Electrochemical reductive silylation

A route to aryltrimethylsilanes was also described by this process [3]. In THF-HMPA, the carbanions Ar generated by cathodic reduction are trapped by MejSiCl to form ArSiMoj. This reaction is possible because MejSiCl is reduced at a more negative potential than aryl halides. In THF-DMPU instead of THF-HMPA the electrochemical reductive silylation of trifluoromethylbenzene led to the corresponding mono-, bis- or tris-trimethylsilyl derivatives, respectively, PhCFjSiMej, PhCF(SiMe3)2 and PhC(SiMe3)3 [4]. 

In the cathodic reduction of activated olefins, chlorosilanes also act as trapping agents of anionic intermediates. Nishiguchi and coworkers described the electrochemical reduction of a,/ -unsaturated esters, nitriles, and ketones in the presence of Me3SiCl using a reactive metal anode (Mg, Zn, Al) in an undivided cell to afford the silylated compounds [78]. This reaction provides a valuable method for the introduction of a silyl group into activated olefins. 

The electrochemical reduction of R2SiCl2 in the presence of 2,3-dimethylbutadiene proceeds smoothly to give the sila-cyclopentene derivatives [Eq. (53)] [193]. Probably the initial reduction of R2SiCl2 produces an silyl anion, which adds to the 1,3-diene. The intramolecular displacement reaction of the resulting allylic carbanion with the chlorosi-lane moiety gives the cyclized product. 

Preparative electrochemical reduction of arylsilanes in methylamine gives Birch-type products [Eq. (56)] [198]. The hydrogen atom is introduced preferentially on the carbon adjacent to the silyl group, and this regioselectivity is explained in terms of the stabilization of the radical anion by the neighboring silyl group. 

Touster, J. Ph.D. Thesis. Electrochemical and Sonochemical Reductive Silylations of Geminal Dihalides (Wesleyan University 1991). 

The silylation of the fluorobenzene can also be achieved by electrochemical reduction (ref. 14). 

Alkenylation of aldehydes. Alkenylchromium(III) species are generated by the electrochemical reaction of CrCl2-NiBr2 with alkenyl bromides in DMF. Their reaction with ArCHO in the presence of Me SiCl gives silyl ethers of allylic alcohols. Instead of electrochemical reduction, aluminum can be employed as the electron source. An alternative promoter is tetrakis(dimethylamino)ethylene. ... 

Reductive silylation. o-Dichlorobenzene undergoes electrochemical trimethyl- ilylation. Depending on reaction conditions, various numbers of the silyl groups can be introduced to the ring. 

However, a new route using silylated ylides (Scheme 2) provides pure compounds and hence trialkyl substituted alkenes. Ylides have been generated from the corresponding phosphonium salts by electrochemical reduction of fluoren-9-ylidenemethanes (15) to the dianions, which then act as bases. Generally the... 

TroU and Wiedemann dimerized allyl acrylates to the dimeric silyl ketene acetals via electrochemical reduction (Scheme 4.61) [61]. The diaUyl adipic diacids were isolated as 1 1 mixtures of diastereomers. 

Under electrochemical reduction conditions allyl acetates undergo Pd-catalyzed reduction to give alkenes in good yield (Scheme 1). The reaction may proceed via two-electron (2 e) reduction of the rr-allylpalladium intermediate. The allyl anion thus formed is protonated selectively at the terminal position to provide an internal alkene predominantly. In the presence of an excess of chlorotrimethylsilane (ca. 13 equiv), trimethylsilylation takes place at the less substituted allylic termini to provide allylsi-lanes in moderate to good yield (Scheme 1). The same products are obtained in similar yield by the protonation and silylation of a-phenylallyl acetate. The reaction may be... 

A divided cell equipped with a sintered glass separator, a carbon fiber anode, and a platinum cathode is used in order to avoid the electrochemical reduction of anodically generated carbocations. Tetrabutylammonium tetrafluoroborate is usually used as supporting electrolyte, and dichloromethane is in most cases suitable as solvent because of less nucleo-philicity and low viscosity at low temperature. Two equivalent of TfOH (trifluoromethane-sulfonic acid) to a cation precursor is added in the cathodic chamber to facilitate the reduction of protons in the cathodic process. The constant current electrolysis (20 mA) was then carried out at —78 °C with magnetic stirring until 2.0-2.5 F/mol of electricity was consumed to give a cation pool. Carbamates (oc-silyl carbamate) (Scheme 2a) [3], a-silyl ethers (Scheme 2b) [4], diarylmethanes (silylated diary Imethanes) (Scheme 2c) [5] can be... 

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

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. 

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. 

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