The first general and selective palladium(II)-catalyzed alkoxycarbonylation of arylboronates: Interplay among benzoquinone-ligated palladium(0) complex, organoboron, and alcohol solvent

Article abstract of DOI:10.1002/adsc.200900836

Methoxycarbonylation of aryl- and alken-ylboron compounds was performed using the palladium (II) acetate/triphenylphosphine [PdACHTUNGTRENNUNG(OAc) ₂/PPh₃] catalyst with p-benzoquinone as a stoichiometric oxidant in methanol at ambient temperature to obtain the corresponding methyl esters in good yields. A wide variety of functional groups including various carbonyl functionalities, nitrile, nitro, sulfone, and unprotected pyrrole rings were tolerated in the methoxycarbonlation, while the use of higher alcohols except for tert-butanol afforded various pchlorobenzoates in moderate to high yields. The catalytic alkoxycarbonylation proceeded without any acid or base additive, and an oxidative transmetalation step is proposed to explain the exceptional efficacy of this protocol. DFT and MP2 calculations support the proposed mechanism.

Full text of DOI:10.1002/adsc.200900836

FULL PAPERS
DOI: 10.1002/adsc.200900836
The First General and Selective Palladium(II)-Catalyzed
Alkoxycarbonylation of Arylboronates: Interplay among
Benzoquinone-Ligated Palladium(0) Complex, Organoboron,
and Alcohol Solvent
Yoshihiko Yamamotoa,b,*
a
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Current address: Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa-ku,
b
Nagoya 464-8603, Japan
Fax : (+81)-52-789-3209; e-mail: yamamoto@apchem.nagoya-u.ac.jp
Received: December 1, 2009; Revised: December 30, 2009; Published online: February 10, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900836.
Abstract: Methoxycarbonylation of aryl- and alken- chloro
ylboron compounds was performed using the palla- alytic alkoxycarbonylation proceeded without any
dium(II) acetate/triphenylphosphine [Pd(OAc)2/ acid or base additive, and an oxidative transmetala-
ACHTUNGTRENUNbNG enzoates in moderate to high yields. The cat-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
PPh3] catalyst with p-benzoquinone as a stoichiomet- tion step is proposed to explain the exceptional effi-
ric oxidant in methanol at ambient temperature to cacy of this protocol. DFT and MP2 calculations sup-
obtain the corresponding methyl esters in good port the proposed mechanism.
yields. A wide variety of functional groups including
various carbonyl functionalities, nitrile, nitro, sulfone,
and unprotected pyrrole rings were tolerated in the Keywords: carbonylation; carboxylic esters; DFT
methoxycarbonlation, while the use of higher alco- calculations; organoboron compounds; palladium
hols except for tert-butanol afforded various p- catalysts
Introduction
bonylation at ambient temperature.[6a] In this report,
they used PdCl2 as a precatalyst with a reoxidant, p-
The palladium(0)-catalyzed alkoxycarbonylation of benzoquinone (pbq), and additives (NaOAc and
aryl, vinyl, and benzyl halides has become a powerful LiCl) in methanol under a CO atmosphere to obtain
synthetic route to useful unsaturated esters since the the desired esters in 66%–95% yields. Phenylboronic
first report by Heck et al.[1] Great progress has been acid was also subjected to this methoxycarbonylation.
made in recent years to achieve high catalytic effi- However, the yield of methyl benzoate was much
ciency and wide substrate scope.[2] In striking contrast, lower (41%). Then, Uemura and co-workers carried
the relevant palladium(II)-catalyzed alkoxycarbonyla- out the methoxycarbonylation of phenylboronic acid
tion of organoboron compounds has remained far less using PdACHTNURTGNE(UNG PPh3)4 and NaOAc at ambient temperature
explored, although other palladium-catalyzed trans- in methanol under a CO atmosphere to obtain methyl
formations of organoboron species such as Suzuki– benzoate in a slightly improved yield of 58%.[6d] They
Miyaura and relevant couplings[3] or other reactions[4] also obtained benzophenone and biphenyl as by-prod-
have been extensively developed.[5] There exist only ucts in 3% yield each. The use of a stoichiometric
few reports on the methoxycarbonylation of vinyl- amount of PdACTHNURGTNEUNG(OAc)2 led to a similar yield of 64%.
and arylboron reagents.[6,7] Miyaura and Suzuki re- An intramolecular alkoxycarbonylation was reported
ported for the first time that alkenylborons derived in 2004 by Zhou and co-workers.[8] They used stoi-
from the hydroboration of alkynes with catecholbor- chiometric PdACTHUNRGTNEUNG(OAc)2 with dimethyl sulfoxide
ane could be transformed into the corresponding (DMSO) as a cosolvent to achieve the carbonylation
methyl acrylates via palladium-catalyzed methoxycar- of 10-hydroxy-10,9-boroxarophenanthrenes in high
478
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The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
yields. They described that a catalytic protocol also room temperature in methanol to give biphenyls.[10,11]
worked well, although the detailed experimental pro- The use of EtOH, THF, or CHCl3 as solvents resulted
cedure was not presented. Surprisingly, no reoxidant in inefficient couplings. These results suggest that the
necessary to restore the palladium(II) catalytic spe- MeOH solvent plays an important role in the catalytic
cies was employed for their catalyst system. There- homocoupling, even though MeOH is not incorporat-
fore, a general and efficient catalytic protocol for the ed into the final product as in the methoxycarbonyla-
alkoxycarbonylation of arylboron compounds has re- tion described above.
mained undeveloped.
In the palladium(II)/palladium(0)-pbq/p-hydroqui-
This author and co-workers previously developed a none (hq) redox couple processes,[12] MeOH might
novel four-component coupling process, where bora- play the role of a proton donor to pbq to induce elec-
ACHTUNGTRENNUNGphthalides, obtained from the Ru-catalyzed [2+2+ tron transfer from a palladium(0) fragment. However,
2]cycloaddition of an alkynylboronate, a propargyl al- Bꢁckvall and co-workers demonstrated that the redox
cohol, and a third alkyne, were converted into phtha- reaction of a pbq-ligated palladium(0) complex re-
lides via a similar carbonylation (Scheme 1).[9] We in- quires stronger proton donors such as acetic acid, tri-
dependently optimized the catalytic carbonylation fluoroacetic acid, or triflic acid.[12a] In fact, the 1,4-di-
procedure by using a catalyst derived from PdACTHNUTRGNEUNG(OAc)2 methoxylation of 1,3-dienes using the palladium(II)/
with PPh3 and pbq in MeOH under a CO atmosphere. pbq catalyst system failed in the absence of such
By applying this protocol to the [2+2+2]cycloadduct acids, but the use of a catalytic amount of MeSO3H in
of a malonate-derived 1,6-heptadiyne and an ethynyl- this catalytic system effected the desired 1,4-dialkoxy-
boronate, the methoxycarbonylation product was also lation.[12b] Therefore, it is hard to believe that a far
successfully obtained in 77% yield (Scheme 2). This weaker acid, MeOH, by itself induces reoxidation of
yield is higher than those previously reported for cat- palladium(0) species with pbq in our methoxycarbo-
alytic methoxycarbonylations of the parent phenylbor- nylation and homocoupling reactions.
onic acid (vide supra).[6]
In this article, the mechanism of these catalytic
Meanwhile, we found that in the absence of CO, transformations of arylboron compounds using the
the homocoupling of arylboronic acids took place palladium(II)/palladium(0)-pbq/hq redox couple
with Pd(OAc)2/O2 or (NHC)Pd(OAc)2/pbq (NHC= system was investigated in detail, and on the basis of
A
ACHTUNGTRENNUNG
N-heterocyclic carbene ligands) catalyst systems at the obtained results, a new general and selective al-
koxycarbonylation protocol with a wider scope was
developed successfully.
Results and Discussion
The aim of this study is first to obtain insights into
the role of an alcohol solvent and second to expand
the scope of the palladium-catalyzed alkoxycarbony-
lation. At the outset, the previously established car-
bonylation protocol was re-examined using p-chloro-
phenylboronic acid (1a) as a typical substrate, because
the previous study had shown that 1a underwent com-
Scheme 1. Synthesis of phthalides via [2+2+2]cycloaddi-
tion and carbonylation.
petitive homocoupling only modestly under aerobic
conditions.[10a] However, the reaction using Pd
ACHTUNGTRENNUNG(OAc)2
with the PPh3 ligand and pbq gave 4,4’-dichlorobi-
phenyl as a major product rather than the desired
benzoate 4a (vide infra). Thus, this catalyst system
favors homocoupling rather than methoxycarbonyla-
tion in contrast to the Miyaura–Suzuki or Uemura
systems. The obtained result is also in sharp contrast
to the successful methoxycarbonylation of the bicyclic
arylboronic acid 2,2-dimethylpropane-1,3-diol ester
(Scheme 2). It is, hence, reasonable to consider that
the difference in the substituents on the boron center
has a significant impact on the product selectivity. In
the next section, the influence of the boron substitu-
1
ents was first examined using H NMR analysis.
Scheme 2. Methoxycarbonylation of the [2+2+2]cycload-
ducts.
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Yoshihiko Yamamoto
FULL PAPERS
Stoichiometric Reactions of pbq-Ligated Palladium(0) esterification of the boronic acid. After the mixture
Complex with p-Tolylboronic Acid and Esters
was stirred for a further 50 min, most of 1b was con-
sumed (trace c), and within ca. 3 h, the reaction was
To obtain insight into the palladium-catalyzed trans- completed (trace d), resulting in the formation of 6b
formation of arylboron compounds, the stoichiometric in 83% and 75% yields based on 7 or 1b, respectively.
reaction of a pbq-ligated palladium(0) complex, On the basis of these observations, it can be assumed
PdACHTUNGTRENNUNG(pbq)ACHTUNGTRENNUNG
(cod) (7, cod=1,5-cyclooctadiene),[13] with p- that MeOH acts as a proton donor in this particular
1
tolylboronic acid (1b) was monitored with H NMR system, even though this assumption is in disagree-
spectroscopy (Figure 1). Initially, 7 (0.1 mmol) and 1b ment with the previous reports of Bꢁckvall and co-
(0.22 mmol, 2.2 equiv.) were stirred in CDCl3 (5 mL) workers describing that the redox reaction of pbq-li-
for 30 min at room temperature. At this stage, only a gated Pd(0) complexes requires a stronger protic
small amount of homocoupling product 6b was ob- acid.[12] In accord with their observations, the redox
served together with a small amount of hydroquinone reaction of pbq complex 7 was reasonably slow in
(hq) (Figure 1, trace a). This indicates that the CD3OD as shown by the formation of a small amount
proton-donor ability of 1b hardly induces the oxida- of p-hydroquinone (see Figure S2 in the Supporting
tion of the palladium(0) fragment with pbq at least Information).
within 30 min. Nevertheless, the addition of CD3OD
Then, in place of 1b, its ester derivatives 2b and 3b
(0.3 mL, ca. 74 equiv.) brought about a dramatic ac- were employed for the stoichiometric reactions with
celeration effect. The marked increase in the aromatic 7, and the reaction progress was again monitored with
peak of 6b was observed just after the addition of 1H NMR spectroscopy. In the absence of a proton
CD3OD together with the high-field shifts of the aro- source, no product was formed from 2,2-dimethylpro-
matic absorptions of both 1b and 6b (trace b). In ad- pane-1,3-diol ester 2b at least within 30 min in CDCl3
dition, two sets of aromatic signals were observed for (Figure S3, trace a in the Supporting Information).
1b in the presence of CD3OD (also see Figure S1 in Even just after addition of CD3OD, no trace of the
Supporting Information). This is probably due to the coupling product was detected, but the homocoupling
coordination of CD3OD to the boron center or partial slowly proceeded over the course of 5 h to finally fur-
1
Figure 1. Stoichiometric reaction of 7 with 1b in CDCl3 in the presence of CD3OD. H NMR spectra were recorded (a) after
stirring for 30 min without CD3OD, (b) just after the addition of CD3OD, (c) and (d) after stirring for further 50 min and 2 h
50 min, respectively.
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The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
Table 2. Methoxycarbonylation of p-chlorophenylboron re-
agents.
nish 6b in 53% and 49% yields based on 7 or 2b, re-
spectively. These yields were lower than those ob-
tained from 1b, and p-tolylbenzoquinone 8 was also
isolated in 10% yield. This side product 8 was pre-
sumably formed by the reductive elimination process
on a pbq-ligated (p-tolyl)Pd(II) intermediate (vide
infra). Similar results were obtained from pinacol
ester 3b, but in this case, the reaction was not com-
pleted within 20 h (see Figure S4 in the Supporting
Information). Consequently, the yield of 6b was lower
(41% and 38% based on 7 or 3b, respectively), and
quinone 8 was also isolated in 11% yield.
Table 1 summarizes the results of the three stoi-
chiometric reactions. Only 1b underwent homocou-
pling in the absence of CD3OD albeit with a very
Table 1. Summary of stoichiometric reactions of [Pd
(cod)] (7) with p-tolylboron reagents.
ACHTUNGTRNE(NUNG pbq)-
ACHTUNGTRENNUNG
Entry
Substrate, Catalyst[a]
Time [h]
,
Product (Yield [%][b])
Substrate
Time
Product (Yield [%][a])
1
2
3
4
5
6
1a, A, 1
2a, A, 5
3a, A, 7
3a, A, 12[c]
3a, B, 7
4a (11), 5a (4), 6a (55)
4a (56), 5a (3), 6a (27)
4a (84), 6a (trace)
4a (10), 5a (3)
4a (79)
1b
2h 50min 6b (83)
2b
5h
6b (53), 8 (10)
p-ClC6H4BF3K, A, 12
4a (10)
3b
20h
6b (41), 8 (11)
[a]
A: 5 mol% PdACHTNUGTRNEN(UG OAc)2/10 mol% PPh3; B: 2.5 mol% 9/
10 mol% PPh3.
Isolated yield.
pbq was omitted.
[a]
[b]
[c]
Isolated yield based on 7.
slow rate. In all cases, the addition of CD3OD facili-
tated the reaction and the reaction rates decreased in consumption was observed as expected. As a result,
the order of 1b>2b>3b, indicating that the steric the reaction was completed in 5 h and the desired me-
congestion around the boron center slows the cou- thoxycarbonylation product 4a was obtained in 56%
pling reaction. In the cases of boronate esters 2b and yield (entry 2). The yield of homocoupling product 6a
3b, (p-tolyl)benzoquinone 8 was isolated as a side was decreased to 27%. The selectivity was further im-
product in ca. 10% yields.
proved by employing a more sterically demanding pi-
nacol ester 3a. The reaction was completed in 7 h to
furnish 4a in 84% yield, and the undesired side reac-
tions were suppressed effectively (entry 3). The im-
portance of the reoxidant in the restoration of the
Optimization of the Methoxycarbonylation
Taking the above results into consideration, the me- palladium(II) catalyst was exemplified by the reaction
thoxycarbonylation of arylboron reagents was revisit- carried out in the absence of pbq (entry 4). In this
ed (Table 2). The reaction was carried out using case, 4a was obtained in only 10% yield together with
5 mol% Pd
ACHTUNGTRENNUNG
pbq in MeOH at room temperature under a CO at-
mosphere for 1 h. The desired p-chlorobenzoate 4a stoichiometric amounts of NaOAc were used, and this
was formed only in 11% yield. Instead, biphenyl 6a is probably because acetate anion was believed to
was obtained as a major product in 55% yield togeth- attack the boron center to facilitate transmetalation.[6]
er with a small amount (4%) of benzophenone 5a Thus, the influence from the acetate ligand on the re-
(entry 1). It can be reasonably assumed that the use action efficiency was examined. To this end, the previ-
of bulky 2,2-dimethylpropane-1,3-diol or pinacol ously synthesized dinuclear palladium(0) complex,
esters sterically encumbers the homocoupling reac- Pd2ACHTNUGTRENNUG(pbq)2ACHUTNGTREN(NUGN nbe)2 (9, nbe=norbornene), was used as a
tion, resulting in the increase in selectivity for the me- precatalyst, because 9 was found to catalyze a linear
thoxycarbonylation. When 2,2-dimethylpropane-1,3- cotrimerization which failed when using a mononu-
diol ester 2a was used, the slower rate of the substrate clear complex 7.[13b] In the presence of 2.5 mol% 9
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Yoshihiko Yamamoto
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(5 mol% Pd) and 10 mol% PPh3, 3a underwent me-
thoxycarbonylation with similar efficiency to furnish
4a in 79% yield (entry 5). Consequently, it is safely
concluded that the acetate ligand has no impact on
the present methoxycarbonylation. For further inves-
tigations, commercially available Pd
as a precatalyst.
ACHTUNGRTENUN(NG OAc)2 was used
In striking contrast to the boronic acid and esters,
the corresponding potassium trifluoroborate was
found to be an ineffective substrate under the optimal
conditions, although aryltrifluoroborates were report-
ed to undergo efficient Suzuki–Miyaura coupling
without any base additive.[14] Only a small amount of
4a (10%) was obtained and most of the starting mate-
Figure 2. Unreactive boronates.
rial remained intact even after a longer reaction time be used to give 4e in a lower yield of 50%, although
of 12 h (entry 6). This result strongly implies that the the reaction time was shortened to 5 h. The reactions
vacant orbital on the boron center plays a critical role of electron-rich boronates 3k–q were relatively more
for the palladium(II)/palladium(0)-pbq/hq redox sluggish than those of the electron-deficient sub-
couple.
strates, requiring longer reaction times (entries 9–11,
As initially assumed, steric crowding around the 14, and 15) and/or increased catalyst loadings (en-
boron center is responsible for suppression of the un- tries 9, 11, 12, 14, and 15). The isolated yields of the
desired homocoupling and, hence, the increase in se- obtained esters 4k–q were also high (>80%). It is
lectivity of methoxycarbonylation. If this is the case, noteworthy that transesterification was not observed
the sterically demanding ortho,ortho-disubstituted for ethoxycarbonyl-substituted boronate 3f, and the
aryl ring should also favor methoxycarbonylation more labile phenyl acetate moiety of 3k was pre-
rather than homocoupling. In fact, 2,6-dimethoxyphe- served under the mild conditions. Indole- and carb-
nylboronic acid (10) was converted into methyl ester
ACHTUNGTRENaNUNG zole-derived boronates 3p and 3q can be directly
11 in 58% yield under the same conditions even used without nitrogen protection in this protocol (en-
though it has no bulky substituent on the boron tries 14 and 15). In striking contrast to the above phe-
center (Scheme 3). A protodeboration product was nylboronates, N-heteroaromatic and ferrocene-de-
detected as a sole by-product.
rived boronates 3r and 3s failed to undergo methoxy-
carbonylation under the optimized conditions
(Figure 2). Benzylic boronate 3t was also found to be
an incompetent substrate for the present methoxycar-
bonylation.
On the other hand, a benzofuran derivative reacted
faster: 3u was consumed within 1.5 h to give ester 4u
in 69% yield together with homocoupling side-prod-
uct 6u in 23% yield (Scheme 4). A similar trend was
observed for p-chlorostyrylboronate 3v, yielding
methyl ester 4v as a major product (71%) together
with homodimer 6v as a minor product (10%).
Double methoxycarbonylation was also attempted
using diborylalkenes 3w and 3x (Scheme 5). Such di-
boron substrates are commercially available or readily
prepared via Pt-catalyzed diboration of alkynes,[15]
and the produced maleates are found in biologically
Scheme 3. Methoxycarbonylation of 2,6-dimethoxyphenyl-
boronic acid (10).
Generality of Methoxycarbonylation of Pinacol
Boronates with the Pd
System
ACHTUNGTREN(NGNU OAc)2/PPh3/pbq Catalyst
The scope of the optimized protocol was examined as active compounds from natural sources.[16] The dibor-
summarized in Table 3. p-Bromo-substituted boronate ylalkenes 3w and 3x were treated with 16 mol%
3c was converted into 4c in a good yield without loss (8 mol%/boron) Pd
(OAc)2, 32 mol% (2 equiv./palladi-
À
of the reactive C Br bond (entry 1). Various sub- um) PPh3, and 2 equiv. (1 equiv./boron) of pbq at
strates 3d–j bearing electron-withdrawing functional room temperature in MeOH under a CO atmosphere.
groups such as an aldehyde, ketone, ester, amide, ni- Although 24 h stirring were required to bring the re-
trile, nitro, and sulfone underwent smooth methoxy- action to completion due to the severe steric conges-
carbonylation to furnish corresponding esters 4d–j in tion, the desired dimethyl maleates 4w and 4x were
yields higher than 70% (entries 2–8). In place of o- successfully obtained in 93% and 90% yields, respec-
acetylphenylboronate 3e, the parent boronic acid can tively.
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The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
Table 3. Methoxycarbonylation of arylboronates 3c–q.[a]
Entry
1
Substrate
Time [h]
6
Product (Yield [%][b])
3c
4c (77)
2
3
3d
5
4d (87)
4e (70)
3e
12
4
5
3f
3g
6
8
4f (90)
4g (84)
6
7
8
9
3h
3i
5
4h (90)
4i (90)
4j (84)
4k (82)
6
3j
4
3k
20
10
3l
18
4l (89)
11
12
3m
3n
14
4
4m (82)
4n (86)
13
14
3o
3p
8
4o (94)
4p (81)
18
15
3q
18
4q (82)
[a]
PdACHTUNGTRENNUNG(OAc)2/2PPh3 (5 mol% or 8 mol% for entries 9, 11, 12, 14, and 15), pbq (1 equiv.), CO (1 atm), 3 (0.5 mmol), MeOH
(10 mL), room temperature.
Isolated yields.
[b]
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Yoshihiko Yamamoto
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Scheme 5. Double methoxycarbonylation of diborylalkenes
3w and 3x.
Impact of Alcohol Solvents
The reaction of 3a was carried out under the standard
reaction conditions except for using EtOH as a sol-
vent, affording the corresponding ethyl ester 13a in a
similar good yield of 86%. In contrast, the reactions
with higher alcohols with more than three carbons
were found to be sluggish. Although 3a was not com-
pletely consumed even after stirring for 24 h, 1-propa-
nol and 1-butanol furnished corresponding esters 13b
and 13d in 58% and 48% yields, respectively (Table 4,
Scheme 4. Methoxycarbonylation of benzofuran and p-chlo-
ACHTUNGTRENNUNG
Entry
Alcohol, Time [h]
ethanol, 7
Product
Yield [%][a]
1
2
3
4
13a
13b
13c
13d
86
58
66
48
1-propanol, 24
2-propanol, 24
1-butanol, 24
5
2-methylpropanol, 24
13e
68
6
7
8
tert-butanol
13f
13g
13h
6
1-pentanol, 24
3-methylbutanol, 24
51
51
9
10
allyl alcohol, 0.25
allyl alcohol, 0.5[b]
13i
13i
76
92
11
12
3-buten-1-ol, 24
13j
13j
53
66
3-buten-1-ol, 24[b]
[a]
Isolated yields. Ar=p-ClC6H4.
2 equiv. of pbq were used.
[b]
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The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
entries 2 and 4). Sterically more demanding branched
alcohols are expected to be much less reactive. More-
over, secondary alcohols are more susceptible to oxi-
dation with transition metals. In other words, they are
superior reductants to primary alcohols. However, 2-
propanol was employed as a solvent under the same
conditions, and the desired ester 13c was obtained in
66% yield, which is higher than that from linear 1-
propanol (entry 3 vs. 2). Moreover, the yield of isobu-
tyl ester 13e from 2-methylpropanol (68%) was much
better than that of n-butyl ester 13d (entry 5 vs. 4).
These observations suggest that the steric factor has a
less significant impact on the reaction efficiency.
However, tert-butyl alcohol almost completely sup-
pressed the alkoxycarbonylation (entry 6). 1-Pentanol
and 2-methylbutanol gave the corresponding esters
13g and 13h in the same yield of 51% (entries 7 and
Scheme 6. Methoxy- and allyloxycarbonylation of o-allyloxy-
phenylboronic acid (13).
8).
Then, the influence of unsaturation in the alcohol
solvents was examined. Surprisingly, the reaction car-
ried out in allyl alcohol proceeded much faster than subjected to methoxycarbonylation under the optimal
those in any other alcohols employed in this study: conditions for 2 h to obtain methyl ester 16 in a
after the reaction mixture had been stirred for 15 min, modest yield of 52% (Scheme 6). In addition, two
the corresponding allyl ester 13i was obtained in 76% types of homocoupling products were obtained as an
yield (entry 9). In this case, a small amount of 3a re- inseparable mixture. Among them, major isomer 14
mained, as indicated by 1H NMR analysis of the was found to be the known biphenyl analogue, while
crude reaction mixture, although pbq was completely the second isomer was tentatively assigned as 15 by
consumed. In addition, hygroscopic oligomeric mate- comparison of its spectral features with those report-
rials were formed concomitantly. These facts suggest ed for the parent 3-benzyl-2,3-dihydrobenzofuran
that allyl alcohol was oxidized to acrolein, to which is (Figure S5 in the Supporting Information).[17] Al-
ascribed the formation of oligomeric side products as though ortho substituents generally suppress homo-
well as overconsumption of pbq. In fact, an increased coupling of arylboron reagents due to their steric in-
loading of pbq (2 equiv.) led to a significant improve- fluence, an internal coordinating group probably sta-
ment of the yield to 92% (entry 10). The rate en- bilized an arylpalladium(II) intermediate such as 17,
hancement was not observed for 3-buten-1-ol, and the and thereby the CO insertion pathway was blocked.
yield of 13j (53%) is similar to that of 13d from the In fact, it is reasonable to consider that dihydrobenzo-
parent 1-butanol (entry 11 vs. 4). An increased load- furan 15 was formed via the insertion of the pendant
À
ing of pbq, however, improved the yield up to 66% alkene into the Pd C bond of 17 and subsequent cou-
(entry 12). In striking contrast to the alkenyl alcohols, pling of the resulted 18 with 17.
benzyl alcohol and propargyl alcohol were found to
In sharp contrast to the reaction in methanol, the
be totally ineffective. In particular, the latter gave in- reaction carried out in allyl alcohol was completed
tractable materials probably due to extensive oligo- within 30 min and desired allyl benzoate 19 was exclu-
merization. It is also assumed that the acidic terminal sively formed in an excellent yield of 95%
À
C H bond might be also detrimental for palladium (Scheme 6). This result can be rationalized providing
catalysis.
that the above proposed bidentate coordination of
The origin of the superior performance of allyl al- allyl alcohol is considered: the stronger chelate coor-
cohol can be ascribed to the coordination ability of its dination of allyl alcohol expels the internally coordi-
olefin moiety: alkenyl alcohols might behave as bi- nated alkene in 17, and as a result, both direct homo-
dentate ligands for the palladium catalyst, and there- coupling and cyclization/homocoupling were sup-
fore, their reactivity varies depending on the distance pressed effectively.
between their olefin and hydroxy moieties. In other
Similar results were obtained from o-alkynylphenyl-
words, allyl alcohol is assumed to make a stronger boronic acid 20 (Scheme 7). When the reaction was
chelate than that of its homologue. This assumption is carried out in methanol, homocoupling product 21
also supported by the difference in chemoselectivity and methyl benzoate 22 were obtained in 31% and
for reactions of ortho-substituted phenylboronic acids 51% yields, respectively. On the other hand, the use
possessing an internal coordinating group such as an of allyl alcohol again resulted in exclusive allyloxycar-
olefin or alkyne. 2-Allyloxyphenylboronic acid 13 was bonylation, leading to 23 in 98% yield.
Adv. Synth. Catal. 2010, 352, 478 – 492
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485

Yoshihiko Yamamoto
FULL PAPERS
2
À
CO into the Pd Csp bond takes place to give rise to
a benzoylpalladium(II) complex, which reacts with an
alcohol in the presence of a base (B), finally yielding
a benzoate. A plausible mechanism of the alkoxycar-
bonylation of arylboronates is much more complicat-
ed (Figure 3b). In contrast to the original Heck ver-
sion, the initial step is the transmetalation of the aryl
group from boron to palladium, and the palladium(II)
catalyst is reduced to the palladium(0) oxidation state
at the end of the catalytic cycle via subsequent CO in-
sertion and reaction with an alcohol. An appropriate
reoxidant is, hence, required to convert the palladi-
um(0) species back into the active palladium(II) cata-
lyst that participates in further transmetalation. In our
studies, the palladium(II)/palladium(0)-pbq/hq redox
couple has been found to be effective toward the al-
koxycarbonylation of arylboronates as well as the ho-
mocoupling of arylboronic acids. In addition, stoichio-
metric reactions of Pd
acid and esters hardly proceeded in the absence of
MeOH(-d4), indicating that the MeOH solvent is in-
ACHUTNGTRENNUG(pbq)ACHTUNGTNER(NUGN cod) with p-tolylboronic
AHCTUNGTRENNUNG
Scheme 7. Methoxy- and allyloxycarbonylation of o-phenyl-
ACHTUNGTRENNUNGethynylphenylboronic acid (20).
dispensible as a proton donor. However, the exact
role of alcohol solvents is vague, because it was re-
vealed that an additional acid additive was required
to have the palladium(II)/palladium(0)-pbq/hq redox
process work for the 1,4-dialkoxylation of 1,3-
Mechanistic Considerations
Figure 3a shows the catalytic cycle of the Heck alk- dienes.[12] Furthermore, the use of base additives is
ACHTUNGTRENNUNG
oxycarbonylation of aryl halides.[1,2] The cycle starts usually essential for the smooth transmetalation of ar-
with the oxidative addition of palladium(0) species ylboronic acids and derivatives in Suzuki–Miyaura
[L2Pd(0)] to aryl halides (Ar X) to form an arylpalla- coupling and other variants,[3] whereas no such addi-
À
dium(II) intermediate. The subsequent insertion of tive is necessary for the present alkoxycarbonylation
protocol. In short, the newly developed protocol
works efficiently without both acid and base additives
even at ambient temperature.
To explain the reason for the exceptional efficiency
of the present method, the following oxidative trans-
metalation mechanism is proposed (Scheme 8). Al-
though MeOH itself is a weak proton donor, the coor-
dination to organoborons would render its acidity suf-
ficiently higher. As a consequence, the arylboron/
MeOH complex is able to donate its proton to the
pbq ligand, inducing electron transfer (25 in
Scheme 8). Simultaneously, the methoxy group of
MeOH is donated to the boron center to facilitate the
transfer of the aryl group onto the palladium center,
giving rise to arylpalladium(II) complex 26 possessing
a protonated h3-quinone ligand.[18] This is why the cat-
alytic reaction never proceeded with the trifluorobo-
rate possessing no vacant orbital on its boron center
(Table 2, entry 6).
From 26, carbonyl complex 27 can be generated via
isomerization of the h3-quinone ligand to p-hydroxy-
phenoxide and the replacement of PPh3 with CO.[12a]
The ligand exchange of the phenoxide by methoxide
would give 28, from which two migratory CO inser-
tion pathways are expected: one is the insertion into
the Pd Caryl bond leading to benzoyl complex 29, and
Figure 3. Catalytic cycle of alkoxycarbonylations of (a) aryl
halides and (b) arylborons.
À
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Adv. Synth. Catal. 2010, 352, 478 – 492

The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
Scheme 8. Proposed mechanism involving oxidative tranmetalation.
À
the other is that into the Pd O bond giving the me- Density Functional Study on the Oxidative
thoxycarbonyl complex 30.[19] In either way, pbq-in- Transmetalation Step
duced reductive elimination finally produces methyl
benzoate and the pbq-ligated palladium(0) complex. In order to support the above mechanism, the oxida-
It is likely that pbq acts as a p-acidic ligand toward tive transmetalation step was briefly studied by theo-
palladium(II) acyl complexes 29 and 30 to facilitate retical calculations. Although a similar oxidative
reductive elimination by depriving the palladium(II) transmetalation of the palladium peroxo complex
center of electron density.[20,21]
with phenylboronic acid was evaluated by DFT calcu-
The formation of (p-tolyl)benzoquinone 8 in the lations,[23] no such study on that of the pbq complex
stoichiometric reaction of Pd(pbq)(cod) (7) with p- has been reported. At the outset, the geometries of
A
ACHTUNGTRENNUNG
tolylboronates 2b or 3b can be reasonably explained three model complexes S-I, TS-I, and P-I were opti-
by assuming the intermediary of a similar h3-quinone mized by the density functional theory (DFT) calcula-
complex 26’ (Scheme 9). The coupling of the quinone tions using the B3LYP hybrid functional[24] with the
ligand and the p-tolyl group via reductive elimination basis set comprising the LanL2DZ basis set[25a] for
of a palladium(0) fragment would be followed by tau- palladium and the 6-31G(d) basis sets for other ele-
tomerization and subsequent air oxidation to furnish ments (BS-I, Figure 4). For computational efficiency,
8.[22]
PPh3 and a pinacol ester were replaced by PH3 and an
ethane-1,2-diol ester, respectively. The energies of
Scheme 9. Plausible mechanism for the formation of p-tolylbenzoquinone 8.
Adv. Synth. Catal. 2010, 352, 478 – 492
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Yoshihiko Yamamoto
FULL PAPERS
Table 5. Energies for TS-I and P-I relative to S-I.
Method[a]
TS-I DE
P-I DE
[kcalmolÀ1] [kcalmolÀ1]
B3LYP/BS-II
MP2/BS-II
+29.5
+14.2
+23.6
+22.6
+22.2
À3.0
À3.9
À5.3
À7.5
À10.5
À8.7
mPW1PW91/BS-II
mPW1PW91/BS-III
mPW1PW91/BS-III (MeOH)[b]
mPW1PW91/BS-III (n-C7H16)[b] +22.5
[a]
BS-II: SDD for palladium, 6-311+G(d) for others; BS-
III: SDD
others.
ACHUTGTNRENNUG(2f1g) for palladium, 6-311++GACHTUNTGREN(NUNG 2d,p) for
[b]
PCM calculations were performed with dielectric con-
stant of 32.63 and 1.92 for MeOH and heptane, respec-
tively.
bearing a p-allyl-type protonated quinone ligand. The
À
final Pd Cipso bond length is 2.07 ꢂ.
The proposed oxidative transmetalation was esti-
mated to proceed with relatively large activation
energy (DH°) of 29.5 kcalmolÀ1 and a small exother-
micity of À3.0 kcalmolÀ1 at the B3LYP/BS-II level
(Table 5). The DH° value seems to be overestimated
with the DFT calculations at this level of theory, be-
cause a much smaller activation barrier of 14.2 kcal
molÀ1 was predicted using the MP2 method.[27] Then,
the DFT calculations were repeated using the
mPW1PW91 functional[28] to obtain
a
DH° of
23.6 kcalmolÀ1, which is closer to that predicted with
Figure 4. Geometries obtained by the DFT culculations at
the B3LYP/BS-I level of theory. Bond lengths are shown in
ꢂ.
the MP2 method.
A
larger exothermicity of
À7.5 kcalmolÀ1 was calculated at the same level with
the larger basis sets consisting of a [6s5p3d2f1g] con-
tracted valence basis set with the SDD pseudopoten-
tial [SDD
+G(2d,p) basis sets for other elements (BS-III). The
(2f1g)][29] for palladium and the 6-311+
ACHTUNGTRENNUNG
each optimized complex were obtained by the single-
ACHTUNGTRENNUNG
point energy calculations at the same level using the influence of the solvent was also examined with the
basis sets consisting of the SDD basis set[26] for palla- PCM method[30] at the mPW1PW91/BS-III level. As a
dium and the 6-311+G(d) basis sets for other ele- result, a polar solvent, MeOH (e=32.63), was found
ments (BS-II). Starting complex S-I has a trigonal ge- to further improve the exothermicity (À10.5 kcal
ometry bearing h2-coordinated pbq and phenyboro- molÀ1). In a non-polar solvent, heptane (e=1.92), the
nate ligands in addition to a PH3 supporting ligand. In reaction is expected to be less exothermic (À8.7 kcal
the presence of one MeOH molecule, S-I undergoes molÀ1). Although the activation barriers calculated by
oxidative transmetalation via transition state TS-I, the DFT methods are possibly overestimated, the ob-
where one of the quinone carbonyl groups is proton- tained results favorably support the proposed oxida-
ated by a MeOH-phenylboronate complex and its tive transmetalation being involved in the alkoxycar-
phenyl ring is simultaneously transferred onto the pal- bonylation of arylboron reagents with the palladi-
À
ladium center. Consequently, the B Cipso bond is
ACHTUNGTRENuNGNU m(II)/palladium(0)-pbq/hq redox couple. It can be
lengthened from 1.56 ꢂ in S-I to 2.08 ꢂ in TS-I but, assumed that multiple MeOH molecules participate
À
instead, the Pd Cipso distance is shortened by 1.01 ꢂ. in hydrogen bonding with the pbq ligand to further
Upon protonation, the C O bond length of the qui- decrease the activation energy.
À
none carbonyl is elongated by 0.1 ꢂ, and the distance
In the stoichiometric reaction of pbq-ligated palla-
between the palladium and this carbonyl carbon is dium complex 7 with p-tolylboronic acid (1b), the for-
shortened from 2.98 ꢂ in S-I to 2.41 ꢂ in TS-I, indi- mation of homocoupling product 6b in small amounts
cating that a part of the palladium electrons is trans- was observed even in the absence of MeOHACHTUNRGTNEUNG(-d4). This
ferred to the electrophilic, protonated carbonyl group. suggests that the arylboronic acid itself can behave as
The resultant P-I is a palladium(II) s-phenyl complex a proton donor albeit with low efficiency. Thus, the
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Adv. Synth. Catal. 2010, 352, 478 – 492

The First General and Selective Palladium(II)-Catalyzed Alkoxycarbonylation of Arylboronates
oxidative transmetalation without an alcohol media- the ortho carbon in S-III, while it behaves as a h2-
tor was examined with theoretical calculations of the ligand in S-I. Then, the single-point energy calcula-
corresponding models S-II, TS-II, and P-II (see Fig- tions for the optimized geometries were carried out at
ure S6 in the Supporting Information). In this case, the ONIOM (mPW1PW91/BS-III:mPW1PW91/SDD)
the boronic acid proton is transferred onto one of the level of theory. These ONIOM calculations predicted
quinone carbonyl groups, initiating the oxidative the slightly improved activation energy of 19.5 kcal
transmetalation. In a similar manner to the case in- molÀ1 and exothermicity of À12.7 kcalmolÀ1, showing
volving one MeOH molecule, the phenyl ring is trans- that the PPh3 supporting ligand behaves in favor of
ferred from the boron center to the pbq-ligated palla- the oxidative transmetalation.
dium via cyclic transition state TS-II. The proton ab-
straction from one of the boronic acid hydroxy groups
À
causes the shortening of the corresponding B O Conclusions
length with the concomitant elongation of the rele-
À
À
À
vant quinone C O bond. The Pd Cipso and B Cipso In conclusion, the optimal protocol for the alkoxycar-
distances in TS-II are shorter and longer, respectively, bonylation of various aryl- and alkenylboronic acids
than those in TS-I, indicating that the oxidative trans- and esters was successfully developed using the palla-
metalation proceeds with the later transition state in dium(II)/palladium(0)-pbq/hq redox couple. The pres-
the absence of an alcohol mediator. The activation ent method well tolerates a wide variety of functional
barrier was estimated as +40.1 kcalmolÀ1 by the DFT groups including various carbonyl functionalities, ni-
method, which is much greater than that expected for trile, nitro, sulfone, and unprotected pyrrole rings. As
the MeOH mediated transition state. Moreover, this for the alcohol part, the esters were obtained in high
reaction was expected to be +25.8 kcalmolÀ1 endo- yields from methanol and ethanol, while the use of
thermic. Because no matter whether MeOH is in- higher alcohols resulted in diminished yields ranging
volved or not, the common palladium(II) s-phenyl from 48% to 68%. As an exception, the reaction
complex bearing a p-allyl-type protonated quinone hardly proceeded with tert-butyl alcohol. It is also
ligand is evolved through oxidative transmetalation, noteworthy that allyl alcohol exhibited exceptionally
the high endothermicity must be ascribed to the con- high reactivity: the reactions were completed in
comitant formation of HOB=O. In fact, the compari- 30 min and the yields of allylic esters were over 90%
son of heats of formation calculated for equations S1 when a small excess of pbq was used.
and S2 (see Supporting Information) at the
The alkoxycarbonylation developed in this study
mPW1PW91/BS-III//B3LYP/BS-I level, where the proceeds without acid or base additives, in striking
common pbq-ligated palladium(0) fragment and pal- contrast to related palladium-catalyzed reactions. To
ladium(II) s-phenyl complex are involved, shows that explain the efficacy of the present catalyst system, an
the former is exothermic but the latter endothermic. oxidative transmetalation mechanism was proposed.
Therefore, it is concluded that the formation of In this mechanism, an arylboron/MeOH complex is
HOB=O renders the reaction thermodynamically un- considered to be a proton donor for the pbq ligand,
favorable. Although other possibility such as, for ex- which triggers the oxidation of the pbq-ligated palla-
ample, the involvement of multiple boronic acid mol- dium center. Simultaneously, the methoxy group of
ecules cannot be excluded, the participation of one or MeOH is donated to the boron center to induce the
multiple alcohol molecule(s) can play a critical role in transmetalation of the aryl group from boron to palla-
alkoxycarbonylation as well as homocoupling of aryl- dium. This novel mechanism is supported by theoreti-
boron compounds using the palladium(II)/palladi- cal calculations using the DFT and MP2 methods.
um(0)-pbq/hq redox couple.
Finally, larger models including the PPh3 ligand, S-
III, TS-III, and P-III (see Figure S7 in Supporting In-
Experimental Section
formation), were analyzed using the ONIOM
method.[31] The DFT method (B3LYP/BS-I) was used
General
for the high-level calculations of the electronically im-
portant parts of the larger models, which are identical
Column chromatography was performed on silica gel (Cica
1
silica gel 60N) with mixed solvents [hexane/AcOEt]. H and
with S-I, TS-I, and P-I, while the Hartree–Fock (HF)
13C NMR spectra were obtained for samples in CDCl3 solu-
method with a minimal basis set, LanL2MB,[25] was
1
tions at 258C. H NMR chemical shifts are reported in terms
used for the low-level calculations. Hereafter, this
method is denoted as B3LYP/BS-I:HF/LanL2MB.
of chemical shift (d, ppm) relative to the singlet at 7.26 ppm
for chloroform. Splitting patterns are designated as follows:
The ONIOM calculations at this level gave geome-
s, singlet; d, doublet; t, triplet; q, quartet; quint, quintet;
tries similar to those obtained for the smaller models.
A slight difference is that the phenylboronate is
sext, sextet; sept, septet; m, multiplet. Coupling constants
are reported in Hz. 13C NMR spectra were fully decoupled
bound to the palladium center with h1 coordination at and are reported in terms of chemical shift (d, ppm) relative
Adv. Synth. Catal. 2010, 352, 478 – 492
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489

Yoshihiko Yamamoto
FULL PAPERS
to the triplet at 77.0 ppm for CDCl3. Melting points were
1.2 Hz, 1H), 5.86 (ddt, J=17.4, 10.2, 6.6 Hz, 1H), 7.41 (d,
J=9.0 Hz, 2H), 7.97 (d, J=9.0 Hz, 2H); 13C NMR (75 MHz,
CDCl3): d=33.2, 64.2, 117.3, 128.4, 128.5, 130.7, 133.6, 139.0,
165.3; IR (CHCl3): n=1716 (C=O), 1596 (Ar C=C), 1285
(C-O), 1276 (C-O) cmÀ1; HR-MS (FAB): m/z=233.0349,
calcd. for C11H11ClNaO2 [M+Na]+: 233.0345.
obtained in capillary tubes. Pd
most of the boronic acids were purchased and used as re-
ceived. Pd(bq)ACHTUNGTRENNUNG(cod), Pd2(bq)2ACHTUNGREN(NUG nbe)2, boronic acids 13 and 20
ACHTUGNRTEN(NUGN OAc)2, PPh3, alcohols, and
were prepared according to the reported methods.[13,32,33]
Boronic acid esters are commercially available or readily
obtained from the corresponding boronic acids through con-
densation with diols in refluxing THF using the Dean–Stark
apparatus. All products except for 13e, j were known com-
pounds previously reported in the literatures.
Acknowledgements
This research was partially supported by the MEXT, Grant-
in-Aid for Scientific Research (B) (20350045), Scientific Re-
search on Priority Area “Chemistry on Concerto Catalysis”
(20037018). I thank Prof. K. Tomooka (Kyushu University)
for use of an NMR spectrometer, Mr. K. Yamashita for HR-
MS measurements, and Profs. H. Suzuki, T. Ikariya, and N.
Iwasawa (Tokyo Institute of Technology) for warm encour-
agement.
Stoichiometric Reactions of Pd(bq)ACHTNUTRGNEUNG(cod) (7) with p-
Tolylboron Reagents; Representative Procedure.
Reaction of 7 with p-Tolylboronate 1b
PdACHTUNGTRENNUNG(pbq)ACHTUNGTRENNUNG(cod) (7) (32.85 mg, 0.102 mmol) and p-tolylboro-
nate 1b (44.99 mg, 0.220 mmol) were dissolved in CDCl3
(5 mL) and degassed at À788C. The solution was stirred at
room temperature under an argon atmosphere. The progress
of the reaction was monitored by 1H NMR spectroscopy.
After stirring for 30 min, CD3OD (0.3 mL) was added to the
reaction mixture and the solution was stirred for 5 h. The so-
lution was concentrated under vacuum, and the residue was
purified by silica gel flash column chromatography (hexane-
AcOEt, 50:1) to give 4,4’-dimethylbiphenyl 6b as a colorless
solid; yield: 9.9 mg (53% based on Pd). Further elution
(hexane-AcOEt 20:1) afforded 2-(p-tolyl)-1,4-benzoquinone
8 as a yellow solid; yield: 1.95 mg (9.7%). The spectral data
of 6b and 8 were in good agreement with those reported
previously.
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To a solution of p-chlorophenylboronate 3a (119.83 mg,
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was concentrated under vacuum. The residue was purified
by silica gel flash column chromatography (hexane-AcOEt,
80:1) to give methyl p-chlorobenzoiate 4a as a colorless oil;
yield: 72.30 mg (84%).
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Analytical data for 13e: Colorless oil: 1H NMR
(300 MHz, CDCl3): d=1.03 (d, J=6.9 Hz, 6H), 2.09 (sept,
J=6.9 Hz, 1H), 4.11 (d, J=6.9 Hz, 2H), 7.41 (d, J=8.6 Hz,
2H), 7.98 (d, J=8.6 Hz, 2H); 13C NMR (75 MHz, CDCl3):
d=19.3, 28.0, 71.2, 128.4, 128.7, 130.7, 139.0, 165.3; IR
(CHCl3): n=1716 (C=O), 1596 (Ar C=C), 1286 (C-O), 1275
(C-O) cmÀ1; HR-MS (FAB): m/z= 235.0501, calcd. for
C11H13ClNaO2 [M+Na]+: 235.0502.
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1
Analytical data for 13j: Colorless oil; H NMR (300 MHz,
CDCl3): d=2.53 (qt, J=6.6, 1.2 Hz, 2H), 4.37 (t, J=6.6 Hz,
2H), 5.12 (dq, J=10.2, 1.2 Hz, 1H), 5.17 (dq, J=17.4,
490
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Adv. Synth. Catal. 2010, 352, 478 – 492

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