Tandem one-pot palladium-catalyzed reductive and oxidative coupling of benzene and chlorobenzene

Article abstract of DOI:10.1021/jo991868e

The in situ combination of oxidative coupling of benzene to biphenyl and reductive coupling of chlorobenzene (also to biphenyl) using palladium catalysts (Pd²⁺/Pd⁰) is described. In each cycle, the reductive process regenerates the catalyst for the oxidative process and vice versa. Kinetic investigations show that the reaction rate depends on [C₆H₆], [C₆H₆Cl], and catalyst loading, with E(a) = 13 kcal mol^-1. The reduced palladium catalyst undergoes deactivation through aggregation and precipitation, but it is observed that during this deactivation process the Pd⁰ becomes an active catalyst for the reductive coupling of chlorobenzene. Accordingly, while Pd⁰/C particles are inactive, Pd⁰ colloids do catalyze the tandem reaction. Conversion is increased in the presence of a phase-transfer catalyst, presumably due to stabilization of the active Pd⁰ clusters. The two halves of the catalytic cycle are examined in the light of previous research, regarding analogous oxidative and reductive coupling reactions, using stoichiometric amounts of PdCl₂ and Pd⁰, respectively. The roles of homogeneous PdCl₂ and Pd⁰ clusters are discussed.

Full text of DOI:10.1021/jo991868e

J . Org. Chem. 2000, 65, 3107-3110
3107
Ta n d em On e-P ot P a lla d iu m -Ca ta lyzed Red u ctive a n d Oxid a tive
Cou p lin g of Ben zen e a n d Ch lor oben zen e
Sudip Mukhopadhyay, Gadi Rothenberg,† Diana Gitis, and Yoel Sasson*
Casali Institute of Applied Chemistry, Hebrew University of J erusalem, 91904, Israel
Received December 6, 1999
The in situ combination of oxidative coupling of benzene to biphenyl and reductive coupling of
chlorobenzene (also to biphenyl) using palladium catalysts (Pd2+/Pd0) is described. In each cycle,
the reductive process regenerates the catalyst for the oxidative process and vice versa. Kinetic
investigations show that the reaction rate depends on [C6H6], [C6H6Cl], and catalyst loading, with
Ea ) 13 kcal mol-1. The reduced palladium catalyst undergoes deactivation through aggregation
and precipitation, but it is observed that during this deactivation process the Pd0 becomes an active
catalyst for the reductive coupling of chlorobenzene. Accordingly, while Pd0/C particles are inactive,
Pd0 colloids do catalyze the tandem reaction. Conversion is increased in the presence of a phase-
transfer catalyst, presumably due to stabilization of the active Pd0 clusters. The two halves of the
catalytic cycle are examined in the light of previous research, regarding analogous oxidative and
reductive coupling reactions, using stoichiometric amounts of PdCl2 and Pd0, respectively. The roles
of homogeneous PdCl2 and Pd0 clusters are discussed.
Sch em e 1
In tr od u ction
Sustainable chemistry and waste effluent minimization
feature prominently on the chemists’ agenda.1 The sub-
stitution of polluting processes that require stoichiometric
amounts of metallic reagents with catalytic reactions may
be viewed as the first step toward total eco-efficiency.2
Unfortunately, ideal, 100% atom-efficient transforma-
tions, e.g., the hydrogenation of benzene to cyclohexane,3a
are usually limited to simple addition or substitution
reactions. In some other cases, e.g., replacement of
stoichiometric CrO3 oxidations with Cr-catalyzed oxida-
tion using air or hydrogen peroxide,3b in situ catalyst
regeneration is relatively simple, but frequently catalyst
regeneration leads to additional byproducts and/or waste
streams (Scheme 1, top).
Advancing one step further, the ideal catalytic process
would involve regeneration of the catalyst using not a
reagent, but rather another, different, substrate molecule
(Scheme 1, bottom); this second substrate would interact
with the “used catalyst”, to give the same product as the
first, and a recycled catalyst.
proposal of numerous reductive7 and oxidative8 pal-
ladium-catalyzed coupling protocols, the common factor
of which is the use of the Pd2+ S Pd0 redox cycle.9 During
our recent investigations into the mechanisms of pal-
Obviously, one of the likely systems that this concept
could be applied to is a circular oxidation-reduction
process. In this study, we examine the application of this
concept to aryl-aryl coupling reactions, which are of
accountable interest in several branches of the chemical
industry.4 The Ullmann5 (homocoupling) and Suzuki6
(cross-coupling) reactions are well-known in this context.
Intensive research over the past decade has led to the
(4) For reviews on biaryl preparation methods and applications,
see: (a) Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem., Int.
Ed. Engl. 1990, 29, 977. (b) Sainsbury, M. Tetrahedron 1980, 36, 3327.
See also (c) Stinson, S. C. Chem. Eng. News 1999, 77 (J uly 19), 65-
87.
(5) (a) Ullmann, F. Ber. 1903, 36, 2389. (b) Fanta, P. E. Synthesis
1974, 9.
(6) (a) Miyamura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981,
11, 513. (b) Suzuki, A. Pure Appl. Chem. 1991, 63, 419. (c) Moreno-
Man˜as, M.; Pe´rez, M.; Pleixats, R. J . Org. Chem. 1996, 61, 2346.
(7) (a) Hennings, D. D.; Iwama, T.; Rawal, V. H. Org. Lett. 1999, 1,
1205. (b) Brenda, M.; Knebelkamp, A.; Greiner, A.; Heitz, W. Synlett
1991, 809. (c) Marques, C. A.; Selva, M.; Tundo, P. J . Org. Chem. 1993,
58, 5256. (d) Hassan, J .; Penalva, V.; Lavenot, L.; Gozzi, C.; Lemaire,
M. Tetrahedron 1998, 54, 13793. (e) Bamfield, P.; Quan, P. M.
Synthesis 1978, 537.
(8) 8.(a) Unger, M. O.; Fouty, R. A. J . Org. Chem. 1969, 34, 18. (b)
Kashima, M.; Yoshimoto, H.; Itatani, H. J . Catal. 1973, 29, 92. (c)
Iataaki, H.; Yoshimoto, H. J . Org. Chem. 1973, 38, 76. (d) Rachk-
ovskaya, L. N.; Gusevskaya, E. V.; Matveev, K. I.; Il’inich, G. N.;
Eremenko, N. K. Kinet. Catal. 1977, 18, 660. (e) Yatsimirsky, A. K.;
Deiko, S. A.; Ryabov, A. D. Tetrahedron 1983, 39, 2381. (f) Yatsimirsky,
A. K.; Deiko, S. A.; Ryabov, A. D. Kinet. Catal. 1984, 25, 272. (g)
Shiotani, A.; Itatani, H.; Inagaki, T. J . Mol. Catal. 1986, 34, 57.
* To whom correspondence should be addressed. e-mail: ysasson@
vms.huji.ac.il.
† Current address: Clean Technology Centre, Department of Chem-
istry, University of York, Heslington, York YO10 5DD, UK. e-mail:
gr8@york.ac.uk.
(1) (a) Sheldon, R. A. Chemtech 1994, 38. (b) Hudlicky, T. Chem.
Rev. 1996, 96, 3.
(2) (a) Sheldon, R. A. Chem. Ind. 1997, 12. (b) Trost, B. M. Science
1991, 254, 1470.
(3) (a) Weissermel, K.; Arpe, H.-J . Industrial Organic Chemistry;
3rd ed., VCH: Weinheim, 1997; p 358. (b) J . Muzart, Chem. Rev. 1992,
92, 113, and refs cited therein.
10.1021/jo991868e CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/26/2000

3108 J . Org. Chem., Vol. 65, No. 10, 2000
Mukhopadhyay et al.
ladium-catalyzed aryl-aryl coupling,10,11 we became in-
terested in combining these two pathways in such a
concerted manner, so that the starting material for the
oxidative coupling would act as the in situ catalyst
regenerator in the reductive coupling and vice versa.
Here, we present the application of the above con-
cept to a redox catalytic cycle, namely the tandem
oxidative coupling of benzene to biphenyl, with in situ
reductive coupling of chlorobenzene, also to biphenyl, and
discuss possible mechanisms for this palladium-catalyzed
“Ullman-type” reaction.
Ta ble 1. Effect of P r ocess P a r a m eter s on Ben zen e
Con ver sion a n d Selectivitya
entry parameter changed benzene conv % biphenyl sel. %b
PdCl2 (M)
1
2
3
4
0.0175
0.035
0.07
14
23
39
45
66
67
67
66
0.14
NaOAc (M)
5
6
7
8
9
0.0
1.5
2.0
2.5
3.0
0
15
21
39
39
0
65
67
67
65
Resu lts a n d Discu ssion
THAC (g)
In a typical reaction (eq 1), a 1:1 equivalent mixture
of benzene and chlorobenzene together with catalytic
amounts of tetrahexylammonium chloride (THAC) and
PdCl2 was stirred in an AcOH/AcONa solvent medium
in an autoclave at 105 °C for 28 h. Biphenyl was found
10
11
12
13
14
0.0
0.2
0.5
1.0
1.2
20
23
31
45
43
69
65
66
67
69
Solvent
16
17
18
AcOH
DMSO
H2O
39
4
0
67
62
0
Catalyst (2.8 mol %)
19
20
21
PdCl2
39
0
13
67
<1
63
Pd0/C
Pd0 colloid
a
Standard reaction conditions: benzene, 7.8 g (100 mmol);
chlorobenzene, 11.3 g (100 mmol); AcONa 13.6 g (100 mmol);
PdCl2, 0.5 g (2.8 mol %); THAC, 1.0 g (2.3 mol %); 105 °C; AcOH
b
(total reaction volume), 40 ml, 28 h. Based on GC area, corrected
by the presence of an internal standard.
performing the reaction at higher temperatures increased
the overall reaction rate. The experimental Arrhenius
energy of activation was found to be Ea ) 55 kJ mol-1
(13 kcal mol-1, r2 ) 0.997 for four measurements at 75,
85, 95, and 105 °C). When the temperature was raised
to 115 °C, however, the rate slowed, which may be
attributed to formation of a large quantity of benzene
vapor that does not participate in the liquid-phase
reaction.
The above system comprises four distinct transforma-
tions, three of which make up the oxidative coupling
pathway (eqs 2-4), and one, the reductive coupling of
C6H5Cl, which allows catalyst recycling (eq 5). As we
showed earlier, it is impossible to obtain the cross-
coupling product via the reductive coupling route.10a The
complexity of the system is increased by the fact that the
homogeneous Pd0 itself undergoes deactivation through
aggregation and precipitation. In addition, the reductive
coupling depends on heterogeneous catalysis. To simplify
matters, we shall consider the oxidation and reduction
reactions separately.
P d -Ca ta lyzed Oxid a tive Cou p lin g of Ben zen es. It
is worthwhile to examine this half of the catalytic cycle
on the basis of the oxidative coupling of benzenes with
stoichiometric amounts of PdCl2 in a similar medium,
namely 2C6H6 + PdCl2 + 2NaOAc f C6H5-C6H5 + Pd0
+ 2NaCl + 2AcOH. This reaction was thoroughly inves-
tigated by VanHelden et al.,12 who suggested the follow-
ing four-step mechanism: (i) reaction between C6H6 and
PdCl2, arising from π-σ interactions;13 (ii) fast reaction
to be the major product, together with some dichlorobi-
phenyl and some of the cross-coupling product, 4-chlo-
robiphenyl. The effects of various reaction parameters
on the conversion and selectivity are shown in Table 1.
No reaction was observed in the absence of benzene, and
only traces of biphenyl (<1%) were observed in the
absence of chlorobenzene. Moreover, no reaction was
observed when Pd(OAc)2 was used as a catalyst instead
of PdCl2, indicating that Cl- ligands do not exchange
readily. When using PdCl2 as catalyst, the reaction ceased
after ca. 40% conversion of benzene, due to catalyst
deactivation. The deactivated catalyst precipitated as
palladium metal particles and could easily be separated
from the reaction mixture by filtration.
Kinetic studies showed the reaction rate to be a
function of benzene concentration, chlorobenzene con-
centration, and catalyst loading, i.e., -d[C6H6]/dt )
k[C6H6][C6H5Cl][PdCl2], wherein first-order dependence
was observed for all three variables. A pseudo first-order
rate law, -d[C6H6]/dt ) kobs[C6H6], was observed for fixed
catalyst loading and chlorobenzene concentrations, with
a typical k value of 4 × 10-6 s-1. (To ensure that the rates
were not mass transfer controlled, agitation speed was
varied from 200 to 950 rpm. Above 800 rpm, no increase
in conversion was detected. Thus, all measurements
pertain to 950 rpm stirring.) As could be expected,
(9) For an excellent recent monograph on palladium catalysis, see:
Tsuji, J . Palladium Reagents and Catalysts; Wiley: New York, 1993.
(10) (a) Mukhopadhyay, S.; Rothenberg, G.; Gitis, D.; Wiener, H.;
Sasson, Y. J . Chem. Soc., Perkin Trans. 2 1999, 2481. (b) Mukho-
padhyay, S.; Rothenberg, G.; Wiener, H.; Sasson, Y. Tetrahedron 1999,
55, 14763.
(12) VanHelden, R.; Verberg, G. Recl. Trav. Chim. Pays-Bas 1965,
84, 1263.
(11) Mukhopadhyay, S.; Rothenberg, G.; Gitis, D.; Sasson, Y. J .
Chem. Soc., Perkin Trans. 2 1999, in press.
(13) . cf. Demsey, J . N.; Baenzinger, N. C. J . Am. Chem. Soc. 1955,
77, 4984.

Pd-Catalyzed Coupling of Benzene and Chlorobenzene
J . Org. Chem., Vol. 65, No. 10, 2000 3109
of the [C6H6]+[PdCl2]- ion pair with an acetate ion; (iii)
dimerization to form complex 1 [analogous to the (π-allyl
PdCl2)2 complex];14 and (iv) disproportionation of 1 to give
the coupling product, Pd0, and PdCl2. Step (i) was
assumed to be rate-determining. The two remaining
oxidative couplings (eqs 3 and 4) can be envisaged in a
similar manner. Note that the requirement for stoichio-
metric amounts of PdCl2 stems from the fact that the
reduced Pd0 species precipitates, and cannot be recycled.
We found, however, that during this deactivation process
the Pd0 species goes through an intermediate state in
which it can catalyze the reductive coupling of C6H5Cl,
and regenerate as PdCl2.
F igu r e 1. Cycle combining oxidative coupling of benzene,
reductive coupling of chlorobenzene, and catalyst deactivation
through aggregation.
iodobenzenes), because the bulky PdBr2 and PdI2 will not
react with benzene to form biphenyl.12
The successful combination of the oxidative and reduc-
tive coupling reactions depends therefore on the forma-
tion of an active heterogeneous catalytic species. The
problem is that when the Pd0 becomes truly heteroge-
neous, i.e., precipitates as Pd metal on the bottom of the
reactor, it is no longer active. However, precipitation of
Pd0 out of the reaction mixture is unlikely to be an
instantaneous process, as the Pd0 atoms are thought to
be formed individually (one Pd0 atom per oxidative
coupling reaction). A similar phenomenon was observed
by Mandai et al. with regard to the reduction of Pd(OAc)2
with n-Bu3P, wherein after 30 min all of the active Pd0
catalyst was deactivated as solid black palladium par-
ticles.17 It is therefore possible to envisage catalyst
deactivation as a two-stage process: clustering of Pd0
atoms to form small, soluble clusters, followed by ag-
gregation of these clusters to form large insoluble Pd0
aggregates which precipitate out of the reaction mixture.
Some indication as to the rate of formation of the inactive
palladium aggregates was obtained from a blank reac-
tion, to which C6H5Cl was added only after 10 min
stirring of the other reaction components. No reaction
was observed in this case, indicating the fast deactivation
of the catalyst in the absence of chlorobenzene.
We suggest that the Pd0 clusters, being soluble and
having a high surface area, could catalyze the reductive
coupling of C6H5Cl, with concurrent regeneration of the
homogeneous PdCl2 (see Figure 1). Evidence supporting
this hypothesis was obtained from experiments using
different kinds of Pd0 catalysts (Table 1, entries 20 and
21). While no reaction was observed in the presence of
Pd/C, equivalent catalytic amounts of a colloid dispersion
of Pd0 particles did catalyze the reaction. (The lower
conversion observed using colloid palladium as catalyst
might be due to facile aggregation of Pd0 colloid particles
compared to that of homogeneous “just formed” Pd0
atoms.) Furthermore, it can be argued that catalytic
P d -Ca ta lyzed Red u ctive Cou p lin g of Ha loben -
zen es. Recently, we demonstrated that heterogeneous
Pd/C is an efficient catalyst for the reductive homocou-
pling of halobenzenes in both aqueous and organic media,
in which catalyst regeneration can be effected using
various reductants, e.g., formate ions,10a H2 gas,10b or zinc
powder.11 Here, too, it is advantageous to examine the
analogous reaction using stoichiometric amounts of Pd/
C, namely 2C6H5X + Pd0 f C6H5-C6H5 + Pd2+ + 2X-
(wherein X ) Cl, Br, I) in which a quantitative yield of
biphenyl is obtained. We believe that this reaction
proceeds via two consecutive single-electron-transfer
(SET) processes from a palladium site to two substrate
molecules, generating two [C6H5X]-• radical anions.
Subsequent ejection of X- anions15 and coupling of the
phenyl radicals affords PdX2 and biphenyl.16 Fortunately,
this system enables coupling of chlorobenzenes (as op-
posed to most other catalytic reductive coupling proce-
dures, which require the use of bromo- or preferably
(14) Dehm, H. C.; Chien, J . C. J . Am. Chem. Soc. 1960, 82, 4429.
(15) For recent studies on this specific bond scission, see (a) Pierini,
A. B.; Duca, J . S., J r.; Vera, D. M. A. J . Chem. Soc., Perkin Trans. 2
1999, 1003. (b) Pause, L.; Robert, M.; Save´ant, J .-M. J . Am. Chem.
Soc. 1999, 121, 7158.
(16) A similar mechanism was suggested for the Pd-catalyzed
coupling of halopyridines: Munavalli, S.; Rossman, D. I.; Szafraniec,
L. L.; Beaudry, W. T.; Rohrbaugh, D. K.; Ferguson, C. P.; Gra¨tzel, M.
J . Fluor. Chem. 1995, 73, 1.
(17) Mandai, T.; Matsumoto, T.; Tsuji, J .; Saito, S. Tetrahedron Lett.
1993, 34, 2513.

3110 J . Org. Chem., Vol. 65, No. 10, 2000
Mukhopadhyay et al.
Exp er im en ta l Section
amounts of THAC (Table 1, entries 10-14) facilitate
conversion without affecting product selectivity because
THAC stabilizes the small palladium clusters and retards
catalyst deactivation caused by aggregation. Indeed,
stabilization of Rh0 and Pd0 clusters using quaternary
ammonium salts has been discussed at length,18a-c and
their roles in regenerating PdCl2 from Pd0 complexes
using 1,2-dichloroethane has also been studied.18d,e The
solvent apparently plays a similar role (Table 1, entries
16-18).
The typical pseudo first-order rate constants observed
in this tandem system (kobs ∼ 10-6 s-1) are significantly
lower than those observed in any of the reductive or
oxidative parent systems. Thus, for the stoichiometric
reaction 2C6H6 + PdCl2 + 2NaOAc f C6H5-C6H5 + Pd0
+ 2NaCl + 2AcOH, kobs ∼ 10-3 s-1 (ref 12); for the
stoichiometric reaction 2C6H5Cl + Pd0 f C6H5-C6H5 +
Pd2+ + 2Cl-, kobs ∼ 10-3 - 10-4 s-1 (ref 10a); and for the
palladium-catalyzed reductive coupling of chlorobenzenes
kobs ∼ 10-3 - 10-4 s-1 (depending on the reagent used to
regenerate the Pd0 catalyst).10b,11 This may reflect the low
concentration of the active palladium clusters required
to catalyze the reductive coupling.
Another route to biphenyl may be envisaged through
formation of PhPdCl, followed by cross coupling as in
PhPdCl + PhH f Ph-Ph + Pd(0) + HCl. However,
though bromobenzene and iodobenzene undergo Heck-
type reactions with a catalytic amount of Pd and with
styrene, chlorobenzene cannot. Indeed, such a reaction
would be potentially useful for mild cross-coupling of
chloroaryls as in PhPdCl + PhMe f Ph-PhMe + Pd(0)
+ HCl, but unfortunately, when we tried it, it did not
work.
Instrumentation, experimental apparatus, and product
isolation and identification methods have been described in
detail previously.10,11
P r ep a r a tion of a Colloid a l Disp er sion of P d 0 in Ben -
zen e. This is a modification of a published procedure.19 An
aqueous solution of 0.53 g of (14 mmol) NaBH4 was added
dropwise over 1 h to a reaction mixture containing 0.50 g (2.8
mmol) of PdCl2, 1.0 g (2.3 mmol) of THAC, and 10 g of C6H6
at 27 °C. The mixture was stirred for 2 h, after which the
benzene layer after phase separation was directly taken for
the reaction.
Gen er a l P r oced u r e for Cou p lin g. Example: biphenyl
from C6H6 and C6H5Cl. Amounts of 7.8 g (100 mmol) of C6H6,
11.27 g (100 mmol) of C6H5Cl, 1.0 g (2.3 mol %) of THAC, 0.5
g (2.8 mol %) of PdCl2, 13.6 g (100 mmol) of NaOAc, and 40
mL of AcOH were charged to an autoclave as above. Reaction
progress was monitored by GC. The mixture was stirred (1000
rpm) at 105 °C for 36 h and cooled, and the solvent was
removed by distillation. Water (30 mL) was added, and the
product mixtures were extracted in of CHCl3 (4 × 15 mL).
Solvent evaporation and recrystallization afforded 4.6 g of
biphenyl (30 mol % based on C6H6; repeating the reaction four
times gave 27%, 29%, 28%, and 28%, respectively), mp 68.5
°C (from cold EtOH) (lit.,20 69-71 °C). Found: C, 93.27; H,
6.68. C12H10 requires C, 93.46; H, 6.54%. δH (CDCl3; Me4Si)
7.37 (2H, tt, aromatic 4,4′-H, J 7.2 and 1.3), 7.46 (4H, tt,
aromatic 3,3′,5,5′-H J 7.1 and 1.0), 7.58 (4H, dq, aromatic
2,2′,6,6′-H J 7.0 and 1.2). Good agreement was found with
literature values.21
Exp er im en ta l P r oced u r e for Kin etic Stu d ies. Condi-
tions and apparatus were similar to general coupling procedure
given above. The following parameters were studied: (i)
benzene concentration, (three experiments at 2.0 M, kobs ) 4.55
× 10-6 s-1, r2 ) 0.995 for 7 observations; 2.5 M, kobs ) 4.97 ×
10-6 s-1, r2 ) 0.999 for 7 observations; and 3.0 M, kobs ) 5.19
× 10-6 s-1, r2 ) 0.998 for seven observations); (ii) chlorobenzene
concentration, (three experiments at 2.0 M, kobs ) 4.58 × 10-6
s-1, r2 ) 0.997 for 7 observations; 2.5 M, kobs ) 4.94 × 10-6
s-1, r2 ) 0.999 for seven observations; and 3.0 M, kobs ) 5.33
× 10-6 s-1, r2 ) 0.994 for seven observations); (iii) initial PdCl2
loading, (four experiments using 0.0175 M, kobs ) 1.25 × 10-6
s-1, r2 ) 0.995 for seven observations; 0.035 M, kobs ) 2.4 ×
10-6 s-1, r2 ) 0.997 for seven observations; 0.07 M, kobs ) 4.97
× 10-6 s-1, r2 ) 0.999 for seven observations; and 0.14 M, kobs
) 9.7 × 10-6 s-1, r2 ) 0.999 for seven observations); and (iv)
reaction temperature, (four experiments at 75 °C, kobs ) 1.13
× 10-6 s-1, r2 ) 0.98 for seven observations; 85 °C, kobs ) 2.0
Con clu sion
Two noncatalytic processes, the homogeneous oxidative
homocoupling of benzene to biphenyl using PdCl2, and
the reductive heterogeneous homocoupling of chloroben-
zene (also to biphenyl) using Pd0, can be combined in situ
to obtain a closed catalytic cycle based on the Pd2+ S Pd0
redox transformation. A key feature of this system is the
interchanging of the palladium catalyst between homo-
geneous (PdCl2) and heterogeneous (Pd0) forms. The
concept of two different substrates “converging” on the
same is a step toward waste minimization. In this study,
the cheaper, but normally inactive, aryl chlorides were
employed. Practical application of this process is limited
(to a certain extent by its own mechanism), by low
catalyst turnover and moderate yields, compared with
either the reductive or the oxidative stoichiometric
processes. Optimizing applications of this concept to other
substrates requires further research, which is currently
in progress in our lab.
× 10-6 s-1, r2 ) 0.997 for seven observations; 95 °C, kobs
kobs ) 4.97 × 10-6 s-1, r2 ) 0.999 for seven observations).
)
2.97 × 10-6 s-1, r2 ) 0.997 for seven observations; and 105 °C,
Ack n ow led gm en t. We are grateful to Dr. Dave J .
Adams (York Green Chemistry Group, The University
of York) for valuable comments and to Dr. J acques
Muzart (University of Reims) for discussions and pre-
prints. G.R. thanks the Royal Society and the Israeli
Academy of Sciences and Humanities for an exchange
fellowship to visit the York Green Chemistry Group.
J O991868E
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