Synthesis of trialkylaluminum derivatives by the reaction of non-solvated aluminum hydride with α-olefins

Article abstract of DOI:10.1023/A:1024765426527

Hydroalumination of α-olefins by non-solvated polymeric aluminum hydride (AlH₃)_n occurs at 120-140°C. Mechanochemical activation accelerates this reaction. The addition of catalytic amounts of the prepared R₃Al forms to the reaction system decreases the temperature of the process to 90-100°C. The greatest initiation effect is observed when ate-complexes of the MAlR₄ type (M = Li, Na) are used: the reaction occurs with a higher rate already at 60-90°C affording R₃Al free of admixtures of carbalumination products and dimers of α-olefins.

Full text of DOI:10.1023/A:1024765426527

Russian Chemical Bulletin, International Edition, Vol. 52, No. 5, pp. 1161—1163, May, 2003
1161
Synthesis of trialkylaluminum derivatives by the reaction
of nonꢀsolvated aluminum hydride with αꢀolefins
V. V. Gavrilenko
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085. Eꢀmail: bre@ineos.ac.ru
Hydroalumination of αꢀolefins by nonꢀsolvated polymeric aluminum hydride (AlH3)n
occurs at 120—140 °C. Mechanochemical activation accelerates this reaction. The addition of
catalytic amounts of the prepared R3Al forms to the reaction system decreases the temperature
of the process to 90—100 °C. The greatest initiation effect is observed when ateꢀcomplexes of
the MAlR4 type (M = Li, Na) are used: the reaction occurs with a higher rate already at
60—90 °C affording R3Al free of admixtures of carbalumination products and dimers of
αꢀolefins.
Key words: trialkylaluminum derivatives, synthesis; hydroalumination of αꢀolefins, mechaꢀ
nochemical activation, catalytic activation.
Scheme 1
Hydroalumination of αꢀolefins is one of the most imꢀ
portant reactions in the synthesis of organoaluminum
compounds. Lithium aluminum hydride and aluminum
hydride (AH) in the form of the (AlH3)3•OEt2 solvate
have first been prepared by Schlesinger.1 Later2 the reacꢀ
tion of these hydrides with αꢀolefins at 100—110 °C was
studied. This reaction affords organoaluminum comꢀ
pounds of the LiAlR4 and LiR3AlH type (for βꢀbranched
olefins). The use of the AH solvate gives a mixture of R3Al
and R3Al•OEt2 (2 : 1). It is known that the elimination of
Et2O from the R3Al etherate poses a certain problem,
which impedes the synthesis of individual trialkylaluꢀ
minum derivatives.
(AlH3)n + 3 H2C=CHR
n (RCH2CH2)3Al
R = nꢀC6H13, nꢀC5H11, Bun, nꢀC7H15
Mechanical activation accelerates the process.
Further it turned out that the prepared R3Al forms
added to the reaction mixture (AH + olefin) decreased
the temperature of the process to 100—110 °C and even
to 80—90 °C under mechanochemical activation.6,7 The
role of the R3Al additives in this reaction is the destrucꢀ
tion of the crystalline lattice of AH and alkylꢀhydride
exchange according to Scheme 2.
Nonꢀsolvated AH in the form of a solid polymeric
substance was prepared3 in 1976 by crystallization from a
mixed solvent (ether—benzene) in the presence of addiꢀ
tives (LiAlH4 and LiBH4, 5 mol.% each). Aluminum hyꢀ
dride is insoluble in many inorganic and most part of
organic solvents and is inert toward αꢀolefins under stanꢀ
dard conditions. In this work, we studied the reaction of
αꢀolefins with AH to develop preparative methods for the
synthesis of R3Al.
Scheme 2
We found that heating of AH to 150—200 °C in a
sealed ampule resulted in its decomposition and formaꢀ
tion of the aluminum mirror. The corresponding R3Al
were observed in the presence of olefins (Scheme 1). The
noncatalytic addition of AH to olefins begins in any case
at 120—140 °C simultaneously with the fast thermal deꢀ
composition of AH. This is confirmed by the kinetic studꢀ
ies of the thermal decomposition of AH 4 and analysis of
the mass spectra of its thermolysis products containing the
The R2AlH formed in this reaction readily reacts with
αꢀolefins already at 60—100 °C. Using the reactions of
AH with isobutylene, hexꢀ1ꢀene, and decꢀ1ꢀene6,7 as exꢀ
amples, the corresponding R3Al were obtained from heptꢀ
1ꢀene, octꢀ1ꢀene, and nonꢀ1ꢀene. Side processes can ocꢀ
cur at the reaction temperature >100 °C: elimination of
αꢀolefins and their dimerization.
Another route of activation of the reaction of AH with
olefins, which simultaneously decreases the temperature
of the process, is associated with the use of catalytic addiꢀ
+
Al+, AlH+, AlH2+, AlH3+, AlH4+, and Al2H6 species.5
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1100—1102, May, 2003.
1066ꢀ5285/03/5205ꢀ1161 $25.00 © 2003 Plenum Publishing Corporation

1162
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
V. V. Gavrilenko
LiAlH4 used for the synthesis of AlH3 was recrystallized from
an ether—toluene mixture. Technical AlCl3 (purity 98—99%)
was purified by heating to 180 °C with addition of 10—15% Al
powder (trade mark Аꢀ1) and 1—8% Mg powder Mg (MPFꢀ4)
and sublimation in vacuo. Aluminum hydride was synthesized
according to a known procedure3 (purity of the product was
95—98%); NaAlEt4 was prepared by the reaction of Et3Al with
metallic Na 8 and recrystallized from toluene. Aluminum was
analyzed by complexometric titration (using eriochrome as an
indicator9) and gravimetrically (precipitation with hydroxyꢀ
quinoline). Hydride hydrogen was determined by the gas voluꢀ
metric method. Purity of olefins and paraffins after R3Al hydrolyꢀ
sis was monitored by GLC on an LKhMꢀ8MD instrument
(2 m × 4 mm, 5% SEꢀ30 on Chromaton (acid washed) with a
particle size of 0.2 mm).
tives of the MAlR4ꢀtype complexes. They are easily synꢀ
thesized by the reaction of alkaline metals with R3Al. In
fact, it turned out that in the presence of NaAlEt4
(5 mol.%) AH readily reacts with hexꢀ1ꢀene already
at 60—70 °C. This can be explained by the fact that
NaAlEt4 reacts with AH to form Et2AlH and NaEt2AlH2
(Scheme 3) likely through the intermediate complex
NaAlEt4•AlH3.
Scheme 3
(AlH3)n + NaAlEt4
(AlH3)n–1 + NaEt2AlH2 + Et2AlH
The syntheses of Bui3Al, (C6H13)3Al and (C10H21)3Al from
(AlH3)n and olefins have been described previously.7
The products of this reaction (see Scheme 3) are furꢀ
ther added to αꢀolefins to form NaAlR4 and R3Al. Then
the cycle repeats but with a sharp increase in the reaction
rate, because both catalysts, viz., R3Al and NaAlR4, begin
to work. This results in the evolution of a great amount of
heat, which should be removed by cooling or controlling
the rate of olefin addition to the reaction mixture in order
to prevent the "thermal explosion." The MR3AlH comꢀ
plexes easily obtained from the corresponding metal hyꢀ
drides and R3Al can be used instead of MAlR4 for the
initiation of olefin hydroalumination. In a separate exꢀ
periment on the reaction of (AlH3)n with NaAlEt4 at
100—125 °C, we detected 64% Et3Al and 36% Et2AlH in
the liquid reaction products after they were distilled in
vacuo. The extraction of the solid residue with toluene
and THF produced NaEt2AlH2 and NaAlH4 (60 : 40),
which confirms the scheme proposed for the process.
It should be mentioned that the LiAlH4 additives
(5—10%) can be used instead of NaAlR4 as the catalyst of
the process. These additives react with αꢀolefins already
at 100—120 °C 2 to form LiAlR4, after which the reaction
smoothly ceases at 60—100 °C. Mixtures of NaAlH4 and
R3Al, which readily form a mixture of NaR2AlH2 and
R2AlH, are also efficient as the catalyst of this process.
Thus, the methods developed for the synthesis of R3Al
from nonꢀsolvated AH and αꢀolefins in the presence of
MAlR4, MR3AlH, and R3Al produce individual triꢀ
alkylaluminum derivatives free of admixtures of R2AlH,
olefin dimers, and carbalumination products. The MAlR4
catalyst, which remained in the reaction mixture, can
easily be transformed into R3Al by the addition of the
calculated amount of aluminum halides.
Tri(nꢀoctyl)aluminum. А (synthesis without promoters).
A 100ꢀmL threeꢀnecked flask equipped with a reflux condenser,
a dropping funnel, and a thermometer was loaded with (AlH3)n
(1.65 g, 0.055 mol, calculated for 100% purity) suspended in
octane (30 mL), and the mixture was heated to 120 °C. At first, a
portion of olefin (33%) was added from the dropping funnel
containing octꢀ1ꢀene (19.3 g, 0.173 mol, ∼10% excess). After
30 min, the temperature of the mixture raised to 125 °C (reflux).
Then the temperature was decreased to 100—110 °C, and the
remained olefin was added during 3—4 h. The mixture was kept
for 1 h at 90—100 °C, cooled, and filtered through a glass filter
no. 4. Octane and excess octene were distilled off in vacuo below
80 °C (7 Torr). A light liquid product was obtained (14.3 g,
91%). Found (%): Al, 7.08. C24H51Al. Calculated (%): Al, 7.36.
The obtained product (13.1 g) was oxidized with dry air in hepꢀ
tane (50 mL), hydrolyzed, dried above Na2SO4, and distilled at
82—83 °C (7 Torr). nꢀOctyl alcohol (12.0 g) was isolated,
20
20
nD 1.4292 (cf. Ref. 10: b.p. 196 °C, nD 1.4303).
B (synthesis with the activation of the process by adding
R3Al in a ball mill). A vertical ball mill11 (capacity 150 mL, 70 g
of balls 3—4 mm in diameter) was loaded with (AlH3)n (1.10 g,
0.036 mol), octane (30 mL), Bui3Al (1 g, 0.005 mol), and octꢀ
1ꢀene (19.5 g, 0.174 mol). The reaction mixture was gradually
heated to 100 °C with permanent stirring (temperature jump to
115 °C), cooled, and stored for 2 h at 90—100 °C. The balls and
blend were separated, and the residue was washed with octane
(2×20 mL). A light filtrate was evaporated at 80—90 °C (7 Torr)
to a constant weight. The target product was obtained in 88.6%
yield (16.9 g). Found (%): Al, 7.11. C24H51Al. Calculated (%):
Al, 7.36.
C (synthesis with activation by NaBui3AlH). A mixture of
(AlH3)n (1.65 g, 0.055 mol), NaBui3AlH (2.25 g, 0.013 mol),
and octane (30 mL) were heated to 80 °C for 1 h, and octꢀ1ꢀene
(9.7 g) was added. The temperature spontaneously raised to
100 °C for 20 min. The reaction mixture was cooled, and octꢀ1ꢀ
ene (15 g) was additionally added (totally 24.7 g) from a dropping
funnel at 90 °C for 0.5—1 h. A light product, viz., (C8H17)3Al +
+ NaAl(C8H17)4 mixture (27.1 g), was obtained after standard
treatment. Found (%): Al, 7.02. C24H51Al. Calculated (%):
Al, 7.36.
Experimental
All experiments were carried out in an inert atmosphere
(nitrogen or argon). Hydrocarbon solvents and ethers were dried
over hydrides (NaH, LiAlH4), distilled, and stored over Na or
Ca. Commercially available olefins were stored over anhydrous
Na2SO4 and distilled with the NaAlEt4 additive (1—5%).
D (synthesis with activation by LiAlH4). A known proceꢀ
dure3 was used to obtain γꢀAlH3 in a mixture with excess LiAlH4
(without washing off LiAlH4 with ether). This mixture (1.60 g,
1.2 g of AlH3 + 0.4 g of LiAlH4), octane (30 mL), and octꢀ1ꢀene

Hydroalumination of αꢀolefins by nonꢀsolvated (AlH3)3 Russ.Chem.Bull., Int.Ed., Vol. 52, No. 5, May, 2003
1163
(8 g) were used for the reaction. The reaction mixture was heated
to 100—110 °C for 1 h, and octꢀ1ꢀene (12 g) was added for 2 h at
90—100 °C. Then the mixture was filtered. Excess octene and
octane were removed in vacuo at 60—70 °C (1 Torr), and AlCl3
(0.45 g) was added to the residue. The mixture was heated at
80 °C, and LiCl was filtered off. Compound (C8H17)3Al was
obtained in 89% (14.3 g). Found (%): Al, 7.12. C24H51Al. Calꢀ
culated (%): Al, 7.36.
distilled off in vacuo (1 Torr) at 70—80 °C. Their composition
after hydrolysis and determination of the amounts of ethane and
hydrogen (7.5 : 1.0) corresponded to the ratio Et3Al : Et2AlH =
1.76 : 1.00. The extraction of a solid residue with toluene gave
NaEt2AlH2 (3.1 g), m.p. 83—85 °C (cf. Ref. 12: 84—88 °C) and
then, after the treatment with THF, NaAlH4 was extracted (2 g).
Found (%): H, 7.25. NaAlH4. Calculated (%): H, 7.41.
Tri(nꢀheptyl)aluminum. A ball mill (150 mL, 70 g of balls
3—4 mm in diameter) was loaded with (AlH3)n (1.65 g,
0.055 mol), heptane (25 mL), heptꢀ1ꢀene (18.0 g, 0.183 mol),
and Bui3Al (1 g, 0.005 mol), and the mixture was stirred for 3 h
at 90—100 °C. The reaction product was filtered off, and a preꢀ
cipitate was washed with heptane (10 mL). Heptane and excess
heptene were removed in vacuo. A light liquid product (16.1 g,
91.0%) was obtained. Found (%): Al, 8.18. C21H45Al. Calcuꢀ
lated (%): Al, 8.32. This product (14.0 g) in heptane (30 mL)
was oxidized with dry air followed by hydrolysis to afford nꢀheptyl
alcohol (14.3 g), b.p. 174—175 °C, nD20 1.4223 (cf. Ref. 10: b.p.
References
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Bunbury, Chapman and Hall, London, 1946.
20
175 °C, nD 1.4231).
Tri(nꢀhexyl)aluminum. A mixture of AlH3 (1.65 g, 0.055 mol),
toluene (10 mL), NaAlEt4 (0.98 g), and hexꢀ1ꢀene (16.8 g,
0.21 mol) was heated in a flask equipped with a reflux condenser
and a thermometer at the reflux temperature of the mixture.
After 20 min, the temperature raised from 80 to 100 °C and then
gradually decreased to 70—80 °C. After 2 h, the reaction mixꢀ
ture was cooled and filtered off, and a precipitate was washed
with hexane (10 mL). The solvents and excess hexene were
removed in vacuo at 60—80 °C (7 Torr), and 15.1 g of the prodꢀ
uct were obtained. Found (%): Al, 9.41. C18H39Al. Calcuꢀ
lated (%): Al, 9.55.
Tri(nꢀnonyl)aluminum. A mixture of (AlH3)n (1.53 g,
0.051 mol), NaBui2AlH2 (1.7 g, 0.01 mol), and nonꢀ1ꢀene
(32.0 g, 0.253 mol) was heated in a ball mill to 100 °C and then
stored for 1.5 h at 90—100 °C. After separation of the balls and
filtration, residues of nonene were removed in vacuo. A transꢀ
parent product (18.5 g) was obtained. Powdered AlCl3 (0.35 g,
0.026 mol) was added to the product, and the mixture was heated
with stirring for 1.5 h at 80—90 °C. The formed NaCl was sepaꢀ
rated by filtration. The probe of the hydrolysis product for haloꢀ
gen (AgNO3) was negative. Found (%): Al, 6.22. C27H57Al.
Calculated (%): Al, 6.61.
11. H. Clasen, Angew. Chem., 1961, 73, 322.
12. N. M. Alpatova, V. V. Gavrilenko, Yu. M. Kessler, O. R.
Osipov, and D. N. Maslin, Kompleksy metalloorganicheskikh,
gidridnykh i galoidnykh soedinenii alyuminiya [Complexes of
Organometallic Aluminum Hydrides and Halides], Nauka,
Moscow, 1970, 297 pp. (in Russian).
Reaction of (AlH3)n with NaAlEt4. A threeꢀnecked flask was
loaded with (AlH3)n (3.1 g) and NaAlEt4 (9.8 g). The mixture
was heated to 125 °C for 1 h (to the homogeneous state) and
then stored for 2 h at 100 °C. Liquid products (5.8 g) were
Received October 7, 2002;
in revised form January 22, 2003