Hydrosilylation of Carbonyl Compounds Catalyzed through a Lithiated Hydrazone Derivative

Article abstract of DOI:10.1021/acs.organomet.8b00315

A well-defined lithiated hydrazone derivative has been synthesized and fully characterized through various analytical platforms, including multinuclear (¹H, ¹³C, ¹⁵N, ⁷Li) and two-dimensional NMR, high-resolution MS spectrometry, IR, and X-ray diffraction crystallography. It behaves as a binuclear species in the solid state and as a monomeric contact ion pair in solution. It has also been tested as a catalyst in hydrosilylation reactions, being the first lithium hydrazone reported to catalyze the full conversion of carbonyls of different nature into alcohols in short reaction times, at room temperature, and with catalyst loadings equal to or below 0.5 mol %. Kinetic studies have proven fractional order dependences with respect to ketone and silane and first order dependence in the case of the catalyst. The proposed reaction mechanism is characterized by the nucleophilic addition of the lithium hydrazonide to the silicon atom of the silane to give a five-coordinate silicon species.

Full text of DOI:10.1021/acs.organomet.8b00315

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Hydrosilylation of Carbonyl Compounds Catalyzed through a
Lithiated Hydrazone Derivative
†,§
†,§,∥
‡
́
́
́
̃
Alvaro Raya-Baron, Pascual Ona-Burgos,
Antonio Rodríguez-Dieguez,
,†
́
and Ignacio Fernandez*
†Department of Chemistry and Physics, Research Centre CIAIMBITAL, University of Almería, Ctra. Sacramento, s/n, Almería
E-04120, Spain
‡Department of Inorganic Chemistry, Faculty of Science, University of Granada, 18071 Granada, Spain
S
* Supporting Information
ABSTRACT: A well-defined lithiated hydrazone derivative has been synthesized
and fully characterized through various analytical platforms, including multinuclear
7
(1H, 13C, 15N, Li) and two-dimensional NMR, high-resolution MS spectrometry,
IR, and X-ray diffraction crystallography. It behaves as a binuclear species in the
solid state and as a monomeric contact ion pair in solution. It has also been tested as
a catalyst in hydrosilylation reactions, being the first lithium hydrazone reported to
catalyze the full conversion of carbonyls of different nature into alcohols in short
reaction times, at room temperature, and with catalyst loadings equal to or below 0.5
mol %. Kinetic studies have proven fractional order dependences with respect to
ketone and silane and first order dependence in the case of the catalyst. The
proposed reaction mechanism is characterized by the nucleophilic addition of the
lithium hydrazonide to the silicon atom of the silane to give a five-coordinate silicon
species.
catalyzed systems described so far require typical catalyst
loadings of 5−10 mol %, which in turn represent some of their
main disadvantages.
To the best of our knowledge, hydrosilylation catalyzed by
lithium hydrazones or lithium amides have not been described.
We therefore describe herein the first time where a lithium
hydrazone of anthraquinonic nature catalyzes the hydro-
silylation of polarized unsaturated bonds with very low catalyst
loadings and at room temperature. The pivotal role of the
anthraquinone moiety will be highlighted.
INTRODUCTION
■
The reduction of organic substrates into value-added products
has found many applications in industrial and academic
settings.1 The addition of silanes to unsaturated functional
groups, the so-called hydrosilylation, has attracted great
interest in the last few years.2 Transition-metal catalysts,
both noble metals2,3 and first-row transition metals,4 have
provided remarkable activity, as well as control of the chemo-,
regio-, and stereoselectivity in the reduction process. Main-
group-element compounds have also been employed since the
discoveries of Volpin et al., where CsF or ammonium fluoride
catalyzed the hydrosilylation of carbonyls.5 Main-group-
catalyzed reductions of unsaturated bonds have been nicely
reviewed by Revunova et al. very recently.6 Electrophilic
boranes have also been used as effective catalysts for the
hydrosilylation of carbonyls and imine functions.7 Studies
related to those described in the present article are those
reported by Hosomi et al. that employed alkali-metal alkoxides
to carry out the chemo-8 and stereoselective9 catalytic
reduction of aldehydes, ketones, and imines. Similarly, the
reduction of hydroxy esters, lactones, and tosylimines catalyzed
by LiOMe has been also described.10 More recently, tertiary
amides were efficiently reduced to their corresponding
enamines under hydrosilylation conditions, using t-BuOK (5
mol %) and (MeO)3SiH or (EtO)3SiH as the reducing agent.11
Simple bases such as t-BuOK and KOH can catalyze
polymethylhydrosiloxane (PMHS) reduction of ketones and
esters to alcohols and the reduction of aldimines to amines, as
described by Nikonov and co-workers.12 Most of the base-
RESULTS AND DISCUSSION
■
Ligand 1 has been synthesized from inexpensive starting
materials in high yields through 1-hydrazinoanthraquinone,
which was prepared following reported methods (Scheme 1).13
NMR, mass spectrometry, IR, and X-ray diffraction crystallo-
graphic data confirmed the formation of (E)-1-(2-(pyridin-2-
ylmethylene)hydrazinyl)anthraquinone (1) in pure form.
1H,15N gHMBC experiments performed on a THF-d8
solution of ligand 1 allowed the identification of N1, N2,
and N3 signals, which were located at δN 149.6, 326.3, and
315.9 ppm, respectively (Figure S15).
Crystals of 1·HBF4 were grown by layering hexane over a
dichloromethane solution of 1 in the presence of a few drops
of fluoroboric acid to assist with crystallization. The X-ray
diffraction structure is given in Figure S4 and confirms, on one
Received: May 15, 2018
© XXXX American Chemical Society
A
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These dinuclear molecules are packed thanks to π-stacking
interactions involving anthraquinone units (Figure S6).
We have occasionally observed that the solution and solid-
state structures of group 1 salts may be different and are
strongly dependent on the choice of solvent.15 Consequently,
we have studied the solution structure of 2 via multinuclear
Scheme 1. Synthesis of Derivatized Anthraquinone 1 and Its
Lithiated Derivative 2 Starting from 1-Chloroanthraquinone
1
7
NMR spectroscopy together with H and Li PGSE NMR
diffusion measurements. The 1H NMR spectrum of 2 at 294 K
exhibited averaged signals for the two hydrazone ligands
incorporated in the complex so that no distinction between the
dimer and monomer could be inferred. The whole set of
signals spans from δH 7.14 to 8.54 ppm, where those coming
from the pyridine moiety and those from the anthraquinone
were distinguished thanks to COSY correlations (Figure S22).
The assignment of the 13C NMR spectrum was accomplished
in combination with DEPT-135 and two-dimensional HMQC
and HMBC experiments (Figures S19−S21). 1H,15N gHMQC
2D NMR allowed the observation of all three nitrogen atoms
N1, N2, and N3, which were located at δN 264.3, 383.5, and
313.3 ppm, respectively. The values for N1 and N2 are clearly
shifted downfield with respect to the neutral ligand (1) with
coordination shifts of Δ(15Ncoord) = +115.0 and +57.6 ppm,
respectively, which matches earlier observations of similar
trends in δN of metal complexes and parallels the effects
induced by alkylation or protonation of a nitrogen lone pair.16
Importantly, N3 did not shift significantly from the value of its
hand, the protonation of the pyridinic nitrogen, and on the
other hand, the chelating potential of the quinonic moiety,
since an intramolecular hydrogen bond between NH and C
O of length ca. 2.623 Å is observed.
The reaction of 1 with 1 equivalent of LiHMDS in THF led
to the quantitative formation of complex 2 (Scheme 1), which
is stable at room temperature over several days or weeks under
an inert atmosphere. This new Li hydrazone derivative has
been fully characterized by NMR in THF-d8 solution and by X-
ray diffraction in the solid state. X-ray diffraction studies for the
lithium material 2 revealed a binuclear coordination compound
where both lithium ions are equivalent and contribute toward
the formation of a 12-membered ring based on Li−N bonds
(Figure 1). The lithium atoms are equivalent and are both
neutral ligand, showing a 15N coordination shift of Δ(15Ncoord
)
= −2.6 ppm, which points to a solution structure where the
binuclear scaffold has been broken up into mononuclear
species and where the pyridine moiety is not chelating the
lithium atom and, therefore, no apparent change in its
nitrogen-15 chemical shift is observed.
To ascertain and verify the real solution structure in
comparison to the dimeric architecture found in the solid
7
state, PGSE diffusion NMR measurements (1H and Li) in
combination with 1H−7Li-HOESY experiments were per-
1
7
formed. The H and Li PGSE NMR results for THF-d8
solutions of 1 and 2 are given in Table S1 (further details are
given in the Supporting Information). The almost equivalent D
values for both the anion and cation in 2 point to a contact ion
pair in THF solution. From the X-ray diffraction structure of 2,
one can estimate a radius of 6.2 Å. Therefore, the 5.2 Å rH
value based on the measured D coefficients is consistent with a
mononuclear contact ion pair at 294 K.17
The relative intensities of the cross-peaks in a 7Li,1H
HOESY experiment are determined by the competition
between the NOE buildup and the decay of signals via
relaxation. For 2 these values were large enough to be adequate
for their observation (Figure 2) and only one cross-peak
between the 7Li nucleus and the proton H15 was detected. No
other cross-peaks were observed, since all other Li···H
through-space distances are greater than 4 Å (Li···H−C5
4.005 Å; see the molecular structure of 2 in Figure 1). In
addition, no cross-peaks between the 7Li nuclei and the
protons of THF were detected, probably because of isotopic
dilution, through which the majority of the protonated THF
molecules are displaced by deuterated molecules and the
correlation signal may disappear under the noise.
Figure 1. Molecular structure of 2 in the solid state. Hydrogen atoms
are omitted for clarity. The perspective of the complex is supported by
an X-diffraction study.
four-coordinated with a LiO2N2 core almost of tetrahedral
geometry. The connectivity of this complex was supported by
an X-ray diffraction study, but unfortunately its quality
prevents its publication. This coordination environment is
established from two nitrogen atoms pertaining to the pyridine
ring and imine group, one oxygen atom from the
anthraquinone unit (CO), and one coordinated tetrahy-
drofuran molecule. Similar heterometallic macrocycles have
been already described by Dyson and co-workers.14 Interest-
ingly, the dimeric scaffold is promoted by the imine-picoline
fragment, which is poorly flexible and has an E geometry. The
Fourier transform infrared (FT-IR) spectrum (Figure S33)
corroborates this statement and shows two distinct CO
stretching bands at 1655 and 1610 cm−1, indicating the
unsymmetrical coordination of the two quinonic oxygens.
Aside from the crystallographic characterization, NMR, and
FT-IR spectroscopy, MS spectrometry was also used to verify
the structural nature of 2 in more detail. In particular, MS
(ESI) spectrometry in THF/dichloromethane solution gave
evidence for the cleavage of the binuclear scaffold in solution.
B
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higher than 99% conversion after another 1 h of reaction
(entry 2). Other silanes were used (entries 3−6), being
phenylsilane the only one that afforded similar conversion
(entry 3).
These results were compared to those obtained using other
Li−N based compounds, such as LiHMDS and LDA (Table 1,
entries 7 and 8), but their performance as catalysts was
remarkably lower.
To prove the relevance of the anthraquinone moiety in our
catalyst design, we tested the performance of (E)-2-((2-
phenylhydrazone)methyl)pyridine (3) by using it under
catalytic conditions in the presence of the same amount of
n-BuLi. This ligand is analogous to 1 but lacks the
anthraquinone scaffold and bears only a phenyl substituent
instead. The obtained conversion was notably lower (60%;
Table 1, entry 9) than that from 2 (92%; entry 1), indicating
that the presence of the anthraquinone substituent in the
catalyst’s structure must have a significant influence on the
hydrosilylation process. It is important to note that no additive
other than our complex 2, in catalytic amount, was needed for
the reactions to take place.
Figure 2. (a) Section of the 7Li,1H-HOESY NMR (194 MHz)
spectrum of compound 2 in THF-d8 at 294 K. The 1D projections
7
1
1
correspond to Li and H NMR spectra. (b) Section of the H,15N
gHMBC NMR (500 MHz) spectrum acquired with a 500 MHz
spectrometer of compound 2 in THF-d8 at 294 K. Chemical shifts are
referenced to NH3.
This strategy was extended to more sterically demanding
substrates, aliphatic ketones and aromatic aldehydes and
ketones. The alkane functionality of ketonic substrates such
as 3,4-dihydronaphthalen-1(2H)-one (entry 2, Table 2) and
cyclohexanone (entry 3, Table 2) produced different out-
comes. The former needed 24 h to reach 84% conversion,
while the latter was complete in 2 h.
The positive ion mode revealed parent ions corresponding to
mononuclear lithium derivatives with two or three bonded
THF molecules ([C20H12N3O2Li(THF)2(OH)]+ or
[C20H13N3O2Li(THF)3]+, respectively), with m/z 494.26821
and 550.23066, respectively. As expected, the observed isotope
pattern matches well with those theoretically calculated for the
exact masses (Figures S36 and S37). Interestingly, a fragment
located at m/z 661.21946 was detected and assigned to
[C40H27N6O4Li], which in fact represents the substitution of
the THF molecules by another molecule of ligand 1,
corroborating the coordination abilities of the ligand itself
(Figure S38).
Hydrosilylation of Carbonyl Compounds. With the
new lithiated hydrazone characterized in solution and the solid
state, we first tested its efficiency in the hydrosilylation of
acetophenone (Table 1). Stirring the reaction mixture of
acetophenone and (EtO)2MeSiH in the presence of 0.5 mol %
of 2 at room temperature for 1 h afforded the corresponding
alcohol (after acidic workup) in 92% conversion (entry 1) and
When aryl aldehydes were employed, quantitative yields in
less than 1 h were obtained in all the cases (Table 2, entries 4−
9). When electron-donating substituents such as methoxy
(entry 6)- and 4-dimethylaminobenzaldehyde (entry 7) were
used, extended reaction times of 37 and 45 min, respectively,
were necessary to reach full conversion. When electron-
withdrawing groups were attached at the para position with
respect to the aldehyde functional group, as in 4-
fluorobenzaldehyde (entry 5) and 4-nitrobenzaldehyde (entry
9), shorter reaction times of 15 and 10 min, respectively, were
required. In the case of 2-pyridinecarboxaldehyde, the metal
coordination abilities of the pyridine could explain its
retardation down to 40 min. The highest TOF value observed
was for 4-nitrobenzaldehyde, at 19.8 min−1 (1188 h−1).
Remarkably, the nitro group, which is often not tolerated by
iron-based catalysts, provides in this case excellent conversion
in a very short reaction time. Hydrosilylation of esters and
amides was attempted, but no reaction product was observed
similar conditions under to those used for ketones and
aldehydes. Thus, these functional groups are not expected to
interfere with the reduction of aldehydes or ketones if they are
present in the same molecule. Overall, the major part of the
catalysis assayed occurs on the order of minutes, and this
therefore represents a straightforward method for the fast
reduction of a great variety of carbonyls.
Table 1. Optimization of the Reaction Conditions for the
a
Hydrosilylation of Acetophenone
b
entry
silane
time (h)
conversn (%)
92
>99 [80]
1
2
3
4
5
6
7
8
9
(EtO)2MeSiH
(EtO)2MeSiH
PhSiH3
Ph2SiH2
Et3SiH
1
2
1
24
24
24
1
c
86
29
0
PMHS
<1
d
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
66
To test the recyclability of the catalyst, we performed six
consecutive catalytic runs on a J. Young NMR tube (entry 2,
Table 1) with only one loading of catalyst (0.5 mol %). No
significant loss of catalytic activity was observed after six
reaction cycles, where conversions of 97, 97, 95, 97, 95, and
e
1
1
82
f
60
a
Reaction conditions unless stated otherwise: carbonyl substrate (2
mmol), hydrosilane (2.5 mmol), 2 (0.5 mol %), THF (1.5 mL), room
1
95% for all of the successive cycles were determined by H
b
temperature. Quenched with HCl(aq) 1 M. Conversions (relative to
c
1
NMR.
acetophenone) determined by H NMR of the reaction crudes. The
d
The reactivities of some of these substrates were monitored
isolated yield is given in brackets. LiHMDS was used as catalyst.
LDA was used as catalyst. A Li hydrazone obtained in situ by mixing
3 and n-BuLi was used as catalyst.
1
e
f
by H NMR spectroscopy in J. Young NMR tubes. These
reaction profiles (Figure 3) allowed us to distinguish initial
C
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Kinetic Study. By measurement of the initial rate of the
reaction while the initial concentration of one of the reaction
components is changed, the order of the reaction with respect
to that component can be determined. This is done following
the methodology described by Bleith and Gade,18 by plotting
the natural logarithm of the initial reaction rate versus the
natural logarithm of the initial concentration of the varying
component (all other initial concentrations are held constant),
which results in a straight line of slope equal to the order for
that component. Figure 4 illustrates these variations for both
silane and ketone as well as for the catalyst 2.
Table 2. Hydrosilylation of Aldehydes and Ketones Using 2
as Catalyst and (EtO)2MeSiH as Reducing Agent
a
a
Reaction conditions: carbonyl substrate (0.74 mmol), (EtO)2MeSiH
(0.83 mmol), Li precursor (0.5 mol %), THF-d8 (0.5 mL), room
b
temperature. Conversions (relative to the carbonyl substrate)
c
determined by 1H NMR of the crude reaction mixture. Isolated
yields are given in brackets.
Figure 4. a) ln(initial rate) as a function of ln(concentration) of
acetophenone (blue circles) and (EtO)2MeSiH (red triangles). (b)
ln(initial rate) as a function of ln(concentration) of catalyst (blue
circles). The concentrations assayed were 0.2 mM (0.1 mol %), 0.6
mM (0.25 mol %), 1.8 mM (0.75 mol %), and 2.4 mM (1 mol %).
rates and culmination steps, though no induction periods were
observed in any of the monitored reactions.
On the basis of the proposed model of coordination it is very
probable that the pyridine moiety has no influence on the
catalysis; however, further experiments are in progress with
modified ligands in order to verify this statement.
In the acetophenone series the concentration ranged from
0.7 to 2.5 M, whereas in the case of the silane the range varied
from 0.8 to 2.8 M. In both cases, linear correlations were
observed and from the slopes we could deduce the orders of
reaction as 0.39 and 0.71, respectively. As is mentioned in the
Experimental Section, the initial rates were obtained by
quenching the catalytic runs 3−10 min after the reactions
started; thus, the straight lines obtained could have some
deviation from the tangent at t = 0. At present, the exact reason
for the observed fractional rate orders is not clear, as the
reaction mechanism has not been clarified yet and will require
further rationalization. When we represented the initial rate of
1-phenylethan-1-ol (acetophenone reduction product) versus
precatalyst concentration in the range of 0.1−1 mol %, we
found again a positive linear correlation but with a first-order
dependence in this case (slope of 0.94 in the ln−ln graph,
Figure 4b).
Figure 3. Reaction profiles of the catalytic hydrosilylation of 3,4-
dihydronaphthalen-1-(2H)-one, 4-methoxybenzaldehyde, 4-dimethy-
laminobenzaldehyde, 2-picolinaldehyde, and benzophenone, all using
(EtO)2MeSiH as silane and 2 as catalyst.
From these kinetic data, the deduced rate law for the lithium
hydrazonide catalyzed hydrosilylation of ketones is given by
D
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d[P]
dt
= k[2]1[(EtO)2MeSiH]0.4[ketone]0.7
In order to detect possible intermediate species in the
catalytic process, stoichiometric experiments were carried out
and monitored by 1H and 7Li NMR. When 2 and
acetophenone (0.01 mmol each) are dissolved in THF-d8 in
a J. Young NMR tube, no reaction is observed in the 1H NMR
spectrum. However, when the same experiment is performed
using 2 and (EtO)2MeSiH, all of the signals associated with
complex 2 except those assigned to the pyridine ring (H17−
H20) were broadened (Figure 5).
Figure 6. Reaction profiles of the catalytic hydrosilylation of
acetophenone using 1 equiv of phenylsilane (purple squares) or
(EtO)2MeSiH (orange circles) and 0.5 mol % of 2 as catalyst.
The order dependences with respect to acetophenone and
(EtO)2MeSiH implies that in the rate-determining step an
intermediate participates that comprises both the ketone and
the silane. The usual wisdom in the field is that a base attacks
the silicon atom to give a pentacoordinated silicate species,
which then transfers the hydride to the carbonyl substrate as in
previously reported alkali-metal-catalyzed hydrosilyla-
tions.6,12,19
Figure 5. 1H NMR (500.13 MHz, THF-d8) spectra showing the
progress of the reaction between 2 (0.01 mmol) and (EtO)2MeSiH
(0.01 mmol).
CONCLUSIONS
This suggests on one hand that the pyridine is not involved
in any coordination, thus supporting the monomeric structure
found for 2, and on the other hand that the averaged signal
detected probably come from a fast equilibrium between 2 and
the silicate species formed from the attack of 2 to the silicon
■
A well-defined lithiated hydrazone derivative bearing an
anthraquinonic unit has been synthesized and fully charac-
terized in the solid and solution states. It behaves as an efficient
catalyst for the hydrosilylation of carbonyls at room temper-
ature, with loadings equal to or below 0.5 mol %, with no need
for special neutral donors or activators, and with excellent
conversions. The substrate scope included aromatic and
aliphatic aldehydes and ketones. As far as we are aware, it is
the first Li hydrazone complex reported to catalyze such
reactions. Kinetic studies have proven 0.39, 0.70, and 0.94
order dependences with respect to acetophenone,
(EtO)2MeSiH, and catalyst, respectively. Further derivatization
of the ligand and its use in enantioselective reactions are in
progress.
7
atom of the silane. The Li NMR spectra provided a broad
singlet during the 24 h evaluated. With regard to the silane
signals, those coming from the ethoxy and methyl moieties
were observed, but the SiH signal was absent probably due to
7
fast T2 relaxation induced by the quadrupolar Li nucleus,
which somehow proves that both nuclei are in close proximity.
At 24 h, a 1H,15N gHMBC NMR experiment was acquired and
revealed only one cross-peak located at δN 313.2 ppm
(Δ(15Ncoord) = 0.1 ppm), which was assigned to N3 (Figure
S40). The other two nitrogens, N1 and N2, did not show any
correlation probably due to fast T2 relaxation as a consequence
1
7
EXPERIMENTAL SECTION
of chemical exchange. Low-temperature H and Li NMR
measurements were also performed, but unfortunately, only
broad signals were observed during the whole range of
temperatures assayed (from 293 down to 173 K) (Figure S41).
We were also interested in testing the influence of the
hydrosilane nature on the rate of the catalytic runs. The
reduction of acetophenone using (EtO)2MeSiH and PhSiH3
■
General Laboratory Procedures. All experiments involving
moisture-sensitive compounds were performed under an inert
atmosphere of N2 using standard Schlenk techniques or in a glovebox.
Deuterated solvents were degassed and dried over activated molecular
sieves prior to use. 1-Chloroanthraquinone and hydrazine hydrate
were purchased from Sigma-Aldrich and used as received. 2-
Pyridinecarboxaldehyde was purchased from Acros and used for the
synthesis of 1 without further purification. THF, hexane, and diethyl
ether used for air-sensitive processes were dried and degassed in a
Solvent Purification System. LiHMDS was purchased from ABCR and
used as received. All hydrosilanes employed in the catalytic runs were
purchased from Acros and used without further purification. All
ketones and aldehydes used for catalysis were purified and dried by
standard methodologies prior to use. NMR spectra were measured
with a Bruker Avance III 500 spectrometer. IR spectra were recorded
with a FT-IR Bruker Alpha spectrometer. Elemental analyses (EA)
were performed on an Elementar vario EL cube in the CHN mode.
Mass spectra were acquired using an Orbitrap Thermo Fisher
Scientific mass spectrometer (ExactiveTM, Thermo Fisher Scientific,
Bremen, Germany) with an electrospray interface (ESI) (HESI-II,
Thermo Fisher Scientific, San Jose, CA, USA).
1
were monitored by H NMR, and similar reaction times were
found in both cases (Figure 6). When (EtO)2MeSiH was
employed, no induction period was detected. In contrast, when
PhSiH3 was the silane of choice, an induction period of 25.1
min was observed. The intersection of the maximum rate
tangent with the abscissa was taken as the time required for the
catalyst activation (Figure 6). These results suggest that the
presence of a Si−O bond in the silane must play a key role in
the activation of the catalyst. The fact that the reaction rate
with PhSiH3 is similar to that with (EtO)2MeSiH could be due
to the better catalytic performance of the silyl ether obtained in
the first cycle that still has two hydride equivalents and
therefore behaves as a more activated reagent.
E
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Synthesis and Analytical Data for Ligand 1. Ligand 1 was
synthesized from 1-hydrazinoanthraquinone, which was prepared
following a method previously described in the literature.13 1-
Hydrazinoanthraquinone (7.2 mmol) was suspended in methanol
(250 mL), and 2-pyridinecarboxaldehyde (0.83 mL, 8.7 mmol) was
added. The mixture was stirred and refluxed for 3.5 h, after which a
red precipitate was observed. The suspension was cooled to room
temperature, and the pure product was obtained as a deep red powder
by filtration under reduced pressure. Yield: 2.29 g (97%). Mp: 247.5
[C12H12N3]+, 198.1026; found, 198.1011 [M + H]+. Anal. Calcd for
C12H11N3: C, 73.07; H, 5.62; N, 21.30. Found: C, 72.98; H, 6.01; N,
21.01. IR (KBr): ν (cm−1) 3226m, 3185m, 3127m, 3057s, 3004s,
2948s, 2893s, 2839s, 2574m, 2422m, 2290m, 1915m, 1837m, 1602s,
1572s, 1495s, 1432s, 1355s, 1271s, 1152s, 1088s, 995s, 907s, 879s,
775s, 736s, 636m, 502m.
General Procedure for the Catalytic Hydrosilylation of
Carbonyl Compounds. Inside the glovebox, compound 2 (0.5 mol
%) was dissolved in THF (1.5 mL) inside a small vial. The
corresponding silane (2.25 mmol) was then added to the solution,
followed by the carbonyl substrate (2.00 mmol), and the vial was
sealed. The resulting mixture was stirred at room temperature for the
times indicated in Table 2, and then the reaction mixture was
quenched by adding HCl(aq) 1 M (2 mL) and stirred for 30 min to
ensure the complete hydrolysis of the silyl ether product. The mixture
was then taken to pH 7 by adding 5 M NaOH(aq), and the organic
products were extracted with ethyl acetate (3 × 10 mL). The organic
layers were dried using anhydrous Na2SO4, filtered, and concentrated
by rotary evaporation. Conversion values relative to the limiting
reagent were calculated from the 1H NMR spectra of the crude
reaction mixtures. Isolated products were obtained after column
chromatography in silica gel using ethyl acetate/hexane or dichloro-
methane as eluent.
Procedure for the Calculation of Initial Rates for the Kinetic
Studies of the Reduction of Acetophenone with (EtO)2MeSiH.
For each of the three components of the hydrosilylation reactions
(acetophenone, (EtO)2MeSiH, and catalyst 2), a series of catalytic
runs was performed, in which the concentration of the component
being studied was changed while those of the other two components
were held constant along that series. Order of reaction of PhCOMe:
[(EtO)2MeSiH] = 0.56 M, [2] = 0.6 mM, [PhCOMe] = 0.74, 0.99,
1.24, 1.49, 1.98, and 2.48 M. Order of reaction of (EtO)2MeSiH:
[PhCOMe] = 0.50 M, [2] = 0.6 mM, [(EtO)2MeSiH] = 0.56, 1.12,
1.67, and 2.23 M. Order of reaction of catalyst 2: [PhCOMe] = 0.50
M, [(EtO)2MeSiH] = 0.56 M, [2] = 0.24, 0.6, 1.8, and 2.4 mM. In
order to ensure that the catalytic runs were in their initial stages,
reactions were quenched just at the beginning (from 3 to 10 min
depending on substrate and concentration). The workup proceeded
as described above. In all of the cases, conversions were determined
by comparing the 1H NMR signals of the methyl group present in the
remaining acetophenone and the methyl group of 1-phenylethanol
product.
1
°C. H NMR (500.13 MHz, CDCl3): δ (ppm) 12.88 (s, 1H, NH),
8.65 (d, J = 4.5 Hz, 1H, H17), 8.34−8.28 (m, 2H), 8.24 (d, J = 8.1
Hz, 1H, H2), 8.16 (s, 1H, H15), 8.06 (d, J = 8.1 Hz, 1H, H20), 7.86−
7.74 (m, 4H), 7.70 (dd, J = 8.0, 8.1 Hz, 1H, H3), 7.28 (m, 1H, H18).
13C NMR (125.77 MHz, CDCl3): δ (ppm) 185.9 (CO), 183.2 (CO),
153.8 (CPy, C16), 149.5 (CPy, C17), 147.7 (CAr, C1), 143.0 (C15),
136.4 (CPy C19), 135.4 (CAr, C3), 134.4, 134.2, 134.2, 134.1, 133.7,
133.0, 127.0, 127.0, 123.5 (CPy, C18), 120.4, 120.28, 119.21 (CAr,
C4), 113.11 (CAr, C13). 15N NMR (500.13 MHz, THF-d8, via
gHMBC): δ (ppm) 326.3 (N2), 315.9 (N3), 149.6 (N1). ESI-MS:
calcd (m/z) for [C20H14N3O2]+, 328.1081; found, 328.1078 [M +
H]+. Anal. Calcd for C20H13N3O2: C, 73.38; H, 4.00; N, 12.84.
Found: C, 73.17; H, 4.39; N, 12.44. IR (KBr): ν (cm−1) 3212w,
3054w, 2922w, 1675s, 1632s, 1596s, 1567s, 1506s, 1471s, 1435m,
1299s, 1270s, 1117s, 1070m, 1005m, 829m, 771m, 732s, 706s, 611m.
Synthesis and Analytical Data for Complex 2. LiHMDS (42
mg, 0.238 mmol) was dissolved in THF (10 mL) and added to a
suspension of 1 (78 mg, 0.238 mmol) in THF (10 mL). The resulting
deep green solution was stirred for 2 h at room temperature, and then
all the volatiles were removed under vacuum. The remaining solid was
washed with hexane (15 mL) and filtered out to yield the pure
product as a dark green powder. This powder is soluble in THF or
dichloromethane. Its purity has been established through NMR and
high-resolution mass spectrometry (see below). Yield: 77 mg (80%).
Crystals were grown by layering a THF solution of complex 2 with
diethyl ether at room temperature. 1H NMR (500.13 MHz, THF-d8):
δ (ppm) 8.54 (d, J = 9.5 Hz, 1H, H2), 8.50 (d, J = 4.0 Hz, 1H, H17),
8.34 (d, J = 8.0 Hz, 1H, H8), 8.32 (s, 1H, H15), 8.19 (d, J = 7.5 Hz,
1H, H5), 8.16 (d, J = 8.0 Hz, 1H, H20), 7.74 (dd, J = 7.3 Hz, 1H,
H7), 7.67 (m, 2H, H6, H19), 7.40 (d, J = 7.0 Hz, 1H, H4), 7.23 (dd, J
= 7.0, 9.0 Hz, 1H, H3), 7.14 (dd, J = 6.0 Hz, 1H, H18). 13C NMR
(125.77 MHz, THF-d8): δ (ppm) 183.2 (CO, C10), 178.4 (CO, C9),
161.4 (CN, C1), 157.1 (CPy, C16), 149.1 (CPy, C17), 144.0 (C15),
137.2 (CAr, C12), 135.2 (CAr, C14), 135.1 (CPy, C19), 133.2 (CAr,
C11), 132.2 (CAr, C7), 131.6 (CAr, C3), 130.9 (CAr, C6), 126.5 (CAr,
C8), 125.7 (CAr, C5), 124.7 (CAr, C2), 121.4 (CPy, C18), 119.1 (CPy,
Procedure for the in Situ Lithiation of 3 Followed by
Catalytic Hydrosilylation. (E)-2-((2-Phenylhydrazone)methyl)-
pyridine (3) was dissolved in THF and the solution cooled until it
froze. As the solution thawed, n-BuLi (1.6 M in hexanes) was added
and the reaction mixture was warmed to room temperature. The
corresponding hydrosilane and ketone were then added, and the
mixture was stirred for 1 h at room temperature. The workup then
proceeded as described above.
7
C20), 116.0 (CAr, C4), 112.0 (CAr, C13). Li NMR (194.37 MHz,
THF-d8): δ (ppm) 1.99 (s). 15N NMR (500.13 MHz, THF-d8, via
gHMBC): δ (ppm) 383.5 (N2), 313.3 (N3), 264.3 (N1). ESI-MS:
calcd (m/z) for [C20H13N3O2Li(THF)3]+, 550.28878; found,
550.23066. IR (KBr): ν (cm−1) 3583w, 3050w, 2921w, 2851w,
1649m, 1608w, 1578m, 1564m, 1509m, 1472m, 1456m, 1427m,
1407m, 1380m, 1342m, 1323m, 1297m, 1249m, 1231m, 1186m,
1143m, 1098m, 1055m, 995m, 891w, 826w, 797w, 773w, 705m.
Synthesis and Analytical Data for (E)-2-((2-
Phenylhydrazone)methyl)pyridine (3). Benzylhydrazine (9.4
mmol) and 2-picolylaldehyde (9.4 mmol) were dissolved in methanol
(20 mL) and the solution was heated to reflux for 4 h. The resulting
mixture was then allowed to cool at room temperature with
concomitant precipitation of a yellow solid. The product was
collected by filtration and washed with cold methanol. The mother
liquors were placed at −20 °C to afford more solid which increased
General Procedure for the 1H NMR Monitoring of the
Catalytic Hydrosilylation of Carbonyl Compounds. Compound
2 (0.5 mol %) was placed in a J. Young NMR tube and dissolved in
0.5 mL of THF-d8. The corresponding silane (0.83 mmol) was then
added to the solution, followed by the carbonyl substrate (0.74
mmol), and the tube was sealed. The sample tube was shaken to
ensure the homogeneity of the mixture and then placed inside the
NMR spectrometer, where it remained for the entire duration of the
experiment.
ASSOCIATED CONTENT
■
1
the overall isolated yield. Yield: 1.23 g (66%). Mp: 181.4 °C. H
S
* Supporting Information
NMR (500.13 MHz, CDCl3): δ (ppm) 8.55 (ddd, J = 1.5, 4.5, 13 Hz,
1H, H6), 8.15 (s, NH, 1H), 8.03 (dt, J = 2.0, 13.0 Hz, 1H, H4), 7.83
(s, 1H, H7), 7.70 (td, J = 2.5, 12.5 Hz, H1, H3), 7.31 (m, 2H, H10,
H12), 7.18 (m, 3H, H5, H9, H13), 6.93 (tt, J = 1.5, 12.5 Hz, 1H,
H11). 13C NMR (125.77 MHz, CDCl3): δ (ppm) 154.4 (CPy, C2),
148.7 (CPy, C6), 144.0 (CAr, C8), 136.8 (C, C7), 136.5 (CPy, C3),
129.3 (CAr, C10, C12), 122.5 (CPy, C5), 120.7 (CAr, C11), 119.7
(CPy, C4), 113.02 (CAr, C9, C13). ESI-MS: calcd (m/z) for
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00315.
Further experimental details, spectral data for the
complexes, and crystallographic data for 1·HBF4
(PDF)
F
DOI: 10.1021/acs.organomet.8b00315
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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Accession Codes
CCDC 1821094 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for I.F.: ifernan@ual.es.
ORCID
̃
Pascual Ona-Burgos: 0000-0002-2341-7867
́
̈
4849−4859. (h) Metsanen, T. T.; Gallego, D.; Szilvasi, T.; Driess, M.;
Oestreich, M. Peripheral mechanism of a carbonyl hydrosilylation
catalysed by an SiNSi iron pincer complex. Chem. Sci. 2015, 6, 7143−
7149. (i) Lipke, M. C.; Liberman-Martin, A. L.; Tilley, T. D.
Electrophilic Activation of Silicon-Hydrogen Bonds in Catalytic
Hydrosilations. Angew. Chem., Int. Ed. 2017, 56, 2260−2294.
(5) Deneux, M.; Akhrem, I. S.; Avetissian, D. V.; Myssoff, E. I.;
Volpin, M. E. Reactions des Trialkylsilanes Catalysees par L’ion
Fluorure. Bull. Soc. Chim. Fr. 1973, 2638.
́
Antonio Rodríguez-Dieguez: 0000-0003-3198-5378
́
Ignacio Fernandez: 0000-0001-8355-580X
Present Address
∥
́
́
P.O.-B.: Instituto de Tecnologıa Quımica, Universitat
Politecnica de Valencia-Consejo Superior de Investigaciones
Cientificas (UPV-CSIC), Avda. de los Naranjos s/n, 46022
Valencia, Spain.
Author Contributions
(6) Revunova, K.; Kikonov, G. I. Main group catalysed reduction of
unsaturated bonds. Dalton Trans. 2015, 44, 840.
§A.R.-B. and P.O.-B. contributed equally to this work.
(7) (a) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Mechanistic
Aspects of Bond Activation with Perfluoroarylboranes. Inorg. Chem.
2011, 50, 12252 and references cited therein. (b) Oestreich, M.;
Hermeke, J.; Mohr, J. A unified survey of Si-H and H-H bond
activation catalyzed by electron-deficient boranes. Chem. Soc. Rev.
2015, 44, 2202−2220.
(8) Hosomi, A.; Hayashida, H.; Kohra, S.; Tominaga, Y. J.
Pentacoordinate silicon compounds in synthesis: chemo- and
stereo-selective reduction of carbonyl compounds using trialkoxy-
substituted silanes and alkali metal alkoxides. J. Chem. Soc., Chem.
Commun. 1986, 1411.
(9) (a) Kohra, S.; Hayashida, H.; Tominaga, Y.; Hosomi, A.
Pentaco-ordinate organosilicon compounds in synthesis: Asymmetric
reduction of carbonyl compounds with hydrosilanes catalyzed by
chiral bases. Tetrahedron Lett. 1988, 29, 89. (b) Hojo, M.; Fujii, A.;
Murakami, C.; Aihara, H.; Hosomi, A. Divergent stereoselectivity in
the reduction of α, β-Epoxy ketones using hydridosilicates.
Tetrahedron Lett. 1995, 36, 571. (c) Schiffers, R.; Kagan, H. B.
Asymmetric Catalytic Reduction of Ketones with Hypervalent
Trialkoxysilanes. Synlett 1997, 1997, 1175. (d) Nishikori, H.;
Yoshihara, R.; Hosomi, A. Optically Active Lithium-Alkoxide
Catalyzed Asymmetric Reduction of Imines with Trimethoxyhydro-
silane. Synlett 2003, 0561.
(10) Hojo, M.; Murakami, C.; Fujii, A.; Hosomi, A. New reactivity
of methoxyhydridosilane in the catalytic activation system. Tetrahe-
dron Lett. 1999, 40, 911.
(11) Volkov, A.; Tinnis, F.; Adolfsson, H. Catalytic Reductive
Dehydration of Tertiary Amides to Enamines under Hydrosilylation
Conditions. Org. Lett. 2014, 16, 680.
(12) Revunova, K.; Nikonov, G. I. Base-Catalyzed Hydrosilylation of
Ketones and Esters and Insight into the Mechanism. Chem. - Eur. J.
2014, 20, 839.
(13) Kim, M.; Mao, Q.; Davidson, B. L.; Wiemer, D. F. Tripeptide
Probes for Tripeptidyl Protease I Production via Gene Transfer. J.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
This work was supported by the Junta de Andalucıa (project
́
number P12-FQM-2668) and the Ministerio de Economıa,
Industria y Competitividad (project number CTQ2017-84334-
́
R). A.R.-B. thanks the University of Almerıa for a Ph.D.
fellowship. P.O.-B. thanks the MEC for his Ramon y Cajal
contract RYC-2014-16620.
́
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H
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