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

03 December 2020, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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.


quantum Mechanical Calculations
Solar Fuel Production
Organic photochemistry
conical intersection
molecular dynamics


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.