Methyl-substituted zirconocene

Farina et al. first prepared hemiisotactic polypropylene in 1982 through the hydrogenation of isotactic poly(2-methylpenta-l,3-diene). The first direct, stereoselective synthesis of hemiisotactic polypropylene was reported by Ewen, et al. in 1991" with the methyl-substituted zirconocene (CH3)2C(3-CH3-C5H3)(Ci3H8)ZrCl2 (h-1) activated with Scheme 2.4 depicts... 

Lithiation of commercially available bromoarenes (281) followed by treatment with zirconocene (methyl) chloride affords zirconocene complexes of substituted benzynes (283), which react with symmetric alkynes and l-(trimethylsilyl)propyne to give single regioisomeric zirconacycles, (284) and (285), respectively (Scheme 66). Both (284) and (285) react with disulfur dichloride to produce benzothiophenes (286) and (287), respectively, in 60-80% isolated yields in a one-pot procedure <89JOC2793>. Protodesilylation of (287) can be accomplished in >90% yield to give the corresponding 2-unsubstituted benzothiophenes by treatment with tetrabutylammonium fluoride in tetrahydrofuran. Application of the procedure to the preparation of 2,3-dihydrobenzo[6]thiophenes was also reported <9iOM537>. 

Alkynes may be converted to the corresponding substituted vinylalanes by the action of zirconocene dichloride and trimethylaluminum (Eq. 42) (254). The regioselectivity for Z-addition of the Al—R bond is generally >95%, but drops significantly if R is larger than the methyl group. 

The zirconocene 1 catalyzed cyclization is also convenient for cyclization of dienes bearing a leaving group in the allylic position with respect to the one of the double bonds. In this process the l-vinyl-2-methylcyclopentanes 107 (Scheme 45) are formed. The reaction can be carried out both in Et20 and THF [58, 59]. Formation of the l-vinyl-2-methyl-cyclohexenes proceeds stereo-specifically to give c/s-substituted products when the leaving group OR is the part of the ds-substituted double bond [53]. 

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