Cations 1a Research Articles

Synthesis and crystal structure of the bromide salt of the inside protonated form of the cage amine [(2.3)3]adamanzane, 1,4,8,12-tetraazatricyclo[6.6.3.24,12]nonadecane and synthesis of the bowl amine [(2.3)2.21]adamanzane, 1,5,9,12-tetraazabicyclo[7.5.2]hexadecane

The reaction of 1,4,7-triazacyclononane with tris(3-chloropropyl)amine affords the inside monoprotonated form of the tricyclic amine 1,4,8,12-tetraazatricyclo[6.6.3.24,12]nonadecane (3), which was isolated as the bromide salt, [H[(2.3)3]adz]Br (yield 38%). The crystal structure of [H[(2.3)3]adz]Br·4H2O has been solved by X-ray diffraction at T = 120 K. In the i+,i,i,i-H[(2.3)3]adz+ cation (3a) the acidic hydrogen atom and the lone pairs of the nitrogen atoms are oriented towards the inside of the cavity. The acidic hydrogen atom is attached to the apical nitrogen atom and both are positioned on a three-fold axis. The distances between the nitrogen atoms are in the range of 2.73(2)–2.99(1) Å. The inside coordinated proton in H[(2.3)3]adz+ (3a) is unusually inert to reaction with base (kdiss < 4 × 10–9 s–1 at 25 °C in 0.01 M NaOD). From 1H and 13C NMR it is concluded that in slightly acidic (pH > 2) and in basic aqueous solutions the dominant form of the cage has all four lone pairs pointing into the cavity. In concentrated strong acid protonation occurs and is believed to involve inversion at one or several of the bridgehead nitrogen atoms. In concentrated boiling hydrochloric acid the cage undergoes a slow cleavage of one trimethylene bridge to give the bowl amine [(2.3)2.21]adz (7), 1,5,9,12-tetraazabicyclo[7.5.2]hexadecane, which was isolated as the triprotonated bromide dichloride salt, [H3[(2.3)2.21]adz]BrCl2·H2O (yield 40%).

A New 1-Alkoxy-2-(chalcogeno)allylic or 1-Alkoxy-2,4-bis(chalcogeno)penta-2,4-dienyl Cation: Highly-Regioselective Allylating or Penta-2,4-dienylating Electrophiles and Their Reactions

2-(Chalcogeno)-1-ethoxyallylic cations 4A are easily generated from the reactions of 2-(chalcogeno)-1-ethoxyprop-1-en-3-ols 2a−e or 2-(chalcogeno)prop-2-enal acetals 3a−c and TMSOTf and reacted with various nucleophiles to give the adducts 5a−8a, 5b−11b, and 5c−e regio and stereoselectively. 2,4-Bis(chalcogeno)penta-2,4-dienal acetals 16a,c and the 2,4,6-tris(phenylthio)hepta-2,4,6-trienal acetal 20c also gave the dienones 17a,c and 21c in good yields. Furthermore, the intramolecular cyclization of the alkenyl alcohols 25a,b and the 2,4-dienal 2,4-dinitrophenyl hydrazones 29a−c afforded the tetrahydrofurans 26a,b and 3,5-bis(chalcogeno)pyridines 30a−c, respectively.

Flash Photolysis of 1,3-Dichloro-1,3-diphenylpropane in Polar Solvents: Generation of a Stabilized γ-Chloropropyl Cation, Subsequent Formation of a Propenyl Cation, and Nucleophilic Trapping of Both Cations

Photolysis of 1,3-dichloro-1,3-diphenylpropane (1) in 2,2,2-trifluoroethanol at 266 nm leads to a γ-chloropropyl cation (2a) following loss of chloride ion. Kinetic evidence has been obtained for the interconversion between the open chain cation and the corresponding cyclic chloronium ion (2b); thermal loss of hydrogen chloride gives the 1,3-diphenyl-2-propenyl cation (3). The formation of this species occurs through a monophotonic process; the involvement of a two-photon process in the photoheterolysis of dihalide 1 has been ruled out by studying the formation of cation 3 from 1 at different laser intensities. The lack of power dependence implies that 3-chloro-1,3-diphenylpropene (4) is not an intermediate in the photoheterolysis of dihalide 1. The reaction of both cationic intermediates (2 and 3) with different nucleophiles has also been examined. Finally, two-laser two-photon experiments have allowed the study of the photoisomerization of E,E-3 into Z,E-3.

Electrochemical reduction of some [( η5-Cyclopentadienyl) ( η6-arene) iron(II)][PF 6] complexes bearing an imine or a nitrone function in benzylic position of the arene ligand

The electrochemical reduction in basic hydro-organic media of mixtures of [( η 5-Cp) ( η 6-benzophenone- N-(aryl)nitrone) Fe] + 1a–d and [( η 5-Cp) ( η 6-benzophenone anil) Fe] + complexes 1′a–d led to the corresponding [( η 5-Cp) ( η 6- α-(arylamino)diphenylmethane) Fe] + cations 2a–d with good yields (85–90%). In a similar manner, various [( η 5-Cp) ( η 6-9-(arylamino)fluorene) Fe] + cations 4a–i were synthesized (70–88% yield) in acidic hydro-organic media from the corresponding [( η 5-Cp) ( η 6-fluorenone anil) Fe] + 3a–i or the [( η 5-Cp) ( η 6-fluorenone- N-(2′-methoxycarbonyl phenyl)nitrone) Fe] + derivative 3′i. According to 1H NMR chemical shifts, the exo isomer is formed in the majority ( exo: endo ca. 75:25) during electrosynthesis of compounds 4a–i. We could explain these unexpected results by an epimerization in the electrolysis media. The amines 4a,b were re-oxidized by oxygen into the starting imine 3a,b under very mild Al 2O 3-catalysis conditions (77–80% yield).

Products from photochemical reactions and electrochemical oxidation of methylenecyclopropane

The reactivity of methylenecyclopropane (MCP, 1) and its radical cation (1+•) have been studied in the presence and absence of a nucleophile (methanol). Photochemical reactions of 1 in the presence of an electron-acceptor (1,4-dicyanobenzene, 6) and a codonor (biphenyl, 7) in acetonitrile (with and without methanol present) or chloroform lead to cycloadditions (ortho, meta, and para; products 12–17) rather than products from photoinduced electron transfer (PET). Based on the measured (cyclic voltammetry, CV) oxidation potential, using the Weller equation, electron transfer (ET) was predicted to occur. It was shown that the measured oxidation potential of 1 represents the adiabatic ionization potential. For PET processes the value for the vertical ionization potential must be used. Electrochemical (EC) generation of 1+• without a nucleophile present results in the formation of one major product: tert-butyl acetamide (25). A series of rearrangements leading to the tert-butyl cation is proposed. Addition of a nucleophile (methanol) to the mixture leads to the formation of 3-methoxy-2-(methoxymethyl)-1-propene (26). This product may arise from trapping of the initially formed ring-opened (trimethylenemethane) radical cation (1a+•), which undergoes a second oxidation and nucleophilic addition (ECE). Keywords: methylenecyclopropane, radical cation, photochemistry, electrochemistry, photocycloaddition.

Cationic Zirconium Dialkyl and Alkyl Complexes Supported by DAC (Deprotonated 4,13-Diaza-18-crown-6) Ligation

The synthesis, characterization, and reactivity of a series of neutral and cationic Zr alkyls supported by DAC (deprotonated 4,13-diaza-18-crown-6) ligation are reported. Reaction of H2DAC with Zr(CH2Ph)4 affords a 1:4 mixture of cis- and trans-Zr(DAC)(CH2Ph)2 (cis-/trans-1a). The pure isomers undergo slow cis−trans isomerization in solution to regenerate the 1:4 cis:trans equilibrium mixture. X-ray crystallographic results are reported for both cis- and trans-1a. Reaction of Zr(CH2Ph)2Cl2 with H2DAC, followed by treatment with LiR (2 equiv), gives cis-Zr(DAC)R2 (R = CH2SiMe3, cis-1b; R = CH2CMe3, cis-1c) exclusively. Alkyl abstraction from cis- or trans-1a using B(C6F5)3 (1 equiv) produces the stable cation [Zr(DAC)(CH2Ph)]+[B(CH2Ph)(C6F5)3]- (2a) as a yellow oil. NMR studies on 2a in CD2Cl2 show no evidence for η2-benzyl formation or anion coordination. Protonation of cis- or trans-1a with [n-Bu3NH]+[BPh4]- similarly yields [Zr(DAC)(CH2Ph)]+[BPh4]- (2b). Cation 2a reacts with t-BuNC to form the vinyl amide complex [Zr(DAC){N(t-Bu)CHCHPh}]+[B(CH2Ph)(C6F5)3]- (3). p-Tolylacetylene undergoes catalytic dimerization to (Z)-1,4-di-p-tolyl-1-buten-3-yne in the presence of 2a.

Electron-transfer reaction of 1,2-disila-3,5-cyclohexadienes

Photolysis of 1,2-disila-3,5-cyclohaxadienes 1a-b in the presence of methylene blue as a sensitizer led to efficient formation of the corresponding siloles as ring contraction products. The reaction mechanism is best rationalized by electron-transfer from 1a-b to the excited state of methylene blue. Semiempirical molecular orbital calculation was also carried out to characterize the geometric and electronic structure of radical cation 1a +•.

Mass spectrometry of tert-butylnaphthalenes–a comparison with the unimolecular fragmentation of tert-butylbenzene

The unimolecular fragmentation of ionized 1- and 2-tert-butylnaphthalenes 1 and 2 have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry and specifically deuterated and 13 C labeled derivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3. Metastable molecular ions of 1, 2 and 3 lose exclusively a methyl radical yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2propyl cations 3a. Further fragmentations of 1a and 2a include, as the main processes, the elimination of C 2H4 and of CH3 • . The loss of C2H4 is associated with a large kinetic energy release giving rise to a broad flat-topped peak in the MIKE spectra (as in the case of 3a) while loss of CH3 gives rise to a Gaussian shaped signal. The latter reaction is not observed for metastable 2-phenyl-2-propyl cations 3a. The fragmentations of labeled derivatives of 1 and 2 show that H/D exchange in 1a and 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C 2H4 and of CH3 • , is much more extensive than in 3a. This H/D exchange decreases for metastable 1a and 2a decomposing in the second field-free region, and the interchange of the C atoms within the C 3H6 is less than for 3a. These observations are explained by H migrations between the side chain and the naphthalene rings of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2-methyl shift known for the reactions of 3a. The differences observed for the reactions of 1a and 2a and of 3a are explained by the more basic aryl group of 1a and 2a. These fragmentation mechanisms are corroborated by an AM1 calculation of the minimum energy reaction pathways (MERP).

Charge-transfer complex formation and photo-induced electron-transfer reaction of dibenzo-7-silabicyclo[2.2.1]hepta-2,5-dienes

Dibenzo-7-silabicyclo[2.2.1]hepta-2,5-dienes ( 1a, 1b) are excellent electron donors because of effective σ-π conjugation between the orbitals of CC π bonds and SiC σ bonds. Some of their donor properties are demonstrated by the reactions with some electron acceptors. When 1a and 1b are mixed with tetracyanoethylene, facile formation of charge-transfer complexes was observed. In the 2,4,6-triphenylpyrylium tetrafluoroborate-sensitized photoreaction of 1b, the corresponding difluorosilane and anthracene were obtained in good yields. The structural and electronic features of radical cation 1a +• were provided by semiemperical molecular orbital calculation. In addition, the structure of 1a in crystals was determined by X-ray crystallography and compared with that obtained by the calculation.

Heterolytic fragmentation of 4-hydroxy-6,6,7,7-tetramethyl-Δ2-dehydro-quinuclidine-2,3-dicarboxylic acid esters

The title quinuclidines undergo cleavage under the action of acids or H2O to give Δ2-dehydro-4-piperidones 4, 5 and isobutylene; the resulting redistribution of bond lengths upon N-protonation has been demonstated by X-ray diffraction methods for picrate 2a as well as by ab initio calculations for quinuclidine la, cation 2a’ and zwitterion 2a”, the latter being the most suitable as an intermediate of the fragmentation.

Protonation of azuleno [1,2-a]acenaphthylene and 7-bromoazuleno[1,2-a]acenaphthylene in superacids: azulenium, acenaphthenium or naphthalenium cations?

Persistent monocation 1aH+ has been formed as the major product of low temperature protonation of azuleno [1,2-a] acenaphthylene 1 with FSO3H·SO2ClF or FSO3H·SbF5(4:1)–SO2ClF superacid media. Protonation at C-7(→1bH+) was observed only as a minor competing process (10–15%). Protonation of the 7-bromoazuleno[1,2-a]acenaphthylene 2 similarly gave the monocation 2aH+; the ipso protonated cation 2bH+ was not observed.The NMR characteristics of the resulting ions and their charge distribution patterns are discussed and compared with those of parent acenaphthemum 3H+ and azulenium 4H+ cations as models. Monocations la and 2a are best viewed as azulenium ions having strong tropylium ion character with limited charge delocalization into the naphthalene moiety. For comparison, the relative energies and changes in charge distribution Δq[qc(ion)–qc(neutral)] were calculated for cations 1a, 1b, 1c and 2a, 2b, 2c by the AM1 and PM3 methods.

Carbon−Carbon Bond Fragmentation in Aminoalcohol Radical Cations. Kinetics, Thermodynamic Correlations, and Mechanism

A detailed study of the kinetics, thermodynamics and mechansim of carbon−carbon bond fragmentation in a series of aminoalcohol radical cations is presented. The compounds that provide the basis for this investigation are derived from the parent structure, erythro-2-(phenylamino)-1,2-diphenylethanol, by substitution at the para position of the N-phenyl with methoxy, methyl, (hydrogen), chloro, and cyano groups (compounds 1a−e, respectively). The rates for C−C bond fragmentation for radical cations 1a−e•+ in CH3CN solution were determined by laser flash photolysis and vary from 3.9 × 104 (1a) to 7.4 × 106 s-1 (1e). The activation parameters for bond fragmentation in 1c−e•+ are characterized by low activation enthalpies and relatively large, negative activation entropies. The bond fragmentation rates increase with the peak potential for anodic oxidation of the neutral aminoalcohols, Ep(1). Correlation of the free energy of activation for bond fragmentation (ΔGBF⧧) with FEp(1) (F is the Faraday constant) impl...

Ring-opening reactions of pyrrolidinylcyclopropenyl cations in alkaline aqueous solution

Pyrrolidinylcyclopropenyl cations 1a,b in alkaline solution at room temperature are converted efficiently into (Z)-and (E)-α,β-unsaturated amides 3 and 4, respectively, with more electron-donating alkylsulfanyl groups at the β-position.

Stabilization of a Planar‐tetracoordinate Carbon Center in an Organometallic Complex Containing Both a Zirconocene and a Hafnocene Moiety

AbstractThe methylhafnocene cation 2a reacts with bispropynylzirco‐nocene to give a single regioisomeric planar‐tetracoordinate carbon compound (4a) that contains the hafnium atom singly‐bonded and the zirconium center η2‐coordinated to the bridging μ‐(η1‐C:η2‐C,C‐2‐butyne) ligand. However, the reaction between the methylzirconocene cation 2b with bis(phenylacetylide)hafnocene (3b) gives the pure regioiso‐mer 4b with an opposite relative arrangement of the two group 4 transition metals (the methyl is at the planar‐tetra‐coordinate carbon atom C2). Complex 4a was characterized by an X‐ray crystal structure analysis.

Radical cations of alkylazulenes: an EPR and ENDOR study

Azulene (1) and its alkyl derivatives 2–15 were oxidized in a UV-irradiated stationary system by mercury(II) trifluoroacetate in dichloromethane. These derivatives are: 4,6,8-trimethyl-(2), 1,4,6,8-tetramethyl-(3), 6-tert-butyl-1,4,8-trimethyl-(4), 2,4,6,8-tetramethyl-(5), 1,3-dimethyl-(6), 1,3-ditert-butyl-(7), 1,3,5-tri-tert-butyl-(8), 1,3,6-trimethyl-(9), 6-tert-butyl-1,3-dimethyl-(10), 1,3,5,7-tetramethyl-(11), 1,3,4,6,8-pentamethyl-(12), 1,3,4,8-tetramethyl-6-propyl-(13), 6-tert-butyl - 1,3,4,8-tetramethyl-(14) and 1,2,3,4,6,8-hexamethyl-azulene (15). Only radical cations substituted in both reactive positions C-1 and -3 (6˙+-15˙+) were sufficiently persistent to be characterized by their hyperfine data with the use of EPR spectroscopy. Those bearing tert-butyl substituents at C-1 and -3 (7˙+ and 8˙+) or having three alkyl groups at the seven-membered ring in addition to the 1,3-dimethyl substituents (12˙+-15˙+) were also amenable to ENDOR and TRIPLE resonance studies. In contrast, the radical cations with none (1˙+, 2˙+ and 5˙+) or only one methyl group in the positions C-1 and -3 (3˙+ and 4˙+) rapidly reacted to yield follow-up products. For 1˙+-4˙+these products were identified by EPR and EN DOR spectroscopy as the radical cations of correspondingly substituted 1, 1′-biazulenyls (1a–4a). A mechanism is proposed for the formation of the primary (6˙+-15˙+) and the secondary radical cations (1a˙+-4a˙+). The failure to detect the radical cation of biazulenyl 5a˙+, starting from 5, must be due to instability of 5a˙+ in which two pairs of methyl substituents sterically interact. The high reactivity of the azulene radical cations in the positions C-1 and -3 is consistent with the unusually high π-spin populations ρ1.3 at these centres, as manifested by the largest by far coupling constants |aH1,3| of the α and methyl β-protons.

Erzeugung von 2‐substituierten 1‐Ethenyl‐Kationen aus Ethylendiazonium‐Salzen

Thermal decomposition of 2‐substituted ethylenediazonium salts 1a–d produces the corresponding 1‐ethenyl cations 2a–d as short‐lived intermediates. The primary vinyl cation 2d or the rearranged and energetically favored analogues 3a–c of 2a–c are intercepted by chloride to give the decomposition products 4d and 5a–c, respectively. The generation of 4d and 5a–c as well as a kinetic study of the thermolysis reaction provide no indication that the elimination of nitrogen from 1a–d is occurring with the intermediate formation of cyclic cations 6 or 7.

Regioselective Rearrangement of Bridgehead-Methyl-Substituted Radical Cations Derived from Bicyclo[2.1.0]pentanes and 2,3-Diazabicyclo[2.2.1]hept-2-enes through Photoinduced Electron Transfer and Radiolytic Oxidation: Product Distribution and Matrix ESR Studies

Cyclopentane-1,3-diyl radical cations were generated from the 1-methyl- and 1,4-dimethyl substituted bicyclo-[2.1.0]pentanes 1b,c through photoinduced electron transfer (PET) and radiolytic oxidation. The unsymmetrical bridgehead-substituted bicyclopentane 1b rearranged spontaneously and exclusively to the 3-methylcyclopentene 3b under PET conditions. ESR studies showed similarly that 3b[sup [sm bullet]+] was the only final oxidation product of 1b; the initial radical cation 1b[sup [sm bullet]+] was not detected because it rearranges rapidly and stereoselectively by a 1,2-hydrogen shift to 3b[sup [sm bullet]+], even at 80 K, and no trace of the more stable 1-methylcyclopentene radical cation 3a[sup [sm bullet]+] was observed. This contra-thermodynamic regioselectivity is rationalized in terms of essential localization of positive charge at the tertiary center as the reaction proceeds in the 1,3-diyl radical cation 1b[sup [sm bullet]+]. The symmetrical dimethyl derivative 1c rearranged much more reluctantly than 1b despite its lower oxidation potential, and this is attributed to the greater persistence of radical cation 1c[sup [sm bullet]+] through its reluctance to undergo a 1,2-H shift. The oxidation of these azoalkanes generates highly reactive transients, presumably diazenyl radical cations, which readily denitrogenate and undergo 1,2-H shifts in either a consecutive or concerted manner to form olefin radical cations. 25 refs., 6 figs., 2more » tabs.« less

Peculiarities of structure and properties of phosphorus-containing cations with two intramolecular donor-acceptor bonds N→P. Synthesis of the first 5-coordinated P-cation with PN bond

cations 1a,b display dual reactivity, and they interact both with electrophilic and nucleophilic reagents. Reaction with chloramine B or benzenesulphonylazide leads to the formation of first representative of 5-coordinated P-cations with PN bond.

Involvement of oxirane intermediates in the electron transfer photooxygenation of 1,1- and 1,2-diarylethylenes sensitized by 2,4,6-triphenylpyrylium tetrafluoroborate

The photooxygenation of trans - and cis -stilbene ( 2a,b) by means of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) led to benzaldehyde ( 5) and diphenylacetaldeyde ( 6). Under the same conditions trans - and cis -1,2-diphenyloxirane ( 4a,b) afforded 6 as major product, together with minor amounts of 5, benzophenone ( 8) and, in the case of 4b, 1,2-diphenylethanone ( 7). These data support the intermediacy of the oxiranes in the photooxygenation of the corresponding stilbenes and reveal that rearrangement of the radical cations 4a,b +. occurs by two alternative pathways: phenyl migration and H migration. The TPT-photosensitized oxygenation of 5-methylene-10,11-dihydro-5 H-dibenzo[a,d]cycloheptene ( 9) produced mainly dibenzosuberone ( 10) and dibenzosuberane ( 11), together with trace amounts of products arising from dehydrogenation of the dimethylene bridge ( 14,15,16) and solvent-derived products ( 12,13,18). The formation of 10 and 11 is justified via the radical cation of the 1,1-diphenylethylene moiety, generated by electron transfer from the olefin to the excited sensitizer. The epoxide 20, aldehyde 17 and dioxetane 19 appear to be key intermediates. Formation of the minor compounds 14, 15 and 16 can be explained through the radical cation of the 1,2-diphenylethane substructure.

Reversed Mills—Nixon effect in benzoborirene and benzocyclopropenyl cations

Abstract The structural properties of benzoborirene (2a) and the benzocyclopropylene cations (3a) are examined at the HF 6-31G* level of theory. It is shown that a pronounced reversed Mills—Nixon (MN) type of deformation takes place in these molecular systems within the aromatic fragment. This feature is rationalized by a strongly increased delocalization of π electrons over the fused three-membered ring and rehybridization at the carbon junction atoms. The results are compared with those for the parent benzocyclopropene (1a) compound where MN distortion is operative.