Development of an axially chiral sp3P/sp3NH/sp2N-combined linear tridentate ligand - Fac-selective formation of Ru(II) complexes and application to ketone hydrogenation

Article abstract of DOI:10.1016/j.tet.2016.02.007

A newly developed chiral linear tridentate ligand, R-PN(H)N (R=H or Ph), possesses Ph₂P and PyCH₂NH groups at C(2) and C(2′) positions of the 1,1′-binaphthyl skeleton without or with a C(3)-Ph substituent. The steric effect of C(3)-Ph and the electronic effect of the DMSO co-ligand realize the facial selective generation of fac-RuCl₂(Ph-PN(H)N)(dmso) and fac-[Ru(H-PN(H)N)(dmso)₃](BF₄)₂, respectively. Both an H-Ru?sp³N-H reaction site responsible for the donor-acceptor bifunctional catalyst (DACat) and a fence/plane chiral context were constructed by means of the following advantageous points: i) the sp³P, sp³N, and sp²N ligating atoms have different electronic properties; ii) DMSO trans to sp³N strongly coordinates to Ru and is fixed by a PyC(6)H?O=S hydrogen bond; and iii) the single NH function simplifies the DACat reaction site. The synergistic effect has led to success in the asymmetric hydrogenation of sterically demanding ketones. Structural characteristics of first-row transition metal complexes of R-PN(H)N have been also investigated.

Full text of DOI:10.1016/j.tet.2016.02.007

Accepted Manuscript
3
3
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Development of an axially chiral sp P/sp NH/sp N-combined linear tridentate
ligand—fac-Selective formation of Ru(II) complexes and application to ketone
hydrogenation
Tomoya Yamamura, Satoshi Nakane, Yuko Nomura, Shinji Tanaka, Masato Kitamura
PII:
S0040-4020(16)30070-9
10.1016/j.tet.2016.02.007
TET 27477
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 24 December 2015
Revised Date: 1 February 2016
Accepted Date: 2 February 2016
Please cite this article as: Yamamura T, Nakane S, Nomura Y, Tanaka S, Kitamura M, Development of
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an axially chiral sp P/sp NH/sp N-combined linear tridentate ligand—fac-Selective formation of Ru(II)
complexes and application to ketone hydrogenation, Tetrahedron (2016), doi: 10.1016/j.tet.2016.02.007.
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Development of an axially chiral
sp3P/sp3NH/sp2N-combined linear tridentate
ligand—fac-Selective formation of Ru(II)
complexes and application to ketone
hydrogenation
Leave this area blank for abstract info.
Tomoya Yamamura, Satoshi Nakane, Yuko Nomura, Shinji Tanaka, and Masato Kitamura*
Graduate School of Pharmaceutical Sciences, Graduate School of Science, and Research Center for Materials
Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan
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Tetrahedron
journal homepage: www.elsevier.com
Development of an axially chiral sp3P/sp3NH/sp2N-combined linear tridentate
ligand—fac-Selective formation of Ru(II) complexes and application to ketone
hydrogenation
Tomoya Yamamura, Satoshi Nakane, Yuko Nomura, Shinji Tanaka, and Masato Kitamura*
Graduate School of Pharmaceutical Sciences, Graduate School of Science, Research Center for Materials Science, Nagoya University, Chikusa, Nagoya
464-8601, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A newly developed chiral linear tridentate ligand, R-PN(H)N (R = H or Ph), possesses Ph2P and
PyCH2NH groups at C(2) and C(2’) positions of the 1,1’-binaphthyl skeleton without or with a
C(3)-Ph substituent. The steric effect of C(3)-Ph and the electronic effect of the DMSO co-
ligand realize the facial selective generation of fac-RuCl2(Ph-PN(H)N)(dmso) and fac-[Ru(H-
PN(H)N)(dmso)3](BF4)2, respectively. Both an H–Ru---sp3N–H reaction site responsible for the
donor–acceptor bifunctional catalyst (DACat) and a fence/plane chiral context were constructed
by means of the following advantageous points: i) the sp3P, sp3N, and sp2N ligating atoms have
different electronic properties; ii) DMSO trans to sp3N strongly coordinates to Ru and is fixed
by a PyC(6)H---O=S hydrogen bond; and iii) the single NH function simplifies the DACat
reaction site. The synergistic effect has led to success in the asymmetric hydrogenation of
sterically demanding ketones. Structural characteristics of first-row transition metal complexes
of R-PN(H)N have been also investigated.
Available online
Keywords:
Chiral ligand
Tridentate
fac-Ru complex
Catalysis
Hydrogenation
2009 Elsevier Ltd. All rights reserved
.
hesitation toward the development of unsymmetrical linear
polydentate ligands even when the denticity is only three. An
intrinsic disadvantage associated with linear tridentate ligands is
the geometry isomerism that generates facial (fac) and
meridional (mer) diastereomers in an octahedral metal complex
1. Introduction
The steric and electronic properties of chiral ligands exert a
significant effect on the performance of asymmetric molecular
metal catalysis.1 The chiral ligand should be appropriately
designed and synthesized in accordance with properties of the
central metal ion, including size, coordination number, and
oxidation state change during a given reaction. The denticity,
hapticity, hybridization of ligating atoms, and geometric
isomerism should also be sufficiently considered so that the
catalytic cycle turns over smoothly with efficient selection of the
enantioface, enantioatom, or enantiomer of the substrate to give
*
the chiral product with high enantiomer ratio (er).
Most privileged chiral ligands2 comprise C2-symmetric
bidentate organic molecules with the same ligating atoms,
thereby theoretically reducing the number of reactive catalytic
species. Increasing the types of ligating atom and denticity, as
well as loss of symmetry, exponentially increases the number of
ligand candidates, which raises the possibility of developing
high-performance asymmetric catalysts. On the other hand, too
many possibilities make the design concept difficult, provoking
———
Fig. 1. (a) Geometry isomerism in an octahedral metal complex of a
linear and flexible tridentate ligand and (b) general solutions to avoid the
problem in the design of chiral tridentate ligands.
* Corresponding author. E-mail address: kitamura@os.rcms.nagoya-u.ac.jp
(M. Kitamura).

2
Tetrahedron
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(Fig. 1a).
With tetrahedral, square planar, or trigonal
3a) to realize, for example, the hydrogenation of ketones.14–16
bipyramidal complexes, the problem is not serious, as
demonstrated in asymmetric reactions using Li, Zn, Cu, or Pd
chemistry.3 Although dependent on ligand structure, the higher
thermodynamic stability of mer isomers as compared with fac
isomers also tends to lessen, but not completely solve, the
problem. There are mainly two types of positive approach to
completely avoid such isomeric ambiguity, as shown in Fig. 1b:
i) abandonment of the linear property; and ii) introduction of
rigidity into the linear ligand skeleton. Geometrical restriction
enables fac- or mer-selective formation of the octahedral metal
complex. The former case i is exemplified by chirally
substituted arene/Cp-type ligands,4 scorpionate-type branched
tripod ligands,5 and crown-type macrocyclic tridentate ligands.6
The latter case ii is represented by Pincer-type ligands
possessing a benzene, pyrrole, or pyridine core with symmetrical
or unsymmetrical side wingtips7 and Schiff base-type ligands
derived from salicylaldehyde, 2-formylpyridine, or a 1,3-
diketone.8
The single sp3NH function in the R-PN(H)N ligand has the
advantage of reducing the possible Intramol-DACat reactive
sites in comparison to the corresponding multiple sp3NH2-
and/or sp3NH-containing systems.17 Assuming an octahedrally
configured fac- and mer-M(R-PN(H)N)L3 complex (L is a
neutral or anionic auxiliary ligand (co-ligand)), however, a
complexity in the reaction site arises even with the single sp3NH
system. In the case that one L differs from the other two Ls,
three different reaction sites are potentially generated both for
the fac complex and for the mer complex. The total
performance in terms of reactivity, selectivity, and productivity
in a given asymmetric catalysis is averaged by the degree of
contribution from these six isomers, which have their own
performances.18 Selective construction of the reaction site is a
key issue for a high-performance catalyst. From this viewpoint,
the three different ligating atoms in the R-PN(H)N ligand — soft
sp3P, hard and highly σ-donative sp3N, and soft sp2N — are
expected to work to its advantage. In particular, the advantage is
most obvious in the fac complex, in which all Ls locate trans to
the sp3P, sp3N, and sp2N atoms, exerting varying influence on
the M–L bond strength according to the degree of σ-donicity
and π-acceptor ability. The L that is trans to sp3P is thought to
be the most labile, being easily replaced by a nucleophile such
as hydride and selectively generating a “H–M---N–H” Intramol-
DACat reaction site (Fig. 3a, blue region). In this fac complex,
unfortunately, no fence/plane-type chiral pocket can be
constructed by the pseudo-axial Ph group on P and the
PyCH2NH moiety, which are located above and below the
sp3P/sp3N coordination plane, respectively. Moreover, as shown
in Fig. 3b, the fac complex of (R)-1a is sterically less favored
than the mer isomer: for an imaginary dicationic Ru(H-
A niche field remaining in the design of chiral tridentate
ligands is the development of a linear and flexible ligand with all
three different types of ligating atom in terms of element and/or
hybridization.9 Although the potential complications of fac/mer
stereoisomers and λ/δ chelating conformations become obvious,
the unsymmetrical situation together with non-identical ligating
atoms has hidden potential for high-performance asymmetric
molecular metal catalysis.
A unique chirality may be
constructed, and both the central metal and the coordination sites
may be synergistically influenced. From this viewpoint, we
have developed a new axially chiral sp3P/sp3NH/sp2N-combined
linear tridentate ligand, R-PN(H)N (1),10 as shown in Fig. 2. In
this article, we report details of the design concept, preparation,
metal complex formation, and structural analyses, and its
application to the asymmetric hydrogenation of ketones.
Fig. 2. A new chiral sp3P/sp3NH/sp2N-combined linear ligand R-
PN(H)N (1).
2. Results and discussion
2.1. Design concept
R-PN(H)N (1) was designed as an extension of the C2-
symmetric diphosphine BINAP.11 The binaphthyl skeleton of
BINAP in octahedral metal complexes is well known to
construct a clear λ/δ seven-membered chelating conformation
((R)-BINAP: λ, (S)-BINAP: δ), and the pseudo-axial Ph
substituent on the sp3P atom is located nearly vertical to the
sp3P/sp3P coordination plane to form a chiral fence.12
Replacement of one of the PPh2 groups of (R)-BINAP with a
PyCH2NH moiety leads to (R)-H-PN(H)N ((R)-1a), in which the
sp3P atom adopts 3D spreading, while the sp2N atom establishes
a 2D planar region. Introduction of a Ph group at the C(3)
position of the PyCH2NH-substituted naphthalene ring gives
(R)-Ph-PN(H)N ((R)-1b). These tridentate ligands can avoid
both ligand dissociation and disproportionation to a 2:1
ligand/metal complex, a problem that bidentate ligands
sometimes face. In both PN(H)N ligands, the sp3NH function
would endow a metal complex with intramolecular donor–
acceptor bifunctional catalyst13 (Intramol-DACat) ability (Fig.
_________
__________________________________
Fig. 3. (a) Characteristics of an octahedral metal complex of (R)-R-
PN(H)N. (b) Stability difference between a dicationic fac-Ru((R)-H-
PN(H)N)(CO)3 and its mer isomer. (c) Stability difference between a
dicationic fac-Ru((R)-Ph-PN(H)N)(CO)3 and its mer isomer.

3
PN(H)N)(CO) complex, in which the one-dimensional CO co-
ligand has little steric effect, the fac complex is 25.3 kJ/mol less
the supporting information of the preceding short
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3
communication.14b In a similar way, (R)-Ph-PN(H)N was
prepared in 80% yield from the triflate intermediate (R)-2b,20
which was obtained in 87% yield by treatment of the
corresponding known phosphine alcohol21 with Tf2NPh. Ligand
(R)-1b was recrystallized from CH3OH and CH2Cl2 (mp 188
°C). The molecular structure is shown in Fig. 4b. The absolute
configuration was confirmed by Flack parameter analysis to be
R.
stable than the mer one. The energy profile is altered by the
introduction of a Ph group at C(3) of (R)-1a, which stabilizes the
corresponding fac complex of (R)-1b by 24.5 kJ/mol (Fig. 3c).
Gauche-type steric repulsion (red arrow) between the C(3)Ph
group and the PyCH2NH group in the mer isomer would shift
the equilibrium to the fac side. Density functional calculations
indicate that Ph-PN(H)N ((R)-1b) inherently forms the fac
isomer with a octahedral-preferred central M, while H-PN(H)N
((R)-1a) requires an appropriate co-ligand L that would favor
2.3. Metal complexes
formation of the fac isomer.
On the basis of these
2.3.1. Octahedral Ru(II) complexes
considerations, we decided to attempt the construction of the fac
complex with an appropriate co-ligand L such as (CH3)2S=O
(DMSO). We hypothesized that the 3D spreading of DMSO
might make a fence region instead of the PPh fence. For
reference, the PPh-fence/Py-plane chiral pocket can be
efficiently constructed in the mer isomer, but there is no DACat
reaction site in the chiral pocket (Fig. 3a).
Complexes of Ru with an oxidation state of II are known to
take
a trigonal bipyramidal (TBP) or octahedral (OC)
geometry,22 depending on the property of the ligands. The
geometry isomerism associated with the sp3P/sp3NH/sp2N-
combined ligand (R)-R-PN(H)N was investigated by using fac-
RuCl2(dmso-S)3(dmso-O) (4)23 as a stereochemically well-
studied Ru precursor. The synthetic procedures, as well as the
structural assignments, are summarized in Fig. 5.20xaaaaaaaaaaa
The reaction of (R)-H-PN(H)N ((R)-1a) with a 1 mol amount
of 4 (THF, 120 °C, 30 min, microwave) quantitatively gave a ca.
3:2:1 stereoisomeric mixture of RuCl2((R)-H-PN(H)N)(dmso)
(5), the 31P NMR signals of which appeared at δ 46.0, 41.1, and
34.5 in CDCl3 (Fig. 5a). The major complex with the 31P signal
at δ 46.0 was separated as crystals, and X-ray crystallographic
analysis determined the molecular structure as mer-RuCl2((R)-
H-PN(H)N)(dmso) (5a) with a DMSO-S/Cl trans arrangement
(Fig. 5b). The seven-membered chelation made by sp3P and
sp3N atoms to Ru takes a λ conformation with a dihedral angle
of 77.7° for the binaphthyl skeleton. Consequently, the five-
membered chelation of the PyCHSHRNH moiety to Ru takes a δ
conformation with the HS–C–N–H and HR–C–N–H dihedral
angles of 167° and 49.0°, respectively. This geometric feature is
2.2. Ligand synthesis
The H-PN(H)N ligand was prepared, as shown in Fig. 4a, in
78% total yield (three steps and two pots) by i) the Staudinger-
type reaction of (R)-2a19 with 2-(azidomethyl)pyridine, ii)
hydrolysis of the resulting iminophosphorane to the
corresponding phosphine oxide, and iii) PhSiH3 reduction of the
phosphine oxide. The detailed procedure has been reported in
1
reflected in the H-NMR spectrum (CDCl3), in which the three
AMX-type H signals at δ 4.22 (J = 14.5 Hz (gem) and 2.1 Hz), δ
5.47 (J = 14.5 Hz (gem) and 11.6 Hz), and δ 7.06 (J = 11.6 Hz
and 2.1 Hz) are assignable to HR, HS, and NH, respectively, on
the basis of the Karplus equation. The second major complex (δ
41.1) was thought to be a DMSO-S/sp3NH-trans fac isomer 5b
on the basis of the similarity of the NMR spectra (31P δ 41.1; 1H
NMR δ 4.13 (PyCHSHRNH, J = 17.2 Hz (gem) and <3 Hz), 4.96
(PyCHSHRNH, J = 17.2 Hz (gem) and 10.3 Hz), 7.09 (NH, J: not
assignable due to signal overlaps), δ 2.40 and 3.60 ((CH3)2SO))
to those of fac-RuCl2((R)-Ph-PN(H)N)(dmso) (6) with a DMSO-
S/sp3NH trans arrangement (see Fig. 5d). The coordination of
DMSO via its S or O atom is affected by the properties of the
trans-located ligating atom and the metal ion.24 In the present
RuCl2 complexes of the sp3P/sp3NH/sp2N-combined ligand,
DMSO coordinates to Ru through the S atom such that DMSO-S
is trans to the ligating atom—either sp3N or Cl—that has a high
σ-donicity. The minor 31P signal at δ 34.5 may be assigned to a
1
DMSO-isomeric mer complex 5c on the basis of the H-NMR
similarity to that of 5b (Fig. 5d).aaaaaaaaaaaaaaaaaaaaaaaaaaaa
Under the same conditions as those for (R)-1a, the C(3)-Ph-
substituted (R)-Ph-PN(H)N ligand ((R)-1b) was allowed to react
with 4 to give a single stereoisomer quantitatively, as estimated
from the 31P-NMR analysis (Fig. 5c). Single crystals were
obtained in 53% yield from CH3OH, and the molecular structure
was determined by X-ray crystallographic analysis as fac-
RuCl2((R)-Ph-PN(H)N)(dmso) (6) with a DMSO-S/sp3N trans
arrangement (Fig. 5d). The sp3P and sp3N atoms coordinate to
Ru to generate a λ conformation with a dihedral angle of 71.8°
(binaphthyl skeleton), and the PyCHSHRNH moiety connecting
to C(2) is flipped up to the Ph2P-connected naphthalene side.
The fac coordination of the Ph-PN(H)N ligand results in the
Fig. 4. (a) Preparation of (R)-H-PN(H)N ((R)-1a) and (R)-Ph-PN(H)N
((R)-1b). (b) Molecular structure of (R)-1b in crystal. P1 (#1), a = 9.088(4)
Å, b = 10.730(5) Å, c = 17.001(7) Å, α = 87.249(13)°, β = 89.841(10)°, γ =
89.889(9)°, V = 1655.9(13) Å3, Z = 2, R = 0.0590, Rw = 0.1278.

4
Tetrahedron
geometrical arrangement of PyCH H NH with H –C–N–H and
coordination site and the other CH group is located obliquely
ACCEPTED MANUSCRIPT
S
R
S
3
HR–C–N–H dihedral angles of 108.5° and 9.2°, respectively, and
with the HS atom in close proximity to the C(8’)H of the
naphthalene ring (2.685 Å). The remaining three coordination
sites are occupied by DMSO and two chloride anions, which are
located trans to sp3N, sp3P, and sp2N, respectively. The highly
electron donative sp3N atom, which has no ability to accept Ru-d
orbital back donation, enhances the orbital interaction between
the Ru-d orbital and the σ*S–O of DMSO. The synergistic effect
of the electrostatic and orbital factors shortens the Ru–S bond
length to 2.265 Å from the length observed in a trans sp3P---Ru--
-S=O system (2.30–2.35 Å).25 The existence of a hydrogen
bond between the sulfur-bound DMSO oxygen atom and
PyC(6)H is suggested by the short distance, 2.639 Å, and the
small sp2N---Ru---S=O dihedral angle, 6.6°. The PyC(6)H---
O=S interaction further stabilizes the complex, and determines
the conformation of the DMSO co-ligand such that one of the
CH3 groups is sticking out toward the sp3P/sp3N in-plane
above the pseudo-equatorial phenyl substituent on the sp3P
atom. The molecular structure observed in the crystal was fully
consistent with the H-NMR behavior in CDCl3 (Fig. 5d). The
1
three H signals in the PyCHSHRNH moiety show an AMX
pattern at δ 4.14 (HS, J = 17.2 Hz (gem) and 2.1 Hz), δ 4.98 (HR,
J = 17.2 Hz (gem) and 10.3 Hz), and δ 7.92 (NH, J = 10.3 Hz
and <3 Hz), and the HS proton has a cross signal with the
naphthalene C(8’)H in the ROESY spectrum (2.4% nOe).20 The
two diastereotopic CH3 groups of DMSO resonate at δ 2.22 and
3.38. The 1.2-ppm downfield shift can be ascribed to the fact
that the CH3 group is located in the deshielding region of the
pseudo equatorial Ph substituent on the sp3P atom. Consistent
with the existence of a PyC(6)H---O=S hydrogen bond, the
PyC(6)H signal appears at δ 9.92, which is 2 ppm lower as
compared with the ligand itself (δ 7.92; see Fig. 4a).26 aaaaa
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Fig. 5. Preparation of octahedral R-PN(H)N–Ru complexes and the structural analyses. Typical 1H-NMR signals in CDCl3 are shown on the chemical
structures. (a) Ru–DMSO complex formation of RuCl2(dmso-S)3(dmso-O) (4) and (R)-H-PN(H)N ((R)-1a) and the 31P-NMR spectrum. (b) Molecular structure
of mer-RuCl2((R)-Ph-PN(H)N)(dmso) (5a) (δ 46.0) in crystal. P21 (#4), a = 11.538(7) Å, b = 11.624(7) Å, c = 13.723(9) Å, β = 97.471(8)°, V = 1825(2) Å3, Z =
2, R = 0.0856, Rw = 0.2564. The supposed structures of the second major complex 5b (δ 41.1) and the minor complex 5c (δ 34.5) are shown. (c) Preparation of
fac-RuCl2((R)-Ph-PN(H)N)(dmso) (6) from 4 and (R)-Ph-PN(H)N ((R)-1b) and the 31P-NMR spectrum. (d) Molecular structure of 6 in crystal. P212121 (#19), a
= 12.359(4) Å, b = 16.791(5) Å, c = 18.602(5) Å, V = 3860(2) Å3, Z = 4, R = 0.0912, Rw = 0.2428. (e) Preparation of fac-[Ru((R)-H-PN(H)N)(dmso)3](BF4)2 (7)
from fac-[Ru(C6H6)((R)-H-PN(H)N)](BF4)227 and the 31P-NMR spectrum.

5
fac-Selective generation of the Ru complex even with the
C(3)-non-substituted (R)-H-PN(H)N ((R)-1a) was attained on
the basis that the trans arrangement of two DMSOs on Ru, O=S-
--Ru---S=O, is energetically disfavored.24 Thus, treatment of
fac-[Ru(C6H6)((R)-H-PN(H)N)](BF4)2 with DMSO at 50 °C for
PN(H)N), yellow blocks; FeCl2((R)-Ph-PN(H)N), yellow
ACCEPTED MANUSCRIPT
prism; FeBr2((R)-Ph-PN(H)N), green chunks). The coordination
geometry of FeCl2((R)-H-PN(H)N) was close to SMP (Fig. 6b),
in which the sp3P, sp3N, sp2N, and Cl ligating atoms occupy the
basal plane and another Cl is located at the apex. The sum of the
angles in the basal plane is 348.8° (sp2N–Fe–sp3N = 75.3°,
sp3N–Fe–sp3P = 84.15°, sp3P–Fe–Cl(basal) = 95.45°, and
Cl(basal)–Fe–sp2N = 93.85°), and the average angle of the axial
Cl from the square plane is 102.3° (sp2N–Fe–Cl(apical) =
109.1°, sp3N–Fe–Cl(apical) = 95.85°, sp3P–Fe–Cl(apical) =
95.2°, and Cl(basal)–Fe–Cl(apical) = 109.15). The difference of
the two angles made by two atoms at the opposite corners of the
basal plane is only 5.5° (τ5 = 0.09) (sp2N–Fe–sp3P = 149.4° and
sp3N–Fe–Cl(basal) = 154.9°), showing that the geometry is
closer to SMP than to TBP. Replacement of Cl with Br had
little effect on the SMP geometry (sp2N–Fe–sp3P = 149.2° and
sp3N–Fe–Br(basal) = 157.7°; τ5 = 0.14) except for the bond
lengths (Fig. 6c). In FeBr2((R)-H-PN(H)N), the bond lengths of
sp3P–Fe and sp2N–Fe are 0.02 and 0.01 Å longer, respectively.
One CH3CN molecule per Fe(II) ion was present in the
crystals,20 but an octahedral geometry involving CH3CN was not
formed. Reaction of FeBr2((R)-H-PN(H)N) with CO (1 atm,
CDCl3, rt, 1 h) resulted in no change. Introduction of a C(3)Ph
substituent in the naphthalene ring changed the coordination
geometry to the TBP side (τ5 = 0.43 (Cl; sp2N–Fe–sp3P = 162.9°
and sp3N–Fe–Cl(basal) = 136.4°) and τ5 = 0.42 (Br; sp2N–Fe–
sp3P = 163.3° and sp3N–Fe–Br(basal) = 138.3°)) (Fig. 6d–e). In
comparison to the H-PN(H)N case, the sp3N–Fe–Cl(basal) angle
(136.4°) was narrowed by 18.5°, while the sp2N–Fe–sp3P angle
(162.9°) was widened by 13.5°. The same tendency was
observed for the FeBr2 complex. Steric repulsion between the
C(3)Ph group and the PyCH2NH group would alter the sp2N–Fe
bond, changing the SMP geometry to TBP-like one.
Reaction of CoCl2 with (R)-1a in a 1:1 ratio (10 mM each,
THF, rt, 18 h), followed by concentration and recrystallization
from a 1:3 THF–Et2O mixture gave single crystals of CoCl2((R)-
H-PN(H)N) (ca. 30% yield; blue prism). NiCl2((R)-H-PN(H)N)
48
h
followed by evaporation gave fac-[Ru((R)-H-
PN(H)N)(dmso)3](BF4)2 (7) (Fig. 5e).27 In view of the NMR
behavior of the above two Ru complexes, NMR analysis in
CDCl3 supported a structure in which the facial stereochemistry
was retained after replacement of the benzene ligand with three
DMSOs. The DMSO ligands showed three sets of signals for
the two methyl H atoms at δ 2.37 and δ 3.23, δ 3.00 and δ 3.05,
and δ 2.65 and δ 2.92; these sets were assigned by cross signal
analysis of the H-H COSY spectrum.27 Furthermore, the DMSO
that was trans to sp3N was found to be stabilized by a six-
membered hydrogen bond between the DMSO oxygen atom and
C(6)H of the Py moiety in the ligand, as supported by the low
C(6)H chemical shift (δ 9.82) similar to that of 6 (δ 9.92). The
1H-NMR behavior of PyCHSHR and NH (δ 4.39 (HS, J = 19.2 Hz
(gem) and <3 Hz), 5.05 (HR, J = 19.2 Hz (gem) and 8.9 Hz), and
7.61 (NH)) was also consistent with the fac-determined Ru
complex 6.
2.3.2. Non-octahedral metal complexes
For reference, Fig. 6 shows the molecular structures of first-
row transition metal complexes of the R-PN(H)N ligands (R)-1a
and (R)-1b. All of the structures of the Cu(I), Fe(II), Co(II), and
Ni(II) complexes were determined by X-ray crystallographic
analyses of single crystals.20 The selected structural parameters
are also summarized in Fig. 6. The coordination geometry
varied from uncommon trigonal monopyramidal (TMP) to
square monopyramidal (SMP) or trigonal bipyramidal (TBP),22
depending on the central metal ion and the presence or absence
of a C(3) substituent. The four- and five-coordinate geometries
were analyzed by using the conventional τ4 and τ5 indexes,
respectively: τ4 = [Σ(basal–M–basal) – Σ(basal–M–axial)]/90°
(τ4 = 0 (tetrahedral (Td)); τ4 = 1 (TMP)). τ5 = (α – β)/60° (α, β
= basal angles; τ5 = 0 (SMP); τ5 = 1 (TBP)).28xxxxxxxxxxxxxxxx
CuCl((R)-H-PN(H)N) was prepared from CuCl and (R)-1a in
a 1:1 ratio (10 mM each, THF, reflux, 18 h). Recrystallization
from a 1:3 CHCl3–Et2O mixture afforded yellow chunky crystals
(ca. 30% yield). As shown in Fig. 6a, the Cu(I) complex takes a
TMP geometry with sp3P, sp2N, and Cl in the basal plane (thin
red lines) and sp3N at the apex. The sum of the angles in the
trigonal plane is 359.7° (sp2N–Cu–sp3P = 132.7°, sp2N–Cu–Cl =
99.6°, and sp3P–Cu–Cl = 127.4°), and the average angle of the
axial sp3N from the trigonal plane is 91.1° (sp3N–Cu–sp3P =
99.2°, sp3N–Cu–sp2N = 78.7°, and sp3N–Cu–Cl = 95.45°). The
τ4 value is calculated to be 0.96, clearly indicating ideal TMP
geometry. The sp3N–Cu bond length of 2.403 Å is longer than
usual. The sp3N is thought to weakly coordinate to Cu in the
basal trigonal plane. TMP complexes are not common, and are
generally prepared by using specially designed ligands,
including tripodal tetradentate ligands29 and PNN-pincer-type
iminophosphorane ligands,30 in combination with Fe(II), Co(II),
Ni(II), Zn(II), and Cu(I). The sp3P/sp3NH/sp2N-combined (R)-
H-PN(H)N ligand may have certain steric and electronic
characteristics that facilitate generation of a Cu(I) complex with
TMP geometry. Such a TMP Cu(I) complex with a τ4 value of
near 1 is the first example of its kind.31xxxxxxxxxxxxxxxxxxxxxxxx
With Fe(II), Co(II), and Ni(II) ions, (R)-R-PN(H)N ((R)-1)
formed a 1:1 complex with SMP or TBP geometry. FeX2((R)-R-
PN(H)N) (X = Cl or Br; R = H or Ph) was prepared by mixing
Fe(II)X2 with (R)-1 in a 1:1 ratio in CH3CN (60 mM each; 100
°C; 12 h). Cooling down to rt afforded single crystals in 30%–
60% yields (FeCl2((R)-H-PN(H)N), yellow prism; FeBr2((R)-H-
was prepared from NiCl2 6H2O and (R)-1a (10 mM each, CH3OH,
•
rt, 18 h) and recrystallized from a 1:3 CH3OH–Et2O mixture (ca.
60% yield; colorless prism). As shown in Fig. 6f–g, the molecular
structures with τ5 values of 0.10–0.11 were nearly the same as the
structure of FeCl2((R)-H-PN(H)N) (τ5 = 0.09).
2.4. Application to Ru-catalyzed asymmetric hydrogenation of
ketones
The catalytic performance of the two fac-Ru complexes, fac-
RuCl2((R)-Ph-PN(H)N)(dmso) (6) and fac-[Ru((R)-H-
PN(H)N)(dmso)3](BF4)2 (7), was investigated with respect to
asymmetric hydrogenation of the ketones 8 to the corresponding
alcohols 9. Four different types of ketone were selected (8a,
non-chelatable and sterically demanding ketone; 8b, non-
chelatable aromatic ketone; 8c, chelatable and sterically
demanding ketone; 8d, chelatable aromatic ketone), and the
solvents were fixed as CH3OH and iPrOH. The standard
conditions were set to [8]0 = 1 M, [6 or 7]0 = 1 mM, [tBuOK]0 =
10 mM, 30 °C, 100 atm H2, and 12 h. Table 1 summarizes the
results. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
With the Ru complex 6, 8a was slowly hydrogenated in
CH3OH to give (S)-9a with a 98:2 S/R er. Replacement of
CH3OH with iPrOH attained full conversion without
deterioration of the enantioselectivity (entries 1 and 2).
Decreasing the amount of tBuOK to a three molar equivalent to
Ru led to a significant decrease in reactivity (entry 3). Aromatic
ketone 8b was quantitatively hydrogenated but with lower
enantioselectivity (entry 4). The complex showed little
reactivity toward chelatable ketones 8c and 8d either in the

6
Tetrahedron
ACCEPTED MANUSCRIPT
x
Fig. 6. Molecular structures of the first-row transition metal complexes with (R)-R-PN(H)N. (a) CuCl((R)-H-PN(H)N). P212121 (#19), a = 9.7661(11) Å, b =
9.9591(11) Å, c = 31.522(4) Å, V = 3065.9(6) Å3, Z = 4, R = 0.0730, Rw = 0.1607. (b) FeCl2((R)-H-PN(H)N). P21 (#4), a = 8.606(3) Å, b = 18.480(5) Å, c
= 10.725(3) Å, β = 97.145(5)°, V = 1692.5(8) Å3, Z = 2, R = 0.0411, Rw = 0.1066. (c) FeBr2((R)-H-PN(H)N). P21 (#4), a = 8.695(3) Å, b = 18.598(6) Å, c =
10.894(4) Å, β = 98.968(4)°, V = 1740.1(9) Å3, Z = 2, R = 0.0626, Rw = 0.1667. (d) FeCl2((R)-Ph-PN(H)N). P21 (#4), a = 9.0539(13) Å, b = 13.605(2) Å, c
= 15.771(3) Å, β = 104.378(2)°, V = 1881.8(5) Å3, Z = 2, R = 0.0514, Rw = 0.1342. (e) FeBr2((R)-Ph-PN(H)N). P21 (#4), a = 9.141(4) Å, b = 13.688(5) Å, c
= 15.888(6) Å, β = 103.625(4)°, V = 1932.0(12) Å3, Z = 2, R = 0.0899, Rw = 0.2566. (f) CoCl2((R)-H-PN(H)N). P1 (#1), a = 19.747(8) Å, b = 8.725(4) Å, c =
21.648(8) Å, β = 97.563(7)°, V = 3698(3) Å3, Z = 4, R = 0.2419, Rw = 0.6189. (g) NiCl2((R)-H-PN(H)N). P1 (#1), a = 8.527(7) Å, b = 18.53(2) Å, c = 10.809(9)
Å, β = 89.604(11)°, V = 1708(3) Å3, Z = 2, R = 0.1972, Rw = 0.5225. τ4 = [Σ(basal–M–basal) – Σ(basal–M–axial)]/90°. τ5 = (α – β)/60° (α, β = basal–M–basal
angles). The thin red lines in the chemical structures indicate the basal parts used for calculation of the τ values.

7
absence of DMSO or in the presence of 1400 mM DMSO
substrate via a hydrogen bond with the C=O oxygen atom to
ACCEPTED MANUSCRIPT
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
(entries
5–8).
move to the transition states TSS and TSR, in which TSR suffers
from steric repulsion between the CH3 group of a coordinating
DMSO trans to sp3N and the tert-alkyl substituent on C=O,
leading to the S product (R2 < tBu) via TSS. A synergistic
electrostatic and orbital effect of the electron donative sp3N
atom strengthens the Ru---S=O bond, and the degree is further
enhanced by the formation of a hydrogen bond between the
hydrogen atom at C(6) of the Py moiety and the oxygen atom of
the DMSO. The PyC(6)H---O=S hydrogen bond fixes the
DMSO conformation, creating a CH3-fence/Py-plane chiral
context. With a chelatable substrate, however, the stability of
the sp3N---Ru---S=O system is overcome, resulting in
deconstruction of the chirally well-defined reactive species.
This would explain why an excess amount of DMSO is required
to attain high reactivity and enantioselectivity. The reason for
the lack of reactivity of 6 toward a chelatable substrate even in
the presence of excess DMSO is unclear. A steric effect of the
C(3)Ph group might be involved. The reaction pathway from
TSS to 10 is controversial. According to the mechanisms
proposed in the hydrogenation of acetophenone using a BINAP–
Ru/diamine catalyst,32–34 the possible pathways can be
categorized into three via species 11–13 as shown in Fig. 7: i)
the two hydrogen atoms in TSS are simultaneously transferred to
the C=O group to generate fac-RuH((R)-R-PNN)(dmso) (11)
with liberation of the alcoholic product, and then the Ru amide
bond of 11 is cleaved by H2 to revive 10; ii) the Ru hydride
transfers to the C=O carbon to generate the alkoxide 12, which
is then cleaved by the H2 molecule via 13 to liberate the product
with concomitant regeneration of 10; iii) the hydride transfer
generates a coordinately unsaturated 16e Ru cation/alkoxide
anion species, to which an H2 molecule coordinates in a η2
manner to give 13. The alkoxide anion is quickly protonated by
The performance of complex 7 was weaker than that of 6 in
iPrOH. The sterically demanding simple ketone 8a was
hydrogenated to give, after 12 h, (S)-9a with an 87:13 er in 96%
yield (entry 11). The conversion was only 10% after 2 h. In
CH3OH, however, the catalytic performance improved markedly
to realize quantitative conversion and high enantioselectivity
(entry 9). The reaction was completed within 2 h. The molar
equivalent of tBuOK could be reduced to three (entry 10). In
addition, aromatic ketone 8b was hydrogenated with low
enantioselectivity by complex 7 (entry 12). With the chelatable
substrate 8c, both the reactivity and enantioselectivity were low
under the standard conditions (entry 13), but addition of DMSO
(1400 mM) led to quantitative generation of (S)-9c with a 99:1
er (entry 14).14b In CH3OH–DMSO solvent, the corresponding
aromatic ketone 8d was slowly hydrogenated to give (R)-9d
with an S/R er of 10:90 (entry 15). Thus, the tertiary alkyl group
is the least requirement to attain high enantioselectivity in the
presentaRu-catalyzedahydrogenation.
Although there is no proof at the present stage, we think that
the Ru dihydride 10 is involved in the hydrogenation (Fig. 7). A
DACat reaction site, Hδ––Ruδ+---Nδ––Hδ+, is constructed by the
proton on the sp3N ligating atom and the hydride trans to the
sp3P ligating atom. The hydride trans to the sp2N atom is a
spectator ligand. The acidic NH proton captures a ketonic
Table 1.
Asymmetric hydrogenation of non-chelatable and chelatable ketones using fac-RuCl2((R)-Ph-PN(H)N)(dmso) (6) and fac-
[Ru((R)-H-PN(H)N)(dmso)3](BF4)2 (7)a
Entry
1
Ru complex
Substrate
8a
tBuOK, mM
DMSO, mM
Solvent
CH3OH
iPrOH
% Conversionb,c
S:Rd
98:2
98:2
—
6
10
10
3
0
19
2
8a
0
>99 (10)
<1
3
8a
0
iPrOH
4
8b
10
10
10
10
10
10
3
0
iPrOH
>99 (10)
<1
86:14
—
5
8c
0
1400
0
iPrOH
6
8c
iPrOH
<1
—
7
8d
iPrOH
<1
—
8
8d
1400
0
iPrOH
<1
—
9
7
8a
CH3OH
CH3OH
iPrOH
>99 (>99)
>99
98:2
98:2
87:13
37:63
85:15
99:1
10:90
10
11
12
13
14
15
8a
0
8a
10
10
10
10
10
0
96 (10)
>99
8b
0
CH3OH
CH3OH
CH3OH
CH3OH
8c
0
22
8c
1400
1400
>99e
8d
32
aConditions: [8]0 = 1 M; [6 or 7]0 = 1 mM; 30 °C; 100 atm H2; 12 h.
bThe 1H-NMR analysis indicates that the conversion is quantitative to the yield of 9.
cValues in parentheses are the results after 2 h.
dDetermined by GC or HPLC analysis.
e24 h.

8
Tetrahedron
ACCEPTED MANUSCRIPT
the highly acidic hydrogen atom on the η2-coordinating H2
changing the overall rate.35,36 A more detailed mechanistic study
is required for complete understanding of the present asymmetric
hydrogenation.
molecule to give the alcohol product and 10. A theoretical
calculation present by Gordon and colleagues strongly supports
pathway iii.35 The reaction pathways via 11, 12, and 13 would be
subtly affected by solvent. For example, differences in polarity,
dielectric constant, and acidity between CH3OH and iPrOH may
change the pathway. The observed reverse solvent effect
depending on the presence or absence of the C(3)Ph substituent
3. Conclusion
A new chiral tridentate sp3P/sp3NH/sp2N-combined ligand, R-
PN(H)N (1), has been developed as an extension of sp3P/sp3P
bidentate BINAP chemistry. The tridentate property provides an
advantage over bidentate ligands by suppressing ligand
dissociation or disproportionation to a 2:1 ligand/metal complex;
by contrast, a serious disadvantage is associated with the linear
and flexible 1 is fac/mer selectivity in terms of the formation of
octahedral transition metal complexes, which constitutes an
essential component of asymmetric molecular catalysis. This
problem of ambiguous geometry has been overcome, by using
both Gauche-type steric repulsion in the ligand and the DMSO
effect that trans coordination of DMSO-S is energetically
disfavored, to realize the selective synthesis of fac-RuCl2((R)-Ph-
PN(H)N)(dmso) (6) and fac-[Ru((R)-H-PN(H)N)(dmso)3](BF4)2
(7). The three different facially arranged ligating atoms—
namely, the soft sp3P, hard and highly σ-donative sp3N, and in-
between sp2N—in R-PN(H)N electronically exert a significantly
different effect on the trans coordination sites, weakening the
Ru–DMSO bond trans to sp3P and strengthening the Ru–DMSO
bond trans to sp3N. Furthermore, the conformation of the sp3N-
trans DMSO is fixed by a PyC(6)H---O=S hydrogen bond that
not only makes the Ru–DMSO bond strong but also constructs a
clear DMSO-CH3-fence/Py-plane chiral context. The presence of
the single sp3NH function in the ligand simplifies the
construction of an Intramol-DACat reaction site, H–Ru---sp3N–
H, in which the RuH will be generated at the most labile
coordination site trans to sp3P. These steric and electronic factors
can be synergistically altered to attain the efficient hydrogenation
of sterically demanding ketones with high enantioselectivity.
aaaaaaaaaaaaaaaaaaaa
would be related both to the preliminary step (k06 and k0 ) toward
7
10 from 6 and 7, and to the catalytic cycle itself via 11, 12, or 13
6
7
with the rate constant k1 for 6 and k1 for 7. The dicationic Ru
complex 7, which has high oxidizing power, would be easily
reduced even by CH3OH with its poor reducing ability, whereas
the neutral RuCl2 complex 6 would require iPrOH with stronger
reducing ability to generate the RuH2 species 10. A steric effect
of the C(3)Ph substituent and iPrOH may also exert a subtle
effect on product generation. In the case of both the C(3)-Ph
substituted 6 and the C(3)-non-substituted 7, the turnover
efficiency of the RuH2 species 10 is thought to not differ
significantly. The higher acidity and the smaller size of CH3OH
in comparison to iPrOH would facilitate generation of the
intermediaries 11, 12, and 13 and stabilize these species via a
hydrogen bond network, leading to the higher reactivity observed
in CH3OH. The rate relationships, k0 > k0 and k1 ≈ k1 , are
thought to be responsible for the reverse solvent effect. In
addition, as compared with CH3OK, the more basic iPrOK would
increase the contribution of the potassium amidate species, fac-
RuH2((R)-R-PN(K)N)(dmso), which has a different reactivity
from fac-RuH2((R)-R-PN(H)N)(dmso) (10) itself, thereby
7
6
7
6
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
_
In addition, a series of first-row transition metal (Cu(I), Fe(II),
Co(II), and Ni(II)) complexes of H-PN(H)N (1a) and Ph-PN(H)N
(1b) were prepared, and their non-octahedral geometries were
analyzed in detail. In particular, the characteristics of the
sp3P/sp3NH/sp2N-combined ligand have realized the first
synthesis of a TMP Cu(I) complex with τ4 = 0.96, and the
presence or absence of the C(3)-Ph substituent changes the SMP
and TBP geometries. We hope that the present study will
stimulate our ideas for the development of further chiral ligands.
Acknowledgments
This work was aided by a Grant-in-Aid for Scientific Research
(No. 25E07B212) and (23005914) from the Ministry of
Education, Culture, Sports, Science and Technology (Japan), and
an Advanced Catalytic Transformation Program for Carbon
Utilization (ACT-C) from Japan Science and Technology
Agency (JST).
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(2'-(diphenylphosphanyl)-3-phenyl-N-(pyridin-2-ylmethyl)-[1,1'-
binaphthalen]-2-amine).
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Tanaka, S.; Suzuki, Y.; Saburi, H.; Kitamura, M. Tetrahedron
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Chem. Int. Ed. 2005, 44, 1730; (h) Tanaka, S.; Saburi, H.;
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10
Tetrahedron
ACCEPTED MANUSCRIPT
Chem. Soc. 2005, 127, 4152; (f) Daley, C. J. A.; Bergens, S. H. J.
Am. Chem. Soc. 2002, 124, 3680.
experimental procedures, NMR spectroscopic data, X-ray
crystallographic data (CIF) for ligands and metal complexes, and
Ru-catalyzed asymmetric hydrogenation. This material is
available free of charge via the Internet at XXX. The X-ray
diffraction data can be obtained free of charge from the
Cambridge
www.ccdc.cam.ac.uk—data_re-quest/cif.
34. (a) Sandoval, C. A.; Ohkuma, T.; Muñiz, K.; Noyori, R. J. Am.
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Chem. Soc. 2014, 136, 3505.
Crystallographic
Data
Centre
via
36. (a) Hartmann, R.; Chen, P. Adv. Synth. Catal. 2003, 345, 1353; (b)
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Supplementary Material
Supplementary data associated with this article can be found
in the online version at XXXX. These data include details of