Utility of osmium(II) in the catalysis of asymmetric diels-alder reactions

Article abstract of DOI:10.1021/om000896g

Half-sandwich complexes of osmium(II) comprising either (S)-BINAP or (S)-BINPO can be used to prepare formally dipositive 16-electron Lewis acidic species. These in turn can be used to catalyze the Diels-Alder condensation of either methacrolein or ethyla

Full text of DOI:10.1021/om000896g

Organometallics 2001, 20, 697-699
697
Utility of Osm iu m (II) in th e Ca ta lysis of Asym m etr ic
Diels-Ald er Rea ction s
J . W. Faller*,† and J onathan Parr*,‡
Departments of Chemistry, Yale University, 225 Prospect Street,
New Haven, Connecticut 06520, and Loughborough University,
Loughborough, Leicestershire LE11 3TU, U.K.
Received October 19, 2000
Half-sandwich complexes of osmium(II) comprising either (S)-BINAP or (S)-BINPO can
be used to prepare formally dipositive 16-electron Lewis acidic species. These in turn can be
used to catalyze the Diels-Alder condensation of either methacrolein or ethylacrolein with
cyclopentadiene in high ee (>90%). The complex containing the non-C2-symmetric bisphos-
phine monoxide was found to be markedly more effective than the C2-symmetric bisphos-
phine.
Sch em e 1a
In tr od u ction
Since the first reports of transition-metal-catalyzed
stereoselective Diels-Alder reactions there has been a
continuous development of new ligands and new com-
plexes which can be used for this reaction,1 and among
these, some ruthenium complexes have proved very
promising.2-4 We have recently reported on the role of
chiral bisphosphine monoxide (BPMO) complexes of
ruthenium(II) in the catalysis of an asymmetric Diels-
Alder condensation reaction5 and upon diastereoselec-
tivity in chiral osmium complexes of a chelating BPMO
ligand.6 Following the observation that complexes of the
type [(η6-Cy)Os(BPMO)(aldehyde)]2+ contain σ-bound
aldehydes in much the same way as the corresponding
ruthenium species, it seemed reasonable to investigate
the potential of such osmium(II) compounds as catalysts
for this class of reaction.
We report the synthesis of the Lewis acidic com-
pounds [(η6-Cy)OsCl(L)](SbF6), where L ) (S)-BINAP
(1) or its monoxide, (S)-BINPO7 (2), and examine the
efficacy of the corresponding dipositive 16-electron
Lewis acids [(η6-Cy)Os(L)](SbF6)2 (L ) (S)-BINAP (3),
(S)-BINPO (4)) as catalysts in the condensation of
methacrolein and ethylacrolein with cyclopentadiene.
a
Key: (i) NaSbF6 (-NaCl), CH2Cl2, 25 °C; (ii) AgSbF6
(-AgCl), CH2Cl2, 25 °C.
very good yield (Scheme 1). In the case of complex 2 the
chelation leads to the formation of a chiral center at the
osmium8 and, hence, the possibility of diastereomeric
products. The spectroscopic data indicate that the
chelation proceeds in a selective manner, giving only
one of the two possible isomers, shown by an X-ray
crystallographic investigation to be ROs,S (Figure 1).
This finding is in contrast to our previous observation
that Ph2PCH(CH3)P(O)Ph2 will chelate initially in both
relative configurations at the metal to the [(η6-Cy)OsCl]
fragment. The reasons for this are not clear but may
lie in the slower rate of chloride abstraction by the
NaSbF6 used in this preparation relative to that of the
silver salt used in the other preparation. Alternatively,
there may be a greater steric difference between the
P(III) phenyls and the P(V) phenyls of the BINPO
compared to that of Ph2PCH(CH3)P(O)Ph2, owing to the
difference in chelate ring size.
Resu lts a n d Discu ssion
Treatment of 1 or 2 with AgSbF6 leads to the abstrac-
tion of the remaining chloride ligand and the formation
of the corresponding dipositive species, 3 or 4. The
successful application of 3 and 4 to catalysis of the
Diels-Alder reactions suggests that there is a prefer-
ence for σ-bonding of aldehyde substrates by these
osmium(II) complexes. Furthermore, there should be a
structural similarity of the aldehyde complex of 4 to a
previously reported BPMO ruthenium analogue.6 An
osmium(II) center would not ordinarily be considered a
hard Lewis acid, but it appears, nevertheless, to be
sufficiently acidic to activate the dienophile in these
reactions.
The interaction of [(η6-Cy)OsCl2]2 with (S)-BINAP or
(S)-BINPO in a 1:2 mole ratio in the presence of NaSbF6
gives the yellow monocationic complexes [(η6-Cy)OsCl-
(η2-BINAP)]SbF6 and [(η6-Cy)OsCl(η2-BINPO)]SbF6 in
† Yale University.
‡ Loughborough University.
(1) For a recent review, see: Carmona, D.; Lamata, M. P.; Oro, L.
A. Coord. Chem. Rev. 2000, 200, 717.
(2) Kundig, E. P.; Saudan, C. M.; Bernardinelli, G. Angew. Chem.,
Int. Ed. 1999, 38, 1220.
(3) Davies, D. L.; Fawcett, S. A.; Garrat, S. A.; Russel, D. R. Chem.
Commun. 1997, 1351.
(4) Faller, J . W.; Grimmond, B.; D’Alliessi, D. Unpublished observa-
tions.
(5) Faller, J . W.; Parr, J . Organometallics 2000, 19, 1829.
(6) Faller, J . W.; Parr, J . Organometallics 2000, 19, 3556.
(7) Grushin, V. V. J . Am. Chem. Soc. 1999, 121, 5831.
(8) Brunner, H. Angew. Chem., Int. Ed. 1999, 38, 1195.
10.1021/om000896g CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/17/2001

698 Organometallics, Vol. 20, No. 4, 2001
Faller and Parr
Ta ble 1. Lew is Acid Ca ta lysis of Diels-Ald er
Con d en sa tion of Cp H w ith Meth a cr olein (R ) Me)
a n d Eth yla cr olein (R ) Et)
catalyst
R
T, °C
de, %c
ee, %d
3a
3a
3b
3b
4a
4a
4b
4b
Me
Me
Et
-24
-78
-24
-78
-24
-78
-24
-78
92
96
89
91
98
99
94
94
9
15
8
12
65
93
86
91
Et
Me
Me
Et
Et
a
Catalyst loading at 4 mol %, with 0.24 mmol of methacrolein
b
and 2.9 mmol of CpH. Catalyst loading at 4.5 mol %, with 0.22
mmol of ethylacrolein and 2.9 mmol of CpH. c Determined from
the 1H NMR and refers to the excess of the exo isomer. For R )
d
Me, the S-(+) isomer is in excess. For R ) Et, it is assumed to be
the same.
F igu r e 1. ORTEP view of the cation in (ROs)-[CyOsCl(η2-
(S)-BINPO-P,O)]SbF6 (2) with 50% probability ellipsoids.
undertaken with the corresponding ruthenium com-
plexes as catalyst and seems to indicate that electronic
asymmetry in a C1-symmetric ligand can be more
effective in inducing selectivity than corresponding C2-
symmetric ligands.4 The utility of C2-symmetric ligands
has been widely reported in the literature in a variety
of reactions,10 and the C2 symmetry has often been
viewed as a key factor in their effectiveness. The recent
success of non-C2-symmetric ligands in this BINPO case,
as well as others,11 suggests that non-C2-symmetric
ligands, as a class, may well be more effective.
Some reference to the metrical data from the X-ray
structure shown in Figure 1 provides some insight into
the diastereomer preference and the enantioselectivity.
The (ROs)-[CyOsCl(η2-(S)-BINPO-P,O)]SbF6 structure
found for 2 shows that the preferred diastereomer has
the relative configuration of R*,S*. Although some
modest flexibility might be expected from rotation about
the bond connecting the naphthyl groups, they are
approximately at right angles with a dihedral angle
between them of 81(1)°. The configuration of the bi-
naphthyl portion of the ligand apparently forces a
configuration of the Ph2P fragment which sterically
controls the preference for the orientation of chelation.
The ligand backbone requires an unusually large Os-
O-P angle of 161.5(5)°. Even though there is an eight-
membered chelate ring, it produces a quite small bite
angle of 82.1(2)°, which is slightly larger than that found
in the ruthenium analogue of 81.5(1)°. Although the
coordination about the metal can be considered pseudo-
tetrahedral for purposes of chirality descriptors, one
should note that all of the angles between P, O, and Cl
are less than 90°. We suggest that a major feature of
the stereocontrol of the Diels-Alder reaction with the
BINPO complex is the differential back-bonding pro-
vided by the d-orbitals aligned with the Os-P and
Os-O bonds. This provides a preferential orientation
of the aldehyde which should be superior to that found
in the BINAP complex, for which the conformation is
determined largely on steric interactions.
Osmium-mediated Diels-Alder reactions are not
entirely new. A protocol involving osmium pentaammine
complexes in the dearomatization of substituted sty-
renes to yield activated diene fragments that then
undergo Diels-Alder reactions has appeared.9 This
procedure, while efficient, is a stoichiometric reaction
in which the osmium complex plays a role altogether
different from that reported here.
Inspection of the results from the catalytic reactions
in Table 1 reveals greater enantioselectivity arising in
reactions mediated by the BPMO complex. This is
consistent with the results from similar reactions
Exp er im en ta l Section
Gen er a l P r oced u r es. All synthetic and spectroscopic
operations were carried out as described previously.5,6
P r ep a r a tion of [(η6-Cy)OsCl(η2-(S)-BINAP )]SbF 6 (1). To
a solution of [(η6-Cy)OsCl2]2 (50 mg, 0.063 mmol) in 5 mL of
dichloromethane was added (S)-BINAP (79 mg, 0.127 mmol)
and NaSbF6 (33 mg, 0.128 mmol). The mixture was stirred
for 4 h, the solvent was removed under reduced pressure, and
the residue was extracted with 2 mL of methylene chloride.
The extract was filtered through Celite and ether added to
the filtrate until precipitation was complete. The yellow
microcrystalline product was collected, washed with diethyl
ether, and dried under vacuum. Yield: 127 mg, 82%. 1H NMR
(CD2Cl2, 293 K, δ): 8.02-5.92 (32H, m, arom), 6.49 (1H, d,
6.5 Hz, Cy H), 6.34 (1H, d, 6.5 Hz, Cy H), 5.83 (1H, d, 6.5 Hz,
Cy H), 5.41 (1H, d, 6.5 Hz, Cy H), 2.95 (1H, sept., 6.5 Hz,
CH(CH3)2)), 1.99 (3H, s, Cy-CH3), 1.36 (3H, d, 6.5 Hz, CH3C-
(H)CH3), 0.98 (3H, d, 6.5 Hz, CH3C(H)CH3). 31P{1H} NMR (δ):
-5.85 (d, 34 Hz), -14.60 (d, 34 Hz). Anal. Calcd for C54H46F6P2-
ClOsSb: C, 53.22; H, 3.72. Found: C, 53.50; H, 3.68.
P r ep a r a tion of [(η6-Cy)OsCl(η2-(S)-BINP O)]SbF 6 (2). To
a solution of [(η6-Cy)OsCl2]2 (50 mg, 0.063 mmol) in 5 mL of
(9) Kolis, S. P.; Chordia, M. D.; Liu, R.; Kopach, M. E.; Harman, W.
D. J . Am. Chem. Soc. 1998, 120, 2218.
(10) Whitesell, J . K. Chem. Rev. 1989, 89, 1581.
(11) Examples of successful asymmetric reactions using complexes
of C1-symmetric ligands are as follows. Phosphinooxazoline: Helmchen,
G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336-345. Sagasser, I.;
Helmchen, G. Tetrahedron Lett. 1998, 39, 261-264. Dawson, G. S.;
Frost, C. G.; Williams, J . M. J .; Coote, S. J . Tetrahedron Lett. 1993,
34, 3149. Thiophenooxazolines: Allen, V. J .; Bowen, J . F.; Williams,
J . M. J . Tetrahedron: Asymmetry 1994, 5, 1895. Phosphinoamines:
Cahill, J . P.; Bohnen, F. M.; Goddard, R.; Kruger, C.; Guiry, P. J .
Tetrahedron: Asymmetry 1998, 9, 3831. Suzuki, Y.; Ogatu, Y.; Hiroi,
K. Tetrahedron: Asymmetry 1999, 10, 1219.

Utility of Os(II) in Diels-Alder Reactions
Organometallics, Vol. 20, No. 4, 2001 699
Ta ble 2. Cr ysta llogr a p h ic Da ta for th e X-r a y
Diffr a ction Stu d y of
dichloromethane was added (S)-BINPO (80 mg, 0,127 mmol)
and NaSbF6 (33 mg, 0.128 mmol). The reaction and workup
(ROs)-[CyOsCl(η2-(S)-BINP O-P ,O)]SbF 6 (2)
1
was as above. Yield: 138 mg, 88%. H NMR (CD2Cl2, 293 K,
δ): 8.16-5.95 (32H, m, arom), 6.49 (1H, d, 5.5 Hz, Cy H), 6.22
(1H, d, 5.5 Hz, Cy H), 5.64 (1H, d, 5.5 Hz, Cy H), 5.22 (1H, d,
5.5 Hz, Cy H), 2.24 (1H, hept, 6.5 Hz, CH(CH3)2)), 1.45 (3H, s,
Cy-CH3), 1.08 (3H, d, 6.5 Hz, CH3C(H)CH3), 0.85 (3H, d, 6.5
Hz, CH3C(H)CH3). 31P{1H} NMR (δ): 50.24 (P(V)), 14.25
(P(III)). Anal. Calcd for C54H46OF6P2ClOsSb: C, 52.51; H, 3.76.
Found: C, 52.29; H, 3.87.
formula
cryst syst
space group
a, Å
SbOsClP2F6OC54H46
monoclinic
P21 (No. 4)
12.3190(4)
15.2290(6)
12.9244(5)
97.539(2)
2403.7(1)
1234.30
1.583 (Z ) 2)
10.60
0.24 × 0.24 × 0.05
Nonius KappaCCD
graphite
Mo KR (0.710 73 Å)
55
-90
19 092
4025
593
0.02
0.038, 0.035
0.00
1.05
1.29
b, Å
c, Å
â, deg
V, Å3
P r ep a r a tion of [(η6-Cy)Os(η2-(S)-BINAP )](SbF 6)2 (3). To
a solution of [(η6-Cy)OsCl(η2-(S)-BINAP)]SbF6 (50 mg, 0.041
mmol) in 4 mL of dichloromethane was added AgSbF6 (13 mg,
0.038 mmol) in 4 mL of dichloromethane. After precipitation
was complete, the reaction mixture was centrifuged and the
supernatant removed by syringe. The solution was used
directly for the catalytic reactions without isolation of the
dipositive complex. 1H NMR (CD2Cl2, 293 K, δ): 8.20-6.08
(32H, m, arom), 6.30 (1H, d, 6.0 Hz, Cy H), 5.92 (1H, d, 6.0
Hz, Cy H), 5.83 (1H, d, 6.0 Hz, Cy H), 5.76 (1H, d, 6.0 Hz, Cy
H), 2.97(1H, hept, 7.0 Hz, CH(CH3)2)), 1.99 (3H, s, Cy-CH3),
1.39 (3H, d, 7.0 Hz, CH3C(H)CH3), 0.99 (3H, d, 7.0 Hz, CH3C-
(H)CH3). 31P{1H} NMR (δ): -6.05 (d, 40 Hz), -16.55 (d, 40 Hz).
P r ep a r a tion of [(η6-Cy)Os(η2-(S)-BINP O)](SbF 6)2 (4).
The dipositive catalyst 4 was prepared in the same way using
the same concentrations and again used directly without
isolation. 1H NMR (CD2Cl2, 293 K, δ): 8.20-6.02 (32H, m,
arom), 6.38 (1H, d, 6.5 Hz, Cy H), 6.25 (1H, d, 6.5 Hz, Cy H),
5.91 (1H, d, 6.5 Hz, Cy H), 5.80 (1H, d, 6.5 Hz, Cy H), 2.92
(1H, hept, 7 Hz, CH(CH3)2)), 1.89 (3H, s, Cy-CH3), 1.04 (3H,
d, 7 Hz, CH3C(H)CH3), 0.84 (3H, d, 7 Hz, CH3C(H)CH3). 31P-
{1H} NMR (δ): 80.36 (P(V)), 27.18 (P(III)).
fw
D
calcd, g/cm3
abs coeff, cm-1
cryst size, mm
diffractometer
monochromator
radiatn
max 2θ, deg
T, °C
no. of rflns measd
no. of data used, F2 > 3σ(F2)
no. of params refined
p factor
final residuals R, Rw
convergence, largest shift/error
GOF
largest ∆(F), e Å-3
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [CyOsCl(η2-(S)-BINP O-P ,O)]SbF 6 (2)
Os(1)-Cl(1)
Os(1)-O(1)
2.396(2)
2.138 (5)
Os(1)-P(1)
P(2)-O(1)
2.386(2)
1.496(4)
Cl(1)-Os(1)-P(1)
P(1)-Os(1)-O(1)
84.95(6)
82.1(2)
Cl(1)-Os(1)-O(1)
Os(1)-O(1)-
84.7(3)
161.5(5)
Ca ta lytic Rea ction s. For both 1 and 2 the catalytic
reactions were carried out on the four fractions a-d as
follows: (a) addition of methacrolein (0.017 g, 0.24 mmol) and
CpH (0.197 g, 2.9 mmol) at -24 °C; (b) as for (a) at -78 °C; (c)
addition of ethylacrolein (0.022 g, 0.21 mmol) and CpH (0.197
g, 2.9 mmol) at -24 °C; (d) as for (c) at -78 °C.
Reactions were allowed to reach completion (18-24 h), the
dichloromethane was removed under reduced pressure, and
the residue was extracted with 2 × 5 mL of pentane to separate
the organic product from the inorganic component. The vola-
tiles were removed under reduced pressure and the product
aldehyde analyzed by NMR.
2-Meth ylbicyclo[2.2.1]h ep t-5-en e-2-ca r boxa ldeh yd e. 1H
NMR (CDCl3, 298 K, 500 MHz, δ): 9.69 exo-CHO (major); 9.39
endo-CHO (minor). The chiral shift reagent Eu(hfc)3 was used
to determine the enantioselectivity of the exo diastereomer.
It was observed that the signal for the S enantiomer was
consistently shifted further downfield than the signal for the
R enantiomer. This was established by comparison of the sign
of the optical rotation to the literature values.12 The enantio-
selectivity was then determined by line shape analysis of the
two signals.
2-Eth ylbicyclo[2.2.1]h ep t-5-en e-2-ca r boxa ld eh yd e. 1H
NMR (CDCl3, 298 K, 500 MHz, δ): 9.70 exo-CHO (major) 9.40
endo-CHO (minor). Use of the chiral europium shift reagent,
Eu(hfc)3 failed to distinguish the enantiomers. The enantiose-
lectivity of the exo diastereomer was determined by derivati-
zation with (2R,4R)-2,4-pentanediol to the corresponding
diastereomeric acetals. Integration of the resonances for the
CHO2 protons at δ 4.85 and 4.82 was used to determine the
enantioselectivity. The signal at δ 4.85 corresponded to the
major enantiomer.4 The configuration of the major enantiomer
was not determined but is presumed to be the same as that
for the major isomer found for methacrolein, i.e, S.
X-r a y Cr ysta llogr a p h y. Single crystals suitable for X-ray
analysis were formed by vapor diffusion of diethyl ether into
a methanol solution of 2. Crystallographic data are sum-
marized in Tables 2 and 3. The structure of 2 was determined
from data collected with a Nonius KappaCCD at -90 °C.
Lorentz and polarization corrections were applied to all data.
An empirical absorption correction was applied using SOR-
TAV.13 Intensities of equivalent reflections, excluding Friedel
pairs, were averaged. The structure was solved by direct
methods (SIR9214) using the teXan crystal structure analysis
package, and the function minimized was ∑w(|Fo| - |Fc|)2.
Hydrogen atoms were placed at calculated positions before
each refinement and were included in the refinement but were
not refined. The absolute configuration was determined by
reference to the configuration of (S)-BINPO.
Ack n ow led gm en t. This research was supported by
a grant from the National Science Foundation (Grant
No. CHE9726423).
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, positional and thermal parameters, and bond lengths
and angles for 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM000896G
(13) Blessing, R. H. Acta Crystallogr. 1995, A51, 33-37. Blessing,
R. H.; J . Appl. Crystallogr. 1997, 30, 421-426.
(14) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J . Appl. Crystallogr. 1994,
27, 435.
(12) Hashimoto, S.-I.; Komeshima, N.; Koga, K. Chem. Commun.
1979, 437.