3,3-Diethyl- and 3,3-dibenzyl-1,2-diferrocenylcyclopropenes

Article abstract of DOI:10.1039/b307408k

Reactions of 2,3-diferrocenylcyclopropenone 1 with ethyl- and benzylmagnesium chlorides afford 3,3-diethyl-and 3,3-dibenzyl-1,2-diferrocenylcyclopropenes 2 and 3, respectively, and products of nucleophilic opening of the three-membered ring resulting from

Full text of DOI:10.1039/b307408k

View Article Online / Journal Homepage / Table of Contents for this issue
3,3-Diethyl- and 3,3-dibenzyl-1,2-diferrocenylcyclopropenes
Elena I. Klimova,a Tatiana Klimova Berestneva,*a Arnaldo Cinquantini,b Maddalena Corsini,b
Piero Zanello,b Ruben A. Toscano,c Simón Hernández-Ortegac and Marcos Martínez Garcíac
a Universidad Nacional Autónoma de México, Facultad de Química, Cd. Universitaria,
Coyoacán, C.P. 04510, México D.F., México. E-mail: klimova@servidor.unam.mx;
Fax: (525) 622-5371; Tel: (525) 622-5371
b Università di Siena, Dipartimento di Chimica, Via Aldo Moro, 53100 Siena, Italy
c Universidad Nacional Autónoma de México, Instituto de Química, Cd. Universitaria,
Coyoacán, C.P. 04510, México D.F., México
Received 27th June 2003, Accepted 16th October 2003
First published as an Advance Article on the web 12th November 2003
Reactions of 2,3-diferrocenylcyclopropenone 1 with ethyl- and benzylmagnesium chlorides afford 3,3-diethyl-
and 3,3-dibenzyl-1,2-diferrocenylcyclopropenes 2 and 3, respectively, and products of nucleophilic opening of
the three-membered ring resulting from the addition of RMgCl to the carbonyl group, viz., saturated ketones
(4,5-diferrocenylheptan-3-ones 4a,b and 3,4-diferrocenyl-1,5-diphenylpentan-2-ones 5a,b as ca. 3 : 1 mixtures
of two diastereomers) and other products. The spatial structures of compounds 2 and 4a were established by X-ray
diffraction analysis of single crystals. Protonation of the cyclopropenes 2 and 3 with tetrafluoroboric acid at Ϫ40 ЊC
yields the corresponding 3,3-dialkyl-1,2-diferrocenylcyclopropylium tetrafluoroborates. Transformation of the latter
into diferrocenylallylic cations upon increasing the temperature to 20 ЊC and their deprotonation under the action
of N,N-dimethylaniline were studied. Electrochemical investigation of 1 and 2 shows that in both complexes the
cyclopropene spacer allows electronic communication between the two outer ferrocenyl groups, this being notably
greater for 2 than for 1.
oxidation products are usually partially or completely delocal-
Introduction
ised mixed-valent derivatives. A typical example is offered by
The diverse and unusual chemistry associated with the cyclo-
propene ring system is well documented.1,2 The reactivity of
this system is dominated by its inherent ring strain, and
reactions involving the cleavage of each of the bonds constitut-
ing the ring are known. Under thermal,1–4 photochemical,1–4
or metal-catalyzed5 conditions, cyclopropenes undergo ring
opening to vinylcarbene intermediates leading to a variety of
different products. In the present work, we describe several
transformations of 1,2-diferrocenylcyclopropenes with alkyl-
(arylalkyl) substituents in position 3, which were prepared
starting from 2,3-diferrocenylcyclopropenone.
diferrocenylethene and diferrocenylethyne.18,19 In this connec-
tion, electrochemical investigations on diferrocenes having
cyclopropenes as spacers are rather scanty. To the best of our
knowledge, the only electrochemical results are concerned with
1,2,3-triferrocenylcyclopropenes.13
Given the considerable and often highly selective effect of
ferrocenyl groups introduced into the small ring, we are inter-
ested in elucidating the specific features of the electronic inter-
actions of the ferrocenyl substituent at the multiple bond of
cyclopropene. This is of both theoretical and practical interest
in relation to the search for selective reactions of ferrocene
compounds in organic synthesis.
It is known that the introduction of ferrocenyl substituents
into the three-membered ring of cyclopropenes markedly
changes their properties. This is most evident in the case of
3-ferrocenylcyclopropenes.6–12 Thus 3-ferrocenyl-3-isopropyl-
cyclopropene rearranges into 3-ferrocenyl-4-methylpenta-
1,3-diene in solution at ca. 20 ЊC.11 The small-ring opening of
3,3-diferrocenylcyclopropene at 20 ЊC results in 3-ferrocenyl-
1H-cyclopentaferrocene.12 Likewise, 3-aryl-1,2,3-triferrocenyl-
cyclopropenes readily undergo rearrangements (50 ЊC, chloro-
form) to give the alkylation products of the aryl and ferrocenyl
substituents.7 Intramolecular stereoselective alkylation of only
the aryl substituents occurs upon transformation of 3-phenyl-
and 3-naphthyl-3-ferrocenylcyclopropenes into the correspond-
ing ferrocenylindenes at higher temperatures (ca. 80 ЊC). Data
on the properties of cyclopropenes containing ferrocenyl
substituents only in positions 1 and 2 of the small ring are
extremely scarce. Only 2-ferrocenyl-1,3-diphenyl-13 and 1-ferro-
cenyl-1,3-diphenylcyclopropenes14 have been described so
far. The latter has been employed for the preparation of the
1-ferrocenylcyclopropyl cation14,15 and the ability of the
ferrocene ring to stabilize the carbocationic centre in the three-
carbon ring has been examined.16,17
Results and discussion
2,3-Diferrocenylcyclopropenone
1 served as the starting
material for the synthesis of 3,3-diethyl- and 3,3-dibenzyl-1,2-
diferrocenylcyclopropenes. Compound 1 was first isolated as a
side product (ca. 7%) upon alkylation of ferrocene with tetra-
chlorocyclopropene in the presence of AlCl3 at a low temper-
ature back in 1975.8 The 1,2,3-triferrocenylcyclopropenylium
salt was the main reaction product in that case. No studies of
the minor 2,3-diferrocenylcyclopropenone 1 have been carried
out.
We managed to simplify the procedure for the alkylation of
ferrocene with tetrachlorocyclopropene by conducting the
reaction in dichloromethane at 20 ЊC and using a smaller
amount of aluminium chloride. The yield of 2,3-diferro-
cenylcyclopropenone 1 was ca. 90% (Scheme 1).
In this work we have studied the reactions of the cyclo-
propenone 1 with organomagnesium compounds (EtMgCl,
BnMgCl). It was found that 1,2-diferrocenylcyclopropenes 2
and 3 and saturated diferrocenylalkyl ketones 4a,b and 5a,b
were formed along with other products (Scheme 2).
Diferrocenes separated by unsaturated carbon–carbon bonds
constitute a classical example of molecules able to exhibit
intramolecular electron mobility in that their one-electron
1
The structures of compounds 2–5 were established from H
and 13C NMR spectroscopic data.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
4458

View Article Online
Fig. 1 (a) Crystal structure of 2. Selected bond lengths (Å): C(1)–C(2) = 1.285(11), C(1)–C(3) = 1.508(12), C(3)–C(2) = 1.494(11). Selected bond
angles (Њ): C(2)–C(1)–C(3) = 64.1(6), C(1)–C(2)–C(3) = 65.2(6), C(2)–C(3)–C(1) = 50.7(5), C(6)–C(3)–C(4) = 112.8(9), C(6)–C(3)–C(1) = 121.5(8).
(b) Crystal packing of 2.
Fig. 2 (a) Crystal structure of 4a. Selected bond lengths (Å): C(1)–C(21) = 1.570(7), C(21)–C(22) = 1.567(5), C(21)–C(23) = 1.539(8), C(25)–O(1) =
1.191(6), C(25)–C(26) = 1.469(7), C(22)–C(11) = 1.468(7). Selected bond angles (Њ): C(22)–C(21)–C(1) = 107.4(4), C(11)–C(22)–C(25) = 112.4(5),
C(25)–C(22)–C(21) = 110.5(5), O(1)–C(25)–C(22) = 120.4(5), O(1)–C(25)–C(26) = 120.9(7). (b) Crystal packing of 4a.
the unit cell, of cyclopropene molecules closely arranged
in pairs (Fig. 1b).
The three-membered ring in molecule 2 is a scalene triangle
extended towards C(3). The C᎐C bond length [d = 1.285(11) Å]
᎐
and the acute angle at C(3) ω = 50.7(5)Њ are similar to those
of 3-aryl-3-ferrocenylcyclopropenes.9,10 The substituted cyclo-
pentadienyl rings of the ferrocene fragments are coplanar with,
and the ferrocene sandwiches are oriented in opposite direc-
tions with respect to, the central planar small ring (Fig. 1a). The
Fe–C bonds and geometrical parameters of the ferrocene
sandwiches are similar to standard values.9–11
Scheme 1
1H and 13C NMR spectroscopic data suggest that compounds
4,5 are formed as mixtures of two diastereomers (4a,5a, and
4b,5b), respectively, in a ratio of ca. 3 : 1. The diastereomeric
4,5-diferrocenylheptan-3-ones (4a and 4b) could be separated
by preparative TLC on silica gel. The spatial structure of the
major isomer 4a was established by X-ray diffraction analysis
of single crystals prepared by crystallization from hexane. The
general view of a molecule of 4a is shown in Fig. 2a, the crystal
packing is shown in Fig. 2b; no special comments are needed.
As follows from the X-ray diffraction data, compound 4a has
the structure of 4R*,5S*-diferrocenylheptan-3-one. By analogy,
erythro-configurations were ascribed to the major isomer 5a.
No difficulties were encountered in the spectroscopic identifi-
Scheme 2
The relative configuration of one of the cyclopropenes, viz.,
3,3-diethyl-1,2-diferrocenylcyclopropene 2, was deduced from
the NMR spectroscopic data and confirmed by X-ray diffrac-
tion analysis. The general view of a molecule of compound 2 is
shown in Fig. 1a, and the crystal packing is given in Fig. 1b. A
specific feature of the crystal structure of 2 is the presence, in
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4459

View Article Online
cation of ketones 4a,b and 5a,b since the positions of all the
signals in the 1H and 13C NMR spectra, their multiplicities and
integral intensities were distinctly different (see Experimental
section).
We have demonstrated that the protonation of the cyclo-
propenes 2 and 3 with tetrafluoroboric acid etherate at Ϫ40 ЊC
afforded tetrafluoroborates of 3,3-diethyl- and 3,3-dibenzyl-
Scheme 5
1,2-diferrocenylcyclopropyl cations
(Scheme 3).
7 and 8, respectively
previously,9,10 but this resulted in ferrocenylindenes. The form-
ation of naphthalene derivatives is the first example of this kind
of reaction in ferrocenylallylic cations.
The formation of 3,3-diethyl- and 3,3-dibenzyl-1,2-diferro-
cenylcyclopropenes 2 and 3 is an unusual outcome of the
reaction of 2,3-diferrocenylcyclopropenone 1 with ethyl- and
benzylmagnesium chlorides, respectively, which probably
occurs via intermediate magnesium enolates 14a,b and their
reaction with a second molecule of RMgCl (Scheme 6).
Scheme 3
The tetrafluoroborates 7 and 8 are finely crystalline, almost
black compounds which can be stored unaltered for several
hours in a dry inert atmosphere at low temperatures (Ϫ30 ЊC to
Ϫ40 ЊC). The 1H NMR spectra of the salts 7 and 8 (in CD2Cl2
solution) confirm completely their structures, since they each
contain one singlet for an alicyclic proton at δ 3.37 and 3.56
ppm, respectively. As the solution temperature was gradually
increased to ca. 20 ЊC, the tetrafluoroborates 7 and 8 converted
into tetrafluoroborates of diferrocenylallylic cations 9 and 10,
respectively. Recording the 1H NMR spectra at intervals allows
this process to be monitored by following the appearance of the
low-field singlets of the protons of the allylic cations 9 and 10
and the decrease in intensities of the more high-field alicyclic
protons of the cations 7 and 8. Complete conversions of the
latter into the allylic cations 9 and 10 takes ca. 4 h.
Scheme 6
Similar processes in which the carbonyl group has been
completely replaced by two alkyl radicals are known only for
the reactions of ferrocenyl ketones with alkylmagnesium
iodides.20,21
It is quite obvious that the diferrocenylalkyl ketones 4a,b and
5a,b resulted from small-ring opening of one of the two
possible products arising from the addition of the Grignard
reagents to the carbonyl group (14a,b) or to the C1 carbon atom
(16a,b) of cyclopropenone 1. In the first case, vinylcarbene
intermediates 15a,b are formed, and in the second one,
carbanionic intermediates 17a,b are obtained (Scheme 7).
The addition of the second RMgCl molecule to the inter-
mediates 15a,b or 17a,b gives rise to the same carbanions 18a,b,
which, when treated with water, abstract protons from the water
molecules resulting in the formation of the final products 4
and 5.
1,1-Diethyl-2,3-diferrocenylallylium tetrafluoroborate
9
undergoes deprotonation upon treatment with N,N-di-
methylaniline to give 3-ethyl-1,2-diferrocenylpenta-1,3-diene
1
11. This was characterized by H and 13C NMR spectra and
by the Diels–Alder adduct (12) formed by its reaction with
4-phenyl-3,5-dihydro-1,2,4-triazole-3,5-dione (Scheme 4).
In order to corroborate the reaction mechanism proposed
in Scheme 7, the reaction mixture of cyclopropenone 1 and
1
EtMgCl was treated with deuterium oxide (D2O). H NMR
spectroscopic data show that 4,5-dideuterioheptanone 4-D was
formed as a mixture of two isomers 4a-D and 4b-D (∼3 : 1,
respectively) (Scheme 8).
This result confirms the formation of ferrocenyl(vinyl)
anionic intermediates 18a,b in the course of the reaction and
their further transformation into saturated ketones 4 and 5
upon treatment with a protic solvent (water). However, which
of the two possible routes of small-ring opening (Scheme 7) is
preferable for the formation of 18a,b cannot be defined yet.
Low deuterium content in position 4 of the compounds 4a,b-
D seems to be due to isotopic exchange during chromato-
graphic purification of the products on Al2O3 (Brockmann
activity III).
Scheme 4
Electrochemistry
Unlike the cation 9, its benzyl analogue 10 undergoes N,N-
dimethylaniline-promoted intramolecular cyclization due to
ortho-alkylation of one of the phenyl groups of the benzyl
fragments to yield 3-benzyl-1,2-diferrocenyl-1,4-dihydro-
naphthalene 13 (Scheme 5).
Similar alkylation of the aryl fragments in arylferrocenyl-
allylic cations prepared by acid-catalyzed ring opening of
3-aryl-3-ferrocenylcyclopropenes has often been observed
In a previous paper we discussed the electrochemical behaviour
of 1 in CH2Cl2 solution.23 Depending upon the coordinating as
well as the ion-pairing abilities of the supporting electrolyte,
compound 1 afforded two reversible, more or less separated,
one-electron oxidations. In fact, in the presence of the classical
[NBu4][PF6] the separation of the two processes is notably
lower than that in the presence of the (relatively) new
[NBu4][B(C6F5)4],24 Table 1.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4460

View Article Online
Table 1 Formal electrode potentials (V, vs. SCE), peak-to-peak separations (mV), and separation between the two sequential oxidations (V) of
complexes 1 and 2 in CH2Cl2 solution
Proposed nature of
the corresponding
mixed-valent
a
Complex
EЊЈ(0/ϩ)/V
∆Ep
EЊЈ(0/ϩ)/V
∆Ep a/mV
∆EЊЈ/V
Kcom
Supporting electrolyte
monocationc
1
ϩ0.66
ϩ0.75
ϩ0.38
ϩ0.45
ϩ0.39
ϩ0.42
60
91
79
139
80
145
ϩ0.81a, b
ϩ1.01
ϩ0.63
ϩ0.83
—
115
85
≈120
260
250
380
≈1.1ؒ102
2.5ؒ104
1.7ؒ104
2.6ؒ106
[NBu4][PF6] (0.2 mol dmϪ3
)
Class I
[NBu4][B(C6F5)4] (0.1 mol dmϪ3
)
)
)
Class II
Class II
Class III
2
[NBu4][PF6] (0.2 mol dmϪ3
[NBu4][B(C6F5)4] (0.1 mol dmϪ3
[NBu4][PF6] (0.2 mol dmϪ3
[NBu4][B(C6F5)4] (0.1 mol dmϪ3
)
145
FcH
)
a Measured at 0.1 V sϪ1
.
b Affected by electrode adsorption. c See text
Scheme 7
Scheme 8
Since such separation is directly linked25 to the value of the
comproportionation constant, Kcom
:
Fig. 3 Cyclic voltammograms recorded at a platinum electrode in
CH2Cl2 solutions of 2 (0.8 × 10Ϫ3 mol dmϪ3) containing: (a) [NBu4]-
[PF6] (0.2 mol dmϪ3) as supporting electrolyte; (b) [NBu4][B(C6F5)4]
which in turn reflects in part26 the extent of charge delocalis-
ation present in the mixed-valent monocation [1]ϩ, it follows
that this latter, under the appropriate conditions (or, in the
presence of the low coordinating and low ion-pairing anion
[B(C6F5)4]Ϫ) would possess a partially delocalised extra charge
(Robin–Day27 Class II).
(0.1 mol dmϪ3) as supporting electrolyte. Scan rate 0.1 V sϪ1
.
By way of comparison, let us hence discuss the electro-
chemical behaviour of 1,1-diethyl-2,3-diferrocenylcyclopropene,
2. Fig. 3 illustrates its redox ability in CH2Cl2 solutions con-
taining [NBu4][PF6] and [NBu4][B(C6F5)4], respectively, as
supporting electrolytes.
As seen, independent of the nature of the supporting
electrolyte, two separated anodic steps are also displayed, both
possessing features of chemical reversibility.
As Fig.
4 shows, exhaustive one-electron oxidation
corresponding to the first anodic process (Ew = ϩ0.5 V; [NBu4]-
[PF6] as supporting electrolyte) causes the red solution of 2
(λmax = 460 nm, very intense) to generate the stable green
monocation [2]ϩ (λmax = 680 nm, broad); subsequent oxidation
(Ew = ϩ0.8 V) in turn affords the stable grey–green dication
[2]2ϩ (λmax = 690 nm, broad).
Fig. 4 Spectrophotometric behaviour of 2 (0.8 × 10Ϫ3 mol dmϪ3) after
step-by-step exhaustive oxidation in CH2Cl2 solution ([NBu4][PF6] (0.2
mol dmϪ3) as supporting electrolyte). (—) Initial; (---) after about one-
electron oxidation; (. . .. . .) after about two-electron oxidation (see text).
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4461

View Article Online
As can be deduced from Table 1, the use of the low co-
ordinating anion [B(C6F5)4]Ϫ in this case significantly increases
the electronic communication between the two ferrocenyl units.
In fact, [2]ϩ looks like a partially delocalized Class II mixed
valent species in the presence of [NBu4][PF6], whereas it can be
classified as a completely delocalized Class III species in the
presence of [NBu4][B(C6F5)4].
(22%), 4,5-diferrocenylheptan-3-one 4, yield 0.26 g (58%), and
3-ethyl-4,5-diferrocenylheptan-3-ol 6, yield 0.052 g (10%).
Compound 2, orange crystals, mp 168–169 ЊC [Found: C,
69.74; H, 6.21; Fe, 23.87. Calc. for C27H28Fe2: C, 69.86; H, 6.08;
Fe, 24.06%]; νmax (KBr)/cmϪ1: 721, 824, 1004, 1120, 1267, 1464,
1609, 1643, 2859, 2920, 3095; δH (300 MHz, CDCI3): 0.98 (6 H,
t, J = 7.5, 2CH3), 1.70 (4 H, q, J = 7.5, 2 CH2), 4.15 (10 H, s, 2
C5H5), 4.33 (4 H, m, C5H4), 4.46 (4 H, m, C5H4); δC (75 MHz,
CDCl3): 12.46 (2 CH3); 29.46 (2 CH2); 32.16 (C); 68.61, 68.94
(2 C5H4); 69.28 (2 C5H5); 75.19 (2 CipsoFc); 117.03 (2 C).
It is noted that comparison between [1]ϩ and [2]ϩ shows that
the presence of the two electron-donating ethyl units in 2
enhances the intramolecular electron mobility, thus allowing
us to hypothesize that in both the diferrocenylcyclopropene
complexes the electronic communication between the two
ferrocene subunits is not only due to through space (or electro-
static) interactions, but it is likely to also involve a through
bond contribution.
Compound 4, orange oil, represents a ca. 3 : 1 mixture of
isomers 4a and 4b (1H NMR data). The isomers were separated
by preparative TLC (hexane–diethyl ether, 4 : 1).
Compound 4a, 0.17 g (38%), orange crystals, mp 176–177 ЊC
[Found: C, 67.41; H, 6.09; Fe, 23.31. Calc. for C27H30Fe2O: C,
67.25; H, 6.27; Fe, 23.16%]; νmax (KBr)/cmϪ1: 762, 815, 1102,
1145, 1250, 1435, 1520, 1709, 2931, 3089; δH (300 MHz,
CDCI3): 0.75 (3 H, t, J = 7.5, CH3), 0.92 (3 H, t, J = 7.2, CH3),
1.80 (2 H, m, CH2), 2.41 (2 H, q, J = 7.2, CH2), 2.46 (1 H, m,
CH), 3.22 (1 H, d, J = 9.0, CH), 3.99 (5 H, s, C5H5), 4.08 (5 H, s,
C5H5), 3.85 (1 H, m, C5H4), 3.92 (2 H, m, C5H4), 4.02 (1 H, m,
C5H4), 4.09 (2 H, m, C5H4), 4.16 (1 H, m, C5H4), 4.36 (1 H, m,
C5H4); δC (75 MHz, CDCl3): 6.88, 12.46 (2 CH3); 26.45, 38.32
(2 CH2); 45.99, 58.36 (2 CH); 66.25, 66.56, 66.86, 67.52, 68.27,
68.56, 69.34, 69.66 (2 C5H4); 68.35, 68.44 (2 C5H5); 84.12, 93.49
Experimental
All the solvents were dried according to standard procedures
and used freshly distilled. IR spectra were obtained for samples
as KBr pellets on a Specord IR-75 instrument. The mass
spectrum of compound 4b was obtained on a Varian-MAT
1
CH-6 instrument (EI, 70 eV). The H and 13C NMR spectra
were recorded on a Unity Inova Varian spectrometer (300 and
75 MHz) for solutions in CDCl3 and CD2Cl2 with Me4Si as the
internal standard. Chemical shifts are given in ppm and
J values in Hz. Chromatographic separations were carried out
on columns of alumina (Brockmann activity III) and on plates
with a fixed SiO2 layer.
The following reagents from Aldrich were employed: tetra-
chlorocyclopropene, 98%; ferrocene, 98%; aluminium chloride,
99.99%; ethylmagnesium chloride, 2.0 M solution in diethyl
ether; benzylmagnesium chloride, 1.0 M solution in diethyl
ether. Tetrafluoroboric acid etherate, 50–52%, was purchased
from Alfa AESAR.
Materials and apparatus used in the electrochemistry
experiments have been described elsewhere.28 [NBu4][B(C6F5)4]
supporting electrolyte was prepared according to the published
procedure.24 All the potential values are referenced to the satur-
ated calomel electrode (SCE).
(2 CipsoFc); 210.95 (C᎐O).
᎐
Compound 4b, 0.05 g (10%), orange crystals, mp 164–165 ЊC;
MS: m/z 482 [M]ϩ; νmax (KBr)/cmϪ1: 768, 824, 1100, 1125, 1231,
1425, 1512, 1689, 2919, 3086; δH (300 MHz, CDCI3): 0.94 (3 H,
t, J = 7.3, CH3), 1.05 (3H, t, J = 7.5, CH3), 1.88 (2 H, m, CH2),
2.45 (2 H, q, J = 7.5, CH2), 2.49 (1 H, m, CH), 3.56 (1 H, d,
J = 5.1, CH), 3.97 (5 H, s, C5H5), 4.04 (5 H, s, C5H5), 3.67 (1 H,
m, C5H4), 3.76 (1 H, m, C5H4), 3.79 (1 H, m, C5H4), 3.94 (1 H,
m, C5H4), 4.03 (2 H, m, C5H4), 4.08 (2 H, m, C5H4); δC (75
MHz, CDCl3): 8.39, 13.61 (2 CH3); 24.73, 37.33 (2 CH2); 46.10,
56.56 (2 CH); 66.68, 67.03, 68.00, 68.74 (2 C); 68.92, 69.57,
69.72 (2 C5H4); 68.40, 68.46 (2 C5H5); 91.32, 94.00 (2 CipsoFc);
210.12 (C᎐O).
᎐
Compound 4-D, 0.51 g (56%), orange oil, represents a ca.
3 : 1 mixture of isomers 4a-D and 4b-D (1H NMR data). MS:
m/z 483, 484 [M]ϩ.
Synthesis of 2,3-diferrocenylcyclopropenone 1
Compound 4a-D, δH (300 MHz, CDCI3): 0.76 (3 H, t, J = 7.3,
CH3), 0.92 (3 H, t, J = 7.2, CH3), 1.81 (2 H, q, J = 7.3, CH2),
2.41 (2 H, q, J = 7.2, CH2), 2.46 (0.13 H, m, CH), 3.22 (0.6 H, s,
CH), 3.99 (5 H, s, C5H5), 4.06 (5 H, s, C5H5), 3.85 (1 H, m,
C5H4), 3.92 (2 H, m, C5H4), 4.02 (1 H, m, C5H4), 4.09 (2 H, m,
C5H4), 4.16 (1 H, m, C5H4), 4.36 (1 H, m, C5H4).
Compound 4b-D, δH (300 MHz, CDCI3): 0.95 (3 H, t, J = 7.2,
CH3), 1.04 (3 H, t, CH3, J = 7.5, CH3), 1.89 (2 H, q, J = 7.2,
CH2), 2.44 (2 H, q, J = 7.5, CH2), 2.49 (0.12 H, m, CH), 3.56
(0.55 H, s, CH), 3.97 (5 H, s, C5H5), 4.04 (5 H, s, C5H5), 3.67
(1 H, m, C5H4), 3.76 (1 H, m, C5H4), 3.79 (1 H, m, C5H4), 3.94
(1 H, m, C5H4), 4.03 (2 H, m, C5H4), 4.07 (2 H, m, C5H4).
To a stirred solution of ferrocene (5.6 g, 0.03 mol) and
tetrachlorocyclopropene (3.6 g, 0.02 mol) in dry dichloro-
methane (200 ml), aluminium chloride (0.67 g, 0.005 mol) was
added portion-wise for 30 min. The mixture was stirred for 1 h
at 20 ЊC and poured into cold water (200 ml). The organic layer
was separated, washed with water (2 × 50 ml), and dried with
MgSO4. The solvent was evaporated in vacuo and the residue
was chromatographed on Al2O3 (hexane–dichloromethane,
3 : 1) to give compound 1, yield 5.8 g (92%), as orange crystals,
mp 182–183 ЊC (lit:8 mp 181 ЊC) [Found: C, 65.71; H, 4.09; Fe,
26.54. Calc. for C23H18Fe2O: C, 65.58; H, 4.28; Fe, 26.36%]; νmax
(KBr)/cmϪ1: 729, 821, 850, 887, 1003, 1100, 1109, 1480, 1602,
1825, 1850, 2917, 3100; δH (300 MHz, CDCl3): 4.25 (10 H, s, 2
C5H5), 4.58 (4 H, m, C5H4), 4.84 (4 H, m, C5H4); δC (75 MHz,
CDCl3): 65.16 (2 CipsoFc); 70.0 (2 C5H5); 70.85, 71.93 (2 C5H4);
The reaction of 2,3-diferrocenylcyclopropenone 1 with benzyl-
magnesium chloride
This was carried out analogously using cyclopropenone 1
(0.42 g, 1 mmol) and benzylmagnesium chloride (1 M solution
in diethyl ether, 6 ml). The reaction mixture was worked up
as described above, subsequent chromatography on Al2O3
(hexane–diethyl ether, 2 : 1) gave 3,3-dibenzyl-1,2-diferro-
cenylcyclopropene 3, yield 0.12 g (20%), and 3,4-diferrocenyl-
1,5-diphenylpentanon-2-one 5, yield 0.33 g (55%).
Compound 3, 0.12 g (20%), orange crystals, mp 112–113 ЊC
[Found: C, 75.39; H, 5.67; Fe, 19.20. Calc. for C37H32Fe2: C,
75.53; H, 5.48; Fe, 18.99%]; νmax (KBr)/cmϪ1: 718, 821, 1003,
1105, 1258, 1470, 1589, 1623, 1645, 2883, 2936, 3085; δH (300
MHz, CDCI3): 2.96 (4 H, s, 2 CH2), 4.10 (10 H, s, 2 C5H5), 4.33
(4 H, m, C5H4), 4.39 (4 H, m, C5H4), 7.15–7.29 (10 H, m,
144.85 (2 C); 152.312 (C᎐O).
᎐
The reaction of 2,3-diferrocenylcyclopropenone 1 with ethyl-
magnesium chloride
A 2 M solution of ethylmagnesium chloride in diethyl ether
(4 ml) was added to a solution of compound 1 (0.42 g, 1 mmol)
in dry benzene (100 ml). The mixture was stirred in an inert
atmosphere for 3 h at 20 ЊC and quenched by addition of water
(100 ml). The organic layer was separated, washed with water
(2 × 20 ml) and benzene was evaporated in vacuo. The residue
was chromatographed on Al2O3 (hexane–diethyl ether, 3 : 1) to
give 3,3-diethyl-1,2-diferrocenylcyclopropene 2, yield 0.10 g
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4462

View Article Online
2 C6H5); δC (75 MHz, CDCl3): 33.31 (C); 43.85 (2 CH2); 68.85,
69.03 (2 C5H4); 69.34 (2 C5H5); 74.37 (2 CipsoFc); 117.69 (2 C);
125.66, 127.90, 129.91 (2 C6H5); 140.90 (2 Cipso).
Compound 5, 0.33 g (55%), orange powder, mp 183–188 ЊC,
represents a ca. 3 : 1 mixture of isomers 5a and 5b (1H NMR
data). MS: m/z 606 [M]ϩ. [Found: C, 73.41; H, 5.39; Fe, 18.31.
Calc. for C37H34Fe2O: C, 73.29; H, 5.65; Fe, 18.42%]; νmax
(KBr)/cmϪ1: 767, 823, 1021, 1110, 1233, 1448, 1525, 1536, 1654,
1710, 2921, 3098.
Compound 5a, δH (300 MHz, CDCI3): 2.61 (1 H, m, CH),
3.15 (2 H, d, J = 6.3, CH2), 3.23 (2 H, s, CH2), 3.54 (1 H, d,
J = 8.4, CH), 4.09 (5 H, s, C5H5), 4.14 (5 H, s, C5H5), 4.05 (1 H,
m, C5H4), 4.12 (2 H, m, C5H4), 4.15 (1 H, m, C5H4), 4.18 (2 H,
m, C5H4), 4.21 (1 H, m, C5H4), 4.46 (1 H, m, C5H4), 6.89–7.54
(10 H, m, 2 C6H5).
Compound 5b, δH (300 MHz, CDCl3): 2.68 (1 H, m, CH),
3.10 (2 H, d, J = 6.6, CH2), 3.30 (2 H, s, CH2), 3.41 (1 H, d,
J = 8.7, CH), 4.00 (5 H, s, C5H5), 4.10 (5 H, s, C5H5), 3.95 (1 H,
m, C5H4), 4.02 (2 H, m, C5H4), 4.13 (1 H, m, C5H4), 4.15 (2 H,
m, C5H4), 4.17 (1 H, m, C5H4), 4.32 (1 H, m, C5H4), 7.04–7.63
(10 H, m, 2 C6H5).
(1 H, m, C5H4), 4.92 (3 H, m, C5H4), 5.51 (1 H, m, C5H4),
6.05 (1 H, m, C5H4), 6.11 (1 H, m, C5H4), 4.87 (5 H, s, C5H5),
5.01 (5 H, s, C5H5), 7.21–7.47 (10 H, m, 2 C6H5), 8.59 (1 H,
s, CH).
3-Ethyl-1,2-diferrocenylpenta-1,3-diene 11
N,N-Dimethylaniline (0.5 ml) was added dropwise to a solution
of the tetrafluoroborate 9 (0.17 g, 0.3 mmol) in dichloro-
methane (50 ml). The mixture was stirred for 1 h at 20 ЊC, then
the excess of the amine was removed by washing with 5% HCl
(3 × 15 ml) and the solvent was evaporated in vacuo. The
residue was chromatographed on Al2O3 (hexane) to give 0.11 g
(80%) of the diene 11 as orange crystals, mp 142–143 ЊC
[Found: C, 69.73; H, 5.91; Fe, 24.25. Calc. for C27H28Fe2: C,
69.86; H, 6.08; Fe, 24.06%]; νmax (KBr)/cmϪ1: 738, 829, 1014,
1102, 1259, 1444, 1580, 1664, 2895, 2930, 3081; δH (300 MHz,
CDCI3): 0.92 (3 H, t, J = 7.5, CH3), 1.82 (3 H, d, J = 6.9, CH3),
2.30 (2 H, q, J = 7.5, CH2), 4.10 (5 H, s, C5H5), 4.11 (5 H, s,
C5H5), 4.18 (2 H, m, C5H4), 4.20 (2 H, m, C5H4), 4.34 (2 H, m,
C H ), 4.38 (2 H, m, C H ), 5.59 (1 H, q, J = 6.9, CH᎐), 6.38
᎐
5
4
5
4
(1 H, s, CH᎐); δ (75 MHz, CDCl ): 12.80, 23.81 (2 CH ); 13.40
᎐
C
3
3
(CH2); 66.54, 68.08, 68.37, 68.83 (2 C5H4); 69.03, 69.48
The action of tetrafluoroboric acid etherate on the cyclopropene 2
(2 C5H5); 83.21, 88.47 (2 CipsoFc); 121.06, 122.87 (2 CH᎐);
138.42, 140.48 (2 C).
᎐
Tetrafluoroboric acid etherate (3 ml) was added dropwise in
an atmosphere of dry nitrogen to a stirred solution of the
cyclopropene 2 (0.32 g, 0.7 mmol) in dry diethyl ether (100 ml)
at Ϫ40 to Ϫ50 ЊC and the mixture was stirred for 15 min at
Ϫ40 ЊC. The black powder that formed was separated by
decanting, washed with several portions of cold dry ether, and
dried in a vacuum desiccator with external cooling. The yield of
tetrafluoroborate 7, a nearly black powder, was 0.25 g (64%),
decomp. at ca. 183 ЊC; δH (300 MHz, CD2CI2): 1.14 (3 H, t,
J = 7.4, CH3), 1.26 (3 H, t, J = 7.2, CH3), 1.88–2.14 (4 H, m,
2 CH2), 2.47 (1 H, s, CH), 4.13 (2 H, m, C5H4), 4.17 (2 H, m,
C5H4), 4.58 (2 H, m, C5H4), 5.74 (2 H, m, C5H4), 4.22 (5 H, s,
C5H5), 4.35 (5 H, s, C5H5).
A solution of the tetrafluoroborate 7 (0.22 g, 0.4 mmol) in
dichloromethane (50 ml) was kept at ambient temperature for
4 h. Then dry diethyl ether (100 ml) was added, the precipitate
formed was filtered off, washed on a filter with several portions
of dry ether, and dried in a vacuum desiccator to give
1,1-diethyl-2,3-diferrocenylallylium tetrafluoroborate 9 as a
dark brown powder, yield 0.17g (80%), decomp. at ca. 200 ЊC
[Found: C, 58.93; H, 5.12; F, 13.64; Fe, 23.45. Calc. for C27H29
BF4Fe2: C, 58.74; H, 5.30; F, 13.77; Fe, 20.23%]; δH (300 MHz,
CD2CI2): 1.12 (3 H, t, J = 6.6, CH3), 1.73 (3 H, t, J = 6.9, CH3),
2.57–3.54 (4 H, m, 2 CH2), 4.20 (2 H, m, C5H4), 4.70 (2 H, m,
C5H4), 5.20 (1 H, m, C5H4), 6.22 (2 H, m, C5H4), 6.41 (1 H, m,
C5H4), 4.27 (5 H, s, C5H5), 4.86 (5 H, s, C5H5) 8.57 (1 H,
s, CH).
3-Benzyl-1,2-diferrocenyl-1,4-dihydronaphthalene 13
Treatment of the tetrafluoroborate 10 (0.38 g, 0.5 mmol) with
N,N-dimethylaniline and subsequent chromatography (Al2O3,
hexane) afforded 0.20 g (65%) of compound 13 as a pale yellow
powder, mp 184–186 ЊC [Found: C, 75.68; H, 5.24; Fe, 19.18.
Calc. for C37H32Fe2: C, 75.53; H, 5.48; Fe, 18.99%]; νmax (KBr)/
cmϪ1: 723, 817, 1009, 1110, 1245, 1452, 1567, 1620, 1667, 2906,
2936, 3087; δH (300 MHz, CDCI3): 3.12 (1 H, d, J = 18.0, CH2),
3.24 (1 H, d, J = 18.0, CH2), 3.66 (1 H, d, J = 15.6, CH2), 3.97
(1 H, d, J = 15.6, CH2), 3.95 (5 H, s, C5H5), 4.06 (5 H, s, C5H5),
3.90 (2 H, m, C5H4), 4.01 (2 H, m, C5H4), 4.18 (2 H, m, C5H4),
4.24 (2 H, m, C5H4), 7.01–7.33 (9 H, m, C6H4, C6H5).
7-Ethyl-5,6-diferrocenyl-8-methyl-2-phenyl-5,8-dihydro[1,2,4]-
triazolo[1,2-a]pyridazine-1,3-dione 12
4-Phenyl-3,5-dihydro-1,2,4-triazole-3,5-dione (0.05 g, 0.3 mmol)
was added to a solution of the diene 11 (0.12 g, 0.25 mmol) in
benzene (20 ml). The mixture was stirred at 20 ЊC for 3 h, the
solvent was evaporated in vacuo, and the residue was chromato-
graphed on Al2O3 (hexane–dichloromethane, 2 : 1) to give 0.12
g (70%) of the adduct 12 as yellow crystals, mp 213–214 ЊC
[Found: C, 65.84; H, 4.98; Fe, 17.73; N, 6.69. Calc. for
C35H33Fe2N3O2: C, 65.75; H, 5.20; Fe, 17.48; N, 6.57%]; νmax
(KBr)/cmϪ1: 742, 834, 1014, 1025, 1140, 1280, 1463, 1620, 1665,
1712, 2915, 2930, 3078; δH (300 MHz, CDCI3): 1.26 (3 H, t,
J = 7.5, CH3), 1.45 (3 H, d, J = 6.3, CH3), 3.73 (2 H, q, J = 7.5,
CH2), 3.91 (5 H, s, C5H5), 4.14 (5 H, s, C5H5), 3.85 (1 H, m,
C5H4), 4.01 (2 H, m, C5H4), 4.07 (1 H, m, C5H4), 4.35 (2 H, m,
C5H4), 4.41 (1 H, m, C5H4), 4.56 (1 H, m, C5H4), 4.59 (1 H, q,
J = 6.3, CH), 5.68 (1 H, s, CH), 7.33–7.50 (5 H, m, C6H5); δC (75
MHz, CDCl3): 13.77, 16.34 (2 CH3); 23.72 (CH2); 52.52, 56.23
(2 CH); 66.87, 67.17, 67.58, 67.60, 67.76, 68.72, 69.15, 71.21
(2 C5H4); 69.00, 69.63 (2 C5H5); 84.30, 87.88 (2 CipsoFc); 125.46,
128.09, 129.16 (C6H5); 126.11, 131.35 (2 C); 135.11 (Cipso);
The action of tetrafluoroboric acid etherate on the cyclopropene 3
Analogous treatment of the cyclopropene 3 (0.44 g, 0.8 mmol)
in dry ether (100 ml) with tetrafluoroboric acid etherate (4 ml)
at Ϫ50 ЊC afforded tetrafluoroborate 8, yield 0.41 g, 60%, as a
black powder, decomp. at ca. 190 ЊC; δH (300 MHz, CD2Cl2):
2.86 (1 H, s, CH), 3.28 (2 H, d, J = 13.5, CH2), 3.38 (2 H, d,
J = 15.3, CH2), 4.19 (2 H, m, C5H4), 4.29 (2 H, m, C5H4), 4.73
(2 H, m, C5H4), 5.61 (2 H, m, C5H4), 4.14 (5 H, s, C5H5), 4.48
(5 H, s, C5H5), 6.95–7.44 (10 H, m, 2 C6H5).
152.66, 152.86 (2 C᎐O).
᎐
A solution of the tetrafluoroborate 8 (0.34 g, 0.5 mmol) in
dry CH2Cl2 (50 ml) was kept at ambient temperature for 4 h and
1,1-dibenzyl-2,3-diferrocenylallylium tetrafluoroborate 10 was
precipitated with dry ether, yield 0.25 g (72%), as a nearly black
fine crystalline powder, decomp. at ca. 210 ЊC [Found: C, 65.89;
H, 4.74; F, 11.42; Fe, 16.68. Calc. for C37H33 BF4Fe2: C, 65.72;
H, 4.92; F, 11.24; Fe, 16.52%]; δH (300 MHz, CD2CI2): 3.71
(2 H, br s, CH2), 3.99 (2 H, br s, CH2), 4.70 (1 H, m, C5H4), 4.79
Crystal structure determination
The unit cell parameters and the X-ray diffraction intensities
were recorded on a Siemens P4/PC/ω diffractometer (com-
pound 2) and on a Bruker Smart Apex CCD area detector/ω
diffractometer (compound 4a). The structures of compounds 2
and 4a were solved by the direct method (SHELXS) and refined
using full-matrix least-squares on F 2.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4463

View Article Online
Crystal data for C27H28Fe2 (2),† M = 464.19 g molϪ1, mono-
clinic P21/c, a = 7.632(2), b = 15.352(4), c = 18.943(8) Å, α = 90,
β = 94.83(3), γ = 90Њ, V = 2211.6(12) Å3, T = 293(2) K, Z = 4,
ρ = 1.394 Mg mϪ3, λ(Mo-Kα) = 0.71073 Å, F(000) = 968,
absorption coefficient 1.322 mmϪ1, index ranges 0 ≤ h ≤ 9, 0 ≤ k
≤ 18, Ϫ22 ≤ l ≤ 22, scan range 2.16 ≤ θ ≤ 25.01Њ, 3884 independ-
ent reflections, Rint = 0.0622, 4195 total reflections, 264 refinable
7 M. A. Battiste, B. Halton and R. H. Grubbs, Chem. Commun., 1967,
907.
8 I. Agratan and E. Aharon-Shalom, J. Am. Chem. Soc., 1975, 35,
3829.
9 E. I. Klimova, T. Klimova Berestneva, L. Ruiz Ramirez,
M. Martinez Garcia, C. Alvarez Toledano, P. G. Espinosa and
R. A. Toscano, J. Organomet. Chem., 1997, 545, 191.
10 E. I. Klimova, M. Martinez Garcia, T. Klimova, C. Alvarez
Toledano, R. A. Toscano, R. Moreno Esparza and L. Ruiz Ramirez,
J. Organomet. Chem., 1998, 566, 175.
11 E. I. Klimova, M. Martinez Garcia, T. Klimova, C. Alvarez
Toledano, R. A. Toscano and L. Ruiz Ramirez, J. Organomet.
Chem., 2000, 598, 254.
parameters, final R indices [I > 2σ(I )] R1 = 0.0711, wR2
0.1517, R indices (all data) R1 = 0.1389, wR2= 0.1872, largest
difference peak and hole 0.604/Ϫ0.384 e ÅϪ3
Crystal data for C27H30Fe2O (4a),† M = 482.21 g molϪ1
=
.
,
monoclinic P21, a = 11.0169(8), b = 7.8226(5), c = 13.1321(9) Å,
α = 90, β = 102.3290(10), γ = 90Њ, V = 1105.63(13) Å3, T = 291(2)
K, Z = 2, ρ = 1.448 g cmϪ3, λ(Mo-Kα) = 0.71073 Å, F(000) =
504, absorption coefficient 1.328 mmϪ1, index ranges Ϫ13 ≤ h ≤
13, Ϫ9 ≤ k ≤ 9, Ϫ15 ≤ l ≤ 15, scan range 1.59 ≤ θ ≤ 25.00Њ, 3864
independent reflections, Rint = 0.0351, 9069 total reflections, 273
refinable parameters, final R indices [I > 2σ(I )] R1 = 0.0390,
wR2 = 0.0749, R indices (all data) R1 = 0.0511, wR2 = 0.0784,
12 E. I. Klimova, M. Martinez Garcia, T. Klimova, J. M. Mendez
Stivalet, S. Hernandez Ortega and L. Ruiz Ramirez, J. Organomet.
Chem., 2002, 659, 56.
13 A. J. Fry, P. S. Jain and R. L. Krieger, J. Organomet. Chem., 1981,
214, 381.
14 T. Klimova, E. I. Klimova, M. Martinez Garcia, C. Alvarez
Toledano and R. A. Toscano, J. Organomet. Chem., 2003, 665, 23.
15 G. K. Surya Pracash, H. Buchholz, V. Pracash Reddy, A. Meijere
and G. A. Olah, J. Am. Chem. Soc., 1992, 114, 1097.
16 G. A. Olah and M. Mayer, J Am. Chem. Soc., 1975, 97, 1539.
17 W. G. Young, S. U. Sharman and S. Winstein, J. Am. Chem. Soc.,
1960, 82, 1376.
largest difference peak and hole 0.478/Ϫ0.183 e ÅϪ3
.
18 G. Ferguson, C. Glidewell, G. Opromolla, C. M. Zakaria and
P. Zanello, J. Organomet. Chem., 1996, 517, 183.
19 C. LeVanda, D. O. Cowan, C. Leitch and K. Bechgaard, J. Am.
Chem. Soc., 1974, 96, 6788.
20 T. S. Abram and W. E. Watts, Synth. React. Inorg. Met.-Org. Chem.,
1974, 4, 335.
21 W. M. Horspool, P. Stanley and R. G. Sutherland, J. Chem. Soc. C,
1971, 1365.
22 A. N. Pushin and V. A. Sazonova, Izv. Akad. Nauk SSSR. Ser.
Khim., 1986, 2769.
Acknowledgements
This work was supported by a grant from CONACyT (Mexico,
grant 34862-E). Thanks are due to M. L. Velasco, J. Perez,
H. Rios, E. R. Patiño, and O. S. Yañez Muñoz for their
technical assistance. P. Z. gratefully acknowledges the financial
support from the University of Siena (PAR 2003).
23 E. I. Klimova, B. T. Klimova, R. L. Ruiz, A. Cinquantini,
M. Corsini, P. Zanello, O. S. Hernández and G. M. Martinez, Eur. J.
Org. Chem., 2003, 4265–4272.
† CCDC reference numbers 215043 and 215044. See http://
www.rsc.org/suppdata/ob/b3/b307408k/ for crystallographic data in .cif
or other electronic format.
24 (a) R. LeSuer and W. E. Geiger, Angew. Chem., Int. Ed., 2000, 39,
248–250; (b) N. Camire, U. T. Mueller-Westerhoff and W. E. Geiger,
J. Organomet. Chem., 2001, 637, 823.
References
25 A. W. Bott, Curr. Sep., 1997, 16, 61.
1 B. Halton and M. G. Banwell, in The Chemistry of the Cyclopropyl
Group, ed. Z. Rappoport, Wiley, New York, 1987, p. 1223.
2 M. S. Baird, Top. Curr. Chem., 1988, 144, 137.
3 M. G. Steinmetz, R. Srinivasan and W. Leigh, Rev. Chem. Intermed.,
1984, 5, 57.
4 D. Boger and C. E. Brotherton, J. Am. Chem. Soc., 1986, 108,
6695.
26 (a) V. Palaniappan, R. M. Singru and U. C. Agarwala, Inorg. Chem.,
1988, 27, 181; (b) D. Astruc, Electron Transfer and Radical Processes
in Transition-Metal Chemistry. Wiley–VCH. New York, 1995;
(c) B. S. Brunschwig and N. Sutin, Coord. Chem. Rev., 1999, 187,
233.
27 M. B. Robin and P. Day, Adv. Inorg. Chem. Radiochem., 1967, 10,
247.
5 P. Binger and H. M. Buch, Top. Curr. Chem., 1987, 144, 77.
6 A. J. Fry, R. L. Krieger, I. Agranat and E. Aharon-Shalom,
Tetrahedron Lett., 1976, 52, 4803.
28 F. Fabrizi de Biani, F. Laschi, P. Zanello, G. Ferguson, J. Trotter,
G. M. O’Riordan and T. R. Spalding, J. Chem. Soc., Dalton Trans.,
2001, 1520.
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 4 5 8 – 4 4 6 4
4464