Zirconium reagents

See page 559, Section 8 for olefin metathesis leading to cycloalkenes. 

Nuc2 = R2CuLi, R2CuLi R2CuLi, ketone enolate R2CuLi, 

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The corresponding zirconium reagents are prepared in a similar manner7,9. 

Table 1. 1,2-Disubstituted 3-Butenols by Addition of 2-Alkenyltitanium or -zirconium Reagents to Aldehydes...

The reactions of various 2-butenyltitanium and -zirconium reagents with 2-(benzyloxy)propanal (and other O-protected derivatives) also proceed with low induced diastereoselectivity90. 

Carbon-carbon bond formation from alkyl and alkenyl zirconium reagents usually involves transmetallation reactions and are discussed in Chapter 8. 

An even more reactive activator of glycosyl fluorides is generated by employing a 1 2 ratio of the metallocene and silver salt in benzene [25]. The zirconium reagent proved... 

For the asymmetric 1,4-addition of alkenyl groups in aprotic media, alkenyl zirconium reagents can be used, which are generated by hydrozirconation of terminal alkynes (Figure 3.30). Under these conditions, alkenyl groups derived from alkylacetylenes are efficiently installed, but those from arylacetylenes are not as effective (entry 3). 

Dicyclopentadienyldihydridozirconiuni, Cp2ZrH2. This zirconium reagent is obtained as a white solid by reduction of (Cp2ZrCl)20 with LiAlH4 in THF in 66% yield.1... 

Alkenyl-aluminum and -zirconium derivatives have been found to couple with allyl halides in the presence of Pd° catalysts (equation 38), although simple alkyl-aluminum and -zirconium reagents fail in the reaction.154 The 1,4-dienes thus generated are important intermediates in organic synthesis. 

Alkyl(tributoxy)zirconium reagents, RZr(OBu), (l).1 These reagents are pre-... 

Reduction of the triple bond is very uncommon and involves equimolar amounts of a zirconium reagent. 

The methylation of pregnenolone acetate 58 to form 59 occurs chemoselectively with 6 83). The zirconium reagent 61, prepared by quenching methyllithium with 60, reacts analogously83), so that no advantages are apparent in this case. 

Unfortunately, 6 and 63 as well as the zirconium reagent 61 also fail. Based on the observation that reagents containing two methyl groups are more reactive and less ... 

Variations due to temperature or solvent effects are small. The tri-phenoxy derivative 127 (Table 6) displays the highest degree of selectivity. The zirconium reagents 61... 

Michael additions of organotitanium or zirconium reagents remain to be explored systematically. Recently, Stork described an interesting stereoselective intramolecular Michael addition in which zirconium enolates appear to be involved113). In another Michael type process, methyltitanium triisopropoxide 6 was added enantioselectively to a chiral a, p-unsaturated sulfoxide, but CH3MgCl was more efficient114). 

Table 9. Threo-selective Addition of Crotyltitanium and Zirconium Reagents to Aldehydes 216...

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). 

A particularly clear example of syn versus anti addition was carried out in stoichiometric fashion using vinyl-zirconium reagents, in direct comparison with the stereochemical outcome of the catalytic ally lie alkylation with soft anions (Scheme 17). The zirconium reagent gives the natural steroid coiffiguration, while the malonate anion gives the opposite... 

Other acyl organometallic reagents are known. An acyl zirconium reagent, such as RCOZr(Cl)Cp2, reacted with allylic bromide in the presence of Cul to give the corresponding ketone, but with allylic rearrangement. ... 

Vinyl organometallic reagents can be added to acyl derivatives. Reaction of an alkyne with Cp2ZrEt2 generates the vinyl zirconium reagent, which react with ethyl chloroformate to give an a,p-unsaturated ester. °°°... 

Whereas ethylmagnesiation of allyl alcohols affords the syn diol with 95 5 diastereoselectivity, the same reaction on the allyl methyl ether affords predominantly the anti diastereomer with 90 10 selectivity [92] (Scheme 7-80). The reversal of stereoselectivity was attributed to the association of the magnesium alkoxide (and not of ethers) with the zirconium reagent (Scheme 7-80). 

Dienyl Titanium and Zirconium Reagents 133.6 Propargylic Titanium Reagents... 

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