Dinuclear allylpalladium complexes of C2-symmetric pyrazolate-bridged bis(oxazoline) ligands (pyrbox's): Structures, dynamic behavior, and application in asymmetric allylic alkylation

Article abstract of DOI:10.1021/om900835f

A series of new chiral binucleating pyrazolate-based N-donor ligands (3a-d) with oxazoline side arms (coined pyrbox's) have been synthesized. Bimetallic methallylpalladium complexes (4a-d) of these ligands were obtained, and the solid-state structures of complexes 4a,c were characterized by X-ray diffraction. NMR spectroscopy revealed that in solution 4a-d exist as three different isomers that differ in the orientation of the two methallyl ligands. Exchange between the isomers (i.e., allyl rotation) was observed in two-dimensional NOESY NMR experiments as syn/syn and anti/anti interconversion of the allylic hydrogen atoms; kinetic parameters for the fluxional behavior have been determined. The catalytic activity of the complexes was tested in palladium-catalyzed allylic alkylation of rac-(E)-1, 3-diphenylallyl acetate. By comparison of the set of complexes 4a-d that feature different ligand scaffolds, both steric and electronic factors were found to be important for enantiocontrol, and a model has been proposed for rationalizing the observed enantioselectivities.

Full text of DOI:10.1021/om900835f

Organometallics 2010, 29, 1117–1126 1117
DOI: 10.1021/om900835f
Dinuclear Allylpalladium Complexes of C2-Symmetric Pyrazolate-
Bridged Bis(oxazoline) Ligands (pyrbox’s): Structures, Dynamic
Behavior, and Application in Asymmetric Allylic Alkylation
Arne Ficks, Connor Sibbald, Michael John, Sebastian Dechert, and Franc Meyer*
€
€
Institut fu€r Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4,
€
D-37077 Gottingen, Germany
Received September 28, 2009
A series of new chiral binucleating pyrazolate-based N-donor ligands (3a-d) with oxazoline side
arms (coined pyrbox’s) have been synthesized. Bimetallic methallylpalladium complexes (4a-d) of
these ligands were obtained, and the solid-state structures of complexes 4a,c were characterized by
X-ray diffraction. NMR spectroscopy revealed that in solution 4a-d exist as three different isomers
that differ in the orientation of the two methallyl ligands. Exchange between the isomers (i.e., allyl
rotation) was observed in two-dimensional NOESY NMR experiments as syn/syn and anti/anti
interconversion of the allylic hydrogen atoms; kinetic parameters for the fluxional behavior have
been determined. The catalytic activity of the complexes was tested in palladium-catalyzed allylic
alkylation of rac-(E)-1,3-diphenylallyl acetate. By comparison of the set of complexes 4a-d that
feature different ligand scaffolds, both steric and electronic factors were found to be important for
enantiocontrol, and a model has been proposed for rationalizing the observed enantioselectivities.
Introduction
somewhat surprising, since recent chemical research has seen
an increasing interest in two-center catalysis that offers the
potential of cooperative effects of the proximate metal
ions.9-11 The principle is nothing new, as naturally occurring
enzymes use this approach in various ways, and binucleating
ligand design is indeed very popular in biomimetic and
bioinspired coordination chemistry.12,13 However, only a
few reports have dealt with bimetallic C2-symmetric com-
plexes of the bis(oxazoline) type: Pfaltz and Fahrni devel-
oped a series of phenol-, 1,8-naphthyridine-, and pyridazine-
bridged ligand scaffolds, each with oxazoline side arms,14
and have studied some aspects of their coordination
C2-symmetric bis(oxazolines), usually called box’s, were
introduced as a new class of ligands for asymmetric catalysis
in the early 1990s.1 Since then, the list of applications of these
ligands has rapidly grown, as has the number of differently
substituted box derivatives.2-4 Further developments in the
field comprise, inter alia, the tris(oxazolines)5,6 as well as box
ligands with a range of bridges linking the chiral oxazoline
rings, including bridges with additional donor atoms.7 Pro-
minent examples of the latter type are the so-called pybox
ligands, which feature a central pyridine and provide a
tridentate binding site.8 Though oxazoline-based ligands
have become very common, binucleating systems where a
central bridging unit with appended oxazoline rings can
host two metal centers have remained scarce so far. This is
(9) (a) Van den Beuken, E. K.; Feringa, B. L. Tetrahedron 1998, 54,
12985–13011. (b) Matsunaga, S.; Shibasaki, M. Bull. Chem. Soc. Jpn. 2008,
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(10) Multimetallic Catalysts in Organic Synthesis; Shibasaki, M.,
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Chem. Soc. 2001, 123, 3367–3368. (b) Ackermann, J.; Meyer, F.; Kaifer, E.;
Pritzkow, H. Chem. Eur. J. 2002, 8, 247–258. (c) Li, H.; Marks, T. J. Proc.
Natl. Acad. Sci. U.S.A. 2006, 103, 15295–15302. (d) Tzur, E.; Ben-Asuly,
A.; Diesendruck, C. E.; Goldberg, I.; Lemcoff, N. G. Angew. Chem. 2008,
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2008, 27, 617–618. (g) Prokofieva, A.; Prikhod'ko, A. I.; Dechert, S.; Meyer,
F. Chem. Commun. 2008, 1005–1007. (h) Kember, M. R.; Knight, P. D.;
Reung, P. T. R.; Williams, C. K. Angew. Chem. 2009, 121, 949–951. Angew.
Chem., Int. Ed. 2009, 48, 931-933.
*To whom correspondence should be addressed. Tel: þ49 551 393012.
Fax: þ49 551 393063. E-mail: franc.meyer@chemie.uni-goettingen.de.
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2010 American Chemical Society
Published on Web 02/01/2010
pubs.acs.org/Organometallics

1118 Organometallics, Vol. 29, No. 5, 2010
Scheme 1. Synthesis of Pyrazole-Bridged Ligands 3a-d
Ficks et al.
chemistry and their use in catalysis.15 More recently, the
structure of a binuclear η3-allylpalladium complex of the
pyridazine-centered pydbox ligand with appended oxazoline
rings was also reported.16
organometallic chemistry are increasingly recognized.22-25
While a pyrazolate-bridged bimetallic array has inherent
2-fold symmetry, however, little is known as yet about
C2-symmetric systems with chiral chelate arms such as the
ligand HL bearing two (R)-2-(methoxymethyl)pyrrolidine
substituents at the central pyrazole core.26
Pyrazole, in its deprotonated form, is a well-known brid-
ging unit that supports the formation of bimetallic com-
plexes.17 The generally adopted metal-metal separations are
in the range of 3.4-4.5 A and are thus suitable for exploiting
cooperative reactivity toward substrate molecules. Further
enhancement of preorganization of the two metal ions can be
achieved by the attachment of chelating side arms to the 3,5-
positions of the pyrazole heterocycle, and a variety of
symmetrical and unsymmetrical 3,5-disubstituted ligands
with mono-, bi-, and tridentate side arms have been devel-
oped over the past decade.18 By varying the lengths of
these side arms and the nature or the quantity of the donor
atoms, a fine tuning of the metal-metal distance can be
achieved.19,20 Such highly preorganized pyrazolate-based bi-
metallic complexes have been successfully used in metallobio-
site modeling,21 and their favorable properties in two-center
In this work we report the synthesis of new chiral pyrazole-
bridged ligands with oxazoline side arms (coined pyrbox’s),
as well as the synthesis and molecular structures of their
binuclear allylpalladium complexes. Mutual interconversion
of different isomers of the allylpalladium complexes in
solution is studied in detail, using two-dimensional NMR
techniques, and initial findings for the application of these
C2-symmetric binuclear pyrbox-palladium complexes in the
asymmetric allylic alkylation of rac-(E)-1,3-diphenylallyl
acetate are discussed in view of the fluxional behavior.
(15) (a) Fahrni, C. J.; Pfaltz, A.; Neuburger, M.; Zehnder, M. Helv.
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Recl. 1997, 130, 605–613. (b) Meyer, F.; Heinze, K.; Nuber, B.; Zsolnai, L. J.
Chem. Soc., Dalton Trans. 1998, 207–213. (c) Konrad, M.; Meyer, F.;
Heinze, K.; Zsolnai, L. J. Chem. Soc., Dalton Trans. 1998, 199–205. (d)
Siegfried, L.; Kaden, T. A.; Meyer, F.; Kircher, P.; Pritzkow, H. Dalton
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Dechert, S. Chem. Eur. J. 2005, 11, 4349–4360. (b) Prokofieva, A.;
Prikhod'ko, A. I.; Enyedy, E. A.; Farkas, E.; Maringgele, W.; Demeshko,
S.; Dechert, S.; Meyer, F. Inorg. Chem. 2007, 46, 4298–4307. (c) M€uller, H.;
Bauer-Siebenlist, B.; Csapo, E.; Dechert, S.; Farkas, E.; Meyer, F. Inorg.
Chem. 2008, 47, 5278–5292.
(21) See for example: (a) Kaden, T. A. Coord. Chem. Rev. 1999,
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Meyer, F. Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley:
Hoboken, NJ, 2009; Vol. 56, pp 487-542.
Results and Discussion
Synthesis of Ligands 3a-d. We prepared a set of four new
ligands that differ in the C4 substituent at the central
pyrazole (H or Ph) and in the substituent of the outer
i
oxazolines (Ph or Pr), which allowed us to investigate the
effect of steric and electronic modifications at both the
bridging unit and the chelate arms. The synthesis was
achieved in a straightforward approach (Scheme 1): in the
first step, the pyrazole compounds 1a27 and 1b28 were reacted
with an appropriate amino alcohol in the presence of a
catalytic amount of acid. It was found beneficial to keep
the amount of solvent very low, as dilution caused longer
reaction times and favored the formation of the monosub-
stituted byproduct. Pyrazole compounds 2a-d were
obtained in moderate yields (55-73%) after recrystallization
or chromatographic workup. Ring closure to form the
oxazolines was then achieved by using thionyl chloride
followed by the addition of sodium methoxide. The crude
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2414-2417; Angew. Chem., Int. Ed. 2002, 41, 2304-2306. (c) Roder, J. C.;
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2636–2648. (d) Sheng, T.; Dechert, S.; Hyla-Kryspin, I.; Winter, R. F.;
Meyer, F. Inorg. Chem. 2005, 44, 3863–3874. (e) Sheng, T.; Dechert, S.;
St€uckl, A. C.; Meyer, F. Eur. J. Inorg. Chem. 2005, 1293–1302.
(26) Roder, J. C.; Meyer, F.; Konrad, M.; Sandhofner, S.; Kaifer, E.;
Pritzkow, H. Eur. J. Org. Chem. 2001, 4479–4487.
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€
€
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Mecking, S.; Meyer, F. Adv. Synth. Catal. 2006, 348, 887–897. (b) Moret,
M.-E.; Chen, P. Organometallics 2008, 27, 4903–4916.
€
(28) Sachse, A.; Penkova, L.; Noel, G.; Dechert, S.; Varzatskii, O. A.;
Fritsky, I. O.; Meyer, F. Synthesis 2008, 800–806.

Article
Organometallics, Vol. 29, No. 5, 2010 1119
Scheme 2. Synthesis of Methallylpalladium Complexes 4a-d
Table 1. CdN Stretching Vibrations for the Free Ligands (νL) and
the Complexes (νC) Obtained from IR Spectroscopy
entry ligand νL(CdN), cm-1 complex νC(CdN), cm-1 Δν, cm-1
1
2
3
4
3a
3b
3c
3d
1652
1656
1664
1659
4a
4b
4c
4d
1633
1623
1640
1624
19
33
24
35
products were already quite pure and could be further
purified by recrystallization or column chromatography,
giving potential ligands 3a-d in moderate to good yields
(58-80%).
Synthesis and Characterization of Complexes 4a-d. The
coordination properties of ligands 3a-d were investigated in
their monocationic bis(methallylpalladium) complexes.
Deprotonation of the pyrazole with potassium tert-butoxide,
addition of methallylpalladium chloride, and anion ex-
change with ammonium tetrafluoroborate afforded com-
plexes 4a-d, which were isolated as pale yellow solids after
aqueous workup in good to excellent yields (80-98%;
Scheme 2).
The products are soluble in polar solvents such as acetone,
dichloromethane, and tetrahydrofuran but insoluble in hex-
ane, pentane, or diethyl ether. Neat samples of the methal-
lylpalladium complexes or solutions stored at -30 °C are
stable in air for several weeks without any decomposition.
However, at room temperature the compounds in solution
gradually decompose within a few days, even when handled
under inert conditions. All four complexes were fully char-
acterized by IR, NMR, and mass spectrometry. As expected,
the CdN stretching frequency is lower in the complexes
4a-d compared to the corresponding free ligands 3a-d due
to coordination of the palladium (Table 1). This effect is
somewhat more pronounced for 4b,d, presumably because
the phenyl substituent in the 4-position of the pyrazole
weakens the electron donation of the pyrazole-N, which is
balanced by a more pronounced binding of the oxazoline.
However, 13C NMR chemical shifts of the oxazoline
CdN group are very similar in all cases (4a, 166.6 ppm; 4b,
167.0 ppm; 4c, 165.9 ppm; 4d, 166.3 ppm) and only margin-
ally larger for complexes bearing the backbone phenyl sub-
stituent (4b,d) compared to the respective complexes with a
backbone H (4a,c).
Figure 1. ORTEP plot (50% probability thermal ellipsoids) of
the molecular structure of the cation of 4a. Hydrogen atoms and
the BF4 anion have been omitted for clarity. Selected bond
-
distances (A) and angles (deg): Pd(1)-N(1) = 2.133(7), Pd(1)-
N(3) = 2.104(8), Pd(1)-C(22) = 2.127(12), Pd(1)-C(23) =
2.130(10), Pd(1)-C(24) = 2.131(11), C(22)-C(23) = 1.42(2),
C(23)-C(24) =1.380(15), Pd(2)-N(2)=2.132(7), Pd(2)-N(4) =
2.100(7), Pd(2)-C(26) = 2.113(8), Pd(2)-C(27) = 2.128(9), Pd-
(2)-C(28) =2.142(8), C(26)-C(27) = 1.412(13), C(27)-C(28) =
1.417(12), Pd(1) Pd(2)=4.6070(9); N(1)-Pd(1)-N(3)=78.2(3),
3 3 3
N(1)-Pd(1)-C(24) = 109.3(4), N(3)-Pd(1)-C(22) = 103.9(5),
C(22)-Pd(1)-C(24) = 68.8(6), N(1)-Pd(1)-C(22) = 176.1(4),
N(3)-Pd(1)-C(24) = 171.3(4), N(2)-Pd(2)-N(4) = 78.0(3),
N(2)-Pd(2)-C(28) = 110.8(3), N(4)-Pd(2)-C(26) = 103.5(3),
C(26)-Pd(2)-C(28)=67.8(3), N(2)-Pd(2)-C(26)=178.5(3), N-
(4)-Pd(2)-C(28)=170.9(3).
Figure 2. ORTEP plot (50% probability thermal ellipsoids) of
the molecular structure of the cation of 4c. Hydrogen atoms and
-
the PF6 anion have been omitted for clarity. Only one of the
two crystallographically independent molecules is shown. Se-
lected bond distances (A) and angles (deg): Pd(2)-N(11) =
2.098(9), Pd(2)-N(12)=2.108(6), Pd(2)-C(29)=2.098(9), Pd-
(2)-C(30)=2.137(8), Pd(2)-C(31)=2.117(8), C(29)-C(30)=
Solid-State Structures of 4a,c. Crystals of 4a,c were ob-
tained by slow diffusion of diethyl ether into dichloro-
methane solutions of the complexes. Molecular structures
are shown in Figures 1 and 2. One [LPd2{(CH2)2CCH3}2]þ
cation per asymmetric unit is found in 4a, while two different
[LPd2{(CH2)2CCH3}2]þ cations with crystallographically
imposed C2 symmetry are present in 4c. In one of the latter
cases the (η3-methallyl)Pd moieties are disordered about two
1.397(14), C(30)-C(31)=1.428(12), Pd(2) Pd(2)0=4.6313(7);
3 3 3
N(11)-Pd(2)-N(12)=78.2(2), N(11)-Pd(2)-C(29)=109.0(3),
N(12)-Pd(2)-C(31)=104.1(3), C(29)-Pd(2)-C(31)=68.7(3),
N(11)-Pd(2)-C(31)
172.7(3). Symmetry transformation used to generate equivalent
= 177.6(3), N(12)-Pd(2)-C(29) =
atoms: (0) 1 - x, y, -z.

1120 Organometallics, Vol. 29, No. 5, 2010
Ficks et al.
Figure 3. Variable-temperature 1H NMR spectra (CDCl3, 500 MHz) of complex 4a.
positions (Figure S4 in the Supporting Information). Com-
plexes 4a,c both consist of a bimetallic monocation with two
methallylpalladium moieties hosted in the two {N2} binding
pockets of the ligand, as anticipated. The bite angle of the
ligand is approximately 78° in both coordination sites, only for
the disordered cation in 4c it differs somewhat. The distance
between the two palladium atoms (∼4.6 A) is relatively long
for binuclear complexes of compartmental pyrazolate-based
ligands,18,24 which is probably due to (i) the short oxazoline
chelate arms that pull the metal ions outward and (ii) steric
crowding between the two methallyl groups in the bimetallic
pocket that pushes the (η3-methallyl)Pd moieties apart. Both
methallyl CH3 groups point in the same direction as the phenyl
groups on the neighboring oxazoline rings in 4a (exo/exo
isomer); this is also the major isomer in solution (vide infra).
Since the (η3-methallyl)Pd moiety is disordered between exo
and endo isomers and the cation is located on a 2-fold axis, all
possible orientations of the η3-bound methallyl group, giving a
C2-symmetric [LPd2{(CH2)2CCH3}2]þ cation, are present
within the unit cell of 4c. This finding suggests that fluxional
behavior due to rotation of the methallyl groups might occur in
solution, as is indeed observed.
Pd-C bond lengths in 4a,c are found as expected for
palladium(II) allyl compounds,29,30 and Pd-N bond lengths
are common for pyrazolate- and oxazoline-derived ligands.
A closer inspection of atom distances reveals that all Pd-N
bond lengths are found in a rather narrow range (2.133(7)-
2.098(9) A), as are the Pd-CH2 bonds (2.142(8)-2.098(9)
A). This suggests similar trans influences of the pyrazolate-N
and oxazoline-N atoms, in accordance with NMR data (vide
infra). Interestingly, a subtle distinction is discernible when
comparing 4a and 4c (although differences are barely signi-
ficant at the 3σ level): in 4a the Pd-N(pyrazolate) bonds are
slightly longer than the Pd-N(oxazoline) bonds, and hence
the Pd-CH2 bonds trans to N(pyrazolate) are slightly short-
er than the Pd-CH2 bonds trans to N(oxazoline), while the
situation is reversed in the case of 4c. Indeed, this reversal is
also reflected in the 13C NMR spectra for the methallyl CH2
groups (vide infra).
Dynamic Behavior of 4a-d in Solution. The behavior of
complexes 4a-d in solution was studied by one- and two-
dimensional NMR experiments at variable temperatures. At
room temperature, the spectrum of 4a in chloroform-d shows
very broad signals in the allylic region, which indicates that
the methallyl groups are dynamic (Figure 3). Cooling to 0 °C
results in single, albeit somewhat broad peaks, and at -50 °C
the methallyl resonances are sufficiently resolved to allow
assignment by two-dimensional NOESY experiments
(Figure S1 in the Supporting Information).
Multiple NMR peaks are due to two possible orientations
of each η3-bound methallyl group, resulting in three distinct
isomers (Scheme 3): two C2-symmetric species (A and C)
with antiparallel oriented allyl groups and a fully asymmetric
species (B) with parallel oriented allyl groups that gives rise
to twice the number of NMR resonances (denoted H and H0).
A similar situation is encountered for the other complexes
4b-d (not shown). From specific NOE correlations between
the oxazoline phenyl protons and H1s(A) and CH3(A) we
could assign the major isomer of complex 4a to structure A,
where the phenyl groups and the adjacent methallyl CH3
group face the same direction. This isomer is also the one
observed in the solid-state structure of 4a (vide supra).
In solution, isomers A-C are populated with a ratio of
approximately 4:2:1 (Table S1 in the Supporting Information).
Also, the phenyl groups induce substantial shielding of the
“outer” protons H1s and H1a relative to the “inner” protons H2s
and H2a.
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At -25 °C, the NOESY spectra contain not only NOE
correlations but also cross-peaks that arise from exchange
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Article
Organometallics, Vol. 29, No. 5, 2010 1121
Scheme 3. Three Possible Isomers of the Allylpalladium Complexes 4a-d
Table 2. Allylic Alkylation of rac-(E)-1,3-Diphenylallyl Acetate
with Dimethyl Malonate
entry
catalysta
yield, %b
ee, %c
1
2
3
4
5
6
4a
4b
4c
4d
40
53
67
54
69
72
68 (R)
28 (R)
44 (S)
5 (S)
58 (R)
56 (R)
d
3a/[(methallyl)PdCl]2
3a/[(allyl)PdCl]2
d
a Reaction of the catalyst (2 mol %) with rac-(E)-1,3-diphenylallyl
acetate (1.0 mmol), dimethyl malonate (3.0 mmol), and NaH (3.0 mmol);
48 h reaction time at 45 °C. b Isolated yields after workup. c Determined
by 1H NMR using Eu(hfc)3 as chiral shift reagent; absolute configura-
tions were determined by comparing the optical rotation values to
literature data.38a d Catalyst was generated in situ from ligand (5 mol
%) and [Pd] source (2 mol %).
Figure 4. Part of the 2D NOESY spectrum of 4c (CDCl3,
500 MHz, -25 °C, mixing time 0.5 s) showing the pz H4 region.
The spectrum was acquired with 957 ꢀ 512 data points and a
spectral width of 0.3 ppm.
Exchange rate constants in chloroform-d were determined
at -25, -12.5, and 0 °C for complexes 4a,b and at -25 °C for
complex 4c (the results are given in Table S2 in the Support-
ing Information) and converted into activation free energies
ΔGq.35 The ΔGq value of the transition B f C is approxi-
mately 65 kJ/mol in complexes 4a,b and about 3 kJ/mol
lower in complex 4c (Figure S3 in the Supporting In-
formation), which is in agreement with values reported for
similar allylic systems.29,31 The temperature dependence
obtained for 4a indicates a negative activation entropy and
thus a highly organized transition state as part of an asso-
ciative mechanism.33 This is further supported by the much
lower activation free energy (56 kJ/mol) observed for com-
plex 4c in the coordinating solvent THF. In acetonitrile the
rate constants could not be determined due to coalescence of
peaks already at -25 °C, but we estimated them to be at
least another 20 times higher (corresponding to another 6-7
kJ/mol less) than in THF. Since apparent allyl rotation is
orbitally forbidden unless the coordination number is altered
in the transition state, these findings suggest that the solvent
serves as a fifth donor to initiate allyl rotation around the
Pd-allyl bond via an associative process.36
between the isomers A and B and between B and C (Figure 4).
For the allyl protons, exchange is strictly syn/syn and
anti/anti (Figure S2 in the Supporting Information) and
therefore indicates a rotation of the allyl plane, consistent
with former reports about allylpalladium systems with other
N-donor ligands.29,31 A η3-η1-η3 mechanism,32 which has
often been observed in the case of phosphorus or N-hetero-
cyclic carbenes coordinated to Pd,25c,33,34 requires syn/anti
exchange of the transiently η1-bound allyl CH2 protons and
can be ruled out here. Quantitative analysis of the peak
integrals with a 3 ꢀ 3 exchange matrix approach yielded
typical values of kAB ≈ kBC ≈ 0.1 s-1 and kBA ≈ kCB ≈ 0.2 s-1
for 4a,b, while the rate constants in 4c are somewhat larger,
presumably due to a less pronounced steric interference with
the peripheral isopropyl groups on the ligand. The values for
kAC and kCA were always close to zero, indicating that no
direct exchange occurs between A and C and that the two
methallyl groups rotate sequentially, independent from each
other.
Palladium-Catalyzed Asymmetric Allylic Alkylation. The
catalytic activity of the bis(methallylpalladium) complexes
was tested in the allylic alkylation of the model substrate
rac-(E)-1,3-diphenylallyl acetate, using dimethyl malonate
as a nucleophile under basic conditions. Palladium-catalyzed
ꢀ
(31) (a) Gogoll, A.; Grennberg, H.; Axen, A. Organometallics 1997,
ꢀ
16, 1167–1178. (b) Jalon, F. A.; Manzano, B. R.; Moreno-Lara, B. Eur. J.
Inorg. Chem. 2005, 100–109. (c) Montoya, V.; Pons, J.; Garcia-Anton, J.;
Solans, X.; Font-Bardía, M.; Ros, J. Organometallics 2007, 26, 3183–3190.
(32) Breutel, C.; Pregosin, P. S.; Salzmann, R.; Togni, A. J. Am.
Chem. Soc. 1994, 116, 4067–4068.
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(33) Ketz, B. E.; Cole, A. P.; Waymouth, R. M. Organometallics
2004, 23, 2835–2837.
(35) Laidler, K. J.; King, M. C. J. Phys. Chem. 1983, 87, 2657–2664.
ꢁ
(36) Pericas, M. A.; Puigjaner, C.; Riera, A.; Vidal-Ferran, A.;
(34) (a) Peng, H. M.; Song, G. Y.; Li, Y. X.; Li, X. W. Inorg. Chem.
2008, 47, 8031–8043. (b) Filipuzzi, S.; Pregosin, P. S.; Albinati, A.; Rizzato,
S. Organometallics 2008, 27, 437–444.
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Gomez, M.; Jimenez, F.; Muller, G.; Rocamora, M. Chem. Eur. J.
2002, 8, 4164–4178.

1122 Organometallics, Vol. 29, No. 5, 2010
Scheme 4. Enantiocontrol Provided by Dinuclear Pyrazole-Bridged Pd Catalyst 4a
Ficks et al.
allylation of carbon nucleophiles is a versatile method for
carbon-carbon bond formation in organic synthesis,37 and
it is a common benchmark reaction for probing new ligands
in enantioselective catalytic reactions.38 Whereas there are
many reports on Pd-catalyzed allylic substitutions employ-
ing chiral phosphine derivatives, it should be noted that
systems with purely N-donor ligands are comparatively
few.36,39 Complexes 4a-d were either used directly or generated
in situ from an allylpalladium precursor and the appropriate
ligand. All palladium complexes showed activity in the
alkylation reaction with isolated (nonoptimized) yields after
48 h of 40-72% (2 mol % catalyst; Table 2). The catalysts
generated in situ seem to have slightly higher activities;
possibly this effect arises from the presence of chloride rather
than tetrafluoroborate as counteranion. The best enantio-
selectivity (68% ee) was achieved with 4a, whereas 4d gave
only 5% ee. The (electron-withdrawing) phenyl group on the
backside of the central pyrazole bridge (4b,d) apparently
reduces the selectivity, as does replacement of the phenyl
group on the oxazoline side arm by an isopropyl group (4a,b
versus 4c,d). Note that changes in the absolute configuration
of the product are caused by different configurations of the
stereocenters on the ligand (4a,b versus 4c,d).
shown in Scheme 4. If one assumes that the syn/syn isomer
of the 1,3-diphenylallyl moiety predominates,40,41 due to
steric repulsion of the bulky phenyl substituents within the
bimetallic pocket the two 1,3-diphenylallyl groups in the
intermediates can only adopt an antiparallel orientation,
which corresponds to isomers A (exo/exo) and C (endo/
endo) of complexes 4a-d (Scheme 3). In the case of 1,3-
diphenylallyl instead of methallyl, however, it is reasonable
to suppose that isomer A is strongly disfavored because of
interference with the oxazoline substituents.
Nucleophilic attack is generally expected to occur at the
allyl C that is trans to the stronger trans-effect donor atom,
which can usually be identified by the lower field 13C NMR
chemical shift of that allylic carbon atom.41-43 In accor-
dance with what has been concluded from the crystallo-
graphic data for 4a,c, however, there seems to be no
substantial difference in ground-state trans influence (and
likely also in kinetic trans effect) between the pyrazolate and
oxazoline donors of the pyrbox ligands. 13C NMR reso-
nances are detected in the very narrow range 58.2-59.4 ppm
for the methallyl CH2 trans to the pyrazolate N and
58.8-59.9 ppm for the methallyl CH2 trans to the oxazoline
N. The locus of nucleophilic attack is therefore not evident
Dinuclear species similar to complexes 4a-d, but with two
1,3-diphenylallyl ligands instead of methallyl ligands, could
be identified by ESI mass spectrometry in the reaction
mixtures. As an initial working model, the observed enan-
tioselectivities may thus be rationalized by the scenario
(37) (a) Trost, B. J. Org. Chem. 2004, 69, 5813–5837. (b) Pfaltz, A.;
Lautens, M. Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz,
A., Yamamoto, H., Springer: Heidelberg, Germany, 1999; pp 833-886.
(38) (a) Leutenegger, U.; Umbricht, G.; Fahrni, C.; von Matt, P.;
Pfaltz, A. Tetrahedron 1992, 48, 2143–2156. (b) Chelucci, G.; Medici, S.;
(40) Boog-Wick, K.; Pregosin, P. S.; Trabesinger, G. Organometallics
1998, 17, 3254–3264.
(41) Dotta, P.; Anil Kumar, P. G.; Pregosin, P. S. Magn. Reson.
€
Saba, A. Tetrahedron: Asymmetry 1997, 8, 3183–3184. (c) Nordstrom, K.;
Chem. 2002, 40, 653–658.
Macedo, E.; Moberg, C. J. Org. Chem. 1997, 62, 1604–1609.
(39) von Matt, P.; Lloyd-Jones, G. C.; Minidis, A. B. E.; Pfaltz, A.;
€
Macko, L.; Neuburger, M.; Zehnder, M.; Ruegger, H.; Pregosin, P. S.
(42) Steinhagen, H.; Reggelin, M.; Helmchen, G. Angew. Chem., Int.
Ed. Engl. 1997, 36, 2108–2110.
(43) Tu, T.; Zhou, Y.-G.; Hou, X.-L.; Dai, L.-X.; Dong, X.-C.; Yu,
Helv. Chim. Acta 1995, 78, 265–284.
Y.-H.; Sun, J. Organometallics 2003, 22, 1255–1265.

Article
Organometallics, Vol. 29, No. 5, 2010 1123
from the spectroscopic and structural data, and it is quite
likely that the lack of electronic difference of the pyrazolate
and oxazoline donors, which propagates itself in a lack of
electronic differentiation of the allylic termini, is a reason for
the only modest enantiomeric excess of the present systems.41
On a more subtle level, it is interesting to note that the allylic
carbon trans to the pyrazolate N resonates at slightly lower
field than the carbon trans to the oxazoline N in the case of
4a,b (13C at 59.3/59.4 versus 58.8/58.8 ppm), while this order
is reversed in 4c,d (58.3/58.2 versus 59.6/59.9 ppm). One has
to bear in mind, though, that various factors, including the
shielding effect of the oxazoline phenyl substituents in 4a,b,
will contribute to these minor differences. According to 13C
NMR data, the presence or absence of a phenyl group at the
pyrazolate C4 has no obvious ground-state electronic effect
on the allyl groups; thus, the significant effect of the back-
bone C4 substituent on enantioselectivities remains un-
explained at this stage.
Starting from the predominant isomer C, nucleophilic
attack trans to the oxazoline N (pathway c in Scheme 4)
would lead to the experimentally observed R enantiomer in
the case of precatalyst 4a. Given a late-transition-state
model, the alternative pathway d might be disfavored be-
cause of repulsive steric interactions with the ligand substit-
uents in the intermediate Pd(0)-olefin complex (Scheme 4).
It has to be noted, though, that this picture represents just a
preliminary working scheme, since it is well-known that
species which are present in only low amounts may react
much more quickly than the major rotational isomer and
may carry most of the reaction flux.40,44 In addition, it is
quite conceivable that syn/syn-syn/anti isomerism of the
bulky 1,3-diphenylallyl ligands45 may play a role in the
present bimetallic systems due to unfavorable steric interac-
tions of the two proximate 1,3-diphenylallyl groups, even
though fluxionality in the methallyl precatalyst complexes
was found to proceed via apparent allyl rotation rather than
syn/anti exchange.
showed similar activities, drastic differences in enantio-
control were observed. Enantioselectivities are much better
for the systems with phenyl groups at the oxazolines (4a,b)
compared to isopropyl substituents (4c,d) and much better
for complexes bearing just a proton instead of an electron-
withdrawing phenyl substituent at the central pyrazole
bridge (4a,c versus 4b,d). Hence, both steric and electronic
factors play a key role in determining enantioselectivities,
which offers a multitude of options for tuning and improving
the new pyrbox ligand scaffolds. Recent reports on enantio-
selective allylic substitution using a binuclear allylpalladium
complex with a single-stranded helicate structure showed
that ee values crucially depend on many factors, such as the
base and the solvent.46 An ee of 68% in our nonoptimized
preliminary experiments thus represents a promising starting
point for further optimization. Studies of tris(oxazoline)-
based and related catalysts for allylic substitution showed
that complexes with the least internal degrees of freedom
display the highest activity and stereoselectivity,6 which may
represent a guiding line for further elaboration of the present
bimetallic systems.
Future work will also focus on the application of the
pyrbox ligands in other enantioselective metal-mediated
reactions and on the study of possible cooperative effects
of the proximate metal ions. In the context of palladium-
catalyzed allylic substitution reactions, these highly pre-
organized bimetallic systems offer interesting prospects in
view of a recent report by the Pfaltz group, where the
reversible formation of allyl-bridged binuclear PdI reservoir
species has been observed under catalytic conditions.47
Experimental Section
General Procedures. All air- and/or water-sensitive reactions
were performed under a nitrogen atmosphere in a glovebox or
using standard Schlenk line techniques. Tetrahydrofuran and
dichloromethane were dried over potassium and calcium
hydride, respectively, and distilled prior to use. Dimethyl 1H-
pyrazole-3,5-dicarboxylate hydrochloride (1a),27 dimethyl
4-phenyl-1H-pyrazole-3,5-dicarboxylate (1b),28 (S)-valinol,48
and (R)-phenylglycinol48 were prepared according to literature
procedures. All other chemicals were used as purchased without
further purification. Flash chromatography was performed on
silica gel from Macherey-Nagel (Kieselgel 60 M, 0.04-0.063
mm, 230-400 mesh ASTM). Thin-layer chromatography was
performed on Fluka aluminum-based plates with silica gel and
fluorescent indicator 254 nm. For indication, UV light (λ 254
nm/366 nm), potassium permanganate solution (1.0 g of
KMnO4, 6.7 g of K2CO3, 0.1 g of NaOH, 100 mL of H2O), or
ninhydrin solution (1.0 g of ninhydrin, 0.2 mL of acetic acid,
100 mL of EtOH) was used. Melting points were determined in
open glass capillary tubes on a Stanford Research Systems
OptiMelt MPA 100 device; values are uncorrected. Optical
rotation values were determined on a Perkin-Elmer 241 polari-
meter device. Infrared spectra were recorded as KBr pellets on a
Digilab Excalibur Series FTS 3000 spectrometer. Mass spectro-
metry was performed using an Applied Biosystems API 2000
(ESI) or a Bruker FTICR (HR-ESI) instrument.
Conclusions
New pyrazolate-bridged ligands 3a-d, coined pyrbox’s,
have been successfully synthesized and characterized. A first
glance at their coordination properties has been obtained
from their methallylpalladium complexes 4a-d. In the solid
state, complex 4a was found as a single isomer, while 4c
displays disorder in the coordinated methallyl fragment with
all possible isomers present. In solution the complexes each
exist as three isomers in a 4:2:1 ratio in chloroform. Flux-
ional behavior was observed and identified as an apparent
allyl rotation process. Rate constants and activation free
energies were determined from NOESY NMR experiments
for selected cases and were found to be strongly solvent
dependent, in accordance with an associative mechanism.
Allyl rotation in systems with isopropyl substituents at the
oxazolines is around 2 times faster than in systems with
phenyl groups at the oxazolines, whereas the substituent on
the pyrazole backbone of the ligand scaffold has only a
minor effect.
X-ray Diffraction. The crystal data and details of the data
collections for 4a,c are given in Table 3. X-ray data were
The set of complexes 4a-d was evaluated in palladium-
catalyzed asymmetric allylic alkylation. While all complexes
(46) Kwong, H.-L.; Yeung, H.-L.; Lee, W.-S.; Wong, W.-T. Chem.
Commun. 2006, 4841–4843.
(47) Markert, C.; Neuburger, M.; Kulicke, K.; Meuwly, M.; Pfaltz,
A. Angew. Chem. 2007, 119, 5996–5999.
(48) (a) Meyers, A. I.; Dickman, D. A.; Bailey, T. R. J. Am. Chem.
(44) Wassenaar, J.; van Zutphen, S.; Mora, G.; Le Floch, P.; Siegler,
M. A.; Spek, A. L.; Reek, J. N. H. Organometallics 2009, 28, 2724–2734.
(45) Blochl, P. E.; Togni, A. Organometallics 1996, 15, 4125–4132.
€
Soc. 1985, 107, 7974–7978. (b) Vogl, O.; Pohm, M. Monatsh. Chem. 1952,
€
83, 541–543.

1124 Organometallics, Vol. 29, No. 5, 2010
Ficks et al.
Table 3. Summary of X-ray Crystallographic Data for Complexes
4a,c
determined manually using MNova, and rate constants were
calculated with an estimated error of 10% using EXSYCalc.52
Synthesis of N3,N5-Bis((R)-2-hydroxy-1-phenylethyl)-1H-pyr-
azole-3,5-dicarboxamide (2a). A solution of 1a (1.93 g, 8.75
mmol), (R)-2-phenylglycinol (3.60 g, 26.2 mmol), and potassium
cyanide (120 mg, 1.84 mmol) in methanol (30 mL) was heated to
reflux for 4 days. The solvent was removed in vacuo, and the
residue was washed thoroughly with dichloromethane. The
crude product was recrystallized from methanol/diethyl ether
to give the intended product as a white solid. Yield: 2.23 g (65%).
Rf = 0.30 (silica gel, CH2Cl2/MeOH, 10:1). Mp: >250 °C.
[R]D20 = þ39.1° (c = 2.3 mg/mL, MeOH). IR (KBr): ν 3406.9
(N-H), 3370.2, 3331.3, 3160.1, 2945.3, 2850.5, 2638.0, 1651.0
(CdO), 1619.4, 1527.5, 1447.4, 1384.5, 1277.1, 1262.1, 1192.0,
1068.1, 1042.8, 852.5, 773.5, 698.9, 570.7, 544.4, 445.6 cm-1. 1H
NMR (MeOD, 300 MHz, 298 K): δ (ppm) 7.46-7.21 (m, 10H,
Ph H), 7.15 (s, 1H, pz H4), 5.17 (t, 3JHH = 6.4 Hz, 2H, R-CH),
4a
4c
empirical formula
C
29H31BF4-
N4O2Pd2
C26H42.50F6-
N4O2.75PPd2
812.91
0.22 ꢀ 0.17 ꢀ 0.07
monoclinic
C2 (No. 5)
28.2448(10); 90
8.7037(4); 97.382(3)
13.8195(5); 90
3369.1(2)
1.603
4
1638
1.179
0.9201/0.6937
formula wt
cryst size, mm
cryst syst
space group
a, A; R, deg
b, A; β, deg
c, A; γ, deg
V, A3
767.19
0.50 ꢀ 0.23 ꢀ 0.22
hexagonal
P61 (No. 169)
13.5487(4); 90
13.5487(4); 90
27.8944(10); 120
4434.5(2)
1.724
6
2292
F
calcd, g cm-3
Z
F(000)
μ, mm-1
Tmax/Tmin
hkl range
1.276
0.5979/0.7586
3
3.85 (d, JHH = 6.4 Hz, 4H, β-CH2). 13C NMR (MeOD, 75
(16, (16, -31 to þ33 (34, (10, (16
MHz, 298 K): δ (ppm) 166.2 (CdO), 148.0 (pz C3, pz C5), 141.6
(Ph C1), 129.5 (Ph C3, Ph C5), 128.4 (Ph C2, Ph C6), 128.1 (Ph
C4), 108.0 (pz C4), 66.6 (β-CH2), 56.8 (R-CH). MS (m/z, ESIþ,
MeOH): 417.1 (100) [M þ Na]þ. HRMS (m/z, ESI-, MeOH):
found 393.1569, calcd for C21H21N4O4- 393.1568.
θ range, deg
no. of measd rflns
no. of unique rflns (Rint
no. of obsd rflns,
I > 2σ(I)
1.74-25.68
38 453
1.45-25.63
13 640
6334 (0.0541)
5630
)
5482 (0.0416)
5283
Synthesis of N3,N5-Bis((R)-2-hydroxy-1-phenylethyl)-4-phen-
yl-1H-pyrazole-3,5-dicarboxamide (2b). A solution of 1b (4.40 g,
16.9 mmol), (R)-2-phenylglycinol (6.96 g, 50.7 mmol), and
potassium cyanide (110 mg, 1.69 mmol) in methanol (13 mL)
was heated to reflux for 4 days. The solvent was removed in
vacuo, and the residue was cleaned by column chromatography
(dichloromethane/methanol, 10:1) on silica media. For purifi-
cation the crude product was washed thoroughly with dichloro-
methane to give the intended product as a white solid. Yield:
4.40 g (55%). Rf = 0.41 (silica gel, CH2Cl2/MeOH, 10:1). Mp:
113 °C. [R]D20 = þ27.3° (c = 2.2 mg/mL, MeOH). IR (KBr): ν
3392.6 (N-H), 2935.2, 1655.5 (CdO), 1535.7, 1454.6, 1293.6,
refined params/
restraints
377/43
386/19
goodness of fit
Abs. structure param.
R1/wR2 (I > 2σ(I))
R1/wR2 (all data)
resid electron
dens, e A-3
1.065
-0.03(5)
0.0547/0.1244
0.0566/0.1256
3.487/-2.309
(near Pd)
1.029
0.00(5)
0.0533/0.1341
0.0611/0.1386
2.399/-1.037
(near Pd)
collected on a STOE IPDS II diffractometer (graphite -mono-
chromated Mo KR radiation, λ = 0.710 73 A) by use of ω scans
at -140 °C. The structures were solved by direct methods and
refined on F2 using all reflections with SHELX-97.49 Most non-
hydrogen atoms were refined anisotropically. Most hydrogen
atoms were placed in calculated positions and assigned to an
isotropic displacement parameter of 0.08 A2. The BF4- anion in
4a was found to be disordered and was refined using SADI
restraints (dB-F and dF 3 3 3 F). One (η3-methallyl)Pd moiety in 4c
was found to be disordered about two positions. The occupan-
cies were set to 0.75 and 0.25. The main component also includes
one diethyl ether solvent molecule with an occupancy of 0.75.
Atoms of the disordered parts (except Pd) were refined iso-
tropically. SAME and, for the diethyl ether molecule, DFIX
restraints (dC-C = 1.51 A and dC-O = 1.43 A) were used to
model the disorder. Allyl protons of the disordered methallyl
moiety were first calculated and later refined at fixed positions.
A DFIX restraint (dC-H = 0.93 A) was applied in the case of the
nondisordered allyl protons. The absolute structure parameters
were determined according to Flack50 with SHELX-97. Face-
indexed absorption corrections were performed numerically
with the program X-RED.51
1240.4, 1189.0, 1071.9, 1030.8, 877.6, 760.9, 699.9 cm-1 1H
.
NMR (MeOD, 300 MHz, 298 K): δ (ppm) 7.43-7.13 (m, 16H,
Ph H, pz H4), 4.99 (t, 3JHH = 5.9 Hz, 2H, R-CH), 3.69 (dd, 3JHH
= 5.9 Hz, 2JHH = 11.0 Hz, 2H, β-CHaHb), 3.69 (dd, 3JHH = 5.9
2
Hz, JHH = 11.0 Hz, 2H, β-CHaHb). 13C NMR (MeOD, 75
MHz, 298 K): δ (ppm) 162.2 (CdO), 140.7 (Ph C1), 132.4 (pz Ph
C1), 131.5 (pz Ph C3, pz Ph C5), 130.0 (pz Ph C4), 129.6 (pz Ph
C2, pz Ph C6), 129.5 (Ph C3, Ph C5), 128.5 (Ph C2, Ph C6), 127.9
(Ph C4), 123.2 (pz C4), 66.0 (β-CH2), 56.7 (R-CH), pz C3 and pz
C5 were not observed. MS (m/z, ESIþ, MeOH): 493.1 (100) [M
þ Na]þ. HRMS (m/z, ESI-, MeOH): found 469.1879, calcd for
C27H25N4O4- 469.1881.
Synthesis of N3,N5-Bis((S)-1-hydroxy-3-methylbutan-2-yl)-
1H-pyrazole-3,5-dicarboxamide (2c). A solution of 1a (2.00 g,
9.07 mmol) and (S)-valinol (2.81 g, 27.2 mmol) in methanol
(15 mL) was heated to reflux for 3 days. The solvent was
removed in vacuo, and the residue was cleaned by column
chromatography (dichloromethane/methanol, 10:1) on silica
media to give the product as a white solid. Yield: 1.62 g (55%).
=
NMR Spectroscopy. H NMR and 13C NMR spectra were
1
20
Rf = 0.67 (silica gel, CH2Cl2/MeOH, 5:1). Mp: 232 °C. [R]D
recorded on Bruker Avance 300 and 500 MHz spectrometers
using the indicated solvent as internal reference. Full-range
NOESY spectra were acquired with 512 ꢀ 1024 data points
and a spectral width of 9.0 ppm; in particular cases the spectral
width was reduced to increase the resolution. Mixing times were
chosen between 50 and 500 ms, depending on the rate of the
exchange. For the measurement of exchange rate constants in
complexes 4a,c the pz H4 proton and in complexes 4a,b alter-
natively the allyl H1S/H01S proton was used. Peak volumes were
-60.5° (c = 2.1 mg/mL, MeOH). IR (KBr): ν 3152.5, 2961.4,
1655.7 (CdO), 1617.1, 1560.4, 1535.1, 1450.6, 1318.8, 1294.7,
1260.7, 1187.1, 1147.6, 1067.8, 1021.6, 870.4, 819.7 cm-1 1H
.
NMR (MeOD, 300 MHz, 298 K): δ (ppm) 7.28 (s, 1H, pz H4),
3.87 (m, 2H, R-CH), 3.69 (m, 4H, β-CH2), 1.97 (octet, 3JHH = 7.0
Hz, 2H, iPr CH), 1.00 (d, 3JHH = 7.0 Hz, 6H, iPr CH3), 0.96 (d,
3JHH = 7.0 Hz, 6H, iPr CH3). 13C NMR (MeOD, 75 MHz,
298 K): δ (ppm) 162.8 (CdO), 106.3 (pz C4), 63.1 (β-CH2), 58.2
(R-CH), 30.3 (iPr CH), 20.1 (iPr CH3), 19.2 (iPr CH3), pz C3 and
pz C5 were not observed. MS (m/z, ESIþ, MeOH): 349.1 (100)
[M þ Na]þ. HRMS (m/z, ESI-, MeOH): found 325.1877, calcd
for C15H25N4O4- 325.1881.
(49) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(50) Flack, H. D. Acta Crystallogr. 1983, A39, 876–881.
(51) X-RED; STOE & CIE GmbH, Darmstadt, Germany, 2002.
(52) Cobas, J. C.; Martı
Research, Escondido, CA, 2007.
Synthesis of N3,N5-Bis((S)-1-hydroxy-3-methylbutan-2-yl)-4-
phenyl-1H-pyrazole-3,5-dicarboxamide (2d). A solution of 1b
(2.00 g, 7.68 mmol), (S)-valinol (2.38 g, 23.1 mmol), and
´
n-Pastor, M. EXSYCalc 1.0; Mestrelab

Article
Organometallics, Vol. 29, No. 5, 2010 1125
trifluoromethanesulfonic acid (133 mg, 0.77 mmol) in methanol
(15 mL) was heated to reflux for 3 days. The solvent was
removed in vacuo, and the residue was cleaned by column
chromatography (dichloromethane/methanol, 10:1) on silica
media to give the intended product as a white solid. Yield:
2.25 g (73%). Rf = 0.34 (silica gel, CH2Cl2/MeOH, 10:1). Mp:
85 °C. [R]D20 = -51.9° (c = 2.1 mg/mL, MeOH). IR (KBr): ν
3398.2, 2961.5, 1651.4 (CdO), 1539.3, 1464.9, 1386.7, 1311.6,
2962.1, 1664.1 (CdN), 1561.6, 1468.0, 1349.7, 1263.7, 1230.2,
1136.3, 1065.9, 988.8, 950.3, 891.1, 801.6, 730.5 cm-1. 1H NMR
(CDCl3, 500 MHz, 298 K): δ (ppm) 14.2 (br s, 1H, NH), 7.11 (s,
1H, pz H), 4.42 (m, 2H, R-CH), 4.17 (m, 4H, β-CH2), 1.81 (m,
3
2H, iPr CH), 0.96 (d, JHH = 6.9 Hz, 6H, iPr CH3), 0.88 (d,
3JHH = 6.9 Hz, 6H, iPr CH3). 13C NMR (CDCl3, 125 MHz,
298 K): δ (ppm) 157.6 (NdC-O), 108.6 (pz C4), 72.0 (β-CH2),
70.7 (R-CH), 32.8 (iPr CH), 18.7 (iPr CH3), 18.2 (iPr CH3), pz C3
and pz C5 were not observed. MS (m/z, ESIþ, CH2Cl2): 289.1
(100) [M - H]-. HRMS (m/z, ESIþ, MeOH): found 313.1636,
calcd for C15H22N4NaO2- 313.1635.
1234.7, 1073.2, 1028.0, 871.8, 794.6, 764.7, 701.7 cm-1 1H
.
NMR (MeOD, 300 MHz, 298 K): δ (ppm) 7.51-7.39 (m, 5H,
Ph H), 3.73 (q, 3JHH = 5.4 Hz, 2H, R-CH), 3.49 (m, 4H, β-CH2),
1.77 (m, 2H, iPr CH), 0.85 (d, 3JHH = 6.7 Hz, 6H, iPr CH3), 0.70
(br s, 6H, iPr CH3). 13C NMR (MeOD, 75 MHz, 298 K): δ (ppm)
162.6 (CdO), 132.7 (pz Ph C1), 131.6 (pz Ph C3, pz Ph C5), 130.1
(pz Ph C2, pz Ph C6), 129.7 (pz Ph C4), 122.7 (pz C4), 62.8
(β-CH2), 57.6 (R-CH), 29.9 (iPr CH), 20.0 (iPr CH3), 18.4 (iPr
CH3), pz C3 and pz C5 were not observed. MS (m/z, ESI-,
MeOH): 401.2 (100) [M - H]-. HRMS (m/z, ESI-, MeOH):
found 401.2193, calcd for C21H29N4O4- 401.2194.
Synthesis of 3,5-Bis((S)-4-isopropyl-4,5-dihydro-2-oxazolyl)-
1H-4-phenylpyrazole (3d). The same procedure was followed
as for 3a, except for purifying the crude product by column
chromatography (dichloromethane/methanol, 10:1) on a silica
media to give the intended product as a white solid. Yield: 1.16 g
(58%). Rf = 0.62 (silica gel, CH2Cl2/MeOH, 10:1). Mp: 76 °C.
[R]D20 = -127.9° (c = 1.9 mg/mL, CHCl3). IR (KBr): ν 2960.6,
1658.8 (CdN), 1607.1, 1556.3, 1514.1, 1475.5, 1425.6, 1341.2,
1266.9, 1122.6, 991.5, 950.7, 894.6, 759.2, 697.0 cm-1. 1H NMR
(CDCl3, 500 MHz, 298 K): δ (ppm) 14.1 (br s, 1H, NH),
7.37-7.30 (m, 5H, Ph H), 4.26 (t, 3JHH = 8.7 Hz, 2H, R-CH),
4.11-3.97 (m, 4H, β-CH2), 1.72 (octet, 3JHH = 6.8 Hz, 2H, iPr
CH), 0.92 (d, 3JHH = 6.8 Hz, 6H, iPr CH3), 0.84 (d, 3JHH = 6.8
Hz, 6H, iPr CH3). 13C NMR (CDCl3, 125 MHz, 298 K): δ (ppm)
157.8 (N=C-O), 130.9, 130.7, 127.4, 127.2, 124.9, 71.9
(β-CH2), 70.6 (R-CH), 33.0 (iPr CH), 18.7 (iPr CH3), 18.4 (iPr
CH3), pz C3 and pz C5 were not observed. MS (m/z, ESI-,
CH2Cl2): 365.2 (100) [M þ H]þ. HRMS (m/z, ESIþ, MeOH):
found 389.1946, calcd for C21H26N4NaO2- 389.1948.
Synthesis of 3,5-Bis((R)-4-phenyl-4,5-dihydro-2-oxazolyl)-
1H-pyrazole (3a). To a solution of 2a (1.36 g, 3.45 mmol) in
1,2-dichloroethane (100 mL) was added thionyl chloride (5 mL).
The mixture was heated to 40 °C for 2 h, followed by the removal
of the solvent in vacuo. The resulting white solid was added to a
solution of sodium methanolate (1.49 g, 27.6 mmol) in methanol
(125 mL) and heated to reflux for an additional 5 h. The solvent
was removed, and the resulting solid was dissolved in dichloro-
methane (50 mL). The organic phase was washed with water
(3 ꢀ 30 mL) and dried over magnesium sulfate. The crude product
was recrystallized from dichloromethane to give a white solid.
Yield: 982 mg (79%). Rf = 0.50 (silica gel, EtOAc). Mp: 187 °C.
[R]D20 = þ81.1° (c = 1.9 mg/mL, CHCl3). IR (KBr): ν 3420.2,
3109.6, 2899.9, 1652.1 (CdN), 1557.6, 1491.0, 1449.1, 1394.5,
1348.4, 1269.2, 1200.4, 1142.2, 1068.6, 990.3, 943.2, 878.4, 754.2,
696.8, 542.0 cm-1. 1H NMR (CDCl3, 500 MHz, 298 K): δ (ppm)
14.0 (br s, 1H, NH), 7.32-7.23 (m, 11H, Ph H, pz H), 5.47 (t,
3JHH = 8.6 Hz, 2H, R-CH), 4.80(t, 3JHH = 8.6 Hz, 2H, β-CHaHb),
Synthesis of Bis(η3-methallyl)[3,5-bis((R)-4-phenyl-4,5-dihy-
dro-2-oxazolyl)-pyrazolate-N,N,N0,N0]dipalladium(II)
Tetra-
fluoroborate (4a). A mixture of 3a (100 mg, 0.28 mmol) and
potassium tert-butoxide (32 mg, 0.28 mmol) in dry dichloro-
methane (30 mL) was stirred for 15 min. (Methallyl)palladium
chloride dimer (110 mg, 0.28 mmol) was then added. The
reaction mixture was stirred at room temperature overnight.
Ammonium tetrafluoroborate (176 mg, 1.68 mmol) was added,
and the reaction mixture was stirred for an additional 5 h. The
organic phase was washed with water (4 ꢀ 20 mL) and dried over
magnesium sulfate. After removal of the solvent the intended
product was obtained as a yellow solid. Yellow needles suitable
for X-ray crystallographic analysis were obtained by slow
diffusion of diethyl ether into a concentrated dichloromethane
solution. Yield: 203 mg (95%). Mp: 204 °C. IR (KBr): ν 3434.1,
3127.0, 2971.2, 2909.7, 1633.0 (CdN), 1535.7, 1518.0, 1495.0,
1471.7, 1455.9, 1431.9, 1380.0, 1320.8, 1277.9, 1241.8, 1192.0,
1055.5, 927.8, 891.8, 837.2, 763.8, 699.4, 645.4, 543.9 cm-1. 1H
NMR (CDCl3, 500 MHz, 223 K): δ (ppm) isomer A (53%),
7.42-7.27 (m, 10H, Ar H), 7.01 (s, 1H, pz H), 5.44 (t, 3JHH = 9.3
Hz, 3JHH = 9.3 Hz, 2H, HR), 5.29 (t, 2JHH = 9.3 Hz, 3JHH = 9.3
Hz, 2H, Hβa), 4.58 (t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 2H, Hβs),
3.80 (s, 2H, H2s), 2.77 (s, 2H, H2a), 2.73 (s, 2H, H1s), 2.62 (s, 2H,
H1a), 1.59 (s, 6H, CH3); isomer B (31%), 7.42-7.27 (m, 10H, Ar
H), 7.03 (s, 1H, pz H), 5.44 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz,
3
4.29 (t, JHH = 8.6 Hz, 2H, β-CHaHb). 13C NMR (CDCl3, 125
MHz, 298 K): δ (ppm) 158.6 (NdC-O), 141.6 (Ph C1), 128.8 (Ph
C3, Ph C5), 127.8 (Ph C4), 126.6 (Ph C2, Ph C6), 109.2 (pz C4),
75.3 (β-CH2), 69.5 (R-CH), pz C3 and pz C5 were not observed.
MS (m/z, ESIþ, CH2Cl2): 359.3 (100) [M þ H]þ. HRMS (m/z,
ESI-, MeOH): found 357.1356, calcd for C21H17N4O2- 357.1357.
Synthesis of 3,5-Bis((R)-4-phenyl-4,5-dihydro-2-oxazolyl)-
1H-4-phenylpyrazole (3b). The same procedure was followed
as for 3a, except for purifying the crude product by column
chromatography (dichloromethane/methanol, 25:1) on a silica
media to give the intended product as a white solid. Yield: 3.04 g
(80%). Rf = 0.23 (silica gel, CH2Cl2/MeOH, 25:1). Mp: 99 °C.
[R]D20 = þ65.0° (c = 3.6 mg/mL, CHCl3). IR (KBr): ν 3126.7,
3060.4, 3029.6, 2900.3, 1950.5, 1883.8, 1809.8, 1655.6 (CdN),
1606.7, 1494.0, 1477.4, 1450.5, 1344.9, 1288.4, 1259.5, 1184.3,
1128.2, 1076.5, 993.9, 943.2, 914.5, 895.1, 757.0, 698.1, 536.9
cm-1. 1H NMR (CDCl3, 300 MHz, 298 K): δ (ppm) 13.9 (br s,
1H, NH), 7.45-7.15 (m, 15H, Ph H), 5.41 (dd, 3JHH = 8.2 Hz,
3JHH = 9.9 Hz, 2H, R-CH), 4.64 (t, 3JHH = 8.2 Hz, 2JHH = 9.9
Hz, 2H, β-CHaHb), 4.12 (t, 3JHH = 9.9 Hz, 2JHH = 9.9 Hz, 2H,
β-CHaHb). 13C NMR (CDCl3, 75 MHz, 298 K): δ (ppm) 158.9
(NdC-O), 141.8 (Ph C1), 130.7, 128.6, 127.7, 127.5, 127.3,
126.5, 74.9 (β-CH2), 69.0 (R-CH), pz C3 and pz C5 were not
observed. MS (m/z, ESIþ, CH2Cl2): 435.1 (100) [M þ H]þ.
HRMS (m/z, ESI-, MeOH): found 433.1677, calcd for
C27H21N4O2- 433.1670.
R
1H, HR), 5.39 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H, H0 ), 5.29
(t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H, Hβa), 5.26 (t, 2JHH = 9.3
Hz, 3JHH = 9.3 Hz, 1H, H0βa), 4.62 (t, 2JHH = 9.3 Hz, 3JHH
=
9.3 Hz, 1H, H0βs), 4.58 (t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H,
2s
Hβs), 3.92 (s, 1H, H0 ), 3.79 (s, 1H, H2s), 3.30 (s, 1H, H01s), 2.85
(s, 1H, H1s), 2.81 (s, 1H, H2a), 2.77 (s, 1H, H02a), 2.64 (s, 1H,
1a
H ), 2.01 (s, 3H, CH3 ), 1.96 (s, 1H, H01a), 1.64 (s, 3H, CH3);
0
isomer C (16%), 7.42-7.27 (m, 10H, Ar H), 7.01 (s, 1H, pz H),
5.39 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz, 2H, HR), 5.26 (t, 2JHH
9.3 Hz, 3JHH = 9.3 Hz, 2H, Hβa), 4.62 (t, 2JHH = 9.3 Hz, 3JHH
=
=
Synthesis of 3,5-Bis((S)-4-isopropyl-4,5-dihydro-2-oxazolyl)-
1H-pyrazole (3c). The same procedure was followed as for 3a,
except for purifying the crude product by column chromato-
graphy (dichloromethane/methanol, 10:1) on a silica media to
9.3 Hz, 2H, Hβs), 3.85 (s, 2H, H2s), 3.59 (s, 2H, H1s), 2.79 (s, 2H,
H2a), 2.31 (s, 2H, H1a), 2.01 (s, 6H, CH3). 13C NMR (CDCl3, 125
MHz, 223 K): δ (ppm) isomer A, 166.6 (N=C-O), 141.4 (Ph
C1), 139.8 (methallyl C), 130.8 (Ph C3, Ph C5), 129.0 (Ph C4),
127.3 (Ph C2, Ph C6), 106.4 (pz C4), 78.9 (β-CH2), 67.7 (R-CH),
give the intended product as a white solid. Yield: 136 mg (62%).
=
20
Rf = 0.87 (silica gel, CH2Cl2/MeOH, 5:1). Mp: 74 °C. [R]D
-94.3° (c = 2.1 mg/mL, CHCl3). IR (KBr): ν 3425.6, 3139.5,

1126 Organometallics, Vol. 29, No. 5, 2010
Ficks et al.
JHH = 7.1 Hz, 6H, iPr CH3 ). 13C NMR (CDCl3, 125 MHz, 223
K): δ (ppm) isomer A, 165.9 (NdC-O), 141.4 (pz C3, pz C5),
131.7 (methallyl C), 105.6 (pz C4), 72.5 (β-CH2), 68.8 (R-CH),
59.6 (methallyl CH2 trans to oxazoline N), 58.3 (methallyl CH2
trans to pyrazolate N), 31.0 (iPr CH), 23.5 (methallyl CH3), 18.3
(iPr CH3), 15.7 (iPr CH3). MS (m/z, ESIþ, CH2Cl2): 613.1 (100)
[M - BF4]þ. HRMS (m/z, ESIþ, MeOH): found 611.0861, calcd
for C23H35N4O2Pd2þ 611.0824.
3
0
59.3 (methallyl CH2 trans to pyrazolate-N), 58.8 (methallyl
CH2), 23.1 (methallyl CH3 trans to oxazoline N), pz C3 and pz
C5 not observed. MS (m/z, ESIþ, CH2Cl2): 681.0 (100) [M -
BF4]þ. HRMS (m/z, ESIþ, MeOH): found 679.0511, calcd for
C29H31N4O2Pd2þ 679.0511.
Synthesis of Bis(η3-methallyl)[3,5-bis((R)-4-phenyl-4,5-dihy-
dro-2-oxazolyl)-4-phenylpyrazolate-N,N,N0,N0]dipalladium(II)
Tetrafluoroborate (4b). The same procedure was followed as for
4a. The intended product was obtained as a yellow solid. Yield: 324
mg (80%). Mp: 166 °C. IR(KBr): ν 3424.5, 3059.0, 3030.0, 2962.1,
2917.4, 1964.4, 1887.1, 1823.6, 1680.0, 1622.8 (CdN), 1531.5,
1496.3, 1444.2, 1365.6, 1308.5, 1263.2, 1178.8, 1052.5, 933.5,
Synthesis of Bis(η3-methallyl)[3,5-bis((S)-4-isopropyl-4,5-di-
hydro-2-oxazolyl)-4-phenylpyrazolate-N,N,N0,N0]dipalladium(II)
Tetrafluoroborate (4d). The same procedure was followed as for
4a. The intended product was obtained as a yellow solid. Yield:
199 mg (94%). Mp: 174 °C. IR (KBr): ν 3061.5, 2961.2, 1624.1
(CdN), 1537.3, 1506.4, 1445.0, 1367.5, 1321.2, 1260.4, 1180.4,
1
837.4, 761.3, 698.2, 574.5, 450.1 cm-1. H NMR (CDCl3, 500
MHz, 223 K): δ (ppm) isomer A (62%), 7.50-7.24 (m, 15H, Ar
3
3
1
H), 5.40 (t, JHH = 9.3 Hz, JHH = 9.3 Hz, 2H, HR), 5.17 (t,
2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 2H, Hβa), 4.43 (t, 2JHH = 9.3 Hz,
3JHH = 9.3 Hz, 2H, Hβs), 3.81 (s, 2H, H2s), 2.79 (s, 2H, H2a), 2.68
(s, 2H, H1s), 2.64 (s, 2H, H1a), 1.59 (s, 6H, CH3); isomer B (23%),
7.50-7.24 (m, 15H, Ar H), 5.40 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz,
1057.0, 931.6, 838.8, 804.6, 700.8 cm-1. H NMR (CDCl3, 500
MHz, 223 K): δ (ppm) isomer A, 7.43-7.33 (m, 5H, Ph H), 4.60 (t,
J
HH = 9.4 Hz, 2H, Hβa), 4.41 (t, 2JHH = 9.7 Hz, 3JHH = 6.9 Hz,
2H, Hβs), 4.14 (m, 2H, HR), 4.04 (s, 2H, H2s), 3.99 (s, 2H, H1s), 3.02
(s, 2H, H2a), 2.99 (s, 2H, H1a), 2.11 (s, 6H, CH3), 2.03 (m, 2H, iPr
CH), 0.95 (d, 3JHH = 6.7 Hz, 6H, iPr CH3), 0.84 (d, 3JHH = 6.7
Hz, 6H, iPr CH3). 13C NMR (CDCl3, 125 MHz, 223 K): δ (ppm)
isomer A, 166.3 (NdC-O), 138.9 (pz C3, pz C5), 131.9 (methallyl
C), 130.4 (Ph C2, Ph C6), 128.4 (Ph C4), 127.8 (Ph C3, Ph C5),
127.5 (Ph C1), 124.1 (pz C4), 72.4 (β-CH2), 68.2 (R-CH), 59.9
(methallyl CH2 trans to oxazoline N), 58.2 (methallyl CH2 trans to
pyrazolate N), 30.9 (iPr CH), 23.5 (methallyl CH3), 18.4 (iPr CH3),
15.6 (iPr CH3). MS (m/z, ESIþ, CH2Cl2): 689.1 (100) [M - BF4]þ.
HRMS (m/z, ESIþ, MeOH): found 687.1160, calcd for
C29H39N4O2Pd2þ 687.1137.
1H, HR), 5.36 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H, H0 ), 5.17 (t,
R
2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H, Hβa), 5.15 (t, 2JHH = 9.3 Hz,
3JHH = 9.3 Hz, 1H, H0βa), 4.48 (t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz,
1H, H0βs), 4.43 (t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 1H, H0βs), 3.94
2s
(s, 1H, H0 ), 3.81 (s, 1H, H2s), 3.29 (s, 1H, H01s), 2.87 (s, 1H, H1s),
2.79 0(s, 1H, H2a), 2.79 (s, 1H, H02a), 2.64 (s, 1H, H1a), 2.04 (s, 3H,
CH3 ), 1.92 (s, 1H, H01a), 1.65 (s, 3H, CH3); isomer C (15%),
7.50-7.24 (m, 15H, Ar H), 5.36 (t, 3JHH = 9.3 Hz, 3JHH = 9.3 Hz,
2H, HR), 5.15 (t, 2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 2H, Hβa), 4.48 (t,
2JHH = 9.3 Hz, 3JHH = 9.3 Hz, 2H, Hβs), 3.86 (s, 2H, H2s), 3.61 (s,
2H, H1s), 2.79 (s, 2H, H2a), 2.27 (s, 2H, H1a), 2.04 (s, 6H, CH3). 13
C
Allylic Alkylation of rac-(E)-1,3-Diphenylallyl Acetate. The
catalyst (0.02 mmol) was dissolved in tetrahydrofuran (2 mL)
and was treated with a solution of rac-(E)-1,3-diphenylallyl
acetate (252 mg, 1.00 mmol) in tetrahydrofuran (1 mL). In a
second flask dimethyl malonate (0.34 mL, 396 mg, 3.00 mmol)
was dissolved in tetrahydrofuran (1 mL) and sodium hydride
(72 mg, 3.00 mmol) was slowly added to give a white solid which
was then added to the solution containing the catalyst. The
reaction mixture was stirred at 45 °C, and the conversion was
monitored by TLC analysis. After the appropriate reaction time
the solution was diluted with diethyl ether (20 mL) and washed
with saturated aqueous ammonium chloride (3 ꢀ 20 mL). The
organic phase was dried over magnesium sulfate, and the
conversion was determined by 1H NMR spectroscopy. The
product was purified by column chromatography (hexane/
EtOAc, 4:1) on a silica media to give the final product as a
colorless oil (in some cases the oil became a white solid after a
few hours). The enantiomeric excess was determined by 1H
NMR spectroscopy (200 MHz, CDCl3, 0.5 equiv of Eu(hfc)3,
for the CO2CH3 singlet at lower field a splitting was observed).
NMR (CDCl3, 125 MHz, 223 K): δ (ppm) isomer A, 167.0
(NdC-O), 140.0 (methallyl C), 139.3 (Ph C1), 131.0 (Ph C3, Ph
C5), 130.5, 129.0 (Ph C4), 128.7, 127.8, 127.5, 127.3 (Ph C2, Ph
C6), 125.1 (pz C4), 79.2 (β-CH2), 66.9 (R-CH), 59.4 (methallyl
CH2 trans to pyrazolate N), 58.8 (methallyl CH2 trans to oxazoline
N), 23.0 (methallyl CH3), pz C3 and pz C5 not observed. MS (m/z,
ESIþ, CH2Cl2): 757.0 (100) [M - BF4]þ. HRMS (m/z, ESIþ,
MeOH): found 755.0831, calcd for C35H35N4O2Pd2þ 755.0824.
Synthesis of Bis(η3-methallyl)[3,5-bis((S)-4-isopropyl-4,5-di-
hydro-2-oxazolyl)-pyrazolate-N,N,N0,N0]dipalladium(II) Tetra-
fluoroborate (4c). The same procedure was followed as for 4a.
The intended product was obtained as a yellow solid. Yield: 940
mg (98%). Mp: 140 °C. IR (KBr): ν 3136.3, 3068.6, 2962.4,
2875.1, 1640.0 (CdN), 1518.9, 1466.8, 1381.0, 1329.0, 1283.5,
1242.2, 1192.0, 1054.2, 930.3, 837.1, 802.6, 761.8, 724.7 cm-1
.
1H NMR (CDCl3, 500 MHz, 223 K): δ (ppm) isomer A (64%),
6.80 (s, 1H, pz H), 4.70 (t, JHH = 9.7 Hz, 2H, Hβa), 4.56 (dd,
2JHH = 9.7 Hz, 3JHH = 6.5 Hz, 2H, Hβs), 4.18 (m, 2H, HR), 4.01
(s, 2H, H2s), 3.99 (s, 2H, H1s), 3.00 (s, 2H, H2a), 2.97 (s, 2H, H1a),
2.09 (s, 6H, CH3), 2.03 (m, 2H, iPr CH), 0.97 (d, 3JHH = 7.1 Hz,
6H, iPr CH3), 0.88 (d, 3JHH = 7.1 Hz, 6H, iPr CH3); isomer B
(27%) 6.80 (s, 1H, pz H), 4.73 (m, 1H, H0βa), 4.70 (t, JHH = 9.7
Acknowledgment. Financial support by the Fonds der
Chemischen Industrie is gratefully acknowledged.
2
3
Hz, 1H, Hβa), 4.56 (dd, JHH = 9.73 Hz, JHH = 6.5 Hz, 1H,
Hβs), 4.54 (m, 1H, H0 ), 4.23 (m, 1H, H0 ), 4.18 (m, 1H, HR),
βs
R
4.09 (s, 1H, H02s), 4.00 (s, 1H, H1s), 3.97 (s, 1H, H2s), 3.94 (s, 1H,
Supporting Information Available: CIF files giving crystal
data for 4a,4c, figures giving 1H and 13C NMR spectra for
compounds 2a-d, 3a-d, and 4a-d, 2D NOESY spectra of 4a
at -50 and -25 °C, tables giving the ratio of the three different
isomers of methallyl complexes 4a-c in CDCl3, rate constants
and activation free energies of the methallyl rotation, and a
figure giving a plot of the molecular structure of the cation of 4c,
emphasizing the disorder of the (η3-methallyl)Pd moiety. This
material is available free of charge via the Internet at http://
pubs.acs.org.
H0 ), 3.01 (s, 1H, H2a), 3.00 (s, 1H, H02a), 2.95 (s, 1H, H1a), 2.92
1s
1a
(s, 1H, H0 ), 2.18 (s, 3H, CH3 ), 2.10 (s, 3H, CH3), 2.03 (m, 1H,
0
iPr CH), 1.96 (m, 1H, iPr CH0), 0.96 (d, 3JHH = 7.1 Hz, 3H, iPr
CH3), 0.96 (d, 3JHH = 7.1 Hz, 3H, iPr CH3 ), 0.88 (d, J0HH = 7.1
3
0
Hz, 3H, iPr CH3), 0.83 (d, 3JHH = 7.1 Hz, 3H, iPr CH3 ); isomer
C (9%), 6.82 (s, 1H, pz H), 4.73 (m, 2H, Hβa), 4.54 (m, 2H, Hβs),
4.32 (m, 2H, HR), 4.06 (s, 2H, H2s), 4.01 (s, 2H, H1s), 3.00 (s, 2H,
H2a), 2.99 (s, 2H, H1a), 2.09 (s, 6H, CH3), 1.96 (m, 2H,
3
iPr-CH0), 0.96 (d, JHH = 7.1 Hz, 6H, iPr CH3 ), 0.75 (d,
0