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Sulphur-stabilized Anions

Sulphur-stabilized Anions.- As in previous years a number of carb-anion reactions have relied on the stabilizing effect of a sulphur substituent a- to the anion centre. N-Alkylcarbazoles (62 X = alkyl), for example, are known to undergo C-1 lithiation, but in contrast the N-(phenylthio)methyl derivative (62 X = SPh) cleanly [Pg.367]

Sulphones are especially powerful anion stabilizing substituents. Both o- and B- -pyranosides have been obtained from 2- [Pg.367]

Dianions based on sulphones are also useful synthetic tools. Thus, the [(phenylsulphonyl)methylene]dilithium species (74) undergoes a series of useful transformations with bifunctional sub-53 [Pg.369]


Aluminium thiophenoxide reacts readily with carboxylic esters to form keten thioacetals. A most important reaction of keten thioacetals has been reported by Cohen. Reduction of keten thioacetals with lithium naphthalide gives the sulphur-stabilized vinyl anions, powerful reagents for ketone synthesis. This type... [Pg.38]

Sulphur.—Continued interest has been shown in sulphoxide-stabilized anions, and Scheme 32 illustrates some of the useful transformations carried out this year, ranging from the synthesis of cyclopropanes (161) to potent dienophiles... [Pg.269]

Yet another general synthesis which relies on sulphur stabilized carbanions is Seebach s method for converting ketones into a-haloesters with the addition of one carbon atom (Scheme 32). The reaction sequence with aldehydes affords aa-dihaloesters for the conversion of aldehydes to the monohaloesters, the disilyl-thio anion (111) must be used. [Pg.138]

Ephraim and Muller 2 describe some results obtained with the groups — S04, — Se04, —Mo04, — W04, and —Cr04. These anions were united with the hexammino-nickel complex, and the temperature measured at which the ammonia tension is equal to atmospheric pressure. The stability of the complex is apparently independent of the atomic volumes of selenium, sulphur, chromium, molybdenum, and tungsten, but increases proportionally with the molecular volume of the corresponding trioxides. [Pg.21]

Very recent studies have examined the possibility of using 33S MAS spectroscopy in the study of some structural problems in cementitious materials107 108 and in sulphur speciation in silicate melts.109 Several processes of the deterioration of cements and more generally lapideous materials can be correlated to the stability of sulphate, to changes in the sulphate phase and to the interaction of sulphate ions with water molecules and hydrates.108 Since sulphate anion is one of the species that can be studied more easily by 33S NMR solid-state spectroscopy, this technique can be a valuable tool in studying... [Pg.42]

Figure 3 Hen egg white lysozyme has 4 disulfide bridges. However, only one of them is easily reduced A marked with an arrow. The detail of its structure Is shown In BJ The disulfide radical anion is stabilized by interaction with the charged end of an Arginine residue (In green). In white the distances (In A) between the central carbon atom and the sulphur atoms. In red the polypeptidic chain. Figure 3 Hen egg white lysozyme has 4 disulfide bridges. However, only one of them is easily reduced A marked with an arrow. The detail of its structure Is shown In BJ The disulfide radical anion is stabilized by interaction with the charged end of an Arginine residue (In green). In white the distances (In A) between the central carbon atom and the sulphur atoms. In red the polypeptidic chain.
Sulphur mustard alkylates the cysteine-34 residue in human serum albumin (Noort et al., 1999, 2004b), which is known to react with a number of electrophiles. This residue is much more accessible than the cysteines in haemoglobin, and its reactivity is promoted by its relatively low pKa, resulting from intramolecular stabilization of the thiolate anion. Aspartic and glutamic acid residues on albumin also react with... [Pg.133]

For S4, the measured frequencies are closer to the calculated values for the Czv and D2h isomers than those of the Czj, form, which is consistent with the relative stabilities calculated. The only frequency measured for the Sg structure (570 32 cm ) is significantly higher than both the calculated and Raman frequencies of the 83 isomer and falls in a pronounced gap of the spectrum for the >3 isomer. Since the vibrational structure was only observed in spectrum Sg- (II), it appears that the neutral cluster has insufficient time to relax to one of the more stable isomers during the measurement. It is also consistent with the observation [69] that all unbranched sulphur rings regardless of the size have no fundamental frequencies above 530 cm This picture is supported by the existence of a totally symmetric (a) vibration with frequency 619 cm for the neutral hexamer with the helical (Cj) geometry found for the Sf anion. [Pg.112]

Additions to the repertoire of anions stabilized by an adjacent sulphur atom include lithiated polymer-bound methylthiophenyl units which can be used for chain-extension of iodoalkanes, " cyclopropylthiobenzene, and doubly lithiated allylthiol, benzylthiol, and ethyl 2-thioacetate, which all react as C-nucleophiles. [Pg.175]

In all the cases considered, 2-mercaptoethanol (HSCH2CH2OH) is used as electron donor. Moreover, since the experiments are usually carried out in a basic solution , the de-protonated anion, namely [HOCH2CH2S] , is our reference species. We do not discuss in detail the results concerning this molecule, since we are interested in differences in reactivity due to changes in the electron acceptor, i.e. the N4-macrocycles. Here, we want only to recall that the HOMO of the anion is a p orbital localised on the sulphur atom, with the correct symmetry for an interaction with a d orbital of the central metal atom of the acceptor. This orbital is strongly stabilized by interaction with a surrounding medium (solvent), as expected in the case of a small, charged species. [Pg.582]

There have been several analyses of second-row ylides in which X = S, and Y = N or C. The compounds (96a) and (96b) are both sulphonyl-stabilized sulphur-nitrogen ylides in which there is the possibility of (d )ir bonding between the N and S+ atoms, and also between the N atom and the sulphonyl groups. The dimensions of the two molecules are virtually identical [S+-N 1.636(8) and 1.628(5) A N -S(sulphonyl) 1.591(8) and 1.598(5) A], and both conformations are characterized by non-syn-planar relationships of the onium and anionic-sulphonyl residues. The carbonyl-stabilized sulphur-carbon ylides (97) [S+-C 1.707(5) A C -C(0)... [Pg.321]

Non-classical Thiophens.—Thiophens which formally require quadricovalent bonding about one or more sulphur atoms are usually called non-classical . Numerous examples have been described during the past decade (see Vol. 5, pp. 282-3). Tetraphenylthieno[3,4-c]thiophen (55) and its radical-cation and -anion have now been subjected to a detailed spectroscopic examination, and the results have been rationalized by M.O. models. Non-classical thiophens usually need several aromatic substituents for reasonable stability examples in which the stability is conferred, at least in part, by fusion to polycyclic nitrogen heterocycles, e.g. (56) and (57), have been described, but the degree of stability is not great (57), for example, has only a transient existence. ... [Pg.281]

Methods Involving Umpolung.—Simple alkyl ethers of 2,6-disubstituted benoic acids may be deprotonated with s-butyl-lithium. Because of the stability of these esters towards acid, suitable tertiary alcoholic adducts afford ketones with sulphuric acid (Scheme 6). 2-Methoxyethyl esters of these hindered acids may also be considered as nucleophilic acetyl units since they undergo deprotonation-elimination-metalation to afford vinyl ester anions [equation (6)]. [Pg.61]

The use of carbon nucleophiles stabilized by SR groups has been a feature of sulphur chemistry for many years now, and this year has seen additional uses for these versatile reagents. Thus the carbanion CH(SPh)2 adds in a 1,2-manner to 2-methylacrolein giving the allyl alcohol (175), which is then converted to the activated diene (176) by standard methodology. The latter has been used in studies towards the total synthesis of the Rubradirin antibiotics. The related anion ArC(SPh)2 also adds to a,/3-unsaturated systems however, in the case of lactone (177) conjugate addition is observed. If the initial ion from (177) is trapped with benzylic bromides then frans-butyrolactones (178) are obtained in good yield. ... [Pg.271]

Just as sulphur and selenium groups stabilize a-anions, so do tellurium substituents, as shown by the formation of the reagents LiCH3- (TeR) . Some uses for these reagents are shown in Scheme 33. [Pg.309]


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Anion stabilization

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