Unprecedented Boron-Functionalized Carborane Derivatives
by Facile and Selective Cobalt-Induced B–H Activation
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Abstract
The
16-electron complex CpCoS<sub>2</sub>C<sub>2</sub>B<sub>10</sub>H<sub>10</sub> (<b>1</b>) is found to react with the alkynes
HCCC(O)R [R = methyl (Me), phenyl (Ph), styryl (St), ferrocenyl
(Fc)] at ambient temperature to give two types of 17-electron cobalt
complexes <b>2a</b>–<b>d</b> and <b>3a</b>–<b>d</b> 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 <b>2a</b>–<b>d</b> undergo alkyl C–S cleavage to give
16-electron complexes <b>4a</b>–<b>c</b> containing
a boron-norbornadienyl moiety, and simultaneous carboranyl C–S
cleavage to afford cobalt-free carborane derivatives <b>5a</b>–<b>d</b> containing a boron-norbornyl unit. Both <b>2a</b>–<b>d</b> and <b>3a</b>–<b>d</b> allow further alkyne insertion into the Co–S bond
to generate cobalt-free boron–norbornyl carborane derivatives
(<i>Z</i>/<i>E</i>)-<b>7a</b>–<b>d</b> and (<i>Z</i>/<i>E</i>)-<b>8a</b>–<b>d</b>, both containing a vinyl sulfido group. Addition
of AlCl<sub>3</sub> not only promotes the conversion of <b>2a</b>–<b>d</b>, but also leads predominantly to (<i>E</i>)-<b>9a</b>–<b>d</b> as retro-Diels–Alder
products. Upon heating, the isomerization from <i>E</i> to <i>Z-</i>configuration of the vinyl group and reorganization of
the norbornyl moiety of (<i>Z</i>/<i>E</i>)-<b>7a</b>–<b>d</b> occur to lead to (<i>Z</i>)-<b>9a</b>–<b>d</b> as well as the unexpected
[1,2]-H shifted products (<i>Z</i>)-<b>10b</b>,<b>c</b>. Thus, the 17-electron complexes <b>2a</b>–<b>d</b> and <b>3a</b>–<b>d</b> 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
