Reaction of zirconium alkoxides with tert-butyl hydroperoxide. Oxidative ability of the Zr(OBu-t)4-t-BuOOH system

Article abstract of DOI:10.1134/S1070363206060235

Oxidation of the isopropoxy group in the Zr(i-PrO)₄·i- PrOH complex involves both direct reaction with tert-butyl hydroperoxide and intermediate formation of zirconium peroxy compound. Zirconium tetra-tert-butoxide reacts with tert-bytyl hydroperoxide to form metal-containing peroxide and trioxide. Decomposition of the latter leads to oxygen evolution and is accompanied by radical formation. The alkoxyl and peroxyl radicals formed were identified by ESR spectroscopy. The nature of the oxidant (oxygen, zirconium-containing peroxide and-trioxide) in the Zr(OBu-t)₄-t-BuOOH system is determined by the structure of the substrate molecule. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070363206060235

ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 6, pp. 980 988.
Pleiades Publishing, Inc., 2006.
Original Russian Text
pp. 1024 1033.
M.V. Gulenova, L.P. Stepovik, V.K. Cherkasov, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 6,
Reaction of Zirconium Alkoxides with tert-Butyl Hydroperoxide.
Oxidative Ability of the Zr(OBu-t)4 t-BuOOH System
M. V. Gulenovaa, L. P. Stepovika, and V. K. Cherkasovb
a Lobachevskii Nizhny Novgorod State University,
pr. Gagarina 23, Nizhny Novgorod, 603950 Russia
e-mail: gulmv@rambler.ru
b Razuvaev Institute of Organometallic Chemistry,
Russian Academy of Sciences, Nizhny Novgorod, Russia
Received September 7, 2005
Abstract Oxidation of the isopropoxy group in the Zr(i-PrO)4 i-PrOH complex involves both direct reac-
tion with tert-butyl hydroperoxide and intermediate formation of zirconium peroxy compound. Zirconium
tetra-tert-butoxide reacts with tert-butyl hydroperoxide to form metal-containing peroxide and trioxide.
Decomposition of the latter leads to oxygen evolution and is accompanied by radical formation. The alkoxyl
and peroxyl radicals formed were identified by ESR spectroscopy. The nature of the oxidant (oxygen, zirco-
nium-containing peroxide and -trioxide) in the Zr(OBu-t)4 t-BuOOH system is determined by the structure of
the substrate molecule.
DOI: 10.1134/S1070363206060235
Systems containing zirconium alkoxides Zr(OR)4
Zr(OR)4 + t-BuOOH + R CH2OH
(R = OPr-n, t-Bu) and hydroperoxides (tert-butyl or,
O
less frequently, cumyl) are effective oxidants for
hydroxyl-containing compounds, such as alcohols and
phenols [1 3]. Primary and secondary benzyl alcohols
convert selectively to the corresponding carbonyl
compounds by treatment with t-BuOOH and catalytic
amounts of zirconium alkoxides [2]. The reactions
occur in methylene chloride at room temperature, the
catalyst-to-peroxide ratio is 1:15. Faster reactions are
with substrates possessing donor substituents than
with those bearing an acceptor group. The yields of
carbonyl compounds in the majority of cases vary
within the range of 90 96%.
t-Bu
Zr(OR)2
O
H
ROH
C
O
R
H
(1)
t-BuOH + R CHO + O=Zr(OR)2.
The metal atom in the complex favors three
processes. The first involves coordination of the sub-
strate and hydride transfer to the tert-butylperoxy
group. The second involves peroxide bond activation,
and the third, activation of the substrate C H bond
[2]. The best dehydration catalyst for alcohols is
Zr(OBu-t)4 due to the higher exchange rate of the
tertiary alkoxy group compared to primary and se-
Aliphatic and cyclic alcohols are also oxidized with
tert-butyl hydroperoxide in the presence of Zr(O-Bu-t)4 condary ones, monomeric nature of tetra-tert-butoxy-
or Zr(OPr-n)4 [1]. Secondary alcohols are quantitati-
vely converted to corresponding ketones. In the case
of primary alcohols, such by-products as esters or
acids are formed together with aldehydes.
zirconium, and resistance of the t-BuO group to
the oxidation presented by Eq. (1).
Naphthols, anthracenols, phenanthrols, and mono-
nuclear phenols with electron-donor substituents are
easily converted to corresponding 1,2-quinones under
the action of tert-butyl hydroperoxide and transition
metal compounds, including zirconium alkoxides [3].
The authors suggest that these reactions proceed
through nonradical oxygen transfer in the ortho posi-
tion of phenol and subsequent dehydration to form
quinone. Oxidation of 1-hydroxyantracene resulted in
These reactions are regarded as dehydration of al-
cohols with hydroperoxide, catalyzed by transition
metal derivatives. They include alkoxy group exchange
between the metal complex, substrate alcohol, and
hydroperoxide. Dehydration takes place in a six-
membered transition state via irreversible hydride
transfer. For example, see Eq. (1).
980

REACTION OF ZIRCONIUM ALKOXIDES WITH tert-BUTYL HYDROPEROXIDE
981
Table 1. Reaction products of (i-PrO)4Zr i-PrOH with
tert-butyl hydroperoxide in benzene at 20 C (mol/mole
zirconium alkoxide).
Table 2. Reaction products of zirconium tetra-tert-
butoxide with tert-butyl hydroperoxide, C6H6, 20 C
(mol/mole metal alkoxide)
Zr(PrO-i)4 i-PrOH
Zr(OBu-t)4 : t-BuOOH molar ratio
Reaction
Reaction
productsa
productsa
1: 1
1: 2
1: 4
1: 1
1: 10
Volatile products
Volatile products
(CH3)2C=O
i-PrOH
t-BuOH
0.80
1.74
0.60
1.92
2.70
0.33
3.27
0.07
O2
0.81
2.45
0.08
3.53
7.91
0.37
0.80
0.78
0.02
t-BuOHb
t-BuOOBu-t
t-BuOOH
Hydrolysis products of nonvolatile residuec
Hydrolysis products of nonvolatile residueb
t-BuOH
H2O2
2.85
0.12
4.30
Not found
i-PrOH
t-BuOH
0.26
0.11
2.90
1.72
0.38
a
b
Average results. Isobutylene oxide was determined quali-
a
b
c
Average results. The content of zirconium in the hydrolysis
products was about 1 mol.
tatively. The amount of zirconium in all experiments about
1 mol.
quantitative isolation of a product that, according to
mass spectral data, was an alkoxy derivative of 1,2-
antracenediol and zirconium. However, in the cited
work [3] no scheme of aromatic C H hydroxylation
was given.
intermediate formation of peroxy compounds, accord-
ing to the scheme offered by Davies and Hall for re-
actions of aluminum alkoxides with hydroperoxides
[6]. In this case, rupture of donor acceptor bonds in
the [(i-PrO)4Zr i-PrOH]2 dimer, coordination of tert-
butyl hydroperoxide at the rupture site, and its reac-
tion with any of the terminal groups of alkoxide II
[Eqs. (2) and (3)] are possible.
The aim of the present work was to study reaction
of zirconium alkoxides containing alkoxy groups of
various structure with tert-butyl hydroperoxide (I).
For the starting compounds we chose Zr(i-PrO)4
i-PrOH (II) and zirconium tetra-tert-butoxide (III).
Previous studies of alcohol oxidation with hydroper-
oxide I in the presence of zirconium alkoxides showed
the presence of dimeric and even oligomeric forms of
Zr(OR)4 does not affect nature of hydride transfer [1].
Alkoxide III is monomeric and volatile [4]. Complex
II is a dimer with each metal atom coordinated with
an alcohol molecule [5], which can create steric
hindrances to reaction with tert-butyl hydroperoxide .
All reactions were carried out in benzene at room
temperature (Tables 1, 2).
[i-PrOH Zr(OPr-i)4]2 + 2t-BuOOH
2[Zr(OPr-i)4 i-PrOH t-BuOOH],
(2)
HOPr-i
O
^
CMe2
> H
(i-PrO)3Zr
HO<
O
Bu-t
Me2C=O + t-BuOH + (i-PrO)3ZrOH i-PrOH. (3)
If follows from Table 1 that the products of the
reaction of alkoxide II with hydroperoxide I are
acetone and tert-butyl and isopropyl alcohols. At any
I:II molar ratio, equimolar amounts of acetone and
t-BuOH were isolated from the volatile fraction, but
the yield of isopropyl alcohol decreased as the amount
of hydroperoxide was increased. The above-mentioned
products can form in two ways. The first includes
formation of zirconium-containing peroxide and its
decomposition [scheme (1)] [1, 2]. This direction is
the most probable at a 1:1 ratio of the starting com-
pounds. However, in view of the steric shielding of
the metal atom with alkoxy groups, we can assume
that oxidation of the i-PrO group may not involve
Zirconium hydroxyl derivatives are known to be
susceptible to intermolecular dehydration, and the li-
berated water can hydrolyze i-PrO Zr bonds. Ho-
wever, the contribution of these processes is insigni-
ficant because of the limited solubility of the reaction
products of alkoxide II with hydroperoxide I in
benzene.
The above data are confirmed by IR spectroscopy.
The IR spectrum of the starting complex corresponds
to published data for (i-PrO)2Zr and i-PrOH [7, 8].
The IR spectra of the solid reaction residues contain a
1
very strong band at 3340 cm , that relates to the
hydroxy group bound with the metal. The weak band
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 6 2006

982
GULENOVA et al.
1
at 1570 cm was assigned to vibrations of water. The
was shown that the system zirconium tert-butoxide
(III) hydroperoxide I in benzene at room temperature
at a 1:2 reagent ratio generates oxygen (Table 2). The
yield of oxygen reaches 0.80 0.90 mol per mole al-
koxide III. The process is catalytic in nature. As the
fraction of hydroperoxide I was increased to 10 mol
per mole alkoxide III, the oxygen amount
reached 3.50 mol (71%). No solvent oxidation
products were found.
1
spectral range 400 600 cm characteristic of Zr O
stretching vibrations in alkoxides is obscured by
strong ill-resolved bands, which can be explained by
the presence of associated Zr O Zr, Zr=O, and
Zr OH groups.
The resulting data show that the oxidation of iso-
propoxy groups in Zr(i-PrO)4 i-PrOH with tert-butyl
hydroperoxide, especially at increased molar fraction
of the latter, proceeds without intermediate formation
of zirconium-containing peroxides.
The detection of oxygen in the Zr(OBu-t)4 t-
BuOOH system makes us to suggest that the reaction
of its components involves formation of zirconium-
containing peroxide and trioxide A [scheme (4)], like
in systems based on aluminum and titanium tert-
butoxides [9, 10].
Contrary to complex II, monomeric tetra-tert-
butoxyzirconium (II) contains no hydrogen atoms at
the -carbon atom of the alkoxy group and cannot
react with t-BuOOH according to Eqs. (2) and (3). It
(t-BuO)4Zr + t-BuOO
t-BuOH + (t-BuO)3ZrOOBu-t
O
O
(t-BuO)3Zr
t-BuO
OBu-t
(t-BuO)3Zr
O
t-BuOH
(4)
H
t-BuO
O
A
Trioxide A can decompose both by the molecular
mechanism with oxygen evolution and homolytically
with formation of alkoxyl and peroxyl radicals
[scheme (5)].
assigned, on the basis of the gi value of 2.0158, to a
peroxyl radical [11] (Fig. 1). Since t-BuOO radicals is
known to be impossible to fix in the absence of spin
traps, we can assume that this signal relates to the
(t-BuO)3ZrOO. radical (IV).
(t-BuO)4Zr + O2,
O1
The isotrope ESR spectrum of the Zr(OBu-t)4
t-BuOOH system in benzene at 20 C in the presence
of 2-methyl-2-nitrosopropane shows, together with
the signal of radical IV, contains a symmetric triplet
(1:1:1; ai(15N) 2.715 mT, gi 2.0057). This signal was
assigned to an adduct of the spin trap with t-BuO
radicals (V) [9]. With N-tert-butyl- -phenylnitrone
under the same conditions, the ESR spectrum contains
a strong singlet of zirconium-contaning peroxyl ra-
dical IV and a triplet (1:1:1) of doublets (1:1) with
asymmetric lines. The nitroxyl signal (gi 2.0066,
ai(15N) 1.445 mT, ai(H) 0.180 mT) should be con-
sidered either to belong to an adduct of N-tert-butyl-
-phenylnitrone with t-BuO [9] or to its superposition
with the (t-BuO)3ZrO radical.
1 2
O2
(t-BuO)3ZrOO + t-BuO ,
(t-BuO)3Zr
3
3 4
t-BuO
(t-BuO) ZrO + t-BuOO .
(5)
3
Oxygen generated by systems based on aluminum
and titanium tert-butoxides and tert-butyl hydroper-
oxide is known to reacts in the singlet form [9, 10].
1
For fixation of O2 and its quantitative determination
we studied the reaction Zr(OBu-t)4 with t-BuOOH by
ESR in benzene in the presence of anthracene, a ty-
pical acceptor of singlet oxygen. The anthracene-to-
t-BuOOH ratio was 1:1:2. Anthraquinone was found
in an amount of up to 0.20 mol/mole Zr(OBu-t)4.
To identify radicals shown in scheme (5), the reac-
tion of alkoxide III with tert-butyl hydroperoxide
was studied by means of ESR both in the absence and
in the presence of spin traps (2-methyl-2-nitrosopro-
pane, N-tert-butyl- -phenylnitrone). In the absence of
spin traps at room temperature, the ESR spectrum of
benzene, taken immediately after mixing components
III and I, contains a very strong singlet that was
Indirect evidence of the formation of peroxyl ra-
dicals in the alkoxide III hydroperoxide I system
comes from the detection of tert-butylperoxide and
bound hydrogen peroxide among the reaction products
(Table 2). Hence, the reaction of zirconium tetra-tert-
butoxide with tert-butyl hydroperoxide involves zir-
conium-containing peroxide and trioxide formation.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 6 2006

REACTION OF ZIRCONIUM ALKOXIDES WITH tert-BUTYL HYDROPEROXIDE
983
Decomposition of the latter proceeds mainly with
oxygen evolution and is accompanied by radical
formation, mostly by the 1 2 pathway [scheme (5)].
Note that the rate of oxygen evolution in the reaction
of Zr(OBu-t)4 with t-BuOOH is higher than with
Ti(OBu-t)4 and Al(OBu-t)3. In the last case, oxygen
begins to evolve in 15 20 min, with Ti(OBu-t)4, in
2 3 min, while with Zr(OBu-t)4, in several seconds.
The main factors operative to accelerate the reaction
of Zr(OBu-t)4 with t-BuOOH are the monomeric form
of the alkoxide and large covalent radius of zirconium
as compared to titanium and aluminum [12], which
results in weaker shielding of zirconium with alkoxy
groups.
0.5 mT
Fig. 1 ESR spectrum of the (t-BuO) t-BuOOH (1:2)
system 5 min after reagents mixing without spin traps
(benzene, 20 C, [Zr(OBu-t) ] 0.05 M).
4
4
It follows from schemes (4) and (5) that the reac-
tion between Zr(OBu-t)4 and t-BuOOH includes three
types of potential oxidants: oxygen and zirconium-
containing peroxide and -trioxide.
spectrum contains a triplet of doublets with ai(15N)
1.490 mT, ai(H ) 0.360 mT, and gi 2.0051, related to
the adduct PhCH(Me)N(O )Bu-t (VI), and two triplet
signals. One of them [gi 2.0057, ai(15N) 2.715 mT]
relates to tert-butoxy-tert-butylnitroxyl (V) [9] and the
second (gi 2.0056, ai(15N) 2.920 mT), to an adduct of
the spin trap with an alkoxyl radical [probably,
(t-BuO)3ZrO (VII)]. The ESR spectrum also contains
a triplet signal belonging to di-tert- butylnitroxyl
(VII) [gi 2.0061, ai(15N) mT] formed by photolysis
of the spin trap.
To find out whether hydrocarbon C H bonds can
be oxidized with the system under study (III:I 1:2),
we reacted it with ethylbenzene. The yield of oxidation
products depends on the metal alkoxide concentration.
Addition of tert-butyl hydroperoxide to the system
containing 0.25 M of Zr(OBu-t)4 causes fast and
strong oxygen evolution Acetophenone, 0.06 mol, and
traces of 1-phenylethanol were detected. When the
reagent concentration was decreased 5 times, the yield
of acetopenone decreased to 0.21 mol, and 0.04 mol
of 1-phenylethyl hydroperoxide was detected. This
reaction was studied by ESR in the presence of spin
traps. In the alkoxide III hydroperoxide I 1-methyl-
1-nitrosopropane system in ethylbenzene at 20 C, a
superposition of five different ESR spectra was ob-
served 15 min after reagent mixing (Fig. 2). Along
with a singlet with gi 2.0158 [(t-BuO)3ZrOO IV], the
H
t-BuO N Bu-t Ph C N Bu-t
O
MeO
V
VI
(t-BuO)3ZrO N Bu-t t-Bu N Bu-t
O
O
VII
VIII
VI
IV
VIII
VI
VIII
VI
VII
VII
VIII
VI
V
V
V
VI
VII
VI
0.25 mT
Fig. 2. ESR spectrum of the Zr(OBu-t) t-BuOOH (1:2) system in ethylbenzene 15 min after reagent mixing in the presence
4
of 3-methyl-1-nitrosopropane at 20 C, degassed sample. The figures in the spectrum correspond to compound numbers.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 6 2006

984
GULENOVA et al.
The ESR spectrum of the alkoxide III hydroper-
We suggest that, by analogy with the Ti(OBu-t)4 t
BuOOH system, the carbonyl-containing products are
formed by decomposition of 1,2-dioxetane formed by
reaction of phenylalkene with zirconium-containing
trioxide.
oxide I N-tert-butyl- -phenylnitrone system in ethyl-
benzene at 20 C contained a triplet of doublets [gi
2.0068, ai (15N) 1.370 mT, ai(H) 0.135 mT] charac-
teristic of the tert-butylperoxyl spin adduct [9].
Simultaneously, a singlet of radical IV was observed.
Ph2C=CH2 + [(t-BuO)3ZrOOBu-t]
(t-BuO)4Zr + Ph2C CH2
The resulting data suggest that the oxidation of
ethylbenzene [9, 13] occurs homolytically by sche-
me (6) that involves formation of 1-phenylethyl hydro-
peroxide and the corresponding zirconium-containing
peroxide.
O
O
(7)
Ph2C=O + CH2O.
To study the oxidative properties of zirconium
peroxy compounds formed in the alkoxide III hydro-
peroxide I system, we reacted the system with -diols
and 3,6-di-tert-benzoquinone. 2,3-Butanediol was
chosen as the glycol. Its reaction with the alkoxide
III hydroperoxide I system was carried out at 20 C
in benzene at a 1:1:2 component ratio. Acetic acid,
1.05 mol, was isolated as the main hydrolysis product
of the nonvolatile residue, and t-BuOH, 5.11 mol, was
the main volatile product. Acetaldehyde was qualita-
tively identified in the latter fraction as 2,4-dinitro-
phenylhydrazone. The formation of zirconium acyloxy
derivative was confirmed by IR spectroscopy. The IR
spectrum of the nonvolatile residue contains a very
PhCH2CH3 + RO
PhCHCH3 + ROH,
R = t-Bu, t-BuO; (t-BuO)3Zr, (t-BuO)3ZrO,
PhCHCH3 + O2
PhCH(OO)CH3
PhCH2CH3
.
(6)
PhCH(OOH)CH3,
PhCHCH3
PhCH(OOH)CH3 + Zr(OBu-t)4
(t-BuO)3ZrOOCH(Ph)CH3
t-BuOH
(t-BuO)3ZrOH + CH3C(O)Ph.
Note that the reaction between the components of
the Zr(OBu-t)4 t-BuOOH system and the oxidative
action the system on organic substrates are analogous
to those observed with the system based on titanium
tetra-tert-butoxide [13]. But the yield of contrary to
the latter, in the case of zirconium alkoxide the yield
of hydrocarbon C H oxidation products is always
considerably lower. This system practically does not
oxidize triphenylmethane. Such a different reactivity
of the Ti(OBu-t)4 I and Zr(OBu-t)4 I systems can be
explained by that in the former system oxygen is
generated very fast [Eq. (5)] and evolved form bulk
solvent. This effect is indirectly evidenced by the fact
that the yield of ethylbenzene oxidation products in-
creases with decreasing concentration of the system.
1
strong band at 1580 cm characteristic of stretching
vibrations of the acylate COO group.
2,3-Butanediol can react with the Zr(OBu-t)4
t-BuOOH oxidative system along two routes. The first
involves consecutive or concurrent oxidation of the
two secondary hydroxy groups via decomposition of
intermediate zirconiumcontaining peroxides to buta-
nedione [1, 2]. However, no -diketone was detected.
It is known that -diketones are oxidized by the
Al(OBu-t)3 t-BuOOH system with preferential forma-
tion of aluminum acylates and percarboxylic acid
esters [17]. Analogous process is also possible in the
case of the system based on zirconium alkoxide. tert-
Butyl peracetate was found, but its amount did not
exceed 0.05 mol.
The Zr(OBu-t)4 t-BuOOH system can react with
phenylalkenes, such as 1,1-diphenyl- and triphenyl-
ethenes. The highest conversion is achieved in the
case of 1,1-diphenylethylene. The reaction of the al-
koxide III hydroperoxide I system with phenylalkene
(1:2:1, benzene, 20 C) involves about 0.5 mol of the
latter. The product contained 0.46 mol of benzophe-
none. Formaldehyde was detected qualitatively as 2,4-
phenylhydrazone. The aldehyde partially oxidizes to
formic acid (0.15 mol) and also enters condensations.
The other possible oxidation route of 2,3-butane-
diol consists of formation of zirconium peroxy com-
pound by one hydroxy group and decomposition of
the product, involving the second hydroxy group, in
an eight-membered reaction complex to give acetal-
dehyde [Eq. (8)].
Acetaldehyde is further oxidized with zirconium-
containing peroxide to acetoxy derivative. Moreover,
acetaldehyde can enter polycondensation reactions
catalyzed with zirconium compounds analogously to
titanium alkoxides [18]. Reaction of the polycondensa-
tion product with t-BuO Zr bonds should lead to
The formation of carbonyl compounds on oxidation
of alkene double bonds occurs via rection of the latter
with singlet oxygen. However, phenylalkenes are
rather inert to free singlet oxygen [14] and are easily
1
oxidized with bound O2 [15, 16].
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REACTION OF ZIRCONIUM ALKOXIDES WITH tert-BUTYL HYDROPEROXIDE
MeCH CHMe + Zr(OBu-t)4 + t-BuOOH
985
OH OH
Bu-t
O
O
^
>
H
O
(t-BuO)2Zr
O <
(8)
2t-BuOH +
t-BuOH + (t-BuO)2Zr O + 2MeCHO.
CH CH
Me
Me
liberation of tert-butanol. The latter was actually de-
tected in the reaction in hand. Starting alkoxide III is
almost completely dealkylated.
tert-butyl hydroperoxide effectively react with 3,6-di-
tert-butylbenzoquinone (IX) due to intermediate
formation of metal peroxides. To provide evidence
for the participation of zirconium-containing peroxide
in the oxidation reaction, we studied the reaction of
3,6-di-tert-butyl-o-benzoquinone with the alkoxide
III hydroperoxide I system in benzene at 20 C and
a 1:1:2 reagent ratio. 3,6-Di-tert-butyl-2-hydroxy-p-
benzoquinone (X), 0.15 mol, and 3,6-di-tert-butyl-
2,3-epoxy-5-hydroxy-p-benzoquinone (XI), 0.63 mol,
were isolated. Hydroxyquinone is the primary product
of quinone oxidation.
Evidence for the above-mentioned process might
be found in reactions of the Zr(OBu-t)4 t-BuOOH
system with bis-tertiary glycols, where -dicarbonyl
compounds are impossible to form. Reactions of
benzopinacol with the above system were studied in
benzene at 20 C and a 1:1:1 reagent ratio. The con-
version of the starting pinacol was about 50%. tert-
Butanol, 2.21 mol, and benzophenone, 0.94 mol, were
isolated as the main products. Steric factors in the
benzopinacol molecule prevent formation of its glyco-
late derivative [19] and favor benzopinacol oxidation
according to Eq. (8).
We suggest that hydroxyquinone X is formed by
nucleophilic 1,4-addition of zirconium-containing
peroxide or hydroperoxide I to the system of con-
jugated bonds of the starting quinone and subsequent
reaction of the hydroxy group with zirconium al-
koxide [scheme (9)].
We previously showed [20] that systems on the
basis of aluminum and titanium tert-butoxides and
a. (t-BuO)3ZrOOBu-t
t-BuOO
O
OZr(OBu-t)3
H
b. t-BuOOH, Zr(OBu-t)4
O
O
OH
OZr(OBu-t)3
O
H2O, H+
O
(9)
t-BuOH
O
O
B
X
Hydrolysis of alkoxy derivative B leads to 3,6-di-
tert-butyl-2-hydroxy-p-benzoquinone (X). The forma-
tion of 3,6-di-tert-butyl-2,3-epoxy-5-hydroxy-p-
benzoquinone (XI), the main product of this reaction,
can be explained by further reaction of alkoxy deriva-
tive B with tert-butyl hydroperoxide (I). It is known
that in solutions o- and p-hydroxyquinones and their
derivatives exist in thermodynamic equilibrium
[21, 22], and the isomer of alkoxy derivative B
undergoes epoxidation. The reaction proceeds at the
allyl position through the step of zirconium peroxide
formation, like with aluminum [23] and titanium [24]
peroxides [Eq. (10)].
Hence, we showed that the Zr(OBu-t)4 tert-
BuOOH system reacts with quinone IX mainly by the
way of nucleophilic 1,4-addition of zirconium per-
oxide. Further reaction of the adduct with hydroxy-
peroxide I leads to epoxidation of the quinone allyl
double bond.
Two conclusions can be drawn from the results
obtained. First, the reactions of the components of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 6 2006

986
GULENOVA et al.
t-BuO
O
O
O
O
O
t-BuOOH
t-BuOH
B
O
(t-BuO)3ZrO
(t-BuO)2ZrO
O
O
O
O
O
O
O
H2O
(10)
Zr(OH)4,
t-BuOH
OH
HO
(t-BuO)3ZrO
O
XI
Zr(OBu-t)4 t-BuOOH system and its oxidative pro-
perties are similar to those characteristic of systems
based on aluminum or titanium tert-butoxides. The
possible reason for this similarity is that these metals
have unoccupied p and d orbitals and their alkoxides
can form metal-containing peroxides and trioxides
(ozonides). Second, the role of the oxidizing agent,
depending on the structure of the substrate, can belong
to oxygen, zirconium trioxide, or zirconium peroxide.
The Zr(OBu-t)4 t-BuOOH system is the most ef-
fective when oxidation involves zirconium-containing
peroxides.
2,6-Di-tert-butyl-2-hydroxy-p-benzoquinone and 3,6-
di-tert-butyl-2,3-epoxy-5-hydroxy-p-benzoquinone
were analyzed at 130 150 C on a 1200 3-mm column
packed with 10% of Reoplex-400 on Inerton AW
HMDC. tert-Butyl peroxide in benzene was analyzed
at 50 C on a 3000 3-mm column packed with 5% of
SP-2401 on Chromaton N-AW-DMCS. Anthracene
and anthraquinone were analyzed at 210 C on a
3000 3-mm column packed with 5% of OV-15 on
Inerton-Super. External calibration technique was used
for quantification. All operations with zirconium-
containing compounds were carried under dry argon
free of oxygen.
EXPERIMENTAL
The number of tert-butoxy groups was evaluated
according to Deniges [25], and aliphatic acids in the
nonvolatile residues were analyzed by the procedure
in [26]. Carbonyl compounds were identified as 2,4-
dinitrophehylhydrazones by melting points and TLC
on Silufol UV-254 plates, eluents benzene or 9:1
benzene diethyl ether. The amount of evolved oxygen
was found from the weight of benzoic acid obtained
in the reaction of oxygen with benzaldehyde [27].
The IR spectra were recorded on a Specord IR-75
spectrometer (liquid film between KBr plates). The
ESR spectra were registered 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. The g
factor was evaluated against diphenylpicrylhydrazyl.
The ESR analysis was carried out in the spectrometer
cell. For better resolution of ESR signals and removal
of oxygen evolving during reaction between com-
ponents I and III, the solutions were degassed.
Chromatographic analysis of the reaction products in
the liquid phase was carried out on a Tsvet-2-65 in-
strument with a flame ionization detector and argon
as the carrier gas. Volatile components (acetone and
isopropyl and tert-butyl alcohols) were analyzed at
55 70 C on a 2400 3-mm column packed with 10%
of PEGA on TZKM. Analysis of high-boiling pro-
ducts (acetophenone, benzophenone, phenol, 1,1-di-
phenylethene, benzopinacol, 2,3-butanediol, etc.) was
carried out at 100 190 C on a 3000 3-mm column
packed with 5% of SE-30 on Inerton AW. tert-Butyl
hydroperoxide was analyzed on an LKhM-80 instru-
ment at 80 C on a 1200 3-mm column packed with
15% of dinonyl phthalate on Cromaton N-AW-DMCS.
The Zr(OPr-i)4 i-PrOH complex was prepared by
treatment of zirconium tetrachloride with isopropyl
alcohol in the presence of NH3 [28]. Found, %: i-PrO
[26] 77.18; Zr 23.96. C15H36O5Zr. Calculated, %:
i-PrO 76.24; 33.55.
The concentration of tert-butyl hydroperoxide was
not less than 99.6 99.8%.
Zirconium tetra-tert-butoxide of 99.9% purity, light
yellow liquid, bp 70 C (2 mm Hg) [29], was prepared
by Sinor (Nizhny Novgorod).
Reaction of Zr(OPr-i)4 i-PrOH with t-BuOOH
(1:4). To a solution of 0.37 g of Zr(OPr-i)4 PrOH in
15 ml of benzene, 0.34 g of tert-butyl hydroperoxide
was added with vigorous stirring. The reaction mix-
ture acquired an orange color. A gel-like precipitate
formed simultaneously. No oxygen evolution was
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REACTION OF ZIRCONIUM ALKOXIDES WITH tert-BUTYL HYDROPEROXIDE
987
observed. The solvent and volatile reaction products
were condensed in a trap cooled with liquid nitrogen.
Chromatographic analysis of the condensed solvent
showed that it contained 0.23 g of t-BuOH, 0.02 g of
i-PrOH, and 0.14 g of acetone. The last component
was qualitatively identified as 2,4-dinitrophenylhydra-
zone (mp 127 C). tert-Butyl hydroperoxide, 0.006 g,
was determined by iodometric titration. The colorless
powdery residue was analyzed for t-BuO (0.03) and
i-PrO (0.10) [26].
system. Benzopinacol, 0.34 g, was suspended in a
solution of 0.36 g of Zr(OBu-t)4 in 7 ml of benzene,
and 0.08 g of tert-butyl hydroperoxide was added.
Even though benzopinacol is poorly soluble in ben-
zene, its fast dissolution and formation of a transparent
solution was observed. The condensed solvent
contained 0.15 g of tert-butanol. The residue was a
solid yellow material. Its IR spectrum contained a
1
strong band at 1670 cm , characteristic of benzo-
phenone C=O stretching vibrations. These data con-
firm the formation of benzophenone before hydrolysis.
The residue was treated as described above. In the
ethereal extract, 0.16 g of benzophenone and 0.16 g
of t-BuOH were found.
Reaction of zirconium tetra-tert-butoxide with
tert-butyl hydroperoxide (1:2). A solution of 0.62 g
of Zr(OBu-t)4 in 10 ml of benzene was treated with
0.29 g of t-BuOOH. Vigorous oxygen evolution was
observed. The reaction solution acquired an intense
yellow coloration. Benzene and volatile products were
removed at 20 C at reduced pressure. Part of the
condensate was treated with 2,4-dinitrophenylhydra-
zine to precipitate acetone and 4-methyl-3-penten-3-
one hydrazones identified by TLC. The volatile frac-
tion contained 0.29 g of t-BuOH, 0.01 g of acetone,
0.02 g of (t-BuO)2, and isobutylene oxide. The latter
was detected as 2-methylpropanal 2,4-dinitrophenyl-
hydrazone (mp 182 C) after treatment of the benzene
solution with BF3 etherate. The residue, dark yellow
tarry material, was hydrolyzed with 10% H2SO4 and
extracted with ether. The ethereal extract contained
0.26 g of t-BuOH. The acidic aqueous layer contained
0.08 g of tert-butanol [25] and 0.007 g of hydrogen
peroxide. The latter was identified qualitatively by
the procedure in [30]. The amount of the evolved
oxygen was established in the parallel experiment
[27]. Benzoic acid, 0.32 g [1.62 mol/mole Zr(OBu-t)4]
was found, which corresponds to 0.04 g of O2
(0.81 mol/mole Zr(OBu-t)4).
To isolate unreacted benzopinacol, the ethereal
hydrolyzate was evaporated. The dry residue contain-
ing benzopinacol and benzophenone was treated with
boiling ethanol to remove benzophenone. Benzopi-
nacol, 0.17 g, was isolated, mp 186 C.
Reaction of 3,6-di-tert-butyl-o-benzoquinone
with the Zr(OBu-t)4 t-BuOOH (1:1:1) system.
The dark green solution of 0.17 g of 3,6-di-tert-butyl-
o-benzoquinone in 15 ml of benzene was treated with
0.30 g of Zr(OBu-t)4 and 0.14 g of t-BuOOH. Heat
evolution was observed, and the reaction mixture
acquired a reddish brown color. The condensate of
volatile products contained 0.27 g of t-BuOH. The
residue, a dark cherry tarry material, was hydrolyzed
in ether with 10% sulfuric acid and extracted with
small portions of diethyl ether.
The presence of hydroxyquinone V in the hydro-
lyzate was confirmed by pH-chromism. When several
drops of 0.5 N NaOH were added to a portion of the
ethereal solution, it changed from yellow to crimson,
and the yellow color recovered on subsequent addition
of H2SO4 [31]. Epoxide VI was detected by qualitative
reactions on the oxirane ring, such as precipitation of
silver iodate [32] and change in color on heating with
pyridine [33].
Oxidation of 1,1-diphenylethylene with the
Zr(OBu-t)4 t-BuOOH (1:1:2) system. To a solu-
tion of 0.63 g of Zr(OBu-t)4 and 0.30 g of Ph2C=CH2
in 10 ml of benzene, 0.30 g of t-BuOOH was added.
One minute after the reaction mixture became light
yellow. The condensate of volatile products contained
0.40 g of t-BuOH. Formaldehyde was identified by
TLC as 2,4-dinitrophenylhydrazone. The residue,
orange-red gel-like material, was hydrolyzed by the
above-mentioned procedure. The ethereal extracts
contained 0.18 g of t-BuOH, 0.13 g of the starting
alkene, and 0.14 g of benzophenone. The latter was
identified as 2,4-dinitrophenylhydrazone, mp 238 C.
The amounts of 3,6-di-tert-butyl-2-hydroxy-p-
benzoquinone (0.03 g), 3,6-di-tert-butyl-2,3-epoxy-5-
hydroxy-p-benzoquinone (0.15 g), and t-BuOH
(0.10 g) in the ethereal solution were determined by
GLC. Hydroxy and epoxy compounds, and unreacted
quinone were separated by column chromatography
on silica gel, eluent 22:1 petroleum ether diethyl
ether (control by TLC). The melting points of V and
VI were 84 [34] and 81 C [35]; mixed samples with
authentic samples gave no melting point depression.
The starting quinone was washed off from the ad-
sorbent with benzene and ethyl acetate in succession;
0.027 g of 3,6-di-tert-butyl-p-benzoquinone was
isolated, mp 207 C.
In the parallel experiment, the content of formic
acid (0.012 g) was determined in the residue after
removal of benzene [26].
Oxidation of benzopinacol with zirconium the
tert-butoxide tert-butyl hydroperoxide (1 :1 :1)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 6 2006

988
GULENOVA et al.
In the acidic aqueous layer, t-BuO groups (0.04 g)
were found by Deniges.
Papers, XI Muzhdunarodnaya konferentsiya po khimii
organicheskikh i elementoorganicheskikh peroksidov
(XI Int. Conf. on Chemstry of Organic and Organo-
element Peroxides), Moscow, 2003, p. 47.
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