Chiral octahedral phosphano-oxazoline iridium(III) complexes as catalysts in asymmetric cycloaddition reactions

Article abstract of DOI:10.1021/om301089c

The synthesis and characterization of cationic iridium(III) aqua complexes of the formula [IrH(H₂O)(PN*)(PP)][SbF₆]₂ (PN* = chiral phosphano-oxazoline ligand; PP = diphosphane) as well as that of the OPOF₂-conta

Full text of DOI:10.1021/om301089c

Article
pubs.acs.org/Organometallics
Chiral Octahedral Phosphano−Oxazoline Iridium(III) Complexes as
Catalysts in Asymmetric Cycloaddition Reactions
́
́
Daniel Carmona,* Joaquina Ferrer,* Nestor Garcıa, Paola Ramırez, Fernando J. Lahoz,
́
́
Pilar Garcıa-Orduna, and Luis A. Oro
̃
́
́
́
́ ́
isis Homogenea (ISQCH), CSIC-Universidad de Zaragoza, Departamento de Quımica
Instituto de Sıntesis Quımica y Catal
Inorganica, Pedro Cerbuna 12, 50009 Zaragoza, Spain
́
S
* Supporting Information
ABSTRACT: The synthesis and characterization of cationic iridium(III)
aqua complexes of the formula [IrH(H2O)(PN*)(PP)][SbF6]2 (PN* =
chiral phosphano−oxazoline ligand; PP = diphosphane) as well as that of
the OPOF2-containing complex [IrH(OPOF2)(PNiPr)(dppp)][SbF6]
(10) are reported. The X-ray molecular structures of [IrH(H2O)-
(PNInd)(dppe)][SbF6]2 (1), [IrH(H2O)(PNInd)(dppen)][SbF6]2 (2),
and 10a have been determined. Dichloromethane solutions of these aqua
complexes efficiently catalyze the enantioselective 1,3-dipolar cyclo-
addition of the nitrone N-benzylidenephenylamine N-oxide to meth-
acrolein and Diels−Alder reactions between cyclopentadiene and trans-β-nitrostyrenes. In the first case, the catalytic reaction
occurs with excellent endo selectivity and ee up to 85%; the Diels−Alder reaction occurs rapidly at room temperature with good
endo:exo selectivity and ee up to 90%. The dipolar cycloaddition intermediates [IrH(methacrolein)(PNInd)(PP)][SbF6]2 (PP =
(S,S)-chiraphos (11), (R)-prophos (12)) have been characterized, and the molecular structure of 11 has been determined by an
X-ray structural analysis.
unsaturated ketones promoted by 2-substituted pyrrolidines.10
Therefore, as far as we know, no metal-catalyzed asymmetric
DA reactions with this type of dienophile have been reported so
far.
INTRODUCTION
■
Asymmetric catalysis plays a decisive role in modern synthetic
organic chemistry, and chiral metal complexes are frequently
employed as effective homogeneous catalysts in order to
achieve both high reactivity and control of the selectivity.1 One
of the most attractive methodologies to obtain optically active
organic compounds involves cycloaddition reactions, in which
several adjacent stereogenic centers can be created in a
concerted fashion.2 Among them, the enantioselective 1,3-
dipolar cycloaddition (DC) reaction of an alkene and a nitrone
affords five-membered isoxazolidines containing up to three
contiguous asymmetric carbon atoms (Scheme 1a)3 and the
Diels−Alder (DA) reaction between an alkene and a diene can
generate up to four contiguous stereogenic centers within a
cyclohexene framework (Scheme 1b).4
To study these metal-catalyzed cycloadditions, alkenes that
enable a bidentate coordination to the metallic Lewis acid, such
as 3-alkenoyloxazolidinones (Scheme 1c), have been frequently
employed as model substrates. In contrast, examples of
activation of monofunctionalized alkenes are scarce. Only in
the last years have some examples of the use of monodentate
enals as dipolarophiles in enantioselective DC5 reactions and as
dienophiles in enantioselective DA reactions, the latter mostly
catalyzed by half-sandwich complexes of Rh, Ir, and Ru,6−9
been reported. However, other activated alkenes capable of
linking one-point-binding metallic moieties in a monodentate
fashion, such as nitroalkenes, remain almost unexplored. In
particular, the only asymmetric DA processes studied involving
nitroalkenes as dienophiles are their reactions with α,β-
On the other hand, chiral phosphano−oxazoline ligands
(PN*) have proven to be useful in several metal-catalyzed
reactions.11 In some instances, for example, when IrI/PN*
systems are applied to the asymmetric hydrogenation of
unfunctionalized olefins,12 they outperform P,P and N,N
ligands.13 In this context, we have reported the preparation
and characterization of Ir(I) compounds with the (4S)-2-[2-
(diphenylphosphanyl)phenyl]-4-isopropyl-4,5-dihydrooxazole
(PNiPr) ligand14 and employed some of them as catalysts for
asymmetric Michael additions.14c It is interesting to point out
that although in many cases Ir(III) species are considered key
intermediates in the catalytic processes involving PN* ligands,
the use of well-defined IrIII/PN* complexes as catalyst
precursors is uncommon, with the exception of (η5-C5Me5)IrIII
derivatives.9e
With all these concerns in mind, in the present paper we
disclose a general preparative route of octahedral iridium(III)
aqua complexes containing chiral chelate phosphano−oxazo-
lines and diphosphanes (PP) of general formula [IrH(H2O)-
(PN*)(PP)][SbF6]2 and their use as catalyst precursors in the
asymmetric 1,3-dipolar cycloaddition of the nitrone N-
benzylidenephenylamine N-oxide to methacrolein and in the
Received: November 13, 2012
© XXXX American Chemical Society
A
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Scheme 1. DC of Nitrones and Alkenes (a), DA Reaction (b), and 3-Alkenoyloxazolidinones (c)
Diels−Alder reaction of trans-β-nitrostyrenes with cyclo-
pentadiene. The complete analytical and spectroscopic
characterization of the new complexes, including the molecular
structure determination by X-ray diffraction of some
representative examples, is also reported.
high field (around −27 ppm). The δ value for the hydride
resonance is indicative of a trans relationship between the
hydride and the oxygen atom of the coordinated water.16 For
the PNiPr-containing complexes 6−9, very low chemical shift
values were registered for the isopropyl methyl protons (from
0.65 to −0.58 ppm). We will turn to this point when discussing
the molecular structure of the related complex 10. On the other
hand, the 31P{1H} NMR spectra of all compounds exhibit ABX
or AMX patterns characteristic of a meridional arrangement
around the metal with a large trans J(P,P) (about 320 Hz) and
two smaller cis J(P,P) coupling constants.
The molecular structure determination by X-ray diffraction
methods for the unique isomer of compounds 1 and 2 confirms
the solution structural data and reveals a C configuration17 for
these diastereomers. Taking advantage of the similarity of the
NMR data for all the major isomers of the PNInd-containing
compounds 1−5, we propose for all of them a C configuration
(Figure 1), according to the stereochemical rules.17 For the
RESULTS AND DISCUSSION
■
Preparation and Characterization of the Aqua
Complexes [IrH(H2O)(PN*)(PP)][SbF6]2. The aqua com-
plexes [IrH(H2O)(PN*)(PP)][SbF6]2 (1−9) were prepared
in high yield by treating the corresponding chlorides15
[IrClH(PN*)(PP)][SbF6] with equimolar amounts of AgSbF6
and excess of water in dichloromethane/acetone mixtures,
according to eq 1.
Figure 1. Proposed structures for the cations of the major
diastereomers of 1−9.
The preparative route employed is highly diastereoselective:
complexes 1, 2, 4, 6, and 7 were obtained as a single isomer,
while complexes containing dppp (3, 8) and (R)-prophos (5,
9) as diphosphanes were isolated as mixtures of two
diastereomers (84:16 (3), 68:32 (5), 85:15 (8), and 62:38
(9) molar ratio mixtures). With labels a and b we represent a
pair of epimers at the metal, and labels a and a′ are used for
coordination isomers (see below); in both cases label a refers to
the major isomer.
major isomer (labeled a) of the PNiPr-containing complexes,
the configuration at the metal was determined by NOE
measurements. Thus, irradiation of the hydride resonance
induces NOE enhancement on the CH proton of the iPr group.
This NOE relationship is only compatible with an A
configuration for these complexes (Figure 1).
By analogy with the related chloride isomers (see ref 34), we
assign an A configuration to the minor isomer of complex 3
(3b) and a C configuration to the b isomer of the PNiPr-
containing complex 8. It is interesting to note that only for
dppp derivatives are both epimers at metal obtained. Most
probably, the higher conformational flexibility of the three-
carbon chain connecting the P atoms of the diphosphane (only
two carbons in the remaining cases)18 allows for the
accommodation of the water molecule at both sides of the
equatorial plane defined by the two phosphorus atoms of the
diphosphane and the phosphorus and nitrogen atoms of the
PN* ligand.
The new complexes were characterized by mass spectrom-
etry, IR and NMR spectroscopy, and microanalyses and, for
compounds 1 and 2, by X-ray crystal structure determinations.
The NMR data were consistent with the presence of PN*, PP,
hydride, and H2O ligands in a 1:1:1:1 molar ratio (see the
Experimental Section). Stereochemical assignments were
accomplished through NOE experiments and two-dimensional
homonuclear and heteronuclear (13C−1H, 31P−1H) correla-
tions. The presence of coordinated water was confirmed by an
IR band in the 3600−3450 cm−1 region, along with a broad 1H
NMR signal at δ ranging from 3 to 5 ppm. The hydride ligand
is denoted by a weak ν(Ir−H) band at high energy (2296−
On the other hand, assignment of the stereochemistry to the
a, a′ pair of isomers of the (R)-prophos-containing compounds
5 and 9 was accomplished by comparison of the 13C NMR
resonance of the asymmetric carbon of the (R)-prophos ligand
1
2326 cm−1) and a H NMR doublet of pseudotriplets at very
B
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with that of the corresponding chlorides (see ref 34). The
phosphorus atom nearest to the asymmetric carbon of the
diphosphane is trans to the phosphorus atom of the PN* ligand
in isomers a′ and cis in the corresponding a isomers (Figure 2).
Table 1. Selected Bond Distances (Å) and Angles (deg) for
1, 2, and 10a
1
2
10a
Ir−P(1)
Ir−P(2)
Ir−P(3)
Ir−O(2)
Ir−N
Ir−H
N−C(45)
N−C(54)/C(47)
P(1)−Ir−P(2)
P(1)−Ir−P(3)
P(1)−Ir−N
P(1)−Ir−O(2)
P(1)−Ir−H
P(2)−Ir−P(3)
P(2)−Ir−N
P(2)−Ir−O(2)
P(2)−Ir−H
P(3)−Ir−N
P(3)−Ir−O(2)
P(3)−Ir−H
N−Ir−O(2)
N−Ir−H
2.353(2)
2.287(2)
2.345(2)
2.238(7)
2.145(7)
1.6036
2.341(2)
2.286(2)
2.335(2)
2.258(6)
2.143(7)
1.599(10)
1.312(11)
1.489(11)
83.79(8)
174.73(8)
95.5(2)
2.3599(18)
2.294(2)
2.3235(18)
2.231(5)
2.120(6)
1.589(10)
1.269(9)
1.524(9)
87.38(8)
174.49(8)
92.83(18)
90.03(13)
89(3)
1.283(11)
1.496(11)
83.52(7)
174.68(10)
94.4(2)
90.8(2)
90.2
a
Figure 2. Proposed structures for the cations of 5a′ and 9a′.
92.9(2)
In an attempt to obtain single crystals of complex 8, we
carried out the reaction depicted in eq 1 but using AgPF6 as
halogen scavenger instead of AgSbF6 (eq 2). From the reaction
medium a pale yellow solid was isolated that, according to
analytical and spectroscopic data, was formulated as the
OPOF2-containing complex [IrH(OPOF2)(PNiPr)(dppp)]-
[SbF6] (10). The difluorophosphate anion most probably
derives from partial hydrolysis of the PF6 anion.9g,19 Complex
10 was obtained as a 92:8 molar ratio mixture of the two
isomers 10a,b. Their spectroscopic properties are comparable
to those of 8a,b, respectively (see the Experimental Section).
Single crystals of the major isomer, 10a, were obtained, and its
molecular structure was solved by X-ray diffraction.
93(5)
100.60(8)
172.6(2)
103.47(19)
84.3
97.22(8)
173.8(2)
103.53(16)
77(3)
98.09(8)
172.78(18)
101.94(18)
86(3)
81.1(2)
91.53(19)
86.9
83.0(2)
81.67(18)
89.48(13)
91(3)
91.9(2)
82(4)
83.7(3)
88.6
82.7(2)
85.3(2)
97(3)
87(3)
O(2)−Ir−H
Ir−N−C(45)
Ir−N−C(54)
172.3
174(5)
172(3)
125.6(6)
126.2(6)
107.9(7)
125.9(6)
128.0(5)
106.0(7)
129.8(5)
123.6(5)
106.4(6)
C(45)−N−C(54)
[IrHCl(PNiPr)(dppp)][SbF ] + AgPF + H2O
6
6
a
Label C(47) corresponds to 10a.
→ [IrH(OPOF )(PNiPr)(dppp)][SbF ] + AgCl
2
6
(2)
10
Molecular Structures of Complexes 1, 2, and 10a.
Single crystals suitable for X-ray diffraction analysis of the three
complexes were obtained by slow diffusion of diethyl ether into
dichloromethane solutions of the compounds. Views of the
three cations are depicted in Figure 3. Selected structural
parameters are summarized in Table 1.
All the three cations display an octahedral coordination
around the iridium, with a hydride trans to the oxygen atom of
coordinated water (1 and 2) or to one of the two oxygens of a
difluorophosphate anion (10a); a phosphano−oxazoline,
coordinated through its nitrogen and phosphorus atoms, and
a diphosphane, coordinated through the two phosphorus
atoms, occupy an equatorial plane of the octahedron. According
to stereochemical nomenclature rules,17 the three cations are
OC-6-24 isomers. The PNInd-containing cations (compounds
1 and 2) present a C configuration; however, the PNiPr-
containing cation (complex 10a) adopts an A configuration, in
good agreement with the observed NOE relationship (see
above). As a consequence of the trans influence of the hydride
ligand, the Ir−O bond distances, 2.238(7) Å (1), 2.258(6) Å
(2), and 2.231(5) Å (10a), are elongated in comparison to the
values observed in octahedral Ir complexes in which the oxygen
atom is not trans to an hydride ligand (mean value 2.134(7)
Å).20 The Ir−P(1) and Ir−P(2) bond distances are in the range
Figure 3. Molecular representations of the cations of 1, 2, and 10a. Selected bond distances (Å) and angles (deg) for the OPOF2 moiety in complex
10a: O(2)−P(4), 1.459(5); O(3)−P(4), 1.449(5); P(4)−F(7), 1.537(5); P(4)−F(8), 1.538(5). Ir−O(2)−P(4), 167.1(4); O(2)−P(4)−O(3),
121.1(4); O(2)−P(4)−F(7), 108.2(3); O(2)−P(4)−F(8), 106.7(4); O(3)−P(4)−F(7), 110.3(3); O(3)−P(4)−F(8), 109.0(3); F(7)−P(4)−F(8),
99.3(3).
C
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Scheme 2. DC of N-Benzylidenephenylamine N-Oxide and Methacrolein
a
Table 2. Catalytic Results for the DC Reaction
b
c
c
d
entry
precatalyst
T (°C)
time (h)
conversn (%)
3,4-endo (%)
3,5-endo (%)
ee (%)
1
1
1
2
2
room temp
−25
room temp
1.5
96
1.5
48
1.5
96
48
96
96
3
47
29
85
20
45
34
98
20
87
82
36
47
54
47
38
29
22
9
8
91
92
−22/−21
−31/−25
−23/−12
−32/−31
−72/−6
−85/−14
−53/−26
−71/−22
77/38
2
e
3
8
90
4
−25
room temp
−25
22
37
41
5
78
63
59
5
3a,b (84:16)
6
3a,b (84:16)
e
7
4
room temp
93
8
4
−25
−25
room temp
7
93
40
9
5a,a′ (68:32)
6
60
7
e
10
11
12
13
14
15
16
17
91
19/10
6
−25
room temp
−25
96
1.5
96
1.5
96
1.5
96
15
14
25
13
17
11
12
85
86
75
87
83
89
88
5/32
7
18/2
7
27/18
8a,b (85:15)
8a,b (85:15)
9a,a′ (62:38)
9a,a′ (62:38)
room temp
8/8
−25
room temp
−25
25/25
13/18
22/35
a
Reaction conditions: catalyst 0.030 mmol, (5.0 mol %), methacrolein 4.2 mmol, nitrone 0.60 mmol, 4 Å molecular sieves (100 mg), in CH2Cl2 (2.5
b
c
d
1
1
mL). Based on nitrone. Determined by H NMR spectroscopy. Determined by integration of the H NMR signals of the diastereomeric (R)-
methylbenzylimine derivatives. Positive ee values correspond to the (3R,4R)-endo adduct. For the 3,5-endo adduct, positive ee values are arbitrarily
assigned to the ee in the lower field 3-H proton H NMR signal. Exo adducts, 2%.
e
1
found for Ir(III) hydride complexes containing diphosphanes,21
the former being longer than the second, probably due to the
higher trans influence of the P(3) phosphorus atom. The bond
distances and angles of PN* ligands are in good agreement with
those of previously reported PN*/iridium(III) complexes.9e
The difference of the two N−C bond distances clearly indicates
the localization of the NC double bond on the N−C(45)
pair, with no evident electron density delocalization. The
bidentate coordination of these ligands leads to the formation
of a six-membered Ir−N−C(45)−C(44)−C(39)−P(3) metal-
lacycle that, according to the Cremer and Pople parameters,22
adopts a highly puckered slightly twisted screw-boat 1S6 (1 and
interactions are considered, the ability of the coordinated
water to act as a potential hydrogen bond donor should be
taken into account. In this context, 1 and 2 cations interact
through O−H···O hydrogen bonds with water and diethyl ether
solvent molecules, respectively (O···O distances 2.613(14) Å
(1) and 2.701(11) Å (2)).
1,3-Dipolar Cycloaddition of N-Benzylidenephenyl-
amine N-Oxide and Methacrolein. Complexes 1−9 were
tested as catalyst precursors for the 1,3-dipolar cycloaddition
between methacrolein and N-benzylidenephenylamine N-oxide
(Scheme 2). Table 2 gives a selection of the results obtained
together with the reaction conditions employed. The collected
results are the average of, at least, two comparable reaction
runs. Before the addition of the nitrone, the precursors were
treated with methacrolein, in the presence of molecular sieves
(see the Experimental Section). Under these conditions, the
formation of active methacrolein complexes is favored at the
expense of the initial aqua complexes (see below). All
complexes generate active systems for the essayed reaction.
An endo preference was showed in all cases, and most of the
reactions occur with perfect endo selectivity. Moderate to good
enantioselectivity was obtained, the PNInd derivatives 1−5
being more enantioselective. Strikingly, the highest ee values
were achieved when mixtures of diastereomers were employed
as precatalysts (entries 5, 6, and 9). Lowering the reaction
temperature had very little effect on the diastereoselectivity, but
significant improvements in the enantioselectivity were
achieved.
6
2) or an analogous inverted S1 conformation (10a). This
structural feature, together with the close values of dihedral
angles between NIrP and N−C(45)−C(44)−C(39)−P planes
(130.8(2)° (1), 136.7(2)° (2), and 135.1(2)° (10a)) seems to
be characteristic of these phosphano−oxazoline fragments. We
have already pointed out the marked rigidity of these ligands
and their coordination toward the metal (see ref 34). In
particular, this conformation forces the pro-R (C(33)−C(38))
and pro-S (C(27)−C(32)) phenyl groups of the PPh2 moiety of
the PN* ligand to occupy pseudo-axial and pseudo-equatorial
positions, respectively. This puckering is most probably
maintained in solution, as evidenced by the shielding of the
methyl isopropyl groups of 10a (see above), due to the
diamagnetic electronic current of the pseudo-axial PPh ring.
It is interesting to note that substitution of chlorine atom by
a water molecule (eq 1) seems to hardly alter the structural
parameters of the metal coordination sphere, as shown by the
very similar geometrical values of 2 and its parent chloride
[IrClH(PNInd)(dppen)]+. However, when intermolecular
Methacrolein Complexes [IrH(methacrolein)(PNInd)-
(PP)][SbF6]2 (PP = (S,S)-chiraphos (11), (R)-prophos (12)).
To get a better insight into the catalytic reaction, we have
D
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studied, by NMR spectroscopy, the solution behavior of
complexes 1−9 in the presence of an excess of methacrolein. In
most cases, complex mixtures of unidentified compounds are
formed, but when at 0 °C 5 equiv of methacrolein was added to
dichloromethane solutions of the PNInd-containing aqua
complexes [IrH(H2O)(PNInd)(PP)]2+ (PP = (S,S)-chiraphos
(4), (R)-prophos (5)), the clean formation of the correspond-
ing new cation [IrH(methacrolein)(PNInd)(PP)]2+ was
the olefinic proton at 5.80 ppm was irradiated is only
compatible with an s-trans conformation for the coordinated
methacrolein.
Although the structural parameters obtained from the X-ray
analysis of 11 are of limited accuracy as a consequence of the
weakly diffracting sample, we present the refined structure as it
supportstogether with the spectroscopic data commented on
abovethe connectivity and conformation of the complex and
allows rationalization of the catalytic results (see below).
In the asymmetric unit, two isostructural independent
molecules (11 and 11′) were present with no significant
differences in their geometrical parameters. A view of the cation
of 11 is depicted in Figure 4. The iridium atom displays a
distorted-octahedral geometry, coordinated to the PNInd and
(S,S)-chiraphos chelate ligands (κ2P,N and κ2P,P coordination,
respectively), to one hydride, and to a methacrolein fragment
(η1 coordinated through its oxygen atom). The hydride and the
oxygen atoms are mutually trans, the resulting diastereomer
adopting an OC-6-24-C stereochemistry. The aldehyde
coordinates as a planar molecule with an s-trans conformation
and an E configuration around the carbonylic double bond.
This molecular structure is compatible with the reported
spectroscopic data, indicating that the solid-state structure is
essentially maintained in solution.
1
observed, according to H and 31P NMR measurements (eq
3). At this temperature, the aqua and methacrolein complexes
are in equilibrium, but this equilibrium can be completely
shifted to the right by the addition of 4 Å molecular sieves. In
fact, from these solutions the methacrolein cations can be
isolated as the hexafluoroantimonate salts [IrH(methacrolein)-
(PNInd)(PP)][SbF6]2 (PP = (S,S)-chiraphos (11), (R)-
prophos (12)). Complex 11 was obtained as the sole
diastereomer; however, due to the C1 symmetry of the (R)-
prophos ligand, two diastereomers, 12a and 12a′, in the molar
ratio 74:26 were isolated for complex 12.23
The puckering amplitude of the six-membered PN*
metallacycle is very similar to that of complexes 1, 2 and
1
10a; it exhibits the same S6 screw-boat conformation already
The complexes were characterized by spectroscopic means
and by the X-ray molecular structure determination of
compound 11. One large and two short P,P couplings and
three cis-type P,H couplings in the NMR spectra strongly
indicated a meridional arrangement for the three phosphorus
atoms. Additionally, the chemical shift of the hydride, about
−26 ppm, close to those observed for this ligand in the parent
aqua compounds, is indicative of a trans relationship between
the hydride and the oxygen atom of the methacrolein ligand.
noted.22 In contrast, significant differences have been observed
in the deviation from planarity and in the conformation of the
PP metallacycles of the complexes reported in this paper (and
in ref 34), reflecting their flexibility.24 In 11 the five-membered
Ir−P(1)−C(13)−C(14)−P(2) metallacycle adopts a twisted
4T3 conformation. This disposition seems to be suitable for the
establishment of intramolecular CH/π interactions involving
the CHO proton and the pro-R phenyl ring connected to the
P(1) atom of the (S,S)-chiraphos ligand (Figure 4) of potential
relevance for its catalytic behavior.5o From the calculated
positions of the hydrogen atoms, short H···Ph interatomic
distancesshorter than the sum of van der Waals radii (H−
C)have been estimated.25 These interactions fix the Ir−O
rotamer and place the aldehydic proton inside the electronic
diamagnetic current of the involved phenyl ring. Most probably,
the CH/π interactions are also operating in solution, giving rise
to the strong shielding observed for the CHO proton in the 1H
NMR spectrum of 11.
1
The H and 13C NMR data show resonances attributable to
coordinated methacrolein. In particular, a 13C NMR peak
around 207 ppm (CHO) and two new 1H NMR resonances at
about 6 ppm (olefinic protons) denote the presence of this
ligand in the molecule. While in complex 11 the aldehyde CHO
proton resonates at 7.39 ppm, about 2.1 ppm shifted to higher
field with regard to the corresponding free methacrolein
resonance (9.53 ppm), the aldehyde proton in 12a and 12a′ is
only shielded by ca. 0.5 and 1.3 ppm, respectively. Finally, in
complex 11, enhancement of the CHO proton resonance when
Figure 4. Molecular representations of the cation of complex 11 showing the CH···π interaction.
E
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Article
a
In the conformation observed for the coordinated enal, its Si
face becomes shielded by the phenyl ring involved in the CH/π
interaction and, therefore, nitrone attack would preferentially
occur through the Re face of this substrate. In particular, a 3,4-
endo nitrone attack will give (3S,4S)-endo enantiomers which
are the major isomers experimentally obtained. Similarly, a 3,5-
endo nitrone attack through the enal Re face would render
(3R,5S)-endo adducts as the major enantiomers.
Table 4. Catalytic Results for the DA Reaction of
Cyclopentadiene with Substituted Nitrostyrenes Catalyzed
by Complex 5
Diels−Alder Reaction between Cyclopentadiene and
Nitrostyrenes. As stated in the Introduction, as far as we
know, no metal-catalyzed asymmetric DA reactions involving
nitroalkenes as dienophiles have been reported so far. As we
have shown, the coordinated molecule of water in complexes
1−9 can be readily displaced by oxygen-donor substrates such
as methacrolein. Then, we considered the possibility of
activating the commercially available nitrostyrene toward the
DA reaction with cyclopentadiene.26 Indeed, complexes 1−9
catalyze this reaction. Table 3 gathers a selection of the results
c
time
(h)
conversn
isomer ratio
ee
bc
,
d
entry
R
(%)
endo:exo (%)
(%)
1
2
3
4
5
6
H
0.75
1.5
17
100
100
100
82
100:0
99:1
98:2
99:1
91:9
97:3
98:2
87:13
99:1
84
79
79
67
87
77
86
70
71
2-OMe
2,3-(OMe)2
2,4-(OMe)2
2-CF3
71
1
100
100
49
2-Cl
0.5
144
144
143
e
7
2-Cl
8
9
2,3-Cl2
2,4-Cl2
98
59
a
Table 3. Catalytic Results for the DA Reaction of
a
Reaction conditions: catalyst 0.030 mmol, (5.0 mol %), nitrostyrene
0.6 mmol, cyclopentadiene 3.60 mmol, 4 Å molecular sieves (100 mg),
in CH2Cl2 (4 mL), at room temperature. Based on nitrostyrene.
Determined by H NMR spectroscopy. Determined by HPLC. At
Cyclopentadiene with Nitrostyrene
b
c
d
e
1
−25 °C.
c
time
(h)
conversn
isomer ratio
ee
(%)
bc
,
d
nitroalkene, although two substituents in this ring strongly slow
down the reaction rate. Taking into account the electrophilic
character of the dienophile in DA reactions, this rate
deceleration has to be based on steric grounds because it is
even stronger for electron-withdrawing groups (entries 8 and
9). In general, excellent endo selectivity was observed and good
ee values were obtained for both types of substituents.
entry
precat.
(%)
endo:exo (%)
1
2
3
4
5
6
2
18.5
4
1
15
96:4
97:3
96:4
96:4
100:0
100:0
97:3
96:4
94:6
95:5
1
2
100
100
43
59
44
20
84
90
53
46
4
4.5
8
3a,b (84:16)
5a,a′ (68:32)
5a,a′ (68:32)
6
0.75
45
5
100
23
e
7
8
97
CONCLUSIONS
■
9
7
6
100
25
In summary, we have prepared phosphano−oxazoline/iridium-
(III) complexes of the formula [IrH(H2O)(PN*)(PP)][SbF6]2
through a highly stereoselective synthetic route. The new
complexes have been completely characterized, including the
assignment of the cationic absolute configuration. The high
trans influence of the hydride ligand originates in the long, and
potentially labile, Ir−O bond distances. The PN* metallacycles
always showed a highly puckered conformation, apparently
quite rigid, internally and in their coordination to the metal.
These aqua complexes are active precursors for the 1,3-dipolar
cycloaddition between the nitrone N-benzylidenephenylamine
N-oxide and methacrolein. The reaction occurs with excellent
diasteroselectivity toward the endo isomers and ee values up to
85%. The methacrolein intermediates [IrH(methacrolein)
(PNInd)(PP)][SbF6]2 (11 and 12) have been isolated and
characterized. The structural analysis has confirmed the planar
coordination of the methacrolein molecule, with a common s-
trans conformation and E configuration, and the presence of the
catalytic relevant CH/π interaction. The aqua compounds also
generate active and selective systems for the Diels−Alder
reaction of trans-β-nitrostyrenes with cyclopentadiene, exhibit-
ing high diastereoselectivity toward the endo isomers and ee
values up to 90%. This is the first example reported up to now
of a metal-catalyzed asymmetric Diels−Alder reaction involving
nitroalkenes as dienophiles.
10
11
8a,b (85:15)
9a,a′ (62:38)
8
7
62
25
a
Reaction conditions: catalyst 0.030 mmol, (5.0 mol %), nitrostyrene
0.6 mmol, cyclopentadiene 3.60 mmol, 4 Å molecular sieves (100 mg),
in CH2Cl2 (4 mL), at room temperature. Based on nitrostyrene.
Determined by H NMR spectroscopy. Determined by HPLC. At
b
c
d
e
1
−25 °C.
obtained at room temperature, together with the remaining
reaction conditions employed. The collected results are the
average of at least two comparable reaction runs. In almost all
cases, quantitative conversions were achieved after a few hours
of treatment at room temperature. The catalyzed reaction
shows endo preference and for complex 5 occurs with perfect
endo selectivity. Moderate to good ee values were achieved, and
as expected, the enantioselectivity increases when the temper-
ature decreases (entries 6 and 7).
As complex 5 gives the best results, i.e. quantitative
conversion in 45 min, perfect endo selectivity, and 84% ee, at
room temperature, we tested this catalyst precursor in the
reaction of substituted nitrostyrenes with cyclopentadiene.
Table 4 gives the results obtained operating at room
temperature. As in the preceding examples, the collected
results are the average of at least two comparable reaction runs.
The catalytic system admits both electron-withdrawing and
electron-donor substituents into the aromatic ring of the
F
dx.doi.org/10.1021/om301089c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
87.38 (C5); 80.77 (d, J = 2.0 Hz, C4); 36.25 (C6); 33.64 bd, J = 42.9,
9.1 Hz, 26.92 dd, J = 36.9, 7.1 Hz (2C, (CH2)2).31P{1H} NMR (121.5
MHz, CD2Cl2, room temperature): δ 36.48 (d, J(P3,P1) = 317.7 Hz,
P1); 12.11 (d, J(P3,P2) = 13.7 Hz, P2); 2.04 (dd, P3).
EXPERIMENTAL SECTION
■
General Comments. All solvents were dried over appropriate
drying agents, distilled under argon, and degassed prior to use. All
preparations have been carried out under argon. Infrared spectra were
obtained as Nujol mulls with a Perkin-Elmer Spectrum One FT IR
spectrophotometer. Carbon, hydrogen, and nitrogen analyses were
2: yield 86%. Anal. Calcd for C54H47F12IrNO2P3Sb2: C, 43.3; H, 3.2;
N, 0.9. Found: C, 43.1; H, 3.6; N, 0.7. IR (Nujol, cm−1): ν(OH) 3600
(b), ν(IrH) 2310 (w), ν(CN) 1598 (w), ν(SbF6) 666 (m). MS: m/z
(%) 1008.2 (100) [M+ − H2O − H]. 1H NMR (300.13 MHz, CD2Cl2,
room temperature): δ 8.3−6.0 (38 H, Ar); 7.3 (bm, 2H, CHCH);
5.61 (pt, J = 5.1 Hz, 1H, H5); 5.35 (d, J = 5.1 Hz, 1H, H4); 3.3 (bs, 2H,
H2O); 3.42, 3.21 (AB part of an ABX system, J(AB) = 18.4 Hz, J = 4.6
Hz, H61, H62); −27.69 (dpt, J(PH) = 23.6, 12.3 Hz, 1H, Ir−H).
13C{1H} NMR (75.5 MHz, CDCl3, room temperature): δ 167.41 (t, J
= 6.1 Hz, C2); 151.85 ddd, J = 55.2, 23.9, 4.6 Hz, 141.06 dd, J = 53.8,
15.3 Hz (2C, CHCH); 138.4−121.3 (48C, Ar); 86.56 (C5); 79.41
(bs, C4); 36.44 (C6).31P{1H} NMR (121.5 MHz, CD2Cl2, room
temperature): δ 44.86 (bd, J = 325.2 Hz, P1); 17.05 (pt, J = 13.4 Hz,
P2); 1.20 (dd, P3).
performed using a Perkin-Elmer 240 B microanalyzer. 1H, 13C, and 31
P
NMR spectra were recorded on Bruker AV-300 (300.13 MHz), Bruker
AV-400 (400.16 MHz), and Bruker AV-500 (500.13 MHz)
1
spectrometers. In both H NMR and 13C NMR measurements the
chemical shifts are expressed in ppm downfield from SiMe4. The 31P
NMR chemical shifts are relative to 85% H3PO4. J values are given in
1
Hz. NOESY and 13C, 31P, H correlation spectra were obtained using
standard procedures. MALDI-TOF+ (dithranol) mass spectra were
recorded on a Bruker Autoflex III spectrometer. Analytical high-
performance liquid chromatography (HPLC) was performed on an
Alliance Waters 2695 (Waters 2996 PDA detector) instrument using a
chiral column Chiralpack AD-H (0.46 cm ×25 cm) or Daicel Chiralcel
OJ-H (0.46 × 25 cm) and OJ-H guard (0.46 cm × 5 cm).
Phosphano−oxazolines PNInd and PNiPr27a and the nitrone N-
benzylidenephenylamine N-oxide27b were prepared using literature
procedures. For the preparation of the starting chloride complexes
[IrClH(PN*)(PP)][SbF6] see ref 34. General labeling for NMR
assignments is given in Scheme 3, and labeling of the (R)-prophos
ligand for NMR assignments is given in Scheme 4.
3a:3b: 86:14 molar ratio. Yield: 88%. Anal. Calcd for
C55H51F12IrNO2P3Sb2: C, 43.6; H, 3.4; N, 0.9. Found: C, 43.4; H,
3.3; N, 0.8. IR (Nujol, cm−1): ν(OH) 3600 (b), ν(IrH) 2302 (w),
ν(CN) 1598 (m), ν(SbF6) 655 (m). MS: m/z (%) 1024.4 (100) [M+
− H2O − H].
1
3a: H NMR (300.13 MHz, CD2Cl2, −25 °C): δ 8.0−6.5 (38H,
Ar); 5.36 (d, J = 5.7 Hz, 1H, H4); 4.56 (pt, J = 8.1 Hz, 1H, H5); 4.0
(bs, 2H, H2O); 3.31, 3.12 (AB part of an ABX system, J(AB) = 18.1
Hz, J = 4.8 Hz, 2H, H61, H62); 3.2−2.8, 2.7−2.3, 1.9−1.5 (3 × bm, 6H,
(CH2)3); −27.05 (dpt, J(PH) = 23.4, 10.0 Hz, 1H, Ir-H). 13C{1H}
NMR (75.5 MHz, CD2Cl2, −25 °C): δ 167.12 (bs, C2); 138.6−123.5
(48C, Ar); 87.71 (C5); 80.95 (C4); 36.19 (C6); 28.28 bdd, J = 42.7,
18.2 Hz, 22.85 dd, J = 34.8, 12.7 Hz, 16.55 s (3C, (CH2)3). 31P{1H}
NMR (121.5 MHz, CD2Cl2, −25 °C): δ −0.22, −8.30 (AB part of an
ABX system, J(AB) = 322.2 Hz, J = 23.6, 15.0 Hz, 2P, P1, P3); −30.20
(dd, P2).
Scheme 3. Labeling for NMR Assignments
1
3b: H NMR (300.13 MHz, CD2Cl2, −25 °C): δ −24.93 (dpt,
J(PH) = 29.6, 14.8 Hz, 1H, Ir-H). 31P{1H} NMR (121.5 MHz,
CD2Cl2, −25 °C): δ −0.48, −10.88 (AB part of an ABX system, J(AB)
= 319.3 Hz, J = 22.6, 16.0 Hz, 2P, P1, P3); −15.09 (dd, P2).
4: yield 76%. Anal. Calcd for C56H53F12IrNO2P3Sb2: C, 44.0; H, 3.5;
N, 0.9. Found: C, 43.9; H, 3.4; N, 0.7. IR (Nujol, cm−1): ν(OH) 3500
(b), ν(IrH) 2301 (w), ν(CN) 1600 (w), ν(SbF6) 666 (m). MS: m/z
(%) 1038.3 (100) [M+ − H2O − H]. 1H NMR (300.13 MHz, CD2Cl2,
room temperature): δ 8.2−5.85 (38H, Ar); 5.40 (pt, J = 5.1 Hz, 1H,
H5); 5.30 (d, J = 5.6 Hz, 1H, H4); 3.4 (bs, 2H, H2O); 3.44, 3.27 (AB
part of an ABX system, J(AB) = 18.4 Hz, J = 4.6 Hz, 2H, H61, H62);
2.62 (bs, 2H, CHMe); 0.90 (dd, J = 13.6, 5.4 Hz, 3H, Me); 0.79 (dd, J
= 14.1, 5.1 Hz, Me); −27.11 (dt, J = 22.70, 11.44 Hz; 1H; Ir-H).
13C{1H} NMR (75.5 MHz, CD2Cl2, room temperature): δ 167.37
(C2); 138.5−120.9 (48C, Ar); 87.17 (C5); 79.55 (d, J = 3.2 Hz, C4);
40.11 (bdd, J = 42.2, 16.7 Hz, CHMe); 36.34 (C6); 32.69 (dd, J = 35.6,
10.3 Hz, CHMe); 13.37 (d, J = 4.0 Hz, Me); 13.15 (d, J = 3.2 Hz, Me).
31P{1H} NMR (121.5 MHz, CD2Cl2, room temperature): δ 38.37 (bd,
J = 314.8 Hz, P1); 11.73 (pt, J = 13.4 Hz, P2); 2.11 (dd, P3).
5a:5a′: 68:32 molar ratio. Yield: 77%. Anal. Calcd for
C55H51F12IrNO2P3Sb2: C, 43.6; H, 3.4; N, 0.9. Found: C, 43.55; H,
3.4; N, 0.8. IR (Nujol, cm−1): ν(OH) 3500 (b), ν(IrH) 2310 (w),
ν(CN) 1613 (w), ν(SbF6) 660 (m). MS: m/z (%) 1024.2 (100) [M+
− H2O − H].
Scheme 4. Labeling of the (R)-Prophos Ligand for NMR
Assignments
Preparation of the Complexes [IrH(H2O)(PN*)(PP)][SbF6]2
(1−9). Under argon at room temperature, in the absence of light, to
a solution of the corresponding chloride [IrHCl(PN*)(PP)][SbF6]
(0.25 mmol) in 15 mL of a mixture of CH2Cl2 and (CH3)2CO (95/5,
v/v) was added 85.9 mg (0.25 mmol) of AgSbF6 and 10 μL (0.55
mmol) of H2O. The suspension was stirred for 1 h (5 h at −25 °C for
compounds 3 and 8), and the AgCl that formed was filtered through a
cannula. The resulting filtrate was partially concentrated under
reduced pressure to ca. 2 mL, and the slow addition of n-hexane led
to the precipitation of a pale yellow solid which was washed with n-
hexane and vacuum-dried.
1: yield 87%. Anal. Calcd for C54H49F12IrNO2P3Sb2: C, 43.2; H, 3.3;
N, 0.9. Found: C, 43.3; H, 3.1; N, 0.9. IR (Nujol, cm−1): ν(OH) 3600
(b), ν(IrH) 2296 (w), ν(CN) 1599 (w), ν(SbF6) 666 (m). MS: m/z
(%) 1010.3 (100) [M+ − H2O − H]. 1H NMR (300.13 MHz, CD2Cl2,
room temperature): δ 8.2−6.1 (38H, Ar); 5.32 (d, J = 5.8 Hz, 1H,
H4); 5.28 (pt, J = 4.8 Hz, 1H, H5); 4.1 (bs, 2H, H2O); 3.45, 3.28 (AB
part of an ABX system, J(AB) = 17.9 Hz, H61, H62); 2.64 dt, J(PH) =
41.5 Hz, J(HH) = 11.1 Hz, 2.54 dt, J(PH) = 36.5 Hz, J(HH) = 11.7
Hz, 2.37 m, 2.30 m (4H, (CH2)2); −26.86 (dpt, J(PH) = 22.6, 10.3
Hz, 1H, Ir−H). 13C{1H} NMR (75.5 MHz, CD2Cl2, room
temperature): δ 167.64 (t, J = 6.1 Hz, C2); 138.6−123.6 (48C, Ar);
5a: 1H NMR (400.16 MHz, CDCl3, room temperature): δ 8.3−6.0
(38H, Ar); 5.20 (m, 1H, H5); 4.28 (d, J = 7.7 Hz, 1H, H4); 3.49 (m,
1H, Hg); 3.40 (bs, 2H, H2O); 3.30, 3.13 (AB part of an ABX system,
J(AB) = 18.4 Hz, J = 5.1 Hz, 2H, H61, H62); 3.15 (m, 1H, Hc); 2.17
(m, 1H, Ht); 1.15 (dd, J(PH) = 14.3 Hz, J(HH) = 6.7 Hz, 3H, Me);
−27.85 (dpt, J(PH) = 22.5, 11.8 Hz, 1H, Ir-H).13C{1H} NMR (75.5
MHz, CD2Cl2, room temperature): δ 166.3 (t, J = 4.3 Hz, C2); 138.3−
121.5 (48C, Ar); 85.45 (C5); 75.67 (C4); 41.60 (bdd, J = 36.3, 13.7
Hz, Ctc); 36.61 (C6); 38.12 (bd, J = 42.1 Hz, CMe); 14.95 (dd, J = 13.5,
3.9 Hz, Me). 31P{1H} NMR (162.0 MHz, CD2Cl2, room temper-
ature): δ 38.72 (bd, J = 323.7 Hz, P1); 2.60 (pt, J = 11.9 Hz, P2); 2.24
(dd, P3).
G
dx.doi.org/10.1021/om301089c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
5a′: H NMR (400.16 MHz, CDCl3, room temperature): δ 5.40
J = 8.4 Hz, 1H, H4); 1.56 (m, MeMeCH); 0.80 (m, 3H, Me); 0.03 (d,
J = 6.9 Hz, 3H, MeMeCH); −0.58 (d, J = 6.4 Hz, 3H, MeMeCH);
−27.11 (dpt, J(PH) = 20.5, 9.1 Hz, 1H, Ir-H). 13C{1H} NMR (75.5
MHz, CD2Cl2, room temperature): δ 166.1 (t, J = 4.9 Hz, C2); 136.0−
120.9 (42C, Ar); 7.06 (C4); 8.64 (C5); 41.39 (bdd, J = 47.5, 16.0 Hz,
Ctc); 29.78 (bd, J = 30.8 Hz, CMe); 28.93 (MeMeCH); 17.88
(MeMeCH); 14.56 (dd, J = 22.7, 3.7 Hz, Me); 13.85 (MeMeCH).
31P{1H} NMR (121.5 MHz, CD2Cl2, room temperature): δ 38.17 (dd,
J = 315.4, 6.8 Hz, P1); 12.26 (dd, J = 14.6 Hz, P2); 2.17 (dd, P3).
(pt, J = 6.7 Hz, 1H, H5); 5.07 (d, J = 6.7 Hz, 1H, H4); 3.4−3.0 (bm,
4H, H61, H62, H2O); 3.10 (m, 1H, Ht), 2.74 (m, 1H, Hc), 2.50 (m, 1H,
Hg); 1.34 (dd, J(PH) = 14.3 Hz, J(HH) = 6.7 Hz, 3H, Me); −27.66
(dpt, J(PH) = 25.1, 13.8 Hz, 1H, Ir-H). 13C{1H} NMR (75.5 MHz,
CD2Cl2, room temperature): δ 167.0 (bs, C2); 86.01 (C5); 77.73 (C4);
41.60 (m, CMe); 37.57 (bs, Ctc); 36.61 (C6); 13.98 (dd, J = 15.9, 4.4
Hz, Me). 31P{1H} NMR (162.0 MHz, CD2Cl2, room temperature): δ
22.30 (d, J(P3,P1) = 317.8 Hz, P1); 15.50 (d, J(P3,P2) = 13.4 Hz, P2);
1.29 (dd, P3).
1
9a′: H NMR (300.13 MHz, CD2Cl2): δ 4.33 (bs, 1H, H52); 4.18
6: yield 93%. Anal. Calcd for C50H51F12IrNO2P3Sb2: C, 41.3; H, 3.5;
N, 1.0. Found: C, 41.0; H, 3.6; N, 0.9. IR (Nujol, cm−1): ν(OH) 3605
(b), ν(IrH) 2325 (w), ν(CN) 1606 (w), ν(SbF6) 659 (m). MS: m/z
(%) 964.3 (100) [M+ − H2O − H]. 1H NMR (400.16 MHz, CD2Cl2,
room temperature): δ 8.3−6.7 (34H, Ar); 4.42 (d, J = 9.2 Hz, 1H,
H52); 4.29 (pt, J = 9.2 Hz, 1H, H51); 3.90 (d, J = 8.2 Hz, 1H, H4); 3.80
(bs, 2H, H2O); 2.56 m, 2.17 m (4H, (CH2)2); 1.65 (bs, 1H,
MeMeCH); 0.11 (d, J = 6.6 Hz, 3H, MeMeCH); −0.43 (d, J = 6.6 Hz,
3H, MeMeCH); −26.98 (dpt, J(PH) = 22.0, 10.2, 1H, Ir-H). 13C{1H}
NMR (100.6 MHz, CD2Cl2, room temperature): δ 166.20 (C2);
135.8−125.3 (42C, Ar); 78.07 (C4); 68.52 (C5); 33.91 ddd, J = 44.3,
11.1, 2.4 Hz, 26.97 dd, J = 37.2, 6.3 Hz (2C, (CH2)2); 29.17
(MeMeCH); 17.68 (MeMeCH); 11.59 (MeMeCH). 31P{1H} NMR
(162.0 MHz, CD2Cl2, room temperature): δ 36.58 (bd, J = 320.1 Hz,
P1); 14.88 (bs, P2); 6.51 (bd, P3).
7: yield 85%. Anal. Calcd for C50H49F12IrNO2P3Sb2: C, 41.3; H, 3.4;
N, 1.0. Found: C, 41.2; H, 3.8; N, 1.0. IR (Nujol, cm−1): ν(OH) 3605
(b), ν(IrH) 2324 (w), ν(CN) 1604 (w), ν(SbF6) 659 (m). FMS: m/z
(%) 962.4 (100) [M+ − H2O − H]. 1H NMR (300.13 MHz, CD2Cl2,
room temperature): δ 8.3−6.6 (34H, Ar); 7.1 (m, 2H, CHCH);
4.45 (pt, J = 9.2 Hz, 1H, H51); 4.35 (dd, J = 9.5, 7.7 Hz, 1H, H52); 4.04
(bd, J = 8.6 Hz, 1H, H4); 3.50 (bs, 2H, H2O); 1.29 (bs, 1H,
MeMeCH); −0.15 (d, J = 6.7 Hz, 3H, MeMeCH); −0.52 (d, J = 6.7
Hz, 3H, MeMeCH); −27.46 (dpt, J(PH) = 21.3, 11.6 Hz, 1H, Ir-H).
13C{1H} NMR (100.6 MHz, CD2Cl2, room temperature): δ 166.38
(C2); 151.09 bdd, J = 55.6, 23.4 Hz, 142.04 dd, J = 52.0, 16.1 Hz (2C,
CHCH); 135.6−125.0 (42C, Ar); 77.64 (C4); 68.26 (C5); 28.86
(MeMeCH); 17.39 (MeMeCH); 11.41 (MeMeCH). 31P{1H} NMR
(121.5 MHz, CD2Cl2, room temperature): δ 44.63 (dd, J = 325.2, 13.4
Hz, P1); 17.29 (pt, P2); 3.15 (dd, J = 13.4 Hz, P3).
(pt, J = 9.0 Hz, 1H, H51); 3.76 (bd, J = 8.4 Hz, 1H, H4); 3.35 (bs, 2H,
H2O); 2.83 (m, 1H, Hg); 2.48, 2.36 (2 × m, 2H, Hc, Ht); 1.56 (m,
MeMeCH); 0.80 (m, 3H, Me); 0.08 (d, J = 6.9 Hz, 3H, MeMeCH);
−0.57 (d, J = 5.7 Hz, 3H, MeMeCH); −27.43 (bm, 1H, Ir-H).
13C{1H} (75.5 MHz, CD2Cl2): δ 166.05 (t, J = 4.9 Hz, C2); 77.74
(C4); 68.54 (C5); 36.13 (CMe); 33.14 (Ctc); 29.20 (MeMeCH); 17.76
(MeMeCH); 14.20 (dd, J = 18.7, 4.6 Hz, Me); 11.45 (MeMeCH).
31P{1H} (121.5 MHz, CD2Cl2, room temperature): δ 20.47 (dd, J =
315.7, 9.3 Hz, P1); −0.05 (dd, J = 7.4 Hz, P2); 2.17 (dd, P3).
Preparation of the Complex [IrH(OPOF2)(PNiPr)(dppp)]-
[SbF6] (10). Under argon in the absence of light, to a solution of
[IrClH(PNiPr)(dppp)][SbF6] (60.0 mg, 0.050 mmol) in 10 mL of a
mixture of CH2Cl2/(CH3)2CO (95/5, v:v) were added 12.1 mg (0.050
mmol) of AgPF6 and 2 μL (0.11 mmol) of H2O. The suspension was
stirred for 45 min and filtered through Celite. The resulting filtrate was
concentrated until ca. 1 mL, and addition of n-hexane afforded a pale
yellow solid which was filtered off, washed with n-hexane, and vacuum-
dried. The solid was recrystallized from CH2Cl2/Et2O.
10a:10b: 92:8 molar ratio. Yield: 53%. Anal. Calcd for
C51H51F8IrNO3P4Sb: C, 46.6; H, 3.9; N, 1.1. Found: C, 46.6; H,
3.9; N, 1.0. IR (Nujol, cm−1):ν(IrH) 2320 (w), ν(CN) 1611 (m),
ν(PO) 1306 (s), ν(SbF6) 654 (m).
1
10a: H NMR (400.16 MHz, CD2Cl2, room temperature): δ 8.4−
6.4 (34H, Ar); 4.05 (dd, J = 9.0, 1.4 Hz, 1H, H52); 3.83 (bd, J = 8.1
Hz, 1H, H4); 2.88 (pt, J = 8.6 Hz, 1H, H51); 3.40, 2.93, 2.33, 2.07, 1.90,
1.61 (6 × m, 6H, (CH2)3); 1.86 (sp, J = 6.7 Hz, 1H, MeMeCH); 0.63
(d, J = 6.7 Hz, 3H, MeMeCH); −0.51 (d, J = 6.7 Hz, 3H, MeMeCH);
−27.09 (ddt, J(PH) = 38.4, 21.0, 10.5 Hz, 1H, Ir-H). 13C{1H} NMR
(100.6 MHz, CD2Cl2, room temperature): δ 165.82 (t, J = 4.4 Hz, C2);
135.3−125.8 (42C, Ar); 78.30 (d, J = 4.4 Hz, C4); 68.06 (C5); 29.91
(MeMeCH); 26.70 dd, J = 41.6, 18.6 Hz, 22.02 dd, J = 33.6, 10.6 Hz,
16.77 s (3C, (CH2)3); 18.39 (MeMeCH); 11.96 (MeMeCH). 31P{1H}
NMR (161.9 MHz, CD2Cl2, room temperature): δ 2.62, −7.19 (AB
part of an ABX system, J(AB) = 345.7 Hz, J = 24.5, 16.9 Hz, P1, P3);
−19.52 (t, J(PF) = 957.9 Hz, PO2F2); −33.44 (dd, P2).
8a:8b: 85:15 molar ratio. Yield: 72%. Anal. Calcd for
C51H53F12IrNO2P3Sb2: C, 41.7; H, 3.6; N, 1.0. Found: C, 41.5; H,
3.9; N, 0.7. IR (Nujol, cm−1): ν(OH) 3604 (b), ν(IrH) 2325 (w),
ν(CN) 1607 (w), ν(SbF6) 659 (m). MS: m/z (%) 978.3 (100) [M+ −
H2O − H].
8a: 1H NMR (300.13 MHz, CD2Cl2, room temperature): δ 8.3−6.0
(34H, Ar); 4.29 (d, J = 9.5 Hz, 1H, H52); 3.95 (d, J = 7.6 Hz, 1H, H4);
3.52 (pt, J = 9.1 Hz, 1H, H51); 3.30 (bs, 2H, H2O); 3.15, 2.67, 2.46,
1.69 (4 × m, 6H, (CH2)3); 2.07 (psp, J = 6.7 Hz, 1H, MeMeCH); 0.65
(d, J = 6.7 Hz, 3H, MeMeCH); −0.50 (d, J = 6.7 Hz, 3H, MeMeCH);
−26.77 (dpt, J(PH) = 22.5, 10.0 Hz, 1H, Ir-H). 13C{1H} NMR (75.5
MHz, CD2Cl2, room temperature): δ 165.82 (t, J = 4.7 Hz, C2);
135.9−123.0 (42C, Ar); 77.96 (C4); 69.18 (C5); 28.24 (dd, J = 43.7,
16.8 Hz), 23.25 (dd, J = 34.1, 12.7 Hz), 16.51 (3C, (CH2)3); 29.87
(MeMeCH); 18.43 (MeMeCH); 11.87 (MeMeCH). 31P{1H} NMR
(121.5 MHz, CD2Cl2, −25 °C): δ 4.07, −7.61 (AB part of an ABX
system, J(AB) = 321.2 Hz, J = 23.5, 16.0 Hz, 2P, P1, P3); −28.23 (dd,
P2).
10b: 1H NMR (400.16 MHz, CD2Cl2, room temperature): δ
−26.50 (m, 1H, Ir-H). 31P{1H} NMR (161.9 MHz, CD2Cl2, room
temperature): δ −4.28, −19.85 (AB part of an ABX system, J(AB) =
353.2 Hz, J = 28.3 Hz, J = 15.1 Hz, P1, P3); −14.85 (dd, P2); −17.27
(t, J(FP)= 954.8 Hz, PO2F2).
Catalytic Procedure for the 1,3-Dipolar Cycloaddition
Reaction. At −25 °C, under argon, the complexes [IrH(H2O)-
(PN*)(PP)][SbF6]2 (0.030 mmol, 5 mol %) were dissolved in CH2Cl2
(1.5 mL). Activated molecular sieves (4 Å, 100.0 mg) and freshly
distilled methacrolein (0.35 mL, 4.2 mmol) were added. The
suspension was stirred for 30 min, and then a solution of the nitrone
N-benzylidenephenyl N-oxide (118.25 mg, 0.60 mmol) in CH2Cl2 (1
mL) was added. After the mixture was stirred at the appropriate
temperature for the reaction time indicated, n-hexane (15 mL) was
added. The suspension was filtered through Celite and the solution
concentrated to ca. 0.3 mL. The resulting residue was purified by
chromatography (SiO2), and a mixture of the corresponding isomers
was obtained. Conversion and regio- and diastereoselectivity were
determined by 1H NMR spectroscopic analysis. Enantioselectivity was
determined as indicated in the footnote of Table 2.
8b: 1H NMR (300.13 MHz, CD2Cl2, room temperature): δ −26.05
(m, 1H, Ir-H). 31P NMR (121.5 MHz, CD2Cl2, −25 °C): δ −1.82,
−10.81 (AB part of an ABX system, J(AB) = 319.5 Hz, J = 23.1, 14.6
Hz, 2P, P1, P3); −15.83 (dd, P2).
9a:9a′: 62:38 molar ratio. Yield: 74%. Anal. Calcd for
C51H51F12IrNO2P3Sb2: C, 41.7; H, 3.6; N, 0.9. Found: C, 42.2; H,
3.9; N, 1.0. IR (Nujol, cm−1): ν(OH) 3600 (b), ν(IrH) 2326 (w),
ν(CN) 1607 (m), ν(SbF6) 658 (m). MS: m/z (%) 978.5 (100) [M+ −
H2O − H].
Preparation of the Complexes [IrH(methacrolein)(PNInd)-
(PP)][SbF6]2 (PP = (S,S)-chiraphos (11), (R)-prophos (12)). At 0
°C under argon, to a solution of the appropriate aqua complex
[IrH(H2O)(PNInd)(PP)][SbF6]2 (4, 5; 0.070 mmol) in CH2Cl2 (10
mL) were added methacrolein (30.0 μL, 0.36 mmol) and molecular
1
9a: H NMR (400.16 MHz, CD2Cl2, room temperature): δ 8.05−
6.4 (34H, Ar); 4.31 (bs, H52); 4.18 (pt, J = 9.0 Hz, 1H, H51); 3.13 (bs,
2H, H2O); 2.53 (m, 1H, Hg); 2.47, 2.24 (2 × m, 2H, Hc, Ht); 3.83 (bd,
H
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Organometallics
Article
sieves (4 Å, 100.0 mg). The solution was stirred for 90 min and then
filtered through a cannula. The solvent was partially evaporated, and
the slow addition of n-hexane (20 mL) afforded a pale yellow solid
which was washed with n-hexane and vacuum-dried. Labeling of the
methacrolein ligand for NMR assignments is given in Scheme 5.
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 effect with the
SAINT-PLUS program.28 The structures were solved by direct
methods with SHELXS-97.29 Refinement, by full-matrix least squares
on F2, was performed with SHELXL-97.30 Hydrogen atoms were
included in calculated positions and refined with displacement and
positional riding parameters. The hydride ligand was included from
electrostatic potential calculations (HYDEX program)31 and refined
with a geometrical restraint in Ir−H distance (except in 1, where its
position was fixed). The absolute configuration was determined on the
basis of previously known internal references, and this assignment was
confirmed using the Flack parameter.32 Particular details concerning
the presence of solvent and specific refinements are listed below.
Crystal data for 1: C54H49F12IrNO2P3Sb2·2C4H10O·H2O, M =
1666.81; colorless plate, 0.077 × 0.064 × 0.009 mm3; monoclinic, P21;
a = 11.6736(8), b = 23.6185(17), c = 11.8942(8) Å; β = 94.5270(10)°;
Z = 2; V = 3269.2(4) Å3; Dc = 1.693 g/cm3; μ = 3.006 mm−1,
minimum and maximum absorption correction factors 0.802 and
0.974; 2θmax = 54.12°; 25174 collected reflections, 14045 unique (Rint
= 0.0538); number of data/restraints/parameters 14045/97/773; final
GOF 1.046; R1 = 0.0605 (11624 reflections, I > 2σ(I)); wR2 = 0.1170
for all data; Flack parameter x = 0.002(6); largest difference peak 2.107
e/Å3. One of the diethyl ether solvent molecules has been found to be
highly disordered. It has been included in the model in two different
positions, with complementary occupancy factors and refined with
geometrical restraints. Some constraints have been included to avoid
unrealistic thermal parameters.
Scheme 5. Labeling of the Methacrolein Ligand for NMR
Assignments
11: yield 57%. Anal. Calcd for C60H57F12IrNO2P3Sb2: C, 45.6; H,
3.6; N, 0.9. Found: C, 45.2; H, 3.9; N, 1.2. IR (Nujol, cm−1): ν(IrH)
2324 (w); ν(CO) 1566 (w); ν(CN) 1607 (s); ν(SbF6) 658 (m). MS:
m/z (%): 1038.3 (100) [M+ − methacrolein − H]. 1H NMR (400.16
MHz, CD2Cl2, −25 °C): δ 8.3−5.9 (38H, Ar); 7.39 (s, 1H, CHO);
6.40 (s, Ha); 5.80 (s, Hb); 5.67 (pt, J = 5.4 Hz, 1H, H5); 5.23 (d, J =
5.8 Hz, 1H, H4); 3.41, 3.22 (AB part of an ABX system, J(AB) = 17.9
Hz, J = 4.6 Hz, 2H, H61, H62); 2.63, 2.50 (2 × m, 2H, 2 × CHMe);
1.41 (s, 3H, Mea); 0.77 (bm, 6H, 2 × CHMe); −26.05 (dpt, J(PH) =
23.8, 9.9 Hz, 1H, Ir-H). 13C NMR (100.61 MHz, CD2Cl2, −25 °C): δ
205.96 (CHO); 166.90 (t, J = 5.1 Hz, C2); 148.77 (CHaHb); 143.51
(CHOC); 139.2−120.5 (48C, Ar); 88.04 (C5); 78.94 (C4); 39.43
(ddd, J = 42.5, 11.7, 4.1 Hz, CHMe); 35.91 (C6); 31.96 (dd, J = 35.9,
9.5 Hz, CHMe); 13.80 (Mea); 13.50, 13.33 (2C, 2 × CHMe). 31P
NMR (161.98 MHz, CD2Cl2, −25 °C): δ 37.19 (dd, J = 314.4, 13.8
Hz, P1); 13.77 (pt, J = 13.8 Hz, P2); 1.04 (dd, P3).
Crystal data for 2: C54H47F12IrNO2P3Sb2·C4H10O·CH2Cl2, M =
1657.58; colorless block, 0.220 × 0.080 × 0.040 mm3; monoclinic, P21;
a = 13.102(5), b = 16.399(5), c = 14.495(5) Å; β = 109.079(5)°; Z =
2; V = 2943.3(18) Å3; Dc = 1.870 g/cm3; μ = 3.423 mm−1, minimum
12a:12a′: 74:26 molar ratio.23
1
and maximum absorption correction factors 0.521 and 0.875; 2θmax =
12a: H NMR (400.16 MHz, CD2Cl2, −25 °C): δ 9.05 (s, 1H,
57.76°; 36442 collected reflections, 13985 unique (Rint = 0.0527);
number of data/restraints/parameters 13985/21/746; final GOF
1.117; R1 = 0.0592 (12765 reflections, I > 2σ(I)); wR2 = 0.1221
for all data; Flack parameter x = 0.018(6); largest difference peak 1.575
CHO); 8.2−5.9 (m, 38H, Ar); 6.50, 6.34 (2 × s, 2H, Ha, Hb); 5.35 (m,
1H, H5); 3.78 (d, J = 9.1 Hz, 1H, H4); 3.7 (m, 1H, Hg); 3.3, 1.8 (2 ×
m, 2H, Hc, Ht); 2.40, 3.02 (AB part of an ABX system, J(AB) = 18.3
Hz, J = 4.7 Hz, 2H, H61, H62); 1.69 (s, 3H, Mea); 1.16 (dd, J(PH) =
13.3 Hz, J(HH) =5.7 Hz, 3H, Me); −26.00 (dpt, J(PH) = 21.9, 12.0
Hz, 1H, Ir-H). 13C NMR (125.77 MHz, CD2Cl2, −25 °C): δ 209.46
(CHO); 165.70 (t, J = 4.2 Hz, C2); 148.39 (CHaHb); 143.90
(CHOC); 143.9−119.9 (48C, Ar); 84.45 (C5); 73.38 (C4); 40.23
(bdd, J = 44.9, 11.5 Hz, Ctc); 39.25 (C6); 37.90 (dd, J = 37.4, 3.5 Hz,
CMe); 14.07 (Me); 14.05 (Mea). 31P NMR (121.98 MHz, CD2Cl2,
−25 °C): δ 37.48 (dd, J = 331.6, 9.4 Hz, P1); 3.79 (bd, P3); 1.28 (pt, J
= 13.0 Hz, P2).
−
e/Å3. Static disorder was observed in one SbF6 anion and in the
dichloromethane and diethyl ether solvent molecules; they have been
modeled in each case with two sets of positions with complementary
occupancy factors and refined with some geometrical restraints.
Crystal data for 10a: C51H51F8IrNO3P4Sb·4C4H10O, M = 1612.31;
colorless prism, 0.110 × 0.096 × 0.074 mm3; orthorhombic, P212121, a
= 14.003(3), b = 18.597(4), c = 22.965(5) Å; Z = 4; V = 5980(2) Å3;
Dc = 1.791 g/cm3; μ = 2.843 mm−1, minimum and maximum
absorption correction factors 0.716 and 0.823; 2θmax = 57.66°; 60679
collected reflections, 14653 unique (Rint = 0.1273); number of data/
restraints/parameters 14653/1/628; final GOF 0.918; R1 = 0.0532
(10043 reflections, I > 2σ(I)); wR2 = 0.1073 for all data; Flack
parameter x = 0.004(6); largest difference peak 1.097 e/Å3. At the last
steps of refinement, there were very large solvent-accessible voids in
the structure and two different zones where residual density peaks
were observed. The solvent was highly disordered, and it cannot be
modeled even assuming several restraints. Therefore, SQUEEZE33
corrections have been applied. The total potential solvent accessible
void volume (1400 Å3) and the number of electrons (636 e in the unit
cell) have been interpreted with the presence of 16 ether molecules.
Crystal data for 11: weakly diffracting single crystals of this
compound were obtained by slow diffusion of n-hexane into
dichloromethane solutions; C60H57F12IrNO2P3Sb2·0.5CH2Cl2·-
1.5C6H14, M = 1752.49; yellow plate, 0.220 × 0.157 × 0.058 mm3;
orthorhombic, P21212, a = 25.703(3), b = 26.433(3), c = 21.329(2) Å;
Z = 8; V = 14491(2) Å3; Dc = 1.619 g/cm3; μ = 2.749 mm−1,
minimum and maximum absorption correction factors 0.505 and
0.744; 2θmax = 40.28°; 68844 collected reflections, 13738 unique (Rint
= 0.107); number of data/restraints/parameters 13738/37/615; final
GOF 1.060; R1 = 0.086 (10041 reflections, I > 2σ(I)); wR2 = 0.2468
for all data; Flack parameter x = 0.057(13); largest difference peak
1.871 e/Å3. Several crystals were tested before selecting the one used
for data collection. In general, they were slightly twinned samples with
1
12a′: H NMR (400.16 MHz, CD2Cl2, −25 °C): δ 8.2−5.9 (m,
38H, Ar); 8.20 (s, 1H, CHO); 6.43, 5.83 (2 × s, 2H, Ha, Hb); 2.9−2.6
(m, 3H, Hg, Hc, Ht); 1.49 (s, 3H, Mea); 1.26 (dd, J(PH) = 14.7 Hz,
J(HH) = 6.2 Hz, 3H, Me); −26.00 (bm, 1H, Ir-H). 13C NMR (125.77
MHz, CD2Cl2, −25 °C): δ 207.09 (CHO); 166.73 (C2); 148.80
(CHaHb); 143.9−119.9 (49C, Ar, CHOC); 86.72 (C5); 77.75 (C4);
35.96 (C6); 14.07 (Me); 14.05 (Mea). 31P NMR (121.98 MHz,
CD2Cl2, −25 °C): δ 26.79 (bd, J = 320.3 Hz, P1); 19.08 (bs, P2); 2.07
(bd, P3).
Catalytic Procedure for the Diels−Alder Reaction between
Cyclopentadiene and trans-β-Nitrostyrenes. Under argon at −25
°C, in a Schlenk flask, the appropriate catalyst (0.030 mmol), 3 mL of
CH2Cl2, molecular sieves (4 Å, 100.0 mg), and the corresponding
trans-β-nitrostyrene (0.60 mmol) were added. After 15 min of stirring,
freshly distilled cyclopentadiene (3.6 mmol) in 1 mL of CH2Cl2 was
added and the system was stirred at the indicated temperature. The
reaction was monitored by TLC chromatography. After the reaction
time, a solution of 19.3 mg (0.125 mmol) of NnBu4Br in 0.1 mL of
methanol was added. The resulting suspension was concentrated, and
Et2O (15 mL) was added to precipitate the catalyst. The suspension
was filtered through Celite and the resulting solution concentrated to
ca. 0.3 mL. The conversion and endo:exo ratio were determined by 1H
NMR. Enantiomeric excesses were determined by HPLC.
Crystal Structure Determination of Complexes 1, 2, 10a, and
11. X-ray diffraction data were collected at 100(2) K with graphite-
I
dx.doi.org/10.1021/om301089c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
highly irregular mosaic structure. Eventually, a very weakly diffracting
crystal was selected, showing no detectable intensity over 2θ > 40°.
Due to the limited quality of data, only Ir, Sb, P, and O atoms have
been refined with a thermal anisotropic model, and several geometrical
restraints have been applied. At this point, two very large voids were
observed in the structure, with several significant residual peaks.
Unfortunately, no clear model of disordered solvent could be
established. Therefore, SQUEEZE33 corrections have been applied.
The solvent-accessible void volumes (2 × 1081 Å3) and the estimated
number of electrons (2 × 370 e) have been interpreted with the
presence of six n-hexane molecules in each of these two apparently
empty regions.
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ASSOCIATED CONTENT
* Supporting Information
́
D.; Lamata, M. P.; Viguri, F.; Rodrıguez, R.; Oro, L. A.; Balana, A. I.;
■
Lahoz, F. J.; Tejero, T.; Merino, P.; Franco, S.; Montesa, I. J. Am.
Chem. Soc. 2004, 126, 2716−2717.
S
Text, figures, and CIF files giving X-ray crystallographic data for
the structural analysis of complexes 1, 2, 10a, and 11, scanned
HPLC traces and spectroscopic data of the Diels−Alder
cycloadducts, and NMR spectra of 12a,a′. This material is
available free of charge via the Internet at http://pubs.acs.org.
(6) (a) Kundig, E. P.; Bourdin, B.; Bernardinelli, G. Angew. Chem.,
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Int. Ed. Engl. 1994, 33, 1856−1858. (b) Bruin, M. E.; Kundig, E. P.
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̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: dcarmona@unizar.es (D.C.); jfecer@unizar.es (J.F.)
Tel: 34-976-762027. Fax: 34-976-761187.
Bernardinelli, G. Angew. Chem., Int. Ed. 2001, 40, 4481−4485.
■
(f) Alezra, V.; Bernardinelli, G.; Corminboeuf, C.; Frey, U.; Kundig,
̈
E. P.; Merbach, A.; Saudan, C. M.; Viton, F.; Weber, J. J. Am. Chem.
Soc. 2004, 126, 4843−4853. (g) Anil Kumar, P. G.; Pregosin, P. S.;
Vallet, M.; Bernardinelli, G.; Jazzar, R. F.; Viton, F.; Kundig, E. P.
Organometallics 2004, 23, 5410−5418. (h) Rickerby, J.; Vallet, M.;
Bernardinelli, G.; Viton, F.; Kundig, E. P. Chem. Eur. J. 2007, 13,
3354−3368. (i) Thamapipol, S.; Bernardinelli, G.; Besnard, C.;
Kundig, E. P. Org. Lett. 2010, 12, 5604−5607.
̈
Notes
The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
We thank the Ministerio de Educacion
2009-10303/BQU) and Gobierno de Aragon
solidado: Catalizadores Organometal
̈
■
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2347−2348. (c) Davenport, A. J.; Davies, D. L.; Fawcett, J.; Krafczyk,
R.; Russell, D. R. Dalton Trans. 2000, 4432−4441. (d) Davenport, A.
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D. R. Organometallics 2001, 20, 3029−3034. (f) Davies, D. L.; Fawcett,
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(g) Davenport, A. J.; Davies, D. L.; Fawcett, J.; Russell, D. R. Dalton
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́
y Ciencia (Grant CTQ
(Grupo Con-
́
icos Enantioselectivos) for
́
financial support. This work was supported by the CON-
SOLIDER INGENIO-2010 program under the project
́
́
“Factorıa de Crystalizacion” (CSD2006-0015).
REFERENCES
■
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