(±)-3,3-Diethoxycarbonyl-2-p-tolylsulfinylacrylonitrile as dienophile: relationship between the barrier for pyramidal inversion of vinyl sulfoxides and the electronic effects of the substituents at the double bond

The dienophilic behavior of (±)-3,3-diethoxycarbonyl-2-p-tolylsulfinylacrylonitrile () is reported and compared with that of trialkoxycarbonyl sulfinylethylene (). The change of a CO2R group by a CN one increases both the reactivity and the stereoselectivity of the cycloaddition. It seems that the linear structure of the CN group increases the planarity of the system reinforcing its electron-withdrawing power and therefore improving its dienophilic behavior. Otherwise, the low configurational stability at room temperature of the sulfoxide , which is obtained as a racemic from its optically pure precursor, must also be a consequence of this strong electronic demand of the substituents which results in an unusually small energetic barrier for pyramidal inversion of the sulfinyl sulfur in compound (ΔG‡=16.3 kcal mol−1 at Tc=25°C, determined by dynamic NMR spectroscopy). This is the first evidence of the existing relationship between racemization barrier of vinyl sulfoxides and the electron-withdrawing power of the groups joined to the double bond.