1,1-carboboration of dialkynyltin compounds using triorganoboranes of greatly different lewis acid strength. 1,4-stannabora-cyclohexa-2,5-dienes and characterization of zwitterionic intermediates

Article abstract of DOI:10.1002/zaac.201300007

Triorganoboranes BR₃, Et-9-BBN, BPh₃, and B(C ₆F₅)₃, were compared in their reactivity towards various dialkynyl(diorgano)tin compounds (R¹ ₂Sn(C≡C-R²)₂ with R¹ ₂ = -(CH₂)₅-, R² = H (a), R ¹ = nBu, R² = H (b), R¹ = Ph, R² = H (c), R¹ = R² = nBu (d)). 1,1-Carboboration took place readily in two consecutive steps (inter- and intramolecular), leading either to stannoles or to 1,4-stannabora-cyclohexa-2,5-dienes, or mixtures thereof. The weakest Lewis-acidic triorganoboranes BEt₃ and Et-9-BBN afford selectively stannoles with diethynyltin compounds, whereas the strongly electrophilic B(C₆F₅)₃ leads selectively to 1,4-stannabora-cyclohexa-2,5-dienes for all dialkynyltin compounds studied. In several cases, zwitterionic intermediates could be detected by multinuclear magnetic resonance spectroscopy (¹H, ¹¹B, ¹³C, and ¹¹⁹Sn NMR), and the molecular structure of such an intermediate as well as that of the final product, an 1,4-stannabora-cyclohexa-2,5-diene, could be determined by X-ray crystallography. Copyright

Full text of DOI:10.1002/zaac.201300007

ARTICLE
DOI: 10.1002/zaac.201300007
1,1-Carboboration of Dialkynyltin Compounds using Triorganoboranes of
Greatly Different Lewis Acid Strength. 1,4-Stannabora-cyclohexa-2,5-dienes
and Characterization of Zwitterionic Intermediates
Bernd Wrackmeyer,*[a] Peter Thoma,[a] Simone Marx,[a] Germund Glatz,[a] and
Rhett Kempe[a]
Dedicated to Professor Heinrich Nöth on the Occasion of His 85th Birthday
Keywords: Organotin compounds; Alkynes; 1,1-Carboboration; Side-on-η2 coordination; Tin; NMR; X-ray spectroscopy
Abstract. Triorganoboranes BR3, Et-9-BBN, BPh3, and B(C6F5)3, were diethynyltin compounds, whereas the strongly electrophilic B(C6F5)3
compared in their reactivity towards various dialkynyl(diorgano)tin
leads selectively to 1,4-stannabora-cyclohexa-2,5-dienes for all dialk-
ynyltin compounds studied. In several cases, zwitterionic intermediates
could be detected by multinuclear magnetic resonance spectroscopy
compounds (R1 Sn(CϵC–R2)2 with R1 = –(CH2)5–, R2 = H (a), R1 =
2
2
nBu, R2 = H (b), R1 = Ph, R2 = H (c), R1 = R2 = nBu (d)). 1,1-
Carboboration took place readily in two consecutive steps (inter- and (1H, 11B, 13C, and 119Sn NMR), and the molecular structure of such
intramolecular), leading either to stannoles or to 1,4-stannabora-cy- an intermediate as well as that of the final product, an 1,4-stannabora-
clohexa-2,5-dienes, or mixtures thereof. The weakest Lewis-acidic
triorganoboranes BEt3 and Et-9-BBN afford selectively stannoles with
cyclohexa-2,5-diene, could be determined by X-ray crystallography.
migration of the metal fragment along the CϵC bond
(Scheme 1b). This reaction turned out to be less prone to side
reactions and rather versatile. For instance it allowed to use
alkynylmetal compounds with up to four alkynyl groups linked
to the metal.[4,5]
For M = Si the 1,1-ethyloboration requires rather harsh reac-
tion conditions (several hours at 100 °C), indicating that
strongly electrophilic triorganoboranes might work better,
eventually even with “non-activated” alkynes, as has been con-
vincingly demonstrated by using B(C6F5)3.[6–8] Nevertheless it
proved possible to obtain siloles in a most straightforward way
from reactions of numerous dialkynylsilanes with the weakly
Lewis-acidic trialkylboranes.[9,10] Under much milder condi-
tions, such siloles could also be prepared using B(C6F5)3.[11]
The facile and quantitative synthesis of stannoles by the reac-
tion of diethynyltin compounds with various trialkylboranes
(Scheme 2) appears to be similarly intriguing. Thus, the stan-
Introduction
Since a great variety of triorganoboranes BR3 (e.g. R = alkyl,
vinyl, aryl, cyclic, non-cyclic) is readily accessible,[1,2] their
use in synthesis to form C–C bonds is attractive. In this respect
1,1-carboboration of alkynes has been developed in several
ways. Early progress had been made by conversion of BR3 into
trialkyl(alkynyl)borates [R3B–CϵC–C–R2]–, followed by their
reaction with electrophiles and salt elimination (Scheme 1a).[3]
Then, it was found that trialkylboranes react with alkynyltin
and some other alkynylmetal derivatives by 1,1-carboboration,
almost quantatively and in many cases stereospecifically by
Scheme 1. (a) Example of the reaction of trialkyl(alkynyl)borates with
an electrophile via salt elimination. (b) Examples of 1,1-ethyloboration
of propyn-1-ylmetal compounds (the intermediacy of the zwitterionic
species is proposed).
* Prof. Dr. B. Wrackmeyer
Fax: +49-921-552157
E-Mail: b.wrack@uni-bayreuth.de
[a] Anorganische Chemie II
Universität Bayreuth
Scheme 2. Examples of 1,1-carboboration reactions of diethynyltin
compounds affording stannoles (R = sBu).
95440 Bayreuth, Germany
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nole 1 was the first example, characterized by multinuclear
magnetic resonance spectroscopy (1H, 11B, 13C, 119Sn NMR)
in solution,[12] and later on, the molecular structure of 2 was
confirmed by X-ray structural analysis.[13]
In the presented work we have set out to compare the reac-
tivity of four dialkynyltin compounds 3 (R1 Sn(CϵC–R2)2
2
with R1 = –(CH2)5–, R2 = H (a), R1 = nBu, R2 = H (b), R1 =
2
Ph, R2 = H (c), R1 = R2 = nBu (d)), towards the triorganobor-
anes BEt3, Et-9-BBN, BPh3, and B(C6F5)3. Particular care was
taken to look for zwitterionic intermediates[14–16] in order to
reveal mechanistic aspects of the 1,1-carboboration reactions.
Scheme 3. Typical 1,1-carboboration reactions of trialkylboranes with
diethynyltin compounds.
Results and Discussion
Synthesis and NMR Spectroscopy
The solution-state characterization of organotin compounds
by NMR spectroscopy is well documented.[17–19] In the pre-
sented work, chemical shifts δ119Sn show the expected pattern
due to different substituents. δ119Sn data are particularly help-
ful for detecting zwitterionic intermediates and to distinguish
between different ring sizes. Moreover coupling constants
nJ(119Sn,13C) or nJ(119Sn,1H) give useful information, espe-
cially if the tin atom may be found in somewhat extreme bond-
ing situations. Structural elucidation, based as usual on 13C
NMR spectroscopy, is further aided by the presence of boron,
since 13C(B–C) NMR signals are usually broadened[20] and
therefore easy to assign.
Scheme 4. 1,1-Ethyloboration of dibutyl(dihexyn-1-yl)tin proceeds via
a zwitterionic intermediate 6d. The activation of the Sn–Cϵ bond in
6dЈ is indicated by a dashed line.
Trialkylboranes react with diethynyltin compounds 3a–c
(Scheme 3) via two fast consecutive 1,1-carboborations in the
Substituents R2 at the CϵC bond other than hydrogen,[21,22]
same way as reported previously e.g. for 1[12] and 2[13] here R2 = nBu in 3d, have a significant influence on the prod-
(Scheme 2), as shown by consistent NMR spectroscopic data uct distribution (Scheme 4). Using a 1.1 ratio of starting mate-
sets (Table 1). Therefore, different organyl groups as substitu- rials, the formation of the fairly long-lived zwitterionic inter-
ents on tin do not exert a major effect on the course of the 1,1- mediate 6d can be readily detected, most likely a result of
carboboration reactions. Attempts failed to observe intermedi- kinetic stabilization due to the nBu groups at both tin and CϵC
ates, and using a large excess, e.g. of BEt3, had no influence.
bonds. NMR signals of 6dЈ, the precursor of 6d (Scheme 4)
a)
Table 1. Selected NMR parameters for stannoles 1 (for comparison), 4, 5, and 7d
δ13C
.
δ119Sn
δ11B
C2
C3
C4
C5
R/BR2
b)
1
127.8 [410.4]
126.6 [382.7]
128.1 [364.9]
124.8 [441.1]
138.6 [373.8]
175.3 br
175.0 br
175.8 br
176.5 br
171.3 br
162.7 [89.9]
162.4 [84.4]
163.3 [79.1]
164.7 [99.6]
170.6 [71.1]
121.0 [484.4]
119.5 [456.0]
121.3 [433.9]
118.4 [517.7]
120.8 [436.6]
30.9 [62.6], 13.2/21.4 br, 9.2 19.5
30.9 [61.9], 13.0/21.2 br, 9.0 –21.7
31.0 [59.0], 13.2/21.3 br, 9.2 22.9 (25.0)
85.0
86.7
83.2
c)
d)
e)
f]
4a
4b
4c
5a
30.5 [67.6], 12.7/21.1 br, 8.9 –46.9 (23.4) 84.5
23.7 br, 11.3/33.5 br, 31.4,
22.2, 33.6, 43.2 [64.0]
23.7 br, 11.3/33.7 br, 31.6,
22.2, 33.7, 43.4 [60.5]
23.9(br), 11.4/33.9 br, 31.5,
22.2, 33.8, 43.2 [70.8]
–37.8 (29.1) 79.9 (1230)
8.1 (18.0) 80.1 (1460)
–59.2 (22.1) 80.9 (1410)
g)]
h]
5b
5c
7d
139.4 [360.5]
136.0 [435.0]
171.9 br
176.1 br
170.0 [66.4]
172.7 [83.9]
122.3 [418.3]
119.8 [503.1]
i)
142.4 [380.8.6] 165.8 br [44.2]
152.4 [104.7] 141.6 [428.6]
27.0 [60.6], 12.5/21.2 br, 9.1 0.4 (17.2)
90.4 (2570)
n
a) In C6D6 at 296 K; br. denotes the broad 13C NMR signal of carbon linked to boron; coupling constants J(119Sn,13C) are given in brackets.
b) Ref. [12]; in CD2Cl2 at 299 K, other 13C NMR spectroscopic data: –9.3 [329.7] (Sn–CH3). c) Ref. [13]; other 13C NMR spectroscopic data:
11.3 [311.5] (Sn–CH2), 28.5 [32.3] (CH2), 32.4 [53.9] (CH2). d) In [D8]tol; other 13C NMR spectroscopic data: 11.9 [337.0] (Sn–CH2), 29.9
[22.4] (CH2), 27.3 [50.8] (CH2), 13.8 (CH3). e) Other 13C NMR spectroscopic data: 138.5 [506.5] (C-i), 137.1 [40.0] (C-o), 128.6 [51.8] (C-
m), 129.0 [11.4] (C-p). f) In CDCl3, other 13C NMR spectroscopic data: 11.1 [313.7] (Sn–CH2), 28.4 [31.7] (CH2), 31.7 [53.1] (CH2). g) In
CDCl3, other 13C NMR spectroscopic data: 11.7 [340.7] (Sn–CH2), 29.6 [22.6] (CH2), 27.0 [50.6] (CH2), 13.7 (CH3). h) In CDCl3, other 13C
NMR spectroscopic data: 138.4 [509.9] (C-i), 137.1 [39.8] (C-o), 128.6 [51.2] (C-m), 129.0 [10.0] (C-p). i) No assignent of 13C(Sn-nBu)
resonances because of overlap with signals for 8d.
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1,4-Stannabora-cyclohexa-2,5-dienes and Their Zwitterionic Intermediates
Figure 2. 100.5 MHz 13C{1H} NMR spectrum (in C6D6, at 296 K;
Figure 1. 149.1 MHz 119Sn{1H} NMR spectra of the reaction mixture
of 3d with BEt3 (in C6D6 at 296 K). (A) 1 h after the mixture has
reached room temp.; (B) after 5 h at room temp.; (C) after 17 h at
room temp. The 119Sn NMR signal of 8d is almost two-times broader
than that of 7d, as a result of unresolved scalar three-bond 119Sn-11B
spin-spin coupling (two equivalent coupling pathways in 8d).
shown is the region of olefinic carbon atoms) of the final mixture
containing 7d and 8d 117/119Sn satellites are marked by asterisks, and
n
coupling constants J(119Sn,13C) in Hz are given in brackets. Note the
broadened 13C NMR signals for 13C nuclei linked directly to boron
owing to partially relaxed 13C-11B spin-spin coupling.[20]
were not observed. Interestingly, the selectivity of the final centration of other species present in solution (compare Fig-
intramolecular 1,1-carboboration is lost, The stannole 7d and ure 1, A and B). All NMR spectroscopic data of 7d (Table 1)
the 1,4-stannabora-cyclohexa-2,5-diene 8d are formed in a and 8d (Table 2) are in support of the proposed structures (see
ratio of about 60:40 by intramolecular 1,1-vinylo- and Figure 2 for 13C NMR spectroscopy of the final mixture of
1,1-ethyloboration, respectively.
products).
The reaction shown in Scheme 4 can be conveniently moni-
In order to increase the Lewis-acid strength with respect to
tored by 119Sn NMR spectroscopy (Figure 1). The 119Sn nu- trialkylboranes, triphenylborane BPh3 was used (Scheme 5).
cleus in 6d is typically deshielded[14–16] when compared with Monoalkynyltin compounds react readily with BPh3 to give
chemical shifts δ119Sn of tetraorganotin compounds.[17] This is clean 1,1-carboboration products.[23] Similarly, the reactions of
attributed to the partial positive charge on tin in the cationic BPh3 with dialkynyltin compounds proceed fast, even at low
fragment, which is stabilized by side-on (η2) coordination to temperature. However, the formation of final major products
the remaining CϵC bond. The 119Sn NMR signal of 6d ap- was accompanied by many side products in minor quantities,
pears to be sensitive to small changes in temperature and con- most of which appeared to be phenyltin compounds (suggested
a)
Table 2. Selected NMR parameters of 1,4-stannabora-cyclohexa-2,5-dienes 8d, 10d, and 12
δ = 13C
.
δ = 119Sn
δ = 11B
=C(2)–Sn
B–C(3)=
3-R
BR
R12Sn
b)
8d
153.4 [421.2]
164.7 [390.1]
163.1 [30.7]
161.0 [36.1]
–134.8 (42.0)
–133.8 (24.1)
72.9 (1500)
60.4 (2110)
c)
d)
e)
f)
10d
12a
12b
12c
143.6 [63.6], 128.9,
127.6, 126.1
121.2 t ʈ19.4ʈ (i)
145.6 br, 131.8,
126.1, 125.1
115.9 br (i)
12.0 [336.6]
179.2 [341.8]
180.0 [319.7]
174.6 [409.5]
150.0 br
11.7 [356.8] 27.8 [34.1],
30.7 [69.4]
12.3 [386.6], 29.3 [27.1]
27.0 [60.6] 13.6
134.4 (i), 137.1 [41.7] (o),
129.8 [59.1] (m), 130.6
[12.8] (p)
–176.5 (35.0)
–134.4 (25.0)
–212.8 (18.5)
55.5 (2500)
57.0 (2100)
58.0 (3300)
149.7 [50.8] br 121.4 t ʈ20.5ʈ [51.7] (i) 116.4 br (i)
150.6 br
121.4 t ʈ 19.9ʈ (i)
116.4 br (i)
g)
12d
197.8 [336.2]
150.0 br
118.4 t ʈ22.3ʈ (i)
br (not observed) 13.7 [357.4] 29.2 [25.1]
27.3 [60.6] 13.6
–129.7 (16.2)
56.8 (2070)
n
n
a) In C6D6 at 296 K; line widths h are given in parentheses, coupling constants J(119Sn,13C) in Hz in brackets; coupling constants J(19F,13C)
½
and J(19F,19F) in Hz in ʈ &par. b) Other 13C NMR spectroscopic data not assigned owing to overlapping signals. c) In CDCl3, other 13C NMR
n
spectroscopic data: 12.0 [336.3] (CH2), 13.6 (CH3), 14.0 (CH3), 22.8, 27.4 [61.7], 29.5 [21.0], 33.2 [11.2], 36.5 [44.8]. d) 19F NMR spectroscopic
data: C6F5 group: –143.1 m (o), –156.5 t ʈ20.5ʈ (p), –162.7 m ʈ7.2ʈ ʈ20.5ʈ (m), B–C6F5 group: –132.3 m (o), –150.7 t ʈ20.5ʈ (p), –161.4 m ʈ7.7ʈ
ʈ 20.5ʈ (m). e) 19F NMR spectroscopic data: C6F5 group: 143.4 m (o), –156.6 t ʈ21.5ʈ (p), –162.7 m ʈ8.4ʈ ʈ 21.5ʈ (m), B–C6F5 group: –132.5 m
(o), –150.7 t ʈ20.5ʈ (p), –161.3 m ʈ8.2ʈ ʈ20.5ʈ (m). f) 19F NMR spectroscopic data: C6F5 group: –143.0 m (o), –156.6 t ʈ21.3ʈ (p), –162.9 m
ʈ8.5ʈ ʈ21.3ʈ (m), B-C6F5-group: –132.3 m (o) –150.7 t ʈ22.6ʈ (p), –161.4 m ʈ8.3ʈ ʈ22.6ʈ (m). g) 19F NMR spectroscopic data: C6F5 group: –140.5
m (o), –156.1 t ʈ21.1ʈ, (p), –162.9 m (m), B–C6F5 group: –132.7 m (o), –153.1 t ʈ21.1ʈ (p), –162.1 m (m).
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Scheme 6. 1,1-Carboboration of dialkynyltin compounds 3a–d using
B(C6F5)3.
Scheme 5. 1,1-Phenyloboration of dialkynyltin compounds.
by 119Sn NMR spectra), most likely as the result of alkynyl/
phenyl exchange. These side reactions hampered the search for
zwitterionic intermediates. Nevertheless, mixtures of stannoles hexane revealed intense 119Sn NMR signals with δ119Sn
[in minor quantity, δ119Sn –23.3 ppm (9a), 24.1 ppm (9b)], around 260 ppm, typical of the stabilized cation Me3Sn+[24]
and 1,4-stannabora-cyclohexa-2,5-diene derivatives [δ119Sn and 11B NMR signals with δ11B = –1 ppm, typical of tetra-
–152.0 ppm (10a), –113.6 ppm (10b)] were clearly identified coordinate boron,[25] pointing towards ionic intermediates. So
by characteristic NMR signals, and in the case of 3d, the 1,4- far, these compounds could not be isolated or identified.
stannabora-cyclohexa-2,5-diene 10d (Table 2) was the pre-
Turning to the 1,1-carboboration of dialkynyltin compounds
ferred product (Ͼ85%). Apparently, increasing Lewis-acid 3 with B(C6F5)3, much better results and clean products were
strength of the borane directs the course of the reaction obtained (Scheme 6). In all cases studied, the zwitterionic in-
towards the six-membered ring, and this tendency is further termediates 11 could be readily identified by NMR spec-
augmented by substituents R2 other than hydrogen.
troscopy (Table 3, Figure 3, and Figure 4) in solution or even
The results for BPh3 prompted the study of B(C6F5)3 as a isolated (11a) and studied by X-ray crystallography (vide in-
strong Lewis acid for 1,1-carboboration reactions of alkynyltin fra).
compounds. However, first experiments with monoalkynyltin
Whereas the zwitterionic intermediate 6d rearranged to a
compounds Me3Sn–CϵC–R2 (R2 = H, Me) did not give clean mixture of the final products 7d and 8d (Scheme 4), the analo-
products, except of Me3Sn–C6F5, [δ119Sn –12.4 ppm, ttd, gous intermediates 11 afforded selectively the 1,4-stannabora-
3J(119Sn,19F) = 23.0, 4J(119Sn,19F) = 8.1, 5J(119Sn,19F) = cyclohexa-2,5-diene derivatives 12. This is clearly evident
8.1 Hz] and only traces (R2 = H) or small amounts (R2 = Me) from all NMR spectroscopic data for solutions of 12 (Table 2,
of 1,1-carboboration products analogous to those in Figure 5) and in the case of 12a, crystalline materials for X-
Scheme 1b. Measuring NMR spectra of reaction solutions in ray structural analysis could be isolated (vide infra).
a)
Table 3. Selected NMR Parameters of zwitterionic intermediates 11
.
δ1H
δ13
R12Sn
C
δ19
C6F5
F
δ11B (h )
δ119Sn (h )
½
½
=C–H
ϵC–H
B(C6F5)2
11a
6.28 br [244.3]
2.57 [7.1]
21.9 [276.6] (Sn-CH2), –141.4 m (o), –157.0 –130.8 m (o) –158.0 t –15.4 (126.0) 225.8 (125.0)
26.9 [38.1] (CH2),
29.6 [72.8] (CH2)
t ʈ21.7ʈ (p), –163.8 dt ʈ20.7ʈ (p), –164.9 dt
ʈ7.7ʈ ʈ21.7ʈ (m) ʈ8.7ʈ ʈ20.7ʈ (m)
b)
c)
11b
11c
11d
6.30 br [220.1]
6.26 br [242.4]
–
2.12 [9.9]
2.34 [11.5]
–
21.8 [306.1] (Sn-CH2), –141.1 m (o), –157.3 –130.8 m (o), –158.3 t –15.3 (116.0)
27.7 [20.6] (Sn-CH2– t ʈ21.9ʈ (p), –163.8 dt ʈ21.2ʈ (p), –164.9 dt
241.5 (120.0)
CH2), 26.8 [68.8]
(CH2), 13.4 (CH3)
134.4 [n.b.] (C-i),
137.1 [41.7] (C-o),
129.8 [59.1] (C-m),
130.6 [12.8] (C-p)
ʈ7.2ʈ ʈ21.9ʈ (m)
ʈ8.1ʈ ʈ21.2ʈ (m)
–140.5 m (o), –157.1 –130.5 m (o) –158.3 t –15.2 (203.0) 68.9 (110.0)
t ʈ21.5ʈ (p), –164.0 dt ʈ21.2ʈ (p) –165.1 dt
ʈ7.5ʈ ʈ21.5ʈ (m)
ʈ8.3ʈ ʈ20.9ʈ (m)
d)
21.1 [279.7] (Sn-CH2), –140.2 m (o), –157.8 –130.3 m (o), –159.6 t –12.4 (141.0) 240.3 (44.3)
28.0 [21.7] (Sn-CH2– t ʈ21.3ʈ (p), –164.1 m ʈ20.7ʈ (p), –165.6 dt
CH2), 27.0 [67.0]
(CH2), 13.4 (CH3)
(m)
ʈ8.3ʈ ʈ20.7ʈ (m)
n
a) In C6D6 at 296 K, line widths h in Hz in parentheses, coupling constants J(119Sn,X) (X = 19F,13C,1H) in Hz in brackets, coupling constants
½
nJ(19F,13C) and nJ(19F,19F) in Hz in ʈ ʈ; (br) denotes broad 13C NMR signals for carbon atoms linked to boron. b) Other 13C NMR spectroscopic
data: 100.8 (br., B–Cϵ), 118.4 [48.9] (ϵC–Bu), 122.2 [55.8] (ϵC–H), 153.8 [603.6] (Sn–CH=). c) Other 13C NMR spectroscopic data: 122.2
[55.8] (ϵC–H), 153.8 [603.6] (Sn–CH=);. d) Other 13C NMR spectroscopic data: 12.9 (CH3), 13.7 (CH3), 20.6 (CH2), 21.4 (CH2), 22.5 [6.3]
(CH2), 30.0 (CH2), 34.8 [32.7] (CH2), 36.1 [96.4] (CH2),100.8 (br., B–Cϵ), 118.4 [48.9] (ϵC–Bu), 119.0 (t, ʈ21.5ʈ], (i), C6F5), 120.6 (br., (i),
B–C6F5), 155.3 [490.5] (Sn–C=), 154.9 (br., =C–B).
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1,4-Stannabora-cyclohexa-2,5-dienes and Their Zwitterionic Intermediates
Figure 3. 399.8 MHz 1H NMR spectrum of the zwitterionic intermedi-
ate 11c (in C6D6 at 296 K). 117/119Sn satellites are marked by asterisks
n
and coupling constants J(119Sn,1H) in Hz are given in brackets.
Figure 4. 100.53 MHz 13C{1H} NMR spectrum of the intermediate
X-ray Structural Analyses of the Zwitterionic Intermediate
11a and the 1,4-Stannabora-cyclohexa-2,5-diene Derivative
12a
11d (the region for unsaturated carbon atoms is shown; in C6D6 at
296 K). 117/119Sn satellites are marked by asterisks and coupling con-
stants nJ(119Sn,13C) in Hz are given in brackets. Note the broadened
13C NMR signals for 13C nuclei linked diretly to boron owingt to paart-
ially relaxed 13C-11B spin-spin coupling.[20]
The molecular structures of 11a and 12a are shown in the
Figure 6 and Figure 8, respectively. Intermolecular interactions
appear to be negligible. In contrast with transition metals, there dinated side-on (η2) to a group-14 metal.[14–16] To the best of
are only few examples known, where an alkynyl group is coor- our knowledge, 11a is the first example with side-on coordina-
Figure 5. 100.5 MHz 13C{1H} NMR spectrum (right) of the 1,4-stannabora-cyclohexa-2,5-diene derivative 12b (in C6D6 at 296 K; shown is the
region for olefinic and aromatic carbon atoms). 117/119Sn-satellites are marked by asterisks, and coupling constants nJ(119Sn,13C) in Hz are given
1
1
2
3
in brackets. The results of two-dimensional (2D) H-13C correlations (HSQC[26]) based on J(13C,1H) (A), J(13C,1H) (B), and J(13C,1H) (C)
are shown on the left side. The 117/119Sn satellites of the cross peaks are connected by dotted lines to indicate the positive tilt in all three cases.
This proves[27] that the signs of J(119Sn,1H) and of J(119Sn,13C) are alike for n = 1 (A), 2 (B), 3 (C). Since for vinyl tin compounds the sign
2
n
of J(119Sn,1H) Ͻ 0 is known,[28] the signs of J(119Sn,13C) were determined here as Ͻ 0.
2
n
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1209

B. Wrackmeyer et al.
ARTICLE
Figure 7. Some calculated non-bonding C–C distances for optimized
geometries of zwitterionic intermediates [Si-1, Si-2: B3LYP/6-
311+G(d,p); Sn-1: B3LYP/LanL2DZ].
Figure 6. Molecular structure of the zwitterionic intermediate 11a
(ORTEP, 50% probability, hydrogen atoms and solvent molecules are
omitted for clarity). Selected bond lengths /pm and angles /°: Sn1–C1
211.0(4), Sn1–C20 252.0(3), Sn1–C21 252.7(4), Sn1–C15 212.2(4),
Sn1–C19 213.0(4), B1–C2 163.7(5), B1–C20 161.9(5), B1–C9
164.2(5), B1–C22 164.9(6), C1–C2 133.1(5), C20–C21 120.3(6), C2–
C3 150.8(5), C1–Sn1–C20 78.04(14), C1–Sn1–C21 105.06(4), C20–
Sn1–C21 27.59(13), C1–Sn1–C15 124.80(16), C1–Sn1–C19
125.65(17), C15–Sn1–C19 103.90(17), Sn1–C1–C2 120.02(3), B1–
C2–C1 124.4(3), C1–C2–C3 116.4(3), B1–C20–C21 175.6(4), C2–
B1–C20 108.1(3), Sn1–C20–B1 106.2(2), C21–Sn1–C15 95.52(15),
C20–Sn1–C19 111.91(15), C21–Sn1–C19 91.33(15), C20–Sn1–C15
106.84(15).
tion of an ethynyl group to tin. Similarly, 12a appears to be
the first example of a structurally characterized 1,4-stannabora-
cyclohexa-2,5-diene.
The surroundings of the tin atom in 11a are pyramidal, al-
though not far from trigonal planar (sum of bond angles at tin:
354.4°), as one would expect for a “free” triorgantin cation.
The bond lengths Sn–C1, Sn–C15, Sn–C19 (about 211 pm) are
slightly shorter than Sn–C bonds in tetraorganotin compounds
(see, for example 12a), whereas for the side-on coordination
the bond lengths Sn–C20 (252.0 pm) and Sn–C21 (252.7 pm)
are elongated and almost identical. In comparable situations
marked differences have been observed.[14–16] Both the C1=C2
and the C20ϵC21 bond lengths are in the expected range, ap-
parently unaffected by the neighborhood to positively charged
tin, the negatively charged borate unit, and side-on coordina-
tion. The surroundings of the boron atom in 11a correspond to
Figure 8. Molecular structure of 12a (ORTEP, 40% probability; hy-
drogen atoms are omitted for clarity), and a side view of the 1,4-
stannabora-cyclohexa-2,5-diene. Selected bond lengths /pm and
angles /°: Sn1–C1 213.3(4), Sn1–C9 213.0(5), Sn1–C21 213.6(4),
Sn1–C17 214.6(4), C1–C2 135.2(6), C2–B1 156.2(7), C9–C10
135.4(6), C10–B1 155.2(7), B1–C22 160.8(6), C2–C3 149.9(6), C10–
C11 149.6(6), C1–Sn1–C9 98.16(17), C9–Sn1–C21 107.89(19), C9–
Sn1–C17 112.11(17), C1–Sn1–C17 117.84(18), C1–Sn1–C21
122.33(18), Sn1–C1–C2 120.4(4), Sn1–C9–C10 122.8(4), B1–C2–C1
126.5(4), B1–C10–C9 124.9(4), C2–B1–C10 127.0(4), C2–B1–C22
115.6(4), C10–B1–C22 117.3(4), C3–C2–B1 116.5 (4), C11–C10–B1
118.9(4), C3–C2–C1 117.0(4), C11–C10–C9 117.0(4).
a slightly distorted tetrahedron, and the B–C distances are in (Figure 7; presumably the precursor of stannole 1) show a dif-
the usual range for tetra-coordinate boron. ferent pattern for the distances between carbon atoms in the
Since 11a rearranges finally into 12a by transfer of one of surroundings of the boron atom, fitting to the formation of
the C6F5 groups (C9, C22) from boron to the neighboring posi- stannole 1. Similar calculations carried out on a higher level of
tively charged alkynyl carbon atom C20, the comparison be- theory [B3LYP/6-311+G(d,p)[30,31]] for the intermediates Si-1
tween the non-bonding distances C2–C20 (263.4 pm), C22– and Si-2 reveal a situation comparable with that in Sn-1 (Fig-
C20 (252.8 pm), and C9–C20 (265.9 pm) is meaningful. The ure 7), in particular for the aforementioned non-bonding dis-
shorter distance for C22–C20 when compared with C2–C20 tances, This is in agreement with the preferred formation of
may be one reason for preferred transfer of the C6F5 group. siloles via 1,1-carboboration of dialkynylsilanes, independent
The experimental geometry and all bond lengths for 11a are of the nature of the triorganoborane.
reasonably well reproduced by calculations (B3LYP/
The surroundings of the tin atom in 12a correspond to a
LANL2DZ [29]) including the intramolecular non-bonding dis- distorted tetrahedron, and those of the boron atom are trigonal
tances. Calculations for the non-detected intermediate Sn-1 planar, within the experimental error (Figure 8). The arrange-
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1,4-Stannabora-cyclohexa-2,5-dienes and Their Zwitterionic Intermediates
30 min at room temp., all volatile materials were removed in a vacuum.
ment of the 1,4-stannabora-cyclohexa-2,5-diene deviates from
planarity, with the tin and the boron atom shifted slightly into
the same direction (Sn: 7.5 pm, B: 11.7 pm) out of the best
plane formed by the olefinic carbon atoms C1,C2,C9,C10. The
bond lengths Sn–C between 213.0 pm and 214.6 pm are in the
typical range for tetraorganotin compounds. The endocyclic
bond lengths B–C (B–C2 156.2 pm, B–C10 155.2 pm ) are
markedly shorter than the exocyclic one (B–C22 160.8 pm),
all being shorter than in 11a.
NMR spectra of the yellow to brown oily products showed the quanti-
tative formation of the stannoles 4 and 5 [Ͼ 97% pure (NMR)]. Fur-
ther attempts at purification by chromatography on silica or various
types of Al2O3 led to decomposition, as was found previously for sim-
ilar stannoles.
4a: δ 1H ([D8]toluene, [J(119Sn,1H)]) = 0.86, 1.18 (t, 6 H, 4 H, q, BEt2),
0.94, 2.14 (t, 3 H, 2 H, m, 4-Et), 1.09 [52.6] (m, 4 H, SnCH2), 1.74
[82.3] (m, 4 H, CH2), 1.31 (m, 2 H, CH2),, 6.04 [164.7] (s, 1 H, H-2),
6.02 [162.6] (s, 1 H, H-5). 4b: δ 1H ([D8]toluene, [J(119Sn,1H)]) = 0.81
(t, 6 H, CH3), 0.89 (4 H, CH2), 0.96 (6 H,), 1.03 (m, 7 H), 1.26 (m, 8
H), 2.23 (2 H, q), 6.07 [151.0] (s, 1 H, H-2), 6.13 [154.2] (s, 1 H, H-
5). 4c: δ 1H (CDCl3, [J(119Sn,1H)]) = 1.26, 0.88 (q, 10 H, t, BEt2),
2.21, 1.01 (q, 5 H, t, 4-Et), 6.04 [163.5] (s, 1 H, H-5), 6.11 [162.2] (s,
1 H, H-2), 7.15–7.27 (m, 6 H, SnPh2), 7.42–7.50 (m, 4 H, SnPh2).
Conclusions
The strongly electrophilic triorganoborane B(C6F5)3 exerts
a remarkable influence on the final products of 1,1-carbobor-
ation of dialkynyltin compounds, when compared with si-
lanes.[10,11] In contrast with weakly Lewis-acidic triethylbor-
ane BEt3 or Et-9-BBN, the reactions of B(C6F5)3 with all di-
alkynyltin compounds studied herein afford selectively 1,4-
stannabora-cyclohexa-2,5-dienes. The proposed mechanism of
1,1-carboboration reactions[14–16] is further supported by iden-
tification and structural characterization of zwitterionic inter-
mediates, and the molecular structure of a 1,4-stannabora-cy-
clohexa-2,5-diene could be established for the first time.
5a: δ 1H (CDCl3, [J(119Sn,1H)]) = No assignment in the range of alkyl
groups because of severe overlap; 6.13 [161.5] (s, 1 H, H-2), 6.43
[162.1] (s, 1 H, H-5). 5b: δ 1H (CDCl3, [J(119Sn,1H)]) = 0.91 (t, 6 H,
CH3), 1.07 (3 H, 3 H, CH3 1.20 (t, 4 H, CH2, 7.6 Hz), 1.29–1.48 (m,
8 H, CH2), 1.50–1.73 (m, CH, CH2, BBN), 1.92–2.11 (m, CH2, BBN),
3.22 (1 H, qt, CH, BBN), 5.93 [154.0] (s, 1 H, H-5), 6.78 [153.5] (s,
1 H, H-2). 5c: δ 1H (CDCl3, [J(119Sn,1H)]) = 0.89, 1.24 (t, 3 H, 2 H,
q, BEt), 1.36–1.51 (m, CH, CH2, BBN), 1.76–1.91 (m, CH2, BBN),
3.10 (1 H, qt, CH, BBN), 5.86 [164.4] (s, 1 H, H-5), 6.65 [163.3] (s,
1 H, H-2), 7.17–7.25 (m, 6 H, SnPh2), 7.33–7.59 (m, 4 H, SnPh2).
Experimental Section
1,1-Ethyloboration of Dibutyl-bis(hexyn-1-yl)tin (1:1 Reaction):
Triethylborane (58.4 mg, 86 μL, 0.5954 mmol) was added through a
syringe to a cooled (–78 °C) solution of 3d (235.5 mg, 0.5954 mmol)
in hexane (20 mL), This mixture was warmed to room temp. Most of
the hexane was removed in a vacuum, and C6D6 (1.5 mL) was added
for preparing of NMR samples (see Figure 1). Finally after 2 d, a mix-
ture of the stannole 7d and the 1,4-stannabora-cyclohexa-2,5-diene 8d
was formed (see Figure 1, Figure 2, and Table 2, Table 3). 1H NMR
spectra show severe overlap of all signals, and are not assigned.
General and Starting Materials: All preparative work as well as
handling of the samples was carried out observing precautions to ex-
clude traces of air and moisture. Carefully dried solvents and oven-
dried glassware were used throughout. Diphenyltin dichloride, dimeth-
yltin dichloride, dibutyltin dichloride, triethylborane, and ethynylmag-
nesium bromide in THF were commercially available and used as re-
ceived. Diethynyl(dimethyl)stannane was prepared as described,[32]
and the synthesis of 3a, 3b, and 3d was carried out in the same way,
using the commercially available diorganotin dichlorides or
(CH2)5SnBr2.[33] 3d was prepared from nBuSnCl2 and two equivalents
of LiCϵC-nBu.[34] NMR measurements in CDCl3, C6D6, and [D3]Tol
(concentration ca. 5–10%) with samples in 5 mm tubes at 23Ϯ1 °C:
Varian Inova 400 MHz spectrometer for 1H, 11B, 13C, and 119Sn NMR;
chemical shifts are given relative to Me4Si [δ1H (CHCl3) = 7.24; δ13C
(CDCl3) = 77.0; δ1H (C6HD5) = 7.15; δ13C (C6D6) = 128.0; δ1H
(C6D5CHD2) = 2.03; δ13C (C6D5CD3) = 20.4; external Me4Sn [δ119Sn
= 0 for Ξ(119Sn) = 37.2906 MHz]; external CFCl3 [δ19F = 0 for Ξ(19F)
1,1-Phenyloboration of Dialkynyltin Compounds: To a stirred solu-
tion of 3a or 3b (1.6534 mmol) in hexane (10 mL), cooled to –78 °C,
a solution of BPh3 in toluene (0.1637 m, 10.1 mL) was added in one
portion. The colorless mixture was warmed to room temp., turns yel-
low, and all volatile materials were removed in a vacuum to leave a
red-brown oil, containing stannoles 9a or 9b and the 1,4-stannabora-
cyclohexa-2,5-dienes 10a or 10b, accompanied by other unidentified
side products.
94.0940 MHz; external BF3–OEt2 [δ11B for Ξ(11B)
= 0 =
δ 119Sn (CDCl3) = 9a: –23.3; 9b: 24.1; 10a: –152.6; 10b: –113.7; δ
11B (CDCl3) = 9a/10a: 68.2; 9b/10b: 67.1
=
32.0840 MHz]. Chemical shifts are given to Ϯ0.1 ppm for 13C and
119Sn, and Ϯ0.4 ppm for 11B; coupling constants are given Ϯ0.4 Hz
for J(119Sn,13C). 119Sn NMR spectra were measured directly by single
pulse methods or by using the refocused INEPT pulse sequence,[35]
based on 2J(119Sn,1H) (50 –100 Hz) after optimizing the delay times
in the pulse sequence. Melting points (uncorrected) were determined
with a Büchi 510 melting point apparatus. All quantum chemical cal-
culations were carried out using the Gaussian 09 program package.[36]
The analogous phenyloboration of 3d gave an orange-yellow oil,
which contained mainly 10d, accompanied by small amounts (Ͻ10%)
of side products, without a stannole.
10d: δ 1H (CDCl3, [J(119Sn,1H)]) = 0.97 (t, 6 H, CH3), 1.03 (t, 6 H,
CH3,), 1.29 (t, 4 H, CH2), 1.40 (q, 4 H, CH2), 1.45–1.81 (m, 12 H,
CH2), 2.52 [57.7] (t, 4 H, 2,5-CH2,), 6.72–7.36 (m, 15 H, Ph).
1,1-Carboboration of Diethynyltin Compounds using Trialkylbor-
anes (BEt3, Et-9-BBN): The synthesis of 1[12] and 2[13] were repro-
duced, and similar conditions were used for the reactions of 3a–c. A
solution of the diethynyltin compound (1.1 mmol) in hexane (20 mL)
1,1-Carboboration of Dialkynyltin Compounds with B(C6F5)3: A
solution of B(C6F5)3 in toluene (0.01875 m, 6 mL) was kept stirring at
–78 °C, and a solution of 3b in hexane (0.1836 m, 510 μL) was added
was cooled to –78 °C. After adding the equimolar amount of the re- through a syringe. The mixture was stirred for 30 min at –78 °C,
spective trialkylborane through a syringe, the stirred colorless mixture warmed to room temp., and stirred for further 30 min. All volatile
was warmed to room temp., by which a yellow color developed. After
materials were removed in a vacuum to leave a yellow solid. This
Z. Anorg. Allg. Chem. 2013, 1205–1213
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B. Wrackmeyer et al.
ARTICLE
a)
material was completely soluble in C6D6 for NMR spectroscopic char-
acterization. This revealed the presence of the zwitterionic intermedi-
ate 11b, which after 16 h rearranged into the 1,4-stannabora-cy-
clohexa-2,5-diene 12b [Ͼ 90% pure (NMR)]. Further prurification
(excpet of 12a by crystallization) by chromatography on silica or
Al2O3 was not succeesful, and, therefore, elemental analyses were not
carried out.
Table 4. Crystallographic data of the zwitterionic intermediate 11a
b)
and the 1,4-stannabora-cyclohexa-2,5-diene 12a.
11a
12a
Formula
C75H49B2F30Sn2
colorless plates
0.42ϫ0.41ϫ0.34 0.64ϫ0.10ϫ0.08
C27H12BF15Sn
Crystal
colorless needles
Dimensions /mm3
Temperature /K
Crystal system
Space group
Lattice parameters
a /pm
b /pm
c /pm
α /°
133
133
monoclinic
P21/c
triclinic
11a, 12a, 11c, 12c, and 11d, 12d were obtained in the same way.
Suitable crystals of 11a (m.p. Ͼ35 °C; rearrangement into 12a) were
grown from a concentrated solution in toluene at –26 °C, whereas crys-
tals of 12a (m.p. 110 °C, decomposition) were isolated from a satu-
rated solution in C6D6 at room temp. In all cases, NMR measurements
indicated quantitative reactions, and the products were yellow solids.
¯
P1
1146.50(7)
1176.90(7)
1386.30(9)
67.414(5)
85.488(5)
89.382(5)
1
1096.70(6)
2141.60(11)
1137.30(6)
90
β /°
γ /°
105.535(4)
90
11a; δ 1H (CDCl3, [nJ(119Sn,1H)] in Hz) = 0.60–1.70 (m, 10 H), 2.50
[6.8] (s, 1 H), 6.22 [243.8] (s, 1 H, br); 12a: δ1H (CDCl3,
[nJ(119Sn,1H]) = 0.81–2.05 (m, 10 H), 7.94 [93.8] (s, 2 H). 11b: δ 1H
(CDCl3, [nJ(119Sn,1H)]) = 0.83 (t, 6 H, CH3, 7.2 Hz), 0.98 (t, 4 H,
CH2), 1.05–1.16 (m, 4 H, CH2), 1.22–1.32 (m, 4 H, CH2), 2.12 [9.9]
(s, 1 H), 6.30 [220.1] (s, 1 H, br); 12b: δ 1H (CDCl3, [nJ(119Sn,1H)])
= 0.89 (t, 6 H, CH3), 1.01 (t, 4 H, CH2), 1.19–1.31 (m, 4 H, CH2),
Z
4
Absorption coefficient μ,
0.852
1.120
/mm–1
Measuring range (ϑ) /deg
Reflections collected
Independent reflections
[I Ն 2σ(I)]
Absorption correction
Refined parameters
1.60–25.66
21493
5380
1.50–25.69
9178
3179
1
1.41–1.52 (m, 4 H, CH2), 7.91 [88.7] (s, 2 H); 11c: δ H (CDCl3,
a)
c)
no
numerical
397
[nJ(119Sn,1H)]) = 2.34 [11.5] (s, 1 H), 6.26 [242.4] (s, 1 H, br), 7.22–
7.28 (m, 1 H, Ph), 7.38–7.43 (m, 4 H, Ph). 12c: δ 1H (CDCl3,
[nJ(119Sn,1H)]) = 6.85–7.60 (m, 10 H, Ph), 7.68 [88.7] (s, 2 H).
486
wR2/R1 [I Ն 2σ(I)]
0.0393/ 0.1034
0.0349/ 0.0750
1.114/–0.497
Max./min. residual electron 1.297/–0.790
density /e·pm–3·10–6
Crystal Structure Determinations of 11a and 12a: Details pertinent
to the crystal structure determinations are listed in Table 4.[37] Crystals
of appropriate size were selected (in perfluorinated oil at room
temp.[38]), and the data collections were carried out at 133 K using
a STOE IPDS II system equipped with an Oxford Cryostream low-
temperature unit. Structure solutions and refinements were ac-
complished using SIR97,[39] SHELXL-97,[40] and WinGX.[41]
a) Diffractometer: STOE IPDS II, Mo-Kα, λ = 71.073 pm, graphite
monochromator. b) Crystallizes with 1.5 molecules of toluene per for-
mula unit. c) Absorption corrections did not improve the parameter
set.
[9] R. Köster, G. Seidel, J. Süß, B. Wrackmeyer, Chem. Ber. 1993,
126, 1107.
[10] a) B. Wrackmeyer, O. L. Tok, A. Khan, A. Badshah, Z. Natur-
forsch. B 2005, 60, 251; b) E. Khan, S. Bayer, R. Kempe, B.
Wrackmeyer, Eur. J. Inorg. Chem. 2009, 4416.
Acknowledgements
[11] G. Dierker, J. Ugolotti, G. Kehr, R. Fröhlich, G. Erker, Adv. Synth.
Catal. 2009, 351, 1080.
[12] L. Killian, B. Wrackmeyer, J. Organomet. Chem. 1977, 132, 213.
[13] B. Wrackmeyer, U. Klaus, W. Milius, E. Klaus, T. Schaller, J.
Organomet. Chem. 1996, 517, 235.
This work was supported by the Deutsche Forschungsgemeinschaft.
We thank Prof. G. Erker and his group for a sample of pure
B(C6F5)3, and the late Prof. R. Köster for samples of Et-9-BBN and
BPh3.
[14] B. Wrackmeyer, K. Horchler, R. Boese, Angew. Chem. 1989, 101,
1563; Angew. Chem. Int. Ed. Engl. 1989, 28, 1500.
[15] B. Wrackmeyer, G. Kehr, R. Boese, Angew. Chem. 1991, 103,
1374; Angew. Chem. Int. Ed. Engl. 1991, 30, 1370.
[16] a) B. Wrackmeyer, S. Kundler, R. Boese, Chem. Ber. 1993, 126,
1361–1370; b) B. Wrackmeyer, S. Kundler, W. Milius, R. Boese,
Chem. Ber. 1994, 127, 333.
[17] a) J. D. Kennedy, W. McFarlane, Rev. Silicon Germanium Tin
Lead Compd. 1974, 1, 235; b) J. D. Kennedy, W. McFarlane, in
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New York 1987, pp. 305–333.
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Received: January 6, 2013
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scal-
Published Online: March 14, 2013
Z. Anorg. Allg. Chem. 2013, 1205–1213
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