Pd(0)-catalyzed chemoselective construction of spirocarbocycles via an alkyne insertion/β-naphthol dearomatization cascade

Article abstract of DOI:10.1039/c4cc09099c

A microwave-assisted Pd(0)-catalyzed alkyne migratory insertion/β-naphthol dearomatization cascade process has been accomplished to access a variety of spirocyclic compounds bearing all-carbon quaternary stereogenic centers in high yields with excellent chemoselectivities.

Full text of DOI:10.1039/c4cc09099c

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Pd(0)-catalyzed chemoselective construction of
spirocarbocycles via an alkyne insertion/β-naphthol
dearomatization cascade†
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Huayu Zheng,‡ Lu Bai,‡ Jingjing Liu, Jiang Nan, Zhijun Zuo, Liu Yang, Yaoyu
Wang, Xinjun Luan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
unpredictable, as one potential side reaction could occur very easily
to form dibenzofuran 4.12 The challenge, therefore, is to find suitable
conditions under which the favorable cyclization between
palladacycle A and alkyne 2 proceeds much faster than the possible
direct reductive elimination of A, thus providing spirocyclic 3 as the
desired product. Moreover, the synthetic difficulty for accessing the
spirocyclic framework in 3 was further increased due to the
prerequisite dearomatization of the naphthyl ring. Remarkably,
several elegant protocols of Pd(0)-catalyzed intramolecular
dearomatization of para-substituted phenols or naphthols have been
reported during the past few years.13 However, the dearomatization
of ortho-substituted phenol or naphthol derivatives under these
A
microwave-assisted Pd(0)-catalyzed alkyne migratory
insertion/β-naphthol dearomatization cascade process has
been accomplished to access variety of spirocyclic
a
compounds bearing all-carbon quaternary stereogenic
centers in high yields with excellent chemoselectivities.
Transition metal-catalyzed annulation of alkynes with readily
available coupling partners has proven to be an extremely powerful
strategy to access a diverse range of heterocycles and carbocycles.1
Indeed, a great number of transition-metal-catalyzed transformations
employing this approach have enabled highly efficient construction
of indoles,2 pyrroles,3 benzofurans,4 isoquinolines,5 indenes,6
polycyclic aromatic hydrocarbons,7 and so on,8 in a single synthetic
operation. In contrast, the development of corresponding processes
for the preparation of spirocarbocyclic compounds that are
inaccessible by conventional cycloaddition methods has been
dramatically limited.9 In this context, we recently disclosed a Ru(II)-
catalyzed oxidative annulation of alkynes with 1-aryl-2-naphthols to
reaction conditions remains
a formidable challenge. To our
knowledge, there is only one example of Pd(0)-catalyzed
transformation in this arena, which was established by You and
coworkers using an asymmetric allylic alkylation strategy.14 In this
paper, we describe the successful execution of our design, which was
realized by Pd(0)-catalyzed sequential migratory insertion of the
alkyne and dearomatization of α-substituted β-naphthol.
generate
a new class of spirocyclic molecules bearing the
spiro[indene-1,1'-naphthalen] skeleton.10 Notably, this intriguing
structural moiety is often present in natural products and functional
materials (Fig. 1).11
Scheme
1 Proposed Pd(0)-catalyzed one-step construction of
spirocarbocycles 3 and potential challenge
Fig. 1 Selected natural products and functional materials containing
spiro[indene-1,1'-naphthalene] core structures.
The investigation commenced by using 2-naphthol 1a and 1,2-
diphenylethyne 2a as the model substrates to optimize the reaction
conditions. The experimental data are summarized in Table 1. First,
with the catalytic system consisting of Pd(OAc)2 (5 mol%), PPh3 (12
mol%), and K2CO3 (1.0 equiv), different solvents were briefly
evaluated (entries 1-5). Satisfactorily, the anticipated spirocyclic
product 3a could be obtained in 79% NMR yield in DMF, albeit
with the concomitant generation of 14% unwanted byproduct 4a. To
Herein we describe the development of a non-oxidative version of
our Ru(II)-catalyzed reaction, which was conceived to be executed
by Pd(0) catalysis. This work was launched by identifying substrate
1 as the coupling partner that might undergo cyclization with alkynes
involving
A as one key intermediate. At the outset, the
chemoselectivity for the prospective transformation was quite
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prevent this side reaction, other palladium sources were examined, an electron-donating group (EDG) such as a methoxy group (3d),
but none of them could enhance the chemoselectivity for the desired and electron-withdrawing groups (EWGs) such as trifluoromethyl
transformation (entries 6-8). K2CO3 was then changed to other bases (3e), chloro (3f), fluoro (3g-h), and ester (3i) groups. Moreover, it
such as Na2CO3, Cs2CO3, KOAc, DBU and Et3N. The catalytic should be noted that the chlorine-containing substrate 1f underwent
results indicated that inorganic bases (Na2CO3, Cs2CO3 and KOAc) envisioned annulation without decomposition to afford compound 3f,
behaved better than organic ones (DBU and Et3N), but they couldn’t in which the variable chlorine group could serve as a highly valuable
give superior results in comparison to K2CO3 (entries 9-13). synthetic handle for further functionalization. Accordingly, the more
Subsequently, various phosphine ligands and one NHC ligand were active aryl iodides often react preferentially in Pd(0)-catalyzed O-
screened with the combination of Pd(OAc)2 (5 mol%) and K2CO3 arylation of phenol derivatives.15 Under the original reaction
(1.0 equiv.) in DMF at 130 ºC for 48 h (entries 14-20). Notably, IPr, conditions, iodo-substrates 1a' and 1b' mainly occurred via an
Xantphos, dppe and dppp favored the formation of the desired intramolecular cyclization pathway to give dibenzofurans 4a and 4b
product 3a, while PCy3, dppf and dppm provided compound 4a as as the major products respectively. To overcome this limitation, one
n
the major product. Based on these observations, it seemed that both equivalent of Bu4NCl, which was used in the Larock’s protocol,2b
steric and electronic properties of the ligands are critical for was quickly recognized as the additive, and the desired products 3a
achieving the title transformation. Overall, dppp was found to be the and 3b were then obtained in high yields (92% and 80%), which are
better ligand, and was able to prevent the formation of byproduct 4a, comparable to the reaction performance of their bromo-counterparts
affording compound 3a in quantitative yield (entry 20). Finally, to (94% and 90%). In sharp contrast to our prior Ru(II)-catalyzed
avoid the high energy cost, the harsh thermal condition (130 ºC, 48 h) oxidative approach,10 this new Pd(0)-catalyzed transformation has
was successfully switched to a microwave irradiation (135 ºC, 15 enabled a great example for expanding the naphthol scope. When
min), which was then identified as the optimal energy parameter to substrate 1j was used, the synthetically challenging compound 3j,
evaluate the reaction scope (entry 21).
which was not able to be accessed via Ru(II)-method,10 was
successfully prepared in 70% yield. In the end, the phenol substrates
(1k-l) were also attempted for the desired transformation, and one
(1k) was totally ineffective, while another one (1k) was able to give
an encouraging yield (8%) of the anticipated product 3l.
Table 1 Optimization of the reaction conditionsa
Table 2 Survey of the scope of the naphthol coupling partner
T
conv. (%)
entry
[Pd]
Ligand
base
solvent
(ºC ) (3a/4a)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Pd(OAc)2
Pd(OAc)2
PPh3
PPh3
K2CO3 THF
K2CO3 MeCN 80
K2CO3 DME
K2CO3 toluene 100 33 (26:7)
60
3 (0:3)
5 (0:5)
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(dba)2
PdCl2
PPh3
PPh3
PPh3
PPh3
PPh3
100 19 (13:6)
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
Na2CO3 DMF
Cs2CO3 DMF
KOAc DMF
130 93 (79:14)
130 93 (49:44)
130 30 (21:9)
130 60 (45:15)
130 83 (66:17)
130 94 (64:30)
130 85 (68:17)
130 90 (45:45)
130 49 (33:16)
130 92 (17:75)
130 85 (68:17)
130 90 (68:22)
130 85 (29:56)
130 88 (15:73)
130 89 (78:11)
130 99 (99:0)
135 94e (97:2)
[Pd(allyl)Cl]2 PPh3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PPh3
PPh3
PPh3
PPh3
PPh3
PCy3
IPr
DBU
Et3N
DMF
DMF
K2CO3 DMF
K2CO3 DMF
15c Pd(OAc)2
16c Pd(OAc)2
17c Pd(OAc)2
18c Pd(OAc)2
19c Pd(OAc)2
20c Pd(OAc)2
21c,d Pd(OAc)2
xantphos K2CO3 DMF
dppf
dppm
dppe
dppp
dppp
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
a0.2 mmol of 1a, 0.3 mmol of 2a, 5.0 mol% of [Pd], 12.0 mol% of PPh3, and
0.2 mmol of base. bDetermined by 1H NMR analysis. c6 mol% of ligand was
used to replace PPh3. dMicrowave, 15 min. eIsolated yield.
a1.0 equiv of nBu4NCl was added. b2.0 equiv of K2CO3 was used. c3.0 equiv
of 2a was used.
Under the optimal reaction conditions, we first examined the
reaction scope with regard to the naphthol coupling partner. As
shown in Table 2, a variety of naphthol derivatives (1a-j) reacted
smoothly with 2a to generate a broad range of spirocyclic
compounds 3a-j in high yields (70-94%) with excellent
chemoselectivities. Satisfactorily, various substituents on the 4- or 5-
position of the phenyl ring were well tolerated in the reaction,
including an electron-neutral group such as a methyl group (3b-c),
Next, we evaluated the reaction performance of a variety of
alkynes bearing different substituents by using this new protocol. As
shown in Table 3, the Pd(0)-catalyzed alkyne insertion/β-naphthol
dearomatization tandem process proceeded smoothly with various
symmetrical alkynes. Annulation of 2-naphthol 1a with 4-
methylphenyl-substituted alkyne 2b or electron-rich 4-
methoxyphenyl-substituted alkyne 2c produced the spirocyclic 3b'
and 3c' in 93% and 80% yields, respectively. Similarly, the electron-
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poor alkynes that are substituted with EWGs such as trifluoromethyl
Based on the catalytic cycles put forth for previous related
(2d), chloro (2e), and fluoro (2f and 2g) groups at the p-, m-, or o- processes,2b,6a,17-18 a plausible mechanism was proposed for this
position of the aromatic ring underwent effective annulations with unprecedented transformation (Scheme 2). Initial oxidative addition
substrate 1a (87-94% yields for 3d'-g'). Notably, the potentially of aryl bromide to zerovalent palladium occurs to afford
removable chlorine substituent in 2e remained intact under the Pd(0) arylpalladium bromide, followed by deprotonation of the acidic
catalysis conditions. Moreover, heterocyclic 2h could adapt to this proton of 2-naphthol by potassium carbonate, thus enabling the
reaction as well, resulting in 3h' in 91% yield. Due to the lack of intramolecular cyclization to access a six-membered palladacycle A.
conjugation, the dialkyl-substituted 2i reacted sluggishly under the Subsequently, coordination and migratory insertion of the alkyne 2a
optimized conditions. However, the successful preparation of 3i' in with intermediate A to form an eight-membered palladacycle B,
good yield (84%) was simply managed by using three equivalents of which then undergoes a ring contraction to give a spirocyclic
2i. Meanwhile, several unsymmetrical alkynes 2j-m were also intermediate C by release the ring strain and steric repulsion of
employed to study the regioselectivity for this new method.16 The intermediate B. Finally, reductive elimination of the spirocyclic
experimental results indicated that the insertion of alkyne 2j with intermediate C takes place to provide spirocyclic compound 3a and
two sterically similar but electronically differential substituents was concomitantly regenerate the Pd(0) catalyst to complete the catalytic
not selective, while the reactions involving alkyl-aryl mixed alkynes cycle. It should be mentioned that the direct reductive elimination of
2k-m could be obtained with moderate regioselectivities (3:1) by intermediate A to generate dibenzofuran 4a, which is a known
giving the same major regioisomer as the prior report.10
pathway in the literature,13 was also observed during our
optimization process. However, careful control of the reaction
conditions could prevent its concomitant formation. Additionally,
the possible seven-membered oxepine byproduct was never observed
in this study, we assumed that the steric repulsion between the
phenyl and naphthyl rings of intermediate B played an important role
on preventing the direct reductive elimination of B.18
Table 3 Survey of the scope of the alkynes coupling partner
Scheme 3 Proposed mechanism
Conclusions
In summary, we have developed
a novel Pd(0)-catalyzed
a3.0 equiv. of alkyne was used.
vinylative dearomatization reaction of β-naphthols with internal
alkynes to access a class of valuable spirocyclic compounds. This
new approach was successfully implemented by avoiding
Notably, the above reactions could also work quite well under the
thermal conditions (see: Table 1, entry 20), and the experimental
results are given in the ESI (Tables S1 and S2).
stoichiometric Cu(OAc)2 oxidant, and it represents
a highly
competitive alternative to the previous Ru(II)-catalyzed oxidative
approach. More importantly, this Pd(0)-catalyzed transformation has
gained significant progress for expanding the naphthol scope in
comparison to the Ru(II)-method.
To evaluate the practicality of this microwave assisted method, a
large-scale experiment was conducted with 1a and 2a, and the
desired product 3a (0.91 g) was obtained in 92% yield (Scheme 2).
This work was supported by the National Science Foundation of
China (21222201, 21271147, 51202192), the “1000 Youth Talents
Plan”, and the Education Department of Shaanxi Provincial
Government (2013JQ2024).
Scheme 2 Gram-scale preparation of 3a
Notes and references
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Key Laboratory of Synthetic and Natural Functional Molecular
Chemistry of the Ministry of Education, College of Chemistry &
Materials Science, Northwest University, Xi’an 710069, China.
E-mail: xluan@nwu.edu.cn
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† Electronic Supplementary Information (ESI) available: Experimental
details and spectroscopic data. See DOI: 10.1039/c000000x/
‡
H. Zheng and L. Bai contributed equally to this work.
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