The Hexadehydro-Diels-Alder Cycloisomerization Reaction Proceeds by a Stepwise Mechanism.
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.
Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
Tao
T
Wang
Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
Dawen
D
Niu
Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
Thomas R
TR
Hoye
D000480:Alkynes / Q000737:chemistry; D002070:Butadienes / Q000737:chemistry; D002625:Chemistry, Organic / Q000379:methods*; D003500:Cyclization; D061565:Cycloaddition Reaction; D004583:Electrons; D005609:Free Radicals; D007700:Kinetics; D007783:Lactones / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D013053:Spectrophotometry
D000480:Alkynes; D002070:Butadienes; D005609:Free Radicals; D007783:Lactones; C031763:1,3-butadiene