Dibornacyclopentadienyltrichlorozirconium1

[126035-93-6]  · C21H29Cl3Zr  · Dibornacyclopentadienyltrichlorozirconium  · (MW 479.07)

(chiral Lewis acid for enantioselective C-C coupling of pyruvic esters with active arenes1)

Physical Data: 1H NMR (CDCl3, 200 MHz): d 2.73, 3.05 (4,4-H), 0.8-1.0, 1.8-2.2, 2.68 (5,5,6,6-H), 6.11 (8-H), 0.26, 0.92, 0.93, 0.95, 1.19, 1.46 (1,1,7,7-CH3). 13C NMR (CDCl3, 50 MHz, 1J(CH) in Hz): d 11.6, 12.5 (1,1-CH3), 19.8, 20.6 (double intensity), 29.8 (7,7-CH3), 25.8 (122) and 00.0 (125; 5,5), 32.1 (131) and 38.5 (137; 6,6), 50.5 (149) and 51.6 (147; 4,4), 55.8 (triple intensity), 68.6 (1,1,7,7), 106.4 (173; 8), 144.9, 146.9, 152.7, 159.7, 159.2 (2,2,3,3).

Solubility: sol dichloromethane.

Preparative Method: reaction of dibornacyclopentadienyllithium with ZrCl4 (40% yield).1

Preparation.

Both the title zirconium catalyst (1), and the corresponding hafnium derivative (2) have been synthesized according to Scheme 1. Addition of 2 equiv of 2-bornen-2-yllithium (4) (generated via the Shapiro reaction2 from (+)-camphor (3)) to ethyl formate gives bis(2-bornen-2-yl)methanol (5). Cyclization of alcohol (5) is carried out with Potassium Hydrogen Sulfate at 100-150 °C to afford dibornacyclopentadiene (6). Deprotonation of (6) with n-Butyllithium followed by reaction with Zirconium(IV) Chloride affords (1). Reaction with HfCl4 affords (2).

Arylation of a-Keto Esters.

Zirconium complex (1) has been used as a catalyst for the synthesis of optically active 2-(2-hydroxyaryl)lactic acid ethyl ester (7) (eq 1).1

It was found that lowering the reaction temperature from rt to -10 °C resulted in an increased enantiomeric excess (from 27% ee to 54% ee). Surprisingly, the addition of small amounts of water (ca. 20 mol %) also increased the chiral induction observed (up to 89% ee).


1. Erker, G.; van der Zeijden, A. A. H. AG(E) 1990, 29, 512.
2. Chamberlin, A. R.; Stemke, J. E.; Bond, F. T. JOC 1978, 43, 147.

Eduardo Peña-Cabrera

Emory University, Atlanta, GA, USA



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