Ketene silyl acetals, arylation

An Q-arylalkanoate is prepared by the reaction of aryl halide or triflate with the ketene silyl acetal 74 as an alkene component. However, the reaction is explained by transmetallation of Ph - Pd—Br with 74 to generate the Pd eno-late 75, which gives the a-arylalkanoate by reductive elimination[76]. 

Silyl enol ethers and ketene silyl acetals react with tetraphenylbismuth fluoride under neutral conditions to give the corresponding a-monophenylated carbonyl compounds in good yields together with triphenylbismuthane and fluorotrimethylsilane (Equation (111)).181 The reaction is likely to be initiated by the nucleophilic attack of the fluoride ion on the silicon atom, and the regiochemistry of arylation is governed by the structure of silyl enolates. 

Unfortunately, attempts to perform this substitution reaction on cyclohexenol and geraniol led to the exclusive formation of the corresponding silyl ethers. It thus would seem that one requirement for effective carbon-carbon bond formation is that allylic alcohols be secondary and have possess y,y-disubstitution. Pearson, however, discovered a method with less restriction on the natiue of the substrate he used allylic acetates with y-mono-substitution or primary alcohols [96]. Not only ketene silyl acetals but also a diverse set of nucleophiles including aUyl silane, indoles, MOM vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propargyl silane participate in the substitution of y-aryl allylic alcohol 90 to give allylated 91 (Sch. 45). Further experimental evidence suggests that these reactions proceed via ionization to allylic carboca-tions—alcohols 90 and 92 both afforded the identical product 93. 

A -Aryl thiourea derivatives were found to behave like Lewis acids to promote the addition of ketene silyl acetals to nitrones and were used in the synthesis of monocyclic and bicyclic isoxazolidinones <2003TL2817>. 

PET reactions of aryl ketones with ketene silyl acetals and cyclo-propanone silyl acetals in acetonitrile afford P-hydroxy- and y-hy-droxy-carbonyl compounds in good yields. - ... 

Both 2- and 3-aryl-P-alanine derivatives have been prepared by the Mukaiyama reaction of imines when catalyzed by titanium tetrachloride. The reaction of ketene silyl acetals with 0-alkyl imines is another source of aryl-substituted P-alanines.20 O-Substituted hydroxylamines (an "0-imine") have been used in this type of reaction. When 4.28 reacted with O-silylketene acetal 4.29, in the presence of... 

Conditions for arylation of enolate equivalents have also been developed. In the presence of ZnF2, silyl enol ethers, silyl ketene acetals, and similar compounds react. For example, the TMS derivatives of /V-acyl oxazolidinones can be arylated. 

Arylacetate esters have been generated by coupling aryl bromides with stannyl enolates generated from silyl ketene acetals. 

Keywords Catalyst, Alkylation, Allylation, Arylation, Mannich reaction, Carbon-nitrogen double bond, Imine, Nitrone, Aldimine, Organozinc reagents, Silyl ketene acetal, Silyl enol ether, Amine, (3-Amino acid... 

The use of aryl-A3-iodanes for C-heteroatom bond formation at the a-carbon atoms of ketones and / -dicarbonyl compounds, and related transformations of silyl enol ethers and silyl ketene acetals, has been exhaustively summarized in recent reviews (Scheme 27) [5,8]. Reactions of this type are especially useful for the introduction of oxygen ligands (e. g., L2 = OH, OR, OCOR, 0S02R, OPO(OR)2), and have been extensively utilized for the synthesis of a-sulfonyl-oxy ketones and a-hydroxy dimethyl ketals. 

The photo-SNl reaction with particular classes of alkenes - namely enamines, silyl enol ethers and silyl ketene acetals - affords a smooth synthesis of a-aryl ketones,... 

The palladium-catalyzed coupling of silyl ketene acetals with aryl bromides in the presence of TlOAc has been applied to the asymmetric synthesis of 2-aryl-propionic acids (40—50% ee) by the use of BPPFA and several other chiral phosphine ligands (Scheme 2-16) [35]. 

Bis(silyl)ketene acetals undergo silatropic ene reaction with nitrosobenzene to give N-hydroxyamino acid derivatives. When allylmagnesium chloride is reacted with nitroarenes, unstable adducts result. Reduction of these adducts with LAH in the presence of palladium on charcoal leads to A -allyl-W -aryl-hydroxylamines (73 Scheme 15). With alkyl Grignard reagents this reaction is negligible. ... 

Oxidations. Silyl ketene acetals undergo methoxylation to give a-methoxycarboxylic esters on treatment with Phl=0 in methanol. Aryl ketones are obtained from the corresponding hydrocarbons in an oxidation catalyzed by a (salen)Mn(III) complex, using Phl=0 as oxidant. ... 

For synthesis of arylacetic esters from aryl bromides and silyl ketene acetals, copper(II) fluoride is added to the reaction media besides the Pd catalyst."... 

Two short syntheses of racemic ipalbidine ( )-(842) are shown in Scheme 109. The synthesis by Jefford et al. commenced with conjugate addition between pyrrole and Ae atropate ester 849 followed by homologation of the acid 850 with diazomethane and rhodium-induced intramolecular carbene cyclization of the resulting diazoketone 851 (574). The bicyclic product 852 was converted into ( )-842 in a further four steps. The approach taken by Danishefsky and Vogel centered on acid-catalyzed cyclocondensation between the silyl ketene acetal 853 and A -pyrroline (854) to give indolizidinone 855 (575). Reduction of the lactam and cleavage of the aryl ether completed the synthesis of ( )-842. 

Enol silanes (181), as reported by Pilli and Russowsky, add to arylimines (182) in the presence of catalytic amounts of TMS-OTf to afford /V-aryl-p-amino ketones (183 equation 20). The reaction is carried out by adding the enol silane to the imine in the presence of IS mol % of TMS-OTf at 0 C in CH2CI2, followed by aqueous work-up. Yields range from 50% to 98% in a series of benzylideneaniline derivatives using the enol silanes derived from acetophenone and t-butyl methyl ketone. The extent to which the reaction may be limited to nonenolizable imines, as in the analogous TMS-OTf-promoted reactions of silyl ketene acetals (see Section is not repotted. The authors describe some... 

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