Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Expedient Synthesis of Thermally Stable Acyclic Amino(haloaryl)carbenes: Experimental and Theoretical Evidence of “Push–Pull” Stabilized Carbenes

“…Thus, 2 behaves essentially as a Lewis base and accounts for the widespread use of NHCs not only as ligands for transition metals but also as organocatalysts for (macro)­molecular synthesis . Since these pioneering discoveries, a myriad of stable singlet carbenes have been isolated, the vast majority being cyclic and thermodynamically stabilized by two nitrogen atoms. − Bertrand and co-workers later showed that only one amino group is sufficient to stabilize the carbene, as in monoaminocarbenes (MACs), which include monosubstituted carbene, acyclic amino­(alkyl)­carbene (AAAC) 3 , amino­(aryl)­carbene (AAArC) 4 , and cyclic­(alkyl)­(amino)­carbenes (CAACs) 5 and 6 (Figure A). − MACs exhibit an intermediate Δ G ST and a rather small HOMO–LUMO gap, with a low-lying LUMO. Hence, MACs are both more nucleophilic (σ-donating) and more electrophilic (π-accepting) than NHCs, i.e., more ambiphilic, which has enabled them to activate strong chemical bonds, such as H–H, B–H, P–H, Si–H, or N–H, a difficult task that has long been exclusively reserved to transition metals. , …”