FACIAL SELECTIVITY IN THE DIELS-ALDER REACTIONS OF 5-CHLORO-1,3-CYCLOPENTADIENE, 5-BROMO-1,3-CYCLOPENTADIENE, AND 5-IODO-1,3-CYCLOPENTADIENE AND DERIVATIVES

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-nonsymmet ric atoms pitted against hydrogen or methyl at C-5. The results were r ationalized in terms of the major factor controlling the facial select ivity being related to steric hindrance between the diene and the dien ophile. Selectivity did not correlate with reactivity. Facial selectiv ity in the reactions with 4-phenyl-1,2,4-triazoline-3,5-dione as the d ienophile was also influenced by a second significant factor, postulat ed to be filled-orbital repulsion with the halogen substituent.