Intermolecular Highly Regioselective Hydroamination of Alkenes with Ligandless Platinum(II) Catalysts in Ionic Solvents:  Activation Role of n-Bu₄PBr

Abstract

The reaction of aniline with 1-hexene catalyzed by PtBr2 (0.3%) in the presence of n-Bu4PBr (65 equiv/Pt) at 150 °C affords the expected hydroamination products with a 95% Markovnikov regioselectivity (N-(2-hexyl)aniline). An activating role of n-Bu4PBr has been evidenced, which is due to the presence of the bromide ions. This accelerating effect is discussed in terms of increased basicity of the platinum center in a zwitterionic intermediate. The ligandless, inexpensive, and nontoxic PtBr2−n-Bu4PBr−H+ catalytic system allows reaching a TON = 243 after 96 h at 150 °C (70% yield, 95% Markovnikov regioselectivity). The same results are obtained for reactions performed in the air.

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

“…Extensive deposition of metallic Pt was observed at the end of the reaction, which has been reported for the aniline addition experiments. [4][5][6] Surprisingly, ethylamine, the formal product of dehydroamination, was also observed as a coproduct in nonnegligible amounts.…”

“…In the last six years, a number of papers have appeared on the hydroamination of ethylene and other α-olefins catalyzed by Pt-based systems. These include the addition of substituted anilines to ethylene [4] and 1-hexene [5] catalyzed by PtBr 2 /nBu 4 PBr (I) and our recent extension to aqueous media (PtBr 2 / NaBr (aq) , IЈ), [6] the addition of carboxamides to RHC=CH 2 (R = H, Me) catalyzed by [PtCl 2 (H 2 C=CH 2 )] 2 /PPh 3 (II) [7] and that of sulfonamides or anilines to more strained olefins catalyzed by (COD)Pt(OTf) 2 (III). [8] For each type of catalyst the turnover number (TON) or product yield was shown to increase as the amine basicity decreases and no activity was found when the conjugate acid of the amine has a pK a value above a certain cut-off value.…”

Electrophilic Activation of Alkenes by Platinum(II): So Much More Than a Slow Version of Palladium(II)

“…The platinum system was effective for the high-temperature hydroamination of norbornene, [80] ethylene, [81] and 1-hexene. [82] The selective Markovnikov hydroamination of 1-hexene [Eq. (19)] is significant, as rapid Pt-catalyzed olefin isomerization is common, and was observed by Tilley and coworkers.…”

Platinum‐Catalyzed Intermolecular Hydroamination of Alkenes: Halide‐Anion‐Promoted Catalysis

“…Hence, the composition of the alkene fraction was examined at the end of several runs ( 4 PBr system is a poor isomerization catalyst (irrespective of the reaction pathway). [58] The promoting effect of nBu 4 PBr for the [PtBr 2 ]-catalyzed hydroamination of alkenes was thus confirmed, but remained to be explained. The study of the hydroamination of norbornene with aniline (vide supra) indicates that for a given cationic moiety, ionic solvents associated with a bromide ion give a higher catalytic efficiency than those associated with a chloride ion.…”