Synthesis of an [(NHC)2Pd(SiMe3)2] complex and catalytic cis-bis(silyl)ations of alkynes with unactivated disilanes

Article abstract of DOI:10.1002/anie.201501764

The novel complex cis-[(ITMe)₂Pd(SiMe₃)₂ (ITMe=1,3,4,5-tetramethylimidazol-2-ylidene) has been synthesized by mild oxidative cleavage of Me₃SiSiMe₃ using [(ITMe)₂Pd⁰]. The use of this complex as precatalyst for the cis-bis(silyl)ation of alkynes using unactivated disilanes is reported. Double the Si: The novel complex cis-[(ITMe)₂Pd(SiMe₃)₂] (1, ITMe=1,3,4,5-tetramethylimidazol-2-ylidene) has been synthesized by mild oxidative cleavage of Me₃SiSiMe₃ using [(ITMe)₂Pd⁰]. The synthesized complex was used as a precatalyst for the cis-bis(silyl)ation of alkynes using unactivated disilanes.

Full text of DOI:10.1002/anie.201501764

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501764
German Edition: DOI: 10.1002/ange.201501764
Homogeneous Catalysis
Synthesis of an [(NHC)2Pd(SiMe3)2] Complex and Catalytic
cis-Bis(silyl)ations of Alkynes with Unactivated Disilanes**
Melvyn B. Ansell, Debbie E. Roberts, F. Geoffrey N. Cloke, Oscar Navarro,* and John Spencer*
In memory of Mike Lappert
Abstract: The novel complex cis-[(ITMe)2Pd(SiMe3)2
(ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) has been syn-
thesized by mild oxidative cleavage of Me3SiSiMe3 using
[(ITMe)2Pd0]. The use of this complex as precatalyst for the
cis-bis(silyl)ation of alkynes using unactivated disilanes is
reported.
the most common phosphines and that NHC/M complexes
(M = metal) are less prone to decomposition by cleavage of
the (NHC)–M bond.[10,11] As part of our ongoing studies of
NHC/palladium-catalyzed reactions, we decided to explore
their potential to activate hexamethyldisilane since there are
no examples in the literature of palladium complexes capable
of this reaction. Herein we report on the synthesis of the novel
complex cis-[Pd(ITMe)2(SiMe3)2] from [(ITMe)2Pd0] and its
inclusion in a catalytic cycle leading to the cis-bis(silyl)ation
of alkynes.
While very recent literature on NHC/Pd complexes
features the use of large NHCs as a common denominator,[12]
we decided to start our studies with one of the smallest NHCs
available, that is, 1,3,4,5-tetramethylimidazol-2-ylidene
(ITMe; 1), which exhibits a very small percent buried
volume and a high s-donor character.[13] The conventional
synthetic route to 1 was established by Kuhn and co-work-
ers.[14] It involves the formation of the corresponding thione
by a ring-forming double condensation of N,N’-dimethyl-
thiourea and acetoin, and subsequent reductive desulfuriza-
tion of the thione using potassium metal, with an overall yield
of about 76%. A thorough modification of the synthetic
protocol, including a microwave-mediated cyclization step,
allowed us to obtain 1 in 86% overall yield even on a gram
scale (Scheme 1).[15]
T
he transition metal catalyzed activation of disilanes for the
synthesis of high-value organosilicon compounds has received
a significant amount of attention from industry and academia.
Applications encompass the formation of silanes,[1] bis-
(silyl)ation of unsaturated compounds,[2] aryl/acyl silane syn-
thesis,[3,4] and protection of alcohols.[5] Since the oxidative
À
cleavage of the Si Si bond by a low-valent platinum-group
transition-metal center is proposed as a vital step for some of
these processes,[6] the isolation of the resulting bis(silyl)
transition-metal complexes is of great interest for elucidating
reaction mechanisms. Unfortunately, the synthesis of such
complexes has been largely limited to the oxidative addition
of strained or activated disilanes.[7]
The cleavage of non-activated hexamethyldisilane is
particularly challenging. Examples of the resulting bis(trime-
thylsilyl) platinum-group metal complexes are rare: only two
have been described in the literature, and they bear either
phosphine or isocyanide ligands. Braun and co-workers
reported that [Pt(PEt3)3] reacted with a large excess of
hexamethyldisilane to yield cis-[Pt(PEt3)2(SiMe3)2] at ambi-
ent temperature, but it only went to 50% completion after
three weeks.[8] Earlier, Ito and co-workers synthesized cis-
[Pt(CNAd)2(SiMe3)2] (CNAd = 1-adamantyl isocyanide)
from [Pt3(CNAd)6] using 30 equivalents of hexamethyldisi-
lane at 808C.[9]
The second step involved the synthesis of [(ITMe)Pd-
(methallyl)Cl] (2). The synthesis of this complex has not been
reported, although unsuccessful attempts were detailed by
Cavell and co-workers.[16] The conventional synthetic route to
[(NHC)Pd(R-allyl)Cl] species involves the reaction of the
It is well documented that N-heterocyclic carbenes
(NHCs) have equivalent or better s-donor character than
[*] M. B. Ansell, Dr. D. E. Roberts, Prof. F. G. N. Cloke, Dr. O. Navarro,
Dr. J. Spencer
Department of Chemistry, University of Sussex
Brighton BN1 9QJ (UK)
E-mail: o.navarro@sussex.ac.uk
j.spencer@sussex.ac.uk
[**] We wish to thank Dr. Iain Day and Dr. S. Mark Roe (University of
Sussex) for helpful discussions. We also thank the EPSRC UK
National Mass Spectrometry Facility at Swansea University for
HRMS measurements. M.B.A. is funded as an EPSRC Standard
Research Student (DTG) under Grant No: EP/L505109/1.
NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501764.
Scheme 1. Synthesis of 1 and 2. 2-Me-THF=2-methyltetrahydrofuran.
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corresponding [{Pd(R-allyl)Cl}2] dimer with a free NHC.[17]
We found that reacting [{Pd(methallyl)Cl}2] with a slight
excess of 1 in toluene, initially at À258C, and then warming to
ambient temperature, led to the formation of 2 in 95% yield
(Scheme 1).
[(ITMe)2Pd0] (3) has been proposed as the active catalytic
species in a number of reactions.[18] The only reported
synthesis of 3 was achieved through metal vapor synthesis
(MVS).[19] Recently, Fantasia and Nolan used [(NHC)Pd-
(allyl)Cl] complexes as precursors to easily synthesize a series
of [(NHC)2Pd0] complexes,[20] suggesting that the solvent
employed in these reactions (isopropanol) was also a reagent
and essential to the mechanism of these transformations.
Unfortunately, the application of this methodology to the
synthesis of 3 resulted in the precipitation of large quantities
of Pd black. Modifying this procedure by using isopropanol in
stoichiometric quantities resulted in the first solution-based
synthesis of 3, which was formed as a yellow crystalline
precipitate. Its isolation, however, proved difficult because of
its limited solubility in toluene, THF, and pyridine, and its
instability in alcohols or halogenated solvents. Consequently,
the reaction mixture was directly reacted with hexamethyldi-
silane at room temperature for 18 hours (Scheme 2). cis-
[(ITMe)2Pd(SiMe3)2] (4) was collected as an off-white solid in
62% yield.
Figure 1. Molecular structure of 4 with thermal ellipsoids at the 50%
probability level.[29] Hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Pd–Si1: 2.3557(6), Pd–Si2 2.3468(6),
Pd–C7 2.102(2), Pd–C14 2.119(2); Si1-Pd-Si2 88.65(2), Si1-Pd-C14
88.44(6), Si2-Pd-C7 88.74(6), C7-Pd-C14 94.74(8).
Single crystals of 4 were isolated from a saturated solution
of toluene at À308C, and X-ray analysis revealed that 4
Scheme 3. Reversible formation/decomposition of 4.
Scheme 2. Synthesis of 4.
displays a marginally distorted square-planar geometry with
the two NHCs in a cis configuration and orthogonal to the Si-
Pd-Si plane (Figure 1). The Pd–Si bond lengths are compa-
rable with those found in similar complexes such as
[(dcpe)Pd(SiMe2H)2] (dcpe = 1,2-bis(dicyclohexylphosphi-
no)ethane).[6a]
Scheme 4. Stoichiometric bis(silyl)ation of diphenylacetylene.
The reactivity of 4 was then investigated. A solution of 4 in
C6D6 was heated to 858C and resulted in an intense yellow
solution in less than 1.5 hours. H NMR analysis showed the
resulted in the quantitative formation of the novel complex
ꢀ
[(ITMe)2Pd(PhC CPh)] (6), which could be easily isolated
1
after an n-hexane extraction. Compound 6 was fully charac-
terized by NMR spectroscopy and elemental analysis. Single
crystals of 6 were obtained from a saturated toluene solution
upon cooling to À308C and the result of X-ray analysis is
depicted in Figure 2, featuring a Y-shaped structure. There is
formation of hexamethyldisilane and 3 (Scheme 3). The
formation of the reductive elimination products was limited
to 69% conversion, even upon increasing the temperature
and heating time, probably because of the recombination of
the two products to form 4.
To our knowledge, the bis(silyl)ation of disubstituted
alkynes using hexamethyldisilane has not been reported. The
stoichiometric reaction of 4 with diphenylacetylene at room
temperature yielded the corresponding cis-bis(silyl)ated
product 5 (Scheme 4) within 30 hours in quantitative yield
(see the Supporting Information).[21] This reaction also
ꢀ
a clear and expected elongation of the C C bond and
ꢀ À
a shortening of the C C C bond when compared to free
diphenylacetylene.[22] To the best of our knowledge, this is the
first example of an NHC/Pd0 complex bearing an h2-bound
alkyne. This species can react with an excess of hexamethyl-
disilane at 508C for over 5 days, resulting in the formation of 4
and 5 (Scheme 5).
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Figure 2. Molecular structure of 6 with thermal ellipsoids at the 50%
probability level.[29] Hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Pd1–C1 2.033(3), Pd1–C6 2.029(3),
C1–C6 1.290(4); C6-C1-C5 147.5(3), C1-C6-C25 146.03(3).
Scheme 6. Catalytic bis(silyl)ation reactions.
lenes[24] or the addition of methyl lithium to a silyldisilacyclo-
butene.[25] The protocol was applied to a terminal alkyne,
phenylacetylene, affording compound 13, which was synthe-
sized in a yield comparable to that of the best catalytic
protocol in the literature.[26] Unfortunatelly, the reaction of
hexamethyldisilane with 2-heptyne under these reaction
conditions gave very low conversion into the desired product
(< 5%).
The results from the catalytic reactions and the isolation
of 4 and 5 prompted us to propose a mechanism for the
catalytic cycle, in which 3 is the catalytically active species
(Scheme 7). This 14 electron species can oxidatively add
hexamethyldisilane, thus yielding 4, followed by a migratory
insertion of a silyl group into the acetylene to give the
corresponding vinyl-palladium-silyl complex.[27] This complex
would be stabilized by a weak interaction between the silicon
from the vinylsilyl moiety and the palladium center,[28] thus
allowing a stereoselective reductive elimination to yield 5.
The coordination of diphenylacetylene to 3 affords 6, which
could be considered the resting state of 3.
In conclusion, we have synthesized the first NHC-bearing
complex resulting from the oxidative addition of hexame-
thyldisilane to a palladium center at room temperature. This
complex was used as a precatalyst for the bis(silyl)ation of
electronically and sterically challenging internal acetylenes
using non-activated disilanes. A series of novel 1,2-disilyl-
stilbenes were synthesized in high yield and with 100%
Z stereoselectivity. Studies on the activity of 4 in this and
other catalytic processes are currently ongoing in our
laboratories.
Scheme 5. Reaction of 6 with hexamethyldisilane.
With all this information in hand, we proceeded to the
inclusion of all these organometallic species into a catalytic
cycle. Diphenylacetylene and hexamethyldisilane were
selected as model substrates for the initial optimization of
the reaction parameters. To our delight, 100% stereoselective
conversion into 5 (yield = 94%) was observed using 1 mol%
of 4 (1008C for 24 h in C6D6).[21] This reaction is the first
reported catalytic synthesis of 5. To test the versatility of 4
towards a range of challenging electronics and sterics
ꢀ
surrounding the C C bond, a series of internal alkynes and
non-activated disilanes were also used as substrates. For
instance, the reaction of diphenylacetylene and an excess of
PhMe2SiSiMe2Ph yielded compound 7 (Scheme 6). The only
synthesis reported for this compound involved the stoichio-
metric reaction of cis-[(PPh2Me)2Pt(SiMe2Ph)2] with diphe-
nylacetylene.[23] The novel compounds 8, 9, 10, and 11 were all
synthesized as Z isomers from the corresponding unsym-
metrical internal acetylenes and excess hexamethyldisilane.
Compound 10 was synthesized with greater than 90%
conversion into the desired product. However, its isolation
from the crude reaction mixture proved troublesome, and
after numerous attempts a maximum of 41% of the desired
compound was obtained. The reaction of hexamethyldisilane
with 1-phenyl-2-trimethylsilylacetylene resulted in the for-
mation of 12 (49% yield), but it required a considerable
increase in catalyst loading and reaction time. The only
previous reported syntheses of 12 required either the
stoichiometric addition of Grignard reagents with acety-
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Received: February 24, 2015
Revised: March 17, 2015
Published online: && &&, &&&&
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Communications
Homogeneous Catalysis
M. B. Ansell, D. E. Roberts,
F. G. N. Cloke, O. Navarro,*
J. Spencer*
&&&& — &&&&
Synthesis of an [(NHC)2Pd(SiMe3)2]
Complex and Catalytic cis-Bis(silyl)ations
of Alkynes with Unactivated Disilanes
Double the Si: The novel complex cis-
[(ITMe)2Pd(SiMe3)2] (1, ITMe=1,3,4,5-
tetramethylimidazol-2-ylidene) has been
synthesized by mild oxidative cleavage of
Me3SiSiMe3 using [(ITMe)2Pd0]. The syn-
thesized complex was used as a precata-
lyst for the cis-bis(silyl)ation of alkynes
using unactivated disilanes.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!