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Article Reduced Reactivity of Amines against Nucleophilic Substitution via Reversible Reaction with Carbon Dioxide Fiaz S. Mohammed and Christopher L. Kitchens * Received: 6 November 2015 ; Accepted: 13 December 2015 ; Published: 23 December 2015 Academic Editor: Jason P. Hallett Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, USA; fiaz.mohammed@chbe.gatech.edu * Correspondence: ckitche@clemson.edu; Tel.: +1-864-656-2131 Abstract: The reversible reaction of carbon dioxide (CO2) with primary amines to form alkyl-ammonium carbamates is demonstrated in this work to reduce amine reactivity against nucleophilic substitution reactions with benzophenone and phenyl isocyanate. The reversible formation of carbamates has been recently exploited for a number of unique applications including the formation of reversible ionic liquids and surfactants. For these applications, reduced reactivity of the carbamate is imperative, particularly for applications in reactions and separations. In this work, carbamate formation resulted in a 67% reduction in yield for urea synthesis and 55% reduction for imine synthesis. Furthermore, the amine reactivity can be recovered upon reversal of the carbamate reaction, demonstrating reversibility. The strong nucleophilic properties of amines often require protection/de-protection schemes during bi-functional coupling reactions. This typically requires three separate reaction steps to achieve a single transformation, which is the motivation behind Green Chemistry Principle #8: Reduce Derivatives. Based upon the reduced reactivity, there is potential to employ the reversible carbamate reaction as an alternative method for amine protection in the presence of competing reactions. For the context of this work, CO2 is envisioned as a green protecting agent to suppress formation of n-phenyl benzophenoneimine and various n-phenyl–n-alky ureas. Keywords: carbamate; urea; benzophenoneimine; green chemistry 1. Introduction It has been previously reported that primary and secondary amines react with CO2 to produce thermally reversible alkylammonium alkylcarbamates as seen in Scheme 1 [1–7]. The CO2 reacts with one primary amine to form carbamic acid, which then further reacts with free amine to form the carbamate product. This mechanism has been extensively studied in literature with a variety of applications in different fields [8–12]. For example, the reversible amine to carbamate transition exhibits a significant change in the ionic character from non-ionic to ionic upon CO2 reaction, which has been employed in systems referred to as reversible ionic liquids, reversible surfactants, and switchable solvents [8,13–19]. These systems have included CO2 reaction with guanidines and amidines reaction with alcohols as two-component systems and primary or secondary amines as single component systems. In these systems, the solvent property changes can be quite appreciable, changing in character from that of hydrophilic methanol to that of hydrophobic chloroform [8,14,15]. The potential implications of such tunable solvent properties are widespread in the realm of reactions and separations, where a change in solvent properties can induce a complete phase separation and facile recovery of products, catalysts, or unreacted reagents. This was initially demonstrated by Jessop et al. with the reversible protonation of DBU (1,8-diazabicyclo-[5.4.0]undec-7-ene) in the presence of an Molecules 2016, 21, 24; doi:10.3390/molecules21010024 www.mdpi.com/journal/moleculesPDF Image | Amines vs Nucleophilic Substitution Reversible Reaction CO2
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