ABSTRACT
Diaryl-substituted vinyl boronates as potent building modules are challenging to synthesize. Herein, we present a convenient strategy based on a gold-catalyzed Hiyama arylation of (Z)-ß-(borylvinyl)silanes which are easily accessible by hydroboration of silylalkynes. By exploiting the highly electronegative nature of the Au(III) intermediate (which is accessed by the light-assisted oxidation with aryl diazonium salts), a selective activation of the silyl group in the presence of the boron moiety is achieved. This opens a route to selectively synthesize diaryl-substituted vinyl boronates. The reaction shows a broad substrate range, excellent functional group tolerance and perfect chemo-selectivity. Experimental studies and DFT calculations allowed us to elucidate the mechanism of the reaction, the synthetic potential was demonstrated by downstream transformations providing a facile route to bifunctional phenanthrenes and triaryl-substituted olefins.
ABSTRACT
We report the direct synthesis of new azulene derivatives through gold-catalyzed cyclization reactions. A five-membered ring as backbone in the applied triene-yne substrates turned out to be crucial to induce the 7-endo-dig cyclization mode necessary to trigger azulene formation. The obtained targets are of high interest due to their potential applications in different fields, like organic materials, medicine or cosmetics. UV/Vis spectra and cyclic voltammetry were measured, based on these the electronic properties were determined. Short two or three step sequences towards the applied starting materials make this approach synthetically highly attractive.
ABSTRACT
We present a protocol to synthesize air stable gem-diaurated gold(III) compounds from 1,3-diketones in a single cycloauration step with tetrachloroauric acid. So far related species were only accessible from phosphonium bis(ylide) ligands which hold the two gold atoms in close proximity. Lacking such a constraint, our compounds show the longest Au-Au distances of all gem-diaurated carbons, ranging from 3.26 to 3.32 Å. Modeling based on results of CCSD(T) calculations shows no stabilization by aurophilic interactions for our gold(III) systems, compared to 9.1 kcal/mol for gold(I) gem-diauration. This demonstrates no aurophilic interactions are needed for the isolation of air stable gem-diaurated gold(III) complexes. We show the new gem-diaurated gold(III) compounds are active in the gold-catalyzed phenol synthesis and highly active in the cycloisomerization of an N-propargylcarboxamide; here, we obtained the so far highest known TON of over 2500 per gold atom with respect to the oxazole formation.
ABSTRACT
Gold(I) complexes are considered active species toward oxidative addition; current understanding indicates a different mechanism in contrast to other late transition metals, but a rational understanding of the reactivity profile is lacking. Herein, we propose that the accessibility of the gold(I) center to tri- or tetra-coordination is critical in the oxidative process involving a tri- or tetra-coordinate gold(I) with the oxidizing reagent as one of the ligands as an intermediate. A computational study of the geometry of (Phen)R3PAu(I)NTf2 complexes shows that the accessibility of such tricoordinate species shows a good correlation with the "trans influence" of phosphine ligands: the weak σ-donating phosphine ligands promote tricoordination of gold(I) complexes. The oxidative addition to the asymmetric tricoordinate (Phen)R3PAu(I)NTf2 complexes with alkynyl hypervalent iodine reagents was built. The kinetic profile of the oxidative addition exhibits a good relationship to the Hammett substituent parameter (ρ = 3.75, R2 = 0.934), in which the gold(I) complexes bearing less σ-donating phosphine ligands increase the rate of oxidative addition. The positive ρ indicates a high sensitivity of the oxidative addition to the trans influence. The reactivity profile of oxidative addition to a linear bis(pyridine)gold(I) complex further supports that the oxidative addition to gold(I) complexes is promoted by ligands with small trans influence.