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Photochemical reactions of dialkylboryltropolonate complexes
Tetrahedron Letters,Vo1.30,No.33,?? Printed in Great Britain
oo40-4039/89 $3.00 + .OO Pergamon Press plc
4395-4398,1989
PHOTOCHEMICAL REACTIONS OF DIALKYLBORYLTROPOLONATE
COMPLEXES
Keiji Okada,* Haruki Inokawa, and Masaji Oda* Department
of
of
Faculty
Chemistry,
Science,
Osaka
University,
Toyonaka, Osaka 560, Japan Irradiation of a benzene solution of dialkylboryltropolonates produced Summary: The product distribution and alkyl migrated tropolone and tropone derivatives. the mechanism of this rearrangement were investigated. Reactions
of organoboranes
interest
in organic
chemical
rearrangement
in their
excited
We have
photochemistry.
states
recently
of dialkylborylacetylacetonate
group migrates
from the boron to the carbonyl
was proposed.
This type of rearrangement
have
complexes
photo-
in which one alkyl pair mechanism
reaction
for the four
In order to know the generality of this
reaction and to obtain further insight into the mechanism,
we have studied photo-
(11,
chemical reactions of dialkyll>oryltropolonate complexes carbon centers and therefore
growing
a novel
A radical
carbon.'
may be a general
coordinated alkylboryl chelate complexes.
reactive
received
reported
which
have
multiple
would be suitable for a test of the radical
pair mechanism. When a benzene
solution
of the borane
chelate
la was irradiated
with a 500-W
Xenon lamp under nitrogen through a glass filter (>350 nm), the yellow color due to The products 2a, 3a, 4a, 5a, and 6a were best separated
la gradually disappeared.
and isolated by preparative TLC on silica gel after 1) oxidative hydrolysis solution
of pH 7 phosphate
2) 0-methylation formed shown
from
These
1.2
initial products 7-9.
(2'&3'),
tropolonate which
oxidative
considered compared
would
hydrolysis,
dehydration
under
chelates products
30% H202
2 and 3 must be formed whereas
these
2(3,6)e
having 2-6
various
Similar products were
alkyl groups
are considered
be derived
through
in the yields
to be derived
the tiopone
conditions. 5.
: 2(3,6)f
from
Two
However,
derivatives precursors
5 and 6 can be formed 7 and
9 can
of via
be formally
high ratio
of le, which
the
tropolones
under the conditions
the distinctly
in the photolysis
from
The isomers
the tautomeric
dehydrogenation
with a
(2_5:1O:l(v/v)), and
Formal reaction pathways are shown in Scheme 1.
for the formation with
THF, and aqueous
with dimethyl sulfate-K2C03 in dry acetone.
the tropolonate
in Table
of methyl
buffer,
of Se : 5f
is shown
later,
suggests that the precursor of 5 is 9 rather than 7.3
The structural discrimina-
tion of 2 and 3 were achieved by 'H NMR NOE experiment:
difference NOE spectrum of
2 shows a clear NOE between
the methoxy protons and tropolone ring proton (H3).
4395
4396
/ R
R\ dB\o
+jMe
hv_--S. 01;
1
*$l
2
vMe
3
j-j
4,
6
5
Table 1. Quantum and Chemical Yields of Photoreactions R
Quantum yielda
2(%)
a: n-C4Hg
0.041 (0.040)b
22
6
b:
0.0039 (0.0039)C
24
c: ally1
0.40 (0.4O)C
d: benzyl
0.18 (0.18jb
c-C6H,,
*bR
3(%)
of 1
4(%)
5(%)
6(%)
6
12
6
4
8
16
4
20
5
11
18
_d
18
7
9
19
_d
a: Quantum yield of disappearance of 1 at 366 nm excitation in benzene argon. b: Under aerated conditions. c: In the presence of piperylene under argon. d: Less than 2%.
Scheme
2+3 A
1 HO H202
,&2
0
a2p--
IMe&,-K2C03 'e
pH 7) \H20 7 R R 'B' d '0
k
5+6
? d8b hY,
HO
1-i 3 HR
H2O2,
_
PH 7
-'-'2
0 > Me2SO4-K2C03 R
8
H2O2
--5
PH 7
? / B\ 1/
R* '6 *
a (12-19%)
-H20
3
+-b(25-34o/~)3
7 X(0%) d (6 -11 "/.) HMO coefficients of SOMO
4
under (2 M)
4398
Scheme
2
Path a noncyclized products pair
Path b
radical radical
(11) to produce in the singlet
recombination
<
cyclized products
the
O-0
cyclized
radical
s-1
).’
The
resonance
relatively
stabilization
(usually high
The cyclization
pair (path c) seems
relatively slow rate of the cyclization rate of recombination
products.
s-' )' or intersystem
of the cyclized
of the tropyl
to be unlikely
radical,
products
No
1.
2. 3.
4. 5.
6. 7.
because
crossing may
the stabilization
coupling in the radical pair to give higher chance of intersystem Acknowledgment:
of 5-hexenyl of its
(k ,= 2 x lo5 s-' at 25 0C)6 compared to the
1O7-1O1o
yield
13
(IO*-IO9
be due to the retarding
the
crossing.
This work was supported by the Grant-in-Aid for Scientific Research
63540397 from Ministry of Education,
Science and Culture, Japan.
References and Notes K. Okada, Y. Hosoda, and M. Oda, J. Am. Chem. Sot., 108, 321(1986). ~11 the products were characterized by spectral data (NMR,,,IR, and Mass); the details will be reported elsewhere. The minor contribution of 7 for the formation 5 can not be excluded. The values in parentheses in A are based on the assumption that the precursor of 5 is only 9. D. Griller and K. U. Ingold, Act. Chem. Res., 13, 317(1980). The values of quantum yields (less than unity) and their insensibility to dissolved oxygen in this system as well as the previous results for dialkylborylacetylacetonate complexes suggest that the reactions do not involve radical chain processes which have been postulated in thermal rearrangement 'of 2-arylH. Takeshita, H. Mametsuka, A. Chisaka, and N. Matsuo, Chem. methoxytropones; Lett., 1981, 73. C. Chatgilialoglu, K. U. Ingold, and J. C. Scaiano, J. Am. Chem. Sot., 103, 7739(1981); D. P.Curran, Synthesis, 1988, 417. R. Kaptein; in "Chemically Induced Magnetic Polarization", ed by L. T. MUUS, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen; D. Reidel; Dordrecht-Holland, 1977; pp l-16. (Received
in Japan
27 April
1989)
oo40-4039/89 $3.00 + .OO Pergamon Press plc
4395-4398,1989
PHOTOCHEMICAL REACTIONS OF DIALKYLBORYLTROPOLONATE
COMPLEXES
Keiji Okada,* Haruki Inokawa, and Masaji Oda* Department
of
of
Faculty
Chemistry,
Science,
Osaka
University,
Toyonaka, Osaka 560, Japan Irradiation of a benzene solution of dialkylboryltropolonates produced Summary: The product distribution and alkyl migrated tropolone and tropone derivatives. the mechanism of this rearrangement were investigated. Reactions
of organoboranes
interest
in organic
chemical
rearrangement
in their
excited
We have
photochemistry.
states
recently
of dialkylborylacetylacetonate
group migrates
from the boron to the carbonyl
was proposed.
This type of rearrangement
have
complexes
photo-
in which one alkyl pair mechanism
reaction
for the four
In order to know the generality of this
reaction and to obtain further insight into the mechanism,
we have studied photo-
(11,
chemical reactions of dialkyll>oryltropolonate complexes carbon centers and therefore
growing
a novel
A radical
carbon.'
may be a general
coordinated alkylboryl chelate complexes.
reactive
received
reported
which
have
multiple
would be suitable for a test of the radical
pair mechanism. When a benzene
solution
of the borane
chelate
la was irradiated
with a 500-W
Xenon lamp under nitrogen through a glass filter (>350 nm), the yellow color due to The products 2a, 3a, 4a, 5a, and 6a were best separated
la gradually disappeared.
and isolated by preparative TLC on silica gel after 1) oxidative hydrolysis solution
of pH 7 phosphate
2) 0-methylation formed shown
from
These
1.2
initial products 7-9.
(2'&3'),
tropolonate which
oxidative
considered compared
would
hydrolysis,
dehydration
under
chelates products
30% H202
2 and 3 must be formed whereas
these
2(3,6)e
having 2-6
various
Similar products were
alkyl groups
are considered
be derived
through
in the yields
to be derived
the tiopone
conditions. 5.
: 2(3,6)f
from
Two
However,
derivatives precursors
5 and 6 can be formed 7 and
9 can
of via
be formally
high ratio
of le, which
the
tropolones
under the conditions
the distinctly
in the photolysis
from
The isomers
the tautomeric
dehydrogenation
with a
(2_5:1O:l(v/v)), and
Formal reaction pathways are shown in Scheme 1.
for the formation with
THF, and aqueous
with dimethyl sulfate-K2C03 in dry acetone.
the tropolonate
in Table
of methyl
buffer,
of Se : 5f
is shown
later,
suggests that the precursor of 5 is 9 rather than 7.3
The structural discrimina-
tion of 2 and 3 were achieved by 'H NMR NOE experiment:
difference NOE spectrum of
2 shows a clear NOE between
the methoxy protons and tropolone ring proton (H3).
4395
4396
/ R
R\ dB\o
+jMe
hv_--S. 01;
1
*$l
2
vMe
3
j-j
4,
6
5
Table 1. Quantum and Chemical Yields of Photoreactions R
Quantum yielda
2(%)
a: n-C4Hg
0.041 (0.040)b
22
6
b:
0.0039 (0.0039)C
24
c: ally1
0.40 (0.4O)C
d: benzyl
0.18 (0.18jb
c-C6H,,
*bR
3(%)
of 1
4(%)
5(%)
6(%)
6
12
6
4
8
16
4
20
5
11
18
_d
18
7
9
19
_d
a: Quantum yield of disappearance of 1 at 366 nm excitation in benzene argon. b: Under aerated conditions. c: In the presence of piperylene under argon. d: Less than 2%.
Scheme
2+3 A
1 HO H202
,&2
0
a2p--
IMe&,-K2C03 'e
pH 7) \H20 7 R R 'B' d '0
k
5+6
? d8b hY,
HO
1-i 3 HR
H2O2,
_
PH 7
-'-'2
0 > Me2SO4-K2C03 R
8
H2O2
--5
PH 7
? / B\ 1/
R* '6 *
a (12-19%)
-H20
3
+-b(25-34o/~)3
7 X(0%) d (6 -11 "/.) HMO coefficients of SOMO
4
under (2 M)
4398
Scheme
2
Path a noncyclized products pair
Path b
radical radical
(11) to produce in the singlet
recombination
<
cyclized products
the
O-0
cyclized
radical
s-1
).’
The
resonance
relatively
stabilization
(usually high
The cyclization
pair (path c) seems
relatively slow rate of the cyclization rate of recombination
products.
s-' )' or intersystem
of the cyclized
of the tropyl
to be unlikely
radical,
products
No
1.
2. 3.
4. 5.
6. 7.
because
crossing may
the stabilization
coupling in the radical pair to give higher chance of intersystem Acknowledgment:
of 5-hexenyl of its
(k ,= 2 x lo5 s-' at 25 0C)6 compared to the
1O7-1O1o
yield
13
(IO*-IO9
be due to the retarding
the
crossing.
This work was supported by the Grant-in-Aid for Scientific Research
63540397 from Ministry of Education,
Science and Culture, Japan.
References and Notes K. Okada, Y. Hosoda, and M. Oda, J. Am. Chem. Sot., 108, 321(1986). ~11 the products were characterized by spectral data (NMR,,,IR, and Mass); the details will be reported elsewhere. The minor contribution of 7 for the formation 5 can not be excluded. The values in parentheses in A are based on the assumption that the precursor of 5 is only 9. D. Griller and K. U. Ingold, Act. Chem. Res., 13, 317(1980). The values of quantum yields (less than unity) and their insensibility to dissolved oxygen in this system as well as the previous results for dialkylborylacetylacetonate complexes suggest that the reactions do not involve radical chain processes which have been postulated in thermal rearrangement 'of 2-arylH. Takeshita, H. Mametsuka, A. Chisaka, and N. Matsuo, Chem. methoxytropones; Lett., 1981, 73. C. Chatgilialoglu, K. U. Ingold, and J. C. Scaiano, J. Am. Chem. Sot., 103, 7739(1981); D. P.Curran, Synthesis, 1988, 417. R. Kaptein; in "Chemically Induced Magnetic Polarization", ed by L. T. MUUS, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen; D. Reidel; Dordrecht-Holland, 1977; pp l-16. (Received
in Japan
27 April
1989)