In exploring the conformational behavior of cyclic tungsten bis-alkyne complexes, two dialkynylamides (14a and 14c) and two dialkynylesters (14b and 14d) derived from 1,1’-ferrocenedicarboxylic acid were prepared. They were subsequently reacted with W(CO)3(dmtc)2 to yield the desired cyclic tungsten bis-alkyne complexes 8-11. In the cyclization of 14a to yield 8 a dimeric macrocyclic complex, 15, featuring two tungsten bis-alkyne complexes in the ring, also was isolated. The conformational behavior of these complexes was assessed by analysis of the 1H NMR resonances for the alkyne hydrogens, which appear around 11 ppm. The spectra for complexes 10, 11 and 15 show multiple singlets of varying integrations for these protons, while the spectra for complexes 8 and 9 show only two resonances of equal integration for the alkyne hydrogens. The spectra for 8 and 9 changed very little when examined at higher temperatures, indicating that the solution conformation is robust. A ROESY spectrum was obtained for 8. It did not show any crosspeaks between the two alkyne hydrogens. The NMR data shows that the alkyne ligands in 10, 11 and 15 are able to rotate about the tungsten-alkyne bond; these complexes adopted several different solution conformations relating to syn and anti arrangements ofmore »
This content will become publicly available on June 2, 2023
Localized Antiaromaticity Hotspot Drives Reductive Dehydrogenative Cyclizations in Bis- and Mono-Helicenes
We describe reductive dehydrogenative cyclizations that form hepta-, nona-, and decacyclic anionic graphene subunits from mono- and bis-helicenes with an embedded five-membered ring. The reaction of bis-helicenes can either proceed to the full double annulation or be interrupted by addition of molecular oxygen at an intermediate stage. The regioselectivity of the interrupted cyclization cascade for bis-helicenes confirms that relief of antiaromaticity is a dominant force for these facile ring closures. Computational analysis reveals the unique role of the preexisting negatively charged cyclopentadienyl moiety in directing the second negative charge at a specific remote location and, thus, creating a localized antiaromatic region. This region is the hotspot that promotes the initial cyclization. Computational studies, including MO analysis, molecular electrostatic potential maps, and NICS(1.7)ZZ calculations, evaluate the interplay of the various effects including charge delocalization, helicene strain release, and antiaromaticity. The role of antiaromaticity relief is further supported by efficient reductive closure of the less strained monohelicenes where the relief of antiaromaticity promotes the cyclization even when the strain is substantially reduced. The latter finding significantly expands the scope of this reductive alternative to the Scholl ring closure.
- Award ID(s):
- 2102579
- Publication Date:
- NSF-PAR ID:
- 10335316
- Journal Name:
- Journal of the American Chemical Society
- ISSN:
- 0002-7863
- Sponsoring Org:
- National Science Foundation
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