Some Singular Features of Gold Catalysis: Protection of Gold(I) Catalysts by Substoichiometric Agents and Associated Phenomena

This study deals with two striking phenomena: the complete protection against decomposition of hypothetically monocoordinated Au^I intermediates [AuL]Y (L = strongly coordinating ligand; Y^– = poorly coordinating anion) by addition of small substoichiometric amounts (5 mol % relative to Au) of not strongly coordinating ligands (e.g., AsPh₃) and the fact that, in contrast, strongly coordinating ligands cannot provide this substoichiometric protection. The two phenomena are explained considering that (i) the existence of real monocoordinated [AuL]Y is negligible in condensed phases and the kinetically efficient existing species are dicoordinated [AuL­(W)]­Y (W = any very weakly coordinating ligand existing in solution, including OH₂, the solvent, or the Y^– anion) and (ii) these [AuL­(W)]Y intermediates give rise to decomposition by a disproportionation mechanism, via polynuclear intermediates formed by associative oligomerization with release of some W ligands. It is also shown that very small concentrations of [AuL­(W)]Y are still catalytically efficient and can be stabilized by overstoichiometric adventitious water, so that full decomposition of the catalyst is hardly reached, although eventually the stabilized concentration can be kinetically inefficient for the catalysis. These results suggest that, in cases of gold catalysis requiring the use of a significant quantity of gold catalyst, the turnover numbers can be increased or the concentration of gold catalyst widely reduced, using substoichiometric protection properly tuned to the case

Pt^II-Catalyzed Hydrophenylation of α‑Olefins: Variation of Linear/Branched Products as a Function of Ligand Donor Ability

The Pt^II complexes [(^<i>x</i>bpy)­Pt­(Ph)­(THF)]⁺ (^<i>x</i>bpy = 4,4′-X₂-2,2′-bipyridyl; <i>x</i> = OMe (<b>1a</b>), ^<i>t</i>Bu (<b>1b</b>), H (<b>1c</b>), Br (<b>1d</b>), CO₂Et (<b>1e</b>) and NO₂ (<b>1f</b>)] catalyze the formation of <i>n</i>-propylbenzene and cumene from benzene and propene. The catalysts are selective for branched products, and the cumene/<i>n</i>-propylbenzene ratio decreases with increasing donor ability of the ^<i>x</i>bpy ligand. DFT­(D) calculations predict more favorable activation barriers for 1,2-insertion into the Pt–Ph bond to give branched products. The calculations indicate that 1,2-insertion of propene should be faster than 2,1-insertion for all Pt­(II) catalysts studied, but they also indicate that cumene/<i>n</i>-propylbenzene selectivity is under Curtin–Hammett control