To further assess the observed reactivity, a computational investigation of the reaction was initiated. We focused on examining the mechanism of the different 2+2 cycloadditions through DFT calculations paying particular attention to the excellent site-selectivity, regioselectivity and diastereoselectivity. For this purpose, the mechanistic pathway of cycloadduct 5 formation was explored at the SMD (MeCN) ωB97xD/Def2TZVPP//ωB97xD/Def2SVP level of theory in Figure (full computational details can be found in the Supporting Information). Upon sensitization of substrate, the generated triplet diradical state can undergo a site-selective bond rotation to generate the more stable 3 1Z structure through a minimal free energy barrier of 1.4 kcal/mol. Excited diradical 3 1Z can then return to the ground state to form the corresponding (Z),(E)isomer via radical recombination. Reexcitation of (Z),(E)-isomer with varying efficiency generates a dynamic photostationary state equilibrium between both geometrical isomers which is substrate dependent. This is in agreement with the rapid, site selective geometric isomerization observed experimentally for the different alkenes explored in Figure . In the triplet excited state, spin density is primarily located at both termini of the diene; at the carbon adjacent to boron and delocalized between the aromatic ring and the benzylic position. While this supports specifically neither isomerization nor 2+2 cycloaddition, rendering spin density as an inaccurate tool for regioselectivity prediction, it provides an excellent basis for the subsequent step-wise reactivity that occurs. The more reactive α-boryl radical for both Z and E states can undergo a radical addition to the double bond of 1,1-diphenylethylene. Interestingly, the addition of the E isomer is much faster than the Z isomer (ΔΔG ‡ = 4.4 kcal/mol) and the reaction is very exergonic forming preferentially 3 2E-anti, and making this step irreversible, defining the selectivity of the final E product in a Curtin-Hammet scenario, where the less stable isomer reacts faster than the most stable one. This is consistent with the fact that only one geometric isomer was observed experimentally. Two different conformers were calculated for this radical addition (see Figure ) and, in both cases, endo-like transition states were more stable than the exo-like ones (10.4 and 14.8 vs 12.8 and 15.9 kcal/mol).