Norcaradienes cycloheptatrienes

Diazomethyl)-1,4-diphenyl-1,2,3-triazole (354) reacts with 1,4-disubstituted benzenes or naphthalenes to give a mixture of carbene derived norcaradiene/cycloheptatriene products (355) and (356) (Scheme 68). Attack is favored adjacent to the more highly branched substituent. When R = = Pr, a mixture of triazole derivatives (357) and (358), formed by addition to benzene and... 

The bicyclic 3,4-dihydropyridines (51) and (53) illustrate that the electrocyclic ring opening is sensitive to substituents (80TL599). The 3,4-dihydropyridine (51) has been observed to be in equilibrium with the azepine (50). Consistent with the analogous carbocyclic system, norcaradiene-cycloheptatriene, the monocycle (50) is the predominant species present in this equilibrium. In contrast, the dimethyl derivatives (52) and (53) exist almost exclusively in the 3,4-dihydropyridine form. A greater steric interaction between these methyl substituents in (52) was given in explanation of these observations. 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

Note that these systems also undergo the electrocyclic norcaradiene-cycloheptatriene interconversion (Section 12.2, p. 654). 

For reviews of the norcaradiene-cycloheptatriene interconversion and the analogous benzene oxide-oxepin interconversion, see Maier, G. Angew. Chem. Int Ed. 1967, 6, 402 Vogel, E. Gunther, H. Angew. Chem. Int. Ed. 1967, 6, 385 Vogel, E. Pure Appl. Chem. 1969, 20, 237. 

C-NMR spectroscopy has also been employed to address the question of the norcaradiene/cycloheptatriene equilibrium. Thus whereas in cycloheptatriene the... 

A whole new field was opened when Vogel tried to synthesize [4.4.1]propella-2,4,7,9-tetraene (24) (alas, this name did not even exist at that time) in order to study the norcaradiene cycloheptatriene equilibrium in this potentially new bis norcaradiene . He was to find that none of the propellane is obtained. The open fluxional isomer 25 is... 

With alkynes as cosubstrates satisfactory yields of cyclotrimers are obtained in the presence of phosphane-free Co(I) catalysts, e.g r 3-cyclooctenyl-r 4-cycloocta-diene cobaltQ). The main products are tricyclo. O.O Joct -ene derivatives 32, sometimes accompanied by small amounts of the norcaradiene/cycloheptatriene derivatives 33 and 34 (Eq. 42)27>. 

Cyclopropanation of monocyclic aromatic rings (benzene and its derivatives) yields 7-acyl-bicyclo[4.1.0]hepta-2,5-dienes that are subject to the norcaradiene/cycloheptatriene valence equilibrium (see Houben-Weyl, Vol.4/3, p509 and Section 1.2.I.2.4.2.6.2.). Whereas 7-mono-... 

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). 

Although nonaromatic cyclic alkaoligoenes always react with transient phosphorus-substituted carbenes to give 1 1 adducts, aromatic derivatives sometimes give rise to a mixture of 1 1 and 1 2 adducts. Numerous bicyclo[4.1.0]hepta-2,4-dienes (norcaradienes) have been prepared by reacting transient phosphorus-substituted carbenes with aromatic compounds in order to study the influence of the substituents on the norcaradiene-cycloheptatriene equilibrium. Benzene itself and its functionalized derivatives, in which the three bonds are not equivalent, will be reviewed in separate sections. 

Maas, G., and Regitz, M., Carbenes. Part 10. Substituent dependence of the norcaradiene/cycloheptatriene-equitibrium investigated in 7-phosphoryl- and7-carbonylsubstituted systems, Chem. Ber, 109,2039,1976. 

Walk rearrangements have been observed in many carbo- and heterocyclic norcaradiene cycloheptatriene systems upon thermal as well as photochemical excitation. Table 4 contains selected examples. In this connection, the very different thermal stability of ll,ll-dimethyl-l,6-methano[10]annulene and its radical anion is worth mentioning (59). The neutral hydrocarbon rearranges to 7,7-dimethyl-l,2-benzocycloheptatriene at temperatures between 150 and 190°C [Table 4, entry 3, X = C(CH3)2 log A = UA Ea = 35.9 kcal/mol]. The corresponding rearrangement of the radical anion produced by reduction of the hydrocarbon with potassium occurs at -110°C already. The activation energy is lowered here by about 25 kcal/mol over that of the hydrocarbon. The norcaradiene walk has also gained some synthetic importance, for example, in the preparation of the unsaturated bicycle 44, a precursor of heptalene (60). 

An experimental distinction among these three alternatives proved to be difficult, particularly as norcaradienes and cycloheptatrienes usually interconvert easily via valence bond isomerization (norcaradiene - cycloheptatriene EA = 6.5 kcal/mol log A = 11.8) (64) and cycloheptatriene ring inversion (1,3,5-cyclo-heptatriene EA - 6.3 kcal/mol) (Figure 4 ref. 65). Therefore it was necessary to resort to such devices as introducing chirality into the molecules by way of an additional stereochemical label. 

Several experimental tests using the chiral norcaradiene cycloheptatriene systems 47-51 (66-70) have been reported. Conflicting results have finally been... 

Figure 4. Exo-endo equilibration via facile norcaradiene-cycloheptatriene valence bond isomerization (64) and cycloheptatriene ring inversion (65).

Among possible alternatives to DPMR we discarded the cx.cx-elimination of benzidine 12 from biradical-zwitterions 2 and 3 and formation of carbene 23 based on the absence of trapped carbene products in experiments with cyclohexene and ethanethiol (Xgxc = 300 nm). However, norcaradiene 6 produces carbene 23 when subjected to 350-nm irradiation. Another plausible alternative to DPMR involves initial C-CN bond homolysis, which is energetically favorable in both excited states, followed by a photochemical reaction of Crystal Violet radical. We discarded this mechanism because none of the expected products of the photochemical reactions of Crystal Violet radical has been detected during the course of our work. Nevertheless, isolation of MGCN in the trapping experiment with ethanethiol points to the possible involvement of the C-CN bond homolysis in S. Finally, although norcaradiene, cycloheptatriene, and norbomadiene are valence tautomers and are easily interconverted into each other [188], products of the latter two kinds have been neither detected nor isolated in our study [66, 30]. 

The scrambling of deuterium in 3,4-benzo starting material was ascribed to a 1,5-hydrogen shift followed by reversible norcaradiene-cycloheptatriene electro-cyclization, and subsequent 1,5-hydrogen shifts could account for the 1,2-benzo isomer (Scheme 12.10). 

Norcaradienes-Cycloheptatrienes. N.m.r. investigation of the norcaradiene-cycloheptatriene equilibrium (564) (565) has established that the relative order of... 

Thermal rearrangements in homoporphyrins proceed by a mechanism resembling the norcaradiene-cycloheptatriene interconversion. ... 

Gunther, H., Tunggal, B.D., Regitz, M., Scherer, H., and KeUer, T, Apphcation of carbon 13 resonance spectroscopy. 2. Two new norcaradiene-cycloheptatriene equhbria, Angew. Chem. Int. Ed. Engl, 10, 563,1971. 

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