Diels alder reaction.power point

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Diels alder reaction.power point

  1. 1. Diels-Alder ReactionOtto Diels Kurt Alder • Method for synthesis of 6-membered ring • One-step, concerted reaction • Termed [4+2] cycloaddition reaction where 4π and 2π electrons react. +
  2. 2. Diels-Alder Reaction• Discovered by O. Diels and K. Alder in 1928.• Occur between a conjugated diene and substituted alkene (dienophile) to form cyclohexene ring system.• Concerted reaction (single step), can be accelerated by heating or using some catalysts.• [ 4+2 ] cycloaddition reaction.• In retro Diels-Alder reaction, the six membered ring is break down to regenerate the diene and dienophile using high temperature usually.• Stereoselective reaction ( mainly one product formed).
  3. 3. Diels-Alder Reaction• Stereospecific reaction ( reactants can keep their stereochemistry).• No transition states or charged intermediates.• All electrons moving in same time to form two new σ bonds.• 100 % economic ( No. of reactants atoms = No. of products atoms).• If one or more of diene or dienophile atoms are not carbon ,the reaction is hetero-Diels-Alder reaction.
  4. 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 Na OMe OTMS R Danishef sky diene R= Me, OMe, CH(Me)OMe
  5. 5. Cyclopentadiene• Cyclopentadiene which is well know as a standard diene can undergoes self D.A at R.T .• This dimer can be cracked by distillation. 25oC
  6. 6. The dienophiles• Wide range of dienophiles can be used including cyclic, acyclic and hetero compounds.• Perfect dienophiles are alkenes conjugated to electron withdrawing groups such as carbonyl, nitro, cyano, halogens…etc• -I groups increase the rate of D.A reaction.• D.A reaction between alkene and diene without any substituents can take place but with low yield.
  7. 7. The dienophiles O O O Br O O NMe Me O 2N O O O O O O O OF3C CF3 N N N N Ph Ph MeO OMe N Ph O O S O Ph NC CN O P OEt SiMe3 OEt NC CN
  8. 8. How to know its D.A reaction?• Since it s {4+2} cycloaddition, the product is, six membered ring, double bond inside the ring , conjugate group outside the ring opposite to double bond. O O + O O O O
  9. 9. How it works?• Concerted reaction via aromatic transition state.• There are two approches, the first depends on the interaction between HOMO of The diene and LUMO of the dienophile.• The second, depends on +I, -I groups affect, since they form negative and positive cherges on the diene and dienophile.
  10. 10. Stereochemistry• The stereochemistry of substituents can be retained in the product.• Cyclic dienes must be in s-cis are highly reactive . CO2 Me CO 2Me CO2 Me CO 2Me + CO2 Me CO 2Me MeO 2C CO 2Me
  11. 11. Endo rule• D.A between cyclic dienes and dienophiles can lead two diasteremers, endo and exo.• Endo product is kinetically controlled product.• Exo product is thermomdynamically product.
  12. 12. Regioselectivity• Diels-Alder reaction can lead to different structural isomers.• Electronic and steric effects of the substituents.• Position of these groups is the main factor.• Usually its ortho and para directiong. X X Z Z ortho Z Z para X X Z= electron-withdrawing gruop x= electron-donating group
  13. 13. Regioselectivity CO 2H HO2 C HO 2C + + OHC OHCCHO ort ho meta O O Me Me + Me Me ort ho
  14. 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. 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 used individually or combined with Lewis Acids).• Imidazolidinone derivatives, thiourea derivatives, BINOL and its derivatives…..
  16. 16. Lewis Acid catalysis• L.A binding with EWD group then make the dienophile more electron deficient.• Then, decrease LUMO energy, strong interaction with diene`s HOMO. - + SnCl O O O 4 O + + without catalyst 71 : 29 with SnCl4 93 : 7
  17. 17. Asymmetric Diels-Alder reaction• Preparation of chiral cyclohexene derivatives .• Using chiral dienes or dienophiles• Using chiral Lewis Acids. CO2 Bn CO2 Bn O N N + O
  18. 18. Chiral dienophiles O OMe N R O S O O R R OMe R N N O O O
  19. 19. Chiral dienes• Using chiral dienes is less developing as the use of chiral dienophiles. OTMS R EtO OMe N O N O O OAc O OAc OAc OAc
  20. 20. Chiral Lewis Acids+ Me CHO CHO 72% OAlCl2 Me endo:exo 98:2O O N O Me + + OH Me OH endo : exo TMSO 93 : 07 O O O O + TiCl4 OTMS
  21. 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

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