Platinum-Catalyzed Ethylene Hydroamination with Aniline: Synthesis, Characterization, and Studies of Intermediates

Abstract

Starting from either K2PtCl4 or K[PtCl3(C2H4)]•H2O (Zeise's salt), complexes (nBu4P)2[PtBr4] (1), nBu4P[PtBr3(C2H4)] (2), nBu4P[PtBr3(PhNH2)] (3), trans-[PtBr2(C2H4)-(PhNH2)] (4), cis-[PtBr2(C2H4)(PhNH2)] (5), and cis-[PtBr2(PhNH2)2] (6) have been obtained by efficient one-pot procedures. All have been fully characterized by microanalysis (C, H, N), multinuclear NMR spectrometry ( 1 H, 13 C, 195 Pt), UV-visible spectroscopy, and single crystal Xray diffraction. Compound 1 slowly loses Brin solution to yield (nBu4P)2[Pt2Br6] (1'), which has also been characterized crystallographically. The relative stability of the various compounds has been probed experimentally by NMR studies in several solvents and computationally by gas phase geometry optimizations followed by C-PCM calculations of the solvation effects in dichloromethane and aniline. The calculations also included the bis(ethylene) complexes [PtBr2(C2H4)2] in the trans (two different conformations 7 and 7') and cis (8) configurations.The solution experiments gave no evidence for a nucleophilic attack of aniline onto coordinated ethylene under mild conditions (T up to 68°C), setting a lower limit of 29 kcal mol -1 for the activation barrier of this process. Therefore, the relative energies computed for the other compounds suggest that all ethylene-containing complexes (2, 4, 5, 7 and 8) are viable candidates for the key nucleophilic addition step of the PtBr2-catalyzed ethylene hydroamination by aniline. Use of the isolated complexes 2, 4 or 5 in combination with nBu4Br as precatalysts for the ethylene hydroamination by aniline yields similar catalytic activities.

Transition‐Metal‐Catalyzed Hydroamination Reactions

“…Considering the important commercial potential of such ethylene hydroamination catalysts, extensive work in recent years has focused on developing a mechanistic understanding of the most promising Pt salt systems, with an eye toward catalyst development to further enhance reactivity, selectivity, and catalyst longevity. Stoichiometric investigations showed that K 2 PtCl 4 or Zeise's salt (K[PtCl 3 (C 2 H 4 )]) could be used to generate a variety of Pt ethylene and amine complexes, thereby illustrating the complexity of the catalytic system in hand [166]. Further investigations using computational approaches were helpful in probing reactivity of high-energy transition states and intermediates to better assemble an informed mechanistic hypothesis [167,168].…”

Modeling the platinum-catalyzed intermolecular hydroamination of ethylene: The nucleophilic addition of HNEt2 to coordinated ethylene in trans-PtBr2(C2H4)(HNEt2)

“…Complex transPtBr 2 (C 2 H 4 )(HNEt 2 ) (1) was synthesized in 75% yield starting from Zeise's salt by adapting the methodology previously described for the preparation of related complex trans-PtBr 2 (C 2 H 4 )(PhNH 2 ), 1a [7].…”

“…The same methodology adopted for the preparation of cis-PtBr 2 (C 2 H 4 )(PhNH 2 ) [7] (ethylene addition to the aminetribromoplatinate precursor) cannot be applied here because the product cannot be kinetically stabilized by precipitation and thus Table 1 Crystal data and structure refinement for compounds 1 and 2. led again to the thermodynamically preferred 1 as a sole product under these conditions. An alternative synthetic pathway, described for a related Cl-NHMe 2 complex [13,41], involves the irradiation of trans-PtCl 2 (C 2 H 4 )(HNMe 2 ) to afford trans-Pt 2 Cl 4 (HNMe 2 ) 2 , from which cis-PtCl 2 (C 2 H 4 )(HNMe 2 ) is obtained by addition of ethylene.…”

“…We have recently reported experimental and theoretical studies on the hydroamination of ethylene by aniline [7,8,10,21]. The catalytic system consisting of PtX 2 /X À shows the best activity for X ¼ Br.…”

Reactions of Diethylamine and Ethylene Catalyzed by PtII or Pt0 – Transalkylation vs. Hydroamination

“…[7] Catalytic system III has been shown to be inactive when using aniline (pK a = 4.63). [8] We have recently explored the details of the PtBr 2 /Br -catalyzed addition of aniline to ethylene experimentally and computationally by isolating a number of low-energy intermediates and off-loop species and studying their solution equilibria [9] and by carrying out a computational investigation of the catalytic cycle, [10][11] of basic amines to Pt-coordinated olefins. [13][14][15] This observation suggests that the catalyst deactivation process is triggered by deprotonation of the intermediate zwitterionic species, and this hypothesis has been confirmed by new catalytic tests: the addition of strong bases (KOH, Et 2 NH, NEt 3 or 1,8-diazabicyclo [5.4.0]undec-7-ene) to the catalytic medium completely suppresses the production of PhNHEt, and the Pt II catalyst is completely converted into metallic platinum.…”