Carbon-carbon bond-forming reactions of organometallics and reductive beta-elimination of dialkylzirconocenes

Thinh Ba Nguyen, Purdue University

Abstract

Thermal decomposition via reductive $\beta$-elimination of dialkylzirconocenes containing one or more $\beta$-hydrogen atoms produces alkanes and zirconocene-alkene complexes identified as their complexes with PMe$\sb3$. The decomposition reaction follows a first-order kinetics independent of trimethylphosphine concentration. The relative reactivity of alkyl groups containing $\beta$-hydrogen atoms is $\beta$-methyl $>$ $\beta$-methylene $>$ $\beta$-methine. However, the relative reactivity order does not correlate well with the overall steric requirements of alkyl groups as follows: s-Bu $>$ t-Bu $\sim$ Et $>$ n-Bu $>$ i-Bu. In the decomposition of Cp$\sb2$Zr(CH$\sb2$CHDCH$\sb2$CH$\sb2$Ph)$\sb2$, the deuterium isotope effect, i.e., k$\sb{\rm H}$/k$\sb{\rm D}$, is 5.7. The relative reactivity order of $\beta$-arylethyl groups with different para substituents is p-Cl $>$ p-H $>$ p-OMe. Also presented is the reactivity of aryl and alkenyl groups versus alkyl groups. The reaction of trialkylboranes with $\omega$-tosyloxy-1-lithio-1-alkynes induces 1,2-migration of an alkyl group from boron atom to the alkynyl carbon atom followed by intramolecular displacement of the $\omega$-tosyloxy group. The reaction produces four- through six-membered exocyclic alkenes. The stereoselective cyclization of the reaction is low (anti/syn = 1.5-1.7). A similar reaction was also observed with $\omega$-halo or $\omega$-tosyloxy-1-alkenylborates to give exocyclic alkenes. The reaction of 6-lithio-5-hexynyl tosylate with diphenyl zirconocene follows the same pattern, a 1,2-migration cyclization, to give phenylmethylenecyclopentane in moderate yield. Stereoselective formation of $\beta$- and $\gamma$-hydroxy di- and tri-substituted alkenes in high yields by the reaction of organolithium with $\alpha$-lithiated cyclic enol ethers was developed. Oxidative addition of alkenyl and aryl halides containing ether moiety with Pd(0) catalyst followed by intramolecular carbopalladation to the double bond produces, after dehydropalladation, bicyclic and polycyclic fused-, spiro-, and bridged-ring heterocycles in high yields. A convenient method to generate in-situ an equivalent to CpZr(H)Cl, Cp$\sb2$Zr(isobutyl)Cl, was developed. The intermediate can hydrozirconate alkynes and alkenes cleanly in high yields upon heating to 50$\sp\circ$C.

Degree

Ph.D.

Advisors

Negishi, Purdue University.

Subject Area

Organic chemistry

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