Enantioselective catalytic diels-alder reactions with enones as dienophiles

Article abstract of DOI:10.1021/om300346s

The aqua complexes (S_M,R_C)-[(η⁵-C ₅Me₅)M(PROPHOS)(H₂O)][SbF₆] ₂ [PROPHOS = (R)-propane-1,2-diylbis(diphenylphosphane); M = Rh (1), Ir (2)] are active catalysts for the asymmetric Diels-Alder reaction between ketones and dienes. At low temperatures, enantioselectivities of up to 89% ee are achieved. The intermediate Lewis acid-dienophile complexes (S _M,R_C)-[(η⁵-C₅Me ₅)M(PROPHOS)(MVK)][SbF₆]₂ (MVK = methyl vinyl ketone; M = Rh (3), Ir (4)) and (S_Ir,R_C)- [(η⁵-C₅Me₅)Ir(PROPHOS)(EVK)][SbF ₆]₂ (EVK = ethyl vinyl ketone (5)) have been isolated and characterized by analytical and spectroscopic means, including the determination of the crystal structure of the iridium complexes 4 and 5 by X-ray diffractometric methods. Structural parameters indicate that the dispositions of the coordinated dienophiles are controlled by the CH/π attractive interactions established between a phenyl group of the PROPHOS ligand and the α-vinyl proton of the ketones. Proton NMR parameters indicate that these interactions are maintained in solution. From these data, the stereoselectivity of the catalytic reaction is discussed.

Full text of DOI:10.1021/om300346s

Article
pubs.acs.org/Organometallics
Enantioselective Catalytic Diels−Alder Reactions with Enones As
Dienophiles
Daniel Carmona,* Fernando Viguri,* Ainara Asenjo, Fernando J. Lahoz, Pilar García-Orduna,
̃
and Luis A. Oro
́ ́ ́
Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), Departamento de Química Inorganica, CSIC−Universidad de
Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The aqua complexes (SM,RC)-[(η5-C5Me5)M(PROPHOS)-
(H2O)][SbF6]2 [PROPHOS = (R)-propane-1,2-diylbis(diphenylphosphane);
M = Rh (1), Ir (2)] are active catalysts for the asymmetric Diels−Alder reaction
between ketones and dienes. At low temperatures, enantioselectivities of up to
89% ee are achieved. The intermediate Lewis acid−dienophile complexes
(SM,RC)-[(η5-C5Me5)M(PROPHOS)(MVK)][SbF6]2 (MVK = methyl vinyl
ketone; M = Rh (3), Ir (4)) and (SIr,RC)-[(η5-C5Me5)Ir(PROPHOS)(EVK)]-
[SbF6]2 (EVK = ethyl vinyl ketone (5)) have been isolated and characterized by analytical and spectroscopic means, including
the determination of the crystal structure of the iridium complexes 4 and 5 by X-ray diffractometric methods. Structural
parameters indicate that the dispositions of the coordinated dienophiles are controlled by the CH/π attractive interactions
established between a phenyl group of the PROPHOS ligand and the α-vinyl proton of the ketones. Proton NMR parameters
indicate that these interactions are maintained in solution. From these data, the stereoselectivity of the catalytic reaction is
discussed.
cyclopentadiene,10 and Harada’s group reported that oxazabor-
INTRODUCTION
■
olidinones are efficient catalysts for the asymmetric DA reaction
of acyclic enones.11 Notably, as far as we know, the only
example of a chiral transition metal Lewis acid catalyst for
asymmetric DA reactions of this type is the ruthenium complex
[(η5-C5H5)Ru(R,R-BIPHOP-F)(acetone)][SbF6] (R,R-BI-
PHOP-F = 1,2-bis[bis(pentafluorophenyl)phosphanyloxy]-1,2-
Asymmetric catalysis is one of the most efficient synthetic
methodologies for the preparation of enantioenriched com-
pounds.1 Among the wide variety of metal-catalyzed asym-
metric processes, the Diels−Alder (DA) reaction is a powerful
and versatile synthetic transformation that plays an important
role in the construction of cyclohexene derivatives with up to
four contiguous stereocenters.2 In particular, cationic half-
sandwich complexes of general formula [(ηn-ring)M(L1L2)
*(Solv)]n+ [M = Rh, Ir, Ru; (L1L2)* = chiral bidentate ligand]
have been used as chiral one-point-binding catalysts in
diphenylethane) recently reported by Kundig’s group.12
̈
Following our studies on enantioselective DA reactions of
enals catalyzed by (SM,RC)-[(η5-C5Me5)M(PROPHOS)-
(H2O)][SbF6]2 [PROPHOS = (R)-propane-1,2-diylbis-
(diphenylphosphane); M = Rh (1), Ir (2)] complexes6a,h,j,m,n
and taking into account the lack of examples of DA reactions of
enones catalyzed by transition metal complexes, we envisaged
the possibility of extending the application of our catalysts to
DA reactions involving this type of dienophiles. In this paper,
we report the results obtained in the reaction of vinyl ketones
(MVK, EVK) with dienes (cyclopentadiene, 2,3-dimethylbuta-
diene, isoprene). The determination of the crystal structure by
X-ray diffractometric methods of the intermediates (SIr,RC)-
[(η5-C5Me5)Ir(PROPHOS)(MVK)][SbF6]2 (4) and (SIr,RC)-
[(η5-C5Me5)Ir(PROPHOS)(EVK)][SbF6]2 (5), in which the
dienophile is coordinated to the metal, allows us to discuss the
observed asymmetric induction.
enantioselective DA reactions by the groups of Kundig,3
̈
Faller,4 Davies,5 and ourselves.6 Olefins with one carbonyl-
containing substituent are well-suited dienophiles to which
these catalysts can be applied: the electron-withdrawing
carbonyl group activates the olefin toward a nucleophilic attack
and, concurrently, provides the dienophile with an oxygen atom
capable of linking the metal in an η1-coordination mode. In fact,
the reaction of enals (mostly methacrolein) with cyclo-
pentadiene can be considered as the DA reaction model for
this type of catalyst. However, enones, dienophiles that fulfill
the two above-mentioned features, have been very scarcely
employed as DA dienophiles. In 2002, MacMillan and
Northrup reported the first enantioselective organocatalytic
DA reaction with enones as dienophiles.7 Subsequently, Corey
et al.8 and Shibatomi and Yamamoto9 reported that activated
chiral oxazaborolidines efficiently mediate the enantioselective
cycloaddition of enones and dienes, Hawkins and co-workers
published the application of a chiral aromatic alkyldichlorobor-
ane compound to the DA reaction between enones and
Received: April 26, 2012
© XXXX American Chemical Society
A
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Article
below). Both rhodium and iridium systems are very active for
the reaction of the vinyl ketones 6 and 7 with cyclopentadiene,
conversions higher than 75% being achieved in 15 min in all
cases (entries 1, 4, 7, and 10); however with dienes 9 and 10,
under the same conditions, low conversions (≤36%) are
obtained after 6 days of reaction. For both metals, good endo
selectivities are obtained in the reactions with cyclopentadiene
(entries 1, 4, 7, and 10), and the 1,4-isomers are obtained
preferentially in the reactions with isoprene (entries 3, 6, 9, and
12); however the enantioselectivities achieved are modest
(≤27% ee).
RESULTS AND DISCUSSION
■
Diels−Alder Reactions of Vinyl Ketones with Dienes.
We first tested the catalytic activity of the complexes (SM,RC)-
[(η5-C5Me5)M(PROPHOS)(H2O)][SbF6]2 [M = Rh (1), Ir
(2)] in the DA reaction between the ketones MVK (6) and
EVK (7) and cyclopentadiene (8), 2,3-dimethylbutadiene (9),
and isoprene (10). As specified in Scheme 1, two pairs of
enantiomers can be formed when cyclopentadiene or isoprene
are used as diene (a, c), but only two enantiomers can be
obtained for 2,3-dimethylbutadiene (b).
The high activity shown by vinyl ketones 6 and 7 with
cyclopentadiene at room temperature together with the
possibility of improving the low ee obtained prompted us to
study these reactions at lower temperatures (Table 2). For
comparative purposes, the values registered at room temper-
ature (RT) are also included. The catalytic systems remain
active at low temperatures. Thus, for example, at −50 °C, after
24 h of reaction, quantitative conversions are achieved with
both catalysts (entries 6, 10, and 13) and, as expected, the
endo/exo selectivity slightly increases when temperature
decreases. For the dienophile MVK, the major adduct obtained
is the endo-(S) isomer, which implies a diene addition to the
Cα-re face. Notably, while for the enone MVK the ee value
increases when temperature decreases (entries 1−4 and 7−11),
unexpectedly, this value remains almost unchanged for the
enone EVK and, furthermore, the endo-(R) product is slightly
more abundant (entries 5, 6 and 12, 13). We will be back to
this point later, when discussing the molecular structures in the
solid state of the enone-containing intermediates 4 and 5. It is
interesting to point out that the enantioselectivity achieved for
the enone MVK, with both catalytic systems, is the highest
reported so far for a metallic catalytic system.12
Scheme 1. Possible Cycloadducts for the DA Reaction
between the Vinyl Ketones 6 and 7 and the Dienes 8−10
Table 1 lists the results obtained and the reaction conditions
employed. All the reactions were carried out at room
temperature, and the collected results are the average of at
least two comparable reaction runs. Catalyst precursors 1 and 2
are treated with the corresponding vinyl ketone, in the presence
of 4 Å MS, before the addition of the diene to generate the
complexes [(η5-C5Me5)M(PROPHOS)(enone)]2+ (see
a
Table 1. DA Reactions of Vinyl Ketones 6 and 7 and Dienes 8, 9, and 10
b
bc
,
d
entry
catalyst
R1
diene/R2
time (h)
conv (%)
selectivity (molar ratio)
90/10
ee (%)
1
1
1
1
1
1
1
2
2
2
2
2
2
Me (6)
Me (6)
Me (6)
Et (7)
Et (7)
Et (7)
Me (6)
Me (6)
Me (6)
Et (7)
Et (7)
Et (7)
HCp (8)
Me (9)
H (10)
HCp (8)
Me (9)
H (10)
HCp (8)
Me (9)
H (10)
HCp (8)
Me (9)
H (10)
0.25
144
77
32
11
89
30
17
96
36
14
91
32
28
27 (S)
2 (S)
2/1
2
3
144
70/30
92/8
4
0.25
144
1 (R)
0
5
6
144
75/25
93/7
0/0
7
0.25
144
17 (S)
5 (S)
3/1
8
9
144
72/28
93/7
10
11
12
0.25
144
1 (R)
0
144
80/20
1/0
a
Reaction conditions: catalyst 0.025 mmol (5 mol %), ketone 0.5 mmol, diene 2.5 mmol, and 100 mg of 4 Å molecular sieves in 4 mL of CH2Cl2.
b
c
1
For diene 8 determined by GC; for dienes 9 and 10 determined by H NMR. For diene 8, endo/exo molar ratio; for diene 10, 1,4/1,3 adducts
molar ratio. Absolute configuration of the major adduct established by comparison with literature data (S at C2)12
d
B
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1
The most striking feature of the H NMR spectra is the
strong shielding observed for the Ha and methyl protons of the
coordinated enones (Scheme 2). The α vinyl proton resonance
Table 2. DA Reaction of Vinyl Ketones 6 and 7 with
Cyclopentadiene at Low Temperatures
a
Scheme 2. Labeling of Selected Protons of the Enones
isomer
b
ratio
b
bc
,
T
conv
(%)
(endo/
exo) (%)
ee
entry catalyst
R1
(°C) time (h)
(%)
1
1
1
1
1
1
1
2
2
2
2
2
2
2
Me (6)
Me (6)
Me (6)
Me (6)
Et (7)
RT
0
0.25
0.5
6
77
76
90/10
91/9
92/8
92/8
92/8
92/8
93/7
93/7
93/7
94/6
94/6
93/7
96/4
27 (S)
47 (S)
56 (S)
71 (S)
1 (R)
2
appears about 1.7 ppm and the methyl protons about 0.8 ppm
shifted in both cases toward high field with respect to the
corresponding free molecule. The solid-state molecular
structure satisfactorily explains these data (see below).
From a catalytic point of view, the conformation of the
coordinated enone is an important feature, and in this regard,
NOESY data give valuable information (Figure 1). NOE
3
−20
−50
RT
−50
RT
0
82
4
24
0.25
24
80
5
89
6
Et (7)
100
96
4 (R)
7
Me (6)
Me (6)
Me (6)
Me (6)
Me (6)
Et (7)
0.25
0.5
6
17 (S)
45 (S)
74 (S)
88 (S)
89 (S)
1 (R)
8
100
91
9
−20
−50
−70
RT
−50
10
11
12
13
24
165
0.25
24
100
85
91
Et (7)
100
4 (R)
a
Reaction conditions: catalyst 0.025 mmol (5 mol %), ketone 0.5
mmol, diene 2.5 mmol, and 100 mg of 4 Å molecular sieves in 4 mL of
b
c
CH2Cl2. Determined by GC. Absolute configuration of the major
adduct established by comparison with literature data (S at C2).12
Enone Compounds (SM,RC)-[(η5-C5Me5)M(PROPHOS)-
(enone)][SbF6]2. To obtain information about the catalytic
outcome, we have isolated and characterized the enone-
containing complexes (SM,RC)-[(η5-C5Me5)M(PROPHOS)-
(enone)][SbF6]2 (enone = MVK, M = Rh (3), Ir (4); enone
= EVK, M = Ir (5)). Addition of an excess of enone to
dichloromethane solutions of the aqua complexes (SM,RC)-[(η5-
C5Me5)M(PROPHOS)(H2O)][SbF6]2, in the presence of 4 Å
molecular sieves as water scavenger, affords the corresponding
enone complexes 3−5 (eq 1).
Figure 1. Selected NOEs for the enone complexes.
enhancements between the alkyl and the Hc protons as well as
between Ha and Hb confirm an s-trans conformation for the
enone ligand. On the other hand, a NOE relationship of Ha
with the C5Me5 and the ortho protons of the pro-S phenyl ring
of the P1Ph2 group, together with an enhancement of the ortho
protons of the pro-R phenyl ring of the P2Ph2 group when the
methyl enone protons are irradiated, indicates a Z configuration
around the CO bond, in an S configuration at the metal
center with a λ conformation for the M-(PROPHOS)
metallacycle. In this context, it is interesting to point out that
an E configuration around the carbonyl bond of coordinated
MVK was found in the ruthenium compound [(η5-C5H5)Ru-
(R,R-BIPHOP-F)(MVK)][SbF ] recently reported by Kundig
̈
6
et al.12 Finally, another NOE interaction was detected between
the methyl group of the EVK ligand and the methyl protons of
the C5Me5 ring.
In the new complexes, the metal is a stereogenic center and
the preparative reaction is completely diastereoselective
because from −70 °C to RT only one set of sharp resonances
Molecular Structure of Compounds 4 and 5. An X-ray
structural analysis of complexes 4 and 5 was undertaken. A
molecular representation of the cationic complexes is shown in
Figure 2. Selected bond lenghts and angles are summarized in
Table 3. In spite of the limited data quality of 5, it can be clearly
established that both cationic structures show similar general
characteristics. In both half-sandwich complexes, the metals
adopt the common pseudotetrahedral coordination mode, with
the metal coordinated to the η5-C5Me5 ring, to the two
phosphorus atoms of the chelate PROPHOS diphosphine, and
to the oxygen atom of the enone ligand. According to the
ligand priority sequence,13 the absolute configuration at the
metal is S in both complexes. The five-membered metallacycle
Ir−P(1)−C(24)−C(23)−P(2) adopts a λ conformation.
Cremer and Pople ring puckering parameters (Q2 = 0.487(3)
1
was observed in the H, 13C, and 31P NMR spectra. The
complexes have been characterized by the usual analytical and
spectroscopic means, including the molecular structure
determination of compounds 4 and 5.
1
The H NMR spectra indicate the presence of the C5Me5,
PROPHOS, and enone ligands in a 1:1:1 molar ratio. In
particular, three groups of signals in the 4.4−6.3 ppm region
together with one 13C resonance around 215−220 ppm and a
band at ca. 1670 cm−1 in the IR spectra denote the presence of
the coordinated enone. The 31P NMR spectra consist of two
double doublets (3) or two doublets (4, 5) with 103Rh−31P
(∼130 Hz) and 31P−31P (∼40 Hz (3) and ∼11 Hz (4, 5))
couplings.
C
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Figure 2. Molecular structures of the cation in complexes 4 and 5. Hydrogen atoms have been omitted for clarity.
Table 3. Selected Bond Legnths (Å) and Angles (deg) for Complexes 4 and 5
4
5
4
5
a
Ir−P(1)
Ir−P(2)
Ir−O
Ir−G
O−C(38)
C(38)−C(39)
C(39)−C(40)
C(38)−C(41)
C(41)−C(42)
P(1)−Ir−P(2)
P(1)−Ir−O
2.3250(10)
2.3443(10)
2.145(2)
1.879(3)
1.229(5)
1.468(7)
1.293(7)
1.497(6)
2.326(8)
2.346(9)
2.140(13)
1.857(18)
1.24(3)
1.52(4)
1.32(4)
1.52(3)
1.61(4)
84.0(2)
86.4(4)
P(1)−Ir−G
P(2)−Ir−O
131.29(11)
81.67(9)
130.32(11)
128.64(13)
138.6(3)
120.2(4)
124.3(5)
118.7(4)
121.1(4)
132.6(6)
81.4(4)
130.2(6)
125.2(7)
140(2)
120(2)
126(3)
117(3)
123(2)
106(2)
a
P(2)−Ir−G
a
a
O−Ir−G
Ir−O−C(38)
O−C(38)−C(39)
C(38)−C(39)−C(40)
O−C(38)− C(41)
C(39)−C(38)−C(41)
C(38)−C(41)−C(42)
84.06(3)
83.10(8)
a
G represents the centroid of the η5-C5Me5 ring.
Å and ϕ2 = 79.6(2)° in 4 and Q2 = 0.45(2) Å and ϕ2 = 78(1)°
in 5) are characteristic of a mixture between 3E and 3T4
conformations.14 Puckering amplitude values (Q2) are very
similar and close to those observed in other [(η5-C5Me5)M-
(PROPHOS)(L)] half-sandwich complexes.6l,15
Table 4. Selected Structural Parameters (Å, deg) Concerning
CH/π Interactions for Complexes 4 and 5
a
H···Ph
(plane)
γ
C−H···C(17)/
C−
complex H···G(Ph)
angle
C(18)
H···C(Ph)
4
5
3.12
2.87
2.89
2.70
22.2
29.6
2.99/2.95
2.77/2.81
3.39−3.82
3.13−3.56
The relative disposition of the enone groups within the metal
coordination sphere, characterized by the G−Ir−O−C(38)
torsion angle (−52.3(5)° in 4 and −53(3)° in 5), corresponds
to an intermediate situation between a parallel and an
orthogonal arrangement of the enone and cyclopentadienyl
planes. This disposition is suitable for the establishment of
intramolecular CH/π interactions involving the α vinyl proton
(H(39) in Figure 3) of the MVK and EVK ligands, in 4 and 5,
respectively, and the pro-S phenyl ring of the P(1)Ph2 group.
Table 4 collects the values of the structural parameters
characteristic for CH/π interactions.16 These interactions fix
the M−O enone rotamer and place the α vinyl proton of the
enone inside the electronic diamagnetic ring current of the
a
H···G(Ph) represents the distance from the H(39) atom to the
centroid of the phenyl ring G(Ph); H···Ph is the separation from the H
atom to the mean plane of the phenyl ring; γ angle is the angle
between the G(Ph)−H vector and the normal to the phenyl ring; C−
H···C is the contact distances between H atom and phenyl carbon
atoms (≤3.05 Å); C−H···C(Ph) is the separation between H and the
rest of the carbon atoms of the phenyl ring.
phenyl ring of the P(1)Ph2 group (Figure 3), and most
probably, they are also operating in solution, giving rise to the
1
strong shielding observed for this proton in the H NMR
spectra. Furthermore, in this conformation the enone Cα-si face
becomes shielded by the phenyl ring involved in the CH/π
interactions.
On the other hand, the shift to higher energy of the enone
methyl protons' resonance can be accounted for by assuming
that these protons are affected by the electronic diamagnetic
ring current of the pro-R phenyl ring of the P(2)Ph2 group. The
structural parameters observed in the solid state exclude any
significant CH/π interaction between these two fragments.
The conformation proposed in solution for the coordinated
enones, on the basis of NOE measurements, is comparable to
that determined in the solid state by means of the X-ray
diffraction structural study. Thus, both enones adopt an s-trans
conformation and the configuration around the CO carbonyl
bond is Z, placing the M(C5Me5) and vinyl groups at the same
Figure 3. CH/π interactions in complex 5.
D
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procedures. Gas chromatography was performed on Hewlet-Packard
3398 and 6890 Series gas chromatographs equipped with a split-mode
capillary injection system and flame ionization detectors using HP
Ultra-1 (25 m × 0.32 mm), CP-Chirasil-DEX CB (25 m × 0.25 mm),
and Beta Dex 120 (30 m × 0.25 mm) columns. (SM,RC)-[(η5-
C5Me5)M(PROPHOS)(H2O)][SbF6]2 (M = Rh (1), Ir (2)) were
prepared according to published procedures.17
side of the double bond. In this disposition and with the M−O
rotamer fixed by the CH/π interactions, the Cα-si face of the
enone is shielded by the pro-S phenyl ring of the P1Ph2 group,
and therefore, the diene attack would take place preferentially
through the re-face, in good agreement with the catalytic
outcome for the MVK/HCp reaction. The comparison of the
structural parameters of the EVK ligand in 5 to those of its
analogue MVK in 4 sheds light on the different catalytic
behavior of 5. The MVK ligand in 4 is essentially planar. The
maximum deviation from the mean plane, 0.030(5) Å,
corresponds to C(40). However, although the O−C(38)−
C(39)−C(40)−C(41) skeleton of the EVK ligand in 5 is also
essentially planar, the remaining CH3 fragment significantly
deviates from planarity (Figure 3). In fact, the C(41)−C(42)
bond is almost perpendicular to the above-defined plane, the
angle between the C(41)−C(42) vector and the normal to this
plane being only 16(1)°. Probably, this methyl fragment adopts
a similar disposition in solution because we have measured a
NOE relationship between these protons and those of the
C5Me5 ring (see above). In this conformation, this methyl
hinders the approach of the diene through the Cα-re face, and
therefore, both faces are similarly accessible by the diene.
Consequently, even at low temperature, enantioselectivity is
eroded and, according to the catalytic outcome, the attack via
the Cα-si face is slightly preferred for this complex.
Scheme 3. Labeling of the Cation of the Complexes for NMR
Assignments
Preparation of (SM,RC)-[(η5-C5Me5)M(PROPHOS)(MVK)][SbF6]2
(M = Rh (3), Ir (4)) and (SM,RC)-[(η5-C5Me5)Ir(PROPHOS)(EVK)]-
[SbF6]2 (5). At −20 °C, under argon, to a solution of the
corresponding (SM,RC)-[(η5-C5Me5)M(PROPHOS)(H2O)][SbF6]2
(0.09 mmol) complex in CH2Cl2 (4 mL) were added enone (0.9
mmol) and 4 Å molecular sieves (100.0 mg). The resulting suspension
was stirred for 20 min and then was filtered through a cannula. The
filtrate was concentrated to ca. 3 mL. The slow addition of 20 mL of
dry n-hexane afforded yellow crystals, which were filtered off, washed
with n-hexane, and vacuum-dried. Recrystallization from CH2Cl2/n-
hexane yielded pure samples of the complexes.
CONCLUSION
■
In summary, the aqua complexes 1 and 2 generate active
systems that efficiently catalyze the Diels−Alder reaction
between the vinyl ketones MVK and EVK and dienes in
good endo/exo ratio and moderate to good enantioselectivity.
From 1 and 2, the catalyst−substrate intermediates (SM,RC)-
[(η5-C5Me5)M(PROPHOS)(enone)][SbF6]2 can be prepared
in a completely diastereoselective manner. From detailed
structural information about these catalyst−substrate inter-
mediates, in both the solid state and solution, it is possible to
explain the catalytic outcome. The coordinated enone adopts
an s-trans conformation and the configuration around the CO
double bond is Z. Particularly relevant is the existence of CH/π
intramolecular interactions in the solid state that, according to
NMR solution data, most probably remain in solution. These
interactions fix the M−O enone rotamer and conform the
disposition of the enone inside the chiral pocket of the catalyst
defined by the (C5Me5)M(PROPHOS) moiety. As a result, the
Cα-si face of both MVK and EVK intermediates becomes
hindered by a PROPHOS phenyl and, additionally, the CH3
fragment of the enone EVK hampers approach of the diene
through the opposite enantioface. All these structural data are
in good agreement with the experimental catalytic results: while
89% ee's are achieved for the MVK/HCp reaction, only 4% ee
is obtained for the related EVK/HCp system.
(SRh,RC)-[(η5-C5Me5)Rh(PROPHOS)(MVK)][SbF6]2 (3). Yield:
85%. Anal. Calcd for C41H47F12RhOP2Sb2: C, 41.3, H, 3.9. Found:
C, 41.4; H, 3.9. IR (KBr, cm−1): ν(CO) 1664 (m), ν(SbF6) 659 (s).
1H NMR (400.16 MHz, CD2Cl2, −50 °C): δ 7.91−7.26 (m, 20H, Ph),
6.07 (d, J = 17.7 Hz, 1H, Hc), 6.03 (d, J = 10.8 Hz, 1H, Hb), 4.65 (dd, J
= 17.6, 10.8 Hz, 1H, Ha), 3.37 (dt, J = 53.2, 14.2 Hz, 1H, H22), 2.59
(m, 1H, H11), 2.55 (m, 1H, H21), 1.48 (s, 3H, COCH3), 1.42 (m, 15H,
C5Me5), 1.19 ppm (m, 3H, Me). 13C NMR (100.61 MHz, CD2Cl2,
−50 °C): δ 215.57 (CO), 141.87 (C4), 131.83 (C3), 134.44−119.42
(24C, Ph), 99.17 (C5Me5), 31.91 (Me), 31.02 (dd, J(PC) = 37.0, 6.8
Hz, C1), 30.35 (dd, J(PC) = 38.0, 13.5 Hz, C2), 26.20 (COCH3),
14.36 (dd, J(PC) = 18.0, 3.0 Hz, Me), 10.07 ppm (C5Me5). 31P NMR
(161.96 MHz, CD2Cl2, −20 °C): δ 74.51 (dd, J(RhP1) = 130.2 Hz,
J(P1P2) = 39.8 Hz, P1), 50.71 ppm (dd, J(RhP2) = 131.3 Hz, P2).
(SIr,RC)-[(η5-C5Me5)Ir(PROPHOS)(MVK)][SbF6]2 (4). Yield: 79%.
Anal. Calcd for C41H47F12IrOP2Sb2: C, 38.4, H, 3.7. Found: C, 38.3;
1
H, 3.9. IR (KBr, cm−1): ν(CO) 1676 (m) ν(SbF6) 659 (s). H NMR
(400.16 MHz, CD2Cl2, −70 °C): δ 7.91−7.23 (m, 20H, Ph), 6.14 (d, J
= 17.1 Hz, 1H, Hc), 6.07 (d, J = 10.7 Hz, 1H, Hb), 4.62 (dd, J = 18.1,
11.2 Hz, 1H, Ha), 3.26 (dt, J = 53.9, 10.8 Hz, 1H, H22), 2.51 (m, 1H,
H11), 2.42 (m, 1H, H21), 1.56 (s, 3H, COCH3), 1.44 (m, 15H,
C5Me5), 1.20 ppm (m, 3H, Me). 13C NMR (100.61 MHz, CD2Cl2,
−50 °C): δ 216.60 (CO), 141.92 (C4), 132.90 (C3), 134.81−118.43
(24C, Ph), 99.17 (C5Me5), 30.98 (dd, J(PC) = 36.8, 7.7 Hz, C1), 30.35
(m, C2), 26.37 (Me), 14.48 (COCH3), 9.66 ppm (C5Me5). 31P NMR
(161.96 MHz, CD2Cl2, −50 °C): δ 45.74 (d, J(P1P2) = 11.6 Hz, P1)
28.49 ppm (d, P2).
EXPERIMENTAL SECTION
■
General Comments. All solvents were dried over appropriate
drying agents, distilled under argon, and degassed prior to use. Dienes
and dienophiles were distilled prior to use. All preparations have been
carried out under argon. Infrared spectra were obtained as KBr pellets
with a Perkin-Elmer Spectrum One FT-IR spectrophotometer.
Carbon, hydrogen, and nitrogen analyses were performed using a
Perkin-Elmer 240C microanalyzer. 1H, 13C, and 31P NMR spectra were
recorded on a Bruker AV 500 (500.13 MHz), AV-400 (400.16 MHz),
or 300 ARX (300.10 MHz) spectrometer. Chemical shifts are
expressed in ppm upfield from SiMe4 or 85% H3PO4 (31P). NOESY
and 13C, 31P, 1H correlation spectra were obtained using standard
(SIr,RC)-[(η5-C5Me5)Ir(PROPHOS)(EVK)][SbF6]2 (5). Yield: 83%.
Anal. Calcd for C42H49F12IrOP2Sb2: C, 38.9, H, 3.8. Found: C, 39.0;
1
H, 3.3. IR (KBr, cm−1): ν(CO) 1677 (m) ν(SbF6) 652 (s). H NMR
(400.16 MHz, CD2Cl2, −70 °C): δ 7.86−7.20 (m, 20H, Ph), 6.29 (d, J
= 17.7 Hz, 1H, Hc), 6.03 (d, J = 10.8 Hz, 1H, Hb), 4.42 (dd, J = 17.7,
11.1 Hz, 1H, Ha), 3.28 (dt, J = 53.2, 14.2 Hz, 1H, H22), 2.43 (m, 1H,
H11), 2.37 (q, J = 8.7 Hz, 2H, COCH2), 2.34 (m, 1H, H21), 1.41 (m,
15H, C5Me5), 0.83 (t, J = 7.0 Hz, 3H, Me), 0.23 ppm (t, J = 7.3 Hz,
3H, COCH2CH3). 13C NMR (100.61 MHz, CD2Cl2, −50 °C): δ
220.54 (CO), 140.79 (C4), 131.16. (C3), 136.60−118.34 (24C, Ph),
99.68 (C5Me5), 33.12 (dd, J(PC) = 40.6, 7.7 Hz, C1), 33.16
E
dx.doi.org/10.1021/om300346s | Organometallics XXXX, XXX, XXX−XXX

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Article
(CH2CH3), 31.41 (dd, J(PC) = 38.3, 9.2 Hz, C2), 14.34 (Me), 9.07
(CH2CH3), 8.74 ppm (C5Me5). 31P NMR (161.96 MHz, CD2Cl2, −50
°C): δ 45.88 (d, J(P1P2) = 11.0 Hz, P1), 28.99 ppm (d, P2).
General Procedure for Catalytic Diels−Alder Reactions
between Enones and Dienes. The corresponding (SM,RC)-[(η5-
C5Me5)M(PROPHOS)(H2O)][SbF6]2 complex (0.025 mmol, 5 mol
%) was dissolved in 3 mL of dry CH2Cl2 under argon at −20 °C, and
100 mg of activated 4 Å molecular sieves and the enone (0.500 mmol)
were added. After 15 min the mixtures were introduced in a cryogenic
bath at the appropriate temperature, and diene (2.5 mmol) in 1 mL of
CH2Cl2 was added. The reaction was monitored by gas chromatog-
raphy (GC) and quenched, by addition of 0.1 mL of MeCN, at the
specified times. Yields and endo/exo ratios were determined by GC
analysis. Finally, the mixture was concentrated to ca. 0.3 mL, filtered
through silica gel, and washed with n-pentane/diethyl ether (9:1).
Liquids were removed under vacuum (ice bath) before the
determination of the enantiomeric purity. Enantiomeric excesses
were determined by gas chromatography (for detailed procedures see
the Supporting Information). The absolute configuration of the major
adduct was assigned by comparison with literature data.12
Crystal Structure Determination of Complexes 4 and 5. X-
ray diffraction data were collected at 100(2) K with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) using narrow ω
rotation (0.3°) on a Bruker SMART APEX CCD diffractometer.
Intensities were integrated and corrected for absorption effects with
the SAINT-PLUS program.18 The structures were solved by direct
methods with SHELXS-97.19 Refinement, by full-matrix least-squares
on F2, was performed with SHELXL-97.20 Hydrogen atoms were
included in calculated positions and defined with displacement and
positional riding parameters. In both structures, in addition to the
internal configuration reference of the (R)-PROPHOS ligand, the
Flack parameter has been refined as a check of the correct absolute
structure determination.21 Particular details concerning the presence of
solvent and specific refinement are listed below.
ASSOCIATED CONTENT
* Supporting Information
Analytical procedures, data of adducts, and X-ray crystallo-
graphic information files containing full details of the structural
analysis of complexes 4 and 5 (CIF format). This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: dcarmona@unizar.es.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Ministerio de Educacion
2009-10303/BTQ) and Gobierno de Aragon
Europeo (Grupo Consolidado: Catalizadores Organometal
Enantioselectivos) for financial support. This work was
supported by the CONSOLIDER INGENIO-2010 program
■
́
y Ciencia (Grant CTQ
and Fondo Social
icos
́
́
́
under the project Factorıa de Cristalizacion
́
(CSD2006-0015).
isis Homogenea
A.A. thanks the Instituto Universitario de Catal
́
́
(IUCH) for a grant. P.G.-O. acknowledges financial support
from the CSIC, “JAE-Doc” program, contract cofunded by the
ESF.
REFERENCES
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Crystal data for 4: C41H47F12IrOP2Sb2·CH2Cl2; M = 1366.35;
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