Reaktionen mit Alkylkationen

Meerwein, Hans; Borner, Peter; Fuchs, O.; Sasse, Hans Jürgen; Schrodt, Heinz; Spille, Jürgen

doi:10.1002/cber.19560890907

Cited by 217 publications

(57 citation statements)

References 12 publications

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“…In 2 these chlorine atoms, Cl(21) and C1 (11), are situated 3.323(4) and 3.495(3) A respectively from C(6), at internuclear distances that are less than the sum of the van der Wrlals radii of neutral C and C1 atoms (3.50 A) (25). These C1 atoms are oriented as if they were undergoing a nucleophilic addition at C(6).…”

Section: Crystal Packingmentioning

confidence: 99%

“…With the appropriate choice of conditions it is possible to generate alkoxyhydroxy substituted carbenium ions as long-lived intermediates (1,11). Despite the large number of studies describing the properties of these cations, there have been few reports of the determination of their structures using direct method^.^ The conformational preferences of some acyclic dialkoxyhydroxy carbenium ions have been determined by NMR studies (17), but, this methodology cannot be extended to the types of cyclic systems used in establishing the concept of stereoelectronic control.…”

Section: Introductionmentioning

confidence: 99%

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Structural studies on 6-ethoxytetrahydropyrylium cations; stereoelectronic control in the reactions of lactonium salts

Childs¹,

Kostyk²,

Lock³

et al. 1991

Can. J. Chem.

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. Can. J. Chem. 69,2024Chem. 69, (1991.The structures of 6-ethoxy-2,3,4,5-tetrahydropyrylium, 1, 6-eth0xy-2-methyl-2,3,4,5-tetrahydropyrylium, 2, and 2-ethoxy-3,4,4a,5,6,7,8,8a-octahydro-1-benzopyrylium, 3, hexachloroantimonates have been determined by X-ray crystallography. In each case the tetrahydropyrylium rings exist in shallow half-chair conformations with the cationic centers C(6), C(5), 0(1), and O(6) having a planar arrangement. The cations all have a Z conformation about the 0 ( 6 j C ( 7 ) bonds and it is shown that this conformation is also preferred in solution by comparison of narrow-line I3C NMR spectra in the solid state and solution. The O ( l W ( 2 ) and 0 ( 6 w ( 7 ) bond distances in 1-3 are significantly longer than those of comparable bonds in neutral esters. The length of the O ( l W ( 2 ) bond is very dependent on substitution at C(2) suggesting a considerable fraction of the positive charge resides on this carbon as well as C(7). The closest cationlanion interactions present in the crystal lattices of these salts are between the chlorine atoms of the anion and the cationic center, C(6). These interactions provide information on the origin of the stereoselectivity observed in nucleophilic attack on these cations. The conformations, details of the C-0 bonding, and closest contacts between cation and anion are discussed in terms of the concept of stereoelectronic control.Key words: 6-ethoxytetrahydropyrylium cations, lactonium salts, structure, X-ray, stereoelectronic control. Chem. 69, 2024Chem. 69, (1991.Les structures des hexachloroantimonates des 6-Cthoxy-2,3,4,5-tttrahydropyrylium (I), 6-tthoxy-2-mCthyl-2,3,4,5-tttrahydropyrylium (2) et 2-~thoxy-3,4,4a,5,6,7,8,8a-octahydro-l-benzopyrylium (3) ont Ct C dCterminCes par diffraction des rayons-X. Dans chaque cas, les cycles tCtrahydropyrylium existent dans des conformations demi-chaises peu profondes dans lesquelles les centres cationiques C(6), C(5), O(1) et O(6) sont dans un arrangement planaire. Les cations possbdent tous une conformation Z autour des liaisons 0 ( 6 w ( 7 ) ; par comparaison des bandes fines des spectres RMN du 13C i i 1'Ctat solide et en solution, on a aussi dtmontrC que cette conformation est aussi la conformation privilCgiCe en solution. Les longueurs des distances 0 ( 1 ) 4 ( 2 ) et O(6)-X(7) des composCs 1-3 sont beaucoup plus longues que celles des liaisons comparables des esters neutres. La longueur de la liaison O ( l W ( 2 ) dtpend beaucoup de la substitution en C(2); ce rCsultat suggkre qu'une fraction importante de la charge positive se trouve sur ce carbone ainsi que sur celui en C(7). Les interactions cation/anion le plus proches observCes dans les rCseaux cristallins de ces sels se retrouvent entre les atomes de chlore de l'anion et le centre cationique C(6). Ces interactions fournissent de l'information sur l'origine de la stCrCosClectivitC observCe lors des attaques nuclCophiles sur ces cations. On fait appel au concept de contr6le stCrCoClectronique pour discuter des conformations, des details de l...

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Section: Crystal Packingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Structural studies on 6-ethoxytetrahydropyrylium cations; stereoelectronic control in the reactions of lactonium salts

Childs¹,

Kostyk²,

Lock³

et al. 1991

Can. J. Chem.

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“…Ethyl carbamate in sake was extracted and purified according to the procedure proposed by Ough4,5) with some modifications, and determined by GC-FID after alkylation of ethyl carbamate with N, N-DMF-DMA. 10,15) Extraction and purification: The pH of 200 ml of the sample was adjusted to 6.5 by addition of 1 N sodium hydroxide, and 50 g of sodium chloride was dissolved in it. Then, the sample was transferred into a 500 ml separatory funnel, and extracted with 180 ml of chloroform for 30 min.…”

Section: Methodsmentioning

confidence: 99%

Determination of Ethyl Carbamate in Sake by Alkylation and Gas Chromatography with a Flame Ionization Detector

Kobayashi

Toyoda

Saitō

1987

J. Food Hyg. Soc. Jpn

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A derivatization method for the determination of ethyl carbamate in sake, a Japanese alcoholic beverage, was developed. The ethyl carbamate was extracted with chloroform, purified on a Florisil column and concentrated. The concentrated sample was alkylated with N, N-dimethyl-formamide dimethylacetal, purified on an Extrelut column and concentrated. The ethyl carbamate was determined by capillary gas chromatography with flame ionization detection (GC-FID). The alkylation procedure made it possible to analyze ethyl carbamate in sake without interference on the gas chromatograms.The recoveries of ethyl carbamate added at a level of 100 pg/kg to sake were 73.70 on average. The chemical structure of the alkyl derivative of ethyl carbamate was confirmed by GC/MS. This alkylation procedure for determination of trace amounts of ethyl carbamate in sake is suitable for routine work.

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“…More interesting and effective is the use of basic, or preferably neutral, conditions. These latter transformations usually are more selective and occur under milder conditions than the acidic modern alternative is to use polymer-bound methyl sulfonate, 8 In aqueous conditions, it is possible to use Meerwein methylation, 9 using the corresponding oxonium salts (Me 3 O•BF 4 ) with NaHCO 3 . However, these salts are rapidly hydrolyzed in water.…”

Section: Classification Of Alkylating Agentsmentioning

confidence: 99%

A comparison of several modern alkylating agents

Lamoureux¹,

Agüero²

2009

Arkivoc

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This review covers the application, mechanisms and limitations of simple alkylating agents that can be used for the conversion of carboxylic acids to esters, phenols to aryl ethers, enols to enol ethers and thiols to thioethers. Specific recent examples are provided to show the selectivity (regio or chemo) in these transformations. It is important to have access to these methods when considering alternatives to classical alkylating techniques.

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Reaktionen mit Alkylkationen

Cited by 217 publications

References 12 publications

Structural studies on 6-ethoxytetrahydropyrylium cations; stereoelectronic control in the reactions of lactonium salts

Structural studies on 6-ethoxytetrahydropyrylium cations; stereoelectronic control in the reactions of lactonium salts

Determination of Ethyl Carbamate in Sake by Alkylation and Gas Chromatography with a Flame Ionization Detector

A comparison of several modern alkylating agents

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