Control of Conformer Population and Product Selectivity and Stereoselectivity in Competitive Photocyclization/Rearrangement of Chiral Donor–Acceptor Dyad

Orbital Control of Photochemical Rearrangement of 4-Aryl-1,1-dicyano-1-butenes through the Hyperconjugative Substitution on the Linker Chain

Hyperconjugative interaction was demonstrated to play a vital role in the photochemistry of 4-aryl-1,1-dicyano-1-butenes. Thus a simple substituent on the benzylic position effectively induced a new photoreactivity to afford an allylic rearrangement product that is not obtained for the parent substrate. The natural bond orbital analysis was employed to reveal the enhanced relative contributions of hyperconjugation in the excited state, which dramatically alter the photochemical outcomes not only by reducing the strength of the allylic/benzylic bond but more crucially by affecting the conformer distribution.

Contrasting Behaviour of Exciplex Ensembles in the Diastereodifferentiating Paternò–Büchi Reaction of Chiral Cyanobenzoate with Naphthyl- and Phenylethenes on Direct or Charge-Transfer Excitation

The diastereodifferentiating Paternò–Büchi reaction of chiral cyanobenzoate with 1-(1-naphthyl)-1-phenylethene was compared with those with 1,1-diphenylethene on direct and charge-transfer excitations. By desymmetrization of the donor, four diastereomeric oxetane products were formed on photolysis in excellent combined yields. Increase in donor strength induced a stronger charge-transfer interaction both in ground and excited states. Thus, the difference in diastereoselectivities with two different excitation modes (i.e. direct versus charge-transfer) became less significant with a naphthyl derivative as donor. A subtle change of donor–acceptor interaction was shown to have profound effect on the nature of the excited-state complexes and thus the product (stereo)selectivities. Despite a small temperature dependence, an Eyring-type study on the diastereoselectivities confirmed that the excited charge-transfer complex is an excited species distinct from the conventional exciplex.

Exciplex ensemble modulated by excitation mode in intramolecular charge-transfer dyad: effects of temperature, solvent polarity, and wavelength on photochemistry and photophysics of tethered naphthalene-dicyanoethene system

Solvent, temperature, and excitation wavelength significantly affected the photochemical outcomes of a naphthalene-dicyanoethene system tethered by different number (n) of methylene groups (1-3). The effect of irradiation wavelength was almost negligible for 2a but pronounced for 3a. The temperature dependence and theoretical calculations indicated the diversity of exciplex conformations, an ensemble of which can be effectively altered by changing excitation wavelength to eventually switch the regioselectivity of photoreactions.