Electrostatic interactions between substituents as regioselectivity control elements in Diels-Alder cycloadditions. A DFT study of cycloadditions of 1-methoxy-4-trimethylsiloxy dienes with acrylonitrile.

Abstract

The regioselectivities of Diels-Alder reactions of 1-methoxy-4-trimethylsiloxy-1,3-butadiene and the corresponding o-xylylene with acrylonitrile were explored with density functional theory. The transition state of the reaction of the diene with acrylonitrile was studied in both the gas phase and in a low dielectric (epsilon = 2.247) solvent. The regioselectivities of these reactions are predicted to be controlled by the direct electrostatic interactions between the diene and dienophile substituents. The electron-donating capacities of methoxy and trimethylsiloxy substituents are shown to be very similar and to not contribute to regioselectivity control. A prediction is made that the cycloadditions of the o-xylylene will be unselective, while 1-methoxy-4-trimethylsiloxy-1,3-butadiene will give a small preference ( approximately 5:1) in favor of the proximal methoxy and cyano groups. The thermochromic behavior of trans-disubstituted disiloxy benzocyclobutene was also explored.