Cycloheptatriene Research Articles

Pd3 Sandwich Complexes with π-Cyclic Ligands: Theoretical Insight into Stability, Face-Capping Coordination Bond, Trans-Influence, and Backside-Ligand Effect.

Triangle Pd3 sandwich complexes with two face-capping π-cyclic ligands, [Pd3(Cm)(Cn)L3]2+ [L = acetonitrile; Cm, Cn = benzene, cycloheptatriene (CHT), cyclooctatetraene (COT), and [2.2]paracyclophane (PCP)], are theoretically investigated. The stability increases in the order [Pd3(C6H6)2L3]2+ ∼ [Pd3(C6H4Me2P)2L3]2+ (C6H4Me2P = p-xylene) < [Pd3(COT)2L3]2+ < [Pd3(CHT)2L3]2+ < [Pd3(PCP)2L3]2+ and [Pd3(C6H6)2L3]2+ ∼ [Pd3(C6H6)(C6H4Me2P)L3]2+ < [Pd3(C6H6)(PCP)L3]2+ < [Pd3(C6H6)(CHT)L3]2+ < [Pd3(C6H6)(COT)L3]2+ < [Pd3(CHT)(PCP)L3]2+ < [Pd3(COT)(PCP)L3]2+, where "bold" represents an experimentally isolated complex. Based on these results, we succeeded in synthesizing a new complex [Pd3(CHT)(PCP)L3]2+. Geometrical features suggest that benzene and PCP interact with one Pd atom using one C═C double bond and with two other Pd atoms using the other C═C double bond in the bridging manner. CHT interacts with three Pd atoms using three C═C double bonds. COT interacts with three Pd atoms using three C═C double bonds in [Pd3(COT)2L3]2+ and using π-allyl type MOs with additional participation of two C═C π bonds in [Pd3(C6H6)(COT)L3]2+. CHT and PCP weaken the benzene coordination bond in [Pd3(C6H6)(CHT)L3]2+ and [Pd3(C6H6)(PCP)L3]2+ like trans-influence. Unprecedentedly, COT strengthens the benzene coordination bond in [Pd3(C6H6)(COT)L3]2+. This "backside-ligand effect" arises from the characteristic coordination bond of the COT, which is elucidated based on the orbital interaction between the COT and Pd3 triangle.

Pyrolysis of norbornadiene: An experimental and kinetic modeling study

Pyrolysis of norbornadiene (NBD) was investigated in a jet-stirred reactor at 760 torr and within the temperature range of 560 to 1100 K. 37 products and intermediates ranging from m/z = 26 to 178 were identified and quantified by using synchrotron radiation photoionization molecular-beam mass spectrometry accompanied by gas chromatography. The C7H8 isomers, including NBD, cycloheptatriene (CHT) and toluene (A1CH3), were differentiated. Acetylene (C2H2), cyclopentadiene (C5H6), C7H8 isomers and other aromatics are abundant during the pyrolysis process. A detailed kinetic model was developed with reasonable predictions against the measured results, involving 264 species and 1259 reactions, which is helpful to understand the kinetic and heat features of initial consumption as well as the synergistic effect of C2H2, C5H6 and A1CH3 to the formation of aromatics. Rate-of-production analysis shows that the pyrolysis of NBD experiences two stages. The first stage involves the isomerization and decomposition of NBD, yielding A1CH3, CHT, C5H6 and C2H2. Branching ratio analysis shows that C5H6 and C2H2 are dominant products in the first stage. Heat release analysis indicates that the first stage is endothermic. The second stage involves the pyrolysis and interactions of first-stage products to produce light hydrocarbons and polycyclic aromatic hydrocarbons like indene, naphthalene, acenaphthylene, fluorene and phenanthrene. Three routes dominate the formation of indene, including C5H6 + C5H5, benzyl radical + C2H2 and cycloheptatrienyl radical + C2H2, the last one of which has been scarcely considered in the previous investigation. Sensitivity analysis indicates the reactions in the first stage exhibit strong promoting effects for the consumption of NBD, while the second-stage reactions play minor roles in NBD consumption. This work would enrich the understanding of NBD consumption, PAH formation and heat release.

A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon.

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)3 (2) with KC8 yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)2] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I2 to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)2)2(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η7-C7H7)(TDA)2(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)4] (1-crypt) and U(COT)2, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)3]2(μ-η8:η3-C8H8) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)4 (4) with KC8 gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)3(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

Synthesis of Cycloheptatrienes, Oxepines, Thiepines, and Silepines: A Comparison between Brønsted Acid and Au‐Catalysis

The cyclization of 1‐(benzyl‐, oxyaryl‐, thioaryl‐, and silaaryl)‐2‐ethynylbenzenes under Brønsted acid‐ and Au(I)‐catalysis is described. Brønsted acid catalysis favors without exception the formation of the products derived from the regioselective protonation of the alkyne to generate the most stable vinyl carbocation intermediate. This determines the type of cyclization following this initial step and therefore, the structure of the products. Contrarily, with only a few exceptions, for example, when the substrate contains a terminal alkyne moiety, Au‐catalysis preferentially promotes the formation of seven‐membered rings via 7‐endo‐dig cyclization. This reaction pathway is imposed even if a priori the natural electronic polarization in the substrates would suggest another operating mechanism. Making use of these reactivity patterns, a series of structurally differentiated dibenzo‐cycloheptatrienes, ‐oxepines, ‐thiepines and ‐silepines have been synthesized with excellent yields and regioselectivities.

Anti dinuclear adducts of cycloheptatriene and cycloheptatrienyl ligands: Anti-[Pd2(μ-C7H8)(PPh3)4][BF4]2 and anti-[M2(μ-C7H7)(PPh3)4][BF4] (M = Pd, Pt)

The reaction of a dinuclear palladium complex [Pd2(CH3CN)6][BF4]2 with cycloheptatriene (CHT) in the presence of triphenylphosphine afforded an anti dinuclear addition product, anti-[Pd2(μ-C7H8)(PPh3)4][BF4]2. Deprotonation of the CHT ligand in the dinuclear CHT complex gave a mixed-valence cycloheptatrienyl (Tr) complex, anti-[Pd2(μ-C7H7)(PPh3)4][BF4]2. A platinum congener of the dinuclear Tr complex was also synthesized by the double addition of Pt0(PPh3)2 moieties to [C7H7][BF4].

Trinuclear palladium addition to unsaturated carbocycles

The trinuclear palladium addition to monocyclic unsaturated hydrocarbons was revealed. The trinuclear adduct of [2.2]paracyclophane, cycloheptatriene (CHT), or cyclooctatetraene (COT) was obtained by treatment of the corresponding tripalladium sandwich complexes with 3 equiv. of 1,10-phenanthroline. In the trinuclear adduct of [2.2]paracyclophane or CHT, a Pd2 unit and a Pd1 unit synfacially coordinate through a (μ-η(3)-allyl):(η(3)-allyl) manner. A trinuclear adduct of COT exhibited an unusual bonding structure in which a formally tetraanionic COT coordinates to three Pd(II) centers through a synfacial η(3):η(1):η(3):η(1) coordination mode. The tetraanionic COT ligand bearing three Pd(II) moieties undergoes a unique intramolecular 1,5-C-C coupling.

Palladium Catalyzed 1,8-Conjugate Addition to Heptafulvene via Bis-π-allyl Palladium Complexes

The palladium catalyzed 1,8-conjugate addition of heptafulvene, an antiaromatic conjugated 8π-electron system, is discussed. The method is utilized for the concise synthesis of bis-functionalized cycloheptatriene (CHT) derivatives. This is the first report on the palladium catalyzed bisfunctionalization of a cyclic cross conjugated system.

Double ionization of cycloheptatriene and the reactions of the resulting C7Hn2+ dications (n = 6, 8) with xenon

The formation and fragmentation of the molecular dication C(7)H(8)(2+) from cycloheptatriene (CHT) and the bimolecular reactivities of C(7)H(8)(2+) and C(7)H(6)(2+) are studied using multipole-based tandem mass spectrometers with either electron ionization or photoionization using synchrotron radiation. From the photoionization studies, an apparent double-ionization energy of CHT of (22.67 ± 0.05) eV is derived, and the appearance energy of the most abundant fragment ion C(7)H(6)(2+), formed via H(2) elimination, is determined as (23.62 ± 0.07) eV. Analysis of both the experimental data as well as results of theoretical calculations strongly indicate, however, that an adiabatic transition to the dication state is not possible upon photoionization of neutral CHT and the experimental value is just considered as an upper bound. Instead, an analysis via two different Born-Haber cycles suggests (2)IE(CHT) = (21.6 ± 0.2) eV. Further, the bimolecular reactivities of the C(7)H(n)(2+) dications (n = 6, 8), generated via double ionization of CHT as a precursor, with xenon as well as nitrogen lead, inter alia, to the formation of the organo-xenon dication C(7)H(6)Xe(2+) and the corresponding nitrogen adduct C(7)H(6)N(2)(2+).

Vibrational cooling in the liquid phase studied by ultrafast investigations of cycloheptatriene

Solvent-mediated vibrational relaxation in UV pump–probe experiments has been studied using cycloheptatriene (CHT) and its perdeuterated counterpart (CHTd8) as model systems. Transient spectra containing absolute ε(t) values are obtained by a modified Sulzer–Wieland ansatz and a global fit function for vibrational relaxation is proposed.

Acid‐Catalyzed Conversion of 7‐Ethynyl‐ and 7‐Vinylcyclohepta‐1,3,5‐trienes to Substituted Benzene Derivatives

AbstractThe acid‐catalyzed rearrangement in THF/TFA (5:1, v/v) of cyclohepta‐1,3,5‐trienes containing an α,β‐unsaturated substituent at C‐7 was investigated. 7‐Ethynylcyclohepta‐1,3,5‐triene (1a) and its 1,4‐di‐tert‐butyl (1b), 1,5‐di‐tert‐butyl (1c), and 2,5‐di‐tert‐butyl (1d) derivatives underwent isomerization to phenylallenes 2, 3, 7, and 3, respectively. 2,5‐Di‐tert‐butyl‐7‐vinylcyclohepta‐1,3,5‐triene (17) gave a mixture (66:34) of (E)‐ and (Z)‐1,4‐di‐tert‐butyl‐2‐(1‐propenyl)benzene (20). A mechanism involving the protonation of the norcaradiene tautomer (NCD), which is in equilibrium with cycloheptatriene (CHT) 1 or 17, followed by the cleavage of a three‐membered ring to give an arenium ion, is proposed. In contrast, 2,5‐di‐tert‐butyl‐7‐cyanocyclohepta‐1,3,5‐triene (12) and bis(cyclohepta‐2,4,6‐trien‐1‐yl)ethyne (21) did not give the expected products, probably due to the unfavorable energetics of the ring‐opening step. A thermal 1,5‐hydrogen shift predominates in these cases. Kinetic studies indicated that the rearrangement of 1a is significantly accelerated by the presence of tert‐butyl groups on the seven‐membered ring. The order of reactivity, 1a &lt; 1b &lt; 1c &lt; 1d, is in agreement with the population of NCD forms at equilibrium, as estimated by the calculated CHT−NCD energy‐difference and the 13C NMR chemical shifts of 1a−d, indicating the importance of the equilibrium concentration of the norcaradiene form as a rate‐controlling factor. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)

Direct time-resolved UV-absorption study on the ultrafast internal conversion of cycloheptatriene in solution

We present the first direct measurement of the internal conversion time constant of cycloheptatriene (CHT) in the liquid phase by transient absorption spectroscopy in the femtosecond time domain. The pump and probe wavelengths were 266 and 400 nm, respectively. The experiments were carried out in solutions of CHT in cyclohexane, methanol and glycol. In all cases the excited state lifetime was found to be τ=(110±10) fs. The internal conversion is therefore, in good approximation, independent of polarity and viscosity of the surrounding solvent. The influence of the different group velocities in the liquid samples on the time resolution was minimized by using highly absorbent samples with CHT concentrations 0.02 mol l-1. In these samples the pump pulse is absorbed completely within a very thin layer of the solution of an optical depth of less than 100 µm. Thus, our time resolution is controlled by the 65 fs widths (FWHM) of the pump and probe pulses only. After the internal conversion of CHT there is a residual absorption due to the vibrationally excited ground state molecules. The corresponding absorption cross-section fully agrees with the absorption cross-section of thermally excited CHT containing an average energy identical to the original electronic excitation energy.

Vibrational Overtone Spectroscopy of Cycloheptatriene Chromium Tricarbonyl, Benzene Chromium Tricarbonyl, and Corresponding Hydrocarbon Ligands

The vibrational overtone spectra of gaseous cycloheptatriene chromium tricarbonyl (CHTCT) and benzene chromium tricarbonyl (BCT) were recorded at the third overtone region using photoacoustic spectroscopy and compared to the spectra of the nonbonded ligands, cycloheptatriene (CHT) and benzene. Analysis of the spectra leads to the conclusion that the metal−ligand bonding lengthens the CH bond by 0.007 Å in the cycloheptatriene complex and by 0.003 Å in the benzene complex. The spectrum of BCT exhibits an additional transition which is absent in the spectrum of free benzene. The experimental peaks are assigned to the pure CH overtones and combination bands. The structural information obtained from the spectra is discussed and compared to the results of gas-phase electron diffraction and microwave spectroscopic studies. Analysis of experimental data indicates that μ6-bonding is more favorable for cycloheptatriene chromium tricarbonyl.

Vibrational Overtone Spectroscopy of Cycloheptatriene-d6 at the Third and Fourth Overtone Regions

In a previous report1 the vibrational overtone spectrum of gaseous cycloheptatriene (CHT) was presented. The main absorption features at the third and fourth overtones were assigned to the CH olefinic oscillators of CHT using a direct correlation method. Assignments were also suggested for the methylenic CH absorptions; however, further confirmation of these spectral assignments was required. In CHT-d6 the CH olefinic transitions are eliminated, and only features due to the methylenic group of the CHT ring will remain in the spectrum. The CHT-d6 was synthesized by refluxing 12 mL of C6D6 (99.6 atom % d, Aldrich) in the presence of CuBr for 3.5 h with dropwise addition of 40 mL of diazomethane (CH2N2) solution in C6D6 as was described by Muler et al.2 The diazomethane solution was generated from nitrosomethylurea according to the procedure proposed by Arndt.3 Nitrosomethylurea was prepared from acetamide and bromine as was recommended by Amstutz and Myers.4 The resulting reaction mixture contained about 2.4% CHT-d6 dissolved in the deuterated benzene. The 1H NMR of the mixture displayed the single broad peak around 2.1 ppm corresponding to the CH2 group of the CHT-d6. The amount of solvent was reduced using rotatory evaporation to bring the CHT-d6 content to 43%. Then the CHT-d6 sample (91%) was collected using preparative gas chromatography. The third and fourth overtone spectra of CHT-d6 were obtained using the photoacoustic spectrometer described previously.1 The sample pressure was 14 Torr, and 300 Torr of argon was added to increase the photoacoustic signal. The spectra of CHT-d6 and CHT at the third and fourth overtone regions are shown in Figures 1 and 2, respectively. Peak positions and vibrational assignments are tabulated in Table 1. At the third overtone the absence of the olefinic CH bond stretch absorptions in the CHT-d6 spectrum around 11 30011 400 cm-1 leaves only two well-resolved peaks associated with the methylenic CH bonds of the CHT-d6 ring (Figure 1). Based on the general pattern reported for the CH methylenic bond in cyclic hydrocarbons5 (rCHax > rCHeq) and ab initio geometry optimization for CHT,1 the lower energy peak at 11 096 cm-1 can be assigned to the CHax of CHT-d6, while the higher energy transition, located at 11 215 cm-1, belongs to the CHeq. Similar arguments can be made for the two peaks recorded in the next overtone region (Figure 2), where the CHax peak appeared at 13 643 cm-1 and the CHeq transition appeared at 13 785 cm-1. Thus, the spectral results for CHT-d6 confirmed the assignments suggested for the CHT third and fourth overtone methylenic peaks. Acknowledgment. The authors wish to thank Dr. Thomas Kinstle for assistance in the synthetic part of work and J. Romanowitz for the GCMS analysis of the samples.

Cycloheptatriene: a new versatile co-ordination ligand. Synthesis and structural characterization of [Ru6C(CO)17] derivatives

The reaction of [Ru6C(CO)17] and cycloheptatriene (CHT) gives rise to the formation of several products which have been fully characterized in solution and by single-crystal X-ray diffraction as [Ru6C(CO)15(ENBD)] 1 (ENBD = ethylenenorbornadiene), [Ru6C(CO)14(C7H8)] 2 and [Ru6C(CO)11(η5-C7H9)(µ3-η2∶η2∶η3-C7H7)] 3. Compounds 2 and 3 provide an example of the co-ordinative variability of carbocyclic rings based on the C7-frame. The ligand in 1 contains nine carbon atoms and is formed from C7H9 by an unknown process. The CHT molecule has also been observed to convert to toluene through a ring contraction activated by the cluster to form the known compound [Ru6C(CO)14(η6-C6H5CH3)] 4.

The reaction of CH2(X̄3B1) with C6H6

AbstractThe kinetics of the reaction CH2(X̄3B1) + C6H6 → products (1) has been studied in a discharge flow reactor using Laser Magnetic Resonance (LMR) detection for CH2. The experiments yielded the overall rate constant kexp1 = (4.7 ± 2.4) · 10t3 exp(−(37.5 ± 2.4) kJ mol−1/RT) in the temperature range 448 K ≦T ≦ 683 K. The product formation was investigated for reaction (1) as well as the corresponding reaction CH2(ã1A1) + C6H6 → products (2) in a stationary photolysis reactor employing gas chromatographic analysis by photolyzing mixtures of CH2CO and C6H6 at λ1 = 366 nm and λ2 = 313 nm. The relative yields of the products cycloheptatriene (CHT) and toluene (T) were found to depend on the wavelength, [T]/([CHT] + [T]) = (0.13 ± 0.02) and (0.23 ± 0.02) at λ1, and λ2, respectively. The implications of these results and the mechanisms are discussed.

Reactions of 11C atoms with benzene. II. Formation of 11C‐labelled toluene and cycloheptatriene

AbstractThe reactions of recoil 11C atoms with liquid C6H6 result in the production of 11C‐labelled cycloheptatriene (CHT) and toluene in equal amounts of approximately 3%. From the effect of O2 on the product yields it is concluded that recoil labelling proceeds via both singlet and triplet 11CH2 intermediates, whereas 11CH may also contribute specifically to the formation of C6H5CH3. Addition of H2S increases the [11C]C6H5CH3 yield by ca. 5%, probably due to reactions of triplet phenyl‐carbene (C6H5CH). The reactions of singlet CH2, produced via the photolysis of CH2N2, with liquid C6H6 were also investigated in the presence of N2, O2 and I2.

ChemInform Abstract: Anodic Transformation of Cycloheptatriene to Tropone and Tropolone.

AbstractAnodic oxidation of cycloheptatriene (I) in the presence of methanol (II) produces the methoxycycloheptatriene (III) and benzaldehyde dimethyl acetal (IV).

Anodic transformation of cycloheptatriene to tropone and tropolone

Cycloheptatriene (CHT) has been directly transformed to 7-methoxycycloheptatriene (7-MCHT), an equivalent compound to a tropylium salt, by the anodic oxidation of CHT in methanol. The further anodic oxidation of 3-MCHT prepared from 7-MCHT has been found to be an effective method of synthesis of tropone and tropolone.

Functionalization of cycloheptatriene: the reaction of cycloheptatrienyltricarbonylferrate with chloroformates

Functionalization of cycloheptatriene: the reaction of cycloheptatrienyltricarbonylferrate with chloroformates

Structural effects on valence tautomerism : VII. An NMR and molecular mechanics study of the steric effects of t-butyl substituents on the cycloheptatriene-norcaradiene equilibrium of 7-cyano-7-m

7-Cyano-7-methylcycloheptatrienes containing one t-butyl group on the 1-position ( 2 ) or two t-butyl groups on the 1- and 3-, 1- and 4-, or 1- and 5-positions ( 3 , 4 , or 5 , respectively) were synthesized and their cyclo- heptatriene (CHT)-norcaradiene (NCD) equilibria measured by variable-temperature 1H NMR for CS 2-CD 2Cl 2 solutions. The 1H NMR chemical shifts of the 7-methyl group indicate that these compounds are composed of essentially one CHT and one NCD tautomer with an endo geometry of the methyl group. The introduction of a t-butyl group at the 1-position of 7-cyano-7-methylcycloheptatriene ( 1 ) markedly shifts the equilibrium to the NCD side and the addition of the second t-butyl group further favors the NCD form, with the NCD populations for 2 , 3 , 4 , and 5 at 25 °C 70.9, 96.5, 92.3, and 99.3%, respectively. An application of molecular mechanics (MMPI) calculations to various t-butylated CHT-NCD systems suggests that the t-butyl groups sterically destabilize the CHT form more than the NCD form, bringing about increased NCD populations.