Pd/C-Catalyzed Carbonylative Esterification of Aryl Halides with Alcohols by Using Oxiranes as CO Sources

Article abstract of DOI:10.1002/chem.201600570

A carbonylative esterification reaction between aryl bromides and alcohols, promoted by Pd/C and NaF in the presence of oxiranes, has been developed. In this process, oxiranes serve as sources of carbon monoxide by their conversion to aldehydes through a palladium-promoted Meinwald rearrangement pathway. Intramolecular versions of this process serve as methods for the synthesis of lactones and phthalimides. CO gas free! A carbonylative esterification reaction between aryl bromides and alcohols, promoted by Pd/C and NaF in the presence of oxiranes, has been developed. In this process, oxiranes serve as sources of carbon monoxide by their conversion to aldehydes through a palladium-promoted Meinwald rearrangement pathway (see scheme).

Full text of DOI:10.1002/chem.201600570

DOI: 10.1002/chem.201600570
Communication
&
Synthetic Methods
Pd/C-Catalyzed Carbonylative Esterification of Aryl Halides with
Alcohols by Using Oxiranes as CO Sources
Byul-Hana Min, Dong-Su Kim, Hyo-Soon Park, and Chul-Ho Jun*[a]
Abstract: A carbonylative esterification reaction between
aryl bromides and alcohols, promoted by Pd/C and NaF in
the presence of oxiranes, has been developed. In this pro-
cess, oxiranes serve as sources of carbon monoxide by
their conversion to aldehydes through a palladium-pro-
moted Meinwald rearrangement pathway. Intramolecular
versions of this process serve as methods for the synthesis
of lactones and phthalimides.
Scheme 1. Carbonylative esterification strategy employing Meinwald rear-
rangements of oxiranes.
volved in this process undergoes Meinwald rearrangement[14]
to form an aldehyde, which serves as the carbon monoxide
source.[15] To the best of our knowledge, this is the first exam-
ple of an ester synthesis protocol, which utilizes oxirane as the
carbonyl source.
Transition-metal-catalyzed carbonylation reactions are among
the most useful and reliable processes for the preparation of
esters.[1] The pioneering studies by Heck led to the develop-
ment of the first palladium-catalyzed carbonylation reactions
between alcohols, aryl halides, and carbon monoxide.[2] Be-
cause carbon monoxide gas is toxic and difficult to handle,
many other one carbon carbonyl surrogates were applied in
An investigation was conducted to determine if oxiranes
function as suitable CO sources. The reaction of 2-phenyloxir-
ane (1a) in the presence of Pd/C (3a, 5 mol%) in 1,4-dioxane
was observed to produce toluene (4a) in 92% yield along with
8% of phenylacetaldehyde (2a) (Table 1, entry 1). This result
suggests that carbon monoxide is generated in 92% yield in
this process by decarbonylation of the intermediate aldehyde
2a. However, when 2a was directly subjected to the decarbon-
ylation conditions, toluene (57%) along with a large amount
(43%) of the aldol product 2b was produced (entry 2). Other
aldehydes, such as benzaldehyde (2c) and cinnamaldehyde
various transition-metal-catalyzed carbonylation reactions.[3]
A
remarkable transition-metal-catalyzed carbonylation involving
ex situ generation of carbon monoxide from organic precur-
sors, such as silacarboxylic acid,[4] acid chloride,[5] formic acid,[6]
and carbon dioxide[7] was developed. However, these reactions
require multistep and complex reaction setups (e.g., two-
chamber reactor). Carbonylation involving in situ generation of
carbon monoxide from carbonyl surrogates, such as chloro-
form,[8] formate,[9] formic anhydride,[10] aldehyde,[11] and alco-
hol,[12] has been also studied, and these reactions demand ho-
mogeneous catalysts (e.g., Pd(OAc)2, [{Rh(CO)Cl(dppp)}2] and
Ru3CO12), which require special phosphine ligands (e.g., dppp,
DPEphos, Xantphos, and dtbpx).
Table 1. CO production by decarbonylation of oxiranes and aldehydes.[a]
Recently, we reported a Pd/C-catalyzed carbonylative esterifi-
cation reaction between alcohols and aryl chlorides, in which
the alcohol serves the role of both the carbonyl source
through dehydroxymethylation and the ether moiety of the
ester.[13] During the course of recent studies targeted at the
design of new ester-forming reactions, we observed that addi-
tion of an oxirane to a mixture of aryl bromide, alcohol, and
Pd/C leads to the production of an alkane along with a cross-
coupled ester (Scheme 1). We assumed that the oxirane in-
[a] B.-H. Min, Dr. D.-S. Kim, H.-S. Park, Prof. Dr. C.-H. Jun
Department of Chemistry
Yonsei University
50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (Republic of Korea)
E-mail: Junch@yonsei.ac.kr
[a] Unless otherwise noted, reactions were carried out with CO sources
1 or 2 (0.2 mmol) and 3a (5 mol%) in 1,4-dioxane (0.1 mL) at 1508C.
[b] All yields were determined by using GC analysis.
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600570.
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(2d), were found to be less efficient CO generators than 1a
under the reaction conditions (entries 3–4).
Table 3. Optimization of various oxiranes as carbonyl sources.[a]
Based on these observations, 2-phenyloxirane was employed
as the carbonyl source in studies aimed at optimizing the
ester-forming process (Table 2). The results show that the reac-
Table 2. Optimization of the carbonylative esterification reaction.[a]
[a] Unless otherwise noted, reactions were carried out with 5a
(0.2 mmol), 1 (0.4 mmol), 6a (0.4 mmol), 3a (5 mol%), and 7a (0.4 mmol)
in 1,4-dioxane (0.1 mL) at 1508C. [b] GC yield based on 1. [c] GC yield
based on 5a. [d] Isolated yield based on 5a.
Moreover, aliphatic oxiranes, such as 2-methyloxirane (1c) and
2-butyloxirane (1d), do not participate in the ester-forming
process (entries 3–4), and the reaction in which the sterically
hindered analog 2,2-dimethyloxirane (1e) serves as the CO
donor generates 9a in only 11 % yield (entry 5).
[a] Unless otherwise noted, reactions were carried out with 5a
(0.2 mmol), 1a (0.4 mmol), 6a (0.4 mmol), 3 (5 mol%), and 7 (0.4 mmol)
in solvent (0.1 mL) at 1508C. [b] GC yield based on 1a. [c] GC yield based
on 5a. [d] Isolated yield based on 5a. [e] 10% palladium on activated
charcoal. [f] 10% palladium on carbon.
Based on the results described above, it is possible to pro-
pose the mechanism outlined in Figure 1 for the carbonylative
esterification process. In this pathway, a palladium-catalyzed
Meinwald-type rearrangement of 2-phenyloxirane (1a) occurs
initially to generate phenylacetaldehyde (2a), which then rap-
idly undergoes a Pd0-promoted decarbonylation to generate
toluene (4a) and Pd0ÀCO complex 10a.[15g,16] In this event,
aldol condensation of 2a would be inefficient owing to its low
concentration. An oxidative addition of 4-bromoanisole (5a) to
10a affords acylÀPdIIÀBr intermediate 11 a, which reacts with
tion of 4-bromoanisole (5a) with 2-phenyloxirane (1a) and
heptanol (6a) in the presence of Pd/C (3a) and NaF (7a) in
1,4-dioxane at 1508C for 6 h generates heptyl 4-methoxyben-
zoate (9a) in 82% isolated yield (entry 1) along with toluene
(4a, 77% GC yield) and a lesser amount of anisole (8a, 12%
GC yield). Among various palladium catalysts tested, homoge-
neous catalysts, such as [Pd(PPh3)4] and Pd(OAc)2, do not pro-
mote the esterification reaction, whereas the process catalyzed
by [Pd2(dba)2] yields only 15% of the target ester 9a (en-
tries 3–5), implying that only a heterogeneous catalyst can be
applied for this reaction. Among the various weak bases ex-
plored, such as NaF (7a), CsF (7b), Na2CO3 (7c), and NaHCO3
(7d), NaF (7a) was found to be the optimal for this process
(entries 1 and 6–8). Importantly, the reaction in which NaOH
(7e) is the base gives only the homocoupling product (a bi-
phenyl derivative, derived from 5a) but none of the corre-
sponding ester. Finally, 1,4-dioxane was found to be better sol-
vent than those tested (CH3CN, toluene, and ClCH2CH2Cl; en-
tries 10–12).
The oxirane diversity of the new ester-forming process was
explored next. The reaction between 4-bromoanisole (5a) and
1-heptanol (6a) employing 2-phenyloxirane (1a) as the carbon-
yl source (carried out under the optimized conditions de-
scribed in Table 2) produces 82% yield of ester 9a (Table 3,
entry 1). The use of 2-(4-chlorophenyl)oxirane (1b) as the CO
source does not lead to an improvement in the yield (entry 2).
Figure 1. Proposed mechanism for the carbonylative esterification process.
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heptanol (6a) to produce acylÀPdIIÀOÀheptyl intermediate
12a. This Pd complex then undergoes reductive elimination to
generate ester 9a along with the catalytically active Pd0.
Table 5. Alcohol substrate scope of the reaction.[a]
Next, the aryl halide substrate scope was explored. Esterifica-
tion reactions of electron-neutral (bromobenzene (5b)), -do-
nating (4-bromoanisole (5a)), and -withdrawing (4-bromoben-
zonitrile (5c)) group-substituted aryl bromides with heptanol
proceed to give the corresponding esters in respective yields
of 85, 82, and 62% (Table 4, entries 1–3). Hydroxyl (5d), N,N-di-
Table 4. Aryl halides substrate scope of the reaction.[a]
[a] Unless otherwise noted, reactions were carried out with 5a
(0.2 mmol), 1a (0.4 mmol), (0.32 mmol), 3a (5 mol%), and 7a
6
(0.4 mmol) in 1,4-dioxane (0.1 mL) at 1508C. [b] GC yield based on 5a.
[c] GC yield based on 5a and isolated yields are given in parenthesis.
[d] 1,4-dioxane (0.05 mL) was used.
the corresponding esters in 53 and 55% yield, respectively (en-
tries 3 and 4). The reaction of thiophene-substituted alcohol
6 f proceed to give the corresponding ester 9n in a moderate
36% yield (entry 5). Steric effects also appear to influence the
efficiency of the process as is reflected by the observation that
secondary alcohol 6g is less reactive than primary alcohol 6b
(entries 1 and 6), and that the tertiary alcohol 6h does not par-
ticipate in the process (entry 7).
[a] Unless otherwise noted, reactions were carried out with 5 (0.2 mmol),
1a (0.4 mmol), 6a (0.4 mmol), 3a (5 mol%), and 7a (0.4 mmol) in 1,4-di-
oxane (0.1 mL) at 1508C. [b] GC yield based on 5. [c] GC yield based on 5,
and isolated yields are given in parenthesis. [d] 1,4-dioxane (0.05 mL) was
used. [e] Reaction was carried out with 5 (0.2 mmol), 1a (0.6 mmol), 6a
(0.6 mmol), 3a (5 mol%), and 7a (0.4 mmol) in 1,4-dioxane (0.05 mL) at
1508C. [f] Phenylacetaldehyde was obtained in 71% yield based on 1a.
The intramolecular version of the carbonylative esterification
reaction serves as a useful method for the preparation of lac-
tones (Table 6).[18] For example, the reaction of 2-bromobenzyl
alcohol (5l) with 2-phenyloxirane (1a) in the presence of 3a
and 7a takes place to generate isobenzofuran-1(3H)-one (13a)
in 94% yield (entry 1).[19] Likewise, 2-bromoaryl alcohols, such
as 2-bromophenyl ethanol (5m) and 2-bromophenyl propanol
(5n), also yield the respective isochroman-1-one (13b) and 4,5-
dihydrobenzo[c]oxepin-1(3H)-one (13c) in high yields (en-
tries 2–3). In contrast, the reaction with 4-(2-bromophenyl)bu-
tan-1-ol (5o) does not produce the expected 8-membered lac-
tone 13d (entry 4), and the yields of the lactone-forming reac-
tion decreases as the ring size increases. This result demon-
strates that the generation of six-membered palladacycle inter-
mediate 14a (Table 5) is most favorable.
Lactonization using styrene and meta-chloroperbenzoic acid
(mCPBA) as a CO source instead of 2-phenyloxirane was also
conducted [Eq. (1)]. The reaction of 5l with styrene (14a) and
mCPBA proceeds in the presence of 3a and 7a to generate
13a in 94% yield. During the reaction, 2-phenyloxirane might
be generated in situ by the reaction of styrene with mCPBA. 2-
Bromophenyl ethanol (5m) was also efficiently converted to
lactone 13b in 92% yield.
methylamino (5e), chloro (5 f), and acetyl (5g) functional
groups are well tolerated under the esterification reaction con-
ditions (entries 4–7). The reaction of polyaromatic analogue, 2-
bromonaphthalene (5h), also gives a similar yield to that of
bromobenzene (5b; entry 1 versus 8). Steric factors appear to
influence the efficiency of the reaction. This is exemplified by
the observation of the reaction of 2-bromo-1,3,5-trimethylben-
zene (5i) under the optimal conditions, which generates ester
9i in only 24% yield. Finally, the reactions of bromoarenes
occur smoothly under the standard reaction conditions, where-
as those of chloro- (5j) and iodoarene (5k) analogs are highly
inefficient (entries 1, 10 and 11).[17]
The participation of various alcohols in the esterification re-
action was also probed. The reaction of 4-bromoanisole (5a)
with aliphatic alcohols, such as octanol (6b) and methanol
(6c), leads to the formation of octyl ester and methyl ester in
84 and 81% yield, respectively (Table 5, entries 1 and 2). Benzyl
alcohol (6d) and phenol (6e) also serve as reactants, forming
Finally, the intramolecular cyclization of 2-bromobenzamide
(5p) with oxirane 1a in the presence of 3a and 7a under the
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bonylative esterification using oxiranes as carbon monoxide
sources are currently under study in our group.
Table 6. Lactonization of various 2-bromoaryl alcohol[a]
Acknowledgements
This work was supported by a grant from the National Re-
search Foundation of Korea (NRF) (Grant 2011-0016830).
Keywords: carbonylation
lactones · palladium
· cross-coupling · cyclization ·
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13e in 68% isolated yield [Eq. (2)].
In summary, we developed a new Pd0-catalyzed carbonyla-
tive esterification reaction, which utilizes 2-phenyloxirane as
a carbon monoxide source. Carbon monoxide, in the form of
the Pd0ÀCO complex, is generated in this process by a pathway
involving sequential palladium-promoted Meinwald rearrange-
ment to form phenylacetaldehyde and decarbonylation. The
new protocol can be used to synthesize 5–7-membered lac-
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Received: February 6, 2016
Published online on March 11, 2016
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