Branched Tertiary Amines from Aldehydes and α-Olefins by Combined Multiphase Tandem Reactions

Abstract

This study presents the transformation of olefins to branched amines by combining a hydroformylation/aldol condensation tandem reaction with the reductive amination in a combined multiphase system that can be recycled 9 times. The products are branched amines that are precursors for surfactants. Since the multiphase hydrofomylation/aldol condensation system has already been studied, the first step was to develop the partial hydrogenation of unsaturated aldehydes together with a subsequent reductive amination. The rhodium/phosphine catalyst is immobilized in a polar polyethylene phase which separates from the product phase after the reaction. Reaction and catalyst recycling are demonstrated by the conversion of the C₁₄ -aldehyde 2-pentylnonenal with the dimethylamine surrogate dimethylammonium dimethylcarbamate to the corresponding tertiary amine with yields up to 88 % and an average rhodium leaching of less than 0.1 % per recycling run. Furthermore, the positive influence of a Bronsted acid and carbon monoxide on the selectivity are discussed. Finally, the two PEG based systems have been merged in one recycling approach, by using the product phase of the hydroformylation aldol condensation reaction for the reductive amination reaction. The yields are stable during a nine recycling runs and the leaching low with 0.09 % over the two recycling stages.

Keywords: amination; homogeneous catalysis; rhodium.

Scheme 1

Different routes to branched tertiary amines.

Scheme 2

Reaction network for the hydrogenation / reductive amination tandem reaction of 2‐ethylhexenal. Displayed are the pathways to the desired amine product and the two major side products.

Figure 1

Phase separation after the reaction.

Figure 2

Recycling experiment of the hydrogenation / reductive amination of C14‐Aldol with TsOH as co‐catalyst in PEG. Conditions: 2‐Pentylnonenal (2 mmol, 0.5 mL), DimCarb (1.0 mmol, 0.13 mL), PEG‐200 (0.4 mL), [Rh(acac)(CO)2] (0.5 mol %), SulfoXantphos (0.75 mol %), TsOH (30 mol %), H2 (50 bar), CO (0.5 bar), 125 °C, 3 h. From run 3 on (black arrow), the time on the vacuum after each run was increased from 1 min to 1 h to remove water.

Figure 3

CO‐pressure influence on the reductive amination of 2‐pentylnonenal with DimCarb. Conditions: 2‐Pentylnonenal (2 mmol, 0.5 mL), DimCarb (1.0 mmol, 0.13 mL), PEG‐200 (0.4 mL), [Rh(acac)(CO)₂] (0.5 mol %), SulfoXantphos (0.75 mol %), 4‐Cl‐PhSO₃H (30 mol %), H₂ (50 bar), CO, 125 °C, 3 h.

Figure 4

Substrate Scope. Yields were determined by GC analysis.

Figure 5

Schematic representation of a general cascade reaction with two multiphase partial reactions. The product phase is shown in orange and the catalyst phases in blue. S=substrate(s), P=product(s), C=catalyst(s).

Figure 6

(a) Combination of hydroformylation/aldol condensation tandem reaction and hydrogenation/reductive amination tandem reaction. (b) Recycling experiment of the hydroformylation/aldol condensation of 1‐hexene. Conditions: substrate: 1‐Hexene (8 mmol, 1.0 mL), catalyst phase: PEG‐200 (0.4 mL), [Rh(acac)(CO)₂] (0.1 mol %), SulfoXantphos (0.2 mol %), Cs₂CO₃ (2.5 mol %), additional parameter: H₂/CO (30 bar each), 5 h at 100 °C then 16 h at 125 °C. Others=hexane and heptanol. (c) Recycling experiment of the hydrogenation/reductive amination using the product mixture from the hydroformylation/aldol condensation. as substrate. Substrate solution: 0.5 ml of the product solution of the corresponding run from experiment (b), catalyst phase: PEG‐200 (0.4 mL), [Rh(acac)(CO)₂] (0.01 mmol), SulfoXantphos (0.015 mmol), 4‐Cl‐PhSO₃H (0.6 mmol), additional parameter: H₂ (50 bar), 125 °C, 3 h. Others=hexane, heptanol.