A nontransmetalation reaction pathway for anionic four-electron donor-based palladacycle-catalyzed addition reactions of arylborons with aldehydes

Article abstract of DOI:10.1016/j.tetlet.2011.04.074

A nontransmetalation reaction pathway for anionic four-electron donor-based (Type I) palladacycle-catalyzed addition reactions of arylborons with aldehydes is described. This new reaction pathway offers new catalysis opportunities for Type I palladacycle-catalyzed addition reactions such as the exceptionally low catalyst loading catalysis, with the catalyst loading as low as 0.0005 mol %. This new pathway may be applicable for other transition metal-catalyzed addition reactions and might lead to the development of new reactions including sequential/tandem reactions.

Full text of DOI:10.1016/j.tetlet.2011.04.074

Tetrahedron Letters 52 (2011) 3324–3328
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A nontransmetalation reaction pathway for anionic four-electron donor-based
palladacycle-catalyzed addition reactions of arylborons with aldehydes
⇑
Yuan-Xi Liao, Chun-Hui Xing, Matthew Israel, 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, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 February 2011
Revised 2 April 2011
Accepted 18 April 2011
Available online 24 April 2011
A nontransmetalation reaction pathway for anionic four-electron donor-based (Type I) palladacycle-cat-
alyzed addition reactions of arylborons with aldehydes is described. This new reaction pathway offers
new catalysis opportunities for Type I palladacycle-catalyzed addition reactions such as the exceptionally
low catalyst loading catalysis, with the catalyst loading as low as 0.0005 mol %. This new pathway may be
applicable for other transition metal-catalyzed addition reactions and might lead to the development of
new reactions including sequential/tandem reactions.
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Type I palladacycle
Arylboroxines
Aldehydes
Addition
Transmetalation
Transmetalation of organometallic reagents with ArPd(II)X(Ln)
complexes comprises one of the key steps in Pd-catalyzed cross-
coupling reactions such as the Kumada coupling, the Stille coupling
and the Suzuki coupling.1,2 In these coupling reactions, it is desir-
able for transmetalation to occur. Transmetalation of organoboronic
acids with transition metal catalysts has also been considered as a
key step in transition metal-catalyzed addition reactions of arylbo-
Achieving such addition reactions (aryl transferring) without trans-
metalation could prevent palladacycle catalysts from decomposi-
tion and thus offer us new catalysis opportunities such as low
catalyst loading catalysis, expanded substrates/nucleophile scope
O
OH
M
O
Ar
ronic acids with aldehydes, imines and
a,b-unsaturated ketones/
addition
ArB(OH) /
2 R
H
aldehydes/esters (Scheme 1).3–7
Ar
R
M
X
(transmetalation)
R
In our laboratory, we are interested in employing anionic four-
electron donor-based (Type I) palladacycles,8,9 which are readily
available and air/moisture-stable, as catalysts for such addition
reactions.10,11 The transmetalation of arylboronic acids with Pd–X
to form A was considered to be a key step for Type I palladacycle-
catalyzed addition reactions (Scheme 2, path a). During our study,
we were bothered by the decomposition of Pd(II) catalyst to Pd(0)
species B via reductive elimination of A (Scheme 2, path b), which
resulted in high catalyst loadings (3 mol % or more) and limited
the scope of substrates/arylboronic acids. Based on the consider-
ation that contacts between Type I palladacycles and arylboronic
acids should occur prior to the transmetalation between them,12
that is, complexes such as C might be formed prior to the formation
of A, we hypothesized a new reaction pathway for Type I palladacy-
cle-catalyzed addition reactions: the addition reactions (aryl trans-
ferring) might take place at this stage to afford addition product
without the occurrence of transmetalation (Scheme 2, path c).
H
M = Rh(I/II), Pd(II), Pt(II), Ni(0)/Ni(II), Cu(II), Fe(III), Ru(III), Co(II)
Scheme 1. Transition metal-catalyzed addition reactions of arylboronic acids with
aldehydes.
addition
C
OH
Ar
Path a
R
L
PdII
O
L
Cross-coupling
and/or
Pd Black
R
C
H
Pd(0)
Ar
Ar
O
Path b
ArB(OH)2
base
R
H
L
B
A
(Reported)
C
PdII
transmetalation
X
Type I
Palladacycles
L
PdII
O
Y
OH
Ar
addition
H
R
Path c
C
without
transmetalation
B
R
"ArB"
base
O
Ar
R
H
C
(This Work)
Y = halide and/or base
⇑
Corresponding author. Tel.: +1 718 982 3901; fax: +1 718 982 3910.
E-mail address: qiaosheng.hu@csi.cuny.edu (Q.-S. Hu).
Scheme 2. Mechanistic considerations for Type I palladacycle catalyzed addition
reactions of arylborons with aldehydes.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.074

Y.-X. Liao et al. / Tetrahedron Letters 52 (2011) 3324–3328
3325
Table 1
Palladacycle 1-catalyzed reactions of phenylboronic acid/phenylboroxine with bromobenzaldehydesa
t-Bu
O(2,4-di-t-BuC6H3)
CHO
O
P
OH
Ph
0.5 mol%
t-Bu
O(2,4-di-t-BuC6H3)
OH
Ph
CHO
Pd
Cl (1)
Br
+
+
toluene, base, rt, 6 h
Br
Ph
II
I
Ph
"PhB"
CHO
Entry
1
‘PhB’
Base (equiv)
H2O (equiv)
—
Conv. (%)
78
Ratio of I:IIb
Br
CHO
(PhBO)3
K3PO4 (A, 1)
100:0
(2a)
Br
2
3
4
5
6
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
Br
(PhBO)3
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
K3PO4 (A, 3)
K3PO4 (B, 3)
K3PO4 (B, 1)
K3PO4 (A, 1)
K3PO4 (A, 3)
K3PO4 (B, 3)
K3PO4 (A, 3)
K2CO3 (A, 1)
K2CO3 (A, 1)
Na2CO3 (A, 1)
KOAc (A, 1)
KF (A, 1)
—
—
—
—
—
1
1
—
1
81
95
71
83
75
77
71
48
89
15
3
100:0
85:15
39:61
77:23
95:5
83:17
82:18
100:0
53:47
100:0
—
7
8
9c
10c
11c
12c
13c
—
—
—
33
100:0
CHO
(2b)
14
(PhBO)3
K3PO4 (A, 3)
—
40
99:1
15
16
17
2b
2b
2b
PhB(OH)2
(PhBO)3
(PhBO)3
K3PO4 (B, 1)
K3PO4 (A, 3)
K3PO4 (A, 3)
—
1
3
67
70
80
14:86
38:62
40:60
CHO
18
(PhBO)3
K3PO4 (A, 3)
—
47
99:1
Br
2c
2c
2c
2c
2a
2a
2a
2a
(2c)
19
20
21
22
23
24
25
26
PhB(OH)2
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
(PhBO)3
K3PO4 (B, 1)
K3PO4 (A, 3)
K3PO4 (A, 3)
K3PO4 (A, 3)
K3PO4 (A, 0)
K3PO4 (A, 0.25)
K3PO4 (A, 0.5)
K3PO4 (A, 0.75)
—
1
3
—
—
—
—
—
60
84
90
0
50:50
36:64
40:60
d
—
0
—
21
44
63
100:0
100:0
100:0
a
Reaction conditions: aldehyde (1.0 equiv), phenylboronic acid (2.0 equiv) or phenylboroxine (0.67 equiv), toluene (2 mL), base (1 or 3 equiv, A: ground and dried,
B: ground), room temperature, 6 h.
b
Based on 1H NMR or GC–MS.
1 mol % catalyst loading in 3 h.
No palladacycle 1 was used, rt or 50 °C, 24 h.
c
d
and new bond-forming reactions. In this Letter, we report our re-
sults to establish such a possibility, specifically Type I palladacy-
cle-catalyzed addition reactions of arylboroxines with aldehydes,
with unprecedented low catalyst loading and expanded nucleo-
phile scope.
results for the same reaction being carried out in the presence of
water: significant amount of cross-coupling product was detected
and Pd black was observed in less than 10 min (Table 1, entries 3–
8). Similar trends were also observed with other bases, which were
less effective than K3PO4 (Table 1, entries 9–13), and with 3-bro-
mobenzaldehyde and 4-bromobenzaldehyde as substrates (Table
1, entries 14–21). These results showed that the addition reactions
could indeed occur under anhydrous conditions and the catalyst
decomposition was observed to be minimal.
The hypothesis of addition reactions taking place without the
occurrence of transmetalation suggested that the decomposition
of the palladacycle catalysts would not occur, even at elevated
temperature. Thus, this nontransmetalation hypothesis implied
that the catalyst loading could be lowered to an unprecedented le-
vel by using arylboroxines and dried bases, and previously unsuit-
able or unreactive arylboronic acids such as o-trifluoromethyl-
containing arylboronic acids could be appropriate for the addition
reactions. We tested such possibilities and our results are summa-
rized in Table 2. We found that 0.001–0.02 mol % catalyst loading
was sufficient enough to effect the addition reactions of arylborox-
ines with aldehydes in high yields (Table 2, entries 1–17). We also
found electron-deficient arylboroxines, generated from arylboronic
acids such as o-trifluoromethyl-containing phenylboronic acids,
were excellent nucleophiles (Table 2, entries 18–21). Further
It has been established that transmetalation of organoboron
compounds with Pd–X complexes would be very slow in the ab-
sence of species such as H2O/OHÀ, ROH/ORÀ,1,13 we surmised that
conditions without these species such as an anhydrous condition
might achieve the addition reaction without the occurrence of
transmetalation. We thus began our study by determining the fea-
sibility to achieve the addition reaction under the anhydrous con-
dition. Dried arylboronic acids (arylboroxines)14 and dried K3PO4
were used. Bromobenzaldehydes were chosen as substrates as they
contain both the addition reaction acceptor (aldehyde carbonyl
group) and cross-coupling acceptor (C–Br moiety). The observation
of Pd black and/or detection of cross-coupling products would indi-
cate the formation of Pd(0) species. We first examined the reaction
of 2-bromobenzaldehyde with phenylboroxine and dried K3PO4
(1 equiv) with Type I palladacycle 1 as the catalyst. We were very
pleased to find that the addition reaction occurred smoothly, with
no cross-coupling reaction product being observed (Table 1, entries
1 and 2). In addition, no Pd black was observed for the reaction
system even after three days. This was in sharp contrast to the

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Y.-X. Liao et al. / Tetrahedron Letters 52 (2011) 3324–3328
Table 2
Palladacycle 1-catalyzed reactions of arylboroxines with aldehydes with low catalyst loadinga
0.02-0.0005 mol % Palladacycle 1
O
OH
Ar
(ArBO)
+
H
3
o
R
R
toluene, K PO , 50
C
3
4
Entry
RCHO
(ArBO)3
BO
Catalyst loading (%)
Time (h)
Yieldb (%)
1
2
3
4
0.01
0.01
0.01
0.01
72
24
24
24
89c
91
93
86
Br
CHO
(2c)
3
3
BO
BO3
2c
2c
2c
MeO
BO3
5
2c
0.01
24
91
BO 3
BO3
Br
(2a)
6
7
8
0.01
0.01
0.01
24
24
24
90
90
90
CHO
BO 3
BO3
2a
Br
(2b)
CHO
(2d)
CHO
BO3
BO3
9
0.01
0.01
24
30
91
87
(2e)
CHO
10
11
2e
0.01
30
89
BO 3
BO
12
13
14
15
16
2e
0.01
0.01
0.02
0.02
0.02
30
24
36
30
30
84
92
83
80
81
3
3
Cl
CHO
BO3
BO
(2f)
MeO
CHO(2g)
(2h)
BO3
BO
CHO
2h
3
CHO
(2i)
BO3
17
18
0.02
0.01
30
12
82
Br
CHO
CF3
BO3
CF3
BO
85d
(2a)
19
20
21
2a
0.01
0.01
0.02
12
12
12
90d
91d
83d
F3C
F3C
F3C
3
3
3
CF3
BO
(2c)
CHO
Br
CF3
BO
(2e)
CHO
BO
22
23
24
2c
2c
2c
0.005
0.001
0.0005
36
60
24
92
3
BO3
BO3
91
83e
a
b
c
Reaction conditions: aldehyde (0.25 mmol), arylboroxine (0.67 equiv), toluene (1 mL), base (1 or 3 equiv), 50 °C.
Isolated yields.
Reaction was carried out at room temperature.
Reaction was carried out at 90 °C.
Reaction was carried out at 80 °C.
d
e
0.01 mol % catalyst loading was achieved,10a our study described
here apparently offered a catalysis protocol with unprecedentedly
low catalyst loading and expanded nucleophile scope under milder
reaction conditions.
Untreated K3PO4 + H2O (1 equiv):
Pd Black observed
in 2 minutes
(BO)3
OAr
O
Toluene/THF
K3PO4, rt
P
1
+
Pd OAr
Dried K3PO4:
Ph
L
Pd Black was not observed
after 2 h
To support that the addition reaction under anhydrous condi-
tion occurred via the nontransmetalation pathway, we first ex-
cluded the possibility of phenylboroxine reacted with 4-
bromobenzaldehyde in the absence of palladacycle 1 catalyst (Ta-
ble 1, entry 22). We examined the addition reaction with different
amounts of the base and found the presence of the base was vital
to the addition reaction (Table 1, entries 23–26), suggesting that
the base likely participated in the addition reaction via either C
or transmetalation to form A. To understand whether K3PO4 facil-
itated the transmetalation of palladacycle 1 with phenylboroxine
to form A, we tested the reaction of palladacycle 1 with phenylbo-
roxine in the presence of K3PO4. We found while Pd black was
D
L = THF
Scheme 3. Reaction of palladacycle 1 with phenylboronic acid/phenylboroxine.
decreasing the catalyst loading was also tested and we found
excellent yield was obtained with the catalyst loading as low as
0.0005 mol % by extending the reaction time and/or raising the
temperature (Table 2, entries 22–24). To our knowledge, this is
the lowest catalyst loading ever reported for such addition reac-
tions. Compared to our previously reported platinacycle-catalyzed
addition reactions of arylboronic acids with aldehydes, in which

Y.-X. Liao et al. / Tetrahedron Letters 52 (2011) 3324–3328
3327
t-Bu
CHO
Ph
Ph
t-Bu
O
O
OAr
OAr
PhMgBr
OAr
OAr
2 mol % 1
O
OH
Ar
Br
45% yield
5% ee
B
O
Pd Black observed
P
O
P
Br
Toluene, K3PO4,
0 oC, 80 h
+
Toluene/THF
in 5 minutes
Pd
t-Bu
Pd
t-Bu
Ph
Ph
(1)
O
O
Cl
L
Ar
F
Ph
B
D
Br
E
Scheme 4. Reaction of palladacycle 1 with phenylmagnesium bromide.
Ph
O
OH
2 mol % 1/
Y
B
(0.5 equiv)
Ph
Toluene, K3PO4, 0 oC, 68 h
CHO
O
Ar
Br
+
(
BO)3
Br
G:Y = 4-CF3C6H4: 55% yield, 0% ee
R
R
(BO)3
H:Y = OEt:
58% yield, 0% ee
0.5 mol % Palladacycle 1
+
Ph
HO
HO
OH
toluene, K3PO4, rt, 6 h
(1 equiv)
Br
2 mol % 1/
BO)3
CHO
Ph
Ar
Br
+
(
Toluene, K3PO4, 0 oC, 60 h
R = CH3
R = COCH3
Br
Conversion: without added H2O:
< 3%
2%
85% yield, 0% ee
50%
with added H2O (1 equiv): 73%
Scheme 7. Palladacycle 1-catalyzed addition reaction of optically active phenyl-
boronate with 2-bromobenzaldehyde.
Scheme 5. Palladacycle 1-catalyzed reaction of 2-bromoarenes with phenylboronic
acid/phenylboroxine.
the occurrence of transmetalation (Scheme 2, path c). We exam-
ined the addition reaction of optically active o-tolylboronate E with
2-bromobenzaldehyde and found that the reaction occurred
smoothly. As in the cases with arylboroxines as nucleophiles, no
Pd black was observed. Importantly, a 5% ee was observed for
the reaction product (Scheme 7). To exclude the possibility that
optically active boronate E or borate F acted as chiral Lewis acids
to induce the enantioselectivity, we tested the use of optically ac-
tive boronate G and borate H, analogues of E and F, as additives for
the addition reaction of 2-bromobenzaldehyde with o-tolylborox-
ine (Scheme 7). No enantioselectivity was observed for the addi-
tion reaction with either optically active boronate G or borate H
as the additive, suggesting that the enantioselectivity observed
with E as the nuclepophile was not due to the Lewis acid function
of optically active boronate E or borate F. As no enantioselectivity
was observed for the addition reactions of o-tolylboroxine with 2-
bromobenzaldehyde in the presence of (R,R)-1,2-diphenyl-1,2-eth-
anediol (Scheme 7), our result with the use of optically active o-tol-
ylboronate E, even though the observed ee% was low, suggested
that the occurrence of transmetalation was unlikely for the addi-
tion reaction.
In summary, based on the hypothesis that the Type I palladacy-
cle-catalyzed addition reactions of arylborons with aldehydes
might occur without transmetalation between palladacycles and
arylborons, we demonstrated that the Type I palladacycle-cata-
lyzed addition reaction of arylboroxines/arylboronates with alde-
hydes occurred smoothly under anhydrous conditions. We found
that Type I palladacycles could be long-lived under anhydrous con-
ditions. This new addition reaction mechanism hypothesis allowed
us to use previously unused o-trifluoromethyl-containing arylbo-
roxines as nucleophiles and to develop an unprecedentedly low
catalyst loading protocol for the transition metal-catalyzed addi-
tion reaction of organoboron reagents with aldehydes. This new
addition reaction pathway might be applicable for other transition
metal-catalyzed addition reactions of organoboron reagents with
carbonyl-containing compounds. Our future work will be focused
on elucidating details of this new addition reaction mechanism,
determining its scope and limitation, and investigating its applica-
tions for the development of new reactions including sequential/
tandem reactions.
OH
0.1 mol % Palladacycle 1
CHO
Br
O
O
+
Ph
Br
91% yield
B
Toluene, K3PO4,
50 oC, 18 h
Scheme 6. Palladacycle 1-catalyzed addition reaction of pinacol phenylboronate
with 2-bromobenzaldehyde.
observed in less than 2 min when this reaction was carried out in
the presence of water, no Pd black formation was observed under
anhydrous reaction condition (Scheme 3). This result suggested
that under anhydrous condition, either no transmetalation be-
tween palladacycle 1 and phenylboroxine occurred or the trans-
metalated intermediate
D was stable. We thus tested the
reaction of palladacycle 1 with phenylmagnesium bromide, in
which D was expected to be the intermediate. We found Pd black
was observed in less than 5 min (Scheme 4), suggesting that the
Pd species formed from the transmetalation of palladacycle 1 with
PhMgBr, D, should not be stable. We also carried out palladacycle
1-catalyzed reaction of 2-bromotoluene with phenylboroxine and
phenylboronic acid. We found the cross-coupling reaction of 2-
bromotoluene and 2-bromoacetophenone with phenylboroxine
occurred much slower under anhydrous condition than in the pres-
ence of H2O (Scheme 5). Because the oxidative addition and reduc-
tive elimination steps for these cross-coupling reactions did not
involve the role of base, the low conversions observed for reactions
under anhydrous condition suggesting that transmetalation be-
tween palladacycle 1 and phenylboroxine occurred very slowly un-
der anhydrous condition. These results suggested that palladacycle
1-catalyzed addition reactions of bromobenzaldehydes with phen-
ylboroxine under anhydrous conditions were likely to have oc-
curred without the transmetalation between Type I palladacycle
1 and phenylboroxines.
The nontransmetalation pathway hypothesis suggested that
other arylborons such as arylboronate esters should also be suit-
able nucleophile sources for the addition reaction in the absence
of water.15 We thus tested the use of pinacol phenylboronate for
the addition reaction and found it indeed served as an excellent re-
agent for the addition reaction and the addition reaction product
was obtained in a high yield (Scheme 6).
Acknowledgments
We then tested the use of optically active o-tolylboronate for
the addition reaction. We reasoned that a racemic reaction product
(0% ee) should be observed if the addition reaction occurred with A,
formed via transmetalation process, as the intermediate (Scheme
2, path a). On the other hand, a nonracemic reaction product
(>0% ee) might be formed if the addition reaction occurred without
We gratefully thank the NSF (CHE0719311) for funding. Partial
support from PSC-CUNY Research Award Programs is also
acknowledged. We thank the Frontier Scientific for its generous
gifts of arylboronic acids.

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I palladacycles are known to exist as bridged dimers and to
dissociate into monomeric forms during reactions. Type I palladacycles in this
Letter were drawn in monomeric forms.
10. (a) Liao, Y.-X.; Xing, C.-H.; He, P.; Hu, Q.-S. Org. Lett. 2008, 10, 2509–2512; (b)
He, P.; Lu, Y.; Dong, C.-G.; Hu, Q.-S. Org. Lett. 2007, 9, 343–346; (c) He, P.; Lu, Y.;
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11. (a) Yu, A.; Cheng, B.; Wu, Y.; Li, J.; Wei, K. Tetrahedron Lett. 2008, 49, 5405–
5407; (b) Suzuma, Y.; Hayashi, S.; Yamamoto, T.; Oe, Y.; Ohta, T.; Ito, Y.
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Eastham, G. R.; Tooze, R. P.; Cole-Hamilton, D. J. Chem. Commun. 2001, 779–
780. also see 5b.
12.
A nontransmetalation pathway was proposed in Rh(II)-catalyzed addition
reaction of arylboronic acids with aldehydes: Trindade, A. F.; Gois, P. M. P.;
Veiros, L. F.; Andre, V.; Duarte, M. T.; Afonso, C. A. M.; Caddick, S.; Cloke, F. G. N.
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Ohta, T.; Ito, Y. J. Organomet. Chem. 2009, 694, 1325–1332; (b) Lu, X.; Lin, S. J.
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N. Organometallics 2004, 23, 4317–4324.
13. For examples: (a) Nakai, H.; Ogo, S.; Watanabe, Y. Organometallics 2002, 21,
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14. For Ni(0)-catalyzed addition reactions of arylboroxines with aldehydes: Xing,
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6. For examples with other transition metal catalysts: (a) Tomita, D.; Kanai, M.;
Shibasaki, M. Chem. Asian J. 2006, 1, 161–166; (b) Yamamoto, Y.; Kurihara, K.;
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transition metal-catalyzed addition reactions in the absence of water: (a)
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