Indi an Journal or Chemi stry
Vol. -l2A. April 2003. pp. 75 1-7 57
Electrochemical reduction of benzyl bromide in the presence of carbon dioxide
Abdiri sa k A (sse & Armando Gennaro*
Department or Ph ys ical Chemi stry. Uni versity of Padova. via L oredan 2. 35 13 1 Padova . It al y
E-mail: A .Gen naro @c hfi .unipd .it
Receil'ed 5 Decelll ber 2002
Th e ele..:trochemi cal reduction o f benzy l brom ide has been in ves ti gated in acetonitril e and CO 2-sa turated aceto nitrile by
cyc li c vo ltammetry and contro ll ed-potenti al elec tro lys i s. Electroreduction of the halide in th e absence or CO 2 lead s to a
va riety of products. th e di stributi on o f whi ch d epe nd ~ on the elec trode material. applied potcnti al and proton ava ilability in
the med ium . Th e elec trocarboxylat ion proresse, ha ve bee n carri ed out at Hg and graphite ca th odes using bo th a twoco mpartment ce ll and an undi vided ce ll w ith di ssol ving AI anode. The best results (87 % pheny laceti c y ield) are ob tain ed at
Hg in th e undivided ce ll . Ca talys is by Co(salen ) all ows th e process to be perform ed at potenti als more positi ve than th ose
required by direc t reducti on but gi ves onl y poor to moderate y ields of acid. Results obta ined rrom experime nts carri ed out
with ben zy l chloride have been compared with th ose obtained in the case of benzy l bromid e.
The electrochem ical fixat ion of CO 2 into organic
substrates is a convenient method o f synthes is of
carboxyl ic ac ids I.
In
particular,
non-steroidal
anriinflammatory agents such as 2-ary lpropanoic
ac ids may be easily prepared by electrocarboxy lation
o f suitabl e benzy l halides. Indeed . thi s process has
been the subject of several investigation s with
particu lar inlerest to th e optim isation of product
y i e ld s 2.~ . To thi s end, chl orides ha ve been largely
preferred to brom ides or iod ides as starting material s
for elec trocarboxy lat ion. However. a drawback to the
use of benzyl ch lorides is that their reduction at the
most common ly used cathodes occurs at very hi ghl y
negative potential s, where concomitant reduction of
CO 2 may tak e place, resulting in undesired products
and a decrease of current efficiency. To circ um vent
thi s difficulty. various cata lytic systems, mainly based
on transition metal comp lexes. have been used 5-8 .
[t is widel y known that alky l bromides and iod ides
reduce much eas ter than th eir corresponding
. Ies '1 ·111 . For example, at a glassy carbon
ClII one
elec trod e in acetonitrile, th e reduction potential of
ben zy l bromide is 0.5 V more positive than that of
benzy l chloride Ill . Th e potential s required for th e
reduction of benzy l brom ides are considerably less
negati ve than the reduction pot ential of CO 2. Hence,
use of benzyl
bromides
in
electrochemica l
carbox y lati ons ;nay have th e advantage of avoiding
concomitant reduction of CO 2.
In thi s paper we describe the results of an
inves ti gation on th e elec troc hemica l carboxylati on of
benzy l bromide in acetonitrile. Th e study W;lS carri ed
out at Hg and graphite electrodes using two different
cell arrangements: a two-compartment ce ll w ith a Pt
anode or an undi vided ce ll w ith a dissolving AI
anode.
Both
direct and
indirect
(med iated)
electroreduct ion of the halide in CO 2-saturated
C H, CN were examined. As a cata lyst a coba lt
co mpl ex (Co(salen), sa len = lI2,2'- 11 ,2-ethanedi y lbi s2
(nitri lomethy lidyne)]bis[p henolatoIJ - ) , which has
been previously reported to have good catal ytic
effec ts towards the reduct ion of benzy l chlorides, was
6
used .
Materials and Methods
Acetonitrile (B OH ) was distilled over Ca H2 and
stored
under
argon
atmosphere.
T etra-I/butylammonium perchlorate (Fluka) was cry stalli zed
twice from EtOH + H 2 0 (2: I ) and dried in a vacuum
oven at 60°C. Co(salen)" and benzyl phenylacetate
(PhC H 2C0 2C H 2 Ph )1 2 were prepared according to
procedures described in th e literature. Carbon di oxide
(99 .998 %) was suppli ed hy Sl AO ( Ital y). A ll other
reagents were commerc iall y availabl e reagents and
we re used as received.
Electrochemical measurements were carri ed ou t by
using
an
EG&G
PARC
(Model
173/179)
potentios tatlcoul ometer co upled w ith a uni versal
programmer (M odel 175) and a LeCroy L T 322
osc i lloscope. For cyc li c vo ltaJl1metry measurements.
glassy carbon or mercury was used as the working
electrode. Th e counter and the reference elec trodes
were a Pt wire and Ag/Agl/O. I M ( I/- C~ H ~)" I in
OMF, respec ti ve ly. A t th e end of th e ex perim ent. th e
752
INDIAN J Cf-IEM . SEC A, APR IL 2003
potential of th e reFerence elec trode was always
measu red versus the saturated calome l electrode
(SCE) , to which all potential s are finally referred.
Con troll ed-potenti al electrol yses were ca rried out
e
either at a Hg pool Os cm ) or at a co mpact graphite
rod (6 cm\ using two different cells: a twocompartment ce ll with a Pt an ode separated from th e
cathodic co mpartment by glass Frits and Tylo se-(IlC~ H (» 4 NCl04 -sa turated brid ge or an undivided cell
w ith a sacrifi cial AI an ode. All ex periments were
performed at 25 °C.
The electrolys is products were analysed by using
ail
HPLC
Perkin -E lmer
Series
4
liquid
ch romatograph. equipped with a UV detector and a
reversed-phase LC 18- DB Supelco column . Th e eluent
was a mixture of C H, CN and H 20 , acidifi ed with
I
CH, C0 2 H (5 g L- ). At th e end of th e elec trolys is a
samp le of so luti on was withdrawn from the
elec troc hemi ca l ce ll and direc tl y analysed by HPLC.
Authentic compounds were used for th e identificati on
and quantl!'ica ti on of the products.
o .
-5 0
- J 00
-2 .0
-1.5
, 1.0
E ( V vs SCE )
Fi g. I - Cyc li c vnltam metry o j" 1.93 mM be ll zy l bromid e ill
C H,CN + 0.1 M (II-C4 H<j)4NCl04 al a glassy carbo ll elec trode at I '
= 0.2 Vs- I .
o
~
Resu lts and Discussion
~--~------~------~--~
2,
50
1.::teClmcl,elllical redllC lioll of Ph CH ]Br
Cyc li c vo ltamlTletry ex perim ents were carried out
in C H, CN + 0 .1 M ( Il -C4 H () ~ NClO~ . using mercury
and glassy ca rbon electrodes (GC). Figures I and 2
show cyc li c vo ltammetric curves for the reduction of
ben zy l brom ide at GC and Hg, respectively. On both
electrodes. th e com pound ex hibits a single irreversibl e
and broad peak. The va lues of th e peak potentials (Ep)
measured at a sean rate (II) of 0.2 V S- I are - 1. 82 V
and - 1.5 1 V \IS SCE at GC and Hg. respectivel y. Ep
va ri es linearl y w ith the logarithm of II and th e slopes,
aE/ alogl', obtai ned at GC and Hg are - 124
m V /decade and - 183 m V /decade , respec ti vely . Th e
tran sfer coefficients (a) ca lculated from th e above
slopes accord i ng to the equati on IJ aEI/dlog v =
- 1.1 5RT/aF are 0.24 and 0. 16 at GC and Hg,
respectively. a was also ca lculated fro m th e peak
w idth. th e di fference between Ep und th e potenti al at
half peak (Ep/c ), accordin g to the equati on 1.1 Ep/2 - Ep =
1.857 RT/a. Th e averages of th e va lues obtained at
different scan rates in the 0.2 - 20 V S- I ran ge are
0.30 and 0.28 for GC and Hg, respec ti ve ly . These data
are typ ica l of a reducti on controlled by the kineti cs of
th e heterogeneous electron trans fer (ET) and are in
agreement with the mechani sm previously reported
for th e clectroreducti on of Ph C H 2 Br at GC, whi ch IS
co nsidered to be an inert electrode materi al 10 .
100
-2.0
-1. 5
-1. 0
-0 .5
E ( V vs SCE)
Fig. 2- Cycli c voltam metry of 2.24 mM benzy l bromide in
C H ICN + O.t M (II -C4Hq)4NCI04 at a Hg electrode ill th e (- )
ab~c ll c e and (- - -) presence 01'0.28 M CO 2. I' 0.2 V S- I.
=
Th e vo ltam metri c beha viour of Ph C H 2 Br IS
stron gly affected by CO c or by proton do nors such as
C H, C0 2 H. For exampl e, when CO 2 is bubbled into a
soluti on containing PhC H 2 Br. a con ~ i de ra bl e increase
of th e reducti on peak of the bromide is observed
(Fig. 2). Add iti on of an ac id brings about a similar
effec t. It seems that, in the absence of carbanion
scavengers, reducti on o f the halide is not trul y a 2eprocess. T o understand better th e exact stoichiometry
of the process, a seri es of con trolled-potential
under different
elec trol ys is was carried out
ex perimental co nditi ons.
The results o f th e elec trolys is are summ ari sed in
T able I . A first observation on the data is that , if an
eff icient pro ton donor is not present in so luti on , the
charge consumption is very close to I e-/ molecul e of
PhC H 2 Br (entri es 1-4). Add iti on o f I fill water
ISSE clal. : ELECTROCARBOXYLATIO
S. No.
OF BENZYL BROt\lID E
75:'
Tabl e I- Elcc trochemi cal reduction or benzy l bromide in CH,CN + 0.1 M (II -C 4 H9 ).:NCI0 4 .
h
Elec trod e IPhCH 2 Bri
II
Product zields (% )"
E"rp
d
ROR d
mM
V vs SCE
ROI-I
RI-I
I-I gR2
RC H 2C
Tota l
Hg
11 .58
- 1. :1 1
1.0
18
78
0
2
()
98
2
Hg
8.42
- 1.40
1.2
33
20
20
2
8
X]
3
Ho
0
8.42
- 1.65
1.1
49
0
15
4
15
88
4
C
- 1.85
U
57
0
10
7
16
90
5
Hg
10.52
8.42,,1
- 1.40
1.0
49
15
4
.'i
15
X8
6
Hg
8.42,,1
- 1.65
1.1
49
()
5
5
:10
Xc)
7
Hg
8.42·,2
- 1.65
1.7
82
0
0
17
()
99
8
C
8.42,,2
- 1.85
1. 7
84
0
0
11
0
96
"Added wa ter: 10. 1 M. 2 1.0 M. hCharge (F/mol) consumed with respect to convert ed PhCH 2 Br. "Yi c ld is ca lculated with respect to
PhC H 2 13r disappeared. dYi eld represent s th e percelllage or th e ori ginal PhC H 2 13r incorporated into th e product
sign ifi ca ntl y increa. es the charge co nsumption of the
process, which now tends to a 2e- reducti on to toluene
(entri es 7-8). Tht' electrolys is gi ves, ri se to a variety
of products th e distribution of which strongly depends
on ex perimental conditions such as cathode material ,
applied potemial and proton ava il ab ility in th e
medium . Wh en the applied potential is negativ e
enough to ensure reducti on of th e intermed iate benzyl
rad ica l, toluene is formed as th e principal reducti on
product. Under such circumstance, th e maj or side
prod ucts are hydrocinnamonitrile, benzy l alcohol and
dibenzy l ether. w hi ch are formed b y nucl eophili c
allack of anion s stemming from the protonation of
PhCH :-- at th e startin g benzy l bromide. In fact, th e
pro ton donor involved in reaction could be H 2 0
(either resi dual or purposely added) or C H 3CN , w hi ch
•nave simi
" 1ar p K" va Iues t4 .
PhCH 2- + C H,C . - ' • PhCH, + T H 2CN
... ( I )
PhC H 2- + H 20 -
. . . (2)
. PhC H, + OH
. .. (4)
I\l so the benzy l alcohol formed in reac tion (4) may
get in vo lved in a pro ton transfer react ion wi th PhCH ~
to give an alkoxide Ion PhC H 2 0 - that l11ay further
react with PhCH 2 Br.
Ph CH 2 - + Ph C H2 0H - -
PilC H, + PhCH 20 -
... (5)
The di :-. tri butiOI1 of such products is strong ly
affected by th e concen tration or 11 2 0 in th e reactio n
medium. In nominal ly dry acetonitril e th e yi eld of
toluene does not exceed 57 % wh ile significant
amounts o f side products are formed (entri es 3-4). It
is noteworth y that under such co nditi ons appreciabl e
amounts of hydrocinnamonitrile are fo rmed. Wh en
H 20 is added to the reacti on medium . the y it' ld o f
toluene increases at th e detriment of the y ield of the
side products, reachi ng ca 84% in the presence of I M
H 2 0. The y ield of hydrocinnamon itril e is particularl y
affected by th e presence of H 2 0 , decreasing wit h
increasing co ncentration of th e latter. In th e presence
of I M H 2 0 , hydrocinn amonitril e form ati on is no
longer observed implying that reac ti on ( I ) is outpaced
by reacti on (2). Thi s mean s th at H 20 is a much more
efficient proton donor than C H, CN. Since, however,
th e two proton donors have compa rable pK" va lu t's l~ .
the difference in reactivity between th e two
compounds should be clue to a signifi ca nt difference
between the intrinsic barri ers of the proton transfer
reactions ( I ) and (2).
The effect of th e appl ied potential (Eapp) on the
di stribution of th e products has been inves ti gated at
the Hg electrode. I\s show n by the data reported in
Table I ·(entri es 1-3), the y ield of toluene increa. e.
with decreasing E ilpp . When the electrol ys is was
carri ed out at - 1.31 V liS SCE, a poten tial
correspo nding to the foo t of the reduction peak o f
PhCH 2 Br (see Fig. 2), dibenzyl mercury was obtain ed
as the pri nci pal reduction prod uc t. Shi rt ing the
electrolysis potential to nlore negative va lues results
in a decrease of the yie ld of (PhCH:!h Hg whil e that of
Ph C H ~ increases . At - 1. 65 V vs SCE no (PhCH :!) ~ H g
is form ed. It is worth noti ng thaI. although radi ca lrad ical coupl ing of benzy l rad ica ls has a rate con , tant
of the order of I O~ M- I~ I(Ref. 15), bibenzyl was
never observed among th e recluction pr\)(lu cts The
ben zyl radica ls arc preferen tiall y captured by th e Hg
electrode'. Depending on the app lied potenti al. th e
754
INDI AN J CHEM. SEC A. APR IL 2003
benzy lmercury radi ca l so form ed may either undergo
I e- reducti on to PhCH 2- or gi ve diben zy lmercury
9
through di sproporti onati on
The electrol ys is at th e graphite elec trode was
ca rri ed out at a po tential co rresponding to the E" of
the halide as measured at GC electrode. In thi s case
th e process in vo lves free benzy l radicals, which are
immed iately reduced at th e very negative potential s
required for th e reduction of th e starting halide. Thu s,
onl y trace amounts of bibenzyl were observed in th e
ex periment s carried out at th e graphite elec trode. It
was also noti ced that th e nature of the cath ode
material has no signifi cant effect on th e se lec ti vit y of
th e process so long as an £ va lue negati ve enough to
ensure immed iate reducti on of th e intermediate
radica ls is app li ed.
EleCfJ'Ocarboxvlafion of Plt C H ]X (X = Br. Cf)
Accord ing to the data obtai ned from the
vo ltammetri c in ves ti gati on, CO 2 is a good scavenger
of benzy l ca rbani ons. In fac t. as show n in Fi g. 2.
bubbling CO 2 into a soluti on of PhCH 2 Br ca uses a
remarkabl e enhancement of th e peak current for th e
red ucti on of the halide. In th e presence o f CO 2 . th e
benzy l carbani ons are rapidly trapped by CO 2 (Eq. 7)
and the overall process tends to become a 2ered ucti on of the halide. Under such ci rcumstances,
ph eny l acetate is ex pec ted to be th e principal
reducti on product.
... (7)
The res ults or preparati ve-sca le electrol ys is of benzy l
bromide in CO 2 -saturated C H:1 C
are reported in
Table 2. Th e elec tro lys is were performed both in
div ided and undi vided ce ll s using both Hg and
graphite (C) cath odes. T he main reducti on products
we re ph eny lacetic acid (RC0 2 H) and tolu ene,
bibenzyl being either absent or detec tabl e onl y at
trace leve ls. M os t of th e side produc ts observed in th e
ex periments performed in th e absence o f CO 2 were
also absent : onl y small quantiti es of benzy l alcohol
could be observed. Besides pheny laceti c ac id and
toluene, ben zy l ph eny l acetate, w hi ch is form ed by
nucleophili c attack of th e carboxy late ion on
Ph C H 2 Br (Eq. 8), was obtained wh en th e electrol ys is
was perform ed in a two-compartm ent cell (Table 2,
entri cs 1-2). In such experiments, th e yie ld
Ph C H 2 Br + PhCH 2CO c- - PhCH 2 C0 2 C H 2 Ph + Br-
.. . (8)
of th e acid is very low, th e maj or prod uct being th e
es ter, which aecoul1lS for up to 70% of the start ing
halide. Wh en the ex periments were performed in
an undi vided cell with a sac ri fi cial A I anode
(entries 4-5), th e y ield of the acid increased up to 68 o/c
w hil e formati on of th e es ter became co mpl etely
suppressed. Th e A IJ + cati ons form ed at th e anode
stabi I ise th e ca rboxy late ion. mak i ng reac tion (8) too
slow to occur in the time scal e of th e experim ent.
Th e
results
of
so me
elec trocarhoxy lati on
ex perim en ts on benly l ch lori de per formecl at a Hg
cathode in CO 2-sa tu rated C H.lC
are inc I uded in
Tabl e 2 (entri es 3, 6). The reduct ion potential of
PhC H 2Ci at th e Hg electrode is very nega ti ve ( 'ee
Fig. 3), so th e ex periments were carri ed out at a
potenti al (- 2. 16 V vs SC E) co rrespond ing to th e foot
of the reducti on wave of th e chloride. i n order to
minimise th e con tributi on of elec troreduction of CO 2 .
A few observation s can be made comparing the
res ults of electrocarboxy lati on of PhC H 2Ci w ith th ose
of the same process, carri ed ou t under simil ar
co nditi ons, for PhCH 2 Br. Firstl y, in both types o f
electrochemi ca l ce ll , electrocarbo xy lation or the
chloride gi ves better chemica l y ields of pheny laceti c
acid. Secondl y, onl y a sma ll quant i ty o f ester is
formed w hen PhC H 2 Ci is used, indi catin g that the SN2
reaction (Eq. 8) on the chl oride is quite slow .
Red ucti on of PhCH 2Ci , however, requires very
nega tive potential , which also in vo lves direct
reducti on of CO 2 at th e elec trode (Fig. 3). To
minimi se th e in vo l ve ment of 5uch undes irable
reac ti on, an appli ed poten tial as much posi ti ve as
poss ibl e
was
selected
for
th e
elec trolys is.
Neverth eless, th e charge co nsumpti on (see Table 2.
entri es 3,6) is co nsiderabl y greater tha ll th e theoretical
va lue or 2e-/molec ul e of PhCH 2Cl req uired for th e
carbo xy lati on or th e halide.
Co(sa len )-colOlysed el eCl rocor!Joxv!of ion
Th e catalytic effect of Co(sal en) 0 11 th e
elec trocarboxy lati on process has also been examined.
Fi gure 4 shows cyc li c vo ltammogram s fo r th e
reducti on of the co mpl ex in th e prese nce of be Illy I
bromi de and COo. In th e absence o f ·Ph C HoBr.
Co " (salen) ex hibits a reversible pea k cOL~p l e
co rresponding to the red uct ion of CoO l ) to Co( l ) .\l ith
C" = - 1.30 V vs SCE. Addition I' benzy l bromide
causes an increase in th e peak curren l or th e reduction
peak which becomes irreversibl e and is shifted to
more posi tive potential s, and a new redu cti on peak
appears at more negativ e potential s (Fig. 4b). The
ISSE
el
al.: ELECTROCA RBO X YLATION OF BE ZYL BROMIDE
Table 2- Elec trochelllical carboxy lati on of benzy l halides in CH,C
S.
o.
Electrode
Ce ll"
RX
fR X I
mM
755
+ 0. 1 M (II -C.j H<j).jNClO.j.
Product yields (* )"
E"pp
V vs SCE
I
2
Hg
A
PhCH , Br
10.51
- 1.65
IA
26
9
.+.+
79
PhCH, Br
10.5 1
- 1.85
1.5
13
14
70
<)7
C
A
3
Hg
A
PhCH ,CI
10.86
- 2. 16
2.6
71
22
5
<)R
4
Hg
i3
PhCH , Br
10.5 1
- 1.65
2A
62
22
0
R.+
5
C
B
PhCH, Br
10.5 1
- 1.85
2.2
68
16
()
X'+
6
Hg
B
PhCH ,CI
10.86
-2 . 16
SA
87
10
()
<) 7
7
C
A
PhCH, i3r"
10.5 1
- l AO
1.5
8
19
44
71
PhCH,CI "
10.86
- 1.62
2. 1
36
37
I
7.+
8
C
A
9
C
i3
PhCH, Br"
I 0.51
- l AO
2.0
47
25
()
72
la c
B
PhCHoCI "
10.86
- 1.62
2A
49
32
0
XI
"The ce ll used fo r elec tro lys is was either a t;o compartm ent cell (A) or an undi vi ded cell with an alulllin iulll sacrifi cial anode ( 13 ). hl n the
presence of co I mM fCo(sa len) I. "Charge (F/ mo l) consumed with respec t to converted PhCH , X. dYi eld is calcu lated with respect 10
convert ed PhCH, X. cYi eld represents the percentage of th e ori ginal PhCH , X incorporated into the product.
20,---------------- -- -,
o
Or-50
-20 r-
-<
~/ / ...
-100
2-
I
-40 r-1 50
-60
, I'
I
I
, ,
/ -"
r- b I /rV
-2.5
-2.0
~
-1.5
E ( V vs SCE)
Fig. 3- Cyc li c vo lt ::lInmetry of benzy l chl oride in C H,CN + 0. 1 M
(II-C.j I-l<j).jNC I04 at a Hg elec trode at II = 0.2 V S- I. (- ) 2.0 mM
PhCH ,CI: (- - -) 0.28 M CO,. Th e arrow indi cates th e appli ed
potential of electrolys is of PhCH ,CI in CO,-saturated CH,CN.
mechanism of th e electrocata lytic reducti on o f benzyl
halides by Co(salen) has been previously described in
detail 6. 16.1 7. T he following reacti on sequence takes
place:
rCo11(salen)] + e-
[Co1(salen) r
[Co1(sa len)r + Ph C H 2 Br - ~
I Ph C H 2Co lll (sa len)1 + Br[ Ph C H 2 Co lll (sa len)] + e- -~
ll
[Ph C H 2Co (salen)r
... (9)
- 100 r-
~.
"
b
"
"
"
"
,;'"
-200 r-
-300 r-
~
~
Ot-
c II1/"
t
~
~
-1 .5
- 1. 0
E (V vs SCE)
Fig. 4- Cycli c vo itammogra ills at a glassy carbon electrode of
Co(sa len) in CH 3 C + 0. 1 M (II -C4 H<».j CI0 4 at v = 0.2 Vs- I : (a)
0.82 mM Co(salen) (b) as (a) + 8A I mM PhCH , i3r and (c) as (b)
in th e presence of 0.28 M CO, .
... ( IOj
... ( I I )
Th e elec trogenerated coba lt(l ) co mpl ex reacts w ith
Ph C H 2 Br according to an SN2 mechani sm. Such a
reac ti on lead s to th e formation of an orga nocobalt
com pl ex (Eq. 10), which is reducible at potenti al s
more negativ e th an that of Co(salen). Th e peak
for
the
reduct ion
of
potential
measured
lll
PhCH 2Co (sa len) is -1.45 V vs SCE at v
0.2 V s· l .
Reducti on of th e new ly formed organometall ic
co mpl ex (Eq. I I ) yie ld s a very unstabl e coba lt( lI )
species, which rapid ly undergoes homolyti c Co-C
bond cleavage to gi ve Ph C H 2 • and [Col(salen)r
(Eq . 12). The chemi stry and electrochemi stry of th e
benzyl rad ica l so formed are essentiall y th ose already
described in th e unmedi ated process.
=
756
INDI AN J CHEM. SEC A. APRIL 2003
Acco rdin g to th e above reaction sequence,
elcc troca tal y ti c reduction o f PhC H 2 Br ca taly sed by
Co(salen) can be achi eved at potenti als co rresponding
to th e reduction of th e orga nocobalt co mplex . In fact,
th e peak current observed for the latter process is
more than twi ce that o f th e I e- reduction of
Co"(sa len) to rCo1(salen)r. Indeed, th e process is
cataly ti c and the exchanged number o f electron s is
signifi ca ntl y greater than I . Th e catal y ti c nature of th e
lll
red uct ion of Ph C H 2 Co (s al en) becomes more
apparent i f th e soluti on is saturat ed w ith CO 2 (see
Fig. 4, curve c). Similar results ha ve been already
found for th e catalyti c reducti on of benzyl chlorides
by Co(sa len) and th e role play ed by CO 2 in th e
elec trocata ly ti c
process
has
been
ex plain ed
elsewhere('.
T he
results
of
so me
preparati ve-sca le
electrocarboxy lati ons of benzyl bromide catal ysed by
Co(salen) are reported in Tabl e 2. Th e experiments
were co nducted both in divided and undi vided cell s
wi th a graphite ca th ode: Hg could not be used for thi s
purpose as direct reducti on o f PhC H 2 Br overlapped
wi th that of th e ca taly ti c process. Contro ll ed-potential
elec trol ys is, in CO 2-sa turated C H, C conta ining co I
mM Co(salen) and a 10- fold excess of PhC H 2 Br, was
carri ed out at - 1.4 V vs SCE. It is to be noted that thi s
potential is in th e ri sing portion of the reduction peak
of the organocobalt com pl ex (Fi g. 4. curve b). Such a
va lue has been chosen for th e electrolys is in order to
avoid unmediated reduction of th e bromide at th e
graphite elec trode. Th e appli ed potential , however.
ensures immediate reduction o f PhC H 2 • at the
electrode since il is co mparabl e w ith th e reducti on
potential of th e radi ca l (- 1.43 VI 'S SCE). The data
show th at bett er res ults. in term s o f product y ields are
obtained by the unmedi ated elec troca rboxy lati on as
compared to the Co(salen)-catalysed process. In
additi on, th e co balt co mpl ex decomposes during the
electrol ys is; at th e end of th e elec trolys is onl y a very
small fraction of th e cata lyst still rema ined in th e
act ive form . It is very likely th at th e benzy l ca rbani on
is involved in th e chemica l process leading to
deac tivation o f the catal ys{" w hi ch ex plains, at least
i n part, why th e overall y ield of th e catal ysed
electrocarbox y lation is alway s small er th an that of the
corresponding unmedi ated process . A n advantage o f
th e Co(salen)-catal ysed process over th e unmediated
one is that th e Fonner can be achi eved at relati ve ly
positive potentials. The poss ibility of ex ploiting thi s
advantage is, however, se verely limited by th e low
turnover number o f th e ca tal yst.
A compari son of th e product di st ributi on in th e
med iated and unmediated processes shows that
catal ys is w ith Co(salen) brin gs about a marked
increase of th e RH to RC0 2 H ratio . Th e add iti onal
amount of toluene formed in the Co(sa lcn)-ca talysed
process may be attributed to th e hyd ro lys is of th e
intermed iate organocobalt comp lex (Eq. 13).
[phC H 2 Co"(sa len)r + H 2 0
PhCH , + Co " (salen) + O H
... ( 13)
The role played by such a reac ti on in the
elec trochemi ca l carboxy lati on o f benzy l chl orides
ca talysed by Co(salen) has been prev ious ly evidenced
by ex periments with D2 0 6. Th e occurrence of reaction
( 13), and hence th e hi gh sensiti vity of th e catal yt ic
process to H 20 , is ye t another point in fa vo ur of the
unmedi ated process as co mpared to the Co(salen )catalysed one.
To
co mplete
th e
co mparI son
of
th e
elec trocarboxy lati on processes of benzy l bromide
with th ose of the chl oride. th e Co(sa len)-catalysed
of
PhC H 2 Ci
was
also
electrocarboxy lation
in vesti gated. Indeed. thi s process has been th e subject
o f a prev ious study, in whi ch modera te y iel ds of
6
ph eny laceti c acid have been found . Since it has been
ev idenced in that study that th e yi eld of th e ac id
depends on th e app li ed potenti al, the best results
being ob tained at E < - 1. 60 V liS SCE. the
ex periments w ith PhC H 2 Ci were perfo rm ed at - 1. 62
V I'S SC E, under oth erwi se identical conditi ons as in
th e case of PhC H:> Br. Th e -results are included in
Tab le 2 (entries 8, I 0). In good agreement w ith th e
unmedi ated processes, in the divided ce ll Ph C H 2CI
gi ves a much bett er ph eny lacetic y ield as co mpared to
th e bromide. In stead, in th e undi vided ce ll th e two
Co(sa len)-catalysed processes gi ve virtuall y the same
results.
T o sum up, ben zy l bro mide may be used as a
startin g materi al in th e electrochemi ca l syn th es is of
ph eny laceti c acid . Th e process in t c undi vided ce ll
gives a sa ti sfactory acid y ield . The chl ori de appears.
hO\A/ever, to be a better ca ndidate i f th e probl ems
related w i th th e very nega ti ve potential req uired for
its reduction are overcome. e.g.. by effic ient
elec trocatal ys is.
Acknowledgements
Financial
support
from
l he
Millisle m
del/'/slm zioll e, de// 'Ulli le rsilit c della Ricerca
(MIUR ) is gratefull y acknowled ged.
ISSE et a l .: EL ECTROCA RBOXYLAT IO
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