Lewis acid platinum complexes of conformationally flexible NUPHOS diphosphines: Highly efficient catalysts for the carbonyl - Ene reaction

Article abstract of DOI:10.1021/om0505716

Enantiopure Lewis acid complexes of conformationally flexible acyclic and monocyclic NUPHOS diphosphines, δ- and λ-[(NUPHOS)Pt(OTf) ₂], are efficient catalysts for the carbonyl-ene reaction between various unsymmetrical 1,1′-disubstituted alken

Full text of DOI:10.1021/om0505716

Organometallics 2005, 24, 5945-5955
5945
Lewis Acid Platinum Complexes of Conformationally
Flexible NUPHOS Diphosphines: Highly Efficient
Catalysts for the Carbonyl-Ene Reaction
Simon Doherty,*,† Peter Goodrich, Christopher Hardacre,* He-Kuan Luo,
Mark Nieuwenhuyzen, and Rakesh K. Rath
Centre for the Theory and Applications of Catalysis, School of Chemistry,
The Queen’s University of Belfast, David Keir Building, Stranmillis Road,
Belfast BT9 5AG, Northern Ireland
Received July 8, 2005
Enantiopure Lewis acid complexes of conformationally flexible acyclic and monocyclic
NUPHOS diphosphines, δ- and λ-[(NUPHOS)Pt(OTf)2], are efficient catalysts for the
carbonyl-ene reaction between various unsymmetrical 1,1′-disubstituted alkenes and
phenylglyoxal or ethyl glyoxylate. While catalyst performance was substrate dependent, ee
values as high as 95% and yields up to 90% have been obtained. In a number of cases catalysts
generated from δ- and λ-[(NUPHOS)Pt{(S)-BINOL}] showed marked enhancements in
enantioselectivity in ionic liquids compared with organic media. Although an enhancement
in enantioselectivity was not obtained for all substrate combinations in such cases, the
enantioselectivities were comparable to those obtained in dichloromethane. Furthermore,
although the ee’s are initially comparable in both the ionic liquid and dichloromethane, a
gradual erosion of ee with time was found in the organic solvent, whereas the ee remained
constant in the ionic liquid. Preliminary kinetic investigations suggest that the decrease in
ee may be due to a faster racemization of the catalyst in dichloromethane compared with
the ionic liquid.
Chart 1
Introduction
Traditionally, ligand design for asymmetric catalysis
has been guided by the principle that a rigid, confor-
mationally restricted, enantiopure ligand is necessary
to obtain high ee’s.1 However, such an approach is often
an iterative, time-consuming process, since the synthe-
sis of enantiopure ligands can involve lengthy multistep
procedures. Recently, alternative strategies in asym-
metric catalysis have begun to emerge which involve
the use of conformationally flexible or meso ligands to
either magnify the effect of another chiral ligand or act
as the only source of asymmetry for an enantioselective
transformation.2 In the latter case, coordination of the
ligand to a substitutionally inert metal slows intercon-
version of its conformations such that a metastable
enantiopure metal-ligand assembly can be resolved and
used as the sole source of asymmetry for catalysis.3 One
such ligand, 2,2′-bis(diphenylphosphino)biphenyl (BI-
PHEP),4 belongs to the tropos class of diphosphine, since
its axial chiral conformations cannot be resolved (Chart
1).5 Gagne´ and co-workers have recently studied dia-
stereointerconversion in chiral complexes of this diphos-
phine6 and shown that δ- and λ-[Pt(BIPHEP){(S)-
BINOL}] can be resolved, the BINOL liberated, and the
resulting enantiopure Lewis acid used to catalyze the
Diels-Alder and glyoxylate-ene reaction with no loss
of enantiopurity of the catalyst.7 Following this, Mikami
(2) For a recent exhaustive and highly informative review see: (a)
Walsh, P. J.; Lurain, A. E.; Balsells, J. Chem. Rev. 2003, 103, 3297.
For additional relevant reviews see: (b) Mikami, K.; Terada, M.;
Korenaga, T.; Matsumoto, Y.; Ueki, M.; Angelaud, R. Angew. Chem.,
Int. Ed. 2000, 39, 3533. (c) Mikami, K.; Terada, M.; Korenaga, T.;
Matsumoto, Y.; Matsukawa, S. Acc. Chem. Res. 2000, 33, 391. For
selected recent examples see: (d) Mikami, K.; Matsukawa, S. Nature
1997, 385, 613. (e) Matsukawa, S.; Mikami, K. Tetrahedron: Asym-
metry 1997, 8, 815. (f) Ding, K.; Ishii, A.; Mikami, K. Angew. Chem.,
Int. Ed. 1999, 38, 497. (g) Mikami, K.; Korenaga, T.; Terada, M.;
Ohkuma, T.; Pham, T.; Noyori, R. Angew. Chem., Int. Ed. 1999, 38,
495. (h) Korenaga, T.; Aikawa, K.; Terada, M.; Kawauchi, S.; Mikami,
K. Adv. Synth. Catal. 2001, 343, 284. (i) Yamanaka, M.; Mikami, K.
Organometallics 2002, 21, 5847. (j) Aikawa, K.; Mikami, K. Angew.
Chem., Int. Ed. 2003, 42, 5455. (k) Aikawa, K.; Mikami, K. Angew.
Chem., Int. Ed. 2003, 42, 5458. (l) Mikami, K.; Aikawa, K.; Korenaga,
T. Org. Lett. 2001, 3, 243. (m) Balsells, J.; Walsh, P. J. J. Am. Chem.
Soc. 2000, 122, 1802. (n) Davis, T. J.; Balsells, J.; Carroll, P. J.; Walsh,
P. J. Org. Lett. 2001, 3, 2161. (o) Garcia, C.; LaRochelle, L. K.; Walsh,
P. J. J. Am. Chem. Soc. 2002, 124, 10970.
* To whom correspondence should be addressed.
† Present address: School of Natural Sciences, Chemistry, Bedson
Building, University of Newcastle upon Tyne, Newcastle upon Tyne
NE1 7RU, U.K.
(1) (a) Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asym-
metric Catalysis: Wiley: New York, 1995. (b) Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley: New York, 1994. (c) Ojima, I.,
Ed. Catalytic Asymmetric Synthesis, 2nd ed.; Wiley-VCH: New York,
2000. (d) Tsuji, J. Palladium Reagents and Catalysts: New Perspectives
for the 21st Century; Wiley: Chichester, U.K., 2004.
10.1021/om0505716 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/25/2005

5946 Organometallics, Vol. 24, No. 24, 2005
Doherty et al.
et al. adopted a similar strategy to resolve palladium
complexes of BIPHEP with (R)-diaminobinaphthyl ((R)-
DABN), liberated the DABN by protonation, and showed
the resulting Lewis acid fragment to be an efficient cata-
lyst for the hetero Diels-Alder reaction between ethyl
glyoxylate and cyclohexadiene.8 Interestingly, [{(R)-
BIPHEP}Pd{(R)-DABN}]2+ behaved as an activated
catalyst giving markedly higher enantioselectivities
than [{(R)-BIPHEP}Pd(MeCN)2]2+ and exhibiting a dra-
matic negative nonlinear effect.9 (R)-DABN has also
been used as a highly effective auxiliary for control of
axial chirality in Ni, Pd, and Pt complexes of bis(diphen-
ylphosphino)ferrocene, with the Ni/dppf combination
proving to be significantly more efficient and enantio-
selective than its Pd or Pt counterparts in the glyoxyl-
ate-ene reaction.10 The same strategy has recently been
employed to resolve a Rh(I) complex of BIPHEP, which
forms a highly efficient and selective catalyst for the
enantioselective ene-type cyclization of 1,6-enynes.11
We have recently reported that acyclic and monocyclic
NUPHOS diphosphines12 1a-c (Chart 1) exhibit tropos
character in much the same manner as BIPHEP, in that
coordination to Pt(II) slows atropinversion13 such that
enantiopure conformations can be resolved. The result-
ing enantiopure Lewis acid fragments λ- and δ-[(NU-
PHOS)Pt(OTf)2] have been used to catalyze Diels-Alder
and hetero Diels-Alder reactions, often giving ee’s
comparable to those obtained with the corresponding
BINAP-based catalysts.14 To evaluate the potential of
the NUPHOS diphosphines, we have initiated an ex-
tensive program to compare the performance of plati-
num group metal complexes of NUPHOS diphosphines
with their BINAP counterparts in a range of asymmetric
transformations.14-16 In this paper, the enantioselective
addition of olefins to glyoxylate esters has been exam-
ined, since it is an important reaction that provides
access to synthetically important R-hydroxy esters17 and
has recently been catalyzed by Lewis acid platinum and
palladium complexes of atropos biaryl diphosphines.18
In one report, Hao et al. examined the influence of
ligand, counterion, solvent, and temperature on the
palladium-catalyzed glyoxylate-ene reaction and dem-
onstrated that [Pd{(S)-Tol-BINAP}(MeCN)2][SbF6]2 is
a highly efficient catalyst at relatively high reaction
temperatures.18a Koh et al. subsequently found that the
platinum catalyzed glyoxylate-ene reaction was subject
to marked anion-dependent additive effects, the addition
of acidic phenols resulting in a substantial increase in
rate by disrupting contact ion pairs and sequestering
traces of water.18b Herein, we report that NUPHOS
diphosphines form highly efficient catalysts for the
glyoxylate-ene reaction, in most cases their perfor-
mance rivaling that achieved with BINAP. In addition,
the catalysts based on NUPHOS diphosphines show
marked enhancements in enantioselectivity in ionic
liquids compared with organic media, the latter point
underpinning an increasing number of reports that ionic
liquids can enhance the enantioselectivity of a catalytic
asymmetric transformation.19 For example, Meracz and
Oh obtained an ee of 96% for the Diels-Alder reaction
between N-acryloyl oxazolidinones and cyclopentadiene
using copper bis(oxazoline) based Lewis acids in 1,3-
dibutylimidazolium tetrafluoroborate, compared with an
ee of 76% in dichloromethane.20 Ruthenium-catalyzed
hydrogenations have also been shown to give higher ee’s
in the ionic liquid compared with methanol.21
Results and Discussion
For the studies reported herein, two types of catalyst
precursors were examined, the BINOL-ate complexes
λ- and δ-[(NUPHOS)Pt{(S)-BINOL}] (λ-2a, δ-2b,c),
since they are crystalline solids and stable with respect
to racemization, and the enantiopure dichlorides λ- and
δ-[(NUPHOS)PtCl2] (λ-3a, δ-3b,c) which can be gener-
ated from the corresponding BINOL-ate complexes by
reaction with hydrogen chloride in diethyl ether, ac-
cording to eq 1. Addition of a slight excess of acid to a
(3) Ringwald, M.; Stu¨rmer, R.; Brintzinger, H. H. J. Am. Chem. Soc.
1999, 121, 1524.
(4) (a) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics
2000, 19, 1567. (b) Desponds, O.; Schlosser, M. J. J. Organomet. Chem.
1996, 507, 257.
(5) For an insightful perspective see: Mikami, K.; Aikawa, K.;
Yukinori, Y.; Jodry, J. J.; Yamanaka, M. Synlett 2002, 1561.
(6) Tudor, M. D.; Becker, J. J.; White, P. S.; Gagne´, M. R. Organo-
metallics 2000, 19, 4376.
(7) Becker, J. J.; White, P. S.; Gagne´, M. R. J. Am. Chem. Soc. 2001,
123, 9478.
(8) (a) Mikami, K.; Yusa, Y.; Aikawa, K.; Hatano, M. Chirality 2003,
15, 105. (b) Mikami, K.; Aikawa, K.; Yusa, Y.; Hatano, M. Org. Lett.
2002, 4, 91. (c) Mikami, K.; Aikawa, K.; Yusa, Y. Org. Lett. 2002, 4,
95.
(9) Mikami, K.; Aikawa, K.; Yamanaka, M. Pure Appl. Chem. 2004,
76, 537.
(10) Mikami, K.; Aikawa, K. Org. Lett. 2002, 4, 99.
(11) Mikami, K.; Kataoka, S.; Yusa, Y.; Aikawa, K. Org. Lett. 2004,
6, 3699.
(12) (a) Doherty, S.; Knight, J. G.; Robins, E. G.; Scanlan, T. H.;
Champkin, P. A.; Clegg, W. J. Am. Chem. Soc. 2001, 123, 5110. (b)
Doherty, S.; Robins, E. G.; Nieuwenhuyzen, M.; Knight, J. G.; Champ-
kin, P. A.; Clegg, W. Organometallics 2002, 21, 1383.
(13) Doherty, S.; Newman, C. R.; Rath, R. K.; van den Berg, J.-A.;
Hardacre, C.; Nieuwenhuyzen, M.; Knight, J. G. Organometallics 2004,
23, 1055.
(14) (a) Doherty, S.; Newman, C. R.; Rath, R. K.; Luo, H.-K.;
Nieuwenhuyzen, M.; Knight, J. G. Org. Lett. 2003, 5, 3863. (b) Doherty,
S.; Knight, J. G.; Hardacre, C.; Luo, H.-K.; Newman, C. R.; Rath, R.
K.; Campbell, S.; Nieuwenhuyzen, M. Organometallics 2004, 23, 6127.
dichloromethane solution of λ- and δ-2a-c resulted in
an immediate and dramatic color change from either
deep yellow or intense orange-red to a pale yellow/
colorless solution associated with rapid and quantitative
formation of λ- and δ-3a-c, as evidenced by the pres-
ence of a single resonance in the 31P NMR spectrum,

Platinum Complexes of NUPHOS Diphosphines
Organometallics, Vol. 24, No. 24, 2005 5947
Scheme 1
shifted downfield from that of its BINOL-ate precursor.
The resulting complexes were separated from the liber-
ated BINOL with repeated precipitation from dichlo-
romethane by addition of hexane and used without
further purification to limit the extent of racemization,
which occurs in solution within hours at room temper-
ature. Prior to the conversion of these complexes into
the active catalysts, their stereochemical purity and
their stereochemical stability in the absence of the
resolving agent was examined.
The stereochemical purity of λ-3a and δ-3b,c was
investigated by reaction with (S,S)-1,2-diphenylethyl-
enediamine ((S,S)-dpeda), which resulted in rapid sub-
stitution of both chlorides (<2 min) to afford diastereo-
pure λ- and δ-[(NUPHOS)Pt{(S,S)-dpeda}]Cl2 (4a-c).
However, crystallization of λ-4a resulted in slow dias-
tereointerconversion, as evidenced by the gradual ap-
pearance of an additional signal in the 31P NMR
spectrum (Scheme 1). Although X-ray-quality crystals
of λ-4a could not be obtained, crystallization of its
perchlorate salt, [(Me4-NUPHOS)Pt{(S,S)-dpeda}][ClO4]2,
prepared by reaction of λ-4a with NaClO4 in methanol,
gave crystals suitable for X-ray analysis. The crystal
used for the data collection contained both λ(S,S) and
Figure 1. Molecular structure of [(Me4-NUPHOS)Pt-
{(S,S)-dpeda}][ClO4]2 (4a), showing the λ(S,S) diastereo-
isomer with a matched combination of NUPHOS and dpeda
stereochemistries. Hydrogen atoms, perchlorate anions,
and dichloromethane molecules of crystallization have been
omitted for clarity. Ellipsoids are at the 30% probability
level. Selected bond lengths (Å) and angles (deg): Pt(1B)-
P(1B), 2.255(3); Pt(1B)-P(6B), 2.252(3); Pt(1B)-N(7B),
2.091(9); Pt(1B)-N(8B), 2.110(8); C(2B)-C(3B), 1.325(14);
C(3B)-C(4B), 1.491(15); C(4B)-C(5B), 1.338(13); P(1B)-
Pt(1B)-P(6B), 89.62(10); N(7B)-Pt(1B)-N(8B), 79.4(3);
P(6B)-Pt(1B)-N(7B), 170.6(2); P(6B)-Pt(1B)-N(8B), 94.6-
(2); P(1B)-Pt(1B)-N(8B), 170.0(3); P(1B)-Pt(1B)-N(7B),
97.5(2); C(2B)-C(3B)-C(4B), 123.7(10); C(3B)-C(4B)-
C(5B), 123.8(10); C(2B)-C(3B)-C(31B), 121.8(10); C(21B)-
C(2B)-C(3B), 122.8(10); C(31B)-C(3B)-C(4B), 114.4(9);
C(41B)-C(4B)-C(5B), 122.5(10); C(3B)-C(4B)-C(41B),
113.5(9); C(4B)-C(5B)-C(51B), 123.3(9).
δ(S,S) diastereoisomers, and the molecular structure of
the λ(S,S) diastereoisomer is shown in Figure 1, along
with a selection of bond lengths and angles in the figure
caption. As Figure 1 shows, the backbone of the coor-
dinated NUPHOS adopts a distinctive λ-skew conforma-
tion with a dihedral angle of 64.5° between the least-
squares planes containing the double bonds and their
substituents, which is comparable to those reported for
complexes of related four-carbon-bridged diphosphines.22
The platinum atom is distorted from an ideal square-
planar geometry with a dihedral angle of 167° (for both
independent molecules) between the PtP2 and PtN2
planes; the latter is distorted toward the pseudoaxial
P-Ph substituents. The four phenyl rings of the diphen-
ylphosphino groups are arranged in the familiar alter-
nating edge-face arrangement.23 The natural bite angle
of 89.62(10)° is close to the ideal value of 90° and is
comparable to those reported for related complexes of
BIPHEP24 and NUPHOS diphosphines,25 while the
angle of 79.4(3)° associated with N(7B)-Pt(1B)-N(8B)
(15) For recent selected example see: (a) Mikami, K.; Miyamoto,
T.; Hatano, M. Chem. Commun. 2004, 2082. (b) Mikami, K.; Yusa, Y.;
Hatano, M.; Wakabayashi, K.; Aikawa, K. Chem. Commun. 2004, 98.
(c) Becker, J. J.; Van Orden, L. J.; White, P. S.; Gagne´, M. R. Org.
Lett. 2002, 4, 727. (d) Chen, G.; Takunaga, N.; Hayashi, T. Org. Lett.
2005, 7, 2205. (e) Taylor, J. J.; Duffey, M. O.; Morken, J. P. J. Am.
Chem. Soc. 2000, 122, 4528.
(16) Doherty, S.; Goodrich, P.; Hardacre, C.; Luo, H.-K.; Rooney, D.
W.; Seddon, K. R.; Styring, P. Green Chem. 2004, 6, 63.
(17) (a) Dias, L. C. Curr. Org. Chem. 2000, 4, 305. (b) Mikami, K.;
Shimizu, M. Chem. Rev. 1992, 92, 1021. (c) Mikami, K.; Nakai, T.
Catalytic Asymmetric Synthesis; Wiley-VCH: New York, 2000; pp 543-
568. (d) Mikami, K.; Terada, M. Comprehensive Asymmetric Catalysis;
Springer-Verlag: Berlin, Heidelberg, Germany, 1999; Vol. III, pp
1143-1176.
(18) (a) Hao, J.; Hatano, M.; Mikami, K. Org. Lett. 2000, 2, 4059.
(b) Koh, J. H.; Larsen, A. O.; Gagne´, M. R. Org. Lett. 2001, 3, 1233. (c)
Becker, J. J.; Van Orden, L. J.; White, P. S.; Gagne´, M. R. Org. Lett.
2002, 4, 727.
(19) For a review on the effect of ionic liquids in enantioselective
reactions see: Baudequin, C.; Baudoux, J.; Levillain, J.; Cahard, D.;
Gaumont, A.-C.; Plaquevant, J.-C. Tetrahedron: Asymmetry 2003, 14,
3081. For a review article on palladium-catalyzed reactions in ionic
liquids see: Welton, T.; Smith, P. J. Adv. Organomet. Chem. 2004,
51, 251.
(22) (a) Schmid, R.; Broger, E.; Cereghetti, M.; Crameri, Y.; Foricher,
J.; Lalonde, M.; Muller, R. K.; Scalone, M.; Schoettel, G.; Zutter, U.
Pure Appl. Chem. 1996, 68, 131. (b) Schmid, R.; Firicher, J.; Cereghetti,
M.; Schonholzer, P. Helv. Chim. Acta 1991, 74, 370. (c) Schmid, R.;
Cereghetti, M.; Schonholzer, P.; Hansen, H.-J. Helv. Chim. Acta 1988,
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1982, B38, 807.
(20) Meracz, I.; Oh, T. Tetrahedron Lett. 2003, 44, 6465.
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Ngo, T. T.; Pollet, P. Green Chem. 2003, 5, 123.

5948 Organometallics, Vol. 24, No. 24, 2005
Doherty et al.
is significantly smaller than the ideal value and mani-
fests itself in an expansion of the remaining N-Pt-P
angles, both of which are larger than 90°. The relative
thermodynamic stability of the λ(S,S) and δ(S,S) dia-
stereoisomers was investigated by monitoring a dichlo-
romethane solution of the pure λ(S,S) diastereoisomer
by 31P NMR spectroscopy. After standing for several
days, resonances corresponding to the δ(S,S) diastereo-
isomer began to appear and eventually reached equi-
librium with a λ(S,S):δ(S,S) ratio of ca. 7:3. The λ(S,S)
diastereoisomer is presumably thermodynamically fa-
vored, because the phenyl substituents of the (S,S)-
dpeda are projected toward the less congested quadrants
that are occupied by the pseudoaxial P-Ph groups.
The carbonyl-ene reaction was considered an ideal
candidate for a comparative study, since the products
are highly versatile chiral building blocks and because
Lewis acid palladium and platinum complexes of biaryl
diphosphines have already been reported to catalyze
this transformation. In this study, the addition of a
range of unsymmetrical 1,1′-disubstituted olefins to
ethyl glyoxylate and phenylglyoxal in [emim][NTf2] (1-
ethyl-3-methylimidazolium trifluorosulfonylimide) and
dichloromethane was compared, the results of which are
summarized in Tables 1-4. It should be noted that,
although the reactions proceeded in 1,2-dichloroethane,
tetrahydrofuran, and acetonitrile as well as in dichlo-
romethane, consistently lower ee’s were found; hence,
dichloromethane was used for all subsequent experi-
ments. The catalysts examined were either generated
in situ from diastereopure λ-2a and δ-2b,c by liberating
the BINOL resolving agent with an acid of a less-
coordinating counterion such as trifluoromethanesulfon-
ic acid or produced by activation of enantiopure λ-3a
and δ-3b,c with 2 equiv of silver triflate prior to addition
of the substrates (eqs 2 and 3). Regardless of the
precursor, activation should typically be carried out at
low temperature to prevent racemization of the result-
ing Lewis acid.
Table 1 summarizes results from the benchmark
Lewis acid catalyzed addition of methylenecyclohexane
to ethyl glyoxylate and phenylglyoxal using catalysts
generated by protonation of the BINOL-ate precursors
λ-2a and δ-2b,c. For each of the resulting catalysts λ-5a
and δ-5b,c, significantly higher ee’s were obtained in
[emim][NTf2] compared with dichloromethane. For ex-
ample, the Lewis acid λ-5a catalyzes the ene reaction
between methylenecyclohexane and ethyl glyoxylate to
afford the corresponding R-hydroxy ester with an ee of
67% in [emim][NTf2] and only 6% in dichloromethane.
Similarly, λ-5a also catalyzed the addition of methyl-
enecyclohexane to phenylglyoxal to give ee’s of 80 and
19% in [emim][NTf2] and CH2Cl2, respectively. While
[emim][NTf2] has been chosen for the majority of our
studies, catalyst λ-5a was shown to perform equally well
in a range of ionic liquids, giving ee’s of 69% in 1-butyl-
3-methylimidazolium trifluorosulfonylimide ([bmim]-
[NTf2]), 70% in N-butyl-N-methylpyrollidinium trifluoro-
sulfonylimide ([bmpyr][NTf2]), and 69% in 1-butyl-3-
methylimidazolium trifluoromethanesulfonate ([bmim]-
[OTf]), for the ene reaction between methylenecyclo-
hexane and ethyl glyoxylate. The performance of cata-
lysts δ-5b,c showed a trend similar to that of λ-5a: i.e.,
increased enantioselectivities in [emim][NTf2] compared
with dichloromethane for the addition of methylenecy-
clohexane to both ethyl glyoxylate (entries 2 and 3) and
phenylglyoxal (entries 6 and 7). It should also be noted
that, in general, higher isolated yields were obtained
in ionic liquid for each substrate-NUPHOS-based
catalyst combination compared with dichloromethane.
These results show that catalyst performance depends
on the NUPHOS diphosphine and that catalysts based
on the monocyclic NUPHOS diphosphine 1c consistently
outperformed those formed from its acyclic counterparts
1a,b.
The performance of catalysts based on (S)-BINAP was
also investigated for the addition of methylenecyclohex-
ane to both ethyl glyoxylate (entry 4) and phenylglyoxal
(entry 8), in order to compare directly the efficiency of
an atropos biaryl diphosphine with that of NUPHOS
diphosphines. The Lewis acid catalyst generated by
activation of [{(S)-BINAP}PtCl2] with silver triflate gave
comparable ee’s in [emim][NTf2] and dichloromethane
for both dicarbonyl substrates. In addition, the ee of 75%
in dichloromethane for the reaction of methylenecyclo-
hexane with ethyl glyoxylate is similar to that of 74%
reported by Koh et al., albeit with [{(S)-MeO-BIPHEP}-
Pt(OTf)2] and conducted at -50 °C.18b Gratifyingly,
(23) (a) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T.; Nishioka,
E.; Yangi, K.; Moriguchi, K. Organometallics 1993, 12, 4188. (b)
Tominaga, H.; Sakai, K.; Tsubomura, T. J. Chem. Soc., Chem.
Commun. 1995, 2273.
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rington, R. W.; Newman, C. R.; Campbell, R.; Amin, H. Organometallics
2005, 24, 2633. (b) Doherty, S.; Newman, C. R.; Hardacre, C.;
Nieuwenhuyzen, M.; Knight, J. G. Organometallics 2003, 22, 1452.

Platinum Complexes of NUPHOS Diphosphines
Organometallics, Vol. 24, No. 24, 2005 5949
Table 1. Asymmetric Carbonyl-Ene Reaction
between Methylenecyclohexane and Ethyl
Glyoxylate or Phenylglyoxal Catalyzed by λ-5a and
δ-5b,c in [emim][NTf2] and CH2Cl2
Table 2. Asymmetric Carbonyl-Ene Reaction
between r-Methylstyrene and Ethyl Glyoxylate
Catalyzed by λ-5a, δ-5b,c, λ-5a′, and δ-5b,c′ in
[emim][NTf2] and CH2Cl2
[emim][NTf2]
CH2Cl2
[emim][NTf2]
% ee
CH2Cl2
yield % ee
yield
% ee
yield
(%)b
% ee
substrate yield
entrya
catalyst
(%)b
(confign)c
(confign)c
entrya
catalyst
R1
(%)b (confign) (%) (confign)
1
2
3
λ-5a
δ-5b
δ-5c
47
10
18
46 (S)
25 (R)
65 (R)
0
4
0
1
2
3
4
5
6
7
8
λ-5a
OEt
OEt
OEt
OEt
Ph
73
82
74
56
30
22
35
35
67 (S)c
30 (R)c
70 (R)c
78 (R)c
80 (S)d
50 (R)d
87 (R)d
82 (R)d
50
79
73
77
10
0
6 (S)c
5 (R)
δ-5b
2 (R)c
δ-5c
61 (R)c
75 (R)c
19 (S)d
Pt{(S)-BINAP}
λ-5a
4
5
6
7
λ-5a′
39
12
47
36
50 (S)
22 (R)
65 (R)
57 (R)
28
18
41
47
10 (S)
5 (R)
28 (R)
53 (R)
δ-5b′
δ-5b
δ-5c
Ph
δ-5c′
Ph
24
78
37 (R)d
84 (R)d
Pt{(S)-BINAP}
Pt{(S)-BINAP}
Ph′
a Reaction conditions: 5 mol % catalyst, R-methylstyrene (0.25
mmol) and glyoxal substrate (0.5 mmol) in 2.0 mL of [emim][NTf2]
or CH2Cl2, room temperature. b Isolated yields after 2 h run.
c Enantiomeric excess determined by chiral HPLC using a Chiral-
pak AS column. Average of three runs.
a Reaction conditions: 5 mol % catalyst, methylenecyclohexane
(0.25 mmol), and ethyl glyoxylate or phenylglyoxal substrate (0.5
mmol) in 2.0 mL of [emim][NTf2] or CH2Cl2, room temperature.
b Isolated yields after 2 h run. c Enantiomeric excess determined
by chiral GLC using a Chrompak Chirasil DEX CB column.
d Enantiomeric excess determined by chiral HPLC using a Diacel
Chiralcel OD-H column. Average of three runs.
to the addition of substrate, the performance of catalysts
generated from the corresponding enantiopure dichlo-
rides λ-3a and δ-3b,c were also examined in order to
determine whether the liberated BINOL was respon-
sible for the poor performance of the BINOL-ate derived
catalysts in dichloromethane. Reassuringly, Lewis acids
λ-5a′ and δ-5b′,c′, generated by activation of λ-3a and
δ-3b,c with 2 equiv of silver triflate (eq 3), catalyzed
the ene reaction between R-methyl styrene and ethyl
glyoxylate in [emim][NTf2] (entries 4-6) and gave iso-
lated yields and enantioselectivities comparable to those
obtained with the corresponding BINOL-ate derived
catalysts. Interestingly, these catalysts performed mark-
edly better in dichloromethane as compared to those
generated from their BINOL-ate counterparts, although
the enantioselectivities were still significantly lower
than those obtained in [emim][NTf2]. A comparative
study between BINAP- and NUPHOS-based catalysts
revealed that [{(S)-BINAP}Pt(OTf)2] gave an enantio-
selectivity of 57% in [emim][NTf2], slightly lower than
that obtained with either δ-5c (65%) or δ-5c′ (65%),
while the ee of 53% in dichloromethane was significantly
higher than that obtained with any of the NUPHOS-
based catalysts. Regardless of the precursor and method
of activation, catalysts based on the monocyclic NU-
PHOS diphosphine 1c afforded markedly higher enan-
tioselectivities in ionic liquid than either of its acyclic
counterparts, as found for methylenecyclohexane (vide
supra).
Lewis acids λ-5a and δ-5b,c also catalyze the carbo-
nyl-ene reaction of phenylglyoxal with 2,3-dimethylbut-
1-ene and 2,4,4-trimethylpent-1-ene, the results of
which are summarized in Tables 3 and 4, respectively.
Table 3 reveals that each of the BINOL-ate precursors
forms a highly efficient catalyst for the addition of 2,3-
dimethylbut-1-ene to phenylglyoxal in [emim][NTf2]
(entries 1-3), giving good to excellent enantioselectivi-
ties of (R)- and (S)-R-hydroxy ester (68-95%). As with
the addition of R-methylstyrene to ethyl glyoxylate,
these catalyst systems did not give any ene product in
dichloromethane, although a low yield of a single
byproduct was consistently isolated after aqueous work-
up and purification by column chromatography. While
these studies have demonstrated that catalysts based
on NUPHOS diphosphines, particularly 1c, can compete
with their BINAP counterpart. The absolute configura-
tions of the ene products listed in Table 1 were deter-
mined by comparing the HPLC or GC retention times
with those reported in the literature. In a recent study,
Yamada et al. established the absolute configuration of
ene products generated with cobalt(III) complexes of
â-ketoiminates derived from (S,S)-1,2-diphenylethanedi-
amine by NMR analysis of the corresponding MPTA
esters26 and concluded that the absolute stereochemistry
was that predicted by the stereochemical model devel-
oped for the corresponding hetero Diels-Alder reac-
tion.27 Lewis acids δ-5b,c both gave the ene product with
R absolute configuration, and in this regard, acyclic and
monocyclic NUPHOS diphosphines with a δ conforma-
tion behave in much the same manner as (S)-BINAP
(vide infra). As expected, the λ-5a-catalyzed ene reaction
between methylenecyclohexane and ethyl glyoxylate or
phenylglyoxal gave R-hydroxy esters of opposite absolute
stereochemistry to those obtained with δ-5b,c.
The efficiency of catalysts λ-5a and δ-5b,c was also
investigated for the addition of a range of other 1,1′-
disubstituted olefins to ethyl glyoxylate and phenylgly-
oxal in [emim][NTf2] and dichloromethane, the results
of which are listed in Tables 2-4. In the case of R-meth-
ylstyrene with ethyl glyoxylate, catalysts generated
from precursors λ-2a and δ-2b,c each gave the R-hy-
droxy ester in moderate isolated yields with low to good
enantioselectivities in [emim][NTf2] (Table 2). In com-
parison, the same reaction in dichloromethane gave
little or no yield of R-hydroxy ester and, although some
starting material was recovered, significant polymeric
material was also isolated. Although the catalysts are
most conveniently stored as their BINOL-ate complexes
and converted into the active catalyst immediately prior
(26) (a) Kezuka, S.; Ikeno, T.; Yamada, T. Org. Lett. 2001, 3, 1937.
(b) Ohtani, I.; Kusumi, T.; Kashnan, Y.; Kakisawa, H. J. Am. Chem.
Soc. 1991, 113, 4092.
(27) Kezuka, S.; Mita, T.; Ohtsuki, N.; Ikeno, T.; Yamada, T. Bull.
Chem. Soc. Jpn 2001, 74, 1333.

5950 Organometallics, Vol. 24, No. 24, 2005
Doherty et al.
it has not been possible to unequivocally identify this
byproduct, a molecular ion of m/z 375, corresponding
to [M + Na]+ (as well as at m/z 727 for [2M + Na]+ and
m/z 1079 for [3M + Na]+) in the electrospray mass
90%), with the monocyclic catalyst δ-5c proving to be
the most efficient, giving an enantioselectivity of 90%
compared with 82 and 39% obtained with their acyclic
counterparts λ-5a and δ-5b, respectively, and (ii) no ene
product was isolated using these catalysts in dichlo-
romethane. It should be noted that, in contrast to 2,3-
dimethylbut-1-ene, no byproduct was formed using
2,4,4-trimethylpent-1-ene. The similarity in catalyst
performance between the two olefins also includes λ-5a′,
δ-5b′,c′, λ-6a, and δ-6b,c, which gave moderate to
excellent enantioselectivities in [emim][NTf2] (55-87%)
and dichloromethane (57-90%), with the exception of
δ-5b′, which gave low enantioselectivities in both sol-
vents. The ee of 88% achieved with catalyst based on
(S)-BINAP is significantly higher than that of 63%
obtained for the corresponding ene reaction with 2,3-
dimethylbut-1-ene, which highlights the marked depen-
dence of catalyst performance on substrate structure.
In the case of unsymmetrical 1,1′-disubstituted olefins
such as 2,3-dimethylbut-1-ene and 2,4,4-trimethylpent-
1-ene, two isomeric ene products are possible29 (eq 4),
and in order to effect a regioselective process the cata-
lyst-glyoxylate complex must discriminate between a
methyl group and isopropyl and neopentyl groups, re-
spectively. For both substrates, 1H NMR analysis of the
1
spectrum and its H/13C NMR spectra are consistent
with a formulation based on 2 mol of phenylglyoxal
combining with 1 mol of 2,3-dimethylbut-1-ene. Full
details of the spectroscopic data associated with this
compound are provided in the Supporting Information.
In contrast, excellent enantioselectivities were obtained
in both [emim][NTf2] (83-95%) and dichloromethane
(80-89%) with catalysts generated from dichlorides λ-3a
and δ-3b,c (Table 3, entries 4-6). Thus, as found for
the reaction between R-methylstyrene and ethyl gly-
oxylate, the precursor and/or the method of activation
has a dramatic influence on catalyst performance. This
was clearly demonstrated in an experiment designed to
investigate whether the BINOL-ate precursor, or more
specifically the liberated BINOL, was responsible for the
formation of the byproduct in dichloromethane. A
catalyst mixture generated by activation of δ-3c with 2
equiv of AgOTf, filtered and treated with 1 equiv of (S)-
BINOL, catalyzed the carbonyl-ene reaction between
2,3-dimethylbut-1-ene and phenylglyoxal to afford the
ene product in moderate isolated yield (40%) and high
enantioselectivity (94%), with no evidence for the forma-
tion of byproduct (Table 3, entry 7). Similarly, a dichlo-
romethane solution of the Lewis acid generated by
activation of δ-3c with AgOTf and treated with (S)-
BINOL also catalyzed the reaction between 2,4,4-
trimethylpent-1-ene and phenylglyoxal to afford the ene
product in 21% isolated yield with an enantioselectivity
of 54% (Table 4, entry 7), whereas only starting material
was isolated using catalyst generated from the BINOL-
ate precursor δ-5c (Table 4, entry 3). Lewis acids λ-6a
and δ-6b,c, generated by activation of δ- and λ-[(NU-
PHOS)PtCl2] with AgSbF6 (eq 3), also catalyzed the
reaction between phenylglyoxal and 2,3-dimethylbut-
1-ene and gave enantioselectivities, in both [emim]-
[NTf2] and dichloromethane, that were comparable to
those obtained with their triflate counterparts (Table
3, entries 8-10). In general, the SbF6--based catalysts
gave higher yields than the corresponding triflate-based
Lewis acids, which is entirely consistent with previous
reports such as the platinum(II)-catalyzed glyoxylate-
ene reaction18b and the ruthenium-catalyzed Diels-
Alder reaction between methacrolein and cyclopenta-
diene.28 In both of these cases the nature of the
counterion showed a marked effect on the rate but not
on the enantioselectivity, the reactivity increasing in the
order OTf- < BF4- < SbF6-. As found for other olefins,
the most selective catalyst was that based on the
monocyclic NUPHOS diphosphine 1c with both the
BINOL-ate and dichloride precursors giving an ee of
95% in [emim][NTf2], a marked improvement on that
of 63% obtained with [{(S)-BINAP}Pt(OTf)2].
crude reaction mixture revealed complete regioselectiv-
ity for all catalysts examined, which is not surprising,
given the relative sizes of the 1,1′-substituents. High
levels of regioselectivity have recently been reported for
these carbonyl-ene reactions catalyzed by optically
active cationic cobalt(III) complexes of â-ketoimines.26a
This study has demonstrated that enantiopure plati-
num metal complexes of NUPHOS diphosphines exhibit
stereoregular behavior in reactions involving carbonyl
substrates capable of two-point binding. While Lewis
acids δ-5b,c both gave an R-hydroxy ester with R
absolute configuration, that based on λ-5a gave the ene
product with the opposite absolute stereochemistry. This
assignment is entirely consistent with that recently
reported by Hao et al., in which Lewis acid palladium-
(II) complexes of (S)-Tol-BINAP catalyzed the asym-
metric glyoxylate-ene reaction to afford the (R)-R-
hydroxy ester.18a In this regard, NUPHOS diphosphines
with a δ conformation behave in the same manner as
atropos biaryl diphosphines in that the absolute stereo-
chemistry of the ene product is the same as that
obtained with (S)-BINAP and its derivatives. The sense
Not surprisingly, the performance of catalysts λ-5a
and δ-5b,c for the addition of 2,4,4-trimethylpent-1-ene
to phenylglyoxal in both [emim][NTf2] and dichlo-
romethane (Table 4) parallels that obtained for 2,3-
dimethylbut-1-ene in that (i) moderate to excellent
enantioselectivities were obtained in [emim][NTf2] (39-
(29) (a) Evans, D. A.; Tregay, S. W.; Burgey, C. S.; Paras, N. A.;
Vojkovsky, T. J. Am. Chem. Soc. 2000, 122, 7936. (b) Evans, D. A.;
Wu, J. J. Am. Chem. Soc. 2005, 127, 8006.
(28) Ku¨ndig, E. P.; Suadan, C. M.; Bernardinelli, G. Angew. Chem.,
Int. Ed. 1999, 38, 1220.

Platinum Complexes of NUPHOS Diphosphines
Organometallics, Vol. 24, No. 24, 2005 5951
Table 3. Asymmetric Carbonyl-Ene Reaction
between 2,3-Dimethylbut-1-ene and Phenylglyoxal
Catalyzed by λ-5a, δ-5b,c, λ-5a′, and δ-5b,c′ in
[emim][NTf2] and CH2Cl2
[emim][NTf2]
CH2Cl2
yield
% ee
yield
(%)b
% ee
entrya
catalyst
(%)b
(confign)c
(confign)c
1
2
3
λ-5a
δ-5b
δ-5c
25
29
31
90 (S)
68 (R)
95 (R)
0
0
0
4
λ-5a′
δ-5b′
δ-5c′
δ-5c′
90
20
34
90 (S)
83 (R)
95 (R)
54
15
60
40
87 (S)
80 (R)
89 (R)
94(R)
5
6
7d
8
9
λ-6ae
95
46
57
6
86 (S)
84 (R)
92 (R)
63 (R)
66
69
90
0
80 (S)
46 (R)
94 (R)
Figure 2. Molecular structure of [(1,4-Et2-2,3-cyclo-C6H8-
NUPHOS)PtCl2] (3c), highlighting the alternating edge-
face arrangement of the diphenylphosphino phenyl rings.
Hydrogen atoms and the dichloromethane molecule of
crystallization have been omitted for clarity. Ellipsoids are
at the 30% probability level. Selected bond lengths (Å) and
angles (deg) not discussed in the text: Pt(1)-P(1), 2.227(3);
Pt(1)-P(6), 2.24(3); Pt(1)-Cl(1), 2.339(3); Pt(1)-Cl(2),
2.351(3); C(2)-C(3), 1.339(14); C(3)-C(4), 1.498(16); C(4)-
C(5), 1.334(16); P(1)-Pt(1)-P(6), 90.28(9); Cl(1)-Pt(1)-
Cl(2), 87.66(11); P(6)-Pt(1)-Cl(1), 92.58(11); P(1)-Pt(1)-
Cl(2), 90.43(9).
δ-6be
10
11
δ-6ce
Pt{(S)-BINAP}
a Reaction conditions: 2 mol % catalyst, 2,3-dimethylbut-1-ene
(0.25 mmol), and phenylglyoxal (0.5 mmol) in 2.0 mL of [emim]-
[NTf2] or CH2Cl2, room temperature. b Isolated yields after 2 h run.
c Enantiomeric excess determined by chiral HPLC using a Diacel
Chiralcel OD-H column. Average of three runs. d Catalyst mixture
treated with 1 equiv of (S)-BINOL prior to addition of substrates.
e Catalyst mixtures prepared as described for λ-5a′ and δ-5b′,c′,
by activation of λ-3a and δ-3b,c with 2 equiv of AgSbF6 in CH2Cl2
for 30 min at room temperature.
Table 4. Asymmetric Carbonyl-Ene Reaction
between 2,4,4-Trimethylpent-1-ene and
Phenylglyoxal Catalyzed by λ-5a, δ-5b,c, λ-5a′, and
δ-5b,c′ in [emim][NTf2] and CH2Cl2
[emim][NTf2]
yield % ee
(%)b (confign)c (%)b (confign)c
CH2Cl2
yield
% ee
entrya
catalyst
λ-5a
δ-5b
δ-5c
1
2
3
42
53
61
82 (S)
39 (R)
90 (R)
0
0
0
4
λ-5a′
δ-5b′
δ-5c′
δ-5c′
84
20
25
80 (S)
26 (R)
55 (R)
61
47
57
21
76 (S)
17 (R)
56 (R)
54 (R)
5
Figure 3. Variation of percent ee for the carbonyl-ene
reaction between ethyl glyoxylate and methylenecyclohex-
ane catalyzed by λ-5a (squares), δ-5b (circles), and δ-5c
(triangles) in [emim][NTf2] (solid symbols) and CH2Cl2
(open symbols) with respect to time at room temperature.
6
7d
8
9
10
11
λ-6ae
84
52
94
85
80 (S)
57 (R)
87 (R)
88 (R)
90
49
90
82
79 (R)
51 (R)
85 (R)
86 (I)
δ-6be
δ-6ce
Pt{(S)-BINAP}
a Reaction conditions: 2 mol % catalyst, 2,2,4-trimethylpent-1-
ene (0.25 mmol) and ethyl phenylglyoxal (0.5 mmol) in 2.0 mL of
either [emim][NTf2] or CH2Cl2, room temperature. b Isolated yields
after 2 h run. c Enantiomeric excess determined by chiral HPLC
using a Diacel Chiralcel OD-H column. Average of three runs.
d Catalyst mixture treated with 1 equiv of (S)-BINOL prior to
addition of substrates. e Catalyst mixtures prepared as described
for λ-5a and δ-5b,c by activation of λ-3a and δ-3b,c with 2 equiv
of AgSbF6 in CH2Cl2 for 30 min at room temperature.
account for the stereochemical outcome of copper-
catalyzed carbonyl-ene reactions.30 Inspection of this
current model (Scheme 2) reveals that the pseudoequa-
torial phenyl ring attached to PB of the δ-NUPHOS
diphosphine is orientated above the PtP2 plane in such
a manner that the Si face of the coordinated glyoxylate
is sterically hindered thus favoring reaction of the olefin
from the more accessible Re face to afford a homoallylic
alcohol with an R configuration. The asymmetric envi-
ronment created by the alternating edge-face arrange-
ment of the P-Ph rings of δ-NUPHOS is evident in the
of asymmetric induction for the ene reaction catalyzed
by λ- or δ-[(NUPHOS)Pt]2+ is consistent with a model
in which ethyl glyoxylate coordinates through both
carbonyl oxygen atoms in a bidentate manner to afford
a square-planar adduct similar to that formed between
[Cu{(S,S)-t-Bu-box}][SbF6]2 and glyoxylate (or pyru-
vate), proposed by Evans and co-workers and used to
(30) (a) Evans, D. A.; Burgey, C. S.; Paras, N. A.; Vojkovsky, T.;
Tregay, S. W. J. Am. Chem. Soc. 1998, 120, 5824. (b) Evans, D. A.;
Kozlowski, M. C.; Burgey, C. S.; MacMillan, D. W. C. J. Am. Chem.
Soc. 1997, 119, 7893.

5952 Organometallics, Vol. 24, No. 24, 2005
Doherty et al.
ee in dichloromethane, since the ee’s in the initial stages
of reaction are similar in both solvents, the enhance-
ment associated with precursors λ-2a and δ-2b is due
to a combination of an intrinsic solvent effect and
erosion in ee. The latter point is clearly evident in the
markedly different ee’s obtained in both solvents in the
early stages of reaction; catalyst δ-5b gives initial ee’s
of 30 and 15% in [emim][NTf2] and dichloromethane,
respectively, while the corresponding ee’s obtained with
λ-5a were 51 and 20%.
It is possible that the observed erosion in ee could be
associated with a faster rate of racemization of the
catalyst in dichloromethane compared with ionic liquid.
Indeed, preliminary kinetic studies, based on quenching
solutions of catalyst with (S,S)-dpeda, revealed that the
rate of racemization of the catalyst is significantly
slower in ionic liquid than in dichloromethane. For
example, in a comparative study the first-order rate
constant for the racemization of δ-5b in dichloromethane,
in the absence of substrate, was 1.1 × 10-5 s-1 at 30
°C, whereas the corresponding rate constant in [emim]-
Figure 4. Variation in percent conversion (closed symbols)
and percent ee (open symbols) for the ene reaction between
ethyl glyoxylate and methylenecyclohexane catalyzed by
δ-5c in [emim][NTf2] (squares) and in dichloromethane
(circles) with respect to time at room temperature.
Scheme 2
[NTf2] was immeasurably low at 30 °C (<1 × 10-6 s-1
)
and only 5.6 × 10-6 s-1 at 45 °C. While these quenching
experiments occur cleanly and quantitatively to give
[(NUPHOS)Pt{(S,S)-dpeda}]2+, both in the absence of
substrate and in the presence of olefin, the correspond-
ing reactions in the presence of ethyl glyoxylate or the
ene product gave rise to a multitude of products, as
evidenced by the presence of several additional signals
in the 31P NMR spectrum, suggesting that the catalytic
reaction mixture most likely comprises a complex set
of equilibria. In general, the carbonyl-ene reactions are
also found to be faster in [emim][NTf2] than in dichlo-
romethane (Figure 4). Thus, as well as the decreased
racemization rate of the catalyst, this increased reaction
rate could also contribute to the enhancement in ee
found in the ionic liquid compared with dichloromethane,
although it is likely that the more important factor is
the stability of the catalyst.
This supposition is supported by the effect of temper-
ature on the variation of enantioselectivity with time
and the results from recycling the ionic liquid-catalyst
mixtures. As expected, the rate of catalyst racemization
may be significantly lowered by reducing the tempera-
ture. For example, the reaction between methylenecy-
clohexane and ethyl glyoxylate using the BINOL-ate
catalyst precursor δ-2c showed little change in ee (71%)
over a 3 h period at -20 °C in dichloromethane,
indicating higher catalyst stability at low temperature
and demonstrating behavior similar to that observed in
[emim][NTf2] at room temperature. Table 5 summarizes
the results of a series of recycling experiments for the
addition of methylenecyclohexane to ethyl glyoxylate
and phenylglyoxal using catalysts λ-5a and δ-5b in
[emim][NTf2]. The extractions were performed using
diethyl ether in air, and the enantioselectivity remained
constant for three successive reactions, indicating little
racemization of the catalyst even under reaction condi-
tions. However, a dramatic reduction in the isolated
yields over three cycles was found. ICP analysis of the
ether extract showed no evidence for leaching of the
platinum complexes, and the reduction in the isolated
yield is likely to be due to the buildup of polymeric
glyoxylate, which deactivates the catalyst, possibly by
molecular structure of 4c (Figure 2) and is similar to
that adopted by palladium and platinum complexes of
(S)-BINAP and BIPHEP. This structure shows that
P-Ph(11B) and P-Ph(61B) adopt pseudoaxial positions
and expose their edges toward the palladium atoms,
while the spatial arrangement of the pseudoequatorial
P-Ph rings P-Ph(11A) and P-Ph(61A) is between edge
and face and that the quadrants occupied by the
pseudoaxial P-Ph substituents are less congested than
those occupied by their pseudoequatorial counterparts.
This model closely resembles that recently proposed to
account for the high level of enantiofacial selection
obtained in palladium and platinum metal catalyzed
Diels-Alder reactions between acryloyl oxazolidinones
and cyclopentadiene.31
To investigate the origin of the enhancement in
enantioselectivity for reactions conducted in ionic liquid
compared with dichloromethane, the variation in ee
with respect to time for the reaction between methyl-
enecyclohexane and ethyl glyoxylate for each of the
BINOLate catalyst precursors, λ-2a and δ-2b,c, in both
[emim][NTf2] and dichloromethane was studied. It is
clear from Figure 3 that there is a gradual descrease in
ee as the reaction proceeds in dichloromethane, whereas
the enantioselectivities remained constant in [emim]-
[NTf2] over the same period, for each catalyst. Interest-
ingly, while the enhancement in enantioselectivity
associated with catalyst generated from δ-2c in [emim]-
[NTf2] appears to be due solely to a gradual erosion of
(31) (a) Oi, S.; Terada, E.; Ohuchi, K.; Kato, T.; Tachibana, Y.; Inoue,
Y. J. Org. Chem. 1999, 64, 8660. (b) Oi, S.; Kashiwaga, K.; Inoue, Y.
Tetrahedron Lett. 1998, 39, 6253. (c) Ghosh, A. K.; Matsuda, H. Org.
Lett. 1999, 1, 2157.

Platinum Complexes of NUPHOS Diphosphines
Organometallics, Vol. 24, No. 24, 2005 5953
Table 5. Recycle Carbonyl-Ene Reactions
between Methylenecyclohexane and Ethyl
Glyoxylate or Phenylglyoxal Catalyzed by δ-5b,c in
[emim][NTf2]
catalysis. Further studies are currently underway to
explore the applications of this class of diphosphine in
a wide range of asymmetric reactions and to prepare
chiral versions for a comparative study. Moreover,
platinum group metal complexes based on biaryl diphos-
phines such as BINAP and its derivatives have recently
been reported to be the catalyst of choice for numerous
achiral transformations such as the chemo- and regi-
oselective intermolecular cyclotrimerization of terminal
alkynes,32 cycloaddition and cycloisomerization of 1,6-
enynes,33 intramolecular amination of aryl bromides,34
rhodium-catalyzed isomerization of secondary propar-
gylic alcohols to R,â-enones,35 and iridium-catalyzed
crosscouplingofterminalalkyneswithinternalalkynes.36
Thus, NUPHOS-type diphosphines could provide a
practical and significantly less expensive alternative to
BINAP for such platinum group metal catalyzed achiral
reactions.
entrya catalyst recyleb substrate R1 yield (%)c % ee (confign)
1
2
3
4
5
6
7
8
9
δ-5c
δ-5b
δ-5c
OEt
OEt
Ph
74
62
43
82
54
37
38
32
13
71 (R)d
70 (R)d
70 (R)d
30 (R)d
28 (R)d
28 (R)d
87 (R)e
87 (R)e
86 (R)e
1
2
1
2
1
2
a Reaction conditions: 5 mol % catalyst, methylenecyclohexane
(0.25 mmol) and ethyl glyoxylate or phenylglyoxal substrate (0.5
mmol) in 2.0 mL of [emim][NTf2], room temperature. b Ionic liquid
extracted with diethyl ether (3 × 5 mL). c Isolated yields after 2 h
run. d Enantiomeric excess determined by chiral GLC using a
Chrompak Chirasil DEX CB column. e Enantiomeric excess de-
termined by chiral HPLC using a Diacel Chiralcel OD-H column.
Average of three runs.
Experimental Section
General Procedures. All manipulations involving air-
sensitive materials were carried out in an inert-atmosphere
glovebox or using standard Schlenk line techniques under an
atmosphere of nitrogen or argon in oven-dried glassware.
Diethyl ether was distilled from potassium/sodium alloy and
dichloromethane from calcium hydride, and [emim][NTf2] was
prepared and dried by following the method of Bonhoˆte et al.37
(S)-BINAP was purchased from Strem. Unless otherwise
stated, commercially purchased materials were used without
further purification. Deuteriochloroform was predried with
calcium hydride, vacuum-transferred, and stored over 4 Å
molecular sieves. The platinum complexes [(NUPHOS)Pt{(S)-
BINOL}] (λ-2a, δ-2b,c)13 and [Pt{(S)-BINAP)Cl2]31a were
prepared as previously described. Ethyl glyoxylate and phen-
ylglyoxal were purchased from Lancaster and distilled im-
mediately prior to use. 1H NMR and 13C{1H} NMR spectra
were recorded on a AMX 300 machine. Purification of reaction
products was carried out by column chromatography on
reagent silica gel (60-200 mesh). Analytical high-performance
liquid chromatography (HPLC) was performed on an Agilent
110 Series HPLC equipped with a variable-wavelength detec-
tor using either a Daicel Chiralcel OD-H or a Chiralpak AS
column. Chiral GLC was performed on a Agilent 6890N series
GC using a Chrompak Chirasil DEX CB column. Enantiomeric
excesses were calculated from the HPLC and GC profiles.
Synthesis of λ-[{1,4-bis(diphenylphosphino)-1,2,3,4-
tetramethyl-1,3-butadiene}PtCl2] (λ-3a). A solution of enan-
tiopure λ-[{1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-
butadiene}platinum{(S)-BINOL}] (λ-2a; 1.1 g, 1.15 mmol) in
dichloromethane (20 mL) was treated with a diethyl ether
solution of HCl (2.5 mL, 2.5 mmol, 1.0 M solution in diethyl
ether). Addition of HCl resulted in an immediate color change
from deep yellow to near colorless. After 10 min the solution
was filtered, the solvent removed under vacuum, and the
resulting residue washed with diethyl ether (5 × 10 mL) and
hexane (5 × 10 mL). Crystallization of the product by slow
diffusion of a dichloromethane solution layered with hexane
entrapment of the catalyst in the polymer matrix. This
is supported by the observation that addition of polymer
to a fresh reaction mixture resulted in a significant
reduction in the isolated yield of ene product. In
addition, it is not straightforward to recycle the catalyst
in dichloromethane, showing a further advantage as-
sociated with the use of the ionic liquid.
In summary, enantiopure Lewis acid complexes of
conformationally flexible acyclic and monocyclic NU-
PHOS diphosphines, δ- and λ-[(NUPHOS)Pt(OTf)2], are
efficient catalysts for the carbonyl-ene reaction between
unsymmetrical 1,1′-disubstituted alkenes and phenyl-
glyoxal or ethyl glyoxylate. This study has shown that
the type of precursor, the method of activation, and the
solvent are all critical factors in obtaining optimum
catalyst performance. Without exception, the BINOL-
ate precursors formed the most efficient catalysts in
ionic liquid, giving marked enhancements in enantiose-
lectivity compared with dichloromethane. In contrast,
the performance of catalysts generated from the dichlo-
ride precursors depended markedly on the substrate
combination, resulting in enantioselectivities at least
equal, and often higher, in the ionic liquid compared
with dichloromethane. Preliminary kinetic studies have
shown that the enhancement in enantioselectivity ob-
tained with catalysts generated from the BINOL-ate
precursors in ionic liquids is likely to be due to a
combination of decreased racemization rate in [emim]-
[NTf2] and an intrinsic solvent effect. The absolute
stereochemistry can be rationalized by a stereochemical
model similar to that developed by Oi et al., which was
used to account for the high level of enantiofacial
discrimination in palladium (S)-BINAP catalyzed hetero
Diels-Alder reactions.31 This study has clearly shown
that the performance of catalysts based on conforma-
tionally flexible NUPHOS diphosphines can rival or
even outperform that achieved with BINAP, confirming
that this easy-to-prepare and relatively inexpensive
class of diphosphine can be used as the sole source of
chirality for efficient platinum group metal asymmetric
(32) (a) Tanaka, K.; Toyoda, K.; Wada, A.; Shirasaka, K.; Hirano,
M. Chem. Eur. J. 2005, 11, 1145. (b) Tanaka, K.; Nishida, G.; Ogino,
M.; Hirano, M.; Noguchi, K. Org. Lett. 2005, 7, 3119.
(33) Kezuka, S.; Okado, T.; Niou, E.; Takeuchi, Org. Lett. 2005, 7,
1711.
(34) Lebedev, A. Y.; Khartulyari, A. S.; Voskoboyonikov, A. Z. J. Org.
Chem. 2005, 70, 596.
(35) Tanaka, K.; Shoji, T. Org. Lett. 2005, 7, 3561.
(36) Hirabayashi, T.; Sakaguchi, S.; Ishii, Y. Adv. Synth. Cat. 2005,
347, 872.
(37) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundram,
K.; Gra¨tzel, M. Inorg. Chem. 1996, 32, 1168.

5954 Organometallics, Vol. 24, No. 24, 2005
Doherty et al.
at -20 °C gave pale yellow λ-3a in 67% yield (0.61 g). 31P{1H}
NMR (121.5 MHz, CDCl3, δ): 6.7 (t, JPtP ) 3660 Hz, PPh2).
1H NMR (300.0 MHz, CDCl3, δ): 8.36 (br, 4H, C6H5), 7.58 (m,
8H, C6H5), 7.27 (m, 8H, C6H5), 1.23 (d, JPH ) 11.2 Hz, 6H,
CH3), 1.18 (s, 6H, CH3). 13C{1H} NMR (75.4 MHz, CDCl3, δ):
149-124 (m, C6H5 + CdC), 19.5 (t, JPC ) 3.3 Hz, CH3), 17.5
(t, JPC ) 5.6 Hz, CH3). Anal. Calcd for C32H32Cl2P2Pt: C, 51.62;
H, 4.33. Found: C, 52.06; H, 4.67. [R]D ) +0.216° (c 1.0,
CHCl3).
δ-[{1,4-bis(diphenylphosphino)-1,2,3,4-tetraphenyl-1,3-
butadiene}PtCl2] (δ-3b). Compound δ-3b was prepared
according to the procedure described above for λ-3a and
isolated as pale yellow crystals in 82% yield by slow diffusion
of hexane into a chloroform solution at -20 °C. 31P{1H} NMR
(121.5 MHz, CDCl3, δ): 1.9 (t, JPtP ) 3607 Hz, PPh2). 1H NMR
(500.13 MHz, CDCl3, 232 K, δ): 9.57 (br m, 4H, C6H5), 7.9 (br
m, 6H, C6H5), 7.2 (m, 6H, C6H5), 6.88, (m, 4H, C6H5), 6.7 (t, J
) 7.3 Hz, 2H, C6H5), 6.65 (m, 6H, C6H5), 6.54 (t, J ) 7.9 Hz,
4H, C6H5), 6.18 (br, 4H, C6H5), 6.11 (d, J ) 7.3 Hz, 4H, C6H5).
Anal. Calcd for C52H40Cl2P2Pt: C, 62.91; H, 4.06. Found: C,
63.23; H, 4.44. [R]D ) -9.07° (c 1.0, CHCl3).
5 min, freshly distilled carbonyl substrate (0.5 mmol) was
added, followed by olefin (0.25 mmol). The resulting mixture
was warmed to room temperature and stirred for a further 2
h, after which time the solution was filtered through a short
plug of silica with diethyl ether, the solvent removed, and the
residue purified by column chromatography over silica gel. The
product was analyzed by 1H NMR spectroscopy, and the
enantiomeric excess was determined by either chiral GC or
HPLC. The absolute configuration of the adduct was assigned
by comparison with the retention times reported in the
literature, as described below.26a,30a,38
General Procedure for Platinum-Catalyzed Enantio-
selective Carbonyl-ene Reactions with Catalyst Precur-
sors λ-3a, δ-3b,c, and [{(S)-BINAP)PtCl2]: Method B. A
solution of [{(S)-BINAP)PtCl2] (0.0111 g, 0.0125 mmol) in
dichloromethane (2 mL) was treated with silver trifluo-
romethanesulfonate (0.0064 g, 0.025 mmol) or silver hexafluo-
roantimonate (0.0086 g, 0.025 mmol) and stirred for 30 min
until a precipitate of silver chloride had formed. The resulting
catalyst solution was filtered to remove AgCl and cooled to 0
°C, and freshly distilled carbonyl substrate (0.5 mmol) was
added, followed by olefin (0.25 mmol). The reaction mixture
was warmed to room temperature and stirred for a further 2
h, after which the solution was filtered through a short plug
of silica with ethyl acetate, the solvent removed, and the
resulting residue purified by column chromatography over
silica gel. The product was analyzed by 1H NMR spectroscopy,
and the enantiomeric excess was determined by either chiral
GC or HPLC.26a,30a,38
General Procedure for Platinum-Catalyzed Enantio-
selective Carbonyl-Ene Reactions with Catalyst Precur-
sors λ-2a and δ-2b,c in 1-Ethyl-2-methylimidazolium Bis-
(trifluoromethanesulfonimide), [emim][NTf2]: Method A.
A flame-dried Schlenk flask charged with δ-2b (0.0154 g,
0.0127 mmol) was cooled to 0 °C and dichloromethane added
(1 mL). The resulting solution was treated with trifluo-
romethanesulfonic acid (2.1 µL, 0.024 mmol) to give an
immediate color change from deep red-orange to near colorless.
After the mixture was stirred for 5 min, [emim][NTf2] (2 mL)
was added and the dichloromethane removed under vacuum,
after which freshly distilled carbonyl substrate (0.5 mmol) was
added, followed by olefin 0.25 mmol). The resulting mixture
was warmed to room temperature and stirred for a further 2
h, after which time the ionic liquid was extracted with diethyl
ether (5 × 3 mL) in air. The crude product was purified by
column chromatography over silica gel. The product was
analyzed by 1H NMR spectroscopy, and the enantiomeric
excess was determined by either chiral GC or HPLC, as
described in the Supporting Information.26a,30a,38
δ-[{1,2-bis((diphenylphosphino)ethylmethylene)cyclo-
hexane}PtCl2] (δ-3c). Compound δ-3c was prepared accord-
ing to the procedure described above for λ-3a and isolated as
yellow crystals in 71% yield by slow diffusion of a chloroform
solution layered with hexane. 31P{1H} NMR (121.5 MHz,
1
CDCl3, δ): 5.2 (t, JPtP ) 3648 Hz, PPh2). H NMR (300 MHz,
CDCl3, δ): 8.19 (br, 4H, C6H5), 7.56-6.98 (m, 16H, C6H5), 1.34
(m, 4H, Cy H2 + CH2CH3), 0.92 (br m, 2H, Cy H2), 0.85 (m,
2H, CH2CH3), 0.71 (br m, 4H, Cy H2), 0.13 (t, JPH ) 7.2 Hz,
CH2CH3). 13C{1H} NMR (125.65 MHz, CDCl3, δ): 156-129 (m,
C6H5), 36.2 (s, Cy CH2), 29.1 (s, Cy CH2), 26.1 (t, JPC ) 5.2 Hz,
CH2CH3), 14.8 (s, CH2CH3). Anal. Calcd for C36H38Cl2P2Pt: C,
54.14; H, 4.80. Found: C, 54.42; H, 5.03. [R]D ) -28.92° (c
1.0, CHCl3).
Reaction of λ-[{1,4-bis(diphenylphosphino)-1,2,3,4-tet-
ramethyl-1,3-butadiene}PtCl2] with (S,S)-DPEN. A dichlo-
romethane solution of (S,S)-1,2-diphenylethylenediamine (0.071
g, 0.34 mmol in 5 mL) was added to a stirred solution of
enantiopure λ-[(Me4-NUPHOS)PtCl2] (0.25 g, 0.34 mmol) in
dichloromethane. After 5 min the reaction mixture was
filtered, the solvent removed, and the residue crystallized from
a dichloromethane solution layered with hexane to give 4a as
a 1:1 mixture of diastereoisomers in 72% yield (0.231 g).
Although it was not possible to grow crystals of the chloride
salt of 4a suitable for X-ray analysis, crystals of the corre-
sponding perchlorate salt, prepared by reaction of λ-4a with
NaClO4 in methanol, were obtained by slow diffusion of hexane
into a concentrated dichloromethane solution at room tem-
perature, again as a 1:1 mixture of diastereoisomers. 31P{1H}
NMR (121.5 MHz, CDCl3, δ): -1.5 (t, JPtP ) 3449 Hz, PPh2),
-1.6 (t, JPtP ) 3388 Hz, PPh2). 1H NMR (500.13 MHz, CDCl3,
δ): 7.76-7.06 (m, 30H, C6H5), 5.10 (d br, J ) 6.0 Hz, 2H,
CHPh), 4.91 (br, 4H, NHaHb + CHPh), 4.33 (br d, J ) 9.4 Hz,
2H, NHaHb), 3.16 (br, 2H, NHaHb), 2.64 (br d, J ) 9.6 Hz, 2H,
NHaHb), 1.78 (d, J ) 12.0 Hz, 3H, CH3), 1.63 (d, J ) 12.0 Hz,
3H, CH3), 1.11 (s, 6H, CH3), 1.09 (s, 6H, CH3). 13C{1H} NMR
(125.65 MHz, CDCl3, δ): 152.8 (d, J ) 7.8 Hz, C6H5), 152.6 (d,
J ) 7.9 Hz, C6H5), 136.0-125.4 (m, C6H5), 64.8 (s, CHPh), 63.4
(s, CHPh), 20.4 (d, J ) 3.6 Hz, CH3), 202. (d, J ) 3.6 Hz, CH3),
18.1 (d, J ) 5.0 Hz, CH3), 10.0 (d, J ) 5.0 Hz, CH3). Anal.
Calcd for C46H48Cl2N2O8P2Pt: C, 50.93; H, 4.46; N, 2.58.
Found: C, 51.22; H, 4.51; N, 2.73.
General Procedure for Platinum-Catalyzed Enantio-
selective Carbonyl-Ene Reactions with Catalyst Pre-
cursors λ-3a and δ-3b,c in 1-Ethyl-2-methylimidazolium
Bis(trifluoromethanesulfonimide), [emim][NTf2]: Method
B. In a typical procedure, a solution of [{(S)-BINAP)PtCl2]
(0.0111 g, 0.0125 mmol) in dichloromethane (2 mL) was treated
with silver trifluoromethanesulfonate (0.0064 g, 0.025 mmol)
or silver hexafluoroantimonate (0.0086 g, 0.025 mmol) and
stirred for 30 min until a precipitate of silver chloride had
formed. The resulting catalyst solution was filtered to remove
AgCl, [emim][NTf2] (2 mL) added, and the dichloromethane
removed under vacuum. The resulting solution was cooled to
0 °C, and freshly distilled carbonyl substrate (0.5 mmol) and
olefin (0.25 mmol) were added. The reaction mixture was
stirred for a further 2 h, after which the solution was filtered
through a short plug of silica with ethyl acetate, the solvent
removed, and the resulting residue purified by column chro-
General Procedure for Platinum-Catalyzed Enantio-
selective Carbonyl-Ene Reactions with Catalyst Pre-
cursors λ-2a and δ-2b,c: Method A. A flame-dried Schlenk
flask charged with δ-2b (0.0154 g, 0.0127 mmol) was cooled
to 0 °C, and dichloromethane was added (2 mL). The resulting
solution was treated with trifluoromethanesulfonic acid (2.1
µL, 0.024 mmol) to give an immediate color change from deep
red-orange to near colorless. After the mixture was stirred for
1
matography over silica gel. The product was analyzed by H
(38) (a) Sekiguti, T.; Iizuka, Y.; Takizawa, S.; Jayaprakash, D.; Arai,
T.; Sasai, H. Org. Lett. 2003, 5, 2617. (b) Pandiaraju, S.; Chen, G.;
Lough, A.; Yudin, A. K. J. Am. Chem. Soc. 2001, 123, 3850.

Platinum Complexes of NUPHOS Diphosphines
Organometallics, Vol. 24, No. 24, 2005 5955
Table 6. Summary of Crystal Data and Structure
Determination Details for Compounds 3c and 4a
NMR spectroscopy, and the enantiomeric excess was deter-
mined by either chiral GC or HPLC, as described in the
Supporting Information.
3c
36H38Cl2P2Pt‚ C46H50NP2Pt‚
0.5CH2Cl2 2CH2Cl2‚2ClO4
4a
General Procedure for Platinum-Catalyzed Enantio-
selective Carbonyl-Ene Reactions Between Ethyl Gly-
oxylate and Methylenecyclohexane with Catalyst Pre-
cursors λ-3a and δ-3b,c in the Presence of Additives. In
a typical procedure a solution of δ-[{1,4-bis(diphenylphos-
phino)-1,2,3,4-tetraphenyl-1,3-butadiene}PtCl2] (δ-3b; 0.0123
g, 0.0125 mmol) in dichloromethane (2 mL) was treated with
silver trifluoromethanesulfonate (0.0064 g, 0.025 mmol) or
silver hexafluoroantimonate (0.0086 g, 0.025 mmol) and stirred
for 30 min, until a precipitate of silver chloride had formed.
After the mixture was filtered and stirred for 5 min at 0 °C,
(S)-BINOL (0.0036 g, 0.0125 mmol), freshly distilled ethyl
glyoxylate (0.0670 mL, 0.5 mmol), and methylenecyclohexane
(0.030 mL, 0.25 mmol) were added. The resulting mixture was
warmed to room temperature and stirred for a further 2 h,
after which time the solution was filtered through a short plug
of silica with ethyl acetate, the solvent removed, and the
residue purified by column chromatography over silica gel (60-
200 mesh, 20% ethyl acetate/80% hexane). The product was
analyzed by 1H NMR spectroscopy and either chiral GC or
HPLC, as described in the Supporting Information.
Ionic Liquid Recycling Experiments. Catalyst mixtures
were prepared from the BINOL-ate precursors λ-2a and δ-2b,c,
as described above in [emim][NTf2]. After 2 h the reaction
mixture was extracted with diethyl ether (3 × 5 mL), the
remaining ionic liquid solution flushed with inert gas and
charged with further portions of ethyl glyoxylate (0.0670 mL,
0.5 mmol) and methylenecyclohexane (0.030 mL, 0.25 mmol),
and the mixture stirred for a further 2 h. The solution was
again extracted using diethyl ether, filtered through a short
plug of silica, and purified by column chromatography over
silica gel (60-200 mesh, 10% ethyl acetate/90% hexane). The
product was analyzed by 1H NMR spectroscopy and either
chiral GC or HPLC as described above.
formula
C
Mr
841.06
orange
0.45 × 0.34 ×
0.24
1254.65
colorless
0.2 × 0.2 ×
0.2
cryst color
cryst size (mm)
temp, K
cryst syst
space group
a, Å
293(2)
monoclinic
P21/n
14.289(5)
13.482(5)
19.369(8)
90
100.15(2)
90
3673(2)
4
153(2)
monoclinic
P21
11.647(3)
23.066(5)
19.835(4)
90
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
102.196(4)
90
5208.3(19)
4
Z
Dcalcd (g cm-3
F(000)
)
1.5
1668
10.149
57.50
4973
4235
0.0685
398
0.0587
0.0582
1.127
1.6
2512
µ(Mo KR) (mm-1
θmax, deg
)
3.116
23.30
no. of rflns measd
no. of unique rflns
Rint (on F2)
14 838
12 403
0.0849
1215
no. of params
Ra (F2 > 2σ(F2))
Rwb (all data)
0.0413
0.0903
0.976
GOFc (S)
max, min diff map (e Å-3
)
1.828, -1.346
1.424, -0.815
a Conventional R ) ∑||Fo| - |Fc||/∑|Fo| for “observed” reflections
2
2
2
having Fo > 2σ(Fo2). b Rw ) [∑w(Fo - Fc )2/∑w(Fo2)2]1/2 for all
data. c GOF ) [∑w(Fo2-Fc )2/((no. of unique rflns) - (no. of
2
params))]1/2
.
Cambridge Crystallographic Data Center comprises relevant
tables of atomic coordinates, bond lengths and angles, and
thermal parameters.
Crystal Structure Determinations of 3c and 4a. Data
were collected on a Bruker-AXS SMART diffractometer at 293
K for 3c and 153 K for 4a using SAINT-NT software39 with
graphite-monochromated Mo KR radiation. Selected crystal
data are given in Table 6. Crystal stabilities were monitored
via re-collection of the first set of frames. There were no
significant variations (<1%). Lorentz and polarization correc-
tions were applied. The structures were solved using direct
methods and refined with the SHELXTL program package,40
and the non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealized
positions, and a riding model was used for subsequent refine-
Acknowledgment. We gratefully acknowledge the
EPRSC (P.G.) and CenTACat (H.-K.L., R.K.R.) for
funding and Johnson Matthey for generous loans of
palladium salts.
Supporting Information Available: Full details of ex-
perimental procedures, characterization data, and representa-
tive GC and HPLC traces for the ene products and for 3c and
4a details of crystal data, structure solution, and refinement,
atomic coordinates, bond distances, bond angles, and aniso-
tropic thermal parameters in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
Observed and calculated structure factor tables are available
from the authors upon request.
2
2
ments. The function minimized was ∑[w(|Fo| - |Fc| )] with
2
reflection weights w-1 ) [σ2 |Fo| + (g1P)2 + (g2P)], where P )
2
2
[max |Fo| + 2|Fc| ]/3. Additional material available from the
(39) SAINT-NT, Program for Data Collection and Data Reduction;
Bruker-AXS, Madison, WI, 1998.
(40) Sheldrick, G. M. SHELXTL Version 5.0, A System for Structure
Solution and Refinement; Bruker-AXS, Madison, WI, 1998.
OM0505716