Reactivity of Titanocene Methylidene with Metal Halides, Alkene Sulfides, and Alkene Oxides

Citation

Park, Joon Won (1989) Reactivity of Titanocene Methylidene with Metal Halides, Alkene Sulfides, and Alkene Oxides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0ds2-5h68. https://resolver.caltech.edu/CaltechTHESIS:06062013-085651174

Abstract

Titanocene metallacyclobutanes show a wide variety of reactivites with organic and inorganic reagents. Their reactions include methylene transfer to organic carbonyls, formation of enolates, electron transfer from activated alkyl chlorides, olefin metathesis, ring opening polymerization. Recently, preparations of heterobinuclear µ-methylene complexes were reported. In this thesis, mechanistic, synthetic, and structural studies of the heterobinuclear µ-methylene complexes will be described. Also, the reaction of titanocene methylidene trimethylphosphine complex with alkene sulfide and styrene sulfide will be presented.

Heterobinuclear µ-methylene-µ-methyl complexes Cp₂Ti(µ-CH₂)(µ-CH₃)M(1,5-COD) have been prepared (M = Rh, Ir). X-ray crystallography showed that the methyl group of the complex was bonded to the rhodium and bridges to the titanium through an agostic bond. The ¹H, ¹³CNMR, IR spectra along with partial deuteration studies supported the structure in both solution and solid state. Activation of the agostic bond is demonstrated by the equilibration of the µ-CH₃ and µ-CH₂ groups. A nonlinear Arrhenius plot, an unusually large kinetic isotope effect (24(5)), and a large negative activation entropy (-64(3)eu) can be explained by the quantum-mechanical tunneling. Calculated rate constants with Bell-type barrier fitted well with the observed one. This equilibration was best explained by a 4e-4c mechanism (or σ bond metathesis) with the character of quantum-mechanical tunneling.

Heterobinuclear µ-methylene-µ-phenyl complexes were synthesized. Structural study of Cp₂Ti(µ-CH₂)(µ-p-Me₂NC₆H₄)Rh(l,5-COD) showed that the two metal atoms are bridged by the methylene carbon and the ipso carbon of the p-N,N-dimethylarninophenyl group. The analogous structure of Cp₂Ti(µ-CHp₂)(µ-o-MeOC₆H₄)Rh(1,5-COD) has been verified by the differential NOE. The aromaticity of the phenyl group observed by ¹H NMR, was confirmed by the comparison of the C-C bond lengths in the crystallographic structure. The unusual downfield shifts of the ipso carbon in the ¹³C NMR are assumed to be an indication of the interaction between the ipso carbon and electron-deficient titanium.

Titanium-platinum heterobinuclear µ-methylene complexes Cp₂Ti(µ-CH₂)(µ-X)Pt(Me)(PMe₂Ph) have been prepared (X = Cl, Me). Structural studies indicate the following:(1) the Ti-CH₂ bond possesses residual double bond character, (2) there is a dative Pt → Ti interaction which may be regarded as a π back donation from the platinum atom to the "Ti=CH₂" group, and (3) the µ-CH₃ group is bound to the titanium atom through a three-center, two-electron agostic bond.

Titanocene (η²-thioformaldehyde)•PMe₃ was prepared from Cp₂Ti=CH₂•PMe₃ and sulfur-containing organic compounds (e.g. alkene sulfide, triphenylphosphine sulfide) including elemental sulfur. Mechanistic studies utilizing trans-styrene sulfide-d₁ suggested the stepwise reaction to explain equimolar mixture of trans- and cis-styrene-d₁ as by-products. The product reacted with methyl iodide to produce cationic titanocene (η²-thiomethoxymethyl) complex. Complexes having less coordinating anion like BF₄ or BPh₄ could be obtained through metathesis. Together with structural analyses, the further reactivities of the complexes have been explored.

The complex [chemical formula; see thesis file (PDF) abstract for details] was prepared from the compound Cp₂Ti=CH₂-PMe₃ and styrene oxide. The product was characterized with ¹H-¹H correlated 2-dimensional NMR, selective decoupling of ¹H NMR, and differential NOE. Stereospecificity of deuterium in the product was lost when trans-styrene oxide-d₁ was allowed to react. Relative rates of the reaction were measured with varying substituents on the phenyl ring. Better linearity (r = -0.98, ρ⁺ = -0.79) was observed with σp⁺ than σ(r = -0.87, ρ = -1.26). The small magnitude of ρ⁺ value and stereospecificity loss during the formation of product were best explained by the generation of biradicals, but partial generation of charge cannot be excluded. Carbonylation of the product followed by exposure to iodine yields the corresponding β-phenyl γ-lactone.

Thesis Files