Pericyclic Reactions (McM chapt 30)

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1 Pericyclic eactions (McM chapt 0) Polar react. (nucleophiles and electrophiles) u E adical react. ' Pericyclic react. (concerted, cyclic TS # ) Electrocyclic react. ycloadditions (i.e. Diels Alder) Sigmatropic rearrangement Pericyclic react. (concerted, cyclic TS # ) Electrocyclic react. ycloadditions (i.e. Diels Alder) Sigmatropic rearrangement earrangement of polyene Termal (react. in ground state) or photochemical (react of exited state) ring strain

2 Electrocyclic react. - Stereospesific react. Termal cond.: E,Z is E,E Trans pposite stereochem. under photochem. cond. Pericyclic react. are symmetry allowed react. Woodward offmann rules Symmetry in reactants are preserved during pericyclic react. Maximum bonding interactions by transferring electrons between molecular orbitals of the same symmetry in reactant and products. The lobes of the reactant Ms must be of the correct algebraic sign for bonding to occur in the TS esults can generally be predicted just by looking at Front Molecular rbitals (FM; M and LUM) - Fukui

3 Symmetry allowed react. Woodward offmann rules Symmetry in reactants are preserved during pericyclic react. esults can generally be predicted just by looking at Front Molecular rbitals (FM; M and LUM) - Fukui Butadiene (ground state) LUM: Lowest uoccupied M M: ighest occupied M Molecular orbitals,-butadiene M LUM "-bond M!-bond onrotatory Both rotate same way Stereospesific react. E,Z is S: Symmetric A: Antisym. E,E Trans

4 Molecular orbitals hexatriene M LUM "-bond LUM "-bond otation opposite way M!-bond E E is E Z Trans Electrocyclic react. -Symmetry allowed o. of electrons 4n 4n + eactions in the ground state onrotatory Photochemical electrocyclic react. M exitet state (= LUM ground state)

5 Photochemical electrocyclic react. - opposite stereochemistry M exitet state (= LUM ground state) E,Z trans E,E cis o. of electrons 4n 4n + eactions in the ground state (termal) onrotatory eactions in exited state (Photochem.) onrotatory conrot. ring-opening 7-dehydrocolestol cyclohexadiene previtamin D vitamin D

6 Applications in synthesis - Synthesis of occidentalol J. rg. hem. 97, 8, 78 Me termal cond. 6", con Me 6",dis + Me Me (+)-carvone MeLi MeLi. Base (epimerization) -MeLi + 7-epi-(-)-occidentalol (+) occidentalol (-)-occidentalol ycloadditions (i.e. Diels Alder) [4+] add. termal cond. cycloadd. [4+] M - Diene LUM - diene LUM - dienophile M - dienophile [4+] A A B B D D Stereospecific react. Both interact. may lead to product In reality, the electron rich species reacts via its M and the electron poor species via its LUM.

7 and antarafacial cycloadd. Antarafacial cycloadd cycloadd. Photochemical M - Diene M - Diene LUM - dienophile rotation of "-systh Difficult! LUM - dienophile DA-prod. DA-prod. ormal electron demand DA - Electron poor dienophile (Michael accept.) LUM -alkene LUM -alkene EWG M - LUM gap EWG M-alkene M-alkene Michael accept. Lower LUM Ethene etc, very low react.

8 endo - exo selectivity (ot in McM) Endo Exo M - Diene Primary interact., bond formation M - Diene Secondary interact., extra TS # stabil. LUM - dienophile egioselectivity in Diels Alder (ot McM) DA is ortho - para directing + Me Me + Me Me + Me Me + Me Me

9 [+] ycloadditions cycloadd. Antarafacial cycloadd. (termal cond.) cycloadd. (photochem. cond.) M - Diene M-alkene M-alkene exited state LUM -alkene LUM -alkene LUM -alkene Geometrical constrains Difficult to make small rings by antrafacial cycloadd o. of electrons 4n 4n + eactions in the ground state (termal) Antarafacial eactions in exited state (Photochem.) Antarafacial Photochemical + isomers [+] ancer Psoralenes - Psoriasis ' '

10 arbene ycloadditions Singlet arbene Empty p-orb LUM of carbene sp hybr orbital with lone pair M of carbene Empty p-orb LUM of carbene M of carbene M alkene LUM alkene ycloadditions with,-dipolar reagents [4+] cycloadd. ot in McM zone ther ex of,-dipoles Synthesis of heterocycles azides diazoalkanes

11 Sigmatropic earrangements!-bonded subst migrates accross a "-electron systhem [,] earrangement (-shift)!-bond to be broken 5!-bond to formed LUM #-bond h$ M!"bond Antarafacial Too strained TS # [,5] earrangement (-shift)!-bond to be broken LUM #-systhem butadiene M!"bond o. of electrons 4n 4n + eactions in the ground state (termal) Antarafacial eactions in exited state (Photochem.) Antarafacial

12 [,7] sigmatropic rearrangement conrot. ring-opening 7-dehydrocolestol cyclohexadiene previtamin D vitamin D [,] earrangements; laisen rearrang. etc. laisen rearrangement Allyl-vinyl ether or Allyl aryl ether!-bond to be broken!-bond to be formed taut ope rearrangement xy-ope rearrangement taut 6 electrons (4n+)