Platinum(II) catalyzed diene cyclizations Public Deposited

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  • March 21, 2019
Creator
  • Campbell, Alison N.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • The development of a new ligand system for Pt(II) catalyzed diene cycloisomerizations was investigated. Deconstructing the tridentate (triphos)Pt(II) first generation catalysts into a mono- and bidentate phosphine combination (P2P) led to the preparation of chiral catalysts which efficiently converted 1,6- and 1,7-dienes into [3.1.0] and [4.10] bicyclic products. When the bidentate phosphine was either xylyl-BINAP or SEGPHOS, enantio-ratios up to 98:2 were observed. The catalyst resting state was compared to the first generation catalyst as well as the second generation achiral dppm/PMe3 catalyst. An X-ray structure of the precatalyst led to a proposed diastereofacial coordination preference. Additionally, the consequences of using a P2 rather than a P2P ligand array were examined and found to affect the stereochemical outcome and the diastereoselectivity. Efforts to trap the cationic intermediates generated in the 1,6- and 1,7-diene cycloisomerizations discussed above led to the surprising observation that in situ generated [(BINAP)(PMe3)Pt][BF4]2 reacts with benzyl alcohol at room temperature to yield [(BINAP)(PMe3)Pt-H][BF4] and benzaldehyde. This reactivity contrasts similarly ligated platinum-alkyl species which are stable to beta-H elimination even at elevated temperatures. Protonolysis of the platinum hydride leads to a species that is readily substituted by weakly coordinating ligands (e.g, acetone, pentafluorobenzonitrile). Finally, the mechanism of a Pt-dication catalyzed oxidative cyclization of dienyl alcohols has been investigated. Activation of the least substituted alkene in the substrate initiates a reversible cyclization cascade to generate a Pt-alkyl which has been identified and isolated. Reversible beta-H elimination produces a putative olefin/hydride intermediate. Although the hydride has not been directly observed, it has been both trapped with norbornene and observed directly in model compounds. Finally, hydride abstraction from the Pt-hydride intermediate and product release turns the catalytic cycle over; in model complexes this step was demonstrably rapid. Pentafluorobenzonitrile was found to profoundly affect the system by stabilizing a catalytic intermediate thus preventing the formation of a catalytically inactive pi-allyl while also providing a means to intercept and study a catalytically relevant intermediate.
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  • In Copyright
Advisor
  • Gagne, Michel
Degree granting institution
  • University of North Carolina at Chapel Hill
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  • Open access
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