Schwartz’ Reagent

Alkenylzirconium reagents generated from alkynes and the Schwartz reagent react with aldimines under mild conditions in the presence of [RhCl(COD)]2 to give homoallylic amine derivatives (Equation (63)).418... 

Hydrozirconation of alkynes with the Schwartz reagent, Cp2Zr(H)Cl (6), and subsequent methylation is also a general method (Eq. 2.5) [16]. 

Patterns for addition of the Schwartz reagent to multiple C—C bonds in terms of regio-and stereochemistry are well-established for most substituents, as is the fundamental chemistry of the newly formed Csp2—Zr and Csp3—Zr bonds. Conversion to halides and... 

Scheme 4.1. Reduction of a tertiary amide with the Schwartz reagent.

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

General procedure for reduction of tertiary amides to aldehydes using the Schwartz reagent... 

Acylzirconocene chloride derivatives are easily accessible in a one-pot procedure through the hydrozirconation of alkene or alkyne derivatives with zirconocene chloride hydride (Schwartz reagent) [Cp2Zr(H)Cl, Cp = cyclopentadienyl] and subsequent insertion of carbon monoxide (CO) into the alkyl— or alkenyl—zirconium bond under atmospheric pressure (Scheme 5.1) [2],... 

The first example of a stable 1,1-bidentate Lewis acid based on boron and zirconium has been reported [35]. The synthesis of 22 is outlined in Scheme 7.12. Treatment of hex-l-yne with HBBr2 Me2S followed by conversion of the dibromoboronic ester to the corresponding alkenyl boronic acid and esterification with propane-1,3-diol provided the alkenyl boronic ester. Hydrozirconation of this compound with 3 equivalents of the Schwartz reagent, Cp2Zr(H)Cl [57], afforded the desired product 22 in 86% yield. 

Preparation of Boriozirconocene Complex 16 (Scheme 7.5) [33] To a stirred ice-cooled suspension of the Schwartz reagent (0.26 g, 1 mmol) in dry CH2C12 (1 mL) was added a solution of B-hexenyl-9-BBN 15 (0.20 g, 1 mmol) in dry CH2C12 (1 mL). The resulting cloudy mixture became a dear yellow solution in 1 h at 0 °C (or 10 min at ambient temperature), indicating the formation of 16. 

Stable acylzirconocene chloride has been utilized by Hanzawa and coworkers [58] for the synthesis of a-hydroxy ketones. Consecutive hydrozir-conation of terminal alkenes or alkynes with Schwartz reagent and in situ carbonylation with gaseous CO at room temperature affords acylzirconocene... 

The following scheme illustrates the different regio- and stereochemical outcomes of the sequential addition of the Schwartz reagent (Cp2Zr(H)Cl) and butyl tellurenyl bromide to acetylenic selenides. ... 

The above-described procedure is advantageous towards the precedent method since (a) the starting lithium alkynyl selenolate is prepared in situ, avoiding the laborious preparation of the acetylenic selenides and (b) the hydrozirconation step is regio- and stereoselective, in contrast with the previously discussed hydrozirconation of acetylenic selenides resulting in a mixtnre of the regioisomers, and requires only 1 equiv of the Schwartz reagent instead of 2 eqniv of the precedent procednre. 

Acyl tri-isopropylsilanes containing alkene or alkyne functionality undergo smooth hydrozirconation with Schwartz reagent, in contrast to the corresponding aldehydes, which also suffer addition to the carbonyl group (Scheme 118)225. 

Schwartz reagent Cp2Zr(H)Cl behaves in a similar way to that described above. 

Chlorodi(cyclopentadienyl)hydridozirconium (Schwartz reagent), Cp2Zr(H)Cl... 

Vinyl cuprates.3 An attractive route to vinyl cuprates involves transmetalla-tion of vinylzirconates, available by hydrozirconation of 1-alkynes with the Schwartz reagent in THF at 25°. Transmetallation can be effected with CH3Li (3 equiv.)... 

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