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Kinetic studies of bromine addition to olefins X. The bromination of 1,1-diphenylethylenes an asymmetrical transition state
TetrahedronLettersN0.32, pp. 3101-3106,1967. PergamonPress Ltd. Printed in Great Britain.
KIEETIC STUDIES OF BROMINE ADDITIOti TO OLEFINS X. THE BROMINATION
OF l,l-DIPHENYLETHPLENES
AN ASYMMETRICAL
TRANSITION STATE
J.E. Dubois and W.V. Wright* Faculti de8 Sciences, Laboratoire
de Chimie Organique Physique
1 rue Guy de la Brosse. Paris 5e (Received10 April 1967) Until recently. phenyl substituted
the high rates of addition of halogen to olefins prealuded a systematic study of these
reactions. Over the last few years, instrumentation oonvanient neasursment
of fast bromination
has been developed in this laboratory.
(1.2) for
reactions in solution
These techniques,
ly employed to study the bromination of some substituted
previousstyrenes
have been used to measure the rates of bromine addition to
(3).
a series of l.l-diphenylethylenes
(4). and the
results
are
presented here. The rate aonstants, listed in Table I, were measured in methanolic
0.2 W sodium bromide solutions at 25'C. These observed
rate constants are therefore composite and reflect the rate of bromination
by both molecular bromine and tribromide ion. While
the respective rate constants for these two processes
nT.0.
Post-Doctoral
Fellow
1965-66
3101
can be
3102
No.32
evaluated by measuring ion conoentration3. successfully reactivity
the rate of addition at varying bromide
the composite rate constants, kg have been used
for studying the effect of molecular
structure on
(5) anti are so used here. TABLE I
Bromination iia:e Constants kg measured at 2555.02oC -e-e--
____
-__._-____
-----_^-___-
l,l-diphenylethylI~ne --_--___l-_I-P
-_--_-
kg laol-lsec-l n % mean deviation --~_-_______-_-____~~-~~~--~_~~__
4.4'-dimethoxy
3.47 x 106
3
1.8
4-met.hoxy
3.77 x 105
4
4.4
4.4'-dimethyl
3.84 x lo4
3
1.7
4-methyl
1.57 x 103
5
6.4
4-fluoro
1.33 x lo2
5
6.6
Y,4'-difluoro
1.23
4
2.2
4-ohloro
4.76 x lo2
6
7.7
C-bromo
3.62 x lo2
5
8.8
4,4'-dichloro
1.28 x lo1
5
6.6
4,4'-dibromo
1.03 x lo1
3
lc.2
x
10~
n is the number of kinetic experiments. --._---a_--_.-_-____-l_________-__________l__l-_ In considering
the effect of structure on reactivity it is
customary to plot the logarithm of the measured rate constants versus the appropiate Hammett substituent
constants a(6).
Applying the Hammott treatment to the present case, log kg versus produced a curve. the p-methoxy
and the p-methyl derivatives
being more reactive than their Hammett dconstants
would imply,
In such cases a linear plot is usually obtained by the use of Brown G+(7)
values, but for these l,l-diphenylethylenes
the
No.32
3103
oonatanta ourred
for
overcorrected
roaonanoe
interactiona
and the
plot
downwards.
Recently Yukawa and Tsuno 18) have demonstrated involving resonance
stabilization
the relative importance
that reactions
of a positive charge in which
of the resonance interaction nechanisa
differs from that in the two relationships oan be handled by a linear combination
defining 6 and 6:
of these two models.
Their
modified lianmett equation takes the form log k = p(6+rAbR+)+log
k,
. .. .... ... .. ....
where r is a nen reaction constant indicative resonanoe in the transition The
Ab:values
zero,
and
Adi
of the degree of
corresponds
for well behaved meta substituents
to (6+-a).
are essentially
these are normally used to evaluate rho. The remaining
parameter. (l/plog
state and
I
r, oan then be obtained by plotting the function
k/k, -6)
versus Aa+ R. In the present series, however,
no neta substituted
derivatives were available at the time and so
equation (I) was rewritten in the forn: log k =
ab+bA<+log
where a and b correspond
k,
. . . .. . . .. . . . . . . . .
II
to
and r respectively. With the aid of p P a computer and using the experimentally determined values for kg. equation
(II) was solved for a and b on
The Yukawa-Tsuno-Hammett l.l-diphenylethylenes
equation (I) for the broaination
(9).
of
was then calculated to be
log kg = -3.61
(6+0.415A++5.11
The good fit of the experimental is shown by the correlation deviation
a least squares basis
. . .. .. . . . . . . .
points to this linear equation
coefficient
(rx0.991)
and standard
(~~0.165).
The rho value is somewhat lower than that obtained for the
III
No.32
brcmlnaticn
of st.yrener measured under
and implies less charge develcpement the brcminaticn
the same conditions
(3)
in the transition state for
of these 1.Ldiphenylethylenes.
Of particular
interest is the low value of 0.415 for r. This lcu value indicate9 reduced resonance interaoticn that in styrene krcninoticn
in the transition state relative to
(10).
The mcleoule cf l,l-diphenylethylene rings simultaneously
cannot have the two phony1
coplanar with the olefinio bond. Scale models
show that the st*ria interference
of the ortho hydrogen9 may be
rslievod eitoer ty equal twisting of both phenyl rings through an angle of apprcxiaately
30c or by rotation of one phenyl ring through
about 60' leaving the other phenyl coplanar with the clefinio moiety a9 in styrene. On the basis of dipole moment data, Sutton (11) assigned an equal twisting of some 30° to both phenyl rings. Thecretical crlculatians hydrogens)
involving eteric repulsion enorgy (crthc-
and lass of delocalizaticn
energy (through phenyl rctat-
ion) terms indicated a similar conformation for the molecule
to be the most stable
(12).
It is generally accepted that effective resonance interaction between a pnra substituted
phenyl ring and an adjacent oarbcnium
centre requires coplanarity of the aromatic ll-orbital9
and the
empty p-orbital of the carbcnium ion. The reduction in resonance interaction
due to twisting through an angle e is to a first
approximation
equal to Cc9 2 e (13). In the transition state of a
1,Ldiphenylethplene through approximately
reaction, an equal twisting of both ring9 30c uould result in a twenty-five percent drop
in resonance for each ring. On the other hand a greater rotation of
3105
No.32 ,ons phenyl ring through 60~.
leaving the other phony1 coplanar,
would produce (for the rotated phenyl) a seventy-five in resonance
interaction
with the carbonium centre. While the latter
case is perhaps favoured by the low value obtained for parameter
r,
peroent drop
the resonance
a transition state having equally twisted phenyl rings
cannot be excluded. Assuming ad hoc that bromination
involves an asymmetrical
ion state, the following facts might be considered p.p'-disubstituted para-substituent Brown 6+constant
l,l-diphenylethylenes
transit-
to apply. In the
the interaction
of the
in the coplanar phenyl will be measured by its and that of the para-substituent
in the rotated
phony1 by its Hammett 6 oonstant since in the latter case little resonance will be possible. SuBstituted
In the monosubstituted
compounds the
phenyl ring will be the coplanar one. Brown d+constants
are always the more negative electrophilic
(or less positive) and since this is an
reaction the above conformation
would minimieo
energy of the transition state. Under these circumstances
the
the rate
data should follow a modified Hammett equation of the form: log k =/0(a+CS+)+log
k,
. . . . . .. . .. . . . . . .. . IV
This in fact was shown to be the case and a least squares calculation revealed an excellent fit (r p 0.995, s m 0.047). the slope of the line,
rho, being -3.29. These results for l,l-diphenylethylsne
seem to indicate that
while equal partial twisting of the phenyl rings is the least energetic conformation
in the ground state, the preferred transition
state
for bromination
is that in which one phenyl is rotated through a
larger angle leaving the other coplanar vith the ethylenic moiety. An extension of these considerations
to the reactions of other
3106
No.32
l.l-diphanyl
,p2 oarbon systems is intended in a more
dotailed
prpar.
REFERPUCBS -1)
J.E. Dubois, P. Alcois and 0. Barbier. J.Eleotroanal.Chor. 0, 359 (19641.
2)
J.E. Duboiz and 0. Mouvier. C.R. Acad.Sci. u, G. Mouvier. p.i38 SC. Thezia, Poriz (1964). -_
3)
J.E. Dubois and A. Schwartz,
4)
W4
are
grateful
of Joruzalom,
1104 (1962).
Totrahodron Lettorr. 2,
2167 (1964).
to Professor E.D. Borgmann. The Hobrow University for kindly donating those oonpoundz.
5)
J.E. Dubois and G. Kouvior,
6)
L.P. R~nmrtt., Phgrloal Organio Chomistr Hill Book Co., Inc., Now York cl9 0 -?i7;s'
pp 184-199. McGraw-
7)
B.C. Brown omd Y. Okasoto, J.An.Chon&
80, 4979 (1958).
8)
Y. Yukawa and Y. Tzuno. Bull.Cham.Soc.
9)
C.A.
Tetrahedron Lettorz. 20, 1325 (1963).
JaDan, 2,
971 (1959).
Bonnott and I.L. Franklin, Statistical Analysiz in Chemistry an tho Chenioal Indurtrx, p. 245. John Wiley & Sons, -_ Inc., Now Yo$i;-?mrl.
10)
Unpublished
clata, J.E. Dubois and H. Roparr.
11)
G.E. Coatos r.nd L.E. Sutton, J.Chom.Soo.,
12)
M. Simonotta and 5. Car&.
13)
B.l4. Wepster, Progross In Stereoohemistr~,
567 (1942).
Tetrahedron, 19, (Suppl.2) 467 (1963). 2, 99 (1958).
KIEETIC STUDIES OF BROMINE ADDITIOti TO OLEFINS X. THE BROMINATION
OF l,l-DIPHENYLETHPLENES
AN ASYMMETRICAL
TRANSITION STATE
J.E. Dubois and W.V. Wright* Faculti de8 Sciences, Laboratoire
de Chimie Organique Physique
1 rue Guy de la Brosse. Paris 5e (Received10 April 1967) Until recently. phenyl substituted
the high rates of addition of halogen to olefins prealuded a systematic study of these
reactions. Over the last few years, instrumentation oonvanient neasursment
of fast bromination
has been developed in this laboratory.
(1.2) for
reactions in solution
These techniques,
ly employed to study the bromination of some substituted
previousstyrenes
have been used to measure the rates of bromine addition to
(3).
a series of l.l-diphenylethylenes
(4). and the
results
are
presented here. The rate aonstants, listed in Table I, were measured in methanolic
0.2 W sodium bromide solutions at 25'C. These observed
rate constants are therefore composite and reflect the rate of bromination
by both molecular bromine and tribromide ion. While
the respective rate constants for these two processes
nT.0.
Post-Doctoral
Fellow
1965-66
3101
can be
3102
No.32
evaluated by measuring ion conoentration3. successfully reactivity
the rate of addition at varying bromide
the composite rate constants, kg have been used
for studying the effect of molecular
structure on
(5) anti are so used here. TABLE I
Bromination iia:e Constants kg measured at 2555.02oC -e-e--
____
-__._-____
-----_^-___-
l,l-diphenylethylI~ne --_--___l-_I-P
-_--_-
kg laol-lsec-l n % mean deviation --~_-_______-_-____~~-~~~--~_~~__
4.4'-dimethoxy
3.47 x 106
3
1.8
4-met.hoxy
3.77 x 105
4
4.4
4.4'-dimethyl
3.84 x lo4
3
1.7
4-methyl
1.57 x 103
5
6.4
4-fluoro
1.33 x lo2
5
6.6
Y,4'-difluoro
1.23
4
2.2
4-ohloro
4.76 x lo2
6
7.7
C-bromo
3.62 x lo2
5
8.8
4,4'-dichloro
1.28 x lo1
5
6.6
4,4'-dibromo
1.03 x lo1
3
lc.2
x
10~
n is the number of kinetic experiments. --._---a_--_.-_-____-l_________-__________l__l-_ In considering
the effect of structure on reactivity it is
customary to plot the logarithm of the measured rate constants versus the appropiate Hammett substituent
constants a(6).
Applying the Hammott treatment to the present case, log kg versus produced a curve. the p-methoxy
and the p-methyl derivatives
being more reactive than their Hammett dconstants
would imply,
In such cases a linear plot is usually obtained by the use of Brown G+(7)
values, but for these l,l-diphenylethylenes
the
No.32
3103
oonatanta ourred
for
overcorrected
roaonanoe
interactiona
and the
plot
downwards.
Recently Yukawa and Tsuno 18) have demonstrated involving resonance
stabilization
the relative importance
that reactions
of a positive charge in which
of the resonance interaction nechanisa
differs from that in the two relationships oan be handled by a linear combination
defining 6 and 6:
of these two models.
Their
modified lianmett equation takes the form log k = p(6+rAbR+)+log
k,
. .. .... ... .. ....
where r is a nen reaction constant indicative resonanoe in the transition The
Ab:values
zero,
and
Adi
of the degree of
corresponds
for well behaved meta substituents
to (6+-a).
are essentially
these are normally used to evaluate rho. The remaining
parameter. (l/plog
state and
I
r, oan then be obtained by plotting the function
k/k, -6)
versus Aa+ R. In the present series, however,
no neta substituted
derivatives were available at the time and so
equation (I) was rewritten in the forn: log k =
ab+bA<+log
where a and b correspond
k,
. . . .. . . .. . . . . . . . .
II
to
and r respectively. With the aid of p P a computer and using the experimentally determined values for kg. equation
(II) was solved for a and b on
The Yukawa-Tsuno-Hammett l.l-diphenylethylenes
equation (I) for the broaination
(9).
of
was then calculated to be
log kg = -3.61
(6+0.415A++5.11
The good fit of the experimental is shown by the correlation deviation
a least squares basis
. . .. .. . . . . . . .
points to this linear equation
coefficient
(rx0.991)
and standard
(~~0.165).
The rho value is somewhat lower than that obtained for the
III
No.32
brcmlnaticn
of st.yrener measured under
and implies less charge develcpement the brcminaticn
the same conditions
(3)
in the transition state for
of these 1.Ldiphenylethylenes.
Of particular
interest is the low value of 0.415 for r. This lcu value indicate9 reduced resonance interaoticn that in styrene krcninoticn
in the transition state relative to
(10).
The mcleoule cf l,l-diphenylethylene rings simultaneously
cannot have the two phony1
coplanar with the olefinio bond. Scale models
show that the st*ria interference
of the ortho hydrogen9 may be
rslievod eitoer ty equal twisting of both phenyl rings through an angle of apprcxiaately
30c or by rotation of one phenyl ring through
about 60' leaving the other phenyl coplanar with the clefinio moiety a9 in styrene. On the basis of dipole moment data, Sutton (11) assigned an equal twisting of some 30° to both phenyl rings. Thecretical crlculatians hydrogens)
involving eteric repulsion enorgy (crthc-
and lass of delocalizaticn
energy (through phenyl rctat-
ion) terms indicated a similar conformation for the molecule
to be the most stable
(12).
It is generally accepted that effective resonance interaction between a pnra substituted
phenyl ring and an adjacent oarbcnium
centre requires coplanarity of the aromatic ll-orbital9
and the
empty p-orbital of the carbcnium ion. The reduction in resonance interaction
due to twisting through an angle e is to a first
approximation
equal to Cc9 2 e (13). In the transition state of a
1,Ldiphenylethplene through approximately
reaction, an equal twisting of both ring9 30c uould result in a twenty-five percent drop
in resonance for each ring. On the other hand a greater rotation of
3105
No.32 ,ons phenyl ring through 60~.
leaving the other phony1 coplanar,
would produce (for the rotated phenyl) a seventy-five in resonance
interaction
with the carbonium centre. While the latter
case is perhaps favoured by the low value obtained for parameter
r,
peroent drop
the resonance
a transition state having equally twisted phenyl rings
cannot be excluded. Assuming ad hoc that bromination
involves an asymmetrical
ion state, the following facts might be considered p.p'-disubstituted para-substituent Brown 6+constant
l,l-diphenylethylenes
transit-
to apply. In the
the interaction
of the
in the coplanar phenyl will be measured by its and that of the para-substituent
in the rotated
phony1 by its Hammett 6 oonstant since in the latter case little resonance will be possible. SuBstituted
In the monosubstituted
compounds the
phenyl ring will be the coplanar one. Brown d+constants
are always the more negative electrophilic
(or less positive) and since this is an
reaction the above conformation
would minimieo
energy of the transition state. Under these circumstances
the
the rate
data should follow a modified Hammett equation of the form: log k =/0(a+CS+)+log
k,
. . . . . .. . .. . . . . . .. . IV
This in fact was shown to be the case and a least squares calculation revealed an excellent fit (r p 0.995, s m 0.047). the slope of the line,
rho, being -3.29. These results for l,l-diphenylethylsne
seem to indicate that
while equal partial twisting of the phenyl rings is the least energetic conformation
in the ground state, the preferred transition
state
for bromination
is that in which one phenyl is rotated through a
larger angle leaving the other coplanar vith the ethylenic moiety. An extension of these considerations
to the reactions of other
3106
No.32
l.l-diphanyl
,p2 oarbon systems is intended in a more
dotailed
prpar.
REFERPUCBS -1)
J.E. Dubois, P. Alcois and 0. Barbier. J.Eleotroanal.Chor. 0, 359 (19641.
2)
J.E. Duboiz and 0. Mouvier. C.R. Acad.Sci. u, G. Mouvier. p.i38 SC. Thezia, Poriz (1964). -_
3)
J.E. Dubois and A. Schwartz,
4)
W4
are
grateful
of Joruzalom,
1104 (1962).
Totrahodron Lettorr. 2,
2167 (1964).
to Professor E.D. Borgmann. The Hobrow University for kindly donating those oonpoundz.
5)
J.E. Dubois and G. Kouvior,
6)
L.P. R~nmrtt., Phgrloal Organio Chomistr Hill Book Co., Inc., Now York cl9 0 -?i7;s'
pp 184-199. McGraw-
7)
B.C. Brown omd Y. Okasoto, J.An.Chon&
80, 4979 (1958).
8)
Y. Yukawa and Y. Tzuno. Bull.Cham.Soc.
9)
C.A.
Tetrahedron Lettorz. 20, 1325 (1963).
JaDan, 2,
971 (1959).
Bonnott and I.L. Franklin, Statistical Analysiz in Chemistry an tho Chenioal Indurtrx, p. 245. John Wiley & Sons, -_ Inc., Now Yo$i;-?mrl.
10)
Unpublished
clata, J.E. Dubois and H. Roparr.
11)
G.E. Coatos r.nd L.E. Sutton, J.Chom.Soo.,
12)
M. Simonotta and 5. Car&.
13)
B.l4. Wepster, Progross In Stereoohemistr~,
567 (1942).
Tetrahedron, 19, (Suppl.2) 467 (1963). 2, 99 (1958).