Note: Descriptions are shown in the official language in which they were submitted.
8::~4~
TITLE
Process for the Preparation of
Tetraarvlborates
BACKGROUND OF THE INVENTION
S Field of the Invention
_
The present invention is directed to an im-
proved process for prepariny tetraarylborates by
reacting an alkali metal, an aryl halide and a borate
ester at specific ratios of halide to ester and within
a range of temperatures.
Description of the Prior Art
A variety of methods have been emplcyed to
prepare organo substituted boranes. U.S. Patent
2,880,242 discloses a process for preparing trisubsti-
tuted boranes by reacting an alkali metal, an organichalide and a boron halide in dry ethereal solution.
The preparation of organo-boron compounds by reacting
an organo-alkali metal with a boron trihalide or an
ester of boric acid in an inert liquid reaction medium
to produce the corresponding organo-boron halide or
organo-boric acid ester is disclosed in U.S. Patent
3,199,857. U.S. Patent 3,187,054 discloses reacting
boron trifluoride, a boron ester or a boron carbon
compound with an organo-sodium compound in an inert
hydrocarbon solvent and also teaches that reactions
of organo-sodium with boron esters tend to favor
production of compounds having one or two boron-
carbon bonds~
U.S. Patent 3,311,662 discloses a method
for preparing tetraarylboron compounds which comprises
reacting a preformed aryl sodium compound in an organic
solvent with a boron compoùnd such as boron tri-
chloride and particularly discloses the reaction of
boron trichloride with phenyl sodium to produce
PI-0340 35 sodium tetraphenylboron.
~2~
The reaction of an alkali metal, an organic
halide and an orthoborate ester to produce triaryl-
boranes wherein the reaction products are contacted
with water to form the hydroxide salt of the substituted
borane and methods for optimizing the recovery of the
thus prepared triarylborane from the aqueous hydroxide
solution are described in U.S. Patents 4,046,815;
4,045,495 and 4,076,756.
Boranes of the types produced by the process
of the present invention are disclosed for a wide
variety of uses including cure-promoters for epoxy and
the like resins, eOg., in U.S. Patent 3,637,572, in
copper complexes as photo-sensitive materials, e.g.,
in ~.S. Patent 3,927,955, in the recovery of cesium
values by forming insoluble complexes therewith, e.g.,
in U.S. Patent 3,114,716 and in the preparation of
deuterobenzene by the reaction of an appropriate
pyridine hydrohalide with a water soluble alkali metal
tetraphenylboron in the presence of heavy water, e.g.,
in U.S. Patent 3,132,188.
SUMMARY OF THE INVENTION
The present invention is a process for the
preparation of an alkali metal salt of a tetraaryl-
borane, e.g., sodium tetraphenylborate which process
comprises reactiny an alkali metal, e.g., sodium; an
aryl halide, e.g., chlorobenzene, and a borate ester,
e.g., isopropylorthoborate while employing a ratio
of halide to ester in the range of 4.0/1 to ~.0/1,
preferably 4.5/1 to 5.5/1 and a temperature in the
range 90-130C, prefera~ly 110-125C, in an inert
organic solvent. The preferred method of product
recovery is to contact the reaction mixture with
water to thereby obtain an aqueous solution of the
sodium salt of the tetraarylborane. Unwanted impuri-
ties can be removed before recoverin~ the salt.
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DETAILED DESCRIPTION OF THE INVENTION
Numerous organics can be employed as theinert organic solvent-reaction medium in the present
process so long as the reactants are sufficiently
soluble in the organic at reaction temperature but
do not react with it. It is pre~erred to employ a
solvent which has a boiling point at atmospheric
pressure near the desired rea~tion temperature in
order to simplify equipment and fac}litate heat re-
moval by reflux of the solvent during the reaction.Examples of suitable solvents include, either singly
or mixed, branched or unbranched alkanes having the
5-8 carbon atoms, e.g., pentane, hexane, octane,
heptane and 3-methylpentane and cycloalkanes having
S-8 carbon atoms, e.g., cyclohexane, methylcyclohexane,
cyclooctane and cyclopentane. Other suitable solvent-
reaction medium will be apparent to one skilled in the
art in view of the foregoing discussion. Cycloheptane
and mixtures ofC8 10 isoalkanes (mostly C8) having a
boiling point in the range 116-14~C are preferred
because the compounds boil at atmospheric pressure near
the optimum temperature range for the conduct of the
present process.
Alkali metals which are operable in the
present process include lithium, potassium, etc. with
sodium being preferred. Preferably the alkali metal
is introduced in a suspension of fine part~cles (1-20~)
in the reaction solvent. If the reaction is conducted
at temperatures of above 100C, e.g., by employing a
cycloheptane solvent, sodium can be introduced in the
molten state directly into the reaction medium. In a
preferred embodiment, the reaction rate and conse-
quently the temperature is controlled by continuously
metering the sodium dispersion or molten sodium to the
reaction or by staged addition o~ the sodium. This
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technique minimizes the cooling requirements for this
highly exothermic react~n.
The arylhalide which is one of the reactants
o the present invention can be any halogen substituted
organic which is compatible in the system and wherein
the halogen i5 available for reaction while other sites
are essentially inert. The arylhalides may be the
same or different depending upon the substituent groups
desired on the final tetra~orane. Aryl and substituted
arylhalides wherein the aryl group has 6-10 carbon
atoms are preferred. Substituent groups can be the
same or different alkyl groups having 1-8 carbon atoms;
alkenyl groups having 2-8 carbon atoms; aryl groups
having 6-10 carbon atoms; alkoxy groups having 1-8
carbon atoms and amino groups having the formula -NR2
wherein R is hydrogen or the above-mentioned substi-
tuent groups except halogen. Preferably the total
number of carbon atoms in the arylhalide does not
exceed 12. Specific examples of arylhalides include
chlorobenzene, bromobenzene, 4-chlorobiphenyl, 2-, 4-
chlorotoluene, dichlorobenzene and bromobenzene.
Chlorobenzene is preferred. Other halo aromatics
should be apparent to one skilled in the art. ~lthough
the amount of halo aromatic that is required to pro-
duce the tetra-substituted boranes can vary depending
upon its reactivity, it is necessary in most cases to
maintain a ratio of arylhalides to borate ester of
at least 4.0/1 and usually not greater than 6.0/1 while
ratios of 4.5/1 to 5.5/1 are preferred.
The borate esters which are operable in the
present invention include those which are deri~ed from
an alcohol containing 1-10 carbon atoms and are repre-
sented by the formula B(OR)3 wherein R is selected
from the group consisting of methyl, ethyl, n-propyl,
isopropyl, n-butyl, sec bu-tyl, sec-amyl, methyl-isobutyl,
~ 4
. :
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octyl, cyclohexyl, cyclopentyl and phen~l The R may
be the same or different. Orthoborate esters derived
from the lower secondary alkyl alcohols, i.e., those
having 3-8 carbon atoms are especially preferred since
the metahorates and pyroborates give lower yields.
The tetraborate can be recovered and purified
by several methods. In one method an aqueous solution
of sodium tetraphenylborate is obtained by quenching
the reaction mixture with water. The sodium tetra-
phenylborate may then be isolated as a precipitateby adding salt to the aqueous solution. This solid
may be further purified by recrystallization from
acetone or some other suitable solvent. Another method
involves the addition of salt and tetrahydrofuran to
the aqueous solution. The sodium tetraphenylborate
enters the THF phase while undesirable inorganic salt
impurities remain in the aqueous phase. The THF
phase is then isolated, mixed with water and the THF
stripped out by heating, affording an aqueous solution
of the purified sodium tetraphenylborate. This
latter method avoids problems associated with solid
handling.
The following examples are presented to
illustrate but not to restrict the present invention.
Parts and percentages are by weight unless otherwise
indicated. Reagents were at least CP grade. The
yields reported are based on the ester employed.
Example l
The apparatus employed consisted of a 250 ml
4-necked round botto~ flask with 4 vertical indentations
to increase the effectiveness of mixing. The necks
were, in turn, fitted with a mechanical stirrer, an
air-cooled reflux condenser, a thermometer and a rubber
septum. The apparatus was flushed thoroughly with dry
35 nitrogen and then connected to a mineral oil bubbler
~ 5
,~i
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to maintain a static nitrogen atmosphere in the re-
actor at a slight positive pressure. After the
apparatus had b~en thoroughly flushed with dry nitroqen,
approximately 5.7 grams of dry sodium particles (10-20~)
were dispersed in 55.1 grams of cycloheptane which had
been dried for 16 hoursin contact with a 4-A molecular
sieve and the dispersion injected into the reactor.
The contents of the reactor were heated to 118C fol-
lowing which a solution containing 5.0 grams of iso-
propylorthoborate, 13.17 grams of chlorobenzene and25.9 grams of dry cycloheptane was introduced into
the reactor using a syringe pump over a period of 50
minutes while maintaining the contents of the vessel
at 118C. The ratio of ester to halide to sodium was
1/4.~/9.3. After the introduction of the aforementioned
solution, samples of the contents of the reactor were
withdrawn over a 3-hour period and analyzed for sodium
tetraphenylborate. The analysis showed that the
reaction was essentially complete after the addition
of the ester and arylhalide. The yield to tetraphenyl-
borate was 80%. The sodium salt of tetraphenylborane
can be recovered in aqueous solution by injecting
water into the reactor after the contents are per-
mitted to cool to room temperature.
Examples 2-9
The procedure of Example 1 was repeated ex-
cept that the solvent, temperature and relative amount
of reactants were varied. The results are reported in
Table I. In Example 5 the borane was recovered by
salting out from aqueous solution and in Example 7
using tetrahydrofuran as described hereinabove.
TABLE I
Ester, Halide,
TempSodium Na~4B
Example Solvent(C)Mole Ratio Yield
2 Cy~lohexane 82 1/4.4/9.3 35%
3 Cyclohexane 82 I/5.6/11.8 47%
4 Methylcyclo-
hexane 1011/4.4/9.3 70%
Methylcyclo-
hexane 1011/S.6/11.8 75
6 Methylcyclo-
hexane 1011/5.6/11.8 76
7 Methylcyclo-
hexane 1011/5.6/11.8 75%
8 Isoalkane* 125 1/4.4/9.3 7396
9 Cyclooctane 152 1/4.4/9.3 69%
*Mixtures of about 65~ C8, 30% Cg and 5% C10 isoalkanes having
a boiling range 116-149C and no signi~icant amount of alkenes
or aromatics.
Example 10
Approximately 3.7 grams of dry sodium parti-
cles (10-20~) were~dispersed in 44.5 grams of dry
cycloheptane and the dispersion was charged t~ the
; ~ apparatus prepared as described in Example 1. Stirring
25 ~was~commenced~and t~e reactor contents gently heated to
approximately 40C following which a solution of 8.52
grams of dry chlorobenzene in 12.1 grams of dry cyclo-
heptane were fed to the reactor over a 25-minute
~ interval while maintaining the contents of the reactor
; 30 at a temperature in the range 45-50C. The cont~nts
of the reactor were then cooled for a period of 30
minutes to approximately 29C. After cooling, the
pot temperature was then~increased to 117C and a
~solution of 33.23 grams of isopropylorthoborate in
12.1 grams of dry cycloheptane was introduced to the
reactor over a~42-minute period. Analysis of the
reactor contents`indicated that the yield of approxi-
~ `mately 61~ of sodium tetraphenylborate was obtained
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essentially upon completion of the introduction of thereactants into the reactor. The contents of the record
were then cooled to room temperature and water injected
to obtain an aqueous solution of sodium
tetraphenylborate.
Example 11
Example 10 was repeated except that cyclo-
hexane was used as a solvent and the reaction tempera-
ture was maintained at about 82C. The yield to sodium
tetraphenylborate was 55%.
When cyclooctane was used as a solvent and
the reaction temperature maintained at about 150C,
the yield to sodium tetraphenylborate decreased to
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