Abstract
Pyrazolium-3-sulfonates and pyrazolium-4-sulfonates possessing the iminiomethanesulfonate and the 3-iminio-pro-1-ene-2-sulfonate partial structure are prepared. Formally, they represent sulfur analogs of cross-conjugated (CCMB) and pseudo-cross-conjugated heterocyclic mesomeric betaines (PCCMB) and sulfur trioxide adducts of abnormal N-heterocyclic carbenes (aNHC) and N-heterocyclic carbenes (NHC), respectively. The theoretical as well as experimental vibrational spectra of these two classes are compared. Results of calculations of HOMO/LUMO profiles and an X-ray single crystal structure analysis are presented.
Highlights
The iminiomethanesulfonate group I and the 3-iminio-prop-1-ene-2-sulfonate group II (Figure 1) are partial structures of hitherto almost overlooked classes of heterocyclic mesomeric betaines
It belongs to the class of N-heterocyclic carbenes (NHC) whereas its isomer, pyrazol-4-ylidene has been described as remote N-heterocyclic carbene,[7] cyclic allene[8] or aromatic zwitterion.[9]
Partial structure VI is characteristic for cross-conjugated mesomeric betaines (CCMB) such as pyrazolium-4-carboxylates, imidazolium
Summary
The iminiomethanesulfonate group I and the 3-iminio-prop-1-ene-2-sulfonate group II (Figure 1) are partial structures of hitherto almost overlooked classes of heterocyclic mesomeric betaines. The partial structure V (Figure 3) can be identified in pseudo-cross-conjugated mesomeric betaines (PCCMB)[11] such as pyrazolium-3carboxylates, imidazolium-2-carboxylates, and 1-methylpyridinium-2-carboxylates which are precursors of N-heterocyclic carbenes.[12] They are formed on cleavage of the union bond between positive and negative portions of the molecule. Partial structure VI is characteristic for cross-conjugated mesomeric betaines (CCMB) such as pyrazolium-4-carboxylates, imidazolium- Imidazolium-2-sulfonate 7 is an example of a cyclic mesomeric betaine possessing partial structure I (Scheme 2).
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