We report here experiments showing that the hexadehydro-Diels-Alder (HDDA) cycloisomerization reaction proceeds in a stepwise manner - i.e., via a diradical intermediate. Judicious use of substituent effects was decisive. We prepared (i) a series of triyne HDDA substrates that differed only in the R group present on the remote terminus of the diynophilic alkyne and (ii) an analogous series of dienophilic alkynes (n-C7H15COC≡CR) for use in classical Diels-Alder (DA) reactions (with 1,3-cyclopentadiene). The R groups were CF3, CHO, COMe/Et, CO2Me, CONMe2/Et2, H, and 1-propynyl. The relative rates of both the HDDA cyclization reactions and the simple DA cycloadditions were measured. The reactivity trends revealed a dramatic difference in the behaviors of the CF3 (slowest HDDA and nearly fastest DA) and 1-propynyl (fastest HDDA and slowest DA) containing members of each series. These differences can be explained by invoking radical-stabilizing energies rather than electron-withdrawing effects as the dominating feature of the HDDA reaction.
Bibliographical noteFunding Information:
This research was supported by the National Institutes of Health (GM65597). NMR spectral data were collected with instrumentation acquired through the NIH Shared Instrumentation Grant Program (S10OD011952). T.W. received support from a Wayland E. Noland Fellowship; T.W. and D.N. each received support from a University of Minnesota Graduate School Doctoral Dissertation Fellowship.
© 2016 American Chemical Society.