A Synthetic Approach to Singlet Phospholo[3,4‐c]thiophene Biradicals via the Conversion from Phosphole to Thiophene

This paper proposes a molecular design for open‐shell singlet biradicals through a synthetic study of phospholo[3,4‐c]thiophenes in which the phosphole ring is fused with thiophene. The sulfur‐bridged diyne was converted into the titanacycle intermediate, and the subsequent treatment with chlorodiphenylphosphine provided the corresponding phosphole via the [2+2+1] cycloaddition process. The −CH₂SCH₂‐bridged phosphole oxide could be isomerized to the corresponding thiophene structure probably because of the larger aromaticity of thiophene rather than phosphole. The 4,6‐dihydrophospholo[3,4‐c]thiophene 5‐oxide derivative was triflated to give the thiophene‐fused cyclic phosphonium salt, which could generate the desired singlet biradical by treatment with potassium hydride. In addition, pai‐extension of the 4,6‐dihydrophospholo[3,4‐c]thiophene 5‐oxide unit using the Pd‐catalyzed cross‐coupling reaction with arylstannanes was accomplished, and the resultant isolable 5,6‐dihydro‐4H‐phospholo[3,4‐c]thiophene derivatives showed potent characters as precursors of singlet biradical. The generation of biradical phospholo[3,4‐c]thiophenes was supported by the [4+2] trapping reaction and the 1,2‐migration leading to a cyclic phosphaalkene.