2-Ferrocenyl-2-thiazoline as a building block of novel phosphine-free ligands

Article abstract of DOI:10.1039/c3dt50451d

New 1,2-disubstituted ferrocenes [5(b-j), in which R = -SMe, -SPh, -SiPr, -SiMe₃, -SePh, -SnBu₃, -B(OH)₂, -Me, -I] with a thiazoline ring in the ferrocene backbone using as key intermediate a ferrocenyl Fischer carbene complex were synthesized. The capability of the 2-thiazoline moiety as an ortho-directed metalation group was demonstrated by subsequent quenching of lithium intermediate with several electrophiles, proving to be an excellent method for synthesizing bidentate ligands. The catalytic scope of the [N,S] ligand 5b as the corresponding palladium complex 5b-PdCl ₂ in a microwave-promoted Heck reaction was also tested. Results obtained showed better catalytic activity of 5b-PdCl₂ compared to other catalytic systems based on a [N,S] ferrocenyl ligand. The Royal Society of Chemistry.

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2-Ferrocenyl-2-thiazoline as a building block of novel
phosphine-free ligands†
Cite this: DOI: 10.1039/c3dt50451d
Ricardo Corona-Sánchez,a Rubén A. Toscano,a M. Carmen Ortega-Alfaro,b
César Sandoval-Cháveza and José G. López-Cortés*a
New 1,2-disubstituted ferrocenes [5(b–j), in which R = –SMe, –SPh, –SiPr, –SiMe3, –SePh, –SnBu3, –B(OH)2,
–Me, –I] with a thiazoline ring in the ferrocene backbone using as key intermediate a ferrocenyl Fischer
carbene complex were synthesized. The capability of the 2-thiazoline moiety as an ortho-directed metala-
tion group was demonstrated by subsequent quenching of lithium intermediate with several electro-
philes, proving to be an excellent method for synthesizing bidentate ligands. The catalytic scope of the
[N,S] ligand 5b as the corresponding palladium complex 5b-PdCl2 in a microwave-promoted Heck reac-
tion was also tested. Results obtained showed better catalytic activity of 5b-PdCl2 compared to other
catalytic systems based on a [N,S] ferrocenyl ligand.
Received 19th February 2013,
Accepted 11th June 2013
DOI: 10.1039/c3dt50451d
www.rsc.org/dalton
with a thiazoline ring markedly changes the reactivity of the
resulting ligand when applied in catalysis.12
Introduction
Since the discovery of ferrocene in 1951,1 this organometallic
In general, the synthesis of thiazolines involves the prepa-
compound continues to be the focus of tremendous attention ration of a thioamide precursor. Although the synthesis of
and, in fact, it has various applications.2 As a result, currently alkyl thioamides is well-known,13 few syntheses of ferrocenyl
a large number of ferrocenyl derivatives are widely used as analogues have been reported, which may be due to the
ligands in homogeneous transition metal catalysts.3
difficulty in obtaining a thiocarbonyl moiety directly bonded
The most widespread method for preparing 1,2-disubsti- to the ferrocene. Reported methods for the synthesis of ferro-
tuted ferrocenes is based on a directed ortho-metalation (DoM) cenyl thioamides lack generality, involve several steps, and
reaction and its subsequent quenching with an appropriate have very low overall yields.14 Nevertheless, we have previously
electrophile.4 On the basis of Ugi’s pioneering work,5 several reported that a sulfurative demetalation reaction of Fischer
1,2-disubstituted ferrocenes can be accessed by a variety of carbene complexes using elemental sulphur/NaBH4 could be a
DoM reactions using a suitable directed metalation group convenient method for producing ferrocenyl thioamides.15
(DMG).6 As a consequence of the great utility of this strategy,
Despite efforts in this area, the synthesis of ferrocenyl thi-
Sammakia,7 Richards8 and Uemura9 simultaneously intro- azolines is seldom reported. In this regard, only some 5-ferro-
duced oxazolines derived from amino alcohols as one of the cenyl-2-thiazolines (Scheme 1) have been designed,16 but to
most successful ligand motifs in homogeneous catalysis.10 date no information on their use in catalysis as DMG or
Similarly, the imidazoline ring has also been used for the ligands has been reported.
same purpose.11 Unlike the widely applied oxazoline ligands,
To the best of our knowledge, there is no precedent in the
thiazolines have rarely been used in catalysis although some literature regarding the use of the 2-thiazoline moiety as a
studies have demonstrated that replacing an oxazoline motif DMG in the preparation of 1,2-disubstituted ferrocenes. Con-
sidering that both oxazoline and imidazoline rings have been
known to be good directing groups in ortho lithiation, we
expect that the thiazoline ring will also promote selective
lithiation of ferrocene. In this context, we report the synthesis
aInstituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior,
Ciudad Universitaria, Coyoacán, C.P. 04360 México, D.F. México.
of 2-ferrocenyl-2-thiazoline as an efficient and easily accessible
DMG for the preparation of a new family of 1,2-disubstituted
ferrocenes using a synthetic strategy based on sulfurative
demetalation of a Fischer aminocarbene complex. The cata-
lytic applications of one of these ligands were tested in the
palladium-catalyzed Mizoroki–Heck reaction.
E-mail: jglcvdw@unam.mx; Fax: +52 55 56162203; Tel: +52 55 56224513
bInstituto de Ciencias Nucleares, Universidad Nacional Autónoma de México,
Circuito Exterior, Ciudad Universitaria, Coyoacán, C.P. 04360 México, D.F. México.
E-mail: Carmen.ortega@nucleares.unam.mx
†Electronic supplementary information (ESI) available. CCDC 923983–923987.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50451d
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Scheme 3 Reagents and conditions. (i) Ethanolamine, Et2O, rt; (ii) S8/NaBH4,
Scheme 1 Reagents and conditions. (i) Et2AlCN, THF, −78 °C; (ii) LiAlH4, THF,
reflux; (iii) LDA, Me3SiCl, THF, −78 °C then NBS; (iv) (P)-+NR3N3−, CH2Cl2, rt; (v)
BH3·SMe, CBS oxazaborolidine, THF, 0 °C; (vi) H2, Pd/C, EtOH, rt; (vii) RCOCl,
CH2Cl2, Et3N, rt; (viii) Lawesson reagent (2 equiv.), THF, reflux; (ix) n-BuLi, THF,
−78 °C then RCOCl; (x) n-Bu3P, THF, rt. Fc = ferrocenyl, CBS = Corey–Bakshi–
Shibata catalyst, (P)-+NR3N3− = polymeric quaternary ammonium azide.
EtOH, rt; (iii) MsCl–Et3N, CH2Cl2,
0 °C; (iv) Ethanolamine, EtOH, rt, then
S8/NaBH4, EtOH, rt.
complex in accordance with a protocol developed earlier by
our group.17 Thereafter, ethanolamine was added to easily
obtain chromium ferrocenylaminocarbene complex 2, which
was isolated in high yields (95%) (Scheme 3).
Complex 2 was subsequently transformed under mild con-
ditions into ferrocenylthioamide 3 by a sulfurative demetala-
tion reaction with S8/NaBH4.15 Compound 3 was characterized
by conventional spectroscopic techniques. The mass spectrum
(EI) of 3 agrees with the molecular formula and confirms the
loss of the metallic fragment [Cr(CO5)]. The 13C NMR spec-
trum exhibits a signal around δ = 198.4 ppm assigned to CvS.
The structural arrangement for 3 was fully established by a
single-crystal X-ray diffraction analysis (Fig. 1).
The thiocarbonyl moiety is directly bonded to ferrocene;
the sum of bond angles around C11 (Σ = 360°) indicates that
this group has a trigonal geometry. The bond distance CvS
[S(1)–C(11) 1.677(2)] is quite similar to that of other reported
thioamides, whereas the bond distance N(1)–C(11) 1.319(3) is
slightly shorter.18 The structure presents disorder of the
unsubstituted Cp ring generating two orientations in 77 : 23
ratio. Only the major contributor is shown in Fig. 1.
Results and discussion
Our retrosynthetic analysis of substituted 2-ferrocenyl-2-thi-
azolines 5 is described in Scheme 2. As detailed above, we
expect that several functional groups might be introduced by a
DoM/electrophile quench process using the 2-thiazoline ring
as a DMG. We envisaged that the 2-thiazoline moiety could be
constructed by an intramolecular cyclization of the corres-
ponding N-(β-hydroxy) ferrocenylthioamide. We believe that
the target thioamide 3 could be synthesized through a sulfura-
tive demetalation reaction of a Fischer aminocarbene complex,
which would in turn be derived from 1 via an aminolysis
reaction with the appropriate β-amino alcohol. Finally, the
Fischer ethoxycarbene complex required 1 to be synthesized
in a straightforward manner using commercially available
ferrocene.
At first, initial compound 1 was prepared from ferrocene,
which was converted to a ferrocenyl-ethoxycarbene chromium
Fig. 1 ORTEP representation of ferrocenyl thioamide 3. Ellipsoids are shown at
30% probability level. Selected bond lengths [Å] and angles [°]: S(1)–C(11)
1.677 (2), N(1)–C(11) 1.319 (3), C(1)–C(11) 1.472 (3); N(1)–C(11)–S(1)
122.93(16), C(1)–C(11)–S(1) 120.87(15), N(1)–C(12)–C(13) 111.35(17).
Scheme 2 Retrosynthetic approach for the synthesis of substituted 2-ferro-
cenyl-2-thiazolines.
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Furthermore, the resulting ferrocenylthioamide 3 was con-
The coordination polyhedron around the palladium atom
verted to 2-ferrocenyl-2-thiazoline 4 using an intramolecular can be best described as being a distorted square-planar, in
cyclization with p-TsOH or MsCl–Et3N in CH2Cl2 at room which the phosphine molecule and the nitrogen atom adopt a
temperature. Acidic conditions for the cyclization step resulted cis arrangement with P(1)–Pd(1)–N(13) angle of 93.72°. The thi-
in a moderate yield (76%), however, carrying out cyclization azoline group is directly bonded to ferrocene at 2-position and
with 1.5 equiv. MsCl and 4 equiv. Et3N raises the yield to 96% adopts an envelope conformation with the carbon atom C(15)
in only 10 minutes, giving almost pure 4. In an attempt to as a flap with a puckering amplitude of 0.300(3) Å out of the
improve the yield of 4, we used an aminolysis–sulfurative Cremer–Pople plane. Additionally, the structure presents dis-
demetalation tandem reaction as the key sequence and as a order of the un-substituted Cp ring generating two orientations
consequence the yield of the intermediate thioamide 3 was in 88 : 12 ratio. Only the major contributor is shown in Fig. 2.
increased. This slight modification enabled 4 to be prepared
from the ferrocenyl ethoxycarbene chromium complex 1 with a DMG in the selective DoM of ferrocene (Table 1).
94% overall yield. The mass spectrum of 4 agrees with the mole- To ascertain the most feasible temperature for the DoM
Moreover, 2-ferrocenyl-2-thiazoline 4 was investigated as a
cular formula expected and confirms the cyclization of 3. The reaction, we developed a deuterium incorporation analysis.
1H NMR spectrum shows two coupled triplet signals at 3.34 and Thus a solution of 4 in dry Et2O was cooled to −78 °C and
4.25 ppm assigned to the methylene groups of thiazoline, two t-BuLi was then added dropwise, the reaction mixture was
broad singlet signals at 4.36 and 4.71 ppm that correspond to warmed to several temperatures followed by consecutive
the hydrogens of monosubstituted Cp ring and finally a simple quenching using an excess of D2O. Finally, deuterium incor-
signal shifted at 4.2 ppm with integration of 5 hydrogens was poration was determined by NMR, comparing the integrations
assigned to the Cp ring. The 13C NMR spectrum exhibits two of the substituted Cp ring protons. When the ferrocenyl thi-
signals at 33.6 and 64.9 ppm for the methylene groups of thi- azoline 4 was subjected to lithiation with this standard reaction
azoline, and a signal at 168.8 ppm assigned to CvS. Charac- protocol at −78 °C, 5a was obtained in moderate yield and dis-
teristic signals for a monosubstituted ferrocene were also seen.
played incomplete deuterium incorporation. However, gradual
In order to confirm the structural arrangement of 4, we con- warming of the reaction at room temperature produces better
ducted a coordination reaction between 4 and Pd(CH3CN)2Cl2/ deuterium incorporation, leading to 5a in 81% yield (entry 5).
PPh3 obtaining the corresponding complex cis-4-Pd(PPh3Cl2),
In view of those results, we carried out the DoM reaction
where the thiazoline moiety behaves as an N-monodentated followed by the quenching of the lithium intermediate with
ligand. Thus, suitable crystals were obtained and the structural (MeS)2 at room temperature; however, 5b was obtained in poor
arrangement for 4 was established by single-crystal X-ray diffr- yields, which could either be because the lithium intermediate
action analysis (Fig. 2).
is not so stable at room temperature, or because the reaction
with (MeS)2 is slower.
We proceeded to conduct this reaction at 0 °C, and
obtained very good yields of 5b (Table 1; entry 6). As part of
our attempts to improve the yield, other reaction conditions
Table 1 DoM reaction under various screening conditionsa
Entry
Solvent
RLi
E+
Temperature
Yieldb (%)
1
2
3
4
5
6
7
8
9
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
THF
Et2O
Et2O
THF
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
t-BuLi
n-BuLi
s-BuLi
t-BuLi
D2O
D2O
D2O
D2O
−78 °C
65
68
71
74
81
87
70
74
−78 °C to −50 °C
−78 to −30 °C
−78 to 0 °C
−78 °C to rt
−78 °C to rt
−78 °C to rt
−78 °C to rt
−78 °C to rtc
−78 °C
Fig. 2 ORTEP representation of ferrocenyl thiazoline 4-Pd(PPh3Cl2). Ellipsoids
are shown at 30% probability level. Selected bond lengths [Å] and angles [°]:
S(11)–C(12) 1.751 (2), S(11)–C(15) 1.803(4), C(12)–N(13) 1.276(3), N(13)–C(14)
1.474(3), C(14)–C(15) 1.519, C(1)–C(12) 1.449(4), Pd(1)–N(13) 2.020(2), Pd(1)–
P(1) 2.244(1), Pd(1)–Cl(1) 2.287(1), 2.360(1); N(13)–Pd(1)–P(1) 93.72(6), N(13)–
Pd(1)–Cl(1) 177.07(6), P(1)–Pd(1)–Cl(1) 87.00(3), N(13)–Pd(1)–Cl(2) 86.88(6),
P(1)–Pd(1)–Cl(2) 178.60(3), Cl(1)–Pd(1)–Cl(2) 94.46(3), C(12)–S(11)–C(15)
89.98(13), N(12)–C(12)–C(1) 125.5(2), N(13)–C(12)–S(11) 115.82(19), C(1)–
C(12)–S(11) 118.67(19), C(12)–N(13)–C(14) 113.5(2), C(12–N(13)–Pd(1)
128.87(17), N(13)–C(14)–C(15) 107.6(2), C(14)–C(15)–S(11) 104.9(2).
D2O
(SMe)2
(SMe)2
(SMe)2
(SMe)2
(SMe)2
51(8)
62(15)
10
a Reaction conditions: 1 equiv. of 4, 1.2 equiv. of RLi, 1.5 equiv. of
electrophile. b Yield of the isolated product after SiO2 column
chromatography. Values in brackets denote recovered starting
materials. c Addition of (SMe)2 at 0 °C.
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were investigated (entries 7–10). Changing the solvent from
Et2O to THF causes a decrease of the desired compound (entry
7). When we changed the t-BuLi to n-BuLi in Et2O, 5b was
obtained in 74% yield (entry 8), while the use of s-BuLi leads
to the poorest outcomes (entry 9). Optimized conditions
involved adding 1.2 equiv. of t-BuLi to an ethereal solution of
4 at −78 °C and warming the reaction mixture to room temp-
erature, followed by the addition of the electrophile at 0 °C.
Compound 5b was characterized by means of conventional
spectroscopic techniques, which confirms the incorporation of
the electrophile in an adjacent position to the thiazoline
group, providing a new 1,2-disubstituted ferrocene. Mass spec-
trum of 5b thus agrees with the expected molecular formula.
The 1H NMR spectrum shows a broad singlet at 2.35 ppm
assigned to CH3S, two multiple signals at 3.33 and 4.35 ppm
assigned to the methylene groups of thiazoline, three multiple
signals at 4.46, 4.77 and 4.78 ppm that correspond to the di-
substituted Cp ring and finally a simple signal shifted at
4.22 ppm with integration of 5 hydrogens was assigned to the
Cp ring. The 13C NMR spectrum exhibits a signal shifted at
19.7 ppm for the SMe, two signals at 33.5 and 64.5 ppm for
Fig. 3 ORTEP representation of ferrocenyl thiazoline 5c. Ellipsoids are shown at
30% probability level. Selected bond lengths [Å] and angles [°]: C(1)–C(11)
1.467(4), C(2)–S(16) 1.752(3), C(11)–N(15) 1.282(6), C(11)–S(12) 1.774(5), S(12)–
C(13) 1.793(13), C(14)–N(15) 1.449(12), S(16)–C(17) 1.777(3); C(2)–C(1)–C(11)
130.1(3), C(1)–C(2)–S(16) 128.4, N(15)–C(11)–S(12) 116.1(7), C(1)–C(11)–S(12)
121.0(4), C(11)–S(12)–C(13) 90.4(5), C(14)–C(13)–S(12) 104.8(7), N(15)–C(14)–
C(13) 107.9(8), C(11)–N(15)–C(14) 114.4(8), C(2)–S(16)–C(17) 102.7(1).
methylene groups of the thiazoline, and a signal at 167.7 ppm observed in bond and angle distances of the thiazoline moiety
assigned to Cv(N)S. The characteristic signals for a 1,2-disub- compared to those reported in the literature.18a The structure
stituted ferrocene are also observed.
of 5c presents disorder in the thiazoline ring generating two
With these conditions at hand, we examined the scope of different envelope conformations (both with C13 as a flap) in
the reaction by using a range of different electrophilic sub- 83 : 17 ratio. Only the major contribution is shown in Fig. 3.
strates. Selected results for DoM-electrophilic quenching of
thiazoline 4 are shown in Table 2.
By and large, the reaction is successful for a wide range of
electrophiles and gives good yields of the resulting substituted
In the case of 5c, suitable crystals make it possible to fully ferrocenyl thiazolines, after flash chromatography on silica gel.
establish the structural arrangement of this compound by This procedure provides access to a diversity of ferrocenyl thi-
single-crystal X-ray diffraction analysis (Fig. 3).
azolines with several valuable functionalities for further use as
The structure indicates that the phenylsulfide moiety is bidentate ligands, or as versatile substrates for cross-coupling
directly bonded to the monosubstituted Cp ring at 2-position, reactions. In that context, we decided to carry out a prelimi-
confirming the behavior of the thiazoline ring as a DMG in the nary study in order to determine the capacity of coordination
selective DoM of ferrocene. No significant differences were of 5b to Pd(II).
Complex 5b-PdCl2 was prepared by treating 5b with
Pd(CH3CN)2Cl2, giving an air-stable orange complex with 93%
yield. This complex was characterized by conventional spectro-
scopic techniques. The structural arrangement for 5b-PdCl2
was unequivocally established by X-ray diffraction analysis
(Fig. 4).
Table 2 Selective DoM of 4a
The most salient feature of the structure is the coordination
mode of 5b as an effective [N,S] bidentate ligand,19 forming a
six-membered chelated ring with a half-chair conformation
(Cremer–Pople puckering parameters: q2 = 0.643(2), q3 = 0.360(2),
Q = 0.737(2) Å, θ = 60.7(2), ϕ = 22.4(2)°), where the methyl
group adopts an axial orientation to prevent steric hindrance
with the ferrocene moiety. The ligand bite angle (91.02°) and
the Cl(1)–Pd(1)–Cl(2) angle (91.27°) are the origins of slight
distortion from square-planar symmetry in the palladium
atom. Data on the crystals and the refining of the compounds
3, 4-Pd(PPh3Cl2), 5c, and 5b-PdCl2 are summarized in Table 6.
To study the catalytic properties of complex 5b-PdCl2, we
performed a series of experiments using as a model reaction:
the cross-coupling between 4-iodotoluene and methyl acrylate
under microwave irradiation and conventional heating.
Entry
Product
Electrophile
−E
% Yieldb
1
2
3
4
5
6
8
9
5b
5c
5d
5e
5f
5g
5h
5i
(SMe)2
(SPh)2
(S-iPr)2
Me3SiCl
(SePh)2
n-Bu3SnCl
B(O-iPr)3
CH3I
–SMe
–SPh
87
78
63
98
79
51
72
86
75
–SiPr
–SiMe3
–SePh
–SnBu3
–B(OH)2
–Me
10
5j
I2
–I
a Reaction condition: 1 equiv. of 4, 1.2 equiv. of t-BuLi, Et2O, −78 °C to
rt, 1.5 equiv. of electrophile. b Yield of the isolated product after SiO2
column chromatography.
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To determine the optimal conditions, a series of experi- performed the Heck reaction under thermal conditions using
ments were performed changing parameters such as temp- a DMF reflux. In this case a slightly lower yield than that
erature, solvent, base and catalyst loading. We found that obtained with the microwave method was found (entry 10).
0.05 mol% of 5b-PdCl2 provides the best result, with optimum Although both methods are efficient, for the Heck coupling,
yields of the coupled product (Table 3, entry 3). Reactions were the microwave-assisted reactions were faster in comparison to
also carried out in DMF, NMP or DMA as solvents using conventional thermal conditions. These results showed that in
different bases (e.g. Et3N, NaOAc, K2CO3, K3PO4).
this reaction, the microwave irradiation is a more efficient
Among the solvents tested, DMF gave the best results in the heating method.
presence of Et3N as a base, whereas under the same conditions
the other bases gave low yields.
Optimized conditions for this cross-coupling involves the
use of Et3N as a base, and 0.05 mol% of 5b-PdCl2 in DMF
The effectiveness of the microwave irradiation and conven- heating at 160 °C under microwave irradiation.
tional heating for Heck reaction has been compared. We
As a further step, we also compared these findings with
known related phosphine free [N,S] bidentate complexes 7a–c
and new oxazoline analogous complex 10 in order to make
evident the role of the thiazoline moiety in the efficiency and
the activity of these complexes.
Firstly, precursor ligands 6a–c were synthesized in excellent
yields using a modified method taken from the literature,20
starting from N,N-dimethylaminomethyl ferrocene and the
corresponding alkyl disulphide, as is shown in Scheme 4.
Compounds were characterized by using various spectro-
scopic techniques and the structure of compound 6b was
further confirmed by single crystal X-ray diffraction analysis
Fig. 4 ORTEP representation of ferrocenyl thiazoline 5b-PdCl2. Ellipsoids are
shown at 30% probability level. Selected bond lengths [Å] and angles [°]: Pd(1)–
N(1) 2030(2), Pd(1)–S(2) 2.2765(8), Pd(1)–Cl(2) 2.2956(8), Pd(1)–Cl(1) 2.3250(8),
S(1)–C(1) 1.742(3), S(1)–C(3) 1.820(3), S(2)–C(8) 1.756(3), S(2)–C(14) 1.807(4),
N(1)–C(1) 1.284(3), N(1)–C(2) 1.477(3), C(1)–C(4) 1.453(4); N(1)–Pd(1)–S(2)
91.02(7), N(1)–Pd(1)–Cl(2) 178.35(6), S(2)–Pd(1)–Cl(2) 87.35(3), N(1)–Pd(1)–Cl(1)
90.31(7), S(2)–Pd(1)–Cl(1) 173.02(3), Cl(2)–Pd(1)–Cl(1) 91.27(3).
Scheme 4 Reagents and conditions. (i) t-BuLi, Et2O, −78 °C then (SR)2, 0 °C; (ii)
Pd(CH3CN)2Cl2, CH2Cl2, rt.
Table 3 Mizoroki–Heck cross-coupling of 4-iodotoluene with methyl acrylate under microwave irradiationa
Entry
Base
Solvent
% Mol [Pd]
Timeb (min)
% Yieldc
TON
TOF
1128
10 440
14 400
18 800
13 500
13 500
6000
1
2
3
4
5
6
7
8
9
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
NaOAc
K2CO3
K3PO4
Et3N
DMF
DMF
DMF
DMF
NMP
DMA
DMF
DMF
DMF
DMF
1%
0.1%
5
5
8
15
8
8
94
87
96
47
90
90
40
37
20
89
94
870
1920
4700
1800
1800
800
740
400
1780
0.05%
0.01%
0.05%
0.05%
0.05%
0.05%
0.05%
0.05%
8
8
5550
3000
712
8
10
150d
a Reaction conditions: 1.0 mmol of aryl iodide, 1.2 mmol of acrylate, 1.5 mmol of base, 5 mL of solvent at 160 °C. b Time reaction based on total
consumption of aryl iodide determined by TLC. c Isolated yield after SiO2 column chromatography. d Conventional thermal conditions.
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Scheme 5 Reagents and conditions. (i) n-BuLi, EtO2, −78 °C, then (SMe)2, 0 °C.
(ii) Pd(CH3CN)2Cl2, CH2Cl2, rt.
thiazolines by a directed metalation reaction, using ferrocenyl-
oxazoline 8 as a DMG and the corresponding alkyl disulfide as
an electrophile (Scheme 5). The desired oxazoline 8 was con-
structed using the conditions developed by Richards.8b
Once complexes 7a–c and 10 are synthesized, their poten-
tial as catalyst precursors in the Mizoroki–Heck reaction was
also evaluated with coupling between methyl acrylate and
4-iodotoluene under the same conditions as 5b-PdCl2 (Table 4).
We found that the catalytic performance was highly influ-
enced by the type of nitrogen donor group: thiazoline, oxazo-
line or amino group as well as thioether substituents on the
Fig. 5 ORTEP representation of 6b. Ellipsoids are shown at 30% probability
level. Selected bond lengths [Å] and angles [°]:C(1)–C(11) 1.577(4), C(11)–N(12)
1.532(4), C(2)–S(15) 1.750(3), S(15)–C(16) 1.794(4), C(2)–C(1)–C(11) 124.9(3),
C(1)–C(2)–S(15) 125.7(2), C(1)–C(11)–N(15) 106.8(2), C(2)–S(15)–C(16) 101.7(2).
(Fig. 5). The structure indicates that the isopropylsulfide dimethylaminomethyl ferrocene moiety.
moiety is directly bonded to the monosubstituted Cp ring at
Comparison of the results revealed that 5b-PdCl2 was the
2-position. The dimethylamino group shows slight differences best catalytic precursor for the reaction giving the coupled
in both angle and distances to those reported in the litera- product in 89% yield (entry 1) while complexes 7a–c and 10
ture.21 In particular, the bond angle between N(12)–C(11)–C(1), gave lower yields when the reactions were performed under
106.8(2)° is smaller to that measured in other ferrocenyl com- thermal conditions.
pounds [(112.5°)22 and (115.8°)23]. The structure presents dis-
Regarding the effect of thioether substituents on complex
order of the unsubstituted Cp ring generating two orientations 7a–c, we found that these groups affected both the efficiency
in 74 : 26 ratio. Only the major contributor is shown in Fig. 5. and the activity of the coupling (Table 4, entries 2–4). We found
Treatment of 6a–c with one equivalent of Pd(CH3CN)2Cl2 in that the presence of methyl thioether substituent as a donor
CH2Cl2 at room temperature gives the corresponding 7a–c group in complex 7a has a negative effect on efficiency, since
complexes as air-stable brown solids. These complexes were the reaction is 12 times slower compared with complex 5b-
fully characterized using conventional spectroscopic tech- PdCl2. Nevertheless, couplings using complexes 7b and 7c were
niques and all data are in agreement with those previously completed faster than the reaction with 5b-PdCl2; but in these
reported in the literature.20
cases, the activity was less and the coupled products were
Similarly, the palladium complex 10 was synthetized from obtained in lower yields. In the same way, complex 10 with an
the oxazoline 9, which was prepared analogously to the oxazoline moiety provides a good efficiency (entry 5) but unfor-
method above described for the synthesis of functionalized tunately the activity decreases compared with 5b-PdCl2.
Table 4 Mizoroki–Heck cross-coupling of 4-iodotoluene with methyl acrylate under conventional heating and microwave irradiationa
Entry
Complex
Type of heating
Timeb (min)
% Yieldc
TON
TOF
712
51
775
759
730
16 286
5580
10 364
1
2
3
4
5
6
7
8
5b-PdCl2
7a
7b
7c
10
5b-PdCl2
7b
10
Conventional
Conventional
Conventional
Conventional
Conventional
Microwave
150
1800
130
100
120
8
89
76
84
63
73
96
93
95
1780
1520
1680
1260
1460
1900
1860
1900
Microwave
Microwave
20
11
a Reaction conditions: 1.0 mmol of aryl iodide, 1.2 mmol of acrylate, 1.5 mmol of base, 5 mL of DMF, 0.05% mol [Pd]. b Time reaction based on
total consumption of aryl iodide determined by TLC. c Isolated yield after SiO2 column chromatography.
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To further study the potential of these complexes, we per- obtained using conventional heating, microwave irradiation
formed the Pd-catalyzed Heck reaction under microwave proved to be suitable for this coupling since the three com-
irradiation (Table 4, entries 6–8). In contrast to the results plexes showed excellent catalytic activity. However, the
complex with a thiazoline fragment was three times more
efficient than 7b and also better than complex 10. To sum up,
Table 5 Scope of Mizoroki–Heck reaction of aryl iodides with methyl acrylate
under microwave irradationa
we can affirm that introducing a 2-thiazoline moiety into the
thioether ferrocene ligand is convenient for the Heck reaction.
Encouraged by these excellent results, we evaluated the
scope of this reaction by studying the effect of the substituent
on the reactivity of the aromatic iodide employing this
complex (Table 5).
Cross-couplings of a wide range of activated and deactivated
Yieldc (%) aryl iodides with methyl acrylate were carried out. In every
Entry
R
Timeb (min)
case, the substrate was cleanly converted into the desired
1
2
3
4
–CH3
–OMe
–NH2
–H
–COOMe
–COCH3
–Br
8
7
6
5
5
3
3
4
7
96
methyl cinnamate with isolated yields, after CC ranging from
92
71 to 99%.
99
84
71
87
89
79
84
The results show that the yields and time of the coupling
5d
6d
7d
8d
9d
reaction depend on the nature of the substituent in aryl
iodide. As shown in Table 5, the aryl iodides with electron-
withdrawing substituents reacted completely in less than
5 minutes under microwave irradiation with the 5b-PdCl2
complex, except the aryl iodide p-substituted with a nitro
group which reacts completely after 7 min (entry 9), while for
the aryl iodides with electron-donor substituents, longer reac-
tion times were needed but excellent yields were achieved.
–CF3
–NO2
a Reaction conditions: 1.0 mmol of aryl iodide, 1.2 mmol of acrylate,
1.5 mmol of base, 5 mL of solvent at 160 °C. b Time reaction based on
total consumption of aryl iodide determined by TLC. c Isolated yield
after SiO2 column chromatography. d The formation of side-products
was observed.24
Table 6 Crystal data and structure refinement for 3, 4-Pd(PPh3Cl2), 5c, 5b-PdCl2 and 6b
3
4-Pd(PPh3Cl2)
5c
5b-PdCl2
6b
Empirical formula
Formula weight
C13H15FeNOS
289.17
C31H28Cl2FeNPPdS
710.74
C19H17FeNS2
379.31
C14H15Cl2FeNPdS2
494.56
C16H23FeNS
317.26
(g mol−1
)
Crystal size (mm)
Color
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.48 × 0.19 × 0.10
Red
Monoclinic
C2/c
17.861(2)
7.376(1)
18.937(2)
90
0.42 × 0.26 × 0.10
Red
Monoclinic
P21/n
12.137(1)
14.818(1)
19.733(2)
90
0.31 × 0.29 × 0.22
Red
Monoclinic
P21/n
7.364(1)
9.245(2)
25.189(5)
90
0.21 × 0.13 × 0.13
Red
Monoclinic
P21/c
9.590(1)
13.685(1)
16.626(1)
90
0.314 × 0.292 × 0.222
Orange
Monoclinic
P21/c
16.558(5)
8.011(2)
12.175(4)
90
α (°)
β (°)
92.073(2)
90
2493.0(4)
8
1.541
8240
105.843(1)
90
3414.0(5)
4
1.615
36 660
91.759(3)
90
1714.1(6)
4
1.470
9131
104.265(1)
90
2114.6(3)
4
1.928
22 995
96.932(4)
90
1603.2(8)
4
1.314
16 388
γ (°)
V (Å3)
Z
Dcalc (g cm3)
Number of collected
reflections
Number of
independent
2258, Rint = 0.0293
0.7452 and 0.6188
2258/205/206
6244, Rint = 0.0236
0.8630 and 0.6109
6244/230/453
3120, Rint = 0.0268
0.8257 and 0.5671
3120/78/245
3889, Rint = 0.0274
0.7833 and 0.5137
3889/48/255
2925, Rint = 0.0905
0.8171 and 0.7133
2925/236/222
reflections (Rint
Maximum and
minimum
transmission
Data/restraints/
parameters
)
Final R indices
[l > 2σ(l)]
R indices (all data)
R = 0.0294,
wR2 = 0.0685
R = 0.0345,
R = 0.0281,
wR2 = 0.0698
R = 0.0329,
R = 0.0432,
wR2 = 0.1009
R = 0.0621,
R = 0.0270,
wR2 = 0.0629
R = 0.0310,
R = 0.0475,
wR2 = 0.1284
R = 0.0534,
wR2 = 0.0707
1.058
Semi-empirical
from equivalents
wR2 = 0.0727
1.035
Semi-empirical
from equivalents
wR2 = 0.1105
1.021
Semi-empirical
from equivalents
wR2 = 0.0650
1.073
Semi-empirical
from equivalents
wR2 = 0.1326
1.059
Semi-empirical
from equivalents
GoF(F2)
Absorption correction
method
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Thus, we have demonstrated that 5b-PdCl2 based on a thi- is achieved by a hydraulic sensor integrated in the swiveling
azoline ring was highly active and efficient as a catalyst precur- cover of the instrument. The reusable 10 mL Pyrex vial is
sor in the Mizoroki–Heck reaction.
sealed with PEEK snap caps and standard PTFE coated sili-
cone septa. Reaction cooling is performed using compressed
air automatically after the heating period has elapsed.
Conclusions
Structure determination by X-ray crystallography
We have established an appropriate method for the synthesis
of 2-ferrocenyl-2-thiazolines, via a three-step procedure. We Suitable X-ray quality crystals of 3, 4-Pd(PPh3Cl2), 5c, 5-PdCl2
have shown that this versatile strategy could generate a wide and 6b were grown by slow evaporation of n-hexane–benzene
range of thiazolines substituted by only changing the β-amino mixture at −5 °C and chloroform at room temperature, respecti-
alcohol used in the aminolysis reaction. Furthermore, we have vely. The crystals of each compound were mounted on a glass
identified the 2-thiazoline moiety as an efficient DoM for the fiber at room temperature, and then placed on a Bruker Smart
selective deprotonation of ferrocene, and we have established Apex CCD diffractometer, equipped with Mo Kα radiation;
the first DoM-electrophilic quenching process for producing decay was negligible in both cases. Details of crystallographic
several 1,2-disubstituted ferrocene derivatives with a 2-thi- data collected on compounds 3, 4-Pd(PPh3Cl2), 5c and 5-PdCl2
azoline moiety.
are provided in Table 6. Systematic absences and intensity stat-
The final results show that the palladium complex 5b-PdCl2 istics were used in space group determination. The structure
might be a convenient and highly efficient catalytic system for was solved using direct methods.25 Anisotropic structure
the Heck coupling reaction of methyl acrylate and aryl iodides refinements were achieved using full matrix, least-squares
compared with other palladium complexes based on [N,S] technique on all non-hydrogen atoms. All hydrogen atoms
ferrocenyl ligands. The asymmetric version of these com- were placed in idealized positions, based on hybridization,
pounds and their possible applications in asymmetric catalysis with isotropic thermal parameters fixed at 1.2 times the value
are now being pursued in our laboratory.
of the attached atom. Structure solutions and refinements
were performed using SHELXTL V6.10.26
Experimental
General considerations
Synthesis of Fischer ethoxy ferrocenyl carbene complex (1),
Fischer hydroxyl ethyl amine ferrocenyl carbene complex (2)
and N-(2-hydroxyethyl) ferrocenyl thioamide (3)
All operations were carried out under an inert atmosphere of
nitrogen or argon gas using standard Schlenk techniques. The preparation of compounds 1, 2 and 3 was carried out
Anhydrous THF was obtained by distillation under an inert using the methodology previously described elsewhere.15,17
atmosphere over sodium benzophenone. Column chromato-
graphy was performed using 70–230 mesh silica gel. All
reagents and solvents were obtained from commercial suppli-
Synthesis of 2-ferrocenyl-2-thiazoline (4)
ers and used without further purification. All compounds were To an ice-cooled solution of thioamide 3 (1 g, 3.5 mmol) in
characterized by IR spectra, recorded on a Perkin-Elmer 283B 15 mL of dichloromethane were successively added methane-
or 1420 spectrophotometer, by means of film and KBr tech- sulfonyl chloride (0.35 mL, 4.5 mmol) and triethylamine
niques, and all data are expressed in wave numbers (cm−1). (1.95 mL, 14 mmol). The reaction mixture was stirred for
Melting points were obtained on a Melt-Temp II apparatus and 10 min at 0 °C, and then the reaction was quenched by adding
are uncorrected. NMR spectra were measured with a JEOL 30 mL of water. The organic phase was washed with the same
Eclipse +300 using CDCl3 as a solvent. Chemical shifts are in amount of water and then dried over anhydrous Na2SO4. The
ppm (δ), relative to TMS. The MS-FAB and MS-EI spectra were solvent was evaporated under vacuum and the crude product
obtained on a JEOL SX 102A, the values of the signals are was purified by flash chromatography using silica-gel and
expressed in mass/charge units (m/z), followed by the relative hexane–ethyl acetate 9 : 1 as an eluent to give 0.89 g of an
intensity with reference to a 100% base peak. Elemental ana- orange solid (96% yield). Mp: 100–102 °C. 1H NMR (CDCl3,
lyses for carbon, hydrogen, nitrogen and sulfur atoms were ppm): δ 3.31 (t, 2H, J = 8.1 Hz, –CH2S–), 4.16 (s, 5H, Cp), 4.20
performed on a Perkin-Elmer 2400 elemental analyzer using (t, 2H, J = 8.1 Hz, –CH2N–), 4.34 (br s, 2H, subst Cp), 4.68 (br s,
cystine as a standard.
1H, subst Cp). 13C NMR (CDCl3, ppm): δ 33.6 (–CH2S–), 64.8
Microwave irradiation experiments were performed using a (–CH2Nv), 69.3 (CH, subst Cp), 70.0 (Cp), 70.4 (CH, subst Cp),
Monowave 300 single-mode microwave reactor. The reaction 77.3 (Cipso, Fc), 168.8 (CvS). IRvmax (KBr, cm−1): 1608, 1453,
temperature is monitored by an internal fiber-optic (FO) temp- 1251. MS-IE+ m/z (rel. intensity %): 271 [M+] (100), 211 (38).
erature probe (ruby thermometer) protected by a borosilicate HRMS (FAB+): calculated for C13H13FeNS: 271.0118. Found:
immersion well inserted directly into the reaction mixture. 271.0111. Elemental analysis (%): calcd for C13H13FeNS:
Reaction times refer to the hold time at the desired set temp- C 57.58, H 4.83, N 5.17, S 11.83; found: C 57.92, H 4.73, N
erature and not to the total irradiation time. Pressure sensing 5.55, S 19.39.
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Synthesis of 2-ferrocenyl-2-thiazoline palladium complex
4-Pd(PPh3Cl2)
(Cipso, Fc), 85.1 (Cipso, Fc), 167.7 (CvS). IRvmax (KBr, cm−1):
1616, 1434, 1243, 994. MS-IE+ m/z (rel. intensity %): 317 [M+]
(100), 302 [M+ − CH3], (58), 242 [Fc-NvCvS]+ (67). HRMS
(FAB+): calculated for C14H15FeNS2: 316.9995. Found:
316.9998. Elemental analysis (%): calcd for C14H15FeNS2: C
53.00, H 4.77, N 4.42, S 20.21; found: C 53.12, H 4.42, N 4.95, S
19.06.
A solution of ligand 4 (0.1 g, 0.37 mmol) in methanol (5 mL)
was added, at room temperature, to a fresh solution of
Li2PdCl4 [65 mg (0.37 mmol), PdCl2 and 16 mg (0.37 mmol) of
LiCl in methanol (5 mL)]. A brown precipitate was formed
when the reaction mixture was stirred overnight at room temp-
erature. After this time, a solution of PPh3 (97 mg, 0.37 mmol)
in 3 mL of dichloromethane was added and the mixture was
stirred for 5 min. The resulting clear solution was filtered
through celite, and then was washed with water and brine. The
organic phase was dried with anhydrous sodium sulfate and
the solvent was evaporated under vacuum, obtaining the
corresponding palladium complex as a brown solid in 87%
yield. Mp: 196–198 °C. MS-FAB+ m/z (rel. intensity %): 710 [M+]
(5). HRMS (FAB+): calculated for C31H28Cl2FeNPPdS: 708.9441.
Found: 708.9445.
1-(2-Thiazolin-2-yl)-2-(phenylthio)-ferrocene (5c). The com-
pound was purified by flash chromatography using hexane–
EtOAc 9 : 1 as an eluent affording an orange solid in 78% yield
1
(163 mg, 0.43 mmol). Mp: 112–113 °C. H NMR (CDCl3, ppm):
δ 3.18–3.37 (m, 2H, –CH2S–), 4.15 (t, 2H, J = 8.4 Hz, –CH2N–),
4.27 (s, 5H, Cp), 4.48 (s, 1H, sust Cp), 4.52 (s, 1H, subst Cp),
5.01 (s, 1H, subst Cp), 7.06–7.19 (m, 5H, Harom). 13C NMR
(CDCl3, ppm): δ 33.9 (–CH2S–), 63.8 (–CH2Nv), 71.1 (CH,
subst Cp), 71.5 (CH, subst Cp), 71.8 (Cp), 77.8 (Cipso, Fc), 78.0
(CH, subst Cp), 80.6. (Cipso, Fc), 125.5 (CHarom), 126.8 (CHarom),
128.9 (CHarom), 139.8 (Cipso, Ph), 167.5 (CvS). IRvmax (KBr,
cm−1): 1590, 1474, 1433, 1246, 997. MS-IE+ m/z (rel. inten-
sity %): 379 [M+] (100), 371 [M+ − SPh] (78). HRMS (FAB+):
Calculated for C19H17FeNS2: 379.0152. Found: 379.0149.
Elemental analysis (%): calcd for C19H17FeNS2: C 60.16, H
4.52, N 3.69, S 16.91; found: C 60.52, H 4.43, N 4.07, S 16.38.
Deuterium incorporation analysis
A solution of 2-ferrocenyl-2-thiazoline 4 (100 mg, 0.37 mmol)
in anhydrous diethyl ether under nitrogen atmosphere was
cooled to −78 °C (dry ice/acetone bath), then t-butyl lithium in
pentane 1.7 M (0.22 mL, 0.45 mmol) was added dropwise. On
completing the t-butyl lithium addition, the deuterium incor-
poration was determined by quenching the reaction mixture
with excess D2O, after warming to several different temp-
eratures. The aqueous mixtures were extracted with dichloro-
methane, and the organic layers were dried with Na2SO4.
Recovered yields of the deuterated product were determined by
1H NMR comparing the integrations of the substituted Cp ring
protons.
1-(2-Thiazolin-2-yl)-2-(isopropylthio)-ferrocene
(5d). The
compound was purified by flash chromatography using
hexane–EtOAc 9 : 1 as an eluent affording an orange solid in
63% yield (120 mg, 0.35 mmol). Mp: 50–52 °C. 1H NMR
(CDCl3, ppm): δ 1.14 (dd, J = 15, 6.6 Hz, 6H, –CH(CH3)2),
2.92–3.01 (m, 1H, –CH(CH3)2), 3.27 (t, 2H, –CH2S–), 4.14–4.18
(m, 7H, –CH2N–, Cp), 4.37 (s, 1H, subst Cp), 4.46 (s, 1H, subst
Cp), 4.87 (s, 1H, subst Cp). 13C NMR (CDCl3, ppm): δ 22.8
(–CH(CH3)2), 23.2 (–CH(CH3)2), 33.8 (–CH2S–), 40.6
(–CH(CH3)2), 63.7 (–CH2Nv), 70.2 (CH, subst Cp), 71.0 (CH,
subst Cp), 71.6 (Cp), 78.2 (CH, subst Cp), 79.4 (Cipso, Fc), 80.8
(Cipso, Fc), 167.9 (CvS). IRvmax (KBr, cm−1): 3098, 1587, 1454,
General procedure for the DoM-electrophile quenching of
2-ferrocenyl-2-thiazoline (4)
A solution of 4 (150 mg, 0.55 mmol) in 3 mL of anhydrous 1242, 998. MS-IE+ m/z (rel. intensity %): 345 [M+] (100), 302
diethyl ether under nitrogen atmosphere was cooled to −78 °C [M+ − CH(CH3)2], (78), 242 [Fc-NvCvS]+ (59). HRMS (FAB+):
(dry ice/acetone bath), then t-butyl lithium in pentane 1.7 M Calculated for C16H19FeNS2: 345.0308. Found: 345.0311.
(0.33 mL, 0.68 mmol) was added dropwise. The mixture was Elemental analysis (%): calcd for C16H19FeNS2: C 55.65, H
gradually warmed to room temperature (approximately 5.55, N 4.06, S 18.57; found: C 56.28, H 5.14, N 4.41, S 15.68.
1.5 hours). After reaching this temperature, the mixture was
1-(2-Thiazolin-2-yl)-2-(trimethylsilyl)-ferrocene
(5e). The
stirred for an additional 5 min. The reaction mixture was then compound was purified by flash chromatography using
cooled at 0 °C and quenched by adding an electrophile hexane as an eluent affording an orange solid in 98% yield
(1.5 equiv.). The solution was further stirred overnight at room (185 mg, 0.54 mmol). Mp: 71–73 °C. 1H NMR (CDCl3, ppm):
temperature, before quenching with a saturated solution of δ 0.26 (s, 9H, –Si(CH3)3), 3.17–3.37 (m, 2H, –CH2S–), 3.93–4.04
NaHCO3. After standard work-up, the crude product was puri- (m, 1H, –CHHN–), 4.18 (s, 5H, Cp), 4.26 (s, 1H, subst Cp),
fied by flash chromatography on silica-gel.
4.32–4.39 (m, 1H, –CHHN–). 4.43 (s, 1H, subst Cp), 4.74 (s, 1H,
1-(2-Thiazolin-2-yl)-2-(methylthio)-ferrocene (5b). The com- subst Cp). 13C NMR (CDCl3, ppm): δ 0.6 (–Si(CH3)3), 33.9
pound was purified by flash chromatography using hexane– (–CH2S–), 65.1 (–CH2Nv), 70.1 (Cp), 71.8 (CH, subst Cp), 72.3
EtOAc 9 : 1 as an eluent affording an orange solid in 87% yield (CH, subst Cp), 74.9 (CH, subst Cp), 76.9 (Cipso, Fc), 83.3 (Cipso
,
(152 mg, 0.48 mmol). Mp: 64–65 °C. H NMR (CDCl3, ppm): δ Fc), 167.9 (CvS). IRvmax (KBr, cm−1): 3089, 1611, 1241, 822.
2.35 (br s, 3H, –CH3), 3.33 (br s, 2H, –CH2S–), 4.22 (br s, 5H, MS-IE+ m/z (rel. intensity %): 343 [M+] (68), 328 (100). HRMS
Cp), 4.35 (br s, 2H, –CH2N–), 4.78 (m, 1H, subst Cp), 4.46 (m, (FAB+): Calculated for C16H21FeNSiS: 343.0513. Found:
1H, subst Cp), 4.77 (m, 1H, subst Cp). 13C NMR (CDCl3, ppm): 343.0515. Elemental analysis (%): calcd for C16H21FeNSiS:
δ 19.7 (–CH3), 33.5 (–CH2S–), 64.5 (–CH2Nv), 69.1 (CH, subst C 55.97, H 6.16, N 4.08, S 9.34; found: C 56.83, H 6.16, N 4.46,
Cp), 71.4 (CH, subst Cp), 71.0 (Cp), 72.7 (CH, subst Cp), 78.3 S 8.92.
1
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1-(2-Thiazolin-2-yl)-2-(phenylseleno)-ferrocene
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(5f). The 269 (47), 242 [Fc-NvCvS]+ (64). HRMS (FAB+): Calculated for
compound was purified by flash chromatography using C13H12IFeNS: 396.9085. Found: 396.9085. Elemental analysis
hexane–EtOAc 8 : 2 as an eluent affording a yellow solid in 79% (%): calcd for C13H12IFeNS: C 39.32, H 3.05, N 3.53, S 8.08;
yield (185 mg, 0.43 mmol). Mp: 98–99 °C. 1H NMR (CDCl3, found: C 39.72, H 2.95, N 3.88, S 7.2.
ppm): δ 4.05–4.07 (m, 1H, subst Cp), 4.15 (s, 5H, Cp), 4.18 (s,
Synthesis of [1-(2-thiazolin-2-yl)-2-(methylthio)-ferrocene
palladium(II) dichloride] (5b-PdCl2)
3H, Harom), 7.70–7.72 (m, 2H, Harom). 13C NMR (CDCl3, ppm): To a solution of 5b (0.1 g, 0.36 mmol) in dichloromethane
δ 33.9. (–CH2S–), 64.3 (–CH2Nv), 70.8 (CH, subst Cp), 71.6 (10 mL) was added [Pd(CH3CN)2Cl2] (0.09 g, 0.36 mmol), then
(Cp), 75.2 (CH, subst Cp) 76.6 (CH, subst Cp), 79.3 (Cipso, Fc), the mixture was stirred for 30 min at room temperature. The
2H, –CH2N–), 4.32 (t, 1H, subst Cp), 4.46 (m, 1H, –CH2S–),
4.72–4.74 (s, 1H, subst Cp), 4.85 (s, 1H, –CH2S–), 7.33–742 (m,
126.9 (CHarom), 129.1 (CHarom), 131.9 (CHarom), 132.9 (Cipso
,
solvent was removed under vacuum, leaving a brown residue
Ph), 167.9 (CvS). IRvmax (KBr, cm−1): 1590, 1472, 1432, 1244, which was dissolved in the minimum amount of dichloro-
998. MS-IE+ m/z (rel. intensity %): 427 [M+] (100), 347 (25), 287 methane. The resulting solution was filtered through celite,
(45). HRMS (FAB+): Calculated for C19H17FeNSeS: 426.9596. and then the slow addition of hexane gave an orange solid
Found: 426.9600.
complex, which was filtered, washed with hexane, and dried
1-(2-Thiazolin-2-yl)-2-(tri-n-butylstannyl)-ferrocene (5g). The under vacuum to give 0.16 g of complex (93% yield). Mp:
compound was purified by flash chromatography using 178–179 °C (dec). 1H NMR (CDCl3, ppm): δ 2.36 (m, 3H,
hexane–EtOAc 9 : 1 as an eluent affording an orange solid in CH3S–), 3.52–3.60 (m, 2H, –CH2S–), 4.07–4.20 (m, 1H,
51% yield (157 mg, 0.28 mmol). Mp: 36–37 °C. 1H NMR –CH2N–), 4.85 (s, 5H, Cp), 4.89–4.92 (m, 1H, –CH2N–),
(CDCl3, ppm): δ 0.9–1.00 (m, 15H, –SnBu3), 1.26–1.38 (m, 6H, 5.04–5.05 (m, 1H, subst Cp), 5.23 (s, 1H, subst Cp), 5.31–5.41
–SnBu3), 1.48–1.58 (m, 6H, –SnBu3), 3.20–3.38 (m, 2H, (m, 1H, subst Cp). IRvmax (KBr, cm−1): 1571, 1412, 1246, 1113.
–CH2S–), 3.90–4.00 (m, 1H, –CH2Nv), 4.11 (s, 5H, Cp), MS-FAB+ m/z (rel. intensity %): 495 [M+] (3), 460 [M+ − Cl] (5).
4.21–4.45 (m, 2H, subst Cp and –CH2Nv), 4.46 (s, 1H, subst Elemental analysis (%): calcd for C14H15Cl2FeNPdS2: C 34.00,
Cp), 4.68 (s, 1H, subst Cp). 13C NMR (CDCl3, ppm): δ 11.6, H 3.06, N 2.83, S 12.97; found: C 34.7, H 3.41, N 2.82, S 11.03.
13.9, 27.6 and 29.4 (–SnBu3), 33.8 (–CH2S–), 64.5 (–CH2Nv),
General procedure for the synthesis of 1-[(dimethylamino)-
methyl]-2-(alkylthio)ferrocenes 6a–c
69.7 (Cp), 71.6 (CH, subst Cp), 72.4 (CH, subst Cp), 72.9 (CH,
subst Cp), 77.1 (Cipso, Fc), 82.6 (Cipso, Fc), 168.5 (CvS). IRvmax
(KBr, cm−1): 1609, 1459, 1106, 1000. MS-IE+ m/z (rel. intensity
A solution of (dimethylamino)methylferrocene (0.5 mL,
%): 504 [M+ − nBu] (100), 390 (40). HRMS (FAB+): Calculated 2.52 mmol) in 5 mL of anhydrous diethyl ether under nitrogen
for C21H30FeNSSn: 504.0470. Found: 504.0475. Elemental ana- atmosphere was cooled to −78 °C, then t-butyl lithium in
lysis (%): calcd for C21H30FeNSSn: C 53.60, H 7.02, N 2.50, pentane 1.7 M (1.76 mL, 3 mmol) was added dropwise. The
S 5.72; found: C 54.28, H 7.12, N 2.83, S 5.77.
mixture was gradually warmed to room temperature. After
1-(2-Thiazolin-2-yl)-2-(methyl)-ferrocene (5h). The com- reaching this temperature, the reaction was cooled at −10 °C
pound was purified by flash chromatography using hexane– and the corresponding alkyl disulphide (2 equiv.) was added
EtOAc 9 : 1 as an eluent affording an orange solid in 86% yield to the reaction mixture. The solution was further stirred over-
(135 mg, 0.47 mmol). Mp: 98–99 °C. 1H NMR (CDCl3, ppm): night at room temperature, before quenching with a saturated
δ 2.27 (s, 3H, –CH3), 3.24–3.33 (m, 2H, –CH2S–), 4.12–4.33 (m, solution of NaHCO3. After standard work-up, the crude
9H, Cp, 2 subst Cp and –CH2Nv), 4.54 (s, 1H, –CH2N–), 4.55 product was purified by flash chromatography on silica-gel to
(s, 1H, subst Cp). 13C NMR (CDCl3, ppm): δ 15.1 (–CH3), 33.2 give the desired compounds 6a–c.
(–CH2S–), 65.2 (–CH2Nv), 67.9 (CH, subst Cp), 70.6 (CH, subst
Cp), 70.8 (Cp), 72.7 (CH, subst Cp), 76.1 (Cipso, Fc), 84.7 (Cipso
1-[(Dimethylamino)methy1]-2-(methylthio)ferrocene
The compound was purified by flash chromatography using
(6a).
,
Fc), 168.7 (CvS). IRvmax (KBr, cm−1): 1609, 997. MS-IE+ m/z hexane–dichloromethane as an eluent affording a yellow solid
(rel. intensity %): 285 [M+] (100), 225 (20). HRMS (FAB+): in 99% yield (721 mg, 2.49 mmol). Mp: 76–77 °C. 1H NMR
Calculated for
Elemental analysis (%): calcd for C14H15FeNS: C 58.96, H 5.30, 3.24 (d, 1H, –CH2N, J = 12.3 Hz), 3.59 (d, 1H, –CH2N, J = 12.3
N 4.91, S 11.24; found: C 58.28, H 5.59, N 4.51, S 9.34. Hz), 4.10 (s, 5H, Cp), 4.20 (m, 1H, subst Cp), 4.30 (m, 2H,
C14H15FeNS: 285.0750. Found: 285.0271. (CDCl3, ppm): δ: 2.20 (s, 6H, –N(CH3)2), 2.26 (s, 3H, –SCH3),
1-(2-Thiazolin-2-yl)-2-(iodo)-ferrocene (5j). The compound subst Cp). 13C NMR (CDCl3, ppm δ: 20.2 (–SCH3), 45.1
was purified by flash chromatography using hexane–EtOAc (–N(CH3)2), 57.1 (–CH2N), 67.5 (CH, subst Cp), 69.9 (Cp), 70.5
9 : 1 as an eluent affording brown oil in 75% yield (159 mg, (CH, subst Cp), 71.7 (CH, subst Cp), 83.78 (Cipso, Fc), 86.2
0.40 mmol). 1H NMR (CDCl3, ppm): δ 3.28–3.43 (m, 2H, (Cipso, Fc). IRvmax (KBr, cm−1): 3078, 2854, 2771, 1421. MS-IE+
–CH2S–), 4.23–4.40 (m, 8H, Cp, subst Cp and –CH2Nv), 4.65 m/z (rel. intensity %): 289 [M+] (21), 245 [M − N(CH3)2]+ (9).
(s, 1H, subst Cp), 4.71 (s, 1H, subst Cp). 13C NMR (CDCl3,
1-[(Dimethylamino)methy1]-2-(isopropylthio)ferrocene (6b).
ppm): δ 33.9 (–CH2S–), 64.9 (–CH2Nv), 70.3 (CH, subst Cp), The compound was purified by flash chromatography using
71.0 (CH, subst Cp), 73.0 (Cp), 77.3 (Cipso, Fc), 78.5 (CH, subst hexane–dichloromethane as an eluent affording a yellow solid
Cp), 78.6 (Cipso, Fc), 166.8 (CvS). IRvmax (KBr, cm−1): 1616, in 90% yield (719 mg, 2.27 mmol). Mp: 82–83 °C. 1H NMR
1434, 1243, 994. MS-IE+ m/z (rel. intensity %): 397 [M+] (100), (CDCl3, ppm): δ: 1.20 (dd, J = 14.1, 6.6 Hz, 6H, –(CH3)2), 2.19
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(s, 6H, –N(CH3)2), 3.02 (m, 1H, –SCH–), 3.17 (d, J = 12.9 Hz, (–CHCH3–), 31.0 (–N(CH3)2), 50.6 (–SCH–), 63.3 (–CH2N), 71.8
1H, –CH2N), 3.60 (d, J = 12.9 Hz, 1H, –CH2N), 4.09 (s, 5H, Cp), (Cp), 70.6 (CH, subst Cp), 72.1 (CH, subst Cp), 74.5 (CH, subst
4.17 (m, 1H, subst Cp), 4.33 (m, 2H, subst Cp). 13C NMR Cp), 84.0 (Cipso, Fc), 84.9 (Cipso, Fc). IRvmax (KBr, cm−1): 2860,
(CDCl3, ppm δ: 22.9 (–CHCH3–), 23.7 (–CHCH3–), 39.6 1148, 728. MS-IE+ m/z (rel. intensity %): 493 [M+] (2), 317
(–SCH–), 45.4 (–N(CH3)2), 57.4 (–CH2N), 67.8 (CH, subst Cp), [M+ − PdCl2]+ (7).
70.1 (CH, subst Cp), 70.9 (Cp), 75.2 (CH, subst Cp), 78.9 (Cipso
,
[1-[(Dimethylamino)methy1]-2-tertbutylthio)ferrocene]-
Fc), 88.1 (Cipso, Fc). IRvmax (KBr, cm−1): 2758, 1737, 1369, palladium(II) dichloride (7c). The compound was purified by
627. MS-IE+ m/z (rel. intensity %): 317 [M+] (100), 273 flash chromatography affording a brown solid in 80% yield
[M − N(CH3)2]+ (39).
(249 mg, 0.49 mmol). Mp: 176 °C. 1H NMR (CDCl3, ppm):
1-[(Dimethylamino)methy1]-2-(tertbutylthio)ferrocene (6c). δ 1.48 (s, 9H, –(CH3)3), 2.66 (d, 1H, –CH2N, J = 14.1 Hz), 3.05
The compound was purified by flash chromatography using (s, 3H, –NCH3–), 3.22 (s, 3H, –NCH3–), 3.32 (d, 1H, –CH2N, J =
hexane–dichloromethane as an eluent affording a yellow solid 14.1 Hz), 4.53 (t, 1H, subst Cp, J = 5.1 Hz), 4.64 (s, 5H, Cp),
in 60% yield (500 mg, 1.51 mmol). Mp: 82–83 °C. 1H NMR 4.84 (s, 1H, subst Cp), 5.33 (t, 1H, subst Cp, J = 5.1 Hz). 13C
(CDCl3, ppm): δ 1.24 (s, 9H, –S(CH3)3), 2.23 (s, 6H, –N(CH3)2), NMR (CDCl3, ppm) δ: 89.1 (Cipso, Fc), 78.3 (Cipso, Fc), 75.0 (CH,
3.18 (d, J = 13.2 Hz, 1H, –CH2N), 3.53 (d, J = 13.2 Hz, 1H, subst Cp), 73.8 (CH, subst Cp), 69.7 (CH, subst Cp), 72.7 (Cp),
–CH2N), 4.08 (s, 5H, Cp), 4.20 (t, J = 5.1 Hz, 1H, subst Cp), 4.36 61.6 (–CH2N), 55.0 (–C(CH3)3), 51.9 (–NCH3–), 52.7 (–NCH3–),
(t, J = 3.9 Hz, 1H, subst Cp), 4.42 (t, J = 3.9 Hz, 1H, subst Cp). 30.9 (–(CH3)3). IRvmax (KBr, cm−1): 3083, 2918, 1454, 1369, 731.
13C NMR (CDCl3, ppm) δ: 22.9 (–NCH3–), 23.7 (–NCH3–), 39.6 MS-IE+ m/z (rel. intensity %): 437 [M+ − 2Cl] + (3).
(–C(CH3)3), 45.4 (–N(CH3)2), 57.4 (CH, subst Cp), 67.8 (CH,
Synthesis of 2-ferrocenyloxazoline (8)
subst Cp), 70.1 (CH, subst Cp), 70.9 (Cp), 75.2 (CH, subst Cp),
78.9 (Cipso, Fc), 88.1 (Cipso, Fc). IRvmax (KBr, cm−1): 2758, 1737, The preparation of compound 8 was carried out using the
1369, 627. MS-IE+ m/z (rel. intensity %): 331 [M+] (53), methodology previously described elsewhere.8b
273 (100).
Synthesis of 1-(2-oxazolin-2-yl)-2-(methylthio)-ferrocene (9)
General procedure for the synthesis of [1-[(dimethylamino)-
methy1]-2-(alkylthio)ferrocene] palladium(II) dichloride 7a–c
A
stirred solution of ferrocenyloxazoline
8
(100 mg,
0.39 mmol) and TMEDA in Et2O (0.07 mL, 0.47 mmol) under
To a solution of 6 (0.62 mmol) in dichloromethane (10 mL) nitrogen was cooled to −78 °C and then n-BuLi (0.2 mL,
was added [Pd(CH3CN)2Cl2] (0.16 g, 0.62 mmol), then the 0.5 mmol) was added dropwise. After stirring at −78 °C for
mixture was stirred for 1 hour at room temperature. The result- 2 h, the mixture was transferred to an ice bath and the stirring
ing solution was filtered through celite, and the crude product was maintained for a further 5 min. To the resultant solution
was purified by flash chromatography on silica-gel to give the was added (SMe)2 (0.07 mL, 0.78 mmol) and the reaction
desired complexes as brown solids. These complexes were mixture was allowed to warm to room temperature and stirred
characterized using conventional spectroscopic techniques overnight. After this time, the reaction was quenched with a
and all data are in agreement with those previously reported in saturated solution of NaHCO3 and diluted with EtO2. The com-
the literature.20
bined organic layer was dried with anhydrous Na2SO4, filtered
[1-[(Dimethylamino)methy1]-2-methylthio)ferrocene]- and evaporated. The crude product was purified by column
palladium(II) dichloride (7a). The compound was purified by chromatography to give 9 as a yellow solid (89 mg, 76%). Mp:
flash chromatography affording a brown solid in 99% yield 111–112 °C. 1H NMR (CDCl3, ppm): δ 2.40 (s, 3H, –CH3),
(283 mg, 0.61 mmol). Mp: 176 °C. 1H NMR (CDCl3, ppm): 3.95–4.03 (m, 2H, –CH2N–), 4.21 (s, 5H, Cp), 4.29 (s, 1H, subst
δ 2.18 (s, 3H, –SCH3), 2.33 (s, 3H, –NCH3–), 3.09 (s, 3H, Cp), 4.30–4.39 (m, 2H, –CH2O–). 4.42 (br s, 1H, subst Cp), 4.75
–NCH3–), 2.82 (d, 1H, –CH2N, J = 13 Hz), 3.97 (d, 1H, –CH2N, (s, 1H, subst Cp). 13C NMR (CDCl3, ppm): δ 18.4 (–CH3), 55.2
J = 13 Hz), 4.32 (s, 5H, Cp), 4.42 (t, 1H, subst Cp, J = 5 Hz), 4.48 (–CH2Nv), 67.0 (–CH2O–), 68.5 (CH, subst Cp), 70.3 (CH,
(s, 1H, subst Cp), 4.61 (s, 1H, subst Cp). 13C NMR (CDCl3, subst Cp), 70.7 (CH, subst Cp), 71.0 (Cp), 86.6 (Cipso, Fc), 166.2
ppm: δ 29.5 (–SCH3), 31.0 (–N(CH3)2), 48.7 (CH, subst Cp), 63.8 (CvS). IRvmax (KBr, cm−1): 1651, 1258, 1185, 1103. MS-IE+ m/z
(–CH2N), 68.5 (CH, subst Cp), 71.4 (Cp), 74.4 (Cipso, Fc), 79.20 (rel. intensity %): 301 [M+] (100), 286 [M+ − CH3] (58).
(Cipso, Fc). IRvmax (KBr, cm−1): 3021, 1741, 1443, 1370, 689.
Synthesis of [1-(2-oxazolin-2-yl)-2-(methylthio)-ferrocene
palladium(II) dichloride] (10)
MS-IE+ m/z (rel. intensity %): 465 [M+] (1), 419 [M − SCH3]+ (3).
[1-[(Dimethylamino)methy1]-2-isopropylthio)ferrocene]-
palladium(II) dichloride (7b). The compound was purified by To a solution of 9 (50 mg, 0.16 mmol) in dichloromethane
flash chromatography affording a brown solid in 86% yield (5 mL) was added [Pd(CH3CN)2Cl2] (66 mg, 0.16 mmol), then
(261 mg, 0.53 mmol). Mp: 174 °C. 1H NMR (CDCl3, ppm) δ: the mixture was stirred for 30 min at room temperature. The
1.67 (d, 3H, –CHCH3–, J = 7 Hz), 1.73 (d, 3H, –CHCH3–, J = resulting solution was filtered through celite and the solvent
7 Hz), 2.51 (s, 3H, –NCH3–), 3.07 (s, 3H, –NCH3–), 2.82 (d, 1H, was removed under vacuum, leading to an orange colored
–CH2N, J = 13.5 Hz), 3.56 (d, 1H, –CH2N, J = 13.5 Hz), 3.75 (m, residue. This residue was dissolved in a small amount of
1H, –SCH–), 4.39 (s, 5H, Cp), 4.45 (s, 1H, subst Cp), 4.68 (s, CH2Cl2 and precipitated with a large excess of hexane. After fil-
2H, subst Cp). 13C NMR (CDCl3, ppm) δ: 23.2 (–CHCH3–), 23.5 tration, the solid was washed with small portions of hexane,
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and dried under vacuum to give 64 mg of complex 10 as an
orange solid (81% yield). Mp: 198 °C (dec). IRv max (KBr, cm−1):
1619, 1248, 1180, 839. MS-FAB+ m/z (rel. intensity %): 479 [M+]
(2), 443 [M+ − Cl], (7). Elemental analysis (%): calcd for
C14H15Cl2FeNOPdS: C 35.14, H 2.96, N 2.93, S 6.70; found:
C 35.77, H 2.96, N 2.94, S 5.68.
R. L. Halterman, Wiley-VCH, Weinheim, 1998;
(c) Ferrocenes: Ligands, Materials and Biomolecules, ed.
P. Stepnicka, Wiley, Chichester, 2008.
3 For reviews see: (a) L.-X. Dai, T. Tu, S.-L. You, W.-P. Deng
and X.-L. Hou, Acc. Chem. Res., 2003, 36, 659;
(b) T. J. Colacot, Chem. Rev., 2003, 103, 3101;
(c) R. G. Arrayás, J. Adrio and J. C. Carretero, Angew. Chem.,
Int. Ed., 2006, 45, 7674.
General procedure for Mizoroki–Heck coupling reactions
under conventional thermal heating
4 For a representative example see: V. Mamane, Tetrahedron:
Asymmetry, 2010, 21, 1019.
5 D. Marquarding, H. Klusacek, G. Gokel, P. Hoffmann and
I. Ugi, J. Am. Chem. Soc., 1970, 92, 5389.
6 W.-P. Deng, V. Snieckus and C. Metallinos, in Chiral Ferro-
cenes in Asymmetric Catalysis, ed. L.-X. Dai and X.-L. Hou,
Wiley-VCH, Weinheim, Germany, 2009, and references
therein.
7 T. Sammakia, H. A. Latham and D. R. Schaad, J. Org.
Chem., 1995, 60, 10–11.
In a 10 mL round-bottomed flask, 4-iodotoluene (880 mg,
4 mmol), methyl acrylate (0.44 mL, 4.8 mmol) and Et3N
(0.84 mL, 6 mmol) were placed in 5 mL of DMF, then the palla-
dium complex (0.05% mol) was added. The reaction mixture
was refluxed for the time stated in Table 4 at 140 °C. The reac-
tion mixture was poured into water (20 mL) and extracted with
hexane (2 × 20 mL). The combined organic layers were dried
over anhydrous sodium sulfate. The crude product was finally
purified by flash column chromatography on silica-gel. Note:
the entire flasks used in each coupling reaction were meticu-
lously cleaned with aqua regia to avoid the presence of unseen
palladium catalyst.
8 (a) C. J. Richards, T. Damalidis, D. E. Hibbs and
M. B. Hursthouse, Synlett, 1995, 74; (b) C. J. Richards and
A. W. Mulvaney, Tetrahedron: Asymmetry, 1996, 7,
1419.
General procedure for Mizoroki–Heck coupling reactions
under microwave irradiation
9 (a) Y. Nishibayashi and S. Uemura, Synlett, 1995, 79;
(b) Y. Nishibayashi, K. Segawa, K. Ohe and S. Uemura, Orga-
nometallics, 1995, 14, 5486.
A 10 mL microwave-transparent process vial was filled with
aryl iodide (4 mmol), methyl acrylate (0.44 mL, 4.8 mmol),
base (6 mmol), 5 mL of solvent and the palladium complex.
The vial was sealed with PEEK snap caps and standard PTFE
coated silicone septa. The reaction mixture was then exposed
to microwave heating for the time stated in Table 5 at 160 °C.
The reaction vial was thereafter cooled to room temperature
and the mixture was diluted with 30 mL of water and extracted
with 3 × 20 mL of ether or hexane. The combined organic
layers were dried over anhydrous sodium sulfate. The crude
product was finally purified by flash column chromatography
on silica-gel to give the isolated products in yields stated in
Table 5. Note: The entire vials used in each coupling reaction
were meticulously cleaned with aqua regia to avoid the pres-
ence of unseen palladium catalysts.
10 For a review see: O. B. Sutcliffe and M. R. Bryce, Tetrahe-
dron: Asymmetry, 2003, 14, 2297.
11 (a) R. Peters and D. F. Fischer, Org. Lett., 2005, 7, 4137;
(b) D. F. Fischer, Z. Xin and R. Peters, Angew. Chem., Int.
Ed., 2007, 46, 7704; (c) S. Jautze and R. Peters, Angew.
Chem., Int. Ed., 2008, 47, 9284.
12 A.-C. Gaumont, M. Gulea and J. Levillain, Chem. Rev., 2009,
109, 1371.
13 (a) R. N. Hurd and G. DeLaMater, Chem. Rev., 1961, 61, 45;
(b) T. S. Jagodzinski, Chem. Rev., 2003, 103, 197.
14 (a) H. Alper, J. Organomet. Chem., 1984, 80, C-29;
(b) P. D. Beer, A. R. Graydon, A. O. M. Johnson and
D. K. Smith, Inorg. Chem., 1997, 36, 2112; (c) C. D. Hall,
I. P. Danks and N. W. Sharpe, J. Organomet. Chem., 1990,
390, 227; (d) R. W. Fish and M. Rosemblum, J. Org. Chem.,
1965, 30, 1253; (e) S. Kato, M. Wakamatsu and M. Mizuta,
J. Organomet. Chem., 1974, 78, 405; (f) S. Kato,
T. Fukushima, H. Ishirara and T. Murai, Bull. Chem.
Soc. Jpn., 1990, 63, 638; (g) J. O. Amupitan, Synthesis, 1983,
730; (h) K. Hamamura, M. Kita, M. Nomoyama and
J. Fujita, J. Organomet. Chem., 1993, 463, 169; (i) S. Jautze,
P. Seiler and R. Peters, Angew. Chem., Int. Ed., 2007, 46,
1260.
15 C. Sandoval-Chávez, J. G. López-Cortés, A. I. Gutiérrez-
Hernández, M. C. Ortega-Alfaro, A. Toscano and C. Alvarez-
Toledano, J. Organomet. Chem., 2009, 694, 3692.
16 (a) A. Tárraga, P. Molina, D. Curiel and D. Bautista, Tetra-
hedron: Asymmetry, 2002, 13, 1621; (b) P. Molina, A. Tárraga
and D. Curiel, Synlett, 2002, 435; (c) L. Bernardi,
B. F. Bonini, M. Comes-Franchini, C. Femoni, M. Fochi and
A. Ricci, Tetrahedron: Asymmetry, 2004, 15, 1133.
Acknowledgements
The authors would like to acknowledge the technical assist-
ance provided by Rocio Patiño, Luis Velasco, Javier Pérez,
Carmen Márquez, Eréndira Garcia, Nayeli López and Victor
Lemus. We would also like to thank the DGAPA IN-201411 and
CONACYT 153310 projects and CONACYT for the Ph.D. grant
extended to R. C.-S.
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