Zirconocene mediated cyclization and isomerization of nonconjugated dienes
Abstract
Competitive $\beta$-hydrogen abstraction of heteroleptic dialkylzirconocenes and kinetic measurements of decomposition of methyl(alkyl)zirconocenes reveal unexpected relative reactivity of alkyl groups as $\beta$-hydrogen donors. The reactivity of alkyl ligands as $\beta$-hydrogen donors correlates well with the degree of substitution at the $\beta$-carbon centers. Generally, it has been found that the rate of decomposition decreases in the order: $\beta$-methyl $>$ $\beta$-methylene $>$ $\beta$-methine, the products of this thermal decomposition being Cp$\sb2$Zr-alkene complexes and alkanes. The reaction of alkylmagnesium derivatives with nonconjugated dienes, catalyzed by zirconocene dichloride, gives cycloalkylmethylmagnesium derivatives. Treatment of non-conjugated dienes with three equivalents of an alkylmagnesium derivative containing at least two $\beta$-hydrogens, such as n-BuMgCl, and a catalytic amount of Cp$\sb2$ZrCl$\sb2$ in THF at 50$\sp\circ$C produces monocyclic monomagnesium products, which contains minor amounts of the corresponding exomethylene derivatives. The stoichiometric reaction of n-Bu$\sb2$ZrCp$\sb2$ with nonconjugated dienes gives the corresponding zirconabicycle. However, nonconjugated dienes containing a mono- or 1,2-disubstituted alkene at one end and a vinylidene or trisubstituted alkene at the other, gives conjugated diene zirconocenes via multipositional regioisomerization. In each case, the diene-zirconocene products are s-cis and the diene moiety is $\ge$95% regio- and stereoisomerically pure, the 1,2-disubstituted alkene moiety being E. Conjugated dienes can be converted to their zirconocene complexes by their reaction with Cp$\sb2$ZrCl$\sb2$ and freshly ground Mg in THF. These complexes can be protonolyzed with 3N HCl to give regioselective monoenes corresponding to partial hydrogenation of the more highly substituted double bond.
Degree
Ph.D.
Subject Area
Organic chemistry|Chemistry
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