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Molecules 2016, 21, 24 6 of 11 very well to the percentage conversion obtained via GC and support our initial hypothesis that CO2 is a viable tool for the protection and inhibition of free primary amines to undergo coupling reactions. Table 3. Determined BP conversion from GC analysis compared to BP conversion calculated from IR spectral C=N/C=O absorbance ratio. Sample Non-protected Non-protected Protected Protected Time (h) Calculated IR Conversion (%) Conversion from GC (%) 33 65 18 20 * 3 35 12 55 3 16 12 26 * denotes a value that was estimated using Figure S7. In a separate set of experiments, imine formation was observed to take place without the presence of mechanical stirring (Figure S7). Here, the benzophenone, Ti catalyst and propylamine were combined in 1.5 mL GC vials and monitored in-situ with the absence of any a titanium complex intermediates. The percentage conversion correlated very well despite running the reaction in separate vials with maximum conversions identical to those obtained above in Figure 2. The non-protected reaction reaches a 75% equilibrium conversion after 15 h, while the protected propylamine yields a 25% equilibrium conversion after 36 h. The de-protected sample obtains an equilibrium conversion of 50% after 5 h, a phenomenon that has been consistent throughout this work. Confirmed by FTIR, a distinct reduction in CH2 and CH3 absorption provides evidence to support the loss of starting amine concentration during de-protection. An alternative explanation for the reduction in imine and urea yield using the de-protected amines is the formation of isocyanate side products during the thermal treatment [48]; however, this theory is not strongly supported. In the presence of methanol, CO2 will preferentially react with the alcohol to form an alkylcarbonic acid that then reacts with the amine to form methyl carbamates, as opposed to the alkylammonium carbamates obtained under aprotic conditions [49]. The resulting carbonic structure has a larger energy requirement to cleave the methyl group and release the carbon dioxide. It is hypothesized that the reduced BP conversion for the de-protected reactions could be due to the increased difficulty to reverse the protection as the protic methanol solvent interacts with the carbamate. Lastly, an explanation into the formation of ureas and benzophenoneimine in spite of CO2 induced carbamate protection needs to be discussed. For example, the 25% conversion of BP to the imine does not represent a complete protection of the propyl amine. According to Salmi et al. [45], the reductive amination of BP proceeds through an imine species with no observable titanium complex intermediates. Hence, the complexation of BP with the Ti is not a cause for the observed reduction in BP concentration, and this conversion is attributed to reactivity of the amine with the BP. Furthermore, control experiments in the absence of propylamine showed negligible loss in BP concentration, further supporting a residual amine reactivity under CO2 vs. complexing of BP to Ti as the source of BP conversion. We conclude that the conversion of starting material under protected conditions is primarily due to the equilibrium which exists between the carbamates and the amine (Scheme 1). As the amine is converted to urea/imine, the equilibrium can shift towards the reactants, resulting in more conversion of the starting material. This theory explains the formation of the urea/imine under the carbamate protection mechanism and an investigation into the kinetics of the carbamate reactivity can also provide useful information on the degree of protection. By fully understanding the relationship that exists between carbamates and the amine-CO2 system, we can calculate expected yields of the product based on the equilibrium constant of the carbamate formation.PDF Image | Amines vs Nucleophilic Substitution Reversible Reaction CO2
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