Diels-Alder catalysts Zinc chloride

Cycloaddition of 2-cyanoalk-2-enones with several conjugated dienes proceeded under zinc chloride catalysis.636 Zinc halides have also shown reactivity with phenylacetylenes.637 Zinc chloride is an effective Lewis acid catalyst in the Diels Alder reactions of the keto esters and the effects on stereochemistry of catalysts used have been examined.638... 

Lewis acids such as zinc chloride, boron trifluoride, aluminum chloride, and diethylaluminum chloride catalyze Diels-Alder reactions.8 The catalytic effect is the result of coordination of the Lewis acid with the dienophile. The complexed dienophile is more electrophilic and more reactive toward electron-rich dienes. The mechanism of the cycloaddition is still believed to be concerted, and high stereoselectivity is observed.9 10 Lewis acid catalysts also usually increase the regioselectivity of the reaction. 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

C13H22O2, Mr 210.32, bpo iy kpa 120-122 °C, df 0.9941, rag0 1.4915, is a fragrance substance that does not occur in nature. It is a colorless, viscous liquid with a sweet odor reminiscent of lily of the valley. The aldehyde can be prepared by a Diels -Alder reaction of myrcenol and acrolein in the presence of a Lewis catalyst (e.g., zinc chloride) [117] ... 

Mechanistically, the cycloaddition reaction is rather complex. Depending on the catalyst or solvent used and the reaction substrates, pericyclic and/or Mukaiyama aldol-like pathways may be involved.43 The pericyclic mechanism, generally favored by zinc chloride and the lanthanide catalysts, tends to produce adducts having the cis relative stereochemistry at C-5,6. It is assumed that chelation of the aldehyde with the Lewis acid occurs in an anti fashion and that the steric bulk of R is less than that of the Lewis acid-solvent complex L [Eq. (11)], thus favoring a Diels-Alder transition state with R endo. 

Heating of the alkynes 179 with ethyl vinyl ether and ethyl glyoxylate under microwave irradiation in the presence of the second-generation Grubbs catalyst 2 directly furnished a 2 1 mixture of the dihydropyrans 180 and 181 in a domino enyne CM/hetero-Diels-Alder process (Scheme 2.63) [36]. These mixtures could be readily equilibrated to give only the cis isomers 181 on treatment with zinc chloride in dichloromethane, subsequently. 

Zinc chloride is often a preferred catalyst since it is less sensitive to water, even though its activity does not compare to that of AICI3 (37). Its mild acidity results in improved control of selectivities in the Diels-Alder reaction. It is also used in haloalkylation of aromatics and the Fischer indol synthesis. 

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