Nitro and nitroso transformations in superacids

Novel transformations involving the nitro and nitroso groups in superacid media are discussed and summarized. NO₂-diprotonation is a key reaction for nitro PAHs, forming N,N-dihydroxyiminium–arenium dications whereas nitrosoarenes are N,O-diprotonated to form hydroxyiminium–arenium dications. In nitropyrenes a facile cyclization involving the nitro group leads to five- and/or six-membered ring heterocyclic cations. Nitro cyclization also occurs in nitroalkylbenzenes but at higher temperatures. Multinuclear NMR data for the resulting persistent NO₂-diprotonated dications and their cyclized analogs are gathered and the structural requirements to bring about nitro cyclization are assessed. Conjugated nitroalkenes such as nitrostyrene and nitroethene are NO₂-diprotonated to form hydroxyiminium–carbenium dications. β-Heteroatom-substituted nitroethylenes are C,O-diprotonated and subsequently form hydroxynitrilium ions. Nitronate salts and unsubstituted nitroalkanes are activated in superacids via their protonated nitronic acid. β-ethoxycarbonyl-substituted nitroalkanes are activated via the O,O-diprotonated aci-nitro species with further O-protonation to give dioxonium–carbenium trications or via hydroxynitrilium ions. The NMR characteristics for the dihydroxyiminium–carbenium dications and hydroxynitrilium ions from various 2-nitroalkenes and their nucleophile quenching-derived cations are also gathered. The mechanistic aspects emphasizing dicationic (and tricationic) intermediates, their interplay and the potential synthetic benefits of these transformations which greatly extend the chemistry of the nitro group are highlighted.     

HETEROCYCLIC PHOSPHONIUM SALTS BY REACTION OF DIMETHYL(1,2-ALKADIENYL)-PHOSPHINE OXIDES WITH ELECTROPHILIC REAGENTS

Abstract

Salts of 2,5-dihydro-1,2-oxaphosphole have been prepared by reaction of dimethyl(1,2-alkadienyl)phosphine oxides with halogens, sulfenyl and selenenyl chlorides. The essential influence of tertiary carbenium ions for realization of oxaphospholic cyclization of phosphorylated allenes has been established.     

Quantum chemical characteristics of cationic active sites forming in the Bcl3-α,ω-dichloroalkyl systems

Taking as examples the cyclic halonium (I) and linear carbenium (II) model cations of equal composition, viz. [Cl(CH₂)₄]⁺, and products of propene addition to them (V and VI), the energetic characteristics of these structures were estimated by the MNDO^(a) method. The energetic preference of I with respect to II was found to be much higher than that of V with respect to VI (ca. 97 and ca. 11 kJ/mole, respectively). This difference is suggested to reflect the decrease in the tendency of linear Cl-containing carbenium ions to cyclization into halonium ions with an increase in the length of the linear hydrocarbon chain. The results obtained are applied to the discussion of the mechanism of polymerization in the 2-methylpropene-BCl₃-2,5-dichloro-2,5-dimethylhexane system.     

Spiroconjugation

Spiro compounds having mutually perpendicular π-electron systems (“spiro-polyenes”) exhibit a new kind of homoconjugation, which is designated as spiroconjugation. Spiro-polyenes are readily accessible, e.g. by cycloaddition, by elimination of dinitrogen from 1,2-diazaspiro-polyenes, and by cyclization of carbenium ions or carbanions. The magnitude of the effect exerted by spiroconjugation is estimated by model calculations. It can be detected, for instance, by comparison of the electronic and photoelectron spectra of spiro-polyenes and analogous compounds which are unable to undergo such interaction.     

Acyl Migration versus Epoxidation in Gold Catalysis: Facile,Switchable, and Atom‐Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold‐catalyzed annulations of anthranils and ynamides. α‐Imino gold carbenes, generated in situ from anthranils and an N,O‐coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π‐bond, followed by unexpected and highly selective 1,4‐ or 1,3‐acyl migrations to form 6‐acylindoles or 5‐acylindoles. With the (2‐biphenyl)di‐tert‐butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3‐hydroxylquinolines, quinolin‐3(4H)‐ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma‐complexes support the observed substitution pattern in the final product.     

Electrophilic and nucleophilic enzymatic cascade reactions in biosynthesis

The biosynthesis of cyclic terpenoids and polyethers involves enzyme-initiated cascade reactions for ring formation. While the former are obtained by electrophilic cascades through carbenium ions as intermediates, cyclic polyethers are formed by nucleophilic cascade reactions of (poly)epoxide precursors. These mechanistically complementary pathways follow common principles via (i) triggering of the cascade by forming a reactive intermediate ('initiation'), (ii) sequential 'proliferation' of the cyclization and finally (iii) 'termination' of the cascade. As analyzed in this concept paper, the multiplicity of precursors, combined with various initiation and termination routes and kinetically favored or disfavored cyclization modes accounts for the enormous diversity in cyclic terpenoid and polyether scaffolds. Although the essential role of enzymes in the triggering of these cascades is reasonably well understood, remarkably little is known about their influence in proliferation reactions, especially those implying kinetically disfavored (anti-Markovnikov and anti-Baldwin) routes. Mechanistic analysis of enzymatic cascade reactions provides biomimetic strategies for natural product synthesis.     

α-imino carbenium ions: Is the α-imino group really destabilizing?

According to ab initio calculations, there is good evidence that α-imino carbenium ions exist as bridged carbocations. Furthermore, the energy differences between planar and bridged structures are much larger than for the corresponding α-acyl carbenium ions. Compared to the other α-electron withdrawing groups studied, the α-imino group does not lead to a destabilized carbenium ion; α-imino carbenium ions would therefore seem to be possibly useful synthetic intermediates.     

Stable mechanistically-relevant aromatic-based carbenium ions in zeolite catalysts

The existence of carbenium ion species is assumed in many zeolite catalysis mechanisms. Using computational techniques that include environmental effects, a benzenium-type carbenium ion is identified in zeolite catalysts for the first time. Localization of nearby transition states indicate that this species may play an important role as an intermediate in the bimolecular m-xylene disproportionation reaction. The barrier to back-donation of the proton from the benzenium ion is at least 50 kJ/mol, meaning that this species may be spectroscopically observable. An additional carbenium ion intermediate, formed by abstraction of a hydride from m-xylene, is also predicted. The stability of this second new carbenium ion suggests that aromatic-based carbenium ions are likely to be intermediates in many zeolite-catalyzed reactions. Two types of fundamentally different fully periodic calculations support the stability predictions.     

Conformational Switching through the One-Electron Reduction of an Acridinium-based, γ-Cationic Phosphine Gold Complex

Our efforts in the chemistry of gold complexes featuring ambiphilic phosphine-carbenium L/Z-type ligand have led us to consider the reduction of the carbenium moiety as a means to modulate the gold–carbenium interaction present in these complexes. Here, it was shown that the one-electron reduction of [(o-Ph₂P(C₆H₄)Acr)AuCl]⁺ (Acr=9-N-methylacridinium) produces a neutral stable radical, the structure of which showed a marked increase in the Au–Acr distance. Related structural changes were observed for the phosphine oxide analogue [(o-Ph₂P(O)(C₆H₄)Acr]⁺, the reduction of which interfered with the P=O→carbenium interaction. These structural effects, driven by a reduction-induced change in the electronic and electrostatic characteristics of the compounds, showed that the charge and accepting properties of the carbenium unit can be modulated. These results highlight the redox-noninnocence of carbenium Z-type ligand, a feature that can be exploited to induce specific conformational changes.     

Skeletal isomerization of butene in ferrierite: assessing the energetic and structural differences between carbenium and alkoxide based pathways

In this study, the results from a systematic analysis of two different mechanisms for the skeletal isomerization of cis-butene to isobutene in ferrierite (FER) are presented. One involves a conventional mechanism that proceeds via stable alkoxide intermediates and the other is one which proceeds via carbenium ions only. A 27T QM cluster model has been used in this study, which is described using the M06-2X DFT functional. It is found that the alkoxide intermediates formed over the course of the conventional pathway are considerably lower in energy than the carbenium ion formed over the course of the alternate pathway. However, the rate determining step in the latter pathway is predicted to be almost 10 kcal/mol lower in energy. The higher barrier for the latter process is due to the inherent stability of the alkoxide intermediates formed within FER. These results appear to suggest that while these intermediates are formed over the course of the reaction, the skeletal isomerization of linear butenes to form isobutene in FER may occur via a carbenium based mechanism. This proposal is consistent with experimental results that show alkoxide intermediates are experimentally observed species.     

Calculated hydride and fluoride affinities of a series of carbenium and silylium cations in the gas phase and in C6H5Cl solution

We report the hydride and fluoride affinities for a group of silylium and carbenium cations. With comparable substituents on the central atom, the silylium cations have the higher fluoride affinity, whereas the carbenium ions have the higher hydride affinity. In the first approximation, the hydride and the fluoride affinities vary in parallel with changes in substitution, but the deviations from linear correspondence of hydride and fluoride affinities are more pronounced for carbenium ions. The hydride and fluoride affinities of silylium cations are very similar, whereas for carbenium ions, the hydride affinities are 35–60 kcal mol^?1 higher than fluoride affinities. These results are placed in the context of studies of hydrodefluorination of aliphatic C? F bonds enabled by silylium carborane catalysts [C. Douvris, O. V. Ozerov, Science 2008 , 321, 1188]. The abstraction of fluoride from perfluoroalkanes by a trialkylsilylium cation is neither thermodynamically favorable nor kinetically accessible and, if at all possible, will require a much more fluorophilic silylium cation.     

Destabilized carbenium ions: α-imidoyl carbenium ions and the electron impact mass spectra of α-haloaldimines and α-haloketimines

A series of α-chloro- and α-bromoketimines compounds (1-9) with different substituents at the α-position and at the imino group has been investigated by electron impact mass spectrometry as possible precursors of the correspondingly substituted α-imidoyl carbenium ion, an important class of destabilized carbenium ions. The main fragmentation of the molecular ions of compounds, 1-9 in the ion source corresponds to an α-cleavage at the imino group; however, fragment ions are also formed by loss of the α-halo substituent. These fragment ions correspond at least formally to α-imidoyl carbenium ions. Their further reactions in dependence on the type of substituents at the imino group and at the α-C atom, were studied by mass-analysed ion kinetic energy and collisional activation mass spectrometry. The results agree with the initial formation of destabilized α-imidoyl carbenium ions but indicate an easy rearrangement of these ions in the presence of suitable alkyl substituents by 1,2- and 1,4-hydrogen shifts to more stable isomers.     

Rearrangements and hydrogen—deuterium exchange in ferrocenylcarbenium ions

The rearrangement of the t-butyl group in t-butylferrocenyl carbenium ions (VI) and the hydrogen—deuterium exchange in the methyl group of methylferrocenyl carbenium ions (XVI) has been studied. The reaction rates depend on the size of the substituents at the carbenium ion center, an increase in the size of the substituents causing an increase in the rate. This effect is attributed, on the basis of the Gleiter and Cais models of ferrocenyl substituted carbenium ions, to increased steric hindrance between the substituents and the unsubstituted cyclopentadienyl ring.     

Testing the validity of the conventional resonance model for protonated carbonyl, imine and thiocarbonyl compounds. An Ab initio valence bond study

The conventional resonance model describes protonated carbonyls, imines, and thiocarbonyls by a superposition of two structures, one pi polar-covalent and the other of carbenium type. The validity of this model is clearly supported by high level valence bond calculations, giving a 32% weight for the carbenium form in protonated carbonyl, 19% in protonated formamine and thioformaldehyde. The carbenium form is further stabilized by pi-donating substituents. Solvation effects do not fundamentally change the gas-phase picture.     

Hyperconjugative stabilization in alkyl carbocations: direct estimate of the beta-effect of group-14 elements

DFT calculations at the BP86/TZ2P level have been carried out for the primary, secondary, and tertiary carbenium ions [H2C-CH(EH3)2](+) (1a-e), [HC{CH(EH3)2}2](+), (2a-e), and [C{CH(EH3)2}3](+) (3a-e) for E = C, Si, Ge, Sn, Pb. The nature of the interaction between the carbenium center H(2-n)C(+) and the substituents {CH(EH3)2}m has been investigated with an energy decomposition analysis (EDA) aiming at estimating the strength of the pi hyperconjugation which electronically stabilizes the carbenium ions. The results of the EDA show that the calculated DeltaEpi values can be used as a measure for the strength of hyperconjugation in carbenium ions arising from the interactions of saturated groups possessing pi orbitals. The theoretical data suggest that the ability of sigma C-E bonds to stabilize positive charges by hyperconjugation follow the order C < Si < Ge < Sn < Pb. Hyperconjugation of C-Si bonds is much stronger than hyperconjugation of C-C bonds while the further rising from silicon to lead is smaller and has about the same step size for each element. The strength of the hyperconjugation in primary, secondary, and tertiary alkyl carbenium ions does not increase linearly with the number of hyperconjugating groups; the incremental stabilization becomes smaller from primary to secondary to tertiary cations. The effect of hyperconjugation is reflected in the shortening of the C-C bond distances and in the lengthening of the C-E bonds, which exhibits a highly linear relationship between the calculated C-C and C-E distances in carbocations 1-3 and the hyperconjugation estimated by the DeltaEpi values.     

Quantum-Chemical Study of Various Pathways to Carbenium Ions from Olefins and Alkyl Fluorides in the Medium of Hydrofluoric Acid

Elementary reactions of carbenium ion generation from olefins and alkyl fluorides in the medium of hydrofluoric acid are studied by the Hartree–Fock method taking into account electron correlation at the MP2 level and by the DFT (B3LYP) method in the 6-31++G** basis set. Based on enthalpies calculated for these reactions, possible pathways to carbenium ions in the olefin–HF system are determined. A conclusion is drawn that carbenium ions can be formed from olefins by protonation and from the corresponding alkyl fluorides and their protonated forms. It is shown that the heterolytic decomposition of alkyl fluoride in the medium of liquid HF is possible due to the stabilization of carbenium and fluoride ions by hydrogen bonds with HF molecules. The discrete model of microsolvation and the polarizable continuum model (PCM) are used to estimate a decrease in the activation barrier of the heterolytic decomposition of alkyl fluorides due to solvation in the medium of liquid hydrofluoric acid.     

Eine Abfangmethode zum Nachweis kurzlebiger Carbenium-Ionen in stark sauren Reaktionsmedien

In a new approach, unstable carbenium ions produced in strong protic media could be trapped by thioethers yielding sulfonium compounds. The method has successfully been applied to the rearrangement reactions of sec-butyl alcohol and pinacol, where carbenium ions could be trapped before their rearrangement. In the case of the pinacol-pinacon rearrangement the carbenium ion 13 has already been discussed but for the first time its existence is proved. Since compound 16 has been found, a new mechanism for the pinacol-pinacon rearrangement is postulated starting with diprotonated pinacol.     

CC Bond Formation by Addition of Carbenium Ions to Alkenes: Kinetics and Mechanism

The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product-like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity-selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.     

Low‐Temperature Alkane C?H Bond Activation by Zeolites: An In Situ Solid‐State NMR H/D Exchange Study for a Carbenium Concerto

Isotopic H/D exchange has been monitored by in situ MAS NMR spectroscopy of 2‐[D₁₄]methylpentane with H‐USY to probe the controversy over the alkane conversion mechanism. The probe molecule has distinct exchangeable sites with different accessibility to the zeolite surface. In the early stages of the process, the regioselectivity of exchange demonstrates that the slow step of the mechanism is controlled by a carbenium ion intermediate. At a later stage of exchange, intramolecular hydride migrations, typical of carbenium chemistry, replace D by H also on other carbon atoms, resulting in a loss of regioselectivity. Therefore, the first and the subsequent steps of the H/D exchange proceed at this temperature through a carbenium intermediate species.     

π-Participation in Diazoketone Hydrolysis II; Exo-endo Cyclization Ratio in the Hydrolyses of 7-syn- and 5-endo-Diazoacetyl-2-norbornene

The rate of the acid-catalysed hydrosis of 7-syn-diazoacetyl-2-norbornene ( 1a ) is enhanced relative to that of the saturated analogue 5a by a factor of 835. In contrast to the behaviour of other primary diazoketones, the substitution step is no longer rate-determining (mechanism A-2), but so much accelerated that the preceding proton transfer becomes the slow step (mechanism A-S_E2, demonstrated by a solvent isotope effect k_H/k_D = 1.76). Product analysis shows 100% cyclization; the product formation is explained in terms of brexyl and brendyl type carbenium ions (or ion pairs). - 5-endo-Diazoacetyl-2-norbornene ( 3a ) shows very slight acceleration, and forms only 27% cyclization products (identical to those formed from 1a). Thus, in spite of the absence of steric hindrance by hydrogen atoms, the exo-endo rate ratio for anchimeric assistance is ≥ 10³.