The synthesis and efficient one-pot catalytic "self-breeding" of asymmetrical NC(sp3)E-hybridised pincer complexes

Article abstract of DOI:10.1039/c6cc00763e

Cyclopalladation of the pyridyl-substituted chiral phosphine sulfide (N-P=S) and oxide (N-P=O) compounds afforded the asymmetric N-C(sp³)?-S and N-C(sp³)?-O pincer complexes. When applied as catalysts in asymmetric hydrophosphination, the newly developed aliphatic pincer catalyst could be recycled over three runs and obtained in large quantities via a one-pot "self-breeding" catalytic protocol.

Full text of DOI:10.1039/c6cc00763e

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The synthesis and efficient one-pot catalytic
‘‘self-breeding’’ of asymmetrical NC(sp3)E-
Cite this:DOI: 10.1039/c6cc00763e
hybridised pincer complexes†
Received 26th January 2016,
Accepted 17th February 2016
Xiang-Yuan Yang, Wee Shan Tay, Yongxin Li, Sumod A. Pullarkat and
Pak-Hing Leung*
DOI: 10.1039/c6cc00763e
www.rsc.org/chemcomm
Cyclopalladation of the pyridyl-substituted chiral phosphine sulfide
Q
Q
(N–P S) and oxide (N–P O) compounds afforded the asymmetric
N–C(sp3)*–S and N–C(sp3)*–O pincer complexes. When applied as
catalysts in asymmetric hydrophosphination, the newly developed
aliphatic pincer catalyst could be recycled over three runs and obtained
in large quantities via a one-pot ‘‘self-breeding’’ catalytic protocol.
Fig. 1 (a) Aromatic C(sp2)- and (b) aliphatic C(sp3)-pincer complexes.
The discovery of carbometalated pincer complexes in the 1970s complexes include CO2 reduction,4h,i hydroamination,4j Negishi,4k
brought about rapid interest and activity in this class of robust Suzuki–Miyaura,4l,m Mizoroki–Heck,3b,4n and Kumada coupling.4o It
organometallic compounds.1 The tridentate EYE0 anionic ligand should be noted that in the literature, C(sp3)-based pincer complexes
coordinates to the transition metal center in a Z3-meridional fashion, are dominated almost exclusively by the symmetrical PC(sp3)P
giving rise to the formation of thermodynamically favorable bicyclo- framework,3b,4d,5 and formation of the complexes via electrophilic
metalated compounds with exceptional thermal stabilities.2 The activation or oxidative addition of low-valent transition metals
most popular EYE0 ligand scaffolds used in the pincer complexes across the C–H bond often results in moderate yields.3d From a
are based on aromatic C(sp2) moieties (Fig. 1a). In contrast, aliphatic stereochemical standpoint, the organometallic C(sp3)-donors in
C(sp3) pincer complexes (Fig. 1b) are scarce in literature3 possibly unsymmetrical EC*E0 pincer complexes are stable stereogenic
due to the challenges faced in the syntheses and structural centers. Similar to the well-established P-chiral phosphorus ligands,
modifications of their architectures. In square-planar pincer C-chiral carbon donors may also control the stereochemistry of a
complexes, the organometallic C(sp2) and C(sp3)-donor atoms catalytic process at the metal center efficiently. Therefore, it is our
offer different electronic trans influences to the sole coordina- judgment that the chemistry and practical applications of aliphatic
tion site on the complexes. A literature review suggested that the C(sp3)-based pincer complexes should be further developed, in
C(sp3)-donors are generally described as strong s-donors and the particular, the chiral unsymmetrical variants which are synthe-
three dimensional geometry around their metal centers exhibited tically challenging yet structurally interesting.
unique properties that are usually not observed in their C(sp2)-
In this article, we report the first one-pot catalytic ‘‘self-breeding’’
metalated counterparts.3a,d,4 For example, a PC(sp3)P Ir(III) complex synthesis of the optically pure NC(sp3)*E (E = O, S) pincer complexes.
was reported to oxidatively add across the N–H bond of ammonia This methodology functions on the concept of ‘‘chirality breeding’’,6
to form an amido hydride complex, but the PC(sp2)P Ir(III) complex which is an efficient technique for producing infinite replicates of a
readily coordinates to ammonia.4a Some PC(sp3)P pincer complexes particular chiral complex with a small amount of the same complex
are efficient catalysts for H/D exchange reaction but their PC(sp2)P acting as a catalyst. Furthermore, the chiral ligand precursor in
analogues were found to be practically inactive under similar this instance is generated via an efficient catalytic asymmetric P–H
conditions.4g Other applications of PC(sp3)P transition metal addition protocol. An optimisation study conducted on the cyclo-
metalation reaction has successfully overcome the traditional
Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore 637371,
Singapore. E-mail: pakhing@ntu.edu.sg
issue of low yields.
We have previously developed an efficient and straightforward
synthesis of a chiral C2-symmetrical PC(sp2)P Pd pincer complex 1.7
Complex 1 was subsequently found to be an active catalyst for
the addition of diarylphosphines to activated dienes, furnishing
the phosphine adducts in high ee’s (94–99%) and excellent
† Electronic supplementary information (ESI) available: Experimental details,
crystallographic information, and spectral and HPLC data. CCDC 1449085 (4a)
and 1449086 (4b). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6cc00763e
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with an amine complex gave complex 4a in 62% yield with 19% of
the unreacted 3a being recovered (entry 8). Further optimisation of
the reaction conditions ultimately resulted in the formation
of complex 4a in high yields (entries 4, 7 and 9). The selected
methods were extended to the cyclometalation of the phosphine
oxide 3b to afford complex 4b in excellent yields (entries 10–12).
The scaled-up reactions of compound 3a/b (0.375 mmol) under the
optimised conditions proceeded smoothly without any significant
differences in terms of yields (entries 13–14). As mentioned earlier,
it is challenging and uncommon to prepare the C(sp3)-pincer complex
in high yields. Clearly, the choice of the Pd(II) sources and the reaction
conditions are crucial to the cyclometalation reaction. In a previous
report, a similar complex was obtained in 35% yield (DCM, RT, 18 h)
in the presence of PdCl2.3e Complex 4a/b was stable when exposed to
air without any noticeable degradation over several weeks. The
absolute stereochemistry and coordination geometry of the complexes
were confirmed by X-ray crystallography. As desired, the chiral
ligand coordinated to the Pd center via its NC(sp3)*S and NC(sp3)*O
donor atoms to form the neutral chloro complexes 4a and 4b,
respectively (see the ESI†, Fig. S43 and S44, for X-ray structures and
Table S7 for selected bond lengths and angles of both complexes).
Both complexes exhibited slightly distorted square planar geometry
around the Pd center. The two Ph groups on the P atom are located
in the equatorial and axial positions. The longer Pd ’ E bond and
larger C–Pd ’ E angle observed in 4a can be directly attributed to
the larger radius of the sulfur atom in 4a than the oxygen donor in
4b. As their frameworks are similar, the difference in the Pd ’ N
distances is clearly a result of the electronic properties originating
from their trans S and O donors. According to the Cahn–Ingold–
Prelog (CIP) sequence rules,9 the new chiral organometallic carbon
center generated from the reaction adopted the (R)-absolute
configuration in both complexes. It is noteworthy that, prior to
cyclopalladation, the absolute configuration of the existing stereo-
genic center in ligand precursor 3a/b is defined as (S). The exclusive
adoption of the (R)-chirality in these complexes could be explained
by the Dreiding model studies (Fig. 2). In conformer A, the alkene
proton adjacent to the phosphine C-chiral center and the Ph moiety
experience unfavorable steric interactions with the CQO group.
However, rotation along the C–C single bond (as indicated in
conformer A) will afford the sterically favorable conformer B, in
which the alkene and Ph moieties will be directed away from the
CQO group. Subsequently, cyclopalladation ensues to generate
the complex with the new C(sp3)*–Pd chiral center adopting the
(R)-absolute configuration (Fig. 2).
Scheme 1 Catalytic hydrophosphination of pyridyl-substituted diene 2.
Scheme 2 Cyclopalladation of NC(sp3)E ligand precursors.
yields (78–92%).8 The methodology was extended to the catalytic
asymmetric hydrophosphination of the pyridyl-substituted
diene 2. Despite the presence of the potentially strong pyridyl-
nitrogen donor which can disrupt the catalytic cycle by chelation
to the catalyst, the asymmetric addition reaction was achieved
efficiently. Upon sulfurisation or oxidation of the product, the
desired NC(sp3)E ligand precursors 3a/b were obtained regio-
selectively with excellent enantioselectivities (499% ee) in high
isolated yields (86–90%) (Scheme 1).
The structural features of ligand precursors 3a/b suggest that
they could function as potential chiral auxiliaries for transition
metal catalysts (Scheme 2). An initial investigation of compound
3a with PdCl2 (Table 1, entry 1) resulted in the formation of the
chiral unsymmetrical pincer complex 4a in a low isolated yield of
4% (58% of unreacted 3a recovered). In the presence of a stoichio-
metric amount of base (NaOAc), a moderate yield of 44% was
obtained, although the reaction was incomplete even after 24 h
(entry 2). With more reactive (and soluble) Pd(II) starting materials
such as PdCl2(NCMe)2, K2PdCl4 and Li2PdCl4, higher yields of
62–83% were observed (entries 3, 6 and 9). Transcyclometalation
Table 1 Cyclopalladation of NC(sp3)E ligand precursorsa
Entry Reagent(s)
4
Solvent
T (1C) t (h) Yieldb (%)
1c
2c
3c
4
PdCl2
PdCl2, NaOAc
PdCl2(NCMe)2
4a DCM
4a DCM
4a DCM
RT
RT
RT
RT
RT
24
24
24
10
24
24
22
4
44
62
76
0
65
83
PdCl2(NCMe)2, NaOAc 4a DCM
5
Pd(OAc)2
K2PdCl4
K2PdCl4
4a MeOH
4a ACN/H2O RT
4a MeOH
6c
7
RT
In order to examine the catalytic properties of complex 4a/b,
they were applied as catalysts for the P–H addition reaction in
the ‘‘self-generation’’ of the parent NC(sp3)E ligand precursor
(Scheme 3 and Table 2). After optimisation of the reaction
Acetone/
4a
8c
50
24
62
AcOH
9
Li2PdCl4
Li2PdCl4
PdCl2(NCMe)2, NaOAc 4b DCM
K2PdCl4
Li2PdCl4
PdCl2(NCMe)2, NaOAc 4b DCM
4a MeOH
4b MeOH
RT
RT
RT
RT
RT
RT
1.5 77
10
11
12
13d
14d
2
12
18
2
69
85
80
78
83
4b MeOH
4a MeOH
12
a
Reaction conditions: ligand precursor 3a/b (22.0 mmol), Pd source
b
(22.0 mmol or 11.0 mmol in entry 8) in solvent (10 mL). Isolated yield.
c
d
Incomplete reaction, unreacted 3a was recovered partially. 0.375 mmol
Fig. 2 Possible stereochemical pathway for the cyclopalladation process.
of compound 3a/b was used.
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Scheme 3 Asymmetric P–H addition of diene 2 catalysed by 4a or 4b.
Table 2 Asymmetric P–H addition of diene 2 catalysed by NC(sp3)*E
complexesa
Fig. 4 Possible stereochemical pathways for the catalytic hydrophosphi-
nation reaction.
Entry
Cat.
Base
3
Solvent
Yieldb (%)
eec (%)
30
1
2
3
4
5
6
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
Cs2CO3
K3PO4
NaOAc
K2CO3
KOAc
KOAc
KOAc
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3a
3b
3a
DCM
Acetone
DEE
Toluene
DCE
ACN
5
60
27
75
19
70
18
85
45
40
33
35
65
82
66
69
6
19
1
27
to the substrate, resulting in higher enantioselectivity. Nevertheless,
a more detailed investigation is necessary to reveal the contribu-
tions of other factors (such as solvent and base) that may influence
the stereocontrol of the complexes.
5
7
DCM/H2Od
THF/H2Od
DCE/H2Od
THF/H2Od
THF/H2Od
THF/H2Od
THF/H2Od
THF/H2Od
THF/H2Od
THF/H2Od
58
The optical rotation measurements confirmed that the stereo-
genic centers in 3a/b generated in Scheme 3 by the pincer catalyst
4a/b bear the same (S)-absolute configuration as in the original
auxiliaries before undergoing cyclopalladation.11 With reference to
the solid state structures of complex 4a/b, a possible stereochemical
pathway for the P–H addition reaction is proposed (Fig. 4). A close
examination of the X-ray molecular structure with the aid of the
Dreiding model revealed that the top face of complex 4a is sterically
inaccessible due to the steric congestions between the Pd–C(sp3)*
proton, the –CQCPh moiety on the ligand and the axially oriented
Ph group on the phosphorus chelate. Based on the solid state
structures of the complexes and the model studies, it appears that
the addition of the –PPh2 moiety favors the Re-face of the substrate
over the Si-face due to significant steric hindrance imparted by
the NC(sp3)*E auxiliary, thus contributing to the formation of the
(S)-product. A distinct X-ray structural feature observed in complex 4a
is the chiral conformation of the two five-membered (N–C–C–C–Pd
and S–P–C–C–Pd) chelate rings. Due to their skewed ring conforma-
tions, the two adjacent protons on the C-stereogenic centers are
therefore located on the axial plane in the trans-position relative to
each other. The Ph substituents on the P atom are non-equivalent and
adopt the axial and equatorial orientations. A rigid S–P–C–C–Pd
chelate ring comprising of the chiral entities is pivotal in controlling
the stereoselectivity of the hydrophosphination reaction. In order to
confirm if the solid state structural features of the pincer catalysts
have remained unchanged in solution, a 2-D 1H–1H nuclear Over-
hauser effect spectroscopy (NOESY) experiment was conducted on
complex 4a/b in CDCl3 (see the ESI,† Fig. S14 and S15). A detailed
analysis confirmed that the E–P–C–C–Pd chelate ring of
complex 4a/b is locked in the d-conformation, both in the solid
state and in solution.
8e
9
93 (499)f
42
22
19
24
10
11
12
13
14e,g
15e
16e,g
12
96 (499)f
45
46
a
Reaction conditions: diene 2 (43.3 mmol, 1.0 eq.), HPPh2 (65.0 mmol,
1.5 eq.), catalyst (2.2 mmol, 5 mol%), base (8.7 mmol, 20 mol%), solvent
b
c
(2.5 mL), RT, 24 h. Isolated yield. Determined by chiral HPLC.
d
e
f
Solvent : H2O = 10 : 1 (v/v). Reaction was stirred for 40 h. Value of
g
ee in parentheses after a single recrystallisation. S8 (13.0 mmol, 0.3 eq.)
was used instead of aq. H2O2.
conditions, complex 4a was found to be an efficient catalyst in
generating compound 3a/b. Both NC(sp3)*S and NC(sp3)*O
ligand precursors were obtained in high yields with excellent ee
(entries 8 and 14). Under the same reaction conditions, complex 4b
is less reactive and demonstrated lower selectivity (entries 15 and 16).
In the absence of 4a or 4b, the reaction was unable to proceed.10 We
postulated that the base functions by replacing the chloride anion to
allow the coordination of Ph2PH and subsequently, the deprotonation
of the coordinated Ph2PH formed the reactive phosphide inter-
mediate. Finally, the –PPh2 moiety is added onto diene 2 to furnish
the phosphine adduct and the base is regenerated following
protonation. From the results presented, we proposed that the
difference in terms of enantioselectivity could be directly attributed
to the choice of the S or O atom on the complex (Fig. 3). Complex 4a
has visibly smaller Cl–Pd ’ E and Cl–Pd ’ N angles as compared
to that of complex 4b. From the viewpoint of stereoselectivity,
the smaller Cl–Pd ’ S and Cl–Pd ’ N angles of complex 4a will
ensure that the stereogenic elements are situated closer to the
metal center, thereby efficiently transmitting the chiral information
The catalytic P–H addition reaction could be conveniently
monitored by NMR spectroscopy. In particular, the completion
of the catalytic process can be unambiguously detected by both
1H and 31P{1H} NMR spectroscopy. Upon completion of reaction,
catalyst 4a/b could be recovered by column chromatography. The
reactivity and the efficiency of the recycled catalysts were examined
in further P–H addition reactions. The results of the recyclability
tests involving complex 4a are given in Table 3. Typically, upon
completion of the first P–H addition reaction, a large excess of
Fig. 3 Geometries around the NC(sp3)E complexes.
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Table 3 Recyclability test of 4a in the asymmetric hydrophosphination of
diene 2a
efficient in generating optically pure complexes of itself under
mild conditions. From the experimental standpoint, both catalysts 1
and 4a show similar efficiency in the asymmetric P–H addition
reaction of the pyridyl-substituted substrate 2. However, the two
catalysts necessarily transmit their stereochemistry to the reaction
intermediates via different pathways. Catalyst 1 controls the
stereochemistry of the reaction via its prochiral P–Ph groups.
Such C2-symmetrical stereochemically projecting groups are
absent in catalyst 4a. In view of their differences in structural
design, we are testing these catalysts in a variety of stereo-
chemically demanding reactions.
Run
2 (mmol)
Yieldb (%)
eec (%)
4a recoveredd (%)
1
2
3
0.133
0.069
0.031
72
62
50
90 (499)
82 (499)
72 (499)
52
45
—
a
Reaction conditions: diene 2 (1.0 eq.), HPPh2 (1.5 eq.), catalyst
b
(10 mol%), KOAc (40 mol%), THF/H2O = 10 : 1 (v/v), RT, 24 h. Isolated
yield of ligand precursor 3b. Determined by chiral HPLC, ee values (in
c
parentheses) were determined after a single recrystallisation from
d
DCM/n-hexane. Complex 4a recovered is expressed as a percentage
of the amount used in each individual run.
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In conclusion, the cyclopalladation of NC(sp3)E ligand precursors
was efficient in preparing the pincer complexes stereoselectively in
excellent yields. The NC(sp3)S pincer outperformed its O-counterpart
and proved to be an effective catalyst in the hydrophosphina-
1
tion of diene 2. The H–1H NOESY NMR spectra of complexes
4a and 4b were essential in determining the structural rigidity
of the five-membered chelate rings. The recyclability of the
complex was examined (with limited success) to demonstrate
the stability of this hitherto unknown pincer architecture. The
one-pot catalytic ‘‘self-breeding’’ of the pincer complex was
9 R. S. Cahn, C. K. Ingold and V. Prelog, Angew. Chem., Int. Ed., 1966, 5, 385.
10 Based on 31P{1H} NMR, no conversion of HPPh2
(
31P{1H} = ꢀ40.0)
was observed in the absence of the catalyst after 24 h.
11 Absolute configurations of the products were determined by chiral
HPLC and optical rotation measurements.
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