Electrochemical preparation of pinacol allylboronic esters

Article abstract of DOI:10.1016/j.electacta.2009.01.015

The electroreduction of allylic halide derivatives in the presence of pinacolborane afforded allylboronic pinacol esters with moderate to good yields (up to 86%) and high regioselectivities. The electrosynthesis was carried out in a single-compartment cel

Full text of DOI:10.1016/j.electacta.2009.01.015

Electrochimica Acta 54 (2009) 5116–5119
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical preparation of pinacol allylboronic esters
Julien Godeau, Christine Pintaric, Sandra Olivero, Elisabet Dun˜ach∗,1
Laboratoire de Chimie des Molécules Bioactives et Arômes UMR 6001, Université de Nice, Sophia Antipolis, CNRS UMR 6001,
Institut de Chimie de Nice, UFR Sciences, 28 Avenue de Valrose, F-06108 Nice, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2008
Received in revised form 6 January 2009
Accepted 7 January 2009
Available online 15 January 2009
The electroreduction of allylic halide derivatives in the presence of pinacolborane afforded allylboronic
pinacol esters with moderate to good yields (up to 86%) and high regioselectivities. The electrosynthe-
sis was carried out in a single-compartment cell with an Al anode, in THF at room temperature and it
constitutes an alternative route for the preparation of allylboronic esters under mild conditions.
© 2009 Elsevier Ltd. All rights reserved.
Keywords:
Allylic halide
Boration
Electrosynthesis
Allylboronic esters
Pinacolborane
1. Introduction
direct synthetic alternative to the transition-metal-catalysed pro-
cedures.
Allylboronic acids and esters constitute important synthetic
intermediates in organic chemistry. Allylboronic derivatives can
undergo Suzuki-type cross-coupling reactions, similarly to aryl-
boronic acids and esters [1,2]. Allylboronic esters have also been
described as partners for the highly stereo- and enantio-selective
coupling with carbonyl compounds for the synthesis of homoal-
lylic alcohols [3–7]. Moreover, the low toxicity of boronic acids and
their ultimate degradation to boric acid allow these derivatives to
be qualified as “green” compounds [8].
The general synthetic access to these intermediates has been
reported via the corresponding allyl Grignard [9–11] or lithium
reagents [12] with trialkyl borates, in two-step reactions at −78 ◦C
(with low functional group compatibility) or via palladium- or
platinum-catalysed coupling processes with allylic derivatives and
pinacolborane or bis(pinacolato)diboron as the borating agents
[13–19].
2. Experimental
General electrolysis procedure: In a single-compartment cell
fitted with a consumable Al anode and a nickel foam cathode
[25,26] the allyl halide 1 (1 mmol) and HBpin (3 mmol) were
added to a distilled THF solution (20 mL) containing (CF3SO2)2NLi
(7 × 10−1 M), under nitrogen atmosphere. The electrolysis was car-
ried out at room temperature and at constant current density
(i = 0.03 A, j = 0.15 A dm−2), the charge involved during the electro-
chemical process being calculated by the time of the electrolysis.
After 2–3 F mol−1 electrolysis, the solvent was evaporated under
vacuum. The crude reaction mixture was hydrolysed with water
(50 mL) and extracted with Et2O (3× 50 mL). The organic phases
were washed with distilled water, then with saturated aqueous
NaCl solution, dried over magnesium sulphate and concentrated
under vacuum. Boronic esters were purified by column chromatog-
raphy on silica-gel with petroleum ether-diethyl ether mixtures
as the eluents. The 1H and 13C NMR spectra were recorded with
BRUKER AC 200 in CDCl3. Mass spectra were recorded on selective
mass detector HP 5971 (electronic impact 70 eV).
Within our interest in developing the electrochemical method-
ology for synthetic purposes [20,21] we reported the electrosyn-
thesis of arylboronic acids [22] and esters [23,24]. An alternative
electrochemical method for the access to benzylboronic derivatives
has also been described [25].
We present here our results on the application of the electrosyn-
thetic methodology for the preparation of allylboronic esters, as a
(E)-2-(3,7-dimethylocta-2,6-dienyl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane, (E)-2a mixture with (Z)-2a (83:17): 1H NMR:
ı = 1.24 (s, 12H), 1.58–1.61 (m, 8H), 1.68–1.69 (m, 3H), 1.98–2.08 (m,
4H), 5.08–5.13 (bt, 1H), 5.23–5.26 (bt, 1H) ppm; 13C NMR: ı = 16.3,
18.1, 25.1, 26.8, 27.2, 40.1, 83.4, 118.9, 124.9, 131.5, and 135.5 ppm;
MS: m/z (%): 264 (M+, 2), 221 (34), 95 (24), 83 (100), 69 (32), 41
(31).
∗
Corresponding author. Tel.: +33 492076142; fax: +33 492076151.
E-mail address: dunach@unice.f (E. Dun˜ach).
1
ISE member.
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.01.015

J. Godeau et al. / Electrochimica Acta 54 (2009) 5116–5119
5117
2-(3,7-dimethylocta-1,6-dien-3-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane, 3a: 1H NMR: ı = 0.76 (s, 3H), 1.24 (s, 12H),
1.59–1.61 (m, 3H), 1.67–1.69 (m, 3H), 1.91–1.98 (m, 2H), 2.00–2.08
(m, 2H), 4.92–4.96 (m, 2H), 5.08–5.14 (bt, 1H), 5.89 (dd, J = 17.5 Hz,
10.5 Hz, 1H) ppm; 13C NMR, ı = 16.3, 18.1, 19.9, 25.0, 38.7, 83.4,
111.4, 125.4, and 145.6 ppm; MS: m/z (%): 264 (M+, 1), 95 (23), 83
(100), 69 (34), 41 (36).
(E)-2-cinnamyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
(E)-2c, mixture with 3c (82:18): 1H NMR: ı = 1.24 (s, 12H),
1.55–1.65 (m, 2H), 6.25–6.40 (m, 2H) ppm; 13C NMR: ı = 24.9, 83.6,
125.6, 126.3, 127.0, 127.1, 128.9, and 136.2 ppm; MS: m/z (%): 244
(M+, 21), 126 (29), 117 (98), 91 (100), 83 (53), 41 (33).
2-(1-phenylallyl)-1,3,2-dioxaborolane-4,4,5,5-tetramethyl,
3c: 1H NMR: ı = 1.24 (s, 12H), 1.55–1.65 (m, 1H), 5.00–5.10 (m, 2H),
6.02–6.15 (m, 1H) ppm; MS: m/z (%): 244 (M+, 85), 217 (63), 145
(52), 117 (100), 105 (63), 91 (90), 83 (46).
2-(3-methylbut-2-enyl)-1,3,2-dioxaborolane-4,4,5,5-
tetramethyl, 2e, mixture with 3e (87:13): 1H NMR, ı = 1.23 (s,
12H), 1.58 (s, 3H), 1.58–1.61 (m, 2H), 1.69 (s, 3H), 5.22 (m, 1H) ppm;
13C NMR, ı = 18.0, 23.8, 25.1, 26.1, 83.5, 118.9, and 131.8 ppm; MS:
m/z (%): 196 (M+, 36), 139 (76), 101 (35), 84 (100), 69 (61), 55 (56),
41 (83).
2-(2-methylbut-3-en-2-yl)-1,3,2-dioxaborolane-4,4,5,5-
tetramethyl, 3e: 1H NMR: ı = 1.23 (s, 12H), 1.23–1.30 (m, 6H),
4.85–4.94 (m, 2H), 5.88–6.02 (m, 1H) ppm; MS: m/z (%): 196 (M+,
9), 139 (92), 111 (22), 101 (33), 95 (42), 84 (92), 69 (85), 55 (67), 41
(100).
2-(hex-1-ene-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
3g: 1H NMR: ı = 0.86 (t, J = 7.2 Hz, 3H), 1.23 (s, 12H), 1.20–1.50
(m, 4H), 1.60–1.70 (m, 1H), 4.90–5.00 (m, 2H), 5.70–5.90 (m, 1H)
ppm; 13C NMR: ı = 14.5, 22.5, 25.1, 30.1, 83.5, 113.5, and 143.5 ppm;
MS: m/z (%): 210 (M+, 1), 153 (42), 84 (100), 69 (54), 55 (63),
41 (85).
(E)-2-(oct-2-enyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
(E)-2h, mixture with 3h (90:10): 1H NMR: ı = 0.86 (m, 3H),
1.10–1.40 (m, 18H), 1.60–1.70 (m, 2H), 1.90–2.10 (m, 2H), 5.30–5.50
(m, 2H) ppm; 13C NMR: ı = 14.5, 22.9, 29.7, 30.1, 31.7, 33.1, 83.5,
125.0, and 131.5 ppm; MS: m/z (%): 238 (M+, 5), 84 (100), 69 (24),
55 (32), 41 (41).
2-(oct-1-ene-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
3h: 1H NMR: ı = 0.86 (m, 3H), 1.10–1.40 (m, 18H), 1.55–1.70
(m, 1H), 4.88–5.01 (m, 2H), 5.65–5.86 (m, 1H) ppm; MS:
m/z (%): 238 (M+, 2), 181 (43), 84 (100), 69 (37), 55 (47), 41
(58).
3. Results and discussion
The general reaction is presented in Eq. (1) and concerns
the electroreductive functionalization of an allylic halide 1 (bro-
mide or chloride) in the presence of pinacolborane (HBpin) as
the borating agent. The preparative electrolyses were carried out
in a single-compartment cell fitted with a consumable anode
[26,27]:
(1)
The boration of non-symmetrical allyl halides such as 1 can
give rise to two regioisomeric pinacol borates 2 and 3. In addition,
boronic ester 2 may present E/Z stereochemistry.
Previous studies indicated that in electrochemical borations,
HBpin afforded superior results as compared to the use of trialkylb-
orates such as B(OMe)3 or B(Oi-Pr)3 [24]. Moreover, the use of HBpin
may directly afford the corresponding pinacol boronic esters, which
are easier compounds to isolate and purify than the corresponding
boronic acids.
2-(cyclohex-2-enyl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane, 2f: 1H NMR, ı = 1.24 (s, 12H), 1.50–1.90 (m,
5H), 1.95–1.99 (m, 2H), 5.67 (m, 2H) ppm; 13C NMR, ı = 22.9, 24.5,
24.9, 25.0–25.2, 25.3, 83.5, 126.4, and 128.0 ppm; MS: m/z (%): 208
(M+, 3), 84 (100), 81 (74), 79 (64), 41 (27).
(E)-2-hex-2-enyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
(E)-2g, mixture with 3g (85:15): 1H NMR: ı = 0.86 (t, J = 7.2 Hz, 3H),
1.23 (s, 12H), 1.20–1.50 (m, 2H), 1.64 (bs, 2H), 1.90–2.10 (m, 2H),
5.30–5.50 (m, 2H) ppm; 13C NMR: ı = 14.0, 23.1, 25.1, 30.1, 35.2,
83.5, 125.2, and 131.2 ppm; MS: m/z (%): 210 (M+, 8), 84 (100), 69
(36), 55 (37), 41 (49).
The optimisation studies were carried out with geranyl bromide
1a as the model substrate (Eq. (2)), and the most representative
results are presented in Table 1:
Table 1
Electroboration of 1a in the presence of pinacolboranea.
Entry
Anode/cathode
F mol−1
Current density (10−3 A cm−2
)
Conversion of 1a
Yield of boration 2a + 3a
Ratio 2a:3a
E/Z ratio of 2a
1b
2
3
4
5
6
7
8
9
Mg/Ni
Mg/Ni
Al/Ni
Zn/Ni
Al/stainless steel
Al/C
Al/Ni
Al/Ni
Al/Ni
2
2
2
2
2
2
3
3
3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.3
3
100%
100%
80%
95%
100%
94%
100%
100%
100%
44%
67%
70%
5%
61%
25%
76%
81%
71%
64:36
75:25
81:19
40:60
81:19
74:26
93:7
(90/10)
(92/8)
(65/35)
(65/35)
(82/18)
(100/–)
(63/37)
(70/30)
(68/32)
90:10
89:11
a
See general electrolysis conditions. Reaction carried out in THF-TFSILi with slow addition of HBpin (3 equiv.), unless stated.
Addition of HBpin (3 equiv.) at the beginning of the electrolysis.
b

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J. Godeau et al. / Electrochimica Acta 54 (2009) 5116–5119
(2)
In a first reaction, the electrolysis of 1a was run in the pres-
ence of 3 equiv. of pinacolborane in THF at room temperature, with
Mg/Ni as the couple of electrodes (entry 1). The use of DMF or
MeCN was not convenient for these reactions, due to the fact that
some solvent reduction occurs in the presence of pinacolborane. As
supporting electrolyte, lithium bis(trifluoromethanesulfonyl)imide
[(CF3SO2)2NLi or TFSILi] was used, for its high solubility in the THF
resulted in a low yield of boration with a high ratio of protohehalo-
genation of 1a.
With an Al anode, the influence of the nature of the cathode
was examined with stainless steel, carbon fiber and nickel foam,
with the results shown in entries 3, 5 and 6. The best yields were
obtained with Al/Ni electrode combination (entry 3). It is to note
that with Al/C as the electrodes (entry 6), although the yield of
coupling was only of 25%, the borate 2a was found exclusively as the
(E)-isomer.
With Al/Ni as the couple of electrodes, the increase of the
amount of current to 3 F mol−1 of 1a allowed its complete con-
sumption with 76% of boration (entry 7). The further change of
the current density from 1.5 to 3 × 10−3 A cm−2 (intensity from 30
to 60 mA, entries 7–9), gave the best results with an intermediate
0.23 × 10−3 A cm−2 current density (entry 8), with a yield of bora-
tion of 81% and a 2:3 ratio of 90:10. The (E/Z) stereoselectivity for
2a resulted in 70/30.
medium. The reaction monitoring was done by GC. After 2 F mol−1
,
the consumption of 1a was of 100% and geranylboronic esters 2a
and 3a were formed in 44% yield with a 2a:3a ratio of 64:36. The
main by-products resulted from the dimerisation of 1a; R–R prod-
ucts were reductively formed from R–Br in 39% yield. To reduce the
formation of such undesired dimers, the electroboration was next
run with a slow addition of the substrate during the electrolysis
time (2 h). The yields of the coupling products 2a and 3a was raised
to 67%, with a 2a to 3a ratio of 75:25 and a (E/Z) stereoselectivity
for 2a of 92/8 (entry 2). The amount of R–R dimers was reduced to
22%.
The influence of the anode was examined by using magnesium,
aluminum and zinc (entries 2–4). The aluminum anode afforded
the best yield of boration of 70%, with a 2a to 3a ratio of 81:19. The
(E/Z) stereoselectivity for 2a was of 65/35. The use of a zinc anode
Under the optimised conditions (Table 1, entry 8), the electro-
chemical boration methodology was then extended to a series of
differently substituted allyl chlorides and bromides, and the results
are summarised in Table 2.
Table 2
Electrosynthesis of allylboronic pinacol esters from allyl halides and pinacolborane.
Entry
Substrate
Products ratio
E/Z ratio of 2
Boration yield (2 + 3)
1
2a:3a, 90:10
70/30
81%
2
3
2a:3a, 91:9
2c:3c, 84:16
74/26
96/4
86%
70%
4
5
2c: 7%, 4d: 90% (PhCH = CHCH2)2
2e:3e, 79:21
–
–
7%
76%
6
2f
–
65%
7
8
2g:3g, 83:17
2h:3h, 68:32
100/–
100/–
74%
64%

J. Godeau et al. / Electrochimica Acta 54 (2009) 5116–5119
5119
The electrochemical boration of geranyl chloride 1b (entry 2) led
to similar results as those obtained for bromide 1a. Boration of 1b
occurred in an overall yield of 86% with a 2a:3a ratio of 91:9. For
major regioisomer 2a, the stereoisomer (E/Z) ratio was of 74/26.
In contrast, the electrochemical boration of cinnamyl chloride 1c
and cinnamyl bromide 1d resulted in very different results (entries
3 and 4). Whereas 1c led to a 70% boration yield with formation of 2c
and 3c in a 86:16 ratio, the corresponding 2c:3c was only obtained
in 7% yield from bromide 1d. In this case, the reaction led mainly
to the formation of dimer 4d, [(PhCH = CHCH2)2], obtained in 90%
yield.
Prenyl bromide 1e afforded borated regioisomers 2e and 3e in
76% yield and a 79:21 ratio, respectively (entry 5). The electrob-
oration of 2-cyclohexenyl bromide 1f afforded a single isomer 2f,
which was isolated in 65% yield (entry 6). The bromoallyl derivative
1g gave a mixture of 2g and 3g in 74% yield and a relative ratio of
83:17 (entry 7). Pinacol ester 2g was of (E)-configuration. Similarly,
1-bromo-2-octene, 1h, afforded the main boration at the terminal
position, with a 2h:3h ratio of 68:32 and an overall yield of 64%.
Derivative 2h was also of (E)-configuration.
the less hindered terminal borated compound 2 is obtained pref-
erentially with selectivities of 68–91%; the (E)-derivative being the
predominant or exclusive stereoisomer.
References
[1] N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 11 (1981) 513.
[2] A. Suzuki, J. Organomet. Chem. 576 (1999) 147.
[3] S.E. Denmark, N.G. Almstead, in: J. Otera (Ed.), Modern Carbonyl Chemistry,
Wiley–VCH, Weinheim, 2000, p. 299.
[4] S.R. Chemler, W.R. Roush, in: J. Otera (Ed.), Modern Carbonyl Chemistry,
Wiley–VCH, Weinheim, 2000, p. 403.
[5] D.R. Williams, S.V. Plummer, S. Patnaik, Angew. Chem., Int. Ed. 42 (2003) 3934.
[6] R.H. Grubbs (Ed.), Handbook of Metathesis, vols. 1–3, Wiley–VCH, Berlin, 2003.
[7] R. Wada, T. Shibuguchi, S. Makino, K. Oisaki, M. Kanai, M. Shibasaki, J. Am. Chem.
Soc. 128 (2006) 7687.
[8] D.G. Hall (Ed.), Boronic Acids, 1st ed., Wiley–VCH, Weinheim, 2006.
[9] B.M. Mikhailov, Organomet. Chem. Rev. Sect. A 8 (1972) 1.
[10] G.W. Kramer, H.C. Brown, J. Organomet. Chem. 132 (1977) 9.
[11] M. Yamaguchi, T. Mukaiyama, Chem. Lett. (1980) 993.
[12] R.W. Hoffmann, B. Kemper, Tetrahedron Lett. 40 (1984) 2219.
[13] M. Satoh, Y. Nomoto, N. Miyaura, A. Suzuki, Tetrahedron Lett. 30 (1989) 3789.
[14] T. Ishiyama, M. Yamamoto, N. Miyaura, Chem. Commun. (1996) 2073.
[15] M. Suginome, T. Matsuda, T. Yoshimoto, Y. Ito, Org. Lett. 1 (1999) 1567.
[16] G.Y. Fang, V.K. Aggarwal, Angew. Chem., Int. Ed. 46 (2007) 359.
[17] G. Dutheuil, N. Selander, K.J. Szabo, V.K. Aggarwal, Synthesis 14 (2008) 2293.
[18] S. Sebelius, V.J. Olsson, K.J. Szabó, J. Am. Chem. Soc. 127 (2005) 10478.
[19] V.J. Olsson, S. Sebelius, N. Selander, K.J. Szabó, J. Am. Chem. Soc. 128 (2006)
4588.
4. Conclusion
In conclusion, the electrochemical boration of allyl halides with
pinacolborane was examined with several examples and led gen-
erally to good yields. The electrosynthesis may therefore constitute
an interesting synthetic alternative to conventional methods.
The procedure is simple, affords generally good yields with
either chlorides or bromides (64–86%), does not need organometal-
lic catalysis and works under mild conditions (room temperature),
in a one-step reaction. Among the two possible allylic regioisomers,
[20] E. Dunach, D. Franco, S. Olivero, Eur. J. Org. Chem. (2003) 1605.
[21] E. Dunach, M.J. Medeiros, S. Olivero, New J. Chem. 30 (2006) 1534.
[22] C. Laza, E. Dunach, F. Serein-Spirau, J.J.E. Moreau, L. Vellutini, New J. Chem. 26
(2002) 375.
[23] C. Laza, E. Dunach, Adv. Synth. Catal. 345 (2003) 580.
[24] C. Laza, C. Pintaric, S. Olivero, E. Dunach, Electrochem. Acta (2005) 4897.
[25] C. Pintaric, C. Laza, S. Olivero, E. Dunach, Tetrahedron Lett. 45 (2004) 8031.
[26] J. Chaussard, J.-C. Folest, J.-Y. Nedelec, J. Perichon, S. Sibille, M. Troupel, Synthesis
5 (1990) 369.
[27] G. Silverstri, S. Gambino, G. Filardo, A. Gulotta, Angew. Chem. 96 (1984) 978.