SYNTHESIS OF SPIRO DIENONES FROM INTERNAL ACETYLENE AND CYCLIC 3-IODO ENONES IN THE PRESENCE OF NICKEL BROMIDE AND ZINC POWDER

Five and six-membered cyclic 3-iodo enones undergo spiroannelation with symmetric internal acetylenes RCCR (R = Me, Et, n-Pr, and Ph) in the presence of NiBr2 and zinc powder to afford spiro[4.4]nonadienones and spiro[4.5]decadienones in good to excellent yields at temperatures of 60-100-degrees-C. While NiBr2 is a catalyst precursor, zinc powder acts as a reducing agent in the reaction. The observed catalysis is solvent-dependent, and best yields are obtained in acetonitrile or DMF. The addition of PPh3 to these nickel-catalyzed reactions inhibits the rates and decreases the yields of spirodienone product. Palladium complexes PdCl2 and PdCl2.PPh3 are also active catalyst precursors for the spiroannelation. A mechanism is proposed on the basis of the well-established nickel and palladium chemistry to account for the observed catalysis. The proposed mechanism gains support from the observation that the oxidative-addition adduct of Pd(PPh3)4 with 3-iodo-2-cyclohexen-1-one reacts with 2 equiv of 3-hexyne to give the corresponding spirodienone in essentially quantitative yield.