Ein einfacher Zugang zu unsymmetrisch substituierten, gesättigten N‐heterocyclischen Carbenen

Abstract

Aus sekundären Aminen und Iminen sind unsymmetrisch substituierte nichtaromatische N‐heterocyclische Carbene wie rac‐2 über Imidazolidin‐2‐thione (rac‐1) als Zwischenprodukte zugänglich. Bereits ein einziger sterisch anspruchsvoller N‐Substituent in rac‐2 verhindert die Dimerisierung zum Entetramin.

Anellated N‐Heterocyclic Carbenes: 1,3‐Dineopentylnaphtho[2,3‐d]imidazol‐2‐ylidene: Synthesis, KOH Addition Product, Transition‐Metal Complexes, and Anellation Effects

“…The shielding by complexation (Dd 16-8 = À31.7 ppm) is markedly smaller than for the AgCl complex. A comparison with other anellated and nonanellated NHC-Rh(cod)Cl complexes indicates Dd values in a small range (À28 to À33.5 ppm) [10,11,45,47] and without clear trends. Similarly, the one-bond coupling constants 1 J( 13 C, 103 Rh) vary only slightly (50-53 Hz).…”

“…This is at considerably lower field than Rh(cod)Cl complexes of benzoanellated NHC (d = 195-196 ppm) [16a, 39b] , but upfield from the C 2 resonances of the Rh(cod)Cl complexes of imidazolin-2-ylidene ligands (d = 212-213 ppm) [45] , of the naphthoanellated pyrimidin-2-ylidene type 2 (d = 213.3 ppm) [10] , and of the isoelectronic but more electron-withdrawing quinoxaline-anellated dineopentyl-imidazol-2-ylidene (d = 219.5 ppm) [46] . The shielding by complexation (Dd 16-8 = À31.7 ppm) is markedly smaller than for the AgCl complex.…”

Heterocyclic Carbenes

“…Such ligands can be obtained by template-controlled cyclization of 2-functionalized phenyl [12] or ethyl isocyanides [13] and subsequent introduction of alkyl bridges between the transiently formed NH,NH-stabilized carbene ligands. [14] The carbenes with a substituted fivemembered N-heterocyclic ring-9, [15] 10, [16] 11, [17] and 12 [18] -have been described as well as the unsymmetrically substituted compound rac-13 [19] and the spirocyclic derivative 14.…”

Recently Reported Crystalline Isothiazole Carbenes: Myth or Reality

“…[12] Since the two substituents of the carbene have similar steric bulk, the formation of only one stereoisomer is very unlikely. [18] Despite their crystalline nature, the "dimers" have only been characterized by 1 H and 13 C NMR spectroscopy, as well as mass spectrometry. Assuming the presence of only one isomer, at least 15 13 C NMR signals are expected, all of them in the same range, which makes any attributions highly debatable.…”