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Reports: UR451677-UR4: Electron Driven Formation of Aromatic and Nonaromatic Substituted Isocyanurates and Isocyanuratocyclophanes from Isocyanates

Steven J. Peters, PhD, Illinois State University

Reduction of Aryl Isocyanate and Triaryl Isocyanurate in Tetrahydrofuran (THF) Previous work has shown that the potassium metal reduction of aryl isocyanates results in the rapid cyclotrimerization and formation of stable triaryl isocyanurate anion radicals. Recently we observed that the addition of 18-crown-6 (a cyclic ether that readily encapsulates K+ ions) alters the outcome of the reduction of the THF/isocyanate solution. Remarkably, the only observed species produced are that of biaryl anion radicals. For example, when a THF solution containing phenyl or para-tolyl isocyanate and a molar excess of 18-crown-6 is exposed to potassium metal the only anion radicals generated are those of biphenyl or 4, 4’-dimethylbiphenyl, respectively. We have also determined that these same results can be obtained if we begin the reduction with the triaryl isocyanurates instead of with the aryl isocyanates. Therefore the first step in the reduction of the aryl isocyanates is the rapid cyclotrimerization to the respective triaryl isocyanurate. Furthermore, the continued addition of electrons (K metal) must result in a reductive decomposition of the isocyanurate ring to facilitate the release of the aromatic rings and the formation of the biaryl anion radical. We are currently performing reduction studies with 13C labeled isocyanates (p-tolyl N13CO and PhN13CO) to gain insight into this decomposition. Observed changes in the 13C carbonyl and 1H NMR resonances, as the THF solution is titrated with electrons, has given some insight into the breakdown products formed from this decomposition. Preliminary results suggests that the isocyanurate opens to generate a triaryl biuret dianion (e.g. [N(Ph)CON(Ph)CON(Ph)]2-) along with the loss of CO. This biuret dianion is the major product formed early on in the reduction. Upon addition of more electrons this biuret dianion is replaced by a new species in solution. Preliminary results indicate this species to be the biuret tetraanion, which eventually forms the biaryl anion radical (e.g., Ph-Ph-.) as the reduction proceeds further. We are now working towards a mechanism to explain the breakdown of this tetraanion and cleavage of the aromatic ipso C-N sigma bonds within the biuret tetraanion with the eventual formation of the carbon-carbon bond associated with the biaryl molecule. The significance of this work is recognizing how electrons can facilitate the decomposition of isocyanurate rings with the eventual formation of biaryl anion radicals. This method could prove to be a simple and straightforward method of producing unique symmetric and nonsymmetric biaryls. 

Synthesis and Reduction of [8]Annulenyl Isocyanate (COTNCO) We are interested in how the reduction chemistry of the isocyanate moiety is impacted with an [8]annulene ring attached. [8]Annulene or cyclooctatetraene (COT) has a high electron affinity where upon reduction the ring exhibits aromatic character. As a result, the COTNCO system should also display much greater solution electron affinity when compared to the arylNCO systems (e.g. PhNCO) we have previously explored. There is great interest to see how the reduction chemistry will differ with COTNCO, and specifically to see how the chemistry of the NCO group is impacted. Since COTNCO is not available, its synthesis was necessary. After the formation of COTBr starting from COT, a Grignard synthesis was performed with Mg metal. The resulting COTMgBr solution was quenched with dry ice and upon acidification gave the carboxylic acid, COTCOOH. The acid was reacted with thionyl chloride followed by NaN3, resulting in formation of the acyl azide, COTCON3. Upon heating this azide it is converted to COTNCO. <span" roman"="Roman"" new="New">Room temperature reductions of COTNCO in hexamethylphosphoramide (HMPA) with potassium or sodium metals gave no observable EPR signal. This was rather surprising since a number of monosubstituted COT anion radicals have been previously generated in HMPA. However, the low temperature reduction of COTNCO in a THF solution has resulted in the formation of a stable anion radical. Computer simulation of the EPR spectrum reveals that the unpaired electron is coupled to 7 hydrogen atoms and to one nitrogen atom obtained, which indicates the electron is localized within a COT ring. However, these EPR results alone are inconclusive as to the exact anion radical present in solution. (Both the COTNCO and the tri-COT isocyanurate anion radicals would exhibit electron coupling to 7 Hs and 1 N with the electron localized in the COT.) To solve this conundrum, the THF solution containing the anion radical was oxidized with iodine. The major product isolated was the tri-COT isocyanurate. Furthermore, when this authentic isocyanurate compound was re-reduced under the same conditions the resulting EPR spectrum was identical to that obtained from reduction of the COTNCO monomer. Two important findings come from these results. 1) The anion radical of the isocyanate was not observed and so there must be sufficient electron charge residing within the NCO moiety to facilitate rapid trimerization. 2) With the aryl isocyanurate anion radicals we found the electron to be localized within the isocyanurate ring and not one of the aryl groups, but with the tri-COT isocyanurate system the [8]annulenyl ring has a much greater affinity for the electron than the heterocyclic ring attached. We are currently preparing a manuscript for submission describing these results.

Impact on my Students The research going being conducted has had a significant positive impact on my students. Mark Servos (B.S. May 2014) was awarded a prestigious internship at Argonne National Laboratory this past summer due in part to his contribution on the aryl isocyanurate project. He is now a MS graduate student in our department and will continue working with me. Two of my other students will graduate in December 2014 and both are making plans to enter into graduate programs.

Impact on my Career This past year, I was awarded a sabbatical leave for the spring 2015 semester. This leave will give me additional time to complete and submit manuscripts for publication on many of the projects outlined in this ACS-PRF grant. Notably, the contributions made during this sabbatical will greatly enhance my research portfolio as I plan to go for promotion to full professor in fall 2015.