title: Structure-reactivity relationships of zwitterionic 1,3-diaza-Claisen rearrangements.
abstract: Bridged bicyclic tertiary allylic amines aza-norbornene 1 and isoquinuclidene 2 add to isocyanates, isothiocyanates, and in situ-generated carbodiimides to form zwitterionic intermediates that undergo 1,3-diaza-Claisen rearrangements to afford highly substituted ureas, thioureas, and guanidines, respectively. Aza-norbornene 1 is significantly more reactive toward 1,3-diaza-Claisen rearrangements than isoquinuclidene 2. This reactivity difference is most likely due to the inherent ring strain in the aza-bicyclo[2.2.1]heptene ring system of aza-norbornene 1. The most apparent reactivity trend of the heterocumulenes is that the most electron-deficient heterocumulenes are more reactive toward 1,3-diaza-Claisen rearrangements. The introduction of a new stereocenter α- to the nucleophilic nitrogen in aza-norbornene 1 and isoquinuclidine 2 decreases the reactivity toward 1,3-diaza-Claisen rearrangements, while the exodiastereomers 3b and 4b are less reactive than the corresponding endodiastereomers 3a and 4a. Isocyanates that bear an electron-withdrawing group react with allylic amines 1-3b to afford mixtures of ureas and isoureas; however, with excess isocyanate and heat, thermodynamic equilibration is possible affording ureas. Inspired by this observation, a one-pot reaction of isocyanates with amines 1, 2, and 3b followed by BF3·OEt2-catalyzed isomerization of the urea/isourea mixture was developed that affords the corresponding ureas in excellent yields.
authors.affiliation: Department of Chemistry, University of Vermont , Burlington, Vermont 05405, United States.
authors.forename: Rachel
authors.initials: R
authors.lastname: Aranha Potter
authors.forename: Amy M
authors.initials: AM
authors.lastname: Bowser
authors.forename: Yanbo
authors.initials: Y
authors.lastname: Yang
authors.forename: José S
authors.initials: JS
authors.lastname: Madalengoitia
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Ziller
mesh_terms: D019791:Guanidine / Q000031:analogs & derivatives / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D013890:Thiourea / Q000138:chemical synthesis* / Q000737:chemistry; D014508:Urea / Q000031:analogs & derivatives / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D014508:Urea; D013890:Thiourea; D019791:Guanidine