As we teach in our 2-day course “Industrial Phase-Transfer Catalysis,” there are three major reasons that phase-transfer catalysis works so much better than non-PTC systems for nucleophilic substitutions using inorganic anions and they are [1] the inorganic nucleophilic anion is actually dissolved in the bulk organic phase by the phase-transfer catalyst where it reacts with the substrate already soluble in the organic phase [2] a looser more reactive ion pair is formed between the inorganic nucleophilic anion and the large quat cation than between the inorganic nucleophilic anion and an alkali metal [3] the anionic nucleophilic anion has little to no hydration when transferred into a non-polar organic phase.
However, in some cases there is no aqueous phase and the advantageous effects are limited to ion pair looseness and solubility.
In US Patent 9,540,455 issued this month, there is described a nucleophilic substitution between ionic azide and allyl bromide groups attached to a butyl rubber. We examined the extensive data in Table 1 in that patent and we make several observations.
First, when sodium azide is used as the azide source, a combination of THF and DMF are used as the solvent. When tetrabutylammonium azide is used as the azide source, DMF is not needed. We may assume that this is due to solubility requirements that are more challenging for the sodium azide.
Secondly, we observe that 6-10 equivalents of NaN3 are used to achieve 76% conversion in 4 days reaction time versus 1.1-1.4 equivalents of tetrabutylammonium azide in 1 day reaction time to achieve 100% conversion.
Clearly, tetrabutylammonium azide greatly outperforms sodium azide in this nucleophilic substitution.
It appears that the polymer is dissolved in the solvent system, so apparently these are homogeneous systems. If so, then we speculate that the loose ion pair of tetrabutylammonium azide is responsible for the greatly increased reactivity versus sodium azide. We also speculate that the sodium azide system requires the DMF since it may not be fully soluble in THF alone.
At a minimum, we expect that the loose ion par of tetrabutylammonium azide is playing a key role in achieving high reactivity in this system.
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