Journal of the American Chemical Society
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C.; Everitt, S. R. L.; Knegtel, R.; Pinder, J. L.; Rutherford, A. P.;
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(13) In the reaction using 5-chlorobenzotriazole as aryl halide
which shows a comparable reactivity to pyrazole halides, we also
tried several bidentate ligands, e.g. BINAP, XantPhos, DPEPhos and
dppe. Only BINAP gave conversions nearly as good as SPhos. All
other ligands fell short of SPhos. Also see Ref. (9m).
(14) A possible catalysis by Pd nanoparticles thus seems
unlikely, even if they are commonly found in substantial amount in
Pd2dba3: Zalesskiy, S. S.; Ananikov, V. P. Organometallics 2012, 31,
2302-2309.
(4)
For a general exception, cf.: (a) Manolikakes, G.;
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(15) All chemicals can be weighed under air and stored on the
benchtop.
(16) Unsubstituted, unprotected 3-, 4- or 5-halopyrazoles have
not been previously reported in literature to successfully undergo
Suzuki-Miyaura, Negishi or Stille cross-coupling reactions. See ref.
7b.
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(19) Catalan, J.; Abboud, J. L. M.; Elguero, J. Adv. Heterocycl.
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(20) Only few pKa values for NH acidic heterocycles have been
reported in H2O as opposed to DMSO as solvent. Please refer to
reference 19 for an in-depth discussion of these compounds’ acidity.
(21) The reaction of deprotonated starting material or product
with the boronic acid could additionally lead to formation of a Lewis
acid/base pair, potentially binding and deactivating the boronic acid:
In the presence of potassium indazolide (2.0 eq.) in THF-d8 (BF3⋅OEt2
(6)
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as standard),
a
distinct shift in the 11B-NMR spectrum of
phenylboronic acid from 29.9 to 5.0 ppm was observed, which is
indicative of a tetrahedral, anionic boronate species. The presence of
indole led to no change, whereas indazole gave two signals at 31.9
(major) and 23.5 ppm (minor).
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(22) The overlaid curves in figure 3, 5 are only meant as visual
guidance. They were not fitted but are simply smoothed using the B-
Spline option, as implemented in Origin 7.0.
(23) Some of the data points were taken twice by setting up the
reaction a second time to insure reproducibility. In those cases, the
measured yield was within 1%.
(24) Complex 15 has been previously isolated by our group.
See: Barder, T. E.; Biscoe, M. R.; Buchwald, S. L. Organometallics
2007, 26, 2183-2192.
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(26) One major signal and two minor ones could be observed by
31P-NMR spectroscopy in CD2Cl2. In THF-d8 as solvent, one major
signal could be detected as well as 3 minor species.
(27) The formation of Pd(II) pyrazolyl species has been
previously reported starting both from [Pd-OH] and [Pd-I]
complexes: Driver, M. S., Hartwig, J. F. Organometallics 1997, 16,
5706-5715.
(28) The reverse reaction starting from 14 to either 15 or 16 led
to no product formation. Other examples of 1,2-azole bridged Pd(II)
complexes have been previously reported: (a) López, G.; Ruiz, J.;
Garcia, G.; Vicente, C.; Casabó, J.; Molins, E.; Miravitlles, C. Inorg.
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(29) 15 was chosen over P2 for comparing reactivity at room
temperature to accommodate for a potential lag in activation of P2.
(10) Recently, the coupling of benzimidazole-derived potassium
trifluoroarylborates was reported: Molander, G. A.; Ajayi, K. Org.
Lett. 2012, 14, 4242-4245.
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