Oxidation alkyne concentration

It is interesting that the product distribution from the reaction of the o-methoxyphenyl complex with diethylacetylene was found to be dependent on alkyne concentration. One equivalent of alkyne, slow addition of alkyne, or low absolute concentration of alkyne favors the indene product. Reactions performed with excess or high concentrations of alkyne favor benzannulation. The observation, reffered to as the allochemical effect, was explained as resulting from coordination of reaction intermediates by the alkyne. Thus excess alkyne acts as a ligand to stabilize intermediates along the path to benzannulation. Finally, it was noted that the product distribution was not substantially altered by addition of phosphines, phosphine oxides or sulfides. 

Hi) Preparation of isoxazoles from nitrile N-oxides The reaction between a nitrile //-oxide and an alkyne is so facile that it is usually sufficient to leave an ether solution of the reactants at room temperature to obtain the desired isoxazole in good yield. The reaction is in general sensitive to the size of the substituent on the alkyne but not on the nitrile -oxide. In the case of poorly reactive alkynes, the difficulty may be overcome by generating the nitrile -oxide in situ and keeping its concentration low. 

A Michael-type addition reaction of phosphine generated from red phosphorus in concentrated aqueous KOH solution has been noted to provide moderate isolable yields of pure organophosphorus products.27 For example, tris-(2-cyanoethyl)phosphine is produced in 45% isolable yield from acrylonitrile, and tris-(2-[y-pyridyl]ethyl) phosphine oxide is isolated in 40% yield from 4-vinylpyridine under these conditions. Excellent yields of the tertiary phosphine oxide, tris-(2-cyanoethyl)phosphine oxide, have been reported using white phosphorus in absolute ethanol with KOH at ice/salt-bath temperatures.28 A variety of solvent systems were examined for this reaction involving a Michael-type addition to acrylonitrile. Similarly, tris-(Z-styryl)phosphine is produced from phenylacetylene under these conditions in 55% isolated yield. It is noteworthy that this last cited reaction involves stereospecific syn- addition of the phosphine to the alkyne. 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

In this chapter the first of the two most important categories of oxidations catalysed by Ru complexes (the other being alkene and alkyne oxidations in Chapter 3) are considered. The approach in this and subsequent chapters differs from that of Chapter 1, concentrating here on the substrate rather than on the oxidant. The text is divided into the categories of alcohols and their oxidations. There are summaries in section 2.3.6 of systems of limited apphcability which are mentioned only in Chapter 1, and in section 2.3.7 of large-scale (>1 g) oxidations. 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

A cyclopropane readily dissolves in concentrated sulfuric acid, and in this resembles an alkene or alkyne. It can be differentiated from these unsaturated hydrocarbons, however, by the fact that it is not oxidized by cold, dilute, neutral permanganate. 

The oxidative coupling reaction of terminal alkynes is critically dependent on the water concentration in the reaction mixture (see Section 2.5.2). Since water is produced during the reaction, careful elimination of it may be required. Challa and Meinders have demonstrated that the polymer catalyst derived from copper(II) chloride and either N,/V-dimethylbenzylamine or N,/V-dimethylaminomethylated atactic polystyrene (37) provides an extra protection of the catalytic copper complexes against water in the coupling reaction of phenylacetylene (equation 23), resulting in a higher reaction rate than the low molecular weight catalyst. 

Nitrile oxides are conveniently generated in situ by dehydration of primary nitro compounds (with phenylisocyanate or ethyl chloroformate or di-tert-butyl dicarbonate) or from a-chloro-oximes (by treatment with a base). The nitrile oxide reacts with an alkene to form an isoxazoUne or with an alkyne to give a heteroaromatic isoxazole (3.131). Nitrile oxides are prone to undergo dimerization, although this can be minimized by maintaining alow concentration of the dipole in the presence of the dipolarophile. 

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