Generation of α-phosphonovinyl radicals and development of a new route to highly functionalized vinylphosphonates and vinylphosphonate-incorporated carbocyclic or heterocyclic compoundsvia a radical trapping sequence

Abstract

The first direct generation of synthetically useful α-phosphonovinyl radicals was achieved by treatment of α-phosphonovinyl halides with a tributyltin radical. The α-phosphonovinyl radicals 2a–d were trapped with electron-rich olefins and an electron-deficient olefin to produce α-functionalized vinylphosphonates 3a–f in 16–55% yields. The α-phosphonovinyl radicals 7e–g containing the YCH₂CHCH₂ (Y = O, CH₂, S) substituent at the β-position afforded mixtures of 5-exo and 6-endo cyclization products, 5e–g and 6e–g, in good yields. The 5-exo/6-endo product ratios increase in the following order of the β-substituent: OCH₂CHCH₂ > CH₂CH₂CHCH₂ > SCH₂CHCH₂. The effects of the β-substituents upon the cyclization reaction were discussed. Radical cyclization of α-phosphonovinyl radicals bearing functional groups such as geranyloxy, geranylthio, and (2-cyclohexen-1-yl)thio groups at the β-position afforded 5-exo, 5-exo and 6-endo, and cis-fused-5,6-ring cyclization products incorporating an α,β-unsaturated phosphonate unit within the ring, respectively, in good yields. The α-phosphonovinyl radical 20 underwent tandem radical cyclization–radical cyclization to produce a mixture of two isomeric bicyclo[4.3.0]nonenes including a vinylphosphonate moiety in high yield.