Copper(ii)-catalyzed protoboration of allenes in aqueous media and open air

Article abstract of DOI:10.1039/d0nj02010a

A method has been developed for the facile Cu(ii)-catalyzed protoboration of monosubstituted allenes in aqueous media under atmospheric conditions. The reaction occurs site selectively, favoring internal alkene protoboration to afford 1,1-disubstituted vinylboronic acid derivatives (up to 93?:?7) with modest to good yields. The method has been applied to a variety of phenylallene derivatives as well as alkyl-substituted allenes. This journal is

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Copper(II)-catalyzed protoboration of allenes in
aqueous media and open air†
Cite this:DOI: 10.1039/d0nj02010a
Russell F. Snead, Jan Nekvinda
and Webster L. Santos
*
A method has been developed for the facile Cu(II)-catalyzed protoboration of monosubstituted allenes
in aqueous media under atmospheric conditions. The reaction occurs site selectively, favoring internal
alkene protoboration to afford 1,1-disubstituted vinylboronic acid derivatives (up to 93 : 7) with modest
to good yields. The method has been applied to a variety of phenylallene derivatives as well as alkyl-
substituted allenes.
Received 21st April 2020,
Accepted 9th June 2020
DOI: 10.1039/d0nj02010a
rsc.li/njc
Ma9 (using phosphine ligands, Fig. 1a) and Hoveyda10 (using
NHC ligands, Fig. 1b) both developed Cu(I)-catalyzed proto-
Introduction
Methods for the synthesis of vinyl organoboron compounds are boration reactions of monosubstituted allenes. In latter, the
in demand due to their versatility as substrates in a wide variety site selectivity was ligand-controlled; decreasing ligand bulk
of functionalizations, and capability to engage in various cross- altered selectivity from terminal to internal alkene protoboration.
coupling reactions1 – most notably, Suzuki–Miyaura coupling.2 The strict ligand control of the reactions as well as the use of
Methods for transition metal-catalyzed hydroboration3 reactions relatively inexpensive/environmentally friendly copper catalysts
have been developed for the efficient synthesis of viable cross- make these methods quite valuable. The site selectivity of both
coupling partners. These hydroboration reactions typically occur methods was dictated by whether or not the putative Cu–B inser-
via formation of a boron-ligated metal complex, which adds in a tion intermediate underwent isomerization to a more reactive form
concerted cis fashion across a double or triple carbon–carbon bond. prior to protonation by a protic additive, e.g. methanol (Curtin–
Vinylboronic acid derivatives are typically synthesized Hammett kinetics, vide infra). Later, Semba et al. demonstrated an
through effective hydroboration2a,3a,4 or diboration4b,c,5 of elec- allene protoboration using pinacolborane, which was purported to
tron rich alkynes or allenes, either by addition of a borane or occur through formation of a Cu(I)-hydridic intermediate (Fig. 1c).6
use of a suitable diboron compound in conjunction with a Finally, Hoveyda developed an enantioselective protoboration of
proton source (e.g. methanol). Because six plausible hydrobora- disubstituted allenes that was achieved by the combination of
tion products may be formed, allenes are both a versatile and chiral ligand–copper complex with bulky alcohol (Fig. 1d).11
challenging substrate to borylate since regio- and stereo-
Previous work in our group demonstrated the Cu(II)-catalyzed
selectivity must be tightly controlled. Furthermore, allenes protoboration of a,b-unsaturated ketones with amine additives
often lack a strong electron-withdrawing moiety to encourage and bis(pinacolato)diboron in aqueous, open-to-air conditions.12
and direct metal–boron addition to specific positions in transi- Through base-assisted activation of water, an sp2–sp3 diboron
tion metal-catalyzed protoboration reactions. However, several reagent was generated in situ, allowing for transmetallation to
examples have been reported in which monosubstituted allenes form boron-ligated copper and subsequent addition across the
can be converted to boronic acid derivatives with varying carbon–carbon double bond. This facile borylation protocol has
degrees of regioselectivity.6 Under classical conditions,7 the since been extended13 to include other substrates, such as
hydroboration of allene double bonds is achieved with the alkynoic esters and amides,4d a,b-unsaturated nitriles,14 and
typical anti-Markovnikov regioselectivity. Miyaura et al. later imines.15 However, these methods are restricted to polarized,
developed a platinum-catalyzed hydroboration reaction8 with electron-deficient p-bonds, which drive selectivity. Most
ligand-controlled regio- and stereoselectivity. More recently, recently, functionalization of allenes producing 1,3-butadienes
in aqueous media in the presence of palladium was disclosed.3l
Herein, we present the extension of this facile and mild protocol
to monosubstituted allenes and demonstrate site selectivity via
ligand control under mild, environmentally friendly conditions
(Fig. 1e).
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA.
E-mail: santosw@vt.edu
† Electronic supplementary information (ESI) available: 1H, 13C, and 11B spectra
for all vinylboronic acid derivative products. See DOI: 10.1039/d0nj02010a
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Table 1 Optimization of reaction conditions
Entry Base
Cu source
CuSO4
Ligand
% yieldb (2a : 3a)
1a
4-Picoline
Proton sponge CuSO4
None
None
None
None
None
PCy3
PCy3
PCy3
PCy3
10 (34 : 66)
13 (40 : 60)
9 (30 : 70)
16 (38 : 62)
24 (35 : 65)
65 (23 : 77)
49 (21 : 79)
0
2a
3a
DMAP
NaOAc
NH2Bn
NH2Bn
NH2Bn
NH2Bn
None
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
None
CuSO4
CuSO4
CuSO4
CuSO4
CuSO4
Cu(acac)2
Cu(BF4)2
4a
5a
6a
7ac
8a
9ad
10
11
12
13
14
15
16
17
18e
19e
20f
21f
68 (33 : 67)
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
NH2Bn
DPEPhos 52 (46 : 54)
Sphos
PPh3
PCy3
PPh3
PPh3
63 (81 : 19)
78 (87 : 13)
63 (31 : 69)
60 (85 : 15)
53 (77 : 23)
32 (81 : 19)
19 (76 : 24)
50 (74 : 26)
57 (27 : 73)
52 (30 : 70)
36 (64 : 36)
C10H6CuN4O4 PPh3
Cu(OH)2
CuSO4
CuSO4
CuSO4
CuSO4
PPh3
PPh3
PCy3
PCy3
PPh3
a
b
1.5 equiv. B2pin2 used. Yields were determined by 1H NMR analysis
c
of the crude reaction mixture after extraction. 1% surfactant TPGS-
d
e
750-M used. 6 hours. 4 : 1 water : toluene mixture used as solvent.
f
4 : 1 toluene : water mixture used as solvent. C10H6CuN4O4 = Cu(II)
2-pyrazinecarboxylate.
Lipshutz et al.16 demonstrated that micellar catalysis mediated by
this surfactant enabled copper-catalyzed silylation reactions; how-
ever, no improvement in yield was observed (entry 7). Fernandez
reported a phosphine-catalyzed protocol17 for the hydroboration of
a,b-unsaturated ketone, thus we performed a control experiment
without CuSO4 (entry 8) and discovered that the reaction was
indeed copper catalyzed. We also tested the reaction efficiency sans
benzylamine, which furnished the product in good yield and
selectivity. Additionally, the reaction time was increased by several
hours due to what appeared to be a long induction time (entry 9).
With the increased reactivity accompanying phosphine addition,
we noted through GC analysis that the 1,1-disubstituted vinylbor-
onate product could be protoborated for second time. This
hypothetically would result in a decrease of product yield as well
as affect apparent selectivity by diminishing amount of product 2a.
To circumvent this, we adjusted the quantity of B2pin2 to
1.1 equivalents, which significantly minimized this side product.
Among the four phosphine ligands tested (entries 10–13), SPhos
and PPh3 performed equally well for selection of the desired vinyl
boronate product 2a. However, neither yield nor selectivity were
improved. Thus, we chose triphenylphosphine for further studies
and restricted our attention solely to product 2a. A short screening
of various copper sources (entries 14–17) demonstrated that
many copper(II) derivatives could function effectively as cata-
lysts, although CuSO4 worked the best. The effect of toluene as
Fig. 1 Cu(I)-catalyzed and ligand-controlled methods for allene
protoboration.
Results and discussion
Using phenylallene 1a as the optimization substrate, we first
tested conditions similar to those previously-developed for
Cu(II)-catalyzed b-borylation of a,b-unsaturated ketones. These
conditions employ an amine as a Brønsted base, copper source,
ligand, and bis(pinacolato)diboron. As shown in Table 1
(entries 1–5), several bases that were highly effective in the
hydroboration of other substrates6,9,10,12 were unsuitable for allene
protoboration. Benzylamine proved most effective, affording bor-
ylation product in 24% yield with moderate selectivity (entry 5).
Based on previous work by Ellman et al., which demonstrated
increased yields with use of phosphine ligands in aqueous imine
borylations,15b we tested the effect of 10 mol% PCy3 additive on the
reaction outcome and were gratified to find both an increase in
yield (65% combined) and site selectivity 77% for terminal proto-
boration to afford 3a (entry 6). Since the relative insolubility of the
starting materials in water might account for the low yields, we
tested the effect of surfactant (TPGS-750-M) on the reaction.
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co-solvent was tested for both systems containing PPh3 and
PCy3 (entries 18–21). This was to evaluate whether (1) increased
solvation of the reactants by toluene would affect the selectivity
and yield and (2) reducing the amount of water in the system
would allow time for borylcuprate intermediate isomerization
before protonation, thus affecting selectivity.
With our best conditions (entry 12) in hand for formation of
product 2a, we applied the protocol to determine the substrate
scope of the reaction (Fig. 2). Phenylallene derivatives bearing para
alkyl substitutions (2b–2c) demonstrated B90 : 10 isomeric ratio
favoring the 1,1-disubstituted vinylboronic ester product, with a
reduction in yield observed with the p-tBu-substituted allene.
Derivatives bearing a chlorine at the ortho, meta, or para positions
underwent protoboration in good yield and high selectivity, regard-
less of position on the phenyl ring (2d–2f). A p-trifluoromethyl or
p-fluoro substitution (1g–1h) reduced protoboration efficiency
but selectivity remained (93% and 483% alkene selectivity,
respectively). Interestingly, phenylallene derivatives with strong
electron-donating groups (2i–2k) had reduced yields and selec-
tivities. However, both the allyloxy and benzyloxy protecting
groups were well-tolerated in this reaction.
Since the presence of a phenyl group may provide stabilization
to the partial negative charge of the Cu–C bond in the proposed
intermediates (vide infra), it was of interest to determine if alkyl-
substituted allenes were suitable substrates. The reaction per-
formed well for cyclohexylallene (1l) and moderately well for alkyl
ethers (1m–1n). In the case of long alkyl chain allenes, there
appeared to be solubility issues and very low conversions when
run under typical conditions. It was found that use of 25% toluene
as a co-solvent resulted in moderate to good yield with minimal
effects on selectivity of the reaction (2o–2q).
Based on the results, we believe the mechanism proceeds as
described previously by Hoveyda and coworkers (Fig. 3).10 The initial
step is Brønsted base-assisted deprotonation of a water molecule (4)
and formation of an activated sp3-hybridized boronate 5.12a Trans-
metallation forms a copper boron complex 6 that coordinates to the
terminal (7) or internal (8) double bond of the allene to afford
allylcopper species 9 and 10, respectively. The large triphenylpho-
sphine ligand promotes preferential binding of the copper complex
on the terminal double bond leading to chair-like transition state 11
to generate 1,1-substituted vinylboronic acid derivative 12 as the
major product. In contrast, insertion of the Cu–B species on the
internal alkene followed by protonation via 13 leads to trisubstituted
vinyl boronic ester 14. We suspect that equilibration between 9 and
10 is minimized and leads to rapid protonation because the solvent
itself acts a proton source. Thus, the isomeric ratio of borylation
products reflects the ratio of intermediates 9 and 10, which is
supported by the observation that a decrease in selectivity occurs
with electron donating substituents where the more electron rich
double bond increases binding with copper.
Fig. 2 Substrate scope of the protoboration reaction. Conditions: allene 1
(1 equiv.), BnNH2 (5 mol%), PPh3 (10 mol%), B2pin2 (1.1 equiv.), CuSO4
(1 mol%). a 1% TPGS-750-M used. b 3 : 1 toluene : water used as solvent.
performs moderately well for a variety of monosubstituted
allenes and is a first step for application of environmentally
Conclusions
In conclusion, we have described the first Cu(II)-catalyzed proto- friendly methodologies to the borylation of relatively inacti-
boration reaction of allenes in aqueous media. The reaction vated carbon–carbon unsaturated bonds.
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Fig. 3 Proposed mechanism for the Cu(II)-catalyzed protoboration.
Experimental
General experimental details
(0–4% EtOAc in hexanes) affording the final products as off-
yellow oils.
4,4,5,5-Tetramethyl-2-(3-phenylprop-1-en-2-yl)-1,3,2-dioxaborolane
(2a) and (Z)-4,4,5,5-tetramethyl-2-(1-phenylprop-1-en-2-yl)-1,3,2-
dioxaborolane (3a). Synthesized by general procedure, isolated
as an off yellow oil, 78% yield, 87 : 13 isomeric ratio 2a to 3a.
1H NMR (500 MHz, CDCl3) d 7.26–7.08 (m, 5H), 5.76 (d, J =
2.8 Hz, 1H), 5.45 (s, 1H), 3.41 (s, 2H), 1.92* (d, J = 1.4 Hz, 3H),
1.23* (s, 12H), 1.13 (s, 12H). 13C NMR (101 MHz, CDCl3)
d 142.5*, 140.8, 129.9, 129.5*, 129.3, 129.3*, 128.2, 128.2*,
127.2*, 125.8, 83.6*, 83.6, 41.5, 25.0*, 24.8, 16.0*. 11B NMR
(128 MHz, CDCl3) d 30.09. EI-MS: [M]+. Calcd for C15H21BO2
244.16, observed 244.1.
4,4,5,5-Tetramethyl-2-(3-(p-tolyl)prop-1-en-2-yl)-1,3,2-dioxaborolane
(2b) and (Z)-4,4,5,5-tetramethyl-2-(1-(p-tolyl)prop-1-en-2-yl)-1,3,2-
dioxaborolane (3b). Synthesized by general procedure, isolated
as an off yellow oil, 67% yield, 89 : 11 isomeric ratio of 2b to 3b.
1H NMR (500 MHz, CDCl3) d 7.31* (d, J = 7.9 Hz, 2H), 7.23*
(s, 1H), 7.16* (d, J = 7.8 Hz, 2H), 7.11–7.07 (m, 4H), 5.84 (s, 1H),
Synthesis of allenes and their precursors5b,e were carried out under
an inert atmosphere of nitrogen. All borylation reactions were carried
out under atmospheric conditions. NMR spectra were obtained on
Bruker 500 MHz spectrometer at 500 (1H) and 125 (13C) MHz or
Unity-plus 400 at 400 (1H) and 100 (13C) MHz. The chemical shifts
are reported in d (ppm), and coupling constants are given in Hz.
* indicates minor isomer. High-resolution ESI mass spectra
were obtained on an Agilent 6220 accurate mass TOF LC/MS.
Low-resolution EI mass spectra were obtained on a 5977A Series
GC/MSD system. Column chromatography was performed
using Silica gel (ZEOprep 60 ECO 40-63. TLC analyses were
performed using Agela Technologies silica gel MF254 plates or
Silicycle aluminum backed silica gel F-254 plates and spots
were visualized with UV light and KMnO4 stain.
General procedure for synthesis of vinylboronic acid derivatives
To a 2-dram vial equipped with a small stir bar was added allene, 5.52 (s, 1H), 3.45 (s, 3H), 2.36* (s, 3H), 2.32 (s, 3H), 2.01* (s, 3H),
bis(pinacolato)diboron (1.1 equiv.), benzylamine (5 mol%), and 1.33* (s, 12H), 1.24 (s, 12H). 13C NMR (126 MHz, CDCl3) d 142.5*,
triphenylphosphine (10 mol%). Milli-Q water and CuSO4 solution 137.7, 137.0*, 135.3*, 135.2, 129.8, 129.6*, 129.1, 128.9, 128.9*,
(1.3 mg mLÀ1, dispensed to add 1 mol% Cu) were added in 83.6, 83.6*, 41.00, 25.00*, 24.9, 21.4*, 21.1, 16.1*. 11B NMR
equal volume amounts to the mixture, and the dark reaction (128 MHz, CDCl3) d 30.16. EI-MS: [M]+ calcd for C16H23BO2
mixture was allowed to stir until completion (typically 3 h) as 258.18, observed 258.3.
monitored by GC or TLC. Upon completion, chloroform was
2-(3-(4-(tert-Butyl)phenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-
added to the reaction mixture, and the water layer was extracted 1,3,2-dioxaborolane (2c) and (Z)-2-(1-(4-(tert-butyl)phenyl)prop-
3Â with chloroform. After removal of the solvent in vacuo, the 1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3c). Synthe-
resulting yellow residue was purified by flash chromatography sized by general procedure, isolated as off yellow oil, 47% yield,
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90 : 10 isomeric ratio of 2c to 3c. 1H NMR (500 MHz, CDCl3) (500 MHz, CDCl3) d 7.38–7.31* (m, 2H), 7.18* (s, 1H), 7.17–7.12
d 7.33* (q, J = 8.6 Hz, 4H), 7.24 (d, J = 4.2 Hz, 2H), 7.18* (s, 1H), (m, 2H), 7.03* (t, J = 8.6 Hz, 2H), 6.94 (t, J = 8.6 Hz, 2H), 5.82
7.10 (d, J = 7.9 Hz, 2H), 5.79 (s, 1H), 5.49 (s, 1H), 3.42 (s, 2H), (s, 1H), 5.53 (s, 1H), 3.44 (s, 2H), 1.97* (s, 3H), 1.31* (s, 12H), 1.20
1.98* (s, 3H), 1.30* (s, 9H), 1.29* (s, 12H), 1.27 (s, 9H), 1.19 (s, 12H). 13C NMR (126 MHz, CDCl3) d 161.5 (d, J = 243.0 Hz),
(s, 12H). 13C NMR (101 MHz, CDCl3) d 150.2*, 148.6, 142.4*, 141.3, 136.5 (d, J = 3.0 Hz), 131.2* (d, J = 8.1 Hz), 130.6 (d, J =
137.7, 129.8, 129.4*, 128.9, 125.1, 83.6, 40.9, 34.7*, 34.5, 31.6, 7.7 Hz), 130.0, 114.9 (d, J = 20.9 Hz), 100.2*, 83.7, 40.8, 25.0*,
31.4*, 25.0*, 24.8, 16.2*. 11B NMR (128 MHz, CDCl3) d 30.07. 24.8, 15.9*. 11B NMR (128 MHz, CDCl3) d 29.98. EI-MS: [M]+.
EI-MS: [M]+. Calcd for C19H29BO2 300.23, observed 300.3.
Calcd for C15H20BFO2 262.15, observed 262.3.
2-(3-(4-Chlorophenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-
2-(3-(4-Methoxyphenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2d) and (Z)-2-(1-(4-chlorophenyl)prop-1-en-2-yl)- dioxaborolane (2i) and (Z)-2-(1-(4-methoxyphenyl)prop-1-en-2-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3d). Synthesized by general 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3i). Synthesized by general
procedure 4, isolated as yellow oil, 87% yield, 90 : 10 isomeric ratio procedure, isolated as an off yellow oil, 46% yield, 67 : 33 isomeric
1
2d to 3d. H NMR (500 MHz, CDCl3) d 7.31* (s, 4H), 7.22 (d, J = ratio 2i to 3i. 1H NMR (400 MHz, CDCl3) d 7.43–7.32* (m, 2H), 7.18*
8.1 Hz, 2H), 7.17* (s, 1H), 7.12 (d, J = 8.1 Hz, 2H), 5.83 (s, 1H), (d, J = 1.8 Hz, 1H), 7.14–7.05 (m, 2H), 6.92–6.85* (m, 2H), 6.85–6.77
5.53 (s, 1H), 3.43 (s, 2H), 1.96* (s, 3H), 1.31 (s, 12H), 1.21 (s, 12H). (m, 2H), 5.81 (dt, J = 3.3, 1.2 Hz, 1H), 5.51 (dt, J = 3.4, 1.6 Hz, 1H),
13C NMR (126 MHz, CDCl3) d 141.1*, 139.4, 131.6, 130.8*, 3.82* (s, 3H), 3.78 (s, 3H), 3.42 (d, J = 1.4 Hz, 2H), 2.00* (d, J =
130.6, 130.3, 128.4*, 128.3, 83.7, 41.00, 25.0*, 24.9, 16.0*. 11B NMR 1.7 Hz, 3H), 1.31* (s, 12H), 1.21 (s, 12H). 13C NMR (101 MHz,
(128 MHz, CDCl3) d 29.92. EI-MS: [M]+. Calcd for C15H20BClO2 CDCl3) d 158.8*, 157.9, 142.1*, 132. 9, 131.1*, 130.9, 130.2*, 129.6,
278.12, observed 278.1.
113.7, 113.6*, 83.6, 83.5*, 55.4, 55.4*, 40.6, 25.0*, 24.8, 16.1*.
2-(3-(3-Chlorophenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2- 11B NMR (128 MHz, CDCl3) d 30.21. EI-MS: [M]+. Calcd for
dioxaborolane (2e) and (Z)-2-(1-(4-chlorophenyl)prop-1-en-2-yl)- C16H23BO3 274.17, observed 274.3.
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3e). Synthesized by general
2-(3-(4-(Benzyloxy)phenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-
procedure, isolated as yellow oil, 62% yield, 89 : 11 isomeric ratio 2e 1,3,2-dioxaborolane (2j) and (Z)-2-(1-(3-(benzyloxy)phenyl)prop-
to 3e. 1H NMR (500 MHz, CDCl3) d 7.16–6.96 (m, 4H), 5.77 (d, J = 1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3j). Synthe-
2.3 Hz, 1H), 5.48 (s, 1H), 3.36 (s, 2H), 1.92–1.86* (m, 3H), 1.23* sized by general procedure, isolated as white solid, 37%,
(s, 12H), 1.13 (s, 12H). 13C NMR (126 MHz, CDCl3) d 143.0, 140.9*, 66 : 34 isomeric ratio 2j to 3j. 1H NMR (500 MHz, CDCl3)
133.9, 130.5, 129.4, 129.4, 127.6*, 127.4, 127.2*, 126.0, 83.7, 41.4, d 7.48–7.30 (m, 5H from 2j and 7H from 3j), 7.20* (s, 1H),
25.0*, 24.8, 16.0*. EI-MS: [M]+. Calcd for C15H20BClO2 278.12, 7.12 (d, J = 7.8 Hz, 2H), 6.97* (d, J = 8.1 Hz, 0H), 6.90 (d, J =
observed 278.1.
7.8 Hz, 2H), 5.82 (s, 1H), 5.53 (s, 1H), 5.08* (s, 2H), 5.05* (s, 2H),
2-(3-(2-Chlorophenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2- 3.44 (s, 2H), 2.02* (s, 3H), 1.32* (s, 12H), 1.22 (s, 12H). 13C NMR
dioxaborolane (2f) and (Z)-2-(1-(2-Chlorophenyl)prop-1-en-2-yl)- (126 MHz, CDCl3) d 158.0*, 157.1, 142.0*, 137.4, 137.1*, 133.2,
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3f). Synthesized by gen- 131.1*, 131.1*, 130.2, 129.6, 128.7*, 128.7, 128.1*, 128.0, 127.7*,
eral procedure, isolated as yellow oil, 87% yield, 90 : 10 isomeric 127.6, 114.7, 114.5*, 83.6, 83.6*, 70.2, 70.1*, 40.7, 25.0*, 24.8,
1
ratio 2f to 3f. H NMR (500 MHz, CDCl3) d 7.44–7.12 (m, 4H), 16.1*. 11B NMR (128 MHz, CDCl3) d 30.13. EI-MS: [M]+. Calcd for
5.92 (s, 1H), 5.47 (s, 1H), 3.62 (s, 2H), 1.89* (s, 3H), 1.35 (s, 12H), C22H27BO3 350.21, observed 350.3.
1.27 (s, 12H). 13C NMR (126 MHz, CDCl3) d 139.4*, 138.41,
2-(3-(4-(Allyloxy)phenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-
136.2*, 134.5, 133.7*, 131.3, 130.8*, 130.7, 129.5*, 129.4, dioxaborolane (2k) and (Z)-2-(1-(4-(allyloxy)phenyl)prop-1-en-2-yl)-
128.4*, 127.4, 126.6, 126.1*, 83.7, 38.2, 25.00*, 24.9, 16.0*. 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3k). Synthesized by
11B NMR (128 MHz, CDCl3) d 30.05. EI-MS: [M]+. Calcd for general procedure, isolated as an off yellow oil, 49% yield,
C15H20BClO2 278.12, observed 278.1.
isomeric ratio 2k to 3k is 75 : 25. 1H NMR (500 MHz, CDCl3)
4,4,5,5-Tetramethyl-2-(3-(4-(trifluoromethyl)phenyl)prop-1-en- d 7.35* (d, J = 8.6 Hz, 2H), 7.17* (s, 1H), 7.10 (d, J = 8.4 Hz, 2H),
2-yl)-1,3,2-dioxaborolane (2g) and (Z)-4,4,5,5-Tetramethyl-2-(1-(4- 6.89* (d, J = 8.6 Hz, 2H), 6.82 (d, J = 8.5 Hz, 2H), 6.06 (ddq,
(trifluoromethyl)phenyl)prop-1-en-2-yl)-1,3,2-dioxaborolane (3g). J = 15.7, 10.3, 5.0 Hz, 1H), 5.81–5.79 (m, 1H), 5.51 (s, 1H), 5.41
Synthesized by general procedure, isolated as a yellow oil, 19% (dd, J = 17.2, 10.3 Hz, 1H), 5.28 (t, J = 11.6 Hz, 1H), 4.55* (d, J =
1
yield, 93 : 7 isomeric ratio 2g to 3g. H NMR (500 MHz, CDCl3) 5.2 Hz, 2H), 4.51 (d, J = 5.2 Hz, 2H), 3.41 (s, 2H), 1.99* (s, 3H),
d 7.43 (d, J = 8.0 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H), 5.80 (d, J = 3.0 Hz, 1.31* (s, 12H), 1.21 (s, 12H). 13C NMR (126 MHz, CDCl3)
1H), 5.48 (d, J = 3.4 Hz, 1H), 3.45 (s, 2H), 1.90* (d, J = 1.9 Hz, 3H), d 157.7*, 156.8, 141.9*, 133.6, 133.3, 133.0, 130.9*, 130.0, 129.4,
1.24* (s, 12H), 1.13 (s, 12H). 13C NMR (126 MHz, CDCl3) d 143.9, 117.7*, 117.4, 114.4, 114.3*, 83.44, 83.4*, 68.9, 68.8*, 40.5, 24.8*,
129.7, 128.3, 127.1 (q, J = 32.2 Hz), 124.5 (q, J = 271.7 Hz), 124.00 24.7, 15.9*. 11B NMR (128 MHz, CDCl3) d 30.02. HRMS: (ESI)
(q, J = 3.8 Hz), 82.6, 40.2, 28.7, 23.6. 11B NMR (128 MHz, CDCl3) [M + H]+ calcd for C18H25BO3 301.20, observed 301.1968.
d 29.80. EI-MS: [M]+. Calcd for C16H20BF3O2 312.15, observed 312.1.
2-(3-Cyclohexylprop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
2-(3-(4-Fluorophenyl)prop-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-
borolane (2l) and (Z)-2-(1-cyclohexylprop-1-en-2-yl)-4,4,5,5-tetramethyl-
dioxaborolane (2h) and (Z)-2-(1-(4-fluorophenyl)prop-1-en-2-yl)- 1,3,2-dioxaborolane (3l). Synthesized by general procedure, iso-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3h). Synthesized by lated as a yellow oil, 84% yield, 78: 22 isomeric ratio 2l to 3l.
general procedure, isolated as a yellow oil, 19% isolated yield, 1H NMR (500 MHz, CDCl3) d 6.11* (d, J = 8.7 Hz, 1H), 5.81–5.70
50% NMR yield, 83: 17 isomeric ratio 2h to 3h. 1H NMR (m, 1H), 5.53 (s, 1H), 2.43–2.26* (m, 1H), 2.03 (d, J = 6.8 Hz, 2H),
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1.74–1.58 (m, 6H), 1.25 (s, 12H), 1.22–1.05 (m, 3H), 0.84 (q, J = 2.12 (t, J = 7.4 Hz, 2H), 1.66* (s, 3H), 1.44–1.16 (m, 34H), 0.87
11.3 Hz, 2H). 13C NMR (126 MHz, CDCl3) d 151.9*, 130.0, 83.4, (t, J = 6.5 Hz, 3H). 13C NMR (126 MHz, CDCl3) d 146.8*, 128.8,
83.1*, 43.4, 37.8, 37.7*, 33.3, 32.4*, 26.8, 26.6, 26.3*, 26.1*, 25.0*, 83.4, 83.1*, 35.5, 32.1, 29.9, 29.8, 29.8, 29.7, 29.5, 29.4, 29.4, 29.0,
24.8, 14.0*. 11B NMR (128 MHz, CDCl3) d 30.16. EI-MS: [M]+. 28.9, 24.9*, 24.9, 22.8, 14.2, 14.0*. 11B NMR (128 MHz, CDCl3) d
Calcd for C15H27BO2 250.21, observed 250.3.
30.15. EI-MS: [M]+. Calcd for C21H41BO2 336.32, observed 336.4.
2-(5-(Benzyloxy)pent-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (2m) and (Z)-2-(5-(benzyloxy)pent-2-en-2-yl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (3m). Synthesized by general
procedure, isolated as a yellow oil, 34%, 93 : 7 isomeric ratio 2m
Conflicts of interest
There are no conflicts to declare.
1
to 3m. H NMR (500 MHz, CDCl3) d 7.38–7.24 (m, 5H), 6.38–
6.28* (m, 1H), 5.79 (d, J = 3.3 Hz, 1H), 5.64–5.60 (m, 1H), 4.53*
(s, 2H), 4.50 (s, 2H), 3.54 (t, J = 7.2 Hz, 2H), 3.48 (t, J = 6.7 Hz, Acknowledgements
2H), 2.48* (q, J = 7.0 Hz, 2H), 2.24 (t, J = 7.6 Hz, 2H), 1.77 (dt, J =
14.1, 6.8 Hz, 2H), 1.71* (s, 3H), 1.26 (s, 12H). 13C NMR
(126 MHz, CDCl3) d 141.9*, 138.8, 138.6*, 129.5, 128.5*, 128.4,
127.8, 127.6, 127.6*, 83.6, 83.3, 73.0*, 72.9, 70.2, 69.3*, 32.0,
29.5*, 29.3, 24.9*, 24.9, 14.2*. EI-MS: [M]+. Calcd for C18H27BO3
302.21, observed 302.3.
We acknowledge the financial support by the National Science
Foundation (CHE-1414458). We thank AllyChem for donation
of B2pin2. RFS is a recipient of the Institute for Critical
Technology and Applied Science Doctoral Scholarship.
2-(4-(Benzyloxy)but-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane (2n) and (Z)-2-(4-(benzyloxy)but-2-en-2-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (3n). Synthesized by general procedure,
isolated as a yellow oil, 53%, 72 : 18 isomeric ratio 2n to 3n.
1H NMR (400 MHz, CDCl3) d 7.39–7.22 (m, 5H), 6.48* (tq, J = 5.8,
1.7 Hz, 1H), 5.85 (dt, J = 3.5, 0.9 Hz, 1H), 5.70 (dt, J = 3.3, 1.4 Hz,
1H), 4.53* (s, 2H), 4.52 (s, 2H), 4.17 (dq, J = 5.8, 1.1 Hz, 2H), 3.56
(t, J = 7.0 Hz, 2H), 2.49 (tt, J = 7.0, 1.1 Hz, 3H), 1.69* (dd, J = 1.8,
0.9 Hz, 3H), 1.26* (s, 12H), 1.24 (s, 12H). 13C NMR (101 MHz,
CDCl3) d 141.9*, 138.8, 131.2, 128.5*, 128.4, 127.9*, 127.8,
127.7*, 127.5, 83.5, 72.8, 72.6*, 70.1, 67.1*, 35.9, 24.93*, 24.9,
14.5*. EI-MS: [M]+. Calcd for C17H25BO3 288.19, observed 288.3.
2-(Dec-1-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2o)
and (Z)-2-(dec-2-en-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(3o). Synthesized by general procedure, isolated as a yellow oil,
48%. Isomeric ratio 2o to 3o is 74: 26. 1H NMR (500 MHz, CDCl3)
d 6.31* (t, J = 5.9 Hz, 1H), 5.83–5.68 (m, 1H), 5.58 (s, 1H), 2.12 (q,
J = 7.3 Hz, 2H), 1.66* (s, 3H), 1.26 (s, 24H), 0.87 (t, J = 6.4 Hz, 3H).
13C NMR (126 MHz, CDCl3) d 146.8*, 128.8, 83.4, 83.2*, 35.5,
32.1, 32.0*, 29.9*, 29.7, 29.6, 29.4, 29.4, 29.0*, 28.9*, 25.0*, 24.9,
22.8, 14.2, 14.0*. 11B NMR (128 MHz, CDCl3) d 30.16. EI-MS: [M]+.
Calcd for C16H31BO2 266.24, observed 266.3.
Notes and references
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