Schwartz's reagent

Schwartz's reagent

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Names
IUPAC name chloridobis(η5-cyclopentadienyl)hydridozirconium
Other names Cp2ZrClH, zirconocene chloride hydride
Identifiers
CAS Number	37342-97-5
ChemSpider	28605366
Jmol 3D model	Interactive image
InChI InChI=1S/2C5H5.ClH.Zr.H/c2*1-2-4-5-3-1;;;/h2*1-5H;1H;;/q2*-1;;+3;/p-1 Key: GBJQOFBMEJYDAU-UHFFFAOYSA-M InChI=1/2C5H5.ClH.Zr.H/c2*1-2-4-5-3-1;;;/h2*1-5H;1H;;/q2*-1;;+3;/p-1/r2C5H5.ClHZr/c2*1-2-4-5-3-1;1-2/h2*1-5H;2H/q2*-1;+2 Key: GBJQOFBMEJYDAU-CFXPZLBWAB
SMILES [cH-]1cccc1.[cH-]1cccc1.Cl[ZrH+2]
Properties
Chemical formula	C10H11ClZr
Molar mass	257.87 g/mol
Appearance	White solid
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Schwartz's reagent is the common name for the chemical compound with the formula (C₅H₅)₂ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Princeton University. This metallocene is used in organic synthesis for various transformations of alkenes and alkynes.

Preparation

The complex was first prepared by Wailes and Weigold. It can be purchased or readily prepared by reduction of zirconocene dichloride with lithium aluminium hydride:

(C₅H₅)₂ZrCl₂ + ​¹⁄₄ LiAlH₄ → (C₅H₅)₂ZrHCl + ​¹⁄₄ "LiAlCl₄"

In practice this reaction also affords (C₅H₅)₂ZrH₂, which is treated with methylene chloride to give the mixed hydride chloride. An alternative procedure that generated Schwartz's Reagent from dihydride has also been reported.

Uses in organic synthesis

Schwartz's reagent can be used for a number of reactions. It has been shown that it can be used to reduce amides to aldehydes. Reducing tertiary amides with Schwartz's reagent can reach efficient yields, but primary and secondary amides will show decreased yields. The use of Schwartz's reagent in this manner will not require any added heat and can be done quickly, and reduction of the alcohol form is not a problematic side reaction as it can be with other reducing agents. Schwartz's reagent will selectively reduce the amide over any readily reducible esters that may be present in the reaction mixture.

Vinylation of ketones in high yields is a possible use of Schwartz's reagent.

Schwartz's reagent is used in the synthesis of some macrolide antibiotics, (−)-motuporin, and antitumor agents.

Hydrozirconation

File:SchwartzAttack.jpg

Two locations of attack are possible

Schwartz's reagent reacts with alkenes and alkynes via the process called hydrozirconation, which results in the addition of the Zr–H bond across the C=C or C≡C bond.

The selectivity of the hydrozirconation of alkynes has been studied in detail. Generally, the addition of the Zr–H proceeds via the syn-addition. The rate of addition to unsaturated carbon-carbon bonds is terminal alkyne > terminal alkene ≈ internal alkyne > disubstituted alkene Acyl complexes can be generated by insertion of CO into the C–Zr bond resulting from hydrozirconation. Upon alkene insertion into the zirconium hydride bond, the resulting zirconium alkyl undergoes facile rearrangement to the terminal alkyl and therefore only terminal acyl compounds can be synthesized in this way. The rearrangement most likely proceeds via β-hydride elimination followed by reinsertion.

Computational studies indicate that hydrozirconation occurs from the interior portion.

External links

• Examples in organic synthesis at the University of Connecticut website

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