Radical metal-centered radicals

The reactions of atoms or radicals with silicon hydrides, germanium hydrides, and tin hydrides are the key steps in formation of the metal-centered radicals [Eq. (1)]. Silyl radicals play a strategic role in diverse areas of science, from the production of silicon-containing ceramics to applications in polymers and organic synthesis.1 Tin hydrides have been widely applied in synthesis in radical chain reactions that were well established decades ago.2,3 Germanium hydrides have been less commonly employed but provide some attractive features for organic synthesis. 

Scheme 9 Uses of a transition-metal centered radical for the cyclization of an epoxyolefin.32...

W. A. Nugent and T. V. RajanBabu, Transition metal-centered radicals in organic synthesis— Ti(lH)-induced cyclization of epoxyolefins, J. Am. Chem. Soc. 770 8561 (1988). 

Stability of the examined complexes is usually explained by the 18-electron rule (details in Sec. 1.2.3) [3,4,21b, 184h,221-223], It has been observed that a large number of coordination compounds are formed by metal-centered radicals containing from seven to 21 electrons [223]. Also important is that many 19-electron compounds are among them, such as, for instance, [FeCp(r 6-arene)]+(PF6)A [CoCp F BR)], and [Co(C5H4BR)2], in which the fragments 135 137 are contained in due order [223] ... 

Class (i) may be divided into compounds in which the unpaired electron is localized principally on the metal M (I) (see Section II) or remote from the metal (II) (see Section III). The former belong to a wider group, the metal-centered radicals, encompassing not only, for example, the or-... 

The major secondary sources of literature are listed in Table I. The single most important item is the two-volume compendium edited by Kochi 126). It will be noticed that the role of organic free radicals in transition metal chemistry, except for Co(II) derivatives, is a rather new area and constitutes a major theme of our article (Sections III-V). With regard to metal-centered radicals (Section II), we concentrate particularly on those that have considerable kinetic stability, e.g., Sn[CH(SiMe3)2].-i, another recent development 48). 

Numerous transient paramagnetic compounds are known and some of these are also shown in Table IV there is an overlap with Section V, and Table IV does not duplicate material which is more conveniently treated later. The distinction is arbitrary, but we shall defer consideration of transient transition metal-centered radicals, e.g., Pt(I), if their formation is primarily of interest in connection with an organometallic mechanistic study, e.g., the oxidative addition of an alkyl halide to a Pt(0) substrate. The designation of metal oxidation state in Table IV is somewhat formal in many cases it might be more appropriate to describe a complex as derived from a paramagnetic ligand, such as a nitroxide or ketyl. 

Metal alkyl or alkoxy radicals in which the unpaired electron is localized on carbon, a or /3 to the metal, are known for Li, Mg, B, Al, Si, Ge, Sn, Pb, and As (for a selection, see Table II) but not at present for a transition metal. Methods of generating such species are by y-irradiation of the solid at low temperature or by hydrogen abstraction, for example by Ph2C—6 or Bu O radicals, or the addition of a metal-centered radical (e.g., by photolysis of the metal hydride with Buj02) to an unsaturated substrate ... 

Metal-centered radical anions have recently received increased interest. Usually formed by SET reactions from derivatives of tetravalent or divalent metal-14 (equation 51)68,69, they have been characterized by ESR and radiofluorescence. 

While these reactions are of limited use in the synthesis of complex molecules, they are very interesting from a mechanistic point of view. It has been firmly established that benzylic or allylic radicals and metal centered radicals are formed as intermediates through hydrogen atom transfer from the M-H bond to the C-C double bond. Interestingly, the addition of the hydrogen atom is reversible as demonstrated by isotope scrambling with deuterated olefins. If the radicals possess multiple sites for hydrogen atom abstraction, olefin isomerization can ultimately occur. HAT to unactivated olefins was not observed. 

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

Finally, the photolytic cleavage of the intermetallic bond in a Ptm species unsupported by covalent bridges, can lead to formation of highly reactive 17-electron metal-centered radicals which can also activate C-M and C-X bonds [14]. 

Baird MC. Seventeen-electron metal-centered radicals. Chem Rev 1988 88 1217-1227. 

Recent interest in metal-centered radicals has prompted numerons studies of the 17-electron monomers C5H5)Cr(CO)3 and (jj -C5Me5)Cr(CO)3, present to varions extents at ambient temperature via dissociation of the parent dimers (e. g. eqnation 10). ... 

---