Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands with C1, Pseudo-meso, and Pseudo-C2 Symmetries: Application in Rhodium(I)-Mediated Arylation

Article abstract of DOI:10.1021/acs.orglett.9b02405

A new family of sulfinamide/sulfoxide derivatives was synthesized as chiral bidentate ligands by stereoselective additions of methylsulfinyl carbanions to N-tert-butylsulfinylimines. The new ligands, with C₁, pseudo-meso, and pseudo-C₂ symmetries, were successfully assayed in Rh-catalyzed additions of arylboronic acids to activated ketones. The sterically dissymmetric C₁ ligand (R_S,S_C,R_S)-N-[1-(phenylsulfinyl)-3-methylbut-2-yl] tert-butylsulfinamide turned out to be the optimal one, allowing the 1,4-additions of diverse arylboronic acids, on different α,β-unsaturated cyclic ketones with high chemical yields and enantioselectivities up to >99% ee.

Full text of DOI:10.1021/acs.orglett.9b02405

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands with C1,
Pseudo-meso, and Pseudo‑C2 Symmetries: Application in
Rhodium(I)-Mediated Arylation
‡
§
Lorenzo G. Borrego,†,∥ Rocío Recio,†,∥ Eleuterio Alvarez, Antonio Sanchez-Coronilla,
́
́
,†
Noureddine Khiar,*,‡ and Inmaculada Fernandez*
́
†
́
́
́
Departamento de Química Organica y Farmaceutica, Facultad de Farmacia, Universidad de Sevilla, C/Profesor García Gonzalez, 2,
41012 Sevilla, Spain
‡
́
Instituto de Investigaciones Químicas, CSIC-Universidad de Sevilla, Avda. Americo Vespucio, 49, 41092 Sevilla, Spain
§
́
Departamento de Química Física, Facultad de Farmacia, Universidad de Sevilla, C/Profesor García Gonzalez, 2, 41012 Sevilla,
Spain
S
* Supporting Information
ABSTRACT: A new family of sulfinamide/sulfoxide deriva-
tives was synthesized as chiral bidentate ligands by stereo-
selective additions of methylsulfinyl carbanions to N-tert-
butylsulfinylimines. The new ligands, with C1, pseudo-meso,
and pseudo-C2 symmetries, were successfully assayed in Rh-
catalyzed additions of arylboronic acids to activated ketones.
The sterically dissymmetric C1 ligand (RS,SC,RS)-N-[1-(phenyl-
sulfinyl)-3-methylbut-2-yl] tert-butylsulfinamide turned out to
be the optimal one, allowing the 1,4-additions of diverse arylboronic acids, on different α,β-unsaturated cyclic ketones with high
chemical yields and enantioselectivities up to >99% ee.
ransition metal enantioselective catalysis remains an
symmetric, and nonsymmetric sulfinyl derivatives as chiral
ligands in enantioselective organic18 and organometallic
catalysis.19 Within the C2-symmetric ligands developed,
Ferbisox (Figure 1), a 1,2-bis(ferrocenylsulfinyl) ethane, has
1−4
Tactive area of development in synthetic methodology.
Despite the great progress made in this field, one of the aspects
that has been and remains a cardinal challenge in this area is
the design and development of new effective ligands.5 The
steric, electronic, and overall symmetry of the ligands are
important elements to be considered in their design and
preparation. Ligands with C2 symmetry dominated the field at
its birth, while electronically dissymmetric C1 ligands gained
importance over the years. Otherwise, modern asymmetric
catalysis requires ligands that are not only highly enantiose-
lective but also structurally simple and chemically stable, and
both enantiomers could be easily accessed by practical and
cost-effective synthetic approximations. Of the few ligands that
meet these characteristics, those derived from sulfinyl
compounds occupy a preponderant place in asymmetric
catalysis.6−9 Indeed sulfinyl-based ligands are air-, oxygen-,
moisture-, and shelf-stable, and in addition, currently a number
of highly efficient and modular approaches allow their rapid
synthesis in both enantiomeric forms.10−13 Moreover, like the
synthetically challenging P-chiral phosphines, they are ideally
suited for the construction of diverse metal−ligand complexes
with a well-defined chiral environment due to the proximity of
the chiral sulfur atom to the coordination sphere of the
Figure 1. Homo and hybrid-bidentate sulfinyl-based ligands.
proven to be an excellent chiral ligand in the enantioselective
rhodium-catalyzed 1,4-addition of boronic acids to electron-
deficient alkenes.20,21 In the case of the most interesting C1-
symmetric ligands, we have recently found that mixed
sulfinamido−olefin ligands, Sulfolefin17,22,23 and Metsulfole-
fin24 (Figure 1), enclosing a chiral sulfur atom as the sole
metal.6,13,14
·
Within our interest in the development of new efficient
chiral catalysts15−17 with the aforementioned advantages, our
research group focused on the study of structurally varied,
Received: July 11, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02405
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
stereogenic center, are excellent precursors for Rh-catalyzed
addition of arylboronic acids to activated ketones, including
trifluoromethyl ketones.
sulfinyl group of the nucleophile, unlike the analogous N-p-
tolysulfinilimines.26 Thus, each pair of diastereoisomers have
identical configurations on the sulfinamide and the stereogenic
carbon and opposite configurations in the sulfoxide.
The different nature of both functional groups, sulfinamide
and sulfoxide, with stereogenic sulfurs as metal coordination
elements, implies a C1 symmetry for all these ligands. However,
by selecting the nature of the R2 group and the stereochemistry
of the starting sulfoxide, we could consider some of the
obtained ligands as pseudo-C2-symmetric (ligand type I, 3h) or
pseudo-meso (ligand type II, 3g) (Figure 2). Such ligands are
Herein, we wished to explore the behavior of electronically
dissymmetric C1 ligands, comparing and contrasting the
sterically symmetric (pseudo-meso- and pseudo-C2-symmetric)
with the sterically disymmetric ones. Ligands fulfilling our
needs are those that belong to the unprecedented family of
sulfinamide/sulfoxide ligands (Figure 1). The target ligands are
accessible in a short, effective, and low-cost synthetic way, by
nucleophilic addition of a methylsulfinyl carbanion to the
corresponding N-tert-butylsulfinylaldimine (Figure 1). The
approach presents the advantage of allowing the modulation of
the stereochemistry of both sulfurs and the structure of the R1
and R2 groups in the nucleophile and electrophile.
The stereoselective synthesis of these ligands is based on the
known sterochemical control of the tert-butylsulfinyl group of
chiral N-tert-butylsulfinylimines in the nucleophilic additions to
the imino double bond.25 However, it must be taken into
account the fact that the methylsulfinyl carbanion used as a
nucleophile also has a stereogenic sulfur that could also
influence (matched and mismatched pairs), or even control,
the stereochemistry of the new stereogenic center generated in
the process, as it has been previously demonstrated in the case
of N-p-tolylsulfinylimines as substrates.30 The additions of the
corresponding racemic tert-butyl and phenyl methyl sulfoxides,
rac-2a or rac-2b, on different (R)-N-tert-butylsulfinilimines,
1a(RS)−1c(RS), were carried out at −78 °C, adding LHMDS
(300 mol %) to a solution of both reagents in THF (Scheme
1). In all cases, the reactions were completed in <0.5 h,
yielding high to quantitative chemical yields, a 1:1 mixture of
two of the four possible diastereoisomers (Scheme 1).
Figure 2. Sterically symmetric, pseudo-C2 (I), pseudo-meso (II), and
sterically dissymmetric (III and IV) sulfinamide/sulfoxide ligands.
of interest as they allow the determination of the role played by
the steric and electronic factors on the stereochemical outcome
of the catalysis. Pseudo-meso- and pseudo-C2-symmetric
ligands have previously been used in catalysis with success,
but the ligands used were mostly mixed ligands with two
different heteroatoms or bimetallic ligands.27 To the best of
our knowledge, no pseudo-meso or pseudo-C2 ligands with a
sole coordinating heteroatom have yet been reported.
Scheme 1. Synthesis of Sulfinamide/Sulfoxide Ligands 3a−
3h
With ligands 3a−3h in hand, they were assayed as
precursors of chiral Rh(I) catalysts in the 1,4-addition of
arylboronic acids to α,β-unsaturated ketones and the results
obtained are discussed below. To determine the optimal
ligand, we used as a model reaction the addition of
phenylboronic acid (5a) to 2-cyclohexen-1-one (6a) under
the previously determined optimal conditions, i.e., 10 mol % of
the chiral ligand 3a−3h, 4.5 mol % of the rhodium precursor
[Rh(C2H4)2Cl]2, and a previously deoxygenated 2.5 M
aqueous solution of potassium hydroxide as the base, in
different solvents (toluene and methanol), and the results are
listed in Table 1.
The results obtained indicate that, in contrast to the
sterically dissymmetric C1-phenyl sulfoxide ligands 3a−3f
(entries 1−10, Table 1), sterically symmetric tert-butylsulf-
oxides, 3g and 3h, are not suitable chiral ligands for this
reaction, because they yield the corresponding addition
products with low chemical yields, in the absence of
enantioselectivity (entries 11 and 12, Table 1). In the case
of tert-butyl derivative 3g-(RS,SC,RS) (entry 11, Table 2), the
absence of stereoselectivity could be justified considering its
pseudo-meso symmetry. However, its epimer, 3h-(RS,SC,SS)
(entry 12, Table 1) with a pseudo-C2 symmetry, is also not
suitable as a chiral ligand. In general, the C1 dissymmetric
phenyl sulfoxides 3a−3f provided the addition product with
In general, the pairs of diastereoisomers obtained had
different chromatographic resolution factors, allowing their
isolation in diastereomerically pure form by column
chromatography. Exceptions to this behavior are isopropyl
diastereoisomers 3e and 3f, which despite being obtained with
an 84% global yield are isolated in pure form in only 30% and
38% yields, respectively, due to their difficult separation by
column chromatography. The configurational assignment,
based on NMR spectroscopy, X-ray diffraction analysis, and
chemical correlations (see the Supporting Information),
showed that the induced stereochemical configuration of the
new stereogenic center is independent of the chirality of the
B
DOI: 10.1021/acs.orglett.9b02405
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Organic Letters
Letter
naphthyl (3c to 3a, respectively), does not improve the
enantioselectivity of the process (96% and 95% ee for entries 5
and 1, respectively, Table 1). However, the enantioselectivity
improves with isopropyl derivative 3e, which we have named
SulfiSox*, yielding the highest enantioselectivity, 99% ee, with
a high chemical yield (entry 9, Table 1).
Table 1. Enantioselective Rhodium-Catalyzed Addition of
Phenylboronic Acid 5a to Cyclohexen-2-one 6a, Using 3a−
a
3h as Chiral Ligands
After establishing the sulfinamide/sulfoxide 3e-(RS,SC,RS) as
the optimal chiral ligand, we achieved its stereoselective
synthesis, using the DAG methodology28 as the key reaction
(Scheme 2). Condensation of racemic methylsulfinyl chloride
b
c
entry
L*
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
8
3a
3a
3b
3b
3c
3c
3d
3d
3e
3f
toluene
MeOH
toluene
MeOH
toluene
MeOH
toluene
MeOH
toluene
toluene
toluene
toluene
61
74
34
66
73
66
49
23
85
90
27
30
95
85
83
26
96
80
82
52
99
78
0
Scheme 2. Stereoselective Synthesis of Sulfinamide/
Sulfoxide Ligand 3e (SulfiSox*)
9
10
11
12
3g
3h
0
a
All reactions were conducted using 10 mol % chiral ligand and 4.5
b
mol % [Rh(C2H4)Cl]2 at rt for 18−60 h. Isolated yield of pure
compound 7a after column chromatographic purification. For HPLC
c
conditions, see the Supporting Information.
yields and enantioselectivities ranging from good to excellent,
always in favor of the R enantiomer, which results from the
insertion of the aryl group of boronic acid on the re face of the
olefin. Enantioselectivities are higher with toluene as a solvent
(compare entries 1, 3, 5, and 7 and 2, 4, 6, and 8, respectively,
in Table 1). Additionally, the higher chemical and stereo-
chemical yields are obtained with the R-sulfoxide ligands 3a,
3c, and 3e (compare entries 1, 5, and 9 and 3, 7, and 10,
respectively). A comparison between the different homochiral
ligands of the (RS,SC,RS) series shows that an increase in the
size of the aromatic ring of the stereogenic carbon, phenyl to
8 on diacetone-D-glucose (DAGOH) afforded RS-DAG-
methane sulfinate 9-(RS) in 87% yield and 96% diastereomeric
excess, which after reaction with phenyl Grignard gave
enantiopure (R)-methyl phenyl sulfoxide 2-(RS). Next, the
addition of 2-(RS) to N-sulfinilimine 1c-(RS) afforded the
corresponding ligand SulfiSox* as a single diastereoisomer in
87% chemical yield.
We next set to examine the substrate scope with this ligand.
The results obtained (Table 2) showed that ligand SulfiSox* is
a
Table 2. Enantioselective Addition of Arylboronic Acids 5a−5h to Enones 6a−6d, Using SulfiSox* as a Chiral Ligand
a
b
c
entry
n
ketone
Ar
boronic acid
product
yield (%)
ee (%)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
0
2
−
6a
6a
6a
6a
6a
6a
6a
6a
6b
6c
6d
Ph
p-Tol
1-Napht
2-Napht
p-F-C6H4
m-Tol
5a
5b
5c
5d
5e
5f
5g
5h
5e
5e
5e
7a
7b
7c
7d
7e
7f
7g
7h
7i
85
86
73
85
quant
67
89
74
88
82
99
88
93
99
>99
99
96
99
82
>99
34
o-Tol
p-OMe-C6H4
p-F-C6H4
p-F-C6H4
p-F-C6H4
9
10
11
7j
7k
64
a
b
All reactions were conducted using 10 mol % ligand together with 4.5 mol % [Rh(C2H4)Cl]2 at rt for 18 h. Isolated yield of pure compound 7a−
c
7k after column chromatographic purification. For HPLC conditions, see the Supporting Information.
C
DOI: 10.1021/acs.orglett.9b02405
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Letter
not specific to a single substrate but is also effective with
different arylboronic acids and unsaturated systems.
First, several arylboronic acids with different steric and
electronic characteristics of the aromatic ring were tested using
2-cyclohexen-1-one 6a as the model substrate. As shown in
Table 2, in all cases the corresponding 1,4-addition products
were obtained with good to excellent chemical yields (64%
quant) and high enantioselectivities (88−99% ee, entries 1−8,
Table 2). The stereoselectivity of the process was independent
of electronic factors, because boronic acids bearing both
electron-withdrawing (entry 5, Table 2) and electron-donating
(entries 2 and 6−8, Table 2) substituents in the aromatic ring
rendered the addition products with very high chemical and
stereochemical yields. With regard to steric factors, it should be
noted that the position of the substituent in the aromatic ring
(ortho, meta, or para) of the arylboronic acid does not
significantly influence the chemical yield of the process. In this
sense, upon comparison of the tolyl derivatives (entries 2, 6,
and 7, Table 2), the lowest yields were obtained in the case of
the meta substitution (67% yield, entry 6, Table 2), yielding
similar chemical yields in the case of ortho and para derivatives
(89% and 86% yields for entries 7 and 2, respectively). With
regard to the enantioselectivity, upon comparison of the results
obtained with 1- and 2-naphthylboronic acids (entries 3 and 4
for 93% and 99% ee, respectively), as a first interpretation it
could be proposed that ortho substitution may erode the
enantioselectivity. However, in the case of tolylboronic acids,
the ortho- and meta-substituted derivatives (entries 7 and 6,
respectively, Table 2) provided enantioselectivities (96% and
99% ee, respectively), which are similar to that of the
unsubstituted phenyl derivative (99% ee, entry 1, Table 2)
and higher than that of the para-substituted derivative (88%
ee). Therefore, the differences in enantioselectivities cannot be
attributed to steric factors.
The reaction scope was further explored by testing various
cyclic ketones with different ring sizes (cyclohexenone 6a,
cyclopentenone 6b, and cycloheptenone 6c), as well as acyclic
ketone [(E)-3-penten-2-one, 6d], using p-fluorophenylboronic
acid 5e. A significant decrease in the enantioselectivity was
observed in the special case of acyclic enone 6d (entry 11,
Table 2), to give addition product 7k with low enantiose-
lectivity. The conversion obtained in the case of other acyclic
ketones, such as nitrostyrene and chalcone, was very low. This
result is not surprising as the rhodium-catalyzed 1,4-addition of
arylboronic acids to acyclic substrates is a very challenging
process and most catalysts described in the literature render
the addition product with only moderate enantioselectivities.
Fortunately, the addition of 5e to cyclopentenone 6b yielded
the corresponding 3-(4-fluorophenyl)cyclopentanone 7i (entry
9, Table 2) with a high yield (88%) and good enantiose-
lectivity (82% ee), although it was lower than that previously
obtained with 2-cyclohexenone 6a (>99% ee, entry 5, Table 2).
Finally, in the case of the seven-membered enone 6c (entry 10,
Table 2), the addition product 7j was also obtained with a high
chemical yield and enantioselectivity (82% and >99% ee,
respectively).
Figure 3. Transition states, A1 and A2, obtained theoretically at the
M06/cc-pVDZ (for C, H, O, S, and N) and LANDL2DZ (for Rh)
levels of theory for the enantioselective addition of arylboronic acids
to α,β-unsaturated ketones.
where the aryl group is placed in an anti position to the sulfinyl
sulfur of sulfinamide, due to its stronger trans effect. This
hypothesis (trans effect, SON > SO) is based on the structural
data of rhodium complexes derived from sulfoxide and
sulfinamide ligands.29−31
Thus, considering the two possible intermediates, A1 and
A2, generated when the substrate is coordinated to the
transition metal, theoretical calculations were carried out using
Gaussian 09,32 and full geometry optimization of the molecules
was performed using density functional theory in the context of
the M06 functional33 (see the Supporting Information).
Theoretical results indicate that A1 is energetically more
favorable than A2 (∼8 kcal/mol), for steric reasons. In this
sense, the square plane structure is lost in the A2 TS, while it is
maintained in the most stable TS, that is, A1, with angles of
∼170° (Figure 3). Therefore, the addition of the aromatic ring
of the arylboronic acid to the double bond in A1, which
implies an attack by the re face, followed by transmetalation,
justifies the formation of the major enantiomer with high ee
(Figure 3).
In conclusion, the addition of methylsulfinyl carbanions to
different N-tert-butylsulfinylimines has allowed the synthesis of
a new family of electronically dissymmetric bidentate ligands,
with different types of symmetries, having two stereogenic
sulfurs as coordination elements to the metal and an additional
chiral center in the carbon backbone. The addition of the
carbanions proceeds with total stereoselectivity and in the
same sense with both enantiomeric sulfoxides, showing that
the N-tert-butylsulfinyl group of the imine controls the
stereochemical course of the addition. The sulfinamide/
sulfoxide ligands were assayed in the Rh-catalyzed addition
of arylboronic acids to activated ketones. From the ligands
assayed, the sterically symmetric (pseudo-meso and pseudo-C2-
symmetric) ones were the least effective, while the sterically
dissymmetric ligands gave better results. Among these last
ligands, the (RS,SC,RS)-N-[1-(phenylsulfinyl)-3-methylbut-2-
yl] tert-butylsulfinamide (SulfiSox*) has proven to be the
optimum. It allows the 1,4-additions of arylboronic acids of a
different nature, on different α,β-unsaturated cyclic ketones
with high chemical yields and enantioselectivities, up to >99%
ee.
Justification of Enantioselectivity
To justify the enantioselectivity of the process, according to
the established catalytic cycle for the rhodium-catalyzed
additions of arylboronic acids to activated ketones, we propose
that the addition of the aryl group to the double bond of the
cyclohexenone occurs in a complex as shown in Figure 3,
D
DOI: 10.1021/acs.orglett.9b02405
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Letter
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for Asymmetric Catalysis. Angew. Chem., Int. Ed. 2015, 54, 5026−
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02405.
■
S
(9) Otocka, S.; Kwiatkowska, M.; Madalinska, L.; Kiełbasinski, P.
Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur
Atom: Applications in Asymmetric Synthesis. Chem. Rev. 2017, 117,
4147−4181.
Experimental data for the synthesis of ligands, substrates,
and products; analytical data concerning the character-
ization of ligands, substrates, and products (NMR,
HRMS, and chiral HPLC); and RX data of the ligands
(PDF)
(10) Wojaczynska, E.; Wojaczynski, J. Enantioselective Synthesis of
Sulfoxides: 2000−2009. Chem. Rev. 2010, 110, 4303−4356.
(11) Carmen Carreno, M.; Hernandez-Torres, G.; Ribagorda, M.;
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Urbano, A. Enantiopure sulfoxides: recent applications in asymmetric
synthesis. Chem. Commun. 2009, 6129−6144.
Accession Codes
CCDC 1912692−1912695 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(12) Senanayake, C. H.; Krishnamurthy, D.; Lu, Z.-H.; Han, Z.;
Gallou, I. Enantiopure sulfoxides and sulfinamides: recent develop-
ments in their stereoselective synthesis and applications to
asymmetric synthesis. Aldrichimica Acta 2005, 38, 93−104.
́
(13) Fernandez, I.; Khiar, N. Recent Developments in the Synthesis
and Utilization of Chiral Sulfoxides. Chem. Rev. 2003, 103, 3651−
3705.
AUTHOR INFORMATION
Corresponding Authors
■
́
́
(14) Delouvrie, B.; Fensterbank, L.; Najera, F.; Malacria, M. The
chemistry of C2-Symmetric bis(sulfoxides): A new approach in
asymmetric synthesis. Eur. J. Org. Chem. 2002, 2002, 3507−3525.
(15) Khiar, N.; Salvador, A.; Valdivia, V.; Chelouan, A.; Alcudia, A.;
*E-mail: khiar@iiq.csic.es.
*E-mail: inmaff@us.es.
ORCID
́
́
Alvarez, E.; Fernandez, I. Flexible C2-Symmetric Bis-Sulfoxides as
Ligands in Enantioselective 1,4-Addition of Boronic Acids to
Electron-Deficient Alkenes. J. Org. Chem. 2013, 78, 6510−6521.
(16) Valdivia, V.; Bilbao, N.; Moya, J. F.; Rosales-Barrios, C.;
Salvador, A.; Recio, R.; Fernandez, I.; Khiar, N. Pseudo enantiomeric
mixed S/P ligands derived from carbohydrates for the 1,4-addition of
phenyl boronic acid to cyclohexenone. RSC Adv. 2016, 6, 3041−3047.
Noureddine Khiar: 0000-0003-4211-7138
́
Inmaculada Fernandez: 0000-0002-3468-387X
Author Contributions
∥L.G.B. and R.R. contributed equally to this work.
́
(17) Valdivia, V.; Fernandez, I.; Khiar, N. ″Sulfolefin″: a mixed
Notes
sulfinamido-olefin ligand in enantioselective rhodium-catalyzed
addition of arylboronic acids to trifluoromethyl ketones. Org. Biomol.
Chem. 2014, 12, 1211−1214.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
́
(18) Fernandez, I.; Valdivia, V.; Leal, M. O.; Khiar, N. C2-
■
Symmetric Bissulfoxides as Organocatalysts in the Allylation of
Benzoyl Hydrazones: Spacer and Concentration Effects. Org. Lett.
2007, 9, 2215−2218.
The authors are grateful to the Spanish “Ministerio de
́
Economia y Competitividad” (MEC) for financial support
(Grant CTQ2016-78580-C2-2R). The authors gratefully thank
́
(19) Khiar, N.; Fernandez, I.; Alcudia, F. C2-Symmetric bis-
́
CITIUS for NMR facilities and CICA-Centro Informatico
sulfoxides as chiral ligands in metal catalyzed asymmetric Diels-Alder
́
́
Cientifico de Andalucia (Spain) for calculation facilities. L.G.B.
reactions. Tetrahedron Lett. 1993, 34, 123−126.
́
thanks the “Junta de Andalucia” for the Ph.D. grant.
(20) Miyaura, N.; Yanagi, T.; Suzuki, A. The palladium-catalyzed
cross-coupling reaction of phenylboronic acid with haloarenes in the
presence of bases. Synth. Commun. 1981, 11, 513−519.
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