Silyl radical trialkylsilyl radicals

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%). 

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

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. 

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. 

Investigations of phthalimide photochemistry in the authors laboratories have concentrated on SET-induced excited-state reactions with a-trialkylsilyl substituted ethers, thioethers, and amines. These efforts have uncovered several interesting photochemical reactions. For example, in an early effort simple, a-silyl-substituted ethers, thioethers, and amines were observed to undergo efficient photoaddition reactions with phthalimide and its N-methyl derivative (Scheme 5.4) [20]. In these processes, thermodynamically/kinetically driven SET from the a-silyl donors 13 to excited phthalimide leads to formation of ion radical pairs 16 (Scheme 5.5). Solvent (MeOH) promoted desilylation of the cation radicals 16 and protonation of the phthalimide anion radicals 15 then provides radical pairs 17, the direct precursor of adduct 14. 

---