Organic compounds -- part of the class 532-570 series – Organic compounds – Boron acids or salts thereof
Reexamination Certificate
2003-06-05
2004-12-21
Barts, Samuel (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Boron acids or salts thereof
Reexamination Certificate
active
06833470
ABSTRACT:
The invention relates to a process for preparing formylphenylboronic acids of the formula (I).
Ortho-, meta- and para-formylphenylboronic acids are versatile building blocks in organic synthesis and are important intermediates in the synthesis of active compounds in the agrochemical and pharmaceutical industries, but the compounds are of especially high efficiency and importance as enzyme stabilizers, inhibitors and bactericides.
Despite the great commercial interest in these compounds because of the abovementioned applications, only a few, expensive routes for preparing them have been described in the literature.
Boronic acids are prepared quite generally by reacting organometallic compounds (e.g. Grignard compounds or organolithium compounds) with boron trihalides or trialkyl borates. Owing to the reactivity of the formyl group toward organometallic compounds, this procedure is only possible for preparing formylphenylboronic acids when the formyl group is protected appropriately beforehand. As raw materials, it is therefore necessary to use p-halobenzaldehydes which are, for example, acetalized and subsequently reacted to form the organometallic reagent.
Nöth et al. (Chem. Ber. 1990, 1841-1843) convert p-bromobenzaldehyde into the diethyl acetal, convert this into the corresponding Grignard reagent by means of magnesium turnings in tetrahydrofuran (THF) and, after reaction with tri-n-butyl borate, obtain formylphenylboronic acid in a yield of 70%. A disadvantage of this synthesis is the high price of bromobenzaldehyde compared to chlorobenzaldehyde, the need for ultrasonic activation in the preparation of the Grignard reagent and the use of the expensive tributyl borate; in addition, complicated purification procedures have to be employed (e.g. via the hydrolysis product 1-butanol).
Jendralla et al. (Liebigs Ann. 1995, 1253-1257) achieve an improvement to 78% by means of process improvements in the same synthetic sequence, but here, too, the abovementioned disadvantages remain.
Significantly better yields (up to 99%) were achieved by Kobayashi et al. by reaction of bromobenzaldehyde diethyl acetal with n-butyllithium and subsequent reaction with triisopropyl borate, but the high prices of bromobenzaldehyde, triisopropyl borate and butyllithium stand in the way of an economically interesting process.
There is therefore a need to develop a process for preparing formylphenylboronic acids which starts out from advantageous starting materials which are readily available commercially and makes it possible to obtain the target products in good yields and high purities by reaction with cheap boron compounds.
It was firstly established that the necessary Grignard compounds cannot be obtained from various chlorobenzaldehyde acetals by reaction with magnesium in various ethers by methods of the prior art. In the German patent application DE-A-199 60 866, which is not a prior publication, it was found that the Grignard compounds can be obtained in good yields by addition of transition metal catalysts and simultaneous mechanical activation of the magnesium. Reaction with trimethyl borate results in corresponding formylphenylboronic acids in good yields. This is an economically very interesting method of preparation which, however, requires high capital costs and places considerable demands on plant construction due to the mechanical activation of the magnesium which is required. At the same time, the products contain traces of the transition metals used in the ppm range which, depending on the application (pharmaceuticals, enzyme inhibitors), have to be removed quantitatively by costly methods.
It is therefore an object of the present invention to provide a simple and economical process for preparing formylphenylboronic acids which starts out from advantageous starting materials which are readily available commercially and makes do without the use of transition metal catalysts and without high capital costs for plant construction. At the same time, the process should give the products in very high yields and purities.
The present invention achieves these objects and provides a process for preparing formylphenylboronic acids of the formula (I) by reaction of protected chlorobenzaldehydes of the formula (II) with lithium metal in an inert solvent to form compounds of the formula (III) and subsequent reaction with a boron compound of the formula BY
3
to give compounds of the formula (I)
where Y is a straight-chain or branched C
1
-C
6
-alkoxy or C
1
-C
5
-dialkylamino group, halogen or a C
1
-C
6
-alkylthio group, and R is H or a C
1
-C
5
-alkyl or C
1
-C
5
-alkoxy radical.
The radical CHX
2
is preferably an acetal of the formula (IV) or (V)
where R
1
to R
4
are identical or different and are each hydrogen, C
1
-C
12
-alkyl or phenyl, or R
1
and R
2
together or R
1
and R
3
together form a 5- or 6-membered aliphatic or aromatic ring; or an oxazolidine of the formula (VI)
or an aminal of the formula (VII)
where R
1
and R
4
are as defined above and R
5
and R′ are each C
1
-C
6
-alkyl or aryl.
As starting compounds of the formula (II), it is possible to use protected ortho-, meta- or para-chlorobenzaldehydes.
Although the lithium metal used according to the invention is an expensive raw material on the basis of its mass, the price difference compared to magnesium on the basis of molar amounts used is comparatively small. In the present process, the metal is placed as a dispersion, powder, turnings, sand, granules, pieces, bars or in another form together with a suitable solvent in a reaction vessel and is reacted with the protected chlorobenzaldehyde. Suitable inert solvents are all solvents which react neither with the lithium metal nor with the lithiated aromatic formed under the conditions of the process of the invention, in particular aliphatic or aromatic ethers, tertiary amines or hydrocarbons, e.g. THF, diethyl ether, diisopropyl ether, di-n-butyl ether, toluene, cyclohexane or dioxane or mixtures of the inert solvents in question.
The reaction of lithium metal with protected chlorobenzaldehydes is carried out at temperatures in the range from −100° C. to +35° C., since the reaction proceeds too slowly at lower temperatures but at higher temperatures the lithium aryls formed attack and cleave the, for example, acetal, aminal or oxazolidine function. Preferred reaction temperatures are therefore in the range from −70 to +10° C., particularly preferably from −55 to +5° C.
The reaction of lithium with compounds of the formula (II) is generally complete after from 3 to 18 hours, in particular from 4 to 10 hours, although in some cases, depending on the nature of the protected chlorobenzaldehyde used and the solvent employed, the reaction can proceed significantly more slowly, resulting in poor space-time yields. The rate of this reaction can be increased considerably by the presence of organic redox catalysts such as biphenyl, naphthalene or other organic compounds which rapidly take up electrons from the Li metal and transfer them quickly and efficiently to the C—Cl bond of the protected chlorobenzaldehyde. The redox catalysts are added in amounts of from 0 to 5 mol %.
The molar ratio of lithium to the compound of the formula (II) is usually in the range from 1.9:1 to 8:1, but larger excesses can also be used when this is, for example, advantageous for reasons of the apparatus employed, e.g. in pumped circulation apparatuses.
The concentration of the lithium compound in the solvent can be from 0.5 to 50% by weight, preferably from 5 to 35% by weight, particularly preferably from 15 to 30% by weight. The protected chlorobenzaldehyde can either be metered in or (especially in the case of relatively large Li pieces) can all be placed in the reaction vessel initially.
The reaction of the resulting organolithium compounds of the formula (III) with the boron compounds is, to achieve high selectivity, carried out at low temperatures in the range from +20 to −110° C., preferably from 0 to −80° C. It is possible either to add t
Erbes Michael
Meudt Andreas
Scherer Stefan
Vollmueller Frank
Barts Samuel
Clariant GmbH
Silverman Richard P.
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