Chiral platinum and palladium complexes containing functionalized C2-symmetric bisaminoaryl 'Pincer' ligands

Article abstract of DOI:10.1016/S0022-328X(01)00667-2

The synthesis of enantiopure (oxo-functionalized) C₂-symmetric NCN pincer ligands is described. A key step is the symmetric functionalization of the benzylic positions, which was achieved by enantioselective ketone reduction and subsequent stereoselective substitution protocols. The introduction of α-alkyl substituents has a pronounced effect on the cavity for metal binding. For example, lithiation of the α-ethyl-functionalized pincer ligand afforded mixed (alkyl)(aryl)lithium aggregates rather than dinuclear bis(aryl) lithium [Li(NCN)]₂ species as usually observed for NCN-lithium complexes. Similar effects were established for the (trans)-metalation reaction, which proceeded significantly slower when the steric demand of the α-substituent is increased. The potential of the corresponding enantiopure palladium and ruthenium complexes as enantioselective catalyst has been probed in the asymmetric aldol condensation and hydrogen transfer reactions. The low enantiomeric excess of the products of both reactions indicated that face-discrimination of substrates is not induced by these catalysts and that the chiral information must be located in closer proximity to the metal center.

Full text of DOI:10.1016/S0022-328X(01)00667-2

Journal of Organometallic Chemistry 624 (2001) 271–286
www.elsevier.nl/locate/jorganchem
Chiral platinum and palladium complexes containing
functionalized C2-symmetric bisaminoaryl ‘Pincer’ ligands
Martin Albrecht a, Betty M. Kocks a, Anthony L. Spek* b,1, Gerard van Koten* a,2
a Debye Institute, Department of Metal-Mediated Synthesis, Utrecht Uni6ersity, Padualaan 8, CH-3584 Utrecht, The Netherlands
b Bij6oet Center for Biomolecular Research, Department of Crystal and Structure Chemistry, Utrecht Uni6ersity, Padualaan 8,
CH-3584 Utrecht, The Netherlands
Received 30 October 2000; accepted 4 January 2001
Dedicated to Professor Jean Normant on the occasion of his 65th anniversary in admiration of his great contribution to the application of
organocopper chemistry.
Abstract
The synthesis of enantiopure (oxo-functionalized) C2-symmetric NCN pincer ligands is described. A key step is the symmetric
functionalization of the benzylic positions, which was achieved by enantioselective ketone reduction and subsequent stereoselective
substitution protocols. The introduction of a-alkyl substituents has a pronounced effect on the cavity for metal binding. For
example, lithiation of the a-ethyl-functionalized pincer ligand afforded mixed (alkyl)(aryl)lithium aggregates rather than dinuclear
bis(aryl) lithium [Li(NCN)]2 species as usually observed for NCN–lithium complexes. Similar effects were established for the
(trans)-metalation reaction, which proceeded significantly slower when the steric demand of the a-substituent is increased. The
potential of the corresponding enantiopure palladium and ruthenium complexes as enantioselective catalyst has been probed in the
asymmetric aldol condensation and hydrogen transfer reactions. The low enantiomeric excess of the products of both reactions
indicated that face-discrimination of substrates is not induced by these catalysts and that the chiral information must be located
in closer proximity to the metal center. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Asymmetric synthesis; Homogeneous catalysis; Pincer N ligand; Ligand tuning; Platinum
Recently, various research groups started to explore
catalysts derived from related organometallic analogs
with monoanionic ‘pincer’ ligands ECE (C=aromatic
carbanion; E=heteroatom, e.g. P, N, S) [4]. This class
of complexes offers interesting opportunities for the
design of chiral catalysts: (i) the metal is bonded to the
ligand via a direct metal-to-carbon s-bond, thus pre-
venting leaching of the metal from the complex; (ii)
both the benzylic carbon and the two N or P donor
sites are potentially stereogenic centers; and (iii) the
active metal center in such complexes has a distinct
steric surrounding as a result of bis(ortho)-chelation of
the ligand (Fig. 1).
Dependent on the choice of the metal M and the
donor E, various reactions were catalyzed, such as
hydrogenation [5], alkane dehydrogenation [6], CꢀC
bond formation [7–13], CꢀC bond cleavage [14], and
polymerization [15]. Early approaches towards asym-
metric catalysis involved the replacement of the NMe2
1. Introduction
Chiral transition metal complexes containing C2-sym-
metric ligands have found widespread application in
asymmetric catalysis [1]. Frequently, the catalyst pre-
cursor consists of a late transition metal bound to a (set
of) bidentate coordinating ligand(s), of which deriva-
tives of chiral bis-oxazolines, salen-, and binap-type
ligands are prominent examples [2]. Terdentate bond-
ing, C2-symmetric ligands having a neutral donor array
(e.g. h3-N,N,N-coordinating pybox ligands) are less
frequently used, although good to excellent (asymmet-
ric) catalyst performance (stereo-, regio-, chemoselectiv-
ity) was established for some inorganic complexes [3].
1 *Corresponding author for crystallographic details. E-mail:
a.l.spek@chem.uu.nl
2 *Corresponding author. Tel.: +31-30-253-3120; fax: +31-30-
252-3615; e-mail: g.vankoten@chem.uu.nl
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00667-2

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M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
groups in the NCN pincer ligand by asymmetrically
substituted pyrrolidines (A in Chart 1) [16]. A nickel
catalyst with this chiral ligand was used in atom trans-
fer radical (Kharasch) addition reaction, which pro-
vided, however, 1:1 addition products with low
enantiomeric excess. Detailed studies have shown that
this ATRA catalysis involves inner sphere radical for-
mation and quenching at the nickel-stabilized halide
atom. According to a crystal structure determination
on the catalyst precursor, this atom is localized remote
from the stereogenic centers of the chiral ligand [16]. In
a similar approach, C2-symmetric Phebox ligands (B in
Chart 1) were prepared and used in asymmetric cataly-
sis, however, with only limited success (ee typically
below 35%) [9–11]. A markedly improved asymmetric
induction was observed by the enantioselective func-
tionalization of the benzylic positions of PCP pincer
ligands (ee up to 80%; C in Chart 1) [12,13,17].
active) chiral complexes to soluble (e.g. dendritic) [19]
or insoluble heterogeneous (e.g. activated oxide surface)
[20] supports as well as for the synthesis of molecular
networks for crystal engineering.
2. Results and discussion
2.1. Enantioselecti6e pincer ligand synthesis
Alkylated benzylic alcohols such as 1 (Scheme 1)
were selected as key intermediates for the synthesis of
different types of chiral NCN pincer ligands [21]. The
various elegant methods [22] for the enantioselective
preparation of such benzylic alcohols include the asym-
metric reductive alkylation of aldehydes with dialkyl
Chart 1.
Recently, we set out to explore the synthesis of a
series of enantiopure a-alkylated NCN pincer ligands
(E=NMe2) containing chiral information on the ben-
zylic carbons, and their platinum(II) and palladium(II)
complexes [18], since the NCN and PCP metal com-
plexes often have different and sometimes complemen-
tary activity [5]. Moreover, the high kinetic stability
generally observed for NCN–platinum species, makes
them ideal candidates for detailed structural analyses,
while the corresponding palladium(II) complexes are
kinetically active and catalyze a wide range of syntheti-
cally important CꢀC coupling reactions. Furthermore,
the functionalization of the aryl moiety of these chiral
NCN pincer ligands is reported, which offers attractive
opportunities for the anchoring of such (catalytically
zinc reagents catalyzed by enantiopure ortho-arenethio-
late zinc catalysts, e.g. [Zn(S-SC6H4{CH2NMe2}-2)]2
[23]. Hence, the diol 1b was prepared from isophtha-
laldehyde and ZnEt2 (Scheme 1). Chiral HPLC analysis
of the products obtained after work-up revealed a
mixture of S,S-1b (74%), R,R-1b (2%) and meso-1b
(24%). Substitution of the NMe2 groups in the catalyst
by N-pyrrolidinyl or N-piperidinyl moieties increased
the stereoselectivity, because these changes enhanced
the rate of the catalyzed reaction and resulted in a more
pronounced chiral pocket [23b]. Using these catalysts,
the stereoisomeric compounds were obtained in 90:2:8
molar ratio (S,S:R,R:meso).
However, when ZnMe2 was used as the alkylating
agent in order to prepare diol 1a, the reaction was
Fig. 1. Basic concepts for steric and electronic tuning of metal properties in complexes containing a pincer-type ligand (E=NR2, PR2, SR).

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
273
[2a,26]. The R,R-configuration is in full accordance
with the mechanism of the CBS reduction as proposed
by Corey and Helal [24]. Stereoselective transformation
[12b] of the diol 1 involved dropwise addition of
MeSO2Cl (MsCl) to a cold (−80°C) solution of 1 in
the presence of NEt3, affording the corresponding bis(-
mesylate) compound (Scheme 2). Subsequent nucle-
ophilic substitution with HNMe2 was performed in situ
and proceeded with inversion at the stereogenic carbon
centers affording the target bisaminoarene S,S-2 [27].
2.2. C2-Symmetric lithium and platinum NCN
complexes
Generally, heteroatom-directed lithiation of the
bisaminoarene pincer ligand precursors and subsequent
transmetalation allows for the preparation of a large
variety of transition metal pincer complexes [4b].
Hence, reaction of S,S-2a with n-BuLi in alkane sol-
vents followed by transmetalation with [PtCl2(SEt2)2]
afforded the chiral arylplatinum(II) complex S,S-3a in
78% yield. While lithiation of S,S-2a was straightfor-
ward, different product formation was observed when
the a-ethyl substituted bisaminoarene 2b was used.
Lithiation of 2b under similar conditions did not exceed
50% even after prolonged reaction times (Scheme 3)
[18a]. It appeared that addition of a second equivalent
of n-BuLi was necessary to obtain complete ortho-lithi-
ation. Quenching experiments with aliquots of the reac-
tion mixture confirmed that quantitative formation of
either 2b-D (D2O added) or 2b-SMe (MeSꢀSMe added)
had occurred. Also changing the lithiating agent to
t-BuLi, which is much more reactive and sterically
significantly more demanding than the n-Bu group in
n-BuLi [28], did not drive the reaction beyond 50%
conversion, i.e. again two mole equivalents of t-BuLi
were necessary to achieve complete litihiation. This
pointed to the formation of similar mixed aryl–alkyl
lithium aggregates, i.e. [(S,S-2b-Li)(t-BuLi)]. Recent
studies of these reactions showed that the reaction of 2b
with one mole equivalent of n-BuLi stops at the stage
of a stable mixed aryl–alkyl lithium dimer [(2b-
Li)2(BuLi)2] (see Fig. 2 for the molecular structures in
the solid state). The two NCN ligands are each bridging
between two Li-atoms via 3c–2e bonds, thus resulting
in two [NCN·Li2]+ motifs which are bridged by two
butyl anions in an electron-deficient CLi2 bonding
[18a]. These structural features are remarkable, since
Scheme 1.
much slower (90% consumption of aldehyde after 7
days); moreover, the optical purity of the formed
product mixture was considerably lower than in the
corresponding ethyl case. This is a result of achiral
product formation via the uncatalyzed (non-stereoselec-
tive) reaction, which also promoted the formation of
difficult to separate byproducts, e.g. monoalkylated
diols.
A method, which circumvents these limitations, is the
borane-mediated reduction of ketones using an opti-
cally active b-amino alcohol as chiral auxiliary (CBS
reduction) [24]. Incubation of BH3 with S-diphenyl
prolinol afforded a chiral oxazoborolidine which was
used for the asymmetric reduction of 1,3-diacetyl ben-
zene, giving the diol R,R-1a in high chemical yield
(92%) and optical purity (\90% of the enantiopure
diasteroisomer) [25]. Application of the same procedure
starting from 1,3-dipropionyl benzene afforded the de-
sired ethyl-substituted diol R,R-1b in slightly lower
optical purity in a typical 85% yield (Scheme 2). The
absolute configuration of the diols was determined by
chiral HPLC and also by comparison of the specific
rotation of the product mixtures with reported data
Scheme 2.

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M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
Scheme 3.
Fig. 2. Lithium adduct formation of NCN pincer ligands in dependence of the a-alkyl substituents: (a) crystal structure of (2a-Li)2, an aryllithium
dimer containing a-methyl substituents; and (b) crystal structure of the lithiated, a-ethyl functionalized ligand 2b showing the formation of a mixed
aryl–alkyl lithium aggregate [(2b-Li)2(n-BuLi)2]. Note that the crystal, obtained from racemic solutions, comprises one pincer ligand in the S,S-
and the other in the R,R-configuration.
the Li(NCN) compounds form typically dinuclear
[Li(NCN)]2 lithium aggregates, both in solution and in
the solid state [29]. Similar aggregation of 2b-Li was
anticipated but appeared to give tetranuclear lithium
aggregates even when t-BuLi was used. Apparently, the
steric demand of the a-alkyl substituents dictates the
aggregation process, since dimer formation as observed
for the a-methyl ligand is obviously prevented by the
larger a-ethyl substituents, which is elevated in the
[(NCNꢀLi)2(RLi)2] structure by the incorporation of
bridging alkyl groups.
For obvious reasons, it was essential to remove the
t-BuLi species in [(2b-Li)2(t-BuLi)2] prior to attempting
any transmetalation. Interestingly, Me3SiCl was found
to react selectively with the t-BuLi groups of the te-
tranuclear aggregates, thus leaving the [Li(NCN)] moi-
eties unaffected [30]. Apparently, the two bulky
ortho-aminomethyl substituents in [Li(NCN)] shield the
CꢀLi bond efficiently (cf. the quantitative reaction of
Me3SiCl with LiPh). Moreover, separate experiments
have shown that the silicon center in Me3SiCl has too
low a electrophilicity to react with [Li(NCN)]2. [31] It is
important to note that the resulting NCNꢀLi reagent
has the composition [(2b-Li)2(LiCl)2] and has structural
features that are most probably similar to those ob-
served in [(2b-Li)2(BuLi)2] (Fig. 2) but with bridging
chloride ligands instead of butyl groups.
Following this BuLi–Me3SiCl protocol, alkyl
lithium-free S,S-2b-Li·LiCl was successfully transmeta-
lated with [PtCl2(SEt2)2] to give the chiral platinum(II)
complex S,S-3b in good yield (Scheme 3). Notably, the
reaction time for complete platination was much longer
(4 days) than is required for the formation of the
corresponding methyl analog S,S-3a (1 day) [18b]. This
is another indication for the way metal binding and
reactivity of a metal center in the cavity of these pincer
ligands can be engineered by small steric modifications
in the ligand array.
1
Both the H and 13C{1H}-NMR spectra of enantiop-
ure S,S-3a show the NMe2 groups as two well-resolved
3
3
signals (lH=3.14, JHPt=36.6 Hz, lH=2.81, JHPt
=
42.4 Hz; lC=51.9 and 47.0). Diastereotopicity of these
groups points to a rigid conformation of the metallacy-
cles, where PtꢀN bond dissociation is negligible and

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
275
The corresponding a-ethyl analog, S,S-3b displayed
similar spectroscopic properties. Rigid conformation of
the benzylic groups, i.e. a frozen wagging of the metal-
lacycles, was clearly indicated by 1H-NMR spec-
troscopy, since the benzylic protons appeared as
doublet of doublets, owing to different and well-defined
vicinal coupling constants with the two CH2 protons of
the ethyl substituent (3JHH=8.7 and 3.9 Hz). This
unambiguously implies a fixed conformation of the
metallacycles. In addition, the absolute stereochemistry
of S,S-3b was assessed by single crystal X-ray analysis
(Fig. 3) and by determining the specific optical rotation.
These results underline the stereoselectivity of the syn-
thetic protocol, i.e. the R,R-enantiomer of the benzylic
alcohol as the product of the CBS reduction and inver-
sion of the stereocenters upon nucleophilic substitution
of the mesylate by HNMe2. Comparison of the molecu-
lar structure of S,S-3b (Fig. 2) and the earlier reported
structure of R,R-3b [18b] does not show significant
differences, and bond lengths and angles as well as the
unit cell parameters of the two enantiopairs are identi-
cal within standard deviations.
Fig. 3. Perspective view (ORTEP, 50% probability) of the C2-symmet-
,
ric arylplatinum complex S,S-3b. Pertinent bond lengths (A) and
angles (°) are: PtꢀCl 2.4165(16), PtꢀC(1) 1.927(5), PtꢀN(1) 2.099(5),
PtꢀN(2) 2.088(5); ClꢀPtꢀC(1) 178.57(18), N(1)ꢀPtꢀN(2) 162.1(2),
C(1)ꢀPtꢀN(1) 80.8(2), C(1)ꢀPtꢀN(2) 81.4(2), ClꢀPtꢀN(1) 99.74(14),
ClꢀPtꢀN(2) 98.06(15).
puckering processes are slow on the NMR time scale,
cf. the different chemical shifts and HꢀPt coupling
constants of the axially and equatorially positioned
methyl groups and benzylic protons.
The NMR spectra of the diastereoisomeric mixture
of 3a showed two qualitatively similar, well-resolved
signal patterns, one for each enantiopair [32]. The
spectroscopic characteristics of R,S:SR-3a were conve-
niently extracted by subtracting the set of signals iden-
tified as S,S-3a. Only small shift differences were
observed (i.e. ꢀlRR:SS−lRS:SRꢀB0.1 ppm), which were
most pronounced for the two singlets assigned to the
diastereotopic NMe2 groups: in R,S:SR-3a they are
located at lH=3.07 and 2.85, and are separated by
only 0.22 ppm (cf. 0.33 ppm in S,S-3a). Moreover, the
coupling constants of these two resonances to platinum
are equal (3JHPt=39.6 Hz for both signals), which
points to a less pronounced axial–equatorial orienta-
tion of the nitrogen-bound methyl groups than in the
C2-symmetric complex, thus approaching a conforma-
tion with mirror-plane symmetry.
2.3. Functionalized C2-symmetric pincer ligands
Recent work in connection with our interest in nano-
size (multisite) immobilized (grafted) homogeneous cat-
alysts [19a,33] has shown that a hydroxy group on the
pincer arene moiety in para position with respect to the
MꢀCaryl bond is a suitable anchoring point to tether
NCN–metal catalysts to dendritic peripheries and focal
points of dendritic wedges [33e,34]. In addition, such
hydroxy groups in combination with metal-bound chlo-
ride ions have been shown to provide a particularly
attractive synthon for hydrogen bonding and crystal
engineering [35].
The synthesis of the oxo-functionalized precursor 7 is
shown in Scheme 4 and starts from 5-siloxyisophthalic
Scheme 4.

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M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
Scheme 5.
acid 4 containing a silyl-protected phenol functional
group as a potential linker unit to a tether (Scheme 4).
The acid functionalities in 4 were selectively converted
into methyl ketones by treatment with an excess of
MeLi without affecting the silyl-protecting group [36].
Subsequent CBS-type oxazoborolidine-mediated reduc-
tion of the diketone 5 using identical conditions as for
the preparation of 1 afforded the C2-symmetric diol
R,R-6a in high optical purity (typically 90% of the
desired stereoisomer). Residual undesired products
from partial reduction as well as minor quantities of
meso-6a (significantly less than 10%) were at this stage
conveniently separated by column chromatography.
Diastereoisomeric mixtures of rac/meso-6 were pre-
pared from the readily accessible primary diol 8 by
oxidation with pyridinium chlorochromate [37] and
subsequent reductive alkylation using EtMgBr or
MeMgI, with retention of the silyl-protecting group on
the phenol. Stereoselective amination, performed fol-
lowing the MsCl–HNMe2 protocol, finally gave S,S-7a
and rac/meso-7, respectively.
7a (Scheme 5) [38]. The second signal pattern is charac-
terized by two singlets in the aromatic region. More-
over, the proton resonances due to the SiMe2 group are
located at lH=0.40, and are shifted considerably
downfield (cf. lH=0.18 in 7a), pointing to a rearrange-
ment of the silyl-protecting group. Lower symmetry
than the parent species 7a was also indicated, e.g. by
two inequivalent benzylic protons and nitrogen-bound
methyl groups. All these data are in good agreement
with the formation of phenol 10-D, which results from
oxygen-to-arene migration of the silyl group (Scheme
5). This migration can be explained by lithiation in
ortho,ortho% position [38], because of competitive direct-
ing properties of the O- and N-heteroatom-containing
aryl substituents. This lithiation is followed by a fast
retro-[1,2]-Brook rearrangement, involving sequential
SiꢀO bond cleavage and SiꢀCaryl bond formation (Eq.
(1)) [39]. The product 10-D showed a diagnostic
downfield shift of the singlet of the silicon-bound
methyl protons. This resonance provided a sensitive
probe for the products formed by a retro-Brook
rearrangement.
2.4. Lithiation of oxo-functionalized chiral pincer
ligands
Surprisingly, following the usually successful reaction
protocol (viz. n-BuLi in hexane), lithiation of the oxo-
functionalized bisaminoarene rac/meso-7a did not oc-
(1)
The choice of solvent and lithiating agent had a
pronounced influence on the regioselectivity of the lithi-
ation of 7a. The use of more reactive t-BuLi instead of
n-BuLi promoted the formation of 10-D rather than
that of 7a-D (after D2O quench, molar ratio 6:1).
Similar effects were observed when the solvent was
changed from hexane to coordinating solvents such as
Et2O, which enhanced the selectivity for 10-D (molar
ratio 11:2). These results point to a kinetically con-
cur
regioselectively.
According
to
1H-NMR
spectroscopy on a sample obtained from D2O quench-
ing experiments of the reaction mixture, which showed
two sets of clearly resolved resonance patterns in a 1:1
molar ratio, two different products were formed during
lithiation. One of these resonance patterns was assigned
to 7a-D, as all signals were identical to those of rac/
meso-7a except for the absence of the resonance at 6.78
ppm due to the proton in mutual ortho,ortho position in

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
277
trolled product distribution and clearly reveal that lithi-
2.5. Brominated ligand precursors
ation of 7 is predominantly directed by oxygen lone
pairs — either intramolecularly present or originating
from the solvent — rather than by tertiary amines
[40], in spite of the steric shielding of the silyl sub-
stituent in, e.g. Ar-O-SiMe2tBu. This is corroborated by
earlier results from studies of the selective lithiation of
NC(H)N (i.e. the achiral version of 2; see Scheme 2,
RꢁH) [41]. Significant lithiation of the thermodynami-
cally less stabilized ortho,ortho% position was noted in
Et2O, whereas in hexane intramolecular nitrogen-assis-
tance by both CH2NMe2 groups directed the regioselec-
tivity, hence giving ortho,ortho-lithiation exclusively.
Obviously, the two lithiated species (Scheme 5) do not
equilibrate and an (in)direct interconversion of the lithi-
ated precursors of 7a-D and 10-D does apparently not
occur [42].
These results prompted us to also reinvestigate the
lithiation of NC(H)N-OSiMe2tBu, i.e. the achiral
analog of 7 (Scheme 4, RꢁH) in hexane [27b]. The
NMR spectrum of samples after quenching of the
reaction mixture with D2O pointed to the presence of
ca. 10% of products originating from a retro-[1,2]-
Brook rearrangement [43]. This is significantly lesser
than that observed during lithiation of 7a under similar
conditions. Obviously, the introduction of substituents
at the benzylic carbon restricts the population of the
Since the selective lithiation of the desired ortho,ortho
position in 7a via H/Li exchange failed, the Br/Li
exchange route was attempted [44]. To this end, a
bromide substituent had to be introduced first in the
designated ipso position. This was achieved by regiose-
lective bromination of diol R,R-6a with pyrH-Br3 and
catalytic amounts of iron (Scheme 6) [45]. The corre-
sponding diol R,R-11 was obtained in moderate yields
(54%), since slow SiꢀO bond cleavage was observed
during the bromination, presumably due to dissolved
HBr. Purification of R,R-11 from undesired side prod-
ucts by column chromatography was necessary prior to
nucleophilic amination (vide supra) [46].
The bis-aminoarene S,S-13 was lithiated at low tem-
perature (−80°C) using two equivalents of t-BuLi in
hexane. According to 1H-NMR spectroscopy of an
aliquot of the reaction mixture that was quenched with
D2O, the Li/Br exchange was complete after 30 min
reaction time. No traces of products raising from lithia-
tion at other positions could be detected. When the
reaction mixture was kept at RT for 2 days, a new
product was formed in minor quantities (less than
10%), which was characterized (after D2O quench) by a
set of proton resonances that was qualitatively related
to that of 7a-D. However, the diagnostic signal for the
SiMe2 group at 0.40 ppm and the conservation of the
overall molecular symmetry (no additional resonances
or couplings were observed) strongly point to partial
formation of a product originating from a retro-[1,4]-
Caryl–Cbenzyl rotamers and consequently decreases the
required conformational flexibility of the nitrogen lone
pairs to become involved in the regioselective lithiation
process.
Scheme 6.

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M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
ent metal-bound halides [34] and suggested the forma-
tion of the chloride complex S,S-14, and the
corresponding bromide complex S,S-15 (Scheme 7).
Indeed, addition of excess NaBr to an acetone solution
of the crude product mixture gave S,S-15 as the single
platinum complex. The same result was obtained when
LiBr was added during the transmetalation reaction.
The driving force for this conversion was attributed to
the better solubility properties of LiBr in Et2O com-
pared to that of the formed LiCl. Accordingly, the
presence of LiI, which is even better soluble in Et2O,
quantitatively generated the iodide complex S,S-16
(Scheme 7). Obviously, the reverse process, i.e. the
exchange of a platinum-bound iodide by chloride (or
bromide) required a different procedure. Halide ab-
straction by addition of stoichiometric amounts of
AgBF4 in wet acetone resulted in instantaneous precipi-
tation of the silver salt AgX and formation of a cationic
platinum aqua complex, which was converted into the
desired neutral species 14, 15 or 16 by treatment with
the corresponding halide anion X−, e.g. as aqueous
NaX solution (Scheme 8).
Scheme 7.
1
The two NMe2 resonances (vide supra) in the H-
NMR spectra are diagnostic for the determination of
the type of halide present [34], since a systematic
downfield shift was observed for these signals in the
series Cl to Br to I (Table 1). A similar trend was
established for the 13C chemical shift values of these
groups. Remarkably, the benzylic carbons showed an
opposite behavior and the resonances shifted systemati-
cally to higher field.
Scheme 8.
Table 1
The presence of a reactive CꢀBr bond in S,S-13 also
provides access to the alternative preparation of S,S-15
via oxidative addition with an equimolar amount of
[Pt(tol-4)2(SEt2)]2 in benzene at reflux temperature (Eq.
(2)) [48]. Residual bitolyl was removed either by re-
peated recrystallization from CH2Cl2–pentane mixtures
or by gradient column chromatography on silica. Obvi-
ously, this oxidative addition route does not suffer from
the halide scrambling as observed during the transmeta-
lation reaction. The fact that no lithium intermediates
are required might become particularly relevant for the
construction of multifunctional species, for example
nanosize metallodendrimers, containing labile groups
that are sensitive towards strong nucleophiles such as
carbanions.
Selected NMR spectroscopic data for the chiral complexes 14–16
Complex
X
1H-NMR
13C-NMR
ArCHN NMe2
ArCHN NMe2
S,S-14
S,S-15
S,S-16
Cl 4.11
Br 4.14
3.07, 2.75
3.18, 2.84
3.26, 2.91
78.8
78.5
78.1
52.0, 47.2
53.1, 47.7
55.1, 49.0
I
4.10
Brook rearrangement [39,47]. These results emphasize
that the LiꢀCaryl bond in complexes such as 13-Li is
inert to migratory processes, since no residues due to,
e.g. ortho,ortho%-lithiation [38] and subsequent retro-
[1,2]-Brook rearrangement were observed even after
prolonged reaction times.
2.6. Tethered chiral organoplatinum complexes
(2)
Transmetalation of the lithium complex S,S-13-Li,
obtained from selective Br/Li exchange, with
[PtCl2(SEt2)2] gave a mixture of two related platinum
complexes (NMR). The observed chemical shift differ-
ences for the NMe2 resonances of these compounds
were typical for platinum complexes containing differ-
Deprotection of the phenol by fluoride-mediated se-
lective SiꢀO bond cleavage was accomplished in high
yields with Bu4NF followed by a few drops of water

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
279
1
(Scheme 8) [49]. The pertinent H-NMR data of S,S-17
(lH=4.17 for the benzylic ArCHN protons; lH=3.12
and 2.81 with 3JHPt=34.9 and 41.6 Hz, respectively, for
the diastereotopic NMe2 groups; absence of any reso-
nance upfield from 2 ppm) confirmed clean removal of
the SiMe2-t-Bu group without affecting the organoplat-
inum site. Interestingly, complex 17 is soluble in aro-
matic solvents (benzene, toluene; solubility \10 g l−1),
which is in marked contrast to the solubility properties
of the parent achiral complex [PtCl(NCNꢀOH)] with-
out a-methyl substituents [34]. The latter self-assembles
in the solid state and in solution via intermolecular
Pt−Cl···H−O hydrogen bonding to a-type networks
[35]. In contrast, IR spectroscopy (CHCl3 or KBr) on
S,S-17 only showed vibrations for the OꢀH bond that
are in the range for free phenolic groups. No signals
were observed, however, in the characteristic region
that would indicate hydrogen-bond association. Obvi-
ously, the a-methyl substituents not only enhance the
solubility but also restrict the hydrogen-bond-mediated
self-assembly. The excellent solubility of S,S-17 is an
unexpected advantage when using the phenolic func-
tionality for the attachment to soluble supports via a
tether. In the case of the achiral analog [Pt-
Cl(NCNꢀOH)], its low solubility slows down most cou-
pling reactions and therefore enhances the probability
of formation of undesired byproducts.
gradient column chromatography. The pure aqua com-
plex S,S-19 was prepared in situ by AgBF4-mediated
halide abstraction and then used as a catalyst in the
palladium-catalyzed aldol condensation reaction of
benzaldehyde with methyl isocyanoacetate [50]. Moni-
toring of the reaction progress by GC indicated that the
catalytic performance (turnover number and frequency)
compares well with those of related achiral catalysts
and is not significantly influenced by the a-methyl
substituents in the organopalladium complex [50].
Analysis of the product solution by NMR [12] however
revealed only 12% ee (trans isomer).
The synthesis of the enantiopure a-ethyl substituted
PCHP pincer ligand S,S-20b followed a modified pro-
cedure which was developed by Zhang et al. [12b] for
the corresponding a-methyl ligand precursor S,S-20a
(Scheme 10). The detailed preparation and characteri-
zation have been reported elsewhere [17]. Acidic re-
moval (HBF4) of the borane-protecting groups afforded
the enantiopure PCHP ligands, which were converted in
situ to the corresponding ruthenium complexes S,S-21
by using a transcyclometalation protocol as established
recently [17,51]. Notably, the formation of S,S-21b was
considerably slower than the transcycloruthenation of
the corresponding methylated analog. This is another
indication for the high sensitivity of the metal binding
pocket (Fig. 1) towards small ligand modifications.
Application of the chiral ruthenium complex as cata-
lyst in the hydrogen transfer reaction of acetophenone
and i-PrOH revealed a very high catalytic activity
similar to that observed for the achiral analog [5].
However, analysis of the product solution by chiral
HPLC showed that the asymmetric induction of the
catalyst is very poor (14% ee).
2.7. Chiral ruthenium(II) and palladium(II) catalysts
An analogous C2-symmetric arylpalladium(II) com-
plex S,S-18 was prepared by oxidative addition of the
aryl bromide S,S-13 to [Pd(dba)2] (Scheme 9). Due to
the stability of NCN–palladium complexes towards
silica, residual dba could be removed from S,S-18 by
Scheme 9.
Scheme 10.

280
M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
3. Conclusions
spectrometer operating at 300 and 75 MHz, respec-
tively. Spectra were obtained in CDCl3 solution at
room temperature, unless specified otherwise, and were
referenced to external SiMe4 (l=0.00 ppm, J in Hz).
Melting points are uncorrected. Optical rotations (hD)
were measured at 22°C on a Perkin–Elmer 241 polar-
imeter. The optical purity of new organic compounds
was determined by chiral HPLC (chiracel OD column,
hexane–i-PrOH (2%) as eluent, UV–vis detection),
those of known compounds were determined by com-
parison of the observed values with those of authentic
samples. Standard procedures have been used for the
catalytic reductive alkylation [23], the palladium-cata-
lyzed aldol condensation [50] and the ruthenium-cata-
lyzed hydrogen transfer reaction [5]. Most of the
non-stereoselective syntheses were carried out as de-
scribed in detail for the corresponding enantiopure
compounds and are therefore not mentioned separately.
The cavity for metal binding of NCN pincer ligands
was tuned by (enantioselective) ligand functionalization
of the benzylic positions. For example, lithiation of
a-methyl substituted pincer ligands results in dimer
formation, similar to non-functionalized pincer ligands,
while a-ethyl groups prevent direct dimerization and
promoted the formation of mixed alkyl–aryl lithium
aggregates. The pronounced effect of a-alkyl substitu-
tion on the accessibility of the ipso carbon is also
reflected by the reaction rates observed for direct cy-
clometalation of PCP pincer ligands or transmetalation
of the NCN–lithium complexes by platinum which are
considerably lower for ethyl- than for methyl-function-
alized pincer ligands.
Preliminary solid state studies on the enantiopure
platinum complex S,S-17 revealed no indication for a
hydrogen-bond-mediated self-assembly of these com-
plexes and no NLO activity was detected with the
material investigated thus far. Moreover, catalytic ap-
plication of the enantiopure complexes S,S-19 and
S,S-21 in the palladium-catalyzed aldol condensation
and the ruthenium-catalyzed hydrogen transfer reac-
tion, respectively, did not show significant asymmetric
induction on product formation (eeB15% in either
reaction). Apparently, benzylic substitution of the pin-
cer ligand affects the metal binding cavity and hence
the accessibility of the ipso carbon, but not the metal
environment, since the chiral pocket is too large for
efficient asymmetric catalysis. Transfer of the chiral
information into closer proximity of the metal center,
i.e. on the nitrogen or phosphorus donor atoms, is
expected to improve the asymmetric induction for
catalysis.
4.2. Syntheses
4.2.1. S,S-C6H4[CH(Me)NMe2]2-1,3 (S,S-2a)
A solution of R,R-1a (2.56 g, 15 mmol) and NEt3
(5.4 ml, 38 mol) were dissolved in CH2Cl2 (30 ml) and
cooled to −80°C. MsCl (3.0 ml, 38 mmol) was added
dropwise within 20 min and the mixture was stirred at
low temperature for 20 min. Subsequently, HNMe2 (10
ml, large excess) was added and then left overnight to
warm up to room temperature. The formed suspension
was poured onto an aqueous HCl solution (2 M, 50 ml)
and extracted. The aqueous layer was washed with
EtO2 (2×30 ml) and made basic (solid KOH, pH\
12). The product was extracted with pentane (3×40
ml) and the combined organic layers were washed with
brine and dried over Na2SO4. Removal of the solvent in
vacuo yielded crude 2a, which was further purified by
bulb-to-bulb destillation. This afforded 2.88 g (85%) of
S,S-2a as a colorless oil.
4. Experimental
1H-NMR: l=7.27–7.14 (m, 4H, ArH), 3.24 (q,
3JHH=7.4 Hz, 2H, ArCHN), 2.09 (s, 12H, NMe2), 1.36
(d, 3JHH=7.4 Hz, 6H, ArCHCH3); 13C{1H}-NMR:
l=143.7 (CarylꢀC), 127.8 (CarylꢀH), 126.5 (CarylꢀH),
125.8 (CarylꢀH), 65.8 (ArCHN), 43.0 (NMe2), 20.2
(ArCHCH3); [h]D= −95.0° (c=0.39, pentane); Anal.
Found: C, 76.22; H, 11.09; N, 12.79. Calc. for
C14H24N2: C, 76.31; H, 10.98; N, 12.71%.
4.1. General
All reactions involving organolithium reagents were
performed using standard Schlenk techniques unless
stated otherwise. Benzene, pentane, THF, and Et2O
were distilled from Na-benzophenone, CH2Cl2 and
NEt3 from CaH2. HNMe2 was freshly distilled and
condensed directly into the reaction vessel. The synthe-
ses of the diols R,R-C6H4[CHR(OH)]2-1,3 (1 and 8)
[22a,25–27], the acid t-BuMe2SiO-C6H3(COOH)2-3,5
(4) [52], the metal precursors [PtCl2(SEt2)2] [34], [Pt(tol-
4)2(SEt2)]2 [48], and [Pd(dba)2] [53] are described else-
where. All other reagents were obtained commercially
and were used without further purification. Elemental
analyses were performed by Kolbe, Mikroanalytisches
4.2.2. S,S-C6H4[CH(Et)NMe2]2-1,3 (S,S-2b)
The procedure was analogous to the preparation of
2a, starting from R,R-1b (5.16 g, 26 mmol), NEt3 (9.0
ml, 65 mmol), MsCl (5.0 ml, 65 mmol) and HNMe2 (30
ml, large excess). After bulb-to-bulb distillation, 4.46 g
(68%) of S,S-2b were isolated as colorless, air-sensitive
oil.
1
3
Laboratorium (Mu¨lheim, Germany). H and 13C{1H}-
1H-NMR: l=7.23 (t, JHH=7 Hz, 1H, ArH), 7.09–
NMR spectra were recorded on a Varian Inova 300
7.05 (m, 3H, ArH), 3.03–2.98 (m, 2H, ArCHN), 2.15

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
281
(s, 12H, NMe2), 1.96–1.64 (m, 4H, CH2CH3), 0.68 (t,
3JHH=7.4 Hz, 6H, CH2CH3); 13C{1H}-NMR: l=
140.3 (CarylꢀC), 128.8 (CarylꢀH), 127.6 (CarylꢀH), 127.2
(CarylꢀH), 72.7 (ArCHN), 43.1 (NMe2), 26.3 (CH2CH3),
10.8 (CH2CH3); [h]D= −40.4° (c=3.0, pentane);
Anal. Found: C, 77.22; H, 11.26; N, 11.15. Calc. for
C16H28N2: C, 77.36; H, 11.36; N, 11.28%.
with a second amount of t-BuLi (2.75 ml, 4.0 mmol).
After stirring for 24 h, Me3SiCl (1.5 ml, 12 mmol) was
added within 5 min and stirring continued for 6 h,
during which a white precipitate formed. All volatiles
were then removed in vacuo, and the residue was
redissolved in EtO2 (30 ml) and [PtCl2(SEt2)2] (2.01 g,
4.5 mmol) was added. This mixture was stirred at room
temperature for 4 days. The volatiles were removed in
vacuo and the residue was washed with pentane (3×40
ml) and extracted with CH2Cl2 (3×20 ml). The com-
bined extracts were evaporated to dryness at the brown
residue was purified by column chromatography (SiO2;
acetone–CHCl3 1:2) and recrystallized from CH2Cl2–
pentane to give S,S-3b as a colorless solid. Yield: 1.15
g, 60%.
4.2.3. S,S-[PtCl{C6H3{CH(Me)NMe2}-2,6)] (S,S-3a)
At −80°C, n-BuLi (3.0 ml, 1.5 M in pentane, 4.5
mmol) was added to a solution of S,S-2a (1.00 g, 4.5
mmol) in pentane (30 ml). The solution was stirred for
12 h, while the temperature was allowed to increase to
ambient temperature. All volatiles were removed in
vacuo and the residue was redissolved in Et2O (40 ml).
To this solution was added [PtCl2(SEt2)2] (2.01 g, 4.5
mmol) and the mixture was stirred at room temperature
over 4 days, during which a precipitate had formed.
The volatiles were removed in vacuo and the residue
was washed with hexane (3×40 ml) and extracted with
CH2Cl2 (3×20 ml). The product was filtered through a
short pad of SiO2 (CH2Cl2–acetone, 3:1), and recrystal-
lized from toluene–pentane to give S,S-3a as a color-
less crystalline solid. Yield: 1.58 g, 78%.
3
1H-NMR: l=6.94 (t, JHH=7.2 Hz, 1H, ArH), 6.78
3
3
(d, JHH=7.2 Hz, 2H, ArH), 3.58 (dd, JHH=3.9 Hz,
3JHH=8.7 Hz, JHPt=73.5 Hz, 2H, ArCHN), 3.00 (s,
3
3JHPt=44.5 Hz, 6H, NMeCH3), 2.92 (s, 3JHPt=32.8
Hz, 6H, NCH3Me), 2.05–1.80 (m, 4H, CH2CH3), 0.97
3
(t, JHH=7.2 Hz, 6H, CH2CH3); 13C{1H}-NMR: l=
146.1 (CarylꢀC), 145.7 (CarylꢀPt), 121.7 (CarylꢀH), 121.0
(3JCPt=35.1 Hz, CarylꢀH), 85.5 (2JCPt=52.2 Hz,
ArCHN), 55.4 (2JCPt=18, NMeCH3), 48.6 (2JCPt=16,
NCH3Me), 26.1 (CH2CH3), 10.4 (CH2CH3); [h]D= +
106.4°, (c=2.0, CH2Cl2); Anal. Found: C, 40.11; H,
5.63; N, 5.81. Calc. for C16H27ClN2Pt: C, 40.21; H,
5.69; N, 5.81%.
3
1H-NMR: l=7.02 (t, JHH=7.5 Hz, 1H, ArH), 6.68
3
3
(d, JHH=7.5 Hz, 2H, ArH), 4.21 (q, JHH=6.9 Hz,
3
2H, ArCHN), 3.14 (s, JHPt=36.0 Hz, 6H, NMeCH3),
2.81 (s, 3JHPt=42.4 Hz, 6H, NCH3Me), 1.39 (d,
3JHH=6.9 Hz, 6H, ArCHCH3); 13C{1H}-NMR: l=
146.9 (CarylꢀC), 145.5 (CarylꢀPt), 122.9 (CarylꢀH), 120.6
(3JCPt=40.8 Hz, CarylꢀH), 78.8 (2JCPt=63.3 Hz,
ArCHN), 51.9 (2JCPt=18.9 Hz, NMeCH3), 47.0
(2JCPt=19.9 Hz, NCH3Me), 12.9 (3JCPt=31.7 Hz,
ArCHCH3); [h]D= −57.1° (c=1.3, CH2Cl2); Anal.
Found: C, 37.29; H, 5.22; N, 6.18. Calc. for
C14H23ClN2Pt: C, 37.38; H, 5.15; N, 6.23%.
4.2.5. t-BuMe2SiO-C6H3(CHO)2-3,5 (9)
The diol 8 (4.0 g, 14.9 mmol) was dissolved in
CH2Cl2 (80 ml) and added to a suspension of pyri-
dinium chlorochromate (9.64 g, 44.0 mmol) in CH2Cl2
(20 ml). The mixture was stirred at RT for 2 h. The
solution was removed from the solids by decantation
and the solvent removed in vacuo. All the solids were
washed with Et2O (5×80 ml). The washings were
passed through a short column of Fluorosil. The sol-
vents were removed in vacuo to give the product as a
colorless oil (3.86 g, 98%), which solidified at −20°C.
1H-NMR: l=10.04 (s, 2H, CHO), 7.97 (t, 1H,
4JHH=1.4 Hz, ArH), 7.58 (d, 4JHH=1.4 Hz, 2H,
ArH), 1.00 (s, 9H, SiCMe3), 0.25 (s, 6H, SiCH3);
13C{1H}-NMR: l=190.9 (CHO), 157.3 (CarylꢀO),
138.5 (CarylꢀC), 125.4 (CarylꢀH), 124.7 (CarylꢀH), 25.5
(SiCMe3), 18.2 (SiCMe3), −4.4 (SiCH3); Anal. Found:
C, 63.46; H, 7.73. Calc. for C14H20O3Si: C, 63.60; H,
7.62%.
1
Spectroscopic data of R,S:SR-3a: H-NMR: l=6.98
3
3
(t, JHH=7.5 Hz, 1H, ArH), 6.69 (d, JHH=7.5 Hz,
3
3
2H, ArH), 4.03 (q, JHH=6.6 Hz, JHPt=35.7 Hz, 2H,
3
ArCHN), 3.07 (s, JHPt=39.3 Hz, 6H, NMeCH3), 2.85
3
3
(s, JHPt=39.6 Hz, 6H, NCH3Me), 1.40 (d, JHH=6.9
Hz, 6H, ArCHCH3); 13C{1H}-NMR: l=147.8
(CarylꢀC), 144.5 (CarylꢀPt), 122.8 (CarylꢀH), 120.2
(3JCPt=38.6 Hz, CarylꢀH), 79.4 (2JCPt=61.9 Hz,
ArCHN), 53.1 (2JCPt=17.7 Hz, NMeCH3), 47.6
(2JCPt=17.7 Hz, NCH3Me), 16.1 (3JCPt=24.3 Hz,
ArCHCH3).
4.2.4. S,S-[PtCl(C6H3{CH(Et)NMe2}2-2,6)] (S,S-3b)
[18b]
4.2.6. t-BuMe2SiO-C6H3[C(ꢁO)Me]2-3,5 (5)
To a solution of S,S-2b (1.00 g, 4.0 mmol) in pentane
(30 ml) was added t-BuLi (2.75 ml, 1.5 M in pentane,
4.0 mmol) at −80°C. The solution was stirred for 16 h,
while the temperature was allowed to raise to room
temperature. At this stage, 1H-NMR indicated only
45% lithiation and therefore, the mixture was treated
MeLi (60 ml, 96 mmol) was added within 1 h to the
diacid 4 (5.95 g, 20 mmol) dissolved in THF (200 ml).
The suspension was stirred for 4 h at reflux temperature
and then poured onto a cold solution of NH4Cl (1 M,
300 ml) under vigorous stirring. EtO2 (50 ml) was
added and the phases were separated. The inorganic

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M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
phase was extracted with EtO2 (2×100 ml) and the
combined organic layers were washed with brine, dried
(Na2SO4) and evaporated to dryness. The residue was
purified by column chromatography (SiO2, hexane–
Et2O 3:1), thus affording analytically pure 5 as a white
solid (3.40 g, 58%).
(CarylꢀC), 116.7 (CarylꢀH), 116.6 (CarylꢀH), 75.8 (ArCH-
O), 31.8 (CH2CH3), 25.7 (SiCMe3), 18.2 (SiCMe3),10.1
(CH2CH3), −4.4 (SiCH3); Anal. Found: C, 66.47; H,
10.08. Calc. for C18H32O3Si: C, 66.62; H, 9.94%.
4.2.9. rac/meso-t-BuMe2SiO-C6H3[CH(Me)NMe2]2-3,5
(rac/meso-7a)
4
1H-NMR: l=8.10 (t, 1H, JHH=1.5 Hz, ArH), 7.59
4
(d, JHH=1.5 Hz, 2H, ArH), 2.62 (s, 6H, C(O)CH3),
The procedure was analogous to the preparation of
2a, starting from rac/meso-6a (0.76 g, 2.6 mmol) in
CH2Cl2 (15 ml), NEt3 (0.84 ml, 6.0 mmol), MeCl (0.46
ml, 6.0 mmol) and HNMe2 (10 ml, large excess). The
resulting suspension was poured onto NaOH (1 N, 50
ml) and extracted with pentane (3×40 ml). After dry-
ing the combined organic layers over Na2SO4 and
evaporation of all volatiles resulted 0.83 g (91%) of
rac/meso-7a as a yellowish oil.
1.00 (s, 9H, SiCMe3), 0.24 (s, 6H, SiCH3); 13C{1H}-
NMR: l=197.0 (CꢁO), 156.4 (CarylꢀO), 138.8
(CarylꢀC), 123.8 (CarylꢀH), 121.2 (CarylꢀH), 26.8
(C(O)CH3), 25.6 (SiCMe3), 18.2 (SiCMe3), −4.4
(SiCH3); m.p.=55–58°C; Anal. Found: C, 65.65; H,
8.20. Calc. for C16H24O3Si: C, 65.71; H, 8.27%.
4.2.7. R,R-t-BuMe2SiO-C6H3[CH(Me)OH]2-3,5
(R,R-6a)
1H-NMR: l=6.78 (s, 1H, ArH), 6.66 (s, 2H, ArH),
To a solution of BH3·SMe2 (3.5 ml, 2 M in toluene,
7 mmol) was added S-diphenyl prolinol (0.13 g, 0.5
mmol) in THF (3.5 ml). The solution was stirred for 16
h at 45°C in a closed system. After 16 h, a solution of
5 (1.36 g, 4.7 mmol) in THF (9 ml) was added dropwise
over 1 h. The reaction mixture was subsequently stirred
1 h at 45°C and 2 h at RT, then quenched with MeOH
(2 ml, dropwise addition). Stirring was continued for 2
h before all volatiles were evaporated under reduced
pressure. Column chromatography (SiO2; hexane–Et2O
4:1) gave pure R,R-6a as a white solid (0.92 g, 66%).
1H-NMR: l=6.96 (s, 1H, ArH), 6.75 (s, 2H, ArH),
3
3.16 (q, JHH=6.6 Hz, 2H, ArCHN), 2.17 (s, 12 H,
NMe2), 1.34 (d, 3JHH=6.6 Hz, 6H, ArCHCH3), 0.98 (s,
9H, t-SiCMe3), 0.18 (s, 6H, SiCH3); 13C{1H}-NMR:
l=155.3 (CarylꢀO), 145.1 (CarylꢀC), 120.2 (CarylꢀH),
117.8 (CarylꢀH), 65.9 (ArCHN), 43.3 (NMe2), 25.7
(SiCMe3), 20.5 (ArCCH3), 18.2 (SiCMe3), −4.4
(SiCH3); Anal. Found: C, 68.36; H, 11.06; N, 8.03.
Calc. for C20H38N2OSi: C, 68.51; H, 10.92; N 7.99%.
4.2.10. rac/meso-t-BuMe2SiO-C6H3[CH(Et)NMe2]2-3,5
(rac/meso-7b)
3
4.82 (q, JHH=6.4 Hz, 2H, ArCH-O), 2.07 (br s, 2H,
The procedure was analogous to the preparation of
2a, starting from rac/meso-6b (3.13 g, 9.6 mmol) in
CH2Cl2 (30 ml), NEt3 (3.1 ml, 22 mmol), MeCl (1.7 ml,
22 mmol) and HNMe2 (15 ml, large excess). The result-
ing suspension was poured onto NaOH (1 N, 40 ml)
and extracted with pentane (3×40 ml). The combined
organic layers were dried (Na2SO4) and evaporated,
resulting in 3.20 g (88%) of rac/meso-7b as a yellowish
oil.
3
OH), 1.46 (d, JHH=6.4 Hz, 6H, ArCHCH3), 0.99 (s,
9H, SiCMe3), 0.20 (s, 6H, SiCH3); 13C{1H}-NMR: l=
156.0 (CarylꢀO), 147.7 (CarylꢀH), 116.0 (CarylꢀH), 115.3
(CarylꢀC), 70.2 (ArCH-O), 31.7 (ArCHCH3), 25.7
(SiCMe3), 18.2 (SiCMe3), −4.4 (SiCH3); m.p.=92–
94°C; [h]D= +21.1° (c=1.3, THF); Anal. Found: C,
64.76; H, 9.44. Calc. for C16H28O3Si: C, 64.82; H,
9.52%.
1H-NMR: l=6.68 (s, 1H, ArH), 6.62 (s, 2H, ArH),
2.98–2.93 (m, 2H, ArCHN), 2.16 (s, 12 H, NMe2),
2.0–1.6 (m, 4H, CH2CH3), 0.97 (s, 9H, t-SiCMe3), 0.69
4.2.8. rac/meso-t-BuMe2SiO-C6H3[CH(Et)OH]2-3,5
(rac/meso-6b)
To a solution of EtMgBr (60 ml, 1 M in Et2O, 60
mmol) was added dropwise a solution of 3 (5.58 g, 21
mmol) in Et2O (50 ml). The reaction mixture was
stirred 2 h at reflux temperature and then poured onto
aqueous NH4Cl (1 M, 100 ml). After phase separation,
the aqueous layer was extracted with EtO (2×60 ml)
and the combined organic phases were washed with
brine, NaHSO3, H2O, and brine, dried over MgSO4 and
evaporated under reduced pressure to leave rac/meso-
6b as a waxy solid (5.91 g, 86%).
3
(t, JHH=7.2 Hz, 6H, CH2CH3), 0.18 (s, 6H, SiCH3);
13C{1H}-NMR: l=155.1 (CarylꢀO), 141.6 (CarylꢀC),
122.4 (CarylꢀH), 119.1 (CarylꢀH), 72.7 (ArCHN), 43.2
(NMe2), 26.6 (CH2CH3), 25.7 (SiCMe3), 18.2 (SiCMe3),
10.9 (CH2CH3), −4.4 (SiCH3); Anal. Found: C, 69.65;
H, 11.12; N, 7.31. Calc. for C22H42N2OSi: C, 69.78; H,
11.18; N 7.40%.
4.2.11. rac/meso-C6H3OD-[CH(Me)NMe2]2-3,5-
SiMe2t-Bu-2 (rac/meso-10-D)
A solution of rac/meso-7a (0.35 g, 1.0 mmol) in
pentane (8 ml) was cooled to −80°C and treated with
n-BuLi (0.63 ml, 1.6 M in hexane, 1.0 mmol) and
allowed to warm to RT during 4 h. The solution was
1H-NMR: l=6.88 (s, 1H, ArH), 6.72 (s, 2H, ArH),
3
4.52 (t, JHH=6.4 Hz, 2H, ArCH-O), 2.01 (br s, 2H,
OH), 1.82–1.62 (m, 4H, CH2CH3), 0.98 (s, 9H,
3
SiCMe3), 0.89 (t, JHH=7.2 Hz, 6H, CH2CH3), 0.19 (s,
6H, SiCH3); 13C{1H}-NMR: l=155.8 (CarylꢀO), 146.3

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
283
stirred additional 12 h and then quenched with D2O
(0.5 ml, large excess). The suspension was dried over
MgSO4, filtered through Celite and evaporated to dry-
ness to afford a mixture of deuterated rac/meso-7a-D
(vide supra) and rac/meso-10-D.
(15 ml), NEt3 (0.64 ml, 4.6 mmol), MeCl (0.36 ml, 4.6
mmol) and HNMe2 (10 ml, large excess). The resulting
suspension was poured onto NaOH (1 N, 20 ml) and
extracted with pentane (3×20 ml). Drying of the com-
bined organic layers over Na2SO4 and evaporation of
all volatiles resulted in 0.64 g (80%) of S,S-13 as a
colorless waxy solid.
1H-NMR: l=7.06 (d, 4JHH=1.4 Hz, 1H, ArH),
6.59 (d, 4JHH=1.5 Hz, 1H, ArH rac and meso), 3.46 (q,
3
3
3JHH=6.2 Hz, 1H, ArCHN), 3.18 (q, JHH=6.6 Hz,
1H-NMR: l=6.93 (s, 2H, ArH), 3.77 (q, JHH=6.5
1H, ArCHN), 2.21 (s, 6 H, NMe2), 2.18 (s, 6 H, NMe2),
1.35 (d, 3JHH=6.6 Hz, 3H, ArCHCH3), 1.29 (d,
3JHH=6.3 Hz, 3H, ArCHCH3), 0.97 (s, 9H, t-SiCMe3),
0.40 (s, 6H, SiCH3).
Hz, 2H, ArCHN), 2.20 (s, 12H, NMe2), 1.22 (d, 3JHH
=
6.5 Hz, 6H, ArCHCH3), 0.95 (s, 9H, SiCMe3), 0.16 (s,
6H, SiCH3); 13C{1H}-NMR: l=155.1 (CarylꢀO), 145.5
(CarylꢀC), 118.9 (CarylꢀH), 117.0 (CarylꢀBr), 64.4
(ArCHN), 43.5 (NMe2), 25.7 (SiCMe3), 20.1
(ArCHCH3), 18.2 (SiCMe3), −4.5 (SiCH3); [h]D= −
83.4° (c=1.5, acetone); Anal. Found: C, 56.08; H, 8.55;
N, 6.46. Calc. for C20H37BrN2OSi: C, 55.93; H, 8.68; N,
6.52%.
4.2.12. R,R-C6H2Br-[CH(Me)OH]2-2,6-OSiMe2t-Bu-4
(R,R-11)
A mixture of R,R-6a (2.50 g, 8.5 mmol) in CH2Cl2
(40 ml) and pyridinium-Br3 (2.90 g, 9.0 mmol) in
MeOH (10 ml) was stirred in the presence of a catalytic
amount of Fe (0.18 g, 3 mmol) until TLC indicated
complete consumption of the starting material. All
volatiles were removed and the residue was purified by
column chromatography (SiO2; hexane–Et2O 2:1) to
give 1.72 g (54%) R,R-11 as a white solid.
4.2.15. [PtCl(S,S-C6H2-{CH(Me)NMe2}2-2,6-
OSiMe2t-Bu-4)] (S,S-14)
To a solution of S,S-15 (88.5 mg, 0.14 mmol) in
acetone (5 ml) was added AgBF4 (35.0 mg, 0.18 mmol)
and the mixture was stirred for 30 min under exclusion
of light. The formed precipitate was filtered off (Celite)
and all volatiles removed under reduced pressure. The
residue was suspended in CH2Cl2 (15 ml) and brine (3
ml) and rigorously stirred for 30 min and then treated
with MgSO4. After filtration and evaporation of the
volatiles, S,S-14 was obtained as a colorless solid (66.3
mg, 81%).
1H-NMR: l=7.03 (s, 2H, ArH), 5.26 (d of q,
3
3JHH=6.3 Hz, JHH=2.7 Hz, 2H, ArCH-O), 1.89 (d,
3
3JHH=2.7 Hz, 2H, OH), 1.45 (d, JHH=6.3 Hz, 6H,
ArCHCH3), 0.99 (s, 9H, SiCMe3), 0.22 (s, 6H, SiCH3);
13C{1H}-NMR: l=155.9 (CarylꢀO), 146.3 (CarylꢀC),
117.3 (CarylꢀH), 112.0 (CarylꢀBr), 69.2 (ArCH-O), 25.7
(SiCMe3), 23.6 (ArCHCH3), 18.2 (SiCMe3), −4.4
(SiCH3); m.p.=137–140°C; [h]D=37.7° (c=1.3,
CHCl3); Anal. Found: C, 51.11; H, 7.20. Calc. for
C16H27BrO3Si: C, 51.19; H, 7.25%.
3
1H-NMR: l=6.18 (s, 2H, ArH), 4.11 (q, JHH=7.0
Hz, 2H, ArCHN), 3.07 (s, 3JHPt=35.8 Hz, 6H,
NMeCH3), 2.75 (s, 3JHPt=42.8 Hz, 6H, NCH3Me),
3
1.30 (d, JHH=6.6 Hz, 6H, ArCHCH3), 0.93 (s, 9H,
4.2.13. C6H2Br-[C(ꢁO)Me]2-2,6-OSiMe2t-Bu-4 (12)
A solution of rac/meso-11 (1.88 g, 5.0 mmol) was
dissolved in CH2Cl2 (40 ml) and added to a suspension
of pyridinium chlorochromate (2.68 g, 12 mmol) in
CH2Cl2 (20 ml). The mixture was stirred at RT for 4 h,
and then, all volatiles were removed under reduced
pressure. The residue was extracted with Et2O (8×30
ml). The ethereal extracts were passed through a short
column of Fluorosil and evaporated to dryness to give
12 as a colorless solid (1.82 g, 98%).
SiCMe3), 0.12 (s, 6H, SiCH3); 13C{1H}-NMR: l=
152.2 (CarylꢀO), 147.3 (CarylꢀC), 136.2 (CarylꢀPt), 112.8
(CarylꢀH), 78.8 (2JCPt=59.7 Hz, ArCHN), 52.0
(NMeCH3), 47.2 (NCH3Me), 25.6 (SiCMe3), 18.0
(SiCMe3), 13.0 (3JCPt=31.5 Hz, ArCHCH3), −4.5
(SiCH3); [h]D= −66.3° (c=1.0, CH2Cl2); Anal.
Found: C, 41.28; H, 6.36; N, 4.82. Calc. for
C20H37ClN2OSiPt: C, 41.41; H, 6.43; N, 4.83%.
4.2.16. [PtBr(S,S-C6H2-{CH(Me)NMe2}2-2,6-
OSiMe2t-Bu-4)] (S,S-15)
1H-NMR: l=6.84 (s, 2H, ArH), 2.58 (s, 6H,
C(O)CH3), 0.97 (s, 9H, SiCMe3), 0.21 (s, 6H, SiCH3);
13C{1H}-NMR: l=201.4 (ArCO), 155.4 (CarylꢀO),
144.8 (CarylꢀC), 120.8 (CarylꢀH), 104.5 (CarylꢀBr), 30.6
(C(O)CH3), 25.5 (SiCMe3), 18.1 (SiCMe3), −4.5
(SiCH3); m.p.=36–37°C; Anal. Found: C, 51.86; H,
6.26. Calc. for C16H23BrO3Si: C, 51.75; H, 6.24%.
A mixture of S,S-13 (0.45 g, 1.05 mmol) and [Pt(tol-
4)2(SEt2)]2 (0.51 g, 1.05 mmol) in C6H6 (8 ml) was
stirred at reflux temperature for 4 h. All volatiles were
removed in vacuo and the residue was purified by
gradient column chromatography (SiO2; CH2Cl2, then
CH2Cl2–acetone 19:1) to yield S,S-15 as a colorless
solid (0.28 g, 43%).
3
4.2.14. S,S-C6H2Br-[CH(Me)NMe2]2-2,6-OSiMe2t-Bu-4
(S,S-13)
The procedure was analogous to the preparation of
1H-NMR: l=6.23 (s, 2H, ArH), 4.14 (q, JHH=6.6
Hz, 2H, ArCHN), 3.18 (s, 3JHPt=36.9 Hz, 6H,
NMeCH3), 2.84 (s, 3JHPt=41.8 Hz, 6H, NCH3Me),
3
2a, starting from R,R-11 (0.70 g, 1.9 mmol) in CH2Cl2
1.36 (d, JHH=6.9 Hz, 6H, ArCHCH3), 0.98 (s, 9H,

284
M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
SiCMe3), 0.18 (s, 6H, SiCH3); 13C{1H}-NMR: l=
152.4 (CarylꢀO), 147.4 (CarylꢀC), 137.0 (CarylꢀPt), 113.0
(CarylꢀH), 78.5 (2JCPt=60.5 Hz, ArCHN), 53.1
(NMeCH3), 47.7 (NCH3Me), 25.7 (SiCMe3), 18.1
(SiCMe3), 13.4 (3JCPt=26, ArCHCH3), −4.4 (SiCH3);
[h]D= −60.7° (c=1.15, CH2Cl2); Anal. Found: C,
38.52; H, 6.05; N, 4.41. Calc. for C20H37BrN2OSiPt: C,
38.46; H, 5.97; N, 4.49%.
[Pd(dba)2] (200 mg, 0.35 mmol) in C6H6 (3 ml) was
stirred at reflux temperature for 4 h. The resulting
suspension was filtered through Celite and evaporated
to dryness. The residue was purified by gradient column
chromatography (SiO2; CH2Cl2, then CH2Cl2–acetone
4:1) to yield S,S-17 as a yellowish solid (98 mg, 52%).
Analytically pure product was obtained by recrystal-
lization from benzene–pentane.
1H-NMR: l=6.21 (s, 2H, ArH), 4.11 (br q, 3JHH not
resolved, 2H, ArCHN), 3.00 (s, 6H, NMeCH3), 2.75 (s,
6H, NCH3Me), 1.38 (d, 3JHH=6.6 Hz, 6H,
ArCHCH3), 0.97 (s, 9H, SiCMe3), 0.17 (s, 6H, SiCH3);
13C{1H}-NMR: l=153.2 (CarylꢀO), 149.8 (CarylꢀC),
113.0 (CarylꢀH), 75.8 (ArCHN), 52.1 (NMeCH3), 46.4
(NCH3Me), 25.7 (SiCMe3), 18.1 (SiCMe3), 15.0
(ArCHCH3), −4.4 (SiCH3), CarylꢀPd not resolved;
[h]D= −29.0° (c=0.32, CH2Cl2); Anal. Found: C,
48.78; H, 7.18; N, 4.75. Calc. for C20H37BrN2OSiPd·2/3
C6H6: C, 49.02; H, 7.03; N, 4.76%.
Prior to the catalytic runs, the neutral organopalla-
dium complex S,S-18 (8.7 mg, 16 mmol) was dissolved
in acetone (5 ml) and water (0.2 ml) and treated with an
equimolar amount of AgBF4 (3.2 mg, 16 mmol, dis-
solved in 1 ml acetone). The suspension was stirred for
30 min in the absence of light and subsequently filtered
through Celite. The filtrate was evaporated to dryness
to give the cationic catalyst precursor S,S-19 in quanti-
tative yield as an off-white solid. This solid was used in
situ for the catalytic experiments [50].
4.2.17. [PtI(S,S-C6H2-{CH(Me)NMe2}2-2,6-
OSiMe2t-Bu-4)] (S,S-16)
A mixture of S,S-14 (125 mg, 0.20 mmol) and NaI
(0.3 g, 2 mmol) in acetone (10 ml) was stirred for 12 h
and subsequently evaporated to dryness. The residue
was extracted with CHCl (3×20 ml) and the organic
fractions concentrated to 5 ml. Addition of pentane
afforded microcrystalline S,S-16 (126 mg, 94%).
1H-NMR: l=6.22 (s, 2H, ArH), 4.10 (q, JHH=6.2
3
3
3
Hz, JHPt=23.0 Hz, 2H, ArCHN), 3.26 (s, JHPt=38.0
Hz, 6H, NMeCH3), 2.91 (s, 3JHPt=40.9 Hz, 6H,
NCH3Me), 1.38 (d, 3JHH=6.7 Hz, 6H, ArCHCH3),
0.97 (s, 9H, SiCMe3), 0.17 (s, 6H, SiCH3); 13C{1H}-
NMR: l=152.4 (CarylꢀO), 147.8 (CarylꢀC), 139.7
(CarylꢀPt), 113.0 (CarylꢀH), 78.1 (2JCPt=60.5 Hz,
ArCHN), 55.1 (NMeCH3), 49.0 (NCH3Me), 25.7
(SiCMe3), 18.1 (SiCMe3), 14.6 (3JCPt=30.6 Hz,
ArCHCH3), −4.3 (SiCH3); [h]D= −33.4° (c=0.1,
CH2Cl2); Anal. Found: C, 35.49; H, 5.48; N, 4.17. Calc.
for C20H37IN2OSiPt: C, 35.77; H, 5.55; N, 4.17%.
4.2.18. [PtCl(S,S-C6H2-{CH(Me)NMe2}2-2,6-OH-4)]
(S,S-17)
4.3. X-ray structure determination of S,S-3b
A solution of Bu4NF (0.11 ml, 1.0 M in THF, 0.11
mmol) was added to a solution of S,S-14 (66.2 mg, 0.11
mmol) in THF (4 ml). After stirring for 10 min, H2O
(60 ml) was added and the mixture stirred for 40 h at
RT. The product was extracted from H2O (20 ml) and
CH2Cl2 (3×15 ml). The combined organic layers were
washed with brine, dried (MgSO4) and evaporated to
dryness. The residue was precipitated twice from
CH2Cl2 (2 ml) and hexane (70 ml) to afford S,S-17 as
an analytically pure, colorless solid (43.7 mg, 82%).
Intensities were measured on an Enraf–Nonius
CAD4-T diffractometer with rotating anode (Mo–Ka,
,
u=0.71073 A) at a temperature of 150 K. The struc-
ture was solved with Patterson methods (DIRDIF-97
[54]) and refined with the program SHELXL-97 [55]
against F2 of all reflections. Non-hydrogen atoms were
refined freely with anisotropic displacement parameters,
hydrogen atoms were refined as rigid groups. All calcu-
lations, graphical illustrations and checking for higher
symmetry were performed with the PLATON [56]
package.
3
1H-NMR: l=6.30 (s, 2H, ArH), 4.16 (q, JHH=6.8
Hz, 2H, ArCHN), 3.12 (s, 3JHPt=35.0 Hz, 6H,
NMeCH3), 2.81 (s, 3JHPt=41.5 Hz, 6H, NCH3Me),
1.36 (d, 3JHH=6.9 Hz, 6H, ArCHCH3); 13C{1H}-
NMR: l=152.8 (CarylꢀO), 147.6 (CarylꢀC), 135.5
(CarylꢀPt), 108.6 (CarylꢀH), 78.8 (2JCPt=63.2 Hz,
ArCHN), 52.0 (NMeCH3), 47.2 (NCH3Me), 12.9
(3JCPt=31.7 Hz, ArCHCH3); [h]D= −67.8° (c=0.73,
CH2Cl2); Anal. Found: C, 35.88; H, 5.09; N, 7.46. Calc.
for C14H23ClN2OPt: C, 36.09; H, 4.98; N, 7.61%.
Crystal data for S,S-3b: [C16H27ClN2Pt], Mr=
477.93, colorless cubes, monoclinic P2l (no. 4), a=
6.6377(11),
b=10.2267(17),
c=12.4420(16),
3
,
i=99.635(11), V=832.7(2) A , Z=2, Dcalcd=1.906
g cm−3, T=150 K, v=8.579 mm−1 (Mo–Ka radia-
,
tion, u=0.71073 A), empirical absorption correction
transmission range 0.225–0.689),
4364 reflections measured of which 2017 unique, Rint
(
PLATON/DELABS,
=
0.035, final R1=0.0208 (for data I\2s(I), 0.0214 for
4.2.19. [PdBr(S,S-C6H2-{CH(Me)NMe2}2-2,6-
OSiMe2t-Bu-4)] (S,S-18)
A mixture of S,S-13 (150 mg, 0.35 mmol) and
all data). wR2 0.0649, S=1.052, Flack parameter
−3
,
0.017(11), residual density −1.43B1.74 e A
.

M. Albrecht et al. / Journal of Organometallic Chemistry 624 (2001) 271–286
285
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Crystallographic data for the structural analysis of
S,S-3b (excluding structure factors) have been de-
posited with the Cambridge Crystallographic Data
Centre, CCDC no. 151430. Copies of this information
can be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www:http://www/ccdc/cam/ac/uk).
[7] Cross coupling; M=Mn, E=NMe2: J.G. Donkervoort, J.L.
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Acknowledgements
[9] Michael addition; M=Pd, E=oxazolinyl: M.A. Stark, G.
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[11] Allylation: M=Rh, E=oxazolinyl: Y. Motoyama, H.
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Drs G. Rodr´ıguez and E. Rijnberg (Utrecht Univer-
sity) are gratefully acknowledged for fruitful discus-
sions and technical assistance. Dr P. Rechsteiner and
Prof. J. Hulliger (University of Bern) are thanked for
performing preliminary NLO measurements. This work
has been supported in part (A.L.S.) by the Dutch
Council for Chemical Sciences (CW-NWO).
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