1,1- and 1,2-allylboration of alkyn-1-ylsilanes bearing Si-H functions. Electron-deficient Si-H-B bridges, and intramolecular hydrosilylation

Article abstract of DOI:10.1515/znb-2006-0303

The reactions of n-hexyn-1-ylsilanes or arylethyn-1-ylsilanes, bearing methyl groups and one (2), two (3) or three hydride functions (4) at the silicon atom, with triallylborane 1 lead primarily to products of 1,1- or 1,2-allylboration. In the alkenes (5,

Full text of DOI:10.1515/znb-2006-0303

1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes Bearing Si-H Functions.
Electron-Deficient Si-H-B Bridges, and Intramolecular Hydrosilylation
Bernd Wrackmeyer and Oleg L. Tok
Anorganische Chemie II, Universita¨t Bayreuth, D-95440 Bayreuth, Germany
Reprint requests to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 61b, 243 – 251 (2006); received December 22, 2005
The reactions of n-hexyn-1-ylsilanes or arylethyn-1-ylsilanes, bearing methyl groups and one (2),
two (3) or three hydride functions (4) at the silicon atom, with triallylborane 1 lead primarily to prod-
ucts of 1,1- or 1,2-allylboration. In the alkenes (5, 9, 13) formed by stereoselective 1,1-allylboration,
with the silyl and the diallylboryl groups in cis-positions at the C=C bond an electron-deficient Si-H-B
bridge is present. The activation of the Si-H bond in these alkenes induces intramolecular hydrosi-
lylation under very mild reaction conditions to give 1,4-silabora-cyclohept-2-enes (7 and 11). The
products of 1,2-allylboration (6, 10, 14) are further transformed into 1-boracyclohex-2-enes (8, 12,
15) and 7-borabicyclo[3.3.1]non-2-enes (16, 17) by intramolecular 1,2-allylboration reactions. The
proposed structures are based on consistent sets of 1H, 11B, 13C and 29Si NMR data.
Key words: Silanes, Alkynes, Triallylborane, Organoboration, Heterocycles, NMR
Introduction
Triallylborane, B(CH-CH=CH2)3 1, well known for
its permanent allylic rearrangement [1], possesses un-
usual reactivity, unparalleled by other triorganobo-
ranes [2]. Among numerous useful applications, 1,2-
allylboration of various alkynes has opened the way
to many new organoboranes [2]. 1,2-Allylboration of
alkynes can be explained by proposing the transi-
tion state A, in which the boron and one terminal
olefinic carbon atom are close to the alkynyl carbon
atoms. If the alkyne bears one or two organometal-
lic substituents, e.g. silyl groups, 1,1-allylboration (in-
termediate B) may compete succesfully with 1,2-
allylboration, as has been observed for alkyn-1-
ylsilanes, -germanes and -stannanes [3 – 5]. Such
organometallically substituted alkynes undergo prefer-
ably or even exclusively 1,1-allylboration, as has been
found in the case of bis(silyl)ethynes [6]. Interme-
diates of type B, short-lived in the case of alkyn-1-
ylsilanes, have been firmly established by NMR spec-
troscopy and X-ray structural analysis in the course of
1,1-organoborationreactions of alkyn-1-yltin and -lead
compounds [7, 8].
Scheme 1. Alkyn-1-ylsilanes employed in the reaction with
triallylborane.
of NMR data [5, 6, 9]. This can be understood as a
boron-induced activation of the Si-H bond causing in-
tramolecular hydrosilylation to take place under mild
conditions without a catalyst. The present work aimed
to find out about the influence of an alkyl or aryl sub-
stituent at the C≡C bond, in addition to the silyl group
(Scheme 1). This should help to compare 1,1- with
1,2-allylboration, and prove the general applicability
of combining 1,1-allylboration and intramolecular hy-
We have reported that 1,1-allylboration also works drosilylation. Furthermore, the influence of the pres-
with some alkyn-1-ylsilanes bearing the Si-H func- ence of two (3) or three Si-H functions (4) has been
tion [5, 6], leading to alkenes containing an electron- studied (Scheme 1). This work was directed more to-
deficient Si-H-B bridge, as shown by a consistent set wards mechanistic aspects and exploring potential re-
c
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244
B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
Table 1. Selected 1H, 13C and 29Si NMR dataa of the alkyn-
1-ylsilanes 2, 3 and 4.
Table 2. Product distribution after the reaction of the alkyn-
1-ylsilanes 2a – c, 3a, and 4a,b with triallylborane 1.
Starting
1,1-Allylboration 1,2-Allylboration
δ
29Si
δ1H
δ
13C(≡C-Si) δ13C(≡C-R1)
[1J(29Si,1H)][1J(29Si,13C)][2J(29Si,13C)]
Alkyn-1-ylsilane
2cb
−37.5
−61.8
−87.7
−85.9
4.37
[201.1]
3.97
[206.8]
3.81
[214.0]
4.09
[223.3]
89.3
[87.0]
77.0
[92.0]
72.1
[97.9]
81.5
[94.9]
b
106.5
[16.9]
110.6
[17.7]
112.8
[19.2]
108.9
[18.1]
R1 = p-MeO-C6H4
3ac
R1 = n-Bu
4ad
2a
95%
25%
5%
75%
(R1 = n-Bu, R = R’ = Me)
2b
R1 = n-Bu
4be
(R1 = Ph, R = R’ = Me)
2c
< 3%
> 97%
R1 = Ph
a
(R1 = p-C6H4OMe,
In C6D6 at RT; coupling constants in Hz; other 13C NMR data:
R = R’ = Me)
δ = −2.9 [56.0] (SiMe2); 55.0 (MeO); 113.5, 133.4, 134.8, 159.9
3a
(R1 = n-Bu, R = Me, R’ = H)
4a
50%
5%
50%
95%
c
(Ph);
other 13C NMR data: δ = −6.9 [56.2] (SiMe); 13.5, 19.6,
d
21.9, 30.4 (n-Bu);
other 13C NMR data: δ = 13.9, 20.2, 22.5,
30.9 (n-Bu); e other 13C NMR data: δ = 122.2 [11.5], 128.3, 129.2,
(R1 = n-Bu, R = R’ =H)
4b
132.0 (Ph).
< 3%
> 97%
(R1 = Ph, R = R’ = H)
arrangements rather than to optimisation of conditions
in order to obtain single products in high yield.
Results and Discussion
Synthesis of the alkyn-1-ylsilanes 2 – 4
The alkyn-1-ylsilanes 2a,b,c were obtained from the
reaction of chloro(diorgano)silanes with the respec-
tive lithium alkynide as reported [10]. The reaction of
hexyn-1-yl-chloro(methyl)silane [11] with LiAlH4 af-
forded the dihydride 3a, and the analogous reaction
of the alkyn1-yl-trichlorosilanes [12] gave the trihy-
drides 4a,b (Scheme 2).
Scheme 3. Allylboration of alkyn-1-yl(hydrido)silanes.
case of 2c, however, the 1,2-allylboration product is
formed selectively. Relevant NMR data of 5 and 6 are
given in the Tables 3 and 4, respectively.
Apparently there is competition between 1,1-
and 1,2-allylboration. The selective 1,2-allylboration
which takes place in the case of 2c sheds some light on
the mechanism. Considering the extreme structures A
(1,2-allylboration) and B (1,1-allylboration), a zwitte-
rionic structure C, related to both A and B, is also
conceivable. If R1 can help to delocalise the positive
charge in C, cleavage of the Si-C≡ bond (required for
1,1-allylboration) might be suppressed to some extent
or even may not take place at all, and this would pre-
vent 1,1-allyboration. For R1 = Ph, the positive charge
in C can be delocalised and therefore, 1,2-allylboration
is more likely to be observed for R1 = Ph than for
R1 = n-Bu. The delocalisation of positive charge in C
Scheme 2. Reduction of alkyn-1-yl(chloro)silanes with
LiAlH4.
The alkyn-1-ylsilanes 2 – 4 are colourless liquids
which could be used either without further purification
or after distillation. They have been characterised by
their NMR data (Table 1 and Experimental Section).
Reactions of the alkyn-1-yl-hydrido(dimethyl)silanes
2a,b,c with triallylborane 1
The alkyn-1-ylsilanes 2a,b,c react readily with tri- should be even more favourable for R1 = C6H4-p-
allylborane 1 (Scheme 3). In the cases of 2a and 2b, OMe. This is confirmed by the observation of selective
mixtures (see Table 2 for the product distribution) 1,2-allylboration to give 6c starting from 2c. The clean
of the products of 1,1-allylboration (5a,b) and 1,2- formation of 6c is evident from the 13C NMR spectrum
allylboration (6a,b) are formed in the beginning. In the (Fig. 1).
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245
Table 3. Selected NMR dataa for the alkenes 5a,b, 9a and 13a.
2
13
13
δ
29Si { ∆}10/11B(29Si)
δ
11B
δ1H [1J(29Si,1H)]
δ
C
[1J(13C,29Si)]
δ C
(=C−Si)
(=C−B)
5ab
5bc
9ad
13ae
R1 = n-Bu
R1 = Ph
R1 = n-Bu
R1 = n-Bu
−2.3 {−73.0}
−9.5 {−49.4}
−33.7 {−29.1}
−63.2 {9.9}
67.6
57.3
78.4
79.2
3.26 [160.5]
3.71 [168.8]
3.99 [184.0]
3.76 [n. o.]
135.3 [64.4]
160.2 br
161.4 br
161.3 br
–
138.4 [66.7]
133.0 [68.1]
–
In C6D6 at RT; coupling constants in Hz; isotope-induced chemical shifts2∆10/11B(29Si) in ppb.
other 13C NMR: δ = −2.4 [49.4]
a
b
(SiMe2); 14.0, 23.0, 29.3, 32.6 (n-Bu); 34.6 (br), 112.8, 137.7 (All2B); 34.9, 115.5 136.2 (All); c other 13C NMR: δ = −3.6 [51.3] (SiMe2);
d
35.7 (br), 114.2, 136.8 (All2B); 47.6 [7.9], 115.9, 135.1 (All); 126.9, 128.0, 128.1, 141.7 [5.2] (Ph);
other 13C NMR: δ = −2.1 [49.4]
(MeSi); 14.7, 23.7, 31.1, 32.8 (n-Bu); 36.5 (br), 114.2, 137.7 (AllB); 44.5 [7.8], 115.8, 136.8 (All); e 13C resonances were not assigned due
to low concentration.
Table 4. Selected NMR dataa for the alkenes 6a – c, 10a and 14a,b (1,2-allylboration products).
13
13
δ
29Si
δ
11B
δ1H [1J(29Si,1H)]
δ
C
δ C
(All)
(B,Si)
6ab
R1 = n-Bu
R1 = Ph
R1 = p-C6H4OMe
R1 = n-Bu
R1 = n-Bu
R1 = Ph
−32.3
−30.6
−30.8
−53.3
−79.0
−75.4
81.3
81.0
80.9
78.9
81.2
81.2
4.25 [194.7]
3.92 [194.4]
3.92 [193.8]
4.18 [191.4]
3.80 [198.7]
3.79 [201.8]
[b]
[b]
150.1
149.7
152.5 n. o
154.4
154.6
6bc
148.6 br
148.4 br
141.2 br
135.0 br
139.8 br
6cd
10ae
14af
14bg
a
In C6D6 at RT; coupling constants in Hz; b 13C resonances are not assigned due to low concentration; br indicates broadening due to
partially relaxed 13C-11B spin-spin coupling.
other 13C NMR signals: δ = −2.9 [51.5] (SiMe2); 36.0 (br), 113.7, 136.7 (All2B); 47.4
c
[7.6], 118.3, 134.8 (All); 127.7, 128.2, 128.6, 144.8 [5.1] (Ph); d other 13C NMR: δ = −2.9 [51.5] (SiMe2); 35.9 (br), 113.5, 136.7 (All2B);
e
47.5 [7.6], 118.0, 134.8 (All); 55.0 (MeO); 113.0, 129.3, 137.3, 158.4 (C6H4);
other 13C NMR signals: δ = −6.0 [51.1] (SiMe); 14.8,
23.8, 31.9, 33.4 (n-Bu); 36.5 (br), 114.6, 137.7 (All2B); 38.5 [8.4], 119.9, 135.6 (All); other 13C NMR signals: δ = 14.8, 23.5, 31.6, 38.9
f
[8.4] (n-Bu); 36.1 (br), 114.4, 137.5 (All2B); 44.3 [8.4], 120.7, 137.4 (All); other 13C NMR signals: δ = 36.1 (br), 114.7, 137.2 (All2B);
g
46.8 [8.2], 121.0, 136.6 (All); 128.4, 128.9, 129.0, 145.3 [5.6] (Ph).
Table 5. Selected NMR dataa of 1-bora-4-silacyclohept-2-enes 7a,b and 11a.
13
13
13
13
13
δ
29Si
δ
11B
δ
C
δ
C
δ
C
δ
C
δ C
(C−2)
(C−3)
(C−5)
(C−6)
(C−7)
[1J(13C,29Si)]
[1J(13C,29Si)]
[1J(13C,29Si)]
7ab
R = Me, R1 = n-Bu
R = Me, R1 = Ph
R = H, R1 = n-Bu
−4.6
−4.9
81.3
81.3
81.2
148.2 [63.7]
151.1 [61.5]
146.2 [64.4]
156.7 br
158.4 br
158.7 br
31.6 br
31.8 br
30.9 br
19.4
19.1
20.3
18.2 [51.3]
17.8 [51.3]
15.5 [51.1]
7bc
11ad
−17.6
a
b
In C6D6 at RT; coupling constants in Hz; br indicates broadening due to partially relaxed13C-11B spin-spin coupling;
other 13C NMR
signals: δ = 0.8 [50.1] (SiMe2); 14.1, 23.1, 31.7, 32.9 (n-Bu); 35.1 [7.2], 114.9, 136.5 (All); 35.4 (br), 113.5, 137.7 (AllB); c other 13C NMR
d
signals: δ = 0.23 [51.5] (SiMe2); 34.4 (br), 114.2, 137.6 (AllB); 37.6 [6.7], 114.8, 135.8 (All); 127.0, 127.6, 127.9, 144.3 (Ph);
other
13C NMR signals: δ = −2.1 [49.4] (SiMe); 14.3, 23.8, 33.4, 35.9 (n-Bu); 35.8 (br), 114.6, 138.2 (AllB); 41.4 [10.3], 116.4, 136.8 (All).
chemical shift 2∆10/11B(29Si), transmitted through the
Si-H-B bridge, is characteristic [5, 6, 9]. The boron-
induced Si-H activation in the alkenes 5a,b acceler-
ates intramolecular hydrosilylation [15, 16] under very
mild conditions to give the seven-member heterocy-
cles 7a,b. The structures proposed for 7 are supported
by a consistent set of NMR data (see Fig. 2 for the
13C NMR spectrum of 7a and Table 5 for relevant
NMR data).
NMR spectroscopic data (Table 3) provide firm evi-
dence for the presence of the electron-deficient Si-H-B
bridge in the compounds 5a,b. There is an increase in
11B nuclear shielding when compared with similar tri-
organoboranes [13], a decrease in 29Si nuclear shield-
The alkenes 6a,b,c, products of 1,2-allylboration,
ing relative to similar alkenylsilanes [14], and the mag- rearrange into substituted 1-boracyclohex-2-enes 8
1
nitude of | J(29Si,1 H)| is reduced, typical of the Si- (relevant NMR data are given in Table 6) by a sec-
H-B bridge [5, 6, 9]. Furthermore, the isotope-induced ond (intramolecular) 1,2-allylboration. This process is
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246
B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
1
Fig. 1. 125.8 MHz 13C{ H} NMR spectrum of the reaction mixture containing the alkene 6c, formed selectively by 1,2-
allylboration, together with a small amount of unreacted 3c (signals marked by asterisks). Note the typically broad and weak
13C NMR signals for the carbon atoms linked to boron [18] and the satellites owing to J(29Si,13C) (data in Hz given in
brackets).
1
Fig. 2. 125.8 MHz 13C{ H} NMR spectrum of the reaction mixture after conversion of the alkene 5a into the seven-member
heterocycle 7a. Note the typically broad and weak 13C NMR signals for the carbon atoms linked to boron [18] and the
satellites owing to J(29Si,13C) (data in Hz given in brackets).
well documented in the chemistry of allylboranes, for The results of the reaction of 3a with 1 (Scheme 4)
example as one of the key steps on the route to 1- correspond in principle to the findings for the monohy-
boraadamantane [2, 17].
drides. The 13C NMR spectra (see Fig. 3) are readily
analysed, showing the presence of the products of 1,1-
and 1,2-allylboration. 29Si NMR spectra (see Fig. 4)
clearly indicate the presence of the Si-H-B bridge
in 9a, which is absent in the isomer 10a. However,
NMR spectroscopic measurements at variable temper-
Reaction of n-hexyn-1-yl-dihydrido(methyl)silane 3a
with triallylborane 1
Will the Si-H-B bridge become stronger or weaker if
there are two hydrogenatoms linked to silicon as in 3a?
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B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
Table 6. Selected NMR dataa for 1-bora-cyclohex-2-enes 8b, 12a and 15a,b.
247
δ
29Si
−28.6
δ
11B
77.9
δ1H
4.43 [191.7]
δ
13C(2)
139.6 br
132.3 184.3 [8.2] 42.4 [7.6]
125.6
δ
13C(3)
178.4
δ
13C(4)
44.9 [5.3]
δ
13C(5)
δ
13C(6)
8bb R1 = Ph (n = 1)
12ac −50.4
33.9
35.1
38.1
30.4 br
30.9 br
29.9 br
R1 = n-Bu (n = 2) 78.9 4.28 [190.6], 4.30 [194.4]
15ad R1 = n-Bu (n = 3)
−77.3
78.1
3.81 [198.7]
185.5
41.9
br
15be R1 = Ph (n = 3)
a
−72.8
77.1
4.07 [195.3]
130.4 br 182.7 [3.0] 44.9 [5.6]
35.1
30.9 br
In C6D6 at RT; coupling constants in Hz; br indicates broadening due to partially relaxed13C-11B spin-spin coupling;
other 13C NMR
b
signals: δ = −2.1 [51.0], −1.2 [50.5] (SiMe2); 34.9 (br), 115.0, 139.1 (AllB); 43.9, 115.8, 137.2 (All); 127.3, 127.8 128.2. 147.2 [4.5]
c
(Ph);
other 13C NMR signals: δ = −5.0 [50.0] (SiMe); 14.7, 23.8, 31.9, 34.9 (Bu); 35.1 (br), 114.7, 138.0 (AllB); 43.6, 115.8, 137.1
d
(All); other 13C NMR: δ = 14.0, 22.9, 30.7 34.0 (Bu); 35.2 (br), 113.5, 136.8 (AllB); 42.7, 115.7, 136.2 (All); e other 13C NMR signals:
δ = 35.4 (br), 115.2, 137.7 (AllB); 43.5, 116.7, 136.5 (All); 127.4, 128.9, 129.0, 147.0 [5.6] (Ph).
1
Fig. 3. 125.8 MHz 13C{ H} NMR spectrum of the reaction mixture containing the alkenes 9a and 10a formed by 1,1-
and 1,2-allylboration, respectively, of the alkyne 3a. A small amount of an excess of triallylborane is still present. Note the
typically broad and weak 13C NMR signals for the carbon atoms linked to boron [18] and the satellites owing to J(29Si,13C)
(data in Hz given in brackets).
ature did not allow distinguishing between the hydro-
gen atoms in the bridging and terminal positions. All
observed NMR data sets for 9a (shifts to higher fre-
1
quencies, compared with monohydrides, of H and 11
B
resonances and to lower frequencies of 29Si reso-
nances, together with the magnitude of 1J(29Si,1 H))
indicate fast exchange of terminal and bridging posi-
tions of the hydrogen atoms. It appears that the bridge
is weaker when compared with the monohydrides.
Scheme 4. Allylboration of alkyn1-yl(dihydrido)silanes.
The formation of the seven-member ring in 11a via
intramolecular hydrosilylation again proceeds readily
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B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
1
Fig. 4. 99.6 MHz 29Si{ H} NMR spectrum of the reaction mixture containing a small amount of the starting alkyne 3a,
the products of 1,1- and 1,2-allylboration 9a and 10a, respectively, as well as the seven-member heterocycle 11a, formed
by intramolecular hydrosilylation of 9a. The isotope-induced chemical shift 2∆10/11B(29Si), is typical of the Si-H-B bridge
in 9a.
(see Fig. 4), corresponding to the finding for 7. At- Thermal rearrangement of the products formed by 1,2-
tempts to induce a second intramolecular hydrosilyla- allylboration
tion of the remaining B-allyl group were not success-
ful, indicating that the Si-H bond in 7a is not activated.
A thermal rearrangement, well documented in the
chemistry of allylboranes [2, 17, 19], takes place in
the case of the 1-bora-cyclohex-2-enes, as shown for
Reaction of the alkyn-1-yl(trihydido)silanes 4a,b with
triallylborane 1
the products 8b and 15b (Scheme 6), leading to the
bicyclic boranes 16b and 17b, respectively. This re-
arrangement requires prolonged heating at 80 C and
◦
The reaction of the trihydrides 4a,b with 1
(Scheme 5) leads to the isomers 13 and 14, analo-
gous to those obtained from the mono- and dihydrides.
However, the amount of the isomers 13 containing the
Si-H-B bridge is rather low, when compared with the
results for dihydrides and monohydrides(Table 2). The
presence of the Si-H-B bridge in 13a follows from the
NMR data (Table 3), although it appears to be a rather
weak interaction.
is accompanied by decomposition.
Scheme 6. Rearrangement of 1-bora-cyclohexa-2-enes into
7-borabicyclo[3.3.1]non-2-enes.
Conclusions
In the title reaction, there is competition between
1,1- and 1,2-allyboration, and the latter dominates for
R1 = Ph and p-MeO-C6H4, since these substituents
can delocalise the positive charge in a zwitterionic
intermediate. The 1,1-allylboration affords products
with defined stereochemistry (boryl and silyl group
Scheme 5. Allylboration of alkyn-1-yl(trihydrido)silanes.
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B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
249
in cis-positions) for various organic groups R1, and
in all these cases electron-deficient Si-H-B bridges
are present. The boryl-induced Si-H-activation leads
to intramolecular hydrosilylation under exceptionally
mild reaction conditions. The combination of 1,1-
allylboration and hydrosilylation is a useful strategy
for the synthesis of novel heterocyclic compounds.
Phenylethynyl-trichlorosilane: b. p. 66 – 68 ◦C/10−3 Torr
[12b].
◦
n-Hexyn-1-yl-trichlorosilane: b. p. 40 – 42 C/10−3 Torr.
◦
1H NMR (500 MHz; CDCl3; 23 C): δ = 0.95, 1.46, 1.60,
2.38 (t, m, m, m, 3H, 2H, 2H, 2H, n-Bu);. 13C NMR
(125.8 MHz; CDCl3; 23 ◦C; [J(29Si,13C)]); δ = 13.4;
19.5; 21.9; 29.5 (n-Bu); 77.4 [177.1] (≡C-Si); 113.6 [35.3]
(≡C-n-Bu). 29Si NMR (99.6 MHz, CDCl3): δ = −31.9.
Experimental Section
Reduction of the silicon chlorides
General and starting materials
The solution of the respective silicon chloride in diethyl
ether was added slowly to a suspension of an excess of
LiAlH4 in Et2O at 0 C. After 1 h the reaction mixture was
All compounds were prepared and handled under dry
argon, observing all necessary conditions to exclude air
and moisture, and by using carefully dried solvents. Start-
ing materials such as triallylborane 1 [20] and silicon
monohydrides 2 were prepared according to literature
procedures. NMR measurements: Bruker ARX 250 and
DRX 500 NMR spectrometers [1H, 11B, 13C, 29Si NMR
(refocused INEPT [21] based on 2J(29Si,1 HMe) = 7 or
1J(29Si,1 H) = 200 Hz)]. The NMR spectra were mea-
sured for solutions (5 – 10%) in C6D6 at 23 ◦C, if not
noted otherwise. Chemical shifts are given with respect to
Me4Si [δ1H (CHCl3/CDCl3) = 7.24; δ1H (C6D5H) = 7.15;
◦
quenched with aqueous HCl (10%), and extracted with Et2O,
and the organic layer was washed twice with water and dried
with Na2SO4. The purity of the silanes was checked, and the
silanes were purified, if necessary, by fractional distillation.
2c: 1H NMR: δ = 0.40 (d, 6H, Me2Si, 3J(H,H) = 3.8 Hz);
3.85 (s, 3H, MeO); 6.89 (m, 2H, C6H4); 7.49 (m, 2H, C6H4).
3a: b. p. 60 – 61 ◦C/50 Torr. 1H NMR: δ = 0.25 (t, 3H, MeSi,
3J(H,H) = 4.2 Hz); 0.90, 1.20 – 1.40, 2.23 (t, m, m 3H, 4H,
2H, n-Bu). IR: ν(Si-H) = 2155 cm−1, ν(C≡C) = 2180 cm−1
.
1
4a: H NMR: δ = 0.69, 1.00 – 1.20, 1.89 (t, m, m, 3H, 4H,
2H, n-Bu) [22]. IR: ν(Si-H) = 2171 cm−1. 4b: 1H NMR:
δ = 7.3 – 7.6 (m, 5H, Ph).
δ
13C (CDCl3) = 77.0; δ13C (C6D6) = 128.0; δ29Si = 0
for Ξ(29Si) = 19.867184 MHz], BF3-OEt2 [δ11B = 0;
Ξ(11B) = 32.083971 MHz]. Assignments in 1H and
13C NMR spectra are based on appropriate 2D 1H/1H COSY,
1H/1H NOESYTP experiments, and 1H/13C and 1H/29Si het-
eronuclear shift correlations. IR spectra: Perkin Elmer, Spec-
trum 2000 FTIR.
Reaction of the alkyn-1-ylsilanes 2, 3 and 4 with trial-
lylborane 1. General procedure: To a solution of the alkyn-1-
ylsilane (1 – 2 mmol) in CDCl3 or C6D6 (0.5 ml) the equimo-
lar amount of 1 was added in one portion at r. t. Then the mix-
ture was kept for several hours at r. t., and the progress of the
reactions was monitored by 1H and 29Si NMR spectroscopy.
5a: 1H NMR: δ = 0.26 (d, 6H, Me2Si, 3J(H,H) = 3.4 Hz);
1.32, 1.40, 2.23 (t, m, m, 3H, 4H, 2H, n-Bu); 2.12 (d, 4H,
BAll2); 2.93 (dt, 2H, All); 4.9 – 5.1 (m, 6H, BAll2, All); 5.76
(ddt, 1H, All).
5b: 1H NMR: δ = 0.24 (d, 6H, Me2Si, 3J(H,H) = 3.4 Hz);
2.37 (d, 4H, BAll2); 2.91 (dt, 2H, All); 5.00 – 5.20 (m, 6H,
BAll2, All); 5.80 – 6.20 (m, 3H, BAll2, All); 7.30 – 7.60 (m,
5H, Ph).
Preparation of the silicon di- (3a) and trihydrides (4a,b)
via the chlorides hexyn-1-yl-chloro(methyl)silane, hexyn-1-
yl-trichlorosilane and phenylethynyl-trichlorosilane
The solution of the lithium alkynide (20 – 40 mmol) in
THF (20 ml), freshly prepared from the respective alkyne and
n-BuLi, was added slowly to the solution of a 5-fold molar
excess of MeSiHCl2 or SiCl4 in THF (40 ml) at −78 ◦C. The
mixture was allowed to warm up to r. t., all volatile materials
were removed in vacuo at r. t., the residue was taken up in
pentane, and insoluble materials were filtered off. Then pen-
tane was distilled off first, and the silanes were purified by
fractional distillation.
n-Hexyn-1-yl-chloro(methyl)silane: b. p. 61 – 65 ◦C/
16 Torr. 1H NMR (500 MHz; CDCl3: 23 ◦C): δ = 0.34
(d, 3H, MeSi, 3J(H,H) = 3.0 Hz); 0.73, 1.18, 1.23, 1.92
(t, m, m, m, 3H, 2H, 2H, 2H, n-Bu); 4.90 (q, 1H, SiH,
3J(H,H) = 3.0 Hz, 1J(29◦Si,1H) = 247.2 Hz). 13C NMR
(125.8 MHz; CDCl3; 23 C; [J(29Si,13C)]); δ = 1.8 [64.6]
6b: 1H NMR:
δ
= −0.04 (d, 6H, Me2Si,
3J(H.H) = 4.0 Hz); 2.41 (d, 4H, BAll2); 3.09 (dt, 2H,
All); 5.00 – 5.20 (m, 6H, BAll2); 5.85 (ddt, 1H, All); 6.04
(ddt, 2H, BAll2); 7.30 – 7.60 (m, 5H, Ph).
6c: 1H NMR:
δ
= −0.05 (d, 6H, Me2Si,
3J(H.H) = 4.0 Hz)); 2.37 (d, 4H, BAll2); 3.04 (dt, 2H,
All); 3.88 (s, 3H, MeO); 5.0 – 5.1 (m, 6H, BAll2, All); 5.81
(ddt, 1H, All); 6.12 (ddt, 2H, BAll2); 6.93 (m, 2H, C6H4);
7.20 (m, 2H, C6H4).
9a: 1H NMR: δ = 0.29 (t, 3H, MeSi, 3J(H,H) = 4.0 Hz);
(MeSi); 13.6; 19.6; 22.0; 30.3 (n-Bu); 78.9 [108.2] (≡C-Si); 1.0, 1.40 – 1.60, 2.3 (m, m, m, 3H, 4H, 2H, n-Bu); 2.3 (m,
112.4 [22.4] (≡C-n-Bu). 29Si NMR (99.6 MHz, CDCl3; 4H, BAll2); 2.98 (dt, 2H, All); 5.0 – 5.2 (m, 6H, BAll2, All);
23 ◦C): δ = −22.2.
5.8 – 6.1 (m, 3H, BAll2, All).
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250
B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
10a: 1H NMR: δ = 0.30 (t, 3H, MeSi, 3J(H,H) = 4.3 Hz); dd, 1H, 1H, C4H2); 2.37 (m, 2H, All); 4.30 (sp, 1H, SiH,
1.03, 1.40 – 1.60, 2.43 (t, m, m, 3H, 4H, 2H, n-Bu); 2.3 (m, 3J(H,H) = 3.8 Hz, 1J(29Si,1H) = 177.9 Hz); 5.00 – 5.10 (m,
4H, BAll2); 2.74 (d, 2H, All); 5.00 – 5.20 (m, 6H, BAll2, 4H, BAll, All); 5.7 – 6.2 (m, 2H, BAll, All); 7.20 – 7.50 (m,
All); 5.81 (m, 1H, All); 6.06 (m, 2H, BAll).
5H, Ph).
12a: 1H NMR: δ = 0.37 (q, 3H, MeSi, 3J(H,H) = 4.3 Hz);
13a: 1H NMR signals were not assigned owing to low
concentration and overlap with signals from the other isomer. 0.77, 1.75 (dd, m, 1H, 1H, C6H2); 1.00, 1.40 – 1.60, 2.31 (t,
m, m, 3H, 4H, 2H, n-Bu); 1.65 (m, 1H, C5H); 1.94 (m, 2H,
BAll); 1.99, 2.14 (m, m, 1H, 1H, C4H2); 2.42 (m, 2H, All);
4.50 (m, 2H, SiH2); 5.00 – 5.20 (m, 4H, BAll, All); 5.80 –
6.10 (m, 2H, BAll, All).
14a: 1H NMR: δ = 1.05, 1.47, 1.56, 2.32 (t, m, m, m, 3H,
2H, 2H, 2H, n-Bu); 2.26 (d, 4H, BAll2); 2.85 (d, 2H, All2);
5.00 – 5.20 (m, 6H, BAll2, All); 5.85 (ddt, 1H, All); 5.99 (ddt,
2H, BAll2).
1
15a: H NMR: δ = 0.82, 1.77 (dd, ddd, 1H,1H, C6H2);
14b: 1H NMR: δ = 2.38 (d, 4H, BAll2); 3.1 (d, 2H, All);
3.84 (s, 3H, SiH3, 1J(29Si,1H) = 201.0 Hz); 5.00 – 5.20 (m,
6H, BAll2, All); 5.86 (ddt, 1H, All); 6.16 (ddt, 2H, BAll2);
7.00 – 7.20 (m, 5H, Ph).
1.05, 1.48, 1.57, 2.37 (t, m, m, m, 3H, 2H, 2H, 2H, n-Bu);
1.64 (m, 1H, C5H); 1.97, 2.15 (dd, m, 1H,1H, C4H2);
2.15 (m, 2H, BAll); 2.51 (m, 2H, All); 4.03 (s, 3H, SiH3,
1J(29Si,1H) = 193.0 Hz); 4.90 – 5.20 (m, 4H, BAll, All); 5.8 –
6.1 (m, 2H, BAll, All).
Conversion of the alkenes 5 and 9 into 1,4-silabora-
cyclohept-2-enes 7 and 11
1
15b: H NMR: δ = 0.80, 1.82 (ddd, m, 1H, 1H, C6H2);
2.09, 2.70 (m, dd, 1H, 1H. C4H2); 2.13 (m, 1H, C5H);
2.70 (m, 2H, BAll); 2.52 (d, 2H, All); 4.07 (s, 3H, SiH3,
1J(29Si,1H) = 195.3 Hz); 5.10 – 5.20 (m, 4H, BAll, All);
5.87, 6.14 (ddt, ddt, 1H, 1H, BAll, All); 7.20 – 7.40 (m, 5H,
Ph).
The complete conversion of 5 into 7 required gentle heat-
◦
ing of the solutions at 50 – 60 C for 2 h. In contrast, the
intramolecular hydrosilylation of 9 to 11 took place already
at r. t.
7a: 1H NMR: δ = 0.05 (s, 6H, Me2Si); 0.72 (m, 2H,
C7H2); 0.95, 1.20 – 1.40, 2.28 (t,m,m, 3H, 4H, 2H, n-Bu);
1.20 – 1.40 (m, 1H, C6H2, n-Bu); 1.78 (m, 2H, C5H2); 2.20
(dt, 2H, BAll); 3.14 (dt, 2H, All); 4.90 – 5.00 (m, 4H, BAll,
All); 5.75, 5.95 (ddt, ddt, 1H,1H, BAll, All).
Intramolecular 1,2-allylboration of 8b and 15b to the 7-
borabicyclo[3.3.1]non-2-enes 16b and 17b
Heating of the solution of the monocyclic boranes 8b
or 15b in C6D6 at 70 – 80 ◦C for 24 – 48 h leads to conversion
(35 – 80%) into the bicyclic compounds 16b and 17b. Pro-
longed further heating induces the formation of an increas-
ing amount of decomposition products rather than complete
rearrangement.
7b: 1H NMR: δ = 0.03 (s, 6H, Me2Si); 0.90 (m, 2H,
C7H2); 1.39 (m, 2H, C6H2); 1.68 (m, 2H, C5H2); 2.23 (d,
2H, BAll); 3.04 (dt, 2H, All); 5.00 – 5.20 (m, 4H, BAll, All);
5.80 – 6.20 (m, 2H. BAll, All); 7.20 – 7.50 (m, 5H, Ph).
11a: 1H NMR: δ = 0.23 (d, 3H, MeSi, 3J(H,H) = 4.0 Hz);
0.73, 1.10 (m, m, 1H, 1H, C7H2); 1.04, 1.40 – 1.60, 2.43 (t,
m, m, 3H, 4H, 2H, n-Bu); 1.40 – 1.60 (m, 2H, C6H2); 1.92
(m, 2H, C5H2); 2.20 – 2.30 (m, 2H, BAll); 3.18, 3.23 (ddt,
ddt, 1H,1H, All); 5.00 – 5.20 (m, 4H, BAll, All); 5.80 – 6.10
(m, 2H, BAll, All).
16b: 1H NMR: No assignment was made owing to se-
vere overlap with signals of 8b. 13C NMR: δ = −2.5 (MeSi,
51.0); −2.4 (MeSi, 51.3); 19.1 (C-9); 20.6 (br., C-6); 20.7
(br., C-8); 36.3 (br., CH2B); 37.7 (C-4, 6.4); 43.1 (C-5); 44.0
(C-1, 5.7); 115.9 (=CH2); 126.9 (Ph); 127.6 (Ph); 127.9 (Ph);
137.2 (=CH-); 146.8 (Ph, 5.2); 175.5 (C-3, 3.8). 29Si NMR
(C6D6; 23 ◦C): δ = −27.3.
Conversion of the 1,2-allylboration products 6, 10 and 14
into the 1-boracyclohex-2-enes 8, 12 and 15
17b: 1H NMR: No assignment was made owing to severe
overlap with signals of 15b. 13C NMR: δ = 18.2 (C-9); 21.2
(br., C-6); 21.3 (br., C-8); 36.4 (C-4, 7.6); 37.6 (br., CH2B);
44.0 (C-5); 44.5 (C-1, 6.2); 116.9 (=CH2); 127.8 (Ph); 128.3
(Ph); 128.9 (Ph); 136.0 (=CH-); 147.0 (Ph, 4.8); 179.7 (C-3,
2.2). 29Si NMR (C6D6; 23 ◦C): δ = −73.3.
Heating of the solutions con◦taining the boranes 6, 10 or 14
in C6D6 for 12 h at 70 – 80 C leads to the formation of
1-bora-cyclohex-2-enes 8, 12 and 15 together with bicyclic
products (vide infra).
8a: 1H NMR signals were not assigned because of the low
concentration of 8a in the reaction mixture.
Acknowledgements
8b: 1H NMR: δ = 0.15, 0.22 (d, d 3H,2H Me2Si,
3J(H,H) = 3.8 Hz); 0.75, 1.42 (dd, dd, 1H, 1H, C6H2);
2.07 (m, 1H, C5H); 2.20 (m, 2H, BAll); 2.28, 2.76 (dd,
Support of this work by Volkswagen-Stiftung, Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen In-
dustrie is gratefully acknowledged.
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B. Wrackmeyer – O. L. Tok · 1,1- and 1,2-Allylboration of Alkyn-1-ylsilanes
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