A Theoretical Stereoselectivity Model of Photochemical Denitrogenations of Diazoalkanes Toward Strained 1,3-Dihalogenated Bicyclobutanes

Photochemical reactions exemplify "green" chemistry and are an essential tool for synthesizing highly strained molecules under mild conditions with light. The light-promoted denitrogenation of bicyclic azoalkanes affords functionalized, stereoenriched bicyclo[1.1.0]butanes. These reactions were revisited with multireference calculations and non-adiabatic molecular dynamics (NAMD) simulations to provide a detailed analysis of the photophysics, reactivities, and unexplained stereoselectivity of a series of diazabicyclo[2.1.1]hexenes. We used complete active space self-consistent field (CASSCF) calculations with an (8,8) active space and ANO-S-VDZP basis set; the CASSCF energies were corrected with CASPT2 (8,8)/ANO-S-VDZP. The nature of the electronic excitation is n → π* and ranges from 3.77 to 3.91 eV for the diazabicyclo[2.1.1]hexenes reported here. Minimum energy path calculations showed stepwise C-N bond breaking and led directly to a minimum energy crossing point, corresponding to a stereochemical "double inversion" product. Wigner sampling of diazabicyclo[2.1.1]hexene provided initial conditions for 692 NAMD trajectories. We identified competing complete stereoselective and stereochemical scrambling pathways. The stereoselective pathways feature concerted bicyclobutane inversion and N₂ extrusion. The stereochemical scrambling pathways involve N₂ extrusion followed by bicyclobutane planarization, leading to stereochemical scrambling. The predicted diastereomeric excess (d.e.) almost exactly matches the experiment (calc.d.e. = 46% vs exp.d.e. = 47%). Our NAMD simulations with 672, 568, and 596 trajectories for 1-F, 1-Cl, and 1-Br predicted a d.e. of 94-97% for the double inversion products. Halogenation significantly perturbs the potential energy surface (PES) toward the retention products due to hyperconjugative interactions. The n_C → σ*_{C-X, X = F, Cl, Br} hyperconjugative effect leads to a broader shoulder region on the PES for double inversion.

Unusually Long-Wavelength Emissions of Cyclopropanes: New Insight into C-C Bond Homolysis

Bond homolysis (BHo) is a fundamental concept in chemical-bonding phenomena. To date, research studies on the BHo concept have provided crucial information for understanding the nature of chemical bonding and reactions. Two potential-energy minima, a σ-bonding isomer and a singlet-diradical isomer, have been known to exist in carbon-carbon BHo. Herein, a third isomer, that is, a puckered singlet diradical exhibiting unstructured long-wavelength fluorescence beyond 460 nm, was first observed in the excited states of 1,4-diarylbicyclo[2.1.0]pentane derivatives. The careful selection of appropriate substituents in the bicyclic structures enabled direct spectral detection. State-of-the-art ab initio quantum chemical calculations quantitatively reproduced the experimental observations. This new finding provides new insight into carbon-carbon bond-breaking and -forming processes.