(±)-1,1-Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium Dichloride

[112243-79-5]  · C20H24Cl2Zr  · (±)-1,1-Ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium Dichloride  · (MW 426.57)

(chiral metallocene complex for stereoselective alkene and silane polymerization;1 asymmetric hydrogenation catalyst for alkenes;2,3 reagent for the asymmetric synthesis of allylic amines;4 asymmetric carbomagnesiation catalyst;5 precursor to chiral Lewis acid catalysts6)

Physical Data: d 1.616 g cm-3 (calc).

Solubility: insol hexanes, diethyl ether; sol CH2Cl2, CHCl3, THF; slightly sol toluene.

Form Supplied in: crystalline white or pale yellow solid; not widely available.

Analysis of Reagent Purity: 1H NMR and/or elemental analysis.

Preparative Methods: by hydrogenation of the corresponding racemic ethylenebis(h5-indenyl)zirconium dichloride7a over PtO2 or 5% Pt/C at 1000 or 50-500 psi, respectively. The indenyl precursor is generally prepared from 1,2-bis(3-indenyl)ethane and ZrCl4 in THF solution via the dilithium7b or dipotassium salt3 of the former compound. In the former case, racemic material can be obtained in pure form (40-50% yields), whereas in the latter case the product (>70% crude) is contaminated with significant amounts of the meso isomer which can be removed by fractional crystallization from toluene. The racemate can be resolved on a gram scale by conversion to the (S)- or (R)-binaphtholate derivative.4,8 Conversion, for example, of the (S,S)-binaphtholate derivative to the dimethylzirconocene derivative (MeLi, ether, or toluene) followed by treatment with either stoichiometric anhydrous dimethylamine hydrochloride (CH2Cl2) or anhydrous HCl (ether) provides optically pure material; [a]435 = +416°(c = 0.048 g 100 mL-1, CH2Cl2).8

Purification: the racemate is best crystallized from hot toluene. Optically pure material, which is much more soluble, is best crystallized from hexanes/toluene.

Handling, Storage, and Precautions: the compound is air and moisture stable but is best stored for long periods of time (years) in a desiccator. No known toxic effects or safety hazards.

Introduction.

While primarily employed in the study of the stereoselective polymerization of alkenes and, to a lesser extent, dehydrogenative polymerization of silanes,1 rac-[CH2(h5-C9H10)]2ZrCl2 (1) has attracted increasing attention as a reagent and/or catalyst in organic synthesis.

Asymmetric Hydrogenation.2,3

In the presence of methylaluminoxane and hydrogen (or deuterium), the dimethyl derivative (2) of (S)-(+)-(1) effects the asymmetric hydrogenation or deuteration of alkenes in up to 65% ee (e.g. eq 1). While this level of asymmetric induction is modest, it should be noted that this is one of the few methods available for asymmetric hydrogenation of unfunctionalized alkenes. The sense of asymmetric induction is reversed from that observed in alkene polymerization; coordination and insertion of the alkenes occurs from the re enantioface.

The active intermediate in these transformations is a cationic zirconium hydride complex generated in situ. More recent work has established that preformed cationic hydride complexes are equally effective.3

Asymmetric Synthesis of Allylic Amines.4

Optically pure (S)-(+)-(1) can be converted to the methyl triflate complex (3) via the dimethyl derivative (2). Reaction of (3) with lithium aryl amides at elevated temperatures generates, in situ, zirconaziridine intermediates which undergo facile insertion of alkynes to generate, on acidic workup, allylic amines in high optical purity (eq 2). Although this process requires stoichiometric amounts of (2), optically pure (1) can be recovered following workup with HCl/ether and recycled. Other substrates include alkenes and aldehydes and furnish saturated amines and b-amino alcohols, respectively, in high optical purity.

Asymmetric Carbomagnesiation of Alkenes.5

Complex (S)-(+)-(1) in the presence of alkyl Grigard reagents effects the catalytic carbomagnesiation of allylic alcohols or ethers in both a diastereo- and enantioselective manner. A mechanism involving an in situ generated metallacyclopropane complex has been invoked to rationalize the sense of asymmetric induction. For example, cyclic allylic ethers undergo carbomagnesiation with excellent levels of stereocontrol (eq 3).

Asymmetric Lewis Acid Catalysts.6

Compound (1) has been transformed into the optically pure cationic alkoxide complex (4) via the intermediacy of the dimethyl derivative (2). Complex (4) is an effective catalyst for the Diels-Alder reaction of simple dienophiles with dienes (eq 4). With the dienophiles studied, the magnitude of the asymmetric induction is modest (<=55% ee) but the sense of asymmetry is consistent with a simple model for dienophile coordination to the metal center.

Related Reagents.

(-)-[Ethylene-1,2-bis(h5-4,5,6,7-tetrahydro-1-indenyl)]zirconium (R)-1,1-Bi-2,2-naphtholate.


1. (a) Kaminsky, W.; Kuelper, K.; Brintzinger, H. H.; Wild, F. R. W. P. AG 1985, 97, 507. (b) Li, H.; Gauvin, F.; Harrod, J. F. OM 1993, 12, 575 and references therein.
2. Waymouth, R.; Pino, P. JACS 1990, 112, 4911.
3. Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. OM 1991, 10, 1501.
4. Grossman, R. B.; Davis, W. M.; Buchwald, S. L. JACS 1991, 113, 2321.
5. (a) Morken, J. P.; Didiuk, M. T.; Hoyveda, A. H. JACS 1993, 115, 6997. (b) Hoyveda, A. H.; Morken, J. P. JOC 1993, 58, 4237.
6. Hong, Y.; Kuntz, B. A.; Collins, S. OM 1993, 12, 964.
7. (a) Wild, F. W. R. P.; Wasiucionek, M.; Huttner, G.; Brintzinger, H.-H. JOM 1985, 288, 63. (b) Collins, S.; Kuntz, B. A.; Taylor, N. J.; Ward, D. G. JOM 1988, 342, 21.
8. An alternative procedure based on fractional crystallization of the bis((R)-O-acetylmandelate) derivatives has also been reported. Schaefer, A.; Karl, E.; Zsolnai, L.; Huttner, G.; Brintzinger, H.-H. JOM 1987, 328, 87.

Scott Collins

University of Waterloo, Ontario, Canada



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