Regio- and stereoselective addition of sterically hindered silylboranes to terminal alkynes

Article abstract of DOI:10.1016/S0022-328X(01)01339-0

Novel stable organosilylboranes possessing dimesityl groups attached to boron were synthesised. They gave an addition reaction with terminal acetylenic hydrocarbons in the presence of a transition metal complex. Thus, (diphenylmethylsilyl)dimesitylborane

Full text of DOI:10.1016/S0022-328X(01)01339-0

Journal of Organometallic Chemistry 646 (2002) 179–190
www.elsevier.com/locate/jorganchem
Regio- and stereoselective addition of sterically hindered
silylboranes to terminal alkynes
Jose´ Carlos Araujo Da Silva a, Marc Birot a,*, Jean-Paul Pillot a,*, Michel Pe´traud b
a Laboratoire de Chimie Organique et Organome´tallique, UMR 5802 CNRS-UB 1, Uni6ersite´ Bordeaux 1, 351 cours de la Libe´ration,
F-33405 Talence Cedex, France
b CESAMO, Uni6ersite´ Bordeaux 1, 351 cours de la Libe´ration, F-33405 Talence Cedex, France
Received 11 July 2001; accepted 26 September 2001
Abstract
Novel stable organosilylboranes possessing dimesityl groups attached to boron were synthesised. They gave an addition reaction
with terminal acetylenic hydrocarbons in the presence of a transition metal complex. Thus, (diphenylmethylsilyl)dimesitylborane
and (diphenyl-tert-butylsilyl)dimesitylborane reacted in good yields with phenylacetylene and alk-1-ynes in the presence of
Pd2(dba)3(etpo)2 as a catalyst. The structures of the products were determined by NMR spectroscopy using INEPT techniques
coupled to computational simulation. Various heteronuclear coupling constants J13
and J29
were determined for the first
C−H
time in this series. The results showed that the dimesitylboryl group added to the terminalSia−ceHtylenic carbon atom and the
organosilyl group to the internal carbon atom, according to a regio- and stereoselective syn-addition. © 2002 Elsevier Science
B.V. All rights reserved.
Keywords: Organosilylboranes; Silaboration; Palladium catalyst; 13C- and 29Si-NMR; Heteronuclear coupling constants; Stereochemistry
electronegative heteroatoms such as oxygen and nitro-
gen, and some authors have emphasised that the SiꢀB
bond was then unexpectedly chemically inert [7]. The
recent discovery that compounds containing SiꢀB
bonds possess many interesting properties has focused
renewed interest in this series. Thus, Ito and co-workers
1. Introduction
The first stable organosilylboranes were reported
about 40 years ago [1–3]. This puzzling class of com-
pounds contains a s-bond between a tetravalent silicon
and a trivalent boron atom. Apart from the pioneering
have reported that organosilylboranes possessing elec-
work of No¨th et al. and scarce subsequent reports in
tronegative substituents on boron added regio- and
the literature, compounds possessing SiꢀBIII bonds have
only focused very little attention over the last decades
[4–8]. Conversely, organosilicon chemistry has been
receiving an explosive development during the same
stereospecifically to ethylenic and acetylenic substrates,
in the presence of platinum or palladium complexes as
catalysts [9–12]. Tanaka and co-workers has subse-
quently reported that a similar silaboration of alkynes
and silaborative diyne cyclisation were promoted by a
palladium catalyst with an etpo ligand [13]. The former
time. Because silicon possesses a marked electropositive
character and boron is an electron-deficient het-
eroatom, compounds containing SiꢀBIII bonds are not
authors have extended this reaction to 1,2-dienes and
very stable. Generally, silylboranes are difficult to syn-
methylene cyclopropanes, using various transition
thesise and there are only few different structural
metal-based catalysts [14,15]. Recently, we have re-
edifices accessible, more particularly when boron is
ported preliminary results on the synthesis and UV–vis
bound to alkyl substituents. Classical routes using
properties of a new class of aromatic silylboranes sta-
organometallic condensations lead to borates, i.e. com-
pounds possessing tetracoordinated boron atoms. But
organosilylboranes are more stable when boron bears
bilised by mesityl groups attached to boron instead of
electronegative heteroatoms [16,17]. Availability of data
on this new series was significant to gain a better
knowledge of the properties of the SiꢀB bond. More-
over, it was interesting to compare the chemical reactiv-
* Corresponding authors.
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01339-0

180
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ity of these new compounds with that of previously
described silylboranes. We report on the addition of
these organosilylboranes to acetylenic hydrocarbons in
the presence of a palladium catalyst. The structures of
the addition products were investigated by NMR spec-
troscopy (1H, 13C and 29Si) using INEPT 1D and 2D
sequences complemented by computational line-shape
simulations.
2.2. Addition of silylboranes 1 and 2 to alk-l-ynes
Ito et al. have found that silylboranes 3–6 containing
oxygen or nitrogen atoms (Scheme 1), react with car-
bon–carbon triple bonds in the presence of platinum-
or palladium-based catalysts, according to a regioselec-
tive syn-addition mechanism.
They have shown that the boryl group added to the
terminal carbon atom in the case of terminal acetylenic
derivatives. Moreover, they have investigated a wide
range of transition metal complexes, showing that pal-
ladium–isonitrile complexes were amongst the most
efficient catalysts [11,12]. On the other hand, Wilkin-
son’s catalyst was totally inactive, although it can be
efficient in other reactions involving boron, such as
hydroboration reactions. In the case of terminal olefins,
the SiꢀB bond added in a reverse way, the boryl group
being linked to the internal carbon atom. In the same
way, Tanaka et al. have reported that Pd2(dba)3(etpo)2
[etpo=P(OCH2)3CEt] is an effective catalyst for the
stereoselective addition of 6 to oct-1-yne (92% yields)
and that the addition of borostannanes under the same
conditions led to similar results [19].
2. Results and discussion
2.1. Synthesis of sterically hindered organosilylboranes
Novel organosilylboranes possessing bulky dimesityl
groups attached to boron were synthesised in good
yields according to our previously reported route by
using a modified experimental procedure (compounds 1
and 2, Eq. (1)) [17].
(1)
We have shown that sterically hindered silylboranes 1
and 2 added to the triple bond of terminal alkynes in
the presence of Pd2(dba)3(etpo)2 as the catalyst (Eq.
(2)).
Compounds 1 and 2 are powders that can be handled
in air for short times. They are reasonably stable but
slowly oxidise over several days. They melt before
decomposition and are kept unchanged for weeks under
inert atmosphere at room temperature. Their 29Si-NMR
spectra exhibited chemical shifts in roughly the same
range as their disilane homologues (−25.0 and −20.1
ppm for 1 and 2, respectively). Conversely, boron nu-
clei are deshielded in 11B-NMR spectroscopy (110 and
103 ppm for 1 and 2, respectively) with respect to
organosilylboranes bearing heteroelements linked to
boron, e.g. l=34.3 ppm for the boron nucleus in the
case of compound 4 (Scheme 1) [8]. But the 11B-NMR
chemical shifts are in the same range as those previ-
ously reported for related diboranes [18].
(2)
Compound 1 and hept-1-yne (R2=C5H11ꢀ) in slight
excess were heated at 120 °C for 12 h in toluene as the
solvent, in the presence of the catalyst. The IR spec-
trum of product 8 revealed that the absorption bands of
the starting acetylenic compounds at 2130 cm−1 (wCꢁC
)
and 3340 cm-1 (wCꢀH
) disappeared. Interestingly,
acetylenic
the 29Si- and 11B-NMR chemical shifts appeared
around −13 and 80 ppm, respectively, whereas no
signal corresponding to unreacted silylborane remained.
Similarly, the reaction was performed with silylborane 2
and phenylacetylene. The results are shown in Table 1.
Thin-layer chromatography (TLC), 1H- and 13C-
NMR showed that one major isomer was formed in
each case. Integration of the ethylenic protons of the
Scheme 1. Organosilylboranes stabilised by electronegative groups.

J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
181
Table 1
ethylenic carbon. These results were also supported by
the strong deshielding effect of the BMes2 group previ-
ously reported for other substrates [20,21].
Addition products of organosilylboranes to terminal acetylenic hy-
drocarbons
R1
R2
Product
Z:E
Overall yield (%)
2.3.1.2. Addition of (diphenylmethylsilyl)dimesitylborane
to phenylacetylene. One of the four possible isomers
7a–d might be expected as the major product from the
addition of 1 to the triple bond of phenylacetylene
(Scheme 2). Assuming that a long-distance coupling
Me
Me
tert-Bu
tert-Bu
Ph
7
8
9
95:5
95:5
95:5
95:5
67
65
55
60
CH3(CH2)4ꢀ
Ph
CH3(CH2)4ꢀ 10
constant through the ethylenic bond should be very
crude reaction mixtures was not possible, because of the
presence of signals of dba in the same region. So the
products were first purified by liquid chromatography.
Owing to the results reported previously in the litera-
ture [8,13], the products were assumed to be the Z/E
isomers resulting from the addition of the dimesitylbo-
ryl group to the terminal acetylenic carbon.
Reaction of oct-1-yne with silylborane 2 under the
same conditions gave similar results, but attempts to
perform addition of silylboranes 1 and 2 to internal
acetylenic hydrocarbons and various ethylenic sub-
strates under similar conditions were unsuccessful.
Other catalysts such as the platinum-based catalysts
used by Ito et al., as well as Cp2TiMe2 and Cp2ZrMe2,
led to the consumption of the silylboranes but did not
give the expected silaboration products.
4
weak and consequently not observed (i.e. J13C − H),
1
5
the 13C-NMR INEPT spectra of the carbon nucleus of
the methyl group bounded to the silicon atom (C5)
should exhibit quite different patterns depending on the
regiochemistry of the reaction, as shown in Scheme 2
(spectra A or B).
The experimental 13C-NMR spectrum of C5, centred
at −0.7 ppm, was obtained by using a non-decoupled
INEPT sequence (Fig. 1).
This spectrum was consistent with structures 7a or
7b, since it exhibited only four lines. If stereoisomers 7c
or 7d were formed, the corresponding 13C-NMR spec-
trum should exhibit eight lines resulting from the cou-
pling with the three neighbouring H (1J13C− H) and the
1
ethylenic H (3J13C − H=0.3–1.6 Hz [22]) (spectrum B).
1
5
However, very weak additional peaks were observed in
the same region, which probably corresponded to the
other isomers in very small proportions. Similar results
were obtained concerning the regiochemistry of the
addition of 1 to hept-1-yne, corroborating that the
dimesitylboryl group added to the terminal acetylenic
carbon.
2.3. NMR studies
Ito and co-workers have inferred the regio- and
stereochemistry of the addition of silylboranes to unsat-
urated substrates from the observation of NOE effects
in a few examples [14,15]. To show direct evidence of
the structure of the silaboration protons, we turned to
13C- and 29Si-NMR spectroscopies to measure the vari-
ous heteronuclear coupling constants thanks to the
INEPT technique.
2.3.1.3. Addition of (diphenyl-tert-butylsilyl)dimesityl-
borane to hept-1-yne. The four plausible silaboration
products are shown in Scheme 3 (10a–d).
The non-decoupled 13C-NMR INEPT spectrum of
the addition product was obtained under the same
conditions as in the previous case. Fig. 2 displays the
region of the aliphatic carbons, namely the Me groups
and the quaternary carbon of the tert-butyl group
linked to silicon (C5).
2.3.1. Regiochemistry of the silaboration
2.3.1.1. Addition of (diphenylmethylsilyl)dimesitylborane
to hept-1-yne. In a first approach, the regioselectivity of
the reaction was inferred from H- and 13C-NMR spec-
If regioisomers 10a or 10b were formed, no long-dis-
tance coupling between C5 and the ethylenic H should
1
troscopies. Besides the expected signals corresponding
to the different aliphatic and aromatic protons, the
1H-NMR spectrum of 8 exhibited a low field singlet at
7.6 ppm (1H) assigned to the ethylenic H. Moreover,
the proton-decoupled 13C-NMR spectrum of this
product showed two deshielded signals at 156 and 157
ppm. A 2D 1H–13C-NMR correlation (HMQC) cor-
roborated that only the more deshielded carbon nucleus
at 157 ppm was coupled with the proton at 7.6 ppm.
This signal was assigned to the ethylenic carbon bearing
both the dimesitylboryl group and the ethylenic proton.
Therefore, the boron atom added to the terminal
4
2
be observed (i.e. J13C− H). Therefore, only J13
1
1
C − H
5
coupling constants between C5 and nine neighbouring
H of the tert-butyl group were expected. Conversely, if
reverse addition products were formed (compounds 10c
3
or 10d), a J13
coupling constant between C5 and
1
C− H
the ethylenic H should be observed. Because a tangled
up spectrum was obtained in this region, no coupling
constant involving C5 could be drawn out. This is due
to the weak response of this nucleus and to the fact that
its spectrum overlapped the signals of the CH3 of the
mesityl groups (1J13C− H=128 Hz).
1

182
J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
Scheme 2. Expected J13
coupling for C5 related to Z/E isomers 7a–d and corresponding 13C-NMR INEPT theoretical spectra.
1
C− H
We reasoned that the use of 2D INEPT BIRD
J-resolved 13C-NMR sequences to observe the aliphatic
carbons should help us overcome this difficulty for the
following reasons [23,24]. (i) The phases of the signals
depend on the signs of the coupling constants and the
systems of spins, for a given value of the polarisation
delay ~. (ii) The observed carbon atoms are coupled at
the first order. This feature takes advantage of an
enhanced sensibility resulting from the most efficient
polarisation transfer. (iii) The phases of the BIRD
Scheme 3. Possible isomers 10a–d for the silaboration of hept-1-yne
by (diphenyl-tert-butylsilyl)dimesitylborane (2).
Fig. 1. Experimental 13C-NMR INEPT spectrum of C5 in product 7.

J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
183
pulses can be chosen in a way to selectively mask any
(polarisation delay ~, refocusing delay Z, coupling con-
stants JX–H and JH–H) [25].
n
undesirable J13
coupling constants.
1
C− H
In our case, the BIRD pulses were chosen to elimi-
Thus, the experimental and simulated spectra of the
quaternary carbon atom C5 in product 10 are displayed
in Fig. 4.
The simulation was achieved by varying J, whereas ~,
Z and the half-height width were fixed by the experi-
ment. A good agreement between the simulated and
nate the coupling constant 1J13C− H (ꢀ128 Hz) of each
1
Me group. The chart giving the response of the differ-
ent carbon nuclei is shown in Fig. 3.
Individual 13C INEPT spectra can be easily picked
up from this chart. On the other hand, the SIMEPT
program enabled us to calculate the theoretical INEPT
spectrum of a nucleus X from a set of parameters
experimental spectra was obtained for a J13
value
1
C− H
of −3.3 Hz (9H). As the phase of the signal was
Fig. 2. Non-decoupled 13C-NMR INEPT spectrum of product 10.
Fig. 3. 13C-NMR 2D INEPT BIRD J-resolved chart of the Me and C5 carbon atoms in product 10.

184
J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
technique. In the case of the addition of (diphenyl-
methylsilyl)-dimesitylborane to phenylacetylene, the
proton-decoupled 29Si-NMR INEPT spectrum of 7 is
shown in Fig. 5 (refocused spectrum).
This spectrum exhibited one major signal at −14.7
ppm. Unexpectedly, three other weak peaks which were
not found from chromatography can be seen at −13.3,
−13.1 and −10.8 ppm, respectively. So, the 29Si-NMR
INEPT spectroscopy revealed very sensitive in this case,
whereas only very weak additional signals were de-
tected in the 13C-NMR spectrum. The highest peak at
−14.7 ppm was assigned to stereoisomer 7a and the
peak at −13.3 ppm to isomer 7b. This assignment was
supported by non-decoupled 29Si-NMR INEPT experi-
ments. Thus, two different complex patterns were ob-
served for these nuclei (Fig. 6, spectra a and b).
Qualitative analysis of each system of spins was
performed thanks to the SIMEPT program (Fig. 6, spec-
tra c and d). These results corroborated the fact that
1
the silicon nuclei were coupled to four different H spin
3
systems, including the cis and trans JSiꢀH with the
2
3
3
ethylenic H. Since Jtrans should be larger than Jcis, the
29Si nuclei signal at −14.7 ppm, with the largest JSiꢀH
3
2
value (16.2 Hz) was assigned to the Z stereoisomer 7a
[22,26]. The signal at −13.3 ppm with the smallest
coupling constant (14.9 Hz) was assigned to the E
stereoisomer 7b. Interestingly, the efficiency of the pro-
gram was confirmed by the involvement of a long-dis-
Fig. 4. (a) Section of the chart in Fig. 3 corresponding to C5. (b)
Simulated spectrum.
4
opposite to that of the methyl groups (where only
tance coupling constant JSiꢀH =0.8 Hz (Table 2). If
8
3J13
could be observed), we can conclude that C5
1
C− H
this parameter is ignored, the simulated spectrum does
not fit correctly the experimental data.
Moreover, this strategy enabled us to assign other
small 29Si-NMR peaks. The simulated spectrum of the
signal at −10.8 ppm revealed a coupling constant JSiꢀH
of 11.7 Hz (Fig. 7). Such a value is consistent with the
2
was coupled with its nine neighbouring H (i.e. J13
1
3
4
= −3.3 Hz) only. No J13
or J13
conCst−anHt
1
1
could be found out. This achievement leCd−usH to unam-
biguously assign the product the structures 10a or
10b, with silicon at the internal position of the double
bond and the dimesitylboryl group at the terminal
carbon.
C− H
structure of one of the two isomers 7c or 7d, resulting
2
from the reverse addition to the triple bond, i.e. JSiꢀH
.
2
We can reasonably postulate that the fourth signal at
−13.1 ppm corresponds to the other isomer.
From a quantitative point of view, it must be empha-
sised that the INEPT sequence cannot afford any direct
2.3.2. Stereochemistry
Evidence of the stereochemistry of the silaboration
was given by 29Si-NMR spectroscopy using the INEPT
Fig. 5. Experimental 1H-decoupled 29Si-NMR INEPT spectrum of borosilylation product 7.

J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
185
Fig. 6. Experimental and simulated 29Si-NMR INEPT spectra of isomers 7a and 7b.
relationship between the intensities of the signals and
3. Experimental
the ratios of the geometric isomers. Indeed, the spectra
resulting from INEPT sequences are tightly dependent
on the parameters ~, Z, J and multiplicity. Therefore,
the quantitative determination of each isomer requires
the knowledge of all the system of spins of each com-
pound, as well as their relative response factors. In this
context, the SIMEPT program has emerged as a very
pertinent tool for the reconstruction of the spectra, as
relative response factors can be easily calculated for
every given parameter sets. The following values for the
response factors were obtained: 4.7 (signal at −10.8
ppm), 2.2 (signal at −13.3 ppm) and 1 for the main
signal (signal at −14.7 ppm).
3.1. General
All manipulations were carried out under Ar atmo-
sphere using standard vacuum techniques. Handling of
air sensitive powders and solids was performed under
controlled atmosphere in a glove box containing dry
nitrogen. The solvents were purified by conventional
means and distilled prior to use. Pd2(dba)3 was pur-
chased from Aldrich.
The NMR spectra were recorded in C6D6 or CDCl3
using Bruker AC 250 (1H and 13C) and a DPX 200
spectrometers (29Si and 11B). Chemical shifts are re-
ported in ppm and are referenced to Me4Si (1H, 13C,
29Si) or BF3·Et2O (11B).
In the case of the product 9, the stereochemistry of
the reaction was determined as in the case of product 7,
thanks to 29Si-NMR spectroscopy. Thus, the main ad-
dition product having the largest coupling constant
2D J-resolved INEPT is a technique to obtain cou-
pled spectra. However, the large value of the coupling
3JSi−H (16.2 Hz) corresponds to the isomer Z 9a.
constant J13
(in the range of 140 Hz) requires a
1
1
C− H
2

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J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
Table 2
J
29 1
Si− H
and J
1
1
H− H
coupling constants used for the simulation of spectra in Fig. 6
broad window in the F1 domain, thus lowering the
resolution. When refocusing pulses are replaced by
BIRD pulses, direct coupling J can be suppressed. The
Elemental analyses were performed by the Service
Central d’Analyses du CNRS, BP 22, 69390 F-
Vernaison.
1
corresponding window is narrowed, so the resolution
increases [23]. The coupling constants were measured
1
by the heteronuclear H–13C J-resolved BIRD 2D IN-
EPT technique, in the phase sensitive mode, using the
following sequence: D1–(90°1xH)–~–(180°x1H)(180°x13C)–
~–(90°19yH)(90°x13C)–d0 (BIRD) d0 acquisition in TPPI
1
x
mode with proton decoupling. BIRD–(90°H)–d6–
(180°1xH)(180°1x3C)–d6–(90°1xH). The spectrum was
recorded using spectral widths of 3550950 Hz in the
F2 and F1 dimensions, respectively. A total of 32
transients were accumulated for the 256 increments of
t1. The relaxation delay D1 was 1 s, ~=0.053 s, d6=
0.00357 s, increment=0.0025 s.
Non-refocused INEPT spectra were simulated on a
PC using the program previously described for 119Sn
and 29Si [25]. The following parameters were used: 29Si
~=0.0357 s, Z=0.02 s; 13C ~=0.053 s, Z=0.00357 s.
IR data were collected using
a Perkin–Elmer
Paragon 1000 spectrometer. The samples were prepared
as KBr pellets or films between KBr plates.
Mass spectra (MALDI-TOFMS) were obtained in a
VG TofSpec-SE Fisons Instruments spectrometer (20
kV) from a THF solution containing the same amounts
of the sample and 5-chlorosalicyclic acid as the matrix.
Low and high resolution LSIMS (+) spectra were
obtained in a VG Autospec-EQ (EBEQQ) spectrometer
using 3-nitrobenzylic alcohol as the matrix.
Fig. 7. Experimental and simulated 29Si-NMR INEPT spectra of
products 7c or 7d.

J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
187
Melting temperatures were obtained in a Kofler hot
The same procedure was used to prepare the prod-
bench or a capillary melting point apparatus.
ucts 8–10.
3.5. Physicochemical characterisation
3.2. Synthesis of organosilylboranes
3.5.1. (Diphenylmethylsilyl)dimesitylborane (1)
Organosilyllithium reagents were obtained according
to previously reported procedures [27–29]. Thus,
methyldiphenylsilyllithium was prepared from finely di-
vided lithium (0.28 g, 0.04 mol) and methyl-
diphenylchlorosilane (4.45 g, 0.02 mol) in THF (40 ml)
at −10 °C. The mixture was degassed using ultra-
sounds prior to addition of the chlorosilane. Other
silyllithium reagents were obtained in a similar way.
Dimesitylfluoroborane (5.4 g, 0.02 mol) and dry THF
(15 ml) were placed in a three-necked Pyrex® flask
equipped with a magnetic bar, a pressure-equalising
dropping funnel, a reflux condenser and an Ar inlet.
The flask was cooled at −60 °C and magnetically
stirred. Methyldiphenylsilyllithium in THF was added
dropwise over 30 min and allowed to heat at room
temperature under stirring over 3 h. After evaporation
of the solvent, the crude solid was washed with 25 ml of
petroleum ether, chiefly to eliminate the excess of
Mes2BF. The remaining solid was extracted with
petroleum ether (2×150 ml) and the solution filtered
under inert atmosphere to separate the lithium salts.
Then, the remaining solid was washed with toluene (25
ml) and both solutions evaporated separately, yielding
in each case a yellow–green powder. The product was
crystallised from a petroleum ether solution. Yield:
64%.
M.p. 105 °C. 29Si-NMR (C6D6, ppm): −25.0. 11B-
1
NMR (ppm): 103.0. H-NMR (ppm): 0.75 (s, 3H, 1),
2.08 (s, 12H, 12), 2.13 (s, 6H, 13), 6.69 (s, 4H, 10), 7.11
(m, 6H, 6, 7), 7.45 (m, 4H, 5). 13C-NMR (ppm): −0.8
(1), 21.2 (13), 24.3 (12), 128.0 (6), 128.8 (7), 129.1 (10),
135.5 (5), 138.8 (9), 139.4 (11), 140 (4), 145.3 (8). Anal.
calc. for C31H35BSi (446.51): C, 83.39; H, 7.90; B, 2.42;
Si, 6.29. Found: C, 82.9; H, 7.07; B, 2.40; Si, 7.05%.
EIMS; m/z (%): 249.1 (100%, Mes2B+); 446.2 (0.5%,
MePh2SiBMes+2 ). MALDI TOF LD+: 511.23 (100%,
[M+Na]+), 527.18 (50%, [M+K]+). Low resolution
LSIMS: 223.1 (28%, MePh2SiBMe+); 249.1 (55%,
Mes2B+); 264.1 (24%, MeBMes+2 ); 301.1 (44%,
MesPh2Si+). High resolution LSIMS; C31H36BSi
([MePh2SiBMes2+H]+):
447.260345
(Found);
447.267935 (Calc.).
3.3. Pd2(dba)3(etpo)2
3.5.2. (Diphenyl-tert-butylsilyl)dimesitylborane (2)
Pd2(dba)3 (28 mg, 0.03 mmol) and etpo (14 mg, 0.085
mmol) were introduced under Ar in a 50 ml Schlenk
tube and capped with a septum. Toluene (2 ml) was
added with a syringe. Then, the Schlenk flask was fitted
with a condenser and refluxed for a few minutes until
formation of a green colour.
3.4. Silaboration products
Organosilylborane 1 (0.447 g, 1 mmol), pheny-
lacetylene in excess (0.0204 g, 2 mmol) and toluene (3
ml) were introduced in a round-bottom flask equipped
with an Ar inlet and a magnetic bar. This solution was
slowly added onto the previously prepared catalyst
solution in the Schlenk tube using a syringe. After
heating at 120 °C under stirring for 12 h, the solvent
and phenylacetylene in excess were eliminated under
vacuum. The crude product 7 was passed through a
neutral alumina column under Ar using petroleum
ether as the eluent.
M.p. 132 °C. 29Si-NMR (C6D6, ppm): −20.1. 11B-
NMR (ppm): 110.0. H-NMR (CDCl3, ppm): 1.38 (s,
1
9H, 14), 2.10 (s, 18H, 12, 13), 6.66 (s, 4H, 10), 7.10 (m,
1
6H, 6, 7), 7.75 (m, 4H, 5). H-NMR (CD2Cl2, ppm):
1.28 (s, 9H, 14), 1.93 (s, 12H, 12), 2.24 (s, 6H, 13), 6.68
(s, 4H, 10), 7.15–7.23 (m, 6H, 6, 7), 7.52–7.56 (m, 4H,
5). 13C-NMR (ppm): 20.6 (1), 21.0 (13), 25.1 (12), 30.2
(14), 127.7 (6), 128.4 (7), 129.1 (10), 136.6 (5), 138.0 (4),
138.9 (9), 139.2 (11), 146.4 (8). Anal. calc. for

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J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
2
3
C34H41BSi (488.59): C, 83.58; H, 8.46; B, 2.21; Si, 5.75.
Found: C, 83.25; H, 7.75; B, 2.13; Si, 5.50%. MS:
MALDI TOF LD+: 511.23 (100%, [M+Na]+);
527.18 (50%, [M+K]+). Low resolution LSIMS: 249.2
(100%, Mes2B+); 301.1 (27%, MesPh2Si+).
29Si-NMR (ppm): −13.3. JSi−H =6.6 Hz, JSi−H
=5.1 Hz, JSi−H =0.8 Hz, JH −H5=7.7 Hz, JSi−H
7
2
4
3
3
7
8
8
=14.90 Hz.
3.5.5. [(Z,E)-2-(Dimesitylboryl)-2-phenylethenyl]-
(methyl)diphenylsilane (7c and 7d)
3.5.3. [(Z)-2-(Dimesitylboryl)-1-phenylethenyl]-
(methyl)diphenylsilane (7a)
2
29Si-NMR (ppm): −10.8, −11.5. JSi−H =6.6 Hz,
5
4
2
3JSi−H =5.1 Hz, JSi−H =0.8 Hz, JSi−H =11.7 Hz,
2
3
29Si-NMR (ppm): −14.7. JSi−H =6.6 Hz, JSi−H
7
8
1
3JH −H =7.7 Hz.
=5.1 Hz, JSi−H =0.8 Hz, JH −H5=7.7 Hz, JSi−H
7
4
3
3
7
8
7
8
8
2
=16.20 Hz. 11B-NMR (ppm): 80. 1H-NMR (ppm): 0.58
(s, 3H, 5), 2.15 (s, 12H, 14), 2.19 (s, 6H, 15), 6.83 (s,
4H, 12), 7.17 (m, 5H, 17, 18, 19), 7.2–7.4 (m, 10H, 7, 8,
9), 7.64 (s, 1H, 2). 13C-NMR (ppm): −0.7 (5), 21.5
(15), 24.1 (14), 126.2 (17), 128.1 (8), 128.4 (18), 128.9 (9,
19), 129.0 (12), 135.4 (7), 137.0 (13), 139.5 (6), 141.1
(11), 142.7 (16), 149.8 (10), 154 (1), 161.1 (2). MS; z/e
(low resolution LSIMS): 249.2 (100%, Mes2B+); 301.1
(17%, MesPh2Si+); 351.2 (21%, [M−Mes−Ph−H]+);
429.2 (31%, [M−Mes]+); 471.3 (23%, [M−Ph]+);
533.3 (4%, [M−Me]+); 548.3 (3%, M+). High resolu-
tion LSIMS; C39H41BSi [M]+: 548.306930 (Found);
548.307060 (Calc.).
3.5.6. [(Z)-2-(Dimesitylboryl)-1-pentylethenyl]-
(methyl)diphenylsilane (8a)
3.5.4. [(E)-2-(Dimesitylboryl)-1-phenylethenyl](methyl)-
diphenylsilane (7b)
2
3
29Si-NMR (ppm): −13.2. JSi−H =6.6 Hz, JSi−H
=5.1 Hz, JSi−H =0.8 Hz, JSi−H 5=16.3 Hz, JH −
7
4
3
3
7
8
2
H8=7.7 Hz. 11B-NMR (ppm): 80.0. H-NMR (ppm):
0.42 (s, 3H, 5), 0.67 (m, 3H, 20), 0.84 (m, 4H, 18, 19),
1.08 (m, 2H, 17), 2.17 (s, 6H, 15), 2.24 (s, 12H, 14), 2.41
(m, 2H, 16), 6.71 (s, 4H, 12), 7.19 (m, 6H, 8, 9), 7.51
(m, 4H, 7), 7.60 (s, 1H, 2). 13C-NMR (ppm): −1.5 (5),
14.3 (20), 21.3 (15), 22.7 (19), 24.0 (14), 29.8 (17), 32.4
(18), 42.4 (16), 128.1 (8), 129.2 (12), 129.5 (8, 14), 135.4
(7), 137.6 (13), 139.1 (6), 140.9 (11), 143.6 (10), 156.3
(1), 156.9 (2). MS; z/e (low resolution LSIMS): 249.2
(100%, Mes2B+); 301.1 (7%, MesPh2Si+); 345.3 (15%,
[M−Mes−Ph−H]+); 423.2 (46%, [M−Mes]+);
1

J. Carlos Araujo Da Sil6a et al. / Journal of Organometallic Chemistry 646 (2002) 179–190
189
465,3 (32%, [M−Ph]+); 527.3 (6%, [M−Me]+); 541.2
(6%, [M−H]+). High resolution LSIMS; C29H36-
BSi ([M−Mes]+): 423.266218 (Found); 423.267935
(Calc.)
2.09 (s, 12H, 14), 2.14 (s, 6H, 15), 2.47 (m, 2H, 16), 6.63
(s, 4H, 12), 7.04–7.17 (m, 6H, 8, 9), 7.51 (m, 4H, 7),
7.62 (s, 1H, 2). 13C-NMR (ppm): 14.4 (20), 19.9 (5),
21.2 (15), 23.0 (19), 24.3 (14), 29.0 (17), 30.3 (21), 32.5
(18), 40.7 (16), 127.7 (8), 128.4 (12), 129.3 (9), 135.2
(13), 137.2 (7), 138.7 (6), 140.8 (11), 143.0 (10), 145.5
(1), 158.1 (2). MS; z/e (low resolution LSIMS): 239.0
(14%, tert-BuPh2Si+); 249.2 (100%, Mes2B+); 301.1
(13%, MesPh2Si+); 465.3 (37%, [M−Mes]+); 507.3
(19%, [M−Ph]+); 527.3 (80%, [M−tert-Bu]+); 569.3
(1%, [M−Me]+); 583.3 (4%, [M−H]+); 607.3 (2%,
[M+Na]+); 623.5 (1%, [M+K]+). High resolution
LSIMS; C37H44BSi ([M−tert-Bu]+): 527.328803
(Found); 527.330535 (Calc.).
3.5.7. [(Z)-2-(Dimesitylboryl)-1-phenylethenyl]-
(tert-butyl)diphenylsilane (9a)
4. Conclusions
Novel stable organosilylboranes bearing dimesitylbo-
ryl groups were synthesised. In spite of their steric
hindrance, they give addition products with terminal
acetylenic hydrocarbons in the presence of
Pd2(dba)3(etpo)2 complex as the catalyst. The reaction
took place smoothly regio- and stereoselectively, giving
one major isomer in good yields. Thus, the dimesitylbo-
ryl group added to the terminal carbon and the
organosilyl group to the internal, according to a syn-
addition. The structures of the products were unam-
biguously demonstrated, thanks to 13C- and 29Si-NMR
spectroscopy using various INEPT techniques. These
results are in good agreement with those previously
reported for the silaboration using silylpinacolboranes
and silyldiaminoboranes.
2
3
29Si-NMR (ppm): −7.1. JSi−H =6.6 Hz, JSi−H
=
=
5
7
2
4
3
3
5.1 Hz, JSi−H =0.8 Hz, JH −H =7.7 Hz, JSi−H
7
8
8
16.2 Hz. 11B-NMR (ppm): 88.0. H-NMR (ppm): 0.97
(s, 9H, 20), 2.1 (s, 12H, 14), 2.37 (s, 6H, 15), 6.78 (s, 4
H, 12), 7.1–7.4 (m, 15H, 7, 8, 9, 17, 18, 19), 7.64 (s, 1H,
2). 13C-NMR (ppm): 20.0 (5), 21.2 (15), 24.3 (14), 29.7
(20), 126.3 (17), 127.3 (8), 127.9 (9, 19), 128.4 (18),
128.5 (12), 135.0 (13), 137.3 (7), 138.7 (6), 140.7 (11),
143.0 (16), 145.0 (10), 150.6 (1), 165.0 (2). MS; z/e (low
resolution LSIMS): 239.0 (17%, tert-BuPh2Si+); 249.2
(100%, Mes2B+); 301.1 (57%, MesPh2Si+); 325.1 (9%,
[MesPh2SiH]Na+); 471.2 (32%, [M−Mes]+); 513.2
(16%, [M−Ph]+); 533.3 (81%, [M−tert-Bu]+); 575.3
(1%, [M−Me]+); 589.3 (5%, [M−H]+); 613.2 (2%,
[M+Na]+). High resolution LSIMS; C38H38BSi ([M−
tert-Bu]+): 533.284410 (Found); 533.283585 (Calc.).
1
Acknowledgements
The authors thank the Conseil Regional d’Aquitaine
(CRA) for financial support.
3.5.8. [(Z)-2-(Dimesitylboryl)-1-pentylethenyl]-
(tert-butyl)diphenylsilane (10a)
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