Electrocatalytic method for producing quinone methides

A method for producing quinone methides by the electrocatalytic oxidation of bis(4-hydroxyphenyl)methanes using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the oxidant. Spent oxidant, in the form of DDQH.sub.2, may be recycled and electrochemically regenerated to active DDQ oxidant.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for the production of quinone 
methides, and more particularly, it relates to a method for 
electrocatalytically oxidizing bis(4-hydroxyphenyl)methanes with 
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to produce hydroxyphenyl 
quinone methides. 
Quinone methides are known to be useful as antioxidants as taught by 
Coppinger U.S. Pat. No. 2,940,988. Coppinger discloses the oxidation of 
dihydroxydiphenylmethane with lead dioxide or lead tetraacetate to produce 
a free radical which is subsequently reduced to quinone methide. Reference 
is also made to Bacha U.S. Pat. No. 4,032,547 which discloses an oxidation 
process for preparing quinone alkides from the corresponding tri-alkyl or 
phenyl hindered phenols. The oxidizing agent of Bacha is ferricyanide as 
the secondary oxidant in combination with persulfate as the primary 
oxidant. 
Quinone methides are also useful starting materials in the preparation of 
dihydroxybenzophenones, as disclosed in our copending application Ser. No. 
816,502. The dihydroxybenzophenones may, in turn, be used as light 
stabilizing agents and precursors for epoxy resins, polycarbonate resins, 
and other thermoplastics. 
While prior art methods of preparing quinone methides exist, to date those 
methods have not found significant commercial utility because of their 
cost, inefficiencies, or other drawbacks. Accordingly, the need exists for 
a process by which large quantities of quinone methides can be produced 
economically and at high yields. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a method for 
economically producing large amounts of quinone methide of the type 
##STR1## 
where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are either alike or different 
and are hydrogen, straight or branched chain alkyl moieties, cyclic alkyl 
compounds, halogen compounds, hydroxy and methoxy groups, and combinations 
thereof. 
The instant method for preparing quinone methides of this type involves the 
oxidation of bis(4-hydroxyphenyl)methanes having the formula 
##STR2## 
where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are as in formula (I). 
The oxidant used to oxidize the bis(4-hydroxyphenyl)methanes of formula 
(II) to the quinone methides of formula (I) is 
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It has the formula 
##STR3## 
Preferably, the process is an electrocatalytic one wherein the DDQ oxidant 
is used in a homogeneous solution in the anode compartment of an 
electrochemical cell, which, for example, may contain a platinum anode. 
The potential applied may vary between 0.65 and 1.4 V vs. SCE, and more 
preferably between 0.75 and 1.2 V vs. SCE, and most preferably from 
0.75-0.95 V vs. SCE. Under these conditions, the DDQ may be used in 
catalytic (i.e. less than stoichiometric) amounts, i.e. between 10 and 100 
mole percent relative to the starting material. Likewise, since DDQH.sub.2 
is electrolytically oxidizable to DDQ it is possible to use DDQH.sub.2 as 
the starting oxidant material. Since DDQH.sub.2 is the form of the spent 
DDQ oxidant, it may be recycled and regenerated in situ in the process. It 
is not necessary to isolate either DDQ or DDQH.sub.2 from the solution. 
DDQH.sub.2 has the formula 
##STR4## 
As such, the overall electrocatalytic oxidation process can be expressed as 
follows: 
##STR5## 
The conversion of the bis(4-hydroxyphenyl)methane of formula (II) to the 
quinone methide of formula (I) is accomplished by the DDQ of formula (III) 
acting as an oxidant in homogeneous solution with the 
bis(4-hydroxyphenyl)methane of formula (II). The spent oxidant, DDQH.sub.2 
of formula (IV) may, then, be recycled and regenerated to DDQ (formula 
III) by electrooxidation at an electrode potential which is insufficient 
to directly oxidize the bis(4-hydroxyphenyl)methane of formula (II). 
The result is an efficient, economical, high yield process for the 
production of the quinone methides of formula (II) which, as mentioned, 
find utility as antioxidants and starting materials for preparations of 
dihydroxybenzophenones which are precursors for the production of epoxy 
resins, polycarbonate resins, and other thermoplastics. 
Accordingly, it is an object of the present invention to provide an 
inexpensive means to produce large quantities of quinone methides. 
Other objects and advantages of the invention will become apparent from the 
following description and the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred method is a catalytic oxidation reaction carried out in an 
electrochemical cell at room temperature and pressure. A divided batch 
electrochemical cell is fitted with working and auxiliary electrodes and a 
suitable reference electrode such as a saturated calomel reference 
electrode (SCE). The cathode (auxiliary) compartment is filled with a 
supporting electrolyte solution. Any number of solvent/supporting 
electrolyte solutions can be used so long as they provide acceptable 
solubilities for bis(4-hydroxyphenyl)methanes, quinone methides, DDQ, and 
DDQH.sub.2. 
The working electrode is the anode, which may be platinum, carbon or any 
other inert electrode material which remains stable at the oxidation 
potential. The anode compartment is filled with the supporting electrolyte 
solution and the starting materials. The required starting materials 
include both the oxidation catalyst and the substrate material. The 
working electrode is then biased to, and maintained at, a constant voltage 
vs. SCE using a three electrode potentiostat. During electrolysis, the 
anolyte solution is rapidly stirred using conventional stirring equipment. 
In one embodiment, the starting oxidation catalyst placed in the anode 
compartment is the DDQ of formula (III). DDQ is a known oxidant as taught 
by U.S. Pat. Nos. 4,518,535; 4,056,539, and 3,102,124. It may be purchased 
from Aldrich Chemical Company. In another embodiment, it may be the 
DDQH.sub.2 of formula (IV). In that instance, the electrolytic oxidation, 
at an electrode potential in the range of 0.65 to 1.4 V vs. SCE, oxidizes 
the DDQH.sub.2 to DDQ which, then, serves as the oxidation catalyst. 
DDQH.sub.2 is available as the reduced form of DDQ. Since DDQH.sub.2 is 
produced by the reaction process, it is thus possible to recycle it in 
situ in the anode compartment. In addition, since the DDQ oxidant is 
regenerated in situ it may be used in non-stoichiometric quantities in the 
range of 10 to 100 mole percent relative to the starting substrate 
material and preferably on the order of 10 mole percent. As a result of 
all of this, a relatively inexpensive source of oxidant is utilized in the 
process. In addition, use of an electrocatalytic process offers energy 
saving advantages over an electrolytic process, for example. 
The starting substrate material placed in the anode compartment is the 
bis(4-hydroxyphenyl)methane of formula (II). Bis(4-hydroxyphenyl)methanes 
of formula (II) where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are hydrogen, 
straight or branched chain alkyl moieties, cyclic alkyl compounds, halogen 
compounds, hydroxy or methoxy groups, or combinations thereof are 
available from Aldrich Chemical Company or Dow Chemical Company. 
The starting materials are dissolved in the supporting electrolyte solution 
in the anode compartment and stirred during application of a constant 
voltage vs. SCE of between 0.65 V and 1.40 V and most preferably of 
approximately 0.75 V-0.95 V vs. SCE. The electrolysis is allowed to 
proceed until integration of the cell current indicates a passage of 2 
Faradays of charge per mole of starting reactant material. At that time, 
the cell circuit is disconnected and the quinone methide isolated and 
separated. This may be accomplished by a two step process in which the 
solvent solution is removed from the quinone methide, DDQH.sub.2 and DDQ 
by evaporation. The quinone methide may, then, be separated from the 
DDQH.sub.2 and DDQ by dissolving it in a solvent which is a non-solvent 
for DDQH.sub.2 or DDQ. For example, quinone methide can be separated from 
DDQ and DDQH.sub.2 by dissolving the quinone methide in methylene 
chloride, filtering out the still solid DDQ or DDQH.sub.2, and, then, 
re-precipitating the quinone methide. 
The following example is illustrative. 
EXAMPLE 
This example illustrates the preparation of the quinone methide of 
bis(3,5-dimethyl-4-hydroxyphenyl)methane. A divided batch electrochemical 
cell was fitted with platinum working and auxiliary electrodes and a 
saturated calomel reference electrode (SCE). The cathode (auxiliary) 
compartment was filled with an electrolyte solution which contained 0.25M 
sodium acetate dissolved in one part by volume of acetic acid and four 
parts of acetonitrile. The anode (working) compartment was charged with 
one volume of water and then filled with five volumes of the electrolyte 
solution to which had been added 40 g. 
bis(3,5-dimethyl-4-hydroxyphenyl)methane and 36 g. DDQH.sub.2 per liter of 
electrolyte to give a 10 ml solution. The anode was then biased to 0.75 
volts vs. SCE. The electrolysis was continued for 148 minutes at which 
time current integration showed that there had passed 2 Faradays of charge 
per mole of bis(3,5-dimethyl-4-hydroxyphenyl)methane in the anode 
compartment. At that time the cell circuit was disconnected. Gas 
chromatographic analysis of the anolyte revealed essentially complete 
conversion of bis(3,5-dimethyl-4-hydroxyphenyl)methane to the 
corresponding quinone methide. 
Additional runs were made in this system with solvent/supporting solutions 
containing 160 mM bis(3,5-dimethyl-4-hydroxyphenyl)methane and DDQH.sub.2 
from 10 to 100 mole percent relative to the 
bis(3,5-dimethyl-4-hydroxyphenyl)methane. Essentially quantitative 
conversion to the quinone methide was observed in each instance. However, 
if the oxidation is allowed to continue past the 2 Faraday/mole stage, 
conversion of the quinone methide to 
3,3',5,5'-tetramethyl-4,4'-dihydroxybenzophenone takes place as disclosed 
in our copending application Ser. No. 816,502. 
While the methods herein described constitute preferred embodiments of the 
invention, it is to be understood that the invention is not limited to 
these precise methods and that changes may be made in the method without 
departing from the scope of the invention, which is defined in the 
appended claims.