Cyclic compounds 1,4-cyclohexadienes

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

Under the influence of heat or light, a conjugated polyene can undergo isomerization to form a cyclic compound with a single bond between the terminal carbons of the original conjugated system one double bond disappears, and the remaining double bonds shift their positions. For example, 1,3,5-hexatrienes yield 1,3-cyclohexadienes ... 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated polyene. One tt bond is broken, the other IT bonds change position, a new cyclic compound results, For example, a conjugated triene can be converted into a cyclohexadiene, and a conjugated diene can be converted into a cydobutene. 

Marked decreases of the primarily formed butenes and butadiene with ethylene conversion suggest that these olefins play an important role forming secondary products. In fact, subsequent experiments showed that the addition of butadiene, 3-5 mole %, to ethylene accelerates the formation of cyclopentene, cyclohexene, cyclohexadiene, and benzene (Table I). It seems reasonable, therefore, to propose the following reaction scheme for the formation of cyclic compounds from olefins. 

Cycloadditions. The ability of TiCU to catalyze [2+2]-, [3+2]-, and [4+2]-cycloadditions is well known. The [2+2]-" and [3+2]-cycloadditions involving allylsilanes are useful synthetic methods for assembling polyfunctional cyclic compounds. In Diels-Alder reactions," the catalyst not only promotes cycloaddition of specified addends, but cyclohexadienes can be obtained directly through jS-elimina-tion of the cycloadducts. ... 

Next, in order to learn more about the rates of dehydrogenation of cyclohexenes resulting from Diels-Alder reactions between butadiene and olefins, VCH, HCH and MCH were earlier subjected to thermal reactions at 530- 665 C ( ). The main reactions in these cases were reverse Diels-Alder reactions and dehydrogenations. Dehydrogenations which are related to the productions of cyclohexa-diene and benzene homologues were 1 10 in selectivity as compared to that of the reverse Diels-Alder reaction. An interesting observation related to cyclic compound formation is that, in the case of MCH pyrolysis, cyclohexadiene and cyclopentene are formed at almost the same rates as butadiene and propylene. So that, in this case, about 60% of MCH is employed in the formation of cyclic compounds. 

Certain polyenes and cyclic compounds can be interconverted through a pericyclic process known as an electrocyclic reaction. Examples include the 1,3-butadiene-cyclobutene and 1,3-cyclohexadiene-l,3,5-hexatriene interconversions (Figs. 20.5 and 20.16). 

Organoruthenium compounds tend to form complexes coordinated by the 10 electrons of cyclic unsaturated compounds similar to ferrocene in organoiron compounds, for example, ruthenocene (r/ -C5H5)2Ru and tf( / -C6Hx)Ru. As shown in Scheme 16.1, ruthenium chloride 3H20 reacts with unsaturated cyclic compounds in the presence of zinc metal to afford cyclic compounds coordinated by 10 electrons by dehydrogenation [15-18]. In the case of cyclopentadiene, ruthenocene is formed by the coordination of two five-electron rings. In the case of 1,3-cyclohexadiene, the complex is obtained to be coordinated with 4-electron 1,3-cyclohexadiene and with 6-electron benzene. In the case of 1,3-cycloheptadiene... 

Cycloalkenes and cyclohexadienes. These unsaturated cyclic aliphatic compounds [49, p. 284] have one and two double bonds, respectively, in the ring. Eixamples are... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

An analogous mechanism was proposed for the conversion of the triflate 416 to the vinyl-, allyl- and allenyl-A2-cephems 448 in yields of 47-71% by the respective tributyltin compounds in the presence of cuprous chloride (Scheme 6.91) [176]. Accordingly, the cyclic allene 417 should be liberated from 416 in the first step. Then, the organocopper species would transfer a hydrocarbon group to the central allene carbon atom of 417, leading to an allyl anion derivative, which is protonated during the workup. These reactions of 416 and 443 indicate that the cyclic allenes 417 and 444 behave toward nucleophiles as 1,2-cyclohexadiene (6) (Schemes 6.11— 13) and its non-polar derivatives such as 215 (Scheme 6.51), 221 (Scheme 6.52), 311 (Scheme 6.67) and 333 (Schemes 6.71 and 6.73), that is, they interact with nucleophiles at the central carbon atom of the allene system exclusively. 

Ozonolysis of cyclic olefins in the presence of carbonyl compounds gives the corresponding cross-ozonides.1329 In the ozonation of 1,2,4,5-tetramethyl-1,4-cyclohexadiene, oxidative dehydrogenation (formation of 1,2,4,5-tetramethylben-zene) was found to compete with oxidative cleavage because of steric hindrance.1330 Secondary ozonides (the 76 1,2,4-trioxolanes) are formed in high yields in the gas-phase, low-temperature ozonation of terminal and disubstituted alkenes.1331... 

Cyclopentadienes and related compounds 5.1322 Cyclohexadiene and other cyclic dienes... 

This approach to cyclohexadienes can be considered an extension of that described for the precursor to type I cyclohexadiene in which the vinyl group is part of a cyclic stmcture. An example is tte low-temperature photochemical generation of the bis-exomethylene compound (196) from the benzocyclobutene (195), which was followed by an electrocyclic reaction to give compound (197). Thermolysis of (198) also leads to an assumed bis-exo-methylene intermediate (199), wldch electrocyclizes with involvement of die pyridinium ring to afford die alkaloid ellipticene (200) after dehydrogenation. ... 

The photochemistry of linear conjugated trienes is closely connected to the isomeric 1,3-cyclohexadienes which are both their photochemical precursors and products. Such systems have been dealt with under cyclic 1,3-dienes in the previous section. The photochemical cyclization of m-stilbene to dihydrophenanthrene and ultimately to phenanthrene and hydrogen is formally analogous to reaction (13). This reaction (17) has been observed in a large class of aromatic compounds under a... 

Finally, the reaction of 374 with cyclic unsaturated substrates such as 1,3-cyclohexadiene, 1,4-dihydronaphthalene 1,4-dioxide, or acenaphthalene resulted in formation of cycloadducts 405, 407, and 409, respectively. These compounds could be subsequently aromatized by the use of DDQ in the case of 405 and 409 or polyphosphoric acid (PPA) in the case of 407 to generate l,3-dithiole-2-thiones 406, 408, and 410 (Scheme 55) <1996TL8085, 1999TL801>. The compound 410 was further treated with potassium /< r7-butoxide to give the dithiolate intermediate 411, which underwent dialkylation leading to 412 (Scheme 56) <1999J(P2)755, 1999TL801>. 

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