分子间自由基正离子反应研究进展

自由基正离子含有一个正电荷和一对未成对电子,是很多有机化学反应的重要的活性中间体。文章综述了近几年自由基正离子反应研究进展,主要包括化学氧化剂诱导的自由基正离子反应、可见光诱导的自由基正离子反应、电诱导的自由基正离子反应等方面的研究。     

Relative Reactivities of Three Isomeric Aromatic Biradicals with a 1,4-Biradical Topology Are Controlled by Polar Effects

Unexpectedly, the 5-dehydroquinoline radical cation was formed in the gas phase from the 5-iodo-8-nitroquinolinium cation upon ion-trap collision-activated dissociation. This reaction involves the cleavage of a nitro group to generate an intermediate monoradical, namely, the 8-dehydro-5-iodoquinolinium cation, followed by rearrangement through abstraction of a hydrogen atom from the protonated nitrogen atom by the radical site. Dissociation of the rearranged radical cation through elimination of an iodine atom generates the 5-dehydroquinoline radical cation. The mechanism was probed by studying isomeric biradicals and performing quantum chemical calculations. The 5-dehydroquinoline radical cation showed greater gas-phase reactivity toward dimethyl disulfide, cyclohexane, and allyl iodide than the isomeric 5,8-didehydroquinolinium cation, which is more reactive than the isomeric 5,8-didehydroisoquinolinium cation studied previously. All three isomers have a 1,4-biradical topology. The order of reactivity is rationalized by the vertical electron affinities of the radical sites of these biradicals instead of their widely differing singlet–triplet splittings.     

Basic concepts of "cation pool" and "cation flow" methods and their applications in conventional and combinatorial organic synthesis

Carbocations have been generally considered to be relatively unstable and transient species. But the "cation pool" method enables the easy accumulation of carbocations in conventional reaction media such as dichloromethane. In the "cation pool" method, carbocations are generated by low-temperature electrochemical oxidation and accumulated in a solution. In the next step, the carbocations thus produced are allowed to react with various nucleophiles. Combinatorial parallel synthesis based upon the "cation pool" method has also been developed. The applicability of the "cation pool" method depends upon the stability of the cation that is accumulated. This problem can be overcome by the "cation flow" method. In the "cation flow" method, carbocations are generated in a microflow electrochemical system. Short residence times and efficient temperature control of the microflow system are advantageous. Combinatorial sequential synthesis has been achieved based on the "cation flow" method.     

Relative cation versus anion stabilities of low-coordinated P(III) π-bonded phosphorus compounds

Relative cation and anion stabilities of a selected variety of low-coordinated π-bonded phosphorus compounds are evaluated for the gas phase by means of group transfer reactions at an ab initio RHF/SCF level with inclusion of electron correlation treatment ( ). The cation stabilities reveal large variations; the largest cation stabilities are possessed by the diaminophosphenium cation and the phosphanetriylammonium cation. Interestingly the diphosphinophosphenium structure is capable of strong stabilization of the corresponding cation as well as the anion. An analysis of the frontier orbitals indicates that the cationic species can be divided into classes of structures, isolobal with a carbene or a cyanide, isocyanide, rsp.     

Defect interactions in alkali halides

Pairwise defect interactions between divalent cation impurities, cation vacancies and anion vacancies have been studied in the extrinsic and intrinsic temperature ranges for the KClSr²⁺ system. Pair correlation functions have been derived for the various defect interactions distinguished by their site symmetries and these show the existence of associated divalent cation—cation vacancy and cation—anion vacancy pairs even at high temperatures. Pair formation gives way to dissociation at low impurity concentrations and high temperatures.     

Charge delocalization in self-assembled mixed-valence aromatic cation radicals

The spontaneous assembly of aromatic cation radicals (D(+?)) with their neutral counterpart (D) affords dimer cation radicals (D(2)(+?)). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography.     

Generation and absolute reactivity of an aryl enol radical cation in solution

Laser flash photolysis of 1-bromo-1-(4-methoxyphenyl)acetone in acetonitrile leads to the formation of the alpha-acyl 4-methoxybenzyl radical that under acidic conditions rapidly protonates to give detectable amounts of the radical cation of the enol of 4-methoxyphenylacetone. This enol radical cation is relatively long-lived in acidic acetonitrile (tau approximately equal to 200 micros), which is on the same order of magnitude as the radical cations of other 4-methoxystyrene derivatives. Rate constants for deprotonation of the radical cation and the acid dissociation constant for the enol radical cation were also determined using time-resolved absorption spectroscopy. Deprotonation is rapid, taking place with a rate constant of 3.9 x 10(6) s(-1), but the enol radical cation is found to be only moderately acidic in acetonitrile having a pK(a) = 3.2. The lifetime of the enol radical cation was also found to be sensitive to the presence of oxygen and chloride. The sensitivity toward oxygen is explained by oxygen trapping the vinyloxy radical component of the enol radical cation/vinyloxy equilibrium, while chloride acts as a nucleophile to trap the enol radical cation.     

Studies of 9-fluorenyl carbocations. intramolecular hydride migration in a substituted 9-fluorenyl carbocation

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.     

ENHANCEMENT OF ENERGY TRANSFER FROM THE TERMINAL CHROMOPHORE OF POLYETHER TO EUROPIUM CATION(Ⅲ)VIA COMPLEXATION

The fluorescence spectra of naphthalene end-labelled polyethylene glycol)(N-P_nN) in methanol solution in the presence of europium cation(Ⅲ) was investigated. Selective excitation of the naphthalene group of N-P_n-N re-sults in the emission of europium cation. suggesting that the polyether chain complexes with europium cation, thus efficient energy transfer from naphthalene chromophore to europium cation occurs. Photoirradiation of N-P-N in solution leads to intramolecular dimerization of the two terminal naphthalene groups to give a crown ether. The complexation of this photochemically synthesized crown ether and europium cation was also studied.     

UV Raman spectrum of 1,3-dimethylcyclopentenyl cation adsorbed in zeolite H-MFI

The first Raman spectrum of an adsorbed carbenium ion has been measured: The 1,3-dimethylcyclopentenyl cation adsorbed in zeolite H-MFI. 1,3-Dimethylcyclopentenyl cation has been observed as a component of the hydrocarbon pool formed during the methanol-to-gasoline process catalyzed by zeolite H-MFI. The Raman shifts recorded for 1,3-dimethylcyclopentenyl cation are in remarkable agreement with computer calculations of the vibrational band positions for the isolated cation. This agreement suggests that the cation is unperturbed by interactions with the zeolite pore walls so that Raman spectra of free or solution-phase hydrocarbons can be used to identify these same species adsorbed in zeolite pores.     

Electronic nature of carbonium ions and their silicon analogues

Nonclassical ions or carbonium ions have multi-center bonding from delocalized sigma or pi electrons. The 2-norbornyl cation, its derivative 6,6-difluoro-2-norbornyl cation, tris-homocyclopropenyl cation, 7-norbornenyl cation, and 4-cyclopentenyl cation and their corresponding silicon analogues were studied in this work. All carbocations have topologically different 3c-2e systems. The magnitude of all delocalization indexes between each atomic pair of the 3c-2e bond can be used to predict homoaromaticity. The silicon analogues have a topologically different 3c-2e bond from their corresponding carbocation.     

Rearrangement and dissociative processes in the [C3H6O]+˙ potential energy surface. Methyl vinyl ether: An example of non-ergodic behaviour?

The unimolecular dissociation reaction of the methyl vinyl ether radical cation to acetyl cation plus methyl radical was studied by means of ¹³C labelling and tandem mass spectrometric techniques. The results are in good agreement, in all major aspects, with previous data for ¹³C-labeled propene oxide. The data are discussed in terms of non-ergodic behaviour. Isomerization of the methoxyethylidene radical cation intermediate to the acetone radical cation is ruled out on the basis of the labelling experiments and comparison with the hydroxymethene radical cation system.     

Synthesis and properties of noncyclic polyether compounds. Part V : The influence of acid and concentration of cations on the uphill transport rates of cations and selectivities through liquid membranes by synthetic,noncyclic polyether ionophores

The effects of acid and cation concentration on the uphill transport rates of cations and selectivities through chloroform liquid membrane by noncyclic polyether ionophores, which are pentaethylene glycol derivatives containing both 8-quinolyt and o-carboxylphenyl terminal groups, have been demonstrated. Using sulfuric, oxalic, or polyphosphoric acid in aqueous solution, the initial transport rates and the amount of cation transported by the ionophores after two days were larger than those using hydrochloric or nitric acid. Using picric acid, the amount of cation transported decreased greatly. It was confirmed that it decreased where an acid could easily be counter-transported by the ionophore through liquid membranes. On the other hand, it was found that the rates, amounts of cation transported, and selectivities, change with the changing of the cation concentration in the aqueous solutions. When the cation concentration in both aqueous solutions is high enough compared with the pH gradient between the two aqueous solutions, the rate, amount, and selectivity of the ionophore for potassium ion increases compared with when the cation concentration is low.     

The C4H7+ cation. A theoretical investigation

The potential energy surface of the C4H7+ cation has been investigated with ab initio quantum chemical theory. Extended basis set calculations, including electronic correlation, show that cyclobutyl and cyclopropylcarbinyl cation are equally stable isomers. The saddle point connecting these isomers lies 0.6 kcal/mol above the minima. The global C4H7+ minimum corresponds to the 1-methylallyl cation, which is 9.0 kcal/mol more stable than the cyclobutyl and the cyclopropylcarbinyl cation and 9.5 kcal/mol below the 2-methylallyl cation. These results are in excellent agreement with experimental data.     

Cation binding properties of photodimerizable polymers bearing benzodiglyme units

The polymers which have glyme units as alkali cation binding sites and photodimerizable cinnamoyl units were prepared by the radical polymerization of corresponding monomers. The alkali cation binding ability and selectivity of the polymers, which were studied by a method of picrate salts extraction, were strongly dependent on the length of glyme chains. When irradiated with ultraviolet light, the cinnamoyl groups caused dimerization in dilute solutions. Although the photodimerization of the polymers with relatively short glyme chains enhanced their cation binding ability, the photodimerization of the polymers bearing long glyme chains reduced their cation binding ability. The use of alkali metal cations as templates emphasized the effect of photodimerization on the cation binding properties. The effect of alkali metal cations on the quantum yields of the photodimerization of the polymers showed that two or more benzodiglyme units took part in the binding of one cation. The polymers bearing benzodiglymes, crown ethers, and cinnamoyl moieties were also prepared by the radical copolymerization of the corresponding monomers. It was found that the crown ether units of the copolymers predominantly participated in the cation binding. The photodimerization of the copolymers with suitable alkali metal cations as templates strongly enhanced their cation binding ability.     

Experimental and Computational Studies on the Formation of Three para‐Benzyne Analogues in the Gas Phase

Experimental and computational studies on the formation of three gaseous, positively‐charged para‐benzyne analogues in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon‐centered σ‐type mono‐ and biradicals are well understood. Cleavage of two iodine–carbon bonds in N‐deuterated 1,4‐diiodoisoquinolinium cation by collision‐activated dissociation (CAD) produced a long‐lived cation that showed nonradical reactivity, which was unexpected for a para‐benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N‐deuterated 2‐ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring‐opening before the second iodine atom loss to form an enediyne instead of a para‐benzyne. Similar results were obtained for the 5,8‐didehydroisoquinolinium cation and the 2,5‐didehydropyridinium cation. The findings for the 5,8‐didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7‐didehydro‐isoquinolinium cation and not the expected 5,8‐isomer.     

The tridurylsilylium and tridurylstannylium cations: free and not so free.

Allyltridurylsilane has been prepared and converted to the tridurylsilylium cation by treatment with electrophiles. Allyltridurylstannane has been prepared and converted to the tridurylstannylium cation under similar conditions. The (29)Si chemical shift of the silylium cation indicates that it is free and tricoordinate, whereas the (119)Sn chemical shift of the stannylium cation indicates that it maintains loose fourth coordination with the solvent or the anion.     

A new rearrangement process in tert-amyl cation

13C NMR spectroscopy of the 2-methyl-2-butyl-1-13C cation (13C-labeled tert-amyl cation) indicates that interchange of the inside and outside carbons occurs via a barrier of 19.5 +/- 2.0 kcal/mol. A plausible mechanism involves hydride migration in the proposed 2-pentyl cation 4 to form 3-pentyl cation 5. Via the protonated cyclopropane intermediate 6, which undergoes degenerate corner-to-corner hydride shift, the secondary 3-pentyl cation 5' with the label shifted to the central carbon atom is formed. The tert-amyl cation obtained from 5' in the reverse process has the 13C label on an inside carbon atom. All intermediates and transition structures were located on the PES theoretically at the MP2/6-31G(d,p) level of theory. The rearrangement rate of the doubly labeled tert-amyl cation (methyl-13C-butyl-1-13C cation), followed by means of 13C NMR, revealed that the process that interchanges inside and outside carbons has the highest barrier. Comparison of the initial rates revealed that isotopomer 1e arises considerably more slowly than other isotopomers, indicating that in the overall rearrangement process transition structure 5-TS has the highest energy.     

Reactivite dans la serie du methylenecyclopropane—II : Etude de l'ouverture électrocyclique de bromo-2dialkyl-1,1 méthylène-3 cyclopropanes

A study of the solvolysis products of monobromomethylenecyclopropanes shows that, like the cyclopropyl cation, the methylenecyclopropyl cation opens to give a “methyleneallyl” cation. The ring-opening is disrotatory as predicted by the Woodward-Hoffmann-DePuy rules.