The 4+2cycloaddition of a conjugated diene and a dienophile (an alkene or alkyne), an electrocyclic reaction that involves the 4 πelectrons of the diene and 2 πelectrons of the dienophile. The driving force of the reaction is the formation of new σbonds, which are energetically more stable than the πbonds. In the case of an alkynyl dienophile, the initial adduct can still react as a dienophile if not too sterically hindered. In addition, either the diene or the dienophile can be substituted with cumulated double bonds, such as substituted allenes
Diels-Alder Reaction The [4+2]-cycloaddition of a conjugated diene and a dienophile (an alkene or alkyne), an electrocyclic reaction that involves the 4 π-electrons of the diene and 2 π-electrons of the dienophile. The driving force of the reaction is the formation of new σ-bonds, which are energetically more stable than the π-bonds. In the case of an alkynyl dienophile, the initial adduct can still react as a dienophile if not too sterically hindered. In addition, either the diene or the dienophile can be substituted with cumulated double bonds, such as substituted allenes. With its broad scope and simplicity of operation, the Diels-Alder is the most powerful synthetic method for unsaturated six-membered rings. A variant is the hetero-Diels-Alder, in which either the diene or the dienophile contains a heteroatom, most often nitrogen or oxygen. This alternative constitutes a powerful synthesis of six- membered ring heterocycles. Mechanism Overlap of the molecular orbitals (MOs) is required: Overlap between the highest occupied MO of the diene (HOMO) and the lowest unoccupied MO of the dienophile (LUMO) is thermally allowed in the Diels Alder Reaction, provided the orbitals are of similar energy. The reaction is facilitated by electron-withdrawing groups on the dienophile, since this will lower the energy of the LUMO. Good dienophiles often bear one or two of the following substituents: CHO, COR, COOR, CN, C=C, Ph, or halogen. The diene component should be as electron-rich as possible. There are “inverse demand” Diels Alder Reactions that involve the overlap of the HOMO of the dienophile with the unoccupied MO of the diene. This alternative scenario for the reaction is favored by electron-donating groups on the dienophile and an electron-poor diene. The reaction is diastereoselective. Cyclic dienes give stereoisomeric products. The endo product is usually favored by kinetic control due to secondary orbital interactions. Recent Literature Chiral Oxazaborolidine-Aluminum Bromide Complexes Are Unusually Powerful and Effective Catalysts for Enantioselective Diels-Alder Reactions D. Liu, E. Canales, E. J. Corey, J. Am. Chem. Soc., 2007, 129, 1498-1499. Diarylprolinol Silyl Ether as Catalyst of an exo-Selective, Enantioselective Diels-Alder Reaction H. Gotoh, Y. Hayashi, Org. Lett., 2007, 9, 2859-2862. Asymmetric Diels-Alder Reactions of α,β-Unsaturated Aldehydes Catalyzed by a Diarylprolinol Silyl Ether Salt in the Presence of Water Y. Hayashi, S. Samanta, H. Gotoh, H. Ishikawa, Angew. Chem. Int. Ed., 2008, 47, 6634-6637. Catalytic Enantioselective Diels-Alder Reaction via a Chiral Indium(III) Complex Y Chua Teo, T P. Loh, Org. Lett., 2005, 7, 2539-2541. Design of Chiral N-Triflyl Phosphoramide as a Strong Chiral Brønsted Acid and Its Application to Asymmetric Diels-Alder Reaction D. Nakashima, H. Yamamoto, J. Am. Chem. Soc., 2006, 128, 9626-9627. A Mild and Efficient Asymmetric Hetero-Diels-Alder Reaction of the Brassard Diene with Aldehydes Q. Fan, L. Lin, J. Liu, Y. Huang, X. Feng, Eur. J. Org. Chem., 2005, 3542-3552. DIANANE-Cr(III)-salen Complexes as Highly Enantioselective Catalysts for Hetero-Diels-Alder Reactions of Aldehydes with Dienes A. Berkessel, N. Vogl, Eur. J. Org. Chem., 2006, 5029-5035. Highly Selective Diels-Alder Reactions of Directly Connected Enyne Dienophiles M. Dai, D. Sarlah, M. Yu, S. J. Danishefsky, G. O. Jones, K. N. Houk, J. Am. Chem. Soc., 2007, 129, 645-657. [AlCl 3 + 2THF]: A New and Efficient Catalytic System for Diels-Alder Cycloaddition of α,β- Unsaturated Carbonyl Compounds under Solvent-Free Conditions F. Fringuelli, R. Girotti, F. Pizzo, L. Vaccaro, Org. Lett., 2006, 8, 2487-2489. Strong Counteranion Effects on the Catalytic Activity of Cationic Silicon Lewis Acids in Mukaiyama Aldol and Diels-Alder Reactions L B. Han, C Q. Zhao, J. Org. Chem., 2005, 70, 10121-10123. Synthesis of Dihydronaphthalenes via Aryne Diels-Alder Reactions: Scope and Diastereoselectivity C. Dockendorff, S. Sahli, M. Olsen, L. Milhau, M. Lautens, J. Am. Chem. Soc., 2005, 127, 15028- 15029. Imidazolinium salts as catalysts for the aza-Diels-Alder reaction V. Jurcik, R. Wilhelm, Org. Biomol. Chem., 2005, 3, 239-244. Catalytic Asymmetric Inverse-Electron-Demand Diels-Alder Reaction of N-Sulfonyl-1-Aza-1,3- Dienes J. Esquivias, R. G. Arrays, J. C. Carretero, J. Am. Chem. Soc., 2007, 129, 1480-1481. Chiral Brønsted Acid-Catalyzed Inverse Electron-Demand Aza Diels-Alder Reaction T. Akiyama, H. Morita, K. Fuchibe, J. Am. Chem. Soc., 2006, 128, 13070-13071. . Diels-Alder Reaction The [4+2]-cycloaddition of a conjugated diene and a dienophile (an alkene or alkyne), an electrocyclic reaction that involves the 4 π-electrons. of the dienophile (LUMO) is thermally allowed in the Diels Alder Reaction, provided the orbitals are of similar energy. The reaction is facilitated by electron-withdrawing groups on the dienophile,. “inverse demand” Diels Alder Reactions that involve the overlap of the HOMO of the dienophile with the unoccupied MO of the diene. This alternative scenario for the reaction is favored by electron-donating