Can Electron Transmission from Aza-Crown Ethers Contribute to Diels-Alder π-Facial Selectivity?†

A pair of isodicyclopentafulvenes that carries an exocyclic phenyl group para-substituted with an aza-15-crown-5 or aza-18-crown-6 ring has been prepared. These molecules (5a and 5b) and their respective NaClO4 (11) and KSCN (12) complexes were examined for the extent of π-facial stereoselectivity operational during their Diels-Alder cycloaddition to (Z)-1,2-bis(phenylsulfonyl)ethylene (2) under conditions of high pressure. Whereas 5a and 5b engage in [4 + 2] condensation with a 2.5–3.1 preference for dienophile capture from the above-plane direction, their complexes show a substantially reduced tendency for doing so (14/13 = 1.2–1.4). However, this response goes contrary to those para-substituent effects previously observed. The linear correlation noted earlier conformed to an increased preference for above-plane cycloaddition to 2 as the para-substituent became progressively more electron-withdrawing. Probable reasons for this seemingly divergent behavior are presented.