Diastereoselective Hydrozirconation

Several trialkoxy(2-butenyl)zirconium(IV)6,7i 18 and 2-butenylbis(cyclopentadienyl)zirco-nium(IV)18,19 124 complexes have been investigated with respect to the diastereoselectivity on addition to aldehydes. Chlorobis(cyclopentadienyl)-(3-tributylstannyl-2-propenyl)zirconium(IV), prepared by hydrozirconation of tributyl-(l,2-propadienyl)tin, accomplishes the (E)-selective, Wittig-like 1,2-propenylidenation of aldehydes and methyl ketones125. 

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

On the other hand, hydrozirconation-transmetalation to zinc, when carried out in THF, can be used to prepare allylic amines. When carried out in CH2CI2, however, a different reaction pathway is followed (Scheme 24). Even more, with a different order of introduction of the reagents, it is possible to obtain diastereoselectively homoaUyhc amines instead of the expected aminocyclopropanes. ... 

With cyclic alkenes, fairly high diastereoselectivity can be achieved. Hydrozirconation of norbomene, followed by TBHP oxidation, gives the exo-alcohol in 95% yield, while the more complex bicyclics camphene (37) and P-pinene (38) give the mixtures shown in equations (39) and (40), respectively. TTie hafnium analog shows the opposite preference with P-pinene and does not react with camphene at all, attributed to significantly greater steric demand. Diastereoselectivity in cyclohexanols obtained by hydrozirconation of ketones has also been examined. ... 

Intramolecular nucleophilic ring-opening reactions of oxazolines are also known. Hydrozirconation of A -allylox-azolidine 141 gave a zirconocene intermediate 142, which reacted with TiCh to afford the corresponding 2-substituted pyrrolidine 143 in good yield and diastereoselectivity (Scheme 36) <2005OL4887>. 

The generation and subsequent reaction of oxy-functionalized allylic zirconium reagents to a wide range of aldehydes proceeds with excellent anti selectivity [188]. Allylic zirconium reagents can also be prepared from the hydrozirconation of allenes [189]. Very high levels of diastereoselectivity for both simple aliphatic and aromatic aldehydes are observed in these reactions for the production of the anti homoallylic alcohol (Scheme 10-97). 

The diastereoselective additions of chlorotitanium enolates of 7V-propionyl thiazolidinethione 133 to various metalloaldimines 134, available from hydrometallation of the corresponding nitriles, furnish a mixture of azetine 135 and tetrahydropyrimidinone 136 <04H(62)217>. Among the three hydrometallation methods evaluated, the hydrozirconation process proves to be the best in terms of the yield and selectivity. 

Several natural products total syntheses used the hydrozirconation-transmetal-lation strategy for the constmction of a structurally elaborated carbon framework such as the preparation of polyene segment in (+)-curacin A and in the manumy-cin family [17]. An interesting diastereoselective transmetallation reaction of functionalized Zr to Zn was also recently reported in the synthesis of fostriedne 18 [18] and an enantioseletive transformation of functionalized Zr to Zn followed by reaction with an aldehyde was also reported at a late stage of the total synthesis of (+)-halichlorine 19 (Scheme 12.15) [19]. 

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