10.1002/anie.201903638
Angewandte Chemie International Edition
COMMUNICATION
is consistent with C(sp3)–OAc coupling from 2-NiIV+ proceeding
by a different (likely outer-sphere) pathway that is much faster
than benzocyclobutane formation.
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from bonding to catalysis, University Science Books: Sausalito, CA
2010 and references therein.
We next examined the reaction of 1-NiIII+ with
tetramethylammonium acetate. As shown in Scheme 1b, the
crude 1H NMR spectrum of this mixture showed the formation of
benzocyclobutane 3 in 41% yield. The Ni-containing products
were paramagnetic and thus could not be readily identified by
NMR spectroscopy. As such, trifluoroacetic acid was added to
protodemetallate any Ni σ-alkyl or σ-aryl nickel analogues of 5 or
6. 1H NMR spectroscopic analysis of the resulting mixture
showed no trace of either 5 or 6, the organic products of C(sp3)–
OAc or C(sp2)–OAc coupling, respectively. These data
demonstrate that C–C bond-forming reductive elimination
outcompetes C–OAc coupling at this cationic octahedral NiIII
complex. We attribute this difference in reactivity to the lower
electronegativity of NiIII versus NiIV, which results in a less
electrophilic Ni–C(sp3) bond and thus decreased reactivity in
SN2-type pathways.
[3]
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For selected reviews on the impact of oxidation state on selectivity see:
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Notably, the assignment of oxidation state to a metal center is a
formalism that does not necessarily represent where the
charges/electrons are distributed on the complex. For example, see: (a)
J. P. Snyder Angew. Chem. 1995, 107, 112-113. Angew. Chem. Int. Ed.
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In summary, these studies demonstrate that outer-sphere
C(sp3)–OAc bond-formation is significantly faster than inner-
sphere C(sp3)–C(sp2) coupling at the NiIV complex 2-NiIV+. This
is consistent with the mechanism of C(sp3)–heteroatom
reductive elimination from other d6 group 10 metals, where SN2-
type pathways have been proposed at PtIV,17 PdIV,18 and NiIV.5
Furthermore, inner-sphere C(sp2)–OAc coupling is not
competitive in this system, likely due to the low lability of the
MeCN ligand, which precludes acetate coordination to the NiIV
center. (An inner sphere mechanism has been proposed for the
vast majority of carbon-carbon and C(sp2)-heteroatom coupling
reactions at PtIV,19 PdIV,4 PdIII,20 NiIV,21 and NiIII centers.3,22) In
contrast, the NiIII analogue 1-NiIII+ reacts to selectively form C–C
coupled product 3 in the presence or absence of the acetate
nucleophile. These results demonstrate that the oxidation state
of high valent Ni can play a key role in dictating both the
mechanism and selectivity of the favored reductive elimination
process in these systems. While this study focuses on model
complexes, ongoing investigations in our lab are probing the
generality of obtaining complementary bond-forming reactions
by manipulating the oxidation state of high valent Ni
intermediates. If these observations prove general, they are
likely to find broader applications in catalysis.
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[12] It is also possible that 1-NiIII+ and 2-NiIV+ undergo C–C coupling via
fundamentally different pathways. As one example,
a reviewer
suggested that 1-NiIII+ may react via ejection of radicals rather than
direct reductive elimination. However, literature precedent suggest that
this pathway would likely result in the formation of neophyl dimers.
These are not detected in our system, which affords the
benzocyclobutane in 95% yield. See: B. Akermark, A. Ljungqvist, J.
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[14] An alternative pathway that is consistent with this data involves
reductive elimination from 2-NiIV+ via slow MeCN dissociation followed
by fast C–C coupling.
Acknowledgements
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This work was supported by the National Science Foundation
(CHE-1664961) and the UM Mcubed program. We acknowledge
Dr. Jeff Kampf for X-ray crystallographic analysis of complexes
1-NiIII+ and 2-NiIV+, and NSF Grant CHE-0840456 for X-ray
instrumentation. EGB thanks NSERC for a Michael Smith
Foreign Study Supplement (CGS-MSFSS).
[16] (a) F. Qu, J. R. Khusnutdinova, N. P. Rath, L. M. Mirica, Chem.
Commun. 2014, 50, 3036-3039. (b) J. M. Racowski, A. R. Dick, M. S.
Sanford, J. Am. Chem. Soc. 2009, 131, 10974-10983. (c) J. B. Gary, M.
S. Sanford, Organometallics 2011, 30, 6143-6149.
Keywords: high valent nickel • C–C coupling • C–O coupling •
NiIII • NiIV
[17] We note that modest yields of organic products are common for C–C
and C–heteroatom coupling reactions from high valent Ni centers (see,
for example, refs 5b, 6, and 12). The origin of these issues is often
difficult to definitively discern, but this remains a common challenge in
the field. In our system, we have been unable to definitively identify
other organic by-products. One possibility is that the yield of 5 may be
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Hazari, Organometallics 2019, 38, 3-35. (d) A. Biffis, P. Centomo, A.
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