Hydroaminomethylation hydrogenation catalyst

In hydroformylation reactions coupled with a hydrogenation step (e.g., hydroaminomethylation. Section 5.4), the simultaneous use of Rh(acac)(CO)2 and [Rh(COD)2l BF, together with a single modifying ligand can be advantageous [66]. The first rhodium complex forms the active hydroformylation catalyst A, whereas the second is a precursor of the hydrogenation catalyst B. Both are in equilibrium (Scheme 1.27). 

Secondary and tertiary amines can be obtained if the hydroformylation of olefins is conducted in the presence of primary and secondary amines under elevated hydrogen partial pressures. Here the rhodium catalyst is involved in both steps, the hydroformylation of an olefin as well as the hydrogenation of the imine or enamine resulting from a condensation of the oxo-aldehyde with the amine (Scheme 14). This combination of hydroformylation and reductive amination is also known as hydroaminomethylation and has been applied to the synthesis of various substrates of pharmaceutical interest [55-57] as well as to the synthesis of macrocycles [60-63] and dendrimers [64,65]. 

Hydroaminomethylation is a simple, efficient and atom-economic method to synthesize various amines. This one-pot reaction consists of three consecutive steps in the first step a hydroformylation of an olefin is performed followed by the reaction of the resulting aldehyde with a primary or secondary amine to give the corresponding enamine or imine. Lastly, this intermediate is hydrogenated to the desired secondary or tertiary amine (Fig. 11) [33-39]. In most cases rhodium salts or complexes are used as the homogeneous catalyst in the hydroaminomethylation. 

Hydroaminomethylation is a promising reaction to functionalize unsaturated compounds with an amino group [13, 48, 49], The tandem reaction was discovered by Reppe in 1949 and has been further developed in recent years by Eilbracht and Beller. Hydroaminomethylation consists of three consecutive reactions which are carried out in the same reaction vessel [48], The first reaction is hydroformylation which is followed by the condensation with an amine. Hydrogenation of the generated enamine/imine to the amine is the last step. The conditions for hydroaminomethylation are related to the hydroformylation reaction but are not similar due to the two other reactions. The reaction is called an auto-tandem reaction because two of the three reactions need the same catalyst [9] (Scheme 16). 

The second double bond is not hydroaminomethylated because of the mild conditions at 80°C and 80 bar and due to steric hindrance [51]. Graebin describes seven different products of the hydroaminomethylation of limonene [52]. The reaction time was reduced to 10 h by optimization of the catalyst including stepwise hydroformylation for 5 h and hydrogenation with pure hydrogen gas for 5 h. Isomerization was reduced by adding triphenylphosphine as ligand. 

Karakhanov and coworkers [41] incorporated hydroformylation with methyl-formate in a hydroaminomethylation tandem reaction by using a dual metal catalyst consisting of homogeneous Ru and Rh complexes (Scheme 3.19). In contrast to rhodium, the ruthenium catalyst was used in excess. The transformation is exemplified in the scheme with 1-pentene as substrate. Also, other terminal and internal olefins could be converted with dimethylamine into the corresponding amines. High temperatures enhanced the formation of the desired amines and lowered the degree of olefin hydrogenation. The reaction proceeded also well. 

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