1
082
Chem. Pharm. Bull.
Vol. 65, No. 11 (2017)
2
6)
27)
28)
stibane, diphenyl stibane, diphenylantimonymagnesium,
tion of α-halo carbonyl compounds in acetonitrile or methanol
2
9)
and NaBH -SbBr reagent are known. Akiba et al. have re- solution. Therefore, we were interested in reactivity of group
4
3
ported that tributylstibane is an efficient reagent for the debro- 15 (pnictogen) reagents and the reaction of 1a using various
2
6)
mination of phenacyl bromides. However, such aliphatic ter- triarylpnictogen reagents such as Ph P 3d, Ph As 3e, and
3
3
tiary stibanes are unstable toward air and moisture. Therefore, Ph Bi 3f was also examined under microwave irradiation in
3
their use is limited in routine organic synthesis. In contrast, water. The debromination product 4a was obtained in good
triarylstibanes are easier to handle, exhibit low toxicity, and yield (83%) using Ph P as the reagent while the yields were 3
3
are readily available. Unfortunately, debromination of phena- and 20% using Ph As and Ph Bi, respectively (entries 18–20).
3
3
cyl bromides using triphenylstibane reportedly requires a long These results including the case of Ph Sb do not follow the
3
reaction time (6d) and the substrate scope of this reaction also periodic table trend of reactivity.
2
6)
remains unclear. Owing to the environmental benefit and
In order to demonstrate the efficiency and generality of
favorable effect of water on chemical transformations, it is this procedure, the reaction of various α-bromoketones 1 was
desirable, albeit challenging, to perform organic reactions in examined in the presence of triphenylstibane 3a under mi-
aqueous media. However, only a limited number of examples crowave irradiation in water. These results are summarized
of dehalogenation reactions of α-halo carbonyl compounds in Table 2. The reactions of 3a with α-bromoketones 1b–f
have been reported in literature with water as the reaction sol- containing electron-donating or electron-withdrawing groups
3
0–33)
vent.
As the microwave irradiation has been successfully on the phenyl ring afforded the corresponding products 4b–f
applied for performing organic syntheses in aqueous solu- in excellent yields (entries 1–5). The reaction of the sterically
3
4–40)
tion,
we investigated the microwave-assisted debromina- hindered substrates 1g and h gave the corresponding alkyl
aryl ketones 4g and h without any difficulty, (entries 6, 7) and
tion of α-bromoketones with triarylstibane reagents in water.
As the starting point for optimization studies, the deha- the reaction of α-bromoketones 1i and j having a heterocyclic
logenation of phenacyl halides 1a and 2a with triarylstibanes ring also resulted in the formation of 4i and j without affect-
3a–c was examined in the presence of a proton source based
on the reaction solvent used. The results of these reactions Table 2. Microwave-Assisted Debromination of α-Bromoketones 1 with
a)
Triphenylstibane 3a
and a comparison between conventional heating and micro-
wave irradiation methods (Methods A and B, respectively) are
summarized in Table 1. In the case of conventional heating
(
1
Method A, entries 1–4), the treatment of phenacyl bromide
a with triphenylstibane 3a at 100°C in dioxane–H O (9:1)
2
afforded acetophenone 4a in 79% yield. However, a long reac-
tion time was required (48h) to obtain the product (entry 4).
On the other hand, when the identical reaction was performed
under microwave irradiation, the progress of the reaction
could be confirmed even after 10min (entry 5). The reac-
tion between 1a and 3a under microwave irradiation (150W,
maximum temperature. 120°C) in water for 10min gave 4a
in excellent yields (entry 6). The optimum amounts of the
triarylstibane reagent, and active substrates, and microwave
irradiation condition for this reaction are shown in entries
5
–17. When water was used as the solvent, superior results
were obtained in comparison with the solvent mixture of diox-
ane–H O by conventional heating (entries 4, 6). This reaction
2
may be considered as a stoichiometric reaction between 1a
and 3a because decreasing the amount of 3a reduces the yield
of 4a (entries 6–8). The reaction is complete in a short time
of 10min (entries 6, 9, 10). In addition, lowering the reaction
temperature and microwave power gave significantly lower
yields of 4a (entries 11, 12). It is noteworthy that the reactiv-
ity of triarylstibane 3b having the electron-donating methoxy
group is higher than that of 3c, which has a chloro group
(entries 13, 14). Inferior yields were obtained when phenacyl
chloride 2a was used as the substrate (entries 15–17). Thus,
under optimum reaction conditions, phenacyl bromide 1a was
treated with 1.1 equiv. of the antimony reagent (3a or b) in
water under microwave irradiation (150W, maximum tempera-
ture of 120°C) for 10min (entries 6, 13). The reagent utilized
for investigating the scope and limitations of this reaction
was the commercially available organoantimony compound
triphenylstibane 3a. Meanwhile, it is known that phosphine
a) All reactions were carried out MW irradiation (150W, max. temp. 120°C,
41)
reagents such as triphenylphosphonium iodide and polymer-
1
(
0min) using a mixture of 1 (1mmol), triphenylstibane 3a (1.1mmol) and H O
2
42)
1mL). b) Isolated yield.
supported triphenylphosphine can be used for dehalogena-
Murata, Yuki
