Theoretical investigation of an unusual substituent effect on the dienophilicity of >C&dbond;P&bond; functionality present in 2-phosphaindolizines.

Effect of the number and positions of the methoxycarbonyl substituents in 2-phosphaindolizine on the feasibility of its Diels–Alder (DA) reaction with 1,3-butadiene has been investigated theoretically at the density functional theory (DFT) level. Among the series of four differently substituted 2-phosphaindolizines, 3-methoxycarbonyl-2-phosphaindolizine does not undergo the DA reaction due to the highest activation barrier (29.49 kcal mol-1) and endothermicity, whereas the activation barrier of the corresponding reaction of 1,3-bis(methoxycarbonyl)-2-phosphaindolizine is lowest (22.43 kcal mol-1) with exothermicity making it possible to occur. This reactivity trend is corroborated by FMO energy gaps as well as by global electrophilicity powers of the reactants. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]