Mechanistic Aspects of the Reaction of Anionic Iron(0)-Olefin Complexes with Organic Halides. Detection and Characterization of Paramagnetic Organometallic Intermediates

The scope and the mechanism of the reactions of [CpFe(COD)] [Li(TMEDA) (Cp = C₅H₅^-; COD = 1, 5-cyclooctadiene; TMEDA = Me₂NCH₂CH₂NMe₂), 1, with a number of organic monohalides and geminal dihalides were investigated. With monohalides organic coupling products were observed, and, in particular, the coupling and cross-coupling of benzyl and allyl halides were studied in detail. The addition of 1 equiv of benzyl halide to 1 in benzene resulted in the initial formation of [CpFe(COD)(CH2Ph)], 7. The predominant pathway involved an initial one-electron transfer from [CpFe(COD)]- to PhCH₂X to form PhCH₂X˙^- and [CpFe(COD)]˙. PhCH₂X˙^- quickly disproportionated to form PhCH₂X˙ and X^-. The benzyl radical then added to [CpFe(COD)]˙. Once formed [CpFe-(COD)(CH₂Ph)] reacted with TMEDA in a disproportionate reaction to form 0.5 equiv of the paramagnetic compound, [(TMEDA)Fe(CH₂Ph)₂], 3, and 0.5 equiv of FeCp₂. When additional benzyl (or allyl) halide was added, it reacted with [(TMEDA)Fe(CH₂Ph)₂] to form the coupled (or cross-coupled) product. This coupling reaction involved the intermediacy of benzyl (and allyl) radicals, and the experimental results were consistent with the product being formed by the coupling of two radicals in solution.