Chemistry of carbon compounds – Miscellaneous organic carbon compounds – C-metal
Reexamination Certificate
2000-01-03
2001-07-17
Vollano, Jean F. (Department: 1621)
Chemistry of carbon compounds
Miscellaneous organic carbon compounds
C-metal
Reexamination Certificate
active
06261482
ABSTRACT:
The invention relates to a process for the preparation of organo-alkali metal compounds by reaction of metallic lithium, sodium or potassium or of alloys of these alkali metals with an organic compound which has at least one acid CH bond in a solvent.
There are diverse possible uses for organo-alkali metal compounds in organic syntheses. Some organo-alkali metal compounds are therefore prepared on an industrial scale and are commercially obtainable in relatively large amounts. Organo-alkali metal compounds are usually prepared by reaction of organic halides with an alkali metal in a solvent in accordance with the equation
R—X+2M→R—M+MX (M=Li, Na, K; R= alkyl or aryl radical; X=Cl, Br, I).
It is furthermore known that acidic hydrogen in organic compounds can be replaced by alkali metals by reaction with strong organometallic bases (e.g. butyllithium), with alkali metal amides (e.g. lithium amide or sodium amide) or with alkali metal hydrides (e.g. sodium hydride).
In the case of sufficiently acidic CH bonds, the metallization can also be carried out directly by means of alkali metals in elemental form. 9-Fluorenyllithium is thus prepared by reaction of fluorene with metallic lithium in tetrahydrofuran (see Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], volume 13/1, Metallorganische Verbindungen [Organometallic compounds], pages 97 to 98, G. Thieme Verlag, Stuttgart, 1970). In the metallization of CH-acid compounds with alkali metals, however, undesirable by-products are as a rule formed if multiple bonds are present in the organic compound to be metallized. These are hydrogenated to a certain extent, which depends on the CH-acid compound, the solvent and the temperature. It has thus been found, for example, that partly hydrogenated fluorenes (e.g. tetrahydrofluorene) are formed as by-products in the lithiumization of fluorene (see Eisch and Kaska, J. Org. Chem. 27, 1962, pages 3745 to 3752). It is furthermore known that the metallization reaction often proceeds only very slowly. For this reason, high excesses of alkali metal are employed as a rule, which is a disadvantage, inter alia, because of the relatively high metal prices. The yields which can be achieved in direct metallization can therefore be described as only moderate for the abovementioned reasons. The yield in the preparation of 9-fluorenyllithium is thus only 71% even if an approx. 6-fold excess of lithium is employed (see Houben-Weyl, above reference).
The invention is therefore based on the object of providing a process for the preparation of organo-alkali metal compounds in which the reaction of the alkali metals lithium, sodium or potassium or alloys thereof with a CH-acid organic compound is carried out such that the formation of undesirable by-products is suppressed, the rate of reaction is increased and the yield of product is increased.
The object underlying the invention is achieved by carrying out that the abovementioned reaction in the presence of a hydrogen acceptor, with 0.5 to 5 mol of the hydrogen acceptor being employed per mol of acid hydrogen which can be replaced by lithium, sodium or potassium, with 1 to 3 mol of lithium, sodium or potassium being employed per mol of acid hydrogen, and with the acid CH bond having a pk
a
value of 10 to 30.
Of the alkali metals mentioned, lithium and sodium are preferred for carrying out the process according to the invention, since these two metals are obtainable both in relatively large amounts and relatively inexpensively. Moreover, the compounds metallized with lithium and sodium are relatively stable both to heat and to the solvents employed. Hydrogen acceptors which are used are organic compounds which add on the hydrogen replaced by the alkali metals. The alkali metals present in the process according to the invention do not act as hydrogen acceptors. It has been found that those CH-acid compounds in which the acidity constants pk
a
are in the range between 10 and 30 can be metallized by the process according to the invention. The numerical values of the acidity constant which are to be found in the literature vary to a relatively high degree, since they depend on the method of determination and specifically on the solvent chosen and the temperature (see Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], volume VIII/I, 1970, pages 31 to 69). For the classes of compounds of relevance here, acidity constants which have been determined in aqueous solutions are unsuitable per se, since the organo-alkali metal compounds prepared by the process according to the invention are decomposed by water. Rather, only the values of the acidity constants which have been determined in investigations in organic aprotic solvents are relevant for carrying out the process according to the invention, it being acceptable to estimate the pk
a
values for a particular solvent if pk
a
values for the solvents which have analogous properties are known. The publication by Houben-Weyl cited contains several tables in which the acidity constants pk
a
for various solvents are stated, so that—also using primary literature—sufficiently accurate information is available in respect of the acidity constants pk
a
of the individual CH-acid compounds.
The process according to the invention has the advantage that the organo-alkali metal compounds can be prepared in a high yield and that the formation of by-products is severely restricted, this being attributed to the synergistic interaction of the CH-acid compounds of particular acidity used and the hydrogen acceptors used. In particular, the undesirable hydrogenation of the multiple bonds present in the molecule of the CH-acid compounds is very severely suppressed in the process according to the invention, and the rate of the metallization reaction is increased in an advantageous manner.
The process according to the invention furthermore has the advantage that it allows the use of elemental alkali metals, which is substantially more inexpensive than the use of the known and conventional metallizing agents, such as butyllithium, lithium hydride or sodium hydride and lithium amide or sodium amide. Furthermore, advantageously no gaseous by-products (e.g. hydrogen) are formed in the process according to the invention, i.e. work safety is increased and emissions are decreased.
According to the invention, it is proposed that the CH-acid organic compound used is cyclopentadiene, indene or fluorene. It is furthermore proposed according to the invention that cyclopentadiene, indene and fluorene are substituted by one or more alkyl radicals and/or monocyclic aryl radicals, each carbon atom of the cyclopentadiene, indene and fluorene having a maximum of only one substituent. It is moreover proposed according to the invention that two molecules of cyclopentadiene and/or indene and/or fluorene or of these compounds substituted by alkyl radicals and/or monocyclic aryl radicals are linked to one another via an alkyl, alkenyl or silyl group. Finally, it is proposed according to the invention that the alkyl substituents and the monocyclic aryl substituents in each case have at least one functional group, the following functional groups being used:
a) di- or trialkylmetal functions; R
2
M═ or R
3
M—; M=Si, Ge, Sn; R=alkyl radical
b) fluorine-containing functions; F—, C
6
F
5
, —CF
3
c) carboxylic acid derivative functions; —COOR, —CN, —CONR
2
; R=H, alkyl radical and/or monocyclic aryl radical
d) amine functions; —NR
2
; R=H, alkyl radical and/or monocyclic aryl radical
e) alkoxy functions; RO—; R=alkyl radical or monocyclic aryl radical
f) nitro function; —NO
2
.
The CH-acid compounds cyclopentadiene, indene and fluorene are used for carrying out the process according to
In these compounds, the hydrogen atoms of the CH
2
group have an acid character and are therefore accessible for metallization. The substitution products of these compounds, which can likewise be used for carrying out the process
Lischka Uwe
Rittmeyer Peter
Wietelmann Ulrich
Fulbright & Jaworski LLP
Metallgesellschaft Aktiengesellschaft
Vollano Jean F.
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