Rhodium (II) compounds with functionalized metalated phosphines as bridging ligands

Article abstract of DOI:10.1016/j.jorganchem.2004.10.062

The reaction of Rh₂(O₂CCH₃)₄ ? 2CH₃OH with the phosphine P(4-BrC₆H ₄)₂(C₆H₅), 2, results in the formation of the monometalated compound Rh_2<

Full text of DOI:10.1016/j.jorganchem.2004.10.062

Journal of Organometallic Chemistry 690 (2005) 4424–4432
www.elsevier.com/locate/jorganchem
Rhodium (II) compounds with functionalized metalated
phosphines as bridging ligands
´
F. Estevan, P. Lahuerta, J. Lloret, D. Penno, M. Sanau, M.A. Ubeda
*
´
´ ´ ´
Departamento de Quımica Inorganica, Facultad de Quımica, Universitat de Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
Received 28 July 2004; received in revised form 4 October 2004; accepted 4 October 2004
Available online 13 February 2005
Abstract
The reaction of Rh2(O2CCH3)4 Æ 2CH3OH with the phosphine P(4-BrC6H4)2(C6H5), 2, results in the formation of the monometa-
lated compound Rh2(O2CCH3)3[PC] Æ 2CH3CO2H (PC representing a metalated P(4-BrC6H4)2(C6H5)). The reaction involves
selective metalation of the phosphine at one Br-substituted ring (12:1 isomer ratio). The reaction of Rh2(O2CCH3)3[(4-BrC6H3)-
P(4-BrC6H4)(C6H5)] Æ 2CH3CO2H, 4, with one additional mol of triphenylphosphine yields a mixture of two main stereoisomers
Rh2(O2CCH3)2[(4-BrC6H3)P(4-BrC6H4)(C6H5)] [(C6H4)P(C6H5)2] Æ 2CH3CO2H, 5a and 5b, that were isolated as pure compounds.
These two compounds were resolved in the corresponding M and P enantiomers as trifluoroacetate derivatives that show good
enantioselectivities in catalytic transformation of a-diazocarbonyl compounds.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Rhodium; P ligands; Chirality; Catalysis; Enantioselectivity
1. Introduction
New chiral orthometalated rhodium (II) catalysts
have been synthesized using chiral phosphine ligands
and their catalytic behaviour has been studied [14].
The good catalytic behaviour of these dirhodium (II)
compounds prompted us to study the supporting of
these homogenous catalysts and the catalytic behaviour
under heterogeneous conditions. The first approach
tested involved the linkage of the dirhodium units via
carboxylate groups introduced in a mesoporous mate-
rial. The catalytic selectivities of the resulting com-
pounds were considerably lower than those observed
for the analogous homogeneous catalysts and important
leaching was also observed [15]. This result is consistent
with the facility of exchange of the carboxylate groups
with free carboxylic acid shown by these metalated com-
pounds [16].
Aryl phosphines undergo orthometalation in a pro-
cess that is well documented [1,2]. Compounds of the
general formula Rh2(O2CR)2[PC]2, representing dirho-
dium (II) compounds with two metalated aryl phos-
phines, [PC], with head to tail configuration are
accessible by direct thermal reaction of the correspond-
ing phosphine with dirhodium tetracarboxylate [3–7].
They are a family of dirhodium (II) compounds and
show good activity and selectivity in the catalytic trans-
formation of a-diazocarbonyl compounds. The inherent
backbone chirality of these compounds has been ex-
plored for enantioselective reactions [8–12] since the
racemic mixture can be separated by standard resolution
methods in the M and P enantiomers [11,13].
A similar exchange between the metalated and free
phosphine has never been observed. Consequently, we
consider, as an alternative strategy, the linkage of the
*
Corresponding author. Fax: +34 963544322.
E-mail address: angeles.ubeda@uv.es (M.A. Ubeda).
´
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.062

F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
4425
catalyst to the inert material via one phenyl group of the
phosphine.
combination of chromatography and crystallization,
the major stereoisomer was isolated in low yield, as an
analytically pure compound, 3. All the efforts to obtain
single crystals of this compound in order to perform the
full structural characterization were unsuccessful, but
according to further studies with the phosphine P(4-
BrC6H4)2(C6H5), included in this paper, we assume that
compound 3 contains the phosphine metalated through
the 4-Br phenyl group, Rh2(O2CCH3)3[(4-BrC6H3)-
P(C6H5)2][(4-CH3C6H3)P(4-CH3C6H4)2] Æ 2CH3CO2H.
Similar results were obtained in the reaction under
photochemical conditions of the monometalated com-
pound Rh2(O2CCH3)3[(C6H4)P(C6H5)2] Æ 2CH3CO2H
with 1.
Until now, we have studied to some extent the reac-
tion of dirhodium (II) carboxylates with several substi-
tuted aryl phosphines. However, most of the
phosphines used, contained groups (CH3, F, Cl, CF3,
OCH3) that are not reactive enough to allow further cat-
alyst linkage [17,18]. The functional groups attached to
the phosphine ligand should not be affected by the acid
medium during the metalation process. Considering that
the best catalytic selectivities have been obtained with
catalysts containing triphenylphosphine or tris p-tolyl-
phosphine, functional groups with strong electron-with-
drawing substituents should be avoided.
Our first choice was the p-bromoarylphosphines. P(4-
BrC6H4)(C6H5)2, 1 and P(4-BrC6H4)2(C6H5), 2. We re-
port here some preliminary studies on the reactivity of
rhodium (II) acetate and 1 and 2. Two new isomeric
compounds of the formula Rh2(O2CCH3)2[(4-BrC6H3)-
P(4-BrC6H4)(C6H5)][(C6H4)P(C6H5)2] Æ 2CH3CO2H, 5a
and 5b, have been isolated and characterized by X-ray
crystallography. The trifluoroacetate derivative com-
pounds, isolated as pure M and P enantiomers, have
been studied in catalytic processes, inter- and intramo-
lecular cyclopropanation reactions.
The poor selectivity observed in the metalation reac-
tions with the phosphine 1, prompted us to use the phos-
phine P(4-BrC6H4)2(C6H5), 2, as an alternative
functionalized phosphine. By refluxing for 4 h, rhodium
(II) acetate and 2 (1:1.1 molar ratio) in a mixture of tol-
uene and acetic acid (3:1), the monometalated com-
pound Rh2(O2CCH3)3[PC] Æ 2CH3CO2H was formed in
high yield (>80%). The 31P NMR spectrum of the puri-
fied compound indicated that the metalation reaction
was highly regioselective as the two possible isomers
were formed in the 12:1 ratio.
As phosphine 1, P(4-BrC6H4)2(C6H5) can undergo
orthometalation by activation of C–H bond in the phe-
nyl or 4-Br substituted aryl group. A center of chirality
is created at the P atom when one of the 4-Br substituted
rings is activated.
2. Results and discussion
We explored first the regioselectivity of the metala-
tion reaction using the phosphine 1 that can be activated
at the ortho C–H bond of the substituted phenyl group
or at one of the C–H bonds of the phenyl groups. In the
last case the P atom becomes a center of chirality.
The reaction of rhodium (II) tetraacetate and 1, un-
der thermal or photochemical conditions, is not selective
and gave the two mono metalated compounds, Rh2-
(O2CCH3)3[(4-BrC6H3)P(C6H5)2] and Rh2(O2CCH3)3-
[(C6H4)P(4-BrC6H4)(C6H5)] in approximately 1:1 ratio.
The 31P NMR spectrum of the reaction mixture shows
Detailed NMR study let us identify the major prod-
uct as Rh2(O2CCH3)3[(4-BrC6H3)P(4-BrC6H4)(C6H5)] Æ
2CH3CO2H, 4, the minor compound is assigned to
Rh2(O2CCH3)3[(C6H4)P(4-BrC6H4)2] Æ 2CH3CO2H.
The high regioselectivity observed in the metalation
of P(4-BrC6H4)2(C6H5) prompted us to prepare bis
orthometalated compounds with a mixed set of phos-
phines, Rh2(O2CCH3)2[[(4-BrC6H3)P(4-BrC6H4)(C6H5)]-
[(C6H4)P(C6H5)2] Æ 2CH3CO2H, 5. When a solution of
4 and P(C6H5)3 (1:1.1 molar ratio) in a mixture of tolu-
ene and acetic acid (3:1 volume ratio) was irradiated for
24 h, two isomeric compounds, 5a and 5b, were formed
(Fig. 1). By a combination of chromatography and crys-
tallization techniques they were separated in 33% and
37% yield, respectively, and were structurally character-
ized by X-ray methods. A third possible isomer,
Rh(O2CCH3)2[(C6H4)P(4-BrC6H4)2][(C6H4)P(C6H5)2] Æ
2CH3CO2H, was detected in solution in minor amount
but was eliminated during the purification process.
The 31P NMR data for 5a and 5b are very similar and
show the presence of two different phosphorus environ-
ments centred at around 19.0 and 20.5 ppm.
two doublets of doublets centred at 18.5 (1JRh–P
=
2
152 Hz, JRh–P = 7 Hz) and 18.9 ppm (1JRh–P = 151 Hz,
2JRh–P = 6 Hz). All the efforts to separate the pure com-
pounds by chromatography and crystallization tech-
niques were unsuccessful. A detailed NMR study
allowed us to assign the signals centred at 18.9 ppm to
Rh2(O2CCH3)3[(4-BrC6H3)P(C6H5)2] compound.
We also studied whether the phosphine 1 can undergo
more selective metalation when it reacts with a mono-
metalated compound. Rh2(O2CCH3)3[(4-CH3C6H3)-
P(4-CH3C6H4)2] Æ 2CH3CO2H was reacted with 1 under
thermal or photochemical conditions, and in both con-
ditions, the reaction yielded several compounds. The
photochemical reaction was relatively cleaner and
according to the 31P NMR of the crude of the reaction
three compounds were formed in a 4:1:1 ratio. By a
As an alternative synthetic route for compounds 5a
and 5b, we reacted Rh2(O2CCH3)3[(C6H4)P(C6H5)2] Æ
2CH3CO2H with equimolar amount of phosphine 2. The
same three compounds were formed but the reaction

4426
F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
Rh2(O2CCH3)4
+ P(4-BrC6H4)2(C6H5)
i)
Br
Br
C6H5
C6H5
4-BrC6H4
P
4-BrC6H4
H3C
O
CH3
O
P
O
O
4
Rh
O
Rh
O
Rh
Rh
O
O
racemic mixture
O
O
CH3
O
O
CH3
CH3
(R)
(S)
CH3
ii)
iii)
Rh2(O2CCH3)2[(4-BrC6H3)P(4-BrC6H4)(C6H5)][(C6H4)P(C6H5)2] isomer mixture
iv)
Br
Br
Br Br
4-BrC6H4
C6H5
4-BrC6H4
C6H5
C6H5
4-BrC6H4
C6H5
4-BrC6H4
CH
O
3 H3C
CH
3 H3C
P
P
P
P
O
O
O
O
O
O
O
Rh
Rh
Rh
C6H5
Rh
Rh
Rh
Rh
C6H5
Rh
O
P
P
P
P
C6H5
C6H5
C6H5
O
O
O
O
C6H5
O
O
O
C6H5
C6H5
CH3
CH3
CH3
CH3
5a (racemic mixture)
5b (racemic mixture)
v)
vi)
v)
vi)
Br Br
Br
Br
4-BrC6H4
4-BrC6H4
C6H5
C6H5
4-BrC6H4
C6H5
4-BrC6H4
C6H5
CF3 F3C
CF3 F3C
P
P
P
P
O
O
O
O
O
O
O
O
Rh
Rh
Rh
Rh
O
Rh
Rh
Rh
Rh
O
P
P
P
P
C6H5
C6H5
C6H5
C6H5
O
C H
6
O
5O
O
C6H5
O
O
C6H5
C6H5
CF3
6b (S)(P)
CF3
CF3
CF3
6a (S)(M)
6a (R)(P)
6b (R)(M)
Fig. 1. Synthesis of pure enantiomeric bis cyclometalated compounds. (i) Reflux in toluene/acetic acid (3:1) for 4 h. (ii) PPh3 (molar ratio 1:1.1). (iii)
Irradiation for 24 h. (iv) Chromatography in SiO2, elution with hexane/ethyl acetate/acetic acid (100:50:1). (v) ProtosH in a 1:10 molar ratio, reflux in
toluene for 300, chromatography in SiO2, elution with CH2Cl2/diethyl ether (95:5 and 10 mg Hprotos/100 mL). (vi) CF3CO2H, chromatography in
SiO2, elution with CH2Cl2/ethyl acetate/trifluoroacetic acid (100:100:1). (R) and (S) are assigned to the configuration of the chiral P atom.
was not as clean as in the previous case. Consequently,
the purification process was tedious and the final yields
were lower (14% and 30%, respectively).
Compounds 5a and 5b with a mixed set of metalated
phosphines are asymmetric molecules (Point Group C1)
and they present backbone chirality in the same way as
the symmetric bis cyclometalated compounds
Rh2(O2CCH3)2[PC]2 Æ 2CH3CO2H (Point Group C2).
This backbone chirality produces enantiomers with M
and P configuration [14]. These four enantiomers (Fig.
1), 6, have been separated via ligand exchange with N-
(4-methylphenylsulphonyl)-L-proline (protosH). Further
chromatography allowed to separate the resulting dia-
stereoisomers. The pure diastereoisomers were reacted

F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
4427
Table 1
Asymmetric cyclopropanation of styrene
with trifluoroacetic acid to exchange the carboxylate li-
gands and obtain the pure enantiomers as trifluoroace-
tate derivatives. For both compounds, the enantiomer
arising from the first eluted diastereoisomer was as-
signed to the M-configuration [11].
S
R
S
R
S
R
N2CHCOOEt
+
Ph
COOEt
Ph
Ph
COOEt
COOEt
Rh2(II)
+
Tris 4-tolylphosphine was also used replacing triphe-
nylphosphine derivative in the above described reac-
tions. Thus the readily available monometalated
R
S
Ph
COOEt
8
9
compound
Rh2(O2CCH3)3[(4-CH3C6H3)P(4-CH3C6-
7
H4)2] Æ 2CH3CO2H and an equimolar amount of the
functionalized phosphine P(4-BrC6H4)2(C6H5) were
irradiated in the experimental conditions already
described. A mixture of the three possible isomers was
obtained, two of formula Rh(O2CCH3)2-[(4-BrC6H3)-
P(4-BrC6H4)(C6H5)][(4-CH3C6H3)P(4-CH3-C6H4)2] Æ
2CH3CO2H and Rh(O2CCH3)2[(C6H4)P(4-BrC6H4)2]-
[(4-CH3C6H3)P(4-CH3C6H4)2] Æ 2CH3CO2H in a ratio
7:7:1. All the efforts to isolated pure compounds were
unsuccessful.
Catalyst
T (ꢁC)
Yielda
8/9
% eeb
Configurationc
8
9
8/9
6a(S)(M)
6b(R)(M)
a
36
36
60
70
45/55
50/50
87
85
86
83
(1S, 2R/1S, 2S)
(1S, 2R/1S, 2S)
Cyclopropanation yield based on diazo ester.
b
ee values calculated in this paper were based on GC analysis with a
2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-b-CDX column.
Configuration assignment was based on the GC retention times.
c
catalyst with orthometalated phosphines. Both catalysts
afforded ketone 11 in high yield (92–95%) (Table 2). The
activity of these catalysts is quite high as they work effi-
ciently using a substrate:catalyst ratio of 1000, without
any decrease in the enantioselectivities. Both the yields
and the enantioselectivities compared well with the val-
ues reported for similar rhodium catalysts [9].
3. Asymmetric catalysis
Compounds 6a(R)(P), 6a(S)(M), 6b(S)(P) and
6b(R)(M) were tested as catalysts in inter- and intramo-
lecular cyclopropanation reactions.
Catalyst 6a(S)(M) and 6b(R)(M) show very similar
asymmetric induction and produce the same ketone with
identical configuration, (1R, 5S), as it was the case for
the reported rhodium catalysts with M configuration.
This confirms that, as expected, the backbone configura-
tion plays a dominant role in the catalytic induction be-
cause of the location of the bromo-substituent in the
phenyl ring, far away from the catalytic centre.
3.1. Intermolecular cyclopropanation
The reaction of ethyl diazoacetate with styrene was
used as model reaction to study the catalytic effective-
ness, as well as the stereo- and enantioselectivity induced
by the catalysts in the intermolecular cyclopropanation.
The results obtained in the cyclopropanation of sty-
rene with ethyl diazoacetate using compounds 6a and
6b deserve a comparison with those reported for related
dirhodium (II) catalysts containing triphenylphosphine
and other p-substituted arylphosphines of formula
Rh(O2CCF3)2[(4-XC6H3)P(4-XC6H4)2]2 [8], (X = H,
CH3, F). The observed yields 60–70% (Table 1), are
among the highest reported for these cyclometalated
dirhodium (II) compound. Only the bulkier catalyst
Rh(O2C(C6H5)3)2[(C6H4)P(C6H5)2]2 [9] showed better
yield (94%) but the enantioselectivity of this catalyst
was very low (39 and 6% for 8 and 9, respectively).
The enantioselectivities observed for 6a and 6b are
among the highest. All these compounds exhibit low
diastereoselectivity.
3.3. Structures of 5a and 5b
Crystals suitable for X-ray structure determination
have been obtained for 5a and 5b (racemic mixture).
Table 2
Intramolecular cyclopropanation reaction
*
CHN2
Rh2(II)
*
O
O
10
11
Catalyst
Ratio (10/cat)
11
Yielda
% eeb
Configurationc
6a(S)(M)
6a(R)(P)
6b(R)(M)
6b(S)(P)
a
1000
150
93
93
92
95
56
45
51
55
1R, 5S
1S, 5R
1R, 5S
1S, 5R
3.2. Intramolecular cyclopropanation
1000
150
For the intramolecular processes, 1-diazo-5-hexen-2-
one, 10 [19] was used as model substrate.
The cyclization of the diazo compound 10 in refluxing
n-pentane was used to compare the catalytic behavior of
6a and 6b with other previously studied dirhodium (II)
Cyclopropanation yield based on diazo ketone.
b
ee values calculated in this paper were based on GC analysis with a
2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-b-CDX column.
Configuration assignment was based on the GC retention times.
c

4428
F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
Views of M configuration of both structures are shown
in Figs. 2 and 3, respectively, together with important
bond distances and angles. In both structures, the two
rhodium atoms are bridged by four ligands: two acetate
groups and two cisoid cyclometalated phosphines,
(C6H4)P(C6H5)2 and (p-BrC6H3)P(p-BrC6H4)(C6H5) in
a head-to-tail arrangement. Two oxygens of two acetic
acid molecules, occupying the axial positions, complete
the slightly distorted octahedral coordination. As ob-
served in all the related compounds, each of these acetic
molecules undergoes an Oꢀ ꢀ ꢀH–O interaction with one
oxygen atom of each bridging acetate group. The values
˚
of the Rh–Rh bond distances (2.51 A in both cases) fall
within the range reported for dirhodium compounds of
comparable structures.
4. Conclusion
Two new orthometalated dirhodium (II) catalysts
with backbone chirality and a center of chirality at
one phosphorus atom have been synthesized and their
catalytic behaviour in asymmetric inter- and intramolec-
ular cyclopropanation reactions has been studied. The
yields and the enantioselectivities observed in the cyclo-
propanation of styrene with ethyldiazoacetate are
among the highest values observed for related dirho-
dium (II) catalysts. In the cyclization reaction of 1-
diazo-5-hexen-2-one, the catalysts gave high yields and
the enantioselectivities compared well with the values
reported for similar dirhodium (II) catalysts.
Fig. 2. Ortep diagram for Compound 5a. Thermal ellipsoids are
˚
drawn at 30% probability. Selected bond distances (A) are: Rh(1)–
In both reactions, the M and P configurations of each
dirhodium compound induced identical enantiocontrol
but with opposite ee values, that supports the generally
accepted idea that the catalytic reaction occurs via a
rhodium-carbenoid species. The chirality at the P atom
has no influence on the catalytic results as the 4-Br-sub-
stituents in the phenyl rings are far away from the cata-
lytic centre.
Rh(2) = 2.5170(6), Rh(1)–P(1) = 2.2202(2), Rh(1)–C(42) = 1.984(7),
Rh(2)–P(2) = 2.2156(2), Rh(2)–C(16) = 1.974(6), Rh(2)–Br(1) = 7.37.
Selected angles (ꢁ) are: P(2)–Rh(1)–Rh(2) = 87.53(4), C(16)–Rh(2)–
Rh(1) = 98.01(2), P(1)–Rh(1)–Rh(2) = 86.97(4). Hydrogen atoms are
omitted for clarity.
5. Experimental section
Commercially available Rh2(O2CCH3)4 Æ (CH3OH)2
was purchased from Pressure Chemical Co. P(C6H5)3,
CF3CO2H, styrene and ethyldiazoacetate were used as
purchased. All solvents were of analytical grade. N-p-
tolylsulfonyl-L-proline (ProtosH) [20], 1-diazo-5-
hexen-2-one [21], Rh2(O2CCH3)3[(C6H4)P(C6H5)2] Æ
2CH3CO2H
[17],
Rh2(O2CCH3)3[4-CH3C6H3)P(4-
CH3C6H4)2] Æ 2CH3CO2H [22], P(4-BrC6H4)(C6H5)2
and P(4-BrC6H4)2(C6H5) [23] were synthesized accord-
ing to the method described in the literature. All the
irradiations were made with an OSRAM (60 w, 230 V)
1
lamp. H, 13C and 31P NMR spectra were recorded on
Fig. 3. Ortep diagram for Compound 5b. Thermal ellipsoids are
˚
drawn at 30% probability. Selected bond distances (A) are: Rh(1)–
a Varian Unity 300 MHz and a Bruker 400 MHz spec-
trometers as solutions in CDCl3. Chemical shifts are re-
ported in ppm. The coupling constants (J) are in Hertz
Rh(2) = 2.5135 (9), Rh(1)–P(1) = 2.195(2), Rh(1)–C(42) = 1.983(8),
Rh(2)–P(2) = 2.214(2), Rh(2)–C(16) = 1.987(9), Rh(2)–Br(1) = 8.13.
Selected angles (ꢁ) are: P(2)–Rh(1)–Rh(2) = 86.83(6), C(16)–Rh(2)–
Rh(1) = 97.9(2), P(1)–Rh(1)–Rh(2) = 87.60(6). Hydrogen atoms are
omitted for clarity.
´
(Hz). Analysis were provided by Centro de Microanali-
sis Elemental, Universidad Complutense de Madrid.
Column chromatography was performed on silica gel

F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
4429
(35–70 mesh). Solvent mixtures are volume/volume mix-
tures, unless specified otherwise. All reactions were car-
ried out in oven-dried glassware under argon
atmosphere, although the isolated solids are air-stable.
Optical rotations were measured on a Perkin–Elmer
241 polarimeter at the Na–D line in 10 cm quartz
cuvettes. The ee values were based on GC analysis
with a 2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-b-CDX
column.
lated), 180.5, 182.6, 182.8. 31P{1H} NMR: d 17.5
(dd; 1JP–Rh = 167 Hz, 2JP–Rh = 7 Hz), 21.2 (dd,
2
1JP–Rh = 173 Hz, JP–Rh = 7 Hz). Anal. Calcd. for
C47H47BrO8P2Rh2: C, 51.90; H, 4.35. Found: C, 51.35;
H, 4.32%.
5.2. Synthesis of Rh2(O2CCH3)3[(4-BrC6H3)P(4-
BrC6H4)(C6H5)] Æ 2CH3CO2H (4)
Rh2(O2CCH3)4 Æ 2 CH3OH (250 mg, 0.494 mmol)
was dissolved under reflux in a mixture of toluene and
acetic acid (3:1, 120 mL) and P(4-BrC6H4)2(C6H5)
(226, 0.538 mmol, 1.1 equiv.) dissolved in toluene
(15 mL), was slowly added to the green solution. The
solution changed from green to red and after 1/2 h of
stirring without further heating, became deep blue. After
refluxing for 4 h the solution was evaporated to dryness
and the crude product was purified by column chroma-
tography (silica gel–hexane, 3 · 30 cm). Elution with
hexane/ethyl acetate/acetic acid (100:50:1) separated a
deep blue band. Removal of the solvent gave a crude
oil which was recrystallized from CH2Cl2/hexane yield-
ing a deep blue solid (378 mg, yield 83%). The 31P
NMR showed that the solid contained a mixture of
monometalated products in a 12:1 ratio. The major
product was identified as the monometalated
Rh2(O2CCH3)3[(4-BrC6H3)P(4-BrC6H4)(C6H5)] Æ 2CH3-
CO2H, 4. 1H NMR: d 1.28 (s, 3H), 1.40 (s, 3H), 2.24 (s,
6H), 2.34 (s, 3H), 6.68 (t, J = 8.9 Hz, 1H), 7.13 (d,
J = 8.0 Hz, 1H), 7.30–7.47 (m, 9H), 8.78 (s, 1H).
13C{1H} NMR: d 21.9, 23.5, 23.7, 24.0, 123.3–143.0
(aromatic signals), 166.2 (dd, J = 34.0 Hz, J = 24.6 Hz,
metalated), 180.8, 184.0, 191.3. 31P{1H} NMR: d 18.8
5.1. Synthesis of Rh2(O2CCH3)2[(4-BrC6H3)P(C6H5)2]-
[(4-CH3C6H3)P(4-CH3C6H4)2] Æ 2CH3CO2H (3)
5.1.1. Method A
Rh2(O2CCH3)3[(4-CH3C6H3)P(4-CH3C6H4)2] Æ 2CH3-
CO2H (100 mg, 0.124 mmol, 1.0 equiv.) and P(C6H5)2-
(4-BrC6H4) (48 mg, 0.141 mmol, 1.1 equiv.) were
dissolved in a mixture of toluene and acetic acid (3:1,
100 mL) to give a red solution. It was stirred and irradi-
ated under an argon atmosphere for 14 h and the color
turned purple. After evaporation to dryness the crude
product was purified by column chromatography (silica
gel–hexane, 2 · 30 cm). Elution with hexane/CH2Cl2/
acetic acid (100:100:1) afforded one purple band which
was collected and evaporated to dryness. The 31P
NMR spectrum showed that the product was a mixture
of different orthometalated compounds. The chroma-
tography was repeated (silica gel–hexane, 2 · 50 cm)
using hexane/ethyl acetate/acetic acid (100:100:1) as elu-
ent. A large band was collected in two equal fractions,
the first one contained the main product. The solvent
was eliminated under vacuum, the residue was solved
in CH2Cl2 and precipitation with hexane gave pure com-
pound 3 as a purple powder (35 mg, yield 26%).
1
2
(dd; JP–Rh = 150 Hz, JP–Rh = 6 Hz). Anal. Calcd. for
C28H29Br2O10PRh2: C, 36.44; H, 3.15. Found: C,
36.98; H, 3.23%.
5.1.2. Method B
Rh2(O2CCH3)3[(4-CH3C6H3)P(4-CH3C6H4)2] Æ 2CH3-
CO2H (50 mg, 0.062 mmol, 1.0 equiv.) and P(C6H5)2 (4-
BrC6H4) (23 mg, 0.067 mmol, 1.1 equiv.) were dissolved
in a mixture of toluene and acetic acid (3:1, 40 mL) to
give a red solution. The solution was refluxed for 3 h
and the colour changed to purple. After evaporation
to dryness the crude product was purified by column
chromatography (silica gel–hexane, 1.5 · 30 cm) follow-
ing the same procedure described in method A, gave
pure compound 3 (12 mg, yield 18%).
1H NMR: d 1.25 (s, 6H), 1.87 (s, 3H), 2.21 (s, 6H),
2.34 (s, 3H) 2.36 (s, 3H), 6.40 (t, J = 8.6 Hz, 1H),
6.58–6.60 (m, 2H), 6.71 (bs, 1H), 6.78–6.82 (m, 4H),
6.88 (d, J = 8.3 Hz, 1H), 6.92–6.98 (m, 3H), 7.02–7.08
(m, 2H), 7.16–7.20 (m, 2H), 7.25 (m, 1H), 7.35–7.44 (s,
3H), 7.56–7.60 (m, 2H), 7.64–7.70 (m, 2H). 13C{1H}
NMR: d 21.3, 21.4, 22.0, 22.4, 22.7, 123.2–145.8 (aro-
matic signals), 165.3 (m, metalated), 170.4 (m, meta-
5.3. Synthesis of Rh2(O2CCH3)2[(4-BrC6H3)P(4-
BrC6H4)(C6H5)][(C6H4)P(C6H5)2] Æ 2CH3CO2H, 5a
and 5b
Rh2(O2CCH3)3[(4-BrC6H3)P(4-BrC6H4)(C6H5)] Æ 2CH3-
CO2H, 4, (100 mg, 0.108 mmol, 1.0 equiv.) and P(C6H5)3
(31.3 mg, 0.119 mmol, 1.1 equiv.) were dissolved in a mix-
ture of toluene and acetic acid (3:1, 100 mL) to give a red
solution. The solution was stirred under an argon atmo-
sphere and irradiated for 20 h during which it turned pur-
ple. The solvent was removed at reduced pressure. The
residual red oil was dissolved in 5 mL of CH2Cl2/hexane
(1:1) and chromatographed on a column (silica gel–hex-
ane, 1.5 · 50 cm). Elution with hexane/ethyl acetate/acetic
acid (100:50:1) separated two red bands in equal amounts
which were collected. Both solutions were evaporated to
dryness and the crude oils were recrystallized from
CH2Cl2/hexane yielding red solids, 5a and 5b.

4430
F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
5a. Yield (45 mg, 37%). 1H NMR: d 1.25 (s, 6H), 2.18
5.5. Rh2(O2CCF3)2[(4-BrC6H3)P(4-
BrC6H4)(C6H5)][(C6H4)P(C6H5)2] Æ 2H2O
[6a(S)(M)] and [6a(R)(P)]
(s, 6H), 6.38 (t, J = 7.8 Hz, 1H), 6.60–6.66 (m, 3H),
6.80–6.88 (m, 5H), 7.02–7.06 (m, 2H), 7.14–7.20 (m,
4H), 7.32–7.44 (m, 7H), 7.56–7.62 (m, 4H). 13C{1H}
NMR: d 22.3 (s, axial CH3COOH), 22.5 (s, bridge
CH3COOꢁ), 22.8 (s, bridge CH3COOꢁ), 122–146 (m,
aromatic), 164.8 (m, metalated), 169.2 (m, metalated),
179.8 (s, axial CH3COOH), 181.7 (d, JP–C = 2 Hz,
bridge CH3COOꢁ), 181.9 (d, JP–C = 3 Hz, bridge CH3
1
Spectroscopic data: H NMR: d 6.45 (m, 1H), 6.60–
6.80 (m, 6H), 6.84–6.98 (m, 4H), 7.04–7.12 (m, 2H),
7.14–7.22 (m, 2H), 7.32–7.46 (m, 7H), 7.50–7.62 (m,
4H). 13C{1H} NMR: d 114.9 (q, CF3, JC–F = 287 Hz)
122–146 (m, aromatic), 160.6 (m, metalated) 164.8 (m,
1
COOꢁ). 31P{1H} NMR: d 19.4 (dd, JP–Rh = 168 Hz,
metalated), 166.5 (q, CF3COO, JC–F = 38 Hz). 31P{1H}
1
2
1
16.5 (bd, JP–Rh = 165 Hz), 18.2 (bd,
2JP–Rh = 8 Hz), 20.9 (dd, JP–Rh = 173 Hz, JP–Rh
=
NMR:
d
7 Hz). Anal. Calcd for C44H40Br2O8P2Rh2: C, 46.98;
H, 3.58. Found: C, 46.00; H, 3.54%.
1JP–Rh = 172 Hz).
20
D
6a(S)(M): Yield 47%. ½aꢂ ¼ ꢁ160 (c = 0.020,
5b. Yield (41 mg, 33%). 1H NMR: d 1.23 (s, 6H), 2.18
(s, 6H), 6.34 (t, J = 8.7 Hz, 1H), 6.65 (m, 3H), 6.76 (m,
1H), 6.88 (m, 4H), 7.06 (m, 2H), 7.16 (m, 4H), 7.38
(m, 7H), 7.63 (m, 4H). 13C{1H} NMR: d 22.2 (s, axial
CH3COOH), 22.6 (s, bridge CH3COOꢁ), 121.3–140.4
(aromatic signals), 164.9 (m, metalated), 169.0 (m, meta-
lated), 179.5 (s, axial CH3COOH), 181.7 (s, bridge
CH3COOꢁ), 181.9 (d, bridge CH3COOꢁ). 31P{1H}
CHCl3). Anal. Calcd. for C40H30Br2F6O6P2Rh2: C,
41.82; H, 2.61. Found: C, 41.46; H, 2.83%.
20
D
6a(R)(P): Yield 73%. ½aꢂ ¼ þ157 (c = 0.021,
CHCl3). Anal. Calcd. for C40H30Br2F6O6P2Rh2: C,
41.82; H, 2.61. Found: C, 41.27; H, 2.75%.
5.6. Rh2(O2CCF3)2[(4-BrC6H3)P(4-BrC6H4)(C6H5)]-
[(C6H4)P(C6H5)2] Æ 2H2O [6b(R)(M)] and
6b(S)(P)
1
2
NMR: d 19.0 (dd, JP–Rh = 168 Hz, JP–Rh = 7 Hz),
1
2
20.5 (dd, JP–Rh = 173 Hz, JP–Rh = 7 Hz). Anal. Calcd
for C44H40Br2O8P2Rh2: C, 46.98; H, 3.58. Found: C,
47.38; H, 3.63%.
1
Spectroscopic data: H NMR: d 6.42 (m, 1H), 6.60–
6.76 (m, 6H), 6.84–7.00 (m, 4H), 7.14–7.22 (m, 4H),
7.32–7.46 (m, 7H), 7.50–7.60 (m, 4H). 13C{1H} NMR:
d 114.8 (q, CF3, JC–F = 287 Hz) 122–146 (m, aromatic),
160.9 (m, metalated) 164.9 (m, metalated), 166.4 (q,
CF3COO, JC–F = 39 Hz). 31P{1H} NMR: d 16.9(bd,
5.4. General procedure for the synthesis of
enantiomerically pure Rh(II) complexes with
orthometalated arylphosphines
1
1JP–Rh = 166 Hz), 18.4 (bd, JP–Rh = 172 Hz).
20
D
6b(R)(M): Yield 61%. ½aꢂ ¼ ꢁ115 (c = 0.020,
Rh(II) diastereoisomers were prepared by a modi-
fied method first published by our group [13]. To a
solution of the bis orthometalated dirhodium (II)
compound (157 mg, 0.140 mmol, 1.0 equiv.) in toluene
(20 mL) was added N-(4-methylphenylsulphonyl)-L-
proline (protosH) (376 mg, 1.40 mmol, 10 equiv.) in a
1:10 molar ratio. The mixture was heated under reflux
for 30 min. The solvent was evaporated under reduced
pressure and the crude product was dried under vac-
uum. This procedure was repeated four times with
new addition of 20 mL of toluene each time. The
resulting red solid was dissolved in CH2Cl2/hexane
(1:1, 5 mL) and the solution was transferred to a col-
umn of chromatography (silica gel–hexane, 2 · 50 cm).
Elution with CH2Cl2/diethyl ether (95:5 and 10 mg
Hprotos/100 mL) separated both diastereoisomers.
The solutions were evaporated under reduced pres-
sure. Each diastereoisomer was dissolved in 10 mL
of CH2Cl2 and five drops of trifluoroacetic acid were
added; the mixture was stirred for half an hour. The
solution was concentrated, transferred to a column
(silica gel–hexane, 1.5 · 50 cm) and eluted with
CH2Cl2/ethyl acetate/trifluoroacetic acid (100:100:1).
The resulting solution was evaporated and the residue
was crystallized from CH2Cl2/hexane.
CHCl3). Anal. Calcd. for C40H28Br2F6O6P2Rh2: C,
41.82; H, 2.61. Found: C, 41.15; H, 2.81%.
20
D
6b(S)(P): Yield 60%. ½aꢂ ¼ þ119 (c = 0.021,
CHCl3). Anal. Calcd. for C40H28Br2F6O6P2Rh2: C,
41.82; H, 2.61. Found: C, 41.46; H, 2.86%.
5.7. Catalytic intermolecular cyclopropanation
The reactions of ethyl diazoacetate with styrene were
performed by slow addition (1.5 mL/h) of the solution
of the diazo compound (81 lL, 0.8 mmol) in pentane
(5 mL) to a refluxing solution (20 mL) containing the
rhodium (II) complex (1 mol%) and the styrene
(230 lL, 2.0 mmol) in the same solvent. After complete
addition, the reaction mixture was stirred at reflux for
2 h and cooled to room temperature. The resulting solu-
tion was filtered through a short plug of silica to remove
the catalyst and the solvent was evaporated under re-
duced pressure. The yield of the reaction was calculated
by 1H and 13C NMR spectroscopy and the enantiopuri-
ties of the products were calculated by chiral gas chro-
matography (oven temperature 100 ꢁC for 5 min, then
2 ꢁC/min to 200 ꢁC). tR: cis-(1S,2R), 22.22 min; cis-
(1R,2S), 22.56 min; trans-(1R,2R), 24.76 min; trans-
1S,2S, 24.98 min.

F. Estevan et al. / Journal of Organometallic Chemistry 690 (2005) 4424–4432
4431
5.8. Intramolecular cyclopropanation
atoms were placed in geometrically generated positions
and refined riding on the carbon atom to which they
are attached except in the water molecule of crystalli-
zation in compound 5a. Details of the data collection,
cell dimensions and structure refinement are given in
Table 3.
CCDC-238509 (5a), and -238510 (5b), contain the
supplementary crystallographic data for this paper.
These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retriving.html [or from the
Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge CB2 1EZ; Fax: (internat) +44-1223/
336-033; E-mail: deposit@ccdc.cam.uk].
Diazo compound 10 was prepared from the corre-
sponding carboxylic acid by reaction with methyl chlo-
roformate, followed by treatment with freshly
prepared diazomethane. Catalytic reactions were per-
formed by addition of a solution of 10 (50 mg) in pen-
tane (5 mL) to a refluxing solution (10 mL) containing
the rhodium (II) complex (molar ratio of 10 to Rh(II)
see Table 2) in the same solvent; the mixture was heated
at reflux for 1 h. The work-up procedure of the reaction
mixture was similar to that mentioned above for the
intermolecular processes. The yield of the reaction was
calculated by 1H and 13C NMR spectroscopy. The
cyclization product was purified by HPLC and the enan-
tiomeric excesses were calculated by chiral gas chroma-
tography (oven temperature 70 ꢁC for 1 min, then
6 ꢁC/min to 200 ꢁC). tR: (1R,5S), 7.42 min; (1S,5R),
8.02 min.
Acknowledgements
We gratefully thank Dra. J. Perez-Prieto for helpful
comments in catalytic reactions and M.P. Quilez for
some preliminary synthetic work. We gratefully thank
Ministerio de Ciencia
MAT2002-0442-1-C02-02) for financial support.
y
Tecnologia (Project
6. X-ray crystallographic study
Well formed crystals of complexes 5a and 5b
were used for X-ray structures determination in a
Kappa CCD diffractometer (Mo Ka radiation, k =
0.71067 A). Unit-cell dimensions were determined by
a least squares fit of 50 reflections. Systematic ab-
sences in both cases were consistent with the space
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5b
Empirical formula
Molecular mass
Wavelength
C
44H38Br2O9P2Rh2 C44H38Br2O8P2Rh2
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Triclinic
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Crystal system
Space group
ꢀ
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P1
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a (A)
11.6790(2)
12.1130(2)
18.6680(3)
74.6090(6)
86.3810(7)
64.1040(7)
2286.29(7)
2
11.4130(3)
11.9410(3)
18.5390(6)
72.5160(19)
79.2410(13)
63.1710(13)
2146.55(10)
2
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Density (calculated)
Absorption coefficient
Reflections collected
Goodness-of-fit on F2
1.654
2.591
1.736
2.757
38715
1.087
37811
1.102
Final R indices [I > 2r(I)] R1 = 0.0650,
wR2 = 0.0982
R1 = 0.0690,
wR2 = 0.1428
´
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M.A. Ubeda, Organometallics 16 (1997) 880.

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