Trialkylsilyl radical

Trialkylsilyl radicals are known to be strongly bent out of the plane (cr-type structure 5). The pyramidal structure of trialkylsilyl radicals (RaSi ) was first indicated by chirality studies on optically active compounds containing... 

The addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is subject to a steric effect [49], This shows itself in the greater Ee0 value for Et3Si addition to RCH=CHR compared with that for the addition of the same radical to CH2=CHR. The contribution of... 

No effect of this type is manifested for the addition of alkyl radicals to the same alkenes. Evidently, the steric effect involved in the addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is due to the repulsion between the carbon and silicon atom, caused by the large size of the silicon atom in the reaction center of the transition state. 

The addition of a trialkylsilyl radical to benzene is much less exothermic than the addition to a non-activated alkene the AH of these reactions has been evaluated to be ca —50kJ/mol [9,18]. However, the rate constants for the addition of EtsSi radicals to aromatic and heteroaromatic compounds are similar to those of non-activated alkenes, i.e., 10 s [24]. Furthermore,... 

Trialkylsilyl radicals add to alkyl isocyanate to form imidoyl radicals 56 (Reaction 5.38). Detailed EPR studies established intermediates 56 to be strongly bent at the carbon bearing the unpaired electron [76], The absolute rate constant for the reaction of Et3Si radical with ieri-butyl isocyanate was found to be 5.5 x 10 s at 27 °C [13], whereas the relative rate of the addition of MesSi radicals to alkyl isocyanates was found to decrease in the... 

Trialkylsilyl radicals prepared from the corresponding silanes add to Cjq to form R3SiCgfl (Scheme 6.10) [63]. The Si-CgQ bond is slightly longer than the C-Cjq bond in an alkyl fullerenyl radical, and the rotahon barrier is also lower. Hindered rotation can not be observed in the ESR spectrum. For ( Bu)3SiC5Q, the hyperfine interaction of the impaired electron with 27 equivalent protons can be seen [10, 63]. 

Rationalization via a thermochemical cycle is not straightforward here. In the case of the largely used silanes, for example, the required oxidation potential of a trialkylsilyl radical is not known. One can resort to the use of gas phase measured ionization potentials instead of the solution oxidation potentials (Eq. 5 instead of Eq. 2) - ... 

Data on the formation of trialkylsilyl radical can be obtained from Russell s data (Table XII) on chain transfer constants for various solvents in styrene polymerization. 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

Phosphoranyl radicals were observed by FPR at the end of the sixties [91]. For a long time, phosphoranyl radicals, particularly the alicyclic ones [59], were considered as elusive species. However, recently. Marque et al. [92] observed the first strongly persistent (ti/2=45 min at RT) alicyclic phosphoranyl radicals (Fig. 10) when they irradiated bis(trialkylsilyl)peroxides in the presence of tris(trialkylsilyl)phosphites. The increased lifetime of the ensuing phosphoranyl radicals is a consequence of the presence of four bulky R3SiO groups around the phosphorus. The bulkiness of the substituents hampers the dimerization and the Sh2 reaction of phosphoranyl radicals with the peroxide initiator. Furthermore, the high strength of the P-0 and 0-Si bonds results in slow a- and p-scissions [93]. 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

Phenil, radical (a generic substituent), nitro, sulpbonic acid, diazo, azide, trialkylsilyl. 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

Table 1.2 EPR data for a variety of tris(trialkylsilyl)silyl radicals...

Trialkylsilyl substituents have proved very useful for directing the course of cy-clizations involving vinyl ethers. The trialkylsilyl group stabilises a radical centre but destabilises a carbonium ion next to the silicon atom. An example of the di-... 

To overcome these difficulties the application ofbis-(trialkylsilyl)-amides was recently recommended [770]. The latter ligands are much weaker Jt-do-nors and, therefore, are easier hydrolizable and higher thermally stable. Due to highly ramified radicals, attached to the nitrogen atom of the amide, they are usually monomeric and highly volatile and thus can be efficiently purified by distillation. 

Similar to the deprotonation of enol radical cations, silyl enol ether radical cations can undergo loss of trialkylsilyl cations (most likely not as ionic silicenium ions [190]). Based on photoinduced electron transfer (PET), Gass-man devised a strategy for the selective deprotection of trimethylsilyl enol ethers in the presence of trimethylsilyl ethers [191]. Using 1-cyanonapthalene (1-CN) ( = 1.84 V) in acetonitrile/methanol or acetonitrile/water trimethylsilyl enol ether 93 ( j = 1.29 V) readily afforded cyclohexanone 64 in 60%. Mechanistically it was proposed that the silyl enol ether radical cation 93 undergoes O-Si bond cleavage, most likely induced by added methanol [192-194], and that radical 66 abstracts a hydrogen from methanol. Alternatively, back electron transfer from 1-CN - to 66 would yield the enolate of cyclohexanone which should be readily protonated by the solvent. 

Reduction of tetrakis(trialkylsilyl) disilene with Li in THF or K in DME gives the corresponding 1,2-dianions, together with the anion radicals [Eq. (31)1 (72). 

One of the most frequently eneountered reactions is that with proton sources, as observed with arenes (Birch reduction) [189], aldehydes [ 190], alkynes [2d], fullerenes [191], ketones [192] (even enantioselective protonation of ketyl radical anions [193]), nitriles [194], nitro [195] and nitroso compounds [196], and olefins [197]. Protons are often replaced as electrophiles by trialkylsilyl chloride [198],... 

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