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Highly Sterically Encumbered Gold Acyclic Diaminocarbene Complexes: Overriding Electronic Control in Regiodivergent Gold Catalysis
preprintsubmitted on 06.12.2020, 17:13 and posted on 08.12.2020, 04:43 by Aaron Ruch, matthew ellison, John Nguyen, Fanji Kong, Sachin Handa, Vladimir Nesterov, LeGrande Slaughter
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 1H 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 (%Vbur) 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.