D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions: FMO Analysis Chem 206 FMO Treatment of Electrocyclic reactions. Photochemical Activation. When light is used to initiate an electrocyclic reaction, an electron is I Examine the interactions that occur in the HOMO as the reaction excited from Y2 to Y3. Treating y3 as the HOMo now shows that I If the overlap is constructive (i.e. of the same phase)then the disrotatory closure is allowed and conrotatory closure is forbidden reaction is"allowed I If the overlap is destructive(i.e of different phases) then the 4 reaction is"forbidden Thermal Activation H HAMe Y2f Conrotatory Closure: (Allowed and observed) y2( HOMO 中3 Constructive ovena Disrotatory Closure: (Allowed and observed Me Me 00 00 e p2(diene HOMO) I3(new HOMO Constructive Disrotatory Closure: (Forbidden and not observed) Conrotatory Closure: (Forbidden and not observed Me y2(diene HOMO) Destructive HOMe B2Re-25 Y3(new HOMO overlap A similar analysis for the hexatriene system proves that under We have so far proven which ring closures are allowed and which are thermal conditions disrotation is allowed and conrotation is forbidden. Do we now have to go back and examine all the ring forbidden NO! The principle of microscopic reversibility says that if the reaction is allowed in one direction it must be allowed in the other directionY4 Y3 Me H H Me hn Me H H Me H H Me Me H Me Me H Me H H Me Me H H Me Me H H Me Y1 Y2 Me H H Me Me H H Me Me H H Me Me H H Me Y2 (HOMO) Me H H Me Y3 (HOMO) H H Me Me H Me Me H Y1 Y4 Y3 Y2 D. A. Evans, B. Breit, T. B. Dunn Chem 206 ■ FMO Treatment of Electrocyclic reactions. ■ Examine the interactions that occur in the HOMO as the reaction proceeds. ■ If the overlap is constructive (i.e. of the same phase) then the reaction is "allowed." ■ If the overlap is destructive (i.e. of different phases) then the reaction is "forbidden." Destructive overlap Constructive overlap Y2 (diene HOMO) Y2 (diene HOMO) Conrotatory Closure: (Allowed and observed) Disrotatory Closure: (Forbidden and not observed) Thermal Activation: A similar analysis for the hexatriene system proves that under thermal conditions, disrotation is allowed and conrotation is forbidden. Photochemical Activation: When light is used to initiate an electrocyclic reaction, an electron is excited from Y2 to Y3. Treating Y3 as the HOMO now shows that disrotatory closure is allowed and conrotatory closure is forbidden. Photon absorption Destructive overlap Constructive overlap Y3 (new HOMO) Conrotatory Closure: (Forbidden and not observed) Disrotatory Closure: (Allowed and observed) Y3 (new HOMO) We have so far proven which ring closures are allowed and which are forbidden. Do we now have to go back and examine all the ring openings? NO! The principle of microscopic reversiblity says that if the reaction is allowed in one direction, it must be allowed in the other direction. Electrocyclic Reactions: FMO Analysis