Radical Addition of Ethers to Terminal Alkynes with High E-Selectivity

Direct radical additions of ethers to terminal alkynes were investigated by using Me2Zn/O-2 as radical initiator to afford 2-vinyl ether derivatives with high E-selectivity, while reversed E/Z selectivity is obtained with Et3B/O-2. Two competitive pathways are suggested for the formation of vinyl radical B: zinc-radical exchange (route a) followed by protonation provides E-configuration products exclusively through Zn(II) complexation. Hydrogen abstraction by vinyl radicals (route b) yields mainly Z isomers. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)