Mechanisms of Sn-to-Zr cyclopentadienyl transfer in the formation of Me2Si-bridged zirconocenes from sila-stanna-tetrahydro-s-indacenes

Article abstract of DOI:10.1016/S0022-328X(02)01653-4

The stereochemistry of Sn-to-Zr transmetalation was studied by reacting ZrCl₄ with that isomer of meso-BnMeSi(3-^tBu- C₅H₃)₂SnMe₂ which has the benzyl group in axial position. Exchange of Sn

Full text of DOI:10.1016/S0022-328X(02)01653-4

Journal of Organometallic Chemistry 663 (2002) 58Á62
/
www.elsevier.com/locate/jorganchem
Mechanisms of Sn-to-Zr cyclopentadienyl transfer in the formation of
Me2Si-bridged zirconocenes from sila-stanna-tetrahydro-s-indacenes
Mario Huttenhofer, Armin Weeber, Hans-Herbert Brintzinger ꢀ
¨
Fachbereich Chemie, Universitat Konstanz, D-78457 Konstanz, Germany
¨
Received 22 April 2002; accepted 25 June 2002
Dedicated to Professor Pascual Royo on the occasion of his 65th birthday
Abstract
The stereochemistry of Sn-to-Zr transmetalation was studied by reacting ZrCl4 with that isomer of meso-BnMeSi(3-tBuÃ
C5H3)2SnMe2 which has the benzyl group in axial position. Exchange of SnMe2 against ZrCl2 generates both isomers of the CS-
symmetric ansa-zirconocene meso-BnMeSi(3-tBuÃ
C5H3)2ZrCl2, but not the C1-symmetric, rac-like isomer. The major product is
/
/
formed under inversion at both Sn-bound C atoms by consecutive ‘back-side’ attacks of the Zr electrophile, while the minor product
appears to be formed, under retention at both Sn-bound C atoms, by a concerted ‘front-side’ attack of ZrCl4.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: ansa-Zirconocene; Tin organyls; Transmetalation
1. Introduction
between TiCl4 and trimethylsilyl-substituted iso-dicy-
clopentadiene, which was shown to occur under inver-
sion of the metal-bound C atom, i.e. by, ‘back-side’
A recurring theme in the scientific work of Professor
Royo and his collaborators concerns the reactivity of
attack of the Ti center at the CÃ
/
Si bond [8]. Metal
exchange under inversion has been postulated by
Nifant’ev et al. also for the stereoselective displacement
of trimethyltin groups from rac- or meso-configurated
Me2Si(1-indenyl-3-SnMe3)2 by reaction with ZrCl4 [4],
although the stereochemistry of this reaction might also
be due to exchange of both Me3Sn groups under
retention of configuration.
Me2Si-bridged metallocene complexes [1Á3]. As syn-
/
thons for complexes of this type, stannylated cyclopen-
tadienyl precursors have recently gained substantial
interest. This interest concerns, in particular, the synth-
esis of chiral ansa-zirconocene complexes, since the
displacement of stannyl groups from an indenyl or
cyclopentadienyl unit by reaction with ZrCl4 has been
In order to establish which circumstances might lead
to cyclopentadienyl transfer from a Sn to a Zr center
under retention of configuration at the metal-bound
carbon atom, we have investigated the stereochemical
course of the highly diastereoselective formation of
ansa-zirconocene complexes by transmetalation of
stanna-tetrahydro-s-indacenes with ZrCl4 according to
Scheme 1 [6,7].
found to occur with high stereoselectivity [4Á7]. Metal
/
exchange reactions of this type, which occur with high
yields and excellent diastereoselectivities, have been
utilized for the preparation of either the racemic or the
meso isomers of substituted ansa-zirconocene com-
plexes.
With regard to the mechanisms responsible for the
diastereoselectivity of these metal exchange reactions,
first clues have been derived by Sivik and Paquette from
studies on the stereochemical course of the reaction
2. Results and discussion
Reaction of the meso-configurated Me2Sn compound
2,6-di-tert-butyl-4,8-tetramethyl-8-sila-4-stanna-tetra-
ꢀ Corresponding author. Tel.: ꢁ
883-137
/
49-7531-882-629; fax: ꢁ49-7531-
/
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 5 3 - 4

M. Huttenhofer et al. / Journal of Organometallic Chemistry 663 (2002) 58Á
/62
59
¨
Scheme 1.
hydro-indacene, (1), with ZrCl4 leads exclusively to the
meso-configurated ansa-zirconocene dichloride (c.f.
Scheme 1) [6]. This stereochemical result can arise
from displacement of Me2Sn by ZrCl2 either under
twofold retention or under twofold inversion, provided
that the reaction modes at both Sn-bound carbon
centers are identical. These two variants are not
distinguishable for the reaction of compound 1 with
ZrCl4 but will diverge if the Si bridge carries two
different substituents: in this case, twofold retention
gives a product different from that resulting from
twofold inversion. Interference of the changed Si sub-
Fig. 1. Crystal structure of the benzylmethylsilyl-bridged compound
3A. Thermal ellipsoids drawn at 50% probability, H atoms (except at
C1, C6 and C22) omitted for clarity.
isolated. It was obtained, by crystallization from diethyl
ether, in 48% yield and identified, by crystal structure
determination, as the 8s-isomer with an axially posi-
tioned benzyl group (Fig. 1). As expected, the core
geometry of 3A is practically identical to that of the
Me2Si-bridged analog 1, i.e. essentially unaffected by the
benzyl substituent (Table 1). The lack of any formation
stituent with the SnÁZr exchange mechanism would be
minimized by use of isotopic labelling; for practical
reasons, however, we have chosen the more conveniently
/
Table 1
˚
Selected bond lengths (A) and angles (8) of compound 3A and, for
comparison, of its dimethylsilyl-bridged analog 1 [4]
accessible methylÁ
/
benzyl-Si bridge as a stereochemical
marker.
meso-2,6-di-tert-butyl-8-benzyl-4,4,8-trimethyl-8-sila-
4-stanna-tetrahydroindacene (3) was obtained by reac-
3A
1
Bond lengths
Sn(1)ÃC(1)
Sn(1)ÃC(6)
Sn(1)ÃC(19)
Sn(1)ÃC(20)
Si(1)ÃC(5)
Si(1)ÃC(10)
Si(1)ÃC(21)
Si(1)ÃC(22)
C(22)ÃC(23)
C(6)ÃC(7)
2.206(2)
2.199(2)
2.146(3)
2.112(3)
1.856(2)
1.862(2)
1.845(3)
1.883(2)
1.501(3)
1.473(3)
1.349(3)
1.455(3)
1.358(3)
1.482(3)
2.214(4)
2.214(4)
2.152(7)
2.108(6)
1.867(4)
1.867(4)
1.868(7)
1.850(7)
tion of benzylÁmethyl-silanediyl-bis(3-tert-butyl-cyclo-
/
pentadiene) (2) with Me2Sn(NEt2)2 [9]. 1H-NMR
spectra of the product mixture indicate that the isomers
3A, 3B and 3C are obtained in a ratio of 8:1:1 (Scheme
2). From this mixture, only the major isomer 3A was
1.465(5)
1.349(5)
1.447(5)
1.348(5)
1.489(5)
C(7)ÃC(8)
C(8)ÃC(9)
C(9)ÃC(10)
C(6)ÃC(10)
Bond angles
C(2)ÃC(1)ÃSn(1)
C(5)ÃC(1)ÃSn(1)
C(6)ÃSn(1)ÃC(1)
C(5)ÃSi(1)ÃC(10)
C(6)ÃC(10)ÃSi(1)
C(1)ÃC(5)ÃSi(1)
C(9)ÃC(10)ÃSi(1)
C(5)ÃSi(1)ÃC(22)
C(10)ÃSi(1)ÃC(22)
C(21)ÃSi(1)ÃC(22)
C(23)ÃC(22)ÃSi(1)
104.7(1)
100.6(1)
100.6(1)
105.6(1)
125.2(2)
125.4(2)
127.8(2)
110.5(1)
109.5(1)
109.4(1)
112.2(2)
106.2(2)
101.5(2)
102.6(2)
105.2(2)
125.1(2)
125.1(2)
127.2(3)
111.0(2)
111.0(2)
108.7(4)
Scheme 2.

60
M. Huttenhofer et al. / Journal of Organometallic Chemistry 663 (2002) 58Á
/62
¨
of 3B or 3C from CDCl3 solutions of pure 3A docu-
ments the configurational stability of this sila-stanna-
tetrahydroindacene compound.
When a toluene solution of 3A was reacted with a
suspension of ZrCl4 in toluene at room temperature,
immediate appearance of the yellowish color of the
ansa-zirconocene product and subsequent 1H-NMR
spectra showed that the starting material was rapidly
consumed and that two new products, 4A and 4B, were
formed in a ratio of ca. 2:1. While it was not possible to
separate and isolate these complexes, complete struc-
tural assignments were possible, based on their NMR
1
spectra. Both 4A and 4B give H-NMR signals assign-
able to CS-symmetric silyl-bridged zirconocenes.
ROESY spectra of these products, which show distinct
Si
cross signals of the CH3Ã
/
Si and the phenylÁ
/
CH2Ã
/
groups with individual ring protons (Fig. 2), document
that the major product 4A has its benzyl and tert-butyl
substituents on the same side of the molecule, while the
minor isomer 4B has the benzyl group on the side
opposite to the tert-butyl substituents (Scheme 3).
Remarkably enough, no trace of the C1-symmetric,
rac-like isomer 4C can be detected by 1H-NMR spectro-
scopy.
Preferential formation of the meso-configurated ansa-
zirconocene isomer 4A shows that ZrCl4 attacks pre-
dominantly- presumably consecutively- at the ‘back-
side’ of both Sn-bound cyclopentadienyl units of
compound 3A, i.e. under inversion at both Sn-bound
carbon atoms, as had been assumed for this type of
metal exchange [4,8]. Prima facie puzzling, however, is
the origin of the minor zirconocene isomer 4B. The
Scheme 3.
primarily to the rac-like isomer 4C. The Sn-to-Zr
transfer of the second cyclopentadienyl unit must thus
occur with the same stereochemistry as that of the first
one in the formation of both 4A and 4B. Such a
stereochemical coherence between the first and the
second transmetallation step can be explained by two
reaction paths.
In principle, a minor, possibly undetectable fraction
of the meso-isomer 3B, present in equilibrium with the
dominant isomer 3A, might give rise to the zirconocene
isomer 4B by Sn-to-Zr exchange steps under inversion at
both Sn-bound C atoms. Any such equilibration be-
tween 3A and 3B appears unlikely, however, since the
configurational stability of both isomers is documented
by the observation that the 8:1 ratio of these isomers in
the initial product mixture as well as the NMR spectra
of pure 3A in CDCl3 solution remain unchanged for
extended periods of time.
absence of the mixed retentionÁinversion product ‘rac’-
/
4C implies that racemization- either of some reaction
intermediate or of the final zirconocene product- cannot
explain the formation of 4B, since stepwise epimeriza-
tion of a metalÁcyclopentadienyl unit would always lead
/
Fig. 2. 1H-ROESY spectrum of the ansa-zirconocene product mixture containing 4A and 4B, in C6D6 solution. Broken lines correlate signals of
isomer 4A, solid lines those of isomer 4B.

M. Huttenhofer et al. / Journal of Organometallic Chemistry 663 (2002) 58Á
/62
61
¨
Therefore, we have to assume that the sila-stanna-
tetrahydro-indacene isomer 3A is amenable to a con-
certed ‘front-side’ attack of the electrophile ZrCl4 at
both Sn-bound C atoms, which leads, under retention of
configuration at both of these centers, to isomer 4B of
the ansa-zirconocene product (Scheme 4). That this
double-retention mechanism occurs with a rate compar-
able to that of the normally preferred inversion mechan-
ism might be due to an accumulation of a rather high
electron density at the ‘front-side’ of 3A and/or to the
steric shielding of its ‘back-side’ by the axially posi-
tioned benzyl substituent.
While the concerted attack of ZrCl4 at the ‘front-side’
of the tin precursor 3A might thus be particular to the
specific sila-stanna-tetrahydro-indacene derivative stu-
died here, our data provide first evidence that this
hitherto scarcely discussed reaction mechanism has to be
taken into account in designing transmetalation reac-
tions for stereoselective syntheses. Studies on further
stannylene derivatives appear necessary to delineate the
conditions under which this ‘front-side’ reaction mode
competes with the normally preferred ‘back-side’ attack
of a metal halide at an Sn-bound C atom, which leads to
an inversion of its configuration.
until the lithium salt just began to precipitate. Upon
dropwise addition of a solution of 13.2 g of benzyl-
methyldichlorosilane (64 mmol) in 20 ml pentane, the
solution turned yellow. After stirring overnight, the
solvent was completely removed in vacuo and 50 ml of
pentane was added. A colorless residue was removed by
filtration, the clear filtrate treated with saturated aqu-
eous NH4Cl solution, neutralized with water and dried
over MgSO4. Removal of solvent gave the product as an
oily, almost colorless residue, for which it was not
1
possible to obtain assignable H-NMR spectra, due to
the presence of multiple isomers, and which was thus
used without further purification for subsequent reac-
tions. Crystals obtained from diethyl ether solution at ꢂ
/
80 8C melt again when brought to room temperature
(r.t.). Yield 22.9 g (63 mmol, 98% of theory).
3.2. meso-2,6-di-tert-Butyl-8s-benzyl-4,4,8-trimethyl-8-
sila-4-stanna-tetrahydro-indacene (3A)
To a solution of benzylmethylsilanediyl-bis(3-tert-
butylcyclopentadiene) (2, 5.8 g, 15.4 mmol) in 50 ml of
diethylether a solution of 4.5 g bis(diethylamino)di-
methylstannane in 25 ml of diethyl ether was slowly
added via a dropping funnel. After stirring the reaction
mixture for 4Á5 h, the volume of the solution was
/
reduced to 15 ml in vacuo. Storage at 0 8C gave, after a
few days, colorless crystals of NMR-spectrally pure 3A,
which were collected by filtration. Yield 3.8 g (7.4 mmol,
48% of theory). H-NMR (CDCl3, 600 MHz, assign-
ments supported by ROESY and HQMC spectra): d
3. Experimental
All manipulations were performed on an argon/
vacuum manifold or in a glovebox under a purified
nitrogen atmosphere. Solvents were dried and distilled
from sodium and benzophenone. Me2Si(3-tBuÃ
[10], MeBnSiCl2 [11] and Me2Sn(NEt2)2 [12] were
prepared essentially as described in the literature.
NMR spectra were obtained on a Bruker DRX 600
spectrometer.
1
7.13 (t, Jꢃ
Jꢃ
17 Hz, 2H), 6.68 (s, J(1H,Sn)ꢃ
2H), 4.14 (s, J(1H,Sn)ꢃ
101 Hz, 2H), 2.40 (s, 2H), 1.16
(s, 18H), 0.52 (s, J(1H,Sn)ꢃ
52 Hz, 3H), 0.41 (s, 3H), ꢂ
1.21 (s, J(1H,Sn)ꢃ54 Hz, 3H). 13C-NMR (CDCl3, 150
MHz): d 152.5, 145.1, 134.0, 128.6, 127.9, 125.9, 123.9,
57.0, 32.0, 31.1, 26.6, ꢂ3.9, ꢂ7.1, ꢂ19.2 ppm. These
/
17 Hz, 2H), 7.03 (t, Jꢃ/17 Hz, 1H), 6.92 (d,
/
C5H4)2
/
/19 Hz, 2H), 6.28 (s,
/
/
/
/
/
/
/
3.1. Benzylmethylsilanediyl-bis(3-tert-
butylcyclopentadiene) (2)
NMR spectra remained unchanged for periods of at
least 3 days, indicating that compound 3A is stable
against any rearrangements.
To a solution of 16.5 g of tert-butyl-cylcopentadienyl
lithium (129 mmol) in 200 ml THF, pentane was added
3.3. meso-Benzylmethylsilyl-bis(3-tert-butyl-
cyclopentadienyl) zirconium dichloride (4)
A solution of 500 mg of 3A (1.0 mmol) in 20 ml of
toluene was added, in the course of 20 min, to a
suspension of 220 mg ZrCl4 in 20 ml of toluene. The
reaction mixture immediately developed the yellowish
color of the ansa-zirconocene product. After stirring for
ca. 3 h at room temperature (r.t.), the reaction mixture
was filtered and the filtrate evacuated to dryness in
vacuo. The solid residue was then subjected, under
exclusion of light, to sublimation in vacuo at 90 8C to
remove all Me2SnCl2. C6D6 solutions of the product
1
Scheme 4.
thus obtained gave H-NMR signals in accord with the

62
M. Huttenhofer et al. / Journal of Organometallic Chemistry 663 (2002) 58Á
/62
¨
presence of both 4A and 4B, in a ratio of 2:1 (c.f. Fig. 2).
1H-NMR for 4A (CDCl3, 600 MHz): d 7.06 (pq, 2H),
6.98 (m, 3H), 6.79 (s, 2H), 5.92 (s, 2H), 5.55 (s, 2H), 2.20
(s, 2H), 1.45 (s, 18H), 0.23 (s, 3H). 13C-NMR for 4A
(CDCl3, 150 MHz): d 148.8, 137.0, 129.7, 129.0, 125.5,
4. Supporting information available
Crystallographic data and structural analysis for
complex 3A have been deposited with the Cambridge
Crystallographic Data Centre no. CCDC 160359. Copies
of this information may be obtained, free of charge, from
The Director, CCDC, 12 Union Road, Cambridge CB2
116.6, 113.1, 103.5, 33.7, 31.1, 20.0, ꢂ
5.5 ppm. 1H-
/
NMR for 4B (CDCl3, 600 MHz): d 7.06 (pq, 2H), 6.98
(m, 3H), 6.88 (s, 2H), 5.87 (s, 2H), 5.63 (s, 2H), 2.39 (s,
2H), 1.42 (s, 18H), 0.04 (s, 3H). 13C-NMR for 4B
(CDCl3, 150 MHz): d 146.7, 136.9, 131.6, 129.0, 125.5,
1EZ, UK (Fax: ꢁ44-1223-336033; e-mail: deposit@
/
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
119.1, 111.5, 103.0, 33.5, 31.2, 23.3, ꢂ8.9 ppm.
/
Acknowledgements
We thank Dr. Armin Geyer and Monika Cavegn for
expert help with obtaining and interpreting ROESY
spectra and Frank Schaper for helpful discussions.
Financial support of this work by BMBF and BASF
AG and by funds of the University of Konstanz is
gratefully acknowledged.
3.4. Crystal structure determination of compound 3A
Suitable crystals of compound 3A were obtained from
diethyl ether. Compund 3A crystallizes as colorless
prisms in the monoclinic space group P21/c (aꢃ
/
˚
13.666(2), bꢃ
105.38(1)8, Vꢃ2729.1(8) A , Zꢃ
F000 1088). X-ray diffraction analysis was carried out
at 229 K on a Siemens P4 four-circle diffractometer
using MoÁKa radiation (71.073 pm) and a graphite
/
14.769(2), cꢃ
/
14.024(3) A, bꢃ
/
3
˚
/
/
4, mꢃ ,
/
0.992 mmꢂ1
ꢃ
/
References
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/
monochromator. Crystal decay was monitored by
measuring three standard reflections every 100 reflec-
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range of 2Á
/
278, of which 5946 were independend (Rint
ꢃ
/
3.29%). The structure was solved using direct methods
[13]. All non-hydrogen atoms were refined anisotropi-
cally by least-squares procedures based on F2. Hydrogen
atoms were located in the difference fourier map and
refined isotropically except for the hydrogen atoms of
the tert-butyl groups, which were refined on calculated
positions with fixed isotropic U, using riding model
[6] M. Huttenhofer, M.H. Prosenc, U. Rief, F. Schaper, H.H.
¨
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¨
¨
techniques [13]. Final reliability factors were R(F)ꢃ
2.94 and Rw(F2)ꢃ
7.47% for 5333 observed reflections
with I ꢀ2s(I) and R(F)ꢃ 7.92% for
3.39 and Rw(F2)ꢃ
/
[10] H. Wiesenfeldt, A. Reinmuth, E. Barsties, K. Evertz, H.H.
Brintzinger, J. Organomet. Chem. 369 (1989) 359.
[11] B. Re´sibois, C. Hode´, B. Picart, J.-C. Brunet, Ann. Chim. 4 (1969)
203.
/
/
/
/
all data. The Goodness-of-Fit was 1.046, the residual
electron density 1.103 eꢂ
from the Sn-atom.
[12] K. Jones, M.F. Lappert, J. Chem. Soc. (1965) 1944.
[13] G.M. Sheldrick, SHELXS-86, SHELXL-93; Universitat Gottingen,
Gottingen, Germany, 1990, 1993.
¨
ꢂ3
˚
A
˚
, located in 1 A distance
¨
¨