Silyl radical species

The generation of silyl radicals, species which have proven to be of great value in both mechanistic and synthetic studies, was reported independently in 1947 by Sommer and Whitmore (equation 63), " and by Barry et al. at Dovvf Chemical. " These are useful for many purposes, including halide reductions. " ... 

From Free Radicals RR R"E This last synthetic route, involving the one-electron oxidation of the free radicals RR R"E with an appropriate Lewis acid such as PhjC, is one of the best methods for the extremely fast and clean formation of the element-centered cations RR R"E+. Although this approach requires the presence of the radical species as readily available starting materials, the recent synthesis of stable silyl-substituted radicals of the type (r-Bu2MeSi)3E (E = Si, Ge, Sn) (see Section 2.2.4.1.2) made such an approach a rather attractive and easily accessible synthetic route to the stable and free (r-Bu2MeSi)3E+ cations (Scheme 2.6)... 

The development of sophisticated new experimental techniques during the last decade has made possible the isolation of stable representatives of the free radical species featuring an nnpaired electron on the heavier group 14 elements, that is, silyl, germyl, and stannyl radicals. This great progress in the isolation of the stable radicals opens unprecedented possibilities for their structural characterization in the crystalline form, which in tnrn enables the direct comparison of the fundamental differences and similarities between the solntion and solid state strnctnres of the free radical species. " ... 

Second, nitroxyl radicals, which are generated either by a one-electron oxidation of SENAs (Eq. 1, Scheme 3.98) or by the addition of radical species to silyl nitronates (Eq. 2, Scheme 3.98), are rather stable and, consequently, can act as kinetically independent species. 

It should be recognized that the stability of cation radicals generated by anodic oxidation is also affected by jS-silyl substitution. Stabilization of car-bocations by a silyl group situated at the -position is well known as the / effect . The interaction of the C Si a orbital with the empty p orbital of the carbon stabilizes the carbocation. Therefore, we can expect similar effects of silicon for cation radical species. The interaction of the filled C-Si a orbital with the half-filled orbital of the carbon may stabilize the cation radical. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

The addition of silyl radicals to thiocarbonyl derivatives is a facile process leading to a-silylthio adducts (Reaction 5.37). This elementary reaction is the initial step of the radical chain deoxygenation of alcohols or Barton McCombie reaction (see Section 4.3.3 for more details). However, rate constants for the formation of these adducts are limited to the value for the reaction of (TMS)3Si radical with the xanthate c-C6HuOC(S)SMe (Table 5.3), a reaction that is also found to be reversible [15]. Structural information on the a-silylthio adducts as well as some kinetic data for the decay reactions of these species have been obtained by EPR spectroscopy [9,72]. 

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. 

Figure 8.2b corresponds to the superimposition of signals from at least two persistent paramagnetic species. The nature of these species is still unclear even if the g factors are consistent only with silyl radicals carrying three silyl groups as in 6. Mechanistic schemes for their formation could be suggested based on the addition of transient silyl radicals to the transient disilenes generated photolytically [14]. 

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

A complete stereocontrol is achieved by addition of the bulky silyl radical (Me3Si)3Si to a chiral and conformationally flexible electron-deficient olefin 163, as shown in equation 68213. Replacement of (Me3Si)3Si with a less sterically hindered (n-Bu)3Sn gives a mixture of syn and anti diastereomeric adducts 164 and 165 in a ratio of 7 3. The A values (kcalmol-1) of the tin, carbon and silicon species follow the order (n-Bu Sn (1.1) < Me (1.7) < Me3Si (2.5), and the bond length for C—Sn (2.2 A) is longer than that for C-Si (1.85 A)214 215. 

The product types vary as a function of the degree and type of alkyl/aryl substitution at silicon. Thus Ph2t-BuSi—SiPh2(t-Bu) only yields silyl radicals and no Si=C species when irradiated139. 

The most likely course of this conversion involves H abstraction by bromine atoms. The resulting radical may undergo homolysis of the fullerene-silicon bond as outlined in Scheme 57. The silyl radical thus formed then undergoes intramolecular cyclization to give 132. While this type of intramolecular reaction readily occurs with radical species, it is not a common one in silicon ring systems. The Si-Si bond of 132 then must react with bromine followed by hydrolysis to give siloxane 131. 

In a study dealing with the construction of a 5 -aza-naphthacene derivative, conditions of the radical reaction did not affect the SMA framework present in the starting material. This result could be explained either by the steric hindrance of the silyl group that inhibits the abstraction of the benzylic hydrogen atom to create the corresponding radical species, or this radical, if created, is too stable to react efficiently.153... 

Silyl radicals (1) are generally tetrahedral species. Deviation angles (y) have been calculated (UHF/DZP the DZP basis set is a double-zeta basis set with polarization functions) by Guerra to range from 13.4°[(H3Si)3Si ] to 22.7°[(PH2)3Si ]5. 

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