Hydrolysis of isobutylaluminum aryloxides studied by 1H NMR and quantum chemical methods

Article abstract of DOI:10.1007/s11172-016-1536-3

The results of ¹H NMR and quantum chemical studies of hydrolysis of isobutylaluminum aryloxides are presented. According to the data of ¹H NMR spectroscopy, the hydrolysis of monomeric diisobutylaluminum aryloxides (2,6-Bu₂ ^t—C₆H₃O)AlBu₂ ⁱ and (2,6-Bu₂ ^t,4-Me—C₆H₂O)AlBu₂ ⁱ occurs selectively at the Al—OAr bond to form the corresponding sterically bulky phenol and polyisobutylaluminoxane. At the molar ratios Al: H₂O = 2, the formed sterically bulky phenol reacts slowly with diisobutylaluminum monoaryloxide to form isobutylaluminum diaryloxide. Dimeric aryloxide [(2-Bu^t—C₆H₄O)AlBu₂ ⁱ]₂ is not hydrolyzed under similar conditions. The quantum chemical calculations confirmed the experimental results: the hydrolysis at the Al—OAr bond has a lower energy barrier than that at the Al—C bond because of the formation of HH2O…OO?Ar hydrogen bonds.

Full text of DOI:10.1007/s11172-016-1536-3

1
958
Russian Chemical Bulletin, International Edition, Vol. 65, No. 8, pp. 1958—1965, August, 2016
Hydrolysis of isobutylaluminum aryloxides studied
1
by H NMR and quantum chemical methods
E. E. Faingol´d, I. V. Zharkov, N. M. Bravaya, and A. V. Chernyak
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1
prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (495) 785 7048. Eꢀmail: nbravaya@cat.icp.ac.ru
1
The results of H NMR and quantum chemical studies of hydrolysis of isobutylaluminum
1
aryloxides are presented. According to the data of H NMR spectroscopy, the hydrolysis of
t
i
t
monomeric diisobutylaluminum aryloxides (2,6ꢀBu —C H O)AlBu and (2,6ꢀBu ,4ꢀMe—
2
6
3
2
2
i
C H O)AlBu occurs selectively at the Al—OAr bond to form the corresponding sterically
6
2
2
bulky phenol and polyisobutylaluminoxane. At the molar ratios Al : H O = 2, the formed
2
sterically bulky phenol reacts slowly with diisobutylaluminum monoaryloxide to form isobutylꢀ
t
i
aluminum diaryloxide. Dimeric aryloxide [(2ꢀBu —C H O)AlBu ] is not hydrolyzed under
6
4
2 2
similar conditions. The quantum chemical calculations confirmed the experimental results: the
hydrolysis at the Al—OAr bond has a lower energy barrier than that at the Al—C bond because
of the formation of H
...OO—Ar hydrogen bonds.
H O
2
1
Key words: hydrolysis, isobutylaluminum aryloxide, organoaluminum compounds, H NMR
spectroscopy, quantum chemical study.
Hydrolysis of alkylaluminum compounds is the simꢀ
alkoxides proceeds via the protonation of the heteroatom
rather than the carbon atom at the Al—alkyl bond. The
authors believe that this hydrolysis route occurs due to
a substantial decrease in the basicity of the Al—alkyl group
in the presence of the heteroatomic donor ligand at the
aluminum atom. The predominant occurrence of the
hydrolysis at the Al—OAr bond was also observed in
the reaction of water with methylaluminum diaryloxide
plest and popular method for the preparation of alumiꢀ
noxanes, viz., compounds containing —Al—O—Al— fragꢀ
ments. This method is used for obtaining methylalumiꢀ
noxane (MAO), modified versions of MAO (MMAO),
and mixed aluminoxanes that find wide use as efficient
activators of catalysts for ionꢀcoordination polymerizaꢀ
tion and oligomerization of olefins.1
—5
t
Aluminoxanes modified by phenols with sterically
bulky or electronꢀwithdrawing substituents are promising
AlMe(2,6ꢀBu —C H O) under the conditions of a tenꢀ
2
6
3
2
fold excess of aryloxide toward water in a medium of
1
6,17
t
from the viewpoint of activators of metallocenes and postꢀ
THF.
The major hydrolysis product is 2,6ꢀBu —
2
metallocenes catalysts for olefin polymerization.6
—14
These
C H OH (1a). The reaction proceeds via the formation of
6
3
t
activators were found to exhibit a higher activation ability
toward the organometallic component of the catalytic sysꢀ
tem than the nonꢀmodified analogs and have a high chemꢀ
ical stability. Several methods for the preparation of these
compounds were described. The modification of alumiꢀ
noxanes by the corresponding substituted phenols is the
simplest and popular method.6
the structurally characterized adduct (2,6ꢀBu —C H O) ꢀ
2 6 3 2
AlMe•H O•2THF, which gradually decomposes at the
1
6
2
temperature above 0 °С to form 1.6 equiv. compound 1a,
t
0.2 equiv. MeAl(2,6ꢀBu —C H O), and only 0.1 equiv.
2
6
3
methane.
—14
1
a
b
c
R1
R2
H
Me
H
The mechanisms of hydrolysis of alkylaluminum arylꢀ
oxides remain yet disputable. As asserted in the works
devoted to the synthesis of aryloxyalkylaluminoxanes and
their application as activators of metallocene catalysts in
the copolymerization of ethylene with hexꢀ1ꢀene,6 the
hydrolysis of alkylaluminum aryloxides affords aryloxyꢀ
alkylaluminoxanes with the evolution of the correspondꢀ
ing alkanes. This route of hydrolysis implies that the reacꢀ
tion proceeds at the Al—C bond. At the same time, it is
shown1 that the hydrolysis of aluminum amides and
t
Bu
But
H
,7
We have previously shown18,19 that isobutylaluminꢀ
oxanes obtained by the hydrolysis of triisobutylaluminum
(TIBA) are efficient activators of metallocene complexes
in reactions of homoꢀ and copolymerization of olefins. It
5
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1958—1965, August, 2016.
066ꢀ5285/16/6508ꢀ1958 © 2016 Springer Science+Business Media, Inc.
1

Hydrolysis of isobutylaluminum aryloxides
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
1959
is also shown20 that monomeric forms of isobutylalumiꢀ
num aryloxides are efficient activators in these processes.
The primary aim of this work was the preparation of arylꢀ
atmosphere of dry argon were placed in tubes 5 mm in diameter.
NMR spectra were recorded on a Bruker AVANCE III FTꢀIR
spectrometer (500 MHz). The signal of residual methyl protons
of tolueneꢀd8 (δ = 2.09) was used for the determination of chemꢀ
ical shifts.
i
oxyisobutylaluminoxane activators (ArO)Al(Bu )ꢀ(μꢀO)ꢀ
i
(
Bu )Al(OAr), which, as we assumed, could be syntheꢀ
Quantum chemical study. Quantum chemical calculations
were performed by the density functional theory (DFT) using
the ωB97XD hybrid functional. The 6ꢀ311+G(d,p) basis set was
used for the H, C, and O atoms. To save computation time, the
oneꢀelectron basis set on the Al atoms was specified using the
sized by the hydrolysis of monomeric forms of diisobutylꢀ
aluminum aryloxides by analogy to the published works,6
and their application in polymerization processes. Howꢀ
ever, it was shown by the study of the hydrolysis of two
monomeric diisobutylaluminum aryloxides, viz., (2,6ꢀ
,7
2
1
ECP10MWB pseudoꢀpotential and the valent electrons of Al
2
1
t
i
t
were described using the corresponding ECP10MWB basis set.
Bu —C H O)AlBu (2a) and (2,6ꢀBu ,4ꢀMe—C H O)ꢀ
2
6
3
2
2
6
2
i
The standard algorithm accomplished in the Gaussian 09 proꢀ
gram was used to search for transition states. The found transiꢀ
tion state was checked for the imaginary frequency. The polarꢀ
AlBu (2b), under mild conditions on copper(II) sulfate
2
pentahydrate (CuSO •5H O) or ice particles at the molar
4
2
1
ratio Al : H O = 2 using the H NMR method that the
2
22
ized continuum model (PCM) approximation was used to take
hydrolysis occurred selectively at the Al—OAr bond to
form the corresponding sterically bulky phenol, which
slowly reacted with an excess of diisobutylaluminum
monoaryloxide to form isobutylaluminum diaryloxide. It
is impossible to obtain aryloxyisobutylaluminoxanes by
this method. The quantum chemical simulation of the
hydrolysis showed that the reaction route was determined
by the kinetics of the process and was related to the formaꢀ
tion of a hydrogen bond with the O—Ar fragment resulting
in a substantial decrease in the activation energy of O—Ar
ligand protonolysis.
into account the effect of the solvent (toluene).
Results and Discussion
The hydrolysis products of diisobutylaluminum arylꢀ
oxides were studied for structurally similar sterically bulky
diisobutylaluminum aryloxides 2a and 2b bearing tertꢀbutyl
radicals in both orthoꢀpositions. Isobutylaluminum arylꢀ
oxides containing two tertꢀbutyl groups in the orthoꢀposiꢀ
tions of the aryl fragment of the compounds exist in the
2
3—25
monomeric form due to steric hindrances.
For comꢀ
parison we studied the hydrolysis of diisobutylaluminum
aryloxide with one tertꢀbutyl substituent in the orthoꢀposiꢀ
tion of the O—Ph fragment (2с), which exists as a dimer,
as will be shown below.
Experimental
Toluene (special purity grade) was used. The solvent was
dried by distillation over LiAlH and stored over molecular sieves
4
(
4 Å) in an argon atmosphere. Tolueneꢀd (Aldrich Chem. Co.)
8
dried by multiple distillation over activated molecular sieves (4 Å)
1
t
was used for Н NMR spectroscopy. Compounds (2,6ꢀBu ,4ꢀR—
2
i
t
C H O)AlBu (R = H (2a), Me (2b)) and [(2ꢀBu —C H O)ꢀ
AlBu ] (2c) were synthesized using a described procedure.
6
2
i
2
6
4
20
2
2
Argon was dried by passing through a column with activated
molecular sieves (5 Å). All manipulations with organoaluminum
compounds were carried out in an atmosphere of purified and
dried argon.
Hydrolysis of diisobutylaluminum aryloxides. Hydrolysis of
toluene solutions of disiobutylaluminum aryloxides was carried
out by two methods: using CuSO •5H O (A) and ice (B).
The primary aim of the study was to establish the posꢀ
sibility of forming aryloxide aluminoxane products during
hydrolysis of these compounds by analogy to the works.
4
2
A. Hydrolysis using CuSO •5H O. A cooled to –78 °С soluꢀ
4
2
6,7
tion of diisobutylaluminum aryloxide in toluene (Al : H O = 2)
2
The hydrolysis was carried out using copper(II) sulfate
was poured to a weighed sample of copper(II) sulfate pentaꢀ
hydrate. The temperature of the obtained suspension was inꢀ
creased to room temperature with stirring. The solution was
stirred for 10 h. The solution of the synthesized aluminoxane was
filtered off from crystalline hydrate CuSO •H O.
pentahydrate (CuSO •5Н O) or ice particles dispersed in
4
2
toluene at the molar ratio Al : H2O = 2 according to the
procedures used by us for the synthesis of tetraisobutylaluꢀ
minoxane (TIBAO) and longerꢀchain isobutylaluminoxꢀ
4
2
B. Hydrolysis using ice. Degassed water was cooled to –78 °С.
A cooled to –78 °С solution of diisobutylaluminum aryloxide in
18
anes (IBAO). The choice of the molar ratio was deterꢀ
mined by the aim at obtaining aryloxide aluminoxane prodꢀ
toluene (Al : H O = 2) was slowly added to the formed ice. The
i
i
2
ucts (ArO)Al(Bu )ꢀ(μꢀO)ꢀ(Bu )Al(OAr), which, as we asꢀ
obtained suspension was gradually heated to room temperature
with continuous stirring until ice was completely dissolved. The
overall hydrolysis time was 1 h.
Analysis of diisobutylaluminum aryloxides and hydrolysis prodꢀ
ucts by the NMR method. Toluene solutions of synthesized diꢀ
isobutylaluminum aryloxides and hydrolysis products in an
sumed, should not manifest a tendency to dimerization,
2
6
unlike TIBAO, and could be used for the activation of
metallocene complexes in olefin polymerization. The hyꢀ
drolysis products were analyzed under the conditions of
1
Н NMR experiments.

1
960
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
Faingol´d et al.
1
The fragments of the aliphatic range of the Н NMR
The spectrum of the starting diisobutylaluminum arylꢀ
oxide is presented in this range by signals of protons of the
spectra of the starting compound 2b (a) and the hydrolysis
products of aryloxide obtained using copper(II) sulfate
pentahydrate in 10 h (b) and in 6 days of the subsequent
storage of a toluene solution at room temperature (c) are
presented in Fig. 1.
methyl group CH —Ar (2.25 ppm), tertꢀbutyl groups
3
(1.47 ppm), and isobutyl groups (1.03 and 0.40 ppm).
After 10 h of hydrolysis, the spectrum of the reaction prodꢀ
ucts contains the signals of the starting compound and
also signals of protons of the tertꢀbutyl groups of the correꢀ
sponding sterically bulky phenol 1b (1.35 ppm) and tertꢀ
butyl (1.57 ppm) and isobutyl groups (0.75 and 0.43 ppm)
of isobutylaluminum diaryloxide, Note that the signals of
the tertꢀbutyl groups of the starting aryloxide and formed
phenol have the same intensity. The spectrum also exhibꢀ
its broad unresolved lowꢀintensity signals of the isobutyl
groups of the aluminoxane structures (0.75—0.5 and
1.4—0.9 ppm, see Fig. 1, d). The prolong storage of the
sample leads to the transformation of the spectrum in the
form of a symbate disappearance of the signals of comꢀ
pound 1b and residual diisobutylaluminum monoaryloxide
reacting to form isobutylaluminum diaryloxide (c). The
spectrum of the polyisobutylaluminoxane structures are
not transformed in the presence of compound 1b, indicatꢀ
ing that the reaction of 1b with diisobutylaluminum
monoaryloxide proceeds selectively and compound 1b does
not react with isobutylaluminoxanes (see Fig. 1, d). The
shapes and positions of the signals of isobutylaluminoxane
are similar to those of the signals of polyisobutylaluminꢀ
oxane obtained by the hydrolysis of TIBA with copper(II)
Toluene
a
1
.47
i
Bu H
CH (Bu )
i
3
1
.03
i
CH —Ar
Bu H
3
i
2
.25
CH (Bu )
2
0
.40
3
.15
18.00
12.36
4.03
2
.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
δ
1
.59
b
1
2
1
8
3
sulfate pentahydrate at the molar ratio Al : H O = 1 (see
2
t
CH (Bu )
Fig. 1, e). A similar pattern of hydrolysis was also observed
for compound 2a. The obtained results indicate the selecꢀ
tive hydrolysis of diisobutylaluminum aryloxide at the
Al—O bond rather than at the Al—C bond. The complete
pattern of the processes occurring at the partial hydrolysis
3
i
CH (Bu )
1
.47
3
1
.36
0
.45 0.41
CH (Bu )
2
.26
i
1
.04
1
2
2
.27
i
Bu ꢀAO
0.77
2
.23
2
1
i
Bu ꢀAO
2
of diisobutylaluminum aryloxides (molar ratio Al : H O = 2)
2
2
.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
δ
is shown in Scheme 1.
As in the case of triisobutylaluminum,18 the hydrolysis
by copper(II) sulfate pentahydrate is slower and more seꢀ
lective than the hydrolysis on ice particles, because the
dehydration of the pentahydrate is the rateꢀdetermining
1
.59
c
d
e
2
6
step in this case. For the hydrolysis on ice particles, it is
more difficult to avoid local overheatings that can be
a reason for side reactions to occur. The fragment of the
1
aliphatic range of the Н NMR spectrum of the hydrolysis
products of compound 2a on ice particles is shown in
2
.27
1.4 1.3 1.2 1.1 1.4 1.3 1.2 1.1 δ
.77
Fig. 2. It is seen that the major hydrolysis product at the
0
molar ratio Al : H O = 2, as well as for the hydrolysis on
0
.45
2
copper(II) sulfate pentahydrate, is 2,6ꢀdiꢀtertꢀbutylphenol
(
1a), which further reacts with an excess of diisobutylꢀ
2
.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 δ
aluminum monoaryloxide to form isobutylaluminum diꢀ
aryloxide (see Scheme 1). A substantial distinction is
a significantly lower intensity of the signal of the tertꢀbutyl
groups of compound 1a compared to that of diisobutylꢀ
aluminum monoaryloxide and an almost complete abꢀ
sence of signals of isobutylaluminoxanes in the ranges
0.5—0.75 and 0.9—1.4 ppm. A comparison of the integral
1
Fig. 1. Fragments of the aliphatic part of the Н NMR spectra of
2
b (a) and its hydrolysis products obtained using copper(II) sulꢀ
fate pentahydrate in 10 h (b) and 6 days (c) after hydrolysis:
1) dimer, (2) monomer, and (3) phenol; d, isobutylaluminoxane
fragments of spectra b and c; e, fragment of Н NMR spectrum of
polyisobutylaluminoxane obtained by the hydrolysis of TIBA with
(
1
1
8
copper(II) sulfate pentahydrate at the molar ratio Al : H O = 1.
2

Hydrolysis of isobutylaluminum aryloxides
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
1961
Scheme 1
R = H, Me
t
i
intensities of the tertꢀbutyl groups of compound 1a,
diisobutylaluminum monoaryloxide, and isobutylalumiꢀ
num diaryloxide suggests a ~16% deficiency of the amount
of water involved in phenol formation. A possible explaꢀ
nation is the partial consumption of water under the condiꢀ
tions of local overheatings in the deeper hydrolysis of aluꢀ
of the O—Ar fragment, [(2ꢀBu —C H O)AlBu ] (2c),
6
4
2 2
was chosen as an example. Compound 2с was synthesized
by the reaction of compound 1с with TIBA at the molar
ratio Al : phenol = 1 using an earlier described procedure.
20
The spectrum of the starting compound 1с in the aliphatic
range is presented by a narrow signal of tertꢀbutyl groups
at 1.43 ppm. The spectrum of product 1с of the reaction
with TIBA in this range recorded at room temperature
(Fig. 3, a) is presented by the signal of the tertꢀbutyl groups
(1.50 ppm) and two sets of broad signals of approximately
equal intensities of the CH (0.98 and 0.79 ppm) and CH
fragments (0.69 and 0.55 ppm) of the isobutyl groups.
This pattern indicates the dimeric structure of compound
2с in which dynamic processes involving isobutyl groups
occur. As the temperature of spectrum recording decreasꢀ
es to 0 °С (see Fig. 3, b), the dynamic processes in arylꢀ
oxide 2c is retarded and the signals of isobutyl groups
are presented by narrow doublets of methyl groups and
doublets of doublets of methylene protons. An additional
isomer of compound 2c is observed at this temperature,
which is indicated by the appearance of a new signal of
tertꢀbutyl groups at 1.53 ppm and a set of signals in a range
of 0.9–1.0 ppm. The further temperature decrease to
–20 °С (c) showed two doublets of methyl protons at 0.97
and 0.95 ppm and two doublets of methylene protons at
0.90 and 0.70 ppm. The multiplet of methyl protons at
2.00 ppm corresponding to this isomer becomes resolved
in the spectrum. The further temperature decrease to
i
minoxanes (Bu AlO) to form isobutylaluminoxane strucꢀ
n
i
tures (Bu 1–xAlO ) insoluble in the reaction mixture.
1+x n
As mentioned above, compounds 2a and 2b are rare
representatives of monomeric forms of alkylaluminum
aryloxides protected from dimerization by the bulky tertꢀ
butyl substituents in the orthoꢀpositions of the O—Ar fragꢀ
ment. It was of interest to establish an analogy or specifics
in the occurrence of hydrolysis of less sterically hindered
isobutylaluminum aryloxides. Diisobutylaluminum arylꢀ
oxide with one tertꢀbutyl substituent in the orthoꢀposition
3
2
1.55
t
CH (Bu )
3
1
1.44
i
CH (Bu )
3
2
i
Bu H
1
3
.33
0.41 0.38
i
CH (Bu )
2
1
.02
2
0.72
1
1
2
–
40 °С (d) somewhat "slurs" over the pattern because of
5
7.7 18.0 7.3
12.0
9.5
the overlapping of the signals of the methyl protons of the
minor isomer (0.98 ppm) and overlapping of one group of
doublets of doublets of the major isomer with one of the
signals of its methyl groups (0.85 ppm). The spectrum of
compound 2c recorded at room temperature after cooling
2
.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 δ
1
Fig. 2. Fragment of the aliphatic range of the Н NMR spectrum of
hydrolysis product 2a in ice 6 days after the synthesis: 1, dimer;
, monomer, and 3, phenol.
2

1
962
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
Faingol´d et al.
1.50
a
sis at the Al—C bond. On heating 2с at 50—60 °С for 1 h,
the compound is partially hydrolyzed to form 2ꢀBu —
t
C H OH (1c).
0
.98
6
3
0
.79
Quantum chemical study. The experimental results preꢀ
0
.69
sented on the hydrolysis of sterically hindered monomeric
isobutylaluminum aryloxides and the data for the hydrolyꢀ
1
.83
0.55
1
5—17
sis of other alkylaluminum aryloxides
show that the
1.53
1.03
0
.82
b
hydrolysis of these compounds proceeds at the Al—O bond
rather than at the Al—C bond, which is the unexpected
result since the energy of the Al—O bond is significantly
0
.76
–
1
higher (~120 kcal mol ) than that of the Al—C bond
0
.58
–
1 28
(
~64 kcal mol ).
There is no theoretical substantiation of hydrolysis for
1.94
6.00
5.89 2.29
1.89
this class of compounds in the literature. Therefore, we
simulated the key structures and estimated the energy paraꢀ
meters of the process by the density function theory (DFT)
for the hydrolysis of aryloxide 2a.
c
At the first stage of the study, we calculated the strucꢀ
i
ture of intermediate AlBu OAr•H O (IM), which repreꢀ
2
2
sents the tendency of aluminum to the tetracoordinated
state (Fig. 4). The Al—O(H O) bond length in the comꢀ
1
.50
2
d
plex is 2.06 Å and exceeds the length of the Al—O(Ar)
bond equal to 1.78 Å. Note that the corresponding disꢀ
tances in the aqua complex of methylaluminum diarylꢀ
0
.81
0
.85
1
.07
1
.54
t
oxide [2,6ꢀBu —C H O] AlMe•H O•2THF determined
2
6
3
2
2
0
.98
0.91
0
.63
1
.85
by the Xꢀray diffraction analysis data (–40 °С) are someꢀ
0
.60
2
.0
0.73
1
6
what shorter and equal to 1.86 and 1.72 Å, respectively.
0
.44 2.02
2.16 9.00
6.10 2.78 7.08 0.47 1.98
.28
At the same time, the distance in the monoaryloxide comꢀ
plexes of alkylaluminums with Lewis bases (LB) are
0
.52
1
2
.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
δ
t
similar: [2,6ꢀBu —C H O]AlR •LB:·Al—P = 2.50,
2
6
3
2
Fig. 3. Fragments of the 1Н NMR spectrum of aryloxide 2c
recorded at different temperatures: ~20 °C (a), 0 °С (b), –20 °С (c),
and –40 °С (d).
25
Al—O = 1.74 Å (R = Me, LB = PMe ) ; Al—N = 2.08 Å,
Al—O = 1.75 Å (R = Me, LB = (2,6ꢀMe —Py) ;
Al—N = 2.04 Å, Al—O = 1.75 Å (R = Et, LB = NH Bu) ;
Al—N = 2.01, Al—O = 1.74 Å (R = Me, LB = NH ) ;
3
29
2
t
29
2
30
3
completely corresponds to the spectrum to the stepped
cooling of the sample. Thus, two isomers presented by two
different tertꢀbutyl groups and two sets of isobutyl groups
were revealed for aryloxide 2с. The ratio of isomers in
a toluene solution is 1 : 5. Probably, the major dimeric form
and Al—O(THF) = 1.91, Al—O = 1.75 Å (R = Et,
LB = THF). The hydrogen bond OH···OAr (2.59 Å) in
3
1
intermediate IM should be mentioned. The H(H O)—C
2
distance is 3.02 Å. On the whole, complex formation leads
to a decrease in the energy of the system by 10.55 kcal mol–
compared to infinitely remote reactants.
1
(
(
as the most thermodynamically stable) is the transꢀform
tertꢀbutyl groups in the orthoꢀposition are located at difꢀ
The transition states were considered for two reaction
routes: at the Al—O bond (TS1) and at the Al—C bond
(TS2) (see Fig. 4). Transition state TS1 is formed due to
the interaction of water hydrogen with the oxygen atom of
the aryloxy group. The angles of the O—Al—O and
H—O —Al bonds in the transition state are smaller by
ferent sides of the Al O plane), and the minor dimeric
2
2
isomer represents the cisꢀform. An indirect confirmation
are the published data2 showing that the structurally simꢀ
7
t
i
ilar complex [(2,4ꢀBu —C H O)Al(Bu ) ] exists in the
2
6
3
2 2
transꢀform, according to the Xꢀray diffraction data, whereꢀ
H O
2
1
as the Н NMR spectrum of the solution at room temperꢀ
21° and 32°, respectively, compared to those in IM. The
ature contains two groups of broadened signals of the
methyl and methylene protons of the isobutyl groups.
An attempt of hydrolysis of compound 2с with
copper(II) sulfate pentahydrate (CuSO •5Н O) and ice
was made to study the character of interaction of associatꢀ
ed diisobutylaluminum aryloxides. It turned out that no
hydrolysis of 2с occurred at room temperature. An interꢀ
esting fact is the inability of these compounds to hydrolyꢀ
Al—O
bond length decreases from 2.06 Å to 1.90 Å,
H O
2
whereas the Al—OAr bond length increases from 1.78 to
1.98 Å. The reaction coordinate corresponds to the hydroꢀ
gen atom transfer from one oxygen atom to another, In
the transition state, the lengths of two O—H bonds are
equal in fact being 1.18 and 1.24 Å (cf. the О—Н bond in
the water molecule equal to 0.96 Å). The potential barrier
4
2
–
1
relative to intermediate IM is 14.35 kcal mol . In this

Hydrolysis of isobutylaluminum aryloxides
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
1963
1.90
1.18
6
8
7
8
2.14
122
+
1.77
TS2 7.32
1.99
Starting substances
1.78
i
AlBu OAr + H O
0.96
2
2
Transition
states
0
TS1 –4.09
Al(OH)Bui2•ArOH
12.72
–
–
18.44
Afterꢀreaction
complexes
Prereaction complex
i
IM: AlBu OAr•H O
2
2
–
33.56
i
i
Al(OH)Bu OAr•Bu H
1.19
0
.96
109 2.59
1.90
8
6
71
1.24
7
7
3
.02
.06 100
1.98
9
2
2
1.78
1.98
1.73
1.96
0.96
i
Fig. 4. Potential energy profile showing the hydrolysis of AlBu OAr at the Al—O (transition state TS1) and Al—C (transition state
2
Ar
TS2) bonds. The relative energies (kcal mol–1) were calculated by the ωB97XD/6ꢀ311+G(d,p) method.
Note. Figures 4 and 5 are available in full color in the onꢀline version on the webꢀpage (http://www.link.springer.com).
case, the hydrolysis products are alkylaluminum hydroxide
O—Al—C and H—OH2O—Al angles in TS2 are decreased
by 22° and 28°, respectively, compared to those in interꢀ
mediate IM. The Al—OH2O and Al—C bond lengths are
1.90 and 2.14 Å, whereas in IM they are equal to 2.06 and
1.98 Å, respectively. The reaction coordinate corresponds
to the hydrogen atom transfer from the oxygen atom of
water to the carbon atom of the methylene group. In this
transition state, the hydrogen atom is remote from oxygen
i
AlBu OH and aryloxide 2. The heat effect of this reaction
2
–
1
is –12.72 kcal mol . However, a significant energy gain
of the reaction is achieved when taking into account furꢀ
ther transformations of AlBu (OH), namely, association
i
2
and formation of isobutylaluminoxanes. A significant heat
release was observed in experiments on hydrolysis. The
heat effect of dimerization characteristic of alkylalumiꢀ
3
2
–1
num hydroxides was –44.27 kcal mol in our case.
Transition state TS2 is formed due to the interaction
of the hydrogen atoms of water with the carbon atom of
one of methylene groups. As in transition state TS1, the
at 1.18 Å and approached to the carbon atom by 1.48 Å.
The potential barrier relative to IM is 25.76 kcal mol–
1
.
For the hydrolysis via this route, the heat effect is
–1
–33.56 kcal mol
.

1
964
Russ.Chem.Bull., Int.Ed., Vol. 65, No. 8, August, 2016
Faingol´d et al.
1.21 1.44
6
8
78
2.21
1.91
1.98
+
Transition state
6.12
Starting substances
AlBui + H O
3
2
0
0
.96
2
.98
0
.96
115
–
17.88
9
5
1.73
2
.00
1.96
2.12
Prereaction complex
i
AlBu 3•H O
2
–
35.4
Starting substances
i
i
Al(OH)Bu •Bu H
2
Fig. 5. Potential energy profile showing the hydrolysis of AlBu . The relative energies (kcal mol–1) were calculated by the ωB97XD/6ꢀ
i
3
3
11+G(d,p) method.
A comparison of two possible routes of hydrolysis shows
bonds, respectively, were considered and optimized. It was
shown that the energy barrier for hydrolysis at the Al—C
bond was significantly higher than that at the Al—O bond
(25.76 and 14.35 kcal mol , respectively), which explains
the kinetic control of the hydrolysis in experiment. A lowꢀ
er barrier for the reaction at the Al—O(Ar) bond is caused
by the formation of the hydrogen bond between the water
proton and oxygen atom of the aryloxy fragment.
that transition state TS1 is characterized by a lower energy
–
1
barrier (by 11.41 kcal mol ), which corresponds to a huge
difference in the reaction rate through transition states
TS1 and TS2. The hydrolysis of the studied compounds
proceeding at the Al—O(Ar) bond is considerably faciliꢀ
tated by the formation of a hydrogen bond between the
proton of the water molecule and the oxygen atom of the
aryloxide group.
–
1
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 13ꢀ03ꢀ01281ꢀa
and 15ꢀ03ꢀ02307ꢀa).
The hydrolysis of TIBA was calculated (Fig. 5) to comꢀ
pare with the obtained results for the hydrolysis of comꢀ
pound 2. In this case, the energy barrier of hydrolysis at
–
1
the Al—C bond was 24.0 kcal mol , which is close to the
–
1
26
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Received February 1, 2016;
in revised form May 24, 2016