2-cyano-iso-propyl radical addition to alkynes

The thermolysis of azobisisobutyronitrile (AIBN) in benzene in the presence of various mono- and di-substituted acetylenes has been investigated in order to ascertain the chemical reactivity of transient 2-cyano-isopropyl radical (CPR) towards carbon-carbon triple bonds. Results show that this rather sluggish and bulky carbon centered radical successfully adds in a regioselective fashion to alkynes bearing an electron acceptor phenyl, methoxycarbonyl and trimethylsilyl substituent, but fails with alkylacetylenes and those sterically hindered. The produced vinyl radicals undergo H-abstraction reaction, alkyne addition, aromatic addition and intramolecular cyclization in a relative proportion strongly-dependent upon the nature of both 1- and 2-substituent. Two unprecedented examples of 4-exo and 6-exo vinyl radical cyclization onto the cyano triple bond are reported.