Heteroatom oxidations halogens

Since dioxiranes are electrophilic oxidants, heteroatom functionalities with lone pair electrons are among the most reactive substrates towards oxidation. Among such nucleophilic heteroatom-type substrates, those that contain a nitrogen, sulfur or phosphorus atom, or a C=X functionality (where X is N or S), have been most extensively employed, mainly in view of the usefulness of the resulting oxidation products. Some less studied heteroatoms include oxygen, selenium, halogen and the metal centers in organometallic compounds. These transformations are summarized in Scheme 10. We shall present the substrate classes separately, since the heteroatom oxidation is quite substrate-dependent. 

Major unknowns in the mechanism by which a hydrocarbon fuel bums concern the pyrosynthesis reactions that lead to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot and the oxidation chemistry of atoms other than carbon and hydrogen (heteroatoms) in the fuel, particularly nitrogen, sulfur, and halogens. 

The (EDT-TTF-I)2Br salt described above [36] and the 1 1 (TTFI4)I salt reported by Gompper [51] were the only structurally characterized salts with simple halide anions until Imakubo recently described an extensive series of Cl" and Br" salts from several ortho-diiodo tetrathiafulvalene, tetraselena-fulvalene and dithiadiselenafulvalene derivatives (Scheme 8) [62], The X-ray crystal structure analysis of the nine salts described there show a variety of halogen bonded motifs, demonstrating the adaptability of the supramolecu-lar interactions to other structural requirements imposed by the nature of the heteroatoms (O, S, Se) in the TTF frame. Indeed, in (EDT-TTF-l2)2X-(H20)2 (X = Cl, Br), a bimolecular motif (Fig. 6) associates two partially oxidized EDT-TTF-I2 molecules with one Br" anion and one water molecule. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

Oxidation of iodoalkanes involves removal of an electron from the halogen nonbonding orbital. The radical-cations of primary and secondary alkyl iodides can be identified in aqueous solution by their absorption spectra and have half-lives of microseconds [1]. They are formed during pulse radiolysis of the iodoalkane in aqueous solution in the presence of nitrous oxide. This system generates hydroxyl radicals, which remove an electron from the iodine atom lone pair. Iodoalkane radical-anions complex with the lone-pair on other heteroatoms to form a lollo three-electron bond. In aqueous solution, the radical-cation of iodomethane is involved in an equlibrium indicated by Equation 2.1. 

Carbon atoms are classified depending on their hybridization and whether their neighbors are carbon atoms or heteroatoms. Halogen atoms are classified by the hybridization and oxidation state of the C atom to which they are attached. O, S, Se, N, and P are classified in different ways. The model uses 120 different atom-type descriptions and has been developed with a training set of 893 compounds. Observed versus calculated log Kow showed a correlation coefficient of 0.926 and a standard deviation of 0.496. This method has been implemented in the Toolkit. Applications are shown in Figures 13.4.5 and 13.4.6 for the same compounds used to illustrate the Broto et al. method (Figs. 13.4.2 and 13.4.3). 

Figure 13.4.4 Classification of H atoms in 4,4-difluorobutanoic acid as defined in the model of Ghose et al. [49,50], The subscript represents hybridization and the superscript is the formal oxidation number. X represents any heteroatom (O, N, S, P, Se, and halogens).

Comparison of the stereochemical outcomes in entries 2 and 3 of Table 1 implies that the heteroatom functionality directs the oxidation reaction more effectively from pseu-doequatorial orientation (the O—C—C=C dihedral angle is 140°). Selective formation of yyw-epoxides is also known for olefins having carbamate18, acetal19, ether20 and halogen groups21 in allylic positions. There are many cases in the literature where the epoxidation... 

Let us now consider the formation of three-electron bonds between different atoms. Stabilization of an oxidized sulfur atom can, in principle, be achieved in cases of its interaction with other heteroatoms if they provide free (preferably p-) electron pairs. Nitrogen, oxygen, and halogens (except fluorine) can be mentioned as such heteroatoms (Anklam et al. 1988 Carmichael 1997). The stability of these bonds is generally not as high as that of a symmetric S.. S system. An important reference for the enhanced stability of symmetrical three-electron bonds is Clark s (1988) calculations. 

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

Two organic chemical compounds possess the same oxidation state if the average oxidation numbers of their C atoms are the same and if any heteroatoms that might be present possess their usual oxidation numbers (Li, +1 Mg, +2 B, +3 N and P, -3 O and S, -2 and -1 for halogen atoms). 

When BrF3 is mixed with bromine, the active species is bromine monofluoride and this can be added to olefins (equation 118)213. Bromine trifluoride also used to prepare compounds with hypervalent heteroatoms, many of them, unattainable otherwise (equation 119)214. A key step in Lemal and coworkers unique synthesis of perfluorocyclopentadiene involved also an oxidative fluorination (equation 120)215. BrF3 can also substitute halogens and was used for the preparation of L-3-fluoroalanine 21 (equation 121)216. 

Bicyclic 5-6 Systems Two Heteroatoms 1 1 Table 43 Halogenation of 1 H-pyrrolo[2,3-Z ]pyridine 7V-oxide. 

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