In the course of systematic studies on the chemistry of metal-complexed propargyl radicals, we found that an electronic effect of an alpha-substituent can attenuate the ionic nature of a C-OTf bond, and thus its reducibility with cobaltocene. pi-Donors, such as phenyl, naphthyl, alkenyl, alkynyl, and alkoxy groups, provide for ionicity, and reducibility of the C-OTf bonds, while sigma-donors (alkyl groups), an H-atom, and sigma-/pi-acceptors (ester groups) preserve the covalent nature of the C-OTf bonds, resisting a one-electron reduction with cobaltocene. Heterolytic BDE values were used to quantitate the impact of alpha substituents, to identify the “ionic” and “covalent” domains for electronically diverse propargyl triflates, and to also make predictions for new types of substituents and new classes of organic compounds. The methodology was applied to inter- and intramolecular reactions, with the former allowing for site differentiation in propargyl polyethers, thus paving way for a long sought after radical-ionic alpha,alpha’-functionalization in propargyl systems, and the latter providing access to alkynylated cyclohexanes integrated into the 1,1’-biphenyl- and 1,1’-binaphthyl carbon frameworks. The reaction scope and its synthetic potential, along with low-temperature, multinuclear NMR studies and semi-empirical/DFT calculations will be discussed.

Metal-mediated radical coupling of propargyl triflates: Acquiring a prognostic power in inter- and intramolecular reactions

253rd National Meeting (2017)

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