Catalytic Lactonization of Unactivated Aryl C-H Bonds with CO2: Experimental and Computational Investigation

Article abstract of DOI:10.1021/acs.orglett.8b01363

The first catalytic lactonization of unactivated aryl C-H bonds with CO₂ to afford important phthalides is reported. Notably, this method features high selectivity, excellent functional group tolerance, smooth scalability, and facile product diversification. DFT calculations reveal that a novel insertion of two CO₂ into the O-Pd bond of a palladacycle might be the key step, providing great potential and a different perspective for carbonylation with CO₂.

Full text of DOI:10.1021/acs.orglett.8b01363

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Catalytic Lactonization of Unactivated Aryl C−H Bonds with CO :
2
Experimental and Computational Investigation
†
‡
†
†
†
†
†
Lei Song, Lei Zhu, Zhen Zhang, Jian-Heng Ye, Si-Shun Yan, Jie-Lian Han, Zhu-Bao Yin,
,‡
,†,§
Yu Lan,* and Da-Gang Yu*
†Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu
6
‡
10064, P. R. China
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, P. R. China
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. China
§
*
S Supporting Information
ABSTRACT: The first catalytic lactonization of unactivated aryl C−H
bonds with CO to afford important phthalides is reported. Notably, this
2
method features high selectivity, excellent functional group tolerance,
smooth scalability, and facile product diversification. DFT calculations
reveal that a novel insertion of two CO into the O−Pd bond of a
2
palladacycle might be the key step, providing great potential and a
different perspective for carbonylation with CO2.
he utilization of CO has been an attractive topic in modern
a new perspective for carbonylation of inert aryl C−H bonds
2
T
chemistry since CO is an inexpensive, abundant, and
with CO .
2
2
renewable C1 feedstock. Although the thermodynamic stability
When we tested the lactonization of aryl C−H bonds in
and kinetic inertness of CO render it challenging to activate and
benzylic alcohols with CO to give important phthalides, which
2
2
utilize in organic synthesis, many efficient transformations have
been achieved. Among them, the direct carboxylation and
are ubiquitous motifs in many natural products and bioactive
1
6
molecules, a number of challenges need to be addressed. First,
7
carbonylation of C−H bonds with CO attract increasing
because the benzylic hydroxyl group has low binding affinity to
2
attention due to the associated high step- and atom-
transition metals, it has been less reported in transition-metal-
2
−5
economies. Many elegant transformations of heteroaryl and
catalyzed C−H activation and has never been utilized as a
4
alkenylC−H bondswithCO have beenreported. Although the
directing group for C−H functionalization with inert CO .
2
2
Kolbe−Schmitt reaction is well-known for the carboxylation of
phenols, the activation and application of more inert simple
aryl C−H bonds in such reactions still lag behind. After
Fujiwara’s pioneering investigation on Pd-catalyzed direct
Second, 1° and 2° alcohols easily undergo β-H elimination, while
3° alcohols can undergo β-C elimination or smooth dehydration
3
a
5
7a,8
to give alkenes. These side reactions could compete with the
desired lactonization. Third, the reaction conditions for C−H
5a
activation and CO activation may not be compatible because
carboxylation of arenes, Iwasawa and co-workers made a
2
the reaction conditions for C−H activation are mainly acidic and
breakthrough in this field to realize the novel Rh(I)-catalyzed
5d,e,h
oxidative, while those for CO activation are mostly basic and
carboxylation of arenes with CO2.
We herein report the first
2
reductive.
catalytic lactonization of unactivated aryl C−H bonds with CO
2
With these challenges in mind, we first focused on Pd(II)
to afford important phthalides (Scheme 1). Notably, the DFT
calculations indicate that lactonization might involve insertion of
catalysis due to its power in C−H activation and CO
2
4g,5i
transformation.
We hypothesized that the generation of
two CO into the O−Pd bond of the palladacycle, which is
2
alkoxide in the presence of strong base might enhance the
directing ability of the alcohol for the C−H activation. The
upcoming challenge of easy reduction of Pd(II) to inactive
Pd(0) complex in basic conditions might be resolved by adding a
mild oxidant, which is compatible with strong base and could
regenerate active Pd(II) catalyst through oxidation. Moreover,
the generation and equilibrium of buffer solution from strong
significantly different from previous carboxylations and provides
Scheme 1. Catalytic Lactonization of Unactivated Aryl C−H
Bonds with CO
2
base and CO might provide a new solution to resolve the
2
Received: April 29, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01363
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
compatibilityissue. Withthis strategyandhugeefforts(Table S1,
see the Supporting Information (SI) for details), we successfully
realized the Pd(II)-catalyzed lactonization to give desired
product 2a in 79% yield with 4-bromo-N,N-dimethylbenza-
Notably, a gram-scale reaction of 1a was carried out to give 2a
in a similarly high yield, demonstrating the potential applications
(Scheme 2). Furthermore, the reaction of 2a with PhMgBr
afforded alcohol 3 in 96% yield, which can be used as a substrate
in this lactonization to afford 4 in 62% yield (eq 1), providing a
9
10
mide as an oxidant and Cs CO as additive. In contrast to
2
3
4g,m,11
previous Pd-catalyzed carboxylations with CO2,
we found
that the addition of PCy or bipyridine as ligand inhibited the
3
reaction, which might indicate different mechanisms.
With the optimal reaction conditions in hand, we first
investigated the substrates with different substituents on the
benzylic position, giving phthalides (2a−d) in moderate to good
yields (Scheme 2). However, the secondaryandprimary benzylic
new method to form such a spiro lactone. In addition, the
divergent reductions of 2a were achieved by using DIBAL or
a
LiAlH , providing alcohol 5 or 6 in excellent yield, respectively
4
Scheme 2. Substrate Scope
(
see the SI for details). Moreover, 2o has been reported as a key
intermediate for preparing important compounds, such as
12a
talpram 7 (a norepinephrine transporter inhibitor), dimethy-
1
2b
12c
lanthrone 8, benzoxepin derivative 9, and 4-isochroma-
12d
none 10 (see the SI for details).
To gain more insight into the lactonization reaction, various
control experiments were conducted (Scheme 3, see the SI for
Scheme 3. Control Experiments
a
b
c
0
.2 mmol scale. Isolated yields are shown. 8 mmol scale. 4-Bromo-
d e f
N,N-dimethylbenzamide (1.2 equiv). Without Cs CO . 48 h. 4-
Bromo-N,N-dimethylbenzamide (3 equiv). Pd(TFA) (20 mol %),
LiO Bu (8 equiv).
2
3
g
2
t
alcohols failed to give the desired products due to easy oxidation
via β-H elimination. Next, we found that carbonylations of
diverse benzylic alcohols bearing electron-withdrawing groups
1
3
details). Compound 2a′ was obtained with 92% C label when
1
3
CO was used, which clearly indicated that the carbonyl was
2
1
8
18
(
EWGs) or electron-donating groups (EDGs) could afford the
from CO (eq 2). Next, when O-labeled substrate 1c′ ( O:
2
18
desired products in 51−91% yields. Besides substrates 1e−n,
simple benzylic alcohols (1o−ab) were also compatible.
Moreover, various functional groups, which may be beneficial
for subsequent transformations, were tolerated under the
reaction conditions, including alkene (2s), ketone (2t), ester
82%) was tested in the reaction, 2c′ was obtained with 80% O
label, which suggested that the alcohol oxygen atom was fully
incorporated into the product (eq 3). Moreover, the C−H
activation step was found to be reversible with H/D exchange
under CO atmosphere (see the SI for details). Although we
2
(
(
2i), fluoro (2l, 2aa), chloro (2k), amine (2w−y), and cyano
failed to isolate a palladacycle in the absence of an external ligand
2j) groups. This reaction was not hampered by ortho
due to the presumed low stability, we prepared palladacycle A
13
substitution (2z−ab). It is worth noting that C−H lactonization
selectively occurred at the less hindered ortho position for meta-
substituted substrates (2u−y). To our delight, benzylic alcohols
bearing disubstitution (1aa, 1ab), naphthalene (1ac) or
heterocycles (1ad−af) were also suitable substrates.
from ortho-iodo benzylic alcohol (eq 4). For the reaction of 1o
with CO in the presence of bipyridine, we detected A by ESI-
2
HRMS (eq 5). Importantly, both stoichiometric and catalytic
reactions of A and CO afforded desired products (eqs 6 and 7),
2
suggesting the palladacycle is a competent intermediate.
B
DOI: 10.1021/acs.orglett.8b01363
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
On the basis of preliminary mechanistic studies and previous
reports, a plausible mechanism is proposed to account for this
only 12.5 kcal/mol, leads to the generation of hemicarbonate
7
palladium intermediate 18 reversibly. The second CO insertion
2
transformation (Figure 1). Furthermore, density functional
into O−Pd bond could form a nine-membered palladacycle
intermediate 20. Then the following intramolecular nucleophilic
addition takes place via transition state 21-ts to generate a spiro-
heterocyclic intermediate 22. The free energy barrier of this
process is only 22.4 kcal/mol due to the flexible conformation of
transition state 21-ts, which is much lower than that of the
corresponding process via transition state 25-ts occurred from
intermediate 18. Further structure analysis illustrates that
transition state 21-ts performs a six-membered-cyclic structure,
while a four-membered-cyclic structure is presented in transition
state 25-ts (seeFigure S4afor details). Itclearlyindicatesthatthe
insertion of another molecular CO results in the decrease of ring
2
strain in the transition state of intramolecular nucleophilic
addition step. Meanwhile, the electrostatic potential surface
calculation suggests that the negative charge is well distributed in
transition state 25-ts, owing to the oxygen atom on reacting
carbonyl moiety is adjacent to Pd center in transition state 21-ts
(
see in Figure S4b for details). It indicates that the intra-
molecular nucleophilic addition of nine-membered palladacycle
intermediate 20 would deserve a more subdued free energy
barrier. Subsequently, the decarbonization releases lactone
product 2o and concomitantly generates carbonate palladium
intermediate 24, in which the active catalyst 11 would be
regenerated after an acid−base exchange.
Moreover, starting from the palladacycle intermediate 16, the
CO insertion into C−Pd bond was also considered. The
2
Figure 1. Proposed mechanism. The values in parentheses represent the
relative free energy of each intermediate and transition state. The values
are given in kcal/mol and represent the relative free energies calculated
using the M06-L method in DMF solvent.
nucleophilic addition with phenyl to CO via transition state 28-
2
ts leads to the generation of palladium benzolate intermediate
2
9. The relative free energy of transition state 28-ts is 28.7 kcal/
mol higher than that of 17-ts, which indicates that the alcoholate
moiety exhibits more enhanced nucleophilicity and CO2
insertion into O−Pd bond is much more favored (see Figure
S3 for details).
1
4
theory (DFT) M06-L was employed to investigate the
1
5
t
mechanism. The proton exchange between solvated LiO Bu
and reactant 1o could afford the alcoholate−Li intermediate 12
with a free energy decrease of 3.8 kcal/mol (see Figure S1 for
In summary, we have developed the first catalyticlactonization
of simple and inert aryl C−H bonds with CO to directly
2
t
construct important phthalides. This reaction features high
selectivity, good functional group tolerance and smooth
scalability. Different from previous carboxylations, this trans-
formation might undergo the novel pathway involving C−H
details). It indicates that LiO Bu is qualified for the activation of
benzylic alcohol reactants. Moreover, the combination of BuO
t
−
ion and CO leads to the formation of hemicarbonate ion, which
2
is −12.9 kcal/mol exergonic (see Figure S1 for details). This
t
bond activation and two sequential nucleophilic CO insertion
energy discrepancy suggests that the LiO Bu would be
2
into O−Pd bond, providing great potential and a different
continuously consumed by CO , thus demonstrating that a
2
t
perspective for carbonylation with CO .
high equivalent of LiO Bu is necessary in this system to keep
enough concentration of LiO Bu to deprotonate benzylic
2
t
alcohol. As shown in Figure 1, active catalytic complex
hemicarbonate−Pd 11 could be generated after ligand exchange,
which is considered as a starting complex (see Figure S1 for
details). Starting from 11, the counteranion exchange with 12
affords lithium cation 13 and alcoholate−Pd intermediate 14,
with a free energy decrease of 16.4 kcal/mol. Following a
concerted-metalation−deprotonation (CMD) process via tran-
sition state 15-ts leads to the palladacycle intermediate 16 (see
Figure S2 for details). The activation free energy of this step was
determined to be 19.9 kcal/mol. Such a moderate activation
energy barrier suggests that the C−H bond cleavage would be
considered as a rapid process ahead the rate-determining step,
which is consistent with the experimentally observed KIE values
of 2.13 and 1.86 (see the SI for details).
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b01363.
Experimental details, spectroscopic data, and copies of
NMR spectra for all products (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: lanyu@cqu.edu.cn.
*E-mail: dgyu@scu.edu.cn.
When the palladacycle intermediate 16 is formed, subsequent
ORCID
CO insertion could occur into either an O−Pd bond or C−Pd
2
(
see Figure S3 for details). The CO insertion into the O−Pd
Yu Lan: 0000-0002-2328-0020
Da-Gang Yu: 0000-0001-5888-1494
2
bond through transition state 17-ts, with a free energy barrier of
C
DOI: 10.1021/acs.orglett.8b01363
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
4181. (o) Zhang, Z.; Ju, T.; Miao, M.; Han, J.-L.; Zhang, Y.-H.; Zhu, X.-
Y.; Ye, J.-H.; Yu, D.-G.; Zhi, Y.-G. Org. Lett. 2017, 19, 396. (p) Park, D.-
A.; Ryu, J. Y.; Lee, J.; Hong, S. RSC Adv. 2017, 7, 52496. (q) Zhang, Z.;
Zhu, C.-J.; Miao, M.; Han, J.-L.; Ju, T.; Song, L.; Ye, J.-H.; Li, J.; Yu, D.-
G. Chin. J. Chem. 2018, 36, 430. (r) Li, Y.; Yan, T.; Junge, K.; Beller, M.
Angew. Chem., Int. Ed. 2014, 53, 10476.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Jason J. Chruma (Sichuan University) for
manuscript revision. Financial support was provided by the
National Natural Science Foundation of China (21502124 and
(
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