ChemComm
DOI: 10.1039/C4CC08P70a3gHe 4 of 4
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plotting of ln(k/T) versus 1/T give a smooth linear relationship
(Figure 4), allowing us to calculate the activation enthalpy (∆H‡ =
56.5 kJ/mol) and activation entropy (∆S‡ = -128 J/mol/K). We
further investigated the reductive coupling reactions using
different diarylethynes. Plotting log(k/kH) versus σp for these
substrates indeed gave a rather linear relation (see Supporting
Information), and the positive slope (ρ = 1.43) can be associated
with a developing negative charge in the transition state.
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10 Figure 4. Eyring plot for rate constants.
The usefulness of this new protocol was demonstrated via the
gram scale reaction in the presence of 0.01 mol% of catalyst with
substoichiometric amount of (30 mol%) B2pin2. High yield (70%)
together with long-lived catalysts (12 days) was observed in this
15 gram scale reaction (see Supporting Information), showing that
the clean reaction created by B2pin2 can improve the catalytic
efficiency via suppressing the formation of intractable black Ni-
solid which was usually observed with Zn or Mn as reducing
reagents.
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20 Conclusions
7
In summary, B2pin2 has been identified, for the first time, as a
novel, efficient, and environmentally benign reductant thanks to
rational design, which enables the nickel-catalyzed reductive
coupling of alkynes to proceed efficiently with very low catalyst
25 loading (0.01 mol%) in the presence of formal substoichiometric
amount of (0.3 equiv) B2pin2. Our mechanistic studies are
consistent with a catalytic cycle in which the formation of
dinuclear Ni-Ni complex is the rate-limiting step and the facile
nature of reductive elimination of pinB-Ni-Ni-Bpin to regenerate
30 Ni(0) is responsible to the high efficiency of this catalytic
reaction. This result gives a clue that B2pin2 has promise as a
valuable reducing reagent for a variety of reductive C-C bond-
forming manifolds. Further investigations on application of the
B2pin2 as environmentally benign reductant in other reductive
35 coupling reactions are currently underway in our laboratory.
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Acknowledgment
This work was supported by the Chinese Academy of Sciences
and the National Natural Science Foundation of China (21222203,
21133011 and 21372231).
10 CCDC 1028364 (2a) and 1028365 (2d) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
40 Notes and references
1
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Doster and S. A. Johnson, J. Am. Chem. Soc. 2007, 129, 810; (c) A.
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