Palladium complexes of chiral planar 1-phosphino-2-sulfenylferrocenes as efficient catalysts in enantioselective diels-alder reactions

Article abstract of DOI:10.1021/om0493762

The readily available dichloride palladium complexes of chiral planar (R)-1-phosphino-2-sulfenylferrocenes (Fesulphos ligands) act as efficient Lewis acids in the catalytic asymmetric Diels-Alder reaction of cyclopentadiene with acryloyl-1,3-oxazolidin-2-one. Very high enantioselectivities, up to 95% ee, have been reached from Fesulphos ligands having a very bulky substitution at both sulfur and phosphorus coordinating atoms.

Full text of DOI:10.1021/om0493762

Organometallics 2005, 24, 557-561
557
Palladium Complexes of Chiral Planar
1-Phosphino-2-sulfenylferrocenes as Efficient Catalysts
in Enantioselective Diels-Alder Reactions
Olga Garc´ıa Manchen˜o, Ramo´n Go´mez Arraya´s, and Juan Carlos Carretero*
Departamento de Quı´mica Orga´nica, Facultad de Ciencias, Universidad Auto´noma de Madrid,
28049 Madrid, Spain
Received August 11, 2004
The readily available dichloride palladium complexes of chiral planar (R)-1-phosphino-2-
sulfenylferrocenes (Fesulphos ligands) act as efficient Lewis acids in the catalytic asymmetric
Diels-Alder reaction of cyclopentadiene with acryloyl-1,3-oxazolidin-2-one. Very high
enantioselectivities, up to 95% ee, have been reached from Fesulphos ligands having a very
bulky substitution at both sulfur and phosphorus coordinating atoms.
Introduction
reactions6 contrast with the much less studied chiral
thioethers with potential P,S-coordination, despite the
appealing fact that the sulfur atom becomes stereogenic
upon coordination with the transition metal, therefore
imposing a unique asymmetric environment in very
close proximity to the reactive metal center.7 Concerning
the ADA, to the best of our knowledge, there was a
single reported catalyst based on P,S-coordination: the
cationic palladium complexes of chiral phosphino-
oxatiane ligands described by Nakano et al.8
We have recently described a highly tunable family
of P,S-bidentate ligands possessing planar chirality as
the only element of asymmetry: the 1-phosphino-2-
sulfenylferrocenes (Fesulphos, 1).9 This new family of
chiral ligands, especially those having a bulky substitu-
tion at sulfur, have provided very high asymmetric
inductions in some Pd-catalyzed reactions, such as
allylic substitutions9b and the desymmetrization of
oxabenzonorbornadienes with dialkylzinc reagents.10 In
The asymmetric Diels-Alder (ADA) reaction consti-
tutes a fundamental C-C bond forming reaction in
modern organic synthesis. Therefore, the search for new
chiral Lewis acids as catalysts for this transformation
has received a great deal of attention in recent years,1
the reaction of cyclopentadiene with the bidentate
acryloyl oxazolidinone becoming the standard bench
reaction for the evaluation of the efficiency of such
catalysts. Among many catalyst systems identified to
date, the vast majority are based on C2-symmetric chiral
bidentate ligands having O,O- (e.g., BINOL deriva-
tives),2 N,N- (e.g., bis-oxazolines),3 or P,P-coordination
modes (e.g., BINAP).4
As an alternative, mixed donor ligands with strong
electronic differentiation have also provided high asym-
metric inductions, most precedents involving the use of
P,N-bidentate chiral ligands, especially phosphino-
oxazolines.5 However, the many applications of biden-
tate P,N-ligands found in asymmetric metal-catalyzed
(5) For recent references on P,N-ligands in ADA: (a) Nakano, H.;
Okuyama, Y.; Suzuki, Y.; Fujita, R.; Kabuto, C. Chem. Commun. 2002,
1146. (b) Hiroi, K.; Watanabe, K. Tetrahedron: Asymmetry 2002, 13,
1841. (c) Hiroi, K.; Watanabe, K. Tetrahedron: Asymmetry 2001, 12,
3067. (d) Faller, J. W.; Grimmond, B. J. Organometallics 2001, 20,
2454.
* To whom correspondence should be addressed. Fax: +34 91
4973966. E-mail: juancarlos.carretero@uam.es.
(1) For recent reviews, see: (a) Corey, E. J. Angew. Chem., Int. Ed.
2002, 41, 1650. (b) Cycloaddition Reactions in Organic Synthesis;
Kobayashi, S., Jørgensen, K. A., Eds.; Wiley-VCH: New York, 2002.
(c) Carmona, D.; Lamata, M. P.; Oro, L. A. Coord. Chem. Rev. 2000,
200-202, 717. (d) Evans, D. A.; Johnson, J. S. In Comprehensive
Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999; Vol. 3, p 1177. (e) Ku¨ndig, P. E.; Saudan, C.
M. In Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.; Wiley-
VCH: New York, 2000; Vol. 2, p 631.
(6) For a recent review on P,N-ligands in asymmetric catalysis:
Guiry, P. J.; Saunders, C. P. Adv. Synth. Catal. 2004, 346, 497.
(7) For recent references on the use of P,S-ligands in asymmetric
metal-mediated reactions, see: (a) Faller, J. W.; Wilt, J. C.; Parr, J.
Org. Lett. 2004, 6, 1301. (b) Nakamura, S.; Fukuzumi, T.; Toru, T.
Chirality 2004, 10. (c) Evans, D. A.; Michael, F. E.; Tedrow, J. S.;
Campos, K. R. J. Am. Chem. Soc. 2003, 125, 3534. (d) Kanayama, T.;
Yoshida, K.; Miyabe, H.; Takemoto, Y. Angew. Chem., Int. Ed. 2003,
42, 2054. (e) Kanayama, T.; Yoshida, K.; Miyabe, H.; Kimachi, T.;
Takemoto, Y. J. Org. Chem. 2003, 68, 6197. (f) Tu, T.; Zhou, Y.-G.;
Hou, X.-L.; Dai, L.-X.; Dong, X.-C.; Yu, Y.-H.; Sun, J. Organometallics
2003, 22, 1255. (g) Verdaguer, X.; Perica`s, M. A.; Riera, A.; Maestro,
M. A.; Mah´ıa, J. Organometallics 2003, 22, 1868. (h) Gladiali, S.;
Grepioni, F.; Medici, S.; Zucca, A.; Berente, Z.; Kolla´r, L. Eur. J. Inorg.
Chem. 2003, 556. (i) Kang, J.; Lee, J. H.; Im, K. S. J. Mol. Catal. A
2003, 196, 55. (j) Die´guez, M.; Pa`mies, O.; Net, G.; Ruiz, A.; Claver, C.
J. Mol. Catal. A 2002, 185, 11. (k) Enders, D.; Peters, R.; Lochtman,
R.; Raabe, G.; Runsik, J.; Bats, J. W. Eur. J. Org. Chem. 2000, 3399.
For a review on chiral thioether ligands, see: Masdeu-Bulto´, A. M.;
Die´guez, M.; Martin, E.; Go´mez, M. Coord. Chem. Rev. 2003, 242, 159.
(8) Nakano, H.; Suzuki, Y.; Kabuto, C.; Fujita, R.; Hongo, H. J. Org.
Chem. 2002, 67, 5011.
(2) Recent references on O,O-ligands in ADA: (a) Corminboeuf, O.;
Renaud, O. Org. Lett. 2002, 4, 1731. (b) Fukuzawa, S.-i.; Fujimoto, K.;
Komuro, Y.; Matsuzawa, H. Org. Lett. 2002, 4, 707. (c) Breuning, M.;
Corey, E. J. Org. Lett. 2001, 3, 1559.
(3) Recent references on N,N-chiral ligands in ADA: (a) Lassaletta,
J. M.; Alcarazo, M.; Ferna´ndez, R. Chem. Commun. 2004, 298. (b) Zhou,
J.; Tang, Y. Org. Biomol. Chem. 2004, 2, 429. (c) Bolm, C.; Martin, M.;
Simic, O.; Verrucci, M. Org. Lett. 2003, 5, 427. (d) Clariana, J.;
Comelles, J.; Moreno-Man˜as, M.; Vallribera, A. Tetrahedron: Asym-
metry 2002, 13, 1551. (e) De Coster, G.; Vandyck, K.; Van der Eycken,
E.; Van der Eycken, J.; Elseviers, M.; Ro¨per, H. Tetrahedron: Asym-
metry 2002, 13, 1673. (f) Sibi, M. P.; Venkataraman, L.; Liu, M.;
Jasperse, C. P. J. Am. Chem. Soc. 2001, 123, 8444. (g) Fukuzawa, S.-
i.; Matsuzawa, H.; Metoki, K. Synlett 2001, 709.
(4) Recent references on P,P-ligands in ADA: (a) Benincori, T.;
Gladiali, S.; Rizzo, S.; Sannicolo`, F. J. Org. Chem. 2001, 66, 5940. (b)
Ku¨ndig, E. P.; Saudan, C. M.; Viton, F. Adv. Synth. Catal. 2001, 343,
51. (c) Pignat, K.; Vallotto, J.; Pinna, F.; Struckul, G. Organometallics
2000, 19, 5160. (d) Ghosh, A. K.; Matsuda, H. Org. Lett. 1999, 1, 2157.
(9) (a) Priego, J.; Garc´ıa Manchen˜o, O.; Cabrera, S.; Go´mez Arraya´s,
R.; Llamas, T.; Carretero, J. C. Chem. Commun. 2002, 2512. (b) Garc´ıa
Manchen˜o, O.; Priego, J.; Cabrera, S.; Go´mez Arraya´s, R.; Llamas, T.;
Carretero, J. C. J. Org. Chem. 2003, 68, 3679.
10.1021/om0493762 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/13/2005

558 Organometallics, Vol. 24, No. 4, 2005
Manchen˜o et al.
Scheme 1. Preparation of Metal Complexes of
1-Phosphino- and
tural information deduced from this crystallographic
study, it should be noted the following four features: (a)
very similar and nearly planar conformation of the P,S-
five-membered metallacycle is found in all the studied
metal complexes,17 determining a high steric hindrance
of the upper face of the metallacycle due to the up
orientation of the tert-butyl group and one of the phenyl
groups at phosphorus; (b) the slightly distorted tetra-
hedral structure of the Cu(I) complex [θ S-Cu-Cl(1) )
113.6°, θ P-Cu-Cl(1A) ) 124.9°]; (c) the perfect square
planar geometry around the metal center in the case of
the Pd (sum of angles around Pd ) 360.2°) and Pt com-
plexes (sum of angles around Pt ) 360.0°); (d) the great
electronic trans effect exerted by the phosphorus donor
atom, compared to the sulfur moiety, in the case of the
square planar complexes. For instance, in 1a‚PdCl2 and
1a‚PtCl2 the M-Cl bond trans to phosphorus is much
longer (2.346 and 2.352 Å, respectively) than that trans
to sulfur (2.302 and 2.308 Å, respectively).
In Table 1 are summarized the results obtained in
the standard ADA reaction of cyclopentadiene with
acryloyl-1,3-oxazolidin-2-one in the presence of 10 mol
% of the dichloride metal complex and 20 mol % of
AgBF4 as chloride scavenger to promote the in situ
generation of the presumed catalytically active dica-
tionic Lewis acid. The reactions were conducted in
CH2Cl2 as optimal solvent,18 in the presence of molec-
ular sieves,19 and at the lowest temperature to allow
them to reach quantitative conversion.20
1-(Dimethylamino)-2-sulfenylferrocenesa
a (a) CuCl, THF-MeOH, rt. (b) Pd(CH3CN)2Cl2, CH2Cl2, rt.
(c) PtCl2, CH2Cl2, rt.
addition, these ligands have also proved to be very
efficient in the enantioselective Cu-catalyzed reaction
of Danishefsky’s diene with N-sulfonylimines.11 Extend-
ing the interest of Fesulphos in enantioselective cataly-
sis, we report herein that metal complexes of such
ligands, especially their palladium complexes, act as
effective catalysts in the ADA reaction of cyclopentadi-
ene with acryloyl oxazoline.
Results and Discussion
As expected, all ADA reactions were highly endo
selective, although very different asymmetric induction
was observed from these complexes. Thus, while the Pt
complex (entry 2) was the least reactive and enantiose-
lective, providing the Diels-Alder adduct 3 in racemic
Initially, we chose as parent ligand (R)-1-diphen-
ylphosphino-2-tert-butylsulfenylferrocene [(R)-1a]. Due
to the high affinity of sulfur to late transition metals,
we focused our attention on its copper(I), palladium-
(II), and platinum(II) chloride complexes as precursors
of the required cationic Lewis acid catalyst. These metal
complexes were readily prepared in nearly quantitative
yield by straightforward treatment of 1a with CuCl,11
Pd(CH3CN)2Cl2,9b and PtCl2, respectively. For compari-
son purposes, the Pd complex of the related N,S-ligand
(R)-1-(dimethylamino)-2-tert-butylsulfenylferrocene12
(2‚PdCl2) was also prepared following the same proce-
dure (Scheme 1). Interestingly, a single epimer at sulfur
was detected by NMR in the four complexes: the one
placing the bulky tert-butyl group in anti arrangement
with regard to the ferrocene unit [(RP,RS) configura-
tion].13 In addition, the chloride-bridged dimer complex
[1a‚CuCl]2 was obtained as a single isomer, the two
phosphines being located at the same face of the
molecule and the two sulfides at the opposite face with
the tert-butyl groups oriented anti with regard to each
other (Figure 1).
(14) Reported in ref 11. Crystal data for C52H54Cl2Cu2Fe2P2S2‚
CH2Cl2 [1a‚CuCl]2: crystal size 0.38 × 0.36 × 0.34 mm3, Mw ) 1202.95,
trigonal, space group P3(2)21, a ) 17.7079(2) Å, b ) 17.7079(2) Å, c )
15.0264(2) Å, R ) 90°, â ) 90°, γ ) 120°, V ) 4080.56(8) Å3, Z ) 3, Dc
) 1.469 g cm-3, µ ) 8.413 mm-1, T ) 296(2) K, Cu KR radiation (λ )
1.54178 Å), 24 724 reflections measured, 5090 independent (Rint
)
0.0718). Refinement on F2 for 5090 reflections and 304 parameters gave
GOF ) 1.072, R ) 0.0360, Rw ) 0.0911 for I > 2σ(I).
(15) Reported in ref 9. Crystal data for C26H27Cl2FePPdS [1a‚PdCl2]:
crystal size 0.08 × 0.25 × 0.25 mm3, Mw ) 635.66, monoclinic, space
group P21/n, a ) 9.7541(5) Å, b ) 15.9433(8) Å, c ) 16.9386(8) Å, R )
90°, â ) 94.8180(10)°, γ ) 90°, V ) 2624.9(2) Å3, Z ) 4, Dc ) 1.609 g
cm-3, µ ) 1.595 mm-1, T ) 296(2) K, Mo KR radiation (λ ) 0.7173 Å),
17 617 reflections measured, 7447 independent (Rint
) 0.0810).
Refinement on F2 for 7447 reflections and 293 parameters gave GOF
) 1.067, R ) 0.0515, Rw ) 0.1076 for I > 2σ(I). CCDC reference number
185296.
(16) Crystal data for C26H27Cl2FePPtS‚CH2Cl2 [1a‚PtCl2]: crystal
size 0.25 × 0.15 × 0.15 mm3, Mw ) 809.27, triclinic, space group P1/n,
a ) 10.19170(10) Å, b ) 10.53430(10) Å, c ) 14.0443(2) Å, R ) 82.5820-
(10)°, â ) 80.6780(10)°, γ ) 70.0560(10)°, V ) 1453.31(3) Å3, Z ) 2, Dc
) 1.849 g cm-3, µ) 17.568 mm-1, T ) 296(2) K, Cu KR radiation (λ )
1.54178 Å), 9358 reflections measured, 7024 independent (Rint
)
This configuration was unequivocally determined by
0.0252). Refinement on F2 for 7024 reflections and 638 parameters gave
GOF ) 1.048, R ) 0.0375, Rw ) 0.0952 for I > 2σ(I). CCDC reference
number 237743. Two slightly different structures were detected in the
crystal, differing mainly in the torsion angle of the unsubstituted Cp
ring (Φ ) -38.474 and -17.440).
X-ray diffraction of [1a‚CuCl]2,14 1a‚PdCl2,15 and
16
1a‚PtCl2 (Figure 1). Among other interesting struc-
(10) Cabrera, S.; Go´mez Arraya´s, R.; Carretero, J. C. Angew. Chem.,
Int. Ed. 2004, 43, 3944.
(17) The copper complex deviates slightly from the pseudo-planarity
of the P,S-five membered metallacycle: θ Cu-S-CCp-CCp ) 8.9° (for
[1a‚CuCl]2), θ Pd-S-CCp-CCp ) 0.1° (for 1a‚PdCl2), θ Pt-S-CCp
CCp ) 5.6° (for 1a‚PtCl2).
(11) Garc´ıa Manchen˜o, O.; Go´mez Arraya´s, R.; Carretero, J. C. J.
Am. Chem. Soc. 2004, 125, 456.
-
(12) Priego, J.; Garc´ıa Manchen˜o, O.; Cabrera, S.; Carretero, J. C.
J. Org. Chem. 2002, 67, 1346.
(18) In other solvents such as toluene, THF, and acetonitrile the
reactivity or/and the enantioselectivity were much lower.
(19) In the absence of molecular sieves the reaction did not go to
completion, likely due to the deactivation of the cationic Lewis acid
with traces of water.
(20) The enantioselectivity of the process strongly decreases at
higher temperatures, likely because of the competitive silver-catalyzed
non-enantioselective reaction. For instance, the reaction of 1a‚PdCl2
at -20 °C, instead of -78 °C, afforded (R)-3 with only 20% ee.
(13) This high control in the formation of the stereogenic sulfur atom
seems to rely on steric effects. Thus, ligands with a less bulky
substituent at sulfur, such as a p-tolyl group instead of the tert-butyl
group, provided mixtures of epimers at sulfur upon complexation with
the metal. For instance, the reaction of 1-diphenylphosphino-2-p-
tolylsulfenylferrocene with Pd(CH3CN)2Cl2 afforded a 5:1 mixture of
epimers at sulfur.

Pd Complexes of 1-Phosphino-2-sulfenylferrocenes
Organometallics, Vol. 24, No. 4, 2005 559
Figure 1. ORTEP plots for [1a‚CuCl]2, 1a‚PdCl2, and 1a‚PtCl2 (30% thermal ellipsoids) showing the atomic numbering
of selected atoms. The hydrogen atoms in [1a‚CuCl]2 have been omitted for clarity. Selected bond lengths (Å) and angles
(deg) of [1a‚CuCl]2: Cu1-S1 ) 2.5785(9), Cu1-P1 ) 2.2075(7), Cu1-Cl1 ) 2.3303(7), Cu1-Cl1A ) 2.3621(8); P1-Cu1-
S1 ) 89.67(3), P1-Cu1-Cl1 ) 126.09(3), P1-Cu1-Cl1A ) 124.87(3), S1-Cu1-Cl1 ) 113.64(3), S1-Cu1-Cl1A ) 106.63-
(3), Cl1-Cu1-Cl1A ) 95.31(3). Selected bond lengths (Å) and angles (deg) of 1a‚PdCl2: Pd1-S1 ) 2.3137(9), Pd1-P1 )
2.2427(9), Pd1-Cl1 ) 2.3461(11), Pd1-Cl2 ) 2.3022(11); P1-Pd1-S1 ) 90.83(3), P1-Pd1-Cl1 ) 176.95(4), P1-Pd1-
Cl2 ) 88.26(4), S1-Pd1-Cl1 ) 89.39(4), Cl2-Pd1-S1 ) 176.23(4), Cl2-Pd1-Cl1 ) 91.72(5). Selected bond lengths (Å)
and angles (deg) of 1a‚PtCl2: Pt1-S1 ) 2.2928(18), Pt1-P1 ) 2.2209(19), Pt1-Cl1 ) 2.352(2), Pt1-Cl2 ) 2.308(2);
P1-Pt1-S1 ) 90.56(7), P1-Pt1-Cl1 ) 178.65(8), P1-Pt1-Cl2 ) 91.32(8), S1-Pt1-Cl1 ) 88.21(8), S1-Pt1-Cl2 ) 177.68-
(8), Cl2-Pt1-Cl1 ) 89.89(8).
Table 1. ADA Reaction of Cyclopentadiene with
N-Acryloyl Oxazolidinone in the Presence of
Fesulphos Metal Complexes
Figure 2. Fesulphos palladium(II) halide complexes pre-
pared by reaction of the P,S-ligand with Pd(CH3CN)2X2 in
CH2Cl2 at room temperature.
time
yield
ee (%)c
entry
complex
T (°C) (days) (%)a endo/exob (conf)
process by studying the effect of the substitution at
phosphorus (entries 5-10 in Table 1). All complexes
1‚PdCl2 (Figure 2) were readily prepared as single
epimers at sulfur from ligands 1 and Pd(CH3CN)2Cl2
following the same procedure as previously described
for 1a‚PdCl2.9b In a similar way 1f‚PdBr2 was isolated
in 94% yield by reaction of 1f with Pd(CH3CN)2Br2.
Figure 3 shows the X-ray crystal structure of the
complex 1e‚PdCl2,23 proving the bidentate character of
the Fesulphos ligands even when bearing very bulky
groups at both sulfur (tert-butyl) and phosphorus (R-
naphthyl) atoms. As minor structural differences be-
tween this complex and the parent complex 1a‚PdCl2,
it should be noted the elongation of the Pd-P bond
(2.270 vs 2.243 Å) and the slight opening of the
P-Pd-Cl(2) angle (93.7° vs 88.3°) as the result of the
important increase in the steric bulkiness around the
phosphorus atom.
1
[1a‚CuCl]2
1a‚PtCl2
1a‚PdCl2
2‚PdCl2
1b‚PdCl2
1c‚PdCl2
1d‚PdCl2
1e‚PdCl2
1f‚PdCl2
1f‚PdBr2
[1f‚CuBr]2
-20
-20
-78
-78
-50
-78
-50
-78
-78
-78
-78
0.7
1
94
91
69
20d
79
66
83
35d
90
90
87
95/5
90/10
95/5
95/5
91/9
90/10
93/7
96/4
98/2
97/3
99/1
25 (S)
0
67 (R)
38 (R)
>2
62 (R)
74 (R)
80 (R)
95 (R)
90 (R)
54 (S)
2
3
6
4
6
5
6
6
6
7
6
8
6
9
1.6
7
10e
11
6
a In pure product after chromatography. b Measured by 1H
NMR. c Determined by HPLC (Chiralcel OD column). d Conversion
yield. e Using AgOTf (20 mol %).
form, the reaction of the Cu complex (entry 1) yielded
(S)-3 in a low 25% ee. By contrast, the Pd complexes
(entries 3 and 4) were much more reactive, allowing
performing the reaction at -78 °C, which produced the
enantiomeric adduct (R)-3 as the major product. The
highest asymmetric induction was obtained in the case
of the P,S-complex 1a‚PdCl2 (67% ee).
Having concluded from this study the superiority of
the P,S-palladium coordination21,22 over other types of
P,S-metal complexes and N,S-ligands, we next under-
took the optimization of the enantioselectivity of the
Evaluation of this set of Fesulphos palladium com-
plexes in the ADA reaction led us to find that the
enantioselectivity was deeply affected by the electronic
and steric properties of the phosphine moiety (Table 1).
(22) For a recent review on enantioselective Pd-catalyzed transfor-
mations, see: Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104,
3453.
(23) Crystal data for C34H31Cl2FePPdS [1e‚PdCl2]: crystal size
0.28 × 0.20 × 0.15 mm3, Mw ) 735.77, orthorhombic, space group
P2(1)2(1)2(1)/n, a ) 10.68560(10) Å, b ) 16.15590(10) Å, c ) 17.8511-
(2) Å, R ) 90°, â ) 90°, γ ) 90°, V ) 3081.75(5) Å3, Z ) 4, Dc ) 1.586
g cm-3, µ ) 11.371 mm-1, T ) 297(2) K, Cu KR radiation (λ ) 1.54178
Å), 9358 reflections measured, 5687 independent (Rint ) 0.0469).
Refinement on F2 for 5687 reflections and 364 parameters gave GOF
) 1.048, R ) 0.0313, Rw ) 0.0804 for I > 2σ(I). CCDC reference number
237744.
(21) For precedents on Pd-based chiral Lewis acids for ADA reac-
tions, see: (a) Celentano, G.; Benincori, T.; Radaelli, S.; Sada, M.;
Sannicolo`, F. J Organomet. Chem. 2002, 643-644, 424. (b) Hiroi, K.;
Watanabe, K. Tetrahedron: Asymmetry 2002, 13, 1841. (c) Pignat, K.;
Vallotto, J.; Pinna, F.; Strukul, G. Organometallics 2000, 19, 5160.
(d) Ghosh, A. K.; Matsuda, H. Org. Lett. 1999, 1, 2157. (e) Oi, S.;
Kashiwagi, K.; Inoue, Y. Tetrahedron Lett. 1998, 39, 6253. See also:
(f) Strukul, G. Top. Catal. 2002, 19, 33.

560 Organometallics, Vol. 24, No. 4, 2005
Manchen˜o et al.
planar) and copper (tetrahedral) complexes of Fesulphos
ligands. The endo approach of cyclopentadiene to the
least hindered face of the dienophile, avoiding the steric
interaction with the tert-butyl group at sulfur and the
bulky aryl groups at phosphorus, could explain the
opposite sense of enantioselectivity displayed by the Pd
(re-face approach) and Cu complexes (si-face approach).26
In summary, the P,S-bidentate character of Fesulphos
ligands (1) has been proved by X-ray diffraction analysis
of several metal complexes. By tuning the substitution
at phosphorus, the palladium catalysts derived from
Fesulphos ligands afforded high enantioselectivities, up
to 95% ee, in the ADA reaction of cyclopentadiene with
N-acryloyl-1,3-oxazolidin-2-one. Interestingly, the op-
posite enantioselectivity observed from Pd and Cu
complexes of Fesulphos ligands could be due to the
different geometry around the metal center. The study
of the effectiveness of these novel P,S-palladium Lewis
acids in other types of cycloadditions is underway.
Figure 3. ORTEP plot for 1e‚PdCl2 (30% thermal ellip-
soids) showing the atomic numbering of selected atoms.
The hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): Pd1-S1 ) 2.3015(11),
Pd1-P1 ) 2.2703(9), Pd1-Cl1 ) 2.3484(13), Pd1-Cl2 )
2.3015(11); P1-Pd1-S1 ) 90.38(3), P1-Pd1-Cl1 ) 175.64-
(5), P1-Pd1-Cl2 ) 93.73(4), S1-Pd1-Cl1 ) 85.49(5),
Cl2-Pd1-S1 ) 174.97(5), Cl2-Pd1-Cl1 ) 90.47(6).
Experimental Section
NMR spectra were recorded on a Bruker AC-200 [200 MHz
(1H), 50 MHz (13C)] or AC-300 [300 MHz (1H), 75 MHz (13C)]
at room temperature in CDCl3 with internal CHCl3 as the
reference (7.26 ppm for 1H and 77.0 ppm for 13C). For
phosphorus- and fluoro-containing compounds the observed list
of peaks is given as the 13C NMR data, except for those cases
where the JP-C has been unequivocally determined. Mass
spectra (MS) were determined on a HP-5985 mass spectrom-
eter under FAB conditions. All reactions were carried out in
anhydrous solvents and under argon atmosphere. CH2Cl2 was
dried and stored over microwave-activated 4 Å molecular
sieves. Single crystals of the four compounds were grown by
slow diffusion of hexane onto dichloromethane solutions. Data
collections were carried out at room temperature on a Bruker
Smart CCD diffractometer using graphite-monochromated Cu
KR (λ ) 1.54178 Å) or Mo KR (λ ) 0.7173 Å) and 2θ/ω scans
method. Absorption correction method SADABS (v. 2.03) was
applied, except for 1a‚PdCl2. All structures were solved by
direct methods (SHELXS-97), and the refinement was made
by full-matrix least-squares on F2 (SHELXL-97).
Figure 4. Proposed models for the ADA reaction.
Electronically rich phosphines (complexes of ligands 1b
and 1c) were found to have a deleterious effect on the
enantioselectivity (entries 5 and 6), especially in the case
of the furyl phosphine 1b. In contrast, complexes with
electronically poor phosphine 1d (entry 7) and, particu-
larly, the bulky phosphines 1e and 1f (entries 8-9)
produced a substantial enhancement of the asymmetric
induction. Considering both reactivity and enantiose-
lectivity, the best result was obtained from the complex
of the o-tolyl phosphine 1f (entry 9), which afforded the
endo cycloadduct (R)-3 in excellent chemical yield (90%)
and enantioselectivity (95% ee). In agreement with the
presumed participation of a cationic palladium complex
as the active catalyst of the process, a similar result was
obtained from the dibromo complex 1f‚PdBr2 and AgOTf
as halogen scavenger (entry 10, 90% ee).24
For the optimal Fesulphos ligand 1f we confirmed the
superiority of the Pd complex over its Cu(I) complex.
Thus, the ADA reaction catalyzed by [1f‚CuBr]2/AgBF4
provided the adduct 3 in a moderate 54% ee, albeit,
interestingly, with opposite enantioselectivity (entry 11).
Assuming a bidentate coordination of the dienophile
to the electrophilic metal center, Figure 4 shows a
tentative stereochemical model for these ADA reactions
based on the different geometry of the palladium (square
General Procedure for the Synthesis of Dichloro and
Dibromo Palladium Complexes [LPdX2].9b A solution of 1
or 2 (0.5 mmol) and Pd(CH3CN)2Cl2 or Pd(CH3CN)2Br2 (0.5
mmol) in CH2Cl2 (10 mL) was stirred for 1 h at room tem-
perature under argon atmosphere. After concentration under
reduced pressure, the resulting solid was triturated with Et2O,
filtered, and air-dried to afford complex LPdX2.
(RP,RS)-{1-(tert-Butylsulfenyl)-2-[bis-(o-tolyl)phosphino]-
ferrocene}(dichloro)palladium(II) [1f‚PdCl2]. Yield: 99%,
1
red solid; [R]20 -255° (c 0.10, CHCl3); mp >200 °C (dec). H
D
NMR (300 MHz, CDCl3): δ 9.40-9.22 (m, 1H), 7.58-7.45 (m,
2H), 7.40-7.30 (m, 1H), 7.25-7.14 (m, 2H), 7.12-6.90 (m, 2H),
5.03 (s, 1H), 4.94 (s, 1H), 4.29 (s, 1H), 4.15 (s, 5H), 2.45 (s,
3H), 1.93 (s, 3H), 1.50 (s, 9H). 13C NMR (75 MHz, CDCl3): δ
143.5 (d, JP-C ) 11.5 Hz), 140.4, 140.1, 139.9, 132.3, 132.2,
132.1, 131.8, 131.7, 131.5, 131.4, 131.3, 131.1, 126.4, 126.2,
126.1, 125.8, 125.7, 125.3, 86.4, 86.0, 85.9, 85.2, 80.2 (d, JP-C
) 5.2 Hz), 75.5 (d, JP-C ) 11.5 Hz), 72.6, 70.8, 59.8, 31.8, 23.5
(d, JP-C ) 4.2 Hz), 22.3 (d, JP-C ) 3.1 Hz). MS (FAB+): m/z
629 (M+ + H - Cl, 6), 591 (16), 534 (17), 307 (24), 154 (100).
Anal. Calcd for C28H31Cl2FePPdS‚2CH2Cl2: C, 42.45; H, 4.18;
S, 3.91. Found: C, 42.29; H, 4.08; S, 3.80.
25
(24) AgBF4 provided a similar reactivity (84% yield), although the
enantioselectivity was somewhat lower (78% ee).
(25) [1f‚CuBr]2 was prepared in 95% yield by treatment of 1f with
CuBr in THF/MeOH.
(26) Examples of reversed enantioselectivities depending on the
geometry of the metal center have been previously described; see for
instance: Evans, D. A.; Johnson, J. S.; Burgey, C. S.; Campos, K. R.
Tetrahedron Lett. 1999, 40, 2879.

Pd Complexes of 1-Phosphino-2-sulfenylferrocenes
Organometallics, Vol. 24, No. 4, 2005 561
(RP,RS)-{1-(tert-Butylsulfenyl)-2-[bis-(o-tolyl)phosphino]-
ferrocene}(dibromo)palladium(II) [1f‚PdBr2]. Yield: 94%
(1:8 mixture of 2 rotamers at 25 °C); red solid; [R]20D -548° (c
9H). 13C NMR (75 MHz): δ 135.2 (d, JP-C ) 30.3 Hz), 134.8
(d, JP-C ) 16.7 Hz), 134.4 (d, JP-C ) 34.5 Hz), 132.5 (d, JP-C
15.7 Hz), 130.0, 128.9, 128.3, 128.2, 128.1, 84.6 (d, JP-C
35.6 Hz), 78.7 (d, JP-C ) 46.0 Hz), 77.6, 73.3 (d, JP-C
)
)
)
1
0.02, CHCl3); mp >200 °C (dec). H NMR (300 MHz, CDCl3):
major rotamer, δ 9.45-9.25 (m, 1H), 7.60-7.42 (m, 2H), 7.40-
7.27 (m, 1H), 7.25-7.15 (m, 2H), 7.15-6.93 (m, 2H), 5.01 (s,
1H), 4.92 (t, 1H, J ) 2.3 Hz), 4.25 (s, 1H), 4.18 (s, 5H), 2.5 (s,
3H), 1.92 (s, 3H), 1.54 (s, 9H); representative data of minor
rotamer, δ 8.90-8.70 (m, 1H), 4.98 (s, 1H), 4.54 (s, 1H), 4.39
(s, 5H), 2.00 (s, 3H), 1.91 (s, 3H), 1.44 (s, 9H). 13C NMR (75
4.2 Hz), 72.0, 71.4, 50.0, 30.3. MS (FAB+): m/z 1079 (M+
+
H - Cl, 32), 458 (100), 402 (50). Anal. Calcd for
C52H54Cl2Cu2Fe2P2S2‚1CH2Cl2: C, 53.06; H, 4.70; S, 5.35.
Found: C, 53.02; H, 4.64; S, 5.39.
Enantioselective Diels-Alder Reaction of N-Acryloyl-
1,3-oxazolidin-2-one with Cyclopentadiene. Typical pro-
cedure: a mixture of [1f‚PdCl2] (7.3 mg, 0.011 mmol), AgBF4
(4.3 mg, 0.022 mmol), and 4 Å molecular sieves (50 mg) in
CH2Cl2 (0.5 mL) under argon was stirred for 2 h at room
temperature. A solution of 3-(2-propenoyl)-2-oxazolidinone
(15.5 mg, 0.11 mmol) in CH2Cl2 (1.0 mL) was added, and the
reaction mixture was stirred 5 min at room temperature. The
reaction mixture was cooled to -78 °C, and cyclopentadiene
(50 µL, 0.59 mmol) was added. Once the starting material was
consumed (40 h), the reaction was diluted with 2 mL of 1:1
EtOAc/n-hexane and filtered through a path of silica gel, and
the filtrate was concentrated to dryness. The residue was
purified by flash chromatography (n-hexane/EtOAc, 2:1) to
afford a 98:2 mixture of endo/exo aducts [20.4 mg, 90% yield,
95% ee endo-(R)]. HPLC:27 Diacel Chiralcel OD, n-hexane/i-
PrOH, 90:10, flow rate 1.0 mL/min, exo tR ) 16.9, 17.6 min,
endo-(R) tR ) 18.7 min, endo-(S) tR ) 20.8 min, 210 nm. Endo
aduct:27 1H NMR (300 MHz, CDCl3): δ 6.25 (dd, J ) 3.0, 5.7
Hz, 1H), 5.88 (dd, J ) 2.8, 5.7 Hz, 1H), 4.50-4.30 (m, 2H),
4.05-3.85 (m, 3H), 3.30 (s, 1H), 2.92 (s, 1H), 1.95 (ddd, J )
11.7, 9.2, 3.7 Hz, 1H), 1.55-1.35 (m, 3H). The same experi-
mental procedure starting from [1f‚CuBr]2 (7.1 mg, 0.0056
mmol) and AgBF4 (2.1 mg, 0.011 mmol) provided a 99:1
mixture of endo/exo aducts [19.7 mg, 87% yield, 54% ee endo-
(S)].
MHz, CDCl3): δ 143.3 (d, JP-C ) 11.5 Hz), 142.0 (d, JP-C
)
4.2 Hz), 141.1, 140.7, 140.1, 138.9, 138.8, 138.5, 137.4, 137.2,
132.5, 132.4, 132.3, 132.2, 131.9, 131.8, 131.7, 131.6, 131.5,
131.4, 131.3, 126.5, 126.2, 126.0, 125.8, 125.7, 125.4, 125.2,
87.1, 86.7, 85.9, 80.1 (d, JP-C ) 5.2 Hz), 79.3 (d, JP-C ) 4.2
Hz), 75.5 (d, JP-C ) 10.5 Hz), 74.7 (d, JP-C ) 11.5 Hz), 73.5,
73.0, 72.7, 70.6, 70.4, 65.8, 59.8, 53.4, 32.2, 31.3, 24.3, 24.2,
23.7, 23.6, 22.5. MS (FAB+) m/z 673 (M+ + H - Cl, 6), 616
(7), 593 (9), 534 (13), 307 (30), 154 (100). Anal. Calcd for
C28H31Br2FePPdS: C, 44.68; H, 4.15; S, 4.26. Found: C, 44.96;
H, 4.19; S, 4.27.
Preparation of Complex [1a‚PtCl2]. A solution of 1a (87
mg, 0.19 mmol) and PtCl2 (52 mg, 0.19 mmol) in CH2Cl2 (10
mL) was stirred for 24 h at room temperature under argon
atmosphere. After filtration of the reaction mixture, followed
by concentration under reduced pressure, the resulting solid
was triturated with n-hexane/Et2O (1:1), filtered, and air-dried
to afford complex [1a‚PtCl2] as a yellow solid (132 mg, 95%),
[R]20D -311° (c 0.10, CHCl3); mp >200 °C (dec). 1H NMR (300
MHz, CDCl3): δ 8.15-7.98 (m, 2H), 7.95-7.80 (m, 2H), 7.60-
7.30 (m, 6H), 5.22-5.10 (m, 1H), 4.95-4.85 (m, 1H), 4.50-
4.42 (m, 1H), 4.12 (s, 5H), 1.28 (s, 9H). 13C NMR (75 MHz): δ
134.2 (d, JP-C ) 10.5 Hz), 132.2 (d, JP-C ) 10.5 Hz), 131.7 (d,
JP-C ) 3.1 Hz), 131.1 (d, JP-C ) 3.1 Hz), 130.2, 128.8, 128.7,
128.6, 128.3, 128.2, 127.8, 87.6 (d, JP-C ) 27.4 Hz), 82.0 (d,
JP-C ) 73.7 Hz), 80.3 (d, JP-C ) 6.3 Hz), 77.2, 72.5, 70.7 (d,
JP-C ) 2.1 Hz), 58.5, 30.0. FAB+ MS: m/z 689 (M+ + H - Cl,
10), 632 (23), 154 (100). Anal. Calcd for C26H27Cl2FePPtS‚2/
3CH2Cl2: C, 41.01; H, 3.66; S, 4.11. Found: C, 40.65; H, 3.64;
S, 4.01.
General Procedure for the Synthesis of Chiral Cu(I)
Complexes.11 A solution of ligand 1 (0.4 mmol) and CuCl or
CuBr (0.41 mmol) in a 2:3 mixture of THF/MeOH (5 mL) was
stirred at room temperature under argon for 5-10 min. The
mixture was concentrated to dryness, a 4:1 mixture of n-
hexane/EtOAc was added, and it was passed through a short
pad of silica gel to obtain pure complexes after evaporation of
the solvent under reduced pressure.
Acknowledgment. Support is provided by the MCyT
(projects BQU2000-0266 and BQU2003-0508). R.G.A.
thanks the MCyT for a Ramo´n y Cajal contract. O.G.M.
thanks the MEC for a predoctoral fellowship. We thank
Johnson Matthey PLC for a generous loan of PdCl2.
Supporting Information Available: Physical and spec-
tral data for complexes (1a-e)‚PdCl2, 2‚PdCl2, and [1f‚CuCl]2.
Tables of atomic coordinates, thermal parameters, all bond
distances and angles, and experimental data for X-ray diffrac-
tion studies of 1a‚PtCl2 and 1e‚PdCl2. Copies of 1H and 13C
NMR spectra of all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
(RP,RS)-2-(tert-Butylsulfenyl)-1-(diphenylphosphino)-
ferrocene Copper(I) Chloride Dimer [1a‚CuCl]2. Yield:
OM0493762
99%, orange solid; [R]20 -330° (c 0.10, CHCl3); mp >200 °C
D
1
(dec). H NMR (300 MHz, CDCl3): δ 8.20-8.05 (m, 2H), 7.70
(27) For HPLC and spectroscopic data of adducts (R)-3 and (S)-3,
see for instance: Evans, D. A.; Miller, S. J.; Lectka, T.; von Matt, P. J.
Am. Chem. Soc. 1999, 121, 7559.
(t, J ) 8.6 Hz, 2H), 7.45-7.35 (m, 3H), 7.35-7.20 (m, 3H),
4.42 (s, 1H), 4.63 (s, 1H), 4.50 (s, 1H), 4.00 (s, 5H), 1.12 (s,