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Kinetically controlled equilibrium in cis-trans isomerization of propenylbenzene
Tetrahedron
Letters No.8, PPO 645-646, 1969. Pergamon Press. Printed in Great Britain.
KINETICALLY
CONTROLLED
EQUILIBRIUM
IN CIS-TRANS
ISOMERIZATION
OF PROPENYLBENZENE A. Ohno, Y. Ohnishi, Saqami Chemical 3100 Onuma,
and '2. Tsuchihashi Research Center
Saqamihara-shi,
Kanagawa,
229, Japan
(Receivedin Japan 24 December 1968; received in IK for publication15 January 1969) The interconversion to chemists
of the cis and trans isaners of olefins has been known
almost ever since the discovery
been extensively
still remaining
studied,
we know that (generally)
while the unstable
some ambiguous
the thermodynamically
more
benzene
forms quite unique
excited
thiobenzophenone
dioxide,
with
and has Nowadays,
isomers;
isomerization
stable isomer over the other,
In this paper, we will report that -cis-trans
irradiated
between
In common, the thermal or catalytic
isomer is favored by photochemical
;!hen a mixture
isomerism
points(l).
there are two types of equilibria
thermal and photochemical. tends to predominate
of qeometrical
isomerization.
isomerization
spectrum when the isomerization
of propenyl-
is catalyzed
by photo-
or by thiyl radicals.
of cis- or trans-propenylbenzene
and thiobenzophenone
5S90A light from a sodium lamp under an atmosphere
2,2,3-triphenyl-4-methylthietane
same time, the recovered
was
of carbon
was formed in 63% yield and, at the
olefin was found to be isomerized(2).
The results on
the isomerization
of propenylbenzene
are listed in Table I, from which the cis/
trans composition
at the equilibrium
state is determined
propenylbenzene
is transparent
at 5890i but thiobenzophenone
n,n* triplet state by the 1i:ht of this wavelenTth(3,4), isomerization
of propenylbenzene
is sensitized
almost the same -cis/trans
composition
of thermal isomerization:
when a mixture
butanethiol, i0
0
and azobisisobutyronitrile
under an atmosphere
as 3.1/96.9.
i's excited to its
it is evident that the
by thiobenzophenone.
can be obtained
Since
However,
at the equilibrium
of propenylbenzene,
benzenethiol
state or
was allowed to stand in a thermostat
of ar;on, the isomerization
643
of the olefin
leads the
of
No.8
644 Table I Thiyl Radical
Tew ,
%cis/%trans Initial
Oc
3(Ph2Cs)n 25 nk
3
in Olefina Final
0.5/99.5
2.2/97.8
(Ph2Cs)n',,*
25
25.7/74.3
3.0/97.0
3(i'hpCs)n'n*
25
9(1.7/ 1.3
4.2/95.8
,
Phs*
52.3
0.6/99.4
3.6/96.4
PhS.
52.3
97.3/ 2.7
4.8/95.2
n-Bus_
52.3
0.6/99.4
4.1/35.9
n-BUS_
52.3
a. Analyzed
by vpc.
at Equilibriumb %trans
3.1 f 0.7
96.9 i 0.7
4.1 Yz 0.4
95.9 i 0.4
3.8/96.2
'97.3/ 2.7 The estimated
Composition %cis
error is iO.5~;.
b. Mean of final composi-
tions. Table II Time,hr
%cisi%trans
Time,hr
in Olefina
Xcis/%trans
in Olefina
0
97.1/ 2.9
0
16
78.6/21.4
Lb
25.5/74.5
31
69.1/30.9
30
34-O/66.0
42
68.3/31.7
42
39.9/60.1
150
55.2/41.6
200
60.4/39.6
5.6/94.4
a. Analyzed
by vpc.
composition
of isomers to 4.1% cis and 95.9% trans at the equilibrium
The estimated
error is *0.5X. state as
shown in Table I. CM the other hand,the can be calculated, -37.874,
equilibrium
by usino, the equilibrium
logutrans = -37.320)(5),
commonly expected, component
thermodynamic
of both isomers
of formation
(logKcis =
as 21.8% cis and 78.2% trans at 271)
the photochemical
than the trans-isomer.
constants
composition
isomerization
malces the cis-isomer
The results of the irradiation
benzene with 3660; light from a hi:h-pressure
mercury
As larger
of propenyl-
lamp with a Pyrex filter
are listed in Table II. Thus, the isomerization the equilibrium
constant
of propenylbenzene
differin:
completely
caused by thiylradicals(6)
has
from those of thermal and photo-
645
No.8
I
Energy
A: $&-olefin 6: cis-type
+ RS. intermediate
C: trans-type
intermediate + RS*
D: trans-olefin
I
Reaction
Coordinate Fi:. 1
chemical
isomerizations.
interpreted
The presence
in terms of fairly stable addition
librium with both isomers
of propenylbenzene.
and products,
lon:er control the reactions
which is in equi-
As schematically
A and 3, thermodynamic
the equilibrium C -
intermediate(s)
constant may be
shown in Fis.1,
say 5, is ener.;etically more stable than systems of
if one of intermediates, reactants
of the third equilibrium
composition
A and C -
stabilities
of both isomers no
but free enersies
D determine
of activation
of
the amounts of isomers at the equi-
librium state:
m=
exp(
LDI
9
where [A]/[31
is equal to [cis-olefinl/Ctrans-olefinl
of the thiyl radical The addition
atom to ethylene
representation
of methanethiyl
.,:=
I
radical
to ethylene
the concentration
is an exothermic
which makes a sharp contrast
(OH = 7kcal/mole)(S).
reaction
to the addition
Coupled with our previous
of a radical center by the participation
of J-sulfur,
of iodine
proposal
on
likely
bein; I(4), this supports the idea of fairly stable intermediate. Mention
XT1
because
cancels.
with atl = -14kcal/mole(7),
stabilization
aF:qi nFf,,
should be added on the fact that the trans-
isomer is unexpectably rolled equilibration
preponderant
in kinetically
than in thermodynamically
cont-
controlled
I one.
The reaction
thiyl radical
C ----,A involves
and the rotation
the elimination
of central
C-C
of
bond and
No.8
646 since the former is nearly the same ener;etic the difference kcal/mole,
in free energies
of activation,
mainly depends on the free-energy
Althou.;h the exact value of the difference to be lar;er than that of formations 0.76kcal/mole
process with the reaction
C -_* D,
AAF* = L1F; - OF* = -1 87 ff -1.96
1.
difference
between j and C.
is not available,
we can estimate
of cis- and -trans-propenylbenzenes
it
(bF" =
at 27')(5).
References 1.
For example:
2. L. Eliel, "Stereochemistry
of Carbon Compounds,"
Book Co., New York, N. Y., 1962, pp 318 - 71 and references
Hc;raw-Hill
cited therein.
2.
A. Ohno, Y. Ohnishi,
3.
0. Korver, J. U. Veenland, and Th. J. deBoer, Rec. 'Trav. Chem. Pays-nas,
and G. Tsuchihashi,
Tetrahedron --
Letters,
in press.
84, 2x9 (1965). 4.
A. Ohno, Y. Ohnishi, I". Fukuyama,
and
;. Tsuchihashi,
.J. Amer. Chem. Sot., --
90, 7038 (1968).
5.
J. E. Kilpatrick,
C. W. Beckett,
i. J. Prosen, K. S. Pitzer, and F. D.
Rossini , .J. Res. Natl. Rur. Stand., 42, 225 (1949). 6.
The n,n+: triplet thiyl radicals.
7.
C. Walling,
state of thiobenzophenone
can be re.;arded as a kind of
See ref. 4.
"Free Radicals in Solution,"
lohn Wiley and Sons, Inc., New
York, N. Y., 1957, p 314.
8.
Iodine atom is known as a true catalyst See p 24 of ref. 7.
for thermal isomerizations.
Letters No.8, PPO 645-646, 1969. Pergamon Press. Printed in Great Britain.
KINETICALLY
CONTROLLED
EQUILIBRIUM
IN CIS-TRANS
ISOMERIZATION
OF PROPENYLBENZENE A. Ohno, Y. Ohnishi, Saqami Chemical 3100 Onuma,
and '2. Tsuchihashi Research Center
Saqamihara-shi,
Kanagawa,
229, Japan
(Receivedin Japan 24 December 1968; received in IK for publication15 January 1969) The interconversion to chemists
of the cis and trans isaners of olefins has been known
almost ever since the discovery
been extensively
still remaining
studied,
we know that (generally)
while the unstable
some ambiguous
the thermodynamically
more
benzene
forms quite unique
excited
thiobenzophenone
dioxide,
with
and has Nowadays,
isomers;
isomerization
stable isomer over the other,
In this paper, we will report that -cis-trans
irradiated
between
In common, the thermal or catalytic
isomer is favored by photochemical
;!hen a mixture
isomerism
points(l).
there are two types of equilibria
thermal and photochemical. tends to predominate
of qeometrical
isomerization.
isomerization
spectrum when the isomerization
of propenyl-
is catalyzed
by photo-
or by thiyl radicals.
of cis- or trans-propenylbenzene
and thiobenzophenone
5S90A light from a sodium lamp under an atmosphere
2,2,3-triphenyl-4-methylthietane
same time, the recovered
was
of carbon
was formed in 63% yield and, at the
olefin was found to be isomerized(2).
The results on
the isomerization
of propenylbenzene
are listed in Table I, from which the cis/
trans composition
at the equilibrium
state is determined
propenylbenzene
is transparent
at 5890i but thiobenzophenone
n,n* triplet state by the 1i:ht of this wavelenTth(3,4), isomerization
of propenylbenzene
is sensitized
almost the same -cis/trans
composition
of thermal isomerization:
when a mixture
butanethiol, i0
0
and azobisisobutyronitrile
under an atmosphere
as 3.1/96.9.
i's excited to its
it is evident that the
by thiobenzophenone.
can be obtained
Since
However,
at the equilibrium
of propenylbenzene,
benzenethiol
state or
was allowed to stand in a thermostat
of ar;on, the isomerization
643
of the olefin
leads the
of
No.8
644 Table I Thiyl Radical
Tew ,
%cis/%trans Initial
Oc
3(Ph2Cs)n 25 nk
3
in Olefina Final
0.5/99.5
2.2/97.8
(Ph2Cs)n',,*
25
25.7/74.3
3.0/97.0
3(i'hpCs)n'n*
25
9(1.7/ 1.3
4.2/95.8
,
Phs*
52.3
0.6/99.4
3.6/96.4
PhS.
52.3
97.3/ 2.7
4.8/95.2
n-Bus_
52.3
0.6/99.4
4.1/35.9
n-BUS_
52.3
a. Analyzed
by vpc.
at Equilibriumb %trans
3.1 f 0.7
96.9 i 0.7
4.1 Yz 0.4
95.9 i 0.4
3.8/96.2
'97.3/ 2.7 The estimated
Composition %cis
error is iO.5~;.
b. Mean of final composi-
tions. Table II Time,hr
%cisi%trans
Time,hr
in Olefina
Xcis/%trans
in Olefina
0
97.1/ 2.9
0
16
78.6/21.4
Lb
25.5/74.5
31
69.1/30.9
30
34-O/66.0
42
68.3/31.7
42
39.9/60.1
150
55.2/41.6
200
60.4/39.6
5.6/94.4
a. Analyzed
by vpc.
composition
of isomers to 4.1% cis and 95.9% trans at the equilibrium
The estimated
error is *0.5X. state as
shown in Table I. CM the other hand,the can be calculated, -37.874,
equilibrium
by usino, the equilibrium
logutrans = -37.320)(5),
commonly expected, component
thermodynamic
of both isomers
of formation
(logKcis =
as 21.8% cis and 78.2% trans at 271)
the photochemical
than the trans-isomer.
constants
composition
isomerization
malces the cis-isomer
The results of the irradiation
benzene with 3660; light from a hi:h-pressure
mercury
As larger
of propenyl-
lamp with a Pyrex filter
are listed in Table II. Thus, the isomerization the equilibrium
constant
of propenylbenzene
differin:
completely
caused by thiylradicals(6)
has
from those of thermal and photo-
645
No.8
I
Energy
A: $&-olefin 6: cis-type
+ RS. intermediate
C: trans-type
intermediate + RS*
D: trans-olefin
I
Reaction
Coordinate Fi:. 1
chemical
isomerizations.
interpreted
The presence
in terms of fairly stable addition
librium with both isomers
of propenylbenzene.
and products,
lon:er control the reactions
which is in equi-
As schematically
A and 3, thermodynamic
the equilibrium C -
intermediate(s)
constant may be
shown in Fis.1,
say 5, is ener.;etically more stable than systems of
if one of intermediates, reactants
of the third equilibrium
composition
A and C -
stabilities
of both isomers no
but free enersies
D determine
of activation
of
the amounts of isomers at the equi-
librium state:
m=
exp(
LDI
9
where [A]/[31
is equal to [cis-olefinl/Ctrans-olefinl
of the thiyl radical The addition
atom to ethylene
representation
of methanethiyl
.,:=
I
radical
to ethylene
the concentration
is an exothermic
which makes a sharp contrast
(OH = 7kcal/mole)(S).
reaction
to the addition
Coupled with our previous
of a radical center by the participation
of J-sulfur,
of iodine
proposal
on
likely
bein; I(4), this supports the idea of fairly stable intermediate. Mention
XT1
because
cancels.
with atl = -14kcal/mole(7),
stabilization
aF:qi nFf,,
should be added on the fact that the trans-
isomer is unexpectably rolled equilibration
preponderant
in kinetically
than in thermodynamically
cont-
controlled
I one.
The reaction
thiyl radical
C ----,A involves
and the rotation
the elimination
of central
C-C
of
bond and
No.8
646 since the former is nearly the same ener;etic the difference kcal/mole,
in free energies
of activation,
mainly depends on the free-energy
Althou.;h the exact value of the difference to be lar;er than that of formations 0.76kcal/mole
process with the reaction
C -_* D,
AAF* = L1F; - OF* = -1 87 ff -1.96
1.
difference
between j and C.
is not available,
we can estimate
of cis- and -trans-propenylbenzenes
it
(bF" =
at 27')(5).
References 1.
For example:
2. L. Eliel, "Stereochemistry
of Carbon Compounds,"
Book Co., New York, N. Y., 1962, pp 318 - 71 and references
Hc;raw-Hill
cited therein.
2.
A. Ohno, Y. Ohnishi,
3.
0. Korver, J. U. Veenland, and Th. J. deBoer, Rec. 'Trav. Chem. Pays-nas,
and G. Tsuchihashi,
Tetrahedron --
Letters,
in press.
84, 2x9 (1965). 4.
A. Ohno, Y. Ohnishi, I". Fukuyama,
and
;. Tsuchihashi,
.J. Amer. Chem. Sot., --
90, 7038 (1968).
5.
J. E. Kilpatrick,
C. W. Beckett,
i. J. Prosen, K. S. Pitzer, and F. D.
Rossini , .J. Res. Natl. Rur. Stand., 42, 225 (1949). 6.
The n,n+: triplet thiyl radicals.
7.
C. Walling,
state of thiobenzophenone
can be re.;arded as a kind of
See ref. 4.
"Free Radicals in Solution,"
lohn Wiley and Sons, Inc., New
York, N. Y., 1957, p 314.
8.
Iodine atom is known as a true catalyst See p 24 of ref. 7.
for thermal isomerizations.