Highly Sterically Encumbered Gold Acyclic Diaminocarbene Complexes: Overriding Electronic Control in Regiodivergent Gold Catalysis

Abstract

Two series of sterically encumbered gold(I)-acyclic diaminocarbene (ADC) complexes were prepared by reaction of mono-and dialkylamines with gold-bound 2-mesitylphenyl isocyanide (monomesityl series) and 2,6-dimesitylphenyl isocyanide (dimesityl series). X-ray crystal structures and solution 1 H NMR data showed that the ADC-gold complexes adopt major rotameric conformations with the bulky biaryl/terphenyl group and one alkyl group located syn to gold. This engenders substantial steric hindrance at the metal, as evidenced by percent buried volume (%V bur ) parameters of 35.7 -37.2 for the monomesityl series and 46.4 -52.4 for the dimesityl series. Modest out-of-plane distortions of the ADC N-substituents in the dimesityl series were attributed to attractive CH•••π interactions between alkyl groups and mesityl rings on the basis of dispersion-corrected density functional theory calculations. Gold-catalyzed regiodivergent domino cyclization/hydroarylation reactions of a 1,6-enyne with indole revealed that the bulky biaryl/terphenyl substituents of the ligands exert a strong influence on product selectivity, with the bulkier dimesityl ADC-Au catalysts inducing a shift away from the cyclopropane-fused product toward the normally disfavored alkene product. Incorporation of a yet bulkier bis(2,6-diisopropylphenyl)-substituted terphenyl moiety into the ADC led to a gold catalyst that provided exclusive selectivity for the alkene product. Computational modeling suggested that bulky terphenyl groups hinder attack at the a carbon in the initially formed organogold intermediate, allowing steric effects to override the intrinsic electronic preference for the cyclopropane product. File list (3) download file view on ChemRxiv Slaughter_Encumbered_ADC-Au_Preprint.pdf (2.05 MiB) download file view on ChemRxiv Slaughter_Encumbered_ADC-Au_SuppInfo_Preprint.pdf (4.44 MiB)

Protic Ferrocenyl Acyclic Diamino Carbene Gold(I) Complexes

“…Addition of secondary amines to isocyanide gold(I) complexes yields the corresponding protic acyclic diamino carbene gold(I) complexes. [16][17][18][25][26][27][28] To install the redox-active ferrocene unit, we first coordinated isocyanoferrocene FcÀ NC [56] to gold(I) chloride via chlorido(dimethylsulfide)gold(I) as gold(I) precursor giving the heterobimetallic complex chlorido (isocyanoferrocene)gold(I) 1 in 27 % isolated yield (Scheme 2). The conceivable inverse route to the envisioned ADC gold(I) complexes starting from an (alkylisocyano) chlorido gold(I) complex and aminoferrocene was unsuccessful due to the low nucleophilicity of aminoferrocene.…”

“…[10] In gold(I) catalysis, typically precatalysts of the type AuCl(L) are employed with phosphanes or carbenes as ligands L (Scheme 1), well suited to stabilise the reactive cationic species [Au(L)] + . [7,9] Beside N-heterocyclic carbenes in gold catalysis, [11,12] cyclic amino alkyl carbenes (CAAC) [13] and acyclic diamino carbenes (ADC) [14][15][16][17][18][19] with their stronger donicity and differing steric bulk [20,21] have demonstrated to be promising candidates. Complexes Au(CAAC)Cl enable new catalytic applications, e. g. the direct hydroamination of alkynes or allenes with hydrazine.…”

“…[24] Gold(I) complexes with ADC ligands are acessible from gold(I) isonitriles with primary or secondary amines. [16][17][18][25][26][27][28] These ADCs with N(H)R substituents can be viewed as protic ADCs, similar to protic NHCs, [29][30][31] which are also decribed as ligands in gold complexes. [32][33][34][35] Commonly, the use of activators, halide scavengers like silver(I) salts, fascilitating the dissociation of the chlorido ligand from AuCl(L) are necessesary to form the active catalyst [Au(L)] + .…”

Gold-catalyzed hydroarylation reactions: a comprehensive overview

“…According to the Slaughter group, the bulky biaryl/terphenyl substituents of the ligands exhibited a considerable influence on product selectivity, with the larger dimesityl gold acyclic diaminocarbene (ADC) catalysts promoting a shift away from the cyclopropane-fused product towards the generally disfavored alkene product (Scheme 80). 101 An additional larger bis(2,6-diisopropylphenyl)-substituted terphenyl moiety was included in the ADC, resulting in a gold-catalyst with exclusive selectivity for the alkene product. The reaction took place between 1,6-enyne 250 and indole 171 to furnish the desired products 251 and 252 in up to 88% yields under the optimized reaction conditions of 5 mol% [L 24 AuCl] catalyst in 1,2-dichloroethane at 25 °C for 3 h. The dimesityl ADCs' bulky terphenyl groups appeared to shift selectivity away from cyclopropane product and towards alkene product, resulting in about identical product ratios for this catalyst series.…”

Planar Chiral 1,3-Disubstituted Ferrocenyl Phosphine Gold(I) Catalysts

“… 21 Buried volumes of 45–53% have been reported for other gold(I) complexes bearing bulky alkoxydiaminophosphine 22 or acyclic diaminocarbene ligands. 23 …”