I. Synthetic and mechanistic studies of a vanadium-catalyzed enantioselective oxidation of di-tert-butyl-disulfide. II. Synthetic and mechanistic studies of strained heterocycle opening reactions and nitro and nitroso metathesis reactions mediated by zirconium(IV) imido complexes.

摘要

Chapter 1. The mechanism of the oxidation of di- tert-butyl disulfide (1) to the corresponding chiral thiosulfinate (2) by H2O2 catalyzed by bis(acetylacetonato)oxovanadium and a chiral Schiff-base ligand (3) has been investigated. The reaction was studied by 51V NMR spectroscopy and different vanadium-containing species were observed under homogeneous conditions from those observed under heterogeneous conditions. A dramatic solvent effect on the enantioselectivity of the reaction was observed, and a competing nonselective oxidation pathway was identified. Strategies for limiting this competing pathway were developed. Additionally, a catalyst precursor (6) was isolated and fully characterized.; Chapter 2. Bis-cyclopentadienyl(tert-butylimido) zirconium complex 1 undergoes ambient-temperature metathesis reactions with nitrotoluene and nitrosoarenes to afford cis-azoxy (2) and cis-azo (7) compounds, respectively. These products isomerize to the trans isomers over the course of 0.6--8 hours. Nitro-tert-butane also undergoes metathesis with imido complex 1 to give trans-tert-butylazoxy- tert-butane 6. The oxozirconocene coproduct was trapped with chlorotrimethylsilane. The reaction was studied kinetically, and the rate was found to be first order in both 1 and substrate, inversely proportional to concentration of tetrahydrofuran, and independent of the concentration of trapping agent. Rates measured over this temperature range gave activation parameters DeltaH‡ = 15 +/- 2 kcal mol -1 and DeltaS‡ = -26 +/- 3 cal mol-1 K-1.; Chapter 3. The reactions of bis-cyclopentadienyl( t-butylimido) zirconium (Cp2Zr=N-t-Bu) complex 1 with epoxides, aziridines, and episulfides were investigated. Heterocycles without accessible beta hydrogens undergo insertion chemistry of the C-X bond, to produce 1,2-amino alcohols (X = O) and 1,2-diamines (X = N) complexed to zirconium, and heterocycles with accessible beta hydrogens undergo elimination chemistry to produce allylic alkoxide and allylic sulfide complexes. Mechanistic investigations support a stepwise pathway most consistent with zwitterionic intermediates for the first reaction class, and a concerted pathway for the second reaction class. Additionally, the feasibility of chirality transfer from the planar-chiral ebthi (ebthi = ethylenebis(tetrahydroindenyl) ligand was demonstrated with a chiral analog, (ebthi)Zr=NAr (Ar = 2,6-dimethylphenyl), 2, through the diastereoselective ring opening of meso epoxides.; Chapter 4. Bis-cyclopentadienyl(tert-butylimido) zirconium complex 3 participated in ambient-temperature displacement reactions with allylic halides, producing allylamido(halo)zirconocene products (4a,b, 7) in good to excellent yields (81--97%). The reaction of 3 with crotyl chloride yielded the product of a SN2 ' pathway (nucleophilic displacement with allylic transposition). Additionally, 3 underwent an unexpected double-displacement reaction with cis-3,4-dichlorocyclobutene, producing zirconocene dichloride and 1-tert-butylpyrrole.