Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts

Article abstract of DOI:10.1021/ja300396h

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(ν²- olefin)]₂ and RhCl(IPr)(py)(ν²-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active cata

Full text of DOI:10.1021/ja300396h

Article
pubs.acs.org/JACS
Ligand-Controlled Regioselectivity in the Hydrothiolation of Alkynes
by Rhodium N-Heterocyclic Carbene Catalysts
Andrea Di Giuseppe,†,‡ Ricardo Castarlenas,*,† Jesus J. Perez-Torrente,† Marcello Crucianelli,‡
́
́
Victor Polo,§ Rodrigo Sancho,† Fernando J. Lahoz,† and Luis A. Oro†
†Instituto de Síntesis Química y Catal
́
isis Homogen
́
ea (ISQCH)-Departamento de Química Inor
́
ganica, CSIC-Universidad de
Zaragoza, Pl. S. Francisco S/N 50009 Zaragoza, Spain
‡Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita
̀
dell’Aquila, Via Vetoio, I-67100-Coppito (AQ), Italy
§Departamento de Química Física, Universidad de Zaragoza, Pl. S. Francisco S/N 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: Rh−N-heterocyclic carbene compounds [Rh(μ-
Cl)(IPr)(η2-olefin)]2 and RhCl(IPr)(py)(η2-olefin) (IPr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine,
olefin = cyclooctene or ethylene) are highly active catalysts for
alkyne hydrothiolation under mild conditions. A regioselectivity
switch from linear to 1-substituted vinyl sulfides was observed
when mononuclear RhCl(IPr)(py)(η2-olefin) catalysts were
used instead of dinuclear precursors. A complex interplay
between electronic and steric effects exerted by IPr, pyridine,
and hydride ligands accounts for the observed regioselectivity.
Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate−hydride active species in which the encumbered
and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming
alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate−alkyne disposition,
favoring formation of 2,2-disubstituted metal−alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl−
hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne
into the rhodium−thiolate bond is the rate-determining step.
Scheme 1. Pathways for Alkyne Migratory Insertion into M−
H or M−X Bonds
INTRODUCTION
■
Hydrothiolation of carbon−carbon multiple bonds is a direct
and an atom economical method for formation of carbon−
sulfur bonds present in many biologically active compounds.1
Among them, vinyl sulfides, in addition to their interesting
biological properties,2 are also useful synthetic intermediates in
organic transformations,3 ranging from enol substitutes,4
Diels−Alder,5 thio−Claisen,6 Michael acceptors,7 or olefin
metathesis,8 among others.9 Several metal catalysts including
Mo,10 Pd,11 Pt,12 Ni,13 Ru,14 Rh,11c,15,16 Ir,16 Cu,17 Au,18 Co,19
In,20 Zr,21 An (Th, U),22 and Ln (La, Sm, Lu, Nd, Y)22b are
effective for hydrothiolation of unsaturated compounds, but the
stereo- and regioselectivity control still remains an important
challenge.
In general, regioselectivity in the X−H addition across
carbon−carbon triple bonds proceeding via X−H activation
arises from a complex interplay between migratory insertion
and reductive elimination steps (Scheme 1).23,24 Although
electronic and steric properties of the alkyne and the catalyst
play an important role, formation of linear metal−alkenyls via
1,2-insertion (a,d) is generally preferred to that of branched
isomers via 2,1 insertion (b,c).25,26 Therefore, in a simplified
overview, insertion into a metal−hydride bond (a) gives rise to
linear olefins (anti-Markovnikov or β-type products) whereas
insertion into metal−heteroatom bond (d) generates branched
olefins (Markovnikov or α-type products). Particularly for
hydrothiolation, insertion into metal−hydride bonds is favored
with regard to that into metal−thiolate; thus, linear vinyl
sulfides are preferably obtained.11c,27 Preparation of the
branched vinyl sulfides, more valuable as synthetic intermedi-
Received: January 24, 2012
Published: April 26, 2012
© 2012 American Chemical Society
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ates,3 could be accomplished in a controlled manner if a
method for directing alkyne insertion into metal−thiolate
bonds is developed.
A common strategy for the previously described selective
preparation of 1-substituted vinyl sulfides catalyzed by a wide
variety of metal complexes is to design active species bearing
thiolate ligands and lacking a hydride moiety (Scheme
2).11−13,15c,21,22 In these catalytic systems the first step is
ligands has also been applied to the design of Ni,13c Cu,17 and
Au18 alkyne hydrothiolation catalysts. Herein, we present a
catalytic system for selective hydrothiolation of alkynes based
on a rhodium N-heterocyclic carbene framework. A complex
interplay between electronic and steric effects exerted by NHC,
hydride, and pyridine-type ligands accounts for the regiose-
lective formation of branched vinyl sulfides.
RESULTS AND DISCUSSION
Synthesis of Rhodium−NHC Catalysts. Dinuclear
rhodium−NHC monolefin complexes of type [Rh(μ-Cl)-
■
Scheme 2. Strategies Directed to Preparation of Branched
Vinyl Sulfides
31
(NHC)(η2-olefin)]2 are adequate precursors for a set of
catalysts via either a controlled substitution of the η2-olefin or
bridge-cleaving reactions by different ligands. In particular,
[Rh(μ-Cl)(IPr)(η2-coe)]2 (1) (coe = cyclooctene; IPr = 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-carbene), described by
James’ group,31b has been successfully applied in our
laboratories as starting material for a variety of RhI and
RhIII−NHC complexes.32 Interestingly, the two coe ligands can
be exchanged by bubbling ethylene through a toluene solution
of 1, resulting in formation of the dimer [Rh(μ-Cl)(IPr)(η2-
CH2CH2)]2 (2) (Scheme 3), which was isolated as a yellow
Scheme 3. Synthesis of Rhodium−NHC Catalysts
thiol-promoted protonolysis of an anionic ligand present in the
precatalyst or alternatively formation of a bisthiolate complex
accompanied with release of molecular hydrogen. However, in
these cases it is difficult to inhibit formation of byproducts such
as disulfide or bis(thio)alkenes. A priori, formation of these
unwanted products should be minimized if a catalytic cycle
involving initial S−H oxidative addition to the metal and
successive alkyne insertion into metal−thiolate bond and
alkenyl−hydride reductive elimination will be operative. Thus,
the following question arises: How does one favor alkyne M−S
insertion in the presence of a hydride ligand? Perhaps the well-
established high trans-director capacity of hydrides may be
useful. Due to this property, coordination of the incoming
alkyne to the unsaturated hydride−thiolate catalytic inter-
mediate may be channeled at the vacant position trans to the
hydride ligand and concomitantly cis to the thiolate, thus
favoring formation of a 2,2-disubstituted alkenyl ligand and
subsequently the branched vinyl sulfide. In particular, RhI
complexes are promising precursors to achieve this task as
they are efficient catalysts for alkyne hydrothiolation.11c,15,16
Interestingly, their general tendency to favor linear vinyl
sulfides can be reversed by ligand control, as elegantly shown
for Tp*Rh(PPh3)2 (Tp* = hydrotris(3,5-dimethylpyrazolyl)-
borate) by Love’s group.15b,d−f
In order to succeed with our proposal, high control over
coordination positions and potential isomerizations within the
metal catalytic intermediates is essential.28 An N-heterocyclic
carbene (NHC) ligand may fulfill the requirements due to its
special stereoelectronic properties.29 The high steric hindrance
and powerful electron-donor capacity of bulky NHCs could
govern the coordination positions of labile ligands and
substrates in the active species, thus determining the selectivity
outcome. Indeed, NHC ligands have been revealed as suitable
ligands not only for stabilization of reactive intermediates but
also for improvement of catalytic activity. In particular,
substitution of typical ancillary ligands such as phosphanes by
a more electron-donating NHC has extended the scope of
many catalytic transformations.30 Interestingly, this type of
solid in 92% yield. During preparation of the manuscript an
alternative method for the synthesis of 2 was reported.33 The
structure of 2 has been determined by X-ray analysis on crystals
obtained from slow diffusion of n-hexane over toluene (see
Supporting Information). This structure shows a very different
unit cell dimension compared to that reported for 2·CH2Cl2.33
1
Complex 2 showed dynamic behavior as evidenced in a H
VT-NMR study. That fact may be ascribed to two rotational
processes involving both the η2-ethylene32b,34 and the carbene
ligands (see Figure 2 for a similar behavior of 4).32b,35 In the
case of IPr, activation parameters were calculated by simulation
of the coalescence of the two signals located at δ 2.93 and 2.60
ppm corresponding to the two types of CH-isopropyl protons
(C2 symmetry). The values obtained from the corresponding
Eyring analysis were ΔH‡ = 13.9 0.5 kcal mol−1 and ΔS‡ =
−1.3 1.1 cal K−1 mol−1 for ethylene rotation and ΔH‡ = 11.8
0.4 kcal mol−1 and ΔS‡ = −1.5 0.9 cal K−1 mol−1 for IPr
rotation.
The chloro bridges in 1 and 2 were easily cleaved by
nucleophilic pyridine ligands at room temperature, resulting in
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formation of the mononuclear complexes RhCl(IPr)(L)(η2-
coe) (L = pyridine (3), 2-picoline (5), 4-picoline (7), 2-
ethylpyridine (9)) and RhCl(IPr)(L)(η2-ethylene) (L =
pyridine (4), 2-picoline (6), 4-picoline (8), 2-ethylpyridine
(10)), which were isolated as yellow solids in 77−86% yields.36
It is noticeable that the alkene ligand was not replaced by
pyridine even at 80 °C in net pyridine overnight.37
Monocrystals of 3 suitable for X-ray analysis were obtained
by slow diffusion of n-hexane over a saturated solution of 3 in
toluene (Figure 1). The complex has distorted square-planar
Figure 2. Variable-temperature 1H NMR spectra of RhCl(IPr)-
(pyridine)(η2-ethylene) (4) in CD2Cl2 showing the coalescence of the
CH−isopropyl and ethylene resonances: experimental (left) and
calculated (right).
explained by hindered rotation of 2-picoline, which breaks the
plane of symmetry.
The presence of a substituent in the 2-position of pyridine
affects not only the rotation of the ligand but also the
coordination to the metallic center. Thus, a temperature-
dependent dynamic equilibrium between 6 and 2, as a result of
the decoordination of 2-picoline, was observed between 293
and 353 K (Figure 3). Thermodynamic parameters calculated
from the Van’t Hoff representations (ln Keq vs 1/T) were ΔHo
= 14.4 0.5 kcal mol−1 and ΔSo = 32.5 1.3 cal K−1 mol−1.
The process is endothermic, whereas the slightly positive ΔSo
agrees well with an increase of internal disorder in the products
Figure 1. Molecular diagram of 3. Selected bond lengths (Å) and
angles (deg): Rh−Cl(1) 2.3886(13), Rh−N(3) 2.115(4), Rh−C(1)
2.149(4), Rh−C(2) 2.113(4), Rh−C(10) 1.986(4); Cl−Rh−N(3)
84.43(10), Cl−Rh−C(10) 86.21(12), Cl−Rh−Ct 166.02(14), N(3)−
Rh−C(10) 169.36(15), N(3)−Rh−Ct 94.42(17), C(10)−Rh−Ct
95.89(18).
geometry with pyridine and chloro ligands disposed mutually
trans to IPr and coe, respectively [N(3)−Rh−C(10)
169.36(15)°; Cl−Rh−Ct 166.02(14)°]. The rhodium−carbon
separation [Rh−C(10) 1.986(4) Å] compares well with
previously reported rhodium−NHC single-bond distances.29b
The wingtips of IPr, the η2-olefin, and the pyridine adopt an
out-of-plane disposition from the square-planar metal environ-
ment.38
1
The H and 13C{1H} spectra of compounds 3−10 confirm
the presence of η2-olefin, pyridine, and IPr ligands. Similarly to
2, the IPr and ethylene ligands in 4 exhibited restricted rotation
(Figure 2). The activation parameters obtained from the rate
constants derived from a 1H VT-NMR study and the
corresponding Eyring analysis were ΔH‡ = 13.9
0.5 kcal
1.2 cal K−1 mol−1 for ethylene
mol−1 and ΔS‡ = −1.7
rotation and ΔH‡ = 14.1 0.6 kcal mol−1 and ΔS‡ = −0.7
1.4 cal K−1 mol−1 for IPr rotation. The values obtained for
rotation of ethylene in 4 are almost identical to that calculated
for dimer 2. In contrast, the barrier for rotation of IPr around
the Rh−C axis in 4 is 2.3 kcal mol−1 higher than in 2.
The rotation barrier for the IPr ligand is also affected by the
nature of the η2-olefin. Thus, substitution of ethylene by coe
slightly hinders the rotational process as it is observed from the
parameters calculated for 3 (ΔH‡ = 15.1 0.7 kcal mol−1 and
ΔS‡ = −0.4 1.9 cal K−1 mol−1). Similar values for IPr rotation
were calculated for complex 6 bearing a 2-picoline ligand (ΔH‡
= 15.0 0.7 kcal mol−1 and ΔS‡ = −1.6 1.6 cal K−1 mol−1).
However, in this case, four CH-isopropyl signals were observed
at low temperature (δ 4.37, 4.09, 3.09, and 2.62 ppm), which
coalesced into two at 4.17 and 2.91 ppm. This fact can be
Figure 3. Variable-temperature 1H NMR spectra in the o-pyridine
proton region for the dynamic equilibrium between 6 and 2.
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due to decoordination of 2-picoline. It seems clear that an ortho
substituent on pyridine hinders coordination of the ligand to
the metallic center. In accordance, the disubstituted 2,6-
dimethylpyridine was found unable to cleave the chloro bridges
in 1 and 2.
Alkyne Hydrothiolation Catalytic Studies. Wilkinson’s
catalyst RhCl(PPh3)3 has been previously revealed as an active
catalyst for alkyne hydrothiolation.11c,15e In view that the
substitution of phosphanes by a more electron-donating NHC
has extended the scope of many catalytic transformations,30 we
studied that effect in the present transformation (eq 1).
temperature with a selectivity switch to 53/47 β-E/α (entry 2).
Although Wilkinson's catalyst is initially more active than
RhCl(IPr)(PPh3)2, the catalyst stability gained by introduction
of a IPr ligand compensates for the loss of activity. Then, other
phosphane-free Rh−NHC derivatives were tested as catalyst
precursors. RhClIPr(cod)39 (cod = 1,5-cyclooctadiene) reacts
very slowly (18% after 24 h, entry 3), but the dinuclear η2-coe
compound 1 surpassed the activity observed for RhCl(IPr)-
(PPh3)2 while maintaining clean full conversion and the
preference for the anti-Markovnikov products (entry 4). The
strong coordination of cod ligand in the former compared to
the labile coe ligands in 1 may be determinant for the very
different catalytic activity.
Exchange of coe ligands by ethylene in dimer 2 enhanced the
catalytic rate with similar preference to the linear isomer (entry
5). Surprisingly, catalysts 3 and 4, bearing a pyridine moiety,
showed reverse regioselectivity although also a moderate
decrease of catalytic activity (entries 6−7). In-situ addition of
1 equiv of pyridine to 1 and 2 gave similar results to 3 and 4,
respectively (entries 11 and 12). It was observed that an
increase of the amount of pyridine in the reaction media
switches the equilibrium to the Markovnikov isomer up to 94%
with the catalytic system 2 + 10 equiv of pyridine (entry 13)
(Figure 4). Only traces of disulfide were detected by GC-MS
Addition of thiophenol to phenylacetylene was chosen as the
benchmark reaction (Table 1). Catalytic reactions using a 1:1
Table 1. Phenylacetylene Hydrothiolation with Thiophenol
a
at 25 °C
Figure 4. Catalytic activity and regioselectivity outcome in hydro-
thiolation of phenylacetylene with thiophenol for catalysts 2 (blue
circles), 4 (green crosses), and 2 + 10 equiv of pyridine (red triangles).
analysis, whereas β-Z product was not observed in any of the
catalytic runs carried out with 1−4. A blank test without metal
catalysts showed after 24 h formation 37% of linear vinyl
sulfides of mainly Z configuration probably via a radical process
(entry 15).40 Addition of 2,6-di-tert-butyl-4-methylphenol
(BHT) to a sample catalyzed by 1 did not affect either the
activity or the selectivity, thus excluding a mechanism via
radical species operating with our catalysts (entry 18). Addition
of pyridine to the blank test did not increase activity but rather
slightly reduced it (entry 16). In order to discard that pyridine
acts as a base, NEt3 was added to a sample catalyzed by 1. The
ratio of the products was only slightly switched to the branched
isomers (compare entries 4 vs 17). To further assess that the
positive role of pyridine is played on coordination to active
species, the catalytic activity of complexes 6, 8, and 10 having 2-
picoline, 4-picoline, and 2-ethylpyridine ligands, respectively,
was studied (entries 8, 9, and 10, respectively). Complex 8 gave
a
A 0.5 mL amount of C6D6 with 2 mol % of catalyst. [subs] = 1 M.
Determined at 50% conversion. Yield of β-Z. A 20 mol % of
b
c
d
pyridine.
thiol:alkyne ratio were monitored in an NMR tube in C6D6 at
25 °C with 2 mol % catalyst loading. A catalytic test with
RhCl(PPh3)3 under our mild standard conditions showed high
initial catalytic activity but also catalyst deactivation at about
80% conversion, displaying a high preference for the β-E isomer
(entry 1). In contrast, RhCl(IPr)(PPh3)2
31b
showed full
conversion to clean addition products after 3 h at room
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similar results to 4, but catalysts 6 and 10, bearing 2-substituted
pyridines, showed a behavior between 2 and 4 in terms of both
activity and selectivity. The hindered coordination of 2-picoline
and 2-ethylpyridine, as shown in Figure 3, can be the reason for
this result. Indeed, displacement of the equilibrium to the
coordinated species by addition of 10 equiv of 2-picoline to 2
resulted in an increase of the regioselectivity to the α-isomer
(compare entries 8 vs 14).
an electron-donating group at the para position decreased both
activity and selectivity (compare entry 8, Table 2, vs entry 11,
Table 1), whereas the presence of an electron-withdrawing
substituent resulted in higher TOF and regioselectivity (entry
9). A propargyl ether was also effective with a similar isomeric
distribution (entry 10).
The scope of the catalysts was investigated using different
thiols (Table 3). Again, the regioselectivity can be tuned by
Compounds 1−4 are versatile catalyst precursors. They
promote addition of thiophenol to different alkylic and
aromatic alkynes (Table 2). The reaction rates for the alkylic
Table 3. Hydrothiolation of Phenylacetylene with Different
Thiols at 25 °C
a
a
Table 2. Alkyne Hydrothiolation with Thiophenol
a
A 0.5 mL amount of C6D6 with 2 mol % of catalyst. [subs] = 1 M.
Measured at full conversion. Using 0.5 mol % of 2. Ratio between
b
c
d
a
A 0.5 mL amount of C6D6 with 2 mol % of catalyst. [subs] = 1 M.
Exclusive formation of (E)-3-phenylsulfanyl-3-hexene.
b
β-E,α and α,α.
pyridine addition. Benzyl hydrosulfide, PhCH2SH, reacted
much faster than PhSH (entries 1−4). In fact, full conversion
ones were lower than those observed for phenylacetylene, and
thus, the reactions were performed at 50 °C. Strikingly, the
“pyridine effect” on the regioselectivity was maintained. The
amount of Markovnikov thioether product in the hydro-
thiolation of 1-hexyne was increased from 39/61 (2) to 22/78
using the pyridine complex 4 as catalyst (entries 1 and 2).
Moreover, up to 96% of branched isomer was obtained using 2
+ 10 equiv of pyridine as the catalytic system with no significant
isomerization to internal vinyl thioether. In the case of the
enyne 1-ethynyl-1-cyclohexene a similar behavior was observed,
but the complete chemoselectivity with exclusive addition of
PhSH to the triple bond is noteworthy. More interestingly, the
internal alkyne 3-hexyne gave exclusively the E isomer (entry 7,
eq 2), resulting from a syn addition of the thiol which, in
1
was attained with 2 after the first recorded H NMR spectrum
(∼3 min) with our standard conditions. A reduction of the
amount of catalyst 2 to 0.5 mol % gave the same result, with a
calculated TOF of 4000 h−1. As previously shown for PhSH,
catalyst 4 and the catalytic system 2 + 10 equiv of pyridine were
less active but much more selective to the α-isomer. A doubly
protected N-tert-butoxycarbonyl (Boc) and methylester cys-
teine was also transformed into the vinyl thioether with high
activity under mild conditions (TOF1/2 of 250−580 h−1)
(entries 5−7). Thus, our catalysts are compatible with other
functional groups, giving good selectivities to the α-vinyl sulfide
isomer. Indeed, functionalization of cysteine is an important
goal for synthesis of biologically active derivatives. Interestingly,
1,6-hexanedithiol undergoes a double-addition process with
good selectivity to the α,α-product (88%, entry 8, eq 3).
Mechanistic Proposal. A number of possible mechanisms
for alkyne hydrothiolation have been postulated. The trans-
addition products, β-Z vinyl sulfides, are mainly formed in the
absence of catalyst via a radical pathway.40 In our catalytic
system inversion of regioselectivity promoted by pyridine
addition does not proceed by radical intermediates as addition
of a radical trap (BHT) did not influence either the activity or
addition to the absence of the β-Z isomer thorough the
different catalytic tests, point to a migratory insertion
mechanism for the alkyne hydrothiolation with these systems.
The effect of modifying the electron density on the aromatic
ring of phenylacetylene has been also studied. Introduction of
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the selectivity. On the other hand, external attack of the thiol
on a coordinated alkyne is unlikely as exclusive formation of the
syn-addition β-E vinyl sulfide product was observed in the
hydrothiolation of internal alkynes. In fact, we fail to detect any
well-defined compound from addition of a stoichimetric
amount of phenylacetylene to 1 or 3. Thus, the above results
suggest a classical S−H oxidative addition and successive alkyne
insertion and reductive elimination processes as the plausible
mechanism operating with our catalysts (Scheme 1).
Figure 5. DFT-computed energies for the pentacoordinated square-
pyramidal structures of 11.
even decoordinates on increasing the steric demand on the
NHC ligand.44
In contrast to the behavior observed for 3, treatment of 1
with 1 equiv of PhSH and 3 equiv of pyridine per metal atom in
tol-d8 gave rise to a mixture of two hydride species in dynamic
equilibrium: the unsaturated complex 11 and the saturated
RhClH(SPh)(IPr)(py)2 (11-py), which exhibited a doublet at δ
−16.95 ppm with JRh−H = 15.6 Hz, in a 1:2.5 ratio at 20 °C. The
new saturated octahedral 11-py results from coordination of
pyridine ligand in 11. A similar downfield shift and reduction of
the JRh−H coupling constant has been previously observed in
related rhodium complexes by coordination of pyridine to the
vacant site of the square-pyramidal hydride precursors.45
In order to shed light on the effect of pyridine coordination
on the regioselectivity outcome, several stoichiometric NMR
experiments were carried out. Treatment of 1 with thiophenol
gave rise to an unidentified mixture of rhodium hydrides,
probably originated from oxidative addition of PhSH.41 In fact,
formation of several mononuclear or dinuclear thiolate- or
chloro-bridged complexes can be envisaged.42 However, when
the pyridine complex 3 was treated with PhSH, a doublet at
−26.55 ppm with JRh−H = 48.5 Hz appeared as the main
hydride species, along with the presence of free coe. The
upfield shift and the high coupling constant of this resonance
suggest a square-pyramidal structure with the hydride
occupying the apical position.32a,43 Unfortunately, complex
RhClH(SPh)(IPr)(py) (11) could not be isolated. Addition of
1 equiv of phenylacetylene at −20 °C to an NMR sample of 11
generated in situ led to smooth formation of the organic
product phenyl(1-phenylvinyl)sulfane, resulting from the
Markovnikov addition of PhSH to the alkyne, and a mixture
of unidentified rhodium species with concomitant disappear-
ance of the hydride signal. The fact that alkenyl intermediates
could not be detected points to the migratory insertion as the
rate-determining step. In accordance, alkyl thiols react faster
than thiophenol. Although the more acidic thiophenol
facilitates the oxidative addition step, the more basic sulfur
atom of alkyl thiolate ligands should favor migratory insertion.
Indeed, the observation that an electron-withdrawing group at
the para position of the aromatic ring of phenylacetylene
increases the activity is in accordance with this proposal.
Insertion of alkyne into the Rh−S bond should be facilitated by
a decrease in the electronic density on the C−C triple bond.
It seems likely that oxidative addition of the thiophenol to 3
generates an unsaturated pentacoordinated complex 11 via
decoordination of the olefin on the electron-poor RhIII
intermediate, with the pyridine occupying the less congested
trans position with regard to the high sterically demanding IPr.
Indeed, DFT calculations on the stability of different
unsaturated square-pyramidal isomers with pyridine (11a),
chloride (11b), and thiolate (11c) located trans to IPr showed
that 11a is 13.5 and 14.3 kcal mol−1 more stable than 11b and
11c, respectively (Figure 5). The trigonal bypiramidal structure
for these species was not found as minima in any case. In
addition, the bulky IPr ligand makes difficult the coordination
of the amine ligands in the equatorial position. In fact, several
studies on saturated complexes having two pyridine ligands
have shown that the pyridine ligand cis to IPr is more labile or
1
A H VT-NMR study confirmed the dynamic equilibrium
between 11 and 11-py (Figure 6). As can be seen in the right
part of the figure, which shows the hydride region of the
spectra, on lowering the temperature from 303 to 243 K the
Figure 6. Stacked plot of variable-temperature 1H NMR spectra in tol-
d8 in the o-pyridine (left) and hydride (right) regions of the
equilibrium between 11 and 11-Py.
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intensity of the hydride signal corresponding to 11 diminishes
and shifts to lower field approaching that of 11-py which
slightly broadens. More informative is the left part of the figure.
The ortho protons of the pyridine ligand of 11 (red circles)
integrate by 2 related to the hydride located at upfield shift,
which suggests free rotation of the pyridine ligand that makes
both o-protons equivalents. This resonance does not change
the 2-ethylpyridine prevents formation of the saturated
complex 13-py, and thus, clean formation of square-pyramidal
13 was observed (δ −26.93 ppm, JRh−H = 47.0 Hz).
The unsaturated square-pyramidal species a similar to 11 is
key for the explanation of the observed regioselectivity outcome
(Scheme 4). The high trans influence of the hydride ligand
1
much with temperature. In contrast, the H NMR spectrum at
Scheme 4. Proposed Catalytic Cycle for the Regioselective
243 K of 11-py displays three signals for the four pyridine
protons. The resonance at 9.06 ppm integrates 2:1 with respect
to the hydride signal at −16.27 ppm, and it is assigned to both
o-pyridine protons of the ligand trans to IPr (blue daggers). In
contrast, the signals corresponding to the pyridine ligand cis to
IPr in 11-py (blue squares) split into two resonances at 9.58
and 9.31 ppm, probably by hindered rotation of the pyridine
due the proximity of bulky IPr. At higher temperature these
resonances coalesce to a single resonance over 263 K and
exchange with that corresponding to free pyridine (green
pound signs). Interestingly, the hydride signal for 11 shifts to
low field at low temperature but remains unchanged over 303
K. This observation suggests that 11 is likely involved in
another equilibrium, probably a dimerization process, resulting
in saturation of the vacant site.42
Alkyne Hydrothiolation Leading to Branched Vinyl Sulfides
Coordination of pyridine cis to IPr is not exclusively
determined by the sterical pressure of the carbene ligand.
Treatment of 1 with 1 equiv of benzylthiol and 3 equiv of
pyridine per metal atom resulted in exclusive formation of a
pyridine-saturated hydride complex RhClH(SCH2Ph)(IPr)-
determines the stereochemistry of the complex and directs
coordination of the incoming ligand (alkyne or pyridine) trans
to it (b). Therefore, the alkyne is now located in cis disposition
regarding thiolate ligand, thus favoring migratory insertion
between both groups to generate a 2,2-disubstituted alkenyl
ligand (c).46 Reductive elimination within c generates a
branched vinyl sulfide. The influence of the pyridine excess
on the activity and regioselectivity should be related to an
increase of the concentration of a in the reaction media.
Although formation of species such as 11-py may retard alkyne
coordination, resulting in an inferior catalytic activity, as
observed for the catalytic system 2 + 10 equiv of pyridine, it
could also help to prevent both decoordination of pyridine and
isomerization of a, thus increasing the selectivity. In fact,
decoordination of pyridine gives rise to a hypothetical square-
pyramidal pentacoordinated species d with the alkyne trans to
IPr and cis to both hydride and thiolate ligands where insertion
into Rh−H or Rh−S bonds is now possible, therefore
accounting for an unselective catalytic outcome. Catalytic
pathway via d is probably operating with the dinuclear catalyst
precursors 1 and 2 or the phosphane catalyst RhCl(IPr)-
(PPh3)2, which exhibit good activity but moderate regiose-
lectivity. In the case of 2-substituted pyridine complexes 5, 6, 9,
and 10 the steric repulsion between the substituent and the
incoming alkyne reduces the ability for these catalyst to
generate b-type intermediates and also favors pyridine
decoordination, as shown in Figure 3, driving the catalytic
reaction through type d intermediates.
1
(py)2 (12-py) (Figure 7). The H VT-NMR spectra of 12-py
1
Figure 7. H NMR spectra in tol-d8 at 223 K for 12-py (octahedral)
and 13 (square pyramidal).
are very similar to that of 11-py. In particular, the hydride
ligand was observed as a doublet at δ −17.31 ppm with JRh−H
=
15.0 Hz at 25 °C. In the o-pyridine region a doublet at 9.28
ppm (JH−H = 5.2 Hz) corresponds to the pyridine ligand trans
to IPr, whereas the cis pyridine ligand splits into two signals at
9.45 and 9.19 ppm at 243 K. However, there is no equilibrium
between 12-py and the potential unsaturated compound 12.
Benzylthiolate is more flexible than phenylthiolate, and thus,
complex 12-py can accommodate a second pyridine ligand
more easily than 11-py. The steric influence of the ancillary
ligands also affects the potential coordination of a N-donor
ligand trans to the hydride ligand, as observed in the in-situ
formation of the 2-ethylpyridine complex RhClH(SCH2Ph)-
(IPr)(Et-py) (13). The increase of the steric bulk exerted by
Consequently, a complex interplay between the stereo-
electronic properties of IPr, pyridine, and hydride ligands
accounts for the good-to-excellent regioselectivity achieved with
these catalytic systems. It is likely that IPr and hydride direct
pyridine coordination to the more favorable coordination site,
that located trans to IPr. Alkyne is now forced to coordinate
trans to hydride and so cis to thiolate, favoring Markovnikov
selectivity. The absence of pyridine allows the alkyne to
coordinate in the preferred site trans to IPr position, and thus,
subsequent unselective migratory insertion takes place. This
catalytic system is an example where pyridine plays an
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important role as additive in a transition-metal-mediated
catalytic transformation.47 The “pyridine effect” in this system
has been rationalized with a molecular basis, and it is further
supported by theoretical calculations.
Theoretical Calculations on the Reaction Mechanism.
To gain more insight about the feasibility of our proposed
mechanism, theoretical calculations on the catalytic cycle were
performed for phenylacetylene and thiophenol (DFT, B3LYP,
kcal mol−1, Figure 8). To our knowledge, theoretical studies
configuration of the final product. Thus, both pathways for
insertion of thiol were calculated. The red line shows the 1,2-
insertion or Markovnikov addition pathway (a), whereas the
blue line corresponds to the 2,1-insertion or anti-Markovnikov
route (b) (Figure 8). The complete energy profile shows that
the reaction is kinetically determined by the migratory insertion
step as the highest hurdle. In accordance with the experimental
results, formation of branched vinyl sulfides obtained via the
Markovnikov pathway is favored. The possibility of migratory
insertion of the alkyne into the rhodium−hydride bond has
been also calculated. In agreement with our mechanistic
proposal, the transition states for both insertion types are
higher in energy than TSIa (25.9 and 27.7 kcal mol−1, see
Supporting Information for details).
It has been determined that coordination of the alkyne to
11a is hindered by sterical means; thus, a stationary point for
the saturated species similar to b (Scheme 4) was not found for
either alkyne orientation. However, we were able to locate the
transition states corresponding to C−S and Rh−C coupling for
both pathways (TSIa and TSIb). An affordable value of 24.0
kcal mol−1 was computed for 1,2-insertion TSIa, which is 4.3
kcal mol−1 lower in energy than TSIb. Either transition state
displays a roughly metallacyclobutene structure including the
metal, sulfur, and the two carbon atoms of the former alkyne
(Figure 9). Rhodium−thiolate and C−C bonds slightly
elongate from 2.355 Å in 11a and 1.210 Å in phenylacetylene
to 2.414 (Rh−S) and 1.276 Å (C−C) in TSIa and 2.399 (Rh−
S) and 1.260 Å (C−C) in TSIb.
Intermediates Ia and Ib bearing a thioalkenyl ligand, similar
to proposed species c, were found to be −10.2 and −6.1 kcal
mol−1 more stable than the starting point. The new Rh−C
bond formed agrees with typical simple bond distances, 2.007 Å
in Ia and 2.010 Å in Ib. In the reductive elimination step, two
new transition states were located at 8.5 (TSIIb) and −2.0 kcal
mol−1 (TSIIa) being again the anti-Markovnikov pathway
Figure 8. Computed potential-energy profile for the alkyne migratory
insertion and reductive elimination for formation of vinyl sulfide.
dealing with alkyne hydrothiolation or insertion of an alkyne
into metal−thiolate bonds are scarce.13d,48 Calculations of the
involved organometallic intermediates were performed without
any simplifying approximation in order to increase the accuracy
of the results. The starting point is the square-pyramidal
thiolate−hydride species 11a (Figure 5) and phenylacetylene.
In 11a only migratory insertion of the alkyne into rhodium−
thiolate is possible, but discrimination between migration into
the α or β carbon atom of the alkyne determines the
Figure 9. Optimized structures for the intermediates and transition states for migratory insertion and reductive elimination steps including selected
distances (Å) and angles (deg).
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graph system using a HP-5MS 5% phenyl methyl siloxane column (30
m × 250 μm with a 0.25 μm film thickness).
unfavorable. The final point corresponding to the unsaturated
species RhCl(IPr) II and the respective coupled organic
products is slightly more favorable for formation of linear vinyl
sulfides (−3.8 vs −0.1 kcal mol−1), although both pathways are
thermodynamically less energetic that the starting point.
Preparation of [Rh(μ-Cl)(IPr)(η2-CH2CH2)]2 (2). A yellow
solution of 1 (300 mg, 0.236 mmol) in 10 mL of toluene at room
temperature was bubbled with ethylene for 15 min. Then, the solvent
was evaporated to dryness, and subsequent addition of hexane caused
precipitation of a yellow solid, which was washed with hexane (3 × 4
mL) and dried in vacuo. Yield: 240 mg (92%). Anal. Calcd for
C58H80N4Cl2Rh2: C, 62.76; H, 7.26; N, 5.05. Found: C, 62.45; H, 7.15;
CONCLUSION
■
Herein we presented highly active Rh−NHC catalysts for
alkyne hydrothiolation under mild conditions. Pyridine-type
ligands easily cleave the chloro bridges in dinuclear derivatives
[Rh(μ-Cl)(IPr)(η2-olefin)]2 to afford stable mononuclear
complexes RhCl(IPr)(L)(η2-coe). The presence of a powerful
electron-donor bulky IPr ligand prevents deactivation of active
species, thus increasing catalytic activity compared to RhCl-
(PPh3)3. It has been found that dinuclear complexes are more
active than their related pyridine mononuclear complexes.
However, a reversion in the regioselectivity from β-E to α was
observed with mononuclear catalysts compared to dinuclear
precursors.
1
N, 4.94. H NMR (300 MHz, tol-d8, 233 K): δ 7.3−7.1 (m, 12H,
HPh−IPr), 6.19 (s, 4H, CHN), 3.27 and 2.90 (both sept, JH−H = 6.6,
8H, CHMeIPr), 2.84 and 2.27 (both d, JH−H = 12.6, 8H, CH2CH2),
1.62, 1.47, 1.00, and 0.96 (all d, JH−H = 6.6, 48H, CHMeIPr). 13C{1H}-
APT NMR plus HSQC and HMBC (75.4 MHz, tol-d8, 233 K): δ
179.7 (d, JC−Rh= 62.3, Rh−CIPr), 146.2 and 145.2 (both s, Cq), 137.0
(s, CqN), 129.1 and 123.6 (s, CHPh‑IPr), 123.7 (s, CHN), 43.4 (d,
JC−Rh = 16.7, CH2CH2), 28.5(s, CHMeIPr), 25.8 and 23.3 (both s,
CHMeIPr).
Preparation of RhCl(IPr)(η2-coe)(py) (3). A yellow solution of 1
(300 mg, 0.236 mmol) in 10 mL of toluene was treated with pyridine
(200 μL, 2.47 mmol) and stirred at room temperature for 15 min.
Then the solvent was evaporated to dryness, and subsequent addition
of hexane caused precipitation of a yellow solid, which was washed
with hexane (3 × 4 mL) and dried in vacuo. Yield: 280 mg (83%).
Anal. Calcd for C40H55N3ClRh: C, 67.07; H, 7.73; N, 5.87. Found: C,
66.83; H, 7.55; N, 6.02. 1H NMR (300 MHz, C6D6, 298 K): δ 8.40 (d,
JH−H = 4.3, 2H, H2‑Py), 7.4−7.2 (m, 6H, HPh−IPr), 6.64 (s, 2H, 
CHN), 6.55 (t, JH−H = 5.4, 1H, H4‑Py), 6.20 (dd, JH−H = 5.4, JH−H = 4.3,
2H, H3‑Py), 4.52 and 2.61 (both sept, JH−H = 6.4, 4H, CHMeIPr), 3.12
(m, 2H, CHCHcoe), 1.81, 1.47, 1.15, and 1.02 (all d, JH−H = 6.4,
24H, CHMeIPr), 1.8−0.8 (br, 12H, CH2‑coe). 13C{1H}-APT NMR plus
HSQC and HMBC (75.6 MHz, C6D6, 253 K): δ 183.3 (d, JC−Rh= 54.0,
Rh−CIPr), 153.9 (s, C2‑Py), 149.3 and 146.6 (both s, Cq), 138.3 (s,
CqN), 135.1 (s, C4‑Py), 129.7, 128.9, 125.5, and 123.1 (all s, CHPh‑IPr),
124.8 (s, CHN), 122.9 (s, C3‑Py), 56.1 (d, JC−Rh = 16.4, CH
CHcoe), 30.6, 30.4, and 27.3 (all s, CH2‑coe), 29.5 and 29.4 (both s,
CHMeIPr), 27.2, 27.1, 24.6, and 23.1 (all s, CHMeIPr).
It has been also determined that the mechanism proceeds via
oxidative addition of the S−H bond to RhI intermediates and
successive alkyne migratory insertion and reductive elimination
steps. The regioselectivity of the catalytic outcome is
determined by the alkyne migratory insertion into the Rh−S
bond. The interplay of electronic and steric effects exerted by
IPr, pyridine, and hydride ligands accounts for regioselective
formation of branched vinyl sulfides as a consequence of the
1,2-insertion into the rhodium−thiolate bond. The encumbered
and powerful electron-donor NHC ligand directs coordination
of the pyridine trans to it, consequently blocking coordination
of the alkyne in this position. Simultaneously, the trans
influence of the hydride paves the way to a cis thiolate−alkyne
disposition that gives rise to the branched vinyl sulfide
regioisomer. This mechanistic proposal is supported by DFT
quantum-mechanical calculations. The rate-determining step is
the alkyne migratory insertion leading to C−S bond formation.
The energy difference of 4.3 kcal mol−1 favoring the 1,2- over
the 2,1-migratory insertion fully accounts for the observed
regioselectivity.
Preparation of RhCl(IPr)(η2-CH2CH2)(py) (4). The complex was
prepared as described for 3 starting with 2 (300 mg, 0.270 mmol) and
pyridine (220 μL, 2.72 mmol). Yield: 290 mg (85%). Anal. Calcd for
C34H45N3ClRh: C, 64.40; H, 7.15; N, 6.63. Found: C, 64.22; H, 7.02;
N, 6.45. 1H NMR (500 MHz, tol-d8, 253 K): δ 8.32 (d, JH−H = 4.5, 2H,
H2‑Py), 7.3−7.1 (m, 6H, HPh‑IPr), 6.54 (s, 2H, CHN), 6.46 (t, JH−H
=
5.5, 1H, H4‑Py), 6.08 (dd, JH−H = 5.5, JH−H = 4.5, 2H, H3‑Py), 4.09 and
3.08 (both sept, JH−H = 6.5, 4H, CHMeIPr), 2.52 and 2.03 (both d,
The findings reported herein could be useful for rational
molecular design of new catalysts with improved regioselectiv-
ity in related addition processes such as hydroalkoxylation,
hydrophosphination, hydroamination, or hydroacylation among
others. With the aim to improve activity and selectivity, catalyst
design by modification of both NHC and ancillary ligands is
currently investigated in our laboratories.
JH−H = 11.5, 4H, CH2CH2), 1.91, 1.57, 1.23, and 1.13 (all d, JH−H
=
6.5, 24H, CHMeIPr). 13C{1H}-APT NMR plus HSQC and HMBC
(125.6 MHz, tol-d8, 253 K): δ 183.3 (d, JC−Rh= 54.5, Rh−CIPr), 151.5
(s, C2‑Py), 148.1 and 145.4 (both s, Cq), 137.4 (s, CqN), 134.5 (s,
C4‑Py), 129.1, 128.4, 124.2, and 122.7 (all s, CHPh−IPr), 123.7 (s, 
CHN), 122.5 (s, C3‑Py), 41.6 (d, JC−Rh = 15.4, CH2CH2), 28.9 and
28.7 (both s, CHMeIPr), 26.3, 26.1, 23.9, and 22.9 (all s, CHMeIPr).
Preparation of RhCl(IPr)(η2-coe)(2-picoline) (5). The complex was
prepared as described for 3 starting with 1 (300 mg, 0.236 mmol) and
2-picoline (230 μL, 2.329 mmol). Yield: 275 mg (80%). Anal. Calcd
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out with
rigorous exclusion of air using Schlenk-tube techniques. 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 starting material
[Rh(μ-Cl)(IPr)(coe)]2 (1) was prepared as previously described in
the literature.31b 1H, 13C{1H}, and 19F NMR spectra were recorded on
a Varian Gemini 2000 Bruker ARX 300 MHz, Bruker Avance 400
MHz, or Bruker Avance 500 MHz instrument. Chemical shifts
(expressed in parts per million) are referenced to residual solvent
peaks (1H, 13C{1H}) or external CFCl3 (19F). Coupling constants, J,
are given in Hertz. Spectral assignments were achieved by combination
of 1H−1H COSY, 13C APT, and 1H−13C HSQC/HMBC experiments.
C, H, and N analyses were carried out in a Perkin-Elmer 2400 CHNS/
O analyzer. GC-MS analysis were run on an Agilent 5973 mass-
selective detector interfaced to an Agilent 6890 series gas chromato-
for C41H57N3ClRh: C, 67.43; H, 7.87; N, 5.75. Found: C, 67.74; H,
1
7.91; N, 5.68. H NMR (300 MHz, C6D6, 293 K): δ 8.43 (d, JH−H
=
5.2, 1H, H6‑Py), 7.49−7.11 (m, 6H, HPh−IPr), 6.71 and 6.61 (both d,
JH−H = 1.4, 2H, CHN), 6.57 (dd, JH−H = 7.9, JH−H = 7.6, 1H, H4‑Py),
6.34 (d, JH−H = 7.6, 1H, H3‑Py), 6.21 (dd, JH−H = 7.9, JH−H = 5.2, 1H,
H5‑Py), 4.96, 4.16, 2.83, and 2.20 (all sept, JH−H = 6.4, 4H, CHMeIPr),
3.07 (m, 2H, CHCHcoe), 2.75 (s, 3H, HCH3‑Py), 1.97, 1.65, 1.49,
1.33, 1.22, 1.11, 1.07, and 0.94 (all d, JH−H = 6.4, 24H, CHMeIPr), 1.8−
0.8 (br, 12H, CH2‑coe). 13C{1H}-APT NMR plus HSQC and HMBC
(75.6 MHz, C6D6, 293 K): δ 185.7 (d, JC−Rh= 53.5, Rh−CIPr), 162.1 (s,
C2‑Py), 153.0 (s, C6‑Py), 149.7, 149.4, 146.9, and 146.7 (all s, Cq), 138.5,
138.0 (both, CqN), 135.0 (s, C4‑Py), 130.1, 129.9, 126.0, 125.5, and
123.4, 122.9 (all s, CHPh‑IPr), 125.2 and 124.9 (both s, CHN), 124.6
(s, C3‑Py), 119.9 (s, C5‑Py), 58.3, 52.5 (both d, JC−Rh = 16.3, CH
CHcoe), 31.3, 30.0, 29.4, 29.3, 27.4, and 27.2 (all s, CH2‑coe), 29.9, 29.7,
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29.2, and 29.0 (all s, CHMeIPr), 27.9, 27.3, 27.1, 26.7, 25.0, 24.1, 23.5,
and 22.5 (all s, CHMeIPr), 26.9 (s, CCH3‑Py).
58.3, 51.6 (both d, JC−Rh = 16.0, CHCHcoe), 32.9 (s, CH2‑Py), 30.9,
30.4, 29.5, 29.4, 26.8, and 26.7 (all s, CH2‑coe), 29.5, 29.2, 28.7, and
28.5 (all s, CHMeIPr), 27.5, 26.6, 26.4, 26.2, 24.7, 23.5, 23.1, and 21.9
(all s, CHMeIPr), 13.4 (s, CH3‑Py).
Preparation of RhCl(IPr)(η2-CH2CH2)(2-picoline) (6). The
complex was prepared as described for 3 starting with 2 (300 mg,
0.270 mmol) and 2-picoline (270 μL, 2.734 mmol). Yield: 270 mg
(77%). Anal. Calcd for C35H47N3ClRh: C, 64.86; H, 7.31; N, 6.48.
Preparation of RhCl(IPr)(η2‑CH2CH2)(2-ethylpiridine) (10). The
complex was prepared as described for 3 starting with 2 (300 mg,
0.270 mmol) and 2-ethylpyridine (310 μL, 2.71 mmol). Yield: 315 mg
(86%). Anal. Calcd for C37H53N3ClRh: C, 65.53; H, 7.88; N, 6.20.
1
Found: C, 65.01; H, 7.45; N, 6.37. H NMR (500 MHz, tol-d8, 253
K): δ 8.21 (d, JH−H = 5.3, 1H, H6‑Py), 7.37−7.09 (m, 6H, HPh‑IPr), 6.59
and 6.49 (both d, JH−H = 1.6, 2H, CHN), 6.47 (dd, JH−H = 8.0, JH−H
= 7.7, 1H, H4‑Py), 6.14 (d, JH−H = 7.7, 1H, H3‑Py), 5.93 (dd, JH−H = 8.0,
JH−H = 5.3, 1H, H5‑Py), 4.32, 4.05, 3.05, and 2.59 (all sept, JH−H = 6.7,
4H, CHMeIPr), 2.58 (s, 3H, HCH3‑Py), 2.56, 2.35, 2.04, and 1.64 (all dd,
JH−H = 11.7, JH−H = 8.8, 4H, CH2CH2), 1.94, 1.70, 1.49, 1.40, 1.21,
1.13, 1.10, and 1.02 (all d, JH−H = 6.7, 24H, CHMeIPr). 13C{1H}-APT
NMR plus HSQC and HMBC (125.6 MHz, tol-d8, 293 K): δ 184.9 (d,
JC−Rh= 53.8, Rh−CIPr), 160.9 (s, C2‑Py), 151.7 (s, C6‑Py), 149.5 and
147.2 (both s, Cq), 137.4 (s, CqN), 134.7 (s, C4‑Py), 129.1, 128.4,
125.4, and 122.8 (all s, CHPh−IPr), 124.1 (s, C3‑Py), 123.9 (s, CHN),
120.5 (s, C5‑Py), 43.7 (d, JC−Rh = 16.6, CH2CH2), 25.4 (s, CCH3‑Py),
29.1 and 28.9 (both s, CHMeIPr), 26.6, 26.1, 23.7, and 23.2 (all s,
CHMeIPr).
1
Found: C, 65.89; H, 7.98; N, 6.33. H NMR (500 MHz, tol-d8, 253
K): δ 8.27 (d, JH−H = 5.1, 1H, H6‑Py), 7.38−6.99 (m, 6H, HPh‑IPr), 6.60
and 6.47 (both d, JH−H = 1.5, 2H, CHN), 6.57 (dd, JH−H = 9.0, JH−H
= 7.7, 1H, H4‑Py), 6.27 (d, JH−H = 7.7, 1H, H3‑Py), 6.09 (dd, JH−H = 9.0,
JH−H = 5.1, 1H, H5‑Py), 4.42, 3.94, 3.15, and 2.45 (all sept, JH−H = 6.6,
4H, CHMeIPr), 3.60 and 3.06 (both dq, JH−H = 15.1,, JH−H = 7.6, 2H,
CH2‑Py), 2.62, 2.31, 2.04, and 1.62 (all dd, JH−H = 11.7, JH−H = 8.4, 4H,
CH2CH2), 1.96, 1.6, 1.51, 1.36, 1.21, 1.13, 1.09, and 1.01 (all d, JH−H
= 6.6, 24H, CHMeIPr), 0.96 (dd, JH−H = 7.6, JH−H = 7.6, 3H, CH3‑Py).
13C{1H}-APT NMR plus HSQC and HMBC (125.6 MHz, tol-d8, 253
K): δ 184.3 (d, JC−Rh= 53.8, Rh−CIPr), 165.5 (s, C2‑Py), 151.6 (s, C6‑Py),
148.7, 148.4, 146.2, and 145.7 (all s, Cq), 137.6 and 137.2 (both s,
CqN), 135.0 (s, C4‑Py), 129.8, 129.5, 128.2, 124.6, 123.0, and 122.6 (all
s, CHPh‑IPr), 124.4 and 123.7 (both s, CHN), 122.1 (s, C3‑Py), 120.9
(s, C5‑Py), 44.5 and 36.9 (both d, JC−Rh = 17.3, CH2CH2), 31.8 (s,
CH2‑Py), 29.4, 29.2, 28.8, and 28.6 (all s, CHMeIPr), 27.2, 26.6, 26.3,
26.1, 24.6, 23.3, 23.0, and 22.2 (all s, CHMeIPr), 13.6 (s, CH3‑Py).
In-Situ Preparation of RhClH(SPh)(IPr)(py) (11). A solution of 3
(23 mg, 0.032 mmol) in C6D6 (0.5 mL, NMR tube) at 20 °C was
Preparation of RhCl(IPr)(η2-coe)(4-picoline) (7). The complex was
prepared as described for 3 starting with 1 (300 mg, 0.236 mmol) and
4-picoline (230 μL, 2.329 mmol). Yield: 280 mg (81%). Anal. Calcd
for C41H57N3ClRh: C, 67.43; H, 7.87; N, 5.75. Found: C, 67.81; H,
1
7.98; N, 5.87. H NMR (300 MHz, tol-d8, 253 K): δ 8.22 (d, JH−H
=
5.7, 2H, H2‑Py), 7.42−6.97 (m, 6H, HPh−IPr), 6.65 (s, 2H, CHN),
6.08 (d, JH−H = 5.7, 2H, H2‑Py), 4.46 and 2.61 (both sept, JH−H = 6.3,
4H, CHMeIPr), 3.05 (m, 2H, CHCHcoe), 1.75, 1.46, 1.14, and 1.03
(all d, JH−H = 6.3, 24H, CHMeIPr), 1.7−0.8 (br, 12H, CH2‑coe), 1.50 (s,
3H, HCH3‑Py). 13C{1H}-APT NMR plus HSQC and HMBC (75.6
MHz, tol-d8, 253 K): δ 185.8 (d, JC−Rh= 53.9, Rh−CIPr), 153.0 (s,
C2‑Py), 149.1 and 146.1 (both s, Cq), 146.2 (s, C4‑Py), 138.0 (s, CqN),
129.7, 128.5, 125.0, and 122.7 (all s, CHPh‑IPr), 124.4 (s, CHN),
123.5 (s, C3‑Py), 55.4 (d, JC−Rh = 16.5, CHCHcoe), 30.3, 30.0, and
27.0 (all s, CH2‑coe), 29.1 and 29.0 (both s, CHMeIPr), 26.8, 26.7, 24.1,
and 22.8 (all s, CHMeIPr), 20.2 (s, CH3‑Py).
1
treated with thiophenol (2.3 μL, 0.034 mmol). H NMR (500 MHz,
C6D6, 293 K): δ 8.96 (d, JH−H = 5.0, 2H, H2‑Py), 7.42−6.88 (m, 11H,
HPh), 6.85 (s, 2H, CHN), 6.26 (d, JH−H = 7.6, 1H, H4‑Py), 5.95 (dd,
JH−H = 7.6−5.0, 2H, H3‑Py), 3.76 and 3.66 (both sept, JH−H = 6.3, 4H,
CHMeIPr), 1.96, 1.53, 1.26, and 1.10 (all d, JH−H = 6.3, 24H,
CHMeIPr), −26.55 (d, JRh−H = 48.5, 1H, Rh−H).
In-Situ Preparation of RhClH(SPh)(IPr)(py)2 (11-py). A solution of
1 (22 mg, 0.017 mmol) in toluene-d8 (0.5 mL, NMR tube) at −30 °C
was treated with pyridine (8 μL, 0.102 mmol) and thiophenol (2.3 μL,
0.034 mmol). 1H NMR was immediately recorded at low temperature.
A mixture of 11 and 11-py was observed in a 1:2.5 ratio (at 293 K).
Preparation of RhCl(IPr)(η2-CH2CH2)(4-picoline) (8). The
complex was prepared as described for 3 starting with 2 (300 mg,
0.270 mmol) and 4-picoline (270 μL, 2.734 mmol). Yield: 277 mg
(79%). Anal. Calcd for C35H47N3ClRh: C, 64.86; H, 7.31; N, 6.48.
Found: C, 65.15; H, 7.45; N, 6.52. 1H NMR (400 MHz, C6D6, 298 K):
δ 8.23 (d, JH−H = 6.0, 2H, H2‑Py), 7.34−7.00 (m, 6H, HPh‑IPr), 6.60 (s,
2H, CHN), 5.97 (d, JH−H = 6.0, 2H, H3‑Py), 4.11 and 3.14 (both br,
4H, CHMeIPr), 2.52 and 2.12 (both br, 4H, CH2CH2), 1.87−1.00
(br, 24H, CHMeIPr), 1.50 (s, 3H, HCH3‑Py). 13C{1H}-APT NMR plus
1
Data for 11-py: H NMR (500 MHz, tol-d8, 243 K): δ 9.58 and 9.31
(both br, 2H, Ho‑Py‑A), 9.06 (br, 2H, Ho‑Py‑B), 7.40−6.40 (br, 11H, HPh
and CHN), 6.62 (br, 1H, Hp‑Py‑A), 6.24 (br, 1H, Hp‑Py‑B), 6.51 and
6.07 (both br, 2H, Hm‑Py‑A), 6.06 (br, 2H, Hm‑Py‑B), 4.37, 4.02, 3.51, and
3.26 (all br, 4H, CHMeIPr), 2.21, 1.67, 1.65, 1.60, 1.33, 1.22, 1.17, and
1.04 (all br, 24H, CHMeIPr), −16.27 (d, JRh−H = 15.4, 1H, Rh−H).
In-Situ Preparation of RhClH(SCH2Ph)(IPr)(py)2 (12-py). A
solution of 1 (22 mg, 0.017 mmol) in toluene-d8 (0.5 mL, NMR-
tube) at −30 °C was treated with pyridine (8 μL, 0.102 mmol) and
benzylthiol (3.6 μL, 0.034 mmol). 1H NMR was immediately recorded
HSQC and HMBC (100.6 MHz, C6D6, 298 K): δ 184.6 (d, JC−Rh
=
53.7, Rh−CIPr), 151.8 (s, C2‑Py), 147.4 and 147.2 (s, Cq), 146.8 (s,
C4‑Py), 138.3 (s, CqN), 130.1 and 128.7 (all s, CHPh‑IPr), 124.8 (s, 
CHN), 124.7 (s, C3‑Py), 42.1 (d, JC−Rh = 16.9, CH2CH2), 29.6 and
29.4 (both s, CHMeIPr), 26.8, 26.6, 24.1, 26.6 (both s, CHMeIPr), 20.5
(s, CCH3‑Py).
1
at low temperature. H NMR (500 MHz, tol-d8, 223 K): δ 9.49 and
9.20 (both d, JH−H = 4.9, 2H, Ho‑Py‑A), 9.44 (d, JH−H = 5.2, 2H, Ho‑Py‑B),
7.40−6.90 (m, 11H, HPh), 6.72 and 6.49 (both br, 2H, CHN), 6.61
(br, 1H, Hp‑Py‑A), 6.43 (br, 1H, Hp‑Py‑B), 6.42 and 5.96 (both br, 2H,
Hm‑Py‑A), 6.24 (br, 2H, Hm‑Py‑B), 4.56, 3.97, 3.55, and 2.85 (all sept,
JH−H = 6.2, 4H, CHMeIPr), 2.45 and 2.20 (both d, JH−H = 11.7, 2H,
Preparation of RhCl(IPr)(η2-coe)(2-ethylpyridine) (9). The complex
was prepared as described for 3 starting with 1 (300 mg, 0.236 mmol)
and 2-ethylpyridine (270 μL, 2.36 mmol). Yield: 285 mg (81%). Anal.
Calcd for C42H59N3ClRh: C, 67.78; H, 7.99; N, 5.65. Found: C, 68.19;
CH2S), 2.06, 1.87, 1.71, 1.34, 1.32, 1.22, 1.16, and 0.93 (all d, JH−H
6.2, 24H, CHMeIPr), −17.10 (d, JRh−H = 15.2, 1H, Rh−H).
=
H, 8.12; N, 5.82. 1H NMR (500 MHz, tol-d8, 253 K): δ 8.43 (d, JH−H
=
In-Situ Preparation of RhClH(SCH2Ph)(IPr)(2-Etpy) (13). A solution
10 (19 mg, 0.026 mmol) in toluene-d8 (0.5 mL, NMR-tube) at −50
5.3, 1H, H6‑Py), 7.38−6.99 (m, 6H, HPh‑IPr), 6.63 (dd, JH−H = 8.9, JH−H
= 7.8, 1H, H4‑Py), 6.62 and 6.54 (both d, JH−H = 1.4, 2H, CHN),
6.39 (d, JH−H = 7.8, 1H, H3‑Py), 6.20 (dd, JH−H = 8.9, JH−H = 5.3, 1H,
H5‑Py), 4.93, 3.98, 3.11, and 2.90 (all sept, JH−H = 6.6, 4H, CHMeIPr),
3.65 and 3.34 (both dq, JH−H = 15.2, JH−H = 7.5, 2H, CH2‑Py), 3.07 and
2.94 (both br, 2H, CHCHcoe), 2.11, 2.02, 1.59, 1.40, 1.29, 1.22, 1.08,
and 0.92 (all d, JH−H = 6.6, 24H, CHMeIPr), 1.8−0.9 (br, 12H,
CH2‑coe), 1.03 (dd, JH−H = 7.5, JH−H = 7.5, 3H, CH3‑Py). 13C{1H}-APT
NMR plus HSQC and HMBC (125.6 MHz, tol-d8, 253 K): δ 184.7 (d,
JC−Rh= 53.4, Rh−CIPr), 166.3 (s, C2‑Py), 152.3 (s, C6‑Py), 148.7, 148.6,
146.2, and 146.1 (all s, Cq), 137.9 and 137.2 (both s, CqN), 134.8 (s,
C4‑Py), 129.6, 129.4, 128.2, 125.6, 122.9, and 122.3 (all s, CHPh‑IPr),
124.4 and 123.9 (both s, CHN), 121.9 (s, C3‑Py), 119.6 (s, C5‑Py),
1
°C was treated with benzylthiol (3 μL, 0.028 mmol). H NMR was
immediately recorded at low temperature. 1H NMR (500 MHz, tol-d8,
223 K): δ 7.68 (d, JH−H = 5.0, 1H, H6‑Py), 7.40−7.00 (m, 11H, HPh),
6.60 (dd, JH−H = 8.7, JH−H = 7.6, 1H, H4‑Py), 6.59 and 6.56 (both br,
2H, CHN), 6.20 (d, JH−H = 7.6, 1H, H3‑Py), 6.02 (dd, JH−H = 8.7,
JH−H = 5.0, 1H, H5‑Py), 3.82, 3.80, 3.50, and 3.31 (all sept, JH−H = 6.2,
4H, CHMeIPr), 2.48−2.42 (m, 2H, CH2‑Py), 2.78 and 2.49 (both d,
JH−H = 12.5, 2H, CH2S), 1.78, 1.73, 1.69, 1.58, 1.22, 1.17, 1.15, and
1.06 (all d, JH−H = 6.2, 24H, CHMeIPr), 0.89 (dd, JH−H = 7.5−7.3, 3H,
CH3‑Py), −26.93 (d, JRh−H = 47.0, 1H, Rh−H).
Preparation of S-(Phenylvinyl)-N-Boc-L-cysteine-methylester. A
Schlenk tube containing 1 mL of toluene was charged with 11.1 mg
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Journal of the American Chemical Society
Article
(0.01 mmol) of catalyst 2; 16 μL of pyridine (15.6 mg, 0.2 mmol), 200
μL (229 mg, 1 mmol) of N-(tert-butoxycarbonyl)-L-cysteine methyl
ester, and 110 μL (102 mg, 1 mmol) of phenylacetylene were added.
The yellow solution was stirred for 30 min at room temperature and
then filtered through silica gel in order to remove the catalyst. Finally,
the solution was concentrated under vacuum and analyzed by NMR.
Conversion was complete, and a ratio of 90/10 of branched/linear
hydrothiolation product was obtained.
empirical absorption correction was applied using the SADABS
program.49 Structures were solved by the Patterson method and
completed by successive difference Fourier syntheses. Refinements
were carried out by full-matrix least-squares on F2 with SHELXL-97,50
including isotropic and subsequent anisotropic displacement param-
eters for all non-hydrogen atoms. Hydrogen atoms for the ethylene
molecules in 2 were obtained from difference Fourier maps and
refined as free isotropic atoms; the rest of the hydrogens were included
in calculated positions and refined as riding atoms. In the case of 3,
two regions of disorder were observed and modeled with two moieties
with complementary occupancy factors. Only two residual peaks above
1.00 e−/Å3 were observed in 3 but located in close proximity of the
rhodium atoms.
S-(1-Phenylvinyl)-N-Boc-L-cysteine-methylester. 1H NMR (500
MHz, C6D6, 298 K): δ 7.58 (d, JH−H = 6.8, 2H, Ho‑Ph), 7.23−7.18
(m, 3H, Hm+p‑Ph), 5.70 (d, JH−H = 8.4, 1H, NH), 5.44 and 5.38 (both s,
2H, CH2), 4.79 (dt, JH−H = 8.4 JH−H = 4.9, 1H, NCH), 3.34 (s, 3H,
OCH3), 3.14 and 2.97 (dd, JH−H = 13.7, JH−H = 4.9, 2H, SCH2), 1.49
(s, 9H, CH3‑tBu). 13C{1H} APT NMR plus HSQC and HMBC (125.6
MHz, C6D6, 298 K): δ 171.0 (s, C−CO), 155.1 (s, N−CO),
144.1 (s, CCH2), 139.3 (s, Cq‑Ph), 128.5 (s, Cp‑Ph), 128.4 (s, Cm‑Ph),
127.5 (s, Co‑Ph), 113.3 (s, CH2), 79.4 (s, Cq‑tBu), 53.2 (s, N−CH),
51.7 (s, O−CH3), 34.3 (s, S−CH2), 28.1 (s, CH3‑tBu).
Crystal data for 2: C58H80Cl2N4Rh2, M = 1109.98, triclinic, space
group P1, orange crystal 0.191 × 0.186 × 0.154 mm, a = 10.2222(4) Å,
̅
b = 12.2208(5) Å, c = 13.5228(5) Å, α = 101.2870(5)°, β =
110.5192(5)°, γ = 111.5633(4)°, V = 1365.44(9) Å3, Z = 1, μ(Mo Kα)
= 0.742 mm−1, Dcalcd = 1.35 g cm−3, min and max transmission factors
0.804 and 0.907, 16 152 reflections measured (1.73 ≤ θ ≤ 30.52°, sin
θ/λ ≤ 0.715 Å−1), 7570 independent reflections (Rint = 0.0161);
number of data/restraints/parameters 7570/0/322. Final R1 and
wR(F2) values were 0.0216 and 0.0553 for I > 2σ(I) and 0.0227 and
0.0560 for all data, respectively; S = 1.035 for all data.
S-(2-Phenylvinyl)-N-Boc-L-cysteine-methylester. 1H NMR (500
MHz, C6D6, 298 K): δ 7.61 (d, JH−H = 6.9, 2H, Ho‑Ph), 7.25−7.15
(m, 3H, Hm+p‑Ph), 6.65 (d, JH−H = 15.6, 1H, S−CH), 6.59 (d, JH−H
=
=
15.6, 1H, HC−Ph), 5.73 (d, JH−H = 8.0, 1H, NH), 4.81 (dt, JH−H
8.0−4.8, 1H, N−CH), 3.38 (s, 3H, O−CH3), 3.23 and 3.11 (dd, JH−H
= 14.1−4.8, 2H, S−CH2), 1.47 (s, 9H, CH3‑tBu). 13C{1H} APT NMR
plus HSQC and HMBC (125.6 MHz, C6D6, 298 K): δ 170.8 (s, C−
CO), 155.0 (s, N−CO), 136.9 (s, Cq‑Ph), 129.1 (Ph-CHCH),
128.6 (s, Cm‑Ph), 127.1 (s, Cp‑Ph), 125.8 (s, Co‑Ph), 121.6 (s, CCH−
S), 79.5 (s, Cq‑tBu), 53.8 (s, N−CH), 51.9 (s, O−CH3), 35.4 (s, S−
CH2), 28.0(s, CH3‑tBu).
Crystal data for 3: C40H55ClN3Rh, M = 716.26, triclinic, space
group P1, yellow crystal 0.213 × 0.161 × 0.057 mm, a = 10.374(3) Å,
̅
b = 10.874(3) Å, c = 17.559(4) Å, α = 85.301(3)°, β = 75.467(3)°, γ =
72.538(3)°, V = 1829.1(8) Å3, Z = 2, μ(Mo Kα) = 0.571 mm−1, Dcalcd
= 1.301 g cm−3, min and max abs correction factors 0.813 and 1.139,
18 989 reflections measured (1.96 ≤ θ ≤ 26.37°, sin θ/λ ≤ 0.625 Å−1),
7439 independent reflections (Rint = 0.0667); number of data/
restraints/parameters 7439/6/455. Final R1 and wR(F2) values were
0.0570 and 0.1263 for I > 2σ(I) and 0.0893 and 0.1372 for all data,
respectively. The goodness of fit on F2 was 1.006.
Preparation of Phenyl(1-(4-(trifluoromethyl)phenyl)vinyl)sulfane.
The product was prepared as described for S-(phenylvinyl)-N-Boc-L-
cysteine-methylester starting with 1-ethynyl-4-(trifluoromethyl)-
benzene (158 μL, 1 mmol). The reaction time was 4 h, conversion
was complete, and a ratio of 97/3 of branched/linear hydrothiolation
1
product was obtained. H NMR (500 MHz, C6D6, 298 K): δ 7.53 (d,
Computational Details. All calculations have been performed
with the Gaussian09 package51 at the B3LYP level.52 The rhodium
atom was represented by the relativistic effective core potential
(RECP) from the Stuttgart group and the associated basis set.53 The
remaining atoms (C, H, N, O, Cl, S) were represented by a 6-31G(d)
basis set. Full optimizations of geometry without any constraint were
performed, followed by analytical computation of the Hessian matrix
to confirm the nature of the stationary points as minima or transition
structures on the potential energy surface.
JH−H = 8.1, 2H, H2‑Ph−CF3), 7.38 (m, 2H, Ho‑Ph‑S), 7.33 (d, JH−H = 8.1,
2H, H3‑Ph−CF3), 7.05 (m, 2H, Hm‑Ph‑S), 6.99 (m, 1H, Hp‑Ph‑S), 5.51 and
5.39 (both s, CCH2). 13C{1H} APT NMR plus HSQC and HMBC
(125.6 MHz, C6D6, 298 K): δ 143.6 (s, C=CH2), 142.3 (s, C1‑Ph−CF3),
133.3 (s, Cq‑Ph‑S), 132.0 (s, Co‑Ph‑S), 130.2 (q, JC−F = 32.4, C4‑Ph−CF3),
129.2 (s, Cm‑Ph‑S), 127.6 (s, C2‑Ph−CF3), 127.5 (s, Cp‑Ph‑S), 125.3 (q, JC−F
= 3.8, C3‑Ph−CF3), 124.5 (q, JC−F = 272.6, CF3), 117.61 (s, C = CH2).
19F NMR (470 MHz, C6D6, 298K): δ −62.39 (s, CF3).
Preparation of 1,6-Bis((1-phenylvinyl)thio)hexane. The product
was prepared as described for S-(phenylvinyl)-N-Boc-L-cysteine-
methylester starting with 1,6-hexanedithiol (40 μL, 0.5 mmol) with
a molar ratio of 1,6-hexanedithiol:phenylacetilene = 1:2. The reaction
time was 0.7 h, conversion was complete, and a ratio of 88/12 of
Determination of Rotational Barriers. Full line-shape analyses
1
of the dynamic H NMR spectra of 2, 3, 4, and 6 were carried out
using the program gNMR (Cherwell Scientific Publishing Limited).
The transverse relaxation time, T2, was estimated at the lowest
temperature. Activation parameters ΔH‡ and ΔS‡ were obtained by
linear least-squares fit of the Eyring plot. Errors were computed by
published methods.48
branched-branched/branched-linear hydrothiolation product was
1
obtained. H NMR (300 MHz, C6D6, 298 K): δ 7.72 (dd, JH−H
=
8.1, JH−H = 1.2, 4H, Ho‑Ph), 7.3−7.1 (br, 6H, Hm+p‑Ph), 5.51 and 5.24
(both s, 4H, CCH2), 2.54, 1,50 and 1,17 (br, 12H, CH2‑alkylic).
13C{1H} APT NMR plus HSQC and HMBC (75.4 MHz, C6D6, 298
K): δ 145.8 (s CCH2), 140.1 (s, Cq‑Ph), 128.4 (s, Cm‑Ph), 128.3 (s,
Cp‑Ph), 127.3 (s, Co‑Ph), 110.3 (s, CCH2), 31.9, 28.4, and 28.3 (all s,
CH2‑Alkylic).
Standard Catalytic Conditions. In a NMR tube 0.01 equiv of
catalyst was dissolved in 0.5 mL of C6D6, and then 0.5 mmol of thiol
and 0.5 mmol of alkyne were added. Alkyne conversion to vinyl sulfide
and conversion was quantified by integration of the 1H NMR
spectrum. Reaction product formation was also monitored at periodic
time intervals using GC-MS analyses.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data and processing parameters for com-
pounds 2 and 3, determination of thermodynamic parameters
of the equilibrium between 6 and 2, Cartesian coordinates for
theoretical calculated compounds, and complete ref 51. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Molecular Structure Determination for Complexes 2 and 3.
Single crystals for X-ray diffraction study of 2 and 3 were grown by
slow diffusion of n-hexane into a saturated solution of the complexes in
toluene. Intensity data for both complexes were collected at low
temperature (100(2)K) on a Bruker SMART CCD area detector
diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) using narrow frames (0.3° in ω). Data
were corrected for Lorentz and polarization effects, and a semi-
AUTHOR INFORMATION
■
Corresponding Author
rcastar@unizar.es
Notes
The authors declare no competing financial interest.
8181
dx.doi.org/10.1021/ja300396h | J. Am. Chem. Soc. 2012, 134, 8171−8183

Journal of the American Chemical Society
Article
(15) (a) Burling, S.; Field, L. D.; Messerle, B. A.; Vuong, K. Q.;
Turner, P. Dalton Trans. 2003, 4181. (b) Cao, C.; Fraser, L. R.; Love,
J. A. J. Am. Chem. Soc. 2005, 127, 17614. (c) Misumi, Y.; Seino, H.;
Mizobe, Y. J. Organomet. Chem. 2006, 691, 3157. (d) Fraser, L. R.;
Bird, J.; Wu, Q.; Cao, C.; Patrick, B. O.; Love, J. A. Organometallics
2007, 26, 5602. (e) Shoai, S.; Bichler, P.; Kang, B.; Buckley, H.; Love,
J. A. Organometallics 2007, 26, 5778. (f) Yang, J.; Sabarre, A.; Fraser, L.
R.; Patrick, B. O.; Love, J. A. J. Org. Chem. 2009, 74, 182. (g) Yang, Y.;
Rioux, R. M. Chem. Commun. 2011, 47, 6557. (h) Liu, J.; Lam, J. W.
Y.; Jin, C. K. W.; Ng, J. C. Y.; Shi, J.; Su, H.; Yeung, K. F.; Hong, Y.;
Faisal, M.; Yu, Y.; Wong, K. S.; Tang, B. Z. Macromolecules 2011, 44,
68. (i) Zhao, H.; Peng, J.; Cai, M. Catal. Lett. 2012, 142, 138.
(16) Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P. Dalton
Trans. 2009, 3599.
ACKNOWLEDGMENTS
■
Financial support from the Ministerio de Ciencia e Innovacion
(MICINN/FEDER) of Spain (Project CTQ2010-15221), the
́
Diputacion
under the program “Jov
́
General de Aragon
́
(E07), the ARAID Foundation
́
enes Investigadores”, and CONSOLIDER
INGENIO-2010, Projects MULTICAT (CSD2009-00050) and
́
Factoria de Crystalizacion
́
(CSD2006-0015) are gratefully
acknowledged. R.C. thanks the CSIC and the European Social
Fund for his Research Contract in the framework of the
“Ramon y Cajal” Program.
́
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