Copper-Mediated SN2′ Allyl-Alkyl and Allyl-Boryl Couplings of Vinyl Cyclic Carbonates

Article abstract of DOI:10.1021/acs.orglett.7b02947

A method for the copper-catalyzed borylmethylation and borylation of vinyl cyclic carbonates through an S_N2′ mechanism is reported. These singular reactions involve selective S_N2′ allylic substitutions with concomitant ring opening of the cyclic carbonate and with extrusion of CO₂ and formation of a useful hydroxyl functionality in a single step. The stereoselectivity of the homoallylic borylation and allylic borylation processes can be controlled, and synthetically useful unsaturated (E)-pent-2-ene-1,5-diols and (E)-but-2-ene-1,4-diols are accessed.

Full text of DOI:10.1021/acs.orglett.7b02947

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Copper-Mediated SN2′ Allyl−Alkyl and Allyl−Boryl Couplings of Vinyl
Cyclic Carbonates
Nuria Miralles,† Jose Enrique Gomez,§ Arjan W. Kleij,*,§,‡ and Elena Fernandez*,†
́
́
́
́
†Department Química Física i Inorgan
̀
ica, University Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain
§Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Catalans 16,
43007 Tarragona, Spain
‡Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Spain
S
* Supporting Information
ABSTRACT: A method for the copper-catalyzed borylmethy-
lation and borylation of vinyl cyclic carbonates through an SN2′
mechanism is reported. These singular reactions involve
selective SN2′ allylic substitutions with concomitant ring
opening of the cyclic carbonate and with extrusion of CO2
and formation of a useful hydroxyl functionality in a single step.
The stereoselectivity of the homoallylic borylation and allylic
borylation processes can be controlled, and synthetically useful unsaturated (E)-pent-2-ene-1,5-diols and (E)-but-2-ene-1,4-diols
are accessed.
of our knowledge, there has only been one example related to the
nucleophilic borylmethylation through an SN2′ mechanism,
based on a copper-catalyzed selective allylic substitution of
primary and secondary allylic chlorides with 1,1-diborylalkanes
(Scheme 1, top right).6a Despite the usefulness of this approach,
for substrates such as alkyl cinnamyl carbamates, the SN2′ allyl−
alkyl coupling reaction proved to be unproductive.
Inspired by this limitation and in order to be able to extend the
nucleophilic borylmethylation reaction through an SN2′
mechanism, we have explored copper(I)-catalyzed SN2′ allylic
alkylation of vinyl cyclic carbonates with diborylmethane (1)
(Scheme 1). This new approach would allow additional
functionality to be retained in the homoallylic borylated product
since a hydroxyl group is generated with the concomitant loss of
CO2, providing access to scaffolds that are not easily prepared
through other routes. For the sake of comparison, the copper(I)-
catalyzed SN2′ allylic borylation of the same allylic cyclic
carbonates with B2pin2 2 has also been studied, and control
over the stereoselectivity of the allylic borylated product was
explored since both E to Z isomers can be formed. Stereo-
selective synthesis of allylboronates with a hydroxyl terminus
would potentially provide an unprecedented route toward
functionalized allylboronates.7
olecular diversity through organoboron chemistry
Mprovides easy-to-handle and shelf-stable materials that
can be utilized in diverse transformations. The great potential of
boron-selective reactions in simplifying experimental operations
is due to the direct generation of C−B bonds formed from
diboron reagents.1 Alternatively, the use of 1,1-diborylalkane
reagents to conduct nucleophilic borylmethylation has been less
studied, despite the tremendous interest that homologated
organoboron products offer as scaffolds in organic synthesis. gem-
Diborylated compounds have been shown to be useful reagents
with alkyl-2 and aryl-based electrophiles,3 as well as with carbonyl
compounds4 mainly via base-induced deborylation. Diboryl-
methane reacts with allylic electrophiles to promote selective
substitution reactions via SN2 pathways under Pd/Cu catalysis or
metal-free conditions (Scheme 1, top left).5 However, to the best
Scheme 1. Allyl−Alkyl Couplings Using Allylic Electrophiles
and gem-Diborylated Compounds (eq 1), and New Allyl−
Alkyl or Allyl−Boryl Couplings Using Vinyl Cyclic
Carbonates and Diborylmethane or B2pin2 (eq 2)
Initially we carried out the reaction between the vinyl cyclic
carbonate 3 and diborylmethane 1 in the presence of MeOH as
solvent and base to in situ generate the Cu−OMe derivative from
CuCl (Table 1). The estimated copper salt loading and ligand
(where required) was 9 and 13 mol %, respectively. At rt,
substrate 3 (0.2 mmol scale) reacted with reagent 1 (1.2 equiv)
providing moderate conversions of the desired homoallylic
Received: September 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02947
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Table 1. Allyl−Alkyl Couplings between Diborylmethane and
Scheme 2. Substrate Scope for the Allyl−Alkyl Couplings
between Diborylmethane and Vinyl Cyclic Carbonates
a
the Vinyl Cyclic Carbonate 3
b
entry
Cu/ligand
base (mol %)
E/Z
yield (E)
1
2
3
CuCl/SIPr
CuCl/PPh3
CuCl
Cs2CO3, 15
Cs2CO3, 15
Cs2CO3, 15
Cs2CO3, 15
t-OBuLi, 15
Cs2CO3, 50
3.9:1
4:1
4:1
4:1
4:1
4:1
35
13
58
40
24
75
c
4
CuCl
5
6
CuCl
CuCl
a
Conditions: carbonate (0.2 mmol), CH2(Bpin)2 (1.2 equiv), CuCl (9
mol %), ligand (13 mol %), base (50 mol %), MeOH (0.10 mL), rt, 16
b
c
h. NMR yield using naphthalene as internal standard. CH2(Bpin)2
(2 equiv).
groups (cf. (E)-9), and an interesting butadiene derivative (E)-
11, which was isolated in high yield (82%).
To further test the viability of the C−B bond formation from
vinyl cyclic carbonates, we carried out the reaction between
substrate 3 and B2pin2 2 in the presence of MeOH as solvent and
base (Table 2). When CuCl (9 mol %) was used (Table 2, entry
borylated product (E)-(5-hydroxy-4-phenylpent-3-en-1-yl)-
boronate ester 4 mediated by CuCl/SIPr or CuCl/PPh3
(Table 1, entries 1 and 2). The exclusive formation of the new
C−C bond at the terminal position exemplifies the regiocontrol
of the allyl−alkyl cross-coupling reaction, but of particular note is
that the SN2′ process allows for simple extrusion of CO2 from the
cyclic carbonate precursor, keeping a synthetically useful OH
functionality. In the absence of any ligand, the unmodified
copper species generated product (E)-4 in up to 58% yield
(Table 1, entry 3). Neither the use of a double amount of
diborylmethane nor the presence of alternative bases such as
LiOtBu improved the reaction outcome (Table 1, entries 4 and
5). A higher Cs2CO3 loading (50 mol %) was optimal to achieve
quantitative conversion, and 4 was obtained in a yield of 75% (E/
Z = 4:1) (Table 1, entry 6). Interestingly, the ratio of E/Z
stereoisomers is higher than the E/Z ratios observed in the cross-
coupling of vinyl cyclic carbonates with arylboronic acids
catalyzed by Pd nanoparticles.6b
Since the only examples known for copper-catalyzed SN2′-
selective allylic substitution reaction between 1,1-diborylalkanes
and allylic chlorides6a were unproductive for allylic acylic
carbonates, the newly developed protocol (Table 1) provides
complementary reactivity. In addition, no sign of SN2-
substitution could be detected, and the proposed copper-
catalyzed SN2′-selective allylic substitution thus represents a
carbonate ring opening reaction under relatively high stereo-
control.
We next explored the allyl−alkyl coupling of a series of
substituted vinyl cyclic carbonates and diborylmethane to further
expand this Cu-catalyzed process (Scheme 2) (conditions: Table
1, entry 6). A general trend is observed in the formation of the
borylated products 5−11 with the E isomer being the favored
stereoisomer. In all crude reaction products, the E/Z ratios were
close to 4:1, independent from the substituent present in the
vinyl cyclic carbonates. Both stereoisomers could be isolated
from the reaction media; the corresponding isolated yields of the
E isomer are shown in Scheme 2 (Supporting Information (SI)
for details on the Z-isomers). Electron-donating or -withdrawing
substituents in the aryl group (as well as their relative position)
did not interfere in the formation of the homoallyl boronates
(E)-5, (E)-6, (E)-7, (E)-8, and (E)-10, with yields of up to 70%.
The reaction is also tolerant toward other functionalities present
in the vinyl cyclic carbonate substrate, including thiophenyl
Table 2. Allyl−Boryl Couplings between B2pin2 and Vinyl
a
Cyclic Carbonate 3
b
b
b
entry
Cu/ligand
base
conv (%) (E)-13 (%)
(Z)-13 (%)
c
1
2
3
4
5
6
7
CuCl/−
CuCl/−
CuOtBu/−
CuCl/SIPr
CuCl/PPh3
CuCl/dppe
CuCl/PP
Cs2CO3
KOtBu
95
61
64
99
99
91
99
62 (60)
41 (40)
47 (37)
69 (65)
57
17
19
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
15
31 (30)
35
36 (24)
52 (39)
72 (45)
a
Conditions: carbonate (0.2 mmol), B2pin2 (1.2 equiv), Cu salt (9 mol
%), ligand (13 mol %), base (15 mol %), MeOH (0.10 mL), rt, 16 h. A
high throughput screening of ligands can be found in the SI.
b
Calculated by 1H NMR (CDCl3) using mesitylene as internal
c
standard. Values in brackets represent isolated yields. <5% degraded
substrate was observed.
1), the conversion of 3 was quantitative with the principal
formation of the allyl boronate (E)-12 (isolated yield 60%)
together with a minor amount of a secondary product.
Interestingly, the latter was isolated as a result of an in situ
intramolecular cyclization process from the Z stereoisomer. The
nucleophilic attack of the boryl moiety onto the vinyl cyclic
carbonate 3 readily takes place at rt through a “Cu−Bpin”
intermediate that is formed in situ from a CuCl/MeOH/base/
B2pin2 combination.8 Notably, the transition-metal-free version
does not allow for the allylic borylation of vinyl cyclic
carbonates.9
The copper catalyzed reaction proceeds regioselectively as the
C−B bond was exclusively formed at the terminal position of the
allylic intermediate confirming the SN2′ mechanism.10 In the
absence of any ligand, the formation of some degraded substrate
could be observed (Table 2, entry 1), and the use of alternative
bases such as t-OBuK in the allylic borylation of 3 reduced both
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the overall conversion and stereoselectivity (entry 2). We also
carried out a reaction with a preformed CuOt−Bu catalyst (entry
3)11 and found that it worked comparably to the in situ formed
catalyst derived from CuCl/t-OBuK in MeOH. Therefore, we
continued with the in situ prepared catalyst in the presence of
B2pin2. The amount of base was optimized to 15 mol %, which is
significantly less than the amount of base used in similar copper-
catalyzed allylic borylations requiring typically 1−3 equiv. The
use of an N-heterocyclic carbene ligand slightly modified the
reaction outcome in the allylic borylation of 3 since the process
was more efficient in terms of total conversion toward the
borylated products (entry 4). In the presence of SIPr, the
formation of the allylboronate (E)-12 also gave an improved
yield of 69%. A CuCl/PPh3 based catalyst gave a mixture of
borylated compounds 12 with an E/Z ratio of 57:35 (Table 2,
entry 5). The use of bidentate phosphine ligands, however, favors
the formation of boracycle (Z)-13. An improved selectivity
toward (Z)-13 was achieved when the diphosphine 1,2-
bis(diphenylphosphino)ethane (dppe) was used, giving an E/Z
ratio of 36:52 (Table 2, entry 6). Interestingly, when the
diphosphine 1,2-bis(di-tert-butylphosphinomethyl)benzene
(PP) was added, exclusive formation of boracycle (Z)-13 could
be achieved (Table 2, entry 7).
While copper-mediated decarboxylative allylic borylation
reactions of acyclic carbonates have been used to obtain
allenylboronates,12,13 vinylboronates,14 and allylboronates,15
those methods lose the whole OCO2R functional group during
the C−B bond formation. Our method permits additional
functionality to be retained in the final product. Taking
advantage of this new methodology, we explored the borylation
of a series of vinyl cyclic carbonates using CuCl/SIPr as the
catalyst system (conditions: Table 2, entry 4) to give the (E)-
allylboronates 12 and 14−19 as the main product (Scheme 3).
When 1,2-bis(di-tert-butylphosphinomethyl)benzene (PP)
was used as ligand, the allylic borylation of alkyl/aryl-substituted
vinyl cyclic carbonates advanced toward the (Z)-stereoisomer
following intramolecular cyclization to afford the boracycles 13
and 20−22 (Scheme 4) (conditions: Table 2, entry 7). (Z)-
Scheme 4. (Z)-Selective Allyl−Boryl Couplings between
B2pin2 and Vinyl Cyclic Carbonates
Boracycles are important in the context of diversity-oriented
organic synthesis,16 as well as in organoboron based drug
discovery.17 Other boracycles have exclusively been obtained
through our copper-catalyzed borylation to allylic cyclic
carbonates, but the isolated yields were relatively low (see SI
for details). The molecular structure of (Z)-13 was also
confirmed by X-ray diffraction (Scheme 4, inset).
A proposed reaction mechanism for the SN2′ allyl−alkyl
coupling (Figure 1 and SI for further details) and SN2′ allyl−
Scheme 3. (E)-Selective Allyl-Boryl Couplings between
B2pin2 and Vinyl Cyclic Carbonates
Figure 1. Proposed mechanism for SN2′ allyl−alkyl coupling.
boryl coupling reactions may involve first activation of the
diborylmethane reagent or B2pin2 to form Cu−CH2Bpin or Cu−
Bpin, respectively. Figure 1 shows that Cu−CH2Bpin
intermediate A coordinates the terminal alkene of substrate to
generate B followed by regioselective addition producing a new
alkyl−Cu intermediate C. Hereafter, elimination of the product
from D in a formal anti-SN2′ pathway releases CO2 and
regenerates the copper complex.
To demonstrate the synthetic use of the homoallylic and allylic
borylated products, we conducted an in situ copper-catalyzed
SN2′ allyl−alkyl and SN2′ allyl−boryl coupling followed by
oxidative workup (H2O2, NaOH). The corresponding (E)-
configured pent-2-ene-1,5-diols and but-2-ene-1,4-diols were
isolated as the main products (Figure 2). The corresponding (Z)-
isomers of the pent-2-ene-1,5-diols could also be isolated in low
The conversion of different carbonate precursors into their
borylated products was almost quantitative in most cases, with
some minor amount of the (Z)-isomers being formed (<10%)
together with some degraded substrate. In general, rather similar
isolated yields were obtained (52−65%) independent from the
type of substrate. The borylation of 3 could also be carried out on
gram-scale in a slightly lower yield (56%, Scheme 3), but the use
of vinyl carbonates with alkyl groups (R = Me, Cy) was
unproductive.
C
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2016, 6, 3381. (e) Ebrahim-Alkhalil, A.; Zhang, Z.-Q.; Gong, T.-J.; Su,
W.; Lu, X.-Y.; Xiao, B.; Fu, Y. Chem. Commun. 2016, 52, 4891. (f) Lu, X.;
Xiao, B.; Zhang, Z.-Q.; Gong, T.-J.; Su, W.; Fu, Y.; Liu, L. Nat. Commun.
2016, 7, 11129. (g) Li, F.; Zhang, Z.-Q.; Lu, X.; Xiao, B.; Fu, Y. Chem.
Commun. 2017, 53, 3551.
(3) (a) Endo, K.; Ohkubo, T.; Hirokami, M.; Shibata, T. J. Am. Chem.
Soc. 2010, 132, 11033. (b) Endo, K.; Ohkubo, T.; Shibata, T. Org. Lett.
2011, 13, 3368. (c) Sun, C.; Potter, B.; Morken, J. P. J. Am. Chem. Soc.
2014, 136, 6534. (d) Potter, B.; Szymaniak, A. A.; Edelstein, E. K.;
Morken, J. P. J. Am. Chem. Soc. 2014, 136, 17918. (e) Potter, B.;
Edelstein, E. K.; Morken, J. P. Org. Lett. 2016, 18, 3286.
Figure 2. One-pot preparation of but-2-ene-1,4-diols and pent-2-ene-
1,5-diols.
(4) (a) Endo, K.; Hirokami, M.; Shibata, T. J. Org. Chem. 2010, 75,
3469. (b) Coombs, J. R.; Zhang, L.; Morken, J. P. Org. Lett. 2015, 17,
1708. (c) Joannou, M. V.; Moyer, B. S.; Goldfogel, M. J.; Meek, S. J.
Angew. Chem., Int. Ed. 2015, 54, 14141. (d) Joannou, M. V.; Moyer, B. S.;
Meek, S. J. J. Am. Chem. Soc. 2015, 137, 6176. (e) Murray, S. A.; Green, J.
C.; Tailor, S. B.; Meek, S. J. Angew. Chem., Int. Ed. 2016, 55, 9065.
(5) (a) Endo, K.; Ohkubo, T.; Ishioka, T.; Shibata, T. J. Org. Chem.
2012, 77, 4826. (b) Hong, K.; Liu, X.; Morken, J. P. J. Am. Chem. Soc.
2014, 136, 10581. (c) Zhang, Z.-Q.; Yang, C.-T.; Liang, L.-J.; Xiao, B.;
Lu, X.; Liu, J.-H.; Sun, Y.-Y.; Marder, T. B.; Fu, Y. Org. Lett. 2014, 16,
6342.
yield (see the SI). Interestingly, the (E)-isomers of such but-2-
ene-1,4-diols are valuable compounds, being about 190 times
more expensive than their corresponding (Z)-isomers.18 There-
fore, our versatile one-pot approach opens a new straightforward
route toward these scaffolds,19 which are useful in organic
synthesis.20
In conclusion, we present a stereoselective copper-catalyzed
selective SN2′ allylic substitutions of vinyl cyclic carbonate to
form allylboranes and homoallylboranes. The stereoselectivity is
catalyst-controlled, and in situ copper-catalyzed C−CH2B and
C−B bond formation followed by oxidative workup provides
direct access to valuable (E)-configured pent-2-ene-1,5-diols and
but-2-ene-1,4-diols.
(6) (a) Kim, J.; Park, S.; Park, J.; Cho, S. H. Angew. Chem., Int. Ed. 2016,
55, 1498. (b) Mao, Y.; Zhai, X.; Khan, A.; Cheng, J.; Wu, X.; Zhang, Y. J.
Tetrahedron Lett. 2016, 57, 3268.
́
(7) (a) Diner, C.; Szabo, K. J. J. Am. Chem. Soc. 2017, 139, 2.
(b) Ramachandran, P. V.; Gagare, P. D.; Nicponski, D. R.; Knochel, P.;
Molander, G. A. Allylborons. Comprehensive Organic Synthesis 2014, 1−
71.
(8) (a) Makoto, Y. Bull. Chem. Soc. Jpn. 2011, 84, 984. (b) Cid, J.;
́
Gulyas, H.; Fernandez, E.; Carbo, J. J. Chem. Soc. Rev. 2012, 41, 3558.
(c) Dewhurst, R. D.; Neeve, E. C.; Braunschweig, H.; Marder, T. B.
Chem. Commun. 2015, 51, 9594.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02947.
(9) For alkoxide-catalyzed borylation of allylic and propargylic
alcohols: (a) Miralles, N.; Alam, R.; Szabo, K. J.; Fernandez, E. Angew.
́
́
Chem., Int. Ed. 2016, 55, 4303. (b) Harada, K.; Nogami, M.; Hirano, K.;
Kurauchi, D.; Kato, H.; Miyamoto, K.; Saito, T.; Uchiyama, M. Org.
Chem. Front. 2016, 3, 565.
Experimental procedures and characterization of all allyl−
alkyl couplings using vinyl cyclic carbonates and diboryl-
methane or B2pin2 (PDF)
(10) For opposite regioselectivity in decaboxylative borylation, see: Ito,
H.; Kosaka, Y.; Nonoyama, N.; Sasaki, Y.; Sawamura, M. Angew. Chem.,
Int. Ed. 2008, 47, 7424.
AUTHOR INFORMATION
Corresponding Authors
■
(11) Bonet, A.; Lillo, V.; Ramírez, J.; Díaz-Requejo, M. M.; Fernan
́
dez,
E. Org. Biomol. Chem. 2009, 7, 1533.
*E-mail: mariaelena.fernandez@urv.cat.
*E-mail: akleij@iciq.es.
(12) Ito, H.; Sasaki, Y.; Sawamura, M. J. Am. Chem. Soc. 2008, 130,
15774.
(13) (a) Zhao, T. S. N.; Yang, Y.; Lessing, T.; Szabo,
Soc. 2014, 136, 7563. (b) Yang, Y.; Szabo, K. J. J. Org. Chem. 2016, 81,
250.
(14) For carboxylic acids: (a) Feng, Q.; Yang, K.; Song, Q. Chem.
Commun. 2015, 51, 15394. (b) Zhao, Y.-W.; Feng, Q.; Song, Q.-L. Chin.
Chem. Lett. 2016, 27, 571.
(15) (a) Ito, H.; Kawakami, C.; Sawamura, M. J. Am. Chem. Soc. 2005,
127, 16034. (b) Ito, H.; Ito, S.; Sasaki, Y.; Matsuura, K.; Sawamura, M. J.
Am. Chem. Soc. 2007, 129, 14856. (c) Guzman-Martínez, A.; Hoveyda,
A. H. J. Am. Chem. Soc. 2010, 132, 10634.
(16) Micalizio, G. C.; Schreiber, S. Angew. Chem., Int. Ed. 2002, 41, 152.
(17) (a) Ramachandran, P. V. Future Med. Chem. 2013, 5, 611.
(b) Baker, S. J.; Ding, C. Z.; Akama, T.; Zhang, Y.-K.; Hernandez, V.;
Xia, Y. Future Med. Chem. 2013, 1, 1275.
́
K. J. J. Am. Chem.
ORCID
́
Arjan W. Kleij: 0000-0002-7402-4764
Elena Fernandez: 0000-0001-9025-1791
Notes
́
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by MINECO through projects
CTQ2016-80328-P and CTQ-2014-60419-R and the Severo
Ochoa Excellence Accreditation 2014−2018 through project
SEV-2013-0319.
(18) (a) Ezawa, T.; Kawashima, Y.; Noguchi, T.; Jung, S.; Imai, N.
Tetrahedron: Asymmetry 2017, 28, 266. (b) Guo, W.; Martínez-
REFERENCES
■
(1) (a) Xu, L.; Zhang, S.; Li, P. Chem. Soc. Rev. 2015, 44, 8848.
(b) Neeve, E.; Geier, S. J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B.
Chem. Rev. 2016, 116, 9091. (c) Cuenca, A. B.; Shishido, R. I.; Ito, H.;
Fernandez, E. Chem. Soc. Rev. 2017, 46, 415.
(2) Latest contributions: (a) Kim, J.; Park, S.; Park, J.; Cho, S. H.
Angew. Chem., Int. Ed. 2016, 55, 1498. (b) Shi, Y.; Hoveyda, A. H. Angew.
Chem., Int. Ed. 2016, 55, 3455. (c) Zhang, Z.-Q.; Zhang, B.; Lu, X.; Liu,
J.-H.; Lu, X.-Y.; Xiao, B.; Fu, Y. Org. Lett. 2016, 18, 952. (d) Zhan, M.; Li,
R.-Z.; Mou, Z.-D; Cao, C.-G.; Liu, J.; Chen, Y.-W.; Niu, D. ACS Catal.
Rodriguez, L.; Martin, E.; Escudero-Adan
Chem., Int. Ed. 2016, 55, 11037.
́
, E. C.; Kleij, A. W. Angew.
(19) (a) Ishino, I.; Wakamoto, K.; Hirashima, T. Chem. Lett. 1984, 13,
765. (b) Klimovica, K.; Grigorjeva, L.; Maleckis, A.; Popelis, J.;
Jirgensons, A. Synlett 2011, 2011, 2849. (c) De Silva, A. N.; Francis, C.
L.; Ward, A. D. Aust. J. Chem. 1993, 46, 1657.
(20) (a) Emayavaramban, B.; Sen, M.; Sundararaju, B. Org. Lett. 2017,
19, 6. (b) Tortosa, M. Angew. Chem., Int. Ed. 2011, 50, 3950.
D
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