Diphenylprolinol-derived symmetrical and unsymmetrical chiral pincer palladium(II) and nickel(II) complexes: Synthesis via one-pot phosphorylation/metalation reaction and C-H activation

Article abstract of DOI:10.1021/om100234q

The symmetrical P-stereogenic bis-phosphoramidite PCP pincer Pd(II) complexes 1 and 2 were easily prepared by a four-component, one-pot phosphorylation/palladation' procedure via C-H bond activation of the related ligands. In this synthetic procedure, (S)-diphenyl(pyrrolidin-2-yl)methanol was first phosphonated with PCl₃ to afford the expected phosphorochloridate adduct, which then reacted in situ with resorcinol or disubstituted resorcinol, followed by treatment with PdCl₂. The first examples of the unsymmetrical P-stereogenic phosphoramidite- and imidazoline-containing PCN pincer Pd(II) complex 3 and Ni(II) complex 4 could be obtained in a similar manner by using a chiral imidazoline-containing m-phenol derivative instead of resorcinol as a backbone. ³¹P NMR of the complexes confirmed the formation of a single diastereoisomer concerning the P-stereogenic center, and its absolute configuration was determined by an X-ray crystal structure determination. Preliminary investigations on the use of these complexes in the asymmetric allylation indicated that the unsymmetrical pincer Pd complex 3 exhibited higher catalytic activity than the related symmetrical ones. On the other hand, the more bulky Pd complex 2 gave better enantioselectivity, especially in the allylation of 4-nitrobenzenesulfonimine (69% ee).

Full text of DOI:10.1021/om100234q

2148 Organometallics 2010, 29, 2148–2156
DOI: 10.1021/om100234q
Diphenylprolinol-Derived Symmetrical and Unsymmetrical Chiral Pincer
Palladium(II) and Nickel(II) Complexes: Synthesis via One-Pot
Phosphorylation/Metalation Reaction and C-H Activation
Jun-Long Niu,† Qing-Tao Chen,† Xin-Qi Hao,† Qing-Xiang Zhao,‡ Jun-Fang Gong,*,† and
Mao-Ping Song*,†
†Department of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry,
Zhengzhou University, Zhengzhou 450052, People’s Republic of China, and ‡College of Materials Engineering,
Zhengzhou University, Zhengzhou 450052, People’s Republic of China
Received March 26, 2010
The symmetrical P-stereogenic bis-phosphoramidite PCP pincer Pd(II) complexes 1 and 2 were easily
prepared by a “four-component, one-pot phosphorylation/palladation” procedure via C-H bond
activation of the related ligands. In this synthetic procedure, (S)-diphenyl(pyrrolidin-2-yl)methanol was
first phosphonated with PCl3 to afford the expected phosphorochloridate adduct, which then reacted in situ
with resorcinol or disubstituted resorcinol, followed by treatment with PdCl2. The first examples of the
unsymmetrical P-stereogenic phosphoramidite- and imidazoline-containing PCN pincer Pd(II) complex 3
and Ni(II) complex 4 could be obtained in a similar manner by using a chiral imidazoline-containing
m-phenol derivative instead of resorcinol as a backbone. 31P NMR of the complexes confirmed the
formation of a single diastereoisomer concerning the P-stereogenic center, and its absolute configuration
was determined by an X-ray crystal structure determination. Preliminary investigations on the use of these
complexes in the asymmetric allylation indicated that the unsymmetrical pincer Pd complex 3 exhibited
higher catalytic activity than the related symmetrical ones. On the other hand, the more bulky Pd complex 2
gave better enantioselectivity, especially in the allylation of 4-nitrobenzenesulfonimine (69% ee).
Introduction
most common ones have an N-based ligand framework using
amino alcohols or other natural products as chiral auxili-
aries, such as those containing oxazoline (type A),2,3 imida-
zoline (B),4 imine (C),5 and amine skeletons (D-G).6-8
Pincer-type complexes have been extensively studied in
recent years and widely applied in organic synthesis, organo-
metallic catalysis, and materials science.1 In these systems
the facility to modify and tune the properties of pincer
ligands provides a wealth of opportunities to influence the
reactivities, stabilities, and other important properties of the
ensuing pincer complexes. Much of the research has focused
on symmetrical YCY-type pincer metal complexes, where
the ligands coordinate to the metal center via two identical
Y-donor groups and one aromatic carbon anion. The chiral
versions of such tridentate ligands and their corresponding
metal complexes have also been developed (Chart 1). The
(3) Recent examples: (a) Stol, M.; Snelders, D. J. M.; Godbole, M. D.;
Havenith, R. W. A.; Haddleton, D.; Clarkson, G.; Lutz, M.; Spek, A. L.;
van Klink, G. P. M.; van Koten, G. Organometallics 2007, 26, 3985. (b)
Bugarin, A.; Connell, B. T. Organometallics 2008, 27, 4357. (c) Kimura, T.;
Uozumi, Y. Organometallics 2008, 27, 5159. (d) Ito, J.-I.; Ujiie, S.;
Nishiyama, H. Chem. Commun. 2008, 1923. (e) Ito, J.-I.; Ujiie, S.;
Nishiyama, H. Organometallics 2009, 28, 630.
(4) (a) Hao, X.-Q.; Gong, J.-F.; Du, C.-X.; Wu, L.-Y.; Wu, Y.-J.;
Song, M.-P. Tetrahedron Lett. 2006, 47, 5033. (b) Wu, L.-Y.; Hao, X.-Q.;
Xu, Y.-X.; Jia, M.-Q.; Wang, Y.-N.; Gong, J.-F.; Song, M.-P. Organometal-
lics 2009, 28, 3369.
(5) (a) Fossey, J. S.; Richards, C. J. Organometallics 2002, 21, 5259.
(b) Fossey, J. S.; Russell, M. L.; Abdul Malik, K. M.; Richards, C. J. J.
Organomet. Chem. 2007, 692, 4843.
(6) (a) Gosiewska, S.; Huis in’t Veld, M.; de Pater, J. J. M.; Bruijnincx,
P. C. A.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M.
Tetrahedron: Asymmetry 2006, 17, 674. (b) Gosiewska, S.; Herreras Martinez,
S.; Lutz, M.; Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. M. Eur. J. Inorg.
Chem. 2006, 4600. (c) Gosiewska, S.; Herreras Martinez, S.; Lutz, M.; Spek,
A. L.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G.; Klein Gebbink,
R. J. M. Organometallics 2008, 27, 2549.
*To whom correspondence should be addressed. Tel: þ86 371
67763869. E-mail: mpsong9350@zzu.edu.cn (M.-P.S.); gongjf@zzu.
edu.cn (J.-F.G.).
(1) For selected reviews on pincer complexes, see: (a) Albrecht, M.;
van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (b) Singleton, J. T.
Tetrahedron 2003, 59, 1837. (c) van der Boom, M. E.; Milstein, D. Chem.
Rev. 2003, 103, 1759. (d) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004,
248, 2239. (e) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105,
ꢀ
2527. (f) Szabo, K. J. Synlett 2006, 6, 811. (g) Pugh, D.; Danopoulos, A. A.
(7) (a) Takenaka, K.; Uozumi, Y. Org. Lett. 2004, 6, 1833. (b)
Takenaka, K.; Minakawa, M.; Uozumi, Y. J. Am. Chem. Soc. 2005, 127,
12273.
(8) (a) van Beek, J. A. M.; van Koten, G.; Dekker, G. P. C. M.;
Wissing, E.; Zoutberg, M. C.; Stam, C. H. J. Organomet. Chem. 1990,
394, 659. (b) van Beek, J. A. M.; van Koten, G.; Ramp, M. J.; Coenjaarts,
N. C.; Grove, D. M.; Goubitz, K.; Zoutberg, M. C.; Stam, C. H.; Smeets,
W. J. J.; Spek, A. L. Inorg. Chem. 1991, 30, 3059. (c) Albrecht, M.; Kocks,
B. M.; Spek, A. L.; van Koten, G. J. Organomet. Chem. 2001, 624, 271.
Coord. Chem. Rev. 2007, 251, 610. (h) The Chemistry of Pincer Com-
pounds; Morales-Morales, D., Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007. (i)
Morales-Morales, D. Mini-Rev. Org. Chem. 2008, 5, 141. (j) Serrano-Becerra,
J. M.; Morales-Morales, D. Curr. Org. Synth. 2009, 6, 169.
(2) For recent reviews on the preparation and catalytic reactions of
NCN bis(oxazolinyl)phenyl metal pincer complexes, see: (a) Nishiyama,
H.; Ito, J.-I. Chem. Commun. 2010, 46, 203. (b) Nishiyama, H. Chem. Soc.
Rev. 2007, 36, 1133.
r
pubs.acs.org/Organometallics
Published on Web 04/16/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 9, 2010 2149
Chart 1. Examples of Chiral Pincer Metal Complexes
Among them, the NCN bis(oxazolinyl)phenyl-metal com-
plexes including Rh, Pd, Pt, Ru, and Ni have been extensively
investigated and widely used in asymmetric catalyses such as
alkylation of aldimines (up to 82% ee), aldol reaction of
isocyanides and aldehydes (up to 75% ee), conjugate reduc-
tion of R,β-unsaturated esters with hydrosilanes (up to 98%
ee), hydrogenation (up to 98% ee), and transfer hydrogena-
tion of ketones (up to 97% ee).2,3 In contrast, the synthesis
and applications of chiral pincer metal complexes with
P-donor ligands for the chiral catalysts have been relatively
unexplored.9-12 The following two reasons may account for
this: (a) finding an appropriate P-donor pincer ligand with
stereochemical parameters has proven to be difficult and
costly in comparison with the case for chiral N-donor ligands
and (b) the air- and moisture-sensitive preligands usually
need to be constructed and isolated under harsh conditions,
thus resulting in laborious synthetic procedures required for
the preparation of their corresponding metal complexes.
Nonetheless, chiral pincer metal complexes with phosphines
bearing either a stereogenic benzylic methylene center or a
chiral phosphorus atom have been successfully synthesized
activation, transmetalation reaction, or oxidative addition.
Among them, the direct C-H activation should be the most
simple and convenient method for the construction of com-
plexes because it does not require prefunctionalization of
the pincer ligands to achieve regioselective metalation. In
previous work, we reported a series of chiral pincer Pt(II)
and Pd(II) complexes with 1,3-bis(20-imidazolinyl)benzenes
(Chart 1, type B),4 achiral and chiral Pd(II) complexes with
unsymmetrical oxazolinyl-containing ligands,13a and parti-
cularly a facile “three-component, one-pot phosphorylation/
palladation” method for the preparation of Pd(II) complexes
with mono- or diphosphinite ligands.13b,c These studies
allowed us to obtain various PCP, NCN, and PCN pincer
Pd(II) or Pt(II) complexes via direct C-H activation of the
related ligands. As a part of our program to further extend
one-pot phosphorylation/palladation, we synthesized P-chiral
square-planar pincer metal complexes, including the symmetri-
cal bis-phosphoramidite PCP pincer Pd(II) complexes 1and 2as
well as the first unsymmetrical P-chiral PCN pincer Pd(II)
complex (3) and Ni(II) complex (4) using easily available amino
alcohols as chiral auxiliaries (Figure 1). In addition, preliminary
investigations on the asymmetric allylation of aldehyde and
sulfonimine catalyzed by these chiral PCP and PCN complexes
are also described. It should be mentioned that during the study
presented here, very recently Klein Gebbink and co-workers
reported the related diphenylprolinol-derived chiral Pd(II) com-
plex 5 via an oxidation addition (Figure 1).14
(types F and G, Y = P).9,10 Additionally, Szabo and Bedford
ꢀ
independently reported chiral phosphite PCP palladium com-
plexes by incorporating axially chiral 1,10-bi-2-naphthol- and
biphenanthrol-derived diols into the PCP-pincer backbone
(type H).11
Generally, the formation of pincer metal complexes can be
achieved by the method of direct metalation via C-H bond
Results and Discussion
(9) (a) Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A.; Lianza, F.
Organometallics 1994, 13, 1607. (b) Longmire, J. M.; Zhang, X.; Shang, M.
Organometallics 1998, 17, 4374. (c) Dani, P.; Albrecht, M.; van Klink, G. P.
M.; van Koten, G. Organometallics 2000, 19, 4468. (d) Kuznetsov, V. F.;
Lough, A. J.; Gusev, D. G. Inorg. Chim. Acta 2006, 359, 2806.
Synthesis of Symmetrical Chiral Bis-Phosphoramidite
PCP-Pincer Palladium Complexes 1 and 2. The chiral pincer
complex 1 was prepared from easily available starting materials
in a four-component (including diphenylprolinol, PCl3, resorci-
nol, and PdCl2), one-pot manner, as shown in Scheme 1. First,
the optically active amino alcohol (S)-diphenyl(pyrrolidin-2-yl)-
methanol was phosphonated with PCl3 inthepresenceoftriethyl-
amine in DCE (1,2-dichloroethane) to afford the expected
(10) (a) Williams, B. S.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G.
Helv. Chim. Acta 2001, 84, 3519. (b) Morales-Morales, D.; Cramer, R. E.;
Jensen, C. M. J. Organomet. Chem. 2002, 654, 44. (c) Medici, S.; Gagliardo,
M.; Williams, S. B.; Chase, P. A.; Gladiali, S.; Lutz, M.; Spek, A. L.; van
Klink, G. P. M.; van Koten, G. Helv. Chim. Acta 2005, 88, 694.
ꢀ
(11) (a) Wallner, O. A.; Olsson, V. J.; Eriksson, L.; Szabo, K. J. Inorg.
Chim. Acta 2006, 359, 1767. (b) Baber, R. A.; Bedford, R. B.; Betham, M.;
Blake, M. E.; Coles, S. J.; Haddow, M. F.; Hursthouse, M. B.; Orpen, A. G.;
Pilarski, L. T.; Pringle, P. G.; Wingad, R. L. Chem. Commun. 2006, 3880.
(13) (a) Hao, X.-Q.; Wang, Y.-N.; Liu, J.-R.; Wang, K.-L.; Gong, J.-F.;
Song, M.-P. J. Organomet. Chem. 2010, 695, 82. (b) Gong, J.-F.; Zhang, Y.-H.;
Song, M.-P.; Xu, C. Organometallics 2007, 26, 6487. (c) Zhang, B.-S.; Wang,
C.; Gong, J.-F.; Song, M.-P. J. Organomet. Chem. 2009, 694, 25545.
(14) Li, J.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten, G.;
Klein Gebbink, R. J. M. Organometallics 2010, 29, 1379.
ꢀ
(c) Aydin, J.; Kumar, K. S.; Sayah, M. J.; Wallner, O. A.; Szabo, K. J. J. Org.
Chem. 2007, 72, 4689.
(12) (a) Benito-Garagorri, D.; Bocokic, V.; Mereiter, K.; Kirchner,
K. Organometallics 2006, 25, 3817. (b) Benito-Garagorri, D.; Kirchner, K.
Acc. Chem. Res. 2008, 41, 201.

2150 Organometallics, Vol. 29, No. 9, 2010
Niu et al.
Figure 1
Scheme 1. Synthesis of Symmetrical Chiral Bis-
Phosphoramidite PCP Pincer Palladium Complexes 1 and 2 via
C-H Bond Activation
Scheme 2. Klein Gebbink’s Oxidative Addition Route for the
Synthesis of the Symmetrical Chiral Bis-Phosphoramidite PCP
Pincer Pd(II) Complex 5
phosphorochloridate adduct. Then the adduct reacted in situ
with resorcinol, followed by treatment with PdCl2. The pure
air- and moisture-stable complex 1 was successfully obtained
in 35% isolated yield (based on resorcinol) as a white solid
after chromatography on silica gel. Bedford and co-workers11b
reported that incorporation of tert-butyl groups into the 4- and
6-positions of resorcinol could increase the rate of metalation in
the preparation of bis-phosphite pincer Pd complexes and also
led to enhanced activity in the asymmetric allylation of aldehydes
in comparison with the less bulky pincer complexes. Therefore,
we tried to synthesize complex 2 with the more hindered tert-
butyl group on the central aromatic ring. The process was carried
out in a fashion similar to that described for complex 1 using
4,6-di-tert-butylbenzene-1,3-diol instead of resorcinol as a back-
bone. Surprisingly, the optically pure complex 2 was obtained in
aloweryield(20%,basedondi-t-Bu-substituted resorcinol). The
formation of a single diastereoisomer for complexes 1 and 2
concerning the P-stereogenic center was confirmed by the singlet
resonances at 155.8 and 159.1 ppm, respectively, in the 31PNMR
spectra.
Klein Gebbink and co-workers recently described an
oxidative addition route for the synthesis of the symmetrical
chiral bis-phosphoramidite 5 (Scheme 2).14 In their synthetic
route, 2-iodoresorcinol, which could be prepared from re-
sorcinol,15 was needed for metalation via oxidative addition.
In addition, both the ligand 6 and the pincer Pd complex 5
were obtained as two diastereoisomers with regard to the
P-stereogenic center (dr = 98:2 and >99:1 after recrystalli-
zation, respectively, estimated by 31P NMR). In fact, they
also tried the C-H activation method, but without success.
In their experiments, when 4,6-di-tert-butylbenzene-1,3-diol
was used to couple with phosphorochloridate adduct 7 to
Scheme 3. Conversion of Pd-Cl Complex 1 to Pd-I Complex 5
by the Halogen Exchange Reaction
form the corresponding bis-phosphoramidite ligand, a complete
loss of stereospecificity on the phosphorus atom occurred,
which was confirmed by two sharp singlets of equal intensity
at 145.3 and 147.6 ppm in the 31P NMR spectra. In contrast,
4,6-di-tert-butylbenzene-1,3-diol could react with the phosphoro-
chloridate obtained from treatment of (S)-(þ)-indoline-
methanol with PCl3 to afford the bis-phosphoramidite
ligand in excellent diastereoselectivity (dr = 96:4). However,
subsequent metalation with PdCl2(MeCN)2 via C-H activa-
tion was found to proceed very sluggishly and a pure sample
of the pincer Pd complex could not be isolated.
We think the racemization of P-chirality may be caused by the
high reaction temperature (refluxing toluene) in the coupling
of 4,6-di-tert-butylbenzene-1,3-diol with phosphorochloridate
adduct 7 in Klein Gebbink’s experiments. In our case, the
coupling was carried out at low temperature (-35 °C to room
temperature). Although the yields of the pincer metal complexes
by this “four-component, one-pot” strategy were not very high,
the synthetic procedure was simplified and had some advan-
tages: (a) it avoided the troublesome step of isolating the air- and
moisture-sensitive P-containing species, (b) the synthesis was
highly stereoselective, yielding a single diastereoisomer with
(15) (a) Weitl, F. L. J. Org. Chem. 1976, 41, 2044. (b) Thomsen, I.;
Torssell, K. B. G. Acta Chem. Scand. 1991, 45, 539. (c) Hamura, T.; Hosoya,
T.; Yamaguchi, H.; Kuriyama, Y.; Tanabe, M.; Miyamoto, M.; Yasui, Y.;
Matsumoto, T.; Suzuki, K. Helv. Chim. Acta 2002, 85, 3589.

Article
Organometallics, Vol. 29, No. 9, 2010 2151
Scheme 4. Synthesis of the Unsymmetrical P-Chiral PCN Pincer Pd(II) Complex 3 and Ni(II) Complex 4
regard to the P-stereogenic center, and (c) the direct metalation
via C-H activation of the related 1,3-disubstituted benzene was
successfully achieved during the reaction by using a cheap and
commercially available Pd(II) salt instead of more expensive
and unstable Pd0 sources and so the preparation of the appro-
priate 1,2,3-trisubstituted benzene was unnecessary. Actually,
we found that the iodopalladium complex 5 could be prepared
in good yield from the chloropalladium complex 1 by treatment
with an excess of KI in acetone-MeOH at room temperature
(Scheme 3). The 31P NMR spectrum of complex 5 shows a
singlet resonance at 159.7 ppm, shifted downfield in comparison
with that of complex 1 (155.8 ppm).
PCN pincer metal complexes 3 and 4 are the first examples of
unsymmetrical chiral pincer complexes containing a P-stereo-
genic center. Both complexes 3 and 4 were stable toward air and
moisture and could be stored for several weeks at ambient tem-
perature in an air atmosphere. Similar to the symmetrical PCP
complexes 1 and 2, 31P NMR spectra of these unsymmetrical
chiral PCN pincer complexes also confirmed the formation of a
single diastereoisomer concerning the P-stereogenic center by the
singlet resonance at 56.7 and 160.3 ppm, respectively.
It was reported in the literature that there was a correla-
2
tion between the magnitude of the JC(8)-P value and the
dihedral angle associated with the lone-pair orbital of the
phosphorus atom and the β-carbon of the group attached to
P (Figure 2).17 With such a correlation, the 13C NMR has
been used to predict the configuration of the P-stereogenic
Synthesis of the Unsymmetrical Chiral PCN-Pincer Pd(II)
Complex 3 and Ni(II) Complex 4. To synthesize potentially
hemilabile hybrid chiral PCN-pincer complexes, especially
those containing a P-chiral moiety, we adopted displacement
of one phospholidine cycle at the 2- or 6-position of the
central aryl ring in the symmetrical complexes 1 and 2 by a
chiral imidazoline unit. Thus, following a synthetic route
similar to that for complexes 1 and 2, the adduct obtained
from treatment of (S)-diphenyl(pyrrolidin-2-yl)methanol
with PCl3 was reacted in situ with the imidazolinyl-contain-
ing m-phenol derivative 8. The subsequent palladation also
proceeded in situ by the addition of PdCl2 (Scheme 4). The
unsymmetrical Pd(II) complex 3 was successfully isolated as
a yellow solid after chromatography on silica gel in 42%
yield, which was higher than that for the symmetrical
bis-phosphoramidite Pd(II) complex 1 (35%). Inspired by
the successful preparation of the Pd(II) complex 3, the
nickelation of the related preligand was tried with NiCl2
instead of PdCl2. We were pleased to find that the corres-
ponding PCN-pincer Ni(II) complex 4 could also be obtained
via C-H activation as a pale yellow solid after chromato-
graphy on silica gel, albeit in a lower yield (20%). It should be
noted that nickelation of the symmetrical bis-phosphorami-
dite ligand with NiCl2 via C-H activation in a similar
procedure did not yield the desired PCP pincer Ni(II) com-
plex. The above results indicated that the nature of the pincer
ligands had a major influence on the metalation step. The
metalation of the imidazoline-containing unsymmetrical
ligands was easier than that of the related symmetrical bis-
phosphoramidite ligand, possibly due to the greater donor
strength of imidazoline, which promoted metal coordina-
tion. To our knowledge, reports on the unsymmetrical and
chiral pincer metal complexes are rather limited,13a,16 and the
2
phosphorus donor.18 The large JC(8)-P values (Table 1) of
triplet signals in the symmetrical PCP Pd(II) complexes 1, 2,
and 5 indicate that the phospholidine cycle at the phosphorus
atom is in a pseudoequatorial position and, therefore, that
the P-stereogenic phosphorus atom has an S absolute con-
figuration (without taking into account the formal priority
of the metal atom), which is agreement with the X-ray
analysis results. In the 13C NMR spectra of unsymmetrical
PCN complexes 3 and 4, the large 2JC(8)-P values (Table 1) of
doublet signals also suggest the S absolute configuration of
the P-stereogenic phosphorus atom.
Molecular Structures of Pincer Complexes 1-4. The mo-
lecular structures of three symmetrical and two unsymme-
trical pincer complexes 1-5 were determined by X-ray
single-crystal analysis. In the unit cell of complexes 1 and
2, there exist two crystallographically independent molecules
which have essentially the same structure, and thus only one
of them is shown in Figures 3 and 4, respectively. Molecular
structures of 3 and 4 are shown in Figures 5 and 6 (that of 5
and the related data are given in Supporting Information).
Selected bond lengths and bond angles for complexes 1-4
are collected in Tables 2 and 3. The general features of the
five pincer complexes are the unique configurations of
N- and P-stereogenic centers, which have S absolute configu-
rations of N and P atoms for each complex (without taking
into account the formal priority of the metal atom). This
arrangement avoids the steric interactions between the exo-
cyclic substituents at the phosphorus atom and the central
(17) Arzoumanian, H.; Buono, G.; Choukrad, M. B.; Petrignani, J. F.
Organometallics 1988, 7, 59.
(18) Gavrilov, K. N.; Tsarev, V. N.; Shiryaev, A. A.; Bondarev,
O. G.; Lyubimov, S. E.; Benetsky, E. B.; Korlyukov, A. A.; Antipin,
M. Y.; Davankov, V. A.; Gais, H.-J. Eur. J. Inorg. Chem. 2004, 629.
(16) Motoyama, Y.; Shimozono, K.; Nishiyama, H. Inorg. Chim.
Acta 2006, 359, 1725.

2152 Organometallics, Vol. 29, No. 9, 2010
Niu et al.
2
Figure 2. Correlation between the JC(8)-P value and the con-
figuration of the P-stereocenter for the phospholidine cycle.
Table 1. Selected NMR Spectroscopic Data for Complexes 1-5
(in CDCl3)
Figure 5. X-ray molecular structure of 3. Hydrogen atoms are
omitted for clarity.
complex
31P NMR (ppm)
2JC(8)-P (Hz)
2JC(5)-P (Hz)
1
2
3
4
5
155.8
159.1
56.7
160.3
159.7
9.5
10.0
18.0
15.4
9.6
0
0
2.8
2.0
0
Figure 6. X-ray molecular structure of 4. Hydrogen atoms are
omitted for clarity.
P-Pd-P (or N-M-P) being around 160° and C-M-X
being 176-179°. The phospholidine cycle occupies a pseudo-
equatorial position at the phosphorus atom.
Including the Pd-I complex 5, all three symmetrical PCP-
pincer Pd(II) complexes have similar bond lengths and
angles around Pd(II), except for shorter Pd-P (2.2454(11)-
2.2632(11) A) and longer Pd-C (around 2.005 A) bond
lengths in complex 2, which is attributable to the steric
hindrance of the 2-tert-butyl moiety. The phospholidine
cycles in complex 1 and 2 adopt slightly twisted conforma-
tions (in the unshown structure of complex 1, the phospho-
lidine cycles take an envelope conformation), while the
pyrrolidine cycles have envelope conformations.
Figure 3. X-ray molecular structure of 1 (representation of one
of the two independent crystal structures). Hydrogen atoms are
omitted for clarity.
˚
˚
In comparison with the corresponding symmetrical bis-
phosphoramidite PCP pincer Pd complex 1 or NCN bis-
(imidazoline) Pd complex,4b the unsymmetrical PCN Pd com-
˚
plex 3 has a slightly shorter Pd-C bond distance (1.949(5) A).
˚
The Pd-N bond distance (2.079(5) A) is longer than the related
distances in the symmetrical NCN complex (Pd(1)-N(1) =
˚
˚
2.031(13) A and Pd(1)-N(3) = 2.078(12) A), while the Pd-P
distance (2.1786(16) A) is shorter than those in the PCP
˚
complex 1, implying an increased trans influence of the phos-
phoramidite moiety compared with the imidazoline ligand. A
comparison between the unsymmetrical PCN pincer Pd(II)
complex 3 and Ni(II) complex 4 reveals that the two complexes
possess the same structural architectures and chiralities except
for the different metal atoms. However, they exhibit signifi-
cantly different bond lengths and angles for the metal coordi-
nation sphere (Table 3). Similarly to the previously reported
results,19 the metal-tridentate ligand bond lengths in the two
complexes follow the expected pattern Ni<Pd. For example,
Figure 4. X-ray molecular structure of 2 CH3COCH3
3
(representation of one of the two independent crystal structures).
Hydrogen atoms and the noncoordinated CH3COCH3 molecule
are omitted for clarity.
aryl ring. The ligand in each complex is coordinated to the
metal in a tridentate manner, and the two five-membered-
ring metallacycles that formed are approximately coplanar
with the central aryl ring. The metal atom adopts a typical
distorted-square-planar configuration with bond angles of
(19) Fossey, J. S.; Richards, C. J. J. Organomet. Chem. 2004, 689,
3056.

Article
Organometallics, Vol. 29, No. 9, 2010 2153
˚
Table 2. Selected Bond Lengths (A) and Bond Angles (deg) for Chiral PCP-Pincer Pd(II) Complexes 1 and 2 CH3COCH3
3
1
2
10
20
Pd(1)-C(35)
Pd(1)-Cl(1)
Pd(1)-P(1)
Pd(1)-P(2)
P(1)-O(1)
P(1)-O(2)
P(2)-O(3)
P(2)-O(4)
P(1)-N(1)
P(2)-N(2)
1.958(5)
2.3561(16)
2.2717(13)
2.2759(13)
1.613(3)
1.612(4)
1.631(4)
1.599(3)
1.649(4)
1.650(3)
2.009(4)
2.3795(13)
2.2454(11)
2.2574(11)
1.635(3)
1.596(3)
1.590(3)
1.625(3)
1.660(4)
1.667(4)
Pd(10)-C(350)
Pd(10)-Cl(10)
Pd(10)-P(10)
Pd(10)-P(20)
P(10)-O(10)
P(10)-O(20)
P(20)-O(30)
P(20)-O(40)
P(10)-N(10)
P(20)-N(20)
1.976(4)
2.3756(14)
2.2989(14)
2.2748(14)
1.611(3)
1.636(3)
1.641(3)
1.590(3)
1.625(4)
1.621(3)
2.000(4)
2.3735(14)
2.2632(11)
2.2575(11)
1.588(3)
1.630(3)
1.582(3)
1.638(3)
1.655(4)
1.657(4)
P(1)-Pd(1)-P(2)
C(35)-Pd(1)-P(1)
C(35)-Pd(1)-P(2)
C(35)-Pd(1)-Cl(1)
P(1)-Pd(1)-Cl(1)
P(2)-Pd(1)-Cl(1)
O(2)-P(1)-O(1)
O(2)-P(1)-N(1)
O(1)-P(1)-N(1)
O(4)-P(2)-O(3)
O(4)-P(2)-N(2)
O(3)-P(2)-N(2)
160.98(5)
80.37(13)
80.98(14)
178.08(14)
100.94(6)
97.61(6)
104.17(16)
105.8(2)
96.65(16)
103.77(17)
96.28(16)
106.21(19)
160.12(4)
80.40(11)
80.99(11)
177.18(12)
98.06(4)
100.24(4)
104.05(16)
97.40(17)
107.05(17)
105.05(16)
104.66(18)
96.87(16)
P(20)-Pd(10)-P(10)
C(350)-Pd(10)-P(10)
C(350)-Pd(10)-P(20)
C(350)-Pd(10)-Cl(10)
P(10)-Pd(10)-Cl(10)
P(20)-Pd(10)-Cl(10)
O(10)-P(10)-O(20)
N(10)-P(10)-O(20)
O(10)-P(10)-N(10)
O(40)-P(20)-N(20)
O(40)-P(20)-O(30)
N(20)-P(20)-O(30)
160.30(4)
79.10(10)
81.24(10)
178.37(10)
102.26(4)
97.42(5)
98.69(17)
104.93(19)
96.58(16)
96.69(17)
102.51(19)
105.8(2)
161.50(4)
80.81(12)
80.80(12)
176.66(13)
99.46(5)
98.79(5)
104.68(16)
105.23(18)
97.18(15)
104.12(17)
105.19(18)
97.16(16)
˚
Table 3. Selected Bond Lengths (A), Bond Angles (deg), and
Torsion Angles (deg) for Chiral PCN-Pincer Pd(II)
Complex 3 and Ni(II) Complex 4
bis(oxazolinyl)phenylrhodium(III) NCN-pincer complexes as
Lewis acid catalysts in the enantioselective allylation of alde-
hydes (up to 80% ee).20 As a preliminary investigation, the
activity and the stereocontrolling potential of the obtained pincer
complexes 1-4 in the asymmetric allylation were evaluated.
Klein Gebbink recently reported that the electron-withdrawing
nitro group on the aryl substituent of sulfonimine led to both
higher product yield and improved enantioselectivity when chiral
amino alcohol derived bis-phosphoramidite pincer Pd com-
plexes were used as the catalysts.14 Accordingly, allylation of
4-nitrobenzaldehyde (9) and 4-nitrobenzenesulfonimine (10)
with allyltributyltin were chosen as the model reactions. The
results are summarized in Table 4.
The enantioselectivities of the symmetrical PCP pincer Pd
complexes 1 and 2 in the allylation of 4-nitrobenzaldehyde
(9) were overall rather moderate (entries 1-4). Nonetheless,
it was notable that the more bulky complex 2 displayed
higher product yield and particularly enhanced enantioselec-
tivity (about 2-fold) in comparison with complex 1 (30%
yield, 11% ee vs 42% yield, 23% ee). This was in accordance
with the findings of Bedford and co-workers. They demon-
strated that both the yield and enantioselectity improved
significantly by the incorporation of tert-butyl groups into
the 3- and 5-positions of chiral 1,10-bi-2-naphthol-bis-phos-
phite pincer Pd complexes in the asymmetric allylation of
benzaldehyde (82% yield, 62% ee vs 18% yield, 6% ee).11b
Further studies indicated that complex 2 exhibited more
promising enantioselectivity in the asymmetric allylation of
4-nitrobenzenesulfonimine (10) (69% ee, entries 5 and 6).
The enantioselectivity was much higher than that of the
related bis-phosphoramidite PCP pincer Pd complex derived
from chiral indolinemethanol reported by Klein Gebbink
and co-workers (12% ee).14 In comparison with the sym-
metrical PCP Pd complex 2, the unsymmetrical chiral imi-
dazoline-containing PCN pincer Pd complex 3 provided
higher product yield (90%) and moderate enantioselectity
(33% ee, entry 7) under the same reaction conditions. Inter-
estingly, the enantiomer of 12 possessed the opposite absolute
3 (M = Pd)
4 (M = Ni)
M(1)-C(1)
M(1)-N(2)
M(1)-P(1)
M(1)-Cl(1)
P(1)-O(2)
P(1)-O(1)
P(1)-N(1)
1.949(5)
2.079(5)
2.1786(16)
2.3687(15)
1.588(4)
1.647(4)
1.655(5)
1.851(6)
1.900(5)
2.0930(18)
2.2171(15)
1.594(4)
1.639(4)
1.650(5)
C(1)-M(1)-N(2)
C(1)-M(1)-P(1)
N(2)-M(1)-P(1)
C(1)-M(1)-Cl(1)
N(2)-M(1)-Cl(1)
P(1)-M(1)-Cl(1)
O(2)-P(1)-O(1)
O(2)-P(1)-N(1)
O(1)-P(1)-N(1)
N(2)-C(24)-N(3)
78.6(2)
82.0(2)
81.71(16)
160.31(13)
179.23(17)
101.15(13)
98.51(6)
103.3(2)
96.7(2)
82.50(17)
164.38(14)
177.17(17)
99.96(15)
95.58(6)
103.1(2)
96.6(2)
105.9(3)
113.8(5)
105.5(2)
114.8(5)
M(1)-P(1)-O(1)-C(6)
C(2)-C(24)-N(2)-M(1)
N(2)-M(1)-C(1)-C(2)
-2.2(4)
2.1(6)
0.0(4)
-0.6(4)
0.1(7)
-0.2(4)
˚
the Ni complex 4 has a Ni-C bond length of 1.851(6) A, a
Ni-N bond length of 1.900(5) A, a Ni-P bond length of
2.0930(18) A, and a Ni-Cl bond length of 2.2171(15) A. The
corresponding values in the Pd complex 3 are 1.949(5), 2.079(5),
2.1786(16), and 2.3687(15) A, respectively. The shorter Ni-Cl
bond length in 4 is likely due to a reduced trans influence of the
metalated carbanion. The bond lengths and angles around the
phosphorus atom in 3 and 4 are almost identical (see Table 3).
Additionally, the imidazoline and pyrrolidine cycles in the two
complexes adopt envelope conformations, and the phospholi-
dine cycles have twisted conformations.
˚
˚
˚
˚
ꢀ
Catalytic Studies. The discovery of Szabo’s and Bedford’s
complexes,11 1,10-bi-2-naphthol- and biphenanthrol-based
PCP chiral pincer palladium complexes, acting as effective
catalysts for the asymmetric allylation of aldehydes or
sulfonimines opened the door for developing this reaction
catalyzed by chiral pincer complexes with P-donor ligands.
Additionally, Nishiyama and co-workers applied the chiral
(20) (a) Motoyama, Y.; Narusawa, H.; Nishiyama, H. Chem. Com-
mun. 1999, 131. (b) Motoyama, Y.; Okano, M.; Narusawa, H.; Makihara, N.;
Aoki, K.; Nishiyama, H. Organometallics 2001, 20, 1580.

2154 Organometallics, Vol. 29, No. 9, 2010
Niu et al.
Table 4. Asymmetric Allylation of Aldehyde 9 and Sulfonimine 10 Catalyzed by Symmetrical and Unsymmetrical P-Chiral Pincer Metal
Complexes 1-4 and the Related Symmetrical Bis(imidazoline) NCN Pd Complex 13a
entry
cat.
substrate
product
solvent
time (h)
yield (%)b
ee (%)c,d
1
2
3
4
5
6
7
8
9
1
1
1
2
1
2
3
13
4
9
9
9
9
10
10
10
10
10
11
11
11
11
12
12
12
12
12
DMF
DMF
CH2Cl2
CH2Cl2
DMF
DMF
DMF
DMF
DMF
24
96
24
24
96
96
96
96
96
34
78
30
42
75
77
90
19
11
7
0
11
23
30
69
33e
0
0
a All reactions were performed with 1 equiv of 9 or 10 and 1.2 equiv of allyltributyltin. A 5 mol % amount of catalyst was used. b Isolated yield.
c Detected by chiral HPLC. d The absolute configurations of the allylic products have not been assigned. e Opposite absolute configuration.
configuration in comparison to that of the product catalyzed
by complex 2. The performance of the related symmetrical
bis(imidazoline) NCN Pd complex 13 in the same reaction
was also investigated. A racemic product in low yield was
obtained (entry 8). The above results indicated that the more
electron-deficient phosphoramidite ligand could improve
the catalytic activity of the corresponding pincer complexes,
and a hemilabile hybrid PCN ligand with phosphoramidite
led to further enhancement of the catalytic activity. On the
other hand, bulky ligand substituents were in favor of good
stereocontrol, and the absolute configuration of the enan-
tiomer of 12 may depend much on the configuration of the
chiral center in the imidazoline moiety. Finally, the unsym-
metrical PCN-pincer Ni(II) complex 4 afforded a racemic
product in a rather low yield (11%, entry 9).
more bulky Pd complex 2 gave better enantioselectivity,
especially in the allylation of 4-nitrobenzenesulfonimine
(69% ee). Further studies will focus on the synthesis of bulky
P-chiral unsymmetrical pincer metal complexes as well as
their applications in asymmetric catalysis.
Experimental Section
General Procedures. Unless otherwise noted, all reactions were
carried out under nitrogen using standard Schlenk and vacuum
line techniques. Solvents were dried with standard methods and
freshly distilled prior to use if needed. (S)-Diphenyl(pyrrolidin-
2-yl)methanol,21 the imidazolinyl-containing m-phenol derivative
8,22 and 4,6-di-tert-butylbenzene-1,3-diol23 were prepared accord-
ing to the literature methods. Other reagents were obtained from
commercial sources and used as received without further purifica-
tion. Melting points were determined using an XT4A melting point
apparatus and are uncorrected. Elemental analyses were deter-
mined with a Thermo Flash EA 1112 elemental analyzer. Optical
rotations were measured with a Perkin-Elmer Model 341 polari-
meter at 20 °C in CH2Cl2. The enantiomeric purity was determined
by HPLC using a chiral column with hexane-propan-2-ol (ratio as
indicated) as the eluent. The column used was a Chiracel AD-H
from Daicel Chemical Ind., Ltd. (Japan). The column was oper-
ated at ambient temperature. 1H, 13C NMR, and 31P{1H} NMR
spectra were recorded on a Bruker DPX-400 or Bruker DPX-300
spectrometer in CDCl3 with TMS as an internal standard for 1H
and 13C NMR and 85% H3PO4 as external standard for 31P{1H}
NMR. J values are given in Hz. IR spectra were determined on a
Thermo Nicolet IR 200 spectrophotometer.
General Procedure for the Synthesis of Chiral PCP- and PCN-
Pincer Metal Complexes. To a stirred mixture of Et3N (2 mmol)
and PCl3 (0.525 mmol) in 1,2-dichloroethane (1 mL) was added
dropwise a solution of (S)-diphenyl(pyrrolidin-2-yl)methanol (0.5
mmol) in 1,2-dichloroethane (3 mL) at -35 °C. After the mixture
was stirred for 3.5 h at room temperature, Et3N (1 mmol) and the
resorcinol-based ligand (0.25 mmol) or compound 8 (0.5 mmol)
were added at -35 °C followed by stirring at room temperature for
15-20 h. MX2 (0.5 mmol of PdCl2 or NiCl2) was then added, and
the reaction mixture was refluxed for 24 h (6 h in the case of NiCl2).
Conclusions
In summary, we have developed an operationally simple
procedure for the highly diastereoselective preparation of
P-chiral symmetrical PCP and particularly unsymmetrical
PCN pincer metal complexes using easily available amino
alcohols as chiral auxiliaries. This “four-component, one-
pot phosphorylation/metalation” procedure avoided the
troublesome isolating step of unstable P-containing species
and achieved the metalation of the related ligands via C-H
bond activation. It was found that the nature of the pincer
ligands has a major influence not only on the metalation step
but also on the properties of the ensuing pincer complexes in
the asymmetric allylation of aldehydes or sulfonimines. The
metalation of the imidazoline-containing unsymmetrical
ligand was easier than that of the corresponding symmetrical
bis-phosphoramidite ligand. Also, its unsymmetrical pincer
Pd complex exhibited catalytic activity higher than that for
the related symmetrical complexes. On the other hand, the
(21) (a) Kanth, J. V. B.; Periasamy, M. Tetrahedron 1993, 49, 5127. (b)
Xavier, L. C.; Mohan, J. J.; Mathre, D. J.; Thompson, A. S.; Carroll, J. D.;
Corley, E. G.; Desmond, R. Org. Synth. 1997, 74, 50.
(22) Zhang, B. S.; Wang, W.; Shao, D. D.; Hao, X. Q.; Gong, J. F.;
Song, M. P. Submitted for publication in Organometallics.
(23) Halfpenny, P. J.; Johnson, P. J.; Robinson, M. J. T.; Ward,
M. G. Tetrahedron 1976, 32, 1873.

Article
Organometallics, Vol. 29, No. 9, 2010 2155
Table 5. Selected Crystallographic Data for Complexes 1, 2 CH3COCH3, 3, and 4
3
1
2 CH3COCH3
3
3
4
formula
fw
C
813.51
291(2)
triclinic
P1
0.20 ꢀ 0.17 ꢀ 0.17
10.274(2)
13.535(3)
14.458(3)
97.62(3)
40H37ClN2O4P2Pd
C96H106Cl2N4O8P4Pd2 C3H6O
3
C36H37ClN3O3PPd
732.51
293(2)
monoclinic
P21
0.20 ꢀ 0.20 ꢀ 0.20
10.277(2)
9.5577(19)
17.088(3)
90
96.50(3)
90
1667.7(6)
2
1.459
C36H37ClN3NiO3P
684.82
293(2)
monoclinic
P21
0.20 ꢀ 0.20 ꢀ 0.20
10.232(2)
9.4811(19)
17.026(3)
90
96.66(3)
90
1640.5(6)
2
1.386
1909.51
291(2)
temp (K)
cryst syst
space group
cryst size (mm)
monoclinic
P21
0.20 ꢀ 0.17 ꢀ 0.16
14.581(3)
15.906(3)
20.557(4)
90
91.33(3)
90
2676.5(8)
2
1.331
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
97.78(3)
108.03(3)
1861.3(6)
2
1.452
0.700
832
3
˚
V (A )
Z
Dcalcd (g cm-3
)
abs coeff (mm-1
F(000)
)
0.558
1984
0.725
752
0.762
716
θ range (deg)
index range
1.45-25.00
-12 e h e 11
0 e k e 16
-17 e l e 16
6097
6097
6097/7/874
1.077
0.99-26.00
0 e h e 17
-19 e k e 19
-25 e l e 25
17 187
16 883
16 883/1/1082
1.018
3.07-26.07
-12 e h e 12
-11 e k e 11
-21 e l e 18
13 383
6334
6334/1/396
1.012
2.00-25.99
-12 e h e 12
-11 e k e 11
-20 e l e 20
10 882
6083
6083/1/407
1.006
no. of rflns coll
no. of indep rflns
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R1 = 0.0395
wR2 = 0.1002
R1 = 0.0447
wR2 = 0.1035
0.442/-0.480
R1 = 0.0406
wR2 = 0.0809
R1 = 0.0519
wR2 = 0.0852
0.336/-0.427
R1 = 0.0470
wR2 = 0.0933
R1 = 0.0591
wR2 = 0.1009
0.338/-0.395
R1 = 0.0598
wR2 = 0.1295
R1 = 0.0799
wR2 = 0.1490
0.671/-0.362
R indices (all rflns)
-3
˚
largest diff peak/hole (e A
)
After cooling, filtration, and evaporation, the residue was purified
by preparative TLC on silica gel plates to afford the corresponding
PCP- or PCN-pincer Pd(II) or Ni(II) complexes.
central Ar H), 4.63-4.70 (m, 1H, Py H), 4.39-4.45 (m, 1H,
Py H), 4.33-4.37 (m, 1H, Im H), 4.01 (app t, J = 10.6 Hz, 1H,
Im H), 3.86 (s, 3H, OCH3), 3.82 (dd, J = 6.0, 10.0 Hz, 1H, Im H),
3.27-3.37 (m, 1H, Py H), 2.76-2.84 (m, 1H, ImH), 2.04-2.13
(m, 1H, Py H), 1.74-1.85 (m, 1H, Py H), 1.61-1.71 (m, 1H,
Py H), 1.45-1.52 (m, 1H, Py H), 0.97 (d, J = 6.8 Hz, 3H,
CH3CHCH3), 0.95 (d, J = 6.8 Hz, 3H, CH3CHCH3). 13C
NMR (100 MHz, CDCl3): δ 169.9 (d, JP-C = 4.0 Hz), 159.2,
155.7 (d, JP-C = 19.0 Hz), 151.8, 143.7, 141.0 (d, JP-C = 7.0
Hz), 135.3, 132.7, 128.2 (t, JP-C = 3.6 Hz), 128.1, 127.5,
126.9, 126.2, 126.0, 120.8, 114.9, 113.7 (d, JP-C = 9.5 Hz),
93.7 (d, JP-C = 10.0 Hz), 71.1 (d, JP-C = 3.2 Hz), 66.3 (d,
Chiral PCP-Pincer Pd(II) Complex 1. 1 was obtained as a
white solid (69 mg, 34% yield): mp 275 °C dec. [R]20D = -150°
(c = 0.044, CH2Cl2). 1H NMR (400 Mz, CDCl3): δ 7.23-7.60
(m, 20 H, Ph H), 6.92 (tt, J = 2.0, 8.0 Hz, 1H, central Ar H), 6.30
(d, J = 8.0 Hz, 2H, central Ar H), 4.67-4.74 (m, 2H, NCH),
4.25-4.35 (m, 2H, NCH2), 3.33-3.39 (m, 2H, NCH2), 1.93-
2.02 (m, 2H, NCH2CH2), 1.75-1.86 (m, 2H, NCH2CH2),
1.57-1.66 (m, 4H, NCHCH2). 13C NMR (100 MHz, CDCl3):
δ 157.2 (t, JP-C = 10.5 Hz), 143.4, 140.8 (t, JP-C = 3.0 Hz),
129.3, 128.3, 128.2, 127.6, 127.0, 126.3, 106.9 (t, JP-C = 9.5 Hz),
94.6 (t, JP-C = 3.5 Hz), 70.7, 47.5 (t, JP-C = 9.5 Hz), 30.2, 26.0.
31P NMR (121 MHz, CDCl3): δ 155.8 (s). IR ν (cm-1): 3060,
2879, 1572, 1445, 1223, 1063, 965, 887, 747, 699. Anal. Calcd for
C40H37ClN2O4P2Pd: C, 59.05; H, 4.58; N, 3.44. Found: C,
59.10; H, 4.64; N, 3.38.
J
P-C = 3.9 Hz), 55.7 (d, JP-C = 5.1 Hz), 55.6, 47.6 (d, JP-C =
17.6 Hz), 30.6, 30.1, 25.8 (d, JP-C = 3.2 Hz), 18.8, 14.6. 31
P
NMR (121 MHz, CDCl3): δ 56.7 (s). IR ν (cm-1): 3455, 2921,
1602, 1534, 1441, 1291, 1249, 1028, 968, 876, 699. Anal. Calcd
for C36H37ClN3O3PPd: C, 59.03; H, 5.09; N, 5.74. Found: C,
59.27; H, 5.56; N, 5.22.
Chiral PCP-Pincer Pd(II) Complex 2. 2 was obtained as a
white solid (46 mg, 20% yield): mp 272 °C dec. [R]20D = -150°
(c = 0.046, CH2Cl2). 1H NMR (400 Mz, CDCl3): δ 7.27-7.58
(m, 20 H, Ph H), 6.89 (t, J = 1.8 Hz, 1H, central Ar H),
4.62-4.68 (m, 2H, NCH), 4.42-4.31 (m, 2H, NCH2), 3.37-
3.45 (m, 2H, NCH2), 2.18-2.23 (m, 2H, NCH2CH2), 1.80-1.86
(m, 2H, NCH2CH2), 1.56-1.66 (m, 2H, NCHCH2), 1.34-1.41
(m, 2H, NCHCH2), 0.89 (s, 18H, CH3). 13C NMR (100 MHz,
CDCl3): δ 152.9 (t, JP-C = 10.0 Hz), 143.4, 141.7 (t, JP-C = 3.5
Hz), 128.8 (t, JP-C = 8.5 Hz), 128.4, 128.3, 128.1, 127.8, 127.4,
126.1, 124.3, 93.7 (t, JP-C = 3.5 Hz), 70.6, 47.8 (t, JP-C = 10.0
Hz), 34.3, 30.8, 29.5, 25.8. 31P NMR (121 MHz, CDCl3): δ 159.1
(s). IR ν (cm-1): 3060, 2956, 1714, 1447, 1277, 1222, 1060, 939,
751, 698. Anal. Calcd for C48H53ClN2O4P2Pd: C, 62.27; H, 5.77;
N, 3.03. Found: C, 62.13; H, 6.01; N, 2.79.
Chiral PCN-Pincer Ni(II) Complex 4. 4 was obtained as a pale
yellow solid (68 mg, 20% yield): mp 273 °C dec. [R]20D = þ21°
(c= 0.044, CH2Cl2). 1HNMR(400Mz, CDCl3):δ7.62 (d, J=8.4
Hz, 2H, Ph H), 7.58 (d, J = 7.2 Hz, 2H, Ph H), 7.22-7.53 (m, 6H,
Ph H), 7.20 (d, J = 8.8 Hz, 2H, NAr H), 6.95 (d, J = 8.4 Hz, 2H,
NAr H), 6.62 (td, J = 1.2, 7.8 Hz, 1H, central Ar H), 6.28 (d, J =
8.0 Hz, 1H, central Ar H), 5.97 (d, J = 7.6 Hz, 1H, central Ar H),
4.53-4.60 (m, 2H, Py H), 4.13-4.19 (m, 1H, Im H), 4.05 (app t,
J = 11.7 Hz, 1H, Im H), 3.86 (s, 3H, OCH3), 3.82 (dd, J = 4.4,
10.0 Hz, 1H, Im H), 3.34-3.43 (m, 1H, Py H), 2.56-2.62 (m, 1H,
Im H), 2.11-2.17 (m, 1H, Py H), 1.75-1.84 (m, 1H, Py H),
1.59-1.69 (m, 1H, Py H), 1.28-1.33 (m, 1H, Py H), 1.01 (dd, J =
2.8, 6.8 Hz, 3H, CH3CHCH3), 0.91 (d, J = 3.2, 6.8 Hz, 3H,
CH3CHCH3). 13C NMR (100 MHz, CDCl3): δ 169.1 (d, JP-C
2.0 Hz), 159.1 (d, JP-C = 3.0 Hz), 158.1 (d, JP-C = 23.0 Hz),
143.8, 141.7 (d, JP-C = 7.0 Hz), 136.4, 132.2, 128.1 (t, JP-C
3.5 Hz), 128.0, 127.3, 126.9, 126.0, 125.9, 119.5, 114.8, 112.5 (d,
P-C = 15.0 Hz), 92.3 (d, JP-C = 12.0 Hz), 71.4 (d, JP-C = 2.1
Hz), 64.3(d, JP-C = 2.5 Hz), 56.3 (d, JP-C = 3.1 Hz), 55.6, 47.4 (d,
JP-C = 15.5 Hz), 30.5, 30.4, 25.8 (d, JP-C = 2.8 Hz), 18.8, 14.5. 31
NMR (121 MHz, CDCl3): δ 160.3 (s). IR (KBr pellet): 3431, 2925,
=
Chiral PCN-Pincer Pd(II) Complex 3. 3 was obtained as a
yellow solid (154 mg, 42% yield): mp 223 °C dec. [R]20D = -29°
(c = 0.53, CH2Cl2). 1H NMR (400 Mz, CDCl3): δ 7.23-7.61 (m,
10H, Ph H), 7.20 (d, J = 8.4 Hz, 2H, NAr H), 6.95 (d, J = 8.4
Hz, 2H, NArH), 6.69 (td, J = 1.6, 8.0 Hz, 1H, central Ar H),
6.51 (d, J = 8.0 Hz, 1H, central Ar H), 6.05 (d, J = 7.6 Hz, 1H,
=
J
P

2156 Organometallics, Vol. 29, No. 9, 2010
Niu et al.
1537, 1443, 1287, 1248, 1029, 971, 873, 785, 702. Anal. Calcd for
C36H37ClNiN3O3P 0.5H2O: C, 62.32; H, 5.52; N, 6.06. Found: C,
62.30; H, 5.54; N, 5.94.
X-ray Crystallographic Studies. Crystals of 1, 2, and 5 were
obtained by recrystallization from CH2Cl2/acetone at ambi-
ent temperature. 3 was recrystallized from CH2Cl2/hexane at
ambient temperature. 4 was recrystallized from ethyl acetate/
hexane at ambient temperature. Crystallographic data for 1
and 2 (the data for complex 5 are shown in the Supporting
Information) were measured on a Rigaku-IV imaging plate
area detector using graphite-monochromated Mo KR radia-
3
Procedure for the Synthesis of PCP-Pincer Iodopalladium
Complex 5. A mixture of complex 1 (0.03 mmol, 24.7 mg) and
potassium iodide (49.7 mg, 0.3 mmol) was stirred in methanol/
acetone (1/1 v/v, 3 mL) at room temperature until a clear yellow
solution was obtained (17 h), and the reaction mixture was
filtered through Celite. Subsequently, the solvent was removed
under reduced pressure, and the residue was purified by pre-
parative TLC on silica gel plates (eluent 3/2 petroleum ether/
dichloromethane) to give the product 5 as a yellow solid (23 mg,
85% yield). A single crystal suitable for X-ray crystal analysis
was obtained by recrystallization from a solution of CH2Cl2/
acetone: mp 270 °C dec. [R]20D = -90° (c = 0.070, CH2Cl2). 1H
NMR (400 Mz, CDCl3): δ 7.23-7.62 (m, 20 H, Ph H), 6.92 (tt,
J = 2.0, 8.0 Hz, 1H, central Ar H), 6.30 (d, J = 8.0 Hz, 2H,
central Ar H), 4.74-4.79 (m, 2H, NCH), 4.20-4.32 (m, 2H,
NCH2), 3.30-3.37 (m, 2H, NCH2), 2.15-2.22 (m, 2H,
NCH2CH2), 1.71-1.87 (m, 4H, NCH2CH2 þ NCHCH2), 1.42-
1.46 (m, 2H, NCHCH2). 13C NMR (100 MHz, CDCl3): δ 156.9
(t, JP-C = 10.0 Hz), 143.5, 141.1 (t, JP-C = 4.0 Hz), 129.5,
128.3, 128.2, 128.1, 127.5, 127.1, 126.0, 106.5 (t, JP-C = 10.0
Hz), 94.1 (t, JP-C = 3.5 Hz), 70.8, 47.8 (t, JP-C = 9.5 Hz), 30.5,
25.5. 31P NMR (121 MHz, CDCl3): δ 159.7 (s). IR (KBr pellet):
3059, 2874, 1571, 1445, 1223, 1065, 966, 844, 744, 698. Anal.
Calcd for C40H37IN2O4P2Pd: C, 53.09; H, 4.12; N, 3.10. Found:
C, 53.13; H, 4.07; N, 3.11.
General Procedure for PCP- and PCN-Pincer Metal Complex
Catalyzed Allylation of 9 or 10. To a mixture of 9 or 10 (0.15
mmol) and the pincer complex (0.0075 mmol, 5.0 mol %)
in 1.0 mL of solvent was added allyltributyltin (0.18 mmol,
56 μL). The resulting solution mixture was stirred for the allotted
times at room temperature (Table 4). Thereafter the reaction
mixture was broken up by aqueous KF (10%, w/v) overnight.
The organic layer was then separated, washed with brine, dried
(Na2SO4), and concentrated. The crude product was purified by
flash column chromatography to give the corresponding homo-
allylic alcohol 11 (CH2Cl2) or amine 12 (7/3 petroleum ether/
EtOAc).11,14 The enantiomeric excess was determined by chiral-
phase HPLC (Daicel Chiracel AD-H column).
˚
tion (λ = 0.710 73 A), respectively. Crystallographic data for
3 and 4 were collected on a Rigaku Saturn 724 CCD diffract-
ometer with graphite-monochromated Mo KR radiation (λ =
0.710 73 A) at ambient temperature, respectively. The struc-
˚
tures were solved by direct methods and expanded using
Fourier techniques and refined by full-matrix least-squares
methods. The non-hydrogen atoms were refined anisotropi-
cally, and the hydrogen atoms were included but not refined.
The absolute configuration was verified by the Flack para-
meter. Their raw data were corrected, and the structures were
solved using the SHELXL-97 program.24 Selected crystal
data are given in Table 5. CCDC-769170 (1), -769172 (2),
-769173 (3), -769174 (4), and -769171 (5) contain supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via http://www.ccdc.cam.ac.uk/data_
request/cif.
Acknowledgment. We are grateful to the National
Natural Science Foundation of China (Nos. 20872133
and 20972141) and the Innovation Fund for Outstanding
Scholar of Henan Province (No. 074200510005) for financial
support of this work.
Supporting Information Available: Figures giving 1H NMR,
13C NMR, and 31P NMR spectra of complexes 1-5, tables and a
figure giving crystallographic data of complex 5, and CIF files
giving crystallographic data for 1-5. This material is available
free of charge via the Internet at http://pubs.acs.org.
(24) Sheldrick, G. M. SHELXL-97, Program for Refinement of
€
Crystal Structure; University of Gottingen, Germany, 1997.