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Triaryl-Like MONO-, BIS- and TRISKITPHOS Phosphines: Synthesis, Solution NMR Studies and a Comparison in Gold-Catalysed Carbon-Heteroatom Bond Forming 5-Exo-Dig and 6-Endo-Dig Cyclizations

Lookup NU author(s): Dr Simon Doherty, Dr Julian Knight, Daniel Perry, Nicholas Ward, Dror Bittner, Emeritus Professor William McFarlane, Dr Corinne Wills, Dr Mike Probert

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Abstract

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.


Publication metadata

Author(s): Doherty S, Knight JG, Perry DO, Ward NAB, Bittner DM, McFarlane W, Wills C, Probert MR

Publication type: Article

Publication status: Published

Journal: Organometallics

Year: 2016

Volume: 35

Issue: 9

Pages: 1265-1278

Online publication date: 27/04/2016

Acceptance date: 18/04/2016

ISSN (print): 0276-7333

ISSN (electronic): 1520-6041

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/acs.organomet.6b00146

DOI: 10.1021/acs.organomet.6b00146


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