Highly Efficient Hydroamination of Phenylacetylenes with Anilines Catalysed by Gold Nanoparticles Embedded in Nanoporous Polymer Matrix: Insight into the Reaction Mechanism by Kinetic and DFT Investigations

The synthesis of aromatic ketimines via hydroamination of phenylacetylenes (PAs) with anilines (ANs) has been accomplished in high yields and with excellent regio- and stereo-selectivity using gold nanoparticles (AuNPs) embedded in crystalline syndiotactic polystyrene-cis-1,4-polybutadiene (sPSB) multiblock copolymer matrix. The performances of the AuNPs-sPSB catalyst exceed those of the other commercial gold catalysts as a result of the physical chemical properties of the nanoporous polystyrenic support which allows excellent activity, thermal stability and recyclability of the catalyst. Electron donating (EDGs) and electron withdrawing (EWGs) substituents onto the aromatic group of ANs and PAs yield, in all examined cases, high selectivity in the formation of the thermodynamic favoured E stereoisomer of the aromatic ketimines. Kinetic investigation of the reaction mechanism in the presence of AN and of the deuterated analogue, AN-N,N-d2, highlighted a new reaction pathway for the hydroamination reaction, which was also supported by DFT calculations. Actually, the formation of AN aggregates stabilized by hydrogen bonding interactions produces a favourable transition state for the nucleophilic attack of AN to PA, coordinated/activated onto AuNPs surface. Moreover, an additional AN molecule cooperatively assists the 1,3-hydrogen shuttling from the N atom to the Cβ of the coordinated enamine intermediate to produce the kinetically favoured Z-ketimine intermediate that in turn evolves into the thermodynamically stable E-aromatic ketimine. The first order kinetics observed for AN, along with the experimental energetic barrier (ΔG‡ = 26.6±0.7 kcal mol−1; ΔH‡ = 13.4±1.8 kcal mol-1; ΔS‡ = -0.04±0.04 kcal mol-1 K-1) found in good agreement with the energy of the transition state calculated for the 1,3-hydrogen shift in the DFT modelling, strongly support that the latter is the rate determining step in the gold catalysed hydroamination of AN with PA.