Improved Hiyama cross-coupling reactions using HOMSi

Article abstract of DOI:10.1016/j.tetlet.2011.10.126

Various parameters in the Hiyama cross-coupling reaction of HOMSi reagents with ethyl bromobenzoate were studied. These included solvent, ligand, palladium source, and added water. DMF and THF were found to be excellent solvents. Palladium chloride was found to be the best palladium source and no added water was required. The use of tri-o-tolylphosphine as the ligand proved to be particularly effective.

Full text of DOI:10.1016/j.tetlet.2011.10.126

Tetrahedron Letters 52 (2011) 7178–7181
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Improved Hiyama cross-coupling reactions using HOMSiÒ1 reagents: a novel
application of a palladacycle
Sebastian M. Marcuccio a,b, Ruwan Epa a, Maisa Moslmani b, Andrew B. Hughes b,
⇑
a Department of Chemistry, La Trobe University, Melbourne, Victoria 3086, Australia
b Advanced Molecular Technologies Pty Ltd, Unit 1/7-11 Rocco Drive, Scoresby, Victoria 3179, Australia
a r t i c l e i n f o
a b s t r a c t
Various parameters in the Hiyama cross-coupling reaction of HOMSiÒ reagents with ethyl bromobenzo-
ate were studied. These included solvent, ligand, palladium source, and added water. DMF and THF were
found to be excellent solvents. Palladium chloride was found to be the best palladium source and no
added water was required. The use of tri-o-tolylphosphine as the ligand proved to be particularly
effective.
Article history:
Received 4 September 2011
Revised 12 October 2011
Accepted 21 October 2011
Available online 29 October 2011
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Keywords:
HOMSiÒ
Hiyama reaction
Palladacycle
Tri-o-tolylphosphine
Cross-coupling
Our interest in the silicon-mediated coupling reaction known as
the Hiyama reaction has developed as a parallel to the much stud-
ied and extremely useful boron-mediated Suzuki reaction.2–8 The
Suzuki reaction relies on the generation of a tetrasubstituted bor-
on-intermediate that is able to transfer a ligand to palladium.
The palladium complex formed then undergoes reductive elimina-
tion to form the desired C–C coupled products. Many of the prop-
erties of boron that contribute to the great success of the Suzuki
reaction are also present in the chemistry of silicon in the Hiyama
reaction. The Hiyama reaction is a silicon-mediated palladium-cat-
alyzed coupling reaction that has developed enormously from its
advent to the current day.9–37 Silicon-mediated coupling chemistry
has some notable advantages that warrant its further develop-
ment. Firstly, there is a broad and growing range of silicon reagents
commercially available. Secondly, silicon is the second most abun-
dant element on Earth, whereas boron has limited supplies.38
Hiyama et al.39,40 introduced carefully designed silicon re-
agents, the alkenyl, and aryl[(2-hydroxymethyl)phenyl]dimethyl
silanes (HOMSiÒ).1 A key design feature of the HOMSiÒ reagents
is the presence of two aryl groups on the silicon in many cases.
In other cases, the two relevant substituents are aryl and vinyl.
There is no evidence in either situation that the hydroxymethyl
aryl group transmetallates to palladium in preference to the intro-
duced aryl or vinyl substituent.41,42 Presumably, this is due to the
bidentate nature of the hydroxymethylaryl ligand at the point of
transmetallation. These reagents incorporate an intramolecular hy-
droxy group for silicon activation under basic conditions, they are
readily prepared and recyclable (Scheme 1).
Hiyama and others have recognized these couplings have a
number of parameters that should be optimized for best reaction.
Also the reactions have alternative modes that can enhance the
range of applications. Despite a number of papers reporting further
work on the Hiyama reaction since the disclosure of the HOMSiÒ
reagents,43,39,44–47,42,48 there is still much work to be done in opti-
mizing their use, reaction conditions, and in exploiting different
reagents and coupling modes including the development of cop-
per-free reactions.49
We herein report our preliminary results from our first foray
into this exciting field of silicon mediated Hiyama coupling chem-
istry using HOMSIÒ reagents.
In this Letter, a number of Hiyama reaction parameters for some
exemplary aryl substrates were investigated. These parameters in-
clude the presence of water, alternative solvents, ligands, and pal-
ladium catalyst.
PdCl2—ligand (cat.)
OH
R
O
R1—I
+
CuI (cat.), 35-80 °C
K2CO3, DMSO
R1
+
R
Si
Si
HOMSi®
R = R1 = alkenyl, aryl
RMgBr, Et2O or THF, 0 °C to rt
⇑
Corresponding author.
E-mail address: andrew.hughes2@optusnet.com.au (A.B. Hughes).
Scheme 1. HOMSiÒ reagent coupling and recycling.
0040-4039/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.126

S. M. Marcuccio et al. / Tetrahedron Letters 52 (2011) 7178–7181
7179
HOMSiÒ reagents have a number of important attributes includ-
K2CO3, 9,
PdCl2, CuI,
CO2Et
CO2Et
OH
Ph
ing good functional group compatibility, reactions can be per-
formed under fluoride-free conditions, the organosilicon ‘by-
product’ can be recycled if required and the cross-coupling reac-
tions can be turned ‘OFF’ by the use of suitable protecting groups
on the 2-hydroxymethyl group.
+
Si
Br
º
Ph
solvent, 80
C
8
10
5
PPh2
70% conversion
N
The HOMSiÒ reagents required for the coupling reactions can be
prepared by standard protocols (Scheme 2)44 or purchased from
commercial sources.50 2-Bromobenzyl alcohol 1 was protected as
its tetrahydropyranyl ether 2 using dihydropyran and hydrochloric
acid. Metal-halogen exchange with n-butyllithium gave the aryl
anion, which reacted with chlorodimethylsilane to give the
dimethylsilane 3. Under acidic conditions the THP ether was
hydrolyzed and cyclization of the alcohol gave 2-oxa-1-silaindan
(4). This 2-oxa-1-silaindan is the central intermediate for the for-
mation of HOMSiÒ reagents. In the current study bromobenzene,
4-chlorobromobenzene and 4-methoxybromobenzene were each
converted into the corresponding Grignard reagent and on reaction
with 2-oxa-1-silaindan (4) gave HOMSiÒ reagents 5, 6, and 7,
respectively.
An initial Hiyama coupling (Scheme 3) was performed to estab-
lish a baseline result for the reaction (Table 1, entry 1). This cou-
pling reaction was designed to be close to established general
conditions for the Hiyama coupling.44 The coupling partners 5
and 8 were mixed with potassium carbonate, cuprous iodide, and
palladium(II) chloride in dry DMSO. An amount of water
(1.5 equiv) and the iminophosphine ligand 9, often employed by
Hiyama, were added. The reaction was heated at 80 °C for 15 h
and work-up gave a 70% conversion to product 10. This result
was determined by the comparison of the integrations of two 1H
NMR signals. Ethyl 4-bromobenzoate (starting material) displayed
a doublet at 7.87 ppm, while the biphenyl product 10 showed a un-
ique doublet at 8.12 ppm in the 1H NMR spectra.
9
Scheme 3. Benchmark Hiyama coupling.
Table 1
Hiyama cross-coupling reactiona varying the solvent
Entry
Solvent
Time (h)
Conversionb (%)
1
2
3
4
5
6
7
8
9
DMSO/1.5 equiv H2O
DMSO/2.0 equiv H2O
DMSO/4.0 equiv H2O
DMSO/20 equiv H2O
DMFc
15
15
15
15
15
15
45
15
15
65–70
71
50
21
75
77
58
9
THFc,d
1,4-Dioxanec
Toluenec
MeOHd
0
a
Reaction as depicted in Scheme 3 using a mixture of 5 (1 equiv), ethyl 4-bro-
mobenzoate (0.84 equiv), ligand 9 (0.03 equiv) and PdCl2 (0.025 equiv).
b
% Conversion was determined by comparison of the integrations of the signals
at 7.87 ppm for the starting material 8 and 8.12 ppm for the product 10 in the 1H
NMR spectra.
c
No added water.
d
Reaction performed at 65–66 °C.
cally dried prior to use and may contain a small amount of adven-
titious water. The ethereal solvent THF (Table 1, entry 7) was
similarly effective (77% conversion) even though this reaction oc-
curred at a lower temperature (66 °C). However, the reaction
seemed to be much slower when the related solvent 1,4-dioxane
was used; there was a marked improvement in conversion at the
longer reaction time (Table 1, entry 7, 45 h, 58% conversion).
Conversion was markedly decreased when toluene was used
(Table 1, entry 10). This is thought to be due to the lower solubility
of the reagents in this solvent. The protic solvent, methanol (Table
1, entry 11) afforded no reaction. Presumably, methanol behaves
much like water in excess and interferes with the reaction, partic-
ularly with ligand exchange processes.
Further work on the use of different solvents is suggested by
these results. Ideal solvents seem to be DMF and THF. Inclusion of
a small amount of water is also convenient but the question re-
mains if this might also be replaced effectively by a small amount
of an alcohol such as methanol, ethanol or 2-propanol which might
still improve potassium carbonate solubility but have less propen-
sity to interfere in other reaction processes. The useful improve-
ment in conversion by including water might also be unnecessary
if a more effective, organic soluble base was implemented instead
of potassium carbonate.
Early HOMSiÒ coupling reactions employed solvents such as
DMSO45,51 and THF.40,39,42 Later, the use of DMF was also re-
ported.47,51 Notably, when DMSO was the solvent it was not always
clear whether water (adventitious or added) was included or not.45
The effect of water on the reaction needed to be considered and
also a broader range of solvents examined. Accordingly, the first
variations (Table 1, entries 2–4) had different amounts of water in-
cluded. The initial reaction (entry 1) included 1.5 equiv of water.
The 2 equiv used in entry 2 had no effect (71% conversion) com-
pared to entry 1. But amounts of water above this caused a signif-
icant decrease in the reaction conversion (entries 3 and 4). In the
case of entry 4 (20 equiv H2O) it is notable that the reaction was
still occurring; however, only 21% conversion was observed. Small
amounts of water (2 equiv or less) do not interfere with this pro-
cess to a great extent, but do have the effect of improving the sol-
ubility of the base K2CO3 needed to deprotonate the HOMSiÒ
reagent and this positive effect gives higher conversions.
Substitution of DMSO/H2O for DMF gave a small improvement
in conversion (Table 1, entry 5, 75% conversion). It should be noted
that the DMF used was a commercial sample that was not specifi-
In the next phase of study, the palladium ligand was considered.
A broad range of different substances have been used as the palla-
dium ligand in Hiyama reactions (Fig. 1). Hiyama’s studies have
found that in general the more sterically bulky ligands give higher
cross-coupling yields.41,42 Two of the better ligands being
930,32,44,45 and RuPhos.47,50
Ligand 9 was available in our laboratory and its use was a
convenient point of comparison. To our delight it was found that
the o-tolyl phosphine ligand 11 gave conversion >90% (Table 2,
entry 9) and this was significantly better than reactions employ-
ing 9 as ligand. In addition, its isomer, the p-tolyl phosphine 12
gave lower conversion (Table 2, entry 8) under the same reaction
conditions.
Scheme 2. HOMSiÒ reagent preparation.

7180
S. M. Marcuccio et al. / Tetrahedron Letters 52 (2011) 7178–7181
attention in this study focused on the use of palladium(II) chloride
this
PPh3
tBu3P
ref. 32,46,52 ref. 50
Cy3P
Me
P
study
and different ligands. PdCl2 and the iminophosphine ligand 9 gave
75% conversion (Table 2, entry 7). In Table 2, entry 8, the ligand
was replaced with tri-p-tolylphosphine 12. The conversion with
this ligand was moderate (53%) and reminiscent of earlier reactions
(Table 2, entries 2–4). It was surprising then that changing the li-
gand to tri-o-tolyl phosphine 11 (Table 2, entry 9) gave a marked
increase in conversion. The conversion (>98%) using ligand 11
was the highest found in this study. The use of the alternative
HOMSiÒ reagents 5b and 5c (entries 10 and 11) showed electron
donation had little effect while 5c caused a reduction in conver-
sion. Interestingly, changing the coupling substrate from 8 to
methyl 4-iodobenzoate (entry 12) gave very high conversion while
use of the chlorobenzoate was poor (result not shown).
3
ref. 50
12
PPh2
N
3P
3P
ref. 30,32,45,46,52
O
Me
11
9
ref. 30,32,45,46
PtBu2
QPhos
ref 49
Fe
Ph
PCy2 RuPhos
Ph
Ph
Ph
Ph
ref. 48,52
iPrO
OiPr
OiPr
Figure 1. Ligands used in the Hiyama reaction.
To conclude the current study, the importance of various reac-
tants was examined through a series of experiments involving
selective omission of specific components (Table 3).
Table 2
Table 3, entry 1 shows omitting the potassium carbonate base
still leads to an 8% conversion. The role of the carbonate base is
to deprotonate the HOMSiÒ reagent to promote formation of the
pentavalent silicon species which then transmetallates to palla-
dium. Not surprisingly, omission of palladium (Table 3, entry 2) re-
sulted in 0% conversion as the requisite complexes for the reaction
could not form. Critical to the complex formation are the phos-
phine ligands and so their omission (Table 3, entry 3) also gives
0% conversion. Lastly, ensuring the absence of water (Table 3, entry
4) still gives >90% conversion indicating, together with earlier re-
sults, that the reaction can tolerate small amounts of water but
added water is not required for formation of the complex necessary
for high conversion into the product.
In conclusion, this work has resulted in the discovery that a
highly efficient phosphine ligand (tri-o-tolyl phosphine) gives gen-
erally higher conversions in Hiyama cross-coupling reactions than
other phosphine ligands. It was found that adventitious water did
not have a deleterious effect on the cross-coupling reaction and
consequently no rigorous drying of solvents was required. DMF
and THF are readily available solvents giving high conversions in
the reactions reported. In this study the best palladium source
was palladium(II) chloride. In our hands palladium(II) acetate
was not effective. Electron-rich and electron-poor substituents
are tolerated by the HOMSiÒ reagents and aryl bromides and io-
dides give excellent results as coupling partners. We are currently
exploring the use of other palladacycles and the role of copper in
these reactions. The results of these studies will be reported in
due course.
Formation of biphenyl 10 by varying the palladium ligand and palladium source
Entry HOMSiÒ Pd source
Liganda Reaction time
(h)
Conversion
(%)
1
2
3
4
5
6
7
8
9
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5a
Pd(OAc)2
(Ph3P)2PdCl2
(Ph3P)2PdCl2
Pd2(dba)3
Pd(dppf)Cl2
10% Pd/C
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
9
30
15
30
30
15
30
30
15
15
15
15
15
0
53
65
52
13
69
75
53
9
9
—
—
—
9
12
11
11b
11c
11
>98
>90
63
10
11
12d
PdCl2
>98e
a
b
c
3 mol % of ligand used.
9 mol % of ligand used.
12 mol % of ligand used.
Methyl 4-iodobenzoate used instead of 8.
Methyl ester formed and not 10 as shown.
d
e
In the present study, a limited range of ligands were trialed with
different palladium sources (Table 2).
Following the results from Table 1, the ligand variations were
conducted using DMF as solvent. For the reactions shown in Table
2 the limiting reactant was 4-bromobenzoate 8 (2.1 mmol). The
common reagents and conditions were cuprous iodide (4 mol %),
potassium carbonate (2 equiv) and the reaction was performed at
80 °C. A common source of palladium is palladium(II) acetate.
When this salt was used (Table 2, entry 1) with ligand 9 the result
was no reaction. This was surprising and may be an effect similar
to the presence of excess water, but in this situation caused by
the acetates. Changing to bis(triphenylphosphine)palladium(II)
chloride (Table 2, entries 2 and 3) gave an immediate improvement
to 53% conversion after 15 h and 65% conversion after 30 h reaction
time. Table 2, entry 4 shows the use of tris(dibenzylideneace-
tone)dipalladium(0). The conversion using this catalyst was mod-
erate (52%). Use of the ferrocene-containing phosphine ligand,
1,10-bis(diphenylphosphanyl)ferrocene (dppf) as its palladium(II)
chloride complex gave a disappointing 13% conversion (Table 2,
entry 5). Table 2, entry 6, shows that 10% palladium(0)-on-charcoal
in the absence of any additional ligand such as 9 can serve as an
effective palladium source that is at least as good as previous en-
tries giving close to 70% conversion after 30 h reaction time. Most
A typical experimental procedure. To a mixture of 5 (611 mg,
2.5 mmol), K2CO3 (582 mg, 4.2 mmol), ligand 9 (31 mg, 84
and PdCl2 (11 mg, 63 mol) in dry DMSO (12 ml) was added ethyl
4-bromobenzoate (8) (480 mg, 2.1 mmol), H2O (75 l), and CuI
(16 mg, 84 mol). The resulting mixture was heated with stirring
lmol),
l
l
l
to 80 °C for 15 h. The mixture was then allowed to cool and was di-
luted with H2O (25 ml). The aqueous mixture was extracted with
Et2O (2 ꢀ 25 ml). The combined organic layers were washed with
brine (25 ml), dried (MgSO4), filtered and the filtrate evaporated
to dryness in vacuo. The residual crude oil was then submitted
for 1H NMR analysis.
Table 3
Effect of omitted reagents on the Hiyama cross-coupling reaction
Entry
Omitted reagent
Conversion (%)
1
2
3
4
K2CO3
8
0
Pd catalyst
(o-Tol)3P
H2O
0a
>90
a
By-products formed.

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