Borole formation by 1,1-carboboration

Article abstract of DOI:10.1021/ja4110396

Bis(trimethylsilylethynyl)diphenylaminoborane was reacted with the strong Lewis acid B(C₆F₅)₃ at ambient temperature to give the borole 9 admixed with a small amount of its thermal follow-up product 12. Compound 9 was subsequently stabilized by adduct formation with pyridine (10). Treatment of bis(trimethylsilylethynyl)phenylborane with B(C ₆F₅)₃ gave the borole 14, which reacted with 3-hexyne to give the [4 + 2] cycloaddition product 15.

Full text of DOI:10.1021/ja4110396

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Communication
Borole Formation by 1,1-Carboboration
Fang Ge, Gerald Kehr, Constantin Gabriel Daniliuc, and Gerhard Erker
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Borole Formation by 1,1-Carboboration
Fang Ge, Gerald Kehr, Constantin G. Daniliuc, Gerhard Erker*
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Organisch-Chemisches Institut, Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany
Supporting Information Placeholder
stronger Lewis acidic R-B(C6F5)2 boranes. We9 and others10
ABSTRACT: Bis(trimethylsilylethynyl)diphenylaminoborane
was reacted with the strong Lewis acid B(C6F5)3 at ambient
temperature to give the borole 9 admixed with a small
amount of its thermal follow–up product 12. Compound 9
was subsequently stabilized by adduct formation with pyri-
dine (10). Treatment of bis(trimethylsilylethynyl)phenyl-
borane with B(C6F5)3 gave the borole 14, which reacted with
3-hexyne to give the [4+2] cycloaddition product 15.
have developed a large series of 1,1–carboboration sequences
with such reagents that are easy to perform and, therefore,
are finding more and more practical application. This in-
cludes11 organometallic examples, but also advanced silole12,
phosphole13 and even dihydroborole syntheses14. We have
now applied the advanced 1,1–carboboration scheme to
borole synthesis and prepared first examples by this route
that showed some interesting reaction behavior.
Scheme 2
Substituted and annulated boroles have found great inter-
est as building blocks in materials science due to their special
physical and optical features.1 The anti-aromatic boroles are
very reactive.2 Therefore, isolated borole examples are often
highly substituted, often with up to five aromatic substitu-
ents or other bulky stabilizing groups. Many borole syntheses
make use of nucleophilic reaction pathways involving multi-
ple transmetallation sequences.3,4,5 Typical examples (1, 2, 4)
that were prepared in this way are depicted in Scheme 1. The
high and unusual reactivity that boroles may exhibit are
illustrated
by
the
reaction
of
Piers’
pen-
ta(perfluorophenyl)borole (4) that even splits dihydrogen to
form 5 (two isomers).6 Wrackmeyer had reported about the
potential use of the unique 1,1–carboboration reaction7 of
suitably substituted alkynes that in a few instances led to
boroles. Compound 3 is a typical example that was generated
by 1,1–carboboration of R2NB(C≡C-SnMe3)2 with BEt3 fol-
lowed by subsequent rearrangement. Unfortunately, such
reaction pathways apparently required the SnMe3 substitu-
ents as migrating groups at the time.8
Scheme 1
We prepared the amidoborane 6 by treatment of (Ph2N)BCl2
with Li-C≡CSiMe3 (for details see the Supporting Infor-
mation). The borylacetylene 6 was then treated with B(C6F5)3
in a 1 : 1 molar ratio at ambient temperature. The reaction
went to completion within a few hours to give the borole 9
admixed with a small amount of its thermal follow–up prod-
uct 12 (see Scheme 2). Removal of the solvent gave 9 as a
sensitive dark red oil [λmax (CH2Cl2) = 360 nm]. Generated in
the in situ experiment it was characterized by NMR spectros-
copy. It shows a 1 : 1 intensity pair of 11B NMR resonances at δ
49.2 (NB) and 58.8 (B(C6F5)2) and 19F NMR signals of the
Meanwhile much progress has been achieved in 1,1–
carboboration chemistry especially by using the much
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Page 2 of 5
carbon bound C6F5 group plus a double intensity set of o, p,
mation.7,12,13 1,1–Carboboration of one of the trimethylacety-
1
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4
5
6
7
8
m-C6F5 resonances of the adjacent B(C6F5)2 substituent. The
borole core shows 13C NMR resonances at δ 180.7, 162.6, 148.4,
and 158.8. We have monitored a pair of 1H NMR SiMe3 signals
with corresponding 29Si NMR resonances at δ -9.0 (Si1) and -
9.6 (Si2)(numbering scheme see Figure 1). The phenyl sub-
stituents at nitrogen are inequivalent and, consequently, give
lide moieties could then lead to the intermediate 7 (see
Scheme 2). This would then react by means of (reversible)
alkynyl shift between the boron atoms to generate 8. Subse-
quent 1,1–vinylboration would then directly lead to the borole
system 9.
Boroles are very reactive heterocycles2,6 and compound 9 is
no exception. Upon heating (100 °C, overnight) it readily
attacked the phenyl group at nitrogen in the proximal posi-
tion to the B(C6F5)2 substituent. From the reaction mixture,
we isolated the annulated dihydroborole product 12 as a pale
yellow solid in 60% yield.
13
rise to two sets of 1H and C NMR signals (for details see the
Supporting Information).
The in situ generated borole was treated with pyridine.
Workup gave the product 10 in 46 % yield as an orange red
solid. The X-ray crystal structure analysis showed a planar
five-membered borole framework with a pronounced dou-
ble–single–double carbon-carbon bond alternation (see Fig-
ure 1). The boron atom bears the Ph2N substituent with a
short N1–B1 bond. A trimethylsilyl substituent is bonded to
the adjacent borole carbon atom C4 and we find the C6F5
group attached at C3. The remaining silyl and boryl substitu-
ents have undergone an unusual CH3/C6F5 exchange (as it is
sometimes observed in silyl/boryl systems).15 Consequently,
we find a SiMe2(C6F5) substituent attached at the borole
carbon atom C1. The resulting BCH3(C6F5) substituent at C2
has the pyridine donor attached to it (∑B2CCC 331.6°). Both
the borole boron atom B1 and its adjacent nitrogen atom N1
exhibit slightly distorted trigonal-planar coordination geom-
etries (∑B1NCC = 356.3°, ∑N1CCB = 359.0°).
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The X-ray crystal structure analysis revealed the formation
of the new benzannulated heterobicyclo[3.3.0]octadiene
framework with a planar-tricoordinate bridgehead boron
atom (∑B1NCC = 356.9°). The B1-N1 bond is short and the
nitrogen atom also shows a planar-tricoordinate geometry (∑
N1CCB = 358.9°). The substituted dihydroborole subunit16
contains a pair of typical B-C(sp3) single bonds, a pair of
C(sp3)-C(sp2) single bonds and the C2=C3 double bond to
which the C6F5 and the B(C6F5)2 substituents are bonded. The
boron atom B2 features a planar-tricoordinate bonding ge-
ometry (∑B2CCC = 359.7°). The boryl plane is markedly rotat-
ed out of conjugation with the adjacent C2=C3 double bond
[Θ C3–C2–B2–C51 -124.5(3)] (see the Supporting Information
for the spectroscopic characterization of compound 12).
Figure 1. A projection of the molecular geometry of com-
pound 10 (thermal ellipsoids are shown with 30% probabil-
ity). Selected bond lengths (Å) and angles (grad.): N1-B1
1.408(3), B1-C1 1.588(3), C1–C2 1.361(3), C2-C3 1.547(3), C3–C4
1.358(3), B1–C4 1.595(3), C2–B2 1.633(3), B2-C7 1.630(3), B2–
N61 1.633(3), B2–C51 1.652(3), C1–B1–C4 106.7(2).
Figure 2. Molecular geometry of compound 12 [thermal ellip-
soids are shown with 30% probability; hydrogen are omitted
due to clarity (exept C4)]. Selected bond lengths (Å) and
angles (grad.): B1–N1 1.416(4), B1–C1 1.565(5), B1–C4 1.557(5),
C1–C2 1.539(4), C2–C3 1.373(4), C3–C4 1.522(4), B2–C2 1.527(5),
N1–B1–C4 135.8(3), B1–N1–C11 108.6(3), C1–B1–C4 112.1(3).
In solution the borole 10 shows a UV/Vis absorption at
In Scheme 2 a possible pathway of the formation of the
product 12 is depicted. One might assume that the reactive
electron-deficient borole heterocycle is intramolecularly
attacked by the electron-rich amino-arene to give a zwitteri-
onic intermediate 11 that contains a borata-diene moiety
(which is just a π-resonance form of a stabilized α-boryl
cabanion17). Proton transfer then would complete this elec-
trophilic aromatic substitution sequence with formation of
the observed product 12.
λ
max(CH2Cl2) = 389 nm (ε = 3400). We have observed the
borole Ph2N–B 10B NMR feature at δ 47.6 and the 10B NMR
signal of the tetracoordinate boron center at δ -2.2. The 29Si
NMR signals of the silyl substituents occur at δ -11.2 (SiMe3)
and -12.0 (SiMe2). Due to the persistent boron B2 chirality
the SiMe2(C6F5) substituent features a pair of diastereotopic
methyl groups [1H NMR: δ -0.27, -0.59 (each 3H)].
We assume that the borole formation from 6 by the reac-
tion with B(C6F5)3 follows a typical sequential pathway as it
had been suggested analogously for silole or phosphole for-
2
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The formation of a borole by the 1,1-carboboration route is
seems to provide a useful synthetic alternative, especially by
using the strongly electrophilic RB(C6F5)2 reagents as we
have shown in our study. Using these current advanced de-
velopments of the “Wrackmeyer-reaction”7,9 seems to pro-
vide an easy and convenient new entry to interesting boryl-
functionalized isolable borole derivatives with a possible
potential of further derivatization or functionalization of the
anti-aromatic borole nucleus.
1
2
3
4
5
6
7
not limited to the example of compounds 9 and 10 described
above. We treated PhBCl2 with two equivalents of lithium
trimethylsilyacetylide. The in situ generated bis(alkynyl)-
borane 13 was then reacted with B(C6F5)3 to give the borole 14
formed by an 1,1-carboboration sequence [red crystalline
11
solid, 43% isolated, B NMR: δ 74.0, 63.0]. Compound 14 was
characterized by X-ray diffraction (see Figure 3).
8
Scheme 3
9
1,1-carbo-
boration
10
11
12
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14
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PhB(C≡CSiMe3)2
+ B(C6F5)3
13
Ph
B
[Si]
[Si]
C6F5
≡
EtC CEt
Et
Et
C6F5
B
r.t.
B(C6F5)2
B(C6F5)2
[Si]
[Si]
14
[Si]: SiMe3
15
Figure 4. Molecular geometry of compound 15 [thermal el-
lipsoids are shown with 30% probability; hydrogens and Me
substituents at Si1 and Si2, respectively, are omitted for clari-
ty]. Selected bond lengths (Å) and angles (grad.): B1–C21
1.577(3), B2–C2 1.543(3), C2–C3 1.354(3), C5–C6 1.392(3), C1–C2
1.528(3), C1–C6 1.512(3), B1–C1–C6 68.4(1), B1–C4–C5 68.4(1),
B1–C1–C2 105.9(2), ΣB1C21C1C4 359.4, ΣB2CCC 359.7.
ASSOCIATED CONTENT
Supporting Information
Figure 3. Molecular geometry of compound 14 [thermal el-
lipsoids are shown with 30% probability]. Selected bond
lengths (Å) and angles (grad.): B1–C11 1.545(5), B1–C1 1.606(4),
B1–C4 1.587(5), C1–C2 1.359(4), C2–C3 1.532(4), C3–C4
1.345(4), B2–C2 1.562(4), ΣB1CCC 360.0, B1–C1–C2-C3 -2.3(3),
C1–C2–C3-C4 1.0(4).
This material is available free of charge via the Internet at
http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
For further chemical characterization compound 14 was
reacted with 3-hexyne18 in a 1:1 molar ratio to finally isolate
the [4+2] cycloadditon product 15 as an yellow crystaline
erker@uni-muenster.de
ACKNOWLEDGMENT
11
solid [50% yield, B NMR: δ 61.6 (B(C6F5)2), -9.9 (BPh)]. The
Financial support from the NRW Graduate School and the
Deutsche Forschungsgemeinschaft is gratefully acknowl-
edged.
X-ray crystal structure analysis showed a trigonal-planar
boron atom inside the bicyclic framework (∑B1CCC = 359.4°)
that is, however, markedly leaning over to the C5=C6 double
bond [respective pairs of bond lenths: B1-C1/C4
1.645(3)/1.644(3) Å, B1-C5/C6 1.782(3)/1.777(3) Å, B1-C2/C3
2.533/2.488 Å, see Figure 4].
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nonen, H. M.; Parvez, M. J. Am. Chem. Soc. 2010, 132, 9604. (c)
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Page 5 of 5
Journal of the American Chemical Society
SYNOPSIS TOC
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9
[Si] B(C6F5)2
C6F5
R: Ph2N
[Si]
B(C6F5)3
[Si]
R B
B
N
C6F5
H [Si]
Ph
Ph
R B
1,1-carbo-
boration
B(C6F5)2
B
[Si]
[Si]
Et
Et
C6F5
B(C6F5)2
[Si]
R: Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EtC CEt
R: Ph2N, Ph; [Si]: SiMe3
[Si]
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