Quantifying the Electronic and Steric Properties of 1,3-Imidazole-Based Mesoionic Carbenes (iMICs)

Abstract

Imidazole-based mesoionic carbenes (iMICs) are a very promising class of carbon-donor ligands in synthesis and catalysis. However, a systematic study of the quantification of stereoelectronic properties of iMICs is lacking. This is most likely due to the absence of rational synthetic routes to suitable iMIC compounds. Herein, we report the synthesis of the appropriate phosphinidene (iMIC Ar )PPh (iMIC Ar = :CCH(NDipp) 2 CAr; Ar = Ph (5a), DMP (5b); DMP = 4-Me 2 NC 6 H 4 ; Dipp = 2,6-iPr 2 C 6 H 3 ), selenium (iMIC Ar )Se (Ar = Ph (6a), DMP (6b)), and nickel carbonyl compounds (iMIC Ar )Ni(CO) 3 (Ar = Ph (7a), DMP (7b)). The π-accepting property of iMIC Ar (Ar = Ph (2a), DMP (2b)) has been evaluated by 31 P and 77 Se NMR spectroscopic analyses of 5 and 6, respectively. The overall donor property of 2 has been assessed by determining the Tolman electronic parameter (TEP) of 7. Studies suggest that iMIC Ar (2) are stronger σ donors and weaker π acceptors compared to classical N-heterocyclic carbenes (NHCs). The steric profile of iMIC Ar (2) has been obtained by calculating the percentage buried volume (%V bur ) for (iMIC Ar

Nickel‐Catalyzed Intramolecular 1,2‐Aryl Migration of Mesoionic Carbenes (iMICs)

“…The % V bur for 5 b (45.0 %) is considerably larger than those of 2 a , b (ca. 33 %) [4c] but compares well with that of IPr (44.5 %), [19] also containing two flanking Dipp substituents.…”

“…The exact mechanism for the formation of 3 a is currently unknown. The stronger σ‐donor property of iMIC Ph ( 2 a ) [4c] probably makes the Ni 0 center in the putative complex A electron‐rich (Scheme 1 b). [16] Oxidative addition of Ni 0 into the adjacent C−N bond via B is likely to form a Ni II species C , which eventually undergoes reductive elimination to yield 3 a .…”

“…[16] Oxidative addition of Ni 0 into the adjacent C−N bond via B is likely to form a Ni II species C , which eventually undergoes reductive elimination to yield 3 a . Note, (iMIC Ar )Ni(CO) 3 complexes comprising π‐acidic CO ligands are stable solids [4c] . The stoichiometric conversion of 2 a into 3 a can be translated into a catalytic process that is also applicable to other iMICs ( 2 b – g ).…”

“…[c] The first proton affinity (1 st PA) calculated at the B3LYP/def2‐TZVPP//def2‐SVP level of theory. [d] Calculated for ( 2 a )CuI and ( 2 b )CuBr [4c] . [e] Calculated for the corresponding AuCl adducts [19] …”

Nickel‐katalysierte intramolekulare 1,2‐Aryl‐Wanderung von mesoionischen Carbenen (iMICs)

“…Der genaue Reaktionsmechanismus für die Bildung von 3 a ist derzeit noch unklar. Wahrscheinlich erhöht die außerordentliche σ‐Donorstärke von iMIC Ph ( 2 a ) [4c] die Elektronendichte am Nickelatom und führt zu dem elektronenreichen Komplex A (Schema 1 b). [16] Die oxidative Addition von der benachbarten C‐N‐Bindung an das Ni 0 ‐Zentrum via B lässt die Ni II ‐Spezies C entstehen, welche durch reduktive Eliminierung 3 a ergibt.…”