Chiral pyridylimidazolines: synthesis, arene ruthenium complexes and application in asymmetric catalysis

Article abstract of DOI:10.1039/b103175a

Condensation of (1S,2S)-1,2-diphenylethylenediamine and 2-cyanopyridine gives the chiral pyridylimidazoline (L¹), deprotonation followed by treatment with methyl iodide gives an NMe derivative (L²). The pyridylimidazolines react with [RuCl2(mes)]2 (mes = 1,3,5-trimethylbenzene) in the presence of NaSbF6 to give [RuCl(L)(mes)][SbF6] (1, 2) which have been characterised by X-ray crystallography. After treatment with AgSbF6 both complexes are enantioselective catalysts for the Diels-Alder reaction of methacrolein and cyclopentadiene.

Full text of DOI:10.1039/b103175a

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Chiral pyridylimidazolines: synthesis, arene ruthenium complexes
and application in asymmetric catalysis
Adam J. Davenport, David L. Davies,* John Fawcett and David R. Russell
1
Department of Chemistry, University of Leicester, Leicester, UK LE1 7RH
Received (in Cambridge, UK) 6th April 2001, Accepted 25th May 2001
First published as an Advance Article on the web 13th June 2001
Condensation of (1S,2S)-1,2-diphenylethylenediamine and 2-cyanopyridine gives the chiral pyridylimidazoline
(L1), deprotonation followed by treatment with methyl iodide gives an NMe derivative (L2). The pyridylimidazolines
react with [RuCl2(mes)]2 (mes = 1,3,5-trimethylbenzene) in the presence of NaSbF6 to give [RuCl(L)(mes)][SbF6]
(1,2) which have been characterised by X-ray crystallography. After treatment with AgSbF6 both complexes are
enantioselective catalysts for the Diels–Alder reaction of methacrolein and cyclopentadiene.
Over the last decade, chiral oxazoline (dihydrooxazole) contain-
ing ligands have been extensively studied and have proved
extremely successful in inducing high enantioselectivity in a
wide range of reactions.1 Imidazolines (dihydroimidazoles) are
five-membered heterocycles analogous to oxazolines (Fig. 1).
Since both heterocycles coordinate through the imine, if the
substituent (R1) is the same they should have similar steric
requirements. However, electronically, imidazolines should be
somewhat more electron rich due to the decreased electro-
negativity of nitrogen relative to oxygen. In addition, changing
the substituent (R2) may alter the electronic properties whilst
leaving the steric requirements almost unaltered, particularly if
there is delocalisation through the amidine (NCN) fragment.
Imidazolines, therefore, are ideal ligands to probe electronic
effects on enantioselectivity.2
Fig. 1
Imidazolines have received comparatively little study as
ligands, notably an arene ruthenium complex has been reported
recently.3 To date, only two metal complexes have been struc-
turally characterised4 and no chiral imidazoline complexes have
ever been isolated and characterised. The synthesis of chiral
imidazolines and their use in controlling diastereoselectivity for
organic reactions has been reported.5 More recently, a bidentate
thioimidazoline ligand has been reported to give high enantio-
meric excess in the palladium catalysed allylic alkylation though
the catalyst was only prepared in situ.6
Scheme 1 (i) Reflux in MeOH with NaOMe (cat); (ii) (1S,2S)-
H2NCH(Ph)CH(Ph)NH2; (iii) LiNiPr2; (iv) MeI.
and two diastereomers are possible differing in configuration
at the metal. Signals due to the pairs of diastereomers can
readily be distinguished by 1H NMR, particularly those due to
mesitylene or the pyridine-6-H, and hence the diastereomer
ratio is easily determined by integration. The 1H NMR spectra
of crude reaction mixtures containing 1 or 2 both contain two
signals for the C6H3Me3 protons, i.e. singlets at δ 5.40 and 5.52
for 1 and δ 5.43 and 5.51 for 2, with diastereomer ratios 53 : 47
and 47 : 53, respectively. The isomer ratios did not change over
a month in d6-acetone, indicating that either the ruthenium
configurations are stable under these conditions or that the
complexes have already reached the equilibrium diastereomer
ratio and the rate of epimerisation is much slower than that of
the NMR timescale (see below). Careful recrystallisation
from mixtures of acetone–ether gave crystals suitable for X-ray
diffraction. The X-ray structures of the cations with selected
bond distances and angles are shown in Figs. 2 and 3. In each
case only one diastereomer is present in the crystal. The
ruthenium atoms have a pseudooctahedral geometry with the
arene occupying three adjacent sites of the octahedron. The
imidazolines are coordinated such that the phenyl substituent
adjacent to the imine nitrogen [C(8) in 1, C(9) in 2] is on the
same side as the chloride rather than the arene; as found
in related pyridyloxazoline complexes.7,8 The Ru–N(1) and Ru–
Results and discussion
Ligand L1 was prepared from 1,2-diphenylethylenediamine and
2-cyanopyridine according to Scheme 1. The 1H NMR spectrum
of L1 at room temperature shows three broad singlets for the
imidazoline ring; cooling to 243 K causes the signals to sharpen
into two well resolved doublets at δ 4.86 and 5.13 due to the
CHPh groups and a singlet at δ 6.62 due to the NH. Solvent
assisted tautomerism has been observed previously for imidazo-
lines.5 Ligand L2 was prepared by deprotonation of L1 followed
by treatment with methyl iodide. The 1H NMR spectrum of L2
contained a singlet at δ 2.99 (3H, NMe) and two well resolved
doublets at δ 4.36 (CHaPh) and δ 4.99 (CHbPh) indicating that
tautomerism is not occurring, as expected after substituting
hydrogen by methyl. The mass spectra of L1 and L2 each show
peaks due to the molecular ion [M ϩ H]ϩ at m/z 300 and 314
respectively.
Treating the ligands with [RuCl2(η6-mes)]2 in refluxing meth-
anol in the presence of NaSbF6 gives [RuCl(L)(η6-mes)]SbF6
(1, 2) in good yield. The complexes contain a chiral metal centre
1500
J. Chem. Soc., Perkin Trans. 1, 2001, 1500–1503
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103175a

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Table 1 Enantioselective Diels–Alder reaction of methacrolein or
2-bromoacrolein with cyclopentadiene in dichloromethane using
catalysts prepared from [RuCl(L)(mes)]ϩ
Catalyst
Dienophile
t/h
Yield (%)a
(exo : endo)
Eeb
1
2
3
1
2
Methacrolein
Methacrolein
Methacrolein
2-Bromoacrolein
2-Bromoacrolein
72
72
72
48
48
90
35
71
66
68
94 : 6
93 : 7
94 : 6
84 : 16
91 : 9
45
31
58
17
26
a Isolated yield. b Enantiomeric excess of the major exo product.
Fig. 2 Molecular structure and atom numbering scheme for the cation
of 1. Selected bond distances (Å) and angles (Њ): Ru–N(1) 2.115(7),
Ru–N(2) 2.097(7), Ru–Cl 2.420(3), N(1)–C(1) 1.348(11), N(1)–C(5)
1.355(11), C(5)–C(6) 1.456(12), N(2)–C(6) 1.301(11), N(2)–C(8)
1.491(11), N(3)–C(6) 1.334(11), N(3)–C(7) 1.464(11), N(2)–Ru–N(1)
76.5(3), N(1)–Ru–Cl 80.5(2), N(2)–Ru–Cl 86.4(2).
is consistent with the increased electron donor properties of
the imidazoline providing greater stabilisation of the presumed
16-electron intermediate than with the oxazoline.
In recent years there has been much interest in using chiral
transition-metal-based Lewis acids as catalysts for Diels–
Alder reactions.9,10 Treatment of 1 or 2 with AgSbF6 generates
dications [Ru(solvent)(L)(mes)]2ϩ which can catalyse the Diels–
Alder reaction between methacrolein (methacrylaldehyde) or
2-bromoacrolein (2-bromoacrylaldehyde) and cyclopentadiene.
Results for these and the related cation 38 are shown in Table 1.
In the reaction with methacrolein the major product was
identified as (1R,2S,4R)-2-methylbicyclo[2.2.1]hept-5-ene-2-
carbaldehyde, by comparison of the sign of the optical rotation
and the GC behaviour of the acetal formed from (2R,4R)-
pentane-2,4-diol with literature values.11 The absolute configur-
ation of the major exo product is consistent with the phenyl
shielding the Si face of the coordinated methacrolein leading to
attack of cyclopentadiene at the Re face as we have discussed
previously for the ruthenium pyridyloxazoline complexes.8
Complex 2 has the most electron-donating chelate ligand and
is therefore the least Lewis acidic, consistent with this, it has the
lowest activity and selectivity for the reaction between metha-
crolein and cyclopentadiene. Complex 1 also gives slightly
reduced enantioselectivity compared to the oxazoline complex
3. In the reaction between 2-bromoacrolein and cyclopenta-
diene, complex 2 is more selective than 1, giving a higher
exo : endo ratio and increased enantioselectivity. (Pyridyl-
oxazoline complexes (e.g. 3) are not catalysts for this reaction).8
A weaker Lewis acid may be advantageous with the more
reactive dienophile 2-bromoacrolein, indeed bromine abstrac-
tion has been reported in a strongly Lewis acidic cyclopenta-
dienyl ruthenium complex.10a In addition, the greater reactivity
means that for complex 1 some of the product may arise from
the thermal reaction which gives an exo : endo ratio of 80 : 20.
In summary, we have synthesised new chiral pyridylimidazo-
lines and have shown that, as expected, their steric requirements
are very similar to the corresponding pyridyloxazolines. Whilst
the enantioselectivities reported here for 1 and 2 are only
modest, they demonstrate that imidazolines can be used in
place of oxazolines as chiral directing groups and that
electronic tuning of Lewis acidity and enantioselectivity is
feasible with such ligands.
Fig. 3 Molecular structure and atom numbering scheme for the cation
of 2. Selected bond distances (Å) and angles (Њ): Ru–N(1) 2.091(6),
Ru–N(2) 2.094(6), Ru–Cl 2.412(2), N(1)–C(1) 1.341(9), N(1)–C(5)
1.373(10), C(5)–C(6) 1.468(10), N(2)–C(6) 1.311(9), N(2)–C(9)
1.500(9), N(3)–C(6) 1.344(9), N(3)–C(8) 1.469(10), N(2)–Ru–N(1)
76.4(2), N(1)–Ru–Cl 81.3(2), N(2)–Ru–Cl 85.5(2).
N(2) distances, to pyridine and imine respectively, are the same
within each complex, and are also statistically the same as the
corresponding distances [2.104(5) and 2.105(5) Å] in the related
cation [RuCl(Ph-pymox)(mes)]ϩ (3) [Ph-pymox = 2-(4-phenyl-
oxazolin-2-yl)pyridine].8 The Ru–Cl distances are 2.420(3) and
2.412(2) Å, and the chelate bite angles 76.5(3) and 76.4(2)Њ, for
1 and 2 respectively, compared with 2.403(2) Å and 76.4(2)Њ
in 3.8 Thus, as described above, the steric requirements of the
imidazoline ligands are very similar to the related oxazolines
and the substituent on C(7) (or C(8) in 2) doesn’t seem to have
much effect on the geometry at the metal. In each of 1 and 2,
the N(3)–C(6) distance is statistically the same as the N(1)–C(1)
and N(1)–C(5) distances in the pyridine ring, being only slightly
longer than the formal double bond N(2)–C(6) distance and
considerably shorter than the formal single bonds N(2)–C(8) in
(1) and N(2)–C(9) in (2); implying a degree of delocalisation
across the amidine N(2)–C(6)–N(3) fragment. In 2, the sum of
the angles around N(3) is 358.3Њ which is also consistent with
delocalisation across the amidine.
In each case, dissolution of crystals of 1 or 2 in CD2Cl2 at
low temperature and recording the 1H NMR spectrum showed
a single diastereomer which is assumed to be the same as
that found in the solid state. In acetone solution the single
diastereomers undergo epimerisation at the metal reaching
equilibrium (ca. 45 : 55 mixture of diastereomers) over a period
of 1–2 days at room temperature compared with about 40 days
at 40 ЊC for cation 3.8 The increased rate of epimerisation
J. Chem. Soc., Perkin Trans. 1, 2001, 1500–1503
1501

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[RuCl(L1)(mes)][SbF6] 1
Experimental
A solution of L1 (38 mg, 0.126 mmol) and NaSbF6 (33 mg,
0.126 mmol) in MeOH (10 cm3) was added to [RuCl2(mes)]2
(37 mg, 0.0631 mmol) and the resulting suspension was heated
to reflux for two hours. An orange–brown coloured solution
was obtained, which was evaporated and the crude residue
was dissolved in CH2Cl2. Filtration through Celite gave a red
solution, which was evaporated to afford 1 (99 mg, 99%). Calc.
for C29H29ClF6N3RuSbؒH2O: C, 43.01; H, 3.86; N, 5.19. Found:
C, 42.78; H, 3.34; N, 4.91%. (Note, although no H2O is found in
the X-ray structure, NMR samples often showed the presence
of water.) 1H NMR (300 MHz, d6-acetone) δ 2.08[2.09] (s, 9H,
C6Me3), 5.16[5.10] (d, 1H, J 11.5[6.5], CHaPh), 5.40[5.52]
(s, 3H, C6Me3), 5.77[5.19] (d, 1H, J 11.5[6.5], CHbPh), 7.35–
7.47* (m, 10H, Ph), 7.95* (t, 1H, J 6, py-H), 8.37* (m, 2H,
py-H), 9.02[9.22] (br s, 1H, NH), 9.44[9.64] (d, 1H, J 5.5, py
6-H). Signals for the solid state isomer are shown first with
the other isomer in square brackets, * indicates that the signal
for the second isomer is coincident with the first. MS (ESϩ) m/z
556 [M]ϩ.
Petroleum ether and diethyl ether were dried by refluxing over
purple sodium–benzophenone under nitrogen, whilst dichloro-
methane was purified by refluxing over calcium hydride and
acetone from calcium sulfate. The reactions described were
carried out under nitrogen; however, once isolated as pure
solids the compounds are air-stable and precautions for their
storage are unnecessary. 1H NMR spectra were obtained using
Bruker 300 or 400 MHz spectrometers in CD2Cl2 unless stated
otherwise, chemical shifts were recorded in ppm (referenced
to tetramethylsilane or residual protons in the NMR solvent)
and J values are given in Hz. FAB mass spectra were obtained
on a Kratos concept mass spectrometer using a 3-nitrobenzyl
alcohol (NOBA) matrix, electrospray mass spectra were
obtained on a Micromass Quattro LC in MeOH or MeCN.
Microanalyses were performed by Butterworth laboratories
Ltd., Middlesex. Polarimetric measurements were made on a
Perkin Elmer 341 instrument at ambient temperature at 589
nm, concentration in g per 100 cm3 solution and are given in
13
units of 10Ϫ1 deg cm2 gϪ1. 2-Bromoacrolein,12 [RuCl2(mes)]2,
pyridine-2-carboximidate14 and (1S,2S)-1,2-diphenylethylene-
diamine15 were prepared using literature procedures; on the
basis of the optical rotation the diamine was at least 97%
optically pure.
[RuCl(L2)(mes)][SbF6] 2
A solution of L2 (99 mg, 0.316 mmol) and NaSbF6 (82 mg,
0.316 mmol) in MeOH (10 cm3) was added to [RuCl2(mes)]2
(92 mg, 0158 mmol) and the resulting suspension was heated
to reflux for two hours. An orange–brown coloured solution
was obtained, which was evaporated and the crude residue
was dissolved in CH2Cl2. Filtration through Celite gave a red
solution, which was evaporated to afford 2 (247 mg, 97%). Calc.
for C30H31ClF6N3RuSbؒH2O: C, 43.74; H, 4.03; N, 5.10. Found:
C, 43.73; H, 3.80; N, 4.59%. (Note, although no H2O is found in
the X-ray structure, NMR samples often showed the presence
of water.) 1H NMR (400 MHz, d6-acetone) δ 2.07[2.10] (s, 9H,
C6Me3), 3.38[3.35] (s, 3H, NMe), 5.00[4.97] (d, 1H, J 12[7.5],
CHaPh), 5.43[5.51] (s, 3H, C6Me3), 5.69[5.10] (d, 1H, J 12[8],
CHbPh), 7.40–7.67* (m, 10H, Ph), 7.96* (ddd, 1H, J 1, 5.5, 7,
py-H), 8.35[8.37] (dt, 1H, J 1.5, 8, py-H), 8.60[8.58] (dt, 1H,
J 1.5, 8, py-H), 9.49[9.70] (ddd, 1H, J 0.5, 1.5, 5.5, py 6-H).
Signals for the solid state isomer are shown first with the other
isomer in square brackets, * indicates that the signal for the
second isomer is coincident with the first. MS (ESϩ) m/z 570
[M]ϩ.
Preparation of L1
A mixture of pyridine-2-carboximidate (303 mg, 2.22 mmol),
(1S,2S)-1,2-diphenylethylenediamine (471 mg, 2.22 mmol) and
CHCl3 (1 cm3) was stirred overnight at 60 ЊC. The resulting pale
yellow paste was evaporated, dissolved in CH2Cl2 and then
washed with three 15 cm3 portions of water. The aqueous layers
were extracted with dichloromethane (40 cm3) and the com-
bined organic layers were dried over MgSO4 and evaporated to
give an off-white solid. Recrystallisation from CH2Cl2–hexane
afforded a white crystalline solid. Yield = 627 mg (85%). Calc.
for C20H17N3: C, 80.24; H, 5.72; N, 14.04. Found: C, 80.06; H,
1
5.84; N, 14.54%. H NMR (400 MHz, CDCl3, 243 K) δ 4.86
(dd, 1H, J 1, 9, CHaPh), 5.13 (d, 1H, J 9, CHbPh), 6.62 (br s,
1H, NH), 7.36 (m, 10H, 2 × Ph), 7.48 (ddd, 1H, J 1, 5, 7.5, py
5-H), 7.88 (dt, 1H, J 2, 7.5, py 4-H), 8.34 (td, 1H, J 1, 8, py
3-H), 8.66 (ddd, 1H, J 1, 2, 5, py 6-H). FAB-MS m/z 300
[M + H]ϩ. [α] Ϫ66 (c = 0.100, CH2Cl2).
Crystal structure determinations
Preparation of L2
Crystal data for 1. C29H29ClF6N3RuSb, M = 791.82, ortho-
rhombic, space group P212121, a = 8.159(3), b = 17.370(5),
To a dry degassed colourless solution of L1 (319 mg, 1.06
mmol) in THF (24 ml) at 243 K was added a cyclohexane
solution of LDA (1.5 M, 2.13 cm3, 2.44 mmol), the solution
immediately turned dark purple. After stirring for four hours
dry MeI (73 µl, 1.17 mmol) was added and the reaction mixture
allowed to slowly warm to room temperature over two hours,
which was accompanied by a colour change to pale yellow–
brown. Evaporation gave a brown crude residue which was
dissolved in CH2Cl2 and then washed with three 20 cm3
portions of water. The aqueous layers were extracted with
dichloromethane (40 cm3) and the combined organic layers
were dried over MgSO4 and evaporated to give a pale brown
oily residue. The oil was chromatographed on silica, with
CHCl3–MeOH–NEt3 (80 : 15 : 5) as eluent. Evaporation
of the fore-run gave an oil; recrystallisation from various
laboratory solvents failed to give a solid product, however
the resultant pale brown oil, L2 (247 mg, 74%) was pure
by 1H spectroscopy. 1H NMR (300 MHz, CDCl3) δ 2.99
(s, 3H, NMe), 4.36 (d, 1H, J 10.5, CHaPh), 4.99 (d, 1H,
J 10.5, CHbPh), 7.32 (m, 11H, 2 × Ph + py 5-H), 7.81 (dt, 1H,
J 2, 8, py 4-H), 8.10 (td, 1H, J 1, 7.5, py 3-H), 8.71 (ddd, 1H, J
1, 2, 5, py 6-H). FAB-MS m/z 314 [M ϩ H]ϩ; the high reso-
lution spectrum of this ion was consistent with the proposed
formulation.
c = 21.182(7) Å, U = 3002.0(17) Å3, Z = 4, Dc = 1.752 g cmϪ3
,
µ = 1.553 mmϪ1
,
F(000) = 1560, graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Data collected on a Siemens
P4 diffractometer at 200 K. 4137 reflections collected with
1.9 < θ < 27.0Њ, 4051 unique (Rint = 0.0192). Final R1[F 2 >
2σ(F 2)] = 0.047, wR2 = 0.106 for all data. Flack parameter
Ϫ0.07(5).
Crystal data for 2. C30H31ClF6N3RuSb, M = 805.85, ortho-
rhombic, space group P212121, a = 8.566(3), b = 17.569(8),
c = 20.345(5) Å, U = 3062(2) Å3, Z = 4, Dc = 1.748 g cmϪ3
,
µ = 1.525 mmϪ1
,
F(000) = 1592, graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Data collected on a Siemens
P4 diffractometer at 200 K. 4130 reflections collected with
2.0 < θ < 27.0Њ, 4049 unique (Rint = 0.0361). Final R1 [F 2 >
2σ(F 2)] = 0.046, wR2 = 0.113 for all data. Flack parameter
Ϫ0.03(4). In both cases an absorption correction based on
psi-scan data was applied and the structures were solved by
Patterson methods and refined using full matrix least squares
on F 2 (SHELXL96).16 Anisotropic displacement parameters
used for all atoms, hydrogens included in calculated positions
(C–H 0.96 Å), with isotropic displacement parameters set to 1.2
Ueq(C).
1502
J. Chem. Soc., Perkin Trans. 1, 2001, 1500–1503

View Article Online
A. Schnyder, L. Hintermann and A. Togni, Angew. Chem., Int. Ed.
Engl., 1995, 34, 931.
3 B. Cetinkaya, B. Alici, I. Ozdemir, C. Bruneau and P. Dixneuf,
J. Organomet. Chem., 1999, 575, 187.
CCDC reference numbers 164495 (1) and 164496 (2). See
http://www.rsc.org/suppdata/p1/b1/b103175a/ for crystallo-
graphic files in .cif or other electronic format.
4 B. Cetinkaya, E. Cetinkaya, P. B. Hitchcock, M. F. Lappert and
I. Ozdemir, J. Chem. Soc., Dalton Trans., 1997, 1359; C.
Navarro-Ranninger, F. Zamora, I. Lopez-Solera, A. Monge and
J. R. Masaguer, J. Organomet. Chem., 1996, 506, 149.
5 C. Dauwe and J. Buddrus, Synthesis, 1995, 171; P. I. Dalko and
Y. Langlois, Chem. Commun., 1998, 331; P. I. Dalko and Y. Langlois,
J. Org. Chem., 1998, 63, 8107 and refs. therein.
6 T. Morimoto, K. Tachibana and K. Achiwa, Synlett, 1997, 783.
7 D. L. Davies, J. Fawcett, S. A. Garratt and D. R. Russell, Chem.
Commun., 1997, 1351.
8 A. J. Davenport, D. L. Davies, J. Fawcett, S. A. Garratt and D. R.
Russell, J. Chem. Soc., Dalton Trans., 2000, 4432.
9 For reviews see for example, D. Carmona, M. P. Lamata and L. A.
Oro, Coord. Chem. Rev., 2000, 200–202, 717; L. C. Dias, J. Braz.
Chem. Soc., 1997, 8, 289.
10 For specific examples catalysed by ruthenium complexes see, (a) E. P.
Kundig, C. M. Saudan and G. Bernardinelli, Angew. Chem., Int. Ed.,
1999, 38, 1220; (b) J. Faller and J. Parr, Organometallics, 2000, 19,
1829; (c) D. Carmona, C. Cativiela, S. Elipe, F. J. Lahoz, M. P.
Lamata, M. P. Lopez-Ram de Viu, L. A. Oro, C. Vega and F. Viguri,
Chem. Commun., 1997, 2351; (d ) D. Carmona, C. Vega, F. J. Lahoz,
S. Elipe, L. A. Oro, P. M. Lamata, F. Viguri, R. Garcia-Correas, C.
Cativiela and M. P. Lopez-Ram de Viu, Organometallics, 1999, 18,
3364.
11 D. A. Evans, J. A. Murry, P. v. Matt, R. D. Norcross and S. J. Miller,
Angew. Chem., Int. Ed. Engl., 1995, 34, 798.
12 R. H. Baker, S. W. Tinsley, D. Butler and B. Riegel, J. Am. Chem.
Soc., 1950, 72, 393.
Catalysis
The relevant complex (1, 2, or 3) (0.02 mmol) and one equiv-
alent of AgSbF6 were stirred in CH2Cl2 : acetone (7 : 1) (8 cm3)
protected from the light for 1 h. After this time the mixture
was filtered through Celite to remove AgCl, the solvent was
removed in vacuo and the residue was dissolved in CH2Cl2
(2 cm3) to provide a yellow–orange solution of the catalyst.
Methacrolein or 2-bromoacrolein (1 mmol) and 2,6-di-tert-
butylpyridine17 (0.02 mmol) were added to this solution
which was cooled to 0 ЊC before addition of cyclopentadiene
(2 mmol). At the end of the reaction, the mixture was passed
through a silica plug (to remove catalyst), the solvent was
removed and the product was obtained as a colourless oil. The
exo : endo ratio was determined by NMR spectroscopy. The
enantiomeric excess was determined by NMR or GC after con-
version to the acetal with (2R,4R)-pentane-2,4-diol, according
to the method of Evans et al.11
Acknowledgements
We thank the EPSRC (AJD) for a studentship and Johnson
Matthey for a loan of RuCl3.
13 J. W. Hull, Jr. and W. L. Gladfelter, Organometallics, 1984, 3,
605.
14 F. G. Schaefer and G. A. Peters, J. Org. Chem., 1961, 26, 412.
15 S. Pikul and E. Corey, Org. Synth., 1993, 71, 22.
16 G. M. Sheldrick, SHELXL 96, University of Göttingen, 1996.
17 The use of 2,6-di-tert-butylpyridine in transition metal catalysed
Diels–Alder reactions was suggested in P. V. Bonnesen, C. L.
Puckett, R. V. Honeychuck and W. H. Hersh, J. Am. Chem. Soc.,
1989, 111, 6070.
References
1 A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron:
Asymmetry, 1998, 9, 1; A. Pfaltz, Acc. Chem. Res., 1993, 26, 339;
A. Pfaltz, Acta Chem. Scand., 1996, 50, 189.
2 Examples of electronic effects on enantioselectivity, H. Nishiyama,
S. Yamaguchi, M. Kondo and K. Itoh, J. Org. Chem., 1992, 57, 4306;
E. Corey and C. Helal, Tetrahedron Lett., 1995, 36, 9153;
J. Chem. Soc., Perkin Trans. 1, 2001, 1500–1503
1503