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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 2a–d 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
B–H activation to prepare boron-functionalized carborane derivatives
that are unavailable by conventional routes.