A novel reductive cyclization of arylmethylidenemalononitrile promoted by samarium diiodide

Carbon]carbon bond formation is the essence of organic synthesis and the reductive dimerization of carbonyl derivatives is a most valuable method for establishing carbon]carbon bonds. Since the time Kagan demonstrated a simple preparation of samarium diiodide from samarium metal and 1,2-diiodoethane, SmI2 has been developed as a mild, neutral and versatile single electron transfer reductant. It has been widely used in synthetic reactions, and is especially useful for ring closure reactions, carbon]carbon bond formation and in reaction stereocontrol. The reactivity of samarium diiodide towards various nitrogen compounds including nitro compounds, azocompounds, hydrazones, oximes, imines, azides 6 and hydroxylamines 7 has been examined. Nitro groups are known to be easily reduced by samarium diiodide. The cyano group however, is relatively more stable to samarium diiodide than a nitro group and could not be reduced by this reagent. Souppe and Kagan 8 reported that aromatic and aliphatic nitriles are inert in the presence of SmI2 and mor p-nitrobenzonitrile could be selectively reduced to the corresponding cyanoanilines in almost quantitative yields. Kang and Hong 9 reported the decyanation of α-cyano substituted nitrile derivatives by samarium diiodide. However, no cyclization products were observed. It has been known that α,β-unsaturated carboxylic acid derivatives are reduced with SmI2 to the corresponding saturated compounds. 10