Unprecedented selectivity via electronic substrate recognition in the 1,4-Addition to cyclic olefins using a chiral disulfoxide rhodium catalyst

Full text of DOI:10.1002/anie.200900429

Communications
DOI: 10.1002/anie.200900429
Asymmetric Catalysis
Unprecedented Selectivity via Electronic Substrate Recognition in the
1,4-Addition to Cyclic Olefins Using a Chiral Disulfoxide Rhodium
Catalyst**
Justus J. Bꢀrgi, Ronaldo Mariz, Michele Gatti, Emma Drinkel, Xinjun Luan, Sascha Blumentritt,
Anthony Linden, and Reto Dorta*
The importance of asymmetric synthesis as a tool for
obtaining enantiomerically pure compounds has been fully
acknowledged by the chemical community. Metal-catalyzed
asymmetric reactions provide one of the most elegant ways to
introduce chiral information into a substrate, and the success
of such systems relies on identification of efficient, chiral
ligand–metal complexes.[1] A survey of the nature of these
ligands shows that the overwhelming majority bind the metal
through phosphorous, nitrogen, or oxygen atoms. In the last
few years, chiral diene ligands have also appeared in
combination with rhodium and iridium catalysis.[2] Perhaps
because organic sulfur-containing compounds often poison
metals, catalysts that contain sulfur–metal bonds have to date
played a relatively minor role in (asymmetric) catalysis.[3]
We very recently started an investigation into the
potential of sulfoxide-based compounds as ligands in asym-
metric late-transition-metal chemistry. Especially appealing
properties of such compounds are their nontoxicity and air
and moisture stability, their inherent chirality at the sulfur
center, as well as their easy synthetic access in enantiomer-
ically pure form. An analogue of binap (1,1’-binaphthalene-
2,2’-diyl-bis(diphenylphosphine)) that we called p-Tol-binaso
(1,1’-binaphthalene-2,2’-diyl-bis(p-tolylsulfoxide)) was syn-
thesized in our group and showed very encouraging results
in the rhodium-catalyzed 1,4-addition of boronic acids to a,b-
unsaturated compounds.[4] Pioneered by Miyaura, Hayashi,
and co-workers a decade ago, this reaction represents a very
straightforward entry into useful chiral organic building
blocks and has emerged as an important methodology in
organic synthesis.[5,6]
biphemp
(dimethylbiphenyl-2,2’-diyl-bis(diphenylphos-
phine)) as our template and synthesized the racemic dibromo
derivative using well-established methods.[7] Subsequent gen-
eration of the di-Grignard species[8,9] and reaction with
commercially available (À or +)-menthyl-(S or R)-p-Tol-
sulfinate led to the isolation of p-Tol-Me-bipheso (1) as a
mixture of diastereomeric pairs. The respective diastereomers
were separated by simple column chromatography to give the
pure ligands 1a/1b and 1c/1d in 50–60% yields (Scheme 1).
Scheme 1. Synthesis of the p-Tol-Me-bipheso ligand and rhodium
complexes thereof.
Ligand (M,S,S)-p-Tol-Me-bipheso (1a) or (P,R,R)-p-Tol-
Me-bipheso (1d) was then treated with the rhodium ethylene
dimer [{Rh(C2H4)2Cl}2] in methylene chloride. Subsequent
concentration and layering with THF gave complexes 2a and
2d as red crystals in high yield (90–95%). Diastereomers 1b
and 1c of the ligand do not bind well to the dimeric rhodium
precursor. In these cases, the relative orientation of the tolyl
groups hinders formation of the dimer, a phenomenon that
was observed earlier for atropisomeric diphosphine ligands
incorporating bulky aromatic groups on the phosphorous
atoms.[10]
To unambiguously assign the stereochemistry of the
present system and to better understand the binding situation
of sulfoxides, crystal structure analyses of one of the ligands
(1a) and its corresponding rhodium complex 2a were
performed (Figure 1). For comparison, we also synthesized
and crystallized the analogous diphosphine complex [({(S)-
biphemp}RhCl)2] (3, see the Supporting Information).[11] As
can be seen from the most important bond lengths and angles
(Supporting Information, Table S1), the structures of the
Building upon our previous results and in analogy to
research done with diphosphines, we decided to modify the
atropisomeric backbone to test its impact on reactivity and
selectivity of the rhodium precatalyst. To do so, we chose
[*] J. J. Bꢀrgi, R. Mariz, M. Gatti, E. Drinkel, X. Luan, S. Blumentritt,
Priv.-Doz. Dr. A. Linden, Prof. Dr. R. Dorta
Organisch-chemisches Institut, Universitꢁt Zꢀrich
Winterthurerstrasse 190, CH-8057 Zꢀrich (Switzerland)
E-mail: dorta@oci.uzh.ch
[**] R.D. holds an Alfred Werner Assistant Professorship and thanks the
foundation for generous financial support. Support for this work
was provided by the Schweizerischer Nationalfonds (SNF, grant to
R.M.) and the Roche Research Foundation (RRF, grant to E.D.). We
thank Dr. M. Scalone (F. Hoffmann-La Roche) for a generous
donation of (S)-biphemp.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900429.
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Chemie
workers that show surprising stoichiometric reactivities with
iridium disulfoxide complexes.[14]
Precatalyst 2a was then tested in the Miyaura–Hayashi
reaction of 2-cyclohexen-1-one (8a) with phenylboronic acid
(9A) using previously established reaction conditions (tolu-
ene/H2O/KOH at 408C). It soon became clear that in terms of
activity towards substrate 8a, the Me-bipheso ligand is
superior to binaso and, as a consequence, an average of
only half of the catalyst loading is needed for full conversion
within short reaction times (Table 2, entries 1–14). In absolute
terms, only 0.25–0.50 mol% 2a (0.5–1 mol% Rh) is required,
a finding that puts this system above most of the catalytic
systems tested to date, which routinely need 3 mol% Rh for
good reactivity. Furthermore, degradation of the aryl boronic
acids does not occur during catalysis and allows the use of
stoichiometric amounts of the coupling partner (normally, 2–
5 equivalents have to be used). Albeit less efficiently, cyclic
substrates 8b, 8c, and 8d were also coupled successfully and
in high yield. Probably more impressive than the high activity
displayed by precatalyst 2a is the selectivity with which this
disulfoxide catalyst operates for cyclic unsaturated substrates.
Figure 1. Displacement ellipsoid views of (M,S,S)-p-Tol-Me-bipheso
(left, 1a) and [({(M,S,S)-p-Tol-Me-bipheso}RhCl)2] (right, 2a). Ellip-
soids set at 50% probability.
disulfoxide and diphosphine compounds are similar. The
sulfur–rhodium and phosphorus–rhodium bond lengths as
well as the intermetallic separations in the two dimers are
almost identical. Probably the most important difference
arises from the slightly shorter bonds between the sulfoxide
moieties and the backbone carbon skeleton. This structural
feature leads to a more open S-Rh-S bond angle in both the
Me-bipheso and the binaso rhodium complex,[4] while it
maintains the dihedral angle between the planes of the
atropisomeric backbone units at a very similar value to that of
the corresponding diphosphine systems (Supporting Informa-
tion, Table S1).[12,13] Finally, coordination of the sulfoxide
Table 2: Disulfoxide rhodium precatalyst 2a in the 1,4-addition of aryl
boronic acids to a,b-unsaturated substrates.
=
moiety to the metal leads to a shortening of the S O bond, a
phenomenon that gives a qualitative indication of the donor
abilities of sulfoxides.
To quantify to what extent sulfoxide ligands are able to
donate electron density to rhodium, we synthesized cationic
carbonyl complexes of general formula [(L-L)Rh(CO)2]+
(where L-L is one of the investigated bidentate sulfoxide or
analogous phosphine ligands) by treating [{Rh(CO)2Cl}2] with
the chelating ligand in the presence of AgBF4 (see the
Supporting Information). The surprising results of this study
(Table 1) show that the carbonyl stretching frequencies of the
Entry
8
9
mol% 2a t [h][c]
Yield [%][d]
ee[e] [%]
1
2
8a 9A
8a 9A
8a 9A
8a 9B
8a 9C
8a 9D
8a 9E
8a 9F
8a 9G
0.5
<0.5
0.5
0.5
1.5
0.5
3
98 (10aA)
98 (10aA)
98 (10aA)
91 (10aB)
88 (10aC)
95 (10aD)
88 (10aE)
>99 (R)
>99 (R)
>99 (S)
>99
99
0.25
0.25
0.5
0.5
0.5
3[f]
4
5
6
7
8
99
99
0.25
0.5
2
0.75 96 (10aF)
>99
>99
99
>99
>99
>99
99
98
98
97
97
Table 1: Comparison of the s-donor properties of disulfoxides and
diphosphines in [(L-L)Rh(CO)2]+ complexes.
9
0.5
<0.5
96 (10aG)
98 (10aH)
80 (10aI)
10
11
12
13
14
15
16
17
18
8a 9H 0.5
1.5
1.5
3
1.5
1
Rhodium complex
n(CO) [cmÀ1 [a]
]
8a 9I
8a 9J
8a 9K
0.25
0.25
0.5
98 (10aJ)
92 (10aK)
95 (10aM)
94 (10bA)
98 (10cA)
82 (10cF)
46 (cis-10dL)
48 (trans-10dL)
43 (10eK)
[{(M,S,S)-p-Tol-Me-bipheso}Rh(CO)2]BF4 (4a)
[{(P,S,S)-p-Tol-Me-bipheso}Rh(CO)2]BF4 (4b)
[{(M,R,R)-p-Tol-binaso}Rh(CO)2]BF4 (5)
[{(S)-biphemp}Rh(CO)2]BF4 (6)
2058
2057
2056
2071
2071
8a 9M 0.5
8b 9A
8c 9A
8c 9F
8d 9L
1
2
0.5
1
6
2
[{(rac)-binap}Rh(CO)2]BF4 (7)
1
1
[a] Thin film, solid IR spectroscopy; average value (ns +nas)/2.
95
20
19[g]
8e 9K
2.5
5.5
[a] 8a=2-cyclohexen-1-one, 8b=2-cyclopenten-1-one, 8c=5,6-dihydro-
2H-pyran-2-one, 8d=6-methyl-2-cyclohexen-1-one, 8e=trans-1,3-
complexes with sulfoxide ligands are lower than for the
corresponding phosphine compounds. Contrary to our
expectations, aryl disulfoxides p-Tol-Me-bipheso and p-Tol-
binaso are therefore clearly more electron-donating than
their aryl diphosphine counterparts biphemp and binap. To
our knowledge, this study represents the first measure of
s donation for sulfoxides and indicates to which degree this
ligand class has been neglected in the past. The results might
also shed some light onto recent studies by Milstein and co-
diphenyl-2-propenone. [b] Ar=Ph (9A), 4-CH3C6H4 (9B), 4-ClC6H4
(9C), 4-FC6H4 (9D), 4-CH3OC6H4 (9E), 3-CH3C6H4 (9F), 3-CF3C6H4
(9G), 3-ClC6H4 (9H), 3-FC6H4 (9I), 3-CH3OC6H4 (9J), 1-naphtyl (9K),
2-naphtyl (9L), 1-pyrene (9M). [c] Reaction is stopped after full
conversion or when no further conversion is observed as determined
by GC-MS. [d] Yield of isolated product after column chromatography.
[e] Determined by HPLC analysis with chiral columns (Daicel Chiralcel
OD-H, OJ-H, OB). [f] Using [({(P,R,R)-p-Tol-Me-bipheso}RhCl)2] (2d) as
catalyst. [g] 2.2 equiv 9K.
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À
The enantiomeric excesses reported in Table 2 are superior to
approach pathways of the olefinic substrate to the [Rh] Ph
any catalytic system we are aware of for the 1,4-addition to
substrate 8a. All of the aryl boronic acids tested can be
coupled with at least 99% ee, and in most cases where
selectivities exceed this value, the minor isomer cannot be
detected at all by HPLC and we can assume that only one
enantiomer is generated. Selectivities for the addition of aryl
boronic acids to 8b, 8c, and 8d are almost as high and among
the best reported to date.[6] Preliminary results concerning the
addition of 1-naphthylboronic acid (9K) to a linear a,b-
unsaturated ketone (trans-1,3-diphenyl-2-propenone, 8 f),
however, were disappointing (Table 2, entry 19). Both the
reactivity and the enantioselectivity drop dramatically under
the reaction conditions used.
species. The model for enantioselection in this and in the
overwhelming majority of metal-mediated asymmetric reac-
tions is based on the assumption that the substrates approach
the metal so as to minimize steric interactions with the
protruding R groups of the chiral ligand structure.[16] How-
ever, the present system is devoid of any significant steric
crowding around the metal center, with both p-tolyl groups on
the sulfoxide units oriented away from the metal center and
parallel to the atropisomeric backbone (see partial view of 2a
in Scheme 2).[17] As a viable working model, we therefore
With these data in hand, we turned our attention to
possible reactivity and selectivity pathways for precatalyst 2a,
especially in view of the fact that the catalytic cycle for the
1,4-addition reaction with binap rhodium has been studied in
detail by Hayashi and co-workers.[15] Important information
was gathered regarding the initial step leading to the active,
monomeric rhodium hydroxo catalyst species by comparing
the activities of disulfoxide and diphosphine rhodium dimers
with chloro and hydroxo bridges (Table 3). Several conclu-
sions can be drawn from the results obtained. First of all, the
diphosphine compounds are distinctly less active and selective
than the systems incorporating disulfoxides. Secondly, with
(S)-biphemp as a ligand, the transformation from the chloro-
bridged dimer 3 to the active species is clearly more difficult
Scheme 2. Partial view of complex 2a (left) and proposed origin of
enantioselectivity (right).
À
than formation of the monomeric [Rh] OH species by dimer
dissociation from 12. The inverse trend is observed with
disulfoxide ligand 1a, where the catalytic run performed using
2a is faster and more efficient than when starting with 11a.
Finally, selectivities with the hydroxo-bridged species are
somewhat lower for both ligand classes.
propose that selectivity arises from favorable or unfavorable
electronic interactions of the prochiral substrate molecules
with the oxygen atoms on the sulfoxide moieties. Accordingly,
the olefinic double bond of 2-cyclohexen-1-one (8a) coor-
dinates to rhodium, placing the carbonyl carbon atom in close
proximity to the sulfoxide oxygen atom, and migratory
insertion would then form the stereogenic carbon center
with absolute configuration (R), as observed. In contrast, an
unfavorable electronic situation arises when the enone
approaches the metal center from its opposite face, and the
(S) product is therefore not produced. Obviously, the model
we propose herein is speculative, and we are currently trying
to synthesize rhodium precursors in which both diastereomers
of a given atropisomeric backbone of ligand 1 can be
compared (1a vs. 1c and 1b vs. 1d).[18,19]
The stereochemical pathway of the Miyaura–Hayashi
reaction is well-documented and arises from the possible
Table 3: Reaction network of the rhodium-catalyzed 1,4-addition of
phenylboronic acid (7a) to 2-cyclohexen-1-one (9A) and catalytic results
obtained.
To conclude, p-Tol-Me-bipheso, a chelating disulfoxide
ligand based on the biphemp structure, shows unprecedented
selectivity in the 1,4-addition of aryl boronic acids to cyclic
a,b-unsaturated ketones and esters while allowing the use of
low catalyst loadings and stoichiometric amounts of expensive
boronic acid. As p-Tol-Me-bipheso represents only the
second chelating chiral sulfoxide ligand to be used success-
fully in asymmetric metal-mediated catalysis, the very fact
that it can outperform well-established ligand entities points
to the enormous potential of these new sulfur-based ligands.
Comparing disulfoxide ligands with their diphosphine
counterparts revealed important trends and differences.
Contrary to our expectation, disulfoxides are better s-
donating ligands than diphosphines for the present rhodium
Precatalyst (0.5 mol%)[a]
t [h]
Yield [%]
ee [%]
2a
<0.5
2
48
98[b,c]
91[b,c]
58[b,d]
96[b,c]
>99
98
90
[{(1a)RhOH}2] (11a)
[({(S)-biphemp}RhCl)2] (3)[e]
[({(S)-biphemp}RhOH)2] (12)
24
58
[a] Conditions employed are identical to Table 2, entry 1. [b] Yield of
isolated product after column chromatography. [c] Reaction is stopped
after full conversion. [d] Incomplete conversion. [e] A run at elevated
temperature (1008C, 1 h) gave 83% yield and 84% ee.
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Chemie
Kurihara, N. Sugushita, K. Oshita, D. Piao, Y. Yamamoto, N.
systems. Furthermore, a study of precatalyst activation
unveiled distinctly different reactivity patterns for the two
ligand classes, showing all the while that the Me-bipheso
ligand is clearly superior to biphemp. Probably most intrigu-
ing is the fact that an unusual electronic recognition between
the sulfoxide moieties and the prochiral substrate appears to
be responsible for the unparalleled selectivity of the present
catalytic system. This latter finding is now being thoroughly
investigated as part of our ongoing efforts in the area of chiral
sulfoxide-mediated catalytic studies.
Miyaura, J. Organomet. Chem. 2007, 692, 428; with chiral dienes,
see Refs. [2b,f–h] and: j) Y. Otomaru, K. Okamoto, R. Shintani,
T. Hayashi, J. Org. Chem. 2005, 70, 2503; k) A. Kina, K. Ueyama,
T. Hayashi, Org. Lett. 2005, 7, 5889; l) F.-X. Chen, A. Kina, T.
Hayashi, Org. Lett. 2006, 8, 341; m) T. Nishimura, M. Nagaosa, T.
Hayashi, Chem. Lett. 2008, 37, 860; n) T. Gendrineau, O. Chuzel,
H. Eijsberg, J.-P. Genet, S. Darses, Angew. Chem. 2008, 120,
7783; Angew. Chem. Int. Ed. 2008, 47, 7669; with chiral P-olefin
ligands, see: o) R. Shintani, W.-L. Duan, T. Nagano, A. Okada, T.
Hayashi, Angew. Chem. 2005, 117, 4687; Angew. Chem. Int. Ed.
2005, 44, 4611; p) E. Piras, F. Lꢁang, H. Rꢀegger, D. Stein, M.
Wꢂrle, H. Grꢀtzmacher, Chem. Eur. J. 2006, 12, 5849; q) P.
Kasak, V. B. Arion, M. Widhalm, Tetrahedron: Asymmetry 2006,
17, 3084; r) W.-L. Duan, H. Iwamura, R. Shintani, T. Hayashi, J.
Am. Chem. Soc. 2007, 129, 2130; s) R. Mariz, A. Briceno, R.
Dorta, R. Dorta, Organometallics 2008, 27, 6605.
Received: January 22, 2009
Published online: March 3, 2009
Keywords: 1,4-additions · asymmetric catalysis ·
.
[7] a) R. Schmid, M. Cereghetti, B. Heiser, P. Schꢂnholzer, H.-J.
Hansen, Helv. Chim. Acta 1988, 71, 897; b) Y. Liang, S. Gao, H.
Wan, J. Wang, H. Chen, Z. Zheng, X. Hu, Tetrahedron:
Asymmetry 2003, 14, 1267.
[8] For reasons we do not understand at present, generation of the
dilithiated species and reaction with menthyl-p-Tol-sulfinate
yields only trace amounts of the desired ligand.
homogeneous catalysis · rhodium · sulfoxide ligands
[1] For reviews, see: a) Catalytic Asymmetric Synthesis II (Ed.: I.
Ojima), Wiley-VCH, New York, 2000; b) Comprehensive Asym-
metric Catalysis, Vols. 1–3 (Eds.: E. N. Jacobsen, A. Pfaltz, H.
Yamamoto), Springer, Berlin, 1999.
[9] Intense heating is necessary to obtain the di-Grignard derivative.
[10] See, for example: T. Ohshima, H. Tadaoka, K. Hori, N. Sayo, K.
Mashima, Chem. Eur. J. 2008, 14, 2060.
[11] CCDC 717489 (1a), 717490 (2a), and 717491 (3) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[12] K. A. Bunten, D. H. Farrar, A. J. Poe, A. Lough, Organometal-
lics 2002, 21, 3344.
[13] Dihedral angles and related P-M-P bond angles of atropisomeric
diphosphines (normally calculated using molecular mechanics
programs) have been invoked as playing a vital part in the
enantioselection pathway, for recent examples, see: a) Z. Zhang,
H. Qian, J. Longmire, X. Zhang, J. Org. Chem. 2000, 65, 6223;
b) T. Saito, T. Yokozawa, T. Ishizaki, T. Moroi, N. Sayo, T. Miura,
H. Kumobayashi, Adv. Synth. Catal. 2001, 343, 264; c) S. Juelin,
S. Duprat de Paule, V. Ratovelomanana-Vidal, J.-P. Genet, N.
Champion, P. Dellis, Angew. Chem. 2004, 116, 324; Angew.
Chem. Int. Ed. 2004, 43, 320, and references therein.
[2] For an excellent recent review on the use of chiral olefin-
containing ligands, see: a) C. Defieber, H. Grꢀtzmacher, E. M.
Carreira, Angew. Chem. 2008, 120, 4558; Angew. Chem. Int. Ed.
2008, 47, 4482; for selected references, see: b) T. Hayashi, K.
Ueyama, N. Tokunaga, K. Yoshida, J. Am. Chem. Soc. 2003, 125,
11508; c) N. Tokunaga, Y. Otomaru, K. Okamoto, K. Ueyama,
R. Shintani, T. Hayashi, J. Am. Chem. Soc. 2004, 126, 13584;
d) R. Shintani, K. Okamoto, Y. Otomaru, K. Ueyama, T.
Hayashi, J. Am. Chem. Soc. 2005, 127, 54; e) C. Fischer, C.
Defieber, T. Suzuki, E. M. Carreira, J. Am. Chem. Soc. 2004, 126,
1628; f) C. Defieber, J.-F. Paquin, S. Serna, E. M. Carreira, Org.
Lett. 2004, 6, 3873; g) J.-F. Paquin, C. R. J. Stephenson, C.
Defieber, E. M. Carreira, Org. Lett. 2005, 7, 3821; h) J.-F. Paquin,
C. Defieber, C. R. J. Stephenson, E. M. Carreira, J. Am. Chem.
Soc. 2005, 127, 10850.
[3] For recent reviews on sulfide and sulfoxide ligands in catalysis,
see: a) J. C. Bayon, C. Claver, A. M. Masdeu-Bulto, Coord.
Chem. Rev. 1999, 193–195, 73; b) A. M. Masdeu-Bulto, M.
Dieguez, E. Martin, M. Gomez, Coord. Chem. Rev. 2003, 242,
159; c) M. Mellah, A. Voituriez, E. Schulz, Chem. Rev. 2007, 107,
5133.
[14] T. Schaub, Y. Diskin-Posner, U. Radius, D. Milstein, Inorg.
Chem. 2008, 47, 6502, and references therein.
[15] a) T. Hayashi, M. Takahashi, Y. Takaya, M. Ogasawara, J. Am.
Chem. Soc. 2002, 124, 5052; b) A. Kina, H. Iwamura, T. Hayashi,
J. Am. Chem. Soc. 2006, 128, 3904.
[16] Sometimes, such as in the present catalytic transformation, only
one bulky substituent is enough for efficient chiral recognition,
see Refs. [6n] and [6s].
[4] R. Mariz, X. Luan, M. Gatti, A. Linden, R. Dorta, J. Am. Chem.
Soc. 2008, 130, 2172.
[5] For the first two reports, see: a) M. Sakai, H. Hayashi, N.
Miyaura, Organometallics 1997, 16, 4229; b) Y. Takaya, M.
Ogasawara, T. Hayashi, M. Sakai, N. Miyaura, J. Am. Chem. Soc.
1998, 120, 5579; reviews on rhodium-catalyzed asymmetric 1,4-
addition: c) T. Hayashi, K. Yamasaki, Chem. Rev. 2003, 103,
2829; d) T. Hayashi, Pure Appl. Chem. 2004, 76, 465.
[6] For selected examples of asymmetric 1,4-addition with binap and
its derivatives, see Ref. [5b] and: a) S. Sakuma, M. Sakai, R.
Itooka, N. Miyaura, J. Org. Chem. 2000, 65, 5951; b) R.
Amengual, V. Michelet, J.-P. Genet, Synlett 2002, 11, 1791;
c) M. Pucheault, S. Darses, J.-P. Genet, Eur. J. Org. Chem. 2002,
3552; d) R. Itooka, Y. Iguchi, N. Miyaura, J. Org. Chem. 2003, 68,
6000; with phosphoramidites and phosphonites, see: e) M. T.
Reetz, D. Moulin, A. Gosberg, Org. Lett. 2001, 3, 4083; f) J.-G.
Boiteau, R. Imbos, A. J. Minaard, B. L. Feringa, Org. Lett. 2003,
5, 681; g) J.-G. Boiteau, A. J. Minaard, B. L. Feringa, J. Org.
Chem. 2003, 68, 9481; h) S. L. X. Martina, A. J. Minaard, B.
Hessen, B. L. Feringa, Tetrahedron Lett. 2005, 46, 7159; i) K.
=
[17] In biphemp (or binap) the S O bond is exchanged for a P-phenyl
group. It is this very group that is invoked as being responsible
for enantioselection in these atropisomeric systems. For recent
reviews on binap and its derivatives, see: a) W. J. Tang, X. Zhang,
Chem. Rev. 2003, 103, 3029; b) M. Berthod, G. Mignani, G.
Woodward, M. Lemaire, Chem. Rev. 2005, 105, 1801; c) H.
Shimizu, I. Nagasaki, T. Saito, Tetrahedron 2005, 61, 5405.
[18] Cationic complexes of the formula [Rh(L-L)(cod)]+ (cod = 1,5-
cyclooctadiene) have been obtained with all four ligands (1a–
1d), but unfortunately, catalytic results with these and the
carbonyl-containing complexes 4a/4b of Table 1 were poor.
[19] It is possible that the lack of reactivity and selectivity for the
open-chain enone 8e is linked to the mode of operation of
catalyst 2a. More detailed studies are underway.
Angew. Chem. Int. Ed. 2009, 48, 2768 –2771
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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