A variety of dienophiles was used to assess the facial selectivity of Diels−Alder reactions in a series of 1,3-cyclopentadiene derivatives (1−3, 6−10) in which chlorine, bromine, and iodine were plane-nonsymmetric atoms pitted against hydrogen or methyl at C-5. The results were rationalized in terms of the major factor controlling the facial selectivity being related to steric hindrance between the diene and the dienophile. Selectivity did not correlate with reactivity. Facial selectivity in the reactions with 4-phenyl-1,2,4-triazoline-3,5-dione as the dienophile was also influenced by a second significant factor, postulated to be filled-orbital repulsion with the halogen substituent.

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