Norcaradienic acid

Ethynylcycloheptatriene (32) and its derivatives cleanly isomerize to phenylal-lenes in the presence of acids according to the carbocationic mechanism depicted in Scheme 4.15. The first step of the transformation is the protonation of the norcaradiene tautomer, which is in equilibrium with 7-ethynylcycloheptatriene. 

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 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. 

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. 

In the case of 2-halotropones, for instance, the reaction is believed to begin with the attack of hydroxide ion onto C-l or C-3, and it results in the formation of benzoic acid or salicylaldehyde, respectively. The balance between the two processes is affected by the alkali concentration More dilute alkali favors the second route. According to Pietra (79ACR132), the key step of the first route is ring closure in the initial hydroxide adduct to give a norcaradiene intermediate. 

Formation of 648 from 647 was experimentally discarded. Since the most basic site of 646 is the carbonyl group, both Bronsted and Lewis acids should first coordinate to this position. In the case of Bronsted acid, protonation could occur at the electron-rich 9-position, and this was followed by deprotonation at the 1 la-position, which is promoted by the carbonyl protonation. In contrast, the Lewis acid cannot add to the 9-position and thus the skeletal rearrangement to give 648 took place. This kind of rearrangement through norcaradiene tautomers, shown in Scheme 128, is called walk rearrangement in thermal reaction of cycloheptatrienes <2002CL260>. 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

The Buchner acids exemplified by 287 were originally formulated as norcaradienes. However, Doering s group, in work which probably kindled modern interest in this subject despite an enigmatic numbering system demonstrated that these compounds were cycloheptatrienes. Subsequently line broadening at low temperatures has been shown to occur for only C(l), C(6) and C(7) in the C-NMR spectrum. ... 

Cyclic exponents of the same elimination type are of particular interest. Thus, numerous cyclopentanones photolytically decarbonylate to give 1,4-dienes (p. 876 ff. in Ref. 108)). With thujone (194) this [l,2,(3)4]-elimination of carbon monoxide proceeds quantitatively to give 195 109). With silver nitrate the norcaradiene 196 yields 197 (95%)110) apparently regioselectively, and the [l,2,(3)4]-elimination of methanethiol from 198 to give 199 was realized thermally (16%), acid-catalyzed (59%), and photochemiciiily (ca. 5%)105). For the acid-catalyzed reaction (acetic acid, 100 °C) a non-stereospecific process has been proved 105). 

The same basic method has also been used to prepare 4,4-dichlorotricyclo[5.4.1.0 ]dodeca-7,9,11-triene (17). The starting dibromide 16 is heated with potassium hydroxide in methanol/ dioxane, and the crude norcaradiene derivative is treated with dilute perchloric acid in dioxane/ water, giving the product in 88% overall yield. 

Similar to the addition of halogens, cycloproparenes react under hydrogenation conditions either by initial attack at the cyclopropene bond or via cleavage of one of the lateral cyclopropane bonds. In the former case, the intermediate norcaradiene opens to a cycloheptatriene which is further reduced to cycloheptane. In the latter, the reduction leads to toluene or methyl-cyclohexane. For example, benzocyclopropene, on reaction with platinum oxide in acetic acid, was hydrogenated to cycloheptane (49%) and methylcyclohexane (51%). 1,1-Difluoroben-zocyclopropene, under the same conditions, gave cycloheptane exclusively (85%). Hydrogenation of benzocyclopropene with Raney nickel gave toluene in 90% yield. 

Oxidation of solutions of benzodiazepines in benzene or acetic acid with a high-pressure mercury arc in oxygen also causes oxidative ring contraction to give quinoxalines.49 Since hydrolytic cleavage is ruled out in this case a photolytic norcaradiene-type rearrangement of the intermediate ketone to a quinoxaline was suggested (Scheme 5). 

Several X-vay structure determinations of three-membered-ring compounds have been reported, including a cyclopropenium salt, 7-PO-substituted norcaradienes (6 X = H, Cl, or Br), chrysanthemic acid derivatives, and 1,6 8,13-cyclopro-panylidene[14]annulene (7). The photoelectron spectrum of (7) has also been reported. ... 

Norcaradienes are intermediates in several types of reaction, including intramolecular and intermolecular additions of carbenes to aromatic rings, the chromic acid oxidation of cycloheptatriene to benzaldehyde, the thermal isomerization of 7-substituted norbornadienes to cycloheptatrienes, °° and the reaction of pyridine iV-oxides with benzynes. An interesting feature of the latter reaction is the transfer of oxygen from the pyridine to the benzene nucleus (Scheme 61). 

Since Theodor Curtius reported the synthesis of ethyl diazoacetate in 1883, Buchner had investigated its reactions with carbonyl compounds, alkenes, alkynes, and aromatic compounds for more than 30 years.His extensive contributions in this area resulted in two reactions named in his honor the Buchner-Curtius-Schlotterbeck reaction (formation of ketones from aldehydes and aliphatic diazo compounds) and the Buchner reaction. The prototypical example of the latter involves the thermal or photochemical reaction of ethyl diazoacetate with benzene to give (via norcaradiene 7) a mixture of four isomeric cycloheptatrienes 8-11. Initially, Buchner believed that a single norcaradiene product 7 was generated from this reaction, but later, he realized that the hydrolysis of the product afforded a mixture of four isomeric carboxylix acids. The norcaradiene formulation persisted until 1956 when Doering reinvestigated this reaction. ... 

The first direct chemical evidence for the formation of the norcaradiene system in the intramolecular Buchner reaction was obtained in the rhodium(II)-catalyzed decomposition of l-diazo-4-(2-naphthyl)butan-2-one 44. This reaction provides the tetracyclic norcaradiene 45 and tricyclic ketone 52 in 71% and 8% yield, respectively. When a catalytic amount of trifluoroacetic acid is added to 45, tricyclic ketone 51 is formed. It is surprising that compound 45 is recovered quantitatively after treatment with triethylamine in dichloromethane under reflux. The formation of 52 is explained in terms of an attack of the carbenoid carbon of 44 on the 2,3-double bond of the naphthalene nucleus followed by double bond migration in the tricyclic nonconjugated ketone 49. 

Norcaradienes. Variable-temperature n.m.r. data have established the existence of Buchner s acid (286) and a number of spironorcaradienes (288), with the equilibrium... 

Boron. The activation parameters have been determined for the equilibrium between [Pr2B(cycloheptatrienyl)] and [7- vc -Pr2B-norcaradiene]. NMR spectroscopy has been used to determine exchange rates of B(C6F5)3 adducts of PhC(0)X. B NMR spectroscopy has been used to examine the binding of acetone to [Bu CH=C B(C6F5)2 2] and the thermodynamic data determined. B NMR spectroscopy has been used to study the formation of borate esters of ascorbic acid and isoascorbic acid. NMR spectroscopy has been used to... 

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