Rhodium nanoflowers stabilized by a nitrogen-rich PEG-tagged substrate as recyclable catalyst for the stereoselective hydrosilylation of internal alkynes

Article abstract of DOI:10.1002/adsc.201400740

Morphology and size controllable rhodium nanoparticles stabilized by a nitrogen-rich polyoxyethylenated derivative have been prepared by reduction of RhCl 3 with NaBH4 in water at room temperature and fully characterized. The flower-like Rh NPs are effective and recyclable catalysts for the stereoselective hydrosilylation of challenging internal alkynes and diynes, affording the (E)-vinylsilanes in quantitative yields for a wide range of substrates. The insolubility of the nanocatalyst in diethyl ether allows its easy separation and recycling.

Full text of DOI:10.1002/adsc.201400740

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DOI: 10.1002/adsc.201400740
Very Important Publication
Rhodium Nanoflowers Stabilized by a Nitrogen-Rich PEG-
Tagged Substrate as Recyclable Catalyst for the Stereoselective
Hydrosilylation of Internal Alkynes
Wusheng Guo,a Roser Pleixats,a,* Alexandr Shafir,a,b,* and Teodor Parellaa,c
a
Departament de Quꢀmica and Centro de Innovaciꢁn en Quꢀmica Avanzada (ORFEO-CINQA), Universitat Autꢂnoma de
Barcelona, Cerdanyola del Vallꢃs, 08193-Barcelona, Spain
Fax : (+34) 93581 2477; Tel: (+34) 93581 2067; e-mail: roser.pleixats@uab.cat or alexandr.shafir@uab.cat
Institute of Chemical Research of Catalonia (ICIQ), Av. Paꢄsos Catalans 16, 43007-Tarragona, Spain.
Servei de Ressonꢅncia Magnꢃtica Nuclear, Universitat Autꢂnoma de Barcelona, Cerdanyola del Vallꢃs, 08193-Barcelona,
b
c
Spain.
Received: July 28, 2014; Revised: October 10, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400740.
Abstract: Morphology and size controllable rhodium quantitative yields for a wide range of substrates.
nanoparticles stabilized by a nitrogen-rich polyoxye- The insolubility of the nanocatalyst in diethyl ether
thylenated derivative have been prepared by reduc- allows its easy separation and recycling.
tion of RhCl3 with NaBH4 in water at room tempera-
ture and fully characterized. The flower-like Rh NPs
are effective and recyclable catalysts for the stereo- Keywords: alkynes; hydrosilylation; polyoxyethylen-
selective hydrosilylation of challenging internal al- ated compound; recyclable catalyst; rhodium nano-
kynes and diynes, affording the (E)-vinylsilanes in particles
Introduction
catalyst families, few catalytic systems (i.e. Trostꢆs
ruthenium catalyst)[6g] perform well with the more
The transition-metal catalyzed hydrosilylation of al- challenging internal alkynes. In this context, although
kynes represents a highly straightforward, convenient rhodium-catalyzed hydrosilylation remains relatively
and atom-efficient route for the preparation of vinyl- unexplored, a handful of reports have appeared de-
silanes, versatile organosilicon compounds with multi- scribing the reaction of both the terminal and internal
ple applications in organic synthesis.[1] Thus, these val- alkynes with silanes under catalysis by Rh(I) com-
uable building blocks are involved in a range of trans- plexes,[7] including [Rh(COD)Cl]2, [RhCl(nbd)]2/dppp
formations, such as oxidative rearrangement to ke- and RhCl(PPh3)3); in addition, Rh-NHC species have
tones, vinyl iodide formation or protodesilylation to been used for the hydrosilylation of ketones[8a] and
produce the corresponding alkene.[2] They are also terminal alkynes,[8b,c] and molecular Rh(0) complexes
substrates for the Hiyama cross-coupling with vinyl confined within carbon nanoreactors have been evalu-
and aryl halides,[3] and for the Tamao–Fleming oxida- ated in the hydrosilylation of phenylacetylene.[8d]
tion to give carbonyl derivatives.[4] In particular, the
In addition to discrete complexes, a few recent ex-
use of vinylsilanes as nucleophilic vinyl precursors amples have appeared dealing with the use of metal
offers advantages over other metal alkenylates in nanoparticles in the addition of silanes to acetylenic
terms of cost, non-toxicity, functional group tolerance compounds.[9] Thus, Yus et al. described the recycla-
and high chemical stability. Certain platinum species, bility of platinum nanoparticles deposited on tita-
particularly H2PtCl6 (the Speierꢀs catalyst) and nia[9a,b] and on magnetite[9c] in the hydrosilylation of
Pt2[(Me2SiCH=CH2)2O]3 (the Karstedtꢀs catalyst) have internal alkynes and diynes. More recently, Pt nano-
been the traditional catalyst choice for the addition of particles in carbon nanoreactors have been used in
[5]
À
silanes to unsaturated C C bonds. More recently, the hydrosilylation of phenylacetylene, allowing for
ruthenium complexes have been introduced as alter- the evaluation of the effect of nanoscale confinement
natives to the Pt-based systems.[6] Although terminal on the process selectivity.[9d] In 2014 we have repor-
acetylenes tend to show higher reactivity with both ted[9e] the hydrosilylation of a range of internal al-
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Wusheng Guo et al.
1 (Figure 1C) was envisaged for the formation of
Rh NPs. This new three-fold molecular architecture
was proposed taking into account both the potential
benefits of the nitrogen-enriched structure on the
nanoparticle stabilization, and the operational con-
venience of bonding via the the CuAAC click linkag-
es.[12]
We describe herein the synthesis of the new materi-
al 1 and its use in the efficient preparation of size and
morphology controllable Rh NPs. The catalytic activi-
ty and recyclability of the Rh NPs thus obtained in
the hydrosilylation of various alkynes is also de-
scribed. To the best of our knowledge, this is the first
report on the use of the Rh NPs in the hydrosilylation
of the challenging internal alkynes.
Figure 1. Stabilizers of metal nanoparticles.
Results and Discussion
kynes catalyzed by Pd nanoparticles stabilized by tris- The preparation of stabilizer 1 and the corresponding
imidazolium salts (Figure 1, stabilizer A) which af- Rh NPs are summarized in Scheme 1. Using a modifi-
fords the syn-addition products in a stereoselective cation of the groupꢆs earlier methodology,[11c] the com-
manner. As a particular feature, however, the system mercially available polyether MeO-PEG-OH was
was found highly sensitive to moisture, and rigorously treated with methanesulfonyl chloride in the presence
anhydrous conditions were required to obtain the vi- of triethylamine to give the corresponding mesylate 2
nylsilanes. In the presence of water a competitive in 94% yield. This was then allowed to react with
transfer hydrogenation took place to give the corre- sodium azide in dimethylformamide at 608C over-
sponding alkenes and/or alkanes.[9e]
night, affording the azide 3 in a quantitative yield.
In addition, supported rhodium,[9f] rhodium-plati- The second coupling partner, 2,4,6-tris(propargylami-
num[9f] and gold nanoparticles[9g,h] have been de- no)-1,3,5-triazine, 5, was easily obtained in one step
scribed as catalysts for the regioselective hydrosilyla- from either cyanuric chloride 4a or cyanuric fluoride
tion of alkynes. During that research we were sur- 4b with 34% and 83% yields, respectively.[13] Despite
prised by the scarcity of reports[9f,i] on catalytic rhodi- the better performance, caution should be taken
um nanoparticles (Rh NPs) for the hydrosilylation of when using the cyanuric fluoride due to its highly un-
internal alkynes, given that the use of such materials pleasant smell. With the trialkyne 5 and the azide 3 in
as catalysts has been rapidly extended to a large hand, we went on to perform a threefold copper-cata-
number of other transformations.[10]
lyzed 1,3-dipolar cycloaddition reaction using a 1:3
Our promising results on the activity of the tris-imi- molar ratio. After testing several catalytic system,[14]
dazolium stabilized Pd nanoparticles (Figure 1A) in best results were achieved using a method developed
alkyne hydrosilylation[9e] encouraged us to investigate by Cattoꢈn, Wong Chi Man and co-workers using
other metal nanoparticles as catalysts for this transfor- CuBr(PPh3) as catalyst under microwave irradia-
mation. As part of this effort, a new nanoparticle sta-
bilizer was developed featuring three PEG chains
bound to a central three-fold symmetric core, with the
hopes that the poly-ether chains would help with the
catalyst recycling.[11] The few existing reports on Rh
complexes[7,8] and the scarce precedents[9f] on the use
of Rh NPs in hydrosilylation led us to design a PEG-
tagged derivative suitable for the development of a re-
cyclable alkyne hydrosilylation catalyst based on
Rh NPs.
Applying our groupꢆs previous experience,[11c]
whereby a PEG-matrix prepared via the copper-cata-
lyzed alkyne-azide cycloaddition (CuAAC) chemistry
(Figure 1B) was used as a stabilizer in the synthesis
of water soluble, but diethyl ether insoluble, Pd NPs,
here a nitrogen-rich polyoxyethylenated derivative Scheme 1. Preparation of Rh NPs stabilized by 1.
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Table 1. Preparation of Rh NPs stabilized with different
amounts of stabilizer 1.
diameter % Rh
yield
Entry[a] Rh NPs 1: Rh
[nm][b] theor exper[c] [%][d]
1
2
3
4
5
6
M1
M2
M3
M4
M5
-
1.00:1 1.6Æ0.2 1.61 1.33
0.30:1 2.1Æ0.6 5.08 3.81
0.10:1 8.5Æ5.6 13.84 8.43
0.05:1 25.6Æ5.5 24.31 17.60
0.02:1 32.3Æ7.5 44.54 34.60
81
69
57
70
76
-
0:1
Bulk Rh 100
-
[a]
Reaction conditions: [Rh]=1 mM; RhCl3·3H2O and
1 stirred in water overnight under N2
see supporting information for mean diameter (by TEM)
and size distributions
determined by ICP
based on RhCl3·3H2O used.
[b]
[c]
[d]
tion.[15] The reaction progress was monitored by IR,
with completion of the reaction signaled by the disap-
pearance of the azide absorption around 2099 cmÀ1.
A chemical reduction method was then applied to
test the stabilizing ability of 1 for the formation of
Rh NPs using RhCl3·3H2O as the metal source and
NaBH4 as a reducing reagent, in water at room tem-
perature (Table 1).
Initially, the nanoparticle formation was tested
using a 1:1 1/Rh ratio (Table 1, entry 1). The addition
of NaBH4 to a well stirred solution of 1 and
RhCl3·3H2O immediately led to a color change from
Figure 2. (a-d) Correspond to TEM images of M2, M3, M4
and M5, respectively; M1 has similar morphology as M2,
but with smaller average particle size; (e) HRTEM image of
the “nanoflower” encircled in (d); (f) SAED pattern of the
same “nanoflower” in (d).
light brown to black, but without any precipitate of contain the high resolution TEM (HRTEM) image
bulk rhodium appearing after stirring overnight. TEM and the selected area electron diffraction (SAED)
analysis of the resulting mixture showed the forma- pattern of the “nanoflower” structure outlined in Fig-
tion of the Rh NPs with a mean diameter of 1.6 nm. ure 2d, showing the face-centered cubic (fcc) Rh(0)
To probe the influence of the stabilizer-to-metal ratio lattice in M5. The powder X-ray diffraction (p-XRD)
on the nanoparticle morphology and reactivity, addi- patterns of M2, M3, M4 and M5, along with that for
tional syntheses were conducted varying the ratio 1/ the commercial PEG-2000 reference, are shown in
Rh from 0.3:1 to 0.02:1 (Table 1, entries 2–5). In all Figure 3. Here, the broad peaks at around 2q=41, 48
cases, the corresponding nanoparticles were obtained and 708 correspond to the fcc rhodium(0) planes
in good yields with respect to the rhodium precursor, (111), (200) and (220), respectively,[17a] and the two
and with%Rh (by ICP) comparable to the theoretical sharper peaks at 2q=19 and 238 are ascribed to the
percentage. As
a
reference, the reduction of (120) and (112) planes of PEG-chains.[17b] The absence
RhCl3·3H2O with NaBH4 in the absence of 1 gave of discernible Rh(0) signals in M2 is due to its low
bulk rhodium precipitate (Table 1, entry 6).
rhodium content.
Notably, upon decreasing the stabilizer-to-metal
With the Rh NPs M1–M5 in hand, the hydrosilyla-
ratio, the particles underwent a diameter increase tion of diphenylacetylene with Et3SiH was chosen as
from 1.6 to 32.3 nm and a concomitant morphology the model reaction under a range of conditions
change from small clusters to flower-like nanostruc- (Table 2). In all cases, the neat silane (4 equiv) was
tures (Figure 2a–d).[16] Specifically, Figures 2e and 2f used as the reaction medium, a feature we found ben-
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ing) at 908C under inert atmosphere for 17 h. Under
these conditions, the alkyne was fully consumed (as
gauged by GC), with the corresponding syn addition
product (E)-6a obtained in quantitative yield
(Table 2, entry 1).
The quantitative formation of (E)-6a was main-
tained upon lowering the temperature to 608C
(Table 2, entry 2); however, further decreasing the
temperature to 408C led to a poor conversion after
20 h (Table 2, entry 3). Notably, lowering the catalyst
loading from 0.5 mol% to just 0.1 mol% Rh at 608C
still allowed for a fully selective conversion (Table 2,
entry 4), with the catalyst reaching a respectable turn-
over number (TON=990). In the latter case the reac-
tion was conducted on a 6 mmol scale. A trial was
also conducted in the presence of added water
(Table 2, entry 5, using 2 mmol, 4 equiv of H2O), in
order to determine whether the new rhodium catalyst
would then switch to a hydrogenation mode, as was
the case for the Pd NPs.[9e] Interestingly, unlike with
the Pd system, no corresponding hydrogenated prod-
uct was detected, and the alkyne was once again
cleanly converted to vinylsilane (E)-6a, showcasing
the catalystꢆs water tolerance. In fact, inert atmos-
phere was shown to be unnecessary (Table 2, entry 6),
and the system was not affected by the addition of
solvents, e.g., ethanol or chloroform (Table 2, en-
tries 7-8). Good activity was also observed for the
Rh NPs corresponding to M4 (Table 2, entry 9); how-
ever, no conversion was observed using M3, M2 or
M1 (Table 2, entries 10-12), likely due to the higher
1:Rh ratio in these materials, as shown previously by
others.[18] A control experiment showed that no reac-
tion took place in the absence of a catalyst (Table 2,
entry 13).
Having established Rh NPs M4 and M5 as the best
catalysts, M5 was chosen to achieve the hydrosilyla-
tion of a variety of symmetrical internal alkynes
under the optimized conditions (0.5 mol% Rh,
4 equiv of silane, 608C),[19] as shown in Table 3. The
protocol proved to be highly efficient and selective,
furnishing the corresponding (E)-alkenylsilanes (E)-6
in excellent yields for substrates bearing electron-
withdrawing or electron-donating substituents on the
aromatic ring (Table 3, entries 1-7), as well as with the
non-aromatic substrates (Table 3, entries 8-9).
Only in one case (Table 3, entry 2), a minor amount
of anti-addition product was detected by GC (8%
and 4% in two different runs). It should be noted
that the hydrosilylation of p-methoxy-, p-chloro- and
p-bromo-substituted diphenylacetylene (Table 3, en-
tries 3-5) showed no reaction at 608C, but worked ef-
ficiently at 908C. Interestingly, we believe that this
need for a higher temperature is not the consequence
Figure 3. p-XRD patterns of PEG-2000, M2, M3, M4 and
M5.
Table 2. Optimization of the reaction conditions for the hy-
drosilylation of diphenylacetylene with triethylsilane under
Rh NPs catalysis.
Entry[a] catalyst Rh
[mol%] [mL]
solvent
T
conv. yield
[8C] [%]
[%][b]
1[c]
2
M5
M5
M5
M5
M5
M5
M5
M5
M4
M3
M2
M1
-
0.5
0.5
0.5
0.1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
-
-
-
-
-
90
60
40
60
>99 99
>99 99
3
19[d] 17[d]
>99 99
>99 99
>99 99
>99 99
>99 99
>99 99
4[e]
5
H2O/0.036 60
-
EtOH/1
CHCl3/1
-
-
-
-
-
6[f]
7
60
60
60
60
60
60
60
60
8
9
10
11
12
13
0
0
0
0
-
-
-
-
[a]
Performed in closed vessels (15 mL) with 0.5 mmol of
alkyne and HSiEt3 (0.320 mL, 2 mmol)
isolated yields
experiments of entries 1-5 were performed under dry N2
determined by GC with undecane as internal standard
performed with 6.0 mmol of alkyne
[b]
[c]
[d]
[e]
[f]
experiments of entries 6-13 were performed under air.
eficial and convenient in Pd-catalyzed hydrosilyla- of a lower reactivity, but rather of a higher melting
tion.[9e] Thus, a mixture of diphenylacetylene and trie- point of these substrates, an important factor since
thylsilane was treated with M5 (0.5 mol% Rh load- the reaction is conducted in neat silane.[20] Next, the
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Table 3. Hydrosilylation of symmetric alkynes under cataly-
sis by M5.[a]
Table 4. The recyclability assay of M5 for the hydrosilylation
of diphenylacetylene.
Entry Product
Time Yield
[h]
[%][b]
1
(E)-6a: X=H
15
19
21
20
19
99
86
97
97
95
2[c]
3[d]
4[d]
5[d]
(E)-6b: X=Me
(E)-6c: X=OMe
(E)-6d: X=Cl
(E)-6e: X=Br
Cycle
1st
99
2nd
99
3rd
99
4th
99
5th
99
6th
99
7th
% Yield[e]
80[f]
6
(E)-6 f
19
93
[a]
Reaction mixture after 18 h
Et2O added, image after centrifugation for 10 min at
4000 rpm
supernatant filtered through a short plug of silica gel
excess reagent in the filtrate was removed by evaporation
under an air flow to afford the oily product
isolated yields
[b]
[c]
[d]
7
8
9
(E)-6g
(E)-6h
(E)-6i
6
97
96
99
[e]
[f]
40
20
corrected GC yield with undecane as internal standard.
ether from the reaction mixture, leaving behind
Rh NPs. The recycling procedure used for catalyst M5
is summarized in Table 4. Each time, upon completion
of the reaction (Table 4, image a), diethyl ether was
added and the catalyst was separated by centrifuga-
tion (@ 4000 rpm for 10 min) (Table 4, image b). The
supernatant was transferred and filtered through
a short silica-gel plug (Table 4, image c) affording the
oily product (Table 4, image d) after removal of the
solvent under an air flow. Promisingly, the catalyst
was recycled 6 times with quantitative isolated yields
of (E)-6a. The 7th cycle resulted in a lower yield
under the same reaction time and an incomplete con-
10
(E)-6j: R=OEt
(E)-6k: R=Ph
36
36
99
99
11[e]
[a]
Performed in closed vessels (15 mL) with 0.5 mmol of al-
kynes and 2 mmol of the corresponding silane under air
isolated yields
a minor amount of anti-addition product was detected by
GC (see text and supporting information)
performed at 908C
[b]
[c]
[d]
[e]
chromatographic purification was needed to remove the
excess of triphenylsilane.
hydrosilylation of diphenylacetylene with other si- version, possibly due to a mechanical loss of the cata-
lanes was examined. The reactions with the triethoxy- lyst.[21]
silane and the triphenylsilane (Table 3, entries 10–11)
Next, the nanocatalyst M5 was used in the hydrosi-
also proceeded with high stereoselectivity, although lylation of various asymmetrical alkynes. Theoretical-
longer reaction times were required for the full con- ly four different vinylsilanes could be envisaged in
sumption of the starting alkynes. In all cases, except this case with two stereoisomers (syn- and anti-addi-
for entry 11 in Table 3, the work-up consisted in filter- tion) possible for each of the two regioisomeric forms.
ing the reaction mixture through a pipette with Although the expected syn preference might reduce
a short plug of silica-gel using hexane as eluent, and this number to two, a reasonable degree of regioselec-
then removing the volatile components by evapora- tivity would be possible for substrates with the two
tion under an air flow, to afford a product that did sides clearly differentiated.
not require any further purification.
We began with the hydrosilylation of asymmetrical
Before turning to the hydrosilylation of asymmetric diaryl alkynes (Table 5, entries 1-7). As already seen
alkynes, the recyclability of M5 was investigated. The in our previous research with Pd NPs,[9e] in most cases
presence of the polyether chains makes the stabilizer (Table 5, entries 1-6) the reactions gave a mixture of
and the nanoparticles insoluble in diethyl ether. To the two syn regioisomeric vinylsilanes with little re-
take advantage of this property, the hydrosilylation gioselectivity; only in the case of the naphthyl-substi-
product (E)-6a was directly extracted with diethyl tuted substrate 7c did the reaction lead to a fairly se-
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Table 5. Hydrosilylation of asymmetric diarylalkynes by M5.
lective (88 to 12) hydrosilylation in favour of the
isomer with the SiEt3 group proximal to naphthyl
(Table 5, entry 3). In good agreement with our previ-
ous finding using Pd NPs,[9e] the use of the electroni-
cally differentiated diaryl substrate 7e gave a 62:38
mixture of the regioisomers (E)-8e and (E)-9e, with
the major isomer corresponding to the silylation next
to the more electron-deficient p-acylphenyl group.
Additional experiments at lower temperature were
performed with this electron-deficient substrate. Thus,
treatment of 7e (0.5 mmol) with triethylsilane
(4 equiv) at 508C did not show any evolution of the
reaction after 20 h, probably due to a solubility prob-
lem. Further attempt under the same conditions of
temperature and molar ratio of reactants, but adding
EtOH as solvent (1 mL), gave full conversion and
quantitative yield after 36 h, although the regioselec-
tivity was not improved significantly (65:35 mixture of
(E)-8e and (E)-9e).
As an exception, the hydrosilylation of the sub-
strate 7g with nitro substituent (Table 5, entry 7) gave
a mixture of the four different products (E)-8g, (E)-
9g, (Z)-8g and (Z)-9g with lower overall isolated
yield (76%) and with (E)-8g as the major product.
Care should be taken to analyze the product distribu-
tion immediately upon completion of the reaction,
since the initially formed mixture of the two syn addi-
tion regioisomers could undergo a slow isomerization
to a thermodynamic mixture of all 4 possible isomers.
Indeed, the mixture of (E)-8e and (E)-9e (Table 5,
entry 5) was observed to experience an isomerization
process forming the four isomers when the mixture
had stood in the air for some days, which could be
recognized from the GC traces and 2D NMR tech-
niques (see supporting information). A similar phe-
nomenon occurred in the mixture of (E)-8 f and (E)-
9 f (Table 5, entry 6).
Entry Sub. Time Products[c]
Yield
[a]
[b]
[h]
[%][d]
1
7a
58
99
2
3
7b
7c
20
60
99
99
4
7d
7e
7 f
30
20
14
99
99
99
5[e,f]
6[e,g]
The selective syn addition took place in the reac-
tion of triethylsilane with prop-1-yn-1-ylbenzene 10a
(Table 6, entry 1), giving a 45:55 mixture of (E)-11a
and (E)-12a. For the alkyne 10b (Table 6, entry 2)
bearing a hydroxy group, preliminary experiments led
to a mixture of the mono- and the disilylated product.
Increasing the excess of Et3SiH to 6 equiv led to an
efficient double hydrosilylation, affording a 1:1 mix-
ture of the regioisomeric (E)-11b and (E)-12b in an
overall 95% yield (see supporting information). The
reaction between methyl 2-octynoate 10c and triethyl-
silane (Table 6, entry 3) was relatively regio- and ste-
reoselective, with the major product (E)-12c arising
from the syn addition that placed the triethylsilyl
moiety in gem to the electron-withdrawing ester
group; smaller amounts of the other isomers were
7[e]
7g
30
76
[a]
Performed in vessels (15 mL) with 0.5 mmol of alkynes
under air
uncommercially available alkynes were prepared according
reported methods (see supporting information for details)
the isomers were confirmed by NMR techniques and the
ratio was determined by GC
[b]
[c]
[d]
[e]
[f]
isolated yields
isomerisation processes happen in the air
see text for additional experiments at lower temperature
difficult to make the assignment of the two isomers by
NMR techniques.
[g]
1
also observed by H NMR and GC (see supporting in-
formation). The reduced overall yield (69%) was at-
tributed to some loss in the isolation process due to
higher volatility. Interestingly, the terminal alkyne
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Table 6. Hydrosilylation of asymmetric alkynes bearing significantly different groups in the two sides under catalysis by M5.
Entry[a]
R1, R2
Time
[h]
Products[b]
Yield
[%][c]
1
10a
10b
10c
Ph, Me
30
37
55
95
95
69
2[d]
nBu, (CH2)4OH
COOMe, Pentyl
3
4
10d
Cyclohexyl, H
33
84
[a]
Performed in vessels (15 mL) with 0.5 mmol of alkynes under air
the isomers were confirmed by NMR techniques and ratio was determined by GC
isolated yields
[b]
[c]
[d]
6 equiv of triethylsilane was utilized and difficult to univocally assign the two isomers by NMR techniques.
ethylnylcyclohexane 10d also proved reactive under same conditions. Rhodium trichloride trihydrate af-
the present reaction conditions affording a mixture of forded 97% yield of (E)-6a upon treatment of diphe-
three isomers (Table 6, entry 4).
nylacetylene with triethylsilane (Table 8, entry 1) but
All vinylsilane regio- and stereoisomers 8–9 and it failed in the reaction with methyl 2-octynoate
11–12 were unambiguously characterized by NMR (Table 8, entry 2). On the other hand Rh/Al2O3 gave
spectroscopy using the same experiments and strat- good performances with some symmetric diarylal-
egies described previously for analog derivatives.[9e]
kynes (Table 8, entries 3-6) but the starting material
In addition, the hydrosilylation of 1,3-diynes was was recovered in the cases of 1,4-dimethoxybut-2-yne
also carried out under the present conditions and 1-nitro-4-(phenylethynyl)benzene (Table 8, en-
(Table 7). To our surprise, the hydrosilylation of a ster- tries 7-8). Thus, although the potential of these cata-
ically crowded 2,2,7,7-tetramethylocta-3,5-diyne 13a lysts should be taken into account and further studies
only gave the monohydrosilylation product 14 with are envisaged, our Rh NPs offer wider scope.
the other corresponding isomers undetected (Table 7,
As a final note, it is interesting to compare the per-
entry 1). In contrast, the first attempt at the hydrosily- formance of the newly developed Rh catalyst, i.e.,
lation of dodeca-5,7-diyne 13b using 4 equiv of trie- Rh@1 discussed in this work, with that of the Pd NP
thylsilane gave a mixture of the mono- and bis-hydro- system reported earlier by our group.[9e] Thus, while
silylation products. Repeating the experiment with 6 both catalytic systems afford excellent yields in the
equiv of the silane led, once again, to the pure dihy- hydrosilylation of internal alkynes, the two differ sig-
drosilylation product 15 with an excellent isolated nificantly when it comes to water tolerance. Specifi-
yield (98%) (Table 7, entry 2). This phenomenon cally, the presence of water when using the Pd catalyst
should be ascribed to the dramatic effect of the hin- leads to the switch from the hydrosilylation to trans-
drance of the reagent, which is also observed by fer hydrogenation, affording the corresponding al-
others when applying Pt NPs as catalysts.[9c]
kenes or even alkane species. In contrast, the new Rh
After our good results with Rh NPs and with the system appears to be completely insensitive to the
above mentioned precedents of some Rh(I) com- presence of water (see Table 2, entry 5). In this
plexes active in this addition reaction, we were inter- regard, the hydrogenation reaction with the Pd cata-
ested to test RhCl3·3H2O and Rh/Al2O3 as catalysts lyst has been previously rationalized via a) silane hy-
on the hydrosilylation of internal alkynes under the
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Table 7. Hydrosilylation of symmetric 1,3-diynes.
Entry[a]
Substrate
HSiEt3 [equiv]
R
Time
[h]
Productsc
Yield
[%][b]
1
13a
13b
4
6
tBu
20
20
95
2
nBu
98
[a]
Performed in closed vessels (15 mL) with 0.5 mmol of alkynes under air
isolated yields
[b]
Table 8. Hydrosilylation of internal alkynes with commercially available rhodium catalysts.[a]
Entry
Substrate
Catalyst
Time
[h]
Prod.
Yield
[%][b]
1
2
RhCl3·3H2O
RhCl3·3H2O
18
26
(E)-6a
97
-
-
3
Rh/Al2O3
Rh/Al2O3
Rh/Al2O3
Rh/Al2O3
18
18
18
26
(E)-6a
(E)-6c
(E)-6e
(E)-6g
99
99
95
84
4[c]
5[c]
6
7
Rh/Al2O3
Rh/Al2O3
26
28
-
-
-
-
8
[a]
Performed in closed vessels (15 mL) with 0.5 mmol of alkynes under air
isolated yields
performed at 90 8C.
[b]
[c]
drolysis to afford H2 and b) a subsequent Pd-cata- detect Et3SiOH or (Et3Si)2O. Taken together, these
lyzed hydrogenation (Scheme 2).[22]
experiments suggest that while both catalysts are
Thus, the insensitivity of the Rh NPs toward water active in hydrogenation, the excellent water tolerance
would either stem from their failure to act as efficient in the Rh system appears to stem from the suppres-
hydrogenation catalysts, or from their inability to cat- sion of the oxidative hydrolysis of silanes to form H2.
alyze the hydrolysis of silane. The preliminary control
Following a suggestion made by a referee, we have
experiments showed that exposing diphenylacetylene also performed preliminary experiments to establish
to H2 (1 atm, balloon) in the presence of the Rh NPs whether the excess of the Si-H reagent could be re-
afforded a mixture of the corresponding alkene and duced from the 4 equiv currently used (i.e. neat) to
alkane after 24 h at 608C, showing the catalysts to be below 2 equiv. Indeed, while the use of a neat silane
competent in hydrogenation. In contrast, no hydroly- is convenient in many instances, reducing the excess
sis of Et3SiH was observed upon addition of H2O in of silane is desirable when using solid and/or more ex-
the presence of Rh NPs, as judged by the failure to pensive silane reagents. We were pleased to find that
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Rhodium Nanoflowers
6 times without any loss of activity. Unlike the Pd
system previously developed in our laboratory, the
Rh NP catalyst was shown to be insensitive to the
presence of water, which was traced to the negligible
silane hydrolysis with this catalyst. Preliminary studies
show that the system might be feasible for the use of
nearly stoichiometric amounts of the Si-H reagent
(with addition of solvent) when the silane cost or its
melting point so require.
Scheme 2. Different water sensitivity of the Rh and Pd NP
systems.
Experimental Section
The general comments and the details for the preparation of
compounds 3, 5 and non-commercially available symmetric
and asymmetric alkynes can be found in electronic support-
ing information.
Synthesis of the stabilizer 1
Scheme 3. A hydrosilylation test using 1.2 equiv of triphe-
To a stirred solution of the trialkyne 5 (7 mg, 0.03 mmol),
the azide 3 (182 mg, 0.09 mmol), CuBr(PPh3)3 (0.0045 mmol,
4 mg, 5 mol%/alkyne function) in THF (1.5 mL) in a micro-
wave tube, triethylamine (1.5 mL) was added, and then the
tube was sealed. After 15 min under microwave irradiation
at 1008C (maximum power=200 W), the reaction mixture
was allowed to cool and the solvents were removed under
vacuum affording a yellow solid. The solid was suspended in
water and subjected to dialysis (MWCO 3500 membrane).
Upon the removal of water, the stabilizer 1 was obtained as
nylsilane in EtOH.
a test reaction involving the hydrosilylation of diphe-
nylacetylene (0.5 mmol) with just 1.2 equiv of triphe-
nylsilane in 1.0 mL of ethanol at 608C yielded the cor-
responding vinylsilane (E)-6k in a quantitative yield
(Scheme 3). Under the same conditions, triethylsilane
provided only 73% conversion after 36 h, probably
due to the volatility of the reactant. This result opens
the door for applying the Rh NP catalyst with an es-
sentially stoichiometric amount of the less accessible
silanes; the finding also illustrates an additional ad-
vantage over the Pd NP developed earlier in our labo-
ratory,[9e] with which neither a stoichiometric amount
of silane nor the use of a green solvent such as EtOH
was possible.
a
light yellow solid (175 mg, 92% yield). 1H NMR
(360 MHz, DMSO) d (ppm): 7.72 (s, 3H, triazole-H), 4.64
(broad s, 3H, NH), 4.50 (t, J=5.0 Hz, 6H, NHCH2), 3.85–
3.82 (m, 6H, CH2), 3.64–3.53 (m, CH2 of PEG chains), 3.48–
3.41 (m, 6H, CH2), 7.72 (s, 9H, OCH3); MALDI-TOF MS:
m/z range for [M + H]+ from 5411 (114 CH2CH2O units +
nitrogen-rich core) to 7175 (154 CH2CH2O units + nitrogen-
rich core), separated from 44 D (CH2CH2 unit); the most in-
tense peak at 6161 (131 CH2CH2O units + nitrogen-rich
core); IR (ATR, neat) n (cmÀ1): 2882, 1571, 1519, 1466,
1098, 959.
Conclusions
General procedure for the preparation of Rh NPs
In summary, a nitrogen-rich polyoxyethylenated de-
rivative 1 was designed and prepared for the stabiliza-
tion of Rh NPs. The nanomaterials were prepared in
an efficient manner via the reduction of rhodium(III)
chloride trihydrate by NaBH4 in water at room tem-
perature and fully characterized. Simply by changing
the ratio of the stabilizer to rhodium it was possible
to achieve size and morphology controlable rhodium
M5 (Table 1, entry 5): to a solution of stabilizer 1 (62 mg,
0.01 mmol) and rhodium trichloride trihydrate (135 mg,
0.50 mmol, commercial source with 38% Rh) in degassed
water (450 mL), was added dropwise under nitrogen a solu-
tion of sodium borohydride (284 mg, 7.5 mmol) in water
(50 mL). The mixture was stirred under nitrogen at room
temperature overnight, and then it was filtered through
a Millipore filter. The filtrate was extracted with dichloro-
nanostructures from small atom clusters to nanoflow- methane (6ꢉ20 mL) and the organic phase was dried over
anhydrous sodium sulfate. After the removal of the solvent,
the Rh NPs were obtained as a black powder (110 mg,
Rh%=34.6%, 76% yield with respect to the rhodium
source).
ers. The flower-like Rh NPs proved to be very effec-
tive to catalyze the stereoselective hydrosilylation of
internal alkynes affording (E)-vinylsilanes in quantita-
tive yields under solvent-free conditions. The corre-
sponding products can be directly extracted with di-
ethyl ether and the insoluble Rh nanoflowers then
reused in the next run. The recyclability tests showed
Typical procedure for the hydrosilylation by Rh NPs
Hydrosilylation of diphenylacetylene (Table 3, entry 1): to
that the rhodium nanoflowers could be recycled up to a mixture of diphenylacetylene (89 mg, 0.5 mmol) and M5
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Wusheng Guo et al.
(0.5 mol% Rh, 0.7 mg) into a screw-top sealable tube, trie-
thylsilane (320 mL, 1=0.73 gmLÀ1, 2 mmol) was added and
then heated at 608C for 15 h until total conversion of the
alkyne (GC monitoring). The mixture was filtered through
a plug of silica-gel eluting with hexane, and the solvent and
excess silane was removed under an air flow to afford the
(E)-(1,2-diphenylvinyl)triethylsilane, (E)-6a (146 mg, 99%)
as a light yellow liquid. 1H NMR (360 MHz, CDCl3) d
(ppm): 7.29 (t, J=7.6 Hz, 2H), 7.20 (t, J=7.1 Hz, 1H), 7.14–
7.05 (m, 3H), 7.04–6.90 (m, 4H), 6.76 (s, 1H), 0.95 (t, J=
7.9 Hz, 9H), 0.64 (q, J=7.9 Hz, 6H). 13C NMR
(100.61 MHz, CDCl3) d (ppm): 144.2, 143.3, 138.9, 137.5,
129.7, 128.8, 128.0, 127.4, 127.1, 125.7, 7.5, 2.9.
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Acknowledgements
Financial support from Ministerio de Ciencia e Innovaciꢁn
(MICINN) of Spain (Projects CTQ2009–07881, CTQ2011–
22649, CTQ2012–32436), Consolider Ingenio 2010 (Project
CSD2007–00006), Ministerio de Ciencia y Competitividad
(MINECO) (Project CTQ2013-46705-R), DURSI-Generali-
tat de Catalunya (SGR2009-1441, SGR2014-1105, SGR2014-
1192)), a Ramꢁn y Cajal scholarship to A. S. (MEC, Spain,
RYC-2006–001425), and China Scholarship Council (CSC)
scholarship to W. G. are gratefully acknowledged.
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12 Rhodium Nanoflowers Stabilized by a Nitrogen-Rich PEG-
Tagged Substrate as Recyclable Catalyst for the
Stereoselective Hydrosilylation of Internal Alkynes
Adv. Synth. Catal. 2014, 356, 1 – 12
Wusheng Guo, Roser Pleixats,* Alexandr Shafir,*
Teodor Parella
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