Bis(cyclopentadienyl)zirconium chloride hydride

1. Reagent for the hydrozirconation of olefins.
2. Reagent for the hydrozirconation of alkynes.
3. Catalyst for the hydroboration of olefins.
4. Mediates the reduction of tertiary amides to aldehydes, without reduction of cyano, nitro, ester, or α,β-unsaturated groups.
5. Catalyst for the asymmetric vinylation and dienylation of ketones.
6. Useful reagent for dimethylzinc mediated additions alkenylzirconocenes to aldimines.
7. Useful reagent for the synthesis of oxidized amides from nitriles.
8. Reagent for the hydrozirconation of propargylic alcohols.

Dicyclopentadienylzirconium hydride chloride, zirconocene hydride chloride, Schwartz’s reagent, Cp2ZrHCl (Cp = C5H5), is a white microcrystalline powder, mp >300℃(dec.). Under the influence of light or heat the color changes to pink, then gradually to dark red. The compound is sensitive to moisture; it is virtually insoluble in the usual organic solvents such as ethers, consistent with its polymeric character. It dissolves in halogenated solvents with evolution of heat, forming Cp2ZrClX, where X = Cl, Br, or I.

Bis(cyclopentadienyl)zirconium hydridochloride is used in the conversion of amides to aldehydes. It is also used as a pharmaceutical intermediate.

Functionalization of unsaturated systems via an organozirconium intermediate. Schwartz’s reagent is used in limited amounts in organic synthesis for the selective hydrozirconation of alkenes and alkynes. The reaction takes place rapidly and quantitatively at room temperature, forming zirconium alkyls or alkenyls that may be converted into useful functionalized derivatives. For example, reaction with carbon monoxide followed by removal of the zirconium-containing part of the molecule yields a variety of carbonyl compounds.

Schwartz’s reagent is produced by reaction of zirconocene dichloride (Cp2ZrCl2) with LiAlH4 or LiAlH[OC(CH3)3]3 in THF solution. The commercial product (Alfa Ventron, Aldrich) is slightly pink.

It is moisture and light sensitive. Determine its purity by reaction with a slight excess of Me2CO whereby the active H reacts to produce Cp2ZrClOPri and the integrals of the residual Me2CO in the 1H NMR will show its purity. The presence of Cp2ZrH2 can be determined because it forms Cp2Zr(OPri)2. For a very active compound, it is best to prepare it freshly from the dichloride (see below) by reduction with Vitride [LiAl(OCH2CH2OH)2H2], the white precipitate is filtered off, washed with tetrahydrofuran, then Et2O and dried in a vacuum. Store it dry in the dark. [Carr & Schwartz J Am Chem Soc 101 3521 1979, Negishi & Takahashi Synthesis 1 1988, Beilstein 16 IV 1770.]