Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity

Abstract

This work describes the synthesis of carbenium-based, γ-cationic phosphines and their coordination to Au(i) cations , leading to carbophilic catalysts whose activity is enhanced by the ligand-enforced convergence of the positively charged moieties.

Placing gold on a π+-surface: ligand design and impact on reactivity

“…Dehydroxylation of 1 xant and 1 acr with HBF 4 proceeded swiftly to afford the corresponding phosphonium derivatives [ 2 xant ] + and [ 2 acr ] + , respectively, rather than the phosphine carbenium derivatives thus illustrating the tendency of phosphines to coordinate to carbenium ions. 6 The tetrafluoroborate salts of both phosphonium cations are colourless air-stable solids. Their 13 C NMR spectra reveal diagnostic resonance at 64.2 ppm ( J C–P = 48.1 Hz) for [ 2 xant ][BF 4 ] and 70.1 ppm ( J C–P = 41.6 Hz) for [ 2 acr ][BF 4 ] corresponding to the phosphine-neutralised C carb atom.…”

Allenic Phosphonium Borate Zwitterions via a Transient Phosphonium Allenylidene

“…To attempt the synthesis of IV, we envisioned a suitable pathway that first involves the generation of a carbenium ion III, accessed via a heterolytic bond cleavage reaction at the g carbon of a propargylsubstituted phosphane. 7 We chose to employ phosphane precursors with a p-electron stabilizing diphenylacetylene substituent, which upon methoxide anion abstraction had the anticipated effect of delocalizing the phosphorus atom lone pair and providing salts akin to IV. We reasoned that the formation of IV over the carbenium ion III could be favourable if the positive charge were delocalized from the main group center over several atoms, possibly counteracting the energetic penalty associated with the planarization of the phosphorus center.…”

“…With 1 in hand, we targeted its transformation to the [R 2 P=C=C=CR 2 ] + cation, initially employing trifluoromethylsulfonate (TMSOTf) as a methoxide abstraction reagent. 7 Addition of a stoichiometric excess of TMSOTf (~ 1.5 equiv) to a chloroform-d solution of 1 at -78 °C, in the absence of light, resulted in a gradual change from colorless to bright yellow. Monitoring the reaction by 1 H NMR spectroscopy indicated conversion to a single major product, identified by the generation of a diagnostic doublet resonance at  = 4.11 ppm with a small 3 J PH coupling constant of 12.5 Hz, further confirmed by 2D 1 H-31 P {H} heteronuclear multiple bond coherence experiments.…”

Metal→Carbon Dative Bonding

“…To explore this possibility, we designed a family of phosphine‐carbenium ligands such as L + in which the Lewis‐opposite functionalities are connected by an ortho ‐phenylene backbone (Figure 1). [9] Crystallographic analysis of the corresponding gold(I) complexes, coupled with theoretical calculations, pointed to the absence of significant Au→C carbenium dative bonding in all isolated complexes [9b] including in the phosphine‐acridinium complex [ L AuCl] + ([ 2 ] + ) [8a] . With the view to demonstrate that such carbenium ligands can indeed behave as genuine Z‐type ligands and accept incremental amounts of electron‐density from an adjacent metal atom, we are now considering the synthesis of complexes possessing a more metallobasic d 10 fragment than Au I .…”