First Examples of Structurally Imposing Eight-Membered-Ring (Diazocanylidene) N-Heterocyclic Carbenes: Salts, Free Carbenes, and Metal Complexes

Abstract

Eight-membered-ring (diazocanylidene) N-heterocyclic carbenes (8-NHCs) are reported for the first time. The precursor salts, readily prepared by the amidine route, are converted to the free carbene by treatment with KHMDS; in general, the free NHCs can be isolated, and in one example the molecular structure was obtained. Silver complexes were prepared by direct reaction of Ag 2 O with the diazocanylidinium salts, and Rh complexes were formed by treatment of in situ formed free carbene with the appropriate Rh precursor complex. The salts and complexes were fully characterized using spectroscopic methods, including in some cases by X-ray crystallography. Key features of these new ring-expanded heterocyclic carbenes (RE-NHCs) are the extreme steric strain they impose on the metal center (they are the most sterically imposing class of NHC reported to date) and their high donor capacity, being some of the most basic NHCs currently available.

Complexes Bearing Protic N-Heterocyclic Carbenes: Synthesis and Applications

“…Few complexes are known that bear a purine base-derived NHC ligand with an N-monoalkylated imidazole ring, although the first examples of complexes with theophylline and caffeinederived NH,NMe-NHC ligands of type [48] were described as early as 1975. 55 They were prepared by the treatment of the purine base with [RuCl(NH 3 ) 5 ] 2+ in the presence of zinc/ amalgam, followed by the addition of hydrochloric acid (Scheme 21), which led, in an acid-catalyzed tautomerization of the N-bound purine base, to the C 8 -bound NH,NMe-NHC ligand. A similar approach yielded the osmium(III) complex [49], also bearing a caffeine-derived NH,NR-NHC ligand.…”

“…1,2 Most of the described NHCs are derived from five-membered azolium heterocycles, while only a few examples for NHCs featuring six-, 3 seven-, 4 or eight-membered diamino-heterocycles 5 have been described. Using different NHC ligands, a large number of NHC complexes have been prepared.…”

[(1,2,5,6-η)-Cycloocta-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-ylidene-κC2)(triphenylphosphane-κP)iridium(I) tetrafluoridoborate

“…The Nheterocyclic carbene (NHC) ligands can be tuned sterically and electronically by having different alkyl groups on the nitrogen atoms (Gusev, 2009). Many related NHC rhodium and iridium complexes have been synthesized and structurally characterized (Kö cher & Herrmann 1997;Wang & Lin 1998;Chianese et al, 2004;Herrmann et al 2006;Nichol et al, 2009Nichol et al, , 2010Nichol et al, , 2011Nichol et al, , 2012Lu et al, 2011;Huttenstine et al, 2011, Idrees et al, 2017. Their catalytic activity in transfer hydrogenation reactions has also been studied and reported (Hillier et al, 2001;Albrecht et al, 2002;Gnanamgari et al, 2007).…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…106 The catalyst is also effective for the multi-Suzuki cross-coupling reaction (Table 8, entries [25][26][27]. The sterically hindered NHC complexes (C73a-f ) smoothly catalyzed the reactions of electron-rich and electron-poor aryl chlorides under mild reaction conditions in aqueous DMF (Table 8, entries [28][29][30].…”

“…• C (Table 12, entries [25][26][27][28][29][30]. 126 Additionally the catalyst could be reused five times without loss of activity (Table 12, entry 29).…”