Ligand Effect in Platinum-Catalyzed Cycloisomerization of 1,6-Enynes: Water or Carbon Monoxide, a Similar Role despite Distinct Electronic Properties?

The PtCl₂-catalyzed cycloisomerization of 1,6-enynes is considerably accelerated in the presence of carbon monoxide. The effect of CO has been explained by the reinforcement of the electrophilicity of the alkyne moiety once ligated to the π-acidic [PtCl₂(CO)] fragment. However, platinum is also described as weakly capable of back-donation toward CO. A theoretical study has been carried out to shed light on this apparent contradiction. An alternative explanation based on the approach of CO onto the [PtCl₂(η⁴-(1,6-enyne))] complex is proposed. When CO is ligated to PtCl₂, the triple-bond coordination is favored, yet there is no positive influence on the formation of the cyclopropylcarbene key intermediate. The role of CO is actually comparable to that of a water molecule

Versatile Access to Martin’s Spirosilanes and Their Hypervalent Derivatives

A new route to Martin’s spirosilanes has been devised. The original synthesis does not allow diversely substituted spirosilane derivatives to be synthesized, and thus their corresponding silicates. In this report, Martin’s spirosilanes bearing alkyl, aryl, halogen, alkoxy, and trifluoromethyl substituents on the aryl ring have been prepared through a versatile four-step route. Addition of fluoride onto these Lewis acids as a prototypical reaction with a nucleophile yielded a library of stable fluorosilicates. Both sets of compounds have been characterized by X-ray crystallography

Versatile Access to Martin’s Spirosilanes and Their Hypervalent Derivatives

A new route to Martin’s spirosilanes has been devised. The original synthesis does not allow diversely substituted spirosilane derivatives to be synthesized, and thus their corresponding silicates. In this report, Martin’s spirosilanes bearing alkyl, aryl, halogen, alkoxy, and trifluoromethyl substituents on the aryl ring have been prepared through a versatile four-step route. Addition of fluoride onto these Lewis acids as a prototypical reaction with a nucleophile yielded a library of stable fluorosilicates. Both sets of compounds have been characterized by X-ray crystallography

Secondary Phosphine Oxide–Gold(I) Complexes and Their First Application in Catalysis

A series of new secondary phosphine oxide (SPO)–gold­(I) complexes have been synthesized and characterized by X-ray crystallography. Complexes exhibited dimeric structures interconnected by O–H···Cl hydrogen bonds. Their first use in homogeneous catalysis is reported and suggests a broad field of application in prototypical enyne cycloisomerization and hydroxy- and methoxycyclization reactions

Gold-Catalyzed Polymerization Based on Carbene Polycyclopropanation

The first polymerization exploiting the carbenic reactivity of homogeneous gold catalysis has been devised. In the presence of a gold catalyst, monomers incorporating both a propargylic ester and an alkene moiety polymerized through a metallocarbene generation/cyclopropanation sequence to afford the corresponding macromolecules. This approach constitutes an unprecedented example of cyclopropanation-based polymerization and allows access to original macromolecule skeletons

Assessing Ligand and Counterion Effects in the Noble Metal Catalyzed Cycloisomerization Reactions of 1,6-Allenynes: a Combined Experimental and Theoretical Approach

1,6-Allenynes are useful mechanistic probes in noble-metal catalysis, since they can give rise to very distinct products in a highly selective fashion. Various cycloisomerization reactions have been described, and discrete mechanisms have been postulated. Of particular interest, whereas Alder-ene types of products can be obtained in a variety of ways using noble-metal catalysts (Au, Pt, Rh, ...), hydrindienes have been reported solely with gold and platinum under specific conditions. It was shown in a previous study that this intriguing transformation required the presence of chloride ligands at the active catalytic species. Herein, the factors governing the fate of 1,6-allenynes under cycloisomerization conditions have been studied more thoroughly, revealing a much more complex scenario. The nature of ligands, counterions, and metals was examined, showing that hydrindienes can be isolated in the absence of halides, using electron-rich, bulky triorganophosphines or carbene ligands. This crucial finding could also be used to access hydrindienes in high yields, not only with gold or platinum but also with silver. On the basis of mass spectrometry, NMR spectroscopy, and computations, refined mechanistic scenarios have been put forward, also rationalizing counterion effects. Notably, a metal vinylidene intermediate has been proposed for the formation of the hydrindiene derivatives. Finally, in the presence of tris­((triphenylphosphine)­gold)­oxonium tetrafluoroborate as catalyst, a new pathway has been unveiled, involving gold alkyne σ,π complexes and leading to previously unobserved [2 + 2] cycloaddition compounds