An Enzymatic Step toward the Energetic Material CL-20 via Laccase-Mediated Oxidation of HBIW to TADBIW (Hexabenzylhexaazaisowurtzitane to Tetraacetyldibenzylhexaazaisowurtzitane)

Abstract

The synthesis of one of the most powerful non-nuclear energetics, CL-20 (HNIW; 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) requires a step that debenzylates the intermediate HBIW (hexabenzylhexaazaisowurtzitane) to TADBIW (tetraacetyldibenzylhexaazaisowurtzitane). In current chemical processes, this transformation requires a palladium catalyst, which is challenging to source and recycle on the industrial scale. Here, we characterize a laccase enzyme in combination with a chemical oxidant to catalyze this step. A commercial Trametes versicolor laccase is tested for its ability to oxidize mediators that, in turn, facilitate the debenzylation of HBIW to form the intermediate DBIW (dibenzylhexaazaisowurtzitane). This intermediate is unstable and is acetylated to TADBIW. We screen six mediators for this reaction and find that ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) is the most efficient mediator. Critically, ABTS generates TADBIW only in the presence of the laccase, at yields of 1.0%, compared to yields of 3.4% from the potassium permanganate positive control alone. High-resolution tandem mass spectrometry supports TADBIW identification, placing both benzyl and acetyl groups on a retained isowurtzitane cage. This work demonstrates the substitution of an inorganic process for a lower-cost, more-sustainable enzymatic one towards an alternative pathway to CL-20.