Complexes cis-[PdCl2(OSMeR)2] (R=Me, Ph, and C6H4Me-4) in attempted asymmetric cyclometalation of dimethylaminomethylferrocene

Article abstract of DOI:10.1016/S0022-328X(00)00021-8

Cyclometalation of dimethylaminomethylferrocene by the sulfoxide complexes cis-[PdCl₂(OSMeR)₂] (R=Me, Ph, and C₆H₄Me-4) occurs in methanol or methylene chloride as solvent, the highest yields being 35, 25 and 15

Full text of DOI:10.1016/S0022-328X(00)00021-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 601 (2000) 51–56
Complexes cis-[PdCl2(OSMeR)2] (R=Me, Ph, and C6H4Me-4) in
attempted asymmetric cyclometalation of
dimethylaminomethylferrocene
Alexander D. Ryabov a,b,*, Irina M. Panyashkina a, Gregory M. Kazankov a,
Vladimir A. Polyakov c, Lyudmila G. Kuz’mina d
a Department of Chemistry, M.V. Lomonoso6 Moscow State Uni6ersity, 119899, Moscow, Russia
b Di6ision of Chemistry, G.V. Plekhano6 Russian Economic Academy, Stremyanny per. 28, 113054, Moscow, Russia
c D.I. Mendelee6 Moscow Academy of Chemical Technology, Miusskaya square 9, 125820, Moscow, Russia
d N.S. Kurnako6 Institute of General and Inorganic Chemistry RAS, Leninsky prosp. 31, 117907, Moscow, Russia
Received 23 October 1999; accepted 12 January 2000
Abstract
Cyclometalation of dimethylaminomethylferrocene by the sulfoxide complexes cis-[PdCl2(OSMeR)2] (R=Me, Ph, and
C6H4Me-4) occurs in methanol or methylene chloride as solvent, the highest yields being 35, 25 and 15% in the series, respectively.
The reaction affords complexes trans(N,S)-[Pd{(2-Me2NCH2C5H3)Fe(C5H5)}Cl(OSMeR)] and the structure of the palladacycle
with R=Me was established in the X-ray crystal study. Bound via sulfur, the sulfoxides dissociate in solution demonstrating, in
contrast to the analogous Pt(II) complexes, extremely low affinity to palladium(II) centers. The latter effect is considered to be
crucial to account for the failure to carry out attempted asymmetric cyclopalladation of dimethylaminomethylferrocene by the
complex with the enantiomerically pure ligand R(+)-methyl p-tolyl sulfoxide. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Palladium; Cyclometalation; Ferrocene; Sulfoxide; Asymmetric reaction
rocene (1). These results are consistent with
a
1. Introduction
mechanism wherein the base (acylated amino acid an-
ion) is coordinated to palladium(II) during electrophilic
metalation [15,16]. In the light of the Sokolov work, it
was challenging to find chiral ligands other than acy-
lated amino acids, which, on coordination with the
metal center, provided a system capable of asymmetric
cyclometalation. We have demonstrated recently [17]
that chiral sulfoxides coordinated to Pt(II) show some
promise. In fact, a reaction between 1 and cis-[PtCl2(S-
OSMeC6H4Me-4)2] affords the two easily separable
cycloplatinated ScRp and ScSp diastereomers, the struc-
ture of which was similar to 2. However, the impact of
the cycloplatination procedure is much lower compared
with the cyclopalladation, since cycloplatinated fer-
rocenes are difficult to functionalize further. It seemed
logical to transfer the knowledge obtained in the plat-
inum chemistry to the corresponding cyclopall-
adated ferrocene complexes because of their recog-
nized potential as starting materials in organic synthesis
Asymmetric reactions of metalacycles are investi-
gated intensively nowadays due to the high potential of
chiral cyclometalated compounds in various areas of
chemistry, including asymmetric catalysis [1–3], resolu-
tion of donor molecules such as amines [4], phosphines
and arsines [5,6], creation of planar-chiral structures via
asymmetric cyclometalation [7–9], and synthesis of chi-
ral organic molecules from chiral metalacyclic precur-
sors [10–13]. Of particular interest is asymmetric
cyclopalladation of prochiral metallocene molecules
that do not contain extra orienting chiral fragments.
Such a strategy was first introduced into synthetic
practice by the Sokolov group [7,9,14]. The sodium salt
of N-acetyl-S-valine has been used to promote asym-
metric cyclopalladation of dimethylaminomethylfer-
* Corresponding author. Fax: +7-95-9395417.
E-mail address: ryabov@enz.chem.msu.ru (A.D. Ryabov)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00 021-8

52
A.D. Ryabo6 et al. / Journal of Organometallic Chemistry 601 (2000) 51–56
[18–20]. Therefore, in this work we report on the
cyclopalladation of by the complexes cis-
[PdCl2(OSRMe)2] (R=Me, Ph, and p-tolyl) which does
lead to the goal palladacycles (2) (Scheme 1), but
without evident asymmetric induction in the case of
(R)-methyl p-tolyl sulfoxide.
hexane, and air-dried to afford 0.070 g of 3 (58.8%).
1H-NMR (l, CDCl3) 2.41(s, CH3), 3.75 (s, CH2), 4.16
(s, C5H5), 4.25 and 4.61 (t, J 2, H2, 5 and H3, 4). The
crimson mother liquor was concentrated three-fold and
allowed to stand at 5°C for 5 days. Analytically pure
orange crystals of 2a formed were filtered off, washed
with hexane and dried (0.013 g). An additional portion
of 2a was isolated from the mother liquor by prepara-
tive TLC on Silufol plates (TLC: Silufol plates, ben-
zene–n-hexane (4:1 v/v, Rf 0.6). Total yield of 2a, 19%.
IR (KBr disk) 1117s (SꢀO). Anal. Found: C, 39.7; H,
4.8; Cl, 7.8; S, 6.9. Anal. Calc. For C15H22ClFeNOPdS:
C, 39.0; H, 4.8; Cl, 7.7; S, 6.9%.
1
2. Experimental
2.1. General
DMSO (Reakhim) was distilled in vacuum before
use. Aryl methyl sulfoxides OSRMe {R=(R,S)-Ph and
(R)-p-tolyl} were purchased from Aldrich and used as
received. Complexes cis-[PdCl2(OSRMe)2] (R=Me,
Ph) were prepared as described elsewhere [21]. 1H-
NMR spectra were recorded on a CXP-200 Bruker
instrument with a residual solvent signal as an internal
standard. All J values are in Hz. Fast atom bombard-
ment mass spectra were obtained on a JEOL SX-102
spectrometer.
2.3. Reaction of [PdCl2(DMSO)2] with 1 in methylene
chloride
Compound
1
(0.166 g, 0.68 mmol) and
[PdCl2(DMSO)2] (0.113 g, 0.34 mmol) were dissolved in
15 ml of dry methylene chloride and the mixture was
refluxed for 5 h. The reaction course was monitored by
TLC using Silufol plates and ethyl acetate–n-hexane
(3:7 v/v) as an eluent (Rf (2a)=0.5). The solution was
evaporated to dryness and the residue recrystallized
from benzene–n-hexane (4:1 v/v). Yellow microcrystals
of 3 (0.055 g (24.5%), Rf=0.0) were filtered off and
dried. The filtrate was evaporated to dryness and the
residue was recrystallized from benzene–n-hexane (1:1
v/v) to afford orange crystals of 2a. The crystals were
collected, washed with hexane and air-dried (0.0553 g,
35.3%).
2.2. Reaction of [PdCl2(DMSO)2] with 1 in methanol
Dimethylaminomethylferrocene (0.073 g, 0.3 mmol)
and [PdCl2(DMSO)2] (0.060 g, 0.18 mmol) were added
to 15 ml of dry MeOH to yield a crimson solution
which was stirred for 6 h at ambient temperature.
Yellow crystals started to precipitate after 5 min. After
5 days, the crystals were filtered off, washed with
Scheme 1.

A.D. Ryabo6 et al. / Journal of Organometallic Chemistry 601 (2000) 51–56
53
2.4. Reaction of complex 2 with pyridine
by TLC using hexane–ethyl acetate (7:3 v/v) as eluent
(Rf (2c)=0.5). The solution was evaporated to yield a
brownish–red oil to which 15 ml of methanol was
added. A dark-brown undissolved material, which was
a mixture of 2c and 3, was filtered off, dried (137 mg),
and small amount of 2c (8.4 mg) was isolated from the
latter by TLC. No attempt was made to isolate com-
pound 3 (Rf=0). The methanol filtrate was kept at 5°C
for 4 days. Brownish–red precipitate of pure complex
2c was filtered off and dried (35.3 mg). The mother
liquor was evaporated to dryness and the residue was
subjected to preparative TLC (silica gel, hexane–ethyl
acetate (7:3 v/v)). The orange band with Rf (2c)=0.5
was separated, the product washed off with CH2Cl2 to
yield 20 mg of pure complex 2c. Total yield 14.8% (63.7
mg). The reaction with pyridine to afford 4 was carried
as described above for complex 2a. Yield 30%.
Complex 2a (32.1 mg, 0.07 mmol) was treated with
pyridine (12 mg, 0.15 mmol) in 5 ml of dry benzene for
2 h at ambient temperature. n-Hexane (5 ml) was then
added to form an orange precipitate (Rf (4)=0.5, Silu-
fol, benzene–acetone (7:3 v/v)). The precipitate was
filtered off, washed with n-hexane and air-dried to yield
23 mg of complex 4 (71%), which was prepared by us
previously from the corresponding chloro-bridged
dimer [22]. 1H-NMR (l, CDCl3) 2.95 and 3.20 (s, CH3),
3.23 (d, J 2, H5), 3.33 and 3.66 (d, J 14, AB quartet,
CH2), 3.91 (t, J 2, H4), 4.07 (d, J 2, H3), 4.17 (s, C5H5),
7.41 (dd, J 6.5, 4.8; H3%, 5%), 7.85 (t, J 6.5, H4%), 9.05 (d,
J 4.8, H2%, 6%).
2.5. Reaction of [PdCl2(OSMePh)2] with 1
Compound
1
(0.587 g, 2.4 mmol) and
[PdCl2(OSMePh)2] (0.548 g, 1.2 mmol) were dissolved
in 30 ml of dry CH2Cl2 by stirring the mixture for 30
min and the resulting crimson solution was refluxed for
5 h. The solvent was removed in vacuum and dry
methanol (20 ml) was added to the residue. The undis-
solved crimson material (300 mg), which was a mixture
of 2b and 3, was separated by filtration, dried, and 45
mg of bright-orange complex 2b was obtained by using
preparative TLC (silica gel, hexane–ethyl acetate (7:3
v/v), orange band Rf (2b)=0.5). No attempt was made
to isolate compound 3 (Rf=0). The filtrate was evapo-
rated to dryness and 20 ml of dry methanol was again
added. Orange crystals of practically pure complex 2b
precipitated on cooling were filtered off and dried (49.7
mg). The solvent of the mother liquor which still con-
tained 2b was evaporated and additional amount of the
complex was isolated (63.1 mg) by preparative TLC
(silica gel, hexane–ethyl acetate (7:3 v/v), orange band
Rf (2b)=0.5). Total yield of 2b was 25.1% (157.8 mg).
The spectrally pure material (104.6 mg, 17%) was ob-
tained by recrystallization from benzene–hexane (1:1
v/v). Reaction with pyridine to afford 4 was carried as
described above for complex 2a. Yield 46%.
3. Results and discussion
3.1. Complex [PdCl2(DMSO)2] as metalating agent
Cyclopalladation of 1 via the CꢁH bond cleavage by
the Na2PdCl4ꢁNaOAC system is characterized by a
modest yield of the target compound [23] and therefore
it was necessary first to test the complex
[PdCl2(DMSO)2] in this reaction under various condi-
tions. The principal results are summarized in Table 1
(runs 1–10). Two solvents, viz. methanol and
methylene chloride, were used. The cyclopalladation
does not occur in MeOH when the ligand-to-complex
ratio equals 1:1 (runs 1–2). Complex 2a is formed in a
low yield at the 2:1 ratio, indicative of the necessity of
an extra molecule of 1, which acts as a base to abstract
the leaving proton. In the methylene chloride solvent,
which is less acidic than MeOH, complex 2a is formed
even at a 1:1 ligand-to-complex ratio at ambient tem-
perature. The highest yield (35%) was nevertheless ob-
tained at the 2:1 ratio at reflux (run 9). It should be
mentioned that increasing the reaction time did not
increase the yield, suggesting that target compound 2a
might be rather labile at reflux. The reaction with
[PdCl2(DMSO)2] proceeds always with the formation of
[PdCl2(1)2] as a by-product (Table 1). This should, of
course, be anticipated, since the starting complex
[PdCl2(DMSO)2] is substitutionally labile, accounting
for the facile substitution of DMSO by a such N-donor
ligand as 1. The absence of the PdꢁC bond and the
2.6. Reaction of [PdCl2(OSMeC6H4Me-4)2] with 1
R(+)-Methyl p-tolyl sulfoxide (249 mg, 1.62 mmol)
and PdCl2 (143 mg, 0.81 mmol) were added to 10 ml of
dry methylene chloride. The mixture was stirred for 2
days at room temperature (r.t.) and then refluxed for 4
h. The reaction course was monitored by TLC using
chloroform–methanol (5:1 v/v) as eluent. To thus pre-
pared in situ orange–red solution of the complex
[PdCl2(OSMeC6H4Me-4)2], a solution of 1 (194.5 mg,
0.8 mmol) in 10 ml of CH2Cl2 was added. Refluxing the
resulting mixture for 1 h was followed by stirring at r.t.
for 3 days. The course of this reaction was monitored
1
existence of the PdꢁN one is suggested by the H-NMR
spectra of [PdCl2(1)2] and free 1. In fact, there are two
triplets arising from the mono-substituted cyclopentadi-
enyl ring in [PdCl2(1)2] and the singlets from the CH2
and CH3 protons are shifted downfield by 0.48 and 0.25
ppm, respectively, as a result of coordination of 1 to
palladium(II) via tertiary amine nitrogen only. The

54
A.D. Ryabo6 et al. / Journal of Organometallic Chemistry 601 (2000) 51–56
Table 1
Yields of palladacycles 2 on the reaction of 1 with complexes [PdCl2(OSMeR)2] under different conditions
Run
R
Ratio [1]/[PdCl2(OSMeR)2]
Solvent
Conditions
Yield of 2 (%)
Yield of 3 (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Ph
Ph
C6H4Me-4
C6H4Me-4
1:1
1:1
2:1
2:1
1:1
1:1
2:1
2:1
2:1
2:1
1:1
2:1
1:1
2:1
MeOH
MeOH
MeOH
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
2 h, 2292°C
2 h, reflux
10 h, 2292°C
120 h, 2292°C
120 h, 2292°C
5 h, reflux
6 h, 2292°C
24 h, 2292°C
5 h, reflux
10 h, reflux
5 h, 2292°C
5 h, reflux
1 h reflux, 18 h 2292°C
3 h reflux, 18 h 2292°C
0
0
15
19
18
5
12
33
35
18
6
25
15
5.3
0
0
67
59
a
a
a
a
25
a
a
32 b
24 b
a
a Was not determined.
b Calculated as (2+3)mixture−2pure
.
complex [PdCl2(1)2] reacts cleanly with the stoichiomet-
ric amount of pyridine to afford [PdCl2(py)2]. The
1H-NMR spectrum of the latter contained resonances
from the coordinated py only.
spectra, complex 2a was converted into pyridine deriva-
tive 4, the H-NMR spectrum of which is less compli-
1
cated and fully consistent with the proposed structure.
The py ligand is cis to the phenyl carbon suggested by
a strong upfield shift of the H5 doublet due to the
anisotropic effect of the py ring current [22]. In addi-
tion, the spectrum contains two singlets from the
diastereotopic N-methyls, as well the AB quartet from
the NꢁCH2 protons. The conversion of other palladacy-
cles 2 into diagnostic complex 4 has been further used
3.2. Assignment of the structure of 2a
Determination of the structure of complex 2a by
investigation of its solutions turned out to be laborious,
since the coordinated DMSO dissociates even in chloro-
form as solvent and equilibrium (1) should be taken
into account.
1
for their structural characterization using the H-NMR
technique.
The lability of coordinated DMSO was also revealed
in the course of the FAB+ mass-spectral study of 2a.
The most intensive peak with m/z 768 corresponded to
the dimer 5, the spectral pattern in the m/z range
760–777 being in a perfect agreement with the spec-
trum calculated for 5. Remarkably, there was no peak
with m/z 462, which corresponded to 2a. Thus, equi-
librium 1, as in solution, occurs in the gas phase where
it is strongly shifted to the right.
A single-crystal structural study of complex 2a, the
details of which will be reported elsewhere [27], con-
firmed the assignment made on the basis of the analyt-
ical and spectral data. The structure of the compound is
shown in Fig. 1 and, as seen, it is very similar to that of
the platinum(II) analog [24] and platina(II) cycle based
on acetyl ferrocene oxime [28]. Palladium(II) has a
practically square-planar environment and DMSO is
coordinated through sulfur which is trans to the amine
nitrogen. The PdꢁS bond is a feature that makes com-
plex 2a unique as compared with the DMSO coordina-
tion in related fully structurally characterized
pallada(II) cycles [29–31] in which Pd(II) is bound with
DMSO via oxygen.
(1)
As a result, the 1H-NMR spectrum of the solution of 2a
in CDCl3 is complicated in contrast to the analogous
Pt(II) complex [24]. In particular, there is a strong
singlet at l 2.61 from free DMSO, whereas dia-
stereotopic methyls from the coordinated DMSO are
seen at l 3.43 and 3.48. The relative integral intensity of
the signals from free and coordinated DMSO is practi-
cally equal. There are two singlets at l 2.85 and 3.11
from the N-methyls. These should be ascribed to com-
plex 2a, since their intensity is comparable to those at l
3.43 and 3.48 from coordinated DMSO. The spectrum
is also rich with a variety of signals of lower intensity
presumably from 5, because the dimer can provide a
number of isomers. Among the latter are the geometri-
cal ab–hg and ab–gh species (syn and anti isomers),
and each generates two isomers with a different orienta-
tion of the (h5-C5H5)Fe-antenna with respect to the
approximately planar {Pd(m-Cl)2Pd} unit [25,26]. To
1
avoid the complications in interpreting the H-NMR

A.D. Ryabo6 et al. / Journal of Organometallic Chemistry 601 (2000) 51–56
55
3.3. Cyclopalladation of 1 by [PdCl2(SOMeR)2]
(R=Ph and p-tolyl)
palladium case. Analysis of crude 2b or 2c by TLC did
not show resolution of the diastereomers as it was
observed in the platinum system. As before, this behav-
ior can be accounted for in terms of Eq. (1) taking
place on a solvated silica gel surface. As a result, the
diastereomers do not trivially exist under such condi-
tions. Finally, the optical activity of complex 4 derived
from 2c was measured in a hope that the cyclopallada-
tion in Scheme 1 could be asymmetric in character. The
values of [h]25 measured were +5.5, +7.7, and +8.8
at 589, 578, and 546 nm, respectively (c 1.8, CHCl3,
l=5 cm) and these are by two orders of magnitude
lower compared to the numbers reported by Sokolov
for 5 and its monomeric acetylacetonate derivative [7].
This comparison shows clearly the minor asymmetric
induction in cyclopalladation of dimethylaminomethyl-
ferrocene by enantiomerically pure Pd(II) sulfoxide
complexes.
These transformations were carried by using initially
an isolated complex [PdCl2(SOMePh)2] with the
racemic sulfoxide, whereas the enantiomerically pure
complex of R(+)-methyl p-tolyl sulfoxide was pre-
pared in situ. Since higher yields for [PdCl2(DMSO)2]
were achieved in methylene chloride, the cyclopallada-
tion by [PdCl2(SOMeR)2] was carried out in the same
solvent. As seen in Table 1, the reaction occurs for both
R=Ph and p-tolyl, the highest yields being 25 and
15%, respectively. These are lower than what was ob-
served in the corresponding cycloplatination [17]. The
reaction is also accompanied by the formation of com-
plexes [PdCl2(1)2].
1
As before, the H-NMR spectra of complexes 2b,c
are very complicated due to equilibrium (1). Thus, aryl
sulfoxides, likewise DMSO, are also characterized by a
low affinity toward Pd(II) centers. The fact of cyclopal-
ladation was established by the conversion of 2b,c into
pyridine complex 4. The spectra of complex 4 derived
from either 2a, on one hand, or 2b and 2c, on the other,
were in fact identical. Naturally, our expectations were
associated with the enantiomerically pure metalating
agent in order to have an easy access to planar chiral
palladacycles. However, the palladium(II) system ap-
peared to be much less promising compared to the
previously studied Pt(II) one [17]. Whereas in the latter
case, one of the diastereomers crystallized from the
reaction mixture as a diastereomerically pure material,
and the second could be isolated by preparative TLC,
no resolving of the diastereomers was achieved in the
4. Conclusions
The results presented here reveal that although the
sulfoxide complexes cis-[PdCl2(SOMeR)2] are capable
of cyclometalation of ferrocene derivatives, their poten-
tial in asymmetric cyclopalladation is minor. In the
latter respect, the behavior of Pd(II) complexes is much
less advantageous compared with the analogous platin-
um(II) derivatives [17]. There is a clear mechanistic
reason why the behavior of Pd(II) and Pt(II) is differ-
ent. Sulfoxide palladium(II) complexes are too much
labile either to assemble a proper transition state for
asymmetric cyclopalladation or to form stable com-
plexes of type 2 to generate a pair of diastereomers
which could then be resolved successfully. Instability of
cyclopalladated sulfoxide complexes of Pd(II) even in
aprotic solvents, which has been demonstrated in this
work, is rather unexpected in view of the absence of the
dissociative processes in the case of analogous platinum
species. The formation of substantial amount of com-
plex 3 as a by-product is also in line with this hypothe-
sis. The intermediates of the type 3 or [PdCl2(1)-
(solvent)] generated rapidly from the starting sulfoxide
species in solution are likely on the reaction coordinate
of cyclopalladation. Hence, there is no principle source
of chirality in these pathways.
Acknowledgements
The research described in this publication was made
possible in part by financial support from the Russian
Foundation for Fundamental Research (98-03-33023a)
and INTAS (97-0166).
Fig. 1. Crystal structure of 2a.

56
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Howard, L.G. Kuz’mina, M.S. Datt, C. Sacht, Organometallics
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