Physical Chemistry

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

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Abstract

Photochemical reactions exemplify ‘green’ chemistry and are essential for synthesizing highly strained molecules with mild conditions with light. The light-promoted denitrogenation of bicyclic azoalkanes affords functionalized, stereoenriched bicyclo[1.1.0]butanes. We revisited these reactions with multireference calculations and non-adiabatic molecular dynamics (NAMD) simulations for a series of diazabicyclo[2.1.1]hexenes to predict the photophysics, reactivities, and stereoselectivities. 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 excitation is consistently n→π* and ranges from 3.77–3.91 eV for the diazabicyclo[2.1.1]hexenes. Minimum energy path calculations showed stepwise C–N bond breaking and led to a minimum energy crossing point, which favors the stereochemical ‘double inversion’ bicyclobutane product. Wigner sampling of 1 provided Franck-Condon points 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 non-stereoselective outcomes. The predicted diastereomeric excess almost exactly match 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 diastereomeric excess (d.e.) of 94–97% for the double inversion products. Halogenation significantly perturbs the potential energy surface (PES) towards the retention products because of powerful hyperconjugative interactions. The nC→σ*C–X, X = F, Cl, Br hyperconjugative interaction leads to a broadened shoulder region on the PES for double inversion.

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