Organic compounds -- part of the class 532-570 series – Organic compounds – Aluminum containing
Reexamination Certificate
2000-01-12
2001-02-20
Nazario-Gonzalez, Porfirio (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Aluminum containing
C556S187000
Reexamination Certificate
active
06191295
ABSTRACT:
DESCRIPTION
This invention relates to a stable trimeric isopropoxyalane, to a method of producing the stable trimeric isopropoxyalane, and to the use thereof.
Aluminium hydride (AlH
3
, alane) and complex metal aluminium hydrides (alanates) are commonly used, strong reducing agents. They are therefore used for reducing carbonyl compounds, esters and nitro compounds. The high reactivity with respect to most functional groups, which is desired on the one hand, in some cases leads to problems due to the lack of selectivity of the reducing agents on the other hand. This is the case for instance when one molecule includes several reducible functional groups, or when the reducing agents not only act as hydrogen donor, but also as base. The partial replacement of the hydrogen in the aluminium hydride and/or in the complex metal aluminium hydrides by suitable substituents leads to a variation of the reduction properties and the basicity of the reducing agents. There is mostly employed the introduction of alkoxy substituents into aluminium hydride and/or into the complex metal aluminium hydrides. For instance, the reactivity of the hydrogen in the lithium-tri(t-butoxy)aluminium hydride is diminished due to the steric shielding to such an extent that under certain conditions carboxylic acid derivatives are only reduced to the aldehyde.
A plurality of alkoxy-substituted aluminium hydrides of the type MH
x
Al(OR)
4−x
and H
y
Al(OR)
3−y
(x=1-3, y=1-2, R=organic residue) are known. Of these, merely NaH
2
Al(OCH
2
CH
2
OCH
3
)
2
is of economic importance. This alanate, however, has the disadvantage of a relatively low hydride-hydrogen content (<1 wt-%), which leads to the fact that when using this alanate for reduction reactions a relatively large amount is required and a correspondingly large amount of waste products is obtained. Above all, however, the byproduct methoxyethanol, which is necessarily produced in the hydrolysis of this alanate, is extremely toxic.
The mono- and dialkoxyalanes produced from unbranched alcohols are completely insoluble in aprotic solvents due to their polymeric structure and can therefore not be used for reduction purposes. The tendency to polymerisation can be suppressed in that the steric space filling of the alkoxy residue is increased. For instance, the mono- and di-tert-butoxyalanes are very easily soluble in the commonly used organic solvents. Tert-butoxyalanes can, however, not economically be produced in every case, as the aluminium alkoxide required for the reaction in accordance with the equation (3−x)Al(OtBu)
3
+xAlH
3
→3H
x
Al(OtBu)
3−x
, (x=1,2) is difficult to prepare and/or not commercially available.
From DE-OS 195 29 241 alkoxyalanes of the general formula Al(OR)
a
H
b
are known, where R is an alkyl residue with 3 to 10 C atoms or a cycloalkyl residue with 5 to 8 C atoms, where the residues R can be the same or different, where a is 1 or 2, b is 1 or 2, and the sum a+b=3. From the reference it can be taken that the alkoxyalanes described there are present in dimeric form. Corresponding to the reference, the known dimeric alkoxyalanes can also include propyl and/or isopropyl residues as residues R. For producing the known alkoxyalanes, the DE-OS 195 29 241 proposes a method wherein AlH
3
is reacted with an alcohol ROH in the presence of an inert organic solvent, where the molar ratio is either substantially 1:1 or substantially 1:2. According to this method, there is then formed either H
2
AlOR or HAl(OR)
2
, where these alkoxyalanes are present as dimers. The alkoxyalanes known from DE-OS 195 29 241 should be used for producing optically variable systems.
Isopropoxyalanes are known for about 30 years. They can be produced by reacting aluminium hydride (AlH
3
) with isopropanol in a molar ratio of 2:1 or 1:1, or by reacting AlH
3
with aluminium isopropoxide. As regards the properties of the isopropoxy-substituted alanes contradictory statements were made, as depending on the conditions of preparation different oligomers are quite obviously obtained. The different statements concerning the properties of the isopropoxy-substituted alanes probably result from the fact that the known isopropoxyalanes of the stoichiometry H
2
Al(OiPr) and HAl(OiPr)
2
have no dissociative stability and/or are not pure. By means of so far not published examinations it was demonstrated that these two compounds are capable of disproportionation. The disproportionation reactions lead to a change in the product composition due to precipitation and consecutive reactions, so that reduction-active compounds of very different reactivity are present at the same time, which prevents a high chemoselectivity. Therefore, the two aforementioned compounds can not even be produced and supplied in the form of stable, salable solutions.
The publication of Nöth and Suchy, Zeitschrift für anorganische und allgemeine Chemie, Vol. 358, 1968, pages 44 to 66, describes the reactions of AlH
3
with isopropanol in a molar ratio of 1:1 and 1:2. The publication comes to the conclusion that the reaction of AlH
3
with isopropanol in a molar ratio of 1:1 does either not take place quantitatively, so that unreacted AlH
3
remains in the reaction solution and the H
2
AlOiPr formed furthermore reacts with isopropanol to form HAl(OiPr)
2
, or that the H
2
AlOiPr develops the following equilibrium:
2H
2
AlOiPr&rlarr2;HAl(OiPr)
2
+AlH
3
From the reaction solution, the dimeric isopropoxyalane (H
2
AlOiPr)
2
could, however, be isolated. According to the publication, during the reaction of AlH
3
with isopropanol in a molar ratio of 1:2 the HAl(OiPr)
2
is formed, which is in equilibrium with the mono- and triisopropoxyalane, which, however, promotes the formation of the diisopropoxyalane. 2HAl(OiPr)
2
&rlarr2;Al(OiPr)
3
+H
2
AlOiPr The diisopropoxyalane is dissolved in benzene for instance in a trimeric form and therefore has the composition H
3
Al
3
(OiPr)
6
. However, the aforementioned disproportionation equilibrium develops slowly in the solution of the trimeric diisopropoxyalane, so that the composition of the solution is changed in a disadvantageous way.
It is the object underlying the invention to create a chemoselective, stable isopropoxyalane, which is easily soluble in commonly used solvents, whose solutions are stable in storage, which has an increased hydride content as compared to available alkoxyalanes, which forms no toxic byproducts during hydrolysis, and which can be prepared from inexpensive as well as available raw materials.
The object underlying the invention is solved by creating the stable trimeric isopropoxyalane of the composition H
5
Al
3
(OiPr)
4
, where iPr is a (CH
3
)
2
CH residue. It was surprisingly found out that this trimeric form of the isopropoxyalane has a high stability; it melts at about 60° C., can be sublimated at 40° C./0.5 mbar, be prepared in pure form by sublimation and be characterized unambiguously by means of spectroscopy (
27
Al-NMR, C
6
D
6
:&dgr;=125 ppm, 60 ppm;
1
H-NMR, C
6
D
6
:&dgr;=1.26 ppm, d24H and 4.18 ppm, sep 4H). The stable trimeric isopropoxyalane has the structural formula
The molar mass corresponds to the structural formula. In contrast to the alanes H
2
AlOiPr and HAl(OiPr)
2
, the trimeric isopropoxyalane in accordance with the invention does not disproportionate, but it is very stable and can therefore be stored in solid or in particular in dissolved form also for an extended period and is therefore available as a salable reducing agent.
In accordance with the invention it is furthermore provided that the stable trimeric isopropoxyalane is dissolved in a solvent or solvent mixture in a concentration of 20 to 80 wt-%, where as solvent there are used ethers, tertiary amines, alkyl phosphates, aromatic hydrocarbons, saturated aliphatic hydrocarbons or mixtures of these solvents. In dissolved form, the trimeric isopropoxyalane is very stable, and therefore these solutions are particularly suited as salable product.
It is particularly
Hauk Dieter
Noeth Heinrich
Rittmeyer Peter
Schlegel Andreas
Wietelmann Ulrich
Fulbright & Jaworski LLP
Metallgesellschaft Aktiengesellschaft
Nazario-Gonzalez Porfirio
LandOfFree
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