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Sulfides cyclic basicities

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. [Pg.145]

ANALYZER (Reagent-Tape). The key to chemical analysis by this method is a tape (paper or fabric) that has been impregnated with a chemical substance that reacts with the unknown to form a reaction product on the tape which lias some special characteristic, e.g., color, increased or decreased opacity, change in electrical conductance, or increased or lessened fluorescence. Small pieces of paper treated with lead acetate, for example, have, been used manually by chemists for many years to determine the presence of hydrogen sulfide in a solution or in the atmosphere. This basic concept forms the foundation for a number of sophisticated instruments that may pietreat a sample gas, pass it over a cyclically advanced tape, and, for example, photo-metrically sense the color of the exposed tape, to establish a relationship between color and gas concentration. Depending upon tile type uf reactiun involved, the tape may he wet or dry and it may be advanced continuously or periodically. Obviously, there are many possible variations within the framework of this general concept. [Pg.98]

These differences due to the steric requirements and lone pair repulsion by 0 electrons of the adjacent bonds may disappear for larger heteroatoms and other effects may prevail. This may be among the reasons for the higher basicity of linear as opposed to cyclic sulfides. [Pg.7]

Cyclic sulfides are known to be less basic than linear sulfides. Because the equilibrium constant K (Eq. (38)) is small, trialkylsulfonium ions cannot be used as initiators for the polymerizar tion of cyclic sulfides l. The relation between cyclic and linear amines resembles that of sulfides rather than etha s. In some cases, however, ammonium ions have been successfully used as initiators, e. g. in the polymerization of conidine or l-benzylaziridine l. [Pg.26]

Cationic polymerization of both cyclic amines and sulfides is combined in this section because of the similarities in their polymerization. This similarity is due to the high nucleophilicities of monomers that belong to these groups, the high stability of the active centers and the relatively higher basicity of heteroatoms in the polymer units than in the corresponding monomers. This combination of features leads to very similar synthetic methods used for polymer preparations. [Pg.186]

In the polymerization of cyclic sulfides, cyclization becomes particularly important because of the enhanced basicity (and nucleophilicity) of the linear sulfides in comparison with their parent monomers43). The simplest cyclic oligomer, formed in the polymerization of thiiranes, is a dimer-1,4-dithiane or its derivatives. 1,4-Dithiane was first observed by Bell in the polymerization of ethylene sulfide 441 later Price isolated the styrene sulfide dimer 45) and recently Goethals obtained dimers of isobutylene and cyclohexene sulfides 46 . Structures of these dimers are ... [Pg.192]

Heterocyclics containing P-atoms are usually strong nucleophiles. For example the basicity of 2-methoxy-ODP (pKa = 3.1) is much higher than that of cyclic ethers or sulfides. Therefore, a wide range of initiators, i.e., carbenium or oxonium salts, Lewis and protonic acids, and relatively nonreactive alkyl halides and organoalu-minum compounds have been used. Usually, reactions were carried out in a N2 atmosphere, although no special precautions (e.g. vacuum) were used to avoid contamination with water. [Pg.227]

Metallation of the most simple vinyl ether H2C=CHOCH3 in the a-position has been carried out with Fbutyllithium in THF at low temperatures [81]. Ethene is deprotonated not at all by FBuLi or other strongly basic systems under the same conditions. Sec-butyllithium has been used to metallate alkyl vinyl sulfides H2C=CH—SR [82]. We found that both vinyl ethers and vinyl thioethers can be metallated with excellent results using the cheaper combination of rc-BuLi and fBuOK [9]. The cyclic vinyl ethers 2,3-dihydrofuran and 2,3-dihydropyran can be successfully metallated under the same conditions ... [Pg.76]

Condensation of 911 with ketophosphonate 913, prepared in 92% yield from 912, provides enone 914 in 88% yield. Subsequent functional group transformation converts 914 to the labile bis-cyclic carbonate 915. Treatment of 915 with a large excess of dimethyl sulfide in the presence of benzoyl peroxide followed by basic hydrolysis provides the tetraol 916. When 916 is treated with trimethylorthoacetate in the presence of catalytic PPTS followed by tri-methylsilyl chloride in a triethylamine buffer, the exclusive product is the diacetoxy dichloride 917. Base-mediated saponification of cmde 917 results in spontaneous cyclization to the bis-trans-QpoxidQ, which after treatment with a large excess of mercury(II) chloride affords (— )-depudicin (918) in 52% yield. The total synthesis involves 22 steps, and provides 918 in 1.4% overall yield [255] (Scheme 198). [Pg.459]

Cyclic acetals are more stable than non-cyclic ones as we explain on p. 228 of the textbook. Hydrolysis needs more vigorous conditions. Thioacetals are much harder to hydrolyse because sulfides are even less basic than ethers. They can be hydrolysed using electrophiles that attack sulfur readily, such as Hg(II) or methylating agents. This is one possible solution ... [Pg.96]

The original Pfittner-Moffatt procedure for alcohol oxidation by activated dimethyl sulfoxide utilized dicyclohexylcarbodiimide (DCC) and a source of protons such as polyphosphoric acid or pyridinium tri-fluoroacetate. The use of strong acids such as the common mineral acids must be avoided since, although acidic conditions are initially required, the reaction must readily become basic in the later stages of the process. Mechanistically it is reasonable to suggest that the activation follows the pattern whereby initial attack of the nucleophilic sulfinyl oxygen of dimethyl sulfoxide, with the protonated carbodiimide, forms a sulfonium isourea. This is followed by displacement of dicyclohexylurea by the alcohol to form an alkoxysulfonium salt. Base treatment of this salt forms an ylide, which collapses via the proven cyclic mechanism to the carbonyl compound and dimethyl sulfide (Scheme 4). [Pg.293]

An efficient and highly enantio- and diastereoselective bromocycliza-tion-desymmetrization of olefmic 1,3-diols used a cyclic sulfide as catalyst. Olefmic 1,3-diols as substrates gave substituted THFs with up to three ste-reogenic centers, with two tetra-substituted carbons. This protocol represents the first case of a monofunctional C2-symmetric Lewis basic sulfide-catalyzed enantioselective bromoetherification reaction, which was applicable to the synthesis of a key intermediate to the orally active antifungal drug posaconazole (Noxafil) (14JA5627). [Pg.220]


See other pages where Sulfides cyclic basicities is mentioned: [Pg.464]    [Pg.364]    [Pg.304]    [Pg.308]    [Pg.140]    [Pg.262]    [Pg.139]    [Pg.460]    [Pg.364]    [Pg.207]    [Pg.191]    [Pg.663]    [Pg.16]    [Pg.293]    [Pg.293]    [Pg.675]    [Pg.152]    [Pg.6]    [Pg.441]    [Pg.110]    [Pg.191]    [Pg.539]    [Pg.4]    [Pg.560]    [Pg.179]    [Pg.260]    [Pg.152]    [Pg.287]    [Pg.14]    [Pg.675]    [Pg.145]    [Pg.311]    [Pg.311]    [Pg.314]   
See also in sourсe #XX -- [ Pg.6 ]




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Cyclic sulfides

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