(Alkynyl)dicobalt Hexacarbonyl-Mediated Radical Cyclizations

Radicals flanked by the –(alkynyl)Co2(CO)6 unit add intramolecularly to carbon–carbon double bonds with unusual stereo- and regioselectivity. Cationic complex 13a (R=CO2Me) reacts with Zn to produce exclusively the trans-5-exo product 14a. Labile propargyl bromide complexes 16a–d, prepared from the alcohols 7a–d, undergo cyclization with Et3B/O2/Ph2SiH2 or upon irradiation. Under the latter conditions atom transfer products (17a–c, 18c,d) were obtained exclusively, whose regio- and stereochemistry were dependant on the olefinic acceptor. Thus, irradiation of the ester trans-16a or the phenyl derivative 16b (E/Z mixture) gives the trans-cyclopentyl derivatives 17a and 17b; the bromide 16c (R=Me) affords a mixture of the trans-cyclopentyl compound 17c and a comparable amount of the cyclohexyl derivative 18c; and the parent bromide 16d (R=H) is converted exclusively to the cyclohexyl derivative 18d. Heptenyl derivative 20d (R=H) cyclizes exclusively to the 7-endo cycloheptyl derivative 21d. Limited mechanistic experiments suggest the operation of a radical chain, atom transfer mechanism with a product-like transition state. A tandem cyclization/allylation reaction has been demonstrated in the reaction of acyclic bromide 16a with CpFe(CO)2(η1-allyl).