Iron-catalyzed cross-coupling of aryltrimethylammonium triflates and alkyl Grignard reagents

Article abstract of DOI:10.1016/j.tet.2013.09.039

Fe(acac)₃ effectively catalyzes reaction of aryltrimethylammonium triflates with β-hydrogen-containing primary or secondary alkyl Grignard reagents in a mixed solvent of THF and NMP at room temperature. A series of functional groups are tolerated under the reaction conditions.

Full text of DOI:10.1016/j.tet.2013.09.039

Tetrahedron xxx (2013) 1e6
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Iron-catalyzed cross-coupling of aryltrimethylammonium triflates
and alkyl Grignard reagents
*
Wang-Jun Guo, Zhong-Xia Wang
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 May 2013
Received in revised form 3 September 2013
Accepted 13 September 2013
Available online xxx
Fe(acac)3 effectively catalyzes reaction of aryltrimethylammonium triflates with b-hydrogen-containing
primary or secondary alkyl Grignard reagents in a mixed solvent of THF and NMP at room temperature. A
series of functional groups are tolerated under the reaction conditions.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Aryltrimethylammonium salt
CeN activation
Alkylation
Grignard reagents
Iron catalyst
Cross-coupling
1. Introduction
aryl Grignard reagents in 2010 using Pd catalyst.4 MacMillan et al.
reported the Suzuki coupling of aryltrimethylammonium triflates
Transition-metal-catalyzed CeN bond activation and trans-
formation of amines are attractive because (i) aliphatic and aromatic
amines are widelyavailable natural or industrial products and hence
are abundant starting materials for organic synthesis; (ii) the amino
groups of aromatic amines are important directing groups, which
direct the introduction of other functional groups onto the aromatic
ring. However, cleavage of the CeN bond in an amine is difficult due
to high bond energy although several examples have been reported.
Kakiuchi et al. reported an example of the Csp2eN cleavage of aniline
derivatives and coupling with organoboronates using Ru as catalyst,
but the existence of ortho carbonyl group in the aromatic amines is
indispensable.1 The cleavage of the Csp3eN bond of tertiary amines
has been carried out through iminium ion intermediates formed by
catalyzed oxidation.2 Transformation of a aliphatic or aromatic
amine into an ammonium salt can make the CeN cleavage easier. As
early as 1988 Wenkert et al. reported nickel-catalyzed Kumada
coupling of aryltrimethylammonium iodides with Grignard re-
agents in low to moderate yields.3 Reeves et al. developed the
coupling reaction between aryltrimethylammonium triflates and
and aryl boronic acids using [Ni(cod)2]/IMes (cod¼1,5-
cyclooctadiene; IMes¼1,3-dimesitylimidazol-2-ylidene) as the cat-
alyst.5 More recently, nickel- or palladium-catalyzed cross-cou-
plings of benzylic or vinyl ammonium salts and aryl boronic acids
were carried out.6 We reported the cross-coupling of aryl-
trimethylammonium salts and organozinc reagents using
(Cy3P)2NiCl2 or pincer nickel complexes as catalysts.7 However, the
effective reaction of aryltrimethylammonium salts with b-hydro-
gen-containing alkyl nucleophiles is extremely scarce. (Dppp)NiCl2
(dppp¼1,3-bis(diphenylphosphino)propane) was shown by Wen-
kert et al. to catalyze cross-coupling of p ꢀ n ꢀ BuC6H4NMeþ3 Iꢀ or
p ꢀ MeOC6H4NMeþ3 Iꢀ with i-PrMgBr, but resulted in only 18% and
20% product yields, respectively.3 On the other hand, iron salts or
iron complexes were proven to be effective catalysts for cross-
coupling reactions, including those using alkyl halides and alkyl
metal reagents as substrates.8 For example, Olsson et al. reported
that Fe(acac)3 (acac¼acetylacetonate) catalyzed cross-coupling of
imidoyl chlorides with alkyl Grignard reagents at room temperature
8i
or ꢀ78 ꢁC. Furstner et al. reported that [Li(TMEDA)]2[Fe(C2H4)4]
€
catalyzed cross-coupling of alkyl halides with aryl Grignard reagents
at ꢀ20 ꢁC.8j Herein we report iron-catalyzed cross-coupling of
aryltrimethylammonium triflates and b-hydrogen-containing pri-
* Corresponding author. Tel.: þ86 551 63603043; fax: þ86 551 63601592; e-mail
address: zxwang@ustc.edu.cn (Z.-X. Wang).
mary and secondary alkyl Grignard reagents.
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.09.039
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2
W.-J. Guo, Z.-X. Wang / Tetrahedron xxx (2013) 1e6
2. Results and discussion
20,40,60-triisopropylbiphenyl), dppp, Ph2P(O)H, t-Bu2P(O)H, and
-proline gave only low to moderate yields (entries 8e18, Table 1).
L
A range of aryltrimethylammonium triflates were prepared for
the following studies through reaction of N,N-dimethylarylamine
with methyl trifluoromethanesulfonate according to reported
procedure. All the ammonium salts are white solids and insoluble
in most organic solvents such as Et2O, THF, and toluene, but soluble
in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF).
The unknown aryltrimethylammonium triflates were characterized
by 1H and 13C NMR spectroscopy and elemental analyses (see
Experimental section).
As mentioned in introduction, (dppp)NiCl2 is not efficient for
the coupling of aryltrimethylammonium salts and i-PrMgBr.3
(Ph3P)2PdCl2 catalyst is suitable for only the cross-coupling reaction
of aryltrimethylammonium salts with aryl Grignard reagents.4 On the
other hand, copper, cobalt, and ironwere proven to be able to catalyze
cross-coupling of alkyl Grignard reagents and alkyl or aryl electro-
philes such as halides and triflates.8e10 Based on these reports, we
tried to employ copper, cobalt, and iron as catalysts for the coupling of
aryltrimethylammonium salts with alkyl Grignard reagents. Reaction
of p ꢀ EtO2CC6H4NMeþ3 OTfꢀ (OTf¼triflate) with CyMgBr was
chosen as a model reaction for screening of catalysts and con-
ditions and the results are listed in Table 1. The combination of CuCl,
Cu(OTf)2, Co(acac)3 or Fe(OTf)3, and IPr$HCl (IPr¼1,3-bis(2,6-
diisopropylphenyl) imidazol-2-ylidene) were almost inactive.
Whereas combination of FeCl3 or Fe(acac)3 and IPr$HCl afforded de-
sired products in moderate yields. This positive result encouraged us
to screen different ligands to improve the reaction. However, com-
bination of Fe(acac)3 and a range of ligands involving IMes, TMEDA,
1,10-phenanthroline, PPh3, PCy3, Xphos (2-Dicyclohexylphosphino-
Further examination proved that Fe(acac)3 catalyzed the reaction in
THFwithoutrequirementof extra ligandsand led to59% product yield
(entry 19, Table 1). Test of solvent effect showed that the reaction in
DME (dimethoxyl ethane) afforded a little lower yield than inTHFand
the reaction in toluene did not form cross-coupling product (entries
20 and 21, Table 1). Addition of NMP (N-methyl-2-pyrrolidone) to
a THF solution as co-solvent dramatically improved the reaction re-
sults. The highest yields were achieved when the ratio of THF to NMP
ranged from 7:1 to 3:1 (entries 22e25, Table 1). The higher ratio of
NMP, e.g., a 1:1 mixture of THF and NMP, gave poorer result (entry 26,
Table 1).11 It has been reported that NMP can coordinate to iron center
to form stable complexes.12 Cahiez et al. indicated that NMP probably
stabilizes the active iron intermediates in a Fe(acac)3-catalyzed cross-
coupling of alkenyl chlorides with alkylmagnesium reagents.13 In our
catalytic system, the role of NMP may be to stabilize the catalytically
active species through coordination to the iron center in the catalytic
cycle.
Buchwald and Bolm indicated that trace quantities of other
metals such as copper in the iron salt can significantly affect out-
comes in some reactions.14 In order to rule out the possibility in our
catalytic reactions, we detected the quantities of other metals in
Fe(acac)3 sample (98% purity) and tested catalysis of the impurity
metals. The analysis by atom emission spectroscopy showed the
presence of nickel (740 ppm) and manganese (450 ppm). No copper
and palladium were found. We applied (DME)NiBr2 and MnCl2 in
respective metal quantities determined by atom emission spec-
troscopy to catalyze above reaction and the results showed only
trace cross-coupling product or no product (entries 1 and 2,
Table 2). Cu(OTf)2 of 0.01 mol % was also tested for the reaction
although no copper was found in Fe(acac)3 and only trace quanti-
ties of product were detected (entry 3, Table 2). (DME)NiBr2
(740 ppm) and MnCl2 (450 ppm) were also added into the reaction
system catalyzed by Fe(acac)3 (10 mol % loading, 99.9% purity). The
results showed that the additives led to decrease of product yields
compared with that using Fe(acac)3 (99.9% purity) without addi-
tives (entries 4e6, Table 2). It seems that Mn and Ni additives in-
hibit the reaction to some extent. However, addition of Cu(OTf)2
(0.01 mol % and 0.001 mol %, respectively) into Fe(acac)3 (10 mol %,
99.9% purity) catalyst system did not affect the reaction results
(entries 4, 7, and 8, Table 2). In addition, it was noted that 99.9%
purity of Fe(acac)3 resulted in higher product yield than 98% purity
of Fe(acac)3 (entry 23 in Table 1 and entry 4 in Table 2) for the re-
action of p ꢀ EtO2CC6H4NMeþ3 OTfꢀ with CyMgBr. But in most
Table 1
Screening of catalyst and conditions of the coupling of p ꢀ COOEtC6H4NMeþ3 OTfꢀ
and CyMgBra,b
NMe3OTf
MgBr
[M] (10 mol%)
Solvent, rt, 1 h
+
EtOOC
COOEt
Entry
Catalyst
Ligand (mol %)
Solvent
Yieldc (%)
1
2
3
4
5
6
7
8
d
d
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
DME
Toluene
THF/NMP (9:1)
THF/NMP (7:1)
THF/NMP (5:1)
THF/NMP (3:1)
THF/NMP (1:1)
trace
d
CuCl
IPr$HCl (10)
IPr$HCl (20)
IPr$HCl (30)
IPr$HCl (30)
IPr$HCl (30)
IPr$HCl (30)
IMes$HCl (30)
TMEDA (20)
TMEDA (120)
1,10-Phen (30)
PPh3 (30)
PCy3 (30)
XPhos (30)
Dppp (30)
Ph2P(O)H (30)
t-Bu2P(O)H (30)
Cu(OTf)2
Co(acac)3
Fe(OTf)3
FeCl3
d
Trace
Trace
60
60
20
20
58
38
61
55
53
38
62
62
63
59
53
d
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Table 2
Effects
of
metal
sꢀalts
and
additives
in
the
cross-coupling
of
p ꢀ EtO2CC6H4NMeþ3 OTf and CyMgBra
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
NMe3OTf
MgBr
Catalyst, Additive
+
THF-NMP (7:1)
rt, 1 h
EtOOC
COOEt
Entry
Catalyst
Additive
Yieldb (%)
L-Proline (30)
d
d
d
d
d
d
d
d
1
2
3
(DME)NiBr2 (740 ppm)
MnCl2 (450 ppm)
d
d
d
d
Trace
d
Cu(OTf)2 (0.01 mol %)
Fe(acac)3 (10 mol %)
Fe(acac)3 (10 mol %)
Fe(acac)3 (10 mol %)
Fe(acac)3 (10 mol %)
Fe(acac)3 (10 mol %)
Trace
95
78
59
93
74
89
89
89
4c
5c
6c
7c
8c
(DME)NiBr2 (740 ppm)
MnCl2 (450 ppm)
Cu(OTf)2 (0.01 mol %)
Cu(OTf)2 (0.001 mol %)
41
95
a
a
Unless otherwise stated, reactions were carried out according to the conditions
indicated by the above equation, 2 equiv of CyMgBr in 3 mL THF was used.
Unless otherwise stated, reactions were carried out according to the conditions
indicated by the above equation, 2 equiv of CyMgBr was used in 3 mL THF.
b
b
Purity of Fe(acac)3 is 98%.
Isolated yields.
Isolated yields.
Purity of Fe(acac)3 is 99.9%.
c
c
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W.-J. Guo, Z.-X. Wang / Tetrahedron xxx (2013) 1e6
3
reactions the two purities of iron complexes gave very close
product yields.
However, 2-naphthyltrimethylammonium triflate and 8-dimeth
ylamino-1-naphthyltrimethylammonium triflate displayed lower
reactivity than 1-naphthyltrimethylammonium triflate. Reaction of
the former with primary alkylmagnesium reagents resulted in
55e62% yield of desired products and longer reaction time cannot
improve the yields (entries 22e25, Table 3). 2-Pyridyltrimethyl
ammonium triflate can also react with primary alkylmagnesium
reagents in relatively low yields (entries 26 and 27, Table 3). Reaction
between 2-pyridyltrimethylammonium triflate and n-C8H17MgBr
required longer reaction time (10 h) and gave 43% yield of 2-n-
C8H17C5H5N.
The reaction conditions optimized for this deamination cross-
coupling were similar to those in the iron-catalyzed cross-cou-
pling of Grignard reagents with aryl halides.8def We assume that
they follow a similar mechanism. Based on the mechanism of Fe-
catalyzed cross-coupling of Grignard reagents with aryl halides
We next examined the substrate scope using 99.9%
purity of Fe(acac)3 in a mixture of THF and NMP (7:1). Various
electron-deficient aryltꢀrimethylammonium triflates involving
p ꢀ EtO2CC6H4NMeþOTf , p ꢀ i ꢀ PrO2CC6H4NMeþOTfꢀ, p ꢀ Et2
3
3
NOCC6H4NMeþ3 OTfꢀ
,
p ꢀ NCC6H4NMeþOTfꢀ
,
and p ꢀ CF3C6H4
NMeþ3 OTfꢀ could effectively couple w3ith
b-hydrogen-containing
primary or secondary alkylmagnesium reagents, affording corre-
sponding cross-coupling products in moderate to excellent yields
(entries 1e18, Table 3). Except p ꢀ CF3C6H4NMeþOTfꢀ, all reactions
3
were performed at room temperature. The reactions also proceeded
rapidly, requiring only 1 h or shorter reaction time. For some sub-
strates, longer reaction time led to bꢀy-products. For example, re-
action between p ꢀ NCC6H4NMeþOTf and alkyl Grignard reagents
was quenched in 10 min to give370e80% yield of desired products
(entries 15e17, Table 3). If reaction time was prolonged to 1 h, the
yields of the desired products decreased to some extent due to side
reactions. Reaction of p ꢀ CF3C6H4NMeþOTfꢀ with n-C8H17MgBr
required elevated temperature and gav3e only 77% product yield
(entry 18, Table 3). In these reactions a series of functional groups
including COOEt, COOi-Pr, CONEt2, CN, CF3, and CeC double bond
groups were tolerated. 1-Naphthyltrimethylammonium triflate
exhibited excellent reactivity. Each of the reaction between the
ammonium salt and PhCH2CH2CH2MgBr, n-C4H9MgBr or n-
C8H17MgBr gave excellent product yields (entries 19e21, Table 3).
€
proposed by Furstner et al., we proposed the mechanism of the
Fe(acac)3-catalyzed reaction of aryltrimethylammonium triflate
with alkyl Grignard reagents as shown in Scheme 1. Thus, Fe(acac)3
is reduced by the Grignard reagent to form a formal composition
[Fe(MgX)2] species, which is active catalyst. Reaction of [Fe(MgX)2]
with aryltrimethylammonium triflate generates ArFe(OTf)(MgX)2
and NMe3, which was monitored by 1H and 13C NMR spectroscopy.
The former releases Mg(X)(OTf) to yield ArFe(MgX).
Action of ArFe(MgX) with RMgX produces ArFe(R)(MgX)2, which
upon reductive elimination gives cross-coupling product and
regenerates the active catalyst. The good reactivity of the aryl-
trimethylammonium salts is probably due to decrease of the
aryleammonium bond energy through CAreN deconjugation.
Table 3
Iron-catalyzed cross-coupling of aryltrimethylammonium triflates and alkyl
Grignard reagentsa
Fe(acac)3 (10 mol%)
RMgX + Fe(acac)3
+
RalkylMgX
Ar NMe3OTf
Entry Ar
Ar Ralkyl
THF-NMP (7:1), rt, 1 h
ArNMe3+OTf-
Ralkyl
Yieldb (%)
Ar
R
Fe(MgX)2
1
2
3
4
5
6
7
8
p-EtO2CC6H4
p-EtO2CC6H4
p-EtO2CC6H4
p-EtO2CC6H4
p-EtO2CC6H4
p-i-PrO2CC6H4
p-i-PrO2CC6H4
p-i-PrO2CC6H4
p-i-PrO2CC6H4
p-i-PrO2CC6H4
p-Et2NOCC6H4
p-Et2NOCC6H4
p-Et2NOCC6H4
p-Et2NOCC6H4
p-NCC6H4
p-NCC6H4
p-NCC6H4
p-CF3C6H4
1-C10H7
1-C10H7
1-C10H7
2-C10H7
PhCH2CH2CH2
n-C4H9
n-C8H17
CH2]CH(CH2)4
i-C3H7
PhCH2CH2CH2
n-C4H9
n-C8H17
Cy
i-C3H7
PhCH2CH2CH2
n-C4H9
n-C8H17
Cy
PhCH2CH2CH2
n-C4H9
n-C8H17
n-C8H17
PhCH2CH2CH2
n-C4H9
n-C8H17
n-C4H9
96
96 (98c)
97
NMe3
81
80
92
99
Ar Fe(MgX)2
OTf
Ar Fe(MgX)2
R
99
9
93
Ar Fe(MgX)
86d
81
RMgX
MgX(OTf)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
81e
81
Scheme 1. Hypothetical mechanism for the cross-coupling of aryltrimethylammonium
triflate and alkyl Grignard reagents catalyzed by Fe(acac)3.
49e
70d
78d
80d
77f
99
3. Conclusion
In summary, we have developed a simple iron catalyst, which
efficiently promotes cross-coupling of aryltrimethylammonium
triflates and alkylmagnesium reagents. In this reaction, NMP is
important co-solvent for improving reaction efficiency. The re-
action proceeds fastly at room temperature and is compatible with
92
99
60
8-Me2N-1-C10H6
8-Me2N-1-C10H6
8-Me2N-1-C10H6
2-Py
PhCH2CH2CH2
n-C4H9
n-C8H17
PhCH2CH2CH2
n-C8H17
62d
55d
60d
65
b
-hydrogen-containing primary and secondary alkyl Grignard re-
agents. A series of functional groups such as COOEt, COOi-Pr,
CONEt2, CN, CF3, and CeC double bond groups as well as pyridyl
ring are tolerated under the reaction conditions.
2-Py
43g
a
Unless otherwise stated, reactions were performed with 0.5 mmol aryl-
trimethylammonium triflates and 1.0 mmol alkyl Grignard regents according to the
conditions indicated by the above equation; the purity of Fe(acac)3 is 99.9%.
4. Experimental
b
Isolated product yields.
The substrate scale was 5 mmol.
The reaction time was 10 min.
c
4.1. General remarks
d
e
A
mixture of the cross-coupling product and reduced product N,N-dieth-
All reactions were performed under nitrogen atmosphere using
standard Schlenk and vacuum line techniques. THF and DME were
distilled under nitrogen over sodium/benzophenone and degassed
prior to use. Toluene was distilled from sodium and degassed prior
ylbenzamide was obtained by column chromatography. The yields were calculated
on the basis of weight and integration ratio of the 1H NMR spectral signals of the
mixtures.
f
The reaction was carried out at 60 ꢁC.
The reaction time was 12 h.
g
ꢀ
to use. NMP was dried over 4 A molecular sieves, fractionally
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4
W.-J. Guo, Z.-X. Wang / Tetrahedron xxx (2013) 1e6
distilled under reduced pressure, and stored under nitrogen at-
mosphere. DMSO-d6 was purchased from Acros Organics and used
as received. CDCl3 was purchased from Cambridge Isotope Labo-
ratories and used as received. Fe(acac)3 (98%) was purchased from
TCI and used as received; Fe(acac)3 (99.9%) was purchased
from Aldrich and used as received. Alkyl Grignard reagents
were prepared accoꢀrding to the reported methods.
NMeþ3 OTfꢀ, and 2 ꢀ PyNMeþ3 OTfꢀ were synthesized according to
literature procedure.4 NMR spectra were recorded at room tem-
perature on a Bruker av300 or a Bruker Avance III 400 NMR spec-
trometer. The chemical shifts of the 1H and 13C NMR spectra were
referenced to TMS or internal solvent resonances. Atom emission
spectroscopy was determined using Perkin Elmer Optima 7300DV
atom emission spectrometry. HRMS data were recorded on an
Agilent6890/Micromass LCT-MS spectrometer (ESI). Elemental
analyses were carried out using an Elementar vario EL cube
instrument.
(0.63 mL), and NMP (0.37 mL). To the solution was added dropwise
a solution of RalkylMgBr (2.0 mL, 0.5 M in THF, 1.0 mmol) at 25 ꢁC
and stirred for 1 h. The reaction was quenched with CH3OH, con-
centrated by rotary evaporation, and purified by column chroma-
tography on silica gel.
4.3.1. Ethyl 4-cyclohexylbenzoate.15 1H NMR (300 MHz, CDCl3):
d
1.28e1.50 (m, 8H), 1.74e1.94 (m, 5H), 2.48e2.63 (m, 1H), 4.36 (q,
p ꢀ EtO2CC6H4NMeþ3 OTf
,
p ꢀ NCC6H4NMeþ3 OTfꢀ
,
p ꢀ F3CC6H4
J¼7.2 Hz, 2H), 7.26 (d, J¼8.1 Hz, 2H), 7.96 (d, J¼8.1 Hz, 2H). 13C NMR
(75 MHz, CDCl3):
d 14.51, 26.21, 26.90, 34.31, 44.85, 60.85, 126.95,
128.27, 129.79, 153.49, 166.87.
4.3.2. Ethyl 4-octylbenzoate.16 1H NMR (300 MHz, CDCl3):
d 0.87 (t,
J¼6.9 Hz, 3H), 1.17e1.34 (m, 10H), 1.38 (t, J¼7.2 Hz, 3H), 1.54e1.70
(m, 2H), 2.64 (t, J¼7.6 Hz, 2H), 4.35 (q, J¼7.2 Hz, 2H), 7.22 (d,
J¼8.1 Hz, 2H), 7.95 (d, J¼8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d
14.15, 14.42, 22.74, 29.32, 29.34, 29.52, 31.24, 31.95, 36.09, 60.77,
128.09, 128.44, 129.67, 148.43, 166.76.
4.3.3. Ethyl 4-butylbenzoate.17 1H NMR (300 MHz, CDCl3):
d 0.92 (t,
4.2. General procedure for preparation of aryl-
trimethylammonium triflates
J¼7.2 Hz, 3H), 1.26e1.40 (m, 5H), 1.55e1.65 (m, 2H), 2.65 (t,
J¼7.8 Hz, 2H), 4.35 (q, J¼7.2 Hz, 2H), 7.22 (d, J¼8.1 Hz, 2H), 7.95 (d,
J¼8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d 13.94, 14.41, 22.37, 33.35,
35.76, 60.77, 128.09, 128.44, 129.66, 148.39, 166.76.
A
dry Schlenk tube was charged N,N-dimethylarylamine
(10.0 mmol) and CH2Cl2 (10 mL). To the stirred solution was added
dropwise methyl trifluoromethanesulfonate (1.18 mL, 11.0 mmol) at
room temperature and then stirred for 4 h. Volatiles were removed
in vacuo. The residue was washed with Et2O (10 mLꢂ3) and dried
under vacuum to give a white solid.
4.3.4. Ethyl 4-(3-phenylpropyl)benzoate. 1H NMR (300 MHz,
CDCl3):
2H), 4.35 (q, J¼7.2 Hz, 2H), 7.15e7.30 (m, 7H), 7.96 (d, J¼8.1 Hz, 2H).
13C NMR (75 MHz, CDCl3):
14.44, 32.71, 35.42, 35.48, 60.83,
d
1.37 (t, J¼6.9 Hz, 3H), 1.90e2.01 (m, 2H), 2.60e2.70 (m,
d
125.95, 128.27, 128.45, 128.50, 129.74, 142.00, 147.78, 166.74. HRMS
4.2.1. p ꢀ Et2NOCC6H4NMeþOTf ꢀ. Yield 93%. 1H NMR (300 MHz,
(ESI) calcd for C18H21O2 [M]þ 269.1542, found 269.15334.
3
DMSO-d6):
d 1.06 (br, 3H), 1.16 (br, 3H), 3.39 (br, 4H), 3.62 (s, 9H),
4.3.5. Ethyl 4-isopropylbenzoate.18 1H NMR (300 MHz, CDCl3):
7.61 (d, J¼9 Hz, 2H), 8.02 (d, J¼9 Hz, 2H). 13C NMR (75 MHz, DMSO-
d
1.26 (d, J¼6.9 Hz, 6H), 1.38 (t, J¼7.1 Hz, 3H), 2.88e3.02 (m, 1H),
4.36 (q, J¼7.1 Hz, 2H), 7.28 (d, J¼8.1 Hz, 2H), 7.97 (d, J¼8.1 Hz, 2H).
13C NMR (75 MHz, CDCl3):
14.44, 23.79, 34.33, 60.79, 126.50,
128.27, 129.79, 154.26, 166.76.
d6):
d 12.77, 13.93, 42.75, 56.37, 120.79, 127.65, 138.94, 147.16,
168.25. Anal. Calcd for C15H23F3N2O4S$0.25CH2Cl2: C, 45.15; H, 5.84;
N, 6.91. Found: C, 45.08; H, 5.92; N, 6.86.
d
4.2.2. 1 ꢀ C10H7NMeþ3 OTfꢀ. Yield 90%. 1H NMR (300 MHz, DMSO-
4.3.6. Isopropyl 4-cyclohexylbenzoate. 1H NMR (300 MHz, CDCl3):
d6):
d
3.92 (s, 9H), 7.66e7.83 (m, 3H), 8.12 (d, J¼8.1 Hz, 1H), 8.23 (t,
J¼7.1 Hz, 2H), 8.62 (d, J¼8.4 Hz, 1H). 13C NMR (75 MHz, DMSO-d6):
d
1.24e1.48 (m, 11H), 1.74e1.85 (m, 5H), 2.47e2.60 (m, 1H),
5.18e5.30 (m, 1H), 7.25 (d, J¼8.1 Hz, 2H), 7.96 (d, J¼8.1 Hz, 2H). 13
C
d
57.09, 120.19, 123.21, 123.64, 124.76, 126.65, 128.12, 130.52, 132.35,
135.39, 141.22. Anal. Calcd for C14H16F3NO3S: C, 50.14; H, 4.81; N,
4.18. Found: C, 49.97; H, 4.89; N, 4.10.
NMR (75 MHz, CDCl3):
d 22.06, 26.16, 26.86, 44.80, 68.08, 126.85,
128.66, 129.73, 153.31, 166.25. HRMS (ESI) calcd for C16H23O2 [M]þ
247.1698, found 247.16887.
4.2.3. 2 ꢀ C10H7NMeþOTfꢀ. Yield 99%. 1H NMR (300 MHz, DMSO-
3.71 (s, 9H), 7.369e7.72 (m, 2H), 8.05e8.15 (m, 3H), 8.22 (d,
4.3.7. Isopropyl 4-octylbenzoate. 1H NMR (300 MHz, CDCl3):
d 0.88
d6):
d
J¼9.3 Hz, 1H), 8.51 (d, J¼1.5 Hz, 1H). 13C NMR (75 MHz, DMSO-d6):
(t, J¼6.6 Hz, 3H), 1.19e1.33 (m, 10H), 1.36 (d, J¼6.3 Hz, 6H),
1.54e1.66 (m, 2H), 2.65 (t, J¼7.7 Hz), 5.18e5.30 (m, 1H), 7.22 (d,
J¼8.1 Hz, 2H), 7.94 (d, J¼8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d
56.33, 117.95, 119.17, 127.55, 127.85, 128.21, 128.79, 130.29, 132.02,
132.36, 144.28, 149.73. Anal. Calcd for C14H16F3NO3S: C, 50.14; H,
4.81; N, 4.18. Found: C, 49.89; H, 4.81; N, 4.11.
d
14.20, 22.09, 22.78, 29.36, 29.56, 31.31, 31.99, 36.13, 68.16, 128.46,
128.54, 129.68, 148.38, 166.33. HRMS (ESI) calcd for C18H29O2 [M]þ
277.2168, found 277.21585.
4.2.4. p ꢀ i ꢀ PrO2CC6H4NMeþ3 OTf ꢀ. Yield 78%. 1H NMR (300 MHz,
DMSO-d6):
d
1.34 (d, J¼6.3 Hz, 6H), 3.64 (s, 9H), 5.11e5.24 (m, 1H),
8.13 (s, 4H). 13C NMR (75 MHz, DMSO-d6):
d 21.52, 56.39, 69.00,
4.3.8. Isopropyl 4-butylbenzoate. 1H NMR (300 MHz, CDCl3):
d 0.92
121.21, 130.63, 131.65, 150.39, 163.83. Anal. Calcd for C14H20F3NO5S:
C, 45.28; H, 5.43; N, 3.77. Found: C, 45.05; H, 5.46; N, 3.73.
(t, J¼7.2 Hz, 3H), 1.29e1.41 (m, 8H), 1.56e1.65 (m, 2H), 2.65 (t,
J¼7.7 Hz, 2H), 5.18e5.30 (m, 1H), 7.22 (d, J¼8.1 Hz, 2H), 7.94 (d,
J¼8.1 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d 14.01, 22.09, 22.41, 33.44,
35.81, 68.16, 128.46, 128.55, 129.68, 148.34, 166.33. HRMS (ESI)
4.2.5. 1 ꢀ ðNMe3Þ ꢀ 8 ꢀ Me2NC10HþOTfꢀ. Yield 70%. 1H NMR
6
(300 MHz, DMSO-d6): d 2.60 (s, 6H), 3.86 (s, 9H), 7.57e7.74 (m, 3H),
calcd for C14H21O2 [M]þ 221.1542, found 221.15321.
7.91 (d, J¼7.5 Hz, 1H), 8.08 (d, J¼7.8 Hz, 1H), 8.22 (d, J¼8.1 Hz, 1H).
13C NMR (75 MHz, DMSO-d6):
45.93, 57.83, 121.97, 122.32, 122.74,
124.74, 126.86, 127.31, 131.71, 137.41, 143.55, 149.96. Anal. Calcd for
16H21F3N2O3S: C, 50.78; H, 5.59; N, 7.40. Found: C, 50.94; H, 5.29;
d
4.3.9. Isopropyl 4-(3-phenylpropyl)benzoate. 1H NMR (300 MHz,
CDCl3):
d
1.35 (d, J¼6.3 Hz, 6H), 1.91e2.01 (m, 2H), 2.61e2.71 (m,
C
4H), 5.18e5.30 (m, 1H), 7.15e7.30 (m, 7H), 7.95 (d, J¼7.8 Hz, 2H). 13
C
N, 7.32.
NMR (75 MHz, CDCl3):
d 22.08, 32.72, 35.52, 68.21, 125.97, 128.49,
4.3. General procedure for iron-catalyzed cross-coupling of
aryltrimethylammonium triflate and alkyl Grignard reagent
128.53, 128.72, 129.74, 142.05, 147.68, 166.27. HRMS (ESI) calcd for
C
19H23O2 [M]þ 283.1698, found 283.16910.
A
Schlenk tube was charged with Fe(acac)3 (17.7 mg,
0.05 mmol), aryltrimethylammonium triflate (0.5 mmol), THF
Please cite this article in press as: Guo, W.-J.; Wang, Z.-X., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.09.039
4.3.10. Isopropyl 4-isopropylbenzoate. 1H NMR (300 MHz, CDCl3):
1.26 (d, J¼6.9 Hz, 6H), 1.35 (d, J¼6.3 Hz, 6H), 2.88e3.02 (m, 1H),
d

W.-J. Guo, Z.-X. Wang / Tetrahedron xxx (2013) 1e6
5
5.18e5.30 (m, 1H), 7.28 (d, J¼8.1 Hz, 2H), 7.96 (d, J¼8.1 Hz, 2H). 13
C
(m, 2H), 2.72 (s, 6H), 3.33 (t, J¼6.9 Hz, 2H), 7.20e7.37 (m, 4H), 7.54
(d, J¼7.8 Hz, 1H), 7.64 (d, J¼7.8 Hz, 1H). 13C NMR (75 MHz, CDCl3):
NMR (75 MHz, CDCl3):
d 22.09, 23.85, 34.37, 68.15, 126.48, 128.38,
128.72, 129.63, 129.80, 154.20, 166.28. HRMS (ESI) calcd for
d 14.20, 23.23, 35.40, 36.96, 45.93, 116.76, 125.05, 125.21, 125.46,
C
13H19O2 [M]þ 207.1385, found 207.13768.
127.39, 127.98, 128.77, 136.76, 140.42, 152.44. HRMS (ESI) calcd for
C
16H22N [M]þ 228.1752, found 228.17427.
4.3.11. 4-Cyclohexyl-N,N-diethylbenzamide. 1H NMR (300 MHz,
CDCl3): 1.17 (br, 7H), 1.40 (br, 3H), 1.73e1.85 (m, 4H), 2.50 (s, 1H),
3.29 (br, 2H), 3.51 (br, 2H), 7.20e7.37 (m, 4H). 13C NMR (75 MHz,
CDCl3): 14.01, 26.17, 26.88, 34.38, 44.42, 44.51, 126.34, 126.43,
4.3.20. N,N-Dimethyl-8-octylnaphthalen-1-amine. 1H NMR (300
MHz, CDCl3):
d
d
0.86 (t, J¼7.5 Hz, 3H), 1.17e1.38 (m, 10H), 1.43e1.56
(m, 2H), 2.72 (s, 6H), 3.32 (t, J¼7.8 Hz, 2H), 7.20e7.37 (m, 4H), 7.54
d
(d, J¼7.8 Hz, 1H), 7.65 (d, J¼7.8 Hz, 1H). 13C NMR (75 MHz, CDCl3):
126.85, 128.43, 129.13, 134.77, 137.39, 149.23, 171.58. HRMS (ESI)
calcd for C17H26ON [M]þ 260.2014, found 260.20044.
d
14.27, 22.85, 29.53, 29.77, 30.28, 32.09, 33.29, 37.35, 45.97, 116.77,
125.06, 125.21, 125.47, 127.40, 127.98, 128.75, 136.77, 140.48, 152.45.
HRMS (ESI): calcd for C20H30N [M]þ 284.2378, found 284.23660.
4.3.12. N,N-Diethyl-4-octylbenzamide.16 1H NMR (300 MHz, CDCl3):
d
0.88 (t, J¼6.5 Hz, 3H), 1.17 (br, 6H), 1.22e1.39 (m, 10H), 1.53e1.67
4.3.21. N,N-Dimethyl-8-(3-phenylpropyl)naphthalen-1-amine. 1H
(m, 2H), 2.61 (t, J¼7.7 Hz, 2H), 3.30 (br, 2H), 3.50 (br, 2H), 7.18 (d,
J¼7.8 Hz, 2H), 7.28 (d, J¼7.8 Hz, 2H). 13C NMR (75 MHz, CDCl3):
NMR (300 MHz, CDCl3):
d 1.76e1.86 (m, 2H), 2.59e2.68 (m, 8H),
3.37 (t, J¼7.8 Hz, 2H), 7.11e7.36 (m, 9H), 7.52 (d, J¼8.1 Hz, 1H), 7.65
d
13.04, 14.12, 22.69, 29.27, 29.48, 31.32, 31.90, 35.83, 39.23, 43.32,
(dd, J¼1.5, 7.8 Hz, 1H). 13C NMR (75 MHz, CDCl3):
d 34.80, 36.55,
126.39, 128.37, 134.60, 144.13, 171.55.
37.00, 45.91, 116.76, 125.04, 125.26, 125.45, 125.63, 127.59, 127.91,
128.29, 128.68, 128.87, 136.76, 139.76, 143.04, 152.31. HRMS (ESI)
calcd for C21H24N [M]þ 290.1909, found 290.18994.
4.3.13. 4-Butyl-N,N-diethylbenzamide. A mixture of 4-butyl-N,N-
diethylbenzamide and N,N-diethylbenzamide was isolated. The
mixture contains about 13.8% N,N-diethylbenzamide.
4.3.22. 1-(Trifluoromethyl)-4-octylbenzene.23 1H NMR (400 MHz,
1H NMR (300 MHz, CDCl3):
d
0.92 (t, J¼7.2 Hz, 3H),1.17 (br, 6.9H),
CDCl3):
2H), 2.65 (t, J¼7.8 Hz, 2H), 7.27 (d, J¼8 Hz, 2H), 7.52 (d, J¼8 Hz, 2H).
13C NMR (100 MHz, CDCl3):
14.23, 22.81, 29.37, 29.38, 29.56, 31.35,
d
0.88 (t, J¼6.8 Hz, 3H), 1.17e1.41 (m, 10H), 1.58e1.65 (m,
1.32e1.39 (m, 2H), 1.55e1.64 (m, 2H), 2.62 (t, J¼7.2 Hz, 2H), 3.29 (br,
2.3H), 3.51 (br, 2.3H), 7.18 (d, J¼6.9 Hz, 2H), 7.28 (d, J¼6.9 Hz, 2H),
7.37 (s, 0.8H). 13C NMR (75 MHz, CDCl3):
d 13.02, 13.90, 14.10, 22.28,
d
32.01, 35.97, 124.59 (q, J¼270 Hz), 125.30 (q, J¼3.7 Hz), 128.15 (q,
33.42, 35.46, 39.33, 43.19, 126.34, 128.33, 134.56, 144.05, 171.51.
HRMS (ESI) calcd for C15H24ON [M]þ 234.1858, found 234.18486.
J¼32 Hz), 128.82, 147.20.
4.3.23. 2-Octylpyridine.24 1H NMR (300 MHz, CDCl3):
d 0.87 (t,
4.3.14. N,N-Diethyl-4-(3-phenylpropyl)benzamide. 1H NMR (300
J¼6.9 Hz, 3H), 1.16e1.43 (m, 10H), 1.67e1.77 (m, 2H), 2.78 (t,
MHz, CDCl3, TMS):
4H), 3.27 (br, 2H), 3.51 (br, 2H), 7.12e7.37 (m, 9H). 13C NMR
(75 MHz, CDCl3): 13.04, 14.17, 32.76, 35.21, 35.35, 39.34, 43.35,
d 1.19 (br, 6H), 1.84e2.01 (m, 2H), 2.52e2.71 (m,
J¼7.8 Hz, 2H), 7.06e7.14 (m, 2H), 7.57 (t, J¼7.5 Hz, 1H), 8.52 (d,
J¼4.5 Hz,1H). 13C NMR (75 MHz, CDCl3):
d 14.20, 22.77, 29.35, 29.54,
29.58, 30.04, 31.98, 38.59, 120.93, 122.77, 136.29, 149.30, 162.69.
d
125.80,126.27, 126.43, 128.33,128.40,128.42,129.08,134.76,142.07,
143.45, 171.45. HRMS (ESI) calcd for C20H26ON [M]þ 296.2014,
found 296.20056.
4.3.24. 2-(3-Phenylpropyl)pyridine.25 1H NMR (400 MHz, CDCl3):
d
2.04e2.11 (m, 2H), 2.69 (t, J¼7.8 Hz, 2H), 2.83 (t, J¼7.8 Hz, 2H),
7.08e7.29 (m, 7H), 7.57 (dt, J¼2, 7.6 Hz, 1H), 8.52e8.54 (m, 1H). 13
C
4.3.15. 1-Octylnaphthalene.19 1H NMR (400 MHz, CDCl3):
d 0.88 (t,
NMR (75 MHz, CDCl3):
d 31.50, 35.63, 37.97, 121.06, 122.85, 125.85,
J¼6.8 Hz, 3H), 1.21e1.38 (m, 8H), 1.40e1.47 (m, 2H), 1.71e1.79 (m,
2H), 3.06 (t, J¼7.8 Hz, 2H), 7.31 (d, J¼6.8 Hz, 1H), 7.37e7.41 (m, 1H),
7.44e7.52 (m, 2H), 7.69 (d, J¼8 Hz, 1H), 7.83e7.85 (m, 1H), 8.02 (d,
128.39, 128.55, 136.34, 142.22, 149.34, 162.05.
4.3.25. 4-Octylbenzonitrile.26 1H NMR (400 MHz, CDCl3):
d
0.88 (t,
J¼8 Hz, 1H). 13C NMR (100 MHz, CDCl3):
d 14.25, 22.82, 29.46, 29.67,
30.02, 31.03, 32.06, 33.28, 124.06, 125.48, 125.68, 125.74, 125.97,
126.51, 128.88, 132.09, 134.05, 139.21.
J¼6.8 Hz, 3H), 1.18e1.36 (br, 10H), 1.57e1.67 (m, 2H), 2.65 (t,
J¼7.6 Hz, 2H), 7.27 (d, J¼8 Hz, 2H), 7.55 (d, J¼8 Hz, 2H). 13C NMR
(75 MHz, CDCl3):
d 14.18, 22.75, 29.29, 29.48, 29.55, 31.07, 31.95,
36.23, 109.63, 119.28, 128.32, 128.69, 129.30, 132.21, 148.73.
4.3.16. 1-Butylnaphthalene.20 1H NMR (300 MHz, CDCl3):
d 0.95 (t,
4.3.26. 4-Butylbenzonitrile.20 1H NMR (400 MHz, CDCl3):
d
0.93 (t,
J¼7.4 Hz, 3H), 1.37e1.49 (m, 2H), 1.67e1.77 (m, 2H), 3.04 (t,
J¼7.8 Hz, 2H), 7.27e7.49 (m, 4H), 7.66 (d, J¼8.1 Hz, 1H), 7.81 (d,
J¼8.1 Hz, 1H), 8.02 (d, J¼7.8 Hz, 1H). 13C NMR (75 MHz, CDCl3):
J¼7.4 Hz, 3H), 1.30e1.40 (m, 2H), 1.55e1.64 (m, 2H), 2.66 (t,
J¼7.8 Hz, 2H), 7.27 (d, J¼8 Hz, 2H), 7.56 (d, J¼8 Hz, 2H). 13C NMR
d
14.15, 23.02, 32.94, 33.15, 124.04, 125.47, 125.66, 125.73, 125.98,
(75 MHz, CDCl3): d 13.97, 22.37, 33.20, 35.94, 109.66, 119.31, 128.34,
126.53, 128.88, 132.10, 134.07, 139.11.
128.71, 129.33, 132.23, 148.70.
4.3.27. 4-(3-Phenylpropyl)benzonitrile.15 1H NMR (400 MHz,
4.3.17. 1-(3-Phenylpropyl)naphthalene.21 1H NMR (300 MHz,
CDCl3):
d
1.93e2.01 (m, 2H), 2.65 (t, J¼7.8 Hz, 2H), 2.70 (t, J¼7.8 Hz,
CDCl3):
d
1.91e2.10 (m, 2H), 2.67 (t, J¼7.5 Hz, 2H), 3.05 (t, J¼7.8 Hz,
2H), 7.17 (d, J¼7.2 Hz, 2H), 7.21 (d, J¼7.2 Hz, 1H), 7.25e7.29 (m, 4H),
2H), 7.05e7.26 (m, 6H), 7.27e7.46 (m, 3H), 7.63 (d, J¼8.1 Hz, 1H),
7.57 (d, J¼8 Hz, 2H). 13C NMR (75 MHz, CDCl3):
d 32.47, 35.36, 35.57,
7.70e7.83 (m, 1H), 7.85e7.95 (m, 1H). 13C NMR (75 MHz, CDCl3):
109.79, 119.18, 126.08, 128.48, 128.52, 129.31, 132.25, 141.67, 148.05.
d
32.33, 32.60, 35.96, 123.89, 125.49, 125.61, 125.79, 125.91, 126.00,
126.68, 128.43, 128.58, 128.87, 132.02, 134.04, 138.48, 142.25.
4.3.28. Ethyl 4-(hex-5-en-1-yl)benzoate.27 1H NMR (400 MHz,
CDCl3):
d
1.27e1.37 (m, 5H), 1.48e1.60 (m, 2H), 1.98 (q, J¼6.8 Hz,
4.3.18. 2-Butylnaphthalene.22 1H NMR (300 MHz, CDCl3):
d 0.94 (t,
2H), 2.56 (t, J¼8 Hz, 2H), 4.27 (q, J¼7.2 Hz, 2H), 4.84e4.93 (m, 2H),
J¼7.4 Hz, 3H), 1.32e1.44 (m, 2H), 1.63e1.73 (m, 2H), 2.75 (t,
5.64e5.75 (m, 1H), 7.14 (d, J¼8 Hz, 2H), 7.87 (d, J¼8 Hz, 2H). 13C
J¼7.8 Hz, 2H), 7.31 (d, J¼8.4 Hz, 1H), 7.36e7.47 (m, 2H), 7.59 (s, 1H),
NMR (100 MHz, CDCl3):
d 14.43, 28.50, 30.63, 33.62, 35.88, 60.79,
7.72e7.83 (m, 3H). 13C NMR (75 MHz, CDCl3):
d 14.13, 22.55, 33.67,
114.63, 128.13, 128.45, 128.49, 129.68, 138.69, 148.15, 166.73.
35.95, 125.11, 125.93, 126.43, 127.54, 127.60, 127.74, 127.86, 132.10,
133.82, 140.56.
Acknowledgements
4.3.19. 8-Butyl-N,N-dimethylnaphthalen-1-amine. 1H NMR (300
This research was supported by the National Basic Research
Program of China (Grant No. 2009CB825300) and the National
MHz, CDCl3):
d
0.89 (t, J¼6.9 Hz, 3H), 1.26e1.38 (m, 2H), 1.42e1.52
Please cite this article in press as: Guo, W.-J.; Wang, Z.-X., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.09.039

6
W.-J. Guo, Z.-X. Wang / Tetrahedron xxx (2013) 1e6
Guenoun, F.; Martinez, J.; Lamaty, F. Synth. Commun. 2009, 39, 1583; (h) Gøgsig,
T. M.; Lindhardt, A. T.; Skrydstrup, T. Org. Lett. 2009, 11, 4886; (i) Ottesen, L. K.;
Ek, F.; Olsson, R. Org. Lett. 2006, 8, 1771; (j) Martin, R.; Furstner, A. Angew. Chem.,
Natural Science Foundation of China (Grant No. 21172208). Authors
thank Mr. Jian-long Tao for monitoring of NMe3.
€
Int. Ed. 2004, 43, 3955.
Supplementary data
9. (a) Wang, X.-J.; Zhang, L.; Smith-Keenan, L. L.; Houpis, I. N.; Farina, V. Org.
Process Res. Dev. 2007, 11, 60; (b) Terao, J.; Todo, H.; Begum, S. A.; Kuniyasu, H.;
Kambe, N. Angew. Chem., Int. Ed. 2007, 46, 2086.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2013.09.039.
10. (a) Hess, W.; Treutwein, J.; Hilt, G. Synthesis 2008, 3537; (b) Gosmini, C.;
ꢂ
Begouin, J.-M.; Moncomble, A. Chem. Commun. 2008, 3221; (c) Yorimitsu, H.;
Oshima, K. Pure Appl. Chem. 2006, 78, 441; (d) Zhou, W.; Napoline, J. W.;
Thomas, C. M. Eur. J. Inorg. Chem. 2011, 2029.
References and notes
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Please cite this article in press as: Guo, W.-J.; Wang, Z.-X., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.09.039