Cycloheptatrienes ring expansion

Carbenes are so reactive that they add to the double bonds of aromatic rings. The products are usually not stable and rearrange to give ring expansion. Carbene reacts with benzene to give cycloheptatriene ... 

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. ... 

Buchner method of ring expansion Thermal or photochemical reaction of ethyl diazoacetate with benzenes and its homologs to give the isomeric esters of cycloheptatriene carboxylic acid. 68... 

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... 

Thermolysis of the diazo compound 395 also resulted in aromatic C—H insertion (to 396), together with addition to a benzenoid double bond and ring expansion to the cycloheptatriene 397.466 The same products were obtained from the reaction catalyzed by (C6F5)4Cu (Eq. 136).467... 

Scheme 14 Indanone (146) was converted to alcohol (147). The acetyl derivative of (147) after demethylation was made to react with glyoxal to obtain lactone (149), which reacted with dibromocarbene and diazomethane to yield (150). Its silyl derivative was subjected to ring expansion by radical pathway to obtain cycloheptatriene (151). Its formyl derivative on dehydration gave the natural product lettucenin A.

As with other diazoalkanes, diazomethane reacts with alkenes to form cyclopropane derivatives (sec. 13.9.C.i).272 Reaction with aromatic derivatives leads to ring expansion to cycloheptatriene derivatives.223 Both of these reactions (addition to an alkene or arene insertion) involve generation of an intermediate carbene and addition to a jt bond they will be discussed below. Many of the reactions of diazomethane tend to be ionic in nature and are, therefore, set aside from the other diazoalkane chemistry in this section. One of the commonest uses of diazomethane itself is esterification of small quantities of acids, especially acids that are precious for one reason or another. The reaction is quantitative and gives good yields of a single product, as in Tadano s conversion of 338 to the methyl ester of 339224 in a synthesis of (-)-verrucarol. 

The reaction of carbenes with aromatic hydrocarbons is related to that with alkenes. Doering and Knox (1950, 1953) investigated this reaction using benzene as substrate even before their work with cyclohexene. They observed, however, ring expansion to give cycloheptatriene besides toluene (8-3, X = H). Norcaradiene as an intermediate was isolated only much later in the addition of dicyanomethylene... 

Besides the general application in synthesis of cycloheptatrienes, different bicyclic products—for example, 0-tetralones and azulenones—can be prepared via the intramolecular Buchner ring expansion. 

In electrocyclizations that involve the participation of three-membered rings, the equilibrium favors the ring-opened species. The venerable Buchner ring expansion provides an apt example. A key step in Mander and co-workers har-ringtonolide synthesis involves a Buchner ring expansion of an aryl ring to a cycloheptatriene (Scheme 19.45). ... 

Sodium dihydrogen phosphate/acetic acid Ring expansion of tosylates to cycloheptatrienes... 

A reverse reaction, namely ring expansion of an arene ligand to a cycloheptatriene ligand, was discovered during attempts to acylate the mixed sandwich complexes Tr-CsHsCr-Ti-CeHe and yr-CsHsMn-Tr-CsHs [17]. 

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