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Steric course, kinetics and mechanism of the 2+2 cycloadditions of alkylphenylketenes to ethyl cis - and trans -propenyl ether
TotrahodronLetteraNo. 34, pp 2y6q - 2972, 1975.
STERlC
COURSE,
KINETICS
AND
ALKYLFHENYLKETENES
MECHANISM
TO
ETHYL
klf lnstitut
Rtr Orgrmis&e
Chemie
(Bcceiredin UK 23 June The reaction 40’
quantitatively
tional
reaction
(Table
by nmr I).
The double
In the preceding the transition
is placed
sure while
it is phenyl
between
3
ond hydrogen.
bulk of R the formation less stable
Karlstr.23,
ethyl
propenyl
of ketenophile
Germany 1975)
ethers in beruonitrile structure.
at
The configum-
1 - $_of the Jr2a
complexes
+n2;l
6’ bonds.
that the orientation complexes
complex
is a good model for
of type -3 and 4- in which
In 1 and 2 the ketene
equal
via 1 and 2 to a nearly __ for the sterically
and other cis disubstituted
On the other hand, duct via the more favored -
5
olkyl
is in the zone
the ketene
of steric
pres-
an increase
: 4’
This is attributed
ethylphenyl-
is analogous
ether
is favored
to the increase
and pmpylphenylketene
andi-butylphenylketene
to the tmns enol ether (2 for methyl
of the enol ether
of the ratio _-3’
cis-pmpenyl -
of van use the
react only via 2. --
to the behavior
over
of cyclopentadiene
The 4
mixed ketenes.
of --3 : 4 rests on the assumption
If methyl and hydrogen
from ethyl
preferred.
isopropylphenyl-
towards
complex
2’
Whereas
cyclobutanone
the cycloaddition orientation
is always
to t-butyl.
extent,
more crowded
ethylenes
of cyclobutanone
product
pathways
armngement,
ETHER
in 2 and 4. -
increasing
ful interpretation
2,
of the orientation
we have demonstmted
R from methyl
dmgen.
isomeric
the incipient
der Waals stmin in 1 on varying
predilection
8 Monchen
the structure
OF
Mayr
with retention -1’ - 4’ -
to deduce
ethoxy
CYCLOADDITIONS
TRANS-PROPENYL
with the cis-trans
communication
2+2
accepted for publication10 July
1975;
arrows indicate
the thermodynamically
x,2.,
G-AND
state and that trans enol ethers choose orientation
substituent
With
allows
THE
Huisgen * and Herbert
produces the cyclobutanones
elucidation
OF
der Universitlft,
of alkylphenylketenes
2
PergoaonPrcoo. Printed in Grvot Britain.
to pmpyl,
that the ketene were to interact
should accompany 2969
furnishes predominantly ,4 for,isopropyI
substituent
is located
with the ketens the bmnching
lf
the more stable
and t_-butyl). between
substituent R in contrast
pro-
A meoning-
ethoxy
and hy-
in the orthogonal to observation.
1970
Table
We.
I.
Orientation
Activation
Complexes,
Free Energies
in Bennronitrile
Pmduct Distribution,
for the C)&additions
Rsrtial
Rate Constants
of Alkylphenylketenes
to Ethyl
I*
(18k2, e-
mol-‘set-‘)
34
and
and
at 40”
P c
czv,
M-5
Y
,Aj= p\
’
‘gH5
CzY\
,H
,C
c=lj=c c/ H’
v’s R
CiS
R :f
3
’
II
0
CH3
0
trons
2
G 1
1
CrH50
CgH5
0
c6”5,8,h
RW
N
o
2
CH3 2’
7. Yldd
I?
kl
T
4
t
01. yuztd
k,
AC+
‘I. yw Ld
0
-
-
16
-
10
0.40
27.54
22
164
67
2.4
26.42
13
0.35
27 62
27
176
16
0.022 29.34
65
0.13
26.24
200
0.028
29.19
183
23 94
100
46
65
24.37
90
CH&HzCHJ
51
65
49
62
24.40
-
-
100
26
24 94 25.95
0
CHKIi3)Z CKH,),
-
-
The combination photometrical composition
addition (Table
-
of kinetic
data with
rate constants
I),
i.e 2
kcis
=
Pmbably, energy
:
two electronic the electron
bmnching
k, + k2,
in the steric
factors coopemte
release by R (Taft’s
results
=
kg+
100
allows
interaction
as well
a quantitative
ether are partitioned
evalution.
according
The
to product
k4.
bulk of R cannot have steric
reasons because R is pointing
of ketene and ketenophile
in reducing d’)
-
pmpenyl
and and ktmns
growing
-
the cycloadditions as the out-of-plane
substituents
rate by lifting twisting
out-
in 2 and 4. -
the ketene LUMO
of phenyl
increase with
of R.
On varying 4 kcal/mol
the stereochemical
for the e-
The decrease of k2 and k4 with wards and is hence not involved
5.1
100
i;-
24.41
130
24.26 24.37
k,
AG:
26.19
12
77
-26.60
k4
3.5
22.66
64
88
.I. ylald
4
61
985
CHzCHJ
CH3
k3
and
nal 3 kcal/mol
the alkylphenylketene
AG2* in
&B,*
from methyl
only by 1 .O kcal,&rol. reflect
With
the greater steric
to isopropyl
one observes thot
the same electronic stmin
in the tmnsition
deactivation
dG,*
increase
by
in both series the additio-
state on replacing
methyl
by isopropyl,
2971
No. 34 Analogously,
.AGrincreases
mom steeply than
In the complexes land
AG,?
3 the ketone olkyl is oriented toward the interior,
mpulsion. The increase of k/k3
the mgion of van der Wools
from 16 for R = methyl to 27 for propyl mirrors the additional
tion in overcoming the interfemnce
energy consump-
of R with ethoxy and hydrogen in 2 compared with thot of R between two
hydrogens in 1.
On the other hand, the cis-tmns rote ratio k2+/k4 stays in the mnge of 180 ; i.e., md to conquer the additional two hydtogens in i.
3.2 kcal/mol
om mqui-
steric mpulsion of phony1 between ethoxy and hydrogen in _4 vs. that of phony1 and
The mtio kfi4
does no longer depend an the size of R which juts outwards and, themfom,
plays no active role in the steric intemction of the substituents in the tmnsition state. position to ethoxy in 1’ and at the tmns location in c,
the constancy of k2/k4
As Rends up at the cis
is only mconciloble
with on
early transition state.
Ethyl e-
ond E-pmpenyl
ether having nearly equal free energies,’
the mte ratios k,/k3
and k/k4
mveol unique steric effects in the tmnsition states. Though the singular pmfemnce for cis 1,2-disubstituted ethylenes or ketenophiles is a significant mechanistic criterion, ‘lT2a +
82s
3
it is not an unequivocal proof for the
process. The findings om also consistent with a concerted pathway [Gs
+n2s +rr2s] 7 for
which a diagonal attack of the ketenophile fl systemon one orbital each of the CC and the CO double bond of the ketone is postulated. While the orientation complexes am still diffemnt, tually indistinguishable as the mhybridizotion
Appamntly,
the two schemes am becoming vir-
proceeds.
the cis pmfemnce does not occur in 2+2 cycloadditions via zwitterionic -
the cyclobutane formation from tetmcyanoethylene to mtios of TCNE with e-
and tmns-l-alkenyl
intermediates ;
and enol ethers may be mgorded as a prototype.
olkyl ethers om close to unity.
9
8
The m-
No. 34
2972
REFERENCES
1)
The new cyclobutanones gave satisfactory CH onalyser and were chamcterired
2)
H. Mayrond
3)
R. Huisgen and H. Mayr,
4)
P.R. Brook, J.M.
Harrison, and A.J.
A. Dieffenbacher,
and A. S. Dreiding,
R. Huisgen, Tetmhedron Lett.,
J.Amer.Chem.Soc., 5)
T. Do-Minh
Tetmhedron Lett.,
6)
T. Okuyama,
7)
R.B. Woodward,
8)
Brady, F.H.
Duke,
s,
J.C.S.
Chem.Comm.,
Helv.Chim.Acta,
2,
Bmdy and E.T. Sot.,
Parry, ond J.D.
g,
417 (1970) ; W.T. Hoff,
J.Org.Chem.,
Rey, S. Roberts,
Brady and R. Roe, g,
3733 (1970).
Brady and R. b,
Stockton, J.Org.Chem.,
5,
36, 1486 (1971) ; R.C. =
2908 (1972).
private communication to R.H.,
1968;
5409 (1969). see R. Huisgen, L. A. Feiler,
and G. Binsch,
102, 3460 (1969).
95, 5056 (1973);
J.Amer.Chem.Soc.,
95, 5054, 5055 (1973) ; G. Steiner and R. Huisgen, =
R. Huisgen, R. Schug, and G. Steiner,
81 (1974). 9)
589 (1970) ; M.
1766 (1970) ; W.T.
T. Fueno, and J. Furukowo, Tetmhedron,g,
R. Huisgen and G. Steiner, ibid., -=
preceding communication.
and 0. P. Stmusz, J.Amer.Chem.
De Selms and F. Delay, ibid.,
Chem.Ber.,
1349 (1975).
92, 4618 (1970) ; W.T.
93, 1662 (1971) ; W.T. -
by their spectra.
R. Huisgen and G. Steiner, Tetmhedron Lett.,
3763 (1973).
Angew.Chem.,
Int.Ed.Engl.,
13, 80, 4
STERlC
COURSE,
KINETICS
AND
ALKYLFHENYLKETENES
MECHANISM
TO
ETHYL
klf lnstitut
Rtr Orgrmis&e
Chemie
(Bcceiredin UK 23 June The reaction 40’
quantitatively
tional
reaction
(Table
by nmr I).
The double
In the preceding the transition
is placed
sure while
it is phenyl
between
3
ond hydrogen.
bulk of R the formation less stable
Karlstr.23,
ethyl
propenyl
of ketenophile
Germany 1975)
ethers in beruonitrile structure.
at
The configum-
1 - $_of the Jr2a
complexes
+n2;l
6’ bonds.
that the orientation complexes
complex
is a good model for
of type -3 and 4- in which
In 1 and 2 the ketene
equal
via 1 and 2 to a nearly __ for the sterically
and other cis disubstituted
On the other hand, duct via the more favored -
5
olkyl
is in the zone
the ketene
of steric
pres-
an increase
: 4’
This is attributed
ethylphenyl-
is analogous
ether
is favored
to the increase
and pmpylphenylketene
andi-butylphenylketene
to the tmns enol ether (2 for methyl
of the enol ether
of the ratio _-3’
cis-pmpenyl -
of van use the
react only via 2. --
to the behavior
over
of cyclopentadiene
The 4
mixed ketenes.
of --3 : 4 rests on the assumption
If methyl and hydrogen
from ethyl
preferred.
isopropylphenyl-
towards
complex
2’
Whereas
cyclobutanone
the cycloaddition orientation
is always
to t-butyl.
extent,
more crowded
ethylenes
of cyclobutanone
product
pathways
armngement,
ETHER
in 2 and 4. -
increasing
ful interpretation
2,
of the orientation
we have demonstmted
R from methyl
dmgen.
isomeric
the incipient
der Waals stmin in 1 on varying
predilection
8 Monchen
the structure
OF
Mayr
with retention -1’ - 4’ -
to deduce
ethoxy
CYCLOADDITIONS
TRANS-PROPENYL
with the cis-trans
communication
2+2
accepted for publication10 July
1975;
arrows indicate
the thermodynamically
x,2.,
G-AND
state and that trans enol ethers choose orientation
substituent
With
allows
THE
Huisgen * and Herbert
produces the cyclobutanones
elucidation
OF
der Universitlft,
of alkylphenylketenes
2
PergoaonPrcoo. Printed in Grvot Britain.
to pmpyl,
that the ketene were to interact
should accompany 2969
furnishes predominantly ,4 for,isopropyI
substituent
is located
with the ketens the bmnching
lf
the more stable
and t_-butyl). between
substituent R in contrast
pro-
A meoning-
ethoxy
and hy-
in the orthogonal to observation.
1970
Table
We.
I.
Orientation
Activation
Complexes,
Free Energies
in Bennronitrile
Pmduct Distribution,
for the C)&additions
Rsrtial
Rate Constants
of Alkylphenylketenes
to Ethyl
I*
(18k2, e-
mol-‘set-‘)
34
and
and
at 40”
P c
czv,
M-5
Y
,Aj= p\
’
‘gH5
CzY\
,H
,C
c=lj=c c/ H’
v’s R
CiS
R :f
3
’
II
0
CH3
0
trons
2
G 1
1
CrH50
CgH5
0
c6”5,8,h
RW
N
o
2
CH3 2’
7. Yldd
I?
kl
T
4
t
01. yuztd
k,
AC+
‘I. yw Ld
0
-
-
16
-
10
0.40
27.54
22
164
67
2.4
26.42
13
0.35
27 62
27
176
16
0.022 29.34
65
0.13
26.24
200
0.028
29.19
183
23 94
100
46
65
24.37
90
CH&HzCHJ
51
65
49
62
24.40
-
-
100
26
24 94 25.95
0
CHKIi3)Z CKH,),
-
-
The combination photometrical composition
addition (Table
-
of kinetic
data with
rate constants
I),
i.e 2
kcis
=
Pmbably, energy
:
two electronic the electron
bmnching
k, + k2,
in the steric
factors coopemte
release by R (Taft’s
results
=
kg+
100
allows
interaction
as well
a quantitative
ether are partitioned
evalution.
according
The
to product
k4.
bulk of R cannot have steric
reasons because R is pointing
of ketene and ketenophile
in reducing d’)
-
pmpenyl
and and ktmns
growing
-
the cycloadditions as the out-of-plane
substituents
rate by lifting twisting
out-
in 2 and 4. -
the ketene LUMO
of phenyl
increase with
of R.
On varying 4 kcal/mol
the stereochemical
for the e-
The decrease of k2 and k4 with wards and is hence not involved
5.1
100
i;-
24.41
130
24.26 24.37
k,
AG:
26.19
12
77
-26.60
k4
3.5
22.66
64
88
.I. ylald
4
61
985
CHzCHJ
CH3
k3
and
nal 3 kcal/mol
the alkylphenylketene
AG2* in
&B,*
from methyl
only by 1 .O kcal,&rol. reflect
With
the greater steric
to isopropyl
one observes thot
the same electronic stmin
in the tmnsition
deactivation
dG,*
increase
by
in both series the additio-
state on replacing
methyl
by isopropyl,
2971
No. 34 Analogously,
.AGrincreases
mom steeply than
In the complexes land
AG,?
3 the ketone olkyl is oriented toward the interior,
mpulsion. The increase of k/k3
the mgion of van der Wools
from 16 for R = methyl to 27 for propyl mirrors the additional
tion in overcoming the interfemnce
energy consump-
of R with ethoxy and hydrogen in 2 compared with thot of R between two
hydrogens in 1.
On the other hand, the cis-tmns rote ratio k2+/k4 stays in the mnge of 180 ; i.e., md to conquer the additional two hydtogens in i.
3.2 kcal/mol
om mqui-
steric mpulsion of phony1 between ethoxy and hydrogen in _4 vs. that of phony1 and
The mtio kfi4
does no longer depend an the size of R which juts outwards and, themfom,
plays no active role in the steric intemction of the substituents in the tmnsition state. position to ethoxy in 1’ and at the tmns location in c,
the constancy of k2/k4
As Rends up at the cis
is only mconciloble
with on
early transition state.
Ethyl e-
ond E-pmpenyl
ether having nearly equal free energies,’
the mte ratios k,/k3
and k/k4
mveol unique steric effects in the tmnsition states. Though the singular pmfemnce for cis 1,2-disubstituted ethylenes or ketenophiles is a significant mechanistic criterion, ‘lT2a +
82s
3
it is not an unequivocal proof for the
process. The findings om also consistent with a concerted pathway [Gs
+n2s +rr2s] 7 for
which a diagonal attack of the ketenophile fl systemon one orbital each of the CC and the CO double bond of the ketone is postulated. While the orientation complexes am still diffemnt, tually indistinguishable as the mhybridizotion
Appamntly,
the two schemes am becoming vir-
proceeds.
the cis pmfemnce does not occur in 2+2 cycloadditions via zwitterionic -
the cyclobutane formation from tetmcyanoethylene to mtios of TCNE with e-
and tmns-l-alkenyl
intermediates ;
and enol ethers may be mgorded as a prototype.
olkyl ethers om close to unity.
9
8
The m-
No. 34
2972
REFERENCES
1)
The new cyclobutanones gave satisfactory CH onalyser and were chamcterired
2)
H. Mayrond
3)
R. Huisgen and H. Mayr,
4)
P.R. Brook, J.M.
Harrison, and A.J.
A. Dieffenbacher,
and A. S. Dreiding,
R. Huisgen, Tetmhedron Lett.,
J.Amer.Chem.Soc., 5)
T. Do-Minh
Tetmhedron Lett.,
6)
T. Okuyama,
7)
R.B. Woodward,
8)
Brady, F.H.
Duke,
s,
J.C.S.
Chem.Comm.,
Helv.Chim.Acta,
2,
Bmdy and E.T. Sot.,
Parry, ond J.D.
g,
417 (1970) ; W.T. Hoff,
J.Org.Chem.,
Rey, S. Roberts,
Brady and R. Roe, g,
3733 (1970).
Brady and R. b,
Stockton, J.Org.Chem.,
5,
36, 1486 (1971) ; R.C. =
2908 (1972).
private communication to R.H.,
1968;
5409 (1969). see R. Huisgen, L. A. Feiler,
and G. Binsch,
102, 3460 (1969).
95, 5056 (1973);
J.Amer.Chem.Soc.,
95, 5054, 5055 (1973) ; G. Steiner and R. Huisgen, =
R. Huisgen, R. Schug, and G. Steiner,
81 (1974). 9)
589 (1970) ; M.
1766 (1970) ; W.T.
T. Fueno, and J. Furukowo, Tetmhedron,g,
R. Huisgen and G. Steiner, ibid., -=
preceding communication.
and 0. P. Stmusz, J.Amer.Chem.
De Selms and F. Delay, ibid.,
Chem.Ber.,
1349 (1975).
92, 4618 (1970) ; W.T.
93, 1662 (1971) ; W.T. -
by their spectra.
R. Huisgen and G. Steiner, Tetmhedron Lett.,
3763 (1973).
Angew.Chem.,
Int.Ed.Engl.,
13, 80, 4