Trialkylsilyl ethers oxidation

Cyclopropanoxyl radicals have also been generated by treatment of cyclopropanols 38 with manganese(III) pyridine-2-carboxylate [Mn(pic)j]. The 3-oxopropyl radicals, formed by rapid -scission, added to a variety of alkenes before a second oxidation transformed them to the corresponding adduct carbocations. Alkenes containing trialkylsilyl ether groups were particularly useful because loss of the trialkylsilyl cation led to efficient formation of 1,5-diones... 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

The protection of alcohols as silyl ethers has been reviewed62, as have the relative stabilities of the different trialkylsilyl groups63. Their stability under alcohol oxidation conditions and their oxidative deprotection have been discussed64. Methods for selective deprotection of the various silyl ethers have been the subject of an excellent review65. 

Carbon-centered nucleophiles can also be used to advantage in the reaction with epoxides. For example, the lithium enolate of cyclohexanone 96 engages in nucleophilic attack of cyclohexene oxide 90 in the presence of boron trifluoride etherate to give the ketol 97 in 76% yield with predominant syn stereochemistry about the newly formed carbon-carbon bond <03JOC3049>. In addition, a novel trimethylaluminum / trialkylsilyl triflate system has been reported for the one-pot alkylation and silylation of epoxides, as exemplified by the conversion of alkenyl epoxide 98 to the homologous silyl ether 99. The methyl group is delivered via backside attack on the less substituted terminus of the epoxide <03OL3265>. 

Secondly it has been found that a number of trialkylsilyl groups can be added stereoselectively to the /j-position of the C —C double bond in dioxanone 10 (derived from enantiomerically pure 3-hydroxybutanoic acid)23. In this case the presumed silylcuprate is generated from a chlorosilane and a higher-order organocuprate. In the example shown, with addition of phenyldimethylsilyl chloride, a diastereomeric ratio of 91 9 was obtained. Improved ratios were obtained with other groups (e g., 98 2 for trimethylsily 1), but unfortunately none of the latter were susceptible to oxidative cleavage. The first two steps of the procedure produce a mixture of 11 and its silyl enol ether, which is then desilylated to 11 by treatment with tetrabutylammonium fluoride. 

Trialkylsilyl enolates can be oxidized to the corresponding a, S-nnsatnrated ketones with trityl tetrafluoborate or trityl methyl ether in the presence of BFsOEta. It has been observed that the use of trityl cations often leaves snbstantial amonnts of saturated ketones. The amonnt of these species can be rednced by the nse of ddq as in the case of... 

This reaction was initially reported by Flood in 1933. It is a synthesis of trialkylsilyl halide involving the treatment of the mixture of di-trialkylsilyl oxide and concentrated sulfuric acid with sodium halide or ammonium halide. The resulting trialkylsilyl halide can be extracted by petroleum ether and then purified via distillation. It was found that when di-trimethylsilyl oxide is mixed with concentrated sulfuric acid, trimethylsilyl sulfate can be isolated as a white crystalline (m.p., Sb-SS C), which forms trimethylsilyl halide when it reacts with ammonium halide. The reaction has been improved by continuous extraction of the reaction mixture with pentane to yield more and purer trialkylsilyl halide and by the addition of ammonium bisulfate to the reaction mixture. ... 

Preparative Method the best procedure for the preparation of trimethylsilylacetone is the reaction of trimethylsilyl-methylmagnesium chloride or bromide with acetic anhydride. Other useful preparative methods for a-trimethylsilyl ketones involve the reaction of trimethylsilylmethylmagnesium chloride with acid chlorides, or with aldehydes followed by oxidation of the resulting /3-hydroxysilane to the ketone. When the trialkylsilyl group is very large, particularly the triisopropylsilyl group, a rearrangement from the silyl enol ether to the a-silyl ketone is possible. ... 

Starting materials, like 141, which are accessible from alkenes by azido-selenenylation, afforded only in a few cases vinyl azides exclusively as shown by the example 141 142 (Scheme 5.19). In most cases, after oxidation of the vicinal phenylseleno azides and elimination reaction on the intermediate selenoxide, mixtures of allyl and vinyl azides were obtained. Ring opening of trialkylsilyl-substituted epoxides was utilized several times for stereoselective synthesis of vinyl azides. Thus, treatment of the frani -contigured oxiranes 143 with azidotrimethylsilane and boron trifluoride etherate yielded cis-conflgured products 145. This result was explained by the intermediate 144 which should undergo anti-elimination. On the other hand, when subjected to sodium azide in dimethylformamide, epoxides 146 were transformed into vinyl azides 148 via... 

PET reactions of a-trialkylsilyl (TMS) methoxy-substituted phthalimides 9 have been developed by Yoon and Mariano (Scheme 3). The introduction of the a-TMS group lowers the oxidation potential of the ether oxygen by about 500 mSP and, consequently, electron transfer from the heteroatom to the electronically excited phthalimide becomes energetically feasible. The incorporation of the trialkylsilyl-leaving group additionally enhances the efficiency and regioselectivity of the entire photocyclization and improves the yields and conversion rates in comparison with N-alkoxyalkylphthalimides 5. ... 

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