Thermal cycloaddition reaction of xanthene-9-thione with allenes: An ESR study
✍ Scribed by J. Kamphuis; H. J. T. Bos; A. H. Huizer
- Publisher
- John Wiley and Sons
- Year
- 1986
- Tongue
- English
- Weight
- 820 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
During the parallel (2 + 2)-and (4 + 2)-cycloaddition reactions of aromatic thiones such as xanthene-9-thione with substituted allenes XCH=C==CH, (X = OR, SR, NMe,, Ph), a strong ESR signal is observed. This signal is associated with the (4 + 2)-cycloaddition reaction, affording thiopyran derivatives 3,1lb-dihydro-2-
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 H,C=C=CHX. This investigation shows that where X = Ph and M e 2 , xanthenyl radicals of structure E rud aEe produced, and where X = 0-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 (h > 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.