Mechanistic Insight into Enantioselective Palladium‐Catalyzed Oxidative Carbocyclization–Borylation of Enallenes. Issue 10 (25th January 2018)

• Record Type: Journal Article
• Authors: Qiu, Youai; Mendoza, Abraham; Posevins, Daniels; Himo, Fahmi; Kalek, Marcin; Bäckvall, Jan E.
• Is Part Of: Chemistry. Volume 24:Issue 10(2018)
• Page Start: 2433
• Page End: 2439
• Publication Date: 2018-01-25
• Subjects: asymmetric catalysis -- chiral phosphoric acid -- computational chemistry -- oxidative carbocyclization -- palladium; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201705239
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: The asymmetric palladium‐catalyzed oxidative carbocyclization–borylation of enallenes, employing a chiral phosphoric acid as co‐catalyst, constitutes an efficient and convenient entry into functionalized building blocks with cyclopentene scaffolds in high enantiopurity. Up till now there has been a lack of knowledge concerning the origin of enantioselectivity of this reaction as well as the absolute configuration of the product. Herein, we report the crystal structure of one of the compounds generated via this carbocyclization, providing the link between the configuration of the products and the configuration of the chiral phosphoric acid used in the reaction. Furthermore, the origin of the enantioselectivity is thoroughly investigated with density functional theory (DFT) calculations. By careful examination of different possible coordination modes, it is shown that the chiral phosphoric acid and the corresponding phosphate anion serve as ligands for palladium during the key stereoselectivity‐determining cyclization step. In addition, we examine reactions wherein an extra chiral reagent, a p ‐benzoquinone containing a chiral sulfoxide, is used. The combined experimental and theoretical studies provide insight into the details of complexation of palladium with various species present in the reaction mixture, furnishing a general understanding of the factors governing the stereoselectivity of this and related catalytic reactions. Abstract : Stereoselectivity explained : The absolute configuration of a product of enantioselective palladium‐catalyzed carbocyclization is determined by X‐ray crystallography. The mechanism of the reaction is thoroughly investigated with density functional theory (DFT) calculations and the origin of the observed enantioselectivity is identified.