Organic Letters
Letter
Pd(dba)2 or Pd2(dba)3 respectively, with the concomitant
disappearance of the O1 oxidation potential. These results
perfectly matched those obtained with 31P NMR, confirming a
rapid and efficient formation of the (Xantphos)Pd(dba)
complex. When Synphos was added to Pd(dba)2, a new
oxidation peak O4 appeared referring to the (Synphos)Pd(dba)
complex. However, only partial conversion was attained as
highlighted by the remaining O2 potentials on the CV (Figure
2A, IV). This finding demonstrates that the formation of
(Synphos)Pd(dba) is not efficient in the binary system of
Pd(dba)2/Synphos. Analogous results were obtained with
Pd2(dba)3, evidencing once again that the formation of
(Synphos)Pd(dba) is not favored in the binary system (Figure
2B, IV), which is in perfect agreement with the 31P NMR
results. Additionally, the CV obtained from the ternary system
with Pd(dba)2/Synphos/Xantphos (Figure 2A, V) closely
resembled that with Pd2(dba)3/Synphos/Xantphos (Figure
2B, V). Most importantly, all oxidation peaks, namely O3, O4,
persisted in the mixed ligand system, and no new oxidation
peaks were detected (as confirmed by deconvoluted CV). This
acknowledges the absence of differences in the catalytic species
formed in the mixed ligand systems containing Pd(dba)2 and
Pd2(dba)3, in line with the results acquired using 31P NMR.
Likewise, equal results were obtained between Pd(dba)2- and
Pd2(dba)3-catalyzed C−S coupling reactions (Figures S2 and
S3), further supporting the notion of analogous catalytic
mechanisms in Pd(dba)2- and Pd2(dba)3-catalyzed coupling
reactions.
To summarize, both Pd(dba)2 and Pd2(dba)3 offer equivalent
catalytic efficiency in the reported Pd-catalyzed C−N and C−S
cross-coupling reactions involving the ternary mixed ligand
catalytic systems. Further combined 31P NMR spectroscopy
and cyclic voltammetry experiments yielded convergent results
and enabled an insightful investigation and precise description
of the underlying mechanism. Indeed, Pd(dba)2 and Pd2(dba)3
offer equivalent catalytic efficiencies and display similar catalytic
mechanisms in these mixed ligand Pd-catalyzed cross-coupling
reactions. This information will be certainly useful and valuable
in performing organic synthesis requiring Pd-catalyzed cross-
coupling reactions and may also be extended to other Pd-
catalyzed reactions due to the importance of these two Pd
reagents.24
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ASSOCIATED CONTENT
* Supporting Information
(24) Pd2(dba)3 is more frequently used in synthesis thanks to its
better stability, whereas Pd(dba)2 is better for mechanism investigation
by virtue of its simple and precise Pd/dba ratio.
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S
Chemical synthesis, analytical data, and NMR spectra. This
material is available free of charge via the Internet at http://
AUTHOR INFORMATION
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Corresponding Author
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We thank financial support from Canceropo
CNRS, and Aix-Marseille Universite.
supported by the CSC fellowship from the China Scholarship
Council. We thank Dr. Damien Herault and Mrs. Roseline
Rosas at Aix-Marseille Universite
for their help in the 31P NMR.
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le PACA, INCa,
M.C., Y.T.F., and J.J.T. are
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dx.doi.org/10.1021/ol501600k | Org. Lett. 2014, 16, 4074−4077