The “Busch Catalyst”: A Remarkably Diverse Oxidation Catalyst
1) Original Goals and Rationale
2) Ethyl Cross-Bridged Cyclams are successful tight-binding ligands
CH3
N
N
N
N
•
•
•
•
•
•
•
H3C
3)
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
Mn(Bcyclam)Cl2 identified as active oxidation catalyst
a) Alkene Epoxidation
catalyst
H2O2
b)
O
Hydrogen Atom Abstraction
Hubin, et al., JACS, 2000, 2512.
Hubin, et al., Inorg. Chem., 2001, 435.
Hubin, et al., Inorg, Chim. Acta, 2003, 76.
catalyst
H2O2
US Patents: 6,218,351 B1; 6,387,862 B2; 6,606,015 B2; 6,906,189; 7,125,832
4)
Synthesis of the Mn(Bcyclam)(OH)22+ complex (activated form of the catalyst)
H2O, H2O2, NH4PF6
Yin, et al., Inorg. Chem., 2006, 8052.
5)
Changing the pH of the aqueous solution changes the major species present
Shi, et al., Angew. Chem. Int. Ed., 2011, 7321.
Yin, et al., JACS, 2007, 1512.
6)
Electron Transfer Mechanism
a) At pH = 1.5, [LMnIV(H2O)(OH)]3+ is the major species (MnIV—OH for short)
b) Oxidation of Ph3P: to Ph3P=O is a commonly studied reaction
Xu, et al., Chem. Eur. J., 2009, 11478.
7)
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.
8)
Hydrogen Atom Abstraction Mechanism
a) Similar to phosphine oxidation, low pH (MnIV—OH) and high pH (MnIV=O)
are active catalysts
b)
c)
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.
9)
The Oxygen Rebound Mechanism
a) Suspected mechanism of oxidation by Cytochrome P450 Enzymes, which
protect organisms against toxic organic compounds
b) Hydroxylation of Hydrocarbons by High-Valent Metal ions
c)
MnIV(Bcyclam) is capable of Oxygen Rebound Oxidations as well
d)
MnIV=O
e)
i) Is capable of Oxygen Rebound to produce hydroxylated products
ii) Does not follow the Electron Transfer Mechanism
MnIV—OH
i) Is an efficient Electron Transfer catalyst
ii) Is incapable of Oxygen Rebound
Shi, et al., Angew. Chem. Int. Ed., 2011, 7321.
10) Epoxidation of Alkenes by a Concerted Oxygen Transfer from the Hydrogen
Peroxide Adduct
a) Organic Peracids (like MCPBA) can epoxidize alkenes
b) “Inorganic Peracids” are known to react similarly (Acc. Chem. Res. 2004, 646)
c) 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.
11) Que has recently studied Fe(Bcyclam) as a catalyst for epoxidation and
cis-dihydroxylation of alkenes (Feng, et al., ACS Catalysis, 2011, 1035.)
a) Naphthalene 1,2-dioxegenase cis-dihydroxylates aromatic groups
b)
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
d)
e)
f)
g)
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.)
Supplementary information from Busch 2006 Inorg. Chem. p8052