Radical cascades converting oligoalkynes to carbon-rich polycyclic structures (GEOSET presentation by Kerry Gilmore)
We took advantage of the new rules for alkyne cyclizations for the design of radical cascades aimed at the preparation of polyaromatic ribbons from conjugated and skipped oligoalkynes. These processes correspond to controlled “polymerization” of alkyne moieties positioned between the two rows of aromatic rings via a selective sequence of “all-exo” cyclizations.
Although the regioselectivity of cycle formation is well controlled by the exo preference for alkyne cyclizations, the key remaining challenge in the design of these cascades involves control over chemoselectivity of initial radical attack. An additional challenge lies in avoiding the formation of pentagonal units at initiation and termination points of the oligoalkyne cascade.
We have recently used “directing groups” for intermolecular control of reactivity and introduced “skipped” alkynes are substrates in these cyclization.
The evolution of molecular design shown below illustrates how a more precise control over shape and functionality of the polyaromatic ribbons can be accomplished by the proper choice of starting materials for the radical cyclization cascades.
Byers, P.; Alabugin, I. V. Polyaromatic Ribbons from Oligo-Alkynes via Selective Radical Cascade: Stitching Aromatic Rings with Polyacetylene Bridges, J. Am. Chem. Soc. 2012, 134, 9609–9614. http://pubs.acs.org/doi/abs/10.1021/ja3023626. (Highlighted in SYNFACTS)
Traceless Directing Groups in Radical Cascades: From Oligoalkynes to Fused Helicenes without Tethered Initiators. Pati, K.; Gomes, G. P.; Harris, T.; Hughes, A.; Phan, H.; Banerjee, T.; Hanson, K.; Alabugin, I. V. J. Am. Chem. Soc. 2015, 137, 1165-1180. http://dx.doi.org/10.1021/ja510563d.
Alkenes as Alkyne Equivalents in Radical Cascades Terminated by Fragmentations: Overcoming Stereoelectronic Restrictions on Ring Expansions For the Preparation of Expanded Polyaromatics. Mohamed, R.; Mondal, S.; Gold, B.; Evoniuk, C. J.; Banerjee, T.; Hanson, K.; Alabugin, I. V. J. Am. Chem. Soc., 2015, 137, 6335-6349. http://pubs.acs.org/doi/abs/10.1021/jacs.5b02373.