Silyl radical benzene

The persistent silyl radicals RsSi were synthesized by the reaction of Si2Cl6 with 6 equivalents of RLi in benzene followed by photolysis of the reaction mixture (Scheme 2.27,... 

The addition of silyl radicals to double bonds in benzene or substituted benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has been detected by both EPR and optical techniques [21,22]. Similar cyclohex-adienyl-type intermediates have also been detected with heteroaromatics like furan and thiophene [23]. 

The adduct of silyl radicals to 4-substituted pyridines and pyrazine monitored by EPR results from the attack at the nitrogen atom to give radicals 52 and 53, respectively [68,69]. The rate constant for the addition of Et3Si radical to pyridine is about three times faster than for benzene (Table 5.3) [24]. 

Disilanaphthalenes can be prepared by the thermolysis of a disilacyclopropane in deuteriated benzene the reaction may be monitored by NMR spectroscopy <83JA7776>. A mechanism of formation is proposed which involves Si—Si bond rupture followed by C—Si bond formation via a silyl radical. Thermolysis of 2-(mesityl)-2-(phenylethynyl)hexamethyltrisilane also results in two isomeric 1,3-disilanaphthalenes <860M1518> but better yields are achieved if the reaction (either thermally or photochemically) is catalyzed by a nickel complex <86JA7417, 890M2050>. Nickel-... 

As mentioned above, a crucial requirement in the photoacoustic experiment is that the net reaction 2c must be fast when compared to the response of the tranducers. The rate constants for the slower reaction, 2b, were available for most of the silanes studied and allowed the concentrations of those substrates to be calculated so as to meet the above condition. Another important issue concerns the possible addition of silyl radicals to solvent molecules. Since this reaction is predicted to be exothermic by ca. 60 kJ/mol, its occurence would have a significant impact on the results. However, parallel experiments involving triethylsilane, carried out in benzene and also using the silane as solvent, led to similar AH values, indicating that the addition to benzene was irrelevant on the timescale of the photoacoustic experiment. 

With the photoirradiation on the pristine Cgg and disilane in benzene, 1,16-addition product was obtained via the silyl radical formation (Scheme 3) [64]. These bissily-lated fullerenes show lower oxidation potential due to the electron-donating nature of silicon addends. Actually, laser flash photolysis study on the photophysical properties of these bissilylated fullerenes shows a substantial change of the photoexcited state properties, such as triplet absorption and fluorescence spectra (Fig. 11) [65]. Thus, bissilylation changes properties of the fullerenes to a great extent. 

An early - but mechanistically interesting - construction of a bicyclo[3.1.0]oxa-hexane by a domino radical cyclization was presented by Luh s group [50]. The addition of tributyl tin and AIBN to a solution of bromides 3-111 in refluxing benzene gave 3-114 as single diastereoisomers in acceptable yields via the intermediates 3-112 and 3-113 (Scheme 3.29). It is important that the cyclopropyl carbinyl radical intermediate has the correct stability and reactivity, which is achieved by the a-silyl substituent. 

Supersilyl substituents also stabilize negative charges extremely well In the radical anion of 1,4-di(tris(trimethylsilyl)silyl)benzene - as proven by ESR/ENDOR coupling constants [5a,c] augmented by HMO estimates for the "blind" centers (2 O) [5a,c] - more than half of the it spin population p is located in its Si(SiR3)3 groups. 

Arylsilane radical anions undergo cleavage and coupling reactions, usually under conditions where excess reducing agent is available. Reduction of phenylsilane, diphenylsilane, or triphenylsilane with sodium-potassium alloy under preparative conditions gives high yields of tetraphenylsilane (7). In the reduction of phenylsilanes, the appearance of 1,4-bis(silyl)benzene radical anions is frequently observed (135, 35, 86, 97, 75, 120, 100). Typical results are shown in Table II. 

The ESR/ENDOR TT-spin populations p in the radical anion of 1.4-di[tris(trimethyl-silyl)silyl]benzene (Scheme 7a) and the corresponding squared HMO coefficients (cjM)2, which allow one to augment the blind centers without a CH bond (Scheme 7b), prove that more than half of the 7r-spin population p7 is located in the [(HjC Si Si half-shell... 

Tsai and coworkers89,91,246,247 reported the synthesis of cyclic silyl enol ethers and silyl ethers by using a radical cyclization followed by the radical Brook rearrangement (equation 111). The cyclization of 4-bromo-4-stannylbutyl silyl ketones 188 in benzene with a catalytic amount of tributyltin hydride and AIBN gave cyclic silyl enol ethers 18989 91 247. The whole catalytic cycle proposed is shown in equation 112. 

Radical cyclization of allylsiloxy derivatives 148 to l-oxa-2-silacycloheptanes 149 was achieved by treatment of 2-(allyldimethylsiloxy)-l,l-dibromoalkanes with Bun3SnH in the presence of a catalytic amount of triethylborane in benzene (Equation 27). An interesting stereochemical outcome was observed in the cyclization of 1-allyldimethylsi-loxy-2,2-dibromo-l-phenylpropane (R1 = H, Rz = Ph) which gave a stereoisomeric mixture of 2,2,6-trimethyl-7-phenyl-l-oxa-2-silacycloheptane (cisftrans = 87/13). These seven-membered cyclic silyl ethers and acetals were stable and could be isolated by silica-gel column chromatography <1997BCJ2255>. 

The photoinduced electron-transfer reactions are usually performed in polar solvents such as acetonitrile and benzonitrile, in which the solvation energy of free ions is greatly increased compared to non-polar solvents such as benzene. Such a difference in the solvation energy of ketene silyl acetal radical cation in benzene and... 

Since the required polymer is a functionalized polystyrene, the most sensible approach would be to co-polymerize styrene and some 3,4-dihydroxystyrene, perhaps protected as an acetal or a silyl derivative. A proportion of the benzene rings in the polystyrene would have the correct functionalization and the crown ether could be built on to them by passing a large excess of a suitable reagent, such as one of those we discussed in Problem 2 of this chapter or in the main text (p. 1456), deprotecting as required. A potassium salt would be used as a base in the final cyclization to take advantage of complexation by the crown ether. The various methods of polymerizing styrene (radical, anionic, etc.) are described in the chapter (pp. 1459-62). 

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