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

Abstract

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.

Is π-Single Bonding (C–π–C) Possible? A Challenge in Organic Chemistry

“…The higher level of theory at CAS(2,2)+DDCI3/6-31G(d) more precisely reproduced the electronic transition of HOMO (ψ S,3b ) to LUMO (ψ A,3b ) in CP3b, 23 in which 581 nm was computed for the first ππ* electronic transition in the π-single bonding (Figure 7a). The molar extinction coefficient (¾) of CP3b at 571 nm was experimentally determined to be ca.…”

“…25 The bond order of C1C3 was determined to be 0.37 by using the computed electron occupation numbers in ψ S (1.37) and ψ A (0.63) at the (2/2)CASSCF/6-31G(d) level of theory for CP3b. 23 The blue-shifted absorption maximum was observed at 514 nm for CP3c (X = O(CH 2 ) 2 O) with the cyclic acetal moiety (Figure 7b), demonstrating a larger HOMOLUMO energy gap and higher π-bonding character in CP3c compared with CP3b because of the spiroconjugation (Figure 7b). …”

“…24 The reactivity of the planar singlet diradicaloid CP3 was reproduced well by the density functional theory calculations for the reactivity of the planar singlet diradicaloid CP3b. 23,24 The kinetic stabilization was realized by using the sterically hindered substituents Ar and OR in CP3l, whose lifetime was found to be ca. 5500 ns at 293 K (Figure 8b).…”

Unusually Long‐Wavelength Emissions of Cyclopropanes: New Insight into C−C Bond Homolysis

“…The first minimum energy state CP1 a* corresponds to the experimentally observed short‐wavelength emission state (Figure A). Interestingly, the second minimum energy structure that shows a long‐wavelength emission (calcd 480 nm) was found to possess a longer d (C 1 −C 2 ) distance of 2.332 Å with a puckered structure, puc‐ 1 DR1 a* (entry 3, θ CP1 =32.7°, and θ CP2 =20.7°). The puc‐ 1 DR1 a* species should be the emissive species at long wavelengths (exptl 515 nm, Figure A).…”

“…Figure 2s hows the:a )excited state PES of CP1 a;b )four selected dihedral angles, q Ph1 , q Ph2 , q CP1 ,a nd q CP2 ;a nd c) two selected bond lengths, d(C 1 ÀC 5 )a nd d(C 2 ÀC 8 ). Interestingly,t he second minimum energy structure that shows al ong-wavelength emission (calcd 480 nm) was found to possess al onger d(C 1 ÀC 2 )d istance of 2.332 with a puckered structure, [27,28] puc-1 DR1 a* (entry 3, q CP1 = 32.78,a nd q CP2 = 20.78). Figure 2a shows that CP1 a has two minimum energy states on the PES in the S 1 state.…”