Recovery of tris(aryl)borane from their tetrahydrofuran complexes

This invention relates to methods for separating tris(aryl)boranes from tris(aryl)boranetetrahydrofuran complexes in hydrocarbon solvents via distillation.

TECHNICAL FIELD 
This invention relates to methods for separating tris(aryl)boranes from 
their tris(aryl)boranetetrahydrofuran complexes in hydrocarbon solvents. 
BACKGROUND 
Methods for dissociating tris(aryl)boraneether complexes are well known. 
These methods include evaporation of the ether with sublimation; reactive 
exchange, in which a compound that has a higher affinity for the ether is 
added; and azeotropic distillation from a higher-boiling hydrocarbon 
solvent. Some of these methods are described in U.S. Pat. Nos. 5,510,536, 
5,600,004, 5,693,261, and EP 838466A2. However, the tris(aryl)boraneether 
complexes formed when tetrahydrofuran is the ether are especially strong 
and difficult to separate. A satisfactory yield of the separated 
tris(aryl)borane is often not obtained by azeotropic distillation when 
tetahydrofuran is the complexed ether. Thus, there remains a need for a 
method which more completely separates tris(aryl)boranes from 
tetrahydrofuran. 
THE INVENTION 
This invention makes possible a more complete separation of 
tris(aryl)boranes and tetrahydrofuran from tris(aryl)boranetetrahydrofuran 
complexes. In fact, this invention enables essentially complete 
dissociation of the tetrahydrofuran complex and essentially complete 
separation of the liberated tetrahydrofuran from the tris(aryl)borane. 
Pursuant to this invention, a distillation is performed in a 
non-complexing solvent and under special temperature conditions. Thus, an 
embodiment of this invention involves a process for removal of 
tetrahydrofuran from a tris(aryl)boranetetrahydrofuran complex, which 
process comprises nonazeotropically distilling tetahydrofuran at a 
temperature in the range of from about 115.degree. C. to about 160.degree. 
C. from a mixture comprising an inert hydrocarbon solvent composition and 
a tris(aryl)boranetetrahydrofuran complex. 
In one preferred embodiment, excellent results can be achieved using as the 
solvent a mixture of paraffinic hydrocarbons boiling in the range of from 
about 114.degree. C. to about 126.degree. C. In this embodiment, the 
operation can be conducted either at atmospheric pressure or at 
superatmospheric pressures. 
In another preferred embodiment, the solvent used is toluene, and the 
operation is conducted at an elevated pressure sufficient to maintain the 
solvent in the liquid state. 
Further embodiments of this invention will be apparent from the ensuing 
description and appended claims. 
The aryl groups of the tris(aryl)borane may be the same or different; it is 
preferred that all three aryl groups are the same. The aryl moiety may be 
phenyl, biphenylyl, naphthyl, anthracenyl, phenanthrenyl, naphthacenyl, 
chrysenyl, perylenyl, terphenyl, binaphthyl, and the like. Most preferred 
as the aryl moiety is a phenyl group. The aryl groups may be substituted 
by one or more fluorine atoms, hydrocarbyl groups, alkoxy groups, and/or 
perfluorinated hydrocarbyl groups. 
The hydrocarbyl group substituents are preferably C.sub.6 to C.sub.24 aryl 
groups or C.sub.1 to C.sub.10 alkyl groups. Examples of suitable 
hydrocarbyl groups are methyl, ethyl, isopropyl, tert-butyl, cyclopentyl, 
methylcyclohexyl, decyl, phenyl, tolyl, xylyl, and naphthyl. The alkoxy 
groups preferably have C.sub.1 to C.sub.6 alkyl moieties. Some examples of 
alkoxy groups are methoxy, ethoxy, propoxy, isopropoxy, 
methylcyclopentoxy, and cyclohexoxy. The perfluorinated hydrocarbyl groups 
include alkyl and aryl perfluorocarbons; suitable perfluorinated 
hydrocarbyl groups are, for example, trifluoromethyl, pentafluoroethyl, 
pentafluorophenyl, and heptafluoronaphthyl. 
It is preferred that at most two substituents on the aryl ring are 
hydrocarbyl, perfluorinated hydrocarbyl, or alkoxy, while the rest of the 
substituents are fluorine atoms. It is highly preferred to have aryl rings 
in which the all of the substituents are fluorine atoms. Examples of such 
groups are pentafluorophenyl, nonafluorobiphenylyl, heptafluoronaphthyl, 
nonafluoroanthracenyl, nonafluorophenanthrenyl, undecafluoronaphthacenyl, 
undecafluorochrysenyl, undecafluoroperylenyl, tridecafluoroterphenyl, 
tridecafluorobinaphthyl, and the like. Most preferred as the aryl group is 
a pentafluorophenyl group; thus, the most highly preferred 
tris(aryl)borane is tris(pentafluorophenyl)borane. 
The hydrocarbon solvent composition may be made up of a mixture of 
hydrocarbons such that a liquid mixture is formed; all of the components 
of the hydrocarbon solvent composition should be inert towards the 
tris(pentafluorophenyl)borane. Linear, branched, and/or cyclic saturated 
hydrocarbons and/or linear, branched, and/or cyclic nonaromatic 
unsaturated hydrocarbons may be used to make the mixture. The hydrocarbon 
solvent composition may instead be made up of aromatic hydrocarbons; such 
a mixture should also be a liquid. 
Some examples of saturated hydrocarbons that may be used in forming a 
hydrocarbon solvent composition are pentane, cyclohexane, 
methylcyclohexane, ethylcyclooctane, decane, 6-propyldodecane, and 
eicosane. Examples of nonaromatic unsaturated hydrocarbons that may be 
used in forming a hydrocarbon solvent composition include cyclopentene, 
1-hexene, 2,5-dodecene, and 1,4-cyclooctadiene. Mixtures of saturated 
hydrocarbons are a preferred hydrocarbon solvent composition; more 
preferred are mixtures of saturated hydrocarbons that are structural 
isomers. A highly preferred hydrocarbon solvent composition is Isopar-E, a 
product of Exxon, which is a mixture of paraffinic C.sub.8 hydrocarbons 
with a boiling point range of 114.degree. C. to 126.degree. C. Some 
examples of aromatic hydrocarbons that may be used in forming a 
hydrocarbon solvent composition are toluene, ethylbenzene, 
isopropylbenzene, xylene, ethylmethylbenzene, diethylbenzene, 
methylisopropylbenzene, and mesitylene. The use of a single aromatic 
hydrocarbon is also a preferred hydrocarbon solvent composition; toluene 
is a highly preferred aromatic hydrocarbon for the hydrocarbon solvent 
composition. The foregoing solvents which may make up the hydrocarbon 
solvent composition do not form azeotropes with tetrahydrofuran in the 
distillation temperature range of from about 115.degree. C. to about 
160.degree. C. 
When the tris(aryl)boranetetrahydrofuran complex is from a crude reaction 
mixture, it will probably contain side products from its formation, such 
as magnesium salts; these side products usually precipitate as 
tetrahydrofuran is driven off. Mixing a large amount of 
tris(aryl)boranetetrahydrofuran complex from a crude reaction mixture with 
the hydrocarbon solvent composition is generally undesirable because an 
intractable amount of solid precipitate can form. Preferably, the 
tris(aryl)boranetetrahydrofuran complex may be up to about fifteen weight 
percent of the mixture, but is more preferably less than ten weight 
percent, and most preferably less than six weight percent of the mixture. 
While it is possible to add the hydrocarbon solvent composition to the 
tris(aryl)boranetetrahydrofuran complex, it is preferred that the 
tris(aryl)boranetetrahydrofuran complex be added to the hydrocarbon 
solvent composition, and it is more preferred that the hydrocarbon solvent 
composition is already at an elevated temperature when the 
tris(aryl)boranetetrahydrofuran complex is added. It is advantageous to 
add the tris(aryl)boranetetrahydrofuran complex to hydrocarbon solvent 
composition at an elevated temperature because any precipitate that forms 
tends to be a fine particulate rather than a clumpy solid. Further, the 
elevated temperature allows the removal of any uncomplexed tetrahydrofuran 
that may be present. The elevated temperature is preferably at least about 
70.degree. C., but less than about 115.degree. C. 
After mixing the tris(aryl)boranetetrahydrofuran complex and the 
hydrocarbon solvent composition, the temperature of the mixture is raised 
to at least about 115.degree. C., and preferably to at least about 
120.degree. C. It is highly preferred to heat the mixture to a temperature 
in the range of from about 120.degree. C. to about 140.degree. C. The 
temperature should not exceed about 160.degree. C., as the 
tris(aryl)borane tends to slowly undergo thermal decomposition at or above 
this temperature. At temperatures above 160.degree. C., the thermal 
decomposition is accelerated in the presence of noninert impuritites. When 
the hydrocarbon solvent composition has a boiling point less than about 
115.degree. C., increased pressure is generally necessary for the 
distillation to occur in the desired temperature range. Distillation under 
increased pressure is preferably conducted at pressures up to about 40 
psig; more preferred are pressures in the range of from about 0.5 psig to 
about 30 psig. It may be necessary in the course of the invention to vary 
from the pressures described herein to attain the desired distillation 
temperature, as deemed necessary by those skilled in the art. 
When the temperature at which the tetrahydrofuran is removed is above the 
lower temperature of the boiling range of the hydrocarbon solvent 
composition, it is often necessary to replace some of the hydrocarbon 
solvent composition during the tetrahydrofuran distillation. Typical 
amounts of replacement hydrocarbon solvent composition are in the range of 
from about 1.0 to about 4.0 times the initial amount of hydrocarbon 
solvent composition used. It is preferred to conduct the tetrahydrofuran 
distillation in a hydrocarbon solvent composition that is boiling. 
The following examples are presented for purposes of illustration, and are 
not intended to impose limitations on the scope of this invention.

EXAMPLE 1 
Tris(pentafluorophenyl)boranetetrahydrofuran, made by mixing 1.0 g of solid 
tris(pentafluorophenyl)borane in 2.0 g of dry tetrahydrofuran, is added 
slowly under nitrogen to 45.0 g of Isopar-E at 110.degree. C. to 
118.degree. C. Isopar-E is a product of Exxon, which is a mixture of 
paraffinic C.sub.8 hydrocarbons with a boiling point range of 114.degree. 
C. to 126.degree. C. The complexed tetrahydrofuran, as well as 37.0 g of 
Isopar-E, is removed by distillation at 125.degree. C. to 132.6.degree. C. 
during 80 minutes. The remainder of the mixture is cooled to 22.degree. 
C., and another 37.0 g Isopar-E is added to the mixture, dissolving the 
tris(pentafluorophenyl)borane. The yield of free 
tris(pentafluorophenyl)borane is 97%, with 79 ppm tetrahydrofuran 
remaining, as determined by NMR. 
EXAMPLE 2 
9 g (0.0176 mol) of tris(pentafluorophenyl)borane in 351 g of Isopar-E is 
transferred to a pressurizable reactor under nitrogen. Isopar-E is a 
product of Exxon, which is a mixture of paraffinic C.sub.8 hydrocarbons 
with a boiling point range of 114.degree. C. to 126.degree. C. 1.4 g 
(0.0194 mol) of dry tetrahydrofuran in 20.6 g of Isopar-E is added to the 
solution in the reactor to make the 
tris(pentafluorophenyl)boranetetrahydrofuran complex to form a 2.7 wt. % 
solution of the complex. The complex is observed as a solid at room 
temperature; as the solution is heated, the complex dissolves. The 
temperature is raised to 160.degree. C., and the pressure is raised to 19 
psig. The tetrahydrofuran as well as the Isopar-E is distilled, and is 
removed via a continuous feed outlet. Isopar-E is added via a continuous 
feed inlet during the distillation, such that a constant volume is 
maintained. The distillation is conducted during 4 hours at these 
conditions; a total of two times the original volume of Isopar-E is added. 
The yield of free tris(pentafluorophenyl)borane is at least 60%, with no 
tetrahydrofuran detected by .sup.1 H NMR. 
It is to be understood that the reactants and components referred to by 
chemical name or formula anywhere in the specification or claims hereof, 
whether referred to in the singular or plural, are identified as they 
exist prior to coming into contact with another substance referred to by 
chemical name or chemical type (e.g., another reactant, a solvent, or 
etc.). It matters not what preliminary chemical changes, transformations 
and/or reactions, if any, take place in the resulting mixture or solution 
or reaction medium as such changes, transformations and/or reactions are 
the natural result of bringing the specified reactants and/or components 
together under the conditions called for pursuant to this disclosure. Thus 
the reactants and components are identified as ingredients to be brought 
together in connection with performing a desired chemical reaction or in 
forming a mixture to be used in conducting a desired reaction. 
Accordingly, even though the claims hereinafter may refer to substances, 
components and/or ingredients in the present tense ("comprises", "is", 
etc.), the reference is to the substance, component or ingredient as it 
existed at the time just before it was first contacted, blended or mixed 
with one or more other substances, components and/or ingredients in 
accordance with the present disclosure. Whatever transformations, if any, 
that occur in situ as a reaction is conducted is what the claim is 
intended to cover. Thus the fact that a substance, component or ingredient 
may have lost its original identity through a chemical reaction or 
transformation during the course of contacting, blending or mixing 
operations, if conducted in accordance with this disclosure and with the 
application of common sense and the ordinary skill of a chemist, is thus 
wholly immaterial for an accurate understanding and appreciation of the 
true meaning and substance of this disclosure and the claims thereof. 
Each and every patent or other publication referred to in any portion of 
this specification is incorporated in toto into this disclosure by 
reference, as if fully set forth herein. 
This invention is susceptible to considerable variation in its practice. 
Therefore the foregoing description is not intended to limit, and should 
not be construed as limiting, the invention to the particular 
exemplifications presented hereinabove. Rather, what is intended to be 
covered is as set forth in the ensuing claims and the equivalents thereof 
permitted as a matter of law.