Olefin complexes hydroaminomethylations

Abstract Aldehydes obtained from olefins under hydroformylation conditions can be converted to more complex reaction products in one-pot reaction sequences. These involve heterofunctionalization of aldehydes to form acetals, aminals, imines and enamines, including reduction products of the latter in an overall hydroaminomethylation. Furthermore, numerous conversions of oxo aldehydes with additional C.C-bond formation are conceivable such as aldol reactions, allylations, carbonyl olefinations, ene reactions and electrophilic aromatic substitutions, including Fischer indole syntheses. 

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. 

Originally discovered by Reppe and co-workers, hydroaminomethylation of olefins has attracted attention as an economical, one-pot synthesis of amines. Alper and co-workers have demonstrated that zwitterionic rhodium complex [Rh (q -cyclooctadiene)(q -CgH5BPh3)-] 74 is an excellent catalyst for hydroaminomethylation of phenylalkenes 75 to afford the corresponding branched methylated amines 76 with high regioselectivity (Scheme 22) [42]. 

Many reactions can be conducted in tandem with hydroformylation, but the most extensively developed process is the combination of hydroformylation and reductive ami-nation. The combination of tttese reactions catalyzed by a single complex is called hydroaminomethylation and is shown in Equation 17.18. These reactions formally add tihe C-H bond of an aminomethyl group aaoss an olefin. Hydroaminomethylation reactions were discovered in the mid-twentieth century, but have been developed into more practical processes in recent years. These reactions rely on a complex that can simultaneously catalyze the hydroformylation of an aUcene and the reduction of an imine or enamine. 

Specific examples of the hydroaminomethylations of olefins with secondary amines are shown in Equations 17.19-17.21. Cyclic and acyclic secondary amines occur in high yield with linear-to-branched ratios exceeding 50 to 1 in most cases when catalyzed by the rhodium complex generated from [Rh(COD)JBF and xantphos. The reaction of pen-tene with piperidine is shown in Equation 17.19. These reactions are also compatible with alcohol (Equation 17.20) and acetal functional groups (Equation 17.21). 

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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