D A. Evans, J. Johnson Rules for Ring Closure: SP3 Carbon Related Systems Chem 206 Tetrahedral Carbon FURST-PLATTNER RULE In this simple model, the transition-state leading to 1 involves the All exo cyclization modes are allowed: n-exo-tet, n=3-) diaxial orientation of nucleophile and leaving group. This orientation affords the best overlap of the anti-bonding C-Y orbital and the nonbonding electron pairs on the nucleophile O In the formation of the diastereomeric epoxide 2, the proper alignment of orbitals may only be achieved by cyclization through the There are stereoelectronic issues to consider for n-exo-tet cyclizations "allowed", there are large rate differences the the rates of ring closure Formation of 3-Membered Rings (3-exo-tet) While the FURST-PLATTNER RULE deals wilth the microscopic H reverse, in the opening of epoxides by nucleophiles, the XCH2+Y stereoelectronic arguments are the same H2 Stereoelectronic Effects in Epoxide Ring Cleavage Conformational Effects in Epoxide Ring Formation/cleavage t e3C fuence ring formation. at operate in ring cleavage also formation. Consider a rigid cyclohexene oxide system 丰 √ faster 8-d slower < chair The diaxial nucleophilic ring cleavage of epoxides For more information on epoxide cleavage see handout 03AD. A. Evans, J. Johnson Rules for Ring Closure: SP3 Carbon & Related Systems Chem 206 Tetrahedral Carbon All exo cyclization modes are allowed: (n-exo-tet, n = 3 ) Y C C Y – exo X X There are stereoelectronic issues to consider for n-exo-tet cyclizations Formation of 3-Membered Rings (3-exo-tet) X C H2 X CH2 C H2 C Y C H2 C H H X Y + Y– ‡ Conformational Effects in Epoxide Ring Formation/cleavage O H H Y O – Y O – H H H H Those stereoelectronic effects that operate in ring cleavage also influence ring formation. Consider a rigid cyclohexene oxide system: O H H O H Y d – d – ‡ O Y ‡ d – d – faster slower In this simple model, the transition-state leading to 1 involves the diaxial orientation of nucleophile and leaving group. This orientation affords the best overlap of the anti-bonding C–Y orbital and the nonbonding electron pairs on the nucleophile O– . In the formation of the diastereomeric epoxide 2, the proper alignment of orbitals may only be achieved by cyclization through the less-favored boat conformer. Accordingly, while both cyclizations are "allowed", there are large rate differences the the rates of ring closure. While the FÜRST-PLATTNER RULE deals wilth the microscopic reverse, in the opening of epoxides by nucleophiles, the stereoelectronic arguments are the same. 1 chair boat 2 Stereoelectronic Effects in Epoxide Ring Cleavage Me3C H O Nu- Me3C H HO Nu Me3C H O Nu- Me3C H Nu HO "The diaxial nucleophilic ring cleavage of epoxides" For more information on epoxide cleavage see Handout 03A. H H H H O Me H H Me HO Nu Nu￾FÜRST-PLATTNER RULE •• •• H H