Effects of the electronic properties of chiral chelating diphosphines in stereoselective Diels-Alder cycloaddition reactions promoted by their transition metals complexes

Article abstract of DOI:10.1016/S0022-328X(01)01459-0

The Pd(II) and Pt(II) complexes (perchlorates and hexafluoroantimoniates, respectively) of a series of C₂ symmetric biheteroaromatic diphosphines differing in their electronic properties at phosphorus have been tested as catalysts in [4 + 2]-cy

Full text of DOI:10.1016/S0022-328X(01)01459-0

Journal of Organometallic Chemistry 643–644 (2002) 424–430
www.elsevier.com/locate/jorganchem
Effects of the electronic properties of chiral chelating diphosphines
in stereoselective Diels–Alder cycloaddition reactions promoted by
their transition metals complexes
Giuseppe Celentano c, Tiziana Benincori b, Stefano Radaelli a, Mara Sada a,
a,
Franco Sannicolo` *
a Dipartimento di Chimica Organica e Industriale dell’Uni6ersita` di Milano e Centro CNR per la Sintesi e Stereochimica di Speciali Sistemi Orga
nici, 6ia Venezian, 21-20133 Milan, Italy
b Dipartimento di Scienze Chimiche, Fisiche e Matematiche dell’Uni6ersita` dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
c Facolta` di Farmacia, Istituto di Chimica Organica dell’Uni6ersita` degli Studi di Milano, CNR, 6ia Venezian, 21-20133 Milan, Italy
Received 4 August 2001; accepted 20 November 2001
Abstract
The Pd(II) and Pt(II) complexes (perchlorates and hexafluoroantimoniates, respectively) of a series of C2 symmetric biheteroaro-
matic diphosphines differing in their electronic properties at phosphorus have been tested as catalysts in [4+2]-cycloaddition
reactions of cyclopentadiene and N-2-alkenoyl-1,3-oxazolidine-2-ones. The best stereoselection results were obtained with the
complexes produced from electron-rich ligands, which were found to give also the kinetically most active catalysts. © 2002
Elsevier Science B.V. All rights reserved.
Keywords: Chiral diphosphines; C2 symmetry ligands; Heteroaromatic ligands; Stereoselective [4+2]-cycloadditions; Asymmetric catalysis
diastereomeric excesses in some reference reactions,
even though metal coordination at nitrogen was found
1. Introduction
equally efficient in ligands difunctionalized with chiral
oxazoline units [5].
The utilization of chiral diphosphine ligands has been
Stereoselective Diels–Alder reaction catalyzed by
complexes of trivalent aluminum, boron and transition
metals with chiral modifiers represents a very efficient
rather poorly considered in the past, probably because
tool to contemporarily generate up to four contiguous
preliminary results with DIOP and CHIRAPHOS [6]
stereocenters under very mild experimental conditions.
Activation of the dienophilic counterpart generally de-
velops trough a Lewis type acid–base face-selective
were found rather unpromising.
Very recently, highly satisfactory results were ob-
tained with BINAP as ligand of Pd(II) and Pt(II) in
interaction between a carbonyl function present on the
cycloaddition reactions between cyclopentadiene and
reactant and the chiral catalytic complex [1].
N-alkenoyl-1,3-oxazolidin-2-ones 1a–c which are in
Several classes of chiral promoters have been tested
in different types of Diels–Alder reactions in order to
standard use for the evaluation of the stereoselection
capacity of all new ligands presented in the chemical
select the most efficient ones from the point of view of
literature (Scheme 1) [7].
stereoselection capacity and catalytic efficiency. It was
In a few cases, enantiomeric excesses up to 99% were
found that oxygenated chelating functions, like those
found, even though cases of quite low stereoselectivity
present in BINOL [2], in 3,3%-diaryl-BINOL [3] and in
are reported. The nature of the counterion seems to
tartrate derivatives [4], give very high enantiomeric and
play a crucial role on this respect, since the best results
were obtained with perchlorate and hexafluoroanti-
monate ions, while the worst data (less than 5% enan-
tiomeric excesses) were found with chloride ion. A
* Corresponding author. Fax: +39-2236-4369.
E-mail address: staclaus@icil64.cilea.it (F. Sannicolo`).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01459-0

G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
425
Scheme 1.
possible interpretation of these extraordinary effects is
based on the consideration that the large electronic
availability of BINAP weakens Lewis acidity of the
metal center in the complex, thus lowering its capacity
to associate to the dienophile. Hard anions restitute the
metal the electronic deficiency necessary to effectively
interact with the dienophilic species.
Since we have synthesized in the past few years a
wide and homogeneous series of chiral C2 symmetric
diphosphines, characterized by a biheteroaromatic at-
ropisomeric backbone [8] displaying different electronic
properties at their phosphorus atoms, we considered to
test them as ligands of palladium(II) and platinum(II)
in the standard cycloaddition reactions reported above.
The diphosphines currently available in an enantiop-
ure state are reported in Plate 1, each one associated
with its own acronym and oxidative potential value, E°
(V). This parameter, determined by voltammetry, is
that we have selected to quantitatively evaluate the
electronic availability at phosphorus of chelating
diphosphine ligands [8b].
evidence that catalysts prepared from electron-rich
diphosphines (N-Me-2-BINP, [9] tetraMe-BITIOP, [8c]
N-MOM-2-BINP [9]) are nearly two order of magni-
tude more active than those resulting from electron-
poor
systems
(BITIANP,
BISCAP)
in
the
hydrogenation of the ketonic function of b-oxoesters.
In the case of acetoacetic ester we found a linear
relationship between E° and the logarithm of the rate
constant [9]. An opposite situation was found in the
inter- [10] and intra-molecular [11] Heck reaction,
where the Pd(0) complexes produced by electron-poor
diphosphines (BITIANP) were found to produce faster
kinetics than those found when electron-rich ligands
(tetraMe-BITIOP, BINAP) were used. In some cases,
however, fast kinetics were found to work against
stereoselectivity [11]. We were convinced that electron-
poor diphosphines could be, at least in principle, more
active than the electron-rich ones since their scarce
donation ability could leave a higher Lewis acid charac-
ter on the metal, thus favoring the coordination of the
catalyst to the dienophile. In this light we planned to
add to the series of biheteroaromatic diphosphines a
new term, namely the 2,2%-bis(dicyclohexylphosphino)-
1,1%-bibenzimidazole (4, Cy-BIMIP), probably display-
ing electronic and steric properties quite different from
those exhibited by the corresponding diphenylsubsti-
tuted ligand BIMIP (Plate 2).
Investigation of the influence of the electronic prop-
erties of phosphorus of diphosphine ligands on the exit
of reaction catalyzed by their transition metal com-
plexes is a challenging point in homogeneous catalysis
research, since it is known that each reaction and
substrate require that metal and ligand be tailored
according to their requirements. We have recently given
Plate 1.

426
G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
thus inhibiting any conjugative interaction with the
other substituents at phosphorus. Their influence would
be limited to inductive effects only and would be, by
consequence, rather modest.
The preparation of enantiopure diphosphines gener-
ally follows two main known strategies. In the case of
electron-rich diphosphines, the resolution process is
performed at level of phosphine-oxides and is based on
fractional crystallization of their diastereomeric adducts
with chiral acids. Alkaline decomplexation of the ad-
ducts affords enantiopure phosphine-oxides, which are
reduced to phosphines with trichlorosilane. N-Me-2-
BINP, [9] tetraMe-BITIOP, [8c] N-MOM-2-BINP, [9]
tetraMe-BITIANP [8a] and BITIANP [8a] were easily
obtained in an enantiopure state according to this
methodology.
Plate 2.
2. Results and discussion
The synthesis of Cy-BIMIP (4) was achieved in good
yields by quenching the dianion of 1,1%-bibenzimidazole
with chloro-dicyclohexylphosphine.
Voltammetry of Cy-BIMIP (Fig. 1) showed an oxida-
tion peak at 1.0 V (vs. Ag/Ag+), 0.15 V less than
BIMIP, as expected by considering that the former
carries dialkylsubstituted phosphine groups, while the
latter is a triaryldiphosphine.
It is interesting to note that the E° value found for
diphosphine 4, bearing the cyclohexyl disubstituted
phosphane groups is only slightly lower than that of the
corresponding diphenyl substituted ligand, BIMIP. It is
evident that the heteroaromatic rings of the backbone
influence the electronic properties of the phosphane
groups much more efficiently than the non stereogenic
substituents. A similar behavior was also found in the
case of C1 symmetric diphosphanes [12]. A possible
interpretation of these findings involves conformational
effects which allow the phosphorus lone pairs to align
and overlap the p system of the biheteroaromatic rings,
In the case of electron-poor systems, resolution can
be performed by fractional crystallization of the
diastereomeric complexes obtained by reaction of
racemic diphosphines with
a
chiral enantiopure
aminopalladium complex, generally the di-m-cloro-bis-
[(R/S) - N,N - dimethyl(a - methylbenzyl)aminato - C2N]-
palladium (II) (5) which can be easily prepared from
N,N-dimethyl-1-phenyl-ethylamine, [13] commercially
available in both enantiopure forms (Plate 3).
In the present case we utilized a technique, previously
successfully experimented for resolution of BIMIP, [14]
which combines kinetic resolution and fractional crys-
tallization. If a fourth of a mole of the resolving
reagent, prepared from R-(+)-N,N-dimethyl-1-phenyl-
ethylamine, is used per mole of racemic (9)-4, both a
15% diastereomeric excess in the complexed and a 15%
enantiomeric excess in the uncomplexed diphosphine
are obtained. Once this unbalanced state is reached, it
was easy to obtain the levorotatory antipode (−)-4 in
an enantiopure state by fractional crystallization of the
free phosphine from toluene in a 6% isolation yield.
Cleavage of the complex with sodium cyanide, purifica-
tion by chromatography, followed by repeated crystal-
lization of the product from toluene gave enantiopure
(+)-4 in an about 8% overall yield. Repetition of this
procedure allowed total resolution of the racemate.
The enantiomeric purity of (+)- and (−)-4 was
checked by chiral HPLC and confirmed by the 31P-
NMR spectra of their complexes with enantiopure (R)-
and (S)-5, respectively (Fig. 2). These spectra, when
compared with those obtained from racemic (9)-4,
which are very complex probably because some antipo-
dal interactions are active in solution when both
diastereoisomers are present, clearly demonstrate that
the samples are constituted by only one diastereo-
isomer.
Fig. 1. Voltammetry of 2,2%-bis(dicyclohexylphosphino)-1,1%-bibenz-
imidazole (4) (Cy-BIMIP).
The investigation of stereoselection ability and cata-
lytic activity of the metal complexes prepared from
biheteroaromatic diphosphines was focused on the reac-
Plate 3.

G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
427
ture; it is very slow at −40 °C and nearly completely
stopped at −60 °C.
Table 1 reports the diastereo- and enantio-selection
data obtained in the reaction of 1a and cyclopentadiene
in the presence of the Pd(II) complexes of different
biheteroaromatic diphosphines at −40 and −60 °C.
The experiments were carried out in parallel with the
analogous complex obtained from BINAP.
The catalysts were prepared from bis(acetonitrile)
Pd(II) chloride and the ligand (1% excess) in
dichloromethane solution at 25 °C. Removal of the
solvent gave a solid complex which was used in a crude
state. The chloride- perchlorate anion exchange was
performed in situ with silver perchlorate.
The data reported in the table demonstrate that:
1. The uncatalyzed non-stereoselective reaction is still
active at −40 °C, since stereoselectivity increases
by further lowering the temperature to −60 °C.
2. Electron-rich biheteroaromatic diphosphines give
catalysts with a high stereoselection ability, while
meager diastereomeric and enantiomeric excesses
follow the use of electron-poor ligands.
3. Also from the kinetic point, electron-rich diphosphi-
nes give more active catalysts than electron-poor
diphosphines, as demonstrated by the observation
that the metal complexes prepared from electron-
poor BIMIP and CyBIMIP do not drive the reac-
tion to completion, even after prolonged times at
−40 °C.
4. Scarce stereoselectivity and poor kinetics are inter-
dependent phenomena. The stereoisomeric excesses
are negatively affected by low catalytic activity,
since it makes the uncatalyzed non-stereoselective
reaction to be competitive with the catalyzed
stereoselective process.
The reaction of cyclopentadiene and N-(2-butenoyl)-
2-oxazolidinone (1b) is very slow at room temperature
in the absence of efficient promoters. The Pt(II) com-
plexes of electron-rich diphosphines, with hexafluoroan-
Fig. 2. 31P-NMR spectrum (CDCl3) of the complex of (−)-CyBIMIP
and (R)-5.
tions described in Scheme 1, in particular on the cy-
cloaddition of cyclopentadiene and the N-acryloyl- and
N-[(E)-2-butenoyl]-1,3-oxazolidin-2-ones (1a) and (1b).
The former reaction was found to be rather fast even
1
in the absence of metal complexes. Kinetic H-NMR
experiments demonstrated that the uncatalyzed reaction
follows a first order kinetic in the dienophile, with a
rate constant=2.6×10-4 s−1 at 25 °C. Thus, the un-
catalyzed non stereoselective reaction highly competes
with the catalyzed stereoselective process at this temper-
ature, with detrimental effects on final diastereo- and
enantiomeric excesses. The uncatalyzed reaction is pro-
gressively inhibited by lowering the reaction tempera-
Table 1
Reactions of cyclopentadiene and N-acryloyl-1,3-oxazolidin-2-one (1a) in the presence of Pd(II) complexes (perchlorates) of biheteroaromatic
diphosphines and BINAP in dichloromethane
Diphosphine ligand a
E° (V)
T (°C)
Time (h)
Conv. (%)
de (%) b
ee (%) c
Abs. conf. d
(−)-S-N-Me-2-BINP
(+)-S-tetraMe-BITIOP
(+)-R-BINAP
(−)-S-BITIANP
(+)-Cy-BIMIP
0.52
0.57
0.63
0.83
1.00
1.15
0.52
0.57
0.63
−40
‘‘
‘‘
‘‘
‘‘
‘‘
−60
‘‘
‘‘
15
15
15
15
110
40
112
112
112
100
100
100
100
50
87
81
86
82
85
59
92
84
90
91
90
89
72
20
48
96
94
99.7
R
R
S
R
R
S
R
R
S
(−)-R-BIMIP
60
(−)-S-N-Me-2-BINP
(+)-S-tetraMe-BITIOP
(+)-R-BINAP
100
100
100
a Dienophile 1a–catalyst molar ratio: 10. Cyclopentadiene–dienophile 1a molar ratio: 5.
b Evaluated by 1H-NMR and HPLC; the endo diastereoisomer 2a prevails on the exo diastereoisomer 3a in all the cases.
c Evaluated by chiral HPLC (Chiracel OD; eluant:hexane–iso-propanol, 85:15).
d Referred to the stereocenter in position 2 of the endo diastereoisomer 2a.

428
G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
Table 2
Reactions of cyclopentadiene and N-[(E)-2-butenoyl]-1,3-oxazolidin-2-one 1b in the presence of Pt(II) complexes of biheteroaromatic diphosphines
and BINAP (hexafluoroantimoniates) in dichloromethane at 0 °C
Diphosphine ligand a
E° (V)
Time (h)
Conv. (%)
de (%) b
ee (%)c
(−)-S-N-Me-2-BINP
(−)-R-tetraMe-BITIOP
(+)-R-BINAP
0.52
0.57
0.63
160
160
160
45
40
40
72
75
71
75
58
73
a Dienophile 1b–catalyst molar ratio: 5. Cyclopentadiene–dienophile 1b molar ratio: 10.
b Evaluated by HPLC and 1H-NMR spectroscopy; the endo diastereoisomer 2b prevails on the exo diastereoisomer 3b in all the cases.
c Evaluated by 1H-NMR spectroscopy with Eu(III)tris[3-heptafluoropropylhidroxymethylene-(+)camphorato] as a shift reagent in CDCl3.
timoniate as anion, were found active catalysts for this
reaction. They were prepared according to the method-
ology described before for the synthesis of the Pt(II)
complexes. Table 2 reports the satisfactory diastereo-
and enantio-selection data obtained in the reaction of
1b and cyclopentadiene in the presence of the Pd(II)
complexes (hexafluoroantimoniates) of N-Me-2-BINP
and tetraMe-BITIOP at 25 °C. The analogous complex
prepared from BINAP was used as reference catalyst.
programmable function generator. Diphosphine 4 con-
centration was 10−3 M in acetonitrile; scan rate: 50 mV
s
−1; 0.1 M tetrabutylammonium perchlorate was the
supporting electrolyte.
4.1. Reagents
Acetonitrile for voltammetric measurements was dis-
tilled twice over phosphorus pentoxide and once over
calcium hydride. Tetraethylammonium perchlorate was
dried at 70 °C before use. (MeCN)2PdCl2 is commer-
cially available, while (EtCN)2PtCl2 was prepared ac-
cording to the literature [15]. N-acriloyl-1,3-oxazolidin-
2-one (1a) and N-[(E)-2-butenoyl]-1,3-oxazolidin-2-one
(1b) were prepared according to literature methods [4].
Cyclopentadiene was freshly prepared by distillation of
dicyclopentadiene before the cycloaddition. Methylene
dichloride for cycloaddition reactions was distilled over
phosphorus pentoxide and degassed with argon.
AgClO4 and AgSbF6 were dried under vacuum before
use for 1 h. All reactions and manipulations were
performed using standard Schlenk techniques.
3. Conclusions
The data reported in this paper give some useful
indications for the choice of the most efficient diphos-
phine ligands to apply in [4+2] cycloaddition reac-
tions: they should possess large electronic availability at
phosphorus. The bond between the metal and electron-
poor phosphine groups is probably too loose to main-
tain the dienophyle and the chiral ligand at the correct
distance to allow the latter to perform an efficient face
selection on the double bond.
This indication is similar to that we had in the
hydrogenation of b-oxoesters where we were able to
find a quantitative relationship between the rate con-
stant and the electrochemical oxidative potential of the
free diphosphine employed to prepare the catalyst [9]. It
is opposite, as anticipated, to the requirements of inter-
and intramolecular Heck reaction needing electron-
poor diphosphines to develop efficient kinetics [10,11].
4.2. (9)-2,2%-Bis(dicyclohexyphosphino)-
1,1%-bibenzimidazole (9)-(4)
A 1.6 M buthyllitium solution in hexane (13 ml, 20.8
mm) is dropped into a stirred solution of 1,1%-bibenzim-
idazole [8b] (2.4 g, 10.3 mm) and N,N,N%,N%-te-
tramethylendiamine (3.2 ml) in dry THF (50 ml) at
−55 °C under nitrogen. Temperature is left to raise
0 °C and maintained during the addition of chloro-di-
cyclohexylphosphine (5 g, 21.5 mm). The mixture is left
to stand for 16 h. Solvent is removed under reduced
pressure and the residue treated with water and methyl-
ene dichloride to afford (9)-4 as a solid (3.5 g) which
was purified by treatment with warm EtOAc under
nitrogen and then crystallized from toluene (20 ml)
(3.41 g, yield: 58%); m.p. 291–294 °C. 31P-NMR
(CDCl3): −21,4 (s). 1H-NMR: 0.9–2.6 (44H, m, 4
cyclohexyl); 6.82 (2H, d); 7.2 (2H, t); 7.35 (2H, t); 7.97
(2H, d); MS m/z 626 (M+).
4. Experimental
Chiral HPLC analyses were performed with a
DAICEL CHIRACEL OD column (210 nm). Electro-
chemical experiments were performed in a three-elec-
trode cell at 25 °C under nitrogen. The working
electrode was a platinum microelectrode (0.003 cm2);
the counter electrode was platinum; the reference elec-
trode was silver/0.1 M silver perchlorate in acetonitrile
(0.34 V vs. SCE). The voltammetric apparatus (Amel,
Italy) included a 551 potentiostat modulated by a 568

G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
429
4.3. Resolution of (9)-2,2%-bis(dicyclohexyphosphino)-
4.5. Cycloaddition reactions
1,1%-bibenzimidazole (9)-(4)
The cycloaddition reaction of 1a and cyclopentadiene
in the presence of (tetraMe-BITIOP)Pd(ClO4)2 is de-
scribed herein as a general procedure.
A solution of di-m-chloro-bis[(R)-dimethyl(a-methyl-
benzyl)aminato-2-C,N]dipalladium (II) (R)-(5) (0.79 g,
1.4 mmol) in benzene (15 ml) was added to a solution
of (9)-(4) (3.41 g, 5.4 mmol) in benzene (100 ml). The
resulting suspension was filtered and the filtrate kept in
the dark for 20 h, then evaporated to dryness under
reduced pressure. The residue was chromatographed on
a silica gel column using an EtOAc–methylene dichlo-
ride (9:1) mixture as eluant. The first fractions eluted
were evaporated to dryness to give enantiomerically
enriched (−)-(4) (1.27 g, [h]2D3= −26, c=0.49, tolu-
ene) which was crystallized from toluene (15 ml) to give
some racemic (9)-(4) (1.09 g). Removal of the solvent
from the mother liquors gave a residue (0.062 g) which
was crystallized from toluene (6 ml) to give enantiopure
(−)-(4) (0.046 g, m.p. 320–325 °C (dec.), [h]2D3=
−176, c=0.5, toluene). The final fractions eluted were
evaporated to dryness to give the diastereomerically
enriched complex between (+)-(4) and (R)-5 (1.66 g).
The complex was dissolved in methylene dichloride (45
ml) and a solution of NaCN (7.5 g) in water (45 ml)
was added under nitrogen atmosphere. The mixture
was stirred for 15 h, then the organic layer was dried
and evaporated to dryness under reduced pressure to
give a residue which was chromatographed on a silica
gel column using an EtOAc–methylene dichloride, 9:1
mixture as eluant. The first fractions eluted were evapo-
rated to dryness to give enantiomerically enriched (+)-
(4) (0.86 g, [h]2D3= +42, c=0.5, toluene) which was
crystallized from toluene (15 ml) to give some 4 in a
nearly racemic form (0.53 g, [h]2D3= +1.5, c=0.5, tolu-
ene). Removal of the solvent from the mother liquors
gave a residue (0.115 g) which was crystallized twice
from toluene (5 and 3 ml) to give enantiopure (+)-(4)
(0.070 g, [h]2D3= +182, c=0.5, toluene).
A solution of (tetraMe-BITIOP)PdCl2 (0.016 g, 0,021
mmol) in methylene dichloride (1 ml) was added to
AgClO4 (0.008 g, 0.019 mmol) and the mixture was
stirred at r.t. for 1 h. A solution of 1a (0.029 g, 0.206
mmol) in methylene dichloride (1 ml) was added and
the resulting solution was cooled at −40/−60 °C in a
cryostat, then cyclopentadiene (85 ml, 1.03 mmol) was
introduced. The progress of the reaction was monitored
by TLC [eluant: C6H12–ethyl acetate, 3:2]. Solvent and
cyclopentadiene were removed in vacuo and residue
was treated with a C6H12–EtOAc 1:1 solution and the
resulting suspension was filtered through a silica gel
column. The fractions containing the cycloaddition
products were combined and submitted to chiral HPLC
analysis, after removal of the solvent.
Acknowledgements
This work was supported by a MURST Cofin 2000/
2001 and by Chemi S.p.A (Via dei Lavoratori, Cinisello
Balsamo, Italy) holder of the patents for synthesis and
use of all diphosphane ligands reported in this paper.
We thank A. Girola (Insubria University) for chiral
1H-NMR shift reagent experiments. Dr T. Benincori
thanks the Dipartimento di Chimica Organica e Indus-
triale dell’Universita` di Milano for hospitality.
References
[1] (a) H.B. Kagan, O. Riant, Chem. Rev. 92 (1992) 1007;
(b) U. Pindur, G. Lutz, C. Otto, Chem. Rev. 93 (1993) 741;
(c) E.J. Corey, S. Sarshar, D.-H. Lee, J. Am. Chem. Soc. 116
(1994) 12089.
[2] K. Mikami, M. Terada, Y. Motoyama, Y. Nakai, Tetrahedron
Asymmetry 2 (1991) 643.
4.4. Preparation of the palladium(II) and platinum(II)
dichlorocomplexes: general procedure
A solution of (MeCN)2PdCl2 or (EtCN)2PtCl2 (one
equivalent) in methylene dichloride was added to a
solution of the diphosphine (1.01 equivalents) in meth-
ylene dichloride under argon atmosphere. The mixture
was stirred at room temperature (r.t.) for 24 h, then the
solvent was removed under reduced pressure to give a
dichlorocomplex which was employed in the cycloaddi-
tion reaction without any further purification. Chemical
shifts of the 31P-NMR (CDCl3) of the complexes are
reported below: (BIMIP)PdCl2: l=13; (Cy-BIMIP)-
PdCl2: l=33.1; 29.8; (BITIANP)PdCl2: l=19.3;
(BINAP)PdCl2: l=29.8; (tetraMe-BITIOP)PdCl2: l=
25.7; (N-Me-2-BINP) PdCl2: l=20.5; (tetraMe-
BITIOP)PtCl2: l=6.5 (s+satellite d, J=3640 Hz);
(BINAP) PtCl2: l=10.5 (s+satellite d, J=3660 Hz).
[3] C. Chapuis, J. Jurczak, Helv. Chim. Acta 70 (1987) 437.
[4] (a) K. Narasaka, M. Inoue, N. Okada, Chem. Lett. (1986) 1109;
(b) K. Narasaka, N. Iwasawa, M. Inoue, T. Yamada, M.
Nakashima, J. Sugimori, J. Am. Chem. Soc. 111 (1989) 5340.
[5] A.K. Ghosh, P. Mathivanan, J. Cappiello, Tetrahedron Asym-
metry 9 (1998) 1.
[6] J.W. Faller, C.J. Smart, Tetrahedron Lett. 30 (1989) 1189.
[7] A.K. Ghosh, H. Matsuda, Org. Lett. 1 (1999) 2157.
[8] (a) T. Benincori, E. Brenna, F. Sannicolo`, L. Trimarco, P.
Antognazza, E. Cesarotti, J. Chem. Soc. Chem. Commun. (1995)
685;
(b) T. Benincori, E. Brenna, F. Sannicolo`, L. Trimarco, P.
Antognazza, E. Cesarotti, F. Demartin, T. Pilati, J. Org. Chem.
61 (1996) 6244;
(c) T. Benincori, E. Brenna, F. Sannicolo`, L. Trimarco, P.
Antognazza, E. Cesarotti, F. Demartin, T. Pilati, G. Zotti, J.
Organomet. Chem. 529 (1997) 445;

430
G. Celentano et al. / Journal of Organometallic Chemistry 643–644 (2002) 424–430
(d) T. Benincori, E. Cesarotti, O. Piccolo, F. Sannicolo`, J. Org.
Chem. 65 (2000) 2043.
[9] T. Benincori, O. Piccolo, S. Rizzo, F. Sannicolo`, J. Org. Chem.
65 (2000) 8340.
[12] F. Sannicolo`, T. Benincori, S. Rizzo, S. Gladiali, S. Pulacchini,
G. Zotti, Synthesis 15 (2001) 2327.
[13] K.N. Roberts, S.B. Wild, J. Chem. Soc. Dalton Trans. (1979)
2015.
[10] L.F. Tietze, K. Thede, F. Sannicolo`, J. Chem. Soc. Chem.
Commun. (1999) 1811.
[11] L.F. Tietze, K. Thede, R. Schimpf, F. Sannicolo`, J. Chem. Soc.
Chem. Commun. (2000) 53.
[14] P. Antognazza, T. Benincori, S. Mazzoli, T. Pilati, F. Sannicolo`,
Phosphorus, Sulphur Silicon 146 (1999) 405.
[15] V.Y. Kukushkin, A. Oskarsson, L.I. Elding, Inorg. Synth. 31
(1997) 279.