Method for producing alkali metal alcoholates

The invention relates to a method for producing alkali metal alcoholates by reacting alcohol with alkali metal in an aprotic, organic solvent in the presence of an H acceptor such as e.g. isoprene, butadiene, styrene or methyl styrene.

The invention relates to a method for the preparation of alkali metal
 alcoholates, in which method alcohol is reacted with alkali metal in an
 aprotic organic solvent using a hydrogen acceptor.
 Alkali metal alcoholates R-OM (R=alkyl, M=Li, Na, K, Rb, Cs) are compounds
 that are susceptible to hydrolysis and are often used in organic synthesis
 on account of their basic properties.
 It is known that alkali metal alcoholates can be prepared by reacting the
 corresponding alcohols with an alkali metal in accordance with the
 reaction equation:
EQU 2R--OH+2M.fwdarw.2R--OM+H.sub.2
EQU R=alkyl
EQU M=Li, Na, K, Rb, Cs
 The speed of this reaction diminishes as the length of the alkyl chain
 increases and also as branching increases. Whilst the reaction can be
 successfully accelerated to a considerable extent on a laboratory scale as
 a result of the use of extremely finely divided alkali metal, which is
 produced with a particle size below 50 .mu.m by means of high-speed
 stirrers, the reaction takes many hours on an industrial scale of
 production. Long reaction times, however, impair the economic efficiency
 of this alcoholate synthesis.
 A method for preparing alkali metal alcoholates is known from FR-PS 1 070
 601, in which alkali metal is finely distributed, in a boiling inert
 hydrocarbon by being stirred, and after cooling the calculated quantity of
 alcohol is added drop by drop to the dispersion. During the preparation of
 the sodium suspension, any agglomeration of the finely distributed sodium
 is prevented by adding a dispersive additive, such as fatty acid,
 surfactants or active carbon. The alkali metal alcoholate in xylene that
 results can be separated.
 With the method known from DE-OS 34 37 152 for the catalyzed preparation of
 alkali metal alcoholates from alkali amalgams and alcohols, lumps of
 anthracite are used as the catalyst, the surface of which is preferably
 coated with a mixture of nickel and molybdenum oxide. Aliphatic alcohols
 having 1 to 4 carbon atoms are preferably used.
 With the method known from DE-PS 08 45 341 for the preparation of alkali
 metal alcoholates that are lean in caustic alkali, amalgamated alkali
 metal is brought into contact with alcohol several times in the presence
 of catalysts, such as graphite. In this connection, it is further known
 from DE-PS 09 28 467 that finely distributed sodium amalgam and alcohol
 can be directed in counterflow with respect to a lumpy catalyst,
 consisting of a mixture of graphite or active carbon and metal filings.
 The methods set out above have the following disadvantages:
 The reaction times cannot be successfully reduced in an economical manner
 on an industrial scale of production by means of the use of lumpy or
 filing-like catalyst substances.
 The necessary separation of the catalysts from the reaction product is
 problematic in many cases.
 The use of a toxic amalgam compound as the alkali metal component is
 problematic on account of the impact on the workplace and environment.
 Since the speed of reaction is often simply insufficient unsatisfactory
 when elemental alkali metal is used, in particular in the case of
 sterically hindered tertiary alcohols, despite the measures described,
 very basic organometal compounds have also been used as the alkali- metal
 source. This holds good in particular for the preparation of lithium
 alcoholates:
EQU R--OH+R'Li.fwdarw.R--OLi+R'H.uparw.
 The disadvantage of this smoothly running reaction is the comparatively
 high price of organo-lithium compounds.
 Further syntheses are based on alkali metal hydrides and amides. Whilst
 these reagents often react somewhat faster or clearly faster than the
 alkali metal in elemental form, the compounds, calculated on a molar
 basis, are clearly more expensive than the alkali metal. In the case of
 the amides, moreover, ammonia develops that has to be removed from the
 waste-gas stream at a cost. When hydrides are used--compared with the use
 of the elemental metals--twice the quantity of hydrogen develops. Whilst
 hydrogen is not an ecologically hazardous product, the resultant gas
 stream is loaded with organic compounds (solvent, alcohol) which, for
 ecological reasons, as far as possible should not reach the environment.
EQU R--OH+MH.fwdarw.R--OM+H.sub.2.uparw.
 R--OH+MNH.sub.2.fwdarw.R--OM+NH.sub.3.uparw.
EQU R=alkyl residue; M=alkali metal
 The object of the present invention is to avoid the disadvantages in
 accordance with the prior art, that is, in particular to set forth a
 method for the preparation of alkali metal alcoholates that starts with
 inexpensive raw materials that are available commercially and which in a
 very rapid reaction supplies water-free alkali metal alcoholates whilst
 forming as few gaseous by-products as possible and without using solid
 catalysts that are difficult to separate.
 The object is achieved in that the alcohol is reacted with the alkali metal
 (Li, Na, K, Rb, Cs) in an aprotic organic solvent, and an H-acceptor in
 the form of a conjugated diene or a 1-arylolefine is moreover added
 thereto. The reaction preferably proceeds in accordance with the following
 reaction scheme:
 ##STR1##
 The presence of an H-acceptor brings about an advantageous reduction in the
 quantity of waste gas, from the point of view of reaction-control and
 environmental-protection, in comparison with the conventional reaction of
 alkali metal alcoholate formation.
 Open-chain or cyclic, unsubstituted or substituted 1,3-dienes or
 unsubstituted or substituted 1-arylolefines can be used as the H-acceptors
 (in the case of the substituted reagents, both in the cis and in the trans
 form). Preferred H-acceptors for this reaction are isoprene, butadiene,
 cyclohexadiene-(1,3), styrene or methyl styrene.
 The quantity of H-acceptor added amounts to 0.2 to 4 times, preferably 0.4
 to 1.5 times, the stoichiometric quantity, that is, 0.2 to 4 mol,
 preferably 0.4 to 1.5 mol, relative to, in each case, 2 mol alcohol. The
 method can consequently even be carried out with the quantity of
 H-acceptor that is added lying below the stoichiometrical relationship,
 this increasing the economic efficiency.
 In particular one of the metals Li, Na or K or mixtures of these metals can
 be used as the alkali metal.
 It is advantageous that the alkali metal can be present in pulverulent
 form, granular form or lumps. In the case of Na, K, Rb or Cs in addition
 preferably a finely divided molten mass can be chosen. On account of its
 high melting point, lithium is preferably used in a solid form.
 In particular in the case of the reaction of secondary or tertiary alcohols
 with an alkali metal, the presence of an H-acceptor results in clearly
 higher speeds of reaction in comparison with methods known hitherto. The
 reactions of i-propanol, t-butanol or t-pentanol are of particular
 commercial interest.
 An aliphatic or aromatic hydrocarbon with 4 to 20 C-atoms or an ether or a
 mixture of these substances can be used as the aprotic organic solvent.
 The reaction can be carried out particularly well in hexane, heptane,
 octane, toluene, ethyl benzene, methyl-tert. butyl ether (MTBE),
 tetrahydrofuran (THF) or 2-methyl-THF. Commercially available hydrocarbon
 mixtures, such as, for example, petroleum ether, paraffin oil,
 high-boiling Shellsol D 70, can also be used in a particularly
 advantageous manner as the solvent.
 The mixture of alcohol and H-acceptor is preferably added to the dispersion
 of alkali metal in the aprotic organic solvent. It is also possible to
 produce a mixture of solvent, H-acceptor and metal, to which the alcohol
 is added in doses. In some cases, it is also possible to add the alkali
 metal in a solid or liquid form to the mixture of solvent, alcohol and
 H-acceptor.
 A solution of lithium tert-butylate in THF can be prepared in this way, for
 example.
 The temperature of reaction is maintained at -20 to 200.degree. C.,
 preferably at 20 to 140.degree. C.

The subject-matter of the invention is explained in greater detail in the
 following with reference to exemplifying embodiments.
 EXAMPLE 1
 Synthesis of Sodium Tert-butylate (STB) in Toluene in the Presence of a
 Stoichiometric Quantity of Styrene
 4.78 g (208 mmol) Na-lumps were placed in 69.8 toluene in a 250-ml
 four-necked flask with a heating mantle, precision glass stirrer and
 reflux condenser, and heated to approximately 100.degree. C. At the start
 of the addition of a mixture of 15.0 g (202 mmol) tert-butanol and 10.5 g
 (101 mmol) styrene, the temperature of the reaction mixture was
 immediately increased to 109.degree. C. During the addition which took
 place in 25 minutes, no significant development of gas could be observed.
 Towards the end of the addition, small quantities of a white deposit could
 be observed on the flask wall, although this disappeared 5 minutes after
 the end of dosing (clear, light yellow solution). The temperature was
 maintained at approximately 100.degree. C. for a further 25 minutes.
 The product solution was filtered in the hot state and dried in a rotation
 evaporator until the weight was constant. 18.2 g (94%) STB in the form of
 a colourless powder was obtained.
 Comparative Example A
 Synthesis of Sodium Tert-butylate in Toluene Without an H-acceptor
 Using the same apparatus as in Example 1, tert-butanol was dosed into a
 boiling Na-dispersion (containing 61.5 g sodium =2.7 mol) in toluene.
 After the addition of 70 g tert-butanol (35 mol %, relative to the
 Na-quantity that is used), 17% of the quantity of hydrogen to be expected
 in theory had been formed after 1.4 hours only, suggesting that the speed
 of reaction was substantially slower than that of Example 1 in accordance
 with the invention.
 EXAMPLE 2
 Synthesis of Lithium Tert-butylate in Tetrahydrofuran in the Presence of a
 Stoichiometric Quantity of Isoprene
 9.62 g (1386 mmol) lithium granules (Na-content 0.43%) were placed in 400 g
 THF in a 1 l double-jacket reactor with a reflux condenser, drip funnel
 and precision glass stirrer, and a mixture of 106 g (1430 mmol)
 tert-butanol and 47.2 g (693 mmol) isoprene was added thereto at 15 to
 35.degree. C. The reaction started immediately at the beginning of the 45
 minute dosing time; intensive cooling was necessary. After a 30 minute
 secondary reaction time, only a very small quantity of Li-metal was still
 present in an otherwise clear, slightly grey-coloured product solution.
 2.35 mmol/g total base concentration (corresponding to a 95% reaction)
 were detected in a sample of the solution. After one further hour at
 30.degree. C., the base concentration rose to 2.43 mmol/g (corresponding
 to 98%). Throughout the reaction time, no significant development of
 hydrogen was observed.
 It was possible to filter the product solution in a problem-free manner
 (glass frit, filtration time approximately 1 minute).
 Comparative Example B
 Synthesis of Lithium Tert-butylate in Tetrahydrofuran Without an H-acceptor
 3.5 g (0.5 mol) Li-granules (0.77% Na-content) were suspended in 100 g THF
 in a 500-ml flask with a reflux condenser, drip funnel and precision glass
 stirrer, and a solution of 37.1 g (0.5 mol) tert-butanol in 60 ml THF was
 added thereto under reflux within 145 minutes. During this time, 28% of
 the H.sub.2 -quantity that was to be expected in theory was formed. After
 a further 3 hours boiling under reflux, a filtered sample was taken and
 tested for the total base (2.04 mmol/g corresponding to an 82% reaction).
 After cooling, filtering was carried out by way of a glass frit. The
 filtration process took 100 minutes and yielded a cloudy, yellow solution.
 The reaction time is also longer in this comparative example than in
 Example 2.
 EXAMPLE 3
 Synthesis of Potassium Tert-amylate (PTA) in Hexane at 60.degree. C. in the
 Presence of a Stoichiometric Quantity of Isoprene
 25.3 g (648 mmol) of purified potassium crusts (Merck) were melted in 305 g
 hexane in a 0.5-1 twice clad reactor with a precision glass stirrer,
 reflux condenser and drip funnel, and then a mixture of 57.1 g (648 mmol)
 tert-amyl alcohol and 22.1 g (324 mmol) isoprene was added thereto at
 approximately 60.degree. C. within 100 minutes. During the addition
 process, the reaction mixture became slightly cloudy, although was easy to
 stir. At the end of the addition, refluxing was effected for 10 minutes,
 with a clear, slightly yellowish solution being formed.
 The filtered solution was concentrated by evaporation in a rotation
 evaporator under vacuum. 74.8 g (92%) of a colourless powder was obtained
 that had the expected composition for PTA.
 EXAMPLE 4
 Synthesis of Lithium Tert-butylate in THF in the Presence of a
 Sub-stoichiometric Quantity of Isoprene
 5.2 g (0.75 mol) lithium metal granules (0.4% Na-content) were suspended in
 300 ml THF in an apparatus like that in Example 3, and a mixture of 16.2 g
 (238 mmol) isoprene and 97.3 g (772 mmol) tert-butanol was added thereto
 within 3 hours. The reaction temperature was maintained at approximately
 40.degree. C. At the end of dosing, the mixture was stirred again for
 approximately 1 hour at 40.degree. C.; afterwards no metallic lithium
 could be detected any more. The slightly cloudy solution was filtered
 without any difficulties by way of a glass frit (filtration time
 approximately 30 sec). A base concentration of 2.18 mmol/g was analyzed in
 the slightly grey-coloured, yet clear filtrate, this corresponding to a
 yield of 98%.
 During the reaction phase, approximately 170 mmol H.sub.2 -gas had been
 formed, that is, approximately 45% of the metal reacted directly with the
 alcohol. In this Example, the quantity of isoprene used corresponded to
 0.62 times the stoichiometric quantity.