Substituted Oxidation

PEDERSEN Crown Ethers Crown ether formation and its use in substitutions, oxidations,etc... 

Alkyl compounds can be synthesized by substitution, oxidative addition and insertion reactions... 

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Anodic nuclear substitution by methoxide or cyanide ions gives acceptable yields only for methoxybenzenes and methoxynaphthalenes. The nucleophile is attached to the point of highest positive charge density in the radical-cation and for many examples this leads to ipio-substitution. Oxidation of 1,4-dimethoxybenzene in methanol containing potassium hydroxide leads to the quinone diketal 8 [69]. The reaction is a general one for 1,4-dimethoxybenzenes [70, 71] and 1,4-... 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

Incorporating an electron-donor alkyl group into position 2 of 2 was shown by foe present authors to facilitate S-oxidation thus, 2-efoyl-thieno[3,2-6]thiophene-l,1-dioxide (214) was prepared at40°-45° from 2-ethylthieno[3,2-6]thiophene, hydrogen peroxide and acetic acid. The thieno[3,2-6]thiophene system undergoes oxidation even if foe second a-position is carboxy-substituted oxidation of 5-efoylthieno[3,2-6]-thiophene-2-carboxylic acid furnished foe 4,4-dioxide (215) subsequently decarboxylated to sulfone (214) [Eq. (70)]. The [2,3-6] isomers, 20 and 55, with foe sulfur atoms bound to foe same carbon atom, do not form sulfones under similar conditions. 

DPT calculations indicated that the mechanism most Hkely involves three steps electrophilic substitution, oxidation and reductive ehmination. The inactivity of the iodine complexes prompted us to investigate the counterion dependence. For the methyl-substituted complexes (Scheme 23, R = CH3) we synthesized the acetate (X = OCOCH3) 22 and the chloride complex (X = Cl) 23. The catalytic conversions are within experimental error identical to the results of the bromide complex 20. This indicates that the dissociation of a counterion is a necessary condition for the activity of the complex [59]. 

The influence of o-, wt-, and p-methoxy substituent is cli-.uly evident from isomerisation of correspondingly substituted oxides Eq. 449). Thus, jB -dimethyl-o-methoxyatyrene oxido and its p-methoxy isomer give only a kotona, whereas tho m-mothuvy isomc- ... 

Reactions of para- or mefa-substituted aromatic compounds hydrolysis, hydration of alkenes, substitution, oxidation, enzyme-catalyzed oxidations some type II photooxidations Hydrolysis and many other reactions of aliphatic organic compounds... 

The same activation that allowed simple electrophilic substitution—oxidation to the N-oxide— can also allow a useful nucleophilic substitution. The positive nitrogen atom encourages nucleophilic attack and the oxygen atom can be turned into a leaving group with PCI3. Our example is nicotinic acid whose biological importance we will discuss in Chapter 50. 

The final structure proposed to date for FeMo-co is the nido-cubane or string-bag cluster V81 it is a compromise between the cubane (I, II) and linear (III, IV) models. Attractive features of this model are the ease in accomodating Fe/Mo ratios from 6-8 by deletion of one or two Fe atoms, the potential for coordinative unsaturation of Mo, and the existence of structurally related inorganic complexes such as Roussin s black salt82. Problems with the model include the presence of six Fe rather than the two or three revealed by EXAFS as neighbors of Mo, and the presence of more than four S= per Mo. Although the latter can be easily overcome by substituting oxide for the sulfur atoms not attached to Mo, it is difficult to see how this structure could be consistent with the Mo EXAFS data. 

This was not observed in the diolate cycloreversion chemistry (Fig. 4).82 The measured (for moderately strained alkenes) and calculated (for more highly strained alkenes) enthalpy of activation for extrusion was approximately constant for a half-dozen examples at 30 1 kcal/mol. (All such examples were disubstituted alkenes to eliminate electronic differences due to substitution.) Oxidations showed the expected decrease in AH with increasing strain the near-unity slope indicates that the reacting carbon is fully sp3 hybridized in the transition state. 

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