1.4- Dicarbonyl compounds from alkynes

The reaction of tnfluoromethyl-substituted A -acyl umnes toward nucleophiles in many aspects parallels that of the parent polyfluoro ketones Heteronucleophiles and carbon nucleophiles, such as enarmnes [37, 38], enol ethers [38, 39, 40], hydrogen cyanide [34], tnmethylsilylcarbomlnle [2,47], alkynes [42], electron-nch heterocycles [43], 1,3-dicarbonyl compounds [44], organolithium compounds [45, 46, 47, 48], and Gngnard compounds [49,50], readily undergo hydroxyalkylation with hexafluoroace-tone and amidoalkylation with acyl imines denved from hexafluoroacetone... 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

V-Aminopyrroles, easily prepared from the reaction of azoalkenes with enamines and /3-dicarbonyl compounds, have been shown to react with electron deficient alkynes to afford substituted benzenes (79TL2969). While the N-methoxycarbonylaminopyrrole (208) reacted with DM AD under rather vigorous conditions to afford (211) in only 13% yield, the N-unsubstituted aminopyrrole (209) prepared from (208) by NaCN treatment reacted with DMAD in CHC13 solution at room temperature to give (211) in 50% isolated yield. The formation of the aromatic system probably occurs by extrusion of the heteroatom bridge from (210) to afford a relatively stable nitrene (212 Scheme 45). 

Hatakeyama et al. [115] have also carried out detailed product studies in the HO-initiated oxidation of the alkynes both in the presence and in the absence of NOx. The major products consisted of a-dicarbonyl compounds, i.e., HC(0)CHO from acetylene, CH3C(0)CH0 from propyne and CH3Q0)C(0)CH3 from 2-butyne, as well as HC(0)OH from acetylene and propyne and CH3C(0)OH from 2-butyne. The formation of these products was attributed to 02-reactions of the hydroxyvinyl radicals resulting from the addition reaction of HO with the alkynes, e.g.,... 

The problem of regioselectivity remains. Monosubstituted alkynes usually react cleanly using the HOMO of the alkyne and the LUMO of the nitrile oxide. The product is exactly that type of isoxazole (72 or 73) that was so difficult to make from dicarbonyl compounds and hydroxylamine. Here regioselectivity is controlled because the two substituents (R1 and R2) are on different reagents. Conditions are very mild. 

Hydroxy radical initiated oxidation of alkynes is important from the point of view of both atmospheric and combustion chemistry. Hatakeyama and coworkers have measured rate constants for the reaction of HO with acetylene, propyne and 2-butyne under atmospheric conditions. It has been suggested, based on product studies, that the jS-hydroxyvinyl radicals further react with molecular oxygen to form the corresponding peroxyl radicals and their subsequent reactions give carboxylic acid, a-dicarbonyl compounds and acyl radicals. 

A variety of substituted radicals may be generated by the electro-oxidation of salts of 1,3-dicarbonyl compounds, aliphatic nitrocompounds, phenols, oximes, alkynes, and organometallic compounds. In all of these reactions, an anionic (or partially anionic) species is formed from which the intermediate radical is generated, e.g., in the anodic oxidation of diethylmalonate... 

B.i.b. Alkynes Containing Proximate 1,3-Dicarbonyl Groups. 2-Propargyl-1,3-dicarbonyl compounds react with alkenyl triflates or alkenyl/aryl/heteroaryl halides to give 2,3,5-trisubstituted-furans (Scheme 4). The process probably proceeds through an oxypalladation step that involves a nucleophilic attack of a stabilized enolate across the activated carbon-carbon triple bond, reductive elimination of a palladium(0) species from the resultant oxypalladation adduct, and isomerization of the initially formed alkylidene derivative. 

When 2-propargyl-l,3-dicarbonyl compounds are treated with aryl iodides under a balloon of carbon monoxide 2,3,5-trisubstituted-furans containing a 5-acylmethyl group (Scheme 7a) or its enol ester (Scheme 7b) can be obtained. Formation of the acyhnethyl derivative or its enol ester depends on the aryl iodide to alkyne ratio. Excess alkyne affords the acyhnethyl derivative as the main product whereas employment of an excess of the aryl iodide favors the formation of the enol ester. The enol ester product is very likely formed from the acyhnethyl product via trapping of the corresponding enolate with an acylpalladium complex. 

In 2014, by taking advantage of the unique redox property and carbophilic. -acidity of gold. You and coworkers accomplished an efficient access to 3-alkynyl polysubstituted furans 15 from readily available 1,3-dicarbonyl compounds 14 and terminal alkynes under mild reaction conditions (Scheme 12.7)... 

Reductive termination of the reaction sequence by hydrogen abstraction is occasionally the desired reaction. This is particularly important in converting vinyl radicals (obtained from addition to alkynes) to alkenes because vinyl radicals are not oxidized to vinyl cations. The hydrogen can come from the solvent or from the R-hydrogen of another molecule of the 3-dicarbonyl compound. Ethanol is... 

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