Grignard reagents-Iron oxide

In 2005, Cahiez and coworkers and Nagano and Hayashi showed that a catalytic amount of iron(III) chloride is efficient when a suitable oxidant is added to the reaction mixture (Figure 3). Nagano and Hayashi used an excess of 1,2-dichloroethane (the stoichiometric amount is 0.5 equivalent) in refluxing diethyl ether to couple various aryl Grignard reagents in high yield (Scheme 46). 

Iron-catalysed homo-coupling of aryl Grignard reagents has been successfully developed.96 A variety of aryl Grignard reagents have been efficiently converted into the corresponding symmetrical biaryls in the presence of FeCl3 and a stoichiometric amount of 1,2-dichloroethane as oxidant (Scheme 18). 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

Ruthenium, the homologue of iron in this group, was also shown to form complexes quite early. Ruthenocene, Ru( 5H5)2, is obtained by treatment of the acetylacetonate of tervalent ruthenium with five times the theoretical quantity of the Grignard reagent (206), or, better, by the action of cyclopentadienyl sodium on ruthenium trichloride in tetrahydrofuran (47). It forms pale yellow scales which sublime at 120° and melt at 200°. Its properties are closely similar to those of ferrocene it is soluble in organic solvents, and in the absence of air is not attacked by bases or by sulfuric or hydrochloric acid. Oxidation converts it into the pale yellow [Ru( 5H6)2] + ion. 

Grignard reagent (144) (R = H or Me) followed by oxidation gave the sulphone (145) which with allyl bromide afforded the acyclic C13 compound (146).92 This gave rise to p-ionone (142) or a- and p-irones (147) and (148), cyclization being achieved with H3P04. p-Cyclogeranyl phenyl sulphide (149) has been prepared... 

Nagano T, Hayashi T (2005) Iron-catalyzed oxidative homo-coupling of aryl grignard reagents. Oig Lett 7 491—493... 

The a-arylation of ethers was further developed by Vishwakarma [86, 87], who reported that in situ-prepared Grignard reagents react with THE to give the corresponding 2-arylated compounds (Eq. 35). Despite its low solubility, iron oxide was the catalyst of choice, and high yields were reported. 

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