D. A. Evans [2, 31-Sigmatropic Rearrangements: Olefin Geometry Chem 206 1, 2-Disubstitution: Good Trans Olefin Selectivity Starting olefin: Trans 75to-50° (E)selectivity: only isomer" Rb CO2H Nakai. Tet. Lett 1981. 22. 69 disfavored R R2(MeO)3P RiNe Ra& rb prefer to orient in pseudo-equatorial positions during rearrangement; RUI Rl-X nevertheless, this is a delicately balanced situation Evans. Accts. Chem. Res. 1974.7. 147-55 Starting olefin: Cis The preceeding transition state models do not explain some of the results u Cis selectivity has been observed: Still JACS 1978, 100, 192 L Rb rato,65:35 J However, Cis selectivity is dependent on starting olefin geometry Conclusions OH only(E)isomer(91%) J Olefin geometry dictates sense of asymmetric induction in rearrangement J( z) Olefin rearrangements might exhibit higher levels of 1, 3 induction J Product olefin geometry can be either(E)or(2)from(E)starting material Several theoretical studies have been published: Good reading J Product olefin geometry will be(E) from 2) starting materia Houk JOC 1991, 56, 5657 (Sulfur ylide transition states) Houk JOC 1990, 55, 1421(Wittig transition states)Me O Ph RLi Y :X Ra Rb X Y: Ra Rb X Y Rb H H Ra H X Y H Rb Ra Rb Ra :X Y Ra Y X H H Rb Y :X Ra Rb H Ra Rb X Y H X Ra Rb Y: Ra Rb :X Y CO2H O Me R1 S Ar O R2 R2 S Ar O R Me O SnBu3 R Bu3Sn O Me RLi R1–X n-BuLi n-BuLi O S R1 Ar R2 HO Me Ph CO2H Me HO R Me OH Me OH R MeOH (MeO)3P R1 R2 OH R OH Me ❏ Product olefin geometry will be (E) from (Z) starting material Houk JOC 1990, 55, 1421 (Wittig transition states) Houk JOC 1991, 56, 5657 (Sulfur ylide transition states) Several theoretical studies have been published: Good reading only (E) isomer (91%) -78 °C ❏ However, Cis selectivity is dependent on starting olefin geometry -78 °C ratio, 65:35 The preceeding transition state models do not explain some of the results: Evans, Accts. Chem. Res. 1974, 7, 147-55 (E) selectivity: >95% Nakai, Tet. Lett 1981, 22, 69 (E) selectivity: 75% 2 LDA -75 to -50 °C -75 to -50 °C (E) selectivity: "only isomer" ❏ Product olefin geometry can be either (E) or (Z) from (E) starting material ❏ (Z) Olefin rearrangements might exhibit higher levels of 1,3 induction ❏ Olefin geometry dictates sense of asymmetric induction in rearrangement Conclusions ‡ favored ‡ highly disfavored ❏ Cis selectivity has been observed: Still JACS 1978, 100, 1927. Starting olefin: Trans Ra & Rb prefer to orient in pseudo-equatorial positions during rearrangement; nevertheless, this is a delicately balanced situation ‡ ‡ D. A. Evans [2,3]-Sigmatropic Rearrangements: Olefin Geometry Chem 206 ■ 1,2-Disubstitution: Good Trans Olefin Selectivity Starting olefin: Cis favored disfavored