Benzimidazole polymer-supported reactions

Cyclization to the desired head-to-taU-linked bis-benzimidazoles can also be performed by reaction of aryl or alkyl isothiocyanates with N,N -dicyclohexylcarbodi-imide (DCC) [83]. In a closely related and more recent study by the same group, mercury chloride was used as a catalyst to perform cyclization to the benzimidazoles [84]. Another application of the bis-hydroxylated polymer support PEG 6000, microwave-accelerated liquid-phase synthesis of thiohydantoins, has been reported [85, 86]. 

C2-Arylated 1,3-Azoles In 1998, Miura reported the first example of the catalytic C-H arylation of (benzo)azoles and thiophenes (at their C2 position) with aryl halides [98, 99a]. In the presence of catalytic Pd(OAc)2 and stoichiometric carbonate base at high temperature (140 °C), 1,3-azoles (oxazoles, N-alkylated imidazoles, fhiazoles) were coupled with various aryl bromides selectively at the C2 position. For the reactions of N-alkylated benzimidazoles, benzoxazoles, and benzothiazoles, addition of Cul was necessary to increase the yield of products. Furthermore, when N-methylbenzimidazole was used, Miura discovered that the coupling with aryl bromides proceeds without the palladium catalyst (i.e., by a copper-mediated C-H/C-X coupling of azoles with aryl halides). Thereafter, Miura enhanced the efficiency of the coupling reaction to allow for multiple arylations of azoles both C2 and C5 positions [99b]. In 2000, Rondo [100] prepared polymer-supported aryl iodides, with which the direct coupling reaction with 1,3-azoles (Miura s conditions) was conducted. 

Bismuth chloride was found to be very efficient to catalyze the benzimidazole cyclization on soluble polymer support by microwave irradiation. In the absence of microwave, it took 2 hours to complete the reaction by conventional heating instead of 4 minutes (Equation 26) [44]. 

Some 5-methyl-l,2-disubstituted benzimidazoles derivatives have been synthesized in an efficient and rapid solid-phase method with help of the phospho-nium linker. The phosphonium linker was prepared by reaction between polymer-supported triphenylphosphine and 4-fluoro-3-nitrobenzyl iodide, which underwent aromatic substitution with primary amines, followed by one pot reaction with aldehydes in the presence of SnCl2 2H20 under microwave irradiation. The products were isolated from resin using NaOH to give high purity and good overall yield (Rios et al., 2013). 

Up to now, only a few catalyst systems based on organic polymers such as molybdenum compounds supported on benzimidazole, polystyrene, or poly(gly-cidyl methacrylate) [9] as well as micelle-incorporated manganese-porphyrin catalysts [66] have been tested in the epoxidation of propene. Molybdenum-doped epoxy resins were also employed in the epoxidation of propene with TBHP and propene oxide yields of up to 88% were obtained [65]. The catalysts were employed repeatedly in up to 10 reactions without significant loss of activity and metal leaching proved to be very low. 

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