Safe Synthesis of Nitramines intermediates found in carbon capture compounds

Subjects: Analytical Chemistry View times: 178
Created by: Yngve Stenstrøm

Three different protocols for the syntheses of hydroxyalkylnitramines are presented and compared. Safety issues regarding the synthesis of nitramines are also discussed.

Carbon capture of today has been implemented as an important tool for controlling the greenhouse effect on our planet. Several technologies for carbon capture and storage (CCS) have been developed, but amine-based technologies have been found to be the most promising [1]; especially the aminoalkanols, such as 2-amino-1-ethanol (MEA; 1) and 2-amino-2-methyl-1-propanol (AMP; 2) [2]. The use of MEA for such a purpose has been implemented on a large scale by Statoil in the North Sea, since the mid-1990s [3]. One major drawback associated with the amine-based CCS is the formation of harmful byproducts: i.e., the reaction of the amino group to give nitramines and nitrosamines [4]. This has spurred a new interest in these compounds [5,6,7,8,9,10]. A recent review summarizes the hazard assessment of these as byproducts of amine-based CCS [7]: nitramines are less carcinogenic than the corresponding nitrosamine [11] and formed in lower concentration in CO2 capture systems [3,12], but they are more persistent in the environment due to the lack of sunlight photolysis [9,13]. However, the conclusion is that more studies are needed.
Although nitramines have been known for a long time, they have been made mostly for the purpose of making explosives (e.g., 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), nitroguanidine and tetryl) and rocket fuels [14,15]. On the other hand, their use in synthesis has been exploited only to a very limited extent. The best example of this is the use of primary nitramines in the Mannich reaction [16].
To learn more about these byproducts, we started a new project aiming for a practical and safe synthesis of pure nitramines. The general method for their syntheses has been to add 70%–100% nitric acid to the corresponding amine. This is a general and fairly efficient method. However, several problems may be encountered with this procedure, such as a lack of chemoselectivity. The use of the obnoxious and highly corrosive concentrated or pure nitric acid is by itself a hazard. Special precautions must be taken to avoid over-nitration, which will pose an increased risk for explosions [17].
One way of avoiding the nitration of the alcohol would, of course, be protection. A simple and efficient protection method for an alcohol is transformation to an ester, usually by adding acetyl chloride or acetic anhydride. However, most of these methods will also give the corresponding amide when amino groups are present, primary amines being especially reactive. This will immediately pose two problems: the amide will be less reactive than the amine, and fairly harsh conditions are needed for the deprotection of both the ester and nitrated amide.
The challenge will be to find protection groups that are both stable to the nitration conditions and should either be chemoselective (i.e., not react with the amine) or the protected amine would have to be reactive towards the nitration agent.
Furthermore, the protection group should be removable without affecting the nitramine and to avoid the formation of metal salts of the nitramines, which are quite acidic (pKa ≈ 6). Metal salts of these are known to be even more prone to detonation, and as such, special precautions have to be taken before handling them [18].
See the full paper: Molecules 2016, 21(12), 1738; https://doi.org/10.3390/molecules21121738