New Zinc Catalyst for Hydrosilylation of Carbonyl Compounds

Article abstract of DOI:10.1055/s-0037-1611824

A new zinc complex was synthesized and applied in the catalytic hydrosilylation of carbonyl compounds. Optimization of the reaction conditions showed that the presence a substoichiometric amount of methanol accelerates the process significantly. The reaction can proceed at very low catalyst load (down to 0.1 molpercent) under mild reaction conditions. The reaction tolerates the presence of C=C bonds, and thus can be useful for the synthesis of allylic alcohols from α,β-unsaturated aldehydes and ketones.

Full text of DOI:10.1055/s-0037-1611824

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2019, 51, A–H
paper
A
en
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
New Zinc Catalyst for Hydrosilylation of Carbonyl Compounds
Iryna D. Alshakova
OH
O
0.5 mol% Zn catalyst
0.25 equiv MeOH
2 equiv HSi(OEt)3
Ph
H
H
Georgii I. Nikonov*
0
0
0
0-
0
0
0
1-6
4
8
9-4
1
6
0
R1
R1
H
R1
R1
H
N
N
N
Zn
O
Brock University, 1812 Sir Isaac Brock Way, St. Catharines,
ON, L2S 3A1, Canada
gnikonov@brocku.ca
OH
toluene, 60 °C
H3C
H
S
R2
R2
Ph
Zn catalyst
R1: Alkyl, Aryl
R2: Aryl
Received: 07.03.2019
Accepted after revision: 18.04.2019
Published online: 07.05.2019
utilizing Zn(2-ethylhexanoate)2. Following the pioneering
work of the Carpentier group,9 chiral diamine ligands have
been used for the zinc-catalyzed enantioselective hydrosi-
lylation of ketones.10 However, the procedures required ei-
ther relatively high loads of zinc catalysts10a–e or large ex-
cess of hydrosilane reagents.10f Westerhausen and co-work-
DOI: 10.1055/s-0037-1611824; Art ID: ss-2019-z0158-op
Abstract A new zinc complex was synthesized and applied in the cata-
lytic hydrosilylation of carbonyl compounds. Optimization of the reac-
tion conditions showed that the presence a substoichiometric amount
of methanol accelerates the process significantly. The reaction can pro-
ceed at very low catalyst load (down to 0.1 mol%) under mild reaction
conditions. The reaction tolerates the presence of C=C bonds, and thus
can be useful for the synthesis of allylic alcohols from ,-unsaturated
aldehydes and ketones.
ers
reported
that
bis(alkylzinc)-hydride-di(2-
pyridylmethyl)amides can catalyze the hydrosilylation of
aldehydes and ketone with mono- and diphenylsilanes. This
reaction is sensitive to the sterics of reagents and no reac-
tion occurred with triphenylsilane.11 In 2010, Driess and
co-workers reported a new zinc complex with the triden-
tate O,S,O-ligand 1 that showed high catalytic activity in the
achiral hydrosilylation of ketones with triethoxysilane
(Scheme 1).12 Zinc complexes, synthesized in situ from di-
ethylzinc and commercially available formamidine ligands,
were also demonstrated to be highly efficient catalysts in
the hydrosilylation of aryl and alkyl ketones with TOFs up
to 1000 h–1, albeit at 10% catalyst load.13 Zinc hydride Dip-
pNacNacZnH 2 was shown to catalyze the hydrosilylation of
aldehydes and ketones at 3 mol% load at room temperature
and tolerated cyano, amino, nitro, and ester groups.14 In
2015, a new type of heteroscorpionate zwitterionic termi-
nal hydride zinc complex 3 was reported to catalyze the hy-
drosilylation of aldehydes with phenylsilane at room tem-
perature at 1 mol% catalyst load.15 Zinc hydride complexes
Key words zinc, catalysis, hydrosilylation, aldehydes, ketones, alco-
hols
Alcohols are important building blocks for pharmaceu-
ticals, agrochemicals, polymers, in natural product synthe-
ses, auxiliaries, and ligands.1 The production of alcohols by
the catalytic hydrogenation of carbonyls is attractive from
an economic perspective, but faces issues with chemoselec-
tivity in the case of multifunctional substrates because oth-
er functional groups can be reduced as well, of which the
C=C bond is the most vulnerable. This problem can be cir-
cumvented by chemoselective hydrosilylation of aldehydes
and ketones to afford alkoxysilanes that can be further hy-
drolyzed to afford primary and secondary alcohols. Fur-
thermore, alkoxysilanes, on their own, have diverse appli-
cations as valuable reagents in organic synthesis2 and mate-
rial chemistry.3 The catalytic hydrosilylation of carbonyl
compounds can be now performed under mild reaction
conditions and by using inexpensive hydrosilanes that are
easy to handle.1,4 Zinc has recently attracted increased at-
tention as a rising star in catalysis, with many applications
in the reduction of functional groups.5 It is an inexpensive,
nontoxic, and earth abundant post-transition metal.
4
16 and 517 supported by N-heterocyclic carbenes have been
also shown to act as efficient catalysts for the hydrosilyla-
tion of carbonyl compounds. Another zinc hydride catalyst
[3-Tptm]ZnH 6 was applied in multiple insertions of alde-
hydes and ketones into PhSiH3 and Ph2SiH2, affording tri-
alkoxy(aryl)silanes and dialkoxydiarylsilanes.18 Zinc com-
plex 7, the first example of a two-coordinate zinc hydride
complex, showed only moderate activity in the hydrosilyla-
tion of benzaldehyde and acetophenone and decomposed
under the reaction conditions.19
Among numerous ligands for transition metal complex-
es, compounds containing heterocyclic moieties (pyrazolyl,
oxazolyl, imidazolyl, etc.) occupy a special position because
of the potential non-innocent and cooperative behavior.20
The pyrazolyl group, in particular, possesses an acidic pro-
The first applications of zinc compounds in the hydrosi-
lylation of carbonyl compounds go back to the work of Calas
and co-workers in the early 1960s.6 But the modern chapter
was opened in 1994 by Noyori and co-workers who gener-
ated a catalytically active zinc species in situ by the reaction
of Zn(OSO2Me)2 with LiH.7 In 1999, Mimoun8 developed a
procedure for zinc-catalyzed hydrosilylation with PMHS,
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

B
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
Ph
P
C
Ph
Ph
N
iPr
iPr
S
H
N
O
O
Zn
N
Zn
N
N
N
Me2N
NMe2
Zn
iPr
iPr
N
H
3, Cui
2, Nikonov
1, Driess
(BPh4)–
2
N
N
Mes
THF
Mes
N
Dipp
H
S
Zn2+
Zn
H
THF
N
H
H
N
Mes
Mes
S
Zn
H
H
S
Zn
Zn
N
N
OTf
N
N
Dipp
N
N
Mes
Mes
4, Rivard
5, Okuda
6, Parkin
H
Ph
Ph
N
Ph
N
H
Zn
N
N
iPr
Zn
Si(iPr)3
S
H3C
Ph
Ph
Ph
7, Jones
8, this work
Scheme 1 Zinc catalysts for hydrosilylation of carbonyl compounds
ton on one of the nitrogen atoms, while the second nitrogen
atom can coordinate to a metal center, which makes the
proton even more acidic.21 Meanwhile, the mutual affinity
of zinc and sulfur is well-recognized not only in general
chemistry and mineralogy, but also in biological systems.
The coordination mode of zinc in hundreds of enzymes has
proven to include not only nitrogen and oxygen (of amino
acids), but also sulfur atoms, so that the Zn–S bond is as
much frequently occurring phenomenon in the bioorganic
world, as in the inorganic.22 Combining these ideas from
non-innocent chemistry and enzymology, we designed a
new ligand to prepare zinc catalyst 8 for the hydrosilylation
of aldehydes and ketones that operates under mild condi-
tions and low catalyst load and tolerates C=C bonds.
and 3.91 for the pyrazolyl proton and isolated methylene
group, respectively, and two triplets at  = 3.11 and 2.92 for
the ethylene bridging group.
Zinc chloride was the first choice of a precursor for the
preparation of the NNS Zn complex. Ligand 9 was deproton-
ated in situ by MeLi to create a driving force for the reaction
with ZnCl2 by elimination of LiCl. However, the experiment
resulted in a complex mixture of products.
To circumvent this problem, dimethylzinc, a commer-
cially available reagent, was reacted with compound 9 in
toluene. Complex 8 and methane (as the only byproduct)
formed almost instantly at room temperature. Under nor-
mal conditions, 8 is a colorless semiliquid, therefore grow-
ing crystals suitable for X-ray analysis was not possible.
Nevertheless, the coordination of the NNS ligand 9 can be
The synthesis of ligand 9 is shown in Scheme 2. Claisen
condensation of diethyl oxalate (10) with acetophenone
(11) led to diketone 12, which reacted with hydrazine to
form the pyrazole 13. Amide 14 was obtained by reacting
13 with 2-aminoethanol in the presence of K2CO3. Exchange
of hydroxy for chloride in 14 led to compound 15. Nucleop-
hilic substitution in chloride 15 with sodium thiophenolate
afforded sulfide 16 that was further reduced by LiAlH4 to
give the target NNS ligand 9 as a colorless semiliquid with
1
ascertained by H NMR spectroscopy. Only one NH signal,
significantly shifted upfield ( = 1.18–1.92) can be observed
in the NMR spectrum. The assignment of this signal to the
NH proton was supported by 1H-1H COSY NMR that showed
correlation of this signal with the nearby methylene pro-
tons. Disappearance of a broad IR signal at 3196 cm–1, com-
pared to the IR spectra of ligand 9, also supports the coordi-
nation via amide formation. The reaction is also accompa-
nied by evolution of gas, which could be assigned to
1
an overall yield 55%. The H NMR spectrum of 9 contains
1
five sets of multiplets at lower field, corresponding to aro-
methane based on appearance of a new H NMR signal at
matic protons ( = 7.18–7.77), as well as singlets at  = 6.46
 = 0.17, suggesting that the pyrazolyl group was most like-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

C
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
Na
O
O
O
O
MeONa, MeOH
O
O
+
O
Ph
Ph
O
O
11
12
10
NH2
O
O
N2H4, AcOH
EtOH
Ph
Ph
HO
MeOH, 90 °C
K2CO3
N
N
O
HN
NH
NH
14
OH
13
O
O
Ph
Ph
SOCl2
DCM
NaSPh
EtOH
N
N
HN
HN
NH
15
NH
16
Cl
SPh
Ph
H
N
N
Ph
N
LiAlH4
THF
ZnMe2
toluene
Zn
N
HN
NH
S
SPh
Ph
9
8
Scheme 2 The synthesis of ligand 9 and zinc complex 8
ly deprotonated to give pyrazolide. The pyrazolyl CH signal
( = 6.13), as well as the resonances for the methylene ( =
3.09) and ethylene bridging groups ( = 2.14 and 2.31),
were also shifted to a stronger field relative to the free li-
gand. The zinc-bound methyl group gave rise to a high-field
1H NMR signal at  = –0.03. Furthermore, an NOE experi-
ment showed a through space interaction between the
methyl group and the protons of amine, methylene, eth-
ylene, and phenyl units, which supports the tridentate co-
ordination mode of the deprotonated NNS ligand.
Because complex 8 is a viscous semiliquid, for simplicity
of operations, it is easier to generate it in situ from stock
solutions of ZnMe2 and ligand 9, prior to applications in ca-
talysis. The formation of 8 was confirmed by 1H NMR spec-
troscopy, after that a substrate and a hydrosilane were add-
ed. For the optimization of catalytic conditions acetophe-
none was chosen as a model substrate, and different
hydrosilanes, containing alkyl, aryl, and/or alkoxy groups,
were investigated for their reducing ability (Table 1).
Triethoxysilane was found to have the best activity, provid-
ing full conversion of the substrate within 10 hours at room
temperature and in 1.5 hours when heated at 60 °C (Table 1,
entries 5 and 7, respectively). To confirm the positive effect
of 9, the reaction was performed under the same conditions
as in entry 5, but in the absence of the ligand, which result-
ed in a very low yield of the hydrosilylation product (Table
1, entry 6).
Table 1 Hydrosilylation of Acetophenone with 8 Formed In Situa
Entry
Hydrosilane
Temp (°C)
Time
Conv.b (%)
1
2
3
4
5
6c
7
PhMe2SiH
PhMeSiH2
PMHS
rt
rt
rt
rt
rt
rt
60
22 h
3 d
–
60
9
3 d
(EtO)2MeSiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
5 d
100
100
12
97
10 h
22 h
1.5 h
a Reaction conditions: acetophenone (50 L, 0.429 mmol), ZnMe2 (5 mol%,
0.022 mmol), ligand 9 (5 mol%, 0.022 mmol), hydrosilane (2 equiv), tolu-
ene (1 mL).
b Conversions were determined by 1H NMR analysis.
c No ligand added.
The Carpentier group previously reported that zinc-cat-
alyzed hydrosilylation of carbonyl compounds can be accel-
erated in methanol/toluene (v/v = 80:20) medium and at-
tributed this effect to the formation of a reactive zinc me-
thoxide intermediate. The same idea was applied to the
hydrosilylation with catalyst 8 (Table 2). When 10 equiva-
lents of methanol (relative to the substrate) were added,
27% conversion was reached almost instantly at room tem-
perature, however, no further conversion was observed. In-
stead, all triethoxysilane was consumed by methanol in a
concurrent alcoholysis reaction to give triethoxy(me-
thoxy)silane (Table 2, entry 2).23 Nevertheless, the high ini-
tial rate of hydrosilylation under these conditions served as
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

D
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
a proof of principle. Further optimization showed that if
one equivalent of methanol is added to the reaction mix-
ture, almost full conversion of acetophenone can be
reached within 5.5 hours at room temperature (Table 2, en-
try 3), which is about twice as fast as the reaction without
addition of methanol (Table 2, entry 1). Further decrease of
the amount of methanol down to 0.25 equivalents leads to
further increase of the reaction rate, with a nearly full con-
version achieved in 2.5 hours (Table 2, entry 6). However,
when an even smaller amount of methanol is used, the re-
action slows down (Table 2, entry 7 and 8).
and ketone groups were present in the molecule, e.g. 17e,
the aldehyde is the first to be reduced. Overall, a tendency
for faster reduction of benzaldehydes containing electron-
withdrawing groups was observed. Thus, 4-cyanobenzalde-
hyde (17b) and 4-bromobenzaldehyde (17c) were fully con-
verted into the primary alcohols 17d and 17c in 1 and 4
hours, respectively, with the retention of cyano and bromo
groups. However, 4-methoxybenzaldehyde (17d) requires a
much longer time, 19 hours. These observations suggest
that the reduced reactivity of aldehydes under these cata-
lytic conditions may be caused by the increased stability of
the corresponding primary alcoholates of zinc relative to
secondary alcoholates. Importantly, the reaction tolerates
the C=C bonds in ,-unsaturated aldehydes 17f and 17g,
leading to primary allylic alcohols 18f and 18g, respectively,
in excellent yields. Here, cinnamaldehyde (17g) contains a
more electron-donating group in the -position, as com-
Table 2 Hydrosilylation of Acetophenone with 8 with Various
Amounts of Methanola
Entry
Catalyst
(mol%)
MeOH
(equiv)
Temp
(°C)
Time
(h)
Conv.b
(%)
1
2
5
–
rt
10
0.5
5.5
4.5
3
100
27c
98
0.5 mol% 8
2 equiv (EtO)3SiH
25 mol% MeOH
5
10
rt
1) KOH
MeOH
OH
R2
O
O[Si]
R2
3
5
1.0
rt
R1
R1
R2
R1
toluene, 60 °C
4
5
0.75
0.50
0.25
0.10
0.05
0.25
0.25
0.25
0.25
0.25
rt
100
99
2) HCl
17a–l
18a–l
5
5
rt
6
5
rt
2.5
4
98
OH
OH
OH
OH
7
5
rt
100
96
H
H
H
H
8
5
rt
8.5
10
0.5
1
NC
Br
MeO
9
2
rt
100
98
18a
10 h: 98% (91%)
18b
18c
18d
10
11
12
13
2
60
60
60
60
1 h: 99% (94%)
4 h: 99% (93%)
19 h: 98% (94%)
1
96
0.5
0.1
2.5
5
97
OH
H
O
OH
H
84
OH
H
a Reaction conditions: acetophenone (50 L, 0.429 mmol), ZnMe2, ligand
9, triethoxysilane (158 L, 0.858 mmol), toluene (1 mL).
b Determined by 1H NMR analysis.
18e
2.5 h: 98% (92%)
18f
18g
c No further conversion.
9 h: 99%
20 h: 97% (89%)
OH
OH
This positive effect of methanol allowed us to reduce the
load of the catalyst. Thus, if the reaction is performed in the
presence of 2 mol% of 8, it requires about 10 hours at room
temperature for the quantitative conversion of the sub-
strate (Table 2, entry 9). When heated with the same
amount of the catalyst, the reaction time drops significant-
ly to 0.5 hour (Table 2, entry 10). If the catalyst load is re-
duced further, the process slows down accordingly. Thus,
considering the catalyst economy and time efficiency, the
conditions in entry 12 were chosen as optimal.
OH
Cl
NC
18h
18i
18j
2.5 h: 97% (92%)
0.8 h: 99% (95%)
0.5 h: 99% (94%)
OH
OH
OH
MeO
EtOOC
Next, the catalytic system was studied for its applicabil-
ity and limitations. It was observed that the reaction rate is
highly dependent on the substrate (Scheme 3). Thus, the
hydrosilylation of benzaldehyde (17a) under the optimized
conditions was significantly slower than in the case of ace-
tophenone (17h), which is opposite to the usually observed
trend for these substrates, but nevertheless finds prece-
dents in the literature.7 However, when both an aldehyde
18k
18l
18m
9 h: 99% (93%)
6 h: 99% (97%)
5 h: 99% (93%)
Scheme 3 Reagents and conditions: substrate 17 (0.429 mmol), ZnMe2
(0.5 mol%), ligand 9 (0.5 mol%), MeOH (4.4 L, 25 mol%), triethoxysi-
lane (158 L, 0.858 mmol), toluene (1 mL). Conversions were deter-
mined by 1H NMR spectroscopy by relative integration of the
corresponding 1H NMR signals of substrate and product, isolated yields
are shown in parentheses.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

E
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
pared to crotonaldehyde (17f), and requires about twice as
much time as 17f for full conversion. Acetophenones 17i
and 17j, containing electron-withdrawing groups, can be
reduced in less than 1 hour, while the substrate with a me-
thoxy substituent in the para-position, 17k, requires 6
hours for full conversion into the product. In addition to the
cyano group in 17j, the reaction tolerates the ester group
(substrate 17l), which remains unhydrolyzed despite isola-
tion under basic conditions. Benzophenone (17m) can be
fully reduced within 9 hours, which is significantly longer
compared to acetophenone (17h); this can be explained by
increased steric hindrance of 17m, as well as by the pres-
ence of two electron-donating phenyl groups.
The mechanism of the hydrosilylation with 8 and the
exact role of methanol in the reaction acceleration remain
unclear. No distinctive zinc species could be isolated or de-
termined in the 1H NMR spectrum when alcohol was added
to a solution of 8. For their zinc catalytic system, the Car-
pentier group previously proposed a mechanism based on
the formation of a zinc–methoxy species, which reacts fast
with a hydrosilane, producing a zinc hydride complex.9a We
believe that a similar process can occur in our system, with
the only difference being that a much reduced amount of
methanol (25 mol%) is the most beneficial for the catalytic
system ZnMe2/9, and in fact stoichiometric amounts of al-
cohol impede the reaction completely. Thus, the role of
methanol is likely to help convert 8 into alkoxide 19
(Scheme 4) that is more reactive towards the hydrosilane to
produce the active zinc hydride species 20. Subsequent in-
sertion of a carbonyl compound into the Zn–H bond results
in a new zinc alkoxide 21. Further reaction with hydrosi-
lane completes the catalytic cycle.
ing transition state by N–H···O hydrogen-bonding interac-
tions rather than by a reversible proton transfer.25 Consider-
ing this finding, we believe that ligand 9 operates as a
directing moiety, as well as by stabilizing the zinc alkoxide
intermediate 21.
New zinc complex 8 was synthesized and applied in cat-
alytic hydrosilylation of carbonyl compounds. The optimi-
zation of reaction conditions revealed that the presence of a
substoichiometric amount of methanol significantly accel-
erates catalysis. This behavior was explained by fast forma-
tion of a zinc alkoxide species, which is believed to be more
active towards hydrosilanes in the production of a zinc hy-
dride intermediate. The reaction can proceed at low catalyst
load (down to 0.1 mol%) under relatively mild reaction con-
ditions. The substrate scope analysis showed the tolerance
to C–Br, C≡N, and CO2Et functionalities, and in particular to
the reactive C=C bond. Thus, this procedure can be useful
for the syntheses of allylic alcohols from ,-unsaturated
aldehydes and ketones.
All manipulations, required inert atmosphere, were carried out using
conventional atmosphere glove-box or N2-line Schlenk techniques.
Benzene, toluene, and THF were dried and purified using a Grubbs-
type solvent purification system. All organic substrates were pur-
chased from Sigma-Aldrich and Alfa Aesar. These reagents were used
without further purification. NMR spectra were obtained with a
Bruker DPX-300, AVANCE III HD 400 MHz, and DPX-600 spectrome-
ters (1H, 400 MHz; 13C, 101 MHz) at rt, then processed and analyzed
with MestReNova software (v10.0.2–15465). IR spectra were mea-
sured on a Perkin-Elmer 1600 FT-IR spectrophotometer. HRMS analy-
sis was carried out on Thermo Scientific DFS (Double Focusing Sector)
mass spectrometer.
The suggested mechanism implies that ligand 9 acts as
an innocent ligand and is not involved in the reaction pro-
cess. Although transition metal catalysts bearing acidic
amine ligands are often considered to act via a bifunctional
mechanism (the NH effect),24 recent DFT calculations
showed that the role of the N–H functionality in close prox-
imity to the metal center is to stabilize the rate-determin-
Sodium 4-Methoxy-3,4-dioxo-1-phenylbut-1-enolate (12)
Solid Na (2.53 g, 0.110 mol) was dissolved in MeOH (100 mL, cooled
by ice bath) in a 500-mL round-bottomed flask equipped with a con-
denser. Acetophenone (11; 11.7 mL, 0.100 mol) was added to this
solution, followed by diethyl oxalate (10; 13.6 mL, 0.100 mol). The
mixture was stirred for 12 h to produce a yellow precipitate, which
was filtered, washed with water (2 × 20 mL), and dried; yield: 22.1 g
(98%).
O
SiR3
R1
H-SiR3
R2
MeOH
H2
H-SiR3
MeOH
R1
O
R2
Zn
Me
Zn
OMe
Zn
H
Zn
20
19
8
21
CH4
MeO-SiR3
O
R1
R2
Scheme 4 Proposed mechanism of hydrosilylation of carbonyl compounds with 8 in the presence of a substoichiometric amount of methanol
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

F
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
IR (KBr): 1712.8 (C=O), 1633.1 (C=O), 1245.2 cm–1 (C–O).
13C {1H} NMR (CDCl3, 101 MHz):  = 155.7 (CONH-CH2), 147.1 (Cpyr),
143.0 (Cpyr), 132.3 (CAr), 130.1 (CAr), 129.5 (CAr), 127.0 (CAr), 106.9 (CH-
pyr), 42.4 (NHCH2CH2Cl), 41.7 (NHCH2CH2Cl).
1H NMR (CDCl3, 400 MHz):  = 8.00–8.07 (m, 2 H, o-ArH), 7.60–7.68
(m, 1 H, p-ArH), 7.50–7.57 (m, 2 H, m-ArH), 7.12 (s, 1 H, CH), 3.97 (s, 3
H, OCH3).
HRMS (EI): m/z [M+] calcd for C12H12ClN3O: 249.6962; found:
249.6957.
13C {1H} NMR (CDCl3, 105 MHz):  = 179.7 (C=O or CH=CO), 178.1 (C=O
or CH=CO), 165.7 (COOCH3), 137.0 (CAr), 133.1 (CAr), 128.6 (CAr), 128.4
(CAr), 92.5 [(C=O)CH=(C–O)], 52.8 (OCH3).
3-Phenyl-N-[2-(phenylthio)ethyl]-1H-pyrazole-5-carboxamide
(16)
HRMS (EI): m/z [M+] calcd for C11H9NaO4: 228.1765; found: 228.1738.
The reaction was performed under a N2 atmosphere. A solution of
NaSPh (1.07 g, 8.10 mmol) in EtOH (50 mL) was carefully added to a
solution of 15 (2 g, 8.01 mmol) in EtOH (50 mL). The mixture instantly
turned dark grey, and was stirred at rt for 24 h until the color became
pale yellow. Then the solution was filtered and EtOH was removed
under reduced pressure to give a creamy solid; yield: (2.49 g (96%).
1H NMR (CDCl3, 400 MHz):  = 11.43 (br s, 1 H, NH), 7.65 (d, J = 7.3 Hz,
2 H, o-ArH), 7.36–7.50 (m, 5 H, ArH), 7.26–7.34 (m, 2 H, m-ArH), 7.17–
7.24 (m, 1 H, p-ArH), 7.05 (s, 1 H, CH), 3.69 (q, J = 6.41 Hz, 2 H, CH2),
3.19 (t, J = 6.56 Hz, 2 H, CH2).
13C {1H} NMR (CDCl3, 101 MHz):  = 161.7 (CONHCH2), 146.5 (Cpyr),
146.1 (Cpyr), 135.0 (CAr), 130.0 (CAr), 129.2 (CAr), 129.1 (CAr), 128.9 (CAr),
128.7 (CAr), 126.5 (CAr), 125.6 (CAr), 103.1 (CHpyr), 38.5 (CH2), 33.6
(CH2).
Methyl 3-Phenyl-1H-pyrazole-5-carboxylate (13)
Compound 12 (10.0 g, 43.8 mmol) was dissolved in EtOH (200 mL)
then AcOH (3.00 mL) was added. A solution of hydrazine monohy-
drate (3.76 mL, 96.0 mmol) in EtOH (20.0 mL) was gently added drop-
wise with stirring. The mixture was stirred for 1 d, and then it was
concentrated under reduced pressure. Toluene (100 mL) was added
and the mixture was dried again under vacuum to remove the residu-
al hydrazine monohydrate. Sat. aq NaHCO3 soln was added to the resi-
due to remove AcOH and the white solid obtained was filtered and
dried under vacuum; yield: 8.50 g (96%).
IR (KBr): 3282.9 (N–H), 1736.1 (C=O), 1255.9 cm–1 (C–O).
1H NMR (CDCl3, 400 MHz):  = 7.73–7.81 (m, 2 H, o-ArH), 7.44–7.51
(m, 2 H, m-ArH), 7.37–7.44 (m, 1 H, p-ArH), 7.15 (s, 1 H, CH), 3.98 (s, 3
H, OCH3).
13C {1H} NMR (CDCl3, 105 MHz):  = 16.3 (COOCH3), 149.5 (Cpyr), 139.5
(Cpyr), 129.0 (CAr), 128.7 (CAr), 126.9 (CAr), 125.7 (CAr), 105.7 (CHpyr),
52.2 (OCH3).
HRMS (EI): m/z [M+] calcd for C18H17N3OS: 323.4121; found:
323.4128.
3-Phenyl-N-[2-(phenylthio)ethyl]-1H-pyrazole-5-methanamine
(9)
HRMS (EI): m/z [M+] calcd for C11H10N2O2: 202.2093; found:
The reaction was performed under a N2 atmosphere. Compound 16
(1.00 g, 3.09 mmol) was dispersed in dry THF and the mixture was
cooled with an ice bath. LiAlH4 (0.153 g, 4.00 mmol) was added care-
fully in two potions. The mixture was allowed to warm to rt and
stirred for 16 h. Excess LiAlH4 was quenched with EtOH. The solution
was filtered and the solvent was removed under reduced pressure.
The product was purified by treatment with aq HCl. The crystalline
product was filtered, washed with hexane and dried. Then it was sus-
pended in DCM and the solution was treated with sat. aq NaHCO3
solution. The organic phase was separated and dried (MgSO4), and the
volatiles were removed under reduced pressure to give a pale yellow
semi-liquid; yield: 0.746 g (78%).
202.2049.
N-(2-Hydroxyethyl)-3-phenyl-1H-pyrazole-5-carboxamide (14)
Compound 13 (5 g, 24.7 mmol) was dissolved in MeOH (100 mL). 2-
Aminoethanol (7.48 mL, 137 mmol) and K2CO3 (1.71 g, 13.7 mmol)
were added to the solution. The mixture was refluxed (90 °C) for 24 h.
Then volatiles were removed on a rotavap, and the oily residue was
treated with brine. The product precipitated from the solution as a
creamy solid and was separated and dried; yield: 4.97 g (87%).
1H NMR (CD3CN, 400 MHz):  = 7.73–7.81 (m, 2 H, o-ArH), 7.47–7.54
(m, 2 H, m-ArH), 7.39–7.47 (m, 1 H, p-ArH), 7.06 (s, 1 H, CH), 3.69 (t,
J = 5.49 Hz, 2 H, CH2), 3.50 (dt, J = 5.49, 5.67 Hz, 2 H, CH2), 2.36 (br s,
NH, OH).
IR (Nujol): 3196, 3055, 1469, 1444, 1415, 1377, 1310, 1259, 1193,
1155, 1087, 1074, 1024, 966, 802, 759, 734, 688 cm–1
.
13C {1H} NMR (CD3CN, 101 MHz):  = 161.4 (CONHCH2), 146.0 (Cpyr),
142.3 (Cpyr), 134.9 (CAr), 134.4 (CAr), 133.9 (CAr), 130.9 (CAr), 107.6 (CH-
pyr), 66.2 (NHCH2CH2OH), 46.9 (NHCH2CH2OH).
1H NMR (CDCl3, 400 MHz):  = 7.70–7.77 (m, 2 H, o-ArH), 7.39–7.45
(m, 2 H, ArH), 7.32–7.38 (m, 3 H, ArH), 7.25–7.31 (m, 2 H, m-ArH),
7.18–7.23 (m, 1 H, p-ArH), 6.46 (s, 1 H, CH), 3.91 (s, 2 H, CH2), 3.11 (t,
J = 6.34 Hz, 2 H, CH2), 2.92 (t, J = 6.34 Hz, 2 H, CH2).
HRMS (EI): m/z [M+] calcd for C12H13N3O2: 231.2505; found:
13C {1H} NMR (CDCl3, 101 MHz):  = 149.4 (Cpyr), 145.8 (Cpyr), 135.4
(CAr), 132.3 (CAr), 129.8 (CAr), 129.0 (CAr), 128.8 (CAr), 128.0 (CAr), 126.4
(CAr), 125.6 (CAr), 101.5 (CHpyr), 47.6 (CH2), 45.0 (CH2), 34.1 (CH2).
231.2512.
N-(2-Chloroethyl)-3-phenyl-1H-pyrazole-5-carboxamide (15)
SOCl2 (1.62 mL, 22.7 mmol) was added to a suspension of 14 (3.5 g,
15.1 mmol) in DCM (100 mL). The mixture was stirred at rt for 24 h.
Then the solution was filtered and neutralized with sat. aq NaHCO3
solution. The organic phase was separated and dried (MgSO4); the
solvent was removed under reduced pressure to give a white solid;
yield: 3.39 g (90%).
1H NMR (CDCl3, 400 MHz):  = 8.89 (br s, 1 H, NH), 7.95–8.02 (m, 2 H,
o-ArH), 7.48–7.62 (m, 3 H, m- and p-ArH), 7.44 (s, 1 H, CH), 3.83–3.91
(m, 2 H, CH2), 3.74–3.82 (m, 2 H, CH2).
HRMS (EI): m/z [M+] calcd for C18H19N3S: 309.4286; found: 309.4278.
Methyl[3-phenyl-5-({[2-(phenylthio-S)ethyl]amino-N}methyl)-
1H-pyrazol-1-yl]zinc (8)
The reaction was performed under the N2 atmosphere. Stock solu-
tions of 1.0 M 9 in dry toluene (0.400 mL, 0.100 g, 0.320 mmol) and
1.2 M ZnMe2 in toluene (0.267 mL, 0.320 mmol) were combined in
toluene (2 mL). The solution was stirred at rt in the glovebox for 20
min. When the reaction was complete, the solvent was removed un-
der reduced pressure to give a colorless semiliquid; yield: 0.123 g
(99%).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

G
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
IR (Nujol): 3057, 1631, 1602, 1556, 1496, 1462, 1438, 1415, 1377,
1350, 1338, 1327, 1301, 1288, 1155, 1085, 1070, 1055, 1024, 1001,
1H NMR (CDCl3, 400 MHz):  = 7.30 (d, J = 8.7 Hz, 2 H, ArH), 6.87 (d, J =
8.7 Hz, 2 H, ArH), 4.56 (s, 2 H, CH2OH), 3.80 (s, 3 H, OCH3), 2.20 (s, 1 H,
OH).
981, 914, 893, 804, 759, 738, 690, 659 cm–1
.
1H NMR (C6D6, 400 MHz):  = 7.99–7.87 (m, 2 H, o-ArH), 7.20–7.33 (m,
2 H, o-ArH), 7.06–7.15 (m, 1 H, p-ArH), 6.85–7.06 (m, 5 H, ArH), 6.13
(s, 1 H, CH), 3.09 (d, JHH = 6.9 Hz, 2 H, CH2), 2.31 (br s, 2 H, CH2), 2.14
(br s, 2 H, CH2), 1.18–1.92 (m, 1 H, NH), –0.03 (s, 3 H, CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 159.7 (CAr), 130.0 (CAr), 129.3
(CAr), 114.1 (CAr), 55.2 (CH2OH), 46.2 (OCH3).
3-(Hydroxymethyl)acetophenone (18e)
13C {1H} NMR (C6D6, 101 MHz):  = 214.2 (ZnCH3), 153.3 (Cpyr), 148.7
(Cpyr), 135.0 (CAr), 134.7 (CAr), 129.7 (CAr), 128.9 (CAr), 128.2 (CAr), 127.8
(CAr), 126.9 (CAr), 126.3 (CAr), 98.9 (CHpyr) , 47.7 (CH2), 46.1 (CH2), 31.5
(CH2).
HRMS (EI): m/z [M+] calcd for C19H21N3SZn: 388.8641; found:
388.8613.
98% Conversion was reached in 2.5 h; yield: 59.3 mg (92%).
1H NMR (CDCl3, 400 MHz):  = 7.94 (s, 1 H, ArH), 7.86 (d, J = 7.7 Hz, 1
H, ArH), 7.56 (d, J = 7.6 Hz, 1 H, ArH), 7.44 (t, J = 7.7 Hz, 1 H, ArH), 4.74
(s, 2 H, CH2OH), 2.59 (s, 3 H, CH3), 2.21 (s, 1 H, OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 198.3 (C=O), 141.6 (CAr), 137.3
(CAr), 131.6 (CAr), 128.8 (CAr), 127.5 (CAr), 126.6 (CAr), 64.6 (CH2OH),
26.7 (CH3).
Hydrosilylation; General Procedure
All manipulations were performed under a N2 atmosphere. Stock
solutions of 1.2 M ZnMe2 in toluene (1.8 L) and a stock solution of 1
M ligand 9 in toluene (2.2 L) were added to benzene (1 mL) in a low
pressure NMR sample tube. After 1 min, aldehyde or ketone (0.429
mmol) was injected into the toluene solution, followed by silane
(0.858 mmol) and MeOH (4.4 L, 0.107 mmol). The sealed NMR tube
was heated at 60 °C. The progress of the reaction was monitored by 1H
NMR spectroscopy.
(2E)-But-2-enol (18f)
Full conversion was reached in 9 h. Due to the high volatility of the
product, it was not isolated. 1H NMR signals of the product in the re-
action mixture are reported below.
1H NMR (C6H6/D2O, 400 MHz):  = 5.50–5.74 (m, 2 H, CH=CH), 4.31–
4.40 (m, 1 H, CH2OSi), 4.22–4.31 (m, 1 H, CH2OSi), 1.49–1.63 (m, 3 H,
CH3).
Product isolation: All operations were performed in air. A 0.5 M KOH
in MeOH solution (1 mL) was added to the reaction solution. The mix-
ture was stirred at rt for 20 min, and then the solvent was removed
under reduced pressure. 1 M Aq HCl (2 mL) was added to the residue,
and product was extracted with DCM (2 × 1 mL). The combined or-
ganic solutions were dried (MgSO4) and the solvent was removed on a
rotavap. The purity of the products was confirmed by NMR spectros-
copy.
(E)-Cinnamyl Alcohol (18g)
97% Conversion was reached in 20 h; yield: 50.8 mg (89%).
1H NMR (CDCl3, 400 MHz):  = 6.96–7.26 (m, 5 H, ArH), 6.42 (d, J =
15.6 Hz, 1 H, CH), 6.09 (dt, J = 14.8, 7.2 Hz, 1 H, CH), 4.01 (d, J = 7.2 Hz,
2 H, CH2OH), 2.38 (s, 1 H, OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 135.8 (CAr), 134.0 (CH), 128.6
(CAr), 128.2 (CAr), 126.6 (CAr), 124.9 (CH), 45.2 (CH2OH).
Benzyl Alcohol (18a)
1-Phenylethanol (18h)
Full conversion was reached in 10 h; yield: 42.2 mg (91%).
1H NMR (CDCl3, 400 MHz):  = 7.85–7.93 (m, 4 H, o- and m-ArH),
7.79–7.84 (m, 1 H, p-ArH), 5.22 (s, 2 H, CH2OH), 2.78 (s, 1 H, OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 141.0 (CAr), 128.5 (CAr), 127.6
(CAr), 127.0 (CAr), 65.3 (CH2OH).
97% Conversion was reached in 2.5 h; yield: 48.2 mg (92%).
1H NMR (CDCl3, 400 MHz):  = 7.26–7.46 (m, 5 H, ArH), 4.84–4.97 (m,
CHOH), 2.16 (s, OH), 1.52 (d, J = 6.5 Hz, CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 146.0 (CAr), 128.6 (CAr), 127.6
(CAr), 125.5 (CAr), 70.5 (CHOH), 25.2 (CH3).
4-Cyanobenzyl Alcohol (18b)
1-(4-Chlorophenyl)ethanol (18i)
Full conversion was reached in 1 h; yield: 53.6 mg (94%).
1H NMR (CDCl3, 400 MHz):  = 7.63 (d, J = 7.9 Hz, 2 H, ArH), 7.47 (d, J =
7.8 Hz, 2 H, ArH), 4.76 (s, 2 H, CH2OH), 1.88 (s, 1 H, OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 146.3 (CAr), 132.3 (CAr), 127.0
99% conversion was reached in 0.8 h; yield: 63.8 mg (95%).
1H NMR (CDCl3, 400 MHz):  = 7.18–7.25 (m, 4 H, ArH), 4.77 (q, J = 6.5
Hz, CHOH), 3.56 (s, 1 H, OH), 1.38 (d, J = 6.5 Hz, CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 144.4 (CAr), 132.7 (CAr), 128.3
(CAr), 126.9 (CAr), 69.26 (CH-OH), 25.17 (CH3).
(CAr), 118.8 (CN), 111.1 (CAr), 64.2 (CH2-OH).
4-Bromobenzyl Alcohol (18c)
1-(4-Cyanophenyl)ethanol (18j)
Full conversion was reached in 4 h; yield: 74.6 mg (93%).
1H NMR (CDCl3, 400 MHz):  = 7.47 (d, J = 8.4 Hz, 2 H, ArH), 7.23 (d, J =
8.3 Hz, 2 H, ArH), 4.63 (s, 2 H, CH2OH), 1.88 (s, 3 H, OH).
99% Conversion was reached in 0.5 h; yield: 59.3 mg (94%).
1H NMR (CDCl3, 400 MHz):  = 7.57 (d, J = 8.2 Hz, 2 H, ArH), 7.45 (d, J =
8.2 Hz, 2 H, ArH), 4.90 (q, J = 6.4 Hz, CHOH), 2.64 (s, 1 H), 1.44 (d, J =
6.5 Hz, CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 139.8 (CAr), 131.6 (CAr), 128.6
(CAr), 121.5 (CAr), 64.6 (CH2OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 151.5 (CAr), 132.3 (CAr), 126.1
(CAr), 118.9 (CN), 110.8 (CAr), 69.4 (CHOH), 25.4 (CH3).
4-Methoxybenzyl Alcohol (18d)
Full conversion was reached in 19 h; yield: 55.7 mg (94%).
1-(4-Methoxyphenyl)ethanol (18k)
99% Conversion was reached in 6 h; yield: 63.3 mg (97%).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2019, 51, A–H

H
I. D. Alshakova, G. I. Nikonov
Paper
Synthesis
1H NMR (CDCl3, 400 MHz):  = 7.30 (d, J = 8.6 Hz, 2 H, ArH), 6.88 (d, J =
8.6 Hz, 2 H, ArH), 4.85 (q, J = 6.4 Hz, 1 H, CHOH), 3.80 (s, 3 H, OCH3),
1.47 (d, J = 6.4 Hz, 3 H, CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 159.1 (CAr), 138.1 (CAr), 126.8
(CAr), 113.8 (CAr), 69.9 (CHOH), 55.4 (OCH3), 25.1 (CH3).
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Ethyl 4-(1-Hydroxyethyl)benzoate (18l)
99% Conversion was reached in 5 h; yield: 77.5 mg (93%).
1H NMR (CDCl3, 400 MHz):  = 8.02 (d, J = 8.2 Hz, 2 H, ArH), 7.44 (d, J =
8.2 Hz, 2 H, ArH), 4.96 (q, J = 6.5 Hz, 1 H, CHOH), 4.37 (q, J = 7.1 Hz, 2
H, CH2CH3), 1.50 (d, J = 6.5 Hz, 3 H, CH3), 1.39 (t, J = 7.1 Hz, 3 H,
CH2CH3).
13C {1H} NMR (CDCl3, 101 MHz):  = 166.6 (COOEt), 150.7 (CAr), 129.7
(CAr), 128.3 (CAr), 125.1 (CAr), 70.1 (CHOH), 60.9 (OCH2), 25.3 (CH3),
14.3 (CH3).
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Diphenylmethanol (18m)
Full conversion was reached in 9 h; yield: 79.1 mg (93%).
1H NMR (CDCl3, 400 MHz):  = 7.70 (d, J = 7.5 Hz, 4 H, o-ArH), 7.64–
7.54 (m, 6 H, ArH), 6.42 (s, 1 H, CH2OH).
13C {1H} NMR (CDCl3, 101 MHz):  = 141.1 (CAr), 128.6 (CAr), 128.1
(CAr), 127.8 (CAr), 64.3 (CHOH).
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Funding Information
(12) Marinos, N. A.; Enthaler, S.; Driess, M. ChemCatChem 2010, 2,
846.
This research was supported by the Natural Sciences and Engineering
Research Council of Canada (Discovery Grant, 2017-05231 to G.I.N.).
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611824.
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