A Radical Mechanism for Frustrated Lewis Pair Reactivity

• Published in: Chem Vol. 3; no. 2; pp. 259 - 267
• Main Authors: Liu, Liu (Leo), Cao, Levy L., Shao, Yue, Ménard, Gabriel, Stephan, Douglas W.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 10.08.2017
• Subjects: frustrated Lewis pairs; heterolytic cleavage; homolytic cleavage; mechanism; radical; SDG12: Responsible consumption and production; tin hydride; homolytic cleavage; tin hydride; heterolytic cleavage; radical; SDG12: Responsible consumption and production; mechanism; frustrated Lewis pairs
• ISSN: 2451-9294; 2451-9294
• DOI: 10.1016/j.chempr.2017.05.022

The frustrated Lewis pairs (FLPs) derived from tBu3P and E(C6F5)3 (E = B, Al) react with pO2C6Cl4 and Ph3SnH to give [tBu3POC6Cl4OE(C6F5)3] (E = B 1, Al 2), [tBu3PSnPh3][HB(C6F5)3] 3, and [tBu3PSnPh3][(μ-H)(Al(C6F5)3)2] 4. These products form via the accepted two-electron process involving a transient “encounter complex.” In contrast, the corresponding reactions of Mes3P and E(C6F5)3 (E = B, Al) with pO2C6Cl4 give [(Mes3POC6Cl4OE(C6F5)3] (E = B 8, Al 9); however, the identification of the intermediates [Mes3P·]2[(C6F5)3EOC6Cl4OE(C6F5)3] (E = B 5, Al 6) supports a mechanism that proceeds via a single-electron transfer process. This is further supported by the reactions of Mes3P and E(C6F5)3 (E = B, Al) with Ph3SnH, which yielded [Mes3PH][HB(C6F5)3] 10 and [Mes3PH][(μ-H)(Al(C6F5)3)2] 11, respectively, with the concurrent formation of Ph3SnSnPh3. [Display omitted] •The observation of radical intermediates in the FLP system of Mes3P/E(C6F5)3•The reactions of Mes3P/E(C6F5)3 with p-O2C6Cl4 proceed via radical intermediates•The FLP system of Mes3P/E(C6F5)3 homolytically splits Sn–H bonds For many years, it was believed that the combination of a Lewis base and Lewis acid resulted in the formation of a Lewis acid-base adduct, and consequently the neutralization quenched the donor-acceptor ability of Lewis pairs. This paradigm changed in 2006 with the discovery of frustrated Lewis pairs (FLPs). This finding paved the way for the rapid development of FLP chemistry and its application relevant to organic syntheses, enzymatic models, and material sciences. Understanding reaction mechanisms is crucial to progress in fundamental chemical research. The accepted mechanism of FLP reactivity involves the polarization of a substrate molecule in the pocket of an “encounter complex,” leading to heterolytic cleavage. The present work shows differing reaction mechanisms for FLPs derived from tBu3P/E(C6F5)3 and Mes3P/E(C6F5)3 (E = B, Al). This finding broadens both our understanding and the potential reactivity of FLP systems. The frustrated Lewis pairs (FLPs) derived from Mes3P and E(C6F5)3 (E = B, Al) react via initial single-electron transfer, generating radical cationic and anionic intermediates in contrast to the corresponding reactions of tBu3P and E(C6F5)3.