5‐Lithio‐2H‐tetrazolum Carbenoids: NMR Spectroscopic Detection and Reactions with Nitrogen Electrophiles

Abstract

2H-Tetrazolium ions of type 2 are part of the tetrazolium/ formazanide redox system, which is distinguished by an excellent charge stabilization within both the cationic and the anionic oxidation state. As a consequence, the 2H-tetrazolio substituent should facilitate redox umpolung."] We have examined nucleophilic reagents for the introduction of the 2H-tetrazolio group into electrophilic substrates ['] and report trapping reactions of the 5-lithiotetrazolium ion 3 and its spectroscopic detection by ' 3C NMR spectroscopy. 2a-cArN::pY'H NMR spectra show that tetrazolium salts 2, dissolved in tetrahydrofuran, are deprotonated quantitatively at -90 "C by n-or rert-butyllithium. The 5-lithiotetrazolium salts 3 thus formed can be considered as car be no id^.^^] At -100°C they are sufficiently stable for their 13C NMR spectra to be recorded.[41 The 13C NMR spectrum of 2 b had no signals between 6 = 165 and 250, whereas, after lithiation, a single unstructured multiplet appeared at 6 = 202.4 and vanished when the solution was heated up to room temperature. IGLO/DZ//6-31G* calculations[51 indicate that this multiplet is the 13C NMR signal of the solvated carbenoid's tetrazolium ring.16] According to the calculations, this shift of 48 ppm to lower field upon lithiation of 2 b can be explained mainly by the decrease of n-electron density at the tetrazolium C atom and by the weakening of the two N-C 0 bonds to this atom. ['] The new tetrazolium salts 6 and 9 b thus formed had not been accessible before, among other reasons because the reactivity'"] of 1,5-diarylformazans towards 5 is too low. On the other hand, this sole method to derivatize tetrazolium ions of type 2 failed when strongly oxidizing electrophiles like chlorine (or bromine) were used. In this case not the desired product but ion 2 as its chloride (bromide) was isolated in a yield of 67% (79%). We assume that 3 was oxidized by the halogen to the corresponding radical cation which abstracted an H atom from the solvent THF to re-form 2. ['']The azide 9 b was identified spectros~opically['~~ and by its reduction with iodide to amine 10 b and subsequent diazotization to the known['4] tetrazolium-5-diazotate 8 b. Reduction of 9 b with triphenylphosphane/water led to fission of the C-N bond. In contrast to Balli's azidinium salts,''s1 9 b did not react with azide ions. Thus, from the intensely yellow, shock-insensitive tetraphenylborate salt 9b, the corresponding colorless chloride or azide salt could be prepared with tetrabutylammonium chloride or azide, respectively. Experimental Procedure 9b: To Zb (4.64 mmol) dissolved in 50 mL of THF, n-butyllithium (5.1 mmol, 1.6 molar in hexane) and then tosyl azide (10 mmol) were added dropwise at -90°C. The initially dark brown solution turned dark red within a few minutes. After warming to 20°C over 10 h the solvent was evaporated. The green brown residue, which contained a small amount of 7b according to mass spectrometry, waschromatographed with dichloromethane/methanol (1 : 0 to 10 : 1) on silica gel. From acetonitr...

Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHCTransition‐Metal Bond Properties

“…[11] However, 1 b has never been synthesised, and 1 d is highly unstable and was isolated at 173 K as a lithium adduct only. [12] In this series, there are both normal (1 a and 1 b) and mesoionic (1 c and 1 d) carbenes, which are related by simply changing the substitution pattern without modifications of the ring atoms. Because mesoionic NHCs have attracted considerable attention, series 1 offers a unique opportunity to study the electronic structure of both mesoionic and normal carbenes in detail, in order to shed light on differences as well as similarities between these two classes of NHCs.…”

“…In the tetrazolin‐ylidene series 1 a – 1 d , four isomers are possible 11. However, 1 b has never been synthesised, and 1 d is highly unstable and was isolated at 173 K as a lithium adduct only 12. In this series, there are both normal ( 1 a and 1 b ) and mesoionic ( 1 c and 1 d ) carbenes, which are related by simply changing the substitution pattern without modifications of the ring atoms.…”

Unsymmetrical Dicarbenes Based onN-Heterocyclic/Mesoionic Carbene Frameworks: A Stepwise Metalation Strategy for the Generation of a Dicarbene-Bridged Mixed-Metal Pd/Rh Complex

“…These “wrong-way” carbenes have been referred to as NHCs as a consequence of their lineage; however, since no reasonable resonance forms containing a carbene can be drawn for the free ligands without additional charges, , they have been described as “abnormal” NHCs ( a NHCs). ,− The fact that these compounds require mesoionic resonance forms to represent the carbene character (i.e., a lone pair on carbon) led Bertrand to suggest that they be grouped into a category known as mesoionic carbenes (MICs) . Subsequently, Bertrand et al isolated a variety of free MICs, such as a NHCs, and “remote” NHCs ( r NHCs, in which the stabilizing heteroatoms are located at positions β to the carbene as opposed to the α position). − Other research groups have focused on other MICs, such as 2,3-substituted tetrazol-3-ylidenes, 2,4-substituted tetrazol-5-ylidenes, − or 1,2,3-triazol-5-ylidenes. − …”