Trimethylsilylation, electrochemical

Trifluoromethylbenzene is also trimethylsilylated electrochemically to provide a, a-difluoro-a-(trimethylsilyl)toluene (equation 85)109. 

S-Trimethylsilyl sulphones, reactions of 629 Trisulphones, electrochemical reactions of 1028-1030... 

Phenylthio-l-trimethylsilylalkanes are easily prepared by the alkylation of (phenylthioXtrimethylsilyl)mcthane as shown in Scheme 10 [40], The treatment of (phenylthio)(trimethylsilyl)methane with butyllithium/tetramethylethylene-diamine (TMEDA) in hexane followed by the addition of alkyl halides or epoxides produces alkylation products which can be oxidized electrochemically to yield the acetals. Since acetals are readily hydrolyzed to aldehydes, (phenylthioXtrimethylsilyl)methane provides a synthon of the formyl anion. This is an alternative to the oxidative transformation of a-thiosilanes to aldehydes via Sila-Pummerer rearrangement under application of MCPBA as oxidant [40, 41]. 

Sulfides having two silyl groups are also oxidized electrochemically in methanol to give the corresponding methyl esters (Scheme 11 [36, 37]. The alkylation of (phenylthio)bis(trimethylsilyl)methane with alkyl halides followed by the anodic oxidation provides a convenient access to esters with one-carbon... 

The potentiality of the present methodology is demonstrated by the synthesis of y-undecalactone, as shown in Scheme 18 [37,47], The treatment of the THP-protected cu-hydroxyalkyl iodide with the anion of methoxybis(trimethylsilyl) methane gave the corresponding alkylation product. Acidic deprotection of the hydroxyl group followed by Swern oxidation produced the aldehyde. The aldehyde was allowed to react with heptylmagnesium bromide, and the resulting alcohol was protected as tm-butyldimethylsilyl ether. The electrochemical oxidation in methanol followed by the treatment with fluoride ion afforded the y-undeealactone. 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

This electrochemical method can be widely applicable as shown in equation 3225,38. It is noted that anodic oxidation in dichloromethane containing water provides the aldehyde directly under neutral conditions. Thus, the methoxy(trimethylsilyl)methyl anion is an equivalent to the formyl anion25,38. 

Trifluoromethyl)trimethylsilane is a highly useful trifluoromethylating reagent. Efficient electrochemical trimethylsilylation of bromotrifluoromethane has been developed (equation 84)108. 

Recently, selective electrochemical trimethylsilylation of tetrachlorocyclopropene has been achieved to provide l-(trimethylsilyl)trichlorocyclopropene, which has been converted into hexakis(trimethylsilyl)-3,3 -bicyclopropenyl upon successive electrolysis (equation 86)110-112. 

Similar electroreductive silylation of a series of unsaturated nitrogen compounds such as trimethylsilyl cyanide, cyanamide or isocyanide, bis(trimethylsilyl)carbodiimide and trimethylsilyl isocyanate provide precursors of bis(trimethylsilyl)methylamine, which is useful for organic synthesis (equations 90 and 91)115. This electrochemical method is a safer and more economical process compared with the chemical process using an alkali metal such as lithium. 

AEM-Disilylenamines of acylsilanes, which are excellent precursors for a-(trimethylsilyl)alkylamine, can be prepared electrochemically using a similar procedure (equation 92)116. 

Electrochemical oxidation of l-(trimethylsilyl)dienes in MeOH affords 129 in moderate to satisfactory yield. It is worth noting that the reaction shows some diastereoselectivity (equation 109)1". 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

Oxidative desilylationf Silyl ethers (r-butyldimethylsilyl and trimethylsilyl) of hydroquinones are converted to quinones in 60-90% yield by PCC. Electrochemical oxidation is also possible. 

Notes LOD, limit of detection MeOH, methanol EtOH, ethanol ACN, acetonitrile EtAC, ethyl acetate SPE, solid phase extraction HLB (hydrophilic lipophilic balanced) TFA, trifluoroacetic acid GC, gas chromatography TMS, trimethylsilyl MS, mass spectrometry HPLC, high-performance liquid chromatography DAD, diode array detector NMR, nuclear magnetic resonance ESI, electrospray ionization APCI, atmospheric pressure chemical ionization CE, capillary electrophoresis ECD, electrochemical detector CD, conductivity detector TLC, thin layer chromatography PDA, photodiode array detector. 

The reductive-trimethylsilylation, via either a chemical (Li/HMPA/THF) or an electrochemical (undivided cell/sacrificial anode) process, led to the synthesis of 1-(trimethylsilyl)alkylamines (RSMAs).191 Thus, l-(trimethylsilyl)f-amylamine was prepared in 67% yield from pivalaldehyde iV-(trimethylsilyl)imine, after hydrolysis of the intermediate silazane. 

Trimethylsilyl chloride has been employed in some synthetically useful reductions of aryl halides. The electrochemical reductive trimethylsilylation of aryl chlorides is a good... 

An improved method of preparation of cotarnine and opianic acid by the electrochemical oxidation of narcotine at a graphite anode in the presence of potassium dichromate has been described in a patent.3 In a study of the pharmacological effects of mescaline, the binding of the alkaloid with rat brain tissue has been examined, using 14C-labelled material.4 A technique for the separation, identification, and estimation of tyramine, methoxytyramine, and related phenethylamines by g.l.c. of their trimethylsilyl derivatives has been described in detail.5... 

Methoxy(trimethylsilyl)methane and methoxybis(trimethylsilyl)niethane have been proposed as new synthons for the formyl anion and the methoxycarbonyl anion, respectively after alkylation, C-Si cleavage is achieved by anodic oxidation. Similar electrochemical oxidative cleavage of acylsilanes reveals their potential as acyl cation synthons. Anodic oxidation of N-silylmethyl carbamates in methanol produces f -methoxymethyl carbamates in high yield. 

Not only aryl fluorides can be prepared from trimethylsilanes. Regioselective electrochemical fluorination of4-(trimethylsilyl)azetidin-2-ones 10 to give 11 has also been reported (Table 5). The electrolyses of compounds 10 are carried Out using acetonitrile as solvent. The best yields of fluorinated compounds (Table 5) are obtained when triethylamine trishydrofluoride is used as the fluorine source (with hydrogen fluoride/pyridine and triethylamine bishydro-fluoride lower yields are obtained). The stereochemistry of alkylidene side chains (Table 5, entries 3 and 4) is retained during the electrolyses. High stereoselectivity is observed in some cases (Table 5, entry 6). 

Table 5. Electrochemical Fluorination of 4-(Trimethylsilyl)azetidin-2-ones Using Triethyl-aminc Trishydrofluoride ...

The pale yellow salt, K2[SN2], is prepared from bis(trimethylsilyl)sulfur diimide in boiling dimethoxyethane (equation 10). It reacts explosively with water. The dilithium salt, Ti2[SN2], is generated from MesSnNSNSnMes and MeLi in the same solvent. The SN2 ion is also formed upon exhaustive electrochemical reduction of S4N4. 

Polymeric sulfur lutride (or polythiazyl) (2) was first reported in 1910. It is prepared by the solid-state polymerization of S2N2 at 0°C over several days (see Section 5.1.3). The polymer is also obtained in high yield from the reaction of (NSC1)3 with trimethylsilyl azide in acetoiufrile or by the electrochemical reduction of S5N5+ salts. 

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