Computational Prediction of Structure and Catalytic Activity of New Organic Superacids

A Unity through Knowledge Fund project for young researchers and professionals

co-financed by the APO Ltd. Zagreb

SCIENTIFIC COLLABORATIONS

Ruđer Bošković Institute

Zagreb, CROATIA

Nat. Inst. of Chemistry

Ljubljana, SLOVENIA

University of Bielefeld

Bielefeld, GERMANY

University of Tartu

Tartu, ESTONIA

N.D.Zelinsky Institute

Moscow, RUSSIA

University of Sussex

Brighton, U.K.

During realization of this project several important and fruitful collaborations were initiated, which already provided some scientific results. Furthermore, all of these direct contacts with eminent scientists abroad guarantee that this project will continue to live even beyond its initial lifetime. Specifically, some portions of the work were done in collaboration with the following scientists:

¨ Prof. Yury Tomilov, a synthetic organic chemists at the Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences in Moscow. In their recent publication (Tetrahedron 2008, 64, 10201–10206) the group of Prof. Tomilov reported on experimental synthesis of the heptamethoxycarbonyl derivative of cycloheptatriene (5). The latter compound is of a great interest to the goals of this project as it consists of a conjugated cyclic organic skeleton polysubstituted by many electron withdrawing groups.

Our extensive calculations revealed that molecule 5 is very acidic having deprotonation enthalpy in between HNO3 and HI. Prof. Tomilov determined its pKa value of 7.7 in DMSO, which makes it one of the strongest organic C–H acids known in the literature. Synthesis of some of the cyano derivatives of molecule 5 is attempted, and the most likely candidate is heksacyano–methoxycarbonyl derivative, which should be around 25 orders of magnitude stronger in acidity than 5.

 

¨ Prof. Martyn Coles, a synthetic inorganic/organometallic chemist at the Department of Chemistry, University of Sussex in Brighton, UK. This combined computational/experimental collaboration revealed bis-guanidine molecule 6 as one of the strongest bases in the gas–phase and in acetonitrile. In addition, difference electron density map of 6H+ generated from high resolution X-ray diffraction studies at 110 K give the first direct experimental evidence for proton transfer in a poly(guanidino) system and partial protonation of the non-protonated guanidine subunit.

¨ Prof. Ivo Leito, employed at the Testing Centre of the University of Tartu in Estonia. Prof. Leito heads one of the leading European research groups specialized in experimental determination of the acidity constants in different solvent. Their expertise is extremely beneficial and highly complementary to the goals of this project involving computational treatment of the acidic compounds.

Prof. Leito’s group performed experimental determination of the acidity constant of conjugate acid of superbasic bis-guanidine 6, which revealed excellent agreement with our theoretically predicted value.

¨ Prof. Dietmar Kuck, working at the Faculty of Chemistry, University of Bielefeld, Germany. Prof Kuck has a long–standing experience in synthesizing polycyclic organic hydrocarbons bearing many cyano or other electron–withdrawing groups. During this project a joint work on some of his compounds and promising candidates for superacids was initiated and is still ongoing. Some of the results are presented below (numerical values indicate the gas-phase deprotonation enthalpies in kcal/mol).

 

¨ Assist. Prof. Janez Mavri, who is the head of the Laboratory for Biocomputing and Bioinformatics at the National Institute of Chemistry in Ljubljana, Slovenia. Prof Mavri is an expert in the dynamics of reactions taking place in the enzyme active sites, dynamics of systems connected with hydrogen bonds and chemical reactivity in condensed phases. Together with Prof. Mavri we started working on the problem of the acid-base properties of some biologically relevant molecules in water as exemplified with the recent study on histamine.

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