Transition Metal- and Electro-Catalyzed C-H and B-H Activation for the Efficient Synthesis of Carborane Derivatives

Abstract

Scheme 6: Manganese-catalyzed access to sillylated imidazoles. Manganese-Catalyzed C−H HydroarylationManganese catalysis has gained ground in selective hydroarylation of alkenes with heterocyclic compounds. The report of Wang and co-workers unveiled a straightforward method for synthesizing aromatic alcohols and ketones via manganese catalysis (Scheme 7a). [39] This report revealed that optimum transformations were achieved in the presence of Me2Zn and ZnBr2 which were responsible for activating the manganese pre-catalyst, thereby forming the more active MnMe(CO)5 complex. In contrast, Ackermann and co-workers reported the manganese catalyzed hydroarylation of ketones with nitrogencontaining heterocycles (Scheme 7b). [40] Furthermore, Glorious recently achieved the formylation of arenes via a manganese(I)/iron(III) sequential catalysis broad substrate scope (Scheme 7a). [41] 1 Introduction 8 Scheme 7. Manganese-catalyzed addition of carbonyls to arenes and heterocycles Manganese catalysis has proven resourceful for functionalizing nitrogencontaining heterocycles. The groups of Ackermann and Wang made enormous contributions to this achievement. Ackermann reported the efficient C-2 selective aminocarbonylation of indoles and pyrroles via manganese catalysis. The reaction proceeded successfully without an additive with industrially relevant imines and isocyanates as reaction partners. The same group also realized the C-2 selective C−H activation of indoles with imines via a manganese(I) regime (Scheme 8a). [42] Wang, in an extension unraveled a chemo-selective coupling of ketones with imines to furnish benzylic amines (Scheme 8b). [43] 1 Introduction 9 Scheme 8. Manganese-catalyzed addition of imines and isocyanates to arenes and heterocycles The manganese(I)-catalyzed C−H hydroarylation was not limited to carbonyl compounds but was expanded to include alkenes and alkynes. Wang and coworkers demonstrated this concept in two reports, with the manganesecatalyzed selective alkenylation of arenes with terminal alkynes. [44] In one account, they established an environmentally-benign protocol to access highly functionalized molecules via a base-promoted mechanism. Further, they developed a reliable regime to construct synthetically relevant nitriles from imidates and terminal alkynes. Interestingly, high regio-selectivity was achieved in both cases (Scheme 9a). Also, Ackermann reported Brønsted acid-assisted C-2 selective alkenylation of indole derivatives with propargylic carbonates under manganese(I) catalysis. This regime enabled the successful synthesis of target molecules within a short reaction time facilitated by a flow 1 Introduction 10 setup (Scheme 9b). [45] In addition, Lei and Li extended the scope of manganese(I) catalyzed hydroarylation of indole derivatives to internal alkynes with high regioselectivity (Scheme 9c). [46] Scheme 9. Manganese-catalyzed C−H activation for hydroarylations.In addition to terminal alkynes, manganese(I) catalysis has proven beneficial for the selective alkylation of electron-defic...