Ru0-catalyzed coupling of vinylboronates with silanols, disilanols, and disiloxanediols: A selective route to borasiloxanes

Article abstract of DOI:10.1002/ejic.201402212

A new highly selective method for the synthesis of borasiloxanes through the O-borylation of silanols, disilanols, and disiloxanediols with vinylboronates in the presence of Ru⁰ complexes (mainly Ru ₃CO₁₂) is described. The method leads to the formation of special compounds with a B-O-Si inorganic framework. When Ru⁰ complexes are used as a catalyst, no vinylboronate homocoupling is observed. A mechanism for this transformation is proposed on the basis of stoichiometric reactions of Ru⁰ with triethylsilanol and vinylboronate. A new, selective approach to borasiloxanes through catalytic O-borylation was developed. Vinylboronates are coupled with silanols, silanediols, and siloxanediols, which yield borasiloxanes as the only products with the evolution of ethylene. A mechanism for this Ru₃(CO)₁₂-catalyzed transformation is also proposed. Copyright

Full text of DOI:10.1002/ejic.201402212

FULL PAPER
DOI:10.1002/ejic.201402212
Ru0-Catalyzed Coupling of Vinylboronates with Silanols,
Disilanols, and Disiloxanediols: a Selective Route to
Borasiloxanes
Dawid Frackowiak,[a] Jedrzej Walkowiak,[b] Grzegorz Hreczycho,[a]
˛
˛
and Bogdan Marciniec*[a,b]
Keywords: Homogeneous catalysis / Borasiloxane / Borylative coupling / Ruthenium / Silanol / Reaction mechanisms
A new highly selective method for the synthesis of borasilox-
anes through the O-borylation of silanols, disilanols, and dis-
iloxanediols with vinylboronates in the presence of Ru0 com-
plexes (mainly Ru3CO12) is described. The method leads to
the formation of special compounds with a B–O–Si inorganic
framework. When Ru0 complexes are used as a catalyst, no
vinylboronate homocoupling is observed. A mechanism for
this transformation is proposed on the basis of stoichiometric
reactions of Ru0 with triethylsilanol and vinylboronate.
ceptors for O-metallation of silanols.[5,6a] However, some of
the reactions catalyzed effectively by Ru–H complexes are
not entirely selective (depending on the substrates and cata-
lysts) and yield small amounts of products of the vinylmet-
alloid homocoupling.
Introduction
The silylative coupling of olefins with vinyl-substituted
organosilicon compounds, which we developed in the last
two decades as a new, effective catalytic activation of the
=C–H bond of olefins and the =C–Si bond of organosilicon
compounds (generally occurring in the presence of com-
plexes containing [M]–H and [M]–Si bonds),[1] has been re-
cently found to be general and is also exhibited by vinyl
derivatives of other p-block elements (e.g., boron[2] and ger-
manium[3]). This mode of catalytic reactivity has also been
successfully extended to the catalytic activation of other sp2
carbon atoms, that is, =Caryl–H bonds and sp-hybridized
carbon–hydrogen bonds[4], as well as of the O–H bonds of
silanols, alcohols, and boronic acids.[5,6] Such reactions, cat-
alyzed predominantly by RuII complexes containing Ru–H
bonds, proceed according to the general equation given in
Scheme 1.
Borasiloxanes are a class of siloxanes with the B–O–Si
moiety as a basic unit in their structure. Borasiloxane units
are thermally robust but prone to hydrolysis, the extent of
which depends on the boron content and the type of sub-
stituents at the silicon atoms.[7] They can be represented by
linear oligomeric and polymeric derivatives[8] as well as cy-
clic and cage structures.[9] Borasiloxanes possess a stable in-
organic framework, as they are analogues to borosilicates,
and this makes them ideal precursors for hybrid organic–
inorganic materials. Such materials have many potential ap-
plications, for example, in the preparation of conjugated
polymer sensors based on borasiloxane cages assembled by
electrode polymerization of eight-membered borasiloxane
rings[10] and in the synthesis of precursors for supramolec-
ular motifs and networks.[11] Moreover, borasiloxanes are
commonly used as polymer additives for hydraulic fluids,
lubricants, antifoaming agents, sealants, and components of
heat-resistant adhesives, among many other applications
(see ref.[7]).
Most methods for the synthesis of borasiloxanes are
based on condensation reactions of chloroboranes or bo-
ronic acids with silanols, silanolates, and chlorosilanes un-
der various conditions (recently, the mechanochemical syn-
thesis of cyclic borasiloxanes has been reported[12]), often
leading to the evolution of dangerous and corrosive by-
products (e.g., HCl) or using air- and moisture-sensitive
substrates (namely chloroboranes). Our group has devel-
oped a catalytic transformation mediated by hydridoru-
thenium complexes, which leads to borasiloxanes through
Scheme 1. General protocol for the activation of C–H and O–H
bonds by vinylmetalloids and other hydrogen donors.
Our recent reports have proved the successful use of vin-
ylmetalloid (Si, B, Ge) compounds as new hydrogen ac-
[a] Adam Mickiewicz University in Poznan, Faculty of Chemistry,
Umultowska 89b, 61-614 Poznan, Poland
E-mail: bogdan.marciniec@amu.edu.pl
http://www.metalorg.amu.edu.pl/
[b] Adam Mickiewicz University, Centre for Advanced
Technologies,
Grunwaldzka 6, 60-780 Poznan, Poland
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the coupling of vinylboronates with silanols (see Scheme 1 effective and selective catalyst in the O-germylation of silan-
and ref.[6a]).
ols and proved that it has virtually no activity towards the
The results from our previous report on the reaction of homocoupling of vinylmetalloids (see ref.[13]).
silanols with vinylgermanes, which is catalyzed selectively To check the efficiency of the Ru3(CO)12-catalyzed O-
by the complex Ru3CO12,[13] prompted us to make use of borylation process, a number of silanols, silanediols, and
this general feature of Ru0 complexes (lack of activity in the siloxanediols were tested. The reaction catalyzed by
homocoupling of vinylmetalloids) in the O-metallation of Ru3(CO)12 proceeded through the activation of the O–H
silanols, silanediols, and siloxanediols by vinylboronates. bond and the elimination of ethylene (Scheme 2).
Moreover, we wanted to extend the scope of substrates for
the O-borylation reaction with dihydroxysilicon derivatives,
such as silanediols and siloxanediols. These processes can
serve as versatile methods for the synthesis of organometal-
lic species based on borasiloxane (–B–O–Si–O–B–) inor-
ganic frameworks with desired well-defined structures for
the precursors of new hybrid inorganic–organic materials.
Scheme 2. General reaction protocol for the O-borylation of silan-
ols.
Vinylboronates proved to be much more reactive sub-
strates in the O-metallation process than vinylgermanes.
With the former, the reaction proceeds smoothly with lower
catalyst loading (2 mol-%), at slightly lower temperature
(110 °C), and a twofold excess of vinylboronate, giving
Results and Discussion
For all the reactions, we chose three vinylboronates (i.e., quantitative conversion of silanols to afford borasiloxanes
vinylboronate esters) (Figure 1). Compound I proved to be as the only products in high yields (see Table 2).
active in the homocoupling reaction even in the presence of
Table 2. O-Borylation of silanols catalyzed by Ru3(CO)12.[a]
RuHCl(CO)(PPh3)3 (which was previously reported to be
inactive in this process with other vinylboronates).
Entry Vinylboronate (ViB)
Silanol
Yield [%]
1
2
3
4
5
6
7
8
9
I
II
III
I
II
III
I
(Me3SiO)3SiOH (A)
(Me3SiO)3SiOH
(Me3SiO)3SiOH
Et3SiOH (B)
Et3SiOH
90[b]
100[c]
100[c]
90[b]
100[c]
100[c]
91[b]
Et3SiOH
iPr3SiOH (C)
iPr3SiOH
Figure 1. Vinylboronates: substrates for the O-borylation reaction.
II
III
100[c]
100[c]
iPr3SiOH
Four Ru0 complexes, Ru(CO)3(PPh3)2, Ru(CO2)(PPh3)2,
Ru3(CO)12, and Ru(COD)(COT), were tentatively screened
in catalytic tests. Only the latter two showed activity in the
O-metallation reaction (Table 1). The activity of Ru(COD)-
(COT) in the coupling reaction was lower than that of
Ru3(CO)12. Our recent studies recognized Ru3(CO)12 as an
[a] Reaction conditions: [ViB]/[SiOH]/[Ru3(CO)12] = 2:1:0.02, tolu-
ene or 1,4-dioxane, 110 °C, 24 h, in a sealed glass tube or Schlenk
reactor. [b] Isolated yield. [c] Yield based on GC.
To obtain more complex borasiloxanes, we applied the
same conditions to the O-borylation of selected silanediols,
disilanols, and siloxanediols (Scheme 3, Table 3). A catalytic
screening of vinylboronates in the presence of a hydridoru-
thenium catalyst in a model reaction with diisopropylsilane-
diol showed the formation of diborylethenes as the main
products and small amounts of borasiloxanes. Therefore,
dodecacarbonyltriruthenium appeared to be an ideal solu-
tion for the selective synthesis of borasiloxanes. Because of
the low solubility of silicon substrates in toluene, all reac-
tions were carried out in 1,4-dioxane. Among the selected
substrates, only diethylsilanediol and 1,4-bis(hydroxydi-
methylsilyl)benzene were unreactive towards O-borylation,
and mainly cyclosiloxanes and polysiloxanes, respectively,
were formed. On the other hand, the reaction of vinyl-
Table 1. Catalyst screening in a model reaction of vinylboronate
(ViB) I with (Me3SiO)3SiOH.[a]
Entry
Catalyst
Silanol conversion [%][b]
1
2
3
4
Ru3(CO)12
100
0
0
Ru(CO)2(PPh3)3
Ru(CO)3(PPh3)2
Ru(COD)(COT)
75
[a] Reaction conditions: [ViB]/[SiOH]/[Ru0] = 2:1:0.02, toluene or
1,4-dioxane, 110 °C, 24 h, in a sealed glass tube or Schlenk reactor.
[b] Conversion determined by GC.
Scheme 3. General reaction protocol for the O-borylation of silanediols and siloxanediols.
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Table 3. O-Borylation of silanediols and disiloxanediol.[a]
Entry
Vinylboronate (ViB)
Silanediol or disiloxanediol
Yield [%][b]
1
2
3
4
5
6
7
8
9
I
I
Et2Si(OH)2
1,4-bis(hydroxydimethylsilyl)benzene
iPr2Si(OH)2 (D)
0
0
90
89
85
I
II
III
I
II
III
I
iPr2Si(OH)2 (E)
iPr2Si(OH)2 (F)
Ph2Si(OH)2
Ph2Si(OH)2
Ph2Si(OH)2
71/29[c]
68/32[c]
60/40[c]
85
HOSi(iPr)2OSi(iPr)2OH (G)
HOSi(iPr)2OSi(iPr)2OH (H)
HOSi(iPr)2OSi(iPr)2OH (i)
10
11
II
III
80
78
[a] Reaction conditions: [ViB]/[R2Si(OH)2]/[Ru3(CO)12] = 4:1:0.04, 1,4-dioxane, 110 °C, 24 h, in a sealed glass tube or Schlenk reactor.
[b] Isolated yields. [c] Monosubstituted product, determined by GC.
boronates with diisopropylsilanediol gave disubstituted
To establish a mechanism for the ruthenium(0)-catalyzed
products exclusively in high yields with complete conversion O-borylation, we examined an equimolar stoichiometric re-
of silanediol. However, when diphenylsilanediol was used, action between triethylsilanol and Ru3(CO)12, which was
only a mixture of mono- and disubstituted products was carried out in deuterated benzene (Scheme 4) and moni-
obtained. Even with a larger excess of vinylboronates, we tored by 1H NMR spectroscopy. Triethylsilanol was se-
could not achieve complete conversion of silanediol to di- lected, because its carbonylruthenium derivative is well
substituted borasiloxane.
The functionalization of 1,1,3,3-tetraisopropyldisiloxane-
1,3-diol gave full conversion of disiloxanediol to the desired
products. In all cases, no products of vinylboronate homo-
coupling were observed. New borasiloxanes were isolated
(see Table 3) and characterized spectroscopically (1H, 13C
NMR) and by GC–MS. Non-isolated compounds were
identified by GC–MS.
Scheme 4. Stoichiometric reaction of Et3SiOH with Ru3(CO)12.
Figure 2. Stoichiometric reaction of Ru3(CO)12 with Et3SiOH (bottom) and bridged hydrido-siloxido complex with vinylboronic pinacol
ester (top).
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characterized[14] and appeared to be a potential intermedi- transfer and the elimination of molecular ethylene. The ob-
ate species in our transformation. tained borylsiloxido complex undergoes reductive elimi-
After heating of the reaction mixture at 100 °C for 12 h nation, and a borasiloxane is formed.
in C6D6 in a closed Young NMR tube, a new signal of low
intensity appeared at δ = –12.3 ppm, which can be arguably
assigned to the presence of a bridged hydrido-siloxido com-
plex (μ-H)Ru3(CO)10(μ-OSiEt3) – the product of an oxidat-
Conclusions
ive addition of silanol to Ru3(CO)12. The chemical shift of
this complex provided in the literature is δ = –12.08 ppm
(in CDCl3). To prove that this complex is an exact catalytic
species, we added an equimolar amount of vinylboronate to
the reaction mixture. Monitoring by 1H NMR spectroscopy
showed the disappearance of the signal at δ = –12.3 ppm,
and the evolution of molecular ethylene was observed (sing-
let at δ = 5.25 ppm) (Figure 2, Scheme 5).
We report a new catalytic route for the effective O-bo-
rylation of silanols, silanediols, and disiloxanediols with
vinylboronate esters in the presence of Ru3(CO)12 [contrary
to hydridoruthenium(II) catalysts] for the highly selective
synthesis of borasiloxanes, compounds with a B–O–Si inor-
ganic framework. This process can be extended to the syn-
thesis of hybrid borasiloxane polymers. The O-borylation
of dihydroxy organosilicon compounds with divinyl-substi-
tuted boronates is now under investigation.
Catalytic and stoichiometric studies of an Ru0 complex
with silanols and vinylboronate allowed us to propose a ra-
tional mechanism for the O-borylation process.
Experimental Section
General: 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra
were recorded with a Varian XL 300 MHz spectrometer in CDCl3
(or C6D6) solution. Chemical shifts (δ) are reported in ppm with
reference to the residual solvent peak (CHCl3) for 1H and 13C. Gas
chromatographic (GC) analyses were performed with a Varian 3300
with a DB-5 fused silica capillary column (30 mϫ0.15 mm) and a
thermal conductivity detector (TCD). Mass spectra of the mono-
mers and products were obtained by GC–MS analyses [Varian Sa-
turn 2100 T, equipped with a BD-5 capillary column (30 m) and an
ion-trap detector]. Elemental analyses were carried out with a Vario
EL III instrument (Elementar GmbH). Column chromatography
was performed with silica gel 60 (70–230 mesh, Fluka). Toluene
was dried by distillation from sodium/potassium alloy; dioxane was
purified and distilled from Na/benzophenone; hexane was distilled
from calcium hydride. All solvents were purified under argon.
Dichlorodiethylsilane, chlorotriisopropylsilane, dichlorodiisoprop-
ylsilane, chlorotris(trimethylsiloxy)silane and 1,4-bis(hydroxydime-
thylsilyl)benzene were purchased from ABCR and used as received.
Ru3(CO)12, diphenylsilanediol, tributylchlorostannane, triethyl-
amine and 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaborolane (I) were
purchased from Sigma–Aldrich. Diethylsilanediol,[15] diisopropyl-
silanediol,[16] and 1,3-dihydroxy-1,1,3,3-tetraisopropyldisiloxane[17]
were prepared according to methods reported in the literature. 2-
Vinyl-1,3,2-dioxaborinane (II) and 2-vinyl-1,3,2-dioxaborolane
(III) were synthesized according to literature procedures with some
modifications.[18] Diphenylsilanediol was purified by recrystalli-
zation. Triethylamine was dried with CaH2.
Scheme 5. Stoichiometric reaction of a bridged hydrido-siloxido
complex with vinylboronic pinacol ester.
The GC–MS analysis of the reaction mixture showed
only one product, the expected borasiloxane, and the mass
spectrum displayed a molecular ion with m/z = 258, which
is in agreement with the analysis of an isolated compound.
On the basis of the experimental data, we propose a
mechanism of the catalytic borylation of silanols in the
presence of Ru3(CO)12 (Scheme 6).
General Procedure for the O-Borylation Reaction: Ru3(CO)12
(2 mol-% for silanols, 4 mol-% for silanediols and disiloxanediol)
was placed in a glass ampoule or small Schlenk reactor, and a small
portion of toluene or dioxane was added. Then, the Si–OH sub-
strate, vinylboronate, and the rest of the solvent were added. The
reaction mixture was heated at 110 °C in an oil bath for for 24 h.
The progress of the reaction was examined by GC and GC–MS
analyses. After the reaction was completed, all volatiles were re-
moved under reduced pressure, and the crude substrate was puri-
fied from the catalyst on a short silica column with hexane or hex-
Scheme 6. Proposed catalytic cycle for the Ru3(CO)12-catalyzed O-
borylation of silanols. Pin = pinacol.
The catalytic cycle involves the oxidative addition of sil-
anol to Ru3(CO)12 and the formation of a cluster hydrido-
siloxido complex. This complex undergoes insertion of vin-
ylboronate to the Ru–H bond with a subsequent β-boryl ane/ethyl acetate (1:1) as eluents.
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Procedure for the Stoichiometric Reactions: A Young NMR tube
filled with argon was charged with C6D6 (0.5 mL), triethylsilanol
(0.01 mL), and Ru3(CO)12 (18 mg). The mixture was heated to
100 °C for 12 h, and then (after NMR examination) 4,4,5,5-tet-
ramethyl-2-vinyl-1,3,2-dioxaborolane was added, and the resulting
mixture was heated at 100 °C for additional 12 h. The resulting
mixture was examined by 1H NMR spectroscopy and GC–MS
analyses.
calcd. C 48.44, H 9.03, B 4.84, O 25.09, Si 12.59; found C 48.39,
H 8.95, B 4.79, O 24.99, Si 12.54.
1,3-Bis[(1,3,2-dioxaborolan-2-yl)oxy]-1,1,3,3-tetraisopropyldisilox-
1
ane (I): H NMR (300 MHz, CDCl3): δ = 1.00 (m, 4 H), 1.24 (d,
24 H), 4.17 (s, 4 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 12.80,
16.69, 64.4 ppm. C16H36B2O7Si2 (418.22): calcd. C 45.95, H 8.68,
B 5.17,O 26.78, Si 13.43; found C 45.90, H 8.62, B 5.09, O 26.71,
Si 13.38.
Analytical Data for Selected Products
Acknowledgments
(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)tris(trimethylsilyl) Or-
1
thosilicate (A): H NMR: δ = 0.11 (s, 27 H), 1.22 (s, 12 H) ppm.
This work was supported by a project of the National Science Cen-
tre (“Maestro”, UMO 2011/02/A/ST5/00472).
13C NMR: δ = 1.4, 24.7, 82.2 ppm. C15H39BO6Si4 (438.19): calcd.
C 41.07, H 8.96, B 2.46, O 21.89, Si 25.61; found C 40.97, H 8.92,
B 2.40, O 21.82, Si 25.56. MS (EI): m/z (%) = 423 (100), 365 (5),
307 (35), 281 (37), 267 (65).
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Triethyl[(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)oxy]silane (B):
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NMR: δ = 5.3, 6.4, 24.5, 81.9 ppm. C12H27BO3Si (258.18): calcd.
C 55.81, H 10.54, B 4.19, O 18.59, Si 10.88; found C 55.76, H
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Triisopropyl[(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)oxy]silane
1
(C): H NMR (300 MHz, CDCl3): δ = 1.06 (br., 21 H), 1.22 (s, 12
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 12.3, 17.6, 24.5, 76.5,
77.0, 77.4, 81.9 ppm. C15H33BO3Si (300.23): calcd. C 59.99, H
11.08, B 3.60, O 15.98, Si 9.35; found C 59.92, H 10.98, B 3.55, O
15.91, Si 9.28. MS (EI): m/z (%) = 331 (5), 301 (50), 285 (35), 257
(45), 215 (40).
Diisopropylbis[(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)oxy]sil-
ane (D): 1H NMR (300 MHz, CDCl3): δ = 1.00 (14 H), 1.22 (s,
12 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 12.47, 16.55, 24.53,
82.29 ppm. C18H38B2O6Si (400.26): calcd. C 54.02, H 9.57, B 5.40,
O 23.99, Si 7.02; found C 53.97, H 9.51, B 5.35, O 23.93, Si 6.96.
MS (EI): m/z (%) = 357 (5), 300 (25), 275 (25), 259 (100), 147 (5).
1
Bis[(1,3,2-dioxaborinan-2-yl)oxy]diisopropylsilane (E): H NMR
(300 MHz, CDCl3): δ = 1.02 (14 H), 1.87 (br., 2 H), 4.04 (t, 3 H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 12.49, 16.58, 27.5,
62.9 ppm. C12H26B2O6Si (316.17): calcd. C 45.60, H 8.29, B 6.84,
O 30.37, Si 8.89; found C 45.57, H 8.23, B 6.79, O 30.32, Si 8.81.
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(300 MHz, CDCl3): δ = 1.06 (14 H), 4.19 (s, 4 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 12.45, 16.54, 64.70 ppm. C10H22B2O6Si
(288.14): calcd. C 41.71, H 7.70, B 7.51, O 33.33, Si 9.75; found C
41.68, H 7.65, B 7.48, O 33.29, Si 9.70.
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1,1,3,3-Tetraisopropyl-1,3-bis[(4,4,5,5-tetramethyl-1,3,2-dioxaborol-
an-2-yl)oxy]disiloxane (G): H NMR (300 MHz, CDCl3): δ = 1.02
1
(m, 4 H), 1.21 (s, 24 H), 1.27 (d, 24 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 12.82, 16.72, 24.39, 82.15 ppm. C24H52B2O7Si2
(530.34): calcd. C 54.34, H 9.88, B 4.08, O 21.11, Si 10.59; found
C 54.30, H 9.82, B 4.02, O 21.08, Si 10.52. MS (EI): m/z (%) = 487
(5), 415 (15), 345 (25), 305 (70), 277 (10).
[18] a) F. E. Brinckman, F. G. A. Stone, J. Am. Chem. Soc. 1960,
82, 6218–6223; b) H. C. Brown, D. Basavaiah, N. G. Bhat, Or-
ganometallics 1983, 2, 1309–1311.
1,3-Bis[(1,3,2-dioxaborinan-2-yl)oxy]-1,1,3,3-tetraisopropyldisilox-
1
ane (H): H NMR (300 MHz, CDCl3): δ = 1.01 (m, 4 H), 1.28 (d,
24 H), 1.9 (m, 2 H), 4.03 (t, 4 H) ppm. 13C NMR (75 MHz,
Received: March 21, 2014
CDCl3): δ = 12.80, 16.69, 27.1, 62.6 ppm. C18H40B2O7Si2 (446.25):
Published Online: June 3, 2014
Eur. J. Inorg. Chem. 2014, 3216–3220
3220
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