Enantiospecific synthesis of a planar chiral fluorenylzirconium ansa-metallocene: Synthesis and structures of 1-(9-fluorenyl)-and [1-(1-methyl-9-fluorenyl)-2-(2-indenyl)naphthaIene]zirconium dichloride

Article abstract of DOI:10.1039/a910246i

Di-Iithiation of +ac(R)-1-methyl-9-[2-(2-indenyl)-1-naphthyl]fluorene, +ac(R)-6, generates an axially chiral fluorenyl ligand, (a-S)-lO, which on reaction with ZrCl₄ enantiospecifically provides the planar chiral ansazirconocene (p-S)-[1-(1-methyl-9-fluorenyl)-2-(2-indenyl)-naphthalene]zirconium dichloride, (p-5)-8; single crystal X-ray structures have been determined for the racemic awsa-zirconocene rac-8 and the related C8-symmetric ansazirconocene 7. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a910246i

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Enantiospecific synthesis of a planar chiral fluorenylzirconium
ansa-metallocene: synthesis and structures of [1-(9-fluorenyl)-
and [1-(1-methyl-9-fluorenyl)-2-(2-indenyl)naphthalene]zirconium
dichloride
Robert W. Baker,* Michael A. Foulkes and Peter Turner
School of Chemistry, The University of Sydney, NSW 2006, Australia
Received 20th December 1999, Accepted 17th January 2000
Di-lithiation
of
؉ac(R)-1-methyl-9-[2-(2-indenyl)-1-
naphthyl]fluorene, ؉ac(R)-6, generates an axially chiral
fluorenyl ligand, (a-S)-10, which on reaction with ZrCl4
enantiospecifically provides the planar chiral ansa-
zirconocene (p-S)-[1-(1-methyl-9-fluorenyl)-2-(2-indenyl)-
naphthalene]zirconium dichloride, (p-S)-8; single crystal
X-ray structures have been determined for the racemic
ansa-zirconocene rac-8 and the related Cs-symmetric ansa-
zirconocene 7.
Planar chiral group IV metallocenes in which the cyclopenta-
dienyl ligands are bridged (ansa-metallocenes) continue to
attract great interest in the areas of stereoregular α-olefin
polymerisation1 and asymmetric organic synthesis.2 In the
search for new complexes with desirable reactivity and stereo-
selectivity, considerable effort has been directed at the chal-
lenges associated with obtaining stereochemically pure metal
complexes.3 In the case of polymerisation catalysts, the
emphasis has been on the selective synthesis of racemic C2-
symmetric ansa-metallocenes at the expense of meso diastereo-
isomers. Applications in the area of asymmetric organic
synthesis impose the additional requirement of enantiomeric
purity. Recently, we demonstrated that a new ligand design—
axially chiral cyclopentadienyl ligands—has the potential to
allow the enantiospecific synthesis of planar chiral cyclopenta-
dienylmetal complexes containing pendant donor atoms.4 Here
we describe the enantiospecific synthesis of a planar chiral
fluorenylzirconium ansa-metallocene, linked through a 1,2-
naphthalene bridge, using an axially chiral fluorenyl ligand.
While fluorenylzirconium ansa-metallocenes have been exten-
sively studied as catalysts for olefin polymerisation,5 chiral
complexes have only ever been prepared as the racemates and
complexes with a C(sp2)–C(sp2) bridge are unprecedented.6
For our initial studies, racemic isopropyl 1-(phenylsulfinyl)-
naphthalene-2-carboxylate (2) was prepared in an overall yield
of 73% from 1-bromo-2-naphthoic acid (1), as outlined in
Scheme 1.† Reaction of the sulfoxide 2 with fluorenyllithium
or 1-methylfluorenyllithium afforded the coupled products sp-3
and ac*(R*)-4 in 91 and 88% yields, respectively. Following the
precedence of Bosnich and co-workers,7 reaction of sp-3 and
ac*(R*)-4 with the di-Grignard reagent derived from 1,2-bis-
(chloromethyl)benzene, followed by acid-catalysed dehydration
of the intermediate 2-indanols, provided the 2-indenyl ligands
sp-5 and ac*(R*)-6 in 70 and 49% overall yields, respectively.
Metallation of both sp-5 and ac*(R*)-6 was achieved by
di-lithiation with BuLi in THF solution, followed by replace-
ment of the solvent with benzene and reaction of the dianions
with ZrCl4. After removal of inorganic salts and concentration,
the ansa-zirconocene complexes 7‡ and rac-8§ crystallised in 55
and 57% yields, respectively. The complexes 7 (Fig. 1)¶ and rac-
8 (Fig. 2)|| were characterised by single crystal X-ray analysis;
the geometry about zirconium is within expected values for
both complexes,8 with (η5-centroid)–Zr–(η5-centroid) angles of
127.94(4) and 126.8(1)Њ, and Cl–Zr–Cl angles of 99.05(5) and
97.9(3)Њ, for 7 and rac-8, respectively. For both complexes, the
Scheme 1 Reagents and conditions: i, SOCl2 (10 equiv.), pyridine
(1 equiv.), benzene, 25 ЊC, 16 h; ii, PriOH (2 equiv.), pyridine (2 equiv.),
CH2Cl2, 25 ЊC, 30 min; iii, PhSNa (1.2 equiv.), DMF, 70 ЊC, 16 h; iv,
OXONE (2 equiv.), acetone (12 equiv.), MeCN/aqueous NaHCO3,
25 ЊC, 7 h; v, fluorenyllithium or 1-methylfluorenyllithium (1.2 equiv.),
THF, 0 ЊC, 30 min; vi, 1,2-C6H4(CH2MgCl)2 (1.5 equiv.), THF, Ϫ78 to
25 ЊC, 16 h; vii, TsOH (0.05 equiv.), benzene, reflux, 20 min; viii, BuLi
(2.4 equiv.), THF, 0 ЊC, 60 min; ix, ZrCl4 (1.1 equiv.), benzene, 25 ЊC,
90 min.
indenyl and fluorenyl Zr–C distances vary considerably; those
for the indenyl groups vary from 2.467(4) to 2.630(4) Å,
and 2.45(2) to 2.69(2) Å, for 7 and rac-8, respectively; the
Zr–C distances for the fluorenyl groups vary from 2.406(3) to
2.713(4) Å, and 2.43(2) to 2.69(3) Å, for 7 and rac-8, respectively.
While the crystal structure of 7 shows only minor deviations
from Cs-symmetry, in complex rac-8 the cyclopentadienyl
moieties display consymmetric rotations,9 with respect to the
ZrCl2 bisector, of Ϫ14.5 and ϩ11.5Њ for the fluorenyl and
indenyl groups, repectively (for the enantiomer illustrated in
Fig. 2); the mean plane of the naphthalene is also rotated from
the orthogonal, with respect to the mean plane of the fluorenyl
group, by 9.4Њ (the indenyl group rotated away from the fluor-
enyl 1-methyl substituent).
DOI: 10.1039/a910246i
J. Chem. Soc., Dalton Trans., 2000, 431–433
This journal is © The Royal Society of Chemistry 2000
431

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Scheme
2
Reagents and conditions: i, 1,2-C6H4(CH2MgCl)2 (1.5
equiv.), THF, Ϫ78 to 25 ЊC, 16 h; ii, TsOH (0.05 equiv.), benzene, reflux,
15 min; iii, BuLi (2.4 equiv.), THF, 0 ЊC, 60 min; iv, ZrCl4 (1.1 equiv.),
benzene, 25 ЊC, 90 min.
Fig. 1 ORTEP plot of 7, with crystallographic numbering; 25% ther-
mal ellipsoids are shown for non-hydrogen atoms. Selected distances (Å)
and angles (Њ): Zr(1)–Cl(1) 2.407(1), Zr(1)–Cl(2) 2.412(1), Zr(1)–C(1)
2.406(3), Zr(1)–C(2) 2.580(3), Zr(1)–C(7) 2.704(4), Zr(1)–C(8) 2.713(4),
Zr(1)–C(13) 2.552(3), Zr(1)–C(24) 2.479(3), Zr(1)–C(25) 2.467(4),
Zr(1)–C(26) 2.620(4), Zr(1)–C(31) 2.630(4), Zr(1)–C(32) 2.496(4);
Cl(1)–Zr(1)–Cl(2) 99.05(5), centroid(1)–Zr(1)–centroid(2) 127.94(4).
asymmetrically substituted 9-(1-naphthyl)fluorene retains an
axial chiral element.10 Di-lithiation of ϩac(R)-6 therefore gen-
erates the fluorenyllithium (a-S)-10, where the descriptor for
absolute configuration refers to the fluorenyl–naphthyl axis. In
the case of the ansa-zirconocene (p-S)-8 derived from (a-S)-10,
the single descriptor for absolute configuration refers to the
planar chirality of the fluorenyl–zirconium moiety as the axial
chirality of the precursor fluorenyllithium is considered to be
latent in the complex.
Notes and references
† For simplicity and consistency with Scheme 2, only the ϩac(R)-
enantiomers of ac*(R*)-4 and ac*(R*)-6, and the (p-S)-enantiomer of
rac-8, are illustrated in Scheme 1.
‡ Selected data for 7: Found C, 67.4; H, 3.5; Cl, 12.55. C32H20Cl2Zr
requires C, 67.8; H, 3.6; Cl, 12.5%. δH (400 MHz, C6D6) 6.03 (2H, s,
indenyl 1- and 3-H), 6.86–7.04 (8H, m, Ar-H), 7.23 (1H, dd, J 7.3, 7.6,
Ar-H), 7.31–7.35 (5H, m, Ar-H) and 7.75–7.79 (4H, m, Ar-H).
§ Selected data for rac-8: Found C, 68.2; H, 3.8; Cl, 11.7. C33H22Cl2Zr
requires C, 68.3; H, 3.8; Cl, 12.2%. δH (400 MHz, C6D6) 1.87 (3H,
s, CH3), 6.04 and 6.08 (each 1H, br s, indenyl 1- and 3-H), 6.80–7.00
(6H, m, Ar-H), 7.10 (1H, d, J 8.4, Ar-H), 7.16–7.38 (5H, m, Ar-H), 7.50
(1H, d, J 8.1, Ar-H), 7.66–7.73 (3H, m, Ar-H) and 7.83 (1H, d, J 8.4 Hz,
Ar-H).
¯
¶ Crystal data for 7: C35H23Cl2Zr, M = 605.69, triclinic, space group P1
(no. 2), a = 12.134(2), b = 12.745(2), c = 10.334(2) Å, α = 98.72(1),
β = 112.29(1), γ = 68.47(1)Њ, V = 1375.4(5) Å3, T = 294 K, Z = 2, µ(Cu-
Fig.
2
ORTEP plot of rac-8, with crystallographic numbering;
Kα) = 5.336 cmϪ1, N = 5673, N(unique) = 4926 (Rint = 0.048), Nobs
=
25% thermal ellipsoids are shown for non-hydrogen atoms. Selected dis-
tances (Å) and angles (Њ): Zr(1)–Cl(1) 2.383(6), Zr(1)–Cl(2) 2.424(8),
Zr(1)–C(1) 2.43(2), Zr(1)–C(2) 2.63(3), Zr(1)–C(7) 2.69(3), Zr(1)–C(8)
2.67(2), Zr(1)–C(13) 2.54(3), Zr(1)–C(24) 2.49(3), Zr(1)–C(25) 2.45(2),
Zr(1)–C(26) 2.59(2), Zr(1)–C(31) 2.69(2), Zr(1)–C(32) 2.54(2); Cl(1)–
Zr(1)–Cl(2) 97.9(3), centroid(1)–Zr(1)–centroid(2) 126.8(1).
3982 [I > 3.00σ(I)]. Data collected from a cut twinned crystal. The
asymmetric unit contains a complex molecule and a benzene solvate
molecule centred on an inversion site; benzene hydrogen atoms were
modelled isotropically with restrained bond lengths. Final R(F) =
0.0353, Rw(F) = 0.0398.
|| Crystal data for rac-8: C33H22Cl2Zr, M = 580.66, orthorhombic, space
group Pbca (no. 61), a = 20.802(5), b = 29.313(9), c = 8.312(6) Å,
V = 5068(3) Å3, T = 294 K, Z = 8, µ(Cu-Kα) = 5.738 cmϪ1, N = 2817,
N(unique) = 2724 (Rint = 0.056). Data collection was stopped when
decay exceeded 50% and data with decay to 30% were subsequently
used in the solution and model refinement. A linear decay correction
factor was applied to the data, of which 39% were classified as observed
with Nobs = 1072 [I > 2.50σ(I)]. Atoms C(2), C(14), C(16), C(20), C(23),
C(26) and C(31) were modelled with isotropic thermal parameters.
Final R(F) = 0.0820, Rw(F) = 0.0733. CCDC reference number 186/
1809. See http://www.rsc.org/suppdata/dt/a9/a910246i/ for crystallo-
graphic files in .cif format.
We have described the asymmetric synthesis of ϩac(R)-9
(Scheme 2†), in 75% de, through a coupling reaction of (1R)-
menthyl (S)-1-(p-tolylsulfinyl)naphthalene-2-carboxylate with
1-methylfluorenyllithium at 0 ЊC; recrystallisation furnishes
ϩac(R)-9 in у96% de.10 Reaction of ϩac(R)-9 with the di-
Grignard reagent derived from 1,2-bis(chloromethyl)benzene,
followed by acid-catalysed dehydration of the intermediate 2-
indanol, then recrystallisation, provided the 2-indenyl ligand
ϩac(R)-6 in 64% overall yield, [α]549 Ϫ382 (c 1.9, benzene). The
enantiomeric excess of ϩac(R)-6 was determined to be >99%
by HPLC analysis.** Metallation of ϩac(R)-6, as described
above for the racemate, provided the ansa-zirconocene complex
(p-S)-8 in 35% yield, [α]549 ϩ30.4 (c 3.0, C6D6). While the enan-
tiomeric excess of (p-S)-8 was not determined directly, treat-
ment of a benzene solution of (p-S)-8 with glacial acetic acid
led to the quantitative recovery of ϩac(R)-6 in >99% ee,**
confirming the enantiomeric purity of (p-S)-8. We have previ-
ously demonstrated that the fluorenyllithium derived from an
** A 4.6 × 250 mm column [Chiralpak OT(ϩ), Daicel] was used with
methanol as eluent at a flow rate of 0.5 ml minϪ1, column temp. 3 ЊC,
detection 254 nm, tR: 18.1 min for ϩac(R)-6 and 22.2 min for Ϫac(S)-6.
1 H.-H. Brintzinger, D. Fisher, R. Mülhaupt, B. Rieger and R. M.
Waymouth, Angew. Chem., Int. Ed. Engl., 1995, 34, 1143.
2 A. H. Hoveyda and J. P. Morken, Chiral Titanocenes and Zir-
conocenes in Synthesis, in Metallocenes, ed. A. Togni and R. L.
Halterman, Wiley-VCH, Weinheim, 1998, ch. 10.
432
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3 R. L. Halterman, Synthesis of Chiral Titanocene and Zirconocene
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4 R. W. Baker and B. J. Wallace, Chem. Commun., 1999, 1405.
5 H. G. Alt and E. Samuel, Chem. Soc. Rev., 1998, 323.
8 D. J. Cardin, M. F. Lappert and C. L. Raston, Chemistry of Organo-
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9 P. Burger, J. Diebold, S. Gutmann, H.-U. Hund and H.-H. Brintz-
inger, Organometallics, 1992, 11, 1319.
6 For examples of group IV cyclopentadienyl and indenyl ansa-
metallocenes with a C(sp2)–C(sp2) bridge see: P. Burger and H.-H.
Brintzinger, J. Organomet. Chem., 1991, 407, 207; R. L. Halterman,
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A. L. Reingold and B. Bosnich, Organometallics, 1993, 12, 4391.
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Communication a910246i
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