UBC Theses and Dissertations
Computational mechanistic studies of decamethyldizincocene formation and the enantioselective reactive nature of a chiral neutral zirconium amidate complex Hepperle, Steven Scott
Computational methods were employed to study the surprising 2004 synthesis of de-camethyldizincocene, Zn2(η5−C5Me5)2, which was the first molecule to have a di- rect, unbridged bond between two first-row transition metals. The computational re- sults show that the methyl groups of decamethylzincocene, Zn(η5−C5Me5)(η1−C5Me5), affect the transition-state stability of its reaction with ZnEt2 (or ZnPh2) through steric hindrance, and this allows a counter-reaction, the homolytic dissociation of Zn(η5−C5Me5)(η1−C5Me5) into Zn(η5−C5Me5)• and (η1−C5Me5)• radicals to occur, and since no such steric hindrance exists when zincocene, Zn(η5−C5H5)(η1−C5H5), is used as a reactant, its dissociation never occurs. Experimentally, it was found that forming decamethyldizincocene is more efficient when using a reducing agent (e.g., KH) and ZnCl2 as opposed to a ZnR2 reagent. The computational results show that the methyl groups of decamethylzincocene have a similar indirect effect on the reaction. When zincocene is used, the reaction with KH favours the route that results in the formation of the zincate, K+[Zn(η1−C5H5)3]−. However, the path of formation for the zincate K+[Zn(η1−C5Me5)3]− is simply not favourable kinetically or hermodynamically, so the formation of decamethyldizincocene is the only option when Zn(η5−C5Me5)(η1−C5Me5)is used. Finally, it had been found that a particular chiral neutral zirconium amidate com- plex makes an effective catalyst for cyclizing primary aminoalkenes in a highly enan- tioselective fashion. The computational analysis indicates that the reason why one enantiomer is favoured is because of steric interference with the catalytic backbone that is non-existent with the other enantiomer, and this affects the major transition states throughout the cycle. This finding agrees with the experimental hypothesis.
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