A palladium NNC-pincer complex: An efficient catalyst for allylic arylation at parts per billion levels

Article abstract of DOI:10.1039/c4cc09726b

Allylic arylation of allylic acetates by sodium tetraarylborates in the presence of ppb to ppm (molar) loadings of a palladium NNC-pincer complex catalyst in methanol at 50°C gave the corresponding arylated products in excellent yields. Total turnover numbers of up to 500 000 000 and turnover frequencies of up to 11 250 000 h^-1 were achieved.

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Palladium NNC-pincer complex: an efficient
catalyst for allylic arylation at parts per billion
levels
Cite this: DOI: 10.1039/x0xx00000x
Go Hamasaka,a,b Fumie Sakuraia,b and Yasuhiro Uozumi*a,b,c,d
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Allylic arylation of allylic acetates by sodium tetraarylborates 1 ppm molar) of the palladium NNC-pincer complex 110,11
in the presence of ppb to ppm (molar) loadings of a palladium efficiently catalyze the allylic arylation of aromatic and aliphatic
NNC-pincer complex catalyst in methanol at 50 °C gave the allylic acetates by sodium tetraarylborates in methanol.
corresponding arylated products in excellent yields. Total Initially, we examined the allylic arylation of cinnamyl acetate
turnover numbers of up to 500,000,000 and turnover
frequencies of up to 11,250,000 h–1 were achieved.
(
2a) with sodium tetraphenylborate (3a) in the presence of 0.1
mol% of the palladium NNC-pincer complex (Table 1, entry
1). The reaction was completed within 1 hour and gave 1,1'-
[(1 )-prop-1-ene-1,3-diyl]dibenzene (4aa) in 91% isolated yield.
Next, we tried the reaction with 1 mol ppm of complex
Fortunately, we found that the reaction of acetate 2a with
borat e 3a in t he p resenc e of mol pp m of
1
The development of highly active (high turnover number)
transition-metal catalysts is an important topic in organic
syntheses and chemical processing, because it permits the use
of reduced amounts of toxic or expensive transition metals.
Recently, pincer-type transition-metal complexes have received
much attention as catalysts in synthetic chemistry because
such complexes often exhibit high catalytic activities in various
organic transformations.1 In particular, pincer complexes of
palladium have been intensively studied as catalysts.2 For
example, low loadings (molar ppm levels) of palladium pincer
complexes efficiently catalyze the Mizoroki–Heck reaction.3
The allylic substitution reaction, sometimes known as the
Tsuji–Trost reaction, has been recognized as a useful method
in the synthesis of natural compounds and pharmaceuticals.4
While a variety of efficient catalysts for the allylic arylation with
arylboron reagents has been developed,5,6 the reaction often
requires a relatively high temperature and a large catalyst
loading (1–10 mol%). Therefore, the development of a highly
active catalyst for allylic arylation with boron reagents is highly
E
1
.
1
desirable. We recently reported that
a
self-assembled
poly(imidazole–palladium) composite [0.8–40 ppm (molar) Pd]
efficiently catalyzes allylic arylation reactions.7 However, the
development of a highly active simple complex as a catalyst
for allylic arylation still remains
a major challenge. In a
continuation of our studies on pincer-palladium complex
chemistry,8,9 we found that extremely small amounts (1 ppb to
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DOI: 10.1039/C4CC09726B
Table 1 The allylic arylation of cinnamyl acetate (2a) with sodium
tetraphenylborate (3a) in the presence of palladium NNC-pincer complex 1
Scheme 1 Allylic arylation of acetate 2a with borate 3a catalyzed by 1 mol ppb of
complex 1. Reaction conditions: 1
(1.0 × 10–8 mmol), 2a (10 mmol), 3a (20 mmol),
MeOH (10 mL). The yields refer to isolated products.
Entry
Catalyst
Catalyst
concentration
0.1 mol%
1 mol ppm
0.1 mol%
none
1 mol ppm
1 mol ppm
1 mol ppm
1 mol ppm
1 mol ppm
1 mol ppm
1 mol ppm
1 mol ppm
Solvent
Time
(h)
1
Yielda
(%)
91
1b
2c
1
1
1
1
1
1
1
1
1
5
6
7
MeOH
MeOH
MeOH
MeOH
THF
H2O
DMF
TCE
toluene
MeOH
MeOH
MeOH
The catalytic activities of the phenanthroline palladium(II)
complexes are affected by the steric hinderance around
24
15
24
24
24
24
24
24
24
24
24
87
3d
57
4c
nre
76
palladium.
Thus, the activities of complexes
6 (TON =
5c
450,000) and
7
(TON = 560,000) were superior to that of
6c
69
7c
nre
1
complex
5 (TON = 160,000) (entries 10–12, Table 1). However,
8c
a noteworthy advance in catalytic activity was achieved with
the NNC-pincer backbone. Thus, the turnover number of
palladium increased by triple digits with the NNC-pincer
9c
nr
16
45
56
10f
11f
12f
complex
1 to realize 270,000,000 of TON within 24 hours
a Isolated yield; b Reaction conditions: 1 (3.0 × 10–4 mmol), 2a (0.3 mmol),
3a (0.6 mmol), MeOH (1 mL), 50 °C, 1 h; c Reaction conditions: 1 (1.0 ×
10–5 mmol), 2a (10 mmol), 3a (20 mmol), solvent (10 mL), 50 °C, 24 h; d
Reaction conditions: 1 (3.0 × 10–4 mmol), 2a (0.3 mmol), phenylborpnic acid
(0.6 mmol), KF (0.75 mmol), MeOH (1 mL), reflux, 15 h; e nr = no reaction;
f Reaction conditions: 5, 6 or 7 (1.0 × 10–5 mmol), 2a (10 mmol), 3a (20
mmol), MeOH (10 mL), 50 °C, 24 h.
(Scheme 1).
We then examined the allylic arylation of various allylic
acetates with sodium tetraarylborates in the presence of
complex
tetraphenylborate
electron-donating or electron-withdrawing substituents 2b
proceeded smoothly to give the corresponding arylated
1
(1 mol ppm) (Scheme 2). The reaction of sodium
3a with cinnamyl acetates bearing
2i
(
)
–
1
at 50 °C for 24 hours in methanol gave the desired arylated
products 4ba
(2-naphthyl)prop-2-en-1-yl acetate
phenylprop-1-en-1-yl]naphthalene (4ja) in 99% yield. Sterically
hindered 2-methyl- and 2-methoxycinnamyl acetates (2k and
2l, respectively) gave 1-methoxy-2-[(1
yl]benzene (4ka) and 1-methyl-3-[(1
yl]benzene (4la) in 93 and 83% yield, respectively. Allylic
arylation of cinnamyl acetate (2a) with sodium tetraarylborates
–
4ia in yields of 77–95%. The reaction of (2
E
E
)-3-
)-3-
product 4aa in 87% isolated yield (entry 2). In this reaction,
the turnover number (TON) was 870,000 and the turnover
frequency (TOF) was 36,250 h–1. When phenylboronic acid was
used as a coupling nucleophile, the reaction gave 57% yield of
(
2j) gave 2-[(1
E
E
)-3-phenylprop-1-en-1-
)-3-phenylprop-1-en-1-
4aa even with 0.1 mol% of the catalyst
3). No reaction occurred in the absence of complex
1
after 15 hours (entry
(entry
1
4). We then investigated the effect of the solvent on the
reaction. The reaction proceeded smoothly in tetrahydrofuran
(THF) or water to give product 4aa in similar yields (entries 5
3b
the corresponding arylated products 4ab
respectively, in yields of 59–84%. Complex
–
3d also proceeded in the presence of complex
4ac
also catalyzed the
1
to give
,
,
and 4ad,
and 6). In contrast, no reaction occurred in
dimethylformamide (DMF), 1,1,2,2-tetrachloroethane (TCE), or
toluene (entries 7–9). The palladium complexes and 7,
N,N-
1
reaction of less-reactive aliphatic 2-alkenyl acetates with
sodium tetraphenylborate (3a). The reactions of hex-2-enyl
acetate (2m), 3-methyl-2-buten-1-yl acetate (2n), cyclohex-2-
en-1-yl acetate (2o), neryl acetate (2p), or geranyl acetate (2q
in the presence of complex (1 mol ppm) gave the
corresponding arylated products 4ma 4qa in yields of 54–78%.
The secondary allylic acetates 2r and 2s also underwent the
reaction to give the phenylated products 4ra and 4sa in 74
and 58% yield, respectively. However, the reactions of (2
(2-thienyl)prop-2-en-1-yl acetate 2r and (2 )-3-pyridin-3-
ylprop-2-en-1-yl acetate (2s) were sluggish, suggesting that
strongly coordinating substrates inhibit allylic arylations
catalyzed by complex 1.
5
,
6
which lack the NNC-pincer structure exhibited lower catalytic
activity to give the desired product 4aa in 16, 45, and 56%
yield, respectively (entries 10–12).
)
1
Next, we attempted to perform an allylic arylation with an
–
even lower loading of complex
acetate 2a with sodium tetraphenylborate
presence of 1 mol ppb of complex in methanol at 50 °C for
1
. The reaction of cinnamyl
(
)
(
3a) in the
1
E)-3-
24 h gave the desired product 4aa in 27% yield (Scheme 1).
When the reaction time was prolonged to 96 h, the yield of
4aa increased to 50%. In this case, the turnover number
increased to 500,000,000 and the turnover frequency was
11,250,000 h–1.
(
)
E
2 | J. Name., 2012, 00, 1-3
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Scheme 3 Mechanistic investigations.
In summary, we found that the allylic arylation of various allylic
substrates with sodium tetraarylborates in the presence of
palladium NNC-pincer complex
1 at loadings of the order of
ppb to ppm (molar) proceeded smoothly under mild
conditions to give the desired arylated products in good-to-
excellent yields. Turnover numbers of up to 500,000,000 and
turnover frequencies of up to 11,250,000 h–1 were achieved.
This work was supported by the JST-CREST (Creation of
Innovative Function of Intelligent Materials on the Basis of
Elemental Strategy).
Scheme
tetraarylborates in the presence of palladium NNC-pincer complex 1. Reaction
conditions: (10 mmol), (20 mmol), MeOH (10 mL), 50 °C.
(1.0 × 10–5 mmol),
The yields refer to isolated products.
2 Allylic arylations of various allylic acetates with sodium
1
2
3
Notes and references
a
Institute for Molecular Science, Okazaki 444-8787, Japan. FAX: +81
We then performed several experiments in an attempt to
identify the reaction pathway for the allylic arylation reaction
564 59 5574; E-mail: uo@ims.ac.jp
b
The Graduate University for Advanced Studies, Okazaki 444-8787,
catalyzed by complex
branched acetate with borate 3a in the presence of complex
gave alkene 4aa in 75% yield (Scheme 3a). This result
1 (Scheme 3). The reaction of the
Japan
c
Green Nanocatalysis Research Team, RIKEN Center for Sustainable
8
Resource Science, Wako 351-0198, Japan
1
d JST-CREST, Okazaki 444-8787, Japan
suggested that a
catalytic cycle. Treatment of complex
π
-allyl Pd intermediate is generated in the
with one equivalent of
borate 3a gave the phenylated complex in 96% yield (NMR;
Scheme 3b).12 The reaction of complex
with acetate 2a gave
†
Electronic Supplementary Information (ESI) available: Experimental
1
details and spectroscopic data for all compounds. See
DOI: 10.1039/c000000x/
9
9
alkene 4aa in 56% yield (NMR; Scheme 3c). We also examined
the reaction of acetate 2a with borate 3a in the presence of
1
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9. The desired arylated product 4aa
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allylic arylation.
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