The Chemistry of N-heterocyclic nitrenium Ions
Abstract: N-heterocyclic nitrenium salts (NHNs) – nitrogen-centered, isoelectronic and
isostructural analogues of N-heterocyclic carbenes (NHCs) – become increasingly featured
in the fields of organometallic chemistry, main-group chemistry and organocatalysis. In the
first study of NHNs conducted by the Gandelman group, NHNs were utilized as the core of
pincer-ligands for transition-metal complexes and their considerable π-accepting capability
was demonstrated. The group later discovered that NHNs, namely triazolium and triazinium
salts, are in fact an uncommon example of nitrogen-based Lewis acids, capable of forming
donor-acceptor adducts with a myriad of Lewis bases, such as organolithium reagents and
both protected and unprotected phosphides. The most acidic NHN investigated in that
study – 1,3-dimethyl-naphthotriazinium 1 (Central molecule in the scheme, R1=R2=Me) –
also formed adducts with tertiary alkyl-phosphines such as Me3P and n-Bu3P. Notably, an
adduct was not detected when tBu3P was combined with 1, thus it was theorized that they
may exemplify a frustrated Lewis pair (FLP). In this talk, I present my findings about the
chemistry of NHNs in 4 different studies:
1. Lewis acids in FLPs: The combination of NHNs and tBu3P generate a reactive FLP
capable of efficiently cleaving Si-H bonds in silanes, generating stable and hydridic N-H
triazanes.
2. Lewis acid catalysts: NHNs composed of N-aryl substituents were synthesized and
characterized. Their superior capabilities as Lewis acid catalysts compared with the N alkylsubstituted
NHNs were demonstrated through the facile hydrosilylation of imines.
3. Intramolecular Lewis pairs: A systematic investigation of the structure, properties
and reactivity of intramolecular Lewis pairs comprising NHNs and phosphines.
4. Electrophilic amination: NHNs were utilized in combination with organolithium and
Grignard reagents for the facile electrophilic amination of various alkyl and aryl groups. 15N
labeled biologically active molecules were synthesized.