Dehydrogenative coupling of aromatic thiols with Et3SiH catalysed by N-heterocyclic carbene nickel complexes

Article abstract of DOI:10.1039/c3dt52052h

A series of new tetramethylcyclopentadienyl-functionalised N-heterocyclic carbene ligands with different wingtip substituents have been prepared and characterised. These ligands have been successfully coordinated to nickel affording complexes of the general type (Cp*-NHC^R)NiX (X = Cl, I). These well-defined nickel complexes selectively catalysed the coupling of aromatic thiols with Et₃SiH to give the corresponding silylthioethers (RSSiEt₃). The nickel complexes bearing ethyl, iso-butyl, and n-butyl wingtips displayed comparable catalytic efficiency, while the nickel complex bearing a methyl substituent on the wingtip was the worst performing catalyst.

Full text of DOI:10.1039/c3dt52052h

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ARTICLE TYPE
Dehydrogenative Coupling of Aromatic Thiols with Et3SiH Catalysed by
N-Heterocyclic Carbene Nickel Complexes
Lorena Postigo, Rita Lopes, and Beatriz Royo*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
A series of new tetramethylcyclopentadienylꢀfunctionalised Nꢀheterocyclic carbene ligands with different wingtip substituents
have been prepared and characterised. These ligands have been successfully coordinated to nickel affording complexes of the
general type (Cp*ꢀNHCR)NiX (X = Cl, I). These wellꢀdefined nickel complexes selectively catalysed the coupling of aromatic
10 thiols with Et3SiH to give the corresponding silylthioethers (RSSiEt3). The nickel complexes bearing ethyl, isoꢀbutyl, and nꢀbutyl
wingtips displayed comparable catalytic efficiency, while the nickel complex bearing a methyl substituent on the wingtip resulted
to be the worst performing catalyst.
R3
N
R5
Introduction
R2
R1
R5
R5
15 NꢀHeterocyclic carbene ligands (NHCs) have become one of
the most popular ligands in organometallic chemistry and
catalysis.1,2 The easy access to NHCs and their potential
application in a large number of catalytic processes, has led to
a rapid development in the design of a wide variety of NHCꢀ
20 containing architectures.3
During the last few years, our research group in
collaboration with Peris and coꢀworkers have developed a new
class of bidentate cyclopentadienyl ligands tethered to Nꢀ
heterocyclic carbenes (NHCs) (A, Scheme 1).4ꢀ12 Related
R5
MLn
R4
N
A
50
Scheme 1 Functionalisedꢀcyclopentadienyl Nꢀheterocyclic carbenes
One of the structural parameters that can be modified to
tune the stereoelectronic properties of the CpꢀNHC ligand is
the Nꢀsubsituent of the imidazolium ring. In this work, we
25 indenylꢀfunctionalised
NHCs
were
described
by
Danopoulos,13ꢀ16 Wang17 and others.18 An important feature of
these CpꢀNHC chelating ligands is the possibility of
independently vary their stuctural components, namely the
cyclopentadienyl ring, the spacer, and the imidazolium ring in
30 order to fine tuning the steric and electronic properties of
ligand. The introduction of chelating NHC ligands to the
coordination sphere of a metal catalyst have interesting
consequences in terms of enhancement of thermal stability
and rigidity. In addition, the introduction of chirality elements
35 in this system may assist in controlling the stereochemistry of
reactions taking place at the metal center. The versatile
coordination chemistry of these new ligands to a variety of
metals ranging from early to late transition metals is also an
interesting feature that we are exploring in our group.10 So far,
40 we have demonstrated the applicability of complexes of
general type A to a variety of catalytic applications such as
hydrogen transfer,4,8 hydrosilylation,8,11,12 epoxidation,6
amination of alcohols with primary amines,4 βꢀalkylation of
secondary alcohols,4,5 and isomerisation of allylic alcohols.7
45 We showed that the introduction of different substituents on
55 have prepared a new series of bidentate CpꢀNHC ligands
containing different substituents in the wingtip (R4) of the
imidazolium ring with the aim to investigate the consequences
that smooth modifications may have in catalysis. As a part of
our ongoing research into the development of new catalytic
60 applications of nickel complexes containing the fragment
“(Cp*ꢀNHC)Ni”,12 we decided to prepare a series of (Cp*ꢀ
i
n
NHCR)NiI (R = Bu, Bu, Et, Me) complexes and apply them
as catalysts for the dehydrogenative coupling of thiols with
silanes.
65 Results and discussion
Compound 1 was prepared following the procedure previously
described by us.4 Alkylation of 1 with the appropriate alkyl
halide in THF at 50 °C affords the corresponding imidazolium
proꢀligands 2a-2e with an overall yield of 70ꢀ87% (Scheme
70 2). Compounds 2 are obtained as a mixture of tautomers
resulting from the different position of the double bonds in the
cyclopentadienyl ring. Alkylation of 1 was limited to primary
alkyl halides as secondary and tertiary alkyls gave unwanted
elimination reactions. This is a general limitation for the
75 synthesis of unsymmetrical substituted NHCs.3a
the cyclopentadienyl ring have
a direct implication in
catalysis.6 Moreover, the introduction of a chiral center in the
CpꢀNHC tether have provided an easy access to chiral metal
complexes.7,9
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Scheme 2 Synthesis of proligands 2aꢀ2e
45 Scheme 3 Synthesis of nickel complexes 3a-Iꢀ3c-I. Complex 3d-Cl has
been already published by us.12
Proligands 2a, 2b and 2c were coordinated to nickel
following a similar method to that described for the synthesis
of (Cp*ꢀNHCMe)NiCl (3d-Cl) (Scheme 3), recently reported
by us.12 Treatment of 2aꢀ2c with two equivalents of nꢀBuLi
With these nickel complexes in our hands, we decided to
investigate the catalytic efficiency of complexes 3a-Iꢀ3d-I
(Scheme 4) in the dehydrogenative coupling of thiols (RSH)
50 with triethylsilane (Et3SiH). This reaction represents a useful
catalytic process for the preparation of thiosilanes
(RSSiEt3).20 To date, the are only very few reported examples
of metal complexes catalysing the coupling of thiols with
silanes.21ꢀ23 In 2011, Nakazawa described the unprecedented
55 dehydrogenative coupling of thiol with silanes catalysed by an
iron complex, the halfꢀsandwich CpFe(CO)2Me.24 In view of
these results, we became interested in exploring the reactivity
of our halfꢀsandwich NiꢀNHC complexes in this reaction.
The catalytic activity of complexes 3a-Iꢀ3d-I (Scheme 4)
60 was explored using thiophenol (PhSH) as a model substrate
with Et3SiH. The reaction was carried out in toluene at 80ºC
over 18 h, in the presence of 1 mol% of catalyst, and using a
Et3SiH: substrate ratio of 4:1.
5
i
generated in situ the corresponding (Cp*ꢀNHCR)Li (R = Bu,
nBu, Et) lithium salts, which were subsequently reacted with
NiCl2(DME) (DME = dimethoxyethane) to afford the novel
i
10 complexes (Cp*ꢀNHCR)NiI [R = Bu (3a-I), nBu (3b-I), Et
(3c-I)] in good yields (Scheme 3). All 3a-Iꢀ3c-I complexes
were isolated as crystalline red solids. The identity of all
compounds was established by analytical and spectroscopic
methods. The 1H NMR spectra of 3a-I, 3b-I and 3c-I show the
15 signals due to the nonꢀequivalent protons of the methyl
substituents of the cyclopentadienyl ring at δ 2.02, 1.93, 1.70
and 1.08 for 3a-I (similar set of signals are displayed for 3b-I
and 3c-I) confirming that coordination of the cyclopentadienyl
ring has occurred. Their 13C NMR spectra provide a direct
20 evidence of metalation as seen by the signal at δ 174 (for
complexes 3a-I, 3b-I and 3c-I), assigned to the NiꢀCCarbene
,
which is in the region of the previously reported (Cp*ꢀ
NHCMe)NiCl (3d-Cl) complex (δ 176),12 and other halfꢀ
sandwich NiꢀNHC complexes described in the literature.19
25 Elemental analysis indicated that the corresponding iodide
complexes 3a-I-3c-I were formed. The formation of the
iodides 3a-I-3c-I instead of the corresponding chlorides was
unexpected because similar reaction of proligand 2d with
NiCl2(DME) afforded the chloride complex (Cp*ꢀ
30 NHCMe)NiCl (3d-Cl) instead of the corresponding iodide.12
However, this halide exchange has been already observed by
us in the coordination reaction of proligand 2d to other
transition metals.6 The iodide complex (Cp*ꢀNHCMe)NiI (3d-
I) was obtained by treating 3d-Cl with KI in THF under reflux
35 for several hours.
Attempts to coordinate proligand 2e to nickel failed. The
reaction of proligand 2e with two equivalents of BuLi did not
afford the expected imidazolium salt as a pure sample. A
mixture of undentified compounds was obtained, probably as
40 a consequence of deprotonation of CH2 protons of the benzyl
wingtip, revealing that 2e is not tolerant to a strong base such
as BuLi.
65
Scheme 4 Nickel complexes investigated as catalysts for the
dehydrogenative coupling of thiols with Et3SiH
As shown in Table 1, the nickel complexes 3a-Iꢀ3d-I
catalysed the dehydrogenation of thiophenol affording the
70 corresponding silylthioether (PhSSiEt3) in good yields. The
reaction was selective to the formation of the silylthioether;
no homoꢀdehydrogenative coupling products such as disulfide
or disilane were detected in the reaction. No coupling was
observed in the absence of catalyst. The catalytic activity of
2
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i
complexes 3a-I, 3b-I, and 3c-I containing the Bu, nBu, and Et
The catalytic scope was evaluated with catalysts 3a-I and
3b-I using different aromatic thiols (Table 3). Thiophenol
derivatives containing functional groups such as methyl
(Table 3, entries 3 and 4) and methoxy (Table 3, entries 5 and
wingtips displayed similar catalytic activities, achieving good
yields of the corresponding PhSSiEt3 after 18 h at 80 °C
(Table 1, entries 1ꢀ3). In contrast, the nickel complex 3d-I
5
containing the methyl group in the wingtip, resulted to be less 50 6) were well tolerated for both 3a-I and 3b-I catalysts. A
active, affording 45
silylthioether under similar reaction conditions.
%
yield of the corresponding
detrimental in the yield was observed for the coupling of 2ꢀ
(trifluoromethyl)benzenethiol with silane, showing that thiols
having electronꢀwithdrawing groups display lower yields than
those containing electronꢀdonating groups (Table 3, entries 7
55 and 8). Low yields were obtained in the coupling of
cyclohexanethiol (26%) and benzyl mercaptan (14%) (Table
3, entries 9 and 10). Similar findings were reported for the
iron catalyst CpFe(CO)2Me.24
Table 1. Dehydrogenative coupling of PhSH with Et3SiH catalysed by
10 3a-Iꢀ3d-I, 3d-Cl, and 4[a]
60 Table 3. Dehydrogenative coupling of thiols with Et3SiH catalysed by
Entry
Catalyst
%Yield[b]
TON[c]
n
(Cp*ꢀNHCR)NiI (R = iBu (3a-I), Bu (3b-I)).[a]
1
2
3
4
5
6
3a-I
3b-I
3c-I
3d-I
3d-Cl
4
84
81
82
45
42
62
84
81
82
45
42
62
Entry
1
Substrate
Catalyst
%Yield[b]
84
SH
3a-I
[a]All reactions were carried out with 1.0 mmol of PhSH and 4 mmol
of Et3SiH using 1 mol% of catalyst in toluene at 80ºC for 18h.
15 [b]Yield determined by 1H NMR spectroscopy using Ph2CH2 (0.5
mmol) as internal standard. [c]TON (turnover number) calculated from
mol product/mol catalyst.
SH
SH
2
3
3b-I
81
80
3aꢀI
In order to investigate the effect of linking the
20 cyclopentadienyl ring to the NHC in the catalytic
performance, the catalytic activity of the related nonꢀlinked
Cp*Ni(NHCMe)I19a (4) was determined using similar reaction
conditions (in toluene at 80 ºC, 18 h). Complex 4 displayed
moderate catalytic activity (62% yield, Table 1, entry 6),
25 slighlty higher than that obtained by 3d-I (45% yield).
Additionally, the chloride complex (Cp*ꢀNHCMe)NiCl (3d-Cl)
was applied as catalyst under similar conditions to check the
influence of the halide in the performance of the catalyst. As
shown in Table 1, entries 4 and 5, no influence of the halide
30 was observed.
Under similar reaction conditions, the catalyst loading of
complex 3a-I could be lowered to 0.1 mol % achieving
turnover numbers of 650 (Table 2, entry 4), showing better
catalytic performance than the previously reported iron
35 complex CpFe(CO)2Me (TON 5.8 after 24 h in toluene at 80
ºC and using 1:10 PhSH:Et3SiH ratio).24
SH
SH
4
5
3b-I
75
73
3aꢀI
MeO
MeO
SH
6
7
3b-I
96
60
SH
3aꢀI
CF3
SH
8
3b-I
38
Table 2. Dehydrogenative coupling of PhSH with Et3SiH catalysed by
(Cp*ꢀNHCiBu)NiI (3a-I).[a]
CF3
SH
40
9
3a-I
3a-I
26
14
Entry
%mol cat.
%Yield[b]
TON[c]
1
2
3
4
1
84
80
78
65
84
0.5
0.25
0.1
160
312
650
SH
10
[
a]All reactions were carried out with 1.0 mmol of PhSH and 4 mmol
of Et3SiH in toluene at 80ºC for 18h. [b]Yield determined by H NMR
spectroscopy using Ph2CH2 (0.5 mmol) as internal standard. [c]TON
(turnover number) calculated from mol product/mol catalyst.
1
[a]All reactions were carried out with 1.0 mmol of PhSH and 4 mmol
of Et3SiH using 1 mol% of catalyst in toluene at 80ºC for 18h.
65 [b]Yield determined by 1H NMR spectroscopy using Ph2CH2 (0.5
45
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DOI: 10.1039/C3DT52052H
mmol) as internal standard.
55
Synthesis of Cp*-NHCnBuI, 2b. A procedure similar to that
used for the preparation of 2a was applied by using
iodobutane (0.98 mL, 8.54 mmol) and 1 (0.50 g, 1.70 mmol)
Yield: 0.69g (86%). 1HꢀNMR (CDCl3): 10.55, 10.43, 10.17,
60 10.12, 10.08, 9.62, 9.58 (s, N=CHꢀN), 7.80ꢀ7.01 (CHPh,
CHImid), 5.90ꢀ5.57 (CHlinker), 4.37ꢀ3.99 (m, CH2nBu), 3.43ꢀ
2.98 (m, CH2linker, CH2iBu, CHCp*), 1.80ꢀ0.88 (C5Me4, nBu).
13C{1H}ꢀNMR (CDCl3): 135.2 (N=CHꢀN), 131.0ꢀ120.3
(CHPh, CHImid), 64.5ꢀ62.1 (CHlinker) 51.8ꢀ48.9 (CHCp*), 49.9ꢀ
65 49.7 (CH2linker, CH2nBu), 32.3ꢀ31.7 (CH2linker, CH2nBu), 14.7ꢀ
10.8 (C5Me4, nBu). Anal. Calc for C24H33N2I (476.44): C,
60.50; H, 6.98; N, 5.88. Found: C, 60.35; H, 6.72; N, 5.54.
MS (ESIꢀTOF): m/z [MꢀI]+ calcd for C24H33N2, 349; found:
349 [MꢀI]+.
Conclusions
In summary, we have described the preparation of new
bidentate Cp*ꢀNHCR ligands and their coordination to nickel.
A series of (Cp*ꢀNHCR)NiX (X = Cl, I) complexes bearing
different substituents on the wingtip have been applied as
catalysts for the dehydrogenative coupling of aromatic thiols
with Et3SiH. Interestingly, nickel complexes bearing ethyl,
10 isoꢀbutyl and nꢀbutyl wingtips displayed comparable catalytic
activity, while the nickel complex containing the methyl
substituent on the wingtip resulted to be the worst performing
catalyst. This is the first report of wellꢀdefined nickel(II)
complexes catalysing the coupling of thiols with Et3SiH.
15
5
70
Synthesis of Cp*-NHCEtI, 2c. A procedure similar to that
used for the preparation of 2a was applied by using
iodoethane (0.55 mL, 6.84 mmol) and 1 (0.40 g, 1.36 mmol).
Experimental Section
1
Materials and methods
Yield: 0.42 g (70%). H NMR (CDCl3): 10.47, 10.40, 10.06,
75 10.03, 9.43 (s, N=CHꢀN), 7.75ꢀ7.01 (CHPh, CHImid), 5.83ꢀ5.53
Compounds 1,4 3d-Cl12 and 419a were synthesised according
to the methods described in the literature. All other reagents
20 were used as received from commercial suppliers without
further purification. All manipulations were carried out under
a nitrogen atmosphere using standard Schlenk techniques and
solvents were purified from appropriate drying agents.
Deuterated solvents were degassed and stored over molecular
25 sieves. 1H, 13C, and 29Si NMR spectra were recorded on a
Bruker Avance III 400 MHz. Assigment of resonances was
made from HSQC and HMBC experiments. Electrospray mass
spectra (ESIMS) were recorded on a Micromass Quatro LC
instrument; nitrogen was employed as drying and nebulising
30 gas.A QTOF I (quadrupoleꢀhexapoleꢀTOF) mass spectrometer
with an orthogonal Zꢀsprayꢀelectrꢀspray interface (Micromass,
Manchester, UK) was used for highꢀresolution mass
spectrometry (HRMS). The drying gas as well as nebulizing
gas nitrogen at a flow of 400 and 80 L/h, respectively.
35 Elemental analyses were performed in our laboratories at
ITQB.
(m, CHlinker), 4.43ꢀ4.10 (m, CH2Et), 3.47ꢀ 2.29 (m, CH2linker
CHCp*), 1.81ꢀ0.83 (C5Me4, Et). 13C{1H}ꢀNMR (CDCl3): 140.0
(N=CHꢀN), 131.0ꢀ119.5 (CHPh, CHImid), 69.4ꢀ64.0 (CHlinker
,
)
57.2ꢀ49.1 (CHCp*), 45.6ꢀ45.4 (CH2Et), 32.6ꢀ31.4 (CH2linker),
80 15.7ꢀ11.5 (C5Me4, Et). Anal. Calc for C22H29N2I (448.38): C,
58.93; H, 6.51; N, 6.24. Found: C, 58.61; H, 6.24; N, 5.86.
Synthesis of Cp*-NHCBzI, 2d. A procedure similar to that
used for the preparation of 2a was applied by using
85 benzylbromide (1.05 mL, 8.85 mmol), NaI (1.32 g, 8.85
mmol) and 1 (0.52 g, 1.77 mmol). Yield: 0.77 g (86%). 1H
NMR (CDCl3): 10.50, 10.50, 10.37, 10.05, 10.02, 9,81 (s,
N=CHꢀN), 7.63ꢀ6.92 (CHPh, CHImid), 5.74ꢀ5.40 (CHlinker
,
CH2Bz), 3.46ꢀ2.20 (m, CH2linker, CHCp*), 1.82ꢀ0.81 (C5Me4).
90 13C{1H}ꢀNMR (CDCl3): 136.4 (N=CHꢀN), 133.2ꢀ120.2
(CHPh
, CHImid), 64.5 (CHlinker), 53.8ꢀ51.8 (CH2Bz), 49.0
(CHCp*), 31.8ꢀ31.4 (CH2linker), 15.2ꢀ10.9 (C5Me4). Anal. Calc
for C27H31N2I (510.15): C, 63.53; H, 6.12; N, 5.49. Found: C,
63.20; H, 5.84; N, 5.25. MS (ESIꢀTOF): (m/z) [MꢀI]+ calcd for
95 C27H31N2 383; found: 383 [MꢀI]+.
Synthesis of Cp*-NHCiBuI, 2a. 1ꢀIodoꢀ2ꢀmethylpropane
(1mL, 8.54 mmol) was added to a solution of 1 (0.56 g, 1.19
40 mmol) in 10 mL of THF. The mixture was stirred at 50ºC for
3 days. All volatiles were removed under vacuum to afford a
yellow solid which was washed several times with ethyl ether
and hexane to yield compound 2a as a yellow solid. Yield:
Synthesis of (Cp*-NHCiBu)NiI, 3a-I. Two equivalents of nꢀBuLi
(0.86 mL, 1.36 mmol) were added to a solution of 2a (0.30 g,
0.62 mmol) in THF (10 mL) at ꢀ60ºC. The mixture was stirred for
100 1 h at room temperature, and a suspension of NiCl2(DME) (0.14
g, 0.62 mmol) in THF (5 mL) was added at once. The reaction
mixture was then stirred overnight at room temperature. The
solvent was removed under vacuum, and the remaining solid was
extracted in a mixture of toluene/hexane (15 mL/5 mL), yielding
1
0.56 g (70%). HꢀNMR (CDCl3): 10.68, 10.55, 10.49, 10.26,
45 10.20, 10.16 (s, N=CHꢀN), 7.66ꢀ7.01 (m, CHPh, CHImid), 5.97ꢀ
5.73 (m, CHlinker), 4.23ꢀ4.00 (m, CH2linker, CH2iBu)), 3.46ꢀ 2.98
(m, CH2linker, CH2iBu), 2.15 (m, CHCp*, CHiBu), 1.82ꢀ1.76
(C5Me4), 1.05ꢀ0.84 (C5Me4, iBu). 13C{1H}ꢀNMR (CDCl3):
1
105 3 as a red crystalline solid. Yield: 0.20 g (74%). HꢀNMR (C6D6):
7.07ꢀ6.90 (m, 5 H, CHPh), 6.05 (s, 1 H, CHImid), 5.85 (s, 1 H,
CHImid), 5.19 (dd, J3 = 8.84, 5.60 , 1 H, CHlinker), 5.08 (dd, 1 H,
140.0 (N=CHꢀN), 131.3ꢀ128.1 (CHPh, CHImid), 63.9 (CHlinker
)
,
2
2
iBu, J = 13 Hz, 3J = 6 Hz), 3.59 (dd, 1 H, iBu, J = 13 Hz, 3J = 6
50 57.2 (CH2linker
,
CH2iBu), 49.0 (CH, iBu), 31.6 (CH2linker
i
i
Hz), 2.90 (m, 1 H, Bu), 2.02 (s, 3 H, C5Me4), 2.12 (m, 2 H,
CH2iBu), 29.7ꢀ29.6 (CHCp*, CHiBu), 19.5ꢀ10.8 (C5Me4, Bu).
Anal. Calc for C24H33N2I (476.44): C, 60.50; H, 6.98; N, 5.88.
Found: C, 60.70; H, 6.97; N, 5.81. HRMS (ESIꢀTOF): m/z
[MꢀI]+ calcd for C24H33N2, 349.2644; found: 349.2642 [MꢀI]+.
110 CH2linker) 1.93 (s, 3 H, C5Me4), 1.70 (s, 3 H, C5Me4), 1.08 (s, 3 H,
3
C5Me4), 0.99 (d, 3 H, J = 7.2 Hz, iBu), 0.80 (d, 3 H, 3J = 7.2 Hz,
iBu). 13C{1H}ꢀNMR (C6D6): 174.5 (NiꢀCCarbene), 137.9 (CiPh),
4
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130.8 (CHPh), 129.5 (CHPh), 128.4 (CHPh), 126.1 (CHPh), 122.9
(s, 3 H, C5Me4), 1.73 (s, 3 H, C5Me4), 1.04 (s, 3 H, C5Me4).
(CHImid), 119.7 (CHImid), 105.1 (C5Me4), 104.4 (C5Me4), 102.8 60 13C{1H}ꢀNMR (C6D6): 176.1 (NiꢀCCarbene), 137.7 (CiPh), 129.0
(C5Me4), 101.2 (C5Me4), 89.1 (C5Me4), 66.6 (CHlinker), 60.1 (CH2,
(CHPh), 128.9 (CHPh), 128.3 (CHPh), 122.4 (CHImid), 120.2
(CHImid), 104.8 (C5Me4), 104.5 (C5Me4), 103.1 (C5Me4), 100.0
(C5Me4), 89.0 (C5Me4), 66.8 (CHlinker), 40.9 (NMe), 30.3
(CH2linker), 12.4 (C5Me4), 10.8 (C5Me4), 10.6 (C5Me4), 9.7
i
i
iBu), 30.2 (CH2linker), 30.1 (CH, Bu), 20.1 (Me, Bu), 20.0 (Me,
iBu), 12.4 (C5Me4), 10.7 (C5Me4), 10.6 (C5Me4), 10.0 (C5Me4).
Anal. Calc for C24H31N2NiI (533.11): C, 54.07; H, 5.86; N, 5.25.
5
Found: C, 53.80; H, 5.52; N, 4.86. HRMS (ESIꢀTOF): m/z [MꢀI]+ 65 (C5Me4). Anal. Calc for C21H25N2NiIꢁC7H8 (582.10): C, 57.67;
calcd for C24H31N2Ni, 405.1841; found: 405.1841 [MꢀI]+.
H, 5.70; N, 4.80. Found: C, 57.60; H, 5.52; N, 5.15.
10
Catalytic dehydrogenative coupling of thiols with Et3SiH.
Synthesis of (Cp*-NHCnBu)NiI, 3b-I. A procedure similar to
Toluene (0.5 mL), (Cp*ꢀNHCR)NiX (1 mol%), thiol (1 mmol),
that used for the preparation of 3a-I was applied by using 70 Et3SiH (4 mmol), and Ph2CH2 (0.5 mmol) used as internal
proligand 2b (0.30 g, 0.62 mmol), nꢀBuLi (0.86 mL, 1.36
standard, were charged in a vial. The vial was tight closed
under nitrogen, and the solution was heated at 80ºC for 18 h.
All volatiles were then removed under vacuum, and the
residue was dissolved in CDCl3. The amount of the
mmol), and NiCl2(DME) (0.14 g, 0.62 mmol). Yield: 0.21 g
1
15 (78%). HꢀNMR (C6D6): 7.17ꢀ6.94 (m, 5 H, CHPh), 6.05 (s, 1
H, CHImid), 5.85 (s, 1 H, CHImid), 5.19 (m, 1 H, CHlinker), 5.08
(m, 1 H, Bu), 4.04 (m, 1 H, Bu), 2.02 (s, 3 H, C5Me4), 2.01 75 corresponding triethylsilylthioether formed was determined by
(m, 2 H, CH2linker), 1.97 (s, 3 H, C5Me4), 1.90 (m, 2H, Bu),
1.70 (s, 3 H, C5Me4), 1.08 (s, 3 H, C5Me4), 1.29 (q, 2 H, Bu),
20 0.97 (m, 3 H, Bu). 13C{1H}ꢀNMR (C6D6): 174.3 (NiꢀCCarbene),
137.9 (CiPh), 129.8 (CHPh), 128.35 (CHPh), 127.2 (CHPh),
121.9 (CHImid), 120.2 (CHImid), 104.9 (C5Me4), 104.4 (C5Me4),
102.7 (C5Me4), 100.8 (C5Me4), 89.2 (C5Me4), 66.7 (CHlinker),
52.7 (CH2, Bu), 33.5 (CH2linker), 30.2 (CH, Bu), 19.7 (Me, Bu),
25 12.4 (Me, Bu), 11.9 (C5Me4), 11.3 (C5Me4), 11.1 (C5Me4),
10.7 (C5Me4). Anal. Calc for C24H31N2NiIꢁC7H8 (625.25): C,
59.55; H, 6.29; N, 4.48. Found: C, 59.30; H, 6.27; N, 4.88.
HRMS (ESIꢀTOF): m/z [MꢀI]+ calcd for C24H31N2Ni,
405.1841; found: 405.1836 [MꢀI]+.
1H NMR by the relative intensity of signals of the product and
the Ph2CH2 used as internal standard. The
triethylsilylthioether produced was identified by comparison
of the NMR spectra with reported data.21e,24
80 Acknowledgements
We gratefully acknowledge financial support from FCT of
Portugal, POCI 2010 and FEDER through projects
PTDC/QUIꢀQUI/110349/2009
QUI/099389/2008. We
and
thank
PTDC/QUIꢀ
FC&T for
85 REDE/1517/RMN/2005 and REDE/1504/REM/2005. We wish
to acknowledge M.C. Almeida and Dr. A. Coelho for
providing data from the Elemental Analyses and Mass
Spectrometry Services at ITQB.
30
Synthesis of (Cp*-NHCEt)NiI, 3c-I. A procedure similar to
that used for the preparation of 3a-I was applied by using
proligand 2c (0.20 g, 0.44 mmol), nꢀBuLi (0.63 mL, 0.98
mmol), and NiCl2(DME) (0.10 g, 0.44 mmol). Yield: 0.15 g
1
90 Notes and references
35 (72%). HꢀNMR (C6D6): 7.08ꢀ6.88 (m, 5 H, CHPh), 6.97 (s, 1
3
H, CHImid), 5.82 (s, 1 H, CHImid), 5.22 (dd, 1 H, J = 10.6 Hz,
Instituto de Tecnología Química e Biológica da Universidade Nova de
Lisboa. Av. da República, EAN,2780-417Oeiras. Portugal.; E-mail:
broyo@itqb.unl.pt
3J = 3.5 Hz, CHlinker), 4.85 (sex, 1 H,CH2Et), 4.17 (sex, 1
H,CH2Et), 2.10 (m, 2 H, CH2linker), 2.07 (s, 3 H, C5Me4), 1.99
(s, 3 H, C5Me4), 1.73 (s, 3 H, C5Me4), 1.30 (t, 3 H, MeEt), 1.05
40 (s, 3 H, C5Me4). 13C{1H}ꢀNMR (C6D6): 174.5 (NiꢀCCarbene),
137.7 (CiPh), 129.8 (CHPh), 129.1 (CHPh), 128.9 (CHPh), 128.5
(CHPh), 121.0 (CHImid), 120.4 (CHImid), 105.0 (C5Me4), 104.5
(C5Me4), 102.8 (C5Me4), 100.2 (C5Me4), 89.3 (C5Me4), 66.3
(CHlinker), 47.8 (CH2, Et), 30.2 (CH2linker), 16.4 (Me, Et), 12.4
45 (C5Me4), 11.1 (C5Me4), 10.7 (C5Me4), 9.9 (C5Me4). Anal. Calc
for C22H27N2NiI (505.05): C, 52.31; H, 5.39; N, 5.55. Found:
C, 51.97; H, 5.08; N, 5.16.
95 † Electronic Supplementary Information (ESI) available: NMR and
HRMS spectra of coumpounds.
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DOI: 10.1039/C3DT52052H
"textual abstract for the contents use only"
Lorena Postigo, Rita Lopes, and Beatriz Royo*
Dehydrogenative Coupling of Aromatic Thiols with Et3SiH catalysed by N-Heterocyclic
Carbene Nickel Complexes
A series of well-defined nickel(II) complexes, (Cp*-NHCR)NiX (R = iBu, nBu, Et, Me; X
= I, Cl), effciently catalysed the selective dehydrogenative coupling of thiols with Et3SiH
affording the corresponding silylthioethers in good yields.
"graphical abstract for the contents use only"
SH
+
S
Ni cat., 80 0C
18h
SiEt3
Et3SiH
+ H2
Well-defined Ni-NHC catalysts
Ph
Ph
Ni
Ni
N
I
N
I
N
N
Me
Ph
Ni
Ph
Ni
N
I
N
I
N
N