From Diphosphino‐Functionalized 1,3‐Dialkylimidazolium Cations to Imidazolones through Dehydrogenative CN Coupling

Abstract

Diphosphino-functionalized 1,3-dialkylimidazolium salts react with KOH affording amine/formamide open-chain products, which fully revert to the imidazolium cation by treatment with a variety of acids or are converted to 2-imidazolones by noncatalyzed intramolecular dehydrogenative C-N coupling, a process that is modulated by coordination of the phosphino functionalities to transition metals.

Benzimidazolium‐ and Benzimidazolilydene‐Capped Cyclodextrins: New Perspectives in Anion Encapsulation and Gold‐Catalyzed Cycloisomerization of 1,6‐Enynes

“…[9] The situation is quite different with the AC regioisomer 16,t he cavity of which is not suited for accommodating the PF 6 À anion.I nt his case, the NÀCH=Np rotonr esonates at am uch higherf ield (d = 9.15 ppm) in keeping with aw eaker CH···F H-bond and the inner-cavity protons are not downfields hifted( d H-5 A,C = 4.70 and 4.78 ppm). This was confirmed by a 1 H- 19 FH OESY experiment, which clearly revealed aw eakc orrelation between fluorine atoms and the benzimidazolium H-2 proton, but strong correlationsb etween the same fluorine atoms and CD outward-oriented protons (6-OMe, H-6 Ao rC )t ogether with other benzimidazolium protons. Clearly,i nt his case the PF 6 À anion is located outsidet he cavity just above the CD primary face.…”

“…Here the d value for the NÀ CH=Np roton (d = 9.68 ppm) lies between that of 12 and 14, which is indicative of some degree of hydrogen bonding between at least one fluorine atom of the PF 6 À anion and the benzimidazolium NÀCH=Np roton. This interaction was confirmed by the presence of cross-peaks arising from troughspace correlations between fluorine atoms and the NHC H-2 proton in the 1 H- 19 FH OESY spectra of 15. [14] Moreover,the only other protons that correlate with the PF 6 À fluorine atoms are inner-cavity ones (some of the H-3, H-5 and OMe-3 protons of the CD unit).…”

“…Experimental details including NMR spectra and X-ray structural data are given in the Supporting Information. CCDC 1863327 (12), 1853979 (15), 1863324 (17)a nd 1851144 (19)c ontain the supplementary crystallographic data for this paper.T hese data can be obtained free of charge from The Cambridge Crystallographic Data Centre.…”

“…Although the mechanism of formation of 18 remains unclear, it is likely to be metal‐mediated as the reaction does not occur without gold. Furthermore, it most probably involves a hydrolysis step followed by dehydrogenation rather than direct oxidation as carbenes are not known to be easily oxidized neither with oxygen nor with other oxidizing agents . Clearly, the heavily encapsulated AD carbene intermediate is less easily metallated than its AC counterpart so that metallation competes with carbene oxidation in the case of the former.…”

“…Although the mechanism of formation of 18 remains unclear,i ti sl ikelyt ob em etal-mediated as the reaction does not occur without gold. Furthermore, it most probably involves a hydrolysis step followed by dehydrogenation [19] rather than direct oxidationa sc arbenes are not knownt ob ee asily oxidized neither with oxygen nor with other oxidizing agents. [20] Clearly,t he heavily encapsulated AD carbene intermediate is less easily metallated than its AC counterpart so that metallation competes with carbene oxidation in the case of the former.A sf or their perbenzylated counterparts, both complexes 17 and 19 display strongly deshielded inner-cavityH -5 AD/AC and H-3 protons (Figure 2, bottom)s hows that the benzimidazolidene unit is not parallel to the CD axis but is tilted by an angle of about1 0.78,avalue that is larger than in both imidazolium salts 12 and 15.…”