The iminium cation is among the most thoroughly studied functional groups in organic chemistry. Considering the research effort devoted to exploring the chemistry of iminium cations, it is intriguing that their phosphorus analogues, known as methylene phosphonium cations, remain sparsely investigated in literature. Compared to iminium ions, methylene phosphonium salts are less stable, and are extremely intolerant to atmospheric moisture, making their synthesis and isolation more difficult. Thus, molecules with methylene phosphonium functional groups remain elusive targets to inorganic chemists. Previous contributions from the research groups of Bertrand, Grützmacher, Dielmann, and others have found that 3-coordinate phosphonium cations exhibit unique reactivity and are highly electrophilic. Our group has begun to explore a different approach towards methylene phosphonium cations by investigating extended cumulenic carbon chains. Unexpectedly, this approach yielded allenic phosphonium borate zwitterions, prepared from a combination of alkynyl phosphane and an equivalent amount of the Lewis acid B(C6F5)3. The experimental and computational evidence in this study indicate that the mechanism for this transformation involves a phosphonium allenylidene akin to a methylene phosphonium cation. Our efforts towards stabilizing and isolating these species will be discussed.

Allenic phosphonium borate zwitterions via a transient phosphonium allenylidene

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ACS Fall 2022

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