Microwave irradiation alkynes

For the microwave-assisted experiments, both solvents were replaced by 1,2-dichlorobenzene, as it couples very effectively with microwaves (loss-tangent (tan 5) at 20 °C 1,2-dichlorobenzene 0.280 as compared to 0.101 for chlorobenzene). Diels-Alder reactions of 3-methoxy or 3-phenyl pyrazinones with DMAD were performed at a pre-selected maximum temperature of 200 °C, whereas the intramolecular reaction of alkyne tethered pyrazinone required a higher temperature (220 °C). The yields obtained under microwave irradiation are comparable with those obtained under conventional conditions, while for the dihydrofuropyridinone the yield was improved from... 

Pt-catalyzed hydration of various aliphatic and aromatic alkynes under phase transfer conditions in (CH2C1)2/H20 in the presence of Aliquat 336 led to either a Markovnikov product, mixtures of two ketones, or ketones with the carbonyl group positioned away from the bulky side.72 In the absence of the phase transfer reagent, Aliquat 336, hardly any reaction took place. Recently, a hydrophobic, low-loading and alkylated polystyrene-supported sulfonic acid (LL-ALPS-SO3H) has also been developed for the hydration of terminal alkynes in pure water, leading to ketones as the product.73 Under microwave irradiation, the hydration of terminal arylalkynes was reported to proceed in superheated water (200°C) without any catalysts.74... 

The final example demonstrates that microwave irradiation allows a perfect fine-tuning of reaction conditions to obtain different products from the same starting materials. In the procedure developed by Garcfa-Tellado and coworkers [41], two domino processes were coupled. The first process consists of a high-yielding synthesis of enol-protected propargylic alcohols 10-111 starting from alkyne 10-109 and aldehyde 10-110 (Scheme 10.28). In the second process, transformation into... 

Microwave irradiation promotes the 1,3-dipolar activity of nitrile oxides generated from hydroximoyl chlorides. They interacted in situ over alumina with alkenes and alkynes (150). The effect was demonstrated in reactions of... 

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

Carbon-carbon multiple bonds are most commonly reduced to saturated hydro carbons by hydrogenation or by other methods, such as the use of hydrides. While a large number of alkenes have been reduced under microwave irradiation, surprisingly little work has been published on microwave-assisted reduction of alkynes. 

Keywords terminal alkyne, Glaser coupling, cupric chloride, microwave irradiation, butadiyne... 

Keywords aryl iodide, terminal alkyne, Sonogashira coupling, KF-alumina, microwave irradiation, aryl alkyne... 

Keywords alkyne, amine, paraformaldehyde, alumina, Mannich condensation, microwave irradiation, aminoalkyne... 

Microwave irradiation considerably improved the reaction speed and yields of 2,4-disubstituted quinolines in a MCR of aldehydes, anilines and alkynes [111]. The cyclocondensation was catalyzed by montmorillonite clay doped with copper bromide and was completed within 3-5 minutes (pulsed irradiation technique—1 min with 20 s off interval), when performed in a household microwave oven. Oil-bath heating at 80 °C for 3-6 hours was necessary to achieve comparable yields of quinolines (71-90%) (Scheme 42). 

Terminal alkynes can be coupled with ortho-iodobenzoic acids, using a Pd/C-PPh3-CuI catalystic system to afford 3-substituted isocoumarins (Equation 327) <2005JOC4778>. Likewise, the copper(l) mediated coupling of ortho-iodobenzoic acid with ethynylbenzene under microwave irradiation forms 3-phenyl-isocoumarin in excellent yield (Equation 328) <2002SC1937>. 

Under microwave irradiation, a mixture of an a,(3-unsaturated ketone, an alkyne dienophile and Lawesson s reagent gives excellent yields of highly substituted 4/7-thiopyrans (Scheme 41). The one pot reaction has been applied to 16-dehydropregnenolone. Initial formation of a 6-membered oxathiophosphinino derivative is postulated which is regarded as a precursor of an unsaturated thioketone <06TL4925>. 

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

Recently, Muller and co-workers [145] have reported a series of 3,5-disubstituted and 1,3,5-trisubstituted pyrazoles 104 and 105 by reacting an acyl chloride, a terminal alkyne and a hydrazine via a consecutive one-pot three-component Sonogashira coupling/Michael addition/cyclocondensation sequence under microwave irradiation. The desired products were obtained in good to excellent yields. These obtained pyrazoles are highly fluorescent, both in solution and in the solid state (Scheme 81). 

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