Schwartz’ Reagent Zirconocene

The trans-vmyi iodide is obtained by hydrozirconation, in which zirconocene dichloride is converted in situ into the analogously reactive i-butylzirconocene chloride. [233] Zirconocene dichloride is readily available, and less expensive than the Schwartz reagent, zirconocene hydridochloride. 

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],... 

Some transition-metal hydrides show promise as synthetic reagents of the same general applicability as the boron hydrides (Section 11-6). An excellent illustration is provided by the work of J. Schwartz with zirconocene chloro-... 

Finally, it should be noted that enantiomerically pure derivatives of the Schwartz reagent are very appealing candidates for reagent controlled enantioselective and diastereoselective radical reductions. This is because a plethora of enantiomerically pure zirconocenes are available from other applications in enantioselective catalysis. 

Butenolides,1 Addition of the Schwartz reagent to a protected propargylic alcohol (2) followed by carbonylation provides an acyl zirconocene complex (3). This is not isolated but treated in situ with iodine to provide an intermediate (a) that cyclizes to a 3,5-disubstituted butenolide (4). Optically active substrates undergo this sequence with no loss of optical purity. 

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). 

Hydrozirconation of alkynes with the Schwartz reagent Cp2Zr(H)Cl yields the chlorovinyl zirconocene 17, which is easily converted to the methyl vinyl zirconocene 18 with either methyllithium in THF or methylmagnesium bromide in CH2C12. Compound 18 loses further methane at room temperature to form a zirconacyclopropene intermediate 19, which couples with a second alkyne and forms the metallacyclopentadiene 20 (Scheme 6) [34]. 

Hydrozirconation. The combination of zirconocene dichloride and t-butylmag-nesium chloride in C,H4-ether effects hydrozirconation of monosubstituted alkenes. The actual reagent may be HZrCp,Cl (Schwartz reagent, 6. 175-177) or t-BuZrCp,Cl. The reaction produces monoalkylzirconium derivatives with the metal attached to the terminal position of the alkene. 

Generation of organozirconium species 18 follows the hydrozirconation of the triple bond using zirconocene hydrochloride (Schwartz reagent) 16. This 16-electron, d° Zr(IV) complex is coordinatively unsaturated, so alkyne 15 coordinates to the electrophilic Zr center followed by insertion of the triple bond into the Zr-H bond. The resulting a-vinyl-Zr(lV)-species 18 is formed with a high cA-selectivity and regioselectivity, such that the bulky zirconocene moiety always adds end-on to the terminal multiple bond. 

Fernandez-Megia, E. Zirconocene hydrochloride, "Schwartz reagent". Synlett 999, 1179. 

Nonanoylzirconocene chloride (1), readily prepared by the reaction of 1-octene and zirconocene hydrochloride (Schwartz reagent) followed by exposure to 1 atm of CO, serves as an umnasked acyl anion and provides ketones or a-diketones in moderate to good yields by the reaction with electrophiles (aryl iodide, benzyl bromide, acyl chloride, allyl chloride, aUyl acetate) in the presence of 5% of PdCl2(PPh3)2 (Scheme 47). ... 

Notwithstanding the heartiness of this group in resisting reduction by Schwartz reagent or its reluctance to serve as a Lewis base toward zirconocene intermediates, its manipulation to other-valued functionality is straightforward. 

Scheme 8-4 Synthesis of zirconocene hydride Cp2Zr(H)OTf alternative to the Schwartz s reagent, ...

In their early studies, Schwartz and co-workers [5, 80] reported the zirconocene hydrido chloride [Cp2Zr(H)Cl] (1) as a reagent capable of reacting under mild conditions with a variey of non-functionalized alkenes to form isolable alkylzirconi-um(lV) complexes Cp2Zr(R)Cl in which the zirconium is attached to the least-hindered terminal primary carbon, irrespective of the original location of the double bond in the olefin chain. As an example, at room temperature in benzene, 1-octene, cis-4-octene and trows-4-octene all yield the n-octylzirconocene derivative (Scheme 8-6) [80]. 

As described in the previous section for unsaturated functional groups, Schwartz s reagent (1) and most zirconocene(IV) hydrides readily deprotonate acidic moieties [183, 218]. 

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... 

Schwartz s Reagent3 is available commercially (from the Aldrich Chemical Company, Inc.) although it is quite expensive. Two literature preparations of this important reagent are available. The first utilizes LiAI(OtBu)3H to reduce zirconocene dichloride.4 The second method utilizes sodium bis(2-methoxyethoxy) aluminum hydride (RED-AL) as the reducing agent.2a The disadvantages of these procedures have been discussed.3... 

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