[10026-11-6] · Cl4Zr · Zirconium(IV) Chloride · (MW 233.02)
Physical Data: subl. 331 °C, mp 437 °C/25 atm; d 2.803 g cm-3.
Solubility: sol alcohol, ether.
Form Supplied in: white solid; widely available.
Purification: crystallize repeatedly from conc hydrochloric acid.
Handling, Storage, and Precautions: extremely hygroscopic; decomposed by water to ZrOCl2 and HCl. Reagent causes burns, and is harmful if inhaled or swallowed or if it comes in contact with skin. It is stable at high temperatures under N2, H2, or O2. Use in a fume hood.
ZrCl4 is used as starting material for other zirconium derivatives. Its Lewis acid character is involved in most of its reactions. As a Lewis acid its catalytic power is comparable with that of Titanium(IV) Chloride in Friedel-Crafts reactions.1
ZrCl4 has been employed as one of the Lewis acid catalysts in the addition of the acrylate of (S)-ethyl lactate with cyclopentadiene,2 and further in asymmetric Diels-Alder cycloaddition reactions with chiral a,b-unsaturated N-acyloxazolidinones.3
ZrCl4 is a good catalyst for the [2 + 2] adduct formation between ethyl propiolate and TMS enol ethers (eq 1).4
Like TiCl4 and other Lewis acids, ZrCl4 is a good catalyst in aldol coupling reactions. ZrCl4 is not superior, however, to other Lewis acids in a stereochemical study of the reactions of allylstannanes and a methyl homolog with aldehydes.5 Corresponding intramolecular cyclization reactions are fast, selective, and insensitive to the nature of the Lewis acid (87-99% syn).6 Stereoisomeric ratios and yields were relatively insensitive to the nature of the Lewis acid catalyst in adduct formation between Allyltrimethylsilane and methyl b-formylcarboxylate (eq 2).7
In the aldol reaction between menthyl pyruvate and 1-naphthol, the de of the menthyl 2-(1-hydroxy-2-naphthyl)propionate adduct was 74%, the yield 92%.8 The adduct of 1-naphthol and ethyl pyruvate using the chiral
dibornacyclopentadienyl ligated zirconium chloride (1) had an ee >84% (eq 3).9
ZrCl4 will promote the addition of Lithium Aluminum Hydride to alkenes under mild conditions to yield hydroaluminated intermediates for further transformations (eq 4).10 The reaction has been extended to allylic alcohols. Deoxygenation accompanied the use of titanium analogs.11
Mild esterification conditions utilizing ZrCl4 and Silver(I) Trifluoromethanesulfonate will effect reaction between silyl carboxylates and silyl ethers.12 Chemoselective amination of the ester function in a-chlorocarboxylic methyl esters can also be effected (eq 5). ZrCl4 and Aluminum Chloride are good Lewis acids for this transformation, which proceeds without significant epimerization of enantiomerically enriched chloropropionate.13
The affinity for oxygenated functions can be used to effect ring-opening reactions of cyclic ethers, leading to 6-hydroxy-2,4-cycloheptadien-1-ones.14
Norwegian College of Pharmacy, Oslo, Norway
Frode Rise & Kjell Undheim
University of Oslo, Norway