Practical Solvent-Free Microwave-Assisted Hydroboration of Alkynes

Article abstract of DOI:10.1002/ejoc.202000110

A simple and rapid protocol for the anti-Markovnikov hydroboration of alkynes assisted by microwave irradiation has been developed. Pinacolborane smoothly reacts with terminal alkynes to obtain (E)-alkenyl boronates in good yields and short reactions times in the absence of solvent. Further transformations on the carbon-boron bond of the adducts can be sequentially achieved without the need of purifying the alkenyl boronates.

Full text of DOI:10.1002/ejoc.202000110

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Practical solvent-free microwave-assisted hydroboration of
alkynes
Authors: Julia Altarejos, David Sucunza, Juan José Vaquero, and
Javier Carreras
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000110
Link to VoR: https://doi.org/10.1002/ejoc.202000110
01/2020

10.1002/ejoc.202000110
European Journal of Organic Chemistry
FULL PAPER
Practical solvent-free microwave-assisted hydroboration of
alkynes
Julia Altarejos,[a] David Sucunza,[a] Juan J. Vaquero,[a] Javier Carreras*[a]
[a]
J. Altarejos, Dr. D. Sucunza, Prof. Dr. J. J. Vaquero, Dr. J. Carreras
Departamento de Química Orgánica y Química Inorgánica, Instituto de Investigación Química “Andrés M. del Río” (IQAR)
Universidad de Alcalá, IRYCIS, 28805 Alcalá de Henares, Madrid, Spain
E-mail: javier.carreras@uah.es
Supporting information for this article is given via a link at the end of the document.
Abstract: A simple and rapid protocol for the anti-Markovnikov
hydroboration of alkynes assisted by microwave irradiation has been
developed. Pinacolborane smoothly reacts with terminal alkynes to
obtain (E)-alkenyl boronates in good yields and short reactions times
in the absence of solvent. Further transformations on the carbon-
boron bond of the adducts can be sequentially achieved without the
need of purifying the alkenyl boronates.
Scheme 1. Microwave assisted synthesis of (E)-alkenyl boronates.
Introduction
Alkenyl boronates are versatile building blocks in organic
synthesis.[1] The development of the Suzuki–Miyaura cross-
coupling reactions set them as privileged substrates in the
formation of new C-C bonds.[2] Moreover, the alkene group of
these compounds has been used in cycloadditions,[3] hydro- and
difunctionalizations[4] and radical reactions[5] among others. The
main interest of these reactions is that the boronate groups are
able to undergo further functionalizations, thus transforming the
C-B bond in new C-C and C-heteroatom bonds.[6]
In this work, we have focused on the development of a
simple, fast and general solvent-free microwave-assisted
hydroboration of alkynes with commercially available
pinacolborane.
Results and Discussion
Initially,
we
investigated
the
hydroboration
of
The preparation of alkenyl boronates can be achieved by
different methodologies.[7] In particular, (E)-alkenyl boronates
have been prepared by olefin metathesis,[8] dehydrogenative
borylation of alkenes,[9] Heck type reactions[10] or
coupling/dehydroboration of epoxides[11] among others.
However, hydroboration of alkynes represents a prominent
strategy to obtain (E)-alkenyl boronates. A plethora of transition
metals has been used to catalyzed and control the regio- and
stereoselectivity of this process.[7,12] More recently, transition
metal-free protocols have been reported to obtain the trans
isomers of alkenyl boronates, including the use of several
phenylacetylene with pinacolborane under microwave irradiation
as a model reaction at a constant temperature. No solvent, or
additive were added, and the reagents were mixed in open air.
Initial experiments showed that phenylacetylene was not fully
consumed below 200 ºC after 20-30 min, when low to moderate
yields were obtained (Table 1, entries 1-2). Remarkably, despite
the slow reaction rate, hydroboration was completely regio- and
stereoselective, yielding (E)‐styryl pinacolboronate (1a) as the
only product. Increasing the temperature to 215 ºC gave rise to a
complete consumption of the alkyne in 20 min, with a slight
increase in the reaction yield (entry 3). Using an excess of two
equivalents of pinacolborane gave a satisfactory 80% yield of
the alkenyl boronate 1a (entry 4). The outcome of the reaction
was similar for a range of different reaction scales (1.75-4.5
mmol, see Supporting Information for more details) and further
excess of pinacolborane had no significant impact on the
reaction yield. For comparison, the same reaction under
identical conditions but using conventional heating instead of
microwave irradiation gave a 25% yield after 20 minutes (entry
6). Finally, the reaction time was optimized to 18 minutes,
leading to a 79% yield of (E)‐styryl pinacolboronate in 4.5 mmol
scale (entry 7).[22]
boranes,[13]
N-heterocyclic
carbenes,[14]
4-
(dimethylamino)benzoic acid,[15] different bases[16] (NaOH,
LiOtBu, NaOtBu/PhI(OAc)2) or main-group metals[17] as catalysts.
Furthermore, non-catalyzed reactions were previously studied
by Brown and Knochel.[18] However, long reactions times are
often necessary for the reaction to be completed. Microwave
irradiation commonly reduces reaction times in organic synthesis,
and some examples of metal-catalyzed hydroborations with
microwave heating have been reported using rhodium and
zirconium complexes (Scheme 1).[19] Very recently, tris(3,4,5-
trifluorophenyl)borane[20] and 4-(dimethylamino)benzoic acid[21]
have been applied as transition metal-free catalysts in the
hydroboration of alkynes.
No additives were added to the reaction mixture, so a
catalyst-free hydroboration is suggested to take place under
1
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10.1002/ejoc.202000110
European Journal of Organic Chemistry
FULL PAPER
Table 2. Substrate scope of the hydroboration reaction of terminal alkynes.[a]
these reaction conditions. However, some type of catalysis
cannot be completely ruled out for this process. Hence,
palladium traces could be present in the used alkynes, and
metal catalyzed reactions have been described to proceed at
ultra-low catalyst concentrations under microwave irradiation.[23]
Additionally, some decomposition of pinacolborane by oxygen or
moisture is expected to occur under these reaction conditions,
and borane derivatives has been previously described as
catalysts on alkyne hydroboration.[13]
Table 1. Hydroboration optimization for phenylacetylene.[a]
Entry
Temp. [
ͦ
C]
time [min]
HBpin [equiv]
Yield [%]
1
2
3
4
5
6
7
175
200
215
215
215
215[d]
215
30
20
20
20
20
20
18
1.5
1.5
1.5
2
35[b]
60[b]
67[c]
80[c]
81[c]
25[c]
79[c]
2.5
2
2
[a] Reaction conditions: phenylacetylene, pinacolborane, microwave
irradiation. Entries 1-3: 3 mmol scale; entries 4-7: 4.5 mmol scale. [b] Yields
determined by 1H NMR analysis in the reaction mixture using 1,3,5-
trimethoxybenzene as an internal standard. [c] Isolated yield. [d] Conventional
heating.
We next investigated the scope of our microwave-assisted
hydroboration protocol with different alkynes (Table 2). We
initially tested phenylacetylene derivatives with ortho‐, meta‐,
and para‐substituents with different steric and electronic
properties, including halogen, trifluoromethyl, ether, ester, amine
and alkyl groups (1a-1j). In all the cases, moderate to excellent
yields were obtained (53-91%) under the optimized
[a] Yield of isolated compounds 1a-1p. Reaction conditions: alkyne (4.5 mmol),
pinacolborane (9 mmol), microwave irradiation, 215 ºC, 18 min. 18 mmol of
pinacolborane were used to get 1p.
The synthesized alkenyl boronates are versatile
intermediates for the transformation of the C-B bond into a
variety of new C-C or C-heteroatom bonds.[6] The versatility of
our optimized alkyne hydroboration protocol was demonstrated
by performing several stereoselective transformations over
hydroboration
conditions
and
complete
regio-
and
stereoselectivity was observed, to obtain exclusively the
corresponding (E)-alkenyl boronates. Electron-withdrawing
substituents in the aromatic ring generally increased the yield of
the reaction, unlike electron-donating substituents, and lower
results were obtained for ortho-substituted substrates (1b,1c). 3-
Ethynyl thiophene was tested as an heteroaromatic alkyne
showing similar results to other electron-rich substituents (1k,
70%). Next, we explored the reactivity of 1-ethynylcyclohex-1-
ene. Notably, the chemoselective hydroboration of the alkyne
group with no trace of reaction on the alkene group was
observed, with (E)-alkenyl boronate 1l being the only product
detected by NMR in the reaction mixture.
In addition to aryl rings and alkenyl substituents, aliphatic
chains including 2-phenylethyl (1m), butyl (1n), and cyclopropyl
(1o) also showed excellent reactivity under the same reaction
conditions. In these alkyl derivatives, the regioselectivity was not
complete (: ≥ 85:15), but good yields of the (E)-alkene
−products could be isolated (69-73%). Finally, with the bis-
alkyne 1,3-diethynylbenzene, the corresponding diboronate
product was obtained in moderate yield (1p, 47%). An internal
alkyne, such as prop-1-yn-1-ylbenzene, did not react under
these reaction conditions.
unpurified (E)-alkenyl boronates, in
a sequential manner
(Scheme 2). Thus, a stilbene derivative could be obtained via
sequential hydroboration/Suzuki-Miyaura cross‐coupling in good
yield (2, 63%, two steps). In a similar fashion, the boronate
group could be exchanged by different heteroatoms (iodine,
nitrogen or oxygen) after the hydroboration process,
successfully getting the corresponding vinyl iodide (3, 65%, two
steps), vinyl azide (4, 52%, two steps) and vinyl ether (5, 33%,
two steps). All these products are valuable alkenes employed on
different organic transformations. Finally, the trifluoroborate salt
was obtained via sequential hydroboration/treatment with KHF2
(62%, two steps), without the need of further purification by
column chromatography.
2
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10.1002/ejoc.202000110
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alkenylboronate. General procedure B. The corresponding alkyne (3
mmol), pinacolborane (9 mmol) and decane (0.5 mL) were mixed in a
microwave vial and heated at 215 ºC for 25 minutes by microwave
irradiation. The crude was purified by flash column chromatography
(hexane/AcOEt) to obtain the (E)-alkenylboronate.
Characterization data 1a-1p, 2-6.
(E)-4,4,5,5-Tetramethyl-2-styryl-1,3,2-dioxoborolane (1a). Prepared
from phenylacetylene (0.49 mL, 4.5 mmol) and pinacolborane (1.31 mL,
9 mmol) following the general procedure A and purified by flash
chromatography (hexane/EtOAc, 95/5). Yellow oil (818 mg, 79%). By
1
using general procedure B, 587 mg (85%) were obtained. H NMR (300
MHz, CDCl3) δ 7.53 – 7.46 (m, 2H), 7.40 (d, J = 18.7 Hz, 1H), 7.36 – 7.28
(m, 3H), 6.17 (d, J = 18.4 Hz, 1H), 1.32 (s, 12H) ppm; 13C NMR (75 MHz,
CDCl3) δ 149.5, 137.4, 128.9, 128.5, 127.0, 116.4 (bs), 83.3, 24.8 ppm;
11B NMR (160 MHz, CDCl3) δ 30.2 ppm. NMR data were in accordance
with those previously reported. [12b]
Scheme 2. Sequential hydroboration-boronate transformation reactions.
Reaction conditions: (a) alkyne (4.5 mmol, 1 equiv), pinacolborane (2 equiv),
microwave irradiation, 215 ºC, 18 min. (b) Pd(OAc)2 (0.05 equiv), tBu3PHBF4
(0.1 equiv), K2CO3 (1.2 equiv), 1-bromo-4-methoxybenzene (1.2 equiv), DMF,
inert atmosphere, 90 ºC, 18 h, 63%. (c) NaOH (3 equiv), I2 (2 equiv), THF, 25
ºC, 2 h, 65%. (d) NaN3 (1.5 equiv), CuSO4 (0.6 equiv), MeOH, 25 ºC, 16 h,
52%. (e) Cu(OAc)2 (2 equiv), NEt3 (4 equiv), allyl alcohol (8 mL), 25 ºC, 16 h,
33% (f) KHF2 (5 equiv) MeOH/H2O, 25 ºC, 24 h, 62%.
(E)-2-(2-Fluorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane
(1b).
Prepared from 1-ethynyl-2-fluorobenzene (0.51 mL, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 94/6). Yellow oil (783
mg, 70%). 1H NMR (300 MHz, CDCl3) δ 7.60 (d, J = 18.7 Hz, 1H), 7.59 –
7.50 (m, 1H), 7.29 – 7.18 (m, 1H), 7.09 (t, J = 7.5 Hz, 1H), 7.05 – 6.97 (m,
1H), 6.25 (d, J = 18.6 Hz, 1H), 1.30 (s, 12H) ppm; 13C NMR (75 MHz,
CDCl3) δ 160.6 (d, J = 251.1 Hz), 141.2 (d, J = 3.9 Hz), 130.2 (d, J = 8.5
Hz), 127.4 (d, J = 3.2 Hz), 125.3 (d, J = 11.6 Hz), 124.1 (d, J = 3.5 Hz),
119.3 (bs), 115.8 (d, J = 22.0 Hz), 83.5, 24.9 ppm; 19F NMR (282 MHz,
CDCl3) δ -107.7 ppm; 11B NMR (160 MHz, CDCl3) δ 30.0 ppm. NMR data
were in accordance with those previously reported. [24a]
Conclusion
In conclusion, we have described a simple, efficient and rapid
protocol for the solvent-free microwave-assisted anti-
Markovnicov hydroboration of terminal alkynes, in a chemo-,
regio-, and stereoselective process. The reaction proceeds in
good to excellent yields without any additives or solvents.
Several organic transformations can be performed sequentially
to the hydroboration reaction in good overall yields, to get vinyl
iodine, azide, ether, cross-coupled products or trifluoroborate
salts without purification of the alkenyl boronates.
(E)-4,4,5,5-Tetramethyl-2-(2-(trifluoromethyl)styryl)-1,3,2-
dioxaborolane (1c). Prepared from 1-ethynyl-2-(trifluoromethyl)benzene
(0.55 mL, 4 mmol) and pinacolborane (1.31 mL, 9 mmol) following the
general procedure
A
and purified by flash chromatography
(hexane/EtOAc, 95/5). Yellow oil (631 mg, 53%). 1H NMR (300 MHz,
CDCl3) δ 7.82 – 7.67 (m, 2H), 7.64 (d, J = 7.8 Hz, 1H), 7.52 (t, J = 7.7 Hz,
1H), 7.37 (t, J = 7.6 Hz, 1H), 6.16 (d, J = 18.1 Hz, 1H), 1.31 (s, 12H)
ppm; 13C NMR (75 MHz, CDCl3) δ 144.8, 137.0, 132.0, 128.3, 127.8 (q, J
= 30.3 Hz), 127.5, 124.4 (q, J = 274.0 Hz) 125.7 (q, J = 5.6 Hz), 121.9
(bs), 83.6, 24.8 ppm; 19F NMR (282 MHz, CDCl3) δ -53.3 ppm; 11B NMR
(160 MHz, CDCl3) δ 30.1 ppm. NMR data were in accordance with those
previously reported. [24a]
Experimental Section
General Experimental Procedures: All commercially available
compounds were used as received unless stated otherwise.
Pinacolborane was purified by distillation before use it and keeped under
inert atmosphere. Microwave reactions were carried out using a Biotage
Initiator 2.5 microwave synthesizer operating at 0−400 W irradiation
(E)-2-(3-Fluorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane
(1d).
Prepared from 1-ethynyl-3-fluorobenzene (0.52 mL, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 95/5). Yellow oil (932
mg, 83%). By using general procedure B, 609 mg (82%) were obtained.
1H NMR (300 MHz, CDCl3) δ 7.32 (d, J = 18.4 Hz, 1H), 7.28 – 7.17 (m,
2H), 7.17 – 7.10 (m, 1H), 6.98 – 6.88 (m, 1H), 6.14 (d, J = 18.4 Hz, 1H),
1.27 (s, 12H); 13C NMR (75 MHz, CDCl3) δ 163.1 (d, J = 245.9 Hz),
148.04 (d, J = 2.4 Hz), 139.90 (d, J = 7.4 Hz), 130.03 (d, J = 8.6 Hz),
123.00 (d, J = 2.4 Hz), 118.3 (bs), 115.7 (d, J = 21.3 Hz), 113.3 (d, J =
power, at
a normal absorption level and controlling the reaction
temperature by an external IR sensor. Reaction times refer to hold times
at the temperatures indicated, not to total irradiation times. Analytical thin
layer chromatography was carried out using TLC-aluminium sheets with
0.2 mm of silica gel (Merck 60 F254) and UV light as visualizing agent or
phosphomolybdic acid solution as developing agent. Chromatography
purifications were carried out using silica gel (40-63 µm, 60 Å). Organic
solutions were concentrated under reduced pressure on a Büchi rotary
evaporator. NMR spectra were recorded at 298 K using either a Varian
Mercury VX-300, Varian Unity 300, or Varian Unity 500 MHz
spectrometer. Chemical shift values for 1H and 13C are reported as δ
values (ppm) relative to the deuterated solvent (CDCl3: 7.26 ppm, 77.16
ppm; CD3CN: 1.94 ppm, 1.32 ppm) and coupling constants (J) in Hz. All
melting points were determined in open capillary tubes using a Stuart
Scientific SMP3 melting point apparatus. High-resolution analysis
(HRMS) was performed using an Agilent 6210.
21.4 Hz), 83.4, 24.8 ppm; 19F NMR (282 MHz, CDCl3) δ -107.7 ppm; 11
B
NMR (160 MHz, CDCl3) δ 30.1 ppm. NMR data were in accordance with
those previously reported. [24a]
(E)-3-(2-(4,4,5,5-Tetramethyl-1,3,2-dioxoborolan-2-yl)vinyl)phenyl
acetate (1e). Prepared from 3-ethynylphenyl acetate (721 mg, 4.5 mmol)
and pinacolborane (1.31 mL, 9 mmol) following the general procedure A
and purified by flash chromatography (hexane/EtOAc, 95/5). Yellow oil
1
(923 mg, 71%). H NMR (500 MHz, CDCl3) δ 7.35 (d, J = 18.3 Hz, 1H),
7.33 – 7.29 (m, 2H), 7.21 – 7.16 (m, 1H), 7.05 – 6.98 (m, 1H), 6.15 (d, J
= 18.4 Hz, 1H), 2.28 (s, 3H), 1.29 (s, 12H) ppm; 13C NMR (126 MHz,
CDCl3) δ 169.3, 150.9, 148.3, 139.2, 129.5, 124.6, 121.9, 119.8, 83.4,
24.8, 21.1 ppm (C-B signal not observed due to low intensity); 11B NMR
(160 MHz, CDCl3) δ 29.9 ppm; HRMS-ESI m/z calcd for C16H22BO4
[M+H]+ 289.1609, found 289.1605.
General procedure A. The corresponding alkyne (4.5 mmol) and
pinacolborane (9 mmol) were mixed in a microwave vial and heated at
215 ºC for 18 minutes by microwave irradiation. The crude was purified
by flash column chromatography (hexane/AcOEt) to obtain the (E)-
3
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(E)-4,4,5,5-Tetramethyl-2-(4-methylstyryl)-1,3,2-dioxoborolane (1f).
Prepared from 1-ethynyl-4-methylbenzene (0.57 mL, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 95/5). Yellow oil (792
mg, 72%). By using general procedure B, 556 mg (76%) were obtained.
1H NMR (300 MHz, CDCl3) δ 7.38 (m, 3H), 7.14 (d, J = 8.0 Hz, 2H), 6.11
(d, J = 18.5 Hz, 1H), 2.34 (s, 3H), 1.31 (s, 12H) ppm; 13C NMR (75 MHz,
CDCl3) δ 149.6, 139.0, 134.9, 129.4, 127.1, 115.3 (bs), 83.3, 24.9, 21.4
ppm; 11B NMR (160 MHz, CDCl3) δ 30.3 ppm. NMR data were in
accordance with those previously reported.[12b]
(E)-2-(2-(Cyclohex-1-en-1-yl)vinyl)-4,4,5,5-tetramethyl-1,3,2-
dioxoborolane (1l). Prepared from 1-ethynylcyclohex-1-ene (0.53 mL,
4.5 mmol) and pinacolborane (1.31 mL, 9 mmol) following the general
procedure A and purified by flash chromatography (hexane/EtOAc, 95/5).
Yellow oil (825 mg, 79%). By using general procedure B, 417 mg (60%)
were obtained. 1H NMR (300 MHz, CDCl3) δ 7.02 (d, J = 18.3 Hz, 1H),
5.97-5.95 (m, 1H), 5.42 (d, J = 18.2 Hz, 1H), 2.14 (m, 4H), 1.69-1.55 (m,
4H), 1.27 (s, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ 153.3, 137.1, 134.2,
111.9 (bs), 83.0, 26.2, 24.8, 23.8, 22.4, 22.4 ppm; 11B NMR (160 MHz,
CDCl3) δ 30.1 ppm. NMR data were in accordance with those previously
reported.[12c]
(E)-2-(4-Chlorostyryl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane (1g).
Prepared from 1-ethynyl-4-chlorobenzene (615 mg, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 95/5). Yellow oil (837
mg, 70%). 1H NMR (300 MHz, CDCl3) δ 7.41 (d, J = 8.6 Hz, 2H), 7.34 (d,
J = 18.5 Hz, 1H), 7.30 (d, J = 8.5 Hz, 2H), 6.13 (d, J = 18.4 Hz, 1H), 1.31
(s, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ 147.9, 135.9, 134.6, 128.8,
128.2, 117.3 (bs), 83.5, 25.0 ppm; 11B NMR (160 MHz, CDCl3) δ 30.0
ppm. NMR data were in accordance with those previously reported. [24a]
(E)-
4,4,5,5-Tetramethyl-2-(4-phenylbut-1-en-1-yl)-1,3,2-
dioxoborolane (1m). Prepared from but-3-yn-1-ylbenzene (0.63 mL, 4.5
mmol) and pinacolborane (1.31 mL, 9 mmol) following the general
procedure A and purified by flash chromatography (hexane/EtOAc, 95/5).
Colorless liquid (848 mg, 73%). 1H NMR (300 MHz, CDCl3) δ 7.31 – 7.24
(m, 2H), 7.21 – 7.17 (m, 3H), 6.71 (dt, J = 18.0, 6.2 Hz, 1H), 5.50 (dt, J =
18.0, 1.6 Hz, 1H), 2.76-2.72 (m, 2H), 2.53 – 2.43 (m, 2H), 1.27 (s, 12H)
ppm; 13C NMR (75 MHz, CDCl3) δ 152.9, 141.3, 128.1, 125.6, 119.1 (bs),
82.7, 37.4, 34.5, 24.8 ppm; 11B NMR (160 MHz, CDCl3) δ 29.8 ppm.
NMR data were in accordance with those previously reported. [24c]
(E)-2-(4-Methoxystyryl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane (1h).
Prepared from 1-ethynyl-4-methoxybenzene (0.58 mL, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 95/5). Yellow oil (822
mg, 70%). By using general procedure B, 549 mg (70%) were obtained.
1H NMR (300 MHz, CDCl3) δ 7.46 – 7.40 (m, 2H), 7.35 (d, J = 18.4 Hz,
1H), 6.85 (d, J = 8.7 Hz, 2H), 6.01 (d, J = 18.4 Hz, 1H), 3.79 (s, 3H), 1.30
(s, 12H) ppm; 13C NMR (75 MHz, CDCl3) δ 160.2, 149.0, 130.4, 128.4,
114.3, 114.0 (bs), 83.2, 55.3, 25.0 ppm; 11B NMR (160 MHz, CDCl3) δ
30.2 ppm. NMR data were in accordance with those previously
reported.[12b]
(E)-2-(Hex-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane
(1n).
Prepared from hex-1-yne (0.55 mL, 4.5 mmol) and pinacolborane (1.31
mL, 9 mmol) following the general procedure A and purified by flash
chromatography (hexane/EtOAc, 95/5). Yellow oil (803 mg, 71%). 1H
NMR (300 MHz, CDCl3) δ 6.63 (dt, J = 18.0, 6.4 Hz, 1H), 5.42 (dt, J =
18.0, 1.5 Hz, 1H), 2.15 (td, J = 8.0, 1.5 Hz, 2H), 1.44 – 1.28 (m, 4H), 1.26
(s, 12H), 0.88 (t, J = 7.1 Hz, 3H) ppm; 13C NMR (75 MHz, CDCl3) δ 154.5,
118.8 (bs), 82.9, 35.6, 30.5, 24.9, 22.4, 14.0 ppm; 11B NMR (160 MHz,
CDCl3) δ 29.7 ppm. NMR data were in accordance with those previously
reported. [12b]
(E)-N,N-Dimethyl-4-(2-(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-
yl)vinyl)aniline (1i). Prepared from 1-ethynyl-N,N-dimethylaniline (0.65
mL, 4.5 mmol) and pinacolborane (1.31 mL, 9 mmol) following the
(E)-2-(2-Cyclopropylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxoborolane
(1o). Prepared from ethynylcyclopropane (0.41 mL, 4.5 mmol) and
pinacolborane (1.31 mL, 9 mmol) following the general procedure A and
purified by flash chromatography (hexane/EtOAc, 95/5). Colorless oil
general procedure
A
and purified by flash chromatography
(hexane/EtOAc, 95/5). Off-white solid (759 mg, 62%). 1H NMR (300 MHz,
CDCl3) δ 7.40 (d, J = 8.8 Hz, 2H), 7.34 (d, J = 18.4 Hz, 1H), 6.67 (d, J =
1
(599 mg, 69%). H NMR (300 MHz, CDCl3) δ 5.88 (dd, J = 17.8, 9.1 Hz,
8.8 Hz, 2H), 5.92 (d, J=18.5 Hz, 1H), 2.98 (s, 6H), 1.31 (s, 12H) ppm; 13
C
1H), 5.28 (d, J = 17.7 Hz, 1H), 1.39 – 1.24 (m, 1H), 1.06 (s, 12H), 0.69 –
0.54 (m, 2H), 0.38 – 0.27 (m, 2H) ppm; 13C NMR (75 MHz, CDCl3) δ
158.2, 115.1 (bs), 82.7, 24.9, 17.0, 7.9 ppm; 11B NMR (160 MHz, CDCl3)
δ 29.6 ppm. NMR data were in accordance with those previously
reported. [24a]
NMR (126 MHz, CDCl3) δ 151.0, 149.9, 128.4, 126.0, 112.0, 83.1, 40.3,
24.9 ppm (C-B signal not observed due to low intensity) ; 11B NMR (160
MHz, CDCl3) δ 30.4 ppm; m.p. 92-93ºC. NMR data were in accordance
with those previously reported. [24b]
(E)-4,4,5,5-Tetramethyl-2-(4-(trifluoromethyl)styryl)-1,3,2-
1,3-Bis((E)-2-(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-
dioxoborolane (1j). Prepared from 1-ethynyl-4-(trifluoromethyl)benzene
(0.74 mL, 4.5 mmol) and pinacolborane (1.31 mL, 9 mmol) following the
yl)vinyl)benzene (1p). Prepared from 1,3-diethynylbenzene (0.6 mL, 4.5
mmol) and pinacolborane (2.66 mL, 18 mmol) following the general
procedure A and purified by flash chromatography (hexane/EtOAc, 95/5).
general procedure
A
and purified by flash chromatography
1
(hexane/EtOAc, 95/5). Yellow oil (1.22 g, 91%). By using general
procedure B, 821 mg (92%) were obtained. 1H NMR (300 MHz, CDCl3) δ
7.57 (s, 4H), 7.41 (d, J = 18.4 Hz, 1H), 6.27 (d, J = 18.4 Hz, 1H), 1.31 (s,
12H) ppm; 13C NMR (75 MHz, CDCl3) δ 147.8, 140.9, 130.6 (q, J = 32.6
Hz), 127.3, 125.6 (q, J = 3.6 Hz), 124.2 (q, J = 271.9 Hz), 119.8 (bs),
83.7, 24.9 ppm; 19F NMR (282 MHz, CDCl3) δ -57.0 ppm; 11B NMR (160
MHz, CDCl3) δ 30.0 ppm. NMR data were in accordance with those
previously reported. [12b]
Pale yellow solid (810 mg, 47%). H NMR (300 MHz, CDCl3) δ 7.56 (s,
1H), 7.42 (d, J = 7.7 Hz, 2H), 7.37 (d, J = 18.4 Hz, 2H), 7.30 (t, J = 7.6 Hz,
1H), 6.16 (d, J = 18.4 Hz, 2H), 1.30 (s, 24H) ppm; 13C NMR (126 MHz,
CDCl3) δ 149.3, 137.9, 129.0, 127.5, 126.3, 83.5, 25.0 ppm (C-B signal
not observed due to low intensity); 11B NMR (160 MHz, CDCl3) δ 30.0
ppm; m.p. 140-141 ºC. NMR data were in accordance with those
previously reported. [24d]
(E)-1-Fluoro-3-(4-methoxystyryl)benzene (2). After hydroboration of 1-
ethynyl-3-fluorobenzene (0.52 mL, 4.5 mmol) following the general
procedure A, the crude was dissolved in deoxygenated DMF (15 mL)
under inert conditions, and Pd(OAc)2 (50.5 mg, 0.23 mmol), tBu3PHBF4
(156 mg, 0.45 mmol) and K2CO3 (746 mg, 5.4 mmol) were added, stirring
at rt for 10 minutes. 1-Bromo-4-methoxybenzene (678 µL, 5.4 mmol) was
then added and the flask was heated at 90 °C for 18 hours. The reaction
was then cooled to rt and the organic phase was extracted with EtOAc
and washed with water and brine. Then it was dried over Na2SO4 and
concentrated under vacuum. The crude was purified by flash
chromatography (hexane/EtOAc, 95/5). White solid (648 mg, 63%). 1H
NMR (300 MHz, CDCl3) δ 7.46 (d, J = 8.7 Hz, 2H), 7.33 – 7.17 (m, 3H),
(E)-4,4,5,5-Tetramethyl-2-(2-(thiophen-3-yl)vinyl)-1,3,2-
dioxoborolane (1k). Prepared from 3-ethynylthiophene (0.48 mL, 4.5
mmol) and pinacolborane (1.31 mL, 9 mmol) following the general
procedure A and purified by flash chromatography (hexane/EtOAc, 95/5).
1
Brown oil (737 mg, 70%). H NMR (300 MHz, CDCl3) δ 7.38 (d, J = 18.4
Hz, 1H), 7.32 – 7.25 (m, 3H), 5.94 (d, J = 18.4 Hz, 1H), 1.30 (s, 12H)
ppm; 13C NMR (101 MHz, CDCl3) δ 143.3, 141.3, 126.2, 125.1, 125.0,
116.2 (bs), 83.4, 24.9 ppm; 11B NMR (160 MHz, CDCl3) δ 30.2 ppm.
NMR data were in accordance with those previously reported. [12b]
4
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10.1002/ejoc.202000110
European Journal of Organic Chemistry
FULL PAPER
7.07 (d, J = 16.3 Hz, 1H), 6.97 – 6.88 (m, 4H), 3.84 (s, 3H) ppm; 13C
NMR (75 MHz, CDCl3) δ 163.4 (d, J = 244.9 Hz), 159.7, 140.2 (d, J = 7.8
Hz), 130.2 (d, J = 8.5 Hz), 129.8, 129.7, 128.1, 125.5 (d, J = 2.6 Hz),
122.3, 114.3, 114.1 (d, J = 21.5 Hz), 112.6 (d, J = 21.7 Hz), 55.5 ppm;
19F NMR (282 MHz, CDCl3) δ -108.0 ppm; m.p. 106-108 ºC. NMR data
were in accordance with those previously reported. [24e]
Acknowledgements
We thank MINECO (CTQ2017-85263-R), Instituto de Salud
Carlos III (FEDER funds, ISCIII RETIC REDINREN
RD16/0009/0015), Comunidad de Madrid and Universidad de
Alcalá (CM/JIN/2019-025 and CCG19/CC-038) for financial
support. J. A. also thank MEFP for a predoctoral contract.
(E)-1-(2-Iodovinyl)-4-(trifluoromethyl)benzene (3). After hydroboration
of 1-ethynyl-4-(trifluoromethyl)benzene (0.74 mL, 4.5 mmol) following the
general procedure A, the crude was dissolved in THF (45 mL), and an
aqueous 3.0 M solution of NaOH (4.5 mL,13.5 mmol) was added, stirring
at room temperature for 10 minutes. I2 (2,28 g, 9 mmol) was added and
the reaction continued stirring for 2 hours. The organic solution was
diluted in Et2O and washed with a saturated solution of Na2S2O3 and
brine. The organic phase was dried over Na2SO4 and concentrated under
vacuum. The crude was purified by flash chromatography (hexane/EtOAc,
98/2). Pale yellow solid (875 mg, 65%). 1H NMR (300 MHz, CDCl3) δ
7.58 (d, J = 8.2 Hz, 2H), 7.47 (d, J = 15.0 Hz, 1H), 7.39 (d, J = 8.3 Hz,
2H), 7.03 (d, J = 15.0 Hz, 1H) ppm; 13C NMR (75 MHz, CDCl3) δ ppm.
143.8, 140.9, 130.2 (q, J = 32.4 Hz), 126.3, 125.9 (q, J = 3.5 Hz), 124.1
(d, J = 267.1 Hz), 80.1 ppm; 19F NMR (282 MHz, CDCl3) δ -57.0 ppm;
m.p. 40-41 ºC. NMR data were in accordance with those previously
reported.[24f]
Keywords: alkynes • hydroboration • microwaves • alkenyl
boronates • solvent-free
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(0.49 mL, 4.5 mmol) following the general procedure A, the crude was
dissolved in MeOH (18 mL), sodium azide (439 mg, 6.75 mmol) and
CuSO4 (431 mg, 2.7 mmol) were added and the reaction was stirred
overnight. The crude was purified by flash chromatography (hexane/
EtOAc, 95/5). Yellow oil (338 mg, 52 %). 1H NMR (300 MHz, CDCl3) δ
7.44 – 7.21 (m, 5H), 6.64 (d, J = 13.8 Hz, 1H), 6.33 (d, J = 13.8 Hz, 1H)
ppm; 13C NMR (75 MHz, CDCl3) δ 135.0, 128.7, 127.3, 126.6, 125.8,
119.7 ppm. NMR data were in accordance with those previously
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(E)-(2-(Allyloxy)vinyl)benzene
(5).
After
hydroboration
of
phenylacetylene (0.49 mL, 4.5 mmol) following the general procedure A,
the crude was dissolved in prop-2-en-1-ol (8 mL) and triethylamine (2.51
mL, 18 mmol) and CuSO4 (1.63 g, 9 mmol) were added and the reaction
was stirred overnight. The reaction was diluted in Et2O and washed with
water and brine. The organic phase was dried over Na2SO4 and
concentrated under vacuum. The crude was purified by flash
chromatography (hexane). Pale yellow oil (238 mg, 33%). 1H NMR (300
MHz, CDCl3) δ 7.43 – 7.30 (m, 4H), 7.29 – 7.20 (m, 1H), 7.10 (d, J = 12.9
Hz, 1H), 6.18 – 6.05 (m, 1H), 6.02 (d, J = 12.9 Hz, 1H), 5.55 – 5.44 (m,
1H), 5.43 – 5.32 (m, 1H), 4.50 – 4.39 (m, 2H) ppm; 13C NMR (75 MHz,
CDCl3) δ 147.4, 136.3, 133.2, 128.5, 125.7, 125.1, 117.7, 106.6, 70.8
ppm. NMR data were in accordance with those previously reported.[6c]
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After
hydroboration of 1-ethynyl-3-fluorobenzene (0.52 mL, 4.5 mmol)
following the general procedure A, the crude was dissolved in MeOH (8
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1
yield 598 mg (62%) of an off-white solid. H NMR (500 MHz, CD3CN) δ
7.29 (dd, J = 14.1, 7.9 Hz, 1H), 7.19 (dd, J = 23.4, 9.4 Hz, 2H), 6.89 (t, J
= 8.5 Hz, 1H), 6.63 (d, J = 18.2 Hz, 1H), 6.35 (d, J = 18.2 Hz, 1H) ppm;
13C NMR (126 MHz, CD3CN) δ 164.1 (d, J = 242.0 Hz), 143.9 (d, J = 7.4
Hz), 137.1 (bs), 135.5 – 134.9 (m), 130.9 (d, J = 8.6 Hz), 123.0 (d, J =
2.4 Hz), 113.8 (d, J = 21.6 Hz), 112.8 (d, J = 21.3 Hz) ppm; 19F NMR
(282 MHz, CD3CN) δ -57.1 ppm; 11B NMR (160 MHz, CD3CN) δ 2.5 ppm;
m.p. 102-103 ºC; HRMS-ESI m/z calcd for C8H6BF4 [M-K]- 189.0506,
found 189.0582.
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5
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10.1002/ejoc.202000110
European Journal of Organic Chemistry
FULL PAPER
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6
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Alkyne hydroboration
A general method for solvent-free hydroboration of alkynes has been developed by microwave irradiation, to get (E)-alkenyl
boronates. Several functionalizations in the C-B bond can be performed sequentially without the purification of the intermediates.
Institute and/or researcher Twitter usernames: @IQAR_UAH
7
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