Electron-Transfer Reactions of Aromatic α,βEpoxy Ketones: Factors That Govern Selective Conversion to β-Diketones and β-Hydroxy Ketones

Article abstract of DOI:10.1021/jo9622439

Photoreaction of trans-1-(4-cyanophenyl)-3-phenyl-2,3-epoxy-1-propanone (trans-4′-cyanochalcone epoxide) with various electron donors was studied. Irradiation of this epoxy ketone with amines produced 1-(4-cyanophenyl)-3-phenyl-1,3-propanedione and 1-(4-cyanophenyl)-3-hydroxy-3-phenyl-1-propanone in various ratios depending on the kinds of amine and solvent used. A reaction mechanism involving an amine cation radical-assisted rearrangement of the epoxy ketone anion radical was proposed to be most consistent with the marked change in the yield of β-diketone. On the other hand, the timing of proton transfer to the anionic intermediates is considered to be a key factor to yield the β-hydroxy ketone. Information obtained from the photochemical study was useful to find the reaction conditions for the conversion of several aromatic epoxy ketones to hydroxy ketones by the use of samarium diiodide. Addition of methanol significantly changed the product distribution. Proper choice of water or methanol as a proton source produced hydroxy ketones in moderate to good yields.

Full text of DOI:10.1021/jo9622439

2
396
J . Org. Chem. 1997, 62, 2396-2400
Electr on -Tr a n sfer Rea ction s of Ar om a tic r,â-Ep oxy Keton es:
F a ctor s Th a t Gover n Selective Con ver sion to â-Dik eton es a n d
â-Hyd r oxy Keton es
Eietsu Hasegawa,* Kenyuki Ishiyama, Takayuki Fujita, Tomoyasu Kato, and Toshiaki Abe
Department of Chemistry, Faculty of Science, Niigata University, Ikarashi, Niigata 950-21, J apan
Received December 3, 1996X
Photoreaction of trans-1-(4-cyanophenyl)-3-phenyl-2,3-epoxy-1-propanone (trans-4′-cyanochalcone
epoxide) with various electron donors was studied. Irradiation of this epoxy ketone with amines
produced 1-(4-cyanophenyl)-3-phenyl-1,3-propanedione and 1-(4-cyanophenyl)-3-hydroxy-3-phenyl-
1
-propanone in various ratios depending on the kinds of amine and solvent used. A reaction
mechanism involving an amine cation radical-assisted rearrangement of the epoxy ketone anion
radical was proposed to be most consistent with the marked change in the yield of â-diketone. On
the other hand, the timing of proton transfer to the anionic intermediates is considered to be a key
factor to yield the â-hydroxy ketone. Information obtained from the photochemical study was useful
to find the reaction conditions for the conversion of several aromatic epoxy ketones to hydroxy
ketones by the use of samarium diiodide. Addition of methanol significantly changed the product
distribution. Proper choice of water or methanol as a proton source produced hydroxy ketones in
moderate to good yields.
In tr od u ction
Carbonyl compounds have been favorably subjected to
Ch a r t 1
thermally or photochemically induced electron-transfer
reactions.1 Although carbonyl anion radicals are common
intermediates, the following chemical pathways differ
depending on reaction conditions. In order to understand
the reactivities of the anion radicals of certain carbonyl
compounds, it is helpful to conduct their reactions under
various electron transfer conditions. From this point of
view, electron-transfer reactions of R,â-epoxy ketones
should be informative. During our study focused on the
reactivities of carbonyl anion radicals generated by
photoinduced electron-transfer (PET) processes, we found
that photoreactions of certain aromatic R,â-epoxy ketones
with triethylamine produced â-diketones as major prod-
ucts instead of â-hydroxy ketones, which were also
obtained as minor components.2a Photoreactions of ke-
tones with amines usually give reduced products of
ketones, such as alcohols and pinacols, through sequen-
tial electron-transfer and proton-transfer processes be-
tween them.3 Therefore, it is quite rare for carbonyl
compounds to undergo isomerization under the PET
conditions with amines.4 At that time, we tentatively
proposed the unimolecular rearrangement of epoxy ke-
tone anion radicals for this unexpected rearrangement.2
However, we have recently encountered several observa-
tions, some of which are certainly inconsistent with such
a mechanism. Apparently, there is only limited informa-
tion on the factors that govern selective conversion of
aromatic epoxy ketones to diketones and hydroxy ke-
tones.2 About 10 years ago, Molander and co-worker
reported that various aliphatic R,â-epoxy ketones were
successfully converted to the corresponding hydroxy
ketones by samarium diiodide.5 In this case, protonation
of the intermediate ketyls seems to be a key step in the
reaction pathway to produce hydroxy ketones. Then, the
question was raised in our minds whether hydroxy
ketones are still major products in the samarium diiodide
reductions of aromatic epoxy ketones that were subjected
to photochemical study. The experimental results and
the discussion focused on the mechanism of the photo-
chemical formation of the diketones as well as hydroxy
ketones will be described below. Samarium diiodide
reductions of some aromatic epoxy ketones will follow.
a
X
Abstract published in Advance ACS Abstracts, March 15, 1997.
(1) (a) Huffman, J . W. Acc. Chem. Res. 1983, 16, 399. (b) Pradhan,
S. K. Tetrahedron 1986, 42, 6351. (c) Eberson, L. Electron Transfer
Reactions in Organic Chemistry: Springer-Verlag: Berlin, 1987. (d)
Photoinduced Electron Transfer, Part C; Fox, M. A., Chanon, M., Eds.;
Elsevier: Amsterdam, 1988.
(2) (a) Hasegawa, E.; Ishiyama, K.; Horaguchi, T.; Shimizu, T. J .
Org. Chem. 1991, 56, 1631. (b) On the contrary, Cossy and co-workers
reported that â-hydroxy ketone was a major isolable product in the
photoreaction of chalcone epoxide 1a with triethylamine: Cossy, J .;
Bouzide, A.; Ibhi, S.; Aclinou, P. Tetrahedron 1991, 47, 7775.
Resu lts a n d Discu ssion
P h otor ea ction s of tr a n s-4′-Cya n och a lcon e Ep -
oxid e w ith Va r iou s Electr on Don or s. When we
irradiated trans-1,3-diphenyl-2,3-epoxy-1-propanone (trans-
chalcone epoxide, 1a ) with an excess of triethylamine
(TEA) in acetonitrile or methanol, dibenzoylmethane (2a )
(3) Yoon, U. C.; Mariano, P. S.; Givens, R. S.; Atwater, B. W., III.
In Advances in Electron Transfer Chemistry; Mariano, P. S., Ed.; J AI
Press: Greenwich, 1994; Vol. 4; pp 117-205.
(4) (a) The observation made by Pandey might be another rare
example in which the endo-exo isomerization of Diels-Alder adducts
took place on irradiation with triethylamine, although the reaction
2a
was the major product in both solvents (Chart 1). There
4
b
mechanism was not fully understood. (b) Pandey, B.; Dalvi, P. V.;
Athawale, A. A.; Pant, B. G.; Kewale, P. P. J . Chem. Soc., Chem.
Commun. 1990, 1505.
(5) Molander, G. A.; Hahn, G. J . Org. Chem. 1986, 51, 2596.
S0022-3263(96)02243-8 CCC: $14.00 © 1997 American Chemical Society

Reactions of Aromatic R,â-Epoxy Ketones
J . Org. Chem., Vol. 62, No. 8, 1997 2397
Sch em e 1
Ta ble 1. P h otor ea ction s of tr a n s-Cya n och a lcon e
Ep oxid e (1b) w ith Electr on Don or s in MeCN
a
b
Epox
V vs SCE
yields, %
,
time,
h
convn,
%
entry
donor
2b
3b
1
2
3
TEA
TMB
DTH
0.89
0.77
1.22
3
3
5
51
7
17
40
0
0
7
0
0
a
1b] ) 21 mM, [donor] ) 42 mM; λ > 340 nm. b Isolated yields
[
based on the conversion of 1b.
Ta ble 2. P h otor ea ction s of tr a n s-Cya n och a lcon e
a
Ep oxid e (1b) w ith Am in es in MeCN a n d Ben zen e
Ch a r t 2
b
Epox
yields, %
,
time, convn,
entry
donor
V vs SCE solvent
h
%
2b
3b
1
2
3
4
5
6
7
8
9
DABCO
DABCO
TEA
TEA
DEA
DEA
TBA
TBA
TMSA
TMSA
0.71
0.71
0.89
0.89
0.76
0.76
1.17
1.17
0.57
0.57
MeCN
benzene
MeCN
benzene
MeCN
benzene
MeCN
benzene
MeCN
benzene
3
3
3
3
5
5
3
3
3
3
27
34
51
95
21
57
45
47
66
83
8
71
40
77
0
7
5
12
1
30
0
0
7
8
5
42
49
50
27
27
are several possible pathways for the formation of 2a as
shown in Scheme 1. Among them is our previously
proposed mechanism that involves a 1,2-hydrogen shift
in the ring-opened anion radicals followed by a back-
electron transfer (BET) to a triethylamine cation radical
1
0
a
[1b] ) 21 mM, [amine] ) 42 mM; λ > 340 nm. b Isolated yields
path a).2
a
(
If any pathway shown in Scheme 1 is
based on the conversion of 1b.
operable, 2a should be always obtained as one of the
major products as long as an initial electron transfer
between the excited state of 1a and the ground state of
the electron donor is accomplished. To determine if
either one of pathways a-c is responsible for the diketone
formation, we chose trans-4′-cyanochalcone epoxide (1b)
as a reactant since 1b appears to be more versatile than
reaction. Subsequently, we made another interesting
observation, viz., that the yield of 2b significantly
increased in the photoreaction with TEA in a nonpolar
solvent, such as benzene (see below).
The above observations encouraged us to study the
photoreactions of 1b with various amines in both aceto-
nitrile and benzene (Table 2). Unexpectedly, the yields
1
(
a for the reaction with a wide range of electron donors
Chart 2). Epoxide 1b has greater quantum yield of the
intersystem crossing and a longer triplet lifetime than
9
of 2b were higher in benzene than in acetonitrile. This
would suggest that the rearrangement of 1b to 2b does
not occur after the diffusion of the initially formed ion
radical pair into free ion radicals but occurs within the
ion radical pair accompanied by an amine cation radical.
The most noteworthy observation presented in Table 2
is that the ratio (2b/3b) was significantly influenced by
the amine structure. For example, 3b was the major
product in the reaction with tribenzylamine (TBA). This
could be explained by the efficient proton transfer from
the cation radical of TBA to the anionic intermediates
6
1
a . In addition to this, the oxidizing ability of the
excited 1b should be stronger than that of 1a in light of
their reduction potentials.7 Therefore, the excited triplet
state of 1b is expected to oxidize all of the electron donors
shown in Chart 2.8
We first conducted the photoreaction of 1b with tri-
ethylamine (TEA), 1,2,4,5-tetramethoxybenzene (TMB),
and 2-(diphenylmethyl)-1,3-dithiane (DTH) as electron
donors in acetonitrile (Table 1). Immediately, we noticed
that the reaction progressed quite slowly, and dibenzoyl-
methane (2b) was not formed in the reactions with TMB
and DTH while it was obtained as the major product in
the reaction with TEA. Particularly noteworthy is that
the stronger electron donor TMB was much less efficient
for the reaction than TEA. This seems to be inconsistent
with any of the mechanisms shown in Scheme 1. These
results would suggest that the electron-donating ability
of the donor is not a sufficient factor to promote the
10
derived from the anion radical of 1b. In the opposite
sense, the nonformation of 3b in the reaction with 1,4-
diazabicyclo[2.2.2]octane (DABCO) could be also under-
stood because the cation radical of DABCO is known to
be exceptionally stable and a poor proton donor, unlike
other aliphatic amine cation radicals.
On the basis of the observations described above, a new
11
mechanism for the formation of 2b has to be considered,
(
9) In most of the cases studied, 1H NMR analyses of the reaction
(
6) (a) Quantum yield of the intersystem crossing: 0.88 for 1a and
mixtures suggested that the peaks for 2b and 3b observed for the
reactions in acetonitrile were significantly smaller than those in
benzene. The isolated yields of 2b and 3b were indeed low in such
cases (compare entries 1 and 5 with 2 and 6 in Table 2, respectively).
Therefore, the relatively low yields in Table 2 would not be mainly
due to the isolation technique although it was, of course, difficult to
isolate the low-yielding products, particularly at the low conversion of
1b in acetonitrile. The formation of benzaldehyde was sometimes
observed in the crude reaction mixture; however, its yield (not
determined) appeared to be low. Eventually, we were unable to isolate
and identify the other products from the photoreaction mixture.
(10) (a) Acidity of the cation radical of TBA is considered to be higher
than that of TEA since deprotonation from the former produces the
6
b
0
.98 for 1b; triplet lifetime: 3.3 ns for 1a and 85 ns for 1b. (b) Kumar,
C. V.; Ramaiah, D.; Das, P. K.; George, M. V. J . Org. Chem. 1985, 50,
818:
7) (a) Reduction potentials of epoxy ketones studied here have been
2
(
2
a,7b
reported.
and for 1b, respectively.
Horaguchi, T.; Shimizu, T. Tetrahedron Lett. 1991, 32, 2029.
8) (a) Electron transfer from the ground states of all donors studied
For example, these are -1.70 V and -1.39 V vs SCE for
7b
1
a
(b) Hasegawa, E.; Ishiyama, K.;
(
3
to the triplet excited state of 1b ( 1b*) must be exothermic since the
oxidation potentials of the donors are smaller than the reduction
8
b
3
potential of 1b* that is calculated to be 1.65 V from the reduction
potential7a and the triplet energy of 1b. Since the triplet energy of 1b
1
0b
is not known, it is assumed to be about 70 kcal/mol from the known
triplet energy of p-cyanoacetophenone. (b) Oxidation potentials (V
vs SCE; scan rate, 100 mV/s or 200 mV/s) were measured in MeCN
R-amino radical possessing a relatively stable benzilic structure. (b)
Zhang, X.; Yeh, S. R.; Hong, S.; Freccero, M.; Albini, A.; Falvey, D. E.;
Mariano, P. S. J . Am. Chem. Soc. 1994, 116, 4211.
8
c
containing 0.1
M
Et
4
NClO
4
.
(c) Wagner, P. J .; Truman, R. J .;
(11) Chow, Y. L.; Danen, W. C.; Nelsen, S. F.; Rosenblatt, D. H.
Chem. Rev. 1978, 78, 243.
Puchalski, A. E.; Wake, R. J . Am. Chem. Soc. 1986, 108, 7727.

2
398 J . Org. Chem., Vol. 62, No. 8, 1997
Hasegawa et al.
Sch em e 2
interfere with the hydrogen abstraction process between
the ion radical pairs but probably enable the amine cation
radical to survive to participate in such a process and
eventually increase the yield of 2b, which was actually
observed.
Consequently, the above solvent effects would be most
consistent with the postulate that both 2b and 3b are
produced through ion radical pairs rather than in free
ion radicals. If the interaction within the ion radical
pairs is responsible for the formation of the desired
products, the choice of less polar solvents for the pre-
parative photoreactions would sometimes be favorable
even though the initial electron transfer appears to be
in which 2b is formed through a pathway that is
ox
ox
promoted by DABCO (E
p
) 0.71) but not by TMB (E
p
)
0.77). We would like to propose the reaction mecha-
nism shown in Scheme 2. In this mechanism, an amine
cation radical abstracts a â-hydrogen from the ring-
opened epoxy ketone anion radical to give the ammonium
and the enolate of 2b. Although such a process is quite
novel within a pair of the photogenerated carbonyl anion
radicals and amine cation radicals, hydrogen abstraction
by an amine cation radical has been well documented (see
Hofmann-L o¨ ffler-Freytag reaction).12 Thus, hydrogen
abstraction from an anion radical by an amine cation
radical could be operable when anion radical intermedi-
ates possess relatively weak carbon-hydrogen bonds as
in the case of 1b. This process should be more favorable
in a less polar solvent because this shifts the equilibrium
of free ion radicals to ion radical pairs. The striking
contrast in the ratio (2b/3b) observed in the reaction with
TEA compared to that with diethylaniline (DEA) should
be also addressed. Although the proton-donating ability
of the cation radical of DEA is considered to be lower than
that of TEA,13 the yield of the reduced product 3b for
DEA was higher than that for TEA. On the other hand,
the yield of 2b for TEA was higher than that for DEA.
This could be consistent with the mechanism in Scheme
1d,3
less efficient.
Since hydroxy ketones are reduced products from epoxy
ketones, the timing of the protonation to the anionic
intermediates generated from epoxy ketones is considered
to be a key factor for the effective formation of these
compounds.16 In other words, inefficient protonation of
anionic intermediates would decrease the yield of hydroxy
ketones. This was exemplified by the observations in the
photoreaction of 1b with DABCO (entries 1 and 2 in
Table 2) and the photoreaction with TEA in the presence
of LiClO
4
. From a synthetic viewpoint, transformation
of epoxy ketones to hydroxy ketones is much more
important than that to diketones.5
,17
Therefore, it would
be interesting to ascertain if the formation of diketones
is a problem, particularly, when certain aromatic epoxy
ketones, such as chalcone epoxides, are subjected to other
electron-transfer conditions. Thus, we conducted the
reactions of aromatic epoxy ketones with samarium
diiodide, which will be discussed in the following section.
2
since the efficiency of the hydrogen abstraction by the
Sa m a r iu m Diiod id e-P r om oted Tr a n sfor m a tion of
Ar om a tic r,â-Ep oxy Keton es. In connection with our
interest in the reactivities of epoxy ketone anion radicals,
we noticed Molander’s report that various epoxy ketones
were converted to the corresponding hydroxy ketones in
good to excellent yields.5 Interestingly, there were no
aryl-substituted epoxy ketones among their substrates.
Therefore, we decided to explore the reactivities of
representative aromatic epoxy ketones 1a ,c-f shown in
Chart 1 under electron-transfer conditions using sa-
marium diiodide. Since samarium diiodide is a well-
known single electron reductant,18 a hypothetical mech-
anism is proposed for the formation of hydroxy ketones
in Scheme 3. Accordingly, efficient proton transfer and
electron transfer to the involved intermediates must be
essential to increase the yield of hydroxy ketones. Pro-
tonation of the anion radicals of epoxy ketones produces
the oxiranylmethyl radicals, which undergo selective
amine cation radical must be qualitatively correlated
with the N-H bond strength of the corresponding am-
monium salt.14 For the effective formation of 2b, the
fragmentation of the amine cation radical is undesirable.
It is known that R-silylamine cation radicals undergo
efficient fragmentation.1
0b
Thus, the yield of 2b signifi-
cantly decreased for the reaction with TMSA compared
to the reaction with TEA.
Other notable observations were provided by conduct-
ing the photoreaction of 1b with TEA in the presence of
salts. Irradiation of an acetonitrile solution of 1b with
4
TEA on addition of LiClO (2 equiv vs 1b) significantly
decreased the yield of 3b (ca. 1%), while the yield of 2b
was increased (69%) at 52% conversion of 1b.15 On the
4 4
other hand, n-Bu NClO had little influence on both the
conversion of 1b and the product yields. These results
would suggest that the complexation of the lithium cation
to the anionic intermediates inhibits proton transfer from
the amine cation radicals. This interaction would not
1
7h
CR-O bond cleavage to give the alkoxy radicals, and,
(
12) (a) See ref 11. (b) Stella, L. Angew. Chem., Int. Ed. Engl. 1983,
2, 337.
13) The order of efficiency for proton transfer from amine cation
radicals to carbonyl anion radicals is known to be as follows: TEA >
(16) (a) On the basis of this concept, we have recently discovered
an effective photochemical method for the conversion of epoxy ketones
to hydroxy ketones by the use of 1,3-dimethyl-2-phenylbenzimidazoline
2
(
1
6b
as a reductant in aqueous solutions.
(b) Hasegawa, E.; Kato, T.;
1
3b
DEA > DABCO.
990, 94, 4540; 1991, 95, 7253.
14) (a) Bordwell has recently reported the bond dissociation en-
(b) Devadoss, C.; Fessenden, R. W. J . Phys. Chem.
Kitazume, T.; Yanagi, K.; Hasegawa, K.; Horaguchi, T. Tetrahedron
Lett. 1996, 48, 7079.
1
(
(17) (a) Robinson, C. H.; Henderson, R. J . Org. Chem. 1972, 37, 565.
(b) Weile, G. R.; McMorris, T. C. J . Org. Chem. 1978, 43, 3942. (c)
Shapiro, E. L.; Gentles, M. J .; Kabasakalian, P.; Magatti, A. J . Org.
Chem. 1981, 46, 5017. (d) Osuka, A.; Takaoka, K.; Suzuki, H. Chem.
Lett. 1984, 271. (e) McChesney, J . D.; Thompson, T. N. J . Org. Chem.
1985, 50, 3473. (f) Miyashita, M.; Suzuki, T.; Yoshikoshi, A. Tetra-
hedron Lett. 1987, 28, 4293. (g) Inokuchi, T.; Kusumoto, M.; Torii, S.
J . Org. Chem. 1990, 55, 1548. (h) Hasegawa, E.; Ishiyama, K.; Kato,
T.; Horaguchi, T.; Shimizu, T.; Tanaka, S.; Yamashita, Y. J . Org. Chem.
1992, 57, 5352. (i) Engman, L.; Stern, D. J . Org. Chem. 1994, 59, 5179.
(18) (a) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96, 307
and references cited therein. (b) Curran, D. P.; Fevig, T. L.; J asperse,
C. P.; Totleben, M. J . Synlett 1992, 943.
thalpies (BDE) of the conjugate acids of various amines in acetoni-
trile: 95.5 kcal/mol for TEA; 82.0 kcal/mol for dimethylaniline; 91.3
kcal/mol for DABCO. Therefore, the corresponding BDE for DEA is
assumed to be smaller than that for TEA.
F. G. J . Org. Chem. 1996, 61, 4778.
1
4b
(b) Liu, W. Z.; Bordwell,
(15) (a) A similar observation has been already made in our previous
2
a
study. Effects of LiClO
reported in several cases.
Chem. Soc. 1983, 105, 4875. (c) Hasegawa, E.; Xu, W.; Mariano, P.
S.; Yoon, U. C.; Kim, J . U. J . Am. Chem. Soc. 1988, 110, 8099. (d)
Salt effects on organic reactions in general: Loupy, A.; Tchoubar, B.;
Astruc, D. Chem. Rev. 1992, 92, 1141.
4
1
on the PET reaction pathways have been
5b,c,d
(b) Simon, J . D.; Peters, K. S. J . Am.

Reactions of Aromatic R,â-Epoxy Ketones
J . Org. Chem., Vol. 62, No. 8, 1997 2399
Ta ble 3. Rea ction s of Ep oxy Keton es 1 w ith Sa m a r iu m
Diiod id e in th e P r esen ce of Meth a n ol or Wa ter in
Tetr a h yd r ofu r a n a
b
yields, %
entry
epoxide
ROH
convn, %
2a
3a
enone
1
2
3
4
5
6
7
8
9
1a
1a
1c
1c
1d
1d
1e
1e
1f
MeOH
H2O
MeOH
H2O
MeOH
H2O
MeOH
H2O
100
96
94
100
100
97
100
100
100
100
11
4
0
0
0
0
0
0
0
38
60
82
79
72
48
66
40
65
53
4
29
0
0
0
0
5
36
0
18
MeOH
H2O
1
0
1f
0
a
1
, 0.67 mmol; THF, 20 mL; SmL2, 2.7-2.9 equiv; ROH, 100
b
equiv; -78 °C; 30 min. Isolated yields based on the conversion
of 1.
F igu r e 1. Effect of MeOH quantity on the yields of diketone
2
a and hydroxy ketone 3a in the reaction of chalcone epoxide
(
1a ) with samarium diiodide (2.2 equiv vs 1a ) in tetrahydro-
achieve better conversion of 1. As mentioned above, in
the case of chalcone epoxide 1a , aqueous conditions were
better than methanolic conditions for the formation of
3a . In the cases of epoxy ketones 1d -f, the yields of
hydroxy ketones 3d -f on addition of methanol were
better than those achieved on addition of water. The
yields of hydroxy ketone 3c were similar for the reactions
of 1c in both aqueous and methanolic solutions.
furan at -78 °C (the vertical axis represents isolated yields
based on the conversion of 1a ).
Sch em e 3
As seen in Table 3, lower yields of hydroxy ketones in
aqueous solutions are apparently due to the significant
formation of enones. To suppress the formation of enone
remains a problem to be solved to attain the better yield
of the hydroxy ketones. Although the reason for the
significant formation of enones under the aqueous condi-
tions is unclear, one possibility is that dehydration of
hydroxy ketones is caused in the presence of Lewis acidic
Sm(III) generated from the decomposition of samarium
diiodide by water.1
9,20
Notably, when 2.2 equiv of sa-
marium diiodide was used for the aqueous reaction (not
much excess against 1a ), the yield of enone was low (5%)
while 17% of 1a was recovered along with 73% of 3a
based on the conversion of 1a . This would suggest that
a substantial amount of samarium diiodide indeed de-
composed to become inefficient as an electron-transfer
reductant.
finally, the hydroxy ketones (path a). On the other hand,
if protonation to the anion radicals of epoxy ketones does
not occur, diketone formation is expected to become a
competitive pathway (path c), particularly for the reaction
of the substrates possessing both a â-aryl group and a
â-hydrogen atom (path b). Thus, we decided to explore
first the reactivity of 1a with samarium diiodide.
When 1a was treated with 2.2 equiv of samarium
diiodide at room temperature, no formation of 3a was
observed while small amounts of 2a (10%) and chalcone
Con clu sion
We have found that irradiation of trans-4′-cyanochal-
cone epoxide with amines produced both â-diketone and
â-hydroxy ketone. Characteristic effects of the electron-
donor structure and solvent on the ratio of those products
let us propose a novel but plausible mechanism in which
â-diketone is formed via the processes involving the
amine cation radical-assisted â-hydrogen abstraction
from the ring-opened form of the epoxy ketone anion
radical. On the other hand, the effective formation of
â-hydroxy ketone requires better proton-donating ability
of the amine cation radical. This would suggest that the
timing of proton transfer to the anionic intermediates is
a key factor to yield the â-hydroxy ketone. On the basis
of the information obtained from the photochemical
study, we were able to determine the proper reaction
(3%) were obtained. Addition of 100 equiv of methanol
did not much affect the yield of 2a (11%). Eventually,
lowering the reaction temperature to -78 °C was found
to be essential for the formation of 3a . As seen in Figure
1
, increasing the quantity of methanol to the reaction
solution increased the yield of 3a and decreased the yield
of 2a , as is expected from the mechanism in Scheme 3.
The maximum yield of 3a was 47% and the minimum
yield of 2a was 10% in these experiments (the conversion
of 1a ) 93-96%). While 1,3-diphenyl-2-propen-1-one
(chalcone) was also isolated (6-20%), its yield was not a
function of the quantity of methanol added. Finally, we
found that addition of water (100 equiv vs 1a ) signifi-
cantly decreased the yield of 2a (ca. 3%) while the yield
of 3a (ca. 60%) was better than that on addition of
methanol.
(
19) (a) Samarium diiodide is considered to be less stable against
19b
water than methanol in tetrahydrofuran.
(b) Hasegawa, E.; Curran,
Next, several aryl-substituted epoxy ketones were
treated with samarium diiodide on addition of methanol
or water (Table 3). In these experiments, an excess of
samarium diiodide (2.7-2.9 equiv vs 1) was used to
D. P. J . Org. Chem. 1993, 58, 5008.
(20) Addition of triethylamine (5 equiv) indeed reduced the yield of
enone to less than 10%, which seems to be consistent with this
consideration. However, the fact that the yield of 3a became low (about
20%) was also witnessed, a result that is not easy to rationalize.

2
400 J . Org. Chem., Vol. 62, No. 8, 1997
Hasegawa et al.
(29 mL). This solution was purged with dry N
for 10 min
conditions for the conversion of some aromatic epoxy
ketones to â-hydroxy ketones in moderate to good yields
by the use of a combination of samarium diiodide and
proton donors.
2
and then irradiated for an appropriate time. Concentration
of the photolysate at ambient temperature gave a residue that
was subsequently separated by column chromatography or
TLC for the reactions with triethylamine, 1,2,4,5-tetramethoxy-
benzene, and 2-(diphenylmethyl)-1,3-dithiane. For the reac-
tions with less volatile amines, such as diethylaniline, tribenz-
ylamine, diazabicyclooctane, and N,N-diethyl(trimethyl-
silyl)methylamine, the residue was extracted with chloroform
or benzene followed by treatment with 2 N HCl, aqueous
Exp er im en ta l Section
Gen er a l P r oced u r es. Triethylamine (Wako) and diethyl-
aniline (Wako) were distilled with CaH
2
. Tribenzylamine
(Tokyo Kasei) and diazabicyclooctane (Tokyo Kasei) were
NaHCO
3 2 4
, aqueous NaCl, and anhydrous Na SO . When the
salts were added to the photoreaction solutions, the residue
was extracted with methylene chloride followed by treatment
purified by sublimation. N,N-Diethyl(trimethylsilyl)methyl-
amine was prepared according to the literature procedure.21
1
,2,4,5-Tetramethoxybenzene was prepared and recrystallized
2 4
with water and anhydrous Na SO . Isolated diketone 2b and
2
2
from methanol.
dithiane was accomplished by the literature procedure.
Lithium perchlorate (Wako) and tetra-n-butylammonium per-
chlorate (Tokyo Kasei) were used as received. Acetonitrile
Preparation of 2-(diphenylmethyl)-1,3-
hydroxy ketone 3b1 were identified by their spectral data.
7h
2
3
Physical and spectral data of diketone 2b: mp 152-154 °C
1
(
7
(
1
1
1
EtOH) (uncorrected); H-NMR (CDCl
3
) δ 6.85 (s, 1H), 7.42-
.62 (m, 3H), 7.70-7.80 (m, 2H), 7.94-8.11 (m, 4H), 16.7
broad s, 1H, enolic OH); 13C NMR (CDCl
) δ 93.9 (d), 15.6 (s),
18.1 (s), 127.4 (d), 127.5 (d), 128.8 (d), 132.4 (d), 133.1 (d),
35.2 (s), 139.4 (s), 182.2 (s), 187.5 (s); IR (KBr) 2224, 1596,
(
Wako) was distilled over P
Benzene (Wako) was treated with H
and then distilled with CaH . Methanol (Wako) was distilled
and dried with molecular sieves 3A. Tetrahydrofuran was
. Water for the
2
O
5
and subsequently with CaH
2
.
3
2
SO , 5% NaOH, and CaCl
4
2
2
-
1
2
554, 1294, 764 cm . Anal. Calcd for C16H11NO : C, 77.10;
distilled from sodium-benzophenone under N
2
H, 4.45; N, 5.62. Found: C, 77.17; H, 4.72; N, 5.67.
Rea ction s of Ep oxy Keton es 1 w ith Sa m a r iu m Diio-
d id e. To a 0.1 M tetrahydrofuran solution of SmI (18 mL,
2
1.8 mmol, in the case of 2.7 equiv vs 1) placed in a dry ice-
acetone bath (-78 °C) was added a proton donor. Subse-
quently, 2 mL of a tetrahydrofuran solution of epoxide 1 (0.67
mmol) was added within 2-3 min. After the resulting mixture
was stirred for 30 min, the reaction was quenched by saturated
reaction was obtained through an ion-exchange column. Sa-
marium diiodide was prepared from Sm (Aldrich) and iodine
2
4
(Wako) in tetrahydrofuran.
Epoxy ketones were known
2
a
compounds, and their preparations were reported: 1a ,c,d ,e,
1
by direct comparison of their H-NMR spectra with those of
authentic samples.
1
7h
17g
b, 1f.
Products obtained for the reactions were identified
1
1H-NMR and 13C-NMR spectra were recorded in CDCl
3
with
NaHCO
and the extract was washed with saturated Na
NaHCO , and saturated NaCl and dried with MgSO
3
or saturated NH
4
Cl. This was extracted with ether,
, saturated
. The
4
Me Si as the internal standard at 90 and 22.49 MHz, respec-
2 2 3
S O
tively. Photoreactions were conducted in a Pyrex tube (2.5 cm
diameter) immersed in a water bath with a 500 W Xe-Hg
lamp as a light source through a glass cut-off filter (Toshiba
UV-37, λ > 340 nm). Column chromatography was performed
with Wakogel C-200 silica gel. Preparative TLC was per-
formed on 20 cm × 20 cm plates coated with Wakogel B-5F
silica gel.
3
4
residue obtained by concentration of the dried etheral solution
was separated by column chromatography or TLC. Isolated
diketone 2, hydroxy ketone 3, and enone were identified by
their spectral data.
P h otor ea ction s of tr a n s-4′-Cya n och a lcon e Ep oxid e
1b) w ith Electr on Don or s. Epoxide 1b (0.60 mmol) and
donor (1.22 mmol) were dissolved in an appropriate solvent
Ack n ow led gm en t. Partial support by a grant from
the Ministry of Education, Culture, Science and Sports
of J apan is acknowledged. We thank Professor Masaki
Kamata (Faculty of Education) for his assistance in
measurement of redox potentials and the donation of
some electron donors. We thank Professors Takahachi
Shimizu (Faculty of Science, emeritus) and Takaaki
Horaguchi (Faculty of Science) for their generous support.
(
(
21) Hasegawa, E.; Brumfield, M. A.; Mariano, P. S.; Yoon, U. C. J .
Org. Chem. 1988, 53, 5435.
22) Benington, F.; Morin, R. D.; Clark, L. C. J . Org. Chem. 1955,
0, 102.
23) Kamata, M.; Murakami, Y.; Tamagawa, Y.; Kato, M.; Hase-
gawa, E. Tetrahedron 1994, 50, 12821.
24) Imamoto, T.; Ono, M. Chem. Lett. 1987, 501.
(
2
(
(
J O9622439