Zirconacyclopentanes synthesis

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

Interestingly, zirconacyclopentane 246 formed by the reaction of 1,6-heptadiene with the Zr complex has the firms ring junction mainly [108]. It should be noted that the preparation of the trans ring junction in the bicyclo[3.3.0]octane system by other means is difficult. Carbonylation of 246 affords trans-fuzed bicyclo[3.3.0]octanone 247 [109,111]. The diacetoxy compound 248 is obtained by oxidative cleavage of 246. Protonation affords the frans-dimethylcyclopentane skeleton. Similar reactions occur with 1,6-enynes, and Pauson Khand-type cyclopentenone synthesis is possible by carbonylation. 

Whitby, Blagg, and coworkers have used a similar method for zirconaaziridine synthesis (Scheme 1). They inserted phenyl isocyanide into a Zr-C bond of the zirconacyclopentane 11 thermal rearrangement gave the zirconaaziridine 12. 

The general area of zirconacyclopentane chemistry has been reviewed.76 Because these molecules are best viewed as zirconium or hafnium dialkyl complexes, and hence as tetravalent metal centers, this renders them beyond the scope of this review. It should be noted that these molecules display a rich reaction chemistry, serving as initiators for olefin polymerization,77-79 reagents for organic methodology,57,80-86 and key intermediates in natural product synthesis.87,88... 

The ethylene moiety of zircona- and hafnacyclopentane can be replaced with more strongly coordinating ligands. Trimethylphosphine induces retro-oxidative cyclization of a zirconacyclopentane and replaces one ethylene ligand to afford a monoethylene trimethylphosphine complex (Scheme 1.63) [82]. Alkynes, nitriles, and aldehydes also substitute the olefin moiety through retro-oxidative cyclization and re-oxidative cyclization to afford thermodynamically more stable zirconacy-cles (Scheme 1.64) [83]. The replacement reaction has been applied to the selective synthesis of polysubstituted benzenes and pyridines [84]. 

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