Intramolecular reaction of silanol and triarylborane: boron-aryl bond cleavage and formation of a Si-O-B heterocyle

Article abstract of DOI:10.1021/om8004405

o-(Hydroxydimethylsilyl)(dimesitylboryl)benzene (1) underwent intramolecular cyclization to give 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5- oxasilaboracyclopentene (4) and mesitylene by boron-aryl bond cleavage. Whereas triethylamine as an additive accelerated the cyclization, DBU stabilized a silyloxyborate complex, 1,8-diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboratacyclopentene, which was characterized by X-ray crystallographic analysis and HF calculations. The deuterium-labeling experiments revealed that the hydrogen migrated from the hydroxyl group in 1 to the leaving mesitylene.

Full text of DOI:10.1021/om8004405

Organometallics 2008, 27, 4691–4696
4691
Intramolecular Reaction of Silanol and Triarylborane: Boron-Aryl
Bond Cleavage and Formation of a Si-O-B Heterocyle
Atsushi Kawachi,* Masatoshi Zaima, and Yohsuke Yamamoto
Department of Chemistry, School of Science, Hiroshima UniVersity, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan
ReceiVed May 14, 2008
o-(Hydroxydimethylsilyl)(dimesitylboryl)benzene (1) underwent intramolecular cyclization to give 5,5-
dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclopentene (4) and mesitylene by boron-aryl bond
cleavage. Whereas triethylamine as an additive accelerated the cyclization, DBU stabilized a silyloxyborate
complex, 1,8-diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilabo-
ratacyclopentene, which was characterized by X-ray crystallographic analysis and HF calculations. The
deuterium-labeling experiments revealed that the hydrogen migrated from the hydroxyl group in 1 to the
leaving mesitylene.
Scheme 1. Dehydrogenative Condensation Reaction of
Hydrosilane 1 with Alcohols
We have reported recently the dehydrogenative condensation
reaction of o-(dimesitylboryl)(dimethylsilyl)benzene (1) with
alcohols: hydrosilane 1 reacted with methanol, ethanol, and
isopropyl alcohol in THF at room temperature in a short time,
giving the corresponding alkoxysilanes 2 in excellent yields,
as shown in Scheme 1.1 The efficiency of this reaction is
ascribed to the electrophilic activation of the Si-H bond2 by
the o-boryl group in 1, because of which the silicon center tends
to be receptive toward nucleophiles. The reaction of 1 with tert-
butyl alcohol also proceeded to give 2d, but the yield was rather
low (59%) (Scheme 1). Precise identification of the byproduct
revealed that disiloxane 3 (34% yield), 3,4-benzo-1,2,5-oxasi-
laboracyclopentene 4 (5% yield), and mesitylene (confirmed by
1H NMR spectroscopy) were formed in the reaction, as shown
in Scheme 2. We are interested in the formation of the
unexpected products 4 and mesitylene and postulate their
formation mechanism as follows (Scheme 2). (i) Hydrosilane 1
reacts with water in a manner similar to its reaction with
alcohols, giving hydroxysilane 5. Although tert-butyl alcohol
used in the present reaction might contain only a trace amount
of H2O,3 the reaction of 1 with bulky tert-butyl alcohol is so
slow that the reaction of 1 with H2O becomes competitive. (ii)
While bimolecular condensation of 5 gives disiloxane 3,
intramolecular cyclization of 5 together with boron-mesityl
bond cleavage results in the formation of 4 and mesitylene.
Scheme 2. Formation and Reaction of Hydroxysilane 5
The intramolecular interaction between boryl groups and
heteroatom-functionalized silyl groups has been precisely
investigated by Wrackmeyer et al. with multinuclear NMR
spectroscopy.4 They prepared a number of compounds in which
* To whom correspondence should be addressed. E-mail: kawachi@
sci.hiroshima-u.ac.jp.
(1) Kawachi, A.; Zaima, M.; Tani, A.; Yamamoto, Y. Chem. Lett. 2007,
36, 362.
a heteroatom-functionalized silyl group and a boryl group were
bonded to an alkene skeleton in a cis fashion and revealed that
such compounds formed unique silicon-heteroatom-boron
bridges. Because of intramolecular interaction between the
oxygen atom and the boron atom, cis-2-boryl-1-(hydroxy)sily-
lalkene I underwent intramolecular cyclization via hydrogen
migration or ethane elimination to give 1,2,5-oxasilaborolane
II or 1,2,5-oxasilaborole III, respectively (eq 1).4c cis-2-Boryl-
1-[(diethylamino)silyl]alkene IV underwent hydrolysis to yield
1,2,5-oxasilaborolane V via the postulated hydroxysilyl inter-
mediate VI (eq 2).4d
(2) For other examples of electrophilic activation of Si-H bonds: (a)
Blackwell, J. M.; Foster, K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem.
1999, 64, 4887. (b) Gevorgyan, V.; Liu, J.-X.; Rubin, M.; Benson, S,.;
Yamamoto, Y. Tetrahedron Lett. 1999, 40, 8919. (c) Gevorgyan, V.; Rubin,
M.; Benson, S.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2000, 65, 6179.
(3) tert-Butyl alcohol was dried with CaH2 and distilled under a nitrogen
atmosphere.
(4) (a) Wrackmeyer, B.; Milius, W.; Tok, O. L. Chem. Eur. J. 2003, 9,
4732. (b) Wrackmeyer, B.; Tok, O. L. Magn. Reson. Chem. 2002, 40, 406.
(c) Ko¨ster, R.; Seidel, G.; Wrackmeyer, B. Chem. Ber. 1991, 124, 1003.
(d) Wrackmeyer, B.; Su¨ss, J.; Milius, W. Chem. Ber. 1996, 129, 147.
10.1021/om8004405 CCC: $40.75
2008 American Chemical Society
Publication on Web 08/26/2008

4692 Organometallics, Vol. 27, No. 18, 2008
Kawachi et al.
Figure 1. Crystal structure of 6 at the 30% probability level. One
of the two independent molecules is shown. H atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): C1-C2,
1.414(3)/1.415(3); Si1-C1, 1.852(2)/1.848(2); Si1-O1, 1.6399(15)/
1.6377(14); B1-O1, 1.561(2)/1.567(2); B1-C2, 1.654(3)/1.654(3);
O1 · · · N2, 2.769(2)/2.920(2); C2-C1-Si1, 107.07(14)/108.28(14);
C1-Si1-O1, 97.58(8)/97.69(8); Si1-O1-B1, 114.71(12)/114.82-
(11); O1-B1-C2, 103.27(15)/103.47(14); C1-C2-B1, 115.42(16)/
114.71(16).
In this work, we prepared hydroxysilane 5 and investigated
whether 5 actually affords 4 and mesitylene under the appropri-
ate reaction conditions. We also performed deuterium-labeling
experiments to obtain further information on the boron-mesityl
bond cleavage. In the course of our study, a silyloxyborate-amine
complex was isolated. The results of the characterization of this
complex by NMR spectroscopy, X-ray crystallographic analysis,
and HF calculations are reported herein.
Scheme 3. Cyclization Reaction of Hydroxysilane 5 in the
Presence of DBU
Results and Discussion
Formation of 3,4-Benzo-1,2,5-oxasilaboracyclopentene
4. Hydroxysilane 5 was prepared in 90% yield by hydrolysis
of 1 with H2O (1 molar equiv) in THF at room temperature for
10 min. Precipitation of 5 from acetonitrile yielded its pure form
as white powder (mp 115-117 °C). The Si-OH functionality
in 5 was confirmed by 1H NMR spectroscopy (δ(Si-OH) 1.53
in C6D6) and IR spectroscopy (ν(O-H) 3579 cm-1). The 11B
NMR signal was observed at δ 74 in the typical region for
triarylboranes. These spectral data indicate that there is no
significant intramolecular interaction between the hydroxyl
oxygen and the boron center in 5 itself, unlike in I and IV, in
which the coordination of the heteroatoms to the boron is evident
from the 11B NMR shifts (δ(I) 16,4c δ(IV) 64d). This may be
due to that the o-phenylene skeleton is rigid compared to the
alkene skeleton, as well as the fact that the boron center is
sterically crowded by the o-methyl groups.
With hydroxysilane 5 in hand, we examined the formation
of 4 under several reaction conditions. The results are sum-
marized in Table 1. Actually, 5 in THF underwent intramo-
lecular cyclization, as expected. When the reaction was con-
ducted at room temperature for 240 h, 5 was completely
consumed to give 3, 4, and mesitylene in 10%, 86%, and 81%
yields, respectively (Table 1, entry 1).5 Furthermore, 4 was
isolated in 64% yield by bulb-to-bulb distillation as a colorless
oil that solidified to a white solid on standing. The formation
of 4 was largely accelerated by heat: the reaction was completed
after 24 h at 80 °C (Table 1, entry 2). The addition of
triethylamine (1 molar equiv) also effectively accelerated the
reaction, even at room temperature (Table 1, entry 3). In
contrast, a highly polar solvent such as acetonitrile completely
inhibited the formation of 4 (Table 1, entry 4).
In order to gain further insights into the effect of amines, we
used a stronger base, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene),
in place of triethylamine. Stirring of 5 and DBU (1 molar equiv)
in THF at room temperature afforded the silyloxyborate-
[DBU · H+] complex 6 in 91% yield as a white solid after
precipitation from hexane, as shown in Scheme 3. The silylox-
yborate complex 6 was identified by NMR spectroscopy. In 11
B
NMR spectra, a signal (δ 6) appears in the typical region for
tetracoordinate borates. In 1H NMR spectra, a broad signal for
the amidinium proton in [DBU · H+] was observed at δ 12. As
far as we know, there is only one example of
a
silyloxyborate-[R3NH+] complex in the literature. The complex
was prepared by treatment of a silica surface ((-O-)3Si-OH)
with B(C6F5)3 and Et2NPh and characterized by IR and
multinuclear solid-state NMR spectroscopy.6
Structure of the Silyloxyborate-[DBU · H+] Complex 6.
Recrystallization of 6 from THF-hexane afforded colorless
crystals that were used in X-ray crystallographic analysis. The
crystal structure of 6 is shown in Figure 1. Two independent
molecules are present in the asymmetric unit: their geometries
are similar to each other, except for the direction of tilt of the
mesityl groups. The borate geometry in 6 is clearly demonstrated
by the sum of the three C-B-C angles of 336.2(25)/336.9(25)°
and the B-O bond length of 1.561(2)/1.567(2) Å. The three
B-C bond lengths are almost the same, and their average is
1
(5) The yields were estimated in H NMR spectra by use of anisole as
an internal standard.
(6) Millot, N,.; Cox, A.; Santini, C. C.; Molard, Y.; Basset, J.-M. Chem.
Eur. J. 2002, 8, 1438.

Intramolecular Reaction of Silanol and Triarylborane
Organometallics, Vol. 27, No. 18, 2008 4693
Table 1. Cyclization of Hydroxysilane 5
NMR yield (%)a
(isolated yield (%))
entry solvent
temp
time (h) additive
4
mesitylene
1
2
3
4
THF
THF
THF
room temp
80 °C
room temp
240
24
24
48
86 (64)
91 (86)
Et3N (×1) 93 (91)
81
89
91
CH3CN room temp
no reacn
a Estimated by use of anisole as an internal standard in 1H NMR
spectra.
2
Figure 3. H NMR monitoring of reaction of 5-d in THF.
Scheme 4. Postulated Mechanism of Cyclization Reaction of
Hydroxysilane 5
Figure 2. Optimized structure of 6 at the HF/6-31+G(d) level.
Selected bond lengths (Å) and angles (deg): C1-C2, 1.4042;
Si1-C1, 1.8357; Si1-O1, 1.6449; B1-O1, 1.5745; B1-C2,
1.6721; O1 · · · N2, 2.8605; O1 · · · H, 1.8451; C2-C1-Si1, 107.85;
C1-Si1-O1, 97.78; Si1-O1-B1, 114.98; O1-B1-C2, 102.37;
C1-C2-B, 115.88.
(Scheme 3).5 Formation of the DBU adduct of 4 was confirmed
by the facts that (i) its 11B NMR signal (δ 42) exhibited an
upfield shift compared to that of 4 itself (δ 50) and (ii) a small
change in the 1H NMR signals corresponding to the DBU part
was observed in comparison to those of DBU itself, although
the 1H NMR signals corresponding to the part of 4 were almost
identical.
1.661(3)/1.661(3) Å. The distance between the oxygen atom in
the borate part and the amidinium nitrogen atom in [DBU · H+]
is 2.769(2)/2.920(2) Å (average 2.845 Å), which falls in the
typical distances of O · · · (H)N hydrogen bonds (2.6-3.1 Å).7
Since the proton bonded to the nitrogen could not be observed
by X-ray crystallographic analysis, the structure of 6 was
computationally generated at the HF/6-31+G(d) level of theory8
to determine the location of the proton. The O · · · N distance
was well reproduced, and the proton was found between the
two atoms, as shown in Figure 2: O · · · N ) 2.8605 Å; O · · · H
) 1.8451 Å; H-N ) 1.0194 Å; O · · · H-N ) 174°. Thus, the
borate moiety in 6 forms a contact ion pair with the counter-
cation [DBU · H+] through the O · · · (H)N hydrogen bond.
For the reaction from 6 to 4 to proceed, 6 needs to be activated
at high temperature. A solution of 6 in THF-d8 was heated at
80 °C for 120 h in a sealed tube to afford the DBU adduct of
4 (4 · DBU) in 87% yield and mesitylene also in 87% yield
Hydrogen Source of Mesitylene. Next, we performed
deuterium-labeling experiments with the aid of 2H NMR
spectroscopy to determine the hydrogen source of mesitylene
formed in the reaction. The deuterated hydroxysilane 5-d was
prepared by treatment of 4 with D2O in a manner similar to the
preparation of 5: the atom % D of 5-d was estimated to be 83%
2
from H NMR spectra using CD3CN as an internal standard.
2H NMR monitoring of 5-d in THF at 80 °C for 96 h showed
that the O-D signal at δ 4.26 disappeared and a new signal
appeared at δ 6.79 in the aromatic region, as shown in Figure
3. A quantitative experiment demonstrated that 4 was formed
in 92% yield5 and deuterated mesitylene (mesitylene-d) was
formed in 100% yield (based on the atom % D of 5-d).9 It is
clear that hydrogen/deuterium of mesitylene comes from OH/
OD bonded to silicon in 5/5-d.
(7) Stout, G. H.; Jensen, L. H. In X-ray Structure Determination: A
Practical Guide; Macmillan: New York, 1968.
(8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Oritz,
J. V.; Stefanov, J. V.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, M. W.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA,
1998.
Mechanistic Considerations. A mechanistic overview of the
formation of 4 is postulated, as shown in Scheme 4. First,
intramolecular nucleophilic attack of the hydroxyl oxygen on
the boron center occurs in 5. In transition state 7, the
boron-mesityl bonds are activated by formation of the borate
complex and the proton on the oxygen migrates to the ipso
carbon of one of the mesityl groups, resulting in elimination of
(9) The yield was estimated from 2H NMR spectra by use of CD3CN
as an internal standard.

4694 Organometallics, Vol. 27, No. 18, 2008
Kawachi et al.
the mesitylene. A similar reaction mechanism was discussed
by Eisch et al. in the reaction of trimesitylborirene and an
alcohol: one boron-carbon (ring) bond in trimesitylborirene was
cleaved by prior coordination of the alcohol to the boron center
and subsequent intramolecular proton transfer.10 Kawashima and
Okazaki et al. reported that a boron-mesityl bond in a borate
complex was easily cleaved by hydrolysis.11,12
Triethylamine polarizes the O-H bond in 5 by forming an
OH · · · N hydrogen bond, which enhances nucleophilicity of the
hydroxyl oxygen and accelerates the cyclization. DBU, a
stronger base than triethylamine, completely abstracts the proton
from the hydroxyl group in 5, resulting in the formation of the
stable silyloxyborate-DBU · H+ complex 6. On the other hand,
the effect of CH3CN remains unclear. Initially we postulated
that CH3CN coordinates to a vacant p orbital on the boron center
in 5, which inhibits the nucleophilic attack of the oxygen on
the boron. However, the 11B NMR resonance of 5 in CD3CN
exhibited no shift change compared to that in C6D6.
give 3,4-benzo-1,2,5-oxasilaboracyclopentene 4 with elimination
of mesitylene. The cyclization was accelerated by heating or
in the presence of triethylamine. The cyclization involves
formation of a transient silyloxyborate complex and subsequent
hydrogen migration from the hydroxyl group to the ipso carbon
of one of the mesityl groups. The former was supported by
isolation of the silyloxyborate-DBU · H+ complex 6, and the
latter was confirmed by 2H NMR monitoring of the reaction of
deuterium-labeled 5.
Experimental Section
1
2
General Considerations. H (400 MHz), H (61.4 MHz), 11B
(128.3 MHz), 13C (100 MHz), and 29Si (79.4 MHz) NMR spectra
1
were recorded with a JEOL EX-400 or AL-400 spectrometer. H
and 13C chemical shifts were referenced to the residual proton (for
CDCl3, δ(1H) 7.26 ppm, δ(13C) 77.0 ppm; for C6D6, δ(1H) 7.20
ppm, δ(13C) 128.0 ppm). H and 13C chemical shifts in THF-d8
1
were referenced to external tetramethylsilane (δ(1H) 0 ppm; δ(13C)
0 ppm). 11B chemical shifts were referenced to external BF3 · Et2O
(δ 0 ppm). 29Si chemical shifts were referenced to external
tetramethylsilane (δ 0 ppm). Mass spectra (EI) were measured at
70 eV with a JEOL SX-102A mass spectrometer at the Natural
Science Center for Basic Research and Development (N-BARD),
Hiroshima University. Melting points were measured with a Yanaco
Micro melting point apparatus and were uncorrected. Elemental
analyses were performed with a Perkin-Elmer 2400CHN elemental
analyzer at our laboratory. Recycling preparative HPLC was
performed with an LC918 instrument equipped with a JAIGEL SIL
SH-043-15 column.
Hexane was distilled under a nitrogen atmosphere over calcium
hydride, or the dehydrated solvent (<10 ppm; Kanto Chemical Co.,
Inc.) was dried through a Solvent Dispensing System (GlassCon-
tour) under a nitrogen atmosphere (99.999%). THF and Et2O were
distilled under a nitrogen atmosphere over sodium diphenylketyl,
or the dehydrated solvents (<10 ppm for THF and <50 ppm for
Et2O; Kanto Chemical Co., Inc.) were dried through the Solvent
Dispensing System (GlassContour) under a nitrogen atmosphere
(99.999%). C6D6, toluene, and THF-d8 were distilled under a
nitrogen atmosphere over sodium diphenylketyl. CDCl3, CH3CN,
CD3CN, triethylamine, and 1,8-diazabicyclo[5.4.0]-7-undecene
(DBU) were distilled under a nitrogen atmosphere over calcium
hydride. All reactions were carried out under an inert gas
atmosphere.
Formation of Disiloxane. The reaction pathway to disiloxane
3 deserves some comments. We initially postulated that disi-
loxane 3 was formed by condensation of two molecules of 5
during the reaction of 1 with tert-butyl alcohol (Scheme 2).13
However, we found that stirring of 5 in THF at room
temperature for 240 h gave rise to disiloxane 3 in only 10%
yield, as described earlier. In contrast, treatment of 5 with an
equimolar amount of 1 in THF at room temperature for 192 h
gave 3 in 91% yield and 4 in 6% yield, as shown in eq 3. These
results indicate that the main pathway to 3 in the present reaction
is condensation of 5 with 1, rather than condensation of two
molecules of 5.14 A similar type of disiloxane formation
involving a hydrosilane was reported by Chojnowski et al. They
disclosed that a hydrosilane (R3SiH) underwent (C6F5)3B-
mediated condensation reaction with an alkoxysilane (R′3SiOR′′)
to yield a mixed disiloxane (R3SiOSiR′3) with elimination of
alkane R′′H.15
o-(Dimesitylboryl)(dimethylsilyl)benzene (1). This compound
was prepared in a manner similar to the procedure reported
previously.1
Reaction of 1 with t-BuOH: Formation of 2d, 3, and 4. A
mixture of 1 (155 mg, 0.40 mL) and t-BuOH (38 mg, 0.52 mmol)
in THF (1.0 mL) was stirred at room temperature for 42 h. Then,
the solvent was removed in vacuo. The resulting oil was diluted
with hexane (1 mL), vigorously stirred for 5 min, and dried in vacuo
to form a white solid containing 2d, 3, and 4. The solid was
dissolved in C6D6 (1.0 mL). The yields of 2d, 3, and 4 were
estimated to be 59%, 34%, and 5%, respectively, by 1H NMR
analysis of the solution using anisole, which was added prior to
the analysis, as an internal standard. Then, the solvent was removed
in vacuo. The residue was washed with CH3CN (1 mL) with
vigorous stirring, and the resulting solid was filtered. The solid on
the filter paper was washed with CH3CN (total volume 4 mL) to
give a mixture of 2d and 3 (118 mg) as a white solid. The mixture
was subjected to HPLC with hexane/CH2Cl2 (5:1) as eluent to give
3 (39 mg, 10% yield) (Rf ) 0.58) and 2d1 (56 mg, 30% yield) (Rf
) 0.5) as white solids.
Conclusion
We prepared hydroxysilane 5 by hydrolysis of hydrosilane 1
and confirmed that 5 underwent intramolecular cyclization to
(10) Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem.
Soc. 1990, 112, 1847.
(11) Kawashima, T.; Kannabe, N.; Tokitoh, R.; Okazaki, Organome-
tallics 1999, 18, 5180.
(12) For an example of an intermolecular reaction between triphenylbo-
rane and isopropyl alcohol: Domaille, P. J.; Druliner, J. D.; Gosser, L. W.;
Read, J. M., Jr.; Schmelzer, E. R.; Stevens, W. R. J. Org. Chem. 1985, 50,
189.
(13) For a review of siloxanes and silanols: (a) Drake, R.; MacKinnon,
I.; Taylor, R. In The Chemistry of Organic Silicon Compounds; Rappoport,
Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, pp 2217-
2244. (b) Lickiss, P. D. In The Chemistry of Organic Silicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3,
pp 695-744.
(14) We cannot exclude the possibility that the alcohol serves as a
Brønsted acid to facilitate the bimolecular condensation of 5.
(15) Chojnowski, J.; Rubinsztajn, S.; Cella, J. A.; Fortuniak, W.; Cypryk,
M.; Kurjata, K.; Kazmierski, K. Organometallics 2005, 24, 6077.
[o-(Dimesitylboryl)phenyl]dimethyldisiloxane (3). Mp: 184-185
1
°C (decomposed in a sealed tube). H NMR (C6D6, δ): 0.13 (br,
6H), 0.40 (br, 6H), 1.78-2.31 (br, 18H), 6.68-6.82 (br, 8H), 7.17

Intramolecular Reaction of Silanol and Triarylborane
Organometallics, Vol. 27, No. 18, 2008 4695
(ddd, 3J ) 8 Hz, 3J ) 8 Hz, 4J ) 1 Hz, 1H), 7.29 (ddd, 3J ) 8 Hz,
in THF (2.0 mL) was stirred at room temperature for 24 h to yield
a white suspension. The solvent was removed in vacuo. In order
to remove volatile materials completely, the resulting solid was
treated twice with the following procedures: (i) dilution with hexane
(1 mL), (ii) vigorous stirring at room temperature for 5 min, and
(iii) evaporation under reduced pressure. The obtained solid was
washed with hexane (2 mL) with vigorous stirring and filtered. The
solid on the filter paper was washed with hexane (total volume 8
mL) to give 6 (530 mg, 91% yield) as a white solid. Recrystalli-
zation of 6 from THF-hexane gave colorless crystals of 6 · 0.5THF.
4
3
3
3J ) 8 Hz, J ) 1 Hz, 1H), 7.54 (d, J ) 8 Hz, 1H), 8.23 (d, J
) 8 Hz, 1H). 13C{1H} NMR (CDCl3, δ): 2.80, 21.22, 22.73-24.03
(br),128.67,128.97,127.92-129.20(br),133.86,135.70,138.49-144.92
(br), 145.21, 155.28. 11B NMR (C6D6, δ): 74 (br). 29Si{1H} NMR
(C6D6, δ): -0.49. MS (EI): m/z 768 (M+ - Me, 5), 457 (M+
-
BPhMes2, 44), 383 ([Mes2B(C6H4)SiMe2]+, 85), 177 (100). Anal.
Calcd for C52H64B2OSi2: C, 79.78; H, 8.24. Found: C, 79.68; H,
8.43.
Hydrolysis of 1 with H2O: Formation of 5. To a solution of 1
(3.07 g, 7.99 mmol) in THF (16 mL) was added H2O (145 µL,
8.02 mmol) dropwise via a syringe at room temperature. Gas was
evolved immediately. After the reaction mixture was stirred at room
temperature for 10 min, the solvent was removed in vacuo. In order
to remove volatile materials completely, the resulting highly viscous
oil was treated twice with the following procedures: (i) dilution
with hexane (8 mL), (ii) vigorous stirring at room temperature for
5 min, and (iii) evaporation under reduced pressure, giving a white
solid. Then, the white solid was suspended in CH3CN (5 mL) by
vigorous stirring for 20 min and the suspension filtered. The solid
on the filter paper was washed with CH3CN (total volume 20 mL)
to give pure 5 (2.94 g, 90% yield) as a white solid.
1,8-Diazabicyclo[5.4.0]-7-undecenium 5,5-dimesityl-2,2-di-
methyl-3,4-benzo-1,2,5-oxasilaboratacyclopentene (6 · 0.5THF).
Mp: 178-180 °C (decomposed in a sealed tube). 1H NMR (CDCl3,
δ): 0.18 (s, 6H), 1.63-1.73 (br, 8H, DBU · H+), 1.86 (m, 2H, THF),
1.94 (s, 12H), 2.14 (s, 6H), 2.20 (m, 2H, DBU · H+), 2.71 (m, 2H,
DBU · H+), 3.20 (m, 2H, DBU · H+), 3.32 (m, 2H, DBU · H+), 3.75
3
(m, 2H, THF), 6.54 (s, 4H), 7.00-7.08 (m, 2H), 7.38 (d, J ) 7
Hz, 1H), 7.42 (d, 3J ) 6 Hz, 1H), 11.6 (br, 1H, DBU · H+). 13C{1H}
NMR (CDCl3, δ): 2.42, 19.78, 20.74, 24.44 (DBU · H+), 25.58
(DBU · H+), 25.86, 26.80 (THF), 29.05 (DBU · H+), 31.90
(DBU · H+), 37.55 (DBU · H+), 48.48 (DBU · H+), 53.91
(DBU · H+), 67.94 (THF), 122.89, 127.56, 128.50, 128.57, 131.26,
131.53, 139.91, 142.43, 155.24, 165.09, 171.98. 11B NMR (C6D6,
δ): 6 (br). 29Si{1H} NMR (C6D6, δ): 10.5. Anal. Calcd for
C38H57BN2O1.5Si: C, 75.55; H, 9.00; N, 4.76. Found: C, 75.29; H,
9.03; N, 4.44.
o-(Hydroxydimethylsilyl)(dimesitylboryl)benzene (5). Mp:
1
115-117 °C (decomposed in a sealed tube). H NMR (C6D6, δ):
0.14-0.37 (br, 6H), 1.51 (s, 1H), 1.79-2.36 (br, 18H), 6.74-6.78
(br, 4H), 7.14-7.26 (m, 2H), 7.52 (ddd, 3J ) 8 Hz, 4J ) 1 Hz, 5J
3
4
5
) 1 Hz, 1H), 7.57 (ddd, J ) 8 Hz, J ) 1 Hz, J ) 1 Hz, 1H).
13C{1H} NMR (CDCl3, δ): 1.41 (br), 21.23, 22.64-23.84 (br),
127.76 (br), 128.60 (br), 128.83, 129.08, 129.58-129.72 (br),
134.11, 134.38, 138.76-144.23 (br), 144.73, 155.24 (br). 11B NMR
(C6D6, δ): 74 (br). 29Si{1H} NMR (C6D6, δ): 6.89. MS (EI): m/z
280 (M+ - Mes, 100), 265 (M+ - 2Me, 42), 161 (M+ - 2Mes,
92). Anal. Calcd for C26H33BOSi: C, 77.98; H, 8.31. Found: C,
77.71; H, 8.25.
Formation of 4 · DBU from 6. A suspension of 6 (173 mg 0.29
mmol) in THF-d8 (0.6 mL) in a J. Young valve NMR tube was
heated at 80 °C for 120 h to afford teh DBU adduct of 4 (4 · DBU)
and mesitylene. The yields of 4 · DBU and mesitylene were
1
estimated to be 87% and 87%, respectively, by H NMR analysis
of the solution using anisole, which was added prior to the analysis,
as an internal standard.
1
4 · DBU. H NMR (C6D6, δ): 0.39 (s, 6H), 1.10-1.15 (m, 2H,
Cyclization of 5: Typical Procedure for Formation of 4. A
solution of 5 (200 mg, 0.50 mmol) in THF-d8 (1.0 mL) was stirred
at room temperature for 240 h to afford 3, 4, and mesitylene. The
yields of 3, 4, and mesitylene were estimated to be 10%, 86%, and
81%, respectively, by 1H NMR analysis of the solution using
anisole, which was added prior to the analysis, as an internal
standard. Then, the solvent was removed in vacuo. In order to
remove volatile materials completely, the resulting highly viscous
oil was treated twice with the following procedures: (i) dilution
with hexane (1 mL), (ii) vigorous stirring at room temperature for
5 min, and (iii) evaporation under reduced pressure. The obtained
oil was washed with CH3CN (1 mL) with vigorous stirring, and
the resulting white solid was filtered. The solid on the filter paper
was washed with CH3CN (total volume 4 mL) to give 3 (10 mg,
5% yield) as a white solid. The filtrate was concentrated in vacuo
to afford a highly viscous oil. The oil was subjected to bulb-to-
bulb distillation (240-250 °C/0.3 mmHg) to give 4 (90 mg, 64%
yield) as a highly viscous colorless oil, which solidified on standing.
DBU), 1.24-1.30 (m, 2H, DBU), 1.39-1.54 (m, 4H, DBU),
2.43-2.45 (m, 2H, DBU), 2.59-2.67 (m, 4H, DBU), 2.28 (s, 6H),
2.36 (s, 12H), 3.58-3.63 (m, 2H, DBU), 6.89 (s, 2H), 7.23 (dd, 3J
) 7 Hz, J ) 7 Hz, 1H), 7.31 (dd, J ) 7 Hz, J ) 7 Hz, 1H),
7.54 (d, 3J ) 7 Hz, 1H), 7.70 (d, 3J ) 7 Hz, 1H). 11B NMR (C6D6,
δ): 42 (br).
3
3
3
Hydrolysis of 1 with D2O: Formation of 5-d. The title
compound was prepared in a manner similar to that for 5 using
D2O. The deuterated ratio of 5-d was determined to be 83 atom %
2
D by H NMR (THF) using CD3CN as an internal standard.
o-(Deuterioxydimethylsilyl)(dimesitylboryl)benzene (5-d). 1H
NMR (C6D6, δ): 0.14-0.38 (br, 6H), 1.79-2.38 (br, 18H),
6.72-6.81 (br, 4H), 7.15-7.26 (m, 2H), 7.53 (ddd, 3J ) 8 Hz, 4J
5
3
4
5
) 1 Hz, J ) 1 Hz, 1H), 7.57 (ddd, J ) 8 Hz, J ) 1 Hz, J )
2
1 Hz, 1H). H NMR (THF δ) 6.79.
Cyclization of 5-d: Formation of 4 and Mesitylene-d. A
solution of 5-d (83 atom % D, 120 mg, 0.30 mmol) in THF (0.60
mL) in a J. Young valve NMR tube was degassed by the
freeze-pump-thaw process. Then, the solution was heated at 80
°C for 96 h in the dark to afford 4 and mesitylene-d. The yield of
mesitylene-d was estimated to be 100% (based on 83 atom % D of
5-d) by 2H NMR analysis of the solution using CD3CN, which
was added after the reaction, as an internal standard. Then, the
solvent was removed in vacuo. In order to remove volatile materials,
the residue was treated twice with the following procedures: (i)
dilution with hexane (1 mL), (ii) vigorous stirring at room
temperature for 5 min, and (iii) evaporation under reduced pressure,
giving a highly viscous oil of 4. The oil was diluted with C6D6 (1
mL), and the yield of 4 was estimated to be 92% by 1H NMR
analysis of the solution using anisole, which was added prior to
the analysis, as an internal standard.
5,5-Dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclo-
pentene (4). Mp: 42.0-44.0 °C (in air). 1H NMR (C6D6, δ): 0.39
3
(s, 6H), 2.29 (s, 6H), 2.37 (s, 12H), 6.90 (s, 2H), 7.23 (ddd, J )
7 Hz, 3J ) 7 Hz, 4J ) 1 Hz, 1H), 7.31 (ddd, 3J ) 7 Hz, 3J ) 7 Hz,
4J ) 1 Hz, 1H), 7.54 (ddd, 3J ) 7 Hz, 4J ) 1 Hz, 5J ) 1 Hz, 1H),
7.70 (ddd, 3J ) 7 Hz, 4J ) 1 Hz, 5J ) 1 Hz, 1H). 13C{1H} NMR
(CDCl3, δ): -0.75, 21.37, 22.09, 127.19 (2C), 129.66, 130.44,
131.03, 134.02, 137.90, 139.44, 149.66 (9 signals were observed
in the aromatic region, although 10 signals were expected). 11B
NMR (C6D6, δ): 50 (br). 29Si{1H} NMR (C6D6, δ): 27.26. MS
(EI): m/z 280 (M+, 100), 265 (M+ - Me, 66), 161 (M+ - Mes,
21). Anal. Calcd for C17H21BOSi: C, 72.86; H, 7.55. Found: C,
72.46; H, 7.40.
Cyclization of 5 in the Presence of DBU: Formation of 6. A
solution of 5 (398 mg, 0.99 mmol) and DBU (150 mL, 1.00 mmol)
Reaction between 1 and 5: Formation of 3. A solution of 1
(191 mg, 0.50 mmol) and 5 (200 mg, 0.50 mmol) in THF (1.0

4696 Organometallics, Vol. 27, No. 18, 2008
Kawachi et al.
mL) was stirred at room temperature for 120 h. Then, the solvent
was removed in vacuo. In order to remove volatile materials
completely, the resulting highly viscous oil was treated twice with
the following procedures: (i) dilution with hexane (1 mL), (ii)
vigorous stirring at room temperature for 5 min, and (iii) evaporation
under reduced pressure, giving a white solid containing 3 and 4.
The solid was dissolved in C6D6 (1.0 mL). The yields of 3 and 4
were estimated to be 91% and 6%, respectively, by 1H NMR
analysis of the solution using anisole, which was added prior to
the analysis, as an internal standard. Then, the solvent was removed
in vacuo. The residue was washed with CH3CN (2 mL) with
vigorous stirring, and the resultant white solid was filtered. The
solid on the filter paper was washed with CH3CN (total volume 8
mL) to give 3 (320 mg, 82% yield) as a white solid.
X-ray Crystallographic Analysis. Crystals of 6 suitable for X-ray
crystallographic analysis were obtained by recrystallization from
THF-hexane. A crystal was mounted using silicon oil to a cryoloop
(Hampton Research). The data were collected at 173 K on a Mac
Science DIP2030 imaging plate equipped with graphite-monochro-
mated Mo KR radiation (λ ) 0.710 73 Å). The data were solved using
SHELX by direct methods. The non-hydrogen atoms were refined
anisotropically. All of the hydrogen atoms were located at the expected
positions by geometrical calculations and refined isotropically.
Computational Methods. Calculations were performed with the
Gaussian 988 program package on the HIT HPC-PA264U-6CPU
model. The initial geometry of 6 was obtained from the crystal
structure of 6, and a hydrogen atom was added to the amidinium
nitrogen. Then the geometry was fully optimized at the HF/6-
31+G(d) level.
Acknowledgment. We thank Professor Seiji Matsubara
2
(Kyoto University) for advice on H NMR measurement.
This work was supported by Grants-in-Aid for Scientific
Research on Priority Area “Advanced Molecular Transfor-
mations of Carbon Resources” (No. 19020048) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy of Japan.
Supporting Information Available: A CIF file, tables, and
figures giving crystallographic data for 6 and computational work
on 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM8004405