Bicyclic (Alkyl)(amino)carbenes (BICAACs): Stable Carbenes More Ambiphilic than CAACs

Abstract

A straightforward strategy allows for the synthesis of storable bicyclic (alkyl)(amino)carbenes (BICAACs), which feature enhanced σ-donating and π-accepting properties compared to monocyclic (alkyl)(amino)carbenes (CAACs). Due to the bicyclo[2.2.2]octane skeleton, the steric environment around the carbene center is different from that of CAACs and similar to that observed in classical N-heterocyclic carbenes. The different electronic properties of BICAACs as compared to CAACs allow for ligand exchange reactions not only at a metal center, but also at main group elements.

The Influence of C(sp3)H–Selenium Interactions on the 77Se NMR Quantification of the π‐Accepting Properties of Carbenes

“…To begin our study, we first considered the five‐membered cyclic (alkyl)(amino) carbenes (CAAC‐5) [17] . We previously reported that the 77 Se{ 1 H} NMR signal of the selenium adduct of Et CAAC‐5 1 a is at 481 ppm [12g] . As observed with the NHC series, we found that the 77 Se{ 1 H} NMR chemical shift of the selenium adducts of the more sterically hindered Menth CAAC‐5 (635 ppm) 1 b and Ad CAAC‐5 (683 ppm) 1 c (Figure 3) appeared considerably downfield.…”

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents

“…However, N-heterocyclic carbenes (NHCs) which are accessed by the deprotonation of an N , N ′-disubstituted imidazolium (or other azolium) salt are known to be stable and have been shown to be strong σ-donors with negligible π-accepting character 7. Recent reports indicate that the substituents on the carbene center play a crucial role on the σ-donor ability as well as π-electron accepting properties 8. Bertrand et al in 2005 reported that cyclic (alkyl)(amino)carbenes (cAACs) which have a nitrogen and a carbon flanking the carbene center have better σ-donor ability as well as better π-electron accepting properties than conventional NHCs 9.…”

The debut of chiral cyclic (alkyl)(amino)carbenes (CAACs) in enantioselective catalysis

“…The direct protonated precursors of CAACs are readily available in one pot from an aldehyde and a primary amine (Scheme 1(1)). 16 We have shown that our versatile synthetic methodology facilitates access to a library of 5-membered (CAAC-5), 6-membered (CAAC-6),12 bicylic (BiCAAC)17 and even bifunctional CAACs (FunCAAC)14 b (Scheme 1(2)). Of particular importance, the CAAC family features a quaternary carbon adjacent to the carbene carbon, thus allowing the introduction of a chiral center in closer proximity to the active site than NHCs.…”