Nonpolar Resonance Controls Polar Cycloadditions through Transient Electromeric Charge Redistribution

The electronic influence of substituents on chemical reactivity has long been described in terms of mesomeric and electromeric effects—concepts introduced nearly a century ago and now broadly unified under the IUPAC term “resonance effect”, which no longer considers them as independent. In 2002, however, the electromeric effect—transient electron redistribution in the presence of the second reaction center—was reinvented by Liu et al. under the name of “nonpolar resonance effect” to explain thermodynamics of nonpolar reactions exhibiting non-Hammett behavior. They noted that low polarity is a prerequisite for its observation. Here, we combine experiments and computations to demonstrate that electromeric effect—we adopt its historic name—controls the closure of zwitterionic intermediate in a reaction between pyridinium ylides and nitroalkenes: it stabilizes the transition state, making the reaction feasible for polarizable substituents in spite of their donor or acceptor nature. Our results imply that polarizable substituents can stabilize transition states even when their electron demands counteract the usual electronic preferences of substituents. We propose that similar electronic and reactivity patterns might be observed in other transformations, suggesting that the recently revived electromeric effect is more widespread than previously recognized.