Photochemical substitution of polyhalogenothiophene and halogenothiazole derivatives †
作者:Maurizio D’Auria、Claudio Distefano、Franco D’Onofrio、Giacomo Mauriello、Rocco Racioppi
DOI:10.1039/b004002i
日期:——
The irradiation of 2,3-diodo-5-nitrothiophene in the presence of aromatic and heteroaromatic compounds gave the corresponding 2-aryl derivatives in high yields. The irradiation of 2,4-diiodo-5-nitrothiophene under the same conditions gave the corresponding 2-aryl derivatives in low yields. The observed difference in the reactivity can be explained on the basis of the hypothesis that the homolytic cleavage of the carbon–iodine bond occurred in a π,σ* triplet state. Computational results showed that the lowest triplet state of the 2,3-diiodo isomer is π,σ*, while that of the 2,4-isomer is π,π*. The irradiation of 2-bromo-5-nitrothiazole in the presence of benzene or indene gave the corresponding 2-bromo-5-arylthiazole. This behaviour can be explained by considering that the lowest excited triplet state cannot allow the cleavage of the carbon–bromine bond thus electron transfer occurs and leads to the substitution of the nitro group. The photochemical substitution reactions on 2,3-diiodo-5-nitrothiophene can be carried out in large scale using a new flow reactor using a PFTE pipe.
在芳香和杂芳香化合物的存在下,用2,3-二碘-5-硝基噻吩进行辐照,可以高产率地得到相应的2-芳基衍生物。在相同条件下,用2,4-二碘-5-硝基噻吩进行辐照,只能低产率地得到相应的2-芳基衍生物。观察到的反应性差异可以通过以下假设来解释:在π,σ*三重态下发生了碳-碘键的均裂断裂。计算结果显示,2,3-二碘异构体的最低三重态为π,σ*,而2,4-异构体的最低三重态为π,π*。在苯或茚存在下,用2-溴-5-硝基噻唑进行辐照,可以得到相应的2-溴-5-芳基噻唑。这种行为可以通过考虑最低激发三重态无法允许碳-溴键的断裂,从而发生电子转移,导致硝基的取代反应来解释。2,3-二碘-5-硝基噻吩的光化学取代反应可以使用新的流动反应器,通过聚四氟乙烯管路大规模进行。