A chiral bis-sulfoxide ligand in late-transition metal catalysis; rhodium-catalyzed asymmetric addition of arylboronic acids to electron-deficient olefins

Article abstract of DOI:10.1021/ja710665q

A bis-sulfoxide with a binaphthyl backbone is introduced as a readily available, chiral ligand entity in late-transition metal catalysis. Ligand p-tol-BINASO [where p-tol-BINASO is 1,1′-binaphthalene-2,2′-diyl-bis-(p-tolylsulfoxide)] is obtained in pure f

Full text of DOI:10.1021/ja710665q

Published on Web 01/26/2008
A Chiral Bis-Sulfoxide Ligand in Late-Transition Metal Catalysis;
Rhodium-Catalyzed Asymmetric Addition of Arylboronic Acids to
Electron-Deficient Olefins
Ronaldo Mariz, Xinjun Luan, Michele Gatti, Anthony Linden, and Reto Dorta*
Institute of Organic Chemistry, UniVersity of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland
Received November 28, 2007; E-mail: dorta@oci.uzh.ch
Asymmetric metal catalysis has seen tremendous developments
in the last decades.1 The overwhelming majority of successful chiral
chelate ligands in late-transition metal catalysis are based on
phosphorus and/or nitrogen. Recently, Hayashi and Carreira have
developed alternative chiral diene ligands for asymmetric rhodium
and iridium catalysis.2,3 Synthesis of these new dienes as well as
the more traditional P/N ligands requires multistep synthetic
procedures with often tedious separation of the enantiomers. A
potentially very readily available chiral ligand class with a well-
known coordination chemistry is represented by sulfoxides.4
Considering that these compounds play an important role as chiral
auxiliaries in asymmetric synthesis,5 it is surprising that few
examples exist in which this ligand class participates in homoge-
neous metal catalysis.6-8 The use of rhodium bis-sulfoxide com-
pounds in catalysis is unknown,9 and successful applications of bis-
sulfoxide ligands in asymmetric transformations have not been
reported.7,8
Figure 1. ORTEP drawings (50% probability) of (M,R,R)-p-tol-BINASO
(left, 1) and [{(P,R,R)-p-tol-BINASO}RhCl]2 (right, 2).
Scheme 1. Synthesis of p-tol-BINASO Ligands
We recently decided to investigate chiral bis-sulfoxides as ligands
in LTM catalysis. In this report, we describe our results on the
preparation of a chiral bis-sulfoxide rhodium(I) complex and its
use as a precatalyst in the asymmetric 1,4-addition of arylboronic
acids to cyclic R,â-unsaturated ketones and esters.10
units remains very similar (74.1° BINASO; 76.0° BINAP). A
comparison of the Rh‚‚‚Rh distances in 2, in [{(R)-BINAP}RhCl]2,
and in Hayashi’s similarly bulky diene complex [{(S,S)-Ph-bod*}-
RhCl]2,16 indicates that the ligating properties of bis-sulfoxides
might lie somewhere between diene and bis-arylphosphine ligand
systems.17
The bis-sulfoxide ligand used in our first catalytic application is
a 1,1′-binaphthyl derivative similar to the well-known BINAP (1,1′-
binaphthalene-2,2′-diyl-bis-diphenylphosphine) ligand developed by
Noyori et al.11 Compared to BINAP, synthesis of the bis-sulfoxide
analogue is extremely straightforward and can be done in one single
step from commercially available starting materials according to
Scheme 1.12 Following the nomenclature for BINAP and its
derivatives, we name this ligand p-tol-BINASO (1,1′-binaphthalene-
2,2′-diyl-bis-(p-tolylsulfoxide, 1). The two atropisomers of the
diastereoisomeric BINASO compounds were easily separated via
column chromatography, independently from whether S- or R-
sulfinates were employed, giving four pure ligands (Scheme 1) in
good overall yield (>70% based on rac-DBBN).13
(P,R,R)-p-tol-BINASO [(P,R,R)-1] (2 equiv) ligand reacts readily
with [RhCl(C2H4)2]2 in a methylene chloride solution at room
temperature.14 Subsequent filtration, concentration and layering with
THF followed by crystallization at -35 °C afforded burgundy
crystals of [{(P,R,R)-p-tol-BINASO}RhCl]2 (2) in high yield
(>90%).
Figure 1 shows the ORTEP drawings of both (M,R,R)-p-tol-
BINASO (left, 1) and [{(P,R,R)-p-tol-BINASO}RhCl]2 (right, 2).
Coordination of p-tol-BINASO to the metal leads to the expected
sulfur-oxygen bond contraction [S-O in 1, 1.4922(16) Å; S-O
in 2, 1.466(4) and 1.473(4) Å], indicating efficient σ-donation of
the sulfoxide moiety. Comparing the solid-state structure of 2 with
its phosphine analogue [{(R)-BINAP}RhCl]2,15 reveals a significant
increase in bite angle for BINASO (98.1°) over BINAP (90.5°),
whereas the dihedral angle between the planes of the two naphthyl
With catalyst precursor 2 at hand, we evaluated its performance
in the 1,4-addition of phenylboronic acid to 2-cyclohexenone.
Experiments carried out in a mixture of dioxane/water/KOH
following Hayashi’s procedure gave excellent selectivities, but low
overall yields. Gratifyingly, substituting dioxane with toluene led
to complete conversion to the product within 30 min at room
temperature while maintaining high ee values (see Supporting
Information for details). Subsequent studies showed that catalyst
loadings could be diminished to 1.5 mol % Rh without significant
loss of reactivity (In general, 3 mol % Rh have to be used with
phosphine or diene ligands). More importantly, our catalytic system
does not require excess of expensive boronic acid (normally 2-5
equiv have to be used) and can be run conveniently with
stoichiometric (1.1 equiv) amounts. This is all the more surprising
since throughout our catalytic studies, commercially available
starting materials were used without purification.
Table 1 shows that the catalytic activity of 2 is excellent with
complete conversion normally achieved within a couple of hours
at 40 °C. The enantioselectivities observed here are among the
highest for the rhodium-catalyzed asymmetric 1,4-addition, the
selectivity being 90% ee and higher in all but one of the reactions
examined (entry 18). Virtually complete selectivity was obtained
with a number of substrates. Using [{(M,S,S)-p-tol-BINASO}RhCl]2
(2) gave the opposite enantiomer with equally high selectivity (entry
2). An interesting possibility arises with chiral, racemic 6-methyl-
2-cyclohexen-1-one (3e). Addition of 2-naphthylboronic acid (4y)
9
2172
J. AM. CHEM. SOC. 2008, 130, 2172-2173
10.1021/ja710665q CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalytic Results with [{(P,R,R)-p-tol-BINASO}RhCl]2 (2)
come limitations associated with phosphines. Studies pertinent to
the above transformations are underway.
Acknowledgment. We dedicate this work to Prof. David
Milstein on the occasion of his 60th birthday. R.D. is the recipient
of an Alfred Werner Assistant Professorship and thanks the
foundation for generous financial support. R.M. and R.D. thank
the SNF and the University of Zurich (OCI) for support.
entry
3
4
time (h)
yieldc (%) of 5
% eed
1
2e
3
4
5
6
7
8
9
10
11f
12
13f
14
15
16
17
18g
19
20g
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
4l
4l
1
1
1
99 (5al)
99 (5al)
98 (S)
98 (R)
96
98
98
99
99
97
99
97
97
90
99
90
96
95
96
Supporting Information Available: Experimental procedures and
CIFs for 1 and 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
4y
4z
4l
97 (5am)
86 (5an)
90 (5ao)
92 (5ap)
93 (5aq)
94 (5ar)
93 (5as)
91 (5at)
55 (5au)
98 (5av)
60 (5aw)
99 (5ax)
89 (5ay)
90 (5az)
99 (5bl)
98 (5cl)
1.5
1.5
1
1.5
1
1.5
1
1
2
1
1
1
1
0.5
3
1
1
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(13) The synthesis of (M,S,S)-1 and (P,S,S)-1 has been reported: Clayden, J.;
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a 3a ) 2-cylcohexen-1-one, 3b ) 2-cyclopenten-1-one, 3c ) 2-cyclo-
hepten-1-one, 3d ) 5,6-dihydro-2H-pyran-2-one, 3e ) 6-methyl-2-cyclo-
hexen-1-one. b Ar ) Ph (4l), 4-CH3C6H4 (4m), 4-ClC6H4 (4n), 4-FC6H4
(4o), 4-CH3OC6H4 (4p), 3-CH3C6H4 (4q), 3-CF3C6H4 (4r), 3-ClC6H4 (4s),
3- FC6H4 (4t), 3-CH3OC6H4 (4u), 2-CH3C6H4 (4v), 2-FC6H4 (4w),
1-naphthyl (4x), 2-naphthyl (4y), 1-pyrene (4z). c Isolated yields. d Deter-
mined by HPLC analysis with chiral columns (Daicel Chiralcel OD, OD-
H, OJ-H, OB-H or Chiralpak IA). e Using [{(M,S,S)-p-tol-BINASO}RhCl]2
(2) as catalyst. f Using 2 equiv of the boronic acid gives better yields (70-
80%). g Reaction run using 3 mol % of catalyst and 2 equiv of boronic
acid.
to 3e gave cis and trans diastereomers in equal amounts and equal
selectivities after separation through silica gel chromatography
(entry 20), a result that is in contrast to the copper-catalyzed 1,4-
addition of alkyl-zinc reagents to 3e.18 More intriguingly, epimer-
ization of the methyl group in cis-5ey under thermodynamic control
(NaOMe/MeOH or HCl/MeOH) to give the trans diastereomer does
not occur (see Supporting Information).
Finally, we should note that derivatives of [{p-tol-BINASO}-
RhCl]2 (2), namely (p-tol-BINASO)Rh(acac) as well as the cationic,
coordinatively saturated η6-tolylsulfoxide bound dimer [{p-tol-
BINASO}Rh]2(PF6)2, are equally effective catalysts for the present
transformation.
In conclusion, we have shown that chiral bis-sulfoxides can be
used successfully as ligands in asymmetric late-transition metal
catalysis. Precatalyst [{(P,R,R)-p-tol-BINASO}RhCl]2 (2) shows
high reactivities and excellent selectivities in the 1,4-addition of
arylboronic acids to cyclic, electron-poor double bonds and we are
currently expanding the scope of our catalyst system to other
substrates. In the present protocol, catalyst loadings can be kept
low and excess boronic acid is not required for efficient catalysis.
The key advantage of p-tol-BINASO over known chiral ligands
for this transformation (and for asymmetric LTM catalysis in
general) lies in its extraordinarily easy synthesis. In addition, p-tol-
BINASO and other bis-sulfoxides should be more versatile than
diene ligands and be able to support catalysis involving oxidative
addition processes (H2, HSiR3, etc.) or carbon monoxide. Further-
more, examples of achiral oxidation catalysis with sulfoxide-
palladium compounds6c-f show that this ligand class might over-
(14) Its enantiomer (M,S,S)-1 also reacts readily with the Rh precursor, whereas
(M,R,R)-1 and (P,S,S)-1 do not react cleanly to give the corresponding
complexes. In this case, the relative orientation of the tolyl groups on the
sulfoxides hinder formation of the dimer.
(15) Bunten, K. A.; Farrar, D. H.; Poe, A. J.; Lough, A. Organometallics 2002,
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43, 7887. (b) Whether epimerization of the methyl group occurs during
catalysis under the reaction conditions used is not clear at this stage.
JA710665Q
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J. AM. CHEM. SOC. VOL. 130, NO. 7, 2008 2173