Ruthenium-catalyzed double-fold C-H tertiary alkoxycarbonylation of arenes using di-tert-butyl dicarbonate

Article abstract of DOI:10.1039/c4cc05173d

An efficient ruthenium-catalyzed double-fold C-H alkoxycarbonylation of arenes was developed using di-tert-butyl dicarbonate as the tertiary esterification reagent, which leads to a direct route to valuable 2,6-dicarboxylated products. This journal is

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Ruthenium-catalyzed double-fold C–H tertiary
alkoxycarbonylation of arenes using di-tert-butyl
dicarbonate†
Cite this:DOI: 10.1039/c4cc05173d
Received 5th July 2014,
Accepted 12th September 2014
Xiaohu Hong,a Hao Wang,a Bingxin Liua and Bin Xu*abc
DOI: 10.1039/c4cc05173d
www.rsc.org/chemcomm
An efficient ruthenium-catalyzed double-fold C–H alkoxycarbony- dioxide,8i and alkyl chloroformate8j as esterification reagents.
lation of arenes was developed using di-tert-butyl dicarbonate as Although these methods are often effective, some of them suffer
the tertiary esterification reagent, which leads to a direct route to from substrate limitation, lack of diversity, poor functional group
valuable 2,6-dicarboxylated products.
tolerance, or the requirement for multistep procedures. In com-
parison with other simple esters, tertiary esters exhibit many
Transition metals-catalyzed C–C bond formation have been advantages,9 for example. t-Butyl esters can be easily converted to
investigated intensively through C–H bond alkenylation, alkyla- the corresponding carboxylic acids under mild acid conditions,
tion, arylation, and carbonylation reactions.1 Among these although their formation is sometimes very difficult due to
reactions, ruthenium-catalyzed C–H functionalization reactions tedious procedures and specialized reagents.8h,10 Therefore, the
have attracted increased attention because of their remarkable development of novel synthetic methods for the tertiary ester
reactivity.1j,2 Recently, ruthenium catalysts have been employed group is still in high demand, in terms of synthetic efficiency and
to couple with various C–H bond activation partners, including starting material availability.
alkenes,3a,d arylhalides,3b alkylhalides,3c arylsulfonyl chlorides,3e
In contrast to widely documented reports on palladium-8a–g and
alkynes,3f and azides.3g Despite their significant progress, it rhodium-catalyzed8h,i carbonylation reactions, the ruthenium-
is necessary to further explore other coupling partners under catalyzed C–H bond alkoxycarbonylation reaction has been much
ruthenium catalysis.
less explored,8j although it has been proven to be a good comple-
Aromatic esters are of considerable contemporary interest with ment to other transition noble metals in terms of substrate scope
respect to their exceptionally rich synthetic possibilities, which and functional group compatibility. In a pioneering study,
allows their use as a versatile building block and effective precursor Kakiuchi and co-workers elegantly devised the first ruthenium-
for various functional group transformations.4,5 Generally, esters are catalyzed introduction of ester groups via C–H bond cleavage
prepared by traditional methods such as the reaction of acid using ethyl chloroformates.8j However, as reported earlier, most
chlorides with alcohols, the insertion reaction of aryl halides with of the reactions focused on the mono-carboxylation reaction and
carbon monoxide,5 and the coupling reaction of di-tert-butyl dicar- less attention has been paid to the successful synthesis of double
bonate with stoichiometric organometallic compounds6 or boronic alkoxycarbonylated products.8a,i This may be due to the electron-
acids.7 Recently, alternative access to ester groups has been achieved withdrawing and steric effects of the ester group, which can
through direct sp2 C–H bond esterification using DEAD,8a,b oxazir- make the second cyclometalation difficult to proceed and to
idine,8c glyoxylates,8d a-keto esters,8e carbon monoxide,8f–h carbon predominantly afford the mono-alkoxycarbonylated product.11
In this event, the development of ruthenium-catalyzed highly
efficient and practical double-fold C–H bond alkoxycarbonyla-
tion methods using easily available esterification reagents
continues to be an active and rewarding research area. We
envisioned that the use of a pyrimidyl substituent as a directing
group, which has two nitrogen atoms and is prone to form a dual
metal complex,12,13 may facilitate the second C–H bond activa-
tion and afford the double alkoxycarbonylated products. Herein,
we disclose a ruthenium-catalyzed protocol for the double tert-
butoxycarbonylation of arylpyrimidines and aldimines, which is
superior to the previously developed methods for the formation
a School of Materials Science and Engineering, Department of Chemistry,
Innovative Drug Research Center, Shanghai University, Shanghai 200444, China.
E-mail: xubin@shu.edu.cn; Fax: +86-21-66132830; Tel: +86-21-66132830
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
c Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, Shanghai 200062, China
† Electronic supplementary information (ESI) available: General experimental
procedures, characterization data and copies of the 1H, 13C and 19F NMR spectra
for all compounds. CCDC 995322 (2n). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc05173d
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Table 2 Scope of heterocyclesa,b
Scheme 1 Ruthenium-catalyzed double tert-butoxycarbonylation with
Boc2O.
of 2,6-dicarboxylated products (Scheme 1). To the best of our
knowledge, the given approach is the first example that utilizes
commercially available di-tert-butyl dicarbonate as the esterifica-
tion reagent in ruthenium-catalyzed C–H bond tertiary alkoxy-
carbonylation reactions.
To determine the feasibility of the chelation effect of a
pyrimidyl group,14 we started our investigation with the bench-
mark reaction between 2-phenyl-pyrimidine 1a and di-tert-butyl
dicarbonate (2.5 equiv.) in the presence of [RuCl2( p-cymene)]2
and K2CO3 in toluene at 120 1C. Intriguingly, the mixture
of di-t-butoxycarbonylation product 2a and mono-t-butoxy-
carbonylation product 2a0 was isolated in a 29% combined
yield at a ratio of 1.1 : 1, with a 35% conversion of 1a (entry 1,
Table 1). An extensive screening of the carboxylic acids used as
cocatalytic additives (entries 2–5), bases (entries 6–8), solvents
(entries 9–12), and ruthenium catalysts (entries 13 and 14)
revealed that the use of 1-AdCOOH (30 mol%) as an additive
in toluene at 120 1C under a nitrogen atmosphere turned out to be
the best choice and resulted in the desired di-ester product 2a in a
86% yield, exclusively. Decreasing the amount of Boc2O to 1.2 equiv.,
the reaction could still give the di-ester 2a as a major product in a
43% combined yield, with 54% conversion of the substrate (entry 15).
a
Reaction conditions: 1 (0.5 mmol), Boc2O (2.5 equiv.), [RuCl2(p-cymene)]2
(2.5 mol%), 1-AdCOOH (30 mol%), and K2CO3 (2.5 equiv.) in toluene
b
c
(1.25 mL), 120 1C, under nitrogen atmosphere. Isolated yield. Mono-t-
butoxy-carbonylation product 2i0 was obtained in a 6% yield, with a 65%
d
e
conversion. Based on 51% conversion. Based on 60% conversion.
Table 1 Condition optimizations for the tert-butoxycarbonylation reactiona
With the optimized reaction conditions in hand, we sought
to explore the reaction with a range of substrates, as summar-
ized in Table 2. Substrates bearing an electron-donating or
electron-withdrawing group at the para-position underwent
di-t-butoxycarbonylation in good yields (2b–2g), in which func-
tional groups such as Me, OMe, halide, and CF3 were all tolerated.
Substitution groups such as fluorine and chlorine at the meta-
position caused no problem and the di-t-butoxycarbonylated pro-
ducts 2h and 2i were furnished in good yields. The introduction of
an ethyl or phenyl group on the 4- or 5-position of the pyrimidine
ring did not affect the efficiency of the reaction and afforded 2j–2l
in moderate to good yields. This newly established protocol
was not limited to pyrimidines, pyridine and pyrazole were also
found to be the effective directing groups for this reaction, and
di-t-butoxycarbonylated products 2m and 2n were obtained in
excellent yields. The identity of 2n was determined by spectra
analysis and further confirmed by X-ray crystallographic analysis.15
Furthermore, when the phenyl group was replaced with a pyrrole
substituent, the reaction proceeded smoothly and the di-t-butoxy-
carbonylation reaction took place in the pyrrole moiety in a high
yield (2o). However, mono-ester products were also achieved in
good to excellent yields (2p–2r), which could contribute to the
electronic or steric effects of the substrates. Moreover, this newly
established protocol could be extended to benzo[h]quinoline and
Conversion Yieldb Ratioc
Entry Additive
Base
Solvent [%]
[%]
[2a : 2a0]
1
2
3
4
5
6
7
8
—
K2CO3
Toluene 35
Toluene 74
Toluene 66
Toluene 100
Toluene 100
Toluene 54
29
63
58
82
86
47
71
72
31
77
—
24
55
48
43
1.1 : 1
14 : 1
6 : 1
420 : 1
420 : 1
5 : 1
5 : 1
420 : 1
5 : 1
8 : 1
—
3 : 1
1.8 : 1
2.3 : 1
5 : 1
CH3COOH K2CO3
PhCOOH K2CO3
MesCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH KOAc
1-AdCOOH K3PO4ꢀ3H2O Toluene 88
1-AdCOOH Cs2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
1-AdCOOH K2CO3
Toluene 91
DMF 38
Dioxane 85
9
10
11
12
13d
14e
15f
DCE
THF
—
41
Toluene 63
Toluene 61
Toluene 54
a
Reaction conditions: 1a (0.2 mmol), Boc2O (2.5 equiv.), [RuCl2(p-cymene)]2
(2.5 mol%), additive (30 mol%), and base (2.5 equiv.) in solvent (0.5 mL),
b
120 1C, 3 h, under nitrogen atmosphere. Isolated yield. c Ratio was
determined by NMR analysis. d RuCl2(PPh3)3 (2.5 mol%) was used. e [RuCl-
2(COD)]n (2.5 mol%) was used. f Boc2O (1.2 equiv.) was used.
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furnish the corresponding mono-t-butoxycarbonylated product 2s
exclusively. To further broaden the substrate scope, we tried to
apply the reaction using various dicarbonates other than Boc2O;
however, none of the expected carbonylation products were
obtained. Furthermore, none of the desired ethoxycarbonylation
or aminocarbonylation products could be achieved, when using
dimethylcarbamic chloride and ethyl chloroformate as the amida-
tion or esterification reagent.
Imine groups were fully investigated in the C–H bond functiona-
lization as directing groups;1b,e,j however, only one example was
reported for the C–H bond esterification of oximes with DEAD.8a To
further explore the generality of this method, various aldimines,
with only one nitrogen atom and acting as a directing group, were
investigated instead of the pyrimidyl group, on the basis of the
previous result on the sterically controlled C–H bond diborylation of
hydrazones.16 To our delight, this protocol could be successfully
applied to the corresponding aldimine counterpart, which repre-
sents the first reported ruthenium-catalyzed C–H bond tert-
butoxycarbonylation of aldimines (Table 3). A wide range of
substrates bearing an electron-donating or electron-withdrawing
group at the para position underwent the di-t-butoxycarbonylation
smoothly in excellent yields under optimized conditions (4b–4e).
A substrate containing fluorine substitution at the meta-position
also underwent this reaction smoothly to generate the corres-
ponding ester product 4g in a high yield, while a sterically bulky
meta-methyl substitution retarded the reaction (4f). When the
methoxyl group was replaced with different substitutions on the
aryl ring, double carboxylation was achieved, to give their
corresponding di-esters (4h–4j). We also investigated the tert-
butoxycarbonylation reaction of ortho-substituted aldimine, and
found that the ortho-carboxylate substituent underwent smooth
carboxylation, producing the corresponding product 4k in an
81% yield, which enabled the preparation of nonsymmetrical
di-ester. It should be noted that, the facile transformation of 3a
with Boc2O could be carried out without difficulty on a 10 mmol
scale, to give 4a in a 96% yield with a ruthenium catalyst loading
of only 1.0 mol%, which expands the synthetic utility of this
catalytic system.
Scheme 2 Further transformations of 4a.
Further efforts were made to explore the applications of this
method (Scheme 2). It was found that the prepared di-ester 4a
resulted in efficient deprotection of the directing group in THF
at 0 1C, affording the synthetically useful 2,6-dicarboxylated
benzaldehyde 5 in an almost quantitative yield.17 Interestingly,
while this hydrolysis reaction was conducted at room tempera-
ture, the 3-hydroxyphthalide derivative 6 was obtained in a high
yield through intramolecular cyclization with removal of the
imine group,18 which is an important skeleton in organic
synthesis. Notably, the intramolecular amination product iso-
indolinone 7 could be furnished smoothly with ZnCl2 and
NaCNBH3 in methanol, which provides a convenient process
to make such a useful skeleton.19
To define the possible intermediates and pathway, the chloro-
ruthenacycle 8 was prepared from 3a and [RuCl2(p-cymene)]2 in
the presence of potassium acetate (Scheme 3).20 When complex 8
was used as a catalyst instead of [RuCl2(p-cymene)]2, the di-ester 4a
was furnished in an 88% yield under otherwise identical conditions.
In stark contrast, product 4a was afforded in a significant lower yield
(15%) in the absence of 1-AdCOOH, which indicated that the
cycloruthenated complex 8 could be the key intermediate, and
which furthermore highlighted the key importance of carboxylate
assistance in this reaction.
Although further studies were required to clarify the detailed
pathway, a plausible mechanism for the formation of carbony-
lated arenes is proposed (Scheme 4). The catalytic cycle is likely
initiated by the formation of a transition state A with the
assistance of carboxylate acid, which could further transfer
to the cycloruthenated complex B through a concerted cyclo-
metalation deprotonation process (CMD).2b Complex B can be
oxidized to the Ru(IV) species C by the addition of Boc2O and
then undergo further reductive elimination to give the mono-
tert-butoxycarbonylated product D, together with expulsion of
carbon dioxide (Path A). Finally, the produced intermediate D
Table 3 Scope of aldiminesa,b
a
Reaction conditions: 3 (0.5 mmol), Boc2O (2.5 equiv.), [RuCl2(p-cymene)]2
(2.5 mol%), 1-AdCOOH (30 mol%), and K2CO3b(2.5 equiv.) in toluene
c
(1.25 mL), 120 1C, under nitrogen atmosphere. Isolated yield. Based
on 48% conversion.
Scheme 3 Cycloruthenated complex 8 as the catalyst.
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Scheme 4 Plausible mechanism.
undergoes a second catalytic cycle to afford the desired di-t-butoxy-
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2,6-dicarboxylated products, which are the precursors for
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We thank the National Natural Science Foundation of China
(No. 21272149) and Innovation Program of Shanghai Municipal
Education Commission (No. 14ZZ094) for financial support.
The authors thank Prof. Hongmei Deng (Laboratory for Micro-
structures, SHU) for NMR spectroscopic measurements.
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