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Molecules 2016, 21, 24 3 of 11 agent in selective Michael additions and acylations [36]. Specifically, the reaction of primary amines was inhibited in favor of normally less reactive sulfonamides, cyclic secondary amines, or β-ketoesters for the Michael addition and complete inhibition of benzylamine acylation was achieved, effectively favoring alcohol conversion without significant byproduct. Ethier et al. investigated the solvent effect and addition of other bases in addition to DBU on the protection of benzylamines with CO2 [37]. In each case, protection of the amine was achieved in the presence of a competitive reaction and removal of the CO2 protecting group was achieved; however, the reactivity of the de-protected amine was not examined. This work further explores the use of the reversible CO2 chemistry as a unique method of protecting amines during simple coupling reactions. All reactions were carried out with (1) the non-protected amine; (2) the carbamate analogs formed in solution via reaction with gaseous CO2; and (3) with the de-protected amine regenerated through a thermal treatment of the carbamate. Significant differences between this work and those of Peeters et al. and Ethier et al. are the lack of a competing reaction and measurement of reactivity following de-protection. The lack of a competing reaction is significant because it measures the decreased reactivity rather than the competitive reactivity. 2. ResultsandDiscussion 2.1. Synthesis of n-Alkyl, n-Phenyl Urea Urea formation via amine coupling with isocyanates has been well-studied and characterized [39,40]. The urea synthesis in this work showed 95% yield from propylamine, while other amines achieved comparable yields of 85% from hexylamine, 91% from decylamine, and 90% from octadecylamine (Table 1). Once protected in the carbamate form, the reactivity was significantly reduced for the shorter chained amines with an average decrease of 62%. After the heat treatment, de-protection of the propyl, hexyl and decyl amine was successful based on the increase in urea yield to values comparable to the non-protected amine. This demonstrates the reversibility of the carbamate protection and requirement of no pressurization or separate reaction conditions. The observed lower product yields for the de-protected amines are attributed to incomplete de-protection of the carbamates along with a loss of starting material during de-protection due to the inherent volatilities of the propyl and hexylamine. TGA showed carbamate degradation temperatures higher than their amine conjugates, thus resulting in a loss of starting material during thermal amine regeneration process (Figure S6). Decylamine, with its low volatility and ease of carbamate reversal displayed the best results, with a 93% urea yield from the de-protected amine comparable to that of the pure or un-protected amine (91%). The octadecyl carbamate showed the lowest reduction in yield and the poorest recovery in the de-protected amine reactivity. This was not an expected result as a protection/de-protection cycle had been achieved separately for the octadecyl amine. A significant difference was that the octadecyl-carbamate formation resulted in a white precipitate, however the de-protection regenerated the soluble amine analog. We hypothesize that the long alkyl chains and precipitation inhibited the complete formation of the carbamate and resulted in insufficient protection. FTIR of the isolated ureas demonstrated high purity through the absence of carbamate or other reaction byproducts for all three scenarios (non-protected, protected and de-protected). In each FTIR spectra (Figure S1), the characteristic carbonyl (C=O) and secondary amine (N-H) absorbance associated with urea formation are consistent within all isolated products while the C-H (2800–3000 cm ́1) absorption levels correlate very well to the alkyl chain backbone length. The three states of amine (non-protected, protected and de-protected) all produced an isolated urea product displaying similar absorbance bands (Figure S1). This indicates that the products obtained were spectrally identical and not side products due to residual carbamate presence. The urea purity was further determined by 1H-NMR, displaying chemical shifts characteristic to our desired products (Figures S2 and S3). The main differences in spectra were the chemical shifts at 1.3 ppm attributed toPDF Image | Amines vs Nucleophilic Substitution Reversible Reaction CO2
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