Intercyclic [π 2+σ2+σ2]-and π2+ π2]-cycloaddition reactions in sterically fixed norbornadiene-/quadricyclane-systems

The stereoelectronic prerequisites for unusual “zipper-type” [_π 2+_σ 2+_σ 2]- and [_π 2+_π 2]-reaction sequences, e.g. in the still unknown C₇H₆-pentamers (1)/(2), are being studied using model compounds with systematically varied geometrical parameters. In line with expectations based on model considerations, in the [3]-series ((6), (7)) $\text{intracyclic}$, in the [5]-series ((10), (11)) $\text{inter}$cyclic processes clearly predominate.     

ESR. Spectra of the Radical Anion of Dimethyl-phenyl-phosphine

ESR. spectra of the radical anion (I^?) produced from dimethyl-phenyl-phosphine (I) both by electrolysis and reaction with alkali metals have been studied upon variation of temperature. The coupling constant assigned to the ³¹P nucleus depends strongly on temperature, whereas the coupling constants attributed to protons do not exhibit such a dependence. The π-spin populations at the benzene ring of I^? give evidence - in accordance with other experimental data [1] [2] – that the dimethylphosphino substituent is electron-attracting. This effect is thought to be due mainly to P ← C_π delocalization, which is analogous to the Si? C_π interaction in trimethylsilyl-substituted π-systems [3]. The ESR. spectrum previously [4] ascribed to I^? is shown to arise from a secondary radical. The formation and structure of this radical are briefly discussed.     

ESR.-Messungen an Radikal-Anionen Trimethylsilyl-substituierter Hetero-π-Systeme

The ESR. spectra of the radical anions of trimethylsilyl-substituted derivatives of p-benzoquinone, phenylketone, diimine, p-benzoquinone-diimide, aniline and p-phenylenediamine are reported. In most cases the coupling constants of ²⁹Si isotopes in natural abundance have been measured. The comparison of the ESR. data for the radical anions of the trimethylsilyl derivatives with those for the radical anions of the corresponding parent hydrocarbons and/or alkyl-substituted compounds provide additional evidence for the overall electron withdrawing effect of the trimethylsilyl substituent (Si←C_π and Si←n_π delocalization).     

Inverse‐electron demand [π2 + σ2 + σ2] cycloaddition reaction of dimethyl oxaquadricyclane‐2,3‐dicarboxylate with cyclooctyne

A first example of an inverse‐electron demand [_π2 + _σ2 + _σ2] cycloaddition reaction of dimethyl oxaquadricyclane‐2,3‐dicarboxylate was reported: cyclooctyne underwent cycloaddition with dimethyl oxaquadricyclane‐2,3‐dicarboxylate to afford the corresponding adducts one of whose structure was confirmed by a single crystal X‐ray analysis.     

Computational study of methane functionalization by a multiply bonded, Ni-bis(phosphine) complex

A computational chemistry study of nickel-catalyzed group transfer to methane is presented. Two mechanisms were evaluated: a one-step mechanism involving [1+2] insertion of E into the C-H bond of methane, and a two-step [2_π + 2_σ] mechanism involving addition of the C-H bond of methane across the NiE bond to a square planar Ni^II intermediate, followed by C-E reductive elimination. Analysis of the energetics for the different mechanistic steps implies a possible competition between the two mechanisms for carbene transfer. For nitrene transfer, the [1+2] pathway is predicted to be the preferred route. Finally, for phosphinidene transfer, the [2_π + 2_σ] mechanism is calculated to be the preferred mechanism. The two mechanisms studied - [1+2] and [2_π + 2_σ] - entail exothermic individual reactions, coupled with reasonable enthalpic barriers. Furthermore, regeneration of the catalyst active species by reaction with a group transfer reagent XE is highly exothermic. The calculations thus indicate that (P ∼ P)NiE (P ∼ P denotes a chelating bis-phosphine ligand) deserve consideration as plausible starting points in the search for improved hydrocarbon functionalization catalysts.     

Radical Anions of Cyclic Azoalkanes: An ESR,ENDOR, and TRIPLE-Resonance Study

Radical anions often monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazoline ( 1 ), 2,3-diaza-bicyclo[2.2.1]hept-2-ene ( 4 ), and 2,3-diazabicyclo[2.2.2]oct-2-ene ( 9 ), as well as by their derivatives 2 , 3 , 5 – 8 , and 10 . For all radical anions 1 ″– 10 ″, the ¹⁴N-coupling constant, a_N, is in the range of +0. 83 to +0. 97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The ¹⁴N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of ⁷Li-, ²³Na-, ³⁹K-, and ¹³³Cs-nuclei point to a close association of the radical anions with their alkali-metal counterions. With the exception of ³⁹K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of 1 ″– 10 ″ are discussed.     

Kinetics and thermochemistry of the [2π+2σ+2σ]-cycloaddition of quadricyclane with 2,3-dicyano-1,4-benzoquinone

Two-stage [2π+2σ+2σ]-cycloaddition of quadricyclane ( 2 ) with 2,3-dicyano-1,4-benzoquinone ( 1 ) with a huge difference in the activity of two reaction centers has been studied. In the first stage (kinetic control), the cycloaddition of 2 takes place on the activated С₂=С₃ bond of 1 to form the monoadduct 3 , and in the second stage the cycloaddition of 2 on the С₅=С₆ bond of the monoadduct 3 occurs by 6 orders of magnitude lower with the formation of bisadduct 4 . The structures of adducts 3 and 4 have been proved by NMR data and the X-Ray method, respectively. The kinetics of the first and second stages, the enthalpy of dissolution of 1 in the π-donor solvents, and the enthalpy of the reaction 1+2→3 have been measured.     

Secododecahedradienes – Syntheses,Reactivity, in‐Plane Homoconjugated 3C/2e Cations, 4C/3e Radical Cations,and σ‐Bishomoaromatic 4C/2e Dications?

Secodecahedradiene 2a , featuring very proximate, perfectly syn‐periplanar and significantly pyramidalized C=C bonds, was synthesized as testing object for in‐plane(σ)‐homoconjugational electron delocalization, starting from the available pagodane 15b . The response of 2a (and in part its diester 2b ) – in π,π‐distance (average 3.08 Å), olefinic pyramidalization (average 26.9°), and π,π‐split (PE, 1.15 eV) intermediate between disecododecahedradiene 1a and 1,16‐dodecahedradiene 3a – to selected 4π‐reagents, electrophiles, and radicals was explored experimentally and by calculations. Intriguing multistep reaction sequences attest to the ease of competing stabilization pathways for the 3C/2e in‐plane homoconjugated cationic intermediates. PE, CV, and ESR measurements and calculations (DFT) characterize the radical cation generated from 2a as in‐plane homoconjugated 4C/3e‐species 2a ^.+, persistent in a Freon matrix, but only very shortly existent in solution (CIDNP). Consequently, NMR control of the two‐electron oxidation in SbF₅/SO₂ClF did not disclose the σ‐bis‐homoaromatic dication 4C/2e (see 2a ^{2 +} ), but a bis‐allylic dication 75 as persistent species. In support of 2a ^{2 +} as intermediate, evidence is presented for very limited kinetic protection offered by the secododecahedral framework to through H‐cage σ‐homoconjugated cations.     

The Radical Anions of [2.2.2.2]Paracyclophane-1,9,17,25-tetraene and Some of its Heteroaromatic Analogues

The radical anions of [2.2.2.2]paracyclophane- 1,9,17,25-tetraene (I), [2] (2, 5)-furano [2]paracyclo [2] (2,5)furano [2]paracyclophane-1,8, 16,23-tetraene (II), [2]-(2,5)thiopheno [2]paracyclo [2] (2,5)thiopheno [2]paracyclophane-1,8,16,23-tetraene (III) and [2.2.2.2](2,5)thiophenophane-1,8,15,22-tetraene (IV) have been studied by ESR. and ENDOR. spectroscopy. The assignment of the proton coupling constants, a_Hμ is to a large extent based on investigations of deuteriated derivatives. These investigations impressively demonstrate the potential of ENDOR. spectroscopy as an analytical tool. The Arrhenius activation energies, E_a, for the rotation of phenylene fragments about the bonds linking them with the ethylenic parts in I ? and II ? are 36±6 and 28±4 kJ/mol, respectively. The value a_Hμ of the olefinic protons in I? appears substantially smaller than expected for the corresponding planar radical anion. The hyperfine data for II ?, III ? and IV ? are consistent with the conformations which should minimize the deviations of the macrocyclic π-systems from planarity. In the case of II ?, tight ion pairs are formed by the radical anion and its counter-ion, K ⊕, in DME , owing to the strong association of the alkali metal cation with one of the furan moieties. An analogous interaction of K ⊕ with a thiophene moiety in III ? must be weaker, since no effects of ion pairing on the ESR. and ENDOR. spectra have been observed for this radical anion.     

Energy Sequence of the Lowest Antibonding Orbitals in Symmetrical Didehydro[n]annulenes with n = 4N+2

A model of a perturbed n-membered perimeter (n = 4N + 2) is used to interpret the ESR.-data for the radical anions of symmetrical tetra-t-butyl-didehydro[n]annulenes with n = 14, 18, 22 and 26 (N = 3, 4, 5 and 6). The singly occupied orbital of such radical anions correlates with one of the doubly degenerate, lowest antibonding perimeter MO's which have been classified as symmetric (Ψ_S) or antisymmetric (Ψ_A) with respect to the mirror plane passing through two opposite centres and perpendicular to the plane of the perimeter. The relevant MO is Ψ_S for n = 14 (N = 3) and 22 (N = 5), but Ψ_A for N = 18 (N = 4) and 26 (N = 6), in accordance with the prediction of the model. It has been shown that the introduction of two symmetrically placed triple bonds into a (4N + 2)-membered perimeter should stabilize Ψ_S and destabilize Ψ_A when N is odd, whereas the reverse should hold when N is even.     

Persistance of the Cyclopnane Radical Anions and its Relation to Structure

Reactions of [2_N]cyclophanes (N = 2, ?6) with solvated electrons in 1,2-di-methoxyethane at 193 K have been studied by ESR. and ENDOR. spectroscopy. All but the two most highly bridged cyclophanes (N = 5 and 6) are reduced to paramagnetic species under these conditions. Whereas the radical anions of [2.2]-paracyclophane and [2₃](1,2,4)- and [2₄](l,2,4,5)cyclophanes are sufficiently persistent to be characterized by their hyperfine data, those of the remaining five cyclophanes undergo a rapid cyclization to the radical anions of 4,5,9,10-tetrahydropyrenes. These have been identified as the unsubstituted tetrahydropyrene (from [2.2]-metacyclophane and [2₃](l,2,3)cyclophane), the 2,7-dimethyl-derivative (from [2₃](1,3,5)- and [2₄](l,2,3,5)cyclophanes) and the 1,8-dimethyl-derivative (from (2₄l,2,3,4)cyclophane). The persistence of the cyclophane radical anions seems to depend on the numbers, n_meta and n_para, of the meta-and para-positions of the bridging ethano groups in the two benzene rings. The prerequisite for the radical anion to be persistent is n_meta?n_para.     

The Radical Anions of [2.2]Paracyclophane-1,9-diene and Some of its Derivatives. An Unexpected Conformational Interconversion

The radical anions of [2.2]paracyclophane-1,9-diene ( 2 ) and its 1,10,12,13,15,16-hexadeuterio derivative 2 -D₆, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-1,9-diene ( 3 ) and its 12,13,15,16-tetradeuterio derivative 3 -D₄, have been studied by ESR spectroscopy. The coupling constants for 2 ^?· at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D_2h symmetry. The ESR spectra of 2 ^?· are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D_2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D₂, i.e., the benzene rings in 2 ^?· are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2 -D₆^?· and 3 ^?·. In the case of 3 ^?·, the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2 ^?· and 3 ^?· gradually convert into the radical anions of [2.2]paracyclophane ( 1 ) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2 ^?· also occurs, with the formation of the radical anion of (E)-4,4′-dimethylstilbene ( 7 ). Both reactions of 2 ^?· must involve the transient radical anion of [2.2]paracyclophane-1-ene ( 4 ) as proved by the observation of the spectra of 1 ^?· and 7 ^?· with 4 as the starting material.     

Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me₃Si-(C≡C)₂-Ru(dppe)₂-C≡C−Ar-C≡C−Ru(dppe)₂-(C≡C)₂-SiMe₃ ( 1–6 ) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ([ 1 ]⁺–[ 6 ]⁺). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1–6 ranging from the UV region even to the NIR region. The cationic species [ 1 ]⁺–[ 6 ]⁺ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in [ 1 ]⁺–[ 6 ]⁺ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.     

Chemistry of alkali metal-unsaturated hydrocarbon adducts : X. Electron-transfer reactions of α, β-unsaturated organosilanes

The chemical behavior of alkali metal adducts of α,β-unsaturated organosilanes was investigated by generating the adducts with lithium or potassium metal in donor solvents at ?78°C and studying the ensuing reduction, bimolecular coupling, cleavage and isomerization processes. Chosen for study were the triphenylvinyl derivatives of silicon, germanium and tin, as well as other vinyl-, phenyl-, allyl-, 1,3-alkadienyl-, 1-alkynyl- and cyclopropyl-silanes. By assessing gradations in substituent reactivity toward alkali metal, an empirical ordering of electron affinity for these substituents on silicon was determined: 1,3-alkadienyl > 1-alkynyl > 1-alkenyl > phenyl > > 2-alkenyl or cyclopropyl. Where available, ESR data were used to explain the reactivity and the reaction pathway observed for radical anion intermediates. Where such information was unobtainable, the behavior of unstable radical anions was ascribed to variations in p_π → d_π stabilization.     

Stereospecific [2+2] cycloaddition reactions of diphosphorus with alkenes

[2+2] Cycloaddition reactions of P₂ with alkenes were predicted to have concerted paths, that is, pseudoexcitation, distorted 2π_s+2π_s, and 2π_s+2π_a processes without any interventions of intermediates. The pseudoexcitation and/or distorted 2π_s+2π_s paths with retention of configuration of alkenes are kinetically preferred to the 2π_s+2π_a path with inversion of configuration. The reactions were predicted from the appreciable difference in the calculated enthalpies of activation to be stereospecific.     

Radical Anions of Cyclic Azoalkanes: An ESR,ENDOR, and TRIPLE-Resonance Study

Radical anions of ten monocyclic and bicyclic azoalkanes containing the azo group in (Z)-conformation, have been fully characterized by their hyperfine data with the use of ESR, ENDOR, and general-TRIPLE-resonance spectroscopy. These azoalkanes are represented by 3,3,5,5-tetramethyl-1-pyrazline ( 1 ), 2,3-diazabicyclo[2.2.1]hept-2-ene ( 4 ), and 2,3-diazabicyclo[2.2.2]oct-2-ene ( 9 ), as well as by their derivatives 2, 3, 5–8 , and 10. For all radical anions $1^{- \atop \dot{}}-10^{- \atop \dot{}}$, the ¹⁴N-coupling constant, a_N, is in the range of +0.83 to +0.97 mT; this finding indicates that the spin population is essentially restricted to the π system of the azo group. The ¹⁴N-hyperfine anisotropy largely affects the width of ESR lines, particularly at low temperatures. Substantial coupling constants of ⁷Li-, ²³K-, and ¹³³Cs-nuclei point to a close association of the radical anions with their alkakimetal counterions. With the exception of ³⁹K, these nuclei give rise to readily observable ENDOR signals which appear along with those stemming from protons. The prominent hyperfine features of $1^{- \atop \dot{}}-10^{- \atop \dot{}}$ are discussed.     

ESR spectra and structure of the CF3Cl−, CF3Br−, and CF3I− radical anions

The anisotropic ESR spectra of the CF₃X^? radical anions (X = Cl, Br, I) have been observed following γ irradiation at 77 K of solid solutions containing up to 5 mole % of the CF₃X parent compound in tetramethylsilane (TMS), neopentane, and 2-methyltetrahydrofuran. The resolution of the line components in the TMS matrix allowed the spectra to be analyzed in detail, the parallel and perpendicular features showing clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit ESR parameters obtained. Confirmation of the CF₃X^? identification was achieved through chemical studies which showed that a similar ESR spectrum was generated by electron attachment to the parent molecule in a photoionization experiment. Also, the spectrum of the CF₃ radical was observed to grow in during the decay of the CF₃X^? spectrum in neopentane above 100 K. The spin density distributions calculated from the ESR parameters of these congeneric radical anions suggest that the unpaired electron resides in an a₁ (σ^*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C_3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p orbitals of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.     

Synthesis, crystal structure and properties of the Cu complex of a tetradentate imidazole-functionalized diazamesocyclic ligand, 1,4-bis(N-1-methylimidazol-2-yl-methyl)-1,4-diazacycloheptane

A new potential tetradentate ligand, 1,4-bis(N-1-methylimidazol-2-ylmethyl)-1,4-diazacycloheptane (l), together with its Cu^II complex [CulCl]ClO₄ (1), has been reported. The crystal structure of 1, determined by single-crystal X-ray analysis, shows that it is in chiral P2₁2₁2₁ space group. The Cu^II centre is penta-coordinated in square pyramidal geometry and the diazacycloheptane (DACH) ring adopts normal boat configuration. The most striking feature of this complex is the formation of a 3D network bridged through the C-H?Cl hydrogen bonds with the perchlorate anions in the cavities, and stabilized via π-π stacking interactions along the a-direction. The solution behaviour of 1 has been further investigated by UV/Vis and ESR techniques.     

Radical anions of 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides in MeCN-H<Subscript>2</Subscript>O solvent mixtures

Radical anions of cyclic dinitrones, 3,3′-bi(2-R-5,5-dimethyl-4-oxopyrrolinylidene) 1,1′-dioxides (R = Me, Ph, Bu^t, CF₃) have been studied by ESR in combination with quantum chemical calculations of geometrical parameters and isotropic hyperfine interaction constants by the DFT/PBE methods in the 3z basis set and UB3LYP in the 6–31+G* basis set with allowance for the solvation in the framework of continual PCM model. The radical anions were generated electrochemically by reduction of their neutral precursors in MeCN-H₂O solvent mixtures and found to possess enough stability for measurement of their ESR spectra in a wide range of the mixture compositions. Radical anions of compounds of nitrone series were generated and characterized in MeCN-H₂O mixtures and water (for the dinitrone with R = Me) for the first time. Differences in the solvent depending behavior of the nitrogen hyperfine interaction constants in the series of radical anions with various substituents R are explained by competition of transsolvation and structural effects.     

ESR.-Spektren des Radikal-Anions des syn-1, 6; 8, 13-Bis-oxido-[14]annulens

The ESR.-spectra of the radical anion of syn-1, 6; 8, 13-bis-oxido-[14]annulene have been recorded. The hyperfine structure of the electrolytically generated anion (solvent: N, N-dimethylformamide; gegenion: Et₄N^⊕) is that of an unassociated species; on the other hand, evidence of strong ion-pairing can be derived from the spectra of chemically prepared anions (solvent: 1,2-dimethoxyethane; gegenion: K⊕ or Na⊕). The distribution of the n-spin population confirms the conclusion previously drawn for the radical anion of 1,6-oxido-[70]annulene that the overall effect of the oxygen bridging is electron repelling.