Evidences of the Stabilization for Weeks of the Benzyl Cation in Zeolites

The benzyl cation is an iconic intermediate in organic chemistry which has not been isolated yet. Here, we show that the incorporation of tropylium cations in commercially available sodium zeolites, after cation exchange, plausibly triggers the spontaneous formation of benzyl cations inside the zeolite pores at room temperature. The zeolite plays a bifunctional role as a host. First, the unimolecular rearrangement reaction occurs by the ability of the zeolite to isolate tropylium molecules and achieve the required energy to transform tropylium into benzyl cations. Then, the compartmentalized nature of the zeolite framework avoids the quenching of the so–formed benzyl cations with each other, or any other nucleophile, allowing lifetimes of weeks for the benzyl cations under ambient conditions. In this way, a customary organic characterization of the zeolite–embedded benzyl cation by absorption ultraviolet–visible spectrophotometry and ¹³C nuclear magnetic resonance (¹³C NMR) has now been possible. Transient absorption spectroscopy and reactivity studies, together with molecular dynamic calculations, further support the formation of the benzyl cation inside the zeolite. These results bring this fundamental carbocation intermediate to our laboratories as a manageable organic compound.