Intramolecular Hydroalkoxylation and Hydroamination of Alkynes Catalyzed by Cu(I) Complexes Supported by N-Heterocyclic Carbene Ligands

Abstract

Intramolecular addition of O–H and N–H bonds across carbon–carbon triple bonds to form 5- or 6-membered rings with exocyclic methylene groups for ether products and exocyclic methyl groups for imine products is catalyzed by (IPr)Cu(Me) (IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). In a competition study, the cyclization of primary amines was found to be faster than that of alcohols. Kinetic studies for the conversion of 4-pentyn-1-ol reveal that the catalytic reaction is first-order in copper catalyst and zero-order in alkynyl alcohol, and an Eyring analysis yields ΔH‡ = 18.7(4) kcal/mol and ΔS‡ = −26(1) eu. The reaction of 5-phenyl-4-pentyn-1-ol provides (Z)-2-benzylidene-tetrahydrofuran in high yield and with quantitative stereoselectivity. Results from combined experimental and DFT studies are consistent with a mechanism that involves alkyne insertion into a Cu–Oalkoxide bond followed by protonolysis upon reaction with free alkynyl alcohol.