Cross-coupling of aryl/alkenyl pivalates with organozinc reagents through nickel-catalyzed C-O bond activation under mild reaction conditions

Article abstract of DOI:10.1002/anie.200803814

Finding the right partner: The catalyst system [NiCl₂(PCy ₃)₂] mediates the title transformation with high efficiency and allows aryl-aryl and vinyl-aryl coupling to proceed. The first example of catalytic cross-coupling u

Full text of DOI:10.1002/anie.200803814

Communications
DOI: 10.1002/anie.200803814
Bond Activation
Cross-Coupling of Aryl/Alkenyl Pivalates with Organozinc Reagents
À
through Nickel-Catalyzed C O Bond Activation under Mild Reaction
Conditions**
Bi-Jie Li, Yi-Zhou Li, Xing-Yu Lu, Jia Liu, Bing-Tao Guan, and Zhang-Jie Shi*
Biaryls have been widely applied in the syntheses of natural
products, polyaromatic molecules, and pharmaceuticals.[1,2]
Transition-metal-catalyzed cross-coupling reactions are
among the most powerful tools used to construct such
structural units. In the last decades, significant effort has
been directed towards cross-coupling reactions, thus allowing
the coupling partners to be extended from aryl iodides,
bromides, and triflates to relatively unreactive chlorides and
tosylates/mesylates.[3] The complexes of first-row transition
metals, such as iron, cobalt, and nickel, also exhibited
promising catalytic reactivities to take the place of late-
transition-metal complexes.[4] Despite these noteworthy
advances, aryl carboxylates (one family of the most easily
available and useful compounds) have not yet been success-
fully employed as coupling partners in cross-coupling reac-
tions.[5] Their simplicity as well as ready availability make
carboxylates particularly attractive for use in synthetic
chemistry. For example, aryl/alkenyl carboxylates can be
easily derived from phenols and carbonyl compounds in a
single step.[6] Also, the application of carboxylates in coupling
reactions could reduce the use of halides, which are environ-
mentally unfriendly. Ironically, the inertness of aryl carboxy-
lates has long been viewed as a vivid testimony of their
functional group tolerance in many reactions. Owing to their
stability, carboxylates such as acetates and pivalates are
generally applied to synthetic chemistry as protecting groups.
After the pioneering studies of Wenkert et al. on nickel-
catalyzed arylation of aryl/vinyl methyl ethers in the late
recent appearance of several impressive examples. It is
apparent that a renaissance in the activation of an unreactive
À
C O bond is on the way. The studies of Dankwardt and co-
workers have greatly broadened the substrate scope to a wide
range of nonactivated aromatic ethers in nickel-catalyzed
coupling with Grignard reagents.[7c] Kakiuchi et al. first
reported chelation-assisted ruthenium-catalyzed coupling of
aromatic ethers with organoboron reagents.[7d] Very recently,
Chatani and co-workers made advances in the nickel-
catalyzed cross-coupling between aromatic methyl ethers
and arylboronic esters.[7e] Our recent studies also showed that
nickel catalyzed the efficient alkylation of aromatic as well as
benzylic methyl ethers with Grignard reagents.[7f,g] As part of
À
a program aimed at the activation of otherwise unreactive C
O bond to enable synthetically useful transformations, we
have disclosed the first successful cross-coupling of aryl
carboxylates with aryl boroxines.[8] However, several issues
remained to be addressed such as high catalyst loading and
elevated temperature. Given their easy preparation and high
functional group tolerance, organozinc reagents have proved
to be quite useful in cross-coupling reactions to construct
biaryl and aryl–vinyl structural scaffolds.[9] To the best of our
knowledge, aryl carboxylates have never been used to couple
with organozinc reagents. Herein, we reported the unprece-
dented cross-coupling of aryl/alkenyl pivalates with arylzinc
À
reagents to construct C C bonds under mild reaction
conditions.
On the basis of our previous studies, 2-naphthyl carboxy-
1970s,[7a,b] the field of C O bond activation remained dormant
lates 1 were chosen as model substrates because of their
À
[8]
À
for nearly three decades. The vast synthetic potential of this
activation-type chemistry has been rarely explored until the
higher reactivity in related C O activation transformations.
Gratifyingly, when 2-naphthyl acetate (1a) was used as a
substrate, the desired coupling product 3a was obtained in the
presence of a catalytic amount of [NiCl2(PCy3)2] in DMA,
albeit in a low yield, accompanied by the hydrolyzed 2-
naphthol as a by-product (Table 1, entry 1). The increased
steric bulk of the carboxylates dramatically improved the
yield of the reaction, presumably by inhibiting the direct
attack of the arylzinc reagents on carbonyl groups (Table 1,
entry 3 versus entries 1 and 2). When 2-naphthyl pivalate (1c)
was applied, the reaction reached full conversion and gave the
isolated product in 84% yield. Other ligands such as PPh3,
P(nBu)3, and P(tBu)3 were tested but gave lower yields
(Table 1, entries 4–7), thus suggesting that the PCy3 ligand
had a balance between both steric and electronic effects.
Furthermore, other nonpolar solvents were not appropriate
for this transformation, meanwhile the polar solvents such as
DMF and NMP were efficient (Table 1, entries 8–12). Nota-
bly, the reaction proceeded at as low as 308C without
[*] Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
Fax: (+86)10-6276-0890
E-mail: zshi@pku.edu.cn
Homepage: http://www.shigroup.cn/
B.-J. Li, Y.-Z. Li, X.-Y. Lu, J. Liu, B.-T. Guan
Beijing National Laboratory of Molecular Sciences (BNLMS), PKU
Green Chemistry Centre and Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of the Ministry of Education,
College of Chemistry, Peking University, Beijing 100871 (China)
[**] Support of this work by a starter grant from the Peking University
and by a grant from the National Sciences of Foundation of China
(Grant Nos. 20542001, 20521202, 20672006) is gratefully acknowl-
edged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803814.
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À
Table 1: Cross-coupling reaction between 1 and 2a under different
Table 2: Nickel-catalyzed C C bond formation with various arylzinc
reaction conditions.[a]
reagents.[a]
Cosolvent T [8C] Yield [%][b]
Entry
ArZnCl
Product (yield [%])[b]
Entry Carboxylate Catalyst
1
2
3
4
R=H, 2a
R=Me, 2b
R=OMe, 2c
R=CH2OMe, 2d
3a (84)
3b (80)
3c (85)
3d (66)
1
2
3
4
5
6
7
8
R=Me, 1a [NiCl2(PCy3)2]
R=iPr, 1b [NiCl2(PCy3)2]
R=tBu, 1c [NiCl2(PCy3)2]
DMA
DMA
DMA
DMA
DMA
70
70
70
70
70
70
70
70
70
70
70
70
50
30
70
70
70
70
70
21
73
100 (84)[c]
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
NiCl2 +2 PCy3
NiCl2 +2 PPh3
NiCl2 +2 PnBu3 DMA
NiCl2 +2 PtBu3 DMA
100 (78)[c]
19
38
47
6
5
6
3e (69)
3 f (81)
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[NiCl2(PCy3)2]
[FeCl3(PCy3)3]
[CoCl2(PCy3)2]
[PdCl2(PCy3)2]
THF
DCE
9
<5
10
11
12
13
14
15[d]
16[e]
17
18
19
toluene
DMF
NMP
DMA
DMA
DMA
DMA
DMA
DMA
DMA
<5
100 (85)[c]
100 (82)[c]
7[c]
3g (<5)
3h (80)
3i (82)
89
71
70
84
0
8
<5
0
9
[a] Reaction conditions: 0.5 mmol of 2-naphthyl carboxylate 1, 1.5 mmol
of PhZnCl 2a, 0.025 mmol of catalyst in 1 mL of THF and 2 mL of
cosolvent. [b] Yield determined by GC analysis. [c] Yield of isolated
product. [d] 1 mol% of catalyst. [e] 2 equivalents of PhZnCl. Cy=cyclo-
hexyl, DCE=1,2-dichloroethane, DMA=dimethylacetamide, DMF=
N,N-dimethylformamide, NMP=N-methylpyrrolidine.
10
3j (84)
11
12
3k (85)
3l (69)
significant loss of catalytic reactivity (Table 1, entries 13 and
14). Lowering the catalyst loading to 1 mol% or the amount
of arylzinc reagent to 2 equivalents led to slightly lower yields
(Table 1, entries 15 and 16). For this transformation other
transition-metal complexes did not show comparable catalytic
reactivity (Table 1, entries 17–19).
[a] Reaction conditions: 0.5 mmol of naphthyl pivalate, 1.5 mmol of
ArZnCl, 0.025 mmol of catalyst in 1 mL of THF and 2 mL of DMA.
[b] Yield of isolated product. [c] Yield determined by GC analysis with the
use of n-dodecane as an internal standard. Piv=pivalate.
With the optimized reaction conditions in hand, the
reactivity of different arylzinc reagents were subsequently
investigated (Table 2). 2-Naphthyl pivalate (1c) was coupled
with a variety of arylzinc reagents in good efficiency. The
electronic nature of the arylzinc reagents did not play a
significant role in the reaction. Arylzinc reagents bearing
para substituents, such as methyl and methoxyl groups, were
suitable for this transformation and the desired products were
obtained in good to excellent yields (Table 2, entries 2–4).
atic before reaching the reductive elimination step.[10]
Unfortunately, functionalized arylzinc reagents such as p-
cyano and p-methoxycarbonyl phenylzinc chlorides, which
can be prepared through halide/Mg exchange and subsequent
Mg/Zn transmetalation,[11] can not successfully undergo the
desired cross-coupling reaction.
Different carboxylates were further explored to examine
the functional group tolerance of the reaction (Table 3).
Several phenol protecting groups including methyl, benzyl,
and MOM were tested and all of them survived (Table 3,
entries 2–4). Notably, the ester functionality remained
untouched in the reaction, thus suggesting an orthogonal
reactivity (for the current system) towards two different types
of ester groups (Table 3, entry 5). Moreover, the coupling
reaction could also be applied to larger conjugated system.
2,4,5-Trimethylphenylzinc chloride (2j) was chosen as the
coupling partner with 6-aryl-2-naphthyl pivalates (1h–k)
because of concern about both the reactivity of the substrate
and solubility of the product (Table 3, entries 6–9). Signifi-
cantly, only 5 mol% of the catalyst and 3 equivalents of
À
À
During this transformation, C O and C F bonds survived,
and they could be further transformed into different func-
tionalities (Table 2, entries 3–5). Interestingly, an ortho sub-
stituent on the phenyl ring of the arylzinc reagent increased
the reaction efficiency and exerted a marked rate acceleration
(Table 2, entries 6, 8–12)—presumably because steric repul-
sion enhanced the reductive elimination step.[10] However,
further increase of the steric bulk led to the termination of the
cross-coupling, as was demonstrated by the reaction of the 2-
mesitylzinc chloride (2g), which gave no desired product
(Table 2, entry 7). This outcome suggests that the transmeta-
lation of extremely bulky arylzinc reagent might be problem-
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Communications
[a]
À
Table 3: Nickel-catalyzed C C bond formation with different 2-naphthyl pivalates.
nickel-catalyzed arylation of aryl
carboxylates proceeds through a
classic cross-coupling mechanism.
First, in situ reduction of NiII by
the organozinc reagent generates
the active Ni0 catalytic species 4,
which was evidenced by the gener-
ation of homocoupling biaryls in the
Entry ArOPiv
1
Product
Yield [%][b]
84
2
3
4
R=Me, 1d
R=Bn, 1e
R=MOM, 1 f
R=Me, 3m
R=Bn, 3n
R=MOM, 3o 79
70
77
presence of ArZnCl and [NiCl2-
[13a]
(PCy3)2].
Subsequently, oxida-
tive addition of aryl carboxylates 1
to Ni0 furnishes aryl nickel species
5, which further undergoes trans-
metalation with the organozinc
reagent to produce the biaryl
nickel intermediate 6.[13b] Finally,
upon reductive elimination, the
desired product 3 is obtained along
with regeneration of the active
catalyst to facilitate the catalytic
5c
82
6[d]
7[d]
8[d]
9[d]
R=H, 1h
R=OMe, 1i
R=F, 1j
R=H, 3q
82
R=OMe, 3r 81
R=Ar, 3s
R=Ar, 3s
81
79
R=Cl, 1k
[a] Reaction conditions: 0.5 mmol of naphthyl pivalate, 1.5 mmol of ArZnCl, 0.025 mmol of catalyst in
1 mL of THFand 2 mL of DMA. [b] Yield of isolated product. [c] Reaction carried out at 508C. [d] Arylzinc
reagent 2j was used. Ar=2,4,5-Me3C6H2, Bn=benzyl, MOM=methoxymethyl.
arylzinc reagent were sufficient for double arylations, thus
highlighting the effectiveness of the catalytic system (Table 3,
entries 8 and 9). Although the concomitant arylation of the
À
À
aryl C Cl bond was not surprising, the activation of the C F
À
bond was somewhat unexpected because the aryl C F bond
survived when it was present in the arylzinc counterpart
(Table 2, entry 5 and see the Supporting Information). On the
À
other hand, activation of the C F bond turned out to be an
additional bonus which represented a rare example for
[12]
normally unactivated aryl C F arylation.
has also set a boundary for tolerance towards aryl halides
used in the current system.
This reaction
cycle.[13c] Similarly, the coupling of alkenyl pivalates with
arylzinc reagents follows the same pathway.
In summary, we have demonstrated the first example of
catalytic cross-coupling using aryl/alkenyl pivalates (as aryl/
alkenyl electrophile surrogates) with arylzinc reagents. The
simple and efficient catalytic system allowed the coupling to
À
Besides naphthyl pivalates, phenyl pivalates were also
suitable substrates as long as an electron-withdrawing sub-
stituent was incorporated on the phenyl ring to activate the
substrates. For example, the coupling of 4-phenylcabonyl-
phenyl pivalate (1l) and 4-methoxylcarbonylphenyl pivalate
(1m) proceeded well [Eqs. (1) and (2)]. The current catalytic
system was by no means limited to aryl–aryl coupling, as it
could also be extended to vinyl–aryl coupling. The coupling of
acyclic as well as cyclic vinyl pivalate (1n and 1o) with
phenylzinc chloride turned out to be highly efficient, and
provided the styryl derivatives in high yields [Eqs. (3) and
(4)].
A proposed mechanistic model for the cross-coupling
reaction is outlined in Scheme 1. It is anticipated that the
Scheme 1. Proposed mechanism for the direct arylation of aryl/alkenyl
pivalates with organozinc reagents.
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further extension of other coupling reactions with aryl
carboxylates as well as deeper mechanistic insight into this
novel reaction.
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Experimental Section
A representative procedure (Table 2): A solution of tBuLi (2.0 mL,
3.0 mmol; 1.5m solution in pentane) was added dropwise to a solution
of aryl bromide (1.5 mmol) in THF (2 mL) in a Schlenk tube at
À788C. The resulting mixture was stirred for 30 min at À788C, and
then for a further 30 min at room temperature. Next, ZnCl2 (230 mg,
1.7 mmol) was added in one portion at 08C to the aryllithium solution
thus obtained. After stirring at room temperature for an additional
15 min, the reaction mixture was charged with [NiCl2(PCy3)2]
(17.0 mg, 0.025 mmol), arylcarboxylate (0.5 mmol), and anhydrous
DMA (2 mL). A balloon containing N2 (1 atm) was connected to the
Schlenk tube before it was immediately heated to 708C, thus ensuring
the pentane evaporated into the balloon. After the reaction was
complete (as evident by TLC), the mixture was quenched with
aqueous HCl (2m, 5 mL) and then EtOAc (80 mL) was added. The
organic phase was washed with water (30 mL ꢀ 3), dried over MgSO4,
concentrated and purified by flash column chromatography on silica
gel.
Received: August 3, 2008
Published online: November 17, 2008
[8] B.-T. Guan, Y. Wang, B.-J. Li, D.-G. Yu, Z.-J. Shi, J. Am. Chem.
Soc. 2008, 130, 14468.
À
Keywords: C O activation · cross-coupling ·
homogeneous catalysis · nickel · organozinc reagents
.
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