Competing Michael/Anti-Michael Addition of Silyl Ketene Acetals to β-Nitrostyrenes Incorporating an Electron-Withdrawing Group

Abstract

β-Nitrostyrenes are archetypal Michael acceptors due to the strong electron-withdrawing nature of the nitro group. However, we found that β-nitrostyrenes substituted with electron-withdrawing groups react with silyl ketene acetals, activated by stoichiometric Lewis bases (K₂CO₃ or TBAF), to produce a mixture of Michael addition (M) and anti-Michael addition (AM) products. The yield strongly depends on the position and nature of the electron-withdrawing group within the phenyl ring (e.g., p-nitro: 8% AM 2% M; o-nitro 70% AM, 26% M). The formation of the unexpected AM product cannot be explained by a higher electrophilicity of the α-carbon compared to the β-carbon (Parr’s indexes). Furthermore, we demonstrated that the formation of the AM product is not influenced by external factors such as light, metal impurities, or the nature of the Lewis base. Instead, it appears from the intrinsic reactivity of the reacting partners. Based on theoretical and experimental evidence, we propose that the AM product is formed via a vinylic S_RN1 mechanism rather than an ionic pathway. In this mechanism, the anionic enolate reduces the nitrostyrene through a single-electron transfer process initiating a radical mechanism that ultimately leads to the formation of the AM product.