Chiral-at-Metal Ruthenium Complexes with Guanidinobenzimidazole and Pentaphenylcyclopentadienyl Ligands: Synthesis, Resolution, and Preliminary Screening as Enantioselective Second Coordination Sphere Hydrogen Bond Donor Catalysts

Article abstract of DOI:10.1021/acs.organomet.0c00073

Pentaphenylcyclopentadienyl ruthenium 2-guanidinobenzimidazole (GBI = HN≠C(NHH′)NHC(NHR)≠NR′) chelate complexes are accessed by treating (η5-C5Ph5)Ru(CO)2(Br) with Me3NO, GBI, and Ag+PF6-. Chromatographic workups give [(η5-C5Ph5)Ru(CO)(GBI)]+PF6- (2+PF6-,

Full text of DOI:10.1021/acs.organomet.0c00073

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Article
Chiral-at-Metal Ruthenium Complexes with
Guanidinobenzimidazole and Pentaphenylcyclopentadienyl
Ligands: Synthesis, Resolution, and Preliminary Screening as
Enantioselective Second Coordination Sphere Hydrogen Bond
Donor Catalysts
Tathagata Mukherjee, Subrata K. Ghosh, Taveechai Wititsuwannakul, Nattamai Bhuvanesh,
and John A. Gladysz*
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Supporting Information
ACCESS
ABSTRACT: Pentaphenylcyclopentadienyl ruthenium 2-guanidinoben-
zimidazole (GBI = HNC(NHH′)NHC(NHR)NR′) chelate com-
plexes are accessed by treating (η5-C5Ph5)Ru(CO)2(Br) with Me3NO,
−
GBI, and Ag+PF6 . Chromatographic workups give [(η5-C5Ph5)Ru-
(CO)(GBI)]+PF6 (2+PF6 , 70%; silica gel) or 2+BArf− (69%; alumina,
−
−
then Na+BArf− (BArf− = B(3,5-C6H3(CF3)2)4 )). Treatment of 2+PF6
−
−
with K+t-BuO− deprotonates the GBI ligand to give the neutral species
(η5-C5Ph5)Ru(CO)(GBI−H) (3; 73%). Complexes 2+PF6 , 2+BArf−, and
−
3 have been characterized by NMR, X-ray crystallography, and other
methods. Protonation of 3 with enantiopure (P)-1,1′-binaphthyl-2,2′-
diyl hydrogen phosphate ((P)-H-4) gives the diastereomeric salts (RRu/
SRu)-2+(P)-4− (93%); (SRu)-2+(P)-4− selectively precipitates from cold
toluene/hexanes (35% or 70% of theory), and addition of Na+BArf−
gives (SRu)-2+BArf− (71%). The absolute configuration is provisionally assigned by CD spectroscopy. (SRu)-2+BArf− is an effective
hydrogen bond donor catalyst for a variety of addition reactions by virtue of a synperiplanar NH triad involving the noncoordinating
NH groups (some reactions with racemic 3 are also explored), but the enantioselectivities are uniformly low. Nonetheless, this
provides insight regarding the mechanism of asymmetric induction for related catalysts with GBI-based carbon stereocenters.
hydrogen-bond donors.6 During the previous 6 years, we have
sought to analogously develop families of chiral transition-
INTRODUCTION
■
There is an extensive literature of “chiral-at-metal” com-
metal-containing catalysts7 with coordinated or noncoordi-
nated NH groups that can function as hydrogen-bond
donors.8−10 Indeed, we have achieved high enantioselectivities
with motifs such as Ic (monofunctional) and IV (bifunctional
per the pendant amine). The related systems III and V, which
so far have only been accessed in racemic form, are also very
effective catalysts, but the enantioselectivities remain to be
assayed. These contain what are termed 2-guanidinobenzimi-
dazole (GBI) ligands,11 which in the usual chelate binding
mode afford directed, roughly planar triads of NH moieties.
plexes.1−3 Major categories include Werner complexes such as
[Co(en)3]3+·3Cl− (Ia; Figure 1)4the first family of chiral
inorganic compounds to be resolved into enantiomersand
“piano stool” species of the formula (η5-C5R5)M(L)(L′)(L″)
(II).1a,b,c,2,3 The latter are often termed pseudotetrahedral, but
they are more precisely octahedral, with the cyclopentadienyl
ligand occupying three sites and the L−M−L angles close to
90°.
There is also a fairly extensive literature of enantiopure
chiral-at-metal complexes that catalyze reactions of achiral
organic substrates and lead to chiral products with appreciable
enantioselectivities.1b,d,3,5 Although they have not proved
superior to metal catalysts that bear only ligand-based
stereocenters, metal-based chirality remains a design element
that can be exploited, even when the catalytically active site is
not the metal center.5
Received: February 3, 2020
Over the last 20 years, many chiral “organocatalysts” have
been developed that activate organic substrates by serving as
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Scheme 1. Enantioselective Catalysis with Diastereomeric
Bifunctional Chiral-at-Ruthenium Complexes IV
Figure 1. Some previously studied chiral-at-metal complexes and
catalysts.
Catalyst IV features ruthenium and carbon stereocenters,
and the SRuRCRC and RRuRCRC diastereomers can both be
prepared. Some representative results for additions of 1,3-
dicarbonyl compounds to trans-nitroalkenes are depicted in
Scheme 1.9b,12 When the alkene substituent is an aryl group,
the product is obtained in 89−99% ee. Importantly, the carbon
stereocenter in IV controls the product configuration.
However, we were also curious as to the degree of
enantioselection possible in related catalysts with only
ruthenium stereocenters.
Given our inability to access enantiopure catalysts III,9a and
the poor computational prognosis for the attendant enantio-
selectivity in the preceding paper,12 we decided to target
pentaphenylcyclopentadienyl analogues. Our thinking was that
this bulky ligand would help extend the chiral environment
farther from the ruthenium, possibly providing superior
enantioselectivities. We report in this paper facile syntheses
of such complexes in enantiopure form, configurational
assignments, crystallographic data, and preliminary screening
of their catalytic activities.13
methodology previously used to transform 1 to the racemic
chiral-at-metal complexes (η5-C5Ph5)Ru(PEt3)(CO)(Br) and
14a
−
[(η5-C5Ph5)Ru(PEt3)(MeCCMe)(CO)]+PF6 .
Accord-
ingly, a CH3CN suspension of 1 was sequentially treated
with the amine oxide Me3NO·2H2O (1.0 equiv), excess GBI,
and Ag+PF6 (1.0 equiv). A workup that included silica gel
−
chromatography gave the racemic chiral complex [(η5-
C5Ph5)Ru(CO)(GBI)]+PF6 (2+PF6 ) as a bright green
−
−
powder in 70% yield.
−
The salt 2+PF6 was soluble in CH2Cl2, CHCl3, CH3CN,
DMSO, and MeOH but insoluble in nonpolar toluene. Like all
−
of the new complexes described below, 2+PF6 was
RESULTS
■
Syntheses of Racemic Complexes. The known
pentaphenylcyclopentadienyl ruthenium complex (η5-C5Ph5)-
Ru(CO)2(Br) (1) can be isolated in 75% yield from the
reaction of commercially available Ru3(CO)12 and 5-bromo-
1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene in refluxing tol-
uene.14 The latter educt can, if desired, be synthesized in
two steps from tetraphenylcyclopentadienone.15,16 Slight
modifications that improve the literature yields have been
described elsewhere.13b
With the complexes III (Figure 1), the GBI ligand was
introduced by simple thermal reactions with the bis-
(phosphine) complexes (η5-C5R5)Ru(PPh3)2(Cl) (R = H,
Me). However, related routes were not successful in the
pentaphenylcyclopentadienyl series. Thus, we turned to a
characterized by NMR (1H, 13C) and IR spectroscopy, as
summarized in the Experimental Section and Tables s1−s3 in
the Supporting Information. On the basis of 1H and 13C NMR
studies of the cyclopentadienyl analogue [(η5-C5H5)Ru(CO)-
(GBI)]+PF6 (IIIa; Figure 1),9a all proton and carbon signals
−
could be unambiguously assigned. The numbering system
depicted in VI in Scheme 2 was employed. The structure was
furthermore verified crystallographically, as described below.
When the preceding reaction was worked up using an
alumina column, a complex we represent as 2+X− was isolated.
Both 31P{1H} NMR and IR spectroscopy showed at most trace
amounts of the PF6− anion (<5%). Thus, X− was presumed to
be derived from the alumina. Efforts to quantitatively
−
reintroduce PF6 were unsuccessful. However, workup of a
B
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Scheme 2. Syntheses of Racemic Chiral Complexes
Scheme 3. Syntheses of Enantiopure Complexes
biphasic reaction of 2+X− and Na+BArf− (BArf− = B(3,5-
Syntheses and Configurations of Enantiopure Com-
plexes. A CH2Cl2 solution of 3 was treated with an equimolar
amount of the commercially available, enantiopure axially
chiral diaryl hydrogen phosphate (P)-H-4 shown in Scheme 3.
The GBI−H ligand was reprotonated to give a 50:50 mixture of
the diastereomeric salts (RRu)-2+(P)-4− and (SRu)-2+(P)-4−,
17
C6H3(CF3)2)4 ) in CH2Cl2/H2O gave 2+BArf− as a hydrate
(2−4 H2O depending upon workup) in 69% overall yield. This
new salt was characterized similarly to 2+PF6− and was soluble
−
in toluene as well as the solvents that dissolved 2+PF6 .
−
A CH2Cl2 solution of 2+PF6 and a H2O solution of K+t-
−
BuO− were combined to give a biphasic yellow suspension. As
shown in Scheme 2, workup gave the neutral racemic chiral
ruthenium complex (η5-C5Ph5)Ru(CO)(GBI−H) (3) as a
bright yellow powder in 73% yield. Here, GBI−H represents
the conjugate base of GBI, which acts as an anionic four-
electron-donor ligand for the [(η5-C5Ph5)Ru(CO)]+ fragment.
Deprotonated GBI ligands also have an extensive coordination
chemistry,18 and the tautomer depicted was verified crystallo-
graphically as described below. This synthesis and all of those
in the following section were carried out aerobically. Complex
3 was moderately soluble in CH2Cl2 and CH3CN and very
soluble in MeOH and DMSO.
1
which was isolated in 93% yield. The H and 13C{1H} NMR
spectra showed two sets of signals that were nearly always
resolvedeven the four NH peaks.
Attempts to separate the salts by silica gel or neutral alumina
chromatography were unsuccessful. However, there was an
appreciable solubility difference in cold toluene/hexanes.
When a 90/10 v/v toluene/hexanes solution of (RRu/SRu)-
2+(P)-4− was kept at −35 °C, a yellow precipitate and greenish
black supernatant formed. Workup of the precipitate gave
(SRu)-2+(P)-4− in 35% yield, or 70% of theory. The NMR
spectra established a minimum diastereomer ratio of >98:<02.
C
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−
Figure 2. Thermal ellipsoid diagram (50% probability level) showing the structure of two molecules of 2+PF6 ·1.5C5H12 with the phenyl hydrogen
atoms and solvate molecules omitted. Hydrogen-bonding interactions are shown in red (<2.5 Å) and cyan (2.5−2.9 Å).
The greenish black filtrate provided a mixture of diastereomers,
with (RRu)-2+(P)-4− predominating, in 60% yield.
of the NMR signals indicate particularly strong (de)shielding
effects from the phenyl groups.
−
Single crystals of the solvate 2+PF6 ·1.5C5H12 could be
Next, (SRu)-2+(P)-4− was treated with Na+BArf− under
biphasic conditions (CH2Cl2/H2O). Workup gave the salt
(SRu)-[(η5-C5Ph5)Ru(CO)(GBI)]+BArf− ((SRu)-2+BArf−),
which contains a single stereocenter, in 71% yield (Scheme
3) as a hydrate (3−4 H2O). Strategically, BArf− salts are
grown, and X-ray data were collected and refined as described
in the Experimental Section and Table s4 in the Supporting
Information. The NH hydrogen atoms were located, and the
others were placed in idealized positions. The resulting
structure is shown in Figure 2, and key metrical parameters
are summarized in Tables s5 and s6 in the Supporting
Information.
−
somewhat preferable to PF6 salts (and greatly preferable to
BF4 or Cl− salts) due to the weaker cation/anion hydrogen
−
bonding and enhanced rates of catalyzed reactions.9a
The configurations of the stereocenters in the cyclo-
pentadienyl complex (SRuRCRC)-IV (Figure 1) have been
established crystallographically,9b and the CD spectrum
exhibits a positive absorption at longer wavelengths (λmax
408 nm).9b The CD spectrum of (SRu)-2+BArf− (Figure s1 in
the Supporting Information) exhibited a similar positive band
(λmax 425 nm; there are minor shoulders in both cases). Hence,
it and the precursor salt were provisionally assigned S
configurations at ruthenium, as represented in the preceding
formulas.
Importantly, the bond lengths of the coordinated CNH
(C1−N1) and CNAr (C2−N3) linkages (1.284(7) and
1.320(7) Å) were shorter than the other four carbon−nitrogen
bonds about C1 and C2 (1.337(7)−1.374(7) Å). As analyzed
in previous papers,9 alternative tautomers of the GBI ligand
would afford different carbon−nitrogen bond length patterns.
The OC−Ru−N and N−Ru−N bond angles ranged from
92.5(2) to 81.5(2)°, per the formally octahedral geometry.
As seen in other GBI complexes, the ligand is slightly
puckered, with the ruthenium atom perched 0.81 Å above the
least-squares plane of the five atoms of the chelate ring. When
one considers all of the Ru(GBI) torsion angles (Table s5 in
the Supporting Information), which would be 0 or 180° for a
planar assembly, the average deviations are 13.2(7) and
16.4(5)°, respectively. The three NH units on the nitrogen
atoms remote from the ruthenium atom, N5−H5, N2−H2,
and N4−H4B, define a roughly synperiplanar triad, as reflected
by H−N−N−H torsion angles that are reasonably close to 0°
(−24.7 and −24.4° for the nearest neighbors). The two other
NH units, N1−H1 and N4−H4A, similarly constitute an
approximately synperiplanar dyad with a torsion angle of 25.2°.
Characterization of New Complexes. The preceding
compounds generate a wealth of spectroscopic data, and
various tabular comparisons are presented in the Supporting
Information and elsewhere.13b In general, there are few
noteworthy trends, although the neutral complex 3 is often a
minor outlier. For example, it exhibits an IR νCO band at 1934
−
cm−1 vs 1977 cm−1 for 2+BArf− and 1948 cm−1 for 2+PF6 . The
trend 2+BArf− > 2+PF6 reflects the greater degree of cation/
−
anion hydrogen bonding in the latter, as analyzed earlier for
the series of compounds III in Figure 1.9a,19 These values are
also 16−6 cm−1 greater than those of the cyclopentadienyl
analogues, consistent with the enhanced electron-withdrawing
character of the pentaphenylcyclopentadienyl ligand.20,21 None
−
As illustrated in Figure 2, each PF6 anion exhibits
numerous hydrogen bonds to each of two neighboring cations.
All of the NH groups participate. The F···H, F···N, and P···N
D
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Figure 3. Thermal ellipsoid diagram (50% probability level) of 2+BArf−·H2O with the phenyl hydrogen atoms omitted. Hydrogen-bonding
interactions are shown in red (<2.5 Å).
distances, summarized in Table s6 in the Supporting
Information, are in typical ranges for such hydrogen bonds.22
The shortest F···H contact involved the Ru−NH moiety
(2.029 Å).
The crystal structure of the hydrate 2+BArf−·H2O could
similarly be determined and is depicted in Figure 3. Key
metrical data are summarized in Tables s5 and s7 in the
Supporting Information. The carbon−nitrogen bond lengths
showed patterns similar to those of 2+PF6 ·1.5C5H12. The
−
Ru(GBI)-based torsion angles exhibited comparable average
deviations from 0 or 180° (10.9(4) or 16.8(4)°). A roughly
synperiplanar NH triad (H−N−N−H torsion angles of nearest
neighbors: 38.4 and −19.1°) and dyad (H−N−N−H torsion
angle −35.0°) were again apparent. The water molecule was
hydrogen-bonded to two of the five NH units (N2−H2, N4−
H4B). Consistent with the extremely poor acceptor properties
of BArf−,19,23 only weak interactions with the cation were
Figure 4. Thermal ellipsoid diagram (50% probability level) of the
dominant conformer of 3 with the phenyl hydrogen atoms omitted.
apparent. Three of the CF3 groups and the three NH groups
not hydrogen-bonded to water exhibited NH···FC− contacts
ranging from 2.31 to 2.58 Å (average, 2.47 Å; sum of van der
The carbon−nitrogen bond length of the coordinated CNH
Waals radii, 2.67 Å). As analyzed in a companion paper, similar
(C1−N1) linkage in 3 is considerably shorter than that of the
coordinated CNAr (C2−N3) linkage (1.295(4) vs 1.348(4)
Å). At the same time, the noncoordinated C2−N5 linkage is
shorter than the C2−N2, C1−N2, and C1−N4 linkages
(1.339(4) vs 1.389(4), 1.365(4), and 1.354(4) Å). For
reference, typical carbon nitrogen double-bond lengths are
1.279 Å24 (Ph(H)CNR: R = Ph, 1.286(8) Å; R = Me,
interactions are often seen in BArf− salts of cations of NH
donor groups.19
The crystal structure of the GBI−H complex 3 was also
determined. One of the phenyl rings was disordered between
two positions but could be refined to a 53:47 occupancy ratio.
The dominant conformer is depicted in Figure 4. This
perspective highlights the puckering of the GBI−H ligand and
the displacement of the ruthenium atom from the five-atom
chelate plane (0.96 Å), which is greater than those in the
preceding GBI complexes as well as in cyclopentadienyl or
cyclopentadienyl/PPh3 analogues characterized earlier (0.21−
0.59 Å).9 Key metrical parameters are provided in Table s5 in
the Supporting Information. The Ru(GBI−H)-based torsion
angles exhibit average deviations from 0 or 180° (16.9(4) or
22.4(4)°) that are slightly greater than those in the two
preceding GBI adducts.
1.284(10) Å)25 and single-bond lengths (Csp −N) range from
2
24
2
2
2
3
1.355 Å (Csp −Nsp ) to 1.416 Å (Csp −Nsp ). These data are
best modeled by a tautomer of 3 that has been deprotonated at
N5 and exhibits the dominant resonance form shown in
Schemes 2 and 3 (CNH for C1−N1, C−NAr for C2−N3,
and CNAr for C2−N5).
Interactions with Organic Molecules. Hydrogen bond-
ing between 2+BArf− and a substrate typical of those that might
be used in catalysis, dimethyl malonate (5a), was probed by 1H
NMR as described for the analogous cyclopentadienyl complex
E
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Figure 5. 1H NMR spectra (rt, 500 Hz, CD2Cl2): 2+BArf−·4H2O (bottom); after addition of 0.5 equiv of 5a (middle); after addition of another 0.5
equiv of 5a (top). Key signals (δ (w1/2, Hz)), bottom, middle, top: orange NH 9.42 (28.6), 10.20 (104.0), 10.50 (63.4); green NH 8.29 (37.7),
8.67 (130.4), 8.81 (96.3); purple NH 5.04 (6.4), 5.27 (8.0), 5.36 (7.2); magenta NH 5.13 (9.2), 5.01 (8.4), 4.98 (8.1); H2O 1.77 (7.8), 1.66
(33.0), 1.61 (27.8) ppm.
IIIb and trans-β-nitrostyrene earlier.9a Thus, a spectrum of
racemic 2+BArf−·4H2O was recorded in CD2Cl2, and two 0.5
equiv aliquots of 5a were sequentially added. As shown in
Figure 5, the protons associated with the NH triad (N5, N2,
N4) shifted progressively downfield. In contrast, the dyad
proton on N1 shifted slightly upfield. The Δδ values (ppm) are
illustrated in Figure 5. It should be kept in mind that the two
protons associated with N4one belonging to the triad and
the other to the dyadalways undergo rapid exchange. Thus,
the downfield shift is attenuated relative to the other triad
protons. All CH signals were essentially unaffected, but the
H2O signal shifted slightly upfield (Δδ = −0.16 ppm). These
data are consistent with an adduct of the type VII (Figure 5),
which is analogous to that found computationally for IIIa and
2,4-pentanedione in the preceding paper.12
be effective earlier, with complete reactions requiring on the
order of 1 h at room temperature.9a Indeed, (SRu)-2+BArf−
exhibited similar reactivity, but unfortunately the product 8
was not detectably enantioenriched.
The story was similar for the Michael addition of diethyl
malonate (5b) to 6 (Scheme 4, middle). Here, the bifunctional
catalysts (SRuRCRC)- and (RRuRCRC)-IV effect highly enantio-
selective reactions on a time scale of 24 h, as shown in Scheme
1.9b While (SRu)-2+BArf− (together with the external base
NEt3) matches this activity, the product 9 is essentially
racemic. As detailed elsewhere,13b similar results were obtained
when 5b was replaced by malononitrile or 2,4-pentanedione.
The focus was then shifted to Michael acceptors containing
an oxazolidinone imide moiety, as exemplified by 10 and 13 in
Scheme 4 (bottom). It was thought that this type of 1,3-
dicarbonyl compound, which is widely employed in
enantioselective syntheses,27 might be particularly effective in
hydrogen bonding to our ruthenium catalysts. There had been
some prior art involving catalysts for moderately enantiose-
lective thiol additions to such acceptors.28 However, with
Next, the efficacy of (SRu)-2+BArf− as a catalyst was probed
in a series of test reactions, several of which are shown in
Scheme 4. The first, involving trans-β-nitrostyrene (6) and 1-
methylindole (7), is often termed a Friedel−Crafts alkylatio-
n.9a,26 The racemic catalyst IIIb (Figure 1) had been shown to
F
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Scheme 4. Some Reactions Catalyzed by (SRu)-2+BArf−
Scheme 5, the additions of dimethyl malonate (5b) and
malononitrile (15) to trans-β-nitrostyrene were efficiently
catalyzed by 3 (10 mol %). NMR monitoring showed clean
reactions, and the products 9 and 16 were isolated in 75−90%
yields.
DISCUSSION
■
One obvious gap in previous studies involving the highly
enantioselective ruthenium hydrogen bond donor catalysts
(SRuRCRC)- and (RRuRCRC)-IV (Figure 1 and Scheme 1)9b was
the inability to assess the relative importance of the ruthenium
and carbon stereocenters. We had surmised that, in the
absence of the carbon stereocenters, enantioselectivities would
decrease, as the ruthenium stereocenter is remote from the
substrate binding sites. However, this could not be tested, as all
efforts to separate the enantiomers of racemic complexes III
(Figure 1) were unsuccessful.9a
In the design of this study, it was sought to compensate for
the remoteness of the ruthenium stereocenter by increasing the
steric influence of the cyclopentadienyl ligand by appending
five phenyl groups. As is readily seen in Figures 2−4, these
extend to the periphery of the NH donor groups, and model
building suggested some promise. Auspiciously, it proved easy
to prepare the catalyst (SRu)-2+BArf− in enantiopure form
(Scheme 3). However, the enantioselectivities in all test
reactions were very poor (Scheme 4), underscoring the
importance of the more proximal carbon stereocenters in
(SRuRCRC)-IV. Later, the computational studies in the
preceding paper provided a detailed mechanistic picture,
highlighting the key roles played by the internal base.12
The feasibility of another approach to catalysis with this
family of ruthenium complexes is demonstrated in Scheme 5.
By starting with the neutral Brønsted basic complex 3, it
should be possible to realize transition state assemblies similar
to those operative in Scheme 4, but with the conjugate base of
the Brønsted acidic substrate replacing the counteranion BArf−.
This would seemingly allow for a more intimate interaction.
However, extensions to enantiopure catalysts were not pursued
as part of this study, as species derived from (SRuRCRC)- and
(RRuRCRC)-IV (Scheme 1) were viewed as the best place to
begin.
(SRu)-2+BArf− nearly racemic products (12, 14) were again
obtained.29
It was not practical to recover the (SRu)-2+BArf− catalyst
following the preceding reactions or otherwise verify its
enantiomeric purity. However, the diastereomeric salts (SRu)-
2+(P)-4− and (RRu)-2+(P)-4− were configurationally stable in
the solvents used on the time scales of these reactions, as
1
assayed by H NMR.
This investigation was concluded with two experiments with
the potential to set the stage for another generation of studies.
Although 3 was only isolated in racemic form, it contains a
deprotonated benzimidazole moiety and might be able to serve
as a bifunctional catalyst. This would obviate the need for the
Brønsted base (NEt3) employed in most of the transformations
in Scheme 4. Thus, 3 could potentially activate malonate type
or thiol nucleophilies and in doing so generate the NH triad
that binds and activates one of the substrates. As shown in
At roughly the same time as our chemistry in Scheme 5 was
developed, Meggers reported an octahedral, chiral-at-metal,
neutral iridium catalyst (VIII) in which a pyrazolyl moiety
could deprotonate a thiol nucleophile to give a cationic
pyrazole complex.30 This generated a NH dyad that could
activate N-pyrazolyl crotonates and related Michael acceptors
for thiol additions. Very high ee values could be realized with
Scheme 5. Bifunctional Catalysts with Internal Brønsted Bases
G
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Acros Organics), and Celite were used as received. Na+BArf−,17 trans-
3-cinnamoyloxazolidin-2-one (10),33 and 3-(2-methyl-2-propenoyl)-
oxazolidin-2-one (13)34 were prepared by literature procedures.
(η5-C5Ph5)Ru(CO)2Br (1).14 A Schlenk flask was charged with 5-
bromo-1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (0.743 g, 1.41
mmol), Ru3(CO)12 (0.300 g, 0.468 mmol), and toluene (20 mL)
with stirring. The mixture was refluxed. After 6 h, the oil bath was
removed. After 1 h, the solvent was removed by rotary evaporation
and the residue was chromatographed on a silica gel column (3 × 20
cm, 80/20 to 20/80 v/v hexanes/CH2Cl2). The solvent was removed
from the product-containing fractions by rotary evaporation and oil
pump vacuum to give (η5-C5Ph5)Ru(CO)2Br as a yellow solid (0.781
g, 1.15 mmol, 82%).
very low catalyst loadings, underscoring the promise of
enantiopure 3. We note in passing that other researchers
have developed effective transition-metal-containing NH
hydrogen bond donor catalysts, but of a monofunctional
nature.31
Many enantiopure cyclopentadienyl complexes have been
prepared that feature a tertiary amine and (1) a carbon
stereocenter (e.g., C5H4CH(R)NMe2) or (2) a second
substituent such that the cyclopentadienyl ligand possesses
planar chirality (e.g., C5H3(X)NMe2).32 These would also
represent strategically attractive ruthenium GBI derivatives for
future study, as they simultaneously introduce both an internal
Brønsted base and an additional stereogenic element.
Comparisons to the bifunctional catalysts (SRuRCRC)- and
(RRuRCRC)-IV with their GBI-based stereocenters would be
especially informative.
1
3
NMR (δ, CD2Cl2): H (500 MHz) 7.23 (apparent tt, 5H, JHH
=
7.2 Hz, 4JHH = 1.5 Hz, p-C6H5), 7.13−7.09 (m, 10H, o-C6H5), 7.08−
7.05 (m, 10H, m-C6H5); 13C{1H} (125 MHz) 197.2 (s, CO), 132.7
(s, o-C6H5), 130.0 (s, i-C6H5), 128.7 (s, p-C6H5), 128.1 (s, m-C6H5),
107.1 (s, C5Ph5).
−
[(η5-C5Ph5)Ru(CO)(GBI)]+PF6 (2+PF6−). A Schlenk flask was
In summary, the structural and NMR data herein establish
−
the facility with which 2+X− (X− = BArf−, PF6 ) can enter into
charged with 1 (0.200 g, 0.293 mmol), CH3CN (10 mL), and (after 2
min) Me3NO·2H2O (0.032 g, 0.29 mmol) with stirring. Within 5 min,
the suspension became an orange solution. Then GBI (0.051 g, 0.29
hydrogen-bonding interactions. Scheme 4 establishes the
efficacy of (SRu)-2+BArf− as a catalyst, but not an
enantioselective catalyst. Nonetheless, we continue to view
the inexpensive GBI ligand as a promising component for new
enantioselective catalysts and our current efforts are focusing
on (1) enantiopure analogues of 3 and (2) enantiopure V
(Figure 1) and bifunctional derivatives. Complex V can be
viewed as a “porcupine” of outwardly directed NH groups that
includes three NH triads. These thrusts are augmented by
ongoing studies of a variety of cobalt(III) tris(1,2-diamine)
complexes, such as that represented by I (Figure 1). Some of
these are spectacularly successful enantioselective cata-
lysts.8b,c,d,e
−
mmol) was added. After 15 min, Ag+PF6 (0.074 g, 0.29 mmol) was
added. After 16 h, the solvent was removed by rotary evaporation.
Then CH2Cl2 (10 mL) was added. The suspension was passed
through Celite, which was washed with CH2Cl2 (3 × 10 mL). The
solvent was removed from the combined filtrates by rotary
evaporation. The residue was chromatographed on a silica gel column
(3 × 25 cm, 100/2 v/v CH2Cl2/CH3OH). The solvent was removed
from the product-containing fractions by rotary evaporation. The
residue was washed with 50/50 v/v CH2Cl2/hexanes (3 × 30 mL)
−
and dried by oil pump vacuum to give 2+PF6 ·C6H14 as a bright green
solid (0.199 g, 0.203 mmol, 70%), dec pt 156 °C (capillary). Anal.
Calcd for C44H34F6N5OPRu·C6H14: C, 61.22; H, 4.93; N, 7.14.
Found: C, 61.29; H, 4.67; N 7.93.35 This reaction was repeated on a
−
1.02 mmol scale and gave a 67% yield of 2+PF6 ·C6H14.
NMR (δ, CD2Cl2):36 1H (500 MHz) 7.35 (d, 1H, JHH = 8.0 Hz,
3
EXPERIMENTAL SECTION
General Data. Reactions in Schlenk flasks were carried under
■
CH7/8), 7.20−7.14 (m, 7H, p-C6H5, CH5/6, and CH8/7), 7.02 (t,
10H, JHH = 7.2 Hz, m-C6H5),37 6.95−6.90 (m, 11H, o-C6H5 and
3
nitrogen atmospheres. Other reactions and workups were carried out
1
in air. H, 13C{1H}, and 31P{1H} NMR spectra were recorded on
CH6/5), 5.48 (br s, 2H, H2NCNH), 4.77 (s, 1H, RuNH), 1.27 (m,
C6H14), 0.98−0.82 (m, C6H14);38 13C{1H} (125 MHz) 205.3 (s, CO),
154.3 (s, C1), 146.0 (s, C2), 141.1 (s, C3), 141.5 (s, C4), 132.3 (s, o-
C6H5), 131.9 (s, i-C6H5), 128.1 (s, m-C6H5 and p-C6H5),39 124.1 (s,
C5), 122.3 (s, C6), 118.9 (s, C7), 111.8 (s, C8), 100.6 (s, C5Ph5),
30.2, 24.1, 14.3 (3 × s, C6H14). IR (cm−1, powder film): 3367 (m),
3057 (m), 2960 (w), 2924 (w), 1948 (s, νCO), 1674 (s), 1558 (m),
1500 (s), 1462 (w), 1444 (m), 1398 (m), 1259 (w), 1076 (m), 1028
(s), 1014 (m), 840 (s), 800 (s), 738 (s), 698 (s).
standard 500 MHz spectrometers at ambient probe temperature (24
1
°C) and referenced as follows (δ, ppm): H, residual internal CHCl3
(7.26), CHD2OD (3.30), or CHDCl2 (5.32); 13C, internal CDCl3
(77.0), CD3OD (49.1), or CD2Cl2 (53.9); 31P{1H}, external H3PO4
(0.0). IR spectra were recorded using a Shimadzu IRAffinity-1
spectrophotometer with a Pike MIRacle ATR system (diamond/ZnSe
crystal). Circular dichroism spectra were obtained using a Chirascan
CD spectrometer (Applied Photophysics). Microanalyses were
conducted by Atlantic Microlab. Melting points were recorded with
a Stanford Research Systems (SRS) MPA100 (Opti-Melt) automated
device. HPLC analyses were conducted with a Shimadzu instrument
package (pump/autosampler/detector LC-20AD/SIL-20A/SPD-
M20A).
[(η5-C5Ph5)Ru(CO)(GBI)]+BArf− (2+BArf−). Complex 1 (0.0200 g,
0.0293 mmol), CH3CN (1.0 mL), Me3NO·2H2O (0.0032 g, 0.029
−
mmol), GBI (0.0051 g, 0.029 mmol), and Ag+PF6 (0.0074 g, 0.029
mmol) were combined in a procedure analogous to that given for
−
2+PF6 . After 16 h, the solvent was removed by rotary evaporation.
Then CH2Cl2 (1.0 mL) was added. The suspension was passed
through Celite, which was washed with CH2Cl2 (3 × 1.0 mL). The
solvent was removed from the combined filtrates by rotary
evaporation. The residue was chromatographed on a alumina column
(3 × 15 cm, 95/5 to 75/25 v/v CH2Cl2/CH3OH). The solvent was
removed from the product-containing fractions by oil pump vacuum
to give 2+X−, where X− is an unidentified anion that is believed to
HPLC solvents (hexanes, Fischer; 2-PrOH, J. T. Baker; both
HPLC grade) were degassed before use. Other solvents were treated
as follows: toluene, hexanes, and CH2Cl2, dried and degassed using a
Glass Contour solvent purification system; CH3CN (99.5% BDH).
pentane (99.7%, J. T. Baker), CH3OH (99.8%, BDH), ethyl acetate
(Macron, ACS grade), CDCl3, CD2Cl2, and CD3OD (3 × Cambridge
Isotope Laboratories), used as received. Ru3(CO)12 (99%, Aldrich),
2-guanidinobenzimidazole (GBI; 95%, Aldrich), 5-bromo-1,2,3,4,5-
pentaphenyl-1,3-cyclopentadiene (90%, Aldrich), (S)- or (P)-1,1′-
binaphthyl-2,2′-diyl hydrogen phosphate ((P)-H-4; 98%, Alfa Aesar),
dimethyl malonate (5a; Alfa Aesar, 98+%), diethyl malonate (5b; Alfa
Aesar, 99%), trans-β-nitrostyrene (6; 99%, Alfa Aesar), 1-methyl-
indole (7; 98%, Acros Organics), thiophenol (11; 97%, Aldrich),
−
originate from the alumina (<5% PF6 ), per similar phenomena noted
with related complexes earlier.9b
A round-bottom flask was charged with 2+X− (0.025 g, 0.033 mmol
using only the mass of the cation), CH2Cl2 (2 mL), H2O (1 mL), and
Na+BArf− (0.029 g, 0.033 mmol)17 with stirring. After 30 min, the
organic layer was separated, washed with H2O (3 × 3 mL), and dried
(Na2SO4). Removal of the solvent by rotary evaporation gave partial
decomposition. Thus, N2 was purged through the solution until
dryness. At this point 30/70 v/v CH2Cl2/hexanes (4 mL) was added
and the mixture was passed through a short plug of Celite. The
−
malononitrile (15; 98%, TCI), NEt3 (99%, Alfa Aesar), Ag+PF6
(99%, Alfa Aesar), Me3NO·2H2O (98%, TCI), Ph2SiMe2 (97%,
Aldrich), Na2SO4 (EMD), K+t-BuO− (97%, Alfa Aesar), silica gel
(SiliFlash F60, Silicycle), neutral alumina (Brockmann I, 50−200 μm,
H
https://dx.doi.org/10.1021/acs.organomet.0c00073
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
−
solvent was removed by purging N2 through the filtrate. Hexanes (2
mL) was added to give a suspension. The solvent was removed by
purging N2 through the mixture. This hexanes/purge cycle was
repeated to give 2+BArf−·4H2O (0.040 g, 0.023 mmol, 69%) as a dirty
green solid, dec pt 105 °C (capillary). The sample appeared to
partially decompose under oil pump vacuum. Hence, it was dried in
air for 1 week. Anal. Calcd for C76H46BF24N5ORu·(H2O)2: C, 55.35;
H, 3.06; N, 4.25. Found: C, 55.76; H, 3.37; N, 3.95. Calcd for
C76H46BF24N5ORu·(H2O)4 (per the integration of the H2O peak in
i-C6H5), 131.6, 130.9, 129.3, 128.7, 128.5 (5 × s, C(P)‑4 ) 128.0/127.8
(s, m-C6H5), 127.9/127.7 (s, p-C6H5), 127.3, 126.4, 125.4 (3 × s,
−
−
C(P)‑4 ), 123.4/123.0 (s, C5), 122.68 (s, C(P)‑4 ), 122.1/121.9 (s, C6),
−
121.6 (s, C(P)‑4 ), 118.4/117.8 (s, C7), 111.9/111.7 (s, C8), 100.6/
100.3 (s, C5Ph5); 31P{1H} (CD2Cl2, 202 MHz) 6.2 (s, P(P)‑4 ).
−
(SRu)-2+(P)-4−. A round-bottom flask was charged with (RRu/SRu)-
2+(P)-4− (0.110 g, 0.100 mmol) and 90/10 v/v toluene/hexanes (5
mL) with stirring. After 2 min, the solution was kept at −35 °C for 24
h. This gave a yellow solid suspended in a green supernatant. The
solid was isolated by filtration, washed with 70/30 v/v toluene/
hexanes (3 × 2 mL), and dried by rotary evaporation. The solvent was
removed from the combined filtrates by rotary evaporation. Then 90/
10 v/v toluene/hexanes (3 mL) was added. The sample was kept at
−35 °C for 24 h. A yellow solid suspended in a green supernatant
again formed. The yellow solid was isolated by filtration and washed
with 70/30 v/v toluene/hexanes (3 × 1 mL). The two crops of solid
were combined and dried by oil pump vacuum to give (SRu)-2+(P)-
4−·CH3C6H5 (0.042 g, 0.035 mmol, 35% or 70% of theory, >98/<02
1
the undried sample used for H NMR): C, 54.17; H, 3.23; N, 4.16.
NMR (δ, CD2Cl2):36 1H (500 MHz) 9.47 (br s, 1H, NH), 8.36 (br
s, 1H, NH), 7.72 (s, 8H, o-B(C6H3(CF3)2)4), 7.56 (s, 4H, p-
3
B(C6H3(CF3)2)4), 7.37 (d, 1H, JHH = 7.8 Hz, CH7/8), 7.27−7.25
3
(m, 2H, CH5/6 and CH8/7), 7.19 (t, 5H, JHH = 7.6 Hz, p-C6H5),
7.04 (t, 10H, 3JHH = 8.7 Hz, m-C6H5),37 7.02−6.95 (m, 1H, CH6/5),
6.92 (t, 10H, 3JHH = 7.6 Hz, o-C6H5), 5.13 (s, 1H, RuNH), 5.05 (br s,
2H, H2NCNH), 1.80 (br s, 8H, H2O); 13C{1H} (125 MHz) 204.5
(s, CO), 162.5 (q, 1JCB = 50.7 Hz, i-C6H3(CF3)2), 152.9 (s, C1), 144.0
(s, C2), 140.9 (s, C3), 135.2 (s, o-C6H3(CF3)2), 132.4 (s, C4), 132.3
1
SRu/RRu as assayed by H NMR using the NH protons at 4.48 and
2
(s, o-C6H5), 131.5 (s, i-C6H5), 129.2 (q, JCF = 29.1 Hz, m-
4.92 ppm) as a bright yellow powder, dec pt 105 °C (capillary). Anal.
Calcd for C71H54N5O5PRu·C6H5CH3: C, 71.71; H, 4.58; N, 5.89.
Found: C, 72.06; H, 4.78; N, 5.66. In repeated experiments on up to
0.3 mmol scales, the solvate level ranged as low as 0.2. The ruthenium
configuration was assigned by CD spectroscopy (see text and the
Supporting Information).
C6H3(CF3)2), 128.4 (s, p-C6H5),37 128.2 (s, m-C6H5),37 125.3 (s, C5),
124.9 (q, 1JCF = 271.7 Hz, C6H3(CF3)2), 124.1 (s, C6), 119.6 (s, C7),
117.9 (s, p-C6H3(CF3)2), 111.5 (s, C8), 100.4 (s, C5Ph5). IR (cm−1,
powder film): 3689 (w), 3649 (w), 3450 (w), 3401 (w), 1977 (m,
νCO), 1681 (m), 1608 (w), 1564 (m), 1354 (s), 1274 (s), 1163 (s),
1114 (s), 1091 (m), 1029 (w), 927 (w), 889 (w), 839 (w), 742 (s),
700 (s), 680 (s), 669 (s).
NMR (δ, CD2Cl2):36 1H (500 MHz) 13.77 (br s, 1H, NH), 12.33
(br s, 1H, NH), 8.01 (d, 2H, 3JHH = 8.9 Hz, H(P)‑4 ), 7.95 (d, 2H, JHH
3
−
(η5-C5Ph5)Ru(CO)(GBI−H) (3). A round-bottom flask was charged
with 2+PF6− (0.462 g, 0.459 mmol), K+t-BuO− (0.360 g, 3.21 mmol),
CH2Cl2 (100 mL), and H2O (30 mL) with stirring. The mixture
became cloudy, and in some cases a yellow suspension formed. After 2
h, CH3OH (10 mL) was added to the organic/aqueous biphase
system. The organic layer was separated, washed with H2O (3 × 4
mL), and dried (Na2SO4). The solvent was removed by rotary
evaporation. The residue was washed with CH2Cl2 (3 × 10 mL) and
dried by oil pump vacuum to give 3 as a yellow powder (0.250 g,
0.334 mmol, 73%), dec pt 160 °C (capillary). Anal. Calcd for
C44H33N5ORu: C, 70.57; H, 4.44; N, 9.35. Found: C, 71.00; H, 4.65;
N, 9.01.
3
−
−
= 8.9 Hz, H(P)‑4 ), 7.52 (d, 2H, JHH = 8.2 Hz, H(P)‑4 ), 7.44 (t, 2H,
3
3
−
−
JHH = 8.2 Hz, H(P)‑4 ), 7.38 (d, 2H, JHH = 8.2 Hz, H(P)‑4 ), 7.33 (d,
3
−
1H, JHH = 7.7 Hz, CH7/8), 7.29−7.23 (m, 4H, H(P)‑4 , CH8/7, and
CH5/6), 7.18−7.10 (m, 10H, p-C6H5 and CH3C6H5), 7.04−6.99 (t,
3
3
10H, JHH = 7.8 Hz, m-C6H5), 6.97−6.91 (d, 10H, JHH = 7.6 Hz, o-
3
C6H5), 6.88 (t, 1H, JHH = 7.7 Hz, CH6/5), 5.70 (s, 2H, NH2), 4.48
(s, 1H, RuNH), 2.34 (s, 3H, CH3C6H5); 13C{1H} (125 MHz) 205.5
−
(s, CO), 155.1 (s, C1), 149.3 (s, C(P)‑4 ), 147.2 (s, C2), 141.5 (s, C3),
138.6 (s, i-C6H5CH3), 132.8 (s, C4), 132.3 (s, o-C6H5), 132.1 (s, i-
C6H5), 131.3 (s, o-C6H5CH3), 131.6, 130.7, 129.3, 128.7, 128.5 (5 ×
s, C(P)‑4 and m-C6H5CH3) 128.0 (s, m-C6H5),37 127.9 (s, p-C6H5),37
−
−
−
127.2, 126.5 (2 × s, C(P)‑4 ), 125.7 (s, p-C6H5CH3), 125.3 (s, C(P)‑4 ),
NMR (δ, CD2Cl2/CD3OD; mixed solvent used for solubility
−
−
123.3 (s, C5), 122.5 (s, C(P)‑4 ), 122.1 (s, C6), 121.9 (s, C(P)‑4 ), 118.4
1
purposes and results in NH/ND exchange): H (500 MHz) 7.16 (d,
(s, C7), 111.9 (s, C8), 100.7 (s, C5Ph5) 21.6 (s, C6H5CH3); 31P{1H}
1H, 3JHH = 7.7 Hz, CH8),37 7.09 (t, 5H, 3JHH = 7.3 Hz, p-C6H5), 7.03
(d, 1H, 3JHH = 7.9 Hz, CH7),37 6.99−6.88 (m, 21H, m-C6H5, o-C6H5,
and CH6), 6.72 (t, 1H, 3JHH = 7.7 Hz, CH5);37 13C{1H} (125 MHz)
207.8 (s, CO), 159.0 (s, C1), 154.4 (s, C2), 143.9 (s, C3), 137.9 (s,
C4), 133.0 (s, i-C6H5), 132.6 (s, o-C6H5), 127.7 (s, m-C6H5),37 127.4
(s, p-C6H5),37 120.7 (s, C5), 119.9 (s, C6), 117.3 (s, C7), 111.7 (s,
C8), 101.4 (s, C5Ph5). IR (cm−1, powder film): 3479 (w), 3369 (w),
3059 (w), 2956 (w), 2922 (w), 2852 (w), 1934 (s, νCO), 1668 (m),
1622 (w), 1602 (w), 1566 (m), 1502 (w), 1444 (w), 1375 (s), 1261
(w), 1240 (s), 1074 (m), 1028 (w), 920 (w), 864 (w), 844 (w), 800
(w), 783 (w), 740 (s), 700 (s).
−
(202 MHz) 6.2 (s, P(P)‑4 ).
The solvent was removed from the combined filtrates by oil pump
vacuum to give (RRu/SRu)-2+(P)-4− (0.064 g, 0.060 mmol, 60%, 80/
20 RRu/SRu as assayed by 1H NMR using the NH protons at 4.45 and
4.92 ppm) as a pale green solid.
(SRu)-2+BArf−. A round-bottom flask was charged with (SRu)-
2+(P)-4−·0.2CH3C6H5 (0.142 g, 0.129 mmol), Na+BArf− (0.115 g,
0.129 mmol), CH2Cl2 (15 mL), and H2O (15 mL) with stirring. After
10 min, the organic layer turned green. After 1 h, the organic layer was
separated, washed with H2O (3 × 5 mL), and dried (Na2SO4). The
solvent was evaporated in the air (48 h) to give (SRu)-2+BArf −·(3.5
(RRu/SRu)-2+(P)-4−. A round-bottom flask was charged with
CH2Cl2 (8 mL), 3 (0.230 g, 0.307 mmol), and (P)-H-4 (0.106 g,
0.307 mmol) with stirring. After 2 h, the solution was filtered through
a short plug of Celite. The filtrate was added dropwise to hexanes (40
mL) with stirring. A light green precipitate formed. The solvent was
removed by rotary evaporation to give (RRu/SRu)-2+(P)-4− as a pale
green solid (0.312 g, 0.285 mmol, 93%) and a (50 2):(50 2)
mixture of diastereomers, as assayed by 1H NMR (4.45 and 4.92 ppm
of NH signals).
1
0.5)H2O as a dirty green solid (0.155 g, 0.092 mmol, 71%; the H
NMR spectrum suggests three H2O molecules but the microanalysis
four). Anal. Calcd for C76H46BF24N5ORu·4H2O: C, 54.17; H, 3.23;
N, 4.16. Found: C, 53.88; H, 3.02; N, 3.88. The hydration level is
higher if the air-dried sample is assayed prior to 48 h and decreases to
two after 168 h.
NMR (δ, CD2Cl2):36 1H (500 MHz) 9.45 (br s, 1H, NH), 8.33 (br
s, 1H, NH), 7.72 (s, 8H, o-B(C6H3(CF3)2)4), 7.56 (s, 4H, p-
3
B(C6H3(CF3)2)4), 7.37 (d, 1H, JHH = 7.8 Hz, CH7/8), 7.27−7.25
(m, 2H, CH5/6 and CH8/7), 7.19 (t, 5H, JHH = 7.6 Hz, p-C6H5),
NMR (δ, CD2Cl2; signals for diastereomers are separated by a slash
3
(/)):36 1H (500 MHz) 13.67/13.16 (br s, 1H, NH), 12.30/10.81 (br
3
3
−
7.04 (t, 10H, JHH = 8.7 Hz, m-C6H5), 7.02−6.95 (m, 1H, CH6/5),
s, 1H, NH), 8.12 (br s, 2H, H(P)‑4 ), 7.95 (d, 2H, JHH = 7.3 Hz,
6.92 (d, 10H, 3JHH = 7.6 Hz, o-C6H5), 5.13 (s, 1H, RuNH), 5.06 (br s,
2H, NH2), 1.73 (br s, 4H, H2O); 13C{1H} (125 MHz) 204.5 (s, CO),
162.6 (q, 1JCB = 50.7 Hz, i-C6H3(CF3)2), 152.9 (s, C1), 144.0 (s, C2),
140.9 (s, C3), 135.2 (s, o-C6H3(CF3)2), 132.4 (s, C4), 132.3 (s, o-
−
−
H(P)‑4 ), 7.59 (br s, 2H, H(P)‑4 ), 7.36−7.23, 7.19−7.13, 7.08−6.98,
6.97−6.90, 6.84−6.82 (5 × m, 6H, 3H, 8H, 13H, and 5H, remaining
−
H(P)‑4 , C6H5, and CH5−8), 6.12/5.56 (br s, 1H, NH), 4.92/4.45 (br
s, 1H, NH); 13C{1H} (125 MHz) 205.6 (s, CO), 155.0/154.7 (s, C1),
2
−
149.05/148.98 (s, C(P)‑4 ), 147.1/146.8 (s, C2), 141.46/141.42 (s,
C6H5), 131.5 (s, i-C6H5), 129.2 (q, JCF = 30.9 Hz, m-C6H3(CF3)2),
C3), 132.78/132.72 (s, C4), 132.3/132.2 (s, o-C6H5), 132.1/132.0 (s,
128.4 (s, p-C6H5),37 128.2 (s, m-C6H5),37 125.3 (s, C5), 124.9 (q, 1JCF
I
https://dx.doi.org/10.1021/acs.organomet.0c00073
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Article
= 271.7 Hz, C6H3(CF3)2), 124.1 (s, C6), 119.6 (s, C7), 117.9 (s, p-
C6H3(CF3)2), 111.5 (s, C8), 100.4 (s, C5Ph5).
and CD2Cl2 (0.3 mL) and cooled to −78 °C. Then (SRu)-2+BArf−
(0.0034 g, 0.0020 mmol, 10 mol %) and NEt3 (0.0022 g, 0.022 mmol,
1.1 equiv; delivered by syringe with the mass corresponding to the
differential weight of the empty/filled syringe) were added. The
reaction was monitored by TLC. After 24 h, the solvent was removed
by rotary evaporation. The residue was taken up in hexanes/ethyl
acetate (30/70 v/v) and passed through a short silica gel column,
which was washed with additional hexanes/ethyl acetate (50/50 v/v,
5 mL). The solvent was removed from the combined filtrates by
rotary evaporation, and the chromatography step was repeated (1 ×
10 cm, CH2Cl2). The solvent was removed from the product-
containing fractions by rotary evaporation to give 14 (0.0037 g, 0.014
mmol, 70%) as a pale yellow oil. The 1H and 13C{1H} NMR
spectra13b agreed with those reported earlier.43 The enantiomeric
excess was determined by HPLC with a Chiralcel OD column,
hexanes/2-PrOH (70/30 v/v), 1.0 mL/min, λ = 254 nm: tR = 22.7
min (major, S), 25.5 min (minor, R).42,43
CD (nm, 1.2 × 10−3 M in CH3CN ([θ], deg L mol−1 cm−1; Δε, L
mol−1 cm−1)): 310 (+13336 and +4.04), 330 (sh, + 828 and +2.50),
345 (sh, +4288 and +1.30), 360 (sh, +8262 and +0.25), 370 (sh,
−299 and −0.09), 400 (sh, +2396 and +0.73), 425 (+3714 and
+1.12), 435 (sh, +3498 and +1.07).
1-Methyl-3-(2-nitro-1-phenylethyl)-1H-indole (8; Scheme
4).26 An NMR tube was charged with (SRu)-2+BArf− (0.0034 g,
0.0020 mmol, 10 mol %), 7 (0.0052 g, 0.040 mmol, 2.0 equiv), 6
(0.0030 g, 0.020 mmol, 1.0 equiv), the internal standard Ph2SiMe2
(0.0021 g, 0.010 mmol), and CD2Cl2 (0.5 mL). The formation of 8
was monitored by 1H NMR vs the standard. After 1 h, the solvent was
removed by rotary evaporation. The residue was taken up in hexanes/
ethyl acetate (30/70 v/v) and passed through a short silica gel
column, which was washed with additional hexanes/ethyl acetate (50/
50 v/v, 5 mL). The solvent was removed from the combined filtrates
by rotary evaporation, and the silica gel chromatography step was
repeated (1 × 10 cm, 90/10 v/v hexanes/ethyl acetate). The solvent
was removed from the product-containing fractions by oil pump
vacuum to give 8 (0.0056 g, 0.020 mmol, >99%) as a colorless oil.
The 1H NMR spectrum (CDCl3)13b agreed within 0.01 ppm with the
literature chemical shift values.26a The enantiomeric excess was
determined by HPLC with a Chiralpak AS-H column, hexanes/2-
PrOH (90/10 v/v), 1.0 mL/min, λ = 210 nm: tR = 14.6, 18.6 min.26b
Ethyl 2-Carboethoxy-4-nitro-3-phenylbutyrate (9). (A;
Scheme 4) A 5 mL vial was charged with CD2Cl2 (0.5 mL), 6
(0.015 g, 0.10 mmol, 1.0 equiv), (SRu)-2+BArf− (0.017 g, 0.010 mmol,
10 mol %), and NEt3 (0.011 g, 0.10 mmol, 1.0 equiv). Then 5b
(0.0192 g, 0.12 mmol, 1.2 equiv) was added with stirring. The
reaction was monitored by 1H NMR and TLC. After 12 h, the mixture
was chromatographed on a silica gel column (1.9 × 14 cm, 9/1 v/v
hexanes/ethyl acetate). The solvents were removed from the product-
containing fractions by rotary evaporation and dried by oil pump
vacuum at room temperature to give 9 (0.0306 g, 0.099 mmol, 99%)
as a colorless oil. The NMR data agreed with those reported
previously,8b and the enantiomeric excess was assayed by chiral HPLC
as described earlier.8b,40
(B; Scheme 5). A J. Young NMR tube was charged with 5b (0.0160
g, 0.100 mmol), Ph2SiMe2 (∼0.0500 mmol; internal standard), 6
(0.0149 g, 0.100 mmol), and CD2Cl2 (0.5 mL). Then 3 (0.0075 g,
0.010 mmol, 10 mol %) was added and the tube was capped. Product
formation was monitored vs the standard by 1H NMR. After 24 h, the
solvent was removed by rotary evaporation. The residue was taken up
in hexane/ethyl acetate (30/70 v/v) and passed through a short silica
gel column, which was washed with additional hexane/ethyl acetate
(50/50 v/v, 5 mL). The solvent was removed from the combined
filtrates by rotary evaporation, and a second silica gel chromatography
step was carried out (1 × 10 cm, 80/20 v/v hexane/ethyl acetate).
The solvent was removed from the product-containing fractions by oil
pump vacuum to give 9 (0.0232 g, 0.0750 mmol, 75%) as a colorless
oil. The NMR data agreed with those reported previously.8b
2-(2-Nitro-1-phenylethyl)propanedinitrile (16; Scheme 5). A
J. Young NMR tube was charged with malononitrile (15; 0.0066 g,
0.100 mmol), Ph2SiMe2 (∼0.050 mmol; internal standard), 6 (0.0149
g, 0.100 mmol), and CD2Cl2 (1.0 mL). Then 3 (0.0075 g, 0.010
mmol, 10 mol %) was added and the tube was capped. Product
formation was monitored vs the standard by 1H NMR. After 1 h, the
solvent was removed by rotary evaporation. The residue was taken up
in hexane/ethyl acetate (30/70 v/v) and passed through a short silica
gel column, which was washed with additional hexane/ethyl acetate
(50/50 v/v, 5 mL). The solvent was removed from the combined
filtrates by rotary evaporation, and a second silica gel chromatography
step was carried out (1 × 10 cm, 60/40 v/v hexane/ethyl acetate).
The solvent was removed from the product-containing fractions by oil
pump vacuum to give 16 (0.0193 g, 0.0900 mmol, 90%) as a pale
yellow oil.
−
Crystallography. (A) A CH2Cl2/pentane solution of 2+PF6 was
kept in an NMR tube. After 24 h, yellow blocks with well-defined
faces had formed. X-ray data were collected as outlined in Table s4 in
the Supporting Information. The integrated intensity information for
each reflection was obtained by reduction of the data frames with the
program APEX2.44 Cell parameters were obtained from 180 frames
using a 0.5° scan. Data were corrected for Lorentz and polarization
factors and, using SADABS,45 absorption and crystal decay effects.
The structure was solved by direct methods using SHELXTL
(SHELXS),46 showing the sample to be the solvate 2+PF6
·
−
1.5C5H12. The NH hydrogen atoms were located from a residual
electron density map, and the others were placed in idealized
positions; both types were refined using a riding model. All non-
hydrogen atoms were refined with anisotropic thermal parameters.
The parameters were refined by weighted least-squares refinement on
F2 to convergence.46
(B) A CH2Cl2/hexanes solution of 2+BArf−·4H2O was kept in an
NMR tube. After 1 week, colorless blocks with well-defined faces had
formed. X-ray data were collected and treated as outlined in Table s4
in the Supporting Information and procedure A (cell parameters, 60
frames, 0.5° scan), showing the sample to be the hydrate
2+BArf−.H2O. Several of the CF3 groups were disordered, and this
could be modeled. Residual electron density peaks close to the
fluorine atoms of the CF3 groups indicated further disorder, but
additional modeling was not successful. Additional residual electron
densities were observed and tentatively assigned to a disordered and/
or partially occupied hexane molecule. This electron density
contribution was extracted with the program PLATON/SQUEEZE.47
The NH hydrogen atoms were located from a residual electron
density map, and the others were placed in idealized positions; both
types were refined using a riding model. All non-hydrogen atoms were
refined with anisotropic thermal parameters. The parameters were
refined by weighted least-squares refinement on F2 to convergence.46
(C) A CHCl3/CH3OH solution (10/1 v/v) of 3 was kept in an
NMR tube. After 1 week, colorless needles of 3 with well-defined
faces had formed. X-ray data were collected and treated as outlined in
Table s4 in the Supporting Information and procedure A (cell
parameters, 180 frames, 0.5° scan). The carbon atoms of one phenyl
3-[1-Oxo-3-phenyl-3-(phenylthio)propyl]oxazolidin-2-one
(12). (Scheme 4) A J. Young NMR tube was charged with 10 (0.0438
g, 0.20 mmol. 2.0 equiv),33 11 (0.0110 g, 0.10 mmol, 1.0 equiv), and
CD2Cl2 (0.1 mL), and cooled to −78 °C. Then (SRu)-2+BArf− (0.017
g, 0.010 mmol, 10 mol %) and NEt3 (0.011 g, 0.015 mL, 0.10 mmol,
1.0 equiv) were added. The reaction was monitored by 1H NMR and
TLC. After 48 h, the solution was chromatographed on a silica gel
column (1.9 × 14 cm, 8/2 v/v hexanes/ethyl acetate). The solvents
were removed from the product-containing fractions by rotary
evaporation and dried by oil pump vacuum at room temperature to
1
give 12 (0.0197 g, 0.0603 mmol, 60%) as a white solid. The H and
13C{1H} NMR spectra13b agreed with those reported earlier.41 The
enantiomeric excess was determined by HPLC with a Chiralpak AS-H
column, hexanes/2-PrOH (90/10 v/v), 1.0 mL/min, λ = 210 nm: tR
= 32.2 min (major), 34.3 min (minor).28,42
3-[(2-Methyl-1-oxo-3-(phenylthio)propyl]oxazolidin-2-one
(14). (Scheme 4) A J. Young NMR tube was charged with 13 (0.0062
g, 0.040 mmol, 2.0 equiv),34 11 (0.0022 g, 0.020 mmol, 1.0 equiv),
J
https://dx.doi.org/10.1021/acs.organomet.0c00073
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
ring (C39−C33) were disordered between two positions and could
be modeled with a 53:47 occupancy ratio. Larger thermal ellipsoids
on C21−C26 suggested disorder, but modeling efforts were not
successful. Additional residual electron densities were observed and
tentatively assigned to disordered and/or partially occupied H2O or
CH3OH molecule sites. Thus, the electron density contribution was
extracted with the program PLATON/SQUEEZE.47 Hydrogen atoms
were placed in idealized positions and refined using a riding model.
All non-hydrogen atoms were refined with anisotropic thermal
parameters. The parameters were refined by weighted least-squares
refinement on F2 to convergence.46
Stereoselective Olefin Metathesis: From Ancillary Transformation to
Purveyor of Stereochemical Identity. J. Org. Chem. 2014, 79, 4763−
4792. (e) Zhang, L.; Meggers, E. Stereogenic-Only-at-Metal
Asymmetric Catalysts. Chem. - Asian J. 2017, 12, 2335−2342.
(2) Gladysz, J. A.; Boone, B. J. Chiral Recognition in π Complexes of
Alkenes, Aldehydes, and Ketones with Transition Metal Lewis Acids;
Development of a General Model for Enantioface Binding
Selectivities. Angew. Chem., Int. Ed. Engl. 1997, 36, 550−583. Chirale
Erkennung in π-Komplexen von Alkenen, Aldehyden und Ketonen
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mit Ubergangsmetall-Lewis-Sauren; Entwicklung eines allgemeinen
Modells fur enantiofaciale Komplexierungsselektivitaten. Angew.
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Chem. 1997, 109, 566−602.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00073.
(3) Amouri, H.; Gruselle, M. Chirality in Transition Metal Chemistry:
Molecules, Supramolecular Assemblies and Materials; Wiley: Chichester,
United Kingdom, 2008.
■
sı
(4) (a) Werner, A. Zur Kenntnis des asymmetrischen Kobaltatoms.
V. Ber. Dtsch. Chem. Ges. 1912, 45, 121−130. (b) Werner, A. Zur
Kenntnis des asymmetrischen Kobaltatoms. IV. Ber. Dtsch. Chem. Ges.
1911, 44, 3279−3284. (c) Werner, A. Zur Kenntnis des
asymmetrischen Kobaltatoms. III. Ber. Dtsch. Chem. Ges. 1911, 44,
3272−3278. (d) Werner, A. Zur Kenntnis des asymmetrischen
Kobaltatoms. II. Ber. Dtsch. Chem. Ges. 1911, 44, 2445−2455.
(e) Werner, A. Zur Kenntnis des asymmetrischen Kobaltatoms. I. Ber.
Dtsch. Chem. Ges. 1911, 44, 1887−1898. V. L. King is listed as an
author for the experimental section.
NMR, CD, and crystallographic data (PDF)
Accession Codes
CCDC 1980215−1980217 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(5) (a) Kromm, K.; Zwick, B. D.; Meyer, O.; Hampel, F.; Gladysz, J.
A. A New Family of Chelating Diphosphines with a Transition Metal
Stereocenter in the Backbone: Novel Applications of ″Chiral-at-
Rhenium″ Complexes in Rhodium-Catalyzed Enantioselective Alkene
Hydrogenations. Chem. - Eur. J. 2001, 7, 2015−2027. (b) Kromm, K.;
Hampel, F.; Gladysz, J. A. A New Family of Chelating Diphosphines
with Transition-Metal and Carbon Stereocenters in the Backbone: A
Second-Generation Rhenium-Containing System. Organometallics
2002, 21, 4264−4274. (c) Kromm, K.; Osburn, P. L.; Gladysz, J. A.
Chelating Diphosphines That Contain a Rhenium Stereocenter in the
Backbone: Applications in Rhodium-Catalyzed Enantioselective
Ketone Hydrosilylations and Alkene Hydrogenations. Organometallics
2002, 21, 4275−4280. (d) Scherer, A.; Gladysz, J. A. A promising new
catalyst family for enantioselective cycloadditions involving allenes
and imines: chiral phosphines with transition metal-CH2-P: linkages.
Tetrahedron Lett. 2006, 47, 6335−6337. (e) Seidel, F.; Gladysz, J. A.
Enantioselective Catalysis of Intramolecular Morita-Baylis-Hillman
and Related Reactions by Chiral Rhenium-Containing Phosphines of
the Formula (η5-C5H5)Re(NO)(PPh3)(CH2PAr2). Synlett 2007,
2007, 0986−0988.
AUTHOR INFORMATION
Corresponding Author
■
John A. Gladysz − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3012, United States;
orcid.org/0000-0002-7012-4872; Email: gladysz@
mail.chem.tamu.edu
Authors
Tathagata Mukherjee − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3012, United States;
orcid.org/0000-0002-6369-8923
Subrata K. Ghosh − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3012, United States;
orcid.org/0000-0002-1048-9108
Taveechai Wititsuwannakul − Department of Chemistry,
Texas A&M University, College Station, Texas 77843-3012,
United States
(6) (a) Doyle, A. G.; Jacobsen, E. N. Small-Molecule H-Bond
Donors in Asymmetric Catalysis. Chem. Rev. 2007, 107, 5713−5743.
Nattamai Bhuvanesh − Department of Chemistry, Texas A&M
University, College Station, Texas 77843-3012, United States
̌
̌
(b) Zabka, M.; Sebesta, R. Experimental and Theoretical Studies in
Hydrogen-Bonding Organocatalysis. Molecules 2015, 20, 15500−
15524.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00073
(7) (a) Ghosh, S. K.; Lewis, K. G.; Kumar, A.; Gladysz, J. A.
Syntheses of Families of Enantiopure and Diastereopure Cobalt
Catalysts Derived from Trications of the Formula [Co-
(NH2CHArCHArNH2)3]3+. Inorg. Chem. 2017, 56, 2304−2320.
(b) Ehnbom, A.; Ghosh, S. K.; Lewis, K. G.; Gladysz, J. A. Octahedral
Werner complexes with substituted ethylenediamine ligands: a
stereochemical primer for a historic series of compounds now
emerging as a modern family of catalysts. Chem. Soc. Rev. 2016, 45,
6799−6811.
(8) (a) Ganzmann, C.; Gladysz, J. A. Phase Transfer of Enantiopure
Werner Cations into Organic Solvents: An Overlooked Family of
Chiral Hydrogen Bond Donors for Enantioselective Catalysis. Chem. -
Eur. J. 2008, 14, 5397−5400. (b) Lewis, K. G.; Ghosh, S. K.;
Bhuvanesh, N.; Gladysz, J. A. Cobalt(III) Werner Complexes with
1,2-Diphenylethylenediamine Ligands: Readily Available, Inexpensive,
and Modular Chiral Hydrogen Bond Donor Catalysts for
Enantioselective Organic Synthesis. ACS Cent. Sci. 2015, 1, 50−56.
(c) Ghosh, S. K.; Ganzmann, C.; Bhuvanesh, N.; Gladysz, J. A.
Werner Complexes with ω-Dimethylaminoalkyl Substituted Ethyl-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the Welch Foundation (Grant A-1656) for
support.
■
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Organometallics
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(35) (a) This sample cannot be represented as analytically pure, but
the microanalytical data are nonetheless presented to illustrate the
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(36) See Scheme 2 or Table s1 in the Supporting Information for
atom numbering.
(37) Assignments were made based upon 1H−1H COSY, DEPT-90,
1H−13C HSQC, and 1H−13C HMBC NMR experiments. These
spectra are depicted in Appendix C of ref 13b.
(38) The remaining NH signals were not observed.
(39) This signal is due to two overlapping resonances. The intensity
was almost 1.5 times that of the o-C6H5 signal.
M
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Organometallics XXXX, XXX, XXX−XXX