Reactions of β-keto sulfones with t-butyl aluminum compounds: Reinvestigation of tri-t-butyl aluminum synthesis

Article abstract of DOI:10.1002/aoc.5961

t-Butyl derivatives play a significant role in the organometallic chemistry of group 13 metals. It was shown on the basis of reactions of t-Bu₃Al·OEt₂ with [p-RC₆H₄S(O)₂C(H)₂C(Ph) = O] (where R = CH₃, Cl) β-keto sulfones that the structure of the reaction products depends on the purity of the aluminum compound used. In the reactions, in addition to the expected complexes [p-RC₆H₄S(O₂)C(H) = C (Ph)-OAl(t-Bu)₂] [where R = CH₃ (2); R = Cl (4)] possessing β-keto sulfone ligands, complexes with β-hydroxy sulfone ligands [p-RC₆H₄S(O₂)C(H)₂-C (Ph)-OAl(t-Bu)₂] [where R = CH₃ (1); R = Cl (3)] were formed. Compounds 1 and 3 were the result of the hydroalumination reaction of the β-keto sulfone ligands with t-Bu₂AlH, which is an impurity of t-Bu₃Al. These compounds are obtained, for the first time, as intermediate products in the hydrogenation reaction of β-keto sulfones. In this work, during t-Bu₃Al·OEt₂ production t-Bu₂AlH·OEt₂ formed as a by-product. Re-examination of reaction conditions of AlCl₃ with t-BuMgCl resulted in a control of the t-Bu₂AlH·OEt₂ by-product content in t-Bu₃Al·OEt₂.

Full text of DOI:10.1002/aoc.5961

Received: 15 May 2020
DOI: 10.1002/aoc.5961
Revised: 8 July 2020
Accepted: 13 July 2020
F U L L P A P E R
Reactions of β-keto sulfones with t-butyl aluminum
compounds: Reinvestigation of tri-t-butyl aluminum
synthesis
Tomasz Wojciechowski1 | Zbigniew Ochal1 | Paweł Socha2
Łukasz Dobrzycki2 | Wanda Ziemkowska1
|
1Faculty of Chemistry, Warsaw University
t-Butyl derivatives play a significant role in the organometallic chemistry of
group 13 metals. It was shown on the basis of reactions of t-Bu3AlÁOEt2 with
[p-RC6H4S(O)2C(H)2C(Ph) = O] (where R = CH3, Cl) β-keto sulfones that the
structure of the reaction products depends on the purity of the aluminum
compound used. In the reactions, in addition to the expected complexes
[p-RC6H4S(O2)C(H) = C (Ph)-OAl(t-Bu)2] [where R = CH3 (2); R = Cl (4)]
possessing β-keto sulfone ligands, complexes with β-hydroxy sulfone ligands
[p-RC6H4S(O2)C(H)2-C (Ph)-OAl(t-Bu)2] [where R = CH3 (1); R = Cl (3)] were
formed. Compounds 1 and 3 were the result of the hydroalumination reaction
of the β-keto sulfone ligands with t-Bu2AlH, which is an impurity of t-Bu3Al.
These compounds are obtained, for the first time, as intermediate products
in the hydrogenation reaction of β-keto sulfones. In this work, during
of Technology, Noakowskiego 3, Warsaw,
00-664, Poland
2Department of Chemistry, University of
Warsaw, Pasteura 1, Warsaw, 02-093,
Poland
Correspondence
Wanda Ziemkowska, Warsaw University
of Technology, Faculty of Chemistry,
Noakowskiego 3, 00-664, Warsaw, Poland.
Email: ziemk@ch.pw.edu.pl
Funding information
Warsaw University of Technology
t-Bu3AlÁOEt2 production t-Bu2AlHÁOEt2 formed as
a by-product. Re-
examination of reaction conditions of AlCl3 with t-BuMgCl resulted in a
control of the t-Bu2AlHÁOEt2 by-product content in t-Bu3AlÁOEt2.
K E Y W O R D S
hydroalumination, hydrogenation, tri-t-butyl aluminum, β-Hydroxy sulfones, β-Keto sulfones
1 | INTRODUCTION
Sinn and Kaminsky synthesized methylalumoxane in
1980,[1] the reaction pathway of trialkylaluminum and
Organometallics with sterically crowded groups have
been found to be useful compounds for determining the
mechanisms of many reactions. The presence of steric
hindrances makes it possible to isolate intermediate prod-
ucts of reactions that proceed violently with compounds
having small substituents. The best example is an investi-
gation of the reaction of group 13 metal trialkyls with
water by Barron and Roesky. The reaction of trimethyl
aluminum with water yields methylalumoxane, which is
commonly used as a highly active cocatalyst for group
4 metallocenes for ethylene and propylene polymeriza-
tion and in many organic synthesis reactions. Although
trialkylgallium compounds with water leading to the for-
mation of alumoxanes and galloxanes was described by
Barron[2,3] and Roesky[4–6] in the 1990s. The use of organ-
ometallic compounds with bulky (Me3Si)C and mesityl
groups gave Roesky and his colleagues the opportunity to
isolate and characterize intermediate products. Although
t-butyl groups are relatively small with respect to the effi-
cient steric shielding of organoaluminum compounds,
Barron and his team synthesized unusual cage aluminum
compounds in reactions of t-Bu3Al hydrolysis.[2,3]
Besides tri-methyl-, tri-ethyl-, and tri-iso-butyl com-
pounds, t-butyl derivatives play a significant role in the
Appl Organomet Chem. 2020;e5961.
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© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5961

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WOJCIECHOWSKI ET AL.
organometallic chemistry of group 13 metals. A large
variety of t-butyl aluminum, t-gallium, and t-indium
compounds were synthesized by Barron et al.,[7–11]
In this paper, based on the reaction of β-keto sulfones
with pure t-Bu3AlÁOEt2 and with t-Bu3AlÁOEt2 contami-
nated with the t-Bu2AlHÁOEt2 by-product, we show a
product structure dependence on the composition of
starting aluminum compounds. Organic sulfones belong
to the very important class of compounds in modern
organic synthesis. The presence of sulfonyl groups (SO2)
in molecules is helpful in designing new complex struc-
tures exhibiting biological activity.[24] β-Keto sulfones
exist as a tautomeric equilibrium, but the equilibrium
is almost completely shifted toward the ketone form
(Scheme 2). We found that the treatment of β-keto sul-
fones with t-Bu2AlHÁOEt2 afforded a hydroalumination
of β-keto sulfones to β-hydroxy sulfone aluminum com-
pounds, whereas t-Bu3AlÁOEt2 was inactive in the hydro-
alumination reaction forming di-t-butyl aluminum
complexes with the β-keto sulfones. In the literature,
methods for the synthesis of pure t-Bu3Al and t-Bu2AlH
have been reported[22,25]; however, the results of our
experiments prompted us to re-examine the effect of reac-
tion conditions on the formation of by-products during
the synthesis of t-Bu3AlÁOEt2.
[[12]]
Lewinski et al.,
and Ziemkowska et al.[[13–17]] The
ꢀ
first report about the synthesis, stability, and reactivity of
t-Bu3Al was published in 1968, in which Lehmkuhl
reported that t-Bu3Al and t-Bu3AlÁD (D = OEt2, thf) are
stable at room temperature and resistant to isomerization
to iso-butyl aluminum compounds.[18] Later kinetic stud-
ies have shown that the second-order reaction of t-Bu3Al
isomerizationꢀrequires a long heating time at the temper-
ature of 140 C. The X-ray crystal structure determined
for t-Bu3Al at low temperatures mainly revealed mono-
meric molecular units throughout with consistently lon-
ger M–C bonds than in the corresponding monomeric
trimethyl derivative.[19,20]
In general, t-Bu3Al can be obtained by the reaction of
AlX3 with t-butyllithium or t-BuMgX (X = Cl, Br).
Lehmkuhl et al. found that the reaction of AlCl3
with t-Bu2AlF leads to t-Bu3Al without isomers as by-
products.[21] By contrast, analysis of the composition of
products obtained from the reaction of AlBr3 with
t-butyllithium indicated that the reaction mixture con-
tains three by-products, including t-Bu2AlH, in addition
to the desired t-Bu3Al product.[22] Based on our many
years of experience, we have found that t-Bu3Al etherate
obtained in the reaction of AlCl3 with t-BuMgCl in the
diethyl ether solution often contains t-Bu2AlH etherate.
The percentage of the t-Bu2AlHÁOEt2 depends on the
temperature of the process and increases with increasing
temperature. It is important that the presence of
t-Bu2AlHÁOEt2 does not have an influence on reactions
with compounds containing such functional groups as
OH, COOH, C(O)NHR, because t-Bu2AlH reacts with
hydrogen evolution yielding the same products as for the
reactions with t-Bu3Al (Scheme 1). However, many
compounds containing C=C, C ≡ C, and C=O bonds
undergo a hydroalumination reaction with t-Bu2AlH to
form reduced products.[23] This means that reactions car-
ried out with t-Bu3Al in admixture with t-Bu2AlH can
lead to a mixture of products.
2 | EXPERIMENTAL DETAILS
2.1 | General remarks
All manipulations were carried out using standard
Schlenk techniques under an inert gas atmosphere.
Methylene dichloride (Avantor, Gliwice, Poland) was
deacidified with basic Al2O3 (Sigma-Aldrich, Poznan,
Poland) and distilled over P2O5 (Sigma-Aldrich, Poznan,
1
Poland) under argon. H and 13C nuclear magnetic res-
onance (NMR) spectra were obtained on a Mercury
400BB Varian spectrometer. Chemical shifts were
referenced to the residual proton signals of CDCl3
(Euriso-top, Saarbrucken, Germany) (7.26 ppm). 13C
NMR spectra were acquired at 100.60 MHz
(standard: chloroform 13CDCl3, 77.20 ppm). All NMR
spectra can be found in the Supporting Information
(Figures SI1–SI16). Anhydrous AlCl3 was from ABCR
Company (Karlsruhe, Germany). It was sublimed under
vacuum before reactions. t-BuMgCl (2.0 M solution in
Et2O) was from Sigma-Aldrich (Poznan, Poland). β-Keto
SCHEME 1 Reactions of t-Bu3Al and t-Bu2AlH with
compounds containing active functional groups
SCHEME 2 An equilibrium of β-keto sulfone tautomers

WOJCIECHOWSKI ET AL.
3 of 9
sulfones were synthesized according literature data.[26]
Hydrolyzable t-butyl groups bonded to Al atoms for
products 1 and 3 were determined by hydrolysis of the
compound (0.2–0.3 g) using HNO3 (Avantor, Gliwice,
room temperature for 30 min. After 24 h, the reaction
mixture was placed at −20 C for 4 days. The precipi-
ꢀ
tated crystalline solid of MgCl2 formed was then
removed. A solid obtained after distillation of the
solvent from the postreaction mixture was subjected to
distillation under vacuum (1.33 Pa) at 140–150 ꢀC
(for some experiments the distillation was repeated). A
thick colorless liquid of t-Bu3AlÁOEt2 was obtained
(Yield, 20%–50%). The 1H NMR spectrum reveals
Poland) solution (10% concentrated,
5
cm3) and
measurement of the volume of gaseous C4H10. Subse-
quently, the sample was transformed into Al2O3 by
mineralization and the obtained white solid was dis-
solved in a diluted water solution of HNO3. The content
of aluminum was determined by complexation of Al3+
cations with versenate anions using an excess of
the titrated solution of calcium disodium versenate
(Sigma-Aldrich, Poznan, Poland). Then, the excess of
calcium disodium versenate was titrated using FeCl3
(Avantor, Gliwice, Poland).
1
signals of t-Bu3AlÁOEt2 and t-Bu2AlHÁOEt2 protons. H
NMR (Figure SI1) (CDCl3) δ: t-Bu3AlÁOEt2 4.04 [q,
broad, 4H, O (CH2CH3)2], 1.36 [t, broad, 6H, O
(CH2CH3)2], 0.97 [s, 27H, C (CH3)3]; t-Bu2AlHÁOEt2
δ: 4.66 (s, 1H, AlH), 4.26 [q, 4H, O (CH2CH3)2], 1.45
[t, 6H, O (CH2CH3)2], 0.96 [s, 18H, C (CH3)3] ppm. The
molar ratio of t-Bu3AlÁOEt2:t Bu2AlHÁOEt2 depends on
the reaction conditions.
2.2 | X-ray crystallography
The X-ray measurements of compounds 1 and 3 were
performed at 100(2) K on a Bruker D8 VENTURE
Photon100 diffractometer equipped with a TRIUMPH
monochromator and a Mo-Kα fine-focus sealed tube
(λ = 0.71073 Å). The total frames were collected with the
Bruker APEX2 program.[27] The temperature of samples
was 100 K. The frames were integrated with the Bruker
SAINT software package[28] using a narrow-frame algo-
rithm. Data were corrected for absorption effects using
the multiscan method (SADABS).[29] The structures were
solved and refined using SHELXTL software package.[29]
The atomic scattering factors were taken from the Inter-
national Tables.[30] All hydrogen atoms were placed in
calculated positions and refined within the riding model.
Detailed crystallographic data are listed in Table SI1.
Crystallographic data in cif format are deposited
in the Cambridge Crystallographic Data Center,
CCDC No. 2003230 (compound 1), CCDC No. 2002681
(compound 3).
2.3.2 | The reaction of t-BuMgCl with
AlCl3 for a 3:1 molar ratio of reagents
According to the general procedure, 1 M solution of
t-BuMgCl (90 cm3, 90 mmol) and AlCl3 (4.00 g, 30 mmol
in 60 cm3) were used. After 3 days of reaction at room
temperature, a white solid of MgCl2 precipitated from the
postreaction mixture. The almost pure t-Bu3AlÁOEt2 was
1
distilled from the solution (yield: 2.86 g, 35%). H NMR
spectrum is presented in Figure SI15.
2.3.3 | The reaction of t-BuMgCl with
AlCl3 for a 4:1 molar ratio of reagents
According to the general procedure, 1 M solution of
t-BuMgCl (120 cm3, 120 mmol) and AlCl3 (4.00 g,
30 mmol in 60 cm3) were used. After 3 days of reaction at
room temperature, a white solid of MgCl2 precipitated
from the postreaction mixture. The solution was
decanted and placed at −20 ꢀC for 3 days. Big transparent
colorless crystals were isolꢀated and subjected to thermal
decomposition at 140–150 C under 1.33 Pa. The mixture
of t-Bu3AlÁOEt2 and t-Bu2AlHÁOEt2 containing 23 mol%
of t-Bu3AlÁOEt2 (on the basis of 1H NMR spectrum
[Figure SI16]) was obtained (2.63 g, conversion of AlCl3:
8.9%). The solvent was then distilled from the solution
above the crystals yielding a white solid. A mixture
of t-Bu3AlÁOEt2 and t-Bu2AlHÁOEt2 with the same
composition as the mixture obtained from the decompo-
sition of crystals was sublimated from the obtained solid
(3.44 g, conversion of AlCl3: 18.5%). Total conversion of
AlCl3 to the mixture of t-Bu3AlÁOEt2 and t-Bu2AlHÁOEt2
was 27.4%.
2.3 | Preparation of compounds
2.3.1 | Reaction of AlCl3 with t-BuMgCl.
Tri-t-butyl aluminum etherate and di-
t-butyl aluminum hydride etherate
synthesis: General procedure
The solution of t-BuMgCl (1.0–1.5 M solution in Et2O,
90–120 mmol) was placed in the three-necked flask
(500 cm3). Freshly sublimed AlCl3 (4.00 g, 30 mmol)
was dissolved in diethyl ether at −76 ꢀC and was
warmed up to room temperature. This solution was
then added dropwise to the solution of t-BuMgCl at

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WOJCIECHOWSKI ET AL.
2.3.4 | Di-t-butyl aluminum complex with
2-hydroxy-2-phenylethyl-4-methylphenyl
sulfone (1) and di-t-butyl aluminum
complex with 2-oxo-2-phenylethyl-
4-methylphenylsulfone (2)
(s, 2H, CH2C=O), 2.45 (s, 3H, CH3). 13C NMR (CDCl3)
(Figure SI7) δ: 188,11 (C=O), 145.37, 135.68, 135.63,
134,32, 129.81, 129.31, 128.81, 128.58 (Caromat), 63.53
[C(H) = CO], 21.71 (CH3) ppm.
A solution of t-Bu3AlÁOEt2 containing t-Bu2AlHÁOEt2
2.3.5 | Di-t-butyl aluminum complex with
2-hydroxy-2-phenylethyl-4-chlorophenyl
sulfone (3) and di-t-butyl aluminum
complex with 2-oxo-2-phenylethyl-
4-chlorophenylsulfone (4)
as by-product (1.60 g) in 10 cm3 of CH2Cl2 was
added to the solution of 0.82
g (3 mmol) of
2-oxo-2-phenylethyꢀl-4-methylphenylsulfone in 10 cm3
of CH2Cl2 at −76 C. A dark orange color of the reac-
tion mixture was observed. After warming to room
temperature and removing the solvent by distillation, a
thick orange liquid was obtained. The 1H NMR spec-
trum revealed proton signals of compounds 1 and
2 (Figures SI2a,b). 1H NMR (CDCl3) δ: compound 1,
5.95 [m, broad, 1H, CH2C(H)O], 3.38 [m, broad, 2H,
CH2C(H)O], 2.50 (s, 3H, CH3); compound 2, 5.72
[s, 1H, C(H) = C], 2.45 (s, 3H, CH3); signals of Et2O,
4.27, 3.49 ppm.
Compounds 3 and 4 were synthesized as described
for
1
and
2
using 1.60
g
of t-Bu3AlÁOEt2
containing t-Bu2AlHÁOEt2 and 0.884 g (3 mmol) of
2-oxo-2-phenylethyl-4-chlorophenylsulfone. The 1H NMR
spectrum revealed proton signals of compounds 3 and
1
4 (Figures SI8a,b). H NMR (CDCl3) δ: compound 3, 5.94
[m, broad, 1H, CH2C(H)O], 3.41 [m, broad, 2H, CH2C(H)
O]; compound 4, 5.72 [s, 1H, C(H) = C)]; signals of Et2O,
4.26, 1.45 ppm.
Crystals of 3 were obtained in a yield of 0.406 g (31%
relative to the β-keto sulfone). Melting point: 205–208 ꢀC.
1H NMR (CDCl3) (Figure SI9) δ: 7.96 (m, 2H, Haromat),
7.68 (m, 2H, Haromat), 7.68 (m, 2H, Haromat), 7.45 (m,
broad, 2H, Haromat), 7.34 (m, 2H, Haromat), 7.25 (m, 1H,
The solution of the postreaction mixture was placed
at 7–10 ꢀC. After few days, transparent crystals of
1
vsuitable for X-ray measurements precipitated
(yield 0.312 g, 25% relative to the β-keto sulfone). Melting
1
ꢀ
point: 204–209 C. H NMR (CDCl3) (Figure SI3) δ: 7.89
(m, 2H, Haromat), 7.47 (m, 4H, Haromat), 7.34 (m, 2H,
H
aromat), 7.25 (m, 1H, Haromat), 5.90 [m, broad, 1H,
Haromat), 5.90 [m, broad, 1H, CH2C(H)O], 3.40 [m, broad,
CH2C(H)O], 3.40 [m, broad, 2H, CH2C(H)O], 2.51 (s, 3H,
CH3), 1.09 [s, 9H, C (CH3)3], 1.02 [s, 9H, C (CH3)3] ppm.
Because of the low solubility of compound 1, the 13C
NMR spectroscopy was not performed. Anal. Al, 6.19;
hydrolyzable t-Bu groups, 26.99; calcd for 1: Al, 6.49; t-Bu
groups, 27.40 wt.%.
2H, CH2C(H)O], 1.09 [s, 9H, C (CH3)3], 1.02 [s, 9H,
C (CH3)3]. 13C NMR (CDCl3) (Figure SI10) δ: 143.06,
134.28, 130.62, 129.08, 128.64, 127.86, 127.81, 125.24
(Caromat), 68.30 (CH2CHO), 64.84 (CH2CHO), 30.20,29.75
[AlC (CH3)3], 15.49 [AlC (CH3)3, broad] ppm. Anal. Al,
5.97; hydrolyzable t-Bu groups, 25.70; calcd for 3: Al,
6.18; t-Bu groups, 26.09 wt.%.
A solution of t-Bu3AlÁOEt2 (0.272 g, 1 mmol) con-
taining traces of t-Bu2AlHÁOEt2 in 5 cm3 of CH2Cl2 was
added to the solution of 0.274 g (1 mmol) of 2-oxo-
2-phenylethyl-4-methylphenylsulfone in 5 cm3 of CH2Cl2
at −76 ꢀC. The solvent was removed under vacuum and a
thick liquid was obtained. Crystallization attempts failed.
On the basis of NMR spectra, the postreaction mixture
mainly contains compound 2 and a small amount of
A solution of t-Bu3AlÁOEt2 (0.272 g, 1 mmol) con-
taining traces of t-Bu2AlHÁOEt2 in CH2Cl2 was added to
the solution of 0.295 g (1 mmol) of 2-oxo-2-phenylethyl-
4-chlorophenylsulfone. Compound 4 was obtained as a
1
thick liquid. H NMR (CDCl3) (Figure SI11) δ: 7.91 (m,
2H, Haromat), 7.81 (m, 2H, Haromat) 7.55–7.44 (m, 5H,
Haromat), 5.71 [1H, C(H) = C], 0.99 [s, 9H, C (CH3)3], 0.92
[s, 9H, C (CH3)3]. 13C NMR (CDCl3) (Figure SI12) δ:
174.59 [C(H)COAl], 140.77, 138.81, 135.70, 132.14,
129.71, 128.61, 127.85, 126.81 (Caromat), 86.38 [C(H)
COAl], 29.52, 29.46 [C (CH3)3], 21.66 (CH3), 15.55, 14.58
[broad, C (CH3)3] ppm.
1
unreacted β-keto sulfone. H NMR (CDCl3) (Figure SI4)
δ: 7.86–7.79 (m, 4H, Haromat), 7.49–7.35 (m, 5H, Haromat),
5.71 [s, 1H, C(H) = C], 2.45 (s, 3H, CH3), 0.99 [s, 9H, C
(CH3)3], 0.93 [s, 9H, C (CH3)3]. 13C NMR (CDCl3)
(Figure SI5) δ: 173.56 [C(H)COAl], 145.30, 137.22,
136.02, 131.81, 129.96, 128.53, 126.74, 126.46 (Caromat),
87.41 [C(H)COAl], 29.52, 29.46 [C (CH3)3], 21.66 (CH3),
15.55, 14.58 [broad, C (CH3)3] ppm.
For comparison: 1H NMR of 2-oxo-2-phenylethyl-
4-chlorophenylsulfone (CDCl3) (Figure SI13) δ: 7.93
(m, 2H, Haromat), 7.83 (m, 2H, Haromat), 7.64 (m, 1H,
For comparison: 1H NMR of 2-oxo-2-phenylethyl-
4-methylphenylsulfone (CDCl3) (Figure SI6) δ: 7.95
(m, 2H, Haromat), 7.77 (m, 2H, Haromat), 7.63 (m, 1H,
Haromat), 7.53–7.47 (m, 4H, Haromat), 4.75 (s, 2H,
CH2C=O). 13C NMR (CDCl3) (Figure SI14) δ: 187,87
(C=O), 141.08, 136.93, 135.46, 134,54, 130.14, 129.49,
129.21, 128.91 (Caromat), 63.23 [C(H) = CO] ppm.
H
aromat), 7.49 (m, 2H, Haromat), 7.34 (m, 2H, Haromat), 4.72

WOJCIECHOWSKI ET AL.
5 of 9
3 | RESULTS
The t-Bu3AlÁOEt2 was synthesized using AlCl3 and
t-BuMgCl in a 1:3.5 molar ratio of reagents. After removal
of the solvent, volatile reaction products containing
t-Bu3AlÁOEt2 were distilled from the reaction mixture.
They were then used in the reactions with β-keto
sulfones [2-oxo-2-phenylethyl-4-methylphenyl sulfone
(A sulfone) and 2-oxo-2-phenylethyl-4-chlorophenyl
sulfone (B sulfone)] (Scheme 3).
In the postreaction mixtures, besides di-t-butyl
aluminum complexes with β-keto sulfones (2 and 4),
unexpected di-t-butyl complexes with β-hydroxy sulfones
(1 and 3) were also present.
A careful analysis
of the starting aluminum reagents showed that
actually t-Bu3AlÁOEt2 contains a certain amount of
t-Bu2AlHÁOEt2, which is responsible for the reduction of
β-keto sulfones to β-hydroxy sulfones.
FIGURE 1 Thermal ellipsoid plot (50% probability) of
compound 1. Hydrogen atoms have been omitted for the sake of
clarity. Selected bond and distances (Å) and angles (ꢀ):
S(1)–O(2) 1.431(1), S(1)–O(1) 1.481(1), S(1)–C(15) 1.751(1),
S(1)–C(23) 1.771(1), Al(2)–O(3) 1.746(1), Al(2)–O(1) 1.908(1), O(3)–
C(22) 1.387(2), C(22)–C(23) 1.551(2), O(2)–S(1)–O(1) 115.97(6),
O(2)–S(1)–C(15) 110.38(6), O(1)–S(1)–C(15) 105.03(6), O(2)–S(1)–
C23 110.36(6), O(1)–S(1)–C(23) 106.48(6), C(15)–
S(1)–C(23) 108.23(6), O(3)–Al(2)–O(1) 97.00(4),
S(1)–O(1)–Al(2) 126.82(6), C(22)–O(3)–Al(2) 133.06(8), O(3)–
C(22)–C(23) 111.2(1), C(22)–C(23)–S(1) 110.80(9)
Compounds 1 and 3 were isolated as crystals suitable
for X-ray measurements that allowed determination of
their molecular structures. Details of data collection
and structure analysis are summarized in Table SI1.
Molecular structures of compounds 1 and 3 are shown in
Figures 1 and 2. The monomeric molecules consist of
β-hydroxy sulfone ligand and the t-Bu2Al unit. The
tetra-coordinate aluminum atom is bonded to two
carbon atoms and two oxygen atoms, one of
which is derived from the sulfonyl group. The
bonds Al(2)–O(3) 1.746(1) (compound 1) and
Al(2)–O(2) 1.747(2) (compound 3) Å are typical for
alkoxy aluminum compounds,[14,15] whereas the
bond lengths Al(2)–O(1) 1.908(1) (compound 1) and
Al(2)–O(1) 1.915(2) (compound 3) Å indicate a coordina-
tion character of the bonds between the aluminum atom
and oxygen atom of the sulfonyl group. The six-
membered central ring AlO2SC2 has a distorted chair-like
configuration (Figure SI15). The presence of single
C–C bonds [C(22)–C(23) 1.551(2)
Å
for
1
and
FIGURE 2 Thermal ellipsoid plot (50% probability) of
compound 3. Hydrogen atoms have been omitted for the sake of
clarity. Selected bond and distances (Å) and angles (ꢀ):
S(1)–O(3) 1.432(2), S(1)–O(1) 1.479(2), S(1)–C(14) 1.766(2), Al(2)–
O(2) 1.747(2), Al(2)–O(1) 1.915(2), O(2)–C(13) 1.384(3), C(13)–
C(14) 1.551(3), O(3)–S(1)–O(1) 116.3(1), O(3)–S(1)–C910 110.1(1),
O(1)–S(1)–C(1) 104.8(1), O(3)–S(1)–C(14) 110.4(1), O(1)–S(1)–
C(14) 106.7(1), C(1)–S(1)–C(14) 108.2(1), O(2)–Al(2)–O(1) 96.71(7),
S(1)–O(1)–Al(2) 126.4(1), C(13)–O(2)–Al(2) 133.9(1), O(2)–C(13)–
C(14) 111.1(2), C(13)–C(14)–S(1) 110.9(2)
SCHEME 3 Reactions of mixtures t-Bu3AlÁOEt2 and
t-Bu2AlHÁOEt2 with β-keto sulfones A and B

6 of 9
WOJCIECHOWSKI ET AL.
C(13)–C(14) 1.551(3) Å for 3] indicates a reduction in the
starting β-keto sulfones.
4 (at 5.71 ppm) is a better indicator of the molar ratio of
the starting t-butylaluminum compounds.
Besides proton signals of t-Bu groups and aromatic
protons, H NMR spectrum of the postreaction mixture
Analysis of the structure of compounds 1–4 showed a
chiral environment of the sulfur atom; however, the split-
ting of the t-Bu group resonances in the NMR spectra
was not observed. There is probably a rapid exchange of
oxygen atoms from the SO2 group to form a coordination
bond with the aluminum atom.
Compounds 1 and 3 are formed as a result of the
hydroalumination of keto-tautomers of β-keto sulfones
with t-Bu2AlHÁOEt2 (Scheme 4).
1
reveals characteristic signals for compounds 1 [multiplet
at 5.95 ppm of CH2C(H)O proton, multiplet at 3.38 ppm
of CH2C(H)O protons, and singlet at 2.50 ppm of CH3
protons] and 2 [singlet at 5.72 ppm of C(H) = C
proton and singlet at 2.45 ppm of CH3 protons;
Figure SI2b]. Based on the integration of the signals at
5.95 and 5.72 ppm, the molar ratio of compounds 1 and
2 was 1:1. Similarly, in the postreaction mixture of 3 and
4 the integration ratio of the proton signals at 5.94 ppm
[CH2C(H)O] and 5.72 ppm [C(H) = C] showed a molar
ratio of 1.4:1 for compounds 3 and 4 (Figure SI8b).
The hydroalumination mechanism is based on the
assumption that there are partial positive charges on the
aluminum and C=O carbon atoms, and partial negative
charges are located on the Al–H hydrogen and C=O
oxygen atoms. This charge distribution allows the forma-
tion of an intermediate state in which the C=O oxygen
atom combines with the aluminum atom, whereas the
Al–H hydrogen atom is transferred to the C=O carbon
atom. This reaction is favored by the fact that the
tautomeric equilibrium in β-keto sulfones is almost
completely shifted toward the keto-tautomer. In addition,
the presence of aromatic rings with a strong electron-
withdrawing effect causes an increase in the polarization
of the C=O bond, which has a positive effect on the
hydroalumination process.
Because of two strongly electron-withdrawing moie-
ties (carbonyl and sulfonyl groups), β-keto sulfones are
fairly strong CH acids, the acidity of which is comparable
with that of β-keto-esters.[31] Therefore, in the reaction
between t-Bu3Al and β-keto sulfones, the acidic proton of
the CH2 group can interact with the carbon atom of the
t-BuAl group to form a gaseous C4H10 product, which
leaves the reaction mixture (Scheme 5). By contrast,
although the enol tautomer is present in the minority, it
can also react with t-Bu3Al. In this mechanism, the O–H
oxygen atom interacts with aluminum atom, while O–H
hydrogen atom is transferred to the Al–C carbon atom of
t-Bu group.
1
The H NMR spectra of the pure compounds 1 and
3 revealed two singlets of protons (1.09 and 1.02 ppm for
1, and 1.09 and 1.02 ppm for 3) assigned to the non-
equivalent t-Bu groups, which was in agreement with
their molecular structures.
Reactions of β-keto sulfones with t-Bu3AlÁOEt2 con-
taining only traces of t-Bu2AlHÁOEt2 have proceeded with
the formation of almost pure compounds 2 and 4 as thick
liquids. Actually, the hydroalumination products 1 and
3 were not observed in the postreaction mixture, which
excludes the formation of Al–H groups by butene elimi-
nation from t-BuAl moieties. 1H NMR spectra of the com-
pounds reveal a singlet of the C(H) = C proton
(at 5.71 ppm for 2 and for 4) characteristic for β-keto sul-
fone ligands (Figures SI4 and SI11). Similar to com-
pounds 1 and 3, there are two non-equivalent t-Bu
groups in 2 and 4, as evidenced by the occurrence of two
singlets at 0.99 and 0.93 ppm for 2, and 0.99 and
0.92 ppm for 4. An integration ratio of signals of aromatic
protons, C(H) = C and t-Bu protons (9:1:18), indicates
one β-keto sulfone ligand per one t-Bu2Al unit.
It should be noted that the molar ratios of the prod-
ucts 1 to 2 and 3 to 4 reflect the ratios t-Bu3AlÁOEt2:
t-Bu2AlHÁOEt2 found for the starting materials. The ratio
t-Bu3AlÁOEt2:t-Bu2AlHÁOEt2 determined based on the
integration of t-Bu group proton signals is burdened with
a large error due to the proximity of both signals. There-
fore, the ratio of the proton signals of the CH(O) group of
compounds 1 and 3 (5.95 and 5.94 ppm) to the proton
signals of the C(H) = C group of compounds 2 and
Considering the impact of the presence of by-products
in t-Bu3AlÁOEt2 on the structure of products formed in
the reaction with β-keto sulfones, we focused our atten-
tion on the synthesis of t-Bu3AlÁOEt2. For the last two
decades, we have synthesized t-Bu3AlÁOEt2 using the
reaction of AlCl3 with t-BuMgCl in 1:3 to 1:4 molar
SCHEME 4 Proposed hydroalumination mechanism of β-keto sulfones

WOJCIECHOWSKI ET AL.
7 of 9
SCHEME 5 Proposed mechanism of formation of di-t-butyl aluminum β-keto sulfone complexes 2 and 4
ratios. Usually, a solution of anhydrous, freshly sublimed
AlCl3 in diethyl ether is added dropwise to the diethyl
ether solution of t-BuMgCl at room temperature. Rapid
addition of the reagent increases the temperature of
the mixture until the boiling of the solvent. To increase
reaction efficiency, the postreaction mixture is sometimes
refluxed for 30 min. After removal of the solvent under
reduced pressure, the resulting gray solid was heated
to 140–220 ꢀC under vacuum of 1.33 Pa to obtain
t-Bu3AlÁOEt2 as a thick liquid.
decomposition to R3Al compounds.[32,33] As an example,
we can show the thermal decomposition of the
ionic compound [Et4Al]−[BrMg (thf)5]+(thf), which is
decomposed quantitatively into Et3Al(thf). It is very
likely that t-Bu3AlÁOEt2 formed in the reaction of AlCl3
with t-BuMgCl reacts with an excess of t-BuMgCl to give
an “ate” complex, which then decomposes into
t-Bu2AlHÁOEt2. This explains the appearance of
t-Bu2AlHÁOEt2 in the postreaction mixture.
Concentration of reagents, reaction temperature,
and distillation temperature of t-Bu3AlÁOEt2 in the
postreaction mixture primarily affect the reaction effi-
ciency. The degree of AlCl3 conversion increased from
25% for reactions performed at temperatures close to
room temperature (Table 1, run 2) to 48% when the reac-
tion mixture was heated to boiling (Table 1, run 4). By
contrast, the selectivity of the t-Bu3AlÁOEt2 production
decreases with increasing temperature, which is observed
in the 1H NMR spectrum as an increase in signal integra-
tion at 0.96 ppm attributed to the t-Bu group protons of
the t-Bu2AlHÁOEt2 by-product. In the Figure SI1 we
The most important factor affecting the selectivity of
the reaction toward the production of t-Bu3AlÁOEt2 is the
ratio of the reagents. For a AlCl3:t-BuMgCl molar ratio of
1: 3, almost pure t-Bu3AlÁOEt2 was obtained (Table 1, run
1
1, H NMR spectrum in Figure SI15), whereas for the
reaction of 1 mol of AlCl3 with 4 mol of t-BuMgCl leads
to a mixture containing t-Bu2AlHÁOEt2 as the main prod-
1
uct (Table 1, run 5, H NMR spectrum in Figure SI16).
Our recent results and literature data demonstrate
that aluminum trialkyls react with alkylmagnesium
halides to give “ate” compounds that undergo thermal
TABLE 1 Impact of reaction conditions on the reaction between AlCl3 and t-BuMgCl and the impact of product isolation method on
product compositions
Molar ratio of
AlCl3:t-BuMgCl
Temperature of
product isolation (ꢀC)
Molar ratio of
t-Bu3Al:t-Bu2AlH[c]
Conversion of
AlCl3 (%)
Run Reagents[a],[b]
1.
2.
3.
4.
0.5 M solution of AlCl3, 1 M
solution of t-BuMgCl, RT
1:3
140–150
140–150
150–200
150–200
0.95:0.05
0.85:0.15
0.65:0.35
0.38:0.62
35
25
40
48
0.5 M solution of AlCl3, 1.5 M
solution of t-BuMgCl, RT
1:3.5
1:3.5
1:3.5
1 M solution of AlCl3, 1.5 M
solution of t-BuMgCl, 25–30 ꢀC
1 M solution of AlCl3, 1.5 M
solution of t-BuMgCl, 30 min
under reflux
5.
0.5 M solution of AlCl3, 1 M
1:4
140–150
0.23:0.77
27.4
solution of t-BuMgCl, RT
RT, room temperature.
aSolvent: diethyl ether.
bThe solvent was distilled off from the postreaction mixture under reduced pressure. Then the product was isolated under 1.33 Pa pressure
and higher temperature.
cOn the basis of the signals at 0.97 and 0.96 ppm integrations.

8 of 9
WOJCIECHOWSKI ET AL.
present the 1H NMR spectrum of the product
consisting of 85 mol% of t-Bu3AlÁOEt2 and 15 mol% of
t-Bu2AlHÁOEt2. The product was obtained in the reaction
of 1.5 M solution of t-BuMgCl with 0.5 M solution of AlCl3
in diethyl ether at 20–25 ꢀC at a t-BuMgCl-to-AlCl3 molar
ratio of 3.5:1 and isolated by distillation from the post-
presence of CH2 acidic hydrogen atoms, tri-t-butyl alumi-
num can also react with keto-tautomers according to the
proposed mechanism (Scheme 5).
AUTHOR CONTRIBUTIONS
ꢀ
reaction mixture under vacuum at 140–150 C (Table 1,
run 2). Additional heating of the reaction mixture
under reflux for 30 min resulted in an increase in the
AlCl3 conversion but the content of the by-product
t-Bu2AlHÁOEt2 increased drastically to 62 mol%.
TW synthesized and characterized by NMR spectroscopy
compounds 1–4; ZO synthesized β-keto sulfones; PS
measured crystal structures of 1 and 3 compounds; ŁD
supervised X-ray measurements; WZ designed the
experiment and supervised the whole research, collected,
and analyzed the obtained results as well as prepared
the manuscript.
4 | CONCLUSIONS
Tri-t-butyl aluminum is a very important compound in
the aluminum chemistry. The presence of sterically hin-
dered groups often makes it possible to obtain intermedi-
ate products in reactions with organic compounds, which
allows the reaction pathways to be determined. Unfortu-
nately, the synthesis of t-Bu3AlÁOEt2 is difficult because
reactions of AlCl3 with both t-BuLi and t-BuMgCl pro-
ceed to form by-products, of which t-Bu2AlH is the main
one. The presence of the t-Bu2AlH by-product can help
one to determine the composition of the reaction prod-
ucts with organic ligands, which we have shown in the
reaction with β-keto sulfones. In the reactions of β-keto
sulfones with t-Bu3Al, in addition to the expected com-
plexes 2 and 4 of di-t-butyl aluminum with β-keto sul-
fones, complexes of di-t-butyl aluminum with β-hydroxy
sulfones 1 and 3 were formed. They were the result of the
hydroalumination reaction between the β-keto sulfone
ligands and t-Bu2AlH. This fact prompted us to re-
examine the conditions of t-Bu3Al synthesis to control
this process. We found that the use of a 1:3 molar ratio of
AlCl3 and t-BuMgCl, mild conditions such as low solu-
tion concentrations of AlCl3 and t-BuMgCl, reaction tem-
perature close to room temperature, and careful isolation
of solvent from the postreaction mixture favor the forma-
tion of t-Bu3AlÁOEt2. The use of a t-BuMgCl excess proba-
bly led to the formation of “ate” compounds, which
during the distillation of t-Bu3AlÁOEt2 from the reaction
mixture undergoes decomposition into t-Bu2AlHÁOEt2.
In the presence of t-Bu2AlHÁOEt2, β-keto sulfones
undergo hydroalumination according to the proposed
mechanism, in which the keto-tautomers interact with t-
Bu2AlH. As a result, di-t-butyl aluminum complexes with
β-hydroxy sulfones are formed, which can be considered
as intermediate products in the hydrogenation reaction
of β-keto sulfones to β-hydroxy sulfones. Although β-keto
sulfones contain small amounts of enol-tautomers,
t-Bu3Al can react with the OH groups, which will cause a
shift of the tautomeric equilibrium. Because of the
ACKNOWLEDGEMENTS
The authors are grateful for financial support from the
Warsaw University of Technology, Warsaw, Poland.
The X-ray structures were determined in the Advanced
Crystal Engineering Laboratory (aceLAB) at the Chemis-
try Department of the University of Warsaw, Poland.
AUTHOR CONTRIBUTIONS
Tomasz Wojciechowski: Investigation. Zbigniew
Ochal: Investigation. Pawel Socha: Investigation.
Lukasz Dobrzycki: Supervision. Wanda Ziemkowska:
Conceptualization; supervision.
CONFLICT OF INTEREST
The authors declare no competing financial interest.
ORCID
Wanda Ziemkowska https://orcid.org/0000-0003-0932-
6384
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How to cite this article: Wojciechowski T,
Ochal Z, Socha P, Dobrzycki Ł, Ziemkowska W.
Reactions of β-keto sulfones with t-butyl
aluminum compounds: Reinvestigation of tri-t-
butyl aluminum synthesis. Appl Organomet Chem.
2020;e5961. https://doi.org/10.1002/aoc.5961
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USA 2013.