Impact of Diradical Spin State (Singlet vs Triplet) and Structure (Puckered vs Planar) on the Photodenitrogenation Stereoselectivity of 2,3-Diazabicyclo[2.2.1]heptanes

Abstract

Versatile transformations of azo compounds are utilized not only in synthetic organic chemistry but also in materials science. In this study, a hitherto unknown stereoselectivity was observed by low-temperature in situ NMR spectroscopy for the photochemical denitrogenation of a cyclic azoalkane (2,3-diazabicyclo[2.2.1]heptane) derivative. Direct (singlet) photodenitrogenation at 188 K afforded two products, the configurationally retained ring-closed compound (ret-CP) and the inverted compound (inv-CP), in a ratio of 82/18 (±3) (ret-CP/inv-CP), with an overall yield of >95%. Triplet-sensitized denitrogenation at 199 K using benzophenone ((3)BP*) or xanthone ((3)Xan*) selectively produced inv-CP, with a ret-CP/inv-CP ratio of 7/93 (±3). Thermal isomerization of inv-CP into ret-CP was observed by low-temperature NMR spectroscopy. Transient absorption spectroscopy revealed that two distinct singlet diradicals are involved in the formation of CP during direct photodenitrogenation, that is, puckered puc-(1)DR and planar pl-(1)DR diradicals. The former produces ret-CP, whereas the latter affords inv-CP. Kinetic analysis using the integrated profiles method was used to determine the molecular absorption coefficient of pl-(1)DR (ε560 = 4900 ± 250 M(-1) cm(-1)) for the first time. The involvement of the puckered singlet diradical resolves the mechanistic puzzle of stereoselective denitrogenation of diazabicycloheptane-type azoalkanes.

Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect

“…Consistent with the destabilization of ring‐closed compound CP2 g , the transition‐state energies from singlet diradical S‐ DR2 g to trans ‐ and cis ‐ CP2 g (68.5 and 50.3 kJ mol −1 , respectively) were larger than those for the reaction from DR2 e without the macrocyclic ring to trans ‐ and cis ‐ CP2 e (63.6 and 44.2 kJ mol −1 , respectively), that is, S‐ DR2 g is kinetically stabilized by the macrocyclic ring. As previously found for S‐ DR2 b , formation of cis ‐ CP2 e and cis ‐ CP2 g was predicted to be energetically more favorable than formation of the trans isomers . Thermal isomerization from the cis to the trans isomer appears to be possible for cis ‐ CP2 g at 298 K; indeed, the activation enthalpy for the transition from S‐ DR2 g to trans ‐ CP2 g is computed to be 68.5 kJ mol −1 , as observed for the thermal isomerization from cis ‐ CP2 b to trans ‐ CP2 b .…”

“…As previously found for S‐ DR2 b , formation of cis ‐ CP2 e and cis ‐ CP2 g was predicted to be energetically more favorable than formation of the trans isomers . Thermal isomerization from the cis to the trans isomer appears to be possible for cis ‐ CP2 g at 298 K; indeed, the activation enthalpy for the transition from S‐ DR2 g to trans ‐ CP2 g is computed to be 68.5 kJ mol −1 , as observed for the thermal isomerization from cis ‐ CP2 b to trans ‐ CP2 b . The singlet ground states of DR2 e and DR2 g were also confirmed by the computations as Δ E ST = E S − E T ≈−17 kJ mol −1 (Table ).…”

“…The computational study clarified that cis isomers cis ‐ CP2 g , h are the primary photoproducts of AZ2 g , h . Low‐temperature in situ NMR analysis at 188 K was conducted for the reaction of AZ2 h (3.4 mg, 9.5 m m ) in [D 8 ]toluene under N 2 with 355 nm light from an Nd:YAG laser (Figure ), which was guided into the NMR cavity by using a quartz rod . At 188 K, thermally labile species (signals a–d) were observed with trans ‐ CP2 h (signals e–l) and unconverted AZ2 h (signals m–p).…”

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

“…The molar extinction coefficient (¾) of CP3b at 571 nm was experimentally determined to be ca. 4900 M ¹1 cm ¹1 in benzene at 293 K. 24 As expected for π-single bonding with a small HOMOLUMO energy spacing (Figure 1), the singlet species Figure 7. (a) First electronic transition, π (ψ S,3b ) to π* (ψ A,3b ), in the singlet diradicaloid CP3b with a π-single bonding character; (b) spiroconjugation effect on π to π* electronic transition in CP3c, π (ψ S,3c ) to π* (ψ A,3c ).…”

“…Indeed, the selective formation of the cis-configured ring-closed product RC3b (OR = OR¤ = OCH 3 ) was observed using in situ NMR spectroscopic analyses at low temperature. 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.…”