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DIELS- ALDER REACTION

DIELS- ALDER REACTION. The Diels-Alder Reaction. concerted. ring-forming reaction. three pairs of electrons move at one time. diene + dienophile cyclohexene. = gain of bond order. NOTE: . = loss of bond order. THE DIENE MUST BE ABLE TO ADOPT

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DIELS- ALDER REACTION

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  1. DIELS- ALDER REACTION

  2. The Diels-Alder Reaction concerted ring-forming reaction three pairs of electrons move at one time diene + dienophile cyclohexene = gain of bond order NOTE: = loss of bond order

  3. THE DIENE MUST BE ABLE TO ADOPT THE S-CIS CONFORMATION rotate + reacts s-trans conformation s-cis conformation does not react reacts normally

  4. push R pull W DIELS-ALDER REACTION HOMO (diene) The reaction is concerted - all of the orbitals are aligned in a 6-ring. 2 The HOMO of the diene donates elec- trons into the LUMO of the dienophile. 1 LUMO (dienophile)

  5. BUTADIENE ETHYLENE p4 The HOMO of the diene donates electrons into the LUMO of the dienophile. p3 LUMO HOMO p2 p* p1 p

  6. EXAMPLE - WITH ELECTRONIC FACTORS pull push D Diene Dienophile A Cyclohexene Works best if the dienophile has electron-withdrawing groups, and the diene has electron-donating groups. The HOMO of the diene donates PUSHES electrons into the LUMO of the dieno- phile PULLS.

  7. FORMATION OF A BICYCLIC COMPOUND D

  8. THE REACTION IS USUALLY STEREOSELECTIVE endo exo The endo product is usually preferred. It has been suggested that the pi systems like to establish maximum overlap during the reaction.

  9. MORE DIELS-ALDER REACTIONS D D

  10. OR Retrosynthesis

  11. ACETYLENES (ALKYNES) If the dienophile is an alkyne a cyclohexadiene is formed.

  12. CARBENES - CYCLOPROPANES

  13. a-ELIMINATIONS REVIEW ( also covered in Section 9.13 ) Alkyl halides which have no b-hydrogens will undergo a-elimination to give carbenes. b-hydrogen no b-hydrogens in these groups a a b a-hydrogen E1 or E2 carbene - alkene

  14. MECHANISM Some halides without a-hydrogen atoms Step 1 fast - Step 2 - .. slow + .. - carbene is a strong base such as NaOH, NaOEt, KOH, NaNH2, or nBuLi

  15. : CARBENES ARE ELECTRON-DEFICIENT Carbenes are neutral, no charge, but have an incomplete octet. They react with alkenes (electron donors) to form cyclopropanes. carbene R R syn addition cis cis R R The reaction is thought to be concerted. a cyclopropane STEPWISE ANALYSIS OF THE CONCERTED REACTION : .. - +

  16. SOME REACTIONS OF DICHLOROCARBENE + + + : + : +

  17. SYNTHESIS OF NORCARANE phenyl sodium - + : + + - strong base : : “ylide” - - a -elimination tetramethylammonium chloride + + : + carbene norcarane (bicyclo[ 4.1.0 ] heptane

  18. two methods various solvents are used explosive if not handled carefully DIAZOMETHANE (gas) UNSTABLE easily decomposed by light or heat - .. .. - + + : : light heat photolysis: :pyrolysis = hnD : : : nitrogen is a very thermodynamically stable molecule, and forms readily nitrogen methylene ( the specific name for this carbene )

  19. DIAZOMETHANE REACTIONS pyrolysis D + trans trans D + cis cis photolysis hn

  20. CARBENOID REACTIONS

  21. SIMMONS-SMITH REACTION “a carbenoid reaction” Zn-Cu + Zn-Cu is a “zinc-copper couple” which is more reactive than either would be alone. Copper activates the zinc. R R + cis R R cis concerted stereospecific carbenoid reactions give a result as if a carbene is involved, but it is not.

  22. WHY USE THE SIMMONS-SMITH ? 1) not hazardous like diazomethane 2) milder reaction conditions 3) doesn’t require strong base like a-elimination 4) fewer side products 5) higher yields ….. what else could you want ?

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