Organic compounds -- part of the class 532-570 series – Organic compounds – Aluminum containing
Reexamination Certificate
2003-02-25
2004-03-30
Nazario-Gonzalez, Porfirio (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Aluminum containing
C556S170000, C556S187000, C568S001000, C568S008000
Reexamination Certificate
active
06713642
ABSTRACT:
The invention relates to a process for the preparation of alkali metal monohydridoboranates and -aluminates of the general formula:
wherein
M=Li, Na, K, Rb or Cs and
E=B or Al, and
X
1
, X
2
, X
3
=in each case independently of one another, is
a 2 to 10 C atom secondary or tertiary alkyl group or
a phenyl group, which in its turn can be alkyl-substituted, or
an alkoxy group.
Alkali metal monohydridoboranates and -aluminates are to some extent known classes of compounds, the members of which have found uses as reagents in chemical synthesis, e.g. as reducing agents. Thus, for example, commercially available lithium tri-tert-butoxyaluminium hydride is employed for the chemoselective reduction of acid chlorides to aldehydes or for the stereoselective reduction of asymmetrically substituted or cyclic ketones to alcohols (P. Galatsis, “Lithium-tri-tert-butoxyaluminiumhydride in L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley & Sons, Chichester 1995, p. 3168-3172).
In a similar manner, trialkyl borohydrides also serve as diversely usable reducing agents in organic synthesis. In general, their stereoselectivity increases with the steric bulkiness of the alkyl substituents. (H. C. Brown, S. Krishnamurthy, J. L. Hubbard, J. Am. Chem. Soc. 1978, 100, 3343; R. Köster, “Anionische Organobor-Wasserstoff-Verbindungen [Anionic Organoboron-Hydrogen Compounds]” in: Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry] 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983; J. L. Hubbard, “Lithium tri-s-butylborohydride” in: L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley & Sons, Chichester 1995, 3172-3176; J. D. Odom, in: Comprehensive Organometallic Chemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol. 1, p. 297).
Some representatives of the alkali metal monohydridoboranates and -aluminates can be obtained by addition of alkali metal hydride (MH) to the EX
1
X
2
X
3
compound. This applies e.g. to the preparation of Li[HBEt
3
] in accordance with:
where Et=ethyl.
Without a solvent or with a hydrocarbon as the solvent, the reaction at 200° C. takes approx. 4 h, and with Et
2
O as the solvent the reaction on heating under reflux takes approx. 24 h (R. Köster, “Anionische Organobor-Wasserstoff-Verbindungen” in: Houben-Weyl, Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983). In THF solution, the addition can be effected within one day at room temperature.
The induction time of the reaction presents problems. This method also fails if boranes with bulky substituents are employed. For example, the reaction of B(
s
Bu)
3
with alkali metal hydrides in boiling THF gives a conversion of only 10% after 24 h, and for this reason this reaction is unsuitable for a commercial synthesis. (H. C. Brown, S. Krishnamurthy, J. L. Hubbard, J. Am. Chem. Soc. 1978, 100, 3343; R. Köster, “Anionische Organobor-Wasserstoff-Verbindungen” in: Houben-Weyl, Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983; J. L. Hubbard, “Lithium tri-s-butylborohydride” in: L. A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley & Sons, Chichester 1995, 3172-3176; J. D. Odom, in: Comprehensive Organometallic Chemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol. 1, p. 297).
Boranes with even bulkier substituents are inert with respect to “normal”, i.e. commercially obtainable, NaH and LiH in boiling THF. This does not apply to the highly reactive form of the binary hydrides, such as are prepared e.g. by decomposition of alkyllithium solutions under a hydrogen atmosphere. (R. Pi, T. Friedl and P. v. R. Schleyer, J. Org. Chem. 1987, 52, 4299-4304). Because the active metal hydride first has to be prepared from expensive organolithium solutions, this process is of little commercial interest.
Another variant of preparing active metal hydride comprises preparing the metal, preferably in finely divided form, in the presence of a trisubstituted boron compound, so that the hydride MH formed in situ can add on to the boron compound, immediately, to form a borohydride of the formula M[R
1
R
2
R
3
B]H. Disadvantages in this case are that the metal must be present in the form of a highly reactive powder which is difficult to handle, and a catalyst combination in the form of a transition metal salt (e.g. FeCl
3
) and/or polyaromatics (e.g. phenanthrene) must be employed to achieve reasonable reaction temperatures and times (U.S. Pat. No. 5,886,229). The product solutions accordingly are contaminated and are discoloured by the transition metal content.
Trialkoxy-element hydrides with bulky substituents also do not react or react only extremely slowly with MH. For example, the preparation of lithium tri-tert-butoxyaluminium hydride (LTTBA) in accordance with:
is unknown (see also Comparative Example A).
Since the direct preparation is not possible, a number of process alternatives have been developed. Thus, LTTBA is prepared by alcoholysis of lithium aluminium hydride in accordance with:
The high preparation costs are a disadvantage, since relatively expensive hydride hydrogen in the LiAlH
4
is destroyed by the alcoholysis.
Trialkyl borohydrides with bulky organic radicals are prepared by one of the following general processes:
Disadvantageous with the process according to (4) are the use of expensive LiAlH(OMe)
3
, which is not commercially obtainable, and above all the fact that large amounts of insoluble aluminium methylate are obtained, which makes preparation of the trialkyl borohydride in a pure form extremely difficult. Similar circumstances apply to process (5), and in addition there are high costs for the donor, such as e.g. 1,4-diazabicyclo[2,2,2]octane (DABCO).
Process (6) has the disadvantage that expensive t-butyllithium is used as the LiH source, a gaseous by-product being formed. Furthermore, the reaction must be carried out at very low temperatures, which is very unfavourable in energy terms.
All the processes (3)-(6) have the disadvantage that they are limited in practice to the preparation of the lithium derivative, since only the corresponding lithium raw materials (and not the Na or K compounds) are commercially obtainable. The addition of higher alkali metal hydrides (NaH, KH, RbH, CsH) to a starting compound EX
1
X
2
X
3
indeed proceeds substantially faster than in the case of LiH, but in these cases also the rate of reaction decreases sharply with increasing volume of the substituents X (see comparison example B).
The object of the invention is to overcome the disadvantages of the prior art and to provide a process for the rapid preparation of alkali metal monohydridoboranates and -aluminates of the general formula:
at mild temperatures which starts from commercially available alkali metal hydride, allows a reaction procedure without increased pressure and avoids the formation of insoluble by-products.
The object is achieved by the process described in claim
1
. Claims
2
to
11
develop the process described. Claim
12
describes preferred process products.
It has been found that the addition described above of alkali metal hydride (MH) on to an EX
1
X
2
X
3
compound is significantly accelerated by a catalyst:
where
M=Li, Na, K, Rb or Cs and
E=B or Al and
X
1
, X
2
, X
3
, in each case independently of one another, =
a secondary or tertiary alkyl group consisting of 2 to 10 C atoms or
a phenyl group, which in its turn can be alkyl-substituted, or
an alkoxy group.
Any boron-containing compound which contains the structural unit BH
3
and which itself or the reaction product of which with MH is capable of acting as a hydride transfer agent can be employed as the catalyst.
X
1
, X
2
and X
3
, in each case independently of one another, can preferably be iso-propyl or sec-butyl or tert-butyl or tert-amyl or siamyl (sec-2-methyl-butyl) or a phenyl group, which in its turn can be alkyl-substituted, or the following alkoxy group:
where R′
1
, R′
2
,
Hauk Dieter
Lischka Uwe
Majdalani Andre
Wietelmann Ulrich
Chemetall GmbH
Fulbright & Jaworski L.L.P.
Nazario-Gonzalez Porfirio
LandOfFree
Method for producing alkali metal monohydridoborates and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for producing alkali metal monohydridoborates and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for producing alkali metal monohydridoborates and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3268185