Preparation of triarylamines

A process for the preparation of triarylamines, wherein the process comprises reacting a diarylamine and a haloaromatic compound in a reaction medium comprising a phase-transfer catalyst and an aromatic solvent.

FIELD OF THE INVENTION 
The present invention relates to improved processes for the preparation of 
triarylamines, and more particularly to processes which may be used for 
the preparation of electronic grade triarylamines. 
BACKGROUND OF THE INVENTION 
Triarylamines have been discovered to be excellent charge transport 
compounds in electrophotographic photoconductors. In electrophotography, a 
latent image is created on the surface of an imaging member which is a 
photoconducting material by first uniformly charging the surface and 
selectively exposing areas of the surface to light. The difference in 
electrostatic charge density is created between those areas on the surface 
which are exposed to light and those areas on the surface which are not 
exposed to light. The latent electrostatic image is developed into a 
visible image by electrostatic toners. The toners are selectively 
attracted to either the exposed or unexposed portions of the unexposed 
portions of the photoconductor surface, depending on the relative 
electrostatic charges on the photoconductor surface, the development 
electrode and the toner. 
Processes for the preparation of certain charge transport compounds are 
known reference for example, U.S. Pat. Nos. 4,299,983; 4,485,260; 
4,240,987; 4,764,625; and 4,299,983, the disclosures of each of these 
patents are incorporated herein by reference in their entirety. These and 
other references illustrate the Ullmann condensation of a diarylamine and 
a haloaromatic compound at high temperatures. 
The most common synthesis of triarylamines involves the coupling of a 
diarylamine and a haloaromatic compound, preferably an iodoaromatic 
compound, in the presence of a base and copper at high temperature (around 
200.degree. C.) as was first reported by F. Ullmann in 1903. These 
reactions are typically conducted in high boiling solvents such as 
nitrobenzene or o-dichlorobenzene. In addition, the reaction times are 
typically quite long. The drawbacks of the high reaction temperature and 
long reaction time limit the application of this reaction for the 
efficient preparation of triarylamines for use as hole transport molecules 
in photoconductors. 
Another process for tertiary amine preparation is disclosed in the Turner 
et al U.S. Pat. No. 4,764,625. Turner et al disclose that tertiary amines 
can be prepared by the condensation of a secondary amine with a mono- or 
diiodoaryl compound in the presence of potassium hydroxide and copper and 
a solvent mixture of C.sub.13 -C.sub.15 aliphatic hydrocarbons having a 
boiling point of at least 170.degree. C. However, in order to complete the 
reaction disclosed by Turner et al, a large excess of secondary amine is 
needed because of the limited solubility of the product in the solvent. In 
addition, increased difficulty is encountered in separating any inorganic 
solids from the desired reaction product, resulting in a reduced yield of 
the triarylamine product. 
Various copper catalysts have been disclosed for use in the condensation 
reaction. A process is disclosed in DE 4,427,121 in which a diarylamine is 
condensed with an iodoaryl compound in the presence of Cu (OAc).sub.2 and 
Zn. N,N'-diphenyl-N,N'-di(3-tolyl)-p-benzidine (TPD) was generated in a 
yield of 71% after the reaction was heated at temperatures of 
220-230.degree. C. for four hours in a large excess of 
3-methyldiphenylamine. 
Recently, the Goodbrand et al U.S. Pat. Nos. 5,654,482 and 5,648,539 
disclosed a process in which aniline is reacted with iodoaryl compounds in 
the presence of an alkali metal hydroxide and a ligated copper catalyst 
(for example, 1,10-phenanthrolato-copper (I) chloride). 
However, there remains a need for an efficient process for the preparation 
of triarylamines at high yield under relatively mild conditions. In 
addition, there remains a need for a simple purification process to obtain 
high purity triarylamines, particularly in good yields. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide novel 
processes for the preparation of triarylamines which overcome one or more 
disadvantages of the prior art. It is a more specific object of the 
invention to provide simplified processes for the preparation of 
triarylamines, to provide processes for preparation of triarylamines in 
high yields, and/or to provide processes for preparation of triarylamines 
of high purity. 
These and additional objects and advantages are provided by processes for 
the preparation of triarylamines of the present invention, which generally 
comprise an enhanced Ullmann reaction using phase-transfer catalysts. 
One embodiment of the present invention relates to a process for the 
preparation of a triarylamine, which process comprises reacting a 
diarylamine and a haloaromatic compound in a reaction medium comprising a 
phase-transfer catalyst and an aromatic solvent, wherein the reaction is 
conducted at a temperature less than 200.degree. C. 
Another embodiment of the present invention relates to a process for the 
preparation of a triarylamine, which process comprises reacting a 
diarylamine and a haloaromatic compound in a reaction medium comprising a 
phase-transfer catalyst and an aromatic solvent, wherein the reaction is 
conducted in less than about 8 hours and results in greater than about 80% 
yield of triarylamine. 
Yet another embodiment of the present invention relates to a process for 
the preparation of a triarylamine, which process comprises reacting a 
diarylamine and a haloaromatic compound in a reaction medium comprising a 
phase-transfer catalyst and an aromatic solvent, wherein the aromatic 
solvent has a boiling point less than 180.degree. C. 
The processes of the present invention are advantageous in providing 
simplified preparation of triarylamines, good yields of triarylamines, 
and/or triarylamines of high purity. These and additional objects and 
advantages will be further apparent in view of the following detailed 
description.

DETAILED DESCRIPTION 
In the processes of the present invention for the preparation of a 
triarylamine, a diarylamine and a haloaromatic compound are reacted in a 
reaction medium comprising a phase-transfer catalyst and an aromatic 
solvent in the presence of a base and copper catalyst. 
Diarylamine compounds for use in the present processes are known in the 
art. In a preferred embodiment, the diarylamine is of the general formula 
Ar.sub.1 --Ar.sub.2 --NH, wherein Ar.sub.1 and Ar.sub.2 each and 
independently comprise substituted phenyl. Substituents for the phenyl 
groups may comprise, but are not limited to, alkyl groups, alkoxy groups, 
halogen atoms, and the like. 
Haloaromatic compounds suitable for use in the invention are also known in 
the art. In a preferred embodiment, the haloaromatic compound employed in 
the process of the present invention comprises an iodoaromatic compound. 
Preferably, the idioaromatic compound comprises a monoiodoaryl compound, a 
diiodoaryl compound, or a mixture thereof. The monoiodoaryl compound may 
have the general formula Ar.sub.3 --I, wherein Ar.sub.3 is a substituted 
or unsubstituted aryl, with suitable substituents including, but not 
limited to phenyl. In another preferred embodiment, the diiodoaryl 
compound may have the general formula I--Ar.sub.3 --Ar.sub.4 --I, wherein 
Ar.sub.3 and Ar.sub.4 are independently substituted or unsubstituted aryl, 
with suitable substituents including, but not being limited to phenyl. In 
further preferred embodiments, the haloaromatic compound is a monoiodoaryl 
compound comprising iodobisphenol, or a diiodoaryl compound comprising 
4'4'-diiodobiphenyl. 
Various phase transfer catalysts are known in the art and are suitable for 
use in the present invention. Generally, a phase-transfer catalyst will 
improve contact between reactants which typically reside in different 
phases of a reaction medium. In one embodiment of the present invention, 
the phase-transfer catalyst comprises a substituted or unsubstituted 
X-crown-Y ether, wherein X ranges from about 11 to about 21 and represents 
the number of carbon and oxygen atoms on the crown and Y ranges from about 
2 to about 7 and represents the number of oxygen atoms on the crown. More 
preferably, the phase-transfer catalyst comprises 18-crown-6 ether or 
dibenzo-18-crown-6 ether. 
The reaction medium is heated to a temperature sufficient to achieve 
reaction of the diarylamine and the haloaromatic compounds. 
Conventionally, temperatures of greater than 200.degree. C. are employed. 
However, in one embodiment of the present processes, the reaction is 
conducted at a temperature of less than about 200.degree. C. Preferably, 
the reaction is conducted at a temperature of less than about 190.degree. 
C.; more preferably less than about 180.degree. C.; and most preferably at 
a temperature of less than about 170.degree. C. These relatively lower 
temperatures advantageously allow simpler preparation of the desired 
triarylamines. 
Various aromatic solvents are known in the art and are suitable for use in 
the present methods In one embodiment of the present invention, the 
aromatic solvent comprises an alkylbenzene. Alkyl substituents of 1 to 
about 8 carbon atoms are preferred, with alkyl groups of 1 to about 4 
carbon atoms being further preferred. In one embodiment, the aromatic 
solvent comprises ethylbenzene or diethylbenzene. In another embodiment, 
the aromatic solvent comprises a di-methylated benzene, tri-methylated 
benzene, or a mixture thereof. Preferably, the di- or tri-methylated 
benzene comprises trimethylbenzene, m-xylene, o-xylene, p-xylene, or a 
mixture thereof. To facilitate reactions at relatively lower temperatures, 
the solvent preferably has a boiling point less than about 180.degree. C., 
although solvents with higher boiling temperatures may be employed. 
Once the triarylamine product has been formed and the reaction is complete, 
the product may be purified according to various techniques. In one 
embodiment, the process of the present invention further comprises 
filtering the reaction medium to remove inorganic components from the 
solution and precipitating triarylamine from the resulting filtrate. 
Advantageously, the inorganic component may be separated from the organic 
materials by simple hot filtration, for example by filtering the reaction 
mixture through a paper filter. The triarylamine may then be precipitated 
from the resulting filtrate by addition of a suitable precipitating agent. 
In a preferred embodiment, the resulting filtrate is precipitated by 
addition of hexane. 
In a further embodiment, the triarylamine precipitate is further purified. 
Preferably, the precipitate is redissolved and mixed with an absorbent 
which absorbs remaining impurities. For example, the triarylamine 
precipitate may be redissolved in toluene and then mixed with an aluminum 
oxide absorbent. The dissolved triarylamine product loaded with aluminum 
oxide is then eluted with a suitable solvent, preferably a mixture of 
solvents. In one embodiment, a solvent mixture comprising toluene and 
heptane is employed. Preferably, the solvent mixture comprises toluene to 
heptane in a weight ratio of about 2:1. The solution is then vacuum 
evaporated to remove the solvent, i.e., the mixture of toluene and 
heptane, and yield the triarylamine as a white powder. 
The processes for reacting a diarylamine and a haloaromatic compound in a 
reaction medium comprising a phase-transfer catalyst and an aromatic 
solvent according to the invention are advantageous in that they allow 
preparation of triarylamines in relatively short reaction times and in 
relatively high yields. In preferred embodiments, the reaction is 
conducted in less than about 8 hours and results in greater than about 80% 
yield of triarylamine. 
The processes of the present invention may be used to prepare various 
triarylamine compounds. In a preferred embodiment, a diarylamine 
comprising 3-methyldiphenylamine is reacted with a haloaromatic comprising 
4'4'-diiodobiphenyl and the resulting triarylamine comprises 
N,N'-diphenyl-N,N'-di(3-tolyl)-p-benzidine (TPD). 
The following examples demonstrate various embodiments and advantages of 
the inventive processes for the preparation of a triarylamine. The 
examples further demonstrate characterization of the triarylamine prepared 
under the processes of the present invention as a charge transport 
material. In the examples and throughout the present specification, parts 
and percentages are by weight unless otherwise indicated. 
EXAMPLE 1 
In this example, triarylamine is prepared according to the present 
invention. Specifically, 
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-p-benzidine (TPD) was prepared 
according to a process of the present invention. 
Into a 500 ml, 3-neck flask equipped with a mechanical stirrer and a 
condenser, 24.3 grams of 4,4'-diiodobiphenyl (60 mmol), 27.6 grams of 
3-methyldiphenylamine (150 mmol), 3.0 grams of 18-crown-6 ether and 30 ml 
of mixed xylenes were placed in the flask. The mixture was then purged 
with nitrogen while heat was applied. After the reaction mixture started 
refluxing, 30 grams of potassium hydroxide and 30 grams of copper powder 
were added to the reaction mixture. Gentle refluxing was maintained for 
about 5-8 hours (at a temperature of about 145.degree. C.) under nitrogen. 
The completion of reaction was indicated by TLC (thin layer 
chromatography) or HPLC (high pressure liquid chromatography) analyses. 
The reaction product was isolated by hot filtration to remove inorganic 
solids. The organic solvent was then removed and the crude product was 
precipitated by addition of hexanes to give a brown powder, which was 
comprised of TPD, unreacted amine and by-products. The crude product was 
further purified by dissolving the product in toluene and adding aluminum 
oxide. The final purified product was obtained by eluting the crude 
product aluminum oxide with a mixture of toluene and heptane (in a weight 
ratio of about 2:1) to yield about 82% TPD as a white colorless powder. 
The chemical composition and structure of the TPD prepared by the process 
of the present invention was analyzed by mass spectrometry, UV analysis, 
and HPLC. A commercial TPD sample was used as a reference against the TPD 
prepared under the present invention. 
EXAMPLE 2 
In this example, triarylamines according to the present invention and 
comparative triarylamines commercially obtained were incorporated into 
organic photoconductors, respectively. Each of the photoconductors 
described in this example was prepared by dip-coating a charge generation 
layer dispersion on an aluminum substrate, followed by dip-coating a 
charge transport layer dispersion on the formed charge generation layer. 
In each of the photoconductors, the charge generation layer comprised 
about 45 weight percent type-IV polymorph of titanyloxyphthalocyanine and 
about 55 weight percent polyvinylbutyral. The TPD samples formulated in 
Example 1 were incorporated into a charge transport layer in an amount of 
about 30 weight percent with about 70 weight percent polycarbonate-A 
(based on the weight of the charge transport layer) in a mixed solvent of 
tetrahydrofuran (THF) and 1,4-dioxane. 
The charge generation layer was coated on an aluminum substrate and dried 
at 100.degree. C. for 15 minutes to form a coating having a thickness of 
about 0.2-0.3 .mu.m. The charge transport solution was dip-coated and 
dried at 100.degree. C. for 60 minutes to form a 20 .mu.m thick uniform 
layer on top of the charge generation layer. 
The photoconductors of this example were subject to measurement of 
discharge voltage as a function of energy. Sensitivity measurements were 
made using an electrostatic sensitometer fitted with electrostatic probes 
to measure the voltage magnitude as a function of light energy shining on 
the photoconductor surface using a 780 nm laser. The drum was charged by a 
corona and exposed-to-develop time for all measurements was 257 
milliseconds. The photosensitivity was measured as a discharge voltage on 
the photoconductor drum previously charged to about -700 volts, measured 
at a light energy ranging from about 0.1 to about 1.8 J/cm.sup.2. 
The results of these measurements are set forth in FIG. 1 and demonstrate 
the surprising results that the triarylamines prepared under the process 
of the present invention perform similarly to commercially available 
triarylamines as charge transport compounds in organic photoconductors. 
The foregoing description of various embodiments of the invention has been 
presented for the purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed. Many alternatives, modifications and variations will be 
apparent to those skilled in the art of the above teachings. Accordingly, 
this invention is intended to embrace all alternatives, modifications and 
variations that have been discussed herein, and others that fall within 
the spirit and broad scope of the claims.