Ruthenium-catalysed multicomponent synthesis of borasiloxanes

Article abstract of DOI:10.1039/c7cc00787f

We present the selective atom economical synthesis of borasiloxanes using a multi-component approach directly by the one-pot ruthenium catalysed reaction of boranes, silanes and water.

Full text of DOI:10.1039/c7cc00787f

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Ruthenium-catalysed multicomponent synthesis of borasiloxanes
Basujit Chatterjee and Chidambaram Gunanathan*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We present the selective atom economical synthesis of
borasiloxanes using a multi-component approach directly from the
one-pot ruthenium catalysed reaction of boranes, silanes and
water.
Boron and silicon compounds have found extensive
applications in synthetic, medicinal and material chemistry.1
Borasiloxanes are used in functional inorganic material
synthesis as they possess inherent molecular properties such
as high stability providing resistance to heat and chemical
reactions.2 Borasiloxanes form cages, act as bifunctional
molecules and used as polymeric sensor for amines.3 The
conventional synthetic methods comprise the reaction of
hydroxyborane with silane derivatives or the reaction of silanol
with borane derivatives leading to the formation of
borasiloxanes.4 Such conventional synthesis of silane and
borane derivatives5 requires stoichiometric reagents, tedious
experimental conditions and work up, which make the
ultimate synthesis of borasiloxanes multi-step.
Scheme 1 Recent advances in the synthesis of borasiloxanes.
Catalyst screening embarked on [(Ru(p-cymene)Cl2)2] 1. Thus,
reaction mixture of
1 (0.5 mol%), pinacolborane (1 mmol),
triethylsilane (1 mmol) and water (3 mmol) in toluene (2 ml)
were stirred at room temperature for 2 h and then heated to
100 °C for 24 h, which provided borasiloxanes in 57% isolated
yield (entry 1, Table 1); similar reaction at 125 °C provided the
product in 63% yield (entry 2). This result indicated that the
elegant catalytic coupling of boranes, silanes and water to
borasiloxanes is indeed feasible. Next, the monohydride
To alleviate these problems, two catalytic methods were
developed in recent times for the synthesis of borasiloxanes.6,7
Marciniec reported the pioneering coupling of vinylboronates
with silanol (Scheme 1a).6 Nakazawa and coworkers have
disclosed a remarkable metal carbonyl complex catalysed
photolytic synthesis of borasiloxanes (Scheme 1b).7 However,
synthesis of borasiloxanes from borane, silane and water in a
direct multicomponent pathway is desirable, highly atom
economical, and remains a challenge (Scheme 1c). Recently,
we have reported the ruthenium-catalysed chemoselective
hydrosilylation8 of aldehydes, and hydroboration9 of carbonyl
compounds, nitriles, imines and pyridines. Herein, we report
the ruthenium-catalysed selective synthesis of borasiloxanes
directly from boranes, silanes and water. This efficient catalytic
process proceeds with the liberation of molecular hydrogen
and water and generates no waste.
bridged dinuclear complex [{(η6-p-cymene)RuCl}2(μ-H-μ-Cl)]
2,
was used as a catalyst leading to the formation of the
borasiloxanes in 80% yield (entry 3). To develop more efficient
catalyst, mononuclear ruthenium complexes 3-7 were
explored.9b While PPh3 ligated ruthenium catalyst
provided 50% yield (entry 4), electron-rich PCy3 coordinated
catalyst provided borasiloxanes in 10% yield (entry 5),
3 (0.2 mol%)
4
perhaps due to strongly bound phosphine ligands encumber
further reaction at the metal centre. Thus, upon using labile
pyridine ligated complex
and 125 °C, borasiloxane was obtained in 88% and 96% yields,
respectively (entries 10-11). Further, catalysts and
5 as a catalyst in toluene at 100 °C
6
7
provided 29% and 80% yields, respectively (entries 12-13).
When RuCl3 is used as a catalyst, the product was isolated in
47% yield (entry 14). Control experiment provided unreacted
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silane and hydroxyborane implying the catalyst is essential for
successful formation of products (entry 15). To test weather
chloride ion play any role in catalysis, TBACl (1 mol%) was
employed in which no reaction was observed (entry 16). Thus,
pyridine ligated complex
5 is preferred as the most suitable
catalyst for the synthesis of borasiloxanes.
Table 1 Screening of catalysts and reaction conditions for the selective synthesis of
borasiloxanesa
Scheme 2 Synthesis of borasiloxanes. Silane (1 mmol), borane (1 mmol), catalyst
5
(0.2 mol%), degassed water (3 mmol) and toluene (2 ml) were stirred at room
temperature for 2 h and then heated to 125 °C for 24 h. Yield corresponds to
isolated pure products.
entry
catalyst
(mol %)
1 (0.5 )
1 (0.5 )
2 (0.2)
3 (0.2)
4 (0.2)
5 (0.2)
5 (0.2)
5 (0.2)
5 (0.2)
5 (0.2)
5 (0.2)
6 (0.2)
7 (0.2)
RuCl3 (0.2)
-
Solvent
temp.
(°C)
100
125
125
100
125
80
pinBOSiEt3
(% yield)b
1
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH3CN
57
63
80
50
10
80
-
3
4
5
6
7
rt
8
80
-
9
THF
80
15
88
96
29
80
47
-
10
11
12
13
14
15c
16d
toluene
toluene
toluene
toluene
toluene
toluene
toluene
100
125
100
100
125
125
125
TBACl (1)
-
aCatalyst, pinacolborane (1 mmol), silane (1 mmol), degassed water (3 mmol) and
solvent (2 ml) were stirred at room temperature for 2 h and then heated at
indicated temperature for 24 h. bYield of isolated product. cControl experiment
without catalyst.dTetrabutylammonium chloride is used as catalyst.
The formation of silanol was highly efficient with catalyst 5 and
the evolution of dihydrogen at room temperature was
observed during the reaction progress.10 Upon reaction with
H2O, pinacolborane produced hydroxyborane quantitatively
with concomitant liberation of dihydrogen, which also
occurred at room temperature. The condensation reaction of
silanol and hydroxyboranes was realized upon heating the
reaction mixture at 125 °C to accomplish the formation of
borasiloxanes selectively, overcoming the self-condensation
Scheme
3 Synthesis of (diboryl)siloxanes and (disilyl)boroxanes. Substrates,
catalyst (0.4 mol%), degassed water (10 mmol, 180μl) and solvent (2 ml) were
stirred at room temperature for 5 h and then heated to 125 °C for 24 h. Yield
corresponds to isolated pure compounds.
Scope of this direct coupling reaction was explored with
disubstituted silanes. Diphenylsilane, diethylsilane, and
methylphenylsilane exhibited efficient reactivity and provided
the corresponding (diboryl)siloxanes in excellent yields
(Scheme 3a). Further, we envisaged the reaction between
boronic acids and silanes. Unexpectedly under the optimized
due to the formation of stronger B−O−
Si bond.3,11 Thus,
applying the optimized condition, the substrate scope (alkyl,
alkoxy and aryl silanes) was investigated and found that the
complex
5 is highly effective in synthesis of a wide-range of
borasiloxanes from boranes, silanes and water (Scheme 2).
condition, catalyst
5
provided the expected products in poor
as a catalyst, reactions occurred
yields. However, upon using
2
effectively and an assortment of aryl and alkyl boronic acids
underwent facile reaction with various silanes to provide a
2 | J. Name., 2012, 00, 1-3
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range of (disilyl)boroxanes in good to excellent yields (Scheme Stoichiometric experiments were performed with complex
5
3b).
and triethylsilane or pinacolborane, which confirmed the
1
When boric acid was subjected to the
2
catalysed borasiloxane formation of complex
2
(monitored by H NMR,
from in presence of
were triethylsilane was sluggish and reaction completed after 12 h,
isolated in good yields. Similarly, triphenylsilane also led to the whereas the reaction was much faster in presence of
formation of tris(triphenylsilyl) borate in 70% yield (Scheme pinacolborane and complete formation of occurred within 30
4); unexpectedly, a minor amount (10%) of 1,1,1,3,3,3- min. On contrary, complex didn’t react with phenylboronic
hexaphenyldisiloxane also formed in this reaction. Further, acid and no formation of complex was observed (Scheme
the structure of tris(triphenylsilyl) borate ( ) was unequivocally 5d). These observations confirm that is the true catalyst
involved in the reactions of pinacolborane as well (Scheme 2
and 3a). However, the relatively less reactivity of (see Table
1) over catalyst is not fully understood at this stage; perhaps
the liberated pyridine from (upon formation of ) act as a
weak base and influences the condensation reaction.15,16 As
the formation of complex from occurs on a diminished rate
in silane, when boronic and boric acids (with them do not
δ
-10.18
−
formation with different silanes (3 equiv.) and H2O (10 equiv.), ppm).8,9 However, the generation of
2
5
the corresponding tris(trialkylsilyl)borate products
8
2
5
9
2
8
2
corroborated by single crystal X-ray analysis (Scheme 4).
2
5
5
2
2
5
5
react, see Scheme 5d) are used as boryl partners, catalyst
resulted in poor yields, whereas direct use of catalyst
provided good reactivity.
5
2
Scheme 4 Synthesis of borasiloxanes using boric acid and crystal structure of
tris(triphenylsilyl) borate
crystallized along with
8
.
1,1,1,3,3,3-hexaphenyldisiloxane
9 also co-
8
, which is removed for clarity.
Series of elementary reactions and labelling studies allowed us
to understand the choice of catalysts ( and ) and the
reaction mechanism (Scheme 5). When the independent
reactions of triethylsilane, water with catalysts and (both
2
5
2
5
0.2 mol%) were monitored by 1H NMR, quantitative
triethylsilanol formation13 was observed after 2 h and 6 h,
respectively (Scheme 5a). 1H NMR spectra of both reaction
mixtures indicated that 2 (δ -10.18 ppm) is the major catalytic
species present in solution; however, a minor hydride signal
was observed at
δ -11.54 ppm (1:10 for 2 and 1:8 for 5, major
Scheme 5 Mechanistic studies.
signal being
2
).13 Monitoring the reaction of triethylsilane and
2
D2O catalysed by
2
(0.2 mol%), formation of HD (t, JHD = 48.0
Silanol and hydroxyboranes were independently heated at 125
°C in toluene for 24 h and further 29Si, 11B and 1H NMR analyses
confirmed that there occur no self-condensations (Scheme 5e).
The cross-condensation between silanol and hydroxyboranes
proceeded with remarkable selectivity. The reaction of isolated
triethylsilanol and hydroxyborane, provided the corresponding
borasiloxane in 92% yield (Scheme 5f). The cross-condensation
Hz) was observed in the 1H NMR (Scheme 5b).14 Control
experiments revealed the necessity of catalyst in the silanol
formation step (Scheme 5a and 5b). The tested silanol
formation with RuCl3 (0.2 mol%) and TBACl (1 mol%) also
resulted in no reaction (Scheme 5a). Further, oxygen labelled
water (H218O) was used and the corresponding 18O labelled
borasiloxane was isolated in 90% yield. The presence of
labelled oxygen in the product was identified by the mass
spectroscopy [(m/z = 299.1521 (M+K)+], which confirmed
water as the source of oxygen atom (Scheme 5c) in the
products.14
reaction was also performed with catalyst
2 (0.2 mol%), which
provided 95% borasiloxane, indicating the insignificant effect
of catalyst in cross-condensation reactions. Further, Reaction
of isolated hydroxyboranes and triethylsilane was performed
in the presence of catalyst
2,
which afforded the
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2
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Chem., Int. Ed., 2000, 39, 3598-3601; (b) T. Fujinami, M. A.
corresponding borasiloxane in only 9% yield and excluded the
possibility of dehydrogenative coupling (Scheme 5g).
Mehta, K. Sugie and K. Mori, Electrochim. Acta., 2000, 45
,
On the basis of above experiments, a plausible mechanism is
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proposed for the synthesis of borasiloxanes catalysed by
(Scheme 6). The complex undergoes oxidative addition with
water17 to form a Ru(IV) intermediate
, which reacts with
2
2
I
silane to generate coordination complex II by liberating a
molecule of hydrogen. Further oxidative addition of silane8,13
provide Ru(IV) intermediate III from which silyl and hydroxyl
ligands undergo reductive elimination to deliver silanol and
Ru(II) intermediate IV, which reacts further with water to close
a catalytic cycle. Hydroxyborane formed from the reaction of
pinacolborane and water undergoes condensation reaction
with catalytically generated silanol and results in borasiloxane
and water (Scheme 6). The ESI-MS analysis of the reaction
3
4
A. Andrianov and T. V. Vasil’eva, Chem. Abstr., 1968, 69
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136, 6183-6186.
mixture of
H)+) was observed, indicating the possible involvement of
intermediate
in the catalytic cycles.14
2 with H2O was performed in which I (m/z = 289 (M-
5
6
I
7
8
9
B. Chatterjee and C. Gunanathan, Chem. Commun., 2014, 50
888-890.
,
(a) A. Kaithal, B. Chatterjee and C. Gunanathan, Org. Lett.,
2015, 17, 4790-4793. (b) A. Kaithal, B. Chatterjee and C.
Gunanathan, Org. Lett., 2016, 18, 3402-3405. (c) A. Kaithal,
B. Chatterjee and C. Gunanathan, J. Org. Chem. 2016, 81
11153-11161.
10 (a) M. Jeon, J. Han and J. Park. ACS Catal., 2012, 2, 1539-
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1549. (b) S. T. Tan, J. W. Kee and W. Y. Fan, Organometallics,
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Adv. Synth. Catal. 2009, 351, 1405-1411.
11 T. R. Spalding, B. J. O’Leary, L. Neville and G. Ferguson in Ref.
Scheme 6 Proposed mechanism for the formation of borasiloxanes
1e, pp. 92-95.
12 As the reaction proceeded, H NMR signal corresponds to
In conclusion, an effective protocol for the selective synthesis
of borasiloxanes catalysed by simple ruthenium catalysts is
demonstrated. High atom-economy, selectivity, catalytic
1
Si
signal
−
H (
δ
= 3.67 ppm, multiplet) disappeared and a singlet
= 2.05 ppm, corresponds to Si-OH emerged.
δ
efficiency, abundant and cheap resources, benign by products 13 This minor intermediate complex remains elusive to isolation
and further characterization. However, the previously
(water and molecular hydrogen) makes this method highly
attractive for synthesis of borosilaxane materials.
observed [Ru(H)2(SiEt3)2(η6-p-cymene)] complex (
ppm) was not involved in this reaction. See Ref. 8.
14 See Supporting Information.
δ -13.53
We thank Science and Engineering Research Board, New Delhi
(SR/S1/OC-16/2012 and SR/S2/RJN-64/2010), NISER and
Department of Atomic Energy for financial support. B.C. thanks
UGC for a research fellowship. C.G. is a Ramanujan Fellow.
15 (a) S. Fioravanti, L. Pellacani, P. A. Tardella and M. C. Vergari,
Org. Lett., 2008, 10, 1449-1451; (b) L. Crombie and R.
Ponsford, J. Chem. Soc., 1971, 788-795.
16 Upon heating a toluene solution of triethylsilanol and hy-
droxyborane in the presence of pyridine (4 mol%), 50%
borasiloxane formation was observed in 4 h, indicating
influence of pyridine in this condensation reaction.
Notes and references
17 (a) B. Chatterjee, V. Krishnakumar and C. Gunanathan, Org.
Lett., 2016, 18, 5892-5895. (b) B. Chatterjee and C.
Gunanathan, Org. Lett., 2015, 17, 4794-4797. (c) C.
1
(a) D. G. Hall (Ed.), Boronic Acids: Preparation and
Applications in Organic Synthesis, Medicine and Materials.
2nd edn, Wiley-VCH, Weinheim, 2011; (b) N. S. Hosmane
(Ed.), Boron Science: New Technology and Applications, CRC
press, Florida, 2012; (c) M. G. Davidson, A. K. Hughes, T. B.
Marder and K. Wade (Eds.) Contemporary Boron Chemistry
RSC, Cambridge, 2000.
Gunanathan, and D. Milstein, Acc. Chem. Res. 2011, 44
,
588−602. (d) C. Gunanathan, and D. Milstein, Chem. Rev.
2014, 114, 12024−12087.
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Ruthenium-catalysed multicomponent synthesis of borosiloxanes directly from
silane, borane and water is reported.
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