Heterogeneous C-H Activation for the Heterocycle Synthesis

The ubiquity and inherent stabiUty of C-H bonds have rendered the use of heterogeneous systems rather difficult. Normally, supported catalysts require more drastic reaction conditions than homogeneous counterparts. By using higher temperatures, the somewhat lower activities can be compensated to some extent. To obtain high efficiency, other approaches include the apphcation of non-traditional activation methods, such as microwave irradiation and photochemistry. Especially, heterogeneous photocatalysis thus established offers numerous perspectives for the heterocycie synthesis via C-H activation processes. 

In this chapter, we aim to give a picture of recent advances of C-H activation for the heterocycie synthesis enabled by heterogeneous catalysis. Scope, limitations, and problems associated with the application of heterogeneous catalysts will be covered. Mechanistic aspects of the catalysis are briefly addressed, but an extensive survey about this field is beyond the scope of this chapter. This hmitation is due to the fact that the mechanisms are not yet fully understood and findings in this field are sometimes contradictory. To an extreme, in many cases it is not even clear whether the genuine catalysis is heterogeneous or homogeneous. 

Transition MetaUCatalyzed Heterocycie Synthes via C—H Activation, First Edition. Edited by Xiao-Feng Wu. 2016 Wiley-VCH Verlag GmbH Co. KGaA. Published 2016 by Wiley-VCH Verkig GmbH Co. KGaA. 

Under the optimized conditions, the scope of intramolecular arylation was investigated (Table 14.1). Structurally diverse aryl iodide and bromide substrates underwent intramolecular arylation reactions and gave the desired products in good to excellent yield. High regioselectivity can also be observed in cases where direct arylation can occur at two chemically different arene positions as illustrated in entry 1. In this case, arylation occurs at the more sterically accessible position in a 13.5 1 ratio as determined by H NMR analysis. 

Pd(0) species can be reoxidized by O2 to regenerate the Pd(II) species for the next catalytic cycle. Meantime, the dissolved Pd species would reattach to the existing Pd nanoparticles to form larger clusters via Ostwald ripening process. 

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Non-metallic homogeneous catalyst systems were also reported for methanol synthesis. Recently, Ashley et al. [49] demonstrated the selective hydrogenation of COj to methanol using a FLP-based nomnetal mediated procedure at low pressures (1-2 atm). N-Heterocyclic carbine (NHC) was found to be an elFective organic catalyst for methanol synthesis from CO2 reduction with silane. Compared to transition metal catalyst, NHC is more efficient at ambient reaction conditions [50,51]. Table 5.1 lists catalytic activities of different heterogeneous catalysts employed for methanol synthesis from CO. It shows that maximum CO conversion of 25.9%, methanol selectivity of 99.5% and methanol yield of378 mg/g-cat h could be achieved. The space velocities were tried between 1800 and 18,000 h and the temperature from 170 to 270 C. 

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