The Pt-Catalyzed Ethylene Hydroamination by Aniline: A Computational Investigation of the Catalytic Cycle
journal contributionposted on 06.10.2010, 00:00 by Pavel A. Dub, Rinaldo Poli
A full QM DFT study without system simplification and with the inclusion of solvation effects in aniline as solvent has addressed the addition of aniline to ethylene catalyzed by PtBr2/Br−. The resting state of the catalytic cycle is the [PtBr3(C2H4)]− complex (II). A cycle involving aniline activation by N−H oxidative addition was found energetically prohibitive. The operating cycle involves ethylene activation followed by nucleophilic addition of aniline to the coordinated ethylene, intramolecular transfer of the ammonium proton to the metal center to generate a 5-coordinate (16-electron) PtIV−H intermediate, and final reductive elimination of the PhNHEt product. Several low-energy ethylene complexes, namely trans- and cis-PtBr2(C2H4)(PhNH2) (IV and V) and trans- and cis-PtBr2(C2H4)2 (VII and VIII) are susceptible to aniline nucleophilic addition to generate zwitterionic intermediates. However, only [PtBr3CH2CH2NH2Ph]− (IX) derived from PhNH2 addition to II is the productive intermediate. It easily transfers a proton to the Pt atom to yield [PtHBr3(CH2CH2NHPh)]− (XX), which leads to rate-determining C−H reductive elimination through transition state TS(XX−L) with formation of the σ-complex [PtBr3(κ2:C,H-HCH2CH2NHPh)]− (L), from which the product can be liberated via ligand substitution by a new C2H4 molecule to regenerate II. Saturated (18-electron) PtIV−hydride complexes obtained by ligand addition or by chelation of the aminoalkyl ligand liberate the product through higher-energy pathways. Other pathways starting from the zwitterionic intermediates were also explored (intermolecular N deprotonation followed by C protonation or chelation to produce platina(II)azacyclobutane derivatives; intramolecular proton transfer from N to C, either direct or assisted by an external aniline molecule) but all gave higher-energy intermediates or led to the same rate-determining TS(XX−L).