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

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-dimet hyl-1,3-propanediones 5, which in turn were readily available by 2,2-d imethylation of the diones 4. The acid-catalyzed cycloaddition of cycl opentadiene to the isopyrazoles 6, a Diels-Alder reaction with inverse electron demand, afforded the 1,4-diaryl-substituted gem-dimethyl azo alkanes 7 of the diazabicyco[2.2.1]hept-2-ene (DBH) type. The cycloadd uct yields were strongly dependent on the nature of the aryl substitue nts and highest for the electron-withdrawing substituents. In acidic s olution, the azoalkanes showed cycloreversion to generate an equilibri um between isopyrazole 6, cyclopentadiene, and azoalkane 7. For the p- methoxy derivative, cycloreversion was essentially quantitative, where as only 20% cycloreversion occurred for the para nitro compound. A pos itive Hammett rho value (rho = 3.24 for 2 equiv of CF3COOH) was determ ined for the equilibrium constants of the acid-catalyzed [4(+) + 2] cy cloaddition. The unsymmetrically substituted isopyrazoles gave two reg ioisomeric cycloadducts with a slight excess of one isomer. The direct and triplet-sensitized photochemical and thermal denitrogenation of t he azoalkanes 7 gave in quantitative yields the 1,4-diaryl-substituted bicyclo[2.1.0]pentanes (BCP) 8 with retention of configuration. The a zoalkanes 7 and the housanes 8 are more persistent than the related 1, 4-diaryl-substituted DBH and BCP derivatives. The stabilizing effect i s rationalized in terms of less favorable benzylic conjugation in the transition states for C-N (azoalkanes) and C-C (housanes) bond cleavag e due to steric interactions between the geminal methyl groups at the methane bridge and the diaryl substituents at the bridgehead sites.