Michel R. Gagné

Mechanistic surprises in the gold(I)-catalyzed intramolecular hydroarylation of allenes.

Gold(I)-catalyzed carbon-carbon bond forming reactions continue to fascinate the synthetic community but are less well investigated than many conventional metal catalysts.[1] In many cases a cationic gold(I)-species activates an unsaturated C-C bond and isomerizes or functionalizes it to build molecular complexity through reactive intermediates that include gold-π-complexes, gold-vinyls, and gold-carbenes. Support for these intermediates is mainly based on gold(I) organometallic chemistry, though computational studies and the isolation of proposed catalytic intermediates have been reported.[2–5]

Distinct Reactivity of Pd(OTs)2: The Intermolecular Pd(II)-Catalyzed 1,2-Carboamination of Dienes

A Pd-catalyzed intermolecular 1,2-carboamination route to indolines from N-aryl ureas and 1,3-dienes that proceeds under mild conditions in relatively nonacidic media, is presented. The in situ generation, or preformation, of a palladium tosylate emerges as a key parameter in gaining the requisite reactivity for the C-H insertion/carbopalladation/nucleophilic displacement process.

Room-temperature palladium-catalyzed C-H activation: ortho-carbonylation of aniline derivatives.

Pd and CO--ureally got me! The title reaction proceeds efficiently at 18 degrees C under CO (1 atm) with 5 % [Pd(OTs)(2)(MeCN)(2)] as precatalyst. Depending on the solvents used, either anthranilates or cyclic imides can be obtained in high yields (see picture, BQ = benzoquinone, Ts = 4-toluenesulfonyl).

Intermolecular Addition of Glycosyl Halides to Alkenes Mediated by Visible Light

Abstract : Catching Photons: Visible light, an amine reductant, and a Ru(bpy)32+ photocatalyst can be used to mediate the addition of glycosyl halides into alkenes to synthesize important C-glycosides. This method highlights the growing potential of photocatalysis to effectively drive useful and difficult chemical transformations.

Mechanistic Analysis of Gold(I)-Catalyzed Intramolecular Allene Hydroalkoxylation Reveals an Off-Cycle Bis(gold) Vinyl Species and Reversible C–O Bond Formation

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C-O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.

Dinuclear Gold–Silver Resting States May Explain Silver Effects in Gold(I)-Catalysis

The resting state of the gold(I)-catalyzed hydroarylation of 1 changes in the presence of Ag(+), with silver free catalysts resting at the dinuclear gold structure 5 and Ag(+) containing solutions resting at a heteronuclear species like 6. Adventitious Ag(+) (typically from LAuCl activation) can therefore intercept key organogold intermediates and effect the catalysis even when it does not effect the reaction in Au free control experiments.

Strong electronic and counterion effects on geminal digold formation and reactivity as revealed by gold(I)-aryl model complexes.

Gold(I) cations have emerged as efficient and often times uniquely effective catalysts for the formation of C–X (X=C, O, N) bonds.[1] Its use as a soft π-acid for the activation of C–C multiple bonds has led to proposed intermediates that include π–gold, Au–vinyl, Au–alkyl, and Au–carbene structures.[2] In many instances a transient Au–C σ-bond is converted into a C–E bond through its reaction with an E+ electrophile.[3] Recently, we provided evidence that the intramolecular hydroarylation of allenes proceeded through two different gold–vinyl intermediates, one mononuclear (A), and one dinuclear (B), with the latter acting as the catalyst’s resting state. The digold structure was proposed to result from the reaction of LAu+ with monogold–vinyl A [Eq. (1)].[4]

Gold(I)-catalyzed intramolecular hydroarylation of allenes.

Gold(I) complexes react with 4-allenyl arenes in an exo fashion to furnish vinyl-substituted benzocycles. Phosphite gold(I) monocations were found to be optimal, and the catalyst was tolerant of ethers, esters, and pyrroles. Reactions proceeded in unpurified solvent at room temperature.

A gold-catalysed enantioselective Cope rearrangement of achiral 1,5-dienes

Since the discovery of the Cope rearrangement in the 1940s, no asymmetric variant of the rearrangement of achiral 1,5-dienes has emerged, despite the successes that have been achieved with its heteroatom variants (Claisen, aza-Cope, and so on). This article reports the first example of an enantioselective Cope reaction that starts from an achiral diene. The new gold(I) catalyst derived from double Cl−-abstraction of ((S)-3,5-xylyl-PHANEPHOS(AuCl)2), has been developed for the sigmatropic rearrangement of alkenyl-methylenecyclopropanes. The reaction proceeds at low temperature and the synthetically useful vinylcyclopropane products are obtained in high yield and enantioselectivity. Density functional theory calculations predict that: (1) the reaction proceeds via a cyclic carbenium ion intermediate, (2) the relief of strain in the methylenecyclopropane moiety provides the thermodynamic driving force for the rearrangement and (3) metal complexation of the transition-state structure lowers the rearrangement barriers. The Cope rearrangement has been known since the 1940s but, until now, no catalytic asymmetric variant has been reported. Here, a gold(I) catalyst is shown to induce an asymmetric Cope rearrangement of achiral 1,5-dienes containing a cyclopropylidene moiety to produce vinyl cyclopropanes in high yield and good to excellent enantioselectivities.

Investigating the rate of photoreductive glucosyl radical generation.

The photoreduction of glucosyl halides to generate glucosyl radicals has been investigated to probe the nature of the photoredox cycle. Amine (the reductant) and catalyst concentration affect the reaction rate at low concentrations but exhibit saturation at higher concentrations. Water and hydrophobic catalysts were found to significantly increase the conversion efficiency.