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l4 - O O A A LU M O 2 new bonds formed at once 4 the diene...

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O + O 2 new bonds f d formed at once! 4 π – the diene 2 π – the dieneophile A A LUMO S LUMO A HOMO LUMO A HOMO HOMO S S S S LUMO HOMO or Arrows can go either way The diene must be in the s-cis configuration. s-trans s-cis 0 kJ 24 kJ 15 kJ +24 kJ +15 kJ O cyclohexa-1,3-diene furan cyclopenta-1,3-diene Some cyclic dienes work very well because they are “locked” into the s-cis conformation. s-trans s-cis H C CH 3 Atoms would H 3 C H 3 C CH 3 collide! But with big subsituents sometimes you can not make s-cis . No Diels Alder possible. s cis s-trans s-cis + 200 o C closed vessel Th i l Di l Ald i b The simplest Diels Alder reaction between butadiene and ethane is not very favorable. It has a very large energy of activation. How can we make the Diels Alder reaction more favorable? We can add substituents that enhance the flow of electrons of electrons.
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A B S LUMO A LUMO Base LUMO A A HOMO S HOMO S HOMO Acid How do you make something a better base? Raise this level Add more electrons. Add El t D ti G (EDG) Add an Electron Donating Group. A B Lower this level S LUMO A Base LUMO LUMO A A HOMO S HOMO S HOMO Acid How do you make something a better acid? Remove electrons. Add El t Withd i G (EWG) Add an Electron Withdrawing Group. The simplest Diels Alder reaction between butadiene and ethane is not very favorable. It has a very large energy of activation It has a very large energy of activation. How can we make the Diels Alder reaction more favorable? W dd El D G h We can add Electron Donating Groups to the Diene and make it a better base. And we can add Electron Withdrawing Groups to the Dienophile and make it a better acid. We can add Electron Donating Groups to the Diene and make it a better base Diene and make it a better base.
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