Reaction of 1-alkynylsilanes with triallylborane - Competition between 1,1- and 1,2-allylboration

Article abstract of DOI:10.1016/S0022-328X(98)01152-8

The reaction of triallylborane (All₃B, 1) with various 1-alkynylsilanes of the type Me₃Si-C=CR¹ [R¹ = H (2a), Me (2b), Ph (2c), C≡C-SiMe₃ (2d), SiMe₃ (2e)], Ph₃Si-C≡CPh (3) MeC≡C

Full text of DOI:10.1016/S0022-328X(98)01152-8

Journal of Organometallic Chemistry 580 (1999) 234–238
Reaction of 1-alkynylsilanes with triallylborane—competition between
1
,1- and 1,2-allylboration
a,
b
b
Bernd Wrackmeyer *, Oleg L. Tok , Yuri N. Bubnov
a
Laboratorium f u¨ r Anorganische Chemie der Uni6ersit a¨ t Bayreuth, D-95440 Bayreuth, Germany
b
A.N. Nesmeyano6 Institute of Organoelement Compounds of Russian Academy of Sciences, Va6ilo6a str. 28, 117813 Moscow, Russia
Received 10 November 1998
Abstract
The reaction of triallylborane (All B, 1) with various 1-alkynylsilanes of the type Me Si–CꢀCR [R =H (2a), Me (2b), Ph (2c),
1
1
3
3
CꢀC–SiMe3 (2d), SiMe3 (2e)], Ph Si–CꢀCPh (3) MeCꢀC–SiMe SiMe –CꢀCMe (4) and Me Si(Cl)–CꢀCPh (5) was studied.
3
2
2
2
Triallylborane 1 turned out to be much more reactive than other triorganoboranes R B (e.g. R=Et, Ph). In the cases of 2 and
3
5, the products are organometallic-substituted alkenes 6 and 11, respectively, with the boryl and silyl group in cis-positions as the
result of selective 1,1-allylboration (via cleavage of the Si–Cꢀ bond) or mixtures of such and other alkenes 7 or 8 because of
competition between 1,1- and 1,2-allylboration (the composition of these mixtures depends on the polarity of the solvent). In the
case of 4, the 1,2-dihydro-1,2-disilaborepine derivative 12 is formed selectively (twofold 1,1-allylboration). The alkyne 3 did not
1
11
13
29
react with 1. The products were characterised by H-, B-, C- and Si-NMR spectroscopy. © 1999 Elsevier Science S.A. All
rights reserved.
Keywords: Silicon; Boron; Triallylborane; Alkenes; Alkynes; 1,1-Allylboration; 1,2-Allylboration; NMR
intermediate by partial cleavage of the Si–Cꢀ bond (B)
1
. Introduction
and finally to the alkenes of type C (Scheme 1). In the
case of certain 1-alkynyltin or -lead compounds, it
proved possible to isolate such zwitterionic intermedi-
ates corresponding to B and characterise them by X-ray
structural analysis [8].
1-Alkynylsilanes are readily available by standard
preparative procedures [1], and much of their synthetic
potential can be exploited by taking advantage of the
reactivity of the CꢀC and/or the Si–Cꢀ bond. Recently
Owing to permanent allylic rearrangement, triallylbo-
it was shown that triorganoboranes R B react with
3
rane (All B, 1) possesses unique properties among tri-
3
1-alkynylsilanes by cleavage of the Si–Cꢀ bond, accom-
organoboranes and its great synthetic potential in
reactions with unsaturated substrates has been well
documented [9]. In the case of alkynes a weak interac-
tion like in A is conceivable, followed in general by
panied by 1,1-organoboration [2]. These reactions re-
quire heating up to \100°C for several hours or days
and lead selectively to organometallic-substituted alke-
nes [3], to siloles [4,5] and other heterocycles [6,7]. The
principal mechanism of such 1,1-organoboration reac-
tions is fairly well understood [2]: the reaction is sup-
posed to start with a weak interaction as indicated in A,
from which a stronger interaction leads to a borate-like
*
Corresponding author: +Tel.: +49-921-52542; fax: +49-921-
5
0
52157.
Scheme 1.
022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01152-8

B. Wrackmeyer et al. / Journal of Organometallic Chemistry 580 (1999) 234–238
235
Scheme 2.
1,2-allylboration for which a six-membered cyclic tran-
sition state D has been proposed [9,10]. However, the
reactivity of 1 towards 1-alkynylsilanes has never been
studied in detail. An early report has claimed that 1
reacts with ethynyltrimethylsilane 2a exclusively by 1,2-
allylboration [11]. In a first more systematic attempt,
we have now used NMR spectroscopy to investigate the
products obtained from the reaction of 1 with various
Scheme 4.
1
observed only for R =H. The heterocycle 9 rearranges
slowly to the bicyclic compound 10. This sequence of
reactions has been described for numerous other termi-
nal alkynes when treated with triallylborane [9]. If the
reaction is carried out in an non-polar solvent (pen-
tane), the amount of 6a is reduced with respect to 7a
and 9.
1-alkynylsilanes 2–5 (Scheme 2).
Scheme 4 summarises the results of the reaction of 1
with the alkynes 2b, c and 5. Interestingly, 6b is formed
selectively by 1,1-allylboration. In the case of 2c, the
reaction with 1 in CHCl affords a 2:1 mixture of 6c
3
2
. Results and discussion
and 7c, whereas a 1:1 mixture is obtained if pentane
serves as solvent. The reaction of 1 with 5 also proceeds
slowly at room temperature via 1,1-allylboration to give
2
.1. Reactions of 1 with the 1-alkynylsilanes 2, 3 and 5
1
1. Previously it has been found that triethylborane
does not react with 1-alkynylsilanes of the type 5, even
after heating for several days at 100°C [7].
In contrast to triethylborane, triallylborane reacts
with most 1-alkynylsilanes already at room tempera-
ture. However, the reaction is less selective because of
competition between 1,1- and 1,2-allylboration. This is
shown in Scheme 3 for the reaction of 1 with 2a. Three
products 6a, 7a and 9 are formed in a ratio of 2:1:1,
where 7a and 9 arise from 1,2-allylboration. The com-
pound 9 results from fast rearrangement of 8a (not
detected in solution), and this type of compound was
Triallylborane 1 reacts with 2d via 1,1-allylboration
to give 6d after heating to 100°C in toluene. However,
the conversion amounts only to ca. 10%. For compari-
son, triethylborane does not react at all with 2d, and it
has been noted that the corresponding alkene, prepared
via a different route, decomposes into Et B and 2d
3
upon heating to \140°C [12].
By increasing the bulkiness of groups attached to the
Si atom, as in 3, the reactivity decreases. Thus, we did
not observe any reaction between 1 and 3, even after
prolonged heating of the mixture at 100°C.
2.2. Reaction of 1 with
1,1,2,2-tetramethyl-1,2-di-1-propynyl-disilane 4
The alkyne 4 is known to react with various tri-
organoboranes (e.g. Et B, Ph B) by heating to 100°C to
3
3
give selectively 1,2-dihydro-1,2-disilaborepine deriva-
tives [6], and the molecular structure of the compound
derived from Ph B has been determined [6b]. In an
3
analogous manner, however already at room tempera-
ture, the reaction of triallylborane 1 affords quantita-
tively and selectively the seven-membered heterocycle
Scheme 3.

236
B. Wrackmeyer et al. / Journal of Organometallic Chemistry 580 (1999) 234–238
12 as a result of consecutive inter- and intramolecular
3. Conclusions
1,1-allylborations.
Triallylborane is much more reactive towards 1-
alkynylsilanes than triethylborane. Cleavage of the Si–
Cꢀ bond, the essential step in 1,1-allylboration, takes
place already at room temperature. In some cases, the
well-known 1,2-allylboration competes with 1,1-allylbo-
ration. However, in most cases 1,1-allylboration is
dominant, in particular in more polar solvents. Owing
to the reactive Si–Cl bond, 11 is an attractive starting
material for further transformations. The borepine
derivative 12 is an example of a new polyene with five
non-conjugated double bonds in one molecule obtained
by a one-pot reaction of a simple alkyne with
triallylborane.
2
.3. Mechanistic implications
The 1,2-allylboration to 8a, with a transition state D%,
appears to be a special case, and is probably kinetically
controlled owing to R =H. For all other products, the
1
question of differences between the intermediate of type
B and the transition state D arises. B is more polar and
should be preferred in a polar solvent, which is sup-
ported by the experimental evidence that 1,1-allylbora-
tion products are favoured if the reactions are carried
out in chloroform instead of pentane.
Table 1
11
B-, 13C- and 29Si-NMR dataa of the alkenes 6, 7, 11, 12
Compound l13C (B–Cꢁ) l13C (Si–Cꢁ) l13C(SiMe)
l29Si
Why is triallylborane more reactive than other tri-
organoboranes? In comparison with trialkylboranes,
triallylborane is a stronger Lewis acid as shown by the
stability of complexes with pyridine derivatives [13].
Therefore, it is conceivable that the interaction in A is
stronger for triallylborane than for trialkylboranes, and
further reactions take place more readily. On the other
hand, triphenylborane, which must be regarded as a
stronger Lewis acid than triallylborane, is less reactive
towards 1-alkynylsilanes than triallylborane (compare
reaction conditions in Section 2.2 and those reported in
Refs [3,6,7]). Hence, the nature of the allyl groups plays
an important role once the interaction between boron
and the CꢀC bond in A is sufficiently strong.
b
6
6
6c
1
a
b
165.0 (9.1)
154.7
157.2
151.6
158.9
129.2 (67.9)
134.0 (68.5)
143.3 (64.9)
143.9 (n.m.)
−0.4 (51.5)
−0.9 (50.5)
−0.6 (51.9)
4.6 (58.6)
−9.3
−5.2
−6.8
14.1
c
d
e
f
1
2
1
135.7 (n.m.) −2.5 (43.5)
−23.9
l13
B(Si)–Cꢁ)
C
l13
C
l13
C
l29Si
(
(All–Cꢁ)
136.5 (8.8)
143.9 (n.m.)
(SiMe)
−0.2 (51.8)
0.9 (51.9)
g
7
7c
a
155.1
151.2
−7.9
−14.8
h
2.4. NMR-spectroscopic results
a
In CDCl3 at 20°C; coupling constants J(29Si,13C)90.3 Hz are
The structural assignments are based on consistent
1
13
29
11
given in parentheses; n.m. means not measured; br. denotes a broad
sets of H-, C- and Si-NMR data (Table 1).
B
1
1
signal of acarbon atom linked to boron; l B 82=1, except for 12
chemical shifts are all found in the typical range for
these types of triorganoboranes [14]. The characteristi-
11
with l B 72.0.
b
Other l13C data: 34.0 (br., CH B); 43.1 (CH ); 114.2 (ꢁCH );
2
2
2
1
3
cally broadened C-NMR signals for boron-bonded
carbon atoms (scalar relaxation of the second kind [15])
help to assign the carbon framework. The latter is
116.4 (ꢁCH ); 135.5 (–CHꢁ); 136.6 (–CHꢁ).
2
c 13
Other l C data: 16.1 (CH
); 34.0 (CH
); 36.1 (br., CH
B); 113.8
3
2
2
(
ꢁCH ); 115.6 (ꢁCH ); 135.2 (–CHꢁ); 136.6 (–CHꢁ).
2 2
d 13
29
Other l C data: 36.7 (br., CH
B); 37.0 (CH ); 114.4 (ꢁCH );
2 2 2
further established by observation of Si satellites cor-
1
15.8 (ꢁCH ); 127.6 (Ph); 128.2 (Ph); 128.8 (Ph); 134.9 (–CHꢁ); 136.3
2
9
13
2
responding to J( Si, C). These parameters can also be
(
–CHꢁ); 145.0 (Ph).
e 13
2
9
measured in the Si-NMR spectra (see Fig. 1), serving
Other l C data: 36.5 (br., CH B); 47.4; 113.9 (ꢁCH ); 117.4
2
2
1
3
29
for mutual assignments of C- and Si-NMR signals.
(ꢁCH ); 127.4 (Ph); 127.9 (Ph); 128.4 (Ph); 134.7 (–CHꢁ); 136.6
2
2
9
There is a distinctive broadening in the Si-NMR
signals if B nuclei are in cis, trans or geminal posi-
(–CHꢁ).
1
1
f
13
Other l C data: 16.3 (CH ); 35.0 (CH ); 37.5 (br., CH B); 113.7
3
2
2
(
ꢁCH ); 115.4 (ꢁCH ); 136.0 (–CHꢁ); 136.4 (–CHꢁ).
2 2
g 13
tions, as has been noted previously [16] and has been
Other l C data: 36.1 (br., CH B); 41.0 (CH ); 113.6 (ꢁCH );
29
11
2
2
2
ascribed to partially relaxed scalar Si– B coupling.
The proposed stereochemistry of the products is also
1
16.9 (ꢁCH ); 136.3 (–CHꢁ); 137.4 (–CHꢁ).
2
h 13
Other l C data: 36.5 (br., CH B); 48.5 (CH ); 113.8 (ꢁCH );
2
2
2
1
1
supported by appropriate experiments for H/ H-NOE
1
16.9 (ꢁCH ); 126.6 (Ph); 132.0 (Ph); 135.6 (–CHꢁ); 136.3 (Ph); 136.7
2
difference spectroscopy [17].
(–CHꢁ); 148.5 (Ph).

B. Wrackmeyer et al. / Journal of Organometallic Chemistry 580 (1999) 234–238
237
Fig. 1. 99.4 MHz 29Si-NMR signals of the mixture containing the products 6a, 7a, 9 and 10 of the reaction of 1 with 2a (impurities are marked
2
9
29
11
by asterisks). Although all Si-NMR signals (except in the case of 10) are markedly broadened by partially relaxed scalar Si– B coupling, it
proved possible to observe 13C satellites, which are marked by arrows (isotope-induced chemical shifts D
Their relative intensities serve for the assignment to J( Si, CMe) and J( Si, C=), and in the cases of 6a and 7a, the C satellites due to
J( Si, C) (n=2,3) are also resolved.
1
12/13 29
C( Si) will be discussed elsewhere).
1
29 13
1
29 13
13
n
29 13
1
1
4. Experimental
6b: H-NMR: l H=5.94, 5.02, 4.89, 2.21 10H,
All B; 5.70, 5.03, 4.91, 2.81 5H, All; 1.75 3H, Me; 0.01
2
The synthesis of all compounds was carried out in an
9H, Me Si.
3
1
1
atmosphere of dry argon, and carefully dried solvents
were used throughout. Starting materials were either
used as commercial products without further purifica-
tion (Chlorosilanes, butyl lithium 1.6 M in hexane) or
prepared as described (alkynylsilanes 2, 3, 5 [1], 4
6c: H-NMR: l H=7.52, 7.36, 7.02 5H, Ph; 6.11,
5.02, 2.39 10H, All B; 5.57, 5.00, 2.74 5H, All; 0.04 9H,
2
Me Si.
3
1
1
7a: H-NMR: l H=5.8–6.0, 4.8–4.9, 2.18 10H,
All B; 5.7–5.8, 4.9–5.1, 2.67 5H, All; 5.84 (t, J=5.9
2
[
6a,b], All B (1) [18]). NMR measurements: Bruker
Hz) 1H, ꢁC–H; 0.09 9H, Me Si.
3
3
1
11
13
29
1
1
ARX 250 or DRX 500 [ H-, B-, C-, Si-NMR
(
Chemical shifts are given with respect to Me Si [l H
(
for J( Si)=19.867184 MHz], BF –OEt [l B=0;
J( B)=32.083971 MHz]. Assignments are based on
7c: H-NMR: l H=7.35–7.15 5H, Ph; 6.11, 5.05–
2
29
1
refocused INEPT [19] based on J( Si, H)=7 Hz).
4.85, 2.39 10H, All B; 5.72, 5.05–4.85, 2.94 5H, All;
2
1
−0.14 9H, Me Si.
4
3
1
3
29
1
1
CHCl /CDCl )=7.24; l C (CDCl )=77.0; l Si=0
10: H-NMR: l H=6.00 1H, H-7; 5.91, 5.1–4.9,
2.05 5H, All; 2.46 1H, H-6; 2.40 1H, H-1; 2.32 1H, H-8;
1.87 1H, H-2; 1.79 1H, H-9; 1.71 1H, H-3; 1.61 1H,
H-4; 1.46 1H, H-10; 1.17 1H, H-11; 1.06 1H, H-5; 0.13
3
2
3
3
9
11
3
2
1
1
1
1
1
13
1
29
2
D
H/ H-COSY, H/ C- and H/ Si-HETCOR
experiments.
.1. Reaction of the 1-alkynylsilanes 2–5 with
9H, Me Si.
3
1
1
1
1: H-NMR: l H=7.4–7.1 5H, Ph, 6.12, 5.05, 2.38
4
1
5
0H, All B; 5.65, 4.95, 2.90 5H, All; 0.13 6H, Me Si.
2
2
triallylborane 1: general procedure
1
1
1
2: H-NMR: l H=5.88, 4.89, 2.29 5H, AllB; 5.73,
.02, 2.98 10H, All; 1.78 6H, Me; 0.14 12H, Me Si.
2
To a solution of 2–5 (about 1 mmol) in 2 ml of
CDCl or pentane the equimolar amount of All B was
3
3
added in one portion at room temperature. The pro-
1
29
Acknowledgements
gress of the reactions was monitored by H- and Si-
NMR spectroscopy. Since most of these products
undergo further rearrangements [9] upon heating, sepa-
ration or purification by fractional distillation is not
successful. However, several products such as 6b, 11
and 12 are formed selectively in high purity and can be
used for further transformations. All compounds are
left as colourless, extremely air- and moisture-sensitive
oils.
The authors gratefully acknowledge support of this
work by the Volkswagen-Stiftung.
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