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Photochemical reactions of aromatic esters. IV The oxetane formation involving an exciplex
Tetrahedron LettersNo. 30, pp. 2887-2690, 1971. PergamonPress. Printedin GreatBritain.
PHOTOCHmICAL REACTIONS OF AROMATIC ESTERS. IV1 THE OXETANE FORMATION INVOLVING AN EXCIPLM
Yasuo Shigemitsu, Yutaka Katsuhara, and Yoahinobu Odaira Department of Petroleum Chemistry, Faculty of Engineering, Osaka University, Suita-shi, Osaka, Japan (Received in Japan24 June 1971;receivedin UK for publication 29 June 1971) The photocycloaddition of ketones to olefins yielding oxetanes has been studied extensively in the recent years2 . Ketone triplets are generally thought to be reactive species in these reactions, except for a few cases of nucleophilic attacks of n--R*singlet states of alkyl ketones3 .
In our previous studies on the
photochemistry of carboxylate esters, we found that both ethyl cyanoformate4 and diethyl oxalate5 as well as ketones also underwent the Paterno-B&hi reaction with several olefins. Furthermore, we have recently reported that aromatic carboxylate esters,
having more than one electron-withdrawingsubstituent such as a
cyano or an alkoxycarbonyl group, photochemically added to l,l-diphenylethylene to give new oxetanes and that n-z* excited states of the aromatic carboxylate esters were nresumed to be reactive species in these reactions6 .
In the present
work, our attention has been paid to the nature of the reactive species of the above interesting Paterno-B&hi reaction of the aromatic carboxylate esters. For this purpose, we have studied photocycloadditions of dimethyl terephthalate (I) and dimethyl isophthalate (II) with 2-methyl-2-butene (III). Irradiation of an acetonitrile solution of I (0.03 mole) and III (0.1 mole) with a 500-W high pressure mercury arc through a Pyrex filter led to the formation of a mixture of two isomeric oxetanes (IV) in 45 % yield based on the unrecovered ester (isomer ratio, 1:1.2). Similar photocycloaddition of II (0.05 mole) with III (0.2 mole) gave a mixture of two isomeric oxetanes (V) in 70 % yield (isomer ratio, 1:l).
All of the oxetanes obtained were characterized by
2087
BO.JO
cope
Q
+
MeYHe
He
h3
Me
*
Me
c!
Me
He0
Qx
IV: Xrp-CC2Me
I: X=p-CC2Me
III
V: X=m-CC2i4e
II: X=m-C02Me
elemental analysis, i.r., n.m.r., and mass spectra. Phosphorescence spectra of I and II were observed at 77°K in methanol-ethanol glass (1:4) (W
band at 73.1, 77.0 Kcal mole-l, respectively). The long radi-
ative lifetime (l-2 set) and the vibrational spacing verify that the lowest lying triplet states of I and II are ~-a* in nature.
The failure of the triplet
sensitization with xanthone (?+,=74.2Kcal mole'l) for the formation of IV would exclude the lowest triplet state of I a8 one of the reactive states. Consequently, an excited singlet state or a higher triplet one must be considered a8 the candidate for the reactive species of this oxetane formation. Broad fluorescence emission spectra of I and II having maximum at around 380 mu were also measured at room
temperature In several solvents. For the fluo-
re8cence emission of II, a small bathochromic shift was observed on changing the solvent from ether to acetonitrile, whereas no solvent effect on that of I was detected. These facts are indicative that the lowest excited singlet states of I and II are also x-n* in nature. These fluOre8CenCe emi8SiOn8 in aCetOnitrile solution were quenched by the olefin III.
The Stern-Volmer plots of FC/F against
the concentration of III were linear, and hence the following equation is valid for the present systems. FC/F = 1 + kq?:[III]
(1)
where F. and F are the fluorescence intensities in the absence and presence of III, kq is the bimolecular quenching rate constant, andx in the absence of III. for I and 2 M-1 for II.
is the sin&let lifetime -1 The slopes, corresponding to kqX in eq. (11, were 55 M From the following facts, to account for this quenching
we would like to propose an exciplex or an excited charge-transfer complex forma-
No.30
2889
tion
between the electron-deficient
state
of I or II and the olefin
transfer
from I or II to III
evidence
for a charge-transfer
matic ester
and the olefin
of the fluorescence
generally
ascribed
it
tion
may also
Paterno-Bichi singlet
current
interest
-transfer
reduced state
state
Wagner has suggested
reactions
oxetane
formation
to the oxetanes higher
the inclusion
triplet
state
;[lexl
two possible
ester
of the complex VI to the oxetanes.
mechanism of this -transfer
interesting
interaction
Acknowledgement. of Osaka University
The other
which path will
The authors for helpful
Path A involves
possible
reaction
and will
The only present
be reported
in
the
evidence
is
Further work on the
involving
wish to thank Professor
support
of VI
an n-z*
Path B Involves
be more favorable.
Paterno-E&hi
is in progress
step of the Present
which may be produced by the inter-
direct
to decide
O1
paths of the collapse
time.
from the complex VI.
not sufficient
of the triplet
pat;;*
PQ-$)*
system crossing collapse
donor.
of the latter9.
at the present
of the aromatic
charge
has been photo-
of the complex VI in the initial
may be suggested
case of the
an excited
interaction
;zthT
has become apparent,
complex forma-
compound with a x-electron
Oxetane c----------
Although
Therefore,
There hae been
that a-trifluoroacetophenone
kster)
are-
of a a-xlowest
involving
through a charge-transfer
hv K-)&
the first
component with an olefin.
of the former with the benzene ring
Ester
charge-transfer
via an interaction
aromatic
is no
and amines 8 is
complex formation.
This is probably
of an excited
there
Furthermore,
compounds by dienes’
which may proceed
by alkylbenzenes
measurements.
that the excited
of a carbonyl
Second,
energy
between the ground state
charge-transfer
in the photochemical
interaction
For example,
of aromatic
in our system.
reaction
excited
from uv spectra
singlet
singlet-singlet
unfavorable.
complex formation
to conclude
occur
an exothermlc
is energetically
to such excited
seema reasonable
ring Of the a+%* excited
First,
III.
III
quenching
aromatic
an excited
charge
shortly. S. Kate and Dr. T. Obno
the measurements of emission
spectra
No.30
2890
and for valuable euggestione.
References 1) Pert III. Y. Kateuhara, Y. Shigemiteu, and Y. Odaira, Bull. Chem. Sot. Japan, 4&
1169 (1971). Part 11. K. Fukui end Y. Odaira, Tetrahedron Lett., 5255
(1970). 2) Recent reviews: D. R. Arnold, Moan.
Photochem., f& 301 (1968); D. C. Neckers,
"FlechanieticOrganic Photochemistry*, Reinhold Publishing Co., New York, 1967, P 131. 3) J. C. Dalton, P. A. Wriede, and N. J. Turro, J. Am. Chem. Sot., 92, 1318 (1970) and references cited therein. 4) Y. Odaira, T. Shimodaira, and S. Teutsumi, Chem. Comm., 757 (1967). 5) T. Tominaga, Y. Odaira, and S. Teutsumi, Bull. Chem. Sot. Japan, 2,
2451
(19671. 6) Y. Shigemitsu, H. Rakai, and Y. Odaira, Tetrahedron, g,
3039 (1969) end
Part.III. 7) L. M. Stephenson, D. G. Whitten, G. F. soC.,&j,
Vesley, and G. S. Hammond, J. Am. Chem.
3665 (1966).
8) N. Mataga and K. Ezumi, Bull. Chem. Sot. Japan, 2,
1355 (1967); W. R. Ware
and H. P. Richter, J. Chem. Phys., l&, 1595 (1968). 9) P. J. Wagner and R. A. Leavitt, J. Am. Chem. Sot., 92. 6808 (1970).
PHOTOCHmICAL REACTIONS OF AROMATIC ESTERS. IV1 THE OXETANE FORMATION INVOLVING AN EXCIPLM
Yasuo Shigemitsu, Yutaka Katsuhara, and Yoahinobu Odaira Department of Petroleum Chemistry, Faculty of Engineering, Osaka University, Suita-shi, Osaka, Japan (Received in Japan24 June 1971;receivedin UK for publication 29 June 1971) The photocycloaddition of ketones to olefins yielding oxetanes has been studied extensively in the recent years2 . Ketone triplets are generally thought to be reactive species in these reactions, except for a few cases of nucleophilic attacks of n--R*singlet states of alkyl ketones3 .
In our previous studies on the
photochemistry of carboxylate esters, we found that both ethyl cyanoformate4 and diethyl oxalate5 as well as ketones also underwent the Paterno-B&hi reaction with several olefins. Furthermore, we have recently reported that aromatic carboxylate esters,
having more than one electron-withdrawingsubstituent such as a
cyano or an alkoxycarbonyl group, photochemically added to l,l-diphenylethylene to give new oxetanes and that n-z* excited states of the aromatic carboxylate esters were nresumed to be reactive species in these reactions6 .
In the present
work, our attention has been paid to the nature of the reactive species of the above interesting Paterno-B&hi reaction of the aromatic carboxylate esters. For this purpose, we have studied photocycloadditions of dimethyl terephthalate (I) and dimethyl isophthalate (II) with 2-methyl-2-butene (III). Irradiation of an acetonitrile solution of I (0.03 mole) and III (0.1 mole) with a 500-W high pressure mercury arc through a Pyrex filter led to the formation of a mixture of two isomeric oxetanes (IV) in 45 % yield based on the unrecovered ester (isomer ratio, 1:1.2). Similar photocycloaddition of II (0.05 mole) with III (0.2 mole) gave a mixture of two isomeric oxetanes (V) in 70 % yield (isomer ratio, 1:l).
All of the oxetanes obtained were characterized by
2087
BO.JO
cope
Q
+
MeYHe
He
h3
Me
*
Me
c!
Me
He0
Qx
IV: Xrp-CC2Me
I: X=p-CC2Me
III
V: X=m-CC2i4e
II: X=m-C02Me
elemental analysis, i.r., n.m.r., and mass spectra. Phosphorescence spectra of I and II were observed at 77°K in methanol-ethanol glass (1:4) (W
band at 73.1, 77.0 Kcal mole-l, respectively). The long radi-
ative lifetime (l-2 set) and the vibrational spacing verify that the lowest lying triplet states of I and II are ~-a* in nature.
The failure of the triplet
sensitization with xanthone (?+,=74.2Kcal mole'l) for the formation of IV would exclude the lowest triplet state of I a8 one of the reactive states. Consequently, an excited singlet state or a higher triplet one must be considered a8 the candidate for the reactive species of this oxetane formation. Broad fluorescence emission spectra of I and II having maximum at around 380 mu were also measured at room
temperature In several solvents. For the fluo-
re8cence emission of II, a small bathochromic shift was observed on changing the solvent from ether to acetonitrile, whereas no solvent effect on that of I was detected. These facts are indicative that the lowest excited singlet states of I and II are also x-n* in nature. These fluOre8CenCe emi8SiOn8 in aCetOnitrile solution were quenched by the olefin III.
The Stern-Volmer plots of FC/F against
the concentration of III were linear, and hence the following equation is valid for the present systems. FC/F = 1 + kq?:[III]
(1)
where F. and F are the fluorescence intensities in the absence and presence of III, kq is the bimolecular quenching rate constant, andx in the absence of III. for I and 2 M-1 for II.
is the sin&let lifetime -1 The slopes, corresponding to kqX in eq. (11, were 55 M From the following facts, to account for this quenching
we would like to propose an exciplex or an excited charge-transfer complex forma-
No.30
2889
tion
between the electron-deficient
state
of I or II and the olefin
transfer
from I or II to III
evidence
for a charge-transfer
matic ester
and the olefin
of the fluorescence
generally
ascribed
it
tion
may also
Paterno-Bichi singlet
current
interest
-transfer
reduced state
state
Wagner has suggested
reactions
oxetane
formation
to the oxetanes higher
the inclusion
triplet
state
;[lexl
two possible
ester
of the complex VI to the oxetanes.
mechanism of this -transfer
interesting
interaction
Acknowledgement. of Osaka University
The other
which path will
The authors for helpful
Path A involves
possible
reaction
and will
The only present
be reported
in
the
evidence
is
Further work on the
involving
wish to thank Professor
support
of VI
an n-z*
Path B Involves
be more favorable.
Paterno-E&hi
is in progress
step of the Present
which may be produced by the inter-
direct
to decide
O1
paths of the collapse
time.
from the complex VI.
not sufficient
of the triplet
pat;;*
PQ-$)*
system crossing collapse
donor.
of the latter9.
at the present
of the aromatic
charge
has been photo-
of the complex VI in the initial
may be suggested
case of the
an excited
interaction
;zthT
has become apparent,
complex forma-
compound with a x-electron
Oxetane c----------
Although
Therefore,
There hae been
that a-trifluoroacetophenone
kster)
are-
of a a-xlowest
involving
through a charge-transfer
hv K-)&
the first
component with an olefin.
of the former with the benzene ring
Ester
charge-transfer
via an interaction
aromatic
is no
and amines 8 is
complex formation.
This is probably
of an excited
there
Furthermore,
compounds by dienes’
which may proceed
by alkylbenzenes
measurements.
that the excited
of a carbonyl
Second,
energy
between the ground state
charge-transfer
in the photochemical
interaction
For example,
of aromatic
in our system.
reaction
excited
from uv spectra
singlet
singlet-singlet
unfavorable.
complex formation
to conclude
occur
an exothermlc
is energetically
to such excited
seema reasonable
ring Of the a+%* excited
First,
III.
III
quenching
aromatic
an excited
charge
shortly. S. Kate and Dr. T. Obno
the measurements of emission
spectra
No.30
2890
and for valuable euggestione.
References 1) Pert III. Y. Kateuhara, Y. Shigemiteu, and Y. Odaira, Bull. Chem. Sot. Japan, 4&
1169 (1971). Part 11. K. Fukui end Y. Odaira, Tetrahedron Lett., 5255
(1970). 2) Recent reviews: D. R. Arnold, Moan.
Photochem., f& 301 (1968); D. C. Neckers,
"FlechanieticOrganic Photochemistry*, Reinhold Publishing Co., New York, 1967, P 131. 3) J. C. Dalton, P. A. Wriede, and N. J. Turro, J. Am. Chem. Sot., 92, 1318 (1970) and references cited therein. 4) Y. Odaira, T. Shimodaira, and S. Teutsumi, Chem. Comm., 757 (1967). 5) T. Tominaga, Y. Odaira, and S. Teutsumi, Bull. Chem. Sot. Japan, 2,
2451
(19671. 6) Y. Shigemitsu, H. Rakai, and Y. Odaira, Tetrahedron, g,
3039 (1969) end
Part.III. 7) L. M. Stephenson, D. G. Whitten, G. F. soC.,&j,
Vesley, and G. S. Hammond, J. Am. Chem.
3665 (1966).
8) N. Mataga and K. Ezumi, Bull. Chem. Sot. Japan, 2,
1355 (1967); W. R. Ware
and H. P. Richter, J. Chem. Phys., l&, 1595 (1968). 9) P. J. Wagner and R. A. Leavitt, J. Am. Chem. Sot., 92. 6808 (1970).