Angewandte
Chemie
DOI: 10.1002/anie.201408341
Synthetic Methods
Transfer of Aryl Halide to Alkyl Halide: Reductive Elimination of
Alkylhalide from Alkylpalladium Halides Containing syn-b-Hydrogen
Atoms**
Wei Hao, Junnian Wei, Weizhi Geng, Wen-Xiong Zhang, and Zhenfeng Xi*
Dedicated to Prof. Tamotsu Takahashi on the occasion of his 60th birthday
Abstract: b-Hydride abstraction is a well-accepted elementary
step for catalytic cycles in organometallic chemistry. It is
usually anticipated that alkylpalladium halides containing syn-
b-hydrogen atoms will undergo b-hydride abstraction to afford
the Heck-type products. However, this study discloses that the
above general knowledge is only conditionally correct. Our
experimental results demonstrate that the reductive elimination
of alkylhalides from alkylpalladium halides containing syn-b-
hydrogen atoms may surpass the b-hydride abstraction or even
become exclusive in certain cases.
A
lkylpalladium(II) halides (A; Scheme 1) containing syn-b-
hydrogen atoms have recently become synthetically useful for
constructing carbon–carbon bonds and carbon–heteroatom
bonds through transition-metal-catalyzed cross-coupling
reactions.[1] b-Hydride abstraction, a well-accepted elemen-
tary step for catalytic cycles in organometallic chemistry, is
the major possible competitive reaction, thus resulting in the
Heck-type alkene product.[2] Therefore, to suppress b-hydride
abstraction and facilitate the following step such as a trans-
metallation reaction, bulky trialkylphosphine ligands such as
P(tBu)2Me, PCy3, and other additives are required.[1c] In
contrast, reductive elimination (RE) of alkyl halides from
alkylpalladium halide species (A’, Scheme 1) has been
recently recognized as a novel elementary step for catalytic
cycles.[3–8] In all those examples in the literature reporting this
RE of A’, the alkylpalladium halides do not have syn-b-
hydrogen atoms.[9] Otherwise, b-hydride abstraction would
take place to afford the alkene products.[6c,10] Herein we
report the first example of reductive elimination of alkyl
Scheme 1. Elementary steps for catalytic cycles involving alkylpalla-
dium(II) complex A, having syn-b-hydrogen atoms, and A’, lacking syn-
b-hydrogen atoms. b-hydride abstraction versus reductive elimination.
halides from alkylpalladium(II) halides containing syn-b-
hydrogen atoms.
After screening various reaction conditions with the
substrate 1a and a 3-alkyne (Scheme 2), the base LiOtBu
was found to be most effective. When no LiOtBu was added,
only trace amounts of the product 2a was formed (see the
Supporting Information for details). The role of LiOtBu is not
clear yet, but the base would probably facilitate the regen-
eration of the active palladium(0) species.[11] During ligand
optimization, PPh3 was found to be the most effective ligand
in this reaction. When additional PPh3 was added, the yield of
2a decreased. Bulky ligands such as dppf and P(tBu)3, which
were used successfully in the previous reductive elimination
[*] W. Hao, J. Wei, W. Geng, Prof. Dr. W.-X. Zhang, Prof. Dr. Z. Xi
Beijing National Laboratory for Molecular Sciences (BNLMS)
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry
[6a,c,7]
Peking University, Beijing 100871 (China)
E-mail: zfxi@pku.edu.cn
À
of C I bonds,
appeared to be ineffective. Other mono-
dentate phosphine ligands such as tri-(2-furyl)phosphine
(TFP), tBuXPhos, and bidentate phosphine ligands such as
dppe and dppp gave much lower yields or no product at all
(see the Supporting Information for details).
Prof. Dr. Z. Xi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Shanghai 200032 (China)
Solvents were also found to be essential for the success of
this reaction (Scheme 2). When the reaction was conducted in
polar solvents such as THF and DMF, the reaction became
messy, thus affording a mixture of products with 1-phenyl-
piperidine (deiodination product of 1a) as the major one. A
non-negligible amount of the alkene product 3a was also
[**] This work was supported by the 973 Program (2012CB821600) and
the Natural Science Foundation of China. We thank Prof. Guosheng
Liu (SIOC) and Prof. Zhangjie Shi (Peking University) for helpful
discussions.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 14533 –14537
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14533