Supported gold nanoparticle-catalyzed cis -selective disilylation of terminal alkynes by σ disilanes

Article abstract of DOI:10.1021/ol402935x

Supported gold nanoparticles on metal oxides (1 mol %) catalyze for the first time the cis-selective disilylation of terminal alkynes by 1,2-disilanes in isolated yields up to 94%. It is likely that the reaction proceeds through oxidative insertion of the

Full text of DOI:10.1021/ol402935x

ORGANIC
LETTERS
2013
Vol. 15, No. 23
6038–6041
Supported Gold Nanoparticle-Catalyzed
cis-Selective Disilylation of Terminal
Alkynes by σ Disilanes
Charis Gryparis, Marios Kidonakis, and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes 71003 Iraklion, Greece
stratakis@chemistry.uoc.gr
Received October 11, 2013
ABSTRACT
Supported gold nanoparticles on metal oxides (1 mol %) catalyze for the first time the cis-selective disilylation of terminal alkynes by 1,2-disilanes
in isolated yields up to 94%. It is likely that the reaction proceeds through oxidative insertion of the σ SiÀSi bond of disilanes on gold followed by
1,2-addition to the alkyne.
The isolobal analogy among the couples Au(I/III) and
Pd(0/II) is currently triggering interest of organic chemists
toward unraveling gold-catalyzed versions of CÀC or
CÀheteroatom bond-forming reactions involving 2e redox
catalysis. So far the progress is slow, and a few examples
have been reported regarding the classical cross-coupling
Suzuki1 and Sonogashira2 reactions. A specific case deal-
ing with CÀF activation in perfluoroarenes3 has also been
reported. The majority of those cases involve catalysis by
supported gold nanoparticles.4 From a mechanistic
point of view, the Au-catalyzed Sonogashira reaction is
controversial,5 as the ability for oxidative insertion from
the carbon halide bond to the metal is questioned. There is
debateregarding theoxidation state of gold. Itwas recently
shown that σ disilanes6 or distannanes7 undergo intramo-
lecular oxidative insertion on Au(I) forming bis(silyl)- or
stannylgold(III) complexes. In general, the activation of σ
SiÀSi bonds through oxidative insertion on metals is well es-
tablished under catalysis by Pd, Pt, or Rh.8 The observation
of spontaneous intramolecular oxidative insertion of a
(1) (a) Gonzalez-Arellano, C.; Corma, A.; Iglesias, M.; Sanchez, F.
J. Catal. 2006, 238, 497. (b) Han, J.; Liu, Y.; Guo, R. J. Am. Chem. Soc.
2009, 131, 2060.
(2) (a) Gonzalez-Arellano, C.; Abad, A.; Corma, A.; Garcia, H.;
Iglesias, M.; Sanchez, F. Angew. Chem., Int. Ed. 2007, 46, 1536. (b) Li,
P.; Wang, L.; Wang, M.; You, F. Eur. J. Org. Chem. 2008, 5946. (c) de
Souza, R. O. M. A.; Bittar, M. S.; Mendes, L. V. P.; da Silva, C. M. F.; da
Silva, V. T.; Antunes, O. A. C. Synlett 2008, 1777. (d) Kanuru, V. K.;
Kyriakou, G.; Beaumont, S. K.; Papageorgiou, A. C.; Watson, D. J.;
Lambert, R. M. J. Am. Chem. Soc. 2010, 132, 8081. (e) Beaumont, S. K.;
Kyriakou, G.; Lambert, R. M. J. Am. Chem. Soc. 2010, 132, 12246. (f)
Kyriakou, G.; Beaumont, S. K.; Humphrey, S. M.; Antonetti, C.; Lambert,
R. M. ChemCatChem 2010, 2, 1444. (g) Qian, D.; Zhang, J. Beilstein J. Org.
Chem. 2011, 7, 808.
Scheme 1. Hydrolysis and Alcoholysis of 1,2-Disilanes
Catalyzed by Au/TiO2
9
(3) Zhan, J.-H.; Lv, H.; Yu, Y.; Zhang, J.-L. Adv. Synth. Catal. 2012,
354, 1529.
(4) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469.
(5) (a) Lauterbach, T.; Livendahl, M.; Rosellon, A.; Espinet, P.;
Echavarren, A. M. Org. Lett. 2010, 12, 3006. (b) Corma, A.; Juarez, R.;
Boronat, M.; Sanchez, F.; Iglesias, M.; Garcia, H. Chem. Commun. 2011,
47, 1446. (c) Robinson, P. S. D.; Khairallah, G. N.; da Silva, G.; Lioe, H.;
O’Hair, R. A. J. Angew. Chem., Int. Ed. 2012, 51, 3812. (d) Boronat, M.;
Combita, D.; Concepcion, P.; Corma, A.; Garcia, H.; Juarez, R.; Laursen,
S.; de Dios Lopez-Castro, J. J. Phys. Chem. C 2012, 116, 24855. (e)
Nijamudheen, A.; Datta, A. J. Phys. Chem. C 2013, 117, 21433.
(6) (a) Gualco, P.; Ladeira, S.; Miqueu, K.; Amgoune, A.; Bourissou,
D. Angew. Chem., Int. Ed. 2011, 50, 8320. (b) Gualco, P.; Ladeira, S.;
Miqueu, K.; Amgoune, A.; Bourissou, D. Organometallics 2012, 31, 6001.
(7) Lassauque, N.; Gualco, P.; Mallet-Ladeira, S.; Miqueu, K.;
Amgoune, A.; Bourissou, D. J. Am. Chem. Soc. 2013, 135, 13827.
r
10.1021/ol402935x
Published on Web 11/08/2013
2013 American Chemical Society

Table 1. Reaction of 1-Heptyne (5) with 1,2-Disilanes 1À4 Catalyzed by Supported Au NPs
substituents
catalyst
solvent
time (h)/temp (°C)
isolated yield (%)
R1, R2, R3 = Me (1)
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/ZnO
Au/Al2O3
Au/TiO2
Au/TiO2
Au/TiO2
EtOAc
DCE
12/65
4/65
55
93
48
40
34
81
65
67
28
72
-
R1, R2, R3 = Me (1)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Me, R3 = Ph (2)
R1, R2 = Ph, R3 = Me (3)
R1, R2 = Ph, R3 = Me (3)
R1, R2, R3 = Ph (4)
EtOAc
THF
6/65
5/65
Toluene
DCE
5/65
2/65
DCE
6/65
DCE
5/65
EtOAc
DCE
16/65
10/65
48/65
DCE
SiÀSi functionality on gold(I) triggered the question by
the authors6a on whether gold-catalyzed bis-silylation
reactions on π systems by σ disilanes is feasible. Our initial
exploration on the possible activation of 1,2-disilanes by
gold revealed that supported Au nanoparticles (especially
Au/TiO2) readily catalyze their hydrolysis or alcoholysis
with release of H2 gas (Scheme 1).9 A series of homo-
geneous Au(I) catalysts were proven significantly less
reactive or even completely unreactive.
Apart from reporting the Au/TiO2-catalyzed hydrolysis
and alcoholysis of disilanes, our challenge was to explore
the question raised by Bourissou and co-workers,6a thus
providing the first example of a Au-catalyzed version of
SiÀSi 1,2-addition10 to alkynes. Highly efficient dehydro-
genative double silylation of alkynes has been recently
achieved by our group with tethered 1,n-dihydrodisilanes,
such as 1,1,3,3-tetramethyldisiloxane, in the presence
of Au/TiO2.11 Notably, analogous attempts to perform
double boronation of alkynes with bis(pinacolato)diboron
in the presence of supported gold nanoparticles were
unsuccessful12 and achievable only under catalysis by Pt
nanoparticles. Initially, we examined the reaction of a model
alkyne (1-heptyne, 5) with a series of symmetrical 1,2-
disilanes, namely 1,1,1,2,2,2-hexamethyldisilane (1), 1,1,2,2-
tetramethyl-1,2-diphenyldisilane (2), 1,2-dimethyl-1,1,2,2-
tetraphenyldisilane (3), and 1,1,1,2,2,2-hexaphenyldisilane
(4), varying the support of the Au catalyst,13 its loading level,
the solvent, and the temperature (Table 1). To our delight,
disilanes 1À3 react smoothly with 1-heptyne in 1,2-dichloro-
ethane (DCE) at 65 °C (1 mol % of Au) and provide the cis-
1,2 addition products in good to excellent isolated yields.
Other solvents provided either lower yield or slow reaction
rates (ethyl acetate, THF, acetonitrile, toluene). Hexaphe-
nyldisilane (4) is completely unreactive, more likely due to
steric reasons. The preferable catalyst is Au/TiO2, as Au/
ZnO and Au/Al2O3, are less effective as also observed in the
catalytic hydrolysis of disilanes by Au NPs.9 The solvent
should be dry; otherwise, an excess of disilane (typically
1.5À2.5 molar equiv) is required to compensate for its Au-
catalyzed hydrolysis but also to avoid occasional formation
of minor β-(E)-hydrosilylation side products.14 The hydro-
silylation products arise from the transient hydrosilanes
generated during the gold nanoparticle-catalyzed hydrolysis
of 1,2-disilanes.9 Although no extended studies were per-
formed, the catalyst (Au/TiO2) was recovered by filtration
after completion of the reaction among alkyne 5 and disilane
1 and reused in a separate experiment with a small deteriora-
tion of its activity. In addition, a series of homogeneous Au(I)
catalysts such as Ph3PAuNTf2, [(2-biphenyl)di-tert-butyl-
phosphine]AuSbF6, or AuCl are completely inefficient.
These observations prompted us to study the scope and
limitations of the 1,2-addition on a variety of alkynes,
working with the reactive 1,2-disilanes 1À3 in DCE as
solvent at 65 °C. The results are presented in Table 2.15
(8) (a) Horn, K. A. Chem. Rev. 1995, 95, 1317. (b) Sharma, H. K.;
Pannell, K. H. Chem. Rev. 1995, 95, 1351. (c) Suginome, M.; Ito, Y.
J. Chem. Soc., Dalton Trans. 1998, 1925.
(9) Gryparis, C.; Stratakis, M. Chem. Commun. 2012, 48, 10751.
(10) (a) Beletskaya, I.; Moberg, C. Chem. Rev. 2006, 106, 2320. (b)
Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221. (c) Beletskaya, I.;
Moberg, C. Chem. Rev. 1999, 99, 3435.
(11) (a) Lykakis, I. N.; Psyllaki, A.; Stratakis, M. J. Am. Chem. Soc.
2011, 133, 10426. (b) Kotzabasaki, V.; Lykakis, I. N.; Gryparis, C.;
Psyllaki, A.; Vasilikogiannaki, E.; Stratakis, M. Organometallics 2013,
32, 665.
(13) The screened catalysts Au/TiO2, Au/Al2O3, and Au/ZnO
(∼1 wt % in Au) are commercially available and have an average gold
crystallite size of ∼2À3 nm.
(12) Grirrane, A.; Corma, A.; Garcia, H. Chem.;Eur. J. 2011, 17,
2467. Very recently, diboration of alkynes was achieved under catalysis
by a nanoporous gold catalyst: Chen, Q.; Zhao, J.; Ishikawa, Y.; Asao,
N.; Yamamoto, Y.; Jin, T. Org. Lett. 2013, DOI: 10.1021/ol4028013.
(14) Psyllaki, A.;Lykakis,I. N.;Stratakis,M.Tetrahedron 2012,68, 8724.
Org. Lett., Vol. 15, No. 23, 2013
6039

case, chromatographic purification is necessary to remove
hydrolysis side products (silanols or 1,3-disiloxanes), while
upon use of hexamethyldisilane its hydrolysis products are
volatile and no further purification is often required. For the
vast majority of products, the stereochemistry is Zas proven
by NOE experiments (see the Supporting Information).
In certain cases, however, E isomers are also seen in minor
amounts, as a result of a gradual Z to E isomerization
during the progress of reaction, a well-known effect in
metal-catalyzed disilylation reactions.16 Surprisingly, the
bis-trimethylsilyl adducts have a higher tendency to iso-
merize, even on standing in CDCl3, e.g., disilylation
products of 14 (see the Supporting Information). Of
particular interest is butynol 10, which provides in the
presence of 2 equiv of 1 trisilylated 10c in high yield,
featuring the double activation of the hydroxyl group
(silyl protection9) and the triple bond (disilylation). In the
case of ethyl propiolate (15), while disilane 2 provides the
Z-adduct 15a, hexamethyldisilane (1) led to the E-adduct
15c in relatively low yield. By monitoring the progress of
the reaction among 15 and 1 by 1H NMR and GCÀMS,
we found that initially the Z-isomer is formed and under
the reaction conditions gradually isomerizes into the
thermodynamically more stable E. We were able to
isolate 15c-Z by careful column chromatography at the
initial stages of reaction (see the Supporting Information). In
the meantime, as the reaction of ethyl propiolate with
1 is slow, the alkyne undergoes partial Au-catalyzed
trimerization17 forming triethyl benzene-1,3,5-tricarboxylate
and triethyl benzene-1,2,4-tricarboxylate in a relative
ratio ∼1/1 and ∼25% relative yield (see the Supporting
Information). Silyl-substituted acetylenes 16 and 17 are
surprisingly more reactive than all acetylenes tested here-
in and provide disilylation adducts in excellent yields, yet
unselectively. Both alkynes lead to the predominant for-
mation of the thermodynamically more stable adducts
(verified by NOE experiments). Furthermore, we found
that there is no essential E/Z isomerization of disilylation
products from 16 or 17 (e.g., 16b-Z to 16b-E) under the
reaction conditions. The peculiar stereochemistry of these
substrates in their disilylation reaction is analyzed in the
accompanying mechanistic analysis.
Table 2. 1,2-Disilylation of Terminal Alkynes by 1,2-Disilanes
Catalyzed by Au/TiO2
(15) General procedure for the Au/TiO2-catalyzed disilylation: To a
dry vial containing the alkyne (0.2 mmol), the disilane (0.24 mmol), and
1 mL of dry DCE were added 40 mg of Au/TiO2 (∼1.0 mol% in Au).
When nondried solvent was used, an appropriate excess of disilane was
added (typically 1.5À2.5 fold molar excess) to compensate for its
competing hydrolytic destruction. After a certain period of time (see
Table 2) at 65 °C, the reaction was complete (TLC, GCÀMS). The slurry
was filtered with the aid of 3 mL of DCM under low pressure through a
short pad of silica gel or celite, and the filtrate was evaporated to afford
the disilylation products. Chromatographic purification was performed,
if necessary. The reaction is heterogeneous in nature, as leaching of gold
into the solution under of the reaction conditions in the specific solvent is
negligible (far below ppm level). See also: Efe, C.; Lykakis, I. N.;
Stratakis, M. Chem. Commun. 2011, 47, 803.
(16) (a) Watanabe, H.; Kobayashi, M.; Higuchi, K.; Nagai, Y.
J. Organomet. Chem. 1980, 186, 51. (b) Matsuda, T.; Ichioka, Y. Org.
Biomol. Chem. 2012, 10, 3175.
(17) The Au/TiO2-catalyzed trimerization of ethyl propiolate (15)
was recently established: Leyva-Perez, A.; Oliver-Meseguer, J.; Cabrero-
Antonino, J. R.; Rubio-Marques, P.; Serna, P.; Al-Resayes, S. I.;
Corma, A. ACS Catal. 2013, 3, 1865.
a Z/E = 68/32. b Z/E = 89/11. c 2 equiv of hexamethyldisilane was
used. d Z/E = 94/6. e Z/E = 81/19. f Z/E = 93/7. On standing in CDCl3,
14c gradually isomerized into the E isomer (after 1 day Z/E = 68/32).
g Only the E isomer was isolated; however, it derives from the initially
formed Z isomer which was detected and isolated at the initial stages of
reaction. h Z/E = 34/66. i Z/E = 23/77. j Z/E = 42/58.
We examined a series of terminal alkynes and found that
the reaction is smooth and proceeds in good to excellent
yield with aliphatic and arylacetylenes but also tolerates a
variety of functional groups on the side chains. Internal
alkynes are unreactive. Regarding experimental details,
under strictly dry conditions 1.2 molar equiv of disilanes
were used to drive the reaction to completion, yet nondried
DCE can also be used, requiring an excess of disilane. In this
6040
Org. Lett., Vol. 15, No. 23, 2013

Scheme 2. Mechanistic Studies in the Au-Catalyzed Disilylation
Scheme 3. Proposed Mechanism for the Au-Catalyzed
1,2-Disilylation of Alkynes
Useful mechanistic information was acquired upon
studying the Au-catalyzed addition of disilane 2 to the
deuterium labeled alkynes 8-d (95% D) and 17-d (97%D).18
At ∼60% conversion, in product 8aÀd the D content was
73% (Scheme 2). Yet, recovered alkyne 8Àd also showed
∼30% isotopic depletion in accordance to previous obser-
vations with other deuterium-labeled terminal alkynes un-
der treatment with Au/TiO2 at elevated temperatures.19
Additionally, labeled 17-d reacts very fast (20À30 min) with
2, without undergoing essential isotopic depletion during
the reaction and provided product 17aÀd with ∼95% D
content. These results exclude the formation of gold ace-
tylides as intermediates. In addition, the lack of Au/TiO2-
catalyzed hydrosilylation of internal disilyl alkyne 1820 with
PhMe2SiH, excludes the possibility of a pathway involving
cross coupling among the terminal alkyne and disilane (e.g.,
16 with 2 forming 18 and PhMe2SiH) followed by hydro-
silylation of 18 from the produced hydrosilane. In Scheme 3
we propose oxidized gold nanoclusters/nanoparticles as the
reactive catalytic sites on Au/TiO2, capable of undergoing
insertion from the SiÀSi bond of disilanes as proposed
earlier9 (intermediate I). There is a growing number of
reports postulating ionic gold being the catalytically ac-
tive sites in metal oxide supported gold nanoparticles.21
Intermediate I then undergoes addition to the alkyne form-
ing either II or III. A similar stereoselective syn insertion of
Ph3PAu-SiR3 to alkynes has been recently documented.22
It is possible that for the reactions of silyl acetylenes 16-17,
the corresponding intermediate II is stabilized through
charge transfer from gold to the double bond forming the
silicon-stabilized dipole IV, capable of undergoing bond
rotation, thus forming the isomerized adduct IIisom. This
concept nicely explains the formation of E/Z adducts 16aÀb
and 17c in thermodynamic ratios. An analogous mechan-
istic argument has been postulated in the Pd(0)-catalyzed
silylstannylation of an alkyne.23
In conclusion, we have presented herein the first exam-
ple of the Au-catalyzed disilylation of alkynes by σ 1,2-
disilanes, thus expanding the ability of gold to catalyze
reactions so far feasible with Pd, Pt, and other late transi-
tion metals. As typical homogeneous ionic gold catalysts
are ineffective, this work provides an additional example
of the powerful nature of supported Au nanoparticles in
catalysis.
Acknowledgment. This work was supported by the
project “IRAKLEITOS II - University of Crete” of the
Operational Programme for Education and Lifelong
Learning 2007-2013 (EPEDVM) of the NSRF (2007-
2013), which is cofunded by the European Union
(European Social Fund) and National Resources.
(18) Labeled alkynes 8-d (95% D) and 17-d (97% D) were prepared
by treatment of 8 and 17, respectively, with n-BuLi (THF, 0 °C, 30 min)
followed by quench with D2O.
(19) Gryparis, C.; Efe, C.; Raptis, C.; Lykakis, I. N.; Stratakis, M.
Org. Lett. 2012, 14, 2956.
(20) Cuadrado, P.; Gonzalez-Nogal, A. M.; Valero, R. Tetrahedron
2002, 58, 4975.
(21) Selected examples: (a) Carrettin, S.; Concepcion, P.; Corma, A.;
Lopez-Nieto, J. M.; Puntes, V. F. Angew. Chem., Int. Ed. 2004, 43, 2538.
(b) Fierro-Gonzaleza, J. C.; Gates, B. C. Chem. Soc. Rev. 2008, 37, 2127.
(c) Brown, M. A.; Fujimori, Y.; Ringleb, F.; Shao, X.; Stavale, F.;
Nilius, N.; Sterrer, M.; Freund, H.-J. J. Am. Chem. Soc. 2011, 133,
10668. (d) Hong, S.; Rahman, T. S. J. Am. Chem. Soc. 2013, 135, 7629.
(22) Joost, M.;Gualco, P.;Mallet-Ladeira, S.;Amgoune, A.; Bourissou,
D. Angew. Chem., Int. Ed. 2013, 52, 7160.
1
Supporting Information Available. Copies of H and
13C NMR of all products and selected NOE experiments
for key compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
(23) Murakami, M.; Yoshida, T.; Kawanami, S.; Ito, Y. J. Am.
Chem. Soc. 1995, 117, 6408.
The authors declare no competing financial interest.
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6041