Spirocyclic stannole derivatives by 1,1-organoboration of 2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes

Article abstract of DOI:10.1016/S0020-1693(99)00299-6

With the exception of 1c, the 2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes (1) [C≡C-R¹: R¹ = Me (a), Bu (b), ^tBu (c), Ph (d), SiMe₃ (e), CH₂NMe₂ (f), CH₂OMe (g)] react with triethylborane, Et₃B (2), to give the spirocyclic stannole derivatives 4a,b,d,e,f,g by intermolecular 1,1-ethylboration, followed by intramolecular 1,1-vinylboration. In the cases of 1f and 1g, zwitterionic intermediates 7f,g, in which the tin atom is coordinated to an alkynylborate C≡C bond, were detected by NMR spectroscopy. In the case of the reaction of 1e with triphenylborane, Ph₃B (3), exchange reactions involving the Sn-N bonds compete with 1,1-organoboration, and the spirocyclic stannole derivative 5e is only part of a complex reaction mixture. In the reaction of 1e with Et₃B, an intermediate 8e with the wrong stereochemistry for ring closure was detected by NMR spectroscopy. Owing to the reversibility of 1,1-organoboration, the spirocyclic stannole 4e is finally formed. Products and intermediates were characterised by ¹H, ¹¹B, ¹³C, ¹⁵N, ²⁹Si and ¹¹⁹Sn NMR spectroscopy. Several absolute signs of coupling constants in 4e were determined.

Full text of DOI:10.1016/S0020-1693(99)00299-6

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Inorganica Chimica Acta 296 (1999) 26–32
Spirocyclic stannole derivatives by 1,1-organoboration of
2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes
Bernd Wrackmeyer a,*, Hendrik Vollrath a, Saqib Ali b
a Laboratorium fu¨r Anorganische Chemie der Uni6ersita¨t Bayreuth, D-95440 Bayreuth, Germany
b Department of Chemistry, Quaid-I-Azam Uni6ersity, 45320 Islamabad, Pakistan
Received 5 February 1999
Abstract
With the exception of 1c, the 2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes (1) [CꢀCꢁR1: R1=Me (a), Bu
t
(b), Bu (c), Ph (d), SiMe3 (e), CH2NMe2 (f), CH2OMe (g)] react with triethylborane, Et3B (2), to give the spirocyclic stannole
derivatives 4a,b,d,e,f,g by intermolecular 1,1-ethylboration, followed by intramolecular 1,1-vinylboration. In the cases of 1f and
1g, zwitterionic intermediates 7f,g, in which the tin atom is coordinated to an alkynylborate CꢀC bond, were detected by NMR
spectroscopy. In the case of the reaction of 1e with triphenylborane, Ph3B (3), exchange reactions involving the SnꢁN bonds
compete with 1,1-organoboration, and the spirocyclic stannole derivative 5e is only part of a complex reaction mixture. In the
reaction of 1e with Et3B, an intermediate 8e with the wrong stereochemistry for ring closure was detected by NMR spectroscopy.
Owing to the reversibility of 1,1-organoboration, the spirocyclic stannole 4e is finally formed. Products and intermediates were
1
characterised by H, 11B, 13C, 15N, 29Si and 119Sn NMR spectroscopy. Several absolute signs of coupling constants in 4e were
determined. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Alkynes; Organotin compounds; Heterocycles; Organoboranes
1. Introduction
with respect to their reactivity towards trialkylboranes.
Again it was observed that 1,1-organoboration is the
preferred course of the reaction, leading first to zwitteri-
onic intermediates and finally to spirocyclic stannoles
[6]. We have now extended this investigation in order to
include a greater variety of alkynyl groups (Scheme 1),
some of which bear functional groups such as 1e,f,g.
All alkynes 1 were treated with triethylborane, Et3B (2).
Compound 1e was also treated with triphenylborane,
Ph3B (3). The application of multinuclear magnetic
resonance measurements was found extremely useful in
The reactivity of alkynyltin compounds towards tri-
organoboranes has been studied in some detail [1,2]. In
general, the 1-alkynyltin compound reacts by cleavage
of the SnꢁCꢀ bond, followed by 1,1-organoboration.
Such reactions become particularly attractive for the
synthesis of heterocycles such as stannoles which are
difficult to prepare by other methods [3,4]. There are
numerous examples in which bis(1-alkynyl)diorganotin
compounds are starting materials [5], including a stan-
nacyclohexane derivative [5h]. In many cases it was
found that stannoles can be obtained selectively in high
yield. Little work has focused on alkynyltin compounds
in which the tin atoms do not bear any organic group
in addition to the alkynyl substituent. So far only two
compounds of type 1, namely 1a,b, have been studied
* Corresponding author. Tel.: +49-921-552542; fax: +49-921-
552157.
E-mail address: b.wrack@uni-bayreuth.de (B. Wrackmeyer)
Scheme 1.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00299-6

B. Wrackmeyer et al. / Inorganica Chimica Acta 296 (1999) 26–32
27
Table 1
13C, 15N and 119Sn NMR data a of 1,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes 1a–g
1g
1a
1b
1c
1d
1e
1f
b
d
R1
Me
Bu
tBu
Ph c
SiMe3
CH2Nme2
CH2OMe
SnꢁCꢀ
R1ꢁCꢀ
R1
86.3 [1009.3]
105.1 [206.0]
4.6 [17.4]
86.9 [1006.5]
109.5 [200.6]
19.9 [15.8]
30.9 [24.5]
55.7 [7.1]
84.9 [998.2]
117.7 [191.0]
30.7 [7.9]
28.3 [14.8]
55.7 [6.9]
96.7 [924.0]
108.3 [192.3]
123.1 [20.3]
132.2 (o)
115.7 [870.2]
116.7 [137.8]
−0.4
91.7 [975.6]
104.1 [188.0]
48.7 [13.8] 43.8
93.2 [956.9]
104.9 [194.1]
64.5 [33.5]
−18.5 (l29Si)
NtBu
55.7 [6.5]
55.9 [9.3] 30.2 55.9 [6.9]
55.7 [6.9]
55.8 [7.9]
30.1 [25.1]
47.9 [6.5]
34.3 [29.4]
−321.4 [100.1]
−265.4
30.1 [25.1]
47.9 [6.5]
34.3 [28.9]
−321.8 [99.8]
−264.1
30.1 [25.6]
47.8 [7.9]
34.2 [30.2]
−320.3 [97.4]
−260.0
[24.6]
30.2 [24.6]
47.7 [5.9]
34.1 [29.5]
−319.5 [95.2]
−280.5
30.1 [25.6]
47.7 [5.9]
34.2 [29.5]
−320.0 e [100.3]
−268.9
30.1 [25.6]
47.8 [7.9]
34.1 [30.5]
−320.5 [100.3]
−268.5
NCH2
CH2
47.9 [8.7]
34.2 [31.6]
n.m.
l
15N
l
119Sn
−260.3
a In C6D6 (10–15%) at 2591°C; coupling constants J(119Sn,13C) and J(119Sn,15N) are given in brackets (90.5 Hz); n.m.=not measured. Data
for 1a,b from Ref. [7].
b Other l13C (R1): 22.1, 13.7.
c Other l13C (R1): 128.7 (m), 128.4 (p).
d 3J(119Sn,29Si)=6.0 Hz.
e l15N(NMe2)=−360.2.
the characterisation of the products and in the elucida-
tion of the nature of intermediates.
served for the l119Sn data (20 ppm) with changes,
analogous to the bis(1-alkynyl)dimethyltin compounds
[11,12].
2. Results and discussion
2.2. 1,1-Organoboration of the alkynyltin compounds 1
2.1. Synthesis of the 2,2-bis(1-alkynyl)-1,3,2-
diazastannacyclohexanes (1)
The reactions of 1a,b with Et3B (2) have been de-
scribed [6]. They proceed under relatively mild condi-
tions, and zwitterionic intermediates (vide infra) could
be detected by NMR spectroscopy. In contrast, 1c does
not react with 2, even after prolonged heating at 80°C.
The compounds 1d,f,g react with 2 under similar condi-
tions as 1a,b, in hexane between −20 and +25°C for
several hours or days (1f), to give the stannoles 4d,f,g.
In the case of 1e, the reaction was rather slow at room
temperature and heating at 90°C for several hours was
required to obtain the stannole 4e as the final product
(Scheme 2). Triphenylborane 3 reacts slowly with 1e at
room temperature to give a mixture of products which
The alkynyltin compounds 1 were prepared as de-
scribed previously [7], taking advantage of the readily
available 2,2-dichloro-1,3-di-tert-butyl-1,3,2-diazastan-
nacyclohexane [8]. The reaction of the latter compound
(Eq. (1)) with two equivalents of the lithiated alkynes
LiCꢀCR1 afforded the desired compounds 1 in 70–90%
yield as colourless, moisture-sensitive solids (1c,d,e) or
as yellow oils (1f,g). The 13C, 15N and 119Sn NMR data
of compounds 1, including those of 1a,b [7] for com-
parison, are given in Table 1.
(1)
All 13C NMR data of the alkynyl carbon atoms [9],
including the magnitude of coupling constants
J(119Sn,13C) [10], are in the expected range. The 13C and
15N NMR data of the 1,3,2-diazastannacyclohexane
moiety are almost independent of the nature of R1,
indicating negligible changes of the bonding situation in
the six-membered ring. The greatest variations are ob-
Scheme 2.

28
B. Wrackmeyer et al. / Inorganica Chimica Acta 296 (1999) 26–32
Table 2
13C, and 119Sn NMR data a of the spirocyclic stannoles 4a,b (for comparison) and 4d,e,f,g
4a
4b
4d
4e
4f
4g
h
i
R1
Me
Bu
Ph
SiMe3
CH2NMe2
CH2OMe j
C(1)
C(2)
C(3)
C(4)
NtBu
138.3 [541.7]
160.5 (br)
147.3 [186.4]
133.4 [621.3]
55.3 [5.4]
144.2 [529.3]
159.1 (br)
145.7 [189.2]
140.8 [611.2]
55.5 [8.7]
29.6 [20.5]
48.2 [5.7]
38.3 [17.9]
27.5 [76.8]
13.0 [11.0]
149.9 [536.8]
161.1 (br)
148.8 [175.0]
142.6 [591.5]
56.0 [8.3]
147.1 [281.2]
176.0 (br)
136.2 [485.6]
173.7 (br)
151.5 [136.9]
141.6 [694.8]
55.6 [7.0]
30.3 [20.1]
48.9 [14.4]
34.9 [31.4]
25.2 [77.2] 14.9 [13.0]
137.7 [569.1]
165.3 (br)
146.4 [130.2]
142.3 [599.6]
55.4 [B6.0]
29.8 [23.6]
48.2 [10.8}
35.0 [n.o.]
25.7 [73.6] 13.0 [12.6]
161.0 [162.5]
137.7 [321.3]
55.8 [10.8]
29.5 [21.0]
48.0 [7.2]
35.5 [13.5]
32.8 [123.0]
14.8 [10.8]
23.2 (br), 9.6
2.7 [12.5] e
30.2 [20.7]
50.0 [7.8]
30.3 [21.5]
48.1 [6.3]
NCH2
CH2
3-Et
36.5 [25.1]
26.7 [74.1]
13.0 [10.9]
22.9 (br), 9.2
16.2, 19.4
36.5 [27.0]
26.4 [70.0]
13.9 [10.8]
23.0 (br), 9.9
143.4 [66.4]
143.0 [77.2] d
−93.9
BEt2
22.7 (br), 9.2
12.5 (br), 11.0
71.7 [121.2]
61.1 [61.0] f
−111.0
15.8 (br), 10.8
86.0 [147.8]
73.0 [46.0] g
−104.5
1-R1, 4-R1 b
c
l
119Sn
−87.1
−93.8
−13.9
a In C6D6 (10–15%) at 2591°C; coupling constants J(119Sn,13C), 1J(119Sn,15N) and 2J(119Sn,29Si) are given in brackets (90.5 Hz); n.m.=not
measured; n.o.=not observed; (br) denotes broad 13C NMR signals owing to partially relaxed scalar 13Cꢁ11B coupling. l11B values are at 8591
(broad signals h1/2\600 Hz). Data for 4a,b from Ref. [6].
b Assignment uncertain.
c Numerous overlapping signals; no assignment: l13C 31.0, 23.4, 14.2.
d Other l13C values (without assignment): 128.6, 128.4, 126.3, 125.4.
e Two signals overlap; l29Si −8.1 [118.4], −9.6 [112.0].
f Other l13C values (without assignment): 49.9 [14.4], 48.9 [14.4] (NMe2).
g Other l13C values (without assignment): 57.7, 55.5 (OMe).
h l15N=−318.7 [70.5]; cf. 4a [6] l15N=−317.2 [27.5].
i l11B 6.4.
j l11B 46.9.
contains the stannole 5e. Exchange reactions at the
SnꢁN bonds compete with 1,1-organoboration. The
final complex reaction mixture contains the desired
stannole 5e among other unidentified products which
could not be separated. All stannoles 4a–g are colour-
less or yellowish, air-sensitive oils which can be stored
in the fridge for several months. We could not purify
these compounds by chromatography, and attempts to
crystallise them from pentane solutions at low tempera-
ture failed. 13C and 119Sn NMR data of the spirocyclic
stannoles are given in Table 2.
All NMR data prove the formation of the five-mem-
bered stannole ring, and the 1,3,2-diazastannacyclohex-
ane ring remains unchanged. The typical pattern of the
13C NMR signals for the olefinic carbon atoms, three
sharp signals with 117/119Sn satellites according to
2
1J(119Sn,13C) and J(119Sn,13C), and one broad signal
for the boron-bonded carbon atom, is observed (Fig.
1). The assignment of the sharp signals is based on their
differences in chemical shifts and on the different mag-
nitude of the coupling constants, in analogy to the
Fig. 1. 125.8 MHz 13C{1H} NMR spectrum of 4e in C6D6 (10%) at
known dimethyltin derivatives [12]. The trend of the
25°C showing the region for olefinic carbon atoms (see text). The
l119Sn values for compounds 4 is also very similar to
assignment is indicated, and 117/119Sn satellites are marked by aster-
isks.
that of the dimethyltin analogues [12].

B. Wrackmeyer et al. / Inorganica Chimica Acta 296 (1999) 26–32
29
The presence of the Me3Si groups in 1,4-positions in
4e takes influence in particular on the 119Sn nuclear
shielding (deshielding by \70 ppm) and on the magni-
tude of the coupling constants 1J(119Sn,13C) and
1
1J(119Sn,15N). The magnitude of ꢀ J(119Sn,13C)ꢀ is much
smaller than in the other compounds 4. The absolute
signs of J(29Si,13C) (B0), J(119Sn,13C) (B0) (reduced
coupling constants 1K(119Sn,13C), 1K(29Si,13C)\0, as
expected [10]), and 2J(119Sn,29Si) (\0) (2K(119Sn,29Si)\
0) in 4e were determined by a series of 2D 13C/1H (one
example is given in Fig. 2) and 29Si/1H HETCOR
experiments as described previously for similar com-
pounds [13]. There are also negative contributions to the
119Snꢁ15N scalar interactions owing to the SiMe3 groups
in 1,4-positions. The larger absolute value
1
1
Scheme 3.
1
ꢀ J(119Sn,15N)ꢀ=70.5 Hz for 4e as compared to 27.5 Hz
1
for 4a indicates a negative sign of J(119Sn,15N) in 4e.
In the presence of Lewis bases such as in 4f and 4g,
coordinative interactions with the boryl groups as well
as with the tin atom must be considered as shown by the
proposed structure of 4f (Scheme 3). The coordinative
NꢁB or OꢁB bonds are clearly evident from the increase
in 11B nuclear shielding [14], the OꢁB bond in 4g (l11B
46.9) being considerably weaker than the NꢁB bond in
4f (l11B 6.4). The 119Sn nuclear shielding is slightly higher
than in the stannoles without such substituents, indicat-
Scheme 4.
Scheme 5.
ing fairly weak coordinative NꢁSn or OꢁSn bonds.
However, there are marked changes in the magnitude
1
of coupling constants ꢀ J(119Sn,13C)ꢀ, as expected for
carbon atoms being preferably in axial or equatorial
positions of trigonal bipyramidal surroundings of a
penta-coordinate tin atom [12,15,16]. In 4f, the coupling
1
constant ꢀ J(119Sn,13C(1))ꢀ has the smallest value in the
series of alkyl substituents in 1,4-positions, whereas
1
ꢀ J(119Sn,13C(4))ꢀ has the largest value (Scheme 3). Since
the NꢁB coordination in 4f follows unambiguously from
the l11B value, the NꢁSn coordination must be exerted
by the NMe2 group of R1 in 4-position. This requires that
C(4) prefers on average in equatorial position, whereas
C(1) is in axial position, in agreement with the observed
trends in the coupling constants. Of course, these
molecules are fluxional and therefore, the differences in
the magnitude of the coupling constants are much less
pronounced when compared with more rigid structures
[15,16].
Fig. 2. Contour plot of the 2D 75.5 MHz 13C–1H HETCOR experi-
ment, based on 3J(13C, Si, C, 1HMe), showing the 13C(1,4) resonances
of 4e. The passive spins are 29Si and 117/119Sn. The negative tilts of the
cross peaks [18] for the 29Si satellites prove that the signs of
2K(29Si, 1Hme) and 1K(29Si, 13C(1,4)) are opposite. The former is
known to be negative. There is a slight positive tilt of the cross peaks
for 117/119Sn satellites (dashed lines). Therefore, the signs of 4K(117/
119Sn, C, Si, C, 1HMe) and 1K(119Sn, 13C(1,4) are alike. By a series of
similar experiments, it can be shown that 4K(117/119Sn,
C, Si, C, 1HMe)\0.

30
B. Wrackmeyer et al. / Inorganica Chimica Acta 296 (1999) 26–32
Table 3
2.3. Intermediates in the 1,1-organoboration reactions
11B, 13C, 119Sn NMR a data of the zwitterionic intermediates 7f and
7a [6] for comparison d
There is clear evidence for the stepwise 1,1-organo-
boration of bis(1-alkynyl)tin compounds [1,2]. In the
present cases, it was already shown [6] that the inter-
molecular 1,1-organoboration leads to intermediates of
type 6 which are in equilibrium with 7 (Scheme 4), and
from 7 the stannole is formed by intramolecular 1,1-
vinylboration.
It is also known [12] that such zwitterionic species are
stabilised by coordinative SnꢁN or SnꢁO interactions
which are possible in the case of R1=CH2NMe2 or
CH2OMe (Scheme 5). Indeed, NMR spectra of reaction
solutions of the 1,1-organoboration of 1f,g showed
characteristic signals for zwitterionic intermediates even
after 24 h at room temperature, and in the case of 7f, a
full set of 11B, 13C and 119Sn NMR data (Table 3) could
be obtained.
The reaction of 1e with triorganoboranes is some-
what more complicated. In general it proved impossible
to detect the zwitterionic intermediates if R1=SiMe3
since these intermediates are particularly short-lived
and, under the reaction conditions (heating is fre-
quently required), rearrange rather fast to the final
products or to other more stable intermediates. Re-
cently, a few exceptions have been observed [17]. A
further problem arises because the 1,1-organoboration
is no longer stereoselective for 1-alkynyltin compounds
with R1=SiMe3 [1,2]. On the other hand, the latter
feature enables one to find such intermediates which do
not have the correct stereochemistry for intramolecular
1,1-vinylboration. This is shown by the 29Si NMR
spectrum (Fig. 3) of the reaction solution which indi-
cates the presence of starting material 1e, the final
product 4e, and a large amount of the intermediate 8e
(l29Si −3.3, 2J(119Sn,29Si)=154.4 Hz, −20.2,
3J(119Sn,29Si)=6.0 Hz; l119Sn −242.4) which cannot
rearrange to a heterocycle. The 1,1-organoboration is a
reversible reaction [1,2], and therefore, upon heating, 8e
loses Et3B to give back 1e. Any time the 1,1-organobo-
ration of 1e produces the compound 6e with the correct
stereochemistry (Scheme 4), fast rearrangement to 7e
takes place, followed by similarly fast intramolecular
1,1-vinylboration to 4e.
7a
7f
R1
Me
CH2NMe2
SnꢁCꢂ 133.2 [984.0]
145.1 [1084.6]
177.5 (br)
130.9 (br) [n.o.]
105.6 [61.6]
61.2 [134.3], 47.4
55.2 [n.o.], 44.0
15.0 (br), 13.5
56.6 [11.6], 30.7 [22.5]
49.0 [12.1]
BꢁCꢂ
BꢁCꢀ
178.0 (br)
114.7 (br) [130]
R1ꢁCꢀ 124.2 [82.8]
ꢂCꢁR1 18.8 [189.6]
ꢀCꢁR1 6.2 [B3.0]
BEt2
17.0 (br), 13.6
56.6 [7.1], 30.5 [22.9]
NtBu
NCH2 48.2 [16.9]
CH2
35.6 [16.9]
−7.1
35.4 [18.3]
l
11B
−9.2 b
l
119Sn −32.8
−125.0 c
a In CD2Cl2 (10–15%) at −1091°C; coupling constants
J(119Sn,13C) are given in brackets (90.5 Hz); n.o.=not observed;
(br) denotes broad 13C NMR signals owing to partially relaxed scalar
13Cꢁ11B coupling.
b h1/2=850 Hz; l11B −9.2 (25°C) with h1/2=280 Hz.
c h1/2=33 Hz; l119Sn −115.0 (25°C) with h1/2=118 Hz; l119Sn
−125.0 (−50°C) with h1/2=37 Hz.
d For 7g in toluene, the 119Sn NMR spectrum showed a signal at
l
119Sn −64.7 (−15°C).
1,3,2-diazastannacyclohexane moiety can be achieved.
Furthermore, reductions of the Sn(IV) to Sn(II) com-
pounds seem feasible by retaining the stannole unit.
4. Experimental
The preparative work and all handling of samples
was carried out under an inert atmosphere (Ar or N2),
using carefully dried glassware and dry solvents. BuLi
(1.6 M) in hexane, Et3B were commercial products.
Triphenylborane was obtained as described [20], 2,2-
dichloro-1,3-di-tert-butyl-1,3,2-diazastannacyclohexane
was prepared as reported [8] (note that the N,N%-di-tert-
butyl-1,3-diaminopropane has to be purified by distilla-
tion prior to use). All bis(1-alkynyl)tin compounds 1
were obtained following the literature procedure [9],
1
and their purity was \95% according to H NMR.
NMR spectra were recorded by using Jeol FX90Q,
Bruker AC 300, ARX 250 and AMX 500 instruments,
all equipped with multinuclear units and variable-tem-
perature control. If not mentioned otherwise, samples
dissolved in C6D6 (10–20%) in 5 mm (o.d.) tubes were
measured at 2591°C. Chemical shifts are given with
respect to solvent signals [l1H (C6D5H)=7.15;
(C6D5CD2H)=2.03; (CDHCl2)=5.33; l13C (C6D6)=
128.0; (C6D5CD3)=20.4; (CD2Cl2)=52.3] and external
references [l11B (BF3−OEt2)=0, J11B=32.083971
MHz; l15N (MeNO2, neat)=0, J15N=10.136767
MHz; l29Si (Me4Si)=0, J29Si=19.867184 MHz;
3. Conclusions
Since various bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-di-
azastannacyclohexanes can be readily converted into
novel stannoles of type 4 by 1,1-organoboration, these
compounds can now be used for further transforma-
tions. This can be done by taking advantage of the
well-known reactivity of SnꢁN, SnꢁC or BꢁC bonds.
Thus, the syntheses of other new stannoles as well as of
other metalloles seem feasible, if the substitution of the

B. Wrackmeyer et al. / Inorganica Chimica Acta 296 (1999) 26–32
31
Fig. 3. 17.8 MHz 29Si NMR spectrum (refocused INEPT [19]) of the reaction mixture resulting from the reaction of 1e with Et3B (2) after 1 h
at room temperature. 117/119Sn satellites are marked by asterisks.
1
l119Sn (Me4Sn)=0, J119Sn=37.290665 MHz]. 15N
NMR spectra were recorded by the refocused INEPT
pulse sequence with 1H decoupling [19], taking advantage
of the scalar coupling 15Nꢁ1H(tBu) across three bonds
with a magnitude of ca. 2.8 Hz. In most cases it proved
possible to detect 117/119Sn satellites after a few hours of
spectrometer time. All 29Si NMR spectra were measured
to H NMR). These compounds decompose on silica or
Al2O3, and all attempts to crystallise them either from
pentane or from pentane/ether mixtures at −78°C failed.
The reaction of 1e with Ph3B (3) in the molar ratio 1:1
was carried out in the same way as for Et3B. 29Si NMR
spectra showed that starting material was still present
although various products, including 5e (l29Si −6.6,
−8.0, 2J(119Sn,29Si)=111.0and112.0Hz;l119Sn −16.9;
full assignment of the 13C NMR spectrum was not
possible) had already been formed. At least 40% of Ph3B
was consumed in exchange reactions (see Scheme 2). By
using a large excess of 3, all of 1e was consumed after
2 h at room temperature, giving again a complex mixture
of compounds, including 5e.
2
using the same technique, based on J(29Si,1HMe). 119Sn
NMR spectra were measured by inverse gated 1H decou-
pling in order to suppress the NOE. By using spectrom-
eters at high field strength (B0\7.1 T), 119Sn nuclear spin
relaxation in the compounds studied is mainly governed
by the mechanism of chemical shift anisotropy. This
means that pulse angles in the order of 50–60° and fast
repetition times (ca. 0.1 s) can be used. The signals are
fairly broad owing to partially relaxed scalar 119Snꢁ14N
couplingandtherefore, short(0.3s)acquisitiontimeswere
used. All 2D experiments were optimised by appropriate
1D polarisation transfer experiments.
In the cases of 1f,g, the experiments were repeated in
CD2Cl2 in order to obtain a complete set of 13C NMR
data of the intermediates 7f,g. This was successful for 7f
(Table 3).
Acknowledgements
4.1. 1,1-Organoboration of the alkynes 1. General
procedure
Support of this work by the Deutsche Forschungsge-
meinschaft, Fond der Chemischen Industrie, Volkswa-
gen-Stiftung and Alexander-von-Humboldt-Stiftung
(Georg Forster Scholarship to Saqib Ali) is gratefully
acknowledged.
A solution of the alkynes 1 (0.1 g) in toluene (5 ml)
was cooled to −78°C, and an excess of Et3B (3) (0.6 ml)
was added in one portion. The mixture was warmed to
room temperature, and the progress of the reaction was
monitored by 119Sn and 29Si NMR (1e), as shown in Fig.
3. After heating for 1–5 h at 80°C, all alkynes, except
those of 1c, were completely converted into the stannoles
4. The solvent and the excess of Et3B were removed in
vacuo, and the stannoles were left as colourless or slightly
yellow oils in quantitative yield (purity \95% according
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