Alkyl migration reactions—the direct observation of the preferential migration of branched over linear alkyl groups

Three binuclear metal alkyl complexes were synthesised, viz. [Cp(CO)2Fe{CH(CH3)CH2CH2}Fe(CO)2Cp] (I), [CpFe(CO)2{CH(CH3)CH2CH2CH2}Fe(CO)2Cp] (II) and [CpMo(CO)3{CH(CH3)CH2CH2}Mo(CO)3Cp] (III) (Cp=η5-cyclopentadienyl). These compounds were then reacted with PPh3 in order to induce alkyl migration (CO insertion) reactions. The products obtained were monosubstituted phosphine acyl species. In all cases, the branched chain alkyl fragment (viz. MCH(CH3)) was observed to react preferentially over the linear fragment (viz. MCH2CH2). These results are rationalised in terms of the relative importance of steric and electronic effects in alkyl migration reactions.