posted 12-01-1999 05:58 PM         

The basic theory and motive for this thread is the fact that Birch
reductions rely on solvated electrons to do the "dirty work" of
reduction. Now if one could do away with the alkali metal and just
have the solvated elctron, one could still perform the Birch reduction
of a benzyl alcohol like ephedrine, for example.

=========================================
US patent # 4187156 - Preparation of dihydroaromatic hydrocarbons

Abstract

Electrolytic reduction at the cathode of aromatic hydrocarbons in an
aqueous emulsive electrolysis medium in an undivided electrolytic cell
yields dihydroaromatic hydrocarbons.

References Cited
-----------------------
U.S. Patent Documents
3682791 Aug., 1972
3682794 Aug., 1972
3684669 Aug., 1972
3699020 Oct., 1972
3700572 Oct., 1972
-------------------------
Claims

1. A process for the preparation of dihydroaromatic hydrocarbons which
comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzene, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide to yield
the dihydroaromatic hydrocarbon, wherein the electrolytic reduction is
conducted in the substantial absence of amine salts.

2. The process of claim 1 wherein the aromatic hydrocarbon is benzene
and the dihydroaromatic hydrocarbon is 1,4-cyclohexadiene.

3. The process of claim 1 wherein the quaternary ammonium hydroxide is
a tetraalkylammonium hydroxide.

4. A process for the preparation of dihydroaromatic hydrocarbons which
comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzene, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide to yield
the dihydroaromatic hydrocarbon, wherein the concentration of the
quaternary ammonium hydroxide is maintained by removal of carboxylate
ions formed in situ in the electrolysis medium and simultaneous
replacement by hydroxide ions.

...

7. A process for the preparation of dihydroaromatic hydrocarbons
which comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzene, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide, in absence
of amines or ammonia to yield the dihydroaromatic hydrocarbon, the
anode in the cell being of conductive material causing very little
reoxidation of dihydroaromatic hydrocarbon, being a de Nora-type
dimensionally stable anode, and in which a cathode selected from
mercury, zinc, lead or cadmium is used.

8. A process for the preparation of dihydroaromatic hydrocarbons which
comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzenes, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide, in
absence of amines or ammonia to yield the dihydroaromatic hydrocarbon,
the anode in the cell being of conductive material causing very
little reoxidation of dihydroaromatic hydrocarbon, being stainless
steel.

9. A process for the preparation of dihydroaromatic hydrocarbons which
comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzene, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide, in absence
of amines or ammonia to yield the dihydroaromatic hydrocarbon, the
anode in the cell being of conductive material causing very little
reoxidation of dihydroaromatic hydrocarbon, being made of precious
metal oxides plated on a titanium substrate.

10. A process for the preparation of dihydroaromatic hydrocarbons
which comprises electrolytic reduction at the cathode in an undivided
electrolytic cell by passing a direct electric current through a basic
aqueous emulsive electrolysis medium comprising an aromatic
hydrocarbon selected from the group consisting of benzene, biphenyl,
naphthalene, and alkyl derivatives thereof having 1 to 4 carbon atoms
and an aqueous solution of a quaternary ammonium hydroxide, in
absence of amines or ammonia to yield the dihydroaromatic hydrocarbon,
the anode in the cell being of conductive material causing very little
reoxidation of dihydroaromatic hydrocarbon, being nickel.
-------------------------------------------------------------------
Description
-------------------------------------------------------------------
BACKGROUND OF THE INVENTION

This invention relates to a process for the preparation of
dihydroaromatic hydrocarbons. More particularly, this invention
relates to the electrolytic reduction at the cathode of aromatic
hydrocarbons in an aqueous emulsive electrolysis medium in an undivided
electrolytic cell to yield the corresponding dihydroaromatic
hydrocarbons.

The electrolytic reduction of aromatic hydrocarbons to the
corresponding dihydroaromatic hydrocarbons is known in the art. As
known processes for effecting this transformation, there are
exemplified the following:

(1) A method for electrolytically reducing aromatic hydrocarbons to the
corresponding dihydroaromatic hydrocarbons in a divided electrolytic
cell by subjecting an organic solvent free catholyte composed of a
heterogeneous mixture of such aromatic hydrocarbon and an aqueous
solution of one or more quaternary ammonium salts to electrolysis at a
temperature from 30.degree. C. to 100.degree. C. This process is
described in Hatayama et al, U.S. Pat. No. 3,700,572.

(2) A process for electrochemically reducing aromatic compounds in the
presence of an aqueous system containing an amine, an inorganic acid
or the ammonium salt thereof, and an electron deficient compound
soluble in the amine, such as boron trifluoride, preferably in an
undivided electrolytic cell to yield the corresponding dihydroaromatic
hydrocarbon, this process being described in Matthews, U.S. Pat. No.
3,684,669.

(3) A process as described in Matthews, U.S. Pat. No. 3,682,791 for
electrochemically reducing aromatic compounds in the presence of a
substantially anhydrous system containing an amine, an inorganic acid
or the ammonium salt thereof, and an electron deficient compound
soluble in the amine, such as boron trifluoride, preferably in an
undivided electrolytic cell.

(4) A process for electrochemically reducing aromatic compounds in the
presence of an amine, an inorganic acid or ammonium salt thereof, and
a hydrophobic quaternary ammonium salt, which process is disclosed in
Matthews, U.S. Pat. No. 3,682,794.

(5) A process for electrochemically reducing aromatic compounds in the
presence of anhydrous methylamine (and other low molecular weight
amines such as ethylamine, ethylenediamine, or the like) containing
lithium chloride in an undivided cell to yield the corresponding
dihydroaromatic compounds, which process is described in Benkeser et
al, Journal of the Amerian Chemical Society, 86, 5272-5276 (1964).

These and other prior art processes, however, have various drawbacks,
such as requiring divided electrolytic cells, complex electrolysis
media, anhydrous conditions, or some combination thereof, none of which
are conductive to economical commercial development.

Thus the present process, whereby the electrolytic reduction of
aromatic hydrocarbons to yield dihydroaromatic hydrocarbons is
effected in a simplified and aqueous emulsive electrolysis medium in
an undivided electrolytic cell, is a decided and useful advance in the
state of the art.

DETAILED DESCRIPTION OF THE INVENTION

Electrolytic reduction of aromatic hydrocarbons in an aqueous emulsive
electrolysis medium in an undivided electrolytic cell yields
dihydroaromatic hydrocarbons.

In accordance with the present process, a direct electric current is
passed through an aqueous emulsive electrolysis medium comprising an
aromatic hydrocarbon selected from the group consisting of benzene,
biphenyl, naphthalene, and alkyl derivatives thereof having 1 to 4
carbon atoms, and an aqueous solution of a quaternary ammonium
hydroxide in an undivided electrolytic cell to yield the corresponding
dihydroaromatic hydrocarbon.

The "aqueous emulsive electrolysis medium" employed herein is a
heterogeneous mixture of an aqueous solution of the quaternary
ammonium hydroxide and the aromatic hydrocarbon subjected to vigorous
agitation in the undivided electrolytic cell to form an emulsion.

The term "emulsion" is employed in its usual recognized sense of a
fluid consisting of a microscopically heterogeneous mixture of two
normally immiscible liquid phases, in which one liquid forms minute
droplets suspended in the other liquid.

Exemplary of the aromatic hydrocarbons which may be employed in the
present process are benzene, biphenyl, naphthalene, and alkyl
derivatives thereof having 1 to 4 carbon atoms such as toluene,
ethylbenzene, n-propylbenzene, isopropylbenzene, n-butylbenzene,
isobutylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene,
4-isopropyltoluene (p-cymene), o-diethylbenzene, m-diethylbenzene,
2-methylbiphenyl, 3-methylbiphenyl, ... and the like.

In carrying out the present process, an aromatic hydrocarbon selected
from among those described hereinabove is charged to an undivided
electrolytic cell fitted with a cathode and an anode, and an
electromotive force is impressed upon the cell whereby the aromatic
hydrocarbon undergoes electrolytic reduction to yield the
corresponding dihydroaromatic hydrocarbon. The reaction involved can be
illustrated using benzene as an example. ##STR1##

The electrolysis is carried out in the absence of ammonia, amines,
various electron deficient compounds commonly employed as
catalysts--boron trifluoride, aluminum chloride, aluminum bromide,
tetracyanoethylene, for example--in an aqueous emulsive electrolysis
medium comprising the aromatic hydrocarbon and an aqueous solution
of a quaternary ammonium hydroxide. This medium, which is greatly
simplified over the media of the prior art, permits the reduction to
occur at the cathode with a minimum of interference by side reactions,
for example, reoxidation at the anode, isomerization and further
reduction to the tetrahydroaromatic hydrocarbon, and the like.

In an exemplary method of conducting the present process, a mixture of
the aromatic hydrocarbon and an aqueous solution of
tetra-n-butylammonium hydroxide having a concentration between about 5
percent and about 50 percent, usually between about 10 percent and
about 30 percent, is charged to an undivided electrolytic cell
maintained at a temperature between about 50.degree. C. and about
85.degree. C. and having a mercury, zinc, lead, or cadmium cathode and
a nickel, stainless steel, or a de Nora-type dimensionally stable
anode. The concentration of the tetra-n-butylammonium hydroxide
(or in general the quaternary ammonium hydroxide) is advantageously
between about 10 percent and about 30 percent in that the current
efficiency is maintained at a high level even at increased percent
conversion of the aromatic hydrocarbon. At lower concentrations, the
current efficiency, while initially high, tends to decrease as the
percent conversion increases. Higher concentrations, of course, do not
suffer from this disadvantage; however, no particular advantage is
gained by employing such higher concentrations.

The aromatic hydrocarbon and the aqueous solution of the
tetra-n-butylammonium hydroxide (or in general, the quaternary ammonium
hydroxide) is advantageously employed in a weight ratio such that the
molar ratio of the aromatic hydrocarbon to the tetra-n-butylammonium
hydroxide is initially between about 1:1 and about 10:1. A weight
ratio sufficient to establish the molar ratio ratio initially between
about 2:1 and about 5:1, plus about a 10 percent excess of the aromatic
hydrocarbon is generally preferred.

Following the charging of the mixture to the undivided electrolytic
cell, the mixture is vigorously agitated to form a fairly homogeneous
dispersion--that is, the aqueous emulsive electrolysis medium--and a
direct electric current is then impressed on the cell by connecting
the cathode and anode to a proper source of direct current with
controls to maintain the current density between about 0.05 and 10 or
more amperes per square centimeter for a time sufficient to cause the
desired reduction of the aromatic hydrocarbon to the corresponding
dihydroaromatic hydrocarbon, which then is isolated as described
hereinbelow.

The aqueous emulsive electrolysis medium employed in the present
process will be basic due to the presence of the quaternary ammonium
hydroxide and no particular provisions are necessary to regulate this
parameter. However, as the reaction proceeds, carboxylic acids, for
example, acetic acid, propanoic acid, butanoic acid, and pentanoic
acid, various other compounds, including N-butylacetamide (when
tetra-n-butylammonium hydroxide is employed as the quaternary ammonium
hydroxide), cyclohexanediol, plus smaller amounts of unidentified
components are produced at the anode. The carboxylic acids gradually
lower the pH by reaction with the quaternary ammonium hydroxide to
form carboxylates. This phenomenon results in an undesirable reduction
in the current efficiency which, as noted hereinabove, occurs even
more readily when low concentrations of quaternary ammonium hydroxide
are employed. ...

...

The temperature at which the process of the present invention is
conducted is not narrowly critical and can range from as low as about
20.degree. C. to as high as about 100.degree. C. or even higher if a
pressure cell is employed, with temperatures between about 50.degree.
C. and about 85.degree. C. being generally preferred. ...

...

The current densities employed in the process of the present invention
can range from as low as 0.01 ampere per square centimeter to 10 or
100 or more amperes per square centimeter of cathode surface area.
However, as noted hereinabove, the range between about 0.05 amperes
per square centimeter and about 10 amperes per square centimeter is
generally preferred.

The type of electrolytic cell employed in the process of the instant
invention is not critical. The cell can consist of a container made of
material capable of resisting action of electrolytes, for example,
glass or plastic, and having one or more cathodes and anodes connected
to a source of direct electric current such as a battery and the
like.

The electrodes, that is, the cathode and anode employed in the process
of the present invention can be constructed of a wide variety of
conductive materials. However, cathode materials suitable for use in
the present process are preferably high hydrogen overvoltage materials,
including for example, mercury, zinc, lead, cadmium, and the like.
Mercury-coated surfaces of metals such as lead, nickel, gold, silver,
and platinum are also suitable.

...

Exemplary of the quaternary ammonium hydroxides which may be employed
in the present process are tetraalkylammonium hydroxides such as
tetramethylammonium hydroxide, tetraethylammonium hydroxide,
tetra-n-propylammmonium hydroxide, tetra-n-butylammonium hydroxide,
methyl-tri-n-butylammonium hydroxide, ethyl-tri-n-butylammonium
hydroxide, and the like. Of these, tetra-n-butylammonium hydroxide,
ethyltri-n-butylammonium hydroxide, and tetra-n-propylammonium
hydroxide are generally preferred because they maintain the current
efficiency at a high level even at increased percent conversion of
the aromatic hydrocarbon.

The term "quaternary ammonium" as employed herein has its usual
recognized meaning of a cation having four organic groups substituted
on the nitrogen.

The reaction time will generally range between about 15 hours and about
30 hours for batch operations. However, it will be recognized that
the actual time of reaction is variable and is determined by variables
such as the particular aromatic hydrocarbon, the quaternary ammonium
hydroxide, concentration of the components in the aqueous emulsive
electrolysis medium, percent conversion of the aromatic hydrocarbon to
the dihydroaromatic hydrocarbon, volume of the aqueous emulsive
electrolysis medium, current density, electrode materials and their
surface area and condition, and the like.

It is, of course, apparent to those skilled in the art that the aqueous
emulsive electrolysis medium must be a conducting medium to obtain the
best flow of current. While media of less than ideal conductivity can
be employed, it is preferred from an economic viewpoint not to have too
high a resistance. The conductivity can, if desired, be enhanced by
the addition of common supporting electrolytes such as electrolyte
salts having sufficiently high discharge potentials to the (aqueous
phase of the) aqueous emulsive electrolysis medium. In general, however
, the aqueous solution of the quaternary ammonium hydroxide in the
present process is highly conductive. Thus the addition of a supporting
electrolyte to the aqueous emulsive electrolysis medium is not
actually necessary. In fact, in most instances the addition of
electrolyte salts to the electrolysis medium is neither preferred nor
desirable because the electrodes of choice tend to exhibit decreased
stability in the presence of such salts. Or conversely stated, the
electrodes of choice tend to exhibit increased stability in the absence
of such electrolyte salts. And from an economic viewpoint, this
increased stability is highly advantageous, particularly in long-term
continuous operations.

The term "supporting electrolyte" as employed herein is an electrolyte
capable of carrying electric current but not discharging under
electrolysis conditions. It will be recognized, of course, that
discharge potentials will vary with electrode materials and their
surface conditions and various materials in the electrolysis medium.

The term "salt" is employed in its generally recognized sense to
indicate a compound composed of a cation and an anion such as produced
by the reaction of an acid with a base.

Exemplary of the supporting electrolytes which can be employed to
enhance the conductivity of the aqueous emulsive electrolysis medium
are quaternary ammonium sulfates, phosphates, perchlorates, and the
like such as tetramethylammonium, tetraethylammonium,
tetra-n-propylammonium, tetra-n-butylammonium,
methyltri-n-butylammonium, and ethyltri-n-butylammonium sulfates,
tetramethylammonium, tetraethylammonium, tetra-n-propylammonium,
tetra-n-butylammonium, methyltri-n-butylammonium, and
ethyltri-n-butylammonium phosphates, and the like.

It will be apparent to those skilled in the art that the pre-formed
quaternary ammonium salt can be employed. However, when the quaternary
ammonium salt is that which corresponds to the quaternary ammonium
hydroxide being employed, which arrangement is preferred, it can
either be charged directly--that is, as the pre-formed salt--to the
aqueous emulsive electrolysis medium (or more particularly to the
aqueous solution of the quaternary ammonium hydroxide), or
alternatively, it can be formed in situ by charging an appropriate
acid, for example, sulfuric acid, in appropriate amounts to the
aqueous solution of quaternary ammonium hydroxide. For example, when
tetra-n-butylammonium hydroxide is the quaternary ammonium hydroxide,
the charging of sulfuric acid to an aqueous solution thereof produces
tetra-n-butylammonium sulfate in situ in an amount equivalent to the
amount (gram equivalent weights) of added sulfuric acid.

The concentration of electrolyte salts in the aqueous phase of the
aqueous emulsive electrolysis medium, when used, can vary widely, for
example, from about 0.5 percent to about 50 percent or more by weight
of the aqueous phase of the aqueous emulsive electrolysis medium.
Suitable concentrations, however, will often be in the range of about
1.0 percent to about 25 percent, with the percent concentration ratio
of the quaternary ammonium hydroxide to the quaternary ammonium salt
usually being between about 4:1 and about 1:4.

It will of course be apparent to those skilled in the art that the
concentration of the quaternary ammonium salt (when used) is to a
certain extent determined by the concentration of the quaternary
ammonium hydroxide. That is, if the concentration of the quaternary
ammonium hydroxide is fixed at a predetermined value, the percent
concentration ratio range will in turn fix the range of concentration
of the quaternary ammonium salt. But as noted hereinabove, the
employment of such electrolyte salts is not a necessary requirement or,
in most instances, even desirable.

...

Thus the present invention provides a significant advance in the state
of the art by effecting the electrolytic reduction of aromatic
hydrocarbons to the corresponding dihydroaromatic hydrocarbons in an
undivided cell by employing a greatly simplified aqueous emulsive
electrolysis comprising the aromatic hydrocarbon and an aqueous
solution of a quaternary ammonium hydroxide.

The following examples illustrate the process of the present
invention. They are not to be construed as limitative upon the
overall scope thereof.

===================

From "Electrons in Liquid Ammonia" by J.C. Thompson, 1976

"The fact that dilute solutions containing equivalents amounts of
alkali and alkaline metals give virtually the same near-infrared
absorption spectra indictaes the presence of a common absorbing species
which must be described without reference to the cation. Indeed,
dilute solutions of solvated electrons electrochemically generated
in the presence of tetraalkylammonium ions with widely-varying
structural parameters are also optically indistinguishable from
those formed by the dissolution of metal atoms."

From "The journal of Chemical Physics", Vol 44, number 6,
page 2297, 15 march 1966.

"...The approximate times necessary at -78C for the optical density
due to the solvated electron to decay to one half its initial value
were 8, 20, 8, and 25 mirco-sec for mathanol, ethanol, isopropanol,
and n-butanol, respectively. These half-times are 10 to 20 times
longer than the corresponding values at room tempature."
"... at -78C, absorptions due to the solvated electron were obtained
for monomethylamine (T[.5]= 3 mirco-sec) and monoethylamine
(T[.5]= 3.5 mirco-sec). At -110C the absorption of the solvated
electron in diethylether was obtained (T[.5]= 2 mirco-sec)."

Here is a condensed table of the half-life of the solvated electron
in various solvents: (From the same work as above)

SUBATANCE/HALF-LIFE IN MIRCO-SECONDS
[all measuements at 25C unless otherwise stated]
100% glycerol / .44
63% glycerol- 37% water / .75
47% " - 53% " / .9
32% " - 68% " / 1.6
19% " - 1% " / 1.5
8.3% " - 92% " / 1.3
53% water - 47% ethanol / 3.4
36% " - 64% " / 2.2
20% " - 80% " / 2.5
10% " - 90% " / .40
50% ethylene glycol - 50% water / .85 @ 20C
10% " - 90% " / .6 @ 20C
70% methanol - 30% water / 2.7
79% isopropanol - 21% water / .7 @ 20C
70% " - 30% " / .65 @ 20C
31% glycerol - 69% ethanol / .5
12% " - 88% " / .75
50% methanol - 50% isopropanol / 10 @ -78C

From "Ionizing Solvents" by I. Junder 1970
from a table and its amendments and then text-

"NaOH....i[meaning insoluble in liquid ammonia]
Na2SO4...i
(NH4)2SO4...i"
"Alcohols: Simple and polyfunctional alcohols are miscible with liquid
ammoina. Phenols are also soluble.

Ethers: Diethylether is moderately soluble [in liquid ammonia].
Ethers having higher molecular weights are not very soluble.

Hydrocarbons: Alkanes are insoluble, while alkenes and alkynes
are slightly soluble. Benzene dissolves readily."

"All solutions of metals in liquid ammonia are metastable, though they
can be stored for long periods in the absence of catalysts
(impurities). Catalysts and in paticular finely divided metals
(platium asbestos, platium sponge, and raney nickel), favour
decompostion in accordence with:
M + x NH3 --> M(NH2)x + (x/2)H2
This decompostion is used for the preparation of alkali and alkaline
earth metal amides (amide reaction). It corresponds to the reaction
of alkali metals with water. The catalytic activity of many metal
salts (particularly iron salts) is due to the fact that the salt is
first reduced and the resulting finely divided metal catalyses the
amide formation."

================================
From "Organic Chemistry: An Introduction and a Guide" , Edited by
Manual M. Baizer, 1973
"Solutions containing solvated electrons may be produced in several
ways,... by dissolving metals in suitable polar solvents,... or by
electrolysis....Absorption and EPR spectroscopy have shown, at least
for dilute solutions, that the properties of the solutions are
strickly similiar, whether these have been prepared by electrolysis or
by dissolving metals.
...In a solution of low concentrations a Birch type of reduction is
found,..."
"The reaction between the solvated electron and a solvent molecule is
generally slow in the absense of a proton donor. The stablity of the
solvated electron in ammonia is well known, and even their reaction
with water in ammonia is slow. The mechanism of the dissapearence of
the solvated electrons in this medium has been found to be as follows:
H2O + NH3 <==> NH4(+) + OH(-)
e(-) in NH3 + NH4(+) --> 2 NH3 + .5 H2
"
===========================
From "Basics of Electroorganic Synthesis", by Demetrios K. Kyriacou,
1981

"If the cathode is made very negative (E = 2.68 V for the hydrated
electrons), electrons will be ejected from the cathode into the
surrounding molecules of the medium.... It is possible to use mixed
solvents provided one component is capable of solvating
electrons.... Solutions containing free electrons are blue."
===================
From "Organic Electrochemistry: An Introduction and a Guide" , Edited
by Manual M. Baizer, 1973 Page 842
" The electrode material also effects the both the current yeild and
the the product distrubiton of reductions run in amine media. However,
the cathode material is not critical as far as the electron release
is concerned since the cathodes made from such different materials as
carbon, Pt, Al, Cu, and Pb have served as electron releasing
electrodes."
=========================
The cathode material is really insignificant because all the reducing
is caused by the solvated electron and not some cathode surface
catalyized reaction. BUT, the cathode material does change the product
distrubition slightly becuase of some reaction at the surface, which
can be good, or can be bad. Read "The Journal of the American
Chemical Soceity" Vol. 91, page 4191 and you will get my drift on this
situtation.