engleski

When soft is better than hard: flexible methylene group as a useful linker in designing neutral organic superbases

Brønsted neutral organic bases and superbases are irreplaceable catalysts in chemical synthesis since they have a number of distinct advantages over their ionic inorganic counterparts. Consequently, the design of novel highly basic organic compounds has been the focus of interest of many experimentalists and theoreticians over several decades.[1] Here we report on the synthesis and the computational/spectroscopic characterization of a novel bis-guanidino compound H2C{;hpp};2 (I) that has been isolated as the monocationic [I-H]+ salt.[2] Calculated gas-phase proton affinity, PA(I) = 270.6 kcal/mol, and the pKa in acetonitrile, pKa(I)MeCN = 28.6, both indicate that compound I is a superbase, the latter being in an excellent agreement with experimental pKa of 28.98 ± 0.05. The high basicity of I is attributed to (i) a high intrinsic basicity of the hpp moiety, PA(hpp) = 254.6 kcal/mol, and (ii) the formation of a strong [N–H·····N]+ intramolecular hydrogen bond (IHB) created upon protonation. The mentioned basicity parameters are higher than for most bisguanidino systems previously developed, [1] which were based on rigid frameworks. The flexibility of the methylene group enables the close approach of the two nitrogen atoms, thereby facilitating the formation of a strong, almost linear HB with no barrier to proton transfer. The solid state CPMAS NMR and X-ray data are also presented. Such efficient use of soft linkers in designing organic superbases remains a challenge for future studies.

superbases

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