The Design, Preparation, and Use of Chiral Organoaluminum dibromide Lewis Acids in Asymetric Reactions
Date
2015-09-02
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Abstract
This dissertation adds to the current knowledge of aluminum-based Lewis acids for use in asymmetric catalysis. A highly regioselective and diastereoselective hydroalumination of olefins was used to generate novel achiral and chiral organoaluminum dibromide compounds in situ. Low-temperature proton NMR binding studies were conducted with several different Lewis bases to prove that the Lewis acids coordinate reversibly to Lewis bases. These Lewis acids were investigated for their ability to promote and catalyze organic reactions including Diels-Alder, intramolecular Diels-Alder, epoxide-opening and Strecker reactions.
Allylic O-benzylated dienophiles substituted with chiral oxazolidin-2-one auxiliaries were reacted with the sterically hindered diene 1,3,3-trimethyl-2-vinyl-cyclohexene to access a substituted drimane skeleton for natural product synthesis. The exo adduct was obtained as the major diastereomer in up to 60 % yield. Similar crotonic acid-derivatized dienophiles that were reacted with the same diene produced the endo adduct as the major product in up to 96 % yield.
The very first example of a chiral organoaluminum dibromide Lewis acid catalyzing a reaction asymmetrically is also described. The Diels-Alder reaction between methacrolein and cyclopentadiene was catalyzed in 80 % yield and 13.3 % ee, with an exo:endo ratio of 25:1. Additionally, the first example of an R*AlBr(OR*)-type Lewis acid catalyzing a reaction asymmetrically is described. These Lewis acids were shown to undergo a dehydroalumination under the conditions of their formation, but nevertheless generated the Diels-Alder adduct in the reaction between methacrolein and cyclopentadiene in 68 % yield and 16 % ee, with a 29:1 exo:endo ratio.
Finally, a novel class of 3,3’-disubstituted BINOL-aluminum bromide Lewis acids is also described, generating the same Diels-Alder adduct in up to 77 % yield and 37.5 % ee, with an exo:endo ratio of 14:1. Asymmetric Strecker and epoxide-opening reactions are also described making use of this novel class of BINOL-aluminum bromide Lewis acids. Chiral Strecker adduct was obtained in up to 72 % yield and up to 9.1 % ee with chiral bromohydrins being obtained from the opening of meso-epoxides in up to 65 % yield and 16.8 % ee.
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Chemistry--Organic
Citation
Warner, T. (2015). The Design, Preparation, and Use of Chiral Organoaluminum dibromide Lewis Acids in Asymetric Reactions (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/28049