Chapter 5
Biomimetic Vanadium Complexes and Oxo Transfer Catalysis Pingsong Wu, Gabriella Santoni, Cornelia Wikete, Falk Olbrich, and Dieter Rehder Downloaded by NATL TAIWAN UNIV on July 1, 2015 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch005
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Institut für Anorganische und Angewandte Chemie, Universität Hamburg, 20146 Hamburg, Germany
Chiral allylamino-diethanols H L , prepared from allylamine and styreneoxide, react with VO(OiPr) to form trigonal— bipyramidal complexes of composition [VO(OMe)L]. The complexes model the structure of the active center of vanadate-dependent haloperoxidases, and they also model the sulfideperoxidase activity of these enzymes. Reaction of H 2 L with NaH/CH3I yields ( C H ) L , which forms chloro{b/s(oxyphenylethyl)propylamine}silicate with H S i C l . 2
3
3
2
3
Introduction Vanadate-dependent haloperoxidases from marine macro-algae (such as Ascophyllum nodosum (/) and Corallina officinalis (2)), and the fungus Curvularia inaequalis (5) contain vanadate(V), linked covalently to a histidine, and by hydrogen bonds to a variety of amino acid residues in the active site pocket, Figure 1 (a). Vanadium is center of a trigonal bipyramid, with an oxo group in the trigonal plane and an O H in one of the apical positions. The second apical position is occupied by the Ne of the His. The enzyme catalyzes the oxidation, by peroxide, of halide to a Hal species, presumably hypohalous acid (eqn I), which can halogenate non-enzymatically a large variety of organic substrates. +
© 2007 American Chemical Society
In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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a
b
Figure 1. Native (a) and peroxo (b) forms of the bromoperoxidase from the A. nodosum enzyme. His 411 is replaced by Phe in the chloroperoxidases from the fungus C. inaequalis.
+
Hal" + H 0 + H -> HOHal + H 0 2
2
2
(1)
In the course of the catalytic reaction, a peroxo intermediate is formed, which attains the structure of a distorted square pyramid (4, 5), Figure 1 (b). The peroxo group is subject to protonation (5) and thus provides a center for nucleophilic attack of the substrate, e. g. bromide, leading to the generation of a hypobromito intermediate (6) which is then released in the form of hypobromous acid. The haloperoxidases also exhibit a sulfideperoxidase activity (7) in as far as (prochiral) sulfides are oxygenated to (chiral) sulfoxides plus some sulfone; eqn. 2. This latter reaction is of particular interest, since chiral sulfides are important synthons in organic synthesis (8). Consequently, several groups have been working on vanadium-based model systems of the haloperoxidases during the past decade (9-12), aiming at the synthesis of chiral enantio-pure sulfoxides. Our approach to this challenge has been the development of chiral, di- to tetradentate aminoalcohols as suitable ligands for the oxovanadium(V) moiety (13). In the present communication, we introduce results on aminoalcohols containing an allyl substituent attached to the nitrogen, allowing for further variations of the ligand and thus in the complex periphery. Selected results on the catalytic potential of the vanadium complexes are also reported.
In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Results and Discussion
Synthesis and Characteristics of Ligands The reaction of R-styreneoxide and allylamine (molar ratio 2:1) in /sopropanol yields, after refluxing the mixture for a couple of days and work-up on silicagel (elutant: hexane/ethylacetate, with gradual increase of the polarity) the ligands H L and H L (cf. Scheme 1), and thus the products of the 1,3- and 1,2/1,3-cleavage of styreneoxide in 56 and 23% yields, respectively. The crystal structure of H L (Figure 2, left) indicates that the stereo centers remain in the R configuration. Reaction of H L with sodiumhydride plus methyliodide affords the dimethyl ether M e L , which reacts with trichlorosilane under N in C H C I and in the presence of catalytic amounts of H [PtCl ]//^opropanol to form the hydrosilylation product with simultaneous formation of two silylether bonds, generating the monochlorosilicon compound C l S i L , Scheme 1, again with retention of the stereo centers; Figure 2, right. C l S i L has also been obtained together with other silicon compounds by hydrosilylation of H L . Characteristic ' H N M R data of the four amines are collated in Table 1. The S i N M R of C l S i L shows a resonance at -112 ppm. The bond lengths rf(Si-Cl) = 2.1699(7), rf(Si-N) = 2.0661(17),
