CHEMICAL TECHNOLOGY Nitration reactions in the manufacture of pharmaceutical intermediates Nitration chemistry provides an excellent tool for accessing complex pharma- ceutical molecules. Dr David Firth, exchem organics he pharmaceutical intermediates sector of of materials ranging from explosives, through the chemical industry uses a diverse range dyestuffs, to active pharmaceutical compounds. Tof materials, manufactured to exacting The formation of the nitronium ion occurs by standards, in order to provide the selection of disproportionation of nitric acid, which is promot- life-enhancing and life-saving medicines that ed by the presence of a second (strong) acid. Most constitute the weapons in the armoury of the commonly, this second acid is either sulphuric acid Ionic nitration modern physician. This short article covers the area or oleum, but may include organic acids such as ... is not of nitration chemistry, as used in the manufacture acetic acid (Table 1). The selection of an of pharmaceutical intermediates; this is a broad appropriate second acid to promote the formation limited to the area and so we can only skim the surface of this of the nitronium ion will be influenced by factors electrophilic fascinating and diverse subject. including the power needed to bring about the nitration, the sensitivity of the substrate molecule Overview of nitration mechanism; to strong (mineral) acids and - of course - health, nucleophilic Nitration is simply defined as the introduction of safety and environmental factors. substitution is the nitro functionality, -NO2, into a molecule - Ionic nitration, though, is not limited to the electrophilic mechanism; nucleophilic well known, most frequently by the electrophilic action of the nitronium ion (NO2+). This type of nitration substitution is well known, although it has although it chemistry has been practised on an industrial scale not been well studied in comparison with its has not been for over 100 years in the manufacture of a variety electrophilic cousin. Among the more commonly well studied in comparison with its electrophilic cousin Table 1. Some examples of electrophilic nitrating agents (1). 132 Innovations in Pharmaceutical Technology CHEMICAL TECHNOLOGY practised nucleophilic nitrations is the substitution Processing of a halide by sodium nitrite. The specialised fine chemicals industry makes Whilst nitration chemistry relies to a large much use of traditional batch reactors for the extent on well practised chemistry, it would manufacture of most of its products (4). Batch be unfair to suggest that there have been no reactors find use in virtually every form of developments in this area. The use of exotic chemistry practised on an industrial scale in the nitration species including dinitrogen pentoxide world today - including nitration. However, no (N O ), together with nitrations using lanthanide 2 5 matter how efficient the process, the flow of triflate catalysts and nitronium tetrafluoroborate, materials through a batch reactor is always and transfer nitrations using alkyl nitrates have disrupted by non-productive activities such as been documented. There has also been research heating, cooling, transfer, isolations and so on. into nitration by radical species derived from the The productive time spent in a batch reactor may, oxides of nitrogen. in fact, be as little as one-third of the total plant The productive The most common form of nitration is that of residence time, representing a significant C-nitration, particularly of aromatic systems. time spent in under-utilisation of what may be a very costly However, it is also possible to carry out derivatisations set of equipment. a batch of oxygen and nitrogen functionalities using However, where the volume of product is large reactor may, nitration chemistry. For example, the formation of or the process is particularly energetic, batch nitrate esters is well documented, and nitroglycerine in fact, be as reactors may not be well suited to manufacture - (used as a vasodilator in the treatment of acute incurring relatively high costs, particularly in terms little as angina, as well as being one of the most famous of the labour associated with operation of even the explosives in the world) is manufactured by the one-third of most advanced plants. At a time when there is a nitration of glycerine using a traditional mixed acid the total plant real drive towards cost-effective plant operation, it reaction. N-nitration is usually followed by comes as no surprise to learn that there is an residence rearrangement of the nitramine intermediate increasing tendency to consider continuous formed by the reaction of the amine substrate with time ... manufacturing. Continuously operated plants are the nitronium ion. not new, and have been utilised within the There are several excellent texts available (1,2,3) petrochemical industry for many years. With the to which the reader is referred for a more in-depth advantages that continuous processing can afford, review of nitration chemistry. Scheme 1. Some synthetic modifications of p-NBA. 134 Innovations in Pharmaceutical Technology CHEMICAL TECHNOLOGY it is starting to attract the attention of the fine chemicals industry. Within the chemical industry, the need for the safe operation of plant is paramount. This is especially so in the case of nitration chemistry, where prolonged residence time can lead to undesirable poly-nitration, competitive reactions (for example, O- versus C- versus N-nitration) and so on, and here continuous processing can afford some distinct advantages over batch manufacture. Consider as an example the manufacture of nitroglycerine, which has been successfully practised by the exchem group for about 100 years. The safety of the process has been radically improved by moving from a batch - to a continuous process. The glycerine and the nitrating acid are mixed by a venturi effect, and the reaction mixture is then allowed to flow along a tube to completion. In this fashion, the residence time for the reaction can be cut by a factor of 50 or more, whilst maintaining a lower inventory within the reactor system. In particular, the use of a tubular reactor, as described above, avoids the use of stirrers and reduces the risk of detonation as a result of shear on the reaction mixture. Another example is provided by the nitration of amides and esters, both of which are particularly prone to competitive side reactions, when the conditions used to bring about the nitration employ acidic media. In particular for esters - such as those of 5-nitro-isophthalic acid (used as intermediates in the manufacture of X-ray contrast media) - the heat liberated during the reaction can lead to competitive hydrolysis of the ester function. This can liberate an alcohol (typically short-chained), which may in turn be prone to O-nitration to form nitrate esters, which are well documented as explosives. Also of potential concern is the formation of NOx from the nitrating medium; this becomes more problematic as the temperature of the reaction mixture is increased. With its ability to act as an oxidant, generation of NOx can accelerate thermal runaway, with the uncontrollable evolution of permanent gas as, for example, nitrogen and oxides of carbon. Indeed, it is this evolution of permanent gas that makes nitrated compounds so useful as high explosives. According to legislation enacted by the British Parliament in 1990, the chemical industry is mandated to employ the Best Available Techniques (BAT) for processes to eliminate waste wherever possible. Where this cannot be achieved, recycling of materials to minimise waste-streams becomes a viable strategy. It is also necessary to give due consideration to the use of energy in the process, and optimisation of the same. Improvement in energy utilisation per unit of production is made possible by the use of a series of heat-exchange systems. In general terms, waste-streams from a Innovations in Pharmaceutical Technology 135 CHEMICAL TECHNOLOGY Scheme 2. Some synthetic modifications of 4-methyl-3-nitroaniline. Consider as an continuous process are easily recycled within the Nitration as a tool in synthesis process, and this can lead to significantly less example the The nitro functionality, introduced via the act of waste-intensive processes. It is also practical to nitration, represents an extremely versatile manufacture of design a process such that the waste-stream may be substituent in itself. Consider as examples the nitroglycerine ... re-utilised for a second and even third reaction. molecules 4-nitrobenzyl alcohol, p-NBA [(1) in The safety of Generally speaking, the inventory of reactants Scheme 1] and 4-methyl-3-nitroaniline [(2) in in a continuous process is lower than in its batch Scheme 2]. the process has reactor analogue. This can present its own p-NBA has two functionalities present, both of been radically advantages in terms of reduced financial risk in the which allow for the preparation of a wide variety of improved by case of a process failure. This effect can be further 1,4-disubstituted molecules by modification of the enhanced by the use of ever more readily available molecule along the oxidation chain (5). Relatively moving from automated monitoring techniques; these may be simple, practical chemistry will allow for the a batch to a employed to improve the process by the use of benzylic alcohol to be oxidised to its benzaldehyde continuous control loops and so on. Data from such (used in the manufacture of chloramphenicol) or automated monitoring systems may be used to further to its corresponding benzoic acid. Simple process form the basis of statistical process control, reduction of the nitro- function will afford the potentially making supply chain management corresponding aminobenzyl alcohol, from which a more cost-effective. starburst of products can be envisaged; these may Of particular concern to the pharmaceutical be accessed by, for example, alkylation, acylation, industry is the requirement for a consistent diazotisation and so on. Partial reduction of the (im)purity profile for a given product.