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Photoreduction of aminobenzophenones. Effects of solvent on intersystem crossing.
TetrahedronLettersNo.49, pp. 4321-4324,1969. PergamonPress. Printed in Great Britain.
PHOTOREDUCTION
OF AMINOBENZOPHENONES.
SOLVENT ON INTERSYSTEW by Saul G. Cohen, Martin D. Saltzman Department
of Chemistry,
p-Aminobenzophenone, at all, by alcohols
and Joseph B. Guttenplan
(2)
with cyclohexane,
rp -
Waltham,
Massachusetts
02154
receividin IR for publication22 September1969)
PAB, is photoreduced
(1,2) and primary amines,
by triethylamine.
the photoreduction
CROSSING.
Brandeis University,
(Receivedin USA 20 August 1969;
EFFECTS OF
The efficiency
very inefficiently,
or not
(2) while it is photoreduced
increases when the amine is diluted
0.6 at 0.1-1.0 E tertiary amine in cyclohexane, but
fails when the amine is diluted with a polar solvent,
(2)
Fluorenone
is also not photoreduced
it is photoreduced
by amines,
(3) cp- 0.1. Photoreduction by triethylamine
acetonitrile.
also becomes much more efficient in cyclohexane, a 2-propanol
or with acetonitrile.
(1,2) and an unreactive
(3) These results had been attributed to
charge transfer
n,n* triplet
media.(1,2)
been shown for fluorenone:
(CT) or rr,n* triplet from P?U3
(4) from fluorenone
(2) and formation of chemically
acter in the non-polar
let
in dilute, - 0.1 M, solution of the amine
cp - 0.9, and less efficient when the amine is diluted with
formation of an unreactive
media,
by alcohols, while
reactive
in the more polar
triplets with n,n* char-
Other causes for these effects have since
neat triethylamine
may quench the excited sing-
(5,6), and the polar solvents, 2-propanol and acetonitrile,
creased
intersystem
decreased
crossing
yields of triplets.
PAB to be photoreduced complete erization
as indicated by increased
lead to de-
fluorescence
and
(7) W e now wish to report that failure of
by or in the polar solvents results from essentially
absence of intersystem
crossing,
as indicated by study of isom-
of trans-stilbene.
Solutions of the ketones gassed and irradiated a G.E. H-85-~3
and stilbene
for 4 minutes
in the several solvents were de-
in Pyrex under argon on a turntable with
lamp, filtered through Corning
4321
O-52 windows.
Analyses
for
4322
No.49
trans and cis stilbene were made by glpc on an 8 ft 2O$ Apieson Rates of isomerisation
of stilbene were obtained
of solutions of the aminobensophenones vent, Table 1.
tion was carried out at
for simultaneous
and of bensophenone
Relative rates of isomerisation
L column. irradiation
in the same sol-
were the same when irradia-
366 nmwith a nonochromator. Table 1
Effects of solvent on isomerisation solutions of para-aminobenzophenone, aminobensophenone,
of trans-stilbene,
St, in irradiation
PAB, meta-aminobenzophenone,
OAB, and bensophenone,
MAB, ortho-
B.
Ketone
Solvent
St. isom., ABh !! /hr
St. isom., B' Y/hr
PAB a,b
2-PrOH
< 0.0001
0.012 k
PAB a,b
CIisCN
0.0006
0.010
PAB a,b
CsHl3
0.0045
0.011
PAB a,b
cBH12
0.0032
0.011
KAB c,d
2 PrOH
BAB c,d
CHsCN
0.0010
0.0087
C&L3
090035
0.0093
Cd312
0.0053
0.0093
KAB c,d BAB e,d
0.012 k
< 0.0001
OAB f,d
2 PrOH
< 0.0001
0.012 k
OAB c,g
Cd-b
< 0.0001
0.0093
a. d. g.
0.002 _M ketone.
b'
0.005 _M trans-stilbene.
0.002 _M trans-stilbene. 0.05 E trans-stilbene.
0.002 _M trans-stilbene,
e' h.
0.003 _M ketone.
0.005 E ketone.
'* 'f.
0.Of.g ketone. _ ,/<'>: j-
aminobensophenones:
irradiation period 2 minutes.
0.05 k.
photoreduction.
hr-'.
M bensophenone,
corrected 7
for
1
The data indicate that irradiation of benzophenone vents led to essentially
of
the same rate of isomerisation
Under these conditions benzophenone
in the several solof stilbene, 0.011 E
was photoreduced
by 2-butylamine
This corresponds
to a at a rate of 0.030 5 hr -l, with quantum yield 1.2. -1 and indicates a quantum yield of rate of triplet formation of 0.026 _M hr
No.49
4333
isomerisation
an efficient
stilbene,
quencher of PAP triplet
irradiation
of PAP in the hydrocarbons
irradiation
of bensophenones,
triplet
from PAP.
This corresponds
essentially
that intersystem
a substantial
to the substantial
in hydrocarbons.
no isomerization
crossing
efficiency
of photo-
In irradiation
unreactive
of PAP in
of stilbene was found, indicating results
CT triplet but from absence of trip-
let, at least of triplet which may isomerize
trans-stilbene.
It may be that
the n,n* singlet and triplet are the states of lowest energy,
and the CT triplet has intermediate ing between
but lower yield of
fails, and the absence of photoreduction
not because of a chemically
in cyclohexane
(2) was isomerized during
at a rate about one third that during
indicating
reduction of PAP by triethylsmine 2-propanol
(8) Trane-
of stilbene - 0.4 in agreement with reported data.
unlike states
(1).
energy, allowing rapid intersystem
In 2-propanol
cross-
the CT singlet and triplet
levels may be below the n,rr* states and intersystem
crossing between
the
like CT states is slow. It is conceivable
that a triplet is formed and is deactivated,
effect is quenched by the alcohol. tent of isomerization
In PAP-acetonitrile
an intermediate
is found, about 0.06 that with bensophenone.
tum yield for reduction of 0.0004 _M PAP in 0.04 _M triethylamine nitrile
is however very low, - 6 x 10-5, indicating
is formed has exceedingly
in acetonitrile.
Irradiation
and to an intermediate
this triplet has relatively
shows substantial
and to no observed
MAP, also led to no isomerisa-
yield in acetonitrile.
low reactivity,
ortho-Aminobenzophenone,
even with tertiary amine in
GAB, led to no isomerisation
triplet either in 2-propanol
interactions
Foundation,
GP-9247.
in
However
of
or in benzene,
of the excited carbonyl with the ortho-
amino group. Acknowledgement.
in
to about half that of the benzophenone
the hydrocarbons,
because of quenching
in aceto-
perhaps due to CT character
of meta-aminobensophenone, in 2-propanol,
stilbene,
Theguan-
(5)
tion of stilbene,
hydrocarbon.
ex-
that the triplet which
Fluorenone with its TI,TT*triplet,
the polar solvent. activity
low reactivity,
and in
This work was supported by the National
Sciukcr
4324
No.49
References 1.
G. S. Porter and P. Suppan, Trans. Faraday Sot., $&, 1664
2.
S. G. Cohen and J. I. Cohen, J. Phys. Chem., z, -
3.
S. G. Cohen and J. B. Guttenplan,
4.
K. Yoshihara
5.
G. A. Davis, J. D. Gresser, P. A. Carapelluci e.,
-s,
Tetrahedron
Letters,
and D. R. Kearns, J. Chem. Phys., 5,
2264
3782
(1965).
(1968). 5353
(1968).
1991 (1966).
and K. Ssoc, J. Am. Chem.
(1969).
6.
R. A. Caldwell, Tetrahedron
7.
J. B. Guttenplan
8.
G. S. Hammond et. al., J. Am. Chem. Sot., 8&,
Letters, 2lZ?1(1969).
and S. G. Cohen, Tetrahedron
Letters, 2125
31%’ (1964).
(1969).
PHOTOREDUCTION
OF AMINOBENZOPHENONES.
SOLVENT ON INTERSYSTEW by Saul G. Cohen, Martin D. Saltzman Department
of Chemistry,
p-Aminobenzophenone, at all, by alcohols
and Joseph B. Guttenplan
(2)
with cyclohexane,
rp -
Waltham,
Massachusetts
02154
receividin IR for publication22 September1969)
PAB, is photoreduced
(1,2) and primary amines,
by triethylamine.
the photoreduction
CROSSING.
Brandeis University,
(Receivedin USA 20 August 1969;
EFFECTS OF
The efficiency
very inefficiently,
or not
(2) while it is photoreduced
increases when the amine is diluted
0.6 at 0.1-1.0 E tertiary amine in cyclohexane, but
fails when the amine is diluted with a polar solvent,
(2)
Fluorenone
is also not photoreduced
it is photoreduced
by amines,
(3) cp- 0.1. Photoreduction by triethylamine
acetonitrile.
also becomes much more efficient in cyclohexane, a 2-propanol
or with acetonitrile.
(1,2) and an unreactive
(3) These results had been attributed to
charge transfer
n,n* triplet
media.(1,2)
been shown for fluorenone:
(CT) or rr,n* triplet from P?U3
(4) from fluorenone
(2) and formation of chemically
acter in the non-polar
let
in dilute, - 0.1 M, solution of the amine
cp - 0.9, and less efficient when the amine is diluted with
formation of an unreactive
media,
by alcohols, while
reactive
in the more polar
triplets with n,n* char-
Other causes for these effects have since
neat triethylamine
may quench the excited sing-
(5,6), and the polar solvents, 2-propanol and acetonitrile,
creased
intersystem
decreased
crossing
yields of triplets.
PAB to be photoreduced complete erization
as indicated by increased
lead to de-
fluorescence
and
(7) W e now wish to report that failure of
by or in the polar solvents results from essentially
absence of intersystem
crossing,
as indicated by study of isom-
of trans-stilbene.
Solutions of the ketones gassed and irradiated a G.E. H-85-~3
and stilbene
for 4 minutes
in the several solvents were de-
in Pyrex under argon on a turntable with
lamp, filtered through Corning
4321
O-52 windows.
Analyses
for
4322
No.49
trans and cis stilbene were made by glpc on an 8 ft 2O$ Apieson Rates of isomerisation
of stilbene were obtained
of solutions of the aminobensophenones vent, Table 1.
tion was carried out at
for simultaneous
and of bensophenone
Relative rates of isomerisation
L column. irradiation
in the same sol-
were the same when irradia-
366 nmwith a nonochromator. Table 1
Effects of solvent on isomerisation solutions of para-aminobenzophenone, aminobensophenone,
of trans-stilbene,
St, in irradiation
PAB, meta-aminobenzophenone,
OAB, and bensophenone,
MAB, ortho-
B.
Ketone
Solvent
St. isom., ABh !! /hr
St. isom., B' Y/hr
PAB a,b
2-PrOH
< 0.0001
0.012 k
PAB a,b
CIisCN
0.0006
0.010
PAB a,b
CsHl3
0.0045
0.011
PAB a,b
cBH12
0.0032
0.011
KAB c,d
2 PrOH
BAB c,d
CHsCN
0.0010
0.0087
C&L3
090035
0.0093
Cd312
0.0053
0.0093
KAB c,d BAB e,d
0.012 k
< 0.0001
OAB f,d
2 PrOH
< 0.0001
0.012 k
OAB c,g
Cd-b
< 0.0001
0.0093
a. d. g.
0.002 _M ketone.
b'
0.005 _M trans-stilbene.
0.002 _M trans-stilbene. 0.05 E trans-stilbene.
0.002 _M trans-stilbene,
e' h.
0.003 _M ketone.
0.005 E ketone.
'* 'f.
0.Of.g ketone. _ ,/<'>: j-
aminobensophenones:
irradiation period 2 minutes.
0.05 k.
photoreduction.
hr-'.
M bensophenone,
corrected 7
for
1
The data indicate that irradiation of benzophenone vents led to essentially
of
the same rate of isomerisation
Under these conditions benzophenone
in the several solof stilbene, 0.011 E
was photoreduced
by 2-butylamine
This corresponds
to a at a rate of 0.030 5 hr -l, with quantum yield 1.2. -1 and indicates a quantum yield of rate of triplet formation of 0.026 _M hr
No.49
4333
isomerisation
an efficient
stilbene,
quencher of PAP triplet
irradiation
of PAP in the hydrocarbons
irradiation
of bensophenones,
triplet
from PAP.
This corresponds
essentially
that intersystem
a substantial
to the substantial
in hydrocarbons.
no isomerization
crossing
efficiency
of photo-
In irradiation
unreactive
of PAP in
of stilbene was found, indicating results
CT triplet but from absence of trip-
let, at least of triplet which may isomerize
trans-stilbene.
It may be that
the n,n* singlet and triplet are the states of lowest energy,
and the CT triplet has intermediate ing between
but lower yield of
fails, and the absence of photoreduction
not because of a chemically
in cyclohexane
(2) was isomerized during
at a rate about one third that during
indicating
reduction of PAP by triethylsmine 2-propanol
(8) Trane-
of stilbene - 0.4 in agreement with reported data.
unlike states
(1).
energy, allowing rapid intersystem
In 2-propanol
cross-
the CT singlet and triplet
levels may be below the n,rr* states and intersystem
crossing between
the
like CT states is slow. It is conceivable
that a triplet is formed and is deactivated,
effect is quenched by the alcohol. tent of isomerization
In PAP-acetonitrile
an intermediate
is found, about 0.06 that with bensophenone.
tum yield for reduction of 0.0004 _M PAP in 0.04 _M triethylamine nitrile
is however very low, - 6 x 10-5, indicating
is formed has exceedingly
in acetonitrile.
Irradiation
and to an intermediate
this triplet has relatively
shows substantial
and to no observed
MAP, also led to no isomerisa-
yield in acetonitrile.
low reactivity,
ortho-Aminobenzophenone,
even with tertiary amine in
GAB, led to no isomerisation
triplet either in 2-propanol
interactions
Foundation,
GP-9247.
in
However
of
or in benzene,
of the excited carbonyl with the ortho-
amino group. Acknowledgement.
in
to about half that of the benzophenone
the hydrocarbons,
because of quenching
in aceto-
perhaps due to CT character
of meta-aminobensophenone, in 2-propanol,
stilbene,
Theguan-
(5)
tion of stilbene,
hydrocarbon.
ex-
that the triplet which
Fluorenone with its TI,TT*triplet,
the polar solvent. activity
low reactivity,
and in
This work was supported by the National
Sciukcr
4324
No.49
References 1.
G. S. Porter and P. Suppan, Trans. Faraday Sot., $&, 1664
2.
S. G. Cohen and J. I. Cohen, J. Phys. Chem., z, -
3.
S. G. Cohen and J. B. Guttenplan,
4.
K. Yoshihara
5.
G. A. Davis, J. D. Gresser, P. A. Carapelluci e.,
-s,
Tetrahedron
Letters,
and D. R. Kearns, J. Chem. Phys., 5,
2264
3782
(1965).
(1968). 5353
(1968).
1991 (1966).
and K. Ssoc, J. Am. Chem.
(1969).
6.
R. A. Caldwell, Tetrahedron
7.
J. B. Guttenplan
8.
G. S. Hammond et. al., J. Am. Chem. Sot., 8&,
Letters, 2lZ?1(1969).
and S. G. Cohen, Tetrahedron
Letters, 2125
31%’ (1964).
(1969).