Selective oxidation of substituents

Selective oxidation of methyl pyrroles 65 possessing an a-carboxylic ester and sensitive p-substituents can be accomplished using cerium triflate in methanol <96TL315>. Moreover, the resultant a-methoxymethylpyrroles 66 may be converted to dipyrrylmethanes 67 in a "one-pot" sequence by treatment with 48% HBr. The dipyrrylmethanes, in turn, can be further oxidized to dipyrryl ketones by ceric ammonium nitrate <96JHC221>. 

The introduction of substituents into the 1- and 2- positions of thianthrene can be achieved by deprotonation at C-1 and subsequent reaction with electrophiles and by formation of 2-lithiothianthrene from the corresponding bromo compound <96JCS(P1)2391>. Selective oxidation of both thianthrene and its 5-oxide have been described <96CEJ255, 96JCS(P1)2693>. 

From the aforementioned, it should be evident that the amino group is one of the most reactive functionalities towards dioxirane oxidation consequently, to achieve a chemo-selective oxidation of a multi-functionalized substrate that possesses an amino group, the latter must be protected. This may be accomplished by masking the amino substituent in the form of its ammonium salt ", or BF3 complex ", even better as an amide functionality (iV-phenylacetamide resists TFD oxidation at room temperature""). This will reduce sufficiently the oxidative reactivity of the amino group, such that another less reactive group may be selectively oxidized" ... 

A series of novel styrene- and siloxane-based silanol polymers and copolymers were synthesized by a selective oxidation of the Si—H bond with a dimethyldioxirane solution in acetone from corresponding precursor polymers. The conversion of the Si—H to Si—OH in the polymer modification proceeded rapidly and selectively. The silanol polymers obtained in situ showed no tendency for self-condensation to form siloxane crosslinks in solution. Moreover, stable silanol polymers in the solid states were obtained by placing bulky substitute groups bonded directly to the silicon atom. It was found that the properties of these novel silanol polymers and copolymers depended largely on substituents bonded directly to the silicon atom and silanol composition in the copolymers as well. 

An application of the deracemization strategy has provided efficient entry to a novel amino acid substituent of the antifungal agents, polyoxins and nikkomycins, as shown in Scheme 8E.20. The versatile five-carbon building block was obtained from phthalimidation of the hydroxymethyl-substituted epoxide in 87% yield and 82% ee. Straightforward synthesis of polyoxamic acid was then accomplished by subsequent dihydroxylation and selective oxidation of the alkylation product. 

Selective oxidation of styrene to acetophenone by a Pd(II) catalyst in the presence of ionic liquids has been achieved (equation 27). Electronic effects on the site ofPd(II) oxidation of substituted styrenes are indicated in equation (28). Strong pi-donation from the phenyl substituent favors oxidation at the alkene carbon alpha to the aromatic ring. 

Molecules with a cross section diameter greater than about 0.60 nm caimot diffuse into the TS-1 channels and are, therefore, not oxidized. This size restriction limits this system essentially to the oxidation of linear molecules and monocyclic aromatic rings with at most, small substituents. Even with these limitations, TS-1, and to a lesser extent TS-2, is an effective catalyst for the selective oxidation of a number of different types of organic compounds. Thirty percent hydrogen peroxide is the most commonly used oxidant. The more bulky alkyl peroxides are not effective because of their inability to diffuse into the zeolite channels to react with the titanium sites. While the oxidation of most primary and secondary alcohols occurs with reasonable ease, methanol is sufficiently inert to oxidation under the common reaction conditions that it is the solvent of choice for most TS-1 catalyzed reactions. 2 26,27... 

Ferey and coworkers [106] have also demonstrated that the coordinatively unsaturated Cr(III) sites of the MIL-101 can catalyze the selective oxidation of aryl sulfides to aryl sulfoxides using H2O2 as the oxidant The activity of the catalyst in the oxidation of arylmethylsulfide compounds (having different ring substituents in the para position) was found to depend on the concentration of open Cr(III) sites in the MOF sample. A direct correlation was found between the intrinsic activity... 

Oxidation of sulfides in the presence of electron-rich double bonds is problematic with many of the traditional oxidants such as MCPBA, NaI04, and oxone because of interference with double bond oxidation (e.g., epoxidation). Koo and coworkers [40] addressed this problem and studied the selective oxidation of allylic sulfides having multiple alkyl substituents. They tested various stoichiometric oxidants and a number of catalytic reactions with 30% aqueous H2O2 as the oxidant. Of all the oxidation systems tested for the sulfoxidation, they found that the use of LiNbMoOg as catalyst with H2O2 as the oxidant gave the best result. With this system no epoxidation took place and a reasonably good selectivity for sulfoxide over sulfone was obtained (Table 8.2). 

The explanation is as follows. Two tertiary sites in the bicyclic substrate above are electronically and sterically deactivated toward oxidation C9 is alpha to a ketone, and C5 is both in an axial orientation and adjacent to a em-ditnethyl group. The two ketones (EWGs) electronically deactivate the side chain and the entire B ring, with only 2° C—H bonds of A ring as the probable sites for methylene oxidation. C-2 is furthest away from the sterically bulky quaternary centers on the A ring. Also, C-2 oxidation may possibly relieve repulsive 1,3-diaxial interactions with two methyl substituents (C15 and C16). Thus, distinct selectivity factors can mutually reinforce in a complex molecular construction, leading to predictable and highly selective oxidation of secondary C—H bonds. 

During the mechanistic studies of estrogen biosynthesis, selective oxidation of androstene-3,17,19-trione (n) to the corresponding carboxylic acid was found to proceed by iron porphyrin complexes (Scheme 14A) [253]. On the basis of substituent effect on the benzaldehyde oxidation and kinetic isotope effect, direct hydrogen abstraction mechanism has been proposed [254]. The relative reactivity of aldehydes and alkenes is as follows cyclooctene, styrene > aldehyde, terminal alkene > a, 3-unsaturated ketone. 

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