Transfer hydrogenation of aldehydes catalyzed by silyl hydrido iron complexes bearing a [PSiP] pincer ligand

Article abstract of DOI:10.1039/c8ra02606h

The synthesis and characterization of a series of silyl hydrido iron complexes bearing a pincer-type [PSiP] ligand (2-R₂PC₆H₄)₂SiH₂ (R = Ph (1) and ⁱPr (5)) or (2-Ph₂PC₆H₄)₂SiMeH (2) were reported. Preligand 1 reacted with Fe(PMe₃)₄ to afford complex ((2-Ph₂PC₆H₄)SiH)Fe(H)(PMe₃)₂ (3) in toluene, which was structurally characterized by X-ray diffraction. ((2-ⁱPr₂PC₆H₄)SiH)Fe(H)(PMe₃) (6) could be obtained from the reaction of preligand 5 with Fe(PMe₃)₄ in toluene. Furthermore, complex ((2-ⁱPr₂PC₆H₄)Si(OMe))Fe(H)(PMe₃) (7) was isolated by the reaction of complex 6 with 2 equiv. MeOH in THF. The molecular structure of complex 7 was also determined by single-crystal X-ray analysis. Complexes 3, 4, 6 and 7 showed good to excellent catalytic activity for transfer hydrogenation of aldehydes under mild conditions, using 2-propanol as both solvent and hydrogen donor. α,β-Unsaturated aldehydes could be selectively reduced to corresponding α,β-unsaturated alcohols. The catalytic activity of penta-coordinate complex 6 or 7 is stronger than that of hexa-coordinate complex 3 or 4.

Full text of DOI:10.1039/c8ra02606h

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Transfer hydrogenation of aldehydes catalyzed by
silyl hydrido iron complexes bearing a [PSiP] pincer
ligand†
Cite this: RSC Adv., 2018, 8, 14092
a
a
Peng Zhang,a Xiaoyan Li,
Xinghao Qi,a Hongjian Sun,
Olaf Fuhrb
*
and Dieter Fenskeb
The synthesis and characterization of a series of silyl hydrido iron complexes bearing a pincer-type [PSiP]
i
ligand (2-R2PC6H4)2SiH2 (R ¼ Ph (1) and Pr (5)) or (2-Ph2PC6H4)2SiMeH (2) were reported. Preligand 1
reacted with Fe(PMe3)4 to afford complex ((2-Ph2PC6H4)SiH)Fe(H)(PMe3)2 (3) in toluene, which was
structurally characterized by X-ray diffraction. ((2-iPr2PC6H4)SiH)Fe(H)(PMe3) (6) could be obtained from
the reaction of preligand 5 with Fe(PMe3)4 in toluene. Furthermore, complex ((2-iPr2PC6H4)Si(OMe))
Fe(H)(PMe3) (7) was isolated by the reaction of complex 6 with 2 equiv. MeOH in THF. The molecular
structure of complex 7 was also determined by single-crystal X-ray analysis. Complexes 3, 4, 6 and 7
showed good to excellent catalytic activity for transfer hydrogenation of aldehydes under mild
conditions, using 2-propanol as both solvent and hydrogen donor. a,b-Unsaturated aldehydes could be
selectively reduced to corresponding a,b-unsaturated alcohols. The catalytic activity of penta-coordinate
complex 6 or 7 is stronger than that of hexa-coordinate complex 3 or 4.
Received 26th March 2018
Accepted 30th March 2018
DOI: 10.1039/c8ra02606h
rsc.li/rsc-advances
Reduction of aldehydes and a,b-unsaturated aldehydes to
alcohols is a fundamental and indispensable process for
1 Introduction
synthesis of a wide range of alcohols because a lot of alcohols
are useful products and precursors for pharmaceutical, agro-
chemical, material and ne chemical industries.11 In most cases
the transformation of aldehydes and a,b-unsaturated aldehydes
to the related alcohols is a metal-catalyzed process. In this
process, both H2 and alcohol can be used as reducing agents. In
2008, a series of new Pt(II) pincer complexes bearing a pincer-
type [PSiP] ligand (2-iPr2PC6H4)2SiH2 were synthesized by Mil-
stein's group. In addition, chloro-[PSiP]Pt complex was used to
prepare silanol Pt(II) pincer complex by hydrolytic oxidation.12
In 2013 Beller and co-workers reported the catalytic hydroge-
nation of aldehyde with H2. This catalytic system is chemo-
selective against ketone.13 However, that reaction required an
Phosphine-based [PSiP] pincer complexes of transition metals
have been studied extensively in recent years.1–5 In particular,
they are involved as key intermediates in a variety of catalytic
reactions of silicon compounds such as hydrosilylation,
hydrocarboxylation of allenes and transfer hydrogenation.6
Because silyl ligands have stronger s-donating characters and
show a more potent trans-inuence than commonly-used
ligands in transition metal chemistry,7 the introduction of
strong electron-donating and trans-labilizing silyl groups into
tridentate ligand architectures may promote the formation of
electron-rich and coordinatively-unsaturated complexes that
exhibit novel reactivity with s-bonds.8 Therefore, it is consid-
ered that silyl coordination compounds have potential appli-
cations in catalytic organic synthesis. In addition, changing
phosphorus ligand would provide transition metal complexes
with unique reactivity in catalytic reactions.9,10
ꢀ
elevated temperature (120 C) and a high H2 pressure (30 bar).
In the same year, three-coordinate iron(II) and cobalt(II)
complexes bearing three new N-phosphinoamidinate ligands
were synthesized by Turculet's group and the iron(II) complexes
as catalysts were used for hydrosilylation of carbonyl
compounds with considerably low catalyst loading using 1
equiv. of PhSiH3.14 In 2014, Morris utilized three kinds of iron
complexes bearing tetradentate PNNP ligands to realize
successfully transfer hydrogenation of ketones and imines.15 In
2015, Hu described new iron pincer complexes. These
complexes could activate H2 and catalyze selective transfer
hydrogenation of aldehydes at room temperature under a low
pressure of H2 (4 bar).16 Compared to the traditional hydroge-
nation reaction by the highly ammable molecular hydrogen
aSchool of Chemistry and Chemical Engineering, Key Laboratory of Special Functional
Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27,
250100 Jinan, People's Republic of China. E-mail: hjsun@sdu.edu.cn
b
¨
Institut fur Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF),
¨
Karlsruher Institut fur Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available: CIF les and a table
giving crystallographic data for 3 and 7 and gures giving the original IR, 1H
NMR, 31P NMR, 13C NMR, 29Si NMR spectra of the complexes and catalytic
products. CCDC 1515026 and 1490870. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8ra02606h
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employing precious metal (such as Au, Pt and Pd) catalysts,17 the
reduction of aldehydes and a,b-unsaturated aldehydes via
transfer hydrogenation using alcohol as both reaction solvent
and source of hydrogen in the presence of cheap transition
metal catalysts would be more promising because this is a safer,
atom-efficient and environmentally-benign method. In most
cases, 2-propanol as a conventional hydrogen donor solvent
with a moderate boiling point (82 ꢀC) serves as a reducing agent
because it is stable and nontoxic. In addition, a strong base
such as KOtBu is usually necessary for most transfer hydroge-
nation processes in 2-propanol. In 2002 Crabtree developed
a number of air-stable and moisture-insensitive Ir catalysts for
efficient transfer hydrogenation.18 In 2006 Rashid and co-
workers published several air-stable Ir complexes as effective
catalysts for transfer hydrogenation of ketones under base-free
conditions.19 In 2012 Colacino reported that four Ir(I) and Ir(III)
N-heterocyclic carbene (NHC) based complexes were used as
catalysts in the reduction of aldehydes and ketones with
glycerol.20
Fig. 1 The hydrido resonance of complex 3 at ꢁ40 ꢀC.
a concentrated THF solution layered with n-pentane at ꢁ20 ꢀC.
In the IR spectrum of 3, the typical n(Fe–H) stretching band of
complex 3 is found at 1836 cmꢁ1 while the n(Fe–H) stretching
band of complex 4 is at 1870 cmꢁ1
. This bathochromic shi
19
(34 cmꢁ1) is caused through the replacement of the Me group in
complex 4 by the H atom in complex 3 because the density of the
electron cloud at the iron center in complex 3 is smaller than
that in complex 4. The n(Si–H) of complex 3 was recorded at
In this contribution, we have developed novel silyl hydrido
iron [PSiP] pincer complexes for catalytic transfer hydrogena-
tion of aldehydes and a,b-unsaturated aldehydes under mild
conditions, using 2-propanol as both solvent and hydrogen
donor. Furthermore, we compared the catalytic effects of these
complexes with different phosphorus groups on the results of
catalytic reactions.
1992 cmꢁ1 while the n(Si–H) of preligand 1 was found at
1
2130 cmꢁ1. In the H NMR spectrum of 3 at ꢁ40 C, the char-
ꢀ
acteristic hydrido signal was found at ꢁ17.12 ppm as a pseudo
td peak with the coupling constants JPH ¼ 20 and 70 Hz (Fig. 1).
The split pattern of the hydrido signal of 3 is same with that of
4.19 The proton signal of the Si–H bond of complex 3 appears at
5.72 ppm as d peak while the resonance of the Si–H bond in free
preligand 1 was found at 5.87 ppm. Two signals at 0.97 and
0.45 ppm for two PMe3 ligands in the 1H NMR spectrum clearly
indicate that the trimethylphosphine ligands are not chemically
identical. It was found that two signals for PMe3 ligands and
2 Results and discussion
2.1 Reaction of Fe(PMe3)4 with (2-Ph2PC6H4)2SiRH (R ¼ H
(1) and Me (2))
In 2013, we reported the synthesis and characterization of
a series of Ni, Co, and Fe complexes bearing a tridentate
bis(phosphino)silyl ligand ((2-Ph2PC6H4)2SiMeH) (2) (eqn (1)).
The silyl hydrido iron(II) complex ((2-Ph2PC6H4)2SiMe)
Fe(H)(PMe3)2 (4) was found to be an excellent catalyst for
hydrosilylation of aldehydes and ketones under mild
conditions.21
(1)
Fig. 2 ORTEP plot of complex 3 at the 50% probability level (most of
ꢀ
hydrogen atoms are omitted for clarity). Selected bond lengths (A) and
angles (deg), Fe1–P1 2.2050(1), Fe1–P2 2.1932(1), Fe1–P3 2.2513(1),
Fe1–P4 2.2510(1), Fe1–Si1 2.2831(1), Fe1–H 1.5778(6), P3–Fe1–H1
172.56(5), P1–Fe1–P3 105.91(4), P2–Fe1–P1 146.42(4), P2–Fe1–P3
103.29(4), P1–Fe1–Si1 81.84(4), P2–Fe1–Si1 82.44(4), P3–Fe1–Si1
89.02(4), Si1–Fe1–H1 83.76(4), C14–Fe1–Si1 110.65(1), C19–Si1–Fe1
109.49(1).
Preligand 1 was treated with one equiv. of Fe(PMe3)4 in
toluene at room temperature (eqn (1)). Complex 3 was isolated
in a yield of 79% from diethyl ether at 0 ꢀC. Orange bulk crystals
of
3 suitable for X-ray diffraction were obtained from
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one signal for –PiPr2 groups in the 31P NMR of complex 3 at 1845 cmꢁ1, a little bit larger than that (1841 cmꢁ1) of complex 6
ꢁ40 ꢀC appeared at 5.2, 6.4 and 88.5 ppm in the integral ratio of because the MeO-group has electron-withdrawing ability. In the
1 (PMe3) : 1 (PMe3) : 2 (–PiPr2), respectively. The solid state 1H NMR of complex 7 at ꢁ40 ꢀC, the hydrido signal appeared at
structure of complex 3 shows a distorted hexa-coordinate octa- ꢁ11.10 ppm and split into a pseudo dddd peak due to the
hedral geometry (Fig. 2). The axial angle P3–Fe1–H1 is 172.6ꢀ, coupling effect of one PMe3 ligand and two chemically-identical
slightly deviating from 180ꢀ. [Si1Fe1P1P4P2] are in the equato- –PiPr2 groups. Moreover, the proton signal of the Si–H bond
rial plane. In comparison with the structural data, the molec- disappeared. There are two types of signals appearing at 22.2
ular structure of complex 3 is similar to that of complex 4.19 Fe1– and 106.2 ppm with a relative integral ratio of 1 (PMe3) : 2 (2ꢂ
H1 distance is 1.5776 A. Owing to the strong trans-inuence of –PiPr2) in the 31P NMR of complex 7. The related Pt chloride
ꢀ
ꢀ
H and Si atom, the distances Fe1–P3 (2.2513(1) A) and Fe1–P4 reacted with strong base to afford silyl ether Pt complex and the
(2.2510(1) A) are signicantly longer than the distances Fe1–P1 similar chemistry of Ru complex was also found by Stobart.12
ꢀ
ꢀ
ꢀ
(2.2050(1) A), Fe1–P2 (2.1932(1) A).
2.2 Reaction of Fe(PMe3)4 with (2-iPr2PC6H4)2SiH2 (5)
(3)
(2)
The molecular structure of complex 7 as a tetragonal
pyramid (s5 ¼ 0.0105)22 with an iron atom in the center was
conrmed by single crystal X-ray diffraction (Fig. 3). In this
molecular structure, P3 is the apex point and [Fe1P1P2Si1H] is
the base plane of this tetragonal pyramid. Fe1–H1 distance is
Complex 6 as pale yellow crystals was obtained from the
reaction of 5 with Fe(PMe3)4 in toluene (eqn (2)). In the IR
spectrum of complex 6, instead of the signal at 2140 cmꢁ1 (n(Si–
H) for preligand 5), a new stretching band of the Si–H bond was
ꢀ
1.60(3) A.
However, the similar reaction between complex 3 and MeOH
did not occur. It is guessed that the difference in the reactivity
between 3 and 6 might be caused by the vacant coordination in
6. This might allow for the coordination of MeOH (Scheme 1),
followed by subsequent hydride protonation with the release of
dihydrogen gas to form intermediate 6A (Scheme 1). The
reductive elimination between Fe–Si and Fe–O bond affords
intermediate 6B. Complex 7 was formed via oxidative addition
of the Si–H bond at the iron(0) center of 6B. Complex 7 as
complex 6 is also a penta-coordinate low-spin iron(II) coordi-
nation compound.
found at 2051 cmꢁ1. This large bathochromic shi (89 cmꢁ1
)
indicates that the activation of the Si–H bond occurred. The
1
n(Fe–H) was registered at 1841 cmꢁ1. In the H NMR spectrum
of complex 6, the characteristic hydrido signal was found at
ꢁ14.23 ppm as a td peak with the coupling constant JPH ¼ 18
and 72 Hz. The proton signal of the Si–H bond as multiplet
appears at 5.91 ppm. Moreover, only one signal was identied at
1.11 ppm for one PMe3 ligand. In the 31P NMR of complex 6, two
sets of signals were distinguished at 29.0 and 120.0 ppm,
respectively, corresponding to the two kinds of P atoms in the
integral ratio of 1 (PMe3) : 2 (–PiPr2). Regrettably, no crystals of
2.4 Catalytic application of iron hydrides 3, 4 and 6, 7 in
transfer hydrogenation of aldehydes
complex
6 suitable for X-ray diffraction were obtained.
Compared with hexa-coordinate complex 3, the difference is
that complex 6 is a penta-coordinated compound. Comparing
isopropyl with phenyl group, the isopropyl group has a larger
steric hindrance with stronger electron-donating ability than
phenyl group. These two reasons make complex 6 penta-
(4)
coordinated. Because complex
6 is a low-spin penta-
coordination compound, which should have a tetragonal
pyramid geometry. This can be further veried by the structure
of complex 7.
At the beginning, complex 7 as a catalyst was used to explore
its catalytic application in the transfer hydrogenation of benz-
aldehyde (eqn (4)). The reaction was conducted with benzalde-
hyde as the test substrate using 2-propanol as the reaction
2.3 Reaction of complex 6 with MeOH
The hydrido pincer iron(II) complex 6 could react with MeOH to solvent and source of hydrogen between 30–80 ꢀC. When the
afford another hydrido pincer iron(II) complex 7 (eqn (3)). In the reaction was performed without catalyst, no reduction product
IR spectrum of complex 7, the Fe–H vibration was found at was obtained in the control experiment (entry 1, Table 1). If the
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uorobenzaldehyde, 2-chlorobenzaldehyde, 2-bromobenzalde-
hyde, 4-uorobenzaldehyde, 4-chlorobenzaldehyde and 4-bro-
mobenzaldehyde could be reduced to the corresponding
alcohols by using 2 mol% of catalyst in 24 hours (entries 3–8,
Table 2). For the dihalogeno substrates, the aldehydes could be
also converted to the corresponding products (entries 9 and 10,
Table 2). When electron-donating group at para-position,
moderate yield of the corresponding alcohol could be obtained
from this catalytic system (entry 11, Table 2). With other
aromatic aldehydes, moderate to good yields could be achieved
(entries 13 and 14, Table 2). In addition, a,b-unsaturated alde-
hydes could be selectively reduced to the corresponding a,b-
unsaturated alcohols in good yields (entries 15–19, Table 2)
Although complex 6 could also be used as catalyst for this
catalytic system, the yields for the same substrates are lower
than those of the reactions with complex 7 as catalyst in most
cases. It is obvious that the introduction of MeO-group
improves the catalytic activity of complex 7. From Table 2, we
also know that the yields of the transformation with complex 3
or 4 as catalyst are signicantly lower than those with complex 6
or 7 as catalyst. This is also caused by the different coordination
number in complex 3 or 4 and 6 or 7. The hexa-coordinate
complexes 3 and 4 are more stable than penta-coordinate
Fig. 3 ORTEP plot of complex 7 at the 50% probability level (hydrogen
atoms are omitted for clarity). Selected bond lengths (A) and angles
ꢀ
(deg): Fe1–Si1 2.2712(6), Fe1–P1 2.1998(6), Fe1–P2 2.1997(6), Fe1–P3
2.2183(7), Fe1–H 1.60(3), Si1–O1 1.672(2), O1–C13 1.410(4); P2–Fe1–
P1 151.67(3), P1–Fe1–P3 101.43(2), P2–Fe1–P3 104.23(2), P1–Fe1–Si1
85.63(2), P2–Fe1–Si1 87.08(2), P3–Fe1–Si1 129.86(3), O1–Si1–Fe1
131.71(9), C13–O1–Si1 121.96(2), Si1–Fe1–H 151.5(1), Fe1–Si1–C14
105.94(7), C8–Fe1–Si1 107.05(7).
catalyst loading was 1 mol%, the conversion declined (entry 4, complexes 6 and 7. As a nal result, complex 6 or 7 has
Table 1). However, an excellent conversion (entry 3, Table 1) was stronger catalytic activity than complex 3 or 4. Under these
observed in the presence of 2 mol% of complex 7. When the optimized catalytic conditions, the ketones could not be
reaction temperature was 30 ꢀC, the lower conversion was found reduced to the corresponding alcohols with complex 3, 4, 6 or 7
(entry 11, Table 1). When the reaction temperature rose to 80 ꢀC, as catalyst (entries 20–22, Table 2). It is considered that the
the conversion declined sharply (entry 13, Table 1). And a grey steric effect plays a decisive role in this case.
precipitate appeared in the solution. It is guessed that the
On the basis of the related report,16 a plausible mechanism
catalyst should have decomposed. Among NaOtBu, Cs2CO3, for this catalytic system is proposed (Scheme 2). At rst,
K2CO3, Na2CO3, NaOH and KOtBu, KOtBu was the best base for complex 7 transforms to intermediate 7A via the coordination of
this catalytic system (entries 3 and 6–10, Table 1). Without base, carbonyl group in the aldehyde substrate. The nucleophilic
the reaction did not occur (entry 2, Table 1). At the given cata- attack of the hydrido hydrogen on the C atom of the carbonyl
lytic conditions, the reduction reaction was completely nished group gives rise to intermediate 7B. Again, the ligand substi-
within 24 h. The conversion was lower when reaction time was tution of RCH2O-group by Me2HC–O- group affords interme-
shorter than 24 h (entry 12, Table 1). According to the experi- diate 7C with the formation of the nal product RCH2OH. b-H
mental results in Table 1, the optimized catalytic reaction elimination of the Me2HC–O-group provides acetone with the
ꢀ
conditions can be summarized as follows: 60 C, 24 hours and recovery of catalyst 7.
2-propanol (5 mL), PhCHO (1.0 mmol) and 7 (0.02 mmol). The
mole ratio the catalyst to base should be 1 : 1.
3 Conclusion
Under the optimized reaction conditions, we expanded the
scope of the aldehyde substrates bearing different functional
The silyl hydrido Fe(II) complexs ((2-Ph2PC6H4)2HSi)
groups (Table 2). As shown in Table 2, the reactions with
Fe(H)(PMe3)2 (3) and ((2-iPr2PC6H4)2HSi)Fe(H)(PMe3)2 (6) were
2 mol% of catalyst 7 at 60 ꢀC in iPrOH led to the corresponding
synthesized by the oxidative addition of the Si–H bond of pre-
alcohols with variable yields within 24 h. The substrates with
i
ligands (2-R2PC6H4)2SiH2 (R ¼ Ph (1) and Pr (5)) to Fe(PMe3)4
the electron-withdrawing
substituents,
such
as 2-
respectively. Treatment of 6 with MeOH resulted in the
Scheme 1
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Table 1 Transfer hydrogenation of benzaldehyde with 7 as a catalysta Table 2 Transfer hydrogenation of aldehydes using 3, 4, 6 or 7 as
catalysta
Entry Loading (mol%) Baseb
T (ꢀC) Time (h) Conv.c (%)
Isolated yield
(%)
1
2
3
4
5
6
7
8
0
2
2
1
5
2
2
2
2
2
2
2
2
KOtBu
None
60
60
60
60
60
24
24
24
24
24
24
24
24
24
24
24
12
24
0
0
$99
81
$99
83
57
44
21
#10
61
Entry
1
Substrate
Catalyst
KOtBu
KOtBu
KOtBu
7
6
3
4
7
6
3
4
7
6
3
4
7
6
3
4
7
6
3
4
7
6
3
4
98
94
70
73
91
83
66
74
89
85
72
70
82
83
70
71
79
82
73
70
85
84
68
72
NaOtBu 60
Cs2CO3
K2CO3
60
60
2
3
4
5
9
Na2CO3 60
10
11
12
13
NaOH
KOtBu
KOtBu
KOtBu
60
30
60
80
47
19
a
i
b
c
PhCHO (1.0 mmol), PrOH (5 mL). 7 : base ¼ 1 : 1. Determined by
GC with n-dodecane as internal standard.
formation of hydrido iron(II) complex ((2-iPr2PC6H4)2(MeO)Si)
Fe(H)(PMe3) (7) via elimination of H2. Furthermore, we
demonstrated transfer hydrogenation of aldehydes to alcohols
i
using 3, 4, 6 and 7 as catalysts with PrOH as both solvent and
6
7
8
hydrogen source in moderate to good yields. This catalytic
system could be operated under mild conditions and has
tolerance for some substrates with different substituents. a,b-
Unsaturated aldehydes could be selectively reduced to corre-
sponding a,b-unsaturated alcohols. The catalytic activity of
penta-coordinate complex 6 or 7 is stronger than that of hexa-
coordinate complex 3 or 4.
7
6
3
4
7
6
3
4
7
6
3
4
84
80
73
71
80
81
71
66
75
77
65
61
4 Experimental section
4.1 General procedures and materials
9
Standard vacuum techniques were used in the manipulation of
volatiles and air-sensitive materials. Solvents were dried by
metal sodium and distilled under nitrogen before use. The
ligand 1 and 5 were prepared according to the literature.9,10,12
Fe(PMe3)4 was prepared according to literature procedures.23
Infrared spectra (4000–400 cmꢁ1), as obtained from Nujol mulls
between KBr disks, were recorded on a Bruker ALPHA FT-IR
7
6
3
4
7
6
3
4
7
6
3
4
7
6
3
4
7
6
3
4
81
83
70
66
80
81
64
61
87
84
73
76
95
91
77
71
77
75
77
75
10
11
12
13
instrument. H, 13C{H}, 31P{H}, and 29Si{H} NMR spectra were
1
recorded using Bruker Avance 300 MHz, 400 MHz, 500 MHz and
600 MHz spectrometers with C6D6 or THF-D8 as the solvent at
the corresponding temperature. Melting points were measured
in capillaries sealed under N2 and were uncorrected. Elemented
analyses were carried out on an Elementar Vario EL III
instrument.
4.2 Synthesis of 3
At ꢁ78 ꢀC, Fe(PMe3)4 (0.31 g, 0.86 mmol) in 20 mL toluene was
added to a solution of 1 (0.47 g, 0.86 mmol) in 40 mL of toluene.
The mixture was warmed to room temperature and the color of
solution has no obvious change. Aer stirred at room temper-
ature for 24 h, the solution was evaporated to dryness at reduced
pressure. The residue was washed by two portions of 10 mL of
14
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Table 2 (Contd.)
Isolated yield
(%)
Entry
15
Substrate
Catalyst
7
6
3
4
82
79
61
67
7
6
3
4
75
71
61
66
16b
17
7
6
3
4
80
76
60
69
7
6
3
4
77
74
58
63
18
Scheme 2 A plausible mechanism for transfer hydrogenation.
7
6
3
4
70
72
55
61
19
20
127.0 (t, 3J(PC) ¼ 1.5 Hz, Ar), 127.4 (s, Ar), 132.8 (t, 3J(PC) ¼
8.3 Hz, Ar), 133.2 (t, 3J(PC) ¼ 9.0 Hz, Ar). 31P NMR (202.5 MHz,
THF-D8, 233 K, d/ppm): 5.2 (q, J ¼ 30.4 Hz, PMe3, 1P), 6.4 (m,
PMe3, 1P), 88.5 (t, J ¼ 20.3 Hz, PPh2, 2P). 29Si NMR (79.45 MHz,
C6D6, 298 K, d/ppm): 68.8 (s).
7
6
3
4
0
0
0
0
7
6
3
4
0
0
0
0
21
4.3 Synthesis of 6
To a brown yellow solution of Fe(PMe3)4 (0.40 g, 1.11 mmol) in
20 mL of toluene was added a solution of 5 (0.42 g, 1.01 mmol)
in 30 mL of toluene. The mixture was stirred at room temper-
ature for 32 h. During this period, the reaction solution turned
yellow. The volatiles were removed by vacuum. The viscous
residue was extracted with n-pentane and diethyl ether. The pale
yellow crystals of 6 were obtained from diethyl ether at 0 ꢀC.
Yield: 480 mg (65%). Crystals suitable for X-ray diffraction were
obtained in pentane solution. dec.: > 127 ꢀC. Anal. calc. for
7
6
3
4
0
0
0
0
22
Substrate (1.0 mmol), KOtBu (0.02 mmol), catalyst (0.02 mmol), iPrOH
a
b
(5 mL), 60 ꢀC, 24 h. The reduced product is 3-phenylpro-2-yn-1-ol
because the elimination occurred during the work-up.
C
27H47FeP3Si (548.51 g molꢁ1): C, 59.12; H, 8.64. Found: C,
58.87; H, 8.51. IR (Nujol mull, cmꢁ1): 3058 (Ar–H), 2051 (Si–H),
1841 (Fe–H), 1554 (C]C), 950 (PMe3). 1H NMR (300 MHz, C6D6,
cold THF. Complex 3 (0.43 g, 0.47 mmol) was isolated as an
orange powder in a yield of 79%. Crystals suitable for X-ray
diffraction were obtained from n-pentane solution through
recrystallization. dec.: > 147 ꢀC. Anal. calc. for C42H48FeP4Si
(760.62 g molꢁ1): C, 66.32; H, 6.36. Found: C, 66.67; H, 6.49. IR
(Nujol mull, cmꢁ1): 3048 (Ar–H), 1992 (Si–H), 1836 (Fe–H), 1583
(C]C), 940 (PMe3) cmꢁ1. 1H NMR (500 MHz, THF-D8, 233 K, d/
ppm): ꢁ17.12 (td, J(PH) ¼ 20.0, J(PH) ¼ 70.0 Hz, 1H, Fe–H),
0.45 (s, PCH3, 9H), 0.97 (s, PCH3, 9H), 5.72 (d, 2J(PH) ¼ 10.0 Hz,
1H, SiH); 6.58 (s, 2H, Ar–H), 7.07–7.37 (m, 20H, Ar–H), 7.63 (s,
4H, Ar–H), 8.39 (s, 2H, Ar–H). 13C NMR (75 MHz, C6D6, 298 K, d/
ppm): 19.3 (s, PCH3), 125.4 (s, Ar), 126.0 (t, 3J(PC) ¼ 3.0 Hz, Ar),
2
2
298 K, d/ppm): ꢁ14.23 (td, J(PH) ¼ 18.0 Hz, J(PH) ¼ 72.0 Hz,
1H, Fe–H), 0.71 (q, 2J(PH) ¼ 6.0 Hz, PCHCH3, 6H), 0.84–0.89 (m,
PCHCH3, 12H), 0.92 (q, 2J(PH) ¼ 6.0 Hz, PCHCH3, 6H), 1.11 (d, J
¼ 3.0 Hz, PCH3, 9H), 1.75–1.89 (m, PCHCH3, 2H), 2.28–2.40 (m,
PCHCH3, 2H), 5.86–5.95 (m, 1H, SiH); 6.95–6.99 (m, 2H, Ar–H),
7.04–7.09 (m, 2H, Ar–H), 7.14–7.16 (m, 2H, Ar–H), 8.03 (d, 2J(PH)
¼ 6.0 Hz, 2H, Ar–H). 13C NMR (75 MHz, C6D6, 298 K, d/ppm):
2
2
3
3
23.3 (dd, J(PC) ¼ 7.5 Hz, J(PC) ¼ 16.5 Hz, PCHCH3), 25.6 (s,
PCHCH3), 26.6 (q, 3J(PC) ¼ 6.0 Hz, PCH3), 126.4 (s, Ar), 131.4 (s,
3
3
Ar), 132.5 (t, J(PC) ¼ 9.0 Hz, Ar), 133.3 (t, J(PC) ¼ 4.5 Hz, Ar),
133.9 (t, 3J(PC) ¼ 4.5 Hz, Ar), 143.5 (s, Ar), 155.0 (t, 3J(PC) ¼
26.3 Hz, Ar). 31P NMR (121 MHz, C6D6, 298 K, d/ppm): 29.0 (t, J ¼
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27.8 Hz, PMe3, 1P), 120.0 (d, J ¼ 27.8 Hz, PiPr2, 2P). 29Si NMR
(79.45 MHz, C6D6, 298 K, d/ppm): 57.7 (s).
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ꢀ
pentane solution. dec.:> 123 C. Anal. calc. for C28H49FeOP3Si
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2J(PH) ¼ 5.0 Hz, PCHCH3, 6H), 0.84 (q, 2J(PH) ¼ 5.0 Hz,
2
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PCHCH3, 6H), 1.08 (q, J(PH) ¼ 5.0 Hz, PCHCH3, 6H), 1.23 (q,
2
2J(PH) ¼ 5.0 Hz, PCHCH3, 6H), 1.43 (d, J(PH) ¼ 5.0 Hz, PCH3,
9H), 1.99 - 2.05 (m, PCHCH3, 2H), 2.72 (t, 2J(PH) ¼ 5.0 Hz,
PCHCH3, 2H), 3.39 (s, –OCH3, 3H), 7.17 (t, 2J(PH) ¼ 5.0 Hz, 2H,
Ar–H), 7.25 (t, 2J(PH) ¼ 5.0 Hz, 2H, Ar–H),7.39 (d, 2J(PH) ¼
10.0 Hz, 2H, Ar–H), 7.83 (d, 2J(PH) ¼ 5.0 Hz, 2H, Ar–H). 31P NMR
(202.5 MHz, THF-D8, 298 K, d/ppm): 22.2 (t, J ¼ 24.3 Hz, PMe3,
1P), 106.2 (dd, J ¼ 8.1 Hz, J ¼ 24.3 Hz, PiPr2, 2P). 13C NMR (100
MHz, C6D6, 298 K, d/ppm): 17.1 (s, PCH3), 19.9 (s, PCHCH3),
30.0 (s, PCHCH3), 67.5 (s, OCH3), 126.48 (s, Ar), 128.83 (s, Ar),
131.2 (t, 3J(PC) ¼ 8.0 Hz, Ar), 143.1 (t, 3J(PC) ¼ 27.0 Hz, Ar),
157.81 (t, 3J(PC) ¼ 23.0 Hz, Ar). 29Si NMR (79.45 MHz, C6D6, 298
K, d/ppm): 42.2 (s).
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¨
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In 25 mL Schlenk tube containing a solution of 7 (0.02 mmol) in
i
5 mL of PrOH were added an aldehyde (1.0 mmol) and KOtBu
(0.02 mmol). The reaction mixture was stirred at 60 ꢀC. The
organic product was extracted with Et2O and further puried by
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Crystallographic data for complexes 3 and 7 are summarized in
the ESI.† Intensity data were collected on a Stoe Stadi Vari Cu
diffractometer. Using Olex2,24 the structure was solved with
ShelXS25 structure solution program using direct methods and
rened with the ShelXL26 renement package using least
squares minimization. CCDC-1515026 (3) and 1490870 (7)
contain supplementary crystallographic data for this paper.
Conflicts of interest
20 A. Azua, J. A. Mata, E. Peris, F. Lamaty, J. Martinez and
E. Colacino, Organometallics, 2012, 31, 3911–3919.
21 S. Wu, X. Li, Z. Xiong, W. Xu, Y. Lu and H. Sun,
Organometallics, 2013, 32, 3227–3237.
There are no conicts to declare.
Acknowledgements
We gratefully acknowledge the nancial support by NSF China
No. 21572119/21372143.
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¨
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This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14092–14099 | 14099