Silyl radical allylic-type

Addition of silyl radicals to cumulenes and their isoelectronic derivatives has mainly been studied by EPR spectroscopy. The adducts of MesSi radical with the two substituted allenes 54 and 55 have been recorded [73,74]. The attack occurs at the central atom affording unconjugated allyl-type radicals. In particular the adduct radical with 55 has been described as a very persistent perpendicular allyl radical [74]. 

Early work was focused to establish the preference for exo- vs endo-mode of cyclization. However, the absence of an effective method for generation of alkyl and/or aryl substituted silyl radicals made this task difficult. The reaction of prototype alkanesilane I with thermally generated t-BuO radicals at 145 °C after 4 h afforded a 48 % yield of unreacted starting material and 19 % yield of a six-membered cyclic product (Scheme 6.1) [1]. Moreover, EPR studies of the same reaction recorded the spectra at temperatures between —30 and 0°C, which were identified as the superimposition of two species having allylic-type (2) and six-membered ring (3) structures, respectively [2]. At higher temperatures radical 2 predominates therefore, the low yield detected in the product studies could derive from the extensive t-BuO attack on the allylic hydrogens. 

A convenient route to triquinanes is based on a strategy of silyl radical addition to conjugated dienes to form allylic type radicals and their subsequent intramolecular addition to C=C double bonds. By exposure of 10 to (TMS)3SiH and AIBN at 80 °C (Reaction 7.16) the triquinane 11 is obtained with an unoptimized 51 %> yield [26]. 

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

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