Allylmetallation of 1-Silylalkynes by 2-(Bromozincmethyl)-2-alkenyl Ethers followed by Pd(0)-catalyzed Cyclization: A one-pot Synthesis of 4-Methylenecyclopentenes

Article abstract of DOI:10.1016/S0040-4020(01)89857-4

Reaction of 2-(bromozincmethyl)-2-alkenyl ethers 1 a,b,c,d with 1-(trimethylsilyl)-1-alkynes 2 afforded carbometallation products 3, which were converted by Pd(0)-catalyzed cyclization to 4-methylenecyclopentenes 5.The rates of both the addition and cyclization step depend on the concentration of the organozinc compound and the preparation of 5 is best performed using a 1,4-1,8 M solution of 1.At lower concentrations reaction times must be longer and, when 1c is reacted, the addition reaction is incomplete and gives rise to a mixture of products (3 and 14), which on Pd(0)-treatment leads to a mixture of isomeric methylenecyclopentenes (5, 15 and 16). - Key Words: alkyne addition; carbometallation; 2-alkenylzinc compounds; Pd(0)-catalysis; cyclopentenes.