Cyclische Alkylaminocarbene (CAACs): Neues von guten Bekannten

Abstract

Ein Kennzeichen der seit 2005 bekannten cyclischen Alkylaminocarbene (CAACs) ist ihre ausgeprägte Nucleophilie (σ‐Donor) und zugleich Elektrophile (π‐Akzeptor). Diese Eigenschaften ermöglichen die Aktivierung einer Vielzahl kleiner Moleküle und starker Bindungen, die Stabilisierung hochreaktiver dia‐ und paramagnetischer Spezies der Haupt‐ und Nebengruppenelemente sowie die Bildung stabiler Bindungen mit Metallen, was zu robusten Katalysatoren führt. Dieser Aufsatz fasst die wichtigsten Ergebnisse vor 2014 kurz zusammen, wobei der Schwerpunkt jedoch auf den Erkenntnissen der vergangenen drei Jahre liegt.

Switching the Electron‐Donating Ability of Phosphines through Proton‐Responsive Imidazolin‐2‐ylidenamino Substituents

“…The TEP value of phosphine 1 (2047.5 cm À1 ) [14a] increasedb y2 2.9 cm À1 upon protonation of the imino Na toms [1·HOTf:2 070.4 cm À1 ]. On the TEP scale this would correspond to switchingf rom the range of cyclic (alkyl)(amino)carbenes (CAACs) [16] to arylphosphines. More remarkably,t he successive protonation of the imino Na toms in 2 stepwise decreased of Scheme4.Synthesis of the neutraland cationic Au I complexes 4, 4·HOTf and 4·2 HOTf.…”

New Aspects of Recently Developed Rhodium(N‐Heterocyclic Carbene)‐Catalyzed Organic Transformations

“…While the phosphine ligands in Ni(CO) 3 (PR 3 ) give an average carbonyl stretching between 2060 and 2070 cm −1 , NHC ligands show a significantly weaker CO stretching band ,. Recently, cyclic alkyl amino carbene (CAAC) ligands have been developed, and they are considered to be stronger σ donors than traditional NHC ligands ,. As a result, NHC ligands can effectively stabilize transiently generated electron‐deficient metal species during catalysis.…”

Experimental Assessment of the π‐Acidity of Anionic and Neutral Malonate‐Derived N‐Heterocyclic Carbenes

“…NHCs are indeed well‐known to be strong σ‐donating compounds, and they can also exhibit a significant π‐acidity character if the vacant antibonding π* orbital centered on the carbene carbon atom (generally the LUMO) is of sufficient low energy . This was mainly achieved by controlling the nature of the carbenic heterocycle …”