Benzimidazolines, formation

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

The reaction of 1,2-diaminonitrobenzenes with aldehydes is a widely accepted synthetic route to nitrobenzimidazoles [57, 62,63,66, 350,372-383], This reaction passes sequentially through a stage of the formation of azomethines (Schiff s base) and benzimidazolines. On oxidation the latter forms the corresponding benzimidazole derivatives (Scheme 2.44). 

The reaction between o-phenylenediamine and an equimolar amount of an aromatic or heterocyclic aldehyde has been shown to proceed by initial formation of a monoanil (74). In the presence of oxidizing agents (e.g. nitrobenzene, which also acts as the solvent) this can form the 2-substituted benzimidazole. With two moles of aldehyde the bis-anil (75) forms, giving rise to a 1,2-disubstituted benzimidazole (Scheme 42). This aldehyde route to benzimidazoles is particularly suited to the synthesis of compounds with a heterocyclic group (e.g. 2-thienyl-, 2-pyridyl-) at C-2. Reaction of 2,2,4,4-tetrakis(trifluoromethyl)-l,3-dithietane with o-phenylenediamine gives 2,2-bis(trifluoromethyl)benzimidazoline (7430785). 

Compound 639 also promotes the photo induced reduction of ketones whereas 2-(2 -hydroxyphenyl)-l,3-dimethyl-benzimidazoline 640 promotes benzpinacol formation (Scheme 153) <2005JOC9632>. Photoinduced single electron transfer (SET) followed by regiospecific proton transfer (PT) led to the formation of either benzhydrol or benzpinacol, depending on the 2-aryl substituent. 

In 1999, the first annulated carbene derived from benzimidazole, l,3-bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene, was prepared by the reduction of A/,N -dialk ylated benzimidazol-2-thione with Na/K in toluene [44]. Frequently, benzimidazolium ions containing less bulky Af-alkyl groups interconvert rapidly into enetetramine derivatives. Even when the formation of these compounds can be avoided by sterically demanding substituents, the final generation of the carbene may fail. 

The mechanism for the formation of complexes 63—66 has not been established unambiguously yet. As mentioned before, related complexes bearing bidentate imidazolin-2-ylidene/donor or benzimidazolin-2-ylidene/donor ligands (Scheme 9.18) have been described as formed by tautomerization of the azole to the NR,NH-substituted NHC that then together with the tethered donor coordinates to the metal center. This rather simple type of reaction mechanism is not very likely to be operative. 

The monoanionic CCC ligand type was also used by Chianese and coworkers to prepare the Ir complexes 47a-f with benzimidazolin-2-ylidene moieties to prevent the formation of abnormal carbenes. They treated the imidazolium salt 8 2HBr with 30 equiv of triethylamine as base and 0.5 equiv of [(p-Cl)Ir(COD)]2 in acetonitrile at 80 °C to obtain the Ir(III) complexes 47 (Scheme 9.8). However, a clean reaction was obtained only in the case of 47 bearing Al-mesityl (a) or A/ -2,6-di-isopropylphenyl (b) substituents at the NHC moiety. With A/ -3,5-dimethylphenyl (c), A/ -3,5-di-tert-butylphenyl (d), N-tert-hutyl (e), and Al-l-adamantyl (f) substituents, they had to use a slight excess of CsF as base to obtain the respective Ir(III) complexes 47 [16]. For complex 47b, the group had to raise the temperature in an autoclave up to 150 °C [16b]. 

Palladium complexes of monodentate and chelating bidentate benzimidazolin-2-ylidene ligands have been prepared in moderate to good yield, via m situ deprotonation, with Pd(OAc)2 in To effect formation of... 

Quinoxalino-benzimidazole rearrangement Chem Heteiocycl Compd 36(7) 882-883 Kalinin AA, Isaykina OG, Mamedov VA (2007) Quinoxaline-benzimidazole rearrangements in the reactions of 3-alkanoylquinoxalin-2-ones with 1,2-phenylenediamines. Chem Heterocycl Compd 43 1307-1314. doi 10.1007/sl0593-007-0198-3 Kametani T, Sota K, Shio M (1970) Studies on the syntheses of azole derivatives. Part I. Formation of l-substituted-3-hydroxy-l//-indazole and 1-substituted benzimidazolin-2-one derivatives by thermal reaction of V-substituted-/V-arylcarbamoyl azides. J Heterocycl Chem 7 (4) 807-813. doi 10.1002/jhet.5570070409... 

The opposite route, i.e., the withdrawal of a potential donor moiety by rearrangement and formation of a heterocycle, may also occur (Scheme 11), as shown for the formation of Si-bound l-(2-methyl-2,3-dihydrobenzothiazol-2-yl)propan-2-one (in 58) [194] and Si-bound benzimidazoline (in 59) [195] out of tridentate chelators. 

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