D. A. Evans Diels-Alder Reaction-Orbital Symmetry Considerations Chem 206 The Alder Endo rule The following observation illustrates an example of the Orbital Symmetry Considerations for Diels Alder Reaction Alder rule which will be defined below If the reaction is referred to as"symmetry-allowed". The Diels-Alder is such a case As illustrated. the homo and lumo of both the diene and dien which in this case are the same, will constructively overlap as indicated in formation of both sigma bonds disfavored HoM-π LUMO-π Endo product Observation: The endo Diels-Alder adduct is formed faster even though the exo product is more stable. There is thus some special stabilization in the transition state leading to the endo product which is lacking the exo transition state HOMo-π a Primary orbital overlap leads directly to the formation of new chemical bonds Frontier MO Explanation for the Endo Rule LUMO-Tt3 a Secondary(transient )orbital overlap can also occcur in the stabilization of certain transition state geometries. Such a transient stabilizing interaction can occur in the endo, but not exo, transition state a Of the two possible transition states, the one having the "greatest accumulation of interacting double bonds will be preferred"(the Alder Endo Rule econdary orbital overlap is noted below The Other Dimerization Possibility for Butadiene orbital over ○■ Note that the termini only match at one end for the HOMO-T HOMO-L retry This does not mean that the rea only that the reaction will not be Such reactions are called Exo Ts t Endo ts t Additional Reading: Lowry RichardsonAdditional Reading: Lowry & Richardson, Chapter 10, theory of Pericyclic Rxns pp 839-900 ■ Note that the termini only match at one end for the HOMO-LUMO pairing. Hence we say that the symmetry requirements for the reaction in question are not met. This does not mean that the reaction will not occur, only that the reaction will not be concerted. Such reactions are called "symmetry-forbidden". LUMO-p3 HOMO-p2 Does the possibility for the following concerted dimerization exist? The Other Dimerization Possibility for Butadiene ■ Secondary (transient) orbital overlap can also occcur in the stabilization of certain transition state geometries. Such a transient stabilizing interaction can occur in the endo, but not exo, transition state: ■ Primary orbital overlap leads directly to the formation of new chemical bonds. LUMO-p3 HOMO-p2 Frontier MO Explanation for the Endo Rule If the symmetries of the frontier MO's of reacting partners are "properly matched" the reaction is referred to as "symmetry-allowed". The Diels-Alder reaction is such a case. As illustrated, the HOMO and LUMO of both the diene and dienophile, which in this case are the same, will constructively overlap as indicated in formation of both sigma bonds. Orbital Symmetry Considerations for Diels Alder Reaction LUMO-p3 LUMO-p3 HOMO-p2 HOMO-p2 Energy Secondary orbital overlap disfavored favored The Alder Endo Rule The following observation illustrates an example of the Alder Rule which will be defined below. + "Endo product" "Exo product" Observation: The endo Diels-Alder adduct is formed faster even though the exo product is more stable. There is thus some special stabilization in the transition state leading to the endo product which is lacking the exo transition state. 2 Exo TS ‡ Endo TS ‡ ■ Of the two possible transition states, the one having the "greatest accumulation of interacting double bonds will be preferred" (the Alder Endo Rule). Secondary orbital overlap is noted below. Exo TS ‡ Endo TS ‡ D. A. Evans Diels-Alder Reaction-Orbital Symmetry Considerations Chem 206 H H H H H H H H C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C D