Double arylation of acetylenedicarboxylate with B(C6F 5)3

Article abstract of DOI:10.1002/ejic.201101413

Dimethyl acetylenedicarboxylate reacts with 2 mol-equiv. B(C ₆F₅)₃ by C₆F₅ transfer to the central acetylenic ≡C bond with formation of a tail-to-tail coupled bis(boron ester enolate) system. The cent

Full text of DOI:10.1002/ejic.201101413

SHORT COMMUNICATION
DOI: 10.1002/ejic.201101413
Double Arylation of Acetylenedicarboxylate with B(C6F5)3
Hiroshi Nakatsuka,[a,b] Roland Fröhlich,[a][‡] Masato Kitamura,[b] Gerald Kehr,[a] and
Gerhard Erker*[a]
Keywords: Boranes / Alkynes / Carboboration / Cycloaddition
Dimethyl acetylenedicarboxylate reacts with 2 mol-equiv.
B(C6F5)3 by C6F5 transfer to the central acetylenic CϵC bond
with formation of a tail-to-tail coupled bis(boron ester enol-
ate) system. The central hexasubstituted butadiene unit of
this unique system reacts with dimethyl maleate in a Diels–
Alder reaction to give a hydroquinone derivative.
groups. One example substrate is dimethyl acetylenedicarb-
oxylate (6), which reacted readily with B(C6F5)3 (1) but
gave a markedly different type of product. Its formation,
characterization and a typical subsequent reaction of the
unusual product 7 is described in this account.
Introduction
The strong boron Lewis acid B(C6F5)3 (1) reacts with
a variety of acetylene substrates by 1,1-carboboration.[1]
A
typical example is the reaction with 1-pentyne (2a) that pro-
ceeds rapidly at room temperature to give the alkenyl bor-
ane 3a in high yield (see Scheme 1).[2] Even the internal alk-
yne 4-octyne (2b) reacts with B(C6F5)3 analogously, albeit
at elevated temperature. The reaction can be viewed to be a
novel method for carbon–carbon bond activation.[3] Under
rather forcing conditions (110 °C, 3 d), we obtained the 1,1-
carboboration product 3b in good yield.[4] Phosphanyl-sub-
stituted alkynes react similarly. At 80 °C, the strong P–C
bond of substrate 4 was cleaved upon treatment with
B(C6F5)3 to yield the respective 1,1-caroboboration product
5 with formation of a new B–C and C–C bond and with a
concomitant 1,2-shift of a phosphanyl group along the cen-
tral former acetylenic C–C moiety.[5]
Results and Discussion
The reaction between B(C6F5)3 (1) and dimethyl acet-
ylenedicarboxylate (6) was carried in cyclopentane solution
at room temperature in a 2:1 ratio. After storing the reac-
tion mixture for 3 d at room temperature, the obtained yel-
low crystals that had formed during this time were isolated
(53% yield). They were suitable for characterization by X-
ray diffraction.
The X-ray crystal-structure analysis of compound 7 re-
veals that a C6F5 group had been symmetrically transferred
from each of the B(C6F5)3 molecules employed to the cen-
tral acetylene moiety of substrate 6. The pair of the remain-
ing –B(C6F5)2 units was found to be bonded to the carbonyl
oxygen atoms of the former acetylenedicarboxylate sub-
strate. Thus, we have not carried out a 1,1-carboboration
reaction, but instead a pair of conjugate C6F5-addition re-
actions had occurred (see Scheme 2) to yield a unique tail–
to-tail connected conjugated bis(boron ester enolate).
Scheme 1.
We have now tried to extend this interesting and useful
RB(C6F5)2 variant of the 1,1-carboboration reaction to or-
ganic acetylenes that bear reactive organic functional
[a] Organisch-Chemisches Institut, Universität Münster,
Corrensstraße 40, 48149 Münster, Germany
Fax: +49-251-8336503
E-mail: erker@uni-muenster.de
[b] Department of Chemistry and Research Center for Materials
Science,
Nagoya University, Chikusa, Nagoya 464-8602, Japan
[‡] X-ray structure analyses
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101413.
Scheme 2.
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SHORT COMMUNICATION
In the crystal the molecule 7 was found in a close to C2- may serve as an active electron-rich diene component in
symmetric conformation. The central hexasubstituted buta- a Diels–Alder reaction. The [4+2]cycloaddition took place
diene unit features characteristically alternating carbon– upon treatment with the electron-poor dienophile dimethyl
carbon bond lengths of 1.337(3) (C2A–C3A), 1.479(3) maleate at elevated temperature in [D6]benzene (120 °C,
(C3A–C3B), and 1.340(3) Å (C3B–C2B). The conjugated 65 h). We isolated yellow crystals of product 9 suitable for
diene moiety was found in a cisoid gauche type conforma- characterization by X-ray diffraction (see Figure 2).
tion [C2A–C3A–C3B–C2B 42.3(3)°]. Consequently, the
pair of C6F5 groups at C3A and C3B is facing each other.
The –O–B(C6F5)2 units are both cis-oriented to the carbon-
bonded C6F5 groups. They exhibit bonding angles at oxy-
gen C2A–O1A–B1A 122.8(2), C2B–O1B–B1B 125.6(2)°
and of bond lengths C2A–O1A 1.385(2), B1A–O1A
1.354(3), C2B–O1B 1.383(2), B1B–O1B 1.353(3) Å. The
pair of OCH3 substituents of the ester enolate moieties
points to the inner positions at the cisoid butadiene frame-
work (see Figure 1).
Figure 2. Molecular structure of cycloaddition product 9.
The X-ray crystal-structure analysis of product 9 shows
that a O-borylated tetrasubstituted hydroquinone was ob-
tained from the reaction of 7 with dimethyl maleate. It
features a central arene ring [C2–C6 1.392(4), C2–C3
1.427(4), C3–C3* 1.369(5), C6–C6* 1.428(5) Å]. The C2-
symmetric structure contains a pair of C6F5 groups at C3/
C3* and methyl ester substituents at the distal C6/C6* vec-
Figure 1. Molecular structure of product 7.
tor. The central hydroquinone moiety is O-borylated [C2–
O1 1.325(3), O1–B1 1.471(4) Å], and the –B(C6F5)2 func-
tion is chelated by both the hydroquinone and the adjacent
ester oxygen atoms [O4–B1 1.548(4) Å, angle O1–B1–O4
105.8(2)°, O4–C5 1.267(3) Å, C5–O7 1.287(3) Å].
So far, product 9 was only characterized by X-ray diffrac-
tion since NMR spectroscopic data could not be collected
as a result of the low solubility of 9. However, its formation
shows that the unusually composed tail-to-tail bis(ester
The product 7 features a 11B NMR resonance (δ =
44 ppm, ν1/2 ≈ 1400 Hz) typical for a tricoordinate boron
centre.[6] We monitored two sets of 19F NMR data in a 2:1
ratio for the –B(C6F5)2 units and –C6F5 substituents. The
former is characterized by a rather large Δδ19F(m,p) separa-
tion of 14.9 ppm. The former carbonyl carbon atom of the
ester enolates shows a 13C NMR signal at δ = 154.8 ppm,
the internal enolate carbon resonance is found at δ =
83.9 ppm.
enolate) 7 can apparently undergo Diels–Alder reactions:
the alleged primary [4+2]cycloaddition product 8 (see
Scheme 2 offers a tentative mechanistic interpretation of
Scheme 3) apparently loses 2 equiv. methanol under the
how compound 7 was formed. We assume that each ester
carbonyl group of substrate 6 is activated by forming an
harsh reaction conditions to form the favoured aromatic
reaction product.
adduct with the B(C6F5)3 Lewis acid.[7] Each of the thus
“self-activated” α,β-unsaturated ynone sections can then
undergo an intramolecular nucleophilic addition by C6F5
transfer from boron to carbon. We assume a stepwise pro-
cess proceeding through the respective allenic boron enolate
intermediate (Scheme 2).[8] A consequence of this pathway
is that any geometrical isomer of the resulting individual
ester enolate moieties may be formed since each nucleo-
philic C6F5 attack takes place at a C(sp) carbon centre. Ap-
parently, the formation of the specific geometric isomer 7
featuring both the –O–B(C6F5)2 units in the “outside” syn
positions at the central conjugated butadiene unit is fav-
oured in this reaction. The unique tail-to-tail connection
of the pair of boron ester enolate moieties in 7 makes this
butadiene derivative very special. Nevertheless, the system Scheme 3.
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Eur. J. Inorg. Chem. 2012, 1163–1166

Double Arylation of Acetylenedicarboxylate with B(C6F5)3
Crystal Data of 9: C46H6B2F30O6, M = 1246.13, monoclinic, C2/c
(No. 15), a = 13.3931(5), b = 18.3589(10), c = 19.2265(6) Å, β =
110.298(5)°, V = 4433.9(3) Å3, Dc = 1.867 gcm–3, μ = 1.893 mm–1,
F(000) = 2440, Z = 4, λ = 1.54178 Å, T = 223(2) K, 9816 reflections
collected (Ϯh, Ϯk, Ϯl), [(sinθ)/λ] = 0.60 Å–1, 3521 independent (Rint
= 0.040) and 2920 observed reflections [IՆ2σ(I)], 380 refined pa-
rameters, R = 0.048, wR2 = 0.140, GoF = 1.048.
Conclusions
The easy formation of 7 and its ability to undergo a
[4+2]cycloaddition reaction leads us to hope that such novel
type of highly functionalized diene systems will develop an
interesting application, complementing the ubiquitous elec-
tron-rich enol-type conjugated dienes in their use as inter-
esting synthetic building blocks.[9,10]
CCDC-859180 (for 7), and -859181 (for 9) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
Supporting Information (see footnote on the first page of this arti-
cle): Details of the X-ray structure analysis and NMR spectra de-
tails are presented.
Preparation of Compound 7: A solution of dimethyl acetylenedi-
carboxylate (57.0 mg, 0.4 mmol) in cyclopentane (3 mL) was added
to a solution of B(C6F5)3 (410 mg, 0.8 mmol) in cyclopentane
(3 mL). The solution was kept in the glove box at room temperature
for 3 d. The obtained yellow crystals were collected, washed with
cyclopentane (2ϫ0.5 mL) and dried under vacuum (53%, 247 mg).
The obtained crystals were suitable for an X-ray crystal-structure
analysis. The yield of product 7 was increased by additional frac-
tional crystallization at –35 °C (68%, 70.6 mg). M.p. 147 °C.
C42H6B2F30O4 (1166.07): calcd. C 43.26, H 0.52; found C 42.83,
H not detected. HRMS (C42H6B2F30O4): calcd. 1165.99732; found
Acknowledgments
Financial support from the Deutsche Forschungsgemeinschaft and
the Japan Society for the Promotion of Science (JSPS) are grate-
fully acknowledged.
1
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1165.99197. H NMR (600 MHz, C6D6, 298 K): δ = 3.35 (s, 1 H,
OMe) ppm. 13C{1H} NMR (151 MHz, C6D6, 298 K): δ = 154.8
1
1
(=C–O), 148.6 (dm, JFC ≈ 249 Hz, BC6F5), 145.7 (dm, JFC
1
≈ 250 Hz, C6F5), 144.4 (dm, JFC ≈ 260 Hz, BC6F5), 141.5 (dm,
1JFC ≈ 256 Hz, C6F5), 138.2 (dm, JFC ≈ 254 Hz, C6F5), 137.7 (dm,
1
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1JFC ≈ 256 Hz, BC6F5), 110.8 (br., i-C6F5), 107.1 (br., i-BC6F5),
83.9 (–C=), 56.2 (CH3) ppm. 19F NMR (564 MHz, C6D6, 298 K):
δ = –131.8 (br., 4 F, o), –145.3 (br., 2 F, p), –160.2 (m, 4 F, m),
[Δδ19F(m,p) = 14.9] (BC6F5), –139.3 (m, 2 F, o), –153.2 (t, JFF
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3
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¯
Crystal Data of 7: C42H6B2F30O4, M = 1166.09, triclinic, P1 (No.
2), a = 10.7615(3), b = 10.8751(7), c = 19.1314(9) Å, α = 96.772(3),
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1.798 gcm–3, μ = 1.859 mm–1, F(000) = 1140, Z = 2, λ = 1.54178 Å,
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= 0.60 Å–1, 7407 independent (Rint = 0.042) and 6282 observed
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Preparation of Compound 9: A solution of dimethyl maleate
(24.8 mg, 172 μmol) in toluene (1.0 mL) was added to a solution
of the dienolate 7 (200.0 mg, 172 μmol) in toluene (1.0 mL). The
reaction mixture was transferred to a 20-mL Schlenk tube and
heated at reflux for 24 h. After it was cooled to room temperature,
the supernatant was removed and the obtained yellow crystals were
washed with toluene (3ϫ 0.5 mL). The yellow crystals were dried
in vacuo (7.9%, 16.9 mg, 13.6 μmol). M.p. Ͼ300 °C. C46H6B2F30O6
(1246.13): calcd. C 44.34, H 0.49; found C 43.96, H 0.81. IR (KBr):
1649, 1590, 1523, 1482, 1405, 1395, 1293, 1121, 1100, 987, 786,
738, 693 cm–1.
NMR Scale: Dimethyl maleate (3.60 mg, 25 μmol) in C6D6
(0.5 mL) was mixed with dienolate 7 (29.2 mg, 25 μmol) in C6D6
(0.5 mL). The reaction mixture was transferred to an NMR tube,
which was sealed under vacuum. The NMR tube was then placed
in an autoclave and heating at 120 °C for 65 h. After the NMR
tube was cooled to room temperature, yellow crystals suitable for
an X-ray crystal-structure analysis were obtained. NMR charac-
terization was not possible because of the low solubility of 9.
Eur. J. Inorg. Chem. 2012, 1163–1166
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SHORT COMMUNICATION
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Received: December 19, 2011
Published Online: February 14, 2012
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