Published on the web July 23, 2013
1227
Identification of Alkenyl- and Arylpalladium Hydrides with the Aid of Hydrosilanes
Tieqiao Chen,1 Yongbo Zhou,1 Shuang-Feng Yin,1 Yalei Zhao,2 Midori Goto,2 and Li-Biao Han*2
1College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China
2National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565
(Received June 25, 2013; CL-130588; E-mail: libiao-han@aist.go.jp)
Well studied hydridopalladiums
For the first time, the long-proposed thermodynamically
unstable catalytic intermediates alkenyl and aryl complexes
sp2C-Pd-H have been isolated through the reduction of the
palladium acetates with hydrosilanes and subsequent stabiliza-
tion of the hydridopalladiums with hydrosilanes. The structures
of these hydridopalladiums were established by X-ray analysis.
PR2
Pd
F
N
F
F
F
PR3
Pd
PR3
H
Z
H
Pd
H
PR3
PR3
PR2
Z = X, RS, (RO)2P(O), R3Sn etc.
stabilized C-Pd-H complexes
Pd-H with a heteroatom functionality
No general C-Pd-H complexes fully characterized
PR3
Pd
PR3
Me Pd
PR3
sp3C-Pd-H
PR3
Pd
PR3
Pd
Regarding efficiency and generality, perhaps no other metal
can compete with palladium, which is widely used as a catalyst
in various chemical transformations.1 The isolation of a
palladium intermediate involved in the catalytic reactions is
not only pivotal for understanding the reaction mechanism, but
also crucial for designing a better catalyst. This kind of research
has been performed extensively in the past decades.1,2
R
H
H
H
H
PR3
PR3
PR3
sp2C-Pd-H
this work
spC-Pd-H
Figure 1. Representative hydridopalladiums.
Hydridopalladium complexes have an exceptional relevance
to palladium-mediated reactions.2,3 Whereas heteroatom Z-Pd-
H complexes (Z: a heteroatom or group) have been isolated
successfully in the past, the hydridopalladium intermediate
bearing a C-Pd-H skeleton remains rather unexplored owing to
its chemically labile character (Figure 1).2,4-6 Needless to say,
these C-Pd-H hydridopalladiums are the key intermediates for
two very important reactions in organic synthesis, i.e., C-H
activation and C-H bond-forming reactions such as reductions
of organohalides, alkenes, and alkynes, isomerization, the Heck
arylation, and others (Figure 2).2,3 Despite extensive studies in
the past, no general example of such a hydridopalladium has
been fully characterized.2 Although some C-Pd-H hydridopal-
ladiums stabilized by a rigid tridentate PCP ligand etc. could be
isolated, they were too stable to undergo reactions such as
reductive eliminations involved in the catalytic sequences and,
therefore, are hardly recognized as representatives of C-Pd-H
intermediates involved in Pd-mediated catalytic reactions.5
Therefore, despite its pivotal relevance to C-H activation and
C-H bond-forming reactions, an unambiguous identification of
such a general C-Pd-H intermediate has not yet been achieved,
not to mention the clarification of its reactivity. Herein, we
report the first successful identification of general sp2C-Pd-H
complexes achieved with the aid of hydrosilanes (Figure 1).
Complex 2a was allowed to react with a variety of reducing
agents such as H2, NaBH4, and Ph3SnH, in the hope that the
corresponding hydridoalkenylpalladium 1a could be observed
(eq 1). However, disappointingly, all these reactions gave the
final reduced products stilbenes, and the expected hydridopalla-
dium intermediate 1a could not be observed at all, indicating the
rapid decomposition of 1a as noticed previously.4a,4g Surpris-
ingly, however, when hydrosilanes R2SiH2 (R = Ph, Et) were
used as the reducing reagents, signals corresponding to the
hydridoalkenylpalladium 1a could be observed clearly. Thus,
Ph2SiH2 (0.1 mmol) was added to (E)-2a (0.05 mmol) dissolved
in 0.5 mL of C6D6 at room temperature. The color of the solution
C-H activation
Pd
R
Pd
H
products
R
RH
C-H bond-forming reaction
Pd
[H]
R
Pd
H
H
RX
+
Pd
[H]
PdH
+
Figure 2. C-Pd-H intermediates involved in catalytic reac-
tions.
turned gradually from light green to blue. As shown by 1H NMR
spectroscopy, a signal assignable to H-Pd was observed at
¹7.9 ppm (td, 1H,
J
P-H = 14.5 Hz,
JH-H = 4.5 Hz (with
C=CH)), indicative of the formation of a hydridoalkenylpalla-
dium complex assignable to (E)-1a. Changes in 31P NMR
spectroscopy were also observed clearly. Thus, as Ph2SiH2 was
added, in addition to the starting material at 10.8 ppm, a new
signal emerged at 20.9 ppm. The full reduction product (Z)-
stilbene (C=CH: 6.5 ppm) was also observed in the reaction
mixture. As estimated from 1H NMR spectroscopy, as (E)-2a
gradually disappeared, the products (E)-1a and (Z)-stilbene
increased, i.e., the ratios of (E)-2a/(E)-1a/(Z)-stilbene as a
function of time were as follows: 1 h, 3.5/1.0/0.41; 2 h, 0.71/
1.0/0.65; 10 h, 0.5/1.0/7.5; 20 h, 0/0/1.0. Thus, although it
could be observed in the reaction, the hydridoalkenylpalladium
(E)-1a gradually decomposed to (Z)-stilbene. This hampers the
isolation of pure (E)-1a from the mixture. Fortunately, we found
that under similar reaction conditions, (Z)-2a also reacts with
Ph2SiH2 to produce the corresponding (Z)-1a (90% NMR yield)
with a characteristic palladium hydride signal at ¹7.3 ppm (td,
1H, JP-H = 12.0 Hz, JH-H = 6.4 Hz (with C=CH)). Perhaps
owing to the steric hindrance around the palladium atom, this
hydridoalkenylpalladium complex (Z)-1a was more stable than
(E)-1a, and only a little decomposition to stilbene was observed
at room temperature during the reaction. Unexpectedly, how-
ever, although stable in solution, the attempted isolation of (Z)-
Chem. Lett. 2013, 42, 1227-1229
© 2013 The Chemical Society of Japan
Chen, Tieqiao
