Problems with Aryl Enediyne Cyclizations?
by mitch on Oct 17 2008 (3515 Views)Enediynes are lovely functional groups that are famous for undergoing Bergman cyclizations as shown below.
Scheme 1. Bergman cyclization.
Vavilala et al. have found an alternate reaction pathway for thermal enediyne cyclizations when bulky aryl groups are attached to the terminal ends; they observed indene formation.
Scheme 2. Pascal cyclization anyone?
Making indenes from enediynes is an old trick, either by photochemical or some exotic initiator, the caveat with this work is it runs under thermal conditions. The reaction they ran is shown below.
Scheme 3. Thermolysis (TCP = 2,4,6-trichlorophenyl)
No Bergman cyclization products were observed. The idea being, the distance of the terminal carbons (C1-C6) is crucial in determining which type of cyclization will occur. An interesting part of the paper is Matzger gets mentioned by name as a referee and as holding a different opinion of the authors, he basically argues the results are due to transfer hydrogenations from bergman cyclization intermediates. Definately an interesting read if you have time.
Link to paper: Thermal C1-C5 Diradical Cyclization of Enediynes
Mitch
I love when experts "colorfully" disagree with each other.
It seems that the presence of the chlorides reverse the electronics of the molecule thus making it a poor candidate for Bergman chemistry.
I'll come back to debate this when I've read the paper.