Rhodium-catalyzed carbonylative synthesis of silyl-substituted indenones

Article abstract of DOI:10.1039/c7cc08210j

A novel and efficient rhodium-catalyzed procedure for the preparation of silyl-substituted indenones has been developed. Using silanes and internal alkynes as the substrates, in the presence of CO, good to excellent yields of the desired indenones were isolated. A wide range of functional groups, encompassing esters, amines, nitriles and halides, is compatible in this system.

Full text of DOI:10.1039/c7cc08210j

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Rhodium-Catalyzed Carbonylative Synthesis of Silyl-Substituted
Indenones
Fengxiang Zhu, Anke Spannenberg, and Xiao-Feng Wu*
Leibniz-Institut für Katalyse e.V. an der Universität Rostock. Albert-Einstein-Straße 29a, 18059 Rostock (Germany), E-mail: Xiao-
Feng.Wu@catalysis.de
detected by GC-MS with TEMPO as the oxidant and 1,10-Phen
Abstract: A novel and efficient rhodium-catalyzed procedure for
the preparation of silyl-substituted indenones has been
developed. Using silanes and internal alkynes as the substrates,
in the presence of CO, good to excellent yields of the desired
indenones were isolated. A Wide range of functional groups,
encompassing esters, amines, nitriles and halides, are
compatible in this system.
as the ligand (Table 1, entry 8).
Since silicon atoms can dramatically change the polarity
and conformational characteristics of an organic compound;[1]
organosilicon compounds have attracted more and more
attention from scientific researchers in recent years. They
occupy a prominent position in the field of organic chemistry,
materials, fragrances, and pharmaceuticals.[2,3] With the
increasing interest in organosilicon compounds, several efficient
synthetic methodologies have been reported for their
preparation. For example, the hydrosilylation or dehydrogenative
silylation of unsaturated compounds have emerged as
a
powerful tool to assemble silicon-containing molecules.[4]
However, methods for producing more functionalized silicon-
containing (hetero)arenes are still very limited.
Figure 1. Carbonylative annulation of silanes with alkynes: reaction design.
On
the
other
hand,
transition-metal-catalyzed
By careful analysis and characterizations, we found the
newly formed compound was 3-(dimethyl(phenyl)silyl)-2-phenyl-
1H-inden-1-one 3 and the yield was 5%. Encouraged by this
result, variations of ligands were carried out subsequently (Table
1, entries 9-14). The yield of the target product can be further
improved by varying the nitrogen-containing ligands (Table 1,
carbonylative transformations are a powerful tool in modern
organic synthesis.[5-8] Carbonylative procedures have been
broadly explored and used in both academia and industry.
Interesting compounds including substituted arenes are easily
prepared with high atom economy.
With this in mind, we turned our attention to the
carbonylative annulation of silanes with alkynes (Figure 1). In
the initial step, the silane may be activated by a transition metal
catalyst with relative ease. Then an addition reaction with an
alkyne occurs to give the intermediates a and aa, which
depending on the configurations would lead to different products
(3 and 3’ respectively). Based on this reaction design we carried
out systematic studies to determine which pathway would
proceed. It’s important to mention that carbonylative cyclization
of alkynes to give indanones have been reported. [9] 2,3-Dihydro-
1H-inden-1-ones were prepared by utilizing cobalt or rhodium
catalysts under water-gas shift reaction conditions. In 1997,
t
entries 9-11) and 10% of indenone can be formed with Bubpy
(4,4′-di-tert-butyl-2,2′-dipyridyl) as the ligand (Table 1, entry 10).
However, no reaction occurred when using phosphines as the
ligand (Table 1, entries 12-14). In order to further improve the
results, different silver salts were checked as well (Table 1,
entries 15-18). Finally, 18% of the product can be achieved
when using AgSbF6 as the additive (Table 1, entry 18). The
influences of bases and acids to the reaction mixture were
checked successively (Table 1, entries 19-22). We found that no
effect could be detected with Li2CO3 or Na2CO3 as the base
(Table 1, entries 19-20), but no product could be formed when
Cs2CO3 or HOAc was added to the reaction solution (Table 1,
entries 21-22). To our surprise, the yield can be increased to
68% by applying 4 equivalents of TEMPO (Table 1, entry 23).
Finally, we found [Rh(COD)Cl2]2 can also be applied as the
catalyst precursor (Table 1, entry 24), but not RhCl3 (Table 1,
entry 25).
Matsuda
and
co-workers
studied
rhodium-catalyzed
carbonylation of alkynes and silanes. Under pressurized carbon
monoxide, β-silyl alkenals were produced selectively from both
terminal and internal alkynes.[10]
Initially,
dimethyl(phenyl)silane
(1a)
and
1,2-
diphenylethyne (2a) were chosen as the model substrates using
[Cp*RhCl2]2 as the catalyst and p-xylene as the solvent to
establish this carbonylation procedure (Table 1). The reaction
was totally ineffective when different commonly applied oxidants
such as Cu(OAc)2, AgOAc, BQ, DDQ, K2S2O8, TBHP, DTBP
were employed (Table 1, entries 1-7). To our delight, new
compound with the same molecule weight as 3 or 3’ can be

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DOI: 10.1039/C7CC08210J
Table 1. Optimization of the reaction conditions.[a]
confirmed as well.[11] Additionally, tert-butyldimethylsilane 1l and
triethylsilane 1m as examples of aliphatic silanes have been
applied successfully as well.
Table 2. Silyl-substituted indenones synthesis: Substrate of scope of silanes.[a]
Entry
Oxidant
Ligand
Additive
Yield(%)[b]
1
2
Cu(OAc)2
AgOAc
BQ
Phen
Phen
-
-
-
-
3
Phen
-
-
4
DDQ
Phen
-
-
5
K2S2O8
TBHP
Phen
-
-
6
Phen
-
-
7
DTBP
Phen
-
-
8
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
Phen
-
5
7
10
8
-
9
Bpy
tBubpy
-
10
11
12
13
14
15
16
17
18
19[c]
20[c]
21[c]
22[c]
23[d]
24[e]
25[f]
-
DMB
-
-
Xantphos
DPEphos
DPPF
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
tBubpy
-
-
-
-
AgOAc
5
-
AgOOCCF3
AgBF4
6
18
18
18
-
AgSbF6
AgSbF6/Li2CO3
AgSbF6/Na2CO3
AgSbF6/Cs2CO3
AgSbF6/HOAc
AgSbF6
-
68
64
-
AgSbF6
AgSbF6
[a] 1a (0.3 mmol), 2a (0.2 mmol), [Cp*RhCl2]2 (4 mol%), additive (20 mol%)
oxidant (0.4 mmol), Ligand (15 mol%), 120 oC, p-xylene (2 mL), 24 h, CO (20
bar). [b] GC yields using hexadecane as the internal standard. (c) Li2CO3 or
Na2CO3 or Cs2CO3 or HOAc (1 equiv.). [d] TEMPO (0.8 mmol). [e] TEMPO
(0.8 mmol), [Rh(COD)Cl2]2 (4 mol%). [f] TEMPO (0.8 mmol), RhCl3 (8 mol%).
3ak
Xantphos:
4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene,
DPEphos:
(Oxydi-2,1-phenylene)bis(diphenylphosphine),
DPPF:
1,1′-
[a] Silanes (0.3 mmol), 1,2-diphenylethyne (0.2 mmol), [Cp*RhCl2]2 (4 mol%),
tBubpy (15 mol%), TEMPO (0.8 mmol), AgSbF6 (20 mol%), CO (20 bar), 120
oC, p-xylene (2 mL), 24 h, isolated yield.
Bis(diphenylphosphino)ferrocene.
Subsequently, with methyldiphenylsilane 1n as the model
substrate, different alkynes were tested (Table 3). Several
internal alkynes can be effectively transformed with
methyldiphenylsilane, and afford the corresponding products in
good to excellent yields. It is worth mentioning that 1,2-di-m-
tolylethyne and heterocycle bis(thiophen-2-yl)ethyne can be
applied as well and gave the corresponding silicon-containing
indenone derivatives in good yield and excellent regioselectivity
(Table 3, 3nf and 3nh). In the case of 1-chloro-4-
(phenylethynyl)benzene, a mixture of two isomers were isolated
(Table 3, 3ng). However, to our delight, the other tested
unsymmetrical alkynes gave good yields of the desired products
with total selectivity (Table 3, 3ni-3nl).
With the optimum reaction conditions in hand, we carried
out the generality and limitation testing of this new procedure.
Firstly, a variety of silanes 1 were tested with 1,2-diphenylethyne
2a under our standard conditions (Table 2). Silanes with a wide
range of functional groups, such as ester, amine, nitrile, and so
on were tested and good to excellent yields of the corresponding
products can be achieved without any further optimizations.
Additionally,
X-ray structure analysis
of
2-phenyl-3-
(triphenylsilyl)-1H-inden-1-one 3ak was performed and

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Table 3. Silyl-substituted indenones synthesis: Substrate scope of alkynes.[a]
Scheme 1. Control experiments.
[a] Methyldiphenylsilane (0.3 mmol), alkyne (0.2 mmol), [Cp*RhCl2]2 (4 mol%),
tBubpy (15 mol%), TEMPO (0.8 mmol), AgSbF6 (20 mol%), CO (20 bar), 120
oC, p-xylene (2 mL), 24 h, isolated yield.
In order to get some insight into the reaction mechanism,
control experiments were carried out. In our EPR measurements,
with or without DMPO, no originally designed radicals could be
detected. In the testing with deuterated diphenylacetylene, a 1.6
KIE value was measured with 30% conversion of the alkyne
under our standard conditions (Scheme 1, eq. 1). Then
vinylsilane derivates were prepared and applied as the starting
materials under our conditions, no or low yield were obtained
which exclude the possibility of them acted as reaction
intermediates of this system (Scheme 1, eq. 2 and 3). A
hydridosilylrhodium complex was synthesized and applied as
well (Scheme 1, eq. 4 and 5).12 In these cases, good yields of
the corresponding products can be obtained with the rhodium
complex as intermediate or catalyst.
Scheme 2. Proposed reaction pathway.
Based on all these results, a possible reaction mechanism
is proposed (Scheme 2). The reaction started with the oxidative
addition of the silane to the active rhodium center to give the
corresponding hydridosilylrhodium complex. Then the in situ
formed rhodium complex reacts with alkyne to product the vinyl
rhodium intermediate A. Under the assistant of TEMPO, the C-H
bond of the arene been activated and give the desired five-
membered rhodacycle B.4 After the coordination and insertion of
CO, the corresponding six-membered rhodacycle C will be

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Chojnowski,), Kluwer, Dordrecht, 2000; d) P. M. Zelisko, Bio-Inspired
formed as the key intermediate. The final product 3 can be
eliminated after reductive elimination step and re-produce the
active rhodium complex for the next catalytic cycle.
Silicon-Based Materials, Springer, Dordrecht, 2014.
[3]
[4]
For selected recent reviews, see: a) E. Langkopf, D. Schinzer, Chem.
Rev. 1995, 95, 1375-1408; b) H. F. Sore, W. R. J. D. Galloway, D. R.
Spring, Chem. Soc. Rev. 2012, 41, 1845-1866; c) A. K. Franz, S. O.
Wilson, J. Med. Chem. 2013, 56, 388–405.
In conclusion, a novel carbonylative annulation reaction for
the direct synthesis of silyl-substituted indenones from silanes
and alkynes has been developed. With rhodium as the catalyst
and TEMPO as the oxidant, various desired silicon-containing
indenones were prepared in good to excellent yields and with
good selectivity and functional group tolerance. Remarkably, this
represents the first example on carbonylative annulation of
simple silanes and alkynes.
For selected recent reviews and papers: a) S. Putzien, O. Nuyken, F. E.
Kghn, Prog. Polym. Sci. 2010, 35, 687-713; d) X. Du, Z. Huang, ACS
Catal. 2017, 7, 1227-1243; c) J. Chen, B. Cheng, M. Cao, Z. Lu, Angew.
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H. Neumann, M. Beller, Chem Rev 2013, 113, 1-35; b) X.-F. Wu, H.
Neumann, M. Beller, Chem. Soc. Rev. 2011, 40, 4986-5009.; c) X.-F.
Wu, H. Neumann, ChemCatChem 2012, 4, 447-458; d) Q. Liu, H.
Zhang, A. Lei, Angew. Chem. Int. Ed. 2011, 50, 10788-10799; e) X.-F.
Wu, H. Neumann, M. Beller, ChemSusChem 2013, 6, 229-241; f) C. H.
Schiesser, U. Wille, H. Matsubara, I. Ryu, Acc. Chem. Res. 2007, 40,
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Res. 2014, 47, 1563-1574; h) B. Gabriele, R. Mancuso, G. Salerno, Eur.
J. Org. Chem. 2012, 6825-6839.
Acknowledgements
The authors thank the Chinese Scholarship Council for financial
Support. The analytic supports of Dr. W. Baumann, Dr. C. Fisher,
S. Buchholz, and S. Schareina are gratefully acknowledged. We
also appreciate the general supports from Professor Matthias
Beller in LIKAT.
[6]
[7]
F. Zhu, Z. Wang, Y. Li, X. -F. Wu, Chem. Eur. J. 2017, 23, 3276–3279.
F. Zhu, Y. Li, Z. Wang, X. -F. Wu, Angew. Chem. Int. Ed. 2016, 55,
14151-14154.
[8]
[9]
a) X. Li, X. Li, N. Jiao, J. Am. Chem. Soc. 2015, 137, 9246-9249; b) F.
Zhu, Y. Li, Z. Wang, X. -F. Wu, Adv. Synth. Catal. 2016, 358, 3350-
3354.
General procedure: Under Ar, a 4 mL screw-cap vial was
charged with [Cp*RhCl2]2 (4 mol%), Bubpy (15 mol%), alkyne
t
(0.2 mmol), silane (0.3 mmol), TEMPO (0.8 mmol), AgSF6 (20
mol%), p-xylene (2 mL) and an oven-dried stirring bar. The vial
was closed by Teflon septum and phenolic cap and connected
with atmosphere with a needle. Then the vial was fixed in an
alloy plate and put into Paar 4560 series autoclave (300 mL). At
room temperature, the autoclave is flushed with carbon
monoxide for three times and 20 bar of carbon monoxide was
charged. The autoclave was placed on a heating plate equipped
with magnetic stirring and an aluminum block. The reaction is
a) K. Doyama, K. Fujiwara, T. Joh, K. Maeshima, S. Takahashi, Chem.
Lett. 1988, 17, 901-904; b) T. Joh, K. Doyama, K. Fujiwara, K.
Maeshima, S. Takahashi, Organometallics 1991, 10, 508-513; c) R.
Takeuchi, H. Yasue, J. Org. Chem. 1993, 58, 5386-5392.
[10] I. Matsuda, Y. Fukuta, T. Tsuchihashi, H. Nagashima, K. Itoh,
Organometallics 1997, 16, 4327-4345.
[11] CCDC 1558126 contains the supplementary crystallographic data for
this paper.
[12] K. Osakada, T. Koizumi, T. Yamamoto, Organometallics 1997, 16,
2063-2069.
o
allowed to be heated under 120 C for 24 hours. Afterwards, the
autoclave is cooled to room temperature and the pressure was
carefully released. After removal of solvent under reduced
pressure, pure product was obtained by column chromatography
on silica gel (eluent: pentane/ethyl acetate = 80:1).
A novel and efficient rhodium-catalyzed procedure for the preparation
of silyl-substituted indenones has been developed. With silanes and
internal alkynes as the substrates in the presence of CO, good to
excellent yields of the desired indenones can be isolated.
[1]
[2]
G. A. Showell, J. S. Mills, Drug Discovery Today 2003, 8, 551.
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