Cyclic alkyl amino carbenes
Like NHCs, CAACs have tunable steric and electronic properties that make them versatile ligands in both transition metal and main group.
Addition of triflic anhydride (TfOTf) closes the cyclic system, producing an aldiminium salt that was deprotonated with LDA to yield the first CAAC "Ca".
[10][11] 6-membered CAACs have been synthesized by slight modification to the CAAC-5 procedure; the main change is manifested in increasing the chain length of the alkene used in step 2.
[4] Compared to Ru-CAAC-5, Six-membered Ru-CAAC-6 complexes also showed higher initiation rates for olefin metathesis, but increased steric bulk limited their catalytic activity.
[12] Classic diamino NHCs have been synthetically modified to produce more ambiphilic carbenes by expanding the size of the backbone.
The bicyclic system forces the substituents on the carbon to adopt a "fan-like" geometry closer to that of a diamino NHC.
The synthesis then follows the hydroiminiumation route in which the cyclization by HCl results in a bicyclic aldiminium salt that is then deprotonated to form the free carbene.
[3] This effect is evident in the higher percent buried volume (%VBur) of CAACs compared to diamino NHCs at a distance of 0 Å from the carbene.
For carbenes bearing a diiopropylphenyl group at the N substituent(s), the %VBur for CAACs (79.0-83.1) is markedly higher than the classical NHC (70.3).
[10][22][24][25] A major benefit of CAACs compared to other carbene or phosphine ligands is in their ability to stabilize highly reactive complexes that could not otherwise be isolated.
The strong σ-donor and π-acceptor properties, as well as the steric bulk offered by CAAC ligands has allowed for the stabilization of numerous low-valent complexes across the periodic table.
[6][26] A stable low valent Mg(I) radical has been reported, supported by a CAAC ligand which localizes the unfavorable spin density.
[43] CAAC-supported ruthenium ethenolysis catalysts to produce linear alpha olefins (LAOs) from biomass-derived compounds.
CAAC-Au(I) complexes have been shown to catalyze the production of allynes via cross coupling, hydroamination, hydroamoniumation, and methylamination reactions.
[11] Inclusion of a chiral center on the CAAC ligand allows for the production of β-substituted α,β-unsaturated esters with moderate enantioselectivities (up to 55%).
[8][11] The ambiphilic nature of CAACs gives them properties previously attributed to transition metals, such as the ability to undergo oxidative addition and reductive elimination.
[7] Given that many transition metals are scarce and expensive, activation of small molecules using CAACs has important implications for the development of sustainable processes.
[50] Copper and gold CAAC complexes exhibit photoluminescence, relevant to organic light emitting diodes (OLEDs).
Two coordinate (linear) Cu-CAAC complexes have weaker intermolecular interactions than other OLED candidates, allowing them to reach quantum efficiencies over 99%.
[52][53] CAAC-Cu(I) complexes are also thermally stable up to 270 °C and emit at ambient temperatures, making them good candidates for OLED devices.
