Imidazolines lateral

Later, imidazolidin-2-ylidenes such as 14, a saturated, more electron-rich and nonaromatic version of the imidazolin-2-ylidenes, were isolated. Isolation of a six-membered tetrahydropyrimid-2-ylidene 15 and of acyclic structures such as 1629,30... 

Oxazolines , imidazolines and tetrazoles can all be laterally lithiated. Oxazolines have been used in this regard rather less than for ortholithiation (Scheme 200). 

Thus, the imidazolin-2-ylidene 87a synthesized by Arduengo et al. in 1991 is now referred to as the first N-heterocyclic carbene to be isolated and characterized. Its preparation was achieved by deprotonation of the corresponding imidazolium salt 91. Later, Arduengo et al. also synthesized stable carbenes with less bulky substituents. It should be noted that the structure of these carbenes is identical with the Wanzlick carbenes. Progress in laboratory methods were finally decisive for the successful isolation of these carbenes (Scheme 21). 

However, the isolation of N,N -dimethyl substituted NHC [54] and the first saturated NHC, l,3-dimethyl-lH-imidazolin-2-ylidene by Arduengo et al. in 1995 [95] cast serious doubt as to the validity of the need for steric or aromatic stabilisation in these carbenes. Two independent theoretical studies by Boehme and Frenking [10] and Heinemann et al. [11] in 1996 as well as a later one by Tafipolski et al. [94] gave an excellent account of the stabilising factors and the differences between saturated and unsaturated NHC. 

The acrylic acid derivative presumably adds, via a Michael Addition Reaction, to the nucleophilic nitrogen atom of the open-ring amidoamine, thus forming an amphopropionate. No salt is formed as a by-product. This reaction was first described in 1963, assuming a cyclic product structure which contained imidazoline (32). Later, it was shown that the reaction in aqueous medium results in a linear product (33). 

Another approach to the synthesis of 2-imidazolines (22) mediated by silica-supported tungstosilicic acid, starting from 1,2-diamines and nitrile derivatives (Scheme 1.14), was reported later by Nasr-Esfahani et al. (2010). In contrast to the previously described method, this approach has proved to be efficient also for the synthesis of ( -imidazolines (23). 

Later, Tamm and coworkers investigated the combination of l,3-bis(di-tert-butylphenyl)imidazolin-2-ylidene with a new the fluorinated borane, tris[3,5-bis (trifluoromethyl)phenyl]borane (B(w-XyFg)3), which offers thermally induced FLP reactivity with, for example, CO2, C—H bonds, and H2. Again, however, this FLP tends to form abnormal NHC-borane adducts [100]. The number of FLP combinations involving carbenes is constantly growing and there are a number that have not been included here. Tamm and coworkers have recently published a detailed review specifically of NHCs in FLP chemistry [101]. 

Later, Adib and coworkers reported the synthesis of 3-amino-2-arylimidazo[l,2-a]pyridines 79 by a solvent-free three-component reaction of 2-aminopyridines 71, benzalde-hydes 53, and imidazoline-2,4,5-triones 72 (Scheme 11.11). The MCR required high temperatures (2(X)°C). The first step of the mechanism of the reaction involves the condensation between benzaldehyde 53 and 2-aminopyridine 71 and between benzaldehyde 53 and imidazoline-2,4,5-trione 72. Next, the imidazohum ion 74 is attacked by the pyridine A-atom of... 

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