Ligandless Regioselective Hydrosilylation of Allenes Catalyzed by Gold Nanoparticles

Article abstract of DOI:10.1021/acs.orglett.5b02236

The first example of Au-catalyzed hydrosilylation of allenes is presented using recyclable gold nanoparticles as catalyst, without the requirement of any external ligands or additives. The hydrosilane addition takes place on the more substituted double bond of terminal allenes in a highly regioselective manner. The observed regioselectivity/reactivity modes are attributed to steric and electronic factors.

Full text of DOI:10.1021/acs.orglett.5b02236

Letter
pubs.acs.org/OrgLett
Ligandless Regioselective Hydrosilylation of Allenes Catalyzed by
Gold Nanoparticles
Marios Kidonakis and Manolis Stratakis*
Department of Chemistry, University of Crete, Voutes, 71003 Iraklion, Greece
S
* Supporting Information
ABSTRACT: The first example of Au-catalyzed hydrosilylation of allenes
is presented using recyclable gold nanoparticles as catalyst, without the
requirement of any external ligands or additives. The hydrosilane addition
takes place on the more substituted double bond of terminal allenes in a
highly regioselective manner. The observed regioselectivity/reactivity
modes are attributed to steric and electronic factors.
ligands.12 The latter conditions as reported by Montgomery’s
mong the various organic transformations catalyzed by
1
Ananoparticulated gold under heterogeneous conditions, group have the advantage that depending on the bulkiness of
ligand, switchable regioselectivity can be observed. More
recently, a highly efficient cationic [(3IP)Pd(allyl)]OTf-
catalyzed protocol was reported,13 which provides mechanisti-
cally different regioselective internal or terminal-addition
modes, depending on the substitution of hydrosilane (tertiary
or secondary). Indirect hydrosilylation of allenes can also be
achieved via their Cu(I)-catalyzed reaction with silylboranes14
at the expense of the boronate moiety, or with silylzincation
followed by quenching with acid.15
the hydrosilylation of π systems has been limited to alkynes.
Thus, the Au nanoparticle-,2 gold film-,3 or nanoporous4 gold-
catalyzed addition of hydrosilanes to alkynes provides β-(E)
adducts regioselectively. Notably, no efficient hydrosilylation
protocol has been reported so far under homogeneous ionic
gold catalysis conditions. Our continuous interest in the
catalytic activation of silanes by supported gold nanoparticles
(Au NPs)2c,5 toward addition reactions urged us to examine the
reaction among hydrosilanes and allenes. To the best of our
knowledge, there are no examples in the literature regarding
any addition reactions on allenes (including hydrosilylation) in
the presence of Au NPs as catalysts. In contrast, allenes are
readily activated under homogeneous ionic gold conditions
through coordination on Au(I),6 which triggers intra or
intermolecular addition reactions by a variety of nucleophiles.7
In the same context, there are two literature reports presenting
the regioselective insertion of a Au(III)−H pincer complex,8 as
well as the gold−silicon bond of the Ph3PAu(I)−SiMe2Ph
complex9 on the less substituted double bond of terminal
allenes. In both cases, the metal (Au) is bonded to the central
sp-C atom of the allene.
While all attempts to achieve allene hydrosilylation using a
series of ionic gold catalysts under homogeneous conditions
failed,16 we were pleased to find that, in the presence of
catalytic amounts of supported gold nanoparticles on the
surface of metal oxides, hydrosilanes react smoothly with
terminal allenes affording addition products (α-vinylsilanes)17
in good to excellent yields (Table 1). Optimizing the
conditions, Au/TiO2 or Au/Al2O3 (1 mol %)18 were proven
as the catalysts of choice and dry 1,2-dichloroethane (DCE) or
benzene the most suitable solvents. The reactants are mixed in
an almost equimolar ratio and the reaction time varies from 30
min to 12 h at 65 °C depending on the substrate or
hydrosilane.19 In the absence of catalyst or in the presence of
the support alone (TiO2 or Al2O3), the reaction does not take
place. Nondried solvent can also be used, yet an excess of
hydrosilane (typically 1.5−2 molar equiv) must be added to
compensate its hydrolytic destruction to the corresponding 1,3-
disiloxanes. A variety of hydrosilanes are compatible
(PhMe2SiH, Ph2MeSiH, Et3SiH, Me3SiOSiMe2H,
HMe2SiOSiMe2H) with the exception of triethoxysilane.
The current hydrosilylation protocol tolerates a variety of
functional groups and the product selectivity is in general high.
For the case of monosubstituted allenes, the addition takes
place regioselectively on the more substituted internal double
bond (80−98% selectivity depending on substituents). On both
The so far reported protocols of hydrosilylation of allenes
(Scheme 1) are rather scarce and include catalysis by Lewis
acid,10 the requirement of stoichiometric amounts of
Co2(CO)8,11 and catalysis by in situ generated Ni(0) or
Pd(0) complexes bearing suitable N-heterocyclic carbene
Scheme 1. Possible Hydrosilylation Adducts in the Case of a
Terminal Monosubstituted Allene
Received: July 31, 2015
© XXXX American Chemical Society
A
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Letter
regioisomers (terminal or internal-adducts), the new C−Si
bond occurs on the former sp-C atom of the allene. The minor
terminal-regioisomer has always the E stereochemistry as
proven by NOE experiments, indicative of a highly
diastereoselective facial hydrosilylation. To our delight, 1,1-
disubstituted allenes form a single regioisomer by addition on
the more substituted internal double bond in excellent yields.
The reaction has limitations, however; 1,3-disubstituted or
trisubstituted allenes are unreactive and the possible reasons are
analyzed in the accompanying mechanistic discussion. As per
product selectivity, only in the case of ester bearing
monosubstituted allene 2 is the side product arising from
terminal double bond hydrosilylation formed in 17−20%
relative ratio. Alkyl, alkoxy, or aryl monosubstituted allenes
form this minor addition side product in 2−5% relative yield,
while in the case of 1,1-disubstituted allenes as commented
above, the terminal double bond addition product is completely
absent. A sound example of this trend is the case of allenes 12
and 13 where an extra substituent (methyl or −CH2COOEt)
relative to 2 has a profound effect in selectivity, with 12 and 13
affording a single product. An interesting reactivity was
observed with vinylidenecyclohexane (11), as substantial
amounts of oxidative hydrosilylation products (11c and 11e,
respectively) were seen with Et3SiH and Me3SiOSiMe2H, but
not with PhMe2SiH. Their formation is attributed to
elimination of H2 in the proposed π-allyl intermediate (see
proposed mechanism in Scheme 3).20 In those specific
examples, a possible mechanism involving isomerization of
allene to 1,3-diene followed by an oxidative hydrosilylation is
excluded, as a series of 1,3-dienes do not undergo any reaction
with hydrosilanes in the presence of Au nanoparticles. Another
interesting aspect is the reaction of 1 with 1,1,3,3-
tetramethyldisiloxane (TMDS), which is known to cycloadd
to alkynes in a dehydrogenative manner.5a We found that no
cycloadduct is formed, and the hydrosilylation pathway solely
operates (product 1e), in contrast to the Pt(0)-catalyzed
reaction between o-bis(dimethylsilyl)benzene, an analogue to
TMDS tethered dihydrodisilane, which provides the 1,2-
disilylation cycloadduct on the terminal double bond of an
allene with elimination of H2.21
Table 1. Hydrosilylation of Allenes Catalyzed by Au/TiO2
We also wish to point out that the supported gold catalysts
used in our hydrosilylation protocol are recyclable and reusable
in three consecutive experiments without loss of activity and
deterioration of conversion yields. The recycling was performed
by filtration of catalyst after each run, washing with solvent and
drying in the oven at 100 °C for 1 h.
The kinetics of hydrosilylation as studied in a series of p-
substituted phenylallenes reveal that electron donating
substituents accelerate the reaction rate. This trend is also
apparent with methoxyallene (3) which reacts much faster than
allene 2 which bears an electron withdrawing substituent. A
Hammett plot of the relative reaction rates among p-X-
substituted phenylallenes and parent phenylallene (log kX/kH)
against the σ+ values of substituents shown in Scheme 2 has a
fair linear fit and a negative slope (ρ = −1.08) which indicates
the development of a partial positive charge on the benzylic
position in the transition state of the rate-determining step of
reaction. The same plot against σ values shows inferior results
in terms of linearity (Supporting Information, p S19), an
observation that once more supports the partial carbocationic
character in the transition state. Note that the kinetic profile of
hydrosilylation of arylallenes is in contrast to the Au/TiO2-
a
b
Isolated yield. Relative ratio 11b (hydrosilylation product)/11c =
c
45/55. Relative ratio 11d (hydrosilylation product)/11e = 65/35.
B
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interactions either invoking a π-allyl complex (I) or an η-1 (II).
We have shown in a recent publication studying the Au NP-
catalyzed silaboration of alkynes22 how steric interactions
among Au NP and the alkyne substituents in intermediate
addition products affect not only the regiochemistry of
addition, but also the reactivity of internal alkynes.
Scheme 2. Hammett Plot in the Competing Hydrosilylation
of 1-Arylallenes
Another evidence of the carbocationic character of the
reaction intermediate was provided upon studying the hydro-
silylation of allene 17 bearing as side chain the sensitive
phenylcyclopropyl moiety. The reaction among 17 and Et3SiH
in the presence of Au NPs proceeded smoothly affording an
inseparable mixture of acyclic 17a along its dehydrogenation
analogue 17b in a relative ratio 1/2 (Scheme 4). In none of the
Scheme 4. Hydrosilylation of Cyclopropyl Allene 17 and a
Possible Mechanism of Product Formation
catalyzed hydrosilylation of arylalkynes,2c where the reaction
rate is retarded by electron donating substituents.
Regarding the proposed mechanism (Scheme 3), two
possible scenarios are suggested as working hypotheses. In
Scheme 3. Possible Mechanisms of Hydrosilylation of a
Monosubstituted Allene Catalyzed by Au NP
products did the cyclopropyl group of reactant remained intact.
Either a π-allyl gold intermediate of type I or an η-1 σ complex
of type II may reasonably undergo ring opening isomerization
to the final products (17a via hydride attack or 17b via H2
elimination). Note that in a blank experiment, allene 17 does
not undergo any reaction or rearrangement in the presence of
Au NPs.
In conclusion, we have presented herein the first example of
gold-catalyzed allene hydrosilylation using as catalysts sup-
ported gold nanoparticles on TiO2 or Al2O3 in low loading level
(1 mol %). The reaction is regioselective, favoring the addition
of hydrosilane on the more substituted double bond of terminal
allenes. We emphasize that the protocol is extremely simple, the
catalyst is recyclable and reusable, and, most importantly, no
external additives or ligands are required as in the so far known
homogeneous metal-catalyzed analogous protocols.
the first, the π-allyl gold intermediate I is postulated,
reminiscent of an analogous intermediate during the hydro-
silylation of allenes catalyzed by a Pd(0) complex in the
presence of bulky NHC ligands.12a The silylmetalation pathway
may be driven in our case due to the bulkiness of the Au NP
itself, just as in the case of Pd(0) catalysis, where bulky ligands
favor silylmetalation while nonbulky ligands favor the hydro-
metalation pathway instead. In intermediate I, the hydride is
selectively delivered on the more substituted sp2-carbon atom
for electronic reasons forming the major or only product.
Formation of a σ bonded intermediate η-1 complex II may also
explain the product selectivity and cannot be excluded. As per
the nature of the active catalytic sites on Au NPs, since our so
far experience in addition reactions to alkynes via σ bond
activation of disilanes,5d hydrosilanes5a and silylboranes22
resembles redox catalysis by Pd(0), we suggest once again
ionic gold(I) species at the interface between nanoparticle and
the support23 as the reacting sites. The oxidative insertion of σ
bonds on Au(I) by analogy to Pd(0) is well established.24
1,3-Disubstituted or trisubstituted allenes (14−16) are
completely unreactive. In those substrates, any possible
product-forming transition state is destabilized by nonbonded
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02236.
1
Copies of H, 13C NMR of all products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: stratakis@uoc.gr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
M.K. thanks the Onassis Foundation for a fellowship.
■
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(20) The formation of oxidative hydrosilylation side products (11c
and 11e) in the hydrosilylation of vinylidenecyclohexane (11) is
attributed to elimination of H2 via a six-membered ring transition state.
Hydrogen elimination is more likely favored in this system due to the
rigidity of reacting C-H bond on the cyclohexyl ring.
REFERENCES
■
(1) (a) Corma, A.; Garcia, H. Chem. Soc. Rev. 2008, 37, 2096.
(b) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Chem. Rev. 2012, 112, 2467.
(c) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469.
(d) Mitsudome, T.; Kaneda, K. Green Chem. 2013, 15, 2636.
(e) Takale, B. S.; Bao, M.; Yamamoto, Y. Org. Biomol. Chem. 2014,
12, 2005. (f) Liu, X.; He, L.; Liu, Y.-M.; Cao, Y. Acc. Chem. Res. 2014,
47, 793.
(2) (a) Corma, A.; Gonzalez-Arellano, C.; Iglesias, M.; Sanchez, F.
Angew. Chem., Int. Ed. 2007, 46, 7820. (b) Aronica, L. A.; Schiavi, E.;
Evangelisti, C.; Caporusso, A. M.; Salvadori, P.; Vitulli, G.; Bertinetti,
L.; Martra, G. J. Catal. 2009, 266, 250. (c) Psyllaki, A.; Lykakis, I. N.;
Stratakis, M. Tetrahedron 2012, 68, 8724.
(3) Shore, G.; Organ, M. G. Chem. - Eur. J. 2008, 14, 9641.
(4) Ishikawa, Y.; Yamamoto, Y.; Asao, N. Catal. Sci. Technol. 2013, 3,
2902.
(5) (a) Lykakis, I. N.; Psyllaki, A.; Stratakis, M. J. Am. Chem. Soc.
2011, 133, 10426. (b) Gryparis, C.; Stratakis, M. Chem. Commun.
2012, 48, 10751. (c) Kotzabasaki, V.; Lykakis, I. N.; Gryparis, C.;
Psyllaki, A.; Vasilikogiannaki, E.; Stratakis, M. Organometallics 2013,
32, 665. (d) Gryparis, C.; Kidonakis, M.; Stratakis, M. Org. Lett. 2013,
15, 6038. (e) Vasilikogiannaki, E.; Titilas, I.; Gryparis, C.; Louka, A.;
Lykakis, I. N.; Stratakis, M. Tetrahedron 2014, 70, 6106. (f) Titilas, I.;
Kidonakis, M.; Stratakis, M. Organometallics 2015, 34, 1597.
(6) Gandon, V.; Lemiere, G.; Hours, A.; Fensterbank, L.; Malacria,
M. Angew. Chem., Int. Ed. 2008, 47, 7534.
(21) Tanaka, M.; Uchimaru, Y.; Lautenschlager, H.-J. J. Organomet.
Chem. 1992, 428, 1.
(22) Gryparis, C.; Stratakis, M. Org. Lett. 2014, 16, 1430.
(23) For the importance of ionic gold species in catalysis of organic
transformations by supported Au NPs, see: Fierro-Gonzalez, J. C.;
Gates, B. C. Chem. Soc. Rev. 2008, 37, 2127.
(24) Joost, M.; Gualco, P.; Coppel, Y.; Miqueu, K.; Kefalidis, C. E.;
Maron, L.; Amgoune, A.; Bourissou, D. Angew. Chem., Int. Ed. 2014,
53, 747.
(7) (a) Yang, W.; Hashmi, A. S. K. Chem. Soc. Rev. 2014, 43, 2941.
(b) Caneque, T.; Truscott, F. M.; Rodriguez, R.; Maestri, G.; Malacria,
M. Chem. Soc. Rev. 2014, 43, 2916. (c) Zimmer, R.; Reissig, H.-U.
Chem. Soc. Rev. 2014, 43, 2888.
(8) Rosca, D.-A.; Smith, D. A.; Hughes, D. L.; Bochmann, M. Angew.
Chem., Int. Ed. 2012, 51, 10643.
(9) (a) Joost, M.; Gualco, P.; Mallet-Ladeira, S.; Amgoune, A.;
Bourissou, D. Angew. Chem., Int. Ed. 2013, 52, 7160. (b) Joost, M.;
Estevez, L.; Mallet-Ladeira, S.; Miqueu, K.; Amgoune, A.; Bourissou,
D. J. Am. Chem. Soc. 2014, 136, 10373.
(10) Sudo, T.; Asao, N.; Gervorgyan, V.; Yamamoto, Y. J. Org. Chem.
1999, 64, 2494.
(11) Huang, G.; Isobe, M. Tetrahedron 2001, 57, 10241.
(12) (a) Miller, Z. D.; Li, W.; Beldrrain, T. R.; Montgomery, J. J. Am.
Chem. Soc. 2013, 135, 15282. (b) Miller, Z. D.; Montgomery, J. Org.
Lett. 2014, 16, 5486. (c) Xie, H.; Zhao, L.; Yang, L.; Lei, Q.; Fang, W.;
Xiong, C. J. Org. Chem. 2014, 79, 4517. (d) Miller, Z. D.; Dorel, R.;
Montgomery, J. Angew. Chem., Int. Ed. 2015, 54, 9088.
(13) Tafazolian, H.; Schmidt, J. A. R. Chem. Commun. 2015, 51,
5943.
(14) Xu, Y.-H.; Wu, L.-H.; Wang, J.; Loh, T.-P. Chem. Commun.
2014, 50, 7195.
(15) Yonehara, M.; Nakamura, S.; Muranaka, A.; Uchiyama, M.
Chem. - Asian J. 2010, 5, 452.
(16) We have tested the following homogeneous gold(I) or (III)
catalysts under various conditions (temperature, catalyst loading) and
different solvents: Ph3PAuNTf2, Ph3PAuCl + Ag(I) salts, [(2-
biphenyl)di-tert-butylphosphine]AuSbF6 and AuCl3.
(17) Recent examples for the synthesis of α-vinylsilanes via catalytic
hydrosilylation of terminal alkynes: (a) Wang, P.; Yeo, X.-L.; Loh, T.-
P. J. Am. Chem. Soc. 2011, 133, 1254. (b) Menozzi, C.; Dalko, P. I.;
Cossy, J. J. Org. Chem. 2005, 70, 10717.
(18) Au/TiO2 and Au/Al2O3 (∼1 wt % in Au) with an average gold
crystallite size of ∼ 2−3 nm are commercially available.
(19) General procedure for the Au/TiO2 or Au/Al2O3-catalyzed
hydrosilylation of allenes: To a vial containing the allene (0.2 mmol)
hydrosilane (0.24 mmol) and 1 mL of dry DCE or benzene are added
40 mg of Au/TiO2 or Au/Al2O3 (1.0 mol % in Au). After a certain
period of time (see Table 1) at 65 °C, the reaction is complete (TLC,
GC−MS). The slurry is filtered with the aid of solvent under a low
pressure through a short pad of silica gel or celite, and the filtrate is
evaporated to afford the hydrosilylation products which can be further
purified by column chromatography.
D
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Org. Lett. XXXX, XXX, XXX−XXX