posted on 2021-05-21, 02:29authored byJed M. Burns, Timothy Clark, Craig M. Williams
The
course of the Barton–Kellogg (BK) reaction for alkyl-
and aryl-substituted substrates has been investigated at the DLPNO-CCSD(T)/def2-TZVPP//ωB97X-D/def2-TZVPP
level of theory, with results compared to available experimental kinetic
data. Through comparison with the unsubstituted parent system, the
preference for the formation of 1,3,4-dihydrothiadiazole over the
isomeric 1,2,3-dihydrothiadiazole was observed to result from reduced
steric repulsion in the relevant transition-state structure. Nitrogen
extrusion [retro-(3 + 2)-cycloaddition] from the intermediate dihydrothiadiazole
was found to be the rate-determining step. The barrier for this process
was, however, significantly lower for aromatic substrates, which is
consistent with the difficulty in isolating aryl-substituted dihydrothiadiazoles.
The electronic structure of the transient thiocarbonyl ylide was also
investigated, highlighting the contradictory results from wave-function
theory- and density functional theory-based methods. Correlation of
unrestricted natural orbital eigenvalues with previous experimental
models suggested that the dipole intermediates possess low diradical
character and are therefore considered to be closed-shell species.
Exergonic conrotatory electrocyclization of the dipole led to sterically
congested thiirane products, even for very bulky systems (di-t-butyl). These results complement the recent work of Mlostoń
et al. Finally, DLPNO-CCSD(T)//ωB97X-D was found to be a reliable
method for estimating the feasibility of the BK reaction, which should
assist experimentalists in the selection of viable substrates.