Catalysis by heteropoly acids (HPAs) and related compounds is a field of growing importance, attracting increasing attention worldwide, in which many new and exciting developments are taking place in both research and technology [1–111, HPAs are polyoxometalates incorporating anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units [ll-141. Among a wide variety of HPAs those belonging to the so-called Keggin series are the most importance for catalysis. They include heteropolyanions (HPANs)

Heteropoly Acids and Related Compounds as Catalysts for Fine Chemical Synthesis

How easy are the syntheses of allenes

As the chemistry of saccharides and related molecular species plays a significant role in the metabolic pathways of living organisms, detecting the presence and concentration of biologically important sugars in aqueous solution is necessary in a variety of medicinal and industrial contexts. The recognition of d-glucose is of particular interest, for example in the monitoring of diabetics. Recent research provides clear evidence that tight control of blood sugar levels in diabetics sharply reduces the risk of long term complications, which include blindness, kidney failure, heart attacks and even gangrene and amputation of the limbs. Current enzymatic detection methods of sugars offer specificity for only a few saccharides; additionally, enzyme based sensors are unstable in harsh conditions. Phenylboronic acid has been known for 120 years. However, it took until 1959 for the first quantitative evaluation of saccharide boronic acid interactions. Boronic acids react with 1,2 or 1,3 diols of saccharides to form five- or six-membered cyclic esters in non-aqueous or basic aqueous media. The stable boronic acid-based saccharide receptors offer the possibility of creating saccharide sensors ‘chemosensors’ which are selective and sensitive for any chosen saccharide.

Artificial Receptors as Chemosensors for Carbohydrates

This critical review describes a class of polymers prepared by electrochemical polymerization that employs the concept of molecular imprinting for chemical sensing. The principal focus is on both conducting and nonconducting polymers prepared by electropolymerization of electroactive functional monomers, such as pristine and derivatized pyrrole, aminophenylboronic acid, thiophene, porphyrin, aniline, phenylenediamine, phenol, and thiophenol. A critical evaluation of the literature on electrosynthesized molecularly imprinted polymers (MIPs) applied as recognition elements of chemical sensors is presented. The aim of this review is to highlight recent achievements in analytical applications of these MIPs, including present strategies of determination of different analytes as well as identification and solutions for problems encountered.

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Electrochemically synthesized polymers in molecular imprinting for chemical sensing

As master craftsmen, modern synthetic chemists are challenged to achieve remarkable feats of efficiency and elegance toward molecular targets. The nature of this pursuit necessitates the collection of synthetic repertoires that are tried and true. With methodologies and pathways increasingly scrutinized, the adept chemist must seek out propitious tools to incorporate into the arsenal. With this in mind, this Account highlights the versatility of alkoxyallenes as precursors to valuable heterocyclic building blocks for such efforts as natural product synthesis. Accessed by the etherification of either propargyl alcohols or propargylic halides, alkoxyallenes are obtained after base-catalyzed isomerizations of the propargylic ethers. A host of umpolung synthons are available through this scheme after metalation, generating C3 nucleophiles synthetically equivalent to vital anionic and zwitterionic synthons. Reactions with a diverse set of heteroatomic electrophiles yield carbohydrates, spiroketals, alkaloids, ...

Sugars, Alkaloids, and Heteroaromatics: Exploring Heterocyclic Chemistry with Alkoxyallenes†

Propargyl ethers HCCCH2OR [R = alkyl or-CH(CH8)(OC2H5)] have been isomerized with good yields into the corresponding allenyl ethers CH2CCHOR by warming with potassium tert.-butoxide at 70°.

These allenyl ethers can be metallated with butyllithium in ether or alkali amides in liquid ammonia. In ether, subsequent alkylation with alkyl halides R′Hal affords α-substituted allenyl ethers CH2CC(R′)OR. Alkylation in liquid ammonia produces a mixture of this same compound and the γ-substituted product R′CHCCHOR. In both cases reasonable yields are obtained. Sodamide and potassium amide quickly convert allenyl ethers CH2CCHOR into metallated propargyl ethers MCC-CH2OR (M = Na or K). If alkylation is not performed almost simultaneously with the metallation with sodamide or potassium amide, the only alkylation product obtained is R′CCCH2OR.

Preparation, metallation and alkylation of allenyl ethers

Chloro-dialkylphosphines, R2PCl (R = branched primary, secondary or tertiary alkyl group), can be easily prepared from phosphorus trichloride and alkylmagnesium chlorides in ether.

A note on the preparation of chloro‐dialkylphosphines

It is possible to reduce non-terminal triple bonds in poly-ynes selectively when the terminal triple bonds are previously protected silylation. Desilylation after the reduction can be easily effected with aqueous alcoholic silver nitrate.

Cleavage of the carbon‐silicon bond in 1‐alkynylsilanes by silver nitrate: Protection of a terminal triple bond in hydrogenation reactions with the trimethylsilyl group

Thioacetals and orthothioformates, compounds having the general formula HC(R)(SR′)2 with R = H, SR′, CH3, or C6H5 and R′ = C2H5 or C6H5, were metallated by means of alkali amides in liquid ammonia. The metallated derivatives were A) alkylated, B) converted with ketones (successfully only when R = H and R′ = C6H5), and C) ethylthiolated by means of diethyldisulphide (only when R = H, SR′, or C6H5). The results of the reactions are summarized in Tables I-VI, VIII, and IX. Similar conversions were impossible with common acetals or orthoformates (notably CH2(OC2H5)2, C6H5-CH(OC2H5)2, and CH(OC2H5)3). The easy metallation of the thiocompounds is attributed to d-orbital participation of sulphur, which probably lowers the activation energy of the metallation reaction and enhances the stability of the carbanions.



The new reactions in principle enable the synthesis of aldehydes from formaldehyde, of methyl ketones from acetaldehyde, of phenyl ketones from benzaldehyde, and of carboxylic acids from formic acid.



Upon standing, solutions of NaC(SC2H5)3 and NaC(SC6H5)3 in liquid ammonia slowly decompose, yielding compounds of the type (R′S)2C = C(SR′)2. The transient occurrence of a carbene: C(SR′)2, formed by splitting-off of thiolate from the carbanion ⊖ C(SR′)3, is discussed.

Syntheses with metallated thioacetals and orthothioformates

The isomeric allenyl thioethers, III, are produced in good yield by treating 1-ethylthio-1-alkynes, I, with an equivalent amount of sodamide in liquid ammonia, followed by hydrolysis of the reaction mixture.



The corresponding allenyl thioethers alkylated in the 1-position, IV, are obtained, also in good yield, by adding primary or secondary alkyl halides to the reaction product of sodamide and 1-ethylthio-1-alkynes, I.

J. F. Arens

Papers

Preparation, metallation and alkylation of allenyl ethers

A note on the preparation of chloro‐dialkylphosphines

Cleavage of the carbon‐silicon bond in 1‐alkynylsilanes by silver nitrate: Protection of a terminal triple bond in hydrogenation reactions with the trimethylsilyl group

Syntheses with metallated thioacetals and orthothioformates

Chemistry of acetylenic ethers, 67: Allenyl thioethers from alkynyl thioethers