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.