Diels-Alder reactions in ionic liquids

Scheme 5.16 Examples of Diels-Alder reactions in ionic liquids...

Earle, M. J., McCormac, P. B. Seddon, K. R. Diels-Alder reactions in ionic liquids a safe recyclable green alternative to lithium perchlorate-diethyl ether mixtures. Green Chem., 1999, 1(1), 23-25 Doherty, S. Goodrich, P. Hardacre, C. et al. Marked enantioselectivity enhancements for Diels-Alder reactions in ionic liquids catalysed by platinum diphosphine complexes. Green Chem., 2004, 6(1), 63-67. 

Oh and Meracz were the first to report asymmetric Diels-Alder reaction in ionic liquids [43]. It has also been shown that Diels-Alder reactions in ionic liquids give unusually high stereoselectivities at room temperature as compared to those in conventional organic solvents, where a low temperature was required to achieve good stereoselectivities. For example, the Diels-Alder reaction of cyclopentadiene and dienophile 25 with Cu(ll)-bisoxazoline complex 24 as catalyst in [dbim][BF4] showed a higher endoselectivity (endo j exo = 93/7) and regioselectivity (26/27 = 96/4) than in CHjCb (endojexo = 79/21, 26/27 = 76/24) (Scheme 7.11). 

Quite recently, palladium-phosphinooxazohdine catalysts have been used for asymmetric Diels-Alder reactions in ionic liquids [45]. By screening various counterions of the Pd catalysts and ionic liquids, a combination of cationic Pd-POZ catalyst 31c with ShF counterion and [bmim][BF4] as the ionic liquid was found to be the most effective. The Diels-Alder reaction of cyclopentadiene with dienophile 32 using 31c as a catalyst provided DA adduct 33 with high chemical and optical yields. The catalyst 31c immobilized in [bmimJfBFJ was successfully recycled eight times to give 33 in 89-99% yields and 88-99% ee-values (Scheme 7.12). 

Catalytic asymmetric Diels-Alder reactions in ionic liquids, using an air- and moisture-stable chiral BlNOL-indium complex prepared from ( S)-BINOL and InClj, have also reported [46]. The reactions of various dienes with 2-methacrolein or 2-bromoacrolein proceeded in an ionic hquid with good yields and excellent enantioselectivities (up to 98% ee). The chiral In(lll) catalyst immobilized in ionic liquid [hmim][PF 5] could be reused seven times without any significant loss in catalytic activity (Scheme 7.13). 

Uwari S, Khupse N, Kumar A (2008) Intramolecular Diels-Alder reaction in ionic liquids effect of ion-specific solvent friction. J Org Chem 73 9075-9083... 

Scheme 5.2-75 The aza-Diels-Alder reaction in ionic liquids [185].

The intramolecular Diels-Alder reaction is most frequently used in natural product total synthesis, and numerous examples will be described in the synthetic utility section. As with the intermolecular variant, intramolecular reactions are highly regioselective and stereoselective and participate in hetero, inverse electron demand, and asymmetric Diels-Alder reactions. One report from 2008 describes the investigation of an intramolecular hetero Diels-Alder reaction in ionic liquids. ... 

Bini, R., Chiappe, C., Mestre, VL., Pomelli, C.S. and Welton, T., A rationalization of the solvent effect on the Diels-Alder reaction in ionic liquids using multiparameter linear solvation energy relationships, Org. Biomol. Chem. 6, 2522-2529 (2008). 

Doherty, S., Goodrich, P., Hardacre, C., Luo, H. K., Rooney, D.W., Seddon, KR. Styring, P. (2004). Marked Enantioselectivity Enhancements for Diels-Alder reactions in Ionic Liquids Catalysed by Platinum Diphosphine Complexes, Green Chem., 6, pp. 63-67... 

Herrmann WA, Goossen LJ, Koecher C (1996) Heterocyclic carbenes. 9. Chiral heterocyclic carbenes in asymmetric homogeneous catalysis. Angew Chem Int Ed 35 2805-2807 Earle MJ, McCormac PB, Seddon KR (1999) Diels-Alder reactions in ionic liquids. Green Chem 1 23-25... 

Acevedo O, Jorgensen WL, Evanseck JD (2007) Elucidation of rate variations for a Diels-Alder reaction in ionic liquids from QM/MM simulations. J Chem Theory Cranput 3 132-138... 

Zulfiqar, F., Kitazume, T. (2000). One-pot aza-Diels-Alder reactions in ionic liquids. Green Chem., 2,137-139, Qune 2000) 1463-9262, ISSN 1463-9262... 

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