On the Z-E photoisomerization of chiral 2-pentenoate esters: stationary irradiations, laser-flash photolysis studies, and theoretical calculations

Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-phenyl)-4-oxo-but-2-enoate Ester Derivatives

To clarify the cis-trans isomerization mechanism of simple alkenes on the triplet excited state surface, the photochemistry of acyclic and cyclic vinyl ketones with a p-methoxyacetophenone moiety as a built-in triplet sensitizer (1 and 2, respectively) was compared. When irradiated, ketone 1 produces its cis-isomer, whereas ketone 2 does not yield any photoproducts. Laser flash photolysis of ketone 1 yields a transient spectrum with λ_max ∼ 400 nm (τ ∼ 125 ns). This transient is assigned to the first triplet excited state (T₁) of 1, which presumably decays to form a triplet biradical (1BR) that is shorter lived than the triplet ketone. In comparison, laser flash photolysis of 2 reveals two transients (τ ∼ 20 and 440 ns), which are assigned to T₁ of 2 and triplet biradical 2BR, respectively. Density functional theory calculations support the characterization of the triplet excited states and the biradical intermediates formed upon irradiation of ketones 1 and 2 and allow a comparison of the physical properties of the biradical intermediates. As the biradical centers in 2BR are stabilized by conjugation, 2BR is more rigid than 1BR. Therefore, the longer lifetime of 2BR can be attributed to less-efficient intersystem crossing to the ground state.

Cycloheximide congeners produced by Streptomyces sp. SC0581 and photoinduced interconversion between (E)- and (Z)-2,3-dehydroanhydrocycloheximides

Three new cycloheximide congeners, 2,3-dehydro-α-epi-isocycloheximide (1), (E)- and (Z)-2,3-dehydroanhydrocycloheximides (2 and 3), together with three known compounds, anhydroisoheximide (4), cycloheximide (5), and isocycloheximide (6), were obtained from the cultures of Streptomyces sp. SC0581. Their structures were elucidated by extensive spectroscopic analysis in combination with theoretical conformational analysis and ECD computations. The photoinduced interconversion between 2 and 3 was observed and verified and the possible reaction path and mechanism were proposed by theoretical computations. The antifungal and cytotoxic activities of 1-6 were evaluated and suggested that 2,3-dehydrogenation results in the loss of the activities and supported that the OH-α is important to the activities of cycloheximide congeners.

Tailoring flavins for visible light photocatalysis: organocatalytic [2+2] cycloadditions mediated by a flavin derivative and visible light

A new application of flavin derivatives in visible light photocatalysis was found. 1-Butyl-7,8-dimethoxy-3-methylalloxazine, when irradiated by visible light, was shown to allow an efficient cyclobutane ring formation via an intramolecular [2+2] cycloaddition of both styrene dienes, considered as electron-rich substrates, and electron-poor bis(arylenones), presumably proceeding via an energy transfer mechanism.

Formation and Direct Detection of Non-Conjugated Triplet 1,2-Biradical from β,γ-Vinylarylketone

Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308 nm results in the formation of triplet 1,2-biradical 2 that has λmax at 370 and 480 nm. Biradical 2 is formed with a rate constant of 1.1 × 107 s–1 and decays with a rate constant of 2.3 × 105 s–1. Isoprene-quenching studies support the notion that biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet 1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.

Theoretical study of the photochemical [2 + 2]-cycloadditions of cyclic and acyclic alpha,beta-unsaturated carbonyl compounds to ethylene

The ground and first triplet excited-state potential energy surfaces of the [2 + 2]-cycloadditions of 2-cyclohexenone, methyl acrylate, and methyl crotonate to ethylene have been studied by means of CASSCF and DFT-B3LYP calculations. The attack of ethylene to the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound leads to the formation of a triplet 1,4-biradical intermediate that evolves to the ground-state potential energy surface. The outcome of the reaction is governed by the competition between the deactivation of the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound itself and its reaction with ethylene to form the triplet 1,4-biradical. For 2-cyclohexenone, the potential energy barrier corresponding to the formation of the biradical intermediate is lower than for the acyclic systems. On the other hand, the energy necessary to reach the crossing point between the (3)(pi-pi) and the ground-state potential energy surfaces is lower for the acyclic systems than for 2-cyclohexenone. For methyl acrylate and methyl crotonate, the decay of the (3)(pi-pi) state of the isolated molecule is therefore expected to be faster than the formation of the 1,4-biradical, so that the [2 + 2]-cycloaddition will not take place. However, for 2-cyclohexenone the formation of the triplet 1,4-biradical is favorable, and the process will lead to the formation of the corresponding cyclobutane derivative.