Addition reaction of 3,3,3-trifluoropropene to tetrahydrofuran. Telomeric growth via free-radical rearrangements

Abstract

Tetrahydrofuran and 3,3,3-trifluoropropene react at 20 °C to give a homologous series of compounds in which one to four trifluoropropyl side chains are linked to THF. The principal product is tetrahydro-2-(3,3,3-trifluoropropyl)furan (1), but tetrahydro-2-(3,3,3-trifluoropropyl)-2-(5,5,5-trifluoro-2-trifluoro-methylpentyl)furan (3) and tetrahydro-2,5-bis(3,3,3-trifluoropropyl)-2-(5,5,5-trifluoro-2-trifluoro-2-trifluoro-methylpentyl) furan (4) are also formed in substantial yields. We propose that the adduct (3) is the end result of a free radical rearrangement in which a hydrogen atom is transferred via a 1,5-shift from C-2 of the tetrahydrofuranyl moiety of 1,1,1-trifluoro-5-(tetrahydrofuran-2-yl)-4-trifluoromethyl-2-pentyl radical (ii). Reaction of the resulting rearranged radical (iii) with 3,3,3-trifluoropropene followed by intermolecular hydrogen atom transfer gives (3). Formation of the adduct (4) requires that there be a 1,5-hydrogen shift from C-5 of intermediate (iv) across the ring to the CH₂ĊHCF₃ side chain on C-2. Two 2 : 1 trifluoropropene–THF adducts, cis- and trans-tetrahydro-2,5-bis(3,3,3-trifluoropropyl)furan (2c) and (2d), are formed via a secondary reaction initiated by hydrogen abstraction from C-5 of (1).