Easier and More Efficient Methods for the Generation of Metathesis Catalysts: Investigations into Group VI and VII

Citation

Wilhelm, Thomas Edward (1999) Easier and More Efficient Methods for the Generation of Metathesis Catalysts: Investigations into Group VI and VII. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3d4p-py55. https://resolver.caltech.edu/CaltechTHESIS:10062023-220054560

Abstract

Chapter 1:

A high yield procedure for generating the ruthenium hydride complexes Ru(H)(H₂)Cl(PR₃)₂ (R=Cyclohexyl, cyclopentyl, isopropyl) in very high yield is presented. Following a novel insertion-elimination pathway, these hydrides can react with propargyl or vinyl halides to make metathesis active vinyl and alkyl carbene species with the general formulas (PR₃)₂Cl₂Ru=CH-CH=CR'₂ and (PR₃)₂Cl₂Ru=CHR', respectively. Tertiary propargyl chlorides like 3-chloro-3- methyl-1-butyne work best, yielding Ru-vinyl carbenes in extremely high yield. An alternate route is to first add an alkyne, and then add HCl to give similar species.

In attempting to learn about the insertion-elimination mechanism, the compounds M(H)Cl(CO)(PR₃)₂ (M=Ru, Os; R=cyclohexyl, isopropyl) were found to react with 3-chloro-3-methyl-1-butyne to produce the metathesis inactive carbenes with general formula cis-Cl₂-trans(PCy₃)₂(CO)M=CHCH=CMe₂. Kinetics ofRu(H)Cl(CO)(PⁱPr₃)₂ can only be analyzed qualitatively, but from all of the available data a mechanism is proposed for the insertion of hydrides into alkynes and rearrangement to give carbenes. The compounds M(H)Cl(CO)₂(PR₃)₂ (M=Ru, Os; R=cyclohexyl, isopropyl) show no alkyne insertion.

The osmium analogs Os(H)₃Cl(PCy₃)₂ and (PCy₃)₂Cl₂Os=CH=CH=CMe₂ were investigated for the ability to generate carbenes. The osmium carbene, however, rapidly transforms to the hydrido-carbyne species (PCy₃)₂Cl₂Os(H)(=CCH= CMe₂). It appears that additional stabilization of the osmium system will be necessary to prevent such rearrangement.

It is also presented that (PCy₃)₂Cl₂Ru=CHR' reacts with dihydrogen to give H₃CR', Ru(H)₂(C1)₂(PCy₃)₂, and Ru(H)(H₂)Cl(PCy₃)₂. Theoretically, all Ru(H)₂(Cl)₂(PCy₃)₂ can be converted to Ru(H)(H₂)Cl(PCy₃)₂. It is thus possible to go from hydrides to carbenes, and back to hydrides.

Chapter 2:

Complexes of the type M(O)Cl₂(PR₃)₃ (M=W, Mo; R₃=PMePh₂, PMe₂Ph) were synthesized using literature procedures, and shown to react with 3,3- diphenylcyclopropene to give the η²-olefin complexes M(O)Cl₂(PR₃)₂(η²- diphenylcyclopropene). Spectroscopic data suggest a distorted octahedral structure for both, with the oxo ligand in the axial position with the olefin cis to it and the two mutually trans phosphines in the equatorial plane, which was confirmed for M=W with an x-ray diffraction study. The olefin complexes react with suitable alkoxides to give the oxo-carbene species M(O)(OR)₂(PR₃)(=CHCH= CPh₂), the first known single component tungsten and molybdenum oxo-alkylidene metathesis catalysts, in which the phosphine is readily displaced with THF. For these complexes, spectroscopic data suggest a distorted trigonal bipyramid with the oxo, alkylidene, and one alkoxide ligand in the equatorial plane, which was confirmed for M=W by a diffraction experiment. These alkylidene species are active in olefin metathesis reactions, showing comparable activity to similar arylimido complexes previously described; polymerization data is presented for norbornene and cyclooctene. In addition, the olefin complexes were shown to be active in olefin metathesis at elevated temperatures.

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