Nicolas Marion

N-Heterocyclic carbenes in gold catalysis

The appealing properties of N-heterocyclic carbenes (NHC) as ancillary ligands and the high potential of gold as an organometallic catalyst have made their encounter inevitable. Still in its infancy, NHC-gold catalysis is nevertheless growing rapidly. In this tutorial review, catalytic transformations involving NHC-containing gold(I) and gold(III) complexes are presented. Particular attention is drawn to the versatility and selectivity of these catalysts.

[(NHC)Au-I]-Catalyzed Acid-Free Alkyne Hydration at Part-per-Million Catalyst Loadings

A highly efficient [(NHC)Au(I)]-based (NHC = N-heterocyclic carbene) catalytic system for the hydration of an array of alkynes that operates under acid-free conditions and at very low catalyst loadings (typically 50-100 ppm and as low as 10 ppm) was developed. Terminal and internal alkynes possessing any combination of alkyl and aryl substituents (alkyl/H, aryl/H, alkyl/alkyl, alkyl/aryl, and aryl/aryl) were found suitable substrates in the present catalytic system.

Gold‐ and Platinum‐Catalyzed Cycloisomerization of Enynyl Esters versus Allenenyl Esters: An Experimental and Theoretical Study

Ester-way to heaven: Unexpected formation of bicyclo[3.1.0]hexene 4 was the main focus of combined experimental and theoretical studies on the Au-catalyzed cycloisomerization of branched dienyne 1 (see scheme), which provided better understanding of the mechanistic details governing the cyclization of enynes bearing a propargylic ester group.Experimental and theoretical studies on Au- and Pt-catalyzed cycloisomerization of a branched dienyne with an acetate group at the propargylic position are presented. The peculiar architecture of the dienyne precursor, which has both a 1,6- and a 1,5-enyne skeleton, leads, in the presence of alkynophilic gold catalysts, to mixtures of bicyclic compounds 3, 4, and 5. Formation of unprecedented bicyclo[3.1.0]hexene 5 is the main focus of this study. The effect of the ancillary ligand on the gold center was examined and found to be crucial for formation of 5. Further mechanistic studies, involving cyclization of an enantioenriched dienyne precursor, (18)O-labeling experiments, and DFT calculations, allowed an unprecedented reaction pathway to be proposed. We show that bicyclo[3.1.0]hexene 5 is likely formed by a 1,3-OAc shift/allene-ene cyclization/1,2-OAc shift sequence, as calculated by DFT and supported by Au-catalyzed cyclization of isolated allenenyl acetate 7, which leads to improved selectivity in the formation of 5. Additionally, the possibility of OAc migration from allenyl acetates was supported by a trapping experiment with styrene that afforded the corresponding cyclopropane derivative. This unprecedented generation of a vinyl metal carbene from an allenyl ester supports a facile enynyl ester/allenenyl ester equilibrium. Further examination of the difference in reactivity between enynyl acetates and their corresponding [3,3]-rearranged allenenyl acetates toward Au- and Pt-catalyzed cycloisomerization is also presented.

Catalytic asymmetric C-N bond formation: phosphine-catalyzed intra- and intermolecular γ-addition of nitrogen nucleophiles to allenoates and alkynoates.

Pin the amine on the gamma: A new method has been developed for the γ-addition of nitrogen nucleophiles to γ-substituted alkynoates or allenoates through intra- and intermolecular processes that are catalyzed by spirophosphine 1. An asymmetric version of this reaction affords enantioenriched pyrrolidines, indolines, and γ-amino-α,β-unsaturated carbonyl compounds.

Mechanism of the [(NHC)Au-I]-catalyzed rearrangement of allylic acetates : a DFT study

The DFT study of the mechanism of the rearrangement of H(2)C=CHC(CH(3))OCOCH(3) to (CH(3))(H)C=CHCH(2)OCOCH(3) catalyzed by [(NHC)Au](+) (NHC = N-heterocyclic carbene) shows that a low energy path exists, with a barrier of 14.2 kcal x mol(-1), going through a six-membered ring acetoxonium intermediate and where gold coordinates one of the carbon atoms in the alkene system. The qualitative features of the mechanism are not affected by the introduction of other NHC ligands, counterions, or solvation effects.

Modified [(IPr)Pd(R-acac)Cl] complexes : influence of the acac substitution on the catalytic activity in aryl amination

The synthesis of a series of [(IPr)Pd(R-acac)Cl] precatalysts (acac=acetylacetonato; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), where the acac ligand on palladium has been systematically modified through terminal substitution, is reported. The following substituted acac ligands are employed in this study: dibenzoylmethanato (dbm), benzoylacetonato (bac), tetramethylheptanedionato (tmhd), and hexafluoroacetylacetonato (hfac). Full spectroscopic characterization of the new complexes is provided along with X-ray studies for three of these. Investigation of their catalytic activity in cross-coupling is also presented through a comparative study in an aryl amination reaction. The catalytic results show a strong correlation between the increased steric bulk of the acac substituents and an increased activation rate of the precatalyst, going from the acac to the tmhd ligand. This observation, along with the inertness of the hfac compound, seems to support our previous proposal for the activation mechanism of these complexes under cross-coupling conditions.