Steric versus stereoelectronic control of the facial selectivity in the Diels-Alder reaction

Abstract

The Diels-Alder reactions of 1,2,3,4,5-pentamethyl-1,3-cyclopentadiene and a number of its derivatives, 5-ethyl-1,2,3,4,5-pentamethyl-1,3-cyclopentadiene, 5-methoxymethyl-1,2,3,4,5-pentamethyl-1,3-cyclopentadiene, 5·chloro-1,2,3,4,5-pentamethyl-1,3-cyclopentadiene, 5-bromo-1,2,3,4,5-pentamethyl-1,3-cyclopentadiene and 5-iodo-1,2,3,4,5-pentamethyl-1,3-cyclopentadiene were studied. Additions to these dienes with a number of dienophiles, such as N-phenylmaleimide, 1,4-naphthoquinone, 1,1,2,2-tetracyanoethene, dimethyl acetylenedicarboxylate and 4-phenyl-1,2,4-triazoline-3,5-dione were studied. The results suggest that the facial selectivity of 5-substituted 1,2,3,4,5-pentamethyl-1,3-cyclopentadienes is controlled primarily by steric interactions between the diene and dienophile rather than some stereoelectronic factor. The facial selectivity of 5-alkyl-substituted 1,2,3,4,5-pentamethyl-1,3-cyclopentadienes demonstrated only a slight dependence upon the dienophile used. Differences in facial selectivity for the 5-halo-substituted 1,2,3,4,5-pentamethyl-1,3-cyclopentadienes when different dienophiles were employed have been attributed to electrostatic repulsions between the lone pair electrons of the C-5 substituent and lone pairs on the attacking dienophile. It was observed that dienes such as 5-ethyl-1,2,3,4,S-pentamethyl-1,3-cyclopentadiene and 5-methoxymethyl-1,2,3,4,5- pentamethyl-1,3-cyclopentadiene showed selectivity that demonstrated a large dependence upon the conformation of the substituents on the diene.