The tungsten dearomatization agent, {TpW(NO)(PMe3)}, has been found to promote novel reactivity when bound through dihapto-coordination to aromatic molecules. This study explored how an electron withdrawing functional group affects this chemical reactivity, specifically with regard to protonation followed by nucleophilic addition. Electron withdrawing groups enhance the metal-benzene bond strength, which results in the metal being more resistant to oxidation. However, many common electron withdrawing functional groups (e.g., esters, amides, ketones) are incompatible with {TpW(NO)(PMe3)} because their π bond can compete with the benzene for coordination to the metal. Trifluoromethyl, sulfonyl, and sulfonamidyl groups were found to be compatible with benzene complexation. Complexes of the form TpW(NO)(PMe3)(η2-arene) were prepared from the benzene precursor TpW(NO)(PMe3)(η2-benzene), where arene = PhSO2Me, PhSO2NR2, and PhCF3. These complexes were then subjected to protonation followed by nucleophilic addition to generate 1,3-cyclohexadiene complexes. Proton, 31P, 13C, and multidimensional NMR analysis were used to determine the regio- and stereochemistry of the resulting products. It was found that in all cases, protonation occurs at the C2 carbon with triflic acid leveled in acetonitrile, which is then followed by the addition of the nucleophile to C3, with addition occurring anti to the metal. The resulting 1,3-diene complexes were able to undergo further functionalization through a 1,4-addition, with the proton adding syn to the metal at C1, and the nucleophile adding to C4, anti to the metal once again. These addition reactions have been shown with a range of nucleophiles including enolates, cyanide, amines, and hydrides. Oxidation of the metal allows for the isolation of functionalized cyclohexenes with the potential to have 3 stereocenters that are set by the metal.

Tungsten dearomatization of electron deficient arenes: Synthesis of functionalized cyclohexenes via tandem additions

2019 Fall National Meeting

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