How To Achieve High Regioselectivity in Barrier-less Nucleophilic Addition to p‑Benzynes Generated via Bergman Cyclization of Unsymmetrical Cyclic Azaenediyne?

Inducing high regioselectivity in nucleophilic addition to p-benzynes, first reported by Perrin and O’Connor et al. (J. Am. Chem. Soc. 2007, 129, 4795−4799) has been a challenge as the reaction involves a very fast barrier-less addition of nucleophile. On the other hand, achieving a high degree of regioselectivity is important as that will make the reaction synthetically useful. Recently, a study has been reported from our group (J. Org. Chem. 2018, 83, 7730−7740), whereby it was shown that nucleophilic addition to p-benzynes derived from unsymmetrical N-substituted cyclic enediynes proceeds with low extent of selectivity by incorporation of groups with divergent electronic characters. Herein, we report that excellent regioselectivity (>99%) can be achieved keeping an ortho alkoxy group in unsymmetrical 1,2-dialkynylbenzene in the form of a cyclic enediyne in quantitative yields. High regioselectivity (∼84%) is also shown by pyridine based enediynes where the pyridine nitrogen is in a 1,3-relationship with the impending radical center, expanding the synthetic scope of this nucleophilic addition. The regioselectivity can be explained in terms of computed electrostatic potentials which are substantially different around two radical centers arising due to the “ortho effect” (conformational alignment of lone pair of the ortho alkoxy oxygen or the nitrogen in pyridine systems).