N-Cyclopropenio-imidazol-2-ylidene: An N-heterocyclic carbene bearing an N-cationic substituent

Abstract

Inspired by the success of α-cationic phosphines, a new cationic NHC, derived from IMes by the formal replacement of an N-mesityl substituent by a cationic cyclopropenium group, is described.

Hybrids of cationic [4]helicene and N-heterocyclic carbene as ligands for complexes exhibiting (chir)optical properties in the far red spectral window

“…First, the s-donor ability of the NHC was assessed by measuring the 1 J CH coupling constant between the carbon and hydrogen atom at the pre-carbenic position of precursor 2ÁH(PF 6 ) 2 . 46,47 The value of 1 J CH = 224 Hz measured for 2ÁH(PF 6 ) 2 was found slightly superior to that of A ( 1 J CH = 222 Hz) bearing the neutral mesityl fragment, and inferior to the 229 Hz reported for the cyclopropylium moiety B, 14 highlighting the somewhat low withdrawing character of cationic [4]helicene substituent. The p-character of NHCs can be tabulated using 77 Se NMR spectroscopy.…”

“…As for NHC B (Fig. 1), 14 the presence of the positive charge on the [4]helicene moiety was not a problem for the construction of organometallic complexes. After the in situ generation of free NHC 2(PF 6 ) using potassium tert-butoxide as a base, standard metalation procedures afforded selenium adduct 3(PF 6 ), rhodium(I) 4(PF 6 ), gold(I) 5(PF 6 ) and gold(III) 6(PF 6 ) 2 complexes in good yields (51-81%, Fig.…”

“…1). 14 In addition, chiral NHCs are readily prepared, [15][16][17][18] some of them using helicenes as stereogenic elements. [19][20][21][22][23][24] Helicenes are orthocondensed polyaromatic molecules, inherently chiral due to the steric repulsion of their skeleton termini.…”

“…48,49 The two isomers of the NHC-Se adduct 3(PF 6 ) exhibited 77 Se resonances at 65 and 49 ppm, downfield shifted compared to A-Se adduct (d Se = 10 ppm) revealing a slight increase of the p-character as compared to this neutral species, but less pronounced than for cationic B-Se adduct (122 ppm). 14 Finally, the Tolman Electronic Parameters (TEP) value of NHC 2(PF 6 ) was also investigated. The TEP allows to describe the effects of the combination of the s-donor and p-acceptor characters modulated by the sterics of the ligand.…”

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis–Ligand Development

“…Canac, Ceśar, and coworkers synthesized an NHC ligand bearing an N-cationic substituent (NHC-41) (Figure 13). 145 The stereoelectronic properties of NHC-41 were characterized. This carbene was found to be less donating than the analogous IMes ligand (NHC-3) (TEP = 2057 vs 2051 cm −1 , respectively) and more π-accepting, according to the δ Se chemical shift of their corresponding selenoureas in the 77 Se NMR spectra (δ Se = 112 and 27 ppm, respectively).…”