Unprecedented Boron-Functionalized Carborane Derivatives by Facile and Selective Cobalt-Induced B-H Activation

The 16-electron complex CpCoS2C2B10H10 (1) is found to react with the alkynes HC equivalent to CC(O)R [R = methyl (Me), phenyl (Ph), styryl (St), ferrocenyl (Fc)] at ambient temperature to give two types of 17-electron cobalt complexes 2a-d and 3a-d containing unique B(3)/B(6)-norbornyl carborane moieties. A formation mechanism via a tandem sequence of metal-induced B-H activation, B-Cp formation, Cp delivery and Diels-Alder addition is proposed on the basis of DFT calculations. The reactivity of these paramagnetic 17-electron complexes has been studied: Exposed to a combination of air, moisture and silica, complexes 2ad undergo alkyl C-S cleavage to give 16-electron complexes 4a-c containing a boron-norbornadienyl moiety, and simultaneous carboranyl C-S cleavage to afford cobalt-free carborane derivatives 5a-d containing a boron-norbornyl unit. Both 2a-d and 3a-d allow further alkyne insertion into the Co-S bond to generate cobalt-free boron-norbornyl carborane derivatives (Z/E)-7a-d and (Z/E)-8a-d, both containing a vinyl sulfido group. Addition of AlCl3 not only promotes the conversion of 2a-d, but also leads predominantly to (E)-9a-d as retro-Diels-Alder products. Upon heating, the isomerization from E to Z-configuration of the vinyl group and reorganization of the norbornyl moiety of (Z/E)-7a-d occur to lead to (Z)-9a-d as well as the unexpected [1,2]-H shifted products (Z)-10b,c. Thus, the 17-electron complexes 2a-d and 3a-d serve as intermediates for synthesis of variety of boron-functionalized carborane derivatives. In this study, efficient routes have been developed through cobalt-mediated BH activation to prepare boron-functionalized carborane derivatives that are unavailable by conventional routes.