Dioxygen activation at mononuclear nonheme iron active sites: enzymes, models, and intermediates.

The semiconductor and the surface-adsorbed antenna molecule (dyes or other color species) can constitute efficient photochemical systems for environmental remediation. The major advantage of these systems is that they are able to achieve the degradation of organic pollutants by using visible light from the sun as energy and O2 in the air as the oxidant under ambient conditions. In this tutorial review, the unique mechanistic characteristics, the constitution of photodegradation systems and their performance are described. The involved radical reactions during the degradation are also discussed.

Semiconductor-mediated photodegradation of pollutants under visible-light irradiation

A new iron(III) halide-promoted aza-Prins cyclization between γ,δ-unsaturated tosylamines and aldehydes provides six-membered azacycles in good to excellent yields. The process is based on the consecutive generation of γ-unsaturated-iminium ion and further nucleophilic attack by the unsaturated carbon−carbon bond. Homoallyl tosylamine leads to trans-2-alkyl-4-halo-1-tosylpiperidine as the major isomer. In addition, the alkyne aza-Prins cyclization between homopropargyl tosylamine and aldehydes gives 2-alkyl-4-halo-1-tosyl-1,2,5,6-tetrahydropyridines as the only cyclic products. The piperidine ring is widely distributed throughout Nature, e.g., in alkaloids,1 and is an important scaffold for drug discovery, being the core of many pharmaceutically significant compounds.2,3 The syntheses of these type of compounds have been extensively studied in the development of new drugs containing six-membered-ring heterocycles.4 Reactions between N-acyliminium ions and nucleophiles, also described as amidoalkylation or Mannich-type condensations, have been frequently used to introduce substituents at the R-carbon of an amine.5 There are several examples that involve an intramolecular attack of a nucleophilic olefin into an iminium cation for the construction of a heterocyclic ring system.6 Traditionally, the use of hemiaminals or their derivatives as precursors of N-acyliminium intermediates has been a common two-step strategy in these reactions.6a Among this type of cyclization is the aza-Prins cyclization,7 which uses alkenes as intramolecular nucleophile. However, cy† X-ray analysis. E-mail address: malopez@ull.es. (1) (a) Fodor, G. B.; Colasanti, B. Alkaloids: Chemical and Biological PerspectiVes; Pelletier, S. W., Ed.; Wiley: New York, 1985; Vol. 23, pp 1-90. (b) Baliah, V.; Jeyarama, R.; Chandrasekaran, L. Chem. ReV. 1983, 83, 379-423. (2) Watson, P. S.; Jiang, B.; Scott, B. Org. Lett. 2000, 2, 3679-3681. (3) Horton, D. A.; Bourne, G. T.; Smythe, M. L. Chem. ReV. 2003, 103, 893-930. (4) Buffat, M. G. P. Tetrahedron 2004, 60, 1701-1729 and references therein. (5) Speckamp, W. N.; Moolenaar, M. J. Tetrahedron 2000, 56, 3187- 3856 and references therein. (6) (a) Hiemstra, H.; Speckamp, W. N. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, O., Heathcock, C. H., Eds.; Pergamon: New York, 1991; Vol. 2, pp 1047-1081. (b) Speckamp, W. N.; Hiemstra, H. Tetrahedron 1985, 41, 4367-4416. (7) (a) Dobbs, A. P.; Guesne, S. J. J.; Hursthouse, M. B.; Coles, S. J. Synlett 2003, 11, 1740-1742. (b) Dobbs, A. P.; Guesne, S. J. J.; Martinove, S.; Coles, S. J.; Hursthouse, M. B. J. Org. Chem. 2003, 68, 7880-7883. (c) Hanessian, S.; Tremblay, M.; Petersen, F. W. J. Am. Chem. Soc. 2004, 126, 6064-6071 and references therein. (d) Dobbs, A. P.; Guesne, S. J. Synlett 2005, 13, 2101-2103. ORGANIC

Iron-Catalyzed Reactions in Organic Synthesis

Direct C-H transformation via iron catalysis.

Two decades have passed since the discovery in liver microsomes of a haemprotein that forms a reduced-CO complex with the absorptive maximum of the Soret at 450 nm (Klingenberg, 1958; Garfinkel, 1958) and the identification of this protein as a new cytochrome: pigment cytochrome, P-450 (Omura and Sato, 1962, 1964a). In the intervening years, the study of cytochrome P-450 dependent monoxygenases has expanded exponentially. From the first crude attempts to solubilise a P-450 (Omura and Sato, 1963, 1964b) to the determination of the primary, secondary, and tertiary structure of cytochrome P-450cam by amino acid sequencing (Haniu et al., 1982a,b) and x-ray crystallography (Poulos et al., 1984) our understanding of this unique family of proteins has been advancing on all fronts. Since, perhaps, the greatest understanding of the structure and mechanism of P-450s has come from concentrated study of P-450cam of the Pseudomonas putida camphor-5-exo-hydroxylase, this review will concentrate on findings with P-450cam; attention will be drawn to parallel and contrasting examples from other P-450s as appropriate.

Structure and Chemistry of Cytochrome P450

Mononuclear iron(II) α-keto carboxylate and carboxylate compounds of the sterically hindered tridentate face-capping ligand TpPh2 (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate) were prepared as models for the active sites of nonheme iron oxygenases. The structures of an aliphatic α-keto carboxylate complex, [FeII(TpPh2)(O2CC(O)CH3)], and the carboxylate complexes [FeII(TpPh2)(OBz)] and [FeII(TpPh2)(OAc)(3,5-Ph2pzH)] were determined by single-crystal X-ray diffraction, all of which have five-coordinate iron centers. Both the α-keto carboxylate and the carboxylate compounds react with dioxygen resulting in the hydroxylation of a single ortho phenyl position of the TpPh2 ligand. The oxygenation products were characterized spectroscopically, and the structure of the octahedral iron(III) phenolate product [FeIII(TpPh2*)(OAc)(3,5-Ph2pzH)] was established by X-ray diffraction. The reaction of the α-keto carboxylate model compounds with oxygen to produce the phenolate product occurs with concomitant oxidativ...

Oxygen Activation by Nonheme Iron(II) Complexes: α-Keto Carboxylate versus Carboxylate

Many nonheme iron-dependent enzymes activate dioxygen to catalyze hydroxylations of arene substrates. Key features of this chemistry have been developed from complexes of a family of tetradentate tripodal ligands obtained by modification of tris(2-pyridylmethyl)amine (TPA) with single α-arene substituents. These included the following:  −C6H5 (i.e., 6-PhTPA), L1; −o-C6H4D, o-d1-L1; −C6D5, d5-L1; −m-C6H4NO2, L2; −m-C6H4CF3, L3; −m-C6H4Cl, L4; −m-C6H4CH3, L5; −m-C6H4OCH3, L6; −p-C6H4OCH3, L7. Additionally, the corresponding ligand with one α-phenyl and two α-methyl substituents (6,6-Me2-6-PhTPA, L8) was also synthesized. Complexes of the formulas [(L1)FeII(NCCH3)2](ClO4)2, [(Ln)FeII(OTf)2] (n = 1−7, OTf = -O3SCF3), and [(L8)FeII(OTf)2]2 were obtained and characterized by 1H NMR and UV−visible spectroscopies and by X-ray diffraction in the cases of [(L1)FeII(NCCH3)2](ClO4)2, [(L6)FeII(OTf)2], and [(L8)FeII(OTf)2]2. The complexes react with tert-butyl hydroperoxide (tBuOOH) in CH3CN solutions to give iron(III...

Mark P. Mehn

Papers

Dioxygen activation at mononuclear nonheme iron active sites: enzymes, models, and intermediates.

Oxygen Activation by Nonheme Iron(II) Complexes: α-Keto Carboxylate versus Carboxylate

Biomimetic aryl hydroxylation derived from alkyl hydroperoxide at a nonheme iron center. Evidence for an FeIV=O oxidant

Mid- to high-valent imido and nitrido complexes of iron

Dioxygen Activation at Mononuclear Nonheme Iron Active Sites: Enzymes, Models, and Intermediates