Mechanism and cis/trans Selectivity of Vinylogous Nazarov-type [6π] Photocyclizations

Rearrangement of 2-(benzofuran-2-yl)-3-phenylpyridines via photoinduced 6π-electrocyclization

By photoinduced 6π-electrocyclization of 2-(benzofuran-2-yl)-3-phenylpyridine derivatives 1, a method for the synthesis of trans-dihydrobenzo[f]quinolines 2, cis-dihydrobenzo[f]quinolines 3 and 8b-methyl-1,8b-dihydrobenzo[f]quinolines 4 was developed. Irradiation of 2-(benzofuran-2-yl)-3-phenylpyridine 1 in acetone-H2O (5 : 1, v/v) with a 313 nm UV lamp under an argon atmosphere at room temperature successfully yielded 2, which was further converted into 3 at elevated temperature (200 °C) in glycerol. However, irradiating 2-(3-methylbenzofuran-2-yl)-3-phenylpyridines 1 in CH2Cl2 with a 254 nm UV lamp gave 4 in good yields. The syntheses of 2, 3 and 4via the 6π-electrocyclization rearrangement of 1 not only offer high atom efficiency but also do not require transition metal catalysts or additives.

Insights into the regioselectivity and diastereoselectivity of the Nazarov cyclization of 3-alkenyl-2-indolylmethanol with tryptophol

Density functional theory calculations of Nazarov cyclizations showed that 3-alkenyl-2-indolylmethanol and tryptophol formed binary and ternary complexes, respectively, in the presence and absence of H2O to produce C–N and C–C products.

Alkynedicobalt mediated vinylogous Nazarov reactions

The SnCl₄ mediated reactions of cross conjugated aryl enynone-Co₂(CO)₆ and dienynone-Co₂(CO)₆ complexes afford benzocycloheptynone complexes or cycloheptenynone complexes in a thermal vinylogous Nazarov process. The rate of ring closure is strongly dependent on the ketone α-substituent.

Photoinduced Skeletal Rearrangement of Diarylethenes: Photorelease of Lewis Acid and Synthetic Applications

The skeletal photorearrangement including 6π-electrocyclization induced by UV light of ortho-halogen-substituted diarylethenes has been studied. It has been found that the reaction pathways leading to bi- or tricyclic frameworks depend on the kind of halogen substituent and solvent. Photocyclization with halogen abstraction leads to bicyclic fused aromatics, while the tricyclic frameworks are formed due to the tandem 6π-electrocyclization/sigmatropic shift reaction. THF is preferred as the solvent in the former process and chloroform in the latter reaction. It was found for the first time that, owing to the ability of this series of diarylethenes to undergo skeletal photorearrangement with the release of the bromide cation, they can be used both as brominating agents and as Lewis acids for catalyzing electrophilic reactions.

Exploring Free Energy Profiles of Enantioselective Organocatalytic Aldol Reactions under Full Solvent Influence

We present a computational study on the enantioselectivity of organocatalytic proline-catalyzed aldol reactions between aldehydes in dimethylformamide (DMF). To explore the free energy surface of the reaction, we apply two-dimensional metadynamics on top of ab initio molecular dynamics (AIMD) simulations with explicit solvent description on the DFT level of theory. We avoid unwanted side reactions by utilizing our newly developed hybrid AIMD (HyAIMD) simulation scheme, which adds a simple force field to the AIMD simulation to prevent unwanted bond breaking and formation. Our condensed phase simulation results are able to nicely reproduce the experimental findings, including the main stereoisomer that is formed, and give a correct qualitative prediction of the change in syn:anti product ratio with different substituents. Furthermore, we give a microscopic explanation for the selectivity. We show that both the explicit description of the solvent and the inclusion of entropic effects are vital to a good outcome-metadynamics simulations in vacuum and static nudged elastic band (NEB) calculations yield significantly worse predictions when compared to the experiment. The approach described here can be applied to a plethora of other enantioselective or organocatalytic reactions, enabling us to tune the catalyst or determine the solvent with the highest stereoselectivity.

Molecular Photochemistry: Recent Developments in Theory

Photochemistry is a fascinating branch of chemistry that is concerned with molecules and light. However, the importance of simulating light-induced processes is reflected also in fields as diverse as biology, material science, and medicine. This Minireview highlights recent progress achieved in theoretical chemistry to calculate electronically excited states of molecules and simulate their photoinduced dynamics, with the aim of reaching experimental accuracy. We focus on emergent methods and give selected examples that illustrate the progress in recent years towards predicting complex electronic structures with strong correlation, calculations on large molecules, describing multichromophoric systems, and simulating non-adiabatic molecular dynamics over long time scales, for molecules in the gas phase or in complex biological environments.

[6π] Photocyclization to cis-Hexahydrocarbazol-4-ones: Substrate Modification, Mechanism, and Scope

Upon irradiation at λ = 366 nm, tertiary N-alkoxycarbonyl-N-aryl-β-enaminones furnished exclusively the trans-hexahydrocarbazol-4-ones by a conrotatory [6π] photocyclization but epimerized on silica to cis-hexahydrocarbazol-4-ones (14 examples, 44–98% yield). The acceptor substitution on the nitrogen atom enhanced the stability of the cyclized products compared to N-alkyl-N-aryl-β-enaminones reported previously. The mechanism of the [6π] photocyclization was investigated by quenching experiments, deuterium-labeling experiments, and DFT calculations, suggesting a triplet pathway for the conrotatory ring closure followed by a suprafacial [1,4] hydrogen migration.