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Relevant academic research and scientific papers

Assessing the influence of phosphine substituents on the catalytic properties of self-stabilised digold(i) complexes with supporting ferrocene phosphinonitrile ligands

Gold(i) phosphine complexes are often used in catalysis, but the role of their auxiliary ligands still remains poorly understood. Thus, building on our previous research, we prepared a series of Au(i) complexes [Au2(μ-R2PfcCN)2][SbF6]2 (fc = ferrocene-1,1

Triaryl-Like MONO-, BIS-, and TRISKITPHOS Phosphines: Synthesis, Solution NMR Studies, and a Comparison in Gold-Catalyzed Carbon-Heteroatom Bond Forming 5-exo-dig and 6-endo-dig Cyclizations

A homologous series of triaryl-like KITPHOS-type monophosphines containing one, two, or three bulky 12-phenyl-9,10-dihydro-9,10-ethenoanthracene (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 has been investigated. Detailed solution NMR studies on Ph-TRISKITPHOS, its oxide, and the corresponding gold(I) chloride adduct identified a conformational exchange process involving a concerted librational motion of the individual anthracene-derived organic substituents about their P-C bonds. The cessation of this motion at reduced temperatures lowers the molecular symmetry such that the two C6H4 rings in each of the KITPHOS units become inequivalent; a lower energy process involving restricted rotation of the biaryl-like phenyl ring has also been 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 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 those 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 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.

Efficient cycloisomerization of propargyl amides by electrophilic gold(I) complexes of KITPHOS monophosphines: A comparative study

Electrophilic gold(I) complexes of diphenyl- and dicyclohexylphosphino- based KITPHOS monophosphines catalyze the 5-exo-dig cycloisomerization of a range of propargyl amides to afford the corresponding alkylidene oxazolines; in all cases catalysts formed from diphenylphosphino-substituted KITPHOS monophosphines outperformed their dicyclohexylphosphino counterparts as well as that based on triphenylphosphine, an indication that the biaryl/biaryl-like framework may be responsible for imparting high catalyst efficiency.