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Journal of the American Chemical Society
(9) Maleczka, R. E., Jr.; Shi, F.; Holmes, D.; Smith, M. R., III J.
isotopic label was incorporated in the pincer under both of
these conditions. These results are consistent with oxidative
addition-reductive elimination of B-H(D) as well as C-H(D)
bonds through a metal-based redox cycle rather than metal-
ligand cooperativity involving reversible ligand dearomatization-
aromatization (ref 26).
(28) An equilibrium constant of 0.52 was calculated for the
interconversion of 2-BPin and 2-(H)2BPin (see S27). The
formation of B2Pin2 and H2 from 2 equivalents of HBPin is
thermodynamically unfavorable (~12 kcal/mol uphill; see ref.
2b). The formation of 2-(H)2BPin and B2Pin2 from 2-BPin and
HBPin is most likely driven by the formation of 2 cobalt-hydride
bonds in 2-(H)2BPin.
(29) (a) A similar study probing the effect of the 4-substituent
on a PNP pincer ligand was also investigated for dinitrogen
reduction: see Kuriyama, S.; Arashiba, K.; Nakajima, K.;
Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. J.
Am. Chem. Soc. 2014, 136, 9719. (b) Electron-donating
groups on the 4- and 4’-positions of 2,2’-bipyridine was shown
to improve C-H borylation activity: see Ishiyama, T.; Takagi, J.;
Hartwig, J. F.; Miyaura, N. Angew. Chem. Int. Ed. 2002, 41,
3056.
(30) Attempts to monitor the reaction profiles of the borylation
of 1,3-bis-(trifluoromethyl)benzene with B2Pin2 using the
different cobalt pre-catalysts using identical conditions were
unsuccessful due to the high rates of these reactions (complete
conversion in 6 minutes for all the pre-catalysts).
Am. Chem. Soc. 2003, 125, 7792. (b) Beck, E. M.; Hatley, R.;
Gaunt, M. J. Angew. Chem., Int. Ed. 2008, 47, 3004. (c)
Fischer, D. F.; Sarpong, R. J. Am. Chem. Soc. 2010, 132,
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S. K.; Sarpong, R. Org. Lett. 2012, 14, 5350. (f) Preshlock,
S.M.; Plattner, D.L.; Maligres, P.E.; Krska, S.W.; Maleczka,
R.E.; Smith, M.R. Angew. Chem. Int. Ed. 2013, 52, 12915. (g)
Han, S.; Morrison, K. C.; Hergenrother, P. J.; Movassaghi, M. J.
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(10) Kawamorita, S.; Miyzaki, T.; Ohmiya, H.; Iwai, T.;
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Kawamorita, S.; Miyzaki, T.; Iwai, T.; Ohmiya, H.; Sawamura,
M. J. Am. Chem. Soc. 2012, 134, 12924. (c) Kawamorita, S.;
Miyzaki, T.; Ohmiya, H.; Iwai, T.; Sawamura, M. J. Am. Chem.
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(11) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.;
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(32) The cobalt precursor, 4-(H)2BPin, was selected for N2
inhibition experiments due to its relative ease of isolation and
ability to measure the effect of vacuum on the rate cobalt-
catalyzed C-H borylation as 4-(N2)BPin as this compound
cannot be isolated in the absence of dinitrogen. Under catalytic
conditions, 4-(H)2BPin is immediately and quantitatively
converted to 4-(N2)BPin before any product is observed (see
the red 31P NMR spectrum on Figure S9). In these experiments
and in the presence of N2, the starting cobalt complex before
heating and degassing is 4-(N2)BPin and not 4-(H)2BPin.
(33) Addition of DBPin to a THF-d8 solution of 6-(H)2BPin
resulted to no isotopic exchange in the hydride(deuteride)
position, indicating that reductive elimination does not occur
these conditions (see S67).
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(35) The reluctance of Ir(III) compounds with a bidentate ligand
to undergo reductive elimination was first proposed by Sakaki
and co-workers (ref. 12a).
(26) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J.
Am. Chem. Soc. 2006, 128, 15390.
(27) Addition of DBPin to a benzene-d6 solution of 4-(H)2BPin
resulted in immediate isotopic exchange in the cobalt-
hydride(deuteride) positions (see page S63). Also, addition of
B2Pin2 to a benzene-d6 solution of 4-(H)2BPin furnished 4-
(H2/D2/HD)BPin arising from the borylation of benzene-d6 and
the residual natural abundance arene (see page S65). No
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