- Email: [email protected]
On the role of the CF3 group in determining the relative stability of E, Z-isomers
Tetrahedron Letters,Vol.27,No.l7,pp Printed in Great Britain
1883-1886,1986
0040-4039/86 $3.00 + .OO 01986 Pergamon Press Ltd.
ON THE ROLE OF THE CF3 GROUP IN DETERMINING THE RELATIVL STABILITY OF E, Z-ISOMERS
Carl L. Bumgardner*, John E. Bunch and M.-H. Whangbo* Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695
Abstract: Although base-catalyzed addition of PhSH to PhCECCF3 under thermodynamic is the thermodynamicallyfavored control gives E_Ph(PhS)C=CH(CF3),.?+Ph(PhO)C=CH(CF3) The preference for the +-configuration product from similar reaction of PhOH with PhCXCF3. of CF3 and PhO on a double bond is rationalized in terms of frontier orbital interactions. While examining the nucleophilic addition of PhSH and PhOH to trifluoromethyl acetylenes, L-CSC-CF3 (L=Ph, PhCO, etc.), we observed a remarkable effect of the CF group on the 3 stereochemistry of addition. As shown in Scheme I, addition of PhSH to PhCXCF in ethanol 4 containing a catalytic amount of t-BuO- gives 1-Z as the sole kinetic product, but 1-E 2 as the major thermodynamic product. In contrast, addition of PhOH to PhCZCF3 under similar reaction condition yields 2-2 under both kinetically and thermodynamicallycontrolled reaction conditions.3 It -is noted that in 2-Z the CF3 and PhO groups are e, but in 1-E the CF3 and PhS groups have the trans-arrangement. A similar pattern emerged when PhSH and PhOH were allowed to react with PhCOCXCF3 under kinetically and thermodynamically controlled 4 conditions. The -cis-preference of CF3 and PhO groups on a double bond can be rationalized in terms of the stabilizing n-type orbital interactions5 between the CF3 and vinyl ether 6 groups as depicted in 2. In _3b the vinyl ether moiety is represented by the highest
d
-sib ‘-
7r*
8
F
L- 8 36
3a 1883
cF3
1884
Scheme
I
PhCXCF3
kinetic
Ph,
krmodynamic
/
.”
,c=c,
PhS
+ PhSH
.CFs Ph, phS,C=C 'H
equilibration
CF3
PhCXCF3
+ PhOH
kinetic/
c=c
\rmodynamic
/H
equilibration
-2-Z t other
c
'CF3
PhO'
regio- and stereoisomers
l_E
-1-z
Ph,
t other
regio- and stereoisomers
-2-z
occupied action
orbital
IT', and the CF3 group
is expected
and PhO because However,
to be more
of the secondary
the thermodynamic
a trans-arrangement
will
group.
have small
of CF3 and PhS.
Therefore contribution
which
interaction
weak
tendency
the interaction
occupied
from the sulfur
indicated
7
The
(IT'-rr*CF) inter-
by a double-headed
from the second-row
This contrasting makes
.
for the -cis- than the taans-arrangement
1-E derived
the highest
n*CF
orbital
3p orbital,
nucleophile
may be ascribed of the sulfur
IT' of the vinyl
Jf CF3 arrow.
PhS- prefers essentially
3p orbital
thioether
so that the secondary
with sulfur is weak and does not override of (77'-7r* ) associated CF for the trans-irrangement of bulky groups on a double bond.
interaction tendency
orbital
product
to the long C-S bond length, the vinyl
stabilizing
by its empty orbital
with
moiety
orbital the usual
1885
References
1.
A mixture
containing
PhCXCF3
15 ml of absolute
ethanol
poured
and extracted
into water
aqueous
NaOH, water
residue
which
Raising
6 6.0
in formation
over silica 7.0 - 7.9
gel.
(m, Ar-H)
(d, J=8 Hz, gem of t-BuOK of l_E, 3.
5.2
of PhCXCF3
and PhOH,
H trans
mixture
to PhO);
of PhOH
of isomers,
constituent
"F
NMR
to PhCSCF3 which
according
2-2, was obtained
left a
1_z, LH NMR (CDC13) 19 F NMR(CFC13) 59 ppm
showed
were
"F
with
separated
NMR
for 24 hrs.
by chromatography
'H NMR(CDC13)
(CFC13)
55 ppm
PhSH and a catalytic
the product
mixture
amount
to consist
of 64%
(97% yield).
6 6.8 - 7.5 (m, Ar-H),
(CFC13) 59 ppm (d, J=9Hz; e
vinyl
out at 150' for 24 hrs.
to GC analysis
dilute
and solvent
to GC was l_E;
H -cis to PhS),
two isomer
was separated
(98% yield)
agent
was then
with
in ref. 1 to 150'
which
according
'H NMR(CDC13)
carried
The mixture
was washed
to that in ref. 1, led to isolation
similar
at 25':
mixture
_1-Z was heated
GC analysis
for 24 hrs,
2-Z as the sole product vinyl
When
to PhS);
isomers,
(q, J=8 Hz, vinyl
CF3, H).
Compound
gel.
N2.
in
in 92% yield.
isomer
23% of -1-Z and 13% of the other
Treatment
addition
product
and 50 mg of t-BuOK
solution
Gf drying
H trans
of the starting
of all four addition
vinyl
to 150'
over silica
(q, 9 Hz, vinyl
The major
The ether Removal
over MgS04.
temperature
(5.00 mmoles)
at 25 for 6 hrs under
ether.
CF3, H) was obtained
vinyl
the reaction
resulted
with
was chromatographed
(d, J=9Hz, e 2.
was maintained
and dried
7.1 - 7.9 (m, Ar-H)
PhSH
(5.00 mmoles),
by chromatography
over
t-BuOK
(94%) of (q, J=9Hz, The
CF3, H).
(see ref. 2), led to a silica
The same product
was -2-Z (89%).
when -2-Z was heated with
5.8
at 150"
gel.
The main
mixture,
89%
for 24 hrs.,
see ref. 2. 4.
Addition
of PhSH
in refs.
1 and 2.
and of PhOH
to PhCOCXCF3
The details,
was ccrried
including
structural
out along
assignments,
the lines
described
will be given
in the
full paper. 5.
(a)
Epiotis, 95,
(b)
Bemardi, z,
N.D.;
Bjorkist,
D.; Bjorkist,
L.; Sarkanen,
S., J. Am. Chem. SOC.,
1973,
7558. F.; Epiotis,
N.D.; Yates
R.L.;
Schlegel,
H.B.,
3. Am. Chem.
Sot., 1976,
2385.
(c)
Schleyer,
P.V.R.;
Dill,
(d)
Whangbo,M.-H.;
Wolfe,
(e)
Forsyth,
Osterman,
J.D.;
Pople,
J.A.; Hehre,
W.J.;
Tetrahedron
, 1977, 22
2497.
D.A.;
S., Irs. J. Chem., V.M.; DeMember,
1980, 3,
36.
J.R., J. Am. Chem.
Sot.,
1985, 107,
818.
1886
6.
7.
-x For simplicity of drawing, the r*CF orbital is represented by n CH . Due to the electronegative nature of fluroine,3" n is expected to be lower ly?ng in energy than * CF3 'pi CH so that the (n'-n*CF) interaction 1s expected to be stronger than (n'-n*CH 1.7 3 3 3 Albright, T.A.; Burdett, J.K.; W'hangbo,M.-H., *"Orbital Interactions in Chemistry", 1985, John Wiley & Sons, N.Y., p. 82, 178. (Received
in USA 30 December
1985)
1883-1886,1986
0040-4039/86 $3.00 + .OO 01986 Pergamon Press Ltd.
ON THE ROLE OF THE CF3 GROUP IN DETERMINING THE RELATIVL STABILITY OF E, Z-ISOMERS
Carl L. Bumgardner*, John E. Bunch and M.-H. Whangbo* Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695
Abstract: Although base-catalyzed addition of PhSH to PhCECCF3 under thermodynamic is the thermodynamicallyfavored control gives E_Ph(PhS)C=CH(CF3),.?+Ph(PhO)C=CH(CF3) The preference for the +-configuration product from similar reaction of PhOH with PhCXCF3. of CF3 and PhO on a double bond is rationalized in terms of frontier orbital interactions. While examining the nucleophilic addition of PhSH and PhOH to trifluoromethyl acetylenes, L-CSC-CF3 (L=Ph, PhCO, etc.), we observed a remarkable effect of the CF group on the 3 stereochemistry of addition. As shown in Scheme I, addition of PhSH to PhCXCF in ethanol 4 containing a catalytic amount of t-BuO- gives 1-Z as the sole kinetic product, but 1-E 2 as the major thermodynamic product. In contrast, addition of PhOH to PhCZCF3 under similar reaction condition yields 2-2 under both kinetically and thermodynamicallycontrolled reaction conditions.3 It -is noted that in 2-Z the CF3 and PhO groups are e, but in 1-E the CF3 and PhS groups have the trans-arrangement. A similar pattern emerged when PhSH and PhOH were allowed to react with PhCOCXCF3 under kinetically and thermodynamically controlled 4 conditions. The -cis-preference of CF3 and PhO groups on a double bond can be rationalized in terms of the stabilizing n-type orbital interactions5 between the CF3 and vinyl ether 6 groups as depicted in 2. In _3b the vinyl ether moiety is represented by the highest
d
-sib ‘-
7r*
8
F
L- 8 36
3a 1883
cF3
1884
Scheme
I
PhCXCF3
kinetic
Ph,
krmodynamic
/
.”
,c=c,
PhS
+ PhSH
.CFs Ph, phS,C=C 'H
equilibration
CF3
PhCXCF3
+ PhOH
kinetic/
c=c
\rmodynamic
/H
equilibration
-2-Z t other
c
'CF3
PhO'
regio- and stereoisomers
l_E
-1-z
Ph,
t other
regio- and stereoisomers
-2-z
occupied action
orbital
IT', and the CF3 group
is expected
and PhO because However,
to be more
of the secondary
the thermodynamic
a trans-arrangement
will
group.
have small
of CF3 and PhS.
Therefore contribution
which
interaction
weak
tendency
the interaction
occupied
from the sulfur
indicated
7
The
(IT'-rr*CF) inter-
by a double-headed
from the second-row
This contrasting makes
.
for the -cis- than the taans-arrangement
1-E derived
the highest
n*CF
orbital
3p orbital,
nucleophile
may be ascribed of the sulfur
IT' of the vinyl
Jf CF3 arrow.
PhS- prefers essentially
3p orbital
thioether
so that the secondary
with sulfur is weak and does not override of (77'-7r* ) associated CF for the trans-irrangement of bulky groups on a double bond.
interaction tendency
orbital
product
to the long C-S bond length, the vinyl
stabilizing
by its empty orbital
with
moiety
orbital the usual
1885
References
1.
A mixture
containing
PhCXCF3
15 ml of absolute
ethanol
poured
and extracted
into water
aqueous
NaOH, water
residue
which
Raising
6 6.0
in formation
over silica 7.0 - 7.9
gel.
(m, Ar-H)
(d, J=8 Hz, gem of t-BuOK of l_E, 3.
5.2
of PhCXCF3
and PhOH,
H trans
mixture
to PhO);
of PhOH
of isomers,
constituent
"F
NMR
to PhCSCF3 which
according
2-2, was obtained
left a
1_z, LH NMR (CDC13) 19 F NMR(CFC13) 59 ppm
showed
were
"F
with
separated
NMR
for 24 hrs.
by chromatography
'H NMR(CDC13)
(CFC13)
55 ppm
PhSH and a catalytic
the product
mixture
amount
to consist
of 64%
(97% yield).
6 6.8 - 7.5 (m, Ar-H),
(CFC13) 59 ppm (d, J=9Hz; e
vinyl
out at 150' for 24 hrs.
to GC analysis
dilute
and solvent
to GC was l_E;
H -cis to PhS),
two isomer
was separated
(98% yield)
agent
was then
with
in ref. 1 to 150'
which
according
'H NMR(CDC13)
carried
The mixture
was washed
to that in ref. 1, led to isolation
similar
at 25':
mixture
_1-Z was heated
GC analysis
for 24 hrs,
2-Z as the sole product vinyl
When
to PhS);
isomers,
(q, J=8 Hz, vinyl
CF3, H).
Compound
gel.
N2.
in
in 92% yield.
isomer
23% of -1-Z and 13% of the other
Treatment
addition
product
and 50 mg of t-BuOK
solution
Gf drying
H trans
of the starting
of all four addition
vinyl
to 150'
over silica
(q, 9 Hz, vinyl
The major
The ether Removal
over MgS04.
temperature
(5.00 mmoles)
at 25 for 6 hrs under
ether.
CF3, H) was obtained
vinyl
the reaction
resulted
with
was chromatographed
(d, J=9Hz, e 2.
was maintained
and dried
7.1 - 7.9 (m, Ar-H)
PhSH
(5.00 mmoles),
by chromatography
over
t-BuOK
(94%) of (q, J=9Hz, The
CF3, H).
(see ref. 2), led to a silica
The same product
was -2-Z (89%).
when -2-Z was heated with
5.8
at 150"
gel.
The main
mixture,
89%
for 24 hrs.,
see ref. 2. 4.
Addition
of PhSH
in refs.
1 and 2.
and of PhOH
to PhCOCXCF3
The details,
was ccrried
including
structural
out along
assignments,
the lines
described
will be given
in the
full paper. 5.
(a)
Epiotis, 95,
(b)
Bemardi, z,
N.D.;
Bjorkist,
D.; Bjorkist,
L.; Sarkanen,
S., J. Am. Chem. SOC.,
1973,
7558. F.; Epiotis,
N.D.; Yates
R.L.;
Schlegel,
H.B.,
3. Am. Chem.
Sot., 1976,
2385.
(c)
Schleyer,
P.V.R.;
Dill,
(d)
Whangbo,M.-H.;
Wolfe,
(e)
Forsyth,
Osterman,
J.D.;
Pople,
J.A.; Hehre,
W.J.;
Tetrahedron
, 1977, 22
2497.
D.A.;
S., Irs. J. Chem., V.M.; DeMember,
1980, 3,
36.
J.R., J. Am. Chem.
Sot.,
1985, 107,
818.
1886
6.
7.
-x For simplicity of drawing, the r*CF orbital is represented by n CH . Due to the electronegative nature of fluroine,3" n is expected to be lower ly?ng in energy than * CF3 'pi CH so that the (n'-n*CF) interaction 1s expected to be stronger than (n'-n*CH 1.7 3 3 3 Albright, T.A.; Burdett, J.K.; W'hangbo,M.-H., *"Orbital Interactions in Chemistry", 1985, John Wiley & Sons, N.Y., p. 82, 178. (Received
in USA 30 December
1985)