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