Azulenes rearrangement

Stirling, A. Iannuzzi, M. Laio, A. Parrinello, M., Azulene-to-naphthalene rearrangement the Car-Parrinello metadynamics method explores various reaction mechanisms, Eur. J. Chem. Phys. Phys. Chem. 2004, 5, 1558-1568... 

As appropriate model compounds for these reactions240 the bridgehead substituted di hydro-4-methyleneazulenes 474 were employed. Allyl-, crotyl- and propargyl-substituted dihydroazulenes 474 and 476 can be easily rearranged to the 4-substituted azulenes 475 and 477 (equations 179 and 180) whereas all attempts to obtain 4-benzylazulene 479 by rearrangement of precursor 478 gave only polymeric products (equation 181). Undoubtedly, this failure can be explained by the fact that the Cope rearrangement becomes very... 

Flash vacuum pyrolysis of tricyclo[7.1.0.04,6]deca-2,7-diene 544 is accompanied by a long cascade of rearrangements leading to various azulenes (equation 214)266. The structures of these products were determined by using the chlorine atoms as labels for the 13C NMR measurements. 

Direct irradiation of the (CH)10 hydrocarbon triquinacene (26) in pentane solution gave five different (CH)10 isomers along with some naphthaline and azulene. The two major products were pentacyclo[4.4.0.02 4.03 i0.05,9]dec-7-ene (27), arising from an intramolecular [2 + 2] cycloaddition, and hexacyclo[4.4.0.02,4.03,10.05,8.07 9]decane ( barettane , 28), which is formed via a di-n-methane rearrangement (see Section l.A.2.2.) followed by an intramolecular [2 + 2] cycloaddition,50... 

We compare the ultrafast dynamics of the l,l(8aH)-azulendicarbonitrile,2-(4-cyanophenyl) derivative (hereafter CN-DHA/CN-VHF) with the related but distinctly different DHA derivative l,2,3,8a,9-pentahydrocyclopent[a]azulene-9,9-dicarbonitril (CP-DHA). The final product formed by irradiation of CN-DHA at its Si-So absorption band around 370 nm is the CN-VHF-trans isomer. Therefore, the complete process involves a ds-trans isomerization besides the ring opening (Fig. 1). The broadband transient absorption spectra [3] reveal that an absorption band around 510 nm is formed via an excited state within the first 15 ps. This ground state absorption band is red shifted by 30 nm compared to CN-VHF-trans and can therefore be attributed to the So state of CN-VHF-cis. The rearrangement to the final trans... 

Decomposition of l-diazo-4-arylbutan-2-ones offers a direct entry to bicyclo[5.3.0]decatrienones and the approach has been extensively used by Scott and coworkers to synthesize substituted azulenes.137 Respectable yields were obtained with copper catalysis,137 but a more recent study24 showed that rho-dium(ll) acetate was much more effective, generating bicyclo[5.3.0]decatrienones (154) under mild conditions in excess of 90% yield (Scheme 34). The cycloheptatrienes (154) were acid labile and on treatment with TFA rearranged cleanly to 2-tetralones (155), presumably via norcaradiene intermediates (156). Substituents on the aromatic ring exerted considerable effect on the course of the reaction. With m-methoxy-substituted systems the 2-tetralone was directly formed. Thus, it appeared that rearrangement of (156) to (154) was kinetically favored, but under acidic conditions or with appropriate functionality, equilibration to the 2-tetralone (155) occurred. 

The preparation of azulene (Expt 6.16) provides an interesting series of rearrangements.9 The key step in the synthesis is the intramolecular insertion reaction, carried out under conditions of high dilution, of the ketocarbene (22) into the 1,2-position of the benzene ring. The unstable norcaradiene (23) which is formed then ring-opens to the bicyclic trienone (24), which isomerises to the more stable cross-conjugated trienone (25) during isolation. Dehydration of the trienone with a mixture of phosphorus pentoxide and methanesulphonic acid yields azulene. 

Highly strained substrates can be transformed into even more strained isomers by ODPM rearrangement. This has been shown by Murata et al. for the synthesis of a valence isomer of azulene [45]. Albeit the photochemical reaction yielded only 20-25% of the bicyclo[1.1.0]butane derivative 15, the synthesis of the precursor cyclobutene (14) is straight-forward from the bicyclo[3.3.0]octenone 13 (Sch. 18). This substrate has obviously a diverse reactivity pattern when directly excited, however, triplet sensitization reduces these competitive pathways because alkene excitation is excluded. Also benzo-annulated azulene valence isomers were generated by this approach [46]. 

Gates and Malchick utilized the Beckmann rearrangement for their purposes (Scheme 24).217) When the Hofmann degradation of 142 and 143, followed by catalytic dehydrogenation over 5 % palladium on asbestos at 300—310 °C, afforded not 1-vinylpentalene but azulene, the workers concluded that the former was thermodynamically unstable relative to the latter. 

Skeletal rearrangements of cycloalkanes containing 9-18 carbon atoms were observed for the first time by Prelog et al. (162) on Pd/C catalysts at 400°C. Under these conditions, polycyclic aromatic and pseudoaromatic hydrocarbons are obtained (indene, azulene, naphthalene, phenanthrene, etc.). By carrying out the reaction on Pt/C under less drastic conditions, Kazanskii et al. (163) could observe the precursors of the aromatics as primary products. The latter are bicycloalkanes resulting from transannular 1-5 or 1-6 dehydrocyclizations (Scheme 84). For instance, cyclooctane yields... 

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

The rearrangement of azulene to naphthalene induced by excitation into the S2 state has been examined under collision free conditions in a molecular beam and an approximate estimate of the rate constant for the rearrangement under these conditions has thereby been determined. ... 

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