygen-centered radi-
cals present in the system the singlet triplet transition
occurs at a high rate, 1 10 mol l s [13, 30,
1]:
{
triplet of doublets at g = 2.0062 with hyperfine
1
0
1
1
i
15
coupling constants a ( N) = 1.496, a (H ) = 0.378,
i
i
3
a [H(CH )] = 0.048 mT} [8, 33] and two triplets is re-
i
3
corded. One of these triplets with the parameters g =
i
1
3
(7)
In the mixture from the reaction of system I II
with anthracene and 1,1-diphenylethylene simultane-
ously (1 : 1) in benzene, we detected benzophenone in
a yield of up to 0.30 mol per mole of the initial titani-
um alcoholate. In the process, the yield of anthraqui-
none did not exceed 0.10 mol. Oxygen evolution was
also observed.
15
2
.0057, a ( N) = 2.715 mT belongs to tert-butoxy-
RO + O
RO + O .
i
2
2
tert-butylnitroxyl X [8], and the other [g = 2.0056,
i
1
5
a ( N) = 2.920 mT], to an adduct of spin trap V with
i
an alkoxy radical, probably (t-BuO) TiON(O )Bu-t.
3
When the spectra are successively recorded in time
(
30 min), the intensity of the signals from adducts of
the -phenylethyl radical remains constant, whereas
the intensity of the signals from tert-butoxy radicals
drastically decreases. The peroxy radicals [PhCH(Me)
OO ] were identified in the presence of C-phenyl-N-
tert-butylnitrone IV. The ESR spectrum is a triplet
According to Eq. (6), the oxidation of phenylal-
kenes should result in the regeneration of alcoholate I.
Indeed, in the mixture from the reaction of 1,1-di-
1
5
of doublets [g = 2.0062, a ( N) = 1.315, a (H) =
i
i
i
phenylethylene with the (t-BuO) Ti t-BuOOH sys-
0.104 mT] [8]. These data suggest the radical charac-
ter of the reaction of the system with ethylbenzene
and prove that carbon-centered radicals escape into the
bulk of the solvent. This conclusion can be confirmed
4
tem, we detected up to 0.50 mol of the starting alco-
holate. The remaining 0.50 mol participates in the
oxidation of formaldehyde (Table 4).
by the results of the reaction of the (t-BuO) Ti
4
Aldehydes are oxidized with titanium-containing
peroxide to metal acyloxy compounds [32] whose
hydrolysis yields the corresponding acids [Eq. (7)]:
t-BuOOH system (1 : 2) with ethylbenzene in atmos-
pheres of argon and oxygen. In the latter case, the
amount (in moles per mole of titanium alcoholate) of
acetophenone increased from 0.43 to 1.38; that of
(
t-BuO) TiOOBu-t + RCHO
3
1
-phenyl-1-ethanol, from 0.13 to 0.60; and that of
(
t-BuO) TiOCH(R)OOBu-t
3
1-hydroperoxy-1-phenylethane, from 0.40 to 1.50.
(
t-BuO) TiOCOR + t-BuOH,
(8)
3
Experiments with (t-BuO) TiOOBu-t showed that
3
R = H, Ph.
this compound does not oxidize hydrocarbons. In the
experiments with ethylbenzene, either acetophenone
was not detected at all, or its amount did not exceed
As noted above [6], system I II oxidizes C H
bonds of hydrocarbons at 20 C. In all the cases, the
reaction products were carbonyl compounds, alcohols,
or their transformation products. Significant amounts
of hydroperoxides were also detected, suggesting the
radical character of the process. It was suggested that
the oxidation is initiated by oxygen-centered radicals
formed by decomposition of peroxytrioxide III
0
.005 mol.
To elucidate the factors affecting the relative con-
tributions of the decomposition pathways of III, we
studied the reactions of system I II with anthracene
and ethylbenzene simultaneously. When the reaction
of the (t-BuO) Ti t-BuOOH system with anthracene
4
[
scheme (1)]. Carbon-centered radicals subsequently
was performed in ethylbenzene as solvent, the preva-
lent pathway was radical oxidation of ethylbenzene to
1-hydroperoxy-1-phenylethane and products of its
further transformations (acetophenone, 1-phenyl-1-
react with oxygen generated by the system to form the
corresponding peroxy radicals responsible for the final
products. With ethylbenzene as model substrate, the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

TITANIUM TETRA-tert-BUTOXIDE tert-BUTYL HYDROPEROXIDE OXIDIZING SYSTEM:
273
ethanol, styrene). Molecular oxidation of anthracene
to anthaquinone is realized to 10 15%. The major
pathway of the reaction of system I II with a 1 : 1
mixture of ethylbenzene and anthracene in benzene
Table 5. Products of the reaction of the Ti(OBu-t)4
t-BuOOH system with anthracene and ethylbenzene
(1 : 2 : 1 : 1) in benzene at 20 C (moles per mole of
titanium alcoholate)
(
Table 5) is the oxidation of anthracene to anthraqui-
none. Products of ethylbenzene oxidation were de-
tected in minor amounts.
Ti(OBu-t)4 concentration, M
Reaction products
0
.05
0.12
0.05a
0.30a
As shown in scheme (1), the first step of the reac-
tion of alcoholate I with hydroperoxide II yields tita-
Volatile reaction products
nium-containing peroxide (t-BuO) TiOOBu-t, which
3
t-BuOH
PhC(O)CH3
3.16
0.04
2.99
0.04
3.22
0.10
3.04
is also an oxidant. For example, it oxidizes aldehydes
0.06
[
Eq. (8)] and alcohols [6] by the molecular mech-
b
Products of hydrolysis of nonvolatile residue
anism. We have monitored the reaction of the
t-BuOH
2.68
0.04
Traces Traces
2.88
0.05
2.90
0.27
0.02
3.13
0.21
0.02
(
t-BuO) Ti t-BuOOH system with PhCH(OH)Me
4
PhCH(OOH)CH3
PhCH=CH2
(1 : 1 : 1, benzene, 20 C) by ESR. The absence of any
radicals confirms the molecular reaction pathway.
System I II (1 : 2) oxidizes 3,6-di-tert-butylpyrocate-
chol to 3,6-di-tert-butyl-o-benzoquinone and the latter
to 2-hydroxy-3,6-di-tert-butyl-p-benzoquinone [34].
The reaction with the quinone occurs as nucleophilic
Anthraquinone
0.33
0.34
0.64
0.88
Starting anthracene 0.68
Starting PhCH CH3 0.80
0.60
0.68
2
a
b
Experiments without anthracene.
1 mol of titanium in all
1
,4-addition of the metal-containing peroxide to the
the experiments; the amount of PhCH(OH)CH did not exceed
3
system of conjugated bonds.
0
.01 mol.
Thus, depending on the structure of molecules of
organic substrates, the active oxidizing species in the
titanium tetra-tert-butoxide tert-butyl hydroperoxide
system are metal-containing peroxide, peroxytrioxide,
and oxygen generated by the system. The reaction of
the titanium-containing peroxytrioxide with substrates
occurs along either molecular (anthracenes, phenyl-
alkenes) or homolytic pathway.
were analyzed on a 2400 3-mm column, stationary
phase 10% poly(ethylene glycol adipate) on TZKM
brick, temperature 55 80 C. For the analysis of titani-
um tetra-tert-butoxide, we used a 2400 3-mm col-
umn, stationary phase 15% Apiezon L, temperature
1
80 C. High-boiling products (benzaldehyde, aceto-
phenone, benzophenone, phenol, 1-phenyl-1-ethanol,
etc.) were identified with a 3000 3-mm column,
stationary phase 5% SE-30, adsorbent Inerton AW,
temperature 100 190 C. Anthracene, 9,10-dimethyl-
anthracene, anthraquinone, 1,1-diphenylethylene, and
triphenylethylene were analyzed at 190 210 C
EXPERIMENTAL
The IR spectra were recorded on a Specord IR-75
device (liquid film between windows of a KBr cell).
The ESR spectra were taken on a Bruker ER-200D-
SRC spectrometer equipped with an ER 4105DR
double resonator (operating frequency 9.5 GHz) and
an ER 4111VT temperature-control block. In determi-
nation of the g-factor, we used diphenylpicrylhydrazyl
as reference. The analysis was performed in the cell of
the ESR spectrometer. To improve the resolution of
ESR lines and remove oxygen released in the reaction
of I with II, the reaction solutions were degassed.
Spin traps IV and V were added in the initial steps of
the reactions. The consumption of the peroxy radical
(
1000 3-mm column, stationary phase 5% OV-17 on
Inerton Super). tert-Butyl hydroperoxide in benzene
was determined with an LKhM-80 device, 1200
3
1
8
-mm column, carrier gas helium, stationary phase
5% dinonyl phthalate on Chromaton N-AW-DMCS,
0 C. The chromatograms were calculated using
authentic samples as external references.
All manipulations with titanium-containing com-
pounds were performed under dry deoxygenated
argon.
The amount of aliphatic acids in nonvolatile resi-
dues was determined according to [35], and the
amount of tert-butoxy groups, by the Denigés method
[36]. Carboxylic acids were identified as methyl esters
after treatment with diazomethane. The quantitative
analysis of hydroperoxides was performed by iodo-
metric titration. 1-Hydroperoxy-1-phenylethane was
identified by products of its acid decomposition
(
t-BuO) TiOO was calculated with a special program,
3
Store 1000.
Chromatographic analysis of reaction products in
the liquid phase was performed with a Tsvet-2-65
device equipped with a flame-ionization detector; the
carrier gas was argon. Readily volatile components
(acetone, tert-butyl alcohol, tert-butyl hydroperoxide)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

274
STEPOVIK et al.
(
phenol and acetaldehyde) in the presence of p-tolu-
In a replicate run, the residue after removing ben-
zene was analyzed for acetic acid (0.008 g) [35].
enesulfonic acid [37]. Carbonyl compounds were
identified in the form of 2,4-dinitrophenylhydrazones
by the melting point and by TLC (comparison of Rf
for the spots of the sample and authentic compounds).
The sorbent, Silpearl, was wide-pore silica gel on
aluminum foil (Silufol UV-254), and the eluent was
benzene or benzene diethyl ether (9 : 1). The amount
of oxygen released in the reactions was determined
from the weight of benzoic acid formed by the reac-
Reaction of anthracene and 9,10-dimethylanthra-
cene with the Ti(OBu-t) t-BuOOH system (1 : 1:
4
1
: 2) in benzene. A flask was charged with 0.34 g of
Ti(OBu-t) , 0.18 g of anthracene, 0.21 g of 9,10-di-
4
methylanthracene, and 0.18 g of t-BuOOH in 35 ml of
benzene. The reaction solution immediately became
yellow-brown. The solvent and low-boiling com-
pounds were condensed in a trap cooled with liquid
nitrogen. The condensate contained 0.20 g of t-BuOH.
tion of O with benzaldehyde [38]. The titanium con-
2
tent was determined after hydrolysis of the deriva-
tives, by precipitation of hydroxides followed by cal-
cination to the oxide.
The residue (yellow solid mass) was hydrolyzed in
benzene with a 10% solution of sulfuric acid and
extracted with freshly distilled benzene. The extract
was dried with sodium sulfate. Chromatographic anal-
ysis revealed the presence of 0.14 g of unchanged
Titanium tetra-tert-butoxide was prepared by treat-
ment of TiCl with tert-butyl alcohol in the presence
of NH [39], bp 82 83 C (2 mm Hg), n 1.4420
4
2
0
9,10-dimethylanthracene, 0.15 g of unchanged anthra-
3
D
cene, 0.04 g of anthraquinone, and 0.15 g of t-BuOH.
Then the solvent was removed at 20 C under reduced
pressure. The residue was a yellow crystalline precipi-
tate containing, along with anthracenes and anthraqui-
none, also 9,10-dimethyl-9,10-epidioxyanthracene
[
40]. Tri-tert-butoxy-tert-butylperoxytitanium was pre-
pared from (t-BuO) TiCl and t-BuOOH in the pres-
3
ence of diethylamine [41]. The activity of the perox-
ide obtained was no less than 95 96%. 2-Methyl-2-
nitrosopropane was prepared by oxidation of tert-
butylamine with hydrogen peroxide in the presence of
sodium tungstate [42]. C-Phenyl-N-tert-butylnitrone
was prepared by rearrangement of 2-tert-butyl-3-phen-
yloxazirane [43]. The melting points of the spin traps
obtained were in agreement with published data [42,
(0.09 g). Its amount was determined by iodometric
titration [13].
In a separate experiment, 9,10-dimethylepidioxy-
anthracene was isolated pure by column chromatog-
raphy (Silicagel-60, 0.060 0.20 mm, eluent hexane)
4
3]. 9,10-Dimethylanthracene was prepared by treat-
1
and characterized by H NMR spectroscopy [CDCl ,
3
ment of 9,10-dilithioanthracene with dimethyl sulfate,
mp 183 184 C (from benzene) [44]. The concentra-
tion of tert-butyl hydroperoxide used in this study was
no less than 99.6 99.8%.
,
ppm: 1.48 s (3H), 7.2 and 7.4 two m (AA BB , 2H)]
[
22].
The aqueous layer was analyzed by the Denigés
method for the content of t-BuO groups (0.09 g).
Reaction of titanium tetra-tert-butoxide with tert-
butyl hydroperoxide (1 : 2) in benzene, 20 C. tert-
Butyl hydroperoxide (0.32 g) was added to 0.62 g of
Reaction of 1,1-diphenylethylene with the
Ti(OBu-t) t-BuOOH system (1 : 1 : 2). A mixture
4
of 1.19 g of Ti(OBu-t) , 0.63 g of t-BuOOH, and
4
Ti(OBu-t) in 10 ml of benzene (room temperature),
4
0
.63 g of Ph C=CH in 10 ml of benzene was allowed
2 2
and evolution of oxygen was observed (19.04 ml,
normal conditions). The reaction solution acquired
intense straw color, and 0.49 g of titanium tetra-tert-
butoxide was found in this solution. On the next day,
benzene and low-boiling products were removed at
to stand at room temperature for 2 h. The reaction
solution became red-orange, and 0.58 g of I was
found in this solution. The condensed solvent con-
tained 0.70 g of t-BuOH. Formaldehyde was identified
qualitatively by TLC in the form of 2,4-dinitrophenyl-
hydrazone. The residue after removing the solvent and
low-boiling products was a red-brown gelatinous mass.
It was hydrolyzed in ether with a 10% H SO solution,
2
0 C under reduced pressure. Treatment of a part of
the condensate with 2,4-dinitrophenylhydrazine gave
a precipitate of the corresponding acetone hydrazone
2
4
(
0
TLC monitoring). In the volatile fraction we found
.48 g of t-BuOH, 0.002 g of acetone, and 0.02 g of
and the mixture was extracted with distilled ether. In
the ether extract we found 0.57 g of t-BuOH, 0.23 g
(t-BuO)2.
of the starting alkene, and 0.36 g of Ph C=O (melting
2
point of benzophenone 2,4-dinitrophenylhydrazone
and its mixture with an authentic sample 240 C).
The residue (dark yellow resinous mass) was hy-
drolyzed with 10% H SO , and the mixture was ex-
2
4
tracted with ether. In the ether extract we determined
In a replicate run, the residue after removing ben-
zene was analyzed for formic acid (0.06 g), which was
identified in the form of methyl formate [35, 37].
0
0
.27 g of t-BuOH, and in the aqueous acid layer,
.02 g of t-BuOH and 0.005 g of hydrogen peroxide.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 2 2008

TITANIUM TETRA-tert-BUTOXIDE tert-BUTYL HYDROPEROXIDE OXIDIZING SYSTEM:
275
The aqueous acidic hydrolyzate contained 0.08 g of
t-BuOH [36].
16. Pokhodenko, V.D., Ganyuk, L.N., and Brodskii, A.I.,
Dokl. Akad. Nauk SSSR, 1962, vol. 145, no. 4, p. 815.
1
7. Campbell, B.T.W. and Coppinger, G.M., J. Am. Chem.
Soc., 1952, vol. 74, no. 6, p. 1469.
The reaction of 1,1-diphenylethylene with system
I II in the presence of -carotene was performed
similarly. After the reaction completion, -carotene
was detected by TLC (eluent hexane petroleum ether,
18. Horswill, E.C. and Ingold, K.U., Can. J. Chem., 1966,
vol. 44, no. 3, p. 269.
7
: 1).
19. Ershov, V.V., Nikiforov, G.A., and Volod’kin, A.A.,
Prostranstvenno-zatrudnennye
Hindered Phenols), Moscow: Khimiya, 1972.
fenoly
(Sterically
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