46355-AC4
Macrocyclic Scavengers of Chloroalkanes
Methylene chloride and 1,2-dichloroethane are two of the major non-fuel products of the petroleum industry. Although extremely useful, these two chloroalkanes are known to be carcinogenic and they can persist in the biosphere for a long time. There is an increasing need for the petroleum industry and its downstream petrochemical partners to invent novel methods of sensing and sequestering these potentially harmful chemicals; however, this is a very challenging task because the chemicals are inherently unreactive and difficult to trap. We have discovered that a simple macrocyclic amine can attack haloalkanes with unprecedented nucleophilic reactivity. For example, the macrocycle reacts with methylene chloride with a half-life of 2 minutes at 25 °C, a rate that is about 50,000 times faster than that observed with structurally related, linear amines. A series of physical organic and supramolecular chemistry indicate that the reaction proceeds through a pre-reaction complex. Likely reasons for the selective enhancement effect are: (a) increased transition-state stabilization due to hydrogen bonding in the macrocyclic pocket, and (b) reduced entropic penalty in the transition-state due to an increased fraction of pre-reaction complexes that are oriented in a near attack conformation. A fluorescent version of the macrocycle was developed and used to monitor the alkylation reaction. Thus, one applied direction of this research is fluorescence detection systems for the presence of these carcinogenic chemicals in the biosphere. Finally, additional chemistry studies have discovered that the macrocycle can also accelerate a mechanistically related elimination reaction, i.e., dehydrohalogenation, of an alkyl halide.
