Mechanistic insight into the pyridine enhanced α-selectivity in alkyne hydrothiolation catalysed by quinolinolate-rhodium(I)-N-heterocyclic carbene complexes

Article abstract of DOI:10.1039/c6cy01884j

RhI-NHC-olefin complexes bearing a N,O-quinolinolate bidentate ligand have been prepared from [Rh(μ-OH)(NHC)(η²-olefin)]₂ precursors (olefin = cyclooctene, ethylene). The disposition of the chelate ligand with regard to the carbene i

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Sci. Technol., 2016, DOI: 10.1039/C6CY01884J.
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REVISED
Received 00th January 20xx,
Accepted 00th January 20xx
Mechanistic Insight into the Pyridine Enhanced α-Selectivity in
Alkyne Hydrothiolation Catalysed by
DOI: 10.1039/x0xx00000x
Quinolinolate−Rhodium(I)−N-Heterocyclic Carbene Complexes
www.rsc.org/
Laura Palacios,a Andrea Di Giuseppe,a María José Artigas,a Victor Polo,b Fernando J. Lahoz,a
Ricardo Castarlenas,*a Jesús J. Pérez-Torrente,a and Luis A. Oroa,c
RhI-NHC-olefin complexes bearing
a N,O-quinolinolate bidentate ligand have been prepared from [Rh(μ-
OH)(NHC)(η2-olefin)]2 precursors (olefin = cyclooctene, ethylene). The disposition of the chelate ligand with regard
to the carbene in the square planar derivatives is strongly influenced by the steric hindrance exerted by the
coordinated olefin. These complexes efficiently catalyzed the addition of thiophenol to phenylacetylene with good
selectivity to α-vinyl sulfides, which can be increased up to 97% by addition of pyridine. Several key intermediates
have been detected including the η1-alkenyl species resulting from alkyne insertion into a Rh-H bond. DFT
calculations on the mechanism support a hydrometallation pathway that entails the oxidative addition of thiol, 2,1-
insertion of the alkyne into the Rh-H bond, and reductive elimination as the rate-determining step. Remarkably,
coordination of pyridine to the β-alkenyl intermediate but not to the α-alkenyl, which results in a net stabilization,
is the key for the Markovnikov selectivity.
methodologies.5a,d,e,f,h,j,n,r,s
Introduction
Precise determination of the mechanism of a catalytic process
is essential in order to control the selectivity outcome.
Particularly, alkyne hydrothiolation is
a valuable atom-
economical transformation for the straightforward synthesis of
vinyl sulfides, but the control of chemo-, regio- and
stereoselectivity still constitutes
a stimulating challenge
Scheme 1. Alkyne hydrothiolation reaction.
(Scheme 1).1 Several methodologies are available for the
addition of the S-H bond to an unsaturated fragment, including
radical initiated,2 and acid3 or base4 promoted, but
organometallic catalysts offer mild reaction conditions and the
potential for rational design of the catalyst.5 Regarding the
regioselectivity of the transformation, research efforts have
been directed towards the preparation of the more valuable α-
vinyl sulfides,6 which are difficult to synthesize by other
Pyridine has demonstrated its versatility as additive for a
broad spectrum of catalytic reactions. Its contribution to the
development of “ligand accelerated catalysis” concept is a
prototypical example.7 The particular nucleophilic character
and its relative small size makes it a suitable ligand for
preparation of catalytic precursors as well as for stabilizing
organometallic
catalytic
intermediate
species.
An
enhancement of catalytic activity has been observed for
oxidation,8 epoxidation,9 C-H bond functionalization
reactions,10 as well as alkene oligomerization,11 among
others.12 Noteworthy, pyridine ligands also play an important
role in some precatalysts for olefin metathesis13 or the Pd-
PEPPSI systems.14 However, in many cases its role has not
been fully understood.
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We have recently disclosed new rhodium(I)-N-heterocyclic
Journal Name
Results and Discussion
carbene (NHC)15 catalysts for which the selectivity towards the
Markovnikov-type adducts is remarkably increased by using
pyridine as additive.16 It has been observed that dinuclear
precursors of type [Rh(μ-Cl)(NHC)(η2-olefin)]2 favour the
formation of linear thioethers, whereas the selectivity is
switched towards branched gem-vinyl sulfides by the
Synthesis of Rh-IPr-Quinolinolate Complexes
Previous studies in our group have shown that dinuclear
species [Rh(μ-Cl)(IPr)(η2-olefin)]2
{
IPr
diisopropylphenyl)imidazol-2-carbene; olefin
(coe) ( ), ethylene (
to form RhIII complexes as a result of the oxidative addition of
the O-H bond.20d,g In contrast, the precursors [Rh(
=
1,3-bis-(2,6-
=
cyclooctene
mononuclear
catalysts
RhCl(NHC)(pyridine)(η2-olefin),
1
2)} readily reacts with 8-hydroxyquinoline
resulting from the bridge-cleavage by the nucleophilic ligand
(Scheme 2). In fact, an equilibrium between both type of
species is established, rendering the concentration of pyridine
essential for the control of regioselectivity. However, catalytic
activity was inhibited when the reaction was performed in
neat pyridine, indicating that the ligand competes with
substrates for coordination to the active species. A ten-fold
ꢀ
-
OH)(IPr)(η2-olefin)]2 {coe (
3), ethylene (4)}, bearing basic
hydroxo ligands, have allowed the preparation of RhI-
quinolinolate species (Scheme 3). Treatment of a yellow
solution of
instantaneous formation of a tile red solution from which
compound Rh{κ2-O,N-(C9H6NO)}(η2-coe)(IPr) (
), was isolated
3 with 8-hydroxyquinoline resulted in the
pyridine/catalyst ratio was chosen in order to get
a
5
compromise between activity and selectivity. We now
hypothesize that the equilibrium shift to the mononuclear
species would increase Markovnikov selectivity. Thus, we
envisage anchoring of the pyridine moiety to the catalyst by
means of a chelating N-donor ligand. Indeed, a chelate ligand
may increase the stability of the catalyst while the structural
rigidity of the system would impart a higher steric influence,
resulting in a better control of selectivity. A promising
candidate that fulfils these requirements is the 8-quinolinolate
ligand, as it generally forms robust chelate complexes17 that
have been extensively used in photoluminescent,18
biological,19 and catalytic20 applications.
in good yield as a 55:45 thermodynamic mixture of two
structural isomers, 5a and 5b. In contrast, the ethylene
precursor
(C9H6NO)}(η2-CH2=CH2)(IPr)
Complex can be alternatively prepared by bubbling ethylene
through a solution of both isomers of
The 1H-NMR spectrum of
shows two set of signals
4
gave rise to the formation of Rh{κ2-O,N-
(6) as a single stereoisomer.
6
5.
5
corresponding to each isomer which have different disposition
of the N,O-quinolinolate ligand. The characteristic H(2) of the
quinolinolate ligand appear as doublets at δ 8.33 ppm (JH-H
=
4.9 Hz) for 5a and 7.84 ppm (JH-H = 4.5 Hz) for 5b, whereas the
olefinic protons of coordinated coe resonate at 3.04 (5a) and
3.44 ppm (5b). The more significant resonances in the 13C{1H}-
APT NMR spectrum are two set of doublets at δ 184.7 (JC-Rh
=
59.2 Hz, Rh-CIPr) and 55.3 ppm (JC-Rh = 15.4 Hz, Rh-Ccoe) for 5a
and 193.0 (JC-Rh = 56.2 Hz, Rh-CIPr) and 65.9 ppm (JC-Rh = 15.0
1
Hz, Rh-Ccoe) for 5b. The H-NMR spectrum of
6 shows a single
resonance for the H(2) proton at δ 6.77 ppm (dd, JH-H = 4.9, 1.0
Hz), while the 13C{1H}-APT NMR spectrum displays two
doublets at 186.8 ppm (JC-Rh = 56.7 Hz) and 36.6 ppm (JC-Rh
=
16.2 Hz) for the carbene and ethylene ligands, respectively,
which is in agreement with the presence of a single isomer. In
addition, a rotational process for the IPr and ethylene ligands
is observed.21
Scheme 2. Strategy for selectivity enhancement by anchorage of
the N-donor ligand.
Herein, we report on the preparation of NHC-RhI-
quinolinolate derivatives as efficient catalyst for alkyne
hydrothiolation. A mechanistic study about the effect of the
nitrogenated ligands into the regioselectivity has been
undertaken.
Scheme 3. Synthesis of RhI-quinolinolate complexes 5-6.
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The 1H 1D-NOESY experiments confirms the structural pyridine shows an electronic preference to coordinate
assignment for 5a 5b and in Scheme 3 (see SI). Irradiation of opposite to the high trans-influence ligand IPr. Indeed, a trans
,
6
the major H(2) quinolinolate signal at δ 8.33 ppm showed NOE disposition between the more π-acceptor ligand, the olefin,
effect with coe ligand, indicating that the nitrogen atom is and the more electron-donating oxygen atom of the
disposed trans to IPr in 5a. Similarly, the interaction between quinolinolate ligand should be also favoured, as it has been
the minor H(2) resonance at δ 7.84 ppm with the substituents observed in related complexes.23a,24 Thus, the formation of the
of the carbene confirms a mutually cis disposition in 5b. In unexpected isomer 5b for the coe derivative could be a
addition, exchange peaks between quinolinolate resonances of consequence of the interplay between steric and electronic
both isomers were observed indicating a dynamic equilibrium. factors. In order to shed light into the influence of the
On the other hand, irradiation of the ethylene resonance of
6
coordinated olefin into the disposition of the quinolinolate
caused NOE effect on H(2) quinolinolate proton pointing to ligand, the synthesis of rhodium complexes with the bulkier 2-
trans disposition of the nitrogen atom and the IPr ligand. The methyl-8-quinolinolate ligand was undertaken (Scheme 4).
1H-15N HMBC spectra also support the above structural Treatment of
3
with 2-methyl-8-hydroxyquinoline in toluene-
assignment. Two signals are observed for the N-donor ligand d8 at -40 °C resulted in the formation of Rh{κ2-O,N-
at δ 252.8 (5a) and 248.3 ppm (5b), comparative more [(C9H5NO)(CH3)]}(η2-coe)(IPr) (
). Unfortunately, decomposes
7
7
shielded than the free ligand (288.0 ppm), as a result of at room temperature and could not be isolated, but it was fully
coordination to the rhodium atom (see SI).22 The quinolinolate characterized at low temperature by NMR, showing the
nitrogen atom of
6
resonates at 253.3 ppm which is in presence of a single stereoisomer. Similarly, the ethylene
precursor
accordance with a stereochemistry similar to 5a
.
4
gave a single isomer of the complex Rh{κ2-O,N-
The solid state structure for 5b was determined by an X-ray [(C9H5NO)(CH3)]}(η2-CH2=CH2)(IPr) (
diffraction analysis (Figure 1). The rhodium atom presents a orange solid in 65% yield.
distorted square-planar geometry with the nitrogen atom of
8) that was isolated as an
the quinolinolate ligand located cis to the IPr {C(1)-Rh-N(1) =
94.51(14)°}. The rhodium-carbon separation {Rh−C(1) =
1.943(4) Å} lays in the range of short distances previously
reported for a Rh-NHC single bond. In contrast, the Rh-N(1)
{2.137(3) Å} and Rh-O(1) {2.104(3) Å} bonds are rather long.
The olefinic separation of 1.390(6) Å agrees with that reported
for other Rh-coe-NHC complexes.16b,21,23 The quinolinic and
imidazole rings are almost planar and disposed perpendicularly
each other forming a dihedral angle of 89.88(11)°.
Scheme 4. Synthesis of 2-methyl-8-quinolinolate complexes 7 and
8.
The molecular structure of compounds
7 and 8 has been
determined by X-ray diffraction analysis (Figure 2). Both
complexes adopt a distorted square planar configuration with
the nitrogen atom of the 2-methyl-8-quinolinolate ligand
coordinated trans to the coe ligand in
101.03(7)°}, as in the isomer 5b, but trans to IPr ligand in
{C(1)-Rh-N(3) = 169.83(6)°}, similarly to complex . The Rh-CIPr
distances are comparative short with regard to previous Rh-
NHC complexes { , 1.9495(18); , 1.9666(15) Å}. The presence
of a methyl substituent causes a lengthening of the Rh-N bond
7 {C(1)-Rh-N(3) =
8
Figure 1. Molecular structure of 5b. Selected bond lengths (Å) and
angles (°): Rh-C(1) = 1.943(4), Rh-N(1)= 2.137(3), Rh-O(1) = 2.104(3),
Rh-C(37) = 2.109(4), Rh-C(38) = 2.108(4), C(37)-C(38) = 1.390(6),
C(1)-Rh-N(1) = 94.51(14), C(1)-Rh-O(1) = 172.91(13), C(1)-Rh-Ct* =
92.50(14), N(1)-Rh-O(1) = 78.88(11), N(1)-Rh-Ct* = 171.18(12) (Ct*
represents the midpoint of the olefinic bond).
6
7
8
from 2.137(3) Å in 5b to 2.2809(16) Å in
7
, and a slight
shortening of the Rh-O bond {2.0785(14) in 7 vs 2.104(3) in
5b}. In addition, the N-Rh-IPr angle increase from 94.51(14)°
(
5b) to 101.03(7)° (
7) showing once again the steric influence.
The existence of two isomers for
5 is a rather unexpected
result. Recent calculations in our group for a series of Rh-IPr
complexes23d have revealed that a N-donor ligand such as
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Figure 2. Molecular structures of 7 and 8. Selected bond lengths (Å)
and angles (°). 7: Rh-C(1) = 1.9495(18), Rh-N(3)= 2.2809(16), Rh-
O(1) = 2.0785(14), Rh-C(38) = 2.0996(19), Rh-C(39) = 2.0905(19),
C(38)-C(39) = 1.396(3), C(1)-Rh-N(3) = 101.03(7), C(1)-Rh-O(1) =
176.75(7), N(3)-Rh-O(1) = 77.58(6), C(1)-Rh-Ct* = 90.04(8). 8: Rh-
C(1) = 1.9666(15), Rh-N(3)= 2.1809(13), Rh-O(1) = 2.0478(11), Rh-
C(39) = 2.0772(17), Rh-C(40) = 2.1293(18), C(39)-C(40) = 1.392(3),
C(1)-Rh-N(3) = 169.83(6), C(1)-Rh-O(1) = 90.33(5), N(3)-Rh-O(1) =
79.51(5), C(1)-Rh-Ct* = 90.28(4) (Ct* represents the midpoint of the
olefinic bond).
Figure 3. DFT-optimized ground state minimum energy structures
for complexes 5-8.
The solid state structures of
7 and 8 are maintained in
solution as inferred from 1H and 13C{1H} NMR data. The methyl
substituent of the quinolinolate ligand exerts a significant
steric effect that influences the intrinsic dynamic behaviour of
the complexes. Thus, in contrast to 5b, the rotation of the IPr
ligand is hindered in complex 7 25
rotation is hampered in which contrast to the free rotation
observed at room temperature in 6 26
Theoretical DFT calculations on the ground state energies
of complexes were performed in order to corroborate the
preferential disposition of the quinolinolate ligand in RhI−π-
olefin complexes (Figure 3). In accordance with the
experimental observations, the configuration with the nitrogen
atom of the quinolinolate ligand trans to the carbene is more
.
In the same way, ethylene
8
.
Figure 4. Comparison of the steric hindrance exerted by coe and IPr
ligands on the 2-methyl-quinolinolate ligand in 7.
5-8
Catalytic Activity in Alkyne Hydrothiolation
stable for
minimum for
for agrees with the observation of both isomers. The
unexpected disposition of the quinolinolate ligand found in
complexes and is a consequence of the steric influence
5
,
6
and
8, whereas the opposite configuration is the
7
. The energy separation of only 0.6 kcal mol-1
Catalytic activity of complexes
5, 6 and 8 for alkyne
5
hydrothiolation was evaluated. The addition of thiophenol to
phenylacetylene using a 1:1 ratio was chosen as benchmark
reaction. Catalytic reactions were monitored in NMR tubes
containing 0.5 mL of C6D6 and 2 mol% of catalyst loading at 25
°C (Table 1, Figure 5).
5
7
imparted by the coe ligand. Careful observation of the putative
complex 7at evidences a harsh steric hindrance between the
methyl group and coe if compared with that exerted by the IPr
in 7bt (Figure 4). The fact that the steric influence of coe in
these complexes is greater than that of the carbene could be
in principle paradoxical due to the bulkiness of the IPr ligand.15
However, the anisotropy of IPr allows the methyl group of the
quinolinolate to accommodate between the bis-
isopropylphenyl wingtips of the carbene.27 In contrast, the
pseudo-spherical cyclooctene ligand do not allow for this steric
relief, thus exerting higher steric hindrance.
The introduction of the quinolinolate ligand into the Rh-
NHC framework significantly affects the catalytic outcome
compared to related catalytic systems previously described by
us (entries 1-4 vs 5-10).16 In general, the catalytic activity is
lower, as observed in the aforementioned systems by the
addition of pyridine (entries 2-4 vs 1-3). As we hypothesized,
the selectivity to α-vinyl sulfides of the quinolinolate-based
catalysts is high (74-80 %), but, unfortunately, lower than
expected (entries 5, 7 and 9). Remarkably, the addition of
pyridine results in an increase of the selectivity up to 97%
(entries 6, 8 and 10) with an upsurge of the catalytic activity
except for catalyst precursor 8. The use of NEt3 as additive
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gave poorer results suggesting that the role played by pyridine
is the coordination to the metallic centre and not as a base
(entry 11). However, it seems that coordinating ability of the
additive is essential for an efficient catalytic system, as
reflected in the lower activity observed when much weaker
coordinating ligands such as CH3CN or 2-ethylpyridine were
used (entries 12 and 13).
Table 1. Phenylacetylene hydrothiolation with thiophenol
promoted by Rh-IPr catalysts.a
Figure 5. Monitorization of phenylacetylene hydrothiolation
catalysed by 5, 6, 8 in the presence or absence of pyridine.
d
Entry Cat Additiveb
t(h)
Conv %c α/β-E
TOF1/2
Mechanistic Studies for Alkyne Hydrothiolation
1e
2e
3f
4f
5
2
-
-
-
-
-
0.4
7
99
99
99
91
58
79
40
70
90
80
31
35
30
33/67
94/6
482
22
41
28
3
The above described results evidence that both N-donor
ligands play a key role on the performance of the catalytic
system. We assume that, similarly to related catalytic systems
based on Rh-NHC catalyst precursors described by us, the first
step in the catalytic reaction should be the oxidative addition
2
4
14
14
14
16
16
15
13
15
19
20
20
82/18
94/6
of the thiol to form
a
RhIII species stabilized by the
coordination of pyridine (Figure 6).16a In previous works, we
have observed related octahedral intermediates bearing two
pyridine ligands located trans and cis to the carbene, which
probably exert different influence on the mechanism, but
could not be analysed independently. The presence of a
quinolinolate ligand makes possible to study the effect of the
coordinated ligands in both positions. In one hand, the
bidentate coordination of the quinolinolate ligand inhibits the
equilibrium with dinuclear species thereby increasing the
selectivity to Markovnikov-type products. However, the
positive effect of pyridine in the regioselectivity suggests that
its coordination cis to the carbene ligand is also significant.
4
5g
5g
6
74/26
97/3
6
11
1.3h
4
7
80/20
93/7
8
6
9
8
79/21
97/3
15
5
10
11
12
13
8
5g
5g
5g
NEt3
63/37
61/36/3i
65/35
-
CH3CN
-
-
a0.5 mL of C6D6 with 2 mol % of catalyst, [alkyne] = [thiol] =
b
c
1.0 M at 25 °C. 10 equivalents. Relative to phenylacetylene.
Figure 6. Role of the N-donor ligands in the active species.
e
dTurnover Frequency in h-1 at 50 % conversion. Data from ref
16a. fData from ref 16b. gMixture of 5a and 5b. hDetermined at
25% of conversion. iβ-Z vinyl sulfide.
A series of stoichiometric reactions were carried out in
order to gain insight into the reaction mechanism. Addition of
thiophenol to a NMR tube containing a toluene-d8 solution of
afforded, after 1 h at room temperature, the Rh-hydride
unsaturated complex RhH{κ2-O,N-(C9H6NO)}(SPh)(IPr) (
) as a
result of the S-H oxidative addition (Scheme 5).28 The hydride
ligand of
was observed in the 1H NMR spectrum as a doublet
5
9
9
at δ -26.74 ppm with JH-H = 51.4 Hz (Figure 7). The large
coupling constant points to a square pyramidal structure with
the hydride ligand in the apical position.20e Addition of pyridine
to a solution of in situ prepared
9
in a NMR tube led to the
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formation of the saturated complex RhH{κ2-O,N- and subsequent insertion of ethylene into the newly formed
(C9H6NO)}(SPh)(IPr)(py) (10). The hydride resonance of 10 is Rh-H bond,30 whereas, in contrast to
, the formation of the
shifted to -16.02 ppm and the Rh-H coupling constant is octahedral pyridine complex was not observed. Traces of and
substantially reduced to 19.7 Hz, in accordance with the 10 were detected. The H NMR spectrum shows a set of three
proposed octahedral structure.29 Compounds
and 10 are in a multiplets for the ethyl ligand at δ 3.20, 2.92, and -0.36 ppm
dynamic equilibrium with a 10 ratio of 5:95 at -60 °C. It is corresponding to the diastereotopic >CH2 and -CH3 protons,
9
9
1
9
9
:
worthy to note that, in spite of starting from a mixture of RhI respectively. In addition, two doublets at δ 20.4 (JRh-C = 27.1
complexes with different orientation of the quinolinolate Hz) and 174.0 ppm (JRh-C = 53.2 Hz) appear in the 13C{1H}-APT
ligand, only a single isomer was observed for
attempts for isolation of and 10 were unsuccessful. DFT attached to rhodium. Complex 11 has also been detected
calculations show that the configuration of with the nitrogen when bubbling ethylene through a solution of
atom of quinolinolate ligand trans to IPr is 10.8 kcal mol-1 more
In the next step, we have studied the insertion reaction of the
9 and 10. Several spectrum, ascribed respectively to ethyl and IPr carbon atoms
9
9
9.
stable than the opposite orientation, as observed previously in alkyne into hydride-thiolate-RhIII species. Treatment of a freshly in-
related RhIII-quinolinolate complexes.20d,g
situ prepared solution of 9 with phenylacetylene led, after 1 h at
room temperature, to the formation of the alkenyl derivatives
Rh(η1-C(Ph)=CH2){κ2-O,N-(C9H6NO)}(SPh)(IPr) (12) and Rh(η1-
CH=CHPh){κ2-O,N-(C9H6NO)}(SPh)(IPr) (13) in a 56:44 ratio, as a
result of the alkyne insertion into the Rh-H bond with Markovnikov
or anti-Markovnikov regioselectivity, respectively (Scheme 6).31
Alkyne insertion into the Rh-S bond was not detected.32 The 1H
NMR spectrum shows the characteristic H(2) resonances for the
quinolinolate ligand at δ 8.86 (12) and 8.64 ppm (13), whereas two
doublets are observed in the 13C{1H}-APT spectrum at 174.5 (JRh-C
=
54.3 Hz) (12) and 172.3 ppm (JRh-C = 51.5 Hz) (13) corresponding to
the Rh-IPr carbon atoms, corroborating the presence of two new
complexes. Methylidene protons of 12 resonate at δ 4.77 and 4.73
ppm as broad signals and correlate in the 1H-13C HSQC spectrum
with the same =CH2 carbon atom at 123.6 ppm. Confirmation of the
η1-α-alkenyl moiety in 12 arises from the correlation in the 1H-13
C
HMBC spectrum between the =CH2 protons and a quaternary
carbon at δ 143.4 ppm, appearing as a doublet (JRh-C = 37.8 Hz),
corresponding to the vinylic carbon attached to rhodium. On the
other hand, the olefinic proton at δ 5.84 ppm correlates with a
carbon at δ 131.2 ppm in the 1H-13C HSQC spectrum and with a
=CH- doublet at 126.2 (JRh-C = 39.4 Hz) in the 1H-13C HMBC spectrum,
which confirms the nature of the η1-β-alkenyl moiety in 13. Heating
of the mixture led to the formation of the vinyl sulfides resulting
from C-S coupling. These results are in sharp contrast with those
Scheme 5. Sequential reaction of 5 and 6 with thiophenol and
pyridine.
previously
observed
for
the
intermediate
species
RhHCl(SPh)(IPr)(py)16a and RhH(SCH2Ph)2(IPr)(py)16b where addition
of the alkyne led directly to the coupled vinyl sulfides without
detecting any intermediate resulting for the alkyne insertion into
the Rh-H bond.
1
Figure 7. Aromatic and hydride regions of the H-NMR spectra of 9
and 10 at -60 °C.
The outcome of the reaction of the ethylene compound
with thiophenol is somewhat different. Treatment of with
the thiol and pyridine in an NMR tube afforded the
unsaturated Rh-ethyl complex
Rh(η1-CH2CH3){κ2-O,N-
6
6
(C9H6NO)}(SPh)(IPr) (11), as a result of S-H oxidative addition
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Rh(η1-CH=CHPh){κ2-O,N-(C9H6NO)}(SPh)(IPr)(py) (14) resulting
from the coordination of pyridine to 13 was detected, but
1
coordination of pyridine to 12 was not observed. The H NMR
spectrum of 14 displays two doublets at δ 8.90 (JH-H = 5.2 Hz)
and 8.62 ppm (JH-H = 4.5 Hz) corresponding to H(2) of pyridine
and quinolinolate ligands, respectively. The two doublets at
8.76 and 7.39 ppm, with a mutual coupling of 15.8 Hz, were
ascribed to CH protons of the η1-β-E-alkenyl ligand. The
12
:
13
:
14 ratio was 54:15:31 at -30 °C. The coordination of
13 14 changed
pyridine to 13 is reversible. Thus, the ratio 12
:
:
to 56:36:8 at 20 °C. Indeed, exchange peaks between the
protons of the η1-β-E-alkenyl ligands of 13 and 14 were
1
observed in a H 1D-NOESY experiment (See SI). Noteworthy,
the same mixture of species was obtained after the addition of
the reagents in the reverse order. Finally, heating of the
mixture gave predominantly rise to the formation of gem-vinyl
sulfide.
Scheme 6. Reactivity of 9 and 10 with phenylacetylene.
In order to study the role of pyridine in the catalytic reaction, 3
equiv of this ligand were added to a NMR tube containing a
mixture of 12 and 13 (Figure 8). A new octahedral species
Figure 8. Representative regions of the 1H NMR spectra for in situ preparation of 12 and 13 and subsequent pyridine addition to form 14.
The fact that pyridine coordinates trans to the hydride (
and β-alkenyl (13) complexes but not to ethyl (11) or α-alkenyl slightly higher than that observed for the β-alkenyl derivative
12) derivatives is intriguing. The origin of this discrepancy may 13 (5.05°), suggesting the interplay of electronic and steric
9
)
the pyridine. The ψ angle for the α-alkenyl complex 12 is 5.95°,
(
arise from electronic or steric reasons. However, the electronic factors in this case. A similar steric effect has been observed in
effects do not seem to be relevant as the trans influence of the similar d6-NHC-ruthenium olefin metathesis precursors. Thus,
hydride is higher than the ethyl ligand.33 Thus, this behaviour whereas pyridine coordinates into the benzylidene species
might be rationalized on steric grounds. The DFT-optimized RuCl2(SIMes)(=CHPh)(py)13
(SIMes
=
2,4,6-
structures for the coordination of pyridine in these complexes (trimethylphenyl)imidazolidin-2-carbene), it does not in
nicely agree with the experimental observations (Figure 9). RuCl2(SIMes)(Ind)(py)35 bearing the bulkier indenylidene (Ind)
Pyridine coordination in 11 and 12 result in a destabilization by ligand.
4.3 and 1.5 kcal mol-1, respectively, whereas it is favoured in
9
and 13 by 12.0 and 2.9 kcal mol-1, respectively. The steric
influence of the ligand trans to vacant site is reflected in the
bending of imidazole ring of the IPr with regard to the Rh-IPr
axis, described as the yaw angle (ψ).34 The values of 6.77° for
11 and 0.49° for
9 are in agreement with the highest steric
hindrance imparted by the ethyl ligand, forcing the IPr to bend
to the vacant site and consequently hampering coordination of
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Journal Name
Theoretical Calculations on the Mechanism
In order to discriminate between the two mechanistic
pathways showed in Scheme 7, a detailed DFT computational
study has been performed using B3LYP method including
dispersion correction (Figure 10). Noteworthy, theoretical
calculations dealing with alkyne hydrothiolation are
scarce.16,5q,37 Full IPr, quinolinolate, thiophenol and
phenylacetylene have been explicitly considered. The starting
point is RhH{κ2-O,N-(C9H6NO)}(SPh)(IPr) (
resulting from S-H oxidative addition to RhI precursors. All
energies are relative to and the corresponding reactants.
A, compound 9),
A
Both hydrometallation (right) or thiometallation (left)
pathways have been computed.
The first step in both pathways is the coordination of the
Figure 9. DFT-optimized ground state minimum energy structures
for the coordination of pyridine to 9, 11, 12 and 13.
alkyne into
the hydride of A with the two possible orientations results in
a stabilization of 2.3 ( G’). However, in
) and 3.1 kcal mol-1
A. Thus, coordination of phenylacetylene trans to
G
(
order to hydrometallation pathway to proceed, the hydride
and the π-alkyne have to be disposed mutually cis, therefore
the minima B and B’ were computed, which are destabilized
by 9.2 and 2.0 kcal mol-1, respectively. Then, insertion step
via the hydrometallation pathway takes place through the
transition states C-TS, for the branched alkenyl, and C’-TS for
Based on the above data, the mechanism depicted in Scheme
7 can be proposed for the formation of α-vinyl sulfide. The first
step is the release of olefin and oxidative addition of thiol to the
RhI quinolinolate precursors to form an unsaturated hydride-
thiolate-RhIII intermediate similar to 9. From this point two
pathways are available. Cycle a starts with an isomerization of the
precursor and subsequent fast 2,1-insertion into the Rh-H bond to
generate an α-alkenyl intermediate. Slow reductive elimination
yields the C-S coupled product. Although generally a 1,2 insertion
of alkynes into M-H bonds is preferred, Markovnikov addition has
been also observed.36 Alternatively, pathway b starts by a slow
1,2-insertion of the alkyne into the Rh-thiolate bond, followed by
fast C-H reductive elimination. It was disclosed previously that
pathway b is operative for the catalytic systems [Rh(μ-X)(NHC)(η2-
olefin)]2/py (X = Cl, OH).16 Although this route cannot be excluded
for rhodium quinolinolate-based catalysts, the detection of alkenyl
intermediates strongly suggests a hydrometallation pathway in
this case.
the linear one, located 9.9 and 10.3 kcal mol-1, higher than
respectively. The pathway evolves to intermediates and D’
A
,
,
D
which lay at -18.7 and -29.9 kcal mol-1, respectively.
Noteworthy, the transition states C-TS and C’-TS are more
stable than those corresponding to thiometalation pathways,
H-TS and H’-TS (17.6 and 21.2 kcal mol-1), in spite of the
connected π-alkyne minima
than and B’
The next step is the reductive elimination starting from
and D’ to give and F’, respectively, that display the same
G and G’ are lower in energy
B
.
D
F
activation barrier of 28.1 kcal mol-1 for branched and linear
vinyl sulfides via E-TS and E’-TS. The high energy barriers for
the reductive elimination step through the hydrometallation
pathway open the possibility for
D and D’ species to revert
into A to follow the thiometallation route that show lower
energetic barrier for the reductive elimination (13.2 and 16.2
kcal mol-1 via J-TS and J’-TS, respectively). However, hurdles
of 36.3 and 51.1 kcal mol-1 have to be surpassed from
D and
D’ to H-TS and H’-TS
.
The calculations indicate that reductive elimination is the
rate-determining step for both Markovnikov and anti-
Markovnikov additions, displaying the same activation barrier
(28.1 kcal mol-1). At this point the selectivity is governed by
the energy difference between the transition states for the
insertion step C-TS and C’-TS of only 0.4 kcal mol-1. This result
does not match well with the 74-80 % of regioselectivity
attained in the absence of pyridine. However, thiophenol is in
excess under catalytic conditions, so it can bind to the
pentacoordinated Rh-alkenyl species. Computation of the
thiol coordination to D and D’ shows that the linear alkenyl
complex D’-HSPh is stabilized by 0.8 kcal mol-1, whereas the
Scheme 7. Mechanistic pathways for the formation of α-vinyl
branched alkenyl complex D-HSPh is destabilized by 1.2 kcal
sulfide: a) hydride insertion, b) thiolate insertion.
8 | J. Name., 2012, 00, 1-3
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mol-1. The activation barrier for the reductive elimination
step leading to the formation of linear vinyl sulfide is now 0.8
kcal mol-1 higher, therefore explaining the moderate
Markovnikov selectivity observed in the absence of pyridine.
In order to explain the positive pyridine effect found in this
catalytic system, the coordination of pyridine to D and D’ was also
computed. Remarkably, the β-alkenyl complex D’-py is stabilized
by 2.9 kcal mol-1, whereas D-py lays 1.5 kcal mol-1 higher. The
difference of almost 3 kcal mol-1 gained by the coordination of
pyridine perfectly explains the observed selectivity (Figure 11).
Although pyridine is ubiquitous as additive in many catalytic
transformations,7-14 its role has not always been clearly
established. Here, the interplay of stereolectronic properties
within Rh-IPr species determines the coordination or not of
pyridine in the key alkenyl intermediates. The coordination of
pyridine in the β-alkenyl complex results in a net stabilization of
this species decreasing the catalytic activity. However, the α-
alkenyl intermediate is not influenced by the pyridine ligand and,
as a consequence, an increment of the α-vinyl sulfide product is
observed.
Figure 10. DFT calculated (ΔG in kcal mol-1) along the energy surface for the formation of vinyl sulfides through thiometallation and
hydrometallation. Structures B to F correspond to α-vinyl sulfide (R1 = Ph, R2 = H, red line) and structures B’ to F’ lead to β-vinyl sulfides
(R1 = H, R2 = Ph, blue line).
Conclusions
In summary, new RhI-NHC-olefin derivatives bearing a N,O-
quinolinolate bidentate ligand are efficient catalysts for the
addition of thiophenol to phenylacetylene with good
selectivity towards the Markovnikov product. It has been
found that selectivity can be increased up to 97% by using
pyridine as additive. Reactivity studies involving the
sequential reaction of the precatalysts with thiophenol,
phenylacetylene and pyridine have revealed the role of the
pyridine in the selectivity of alkyne hydrothiolation. Addition
of thiophenol to the metal complex gives
a
RhIII-H
unsaturated derivative, as a result of the S-H oxidative
addition. Further reaction with phenylacetylene affords key
RhIII-alkenyl species resulting from alkyne insertion into Rh-H
bond. The particular stereolectronic properties within the
pentacoordinated intermediates result in
a different
behaviour towards pyridine. It has been observed that
pyridine binds to the η1-β-alkenyl species but not to the
Figure 11. Pyridine effect over the reductive elimination energy
barriers through the hydrometallation pathway.
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ARTICLE
Journal Name
bulkier η1-α-alkenyl which is also supported by theoretical
C4a-quin), 128.0 (s, C6-quin), 124.2 (s, =CHN), 119.4 (s, C3-quin),
calculations. The full theoretical analysis of both 114.9 (s, C7-quin), 106.7 (s, C5-quin), 55.3 (d, JC-Rh = 15.4, =CHcoe),
hydrometallation and thiometallation pathways supports a 30.6, 29.4, and 27.0 (all s, CH2-coe), 28.6 (s, CHMeIPr), 26.6 and
hydrometallation mechanism that is in full agreement with 22.9 (both s, CHMeIPr). 1H-15N NMR HMBC (50.7 MHz,
the experimental results. The preferred pathway starts by toluene-d8, 243 K): δ 248.3 (Nquin), 192.9 (NIPr). 5b (N-cis-IPr):
oxidative addition of the thiol and subsequent 2,1-insertion 1H NMR (400 MHz, C6D6, 298 K): δ 7.84 (d, JH-H = 4.5, 1H, H2-
of the alkyne into the Rh-H bond with the reductive
quin), 7.72 (dd, JH-H = 8.3, 1.3, 1H, H4-quin), 7.48 (dd, JH-H = 8.5,
elimination as rate-determining step. Remarkably, the 7.1, 1H, H6-quin), 7.3-7.1 (m, 7H, HPh-IPr, H7-quin), 6.70 (s, 2H,
coordination of pyridine to the β-alkenyl intermediate, which =CHN), 6.67 (d, JH-H = 7.1, 1H, H5-quin), 6.61 (dd, JH-H = 8.3, 4.5,
results in a net stabilization, but not to the α-alkenyl 1H, H3-quin), 3.69 (sept, JH-H = 6.8, 4H, CHMeIPr), 3.44 (m, 2H,
determines the Markovnikov selectivity. We hope that the =CHcoe), 2.73, 1.89, and 1.44 (all m, 12H, CH2-coe), 1.03 and
understanding of the pyridine effect in this catalytic system 0.56 (both d, JH-H = 6.8, 24H, CHMeIPr). 13C{1H}-APT NMR
will be helpful for the design of new pyridine-based catalytic (100.6 MHz, C6D6, 298 K): δ 184.7 (d, JC-Rh = 56.2 Rh–CIPr),
precursors for other useful transformations.
171.0 (s, C8-quin), 146.1 (s, Cq-IPr), 146.0 (s, C2-quin), 147.0 (s, C8a-
quin), 137.2 (s, C4-quin), 137.1 (s, CqN), 128-123 (CHPh-IPr), 131.1
(s, C4a-quin), 129.5 (s, C6-quin), 124.7 (s, =CHN), 118.7 (s, C3-quin),
113.6 (s, C7-quin), 109.3 (s, C5-quin), 65.9 (d, JC-Rh = 15.0, =CHcoe),
30.5, 28.1, and 26.6 (all s, CH2-coe), 28.7 (s, CHMeIPr), 25.4 and
22.2 (both s, CHMeIPr). 1H-15N NMR HMBC (50.7 MHz,
toluene-d8, 243 K): δ 252.8 (Nquin), 192.6 (NIPr).
Experimental Section
General Considerations. All reactions were carried out with
rigorous exclusion of air using Schlenk-tube techniques. The
reagents were purchased from commercial sources and were
used as received, except for phenylacetylene that was
distilled and stored over molecular sieves. Organic solvents
were dried by standard procedures and distilled under argon
prior to use or obtained oxygen- and water-free from a
Solvent Purification System (Innovative Technologies). The
Preparation of Rh{κ2-O,N-(C9H6NO)}(IPr)(η2-CH2=CH2)(IPr)
(6). The complex was prepared as described for
5 starting
from 4 (78 mg, 0.073 mmol) and 8-hydroxyquinoline (21 mg,
0.14 mmol) and obtained as an orange solid. Yield: 69 mg
(71%). Anal. Calcd. for C38H46N3ORh.H2O: C, 66.95; H, 7.10; N,
1
6.16. Found: C, 66.54; H, 7.02; N, 6.22. H NMR (300 MHz,
organometallic precursors
3 and 4 were prepared as
previously described.16b Chemical Shifts (expressed in parts
per million) are referenced to residual solvent peaks (1H,
13C{1H}) or liquid NH3 (15N). Coupling constants, J, are given in
Hz. Spectral assignments were achieved by combination of
1H-1H COSY, 13C{1H}-APT and 1H-13C HSQC/HMBC
experiments. H, C and N analysis were carried out in a Perkin-
Elmer 2400 CHNS/O analyzer. GC-MS analysis were recorder
on an Agilent 5973 mass selective detector interfaced to an
Agilent 6890 series gas chromatograph system, using a HP-
5MS 5% phenyl methyl siloxane column (30 m x 250 mm with
a 0.25 mm film thickness).
C6D6, 298 K): δ 7.46 (dd, JH-H = 7.9, 7.9, 1H, H6-quin), 7.33 (dd,
JH-H = 8.3, 1.0, 1H, H4-quin), 7.21 (dd, JH-H = 7.9, 1.0, 1H, H5-quin),
7.2-7.1 (6H, HPh-IPr), 6.77 (dd, JH-H = 4.9, 1.0, 1H, H2-quin), 6.64
(dd, JH-H = 7.9, 1.0, 1H, H7-quin), 6.19 (dd, JH-H = 8.3, 4.9, 1H, H3-
quin), 6.60 (s, 2H, =CHN), 3.44 (sept, JH-H = 6.8, 4H, CHMeIPr),
2.31 (br, 4H, CH2=CH2), 1.54 and 1.13 (both d, JH-H = 6.8, 24H,
CHMeIPr). 13C {1H}-APT NMR (75.1 MHz C6D6, 298 K): δ 186.8
(d, JC-Rh = 56.7, Rh–CIPr), 171.3 (s, C8-quin), 146.6 (s, Cq-IPr), 145.2
(s, C8a-quin), 140.4 (s, C2-quin), 136.8 (s, CqN), 134.8 (s, C4-quin),
130.2 (s, C4a-quin), 130.0(s, C6-quin), 124.0 (s, =CHN), 129.5 and
123.8 (both s, CHPh-IPr), 119.9 (s, C3-quin), 112.7 (s, C5-quin),
109.1 (s, C7-quin), 36.6 (d, JC-Rh = 16.2, CH2=CH2), 28.7 (s,
CHMeIPr), 26.2 and 23.1 (both s, CHMeIPr). 1H-15N NMR HMBC
(50.7 MHz, toluene-d8, 298 K): δ 253.3 (Nquin), 191.9 (NIPr).
In situ formation of Rh{κ2-O,N-[C9H5NO(CH3)]}(η2-coe)(IPr)
Preparation of Rh{κ2-O,N-(C9H6NO)}(η2-coe)(IPr) (5).
yellow solution of (100 mg, 0.081 mmol) in toluene (10 mL)
A
3
was treated with 8-hydroxyquinoline (23 mg, 0.16 mmol) and
stirred for 2 h at room temperature to give a tile red solution.
Then, the solution was concentrated to ca. 1 mL and n-
hexane added to induce the precipitation of a tile red solid,
which was washed with n-hexane (3 x 3 mL) and dried in
vacuo. Yield: 92 mg (76%). The compound was obtained as
two structural isomers 5a and 5b in a 55:45 ratio. HRMS (ESI)
m/z calcd for RhC36H41N3O (M – coe – H)+ 634.2299 found
634.2270. 5a (N-trans-IPr): 1H NMR (400 MHz, C6D6, 298 K): δ
8.33 (d, JH-H = 4.9, 1H, H2-quin), 7.44 (d, JH-H = 8.2, 1H, H4-quin),
7.3-7.0 (m, 7H, HPh-IPr, H6-quin), 6.99 (d, JH-H = 7.6, 1H, H7-quin),
6.60 (d, JH-H = 7.2, 1H, H5-quin), 6.52 (s, 2H, =CHN), 6.31 (dd, JH-
(7). A toluene-d8 solution of
3 (30 mg, 0.023 mmol) in an
NMR tube at 233 K was treated with 2-methyl-8-hydroxy-
quinoline (7.7 mg, 0.047 mmol). NMR spectra were
immediately recorded at low temperature. 1H NMR (400
MHz, toluene-d8, 233 K): δ 7.60 (d, JH-H = 8.4, 1H, H4-quin), 7.26
(dd, JH-H = 8.5, 7.2, 1H, H6-quin), 7.1-7.0 (m, 7H, HPh-IPr, H7-quin),
6.63 (d, JH-H = 8.5, 1H, H3-quin), 6.61 (d, JH-H = 8.4, 1H, H5-quin),
6.40 (s, 2H, =CHN), 4.14 and 2.05 (both sept, JH-H = 6.6, 4H,
CHMeIpr), 2.85 (m, 2H, =CHcoe), 2.00 (s, 3H, Mequin), 1.7-1.1
(br, 12H, CH2-coe), 1.23, 1.00, 0.98, and 0.86 (all d, JH-H = 6.6,
24H, CHMeIpr). 13C{1H}-APT NMR (100.6 MHz, toluene-d8, 233
K): δ 196.4 (d, JC-Rh= 52.3, Rh–CIPr), 170.2 (s, C8-quin), 146.5 and
145.8 (both s, Cq-IPr), 153.6 (s, C2-quin), 146.1 (s, C8a-quin), 138.0
(s, C4-quin), 137.2 (s, CqN), 128.4, 124.2, and 123.9 (all s, CHPh-
IPr), 129.2 (s, C4a-quin), 128.9 (s, C6-quin), 123.9 (s, =CHN), 121.7
H = 8.2, 4.9, 1H, H3-quin), 3.04 (m, 2H, =CHcoe), 2.26 (sept, JH-H
=
6.7, 4H, CHMeIPr), 2.1-1.7 (m, 12H, CH2-coe), 1.37 and 0.97
(both d, JH-H = 6.7, 24H, CHMeIPr). 13C{1H}-APT NMR (100.6
MHz, C6D6, 298 K): δ 193.0 (d, JC-Rh= 59.2, Rh–CIPr), 170.5 (s,
C8-quin), 147.9 (s, C2-quin), 146.6 (s, Cq-IPr), 144.8 (s, C8a-quin),
137.0 (s, CqN), 136.1 (s, C4-quin), 128-123 (CHPh-IPr), 130.8 (s,
(s, C3-quin), 115.9 (s, C7-quin), 107.2 (s, C5-quin), 61.2 (d, JC-Rh
=
10 | J. Name., 2012, 00, 1-3
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16.9, =CHcoe), 30.2, 27.7, and 26.8 (all s, CH2-coe), 28.6 and 7.8, 1H, H5-quin), 5.92 and 5.41 (both m, 2H, H3-py, H5-py), 5.64
26.4 (both s, CHMeIPr), 28.4 (Mequin), 26.1, 25.8, 24.0, and (dd, JH-H = 8.1, 4.2, 1H, H3-quin), 3.4-3.0 (br, 4H, CHMeIpr), 1.6-
23.0 (all s, CHMeIPr).
Preparation of Rh{(κ2-O,N-[C9H5NO(CH3)]}(η2-CH2=CH2)(IPr) 13C{1H}-APT NMR (125.6 MHz, toluene-d8, 213 K): δ 177.1 (d,
(8). A yellow solution of (100 mg, 0.081 mmol) in toluene (5 JC-Rh = 53.6, Rh–CIPr), 171.1 (s, C8-quin), 150.0 (br, C2-py), 147.3,
0.7 (br, 24H, CHMeIpr), –16.02 (d, JH-Rh = 19.7, 1H, Rh−H).
4
mL) was treated with 2-methyl-8-hydroxy-quinoline (26 mg, 147.0, 146.5, and 146.3 (all s, Cq-IPr), 145.6 (s, C2-quin), 144.7 (s,
0.16 mmol) and stirred for 1 h at room temperature to give C8a-quin), 137.3 and 137.1(both s, CqN), 137-125 (s, CPh), 135.5
an orange solution. Then, the solution was concentrated to (s, C4-quin), 134.7 (s, C4-py), 130.5 (s, C4a-quin), 130.2 (s, C6-quin),
ca. 1 mL and n-hexane added to –20 ⁰C to induce the 125.0 (s, =CHN), 122.4 y 121.9 (ambos s, C3-py, C5-py), 120.4 (s,
precipitation of an orange solid, which was washed with cold C3-quin), 113.9 (s, C7-quin), 108.2 (s, C5-quin), 29-21 (CHMeIPr).
n-hexane (3 x 3 mL) and dried in vacuo. Yield: 71 mg (65 %). In
situ
formation
of
Rh(η1-CH2CH3){κ2-O,N-
(33 mg,
Anal. Calcd. for C39H48N3ORh.2H2O: C, 65.62; H, 7.34; N, 5.88. (C9H6NO)}(SPh)(IPr) (11). A toluene-d8 solution of
6
1
Found: C, 65.31; H, 7.15; N, 5.85. H NMR (400 MHz, C6D6, 0.050 mmol) in a NMR tube was treated with thiophenol (6
298 K): δ 7.43 (dd, J = 8.0, 7.7, 1H, H6-quin), 7.28 (d, JH-H = 8.3, µL, 0.060 mmol) and pyridine (15 µL, 0.18 mmol). After 15
1H, H4-quin), 7.32 (d, JH-H = 8.0, 1H, H7-quin), 7.2-7.0 (m, 6H, HPh- min at room temperature, NMR spectra were recorded at
IPr), 6.65 (d, JH-H = 7.7, 1H, H5-quin), 6.56 (s, 2H, =CHN), 6.06 (d, low temperature. 1H NMR (400 MHz, toluene-d8, 223 K): δ
JH-H = 8.3, 1H, H3-quin), 3.48 (br, 4H, CHMeIPr), 2.75 and 2.10 8.62 (d, JH-H = 5.0, 1H, H2-quin), 7.47 (dd, JH-H = 7.6, 7.5, 1H, H6-
(both d, JH-H = 10.4, 4H, CH2=CH2), 1.76 (s, 3H, Mequin), 1.55 quin), 7.30 (d, JH-H = 7.6, 1H, H7-quin), 7.3-6.7 (m, 14H, HPh-IPr
,
and 1.12 (both d, JH-H = 6.7, 24H, CHMeIPr). 13C{1H}-APT NMR HPh-SPh, H4-quin, =CHN), 6.64 (d, JH-H = 7.5, 1H, H5-quin), 6.17 (dd,
(100.6 MHz, C6D6, 298 K): δ 183.9 (d, JC-Rh = 59.3, Rh-CIPr), JH-H = 8.2, 5.0, 1H, H3-quin), 3.7-3.5 (m, 4H, CHMeIpr), 3.20 and
171.0 (s, C8-quin), 157.7 (s, C2-quin), 146.6 (s, Cq-IPr), 144.4 (s, C8a- 2.92 (both ddq, JH-H = 7.4, 7.0 JH-Rh = 2.5, 2H, CH2-ethyl), 1.8-1.1
quin), 136.3 (s, C4-quin), 137.0 (s, CqN), 129.6 (s, CHPh-IPr), 128.6 (24H, CHMeIpr), –0.36 (dvt, JH-Rh = 1.0, N = 14.0, 2H, CH3-ethyl).
(s, C4a-quin), 128.6 (s, C6-quin), 124.8 (s, =CHN), 123.1 (s, C3-quin), 13C{1H}-APT NMR (100.6 MHz, toluene-d8, 223 K): δ 174.0 (d,
113.9 (s, C5-quin), 109.4 (s, C7-quin), 38.3 (d, JC-Rh = 15.4, JC-Rh = 53.2, Rh–CIPr), 170.5 (s, C8-quin), 151.2 (s, CqS), 148.2 (s,
CH2=CH2), 28.8 (s, CHMeIPr), 26.2 and 23.3 (both s, CHMeIPr), C2-quin), 147.0, 146.6, 146.1, and 145.8 (all s, Cq-IPr), 144.7 (s,
22.1 (Mequin).
In situ formation of RhH{κ2-O,N-(C9H6NO)}(SPh)(IPr) (9). A (s, C4a-quin), 129.8 (s, C6-quin), 129-123 (CHPh, =CHN), 121.3 (s,
toluene-d8 solution of (25 mg, 0.033 mmol) in an NMR tube C3-quin), 114.8 (s, C7-quin), 110.3 (s, C5-quin), 29-23 (CHMeIPr),
at 298 K was treated with thiophenol (5 μl, 0.049 mmol). 20.4 (d, JC-Rh = 27.1, CH2-ethyl), 18.7 (s, CH3-ethyl).
NMR spectra were recorded after 1 h at 298 K. HRMS (ESI) In situ formation of
Rh(η1-C(Ph)=CH2){κ2-O,N-
m/z calcd for RhC42H47N3OS (M - H)+ 744.2489 found (C9H6NO)}(SPh)(IPr) (12) and Rh(η1-CH=CHPh){κ2-O,N-
744.2473. 1H NMR (500 MHz, toluene-d8, 298 K): δ 8.58 (d, JH- (C9H6NO)}(SPh)(IPr) (13)
freshly prepared toluene-d8
= 4.6, 1H, H2-quin), 7.27 (dd, JH-H = 8.0, 7.9, 1H, H6-quin), 7.13 solution of in an NMR tube starting from (25 mg, 0.033
C8a-quin), 137.3 and 136.0 (both s, CqN), 136.8 (s, C4-quin), 130.5
5
.
A
9
5
H
(d, JH-H = 8.2, 1H, H4-quin), 7.06 (d, JH-H = 8.0, 1H, H7-quin), 7.1-6.7 mmol) and thiophenol (5 µL, 0.049 mmol), was treated with
(m, 11H, HPh), 6.74 (s, 2H, =CHN), 6.51 (d, JH-H = 7.9, 1H, H5- phenylacetylene (5 µL, 0.045 mmol). After 1 h at room
quin), 6.14 (dd, JH-H = 8.2, 4.6, 1H, H3-quin), 3.52 (sept, JH-H = 6.5, temperature the spectra were recorded at low temperature.
1H, CHMeIpr), 3.40 (sept, JH-H = 6.5, 2H, CHMeIpr), 2.03 (br, 1H, A mixture of 12:13 in 56:44 ratio was obtained. HRMS (ESI)
CHMeIpr), 1.49, 1.41, 1.40, 1.37, 1.35, 1.16, 1.13, and 1.12 (all m/z calcd for RhC50H53N3OS (M - H)+ 846.2959 found
1
d, JH-H = 6.5, 24H, CHMeIpr), –26.74 (d, JH-Rh = 50.1, 1H, Rh
−
H). 846.2943. 12: H NMR (500 MHz, toluene-d8, 243 K): δ 8.86
13C{1H}-APT NMR (125.6 MHz, toluene-d8, 298 K): δ 184.1 (d, (d, JH-H = 4.7, 1H, H2-quin), 7.23 (dd, JH-H = 7.7, 7.6, 1H, H6-quin),
JC-Rh = 50.7, Rh–CIPr), 175.4 (s, C8-quin), 155.8 (s, CqS), 152.4 and 7.19 (d, JH-H = 7.7, 1H, H7-quin), 6.69 (d, JH-H = 8.1, 1H, H4-quin),
152.3 (both s, Cq-IPr), 152.2 (s, C2-quin), 150.1 (s, C8a-quin), 142.3 7.1-6.2 (16H, HPh), 6.70 (s, 2H, =CHN), 6.43 (d, JH-H = 7.6, 1H,
(s, CqN), 141.5 (s, C4-quin), 136.6 (s, Co-SPh), 135.6 (s, C4a-quin), H5-quin), 6.12 (dd, JH-H = 8.1, 4.7, 1H, H3-quin), 4.77 and 4.73
135.1 (s, C6-quin), 132.4, 128.8, and 125.8 (all s, CHPh), 129.6 (s, (both br, 2H, =CH2), 3.55 and 3.43 (both sept, JH-H = 6.5, 4H,
Cm-SPh), 129.3 (s, =CHN), 125.7 (s, Cp-SPh),125.2 (s, C3-quin), CHMeIpr), 1.85, 1.18, 1.11, and 1.03 (all d, JH-H = 6.5, 24H,
119.2 (s, C7-quin), 115.2 (s, C5-quin), 34.1 and 33.9 (both s, CHMeIpr). 13C{1H}-APT NMR (125.6 MHz, toluene-d8, 243): δ
CHMeIPr), 31.5, 31.2, 28.0, and 27.7 (all s, CHMeIPr).
In situ formation of RhH{κ2-O,N-(C9H6NO)}(SPh)(IPr)(py) 148.8 (s, C2-quin), 147.5 and 145.6 (both s, Cq-IPr), 145.7 (s, Cq-
(10). A freshly prepared toluene-d8 solution of in an NMR Ph), 145.1 (s, C8a-quin), 143.4 (d, JC-Rh = 37.8, CPh=CH2), 143.0 (s,
tube starting from (25 mg, 0.033 mmol) and thiophenol (5 CqN), 136.9 (s, C4-quin), 131.0 (s, C4a-quin), 130-120 (CPh), 129.6
174.5 (d, JC-Rh = 54.3, Rh–CIPr), 170.4 (s, C8-quin), 151.0 (s, CqS),
9
5
µL, 0.049 mmol), was treated with pyridine (9 µL, 0.11 mmol) (s, C6-quin), 123.6 (br, CPh=CH2), 120.8 (s, C3-quin), 120.4 (s,
1
at 213 K. NMR spectra were recorded immediately. H NMR =CHN), 116.1 (s, C7-quin), 112.0 (s, C5-quin), 30.2 and 30.0 (both
1
(500 MHz, toluene-d8, 213 K): δ 9.59 and 7.52 (both br, 2H, s, CHMeIPr), 27.9, 27.4, 23.3, and 23.0 (CHMeIPr). 13: H NMR
H2-py), 8.14 (d, JH-H = 4.2, 1H, H2-quin), 7.34 (br, 2H, Ho-SPh), 7.03 (500 MHz, toluene-d8, 243 K): δ 8.64 (d, JH-H = 4.8, 1H, H2-quin),
(dd, JH-H = 8.1, 7.8, 1H, H6-quin), 6.92 (d, JH-H = 8.1, 1H, H7-quin), 7.41 (dd, JH-H = 8.1, 7.3, 1H, H6-quin), 7.33 (d, JH-H = 8.1, 1H, H7-
7.0-6.0 (m, 12H, HPh-IPr, HPh-SPh, H4-quin), 6.38 and 6.17 (both s, quin), 7.02 (d, JH-H = 8.2, 1H, H4-quin), 7.2-6.2 (17H, HPh,
2H, =CHN), 6.37 (dd, JH-H = 7.6, 7.3, 1H, H6-py), 6.10 (d, JH-H
=
CH=CHPh), 6.55 (d, JH-H = 7.3, 1H, H5-quin), 6.05 (dd, JH-H = 8.2,
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C6CY01884J
ARTICLE
Journal Name
4.8, 1H, H3-quin), 5.84 (dd, JH-H = 12.5, 1.9, 1H, CH=CHPh), 3.5- to terminal methyl groups C(11), C(12), C(14) and C(27)) have
3.3 and 1.9-0.9 (28H, CHMeIpr), 1.85, 1.18, 1.11, and 1.03 (all required a riding displacement parameters to get sensible
d, JH-H = 6.5, 12H, CHMeIpr). 13C{1H}-APT NMR (125.6 MHz, values.
toluene-d8, 234 K): δ 172.3 (d, JC-Rh = 51.5, Rh–CIPr), 170.3 (s, Crystal data for 7: C45H58N3ORh; M = 759.85; red block 0.186
C8-quin), 150.9 (s, CqS), 148.1 (s, C2-quin), 144.7 (s, C8a-quin), 136.8 × 0.181 × 0.134 mm3; triclinic, P21/n; a = 11.3475(14), b =
(s, C4-quin), 131-120 (s, CPh), 130.5 (s, C4a-quin), 130.0 (s, C6-quin), 127.783(2), c = 19.583(2) Å, β = 103.994(2)°; Z = 4; V =
131.2 (s, CH=CHPh ), 130-120 (CPh), 126.2 (d, JC-Rh = 39.4, 3834.4(8) Å3; Dc = 1.316 g·cm-3; μ = 0.484 mm-1; min. and
CH=CHPh ), 120.4 (s, C3-quin), 115.8 (s, C7-quin), 111.8 (s, C5-quin), max. absorption correction factors 0.792 and 0.914; 2θmax
29-23 (CHMeIPr). 59.00°; 40863 reflections collected, 9929 unique (Rint
In situ formation
=
=
of
Rh(η1-CH=CHPh){κ2-O,N- 0.0338); number of data/restraints/parameters 9929/6/676;
(C9H6NO)}(SPh)(IPr)(py) (14). A freshly prepared toluene-d8 final GOF 1.029; R1 = 0.0323 (8011 reflections, I > 2σ(I));
solution of 12 and 13 in an NMR tube was treated with wR(F2) = 0.0822 for all data. A static disorder was observed
pyridine (9 µL, 0.11 mmol). A mixture of 12:13:14 in 54:31:15 for two carbon atoms of the cyclooctene (C42 and C43); they
1
ratio was obtained. 14: H RMN (300 MHz, tol-d8, 243 K): δ were included in two equivalent positions with bond distance
8.90 (d, JH-H = 5.2, 2H, H2-py), 8.76 (d, JH-H = 15.8, 1H, restrictions for all C-C distances involving these disordered
CH=CHPh), 8.62 (d, JH-H = 4.5, 1H, H2-quin), 7.39 (d, JH-H = 15.8, atoms. Hydrogen atoms were observed from difference
1H, CH=CHPh), 5.80 (dd, JH-H = 5.2, 5.4, 2H, H3-py).
Fourier maps and refined as free isotropic atoms.
Catalytic hydrothiolation of phenylacetylene. A NMR tube Crystal data for 8: C39H48N3ORh·4 C6H6; M = 990.14; orange
containing a solution of 0.01 mmol of catalyst in 0.5 mL of prism 0.374 × 0.182 × 0.122 mm3; monoclinic; P21/n; a =
C6D6 was treated with 0.5 mmol of thiophenol and 0.5 mmol 11.4003(6), b
= 29.8414(15), c = 16.2873(8) Å, β =
of phenylacetylene. The reaction was monitored by H NMR 105.5120(10)°; Z = 4; V = 5339.1(5) Å3; Dc = 1.232 g·cm-3; μ =
at 25 ºC and the conversion was determined by integration of 0.363 mm-1; min. and max. absorption correction factors
the corresponding resonances of the alkyne and the 0.833 and 0.941; 2θmax = 57.33°; 111158 reflections collected,
1
products. The final reaction products was analysed by GC-MS 13095
unique
(Rint
=
0.0399);
number
of
analysis.
data/restraints/parameters 13095/0/892; final GOF 1.053; R1
= 0.0327 (11604 reflections, I > 2σ(I)); wR(F2) = 0.0807 for all
Crystal Structure Determination for complexes 5b, 7 and 8
.
Single crystals for the X-ray diffraction studies were grown by data. Hydrogens of the metal complex were included from
slow diffusion of n-hexane into toluene (5b and ) or benzene observed positions and refined as free isotropic atoms. Four
) solutions of the complexes. X-ray diffraction data were clear benzene solvent molecules were observed in the crystal
collected at 100(2) K on a Bruker SMART APEX (5b and ) or structure; their hydrogen atoms were also included in the
APEX DUO CCD diffractometers with graphite- structure, although some of them with riding parameters.
monochromated Mo-Kα radiation (λ = 0.71073 Å) using Computational details. All DFT theoretical calculations have
7
(
8
8
(7)
narrow ω rotations(0.3°). Intensities were integrated and been carried out using the Gaussian program package.42 The
corrected for absorption effects with SAINT-PLUS,38 and B3LYP method43 has been employed, including the D3
SADABS39 programs, both included in APEX2 package. The dispersion correction scheme developed by Grimme45 for
structures were solved by Patterson methods with SHELXS- both energies and gradient calculations and the “ultrafine”
9740 and refined, by full matrix least-squares on F2, with grid. The def2-SVP basis set45 has been selected for all atoms.
SHELXL-97.41 Both structures were refined first with isotropic The nature of the stationary points has been confirmed by
and later with anisotropic displacement parameters for non- analytical frequency analysis. In the Table XX (see supporting
disordered non-H atoms. Specific relevant details on each information) we have reported the computational results
structure are described below. CCDC 1485401-1485403 including the cartesian coordinates (in Å), absolute energy (in
contain the supplementary crystallographic data for this a.u) and graphical representation.
paper. These data can be obtained free of charge from The
Cambridge
www.ccdc.cam.ac.uk/data_request/cif.
Crystal data for 5b: C44H56N3ORh; M = 745.82; red block
0.254 0.164
0.130 mm3; monoclinic, P21/n; a
Crystallographic
Data
Centre
via
Acknowledgements
Financial support from the Spanish Ministerio de Economía y
Competitividad (MEC/FEDER) of Spain (Project CTQ2013-
42532-P), the Diputación General de Aragón (E07) and
CONSOLIDER INGENIO-2010, under the Project MULTICAT
(CSD2009-00050) are gratefully acknowledged. The support
under the KFUPM–University of Zaragoza research
agreement is also highly appreciated. ADG thanks the
“Subprograma de Formación Posdoctoral” from the
Ministerio de Economia y Competitividad.
×
×
=
11.4926(16), b = 17.417(2), c = 19.109(3) Å, β = 103.684(2)°; Z
= 4; V = 3716.5(9) Å3; Dc = 1.333 g·cm-3; μ = 0.497 mm-1; min.
and max. absorption correction factors 0.407 and 0.528;
2θmax = 56.82°; 42370 reflections collected, 8738 unique (Rint
=
0.0801);
number of
data/restraints/parameters
8738/0/649; final GOF 1.077; R1 = 0.0618 (6104 reflections, I
> 2σ(I)); wR(F2) = 0.1242 for all data. Most of the hydrogen
atoms were observed in the difference Fourier maps and
refined as free independent isotropic atoms. Some of them
(H(2), H(3), H(10), H(13), H(25) and H(34), and those bonded
Notes and references
12 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/AC6RCTYIC01L8E84J
1
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High Markovnikov-selective RhI-NHC-quinolinolate catalysts in
alkyne hydrothiolation with in-deep mechanistic analysis is
reported.
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