Ring Enlargement of Three-Membered Heterocycles by Treatment with In Situ Formed Tricyanomethane

Although the chemistry of elusive tricyanomethane (cyanoform) has been studied during a period of more than 150 years, this compound has very rarely been utilized in the synthesis or modification of heterocycles. Three-membered heterocycles, such as epoxides, thiirane, aziridines, or 2H-azirines, are now treated with tricyanomethane, which is generated in situ by heating azidomethylidene-malonodinitrile in tetrahydrofuran at 45 °C or by adding sulfuric acid to potassium tricyanomethanide. This leads to ring expansion with formation of 2-(dicyanomethylidene)oxazolidine derivatives or creation of the corresponding thiazolidine, imidazolidine, or imidazoline compounds and opens up a new access to these push–pull-substituted olefinic products. The regio- and stereochemistry of the ring-enlargement processes are discussed, and the proposed reaction mechanisms were confirmed by using ¹⁵N-labeled substrates. It turns out that different mechanisms are operating; however, tricyanomethanide is always acting as a nitrogen-centered nucleophile, which is quite unusual if compared to other reactions of this species.     

Tricyanomethane and Its Ketenimine Tautomer: Generation from Different Precursors and Analysis in Solution,Argon Matrix,and as a Single Crystal

Solutions of azidomethylidenemalononitrile were photolyzed at low temperatures to produce the corresponding 2H‐azirine and tricyanomethane, which were analyzed by low‐temperature NMR spectroscopy. The latter product was also observed after short thermolysis of the azide precursor in solution whereas irradiation of the azide isolated in an argon matrix did not lead to tricyanomethane, but to unequivocal detection of the tautomeric ketenimine by IR spectroscopy for the first time. When the long‐known “aquoethereal” greenish phase generated from potassium tricyanomethanide, dilute sulfuric acid, and diethyl ether was rapidly evaporated and sublimed, a mixture of hydronium tricyanomethanide and tricyanomethane was formed instead of the previously claimed ketenimine tautomer. Under special conditions of sublimation, single crystals of tricyanomethane could be isolated, which enabled the analysis of the molecular structure by X‐ray diffraction.     

The Existence of Tricyanomethane

Calcium tricyanomethanide reacts with hydrogen fluoride under formation of tricyanomethane and Ca(HF₂)₂. Tricyanomethane is stable below ?40 °C and was characterized by IR, Raman, and NMR spectroscopy. The vibrational spectra were compared to the quantum‐chemical frequencies at the PBE1PBE/6‐311G(3df,3dp) level of theory and confirm the predicted C_3v symmetry of the molecule with regular C? H (109.8 pm), C? C (146.7 pm), and C?N (114.7 pm) bonds.     

The Search for Tricyanomethane (Cyanoform)

Although listed in organic chemistry textbooks as one of the strongest carbon acids, and in spite of more than a hundred years of attempts to prepare the compound, tricyanomethane (cyanoform) has resisted isolation and characterization, either as the carbon‐acid 1 or as the dicyanoketenimine tautomer 2 . Only in the vapor phase at very low pressure has the compound been identified from its microwave spectrum. Here we review and partially repeat the preparative work. With the aid of spectroscopic and diffraction methods (including powder diffraction) we have identified some of the products obtained as: hydronium tricyanomethanide ( 3 ), (Z)‐3‐amino‐2‐cyano‐3‐iminoacrylimide ( 4 ), a co‐crystal of 4 with sulfuric acid (or corresponding iminium salt), and an addition product of 2 with hydrochloric acid ( 5 / 6 ). Quantum‐mechanical calculations at the MP2/6‐311++g(2d,2p) level have been made to assess the relative energies of some of the molecules involved.