Electrochemical properties and reactions of organoboronic acid esters containing unsaturated bonds at their α-position

Article abstract of DOI:10.1149/2.0561702jes

Electrochemical analyses of 2-(cynnamyl)boronic acid pinacol ester and (3-phenyl-2-propynyl)boronic acid pinacol ester, and their trimethylsilyl analogues as well as their parent compounds were comparatively studied by cyclic voltammetry measurements. We

Full text of DOI:10.1149/2.0561702jes

Journal of The Electrochemical Society, 164 (2) G23-G28 (2017)
G23
0013-4651/2017/164(2)/G23/6/$33.00 © The Electrochemical Society
Electrochemical Properties and Reactions of Organoboronic Acid
Esters Containing Unsaturated Bonds at Their α-Position
Kazuhiro Ohtsuka,a Shinsuke Inagi,b,∗ and Toshio Fuchigamia,∗∗,z
aDepartment of Electronic Chemistry, Tokyo Institute of Technology, Yokohama 226-8502, Japan
bDepartment of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of
Technology, Yokohama 226-8502, Japan
Electrochemical analyses of 2-(cynnamyl)boronic acid pinacol ester and (3-phenyl-2-propynyl)boronic acid pinacol ester, and their
trimethylsilyl analogues as well as their parent compounds were comparatively studied by cyclic voltammetry measurements. We
found remarkable negative shifts of the oxidation potentials of the organoboronic acid pinacol esters in the presence of fluoride ions
compared to those in the absence of fluoride ions. The negative shift observed was more pronounced than that of the corresponding
trimethylsilyl compounds. Such marked negative shift seems to be derived from the formation of negatively charged boron-ate
complex with fluoride ions as well as β-effect of organoborate. Anodic acetoxylation of organoboronic acid pinacol esters was
achieved in NaOAc/AcOH, while their alkoxylation was successfully carried out in the presence of fluoride ions.
© 2016 The Electrochemical Society. [DOI: 10.1149/2.0561702jes] All rights reserved.
Manuscript submitted November 7, 2016; revised manuscript received December 2, 2016. Published December 14, 2016.
Organoboron compounds are highly useful in various fields such
as synthetic organic chemistry, pharmaceutical chemistry, and mate-
rials chemistry1–7 as well as analytical chemistry.8–16 For example,
much attention has been paid to electrochemistry and electrogener-
ated chemiluminescence of BODIPY (boron dipyrromethene) dyes13
and also recently, a new class of fluorescence PET sensors based on
anthraceneboronic acid ester for detection of a trace amount of water
in organic solvents has been developed.14 The preferential boron-ate
complex formation with fluoride ions is applicable to fluoride ion
sensors.8–11,15 Shinkai and his co-worker reported the molecular de-
sign of artificial sugar sensing systems.16 Electrodes modified with
carotenoids bearing a B(OH)2 group were also shown to be potential
sugar sensors.12 Furthermore, carboranes exhibit reversible reduction
behavior and their reduction potential is easily tunable with func-
tional groups on the carbon atoms.7,17 We have recently found that
carboranes are high-performance electrocatalysts for cathodic dehalo-
genation of alkyl halides.18,19 Waldvogel and his co-workers reported
quite unique boron-templated electrosynthesis of 2,2ꢀ-biphenols from
sodium tetraphenoxy borates.20 However, this report does not deal
with carbon-boron bond cleavage.
Quite recently, we found marked negative shifts of the oxidation
potentials of alkyl and arylboronic acids in the presence of fluoride
ions compared with those in the absence of fluoride ions.33 It was
also clarified that the negative shift was derived from the formation
of negatively charged boron-ate complex with fluoride ions.33 On the
other hand, it is well-known that allylsilane and benzylsilane exhibit
less positive oxidation potentials compared with the parent compounds
devoid of a silyl group owing to effective C-Si σ orbital overlapping
with a neighboring π orbital, which causes a significant increase in the
HOMO level (β-effect).34,35 The presence of a silyl group also assists
anodically selective cleavage of a C-Si bond.34,35
These facts prompted us to investigate the anodic oxidation behav-
ior of organoboronic acid pinacol esters having a neighboring double
or triple bond such as 2-(cynnamyl)boronic acid pinacol ester and
(3-phenyl-2-propynyl)boronic acid pinacol ester in organic solvents
in the absence and presence of fluoride ions using cyclic voltamme-
try as well as anodic substitution reaction in this work. Furthermore,
anodic oxidation behavior of their trimethylsilyl analogues was also
comparatively studied.
On the other hand, limited examples of electrochemical reaction of
triorganoboranes have been reported to date.6,17,21,22 Anodic oxidation
of triorganoboranes generally requires the activation with nucleophiles
to generate negatively charged boron-ate complex.23–26 Scha¨fer and
Koch reported anodic oxidation of trihexylborane in NaOMe/MeOH
in the presence of butadiene.23 They proposed that electron transfer
takes place from trihexylmethoxyborate formed in situ to generate
hexyl radical intermediates, which reacts with butadiene. However,
in this reaction, many products were formed by methoxylation and
dimerization.
Anodic oxidation of alkylboronic acids was also carried out in an
aqueous NaOH solution to afford the corresponding dimers or olefins
via radical or cation intermediates.27–29 Becker and his coworkers re-
ported anodic oxidation of arylboronic acid esters in the presence of
bromide ions leading to the cleavage of the carbon-boron bond to
provide the corresponding aryl bromides in moderate yields.30 In this
reaction, the generation of bromonium ion was proposed for the bond
cleavage. Mitsudo, Tanaka and their co-workers developed a facile
electrooxidative method for synthesizing biaryls from arylboronic
acids or arylboronic esters using a catalytic amount of Pd(OAc)2
and TEMPO.31 On the other hand, we have reported cathodic selec-
tive hydroxylation of various substituted phenylboronic acids under
oxygen atmosphere to provide phenol derivatives.32 In this case, elec-
trogenerated superoxide ions caused the C-B bond cleavage.
Experimental
General.—1H, 13C and 19F NMR spectra were recorded on JEOL
JNM EX-270 (1H: 270 MHz, 13C: 67.8 MHz, 19F: 254.05 MHz) spec-
trometer in CDCl3. The chemical shifts for 1H, 13C and 19F NMR spec-
tra were given in δ (ppm) from internal TMS (0.00), CDCl3 (77.0) and
monofluorobenzene (−36.5), respectively. Cyclic voltammetry mea-
surement was performed using BAS ALS Instruments model 600 A.
Preparative electrolysis experiments were carried out with Metronnix
Corp. (Tokyo) constant current power supply model 5944 by mon-
itoring electricity with Hokutodenko Coulomb/Ampere-hour meter
HF-201.
Measurement of cyclic voltammetry.—Cyclic voltammetry mea-
surement was carried out in a glass cell. A platinum disk electrode
(φ = 0.8 mm) was used as a working electrode. A platinum plate
(1 cm × 1 cm) was used as a counter electrode. A saturated calomel
electrode was used as a reference electrode. Electrolyte solutions for
cyclic voltammetry were deoxygenated with bubbling N2 gas before
use.
Materials.—4,4,5,5-Tetramethyl-2-cinnamyl-1,3,2-dioxaborolane
(1) was prepared in a similar manner to the reported procedure.36,37
The spectral data were identical to those of the authentic sample.38
Tetra-n-butylammonium cinnamyltrifluoroborate (2) was prepared
by the cation exchange reaction of the corresponding potassium
salts39 similarly to our previous report40 according to the literature
method.41
∗
Electrochemical Society Member.
Electrochemical Society Fellow.
∗∗
zE-mail: fuchi@echem.titech.ac.jp
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Journal of The Electrochemical Society, 164 (2) G23-G28 (2017)
Table I. Oxidation peak potentials (Epox) of cynnamyl boronate
ester, borate and related compounds.a
Tetra-n-butylammonium cinnamyltrifluoroborate (2).—Color-
less oil. 1H NMR (270 MHz, CDCl3) δ 7.25–7.13 (m, 4H), 7.00–6.95
(m, 1H), 6.61–6.55 (m, 1H), 6.07–6.01 (m, 1H), 3.42 (t, J = 8.6 Hz,
8H), 1.83–1.74 (m, 8H), 1.49–1.41 (m, 8H), 1.35 (br, 2H), 0.97 (t,
J = 7.3 Hz, 12H); 13C NMR (67.8 MHz, CDCl3) δ 139.4, 136.2,
128.2, 124.9, 124.8, 57.9, 53.4, 23.5, 19.4, 13.5; 19F NMR (254 MHz,
CDCl3) δ −62.4; HRMS calcd. for C23H45BF3N: 427.3597. Found;
427.3594.
ox
Compound
Ep (V vs. SCE)
1.90
1
1.85
Synthesis of 4,4,5,5-tetramethyl-(3-phenylpropyn-2-yl)-1,3,2-
dioxaborolane (4).—To a stirred solution of THF (20 ml) containing
ethynylbenzene (20 mmol), n-buthyl lithium in hexane (20 mmol) was
added dropwise at −78◦C. The resulting solution was stirred at the
same temperature for 1 h, and then (bromomethyl)pinacolborane (20
mmol) was added dropwise and the mixture was stirred at room tem-
perature for 2 h. After the reaction, 1 M HCl was added to the reaction
mixture at 0◦C, and the solution was stirred for 20 min. The solution
was mixed with water and the product was extracted repeatedly with
ether. The extracts were washed with brine and the ether layer was
separated and dried over anhydrous MgSO4. The solvent was removed
by evaporation and the residue was distilled under reduced pressure to
give the product (bp: 130–135◦C/0.3 mmHg), which was further pu-
rified with silica gel column chromatography (eluent: hexane/AcOEt
= 5:1) provide a pure product 4 as a yellow oil in 12% yield.
1H NMR (270 MHz, CDCl3) δ 7.48–7.38 (m, 3H), 7.30–7.22 (m,
2H), 2.04 (s, 2H), 1.32 (s, 12H); 13C NMR (67.8 MHz, CDCl3) δ
132.4, 131.5, 128.2, 127.1, 84.1, 83.2, 65.5, 24.8; MS: m/z = 242
(M+), 227 (M+-Me), 198, 169, 143, 128, 115, 89; HRMS calcd. for
C15H19BO2: 242.1478. Found; 242.1480.
1
2
3
0.95b
0.94
1.40
a0.1 M Bu4NClO4/MeCN, Pt disk electrode (φ = 0.8 mm), scan rate:
100 mV/s.
b0.1 M Et4NF-4HF/MeCN.
Similar marked negative shift of the oxidation potentials was also ob-
served in the case of oxygen-containing organoboronic acid pinacol
esters.51 As already mentioned, it is well-known that allylsilane and
benzylsilane exhibit less positive oxidation potentials compared with
their corresponding compounds devoid of a silyl group. Therefore, the
oxidation potential of cinnamyltrimethylsilane (3) was also measured
under the same conditions as the measurement of 2. The oxidation
potential of 3 was found to be 1.40 V vs. SCE, which is less positive
compared with β-methylstyrene probably due to a β-effect of the silyl
group. However, 3 showed much more positive oxidation potential
compared with cinnamyltrifluoroborate 2 although allyltrimethylsi-
lane and allyltrifluoroborate exhibited quite similar oxidation poten-
tials (around 1.40 V vs. SCE).52
In order to further understand the difference in oxidation poten-
tials of 1, 2 and 3, DFT calculation was carried out using Gaussian 03
with the B3LYP/6-31G(d,p) method (Figure 1). The highest occupied
molecular orbitals (HOMO) of these compounds were mainly located
on the C-C double bonds. The optimized structures indicate that the
orbital on the C-B bond or C-Si bond can overlap that of the C-C
double bond. This result strongly supports that the oxidation poten-
tials of the substrates are decreased by the β-effect. Moreover, the
A typical procedure for anodic acetoxylation and alkoxylation.—
Constant current anodic oxidation of 1, 4 or 5 (0.3 mmol) was carried
out with a graphite anode (2 cm × 2 cm) and a platinum cathode (2 cm
× 2 cm) in 10 ml of 0.2 M NaOAc/AcOH, 0.1 M NaOMe/MeOH, 0.1
M Et4NOTs/MeOH, or 0.3 M Et3N-3HF/alcohol in an undivided cell at
room temperature. After electrolysis, 80 ml of water was added to the
mixture and extracted with ether (40 ml × 3). The organic phase was
washed with an aqueous solution of NaHCO3 and brine, and then dried
over MgSO4. The solution was evaporated under reduced pressure
and the product was isolated by silica gel column chromatography
1
(eluent: hexane/AcOEt = 20∼5:1). H NMR yields were estimated
using nitromethane or dichloromethane as an internal standard.
The known products (6a, 6b,42 7a,43 7b,44 7c,,44 7d,45 8a,46 8b,44
8c,44 9, 10,47 11a,48 11b,49 11c,50 12, and 13) were identified by com-
parison with the spectral data of commercially available or reported
authentic samples.
Epox (V vs. SCE)
HOMO diagram
Results and Discussion
1.85
Cyclic voltammetry measurement.—At first, cyclic voltamme-
try measurement of 2-(cinnamyl)boronic acid pinacol ester 1, tetra-
n-butylammonium cinnamyltrifluoroborate (2), cinnamyltrimethyl-
silane (3), and β-methylstyrene was carried out in 0.1 M n-
Bu4NClO4/MeCN. Their oxidation peak potentials are summarized
in Table I.
The oxidation potentials of β-methylstyrene and cinnamylboronic
acid pinacol ester 1 are almost the same, which indicates the in-
troduction of a boryl group does not affect the oxidation potential.
Interestingly, when the supporting electrolyte was changed from n-
Bu4NClO4 to Et4NF-4HF, the oxidation peak potential of 1 was dra-
matically shifted from 1.85 V to 0.95 V vs. SCE. Thus, it was found
that the presence of fluoride ions markedly decreased the oxidation
potential of 1 by 0.9 V. Notably, the oxidation potential of cinnamyl-
trifluoroborate 2 was found to be almost the same as that of 1 in the
presence of fluoride ions. Previously, we clarified the facile formation
0.94
1.40
of trifluoroborate species from organoboronic acid and HF salts by 19
F
NMR and 11B NMR spectroscopic study.33 Therefore, the remarkable
negative shift of the oxidation potential of 1 seems to be attributable to
negatively charged trifluoroborate 2 derived from 1 and fluoride ions.
Figure 1. Left: Oxidation peak potentials (Epox) of cynnamyl derivatives
recorded in Bu4NClO4/MeCN and right: HOMO diagrams of 1, 2, and 3.
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Journal of The Electrochemical Society, 164 (2) G23-G28 (2017)
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Table II. Oxidation peak potentials (Epox) of boronate ester having
Table III. Anodic acetoxylation of 1 under various conditions.
a triple bond, and related compoundsa
ox
Compound
Ep (V vs. SCE)
2.32
4
4
5
2.30
Entry
Anode material
Electricity (F/mol)
Yield (%)a
1
2
3
4
5
Pt
G
G
G
G
2
2
2.5
3
2b
17
20
26
15
1.28b
1.72
4
aIsolated yield.
a0.1 M Bu4NClO4/MeCN, Pt disk electrode (φ = 0.8 mm), scan rate:
b0.1 M Et4NF-4HF/MeCN.
bDetermined by 1H NMR.
100 mV/s.
When a theoretical amount of electricity (2 F/mol) was passed at
a graphite anode, acetoxylation proceeded with eliminating a boryl
group. However, expected acetoxylated product 6a was obtained in
poor yield (entry 2). Platinum was not suitable as an anode material
for the acetoxylation (entry 1). Since considerable amount of starting
material 1 remained at 2 F/mol of electricity, the amount of charge
was increased. As expected, the yield of 6a increased up to 26% with
increase of electricity (entries 2–4). However, at 4 F/mol of electricity,
the yield decreased probably due to the overoxidation of the product
6a even though starting compound 1 remained. Interestingly, 6a was
obtained solely and regioisomer was not formed at all. In compari-
son, anodic acetoxylation of β-methylstyrene was carried out using a
graphite anode under the same conditions. The electrolysis at 2 F/mol
provided two acetoxylated products 6a and 6b as a regioisomeric mix-
ture as shown in Scheme 1. In this case, 1-phenylallyl acetate (6b)
was mainly formed and 6a was a minor product. Since a benzylic
cation is more stable than a terminal cation, 6b seems to be formed
mainly. These results indicate that a boryl group would control the
regioselectivity for the anodic acetoxylation.
Next, anodic methoxylation of 1 was investigated. When a neutral
supporting electrolyte like Et4NOTs was used, the reaction did not
proceed at all and starting material 1 was mostly recovered. In this
case, methanol was oxidized predominantly due to the high oxida-
tion potential of 1. It was expected that 1 would form a boron-ate
complex with a base resulting in decrease of its oxidation potential
as reported by Sha¨fer and Koch.23 Therefore, NaOMe was used as a
basic supporting electrolyte, and anodic oxidation of 1 was carried out
at platinum and graphite anodes. As expected, the methoxylation pro-
ceeded regardless of anode materials to afford methoxylated products
7a and 8a as a regioisomeric mixture as well as cinnamylaldehyde 9
as shown in Scheme 2. However, the total yield was unsatisfactory.
Since we found that the oxidation potentials of 1 was decreased sig-
nificantly in the presence of fluoride ions, anodic methoxylation was
performed using Et3N-3HF as a supporting electrolyte, which has been
observed oxidation potentials well corresponded to the contribution
of the orbital on C-B or C-Si bond to the HOMO diagrams.
Next, cyclic voltammetry measurement of boronic acid pina-
col ester having a triple bond like (3-phenyl-2-propynyl)boronic
acid pinacol ester 4, its trimethylsilyl analogue 5, and 1-methyl-2-
phenylacetylene was carried out similarly. The observed oxidation
potentials are summarized in Table II.
The oxidation potentials of boronic acid pinacol ester 4 and 1-
methyl-2-phenylacetylene were found to be rather positive and quite
similar each other. The introduction of a boryl group to the α-position
of 1-methyl-2-phenylacetylene did not affect the oxidation potential
similarly to the case of the corresponding cinnamyl analogue 1. In
sharp contrast, the oxidation potential of silyl derivative 5 decreased
significantly to 1.72 V vs. SCE by 0.6 V compared with 1-methyl-
2-phenylacetylene, which was quite similar to the case of cinnamyl
analogues 1 and α-methylstylene. Interestingly, the oxidation potential
of boronic acid pinacol ester 4 was changed dramatically from 2.30 to
1.28 V vs. SCE in the presence of fluoride ions similar to the case of
1. It is notable that such large negative shift of the oxidation potentials
of 1 and 4 was observed regardless of neighboring π-systems.
Again, DFT calculation for 4, 5 and BF3 analogue of 4 was per-
formed using the same method as discussed above. The HOMOs were
delocalized through the π-system and the C-B or C-Si bond in a sim-
ilar manner to the result shown in Figure 1 (data not shown). The
level of oxidation potential was dependent on the degree of β-effect
observed with different functional groups.
Anodic substitution reactions of organoborates with oxygen
nucleophiles.—Anodic acetoxylation of 2-(cynnamyl)boronic acid
pinacol ester 1 was carried out at a constant current using graphite and
platinum anodes in 0.2 M NaOAc/AcOH. The results are summarized
in Table III.
Scheme 1. Anodic acetoxylation of β-methylstyrene.
Scheme 2. Anodic methoxylation of 1 with different anode materials.
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Journal of The Electrochemical Society, 164 (2) G23-G28 (2017)
Table IV. Anodic alkoxylation of 1 in the presence of fluoride ions.
Yield (%)a
Entry
1
2
R
7
8
9
Total
50
35
34
33
Me
Et
15
12
13
13
(7a)
(7b)
(7c)
(7d)
18
12
9
(8a)
(8b)
(8c)
(8d)
17
21
12
20
3
iPr
Bn
4b
0
aDetermined by 1H NMR.
b0.3 M BnOH/MeCN.
Scheme 3. Plausible reaction mechanism for anodic alkoxylation of 1 in the presence of fluoride ions.
Scheme 4. Formation of 9 via anodic alkoxylation of 7 and hydrolysis.
used for anodic fluorination of various organic compounds.40,53–57 As
expected, methoxylation proceeded well, and products, 7a, 8a, and 9
were obtained in moderate total yield (50%) at 2F/mol as shown in
Table IV (entry 1). In order to investigate scope and limitation as well
as regioselectivity of alkoxylation, we carried out anodic alkoxylation
similarly, using various alcohols. The results are summarized in Table
IV. Regardless of alcohols, alkoxylation proceeded to provide three
kinds of products in moderate yields except for benzyl alcohol. In all
cases, starting compound 1 remained appreciably. When benzyl al-
cohol was used as a nucleophile, benzylic substituted regioisomer 8d
was not formed at all. This is interesting from a mechanistic aspect.
In all cases, no fluorinated product was formed at all.
The anodic alkoxylation of 1 seems to proceed as shown in Scheme
3. At first, a fluoride ion coordinates to the boron atom to form a
Table V. Anodic acetoxylation of 4 and 5 having a triple bond.
Entry
Substrate
Anode material
Yield (%)a
1
2
3
4
4
4
5
5
Pt
G
Pt
G
4b
44
trace
53
aIsolated yield.
bDetermined by 1H NMR.
Figure 2. Regioselectivity of anodic alkoxylation of 1.
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Journal of The Electrochemical Society, 164 (2) G23-G28 (2017)
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Table VI. Anodic alkoxylation of 4 and 5 under various conditions.
Yield (%)a
Entry
Substrate
R
Me
Me
Me
Allyl
iPr
Supporting electrolyte
0.1 M Et4NOTs
11
12
4
3
10
7
12
13
0
4
11
9
7
Total
13
10
53
44
1
2
3
4
5
4
5
4
4
4
9
3
32
28
8
(11a)
(11a)
(11a)
(11b)
(11c)
0.1 M Et4NOTs
0.3 M Et3N-3HF
0.3 M Et3N-3HF
0.3 M Et3N-3HF
27
aDetermined by 1H NMR.
boron-ate complex A like trifluoroborate. As already mentioned, we
confirmed the formation of trifluoroborate from organoboronic acid
and fluoride ions by 19F NMR and 11B NMR spectroscopic studies.33
The boron-ate complex A is readily oxidized at an anode to generate
a radical intermediate, followed by one more electron oxidation to
generate cationic intermediate B in a manner similar to anodic oxi-
dation of trifluoroboate compounds as reported.52 An alcohol attacks
the terminal carbon or inner carbon of B to form 7 or 8, respectively.
Further anodic oxidation of 7 affords acetal products, which are an
equivalent to cinnamyl aldehyde 9 (Scheme 4). Even in the presence
of fluoride ions in the anodic alkoxylation, any fluorinated products
were not formed at all. This can be explained in terms with much lower
nucleophilicity of a fluoride ion compared with methanol. Supporting
this, we have reported fluoride ion mediated anodic methoxylation of
organosulfur and organoselenium compounds.58–60
In consideration to both Table IV and Scheme 3, the regioselec-
tivity of anodic alkoxylation can be summarized in Figure 2, and the
regioselectivity was found to depend on the bulkiness of alcohols.
Namely, it was found that the selectivity of terminal alkoxylation
increased with an increase of bulkiness of alkyl group of alcohols.
Next, anodic acetoxylation of (3-phenyl-2-propynyl)boronic acid
pinacol ester 4 and its trimethylsilyl analogue 5 was comparatively
investigated in NaOAc/AcOH using platinum and graphite anodes. As
shown in Table V (entry 2), anodic acetoxylation of 4 at a graphite
anode proceeded with eliminating a bory group to afford the desired
acetoxylated product 10 in moderate yield (44%). Anodic acetoxyla-
tion of 5 at a graphite anode also proceeded with eliminating a silyl
group to provide 10 in better yield (53% in Table V, entry 4) compared
with the case of 4. In both cases of 4 and 5, a platinum anode was
not suitable due to anodic passivation (formation of non-conducting
polymer film on the anode surface).
Finally, anodic methoxylation of 4 and 5 was comparatively in-
vestigated at a platinum anode in Et4NOTs/MeOH (Table VI). Quite
different from 1, 4 underwent anodic methoxylation even in a neutral
electrolytic solution to afford methoxylated product 11a mainly in low
yield (entry 1) although the oxidation potential of 4 is much higher
compared with 1. In contrast to 4, anodic methoxylation of 5 gave
three products including 11a without product selectivity (entry 2). In
both cases, considerable amount of starting materials remained due
to their high oxidation potentials. Therefore, anodic methoxylation 4
was carried out in the presence of fluoride ions similarly to the anodic
methoxylation of 1. As shown in Table VI (entry 3), methoxylation
proceeded well to provide α-methoxylated product 11a mainly as well
as α-hydroxyl product 12 and aldehyde derivative 13 in moderate total
yield (53%). Next, anodic oxidation of 4 in allyl alcohol and isopropyl
alcohol was also performed similarly. Although allyl alcohol is readily
oxidized, anodic substitution with allyl alcohol proceeded to give the
desired product 11b in reasonable yield (entry 4). Anodic isopropoxy-
lation also took place although the yield of 11c was low. In all cases,
α-hydroxylated product 12 was formed considerably or mainly (entry
5). A small amount of water contaminated in an electrolytic solution
seems to be a reason for the anodic hydroxylation.
Anodic alkoxylation and hydroxylation of 4 in the presence of flu-
oride ions seem to proceed through electron transfer from the corre-
sponding trifluoroborate derivative in situ formed from 4 and fluoride
ions in a similar manner to the case of 1. Aldehyde derivative 13 seems
to be formed by further anodic oxidation of 11 and 12.
Conclusions
We examined the electrochemical properties of organoboronic acid
pinacol esters having a double or triple bond at the α-position. Their
oxidation potentials are much higher than those of the corresponding
trimethylsilyl compounds. However, it was found that the presence
of fluoride ions markedly decreased the oxidation potentials of the
organoboronic acid pinacol esters. Such remarkable cathodic shift of
the oxidation potentials can be explained by a negatively charged tri-
fluoroborate moiety in situ formed as well as β-effect due to well
overlapping of a C-B σ bond with unsaturated bond, which was con-
firmed by calculations. These effects enabled to achieve regioselective
anodic acetoxylation and alkoxylation. Thus, we found that a boronic
acid ester group in a molecule having a neighboring unsaturated bond
serves as an efficient electroauxiliary similar to a well-known silyl
group.
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