Hydroxyferrocene as a prochiral analog of phenols: cyclopalladation of a mixed phosphite ester of hydroxyferrocene

Article abstract of DOI:10.1016/S0022-328X(01)01228-1

Hydroxyferrocene (ferrocenol), a metallocene analog of phenol, has been converted into a phosphite ester with chiral (racemic) butane-1,3-diol which undergoes cyclopalladation similarly to hydroxyarene phosphites. In this case, a planar chirality emerges

Full text of DOI:10.1016/S0022-328X(01)01228-1

Journal of Organometallic Chemistry 642 (2002) 191–194
www.elsevier.com/locate/jorganchem
Hydroxyferrocene as a prochiral analog of phenols:
cyclopalladation of a mixed phosphite ester of hydroxyferrocene
Ludmila L. Troitskaya *, Svetlana T. Ovseenko, Yurii L. Slovokhotov,
Ivan S. Neretin, Viatcheslav I. Sokolov
A.N. Nesmeyano6 Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Va6ilo6 Street, 119991 Moscow, Russian Federation
Received 10 July 2001; received in revised form 3 September 2001
Abstract
Hydroxyferrocene (ferrocenol), a metallocene analog of phenol, has been converted into a phosphite ester with chiral (racemic)
butane-1,3-diol which undergoes cyclopalladation similarly to hydroxyarene phosphites. In this case, a planar chirality emerges
but no diastereoselectivity has been observed. The molecular structure of the cyclopalladated product as pyridine adduct has been
established by the X-ray study of a single crystal. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Hydroxyferrocene; Cyclopalladation; Phosphites; Butane-1,3-diol; Planar chirality; X-ray molecular structure
1
. Introduction
their organometallic analogs built on p-complex frame-
works in which the stereochemical aspect may be in-
volved because in such systems planar chirality can
arise.
Cyclopalladation is one of the most studied
organometallic reactions [1] whose exceptionally high
regioselectivity is based on the formation of the
chelated five- or six-membered metallocycles depending
on some directing atom which becomes a ligand at the
metal. The following atoms are known to usually serve
as directing ones: nitrogen in amines or imines, phos-
phorus in tertiary phosphines and sulfur in sulfides.
Recently we have extended cyclopalladation as a
preparative method on mixed phenol phosphites [2,3]
using the ability of P(III) in these compounds to ligate
transition metals, in part, palladium. Cyclopalladation
was earlier known to occur with triarylphosphites [4,5],
however, only one aryl group of three was available for
metalation. In our approach, the complete usage of aryl
groups has been ensured by starting with mixed
monoaryldialkylphosphites that makes this approach
applicable to the whole phenolic family including the
important natural representatives like tyrosine and es-
trone [3].
2
. Results and discussion
In this communication, we wish to report that a
metallocenyl analog of phenol, hydroxyferrocene, or
ferrocenol (1), first prepared [6] by Nesmeyanov et al.
4
0 years ago, can be involved in the same reaction as a
mixed phosphite ester. Chemistry of readily oxidisable
ferrocenol is little explored [7]. However, tris(ferro-
cenyl)phosphite was recently prepared by Herberhold et
al. [8]. According to our approach, a dialkyl(ferro-
cenyl)phosphite was required.
Hydroxyferrocene 1 was reacted with chloro-cyclo-
phosphite of 1,3-di(hydroxy)butane, or 2-chloro-4-
methyl-1,3,2-dioxaphosphane
(2),
prepared
in
accordance with the general procedure derived by Ar-
buzov et al. [9], to give the phosphite ester 3. Cy-
clophosphite 2 was chosen because of the presence of a
chiral carbon that might allow to use the enantiomeric
compound in future, if necessary. Mixed phosphite 3 is
very prone to oxidation and, as obtained, without any
purification, was reacted with (PhCN) PdCl in CH Cl
It was interesting to try to extend the scope of this
reaction outside the classical purely organic phenols for
*
Corresponding author. Fax: +7-095-1355085.
E-mail address: sokol@ineos.ac.ru (L.L. Troitskaya).
2
2
2
2
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01228-1

192
L.L. Troitskaya et al. / Journal of Organometallic Chemistry 642 (2002) 191–194
Fig. 2.
Table 1
Fig. 1.
Crystal data and structure refinement parameters for compound 6
at room temperature to afford complex 4 which was
Single crystal (110 K)
Powder (298 K)
3
1
cyclopalladated. It was not possible to get P-NMR
spectrum of 3, instead chemical shift was measured
characteristic for the corresponding phosphate: −9.30
ppm which is similar to that observed by Herberhold et
al. [8] for a related tris(ferrocenyl)phosphate: −14.1
ppm. Reaction sequence is shown on Fig. 1.
Alkoxy analogues of 3 are known [10] to exist in
solution as trans-isomers with axial ROꢀP and equato-
rial Me group that was explained by anomeric effect.
At the same time 3 may be considered as pure
diastereomer with two chiral centres, phosphorus atom
and carbon atom C(4). However, configuration of
three-coordinated phosphorus is labile and can be
changed during coordination with the metal.
Unit cell parameters
a (A
,
)
)
)
7.332(1)
10.483(2)
26.316(6)
90
93.261(5)
90
7.25(1)
10.76(2)
27.58(3)
90
93.3(1)
90
b (A
,
c (A
,
h (°)
i (°)
k (°)
Temperature (K)
Crystal system
110
Monoclinic
Space group
P2 /n
1
3
V (A
Z
,
)
2019
4
2148
Dcalc (g cm−3)
v (mm
1.778
1.83
Bruker SMART
Mo–Ka
16644
5851
3774
244
0.056
−1
)
X-ray device
X-ray wavelength
Reflections
In the course of cyclopalladation in the ferrocene
series specifically [11,12] replacement of hydrogen atom
next to the substituent in Cp ring by palladium occurs,
Independent
Observed (F\4s)
Parameters refined
R (F\4s)
5
so 1,2-disubstituted h -cyclopentadienyl ligand is
formed and, subsequently, planar chirality arises, in
addition to chiral centres. In general, formation of four
racemic diastereomers of a product is possible, but
chiral carbon and phosphorus can induce the configura-
tion of chiral planes in 5 and 6.
atoms were refined within the riding atom approxima-
tion. Dioxaphosphane cycle was refined as a superposi-
tion of two orientations with common P and O atoms.
Powder XRD analysis was performed at room tempera-
ture on DRON-3 diffractometer in Bragg–Brentano
geometry. The measured interval was 4–54° in 2q. Unit
cell parameters were refined with the PIRUM program.
The palladium atom has a planar square coordina-
tion environment with neighbouring atoms displaced
NMR and single-crystal X-ray diffraction data ob-
tained for 6 correspond completely to the structure
expected. Compound 6 was crystallised in the form of
orange plates. A view of the molecule 6 is shown in Fig.
2. The molecule occupies a general position in the
crystal. Ferrocenyl groups form layers perpendicular to
the c-axis. Intermolecular contacts have typical van der
Waals lengths. Powder X-ray analysis at room tempera-
ture has shown that 6 is a single phase with unit cell
parameters corresponding well to the single-crystal
data, taking into account the thermal expansion (see
Table 1). Structure was solved with direct methods and
from the mean plane for less than 0.03 A. The pyridine
,
ligand occupies a trans position in respect to the phos-
phorus atom. The bond lengths PdꢀCl, PdꢀP, PdꢀN,
PdꢀC have typical values. The ferrocenyl fragment at-
tains eclipsed conformation with the usual FeꢀC dis-
tances (see Fig. 2). The metallocycle PdꢀPꢀOꢀCꢀC is
2
refined by least squares against F using SHELXTL pro-
grams, R =0.056 (for 3772 reflections with I\2|(I),
planar within 0.03 A; the PꢀPdꢀC angle is diminished
,
to 80.2°. The sum of the angles in the cycle (539.26°) is
practically equal to the value for a flat pentagon (540°).
1
wR =0.167 (for all reflections). All ordered non-hydro-
2
gen atoms were refined anisotropically. Hydrogen

L.L. Troitskaya et al. / Journal of Organometallic Chemistry 642 (2002) 191–194
193
Correspondingly, the flip angle about the line PdꢀO
that is the dihedral angle between planes PdCCO and
phosphite ester 3. The cyclopalladated product 5 owes
planar chirality but no diastereoselectivity was ob-
served. The molecular structure of the cyclopalladated
complex with pyridine ligand 6 has been determined
using an X-ray method.
(
PdPO) is as low as 7.2°. This is probably characteristic
of this group of complexes since very close values (10.3
and 11.2°) were found for the first representative of this
family, viz. cyclopalladated phosphite of estrone as
dimeric chloride [3]. It is very similar to the molecular
geometry of the cyclopalladated ferrocenylketimine [13]
wherein the five-membered ring is almost flat, but con-
trasts sharply with the structure of the palladium
chelate derived from FcCH NMe in whose acetylacet-
4
. Experimental
All reactions were carried out under Ar. Hydroxyfer-
rocene (1), very susceptible to oxidation, was prepared
according to [6]. NMR spectra were registered with a
Bruker AMX-400 instrument. 1% H PO was used as
2
2
onate complex the corresponding dihedral angle about
the line PdꢀC was found [14] to be 41 and 43°.
3
4
31
31
external standard for P-NMR.
Two diastereomers can be recognised in P- and
H-NMR spectra of 6. In the latter, diastereomeric
1
4.1. Synthesis of
differences have been observed for the phosphite part
of the molecule only, namely, in Me groups (l 0.89 and
2
-chloro-4-methyl-1,3,2-dioxaphosphane (2)
0
.99 ppm), methine proton and one of methylene pro-
An ethereal solution (5 ml) of PCl3 (4.44 g) was
added dropwise at 0 °C on stirring to a mixture of
,3-dihydroxybutane (2.9 g) and dimethylaniline (7.8 g)
tons in CH O (l 5.44 and 5.54 ppm). Basing on the
2
molecular structure of 6, the chemical shifts observed
may be due to shielding of Cp protons by the neigh-
bouring pyridine ring situated perpendicularly to the
palladacycle, whereas the protons of CH O fragment
are held in the deshielding zone of the molecule.
1
in 10 ml of abs. Et O. After 2 h, stirring was stopped
2
and the reaction mixture was left overnight. After filtra-
2
tion and evaporation, distillation afforded 3.5 g (70%)
31
of 2, b.p. 73°/14 mmHg; P {l, CDCl } 151.2 ppm.
3
An X-ray study revealed the presence of the two
diastereomers 6. As was mentioned above the molecule
contains three chiral elements, viz. the methyl-bearing
carbon of the dioxaphosphane cycle, the phosphorus
atom and the chiral plane of ferrocenyl fragment. The
dioxaphosphane cycle is disordered between two orien-
tations with almost equal occupancies, both in chair
conformation. The ferrocenol oxygen atom and the
methyl group are equatorial substituents in cis position
to each other, while the Pd atom is in the axial position
and in the trans position to them. In both orientations
of the disordered group, the chiral plane has the same
absolute configuration. However, the two orientations
differ in the configuration of the chiral carbon and the
phosphorus atoms, resulting in the molecular configura-
tions R R(P)R(C) and R S(P)S(C). As the crystal is
4
.2. Synthesis of the phosphite ester of
hydroxyferrocene, 2-ferrocenyloxy-4-methyl-1,3,2-
dioxaphosphane 3 and of its PdCl complex 4
2
In a flask supplied with magnetic stirring and Ar inlet
suspension of I (0.328 g) in 30 ml of THF and 0.164 g
of Et N were placed. Solution of 2 (0.275 g) in THF (15
ml) was added slowly at −78 °C under Ar. The reac-
tion mixture was left overnight without cooling and
after usual workup 0.424 g (81.7%) of crude 3 was
3
31
obtained ( P-NMR −9.30 ppm for the corresponding
phosphate, cf. −14.1 ppm for tris(ferrocenyl)-
phosphate [8]) which, without further purification, was
reacted with (PhCN) PdCl in CH Cl at room temper-
2
2
2
2
p
p
ature (r.t.) for 15 min. Complex 4 was obtained in 84%
centrosymmetric, the enantiomorphs of both configura-
tions are present too. Therefore, it may be concluded
that crystal structure contains two diastereoisomers
with the same configuration of chiral plane and oppo-
site configurations of phosphite fragments. Because
starting 3 was racemic this points to the non-
diastereoselectivity of cyclopalladation. Hence the re-
mote methyl group in the dioxaphosphane ring does
not influence the stereochemistry of cyclopalladation.
yield. Anal. Found: C, 40.33; H, 4.42; P, 8.09. Calc. for
31
C H Cl Fe O P Pd: C, 41.14; H, 4.19; P, 7.58%. P-
28
34
2
2
6 2
NMR (l, CDCl , ppm): 88.26.
3
4.3. Synthesis of cyclopalladated deri6ati6es 5 and 6
Cyclopalladation was carried out as follows. Com-
pound 4 (0.240 g, 0.29 mM), PdCl (0.055 g, 0.31 mM)
2
and K CO3 (0.08 g, 0.58 mM) were suspended in
2
C H CH or C H (60 ml) and refluxed for 3 h under
6
5
3
6
6
Ar. After filtration and evaporation 0.112 g (41.4%) of
crude cyclopalladated dimeric chloride 5 was obtained.
This crude product 5 in all cases was crystallised from
a benzene to heptane mixture. Anal. Found: C, 41.06;
H, 4.22. Calc. for [C H ClFeO PPd] . C H : C, 40.84;
3
. Conclusions
It has been demonstrated that an organometallic
analog of phenols, hydroxyferrocene (ferrocenol) 1 be-
haves like phenols in the cyclopalladation of the mixed
14
16
3
2
6
6
31
H, 3.83%. P-NMR (l, CDCl , ppm): 129.41.
3

194
L.L. Troitskaya et al. / Journal of Organometallic Chemistry 642 (2002) 191–194
Metal chloride bridges in the dimer 5 were cleaved
with pyridine in CH Cl to give a monomeric pyridine
complex 6 characterised by X-ray study.
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2
2
[
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