Hydrozirconation Zr

In hydroboration, a boron hydride (R2BH) adds across an alkene (R CH=CH2) to give R CH2CH2BR2. The 16-electron, d° Zr(IV) complex Cp2Zr(H)Cl, popularly known as Schwartz reagent, undergoes a closely related reaction. The mechanism involves coordination of an alkene to the electrophilic Zr center followed by migratory insertion of the alkene into the Zr-H bond. The reaction proceeds for alkynes also, by exactly the same mechanism. 

As mentioned earlier, hydrozirconation of internal alkenes gives terminal alkylzir-conium compounds. The isomerization occurs so quickly that no intermediates are observed. The mechanism of the reaction is a series of insertions and /3-hydride eliminations. 

Problem 6.4. When internal alkynes are hydrozirconated, the Cp2ZrCl group 

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It was shown (654) that the sequence terminal alkyne hydrozirconation, Zr to Zn exchange and addition to nitrones, is a good method to the stereoselective synthesis of (E)-N-allylhydroxylamines, under mild conditions and in good yield. 

Scheme 17 Application of a one-pot hydrozirconation, Zr Zn transmetalation, aldehyde addition toward the total synthesis of ( )-nisamycin...

The alkenylzirconium 685, prepared by hydrozirconation of a terminal alkyne with hydrozirconocene chloride, reacts with alkenyl halide to afford the conjugated diene 686(545]. The Zr reagent can be used even in the presence of the carbonyl group in 687, which is sensitive to Al and Mg reagents. 

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

SCHWARTZ Hydrozirconalion Hydrozirconation with Cp2Zr(CI)H and Michael addition of the Zr reagent... 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

Hydrozirconation of iminoboranes -B=N-R leads to a four-membered ring with a Zr-H-B hydride bridge [209]. 

Since the initial reports of Schwartz s reagent ([Cp2Zr(H)Cl]n, 1) over 30 years ago there has been explosive growth of zirconocene chemistry. Hydrozirconation is an efficient method of hydrometalation because of (i) the mild nature of the conditions involved, (ii) the excellent regio- and stereo-chemical control of hydrozirconation, (iii) the one-pot nature of the procedure, and (iv) the price of Zr, which is one of the... 

Derivatives of zirconium with a Zr-H bond also can add to alkenes and alkynes. This reaction is known as hydrozirconation.230 The reagent that is used most frequently... 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

The E configuration of the products, assessed by NOE experiments, is a consequence of the known 100% regio- and ii-stereoselectivity of the hydrozirconation of acetylenic stan-nanes, and the retained configuration in the Zr/Te exchange. 

The hydrozirconation of alkenes will generally result in positional isomerization of the Zr to the least-hindered position. The Zr-intermediate can be replaced with Br (Br2, NBS), I (I2), Cl (NCS). OH (MCPBA, or basic aq. hydrogen peroxide). 

Besides hydrozirconation of terminal triple bonds Cp2Zr(H)Cl (16) also reacts with double bonds.8 The mechanism is similar to that described for alkynes. After coordination of alkene 5 to the Zr center giving -complex 23 the terminal double bond inserts into the Zr-H bond to form the stable (T-alkyl complex 24. The bulky zirconocene moiety again adds to the end-position of the terminal double bond. 

The use of Cp2Zr(H)Cl in hydrozirconation reactions has been referred to (80), and consists of addition to the Zr-H moiety to unsatu-... 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

An important use of Cp2ZrHCl is in the so-called hydrozirconation reaction with alkenes and alkynes which generates a Zr-alkyl group which can then be converted to alcohols or other products.67 In the reactions of CO with Zr and Hf compounds the oxophilicity of these elements is important, because it leads to the formation of a variety of compounds that have M—O bonds. 

Addition of Cp2Zr(H)Cl to an alkenylzinc hahde affords relatively unstable 1,1-bimetallics of Zn and Zr. Addition of an aldehyde or a ketone is a general way to synthesize ( )-alkenes. Remarkably, the hydrozirconation can be applied to functionahzed alkenylzinc reagents bearing an ester, chloride or cyano functionahty (equation 43). ... 

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