Trienes Cycloheptatrienes

It is evident that substituents play an important role in the photo-chemically induced cydization and migration reactions of conjugated trienes. Cycloheptatrienes isomerize to other trienes by migration and the direction of migration is frequently controlled by substituents present in the ring. The direction of cydization to the bicyclo[3.2.0]-heptadiene system is also dependent upon substituents. The relative competition of these two types of reactions has been shown to depend on the type of substituent present in the ring system. Most evidence indicates that these reactions are most likely excited singlet state reactions. 

Bridged Peroxides.—Routine procedures have been extended to the synthesis and characterization of photo-adducts of singlet oxygen with cyclo-octa-1,3,5-triene, cycloheptatrienes, 3,5-cycloheptadienone, cyclohepta-1,3-dienes, ° naphthalenes, anthracenes, a-pyrones, pyrazines and pyrimidines,imidazoles, and indenes. Transformation of some of these... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

Arene (e.g. benzene, substituted benzenes) cycloheptatriene, cycloocta-l,3,5-triene Tropylium (cycloheptatrienyl)... 

An interesting example of l,3-cyclohexadiene-13 5-triene interconversion is the reaction of norcaradienes to give cycloheptatrienes. Norcaradienes give this reaction so readily (because they are cw-1,2-divinylcyclopropanes, see p. 1445)... 

Cycloheptatriene, as an example of a conjugated triene, is mainly cyclopro-panated at an outer double bond (Scheme 6). This is true for Rh2(OAc)4, Cu(OTf)2 and Pd(OAc)2, but the highest yield is obtained again with the rhodium catalyst72>. Twofold cyclopropanation occurs to only a minor extent, as long as an excess of olefin is applied. With equal amounts of diazo ester and cycloheptatriene, double cyclopropanation increases and even traces of the triply cyelopropanated triene are found with Rh2(OAc)4 and Cu(OTf)2. This behavior essentially parallels the earlier... 

Conditions 22 °C molar ratio 800 (triene)/200 (EDA)/1 (catalyst) values in parentheses are yields obtained from equimolar amounts of cycloheptatriene and EDA... 

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.

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

Thus, heptafulvalene (522) was isolated in 33 and 65% yield after thermolysis of 517 in diglyme and its photolysis in THF, respectively [193]. An almost quantitative yield of 522 resulted when a mixture of 1-, 2- and 3-chloro-l,3,5-cycloheptatriene (518a) was treated with KOtBu in THF [206]. Even on variation ofthe concentration of the starting material and the temperature of the reaction, 522 turned out to be the exclusive product [207]. Also, the treatment of (trimethylsilyl)tropylium tetrafluoro-borate (519) with tetrabutylammonium fluoride [208] and the gas-phase pyrolysis of 7-acetoxynorbornadiene and 7-acetoxy-l,3,5-cycloheptatriene [209] afforded high yields of 522. Further, 522 was observed on FVT of N-nitroso-N-(7-norbornadienyl)-urea at 350 °C, which is believed to be converted into 7-diazonorbornadiene initially. Its decomposition should proceed via 7-norbornadienylidene to bicyclo[3.2.0]hepta-l(2),3,6,-triene (514) (Scheme 6.103) and then on to 5 [210]. The intermediacy of 514 is also suspected in the formation of 522 from 7-acetoxynorbornadiene. 

Simonetta and Heilbronner (1964) recently carried out calculations by the valence bond (VB) method for some simple cations, and compared the results obtained by this method, inter alia, with the results of Colpa and collaborators (1963) and of Koutecky and Paldus (1963). In the case of the proton addition complexes of mesitylene and cyclohepta-triene, the electron excitation energies calculated by the VB method agree very well with experiments, and also agree to a good approximation with the results of Cl calculations. The calculations also successfully reproduce the electron density of the cycloheptatriene cation. In this, a perturbation calculation allowed for the AO s adjoining the —CHg—CH2-lihkage. 

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. " ... 

Cycloheptatriene likewise gives the complex [Cr(CO)3(C7H8)] (VII R = H), in which all six -electrons of the triene are used in the metal-hydrocarbon bond (2). In the complex of 1-phenylcycloheptatriene the... 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

In addition to benzene rings, cycloheptatriene is activated or protected by forming the stable if complex 300. An example of the strong stabilization effected by coordination is shown by isolation of the optically active l,3,5-cycloheptatrien-3-ols 301, 304 and 305 as their enol forms. l,3,5-Cycloheptatrien-3-ol was isolated as complex 301 by hydrolysis of silyl enol ether 300. The triene system is stabilized by coordination,... 

While 154-157 are specific products for la, complexes of the type of 153a are the predominate products when 152 is reacted photochemically with acetylene (221), olefins (750,222,225), dienes, trienes, and tetraenes (224-226). In contrast to a previous report (227), 1,3,5-cycloheptatriene reacts with 152 like the other unsaturated hydrocarbons. Similar reaction products are obtained with lc as the diene component [Eq. (76)]. Although complexes comparable with 154,156, and 157 are missing, a trinulear species is formed... 

In the case of trapping with anthracene, a second product (86) is also isolated which apparently arises by trapping of the cycloheptatriene 87 it does not arise from 88203. At 80 °C the adducts of the triene with dienes is obtained exclusive of analogues 88 in 30-78% yield. 

Resonance, as introduced in Chapter 2, explains stability of anions and rationalizes trends in pKa values. However, resonance can also be used to rationalize the stability of cations (positively charged ions). As shown in Scheme 4.7, the stability of the cyclohepta-triene cation is explained by its resonance forms. There is, of course, another reason for the stability of the cycloheptatriene cation, which relates to the principles of aromaticity and which will not be discussed in detail in this book. 

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