Thermal Cycloaddition Reaction of Xanthene-9-thione with Allenes: an ESR Study

Article abstract of DOI:10.1002/mrc.1260240711

During the parallel (2 + 2)- and (4 + 2)-cycloaddition reactions of aromatic thiones such as xanthene-9-thione with substituted allenes XCH=C=CH2 (X = OR, SR, NMe2, Ph), a strong ESR signal is observed.This signal is associated with the (4 + 2)-cycloaddition reaction, affording thiopyran derivatives 3,11b-dihydro-2-(X-methylene)-2H-thiopyrano<4,3,2-kl>xanthene F or 3,11b-dihydro-3X-2-methylene-2H-thiopyrano<4,3,2-kl>xanthene E.Computer analyses of this signal made possible its identification as emanating from a 9-xanthenyl radical substituted at the 1- and 9-positions.The type of xanthenyl radical that is formed in the thermal reaction E rad or F rad depends on the nature of the substituent X at the allenic system H2C=C=CHX.This investigation shows that where X = Ph and NMe2, xanthenyl radicals of structure E rad are produced, and where X = O-t-Bu and S-t-Bu xanthenyl radicals of structure F rad are produced.It is striking that this production is so selective.On irradiation (λ > 254 nm), the production of xanthenyl radicals is enhanced.The conformation of xanthenyl radicals E rad and the role of E rad and F rad in the formation of addition products E and F are discussed.