Arenediazonium Cations Research Articles

Synthesis of a library of 2-aryl-2H-tetrazole-5-carboxamides for photoaffinity labeling of aminergic G-protein coupled receptors

A four-step approach to 2-aryl-2H-tetrazole-5-carboxamides bearing GPCR-focused N-substituted piperazine residues involves ‘SAFE’ diazotransfer onto 1-(piperazin-1-yl)butane-1,3-diones followed by [3+2] cycloaddition of arenediazonium cations at the intermediate ααα-diazo acetamides. The compounds prepared are intended for photoaffinity labeling of aminergic G-protein coupled receptors.

Heck arylation of acyclic olefins employing arenediazonium salts and chiral N,N ligands: new mechanistic insights from quantum-chemical calculations

A quantum-chemical study based on the Kohn–Sham density functional theory (DFT) has been performed in order to investigate the asymmetric Heck–Matsuda (HM) reactions involving a chiral N,N ligand pyrimidine-oxazoline. The oxidative addition (OA) of arenediazonium cation was analyzed to provide further insights in the unexpected high reactivity of N,N-ligated palladium(0)-based catalysts observed in these HM processes. DFT calculations suggest that the ionogenic nature of metal center promoted by the arenediazonium cation addition increases the electronic reactivity of palladium(0) toward the donor N,N ligand. The concerted and multi-step mechanisms were computed for the OA step, in which the cationic aryl-palladium(II) complex formation was revealed as the thermodynamic driving force for this process. The importance of conformational analyses was highlighted, particularly when an acyclic alkenol olefin is taken as substrate, since its absence could lead to misleading predictions about the selectivity of these HM systems. While the enantioselectivity is controlled by the combination of steric and dispersion interactions between ligand and substrate, the regioselectivity is mostly oriented by the electronic nature of olefinic carbons. DFT predictions on the selectivity were shown to be in quantitative agreement with experimental results.

ТРИКОМПОНЕНТНІ РЕАКЦІЇ НЕНАСИЧЕНИХ СПОЛУК З СОЛЯМИ АРЕНДІАЗОНІЮ І НЕЙТРАЛЬНИМИ НУКЛЕОФІЛАМИ. АРИЛСУЛЬФОНІЛЮВАННЯ

The results of recent achievements on the interaction of arenediazonium salts with unsaturated compounds in the presence of neutral nucleophiles are summarized. New examples of multicomponent transformations with the participation of such neutral nucleophiles as CO (arylcarbonylation), NO (arylnitrosylation), aceto¬nitrile (aminoarylation), SO2 (arylsulfonylation) and others are given. These reactions can be applied to alkenes, alkynes, aromatic compounds. Mild reaction conditions allow the use of reagents with different functional groups. Reactions of this type open up the possibility of one-step production of complex poly¬functional compounds. Catalytic systems are quite diverse for these transformations: it is catalysis involving transition metals, platinum group metals. Prospects for the use of arenediazonium salts in multicomponent transfor¬ma-tions according to the concepts of «green» chemistry are outlined – it is photoinitiation by visible and ultraviolet radiation, acid-base catalysis. The role of catalysis in the process, the role of complex intermediates and reaction mechanisms are analyzed. For most processes, the SET reaction mechanism is implemented through the formation of an alkene intermediate ion radical, the stability of which depends on the nature of the substituent near the double bond. Particular attention is paid to arylsulfonylation reactions, because the arylsulfonyl group is one of the many important biologically active molecules. Arylsulfonylation reactions of alkenes with the participation of transition metals or under conditions of metal-free catalysis are considered. Examples of arylsulfonylation of the C=C bond using sulfinic acids, their salts and hydrazides are given. An available method for producing functionalized sulfones is the multicomponent interaction of arenediazonium salts, alkenes and SO2. The arylsulfonylation reaction occurs as a series of successive reactions involving the generation of a catalyst, the decomposition of arenediazonium cations, the addition of an aryl group, a nucleophile to a multiple bond, and the formation of the final products. The use of functionalized alkenes allows to obtain functionalized arylsulfones in one step. This functionalization expands the scope of use of arylsulfones, in particular, for studies of biological activity. The progress made in the development of effective strategies for the production of arylsulfones opens new opportunities for further research.

DFT perspective on the selectivity and mechanism of ligand-free Heck reaction involving allylic esters and arenediazonium salts

A density functional theory (DFT) study has been carried out with respect to the cationic and neutral mechanism of ligand-free Heck-Matsuda cross-coupling reaction involving allylic esters and arenediazonium salts. Full Heck-Matsuda (HM) catalytic cycles were explored in the presence of different possible combinations of ligands based on previous mass-spectrometry studies. DFT calculations were performed to examine the olefinic substrate influences on the regioselectivity of this ligand-free Heck reaction. A multi-step mechanism was revealed for the oxidative addition of palladium (0) center to carbon-nitrogen bond of arenediazonium cation. The cationic migratory insertion pathway is favored by a chelated coordination involving the palladium center and the allylic ester. In contrast, for neutral complexes, a bidentate coordination of acetate ligand is the favored pathway. Regarding the hydride elimination step, the resulting free energy profile is quite flat, revealing that the Heck product and the hydride complex are easily formed for both cationic and neutral cycles. The reductive elimination step assisted by acetate is a very exothermic process, going through small relative reaction barriers. These results constitute new insights to a better understanding of the selectivity and the mechanism of ligand-free Heck-Matsuda reactions.

First Heck Reaction with Arenediazonium Cations with Recovery of Pd‐Triolefinic Macrocyclic Catalyst.

AbstractFor Abstract see ChemInform Abstract in Full Text.

First Heck reaction with arenediazonium cations with recovery of Pd-triolefinic macrocyclic catalyst.

The air- and moisture-stable phosphine-free palladium(0) complex 1 is a highly active and recoverable catalyst for Heck olefination of aryl diazonium tetrafluoroborates. The reactions were performed under aerobic conditions at room temperature to give the coupling products in excellent yields. [reaction: see text]

Ab initio calculation of inner-sphere reorganization energies of arenediazonium ion couples.

The geometries of a series of substituted arenediazonium cations (p-NO2, p-CN, p-Cl, p-F, p-H, m-CH3, p-CH3, p-OH, p-OCH3, p-NH2) and the corresponding diazenyl radicals were optimized at the HF/6-31G, MP2/6-31G, B3LYP/6-31G, B3LYP/TZP, B3PW91/TZP, and CASSCF/6-31G levels of theory. Inner-sphere reorganization energies for the single electron-transfer reaction between the species were computed from the optimized geometries according to the NCG method and compared to experimental values determined by Doyle et al. All levels of theory predicted a CNN bond angle of 180 degrees in the cation. A bent neutral diazenyl radical was predicted at all levels of theory excepting B3LYP/TZP and B3PW91/TZP for the p-Cl-substituted compound. Inner-sphere reorganization energies determined at the HF, MP2, and CASSCF levels of theory correlated poorly with both experimental results and calculated geometries. Density functional methods correlated best with the experimental values, with B3LYP/6-31G yielding the most promising results, although the ROHF/6-31G survey also showed some promise. B3LYP/6-31G calculations correctly predicted the order of the inner-sphere reorganization energies for the series, excluding the halogen-substituted compounds, with values ranging from 42.8 kcal x mol(-1) for the p-NO2-substituted species to 55.1 kcal x mol(-1) for NH2. The magnitudes of these energies were lower than the experimental by a factor of 2. For the specific cases examined, the closed-shell cation geometries showed the expected geometry about the CNN bond, with variations in the CN and NN bond lengths correlating with the electron-donating/withdrawing capacity of the substituent. As predicted by Doyle et al., a large geometry change was observed upon reduction. The neutral diazenyl radicals showed a nominal CNN bond angle of 120 degrees and variations in the CN and NN bond lengths also correlated with the electron-donating/withdrawing capacity of the substituent. Changes in theta(CNN) and r(CN) both correlated well with calculated lambda(inner). The key parameters influencing inner-sphere reorganization energy were the CN and NN bond lengths and the CNN bond angle. This influence is explained qualitatively via resonance models produced from NRT analysis and is related to the amount of CN double bond character. Based on these observations, B3LYP/6-31G calculations are clearly the most amenable for calculating inner-sphere reorganization energies for the single electron-transfer reaction between cation/neutral arenediazonium ion couples.

Competitive homolytic and heterolytic dediazoniation mechanisms: Rate constants and product distribution of methoxy-, hydroxy-, and hydro-dediazoniation of 3- and 4-methylbenzenediazonium salts in acidic MeOH/H2O mixtures

The rates and product distribution for methoxy-, hydroxy- and hydro-dediazoniation and the rate constants for disappearance of 3- and 4-methylbenzenediazonium tetrafluoroborate in acidic MeOH/H2O mixtures, in the presence and absence of electrolytes like HCl, NaCl, and CuCl2, are reported. Data were obtained by using a combination of VIS-UV and HPLC techniques. The kinetics and product distributions are completely consistent with competitive homolytic and heterolytic mechanisms, the heterolytic one being predominant at any solvent composition. Heterolytic data are in agreement with the predictions of a DN + AN mechanism; that is, rate determining formation of an aryl cation that reacts immediately with available nucleophiles. Selectivity values, determined from product yields, are low and independent of solvent composition. Product formation is discussed in terms of a preassociation step between aryl cations and the nucleophile, which does not account for much of the trapping, and a nucleophilic attack on a “free” arenediazonium cation. Activation parameters were also determined at 99.5% MeOH: enthalpies of activation are high and entropies of activation are positive, and they are similar to those reported for pure water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 210–220, 2000

X-ray Crystal Structures and the Facile Oxidative (Au−C) Cleavage of the Dimethylaurate(I) and Tetramethylaurate(III) Homologues

Dimethylaurate(I) has been prepared as the crystalline tetrabutylammonium salt for comparison with the known tetramethylaurate(III) analogue. The linear structure of dimethylaurate(I) and the square-planar structure of tetramethylaurate(III) have both been confirmed by X-ray crystallography. One-electron oxidation of dimethylaurate(I) by either ferrocenium or arenediazonium cations produces the metastable dimethylgold(II) intermediate, which can be trapped as the paramagnetic 9,10-phenanthrenequinone (PQ) adduct. Otherwise, dimethylgold(II) is subject to rapid reductive elimination of ethane and affords metallic gold (mirror). The analogous oxidation of tetramethylaurate(III) by ferrocenium, arenediazonium, or nitrosonium cations also proceeds via electron transfer to generate the putative tetramethylgold(IV) intermediate. The highly unstable (CH3)4AuIV spontaneously undergoes homolytic cleavage to produce methyl radical and the coordinately unsaturated trimethylgold(III), which can be intercepted by adde...

Formation of η2-aroyl, η1-halogenocarbyne or sterically crowded aryldiazenide ligands in the reactions of ring-substituted tricarbonyl[hydrotris(pyrazolyl)borato]-molybdate and -tungstate anions with arenediazonium cations and related oxidants

Although the hydrotris(pyrazolyl)borato complex [Mo{HB(pz)3}(CO)3]– reacted with 3- or 4-substituted arenediazonium cations [R′N2]+ yielding carbonyl-substitution (i.e. aryldiazenido) products [Mo{HB(pz)3}(CO)2(N2R′)]. reaction of the methylsubstituted analogue [ML*(CO)3]–[L*= tris(3,5-dimethylpyrazolyl)hydroborate; M = Mo or W] led, via oxidative formation of aryl radicals and [ML*(CO)3]˙, to η2-aroyl complexes [ML*(CO)2(η2-COR′)][R′= C6H4X-4 (X = NO2, CN, COMe, CF3, H, Me, OMe or NMe2) or C6H4X-3 (X = NO2 or OMe)] in acetonitrile or to the halogenocarbyne complexes [ML*(CO)2(CX)](X = Cl, Br or I) in the presence of the halogenoalkanes CH2Cl2 or CHX3(X = Br or I). The complex [MoL*(CO)3]– reacted similarly with diphenyliodonium or triphenylsulfonium cations, but in the latter case anion–cation redox is very slow in the dark but rapid upon irradiation with sunlight. Comparison of these results with those obtained for [ML*(CO)3]– analogues with different substituents in the pyrazolyl rings demonstrates that oxidation of the former by arenediazonium cations occurs in response to the steric rather than the electronic effect of the 3-methyl substituents. However further steric crowding in either the hydrotris(pyrazolyl)borate ligand or the diazonium cation promotes a reversion to the carbonyl-substitution pathway. A mechanism to account for these observations is proposed. Attempts to extend the chlorocarbyne synthesis to systems other than [ML*(CO)3]– met with only limited success. Spectroscopic data for the new complexes are reported and discussed. Two aryldiazenido complexes, [MoL*(CO)2(N2R′)](R′= 2,6-Me2C6H3 or 2,3-dimethyl-5-oxo-1-phenyl-3-pyrazolin-4-yl) have been characterised by single-crystal X-ray diffraction studies and are found to differ in the manner in which the aryldiazenide ligand accommodates to steric crowding in the molybdenum co-ordination sphere.

Kinetic studies of the azo coupling reaction between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and substituted benzenediazonium salts

Kinetic studies have been carried out on the azo coupling reactions between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and seven substituted benzenediazonium salts. The effects of acidity and temperature upon the rate have been investigated and activation parameters determined, and the involvement of deprotonated substrate in the rate determining step demonstrated. Acid dissociation equilibrium of the substrate is characterized by pKaHA= 5.07 ± 0.16 at 293 K and µ= 0.2. Close activation enthalpies, between 48 and 59 kJ mol–1 and negative entropies between –22 and –53 J mol–1 K–1 have been found, depending on the electronic effects of substituents on the benzene nucleus of the diazonium salts. A Hammett relationship log10k2= 1.0 ± 0.1 +(3.3 ± 0.3)σ holds for coupling reactions. The greater the electron-withdrawing effect, the faster is the coupling rate. 1-(2-Pyrimidinyl)pyrazolin-5-one behaves similarly to 1-arylpyrazolin-5-one and phenols in reactions with arenediazonium cations.

Mechanisms in the reaction of arenediazonium cations with 2-nitropropanide anion

Lithium 2-nitropropan-2-ide reacts, in MeOH solvent, with benzenediazonium and p-methoxybenzenediazonium cations to give diazo-coupled products, but with p-nitrobenzenediazonium cation by competing electron transfer and diazo-coupling reactions.

Structure/activity investigations in eight arylalkyltriazenes comparison of chemical stability, mode of decomposition, and SCE induction in Chinese hamster V79-E cells

A series of seven 1-aryl-3.3-dialkyltriazenes, including 1-phenyl-3.3-dimethyltriazene (DMPT), 1-phenyl-3.3-di-(trideuteromethyl)-triazene (DMPT-ds), 1-p-methylphenyl-3.3-dimethyltriazene (DMpMPT), 1-p-nitrophenyl-3.3-dimethyltriazene (DMpNPT), 1-phenyl-3.3-diethyltriazene (DEPT), 1-phenyl-3.3-di-n-propyltriazene (DnPrPT) and 1-phenyl-3.3-diisopropyltriazene (DiPrPT) and 1.3-diphenyl-3-methyltriazene (DPMT), was synthesized and characterized by UV/VIS, IR and 1H-NMR spectroscopy. Chemical half-life was determined in phosphate buffer at 37 degrees using UV/VIS spectroscopy. With the exception of DMpNPT, which was stable, the triazenes underwent pH-dependent hydrolytic decomposition (acid catalysis). By means of UV/VIS spectra, TLC and HPLC, phenol, aniline and secondary azocoupling products were identified after complete hydrolytic cleavage of the parent compounds. Pathways of spontaneous hydrolysis are proposed and discussed. Genotoxic activity of the triazenes was assayed by measurement of sister chromatid exchanges (SCE) in V79-E cells without and with rat liver S9 mix as an exogenous metabolizing system. In the direct SCE assay (without S9 mix), all triazenes except DMpNPT exerted a toxic action (cell cycle delay) in a narrow concentration range between no effect and overt cytotoxicity. This non-specific toxicity depended on the pH of the incubation system and was inversely proportional to chemical half-life. The toxicity of these agents is most likely due to the arenediazonium cation which is a relatively stable intermediate. In a sublethal concentration range most triazeness induced significant increases of SCE rates. These are interpreted as an indirect consequence of cytotoxicity. Upon metabolic activation, the compounds were genotoxic in a dose-dependent fashion. Their SCE-inducing capacity depended on the nature of the alkylating species generated, i.e., the alkyldiazonium cation, and on chemical stability. Surprisingly, no deuterium isotope effect was observed in DMPT-d6. The order of genotoxic activity among the aryldialkyltriazenes was DMpNPT much greater than DMPT = DMPT-ds greater than DMpMPT much greater than DEPT greater than DnPrPT greater than or equal to DiPrPT. DPMT was a marginal SCE inducer but very toxic upon metabolic activation. As monooxygenation of DPMT, like spontaneous hydrolysis, should generate a phenyldiazonium cation, the results suggest that arylation of DNA causes a very low SCE induction, if any.

Reactivity of new osmium dihydrides with arenediazonium cations: preparation and properties of bis(aryldiazene) and mono(aryldiazenido) complexes. A comparison with analogous iron and ruthenium derivatives

Hydride complexes [OsH2L4][L = P(OEt)3 or PPh(OEt)2] were prepared. Their reactions with arenediazonium cations led to the synthesis of [Os(RNNH)2L4][BPh4]2 and [OsH(RNNH)L4]BPh4(R = Ph, 4-MeC6H4, or 4-MeOC6H4) complexes which were characterized by 1H and 31P n.m.r. data (with 15N isotopic substitution). The compounds [Os(4-MeC6H4NNH)(4-MeC6H4NC)L4][BPh4]2 were also prepared. The reaction of the bis(aryldiazene) derivatives with NEt3 led to new five-co-ordinate aryldiazenido complexes [Os(RN2)L4]BPh4(R = Ph or 4-MeC6H4). Protonation of the diazenido compounds with CF3CO2H and HBF4 gave the six-co-ordinate mono(diazene)[Os(CF3CO2)(RNNH)L4]BPh4 and the five-co-ordinate [Os(RNNH)L4]BPh4(R = 4-MeC6H4) derivatives, respectively. Infrared and 1H and 31P n.m.r. data for these complexes are reported.

Synthesis, structure-activity relationships and a reaction mechanism for mutagenic N-nitroso derivatives of glycosylamines and Amadori compounds—Model substances for N-nitrosated early Maillard reaction products

A series of nine glycosylamines and an Amadori compound were synthesized, together with their N-nitroso derivatives. Their structures were established by physico-chemical and spectroscopic data and elemental analyses. The N-nitroso compounds were further characterized by denitrosation with hydrogen bromide-acetic acid, followed by detection of the liberated NO by a chemiluminescence detector. N-Nitroso derivatives of N- p-nitrophenyl/ p-methylphenyl/ p-carboxyphenyl pentopyranosylamines, N-p- methylphenyl-I-deoxy- d-fructosylamine (the Amadori compound) and N-3- thylindole- d-xylopyranosylamine were shown to be direct-acting mutagens in Salmonella typhimurium TA100. The activity of some of the compounds was similar to that of N-ethyl- N-nitrosourea. Their mutagenic activity was shown to depend on the structure of the amine and the sugar moieties and to require the presence of free hydroxyl groups in the sugar. The mutagenicity of N-nitrosoglycosylamines was attributed to their hydrolysis to arenediazonium cations. The formation of these compounds was detected by azo-coupling with N-ethyl-1-naphthylamine, using spectrophotometric and mass spectrometric analyses. These data implicate arene(alkyl)diazonium cations as the ultimate mutagens of N-nitrosoglycosylamines (and possibly of N-nitroso Amadori compounds), a little-explored class of N-nitroso compounds that may be formed in vivo.

ChemInform Abstract: Applications of Phase Transfer Catalysis to Arenediazonium Cation Chemistry

Arenediazonium cations have been known for many years but the synthetic utility has been diminished by their lack of stability and by their proclivity for undergoing sundry side reactions. Arenediazonium BF4 and PF6 salts are generally stable solids, but are relatively insoluble in nonpolar organic media. Crown complexation and other forms of phase transfer catalysis increase solubility and allow numerous synthetic transformations to be conducted in good yield. The yield improvements result from a reduction in competing side reactions rather than any fundamental change in mechanism. The reactions to which the phase transfer technique is successfully applied are diazo coupling, reduction, halogenation, arylation and cyclization. In the latter case, the use of potassium superoxide is sometimes a more efficacious reagent than potassium acetate. The phase transfer approach can be used in the high yield synthesis of arenediazosulfones and arenediazocyanides. The latter undergo cycloaddition with many dienes to yield pyridazine derivatives.

Investigation of the decomposition of 1,3-diaryl-5-(3-chloro-2-quinoxalyl)-formazans by PMR and MASS spectrometry

In contrast to triarylformazans, 1,3-diaryl-5-(3-chloro-2-quinoxalyl)formazans are unstable in ordinary organic solvents. When they are heated in chloroform, they undergo acidic cleavage, which leads to the formation of 3-chloro-2-quinoxalylhydrazones of p-substituted benzaldehydes and arenediazonium cations. These compounds, as a result of a redox reaction with the participation of the solvent, are converted to 4-chloro-1-(4-Y-phenyl)-1,2,4-triazolo[4,3-a]quinoxalines, substituted benzenes, nitrogen, and hydrogen chloride. The formation of the latter transforms the entire decomposition process into an autocatalytic process. Effects of chemical polarization of the nuclei (CPN), which unambiguously indicate the intermediate formation of diazoaryl radicals during the process, are observed in the PMR spectra of the final products. Such CPN effects, which were also observed in dimethyl sulfoxide (DMSO) and glacial acetic acid, indicate a process involving the oxidative formation of annelated triazoles from α-azahetarylhydrazones via a radical pathway within a solvent “cage.”

Applications of Phase Transfer Catalysis to Arenediazonium Cation Chemistry

AbstractArenediazonium cations have been known for many years but the synthetic utility has been diminished by their lack of stability and by their proclivity for undergoing sundry side reactions. Arenediazonium BF4 and PF6 salts are generally stable solids, but are relatively insoluble in nonpolar organic media. Crown complexation and other forms of phase transfer catalysis increase solubility and allow numerous synthetic transformations to be conducted in good yield. The yield improvements result from a reduction in competing side reactions rather than any fundamental change in mechanism. The reactions to which the phase transfer technique is successfully applied are diazo coupling, reduction, halogenation, arylation and cyclization. In the latter case, the use of potassium superoxide is sometimes a more efficacious reagent than potassium acetate. The phase transfer approach can be used in the high yield synthesis of arenediazosulfones and arenediazocyanides. The latter undergo cycloaddition with many dienes to yield pyridazine derivatives.

Photoinduzierte Elektronenübertragungsreaktionen zwischen aromatischen Diazoniumkationen und Anionen

Photoinduced Electron Transfer between Arenediazonium Cations and AnionsAnions X⊖ can act as donors in electron transfer reactions to photoexcited arenediazonium ions. The yield of the arene formed in this reaction increases with decreasing electrochemical oxidation potential E of X⊖ (X⊖ = BPh4⊖, Br⊖, HOOCCOO⊖, Cl⊖, BF4⊖). The oxidized donor X· (Cl·, ·COO⊖) and aryl radicals Ar· benig intermediates of the reductive dediazoniation of ArN2⊖ can be detected by spin trapping with nitrosodurene (ND) and phenyl‐tert‐butylnitrone (PBN). A solvent effect in the electron transfer reaction is interpreted in terms of ion pairing, where the electron transfer is favored by preorientation and a short distance between the electron donor and the acceptor previous to excitation of ArN2⊖ by light.

The origin of the remarkable stability of the 1H-3,5-dimethylpyrazole-4-diazonium cation: an X-ray crystallographic and MNDO theoretical investigation

The unusual stability 1H-3,5-dimethylpyrazole-4-diazonium chloride has been investigated by a combination of X-ray crystallography and MNDO calculations. Crystals of the diazonium chloride are orthorhombic, space group Pnma, with four molecules in a unit cell of dimensions a= 7.964(1), b= 6.308(2), c= 15.060(3)A. The structure was solved by direct methods and refined by full-matrix least-squares calculations to R= 0.032 for 539 reflections with I > 3σ(I) measured by diffractometer. The cation lies on a crystallographic mirror plane and the diazonium moiety is essentially linear [C(4)–N(7)–N(8) 177.5(4)°]. While the diazonium N–N distance [N(7)–N(8) 1.096(4)A] is typical of that in arenediazonium cations, the C(4)–N(7) distance is short [1.351(4)A] suggesting that the stability of the cation is due to a contribution from a diazo resonance form in which this bond acquires pπ–pπ character. The MNDO-optimised geometry of the cation agrees well with the experimental data. Calculated energy profiles for the dissociation of the heterocyclic diazonium cation and the benzenediazonium cation to the corresponding carbonium ion and N2 show that the strength of the ipso-C–N2 bond is the major distinguishing feature. The difference in bond strength arises from greater availability of π-electron density on the ipso carbon of the pyrazolyl cation and hence greater ipso-C→N2π* back-donation. Both carbonium ions have large π density on the ipso-carbon but in the phenyl cation this is predominantly located in the lowest energy π orbital while in the pyrazolyl cation it is located in the HOMOs and hence available for interaction with the diazonium function.