Dehydrogenative condensation of (o-borylphenyl)hydrosilane with alcohols and amines

Article abstract of DOI:10.1246/cl.2007.362

o-(Dimesitylboryl)phenyl]dimethylsilane (la) undergoes dehydrogenative condensation with alcohols and amines, giving the corresponding alkoxysilanes 4-7 and aminosilanes 8 and 9 in moderate to high yields. It is plausible that the ortho-boryl group in la

Full text of DOI:10.1246/cl.2007.362

362
Chemistry Letters Vol.36, No.3 (2007)
Dehydrogenative Condensation of (o-Borylphenyl)hydrosilane with Alcohols and Amines
Atsushi Kawachi,ꢀ Masatoshi Zaima, Atsushi Tani, and Yohsuke Yamamoto
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526
(Received December 21, 2006; CL-061491; E-mail: kawachi@sci.hiroshima-u.ac.jp)
o-[(Dimesitylboryl)phenyl]dimethylsilane (1a) undergoes
SiMe2H
BMes2
SiMe2OR
BMes2
ROH (×1.1)
dehydrogenative condensation with alcohols and amines, giving
the corresponding alkoxysilanes 4–7 and aminosilanes 8 and 9
in moderate to high yields. It is plausible that the ortho-boryl
group in 1a electrophilically activates the Si–H bond.
THF
room temp.
– H2
1a
4 - 7
Yield
ROH
Time
Product
a
b
1H NMR
Isolated
MeOH
EtOH
10 min
20 min
4 h
4
5
6
7
94%
94%
87%
59%
63%
69%
49%
Dehydrogenative condensation of hydrosilanes with
alcohols1 is a facile and clean reaction for the synthesis of
alkoxysilanes because only dihydrogen is formed as the by-prod-
uct of the reaction. This reaction has received much attention in
organic synthesis in terms of protection of alcohols by silyl
groups. Since the Si–H bond is rather inert toward nucleophilic
attack by alcohols, its activation is generally required. There
are mainly three modes of Si–H bond activation reported so
far. The first one is transition-metal-catalyzed reaction.2 Transi-
tion-metal catalysts activate the Si–H bond via oxidative addi-
tion or formation of a ꢀ-complex. The second one is hypercoor-
dination of hydrosilanes.3 Inter- or intramolecular coordination
of a ligand to the silicon produces a pentacoordinate silicon spe-
cies, in which the Si–H bond is polarized and the silicon center is
sufficiently electrophilic to receive the alcohols. The third one
is electrophilic activation of the Si–H bond by a strong Lewis
acid such as B(C6F5)3.4 The highly electrophilic boron center
interacts with the hydrogen on the silicon, which renders the
Si–H bond polarized and the silicon center susceptible to nucleo-
philic attack by the alcohols. We disclose here the first example
of the intramolecular electrophilic activation of a Si–H bond
by an ortho-boryl group in arylhydrosilane 1a, which leads to
its dehydrogenative condensation with alcohols and amines.
(o-Borylphenyl)hydrosilanes 1 were prepared as follows
(Scheme 1). The halogen–lithium exchange reaction of (o-bro-
mophenyl)hydrosilane 2 with tert-BuLi (2 molar amounts) in
i-PrOH
t
-BuOH
42 h
30%c
aEstimated by use of anisole as an internal standard. bPre-
cipitated from CH3CN. cObtained by HPLC.
Scheme 2.
Et2O at ꢁ78 ꢂC produced ortho lithiated product 3, which was
reacted with fluorodimesitylborane to give 1a in 81% yield.5
The reaction of 3 with (isopropoxy)pinacolborane also afforded
pinacolborane derivative 1b as a colorless oil in 70% yield.5
Upon treatment of 1a with MeOH in THF at room temper-
ature, gas evolution occurred immediately. After 10 min, me-
thoxysilane 4 was formed in 94% yield (1H NMR) (Scheme 2).
1H NMR analysis demonstrated that the septet at ꢁ 4.54 (H–Si)
disappeared and the singlet at ꢁ 3.15 (Me–O) appeared. The re-
action of 1a with EtOH and i-PrOH also produced corresponding
alkoxysilanes 5 in 94% yield (1H NMR) and 6 in 87% yield
(1H NMR), respectively, within a few hours. Precipitation of
the obtained alkoxysilanes from CH3CN gave their pure form
in good yields (Scheme 2).6 The reaction of 1a with tert-BuOH
took a longer time, affording tert-butoxy derivative 7 in 59%
yield (1H NMR).6 In contrast to the high reactivity of 1a, pina-
colborane derivative 1b did not react with even MeOH for 24 h.
The dehydrogenative condensation of 1a with amines
was also successful.7 The reaction of 1a with a secondary amine
(Et2NH) and a primary amine (PhNH2) at room temperature
afforded corresponding aminosilanes 8 in 98% yield and 9 in
82% yield, respectively (1H NMR) (Scheme 3). The products
were isolated by precipitation from CH3CN in good yields.6
SiMe2H
Br
SiMe2H
Li
a)
2
3
SiMe2H
BMes2
SiMe2NRR'
SiMe2H
RR'NH (×1.1)
b)
BMes2 = B
THF
room temp.
– H2
2
BMes2
BMes2
1a 81%
SiMe2H
1a
8, 9
Yield
O
O
RR'NH
c)
Time
Product
1H NMRa
Isolatedb
B
B(pin) =
B(pin)
1b 70%
Et2NH
PhNH2
20 min
30 min
8
9
98%
82%
63%
73%
aEstimated by use of anisole as an internal standard. bPre-
cipitated from CH3CN.
Scheme 1. (a) tert-BuLi (ꢃ2)/Et2O/ꢁ78 ꢂC, 2 h. (b) Mes2BF
(ꢃ1:5)/0 ꢂC–r.t., 12 h. (c) (i) i-PrOB(pin) (ꢃ1:5)/ꢁ78 ꢂC. (ii)
Me3SiCl (ꢃ2), 0 ꢂC–r.t., 17.5 h.
Scheme 3.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.3 (2007)
363
tions of Carbon Resources,’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Si1
H1
References and Notes
1
B1
‘‘Dehydrogenative silylation of alcohols’’ may be a more common
designation in the field of organic synthesis.
B2
2
Some recent examples: a) S. Rendler, G. Auer, M. Oestreich,
Angew. Chem., Int. Ed. 2005, 44, 7620. b) H. Ito, K. Takagi,
T. Miyahara, M. Sawamura, Org. Lett. 2005, 7, 3001. c) H. Ito,
A. Watanabe, M. Sawamura, Org. Lett. 2005, 7, 1869. d) R. L.
Miller, S. V. Maifeld, D. Lee, Org. Lett. 2004, 6, 2773, and
references cited therein.
Selected reviews: a) C. Chuit, R. J. P. Corriu, C. Reye, J. C. Young,
Chem. Rev. 1993, 93, 1371. b) D. Kost, I. Kalikhman, in The Chem-
istry of Organic Silicon Compounds, ed. by Z. Rappoport, Y.
Apeloig, Wiley, Chichester, 1998, Vol. 2, Chap. 23, p. 1339.
Recent examples of B(C6F5)3-catalyzed reaction of hydrosilanes
with alcohols: a) J. M. Blackwell, K. L. Foster, V. H. Beck,
W. E. Piers, J. Org. Chem. 1999, 64, 4887. b) V. Gevorgyan,
J.-X. Liu, M. Rubin, S. Benson, Y. Yamamoto, Tetrahedron
Lett. 1999, 40, 8919. c) V. Gevorgyan, M. Rubin, S. Benson,
J.-X. Liu, Y. Yamamoto, J. Org. Chem. 2000, 65, 6179. d) J.
Chojnowski, S. Rubinsztajn, J. A. Cella, W. Fortuniak, M. Cypryk,
Si2
H2
Figure 1. Crystal structure of 1a with 30% probability level. H
atoms except for those on Si are omitted for clarity. Selected
3
4
ꢂ
˚
bond lengths (A) and angles ( ): Si1–H1, 1.47(2); Si2–H2,
1.55(2); H1ꢄꢄꢄB1, 3.22(2); H2ꢄꢄꢄB2, 3.25(2); ꢀ(= C–B1–C),
ꢁ
359.9; ꢀ(= C–B2–C), 359.8.
ꢁ
H
O
R
δ+ Me
Si Me
Hδ–
B
´
J. Kurjata, K. Kazmierski, Organometallics 2005, 24, 6077.
5
Compound 1a: colorless crystals (from hexane). mp 134–136 ꢂC.
1H NMR (benzene-d6, ꢁ) ꢁ0:09 (br, 3H), 0.24 (br, 3H), 1.75–
2.44 (br, 12H), 2.19 (s, 6H), 4.54 (sept, 3J ¼ 4 Hz, 1H), 6.79 (br,
4H), 7.17 (ddd, 3J ¼ 7 Hz, 3J ¼ 7 Hz, 4J ¼ 2 Hz, 1H), 7.24 (ddd,
3J ¼ 7 Hz, 3J ¼ 7 Hz, 4J ¼ 2 Hz, 1H), 7.47 (d, 3J ¼ 7 Hz, 1H),
7.56 (d, 3J ¼ 7 Hz, 1H). 13C{1H} NMR (CDCl3, ꢁ) ꢁ2:9 (br),
21.25, 22.0–24.0 (br), 127.5–129.5 (br), 128.56, 128.97, 133.22,
Mes Mes
Figure 2. Intramolecular electrophilic activation of the Si–H
bond by the ortho-boryl group.
The reaction time required to consume 1a increased with
increasing steric bulkiness of the alcohols (Scheme 2). This is
in clear contrast to the trend observed by Piers et al. in the
B(C6F5)3-catalyzed system:4a primary alcohols react with hy-
drosilanes more slowly than secondary or tertiary ones because
the less bulky alcohols more readily form adducts with B(C6F5)3
in situ, which inhibits the electrophilic Si–H activation by
B(C6F5)3. In our system, interactions of the boron site in 1a with
alcohols are less likely because of steric congestion around the
boron bearing the two mesityl groups and the ortho-silylphenyl
group. Thus, the primary alcohol reacts with 1a faster than
secondary or tertiary ones.
It is plausible that the electrophilic activation of the Si–H
bond by the ortho-boryl group renders the silicon center in 1a
susceptible to nucleophilic attack by the alcohols. Actually, less
acidic pinacolborane derivative 1b did not react with MeOH.
The physical properties of the Si–H bond in 1a5 including
1JSi{H (193 Hz), ꢂSi{H (2148 and 2167 cmꢁ1), and the Si–H
˚
bond lengths (1.47(2) and 1.55(2) A) in the crystal structure
134.31, 137.0–145.0 (br), 142.59, 156.7 (br). 11B NMR (THF-d8,
1
ꢁ) 75 (br). 29Si NMR (toluene-d8, ꢁ) ꢁ18:0 (d of sept, JSi{H
¼
193 Hz, JSi{H ¼ 7 Hz). MS(EI): m=z 384 (Mþ, 34), 369 (Mþ ꢁ
Me, 99), 204 (100). IR (KBr, cmꢁ1) 2148 (ꢂSi{H), 2167 (ꢂSi{H).
Anal. Calcd for C26H33BSi: C, 81.23; H, 8.65%. Found: C,
81.51; H, 8.50%. Compound 1b: colorless oil. bp 80–84 ꢂC/
0.4 mmHg. 1H NMR (CDCl3, ꢁ) 0.38 (d, 3J ¼ 4 Hz, 6H), 1.35 (s,
2
3
3
3
6H), 4.52 (sept, J ¼ 4 Hz, 1H), 7.36 (ddd, J ¼ 7 Hz, J ¼ 7 Hz,
4J ¼ 2 Hz, 1H), 7.41 (ddd, J ¼ 7 Hz, 3J ¼ 7 Hz, 4J ¼ 2 Hz, 1H),
3
7.65 (dd, 3J ¼ 7 Hz, 4J ¼ 2 Hz, 1H), 7.87 (dd, 3J ¼ 7 Hz, 4J ¼
2 Hz, 1H). 13C{1H} NMR (CDCl3, ꢁ) ꢁ2:14, 24.96, 83.88,
128.09, 129.76, 135.00, 135.41, 144.51. 11B NMR (CDCl3, ꢁ) 31
(br). 29Si NMR (CDCl3, ꢁ) ꢁ13:8 (d of sept, JSi{H ¼ 193 Hz,
1
2JSi{H ¼ 7 Hz). MS(EI): m=z 261 (Mþ ꢁ H, 34), 247 (Mþ ꢁ Me,
48), 84 (100). IR (KBr, cmꢁ1) 2105 (ꢂSi{H), 2138 (ꢂSi{H). Anal.
Calcd for C14H23BO2Si: C, 64.12; H, 8.84%. Found: C, 64.28; H,
9.04%.
6
The high solubility of the prepared alkoxy- and aminosilanes in
common organic solvents lowered their isolated yields by recrystal-
lization or precipitation. Precipitation from CH3CN was found to
suppress loss of the products to as little as possible. However, the
precipitate of 7 from CH3CN was contaminated with a small
amount of by-products, which are under characterization. Pure 7
was obtained by recycling preparative HPLC in 30% yield.
Some recent examples of transition-metal-catalyzed reactions:
a) A. Iida, A. Horii, T. Misaki, Y. Tanabe, Synthesis 2005, 2677.
b) K. Takaki, K. Komeyama, K. Takehira, Tetrahedron 2003, 59,
10381. c) K. Takaki, T. Kamata, Y. Miura, T. Shishido, K.
Takehira, J. Org. Chem. 1999, 64, 3891.
8
(Figure 1) are within normal values of tetracoordinate Si–H
bonds. In addition, the HꢄꢄꢄB atomic distances (3.22(2) and
˚
3.25(2) A) are almost equal to the sum of the van der Waals radii
7
8
9
˚
of hydrogen atom and boron atom (3.28 A). Although these data
suggest that there is no remarkable Si–HꢄꢄꢄB interaction9 in 1a
itself, activation of the Si–H bond may be facilitated in a
push–pull manner with the aid of the attacking alcohol, as shown
in Figure 2.
Crystal data for 1a: C26H33BSi, fw 384.42, orthorhombic, Pbca
˚
(No. 61), a ¼ 17:6410ð6Þ, b ¼ 27:6290ð8Þ, c ¼ 19:7830ð8Þ A,
In summary, the ortho-boryl group in arylhydrosilane 1a
electrophilically activates the Si–H bond in a intramolecular
manner, which leads to the dehydrogenative condensation
with alcohols and amines. Details of the reaction mechanism
are under study in our laboratory.
3
˚
V ¼ 9642:3ð6Þ A ,
Z ¼ 16,
Dcalcd ¼ 1:059 g cmꢁ3
,
RðI >
2ꢀðIÞÞ ¼ 0:0792, Rw(all data) = 0.2514, GOF ¼ 1:065, T ¼
173 K (CCDC No. 632074).
Representative examples of Si–HꢄꢄꢄB intramolecular interaction:
a) B. Wrackmeyer, O. L. Tok, Y. N. Bubnov, Angew. Chem., Int.
Ed. 1999, 38, 124. b) B. Wrackmeyer, W. Milius, O. L. Tok,
Chem.—Eur. J. 2003, 9, 4732. c) B. Wrackmeyer, O. L. Tok,
Y. N. Bubnov, Appl. Organomet. Chem. 2004, 18, 43. d) B.
Wrackmeyer, O. L. Tok, W. Milius, A. Khan, A. Badshah, Appl.
Organomet. Chem. 2006, 20, 99.
This work was supported by Grant-in-Aid for Scientific
Research, Nos. 17550038 and 18037052, the latter of which cor-
responds to Priority Areas ‘‘Advanced Molecular Transforma-
Published on the web (Advance View) January 30, 2007; doi:10.1246/cl.2007.362