Silylated cyclohexadiene

Scheme 4.8 shows that the CH2 of cyclohexadiene moiety acts as the H donor with formation of cyclohexadienyl radical as the intermediate, which rapidly ejects the silyl radical upon re-aromatization. The silyl radical is able to propagate the chain by reaction with a starting halide. The hydrogen donation of silylated cyclohexadienes toward primary alkyl radicals is reported to be 1 X 10 M s at 70 °C [120], which is in accord with the reported range of 10 -10" s at room temperature for the reaction of primary and second-... 

Scheme 4.8 Silylated cyclohexadienes as radical-based reducing agents...

As we anticipated in Section 5.1.1 the class of silylated cyclohexadienes has recently been used as radical transfer hydrosilylating agents for some alkenes... 

Intramolecular hydrosilylation of alkenyloxysilyl radicals has also been investigated using silylated cyclohexadienes as the starting substrates [6]. Scheme 6.5 shows the reaction of 21 in hexane at 80-85 °C and in the presence of di-tert-hvAy hyponitrite as radical initiator. The crude reaction mixture was treated with an excess of PhLi to provide alcohol 22 in moderate yields. Intermediates 23 and 24 are the expected species involved in the 5-endo-trig cyclization. 

Analogously silylated cyclohexadiene 25 having propargyl alcohol as a pendant was used in the radical intramolecular hydrosilylation followed by ionic ring opening to provide alcohol 26 in 55 % yield (Scheme 6.6) [6]. The cyclization of silyl radical 27 to radical 28 represents an example of a 5-endo-dig process. 

Studer, A., Amrein, S. Silylated cyclohexadienes new alternatives to tributyltin hydride in free radical chemistry. Angew. Chem., Int. Ed. Engl. 2000, 39, 3080-3082. 

Studer, A., Amrein, S., Schleth, F., Schulte, T., Walton, J. C. Silylated Cyclohexadienes as New Radical Chain Reducing Reagents Preparative and Mechanistic Aspects. J. Am. Chem. Soc. 2003, 125, 5726-5733. 

Tributyltin hydride has been replaced by the silylated cyclohexadienes as the proton source7 In addition, zinc and indium have been applied to initiate similar ring-expansion reactions 7 ... 

Methyl l-bromomethyl-2-oxo-cyclopentanoate (235 mg, 1 mmol), 349 mg silylated cyclohexadiene (1.3 mmol), and 50 mg AIBN (0.3 mmol) were dissolved in 5 mL hexane under argon. The reaction mixture was refluxed for 7 h. Removal of the solvent in vacuo and purification by flash chromatography (Et20/pentane, 3 8) afforded 76 mg methyl 3-oxo-cyclohexane-l-carboxylate as a colorless oil, in a yield of 49%. 

In general, radical hydrosilylation of alkenes cannot be conducted with tri-alkylsilanes, which is due to a rather strong Si—H bond in the latter. However, the hydrosilylation of carbon-carbon multiple bonds with modified silanes such as tris(trimethylsilyl)silane has been successfully used in radical hydrosilylation (16). The reversible addition of tris(trimethylsilyl)silyl [(TMSlsSi] radical to the C=C bonds is due to the ability of this radical to isomerize alkenes. The hydrosilylation of monosubstituted and gem-disubstituted olefins are efficient processes and have been shown to proceed with high regioselectivity for both electron-rich and electron-poor olefins (140). Walton and Studer presented the results of the radical hydrosilylation with silylated cyclohexadienes as radical initiators (141). The bisvinylic methylene group acts as the hydrogen donor in these reactions. H-transfer leads to a cyclohexadienyl radical (2) that subsequently rearranges to provide er -butyldimethylsilyl radical and arene (3) (see Scheme 20) (141). 

Studer and co-workers have used silylated cyclohexadienes as radical chain reducing agents for the reduction of 44 to 45. Release of RjSi followed by re-aromatization is the driving force of the reaction. These reagents can be easily prepared and initiation can be performed with AIBN, triethylborane/02 or by simply using an air atmosphere. 

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