A Puckered Singlet Cyclopentane-1,3-diyl: Detection of the Third Isomer in Homolysis

In the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane (AZ6) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low-temperature matrix conditions, together with the previously well-characterized planar singlet diradical pl-(1) DR6 with λmax =≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)-silent. Instead of generating the planar diradical pl-(1) DR6 and the precursor azoalkane AZ6 upon warming, the ring-closed bicyclo[2.1.0]pentane derivative SB6, that is, the AZ6 denitrogenation product was identified. Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effect, and laser-flash photolysis, the puckered singlet diradicaloid puc-(1) DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl-(1) DR6. Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane-1,3-diyl diradicaloid puc-(1) DR6, that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.