D. A. Evans. B. Breit. T.B. Dunn [1, 3-Sigmatropic Rearrangements: FMO Analysis Chem 206 The stereochemical issues The migrating group can migrate across the conjugated pi system in a[1, 3]Sigmatropic Rearrangements(H migration one of two ways. If the group migrates on the same side of the system, Construct TS by considering an ally anion and the proton (or radical it is said to migrate suprafacially with respect to that system. If the 丰 group migrates from one side of the pi system to the other, it is said to migrate antarafacially with respect to that system Suprafacial migration: The group moves across the same face A..B 8 xr Antarafacial migration: The group moves from one face to the other Proton 1S(LUMO) ￥Y ￥Y p2(allyl anion HOMO bondi Sigmatropic Rearrangements: FMO Analysis Suprafacial Geometry Antarafacial Geometry IImagine the two pieces fragmenting into a cation/anion pair, (or a a The analysis works if you consider the other ionic reaction, or pair of radicals)and examine the HOMO/LUMO interaction consider a radical reaction. In each case it is the same pair of orbitals a If the overlap is constructive at both termini then the reaction is interacting allowed. If the overlap is destructive at either terminus then the reaction is forbidden a The suprafacial migration is forbidden and the bridging distance too great for the antarafacial migration. Hence, [1, 3] hydrogen migrations a If the migrating atom is carbon then we can also entertain the are not obseryed under thermal conditions possiblity of the alkyl group migrating with inversion of configuration (antarafacial on the single atom a Under photochemical conditions, the[1, 3] rearrangement is allowed suprafacially. How would you predict this using FMO? L If the migrating atom is hydrogen, then it cannot migrate wit nversionD. A. Evans, B. Breit, T. B. Dunn [1,3]-Sigmatropic Rearrangements: FMO Analysis Chem 206 The Stereochemical issues: The migrating group can migrate across the conjugated pi system in one of two ways. If the group migrates on the same side of the system, it is said to migrate suprafacially with respect to that system. If the group migrates from one side of the pi system to the other, it is said to migrate antarafacially with respect to that system. A B A B A B A B A B A B Suprafacial migration: The group moves across the same face. A B A B A B A B A B A B Antarafacial migration: The group moves from one face to the other. ■ If the overlap is constructive at both termini then the reaction is allowed. If the overlap is destructive at either terminus then the reaction is forbidden. ■ Imagine the two pieces fragmenting into a cation/anion pair, (or a pair of radicals) and examine the HOMO/LUMO interaction. ■ If the migrating atom is carbon, then we can also entertain the possiblity of the alkyl group migrating with inversion of configuration (antarafacial on the single atom). ■ The suprafacial migration is forbidden and the bridging distance too great for the antarafacial migration. Hence, [1,3] hydrogen migrations are not observed under thermal conditions. Suprafacial Geometry Antarafacial Geometry bonding Y2 (allyl anion HOMO) bonding antibonding bonding ■ Construct TS by considering an allyl anion and the proton (or radical pair): ■ [1,3] Sigmatropic Rearrangements (H migration) Y X Y X ■ Under photochemical conditions, the [1,3] rearrangement is allowed suprafacially. How would you predict this using FMO? H ■ The analysis works if you consider the other ionic reaction, or consider a radical reaction. In each case it is the same pair of orbitals interacting. Proton 1S (LUMO) X X H H Y Y X Y •• ■ If the migrating atom is hydrogen, then it cannot migrate with inversion. H X X H H Y Y X Y ■ Sigmatropic Rearrangements: FMO Analysis H H