Nitrogen‐Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation

Abstract

A room-temperature-stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO-LUMO gap compared to that of a 1,2,3-triazolium 5 (an N-heterocyclic nitrenium cation). The low-lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me P, nBu P, and IiPr. The N-based Lewis acid 2 also forms an FLP with tBu P but subsequently reacts with (PrS) to cleave the S-S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N analogues.

Lewis Superacids: Classifications, Candidates, and Applications

“…[159] Nitrogen:O nly recently,N -heterocyclic nitrenium (NHN) or cyclic alkylamino nitrenium (CAAN)i ons have been demonstrated as the first nitrogen-centeredL ewis acids. [160] Even thought he Lewis acidity is far from being strong, it might be increased in the future by electronic or geometricall igand effects. Phosphorous:N eutral phosphorous(III) speciesa re typical Lewis bases.…”

The Road Travelled: After Main‐Group Elements as Transition Metals

“…Many more donor/acceptor combinations of main‐group elements have been isolated (as predicted by Power) and have led to a variety of new reactions through new mechanistic pathways . For example, the use of phosphonium cations have been shown to catalyse a range of reactions, whilst a very recent example has shown group 15’s lightest element, nitrogen, is also capable of FLP type reactivity due to the Lewis acidic nitrenium cation . This rapid ascent of FLP chemistry has even made transition metal chemists take note.…”

Nitrogen‐Based Lewis Acids Derived from Phosphonium Diazo Cations

“…More recently we have described a strategy exploiting the analogy between CAACs (cyclic alkyl amino‐carbenes) and CAAN (cyclic diazenium cations—cyclic (alkyl)(amino)nitrenium cations) to enhance the Lewis acidity at nitrogen (Figure ). We showed that such readily accessible species form adducts with phosphine or carbene donors and also participate in FLP reactivity . In addition, we have also shown that a cyclic (amino)(aryl)nitrenium cation derived from spiro[fluorene‐9,3′‐indazole] acts as a Lewis acid, while deprotonation provides a Lewis basic species …”