The redox behavior of gem-dihalo compounds: 9,9-dibromofluorene, 9,9-dichlorofluorene and dichlorodiphenylmethane

The redox behavior of several gem-dihalo compounds has been examined at platinum and vitreous carbon electrodes in dimethylformamide. The reduction of 9,9-dichlorofluorene is initially an overall two-electron process which involves cleavage of chloride ion and the formation of 9-chlorofluorenyl anion. The final products and their distribution are then dependent upon the relative rates of reduction of the parent compound, nucleophilic attack of 9-chlorofluorenyl anion on the parent compound, and proton availability. If reaction by substitution is permitted to predominate, 9,9′-dichlorobifluorenyl results. This species is electroactive at the applied potential and undergoes reductive dechlorination to give bifluorenylidene. In contrast, if either the rate of reduction of 9,9-dichlorofluorene of the rate of protonation of 9-chlorofluorenyl anion exceeds the rate of substitution, the predominant product becomes 9-chlorofluorene. Reduction of this species then gives a mixture of fluorene and bifluorenyl when electrolysis is effected in an aprotic medium and fluorene when electrolysis is performed in the presence of diethyl malonate, a weak proton donor. Dichlorodiphenylmethane and 9,9-dibromofluorene also undergo reductive dehalogenation to give monomeric and dimeric products by pathways analogous to those observed for dichlorofluorene. In the case of dibromofluorene, however, the product distribution is also potential dependent since the intermediate 9-bromofluorenyl radical may not be reduced at the applied potential. No evidence was obtained in these studies to support previous claims of carbenes and/or carbene radical anions in these reductions.     

, B.E. Conway,J.O'M. Bockris (Eds.)Modern Aspects of Electrochemistry,No. 13, Plenum Press,New York and London (1979)

The electrochemical reductions of fluorenone hydrazone (Fl=NNH₂), fluorenone fluorenylhydrazone (FIHNHN=Fl), fluorenone azine (Fl=N?N=Fl) and benzophenone analogs have been studied at platinum cathodes in DMF?0.1 M(n-Bu)₄NClO₄ in the absence and presence of an added proton donor. Fl=NNH₂ undergoes reduction to give an unstable anion radical which decomposes by an unidentified pathway to afford Fl=NH. The latter species is electroactive at the applied potential and is reduced to the corresponding amine, FlHNH₂. Four electrons per molecule of Fl=NNH₂ are required for this process when electroreduction is effected in the presence of diethyl malonate (DEM), an electroinactive proton donor. Unreacted Fl=NNH₂ serves as the source of protons when electroreduction is conducted in the absence of DEM.Dual reaction channels are observed for the reduction of FlHNHN=Fl. If the corresponding anion radical is not protonated by either an added proton donor or unreacted starting material, decomposition occurs by carbon-nitrogen bond cleavage to give FlH₂ and Fl=NNH₂ as products. Reduction of the latter species occurs at the same potential as FlHNHN=Fl, ultimately affording FlHNH₂. The reaction channel involving carbon-nitrogen bond cleavage is replaced by a pathway involving nitrogen-nitrogen bond cleavage in the presence of an added proton donor. The final reduction product by this route is FlHNH₂.Fl=N?N=Fl is reduced stepwise and reversibly to its dianion in an aprotic medium. In the presence of a relatively strong proton donor such as hexafluoro-2-propanol, reduction gives FlHNHN=Fl. The redox behavior of the benzophenone analogs, Ph₂C=N?N=CPh₂, Ph₂CHNHN=CPh₂ and Ph₂C=NNH₂, parallels that of their counterparts in the fluorene series.     

Evidence for the formation of a cis-dichlorovinyl anion upon reduction of cis-1,2-dichlorovinyl(pyridine)cobaloxime

The reduction of cis-1,2-dichlorovinyl(pyridine)cobaloxime, a model complex for the organometallic intermediate proposed in the dechlorination of trichloroethylene by cobalamin, was studied. Two mechanisms were considered for the Co-C bond cleavage following reduction. In the first, the Co-C bond cleaves to produce Co(I) and a chlorovinyl radical, while the second pathway results in the formation of Co(II) and a chlorovinyl anion. Four reducing agents, cobaltocene, decamethylcobaltocene, cob(I)alamin, and chromium(II), were used in the presence of H atom and proton donor species to identify the presence of chlorovinyl radical or chlorovinyl anion intermediates. Mechanistic conclusions were based on comparisons of the final product ratios of cis-dichloroethylene (cDCE) and chloroacetylene, which were found to have a direct relationship to the amount of proton donor available, with increased proton donor leading to increased cDCE production. The results support the intermediacy of a cis-1,2-dichlorovinyl anion.     

Reversed Electron Apportionment in Mesolytic Cleavage: The Reduction of Benzyl Halides by SmI2

The paradigm that the cleavage of the radical anion of benzyl halides occurs in such a way that the negative charge ends up on the departing halide leaving behind a benzyl radical is well rooted in chemistry. By studying the kinetics of the reaction of substituted benzylbromides and chlorides with SmI₂ in THF it was found that substrates para‐substituted with electron‐withdrawing groups (CN and CO₂Me), which are capable of forming hydrogen bonds with a proton donor and coordinating to samarium cation, react in a reversed electron apportionment mode. Namely, the halide departs as a radical. This conclusion is based on the found convex Hammett plots, element effects, proton donor effects, and the effect of tosylate (OTs) as a leaving group. The latter does not tend to tolerate radical character on the oxygen atom. In the presence of a proton donor, the tolyl derivatives were the sole product, whereas in its absence, the coupling dimer was obtained by a S_N2 reaction of the benzyl anion on the neutral substrate. The data also suggest that for the para‐CN and CO₂Me derivatives in the presence of a proton donor, the first electron transfer is coupled with the proton transfer.     

A comparative ab initio study of intramolecular proton transfer in model alpha-hydroxyalkoxides

A comparative ab initio study was performed on the intramolecular proton-transfer reaction that occurs in alpha-hydroxyethanoxy, alpha-hydroxyphenoxide, and alpha-hydroxyethenoxy anions. The intramolecular proton transfer occurs in a five-member atom arrangement, between two oxygen atoms separated by a carbon-carbon bond. The chosen systems serve as models for alpha-hydroxyalkoxide molecules where the carbon-carbon bond varies from a single bond (the glycolate anion or alpha-hydroxyethanoxide anion) to a part of an aromatic ring (the alpha-hydroxyphenoxide anion), and finally to a double bond (the alpha-hydroxyethenoxide anion). Particular attention was given to the evolution along the intrinsic reaction coordinate of such properties as energies, relevant structural parameters, Mulliken charges, dipole moments, and 1H-NMR chemical shifts to reveal the similarities and differences for the proton transfer in the model systems.     

Fragmentation of conjugate bases of esters derived from multifunctional alcohols including triacylglycerols

Enolate anions of esters from 1,2 and 1,3 diols undergo an internal nucleophilic substitution reaction that produces a β-ketoester and an alkoxide ion within the molecular species. These intermediate ions undergo two competitive fragmentation pathways. The first pathway corresponds to a second nucleophilic substitution of the ketoester by the alkoxide that yields a neutral cyclic ether and the β-ketoacid carboxylate. The latter then loses carbon dioxide and produces the enolate anion of the corresponding ketone. The second proposed pathway is stepwise: it starts with a proton transfer from the methylene group between the two carbonyls to the alkoxide anion that produces an alcohol and the enolate ion of the β-ketoester inside the molecular species. The latter undergoes cleavage of the ester bond induced by the negative charge to yield an ion-dipole complex composed of a neutral acylketene and an alkoxide ion. The direct dissociation of this ion-dipole complex competes with an internal proton exchange to yield a new complex that consists of an alcohol molecule and the anion of the acylketene, which can also dissociate. The fragmentation pathway that leads to the ketone enolate is sensitive to the relative positions (1,2 or 1,3) of the esters on the molecular backbone. This position-sensitive reaction is useful for the assignment of the primary and secondary positions in triacylglycerols, even in mixtures, as shown by some examples.     

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.     

The effect of charge on hydroxyl loss from ortho-substituted nitrobenzene ions

An abundant loss of hydroxyl in decompositions of ortho-substituted nitroarene cations is commonly observed when the substituent contains one or more labile hydrogen atoms. The major loss of hydroxyl also takes place from many but not all of the corresponding molecular anions. Data are reported for the collisionally activated decompositions of the cations and anions of o-nitrotoiuene, o-nitrophenol and o-nitroaniline. Data for some dinitro ions are also reported. The results can be rationalized on the basis of a greater degree of charge developed at the substituent in the transition state of the anions that leads to a rearranged ion. It is from this structure tint hydroxyl is lost via simple bond cleavage. This can be viewed most simply as a proton transfer from the substituent to the nitro group in the anion as opposed to hydrogen transfer in the analogous step for the cation. The degree to which hydroxyl loss occurs is therefore largely determined by the tendency for hydrogen (cations), or proton (anions), transfer from the substituent to the nitro group.     

Anthralin: primary products of its redox reactions

One-electron reduction significantly enhances the ability of anthralin, 1, to act as a hydrogen atom donor. On annealing of an MTHF glass in which the radical anion of anthralin, 1*-, is generated radiolytically, this species decays mainly by loss of H* to give the anthralyl anion, 2- . On the other hand, radicals formed on radiolysis of matrices that are suitable for the generation of radical anions or cations are capable to abstract H* from anthralin to give the anthralyl radical, 2* . Both 2- and 2* are obtained simultaneously by mesolytic cleavage of the radical anion of the anthralin dimer. Contrary to general assumptions, the anthralyl radical is found to be much more reactive toward oxygen than the anion. All intermediates are characterized spectroscopically and by reference to quantum chemical calculations. Attempts to generate the radical cation of anthralin by X-irradiation of an Ar matrix containing anthralin led also to significant formation of its radical anion, i.e., anthralin acts apparently as an efficient electron trap in such experiments.     

Electron transfer dissociation of peptide anions

Ion/ion reactions of multiply deprotonated peptide anions with xenon radical cations result in electron abstraction to generate charge-reduced peptide anions containing a free-radical site. Peptide backbone cleavage then occurs by hydrogen radical abstraction from a backbone amide N to facilitate cleavage of the adjacent C-C bond, thereby producing a- and x-type product ions. Introduction of free-radical sites to multiply charged peptides allows access to new fragmentation pathways that are otherwise too costly (e. g., lowers activation energies). Further, ion/ion chemistry, namely electron transfer reactions, presents a rapid and efficient means of generating odd-electron multiply charged peptides; these reactions can be used for studying gas-phase chemistries and for peptide sequence analysis.     

Structures and energetics of the deprotonated adenine-uracil base pair, including proton-transferred systems

The B3LYP/DZP++ level of theory has been employed to investigate the structures and energetics of the deprotonated adenine-uracil base pairs, (AU-H)-. Formation of the lowest-energy structure, [A(N9)-U]- (which corresponds to deprotonation at the N9 atom of adenine), through electron attachment to the corresponding neutral is accompanied by proton transfer from the uracil N3 atom to the adenine N1 atom. The driving force for this proton transfer is a significant stabilization from the base pairing in the proton transferred form. Such proton transfer upon electron attachment is also observed for the [A(N6b)-U]- and [A(C2)-U]- anions. Electron attachment to the A-U(N3) radical causes strong lone pair repulsion between the adenine N1 and the uracil N3 atoms, driving the two bases apart. Similarly, lone pair repulsion in the anion A(N6a)-U causes the loss of coplanarity of the two base units. The computed adiabatic electron attachment energies for nine AU-H radicals range from 1.86 to 3.75 eV, implying that the corresponding (AU-H)- anions are strongly bound. Because of the large AEAs of the (AU-H) radicals, the C-H and N-H bond dissociation in the AU- base pair anions requires less energy than the neutral AU base pair. The computed C-H and N-H bond dissociation energies for the AU- anion (i.e., the AU base pair plus one electron) are in the range 1.0-3.2 eV, while those for neutral AU are 4.08 eV or higher.     

Bond Dissociation Free Energies (BDFEs) of the Acidic H-A Bonds in HA(*)(-) Radical Anions by Three Different Pathways

Cleavage of radical anions, HA(*)(-), have been considered to give either H(*) + A(-) (path a) or H(-) + A(*) (path b), and factors determining the preferred mode of cleavage have been discussed. It is conceivable that cleavage to give a proton and a radical dianion, HA(*)(-) right harpoon over left harpoon H(+) + A(*)(2)(-) (path c), might also be feasible. A method, based on a thermodynamic cycle, to estimate the bond dissociation free energy (BDFE) by path c has been devised. Comparison of the BDFEs for cleavage of the radical anions derived from 24 nitroaromatic OH, SH, NH, and CH acids by paths a, b, c has shown that path c is favored thermodynamically.     

Ion trap collision-induced dissociation of multiply deprotonated RNA: c/y-ions versus (a-B)/w-ions

The dissociation of model RNA anions has been studied as a function of anion charge state and excitation amplitude using ion trap collisional activation. Similar to DNA anions, the precursor ion charge state of an RNA anion plays an important role in directing the preferred dissociation channels. Generally, the complementary c/y-ions from 5′ P-O bond cleavage dominate at low to intermediate charge states, while other backbone cleavages appear to a limited extent but increase in number and relative abundance at higher excitation energies. The competition between base loss, either as a neutral or as an anion, as well as the preference for the identity of the lost base are also observed to be charge-state dependent. To gain further insight into the partitioning of the dissociation products among the various possible channels, model dinucleotide anions have been subjected to a systematic study. In comparison to DNA, the 2′-OH group on RNA significantly facilitates the dissociation of the 5′ P-O bond. However, the degree of excitation required for a 5′ base loss and the subsequent 3′ C-O bond cleavage are similar for the analogous RNA and DNA dinucleotides. Data collected for protonated dinucleotides, however, suggest that the 2′-OH group in RNA can stabilize the glycosidic bond of a protonated base. Therefore, base loss from low charge state oligonucleotide anions, in which protonation of one or more bases via intramolecular proton transfer can occur, may also be stabilized in RNA anions relative to corresponding DNA anions.     

Fluoride ion transfer and stabilisation of reactive ions

Some general guidelines for the generation of salts with high fluoride ion donor ability are discussed. A preliminary scale for a limited number of fluoride ion donors is presented, cations are classified according to their anion-cation interaction properties. Examples for fluoride ion transfer reactions are given and the influence of anion-cation interaction on the stabilisation of reactive anions is discussed. In our work mainly TAS fluoride (Me₂N)₃S⁺Me₃SiF₂⁻ has been used as fluoride ion source: (a) for fluoride ion transfer to coordinatively unsaturated sulfur species, to SN and SO multiple bond systems, to SN, PN and CN heterocycles, (b) for the stabilisation of primary products of nucleophilic attacks and of intermediates in isomerisation processes or of intermediates in polymerisation processes, (c) for the generation of “naked” anions by silicon-element bond cleavage, and (d) for the activation of element-fluorine bonds by anion formation.     

Moderate and advanced intramolecular proton transfer in urea-anion hydrogen-bonded complexes

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.     

Organoiron activation combined with electron- and proton transfer: implications in biology, organic synthesis, catalysis and nanosciences

This Account summarizes the results obtained in our research group on the intra- and intermolecular organoiron activation of substrates by combining the coordination of arenes by CpFe^+/0 and electron and/or proton transfer. The concepts involved are those of electron and proton reservoirs, activation of O₂ by single electron transfer in solution, mimic and inhibition of the reactivity of superoxide radical anion, materials synthesis (for instance fullerene anions), electronic communication between two metals connected by a hydrocarbon bridge, activation of arene ligands for multiple functionalization, giant dendrimer synthesis and electron transfer in catalysis (redox and electron-transfer-chain).     

Photoinduced Color Change and Photomechanical Effect of Naphthalene Diimides Bearing Alkylamine Moieties in the Solid State

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical‐anion species, which may be formed through a photoinduced electron‐transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed‐valence stacks and radical‐anion stacks, as shown by the broad absorption bands in near‐IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed‐valence stacks, and radical‐anion stacks.     

Methyl and silyl mesolytic dissociations in the radical cations and radical anions of but-1-ene,allylsilane, hexa-1,3-diene,and penta-2,4-dienylsilane. CAS-MCSCF and coupled cluster theoretical study

Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.     

Crystal and molecular structures of (9-methylfluoren-9-yl)trimethyltin and (fluoren-9-yl)trimethyltin

Conclusions An increase in the length of the Sn-C (fluorene) bond is not observed in crystals of 9-fluorenyl derivatives of tin, and the variation in the lengths of the Sn-Me bonds is similar to that found in other types of organotin compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2054–2057, September, 1980.