Tricoordinated radicals

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Tn this contribution we will deal only with the low-coordinated (tricoordinated) cations, radicals, and anions. The vast number of hypercoordinated (penta-, hexacoordinated) species is outside the scope of the present review and will not be considered. 

Acyclic Tricoordinate Radicals As one can easily note, the above examples of the cyclic radicals of Si and Ge atoms have rather particular structures featuring the cyclic delocalization of the odd electrons. The simple tricoordinate acyclic radicals of the type RsE (E = Si, Ge, Sn, Pb), lacking the stabilization effects of the cyclic Jt-delocalization, constitute another, more general and even more challenging, class of stable organometallic radicals. " Consequently, the search for such highly symmetrical species appeared to be of primary importance for synthetic chemists. 

A family of tricoordinated persilyl substituted anions of the heavier group 14 elements (RaSOaE (R = alkyl E = Si, Ge, Sn) was synthesized in recent years. Sekiguchi and co-workers employed the straightforward one-electron rednction of the corresponding radicals (r-Bu2MeSi)aE 43-45 with metallic Li or K to form anionic derivatives (r-Bu2MeSi)aE ]VI (E = Si, Ge, Sn M = Li, K) 58 M (Scheme 2.55). ... 

From these data, and the similarity of the data for the other radicals contained in Table 1, it therefore appears that, unlike carbon-centred radicals, the tricoordinate trialkyl radicals of Group IV elements have the tetrahedral structure 1. 

In this section we will review mainly the theoretical studies of tricoordinated MR3 cations, anions and radicals of group 14 elements. Relevant experimental data will be mentioned briefly to supplement and complete the theoretical discussion. 

Theoretical aspects of silyl radicals RsSi were reviewed earlier by Apeloig7. Recent developments in the chemistry of silyl radicals are reviewed in a chapter by Chatgilialoglu and Schiesser in this book29. Experimental studies of tricoordinate MR3 radicals of heavier group 14 elements (M = Ge, Sn and Pb) are reviewed by Mochida482. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

In his account, Moriarty suggested that reactions of nucleophiles with diaryliodonium salts led to a tricoordinate iodine derivative which subsequently decomposed to coupled products by either one of two mechanisms. 1 In the first one (path a), a direct ligand coupling was suggested to occur on the iodane itself. In the second mechanism (path b), the iodane collapsed to a pair of radicals which then evolved to the products. (Scheme 2.12)... 

The tricoordinate radicals of the heavier group 14 elements can be generated in one of the following ways (1) by the photolysis or thermolysis of the corresponding hydrides R3EH (E = heavier group 14 element) in the presence of the radical initiators, (2) by the y-irradiation of R4E or photolysis of RsE-Hg-ERs, or (3) by the thermolysis or photolysis of either R3E-ER3 or R3E-ER2-ER3 (4), (4) one-electron reduction of silicon cation (5), (5) one-electron oxidation of silyl anion (6), as well as by (6) one-electron reduction of alkylhalogenide (7). 

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