Trivalent organophosphorus reagent induced pinacol rearrangement of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one

In this Letter we report on the reaction of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one with various trivalent organophosphorus derivatives, with an emphasis on the final products depending on the applied reagents. No reaction occurred upon treatment with either triphenyl- or tricyclohexylphosphine, whereas addition of triethyl phosphite or tri(n-butyl)phosphine resulted in pinacol rearrangement. Structure elucidation of the isolated 5H-spiro(benzo[1,2-b:6,5-b′]dithiophene-4,4′-cyclopenta[2,1-b:3,4-b′]dithiophen)-5-one was achieved by combined NMR experiments, theoretical chemical shift and geometry calculations, and single crystal X-ray analysis.