Modification of 1-substituents in the 2-azabicyclo[2.1.1]hexane ring system; approaches to potential nicotinic acetylcholine receptor ligands from 2,4-methanoproline derivatives

Successful nucleophilic substitution at a methylene attached to the bridgehead (1-) position of the 2-azabicyclo[2.1.1]hexane ring system opens the way to construction of novel derivatives having a wider range of functional groups attached to the 1-position via a methylene "spacer" (including the beta-amino acid homologue of 2,4-methanoproline) and provides access to epibatidine analogues containing heterocyclic substituents and also to further homologation at the 1-position. Displacements with loss of a nucleofuge (e.g., loss of mesylate anion from the 1-mesyloxymethyl derivative) require thermal activation but proceed without the rearrangement initially anticipated in such a strained bicyclic ring system. A novel tricyclic carbamate intermediate 17 has been isolated; nucleophilic attack on 17 leads directly to the isolation of N-deprotected substitution products (with concomitant decarboxylation).     

Chlorination of 2-phenoxypropanoic acid with NCP in aqueous acetic acid: using a novel ortho-para relationship and the para/meta ratio of substituent effects for mechanism elucidation

Rate constants were measured for the oxidative chlorodehydrogenation of (R,S)-2-phenoxypropanoic acid and nine ortho-, ten para- and five meta-substituted derivatives using (R,S)-1-chloro-3-methyl-2,6-diphenylpiperidin-4-one (NCP) as chlorinating agent. The kinetics was run in 50% (v/v) aqueous acetic acid acidified with perchloric acid under pseudo-first-order conditions with respect to NCP at temperature intervals of 5 K between 298 and 318 K, except at the highest temperature for the meta derivatives. The dependence of rate constants on temperature was analyzed in terms of the isokinetic relationship (IKR). For the 20 reactions studied at five different temperatures, the isokinetic temperature was estimated to be 382 K, which suggests the preferential involvement of water molecules in the rate-determining step. The dependence of rate constants on meta and para substitution was analyzed using the tetralinear extension of the Hammett equation. The parameter lambda for the para/meta ratio of polar substituent effects was estimated to be 0.926, and its electrostatic modeling suggests the formation of an activated complex bearing an electric charge near the oxygen atom belonging to the phenoxy group. A new approach is introduced for examining the effect of ortho substituents on reaction rates. Using IKR-determined values of activation enthalpies for a set of nine pairs of substrates with a given substituent, a linear correlation is found between activation enthalpies of ortho and para derivatives. The correlation is interpreted in terms of the selectivity of the reactant toward para- or ortho-monosubstituted substrates, the slope of which being related to the ortho effect. This slope is thought to be approximated by the ratio of polar substituent effects from ortho and para positions in benzene derivatives. Using the electrostatic theory of through-space interactions and a dipole length of 0.153 nm, this ratio was calculated at various positions of a charged reaction center along the benzene C1-C4 axis, being about 2.5 near the ring and decreasing steeply with increasing distance until reaching a minimum value of -0.565 at 1.3 nm beyond the aromatic ring. Activation enthalpies and entropies were estimated for substrates bearing the isoselective substituent in either ortho and para positions, being demonstrated that they are much different from the values for the parent substrate. The electrophilic attack on the phenolic oxygen atom by the protonated chlorinating agent is proposed as the rate-determining step, this step being followed by the fast rearrangement of the intermediate thus formed, leading to products containing chlorine in the aromatic ring.     

Aromatic nucleophilic substitution (S<Subscript>N</Subscript>Ar) Reactions of 1,2- and 1,4-halonitrobenzenes and 1,4-dinitrobenzene with carbanions in the gas phase

In the gas-phase reactions of halonitro- and dinitrophenide anions with X (X = F, Cl, Br, NO(2)) and NO(2) groups in ortho or para position to each other with selected C-H acids: CH(3)CN, CH(3)COCH(3), and CH(3)NO(2), products of the S(N)Ar-type reaction are formed. Nitrophenide anions are generated by decarboxylation of the respective nitrobenzenecarboxylate anions in ESI ion source and the S(N)Ar reaction takes place either in the medium-pressure zone of the ion source or in the collision chamber of the triple quadrupole mass spectrometer. In the case of F, Cl, and NO(2) derivatives, the main ionic product is the respective [NO(2)-Ph-CHR](-) anion (R = CN, COCH(3), NO(2)). In the case of Br derivatives, the main ionic product is Br(-) ion because it has lower proton affinity than the [NO(2)-Ph-CHR](-) anion (for R = CN, COCH(3)). For some halonitrophenide anion C-H acid pairs of reactants, the S(N)Ar reaction is competed by the formation of halophenolate anions. This reaction can be rationalized by the single electron-transfer mechanism or by homolytic C-H bond cleavage in the proton-bound complex, both resulting in the formation of the halonitrobenzene radical anion, which in turn undergoes -NO(2) to -ONO rearrangement followed by the NO(.) elimination.     

Gas-phase identity nucleophilic substitution reactions of cyclopropenyl halides

The gas-phase identity nucleophilic substitution reactions of halide anions (X = F, Cl, and Br) with cyclopropenyl halides, X(-) + (CH)(3)X <= => X(CH)(3) + X(-), are investigated theoretically at four levels of theory, B3LYP/6-311+G**, MP2/6-311+G**, G2(+)MP2//MP2/6-311+G**, and G2(+)//MP2/6-311+G**. Four types of reaction paths, the sigma-attack S(N)2, pi-attack S(N)2'-syn, and S(N)2'-anti and sigmatropic 1,2-shift, are possible for all the halides. In the fluoride anion reactions, two types of stable adducts, syn- and anti-1,2-difluorocyclopropyl anions, can exist on the triple-well-type potential energy surface of the identity substitution reactions with rearrangement of double bond (C=C), S(N)2'-syn, and S(N)2'-anti processes. The TSs for the sigma-attack S(N)2 paths have "open" (loose) structures so that the ring positive charges are high rendering strong aromatic cyclopropenyl (delocalized) cation-like character. In contrast, in the pi-attack S(N)2' paths, a lone pair is formed at the unsubstituted carbon (C3), which stabilizes the 1,2-dihalocyclopropyl (delocalized) anion-like TS by two strong n(C)-sigma*(C-F) vicinal charge-transfer delocalization interactions. The barrier height increases in the order S(N)2'-anti < sigma-attack S(N)2 < S(N)2'-syn for X = Cl and Br, whereas for X = F the order is changed to S(N)2'-anti < S(N)2'-syn < sigma-attack S(N)2 due to the stable difluoro adduct formation. The sigmatropic 1,2-shift (circumambulatory) reactions have high activation barriers and cannot interfere with the substitution reactions.     

Ring-hydrogen participation in the keto--enol isomerization of the acetophenone radical cation

Molecular ions obtained from acetophenone have been observed to undergo proton transfer reactions in competition with unimolecular blackbody dissociation in a Fourier transform ion cyclotron resonance spectrometer provided with an in situ high temperature blackbody source. The ionizing energy dependence of these two processes and generation of the enol molecular ion by fragmentation of butyrophenone reveal that the keto ion undergoes blackbody dissociation exclusively while the enol ion promotes fast proton transfer reactions and undergoes very slow blackbody induced dissociation. Experiments with labeled acetophenone either on the methyl group or on the ring reveal that the enol ions can transfer both H+ and D+ suggesting that the mechanism responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mechanism. Theoretical calculations at the B3LYP level predict that the most favorable pathway for unimolecular isomerization of the keto ion involves initial migration of an ortho hydrogen to the carbonyl. The subsequent rearrangement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the experimental results. The possibility that isomerization may also occur by a direct 1,3-hydrogen migration is also explored in terms of possible excited electronic states of the ion.     

Diheteroarylmethanes. 5.(1) E-Z Isomerism of Carbanions Substituted by 1,3-Azoles: (13)C and (15)N pi-Charge/Shift Relationships as Source for Mapping Charge and Ranking the Electron-Withdrawing Power of Heterocycles

Previously proposed pi-charge/shift relationships have been applied to (13)C and (15)N shifts of the carbanions of 2-benzylazoles (thiazole, oxazole, and imidazole), their corresponding benzo-fused analogs, and bis(2-azolyl)methanes (azolyl groups as above). In this way it is possible to rank the pi electron-withdrawing power of these heterocycles in terms of charge demands c(X), a quantity representing the fraction of pi negative charge withdrawn (delocalized) by the ring. The results indicate that c(thiaz) > c(oxaz) > c(imidaz); furthermore, benzoazoles are more efficient than monocyclic systems in delocalizing the negative charge. The charge demand c(X) of imidazole is the smallest among the heteroaromatics so far considered, being even smaller than that of the phenyl ring. As a consequence, the negative charge in the anion of 2-benzyl-N-methylimidazole is predominantly transferred from the carbanionic carbon to the phenyl group rather than to the imidazolyl residue. The high double bond character of the bond linking the carbanionic and ipso phenyl ring carbons leads to room temperature (13)C shift anisochrony of the meta and meta' and ortho and ortho' positions of the phenyl ring. In all of the other cases, hindered rotation is observed at room temperature between the carbanionic carbon and position 2 of the heterocycle. A single set of resonances is presented by the bis(heteroaryl)methyl carbanions. pi-Charge/shift relationships allow for the accurate pi-charge mapping in these carbanionic systems, and the results point to considerable delocalization of the electron pair(s) of the oxygen and pyrrolic nitrogen atoms at position 1 in oxazole and imidazole toward the pyridic nitrogen at position 3 of the rings (in both the neutrals and the carbanionic species). On the contrary, not only does the sulfur atom in thiazole derivatives not delocalize any negative charge in the anions but it is barely involved in any pi-donation to the pyridic nitrogen atom at position 3 also in the neutrals.     

氢键活化的苯环上C-F键的亲核取代反应研究

以2,4,5-三氟苯乙酮在氢化钠作用下与碳酸二乙酯反应制得3,4-二氟-6-乙氧基苯甲酰乙酸乙酯(2);2经对甲基苯磺酸催化水解脱羧,合成3,4-二氟-6-乙氧基苯乙酮(3)路线为模型,研究了苯环上C-F键的亲核取代反应.实验结果证明,在形成分子内氢键的活化作用下,苯环上的C-F键能够被烷氧负离子亲核取代.2和3的结构经1H NMR, 19F NMR和MS表征.     

Loss of benzene to generate an enolate anion by a site-specific double-hydrogen transfer during CID fragmentation of o-alkyl ethers of ortho-hydroxybenzoic acids

Collision-induced dissociation of anions derived from ortho-alkyloxybenzoic acids provides a facile way of producing gaseous enolate anions. The alkyloxyphenyl anion produced after an initial loss of CO(2) undergoes elimination of a benzene molecule by a double-hydrogen transfer mechanism, unique to the ortho isomer, to form an enolate anion. Deuterium labeling studies confirmed that the two hydrogen atoms transferred in the benzene loss originate from positions 1 and 2 of the alkyl chain. An initial transfer of a hydrogen atom from the C-1 position forms a phenyl anion and a carbonyl compound, both of which remain closely associated as an ion/neutral complex. The complex breaks either directly to give the phenyl anion by eliminating the neutral carbonyl compound, or to form an enolate anion by transferring a hydrogen atom from the C-2 position and eliminating a benzene molecule in the process. The pronounced primary kinetic isotope effect observed when a deuterium atom is transferred from the C-1 position, compared to the weak effect seen for the transfer from the C-2 position, indicates that the first transfer is the rate determining step. Quantum mechanical calculations showed that the neutral loss of benzene is a thermodynamically favorable process. Under the conditions used, only the spectra from ortho isomers showed peaks at m/z 77 for the phenyl anion and m/z 93 for the phenoxyl anion, in addition to that for the ortho-specific enolate anion. Under high collision energy, the ortho isomers also produce a peak at m/z 137 for an alkene loss. The spectra of meta and para compounds show a peak at m/z 92 for the distonic anion produced by the homolysis of the O--C bond. Moreover, a small peak at m/z 136 for a distonic anion originating from an alkyl radical loss allows the differentiation of para compounds from meta isomers. Copyright (c) 2008 John Wiley & Sons, Ltd.     

1H, 13C and 19F NMR spectroscopy of polyfluorinated ureas. Correlations involving NMR chemical shifts and electronic substituent effects

Seventeen N-(mono-, di-, tri-, tetra- and penta-fluorophenyl)-N'-(3-nitrophenyl)ureas were prepared and characterized. Complete assignment of their (1)H, (13)C and (19)F NMR data was undertaken and the correlation of the chemical shifts of the ureido protons with field-inductive and mesomeric electronic substituent parameters was studied using the Swain-Lupton model. The best correlations were obtained when the study was limited to certain substitution patterns, e.g. non-ortho, mono-ortho- and di-ortho-fluorinated ureas, which reveal probable changes in conformations caused by the degree of ortho fluorination at the phenyl ring. Additionally, there is an excellent linear cross-correlation between the chemical shifts of the fluorine atoms and the ipso carbon atoms for the whole group of fluorinated ureas.     

Alkylating agents stronger than alkyl triflates

A new class of potent electrophilic "R(+)" alkylating agents has been developed using weakly nucleophilic carborane anions as leaving groups. These reagents, R(CHB(11)Me(5)X(6)) (R = Me, Et, and i-Pr; X = Cl, Br), are prepared via metathesis reactions with conventional alkylating agents such as alkyl triflates, using the high oxophilicity of silylium ion-like species, Et(3)Si(carborane), as the driving force to obtain increased alkyl electrophilicity. The crystal structure of the isopropyl reagent, i-Pr(CHB(11)Me(5)Br(6)), has been determined, revealing covalence in the alkyl-carborane bonding. This contrasts with the free i-Pr(+) carbocation observed when the anion is less coordinating (e.g. Sb(2)F(11)(-)) or with tertiary alkyl centers, as in [tert-butyl][carborane] salts. In solution, the reagents exist as equilibrating isomers with the alkyl group at the 7-11 or 12 halide positions of the CB(11) icosahedral carborane anion. These alkylating agents are so electrophilic that they (a) react with alkanes at or below room temperature via hydride extraction to produce carbenium ions, (b) alkylate benzene without a Friedel-Crafts catalyst to give arenium ions, and (c) alkylate electron-deficient phosphorus compounds that are otherwise inert to conventional alkylating agents such as methyl triflate.     

Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chemical calculations

The molecular structures of ethynylbenzene and s-triethynylbenzene have been accurately determined by gas-phase electron diffraction and ab initio/DFT MO calculations and are compared to that of p-diethynylbenzene from a previous study [Domenicano, A.; Arcadi, A.; Ramondo, F.; Campanelli, A. R.; Portalone, G.; Schultz, G.; Hargittai, I. J. Phys. Chem. 1996, 100, 14625]. Although the equilibrium structures of the three molecules have C2v, D3h, and D2h symmetry, respectively, the corresponding average structures in the gaseous phase are best described by nonplanar models of Cs, C3v, and C2v symmetry, respectively. The lowering of symmetry is due to the large-amplitude motions of the substituents out of the plane of the benzene ring. The use of nonplanar models in the electron diffraction analysis yields ring angles consistent with those from MO calculations. The molecular structure of ethynylbenzene reported from microwave spectroscopy studies is shown to be inaccurate in the ipso region of the benzene ring. The variations of the ring C-C bonds and C-C-C angles in p-diethynylbenzene and s-triethynylbenzene are well interpreted as arising from the superposition of independent effects from each substituent. In particular, experiments and calculations consistently show that the mean length of the ring C-C bonds increases by about 0.002 A per ethynyl group. MO calculations at different levels of theory indicate that though the length of the C[triple bond]C bond of the ethynyl group is unaffected by the pattern of substitution, the C(ipso)-C(ethynyl) bonds in p-diethynylbenzene are 0.001-0.002 A shorter than the corresponding bonds in ethynylbenzene and s-triethynylbenzene. This small effect is attributed to conjugation of the two substituents through the benzene ring. Comparison of experimental and MO results shows that the differences between the lengths of the C(ipso)-C(ethynyl) and C(ipso)-C(ortho) bonds in the three molecules, 0.023-0.027 A, are correctly computed at the MP2 and B3LYP levels of theory but are overestimated by a factor of 2 when calculated at the HF level.     

Nitropyrazoles 17. Synthesis of 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole and the study of its chemical transformations

A new one-step method for the synthesis of 4-methyl-3(5)-nitropyrazole by nitration of 4-methylpyrazole is developed. Arylation of 4-methyl-3(5)-nitropyrazole with 1,3,5-trinitrobenzene gives 1-(3,5-dinitrophenyl)-4-methyl-3-nitropyrazole, nitration of which leads to 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole. The action of 1 equiv. of an O- or S-nucleophile (phenolate, p-chlorobenzenethiolate and ethoxide ions; anions of glycolanilide and thioglycolanilide) on 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole led to the substitution of the 5-NO₂ group of the pyrazole ring; under the action of one more equivalent of a nucleophile the NO₂ group of the benzene ring was substituted. The substitution product of the anion of thioglycolanilide for the 5-NO₂ group undergoes the intramolecular cyclization — oxidative nucleophilic hydrogen substitution in the benzene ring under the action of K₂CO₃.     

The effects of sulfur substitution for the nucleophile and bridging oxygen atoms in reactions of hydroxyalkyl phosphate esters

The effects of sulfur substitution on the reactions of hydroxyalkyl phosphate esters are examined. These compounds are models for the intramolecular phosphoryl transfer reaction involved in the cleavage of the internucleotide bond in RNA. The models studied here lack the ribose ring and their conformational flexibility results in greater stability and the availability of different reaction pathways. Sulfur in the nucleophilic position shows no nucleophilic reaction at phosphorus, instead rapidly attacking at the beta carbon atom, forming thiirane with departure of a phosphomonoester. Sulfur substitution at either of the two bridging positions leads to cleavage of the diester via formation of a cyclic intermediate, but with significant rate acceleration when compared to the oxygen analogues. The bridge-substituted models react substantially slower than the analogous ribose compounds with sulfur substitution at comparable positions. Kinetic isotope effects reveal significant differences in the transition state depending on which bridging position sulfur occupies. When sulfur is in the scissile bridging position, a highly associative transition state is indicated, with a largely formed bond to the nucleophile and the scissile P-S bond is little changed. When sulfur occupies the other bridging position, the isotope effects imply a very early transition state in a concerted reaction.     

羧基导向C―H官能团化/脱羧偶联反应研究进展

以邻或对位取代苯甲酸为原料,通过羧基导向的芳香羧酸邻位碳氢键官能团化继而发生脱羧反应,在原羧基的邻位引入官能团,可以合成传统付-克反应难以合成的间位取代芳香化合物。在此类反应中,羧基充当无痕导向基的功能。本文综述了基于过渡金属催化羧基无痕导向的芳香羧酸脱羧偶联策略,形成新C―C、C―杂键的研究进展。     

Palladium-arene interactions in catalytic intermediates: an experimental and theoretical investigation of the soft rearrangement between eta1 and eta2 coordination modes

A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C(ipso) atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C(ipso) = 2.22(1) A (average)], and due to the arene's tilting, the substituted C(ortho) atom is relatively close to the metal atom [2.54(1) A (average)]. The coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) and its related substituted derivatives. The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE < or = 1 kcal x mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C(ortho) bond in the seemingly eta2-type conformer. (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible. A significant strain affects the transition-state structure featuring a Pd-HC(aryl) agostic-type bond. The calculated destabilization of 10.3 kcal x mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects. For all of the other cases, a slipped eta2 coordination may be described. As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C(ipso) atom. The effect may be slightly improved if the C(ortho) atom also gets closer to the metal, but in no case, does the slipped eta2 coordination seem to be crucial for the stability of the system.     

Quantum Chemical Study of Mechanisms of Organic Reactions: VII. Reaction of Ethane-1,2-dithiol with 1,3-Dichloropropene in the System Hydrazine Hydrate–KOH

A mechanism has been proposed for the reaction of 1,3-dichloropropene with ethane-1,2-dithiol in the system hydrazine hydrate–potassium hydroxide on the basis of DFT quantum chemical calculations [B3LYP/6-311++G(d,p)]. The proposed mechanism involves several consecutive steps, in particular nucleophilic substitution of chlorine at the sp³-hybridized carbon atom by sulfur, prototropic allylic rearrangement of the monosubstitution product with double bond migration toward the sulfur atom, dithiolane ring closure via nucleophilic attack of the second thiolate group on the carbon atom in the γ-position with respect to the second chlorine atom, and prototropic allylic rearrangement of 2-vinyl-1,3-dithiolane to more stable 2-ethylidene-1,3-dithiolane.     

Preparation of amphiphilic organoiron(1+) complexes having two long-chain alkyl groups and their molecular assemblage characteristics

[o-, m- and p-Bis(alkylamino or alkyloxy)benzene] (cyclopentadienyl)iron(1+) hexafluorophosphates {2 and 4; [(C_nH_2n+1X)₂C₆H₄](C₅H₅)Fe⁺PF₆^?, X?NH or O} were prepared by aromatic nucleophilic substitution of the (dichlorobenzene)iron cationic complexes (1). Critical micelle concentrations of the complex chlorides (3), prepared from 2 (n=8, X?NH) by anion exchange and soluble in water, gave much smaller values than those of bis(long-chain alkyl)dimethylammonium surfactants. Furthermore, the substitution positions scarcely affected their surface activites. However, the surface pressure-molecular area isotherm of the hexafluorophosphates (2 and 4, n=18, X?NH; insoluble in water) were severely transformed by change in the substitution position of the long-chain alkyl groups on the benzene ligand in the iron cationic complexes: the o-substituted complex gave a molecularly assembled film by the Langmuir-Blodgett (LB) method, but the P-substituted one did not.     

Sigma-complex formation and oxidative nucleophilic aromatic substitution in 4-nitro-2,1,3-benzoxadiazoles

The feasibility of carrying out nucleophilic displacement of hydrogen from highly electron-deficient heteroaromatics has been addressed through a detailed investigation of the interaction of a series of nitrobenzofuroxans 3a-i and two related heterocycles 5 and 6, with the 2-nitropropenide anion. Although this series corresponds to a large modulation in the electrophilic properties of the six-membered ring, all reactions first lead to the quantitative formation of the sigma-adducts C-3a-i, C-5 and C-6 arising from covalent addition of the nucleophile to the C-7 carbon. With the exception of the 4-aza substituted member C-5, all the adducts have been isolated as pure and very stable alkali salts. Measurements of the oxidation potentials by cyclic voltammetry reveal that the ease of subsequent hydrogen substitution at carbon-7 strongly decreases with increasing electron deficient character of the six-membered ring. The measured E0 values are in the range 0.5-0.6 V (vs. SCE) for the 4-nitro-benzofuroxan and -benzofurazan adducts (C-3e, C-3i) but they go up to 1.20-1.33 V for the 4,6-dinitro- and 4-nitro-6-trifluoromethanesulfonylbenzofuroxan adducts (C-3a,d) in acetonitrile. Consistently with these E0 values, only very powerful oxidants such as the Ce4+/Ce3+ or the MnO4-/Mn2+ couples can successfully oxidize the adducts leading to the expected substitution products in moderate to good yields (35-72%). Interestingly, the rearomatization of the 4-nitro substituted benzofuroxan adducts proceeds with a partial Boulton-Katritzky rearrangement of the resulting products. Another noteworthy result is that the 4-nitrobenzofuroxan and 4-nitrobenzofurazan molecules suffer competitive addition of the (CH3)2C-NO2- anion to the 5- and 7- positions under some experimental conditions. This represents the first well-defined example of isomeric addition of a carbon nucleophile to these heterocycles.     

Effect of chalcogenyl substituent on the course of allyl rearrangement at chalcogenation of 1,3-dichloropropene

Formation of 1,3-dichalcogenylpropene at the treatment of 1,3-dichloropropene with organic dichalcogenides in a redox system hydrazine hydrate–KOH is governed by the possibility of an allyl rearrangement. In the presence of bases this rearrangement proceeds via carbanion partially stabilized by the chalcogenyl substituent. The effectivity of the stabilization and consequently the probability of the rearrangement varies in the series of substituents PhS > BnS > PhSe. In the stage of the direct nucleophilic substitution of chlorine the anion PhSe^? possesses the highest activity.     

Doubly protonated benzene in the gas phase

Structural aspects and the unimolecular fragmentations of doubly protonated benzene are studied by means of tandem-mass spectrometry. The corresponding dications are generated by electron ionization (EI) of 1,3- and 1,4-cyclohexadienes, respectively. It is suggested that EI of 1,3-cyclohexadiene leads to the singlet state of doubly protonated benzene, whereas EI of 1,4-cyclohexadiene yields a mixture of singlet and triplet states. Unimolecular fragmentation of doubly protonated benzene exclusively proceeds via dehydrogenation leading to the benzene dication. The proton affinities (PAs) of protonated benzene amount to PA(C(6)H(7)(+))(meta) = 1.9 +/- 0.3 eV for protonation taking place at the meta-position, PA(C(6)H(7)(+))(ortho) = 1.5 +/- 0.2 eV, and PA(C(6)H(7)(+))(para) = 0.9 +/- 0.2 eV, respectively. Various facets of the experiments are compared with density functional theory calculations and generally good agreement is found.