Reductive silylations aromatic rings

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

C-H activation at a primary benzylic site was the key step in very short syntheses of lig-nans 206 and 207 (Scheme 14.27) [138]. Even though both the substrate 203 and the vinyl-diazoacetate 204 contain very electron-rich aromatic rings, C-H activation to form 205 (43% yield and 91% ee) is still possible because the aromatic rings are sterically protected from electrophilic aromatic substitution by the carbenoid. Reduction of the ester in (S)-205 followed by global deprotection of the silyl ethers completes a highly efficient three-step asymmetric total synthesis of (-i-)-imperanene 206. Treatment of (R)-205 in a more elaborate synthetic sequence of a cascade Prins reaction/electrophilic substitution/lacto-nization results in the total synthesis of a related lignan, (-)-a-conidendrin 207. 

Polytrimethylsilylated piperidines have been obtained through the reductive silylation of quinoline (Section III.B.2.d). Among these compounds are SMA derivatives that are readily oxidized in the presence of air and hydrolyzed into pyridine derivatives. Trimethylsilyl groups on the non-aromatic ring were found to be in an all-fraws relationship.179... 

Nickel-catalyzed transformations of SCBs have been studied by Oshima and co-workers <20060L483>. Nickel-catalyzed ring opening of SCBs with aldehydes affords the corresponding alkoxyallylsilanes (Scheme 55). This transformation represents a hydrosilane-free reductive silylation of aldehydes. A wide range of aldehydes (aliphatic, aromatic, electron-rich, and electron-deficient) can be converted to akoxyallylsilanes. 

Arylsilanes serve as a typical example of this system. The reduction potentials of arylsilanes are slightly less negative than those of the parent aromatic hydrocarbons [199-203]. This seems to be attributed to the dj -pj interaction between the aromatic ring and the silicon atom. The electrochemical behavior of silyl-substituted cyclooctatetraene is interesting [204]. The second reduction potential becomes less negative by the silyl substitution. The stabilization of the dianion (aromatic 10 7r-system) by dj -p interaction seems to be responsible for this phenomenon. 

Group IV substituents, especially the trimethylsilyl group, apparently enhance the electron affinity of aromatic systems. The effect is particularly noticeable in aniline derivatives. The strong electron-releasing effect of the amino group decreases the electron affinity of the aniline derivatives and hinders reduction to the radical anions. Nitroanilines may be reduced to radical anions (65). The only other aniline radical anions that have been reported bear silyl substituents either at nitrogen (62) or on the ring (83, 85, 86). 

The hydrosilylation of methylenecyclopropanes is proposed to proceed via oxidative addition to the olefin, followed by rhodium migration across the strained cyclopropane ring, and eventual reductive elimination to give the silyl-substituted olefins. The process is compatible with aromatic and aliphatic substitution on the olefin and often requires heating. Additionally, cyclopropyl-substituted methylenecyclopropanes may be selectively silylated to give alkenes containing one, two, or three /3-silylated olefin chains. 

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