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

• Published in: Journal of the American Chemical Society Vol. 140; no. 15; pp. 4986 - 4990
• Main Authors: Goh, Shermin S, Champagne, Pier Alexandre, Guduguntla, Sureshbabu, Kikuchi, Takashi, Fujita, Makoto, Houk, K. N, Feringa, Ben L
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 18.04.2018; American Chemical Society (ACS); Amer Chemical Soc
• Subjects: ALLYLCARBINYL DERIVATIVES; ALLYLIC SUBSTITUTION-REACTIONS; BETA-LACTONES; BUTYROLACTONES; catalysts; cations; Chemical Sciences; Chemistry; Chemistry, Multidisciplinary; Communication; CONJUGATE ADDITION; CYCLOPROPYLCARBINYL; ENANTIOSELECTIVE SYNTHESIS; General Chemistry; GRIGNARD-REAGENTS; INTERCONVERSION REACTIONS; Lewis acids; Physical Sciences; Science & Technology; SYMMETRY CONTROLLED REACTIONS; CYCLOBUTYL; SYMMETRY CONTROLLED REACTIONS; ENANTIOSELECTIVE SYNTHESIS; BETA-LACTONES; ALLYLIC ALKYLATION; INTERCONVERSION REACTIONS; CYCLOPROPYLCARBINYL; DISCOVERY; CONJUGATE ADDITION; BUTYROLACTONES
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.8b00821

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.