Nucleophilic addition of silyl enol ethers to aromatic nitro compounds: scope and mechanism of reaction

Abstract

In sharp contrast to alkali-metal enolates, silyl enol ethers and ketene silyl acetals add to aromatic nitro compounds in the presence of a fluoride ion source to give the intermediate dihydroaromatic nitronates, which can be observed by NMR. In situ oxidation of the intermediate with bormine or DDQ yields a-nitroaryl carbonyl compounds in moderate-to-high yields. The reaction is applicable to alkyl-, alkoxy-, and halogen-substituted nitrobenzenes as well as to heterocyclic and condensed nitroaromatic compounds. While substitution ortho to the nitro group predominates with sterically undemanding silyl reagents, para-substitution products are exclusively obtained with bulky reagents. However, by blocking the para position with an appropriate group such as chlorine, the addition can be directed to the ortho position. Halogen atoms of halogenated nitroaromatics and p-nitrocumenyl chloride are not displaced in the reaction, suggesting the absence of radical ion intermediates. Dihydroaromatic nitro derivatives can be isolated in some cases, such as anthracene and naphthalene systems which are less prone to rearomatize. The use of silicon reagents in organic synthesis has been ex- panding rapidly in the past few year^,^-^ and versatile methods for carbon-carbon bond formations have been developed based on silyl enol ethers activated by fluoride ion sources. Various groups have reported alkylation^,^^^^ arylati~ns,~~ aldol conden- sation~,~ a~ylations,~ and Michael additions.&* Our own interest in this area originated with a notion that the nucleophilic reactivity of such reagent combinations might be distinctively different from that of the classical metal enolates and, in particular, that the enhanced nucleophilicity might be attained without significantly increasing basicity to lead to novel carbon-carbon bond-forming reactions. For example, we have reported (Scheme I) that tri- alkylketene silyl acetals can be added to a,@-unsaturated ketones in the presence of tris(dimethylamino)sulfonium difluorotri- methylsili~onate ~ (TASF) to give products equivalent to 1,4-ad- ditions of ester en~lates.~ This same reagent combination also initiates group-transfer polymerization of methacrylate mono- mers via sequential Michael additions* (Scheme I). In this paper, we wish to record our results on the fluoride-assisted addition of silyl enol ethers to aromatic nitro compounds. Oxidation of the resulting intermediate nitronate gives highly versatile a-nitroaryl