Diels ionic liquid-doped

Scheme 23 Microwave-enhanced Diels-Alder reactions of alkene-tethered 2(lff)-pyra-zinones in ionic liquid doped solvents...

Scheme 6.91 Diels-Alder cycloaddition reactions in ionic liquid-doped solvents.

In 2002, Leadbeater and Torenius reported the base-catalyzed Michael addition of methyl acrylate to imidazole using ionic liquid-doped toluene as a reaction medium (Scheme 6.133 a) [190], A 75% product yield was obtained after 5 min of microwave irradiation at 200 °C employing equimolar amounts of Michael acceptor/donor and triethylamine base. As for the Diels-Alder reaction studied by the same group (see Scheme 6.91), l-(2-propyl)-3-methylimidazolium hexafluorophosphate (pmimPF6) was the ionic liquid utilized (see Table 4.3). Related microwave-promoted Michael additions studied by Jennings and coworkers involving indoles as heterocyclic amines are shown in Schemes 6.133 b [230] and 6.133 c [268], Here, either lithium bis(trimethylsilyl)amide (LiHMDS) or potassium tert-butoxide (KOtBu) was em-... 

The intramolecular hetero-Diels-Alder reactions in functionalized 2(17/)-pyrazinones to give bicyclo adducts were found to undergo a significant rate enhancement using controlled microwave irradiation in ionic liquid doped solvents <2002JOC7904>. 

Van der Eycken, E., Appukkuttan, P., De Borggraeve, W., Dehaen, W., Dallinger, D. et al, High-speed microwave-pro mo ted hetero -diels-alder reactions of 2 (1H) pyrazinones in ionic liquid doped solvents,... 

Applying the concept of using solvents doped with ionic liquids in order to allow microwave heating to high temperatures (see Section 4.3.3.2), Leadbeater and Tore-nius studied the Diels-Alder reaction between 2,3-dimethylbutadiene and methyl acrylate (Scheme 6.91) [190]. This reaction is traditionally performed in toluene or... 

Inter- and intramolecular hetero-Diels-Alder cycloaddition reactions in a series of functionalized 2-(lH)-pyrazinones have been studied in detail by the groups of Van der Eycken and Kappe (Scheme 6.95) [195-197]. In the intramolecular series, cycloaddition of alkenyl-tethered 2-(lH)-pyrazinones required 1-2 days under conventional thermal conditions involving chlorobenzene as solvent under reflux conditions (132 °C). Switching to 1,2-dichloroethane doped with the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) and sealed-vessel microwave technology, the same transformations were completed within 8-18 min at a reaction temperature of 190 °C (Scheme 6.95 a) [195]. Without isolating the primary imidoyl chloride cycloadducts, rapid hydrolysis was achieved by the addition of small amounts of water and subjecting the reaction mixture to further microwave irradia-... 

Very short reaction times were also observed in microwave-assisted hetero-Diels-Alder reactions carried out in solutions doped with an ionic liquid.[115]... 

The hetero Diels-Alder reaction of a series of functionalized 2(li-f)-pyrazinones was studied in detail by Van der Eycken et al. [58, 65]. For example, in a series of intramolecular cycioadditions of alkenyl-tethered 2(li-f)-pyrazinones 27 the reaction required 1-2 days under conventional thermal conditions (chlorobenzene, reflux, 132 °C) whereas use of 1,2-dichloroethane doped with the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPFe) and use of micro-waves up to a temperature of 190 °C (sealed vessels) enabled the same transformations to be completed within 8-15 min. The primary imidoyl chloride cycloadducts were not isolated, but were rapidly hydrolyzed under the action of microwaves by addition of small amounts of water (130 °C, 5 min). The overall yields of 28 were in the same range as reported for the conventional thermal procedures (Scheme 11.8) [58]. 

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