Cycloheptatriene from benzene

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

In a most elegant manner, Srinivasan has prepared hydrocarbon 490 from benzene and cycloheptatriene in only three steps.356 Thus, irradiation of a benzene solution of bicyclo[3.2.0]hepta-2,6-diene gave a mixture of 488 and 489 as major... 

Tetrafluorenide of cerium has been synthesized in the interaction of (C 3H9)Na with dipyridinehexachloride of cerium in THF at room temperature [13]. The cyclohepta-trienyl complex (C7H7)2CeCl2 has been isolated in 50% yield from the reaction of (C5H5NH)2CeCl6 with cycloheptatriene in benzene medium [14]. The product is dark brown crystals, soluble in common organic solvents and stable in relation to dilute acids and alkalis. 

Cyanuric chloride, 298, 305, 315 Cycloheptatriene, formation from benzene, 61... 

The thermal decomposition of diazo(phenylsulfonyl)methane 223 under a nitrogen atmosphere generates phenylsulfonylcarbene which is trapped by olefin such as cyclohexene to give norcaranes 224 and 225 (equation 138)132. No cycloheptatriene derivative is isolated from the thermolysis of223 in benzene133. In contrast, intramolecular insertion of sulfonylcarbenes into a benzene ring is observed in the thermolysis of 226 (equation 139)134. 

Scheme 31. Isomer distribution [%] of Rh CFjCOO -catalyzed cyclopropanation of substituted benzenes with methyl diazoacetate at 22 °C. The numbers refer to the percentage of 1,3,5-cyelohepta-triene-7-carboxylate from the total cycloheptatriene isomers.

Ring expansion of cycloproparenes to cycloheptatrienes or tropones has been discussed in the context of electrophilic addition to cycloproparenes. When 1,1 -di-chloro-2,5-diphenylbenzocyclopropene (22) is thermolyzed in refluxing benzene, the dimer 373 is formed as a mixture of /Z-isomers. It is believed to arise via dimerization of the carbene 372, which, in turn results from an allylic rearrangement of22to371. ... 

Reaction of CF with benzene generates the 7-fluoronorcaradien-7-ly radical (39), which abstracts hydrogen (from added isobutane) and opens to 7-fluorocyclohepta-triene (40). Cycloheptatriene (10) is trapped as tropylium fluoroborate (41) by the addition of BF3 (Eq. 21)P An additional product of CF + benzene is fluorobenzene (42), in which labeling studies demonstrate that the attacking carbon contains the fluorine in 42. The interesting transfer of CH in Eq. 28 is proposed to account for the formation of 42. " ... 

Terao and Shida130 have reported that CH2 from ketene photolysis in a quartz or Pyrex vessel reacted with benzene to give cycloheptatriene and toluene in the ratio 2.3 1, with evidence of at least one other unidentified product. The product ratio was independent of pressure in the range 16-1000 mm., including 4r-60 mm. CH2CO, 12-60 mm. C6H6, and added N2 or C02. 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

The overall mechanistic picture of these reactions is poorly understood, and it is conceivable that more than one pathway may be involved. It is generally considered that cycloheptatrienes are generated from an initially formed norcaradiene, as shown in Scheme 30. Equilibration between the cycloheptatriene and norcaradiene is quite facile and under acidic conditions the cycloheptatriene may readily rearrange to give a substitution product, presumably via a norcaradiene intermediate (Schemes 32 and 34). When alkylated products are directly formed from the intermolecular reaction of carbenoids with benzenes (Scheme 33 and equation 36) a norcaradiene considered as an intermediate alternatively, a mechanism may be related to an electrophilic substitution may be involved leading to a zwitterionic intermediate. A similar intermediate has been proposed143 in the intramolecular reactions of carbenoids with benzenes, which result in substitution products (equations 37-40). It has been reported,144 however, that a considerable kinetic deuterium isotope effect was observed in some of these systems. Unless the electrophilic attack is reversible, this would indicate that a C—H insertion mechanism is involved in the rate-determining step. 

Another aromatic molecule containing six n electrons is C7H7+, the tropylium ion, derived from cycloheptatriene. This positive ion forms fewer complexes than does benzene, and they are less thoroughly studied. A molecule that has 10 electrons and has an aromatic structure is the cyclooctatetraenyl ion, C8H82. Some sandwich compounds containing this ligand are known as well as complexes of the type... 

Although photodecarboxylations of esters and carboxylic acids are widely known, loss of CO2 from lactones is usually only a minor photochemical pathway. When pyrone (64) (Scheme 15) was irradiated in benzene solution with 356 nm light under conventional, low-intensity conditions, no products were formed, but under high-intensity, laser-jet conditions, the carboxylic acid (66) and cycloheptatriene (67) were obtained. The products can be accounted for, as shown in Scheme 15, on the assumption of initial C-O bond cleavage to... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

Silver complexes have also been described for the cyclopropanation reaction. When using benzene, the use of Tp Ag(thf) (where Tp "" =hydrotris(3, 5-bis(trisfluoromethyl)pyrazolyl)borate for the rules of nomenclature of Tp" ligands see reE ) provided products derived from the addition of the carbene moiety to the arene ring (Scheme 7a), followed by ring expansion into a cycloheptatriene, in the... 

Cu(TpMs)] catalyzes the addition of the iCHCC Et unit (derived from N2HCHCC>2Et) to benzene which yields a cycloheptatriene ring, in analogy with the Buchner reaction. When alkyl groups are linked to the aromatic rings, the selectivity of the reaction can be oriented toward insertion into an alkyl C-H bond by using [Cu(TpMs)].48 On the other hand, [Cu(Tp )] efficiently catalyzes the insertion of... 

A secondary aspect of the solid-state interactions in tropolone compounds is the ability to form rr-stacking units. This effect is due largely to the delocalisation of the cycloheptatriene ring system over the total molecule. Typical tt-tt interactions in benzene molecules range from 3.5. 0 A (mean 3.81 A) and with a nearly parallel orientation of 0 ° (mean 0.67°) between individual benzene molecules. In tropolone compounds the interaction is of order 3.72(3) A with an inter-planar angle between tropolone molecules 0.7(8)°. We present in Table 15-3 a selection of tropolone compounds [in total 21 entries]. 

In 1945 Michael J. S. Dewar suggested that the tropylium ion (the cation derived from cycloheptatriene) should also be aromatic (Figure 9). This was confirmed in 1954 since then, the dianion of butadiene and the dication of cyclooctatetraene have also been shown to be aromatic. Like benzene, all four of these ions are planar rings with six tt electrons. According to Hiickel s rule the cyclopropene cation should also exhibit aromaticity, and it does. (In this case n = 0, and 4n + 2 = 2.) The planar anion of cyclononatetraene and the dianion of cyclooctatetraene should also be aromatic (n = 2, and 4n + 2 = 10), and both of them are. 

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