Direct B–H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer

Abstract

The efficient and selective functionalization of icosahedral carboranes (C 2 B 10 H 12 ) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B−H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B−H functionalization of icosahedral carboranes via B−H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B−H bond and the resulting boroncentered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B−H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B−H heterolytic mechanism in the traditional strategies.

Photoinduced Selective B–H Activation of nido-Carboranes

“…We have explored the photocatalytic reactions for the B–H functionalization of carboranes For example, the synthetic strategies through decarboxylation coupling c and hydrogen atom transfer have been developed to generate highly active boron-center carborane radicals, which further react to lead to the B–H functionalization. In this study, we still used a photoinduced fashion but introduced a new type of noncovalent cage···π interaction to develop an innovative reaction mode aiming to achieve the straightforward and site-selective B–H functionalization of nido -carboranes.…”

From Carbene-Dithiolene Zwitterion Mediated B–H Bond Activation to BH3·SMe2-Assisted Boron–Boron Bond Formation

“…While main-group-species − -mediated B–H bond activation mainly involves 1,1-addition at a low-valent main-group species (such as carbenes, carbenoid species, and amidosilylenes), frustrated Lewis pairs (FLPs) , and boron-containing heterocycles , have also been reported for this utility. In addition, boranes may demonstrate reduced B–H BDE values when being complexed by Lewis bases (such as NHCs). , Consequently, NHC-complexed boranes have been employed as hydrogen atom donors via homolytic B–H cleavage. − Notably, nitrogen-centered radical-mediated B–H activation of icosahedral carboranes via hydrogen atom transfer (HAT) has recently been reported . This laboratory recently reported a cyclic (alkyl)­(amino)­carbene (CAAC)-stabilized dithiolene zwitterion ( 1 , in Scheme ), a new type of metal-free bifunctional molecular system which has been shown to activate ammonia via both single-electron-transfer (SET) and HAT processes .…”

Synthesis of Charge-Compensated nido-Carboranyl Derivatives of Sulfur-Containing Amino Acids and Biotin

“…The most important method for the formation of the closo - o -carborane cluster is the interaction of alkynes with decaborane (B 10 H 14 ). The closo - o -carborane derivatives are easily transformed into the corresponding nido -carboranes under the action of bases (metal alkoxides and hydroxides, primary and secondary amines), fluoride ion, and some other reagents. , At present, approaches to the functionalization of closo - and nido -carboranes as a result of catalytic activation of the B–H bond are being actively developed …”