(s-trans-η4-Dien)zirconocen-Komplexe

Reactions of five-membered zirconacycloalkynes and zirconacycloallenes with Cp2Zr(H)Cl; formal hydrogenation by metal hydrides

Reactions of five-membered zirconacycloalkynes and zirconacyloallenes (1-zirconacyclopent-3-ynes and 1-zirconacyclopenta-2,3-dienes) with an excess of Cp₂Zr(H)Cl, known as the Schwartz reagent, were studied. Both reactions gave five-membered zirconacycloalkenes, 1-zirconacyclopent-3-enes without subsequent work-up such as protonolysis and hydrogenolysis. The product was identical to the zirconocene-diene complex that was prepared from Cp₂Zr(n-Bu)₂ (Negishi reagent) and the corresponding 1,4-disubstituted 1,3-dienes. These results indicate that formal hydrogenation by metal hydride took place. The use of diisobutylaluminum hydride or 9-borabicyclo[3.3.1]nonane also gave the same product, albeit in lower yields. The reactions starting from deuterated compounds suggested that double hydrozirconation followed by elimination of a dinuclear zirconium complex resulted in the hydrogenated products.

Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners established that this step occurs without redox involvement of the pyridine(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C(sp³)-C(sp³) reductive coupling. A series of labeling experiments with pyridine(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C(sp³)-C(sp³) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure-activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as the promotion of chemical recycling of cycloaddition polymers.

Mechanistic insights into dehydrocoupling of amine boranes using dinuclear zirconocene complexes

Catalytic dehydrocoupling of H₃B·NMe₂H using Cp₂Zr(Cl)(μ-Me₃SiC₃SiMe₃)Zr(Cl)Cp₂ (1)/MeLi was studied. Spectroscopic monitoring and stoichiometric experiments show the formation and interconversion of several catalytically active Zr species.

Pyridine(diimine) Iron Diene Complexes Relevant to Catalytic [2+2]-Cycloaddition Reactions

The synthesis, characterization, and catalytic activity of pyridine(diimine) iron piperylene and isoprene complexes are described. These diene complexes are competent precatalysts for (i) the selective cross-[2+2]-cycloaddition of butadiene or (E)-piperylene with ethylene and α-olefins and (ii) the 1,4-hydrovinylation of isoprene with ethylene. In the former case, kinetic analysis implicates the diamagnetic η4-piperylene complex as the resting state prior to rate-determining oxidative cyclization. Variable temperature 1H NMR and EXSY experiments established that diene exchange from the diamagnetic, 18e- complexes occurs rapidly in solution at ambient temperature through a dissociative mechanism. The solid-state structure of (Me(Et)PDI)Fe(η4-piperylene) (Me(Et)PDI = 2,6-(2,6-Me2-C6H3N═CEt)2C5H3N), was determined by single-crystal X-ray diffraction and confirmed the s-trans coordination of the monosubstituted 1,3-diene. Possible relationships between ligand-controlled diene coordination geometry, metallacycle denticity, and chemoselectivity of iron-mediated cycloaddition reactions are discussed.

A quarter of a century of explorations in organozirconium chemistry

Systematic explorations of organozirconium chemistry over the past quarter of a century have led to the discoveries and development as well as structural and mechanistic clarifications of novel Zr-catalyzed and -promoted carbon-carbon bond-forming reactions including (i) Ni- or Pd-catalyzed cross-coupling reaction of organozirconiums, (ii) Zr-catalyzed carboalumination of alkynes, (iii) Zr-catalyzed asymmetric carboalumination of alkenes, (iv) generation and carbometallative ring expansion of zirconacyclopropanes and zirconacyclopropenes and a myriad of their transformations and (v) various organozirconium migratory insertion reactions.