Tricyclic Sulfoxide-Alkene Hybrid Ligands for Chiral Rh(I) Complexes: The "Matched" Diastereomer Catalyzes Asymmetric C-C Bond Formations

Article abstract of DOI:10.1021/acs.organomet.0c00094

Deprotonation of phenyldibenzo[b,f]tropylidene (8) with LDA/t-BuOK followed by quenching with either diastereomer of inexpensive glucose-based t-Bu-sulfinate (R)- or (S)-11 affords a sulfoxide-alkene hybrid ligand as the diastereomeric pairs (SS,SC)-9/(SS,RC)-10 and (RS,RC)-9/(RS,SC)-10, respectively, which via chromatographic/recrystallization may be separated into the four isomers. The optically pure diastereomeric ligands (SS,SC)-9 and (SS,RC)-10 react with [RhCl(coe)2]2 to form the dinuclear complexes (RS,SC)-11 and (RS,RC)-12, respectively, in which the bidentate ligands coordinate the metal centers through the sulfur and alkene donor functions. These complexes catalyze the conjugate addition of arylboronic acids to cyclic Michael acceptors with enantioselectivities of up to 99% ee. DFT calculations show the preponderant influence of planar chirality of the ligand alkene function. The enantioselectivity switch observed between (RS,SC)-11 and (RS,RC)-12 is explained by the inverted cis-trans coordinations of the substrate molecules in catalytic steps.

Full text of DOI:10.1021/acs.organomet.0c00094

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Article
Tricyclic Sulfoxide−Alkene Hybrid Ligands for Chiral Rh(I)
Complexes: The “Matched” Diastereomer Catalyzes Asymmetric
C−C Bond Formations
Alexander Nikol, Ziyun Zhang, Ahmed Chelouan,* Laura Falivene, Luigi Cavallo, Alberto Herrera,
Frank W. Heinemann, Ana Escalona, Sibylle Frieß, Alexander Grasruck, and Romano Dorta*
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Supporting Information
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ABSTRACT: Deprotonation of phenyldibenzo[b,f ]tropylidene (8) with
LDA/t-BuOK followed by quenching with either diastereomer of inexpensive
glucose-based t-Bu-sulfinate (R)- or (S)-11 affords a sulfoxide−alkene hybrid
ligand as the diastereomeric pairs (SS,SC)-9/(SS,RC)-10 and (RS,RC)-9/
(RS,SC)-10, respectively, which via chromatographic/recrystallization may be
separated into the four isomers. The optically pure diastereomeric ligands
(SS,SC)-9 and (SS,RC)-10 react with [RhCl(coe)2]2 to form the dinuclear
complexes (RS,SC)-11 and (RS,RC)-12, respectively, in which the bidentate
ligands coordinate the metal centers through the sulfur and alkene donor
functions. These complexes catalyze the conjugate addition of arylboronic
acids to cyclic Michael acceptors with enantioselectivities of up to 99% ee.
DFT calculations show the preponderant influence of planar chirality of the
ligand alkene function. The enantioselectivity switch observed between
(RS,SC)-11 and (RS,RC)-12 is explained by the inverted cis−trans coordinations of the substrate molecules in catalytic steps.
[Rh(1)2][BF4]4 and [Ir(1)2Cl],5 high selectivity, activity, and
INTRODUCTION
■
stability. Apart from P-alkene ligands, chiral bis-alkenes,6 bis-
The development of new ligand designs for chiral metal
sulfoxides,7 and in particular S(O)-alkene8 hybrids have
complexes is paramount for the advancement of asymmetric
emerged as the ligands of choice for the asymmetric Hayashi−
catalysis.1 For some time, we have been interested in hybrid P-
Miyaura reaction.9 The ease of synthesis and high stability of
alkene ligands such as 1 and 2 based on the tricyclic
enantiopure sulfoxides and the recent finding that polarized
dibenz[b,f]azepine scaffold (see Table 1).2 The azepine−alkene
R2S+−O− ligand functions appear to induce chirality also
function has been proven to be hemilabile,3 a property imparting
metal catalysts, even with a L/M stoichiometry of 2/1 such as
through electrostatic effects10 prompted us to extend the S(O)-
alkene ligand architecture to tricyclic systems. Inspired by
Knochel’s highly effective sulfoxide−alkene 311 and Liao’s
stereoselectivity-switching S(O)-alkenes (both planar-chiral
systems),12 we recently disclosed the planar-chiral sulfonamide
4.13 Since this ligand proved stereochemically stable only at
specific pH values, we were interested in replacing the
sulfinamido S−N bond14 with the more robust sulfoxide S−C
bond of the dibenzotropylidene analogue (i.e., replacing the N
atom with a C−H moiety; see Table 1).15 Here, we describe a
simple protocol for the stereodivergent synthesis of this new
class of enantiopure, tricyclic sulfoxide−alkene ligands, which
Table 1. Inspiring Chiral P-Alkene and S-Alkene Ligands and
the Targeted Tricyclic Sulfoxide−Alkene
Received: February 10, 2020
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are a nice bidentate fit for Rh(I). We show that the rhodium
complex of the matched ligand diastereomer16 is a highly
selective catalyst for the asymmetric Hayashi−Miyaura reaction
and provide mechanistic insight and a stereochemical model by
DFT calculations.
Scheme 2. Symmetric Synthesis of the t-Bu-Sulfoxide−
Alkene Ligand Giving Rise to Diastereomeric Pairs of
Enantiomers rac-9 + rac-10 in a ca. 1/1 Ratio and Their
a
Separation by Chiral Stationary Phase HPLC
RESULTS AND DISCUSSION
■
Exploring ways to construct the C−S bond of the sought-after
sulfoxide-tropylidene analogue of 4, we soon found out that
common sulfinate electrophiles reacted cleanly only with in situ
deprotonated phenyldibenz[a,d]tropylidene (8) (at C9; see
Scheme 1), while the use of more straightforward Grignard or
Scheme 1. Multigram Synthesis of 8 and Its Planar-Chiral
a
Structure in the Crystal
a
The trace shows area/retention times in minutes. Absolute
configurations were assigned by single-crystal XRD analysis of
optically pure ligands (vide infra and the Supporting Information).
Conditions: Daicel Chiralpak AD-H; flow rate: 0.7 mL/min; n-
hexane/i-PrOH = 8/2.
Scheme 3. Stereodivergent Syntheses of the Diastereomeric
S(O)-Alkenes 9 and 10
a
Conditions: (i) PhB(OH)2 (1 equiv), Pd(PPh3)4 (5 mol %), DME,
reflux, 18 h; (ii) HCl, THF, reflux, 72 h; (iii) AlCl3, LiAlH4, Et2O/
THF, reflux, 16 h. Crystal structure of 8 (50% displacement ellipsoids;
H, atoms omitted). Selected distances (Å) and angles (deg): C(1)−
C(2) 1.3465(19), C(1)−C(2)−C(3) 128.71(13), C(2)−C(1)−
C(15) 123.68(12), C8−C9−C10 110.97(11).
lithium nucleophiles obtained from phenyldibenz[a,d]-
tropylidenyl chloride invariably led to impure products.
However, the limited scalability of reported synthetic routes to
817 prompted us to develop a 50 g scale Suzuki coupling of
phenylboronic acid with the bromoalkene precursor 5.18
Deprotection and reduction of product 6 routinely yields 20 g
quantities of white crystalline 8 with a high melting point. In the
crystal, 8 is a planar-chiral racemate with a boat-shaped
tropylidene moiety, while in solution at room temperature it
inverts rapidly via a planar transition state with a DFT-calculated
barrier of 13.1 kcal/mol.19
Before tackling the asymmetric synthesis of the ligand, we
isolated its racemic form: reacting deprotonated 8 with rac-tBu-
S(O)Cl affords the diastereomeric pairs rac-9 + rac-10 because
of the stereogenic benzyl C atom.20 The isomers turned out to
be readily separable by enantioselective HPLC (see Scheme 2),
which indicated that stereochemically stable molecules should
be amenable to asymmetric synthesis. Indeed, deprotonation of
8 with LDA/KOtBu21 followed by in situ quenching at −78 °C
with the inexpensive glucose-derived sulfinate (R)-1122 in THF
gives the alkene−sulfoxides (SS,SC)-9 and (SS,RC)-10 in a ca. 5/3
diastereomeric mixture23 (see Scheme 3). The diastereomers
are separated by flash column chromatography/crystallization24
and isolated on gram scales and in excellent optical purity
(verified by HPLC; see Figures S4 and S5 in the Supporting
Information). (SS,SC)-9 and (SS,RC)-10 are best identified by the
characteristic singlet resonances of the t-Bu groups at 1.24 and
1.19 ppm in their respective H NMR spectra. The ligands do
1
not epimerize in C6D6 solution on prolonged heating at 60 °C.
In order to gain precise structural information for
unambiguous assignment of the absolute configurations of the
stereocenters in 9 and 10 and for identification of the HPLC
peaks in Scheme 2, single-crystal X-ray diffraction analyses were
performed. The molecular structures are depicted in Figure 1
and confirm the expected S-configured sulfur atoms (C−S bond
formation takes place with inversion of configuration) and, most
importantly, reveal the absolute configurations of the carbon
stereocenters, which are S and R, respectively. The stereogenic
sulfur and carbon atoms are pyramidal with average bond angles
of 104.6 and 110.9° for 9 and 104.5 and 110.6° for 10,
respectively. On the other hand, the phenyl-substituted atoms
C9 and C8 are perfectly trigonal planar in both structures
B
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reacted with the (S)-11 diastereomer of the DAG-sulfinate to
afford diastereomers (RS,RC)-9 and (RS,SC)-10 (Scheme 3).25
The crystal structure of (RS,RC)-9 is indeed the enantiomorph of
(SS,SC)-9 (see Figure S7 in the Supporting Information),
showing consistent stereochemistry and further confirming the
HPLC peak assignment.
The coordination behavior of (SS,SC)-9 and (SS,RC)-10 with
Rh(I) was assessed by reacting 2 equiv of the respective ligands
with [RhCl(coe)2]2 (coe = cyclooctene) in benzene or toluene
solution to yield dinuclear complexes (R,S)-11 and (R,R)-12
almost quantitatively as yellow-orange powders (Scheme 4).
The complexes are soluble in CH2Cl2 but much less so in
aromatic solvents. NMR spectra of complex 11 in CD2Cl2
solution show a mixture of syn and anti isomers in a 1/2 ratio,
while complex 12 forms exclusively as the anti isomer. The
perfect anti selectivity of ligand 10 in the synthesis of complex 12
parallels its superior enantioselectivity in catalysis (vide infra)
and the strong diastereoselective interaction with the chiral
stationary phase in HPLC separation causing a large difference
in retention times (Scheme 2, vide supra). Single-crystal X-ray
diffraction analyses of the complexes confirm the bidentate
coordination mode of the S-alkene ligands and show square-planar
coordination geometries around the chloro-bridged Rh centers
(see Figures 2 and 3).26 The butterfly-shaped Rh2Cl2 cores in 11
and 12 span wing angles of 120.4 and 167.5°, respectively.
Exclusive anti coordination is observed in both crystals. In both
complexes, the trans influences exerted by the alkene vs S-donors
differ significantly: Rh−Cl distances trans to the S atoms are
0.06−0.08 Å longer than the corresponding bonds trans to the
alkene donors. The bite angles of the ligands, measured between
the centroids of the alkene functions and the S donors, are very
similar at 93.7° in 11 (average value of two independent ligands)
and 94.5° in 12. The coordinated alkene bonds (C9−C8
1.432(9) Å and C34−C33 1.434(9) Å in 11; C7−C8 1.429(11)
Å in 12) are ca. 0.08 Å longer than those in the respective free
ligands 9 and 10 (1.353(3) and 1.347(2) Å; see Figure 1), and
the C atoms bearing the phenyl substituents are slightly
pyramidalized (sum of angles between C−C bonds is in the
range 354.1−355.1°). These observations reflect metal to alkene
π back-bonding.27 Furthermore, the Rh−C distances to the
phenyl-substituted C atoms are on average 5 pm (for 11) and 8
Figure 1. Molecular structures of (SS,SC)-9 (top) and (SS,RC)-10
(bottom) in the respective chiral crystals (50% displacement ellipsoids;
H atoms are omitted except on C1). Selected distances (Å) and angles
(deg): for (SS,SC)-9, S1−O1 1.4983(17), S1−C1 1.894(3), S1−C22
1.856(3), C8−C9 1.353(3), C9−C16 1.499(4), C22−S1−C1
101.82(11), O1−S1−C1 106.73(11), O1−S1−C22 105.15(11),
C2−C1−S1 110.63(16), C8−C9−C16 116.6(2), C2−C1−C15
114.0(2); for (SS,RC)-10, S1−O1 1.5006(13), S1−C1 1.8714(17),
S1−C16 1.8534(17), C8−C9 1.347(2), C8−C20 1.491(2), C16−S1−
C1 100.32(8), O1−S1−C1 106.74(8), O1−S1−C16 106.37(8), C2−
C1−S1 109.64(11), C9−C8−C20 119.46 (15), C2−C1−C15
111.5(14).
(average bond angles of 120.0°). As a proof of principle that all
four stereoisomers may be accessible, the anion of 8 was also
Scheme 4. Synthesis of Dinuclear Rh(I) Complexes Bearing Ligands 9 and 10
C
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with the two stereocenters working in synergy. The observation
that enantioselection is predominantly governed by the planar
chirality of the coordinated alkene function (and not the
chirality of the S donor) has previously been made by Knochel
and co-workers,11 by Liao and co-workers,12 and by ourselves13
with similar S(O)-alkene ligand systems. In general, while
catalyst 12 gives satisfactory enantioselectivities in additions of
aryl nucleophiles to cyclohexenone, it compares favorably with
the best S(O)-alkene ligands for additions to cyclopentenone
and dihydropyranone.
In order to get a precise mechanistic picture of this reaction
and to rationalize the disparate enantioselectivities observed for
catalysts 11 and 12 (entries 1 and 2, 4 and 5, and 14 and 15 in
Table 2), the prototypical reaction of cyclohexenone 14a with
phenylboronic acid 15a was investigated by DFT calculations.
The first step of the catalytic cycle is well established and consists
of transmetalation of the phenyl nucleophile from the boronic
acid to the metal, affording the nucleophile−rhodium
intermediate (A in Figure 4).29 With this species as the starting
point, set as the zero point energy, the cyclohexenone
electrophile presents its re or si face to the Rh−Ph bond via a
[2 + 2] transition state, leading to the R- or S-configured ketone,
respectively, after hydrolysis. We focused on the coordination/
insertion step affecting the enantioselectivity of the reaction.
The energy profiles for 11 and 12 in Figure 4 show that phenyl
coordination trans to the π-accepting alkene donor is favored in
both cases (11-A and 12-A), while the alternative species A1
with the phenyl groups lying trans to the sulfoxide ligand are
approximately 7 kcal/mol higher in energy. The approach of the
enone to A occurs trans to the sulfoxide ligand via transition state
A-B with energy barriers of 8.8 and 5.0 kcal/mol for 11 and 12,
respectively. From intermediate B the two catalysts behave
differently: in the case of 11, the system prefers first to isomerize
to the lower energy intermediate B1 with the phenyl cis to the
alkene ligand, thereby gaining 5.0 kcal/mol, and then to insert
via transition state B1-C, rather than to insert directly from B. In
fact, both the B → B1 cis/trans isomerization barrier (12.6 kcal/
mol calculated from A) and the following insertion barriers
(16.5 and 19.0 kcal/mol leading to the R- and S-configured
products, respectively), are lower than the direct insertion
barriers from B (18.4 and 20.7 kcal/mol for R- and S-configured
products, respectively), due to a strongly distorted square planar
geometry of the corresponding transition states ascribed to steric
repulsions between the ligand phenyl ring and the phenyl
nucleophile. The free energy difference between the two
competing transition states of 2.5 kcal/mol in favor of pro-(R)
B1-C for 11 implies predominant insertion of the re face of the
enone to the R product, in agreement with experiments. In the
case of catalyst 12, on the contrary, the favored insertion step
occurs from 12-B with the phenyl cis to the sulfoxide ligand and
with energy barriers of 14.9 and 10.6 kcal/mol for the R- and S-
configured products, respectively. The free energy difference
between the two competing transition states of 4.3 kcal/mol in
favor of pro-(S) B-C explains the observed high enantiose-
lectivity for the S product. The alternative pathway with the
phenyl group trans to the sulfoxide ligand was ruled out because
it has higher energy barriers of 15.5 and 17.4 kcal/mol for R- and
S-configured products, respectively (see Figure S40 in the
Supporting Information for a detailed discussion). It is worth
noting that the opposite cis/trans arrangements of the phenyl
and enone substrate molecules in the favored intermediates and
transition states for 11 and 12 are the result of steric effects of the
opposite planar chirality of the phenyl-alkene ligand function.
Figure 2. Molecular structure of anti-(R,S)-11 in the chiral crystal (50%
displacement ellipsoids; H atoms are omitted). Selected distances (Å)
and angles (deg): Rh1−S1 2.1670(15), Rh1−Cl1 2.3777(15), Rh1−
Cl2 2.4535(15), Rh1−C9 2.118(6), Rh1−C8 2.167(6), Rh2−Cl1
2.4414(15), Rh2−Cl2 2.3603(15), Rh2−C34 2.109(6), Rh2−C33
2.150(6), C9−C8 1.432(9), C34−C33 1.434(9), S1−O1 1.485(4),
S1−C1 1.842(6), S1−C22 1.875(6).
Figure 3. Molecular structure of (R,R)-12 in the chiral crystal (50%
displacement ellipsoids; H atoms are omitted). Selected distances (Å)
and angles (deg): Rh1−Cl1 2.4305(16), Rh1−Cl1A 2.3723(16), Rh1−
C8 2.109(7), Rh1−C9 2.192(7), Rh1−S1 2.1590(17), S1−O1
1.481(5), S1−C1 1.846(7), S1−C22 1.880(7), C8−C9 1.429(11),
C9−C16 1.482(10), Rh1A−Cl1−Rh1 98.68(6), S1−Rh1−Cl1A
92.56(6), C8−Rh1−Cl1 89.2(2), C9−Rh1−Cl1 93.96(19).
pm (for 12) longer than the distances to the unsubstituted C
atoms.
With well-characterized optically pure complexes 11 and 12 in
hand, their performance in asymmetric catalysis was bench-
marked in the Hayashi−Miyaura conjugate addition of
arylboronic acids to enones (see Table 1). Standard reaction
conditions consisted of a 1,4-dioxane/H2O solvent system,
Cs2CO3 additive, 40 °C, and a 3 mol % catalyst loading to ensure
complete conversions (>95% isolated yields; optimization of
catalyst activity was not a priority in this study).28 Even though
catalysts (R,S)-11 and (R,R)-12 bear identically configured
sulfur donor atoms, entries 1 and 2, 4 and 5, and 14 and 15 reveal
a preference for opposite configurations in the addition
products. Clearly, the opposite planar chirality of the
coordinated alkene functions that characterizes the two
complexes is the overwhelming factor that determines the
stereochemical outcome in these reactions. Additionally,
complex (R,R)-12 displays vastly superior enantioselectivity,
which identifies ligand 10 as the “matched” ligand diastereomer
D
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Table 2. Catalytic Performance of Complexes 11 and 12 in the Hayashi−Miyaura Conjugate Addition Reaction
a
b
c
Commercial arylboronic acids were used as received. Determined by enantioselective HPLC (see the Supporting Information). Absolute
d
e
configurations are assigned by comparison with reported data. Toluene used instead of dioxane. Assumed configurations.
Thus, the theoretical comparison between the two catalysts is in nice
agreement with experiments (even though the respective absolute
hexenone ring occurs by interaction with only one carbon atom
of the cis-positioned S-t-Bu group, with the planar-chiral
phenyl−alkene ligand function trans to the enone having no
influence (see Figure 5, top right). In 12, the cyclohexenone ring
is cis to the phenyl−alkene ligand and, thus, this function
becomes the determining steric factor with multiple steric
clashes at short distances between the cyclohexenone and the
ligand phenyl group (see Figure 5, bottom left).
Figure 6 schematically summarizes the differences between
the two catalysts in the enantioselective insertion step30 and
shows the steric maps around Rh calculated by the SambVca
software.31 The maps allow rationalizing the results discussed
above as a function of the symmetry of the catalytic pocket. In
both cases, the phenyl group of the alkene donor of the ligand
ΔΔG⧧ values are somehow overestimated by the calculations):
resi
the enantioselectivity of 12 in favor of the S-configured insertion
product is almost 2 kcal/mol higher than that displayed by 11 in
favor of the R product.
An analysis of the favored transition state geometries 11 pro-
(R) B1-C and 12 pro-(S) B-C in Figure 5 (top left and bottom
right structures) highlights the cyclohexenone ring carbons
placed in a rather open space, anti to the S-t-Bu and the alkenyl
phenyl groups, respectively. In contrast, the disfavored transition
states 11 pro-(S) B1-C and 12 pro-(R) B-C are destabilized by
steric repulsion between the cyclohexenone ring and ligand
donor functions. In 11, enantiodifferentiation of the cyclo-
E
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Figure 4. Energy profiles for the insertion step of the Hayashi−Miyaura conjugate addition of cyclohexanone and phenylboronic acid in the presence of
11 (in blue) and 12 (in red). Free energies are given in kcal/mol in 1,4-dioxane solvent.
Figure 5. Optimized geometries of the insertion transition states with 11 (top) and 12 (bottom).
causes more encumbrance (northwest (NW) and southwest
(SW) quadrants for 11 and 12, respectively) in comparison to
the S-t-Bu donor (orange contours in the southeast quadrants
(SE) denote one out of three methyl groups, which in solution
rotate freely). The coordination environment for 11 (left
diagram) is quasi C2 symmetric, while that for 12 (right
diagram) is quasi Cs symmetric, with both ligand donors
obstructing the south quadrants.32 As a consequence, with 12,
the pro-(S) enantiospecific coordination of the enone is more
forced than with 11 in order to avoid the clash of the
cyclohexanone ring with the highly hindered alkene ligand in the
SW quadrant (compare the substrate schemes in Figure 6c and
the hindered quadrants in the maps).
CONCLUSION
■
The stereoselective, divergent synthesis of the four possible
isomers of a new tricyclic sulfoxide−alkene hybrid ligand has
been demonstrated. Diastereomers 9 and 10 are excellent
F
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Figure 6. (a) Stereochemical model (top view) for the prediction of the sense of addition of 14a to 13a when ligand (SS,SC)-9 (left) or ligand (SS,RC)-
10 (right) is used. (b) Steric maps of the respective Rh(I) complexes (RS,SC)-11 and (RS,RC)-12 from crystallographic data. The steric maps are viewed
down the z axis; the orientation of the complexes is indicated on the left. Rh atoms, H atoms, and secondary ligands were excluded in the calculations.
Sphere radii are 3.5 Å, and Bondi radii are scaled at 1.17. The isocontour scheme, in Å, is shown in the middle. Red and blue zones indicate the more
and less hindered zones in the catalytic pocket, respectively. (c) Ball-and-stick representation of the R- and S-configured product formations as in the
favored transition states reported in Figure 5.
Et2O, and benzene were distilled from purple Na/Ph2CO solutions,
toluene was distilled from Na, pentane, C6D6, and THF-d8 were
distilled from Na2K alloy, CH3CN, CH2Cl2, and CD2Cl2 were distilled
from CaH2, and NEt3 and 1,4-dioxane were distilled from K. CD3CN
and CDCl3 were degassed with three freeze−pump−thaw cycles and
then kept in a glovebox over activated molecular sieves (3 and 4 Å,
respectively). Arylboronic acids, LDA (purchased from Sigma-
Aldrich), and DME (from TCI) were used as received. 5,18 (RS)- and
(SS)-1,2:5,6-di-O-isopropylidene-α-D-glucofuranosyl tert-butylsulfinate
((R)-11 and (S)-11),34 PhLi,35 [RhCl(coe)2]2,36 and tert-butanesul-
finyl chloride37 were prepared according to published procedures.
LiAlH4 (Sigma-Aldrich) was extracted in Et2O and used as a snow-
white crystalline powder. Elemental analyses (EA) were performed on a
Euro EA 3000 analyzer, and air-sensitive samples were handled and
prepared in a glovebox. NMR spectra were recorded on Jeol EX 270,
ECP 400, and ECX 400 instruments operating at 269.71, 399.78, and
400.18 MHz for 1H, at 67.82, 100.52, and 100.62 MHz for 13C, and at
161.83 and 162.00 MHz for 31P, respectively. Chemical shifts are given
in ppm and are reported relative to residual solvent peaks as secondary
standards.38 Jeol’s Delta NMR Processing and Control Software was used
to process and visualize the NMR data.39 HPLC was performed on a
Shimadzu LC10 series instrument.
10-Phenyl-5,5-ethylenedioxy-5H-dibenzo[a,d]cycloheptene
(6). 5 (55.0 g, 168 mmol), phenylboronic acid (24.4 g, 197 mmol),
Pd(PPh3)4 (5.58 g, 4.80 mmol), degassed DME (1.38 L), and Na2CO3
(26.2 g, 247 mmol, 2 M in H2O, 123.75 mL) were charged into a
Schlenk vessel, and the reaction mixture was heated to reflux for 40 h.
After the mixture was cooled to room temperature, CH2Cl2 (500 mL)
and water (300 mL) were added. The organic phase was separated and
the aqueous phase extracted with methylene chloride (3 × 150 mL).
The combined organic phases were dried over anhydrous Na2SO4, and
evaporation of the solvent provided solid crude 5, which was slurried in
pentane (500 mL), filtered, and washed with additional pentane (3 ×
150 mL) to yield a yellow solid (52 g, 95%). Mp: 145 °C. Anal. Found:
bidentate ligands for Rh(I), and the respective isolated
complexes 11 and 12 catalyze the asymmetric Hayashi−Miyaura
reaction. The superior enantioselectivity displayed by complex
12 bearing the “matched” ligand diastereomer (SS,RC)-10 and
the reversal and lowering of chiral induction observed with
complex 11 bearing the ligand diastereomer (SS,SC)-9 (with a
sulfur donor of chirality identical with that of the ligand (SS,RC)-
10) is explained by DFT calculations. 11 and 12 promote similar
catalytic cycles with enantioselection occurring in the insertion
step, with the difference that the favored insertion transition
states in 11 and 12 present opposite cis/trans arrangements of
the enone/phenyl substrates with respect to the ligand sulfur
and alkene donors. We conclude that in electronically
dissymmetric S-alkene ligand systems sterics and electronics
(leading to trans effects in the square-planar Rh(I) inter-
mediates) are operative in the Hayashi−Miyaura reaction and
that ligand geometries that create quasi C2 coordination
environments on Rh should be avoided. This is in contrast to
well-established electronically and C2-symmetric bis-alkene, bis-
sulfoxide, and bis-phosphine systems.33 Finally, the remarkable
enantioselectivities achieved with ligand 10 in its prototype
version for the Hayashi−Miyaura reaction bodes well for other
applications, not least because the simple synthetic protocol for
this ligand is amenable to modification for steric and electronic
optimization at both donor functions.
EXPERIMENTAL SECTION
■
Experiments involving sensitive compounds were carried out under
anaerobic and anhydrous conditions, using standard Schlenk and inert-
gas glovebox techniques. Technical grade EtOAc and hexanes for flash
column chromatography were purified by rotary evaporation. THF,
G
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1
C, 84.86; H, 5.60. Calcd for C23H18O2: C, 84.64; H, 5.56. H NMR
[α]D25 = −165° (c = 1.0, CH2Cl2). Anal. Found: C, 80.23; H, 6.50; S,
1
(400 MHz, CDCl3): δ 7.92−7.86 (m, 2H, ArH), 7.52−7.17 (m, 12H,
8.31. Calcd for C25H24OS: C, 80.61; H, 6.49; S, 8.61. H NMR (600
3
ArH), 4.25 (t, JH,H = 8.0 Hz, 2H, OCH2R), 3.84−3.74 (m, 2H,
MHz, CDCl3): δ 7.67 (d, JH,H = 6.0 Hz, 2H, ArH), 7.53−7.26 (m, 9 H,
ArH), 7.24 (m, 2H, ArH), 7.16 (d, JH,H = 6.0 Hz, 1H, ArH), 5.27 (s, 1H,
S−CHR2), 1.24 (s, 9H, C(CH3)3) ppm. 13C NMR (151 MHz, CDCl3):
δ 145.5, 144.2, 137.5, 136.6, 136.0, 135.4, 130.8, 130.7, 129.9, 129.7,
129.6, 129.3, 129.2, 128.5, 128.4, 127.9, 127.8, 127.6, 71.6, 55.7, 23.6
ppm. HPLC (Daicel Chiralpak AD-H column, hexane/iPrOH 9:1, 0.7
mLmin−1): tR2 = 23.34 min (major). Recrystallization of (SS,RC)-10 by
layering a saturated and filtered EtOAc solution with hexane affords
material with ee > 99.5% (1.03 g, 18%). Mp: 136 °C. [α]D25 = −20.6° (c
= 1.0, CH2Cl2). Anal. Found: C, 80.76; H, 6.48; S, 8.49. Calcd for
C25H24OS: C, 80.61; H, 6.49; S, 8.61. 1H NMR (600 MHz, CDCl3): δ
7.42−7.26 (m, 12H, ArH), 7.14−7.08 (m, 2H, ArH), 5.16 (s, 1H, S-
CHR2), 1.19 (s, 9H, C(CH3)3) ppm .13C NMR (151 MHz, CDCl3): δ
143.8, 142.8, 137.2, 136.6, 135.8, 135.3, 131.8, 131.0, 130.9, 129.9,
129.3, 129.2, 129.0, 128.7, 128.5, 128.2, 127.8, 127.6, 70.8, 55.5, 23.8
ppm. HPLC (Daicel Chiralpak AD-H column, hexane/iPrOH 8/2, 0.7
mLmin−1): tR1 = 26.45 min (major).
OCH2R) ppm. 13C NMR (151 MHz, CDCl3): δ 144.2, 143.4, 140.3,
139.3, 134.8, 133.4, 130.3, 129.5, 129.4, 129.1, 128.3, 128.1, 127.5,
127.3, 127.1, 123.5, 123.3, 106.5, 64.8, 64.2 ppm.
10-Phenyl-5H-dibenzo[a,d]cyclohepten-5-one (7). Aqueous
HCl (0.56 L, 6.0M, 3.4 mol) was added to a solution of 6 (46.0 g, 141
mmol) in THF (500 mL) and the mixture refluxed for 3 days. After the
mixture was warmed to RT, 1.0 M NaOH was added until pH >7. The
phases were separated, and the aqueous phase was extracted three times
with EtOAc (350 mL). The combined organic phases were dried over
anhydrous Na2SO4. The solvent was removed in vacuo. The crude liquid
was distilled to remove glycol, the resulting solid was dissolved in EtOH
(200 mL), and the suspension was heated to reflux. After 30 min, the
mixture was filtered over Celite 545 (hot filtration). The orange
solution was evaporated until crystals formed. To favor further
crystallization, the suspension was chilled in the refrigerator to yield
the product 5 (36.0 g, 92%). Mp: 120 °C. Anal. Found: C, 89.11; H,
1
5-(RS)-tert-butylsulfinyl-10-phenyl-5H-dibenzo[a,d]-
cycloheptene ((RS,RC)-9 + (RS,SC)-10).25 A solution of LDA (883
mg, 8.24 mmol) in THF (10 mL) was added slowly via syringe to a
stirred solution of 8 (2.21 g, 8.24 mmol) in THF (40 mL) at −78 °C,
followed by slow addition of a solution of t-BuOK (925 mg, 8.24 mmol)
in THF (10 mL). The reaction mixture was stirred at −78 °C for 4 h and
then transferred via cannula to a solution of (S)-11 (2.50 g, 6.86 mmol)
in THF (20 mL) at −78 °C. The reaction mixture was stirred at −78 °C
and monitored by TLC (hexane/diethyl ether 1/1). Once the reaction
was completed, it was quenched with saturated aqueous NH4Cl (75
mL). The aqueous phase was separated and extracted with EtOAc (3 ×
60 mL). The combined organic phases were washed with brine (60 mL)
and dried over Na2SO4, and the volatiles were evaporated in vacuo to
give a crude diastreoisomeric mixture (4.25 g, 74%, dr 5/3). This
mixture was separated by column chromatography (hexane/AcOEt 2/
1) and each of the diastereomers recrystallized by layering a saturated
and filtered THF solution with pentane to yield (RS,RC)-9 (1.22 g, 48%,
ee = 89%) and (RS,SC)-10 (0.62 g, 24%, ee = 80%) as white solids.
Data for (RS,RC)-9 are as follows. HPLC (Daicel Chiralpak AD-H
column, hexane/iPrOH 8/2, 0.7 mL min−1): tR = 7.45 min (major), tR =
20.81 min (minor). Anal. Found: C, 80.74; H, 6.48; S, 8.28. Calcd for
C25H24OS: C, 80.61; H, 6.49; S, 8.61. 1H NMR (600 MHz, CDCl3): δ
7.54 (d, JH,H = 7.34 Hz, 2H, ArH), 7.40−7.20 (m, 9H, ArH), 7.13 (m,
2H, ArH), 7.02 (d, JH,H = 7.7 Hz, 1H), 5.14 (s, 1H, S−CHR2), 1.15 (s,
9H, C(CH3)3) ppm. 13C NMR (151 MHz, CDCl3): δ 145.341, 144.12,
137.43, 136.46, 135.83, 130.72, 130.66, 129.74, 129.50, 129.23, 129.09,
128.34, 127.67, 127.48, 127.74, 71.49, 55.55, 23.42 ppm.
Data for (RS,SC)-10 are as follows. HPLC (Daicel Chiralpak AD-H
column, hexane/iPrOH 8/2, 0.7 mL min−1): tR = 22.32 min (minor), tR
= 69.76 min (major). 1H NMR (400 MHz, CDCl3): δ, 7.47−7.31 (m,
12H, ArH), 7.31−7.15 (m, 2H, ArH), 5.22 (s, 1H, S−CHR2), 1.24 (s,
9H, C(CH3)3) ppm. 13C NMR (151 MHz, CDCl3): δ 145.5, 144.2,
143.8, 142.8, 137.5, 137.2, 136.5, 135.9, 135.8, 135.4, 135.3, 131.8,
131.0, 130.9, 130.8, 129.9, 129.7, 129.6, 129.3, 129.3, 129.2, 129.2,
129.0, 128.7, 128.5, 128.5, 128.2, 127.9, 127.8, 127.8, 127.6, 127.6, 71.6,
70.7, 55.7, 55.5, 23.8, 23.5 ppm.
[((SS,SC)-9)RhCl]2 ((R,S)-11). A solution of (SS,SC)-9 (286 mg,
0.766 mmol) in benzene (2.0 mL) was added dropwise to a stirred
solution of [RhCl(COE)2]2 (275 mg, 0.383 mmol) in benzene (2.0
mL). The mixture was stirred for 2 h, and then the volatiles were
evaporated and the crude product was slurried in hexane (10 mL). The
solid was separated by filtration and dried in vacuo to yield a yellow
powder (379 mg, 97%). Anal. Found: C, 58.88; H, 4.78; S, 6.01; Calcd
for C21H16Cl2S2O2Rh2: C, 58.77; H, 4.73; S, 6.28. NMR spectra
indicate the presence of syn and anti isomers in a ratio of approximately
1/3. 1H NMR (400 MHz, CD2Cl2): δ 8.56 (d, JH,H = 4 Hz, 2H, ArH,
major isomer), 8.20 (d, JH,H = 8 Hz, 2H, ArH, minor isomer), 7.26−
6.44 (m, 26H, ArH, major isomer), 7.26−6.44 (m, 26H, ArH, minor
isomer), 5.16 (s, 2H, S-CHR2, minor isomer), 5.08 (s, 2H, S-CHR2,
major isomer), 1.20 (s, 18H, C(CH3)3, minor isomer), 1.04 (s, 18H,
C(CH3)3, major isomer) ppm. 13C NMR (400 MHz, CD2Cl2): δ
4.89. Calcd for C21H14O: C, 89.34; H, 5.00. H NMR (270 MHz,
CDCl3): δ 8.02−7.98 (m, 2H, ArH), 7.61−7.38 (m, 9H, ArH), 7.24−
7.17 (m, 2H, ArH) ppm. 13C NMR (151 MHz, CDCl3): δ 195.7, 144.1,
142.8, 141.1, 139.3, 135.6, 134.4, 131.6, 131.1, 130.7, 130.33, 130.28,
129.38, 128.92, 128.69, 128.48, 128.45, 128.39, 127.72 ppm.
10-Phenyl-5H-dibenzo[a,d]cycloheptene (8).40 A solution of
AlCl3 (11.9 g, 89.5 mmol) in Et2O (140 mL) was added to a solution of
LiAlH4 (3.39 g, 89.5 mmol) in Et2O (140 mL). The resulting
suspension was stirred for 15 min at RT, followed by cooling to 0 °C.
Then a solution of 7 (25.0 g, 88.6 mmol) in THF (115 mL) was added
dropwise over 20 min and the mixture heated to reflux overnight. The
resulting yellowish suspension was cooled to 0 °C and quenched with
water (80 mL). The aqueous layer was separated and extracted with
Et2O (3 × 200 mL). The combined organic phases were washed with
water (3 × 200 mL), dried over Na2SO4, and evaporated in vacuo (23.3
g, 98%). Mp: 117 °C. Anal. Found: C, 93.69; H, 5.93. Calcd for C21H16:
C, 93.99; H, 6.01. 1H NMR (270 MHz, CDCl3): δ 7.48−7.31 (m, 11H,
ArH), 7.18−6.98 (m, 2H, ArH), 3.77 (s, 2H, RCH2RI) ppm. 13C NMR
(101 MHz, CDCl3): δ 144.1, 143.6, 140.2, 139.3, 136.4, 135.3, 129.9,
129.5, 129.2, 128.6, 128.3, 128.1, 127.4, 127.3, 127.1, 125.9, 125.6, 41.5
ppm.
rac-5-tert-Butylsulfinyl-10-phenyl-5H-dibenzo[a,d]-
cycloheptene (rac-9 + rac-10). LDA (321 mg, 3.00 mmol) in THF
(10 mL) was added dropwise to a stirred solution of 8 (400 mg, 1.50
mmol) in THF (15 mL). After the reaction mixture was stirred
overnight, rac-2-methylpropane-2-sulfinic chloride (211 mg, 1.50
mmol) in THF (30 mL) was added. Stirring was continued for 4 h,
and then the solvent was removed in vacuo, the crude product purified
by FLASH column chromatography (EtOAc), and the resulting yellow
oil washed and slurried with pentane (75 mg, 13%). HPLC (Daicel
Chiralpak AD-H column, hexane/i-PrOH 8/2, 0.7 mLmin−1): tR1
10.70 min, tR2 = 26.78 min, tR3 = 28.33 min, tR4 = 84.65 min.
=
5-((S)-tert-Butylsulfinyl)-10-phenyl-5H-dibenzo[a,d]-
cycloheptene ((SS,SC)-9 and (SS,RC)-10). To a solution of 8 (5.00 g,
18.6 mmol) in THF (80 mL) at −78 °C under Ar was added a solution
of LDA (2.00 g, 18.7 mmol) in THF (25 mL) slowly via a syringe in one
portion, followed by slow addition of a solution of t-BuOK (2.09 g, 18.7
mmol) in THF (25 mL). The reaction mixture was stirred at −78 °C for
4 h. The reaction mixture was transferred via cannula to a solution of
(R)-11 (5.65 g, 15.5 mmol) in THF (100 mL) at −78 °C. The reaction
mixture was stirred at −78 °C for 3 h and was slowly warmed to RT with
stirring overnight. After the reaction was quenched with aqueous
NH4Cl (75 mL, saturated), EtOAc (125 mL) was added. The aqueous
phase was extracted with EtOAc (3 × 60 mL). The combined organic
phases were washed with brine (60 mL) and dried over Na2SO4. The
solvent was removed in vacuo to afford 4.25 g of the crude
diastreoisomeric mixture, which was separated by column chromatog-
raphy (hexane/AcOEt 2/1) to give (SS,SC)-9 (2.65 g, 46%, ee 95%) and
(SS,RC)-10 (1.59 g, 28%, ee 80%) as white solids. Recrystallization of
(SS,SC)-9 by layering a saturated and filtered THF solution with
pentane affords material with ee > 99.5% (2.10 g, 36%). Mp: 143 °C.
H
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146.31, 146.18, 139.91, 139.62, 138.91, 136.10, 136.00, 134.35, 131.57,
131.30, 130.87, 129.38, 128.92, 128.12, 127.74, 127.59, 127.34, 127.34,
125.65, 79.83, 78.00, 71.92, 71.45, 68.55, 63.04, 62.00, 26.39, 26.41
ppm. X-ray diffraction quality single crystals were grown from a filtered
THF solution of the complex, which had been layered with pentane.
[((SS,RC)-10)RhCl]2 ((R,R)-12). A solution of (SS,RC)-10 (400 mg,
1.07 mmol) in benzene (4.0 mL) was added dropwise to a stirred
solution of [RhCl(COE)2]2 (385 mg, 0.537 mmol) in benzene (4.0
mL), and the mixture was stirred overnight. After the solvent was
removed under reduced pressure, the residue was slurried in pentane
(15 mL), filtered, and vacuum-dried to afford a red-orange powder (566
mg, 97%). Anal. Found: C, 58.84; H, 4.85; S, 6.04. Calcd for
Information). HPLC (Daicel Chiralpak AS-H column, hexane/iPrOH
6/4, 0.7 mLmin−1): ee = 94%, tR1 = 35.80 min, tR2 = 44.94 min (major).
Crystallographic Information. CCDC-1965291 for rac-8,
CCDC-1965292 for (RS,RC)-9, CCDC-1965293 for (SS,SC)-9,
CCDC-1965294 for (SS,RC)-10, CCDC-1965295 for (R,S)-11, and
CCDC-1965296 for (R,R)-12 contain supplementary crystallographic
data for this paper. The data can be obtained free of charge from
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge,
CB2 1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.
uk).
Intensity data of 8 were collected using Cu Kα radiation (λ = 1.54184
Å) on an Oxford Diffraction SuperNova dual radiation diffractometer
with mirror optics. Intensity data of single crystals of the other
compounds were collected using Mo Kα radiation (λ = 0.71073 Å) on
either a Bruker Smart APEX 2 diffractometer (curved graphite
monochromator) for (RS,RC)-9 and (SS,SC)-9 or a Bruker Kappa
APEX 2 IμS Duo diffractometer equipped with QUAZAR focusing
Montel optics for (SS,RC)-10, (R,S)-11, and (R,R)-12. Data were
corrected for Lorentz and polarization effects, and semiempirical
absorption corrections were performed on the basis of multiple scans
using SADABS.41 The structures were solved by direct methods
(SHELX XT 2014/5)42 and refined by full-matrix least-squares
procedures on F2 using SHELXL 2016/6.43 All non-hydrogen atoms
were refined with anisotropic displacement parameters. All hydrogen
atoms were placed in positions of optimized geometry, and their
isotropic displacement parameters were tied to those of the
corresponding carrier atoms by a factor of either 1.2 or 1.5. Compound
(R,S)-11 crystallized with four molecules of tetrahydrofuran in its
asymmetric unit. In (R,R)-12 the complex molecule was situated on a
crystallographic 2-fold rotation axis. This compound crystallized with a
total of seven molecules of benzene per formula unit. Three out of the
five independent benzene molecules were situated on crystallographic
2-fold rotation axes. Similarity restraints were applied to the anisotropic
displacement parameters of the atoms of some solvent molecules.
Pseudoisotropic restraints were applied to the anisotropic displacement
parameters of all carbon atoms. The overall crystal quality was rather
poor. There were two significant residual electron density maxima
observed. These were attributed to truncation effects, as they could not
be attributed to any disorder. There were also no signs of twinning (see
also K value statistics).
1
C21H16Cl2S2O2Rh2: C, 58.77; H, 4.73; S, 6.28. H NMR (270 MHz,
benzene-d6): δ 8.18 (d, JH,H = 7.8 Hz, 2H, ArH), 7.62 (d, JH,H = 7.8 Hz,
2H, ArH), 7.44−6.80 (m, 24H, ArH), 5.38 (s, 2H, S-CHR2), 1.19 (s,
18H, C(CH3)3) ppm. X-ray diffraction quality single crystals were
grown from a saturated and filtered benzene solution of the complex.
General Procedure for the Asymmetric Hayashi−Miyaura
Reaction. In an inert-gas glovebox, a capped 20 mL vial with a
magnetic stir bar was charged with 1.0 equiv of the enone, 2.0 equiv of
boronic acid, and 0.03 equiv of either catalyst 11 or 12. The reactions
were performed on a 1−2 mmol scale. 1,4-Dioxane (3.0−6.0 mL) was
added, and the reaction mixture was stirred for 30 min at 40 °C. A 0.5
equiv portion of aqueous Cs2CO3 (1.0 M, 0.5−1.0 mL) was added by
syringe and the reaction mixture stirred for 24−28 h at 40 °C. Aqueous
workup was performed by the addition of H2O (5−10 mL) and EtOAc
(5−10 mL). The phases were separated, and the aqueous phase was
further extracted with EtOAc (3 × 15 mL). The combined organic
phases were washed with brine (20 mL), dried over MgSO4, filtered,
and evaporated to dryness. The crude products, obtained as colorless or
pale yellow oils or as white or off-white solids, were impregnated on
silica G60 and purified by flash chromatography using n-hexane/EtOAc
in varying ratios as eluent. This procedure affords practically
quantitative isolated yields (>95%). Enantioenriched products 16aa−
16ai, 16ba−16be, 16bg−16bi, 16ca, 16cb, and 16cf−16ci have been
previously reported. For references, pertinent HPLC traces, and a
general protocol for the synthesis of racemic reference substances, see
the Supporting Information.
3-(3-Methoxyphenyl)cyclopentan-1-one (16bf). The general
procedure outlined above was followed using catalyst 12 and 1.08
mmol of 2-cyclopenten-1-one. Purification by flash chromatography
(hexane/EtOAc 9/1) afforded a colorless oil (202 mg, 98%). NMR
spectroscopic data correspond to the racemic reference substance (see
the Supporting Information). HPLC (Chiracel AS-H column, hexane/
iPrOH 99/1, 0.6 mLmin−1): ee = 96%, tR1 = 65.71 min, tR2 = 69.98 min
(major).
4-(4-(tert-Butyl)phenyl)tetrahydro-2H-pyran-2-one (16cc). The
general procedure outlined above was followed using catalyst 12 and
0.94 mmol of 5,6-dihydro-2H-pyran-2-one. Purification by flash
chromatography (hexane/EtOAc 2/1) afforded a yellowish oil that
slowly solidified at 0 °C (218 mg, 99%). NMR spectroscopic data
correspond to the reference substance (see the Supporting
Information). HPLC (Daicel Chiralpak AD-H column, hexane/
iPrOH 95/5, 0.7 mL min−1): ee = 97%, tR1 = 16.28 min (major), tR2
= 17.24 min.
Olex2 was used to prepare material for publication.44 Crystallo-
graphic data, data collection, and structure refinement details are given
in Table S1 in the Supporting Information.
Computational Details. Geometries were optimized with the
Gaussian09 package using the PBE0-D3 functional. The electronic
configuration of the system was described with the split-valence SVP
basis set for main-group atoms (C, H, S, and O) and the relativistic
Stuttgart−Dresden effective core potential with the associated valence
triple-ζ basis set for Rh. All geometries were confirmed as a minimum or
transition state through frequency calculations. The reported free
energies were built through single-point energy calculations on the
PBE0-D3 geometries using the PBE0-D3 functional and the triple-ζ
TZVP basis set for main-group atoms. Solvent effects were included
with the PCM model using 1,4-dioxane as the solvent. To this PBE0-
D3/TZVP electronic energy in solvent were added thermal corrections
from the gas-phase frequency calculations at the PBE0-D3/SVP level.
4-(4-Fluorophenyl)tetrahydro-2H-pyran-2-one (16cd). The gen-
eral procedure outlined above was followed using catalyst 12 and 1.06
mmol of 5,6-dihydro-2H-pyran-2-one. Purification by flash chromatog-
raphy (hexane/EtOAc 2/1) afforded a colorless oil that slowly solidified
(196 mg, 95%). NMR spectroscopic data correspond to the racemic
reference substance (see the Supporting Information). HPLC (Daicel
Chiralpak AS-H column, hexane/iPrOH 8/2, 1.0 mL min−1): ee = 94%,
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00094.
t
R1 = 25.28 min, tR2 = 27.47 min (major).
4-(4-Methoxyphenyl)tetrahydro-2H-pyran-2-one (16ce). The
NMR spectra of ligands, complexes, and catalysis
products, ORTEP of (SS,SC)-9, HPLC traces of ligands
and catalysis products, and synthetic procedures for
racemic reference substances (PDF)
general procedure outlined above was followed using catalyst 12 and
1.03 mmol of 5,6-dihydro-2H-pyran-2-one. Purification by flash
chromatography (hexane/EtOAc 2/1) afforded a colorless oil that
solidified on prolonged standing (203 mg, 95%). NMR spectroscopic
data correspond to the racemic reference substance (see the Supporting
Cartesian coordinates for the calculated structures (XYZ)
I
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Article
Accession Codes
computer time, this research used the resources of the
Supercomputing Laboratory (KSL) at KAUST.
CCDC 1965291−1965296 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
REFERENCES
■
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Ahmed Chelouan − Department Chemie und Pharmazie,
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■
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fau.de
Authors
Alexander Nikol − Department Chemie und Pharmazie,
Anorganische und Allgemeine Chemie, Friedrich-Alexander-
̈
̈
Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany
Ziyun Zhang − Catalysis Research Center, Physical Sciences and
Engineering Division, King Abdullah University of Science and
Technology, Thuwal, Saudi Arabia
(3) For crystallographically authenticated ligand hemilability, see: (a)
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1509-6191
Luigi Cavallo − Catalysis Research Center, Physical Sciences and
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Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany;
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Ana Escalona − Department Chemie und Pharmazie,
Anorganische und Allgemeine Chemie, Friedrich-Alexander-
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Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany
Sibylle Frieß − Department Chemie und Pharmazie,
Anorganische und Allgemeine Chemie, Friedrich-Alexander-
̈
̈
Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany
Alexander Grasruck − Department Chemie und Pharmazie,
Anorganische und Allgemeine Chemie, Friedrich-Alexander-
̈
̈
Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00094
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Anthony Linden (University of Zurich) for
solving the crystal structure of 8 and Ms. Christina Wronna for
carrying out the elemental analyses. Financial support by
Friedrich−Alexander University and King Abdullah University
of Science and Technology (KAUST) is acknowledged. For
Tenten, C.; Lutzen, A.; Strand, D. Control of Enantioselectivity in
̈
Rhodium(I) Catalysis by Planar Chiral Dibenzo[a,e]-
J
https://dx.doi.org/10.1021/acs.organomet.0c00094
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