cis-β-Ruthenium Complexes with Sterically Bulky Salen Ligands: Enantioselective Intermolecular Carbene Insertion into Si-H Bonds and X-ray Crystal Structure of cis-β-[RuII(salen)(CO)(CPh2)] Complex

Article abstract of DOI:10.1021/acs.organomet.0c00268

The synthesis, spectroscopy, crystal structure, and electrochemical behavior of cis-β-RuII(CO)2, cis-β-RuII(CO)(H2O), and cis-β-RuII(CO)(carbene) complexes supported by sterically bulky salen ligands are described, along with the catalytic activity of chi

Full text of DOI:10.1021/acs.organomet.0c00268

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Article
cis‑β-Ruthenium Complexes with Sterically Bulky Salen Ligands:
Enantioselective Intermolecular Carbene Insertion into Si−H Bonds
and X‑ray Crystal Structure of cis‑β-[RuII(salen)(CO)(CPh2)] Complex
Chi Lun Lee,⊥ Daqing Chen,⊥ Xiao-Yong Chang, Zhou Tang, and Chi-Ming Che*
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ABSTRACT: The synthesis, spectroscopy, crystal structure, and
electrochemical behavior of cis-β-RuII(CO)2, cis-β-RuII(CO)-
(H2O), and cis-β-RuII(CO)(carbene) complexes supported by
sterically bulky salen ligands are described, along with the catalytic
activity of chiral cis-β-[RuII(salen)(CO)2] complexes toward
enantioselective carbene Si−H insertion using N2C(Ar)CO2R as
the carbene source under light irradiation with product yields up to
96% and ee up to 84%. The cis-β-RuII(CO)(H2O) complex was
isolated by using a binaphthyl salen ligand bearing bulky CPh3
substituents with the H2O molecule coordinated at the site trans to
an Nimine atom of the salen ligand. Light irradiation of a cis-β-
RuII(CO)2 complex, supported by a binaphthyl salen ligand with
tBu substituents, followed by treatment with N2CAr2 (Ar = Ph, p-
ClC6H4) gave cis-β-[RuII(salen)(CO)(CAr2)], which features a coordinated carbene ligand trans to a salen Nimine atom and
undergoes carbene transfer to nitrosobenzene to give the corresponding nitrone product. These findings and related HR-ESI-MS
studies provide evidence for the involvement of salen-supported cis-β-RuII(CO)(C(Ar)CO2R) active species in the intermolecular
carbene Si−H insertion reaction catalyzed by cis-β-[RuII(salen)(CO)2] complexes.
experimentally, in contrast to the isolation and X-ray crystal
structural determination of several trans-Ru−salen carbene
complexes.3 Subsequently, Katsuki and coworkers reported
asymmetric sulfimidation catalyzed by cis-β-[RuII(salalen)-
(CO)2] complexes (salalen ligand resembles salen ligand
except that one of the two imine groups in the latter becomes
an amine group in the former);4 we reported asymmetric
intramolecular alkene cyclopropanation catalyzed by cis-β-
[RuII(salen)(CO)2] (e.g., cis-β-[RuII(salenL1)(CO)2] (1) and
cis-β-[RuII(salenL2)(CO)2] (2) shown in the inset of Figure 2)
under light irradiation, which exhibits high enantioselectivity
and possibly involves proposed elusive cis-β-Ru−salen carbene
active intermediates I and/or II (inset of Figure 2).5
INTRODUCTION
■
Chiral ruthenium-salen complexes offer versatile catalysts for
various asymmetric organic transformation reactions,1 includ-
ing atom/group transfer reactions such as epoxidation,1a,c,h,i
cyclopropanation,1a,c,d,f−i and aziridination1h,i of alkenes
catalyzed by various Ru−salen complexes which bear
tetradentate salen ligands adopting planar N2O2 arrangement
as in trans-octahedral metal−salen complexes (Figure 1). In
contrast, the synthesis and catalytic studies of Ru−salen
complexes bearing nonplanar N2O2 salen ligands, i.e. adopting
a cis (cis-α or cis-β) configuration of octahedral metal−salen
complexes,1d,g,i (Figure 1, inset), are scarce.1d,g Different from
trans metal−salen complexes, cis metal−salen complexes have
two unique characteristics: (i) the metal atom is stereogenic,
which promises an effective chirality transfer for organic
reactions due to close proximity of the metal to the substrate in
the inner coordination sphere, and (ii) there are two mutually
cis coordination sites which allow substrate binding and
activation via an η2 or η1 coordination mode. In 2001, Scott
and coworkers reported that a cis-β-[RuII(salen)(MeCN)2]
complex (inset of Figure 2) is an active catalyst for
intermolecular alkene cyclopropanation with high diastereose-
lectivity and enantioselectivity via a proposed cis-β-Ru−salen
carbene active intermediate (species I in the inset of Figure
2);2 this cis-β species I has not been directly detected
To obtain an isolable or directly detectable example of cis-β-
Ru−salen carbene species and also explore new carbene
transfer reactions catalyzed by cis-β-Ru−salen complexes, we
directed our attention to sterically bulky salen ligands and
Received: April 20, 2020
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© XXXX American Chemical Society
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A

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carbene insertion into Si−H bonds giving silyl compounds.
Construction of the compounds possessing silyl group(s) has
received considerable attention.6 Previously reported metal-
catalyzed enantioselective carbene Si−H insertion reactions
mostly employ, for example, chiral Cu(I), Rh(II), and Ir(III)
catalysts;7−13 for Ru-catalyzed carbene Si−H insertion, there is
only one report,14 which uses arylsilane substrates and a chiral
Ru(II)-pheox catalyst (pheox: a bidentate cyclometalating
phenyloxazoline C^N ligand), in contrast to a considerable
number of reports on Ru-catalyzed other types of carbene
transfer reactions, particularly cyclopropanation of al-
kenes.15−21
Herein, we report the synthesis and characterization of new
chiral cis-β-Ru−salen complexes cis-β-[RuII(salenL3)(CO)-
(H2O)] (3) and cis-β-[RuII(salenL4)(CO)2] (4) bearing
sterically bulky salen ligands and cis-β-Ru−salen carbene
complexes cis-β-[RuII(salenL2)(CO)(CAr2)] (5; Ar = Ph: 5a,
p-ClC6H4: 5b), electrochemical properties of 1−5, and the
catalytic properties of 1−4 toward carbene insertion into X−H
(X = Si, N) bonds, including detection of the corresponding
Ru−carbene intermediate by ESI-MS analysis, together with
stoichiometric carbene transfer reactions of 5 with a nitro-
soarene. The structures of 3 and 5a were determined by X-ray
crystallographic studies. Complex 4 catalyzed enantioselective
carbene Si−H insertion with product yields up to 96% and
enantioselectivities up to 84% ee. This work, to the best of our
knowledge, provides the first examples of isolated cis-β-Ru−
salen carbene complexes and Si−H insertion catalyzed by a
Ru−salen complex.
Figure 1. Literature reported examples of Ru−salen catalysts featuring
planar N2O2 salen ligands. Inset: three possible configurations (trans,
cis-α, and cis-β) of octahedral metal−salen complexes.
RESULTS
■
Synthesis. The ligands H2salenL1, H2salenL2, and
H2salenL4 were prepared by refluxing the corresponding
diamine and salicylaldehyde in EtOH, as reported in the
literature, and the new ligand H2salenL3 was prepared by a
similar procedure (see the Supporting Information). Like the
preparation of bis(carbonyl) complexes (1R,2R)-1 and rac-2,5
reactions of Ru3(CO)12 with enantiopure (1S,2S)-H2salenL1,
(S)-H2salenL2, and (R)-H2salenL4 in 1,2,4-trichlorobenzene at
185 °C under argon in the dark for 6 h afforded chiral cis-β-
[RuII(salen)(CO)2] complexes (1S,2S)-1, (S)-2, and (R)-4,
respectively (or their racemic counterparts if racemic H2salen
ligands were used; see, for example, Scheme 1). Interestingly,
Scheme 1. Synthesis of cis-β Ru−Salen Complexes 3 and 4
Figure 2. Examples of cis-β ruthenium−salen complexes. The
previously reported ones,2,5 together with proposed cis-β Ru−salen
carbene intermediates, are depicted in the inset. Complexes 3−5 were
first synthesized in this work.
B
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upon changing the H2salen ligand to rac-H2salenL3 bearing
bulky CPh3 substituents, the reaction under the same
conditions gave a mono(carbonyl) complex, cis-β-
[RuII(salenL3)(CO)(H2O)] (rac-3, Scheme 1; the correspond-
ing cis-β-[RuII(salenL3)(CO)2] complex was not obtained). At
the end of the reactions for all of these preparations,
salicylaldehydes of the corresponding H2salen ligands were
1
detected by TLC and H NMR, indicating partial decom-
position of the H2salen ligands under the reaction conditions.
cis-β-[RuII(salenL2)(CO)(CPh2)] (5a) and cis-β-
[RuII(salenL2)(CO)(C(C6H4-p-Cl)2)] (5b) were synthesized
by irradiation of a refluxing solution of cis-β-[RuII(salenL2)-
(CO)2] (2) in MeCN with an incandescent lamp (300 W) for
3 h to remove one of the two coordinated CO groups in 2,
followed by treatment with solutions of diazo compounds
N2CPh2 and N2C(C6H4-p-Cl)2, respectively, in dichloro-
methane (DCM) at room temperature (Scheme 2). Slow
Figure 3. HR-ESI-MS spectrum of 5a in DCM.
Scheme 2. Synthesis of cis-β-Ru−Salen Carbene Complexes
5a and 5b
addition of the diazo compound solutions (via a syringe pump
for 30 min) was required to minimize catalytic decomposition
of the diazo compounds. When the solution of 2 was directly
treated with the diazo compound N2CPh2 or N2C(C6H4-p-
Cl)2 without preirradiation with an incandescent lamp, no cis-
β-Ru−salen carbene complex (5a or 5b) was detected from the
reaction mixture.
Stability and Spectroscopy. Complexes 1, 2, and 4 are
stable in the absence of light; in CDCl3 solutions under dark
conditions, neither isomerization nor decomposition of these
complexes to mono(carbonyl) analogues, such as cis-β-
[RuII(salen)(CO)(H2O)], was observed upon standing at
1
Figure 4. H NMR spectra of (a) 3 and (b) 4 in CDCl3.
comparable to that reported for the salenL1 counterpart 1
(ΔδH = 0.31) and the salenL2 counterpart 2 (ΔδH = 0.57),5 all
being significantly larger than that (ΔδH = 0.08 ppm) for the
salenL1 adopting a planar N2O2 configuration in structurally
characterized trans-[RuII(salenL1)(NO)(Cl)].22
1
room temperature for at least 3 days, as revealed by H NMR
and ESI-MS analyses. Complexes 5a and 5b are stable in open
atmosphere in the solid state (for at least 1 month) in the
absence of light, as revealed by UV−vis absorption spectros-
The cis-β-Ru−salen carbene complexes 5a and 5b in CDCl3
also exhibit well-resolved diamagnetic 1H NMR spectra
(Figure S1 in the Supporting Information), like other Ru−
carbene complexes such as those bearing porphyrin18 or
bis(oxazolinyl)pyridine ligands.23 The salenL2 ligand in 5a and
5b gave signals analogous to, but distinct from, that in 2, with
the most downfield Himine signal appearing at δH 8.18 (5a),
8.19 (5b), and 8.13 (2) ppm. The 13C NMR spectra of 5a and
5b show a low-field signal at δC 337.12 and 329.95 ppm,
respectively, attributable to the coordinated carbene C
atom.3,18,23
1
copy, H NMR, and ESI-MS analyses.
In the FAB mass spectra, cis-β-[RuII(salen)(CO)2] com-
plexes 1, 2, and 4 (in MeOH) each show three prominent
signals assignable to M+, [M − CO]+ and [M − 2CO]+. For
cis-β-[RuII(salenL3)(CO)(H2O)] (3, in MeOH), two prom-
inent signals assignable to [M − H2O]+ and [M − H2O −
CO]+ were observed. The HR-ESI-MS analysis of 5a revealed
a signal at m/z 1011.4066 with the m/z value and isotope
pattern matching that simulated for [M + H]+, together with a
signal at m/z 1033.3888 attributable to [M + Na]+ (Figure 3).
Similar results were obtained for the HR-ESI-MS analysis of 5b
(e.g., m/z 1079.3179 assignable to [M + H]+).
Variable-temperature 1H NMR spectra in CDCl3 were
measured using 1 and 5b as examples (Figures S2 and S3 in
Supporting Information), which revealed that the signals did
not significantly change upon changing the temperature over
the range of −50 to 50 °C.
1
The H NMR spectra of 3 and 4 in CDCl3 (Figure 4), like
that of 1 and 2,5 show well-resolved signals in diamagnetic
region (0−10 ppm). The large splitting of the two Himine
(N=C−H) signals of the coordinated salenL4 ligand in the cis-
β-bis(carbonyl) complex 4 (Figure 4b), with Δδ = 0.53 ppm, is
The UV−vis spectral data of complexes 1−5 are presented
in Table S1, and the corresponding spectra are depicted in
C
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Figure 5 for 2, 3, and 5a bearing binaphthyl salen ligands and
in Figures S4−S6 for 1, 4, and 5b. Complexes 1−4 exhibit
Figure 5. UV−vis absorption spectra of 2, 3, and 5a in DCM. Inset: a
clearer view of the shoulder absorption band at 630−700 nm for 5a.
Figure 6. ORTEP diagram (50% probability) of 3. Hydrogen atoms
and solvent molecules have been omitted for clarity.
broad absorption bands with λmax between 419 and 442 nm,
which could be assigned to MLCT transitions. For 4, there is
also an absorption band with λmax at 366 nm (see Figure S5)
attributable to intraligand charge transfer transitions.24,25 In
the case of 5a and 5b, their absorption bands are similar. For
example, the spectrum of 5a (Figure 5) shows a broad band
with λmax at 410 nm, together with a weak low-energy shoulder
absorption band at ca. 630−700 nm (log ε ∼ 2.69 dm3 mol−1
cm−1, see Discussion section).
In the IR spectra of complexes 1−5 (Figures S8−S13 and
Table S2), intense bands assignable to ν(CO) were observed.
The cis-β-bis(carbonyl) complexes 1, 2, and 4 each exhibit two
intense ν(CO) bands at ∼2040 and ∼1970 cm−1.26 For cis-β-
mono(carbonyl) complex 3, a single intense ν(CO) band at
∼1940 cm−1 was observed (Figure S10). The cis-β-mono-
(carbene) complex 5a or 5b, each bearing one coordinated CO
ligand, also exhibits a single intense ν(CO) band, which
appears at 1977 or 1976 cm−1, respectively (Figures S12 and
S13).
X-ray Crystal Structures. Diffraction-quality crystals of 1·
0.25MeOH and 5a·0.5DCM were obtained by slow evapo-
ration of their solutions in DCM/MeOH, and a diffraction-
quality crystal of 3·2DCM was grown by layering n-hexane on
the top of a solution of 3 in DCM. The structures of the three
complexes determined by X-ray crystallography (Figures 6 and
7, Figure S14) all feature a cis-β configuration. Selected bond
distances and angles are given in Table 1.
For 1·0.25MeOH (Figure S14), its Ru−Nimine and Ru−OOAr
distances are 2.0517(15)−2.0735(17) Å and 2.0602(13)−
2.1008(14) Å, respectively; the two cis CO ligands make a
CCO−Ru−CCO angle of 88.26(9)°, and the Ru−CCO distance
for the CO ligand trans to the OOAr atom of the salenL1 ligand
(1.858(2) Å) is shorter than that trans to the Nimine atom of the
salenL1 ligand (1.912(2) Å). These structural features are
similar to those of the previously reported 1·3MeOH.5
In the crystal structure of 3 (Figure 6), the Ru−Nimine and
Ru−OOAr distances fall within the ranges of 2.027(4)−
2.050(4) Å and 2.071(3)−2.081(3) Å, respectively, which
are comparable to those in 1 and 25 (Ru−N 2.028(7)−
2.090(4) Å, Ru−O 2.044(5)−2.1008(14) Å). The Ru−CCO
Figure 7. ORTEP diagram (50% probability) of 5a (only one of the
two independent molecules in the crystallographic asymmetric unit is
shown). Hydrogen atoms and solvent molecules have been omitted
for clarity.
Table 1. Selected Bond Distances (Å) and Angles (Deg) of
cis-β-Ruthenium−Salen Complexes 1, 3, and 5a
1
3
5a
Ru−O1
Ru−O2
Ru−N1
Ru−N2
Ru−C1
dihedral angle
2.0602(13)
2.1008(14)
2.0735(17)
2.0517(15)
1.858(2)
2.081(3)
2.071(3)
2.027(4)
2.050(4)
1.842(5)
74.0°
2.064(2)
2.060(2)
2.031(3)
2.202(2)
1.859(3)
75.5°
a
b
50.7°
a
C21, C35, and C51 atoms in the ORTEP drawings of 1, 3, and 5a,
which are generated by repeating the symmetric unit to form the
b
whole structure, are presented as C1 atom. Dihedral angle is the
angle between the two phenyl planes of the salen ligand.
bond is trans to the OOAr atom of the salenL3 ligand, with a
bond distance of 1.842(5) Å comparable to the Ru−CCO
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distance of 1.837(26) Å in 2.5 The Ru−Owater bond in 3 is
trans to a Ru−Nimine bond and has a distance of 2.164(3) Å,
which is longer than that of the Ru−OOAr bonds in the same
complex. The cis CO and H2O ligands in 3 make a CCO−Ru-
Owater angle of 96.75(18)°, larger than the CCO−Ru-CCO angles
in 1 and 2 (88.26(9)°−91.4(3)°).5 Notably, the angle formed
by the two phenyl planes of the salenL3 ligand in 3 is 74.0°,
considerably larger than the corresponding angles in 1 and 2
(50.7−58.9°).5
Complex 5a features a coordinated CPh2 carbene ligand
trans to the Nimine atom and a coordinated CO ligand trans to
the OOAr atom (Figure 7), corresponding to the cis-β-Ru−salen
carbene species II depicted in the inset of Figure 2. There are
two independent molecules of 5a in the crystallographic
asymmetric unit, only one of which is depicted in Figure 7.
The bond angles around the Ccarbene atom (C116) are
113.7(3)o (C117−C116−C123), 119.1(2)o (C117−C116−
Ru2), and 127.0(2)o (C123−C116−Ru2), which sum up to
359.8(3)o (the corresponding sum for the Ccarbene atom C52 in
the other independent molecule of 5a, not shown in Figure 7,
is 359.9(3)o), indicating sp2 hybridization of the Ccarbene atom.
The Ru−Ccarbene distance of 5a is 1.939(3) Å (Ru2−C116)
and 1.967(3) Å (Ru1−C52), comparable to that of trans-
[Ru(salen)(CAr2)(L)] (1.910(2)−1.921(12) Å; Ar = Ph, p-
MeOC6H4; L = MeIm, py),3 but significantly longer than that
of trans-[Ru(Por)(CPh2)(L)] (e.g., 1.845(3)−1.876(3) Å; Por
Figure 8. Cyclic voltammograms of 1−5 in DCM (0.1 M
[nBu4N]PF6). Scan rate: 100 mV s−1.
For cis-β-RuII(CO)(CAr2) complexes 5a and 5b, two
oxidation waves (irreversible or quasi-reversible) at Epa
=
0.81−0.83 V and E1/2 = 1.16−1.19 V and a reduction wave
(reversible) at E1/2 = −1.20 to −1.29 V appear in each of their
cyclic voltammograms. The oxidation waves could result from
processes similar to those of 1, 2, and 4. The reversible
reduction wave, which is absent in the cyclic voltammograms
of the cis-β-RuII(CO)2 or cis-β-RuII(CO)(H2O) complexes
(1−4, particularly 2 bearing the same salenL2 ligand as that in
5a and 5b), is tentatively assigned to a carbene-based process.
This assignment is consistent with the anodic shift of the
reduction wave upon introducing electron-withdrawing p-Cl
= porphyrin ligand TTP or F20-TPP; L = MeOH, MeIm27,28
)
and trans-[Ru(pybox)(C(CO2Me)2)Cl2] (1.880(7) Å, pybox =
bis(oxazolinyl)pyridine ligand).23a The two phenyl planes of
the salenL2 ligand form a considerably larger angle of 75.5° in
the cis-β-Ru(CO)(CPh2) complex 5a than in the cis-β-
Ru(CO)2 complex 2 (58.9°).5
Electrochemistry. The electrochemical properties of
complexes 1−5 were investigated by cyclic voltammetry in
DCM containing 0.1 M [nBu4N]PF6. The observed redox
potentials (vs Ag/AgCl) are listed in Table 2, and the cyclic
voltammograms are shown in Figure 8.
substituents on the CPh2 carbene ligand in 5a to give 5b (E1/2
:
from −1.29 V for 5a to −1.20 V for 5b).
Catalytic Carbene Si−H Insertion Reactions. At the
outset, we examined the reaction of dimethylphenylsilane (6a)
with methyl α-diazophenylacetate N2C(Ph)CO2Me (7a) in
1,2-dichloroethane (DCE) at room temperature in the
presence of cis-β-Ru−salen complexes 1−4 (5 mol %) under
irradiation of an incandescent lamp (300 W) for 3 h. The
reactions using chiral catalysts (1S,2S)-1 and (S)-2 gave
product 8a in similar yields (91−92%), but (S)-2 resulted in a
higher enantioselectivity of 31% ee (Table 3, entries 1 and 2; in
control experiment using catalyst (1S,2S)-1 under dark
condition, neither 8a nor decomposition of 7a was detected
(Table 3, entry 8)). Complex rac-3 exhibited markedly lower
catalytic activity, affording 8a in 65% yield (Table 3, entry 3).
With use of chiral complex (R)-4 as catalyst, 8a was obtained
in 96% yield with 55% ee (Table 3, entry 4). Upon changing
the solvent from DCE to hexane, the reactions catalyzed by
(1S,2S)-1 and (S)-2 gave 8a in 83−84% yields with 23−36%
ee (Table 3, entries 5 and 6). Notably, by using hexane as the
solvent, the reaction catalyzed by (R)-4 resulted in isolation of
8a in 95% yield with 71% ee (Table 3, entry 7).
a
Table 2. Electrochemical Data of 1−5
complex
reduction (V)
oxidation (V)
b
b
b
1
2
3
0.85 and 1.22
b
0.96 and 1.29
b
0.59
b
c
4
5a
5b
0.97 and 1.26
b
d
b
b
−1.29
0.83 and 1.16
b
d
−1.20
0.81 and 1.19
a
In CH2Cl2 with 0.1 M [nBu4N]PF6 as the electrolyte. Glassy carbon
was the working electrode, Ag/AgCl was the reference electrode, and
platinum wire was the counter electrode. Cp2Fe+/0 = 0.40 V.
b
c
d
Reversible, E1/2. Quasi-reversible, E1/2
. Irreversible, Epa.
cis-β-RuII(CO)2 complexes 1, 2, and 4 each exhibits two
oxidation waves (reversible or quasi-reversible) with E1/2
=
0.85−0.97 and 1.22−1.29 V. The first oxidation is likely to be
associated with oxidation of Ru(II) to Ru(III). The second
oxidation is attributable to the ligand-centered oxidation
process. This assignment is supported by the first oxidation
couple of the cis-β-RuII(CO)(H2O) complex 3 with E1/2 being
less anodic than those of 1, 2, and 4, as replacing CO (a π-
acceptor) by H2O (a σ-donor) is expected to increase the
electron density of Ru(II) and thus cathodically shift the E1/2
of the Ru(III)/Ru(II) couple.
With (R)-4 as catalyst and hexane as solvent, changing the
diazo compound 7a to the more bulky N2C(Ph)CO2But (7b)
slightly increased the enantioselectivity to 75% ee, with the
corresponding product 8b obtained in 94% isolated yield
(Table 4, entry 2; the enantioselectivity slightly decreased from
75 to 73% ee upon lowering the reaction temperature from
room temperature to 0 °C (Table S3)).
E
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Table 3. Solvent and Catalyst Screening for Carbene Si−H
Table 4. Enantioselective Carbene Si−H Insertion
a
Insertion Catalyzed by cis-β-RuII−Salen Complexes
Catalyzed by cis-β-RuII−Salen Complex (R)-4 for Various
a
Silanes and Diazo Compounds
b
c
entry
cat.
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
(1S,2S)-1
(S)-2
rac-3
(R)-4
(1S,2S)-1
(S)-2
DCE
DCE
DCE
DCE
hexane
hexane
hexane
DCE
92
91
65
96
84
83
95
24
31
55
23
36
71
(R)-4
(1S,2S)-1
d
e
8
nil
a
Reaction conditions: 6a (1.5 mmol), 7a (0.3 mmol), catalyst (5 mol
%), solvent (1 mL), stirring at room temperature for 24 h under argon
b
after irradiation with an incandescent lamp (300 W) for 3 h. Isolated
c
d
yield. Determined by HPLC. Control experiment under dark
e
conditions. Product 8a was not detected by 1H NMR analysis of the
crude reaction mixture.
Besides dimethylphenylsiliane (6a), other silanes including
triethylsilane (6b), triphenylsilane (6c), tripropylsilane (6d),
and tert-butyldimethylsilane (6e) were also used as substrates
under the same reaction conditions. For the aliphatic silanes
6b, 6d, and 6e, their reactions with 7b in hexane catalyzed by
(R)-4 afforded products 8c, 8e, and 8f, respectively, in yields of
84−88% and with 27−73% ee (Table 4, entries 3, 5, and 6). In
the case of the bulky triarylsilane 6c, no product 8d was
detected from the corresponding reaction (Table 4, entry 4).
The scope of diazo compounds for the (R)-4-catalyzed
reaction of 6a in hexane was examined as well, by using
N2C(Ar)CO2But (Ar = o-MeC6H4: 7c, p-MeOC6H4: 7d, p-
MeC6H4: 7e, p-ClC6H4: 7f, p-NO2C6H4: 7g). For the diazo
compounds 7d−f bearing p-MeO, p-Me, and p-Cl substituents,
respectively, high product yields (89−92%) and moderate-to-
good enantioselectivities (60−84% ee) were obtained (Table
4, entries 8−10). The diazo compound with an o-Me
substituent (7c) or with a strong electron-withdrawing p-
NO2 substituent (7g) on the phenyl group gave no detectable
Si−H insertion product (Table 4, entry 7 or 11).
a
Reaction conditions: silane (1.5 mmol), diazo compound (0.3
Catalytic Carbene N−H Insertion Reactions. The cis-β-
[RuII(salen)(CO)2] complexes 1, 2, and 4 were also found to
be active catalysts for carbene insertion into N−H bonds. The
reactions of tert-butylamine with N2C(Ph)CO2Me in DCE at
50 °C for 24 h under argon catalyzed by these complexes (after
irradiation with an incandescent lamp (300 W) for 3 h)
afforded the carbene N−H insertion product in 72−80% yields
(Table 5, entries 1−3; when (R)-4 was used as the catalyst
under the same conditions, the carbene N−H insertion
product was obtained in 80% yield albeit without notable
enantioselectivity). For the n-aliphatic amine substrate n-
hexylamine, the corresponding carbene N−H insertion
product was obtained in 46−55% yields under similar
conditions except for a higher reaction temperature (Table 5,
entries 4−6).
Stoichiometric Carbene Transfer Reaction. We exam-
ined the carbene transfer reactivity of the cis-β-Ru−salen
carbene complexes 5 using 5a as an example. Treatment of 5a
with excess dimethylphenylsilane (6a) or styrene (20 equiv) in
DCE under reflux condition or light irradiation for 24 h did
not give the carbene transfer product (i.e., neither Si−H
mmol), (R)-4 (5 mol %), hexane (1 mL), stirring at room
temperature for 24 h under argon after irradiation with an
b
c
incandescent lamp (300 W) for 3 h. Isolated yield. Determined
d
by HPLC. Carbene Si−H insertion product was not detected by 1H
NMR analysis of the crude reaction mixture.
insertion nor styrene cyclopropanation was observed), as
1
revealed by H NMR analysis of the reaction mixture.
Interestingly, treatment of 5a with excess nitrosobenzene
(PhNO) at 70 °C in DCE resulted in carbene transfer to
PhNO, affording the nitrone product Ph2CNOPh in 63%
yield (Scheme 3; the nitrone product was characterized as
reported in the literature29). After carbene transfer, 5a was
converted to the Ru(CO)(PhNO) complex cis-β-
[RuII(salenL2)(CO)(PhNO)], which was characterized by
NMR and HR-ESI-MS (see the Supporting Information).
1
We monitored the reaction between 5a and PhNO by H
NMR spectroscopy; the resulting time course plot showed no
appreciable induction period before the Ph2CNOPh and cis-
F
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Detection of Reaction Intermediates in the cis-β-
[RuII(salen)(CO)2]-Catalyzed Carbene Si−H Insertion
Reaction under Light Irradiation. The carbene Si−H
insertion reactions with N2C(Ar)CO2R catalyzed by cis-β-
[RuII(salen)(CO)2] complexes 1, 2, and 4 under irradiation of
an incandescent lamp are likely to involve photolytic
decarbonylation of the catalysts to form cis-β-[RuII(salen)-
(CO)(solv)] followed by reaction with the diazo compound to
generate Ru−carbene active intermediate cis-β-[RuII(salen)-
(CO)(C(Ar)CO2R)]. Our attempts to isolate such cis-β-
RuII(CO)(solv) and cis-β-RuII(CO)(C(Ar)CO2R) intermedi-
ates have not been successful, except for the isolation of the cis-
β-RuII(CO)(H2O) complex 3 stabilized by bulky CPh3
substituents on the salen ligand.
Table 5. Carbene N−H Insertion Reactions Catalyzed by
a
cis-β-[RuII(salen)(CO)2] Complexes
To help identify the reaction intermediates, HR-ESI-MS
and/or UV−vis analyses of the corresponding reaction
mixtures were performed using complex 2 as example. When
a solution of 2 in DCE was irradiated with an incandescent
lamp (300 W) under argon, a marked change of the UV−vis
spectrum of this solution was observed after about 10 min
(Figure S7). HR-ESI-MS measurements revealed that irradi-
ation of a solution of 2 in refluxing MeCN with an
incandescent lamp (300 W) for 15 min led to transformation
of 2 to a species (m/z 885.3424, Figure 10b) assignable to the
Ru(CO)(solv) complex cis-β-[RuII(salen)(CO)(MeCN)] (m/
z calcd for M+: 885.3443), and subsequent treatment with a
solution of N2C(Ph)CO2But (4 equiv) in DCM (added via a
syringe pump over 5 min, with the reaction mixture stirred for
additional 10 min) generated a species exhibiting m/z
1035.4270 (Figure 10c) which is assignable to the Ru(CO)-
(C(Ar)CO2R) complex cis-β-[Ru(salenL2)(CO)(C(Ph)-
CO2But)] (m/z calcd for [M + H]+: 1035.4262). Upon
treatment with dimethylphenylsilane (6a, 50 equiv) for 15 min,
the species with m/z 1035.4270 vanished, suggesting that this
species is reactive with the silane substrate 6a.
a
Reaction conditions: amine (1.5 mmol), diazo compound (0.3
mmol), catalyst (5 mol %), DCE (1 mL), stirring at 50 °C (for t-
butylamine) or at reflux (for n-hexylamine) for 24 h under argon after
irradiation with an incandescent lamp (300 W) for 3 h. Determined
by H NMR using 1,1-diphenylethylene as standard.
b
1
Scheme 3. Stoichiometric Reaction between 5a and
Nitrosobenzene
DISCUSSION
■
cis-β-Bis(carbonyl) complexes cis-β-[RuII(salen)(CO)2] such
as 1 and 2 were previously reported as catalysts for
intramolecular cyclopropanation of alkenes under light
irradiation5 via proposed cis-β-mono(carbonyl) species
(formed by photolytic decarbonylation) and cis-β-Ru−salen
carbene species cis-β-[RuII(salen)(CO)(CHCO2R)], neither of
which has been isolated or clearly identified.5 In contrast to the
good stability of isolated cis-β-[RuII(salen)(CO)2] complexes
1, 2, and 4 bearing salenL1, salenL2, and salenL4 ligands,
respectively, (e.g., aerobic oxidation of these complexes in
CDCl3 solutions is quite slow, with their yellow color gradually
turning deep green over a week possibly due to the slow
oxidation of Ru(II) to Ru(III)30), the corresponding cis-β-
mono(carbonyl) counterparts for the three salen ligands,
generated by photolytic decarbonylation (like dissociation of
the CO ligand in M(CO)n-type complexes facilitated by
irradiation31), were found not sufficiently stable for isolation.
The cis-β-RuII(CO)(H2O) complex 3 bearing bulky salenL3
ligand is a unique cis-β-mono(carbonyl) Ru−salen complex
that has been isolated, probably owing to stabilization by the
bulky CPh3 substituents of the salenL3 ligand. The X-ray
crystal structure of 3, featuring a H2O ligand trans to the Nimine
atom and a CO ligand trans to the OOAr atom, is in agreement
with the expected relative strength/reactivity of the two cis
Ru−CCO bonds in cis-β-[RuII(salen)(CO)2] complexes, i.e.,
the Ru−CCO bond trans to the imine group (a π-acceptor
β-[RuII(salenL2)(CO)(PhNO)] products were formed (Figure
9).
Figure 9. Time course plot of the stoichiometric reaction between 5a
and PhNO.
G
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The successful isolation of stable cis-β-[RuII(salenL2)(CO)-
(CAr2)] complexes (5a and 5b) indicates a substantial stability
enhancement of a cis-β-Ru−salen carbene complex upon
changing the carbene group from CHCO2R or C(Ar)CO2R to
CAr2. Complexes 5a and 5b adopting cis-β-configuration
constitute a novel type of isolated Ru−carbene complexes
supported by salen ligands.
The characterization of 5a and 5b was based on HR-ESI-
MS, UV−vis, and NMR measurements and, for 5a, also by X-
ray crystal structure determination and stoichiometric carbene
transfer reactivity. The Ccarbene signal of 5a and 5b at δC
337.12−329.95 ppm in their 13C NMR spectra is comparable
to, though appreciably downfield from, that reported for trans-
[Ru(salen)(CAr2)(MeIm)] complexes (δC 316.3−317.3 ppm;
Ar = Ph, p-MeOC6H4).3 The weak low-energy absorption at
ca. 620−700 nm (log ε ∼2.51−2.69 dm3 mol−1 cm−1) in the
UV−vis spectra of 5a and 5b was tentatively assigned to d-d
transition. Such transitions have previously been reported at
520 nm (log ε 2.56 dm3 mol−1 cm−1) for [RuCl2(CHPh)-
(PCy3)2] (Cy = cyclohexyl)32 and at ca. 630−660 nm (log ε
2.80−3.12 dm3 mol−1 cm−1) for trans-[Ru(salen)(CPh2)(L)]
(L = MeIm, py).3 In the X-ray crystal structure of cis-β-
Ru(CO)(CPh2) complex 5a, the CPh2 carbene ligand is trans
to the imine group, like the H2O ligand in the cis-β-
Ru(CO)(H2O) complex 3; this is supportive of the formation
of 5a from a cis-β-Ru(CO)(solv) intermediate (generated by
photolytic decarbonylation of the cis-β-Ru(CO)2 complex 2 in
solution) with its coordinated solvent molecule being replaced
by the carbene group. The relatively long Ru−Ccarbene distance
of 5a (1.953(3) Å, averaged value for the two crystallo-
graphically independent molecules) is possibly associated with
the presence of a trans imine group which competes for the π-
back bonding with Ru and would weaken the pπ(Ccarbene)-
dπ(Ru) interaction.
Considering the experimental findings obtained in this work,
particularly the isolation of cis-β-Ru(CO)(CAr2) complexes 5a
and 5b and HR-EI-MS detection of a species assignable to cis-
β-[Ru(salenL2)(CO)(C(Ph)CO2But)], a mechanism of the
chiral cis-β-[RuII(salen)(CO)2]-catalyzed enantioselective car-
bene Si−H insertion reactions was proposed and depicted in
Scheme 4, using catalyst 2 as example. This catalyst could
undergo decarbonylation under light irradiation to give a cis-β-
RuII(CO)(solv) species (a structural analogue of 3).
Subsequent reaction of this species with diazo compound
N2CR1R2 is likely to generate a cis-β-Ru(CO)(CR1R2)
intermediate which is proposed to be the active species
undergoing carbene transfer reactions to give the correspond-
ing Si−H insertion products.
Figure 10. High-resolution ESI-MS spectra of (a) cis-β-[Ru(salenL2)-
(CO)2] (2), (b) 2 after irradiation with an incandescent lamp (300
W) in MeCN, (c) the reaction mixture of (b) and N2C(Ph)CO2But
(4 equiv), (d) the reaction mixture of (c) and dimethylphenylsilane
(6a). Insets: (A) Calculated m/z values and (B) observed isotope
pattern for the peak at m/z 1035.4270 and simulated isotope pattern
for {[Ru(salenL2)(CO)(C(Ph)CO2But)] + H}+.
reducing Ru → CO π-back bonding) is weaker than that trans
to the phenoxide group (a π-donor enhancing Ru → CO π-
back bonding) and would more readily undergo ligand
substitution reaction. Accordingly, the two intense ν(CO)
bands at ∼2040 and ∼1970 cm−1 in the IR spectra of 1, 2, and
4 (Table S2) can be assigned to the CO ligands trans to the
imine and phenoxide ligands, respectively, consistent with the
longer Ru−CCO bond trans to the imine group than trans to
the phenoxide group in the X-ray crystal structures of 1 and 2.5
With regard to the proposed elusive cis-β-Ru(CO)-
(CHCO2R) or cis-β-Ru(CO)(C(Ar)CO2R) species supported
by salen ligands, an isolable example has yet to be obtained,
though the HR-ESI-MS measurements depicted in Figure 10
provide evidence for the possible formation of cis-β-[Ru-
(salenL2)(CO)(C(Ph)CO2But)].
CONCLUSIONS
■
We synthesized and characterized new cis-β-ruthenium−salen
complexes 3, 4, 5a, and 5b, including their electrochemical
behavior, and also examined the catalytic properties of 1−4
toward carbene insertion into Si−H and/or N−H bonds.33
Complex 3 represents a cis-β-[Ru(salen)(CO)(solv)] analogue
stabilized by the binaphthyl salen ligand with bulky CPh3
substituents, and complexes 5a and 5b are unique Ru−salen
carbene complexes adopting a cis-β configuration and
exhibiting carbene transfer reactivity toward nitrosobenzene
to give a nitrone product.34 The chiral cis-β-[RuII(salen)-
(CO)2] complexes (1S,2S)-1, (S)-2, and (R)-4 can catalyze
asymmetric intermolecular carbene Si−H insertion; using
t
catalyst (R)-4 which bears a biphenyl salen ligand with Bu
H
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and Innovation, Shenzhen 518057, P.R. China; orcid.org/
0000-0002-2554-7219; Email: cmche@hku.hk
Scheme 4. Proposed Mechanism for cis-β-
[RuII(salen)(CO)2]-Catalyzed Intermolecular Carbene
Insertion into Si−H Bond
Authors
Chi Lun Lee − State Key Laboratory of Synthetic Chemistry and
Department of Chemistry, The University of Hong Kong, Hong
Kong, P. R. China
Daqing Chen − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The University of Hong Kong,
Hong Kong, P. R. China; School of Environment and Ecology,
Jiangsu Open University, Nanjing 210000, P.R. China
Xiao-Yong Chang − State Key Laboratory of Synthetic
Chemistry and Department of Chemistry, The University of
Hong Kong, Hong Kong, P. R. China; orcid.org/0000-
0003-1394-6164
Zhou Tang − State Key Laboratory of Synthetic Chemistry and
Department of Chemistry, The University of Hong Kong, Hong
Kong, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00268
Author Contributions
⊥C.L.L. and D.C. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Hong Kong Research Grants
Council (17303815 and 17301817), 13th 5-year program of
Jiangsu Open University (Project No. 17SSW-Z-Q-028), and
B a s i c R e s e a r c h P r o g r a m - S h e n z h e n F u n d
(JCYJ20170412140251576 and JCYJ20170818141858021).
substituents, the reactions of an aryl silane and aliphatic silanes
with aryldiazoacetates N2C(Ar)CO2R as the carbene sources
in hexane at room temperature upon light irradiation afforded
carbene Si−H insertion products in up to excellent yield and
with moderate-to-good enantioselectivity. The isolation and
spectroscopic/structural studies of 3 and 5a, together with HR-
ESI-MS analysis of a solution of 2 under light irradiation and
its reaction mixture with N2C(Ar)CO2R before and after
addition of a silane substrate, provide evidence for the
involvement of cis-β-ruthenium−salen mono(carbene) com-
plexes as the key intermediates in the cis-β-[RuII(salen)-
(CO)2]-catalyzed intermolecular Si−H insertion reactions.
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* Supporting Information
The Supporting Information is available free of charge at
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■
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AUTHOR INFORMATION
Corresponding Author
Chi-Ming Che − State Key Laboratory of Synthetic Chemistry
and Department of Chemistry, The University of Hong Kong,
Hong Kong, P. R. China; HKU Shenzhen Institute of Research
■
I
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K
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