Enantiocontrol in the intermolecular cyclopropanation reaction catalyzed by dirhodium(II) complexes with ortho-metalated aryl phosphine ligands

Article abstract of DOI:10.1039/b010145c

(P) and (M) dirhodium(II) complexes with ortho-metalated aryl phosphines are assessed as chiral catalysts in the enantioselective cyclopropanation of styrenes by ethyl diazoacetate; enantioselectivities up to 91% and up to 87%, respectively, for cis- and

Full text of DOI:10.1039/b010145c

Enantiocontrol in the intermolecular cyclopropanation reaction catalyzed by
dirhodium(II) complexes with ortho-metalated aryl phosphine ligands
a
b
a
b
Mario Barberis, Pascual Lahuerta,* Julia Pérez-Prieto* and Mercedes Sanaú
a
Departamento de Química Orgánica/Instituto de Ciencia Molecular, Facultad de Farmacia Universidad de
Valencia, Vicént Andrés Estellés s/n, Burjassot, Valencia, 46100 Spain. E-mail: julia.perez@uv.es;
Fax: (34)963864939
Departamento de Química Inorgánica, Facultad de Químicas, Universidad de Valencia, Dr. Moliner 50, 46100
Burjassot,Valencia, Spain. E-mail: pascual.lahuerta@uv.es; Fax: (34)963864322
b
Received (in Liverpool, UK) 8th December 2000, Accepted 25th January 2001
First published as an Advance Article on the web 14th February 2001
(
P) and (M) dirhodium(II) complexes with ortho-metalated
Use of catalysts 4–8 for cyclopropanation of styrene with
ethyl diazoacetate gave the results reported in Table 1 (Fig. 1).
Catalysts 4–7 showed low diastereoselectivity in the obtention
of both diastereoisomers, 2a and 3a. However, all of them
induced high asymmetry in the cyclopropanation of styrene;
those catalysts with a more basic phosphine (catalysts 4–6
compared to 7) led to the best ee values. The influence of the
olefin on enantioselectivity was studied using styrenes of
varying nucleophilicity (Table 2). Product yields were higher
with the more nucleophilic olefins 1c and 1d. However, no
remarkable differences on diastero- and enantioselectivity were
found for catalysts 4–7. These results confirm that degradation
to an achiral rhodium catalyst is not a major competing
pathway.
aryl phosphines are assessed as chiral catalysts in the
enantioselective cyclopropanation of styrenes by ethyl di-
azoacetate; enantioselectivities up to 91% and up to 87%,
respectively, for cis- and trans-2-arylcyclopropanecarboxy-
lates are observed.
A great deal of effort is presently being devoted to the
development of new chiral catalysts to induce enantiocontrol in
carbene transfer reactions.1 The catalytic reaction of ethyl
diazoacetate with styrene is a model reaction with which stereo-
and enantioselectivity for intermolecular cyclopropanation is
measured and catalytic effectiveness is determined. Chiral
copper catalysts,4 especially those with bis-oxazoline ligands
–3
–8
9
and ruthenium catalysts, have been found to induce the highest
levels of enantiocontrol. Thus, enantioselectivities up to 99%
for ethyl 2-phenylcyclopropanecarboxylates have been ob-
tained in the cyclopropanation of styrene with ethyl diazoace-
tate catalyzed by Cu catalysts. In general, chiral dirhodium(II
)
catalysts do not provide high enantioselectivities; there is a
remarkable exception in chiral azetidine-4-carboxylate-ligated
dirhodium(II) catalysts.10 They produce enantioselectivities up
to 76 and 52% ee, respectively, for 2a and 3a (Scheme 1).†
Dirhodium(II catalysts
)
of
general
formula
Rh (O CR) (PC) , containing two ortho-metalated aryl phos-
2
2
2
2
11
phines (PC) in a head to tail arrangement, (Fig. 1), have
backbone chirality and they can be isolated as pure enantiomers
by conventional resolution methods.12 Until now, all
approaches to the design of enantiomerically pure Rh(II
)
catalysts have depended on the attachment of enantiomerically
pure ligands to the dirhodium core.
The racemic Rh
in intramolecular processes.
2
(O
2
CR)
2
(PC)
2
compounds have been used
Those with high electron-
1
3,14
3 2
withdrawing carboxylate groups (CF CO ) have shown the best
selectivity values.
We now report, from results obtained with a series of Rh(II
)
Fig. 1 List of Rh(II) catalysts with ortho-metalated aryl phosphine
ligands.
catalysts, Rh (O CCF (PC) , that they induce low dia-
2
2
3
)
2
2
stereoselectivity in the intramolecular cyclopropanation of
styrenes 1a–1d (Scheme 1), but both cyclopropane diastereoi-
somers are obtained with high optical purity.
Table 1 Asymmetric cyclopropanation of styrene catalyzed by the (M)-
enantiomer
%
ee
Configuration
Rh
Yield %a 2a/3ab
2ac
3ac
2ad
3ad
4
5
6
7
8
a
55
40
36
71
94
48+52
61+39
51+49
47+53
43+57
91
87
88
74
39
87
75
81
74
6
1S, 2R
1S, 2R
1S, 2R
1S, 2R
1R, 2S
1S, 2S
1S, 2S
1S, 2S
1S, 2S
1R, 2R
Cyclopropanation yield based on ethyl diazoacetate. b Determined by GC
c
analysis. Ee values were based on GC analysis with a 2,3-di-O-acetyl-6-O-
tert-butyldimethylsilyl-beta-CDX column. Configuration was determined
d
by correlation of the sign of the rotation of polarized light with that of the
known enantiomer (ref. 15).
Scheme 1 Catalyzed cyclopropanation of styrenes.
DOI: 10.1039/b010145c
Chem. Commun., 2001, 439–440
This journal is © The Royal Society of Chemistry 2001
439

Table 2 Influence of the olefin on enantioselectivity in the cyclization
catalyzed by (M)-4
%
ee
Olefin
Yield %
2/3
2
3
1
1
1
1
a
b
c
55
62
88
90
48+52
49+51
51+49
51+49
91
86
—
84
87
—
85
—
a
a
a
d
a
Not determined.
We suggest that the olefin approaches to the carbenoid
through its less substituted carbon and also that the carbene
transfer to olefin occurs through an early transition state. The
2
interaction between the carbenoid ester group (CO Et) and the
olefinic substituents (R, Ar) would be weak in such a case.
These two mechanistic details, altogether, could explain the low
influence of the olefin on diastereoselectivity observed in our
experiments.
A tentative rationale for the observed high ee values for both
stereoisomers, and for the sense of asymmetric induction in the
cyclopropanation of styrenes with Rh(II) catalysts 4–7 is based
on the model depicted in Fig. 2 for the M-catalyst. The non-
metalated aryl substituents of the phosphorus atom protrude into
the region where carbene transfer takes place, thereby limiting
the possible orientations of the coordinated carbene and
favoring those orientations placing the ester group at the less
Fig.
Rh [(C
Rh(1)–Rh(1A)
2.164(3), Rh(1)–P(2)
angles (°) are C(41)–Rh(1)–O(2)
3
Molecular
[(C
2.5587(7), Rh(1)–O(2)
2.280(13), Rh(1)–N(2)
view
)P] [(M)-8]. Selected distances (Å) are
2.151(3), Rh(1)–O(1) =
of
the
complex
2
H
6 5
)
3
CCO
]
2 2
6
H
) (C H
5 2 6 4
2
=
=
=
=
2.302(4). Selected
=
88.08(15), O(2)–Rh(1)–O(1)
=
=
8
5.65(12), O(2)–Rh(1)–P(2)
=
171.46(9), N(2)–Rh(1)–Rh(1A)
1
62.02.
[
6 5 3 2
(C H ) CCO ] (Fig. 3). In this case two relevant observations
are made: the enantioselectivity was much lower (almost zero
for one of the diastereoisomers) and there was a reversal of
induction. These two facts may be attributed to increased steric
interactions in the area of the carboxylate ligands, due to the
sterically demanding quadrants (orientation A
). The cyclopropanation step, A ? A appears to be more
favored than B , since in the latter, a repulsive interaction
1 1
and B in Fig.
2
1
2
1
? B
2
higher steric requirements of the [(C
to (CF CO ) in 4–7, that make a carbenoid intermediate A
favored than B (Fig. 2). Additional studies oriented to confirm
6
H
5
)
3
CCO
2
] in 8 compared
builds up between the ester group and the metalated aryl group
at some level of the proccess.
3
2
1
less
1
Additional evidence supporting such a model was obtained
from the crystal structure determination of the enantiomerically
pure catalyst (M)-8 having bulky carboxylate ligands
the reliability of the model depicted in Fig. 2 are in progress.
In summary, results indicate a high chiral recognition in the
intermolecular cyclopropanation of styrenes showed by these
dirhodium(II) compounds. The ee values clearly exceed those
previously observed with other Rh(II) catalysts for this partic-
ular intermolecular reaction.
We thank the Dirección General de Investigación Científica
y Técnica (DGICYT) (Project PB98-1437) and the EC (Project
TMR Network ERBFMRXCT 60091).
Notes and references
†
CCDC 155434. See http://www.rsc.org/suppdata/cc/b0/b010145c/ for
crystallographic files in .cif format.
1
2
3
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440
Chem. Commun., 2001, 439–440