Enamine/butadienylborane cycloaddition in the frustrated Lewis pair regime. Issue 42 (2nd September 2015)

• Record Type: Journal Article
• Authors: Chen, Guo-Qiang; Türkyilmaz, Fatma; Daniliuc, Constantin G.; Bannwarth, Christoph; Grimme, Stefan; Kehr, Gerald; Erker, Gerhard
• Is Part Of: Organic & biomolecular chemistry. Volume 13:Issue 42(2015)
• Page Start: 10477
• Page End: 10486
• Publication Date: 2015-09-02
• Subjects: Chemistry, Organic -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1039/c5ob01602a
• Languages: English
• ISSNs: 1477-0520

Abstract

Abstract : The dienylborane2a underwent a stepwise cycloaddition reaction with enamines followed by amidoborane elimination. Abstract : The dienylborane2a was prepared by regioselective alkyne hydroboration of the conjugated enyne1a with Piers' borane [HB(C6 F5 )2 ]. Its reaction with a series of acetophenone derived enamines3 resulted in the formation of the strong enamine β-carbon adduct with the borane Lewis acid (4 ). In contrast B–C adduct formation between the dienylborane2a and a series of much more bulky cyclohexanone derived enamines (6 ) is rapidly reversible above ca. −30 °C and then leads to the formation of the [4 + 2]cycloaddition products8 . A DFT study revealed that this reaction is probably taking a stepwise route, proceeding by means of enamine addition to the dienylborane terminus to generate a zwitterionic borata–alkene/iminium ion intermediate that undergoes rapid subsequent ring closure. Heating of the products8 led to amidoborane elimination from the vicinal amino/borane pair at the product framework to give the respective hexahydronaphthalene product10 . Subsequent treatment with TEMPO (2 equiv.) resulted in selective oxidation of the unsaturated ring to give the respective tetrahydronaphthalene derivative12 .