Persistent N-chirality as the only Source of asymmetry in nonracemic N 2PdCl2 complexes

Article abstract of DOI:10.1021/om0498495

The combination of prochiral 1,2-diamines, Pd(OAc)₂, and (R)-K₂-3,3′-Me₂BINOL leads to enantio- and diastereopure N₂Pd(3,3′-Me₂BINOL) complexes. HCl removes the Me₂BINOL resolving ligand and provides a family of enantiopure N₂PdCl₂ complexes whose only stereochemistry resides on the stereogenic nitrogen centers. In effect, nitrogen inversion (>10⁵ s^-1) is halted by metal coordination and utilized to generate enantiopure complexes. When the diamine substituents are relatively small, the N-chirality is stable; however, large substituents accelerate N-dissociation processes and concomitant racemization. Enantiopure N ₂Pd²⁺-Lewis acid catalysts can be generated for the Diels-Alder reaction, and although enantioselectivities are low (<25% ee), this is due to inefficient stereochemical transfer and not a degradation of the catalyst's chirality.

Full text of DOI:10.1021/om0498495

3210
Organometallics 2004, 23, 3210-3217
P er sisten t N-Ch ir a lity a s th e On ly Sou r ce of Asym m etr y
in Non r a cem ic N2P d Cl2 Com p lexes
Kathryn A. Pelz, Peter S. White, and Michel R. Gagne´*
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received March 1, 2004
The combination of prochiral 1,2-diamines, Pd(OAc)2, and (R)-K2-3,3′-Me2BINOL leads to
enantio- and diastereopure N2Pd(3,3′-Me2BINOL) complexes. HCl removes the Me2BINOL
resolving ligand and provides a family of enantiopure N2PdCl2 complexes whose only
stereochemistry resides on the stereogenic nitrogen centers. In effect, nitrogen inversion
(>105 s-1) is halted by metal coordination and utilized to generate enantiopure complexes.
When the diamine substituents are relatively small, the N-chirality is stable; however, large
substituents accelerate N-dissociation processes and concomitant racemization. Enantiopure
N2Pd2+-Lewis acid catalysts can be generated for the Diels-Alder reaction, and although
enantioselectivities are low (<25% ee), this is due to inefficient stereochemical transfer and
not a degradation of the catalyst’s chirality.
In tr od u ction
off-metal C-C bond rotation,6 but can when coordinated
to a metal center.7,8
The development of new approaches for achieving
enantiomer selective catalysis continues to stimulate
research in the area of fundamental coordination chem-
istry. One intriguing direction of research is the design
of new catalysts for asymmetric synthesis that are
composed of metal-ligand assemblies whose stereo-
chemistries are meta-stable;1 that is, the ligand stereo-
chemistry is held in a specific absolute configuration
solely due to metal coordination. In some of these cases,
the ligands (e.g., BIPHEP, NUPHOS, cyclo-NUPHOS)
were held in nonracemic states because the barrier to
atropinversion was enhanced upon coordination to a
substitutionally inert metal such as Pt(II).2,3 Under
other circumstances where the on-metal rotation rates
were still too high to enable resolution, chiral diamines
could be reliably employed to reversibly bind to the
metal (Ru, Pd) and favor a single enantiomeric form of
the dynamic ligand on the metal (e.g., BIPHEP, dppf).4,5
The term tropos has been coined4 for ligands whose axial
chirality cannot otherwise be resolved because of rapid
In addition to ligands made configurationally meta-
stable by metal coordination, we considered the pos-
sibility that ligands that rapidly inverted their configu-
rations when off-metal might also be amenable to
resolution via this strategy. In particular we wished to
determine if prochiral amines could be locked into
nonracemic N-chiral forms by metal coordination and
used in asymmetric catalysis. At the onset of this work
we noted that there were no known examples of asym-
metric catalysis with solely N-chiral ligands, since the
high rate of inversion (5 × 105 s-1, 25 °C)9 precludes
(1) Walsh, P. J .; Lurain, A. E.; Balsells, J . Chem. Rev. 2003, 103,
3297-3344.
(2) (a) Becker, J . J .; White, P. S.; Gagne´, M. R. J . Am. Chem. Soc.
2001, 123, 9478-9479. (b) Tudor, M. D.; Becker, J . J .; White, P. S.;
Gagne´, M. R. Organometallics 2000, 19, 4376-4384.
(3) (a) Doherty, S.; Newman, C. R.; Rath, R. K.; van den Berg, J .-
A.; Hardacre, C.; Nieuwenhuyzen, M.; Knight, J . G. Organometallics
2004, 23, 1055-1064. (b) Doherty, S.; Newman, C. R.; Rath, R. K.;
Luo, H.-K.; Nieuwenhuyzen, M.; Knight, J . G. Org. Lett. 2003, 5, 3863-
3866. (c) Doherty, S.; Robins, E. G.; Nieuwenhuyzen, M.; Knight, J .
G.; Champkin, P. A.; Clegg, W. Organometallics 2002, 21, 1383-1399.
(4) (a) Mikami, K.; Aikawa, K.; Yusa, Y.; Hatano, M. Org. Lett. 2002,
4, 91-94. (b) Mikami, K.; Aikawa, K.; Yusa, Y. Org. Lett. 2002, 4, 95-
97. (c) Mikami, K.; Aikawa, K. Org. Lett. 2002, 4, 99-101. (d) Mikami,
K.; Aikawa, K.; Korenaga, T. Org. Lett. 2001, 3, 243-245.
(6) For related references see: (a) Tissot, O.; Gouygou, M.; Dallemer,
F.; Daran, J .-C.; Balavoine, G. G. A. Angew. Chem., Int. Ed. 2001, 40,
1076-1078. (b) Ringwald, M.; Stu¨rmer, R.; Brintzinger, H. H. J . Am.
Chem. Soc. 1999, 121, 1524-1527.
(7) A conceptually fascinating set of biaryl phosphines has recently
been disclosed wherein the phosphines are atropos, but achiral until
coordinated to a metal. (a) Aikawa, K.; Mikami, K. Angew. Chem., Int.
Ed. 2003, 42, 5458-5461. (b) Aikawa, K.; Mikami, K. Angew. Chem.,
Int. Ed. 2003, 42, 5455-5458.
(5) These experiments are
a
subset of a broader program on
(8) Examples of selective coordination of diamines with an axis of
chirality are also known; see for example: (a) Ashby, M. T.; Alguin-
digue, S. S.; Schwane, J . D.; Daniel, T. A. Inorg. Chem. 2001, 40, 6645-
6650. (b) Alguindigue, S. S.; Khan, M. A.; Ashby, M. T. Organometallics
1999, 18, 5112-5119.
asymmetric activation of achiral or racemic catalysts, see: (a) Mikami,
K.; Terada, M.; Korenaga, T.; Matsumoto, Y.; Matsukawa, S. Acc.
Chem. Res. 2000, 33, 391-401. (b) Mikami, K.; Terada, M.; Korenaga,
T.; Matsumoto, Y.; Ueki, M.; Angelaud, R. Angew. Chem., Int. Ed. 2000,
112, 3532-3556.
(9) Saunders, M.; Yamada, F. J . Am. Chem. Soc. 1963, 85, 1882.
10.1021/om0498495 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/20/2004

Nonracemic N2PdCl2 Complexes
Organometallics, Vol. 23, No. 13, 2004 3211
Sch em e 1
free-base resolution. This contrasts the many successful
P-chiral ligands that have been reported since the
higher barrier to P-inversion allows P-enantiomers to
be resolved under ambient conditions.10,11
Sch em e 2
Numerous examples exist of chiral diamine ligands
wherein the chirality in the backbone of a 1,2-diamine
favors the coordination of a single N-epimer.1,12 These
ligands can be exquisitely selective, at least partly due
to the close proximity of the N-stereocenter to the
reactive metal.13 Interligand interactions between chiral
ligands and achiral or meso ligands (amines) have also
been utilized to magnify asymmetric induction in chiral
catalysts.1,14 In contrast to both intra- and interligand
control of N-chirality, cases where the N-chirality is the
only source of stereochemistry have not been explored.
This situation could be problematic since the experi-
mental challenges include solving the issue of resolving
the stereogenic N atoms with also preventing N-dis-
sociation (and subsequent fast N-inversion).
In this paper we report that enantiopure N-chiral N2-
PdCl2 complexes can be synthesized and that the
N-chirality can be stable enough to survive activation
and use in catalysis. The focus of this paper is the
development of a method for on-metal dynamic resolu-
tion15 of the N-chirality, determining conditions neces-
sary for maintaining it, and the use of N-chiral com-
pounds in catalysis.
to overalkylation, the reaction of a variety of benzyl
chlorides with NaH in DMF (100 °C) led to reproducibly
high yields of the desired dialkylated ligands. Scheme
1 shows the synthesis of diamine ligands where increas-
ing bulk around the nitrogen was achieved by changing
the third N-substituent from benzyl to 1-naphthyl
1
methyl and 9-anthracenyl methyl. H NMR spectra of
the crude diamines showed clean, complete conversion
to the desired product with no apparent mono- or over-
benzylated product.
Syn th esis of N2P d X2 Com p lexes. To provide an
initial test for the coordinating ability of the new
prochiral diamines, the ligands 1a -c were stirred with
(COD)PdCl2 in dichloromethane (Scheme 2). The family
of N2PdCl2 complexes 2a -c was isolated, each as a
bright orange powder, by precipitating with hexanes.
1H NMR analysis of 2a indicated a 2:1 mixture of
isomers, the diastereotopic resonances of the benzylic
protons between 3 and 5 ppm being particularly diag-
nostic. The major product was obtained in pure form
by fractional crystallization and confirmed to be the dl
isomer by single-crystal X-ray diffraction (Figure 1).16
Resu lts a n d Discu ssion
Dia m in e Liga n d Design a n d Syn th esis. On the
basis of the utility and general availability of N2PdX2
complexes wherein N2 is a chelating dinitrogen ligand,
we chose to first examine 1,2-diamines made prochiral
by virtue of two different nitrogen substituents. Homo-
chiral coordination of the two amines would lead to a
dl ligand array (R,R and S,S), while the heterochiral
coordination mode would lead to the meso ligand ar-
rangement (R,S).
1
Comparison of the H NMR spectrum of the crystals to
that of the mixture enabled the benzyl CH’s for dl-2a
to be assigned to the pair of doublets at δ 3.15 and 4.71
ppm (J ) 12.8 Hz) and meso-2a to the doublets at δ 3.62
and 4.11 ppm (J ) 13.2 Hz). Compounds 2b and 2c were
obtained in similar dl:meso ratios of 2:1 and 3:1,
(12) For a recent report utilizing 1,2-diamines with a chiral backbone
that are prochiral at nitrogen, see: Kobayashi, S.; Matsubara, R.;
Nakamura, Y.; Kitagawa, H.; Sugiura, M. J . Am. Chem. Soc. 2003,
125, 2507-2515.
(13) A typical Pd-NR3 bond length (∼2.08 Å) is significantly shorter
The prochiral 1,2-diamine ligands used in this study
were synthesized by a double N-alkylation of N,N′-
dimethylethylenediamine (Scheme 1). While a number
of protocols were found to be either sluggish or prone
than a typical Pd-PR3 bond (∼2.27 Å).
(14) Costa, A. M.; J imeno, C.; Gavenonis, J .; Carroll, P. J .; Walsh,
P. J . J . Am. Chem. Soc. 2002, 124, 6929-6941. For a review of additive
effects in asymmetric catalysis, see footnote and: Vogl, E. M.; Gro¨ger,
H.; Shibasaki, M. Angew. Chem., Int. Ed. 1999, 38, 1571-1577.
(15) Pellisier, H. Tetrahedron 2003, 59, 8291-8327.
(10) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of
Organic Compounds; J ohn Wiley & Sons: New York, 1994.
(11) Pietrusiewicz, K. M.; Zablocka, M. Chem. Rev. 1994, 94, 1375-
1411.
(16) The orientation of the N-substituents is remarkably similar to
those in the monoprotonated form of N,N′-Me2-N,N′-dibenzyl proton
sponge; see: Charmant, J . P. H.; Lloyd-J ones, G. C.; Peakman, T. M.;
Woodward, R. L. Tetrahedron Lett. 1998, 39, 4733-4736.

3212 Organometallics, Vol. 23, No. 13, 2004
Pelz et al.
van Koten for N2Pd(OAr)2 compounds.17 Stirring Pd-
(OAc)2 with diamine 1a and 1 equiv of (R)-Na2BINOL
provided air and moisture stable 3 as an inseparable
1:4:3 mixture of three diastereomers, two symmetric
(S,S/ R and R,R/ R) and one asymmetric (S,R/ R) (eq 1);
that is, the chiral BINOL ligand failed to significantly
influence the diamine configuration. Previous studies
have shown that methyl substituents at the 3- and 3′-
positions of BINOL more strongly interact with ligands
coordinating across the square plane.4a,18 Synthesis of
N2Pd(R)-(Me2BINOL) complexes 4a -c supported this
notion, as the products were obtained with significantly
improved diastereoselectivities (Scheme 3).
F igu r e 1. X-ray structure representation of one indepen-
dent molecule (Z′ ) 2) in the structure of dl-2a . Bond
distances (Å) and angles (deg) are averaged. Pd-N ) 2.075-
(3), Pd-Cl ) 2.3018(9), N-Pd-N ) 85.2(1), and Cl-Pd-
Cl ) 90.8(1).
Sch em e 3
F igu r e 2. X-ray structure representation of one indepen-
dent molecule (Z′ ) 2) in the refined structure of meso-2c.
Bond distances (Å) and angles (deg) for this molecule
include N1-Pd1 ) 2.083(5), N2-Pd 2.088(5), Cl1-Pd )
2.3085(15), Cl2-Pd 2.3207(16), N1-Pd-N2 ) 85.8(2), and
Cl1-Pd-Cl2 ) 89.80(6).
respectively (Scheme 2). The major diastereomer, which
was obtained by a selective precipitation procedure, was
also shown to be dl in both cases (vide infra).
In the case of 4a , two diastereomers were obtained
in a 5:1 ratio and H NMR analysis revealed that both
In the case of 2c, the minor diastereomer turned out
to be the more crystalline isomer and provided crystals
suitable for X-ray analysis (Figure 2). As Figures 1 and
2 show, coordination of the 1,2-diamine provides a
distinctive skew conformation to the backbone, a result
of the N-substituents adopting pseudoaxial and -equato-
rial orientations. In dl-2a , the sterically subordinate
methyl groups position in the axial sites and the
molecule has C2-symmetry, whereas in meso-2c one
anthracenyl methyl occupies an axial position and the
other an equatorial position.
Following the procedures developed for the on-metal
resolution of Biphep,2 NUPHOS,3 and cyclo-NUPHOS,3
we envisioned synthesizing N2Pd(BINOL) complexes
wherein BINOL stereochemistry could dynamically
resolve15 the N-stereochemistry. BINOL removal would
then lead to new complexes whose stereochemistry was
solely located at the stereogenic nitrogen centers. Sug-
gesting the feasibility of this approach is Walsh’s X-ray
crystal structure of (3,3′-Ph2BINOL)Zn(tBuNHCH2-
NHtBu), which clearly shows how the BINOL ligand
effectively resolves the 1,2-diamine in a tetrahedral
environment.14
1
were symmetric, i.e., (R,R)/(R) and (S,S)/(R).19,20 Rep-
resentative of the complex’s symmetry, the benzylic
CH’s of the major diastereomer appeared as a pair of
doublets at δ 2.73 and 4.26 ppm (J ) 12.4 Hz), as did
the diamine backbone CH’s (δ 1.58 and 2.94 ppm; J )
8.8 Hz). Singlets were observed for the Me2BINOL (2.46
ppm) and ligand (2.26 ppm) methyl groups. The minor
dl-diastereomer revealed a similar though slightly
broadened pattern in the 1H NMR spectrum. The
asymmetric (R,S)/(R) diastereomer was not observed.
The major diastereomer of the 4a mixture was
obtained in pure form by recrystallization (CH2Cl2/
hexanes). As shown in Figure 3, single crystals of the
1
major diastereomer (confirmed by H NMR) were ob-
tained and X-ray analysis revealed it to be the dl-(R,R)/
(17) Kapteijn, G. M.; Grove, D. M.; Kooijman, H.; Smeets, W. J . J .;
Spek, A. L.; van Koten, G. Inorg. Chem. 1996, 35, 526-533.
(18) (a) Becker, J . J .; White, P. S.; Gagne´, M. R. Organometallics
2003, 22, 3245-3249. (b) Brunkan, N. M.; White, P. S.; Gagne´, M. R.
J . Am. Chem. Soc. 1998, 120, 11002-11003.
(19) Although conversion to product does not require the elevated
temperature, it enhances the rate of isomer equilibration and the
product diastereomeric ratio.
Synthesis of the desired N2Pd(BINOL) complexes
followed directly from a one-pot procedure developed by
(20) The bracketed pair of stereochemical labels represents the
N-stereocenters, while the third refers to the BINOL’s axial chirality.

Nonracemic N2PdCl2 Complexes
Organometallics, Vol. 23, No. 13, 2004 3213
dichlorides, dl/ meso-2c. The presence of the meso
isomer is clearly indicative of at least partial epimer-
ization and/or racemization during the Me2BINOL/Cl-
exchange.
Optical rotation measurements on (R,R)-2a and (R,R)-
2b provided specific rotations ([R]D) of +195° and +89°,
respectively,23 whereas [R]D for dl/meso-2c was signifi-
cantly lower (+8°). The large nonzero value for (R,R)-
2a and (R,R)-2b suggested, along with the lack of meso-
F igu r e 3. Structural representation of the refined X-ray
structure of (R,R)/(R)-4a . Relevant bond distances (Å) and
angles (deg) include Pd-N1 ) 2.060(5), Pd-N2 ) 2.062-
(5), Pd-O1 ) 2.034(4), Pd-O2 ) 2.028(4), N1-Pd-N2 )
85.91(24), and O1-Pd-O2 ) 94.97(14).
1
2a and meso-2b in the H NMR, that one enantiomer
was predominant. The observation of significant optical
activity clearly showed that the dl dichlorides 2a and
2b were highly enantioenriched.
(R) stereoisomer. This isomer projects the BINOL 3,3′-
methyl substituents into the sterically less crowded
quadrants occupied by the N-methyl groups,2-4,18,21
which additionally causes a counterclockwise rotation
of 15° for the O-Pd-O plane relative to the N-Pd-N
plane.22 The minor trans diastereomer must therefore
project the BINOL 3,3′-methyl groups into the more
crowded N-benzyl-containing quadrants of space. On the
basis of this crystallographic analysis, the major and
minor trans diastereomers will be referred to as being
stereochemically “matched” and “mismatched”, respec-
tively.
Although the optical rotation measurements clearly
indicated an excess of one enantiomer, a quantitative
measurement of the enantiomer ratio (er) was desired.
To measure the er we drew from the fact that all three
diastereomers of 3 and two diastereomers of 4a -c were
observable in the 1H NMR. Thus, converting an N2PdCl2
set of enantiomers into an N2Pd(R)-BINOL set of
diastereomers would indirectly provide the er of the N2-
PdCl2 compound, provided that no N-isomerization
occurred during the reaction. In a typical experiment,
a solution of NaOtBu and BINOL or KOtBu and 6,6′-
Br2BINOL24 in THF was added to a solution of the 2a -c
cleavage product in CH2Cl2 and stirred for 4 h (eq 3).
The mixture was then filtered through Celite, the
solvent removed, and the crude product directly ana-
Repeating the 4a synthesis procedure with ligands
1b and 1c led to similarly stable complexes but with
higher levels of diastereoselectivity, presumably reflect-
ing the larger size of the naphthyl and anthracenyl
substituents (Scheme 3). Again, C2 symmetry was
1
1
apparent in the H NMR spectra for both compounds.
lyzed by H NMR. The results of these experiments for
The major diastereomers of 4b and 4c were purified by
passing the crude mixture through a plug of silica gel
(9:1 EtOAc/hexanes) and precipitating from CH2Cl2/
hexanes, although this method resulted in reduced
yields (∼60%).
the different BINOL and N2 ligands are collected in
Table 1. Addition of (R)-Na2BINOL to dl-2a (entry 1)
provided a 1:1 mixture of matched and mismatched
BINOL diastereomers, confirming that no N-epimer-
ization occurred.25 Addition of (R)- and (S)-Na2BINOL
to (R,R)-2a (entries 2 and 3) resulted in the matched
and mismatched diastereomers of 3, respectively (Figure
4). Since (R,R)-2a and (R)-Na2BINOL regenerated the
matched diastereomer (previously shown to be (R,R)/
(R) for 1a -Pd(Me2BINOL)), this further indicates that
the protonolysis of the Me2BINOL ligand goes with
retention of nitrogen stereochemistry. Similarly, rebind-
ing (R,R)-2b with (R)- and (S)-K2Br2BINOL resulted in
clean conversion to the matched and mismatched dia-
stereomers, respectively (entries 4-6). Obtaining dia-
stereomerically pure products showed that (R,R)-2a and
(R,R)-2b are enantiomerically pure (>95:5) and that
N-chirality is indeed viable.
Revea lin g th e N-Ch ir a lity. To assess the stability
of the N-stereocenters in the absence of a chiral control-
ling group, the Me2BINOL ligands of the dark red (R,R)/
(R)-4a , (R,R)/(R)-4b, and (R,R)/(R)-4c were removed
(HCl) (eq 2). The bright yellow dichloride complexes
were precipitated away from cleaved Me2BINOL by the
addition of hexanes, isolated, and analyzed for retention
of stereochemistry at the nitrogen centers. 1H NMR
analysis of the crude products from 4a and 4b showed
no meso product, consistent with a BINOL cleavage that
does not randomly epimerize the nitrogen stereocenter.
On the other hand, the 1H NMR spectrum of the 4c
cleavage product showed a mixture of dl and meso
(21) (a) Koh, J . H.; Larsen, A. O.; White, P. S.; Gagne´, M. R.
Organometallics 2002, 21, 7-9. (b) Brunkan, N. M.; White, P. S.;
Gagne´, M. R. Angew. Chem., Int. Ed. 1998, 37, 1579-1582. (c)
Brunkan, N. M.; White, P. S.; Gagne´, M. R. J . Am. Chem. Soc. 1998,
120, 11002-11003.
(23) Optical rotation measurements were taken in dichloromethane.
Analysis of the authentic dl mixtures confirmed that [R]D was 0.
(24) 6,6′-Br2BINOL complexes are more soluble than BINOL com-
plexes.
(25) A similar experiment with (R)-3,3′-Me2BINOL caused the
mismatched diastereomer to isomerize to the matched, a result of steric
interactions with the 3,3′ substituents.
(22) This is in contrast to the nearly perfect square plane of the
N2PdCl2 complexes.

3214 Organometallics, Vol. 23, No. 13, 2004
Pelz et al.
Ta ble 1. Assa yin g th e er of N2P d Cl2 Com p lexes by
Con ver sion to Dia ster eom er ic N2P d (BINOL)
Com p lexes
Ta ble 2. Ap p lica tion of N-Ch ir a l Ca ta lysts to th e
Diels-Ald er Rea ction (eq 4)
%
entry N2PdCl2 source BINOL sourcea
resultb
activation conversion
endo
entry
catalysta
tempb (h)c
endo:exo % eed
1
2
3
4
5
6
7
8
dl-2a
(R)-BINOL
(R)-BINOL
(S)-BINOL
(R)-6,6′-Br2BINOL (R,R)/(R):(S,S)/(R) (1:1)
(R)-6,6′-Br2BINOL (R,R)/(R)
(R,R)/(R)-3:(S,S)/(R)-3 (1:1)
(R,R)/(R)-3
(R,R)/(S)-3
2+
(R,R)-2a
(R,R)-2a
dl-2bc
(R,R)-2b
(R,R)-2b
dl-2cd
1
2
3
4
5
6
7
8
9
1a -Pd(OTf)2
1a -Pd(OTf)2
1b-Pd(OTf)2
1b-Pd(OTf)2
1c-Pd(OTf)2
1c-Pd(OTf)2
rt
-78
rt
-78
rt
-78
rt
98 (5)
97:3
96:4
95:5
93:7
77:23
93:7
96:4
95:5
95:5
90:10
21
23
17
20
9
25
18
17
9
2+
80 (20)
100 (4)
100 (22)
87 (6)
100 (12)
96 (6)
98 (6)
75 (6)
95 (6)
2+
2+
(S)-6,6′-Br2BINOL (R,R)/(S)
2+
(R)-6,6′-Br2BINOL (R,R)/(R):(S,S)/(R) (∼1:1)
2+
dl-2c
(S)-6,6′-Br2BINOL (R,R)/(S):(S,S)/(S) (∼1:1)
(R,R)-2a /AgSbF6
(R,R)-2a /AgSbF6
(R,R)-2b/AgSbF6
a
The BINOL or Br2BINOL was treated with NaOtBu or KOtBu,
-78
rt
respectively, and transferred via cannula to the N2PdCl2 source
b
1
10 dl/ meso-2c/AgSbF6 rt
15
(see text and Experimental Section). The dr was obtained by H
NMR analysis of the crude reaction mixture. The NCH2Ar
resonances were diagnostic. c Obtained from the reaction in Scheme
a
The ditriflate catalysts were obtained by first treating the
N2Pd(3,3′-Me2BINOL) complex with HOTf (1.9 equiv) at the
indicated temperature and then cooling to -50 °C to carry out
d
2. The 2c obtained from eq 2 was a mixture of dl diastereomers.
b
the reaction. Activation was carried out for 15 min prior to
preequilibrating at -50 °C and adding CpH. c Conversion was
monitored by passing an aliquot through a plug of silica gel, eluting
with EtOAc to remove the catalyst, then assaying by HPLC.
d
Chiracel OD-H.
isolated dichloride ((R,R)-2a , (R,R)-2b, and dl/ meso-2c)
and generated the dication by halide abstraction with
AgSbF6. As shown in Table 2, the catalysts were
successful and provided the product after several hours
at -50 °C.
General observations include typically high endo:exo
ratios, the exception being 1c-Pd(OTf)22+ (entry 5), and
higher reactivities for the catalysts activated with
AgSbF6. Regarding enantiomeric excess, however, cata-
lysts typically performed poorly, with the highest se-
lectivities reaching into the mid-20s.
F igu r e 4. Crude 1H NMR spectra of the reaction products
of (A) dl-2a and (R)-Na2BINOL to form the matched and
mismatched diastereomers; (B) (R,R)-2a and (R)-Na2-
BINOL to form the matched diastereomer (R,R)/(R)-3; and
(C) (R,R)-2a with (S)-Na2BINOL to form the mismatched
diastereomer (R,R)/(S)-3.
In the case of the ditriflate catalysts we considered
the possibility that HOTf protonolysis of Me2BINOL at
ambient temperature might lead to in situ racemization
prior to the addition of substrate; therefore a series of
low-temperature (-78 °C) activations were carried out.
In the case of 1a -Pd(OTf)2 and 1b-Pd(OTf)2, the product
selectivities were nearly identical, suggesting that
generation of the ditriflate is stereospecific regardless
of activation temperature. The increase in selectivity
between low and ambient HOTf activation in the most
labile 1c-Pd(OTf)2 catalysts suggests that the ambient
conditions used to remove Me2BINOL causes partial
racemization of the catalyst.
To determine if the N-chirality was maintained dur-
ing catalysis, a series of chloride quenches were exam-
ined, with the expectation that diastereopure N2PdCl2
complexes would be obtained if the N-chirality survived
activation and turnover. Addition of brine to the HOTf-
activated catalysts inexplicably produced no N2PdCl2;
however, addition of brine to solutions containing the
catalyst activated with AgSbF6 generated N2PdCl2
complexes, which were isolated for 1H NMR analysis.
In the case of (R,R)-2a /AgSbF6 and (R,R)-2b/AgSbF6 2a
and 2b were recovered free of the meso isomer, sug-
gesting that N-epimerization had not occurred. The
obvious conclusion of these experiments is that the low
In contrast, the complexes synthesized using ligand
1c were found to be insufficient at maintaining N-
chirality. This was first suggested by the appearance
of the meso diastereomer in eq 2 and further confirmed
in entries 7 and 8, wherein rebinding of a single Br2-
BINOL enantiomer resulted in a mixture of diastereo-
mers. In summary, enantiopure N2PdCl2 complexes
could be prepared and isolated with ligands 1a and 1b,
but the larger size of the anthracenyl groups in 1c
caused an instability that led to N-epimerization and
eventual racemization.
Asym m et r ic Ca t a lysis w it h N-Ch ir a l Liga n d s.
Once the retention of stereochemistry was confirmed,
the compounds were tested as catalysts in the bench-
mark Diels-Alder reaction between cyclopentadiene
(CpH) and N-acryloyl oxazolidinone (eq 4).2,26 Two
protocols were investigated for generating N2Pd2+ cata-
lysts. The first involved directly treating the diastereo-
pure complexes 4a -c with a slight deficiency of triflic
acid (∼1.9 equiv), to provide a reactive N2Pd(OTf)2 Lewis
acid and free Me2BINOL. The second method took the
(26) (a) Brunkan, N. M.; Gagne´, M. R. Organometallics 2002, 21,
1576-1582. (b) Ghosh, A. K.; Matsuda, H. Org. Lett. 1999, 1, 2157-
2159.

Nonracemic N2PdCl2 Complexes
Organometallics, Vol. 23, No. 13, 2004 3215
sponds to 100 mg/10 mL. Elemental analyses were performed
by Complete Analysis Laboratories, Inc. THF and CH2Cl2 were
dried by passage through a column of activated alumina before
use. DMF was dried over CaSO4, distilled, and stored in the
drybox. Pd(OAc)2 was purchased from Strem and stored in the
drybox. (R)- and (S)-BINOL were purchased from Kankyo
Kagaku Center Co., Ltd., J apan, and used as received. (R)-
and (S)-3,3′-Me2BINOL and (R)- and (S)-6,6′-Br2BINOL were
synthesized according to literature procedures.28,29 KOtBu was
purchased from Aldrich, purified by sublimation at 100 °C,
and stored in a drybox. All other chemicals were purchased
from Aldrich or Acros and used with no further purification.
N,N′-Diben zyl-N,N′-dim eth yleth ylen ediam in e (1a). N,N′-
Dimethylethylenediamine (2.00 mL, 18.8 mmol) was added to
a suspension of NaH (1.35 g, 56.2 mmol) in dry DMF (75 mL).
The flask was vented, and benzyl chloride (4.75 mL, 40.8
mmol) was added to the reaction mixture. The reaction mixture
was heated at 100 °C for 4 h, after which the cooled mixture
was filtered through a pad of Celite. Solvent was removed in
vacuo with gentle heating to yield a viscous yellow product.
The product was dissolved in CH2Cl2 and washed twice with
water and once with brine. The aqueous layer was back-
extracted with CH2Cl2, and the combined organic layers were
stirred over MgSO4. Filtering and removal of the solvent under
vacuum gave pure product in 90% yield. 1H NMR (CDCl3, 400
MHz): δ 2.19 (s, 6H, CH3), 2.54 (s, 4H, NCH2CH2N), 3.49 (s,
4H, NCH2Ph), 7.21-7.28 (m, 10H, ArH). 13C{1H} NMR (CDCl3,
100 MHz): δ 43.1, 55.6, 63.1, 127.3, 128.6, 129.5, 139.4. This
material was spectroscopically identical to a commercial
sample purchased from Lancaster. Unfortunately, 1a ceased
to be available midway through our studies.
enantioselectivities reflect a poor stereochemical trans-
fer of information during turnover, and not compro-
mised catalyst.
Su m m a r y
To the broader question of whether N-chirality is
viable and usable, the results described herein clearly
indicate that asymmetric catalysis can be performed
with enantiopure catalysts whose chirality resides solely
on stereogenic nitrogen centers, although stereochemical
induction in the present case is inefficient (<25% ee).
The chirality of the catalyst has been shown to be robust
enough to confidently assess the overall feasibility of
exclusive N-chirality, and since some ligand systems
(with chiral backbones) have successfully utilized this
feature in asymmetric catalysis,1,12 we suggest that
other transformations will be better suited to these
catalysts.
On the other hand, these studies also revealed a
weakness in the strategy of relying exclusively on
N-chirality. First and foremost were issues related to
inversion of stereochemistry. Two unambiguous experi-
ments attested to the sensitivity of nitrogen coordination
to excessive steric hindrance: (1) the rapid isomeriza-
tion of diastereopure N2PdCl2 complexes containing
bulky N-anthracenyl substituents and (2) the conversion
of dl-2a to the matched diastereomer (exclusively) on
(R)-Me2BINOL coordination.25 That is, bulky metal
centers enhance the propensity for dissociation of the
amine ligand via either pure dissociation or associative
Pd-N bond breakage and lead to racemization of the
N-chirality. Since associative ligand substitution is
normally the operative mechanism in square planar d8
metals,27 the stability of the N-chirality is clearly in the
hands of a complex combination of steric and electronic
factors.
In summary, enantio- and diastereopure N2Pd(Me2-
BINOL) complexes can be synthesized using prochiral
diamine ligands. In the cases of 4a and 4b, cleavage of
the chiral resolving agent resulted in the corresponding
enantiopure N2PdCl2 complexes, wherein the N-stereo-
chemistry had been retained. Compound 4c was found
to be significantly less stable as N-racemization occurred
upon removal of the chiral resolving agent. Use of
diastereo- and enantiopure catalysts in a benchmark
Diels-Alder reaction resulted in high endo:exo selectiv-
ity and moderate reactivity, but low ee’s. Quenching
studies showed no racemization was occurring during
catalysis and that these low inductions reflected poor
stereochemical transfer of information rather than
unachievable N-chirality.
N,N′-Dim eth yl-N,N′-bis(n aph th alen -1-ylm eth yl)eth an e-
1,2-d ia m in e (1b). Compound 1b was prepared by a procedure
analogous to 1a to yield an off-white solid. The crude product
was recrystallized from CH2Cl2/hexanes to give off-white
crystals in 85% yield, mp 99-102 °C. Anal. Calcd for
C
26H28N2: C, 84.74; H, 7.66; N, 7.60. Found: C, 84.06; H, 8.10;
N, 6.97. 1H NMR (CDCl3, 400 MHz): δ 2.21 (s, 6H, CH3), 2.72
(s, 4H, NCH2CH2N), 3.89 (s, 4H, NCH2Ph), 7.33-7.43 (m, 8H,
ArH), 7.73 (m, 2H, ArH), 7.81 (m, 2H, ArH), 8.25 (m, 2H, ArH).
13C{1H} NMR (CDCl3, 100 MHz): δ 43.1, 56.1, 61.1, 125.0,
125.5, 125.9, 126.2, 127.9, 128.3, 128.8, 132.8, 134.2.
N,N′-Bis(a n th r a cen -1-ylm eth yl-N,N′-d im eth yl)eth a n e-
1,2-d ia m in e (1c). Compound 1c was prepared by a procedure
similar to 1a to yield a bright yellow solid. The crude product
was recrystallized from CH2Cl2/hexanes to give thin needlelike
crystals in 70% yield, mp 190-193 °C. Anal. Calcd for
C
34H32N2: C, 87.14; H, 6.88; N, 5.98. Found: C, 87.03; H, 7.06;
N, 5.89. 1H NMR (CDCl3, 400 MHz): δ 2.19 (s, 6H, CH3), 2.87
(s, 4H, NCH2CH2N), 4.40 (s, 4H, NCH2Ph), 7.40 (m, 8H, ArH),
7.95 (m, 4H, ArH), 8.37 (s, 2H, ArH), 8.41 (m, 4H, ArH). 13C-
{1H} NMR (CDCl3, 100 MHz): δ 42.8, 54.4, 56.5, 125.2, 125.5,
126.0, 127.8, 129.3, 131.7, 131.8.
d l-2a . N,N′-Dibenzyl-N,N′dimethylethylenediamine (590
µL, 1.76 mmol) was added to a solution of (COD)PdCl2 (500
mg, 1.75 mmol) in CH2Cl2, and the mixture was stirred for 1
h. The solution was concentrated to ∼5 mL, and the product
was precipitated from solution with hexanes. The sticky solid
was isolated on a frit and washed with hexanes to remove
excess COD to give the dl:meso mix (2:1) of N2PdCl2 in 95%
yield. Recrystallization of the crude mixture from CH2Cl2/
Exp er im en ta l Section
Gen er a l Meth od s. Manipulations of all air/water sensitive
compounds were performed under an N2 atmosphere using
standard Schlenk techniques or in an MBraun Labmaster
drybox. NMR spectra were obtained on a Bruker AMX 400 or
AMX 300 spectrometer using deuterated solvents (CD2Cl2,
CDCl3) purchased from Cambridge Isotope Labs. 1H spectra
were referenced to residual solvent resonances. Optical rota-
tion measurements were obtained on a J asco DIP-1000 digital
polarimeter in CH2Cl2, where a concentration of 1.0 corre-
hexanes provided pure dl material. Anal. Calcd for C18H24
-
Cl2N2Pd: C, 48.50; H, 5.43; N, 6.28. Found: C, 48.32; H, 5.47;
1
N, 6.29. H NMR (CD2Cl2, 400 MHz): δ 1.81 (d, J ) 9.2 Hz,
2H, NCHHCHHN), 2.48 (s, 6H, CH3), 2.93 (d, J ) 9.6 Hz, 2H,
NCHHCHHN), 3.15 (d, J ) 12.8 Hz, 2H, NCHHPh), 4.71 (d,
(28) Peacock, S. S.; Walba, D. M.; Gaeta, F. C. A.; Helgson, R. C.;
Cram, D. J . J . Am. Chem. Soc. 1980, 102, 2043-2052.
(29) Supporting Information from: Sasai, H.; Tokunga, T.; Wa-
tanabe, S.; Suzuki, T.; Itoh, N.; Shibasaki, M. J . Org. Chem. 1995, 60,
7388-7389.
(27) Cross, R. J . Adv. Inorg. Chem. 1989, 34, 219-292.

3216 Organometallics, Vol. 23, No. 13, 2004
Pelz et al.
1
J ) 12.8 Hz, 2H, NCHHPh), 7.45-7.52 (m, 6H, ArH), 8.04
(m, 4H, ArH). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 49.3, 57.7,
66.5, 129.2, 129.7, 132.6, 133.0.
d l-2b. This compound was prepared by the same procedure
described for dl-2a . A mixture of the product (dl:meso, 2:1)
was synthesized in 93% yield, which was selectively precipi-
tated to the (()-trans product in 55% yield. Anal. Calcd for
N, 3.16. Found: C, 75.76; H, 5.44; N, 3.12. H NMR (CDCl3,
400 MHz): δ 1.07 (d, J ) 8.8 Hz, 2H, NCHHCHHN), 1.96 (d,
J ) 9.2 Hz, 2H, NCHHCHHN), 2.33 (s, 6H, NCH3), 2.50 (s,
6H, CCH3), 4.02 (d, J ) 14 Hz, 2H, NCHHAr), 5.53 (d, J )
14.4 Hz, 2H, NCHHAr), 6.38 (t, J ) 8.0 Hz, 2H, ArH), 6.72 (d,
J ) 8.4 Hz, 2H, ArH), 6.90 (t, J ) 8.0 Hz, 2H, ArH), 7.15 (t, J
) 6.8 Hz, 2H, ArH), 7.34 (m, 4H, ArH), 7.41 (t, J ) 6.8 Hz,
2H, ArH), 7.72 (s, 2H, ArH), 7.80 (m, 4H, ArH), 7.95 (d, J )
9.2 Hz, 4H, ArH), 8.45 (s, 2H, ArH), 10.23 (d, J ) 8 Hz, 2H,
ArH). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 20.7, 50.1, 55.4,
56.1, 121.6, 123.7, 124.3, 124.5, 124.9, 125.2, 125.4, 126.0,
126.4, 126.9, 127.2, 127.5, 127.7, 128.3, 128.4, 129.3, 130.0,
130.2, 131.3, 131.7, 131.9, 132.6, 133.0, 135.5, 163.6. [R]D +42°
(c 0.98, CH2Cl2, 298 K).
C
26H28Cl2N2Pd: C, 57.21; H, 5.71; N, 5.13. Found: C, 56.96;
1
H, 5.49; N, 5.22. H NMR (CD2Cl2, 400 MHz): δ 1.81 (d, J )
9.2 Hz, 2H, NCHHCHHN), 2.48 (s, 6H, CH3), 2.93 (d, J ) 9.6
Hz, 2H, NCHHCHHN), 3.15 (d, J ) 12.8 Hz, 2H, NCHHPh),
4.71 (d, J ) 12.8 Hz, 2H, NCHHPh), 7.45-7.52 (m, 6H, ArH),
8.04 (m, 4H, ArH). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 49.3,
57.7, 66.5, 129.2, 129.7, 132.6, 133.0.
d l-2c. This compound was prepared by the same procedure
described for dl-2a . A mixture of the product (trans:cis, 3:1)
was synthesized in 91% yield, which was selectively precipi-
(R,R)-2a . To a solution of (R,R)/(R)-N2Pd(Me2BINOL) in
CH2Cl2 was added 2 drops of concentrated HCl. Upon stirring
for ∼15 min the solution turned from dark red to bright orange,
indicating Me2BINOL cleavage. The solution was filtered
through Celite and concentrated to a few milliliters. The
product was precipitated with hexanes, isolated, and washed
with hexanes to remove excess HCl. [R]D +195° (c 0.984, CH2-
Cl2, 298 K).
tated to the dl product in 60% yield. Anal. Calcd for C34H32
-
Cl2N2Pd: C, 63.22; H, 4.99; N 4.34. Found: C, 62.87; H, 5.05;
1
N, 4.24. H NMR (CD2Cl2, 300 MHz): δ 0.75 (br d, J ) 10.2,
2H, NCHHCHHN), 1.97 (br d, J ) 10.2, 2H, NCHHCHHN),
2.13 (s, 6H, CH3), 4.75 (d, J ) 14.4, 2H, NCHHPh), 5.42 (d, J
) 14.4, 2H, NCHHPh), 7.43 (m, 4H, J ) 7.8, ArH), 7.61 (t, J
) 7.5, 2H, ArH), 7.87 (t, J ) 6.6, 2H, ArH), 7.97 (d, J ) 7.5,
2H, ArH), 8.04 (d, J ) 8.7, 2H, ArH), 8.12 (d, J ) 8.4, 2H,
ArH), 8.53 (s, 2H, ArH), 9.75 (d, J ) 8.4, 2H, ArH). 13C{1H}
NMR (CD2Cl2, 100 MHz): δ 57.3, 60.7, 123.7, 124.1, 125.6,
125.9, 126.7, 127.6, 127.8, 128.7, 129.3, 130.0, 130.6, 131.7,
132.3, 132.6.
(R,R)-2b. The compound was prepared with a procedure
analogous to (R,R)-2a . Yield: 80%. [R]D +90° (c 0.980, CH2-
Cl2, 298 K).
d l/m eso-2c. The compound was prepared with a procedure
analogous to (R,R)-2b. Yield: 65%. [R]D +8° (c 0.56, CH2Cl2,
298 K).
Gen er a l Meth od for Diels-Ald er Ca ta lysis. Diels-Alder
catalysis reactions were kept at a constant temperature in a
Neslab CB-80 low-temperature cryobath equipped with a
Cryotrol temperature controller. Gas chromatography (conver-
sion) was performed on a DB-1 column (column temperature
180 °C, injection port temperature 200 °C, N2 as the carrier
gas). HPLC data were obtained on an Agilent 1100 series
instrument using a Diacel Chiracel OD-H column (0.75 mL/
min flow rate, 90% hexanes, 8% 2-propanol, 2% ethanol).
Enantiomeric excess and endo:exo ratio were calculated from
the HPLC profile. HOTf was distilled prior to use (35 °C/30
mTorr). Cyclopentadiene (CpH) was collected by distillation
during thermal cracking of dicyclopentadiene and used im-
mediately. The dienophile (N-acryloyl oxazolidinone) was
synthesized by a literature procedure30 and stored in the
drybox freezer.
1a -P d (R-Me2BINOL) (4a ). To a solution of Pd(OAc)2 (400
mg, 1.80 mmol) and 1a (495 µL, 1.81 mmol) in 50 mL of CH2-
Cl2 under a N2 atmosphere was added a solution of (R)-Me2-
BINOL (562 mg, 1.81 mmol) and KOtBu (400 mg, 3.64 mmol)
in 10 mL of THF. The dark red solution was stirred at 50 °C
for 5 h. After cooling, the solution was filtered through Celite
and the solvent was removed in vacuo to yield a dark red solid
in 90% yield as a mixture of two diastereomers. The major
diastereomer was isolated via crystallization of the mixture
from CH2Cl2/hexanes, which also provided crystals of X-ray
quality. Yield: 85%. Anal. Calcd for C40H40N2O2Pd: C, 69.91;
1
H, 5.87; N, 4.08. Found: C, 69.49; H, 5.93; N, 4.11. H NMR
(CD2Cl2, 400 MHz): δ 1.58 (d, J ) 8.8 Hz, 2H, NCHHCHHN),
2.26 (s, 6H, NCH3), 2.46 (s, 6H, CCH3), 2.73 (d, J ) 12.4 Hz,
2H, NCHHPh), 2.94 (d, J ) 8.8 Hz, 2H, NCHHCHHN), 4.26
(d, J ) 12 Hz, 2H, NCHHPh), 6.72 (d, J ) 8.4 Hz, 2H, ArH),
6.87 (t, 2H, ArH), 7.1 (m, 6H, ArH), 7.34 (t, 2H, ArH), 7.64 (s,
2H, ArH), 7.70 (d, J ) 8.0 Hz, 2H, ArH), 7.95 (d, J ) 7.2 Hz,
4H, ArH). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 20.3, 47.8, 55.2,
64.5, 121.1, 123.8, 124.8, 125.7, 126.5, 127.1, 128.5, 129.0,
131.0, 132.3, 134.6, 162.7. [R]D +39° (c 0.99, CH2Cl2, 298 K).
1b-P d (R-Me2BINOL) (4b). 4b was prepared by the same
procedure described above for 4a . The major diastereomer was
purified by passing the mixture through a plug of silica gel
(EtOAc/Hex, 9:1), albeit in a diminished yield of 60%. Anal.
Calcd for C48H44N2O2Pd: C, 73.23; H, 5.63; N, 3.56. Found:
Diels-Ald er Ca ta lysis, HOTf Meth od . A typical proce-
dure for catalysis is as follows. (R,R)/(R)-N2Pd(Me2BINOL)
(17.1 mg, 25.0 µmol) and dienophile (35.3 mg, 250 µmol) were
loaded into a flame-dried flask under an N2 atmosphere and
dissolved in 3 mL of CH2Cl2. HOTf (4.0 µL, 45 µmol) was added
at either rt or -78 °C, and the solution was allowed to stir for
15 min at the respective temperatures. The flask was then
transferred to a cryobath set at -55 °C, and freshly distilled
CpH (311 µL, 373 µmol) was added. Aliquots were taken (0.2
mL) and quenched by filtering through silica gel, eluting with
EtOAc, followed by immediate injection onto the GC column.
After ∼100% conversion was reached, the reaction mixture was
quenched by filtering through silica gel, eluting with EtOAc.
Solvent was removed in vacuo, and the residue was taken up
in HPLC grade 2-propanol for analysis.
1
C, 73.39; H, 5.81; N, 3.80. H NMR (CDCl3, 400 MHz): δ 1.49
(d, J ) 8.8 Hz, 2H, NCHHCHHN), 2.21 (s, 6H, NCH3), 2.51
(s, 6H, CCH3), 2.56 (d, J ) 8.8 Hz, 2H, NCHHCHHN), 3.43
(d, J ) 12.8 Hz, 2H, NCHHAr), 4.44 (d, J ) 12.8 Hz, 2H,
NCHHAr) 6.78 (d, J 8.8 Hz, 2H, ArH), 6.91 (t, J ) 7.6 Hz, 2H,
ArH), 7.13-7.21 (m, 4H, ArH), 7.39-7.47 (m, 4H, ArH), 7.73-
7.77 (m, 4H, ArH), 7.88 (d, J ) 3.2 Hz, 6H, ArH), 9.18 (d, J )
7.2 Hz, 2H, ArH). 13C{1H} NMR (CDCl3, 100 MHz): δ 20.8,
48.8, 56.3, 58.9, 121.6, 122.8, 124.4, 125.3, 125.8. 126.5, 126.9,
127.0, 127.7, 129.3, 129.8, 130.2, 131.6, 133.5, 133.8, 134.8,
135.0, 160.3. [R]D +43° (c 0.99, CH2Cl2, 298 K).
1c-P d (R-Me2BINOL) (4c). 4c was prepared by the same
procedure described above for 4a . The major diastereomer was
obtained by passing the mixture through a plug of silica gel
(EtOAc/Hex, 9:1) and then precipitating from CH2Cl2/hexanes
(61% yield). Anal. Calcd for C56H48N2O2Pd: C, 75.79; H, 5.45;
Diels-Ald er Ca ta lysis, AgSbF 6 Meth od . A typical pro-
cedure for the catalysis is as follows. (R,R)-2a (11.0 mg, 25.0
µmol) and dienophile (35.3 mg, 250 µmol) were added to a dry
flask under N2 and dissolved in 3 mL of CH2Cl2. AgSbF6 (19.0
mg, 55.0 µmol) was added and the mixture stirred for 10 min.
The reaction mixture was cooled to -78 °C and cannula filtered
to remove excess AgSbF6 and precipitated AgCl. The filtrate
was transferred to a cryobath cooled to -55 °C, and CpH (310
(30) Guo-J ie, H. M.; David J . J . Org. Chem. 1995, 60, 2271-2273.

Nonracemic N2PdCl2 Complexes
Organometallics, Vol. 23, No. 13, 2004 3217
Ta ble 3. Cr ysta llogr a p h ic Da ta a n d Collection P a r a m eter s for 4a , (R,R)-2a , a n d m eso-2c
4a ‚2CH2Cl2
(R,R)-2a
meso-2c‚CH2Cl2
empirical formula
fw
PdC42H44N2O2Cl4
857.02
PdC18H24Cl2N2
445.70
PdC35H34N2Cl4
730.87
space group
a, Å
b, Å
trigonal, P32
10.4804(6)
monoclinic, Pc
14.8129(3)
9.5230(2)
14.1217(3)
1923.49(7)
4
P21/c
26.1762(6)
12.9859(3)
19.9658(5)
6340.3(3)
8
c, Å
31.8116(17)
3026.0(2)
3
V, Å3
Z
T, °C
Dc, g/cm3
λ, Å
-100
-100
-100
1.411
1.539
1.531
0.71073
0.76
0.046
0.057
1.8799
0.71073
1.24
0.028
0.034
1.6535
0.71073
0.95
0.054
0.055
1.8190
µ, mm-1
a
Rf
b
Rw
GoFc
a
b
2 1/2
Rf ) ∑(Fo - Fc)/∑Fo. Rw ) [∑w(Fo - Fc)2/∑wFo
] .
c GoF ) [∑w(Fo - Fc)2/(n - p)]1/2, where n ) number of reflections and p )
number of parameters.
µL, 373 µmol) was added. Workup and analysis of the catalysis
reaction is the same as previously described for the HOTf
method.
NRCVAX system.31 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in calculated
positions (C-H ) 0.96 Å) and allowed to ride on the atoms to
which they were bonded. Crystal data, data collection, and
refinement parameters are listed in Table 1. Absorption
corrections were made using SADABS.
Ca ta lyst Isola tion a fter Diels-Ald er Ca ta lysis. After
∼100% conversion the reaction mixture was quenched with
brine. The mixture was filtered through Celite, the organic
layer separated, and solvent removed in vacuo. The residue
Ack n ow led gm en t. Financial support for this work
was partially provided by the NSF (CHE-0315203 and
CHE-0075717). K.A.P was the recipient of a GAANN
fellowship, which provided additional support. M.R.G.
is a Camille Dreyfus Teacher Scholar.
1
was dissolved in CD2Cl2 for analysis by H NMR.
Cr yst a llogr a p h y. Crystals suitable for X-ray crystal-
lography of 4a were obtained from a saturated CH2Cl2 solution
layered with hexanes, which was cooled to 0 °C. Crystals of
(R,R)-2a were grown at room temperature by slow diffusion
of hexanes into a saturated CH2Cl2 solution. Crystals of cis-
2c were grown at room temperature in a solution of CH2Cl2/
hexanes. Single crystals were mounted in oil on the end of a
fiber. Intensity data were collected on a Bruker-AXS SMART
1K diffractometer with CCD detector using Mo KR radiation
of wavelength 0.71073 Å (ω scan mode). The structures were
solved by direct methods and refined by least-squares tech-
niques on F using structure solution programs from the
Su p p or tin g In for m a tion Ava ila ble: Tables of bond
lengths and angles for structures 4a , (R,R)-2a , and meso-2c.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0498495
(31) Gabe, E. J .; Le Page, Y.; Charland, J . P.; Lee, F. L.; White, P.
S. J . Appl. Crystallogr. 1989. 22, 384.