Alkynes oxidative

This kind of reactivity turned out to be particularly attractive when applied to suitably functionalized terminal alkynes. Oxidative monoaminocarbonylation of 4-yn-l-ols led to the formation of tetrahydrofuran derivatives through intramolecular conjugate addition of -OH group to the triple bond of the initially formed 6-hydroxy-2-ynamide intermediates (Scheme 38) [310]. 

There is a rich chemistry of alkene and alkyne oxidation by RuO. The main application lies in alkene cleavage, bnt there is growing interest in cw-dihydroxylation by the reagent. In the sections below we first consider oxidations which do not sever the C=C bond (epoxidation, ctT-dihydroxylation, ketohydroxylation), and then alkene cleavage reactions. 

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. 

As with alkene cleavage the main reagent for alkyne oxidations is RuO. Oxidative cleavage of alkynes by a variety of reagents has been reviewed [4, 6, 12, 14, 15], The first oxidation of alkynes was noted by Pappo and Becker in 1956 they showed that l,2-fc/x(l-acetoxycyclohexyl)ethyne (2) (Fig. 1.5) gave the diketone. Minimal experimental details were given [195],... 

For other alkyne oxidation systems mentioned in Chapter 1 but not considered here cf. 3.2.23 and for large-scale alkyne oxidations cf. 3.2.2.4. 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

Zirconocene 1,9-anthracenediyl complex 69 presumably undergoes rearrangement to an isomeric benzyne complex prior to the insertion of external alkyne (Equation 26). The isomerization can be understood as a /3-hydrogen elimination/reductive elimination process, resulting in a formal reduction to Zr(ll), followed by a typical alkyne/ alkyne oxidative cyclization to the observed zirconacyclopentadiene product 70. The coordinated benzyne intermediate can be observed spectroscopically as a trimethylphosphine adduct <2000JA9880>. 

Alkyne oxidation reactions are of little value now but were used bi > tortcally in Uie structure -detemiination of substances isolated fron) nau-ral sources. For example, the location of the triple bond in the chain of tariric acid was established by finding that oxidation with KMnO., gax e dodecar. c add and hexanedioic acid ... 

Chlorination and oxidation. This reagent is stable and easy to handle. It can be used to introduce chlorine atoms to C-2 of 2-substituted 1,3-dioxolanes, the a-position of aldehydes besides alkenes and alkynes. Oxidation of alcohols such as benzyl alcohol and cyclooctanol in MeCN requires pyridine-DABCO (4 1) as acid scavenger. 

Oxidation of alkynes Oxidation of alkynes with ruthenium tetroxide leads to the corresponding diketones terminal alkynes give only the corresponding acids. Examples ... 

At low temperature, in the presence of alkynes, the number of free sites must be extremely low. The number of free sites increases with temperature so that oxidizing species can adsorb and allows alkyne oxidation above 250°C. Above 300°C, when alkyne oxidation is rapid, the number of free sites becomes sufficiently high to enable CO oxidation. 

The subject of this chapter is the hydroxylation of all kinds of aromatic rings, either substituted or unsubstituted. The ring(s) may be in an external substrate added to a metal complex - dioxygen system, or may be part of a ligand coordinated to the metal ion within the catalyst complex. Aromatic side-chain oxidations are treated in the sections for alkane, alkene or alkyne oxidation, depending on the substituent. 

Reaction conditions have been developed for a metallonitrene-initiated alkyne oxidation cascade with intermolecular cascade termination by ylide formation/[2,3] Wittig rearrangement upon reaction of alkyne 245 with enantioenriched allyl ethers 246 to provide heterocyclized AT-sulfonyl imine products 247 efEciendy (13AGE5836). 

Rhodium catalyzed alkyne oxidation is proposed to effect ketene generation from 4-methoxyphenylacetylene with oxygen transfer from pyridine N-oxides, with capture of the rhodium complexed ketene 32 by nucleophiles, including phenols and amines (Scheme 4.11). In the presence of... 

Scheme 4.11 Ketene formation by alkyne oxidation with rhodium catalysis.

Scheme 4.12 Ketene formation from alkyne oxidation with a ruthenium catalyst.

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