An organocatalytic strategy for the synthesis of tetrasubstituted pyrrolidines with monoactivated azomethine ylides in high enantiomeric excess and excellent exo/endo selectivity is presented. The key to success is the intramolecular activation via hydrogen bonding through an o-hydroxy group, which allows the dipolar cycloaddition to take place in the presence of azomethine ylides bearing only one activating group. The intramolecular hydrogen bond in the azomethine ylide and the intermolecular hydrogen bond with the catalyst have been demonstrated by DFT calculations and mechanistic proofs to be crucial for the reaction to proceed.

In this work, a thorough theoretical study of the [2+2] cycloaddition reaction between asymmetric substituted alkenes is presented. The simulations evidence a concerted asynchronous or stepwise mechanism, depending on the nature of the electron‐deficient alkene. We have found a direct relation between the interaction of the frontier‐orbitals (HOMO‐LUMO) and the distortion energy with the activation energy (whose values oscillate between ∼4 and ∼35 kcal/mol in the studied cases). We thus conclude that the interaction between the frontier‐orbitals govern the reaction, through changes in the distortion energy. We also show that the lifetime of the zwitterionic intermediate determine the relative stereochemistry of the products.

Quantum chemical modelling shows that energy barriers for excited-state proton transfer reactions involving 2-pyridone are governed by the transient onset of antiaromaticity upon interaction between the photoexcited state and a second excited state.

Excited-state aromaticity (ESA) and antiaromaticity (ESAA) are by now well-established concepts for explaining photophysical properties and photochemical reactivities of cyclic, conjugated molecules. However, their application is less straightforward than the...

Today’s genetic composition is the result of continual refinement processes on primordial heterocycles present in prebiotic Earth and at least partially regulated by ultraviolet radiation. Femtosecond transient absorption spectroscopy and state-of-the-art ab initio calculations are combined to unravel the electronic relaxation mechanism of pyrimidine, the common chromophore of the nucleobases. The excitation of pyrimidine at 268 nm populates the S1(nπ*) state directly. A fraction of the population intersystem crosses to the triplet manifold within 7.8 ps, partially decaying within 1.5 ns, while another fraction recovers the ground state in >3 ns. The pyrimidine chromophore is not responsible for the photostability of the nucleobases. Instead, C2 and C4 amino and/or carbonyl functionalization is essential for shaping the topography of pyrimidine’s potential energy surfaces and results in accessible conical intersections between the initially populated electronic excited state and the ground state.