Dual Gold-Catalyzed Three-Component Reaction: Efficient Synthesis of Indene-Fused Esters, Acids, and Lactones through Gold Vinylidene Intermediates

Article abstract of DOI:10.1002/ejoc.201700084

A dual gold(I)-catalyzed three-component reaction was developed to prepare indene-fused carboxylic acid derivatives from diynes, alcohols, and pyridine N-oxides in both inter- and intramolecular fashions. The pyridine N-oxides were found to exhibit distinct selectivity unlike the α-oxo gold carbene intermediates in the well-developed gold-catalyzed oxidative functionalization of alkynes. Experimental studies and DFT calculations support double nucleophilic substitution of a gold vinylidene intermediate.

Full text of DOI:10.1002/ejoc.201700084

DOI: 10.1002/ejoc.201700084
Communication
Gold Catalysis
Dual Gold-Catalyzed Three-Component Reaction: Efficient
Synthesis of Indene-Fused Esters, Acids, and Lactones through
Gold Vinylidene Intermediates
[
a]
[a]
[a]
[c]
[b]
Congjun Yu, Xiaodong Ma, Bin Chen, Bencan Tang, Robert S. Paton,* and
Guozhu Zhang*[
a]
Abstract: A dual gold(I)-catalyzed three-component reaction termediates in the well-developed gold-catalyzed oxidative
was developed to prepare indene-fused carboxylic acid deriva- functionalization of alkynes. Experimental studies and DFT cal-
tives from diynes, alcohols, and pyridine N-oxides in both inter- culations support double nucleophilic substitution of a gold
and intramolecular fashions. The pyridine N-oxides were found vinylidene intermediate.
to exhibit distinct selectivity unlike the α-oxo gold carbene in-
Introduction
unique oxygen-transferring agent to gold vinylidenes to give
highly functionalized indene-fused esters, acids, and lactones.
Temporary mechanistic studies and theoretical calculations sug-
gest ketene formation is a viable reaction pathway [Equa-
tion (4)].
Multicomponent transformations have emerged as powerful
tools in organic synthesis owing to their innate simplicity, effi-
ciency, and elegance. Among these transformations, transi-
tion-metal-catalyzed three-component reactions have attracted
[
1]
[
2]
broad research interest, as they show unique reactivities.
Clearly, the development of novel multicomponent reactions
for the efficient synthesis of synthetically useful molecular scaf-
folds remains in high demand.
(
1)
The past decade has seen rapid developments in the area
[
3]
of homogeneous gold catalysis. In several early studies, gold
[
4–8]
vinylidenes were proposed as reactive intermediates
that
underwent facile C–H or Nu–H insertion [Equation (1)]. Only a
few reports appeared in which a gold vinylidene was trapped
(2)
[
9]
[10]
with one nucleophile such as water or a methoxy group
Equation (2)]. In line with our interests in exploring new react-
ivities of Au vinylidenes, we continue to be inspired by other
[
[
11]
metal vinylidene complexes such as Ru, Rh, and W.
Kim and Lee reported an elegant pyridine N-oxide mediated
oxidation of a rhodium vinylidene to a ketene,
In 2013,
[
12]
(3)
(4)
which led to
a variety of carboxylic acid derivatives [Equation (3)]. We won-
dered if a similar strategy could be applied to gold vinylidenes.
Herein, we report that pyridine N-oxide indeed serves as a
[
a] State Key Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences,
3
45 Lingling Road, Shanghai 200032, P. R. China
E-mail: guozhuzhang@sioc.ac.cn
[
[
b] Chemistry Research Laboratory, University of Oxford,
Results and Discussion
1
2 Mansfield Road, Oxford OX1 3TA, UK
E-mail: robert.paton@chem.ox.ac.uk
Our study began with evaluating the feasibility of a reaction
to induce ethanol addition to a dialkyne with a commercially
available pyridine N-oxide as the oxygen-transferring agent in
c] Department of Chemical and Environmental Engineering,
Faculty of Science and Engineering, University of Nottingham Ningbo
China,
the presence of IPrAuNTf [IPr = 1,3-bis(diisopropylphenyl)imid-
2
Ningbo 315100, China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201700084.
azol-2-ylidene, Tf = trifluoromethylsulfonyl]. To our delight, the
desired indene-fused ester product was isolated. After numer-
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Communication
ous trials, the optimized reaction conditions were found to in- products 2k and 2l). A synthetically useful acetone functionality
clude IPrAuNTf2 (5 mmol-%), 3,5-dichloropyridine N-oxide was also successfully engaged in the addition reaction (see
(
1.5 equiv.), and 1,2-dichloroethane (DCE)/ethanol (1:1, 0.03
M
)
product 2m). Variations on the diyne aromatic backbone were
also briefly investigated. Substituents such as Me, Cl, and OMe
on the benzene ring were well tolerated (see products 2n–p).
at 70 °C to give 2a in 88 % yield [Equation (5)].
1
Upon changing the methyl group on the alkyne (i.e., R ) to
another group such as a bromo or cyclopropyl group, the
reactions also proceeded well (see products 2q and 2r). To
broaden the universality of this reaction, many other nucleo-
(
5) philes were explored. We were pleased to find that water and
ethyl acetoacetate were also reactive nucleophiles, and the lat-
ter gave 2v in vinyl ester form. However, amino alcohols and
thiols were not effective nucleophiles under the current reac-
tion conditions.
With the optimal reaction conditions in hand, the substrate
scope was promptly investigated by examining various alkynes
and alcohols nucleophiles. As summarized in Table 1, almost all
natures of aliphatic alcohols, including primary, secondary, and
tertiary alcohols, were good substrates for the three-compo-
nent oxygenative cyclization reactions (see products 2a–h). The
reaction also tolerated benzylic and allylic alcohols (see prod-
ucts 2i and 2j). Pharmaceutically interesting monofluoro and
trifluoromethyl groups did not interfere with the reaction (see
Lactones are a diverse group of natural products with a wide
range of biological activities. We envisioned that an intramolec-
ular version of our protocol might offer an approach to the
lactone scaffold. We first subjected 3a′ to the optimized condi-
[5a]
tions; however, a known furan was isolated, presumably as a
result of downstream aromatization once the gold vinylidene
was trapped by the secondary alcohol [Equation (6)].
Table 1. Intermolecular reactions of gold vinylidenes.[a]
(
6)
To inhibit the aromatization, tertiary alcohol 3a was pre-
pared, and it gave the indene-fused lactone product under the
established intermolecular reaction conditions. Optimization of
the conditions finally led to product 4a in 82 % yield (Table 2).
The scope and limitations of this intramolecular oxygenative
Table 2. Intramolecular reactions of gold vinylidenes.[a]
[
a] Yields of the isolated products are given. [b] About 10 % of the diyne was
recovered.
[a] Yields of the isolated products are given.
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Communication
lactonization were subsequently investigated (Table 2). Tertiary
alcohols with different ring sizes and skeletons reacted well (see
products 4b–d). The benzene backbone could be substituted
with CF , Me, and Cl groups (see products 4e–g). To our delight,
3
noncyclized tertiary alcohols were successfully engaged in the
oxidative lactonization (see products 4h–k). The enhanced syn-
thetic utility of this methodology was demonstrated in the
preparation of seven-membered lactones, which represent
another important molecular motif (see products 4l–n). Unfor-
tunately, our attempts to generate six-membered or even larger
macrolactones were not successful.
Next, we turned our attention to mechanistic studies. Several
control experiments were performed. Upon using deuterated
ethanol (EtOD) as a cosolvent, the methylene group of the
product was deuterated by 95 % [Equation (7)], which sug-
gested a dual gold catalysis nature. Vinyl ethyl ether 6 was iso-
lated from the reaction mixture in the absence of a pyridine N-
oxide, and no desired ester was observed from the reaction of
6
[Equation (8)]. This result indicated that oxygen transfer from
the pyridine N-oxide might have occurred first.
Scheme 1. Proposed mechanism for the dual gold-catalyzed three-compo-
nent reaction.
(
(
7)
8)
[
20]
pre- and postreaction complexes.
Computed structures are
[
21]
displayed with CYLview.
We started the calculations from gold vinylidene intermedi-
The activation barrier for nucleophilic attack by
the pyridine N-oxide was calculated to be 9.9 kcal mol (see
Figure 1, TS1). This is comparable to the reported intramolec-
ate II.[
5a,5f,18]
–1
3
ular C(sp )–H insertion reaction in II, for which the calculated
free energy barrier was 9.2 kcal mol .
–1 [5a,5f]
The reaction was
Therefore, a tentative mechanism is proposed (Scheme 1). also investigated with vinylidene intermediate analogues II_a
The generation of gold–vinylidene intermediate II initiates the and II_b (Figure 1), and the activation barriers were found to
–1
reaction cascade. Intermediate II can be trapped with a pyridine be 7.8 and 7.7 kcal mol , respectively, which are lower than
N-oxide as a nucleophile; this is followed by leaving of the that for II itself. In transition state TS1, the bond length for the
pyridine, which leads to species IV. Then, formation of key newly formed C–O interaction is 2.63 Å. The C=C=Au angle is
[
11i,12]
ketene V
and nucleophilic addition of the alcohol and de- 167.3°, which bends by 12.7° from the linear structure in vinyl-
metalation provides 2a.
idene II. This leads to the formation of intermediate III, which
To gain further insight and to support our outlined mecha- then undergoes decomposition to form intermediate IV with
nism, we performed DFT calculations to investigate the ener- a barrier of 8.5 kcal mol– (see TS2). In intermediate IV, Au is
getics of the proposed mechanism; geometry optimizations complexed to the C1 carbon center directly, the C1=C2 bond
1
[
13]
and transitions-state searches were performed at the B3LYP
length is 1.36 Å, the C2=O bond length is 1.15 Å, and the C1–
level with the 6-31+G(d) basis set for the C, H, N, O, and Cl Au bond length is 2.21 Å (Figure 1, IV). Moreover, the C2–C1–
[
14]
atoms and with LANL2DZ
with relativistic effective core po- Au bond angle is 98.8°, and the O=C=C bond is out of the
tentials for gold, as B3LYP was recently shown to give reason- phenyl cyclopentene plane by 47.6°. The bond lengths of the
[
22]
able energy and geometry predictions for homogeneous gold O=C=C moiety are similar to those for a ketene structure;
[
15]
catalysis; these basis sets were recently proven to be suitable therefore, we believe cleavage of the O–N bond in III can lead
for geometry predictions of similar systems involving gold to the formation of ketene IV directly. The free energy of IV is
atoms. Single-point calculations were performed at the M06 much lower (by 72 kcal mol ) than that of III, which makes this
[
16]
–1
[
17]
level with a mixed basis set of SDD for gold and 6-311+G(d,p) a very favorable process. The Au complex in IV was found to
for all other atoms, as this was proven to provide reliable energy be able to migrate to the benzene ring through C3 and C4 to
[
18]
evaluation for similar gold catalysis. Solvation energy correc- form V directly. This process happens through transition TS3,
–1
tions were calculated by using the SMD model and with DCE with a very low activation barrier of 5.8 kcal mol . This serves
as the solvent. All calculations were performed with the Gaus- as one of the intermediate structures for gold slippage in the π
[
19]
sian 09 program package.
All transition states were con- system, and it can be transformed into gem-diaurated species
firmed by intrinsic reaction coordinate (IRC) toward a set of VI (Scheme 1) directly or after nucleophilic attack by the
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Figure 1. Computed free-energy profile for the reaction of 3,5-dichloropyridine N-oxide with a gold vinylidene.
alcohol, which, followed by gold and hydrogen exchange with were added. The mixture was stirred at 70 °C until complete conver-
sion was detected by TLC (3 h). Then, the mixture was concentrated,
1
a, completes the catalytic cycle.
and the residue was purified by flash column chromatography (hex-
ane/ethyl acetate = 20:1) to afford 2.
Conclusions
Acknowledgments
In summary, we were able to realize a dual gold-catalyzed
three-component coupling by relying on new reactivity of a
gold vinylidene intermediate. A pyridine N-oxide served as a
unique oxygen-transfer agent, and it exhibited distinct selectiv-
ity unlike α-oxo gold carbenes. The oxygenative cyclized 1,1-
difunctionalization reaction readily took place in both inter- and
intramolecular settings, which led to a variety of indene-fused
esters, acids, and lactones. Further investigations concerning
gold vinylidene intermediates are currently underway in our
laboratory.
We are grateful to the National Natural Science Foundation of
China (NSFC) (21421091) and the “Thousand Plan” Youth Pro-
gram, the State Key Laboratory of Organometallic Chemistry,
the Shanghai Institute of Organic Chemistry, and the Chinese
Academy of Sciences. We acknowledge the use of the EPSRC
UK National Service for Computational Chemistry Software
(CHEM773) while performing this work. We are also grateful
for access to the University of Nottingham High Performance
Computing Facility.
Keywords: Esters · Gold · Lactones · Multicomponent
reactions · Vinylidenes
Experimental Section
General Procedure for the Synthesis of Compound 2: In a flask,
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IPrAuNTf (5 mmol-%) and 3,5-dicholoropyidine N-oxide (1.5 equiv.)
2
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Received: December 20, 2016
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