Catalytic functionalization of methyl group on silicon: Iridium-catalyzed C(sp3)-H borylation of methylchlorosilanes

Article abstract of DOI:10.1021/ja307956w

A methyl group of methylchlorosilanes undergoes C-H borylation in an iridium-catalyzed reaction with bis(pinacolato)diboron in cyclohexane at 80 °C, giving (borylmethyl)chlorosilanes selectively.

Full text of DOI:10.1021/ja307956w

Communication
pubs.acs.org/JACS
Catalytic Functionalization of Methyl Group on Silicon: Iridium-
Catalyzed C(sp3)−H Borylation of Methylchlorosilanes
Toshimichi Ohmura,* Takeru Torigoe, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
(3a, 4 equiv) was reacted with 1 in cyclohexane at 80 °C in the
ABSTRACT: A methyl group of methylchlorosilanes
undergoes C−H borylation in an iridium-catalyzed
reaction with bis(pinacolato)diboron in cyclohexane at
80 °C, giving (borylmethyl)chlorosilanes selectively.
presence of [Ir(OMe)(cod)]2 (5 mol %, 10 mol % Ir) and a
ligand (10 mol %) (Table 1). An iridium catalyst bearing 4,4′-di-
Table 1. Screening of Reaction Conditions in C−H Borylation
a
of 3a
ince the establishment of Rochow’s direct method in the
1
S
1940s, methylchlorosilanes (MenSiCl4−n, n = 1−3) have
been important feedstocks in the industrial production of silicon-
based materials including silicone resins, elastomers, and oils. In
addition to these oligomers and polymers, a number of
organosilicon compounds have been synthesized from methyl-
chlorosilanes and utilized in organic and inorganic synthesis.2In
these applications, the conversion of methylchlorosilanes is
based on reactions at their Si−Cl bonds, which are efficiently
converted into Si−O, Si−N, and Si−C bonds. In contrast, the
methyl groups of methylchlorosilanes are always left intact in
these conversions, except when radical chlorination gives
(chloromethyl)chlorosilanes.3 It would be highly desirable to
explore reactions at their methyl groups through catalytic
C(sp3)−H bond functionalization. Although such an innovative
transformation has never been shown to date, recent rapid
progress in transition-metal-catalyzed C−H bond functionaliza-
tion may help our exploitation.4,5
b
entry
boron reagent
ligand
yield (%)
1
2
3
4
5
6
7
8
9
(pin)B−B(pin) (1)
1
dtbpy
phen
18
17
15
67
1
1
5,6-Me2-phen
1
4,7-Me2-phen
1
2,9-Me2-phen
1
4,7-(OMe)2-phen
3,4,7,8-Me4-phen
3,4,7,8-Me4-phen
3,4,7,8-Me4-phen
3,4,7,8-Me4-phen
45
74
15
36
84
1
H−B(pin) (2)
Et3Si−B(pin) (5)
1
In sharp contrast to the catalytic borylation of the C(sp2)−H
bond of aromatic and heteroaromatic compounds using
bis(pinacolato)diboron (1) or pinacolborane (2),6 much less
progress has been made in the catalytic borylation of the C(sp3)−
H bond.7 Indeed, the terminal methyl groups of aliphatic
hydrocarbons,8−10 benzylic methyl groups,11−13 and allylic
methylenes of cycloalkenes14 have been reported as limited
C(sp3)−H bonds that can undergo catalytic borylation with high
efficiency. Quite recently, borylations of the C(sp3)−H bonds
located β to the oxygen atom of aliphatic cyclic ethers15 and α to
the nitrogen atom of amides, ureas, and 2-aminopyridines16 have
been reported. Herein, we describe an iridium-catalyzed C(sp3)−
H borylation at the methyl groups of methylchlorosilanes to give
(borylmethyl)chlorosilanes. The reaction achieves direct cata-
lytic functionalization of methyl groups on silicon,5 in which the
chlorine acts as a directing group. It is noted that the reaction
achieves direct catalytic functionalization of methylchlorosilanes,
keeping the chlorine atom at the silicon atom left untouched.
In the course of our attempted reactions of chlorosilanes with
boron reagents, we found that a C(sp3)−H bond of a methyl
group on silicon underwent borylation in the presence of a
transition metal catalyst. For instance, butylchlorodimethylsilane
c
10
a
3a (0.60 mmol), a boron reagent (0.15 mmol), [Ir(OMe)(cod)]2
(0.0075 mmol), and ligand (0.015 mmol) were stirred in cyclohexane
b
(0.6 mL) at 80 °C for 12 h unless otherwise noted. 1H NMR yield
c
based on the boron reagent. Modified procedure: 1 (0.15 mmol),
[Ir(OMe)(cod)]2 (0.0038 mmol), and 3,4,7,8-Me4-phen (0.0075
mmol) were stirred in cyclohexane (0.6 mL) at 80 °C for 3 min. 3a
(0.60 mmol) was added to the solution, and the resulting mixture was
stirred at 80 °C for 12 h.
tert-butyl-2,2′-bipyridine (dtbpy), the most effective catalyst for
C(sp2)−H borylation,6c,d afforded (borylmethyl)chlorosilane 4a
(18% based on 1) selectively with 2 after 12 h (entry 1). It is also
noted that the borylation took place selectively at the methyl
group with no borylation of the butyl group. This result indicates
that the methyl group of 3a underwent C−H borylation under
rather mild conditions, even though the methyl group is sterically
hindered by the α-branch at the silicon atom. To improve the
Received: August 9, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Communication
a
Table 2. Iridium-Catalyzed C−H Borylation of Methylchlorosilanes
a
1 (0.30 mmol), [Ir(OMe)(cod)]2 (0.0075 mmol), and 3,4,7,8-Me4-phen (0.015 mmol) were stirred in cyclohexane (1.2 mL) at 80 °C for 3 min. 3
(1.2 mmol) was added to the solution, and the resulting mixture was stirred at 80 °C for 12 h. The product 4 was treated with i-PrOH and Et3N to
b
c
d
convert to the isopropoxysilane 6. Isolated yield based on 1. 1H NMR yield before isolation. 10 mol % of Ir-3,4,7,8-Me4-phen catalyst was used.
yield of 4a, iridium catalysts bearing various 1,10-phenanthroline
ligands were examined (entries 2−7). The yield was improved to
67% with a 4,7-dimethyl derivative (4,7-Me2-phen, entry 4),
while the parent 1,10-phenanthroline (phen) and 5,6-dimethyl
derivative (5,6-Me2-phen) gave 4a in low yields (entries 2 and 3).
2,9-Dimethyl-1,10-phenanthroline (2,9-Me2-phen) completely
suppresses the borylation (entry 5). Product 4a was obtained in
moderate yield in the presence of 4,7-dimethoxy-1,10-phenan-
throline (entry 6). We finally found that an iridium catalyst
bearing 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me4-
phen) was most effective for the borylation of 3a with 1, giving
4a in 74% yield (entry 7).17 Other boron reagents were checked
in the reaction of 3a using the Ir-3,4,7,8-Me4-phen catalyst
(entries 8 and 9). Hydroborane 2 also participated in the
borylation, although the yield of 4a was low (entry 8). This result
indicates that diboron 1 can afford both of the boron atoms for
borylation, although incorporation of the second boron atom
from 2 is not efficient. We also found that Et3Si−B(pin) (5)13
served as a boron source to give 4a in moderate yield (entry 9).
An optimized experimental procedure using 1 and the Ir-3,4,7,8-
Me4-phen catalyst was finally established as follows: A cyclo-
hexane solution of [Ir(OMe)(cod)]2, 3,4,7,8-Me4-phen, and 1
was prepared and stirred at 80 °C for 3 min. 3a was then added to
the solution, and the resulting mixture was stirred at 80 °C. This
modified procedure improved the yield of 4a to 84% even with a
reduced catalyst loading (5 mol % Ir) (entry 10).
borylated products 4 were converted to the corresponding
isopropoxysilanes 6 via reaction with i-PrOH in the presence of
Et3N. Chlorotrimethylsilane (3b) was borylated efficiently to
give 6b in high yield, although the isolated yield dropped because
of the instability of 6b against silica gel column chromatography
(entry 1). Borylation of n-alkylchlorodimethylsilanes afforded
the corresponding products in high yields, except for the reaction
of ethyl-substituted 3c, which gave 4c in moderate yield (entries
2−5). The reaction was applicable to 3f bearing a chloromethyl
group (entry 6). Probably because of steric hindrance around the
methyl group, borylations of isopropyl-substituted 3g and
dihexyl-substituted 3h were sluggish, giving borylated 6g and
6h in moderate yields by using 10 mol % of the catalyst (entries 7
and 8).18
Dichloromethylsilane derivatives also underwent borylation
(entries 9−12). The reaction of dichlorodimethylsilane (3i) gave
the borylated product 6i in good yield after conversion to
diisopropoxysilane (entry 9). Dichloromethylsilanes 3j, 3k, and
3l bearing hexyl, 3-chloropropyl, and cyclohexyl groups were
effectively borylated to give the corresponding (borylmethyl)-
silanes 6j, 6k, and 6l in 70−72% yields (entries 10−12).18
Substrates including two chlorodimethylsilyl groups were also
borylated. Selective monoborylation of 1,2-bis-
(chlorodimethylsilyl)ethane 3m took place to give 6m in high
yield (entry 13). The borylation was applicable to 1,3-dichloro-
1,1,3,3-tetramethyldisiloxane 3n to give 6n in good yield (entry
14).18
A variety of chlorosilanes 3 were subjected to borylation using
1 in the presence of the Ir-3,4,7,8-Me4-phen catalyst (Table 2).
To avoid handling of the moisture-sensitive chlorosilanes, the
Borylation of 3i could be carried out on a multigram scale
using a reduced amount of the catalyst (Scheme 1). The
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Journal of the American Chemical Society
Communication
borylation took place efficiently using 2 mol % of the Ir-3,4,7,8-
Me4-phen catalyst to give 6.0 g of 4i (78%) after distillation.
Scheme 2. Possible Models for Chlorine Atom Directed
C(sp3)−H Activation
Scheme 1. Multigram Scale Synthesis of 4i Using Lower
Catalyst Loading
assumed to be Ir[B(pin)]3(3,4,7,8-Me4-phen),22 the former may
be ruled out in the present C−H borylation due to the vacant
coordination site. On the other hand, the latter may be
reasonably rationalized on the basis of recent reports on the
ortho-selective borylation of N-(tert-butoxycarbonyl)anilines23
and the β-selective borylation of cyclic ethers.15 In the reaction of
chlorodimethyl(phenyl)silane (3p), the borylation took place
preferentially at the C(sp2)−H on the phenyl group, leaving the
C(sp3)−H of the methyl group untouched (entry 7, Table 3).
This result indicates that the directing effect of the chlorine is
rather weak.
To understand details of the C−H borylation, some substrates
were subjected to the borylation conditions using 1 with the Ir-
3,4,7,8-Me4-phen catalyst (Table 3). The reaction of t-BuCl (7),
Table 3. Reaction of Various Substrates under C−H
a
Borylation Conditions
Synthetic applications of (borylmethyl)chlorosilanes were
demonstrated (Schemes 3 and 4). In addition to the conversion
Scheme 3. Conversion of the Chlorine on 4a
Scheme 4. Conversion of the Boryl and Silyl Groups on 6a
a
Reactions were carried out at 80 °C according to the procedure
b
described in footnote a in Table 2. Yield based on 1 and isomer ratio
c
were determined by 1H NMR. 10 mol % of Ir-3,4,7,8-Me4-phen
d
catalyst was used. Isolated yield.
a carbon analogue of chlorotrimethlsilane (3b), did not give the
desired borylation product at all, indicating that a silicon atom is
essential for the C−H borylation (entry 1). As Et3SiCl (8)
resulted in no reaction (entry 2), the C−H borylation takes place
selectively at the methyl group on the silicon atom. No reaction
took place with trimethyloctylsilane (9) at 80 °C (entry 3),19
while the reaction of butyl(methoxy)dimethylsilane (10) gave
the corresponding borylation product albeit in low (<5%) yield
(entry 4). It is also noted that the reaction of (chloromethyl)-
trimethylsilane (12) gave the borylated product 13, although the
yield was low (20%, entry 5).20 On the basis of these
observations, we propose that the chlorine atom acts as a
directing group in the C−H bond activation step in the catalytic
cycle.21 Indeed, borylation of deuterium-labeled 3o gave a
mixture of 6o-H and 6o-D in the ratio 73:27 (kH/kD = 2.7),
indicating that cleavage of the C−H bond in the catalytic cycle is
involved in the rate-determining step (entry 6). Two possible
directing modes can be proposed (Scheme 2): one is
coordination of chlorine to the iridium center (A to B), and
the other is its coordination to the boron atom bound to the
iridium center (C to D). Because the active catalytic species is
to the isopropoxy derivative 6a as shown in Table 2, MeOH,
EtOH, t-BuOH, and H2O reacted with 4a in the presence of
triethylamine to give the corresponding (borylmethyl)-
alkoxysilanes 11, 15, and 16 and silanol 17 in high yields
(Scheme 3, top). Reactions of 4a with carbon nucleophiles, such
as MeMgI and PhMgBr, resulted in low-yield formation of the
desired tetraorganosilanes, probably because attack of the
nucleophiles to the boron center took place competitively. We
found that successful ethynylation of 4a was achieved by the
reaction with ethynylbenzene in the presence of Zn(OTf)2 and
Et3N (Scheme 3, bottom).24 Conversion of the boryl and silyl
groups was then examined using 6a as a model substrate
(Scheme 4). One-carbon homologation of 6a with ClCH2Li,
which was generated by the reaction of ClCH2Br with n-BuLi,
gave (borylethyl)silane 19 in 74% yield (Scheme 4, top).25
A
benzylic silane 20 was synthesized in good yield via Suzuki−
Miyaura coupling of 6a with 4-bromotoluene using
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Journal of the American Chemical Society
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(6) Selected seminal contributions, see: (a) Iverson, C. N.; Smith, M.
R., III J. Am. Chem. Soc. 1999, 121, 7696. (b) Cho, J.-Y.; Tse, M. K.;
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PdCl2(dppf)·CH2Cl2 as a catalyst and CsOH·H2O as a base
(Scheme 4, bottom).4g The silyl group of 20 could be utilized for
C−O and C−C bond formations,26,27 leading to synthesis of
alcohols 21 and 22.
In conclusion, we have established an efficient functionaliza-
tion of the methyl groups of methylchlorosilanes via iridium-
catalyzed C−H borylation. The reaction is promoted by the
directing effect of the chlorine atom on the silicon atom. This
catalytic reaction provides a new opportunity to supply
organosilicon feedstocks on the basis of conversion of the C−
H bond of a methyl group in methylchlorosilanes. In this regard,
application of related C−H activation chemistry is currently
being pursued in this laboratory.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and characterization data of the products.
This material is available free of charge via Internet at http://
pubs.acs.org.
(11) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.
Angew. Chem., Int. Ed. 2001, 40, 2168.
(12) Ishiyama, T.; Ishida, K.; Takagi, J.; Miyaura, N. Chem. Lett. 2001,
30, 1082.
(13) Boebel, T. A.; Hartwig, J. F. Organometallics 2008, 27, 6013.
AUTHOR INFORMATION
■
Corresponding Author
ohmura@sbchem.kyoto-u.ac.jp; suginome@sbchem.kyoto-u.ac.
jp
(14) (a) Olsson, V. J.; Szabo,
6891. (b) Caballero, A.; Sabo-Etienne, S. Organometallics 2007, 26,
1191. (c) Olsson, V. J.; Szabo, K. J. J. Org. Chem. 2009, 74, 7715.
́
K. J. Angew. Chem., Int. Ed. 2007, 46,
́
Notes
(15) Liskey, C. W.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 12422.
(16) Kawamorita, S.; Miyazaki, T.; Iwai, T; Ohmiya, H.; Sawamura, M.
J. Am. Chem. Soc. 2012, 134, 12924.
The authors declare no competing financial interest.
(17) The efficiency of an Ir-3,4,7,8-Me4-phen catalyst has also been
demonstrated recently in C(sp3)−H silylation and borylation. See refs
4k and 15.
(18) Double borylated products were not observed at all under the
conditions using an excess amount of 3 (4 equiv). When the borylation
of 3b was carried out with 2.2 equiv of 1 in cyclooctane at 110 °C for 12
h, the double borylated Me(i-PrO)Si[CH2B(pin)]2 (28%) was obtained
with 6b (35%) after treatment with i-PrOH/Et3N.
(19) We found that the reaction of 9 at elevated temperature (110 °C)
resulted in formation of a C−H borylated product, (n-C8H17)
Me2SiCH2B(pin) (10%). This result indicates that reactivity of the
C(sp3)−H bond located α to silicon is increased also by an electronic
effect of the silicon atom. See also ref 5b.
(20) No C−H borylation took place with Me3SiBr and Me3SiI, while
Me3SiOH resulted in selective O-borylation at the hydroxyl group to
form Me3SiOB(pin) (170% yield based on 1).
ACKNOWLEDGMENTS
■
This work is supported by a Grant-in-Aid for Young Scientists
(A) (No. 22685010) from JSPS.
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