Lookup NU author(s): Dr Simon Doherty,
Dr Julian Knight,
Emeritus Professor William McFarlane,
Dr Corinne Wills,
Dr Mike Probert
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND).
A homologous series of triaryl-like KITPHOS-type monophosphines containing either one, two or three bulky (E)-9-(1-phenylprop-1-en-1-yl)-9,10-dihydroanthracene (KITPHOS) units have been developed and the influence of increasing steric bulk on their efficacy as ligands in gold(I)-catalyzed carbon-heteroatom bond-forming cyclizations investigated. Detailed solution NMR studies on Ph-TRISKITPHOS, its oxide and the corresponding gold(I) chloride adduct identified an exchange process involving a concerted librational motion of the individual anthracene-derived organic substituent about their P-C bonds which lowers the molecular symmetry such that the two C6H4 rings in each of the (E)-9-(1-phenylprop-1-en-1-yl)-9,10-dihydroanthracene (KITPHOS) units become inequivalent; a lower energy process involving restricted rotation of the biaryl-like phenyl ring has also identified. Electrophilic gold(I) complexes of these triaryl-like KITPHOS monophosphines catalyze the 5-exo-dig cycloisomerization of propargyl amides to afford the corresponding methylene oxazolines which were used in a subsequent tandem carbonyl-ene reaction to afford highly functionalized 2-substituted oxazolines. A comparative survey revealed that catalyst efficiency for cycloisomerization decreases in the order MONOKITPHOS = BISKITPHOS > PPh3 > TRISKITPHOS. The optimum system also catalyzes the selective 6-endo-dig cyclization of 2-alkynylbenzyl alcohols, 2-alkynylbenzoic acid and 2-(phenylethynyl) benzamides; gratifyingly, in several cases the yields obtained are markedly higher and/or reaction times significantly shorter than previously reported for related gold catalysts. Moreover, these are the first examples of gold(I)-catalyzed 6-endo-dig cycloisomerizations involving 2-(phenylethynyl) benzamides and, reassuringly, the optimum gold(I)/MONOKITPHOS systems either rivaled or outperformed existing silver or palladium-based catalysts. The steric parameters of this homologous series of phosphines have been quantified and compared with selected relevant triarylphosphines using a combination of Solid-G calculations, to determine the percentage of the metal coordination sphere shielded by the phosphine (the G parameter), and Salerno molecular buried volume calculations (SambVca) to determine the percent buried volume (%Vbur); the corresponding Tolman cone angles have also been determined from correlations.
Author(s): Doherty S, Knight JG, Perry DO, Ward NAB, Bittner DM, McFarlane W, Wills C, Probert MR
Publication type: Article
Publication status: Published
Online publication date: 27/04/2016
Acceptance date: 18/04/2016
ISSN (print): 0276-7333
ISSN (electronic): 1520-6041
Publisher: American Chemical Society
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