Regulation of an Ambient-Light-Induced Photocyclization Pathway (Norrish–Yang Versus 6π) by Substituent Choice

Photocyclization, irrespective of whether multiple steps (e.g., Norrish–Yang cyclization) or a single concerted step (e.g., 6π photocyclization) are involved, is an intramolecular photochemical process resulting in the formation of one new single bond to afford a ring system. In particular, visible-light-induced photocyclization offers a green and sustainable route to organic cyclic compounds that are difficult to access by thermal reactions. Herein, we describe the ambient light-induced intramolecular photocyclization of a series of donor/acceptor chromophores 1 d – 3 d containing two types of photoresponsive motifs, namely an electron-deficient BF₂-chelated ketone fused with an electron-rich thiophene, and probe the solution-phase and solid-state photochromic performance of these compounds. The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., under ambient light, methyl-substituted 1 d and 2 d undergo reversible 6π photocyclization, whereas ethyl-substituted 3 d exclusively undergoes irreversible Norrish–Yang photocyclization. Single-crystal X-ray analysis of Norrish–Yang cyclization products reveals the formation of four pairs of conformational enantiomers differing in the dihedral angle between benzothiophene and the BF₂ core, namely (±) N -3 d @68°, (±) N -3 d @-77°, (±) N -3 d @-78°, and (±) N -3 d @-102°. The UV/Vis absorption spectra of 1 d – 3 d cover a broad visible-light region (380–572 nm), while DFT and TD-DFT calculations reveal that absorption in this region is dominated by the charge-transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient π-conjugated BF₂-chelated unit and the n→π* and π→π* transitions within the latter unit. The spatial separation of the HOMO and LUMO of these dyes promotes triplet-state generation and self-photosensitizes intramolecular photocyclization in the visible-light region. Three-dimensional time-resolved and steady-state emission spectra of 3 d show that the Norrish–Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).     

Photochemical Studies on Bicyclo[2.1.1]hexyl Derivatives: Chemical Behavior and Asymmetric Induction

The photochemical behavior of bicyclo[2.1.1]hexyl derivatives was investigated by irradiation with a 450 W medium‐pressure mercury lamp in acetonitrile solution. The irradiation of methyl bicyclo[2.1.1]hexane‐5‐carbonylbenzoate ( 1a ) led to both Norrish type II cyclization and cleavage products with a molar ratio of 1:2.2, whereas the irradiation of methyl 5‐methylbicyclo[2.1.1]hexane‐5‐carbonylbenzoate ( 1b ) afforded the only Norrish/Yang photocyclization compound as the sole product. Such results were illustrated by several geometric parameters for Norrish/Yang photoreaction as ?₁, ?₄ and β obtained from the crystal structures. Furthermore, asymmetric photochemical studies using ionic chiral auxiliary technique were also conducted in the solid state.     

Photochemical studies on exo-bicyclo[2.1.1]hexyl and bicyclo[3.1.0]hexyl aryl ketones: two approaches for synthesis of enantiomerically enriched cyclopentene derivatives

Two approaches for the photochemical synthesis of cyclopentene derivatives through the Norrish type II cleavage reaction were described. Asymmetric studies using ionic chiral auxiliaries afforded enantiomeric excesses of up to 98% at the conversion of 85%. The results were rationalized by single X-ray crystal structures.     

Photochemistry of Syn- and Anti-A8,9-Dimethoxycarbonyl-1,5-Dipropyl-3-Oxatricyclo[3.2.2.02,4]Non-8-Ene-6,7-Dione

In the photochemistry of the title compounds, the anti isomer 6 afforded oxepine 9 whereas the syn isomer yielded 9 and 10 ; the reactions proceeded partly via cyclobutanediones 12 and 13 . The distinct photochemical behavior are explained in terms of stereorelectronic effects of the epoxy rings.     

Asymmetric Photochemical Reaction of 5-Methylbicyclo[1.1.1]-pentanyl Ketone

The density functional theory(DFT) calculations were performed to investigate a typical Norrish/Yang type II photoreaction of 5-methylbicyclo[1.1.1]-pentanyl ketone. The results reveal the essential correlation between structures on the one hand and energies, on the other hand, of the reactants, transition states and products based on both singlet ground(S₀) and triplet excited(T₁) potential energy surfaces. The feasible mechanism indicates that an intramolecular Norrish/Yang cyclization reaction takes place via H-abstraction to obtain the sole chiral cyclobutanol photoproduct. The located crossing point plays an important role in the cyclization process, which permits intersystem crossing(ISC) from T₁ to S₀ state. The rate-determining step may be to experience ISC between two different potential energy surfaces, requiring sufficient time for electron spin reversion, i.e., spin multiplicity alteration. These conclusions are further confirmed by the second-order M鴏ler-Plesset perturbation theory(MP2) calculations.     

二环[3.1.0]己烷酮衍生物的固相不对称光化学反应研究

由二环庚二烯为起始原料合成二环己烷酮类衍生物,以手性胺作为手性辅助剂进行固相不对称光化学反应研究,探讨了反应时间和反应温度对光化学裂解产率和对映选择性的影响,ee值最高达到75%。