Different radical initiation techniques of hydrosilylation reactions of multiple bonds in water: Dioxygen initiation

Article abstract of DOI:10.1002/poc.1703

The relevance of radical initiation methodologies for the classical hydrosilylation reactions of organic compounds bearing C-C multiple bonds is due to the need to come up with newer and more efficient methods to effect this reaction, on account of its ap

Full text of DOI:10.1002/poc.1703

Special Issue Articles
Received: 10 November 2009,
Revised: 21 January 2010,
Accepted: 4 February 2010,
Published online in Wiley Online Library: 18 June 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1703
Different radical initiation techniques of
hydrosilylation reactions of multiple bonds
in water: dioxygen initiationy
a
b
*
Al Postigo and Norma Sbarbati Nudelman
The relevance of radical initiation methodologies for the classical hydrosilylation reactions of organic compounds
bearing C—C multiple bonds is due to the need to come up with newer and more efficient methods to effect this
reaction, on account of its applications on surface chemistry. In the past, when organic solvents were employed, thermal
and photochemical methods for the chain initiation reaction have been documented (thermal and photochemical
decompositions of azo compounds). We herein present the dioxygen–initiation technique of the classical radical
hydrosilylation reaction of C—C triple bonds with tris(trimethylsilyl)silane ((Me3Si)3SiH) in water. This initiation
technique is confronted with the photochemical radical initiation in the absence of a chemical radical precursor other
than the silane and also confronted with the classical thermal initiation triggered by the decomposition of an azo
compound, both performed in water. The radical-based dioxygen initiation methodology studied in water is shown to
afford the highest Z:E stereoselective ratios of hydrosilylated alkenes. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: dioxygen initiation; hydrosilylation; radical reactions in water; reaction mechanisms; tris(trimethylsilyl)silane
Scheme 1 are expected. That is, the alkyl radicals abstract
INTRODUCTION
hydrogen from the thiol and the resulting thiyl radicals abstract
hydrogen from the silane, so that the thiol is regenerated along
with the chain carrying silyl radical for a given RX.[10,11]
Recently, Postigo et al.[12] tested the reducing agent
(Me3Si)3SiH in water and observed its high stability in deareated
aqueous media and high temperatures. They subjected a series
of organic halides to reduction with (Me3Si)3SiH in water with
different initiators, azo compounds, and Et3B.
The same authors[13] also effected the hydrosilylation reaction
of unsaturated bonds in water, using different hydrophobic
compounds such as aldehydes, alkenes, and alkynes under light
initiation and by thermal decomposition of azo compounds.
Again, the efficiency of the reaction was very good, and in all
cases, good to quantitative formation of the hydrosilylation
The majority of organic radical transformations employing
Si-centered radicals take place in organic solvents, neat media,
and on surface chemistry,[1–3] and only recently has water been
used as a convenient reaction medium for these radicals.
The development of novel water-soluble organosilane com-
pounds and their applications to radical reactions in water is
constantly being paid attention to, as a means to producing
effective radical organic reactions in water, such as reductions of
hydrophilic organic halides, and hydrosilylation of multiple
bonds.[4–6]
In the radical chain processes, the initial silyl radicals are
generated by some initiation. The most popular initiator is
´ ´
2,2-azobisisobutyronitrile (AIBN), with a half life of 1 h at 818C,
generating the incipient radicals that commence the radical
chain reaction. Other azo-compounds are used from time to
time[4] as well as the thermal decomposition of di-tertbutyl
peroxide[5] depending on the reaction conditions. Et3B [6] in the
presence of very small amounts of oxygen is an excellent initiator
for low temperature reactions (down to – 788C). Also air-initiated
reactions have recently been reported.
It is known that in organic solvents, tris(trimethylsilyl)silane,
(Me3Si)3SiH, is both an efficient reducing agent for organic halides
and an excellent hydrosilylation agent of multiple bonds. Also,
the reported methodology of polarity-reversal catalysis is well
documented in organic solvents. The thiol/silane couple not only
shows an efficient synergy of radical production and regeneration,
but could also provide for the use of an amphiphilic thiol, in order
to enhance the radical reactivity at the interface. For the reduction
of an organic halide (RX) by the couple (Me3Si)3SiH/HOCH2CH2SH
under radical conditions, the propagation steps depicted in
*
Correspondence to: A. Postigo, Faculty of Science, University of Belgrano,
Villanueva 1324 CP 1426, Buenos Aires, Argentina.
E-mail: alberto.postigo@ub.edu.ar
a
b
A. Postigo
Faculty of Science, University of Belgrano, Villanueva 1324 CP 1426, Buenos
Aires, Argentina
N. S. Nudelman
Departamento de Qu´ımica Organica, Facultad de Ciencias Exactas
´
y
Naturales, Universidad de Buenos Aires. Ciudad Universitaria, Pabellon II,
3er piso, CP1428, Buenos Aires, Argentina
´
y
This article is published in Journal of Physical Organic Chemistry as a special
issue on Tenth Latin American Conference on Physical Organic Chemistry,
edited by Faruk Nome, Dept de Quimica, Universidade Federal de Santa
Catarina, Campus Universitario – Trindade 88040-900, Florianopolis-SC, Brazil.
J. Phys. Org. Chem. 2010, 23 910–914
Copyright ß 2010 John Wiley & Sons, Ltd.

DIOXYGEN INITIATION
Scheme 1. Polarity reversal catalysis of silanes with thiols
products was achieved. It is worth mentioning the higher cis
stereoselectivity observed in water vs. toluene at 90 8C.
These results showed that the nature of the reaction medium
does not play an important role either in influencing the
efficiency of the radical transformation or in the ability to dissolve
the reagents. The authors attribute the success of the radical
transformations of all water-insoluble materials suspended in
the aqueous medium to the vigorous stirring that creates
an efficient vortex and dispersion. Probably, the radical
initiation benefits from the enhanced contact surface of tiny
drops containing (Me3Si)3SiH and the azo initiator 1,10-azobis
(cyclohexyl)carbonitrile (ACCN).[13]
Scheme 2. Dioxygen initiation in (Me3Si)3SiH-mediated reactions in
benzene
the absence of an azo compound or light. Thus, it was shown that
(Me3Si)3SiH mediates in the radical addition reaction of aryl
iodides to benzene, and the rearomatization is achieved through
oxygen. The mechanism of this reaction in benzene is described
in Scheme 2.
When hydrophilic substrates were subjected to hydrosilylation
reaction with (Me3Si)3SiH in water, and ACCN as an initiator,
excellent results of hydrosilylation of multiple bonds were
achieved by adding the amphiphilic thiol HOCH2CH2SH to the
system. A heterogeneous mixture of water-soluble starting
material, the system (Me3Si)3SiH/HOCH2CH2SH, and ACCN is
flushed with Ar and heated. The treatment of hydrophilic
substrates in water has the additional advantage of an easy
separation of the silane by-products by partition between water
and organic phases.
In another report[14] by the same authors, alkynes such as
1-octyne, 1-cyclohexylacetylene, 1-phenylacetylene, and propio-
lic acid were treated with (Me3Si)3SiH in water under dioxygen
initiation and yielded the respective alkenes stereoselectively in
high yields. Normally Z alkenes (Z:E ratios >99:1, isolated alkene
yields >95%) are formed. In the case of propiolic acid,
2-mercaptoethanol is needed as a chain carrier. Comparison of
these data with the analogous reactions carried out in toluene at
80–90 8C and AIBN as an initiator,[15] not only shows better
product yields but also a higher stereoselectivity in favor of the Z
isomer.
Recently, Zhou and coworkers[16] reported that phenylacety-
lene is hydrosilylated in water in 1 h through air initiation to
render the hydrosilylated product in 86% yield.
(Me3Si)3SiH as a pure material or in solution reacts spon-
taneously and slowly at ambient temperature with molecular
oxygen from air, to form the siloxane. The mechanism of this
unusual process has been studied in some detail.[17] Absolute rate
constants for the spontaneous reaction of (Me3Si)3SiH with
molecular oxygen (reaction 1) have been determined to be
3.5 ꢀ 10ꢁ5 Mꢁ1 sꢁ1 at 70 8C, and theoretical studies elucidated
the reaction coordinates.[17]
Silyl radicals, generated from reaction of (Me3Si)3SiH with
oxygen, abstract the iodine atom from iodobenzene generating
aryl radicals that suffer intramolecular addition to benzene
(solvent) to form the cyclohexadienyl radical. Oxygen-induced
rearomatization affords products along with hydroperoxyl
radicals (HOOꢂ). This radical abstracts hydrogen from the silane
yielding (Me3Si)3Siꢂ radicals completing the chain reaction.
Though it has been established that the thermal decompo-
sition of ACCN is an excellent method for initiating the
hydrosilylation reaction of unsaturated organic compounds in
water, the high temperature needed for the decomposition of the
initiator (ACCN, 70–100 8C) precludes the treatment of thermally
labile substrates and compromises the stability of products. Also,
the photochemical initiation is very sensitive to the nature of the
alkyne studied, since only alkynes with low absorptivities at
254 nm can be employed. The photostability of the hydrosilylated
alkenes at the irradiation wavelength is also a compromising
and limiting factor in choosing this latter initiation technique.
Therefore, the need to explore and expand another different and
milder radical initiating technique of the well-known radical
hydrosilylation reaction with (Me3Si)3SiH, this time performed in
water, becomes a challenging synthetic goal. We herein present
a series of light and thermal sensitive alkyne substrates and
hydrosilylated products where the dioxygen initiation is deemed
the most convenient radical triggering event.
METHODS OF RADICAL INITIATION IN
WATER
Dioxygen initiation
A balloon filled with pure oxygen connected to the vessel where
no apparent bubbling resulted allowed dioxygen to be
introduced up to its solubility limits in water.
The dioxygen-initiated radical-induced hydrosilylations of
lipophilic alkynes in Ar-degassed water are carried out by adding
subsequently (Me3Si)3SiH (6 ꢀ 10ꢁ5 mol) and the substrate
ðMe3SiÞ3SiH þ O2 ! ðMe3SiÞ3Siꢂ þ HOOꢂ
(1)
In organic solvents (benzene), Curran and Keller[18] reported
the (Me3Si)3SiH/dioxygen-mediated radical initiation reaction in
J. Phys. Org. Chem. 2010, 23 910–914
Copyright ß 2010 John Wiley & Sons, Ltd.
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A. POSTIGO AND N. S. NUDELMAN
(5 ꢀ 10ꢁ5 mol). The vessel is tight-sealed, connected with a
balloon filled with 99.99% dioxygen, and vigorously stirred at
20 8C (24 h). As a slight positive oxygen pressure is exerted on the
reaction vessel, air does not leak in the system. For hydrophilic
alkynes, HOCH2CH2SH (0.3 equiv.) is employed as the chain
propagating agent. The isolation and purification of products are
described in the Experimental section.
It should be pointed out that a small minor by-product (ca. 5%)
detected from the oxygen-initiated reactions in water corresponds
to a compound observed by mass spectrometry of mass 280 and
formula minima C9H28SiO2 whose structure has been assigned to
(Me3SiO)2Si(H)SiMe3, arising from the autoxidation of silane.[15] This
by-product was confirmed by GC-co-injection with an authentic
sample which has been synthesized according to a reported
procedure.[15]
When hydrosilylation reactions of substituted phenylacety-
lenes (5–9) are attempted under dioxygen initiation (Table 1,
column 3, entries 5–9) in water, the respective hydrosilylated
styrene derivatives are obtained in high yields, ranging from 88%
yield (that derived from 1-chloro-4-ethynylbenzene (9)) to 96%
yield (that from 4-ethynyltoluene (5)), and 75% yield from
3,4-dimethyl-ethynylbenzene (7). Notably, the stereoselectivity
observed in all tris(trimethylsilyl)silyl-substituted styrenes
is ꢃ95% in favor of the Z isomers. For comparison, the
hydrosilylation in water of the parent phenylacetylene (10)
initiated by dioxygen is shown in Table 1, where the
hydrosilylated styrene is obtained quantitatively in the Z
geometric isomer exclusively (Table 1, column 3, entry 10).[14]
Hydrosilylated styryl derivatives are isolated and purified by
silica-gel column chromatography (CH2Cl2:ethyl acetate 78:22).
The hydrosilylation reactions of water-soluble[13] (1) and (3)
initiated by ACCN have been tested under the conditions
described in subsection ‘Photochemical initiation and thermal
initiation’, although in this case, 2-mercaptoethanol is added as
the chain carrier. The reactions proceed efficaciously affording
the respective hydrosilylated alkenes in optimal reaction yields
(98 and 89%, respectively, Scheme 1, Table 1, column 4, entries
1 and 3). These alkenes are obtained with Z:E isomeric ratios
equal to 88:12 in both cases (determined by 1H NMR
spectroscopy).
Photochemical initiation and thermal initiation
These methods have been described previously.[12–14]
RESULTS
In this account, we report an expansion on the hydrosilylation
reactions with a new series of alkynes in water using dioxygen
initiation, and provide a piece of mechanistic detail.
The water-soluble prop-2-yn-1-ol[13] (1) and prop-2-yn-1-amine
(3) have been tested under dioxygen initiation conditions
employing the amphiphilic 2-mercaptoethanol as the chain
carrier. The reactions proceed efficaciously (24 h) affording
the respective hydrosilylated alkenes in optimal reaction yields
(98%, Scheme 1, Table 1, column 3, entries 1 and 3). These
hydrosilylated alkenes are obtained with Z:E isomeric ratios equal
to 99:1 and 96:4, respectively. Water-soluble hydrosilylated alkene
products are rinsed with pentane (to discard silane excess),
lyophilized, and column-chromatographed by reverse-phase
silica gel with ethyl acetate-methanol (90:10) as eluant. In some
cases, the products are obtained sufficiently pure for spectral
characterization (Supplementary Material).
For comparison, the hydrosilylation reaction of propiolic acid
(4) with (Me3Si)3SiH in water initiated by dioxygen is reported,
where a high Z:E stereoselectivity ratio is obtained (Table 1,
column 3, entry 4).[14]
The hydrosilylation of water-insoluble 3-chloroprop-1-yne (2)
with (Me3Si)3SiH initiated by dioxygen (24 h) affords a high yield
of the corresponding hydrosilylated alkene product (97%), in a Z:E
ratio equal to 99:1 (Table 1, column 3, entry 2). Retention of the
chlorine atom is observed in this hydrosilylated alkene product.
The hydrosilylation of (2) in water with (Me3Si)3SiH initiated by
ACCN affords a high yield of the corresponding hydrosilylated
alkene product (89%), in a Z:E ratio equal to 89:11 (Table 1,
column 4 entry 2).
When substituted phenylacetylenes (5–9) are subjected
to hydrosilylation reactions in water initiated by ACCN, the
respective hydrosilylated styryl derivatives are obtained in good
Table 1. Hydrosilylation reactions of C—C triple bonds. Organic solvent-soluble substrates and hydrophilic substrates (10 mm) in
water with (Me3Si)3SiH (12 mm) under different initiation conditions
O2 (R.T.) isolated yield,
% (Z:E ratio)
ACCN isolated yield,
% (Z:E ratio)
hn isolated yield,
% (Z:E ratio)
Entry
Substrate
1a, [10]
2
3a
4[13,11]
5[16,20]
6[17,21]
7b
Prop-2-yn-1-ol (1)
98 (99:1)
97 (99:1)
98 (96:4)
99 (99:1)
96 (99:1)
89 (95:5)
75 (99:1)
95 (99:1)
88 (95:5)
99 (99:1)
98 (88:12)
89 (89:11)
89 (88:12)
95 (99:1)
90 (85:15)
85 (87:13)
71 (80:20)
89 (85:15)
90 (71:29)
93 (70:30)
99 (95:5)
95 (93:7)
99 (91:9)
95 (97:3)
83 (79:21)
80 (85:15)
67 (66:34)
90 (79:21)
91 (79:21)
98 (75:25)
3-chloroprop-1-yne (2)
Prop-2-yn-1-amine (3)
Propiolic acid (4)
4-ethynyl toluene (5)
1-ethnyl-4-fluorobenzene (6)
3,4-dimethyl-ethynyl benzene (7)
4-ethynyl anisole (8)
1-chloro-4-ethynylbenzene (9)
Phenylacetylene (10)
8
9
10[13,11]
a HOCH2CH2SH (0.3 equiv.) was added as a chain carrier.
b 24 mM (Me3Si)3SiH used, 4 h-reaction. Isolated yield.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 910–914

DIOXYGEN INITIATION
Scheme 3. Hydrosilylation ((Me3Si)3SiH) reactions of triple-bonded sub-
strates in water by different initiation methods
Scheme 4. Mechanism for the dioxygen-initiated hydrosilylation reac-
tions of C—C triple bonds in water
yields albeit with lower stereoselectivities than those observed
under dioxygen initiation (Table 1, column 4, entries 5–10). The
stereoselectivities are nevertheless in favor of the Z alkenes,
ranging from 71:29 to 87:13. For comparison, the hydrosilylation
in water of the parent phenylacetylene (10) initiated by ACCN is
reported in Table 1 (entry 10, column 4) from previous studies,
where the hydrosilylated styrene is obtained almost quantitat-
ively (93%) with a Z:E stereoisomeric ratio equal to 70:20.[13]
The hydrosilylation reactions of water-soluble (1) and (3) have
also been tested under photochemical initiation conditions
employing 2-mercaptoethanol as the chain carrier. The reactions
proceed efficaciously affording the respective hydrosilylated
alkenes in optimal reaction yields (99%, Scheme 1, Table 1,
column 5, entries 1 and 3). These hydrosilylated alkenes are
obtained with Z:E isomeric ratios equal to 95:5 and 91:9,
Hydroperoxyl radicals are known to be poorly reactive towards
closed-shell compounds, and probably expected to recombine to
form oxygen and hydrogen peroxide (or else abstract hydrogen
from the silane to render (Me3Si)3Siꢂ radicals that add to C—C
triple bonds). No product derived from an addition reaction of
hydroperoxyl radicals on C—C triple bonds has been detected
under our reaction conditions. Nevertheless, hydrogen peroxide
is expected to be a by-product of the reaction. The latter was
detected by adding to the aqueous reaction mixture (24 h
reaction of 2-chloro-1-propyne or phenylacetylene, (Me3Si)3SiH,
dioxygen and water, where the hydrosilylated alkene product
was previously detected by gc) a fresh colorless solution of
Fe(SCN)þ (10 mM) which immediately turned light orange due to
the formation of Fe(SCN)þ þ. A blank experiment considering a
freshly prepared oxygen-saturated aqueous mixture of the
respective alkyne and (Me3Si)3SiH did not lead to a change in
color when Fe(SCN)þ was added to the aqueous mixture,
purporting that the chain reaction initiates very slowly. As a
matter of fact, the radical chain reaction under oxygen initiation is
known to have a poor initiation yield, and the effectiveness of the
overall radical hydrosilylation transformation relies on a very
efficient propagation step. Another experiment carried out by
adding H2O2 (5%) to a 10 mM solution of Fe(SCN)þ, isolated
hydrosilylated alkene product (10 mM) and (Me3Si)3SiH (10 mM)
produced an orange solution.
The silyl radicals perform the well-known addition reaction to
C—C triple bonds. Unconjugated s vinyl radicals are known to be
sp2 hybridized and to invert with a very low barrier (Eqn 2).
Although (Me3Si)3Siꢂ radicals have been shown to isomerize
alkenes, the post-isomerization of the hydrosilylation adduct
is not observed due to steric hindrance. The higher
Z-stereoselectivity observed upon the addition of (Me3Si)3Siꢂ
to triple bonds is rationalized in terms of the E s vinyl silylated
radical being more hindered to abstract hydrogen from
(Me3Si)3SiH than the Z s vinyl silylated radical (Eqn 2). The
higher stereoselectivity in favor of the Z isomer observed in water
versus organic solvents, under the experimental conditions,
suggests that additional factors play a role in water. It could be
hypothesized that the hindrance of approach of the bulky silane
to the radicals may also be influenced by the organization of
the organic material dispersed in water. Further work is in
1
respectively, determined by H NMR spectroscopy. For compari-
son, the hydrosilylation reaction of (4) with (Me3Si)3SiH in water
initiated by light is also reported, where a high Z:E stereo-
selectivity ratio is obtained (Table 1, entry 4).[13,14]
The hydrosilylation of (2) with (Me3Si)3SiH initiated by light
affords a high yield of the corresponding hydrosilylated alkene
product (95%), in a Z:E ratio equal to 93:7 (Table 1, column 5 entry
2). Retention of the chlorine atom is again observed in this
hydrosilylated alkene product obtained under light-induced
initiation.
When substituted phenylacetylenes (5–9) are subjected to
hydrosilylation reactions in water initiated by light, the respective
hydrosilylated styryl derivatives are obtained in good yields
albeit with poorer stereoselectivities as compared to those
obtained under dioxygen (Table 1, column 5, entries 5–10). The
stereoselectivities in favor of the Z alkenes range from 66:34 to
85:15. For comparison, the hydrosilylation yield in water of the
parent phenylacetylene (10) induced by light is shown in Table 1
(entry 10, column 5), where the hydrosilylated styrene is obtained
almost quantitatively (98%) with a Z:E stereoisomeric ratio equal
to 75:25.[14]
DISCUSSION
For the dioxygen initiation, we postulate a mechanism as shown
in Scheme 4 as operative in water for the hydrosilylation of C—C
triple bonds.
Upon reaction of dioxygen with (Me3Si)3SiH (in water), silyl
radicals along with hydroperoxyl radicals are generated.
J. Phys. Org. Chem. 2010, 23 910–914
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com

A. POSTIGO AND N. S. NUDELMAN
progress in our laboratory to disclose the effect of water on the
higher stereoselectivity observed in this medium in the
hydrosilylation reaction of alkynes. It is evident that the higher
Z stereoselectivity observed upon dioxygen initiation in water (as
compared to light-induced or thermal) is related to the stability of
substrates and products under the experimental conditions. The
milder dioxygen-initiated hydrosilylation reaction in water, as
compared to the thermal and light-induced methods, circum-
vents issues associated with the stability of substrates and
products under the reaction conditions. In the photochemical
initiation, light is partially absorbed by the substrate (e.g.,
phenylacetylenes).
Acknowledgements
Government grant PICTO-CRUP 30891 from Agencia Nacional de
´
Promocion Cient´ıfica y Tecnica, Argentina is gratefully acknowl-
edged. Conicet is gratefully acknowledged.
´
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CONCLUSIONS
The scope of the hydrosilylation reaction of C—C triple
bonds in water with (Me3Si)3SiH has been expanded from
the use of the classical radical initiation attained through
the thermal decomposition of azo compounds and photo-
chemical initiation to the radical initiation reactions
employing dioxygen.
Unlike the results known from air initiation of silanes in organic
solvents[19] and in the absence of solvent,[16] it is likely that, in
water, the silyl radical production represents an important and
clean route for generating these species.[19] We are currently
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J. Phys. Org. Chem. 2010, 23 910–914