Photoinduced synthesis of unsymmetrical diaryl selenides from triarylbismuthines and diaryl diselenides

A novel method of photoinduced synthesis of unsymmetrical diaryl selenides from triarylbismuthines and diaryl diselenides has been developed. Although the arylation reactions with triarylbismuthines are usually catalyzed by transition-metal complexes, the present arylation of diaryl diselenides with triarylbismuthines proceeds upon photoirradiation in the absence of transition-metal catalysts. A variety of unsymmetrical diaryl selenides can be conveniently prepared by using this arylation method.


Results and Discussion
First, we investigated the photoinduced reaction of diphenyl diselenide with triphenylbismuthine. Diphenyl diselenide (1a, 0.1 mmol) and triphenylbismuthine (2a, 0.5 mmol) were placed in a Pyrex test tube ( = 9 mm) with CHCl 3 (4 mL), and the mixture was irradiated by a xenon lamp for 5 h at room temperature. As a result, 0.042 mmol (21% yield based on the amount of selenium atoms) of diphenyl selenide (3aa) was obtained after the isolation by silica gel chromatography (the yield was determined by HPLC). Next, optimization of the reaction conditions was investigated as shown in Table 1. Irradiation by a tungsten lamp instead of a xenon lamp did not in-duce the desired arylation reaction (Table 1, entry 2), and in the dark, the reaction did not proceed at all (Table 1, entry 3). When 2,2′-azobis(isobutyronitrile) (AIBN) was used as a radical initiator, the desired reaction proceeded ineffectively ( Table 1, entry 4). Among several solvents, such as benzene, DMSO and CH 3 CN, the use of CH 3 CN improved the yield of 3aa (Table 1, entries 5-7). Although the solubility of 2a is different depending on the solvent, the yield of 3aa is not correlated with the solubility of 2a. It may be more important to choose a solvent that does not react with the generated aryl radical. Moreover, a lower amount of solvent and the utilization of a quartz test tube ( = 9 mm) contributed to the increase in the yield of 3aa (Table 1, entries 8 and 9).
In the case of the reaction of triphenylbismuthine with diphenyl disulfide (4) instead of diphenyl diselenide, diphenyl sulfide 5 was obtained in lower yield with unidentified byproducts, unlike in the case of diphenyl selenide 3aa (Scheme 2).
Additionally, air is entrained in the reaction system, since a test tube with a septum was used in which a needle was inserted. When the reaction of diaryl diselenide with triarylbismuthine was conducted with a strictly sealed tube in Ar atmosphere, a bismuth mirror was observed and the yield of 3aa decreased. We assume that the reaction proceeds with bismuth residue getting oxidized.
A plausible reaction pathway for the photoinduced reaction of diaryl diselenide with triarylbismuthine is shown in Scheme 3. First, an aryl radical is generated from triarylbismuthine by near-UV light irradiation [33,54,55]. The generated aryl radical is captured by diaryl diselenide to produce diaryl selenide and a  seleno radical. The seleno radical may dimerize to re-form diselenide. Diphenyl diselenide has its absorption maximum (λ max ) at 340 nm (ε = 10 3 ) [66] and accordingly, the seleno radical could be produced by the irradiation with a tungsten lamp. However, the irradiation by a tungsten lamp instead of a xenon lamp did not result in the desired reaction (Table 1, entry 3). This fact strongly suggests that the formation of a phenylseleno radical is not important for the formation of diphenyl selenide. Conceivably, when the reaction proceeds, a phenyl radical may be formed directly from triphenylbismuthine upon photoirradiation. Moreover, the use of an excess amount of 1, which has a relatively high carbon-radical-capturing ability, increased the yield of 3, and the use of diphenyl disulfide (4), which has a lower carbon-radical-capturing ability than diselenide, decreased the yield of 5. (The exact capturing abilities of diselenide and disulfide toward the phenyl radical are not known, but they have been reported toward vinyl radicals, where diselenide has a higher capturing ability than disulfide: k Se /k S = 160 [57][58][59].) These facts also support that the reaction starts from the generation of an aryl radical. On the other hand, a pale yellow solid, insoluble in organic solvents, was obtained as a byproduct after the reaction. We assume that this solid is a bismuth residue, which can consist of bismuth oxides and/or bismuth selenides. Moreover, it may form biaryls (Ar-Ar) as byproducts, but no biaryl was observed after the reaction.

Scheme 3:
A plausible reaction pathway for the photoinduced reaction of diaryl diselenide with triarylbismuthine.

Conclusion
We have found that the photoinduced reaction of diaryl diselenides with triarylbismuthines affords unsymmetrical diaryl selenides in good yields. This method is efficient, because two arylseleno groups from diaryl selenides can be used as a selenium source, and its advantage is that the reaction proceeds in the absence of transition-metal catalysts.

Supporting Information
Supporting Information File 1 Spectral and analytical data of the new compound 3cb.