D. A. Evans. B. Breit. B. Dunn Pericyclic Reactions: Major classes Chem 206 (2)CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS:(4)SIGMATROPIC REARRANGEMENTS A cycloaddition reaction is the A sigmatropic rearrangement is the migration of a sigma bond from one position igma bonds are created at the expense of pi bonds. a cycloaddition can occur in a conjugated system to another position in the system, accompanied cycloaddition reactions are referred to as [m+n] additions when a system of m remains constant. The rearrangement is an(m, n] shift when the sigma tmn in an intramolecular sense, but it must be between two independent pi systems. reorganization of the connecting pi bonds. The number of pi and sigma bond conjugated atoms combines with a system of n conjugated atoms. A migrates across m atoms of one system and n atoms of the second system cycloreversion is simply the reverse of a cycloaddition Examples Examples [1, 3]-shift [1, 5]-shift /(3, 3-shift X=CR2, Cope rearrangement The Diels-Alder reaction (5)GROUP TRANSFER REACTIONS (3)CHELETROPIC REACTIONS In a group transfer reaction one or more groups get transferred to a second reaction partner Cheletropic reactions are a special group of cycloaddition/cycloreversion reactions. Two bonds are formed or broken at a single atom. The nomenclature for cheletropic reactions is the same as for cycloadditions Examples Hydrogen Transfer 4+ N2 =0 Ene reaction: R[4+1] [4+1] [2+1] R R R 1 R 2 O O S O O O CR2 R R C O S O O X R X R R H H R N N H H R R' N2 H H D H H H R H R' R 1 H R 2 D. A. Evans, B. Breit, T. B. Dunn Pericyclic Reactions: Major Classes Chem 206 (2) CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS: + [2+2] + [4+2] A cycloaddition reaction is the union of two smaller, independent pi systems. Sigma bonds are created at the expense of pi bonds. A cycloaddition can occur in an intramolecular sense, but it must be between two independent pi systems. Cycloaddition reactions are referred to as [m + n] additions when a system of m conjugated atoms combines with a system of n conjugated atoms. A cycloreversion is simply the reverse of a cycloaddition. Examples: A 2+2 cycloaddition. The Paterno-Büchi reaction. A 4+2 cycloaddition. The Diels-Alder reaction. (3) CHELETROPIC REACTIONS: Cheletropic reactions are a special group of cycloaddition/cycloreversion reactions. Two bonds are formed or broken at a single atom. The nomenclature for cheletropic reactions is the same as for cycloadditions. + + + Examples: (4) SIGMATROPIC REARRANGEMENTS: 1 2 3 1 2 3 [1,3]-shift 1 [1,5]-shift 2 3 4 5 A sigmatropic rearrangement is the migration of a sigma bond from one position in a conjugated system to another position in the system, accompanied by reorganization of the connecting pi bonds. The number of pi and sigma bonds remains constant. The rearrangement is an [m,n] shift when the sigma bond migrates across m atoms of one system and n atoms of the second system. Examples: [3,3]-shift 3 2 1 3' 1' 2' 3 2 1 3' 1' 2' X=CR2, Cope rearrangement X=O, Claisen rearrangement (5) GROUP TRANSFER REACTIONS: In a group transfer reaction one or more groups get transferred to a second reaction partner. Hydrogen Transfer: Ene Reaction: + + + + + hn Examples: