Cyclic (Amino)(aryl)carbenes (CAArCs) as Strong σ‐Donating and π‐Accepting Ligands for Transition Metals

Abstract

Cyclic (amino)(aryl)carbenes (CAArCs) result from the replacement of the alkyl substituent of cyclic (alkyl)(amino) carbenes (CAACs) by an aryl group. This structural modification leads to enhanced electrophilicity of the carbene center with retention of the high nucleophilicity of CAACs, and therefore CAArCs feature a small singlet-triplet gap. The isoindolium precursors are readily prepared in good yields, and deprotonation at low temperature, in the presence of [RhCl(cod)]2 and [(Me2S)AuCl] lead to air-stable rhodium and gold CAArC-supported complexes, respectively. The rhodium complexes promote the [3+2] cycloaddition of diphenylcyclopropenone with ethyl phenylpropiolate, and induce the addition of 2-vinylpyridine to alkenes by CH activation. The gold complexes allow for the catalytic three-component preparation of 1,2-dihydroquinolines from aniline and phenyl acetylene. These preliminary results illustrate the potential of CAArC ligands in transition-metal catalysis.

Cobaltocenylidene: A Mesoionic Metalloceno Carbene, Stabilized in a Gold(III) Complex

“…The calculated singlet‐triplet gap is in between 20.0 and 27.0 kcal mol −1 , depending on the density functional employed . Thus, it is very small compared to those of NHCs, cyclic (alkyl)aminocarbenes (CAACs), and even cyclic (amino)(aryl)carbenes (CAArCs) . However, the triplet species of CcC is the organometallic biradical cobaltocenyl [Cc=η 5 ‐(C 5 H 5 )‐η 5 ‐C 5 H 4 )Co] containing a cyclopentadienyl radical and a paramagnetic cobalt(II) center, clearly not directly comparable to triplet species of NHCs.…”

“…Experimental evaluations of the σ‐donor character and the π‐backbonding properties of NHCs are commonly made by measurements of the IR stretching frequencies of their metal carbonyl complexes, [Ni(CO) 3 (NHC)], [RhCl(CO) 2 (NHC)], or [IrCl(CO) 2 (NHC)], 18b and by 31 P or 77 Se NMR measurements of their phosphinidene derivatives, NHC–P(C 6 H 5 ), or selenium adducts, NHC–Se . Unfortunately, Ni 0 /Rh I /Ir I carbonyl complexes and phosphinidene or selenium derivatives containing the CcC ligand are not accessible by an oxidative addition methodology.…”

“…[15] Thus, it is very small compared to those of NHCs, [2] cyclic (alkyl)aminocarbenes (CAACs), [16] ande ven cyclic (amino)(aryl)carbenes (CAArCs). [17] However,t he triplet specieso f CcC is the organometallic biradical cobaltocenyl [Cc = h 5 -(C 5 H 5 )-h 5 -C 5 H 4 )Co] containing ac yclopentadienyl radical and a paramagnetic cobalt(II) center,c learly not directly comparable to triplet species of NHCs. To provide am etric for the characterization of CcC in comparison to other NHCs, the To lman electronic parameter (TEP) [18] was calculated and av alue of 2037.1 cm À1 was obtained,t hereby ranking CcC with respect to its ligand properties in between CAACsa nd CAArCs (Scheme 3).…”

“…Experimental evaluations of the s-donor character and the p-backbonding properties of NHCs are commonly made by measurements of the IR stretching frequencies of their metal carbonyl complexes,[ Ni(CO) 3 (NHC)],[ RhCl(CO) 2 (NHC)],o r[ IrCl(-CO) 2 (NHC)], 18b and by 31 Po r 77 Se NMR measurements of their phosphinidene derivatives, NHC-P(C 6 H 5 ), [16,17] or selenium adducts, NHC-Se. [19] Unfortunately,N i 0 /Rh I /Ir I carbonyl complexes and phosphinidene or selenium derivatives containing the CcC ligand are not accessible by an oxidative addition methodology.H owever, given the fact that cationic cobaltoceniumyl is a strong electron-accepting aromatic moiety,i tw as proved to be possible to synthesize CcC-Se adduct (5)b ya ne lectrophilic aromatic substitution reactiono fs odium selenide 20 with iodocobaltocenium iodide (4;S cheme 4).…”

Cyclic (Alkyl)(amino)carbenes (CAACs): Recent Developments

“…Noteworthy is the synthesis of carbenes closely related to CAACs, namely acyclic (alkyl)(amido)carbene (CAAmC) [47] and cyclic (amino)(aryl)carbenes (CAArCs) [48] (Scheme 3).…”