Effective Synthesis of 1,4-Diarylbutadienes via Reductive Desulfonylation of 1,3-Butadienyl Sulfones by Proper Choice of Regioisomeric π-Expanded Pyrene Photocatalysts

Abstract For the reductive desulfonylation of 1,3-butadienyl sulfones, we synthesized 1,3,6,8-tetra(phenylethynyl)pyrenes possessing (S)-citronellyloxy groups on the terminal phenyl rings at the ortho-, meta-, and para-positions to serve as photocatalysts. All of these pyrenes exhibited catalytic activity in the reductive desulfonylation of butadienyl sulfones when exposed to either green (514 nm) or blue LEDs (447 nm) in the presence of the sacrificial reducing agent i-Pr2NEt. The photocatalytic activities of these compounds could be fine-tuned by altering the position of the (S)-citronellyloxy group. Under green LEDs illumination, the ortho-(S)-citronellyloxy pyrene photocatalyst (S)-1 proved to be the most effective in the reductive desulfonylation of 1,4-diphenylbutadien-1-yl sulfone 4b to produce 1,4-diphenylbutadiene (5b) with an 88% yield. Conversely, in the reductive desulfonylation of 1,2-diphenyethen-1-yl sulfone 4a, the para-(S)-citronellyloxy photocatalyst (S)-3 demonstrated high performance, stilbene (5a) with a 92% yield. Furthermore, the trio of pyrene photocatalysts proved to be highly efficient in promoting the reductive desulfonylation of functionalized dienyl sulfones as well as π-expanded dienyl sulfones. By judiciously selecting the suitable photocatalyst from the trio of (S)-1-3, these desulfonylation reactions could be rapidly and effectively accomplished.