Substituent Effects in the Diels-Alder Reaction of Substituted Benzynes with Hexamethylcyclohexa-2,4-dienone

The Diels-Alder reaction of unsymmetrically substituted benzynes that had been generated from 3-Me-, 4-Me-, 5-Me-, 6-Me-, 3,4-Me2-, 3-NO2-, 4-NO2-, 5-NO2-, and 3-Cl-benzenediazonium-2-carboxylates, with hexa-methylcyclohexa-2,4-dienone gave a mixture of 5- or 6-substituted-1,2,3,4,10,10-hexamethyl-1,4-dihydro-1,4-. ethanonaphthalene-9-one (Type 7) and its 8- or 7-substituted isomer (Type 8). The 7/8 isomer ratios varied from 1.0 to 2.0 depending substantially on the size and the position of substituents of the benzynes and also on the temperature and the solvent. The ratios were the same when two positionally isomeric acids, i.e., 3- and 6-, or 4- and 5-substituted acids, were used as the precursors. Thus, the identity of the benzynes generated from these paired isomeric precursors was proved, ruling out the possibility of an inherent memory effect of the precursors. The factor that controls the isomer ratios is discussed in terms of the steric repulsive interaction between the substituents of benzynes and the gem-dimethyl groups of the dienone.