The Benzylation and Phenylation Reactions of Unsaturated Compounds Using Organo Monohalides and Polynuclear Iron Carbonyl

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Abstract
The reaction of iodobenzene and benzyl halides (Cl, Br and I) with Fe3(CO)12 (mole ratio; Fe3(CO)12/R–X=1/3) gave the corresponding symmetrical ketones (18–55.2%). The increasing order of the reactivity of the organo halides to Fe3(CO)12 was as follows: iodobenzene<benzyl chloride<benzyl bromide<benzyl iodide. In the reaction of benzyl chloride with Fe3(CO)12, when two-thirds of the Fe3(CO)12 had been used, or moles equal to benzyl chloride, the yields of the ketones were increased to 87% (total yield). The same reaction in the presence of olefin produced benzylation or phenylation products of the olefin; for example, the reaction of benzyl iodide with Fe3(CO)12 in the presence of acrylonitrile (in tetrahydrofuran) gave 4-phenylbutyronitrile (30.2%) and 4-cyano-6-phenylhexanonitrile (10.0%). When olefin with a higher coordinating ability to iron carbonyl was used, the yields of the benzylated adducts of the olefin decreased, while when the more reactive organo halide was used, the yields increassed. Thus, it was postulated that organo halide reacted with Fe3(CO)12 to form the σ-complex, benzyl(or phenyl)iron carbonyl halide, as an intermediate to the benzylation (or phenylation) products. The benzylation and phenylation reactions proceeded smoothly in a polar solvent (dimethylformamide or ethanol) to give high yields. The benzylation of phenylacetylene did not occur in benzene, but in a polar solvent it proceeded, giving 1,3-diphenylpropane and -propene. The mechanistic aspects of these reactions are discussed below.