Use of bis-phosphine platinum-dications as highly electrophilic catalysts for generation of intermediate carbocations in catalysis

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  • March 22, 2019
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  • Mullen, Charles A.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Chiral Pt-dicationic catalysts capable of C-C bond formation with the intermediacy of carbocations were developed. Similar carbocations are key intermediates in the biosynthesis of terpenoid natural products, and are normally generated by protonation of an alkene or an epoxide under careful enzymatic guidance. Under non-enzymatic conditions such activations of alkenes are much more difficult to control. Synthetic Ptdicationic complexes were developed for selective generation of these key intermediates in two specific types of processes. One, the asymmetric Prins cyclization reaction, and second an asymmetric oxidative cation-olefin cascade cyclization reaction that converts polyolefin substrates into complex polycyclic products, an analogy to steroid biosynthesis. Highly electrophilic P2Pt2+ catalysts proved to be uniquely able to catalyze a Prins cyclization reaction in the reaction of alkenyl phenols and glyoxylate esters. Other chiral Lewis acids provided the products of a concerted glyoxylate-ene reaction. The uniqueness of the reactivity to Pt-dicationic catalysts suggested that they were able to access trappable ionic intermediates. This reaction was made highly enantioselective by employing (tol-BINAP)Pt2+ catalysts and tBu glyoxylate with various phenol substrates. The P2Pt2+ catalysts also proved capable of mediating regio- and diastereoselective oxidative polycyclization reactions of dienol and trienol susbtrates. This biomimetic cyclization is initiated by Pt(II) activation of a less substituted alkene at the terminus of a polyene susbtrate and is terminated by <-hydride elimination. This transformation was rendered catalytic by employing trityl cation to abstract hydride from the putative cationic Pt-hydride to regenerate the Pt2+ catalyst. Good enantioselectivity could be achieved in this reaction by employing xylyl-PHANEHPOS as the bisphosphine ligand.
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  • In Copyright
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  • Gagne, Michel
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  • October 19, 2010

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