Effect of Chalcogen-Phosphorus Substituents on Enediynes Undergoing the Bergman Cyclization.

We have theoretically characterized electrocyclizations of chalcogen-phosphorus-containing enediynes. We performed quantum calculations at the BS-(U)CCSD/cc-pVDZ level to analyze the geometries and energetics of two different cyclization pathways, each consisting of the first three chalcogens (oxygen, sulfur, and selenium). The first pathway involved the cyclization of the chalcogen-phosphorus substituents followed by Bergman cyclization, while the second pathway proceeded via Bergman cyclization followed by chalcogen-phosphorus cyclization. To more accurately understand the energies of the diradicals involved in each pathway, we also performed spin-flip characterizations using UHF reference wave functions and the spin-flip formulation of the equation-of-motion coupled cluster theory with singles and doubles method. The addition of the chalcogen-phosphorus substituents to the six-membered acyclic enediyne leads to a lowering in the reaction energy of the Bergman cyclization, from +7.84 kcal/mol for (Z)-hexa-3-ene-1,5-diyne to +6.07, +3.71, and +3.47 kcal/mol for the oxygen, sulfur, and selenium congeners, respectively. Additionally, the formation of the doubly cyclized product is slightly unfavorable for the oxygen species (+0.70 kcal/mol) and energetically favorable for S and Se (-5.50 and -9.05 kcal/mol, respectively). The chalcogen cyclization is energetically favorable whether or not the p-benzyl diradical moiety is present. We also confirmed the aromaticity of these structures as well as the nature of their ground-state wave functions.