Hydrozirconation of Allenes

AUylzirconium complexes are conveniently obtained by the regio- and stereoselective hydrozirconation of allene [127-133] and can be, for example, used subsequently for the MAO-catalyzed allylzirconation of alkynes to prepare enyne derivatives [132]. Alternatively, the preparation of (E)-l,3-dienes from aldehydes and the appropriate allylstannane zirconocene derivative (R = SnBu,) is accomplished (Scheme 8-17) [131], Note that addition of [Cp2Zr(H)Cl[n (1) on the aUenyl reagent with the... 

Hydrozirconation of allenic systems preferentially leads to allylic zirconocenes, which are highly reactive and thus very useful organometallic reagents. Allenic sulfides react in the expected fashion to give the (E)-y-thiophenylallylzirconocene chloride 20 (Scheme 4.18) [47]. These intermediates, upon introduction of an aldehyde or methyl ketone, give predominantly the anti isomer (ratios from 82 18 to > 97 3). Exclusive 1,2-addition was observed by Suzuki et al. in the case of an a,f5-unsaturated aldehyde. As long as the steric demands of the two substituents attached to the ketone carbonyl are significantly different, synthetically useful levels of selectivity can be achieved. 

Allylic zirconocenes, generated by hydrozirconation (see Hydrozirconation) of allenic systems, react with terminal alkynes when activated with MAO to regioselectively afford 1,4-dienes. The same MAO catalyzed process, applied to haloalkynes, leads to excellent yields of the 1,4-enyne (100) (Scheme 23). ... 

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). 

Hydrozirconation of monosubstituted allenes offers easy access to allylzircono-cene chlorides, which react with carbonyl compounds to afford homoallylic alcohols in a highly regio- and stereoselective manner (Scheme 16.68) [73-75],... 

Substituted 1,4-dienes are obtained by reductive coupling of allenes and 1-alkynes via hydrozirconation of the allenes. Greatly enhanced regioselectivity is attained by using methoxyaluminoxane as catalyst. 

Alkynylzinc bromides 133 are also cleanly hydrozirconated with Cp2Zr(H)Cl leading to 1,1-bimetalloalkenes of zinc and zirconium (134), which react smoothly with aldehydes to afford allenes in satisfactory yields (Scheme 7.40) [232,233]. 

Unusual regioselectivity has been observed180 during the reductive coupling of alkynes and allenes by hydrozirconation and zinca-Claisen rearrangement. 

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