Diels alder reaction.power point

  • Published on

  • View

  • Download

Embed Size (px)


Organic chemistry


<ul><li> 1. Diels-Alder ReactionOtto Diels Kurt Alder Method for synthesis of 6-membered ring One-step, concerted reaction Termed [4+2] cycloaddition reaction where 4 and 2 electrons react.+</li></ul> <p> 2. Diels-Alder Reaction Discovered by O. Diels and K. Alder in 1928. Occur between a conjugated diene and substitutedalkene (dienophile) to form cyclohexene ring system. Concerted reaction (single step), can be accelerated byheating or using some catalysts. [ 4+2 ] cycloaddition reaction. In retro Diels-Alder reaction, the six membered ring isbreak down to regenerate the diene and dienophile usinghigh temperature usually. Stereoselective reaction ( mainly one product formed). 3. Diels-Alder Reaction Stereospecific reaction ( reactants can keep theirstereochemistry). No transition states or charged intermediates. All electrons moving in same time to form two new bonds. 100 % economic ( No. of reactants atoms = No. ofproducts atoms). If one or more of diene or dienophile atoms are notcarbon ,the reaction is hetero-Diels-Alder reaction. 4. The dienes Can be cyclic , acyclic carring many kinds of substituents. Must have s-cis conformation. +I substituted dienes are more reactive than others.CO2 NaOMeOTMSR Danishef sky diene R= Me, OMe, CH(Me)OMe 5. Cyclopentadiene Cyclopentadiene which is well know as a standard dienecan undergoes self D.A at R.T . This dimer can be cracked by distillation. 25oC 6. The dienophiles Wide range of dienophiles can be used including cyclic,acyclic and hetero compounds. Perfect dienophiles are alkenes conjugated to electronwithdrawing groups such as carbonyl, nitro, cyano,halogensetc -I groups increase the rate of D.A reaction. D.A reaction between alkene and diene without anysubstituents can take place but with low yield. 7. The dienophilesO OOBrOONMeMe O 2NO OOO O O OOF3CCF3N NNN Ph Ph MeO OMeN PhO O SOPhNCCN OP OEt SiMe3OEt NCCN 8. How to know its D.A reaction? Since it s {4+2} cycloaddition, the product is, sixmembered ring, double bond inside the ring ,conjugate group outside the ring opposite todouble bond. O O+OO O O 9. How it works? Concerted reaction via aromatic transition state. There are two approches, the first depends on theinteraction between HOMO of The diene and LUMO ofthe dienophile. The second, depends on +I, -I groups affect, since theyform negative and positive cherges on the diene anddienophile. 10. Stereochemistry The stereochemistry of substituents can be retained inthe product. Cyclic dienes must be in s-cis are highly reactive .CO2 Me CO 2MeCO2 Me CO 2Me +CO2 MeCO 2Me MeO 2C CO 2Me 11. Endo rule D.A between cyclic dienes and dienophiles can leadtwo diasteremers, endo and exo. Endo product is kinetically controlled product. Exo product is thermomdynamically product. 12. Regioselectivity Diels-Alder reaction can lead to different structuralisomers. Electronic and steric effects of the substituents. Position of these groups is the main factor. Usually its ortho and para directiong. X XZZ ortho Z Zpara XX Z= electron-withdrawing gruop x= electron-donating group 13. RegioselectivityCO 2HHO2 C HO 2C + + OHC OHCCHOort ho metaO O Me Me +MeMeort ho 14. Catalysed Diels-Alder reaction To increase the rate of the reaction. To reduce high temperarture and long reaction times. To improve the regioselectivity. As a source of enetioselectivity. 15. Catalysed Diels-Alder reaction Using Lewis acids catalysts (including chiral members). AlCl3 and its derivatives, M(OTf), BF3 .OEt.. Using organic catalysts (organic molecules can be usedindividually or combined with Lewis Acids). Imidazolidinone derivatives, thiourea derivatives, BINOLand its derivatives.. 16. Lewis Acid catalysis L.A binding with EWD group then make the dienophilemore electron deficient. Then, decrease LUMO energy, strong interaction withdiene`s HOMO. - + SnClOO O4 O + + without catalyst 71 :29 with SnCl4 93: 7 17. Asymmetric Diels-Alder reaction Preparation of chiral cyclohexene derivatives . Using chiral dienes or dienophiles Using chiral Lewis Acids.CO2 Bn CO2 BnONN + O 18. Chiral dienophiles OOMe NROS OO R ROMe R NNO OO 19. Chiral dienes Using chiral dienes is less developing as the use ofchiral dienophiles. OTMS REtO OMeN O N OO OAc O OAc OAc OAc 20. Chiral Lewis Acids+ MeCHOCHO72%OAlCl2Me endo:exo98:2O ON O Me++ OH Me OH endo : exoTMSO 93 : 07OOOO + TiCl4 OTMS 21. References Organic chemistry; Clayden, J.; Greeves, N.; Warren, S.;Wothers, P. Oxford University Press, Oxford, 2006. Lewis Acids and Selectivity in Organic Synthesis;Santelli, M.; Pons, J. CRS Press, USA, 1995. Wikipedia.co.uk</p>