Dioxan, atom polarization

Chloramine-B (CAB, PhS02NClNa) and chloramine-T (CAT, p-Me-C6H4S02NClNa) have also been used for the oxidation of sulphoxides107-115. The required sulphone is produced after initial attack by the sulphoxide sulphur atom on the electrophilic chlorine-containing species, forming a chlorosulphonium intermediate as shown in equation (34). These reactions take place at room temperature, in water and aqueous polar solvents such as alcohols and dioxane, in both acidic and basic media. In alkaline solution the reaction is slow and the rate is considerably enhanced by the use of osmium tetroxide as a catalyst115. 

Pyrido[2,3-, pyridazine derivatives 48 have been synthesized by refluxing equimolar amounts of an appropriate 5-benzylidene-2,2-dimethyl-l,3-dioxane-4,6-dione 47 with 5-amino-6-phenylpyridazin-3(2/7)-one 46 in methanol or a methanol acetic acid mixture. The electron-poor carbon atom of the polarized carbon-carbon double bond of 47 is the electrophile attacking C-4 of the 5-aminopyridazinone 46. Imino-enamine tautomerization of the intermediate is followed by ring closure and subsequent loss of acetone and carbon dioxide affording the reaction products 48 as stable crystalline solids in 70-90% yield (Scheme 9) <2000T2473>. 

In solvents from which a hydrogen atom can be easily abstracted (methanol, dioxane),195 dediazoniation of diazonium tetrafluoroborates RC6H4N2+ BF4 becomes a radical process and arenes RC6H4H are significant byproducts for example, when R = 4-f-Bu, the ratio ArF/ ArH ranges from 5 13 (MeOH) to 55 26 (dioxane). The same phenomenon is observed in aprotic polar solvents,5 179,226 227 such as acetonitrile, hexamethylphosphoric triamide or dimethylformamide, especially when a stabilizing substituent (N02, Cl, OMe) is present in the ortho position. 

Similar results have been obtained in the reactions of 1 and sodium salts of p-cresol and - and /J-naphthols. Under SN2 conditions (DMF or DMSO solvent), the alkylation of sodium cresolate occurs exclusively at the oxygen atom. The addition of a protic solvent causes C-alkylation, though the yields of C-alkylated products are low. Thus in acetone-water or dioxane-water, the yield of C-alkylated products 251 and 252 increases only up to 2%. C-Alkylation has also been observed in the reactions catalyzed by trifluoroacetic acid or boron trifluoride etherate at room temperature. The observed C-alkylation in protic media may be a reflection of a mechanism that involves a protonated epoxide or a more polarized transition state than in an SN2 pathway. 

The hydrolysis of a series of spiro-/4-sulfanes such as (330)-(332) leads to sulfoxides in dioxane-water solutions. A mechanism involving slow nucleophilic attack of water on the positively polarized sulfur atom and simultaneous 0—H and S—N bond cleavage is proposed.298 The photo-oxidation of the sulfenamides (333 R = Me, Et, Ph, Bn, etc.) has been reported.299... 

Nowadays, however, molecular dynamics has been applied to study the dynamics of solvation of Cl53 as the solute in some polar solvents such as 1,4-dioxane and fluoroform. Cinacchi et al. [143] used an all-atom chemically detailed model of both... 

Dioxanes Heterocyclic ethers with two oxygen atoms in a six-membered ring are called dioxanes. The most common form of dioxane is the one with the two oxygen atoms in a 1,4-relationship. 1,4-Dioxane is miscible with water, and it is widely used as a polar solvent for organic reactions. 

The dipole moments of nitroazoles measured in chloroform are lower than the values obtained in dioxane (Table 3.72) [1268], This effect is supposed to be caused by mutual orientation of the substrate and chloroform dipoles, which leads to partial compensation of charges and, hence, to the reduction of polarization. The substitution of hydrogen atom of the NH-fragment by a methyl group does not influence much the dipole moment value of nitroazole. Nevertheless, the dipole moment is, for example, sensitive to substitution in position 5 of the 1,2,4-triazole cycle [1268], The introduction of electron-donating substituent (methyl group)... 

Pyrazine, 1,4-dioxane, dimethyl sulfide, DMSO and so on were found to be good templating molecules, whereas JV-methyl-2-pyrrolidene (NMP), dimethylformamide (DMF), DMA and so on were poor templating molecules. The general observations which were made are as follows (1) for the reported carcerand a maximum of seven nonhydrogen atoms is allowed (2) the guest should be neither too polar nor too nonpolar (3) higher symmetry results in easier carcaplex formation (4) secondary amines are not encapsulated and (5) cyclic molecules act as better templates than acyclic molecules. 

The presence of a reverse anomeric effect was suggested (29) for chlo-romethyl, CICH2—, and bromomethyl, BrCHj—, groups located at the anomeric carbon atom of a 1,3-dioxane ring. This claim was based on the observed reversed dependence of axial preference on solvent polarity, that is, more polar solvents increased the population of axial conformers. This observation is in line with the fact that a-glucopyranosylimidazoles in water (very polar solvent) do not change conformation on protonation (162). 

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