Enantioselective catalysts based on the chiral fragment (η5-C5Me5)Ir(Prophos) for Diels-Alder reactions

Article abstract of DOI:10.1016/j.molcata.2014.01.021

The aqua complex (S_Ir,R_C)-[(η⁵-C ₅Me₅)Ir(Prophos)(H₂O)][SbF₆] ₂ [Prophos = (R)-propane-1,2-diyl-bis(diphenylphosphane)] is an active precursor for the asymmetric Diels-Alder reaction of acyclic enals with cyclopentadiene, 2,3-dimethylbutadiene and isoprene. Enantioselectivities up to 78% ee are achieved. The intermediate Lewis acid-dienophile complex (S _Ir,R_C)-[(η⁵-C₅Me ₅)Ir(Prophos)(ethyl acrolein)][SbF₆]₂ has been isolated and completely characterized, including the X-ray crystal structure determination. Structural parameters indicate that the disposition of the coordinated dienophile is controlled by CH/π attractive interactions established between a phenyl group of the Prophos ligand and the aldehyde proton of the coordinated enal. Proton NMR data indicate that these interactions are maintained in solution. From diffractometric and spectroscopic data, the origin of the enantioselectivity is discussed.

Full text of DOI:10.1016/j.molcata.2014.01.021

Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Enantioselective catalysts based on the chiral fragment
(ꢀ5-C5Me5)Ir(Prophos) for Diels–Alder reactions
Daniel Carmona∗, Fernando Viguri∗, Ainara Asenjo, Fernando J. Lahoz,
Pilar García-Ordun˜a, Luis A. Oro
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC – Universidad de Zaragoza, Departamento de Química Inorgánica, Pedro Cerbuna 12,
50009 Zaragoza, Spain
a r t i c l e i n f o
a b s t r a c t
The aqua complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(H2O)][SbF6]2 [Prophos = (R)-propane-1,2-diyl-
bis(diphenylphosphane)] is an active precursor for the asymmetric Diels–Alder reaction of acyclic
enals with cyclopentadiene, 2,3-dimethylbutadiene and isoprene. Enantioselectivities up to 78% ee
are achieved. The intermediate Lewis acid-dienophile complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(ethyl
acrolein)][SbF6]2 has been isolated and completely characterized, including the X-ray crystal structure
determination. Structural parameters indicate that the disposition of the coordinated dienophile is
controlled by CH/␲ attractive interactions established between a phenyl group of the Prophos ligand
and the aldehyde proton of the coordinated enal. Proton NMR data indicate that these interactions are
maintained in solution. From diffractometric and spectroscopic data, the origin of the enantioselectivity
is discussed.
Article history:
Received 4 November 2013
Received in revised form 20 January 2014
Accepted 26 January 2014
Available online 3 February 2014
Keywords:
Asymmetric catalysis
Diels–Alder reaction
˛,ˇ-Unsaturated aldehydes
(R)-Prophos
© 2014 Elsevier B.V. All rights reserved.
Iridium
1. Introduction
studied system was the reaction between methacrolein and
cyclopentadiene and, in particular, we have shown that the aquo
Asymmetric catalysis is an indispensable and highly valuable
tool for the synthesis of enantioenriched intermediates and prod-
ucts [1–3]. Among the wide variety of metal-catalyzed asymmetric
processes, the Diels–Alder (DA) reaction is a powerful and versa-
tile synthetic transformation that plays an important role in the
construction of cyclohexene derivatives with up to four contigu-
ous stereocenters [4–6]. In this methodology, coordination of the
alkene to the metal lowers the barrier energy of the reaction and
an enantiopure chiral ligand, also coordinated to the metal, is the
source of chirality. In some instances, the metal becomes a chi-
ral center after the coordination of the ligand as, for example, in
half-sandwich three-legged piano stool complexes containing chi-
ral bidentate ligands of C1 symmetry (Scheme 1) [7].
During the last years, examples of one point binding cata-
lysts for the asymmetric DA have been developed by the groups
of Kündig [8–16], Faller [17–24] Davies [25–34] and ourselves
[35–50]. When the alkene was an enal, the CHO group, apart
from activating the double C C bond, provides an anchoring
atom able for the coordination to the metal. By far, the most
complexes [(ꢀ5-C5Me5)M(Prophos)(H2O)][SbF6]2 [Prophos = (R)-
propane-1,2-diyl-bis(diphenylphosphane); M = Ir (1), Rh (2)]
are active catalyst precursors for this reaction [35–45]. How-
ever, the catalytic activity of half-sandwich complexes with
acyclic ˛,ˇ-unsaturated enals other than methacrolein has
been much less studied. At this respect, we have investi-
gated the reaction of acrolein with cyclopentadiene catalyzed
by cations of formula [(ꢀ5-C5Me5)Rh(N–N*)(H2O)]2+ or [(ꢀ6-
p-cymene)Ru(N–N*)(H2O)]2+
, where N–N* is an enantiopure
chiral pyridilimino ligand, obtaining good exo:endo-selectivity
and poor enantioselectivity [38], Kündig’s group has reported
that the ruthenium complex [(ꢀ5-C5H5)Ru(R,R-BIPHOP-F)
(acetone)][SbF6]2 [(R,R-BIPHOP-F) = 1,2-bis[(pentafluorophenyl)
phosphanyloxy]-1,2-diphenyl)] catalyzes efficiently the reaction
between ethyl acrolein and cyclopentadiene [8] and Faller has
achieved good yield and enantioselectivity for the latter reaction
by using half-sandwich chiral Ru(II) and Os(II) complexes with
(S)-BINPO [2,2ꢀ-(diphenyl phosphino-1,1ꢀ-binaphthyl)] ligand as
catalyst [19,21].
Herein we report on: (i) the catalytic asymmetric DA reac-
tion of enals 3–7 (Scheme 2) with cyclopentadiene using the aqua
complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(H2O)][SbF6]2 (1) as cat-
alyst precursor; (ii) the DA reaction of other acyclic dienes with
methacrolein (see Table 2) catalyzed by 1 and by the rhodium
∗
Corresponding authors. Tel.: +34 976 762027; fax: +34 976 761187.
E-mail addresses: dcarmona@unizar.es (D. Carmona), fviguri@unizar.es
(F. Viguri).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.021

120
D. Carmona et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
2+
n+
*
M
H
Ir
A
Ha
Hb
Ph2P1
O
*
B
P2Ph2
3
4
H11
Me
1
2
H2C
Scheme 1. Chiral fragment [(ꢀn-ring)M(AB*)]n+
.
H21
CH3
H22
analogue (SIr,RC)–[(ꢀ5-C5Me5)Rh(Prophos)(H2O)][SbF6]2 (2); (iii)
the isolation and spectroscopic and diffractometric characteri-
zation of the intermediate (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(ethyl
acrolein)][SbF6]2.
Scheme 3. Labeling of the cation of 20 for NMR assignments.
monitored by gas chromatography (GC) and quenched, by addition
of excess of Me4NCl in CH2Cl2, at the specified times. Conversion
and exo/endo 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/diethylether (9/1, v/v). Liquids were
removed under vacuum before the determination of the enan-
tiomeric purity. Enantioselectivity was determined as indicated in
the footnote of Table 1.
2. Experimental
2.1. Material and instrumentation
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 KBr pel-
lets with a Perkin Elmer Spectrum One FT IR spectrophotometer.
Carbon, hydrogen and nitrogen analyses were performed using a
Perkin-Elmer 240 B microanalyzer. 1H, 13C and 31P NMR spectra
were recorded on Bruker AV-300, Bruker AV-400 or Bruker AV-500
spectrometers. Chemical shifts are expressed in ppm upfield from
2.3. Preparation of (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(ethyl
acrolein)][SbF6]2 (20)
Under argon, to a solution of 1 (114.0 mg, 0.091 mmol) in CH2Cl2
˚
(4 mL), excess of ethyl acrolein (88.1 ␮L, 0.900 mmol) and 4 A
SiMe4 or 85% H3PO4 (
31P). NOESY and 13C, 31P, 1H correlation spec-
molecular sieves (100.0 mg) were added. The resulting yellow sus-
pension was stirred for 20 min, at −20 ◦C and then was filtered
through a cannula. The filtrate was concentrated to ca. 1 mL. The
slow addition of 10 mL of dry n-hexane afforded yellow crystals,
which were filtered off, washed with n-hexane and vacuum-dried.
Recrystallization from CH2Cl2/n-hexane yields pure of 20. Yield:
62.5 mg, 59% (Scheme 3).
tra were obtained using standard procedures. Gas chromatography
was performed on Hewlet-Packard 3398 Series gas chromatograph
equipped with a split-mode capillary injection system and flame
ionization detectors using HP Ultra-1 (25 m × 0.32 mm).
Complexes
(SM,RC)–[(ꢀ5-C5Me5)M(Prophos)(H2O)][SbF6]2
(M = Ir (1), Rh (2)), were prepared using literature procedures
[51,52].
1H NMR (400 MHz, CD2Cl2, −50 ◦C): ı = 7.90–7.22 (m, 20H, Ph),
7.28 (s, 1H, CHO), 6.23 (s, 1H, Hb), 5.73 (s, 1H, Ha), 3.56 (dddd,
J = 48.3, 15.6, 11.2, 4.8 Hz, 1H, H22), 2.80 (m, 1H, H11), 2.67 (m, 1H,
H21), 1.42 (m, 18H, C5Me5, Me) 1.37 (m, 2H, CH2CH3). 0.71 ppm
(t, J = 7.4 Hz, 3H, CH2CH3). 13C NMR (100.61 MHz, CD2Cl2, −50 ◦C):
ı = 209.25 (CHO), 149.69 (C3), 147.43 (C4), 134.05–101.32 (Ph),
101.43 (t, J(P,C) = 1.7 Hz, C5Me5), 34.36 (dd, J(P,C) = 40.7, 12.3 Hz,
C2), 31.60 (dd, J(P,C) = 36.9, 7.7 Hz, C1), 19.67 (CH2CH3), 15.16 (dd,
J(P,C) = 17.4, 4.3 Hz, Me), 10.68 (CH2CH3), 8.77 ppm (C5Me5). 31P
NMR (161.96 MHz, CD2Cl2, −25 ◦C): ı = 48.17 (d, J(P1P2) = 8.3 Hz,
P1), 27.64 ppm (d, P2). IR (KBr pellets, cm−1): ␯(CO) 1585m, ␯(SbF6)
659s. Anal. Calcd for C42H49F12IrOP2Sb2: C, 38.9; H, 3.8. Found: C,
39.3; H, 3.6.
2.2. Catalytic procedure
To the complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(H2O)][SbF6]2
(0.025 mmol, 5 mol%) dissolved in CH2Cl2 (3 mL), 100 mg of acti-
˚
vated 4 A molecular sieves and the corresponding freshly distilled
˛,ˇ-unsaturated enal (0.50 mmol) were added. The mixture was
stirred for 15 min at −20 ◦C. Then it was introduced in a cryo-
genic bath at the appropriate temperature and a solution of the
diene (3.00 mmol) in CH2Cl2 (1 mL) was added. The reaction was
O
O
O
2.4. X-ray crystallography
X-ray diffraction data for 20 were collected at 100(2) K using
narrow ω rotation (0.3◦) on a Bruker SMART APEX CCD diffractome-
ter (Mo-K␣ graphite-monochromated radiation, ꢁ = 0.71073 A).
3
4
5
˚
acrolein
ethyl acrolein
2-butenal
Images were processed with SAINT+ [53] and data were corrected
for absorption by numerical methods based on the crystal face
indexing.
O
O
The structure was solved by direct methods with SHELXS-97
[54]. Refinement, by full-matrix least-squares on F2, was performed
with SHELXL-97 [55]. Anisotropic displacement parameters were
included for all non-H non disordered atoms. Hydrogen atoms were
included in calculated positions and refined with displacement
and positional riding parameters. Additionally to the internal con-
figuration reference of the Prophos ligand, the Flack parameter has
6
7
2-methyl-2-butenal
2-methyl-2-pentenal
Scheme 2. ˛,ˇ-Unsaturated aldehydes employed.

D. Carmona et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
121
Table 1
DA reaction of ˛,ˇ-unsaturated enals with cyclopentadiene catalyzed by 1a.
O
CHO
R1
R1
Catalyst
+
R2
3 - 7
R2
9
10 11 12 13 14
R1
R2
H
H
Et
H
H
Me Me
Me Me Et
Entry
Enal
Adduct
T (◦C)
t (h)
0.5
1
1
7
1
Conv. (%)b,c
Isomer ratio (exo/endo) (%)c
ee (%)d,e
1
2
3
4
5
6
7
8
9
10
11
12
13
14
3
3
3
3
4
4
4
5
5
6
6
6
7
7
10
10
10
10
11
11
11
12
12
13
13
13
14
14
RT
0
70
82
70
85
91
87
87
92
88
92
74
56
69
21
55/45
50/50
46/54
30/70
97/3
97/3
98/2
70/30
71/29
97/3
14(–)/10(S)
14(–)/5(S)
10(–)/rac
rac/6(R)
64(S)/(–)
71(S)/(–)
78(S)/(–)
18(–)/rac
15(–)/10(R)
57/(–)
−20
−50
0
−20
−50
0
1
24
24
66
48
102
120
168
148
−20
RT
0
98/2
99/1
98/2
98.5/1.5
58/(–)
64/(–)
41/(–)
48/(–)
−20
0
−20
a
˚
Reaction conditions: catalyst 0.025 mmol (5% mol), enal 0.5 mmol, 100 mg of 4 A molecular sieves and HCp 3 mmol, in 4 mL of CH2Cl2.
Based on enal.
Determined by GC.
b
c
d
e
Determined by acetalization with (–)-(2R,4R)-2,4-pentanediol and 1H NMR analysis.
The absolute configuration was established by comparison with literature data [57,58].
been refined as a check on the correct absolute structure determi-
5 smaller or no exo preference was observed (entries 1–4 and 8,
9). Poor to good enantomeric excesses were achieved. Again, the
substitution pattern strongly affects the catalytic outcome. Thus
while ˛-substituted ethyl acrolein (4), 2-methyl-2-butenal (6), and
2-methyl-2-pentenal (7) give ee values ranging from 48% to 78%
(−20 ◦C, entries 5, 11, and 14) enantioselectivities smaller than 18%
ee were obtained for the ˛-unsubstituted enals acrolein (3) and
2-butenal (5). As expected, lowering temperature slows down reac-
tion rates and, in general, increases ee. However, the influence of
temperature changes on the ee for enals 3 and 5 is more complex,
inversion of the configuration of the endo isomer occurring in some
cases (entry 4). The reasons of this influence remain unclear.
nation [56]. Static disorder has been observed in a phenyl group, in
the dichloromethane solvent molecule and in the terminal methyl
group of the ethyl acrolein. Disordered atoms have been included
in the model in two different positions with complementary
occupancy factors, and have been refined with isotropic thermal
parameters. Bond lengths in the two disordered C–C bonds of the
ethyl acrolein ligand (C(41)–C(42A) and C(41)–C(42B)) have been
restrained to be identical. Hydrogen atoms of the solvent molecule
have not been included in the model. Information concerning
crystallographic data collection and structure refinement is sum-
marized in Table 1S (Supplementary material). CCDC-969379
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif.
3.2. Diels–Alder reactions of methacrolein with acyclic dienes
We also tested the DA reaction between methacrolein (15) and
acyclic dienes 2,3-dimethylbutadiene (16) and isoprene (17), using
the aqua complexes [(ꢀ5-C5Me5)M(Prophos)(H2O)][SbF6]2 (M = Ir
(1), Rh (2)) as catalyst precursors. A list of the obtained results is
collected in Table 2. Both complexes are active but to get good con-
versions, at −20 ◦C, reactions take several days. At this temperature,
moderate enantioselectivities are obtained (about 60% ee). Lower-
ing temperature slightly improves enantioselectivity (63%, 68% ee,
entries 2 and 5, respectively). For asymmetrical isoprene (17) the
1,4-dimethyl regioisomer 19a is preferentially obtained (95%).
Under the reaction conditions depicted in Table 1, neither 1 nor
2 catalyze the DA reaction between methacrolein and cyclohexa-
diene.
3. Results and discussion
3.1. Diels–Alder reaction of the ˛,ˇ-unsaturated enals 3–7 with
cyclopentadiene
The complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(H2O)][SbF6]2 (1)
was tested as catalyst precursor for the DA reaction between the
˛,ˇ-unsaturated enals 3–7 (Scheme 2) and cyclopentadiene (9).
Table 1 lists a selection of the results together with the reaction
conditions employed. The collected results are the average of at
least two comparable reaction runs.
The iridium complex efficiently catalyzes the DA reactions; high
conversions were achieved in all cases. However, the reaction is
sensible to the enal, ˇ-unsubstituted enals being much more reac-
tive than ˇ-substituted ones. Thus, for example, while for enal
4 a conversion of 91% was achieved after 1 h of reaction (entry
5) for enal 5, 24 h of treatment, under similar conditions, were
necessary to achieve 92% of conversion (entry 8). On the other
hand, ˛-substitution strongly affects the exo–endo selectivity: exo
isomers were readily favored for ˛-substituted enals 4, 6, and 7
(entries 5–7 and 10–14) while for ˛-unsubstituted enals 3 and
3.3. Isolation of the intermediate
(SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)(ethyl acrolein)][SbF6]2 (20)
It is commonly assumed that the pathway for the DA reac-
tion between enals and dienes catalyzed by one-point-binding
half-sandwich transition metal complexes (Scheme 4) involves the
diene attack to the coordinated enal (step 1) followed by displace-
ment of the coordinated adduct by the enal (step 2). Therefore, to

122
D. Carmona et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
Table 2
DA reaction of methacrolein with acyclic dienesa.
O
CHO
CHO
Me
R
R
Me
Me
Catalyst
+
+
Me
Me
R
Me
15
R = Me (16)
R = H (17)
R = Me (18)
R = H (19a)
R = H (19b)
Entry
Catalyst
Diene
Adduct
T (◦C)
t (h)
Conv. (%)
(ee) (%)
1
2
3
4
5
1
1
1
2
2
16
16
17
16
16
18
18
19
18
18
−20
−50
−20
−20
−50
72
337
72
95
353
92
74
60
68
42
60
63
91.5b
85
65
a
For reaction conditions and determination of results, see footnote in Table 1.
Isomer ratio 19a/19b:95/5.
b
get a deeper insight into the mechanism of the catalytic reaction, we
attempted to characterize the involved metal–enal intermediate
complex.
spectrum consists of two doublets centered at 27.64 and 48.17 ppm
with a J(P1P2) of 8.3 Hz.
On the other hand, the 1H–1H NOESY spectrum of complex 20
give us important stereochemical information. It shows NOE rela-
tionships between the Ha and formyl protons and between the
Hb and CH3 protons of the ethyl group of the coordinated ethyl
acrolein (see Scheme 3). These correlations strongly indicate an
s-trans conformation for the enal ligand. Moreover, irradiation of
the H11 proton enhances the formyl and methylene ethyl group
protons. With the ethyl acrolein in an s-trans conformation, the lat-
ter NOE relationships are only compatible with a ꢁ conformation
for the Ir–P1–C1–C2–P2 metallacycle together with an S config-
uration for the iridium atom, according to the priority sequence
ꢀ5-C5Me5 > P1 > P2 > O [59], and an E configuration around the car-
bonyl double bond of the enal.
At −20 ◦C, addition of an excess of ethyl acrolein to a solution
˚
of the aqua-complex 1 in dichlorometane, in the presence of 4 A
molecular sieves as water scavenger, affords complex 20 in good
yield (Eq. 1).
[(ꢀ5-C5Me5)]lr(Prophos)H2O)][SbF6]2 + ethyl acrolein
1
4
2
2
−→
CH Cl ,4 Å MS[(ꢀ5-C5Me5)]lr(Prophos)(4)][SbF6]2
(1)
−H
O
2
20
The preparative route is completely diastereoselective: from
−70 ◦C to RT only one set of sharp resonances was observed in the
1H, 13C and 31P NMR spectra of complex 20, indicating that only
one diastereomer was obtained. The complex was characterized
by analytiscal and spectroscopic means, including two dimensional
homonuclear (COSY, NOESY) and heteronuclear (31P–H, 13C–H)
correlations. In addition, its crystal structure was determined by
X-ray diffractometric methods (see below).
3.4. Molecular structure of compound 20
Single crystals, suitable for X-ray diffraction analysis, were
obtained from CH2Cl2/n-hexane solutions of complex 20. Fig. 1
shows a view of the cation of the complex and the most relevant
bond lengths and angles are collected in Table 3.
The metal environment exhibits the “three-legged piano-stool”
geometry commonly encountered for half-sandwich d6 cations: an
ꢀ5-C5Me5 ring occupies three fac coordination positions and the
two phosphorous atoms of the Prophos and the oxygen atom of the
ethyl acrolein ligand complete the coordination sphere. Accord-
ing to the ligand priority sequence ꢀ5-C5Me5 > P(1) > P(2) > O [59]
the absolute configuration at the metal is S. The five-membered
metallacycle Ir–P(1)–C(24)–C(23)–P(2) exhibits a ꢁ conformation.
The 1H and 13C NMR spectra, besides the peaks of the Prophos
and C5Me5 groups, show resonances corresponding to coordinated
ethyl acrolein. Notably, the aldehyde proton is strongly shielded:
it resonates at 7.28 ppm, about 2.3 ppm shifted to higher field with
respect to the corresponding free molecule resonance. A peak at
209.25 ppm, in the 13C NMR spectrum, is attributed to the formyl
carbon. Moreover, an intense ꢂ(CO) band at ca. 1585 cm−1, in the IR
spectrum, confirms the presence of coordinated enal. The 31P NMR
[( 5-C5Me5)Ir(Prophos)(H2O)]2+
H2O enal
[( 5-C5Me5)Ir(Prophos)(enal)]2+
diene
adduct
Step 1
Step 2
enal
[( 5-C5Me5)Ir(Prophos)(adduct)]2+
Fig. 1. Molecular structure of the cation of 20. Only one position of disordered parts
(C(26)–C(32) phenyl ring and C(42) atom) has been represented. Hydrogen atoms
have been omitted for clarity.
Scheme 4. Plausible catalytic cycle.

D. Carmona et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
123
Fig. 2. Schematic representation of CH/␲ interactions observed in complex 20: (a) view perpendicular to the phenyl plane; (b) view nearly parallel to the phenyl plane.
Cremer and Pople ring puckering parameters (Q2 = 0.466(9) Å and
ꢃ2 = 75.5(6)◦) [60] are characteristic for an envelope 3E confor-
mation. The observed puckering amplitude value (Q2) is close to
those reported in other related [(ꢀ5-C5Me5)Ir(Prophos)L]2+ half-
sandwich complexes with coordinated methacrolein [52] or enones
[49].
G
G
O
The conformation proposed in solution for the ethyl acrolein, on
the basis of NOESY measurements, is identical to that determined
Ir
O
Ir
C(38)
in the solid state. Thus, the enal adopts an s-trans conformation
with respect to the C(38)–C(39) single bond and an E configura-
tion around the double O–C(38) bond. The O–C(38)–C(39)–C(40)
skeleton of the ethyl acrolein is essentially planar. The termi-
nal methyl group (C(42)) is disordered over two positions with
complementary occupancy factors; one of them corresponds to
C(38)
(a)
(b)
Scheme 5. G–Ir–O–C(38) torsion angle for enal dispositions: (a) −90◦, (b) 180◦.
˚
a planar disposition while the other position lies 1.11(2) A out
of the mean plane determined by the remaining O, C(38), C(39),
C(40) and C(41) atoms. The relative disposition of the enal frag-
ment within the metal coordination sphere can be characterized
by the G–Ir–O–C(38) torsion angle that relates the plane of the
ethyl acrolein to that of the sterically demanding C5Me5 ligand.
Values close to 0 (or 180◦) point to a relative perpendicular dispo-
sition, while angles close to 90◦ indicate a parallel arrangement
(Scheme 5). The measured value of −59.2(10)◦ indicates an inter-
mediate disposition, close to those found in related methacrolein
cations [(ꢀn-arene)M(PP*)(methacrolein)]2+ (absolute values in the
range 45.5(6)–65(2)◦) [47,48,51,52]. This intermediate arrange-
ment favors the establishment of intramolecular CH/␲ interactions
between the pro-S phenyl ring of P(1) and the formyl proton of the
(C(17)–C(22) ring) (Fig. 2). The geometrical parameters character-
izing this interaction in 20 are collected in Table 4.
As it has been suggested in other closely related Ir or Rh
complexes [47,51,52], these attractive intramolecular interactions
hinder the rotation of the organic substrate within the chiral
pocket conformed into the (ꢀ5-C5Me5)Ir(Prophos) fragment and,
consequently, they stabilize a preferential conformation for the
coordinated aldehyde. Most probably, this interaction is also oper-
ating in solution and explains the strong shielding observed for the
formyl proton in the 1H NMR spectrum. In this conformation, the
C␣-si-face of the enal is sheltered by the phenyl ring involved in the
CH/␲ interactions and, therefore, the cyclopentadiene attack would
take preferentially through the C␣-re-face, in good agreement with
the catalytic outcome.
˚
organic substrate. In 20, a small H. . .phenyl plane distance (2.80 A),
˚
shorter than the sum of van der Waals radii (3.05 A), evidences the
existence of this interaction between the CHO proton, H(38), and
the pro-S phenyl ring linked to the P(1) atom of the Prophos ligand
Table 3
Table 4
Selected bond lengths (Å) and angles (◦) for complex 20.
Selected structural parameters (Å, deg) concerning CH/␲ interactions for complex
20a.
Ir–P(1)
Ir–P(2)
2.319(2)
2.319(2)
2.158(6)
1.871(8)
1.854(8)
1.826(11)
1.196(13)
1.484(18)
1.365(18)
1.41(2)
P(1)–Ir–O
84.11(17)
133.2(3)
86.85(18)
130.4(3)
122.3(3)
127.2(8)
123.6(13)
110.8(13)
113.9(13)
134.4(14)
P(1)–Ir–Ga
H. . .G(Ph)
H. . .Ph plane
␥ angle
C–H. . .C
C–H. . .C(Ph)
Ir–O
P(2)–Ir–O
Ir–Ga
P(2)–Ir–Ga
3.04
2.80
23.0
C17: 2.80
C18: 3.08
3.08–3.79
P(1)–C(24)
P(2)–C(23)
O–C(38)
C(38)–C(39)
C(39)–C(40)
C(39)–C(41)
P(1)–Ir–P(2)
O–Ir–Ga
Ir–O–C(38)
a
H. . .G(Ph): distance between the H(38) atom and the centroid of the phenyl
ring G(Ph); H. . .Ph plane: separation between the hydrogen atom and the phenyl
ring mean plane; ␥ angle: angle between the G(Ph)–H vector and the normal to the
phenyl ring; C–H. . .C: contact distances between hydrogen atom and the phenyl
O–C(38)–C(39)
C(38)–C(39)–C(40)
C(38)–C(39)–C(41)
C(40)–C(39)–C(41)
˚
carbon ring under the assumed criterion ≤3.05 A for CH/␲ interaction; C–H. . .C(Ph):
83.99(8)
distances between the hydrogen atom and the rest of the carbon atoms of the phenyl
ring.
a
G represents the centroid of aromatic ring of the ꢀ5-C5Me5 ligand.

124
D. Carmona et al. / Journal of Molecular Catalysis A: Chemical 385 (2014) 119–124
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In summary, the aqua-complex (SIr,RC)–[(ꢀ5-C5Me5)Ir(Prophos)
(H2O)][SbF6]2 efficiently catalyzes the Diels–Alder reaction
of simple acyclic enals with cyclopentadiene. The catalyst-
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diastereoselective manner. From the stereochemistry of the
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We thank the Ministerio de Educación y Ciencia (Grant CTQ
2009-10303/BTQ), the Ministerio de Economía y Competitividad
(Grant CTQ 2012-32095), and Gobierno de Aragón (Grupo Con-
solidado: Catalizadores Organometálicos Enantioselectivos) for
financial support. A. A. thanks the IUCH for a grant. P. G.-O. acknowl-
edges the CSIC, “JAE-Doc” program (contract co-funded by the ESF)
and the “Factoría de Cristalización” project (CONSOLIDER INGENIO-
2010, CSD 2006-0015 program) for financial support.
Appendix A. Supplementary data
[39] D. Carmona, F.J. Lahoz, S. Elipe, L.A. Oro, M.P. Lamata, F. Viguri, F. Sánchez,
S. Martínez, C. Cativiela, M.P. López-Ram de Víu, Organometallics 21 (2000)
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Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.01.021.
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