Axially chiral molecules are among the most valuable substrates in organic synthesis. They are typically used as chiral ligands or catalysts in asymmetric reactions. Recent progress for the construction of these chiral molecules is mainly focused on the transition-metal-catalyzed transformations. Here, we report the enantioselective NHC-catalyzed (NHC: N-heterocyclic carbenes) atroposelective annulation of cyclic 1,3-diones with ynals. In the presence of NHC precatalyst, base, Lewis acid and oxidant, a catalytic C–C bond formation occurs, providing axially chiral α-pyrone−aryls in moderate to good yields and with high enantioselectivities. Control experiments indicated that alkynyl acyl azoliums, acting as active intermediates, are employed to atroposelectively assemble chiral biaryls and such a methodology may be creatively applied to other useful NHC-catalyzed asymmetric transformations.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D₁ surface with little excess energy at the equilibrium neutral geometry. These D₁ ions are preferably formed because in this case D₁ and D₀ switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D₁ surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D₁/D₀ conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D₀ becomes populated with molecules that are energized mainly along the cis–trans isomerization coordinate, with the kinetic energy above the cis–trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D₀ surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole–lone pair interaction that drives the molecule along the cis–trans isomerization coordinate to facilitate the transition between the involved electronic states

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The hydroxy-substituted alkyl phenyl ketones 2'-, 3'- and 4'- (ortho, meta, and para) hydroxyacetophenone were excited in the strong-field regime with wavelengths ranging from 1200-1500 nm to produce the respective radical cations. For 2'- and 3'-hydroxyacetophenone, the parent molecular ion dominated the mass spectrum, and the intensity of the fragment ions remained unchanged as a function of excitation wavelength. In contrast, 4'-hydroxyacetophenone exhibited depletion of the parent molecular ion with corresponding enhanced formation of the benzoyl fragment ion upon excitation with 1370 nm as compared with other excitation wavelengths. Density functional (DFT) calculations suggest that dissociation occurs when the acetyl group in 4'-hydroxyacetophenone radical cation twists out-of-plane with respect to the phenyl ring, enabling a one-photon transition between the ground cation state D0 and the excited cation state D2 to occur. The DFT calculations also suggest that the lack of dissociation in the wavelength-resolved strong-field excitation measurements for 2'- and 3'-hydroxyacetophenone arises because both isomers have a barrier to rotation about the carbon-carbon bond connecting the phenyl and acetyl groups. These results help elucidate the effects of substituents on the torsional motion of radical cations and illustrate the potential for controlling molecular dissociation through the addition of substituents.