D. A. Evans Sigmatropic Rearrangements-1 Chem Sigmatropic rearrangements are those reactions in which a sigma bond[1,3] Sigmatropic Rearrangements(C migration) ( associated substituent) interchanges termini on a conjugated pi system consider the 13-migration of Carbon CH3 [1,3] Sigmatropic rearrangement Consider the orbitals needed to contruct [23]Sg o Construct TS by uniting an allyl and Me radicals Retention at carbon Inversion at carbon [13, 3]Sigmatropic rearrangement bonding bondin bonding [1, 5] Sigmatropic rearrangement Suprafacial on ally l fragment a[1, 3] Sigmatropic Rearrangements(H migration) Sychronous bonding to both termini Sychronous bonding to both termini cannot be achieved from this geometry is possible from this geometry consider the 1, 3-migration of H eochemical constraints on the suprafacial migration of carbon Consider the orbitals needed to contruct with inversion of configuration is highly disfavored on the basis of strain the transition state Ts) [1, 3-Sigmatropic rearrangements are not common a Construct Ts by uniting an ally l and H radical A no observed scrambling of labels X-H HY Y2(ally! HOMO 120°C Suprafacial Geometry Antarafacial Geometry Bridging distance too great for antarafacial migration. These rearrangements are only seen in systems that are highly strained, an attnbute that lowers the activation for rearrangementX H H C H X R C R H X C Me H 1 3 R X Y:– X R X X H D D D D Y X H H X H Y H X Y X Y D R Y –:X H X X X R X H Y Me H H X H Y D Y CH3 X D D Y X H H Y H3 C X Me H Y CH3 X D D. A. Evans Sigmatropic Rearrangements-1 Chem 206 Bridging distance too great for antarafacial migration. Suprafacial Geometry Antarafacial Geometry bonding Y2 (allyl HOMO) antibonding bonding bonding ■ Construct TS by uniting an allyl and H radical: Consider the orbitals needed to contruct the transition state (TS). ‡ consider the 1,3-migration of H ■ [1,3] Sigmatropic Rearrangements (H migration) [3,3] Sigmatropic rearrangement [2,3] Sigmatropic rearrangement [1,3] Sigmatropic rearrangement [1,5] Sigmatropic rearrangement Sigmatropic rearrangements are those reactions in which a sigma bond (& associated substituent) interchanges termini on a conjugated pi system ■ Examples: Sychronous bonding to both termini is possible from this geometry ❐ The stereochemical constraints on the suprafacial migration of carbon with inversion of configuration is highly disfavored on the basis of strain. bonding bonding Inversion at carbon Suprafacial on allyl fragment Retention at carbon Sychronous bonding to both termini cannot be achieved from this geometry bonding ■ [1,3] Sigmatropic Rearrangements (C migration) consider the 1,3-migration of Carbon ‡ Consider the orbitals needed to contruct the transition state (TS). ❐ Construct TS by uniting an allyl and Me radicals: antibonding Suprafacial on allyl fragment ‡ 1 3 These rearrangements are only seen in systems that are highly strained, an attribute that lowers the activation for rearrangement. 120 °C 3 1 no observed scrambling of labels ✻ ✻ [1,3]-Sigmatropic rearrangements are not common