Alkyl Zinc Halides
FULL PAPER
THF (2 mL) was added through a syringe. After stirring for 3–4 min at
room temperature, the corresponding alkyl zinc bromide (0.5m in THF,
0.854 mmol) was added, and the reaction mixture was stirred at RT (see
the Supporting Information for details on the reaction times). The reac-
tion mixture was then transferred with diethyl ether (20 mL) to a separat-
ing funnel containing water (20 mL), the product was extracted with
ether (2ꢄ30 mL) and the combined organic extracts were washed with
water (20 mL) and brine (20 mL). After drying (anhydrous Na2SO4), the
solution was filtered and concentrated, and the residue was purified by
using silica gel column chromatography (hexane/EtOAc).
Kantchev, C. Valente, N. Hadei, G. A. Chass, A. Lough, A. C. Hop-
kinson, M. G. Organ, Chem. Eur. J. 2006, 12, 4743–4748; j) M. G.
Organ, S. Avola, I. Dubovyk, N. Hadei, E. A. B. Kantchev, C. J.
[8] a) D. J. Cꢀrdenas, Angew. Chem. 2003, 115, 398–401; Angew. Chem.
Int. Ed. 2003, 42, 384–387; b) D. J. Cꢀrdenas, Angew. Chem. 1999,
111, 3201–3203; Angew. Chem. Int. Ed. 1999, 38, 3018–3020.
Computational methods: Calculations were performed with the Gaussi-
an 03 program at the DFT level.[23] The geometries of all complexes re-
ported here were optimised using the B3LYP hybrid functional.[24] Opti-
misations were carried out using the standard 6-31G(d) basis set for C,
H, N and O. The LANL2DZ basis set, which includes the relativistic ef-
fective core potential (ECP) of Hay and Wadt and employs a split-va-
lence (double-z) basis set, was used for Ni, Zn and I.[25] Harmonic fre-
quencies were calculated at the same level to characterise the stationary
points and to determine the zero-point energies (ZPE). The starting ap-
proximate geometries for the transition states (TSs) were graphically lo-
cated. Intrinsic reaction coordinate (IRC) studies were performed to con-
firm the relation of the transition states with the corresponding minima.
Solvent effects were considered by performing single-point calculations
for samples in THF using the polarised continuum model (PCM) on the
optimised structures.
[10] In contrast to the previously reported couplings of alkyl electro-
philes, terdentate nitrogen ligands were not so effective. The use of
[NiACHTNUTRGNENG(U cod)2] (cod=1,5-cyclooctadiene) was less convenient too. See
the Supporting Information for details on the optimisation of the re-
action conditions.
[12] Recently, it has been shown that complexes of bidentate bis-
AHCTUNGRTEG(NNNU trialkyl)phosphines with CF3 and aryl ligands do not afford trifluor-
otoluenes, even in the presence of oxidants: G. G. Dubinina, W. W.
[13] Although DFT calculations with the basis set used cannot be em-
ployed for calculation of reliable absolute energies, the calculated
activation energy values are much higher than those we obtained
with the same approximations in our previous studies. For that
reason and for the low activation energies we have found for the al-
ternative pathways, we may disregard these mechanisms.
Acknowledgements
We thank the Ministerio de Ciencia e Innovaciꢂn for financial support
under project CTQ2007-60494/BQU, the MAE-AECI for a fellowship to
V.B.P. and the Universidad Autꢂnoma de Madrid for a FPU-UAM fel-
lowship to M.G.-C. We also thank Centro de Computaciꢂn Cientꢁfica-
UAM for computation time.
[1] Negishi reactions are usually catalysed by Pd complexes. We do not
intend to survey such processes in this article. Recent reports can be
found in ref. [7]. For general reviews on the Negishi reactions, see:
a) Metal-Catalysed Cross-Coupling Reactions (Eds.: F. Diederich, P.
Stang), Wiley-VCH, Weinheim, 1998; b) Metal-Catalysed Cross-Cou-
pling Reactions (Eds.: A. de Meijere, F. Diederich), Wiley-VCH,
Weinheim, 2004.
[3] For a review on metal-catalysed couplings of alkyl halides, see:
[4] For a recent review on Ni-catalysed Negishi cross-coupling reactions,
[5] V. B. Phapale, E. BuÇuel, M. Garcꢁa-Iglesias, D. J. Cꢀrdenas, Angew.
[17] a) R. Giovannini, T. Stꢆdemnann, A. Devasagayaraj, G. Dussin, P.
J. L. Martin, C. McFarland, O. R. All en, R. E. Hall, A. D. Haley,
R. J. Brandon, T. Konovalova, P. J. Desrochers, P. Pulay, D. A. Vicic,
4201; b) I. A. S. Walters, Tetrahedron Lett. 2005, 46, 341–344;
[20] In fact, homocoupling products of the alkyl zinc bromides are ob-
served in all reactions in approximately 6% yield, which supports
the formation of Ni0 intermediates.
[21] Free energies can be found in the Supporting Information. These
values are affected by errors derived from the calculation of entropy
and do not reflect the solvation processes involved. It is important
to note that the alkyl zinc reagent and the resulting ZnI2 will be co-
ordinated to THF.
[22] Pentacoordinated complexes V and II show similar stabilities (V is
2.2 kcalmolÀ1 more stable than II in the gas phase, and the II is cal-
culated to be 2.2 kcalmolÀ1 more stable in THF), and facile isomeri-
sation could take place according to the behaviour of different start-
ing structures during the geometry-optimisation calculations. We
[7] For recent reports on Pd-catalysed Negishi couplings of aryl halides
and alkyl zinc halides, see: a) C. Han, S. L. Buchwald, J. Am. Chem.
M. A. Schade, C. MuÇoz-Hernandez, H. Mayr, P. Knochel, Org.
Chem. Eur. J. 2009, 15, 12681 – 12688
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