Silyl anions carbon oxides

An efficient preparation of hexahydro-isoxazolo[2,3- ]pyridin-2-ones relies on the anionic addition of nucleophiles at the electrophilic carbon of the nitrone followed by cyclization of the resulting Ar-oxide. As shown by results collected in Scheme 31, various nucleophiles can be engaged in the reaction and include enolates 95 <20020L3119> or 98 <2000BML1811>, silyl acetals 101 <2003TL2817>, or ynolates 103 <20020L3119> (Scheme 31). 

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

Employing the lactone 279, the trimethylsilyl cation generates a positive charge at the anomeric carbon position of 280. The corresponding triflate anion recombines with one of the silyl ethers in 281 and yields the alk oxide 282 and a molecule of catalyst. After nucleophilic attack, 283 is formed, and by a subsequent intramolecular reaction, the orthoester 284 is obtained simultaneously releasing a molecule of hexamethyldisil-oxane. 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

On the other hand, elimination under basic conditions should proceed in a syn manner. Two possible pathways have been postulated for the elimination of a silyl-oxide moiety after deprotonation of a hydroxy group with an equimolar amount of base (Scheme 2.4). One is the stepwise 1,3-migration of a silyl group from carbon to oxygen, followed by elimination of a trimethylsilyloxide moiety. The other involves the formation of a pentacoordinate 1,2-oxasiletanide 2, which is in equilibrium with the j8-silylalkoxide anion 1, and extrusion of the trimethylsilyloxide moiety therefrom. The reaction mechanism has not yet been entirely elucidated. 

The proposed catalytic cycle [42] is analogous to that shown in Scheme 5.6, except for the additional release of an enolate anion due to the fluoride-induced desilylation. Oxidative addition of allyl carbonates leads to the formation of the allyl complex 78, COj, and an alkoxide RO . The fluoride source and the alkoxide RO are capable of liberating an enolate anion by desilylation. This explains why substoichiometric amounts of Bu4NPhgSiF2 are sufficient to maintain the catalytic cycle that is displayed in Scheme 5.25 for the allylation of 2-methylcyclohexanone through the silyl enol ether. The carbon-carbon bond-forming step is assumed to occur by a collapse of the ion pair 79 consisting of the cationic allylpalladium complex and the enolate anion. Aside from these ionic species, covalently bound palladium enolates were also discussed. 

Carbon-14-labeled TMS acetates are most directly prepared from the corresponding haloacetates, as preparation by the reaction of lithiated acetate esters with trimethylsilyl chloride (TMSCl) would result in a mixture of C- and O-silylated products. The preferred procedme involves reaction of TMSCl with the organozinc reagent of the haloacetate in a modified Reformatsky reaction. An application of this Peterson-olefination sequence is illustrated by the preparation of the antipsoriatic vitamin D derivative 204. Reaction of the anion of benzyl trimethylsilyl[l, 2-"C2]acetate (20ft) with TBS-protected cyclohexa-none-3,5-diol afforded the expected benzyl cyclohexylidene[l,2-"C2]acetate t201i in moderate yield. The subsequent transformation of 201 to the phosphine oxide 202 followed... 

---