Synthesis of Branched Alkylboronates by Copper-Catalyzed Allylic Substitution Reactions of Allylic Chlorides with 1,1-Diborylalkanes

Article abstract of DOI:10.1002/anie.201509840

Reported herein is a copper-catalyzed S_N2'-selective allylic substitution reaction using readily accessible allylic chlorides and 1,1-diborylalkanes, a reaction which proceeds with chemoselective C-B bond activation of the 1,1-diborylalkanes. I

Full text of DOI:10.1002/anie.201509840

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International Edition: DOI: 10.1002/anie.201509840
German Edition: DOI: 10.1002/ange.201509840
Allylic Compounds
Synthesis of Branched Alkylboronates by Copper-Catalyzed Allylic
Substitution Reactions of Allylic Chlorides with 1,1-Diborylalkanes
Junghoon Kim, Sangwoo Park, Jinyoung Park, and Seung Hwan Cho*
À
Abstract: Reported herein is a copper-catalyzed SN2’-selective
allylic substitution reaction using readily accessible allylic
chlorides and 1,1-diborylalkanes, a reaction which proceeds
development of chemoselective C C bond-forming reactions
of 1,1-diborylalkanes.[15,16] However, the regioselective cou-
pling of allylic electrophiles with 1,1-diborylalkanes has rarely
been studied. In 2013, Shibata et al. reported the Suzuki–
Miyaura cross-coupling of allylic bromides and diborylme-
thane, thus affording SN2-selective products in the presence of
a palladium catalyst.[14c] Morken and co-workers reported the
alkoxide-promoted SN2-type alkylation of allylic chlorides
with alkyl-1,1-diboronates, thus providing linear alkylboro-
nates.[16g] Fu and Xiao also reported that cinnamyl phosphate
and diborylmethane participated in the copper-catalyzed
allylic alkylation reaction to form linear alkylboronates
(Scheme 1a).[17] Herein, we report a Cu/(NHC)-catalyzed
(NHC = N-heterocyclic carbene) SN2’-selective allylic alkyla-
tion reaction of allylic chlorides with 1,1-diborylalkanes. This
method has a wide substrate scope, is highly regioselective for
the SN2’ product, and paves a path for the future development
of enantioselective and diastereoselective variants (Sche-
me 1b).
À
with chemoselective C B bond activation of the 1,1-diboryl-
alkanes. In the presence of a catalytic amount of [Cu(IMes)Cl]
[IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene]
and LiOtBu as a base, a range of primary and secondary allylic
chlorides undergo the SN2’-selective allylic substitution reaction
to produce branched alkylboronates. The synthetic utilities of
the obtained alkylboronates are also presented.
T
ransition-metal-catalyzed allylic alkylation is one of the
À
most powerful tools for C C bond formation in organic
synthesis.[1] Among these reactions, copper-catalyzed allylic
alkylation is an efficient strategy for synthesizing branched
alkyl compounds.[2,3] Although significant progress has been
made in the copper-catalyzed allylic alkylations of allylic
electrophiles with organometallic reagents (e.g. dialkylzinc,
alkyllithium, alkylaluminum, alkylzirconium and Grignard
reagent), limited functional-group compatibility and the air/
moisture sensitivity of organometallic reagents often limit
their synthetic applications.[4,5] Moreover, these reactions
typically require cryogenic temperatures to achieve high
levels of selectivity.
To test the viability of the envisioned strategy, we
identified an appropriate allylic electrophile for reacting
with the diborylmethane 2a in the presence of a catalytic
amount of [Cu(IMes)Cl] [IMes = 1,3-bis(2,4,6-trimethylphe-
nyl)imidazole-2-ylidene] and LiOtBu as a base. Although
only a negligible amount of product was formed in the
presence of cinnamyl acetate (Table 1, entry 1) and tert-butyl
cinnamyl carbamate (entry 2), the use of methyl cinnamyl
carbamate afforded an almost 1:1 mixture of the SN2’ and SN2
products 4a and 5a, respectively, in a low yield (entry 3).
When cinnamyl ethyl phosphate was used, a 92:8 mixture of
products 4a and 5a was obtained, with 4a as the major isomer
Organoboron compounds are versatile reagents in organic
synthesis because of their wide availability and air stability.[6]
Although several catalytic methods have been reported for
SN2’-selective allylic substitution using organoboron com-
pounds,[7] most of them use aryl,[8] alkenyl,[9] allenyl,[10]
propargyl,[11] and allyl boron[7,12] derivatives, while only
a
few examples using alkylboron reagents have been
reported. Only in the last few years SN2’-selective allylic
alkylation processes have been developed. However, they are
limited to alkyl-9-BBN (generated in situ from alkenes).[13]
Despite these advances, because of the structural diversity,
new sources of alkylboron reagents which are readily
accessible, scalable, and air-stable should be discovered.
Recently, multiborylated compounds, including 1,1-
diborylalkanes, have emerged as efficient substrates for
synthesizing alkylboron compounds through chemoselective
transformations. Since the pioneering work by Shibata and
co-workers,[14] substantial progress has been made in the
[*] J. Kim, S. Park, J. Park, Prof. Dr. S. H. Cho
Department of Chemistry and Division of Advanced Nuclear
Engineering, Pohang University of Science and Technology (POST-
ECH), Pohang, 790-784 (Republic of Korea)
E-mail: seunghwan@postech.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509840.
Scheme 1. Chemo- and regioselective allylic alkylation of allylic electro-
philes with 1,1-diborylalkanes. pin=pinacol.
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Table 1: Optimization study.[a]
afforded the corresponding alkylboronates in good yields and
with good to excellent SN2’ selectivity. The reactions with
cinnamyl chlorides bearing methyl (entry 2) and chloro
(entry 3) substituents in the para-position of the arene ring
afforded the products 4b (68%) and 4c (86%) in good
selectivities of 95:5 and 96:4 SN2’/SN2, respectively.
Notably, aliphatic allylic chlorides underwent allylic
alkylation with a high yields and greater than 99:1 SN2’/SN2
selectivity. The reactions of acyclic and cyclic allylic chlorides
(1d–f) afforded the products 4d–f with a high level of SN2’
selectivity (SN2’/SN2 > 99:1). Various functional groups,
including silyl ether (4g), pivalate (4h), and chloride (4i),
were well tolerated, thus affording the corresponding
branched alkylboronates in moderate to good yields (76–
82%) and with excellent selectivities (SN2’/SN2 > 99:1). A
boronate bearing an exocyclic methylene, 4j, was synthesized
in 78% yield with complete SN2’ selectivity by the reaction of
the cyclohexenyl derivative 1j with 2a under copper catalysis.
That the developed catalytic conditions are applicable to
secondary allylic chlorides is of particular note. The reaction
of the allylic chloride (E)-1k with 2a in the presence of
20 mol% copper catalyst afforded 4k in 62% yield with
complete SN2’ selectivity and moderate stereoselectivity (E/Z
86:14). In contrast, when the Z-allylic chloride 1l was used,
(E)-4l was obtained exclusively. Importantly, the reaction
with an isomeric mixture of a secondary allylic chloride 1m
(E/Z 1:2.6) afforded the corresponding alkylboronate 4m in
80% yield and with excellent regio- (SN2’/SN2 > 99:1) and
stereoselectivity (E/Z > 99:1).
Next, the reactions of substituted 1,1-diboronates were
investigated. However, when the allylic chloride 1d and ethyl-
1,1-diboronate 2b (R3 = Me) were subjected to the optimized
reaction conditions, only a trace amount of the desired
product was formed. Therefore, we immediately reinvesti-
gated the reaction parameters using 1d and 2b as model
substrates. After extensive re-optimization,[18] we were
pleased to find that the reaction required a higher catalyst
loading (20 mol%) and a 5:1 mixture of 1,4-dioxane and
toluene as the solvent to afford the corresponding product 4n
in 80% yield and with 96:4 SN2’/SN2 selectivity. It should be
emphasized that this avenue opens up new possibilities for the
development of a copper-catalyzed diastereoselective allylic
substitution reaction. By using these modified reaction
conditions, the reactions of aliphatic allylic chlorides 1e and
1 f with 2b afforded the products 4o and 4p, respectively, in
satisfactory yields (55–71%) and good SN2’ selectivity (SN2’/
SN2 95:5).
Entry cat. [Cu] LG
2a (equiv) Yield [%][b] SN2’/SN2[c]
1
2
3
4
5
6
7
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
–
OAc
OCO2tBu
OCO2Me
OP(O)(OEt)2
1.5
1.5
1.5
1.5
1.5
1.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
trace
trace
17
68
84
–
–
53:47
92:8
92:8
74:26
93:7
84:16
51:49
85:15
39:61
87:13
58:42
–
Cl
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
43
94 (78)[d]
82
8[e]
9[f]
73
80
99
87
96
<1
10
11
12
13
14
[a] Reaction conditions: 1 (0.2 mmol), 2a (1.5–2.0 equiv), cat. [Cu]
(10 mol%), LiOtBu (3.0 equiv) in toluene at 508C for 24 h. [b] Deter-
mined by 1H NMR analysis with anisole as the internal standard. [c] SN2’/
SN2 selectivity was determined by 1H NMR analysis of the crude reaction
mixture. [d] Yield of isolated product is shown within parentheses.
[e] NaOtBu was used as the base. [f] KOtBu was used as the base.
LG=leaving group.
in 68% yield (entry 4). Comparable selectivity (SN2’/SN2 =
92:8) was obtained when cinnamyl chloride (entry 5) was used
as the allylic electrophile, but with an improved yield (84%).
However, when cinnamyl bromide was used, a poor yield was
obtained (entry 6). To our delight, using 2 equivalents of 2a
with cinnamyl chloride provided the best yield (94%) and
highest SN2’ selectivity (entry 7). The use of other alkoxide
bases (entries 8 and 9) and solvents resulted in lower yields
and reduced SN2’ selectivity.[18] Moreover, the replacement of
IMes with other NHC ligands such as IPr, ICy, and SIMes
(entries 10–12) resulted in lower SN2’/SN2 ratios. When the
NHC was replaced with a phosphine ligand, poor regioselec-
tivity was obtained (entry 13). This result indicated that
SN2’ selectivity is highly favored by the presence of a NHC
ligand. No reaction occurred when the copper catalyst was
absent (entry 14).
To demonstrate the versatility of the products obtained in
this study, further transformations of 4e were investigated
(Scheme 2). The oxidation of 4e in the presence of sodium
perborate in a mixture of THF/H2O at room temperature
afforded the corresponding alcohol 6 in 92% yield. Product
4e was transformed into the 1,4-diol 7 by a sequence of
hydroboration and oxidation reactions (62% yield over two
steps). Moreover, treatment with iodochloromethane and
nBuLi afforded the one-carbon homologated product 8 in
81% yield. The product 4e was hydrogenated with TsNHNH2
and NaOAc, thus affording the aliphatic alkylboronate 9 in
83% yield, in which the Bpin unit remained intact.
With optimized reaction conditions in hand, the substrate
scope with respect to the allylic chlorides 1 and 1,1-
diborylalkanes 2 was investigated (Table 2). To our delight,
both aromatic and aliphatic allylic chlorides successfully
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Table 2: Substrate scope.[a]
Scheme 2. Transformations of the allylic substitution product:
a) NaBO3·4H2O, THF/H2O (1:1 v/v), RT, 5 h. b) (9-BBN)2, toluene,
1208C, 12 h, then NaOH/H2O2, EtOH, RT, 5 h. c) ICH2Cl, nBuLi, THF,
À788C to RT, 3 h. d) TsNHNH2, NaOAc, THF/H2O (1:1 v/v), reflux,
24 h. THF=tetrahydrofuran, Ts =4-toluenesulfonyl.
Scheme 3. Proposed reaction mechanism.
Although more comprehensive studies are still required to
elucidate the mechanistic details, a plausible mechanism is
shown in Scheme 3. On the basis of the incomplete
SN2’ selectivity and literature precedent,[4l,19] we assume that
a monoalkylalkoxycuprate (D) is involved as an active species
for the transformation. First, the copper alkoxide complex B
is generated from [Cu(IMes)Cl] in the presence of LiOtBu.
The copper complex B then undergoes transmetalation with
the monoborate C[20] to afford the heterocuprate D. Subse-
qeunt SN2’ substitution of 1 with the copper species D via
a allylcopper(III) complex[19] affords the SN2’ product 4 and
the copper(I) complex A. Finally, B is regenerated by the
reaction of A with LiOtBu.
In summary, we have developed the first copper-catalyzed
SN2’-selective allylic substitution reaction which utilizes read-
ily accessible 1,1-diborylalkanes as the coupling partners. This
new protocol provides an efficient strategy to synthesize
branched alkylboronates in good yields and with a high
functional-group compatibility. Further studies to develop an
enantio- and diastereoselective version of this transformation
are underway.
[a] Reaction conditions: 1 (0.2 mmol), 2a (2.0 equiv), Cu(IMes)Cl
(10 mol%), and LiOtBu (3.0 equiv) in toluene at 508C for 24 h. [b] SN2’/
SN2 selectivity was determined by 1H NMR analysis of the crude reaction
mixture. [c] Yield of isolated product. [d] [Cu(IMes)Cl] (20 mol%) was
used. [e] A mixture of 1,4-dioxane/toluene (5:1 v/v) was used as the
solvent instead of toluene. [f] Mixtures of diastereomers were obtained
(1:1–1:1.5). MOM=methoxy methyl ether.
Acknowledgments
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
1500
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[8] Examples of metal-catalyzed allylic substitution with arylbor-
(NRF) funded by the Ministry of Science, ICT & Future
Planning (NRF-2015R1C1A1A02036326) and start-up funds
from POSTECH. We thank Prof. Yunmi Lee and Prof.
Byunghyuck Jung for helpful discussions.
onic acid derivatives: For Cu, see: a) A. M. Whittaker, R. P.
Rucker, G. Lalic, Org. Lett. 2010, 12, 3216; b) R. Shintani, K.
Takatsu, M. Takeda, T. Hayashi, Angew. Chem. Int. Ed. 2011, 50,
8656; Angew. Chem. 2011, 123, 8815; c) M. Takeda, K. Takatsu,
R. Shintani, T. Hayashi, J. Org. Chem. 2014, 79, 2354; For Pd,
see: d) H. Ohmiya, Y. Makida, T. Tanaka, M. Sawamura, J. Am.
Chem. Soc. 2008, 130, 17276; e) D. Li, T. Tanaka, H. Ohmiya, M.
Sawamura, Org. Lett. 2010, 12, 3344; f) Y. Makida, H. Ohmiya,
M. Sawamura, Chem. Asian J. 2011, 6, 410. For Rh, see: g) F.
Menard, D. Perez, D. S. Roman, T. M. Chapman, M. Lautens, J.
Org. Chem. 2010, 75, 4056; h) F. Menard, T. M. Chapman, C.
Dockendorff, M. Lautens, Org. Lett. 2006, 8, 4569; i) H. Kiuchi,
D. Takahashi, K. Funaki, T. Sato, S. Oi, Org. Lett. 2012, 14, 4502;
j) B. Yu, F. Menard, N. Isono, M. Lautens, Synthesis 2009, 853.
[9] F. Gao, J. L. Carr, A. H. Hoveyda, Angew. Chem. Int. Ed. 2012,
51, 6613; Angew. Chem. 2012, 124, 6717.
[10] B. Jung, A. H. Hoveyda, J. Am. Chem. Soc. 2012, 134, 1490.
[11] Y. Shi, B. Jung, S. Torker, A. H. Hoveyda, J. Am. Chem. Soc.
2015, 137, 8948.
[12] a) L. A. Brozek, M. J. Ardolino, J. P. Morken, J. Am. Chem. Soc.
2011, 133, 16778; b) P. Zhang, H. Le, R. E. Kyne, J. P. Morken, J.
Am. Chem. Soc. 2011, 133, 9716; c) P. Zhang, L. A. Brozek, J. P.
Morken, J. Am. Chem. Soc. 2010, 132, 10686.
Keywords: allylic compounds · boron · copper ·
N-heterocyclic carbenes · regioselectivity
How to cite: Angew. Chem. Int. Ed. 2016, 55, 1498–1501
Angew. Chem. 2016, 128, 1520–1523
[1] For reviews on transition metal-catalyzed allylic substitution,
see: a) B. M. Trost, M. L. Crawley, Chem. Rev. 2003, 103, 2921;
b) Z. Lu, S. Ma, Angew. Chem. Int. Ed. 2008, 47, 258; Angew.
Chem. 2008, 120, 264; c) J. F. Teichert, B. L. Feringa, Angew.
Chem. Int. Ed. 2010, 49, 2486; Angew. Chem. 2010, 122, 2538;
d) J. D. Weaver, A. Recio, A. J. Grenning, J. A. Tunge, Chem.
Rev. 2011, 111, 1846.
[2] For reviews on copper-catalyzed allylic substitution, see:
a) A. H. Hoveyda, A. W. Hird, M. A. Kacprzynski, Chem.
Commun. 2004, 1779; b) H. Yorimitsu, K. Oshima, Angew.
Chem. Int. Ed. 2005, 44, 4435; Angew. Chem. 2005, 117, 4509;
c) S. R. Harutyunyan, T. den Hartog, K. Geurts, A. J. Minnaard,
B. L. Feringa, Chem. Rev. 2008, 108, 2824; d) C. A. Falciola, A.
Alexakis, Eur. J. Org. Chem. 2008, 3765; e) A. Alexakis, J. E.
Bäckvall, N. Krause, O. Pàmies, M. DiØguez, Chem. Rev. 2008,
108, 2796.
[13] a) H. Ohmiya, U. Yokobori, Y. Makida, M. Sawamura, J. Am.
Chem. Soc. 2010, 132, 2895; b) Y. Shido, M. Yoshida, M. Tanabe,
H. Ohmiya, M. Sawamura, J. Am. Chem. Soc. 2012, 134, 18573;
c) K. Nagao, U. Yokobori, Y. Makida, H. Ohmiya, M. Sawamura,
J. Am. Chem. Soc. 2012, 134, 8982; d) K. Hojoh, Y. Shido, H.
Ohmiya, M. Sawamura, Angew. Chem. Int. Ed. 2014, 53, 4954;
Angew. Chem. 2014, 126, 5054.
[3] Copper-catalyzed SN2-selective allylic alkylation was also
reported. See: A. M. Lauer, F. Mahmud, J. Wu, J. Am. Chem.
Soc. 2011, 133, 9119, and references therein.
[14] a) K. Endo, T. Ohkubo, M. Hirokami, T. Shibata, J. Am. Chem.
Soc. 2010, 132, 11033; b) K. Endo, T. Ohkubo, T. Shibata, Org.
Lett. 2011, 13, 3368; c) K. Endo, T. Ohkubo, T. Ishioka, T.
Shibata, J. Org. Chem. 2012, 77, 4826; d) K. Endo, T. Ishioka, T.
Shibata, Synlett 2014, 25, 2184.
[15] a) J. C. H. Lee, R. McDonald, D. G. Hall, Nat. Chem. 2011, 3,
894; b) X. Feng, H. Jeon, J. Yun, Angew. Chem. Int. Ed. 2013, 52,
3989; Angew. Chem. 2013, 125, 4081.
[16] a) S. H. Cho, J. F. Hartwig, Chem. Sci. 2014, 5, 694; b) C. Sun, B.
Potter, J. P. Morken, J. Am. Chem. Soc. 2014, 136, 6534; c) J. R.
Coombs, L. Zhang, J. P. Morken, J. Am. Chem. Soc. 2014, 136,
16140; d) H. Li, X. Shangguan, Z. Zhang, S. Huang, Y. Zhang, J.
Wang, Org. Lett. 2014, 16, 448; e) B. Potter, A. A. Szymaniak,
E. K. Edelstein, J. P. Morken, J. Am. Chem. Soc. 2014, 136,
17918; f) H. Li, Z. Zhang, X. Shangguan, S. Huang, J. Chen, Y.
Zhang, J. Wang, Angew. Chem. Int. Ed. 2014, 53, 11921; Angew.
Chem. 2014, 126, 12115; g) K. Hong, X. Liu, J. P. Morken, J. Am.
Chem. Soc. 2014, 136, 10581; h) M. V. Joannou, B. S. Moyer, S. J.
Meek, J. Am. Chem. Soc. 2015, 137, 6176.
[17] Only one cinnamyl phosphate was used as the coupling partner
for the synthesis of linear alkylboronates using CuI as the
catalyst and LiOtBu as the base. For details, see: Z.-Q. Zhang,
C.-T. Yang, L.-J. Liang, B. Xiao, X. Lu, J.-H. Liu, Y.-Y. Sun, T. B.
Marder, Y. Fu, Org. Lett. 2014, 16, 6342.
[4] For selected examples on copper-catalyzed enantioselective
allylic substitution with organometallics, see: a) M. van Klave-
ren, E. S. M. Persson, A. del Villar, D. M. Grove, J.-E. Bäckvall,
G. van Koten, Tetrahedron Lett. 1995, 36, 3059; b) F. Dübner, P.
Knochel, Tetrahedron Lett. 2000, 41, 9233; c) C. A. Luchaco-
Cullis, H. Mizutani, K. E. Murphy, A. H. Hoveyda, Angew.
Chem. Int. Ed. 2001, 40, 1456; Angew. Chem. 2001, 113, 1504;
d) H. Malda, A. W. van Zijl, L. A. Arnold, B. L. Feringa, Org.
Lett. 2001, 3, 1169; e) M. A. Kacprzynski, A. H. Hoveyda, J. Am.
Chem. Soc. 2004, 126, 10676; f) K. E. Murphy, A. H. Hoveyda,
Org. Lett. 2005, 7, 1255; g) D. J. Vyas, M. Oestreich, Angew.
Chem. Int. Ed. 2010, 49, 8513; Angew. Chem. 2010, 122, 8692;
h) J. A. Dabrowski, F. Gao, A. H. Hoveyda, J. Am. Chem. Soc.
2011, 133, 4778; i) M. PØrez, M. FaÇanµs-Mastral, P. H. Bos, A.
Rudolph, S. R. Harutyunyan, B. L. Feringa, Nat. Chem. 2011, 3,
377; j) J.-B. Langlois, A. Alexakis, Angew. Chem. Int. Ed. 2011,
50, 1877; Angew. Chem. 2011, 123, 1917; k) J. K. Park, H. H.
Lackey, B. A. Ondrusek, D. T. McQuade, J. Am. Chem. Soc.
2011, 133, 2401; l) D. Li, H. Ohmiya, M. Sawamura, J. Am.
Chem. Soc. 2011, 133, 5672; m) M. PØrez, M. FaÇanµs-Mastral, V.
Hornillos, A. Rudolph, P. H. Bos, S. R. Harutyunyan, B. L.
Feringa, Chem. Eur. J. 2012, 18, 11880; n) L. B. Delvos, D. J.
Vyas, M. Oestreich, Angew. Chem. Int. Ed. 2013, 52, 4650;
Angew. Chem. 2013, 125, 4748; o) H. Li, A. Alexakis, Angew.
Chem. Int. Ed. 2012, 51, 1055; Angew. Chem. 2012, 124, 1079;
p) V. Hornillos, M. PØrez, M. FaÇanµs-Mastral, B. L. Feringa,
Chem. Eur. J. 2013, 19, 5432.
[18] See the Supporting Information for details.
[19] a) M. Yamanaka, S. Kato, E. Nakamura, J. Am. Chem. Soc. 2004,
126, 6287; b) N. Yoshikai, S.-L. Zhang, E. Nakamura, J. Am.
Chem. Soc. 2008, 130, 12862.
[5] Recently, copper-catalyzed enantioselective allylic alkylations
using alkylzirconium reagents as nucleophiles have been
reported. See: a) H. You, E. Rideau, M. Sidera, S. P. Fletcher,
Nature 2015, 517, 351; b) M. Sidera, S. P. Fletcher, Chem.
Commun. 2015, 51, 5044.
[20] Using 11B NMR analysis, the formation of monoborate was
detected by adding LiOtBu to 2a in [D8]toluene. For details, see
the Supporting Information.
[6] D. G. Hall, Boronic Acids, Wiley-VCH, Weinheim, 2005.
[7] For a review on transition metal catalyzed allylic substitution
with allylboron reagents, see: F. C. Pigge, Synthesis 2010, 1745.
Received: October 21, 2015
Published online: December 15, 2015
Angew. Chem. Int. Ed. 2016, 55, 1498 –1501
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