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

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 H-1 NMR data showed that the gold ADC 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 the 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 center dot center dot center dot pi 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 Au ADC catalysts inducing a shift away from the cyclopropane-fused product toward the normally disfavored alkene product. Incorporation of an even 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.

Automated summary

The study synthesized sterically encumbered gold(I) acyclic diaminocarbene (ADC) complexes with different ligands and found that the bulky biaryl/terphenyl groups strongly influence product selectivity in gold-catalyzed reactions. The research also revealed how large aryl groups impact reaction mechanisms.