5257-46-5Relevant articles and documents
CATALYSIS OF DIELS-ALDER REACTIONS BY METALLOPORPHYRINS
Bartley, David W.,Kodadek, Thomas J.
, p. 6303 - 6306 (1990)
Aluminium (III) tetraphenyl porphyrin chloride (AITPPCl) has been shown to catalyze Diels-Alder reactions of α,β-unsaturated carbonyl compounds in moderate yield.Selectivity based on electronic effects is exhibited which is different from that seen with catalysts such as diethylaluminium chloride.
Chiral protic imidazolium salts with a (-)-menthol fragment in the cation: Synthesis, properties and use in the Diels-Alder reaction
Janus, Ewa,Gano, Marcin,Feder-Kubis, Joanna,So?nicki, Jacek
, p. 10318 - 10331 (2018/03/26)
New chiral protic imidazolium salts containing a (1R,2S,5R)-(-)-menthol substituent in the cation and four different anions (chloride, hexafluorophosphate, trifluoromethanesulfonate and bis(trifluoromethylsulfonyl)imide) were efficiently prepared and exte
Silylium ion-catalyzed challenging Diels-Alder reactions: The danger of hidden proton catalysis with strong Lewis acids
Schmidt, Ruth K.,Muether, Kristine,Mueck-Lichtenfeld, Christian,Grimme, Stefan,Oestreich, Martin
supporting information; experimental part, p. 4421 - 4428 (2012/04/23)
The pronounced Lewis acidity of tricoordinate silicon cations brings about unusual reactivity in Lewis acid catalysis. The downside of catalysis with strong Lewis acids is, though, that these do have the potential to mediate the formation of protons by various mechanisms, and the thus released Bronsted acid might even outcompete the Lewis acid as the true catalyst. That is an often ignored point. One way of eliminating a hidden proton-catalyzed pathway is to add a proton scavenger. The low-temperature Diels-Alder reactions catalyzed by our ferrocene-stabilized silicon cation are such a case where the possibility of proton catalysis must be meticulously examined. Addition of the common hindered base 2,6-di-tert-butylpyridine resulted, however, in slow decomposition along with formation of the corresponding pyridinium ion. Quantitative deprotonation of the silicon cation was observed with more basic (Mes)3P to yield the phosphonium ion. A deuterium-labeling experiment verified that the proton is abstracted from the ferrocene backbone. A reasonable mechanism of the proton formation is proposed on the basis of quantum-chemical calculations. This is, admittedly, a particular case but suggests that the use of proton scavengers must be carefully scrutinized, as proton formation might be provoked rather than prevented. Proton-catalyzed Diels-Alder reactions are not well-documented in the literature, and a representative survey employing TfOH is included here. The outcome of these catalyses is compared with our silylium ion-catalyzed Diels-Alder reactions, thereby clearly corroborating that hidden Bronsted acid catalysis is not operating with our Lewis acid. Several simple-looking but challenging Diels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported. Another indication is obtained from the chemoselectivity of the catalyses. The silylium ion-catalyzed Diels-Alder reaction is general with regard to the oxidation level of the α,β-unsaturated dienophile (carbonyl and carboxyl), whereas proton catalysis is limited to carbonyl compounds.
