Chemical Science
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presented, providing a key entry into the library of rst-row
transition metal complexes capable of catalytic two-electron
transformations. Both stoichiometric and kinetic studies
support the assignment of a closed-shell mechanism for the
catalytic amination of aryl halides with (PPh3)3CoCl as the
catalyst. Reactions with various trapping agents disfavored
mechanisms invoking uncaged radicals or aryne intermediates.
Differentiating between a caged radical intermediate and a two-
electron pathway was less straightforward, and was investigated
in two ways.
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Stoichiometric reactivity was examined by assessing the
reactivity of Co(II) intermediates. In these cases, the product
distributions included biaryl side products, which arise from a
known radical pathway. Any productive catalytic reactivity likely
arises from in situ reduction of a Co(II) intermediate to Co(I). The
addition of a reductant such as Zn prevented the formation of
biaryl, implying that a Co(II) intermediate is unlikely to be
operative in the amination reaction. Additional support for this
came via a Hammett study, which clearly correlates a multi-step,
closed-shell mechanism. While work is still ongoing to isolate
the key Co(III) intermediate, the collection of results support a
closed-shell mechanism catalyzed by a Co(I) species. In conclu-
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Acknowledgements
The authors thank the University of Illinois at Urbana-Cham-
paign for nancial support, Dr Danielle Gray for crystallo-
graphic assistance and Prof. Per-Ola Norrby for insightful
discussions.
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