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

TL;DR

This study explores how electron-withdrawing groups affect the reaction of β-nitrostyrenes with silyl ketene acetals, producing unexpected anti-Michael addition products.

Contribution

The paper reveals a novel vinylic SRN1 mechanism for anti-Michael addition products in reactions involving β-nitrostyrenes and silyl ketene acetals.

Findings

The yield of anti-Michael addition products depends on the position and nature of electron-withdrawing groups in the phenyl ring.

The anti-Michael product forms via a vinylic SRN1 mechanism, not an ionic pathway.

External factors like light or metal impurities do not influence the formation of the anti-Michael product.

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 SRN1 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.

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