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Organic & Biomolecular Chemistry
Organic & Biomolecular Chemistry
DOI: 10.1039/C4OB02689F
center bond with Mg. A similar structure was found in occurring leading to an intermediate which is best described as
calculations for the SRN1ꢀtype substitution of the iodide in a neutral biphenyl which acts as a weak ligand for a Mg(I)ꢀ
iodobenzene with phenylꢀMgBr.10b During the CꢀC bond species. Since we have experimental indications of the biaryl
formation, this delocalized electron is initially transferred into radical anion character of such intermediates we assume that
the π system of one of the aryl groups (9-TS1/10-TS2). the Mg(I)ꢀatom is close enough to the biaryl moiety to allow
However, during the release of Ph2, a second SET leads to a intramolecular back electron transfer to the weakly bound
Mg(I) cation (9-Pr1
of the uncharged biaryl product with a polar molecule such as transfer to another TEMPO equivalent. This reaction will be
THF or TEMPO. highly exothermic and will not contribute to the barrier of the
A quantitative determination of the barrier of the SET process overall reaction. Therefore, we did not further analyze that
9-Cpx1 9-Int1, 10-Cpx4 10-Int2) is beyond the scope second electron transfer process.
/10-Pr2), thus facilitating the replacement biaryl ligand. The reaction is completed by a second electron
(
→
→
of our study. Since the geometry required to transfer the
electron is probably very similar to the SET product, in which
the two ipso carbon atoms C1/C1’ are in closer contact and the
nitrogen atom of TEMPO coordinates Mg, we expect that the
SET barrier will not exceed the relative energy of 9-Int1/10-
Int2 by more than a few kcal/mol. With this assumption, we
arrive at an overall electronic barrier of the biaryl formation
from MgPh2 of ca. 10ꢀ15 kcal/mol for the dimer process (Figure
3).
Comparing all three possible reaction paths we have presented,
it is most likely that the reaction occurs via a dimer of MgBrPh
in which TEMPO is coordinated by two Mg cations. One of the
metal ions acts as the center mediating the PhꢀPh coupling. The
Acknowledgements
We thank Deutsche Forschungsgemeinschaft (DFG) and the
WWU Münster for financial support.
Notes and references
OrganischꢀChemisches Institut, Westfälische WilhelmsꢀUniversität,
Corrensstraße 40, 48149 Münster, Germany. EꢀMail: studer@uniꢀ
muenster.de and cml@uniꢀmuenster.de
†
Electronic Supplementary Information (ESI) available: Detailed
experimental procedures, characterization data for all new compounds
and computational data. See DOI: 10.1039/c000000x/
presumably
low
equilibrium
concentration
of
(MgBrPh)2(THF)(TEMPO) is supposed to be sufficient to
allow the fast coupling reaction to occur under the experimental
conditions.
‡ Present address: Max Planck Institute of Molecular Physiology, Ottoꢀ
HahnꢀStrasse 11, 44227 Dortmund, Germany.
1
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2
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Figure 4 Spin densities (B3LYPꢀD3/def2ꢀTZVP, 0.003 a.u.
isosurfaces) of the intermediate structures in the formation of
biphenyl
from
MgPh2(THF)(TEMPO)
(left)
and
(MgBrPh)2(THF)(TEMPO) (right).
Conclusions
We have experimentally shown that the TEMPOꢀmediated arylꢀ
Grignard homocoupling reaction does not proceed via free aryl
radicals. Typical radical clock experiments to proof the aryl
radical character of an intermediate did not show any indication
for their occurrence. However, we found strong evidence that
biaryl radical anions are intermediates in these homocoupling
processes. A necessity for successful homocoupling is the
population of the biarylꢀMg species in the Schlenk equilibrium.
Moreover, we found that the Ph2Mg(THF)2ꢀcomplex does not
undergo oxidative homocoupling in the presence of TEMPO,
which shows that additional MgBr2 is necessary for successful
coupling to proceed. DFTꢀstudies revealed that homocouling
3
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can
occur
easily
from
the
dimeric
Aryl2MgBr2Mg(THF)(TEMPO) species. Coupling processes
are induced by initial single electron transfer from the
Aryl2Mgꢀmoiety to the noninnocent TEMPO ligand rendering
the ligand to become an anionic ligand. CꢀC coupling is then
5
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