Coupling of Alkyl Iodides with a,-Unsaturated Compounds

1 Coupling of Alkyl Iodides with a, Unsaturated Esters 15  

A small amount of nickel boride (0.05 equiv) is enough to fulfill the coupling reaction at ambient temperatures with methyl crotonate 31. However, the coupling reaction with methyl acrylate 32 requires larger amounts of nickel boride (0.2 equiv) for better yields, presumably because alkyl iodides cannot compete with acrylates for the small amount of nickel boride catalyst, since the double bond of acrylates is rapidly hydrogenated even at 0°C by this system [16]. 

The same tendency was also observed in the coupling reactions with a,y -unsaturated nitriles and ketones. Equally good yields were observed in the couplings of alkyl iodides 33 and 34 which contain acetal or nitrile functional groups. 

The slowness of the reaction at room temperature is probably due to the preferential adsorption and/or reduction of crotononitrile on nickel boride, which hinders the radical formation reaction of cyclohexyl iodide at room temperature. Cyclohexyl iodide is quantitatively reduced to cyclohexane by BER-Ni2B (cat.) within 1 h at room temperature in the absence of excess crotononitrile [12a]. 

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