Stereospecific Ring Contraction of Bromocycloheptenes through Dyotropic Rearrangements via Nonclassical Carbocation–Anion Pairs

Abstract

Experimental and theoretical evidence is reported for a rare type I dyotropic rearrangement involving a [1,2]-alkene shift, leading to the regio- and stereospecific ring contraction of bromocycloheptenes. This reaction occurs under mild conditions, with or without a Lewis acid catalyst. DFT calculations show that the reaction proceeds through a nonclassical carbocation–anion pair, which is crucial for the low activation barrier and enantiospecificity. The chiral cyclopropylcarbinyl cation may be a transition state or an intermediate, depending on the reaction conditions.

Absolute Configuration Determination from Low ee Compounds by the Crystalline Sponge Method. Unusual Conglomerate Formation in a Pre‐Determined Crystalline Lattice

“…In the large asymmetric unit of the 1 ⋅ 2 structure, eight independent guest molecules were observed at sites A to H (Figure 2). [25] Seven of them at sites A to G showed R , R configuration. Interestingly, S , S configuration was observed for the guest trapped at the site H .…”

“…The CS method was applied to chiral homoallylic bromide 2 , which was synthesized in 88 % ee in the asymmetric synthesis study by the Feringa group [25] . After guest‐soaking experiment, the space group ( C 2/ c ) turned into non‐centrosymmetric C 2 and the crystal structure of guest‐absorbed 1 ⋅ 2 was solved in this space group.…”

Absolute Configuration Determination from Low ee Compounds by the Crystalline Sponge Method. Unusual Conglomerate Formation in a Pre‐Determined Crystalline Lattice

“…In the large asymmetric unit of the 1•2 structure, eight independent guest molecules were observed at sites A to H (Figure 2). [25] Seven of them at sites A to G showed R,R configuration. Interestingly, S,S configuration was observed for the guest trapped at the site H. All the guests show normal pyramidalization angles at the sp 3 stereogenic centers, indicating that the disorder of S,S and R,R enantiomers at every site is negligible.…”

“…The CS method was applied to chiral homoallylic bromide 2, which was synthesized in 88 % ee in the asymmetric synthesis study by the Feringa group. [25] After guest-soaking experiment, the space group (C2/c) turned into non-centrosymmetric C2 and the crystal structure of guest-absorbed 1•2 was solved in this space group. In the large asymmetric unit of the 1•2 structure, eight independent guest molecules were observed at sites A to H (Figure 2).…”

Desymmetrization of meso-Dibromocycloalkenes through Copper(I)-Catalyzed Asymmetric Allylic Substitution with Organolithium Reagents

“…A detailed structural analysis and mechanistic and theoretical study to elucidate this remarkable ring contraction are reported separately. 14 …”