The potential of hydroaminomethylation : directing the cascade

Amino compounds are important building blocks or end products in a broad range of durable and consumer goods in everyday life such as polymers, airbags, textiles, insecticides, and pharmaceuticals. Classical syntheses of amines often lead to large amounts of waste, mainly inorganic salts. One of the most promising new reactions for the production of amines in terms of atom-efficiency, activity, selectivity, and applicability is the hydroaminomethylation of alkenes in which water is the only side product. Especially the possibility to synthesise primary amines atom-efficiently from cheap alkene feedstocks and ammonia by hydroaminomethylation makes this an interesting reaction from an industrial point of view. Although the hydroaminomethylation has been discovered already in 1949 by Reppe at BASF, most research with respect to this reaction has been performed during the last 15 years. In Chapter 1, the most relevant and interesting literature with respect to the hydroaminomethylation reaction, is reviewed. Chapter 2 deals with catalyst recycling in a biphasic ionic liquid system. Hydroaminomethylation reactions were performed successfully in an imidazolium-based ionic liquid using a rhodium/Sulfoxantphos system by reacting piperidine with different n-alkenes, affording yields higher than 95% of the resulting amine with turnover frequencies of up to 8400 h-1, along with high regioselectivity for the linear amines with l/b ratios up to 78. Additionally, facile quantitative catalyst recovery was accomplished and recycling of the catalyst and product separation were achieved by a fast phase separation after the reaction. The product distribution was monitored in time at different temperatures both in an organic solvent and in the ionic liquid in order to investigate and compare the course of the formation of (side) products and intermediates in these reactions. Furthermore, it was shown that the nature of the Rh-precatalyst has a profound effect on the activity and selectivity. Protic organic solvents and ionic liquids containing a C-H acidic bond in the imidazolium part have a beneficial effect on the hydrogenation activity of the catalyst systems. Chapter 3 is dedicated to the very fast and selective hydroaminomethylation with a novel class of ligands. In order to increase the activity and to maintain a good selectivity in the hydroaminomethylation reaction in comparison to Rh/phosphine-catalysed systems, a new p-acidic ligand, the bis-[(dipyrrolyl)phosphino]xanthene, was synthesised. In combination with rhodium, this ligand leads to outstanding activities and selectivities with turnover frequencies of 6200 h-1 and very high l/b ratios exceeding 200. Furthermore, it was shown that the pKa value of the alcohol used in the solvent mixture has a profound effect on the performance of the catalytic systems. Acidic media enhance the activity, while less acidic media increase the regio- and chemoselectivity, as well as the degree of double bond isomerisation. Chapter 4 describes the Rh-catalysed hydroaminomethylation of internal alkenes towards linear amines is described using amino-functionalised ligands. Bulky and rigid substituents were introduced and the ligand backbone was functionalised with a (bisindolyl) phosphine moiety in order to increase the regioselectivity in this process. However, bis-[(dipyrrolyl)phosphino]xanthene, introduced in Chapter 3, again turned out to be the best performing ligand in combination with rhodium. Although the reaction is slower than in case of n-alkenes, catalyst activities are still reasonably high. The influence of catalyst preformation, reaction temperature, solvent mixture, and syngas ratio are described. Furthermore, the effect of adding a monodentate phosphorus ligand (phosphines or phosphites) to the reaction mixture was investigated. Interestingly, the regioselectivity could be increased considerably by addition of triphenylphosphine to the catalyst mixture, which can be explained by changing the isomerisation rate related to ß-hydrogen elimination in this particular way. Chapter 5 involves the coordination chemistry of the novel xanthene-based aminofunctionalised ligands, which were discussed in Chapters 3 and 4, to rhodium and platinum. In combination with rhodium, these compounds display interesting catalytic results in the hydroaminomethylation reaction. In order to clarify their structure/performance relationship, the coordination behaviour was investigated. The structural properties of the ligands were studied by NMR spectroscopy of the corresponding rhodium and platinum complexes, while the electronic properties were examined by studying the IR frequencies of the CO stretch vibrations in the particular rhodium-carbonyl complexes. For two ligands, the corresponding selenides were synthesised. The NMR coupling constant JSe-P can be used as a measure for the s-donor ability of a ligand. Furthermore, the coordination behaviour of the ligands was investigated by high pressure NMR and IR spectroscopic measurements under actual hydroformylation reaction conditions. The ligands have been compared to the diphosphine ligand Xantphos, which performs very well in regioselective hydroformylations. An X-ray crystal structure was determined for a rhodium complex with Xantphos. Although the hydroaminomethylation reaction is a promising and atom-efficient alternative for the classical production process towards amines, this reaction has not been applied on a large scale in industry to date. On the other hand, in recent (patent) literature, more and more publications concerning this interesting reaction can be found. The near absence of hydroaminomethylation in industry might be explained by the fact that most publications mention rhodium, which is a very expensive metal. Moreover, no chemo- and regioselective synthesis of linear primary amines via hydroaminomethylation with NH3 has been reported up to now. Most probably, this reaction will first be applied in fine chemical or pharmaceutical industry, since smaller product volumes and higher added value are common practice in these industries. For bulk chemical application, this reaction needs further optimisation for which intensive and challenging research is necessary. Chapter 6 deals with these possibilities and the future of hydroaminomethylation. Hydroaminomethylation with protected amines and the opportunities of primary amine protection by using ammonium carbamate as a combined substrate/dynamic protection group has been presented.