Mechanistic insights into the B(C6F5)3-initiated aldehyde-aniline-alkyne reaction to form substituted quinolines

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Sub-stoichiometric quantities of the Lewis acid B(C6F5)3 is sufficient to initiate the aldehyde-amine-alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high yielding tri-component reaction initiated by B(C6F5)3 at raised temperatures include an excess of the in-situ generated imine (which acts as hydrogen acceptor) and an alkyne substituent able to stabilize positive charge build up during the cyclization. Mechanistic experiments revealed that under these conditions B(C6F5)3 is acting as a Lewis acid assisted Brønsted acid, with H2O-B(C6F5)3 the key species enabling catalytic quinoline formation. This was indicated by deuterium labelling studies and the observation that the cyclization of N-(3-phenyl-propargyl)aniline using B(C6F5)3 under anhydrous conditions afforded the zwitterion [(N-H-3-B(C6F5)3-4-Ph-quinolinium] that does not undergo protodeboronation to release B(C6F5)3 and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.

Bibliographical metadata

Original languageEnglish
Pages (from-to)1623-1629
Number of pages7
Issue number8
Early online date12 Apr 2017
StatePublished - 2017