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The “Busch Catalyst”: A Remarkably Diverse Oxidation Catalyst Original Goals and Rationale

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The “Busch Catalyst”: A Remarkably Diverse Oxidation Catalyst Original Goals and Rationale Ethyl Cross-Bridged Cyclams are successful tight-binding ligands Mn ( Bcyclam )Cl 2 identified as active oxidation catalyst Alkene Epoxidation Hydrogen Atom Abstraction

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slide1
The “Busch Catalyst”: A Remarkably Diverse Oxidation Catalyst
  • Original Goals and Rationale
  • Ethyl Cross-Bridged Cyclams are successful tight-binding ligands
  • Mn(Bcyclam)Cl2 identified as active oxidation catalyst
    • AlkeneEpoxidation
    • Hydrogen Atom Abstraction

US Patents: 6,218,351 B1; 6,387,862 B2; 6,606,015 B2; 6,906,189; 7,125,832

  • Topologically constrained like a cryptate
  • Short cross-bridge rigidifies the macrocycle
  • Tunable: ring size and Me group can be modified
  • Simple, high yielding organic synthesis
  • Leaves octahedral metal ions coordinatively unsaturated
  • Neutral ligand giving charged complexes
  • Resistant to oxidation
    • Tertiary amines
    • Saturated

Hubin, et al., JACS, 2000, 2512.

Hubin, et al., Inorg. Chem., 2001, 435.

Hubin, et al., Inorg, Chim. Acta, 2003, 76.

slide2

Synthesis of the Mn(Bcyclam)(OH)22+ complex (activated form of the catalyst)

  • Changing the pH of the aqueous solution changes the major species present

H2O, H2O2, NH4PF6

Yin, et al., Inorg. Chem., 2006, 8052.

Shi, et al., Angew. Chem. Int. Ed., 2011, 7321.

Yin, et al., JACS, 2007, 1512.

slide3

Electron Transfer Mechanism

    • At pH = 1.5, [LMnIV(H2O)(OH)]3+ is the major species (MnIV—OH for short)
    • Oxidation of Ph3P: to Ph3P=O is a commonly studied reaction
  • Concerted Oxygen Transfer

a) Oxidation of Ph3P: at pH = 13.4 [LMnIV(=O)2]o (MnIV=O for short)

Xu, et al., Chem. Eur. J., 2009, 11478.

Xu, et al., Chem. Eur. J., 2009, 11478.

slide4

Hydrogen Atom Abstraction Mechanism

    • Similar to phosphine oxidation, low pH (MnIV—OH) and high pH (MnIV=O) are active catalysts
    • 9,10-Dihydroanthracene is a common substrate; oxidizes to anthracene
    • Two successive H-atom abstractions occur; the first step is rate limiting

Yin, et al., JACS, 2008, 16245.

slide5

The Oxygen Rebound Mechanism

    • Suspected mechanism of oxidation by Cytochrome P450 Enzymes, which protect organisms against toxic organic compounds
    • Hydroxylation of Hydrocarbons by High-Valent Metal ions
slide6

MnIV(Bcyclam) is capable of Oxygen Rebound Oxidations as well

d) MnIV=O

    • Is capable of Oxygen Rebound to produce hydroxylated products
    • Does not follow the Electron Transfer Mechanism

e) MnIV—OH

    • Is an efficient Electron Transfer catalyst
    • Is incapable of Oxygen Rebound

Shi, et al., Angew. Chem. Int. Ed., 2011, 7321.

slide7

Epoxidation of Alkenes by a Concerted Oxygen Transfer from the Hydrogen Peroxide Adduct

    • Organic Peracids (like MCPBA) can epoxidize alkenes
    • “Inorganic Peracids” are known to react similarly (Acc. Chem. Res. 2004, 646)
    • MnIV(Bcyclam), epoxidizes alkenes in the presence of H2O2 by this mech.

Yin, et al., JACS, 2005, 17170.

Yin, et al., Inorg. Chem., 2006, 3467.

slide8

Que has recently studied Fe(Bcyclam) as a catalyst for epoxidation and

cis-dihydroxylation of alkenes (Feng, et al., ACS Catalysis, 2011, 1035.)

    • Naphthalene 1,2-dioxegenase cis-dihydroxylates aromatic groups
    • Que wanted to determine if two available cis sites on the metal were required

Fe(Bcyclam)Cl2 + AgOTf

c) Tested against Fe(TMC) (tetramethylcyclam) due to cis/trans only difference

slide9

Fe(Bcyclam) was a much better catalyst than Fe(TMC)

  • Mechanistic investigation showed activation with Acetic Acid, but loss of dihydroxylation and only epoxidation
  • Without Acetic Acid, dihydroxylation is favored
  • Que suggests an FeV=O active catalyst for both

Feng, et al., ACS Catalysis, 2011, 1035.)