D. A Evans. B. Breit.T B. Dunn [1, 3-Sigmatropic Rearrangements Chem 206 [1, 3] Sigmatropic Rearrangements(C migration) Sigmatropic Rearrangements: Dewar-Zimmerman Dewar-Zimmerman also predicts the [1, 3] suprafacial migration to be H3 forbidden The basis set of s and p orbitals with arbitrary phase Two Phase Inversions a Construct TS by considering an allyl anion and the methyl cation Huckel Topology Four Electrons Forbidden therma Retention at carbon Inversion at carbon 2p on Carbon H Orbital interactions in the Completing the circuit antibonding bondi ing parent syster Y2(allyl anion HOMO) Suprafacial on allyl fragment Suprafacial on allyl fragment The [1.5 shit of a hydrogen atom across a diene u 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 I Under photochemical conditions, the [1, 3] rearrangement is allowed Zero phase Inversions suprafacially with retention of stereochemistry Huckel Topology Six Electrons a The stereochemical constraints on the migration of carbon Allowed thermally with inversion of configuration is highly disfavored on the basis of strain uch rearrangements are rare and usually only occur in highly strained Orbital interactions in the Completing the circuit systems parent system across the bottom face Using a similar analysis, one can prove that [1, 5] hydrogen and alk shifts should be allowed when suprafacial on the pi component and proceeding with retention. Please refer to Fleming for more applications of FMo theory to [1, n] sigmatropic shiftsD. A. Evans, B. Breit, T. B. Dunn [1,3]-Sigmatropic Rearrangements Chem 206 Y2 (allyl anion HOMO) ■ Construct TS by considering an allyl anion and the methyl cation: ■ Under photochemical conditions, the [1,3] rearrangement is allowed suprafacially with retention of stereochemistry. ■ The stereochemical constraints on the migration of carbon with inversion of configuration is highly disfavored on the basis of strain. Such rearrangements are rare and usually only occur in highly strained systems. bonding bonding Inversion at carbon Suprafacial on allyl fragment Retention at carbon bonding antibonding Suprafacial on allyl fragment C H H X H C H H Y X H Y H H H H 2p on Carbon ■ 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. Using a similar analysis, one can prove that [1,5] hydrogen and alkyl shifts should be allowed when suprafacial on the pi component and proceeding with retention. Please refer to Fleming for more applications of FMO theory to [1,n] sigmatropic shifts. ■ Sigmatropic Rearrangements: Dewar-Zimmerman The [1,5] shift of a hydrogen atom across a diene. H H Orbital interactions in the parent system The basis set of s and p orbitals with arbitrary phase: Completing the circuit across the bottom face Zero Phase Inversions Hückel Topology Six Electrons Allowed thermally Dewar-Zimmerman also predicts the [1,3] suprafacial migration to be forbidden. Orbital interactions in the parent system Completing the circuit across the bottom face Two Phase Inversions Hückel Topology Four Electrons Forbidden thermally CH3 X X CH3 CH3 Y Y X Y •• ■ [1,3] Sigmatropic Rearrangements (C migration) H H