The iridium-catalysed borylation of aromatic C-H bonds has become the preferred method for the synthesis of aromatic organoboron compounds. The reaction is highly efficient, tolerant of a broad range of substituents and can be applied to both carbocyclic and heterocyclic substrates. The regioselectivity of C-H activation is dominated by steric considerations and there have been considerable efforts to develop more selective processes for less constrained substrates. However, most of these have focused on benzenoid-type substrates and in contrast, heteroarenes remain much desired but more challenging substrates with the position and/or nature of the heteroatom(s) significantly affecting reactivity and regioselectivity. This review will survey the borylation of heteroarenes, focusing on the influence of steric and electronic effects on regiochemical outcome and, by linking to current mechanistic understandings, will provide insights to what is currently possible and where further developments are required.
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The selective amination of aromatic C-H bonds is a powerful strategy to access aryl amines, functionalities found in many pharmaceuticals and agrochemicals. Despite advances in the field, a platform for the direct, selective C-H amination of electronically diverse (hetero)arenes, particularly electron-deficient (hetero)arenes, remains an unaddressed fundamental challenge. In addition, many (hetero)arenes present difficulty in common selective pre-functionalization reactions, such as halogenation, or metal-catalyzed borylation and silylation.
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Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
The synthesis and application of aryl-substituted pyridine(diimine) iron complexes (PDI)FeCH to the catalytic borylation of heteroarenes under thermal conditions is described. Improvements in catalyst design and performance were guided by precatalyst activation studies, where investigations into stoichiometric reactivities of iron borohydride (4- Bu- PDI)Fe(HBPin) and iron furyl (4- Bu- PDI)Fe(2-methylfuryl) complexes revealed facile C(sp)-H activation and a slower and potentially turnover-limiting C(sp)-B formation step. Formation of the flyover dimer, [(4- Bu- PDI)Fe] was identified as a catalyst deactivation pathway and formally iron(0) complexes were found to be inactive for borylation.
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Department of Chemistry, Inha University, Incheon, Republic of Korea.
Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of -heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, -silyl enamines and -boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process.
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