Metal-assisted reactions. Part XXX.* control of rates of heterogeneously catalyzed transfer hydrogenolysis through changes in solvent composition

Article abstract of DOI:10.1071/CH02252

Adsorption isotherms in the liquid phase can be used to determine the relative strengths of adsorption of reactants and solvent at a catalyst surface. Such isotherms can then be used to indicate which type of solvent would be most suitable for a heterogeneously catalyzed reaction in the liquid-phase. Solubility in any chosen solvent is also important. As examples, rates of heterogeneously catalyzed liquid-phase transfer hydrogenolyzes of aryl tetrazolyl ethers (1) have been shown to be highly dependent on both the nature of the solvent and on the solution concentrations of the reactants. The rate of reaction can be varied from zero to a maximum and then back to zero simply by adjusting the solubility of the redundant through changes in the proportion of water in a mixed-solvent system.

Full text of DOI:10.1071/CH02252

CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2003, 56, 423–428
∗
Metal-Assisted Reactions. Part XXX. Control of Rates of
Heterogeneously Catalyzed Transfer Hydrogenolysis through
Changes in Solvent Composition
José A. C. Alves,A,B Amadeu F. BrigasA,C and Robert A. W. JohnstoneA,D
A
Department of Chemistry, University of Liverpool, Liverpool, L69 3BX, UK.
FCT, Campus de Gambelas, Universidade do Algarve, Faro 8000, Portugal.
Corporacao Industrial do Norte, S.A., Estrada Nacional 13, Km 6, Maia 4470, Portugal.
Author to whom correspondence should be addressed (e-mail: rj05@liverpool.ac.uk).
B
C
D
Adsorption isotherms in the liquid phase can be used to determine the relative strengths of adsorption of reactants
and solvent at a catalyst surface. Such isotherms can then be used to indicate which type of solvent would be
most suitable for a heterogeneously catalyzed reaction in the liquid-phase. Solubility in any chosen solvent is also
important. As examples, rates of heterogeneously catalyzed liquid-phase transfer hydrogenolyzes of aryl tetrazolyl
ethers (1) have been shown to be highly dependent on both the nature of the solvent and on the solution concentrations
of the reactants.The rate of reaction can be varied from zero to a maximum and then back to zero simply by adjusting
the solubility of the reductant through changes in the proportion of water in a mixed-solvent system.
Manuscript received: 5 December 2002.
Final version: 13 February 2003.
are not generally a serious option for most liquid-phase chem-
Introduction
istry, particularly when chirality or the integrity of other
functional groups must be conserved. For a catalyzed reac-
tion to proceed at a reasonable rate in the liquid phase it needs
to be efficient at temperatures in a restricted range of about
Understanding the basis of heterogeneous catalysis in the
gas phase is of considerable economic importance to large-
scale chemistry and is examined widely, being supported
[1]
◦
by a galaxy of instrumental techniques. In contrast, fun-
damental theoretical and experimental approaches to the
economically more important heterogeneous catalysis of a
multitude of much smaller-scale reactions in the liquid phase
receive far less attention, possibly because of the extra diffi-
culties introduced by solvents in an already complex area of
0–150 C. For the reasons just outlined, even small changes
in the type and composition of a solvent system can have
a significant impact on the efficiency of a catalyzed liquid-
phase process. Accordingly, an ability to enhance catalytic
activity at modest temperatures in the liquid phase by simply
altering the composition of a solvent system can be of crucial
importance.
To help fill the gap in fundamental knowledge concern-
ing heterogeneously catalyzed liquid-phase processes, results
available from homogeneous catalytic liquid-phase chem-
istry are frequently transferred conceptually to notionally
corresponding heterogeneous reactions, mostly through intu-
ition rather than through any fundamental conviction of the
‘fitness’ of such applications. Solvation effects in solution
are unlikely to be helpful in describing solvent effects at the
surface of a heterogeneous catalyst.
[2]
research. For heterogeneous catalysis in the liquid phase,
the solvent can influence rates of reaction through solvation
of reactants and intermediates in solution, but it can also
affecttheratebycompetingwithreactantmoleculesforactive
sites on the surface of a heterogeneous catalyst. Solvent may
also stabilize or destabilize transition states or intermediates
formed on the catalyst surface. The competition for surface
sites can be selective in that the effect of solvent on reactant
(
A) is unlikely to be same as its effect on reactant (B). When
a combination of solvents is used, their effects on binding of
the two reactants become more complex.
In one particular area, the heterogeneous catalysis of
liquid-phase reduction, there has been some consideration of
The temperature range available for liquid-phase reac-
tions is usually much more restricted than it is for gas-phase
chemistry, for which even modest catalysts at near ambient
temperatures can become important if the reaction tempera-
ture is increased sufficiently. Large increases in temperature
[2]
the importance and influence of solvent on reactivity. It has
beenshownthatratesofheterogeneoushydrogenolysisofaryl
[3]
ethers may be controlled by use of biphasic systems. It is
also clear that, in aqueous/organic solvent biphasic systems,
∗
Part XXIX: A. F. Brigas, W. Clegg, C. J. Dillon, C. F. C. Fonseca, R. A. W. Johnstone, J. Chem. Soc., Perkin Trans. 2 2001, 1315.
CSIRO 2003 10.1071/CH02252
©
0004-9425/03/05423

4
24
J. A. C. Alves, A. F. Brigas and R. A. W. Johnstone
Ph
1000
Ph
Ph
N
N
N
N
N
N
N
N
Pd/C
N
N
N
N
ArO
ArH ϩ HO
O
NaHPO2
8
6
4
2
00
00
00
00
0
(
1); (1a) Ar = 2-naphthyl
(2)
H
Scheme 1.
even the hydrophobicity or hydrophilicity of the heteroge-
neous catalyst is very important in improving or impeding
[
4]
reactivity.
Toluene
In the present work, the influence of solvent composition
on rates of heterogeneous catalytic transfer hydrogenolysis
has been examined. The approach is aimed at real working
catalysts and commercially available catalysts have been
used in the present work rather than idealized single-crystal
surfaces.
THF
Ethanol
0
5
10
15
20
Results and Discussion
6
Amount of ether (1) adsorbed (ϫ10 mol)
Hydrogenolysis of Aryl Tetrazolyl Ethers to Arenes
Fig. 1. Typical adsorption isotherms for ether (1a) adsorbing onto a
Phenols can be converted easily into aryl tetrazolyl ethers (1),
which in turn are easily hydrogenolyzed to an arene
2
−1
Pd/C (10% w/w) catalyst having a specific metal area of 26 m g
.
−6
Solutions (L, M, N; 20 × 10 M) of ether (1a) were prepared for each
of the solvents toluene (L),THF (M), and ethanol (N). In a typical exper-
iment, an amount of catalyst (x mg) was weighed out and stirred with
an aliquot of solution L for 15–20 min. The catalyst was filtered off
and the amount of ether (1a) remaining in solution was determined by
UV/visible absorption measurements. The quantity of catalyst was var-
ied between 0 < x < 1000 mg. Finally, the amount of ether (1a) adsorbed
on the catalyst was plotted against the weight of catalyst used. The
determinations were repeated for solutions M and N, giving the three
isotherms shown.
(
(
2) through use of a hydrogen donor and Pd/C catalyst
Scheme 1). This reaction provides a simple rapid means for
[5]
ipso replacement of a phenolic OH by H.
The influence of the heterocyclic part of these ethers
on C O bond strength has been revealed through X-ray
[6]
studies. On the basis of these results, it has been possible
to extend Scheme 1 to the hydrogenolysis of similar pseudo-
[7]
saccharyl ethers and to devise a novel cross-coupling
reaction with organometallic reagents, in which a phenolic
[
8]
C O bond is replaced by a C C bond (C-alkyl or C-aryl).
In the present work, the role of solvent composition on the
rate of hydrogenolysis (Scheme 1) is examined. For conve-
nience, 5-(2-naphthoxy)-1H-tetrazole (1a) was chosen for
hydrogenolysis because the rate of formation of the naphtha-
lene product of this high yielding, rapid reaction is easily
monitored by gas chromatography. Water-soluble sodium
phosphinate was used as the hydrogen donor.
The observed relative binding efficiencies of solvent
and reactants, as measured by binding constants K,
[9–11]
suggested that it might be possible to change the rate of
hydrogenolysis(Scheme1)inapredictablemannerbymanip-
ulation of the type of solvent and also by regulation of the
solubilities of reactants through changes in solvent compo-
sition. This reaction is one in which reactant A (the naphthyl
ether (1a)) and reactant B (the hydrogen donor, sodium phos-
phinate) compete for sites on the catalyst surface before
hydrogen is transferred to complete the hydrogenolysis. The
reaction is known to be one in which H2 gas is not formed
but rather one in which H is transferred from reactant (B) to
Isotherms for Adsorption of Aryl Ethers (1) onto a Pd/C
Catalyst Surface
To gain some understanding of the role of solvent, a new type
of ‘Langmuir’isotherm has been used to measure the relative
strengths of adsorption of a variety of organic compounds
[
12]
(A) on the catalyst metal surface.
[
9–11]
onto commercial catalysts from a range of solvents.
In a first set of experiments, a monophasic mixed solvent
system was used and consisted of THF containing water, the
water content varying from 0 to 50% (v/v).The ether (1a) was
solubleinallcases.Thehydrogendonor, sodiumphosphinate,
is not soluble inTHF but, as more water is added to the solvent
system, its solubility increases. By changing its solubility, it
is then possible to alter the amount of phophinate that can
adsorb onto the catalyst surface so that the phosphinate then
competes increasingly with ether (1a) for a limited number of
sites on the catalyst surface. For each change in theTHF/H2O
composition, the initial rate of formation of naphthalene was
measured.
Some typical isotherms are shown in Figure 1 for ether (1a).
The ability of ethers (1) to adsorb onto the Pd metal of a 10%
Pd/C catalyst surface is strongly dependent on the solvent.
For example, in ethanol, adsorption of ether (1a) is suffi-
ciently strong that it almost entirely prevents co-adsorption
of solvent onto the catalytic metal but, for toluene, the solvent
competes significantly with the ether for sites on the metal
surface.The behaviour of tetrahydrofuran (THF) lies between
those of toluene and ethanol (Fig. 1). Solvents clearly play an
important role in the degree of adsorption of reactants from
solution onto a catalyst surface.

Rate-Controlled Heterogeneously Catalyzed Transfer Hydrogenolysis
425
Forcompletecoverageofthecatalystsurface(highlylikely
in a real system in solution for which the concentrations of
solutes are high), θA + θB = 1. Thus, by rearrangement of the
Equations (3a) and (3b), Equation (4) for θB is obtained. A
similar expression may be obtained for θA.
C
1
ϩ
A
CA*
Initial values
sA
Ϫ
At equilibrium 1Ϫu ϪuB
s –s '
u
A
A
A
A
C
ϩ
B
CB*
ꢀ
KB(s − s )
B
B
θB =
(4)
Initial values
1
sB
Ϫ
ꢀ
ꢀ
A
KB(s − s ) + KA(s − s )
B
B
A
At equilibrium 1Ϫu ϪuB
s –s '
u
B
A
B
B
ꢀ
A
Usually, in solution during most of any reaction, s ꢁ s and
A
Scheme 2.
ꢀ
B
s ꢁ s , and this leads to new expressions for θA and θB, in
B
In a second set of experiments, the solvent composi-
tion remained constant (benzene/ethanol/water). Ether (1a)
is soluble in this system but sodium phosphinate is only spar-
ingly soluble. In successive experiments, the solubility of the
sodium phosphinate was increased by the addition of aliquots
of [18]crown-6, a phase-transfer agent that is known to solu-
bilize sodium salts into organic solvents.Thus, as more crown
ether was added, more phosphinate appeared in the monopha-
sic mixed solvent system and was able to compete with ether
which K = KA/KB (Equations (5a) and (5b)).
KsA
θA =
(5a)
(5b)
sB + KsA
KsB
θB =
sA + KsA
If r(= dP/dt) is the rate of reaction, the rate can be esti-
mated from Equation (1) after substituting for θA and θB from
Equations (5a) and (5b), as in Equation (6).
(
1a) for sites on the catalyst surface. Again, the rate of forma-
tion of naphthalene was monitored for each addition of crown
ether. The results of these experiments are described below.
ꢀ
k KsAsB
r = (
(6)
2
sB + KsA)
Expected Variation in the Rate of Hydrogenolysis
By differentiation of Equation (6) with respect to either
sA or sB, it is found that the maximum rate (rmax) is obtained
when KsA = sB so that the maximum rate of reaction on the
(
Scheme 1) with Variation in the Coverage of the
Catalyst Surface by Two Reactants
For two substances A and B reacting at the surface of a cat-
alyst, the rate of formation of product P can be written as in
ꢀ
catalyst surface is given by rmax = k /4.
The equations show that, for θA or θB = 0, the rate of reac-
ꢀ
Equation (1), in which k is a general rate constant and θA and
tion becomes zero, as expected, but reaches a maximum at
θB represent fractional equilibrium coverages of the catalyst
ꢀ
k /4, when KsA = sB. The last relationship implies that, for
metal surface by A and B, respectively. (In a heterogeneous
maximum rate of reaction, the reactants should cover equal
fractions of the catalyst surface. However, the fractions are
themselves controlled by both the concentrations of the two
reactants in solution (sA and sB) and their relative binding
strengths (K = KA/KB). Thus, if KA ꢁ KB or KB ꢁ KA,
then the solution strengths (sA and sB) of A or B need to
be adjusted if θA is to be about equal to θB.
ꢀ
reaction, the rate constant k is also dependent on the quantity
of catalyst used.)
dP
=
kꢀ × θA × θB
(1)
dt
In the equilibria shown in Scheme 2, KA and KB are bind-
ing constants measuring the respective adsorptions of each
reactant onto the surface of a catalyst C, with CA* and CB*
representing the adsorbed complexes of the reactants on the
catalyst metal surface.
Equations (2)–(6) reveal the importance of relative bind-
ing strengths and of the type of solvent used in these
heterogeneous reactions. Binding strength depends on sol-
vent and on the catalyst surface. It is a measure of how
well one reactant can compete with other reactants or with
solvents for sites on the catalyst surface. For example, with
a strongly binding component B in solution above a catalyst
surface (both KB and sB large), then sB ꢁ KsA, thus reactant
A would be excluded from the catalyst surface. In such cir-
cumstances, no reaction should be observed. If the amount of
Binsolutionisreducedtosuchalevelthat sB isapproximately
equal to KsA, then reactant A will adsorb onto the catalyst
surface to about the same extent as B and allow the reaction to
proceed. When sB ꢂ KsA, reactant B will be excluded from
the surface and the reaction rate will fall to zero again. It
can be seen that in situations in which both reactants bind
to a catalyst surface with similar binding energies, the rate
of reaction will then be critically dependent on solution con-
centrations of A and B and also on the nature of the solvent.
As described below, the rate of hydrogenolysis (Scheme 1)
The concentrations of A and B in solution are sA and sB,
ꢀ
ꢀ
respectively, and s and s represent the amounts of A and
A
B
B adsorbed from solution onto the catalyst. The two bind-
ing constants give Equations (2a) and (2b) from which may
be obtained the ratio, KA/KB or θA/θB (Equations (3a) and
(
3b)).
θA
KA =
KB =
(2a)
(2b)
ꢀ
A
(
1 − θA − θB)(s − s )
A
θB
ꢀ
B
(
1 − θA − θB)(s − s )
B
ꢀ
KA
=
θA
(s − s )
B
B
×
or
(3a)
(3b)
ꢀ
KB
θB
(s − s )
A
A
ꢀ
θA
=
KA
KB
(s − s )
A
A
×
ꢀ
θB
(s − s )
B
B

4
26
J. A. C. Alves, A. F. Brigas and R. A. W. Johnstone
could be maximized by ensuring that sodium phosphinate,
though all components were in solution. These experimental
results are entirely in keeping with previous observations
[
9–11]
[13]
an anion known to bind strongly to palladium,
could not
reach a sufficiently high solution concentration that it could
exclude ether (1a) from the catalyst surface. For these exper-
iments, the concentration of A (ether (1a)) was kept constant
and the concentration of B (sodium phosphinate) in solution
was varied, either by changing a mixed solvent composition
or by use of a phase-transfer catalyst.
on the importance of donor concentration on reaction rate.
The rate variations in THF/water illustrate the importance of
solvent composition in controlling heterogeneously catalyzed
reactions through control of the relative proportions of two
reactants on the surface of a catalyst.
ꢀ
Rate of Formation of Naphthalene as the Concentration of
Hydrogen Donor in Solution is Varied
The rate constant k includes the amount of catalyst used
and, therefore, for a situation in which θA = θB, the maximum
rate increases if more catalyst is used (assuming temperature
and pressure to be constant).
In a second set of experiments, the concentration of hydro-
gen donor, again in a monophasic solvent system, was
adjusted through use of a phase-transfer agent. Ether (1a)
was dissolved in an organic phase consisting of the upper
layer from the azeotropic mixture of benzene/ethanol/water
Rate of Formation of Naphthalene as the Composition of
THF/H2O Mixtures is Varied
[
12]
In this series of experiments, hydrogenolysis of ether (1a)
was carried out in a monophasic aqueous tetrahydrofuran
system, in which the percentage of water was varied from
(86.0 : 12.7 : 1.3, v/v/v). Pd/C catalyst was added, together
with solid sodium phosphinate, and the mixture was refluxed;
most of the sodium phosphinate remained undissolved. At
the low concentration of hydrogen donor, formation of
naphthalene was slow (20% yield in 30 min) compared with
0
to 50% (v/v). Initial rates of formation of naphthalene
were monitored by gas chromatographic analysis of aliquots
◦
of the reactant solution. Normally, at about 60–80 C, this
the normal biphasic solvent conditions (complete reaction in
[
5]
[10]
hydrogenolysis can be completed in about 10–20 min. In
the complete absence of water, with THF, no naphthalene
was formed. With some added water, hydrogenolysis was
found to proceed slowly. As more water was included in the
monophasic solvent system, the rate of formation of naphtha-
10 min).
In a separate experiment, addition of [18]crown-
6 at a 0.1 molar ratio to the hydrogen donor caused a large
increase in the rate of formation of naphthalene, a 40% yield
being produced in 30 min and 80% in 4 h. For further experi-
ments, itwasexpectedthatadditionofevenmore[18]crown-6
should again increase the concentration of hydrogen donor
and lead to another increase in the rate of formation of naph-
thalene. However, at a 0.4 molar ratio of crown ether to
hydrogen donor, the rate of reaction fell significantly and,
after 4 h, only about 45% of naphthalene had been formed.
Addition of more phase-transfer catalyst led to a further fall in
the rate of hydrogenolysis. These results are consistent with
those described for the THF/H2O experiment and with oth-
−7
−1 −1
lene increased to a maximum of 2.24 × 10 mol
s when
there was about 30% of added water. However, as even more
water was added, the rate of hydrogenolysis began to decrease
until, with 50% of added water, it had ceased entirely.
Reference to the equations in the section above explains
this behaviour. With no water present in theTHF, there can be
no significant dissolution of sodium phosphinate and, there-
fore, it can not be adsorbed onto the catalyst surface, which
is covered by ether (1a) and solvent. No reduction can be
expected and none was observed. As the proportion of water
is increased, the concentration of sodium phosphinate in the
solvent system increases and so too does its adsorption onto
the surface of the catalyst. The observed increase in rate
of hydrogenolysis with increasing water content of the sol-
vent system reflects the increasing coverage of the catalyst
surface by sodium phosphinate. The fractional coverages (θA
and θB) of the catalyst surface by ether (1a) and sodium phos-
phinate change as the hydrogen donor becomes more soluble.
With both the ether substrate and the hydrogen donor present
together on the catalyst surface, hydrogenolysis proceeds and
reaches a maximum when the relative coverages of ether and
hydrogen donor are about equal. Because the hydrogen donor
has a much greater binding constant than does ether (1a) in
[
3]
ers reported earlier. Increasing the concentration of sodium
phosphinate in solution for the heterogeneously catalyzed
hydrogenolysisreaction(Scheme1)leadstoincreasingexclu-
sion of the ether from the catalyst surface and eventually to
a reduction in reaction rate or even complete cessation of
reaction. This behaviour is quite unlike solution chemistry,
for which continuously increasing the concentrations of reac-
tants in solution generally leads to a continuous increase in
reaction rate.
Hydrophobicity or Hydrophilicity of the
Heterogeneous Catalyst
As shown above, adjustment of solvent composition can
have a marked effect on the rate of hydrogenolysis through
control of the relative coverages of the catalyst surface
by the reactants. In those experiments, the mixed solvents
formed a monophasic system with the catalyst dispersed in
it. Inorganic, water-soluble hydrogen donors are not nor-
mally soluble in organic solvents. In a biphasic water/organic
solvent system (for example, benzene or toluene/water), the
concentration of the substance to be reduced (e.g. ether (1a))
is high in the organic layer and low in the aqueous layer.
Conversely, the hydrogen donor concentration (e.g. sodium
phosphinate) is high in the aqueous layer and low in the
[9–11]
THF/water,
it is to be expected that as the hydrogen
donor concentration in solution increases, it will displace
more and more ether (1a) from the surface of the catalyst. In
these circumstances, the reaction rate begins to fall and even-
tually reaches zero when all ether (1a) has been excluded from
the catalyst surface. With the quantities of catalyst and the
solution strengths used here, a maximum rate was observed
ataTHF/waterratioofabout70 : 30 (v/v).Atacompositionof
5
0 : 50, hydrogenolysis (Scheme 1) would not even start, even

Rate-Controlled Heterogeneously Catalyzed Transfer Hydrogenolysis
427
organic layer. Since the amounts of substrate and donor
adsorbed onto the catalyst surface depend on both binding
constants and concentrations, the environment surrounding
the catalyst becomes important. Pd/C catalyst is hydropho-
bic. Even with vigorous stirring, it remains suspended in the
organic hydrophobic layer of a water/benzene system and not
the aqueous. The hydrogen donor is dissolved almost entirely
in the aqueous phase and the substance to be reduced is dis-
solved in the organic phase. Because the catalyst lies in the
organic phase, access to its surface by the substance to be
reduced is unimpeded but, for the hydrogen donor, access is
restricted since there is little of the hydrogen donor in the
organic layer. Effectively, access to the catalyst is controlled
by having two slightly miscible solvents, with each of the
reactants in a different phase and the catalyst favouring one
phase over the other.
to regulate the effective concentration of the hydrogen donor
at the catalyst surface by adjusting its solubility in solution. In
one set of experiments with ether (1a), the latter was dissolved
in a monophasic aqueous/organic solvent mixture. By adjust-
ing the proportion of water present, it was possible to control
the amount of hydrogen donor dissolved in the solvent. The
catalyst (Pd/C) remained suspended in the organic layer. As
more water was added, the solubility of the sodium phosphi-
nate hydrogen donor in the organic layer increased, leading
first to an increase in the rate of reaction and then to a cessa-
tion. The control of rate was such that, with either no water or
a lot of water in the organic layer hydrogenolysis ceased, but
with intermediate amounts the rate of hydrogenolysis passed
through a maximum. Other experiments designed to control
adsorption of the sodium phosphinate onto the catalyst sur-
face through changes in solvent and access of reactants to
the catalyst surface have led to similar conclusions. Rates of
heterogeneously catalyzed hydrogenolysis of ethers (1) can
be changed easily from zero to a maximum by simple
variations of the solvent system.
To demonstrate the effect of changing environment on cat-
alyst behaviour, ether (1a) was dissolved in the upper phase of
a typical biphasic mixture of benzene or toluene, ethanol, and
water and was treated with sodium phosphinate in the pres-
◦
ence of Pd/C catalyst at 65 C. Hydrogenolysis proceeded
normally.
[13]
Experimental
After about 5 min when some 57% yield of
naphthalene had been produced, stirring was stopped and,
immediately, a sonicator was switched on so as to effect
greater mixing of the phases. After less than 2 min, son-
ication had produced a ‘one-phase’ emulsified system of
the organic solvent and water, with the catalyst finely dis-
Sodium phosphinate and THF were purchased (Aldrich). The supply of
sodium phosphinate was refreshed regularly and its purity was checked
[
15]
5-(2-
by titration with ceric sulfate/ferrous ammonium sulfate.
Naphthoxy)-1H-phenyltetrazole was prepared by a literature method.
Generally, the 10% Pd/C catalyst was a fresh standard commercial
[
6]
2
−1
sample, having a metal dispersion area of about 26 m g (Johnson
Matthey). For consistency, all experiments were carried out on the
same batch of catalyst, except for the sonication experiment, for which
a different 10% Pd/C catalyst was used (Engelhardt). Gas chromato-
graphic monitoring was carried out on a Dani instrument, using an FFAP
capillary column and dodecane as internal standard.
[14a]
persed throughout it.
at 65 C was continued but no further formation of naph-
thalene occurred. The emulsion remained stable overnight
Sonication was stopped and heating
◦
(
18 h) and still no more naphthalene was formed during this
period. This abrupt cessation of catalytic activity upon soni-
cation of a heterogeneously catalyzed reaction is in contrast
to the usual finding that sonication is generally beneficial for
Adsorption Isotherms
These were determined for the ether (1a) and for phosphinate by the
method briefly described in Figure 1. Greater detail can be found in
[14b]
many catalytic processes, even hydrogenation.
Although
[
9–11]
earlier publications.
deleterious changes in the nature and activity of a hetero-
geneous catalyst have been noted during extended sonication,
the complete cessation of reaction observed here was virtu-
ally instantaneous on starting sonication. It is unlikely in so
short a time that the catalyst could have been so damaged
as to no longer act catalytically. The result is explicable in
terms of a greatly increased access of the strongly binding
hydrogen donor to the catalyst surface through rapid emulsi-
fication and dispersal of the catalyst into a thoroughly mixed
benzene/ethanol/water emulsion phase. Such a situation
allows easy access of the hydrogen donor to the catalyst
surface.
Hydrogenolysis of 5-(2-Naphthoxy)-1H-phenyltetrazole (THF, with
Increasing Proportions of Water)
In a typical experiment, the catalyst (10% Pd/C, 100 mg; Johnson
Matthey) and sodium phosphinate (15 mg, 0.17 mmol) were added to
a solution of the ether (1a) (50 mg, 0.17 mmol) and dodecane (50 µL)
in THF (10 mL), to which had been added x (mL) of water (in the dif-
ferent experiments, x varied from 0 to 10). The mixture was vigorously
◦
stirred and heated under reflux (65 C). The formation of naphthalene
was monitored by analysing aliquots withdrawn at intervals. Initial reac-
tion rates were determined (to about 20% of reaction). The maximum
−
7
−1
observed initial rate in the series was 2.24 × 10 mol s . No attempt
was made to find the true maximum. For the case of x = 0 and of x = 10,
no formation of naphthalene was observed over a period of 45 min and
starting material was recovered. For the case of x = 0, even though no
reaction had been observed under reflux over a period of 45 min, it could
be made to proceed normally to give naphthalene simply by the addition
of some water (2–3 mL).
Conclusions
Adsorption isotherms were used to assess relative binding
strengths of hydrogen-acceptor and hydrogen-donor reac-
tants to the catalyst surface in different solvents. For the
heterogeneously catalyzed reaction in the liquid phase using
a water-soluble hydrogen donor, access of the reactants to the
catalyst surface can be controlled through selection of the sol-
vent system. This choice is made so as to obtain favourable
binding of the reactants to the surface and, at the same time,
Hydrogenolysis of 5-(2-Naphthoxy)-1H-phenyltetrazole (1a) in
a Benzene/Ethanol/Water Azeotrope, with Increasing Amounts
of Added [18]Crown-6
◦
Thesolventwastheupperorganicphase(74.1 : 18.5 : 7.4 v/v/v;bp65 C)
[13]
obtained by azeotroping a mixture of benzene, ethanol, and water.
The hydrogen donor used in these experiments was not significantly
soluble in this organic phase. In a typical experiment, a stirred solution of

4
28
J. A. C. Alves, A. F. Brigas and R. A. W. Johnstone
−
4
5
-(2-naphthoxy)-1H-phenyltetrazole (44.3 mg, 1.54 × 10 mol) and
durene (15.4 mg, internal as chromatography standard) in the azeotrope
40 mL), containingPd/Ccatalyst(10% w/w, 25.2 mg)andsodiumphos-
T. A. Delchar, in Modern Techniques in Surface Science—
Cambridge Solid State Series 1986 (Cambridge University
Press: New York, NY).
(
−
4
phinate (42.6 mg, 4.8 × 10 mol), was refluxed and aliquots of the
reaction mixture (ca. 0.1 mL) were taken at intervals to measure the for-
mation of naphthalene. Experiments were performed with and without
[2] (a) J. H. Clark, J. D. Macquarrie, Org. Process Res. Dev. 1997,
1, 149. (b) R. A. W. Johnstone, A. H. Wilby, I. D. Entwistle,
Chem. Rev. 1985, 85, 129.
[
18]crown-6, used as a phase-transfer catalyst to increase the solubility
[3] A. F. Brigas, R. A. W. Johnstone, J. Chem. Soc., Chem. Commun.
1991, 1041.
[4] V. F. D. Alvaro, Ph.D. Thesis 1998 (University of Liverpool:
Liverpool).
of the sodium phosphinate. The amount of crown ether ranged from 0
to 8.1 mg (4.5 × 10− mol) to 32 mg (1.78 × 10 mol).
5
−4
Sonic Irradiation of the Hydrogenolysis Reaction
[5] B. J. Hussey, R. A. W. Johnstone, I. D. Entwistle, Tetrahedron
1
982, 38, 3775.
To a solution of ether (1a) (200 mg, 0.7 mmol) in a two-phase mixture
[
16]
[6] J. A. C. Alves, J. V. Barkley, A. F. Brigas, R. A. W. Johnstone,
of benzene/ethanol/water (7 : 3 : 2; 25 mL),
containing sodium phos-
J. Chem. Soc., Perkin Trans. 2 1997, 669.
phinate (220 mg, 2.5 mmol) was added Pd/C catalyst (10%, 200 mg;
Engelhard). The mixture was stirred while being heated under gentle
reflux. The formation of naphthalene was monitored by gas chromato-
graphy after removing aliquots of the upper phase. After 5 min, the
yield of naphthalene reached about 57%. At this stage, the solution
was sonicated at 20 kHz, using a piezoelectric horn having a titanium
extension which projected into the reaction medium and delivered about
[
[
[
7] A. F. Brigas, R. A. W. Johnstone, Tetrahedron Lett. 1990, 31,
789.
8] A. F. Brigas, R. A. W. Johnstone, J. Chem. Soc., Perkin Trans. 1
000, 1735.
5
2
9] J. A. C. Alves, Ph.D. Thesis 1996 (University of Liverpool:
Liverpool).
−2
[10] A. F. Brigas, Ph.D. Thesis 1993 (University of Liverpool:
2
0W cm . Within less than 2 min, the mixture had completely emulsi-
Liverpool).
fied, the catalyst becoming uniformly distrubuted throughout the liquid.
Sonication was stopped and, with continued heating under gentle reflux,
the formation of naphthalene was monitored for a further 18 h. There
was no observable increase in the amount of naphthalene that had been
formed before sonication had started.
[
11] C. F. C. Fonseca, Ph.D. Thesis 2002 (University of Liverpool:
Liverpool).
12] R. A. W. Johnstone, P. J. Price, J. Chem. Soc., Chem. Commun.
[
1
984, 845.
[
[
13] A. F. Brigas, R. A. W. Johnstone, Tetrahedron 1992, 48, 7735.
14] (a) T. J. Mason, Practical Sonochemistry 1991, pp. 30–32;
Acknowledgments
6
0 (Ellis Horwood: London) and references cited therein; (b)
T. J. Mason, Practical Sonochemistry 1991, pp. 164–166 (Ellis
Horwood: London).
15] A. I. Vogel, A Textbook of Quantitative Inorganic Analysis, 3rd
ed. 1961, p. 411 (Longmans: London).
[16] R. C. Weast (Ed.), Handbook of Chemistry and Physics, 57th
ed. 1976/7, D-37 (CRC Press: Boca Raton, FL).
The authors thank Denis Webster and Ivor Dodgson (Johnson
Matthey) for generous help and information, FCT (Portugal)
for grants (J.A.C.A., A.F.B., R.A.W.J.) and the Eschenmoser
Trust and the Royal Society of Chemistry for financial
assistance (A.F.B.).
[
References
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1] (a) J. R. Anderson, M. Boudart (Eds), Catalysis: Science
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