Phosphinite- and phosphite-based type I palladacycles as highly active catalysts for addition reactions of arylboronic acids with aldehydes, α,β-unsaturated ketones, α-ketoesters, and aldimines

Article abstract of DOI:10.1016/j.tetlet.2007.05.119

Phosphinite- and phosphite-based type I palladacycle-catalyzed additions of arylboronic acids with aldehydes, α,β-unsaturated ketones, α-ketoesters, and aldimines are described. Our study showed that readily available phosphinite- and phosphite-based type I palladacycles could possess higher catalytic activity than phosphine-based palladacycles and were highly active, practical catalysts. Our study may pave the road for development of optically active phosphinite- and phosphite-based palladacycles for asymmetric catalysis.

Full text of DOI:10.1016/j.tetlet.2007.05.119

Tetrahedron Letters 48 (2007) 5283–5288
Phosphinite- and phosphite-based type I palladacycles as highly
active catalysts for addition reactions of arylboronic acids with
aldehydes, a,b-unsaturated ketones, a-ketoesters, and aldimines
Ping He, Yong Lu and Qiao-Sheng Hu*
Department of Chemistry, College of Staten Island and the Graduate Center of the City University of New York,
Staten Island, NY 10314, United States
Received 24 April 2007; revised 14 May 2007; accepted 21 May 2007
Available online 24 May 2007
Abstract—Phosphinite- and phosphite-based type I palladacycle-catalyzed additions of arylboronic acids with aldehydes, a,b-unsat-
urated ketones, a-ketoesters, and aldimines are described. Our study showed that readily available phosphinite- and phosphite-based
type I palladacycles could possess higher catalytic activity than phosphine-based palladacycles and were highly active, practical
catalysts. Our study may pave the road for development of optically active phosphinite- and phosphite-based palladacycles for
asymmetric catalysis.
Ó 2007 Elsevier Ltd. All rights reserved.
Palladium-catalyzed cross-coupling reactions, for exam-
ple, the Suzuki coupling, the Stille coupling, the Heck
coupling, the Sonogashira coupling, and the amination
of aryl halides with amines, etc., have emerged as pow-
Ar-Ar'
Ar-X
Pd(0)
Oxidative Reductive
Addition
Elimination
Ar
Pd(II) Ar'
1
,2
erful tools for organic synthesis over the past decades.
Ar Pd(II)
X
Extensive studies established that there are three key ele-
mentary steps in the catalytic cycles of Pd(0)-catalyzed
cross-coupling reactions: oxidative addition of Pd(0)
Transmetallation
1
Ar'-M
M-X
with an aryl halide to form Pd(II) complex; transmetala-
tion of the Pd(II) complex with an organometallic
reagent to form a diorganopalladate complex; and
reductive elimination of the diorganopalladate complex
to form the cross-coupling product and regenerate the
Pd(0) catalyst (Fig. 1). In our laboratory, we are inter-
ested in developing highly active transition metal cata-
lysts and new reactions/processes to further heighten
Figure 1. Generalized mechanism for Pd-catalyzed cross-coupling
reactions.
already powerful transition metal-catalyzed cross-
coupling reactions be even more powerful for organic
synthesis. Toward this end, we have recently documen-
ted Pd(0)/t-Bu P-catalyzed Suzuki cross-coupling of
3
3
–6
5
the efficiency of transition metal catalysis.
Based on
dihaloarenes with arylboronic acids, a process that
our understanding of the generalized mechanism
depicted in Figure 1, we envisioned that the individual
elementary steps in each catalytic cycle might be con-
trolled. Such a control, especially combined with other
bond forming processes, could provide us opportunities
to develop new reactions/processes and thus make the
relies on the controlling of oxidative addition step. We
have also reported Pd-catalyzed reaction of o-dihalo-
arenes and alkynes with hindered Grignard reagents to
6
form substituted fluorenes and indenes, cyclization pro-
cesses that combines the control of the transmetalation
3
step with sp C–H activation.
Our study also included the control of a reductive
elimination elementary step. We envisioned that by con-
trolling the reductive elimination process, anionic four-
*
0
Corresponding author. Tel.: +1 718 982 3901; fax: +1 718 982
910; e-mail: qiaohu@mail.csi.cuny.edu
7
,8
3
electron donor-based (type I) palladacycles,
rather
040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.05.119

5284
P. He et al. / Tetrahedron Letters 48 (2007) 5283–5288
L
Pd
R
OAr
OAr
Cl
M-X
II
Ph
Ph
O
Ar-M
6: R = H
7: R = t-Bu
Y
C
O
4: R = H
5: R = -Bu
P
Ar
P
Pd
R
C
Z
L
Y
R
Pd
t
R
(
Insertion)
Cl
R
R'
(
Transmetalation)
Figure 3. Phosphinite- and phosphite-based type I palladacycles.
L
Pd
L
II
II
II
C
Pd
Pd
C
Y
X
Ar
(
(
Reductive
Elimination)
Ar
Transmetalation)
R
R'
a
Table 1. Catalyst comparison
L
M Y
R
C
Pd(0)
Ar'
Z
OH
Ar-M
5
% palladacycle
Ar
CHO
R
Ph
R
PhB(OH)2
+
Toluene, K PO , rt
3
4
Cross-Coupling
Reaction Cycle
Addition Reaction
Cycle
b
Entry Palladacyde
R
Reaction time Conversion (%)
1
2
3
4
5
6
7
8
9
3
4
5
6
7
3
4
5
7
NO
2
2
2
2
2
30 min
30 min
30 min
30 min
30 min
4 h
4 h
4 h
4 h
88
100
100
49
100
26
72
59
83
Figure 2. Cross-coupling reactions versus addition reactions for type I
palladacycle catalysts.
NO
NO
NO
NO
OCH
OCH
OCH
OCH
than functioning as cross-coupling catalyst precursors,
could catalyze addition reactions of organometallic
reagents such as arylboronic acids with carbonyl group-
containing compounds and analogs (Fig. 2). We have
demonstrated that phosphine-based type I pallada-
cycles 1–3 (Chart 1) were indeed effective catalysts for
addition reactions of arylboronic acids with a,b-unsatu-
3
3
3
3
a
Reaction conditions: aldehyde (1.0 equiv), phenylboronic acid
2.0 equiv), toluene (2 mL), K PO (1 equiv), room temperature.
Based on H NMR.
(
3
4
b
1
9
rated ketones, aldehydes, and a-ketoesters. Although
palladacycle 3 was found to be a very efficient catalyst
for such addition reactions, it was prepared from non-
commercially available phosphine precursor. In addi-
tion, as the catalytic activity of a type I palladacycle
is expected to be influenced by the steric and electronic
nature of palladium-bound aromatic part and the
P-ligand part, we envisioned that phosphine-based pal-
ladacycle 3 may not be the one with the highest catalytic
activity and type I palladacycles with other Pd-bound
aromatic parts and P-ligand part might possess higher
catalytic activity than that of 3. These considerations
prompted us to examine palladacycles derived from
readily available and more p-acidic phosphinites and
and our results are listed in Table 1.12 We found that
palladacycles 4, 5, and 7 exhibited a higher catalytic
activity than 3 (Table 1, entries 1–3, 5, and 6). Lower
catalytic activity was observed for phosphite-based pal-
ladacycle 6 (Table 1, entry 4), suggesting that the steric
hindrance of Pd-bound aromatic part also plays an
important role for the catalytic activity of palladacycles.
We have further employed less reactive 2-methoxybenz-
aldehyde as substrates and palladacycles 4, 5, and 7
again exhibited a higher catalytic activity than 3, with
phosphite-based palladacycle 7 being the most active
one (Table 1, entries 6–9).
1
0,11
phosphites, that is, 4–7 (Fig. 3),
for addition reac-
tions of arylboronic acids with carbonyl group-contain-
ing compounds. In this Letter, we report that
phosphinite- and phosphite-based palladacycles could
indeed be more active than phosphine-based pallada-
cycle 3, and are highly efficient catalysts for the addition
reactions of arylboronic acids with aldehydes, a-keto-
esters, a,b-unsaturated ketones, and aldimines.
Having established that phosphinite-based palladacycles
4
, 5 and phosphite-based palladacycle 7 possess a higher
catalytic activity than phosphine-based palladacycle 3,
we next briefly examined the addition of arylboronic
acids with aldehydes, a,b-unsaturated ketones, and a-
ketoesters by employing palladacycles 4, 5, and 7 as cat-
alysts. Our results are listed in Tables 2–4, respectively.
We found palladacycles 4, 5, and 7 in general were
highly active catalysts for these addition reactions. High
yields were observed for tested aldehydes including aro-
We began our study by comparing the catalytic activities
of phosphinite-based palladacycles 4, 5 and phosphite-
based palladacycles 6, 7 with that of phosphine-based
palladacycle 3, under the reaction conditions we previ-
ously established (toluene as the solvent and K PO as
1
2,13
matic and aliphatic ones (Table 2),
a,b-unsaturated
1
4
ketones including cyclic and acyclic ones (Table 3),
3
4
15
and a-ketoesters (Table 4). In terms of substrate reac-
tivity, a,b-unsaturated ketones were found to be the
most reactive, with aldehydes being next and a-keto-
esters the least reactive, a trend similar to what was
observed in our study with palladacycle 3 as catalyst.
9
the base). The room temperature addition of phenyl-
boronic acid with 2-nitrobenzaldehyde was first studied
Pt
-Bu2
Pd OAc
PPh2
Pd
We have also examined the addition reaction of aryl-
boronic acids with aldimines and our results are listed
P
Pd
OAc
Fe
o
-Tolyl
o-Tolyl
OAc
1
6,17
in Table 5.
We found 4 and 7 were also effective
1
2
3
catalysts for N-Ts- and N-Bs-containing aldimines in
Chart 1. Phosphine-based type I palladacycles.
general. We found that when aldimines derived from

P. He et al. / Tetrahedron Letters 48 (2007) 5283–5288
5285
a
Table 2. Phosphinite- and phosphite-based palladacycle-catalyzed addition reactions of arylboronic acids with aldehydes
OH
5
% Palladacycle
ArB(OH)2
+
RCHO
Ar
R
K3PO4/Toluene, rt
b
Entry
1
ArB(OH)
2
RCHO
O N
Palladacycle
Time (h)
1
Yield (%)
B(OH)2
B(OH)2
CHO
CHO
7
94
2
c
2
3
4
5
6
O N
2
7
5
86
CHO
NO2
B(OH)2
B(OH)2
B(OH)2
5
7
7
7
0.5
0.5
0.5
0.5
91
94
92
93
CHO
NO2
CHO
NO2
CHO
NO2
MeO
B(OH)2
CHO
NO2
7
8
9
7
7
7
7
7
4
7
7
7
0.5
48
36
48
48
14
10
48
48
92
88
84
91
90
93
90
93
90
B(OH)2
B(OH)2
Cl
CHO
CHO
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
1
1
1
1
1
1
0
1
2
3
4
5
CHO
MeO
CHO
CHO
OCH3
CHO
OCH3
CHO
CHO
a
b
c
Reaction conditions: aldehyde (1.0 equiv), arylboronic acid (1.5–2.0 equiv), K
Isolated yields (average of two runs).
3
PO
4
(1–3 equiv), toluene (2 mL), room temperature.
5
mmol scale, 1% 7 was used.
a
Table 3. Palladacycles 4/7-catalyzed 1,4-addition of arylboronic acids with a,b-unsaturated ketones
O
Ar O
5
% Palladacycle
Ar-B(OH)2
+ R
R
R'
R
R'
K PO /Toluene, rt
3
4
b
Entry
ArB(OH)
2
O
Palladacycle
Time (h)
Yield (%)
R'
O
1
2
B(OH)2
B(OH)2
4
7
0.5
0.5
95
98
Ph
Ph
Ph
O
O
O
Ph
Ph
Ph
c
3
4
B(OH)2
7
7
5
96
Ph
Ph
MeO
B(OH)2
0.5
94
(continued on next page)

5286
P. He et al. / Tetrahedron Letters 48 (2007) 5283–5288
Table 3 (continued)
b
Entry
ArB(OH)
2
O
Palladacycle
Time (h)
Yield (%)
R
R'
O
5
6
B(OH)2
B(OH)2
7
7
0.5
0.5
93
91
Ph
Ph
Ph
O
O
7
8
B(OH)2
B(OH)2
B(OH)2
7
4
7
1
24
24
98
92
92
O
O
9
a
Reaction conditions: ketone (1.0 equiv), arylboronic acid (2.0 equiv), 4 or 7 (5%), K
Isolated yields (average of two runs).
The reaction was carried out at a 4 mmol scale, 1% 7 was used.
3 4
PO (1 equiv), toluene (2 mL), room temperature.
b
c
Table 4. Palladacycles 4/7-catalyzed 1,4-addition of arylboronic acids
with a-ketoesters
Table 5. Palladacycles 4/7-catalyzed addition reactions of arylboronic
acids with aldimines
a
a
OH
NR
O
CO Et
5% Palladacycle
5% Palladacycle
NHR
R
2
ArB(OH)2
+
ArB(OH)2
+
Ar'
H
R
Ar Ar'
2
K PO /Toluene, rt, 48 h
Ar
3
4
CO Et
K PO /Toluene, rt, 24-48 h
3
4
0
b
b
Entry
1
ArB(OH)
2
R
Ph
Palladacycle
Yield (%)
Entry ArB(OH)
2
R
Ar
Pallada- Yield
cycle
(%)
B(OH)2
4
83
1
B(OH)2
Bs
C
6
H
5
7
80
c
2
3
4
5
B(OH)2
Ph
Ph
Ph
CH
7
7
7
7
68
2
3
4
5
6
7
8
9
0
B(OH)2
B(OH)2
B(OH)2
Bs
Bs
C
C
6
H
H
5
5
7
90
87
68
79
MeO
B(OH)2
94
92
60
6
4
B(OH)2
Bs p-MeO–C H
6
4
4
7
B(OH)2
3
MeO
B(OH)2
Bs p-MeO–C H
6
7
a
Reaction conditions (not optimized): a-ketoester (1.0 equiv), aryl-
c
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
Bs p-NO
Bs p-NO
Ts p-NO
Ts p-NO
2
2
2
2
–C
–C
–C
–C
6
H
6
H
6
H
6
H
4
7
67
3 4
boranic acid (2.0 equiv), K PO (3 equiv), toluene (2 mL), room
temperature.
b
c
c
Isolated yields (average of two runs).
2% conversion.
4
4
4
7
63
7
c
7
69
p-nitrobenzaldehyde were used as substrates, (4-nitro-
phenyl)phenylmethanol was isolated in 21–29% yields
(
likely underwent hydrolysis during the reaction. We also
found that the aldimine derived from aniline and benz-
aldehyde was not reactive, with either 4 or 7 as catalyst.
c
7
72
Table 5, entries 6–9), suggesting that the aldimines
1
Ph
C
6
H
5
4 or 7
0
a
Reaction conditions (not optimized): aldehyde (1.0 equiv), aryl-
boronic acid (2.0 equiv), K
3 4
PO (1 equiv), toluene (2 mL), room
temperature.
Isolated yields (average of two runs).
(Aryl)(4-nitrophenyl)methanol was also isolated in 21–29% yields.
b
c
In summary, we have demonstrated that readily avail-
able phosphinite- and phosphite-based type I pallada-
cycles 4, 5, and 7 exhibited a higher catalytic activity
than that of phosphine-based type I palladacycle 3,
and were highly active catalysts for the addition reac-
tions of arylboronic acids with aldehydes, a,b-unsatu-
rated ketones, a-ketoesters, and aldimines. Our study
may pave the road for the development of optically
active phosphinite- and phosphite-based type I pallada-
Acknowledgments
We gratefully thank the NIH (GM69704) for funding.
Partial support from PSC-CUNY Research Award Pro-
gram is also gratefully acknowledged. We also thank
Frontier Scientific, Inc. for its generous gifts of arylbo-
1
8
cycles for asymmetric catalysis. Work toward this
direction is actively underway.
ronic acids and Pd(OAc) .
2

P. He et al. / Tetrahedron Letters 48 (2007) 5283–5288
5287
Supplementary data
11819; One example of 1,2-addition products of phen-
ylboronic acid with p-chlorobenzaldehyde, formed as
byproducts during the cross-coupling reaction catalyzed
by a palladacycle at 130 °C, was previously reported: (c)
Gibson, S.; Foster, D. F.; Eastham, G. R.; Tooze, R. P.;
Cole-Hamilton, D. J. Chem. Commun. 2001, 779–780; For
a recent report on cationic palladium complex-Catalyzed
intramolecular addition of arylboronic acids to ketones:
General procedures and product characterization for
palladacycle-catalyzed addition reactions. Supplemen-
tary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2007.05.119.
(
d) Liu, G.; Lu, X. J. Am. Chem. Soc. 2006, 128, 16504–
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1. (a) Bedford, R. B.; Hazelwood, S. L.; Limmert, M. E.;
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1
3 3
2. For Pd(0)/PPh /CHCl -catalyzed 1,2-addition of arylbo-
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4
155; A 3% yield of 1,2-addition product was observed
during the cross-coupling of phenylboronic acid with 3-
methoxy-4-tosyloxybenzaldehyde catalyzed by Pd(OAc)
2
/
Buchwald’s biarylphosphine: (b) Nguyen, H. N.; Huang,
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(
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1
7. The addition reaction of aryl/alkenylboronic acids with
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