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Tetrahedron xxx (2013) 1e10
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Tetrahedron
Non-heme iron catalysis in C]C, CeH, and CH2 oxidation reactions.
Oxidative transformations on terpenoids catalyzed by
Fe(bpmen)(OTf)2
*
ꢀ
David Clemente-Tejeda, Alejandro Lopez-Moreno, Francisco A. Bermejo
ꢀ
Departamento de Química Organica, Universidad de Salamanca, 37008 Salamanca, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of terpene olefins with hydrogen peroxide in the presence of the non-hemo catalyst 5a
Received 22 October 2012
Received in revised form 25 January 2013
Accepted 1 February 2013
Available online xxx
afforded mixtures of epoxides whose composition was dependent upon the oxidation protocol used in
each case. With terpenoid enones, the mixtures obtained evolved from clean epoxidation of a-ionone 23
to the clean allylic oxidation of damascone 28 due to the progressive deactivation of the electron density
on the double bonds present in this series.
The oxidation of bicyclic and tricyclic terpenoids afforded oxidation products coming from epoxidation,
to olefin degradation, methyne and methylene activation products. Probably, the most attractive result
was the synthesis of the Magnus lactone 46, from the tricyclic ether 45, with 88% yield and 100%
conversion.
Keywords:
Oxidation
Fe(bpmen)(OTf)2
Monoterpenes
Magnus lactone
Cytochrome P-450
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
to tertiary alcohols.17 Furthermore, iron salts in combination with
peroxides and appropriate additives also show catalytic activity in
Iron catalysis has emerged as an attractive research area in recent
years.1 Iron has a number of advantages over other transition metals
usually employed in catalysis; it is relatively non-toxic, abundant,
cheap, and environmentally friendly.2 As a consequence, iron-based
catalyst systems have been applied in numerous organic trans-
formations such as carbonecarbon,3 carboneheteroatom,4 hetero-
eheteroatom bond-forming reactions,5 atom transfer radical
polymerizations,6 reductions,7 and oxidations.8
The selective oxidation of hydrocarbons under mild conditions
constitutes a major challenge of modern chemistry.9 Non-heme iron
enzymes, such as methane monooxygenase10 and Rieske dioxyge-
nases,11 catalyze such reactions and have inspired the development
of synthetic models as alkane oxidation catalysts.12 Exceptional non-
heme iron catalysts are those that are architecturally reminiscent of
the active sites in non-heme enzymes found in nature. Mononuclear
iron complexes synthesized from tris(picolyl)amine (tpa, 1. Fig. 1),9b
the tetradentate bpmen ligand scaffold (2a),12c,13 or from substituted
pyridine ligand 3,14 act as very efficient catalysts, using the envi-
ronmentally friendly H2O2 as the oxidant in several oxidative
transformations, such as the oxidation of alkanes15 or alkenes.16
Complex 4 has been shown to selectively oxidize methine groups
oxidation reactions of alkanes, alkenes or arenes and in epoxidation
reactions.18
Terpenes are cheap and are often chiral precursors to fragrances,
flavors, drugs, and agrochemicals.19 Oxy functionalization of ter-
penes frequently starts with a selective epoxidation. Although cata-
lytic epoxidation is currently the main pathway used to obtain many
commodities and fine chemicals, the synthesis of major terpene
oxides still employs the stoichiometric peracid route. In designing
a catalytic alternative, we assumed that the use of H2O2 in the
presence of Fe(bpmen)(OTf)2 (5a)13c (bpmen: [N,N0-dimethyl-N,N0-
bis(2-pyridylmethyl)-1,2-diaminomethane]; OTf: trifluoromethane-
sulfonate) could be appropriate for the development of an alterna-
tive, more affordable and sustainable method to access functional-
ized terpene derivatives.
2. Results and discussion
As part of a research project aimed at the study of hydrogen
peroxide-promoted oxidative transformations of highly valuable
starting materials in the presence of Fe(II) non-heme complexes,
we recently reported our results on the oxidative transformations
of steroid enones.20 We now report our results on terpenoid olefins,
enones and polycyclic substrates. The oxidation reactions were
carried out at room temperature with the progressive addition of
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.