Imidazole-2-ylidenes, aromaticity

Fig. 5 Isomerization stabilization energy, an isodesmic reaction applied to evaluate the aromaticity of imidazole-ylidenes...

However, similar NHC architectures employing aromatic side chains have shown more encouraging results. In 2000, Nolan and co-workers reported the synthesis and characterisation of the NHC-Ru complex 20 bearing a sterically more demanding N,N -bis-[2,6-(di-/xo-propyl)phenyl]imidazol-2-ylidene (IPr) ligand [27, 28] (Fig. 3.5). Standard RCM substrate 1 was used to test the catalytic performance of 20. The ring closure was found to be complete after 15 min by using 5 mol% 20 as catalyst at room temperature. Under identical conditions, 15... 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

The previously published results [142, 143] devoted to the synthesis of thiazolo [3,2-a]imidazol-2-ones attracted attention of Krasovskii et al. [144] to synthesize corresponding ylidene derivates and to investigate biological activities, promising to be interesting for the medicinal chemistry. By heating 2-aminothiazoles with ethyl bromacetate and a variety of aromatic aldehydes at the appropriate temperature (either 80°C or reflux) in the glacial acetic acid with anhydrous sodium acetate, ylidene derivates of structures 102-104 were isolated (Scheme 48). 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Heinemann, C., Mueller, T., Apeloig, Y. et al.. On the question of stability, conjugation, and aromaticity in imidazol-2-ylidenes and their silicon analogs, /. Am. Chem. Soc., 118, 2023,1996. 

The occurrence of cyclic 67T-electron delocalization in imidazol-2-ylidene-type systems (75) is supported by a variety of structural, energetic, and magnetic criteria, as well as by the charge distributions and low-energy ionization processes which have been carefully analysed theoretically96,98,100 102 103. A summery of the conclusions of the theoretical studies regarding the degree of aromaticity in imidazol-2-ylidene-type systems is presented in Table 9. 

The reaction usually requires palladium catalysis. In the case of aromatic tosylates [23] or arylchlorides, Ni-catalysts [20, 21] or Pd-imidazole-2-ylidene complexes had to be employed ]33]. The latter catalyst is generated in situ from 1,3-bisaryl or 1,3-bisalkyl imidazolidinium chloride ]34, 35] and a base, such as CS2CO3 or KOtBu ]36]. 

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