Phenylboration of monoalkyn-1-yltin compounds

Article abstract of DOI:10.1515/znb-2007-1207

The 1: 1 reactions of triphenylborane 1 with monoalkyn-1-yltin compounds Me₃Sn-C≡C-R¹ 2 [R¹ = ¹Bu (a), Ph (b), ferrocenyl (c), Si(H)Me₂ (d), SnMe₃ (e)] afford mainly (> 80%) the corresponding alkene derivatives 3 by 1,1-phenylboration. Exchange B-Ph/Sn-C≡C-R¹ takes place as a side reaction. The corresponding 1: 2 reaction with 2b leads to the dialkenylborane 4b (R ¹ = Ph), of which the molecular structure could be determined by X-ray analysis. In contrast, the 1:2 reaction with 2e gave an aliene derivative 5e. The solution-state structures of compounds 3-5 have been confirmed by ¹H, ¹¹B, ¹³C and ¹¹⁹Sn NMR spectroscopy.

Full text of DOI:10.1515/znb-2007-1207

Phenylboration of Monoalkyn-1-yltin Compounds
Bernd Wrackmeyer, Oleg L. Tok, and Wolfgang Milius
Laboratorium fu¨r Anorganische Chemie, Universita¨t Bayreuth, D-95440 Bayreuth, Germany
Reprint requests to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 2007, 62b, 1509 – 1513; received August 22, 2007
The 1 : 1 reactions of triphenylborane 1 with monoalkyn-1-yltin compounds Me3Sn–C≡C–R1 2
[R1 = tBu (a), Ph (b), ferrocenyl (c), Si(H)Me2 (d), SnMe3 (e)] afford mainly (> 80 %) the corre-
sponding alkene derivatives 3 by 1,1-phenylboration. Exchange B-Ph/Sn–C≡C–R1 takes place as a
side reaction. The corresponding 1 : 2 reaction with 2b leads to the dialkenylborane 4b (R1 = Ph), of
which the molecular structure could be determined by X-ray analysis. In contrast, the 1 : 2 reaction
with 2e gave an allene derivative 5e. The solution-state structures of compounds 3 – 5 have been con-
firmed by 1H, 11B, 13C and 119Sn NMR spectroscopy.
Key words: Triphenylborane, Organoboration, Alkynes, Organotin, NMR, X-Ray
Introduction
LnM–C≡C–R1. Moreover, considering the intermedi-
ate A(1), other reaction pathways are conceivable, de-
pending mainly on R, e. g. exchange reactions leading
to R–MLn and alkyn-1-ylboranes R2B–C≡C–R1. In
this work, we report on the reaction of triphenylborane
1 with several monoalkyn-1-yltin compounds 2, car-
ried out in a 1 : 1 or 1 : 2 ratio.
1,1-Alkylboration of various alkyn-1-yl metal com-
pounds as a versatile method for forming new carbon-
carbon bonds has been extensively studied [1, 2]. The
products are attractive for various catalysed carbon-
carbon coupling reactions, making use e. g. of the
Sn–C(sp2) [3, 4] or the B–C(sp2) bond [5]. The
1,1-alkylboration proceeds by cleavage of the M–C
(alkyne) bond via an alkyn-1-ylborate-like intermedi-
ate A(1) [1] towards an alkene B, in which usually
the boryl group and the metal fragment are in cis
positions at the C=C bond [1] (Scheme 1). In con-
trast with numerous trialkylboranes, triphenylborane 1
has not been studied in a systematic way in this con-
text. Except for qualitative experiments with alkyn-1-
yltin compounds [1], this borane 1 has been used only
occasionally in reactions with some alkyn-1-ylsilanes
[6 – 8].
Scheme 2. 1,1-Phenylboration of monoalkyn-1-yltin com-
pounds 2.
Results and Discussion
Reaction of triphenylborane 1 with monoalkyn-1-yltin
compounds 2 (1 : 1 ratio)
In all cases studied, the reaction shown in Scheme 2
proceeds mainly to the alkenes 3. The formation of
3, monitored by 119Sn NMR spectroscopy, was ac-
companied by side reactions (ca. 10 – 15 %). Since
Me3Sn–Ph (δ119Sn = −28.4 [9]) was identified as one
of the side products from the beginning in the reac-
tion solutions, Ph/C≡C–R1 exchange competes with
the 1,1-phenylboration. The alkyn-1-ylborane Ph2B–
C≡C–R1 could not be detected, since it is probably
even more reactive towards 2 than BPh3, and therefore
Scheme 1. 1,1-Alkylboration of alkyn-1-ylmetal compounds.
It is known that the formation of alkenes of the type
B is reversible, depending on the substituents R and
R1 [1]. Since B is also a triorganoborane, it may com-
pete, once it is formed, with BR3 in the reaction with
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B. Wrackmeyer et al. · Phenylboration of Monoalkyn-1-yltin Compounds
Scheme 3. Reactions of triphenylborane
1 with two equivalents of 2b and 2e.
Fig.
1.
186.5
MHz
119Sn{ H} NMR spec-
trum (refocused INEPT,
based on 2J(119Sn, 1H) =
55 Hz) of the alkene deriva-
tive 3e showing 117Sn
(arrows) and 119Sn (aster-
isks) satellites. In contrast
to the 117Sn satellites (AX
spin system), only the inner
1
lines of the 119Sn satellites
(AB spin system) are visible.
Note the broad and sharper
lines for Sn(1) and Sn(2),
respectively, owing to trans
and cis positions with respect
to the 11B nucleus.
may be the source of numerous other side products. wise or in one reaction. Two major reaction path-
After several hours unassigned 119Sn NMR signals be- ways are conceivable, considering again an alkyn-1-
gan to grow. The results for the reaction of 1 with 2d ylborate-like intermediate A(2) as shown in Scheme 3.
were reported previously [8]. In the case of 3e, some of Migration of one of the remaining B-phenyl groups
the new growing 119Sn NMR signals were in the region from boron to the neighbouring alkynyl-carbon atom
typical of allenyltin compounds [10 – 12] (vide infra).
in A(2) affords the dialkenylborane 4b (R1 = Ph). This
compound crystallised readily from the reaction mix-
ture, and its molecular structure could be determined
by X-ray analysis (vide infra). The analogous reac-
tion starting from triethylborane and two equivalents
of trimethyl(propyn-1-yl)tin led to a substituted borol-
3-ene [13]. A dialkenylborane, comparable with 4b,
Reaction of triphenylborane 1 with monoalkyn-1-yltin
compounds 2 (1 : 2 ratio)
The alkenylboranes 3 are likely to react with a sec-
ond equivalent of 2, and this can be achieved step-
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B. Wrackmeyer et al. · Phenylboration of Monoalkyn-1-yltin Compounds
1511
1
Fig. 2. Parts of the 125.8 MHz 13C{ H} NMR spectrum of the allene derivative 5e (the crowded region for the phenyl groups
is not shown). The expansions for the 13C(1-4) signals show the 117/119Sn satellites corresponding to the respective coupling
constants.
Table 1. 119Sn, 11B and 13C NMR dataa for the alkene deriva-
ing alkynyl-carbon atom leading to a buta-1,3-diene
derivative as shown. Such boranes are known to un-
dergo a fast and in these cases irreversible allylic rear-
rangement [14] to give organometallically substituted
allenes [10 – 12] such as 5e.
tives 3a – e, 4b (Schemes 2, 3).
13
13
δ
C
,
δ
C
,
(=C−Sn)
(=C−B)
δ
119Sn δ11B [1J(119Sn, 13C), Hz]
[2J(119Sn, 13C), Hz]
160.4 (br)
162.0 (br) [61.0]
161.9 [65.4]
3ab −56.9 64.6 163.2 [514.8]
3bc −41.3 66.4 153.1 [455.2]
3cd −52.4 66.1 149.9 [525.2]
3de −47.0 71.0 153.4 [323.3],
NMR spectroscopic studies
182.3 (br) [29.3]
[1J(29Si, 13C) = 52.6 Hz]
The proposed solution-state structures of com-
pounds 3, 4 and 5 are consistent with the set of
NMR data (Table 1 and Experimental Section). The
11B NMR signals are rather broad, typical of phenyl-
boranes, with characteristic δ11B values [15]. The
3ef −46.5 65.0 159.3 [314.9], [274.9]
184.1 (br)
160.0 (br) [57.1]
4bg −38.7 67.3 156.3 [454.9]
In C6D6 at 296 K; coupling constants J(119Sn,13C) are given
a
in brackets [ 0.3 Hz]; (br) denotes broad 13C NMR signals ow-
ing to partially relaxed scalar 13C-11B spin-spin coupling;
other
b
13C NMR data: δ [J(119Sn, 13C) = −2.0 [317.9] (Me3Sn); 34.1 olefinic 13C NMR signals of 3 and 4 are broad (B–C=;
[23.5], 43.0 [51.8] (t Bu); 126.3, 128.2, 131.0 [7.6], 144.6 [106.4]
scalar relaxation of the second kind [16]), and sharp
c
(Ph); 128.4, 132.1, 139.6, 141.8 (br) (BPh2);
other 13C NMR
(Sn–C=), and the latter are accompanied by 117/119Sn
data: δ [J(119Sn, 13C)] = −3.7 [327.4] (Me3Sn); 68.6, 70.9 [26.2],
1
satellites corresponding to J(Sn, 13C) [9, 17]. Other
91.2 [51.6] (C5H4); 69.4 (C5H5); 126.0, 128.6, 129.9, 145.9 [88.6]
n
satellites for J(Sn, 13C) (n > 1) support the assign-
d
(Ph); 128.0, 132.0, 139.3, 141.8 (br) (BPh2);
other 13C NMR
data: δ [J(119Sn, 13C)] = −6.7 [335.5] (Me3Sn); 125.2, 126.0, 128.2,
ments. The 119Sn NMR signals for 3 are broadened as
a result of partially relaxed three-bond 119Sn-11B spin-
spin coupling. This is most particularly evident for 3e,
where the stannyl groups are in trans and cis positions
with respect to the boryl group (Fig. 1). The greater
line width of one of the 119Sn NMR signals indicates
the trans position of this Me3Sn group relative to the
128.4, 128.6, 130.2, 143.1 [79.2], 146.8 [37.4] (Ph); 128.5, 132.8,
e
139.6, 141.9 (br) (BPh2);
other 13C and 29Si NMR data, ref.
[8]; f other 13C NMR data: δ [J(119Sn, 13C)] = −5.0 [314.9], −4.9
[318.7] (Me3Sn); 127.4, 128.5, 129.3, 149.9 [128.6] [86.9] (Ph);
128.6, 132.7, 139.7, 141.6 (br) (BPh2); other 119Sn NMR data: δ =
−45.7 (2J(119Sn, 117Sn) = 774.6 Hz);
other 13C NMR data:
g
δ [J(119Sn, 13C)] = −7.6 [338.5] (Me3Sn); 124.9 [8.0], 128.2, 142.2,
[76.3], 147.2 [36.8] (Ph); 128.8, 130.6, 141.0, 141.2 (br) (BPh2).
3
3
boryl group, since | J(119Sn, 11B)trans| > | J(119Sn,
11B)cis| [9, 17]. In the case of 4b, the data correspond to
structural features determined for the solid-state struc-
ture (vide infra).
although not detected, was considered as an interme-
diate [13]. Heating of 4b for prolonged times at 110 ◦C
in toluene caused decomposition instead of rearrange-
ment into a borol-3-ene.
The allene derivative 5e provides a large set of 13C
Alternatively (R1= SnMe3), the B-alkenyl group and 119Sn NMR data, all of which support the proposed
may be transferred from the boron to the neighbour- structure. The 13C NMR spectrum (Fig. 2) shows the
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1512
B. Wrackmeyer et al. · Phenylboration of Monoalkyn-1-yltin Compounds
imises steric interactions between phenyl groups and
Me3Sn groups.
Experimental Section
General
The preparative work and the handling of samples were
carried out observing necessary precautions to exclude traces
of air and moisture. Carefully dried solvents and oven-dried
glassware were used throughout. Triphenylborane 1 [18]
and the alkyn-1-yltin compounds (2a [19], 2b [20], 2c [21],
2e [20]) were prepared following literature procedures. NMR
measurements (5 mm o. d. tubes, 23 ◦C; ca. 5 – 10 % in
1
C6D6): Bruker ARX 250 and DRX 500: H, 11B, 13C, and
119Sn NMR (refocused INEPT [22] based on 2J(119Sn,1H) =
55 Hz); Varian INOVA 400: H, 13C NMR; chemical shifts
1
Fig. 3. Molecular structure of the dialkenylborane 4b (ball
and stick model; hydrogen atoms omitted for clarity).
Selected bond lengths (pm) and angles (◦): B–C1 157.4(4),
B–C15 156.2(6), C1–C2 134.4(4), C1–C3 150.4(5),
are given with respect to Me4Si [δ1H (C6D6) = 7.15; δ13
C
(C6D6) = 128.0]; δ11B = 0 for BF3-OEt2 with Ξ(11B) =
32.083971 MHz; and δ119Sn = 0 for SnMe4 with Ξ(119Sn) =
C2–C9 150.1(4), Sn–C2 217.9(3), Sn–C19 213.8(5), 37.290665 MHz. The melting point (uncorrected) was deter-
Sn–C20 215.5(5), Sn–C21 212.9(3); C1–B–C1A 119.0(4),
C1–B–C15 120.5(2), B–C1–C2 124.2(3), B–C1–C3
113.9(2), C2–C1–C3 121.8(3), C1–C2–C9 122.0(3),
C1–C2–Sn 126.0(2); B–C1–C2–Sn 2.2(5), C3–C1–C2–C9
−8.2(5), C15–B–C1–C2 65.4(3).
mined using a Bu¨chi 510 melting point apparatus.
1,1-Phenylboration of the alkyn-1-yltin compounds 2a – e
with triphenylborane 1 (general procedure)
To the solution of triphenylborane 1 (1.0 – 1.5 mmol) in
5 mL of THF a solution of 1 or 2 eqvivalents of the respective
alkyn-1-yltin compound 2 in THF (5 – 10 mL) was added at
r. t. After 30 – 60 min the solvent was removed in a vacuum,
and a pale yellow or dark red (3c) oil remained. According
to NMR spectra, the alkenes 3 and the allene 5e were pure
typical signals for the C=C=C–C unit together with
the required 117/119Sn satellites. The 13C NMR signals
of the Ph2B group are broad as a result of restricted
rotation about B–C bonds. This is confirmed by the
broad and featureless 119Sn NMR signals in the region
typical of allenyltin compounds. The 13C(p) resonance
of the C(3)-phenyl group reveals 117/119Sn satellites
with an integral intensity corresponding to the four tin
atoms present, indicating a long-range coupling across
six and seven bonds, respectively, with 6,7J(Sn,13C) =
7.7 Hz. These and other long-range coupling constants
are typical of the allene system.
◦
(ca. 85 – 95 %). The dialkenylborane 4b [m. p. 199 – 203 C
(decomp.)] was purified by crystallisation from benzene.
3a: 1H NMR (500 MHz): δ1H [J(119Sn,1H)] = 0.21
t
[49.8] (s, 9H, SnMe3); 1.34 (s, 9H, Bu); 6.86 (t, 1H, Ph,
3J(H,H) = 7.5 Hz); 6.98 (t, 2H, Ph, 3J(H,H) = 7.5 Hz); 7.18
(d, 2H, Ph, 3J(H,H) = 7.5 Hz); 7.3 – 8.1 (m, 10H, BPh2). 3b:
1H NMR (500 MHz): δ1H [J(119Sn,1H)] = −0.49 [52.9] (s,
9H, SnMe3); 7.1 – 7.3, 7.6 – 7.8, 8.55 (m, 15H, Ph, BPh2).
3c: 1H NMR (500 MHz): δ1H [J(119Sn,1H)] = 0.26 [51.8] (s,
9H, SnMe3); 4.08 (m, 2H, C5H4); 4.14 (m, 2H, C5H4); 4.24
(s, 5H, C5H5); 6.9, 8.1 (m, 15H, Ph, BPh2). 3e: 1H NMR
(500 MHz): δ1H [J(119Sn,1H)] = 0.17 [50.7], 0.25 [53.0] (s,
18H, SnMe3); 6.97, 7.07, 7.77 (t, t, d, 5H, Ph); 7.3, 8.1 (m,
10H, BPh2). 4b: 1H NMR (500 MHz): δ1H [J(119Sn,1H)] =
−0.28 [52.3] (s, 18H, SnMe3); 7.1 – 8.3 (m, 15H, Ph, BPh2).
X-Ray structural study of the dialkenylborane 4b
The molecular structure of 4b is shown in Fig. 3
together with selected structural parameters. The sur-
roundings of the boron atom are trigonal planar within
the experimental error, which is also true for all three-
coordinate carbon atoms in this molecule. All bond
lengths and angles are in the expected range. Although
the plane of the B-phenyl group is only slightly twisted
against the B,C1,C15,C1A plane, ideal for BC(pp)π
interactions, the C–C(B–Ph) distances show the usual
pattern, similar to that of the C-Ph groups in 4b. The
1
5e: H NMR (500 MHz): δ1H [J(119Sn,1H)] = 0.28 [50.8],
0.43 [53.1] (s, s, 36H, SnMe3); 7.0 – 8.4 (m, 15H, Ph,
BPh2). 13C NMR (125.8 MHz): δ13C [J(119Sn,13C)] =
−4.9 [326.5], [4.6] (SnMe3); −1.9 [321.3], [5.6] (SnMe3);
46.4 (br) [160.9], [22.4] (C-B); 87.8 [253.2], [7.9] (=C-Sn);
94.8 [71.0], [24.1] (=C-Ph); 124.7 [7.7] (p), 126.6 [9.6] (o),
orientation of the C-Ph and the alkenyl groups min- 127.6 (m), 142.4 [31.5], [28.1] (i) (Ph); 128.5 (br) (m), 129.4
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B. Wrackmeyer et al. · Phenylboration of Monoalkyn-1-yltin Compounds
1513
(p), 134.9 (br) (o), 147.0 (br) (i) (BPh2); 207.2 [39.4], [35.1] ing a riding model with fixed isotropic displacement pa-
(=C=). 119Sn NMR (186.5 MHz): δ119Sn = −3 to −7 (br).
rameters. C40H43B1Sn2: Colourless irregular crystal with di-
mensions 0.16 × 0.12 × 0.12 mm3, monoclinic space group
C2/c with the lattice parameters a =1332.3(3), b = 1762.2(4),
c =1590.2(3) pm, β = 90.29(3)◦, V = 3733.2(13)×106 pm3,
Z = 4, µ = 1.362 mm−1; a total of 13065 reflections collected
in the range 2.30◦ ≤ θ ≤ 28.05◦, 4164 independent reflec-
tions, 2280 assigned to be observed [I ≥ 2σ(I)]; full-matrix
least squares refinement on F2 with 196 parameters con-
verged at R1/wR2 values of 0.037/0.083; the max./min. resid-
ual electron density was 0.73/−0.32×10−6 e pm−3 [23].
Crystal structure determination of the dialkenylborane 4b
A crystal of appropriate size was sealed under argon in
a Lindemann capillary, and the data collection was carried
◦
out at 20 C. The reflection intensities were collected on a
Siemens P4 diffractometer (MoKα radiation, λ = 71.073 pm,
graphite-monochromated). Structure solution and refinement
were carried out with the program package SHELXTL-
PLUS V.5.1. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms
were placed in calculated positions and refined by apply-
Acknowledgement
This work was supported by the Deutsche Forschungsge-
meinschaft.
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