Rhodium(I) Complexes with Ligands Based on N-Heterocyclic Carbene and Hemilabile Pyridine Donors as Highly e Stereoselective Alkyne Hydrosilylation Catalysts

Article abstract of DOI:10.1021/acs.organomet.7b00361

Cationic rhodium(I) complexes containing picolyl-NHC (NHC = N-heterocyclic carbene) ligands that differ in the substitution at the 6-position of the pyridine donor serve as efficient E-selective alkyne hydrosilylation catalyst precursors. Particularly, wh

Full text of DOI:10.1021/acs.organomet.7b00361

Article
pubs.acs.org/Organometallics
Rhodium(I) Complexes with Ligands Based on N‑Heterocyclic
Carbene and Hemilabile Pyridine Donors as Highly E Stereoselective
Alkyne Hydrosilylation Catalysts
Judith P. Morales-Ceron,† Patricia Lara,‡ Joaquín Lopez-Serrano,‡ Laura L. Santos,‡ Veronica Salazar,*,†
́
́
́
‡
,‡
́
Eleuterio Alvarez, and Andres Suarez*
́
́
†
́
Area Academ
́
ica de Químicas, Universidad Auton
‡Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgan
(ORFEO-CINQA), CSIC and Universidad de Sevilla, 41092 Sevilla, Spain
́
oma del Estado de Hidalgo 42184 Mineral de la Reforma, Hidalgo, Mexico
ica and Centro de Innovacion en Química Avanzada
́
́
S
* Supporting Information
ABSTRACT: Cationic rhodium(I) complexes containing picolyl-NHC
(NHC = N-heterocyclic carbene) ligands that differ in the substitution at
the 6-position of the pyridine donor serve as efficient E-selective alkyne
hydrosilylation catalyst precursors. Particularly, when the steric
hindrance of the picolyl fragment is increased, a catalyst precursor
exhibiting high catalytic activities (TOF up to 500 h−1 at S/C ratios of
1000) and excellent E selectivities (E/α ratio ≥95/5) in the
hydrosilylation of a series of aryl, alkyl, and functionalized terminal
alkynes with both carbo- and alkoxysilanes has been obtained. The
picolyl-NHC ligands in the Rh complexes exhibit a dynamic behavior in
solution due to the hemilabile coordination of the pyridine fragment.
Preliminary mechanistic studies support the involvement of Rh silyl hydrido species, which are generated in low concentrations
from Rh complexes and the silane, in the hydrosilylation of alkynes in agreement with the assumption of Chalk−Harrod-type
mechanisms.
cally stable E isomer has been observed.3a,6a,b,f,8,10 Despite the
INTRODUCTION
■
significant advances made in the last few years for the selective
synthesis of the β-(Z) and α isomers, few reliable alternatives to
Pt(0) catalysts based on phosphine or N-heterocyclic carbene
ligands have been envisaged for the synthesis of (E)-vinylsilanes
(with the exception of some recently reported Au17 and Co18
catalysts), whose use is usually hampered by both the high cost
and air and moisture sensitivity of the catalysts. Therefore, since
highly regio- and stereoselective predictable catalytic systems
are limited, even for the more commonly accessible E isomer,
the development of new catalysts for this transformation is still
desirable.
Vinylsilanes are widely employed reactants in organic synthesis
that can be conveniently accessed by the atom-economical,
metal-catalyzed hydrosilylation of alkynes.1,2 Although tran-
sition-metal catalysts for the addition of hydrosilanes to alkynes
have been known for a long time, the search for active and,
more importantly, selective catalysts still represents a challenge.
For example, with respect to the selectivity issue, hydro-
silylation of terminal alkynes produces up to three stereo-
isomers, namely β-(Z), β-(E), and α isomers (Scheme 1).
Scheme 1. Hydrosilylation of Terminal Alkynes
In the case of Rh(I) catalysts, neutral species normally show
selectivity toward the formation of β-(Z)-vinylsilanes, while
cationic complexes provide predominantly β-(E)-alkenylsi-
lanes.7−9 For example, in the hydrosilylation of alkylalkynes
the cationic catalysts formed from [Rh(COD)2]BF4 + 2PPh3 or
[Rh(COD)Cl]2 + 2PPh3 in polar solvents achieve high regio-
and stereoselectivity for β-(E)-vinylsilane formation,7,10 where-
as neutral rhodium complexes such as RhCl(PPh3)3 usually
afford (Z)-alkenylsilanes with moderate to high selectivi-
ties.6b,11 Moreover, Rh-catalyzed hydrosilylation reactions
usually depend on the addition order of the reagents, up to
Among the various catalysts employed for alkyne hydro-
silylation, those based on Pt give predominantly β-(E)-
alkenylsilanes,3 while the β-(Z) or α isomers are preferentially
formed when Ru catalysts are employed.4,5 Other catalytic
systems, such as those based on Ir6 and Rh,3a,7−16 provide E
and Z isomers depending on the precise nature of the catalysts,
substrates, and reaction conditions. In addition, metal-catalyzed
isomerization of (Z)-vinylsilanes to the most thermodynami-
Received: May 9, 2017
© XXXX American Chemical Society
A
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the point that some degree of regiocontrol was obtained and a
stereodivergent synthesis of the anti-Markovnikov (Z)- and
(E)-alkenylsilanes has been reported using neutral rhodium(I)
iodide complexes.12 In addition, a significant influence of other
reaction parameters on selectivity such as concentration,11
solvent,10a and alkyne/silane ratio8 have been observed.12,13
Another important factor affecting the reaction selectivity is the
nature of the silane:11 electron-rich silanes tend to give (Z)-
olefins with Rh catalysts, while E products are obtained with
electron-poor hydrosilanes such as (RO)3SiH. Hence, many E-
selective rhodium catalysts are limited to the use of R3SiH
silanes that lead to products of low synthetic value.2d Finally, an
undesired drawback of Rh catalysts is also the occurrence of
competitive processes such as the oligomerization19 and
polymerization of arylalkynes9,10b,12b,16a,20 and the formation
of alkynylsilanes resulting from the dehydrogenative silylation
of the alkynes.6b−d,14g
the catalyst activity (“anchimeric assistance”).16c Finally,
cationic Rh complexes based on tridentate NHC-pyrazole
ligands have been shown to catalyze the hydrosilylation of
phenylacetylene with triethylsilane, providing modest con-
versions and selectivities.16d
Herein, we report our studies on the hydrosilylation of
acetylenes with a series of new cationic picolyl-NHC-containing
Rh complexes that differ in the steric hindrance at the 6-
position of the hemilabile pyridine donor. These derivatives
provide high catalytic activities and E selectivities in the
addition of different silanes to a wide range of terminal alkynes.
Furthermore, a preliminary mechanistic investigation of the
catalytic reactions is also described.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Rh Complexes.
Imidazolium salts 1a−c were easily obtained by alkylation of 1-
mesityl-1H-imidazole with the corresponding 2-halomethylpyr-
idines, as previously reported.22 NMR spectra of the salts are in
agreement with the proposed structures and are consistent with
previously reported data for analogous derivatives.
In the last few years there has been a significant interest in
the development of Rh catalysts based on N-heterocyclic
carbene (NHC)21 ligands for alkyne hydrosilylation.13−16
Particularly, focus has been placed, with limited success, on
the use of NHC ligands containing hemilabile N-donor groups
(Figure 1).15,16 For example, initial work by Peris and co-
In a subsequent step, the corresponding silver complexes
(CN)(AgX) (X = Cl, Br) were prepared to be employed as
NHC transfer reagents.23 Thus, reaction of imidazolium salts 1
with Ag2O in CH2Cl2 at room temperature resulted in the clean
formation of silver complexes 2 (Scheme 2),22 as noted by the
Scheme 2. Synthesis of Silver (2) and Rhodium (3)
Complexes
Figure 1. Rh(I) complexes with NHC ligands containing hemilabile N
donors for the hydrosilylation of terminal alkynes.
disappearance of the imidazolium proton signals at ca. 10 ppm
1
in the H NMR spectra and the appearance of broad signals at
ca. 180 ppm in the 13C{1H} NMR experiments due to the C2
carbon of the NHC moieties.
workers has demonstrated the catalytic activity of a picolyl-
carbene Rh complex in the addition of HSiMe2Ph to
phenylacetylene.15 A temperature-dependent Z selectivity was
observed in this reaction, which was attributed to the
isomerization of the initially formed (Z)-vinylsilane to the
most stable E isomer. Neutral and cationic Rh complexes with
NHCs containing dimethylamino groups have been reported
In turn, rhodium complexes 3a−c were conveniently
prepared by carbene transfer from the appropriate silver
complex 2 to [Rh(COD)2]BF4 in CH2Cl2 at room temperature
and were isolated with good yields as yellow-orange crystalline
solids after recrystallization from CH2Cl2/Et2O solutions
(Scheme 2). Complexes 3 were stable in the solid state
under atmospheric conditions. Electrospray mass spectrometry
investigation of derivatives 3 produced peaks consistent with
the expected molecular ion [M+]+. Moreover, complexes 3 have
been fully characterized by NMR techniques, and all of the
complexes were found to show very similar features. For
by Jimen
́
ez, Oro, and co-workers.16a Despite the fact that high
Z selectivity for the reaction of 1-hexyne with HSiMe2Ph was
obtained, the catalysts show an alkyne-dependent selectivity
and catalyze the Z to E isomerization of the resulting
vinylsilanes upon prolonged reaction times. In addition, in
the reactions of phenylacetylene, extensive polymerization was
observed. Similarly, the Cassani and Mazzoni group has
reported neutral rhodium(I) BOC-protected amino NHC
complexes that are active in the hydrosilylation of terminal
alkynes with HSiMe2Ph.16b Whereas no significant levels of
selectivity were observed in the hydrosilylation reaction due to
the Z to E isomerization process, the presence of an hemilabile
donor group has been shown to provide a beneficial effect on
1
example, in the H NMR spectrum in CD2Cl2, the olefinic
protons of the COD ligand produce four different signals
between 2.3 and 4.8 ppm. In addition, the coordination of the
pyridinyl moiety is evidenced from the observation of two
doublets for the diastereotopic protons of the CH2 bridge
appearing in the range between 5.7 and 6.5 ppm (2JHH = 14−15
Hz). The 13C{1H} NMR spectra show one doublet signal for
the C2(NHC)-Rh carbon at ca. 178 ppm (JCRh = 52 Hz).
B
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To gain further insight into the structure of coordinated
picolyl-NHC ligands in complexes 3, crystals suitable for X-ray
diffraction analysis of 3a,c were obtained by slow diffusion of
Et2O into saturated solutions of CH2Cl2 (Figure 2 and Table
Dynamic Behavior of Complexes 3 in Solution.
Dynamic hemilabile coordination of heteroditopic ligands has
been shown to enhance the catalytic performance by creating
transient vacant coordination sites and stabilizing unsaturated
catalytic intermediates.25 Since the hemilabile character of the
picolyl-NHC ligands can be anticipated due to the presence of a
strong σ-donor NHC fragment and a more weakly bound sp2-N
function, the solution dynamics of complexes 3a−c in CD2Cl2
1
and CD3CN have been studied by H,1H-EXSY and VT-1H
NMR spectroscopy.
In CD2Cl2, the 1H NMR spectra registered between −10 and
55 °C of complex 3a show the existence of fluxional behavior.
At −10 °C the presence of two doublet signals at 6.01 and 5.65
ppm (2JHH = 14.7 Hz) corresponding to the protons of the
methylene backbone is observed. These signals significantly
broaden upon heating and finally coalesce at 55 °C. Line-shape
1
analysis of the H NMR spectra gives activation parameter
values of ΔH⧧ = 11.4 kcal/mol and ΔS⧧ = −13 eu (Table 2). In
1
the H,1H-EXSY spectrum of 3a, intense cross peaks due to
chemical exchange are observed between the signals of the two
olefinic protons of the CHCH fragment trans to the NHC
and between the alkene protons trans to the pyridine. However,
no exchange signals involving the CH protons trans to NHC
and those trans to the N donor were observed. Alternatively,
1H NMR spectra of CD2Cl2 solutions of complexes 3b,c do not
show significant line broadening up to 55 °C and no exchange
1
cross-peaks are observed in their corresponding H,1H-EXSY
spectra (50 °C), suggesting that the dynamic behavior observed
for 3a in CD2Cl2 is absent in the case of 3b,c. Hence, the
fluxional behavior of 3a can be attributed to the interconversion
of the two limiting forms resulting from the inversion of the
chelate ring through the C−Rh−N coordination plane (Scheme
3).26 In addition, since solid-state Rh−N distances are
significantly longer in 3c than in 3a, pyridine decoordination
should be easier for the former complex, and consequently the
CH2 bridge flipping in 3a may occur in CD2Cl2 without
decoordination of the N-donor fragment.
Figure 2. ORTEP drawings at 30% ellipsoid probability of the cationic
fragments of complexes 3a (top) and 3c (bottom). Hydrogen atoms
have been omitted for clarity.
In contrast, in the presence of coordinating solvents, facile
pyridine decoordination can be expected.27 Addition of
increasing amounts of MeCN (2−200 equiv) to CD2Cl2
solutions of 3b produces significant broadening of the signals
caused by the COD olefinic hydrogens and the CH2 protons of
1).22a In these complexes, the rhodium atom displays a square-
planar geometry with the pyridine-imidazolinylidene ligand
coordinated as a chelate. The bite angle of the picolyl-NHC
ligand is ca. 84°. The C2(NHC)−Rh and N−Rh bond lengths
are similar to previously found distances,22a,24 although a
significantly shorter Rh−N distance for 3a than for 3c (ca. 0.1
Å) has been found, probably as a consequence of the higher
steric encumbrance due to the aryl substituent of the pyridine
fragment. The average distances of the CCOD−Rh trans to the
NHC are longer than those in the cis arrangement (Δd(Rh−
centroid CC) = 0.09 Å (3a) and 0.13 Å (3c)), as expected
from the greater trans influence of the carbene donor. Also of
interest, the chelate ring adopts a boat conformation that makes
the two CH2 bridge protons nonequivalent, as previously
inferred from the solution NMR study.
1
the picolyl-NHC ligand in the H NMR spectra. Thus, the
dynamic process for 3a−c in CD3CN has also been studied by
VT-1H NMR spectroscopy. At room temperature, the 1H NMR
spectrum of 3a is characterized by the presence of a broad
signal for the CH2 bridge and two broad signals for the CH
hydrogens, in agreement with a slow interconversion between
the two limiting isomers of the complex (Scheme 3). Line-
shape analysis of the 1H NMR spectra registered in the
temperature range between −35 and 50 °C indicates that the
energy barrier for the dynamic process in CD3CN is lower than
in CD2Cl2 (Table 2). In addition, VT-1H NMR spectroscopy in
CD3CN of 3b shows that a similar dynamic behavior is
Table 1. X-ray Diffraction Bond Lengths (Å) and Angles (deg) for 3a,c
Rh−CNHC
Rh−NPy
Rh−C(trans-NHC)
Rh−C(cis-NHC)
CNHC−Rh−NPy
3a
3c
2.013(3)
2.115(2)
2.187(3)
2.252(3)
2.208(3)
2.238(3)
2.139(3)
2.131(3)
2.107(3)
2.100(3)
84.07(10)
2.039(3)
2.213(2)
84.62(9)
C
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a
Table 2. Thermodynamic Parameters for the Dynamic Behaviour of Complexes 3a,b
solvent
ΔH⧧ (kcal mol−1
)
ΔS⧧ (cal mol−1
)
ΔG⧧ (kcal mol−1
)
300
3a
3a
3b
CD2Cl2
CD3CN
CD3CN
11.4(1.6)
10.1(0.7)
7.4(1.3)
−12.8(5.4)
−9.5(2.7)
−28.6(4.5)
15.3(0.3)
12.9(0.9)
16.0(0.1)
a
Estimated standard deviations in parentheses.
n
Scheme 3. Proposed Dynamic Behavior of Complexes 3
Table 3. Hydrosilylation of Phenylacetylene with Pr3SiH
Catalyzed by Complexes 3
a
b
entry
catalyst
S/C
solvent
yield (%)
E/α
1
2
3
3a
3b
3c
3c
3c
3c
3c
3c
3c
200
200
CD2Cl2
CD2Cl2
CH2Cl2
THF
100 (4)
100 (4)
100 (2)
99 (2)
85/15
89/11
99/1
99/1
99/1
99/1
98/2
99/1
99/1
200
c
4
200
d
5
d
200
MeCN
t-BuOH
2Me-THF
CD2Cl2
CD2Cl2
87 (2)
6
200
97 (2)
operative for this complex, although the coalescence temper-
ature was not reached at the higher studied temperature (70
7
8
9
200
81 (2)
1000
5000
100 (2)
>99 (24)
1
°C). Also, the H,1H-EXSY of 3a,b in CD3CN only shows
pairwise cross peaks for the olefinic protons of the same CH
CH moiety. Therefore, the fluxionality shown by complex 3b,
which is not observable in CD2Cl2, as well as the lower barrier
obtained for 3a in CD3CN may be ascribed to a fast
interconversion between the two limiting isomers facilitated
by the lability of the picolyl fragment. Moreover, the negative
entropy values found suggest an associative mechanism
involving the coordination of CD3CN.
Alternatively, the dynamic behavior in CD3CN of the more
sterically encumbered complex 3c differs from that of 3a,b, as
demonstrated by VT-1H NMR spectroscopy. The initial AX
system observed for the methylene protons of the picolyl-NHC
ligand at low temperature is gradually converted to an AB
system upon raising the temperature, while the signal from the
CH moieties broadens. Further heating of the sample makes
the CH2 protons equivalent, suggesting that pyridine fragment
decoordination occurs leading to a κ1-C coordination of the
ligand.
a
Reactions were carried out at 60 °C (oil bath), using 0.28 mmol of
n
phenylacetylene and 0.31 mmol of Pr3SiH. [phenylacetylene] = 0.6
M. Yield and E/α ratio were determined by 1H NMR spectroscopy or
b
GC-MS using mesitylene as internal standard. Reaction time (in h) in
parentheses. Oligomerization of phenylacetylene was detected by GC-
MS. Formation of poly(phenylacetylene) was observed.
c
d
evidencing a significant influence of the size of the hemilabile
ligand fragment on the E/α ratio.29 Follow-up of the reaction in
CD2Cl2 by H NMR spectroscopy showed that the E/α ratio
1
remained practically constant during the reaction course,
suggesting that the (Z)-vinylsilane is not an intermediate in
the formation of the (E)-olefin or that there is not a significant
accumulation of this isomer. In addition, unlike with previously
studied Rh-NHC catalysts,16a formation of other products such
as silylacetylenes, resulting from the dehydrogenative silylation
of the alkyne, or triphenylbenzenes, formed by trimerization of
phenylacetylene, was not observed.
Examination of other solvents showed that there is not a
marked influence of the reaction media on the selectivity,
although the catalyst activity is somewhat affected (entries 4−
7). Trace amounts of acetylene dimerization and cyclo-
trimerization were observed by GC-MS analysis when THF
was employed as solvent, whereas some polymerization of
Overall, these studies point out the hemilabile character of
the pyridine fragment when other donating species (i.e.,
MeCN) are present and demonstrate the ability of the ligands
to exhibit both κ1-C and κ2-C,N coordination modes.
Catalyst Optimization and Scope. NHC-rhodium
catalysts containing N-donor functionalities have provided
good levels of activity in the hydrosilylation of alkynes,
although low to moderate selectivities have usually been
obtained.15,16 Thus, the catalytic performance of Rh complexes
3 was examined in the addition of silanes to terminal alkynes.
t
phenylacetylene was evidenced when MeCN and BuOH were
used. Interestingly, in CD2Cl2 the substrate/catalyst ratio could
be increased without erosion of the selectivity, providing TOF
values of up to 500 h−1 at an S/C ratio of 1000 and 206 h−1 at
an S/C ratio of 5000 (entries 8 and 9).
n
Reaction of Pr3SiH with phenylacetylene in CD2Cl2 catalyzed
by 3a yielded the corresponding β-(E)-alkenylsilane accom-
panied by lower amounts (15%) of the Markovnikov silane
addition product (Table 3, entry 1). With the exception of the
NHC wingtip, the NHC ligand of complex 3a is structurally
similar to that employed by Peris et al., which showed a
preference for the formation of the Z isomer. This result
suggests that the absence of the chloride ligand is beneficial for
achieving high selectivity toward the (E)-vinylsilane. In
addition, the level of stereoselectivity obtained with complex
3a prompted us to examine the catalytic performance of the
other complexes. Thus, the reaction with 3b led to a slight
increase of the selectivity toward the (E)-vinylsilane, while the
catalyst derived from 3c completed the reaction with an
exquisite regio- and stereoselectivity (entries 2 and 3),28
Having determined that complex 3c provides the highest
selectivity among the series of Rh complexes, other silanes were
tested in the hydrosilylation of phenylacetylene (Table 4).
Good activity and selectivity were observed in the addition of
PhMe2SiH and 1,4-bis(dimethylsilyl)benzene (entries 1 and 2).
In addition, since vinylalkoxy- and vinylsiloxysilanes are
versatile substrates in organic synthesis, particularly in Hiyama
cross-coupling reactions,1d,e,h,i the addition of (EtO)3SiH and
(TMSO)3SiH was examined. In both cases the selectivity was
high, although the reaction with the hydrosiloxane was more
sluggish. It should be noted that a marked decrease in E
selectivity for heteroatom-substituted silanes is usually observed
with Rh and Pt catalysts.2d,3b,8,12
D
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n
Table 4. Hydrosilylation of Phenylacetylene Catalyzed by
3c
Table 5. Hydrosilylation of Acetylenes with Pr3SiH and
(EtO)3SiH Catalyzed by 3c
a
a
b
entry
1
silane
PhMe2SiH
S/C
yield (%)
E/α
2000
2000
2000
500
100
99
97/3
97/3
96/4
99/1
c
2
1,4-(HSiMe2)2C6H4
(EtO)3SiH
3
4
100
100
(TMSO)3SiH
a
Reactions were carried out at 60 °C (oil bath) in CD2Cl2 using 0.28
mmol of phenylacetylene and 0.31 mmol of silane. [phenylacetylene]
= 0.6 M. Yield and E/α ratios were determined by 1H NMR
b
c
spectroscopy. Reaction time 4 h. 0.56 mmol of phenylacetylene was
used.
Next, the hydrosilylation of a series of acetylenes was
examined using two representative silanes, nPr3SiH and
(EtO)3SiH (Table 5). Reactions of substituted phenyl-
acetylenes with electron-donating and -withdrawing groups
yielded the corresponding (E)-vinylsilanes with high selectivity
(≥96%) (entries 1−8). As previously reported, the regiose-
lectivity obtained with the hydroalkoxysilane is slightly lower
n
than that of Pr3SiH, although the difference observed is less
pronounced than with other catalytic systems.2d,3b,8,12
Hex-1-yne was chosen as an example of an alkyl-substituted
alkyne, and their reactions with nPr3SiH and (EtO)3SiH
catalyzed by 3c proceeded with good conversions and E
selectivities higher than 96% (Table 5, entries 9 and 10). In
addition, it is worth noting that isomerization of the vinylsilane
to the corresponding allylsilane, a process previously observed
with Pt and Rh catalysts in the hydrosilylation of alkyl-
substituted acetylenes, is not observed.3a,6b,16a,b However, the
catalyst seems to be sensitive to steric effects, since no reactivity
was observed in the reaction of 3,3-dimethylbutyne with
nPr3SiH.
Furthermore, the catalytic system is compatible with a
number of functional groups. For example, hydrosilylation of
enynes, with both terminal and internal CC bonds, occurs
chemoselectively at the triple C−C bond (Table 5, entries 11−
14). In addition, the hydrosilylation of heteroatom-function-
alized alkynes was examined. Hydrosilane addition to a
propargylic alcohol occurs at the triple bond with high E
selectivity, while no silylation of the oxygen atom was observed
(entries 15 and 16).30 Finally, the presence of potentially
coordinating sulfur atoms did not interfere with the hydro-
silylation, although a sluggish reaction was observed with
nPr3SiH (entries 17 and 18).
Mechanistic Considerations. The commonly accepted
steps for the hydrosylilation of alkynes are based on the Chalk−
Harrod mechanism for alkene hydrosilylation,2d,31 which
involves the oxidative addition of the silane Si−H bond to
the metal, migratory insertion of the coordinated alkyne into
the M−H fragment, and reductive elimination of the vinyl and
silyl ligands. For some catalytic systems, however, a modified
Chalk−Harrod mechanism has been proposed to explain the
formation of the Z derivatives and dehydrogenative silylation
products.2d In this alternative mechanism, alkyne insertion
takes place into the Rh−Si bond, which might be followed by
cis/trans isomerization of the resulting vinyl complex through
the involvement of metallacyclopropene6a,b or zwitterionic
carbene11 species before the reductive elimination step.
To shed light on the mechanism of the reactions catalyzed by
complexes 3, the hydrosilylation of phenylacetylene with
nPr3SiH catalyzed by 20 mol % of complex 3b in CD2Cl2 was
a
Reactions were carried out at 60 °C (oil bath) in CD2Cl2 using 0.28
n
mmol of acetylene and 0.31 mmol of Pr3SiH or (EtO)3SiH. [alkyne]
= 0.6 M. Yield and E/α ratios were determined by 1H NMR
b
c
spectroscopy. Reaction time in parentheses. Minor amounts, less
than 2%, of the β-(Z)-vinylsilane were observed. β-(E)/β-(Z)/α =
91/2/7. [alkyne] = 0.3 M. A longer reaction time (72 h) did not
increase conversion.
d
e
f
followed up by 1H NMR spectroscopy. Interestingly, no
intermediates, to the detection limit of H NMR spectroscopy,
were observed, since the signals corresponding to the Rh
precursor appeared unchanged. This observation suggests that
1
E
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the catalyst is formed in a very low concentration, below the
NMR detection limit, from the catalyst precursor 3b.
Scheme 5. Reaction of Complex 3c with HSi(OEt)3 in
CD3CN
Next, the reactivity of the Rh precursors with the coupling
partners was studied individually. Addition of a slight excess of
phenylacetylene (5 equiv) to a solution of 3b in CD2Cl2 or
CD3CN produces no changes, to the detection limit of the
1
technique, in the signals of the H NMR spectrum of the Rh
complex, whereas the resonances of phenylacetylene gradually
disappear and new broad signals are observed in the aromatic
region. These changes are accompanied by the formation of an
insoluble red solid resulting from alkyne polymerization.32
On the other hand, prolonged heating of a solution of 3b and
triethoxysilane (15 equiv) in CD2Cl2 yields complex 4 (Scheme
4). A similar reaction outcome is observed when tripropylsilane
of a doublet at −17.46 ppm (1JHRh = 30.8 Hz) attributable to
the Rh−H functionality and two doublets at 5.36 and 6.00 ppm
(2JHH = 14.8 Hz) for the diastereotopic protons of the CH2
linker. The signals corresponding to the silyl ligand Si-
(OCH2CH3)3 appear at 0.99 and 3.60 ppm as a triplet and a
quartet (3JHH = 7.0 Hz), respectively. The 13C{1H} NMR
spectrum shows the signal corresponding to the carbenic
carbon at 173.6 ppm as a doublet (1JCRh = 54 Hz). In addition,
the 2D NMR heteronuclear correlation 1H,29Si HMBC exhibits
a cross-peak doublet signal at −34.3 ppm (1JSiRh = 62 Hz)
between the hydride and the silicon atom of 5. Finally, an
intense cross peak was observed in the 1H,1H NOESY
experiment between the Rh−H and SiOCH2CH3 hydrogens,
suggesting a cis arrangement for the hydrido and silyl ligands.
Analysis of 5 by ESI-MS provides a peak at m/z 680
attributable to the ion [M − 2MeCN − BF4]+. In support of
the proposed structure of 5, it should be mentioned that an
analogous Ir complex has been previously reported by Peris and
co-workers.6d Finally, in order to examine the catalytic
performance of 5, a 6-fold equimolar amount of triethoxysilane
and phenylacetylene was added to a solution in CD2Cl2 of 5,
yielding the hydrosilylation product with a E/α ratio of 96/4
along with the starting complex and therefore suggesting that 5
is involved in the catalytic cycle.
Scheme 4. Reaction of Complex 3b with HSi(OEt)3 in
CH2Cl2
is employed; however, the reactions are accompanied by
1
abundant formation of a black metallic precipitate. In the H
NMR spectrum of 4 registered in CD2Cl2, four different signals
between 3.0 and 4.8 ppm attributable to the olefinic protons of
the COD ligand are observed, as well as two doublets for the
diastereotopic protons of the CH2 bridge appearing at 5.74 and
6.77 ppm (2JHH = 15.7 Hz) that reflect a hindered rotation of
the NHC ligand around the C2(NHC)−Rh bond. The 13C{1H}
NMR spectrum shows one doublet signal for the C2(NHC)
carbon at δ 184 ppm (JCRh = 52 Hz) and two pairs of doublets
for the olefinic carbons, one in the range 68−69 ppm (JCRh = 14
Hz) and other at 96−98 ppm (JCRh = 7 Hz). Electrospray mass
spectrometry investigation of 4 produced peaks consistent with
the expected ion [M − Cl]+. The structural assignment for
complex 4, having a coordinated NHC fragment and a dangling
pyridyl moiety, is further supported by comparison of their
NMR data with those reported for similar complexes by Liu
and co-workers.22a Testing of complex 4 in the hydrosilylation
Next, we studied the influence of electronic effects on the
rate of hydrosilane addition to substituted phenylacetylenes.
Preliminary experiments showed significantly faster reactions
for arylalkynes containing electron-donating groups than for
alkynes having electron-withdrawing substituents. However,
this observation might be caused by intrinsically different
catalytic cycle rates or be due to differences in precatalyst
activation if the alkyne plays a role in the formation of the
active species: for example, by promoting the decoordination of
the pyridine fragment of the picolyl-NHC ligand in analogy to
the observed influence of MeCN on the dynamic properties of
complexes 3 (see above). In order to eliminate the influence of
the catalyst formation step on the reaction rate, a series of
competition reactions involving the hydrosilylation of equi-
molar amounts of phenylacetylene and X-C6H4CCH (X = p-
n
of phenylacetylene with Pr3SiH (S/C = 100, 60 °C, CD2Cl2)
resulted in a significantly different catalytic activity and product
distribution in comparison to that provided by 3b, since the
product ratio E/Z/α/styrene/silylacetylene = 84/3/5/3/5 was
obtained (81% conversion, 24 h), in line with the negligible
formation of 4 from 3b under catalytic conditions and making
evident the beneficial effect of using a catalyst precursor with a
noncoordinating counteranion.
n
MeO, p-Br, m-Cl) with Pr3SiH were investigated (Scheme 6).
In all of the experiments, both acetylenes reacted at the same
rate, demonstrating that the catalytic reactions are insensitive to
the electronic nature of the alkyne.
The study of kinetic isotope effects (KIEs) provides a
straightforward method to obtain relevant mechanistic
information and support the proposal of reaction mecha-
nisms.33 Thus, catalytic reactions with selectively deuterated
substrates were performed. From the reaction of phenyl-
acetylene (1.0 equiv) with nPr3SiH (1.0 equiv) and nPr3SiD (1.0
equiv) (Scheme 7), KH/KD = 1.0 was found, suggesting that
oxidative addition of the hydrosilane is not involved in the rate-
determining step. Alternatively, for the reaction of C6H5C
Interestingly, alternative heating of 3c in CD3CN with an
excess of triethoxysilane shows the formation of a new species
that has been assigned to the silyl hydrido Rh complex 5,
resulting from the formal activation of the silane Si−H bond
(Scheme 5). GC-MS analysis of the reaction mixture shows the
formation of cyclooctene as well as Si(OEt)4 and minor
1
amounts of other unidentified silicon species. The H NMR
spectrum of the silylrhodium hydride 5 shows the appearance
F
DOI: 10.1021/acs.organomet.7b00361
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
plausible on the basis of the above experimental data and
literature precedents (Figure 3). After initial formation of the
hydrido-silyl Rh species A (i.e., 5), alkyne coordination should
take place to yield B. From this intermediate, two different
outcomes involving the migratory insertion of the alkyne into
the Rh−H bond are possible, depending on the orientation of
the insertion step (1,2 or 2,1). An alkyne 1,2-insertion would
lead to vinyl Rh species CH(E), which after reductive
elimination and activation of a new silane molecule would
regenerate A and afford the major (E)-vinylsilane derivative.
Alternative 2,1-insertion into the Rh−H bond would produce
CH(α), which ultimately would yield the α-vinylsilane. Similar
pathways involving intermediates CSi(E) and CSi(α) can be
regarded if, instead of the migratory insertion of the alkyne into
the Rh−H bond, the silyl group is delivered to the coordinated
alkyne (modified Chalk−Harrod mechanism). Since the
reaction regioselectivity is given by the migratory insertion
step, analysis of intermediates C can provide clues about the
operative mechanism. If insertion into the Rh−H bond is
considered, formation of intermediate CH(α) should be
disfavored on the basis of steric grounds, and an increase in
the steric crowding of the pyridine fragment of the picolyl-
NHC ligand would provide higher anti-Markovnikov selectivity,
as observed in the reactions catalyzed by complexes 3 (Table
3). Alternatively, if silyl delivery to the coordinated alkyne takes
place, steric hindrance should disfavor the formation of CSi(E)
over CSi(α). Hence, we hypothesized that a standard Chalk−
Harrod mechanism is operative with complexes 3. However,
since formation of minor amounts of Z isomers has been
observed in some reactions (Table 5, entries 3 and 14), which
may be explained by the partial isomerization of CSi(E) to
CSi(Z), the occurrence of a modified Chalk−Harrod mecha-
nism cannot be completely excluded.
Scheme 6. Determination of Electronic Effects on the
Hydrosilylation Rates
Scheme 7. Determination of KIE using Deuterated Silane
n
CH (1.0 equiv) and C6H5CCD (1.0 equiv) with Pr3SiH
(1.0 equiv) (Scheme 8), the secondary kinetic isotope effect
Scheme 8. Determination of KIE using Deuterated Alkyne
CONCLUSIONS
■
KH/KD = 1.1 was determined. In addition, from the study of the
geometry of the obtained deuterated products in these
experiments, syn addition of the silane to the alkyne was
evidenced.
Although further mechanistic studies are required, the
assumption of Chalk−Harrod mechanisms for the hydro-
silylation of alkynes catalyzed by Rh complexes 3 seems
A series of Rh(I) complexes 3 containing picolyl-NHC ligands
differing in the steric crowding at the 6-position of the pyridine
moiety has been synthesized, and the hemilabile nature of the
N-donor fragment has been demonstrated by NMR spectros-
copy. Complexes 3 activate the H−Si bonds of silanes to yield
silyl hydrido Rh species, such as 5, that are efficient catalysts in
Figure 3. Proposed pathways for the standard and modified Chalk−Harrod mechanisms for alkyne hydrosilylation catalyzed by 3.
G
DOI: 10.1021/acs.organomet.7b00361
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(CH2 COD), 31.8 (CH2 COD), 34.9 (CH2 COD), 56.8 (py-CH2),
73.9 (d, JCRh = 12 Hz, CH COD), 75.7 (d, JCRh = 14 Hz, CH
COD), 92.8 (d, JCRh = 7 Hz, CH COD), 100.7 (d, JCRh = 8 Hz,
CH COD), 123.4 (CH arom), 123.7 (CH arom), 124.3 (CH
arom), 125.0 (CH arom), 127.0 (2 CH arom), 128.9 (CH arom),
129.5 (3 CH arom), 131.6 (CH arom), 135.3 (Cq arom), 135.4 (Cq
arom), 135.8 (Cq arom), 139.4 (Cq arom), 140.0 (CH arom), 140.3
(Cq arom), 155.1 (Cq arom), 161.1 (Cq arom), 177.0 (d, JCRh = 52 Hz,
C-2 NHC).
the E-selective hydrosilylation of terminal alkynes. Although the
catalytic activities provided by these catalysts are still far from
that of the very active Pt(0) species, to the best of our
knowledge, this catalytic system is only surpassed in terms of
activity by only two other rhodium(I) E-selective cata-
lysts,10b,12b while the high selectivity toward the formation of
(E)-vinylsilanes and wide alkyne and silane scope make these
alkyne hydrosilylation catalysts of practical interest. In addition,
this study highlights the importance of ligand design for the
development of selective Rh catalysts for the hydrosilylation of
alkynes, since a significant influence of the steric hindrance of
the hemilabile ligand fragment on the E/α ratio has been
evidenced.29
Complex 3c. Yellow-orange solid (63%). Anal. Calcd for
C34H39BF4N3O2Rh: C, 57.40; H, 5.53; N, 5.91. Found: C, 57.25; H,
1
5.53; N, 5.85. MS (ESI, CH2Cl2/MeOH): m/z 624 ([M+]+, 100). H
NMR (CD2Cl2, 400 MHz): δ 1.31 (m, 1H, CHH COD), 1.45 (m, 2H,
2 CHH COD), 1.91 (m, 4H, 4 CHH COD), 1.92 (s, 3H, CH3), 2.05
(s, 3H, CH3), 2.35 (s, 3H, CH3), 2.37 (m, 1H, CHH COD), 3.16 (m,
1H, CH COD), 3.34 (m, 1H, CH COD), 3.62 (s, 3H, OCH3),
3.63 (s, 3H, OCH3), 3.77 (m, 1H, CH COD), 4.82 (m, 1H, CH
EXPERIMENTAL SECTION
■
2
2
COD), 5.78 (d, JHH = 14.3 Hz, 1H, py−CHH), 6.42 (d, JHH = 14.4
Hz, 1H, py−CHH), 6.58 (d, 3JHH = 8.3 Hz, 1H, H arom), 6.75 (d, 3JHH
= 8.7 Hz, 1H, H arom), 6.76 (s, 1H, H arom), 6.96 (s, 1H, H arom),
7.03 (s, 1H, H arom), 7.25 (d, 3JHH = 7.5 Hz, 1H, H arom), 7.44 (dd,
3JHH = 8.3 Hz, 3JHH = 8.3 Hz, 1H, H arom), 7.64 (s, 1H, H arom), 7.86
General Procedures. All reactions and manipulations were
performed under nitrogen or argon, either in a Braun Labmaster
100 glovebox or using standard Schlenk-type techniques. All solvents
were dried over appropriate drying agents and distilled under nitrogen.
NMR spectra were obtained on Bruker DPX-300, DRX-400, or DRX-
500 spectrometers. 13C{1H} and 1H shifts were referenced to the
residual signals of deuterated solvents. All data are reported in ppm
downfield from Me4Si. ESI-MS experiments were carried out in a
Bruker 6000 apparatus by the Mass Spectrometry Service of the
3
3
3
(d, JHH = 7.5 Hz, 1H, H arom), 7.91 (dd, JHH = 7.5 Hz, JHH = 7.5
Hz, 1H, H arom). 13C{1H} NMR (CD2Cl2, 126 MHz): δ 18.0 (CH3),
18.9 (CH3), 21.1 (CH3), 26.9 (CH2 COD), 30.1 (CH2 COD), 31.1
(CH2 COD), 34.7 (CH2 COD), 55.8 (py-CH2), 57.0 (OCH3), 57.1
(OCH3), 69.3 (d, JCRh = 13 Hz, CH COD), 73.7 (d, JCRh = 15 Hz,
CH COD), 96.8 (d, JCRh = 7 Hz, CH COD), 99.1 (d, JCRh = 7
Hz, CH COD), 103.9 (CH arom), 105.0 (CH arom), 117.6 (Cq
arom), 122.2 (CH arom), 124.2 (2 CH arom), 129.4 (CH arom),
129.5 (CH arom), 129.6 (CH arom), 132.0 (CH arom), 135.0 (Cq
arom), 135.6 (Cq arom), 135.8 (Cq arom), 139.2 (CH arom), 139.7
(Cq arom), 154.8 (Cq arom), 158.6 (Cq arom), 158.7 (Cq arom), 159.4
(Cq arom), 177.8 (d, JCRh = 52 Hz, C-2 NHC).
́
Instituto de Investigaciones Quimicas. Elemental analyses were run by
́
the Analytical Service of the Instituto de Investigaciones Quimicas in a
Leco CHNS-932 elemental analyzer.
Synthesis of Rh Complexes 3. A solution of the corresponding
silver complex 2 (0.24 mmol) and [Rh(COD)2]BF4 (0.094 g, 0.24
mmol) in CH2Cl2 (8 mL) was stirred for 4 h. The mixture was filtered
through a short pad of Celite, and the solution was brought to dryness.
The resulting solid was washed with Et2O (3 × 10 mL) and
recrystallized from a CH2Cl2/Et2O mixture.
Procedure for the Generation of Complex 5. A solution of
complex 3c (0.025 g, 0.03 mmol) and HSi(OEt)3 (32 μL, 0.17 mmol)
in CD3CN (0.7 mL) was heated at 60 °C for 16 h. Volatiles were
removed under vacuum, and the residue was dissolved in CD3CN and
analyzed by NMR spectroscopy. Further attempts to purify complex 5
only produced significant decomposition. Signal assignations in the 1H
and 13C{1H} NMR spectra were made with the assistance of 2D NMR
spectroscopy, including 1H,1H-COSY, 1H,1H-NOESY, 1H,13C{1H}-
Complex 3a. Yellow-orange solid (63%). Anal. Calcd for
C26H31BF4N3Rh (%): C 54.29; H 5.43; N 7.30. Found: C 54.29; H
1
5.50; N 7.09. MS (ESI, CH2Cl2/MeOH): m/z 488 ([M+]+, 100). H
NMR (CD2Cl2, 400 MHz): δ 1.75−2.48 (m, 8H, 8 CHH COD), 2.02
(s, 6H, 2 CH3), 2.38 (s, 3H, CH3), 3.30 (br, 1H, CH COD), 4.25
(br, 1H, CH COD), 4.45 (br, 1H, CH COD), 4.51 (br, 1H,
CH COD), 5.72 (br d, 2JHH = 13.5 Hz, 1H, py−CHH), 6.05 (br d,
2JHH = 13.8 Hz, 1H, py−CHH), 6.81 (d, 3JHH = 1.8 Hz, 1H, H arom),
HMQC, and H,13C{1H}-HMBC experiments.
1
1
3
7.01 (br s, 1H, H arom), 7.09 (br s, 1H, H arom), 7.43 (ddd, JHH
=
1H NMR (CD3CN, 400 MHz): δ −17.46 (d, JHRh = 30.8 Hz, 1H,
5.8 Hz, 3JHH = 5.8 Hz, 4JHH = 3.1 Hz, 1H, H arom), 7.59 (d, 3JHH = 1.8
Hz, 1H, H arom), 7.95 (m, 2H, 2 H arom), 8.35 (d, 3JHH = 5.4 Hz, 1H,
H arom). 13C{1H} NMR (CD2Cl2, 101 MHz): δ 17.9 (br s, CH3),
19.1 (br s, CH3), 21.2 (CH3), 28.2 (br s, CH2 COD), 30.4 (br s, CH2
COD), 31.4 (br s, CH2 COD), 33.3 (br s, CH2 COD), 56.2 (py-CH2),
77.0 (br m, CH COD), 79.1 (br m, CH COD), 97.0 (br m,
CH COD), 98.9 (br m, CH COD), 122.7 (CH arom), 123.5
(CH arom), 125.8 (CH arom), 126.2 (CH arom), 129.0 (Cq arom),
129.5 (br s, 2 CH arom), 134.9 (br s, Cq arom), 135.7 (br s, Cq arom),
140.0 (Cq arom), 140.1 (CH arom), 151.3 (CH arom), 154.5 (Cq
arom), 177.7 (d, JCRh = 52 Hz, C-2 NHC).
RhH), 0.99 (t, 3JHH = 7.0 Hz, 9H, 3 SiOCH2CH3), 1.70 (s, 3H, CH3),
2.25 (s, 3H, CH3), 2.37 (s, 3H, CH3), 3.60 (q, JHH = 7.0 Hz, 6H, 3
3
2
SiOCH2), 3.84 (overlapped, 6H, 2 OCH3), 5.36 (d, JHH = 14.8 Hz,
1H, py-CHH), 6.00 (d, 2JHH = 14.8 Hz, 1H, py-CHH), 6.80 (br d, 3JHH
3
= 8.8 Hz, 2H, 2 H arom), 7.00 (d, JHH = 1.9 Hz, 1H, H arom), 7.02
(s, 1H, H arom), 7.11 (s, 1H, H arom), 7.44 (t, 3JHH = 8.8 Hz, 1H, H
3
3
arom), 7.46 (d, JHH = 1.9 Hz, 1H, H arom), 7.50 (d, JHH = 7.8 Hz,
3
4
1H, H arom), 7.70 (dd, JHH = 7.7 Hz, JHH = 1.2 Hz, 1H, H arom),
8.03 (dd, 3JHH = 7.7 Hz, 3JHH = 7.7 Hz, 1H, H arom). 1H,29Si-HMQC
(CD3CN): δ −34.3 ppm (d, JSiRh = 62 Hz). 13C{1H} NMR (CD3CN,
101 MHz): δ 18.0 (CH3), 18.3 (3 SiOCH2CH3), 21.1 (CH3), 56.4
(py-CH2), 58.6 (3 SiOCH2CH3), 106.1 (CH arom), 123.0 (CH
arom), 124.0 (CH arom), 124.8 (CH arom), 129.7 (2 CH arom),
130.3 (2 CH arom), 132.5 (CH arom), 136.7 (Cq arom), 137.2 (Cq
arom), 137.3 (Cq arom), 139.9 (CH arom), 140.2 (Cq arom), 155.6
(Cq arom), 157.5 (Cq arom), 159.4 (2 Cq arom), 173.6 (d, JCRh = 54
Hz, C-2 NHC). Signals corresponding to one CH3 from the mesityl
fragment, the two OCH3, and one quaternary aromatic carbon could
not be unambiguously assigned. MS (ESI, MeCN): m/z 680 ([M −
2MeCN − BF4]+, 100). Fragmentation of ion m/z 680: m/z 516 ([M
− 2MeCN − BF4 − HSi(OEt)3]+, 100).
Complex 3b. Yellow-orange solid (59%). Anal. Calcd for
C32H35BF4N3Rh: C, 59.01; H, 5.42; N, 6.45. Found: C, 59.19; H,
1
5.57; N, 6.41. MS (ESI, CH2Cl2/MeOH): m/z 564 ([M+]+, 100). H
NMR (CD2Cl2, 400 MHz): δ 1.23 (m, 2H, 2 CHH COD), 1.56 (m,
1H, CHH COD), 1.89 (s, 3H, CH3), 2.05 (s, 3H, CH3), 2.07 (m, 4H,
4 CHH COD), 2.47 (s, 3H, CH3), 2.49 (m, 2H, CHH COD + CH
COD), 3.70 (m, 1H, CH COD), 4.03 (m, 1H, CH COD), 4.29
2
(m, 1H, CH COD), 5.99 (d, JHH = 14.8 Hz, 1H, py−CHH), 6.48
(d, 2JHH = 14.8 Hz, 1H, py−CHH), 6.88 (s, 1H, H arom), 7.08 (s, 1H,
3
3
H arom), 7.19 (s, 1H, H arom), 7.63 (dd, JHH = 7.5 Hz, JHH = 7.5
Hz, 2H, 2 H arom), 7.71 (t, 3JHH = 7.0 Hz, 1H, H arom), 7.76 (d, 3JHH
= 7.0 Hz, 1H, H arom), 7.77 (s, 1H, H arom), 8.03 (d, 3JHH = 7.2 Hz,
Representative Procedure for Catalytic Hydrosilylation
Reactions. In a glovebox, an NMR tube was charged with a solution
of complex 3c (0.4 mg, 0.56 μmol), phenylacetylene (31 μL, 0.28
mmol), and tri-n-propylsilane (57 μL, 0.31 mmol) in CD2Cl2 (0.5
mL). The reaction mixture was heated to 60 °C (oil bath) for 4 h.
3
3
1H, H arom), 8.07 (dd, JHH = 7.5 Hz, JHH = 7.5 Hz, 1H, H arom),
8.32 (d, JHH = 7.4 Hz, 2H, 2 H arom). 13C{1H} NMR (CD2Cl2, 101
3
MHz): δ 17.8 (CH3), 17.9 (CH3), 21.3 (CH3), 25.9 (CH2 COD), 29.6
H
DOI: 10.1021/acs.organomet.7b00361
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Organometallics
Article
1
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00361.
■
́
S
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Cambridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for V.S.: salazar@uaeh.edu.mx.
*E-mail for A.S.: andres.suarez@iiq.csic.es.
■
ORCID
Olivan
́
, M.; Oro, L. A. Organometallics 1996, 15, 814−822. (d) Vicent,
E.; Sanau, M.; Peris, E. Organometallics
Andres Suarez: 0000-0002-0487-611X
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C.; Viciano, M.; Mas-Marza,
́
́
Notes
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The authors declare no competing financial interest.
́
́ ́
J.; Polo, V.; Fernandez-Alvarez, F. J.; Perez-Torrente, J. J.; Oro, L. A.
ACKNOWLEDGMENTS
■
Chem. - Eur. J. 2013, 19, 17559−17566. (h) Per
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Gomez-Bautista, D.; Iglesias, M.; Oro, L. A. Organometallics 2016, 35,
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ez-Torrente, J. J.;
Financial support (FEDER contribution) from the Spanish
MINECO (CTQ2013-45011-P, CTQ2016-80814-R, and
CTQ2016-81797-REDC) is gratefully acknowledged. P.L.
thanks the Spanish MINECO for a Juan de la Cierva contract.
Prof. Ernesto Carmona (IIQ) is thanked for a generous loan of
catalyst for silane deuteration.
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