Magnetic Resonance in Chemistry p. 616 - 623 (1986)
Update date:2022-08-05
Topics:
Kamphuis, J.
Bos, H. J. T.
Huizer, A. H.
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.
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