ELECTRONIC SUBSTITUENT EFFECTS ON THE ACID-CATALYZED [4(+)+2] CYCLOADDITION OF ISOPYRAZOLES WITH CYCLOPENTADIENE AND THE PHOTOCHEMICAL AND THERMAL DENITROGENATION OF THE RESULTING 1,4-DIARYL-7,7-DIMETHYL-2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE AZOALKANES TO BICYCLO[2.1.0]PENTANES

Eight symmetrically disubstituted 3,5-diaryl-4,4-dimethylisopyrazoles 6 with para and meta substituents (OMe, Me, H, F, Cl, Br, CN, NO2) and two unsymmetrically para-substituted derivatives (OMe and NO2; Me and CO(2)Me) were synthesized from the corresponding 1,3-diaryl-2,2-dimethyl-1,3-propanediones 5, which in turn were readily available by 2,2-dimethylation of the diones 4. The acid-catalyzed cycloaddition of cyclopentadiene to the isopyrazoles 6, a Diels-Alder reaction with inverse electron demand, afforded the 1,4-diaryl-substituted gem-dimethyl azoalkanes 7 of the diazabicyco[2.2.1]hept-2-ene (DBH) type. The cycloadduct yields were strongly dependent on the nature of the aryl substituents and highest for the electron-withdrawing substituents. In acidic solution, the azoalkanes showed cycloreversion to generate an equilibrium between isopyrazole 6, cyclopentadiene, and azoalkane 7. For the p-methoxy derivative, cycloreversion was essentially quantitative, whereas only 20% cycloreversion occurred for the para nitro compound. A positive Hammett rho value (rho = 3.24 for 2 equiv of CF3COOH) was determined for the equilibrium constants of the acid-catalyzed [4(+) + 2] cycloaddition. The unsymmetrically substituted isopyrazoles gave two regioisomeric cycloadducts with a slight excess of one isomer. The direct and triplet-sensitized photochemical and thermal denitrogenation of the azoalkanes 7 gave in quantitative yields the 1,4-diaryl-substituted bicyclo[2.1.0]pentanes (BCP) 8 with retention of configuration. The azoalkanes 7 and the housanes 8 are more persistent than the related 1,4-diaryl-substituted DBH and BCP derivatives. The stabilizing effect is rationalized in terms of less favorable benzylic conjugation in the transition states for C-N (azoalkanes) and C-C (housanes) bond cleavage due to steric interactions between the geminal methyl groups at the methane bridge and the diaryl substituents at the bridgehead sites.