A Lewis Acidic Stroll Through the p-Block
Since the inception of frustrated Lewis pair (FLP) chemistry more than a decade ago, the development of an assortment of main group Lewis acids in the literature has contributed to the enrichment and diversification of FLP reactivity to achieve transition metal-like chemistry. While most of the employed Lewis acids were initially boron-based, exploration of the remainder of the p-block has led to the expansion of the boundaries of reactivity. The present work focuses on the exploration of Lewis acids based on the scantly explored p-block elements in the context of FLP and Lewis acid chemistry. Fluorosulfoxonium cations containing a Lewis acidic sulfur(VI) center were prepared from sulfoxides in an analogous fashion to previously reported fluorophosphonium cations. These cations were shown to enact Lewis acid catalysis via a S-F σ*-orbital and are among the first sulfur-based Lewis acids to be employed in such chemistry. Azophosphonium cations were prepared from the reaction between diazonium cations and phosphines. Their Lewis acidity is derived from the masked nitrenium character of one of the nitrogen centers. The azophosphonium cations act as one-electron acceptors to form paramagnetic species, and a combination of the bulky phosphine PtBu3 and one of the azophosphonium cations acted as an FLP to effect the cleavage of disulfides. Phosphorus(V) dications containing both an ortho-quinone and terpyridine backbone were prepared and shown to effect the Lewis acid-mediated hydrodefluorination (HDF) of fluoroalkanes with silanes, despite the phosphorus center being coordinately saturated. Computational data support the notion that these dications are in fact Lewis acidic at the para-carbon of the central ring of the terpyridine ligand, with the phosphorus center not directly participating in the chemistry. Following these results, ethylene-bridged bipyridinium dications were prepared to verify their reactivity without a phosphorus center. Indeed, these dications also displayed similar Lewis acidic behavior, acting as one-electron acceptors and effecting the same HDF catalysis, further supporting the notion of the para-carbon center acting as the electrophilic site of reactivity.
acid, catalysis, FLP, group, lewis, main
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